CA1049257A - Two stage drying with single auger - Google Patents
Two stage drying with single augerInfo
- Publication number
- CA1049257A CA1049257A CA270,886A CA270886A CA1049257A CA 1049257 A CA1049257 A CA 1049257A CA 270886 A CA270886 A CA 270886A CA 1049257 A CA1049257 A CA 1049257A
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- drying
- gas
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- zone
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Abstract
ABSTRACT OF THE DISCLOSURE
A relatively low temperature, modestly sub-atmospheric pressure, double or more pass and controlled fall multiple zone technique for drying waste materials involves feeding the wet disintegral material through conveyance means with agitation and forwarding through at least two distinct and separate, yet inter-connected, drying zones wherein the material is sequentially subjected to the drying, action and influence of respectively lower inlet temperatured hot fluid or gas streams which are at least substantially uniformly passed through the material that each of said streams are drying in such a manner (resulting from appropriate adaptation of the bed-forming means containing the material in its passage through the assembly and due to the expanding, increasing cross-sectional area of the material con-tained and being treated for drying in the assembly) as to physically pass through the material being dried at a constantly reducing velocity of the gases. The outlet temperature of the gas in the last zone (from which the finally dried material is removed) is greater than that, in decreasing sequence, of that from the preceding zone(s) when the solid material being dried is below its critical moisture content which is, of course, when the moisture rate is limited by its diffusion within the material being dried. Otherwise, enough flow can be put into the system to cause desired temperature effects with heat trans-fer limitations then being the controlling factor.
A relatively low temperature, modestly sub-atmospheric pressure, double or more pass and controlled fall multiple zone technique for drying waste materials involves feeding the wet disintegral material through conveyance means with agitation and forwarding through at least two distinct and separate, yet inter-connected, drying zones wherein the material is sequentially subjected to the drying, action and influence of respectively lower inlet temperatured hot fluid or gas streams which are at least substantially uniformly passed through the material that each of said streams are drying in such a manner (resulting from appropriate adaptation of the bed-forming means containing the material in its passage through the assembly and due to the expanding, increasing cross-sectional area of the material con-tained and being treated for drying in the assembly) as to physically pass through the material being dried at a constantly reducing velocity of the gases. The outlet temperature of the gas in the last zone (from which the finally dried material is removed) is greater than that, in decreasing sequence, of that from the preceding zone(s) when the solid material being dried is below its critical moisture content which is, of course, when the moisture rate is limited by its diffusion within the material being dried. Otherwise, enough flow can be put into the system to cause desired temperature effects with heat trans-fer limitations then being the controlling factor.
Description
104925~7 BACKG~OUND OF THE INVENTION
In many thermal processes, particularly those involv-~ ing solids treatment or utilization, the amount of moisture present in the feed has a significant effect on the processing rate and process control, and, consequently, on the overall practicality, attractiveness and economics of the process.
For example, most carbonizing furnaces can be operatedat a much higher through-put if the input material, such as wood waste, is pre-dried before feeding into the unit.
Currently, many processes are under development involv-ing the processing by pyrolysis and other means of various waste materials into useful products and useful fuels. Solid wastes, generally predominantly cellulosic in compositional character, such as wood chips and the like, sawdust, paper and other cellulosic goods, agricultural grain, fruit, veyetable and meat materials, animal manures, municipal wastes, which advantageously may be shredde~ and/or air classified, and so forth including mixtures of such materials with one another (or even other substances such as waste petroleum products and oil stocks) are prominent amongst the varieties of things of interest for utilization according to such recovery procedures and techniques.
In practically all of these processes, the question of whether or not the waste rnaterial is dry or wet (as regards water content, of course) plays an important role in the overall J performance of the system and on the desirability and economics of the operation. For these reasons and others, the importance - of drying of waste materials prior to conversion to valuable recovery products is increasing. At the same time, the drying ~ 30 of waste solids for subsequent recovery and/or efficient and beneficial disposal thereof relates to the environment and to energy consumption. These are very important aspects of more beneficial and efficient recovery and profitable accountability of resources.
Many of the problems involved in and with good drying of waste prior to treatment are well brought forth and illus-trated in the article entitled "Gasification Of Solid Wastes In Fixed Beds" by A.C.W. Eggen and Ronald ~raatz appearing as Publication 74-~A/Pwr-10 in the Transactions Of The American Society Of Mechanical Engineers. Another interesting presen-tation (which, in effect, serves to high-light the surprising and unexpected results of the present invention) is in the article entitled "Fuel Preparation Systems Using A Rotary Drier"
by Stanley P. Thompson which was presented in Denver, Colorado during September 3-5, 1975 during a Conference there on Wood Residues As An Energy Source which was sponsored by and is available from the Forest Products Research Society. Other references of interest include U.S. Patents Nos. 403,411; 920,-050; 1,554,854; 1,642,469; 1,749,451; 2,057,681; 2,242,702;
In many thermal processes, particularly those involv-~ ing solids treatment or utilization, the amount of moisture present in the feed has a significant effect on the processing rate and process control, and, consequently, on the overall practicality, attractiveness and economics of the process.
For example, most carbonizing furnaces can be operatedat a much higher through-put if the input material, such as wood waste, is pre-dried before feeding into the unit.
Currently, many processes are under development involv-ing the processing by pyrolysis and other means of various waste materials into useful products and useful fuels. Solid wastes, generally predominantly cellulosic in compositional character, such as wood chips and the like, sawdust, paper and other cellulosic goods, agricultural grain, fruit, veyetable and meat materials, animal manures, municipal wastes, which advantageously may be shredde~ and/or air classified, and so forth including mixtures of such materials with one another (or even other substances such as waste petroleum products and oil stocks) are prominent amongst the varieties of things of interest for utilization according to such recovery procedures and techniques.
In practically all of these processes, the question of whether or not the waste rnaterial is dry or wet (as regards water content, of course) plays an important role in the overall J performance of the system and on the desirability and economics of the operation. For these reasons and others, the importance - of drying of waste materials prior to conversion to valuable recovery products is increasing. At the same time, the drying ~ 30 of waste solids for subsequent recovery and/or efficient and beneficial disposal thereof relates to the environment and to energy consumption. These are very important aspects of more beneficial and efficient recovery and profitable accountability of resources.
Many of the problems involved in and with good drying of waste prior to treatment are well brought forth and illus-trated in the article entitled "Gasification Of Solid Wastes In Fixed Beds" by A.C.W. Eggen and Ronald ~raatz appearing as Publication 74-~A/Pwr-10 in the Transactions Of The American Society Of Mechanical Engineers. Another interesting presen-tation (which, in effect, serves to high-light the surprising and unexpected results of the present invention) is in the article entitled "Fuel Preparation Systems Using A Rotary Drier"
by Stanley P. Thompson which was presented in Denver, Colorado during September 3-5, 1975 during a Conference there on Wood Residues As An Energy Source which was sponsored by and is available from the Forest Products Research Society. Other references of interest include U.S. Patents Nos. 403,411; 920,-050; 1,554,854; 1,642,469; 1,749,451; 2,057,681; 2,242,702;
2,365,890; 2,116,059; 2,981,528; 3,044,182; 3,289,318; 3,303,-578; 3,475,832; 3,646,689; 3,827,158; and 3,837,271. Additional are in the overall field appears, inter alia, in U.S. Search Classes 34, 99, 110 and 432 and International Search Classes F26b and A231.
Amongst the more salient problems which are most frequently encountered in high capacity waste material (or equivalent) driers are: undesirable degradation or decomposi-., ; tion of the material being treated; gas and solids pollution from the drying operation; provision of dried material in an optimum and most suitable condition for use as the pre-dried feed stock to subsequent treatment and/or recovery applications 30 and operations; heat transfer achievement leaving something to be desired as to results and effectiveness; deleteriously and/or undesirably high temperature levels involved in the procedure;
impracticable and cumbersome materials handling requirements and involvements; and overall efficiency and economy of both equipment and the operation involved.
FIELD AND OBJECTIVES OF THE INVENTION
The present invention pertains to and resides in the overall field(s) of an improved, high-capacity and large volume drier and drying method for reducing and eliminating moisture in various generally particulate or disintegral materials, parti-cularly waste materials of this sort and most especially waste intended for processing and treatment (including chemical under-goings and transformations) in subsequent thermal steps and operations such as, but not limited to, pyrolysis.
Amongst the essential and more predominant purposes and objectives of the invention are to provide a technique, including both apparatus and procedure therefor, for better and more attractively and desirably drying waste or other equivalent -materials to most beneficially and most sensitively condition them for subsequent handling and/or treatment and processing, especially as prepared optimally-dry feed stock for thermal processes, wherein and whereby there is materially and r.lost advantageously reduced if not entirely avoided or eliminated the foregoing alluded-to problems of possible pollution, moisture-removal ineffectiveness, undesirable heat transfer efficacies, excessive heat and temperature requirements, diffi-cult materials handling and inefficiency and/or impractical and ~ exhorbitant apparatus and processing costs and other disadvan-- tageous and/or detrimental incidences.
Practice of the present invention generally provides and achieves the indicated novel and heretofore realistically ~ 30 unobtainable results and possibilities, with great attendant practicality and expanded utility thereabout; all this being achieved with great ease, simplicity, versatility, economy and effectiveness; and all predictably according to well-engineered and basically sound concepts and applications of fundamental technological postulations.
- The above-indicated and many more and other of the benefits and advantages in the obtainable by and upon practice of the present invention are more particularly set forth and easily evident in the ensuing Specification, taken in conjunc-tion with the accompanying Drawing (inwhich, insofar as possible, like reference nurnerals and letters refer to like or analogous parts and constituents and all the representations are depicted in more or less generally schematic and/or symbolic and/or fanciful portrayal with many obviously purposefully simplified exaggerations or unnecessary replications of parts or elimina-tion of details non-essential to or especially meaningful for expostulation of the ru~iments of the invention), wherein:
Figure 1 is an exploded, perspective view of the essential components of a drier assembly according to the invention;
Figure 2 is a schematic lay-out view in side elevation of an associated drier assembly;
Figure 3 is a.schematic cross-sectional view of the screw conveyor means and trough taken along Line 3-3 in Figure 2;
Figure 4 is a broken-out cross-sectional view of a plate and screen combination providing both bottom support for the material being dried in apparatus according to the invention as well as admission for passage through the material being - dried of the drying fluid utilized for the purpose in practice of the invention;
Figure 5, which appears on the same sheet of drawings ~ 30 as Fig. 1, is a fanciful flow-diagram type of view depicting, in :; side elevation showing, the general processing scheme and approach for drying material pursuant to the invention; and 104925~
Figure 6 in graphical representation illustrates typical advantageous results obtained in use of the drying assembly and method of the invention.
GENERALIZED REVELATION OF THE INVENTION
In basic essence, the present invention and the findings and discoveries upon which it is based involve a relatively low temperature, modestly sub-atmospheric pressure technique in which the wet, disintegral material feed is conveyed by agitating and forwarding screw conveyor and/or auger-like handling and transfer means through at least two distinct and separate, yet interconnected, multiple dryin~ zones~
The material being dried is sequentially subjected to the drying action and influence of hot gas streams of various temperatures (the total heat content of which will generally decrease in sequential zones). The hot gas streams are substantially uni-formly passed through the material (resulting from appropriaté
adaptation of the bed-forming means containing the material and the gas flow control means) at a predetermined and optimum transverse velocity with the outlet temperature of the gas in the last zone (from which the finally dried material is removed) being fixed to be most efficient according to the critical moisture level of the material being treated.
With greater particularity, the salient features and advantageous characteristics of associated and interacting components and elements utilized in the distinctive cooperative apparatus assembly and means combination with and upon which the associated method steps and procedure are followed in embodi-ments and practice of the invention are as in the following amplified delineations:
I. In general, the invention contemplates assembly and use of a conveyor dryer in which the auger-like or screw conveying mechanism serves to very thoroughly mix the material while it is being conveyed. This avoids stratified drying as so frequently occurs with a batch operation or in such conveyor-type dryers as belt or vibrating bed conveyors where the material is not thoroughly mixed while it is being conveyed and dried. This is of particular importance with combustible type materials, such as wood and paper wastes. It is of significance that in the screw-type conveyer dryers employed pursuant to the present , invention, the mixing can be further improved or increased by ; providing and positioning various types of supplemental agita-~ 10 tion devices along the length of the screw. A noteworthy and ; meritorlous aspect of operation according to the invention is that moisture variation from the top to the bottom of the material in the bed moving along in its drying passage along the screw length, as a result of the thorough mixing thereby imparted, results in only small and literally negligible variations in moisture content measured vertically with bed depth.
; II. The present conveyor dryer operates at sub-atmospheric pressure. Drying gases are sucked through the dryer by a fan ; 20 or equivalent fluid suction means located on the off gas side of the dryer. This particular arrangement offers several advan-tages over pressurized systems, namely: (a) The physical size of the fan or fluid moving and withdrawing installation, since the same is used on the "cold" side of the dryer, is significan-tly reduced. The volumetric quantity of drying gases is signif-icantly less than that which is supplied on the "hot" side thus ; allowing a smaller fan to be utilized and one suitable for low temperature operations. (b) It greatly simplifies and enhances dryer design from a sealing standpoint. The dryer does not have to contain a pressurized gas, and small leaks into the dryer are not detrimental to the dryer's operation. (c) Operations at sub-atmospheric conditions allows floating (or unlocked or unsealed) access doors to be utilized so that quick and efficient pressure release is available to prevent or avoid damage in cases of ~ explosion and the like.
III. The design of the screw trough in the dryer, as mentioned, I is altered to reduce the amount of entrained material that leaves the top of the bed in thedrying gases. One side of the screw trough is flared out from the base so that in a most beneficial embodiment the gas flow area is approximately about three times that of a standard screw trough to reduce the velo-10 city of the leaving gases. The flaring of one side of the trough is coupled with the inherent conveying characteristics of a screw conveyor ! The screw tends to cause the conveyedmaterial to ride up on one side of the trough considerably higher than on hl the other side. Consequently, the side that is flared out is the low side; and the flaring has no effect on the conveying characteristics of the screw trough arrangement. In addition, this inherent feeding characteristic of a screw conveyor (i.e., the drying material riding high on one side of the trough and I low on the other side) must be accorded proper account in designing and positioning the porous portion of the base of the screw trough so that ~he gases will flow uniformly through the bed of materials.
IV. The use of a screw conveyor for a dryer most advantageously enables the dryer to be operated with various bed depths in different compartments or sections of the dryer. For example, a very shallow bed can be operated in the first portion of the dryer by a suitable selection of screw pitch. In another section of the dryer, the screw pitch can be changed so that material will run deeper so as to thereby considerably increase the residence time. These features can be included in the design of the dryer to give most advantageous operating conditions for any given instance and to meet particular requirements in practice ! -7-of the invention; considerin~ in and f~r all of this the type of material to be dried and the inherent drying chaxacteristics of the material itself which dictate the conditions under which optimum drying is to be accomplished. Contemplation must be ¦ made of the fact, however, that a bed which is too deep will not ¦ mix as thoroughly as desired without involving necessity for ¦ additional agitation.
V. The dryer has a porous base in the material handling trough.
The design of this generally foraminous substrate and its position relative to the bed depth is an important feature in , practice of the invention. Firstly in this connection, the part of the base through which the drying gases flow is located to one side of the dryer trough so that it is centered with respect ; to the bed depth of the material being conveyed down the dryer.
The position is usually better off-center with respect to the trough, since the material tends to ride up on one side of the j~ trough and low on the other side. The foraminous base is made j from a perforated plate with benefically uniformly distributed ~ and patterned holes, slots or other open or protected (as in ¦ 20 forms of expanded metal lathe and the like) apertures of any suitable opening geometry which cover anywhere from say 25 or so to 75 or so, most advantageously approximately 40, percent of the overall total area of the porous base. The plate serves as both a structural part of the trough and as a means of passing j the drying gases into the bed. The holes or other openings, which may be of any desired effective diameter or other opening size, are ordinarily larger than the particle sizes of much of i the material being conveyed above the perforated plate.
¦ Ordinarily, actual individual hole sizes (or equivalent open areas) of between about 1/6th of an inch to about l/lOth of an inch - advantageously 1/8 or so inch - in diameter are effective for most materials being handled for drying. The open area for !
, -8-flow of the drying gases through these holes or the like is small, so that the gas velocity is high and prevents the holes from plugging. This is a self-cleaning feature of dryers according to the invention. The perforated plate alone ordinar-..
ily does not provide enough pressure drop to properly distribute the drying gases along the base of the dryer. To increase the pressure drop across the foraminous base, a fine mesh screen is stretched across the outside of the trough area over the perfor-ated plate. This mesh is sized so that the pressure drop across 10 the base region is generally and for a preponderance of the materials to be treated in the approximate neighborhood of 1-1/2 to 2 inches of water, whereas the pressure drop across the bed itself is typically of 1/2 to 1 inch of water. This can of ,i course be varied to best meet the exigencies of any given ~ situation, largely depending on the characteristics of the mater-j ial being dried as will be apparent to and readily determinable
Amongst the more salient problems which are most frequently encountered in high capacity waste material (or equivalent) driers are: undesirable degradation or decomposi-., ; tion of the material being treated; gas and solids pollution from the drying operation; provision of dried material in an optimum and most suitable condition for use as the pre-dried feed stock to subsequent treatment and/or recovery applications 30 and operations; heat transfer achievement leaving something to be desired as to results and effectiveness; deleteriously and/or undesirably high temperature levels involved in the procedure;
impracticable and cumbersome materials handling requirements and involvements; and overall efficiency and economy of both equipment and the operation involved.
FIELD AND OBJECTIVES OF THE INVENTION
The present invention pertains to and resides in the overall field(s) of an improved, high-capacity and large volume drier and drying method for reducing and eliminating moisture in various generally particulate or disintegral materials, parti-cularly waste materials of this sort and most especially waste intended for processing and treatment (including chemical under-goings and transformations) in subsequent thermal steps and operations such as, but not limited to, pyrolysis.
Amongst the essential and more predominant purposes and objectives of the invention are to provide a technique, including both apparatus and procedure therefor, for better and more attractively and desirably drying waste or other equivalent -materials to most beneficially and most sensitively condition them for subsequent handling and/or treatment and processing, especially as prepared optimally-dry feed stock for thermal processes, wherein and whereby there is materially and r.lost advantageously reduced if not entirely avoided or eliminated the foregoing alluded-to problems of possible pollution, moisture-removal ineffectiveness, undesirable heat transfer efficacies, excessive heat and temperature requirements, diffi-cult materials handling and inefficiency and/or impractical and ~ exhorbitant apparatus and processing costs and other disadvan-- tageous and/or detrimental incidences.
Practice of the present invention generally provides and achieves the indicated novel and heretofore realistically ~ 30 unobtainable results and possibilities, with great attendant practicality and expanded utility thereabout; all this being achieved with great ease, simplicity, versatility, economy and effectiveness; and all predictably according to well-engineered and basically sound concepts and applications of fundamental technological postulations.
- The above-indicated and many more and other of the benefits and advantages in the obtainable by and upon practice of the present invention are more particularly set forth and easily evident in the ensuing Specification, taken in conjunc-tion with the accompanying Drawing (inwhich, insofar as possible, like reference nurnerals and letters refer to like or analogous parts and constituents and all the representations are depicted in more or less generally schematic and/or symbolic and/or fanciful portrayal with many obviously purposefully simplified exaggerations or unnecessary replications of parts or elimina-tion of details non-essential to or especially meaningful for expostulation of the ru~iments of the invention), wherein:
Figure 1 is an exploded, perspective view of the essential components of a drier assembly according to the invention;
Figure 2 is a schematic lay-out view in side elevation of an associated drier assembly;
Figure 3 is a.schematic cross-sectional view of the screw conveyor means and trough taken along Line 3-3 in Figure 2;
Figure 4 is a broken-out cross-sectional view of a plate and screen combination providing both bottom support for the material being dried in apparatus according to the invention as well as admission for passage through the material being - dried of the drying fluid utilized for the purpose in practice of the invention;
Figure 5, which appears on the same sheet of drawings ~ 30 as Fig. 1, is a fanciful flow-diagram type of view depicting, in :; side elevation showing, the general processing scheme and approach for drying material pursuant to the invention; and 104925~
Figure 6 in graphical representation illustrates typical advantageous results obtained in use of the drying assembly and method of the invention.
GENERALIZED REVELATION OF THE INVENTION
In basic essence, the present invention and the findings and discoveries upon which it is based involve a relatively low temperature, modestly sub-atmospheric pressure technique in which the wet, disintegral material feed is conveyed by agitating and forwarding screw conveyor and/or auger-like handling and transfer means through at least two distinct and separate, yet interconnected, multiple dryin~ zones~
The material being dried is sequentially subjected to the drying action and influence of hot gas streams of various temperatures (the total heat content of which will generally decrease in sequential zones). The hot gas streams are substantially uni-formly passed through the material (resulting from appropriaté
adaptation of the bed-forming means containing the material and the gas flow control means) at a predetermined and optimum transverse velocity with the outlet temperature of the gas in the last zone (from which the finally dried material is removed) being fixed to be most efficient according to the critical moisture level of the material being treated.
With greater particularity, the salient features and advantageous characteristics of associated and interacting components and elements utilized in the distinctive cooperative apparatus assembly and means combination with and upon which the associated method steps and procedure are followed in embodi-ments and practice of the invention are as in the following amplified delineations:
I. In general, the invention contemplates assembly and use of a conveyor dryer in which the auger-like or screw conveying mechanism serves to very thoroughly mix the material while it is being conveyed. This avoids stratified drying as so frequently occurs with a batch operation or in such conveyor-type dryers as belt or vibrating bed conveyors where the material is not thoroughly mixed while it is being conveyed and dried. This is of particular importance with combustible type materials, such as wood and paper wastes. It is of significance that in the screw-type conveyer dryers employed pursuant to the present , invention, the mixing can be further improved or increased by ; providing and positioning various types of supplemental agita-~ 10 tion devices along the length of the screw. A noteworthy and ; meritorlous aspect of operation according to the invention is that moisture variation from the top to the bottom of the material in the bed moving along in its drying passage along the screw length, as a result of the thorough mixing thereby imparted, results in only small and literally negligible variations in moisture content measured vertically with bed depth.
; II. The present conveyor dryer operates at sub-atmospheric pressure. Drying gases are sucked through the dryer by a fan ; 20 or equivalent fluid suction means located on the off gas side of the dryer. This particular arrangement offers several advan-tages over pressurized systems, namely: (a) The physical size of the fan or fluid moving and withdrawing installation, since the same is used on the "cold" side of the dryer, is significan-tly reduced. The volumetric quantity of drying gases is signif-icantly less than that which is supplied on the "hot" side thus ; allowing a smaller fan to be utilized and one suitable for low temperature operations. (b) It greatly simplifies and enhances dryer design from a sealing standpoint. The dryer does not have to contain a pressurized gas, and small leaks into the dryer are not detrimental to the dryer's operation. (c) Operations at sub-atmospheric conditions allows floating (or unlocked or unsealed) access doors to be utilized so that quick and efficient pressure release is available to prevent or avoid damage in cases of ~ explosion and the like.
III. The design of the screw trough in the dryer, as mentioned, I is altered to reduce the amount of entrained material that leaves the top of the bed in thedrying gases. One side of the screw trough is flared out from the base so that in a most beneficial embodiment the gas flow area is approximately about three times that of a standard screw trough to reduce the velo-10 city of the leaving gases. The flaring of one side of the trough is coupled with the inherent conveying characteristics of a screw conveyor ! The screw tends to cause the conveyedmaterial to ride up on one side of the trough considerably higher than on hl the other side. Consequently, the side that is flared out is the low side; and the flaring has no effect on the conveying characteristics of the screw trough arrangement. In addition, this inherent feeding characteristic of a screw conveyor (i.e., the drying material riding high on one side of the trough and I low on the other side) must be accorded proper account in designing and positioning the porous portion of the base of the screw trough so that ~he gases will flow uniformly through the bed of materials.
IV. The use of a screw conveyor for a dryer most advantageously enables the dryer to be operated with various bed depths in different compartments or sections of the dryer. For example, a very shallow bed can be operated in the first portion of the dryer by a suitable selection of screw pitch. In another section of the dryer, the screw pitch can be changed so that material will run deeper so as to thereby considerably increase the residence time. These features can be included in the design of the dryer to give most advantageous operating conditions for any given instance and to meet particular requirements in practice ! -7-of the invention; considerin~ in and f~r all of this the type of material to be dried and the inherent drying chaxacteristics of the material itself which dictate the conditions under which optimum drying is to be accomplished. Contemplation must be ¦ made of the fact, however, that a bed which is too deep will not ¦ mix as thoroughly as desired without involving necessity for ¦ additional agitation.
V. The dryer has a porous base in the material handling trough.
The design of this generally foraminous substrate and its position relative to the bed depth is an important feature in , practice of the invention. Firstly in this connection, the part of the base through which the drying gases flow is located to one side of the dryer trough so that it is centered with respect ; to the bed depth of the material being conveyed down the dryer.
The position is usually better off-center with respect to the trough, since the material tends to ride up on one side of the j~ trough and low on the other side. The foraminous base is made j from a perforated plate with benefically uniformly distributed ~ and patterned holes, slots or other open or protected (as in ¦ 20 forms of expanded metal lathe and the like) apertures of any suitable opening geometry which cover anywhere from say 25 or so to 75 or so, most advantageously approximately 40, percent of the overall total area of the porous base. The plate serves as both a structural part of the trough and as a means of passing j the drying gases into the bed. The holes or other openings, which may be of any desired effective diameter or other opening size, are ordinarily larger than the particle sizes of much of i the material being conveyed above the perforated plate.
¦ Ordinarily, actual individual hole sizes (or equivalent open areas) of between about 1/6th of an inch to about l/lOth of an inch - advantageously 1/8 or so inch - in diameter are effective for most materials being handled for drying. The open area for !
, -8-flow of the drying gases through these holes or the like is small, so that the gas velocity is high and prevents the holes from plugging. This is a self-cleaning feature of dryers according to the invention. The perforated plate alone ordinar-..
ily does not provide enough pressure drop to properly distribute the drying gases along the base of the dryer. To increase the pressure drop across the foraminous base, a fine mesh screen is stretched across the outside of the trough area over the perfor-ated plate. This mesh is sized so that the pressure drop across 10 the base region is generally and for a preponderance of the materials to be treated in the approximate neighborhood of 1-1/2 to 2 inches of water, whereas the pressure drop across the bed itself is typically of 1/2 to 1 inch of water. This can of ,i course be varied to best meet the exigencies of any given ~ situation, largely depending on the characteristics of the mater-j ial being dried as will be apparent to and readily determinable
3 by those skilled in the art. The gases can be uniformly dis-tributed along the bottom of the bed of the material being dried.
This means prevents blow-through or channeling in the material.
20 The size of the mesh can be varied so that a desired and most effective and efficient flow distribution can be produced along the length of the trough. A great benefit of drying practice in accordance with the present invention results from the fact that ¦ wire mesh can be cleaned periodically by simple brushing. Over a long period of time, dust and other foreign material may get sucked into the hot gas system and lodge on the dryer screen.
The simple servicing procedure is to then brush the screen free of these particles. Alternatively, resort may be had to a type ¦ of vacuum system to clean the material from the screen. In this a 30 connection, the material to be cleaned generally collects on the hot gas side of the trough which is on its outside, this greatly facilitating the cleaning operation. It is feasible in the _ g _ . .
104925'7 i practice of the invention t~ use any appropriate type of fine ¦ mesh or other style l~w pressure drop fllter at the base of the unit and in the inlet gas stream; although replaceable and easily cleaned filter rigs are most beneficial.
VI. A very unique and most advantageous feature of dryers according to the invention is the overall drying efficiency i attained with relatively low temperature and consequently much easier to handle drying gases. This is accomplished by having a low bulk temperature of the off-gases from the dryer, while at the same time drying to a very low moisture content in the outlet feed. Contrary to initial impression (which offhand seems in-consistent with equilibrium conSiderations of moisture content, relative humidity, and temperatures of exiting drying gases), such a low final moisture content is attained by passing only a small fraction of the total drying gas flow through the last . section or zone of the screw conveyor dryer. The temperature at which the gas is taken off from the last drying zone, so as to achieve the desired level of dryness in the product, is dictated by the nature of the material (particularly its water-diffusion characteristic, since its diffusion rate is limiting when the material is relatively dry), its water content, the inlet humid-ity of the drying gas, and the temperature of the material being dried and of the drying gas (which are assumed to be at equil-ibrium). For every material of a given moisture content, there is only one temperature at which it can exist in equilibrium with a ~as of a given humidity; therefore, if the desired moisture content is to be attained in that material, the drying gas must be taken off at a temperature above the equilibrium value. Assuming, however, adequate flow rates and residence times, it has been found that highly effective drying to rela-tive low moisture levels can be achieved even though the tem-¦ perature of the dryer off-gas is only somewhat higher than the ., --10--equilibrium temperature discussed above. For a typical wood waste, in which the final moisture content attained is about five percent, the gas is usually taken off the top of the bed in the final zone at approximately 200F. to 250F. This gas is mixed back with the bulk of the flow coming from the other ~ sections of the dryer. The overall bulk temperature is ordinar-¦~ ily in the range of only 135F. to 150F. or so. This is extremely low for a dryer operating with a typically achievable output moisture of 5 or so percent (and frequently less) mois ture in the waste, such as wood waste. Also, the moisture carrying capability of air is a very strong function of temper-ature. Consequently, the bulk off-gas temperature should usually be maintained in the region of, say, 120F. to 150F. or so for highest efficiency. At off-gas temperatures in this relatively low range, the effect of moisture concentration in the inlet drying gas will be very significant on the amount of moisture picked up from the solids per pound of drying gas flow-ing. The economically keneficial effect of this is to greatly ¦ reduce the losses which would normally be associated with ¦ 20 diluting hot gases with atmospheric air for low temperature . drying. It is well known that dry~ng gases are often combustion gases having relatively high moisture content. When mixed with atmospheric air, the moisture content is, of course, greatly reduced on a per pound basis; and more moisture can thereby be picked up as a result of greater air flow. Efficiency is not j affected significantly by adding air, due to the extremely low temperatures of the bulk off-gas and the increase of total gas flow. Were the gases to exit at a temperature in the range of ¦ 300F. to 400F. the effect of mixing and increasing mass flow as a result would be detrimental to efficiency, because of the wasted energy content of the off-gases.
PARTICULARIZED EXEMPLIFICATION OF THE INVENTION
The further features and characteristics of the im-proved and much more efficient and practicable technique for the dryer assembly and the drying procedure of the present invention as well as the way in which the same so nicely achieve(s) and 1 fulfill(s) the aims and objectives of the invention and con-j tribute to the relevant art are brought out in the Drawing, of which initial reference is had to Figures l, 2, 3, and 4 thereof in which a two-stage unit is shown. In this connection and 10 since the same reference numerals are generally used throughout, the identified Figures are elucidated simultaneously by gener-alized explanation, as follows, of the reference characters I associated with each of the thereby-designated parts or elements ;¦ (whensoever set forth in one or more of Figures 1, 2, 3, and 5) of the depicted assembly as it is to be operated.
Reference DESCRIPTION AND FUNCTION
Numeral Or ! Letter 9. Generally identifies the apparatus assembly.
10. Identifies housing.
20 ll. Designates the supply duct for the hot gases ~, used in the drying process. These gases may be supplied from:
(a). A suitable combustion stack with ambient air mixing to generate the desired tem-. perature;
j (b). A separate combustion burner fired with oil, gas, or other fuel;
(c). Exhaust from a boiler; or ¦ (d). Any hot process gas stream, including such things as incinerator gases and the like.
12. Locates the high temperature plenum. This is positioned on the inlet end of the screw 17 so _ .
Reference DESCRIPTION AND FUNCTION (cont.) Numeral Or Letter (cont.) . that the hottest gases are exposed to the solids which are highest in moisture.
13. Locates the low temperature plenum which is positioned on the exit end of the screw 17 so that lower temperature gases are used in drying the solids which are already partially dried. This . serves to reduce smoke and/or haze and greatly diminishes the possibility of unwanted decomposi- -tion and fires.
14. This indicates the damper used to distribute the flow between plenums 12 and 13 to optimize the drying process.
15. Indicates the damper used to supply additional ambient air to the hot gases entering plenum 13 to lower the temperature to the desired value.
. In connection with this, good mixing in plenum 13 of the flows through dampers 14 and 15 can be achieved by baffles (not shown).
16. Generally designates the overall porous ba'se. The base of the trough is formed of a metal plate 16a with, as mentioned, the generally regular holes or equivalent openings 16p drilled or punched so that it has the desired 40 or other percent opcn area.
~s indicated, l/a inch diameter holes are usually quite satisfactory for such things as pine bark or sawdust. The gas flow upward through these holes ¦ prevents clogging by the material. The size and ¦ 30 distribution of the openings 16p can be varied in j accordance with the type of material, the particle size, and density of the solids being dried so as .
Reference DESCRIPTION AND ~UNCTION (cont~) l~umeral Or Letter (cont.) 16. to accomplish this desirable effect. In many instances, it is desirable to better accommodate the changing characteristics of the drying mater-~ ial - during its complete traverse through the ¦ dryer - to have the downstream apertures more or less slanted or slotted even if those upstream are circular- This materially helps to avoid clogging ¦ l0 of the material in the porous base in the final drying zones(s). At least one metal screen 16s is attached to the bottom of the perforated plate 16a.
This serves to produce a pressure drop approxim-ately as high, and preferably higher, across the base as across the bed of solids being conveyed through the trough 24 by means of the advancing screw 17 in the screw. This prevents channelling and assures a good distribution of gas flow ~ through the material in the screw. The pressure ¦ 20 drop through the bed of solids in the screw de-creases with decreasing moisture content, which can cause a flow variation with axial position i~
the "base" pressure drop is too low. Usually, I this flow variation is small. However, it varies ¦ with particle size, moisture, density, bed depth, etc. It is preferable to make the "base" pressure drop high so that the effects of changes in feed are minimized and channelling is avoided.
17. This identifies the screw that is used to convey i 30 the solids through the dryer 9. Residence time of the material in the dryer is determined by the rate of rotation of the screw. Usually, the Reference i Numeral Or Letter(cont.) DESCRIPTION AND FUNCTION (cont.) 17. clearance between screw tips and trough should be on the order of 1/2 inch or so; this, or course, ` depending on particle size being handled. Bed ¦ depth is controlled by the input feed rate and rotation rate of the screw 17. AS illustrated in Figure 2, one or more portions 17f of the screw may have varied flight pitch to alter the movement and passage of the material in portions of its travel through the dryer 9. The screw also serves to continually stir and mix the material so that the stratified drying problems which occur when ¦ using a deep bed are avoided. These problems, as has been noted, include complete drying low in the bed with attendant decrease in drying rate, increase in fire hazard and smoke, and a conse-quent limitation on inlet temperature as the lower portion of the bed dries. Along this line, it is understood in the art that a so-called "deep" bed is any bed wherein the physical and/or chemical composition of the transitting drying gas is sub-stantially and/or appreciably changed upon passage, regardless of actual bed depth dimensions.
i All this has bearing on heat transfer rates; and property variations in the drying gases can occur . in a "deep" bed of any thickness appropriate to cause the phenomenon even when relatively shallow material layers are being treated. In dryers according to the invention, the solids in any short section taken along the axis of the screw have a relatively constant moisture content, i.e., ,.
~eference Numeral Or Letter(cont.) DESCRIPTION AND FUNCTION (cont.) . 17. the moisture content does not vary appreciably in the vertical direction at a fixed axial position.
It must be noted that proper agitation is the best means to avoid stratification, and different j drive ratios imparted to discontinuously connected segments of a given overall flight pitch screw can also be utilized to make desired regional varia-tions in residence time and/or agitation intensity along the length of the screw. It should be observed that when the bed depth in the trough is about half full or less a screw is sufficient and usually desirable for accomplishing the desired conveying and mixing.
18. These are plenums to aid in keeping the flow from and the bed relatively uniform and at a 19. low velocity to reduce particulate entrainment.
¦ 20. Represents the duct for connecting plenums 18 and 19 to the exhaust fan 21.
21. Designates an exhaust fan or equivalent fluid suction means which is connected here so that the entire dryer 9 operates at below atmospheric pressure. This allows cooling air to be admitted by damper control. Other advantages of having the fan 21 on the downstream, or "cool" side of the ` dryer are the reduced fan size and operating temp-erature requirements. The cubic feet per minute ("cfm") of gas on the "hot" side in Zone "A" will be much higher due to the high temperature there encountered as compared in Zone "B" to the "cool"
side. This is the case even when the additional .
Reference DESCRIpTION AND FUNCTION (cont.) Numeral Or I Letter (cont.) 21. moisture is added to the gas stream on and from drying. Thus, a smaller fan or equivalent can be used; and it can be one that does not have to ~ withstand the higher temperatures on the inlet j side of the dryer.
22. Identifies air lock or seal on the inlet feed to the dryer, which feed is shown by the arrow ¦ 10 designated "FI".
23. Identifies air lock or seal on the exit feed from the dryer, which feed is shown by the arrow desig-y nated "FX".
24. With particular reference to Figure 3, represents the cross-section of the screw trough. The trough 24 is inclined on one side 24s as shown. This 3 reduces the velocity of the gases coming off the 3 bed of solids, designated by reference letter "B", in the screw 17 which consequently reduces the entrainment of particulate matter. The inclined disposition of trough side 24s may be between about 30 and 60 from the vertical, although a slope of about 45 or so is generally desirable and utilized. Usually the drying gas velocities are thus reduced to a few feet/second at the top Ç of the trough. The requirements for this are determined by the material being dried, the part-¦ icle size and density, and the flow rate of drying ¦ gases. Since a screw inherently lifts the material to one side as it pushes the material forward, it should be operated so that the material being dried "rides up" on the vertical wall and not on ! -17-Reference DESCRIPTION AND FUNCTION (cont,) Numeral Or Letter(cont.) 24. the inclined wall. The lower the bed depth, the more pronounced will this become.
j 25. Identifies a moisture-laden gas outlet damper.
! This used to increase or decrease the total flow of drying gases through the bed of material in the screw 17.
26. Designates a drive pulley which provides means for ¦ 10 turning the dryer screw 17.
27. r~ith particular reference to Figure 1, a hanger bearing is identified for supporting and coupling the screw 17 which can be made up in two sections respectively extending through Zones "A" and "B".
Directional arrows "M" (shown in Figure 1) illustrate the direction of movement in the screw of the material passing through trough 21. Arrows "HA" and "HZ" in Figures 2 and 5 illustrate hot gas inlet flow in Zones A and Z, respectively, of dryer 9; while the analogy applys to the flow of exit gases illustrated by arrows "CA" and "CZ". Arrows "OG", also in - Figures 4 and 5, point out the off-gas flow in duct 20 to the fan or other suction device to the outlet; it being frequently desirable before venting or other diversion for use to pass the cool off-gas stream through a cyclone or other suitable particle collector and/or gas cleaner or scxubber 29.
Figure 4 particularly illustrates an advantageous construction for the porous base 16. As shown, the porous base is comprised of the foraminous metal plate 16a which is covered on the bottom by two conforming screens 16s separated by a glass fiber cloth or the like 16f. Advantageously, the screen mater-ial is stainless steel of the 40 mesh size; although coarser or finer mesh sizes may also be employed for the screen over a !
general 10-20 mesh range encompassing the 40 mesh construction.
Asbestcs and other woven and nonwoven cloth constructions may be utilized for the interscreen separater layer 16s. It should go without mention that, so long as a sufficiently high gas inlet pressure drop is achieved, metal screens alone or those separ-ated by mechanical means other than cloth separators can also be ~ utilized.
q There is no absolute necessity for the screen underlayto the foraminous plate 16a in the porous base unit 16, despite the great advantage in utilization of same. The screen and its particular mesh size hàs an obviously significant influence on the inletting pressure drop imparted to the hot drying gas feed during its entry into the trough. Likewise, the separating layer and/or employment of a double screen may also be eliminat-ed, even though the savings in so doing may not compensate for j the benefits of a complete construction.
While the porous base unit 16 may extend the complete distance of the underside portion of the trough 24, it is generally more desirable for the base to be in more than a i 20 single separate porous base section constituting the trough or screw-confining vessel through which the material being dried is advanced~ Thus, two and even more porous base sections can be provided to constitute the bottom of trough 24 in each drying zone or gas flow region of dryer 9. This allows for easier handling of same.
Most advantageously, each porous base unit is readily-detachably affixed, or at least hingedly and swingably mounted, ¦ in the bottom portion of trough 24 over longitudinaly extending ¦ rectangular openings 24a. Quick opening couplers and/or non-J 30 permanent-type fastening means such as screws 16h bayonet-fittings, dog-like clips or latches and other snap fasteners may ¦ be well utilized to mount detachable porous base units on the s .: .
perforated plate bottom of the trough, especially when screens I are involved. This greatly ameliorates interior accessibility J to the dryer and substantially facilitates unit maintenance 3 including porous base cleaning and/or replacement. It also and with at least as great benefit allows ready change of the screen mesh size utilized in a given porous base so as to accommodate desired inlet gas pressure drop(s) under different hot gas feedin~ conditions and when changes are made in the material being" ried in and with a given apparatus assembly and corres-ponding method procedure according to the invention. In this i connection, detatchable porous base units further allow for the ~' possibility, frequently of great advantage,,to utilize different '¦ gas-pressure-drop-controlling screen mesh sizes for processing .., , the same material through different sequential drying zones or regions (or parts of same) in the dryer.
Although placement in a centered position on the under-~' side of the trough is tolerable, it is usually more desirable , and beneficial for the porous base 16 to be offset from the ver-~ tical centerline of trough 24 by about 30 to 60, most advan-,~ ~ 20 tageously 45 or so, from the vertical centerline in a direction ~ upwards and away from the general exact underside opposite the ,j:, sloped or inclined side wall 24s of the trough or material bed container 24. Such off-centered porous base situation gives between gaspassage through the material bed to most nicely balance an even dehumidifying fluid flow through the drying mass given the bed configuration imparted thereto as a consequence of influence by and from the sloped through side wall.
The off-centered or cocked disposition of the porous base, a most significant feature in preferred embodiments of the invention, also minimizes the necessity to have excessively deep bed levels and fillings of the material being processed through the dryer in order to approach utmost heat transfer and ~ .. , ~
-moisture removal efficiencies. Greater trough filling does tend to avoid the possibility of encountering areas in the bed of relatively sparse material volume; but overfilling ordinarily sacrifices optimum mixing and causes unevenness and stratifica-tion, giving rise to problems in good treatment of the drying j material in the bed. This is usually much more readily achieved j when shallower bed depths are utilized; although in the overall uniformity of bed depth is also very important in order to best achieve all desired influences and results.
In this connection and as is schematically portrayed in Figure 3, the depth of material bed "B" should advantageously ¦ be not more than about 2/3, and more advantageously less than ~¦ 1/2 or so, of the effective total action height (such as diame-ter of the flights of screw 17) of the mechanical means utilized to agitate and forward the bed "B" through dryer 9. It is very advantageous, along this line, to so combine the pitch or slope angulation of the inclined wall 24s in the trough 24 with _ the particular offset positioning of the porous base 16 in trough 24 so that the upper surface contour of bed "B", at any given combination(s) of drying gas velocity and vigor of mixing ~ agitation plus speed of drying material advance per particular ¢ involved volume for involved unit residence time relationships,is such that the surface at least roughly lies and terminates in l a generalapproximate plane that is more or less normally (i.e., j right angularly) disposed to the slope angle of inclined wall 24s of trough 24.
Although in most instances it will be highly advanta-geous to have only one of the side walls of trough 24 set in an inclined or flared disposition, it is possible to have both trough walls so sloped. Opening-up relief of both sides of the trough will have an influence on entrainment by the hot drying fluid of the drying material in the bed "B"; but such construc-~049257 tion also unfortunately and somewhat disadvantageously tends to cause occurrence of "dead" spaces in the bed due to the conse-quence excessive spacing around too much of the screw periphery between the circumference of a large portion of the screw flights and the adjacent inner trough surfaces; this being particularly pronounced because of gradual upward spacing increase between the lower semi-circle cross-section of the screw as its contiguity with trough wall progressively decreases j upwardly with respect to the outwardly flared trough wall(s).
Needless to mention, for most efficient material forwarding action the spacing between screw flight periphery and inner trough surface should be as close as possible and practicable (even as little as 1/4 inch or so) in order to effectively move the drying material in the screw and without unnecessary scrap-ing through the trough; taking into account in the mean or J average disintegral particle or individual chunk or piece size of the material being dried. Naturally, all of this does depend on involved particle size; it being obvious that larger particles may need bigger conveyor means for best results. In actuality, a near scraping relative spacing is oftentimes most advantageous to utilize for the purpose.
The present invention lends many unprecedented and heretofore unattainable benefits, advantages and desiderata to material drying means and operations. In this, the fundamental requisites to be achieved are to heat the material to be dried and supply a properly conditioned gas which will carry away the moisture evaporated, so as to most efficiently accomplish the wanted drying. In this, most efficient and effective heat ¦ transfer becomes a controlling factor, especially when desirably ~ 30 relatively low drying temperatures are involved. For the 3 purpose, one needs to supply a properly conditioned drying gas which will effectively carry the moisture from the material ... .
being treated, Greater specific heat content of the gas with minimized saturation levels thereof obviously facilitates dryïng. This, as is readily appreciable, provides the most facile means of eliminating moisture from and most properly conditioning the material being treated. Practice of the pres-! ' ent invention, as is evident in and from the foregoing, gives pragmatic realization of this.
By way of some amplification of the above, it is rudimentary that at any given off-gas temperature, the off-gas ¦ 10 can only have a certain maximum moisture content. Also, the higher the moisture content of the material being dried the higher the temperature which can be utilized in the drying gas while, at the same time, minimizing problems of stratification and/or fire and explosion hazards. Following of the present contribution to the art permits utilization of varying temper-~ atured inlet drying gases in the sequential drying zones j employed enabling the temperature of the gas supplied to each !~ zone to be tailored to the characteristics of the material therein. While equal quantities of moisture could be removed by use of a smaller volume, single stream of gas at a higher temperature, the latter has an inordinate propensity for and likelihood of heat loss, not to mention possibility of undesir-able deterioration or decomposition of the drying material.
The instant invention materially avoids or at least vastly diminishes these disadvantageous factors. A relatively low 3 temperature operation is always involved in practice of the invention so that heat loss consequences are relatively trivial.
The efficient and effective high volume drying fluid throughout achieved in embodiments of and by practice in accordance with the present invention and all the enumerated factors and asso-ciations constituting and giving rise thereto permits the ! very desirable, albeit most efficacious and easily obtainable, low temperature conduction of material drying pursuant to and characterizing of this inVention.
In further substantiation of the foregoing and with particular reference again back to Figure 5 of the Drawing, a very typical drying operation with a mixed pine bark and sawdust material practiced in accordance with the present invention will ¦ utilize:
.
(i) In Zone "A", hot inlet air (or equivalent gas) "HA" of relatively low humidity is fed in at a temperature for drying of ¦ 10 only between about 550F. and about l,000F.; and amost satur-ated outlet gas "CA" at an approximate 120F.;
(ii) In Zone "B", hot "dry" inlet air "HZ" of relatively low humidity fed in at a temperature less than that of Zone A and between about 250F. and 550F.; and relatively dry outlet gas at about 200F.; and (iii) A total off-gas in stream "OG" output through duct 20 drawn out of the assembly at a mean, and obviously quite cool, temperature of only about 140F.
As appears in the foregoing, the present invention provides many associated advantages and benefits in the tech-- nology of drying waste materials for subsequent utilization.
For most desirable benefits in adaptations of the invention, as is herein evident, relatively hot drying gases are utilized in preliminary zones or stages to remove significant portions of the moisture in the waste product being pre-dried. In later stages (although, as explained, variations depending on involved conditions may be effectuated), the temperature is usually lower to avoid excessive smoking and/or actual decomposition of the drier material as it is obtained in the final zone(s) of the operation. Gas flow through the materials being dried is so controlled as to substantially, if not entirely, eliminate channeling.
Embodiments of apparatus according to the present invention provide siqnif~cant capital investment advantages;
these frequently being to an extent of reduction by as much as half or more of the cost of that required for the heretofore known and conventional waste driers. A good part of the reason for this is that, in practice of the present invention, a J relatively low temperature drying assembly is taken advantage ofwhich can, in actuality and for practical purposes, utilize available waste heat at relatively low temperature levels (for example, in the 300-500F. input range) while, at the same time, being capable of using only slightly modified standard conveyor systems (such as rotary drums, rotary screens, drag chains, mesh belt conveyers) and small fan or equivalent suction devices for drawing the drying gases through and out of the assembly.
Equipment according to and the method of the invention can readily handle as much as 8,000 or more lbs./hr. of waste containing as much as 50~ water, yielding, as dried product, ¦ 4,000 lbs. or more per hour of 5~ moisture content waste.
Typical (and overall expectable without limitations thereon) screw diameter, length, trough sizes, screw speeds (including seg~ental variations) etc. include apparatus as long as 25 or more feet with a screw diameter of about 2 feet.
Anticipatable material volume/unit time figures for different operable types of materials include slower drying garbage taken from about 25~ to about 10% (wet basis) moisture content and wood chips dried to about 3-10% (wet basis) moisture, both with residence times of 15 to 20 minutes or so. Some materials such as pine bark can be dried to about 5% (wet basis) in as little as 5 or so minutes.
3 In a dryer having 2 drying zones typically approximat-ely 75%-80% of the drying gas goes through the first zone and 104~ZS7 the remainder through the second zone.
The usual ratio of porous base area to trough semi-half bottom is about 1/2 to 3/4. This, of course, depends to some extent on the particular material being handled and its j precise particle size, density and initial moisture content.
I Overall and zone-by-zone operation can utilize as many ¦ as 3 or 4 (or even more) zones, particularly if it is desired . , .
to operate at higher temperatures while avoiding smoking problems.
In general, moisture removal expectabilities are such - that about 1/3 to 1/2 lb. of water per minute per pound of dry ;~ solid can be realized, except in the final zone where critical moisture content is encountered causing significant reduction in j the drying rate. Expectable dryness (as to precise moisture content) of dried product is usually in the 5% or better range for wood and the like waste and 8-10% for garbage.
Except for the screens, mild carbon steel materials of construction can be employed for the apparatus of the invention.
Parallel trough arrangements can be made to increase f 20 capacity.
PRACTICAL ILLUSTRATION OF THE INVENTION
¦ Using a two zone apparatus of the type illustrated in the Drawing, an equal weight mixture of pine bark and saw-¦ dust containing about 47% water (wet basis) was processed with a 1/2 flight pitch screw of 2 ft. diameter and 25ft. length operated at 5 RPM through both zones. The output volume of dried material (having about 5% moisture) was 4,000 lbs./hr.
The rate of drying gas flowing into the dryer was about 24,000 cfm at about 550F. proportioned in a ratio of about 3:2 as between the first and second zones. The excellent results obtained are set forth and discernible in the graphical repres-', entation made in Figure 6 of the Drawing. The Figure also shows ~04925~
the excellent degree of homogeniety which is attainable in the I apparatus due primarily to the highly effective mixing action ! of the screw.
Analogous good results are obtainable with other ~' materials and under varied operating conditions within the explained parameters of practice in accordance with the present invention.
~ Many changes ànd modifications can readily be made and 1 adapted in embodiments in accordance with the present invention ¦ 10 without substantially departing from its apparent and intended spirit and scope, all in pursuance and accordance with same as it is set forth and defined in the hereto appended Claims.
In any event and as is apparent in and from the fore-going Specification and by virtue of any objective appraisal and realization of the intrinsic great value of and most merit-~ orious moment associated with the instant contribution to the i art, the present invention obviously fulfills a pressing and , heretofore unsatisfied need and provides many attractive and . very desirable opportunities and possibilities in and for very many major uses and requirements for relatively low cost andextremely efficient and very adaptable material drying opera-tions; this bearing particular significance and importance in ` view of currently impending fuel and energy problems that must ¦ necessarily be faced and hopefully satisfactorily resolved.
I
1 .
,, : -27-
This means prevents blow-through or channeling in the material.
20 The size of the mesh can be varied so that a desired and most effective and efficient flow distribution can be produced along the length of the trough. A great benefit of drying practice in accordance with the present invention results from the fact that ¦ wire mesh can be cleaned periodically by simple brushing. Over a long period of time, dust and other foreign material may get sucked into the hot gas system and lodge on the dryer screen.
The simple servicing procedure is to then brush the screen free of these particles. Alternatively, resort may be had to a type ¦ of vacuum system to clean the material from the screen. In this a 30 connection, the material to be cleaned generally collects on the hot gas side of the trough which is on its outside, this greatly facilitating the cleaning operation. It is feasible in the _ g _ . .
104925'7 i practice of the invention t~ use any appropriate type of fine ¦ mesh or other style l~w pressure drop fllter at the base of the unit and in the inlet gas stream; although replaceable and easily cleaned filter rigs are most beneficial.
VI. A very unique and most advantageous feature of dryers according to the invention is the overall drying efficiency i attained with relatively low temperature and consequently much easier to handle drying gases. This is accomplished by having a low bulk temperature of the off-gases from the dryer, while at the same time drying to a very low moisture content in the outlet feed. Contrary to initial impression (which offhand seems in-consistent with equilibrium conSiderations of moisture content, relative humidity, and temperatures of exiting drying gases), such a low final moisture content is attained by passing only a small fraction of the total drying gas flow through the last . section or zone of the screw conveyor dryer. The temperature at which the gas is taken off from the last drying zone, so as to achieve the desired level of dryness in the product, is dictated by the nature of the material (particularly its water-diffusion characteristic, since its diffusion rate is limiting when the material is relatively dry), its water content, the inlet humid-ity of the drying gas, and the temperature of the material being dried and of the drying gas (which are assumed to be at equil-ibrium). For every material of a given moisture content, there is only one temperature at which it can exist in equilibrium with a ~as of a given humidity; therefore, if the desired moisture content is to be attained in that material, the drying gas must be taken off at a temperature above the equilibrium value. Assuming, however, adequate flow rates and residence times, it has been found that highly effective drying to rela-tive low moisture levels can be achieved even though the tem-¦ perature of the dryer off-gas is only somewhat higher than the ., --10--equilibrium temperature discussed above. For a typical wood waste, in which the final moisture content attained is about five percent, the gas is usually taken off the top of the bed in the final zone at approximately 200F. to 250F. This gas is mixed back with the bulk of the flow coming from the other ~ sections of the dryer. The overall bulk temperature is ordinar-¦~ ily in the range of only 135F. to 150F. or so. This is extremely low for a dryer operating with a typically achievable output moisture of 5 or so percent (and frequently less) mois ture in the waste, such as wood waste. Also, the moisture carrying capability of air is a very strong function of temper-ature. Consequently, the bulk off-gas temperature should usually be maintained in the region of, say, 120F. to 150F. or so for highest efficiency. At off-gas temperatures in this relatively low range, the effect of moisture concentration in the inlet drying gas will be very significant on the amount of moisture picked up from the solids per pound of drying gas flow-ing. The economically keneficial effect of this is to greatly ¦ reduce the losses which would normally be associated with ¦ 20 diluting hot gases with atmospheric air for low temperature . drying. It is well known that dry~ng gases are often combustion gases having relatively high moisture content. When mixed with atmospheric air, the moisture content is, of course, greatly reduced on a per pound basis; and more moisture can thereby be picked up as a result of greater air flow. Efficiency is not j affected significantly by adding air, due to the extremely low temperatures of the bulk off-gas and the increase of total gas flow. Were the gases to exit at a temperature in the range of ¦ 300F. to 400F. the effect of mixing and increasing mass flow as a result would be detrimental to efficiency, because of the wasted energy content of the off-gases.
PARTICULARIZED EXEMPLIFICATION OF THE INVENTION
The further features and characteristics of the im-proved and much more efficient and practicable technique for the dryer assembly and the drying procedure of the present invention as well as the way in which the same so nicely achieve(s) and 1 fulfill(s) the aims and objectives of the invention and con-j tribute to the relevant art are brought out in the Drawing, of which initial reference is had to Figures l, 2, 3, and 4 thereof in which a two-stage unit is shown. In this connection and 10 since the same reference numerals are generally used throughout, the identified Figures are elucidated simultaneously by gener-alized explanation, as follows, of the reference characters I associated with each of the thereby-designated parts or elements ;¦ (whensoever set forth in one or more of Figures 1, 2, 3, and 5) of the depicted assembly as it is to be operated.
Reference DESCRIPTION AND FUNCTION
Numeral Or ! Letter 9. Generally identifies the apparatus assembly.
10. Identifies housing.
20 ll. Designates the supply duct for the hot gases ~, used in the drying process. These gases may be supplied from:
(a). A suitable combustion stack with ambient air mixing to generate the desired tem-. perature;
j (b). A separate combustion burner fired with oil, gas, or other fuel;
(c). Exhaust from a boiler; or ¦ (d). Any hot process gas stream, including such things as incinerator gases and the like.
12. Locates the high temperature plenum. This is positioned on the inlet end of the screw 17 so _ .
Reference DESCRIPTION AND FUNCTION (cont.) Numeral Or Letter (cont.) . that the hottest gases are exposed to the solids which are highest in moisture.
13. Locates the low temperature plenum which is positioned on the exit end of the screw 17 so that lower temperature gases are used in drying the solids which are already partially dried. This . serves to reduce smoke and/or haze and greatly diminishes the possibility of unwanted decomposi- -tion and fires.
14. This indicates the damper used to distribute the flow between plenums 12 and 13 to optimize the drying process.
15. Indicates the damper used to supply additional ambient air to the hot gases entering plenum 13 to lower the temperature to the desired value.
. In connection with this, good mixing in plenum 13 of the flows through dampers 14 and 15 can be achieved by baffles (not shown).
16. Generally designates the overall porous ba'se. The base of the trough is formed of a metal plate 16a with, as mentioned, the generally regular holes or equivalent openings 16p drilled or punched so that it has the desired 40 or other percent opcn area.
~s indicated, l/a inch diameter holes are usually quite satisfactory for such things as pine bark or sawdust. The gas flow upward through these holes ¦ prevents clogging by the material. The size and ¦ 30 distribution of the openings 16p can be varied in j accordance with the type of material, the particle size, and density of the solids being dried so as .
Reference DESCRIPTION AND ~UNCTION (cont~) l~umeral Or Letter (cont.) 16. to accomplish this desirable effect. In many instances, it is desirable to better accommodate the changing characteristics of the drying mater-~ ial - during its complete traverse through the ¦ dryer - to have the downstream apertures more or less slanted or slotted even if those upstream are circular- This materially helps to avoid clogging ¦ l0 of the material in the porous base in the final drying zones(s). At least one metal screen 16s is attached to the bottom of the perforated plate 16a.
This serves to produce a pressure drop approxim-ately as high, and preferably higher, across the base as across the bed of solids being conveyed through the trough 24 by means of the advancing screw 17 in the screw. This prevents channelling and assures a good distribution of gas flow ~ through the material in the screw. The pressure ¦ 20 drop through the bed of solids in the screw de-creases with decreasing moisture content, which can cause a flow variation with axial position i~
the "base" pressure drop is too low. Usually, I this flow variation is small. However, it varies ¦ with particle size, moisture, density, bed depth, etc. It is preferable to make the "base" pressure drop high so that the effects of changes in feed are minimized and channelling is avoided.
17. This identifies the screw that is used to convey i 30 the solids through the dryer 9. Residence time of the material in the dryer is determined by the rate of rotation of the screw. Usually, the Reference i Numeral Or Letter(cont.) DESCRIPTION AND FUNCTION (cont.) 17. clearance between screw tips and trough should be on the order of 1/2 inch or so; this, or course, ` depending on particle size being handled. Bed ¦ depth is controlled by the input feed rate and rotation rate of the screw 17. AS illustrated in Figure 2, one or more portions 17f of the screw may have varied flight pitch to alter the movement and passage of the material in portions of its travel through the dryer 9. The screw also serves to continually stir and mix the material so that the stratified drying problems which occur when ¦ using a deep bed are avoided. These problems, as has been noted, include complete drying low in the bed with attendant decrease in drying rate, increase in fire hazard and smoke, and a conse-quent limitation on inlet temperature as the lower portion of the bed dries. Along this line, it is understood in the art that a so-called "deep" bed is any bed wherein the physical and/or chemical composition of the transitting drying gas is sub-stantially and/or appreciably changed upon passage, regardless of actual bed depth dimensions.
i All this has bearing on heat transfer rates; and property variations in the drying gases can occur . in a "deep" bed of any thickness appropriate to cause the phenomenon even when relatively shallow material layers are being treated. In dryers according to the invention, the solids in any short section taken along the axis of the screw have a relatively constant moisture content, i.e., ,.
~eference Numeral Or Letter(cont.) DESCRIPTION AND FUNCTION (cont.) . 17. the moisture content does not vary appreciably in the vertical direction at a fixed axial position.
It must be noted that proper agitation is the best means to avoid stratification, and different j drive ratios imparted to discontinuously connected segments of a given overall flight pitch screw can also be utilized to make desired regional varia-tions in residence time and/or agitation intensity along the length of the screw. It should be observed that when the bed depth in the trough is about half full or less a screw is sufficient and usually desirable for accomplishing the desired conveying and mixing.
18. These are plenums to aid in keeping the flow from and the bed relatively uniform and at a 19. low velocity to reduce particulate entrainment.
¦ 20. Represents the duct for connecting plenums 18 and 19 to the exhaust fan 21.
21. Designates an exhaust fan or equivalent fluid suction means which is connected here so that the entire dryer 9 operates at below atmospheric pressure. This allows cooling air to be admitted by damper control. Other advantages of having the fan 21 on the downstream, or "cool" side of the ` dryer are the reduced fan size and operating temp-erature requirements. The cubic feet per minute ("cfm") of gas on the "hot" side in Zone "A" will be much higher due to the high temperature there encountered as compared in Zone "B" to the "cool"
side. This is the case even when the additional .
Reference DESCRIpTION AND FUNCTION (cont.) Numeral Or I Letter (cont.) 21. moisture is added to the gas stream on and from drying. Thus, a smaller fan or equivalent can be used; and it can be one that does not have to ~ withstand the higher temperatures on the inlet j side of the dryer.
22. Identifies air lock or seal on the inlet feed to the dryer, which feed is shown by the arrow ¦ 10 designated "FI".
23. Identifies air lock or seal on the exit feed from the dryer, which feed is shown by the arrow desig-y nated "FX".
24. With particular reference to Figure 3, represents the cross-section of the screw trough. The trough 24 is inclined on one side 24s as shown. This 3 reduces the velocity of the gases coming off the 3 bed of solids, designated by reference letter "B", in the screw 17 which consequently reduces the entrainment of particulate matter. The inclined disposition of trough side 24s may be between about 30 and 60 from the vertical, although a slope of about 45 or so is generally desirable and utilized. Usually the drying gas velocities are thus reduced to a few feet/second at the top Ç of the trough. The requirements for this are determined by the material being dried, the part-¦ icle size and density, and the flow rate of drying ¦ gases. Since a screw inherently lifts the material to one side as it pushes the material forward, it should be operated so that the material being dried "rides up" on the vertical wall and not on ! -17-Reference DESCRIPTION AND FUNCTION (cont,) Numeral Or Letter(cont.) 24. the inclined wall. The lower the bed depth, the more pronounced will this become.
j 25. Identifies a moisture-laden gas outlet damper.
! This used to increase or decrease the total flow of drying gases through the bed of material in the screw 17.
26. Designates a drive pulley which provides means for ¦ 10 turning the dryer screw 17.
27. r~ith particular reference to Figure 1, a hanger bearing is identified for supporting and coupling the screw 17 which can be made up in two sections respectively extending through Zones "A" and "B".
Directional arrows "M" (shown in Figure 1) illustrate the direction of movement in the screw of the material passing through trough 21. Arrows "HA" and "HZ" in Figures 2 and 5 illustrate hot gas inlet flow in Zones A and Z, respectively, of dryer 9; while the analogy applys to the flow of exit gases illustrated by arrows "CA" and "CZ". Arrows "OG", also in - Figures 4 and 5, point out the off-gas flow in duct 20 to the fan or other suction device to the outlet; it being frequently desirable before venting or other diversion for use to pass the cool off-gas stream through a cyclone or other suitable particle collector and/or gas cleaner or scxubber 29.
Figure 4 particularly illustrates an advantageous construction for the porous base 16. As shown, the porous base is comprised of the foraminous metal plate 16a which is covered on the bottom by two conforming screens 16s separated by a glass fiber cloth or the like 16f. Advantageously, the screen mater-ial is stainless steel of the 40 mesh size; although coarser or finer mesh sizes may also be employed for the screen over a !
general 10-20 mesh range encompassing the 40 mesh construction.
Asbestcs and other woven and nonwoven cloth constructions may be utilized for the interscreen separater layer 16s. It should go without mention that, so long as a sufficiently high gas inlet pressure drop is achieved, metal screens alone or those separ-ated by mechanical means other than cloth separators can also be ~ utilized.
q There is no absolute necessity for the screen underlayto the foraminous plate 16a in the porous base unit 16, despite the great advantage in utilization of same. The screen and its particular mesh size hàs an obviously significant influence on the inletting pressure drop imparted to the hot drying gas feed during its entry into the trough. Likewise, the separating layer and/or employment of a double screen may also be eliminat-ed, even though the savings in so doing may not compensate for j the benefits of a complete construction.
While the porous base unit 16 may extend the complete distance of the underside portion of the trough 24, it is generally more desirable for the base to be in more than a i 20 single separate porous base section constituting the trough or screw-confining vessel through which the material being dried is advanced~ Thus, two and even more porous base sections can be provided to constitute the bottom of trough 24 in each drying zone or gas flow region of dryer 9. This allows for easier handling of same.
Most advantageously, each porous base unit is readily-detachably affixed, or at least hingedly and swingably mounted, ¦ in the bottom portion of trough 24 over longitudinaly extending ¦ rectangular openings 24a. Quick opening couplers and/or non-J 30 permanent-type fastening means such as screws 16h bayonet-fittings, dog-like clips or latches and other snap fasteners may ¦ be well utilized to mount detachable porous base units on the s .: .
perforated plate bottom of the trough, especially when screens I are involved. This greatly ameliorates interior accessibility J to the dryer and substantially facilitates unit maintenance 3 including porous base cleaning and/or replacement. It also and with at least as great benefit allows ready change of the screen mesh size utilized in a given porous base so as to accommodate desired inlet gas pressure drop(s) under different hot gas feedin~ conditions and when changes are made in the material being" ried in and with a given apparatus assembly and corres-ponding method procedure according to the invention. In this i connection, detatchable porous base units further allow for the ~' possibility, frequently of great advantage,,to utilize different '¦ gas-pressure-drop-controlling screen mesh sizes for processing .., , the same material through different sequential drying zones or regions (or parts of same) in the dryer.
Although placement in a centered position on the under-~' side of the trough is tolerable, it is usually more desirable , and beneficial for the porous base 16 to be offset from the ver-~ tical centerline of trough 24 by about 30 to 60, most advan-,~ ~ 20 tageously 45 or so, from the vertical centerline in a direction ~ upwards and away from the general exact underside opposite the ,j:, sloped or inclined side wall 24s of the trough or material bed container 24. Such off-centered porous base situation gives between gaspassage through the material bed to most nicely balance an even dehumidifying fluid flow through the drying mass given the bed configuration imparted thereto as a consequence of influence by and from the sloped through side wall.
The off-centered or cocked disposition of the porous base, a most significant feature in preferred embodiments of the invention, also minimizes the necessity to have excessively deep bed levels and fillings of the material being processed through the dryer in order to approach utmost heat transfer and ~ .. , ~
-moisture removal efficiencies. Greater trough filling does tend to avoid the possibility of encountering areas in the bed of relatively sparse material volume; but overfilling ordinarily sacrifices optimum mixing and causes unevenness and stratifica-tion, giving rise to problems in good treatment of the drying j material in the bed. This is usually much more readily achieved j when shallower bed depths are utilized; although in the overall uniformity of bed depth is also very important in order to best achieve all desired influences and results.
In this connection and as is schematically portrayed in Figure 3, the depth of material bed "B" should advantageously ¦ be not more than about 2/3, and more advantageously less than ~¦ 1/2 or so, of the effective total action height (such as diame-ter of the flights of screw 17) of the mechanical means utilized to agitate and forward the bed "B" through dryer 9. It is very advantageous, along this line, to so combine the pitch or slope angulation of the inclined wall 24s in the trough 24 with _ the particular offset positioning of the porous base 16 in trough 24 so that the upper surface contour of bed "B", at any given combination(s) of drying gas velocity and vigor of mixing ~ agitation plus speed of drying material advance per particular ¢ involved volume for involved unit residence time relationships,is such that the surface at least roughly lies and terminates in l a generalapproximate plane that is more or less normally (i.e., j right angularly) disposed to the slope angle of inclined wall 24s of trough 24.
Although in most instances it will be highly advanta-geous to have only one of the side walls of trough 24 set in an inclined or flared disposition, it is possible to have both trough walls so sloped. Opening-up relief of both sides of the trough will have an influence on entrainment by the hot drying fluid of the drying material in the bed "B"; but such construc-~049257 tion also unfortunately and somewhat disadvantageously tends to cause occurrence of "dead" spaces in the bed due to the conse-quence excessive spacing around too much of the screw periphery between the circumference of a large portion of the screw flights and the adjacent inner trough surfaces; this being particularly pronounced because of gradual upward spacing increase between the lower semi-circle cross-section of the screw as its contiguity with trough wall progressively decreases j upwardly with respect to the outwardly flared trough wall(s).
Needless to mention, for most efficient material forwarding action the spacing between screw flight periphery and inner trough surface should be as close as possible and practicable (even as little as 1/4 inch or so) in order to effectively move the drying material in the screw and without unnecessary scrap-ing through the trough; taking into account in the mean or J average disintegral particle or individual chunk or piece size of the material being dried. Naturally, all of this does depend on involved particle size; it being obvious that larger particles may need bigger conveyor means for best results. In actuality, a near scraping relative spacing is oftentimes most advantageous to utilize for the purpose.
The present invention lends many unprecedented and heretofore unattainable benefits, advantages and desiderata to material drying means and operations. In this, the fundamental requisites to be achieved are to heat the material to be dried and supply a properly conditioned gas which will carry away the moisture evaporated, so as to most efficiently accomplish the wanted drying. In this, most efficient and effective heat ¦ transfer becomes a controlling factor, especially when desirably ~ 30 relatively low drying temperatures are involved. For the 3 purpose, one needs to supply a properly conditioned drying gas which will effectively carry the moisture from the material ... .
being treated, Greater specific heat content of the gas with minimized saturation levels thereof obviously facilitates dryïng. This, as is readily appreciable, provides the most facile means of eliminating moisture from and most properly conditioning the material being treated. Practice of the pres-! ' ent invention, as is evident in and from the foregoing, gives pragmatic realization of this.
By way of some amplification of the above, it is rudimentary that at any given off-gas temperature, the off-gas ¦ 10 can only have a certain maximum moisture content. Also, the higher the moisture content of the material being dried the higher the temperature which can be utilized in the drying gas while, at the same time, minimizing problems of stratification and/or fire and explosion hazards. Following of the present contribution to the art permits utilization of varying temper-~ atured inlet drying gases in the sequential drying zones j employed enabling the temperature of the gas supplied to each !~ zone to be tailored to the characteristics of the material therein. While equal quantities of moisture could be removed by use of a smaller volume, single stream of gas at a higher temperature, the latter has an inordinate propensity for and likelihood of heat loss, not to mention possibility of undesir-able deterioration or decomposition of the drying material.
The instant invention materially avoids or at least vastly diminishes these disadvantageous factors. A relatively low 3 temperature operation is always involved in practice of the invention so that heat loss consequences are relatively trivial.
The efficient and effective high volume drying fluid throughout achieved in embodiments of and by practice in accordance with the present invention and all the enumerated factors and asso-ciations constituting and giving rise thereto permits the ! very desirable, albeit most efficacious and easily obtainable, low temperature conduction of material drying pursuant to and characterizing of this inVention.
In further substantiation of the foregoing and with particular reference again back to Figure 5 of the Drawing, a very typical drying operation with a mixed pine bark and sawdust material practiced in accordance with the present invention will ¦ utilize:
.
(i) In Zone "A", hot inlet air (or equivalent gas) "HA" of relatively low humidity is fed in at a temperature for drying of ¦ 10 only between about 550F. and about l,000F.; and amost satur-ated outlet gas "CA" at an approximate 120F.;
(ii) In Zone "B", hot "dry" inlet air "HZ" of relatively low humidity fed in at a temperature less than that of Zone A and between about 250F. and 550F.; and relatively dry outlet gas at about 200F.; and (iii) A total off-gas in stream "OG" output through duct 20 drawn out of the assembly at a mean, and obviously quite cool, temperature of only about 140F.
As appears in the foregoing, the present invention provides many associated advantages and benefits in the tech-- nology of drying waste materials for subsequent utilization.
For most desirable benefits in adaptations of the invention, as is herein evident, relatively hot drying gases are utilized in preliminary zones or stages to remove significant portions of the moisture in the waste product being pre-dried. In later stages (although, as explained, variations depending on involved conditions may be effectuated), the temperature is usually lower to avoid excessive smoking and/or actual decomposition of the drier material as it is obtained in the final zone(s) of the operation. Gas flow through the materials being dried is so controlled as to substantially, if not entirely, eliminate channeling.
Embodiments of apparatus according to the present invention provide siqnif~cant capital investment advantages;
these frequently being to an extent of reduction by as much as half or more of the cost of that required for the heretofore known and conventional waste driers. A good part of the reason for this is that, in practice of the present invention, a J relatively low temperature drying assembly is taken advantage ofwhich can, in actuality and for practical purposes, utilize available waste heat at relatively low temperature levels (for example, in the 300-500F. input range) while, at the same time, being capable of using only slightly modified standard conveyor systems (such as rotary drums, rotary screens, drag chains, mesh belt conveyers) and small fan or equivalent suction devices for drawing the drying gases through and out of the assembly.
Equipment according to and the method of the invention can readily handle as much as 8,000 or more lbs./hr. of waste containing as much as 50~ water, yielding, as dried product, ¦ 4,000 lbs. or more per hour of 5~ moisture content waste.
Typical (and overall expectable without limitations thereon) screw diameter, length, trough sizes, screw speeds (including seg~ental variations) etc. include apparatus as long as 25 or more feet with a screw diameter of about 2 feet.
Anticipatable material volume/unit time figures for different operable types of materials include slower drying garbage taken from about 25~ to about 10% (wet basis) moisture content and wood chips dried to about 3-10% (wet basis) moisture, both with residence times of 15 to 20 minutes or so. Some materials such as pine bark can be dried to about 5% (wet basis) in as little as 5 or so minutes.
3 In a dryer having 2 drying zones typically approximat-ely 75%-80% of the drying gas goes through the first zone and 104~ZS7 the remainder through the second zone.
The usual ratio of porous base area to trough semi-half bottom is about 1/2 to 3/4. This, of course, depends to some extent on the particular material being handled and its j precise particle size, density and initial moisture content.
I Overall and zone-by-zone operation can utilize as many ¦ as 3 or 4 (or even more) zones, particularly if it is desired . , .
to operate at higher temperatures while avoiding smoking problems.
In general, moisture removal expectabilities are such - that about 1/3 to 1/2 lb. of water per minute per pound of dry ;~ solid can be realized, except in the final zone where critical moisture content is encountered causing significant reduction in j the drying rate. Expectable dryness (as to precise moisture content) of dried product is usually in the 5% or better range for wood and the like waste and 8-10% for garbage.
Except for the screens, mild carbon steel materials of construction can be employed for the apparatus of the invention.
Parallel trough arrangements can be made to increase f 20 capacity.
PRACTICAL ILLUSTRATION OF THE INVENTION
¦ Using a two zone apparatus of the type illustrated in the Drawing, an equal weight mixture of pine bark and saw-¦ dust containing about 47% water (wet basis) was processed with a 1/2 flight pitch screw of 2 ft. diameter and 25ft. length operated at 5 RPM through both zones. The output volume of dried material (having about 5% moisture) was 4,000 lbs./hr.
The rate of drying gas flowing into the dryer was about 24,000 cfm at about 550F. proportioned in a ratio of about 3:2 as between the first and second zones. The excellent results obtained are set forth and discernible in the graphical repres-', entation made in Figure 6 of the Drawing. The Figure also shows ~04925~
the excellent degree of homogeniety which is attainable in the I apparatus due primarily to the highly effective mixing action ! of the screw.
Analogous good results are obtainable with other ~' materials and under varied operating conditions within the explained parameters of practice in accordance with the present invention.
~ Many changes ànd modifications can readily be made and 1 adapted in embodiments in accordance with the present invention ¦ 10 without substantially departing from its apparent and intended spirit and scope, all in pursuance and accordance with same as it is set forth and defined in the hereto appended Claims.
In any event and as is apparent in and from the fore-going Specification and by virtue of any objective appraisal and realization of the intrinsic great value of and most merit-~ orious moment associated with the instant contribution to the i art, the present invention obviously fulfills a pressing and , heretofore unsatisfied need and provides many attractive and . very desirable opportunities and possibilities in and for very many major uses and requirements for relatively low cost andextremely efficient and very adaptable material drying opera-tions; this bearing particular significance and importance in ` view of currently impending fuel and energy problems that must ¦ necessarily be faced and hopefully satisfactorily resolved.
I
1 .
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Claims (29)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. Apparatus for drying a particulate material, com-prising: a generally horizontally-disposed, elongated trough .
having a material feed inlet adjacent one end and a material withdrawal outlet adjacent the other end thereof, the bottom portion of said trough having an elongated foraminous section extending longitudinally therein; longitudinally-extending conveyor means mounted in said trough; a first gas inlet conduit adapted to deliver drying gas from a source thereof to the interior of said trough, and communicating therewith through a first elongated area of said foraminous section disposed adjacent said one end thereof; a second gas inlet conduit adapted to deliver drying gas from a source thereof to the interior of said trough, and communicating therewith through a second elongated area of said foraminous section disposed at a location longitudinally spaced from said one end thereof, said second conduit having an opening with adjustable means thereon affording variable gas flow communication from a source other than said drying gas source; means substantially closing said trough, said closing means effectively dividing said trough into at least first and second drying zones corresponding, respective-ly, to said first and second areas of said foraminous section, and having gas flow outlets from each of said zones; and means for inducing gas flow from said trough through said outlets, whereby, as material is conveyed through said trough by said conveyor means, said flow-inducing means may be employed to convey drying gas from a source thereof sequentially through each of said first and second inlet conduits, through said first and second areas of said foraminous section, upwardly through the material being conveyed, through said first and second drying zones and through said gas flow outlets, and whereby said adjustable means on said opening of said second inlet conduit may be adjusted to vary the temperature and composition of gas flowing into said second zone from that flowing into said first zone of said trough.
having a material feed inlet adjacent one end and a material withdrawal outlet adjacent the other end thereof, the bottom portion of said trough having an elongated foraminous section extending longitudinally therein; longitudinally-extending conveyor means mounted in said trough; a first gas inlet conduit adapted to deliver drying gas from a source thereof to the interior of said trough, and communicating therewith through a first elongated area of said foraminous section disposed adjacent said one end thereof; a second gas inlet conduit adapted to deliver drying gas from a source thereof to the interior of said trough, and communicating therewith through a second elongated area of said foraminous section disposed at a location longitudinally spaced from said one end thereof, said second conduit having an opening with adjustable means thereon affording variable gas flow communication from a source other than said drying gas source; means substantially closing said trough, said closing means effectively dividing said trough into at least first and second drying zones corresponding, respective-ly, to said first and second areas of said foraminous section, and having gas flow outlets from each of said zones; and means for inducing gas flow from said trough through said outlets, whereby, as material is conveyed through said trough by said conveyor means, said flow-inducing means may be employed to convey drying gas from a source thereof sequentially through each of said first and second inlet conduits, through said first and second areas of said foraminous section, upwardly through the material being conveyed, through said first and second drying zones and through said gas flow outlets, and whereby said adjustable means on said opening of said second inlet conduit may be adjusted to vary the temperature and composition of gas flowing into said second zone from that flowing into said first zone of said trough.
2. The apparatus of Claim 1, wherein said foraminous section comprises a disengageably secured screen.
3. The apparatus of Claim 2, wherein said foraminous section comprises a fixed perforated portion having larger openings than said screen.
4. The apparatus of Claim 3, wherein the downstream perforations in the direction of material conveyance are slotted.
5. The apparatus of Claim 1, wherein said foraminous section is substantially coextensive with said conveyor means.
6. The apparatus of Claim 5, wherein said first and second areas of said foraminous section are longitudinally spaced from one another.
7. The apparatus of Claim 1, wherein said second gas inlet conduit is connected to said first inlet conduit with adjustable gas flow control means interposed there-between, whereby said second conduit receives drying gas indirectly from the source thereof through said first conduit, and wherein said flow inducing means is a vacuum source.
8. The apparatus of Claim 1, wherein said other source is the atmosphere, and wherein the gas provided through said adjustable means is air.
9. Apparatus for drying a particulate material, com-prising: a generally horizontally-disposed, elongated trough having a material feed inlet adjacent one end and a material withdrawal outlet adjacent the other end thereof, the bottom portion of said trough having an elongated foraminous section extending longitudinally therein; longitudinally-extending screw conveyor means rotatably mounted adjacent the bottom of said trough; a first gas inlet conduit adapted to deliver drying gas from a source thereof to the interior of said trough, and communicating therewith through a first elongated area of said foraminous section disposed adjacent said one end thereof; a second gas inlet condiut adapted to deliver drying gas from a source thereof to the interior of said trough, and communicating therewith through a second elongated area of said foraminous section disposed at a location longitudinally spaced from said one end thereof, said second conduit having an opening with adjustable means thereon affording variable gas flow communication from a source other than said drying gas source; means substantially closing said trough, said closing means effectively dividing said trough into at least first and second drying zones corresponding, respec-tively, to said first and second areas of said foraminous section, and having gas flow outlets from each of said zones;
and vacuum-inducing means for drawing gas from said trough through said outlets, whereby, as material is conveyed through said trough by said conveyor means, said vacuum-inducing means may be employed to draw drying gas from a source thereof sequentially through each of said first and second inlet conduits, through said first and second areas of said foraminous section, upwardly through the material being conveyed, through said first and second drying zones, and through said gas flow outlets, whereby said adjustable means on said opening of said second inlet conduit may be adjusted to vary the temperature and compo-sition of gas flowing into said second zone from that flowing into said first zone of said trough.
and vacuum-inducing means for drawing gas from said trough through said outlets, whereby, as material is conveyed through said trough by said conveyor means, said vacuum-inducing means may be employed to draw drying gas from a source thereof sequentially through each of said first and second inlet conduits, through said first and second areas of said foraminous section, upwardly through the material being conveyed, through said first and second drying zones, and through said gas flow outlets, whereby said adjustable means on said opening of said second inlet conduit may be adjusted to vary the temperature and compo-sition of gas flowing into said second zone from that flowing into said first zone of said trough.
10. The apparatus of Claim 9, wherein said bottom portion of said trough is of arcuate cross section, and wherein said trough has generally planar sidewalls extending upwardly from said bottom portion.
11. The apparatus of Claim 9, wherein at least one of said sidewalls extends in an outward direction from said bottom portion, at an angle of about 30° to 60° from vertical.
12. The apparatus of Claim 11 wherein said one sidewall extends more outwardly than the other of said sidewalls to provide an upwardly-directed, assymet-rical flare to said trough, said other sidewall being that on which the material being conveyed tends to rise under the action of said screw conveyor means.
13. The apparatus of Claim 9, wherein said foraminous section is displaced from alignment under said conveyor means, with the longitudinal axis of said section being at an angular displacement of 30° to 60° from vertical.
14. The apparatus of Claim 12, wherein said foram-inous section is displaced from alignment under said conveyor means, the direction of displacement being toward said other sidewall of said trough.
15. The apparatus of Claim 9, wherein said foraminous section comprises a disengageably secured screen.
16. The apparatus of Claim 15, wherein said foraminous section comprises a perforated portion having larger openings than said screen.
17. The apparatus of Claim 16, wherein the downstream perforations in the direction of material conveyance are slotted.
18. The apparatus of Claim 9, wherein said foraminous section is substantially coextensive with said conveyor means.
19. The apparatus of Claim 18, wherein said first and second areas of said foraminous section are longitudinally spaced from one another.
20. The apparatus of Claim 9, wherein said second gas inlet conduit is connected to said first inlet conduit with adjustable gas flow control means interposed there-between, whereby said second conduit receives drying gas indirectly from the source thereof through said first conduit.
21. In a method for drying a particulate material, the steps comprising: a) generating a hot drying gas; b) con-veying a moisture-containing particulate material along a travel path through a drying unit having first and second drying zones defined in series therein; c) passing a first portion of said hot drying gas upwardly through said material in said first dry-ing zone; d) admixing with ambient air at a lower temperature a second portion of said hot drying gas, and passing the resultant admixture upwardly through said material in said second drying zone; e) withdrawing gases from said first and second drying zones; and f) selecting the final moisture content value which is desired in said material as it exits from said second zone, the amounts of ambient air and of said second gas portion admixed in said step "d" being proportioned to maintain the temperature of the gas withdrawn from a location near the surface of said exiting material at about the temperature at which a theoretical equilibrium condition exists between said exiting material having said final moisture content and said withdrawn surface gas.
22. The method of Claim 21, wherein said first and second portions of said hot drying gas are obtained from a common stream by diverting one of said portions from said stream by adjustment of adjustable gas flow control means.
23. The method of Claim 21, wherein said admixture with air is produced by adjustment of adjustable gas flow control means, admitting ambient air into conduit means carrying said second drying gas portion to said second drying zone.
24. The method of Claim 21, wherein a vacuum is induced at the gas outlet side of the drying unit, to effect said gas passage and withdrawal steps.
25. The method of Claim 21, wherein pressure drops in said first drying gas portion and said admixture are induced beneath said material, which pressure drops are at least of the magnitude of those which occur across the corresponding areas of the bed of material.
26. The method of Claim 21, wherein said material is selected from the group consisting of agricultural waste, wood, wood waste, and garbage.
27. The method of Claim 23, wherein the flow rates of said first and second drying gas portions and of said ambient air through said material are adjusted, relative to the moisture content and mass transfer characteristics of said material respectively in each of said drying zones, to achieve maximum dryness without substantial burning, smoking, or decomposition thereof.
28. The method of Claim 21, wherein said material comprises wood waste, wherein said temperature of said withdrawn surface gas is about 200°F. to 250°F., and wherein said selected final moisture content is about 5.0 percent (wet basis).
29. The method of Claim 21, wherein the temperature of said withdrawn surface gas is not more than about 30°F. above said theoretical equilibrium temperature.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US65478276A | 1976-02-03 | 1976-02-03 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1049257A true CA1049257A (en) | 1979-02-27 |
Family
ID=24626220
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA270,886A Expired CA1049257A (en) | 1976-02-03 | 1977-02-02 | Two stage drying with single auger |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1049257A (en) |
-
1977
- 1977-02-02 CA CA270,886A patent/CA1049257A/en not_active Expired
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