US2575119A - Spray drying equipment and method - Google Patents

Description

1951 D. D. PEEBLES ET AL 2,575,119

SPRAY DRYING EQUIPMENT AND METHOD Filed Aug. 22, 1949 2 $HEETSSHEET l INVENTORS Da via 0. Peeb/cs Peg/na/a E eaae BY 7 I ATTORNEYS Patented Nov. 13, 1951 2,575,119 SPRAY p fmc: EQUIPMENT AND METHOD David DJeebles, Hillsborough, Calif., and Regb nald E; Meade, Appleton, Wis., assignors to Western Condensing Company, San Francisco, Calif., a corporation of California Application August 22, 1949, Serial No. 111,638

3 Claims. (01. 159-41b' This invention relates generally to spray drying equipment and methods such as are used to convert various liquids and slurries to powdered Conventional types of spray drying equipment utilize a desiccating chamber equipped with a suitable atomizing device. A source of hot' air or other drying gas is continuously introduced into the desiccating chamber, whereby the atomized particles are suspended in swirling currents of the drying gas, thereby removing suflicient water to form a divided or powdered material. In some instances the powder is centrifugally separated out in the lower part of the desiccating chamber (see Gray and Jensen Patent No. 1,107,784) and the spent drying gas is removed through a separate exhaust conduit. In other instances both exhaust gases and the powder are removed together, and the powder separated externally of the desiccating chamber. Because of their capaciy and ability to handle a wide range of liquid material and slurries, centrifugal atomizing devices have found increasing favor for such equipment. The centrifugal atomizing device may consist of a disc mounted on a vertical shaft, and driven at a relatively high speed of rotation. The liquid to be atomized is delivered to theupper face of this disc, and the periphery of the disc is provided with a plurality of circumferentially spaced teeth from which atomized particles are discharged.

Stationary atomizing nozzles may be used in place of centrifugal devices, particularly where the material is such that the problem of atomization is not difficult, or where the capacity requirements are such that a stationary atomizer is feasible. The hot air or other dry gas is generally introduced into the desiccating chamber tangentially, to promote the desired swirling movement, and as the hot air enters the chamber it is immediately guided in a curved path by engaging adjacent curved walls offlthe chamber.

Various difiiculties have been experienced with conventional desiccators and methods of the above type. For example it frequently happens that the atomized particles of material vary considerably as to particle size, and this may result in an unsatisfactory final product. Should the drier be operated in such a manner as to leave a desired moisture content in the larger size particles, then the smaller particles tend to dry to anhydrous condition, thus resulting in a final product in which the moisture content is not unlformly distributed. If it is attempted to operate dried to a desired free moisture content, then the larger particles tend to have too much excess free moisture, thus likewise resulting in an unsatisfactory final product. These difiiculties are particularly noticeable when handling material which is to be dried to leave a desired percentage of water of hydration in the final material, to render the same nonhygroscopic. Because of these difficulties it has become customary practice to operate a spray drier in such a fashion that the majority of the atomized particles are of relatively small size. In the case of centrifugal atomizers this is accomplished by operating the atomizing device at relatively high speeds, while in the case of stationary atomizers it is accomplished by utilizing relatively high atomizing pressures. Atomizing to the extent of causing the bulk of theatomized particles to be relatively finely divided has the disadvantage that the final product may have an objectionably small particle size, and in addition it is well known that excessive atomizer speeds or pressures, are objectionable.

In addition to the above it has been noted that conventional spray driers equipped with either centrifugal or stationary atomizing devices do not have the capacity theoretically attainable. This has been attributed to lack of proper distribution of the suspended atomized particles in the desiccating chamber, whereby there is considerable lack of uniformity with respect to the number of particles present at a: given instant in a unit of space, at various points within th desiccating chamber.

In general it is an object of the present invention to provide a new desiccating apparatus and method which avoids the aforementioned difficulties and limitations of prior conventional spray drying equipments and methods.

Another object of the invention is to provide a new desiccating apparatus and method which is relatively flexible with respect to the sizeof the atomized particles which may be satisfactorily dried, and which will facilitate production of a dried product to a desired moisture content, with a minimum variation in moisture content of the various particles. I

Another object of the invention is to rovide a new desiccating apparatus and'metliodfcharacterized particularly in that it has relatively high capacity and will enable the use of relatively high inlet gas temperatures without causing burning or heat injury to the material being dried.

the drier so that the smaller particles will be Another object of the invention is to provide a desiccating apparatus and method which serves to maintain free flight of the atomized material within the desiccating chamber, without permitting direct impact of material discharged at high velocity in a horizontal plane from the atomizin device on heated walls of the chamber.

Another object of the invention is to provide a desiccating apparatus and method making possible optimum free flight distribution of the atomized particles in the drying gas, thereby promoting high capacity and uniformity with respect to the spray dried product.

Additional objects of the invention will appear from the following description in which the preferred embodiments have been set forth in detail in conjunction with the accompanying drawmg.

Referring to the drawing:

Figure 1 is a side elevational view, diagrammatically illustrating a .desiccating apparatus incorporating the present invention.

Figure 2 is a plan view of the apparatus shown in Figure 1.

Figure 3 is a side elevational view of another embodiment of the invention.

Figure 4 is a plan view of the apparatus shown in Figure 3.

That form of the invention illustrated Figures 1 and 2 of the drawing consists of a desiccating chamber III which is formed symmetrical about the vertical axis H, and which may be annular in transverse cross-section. That portion Hla of the desiccating chamber is conical shaped as illustrated, to facilitate separating powder from the drying gas. Conduit I2 connects with the lower end of the chamber, and enables removal of the dried powder together with a small amount of conveying gas. Exhaust conduit l3 has an open inlet end I located concentric with the axis H, and at a level considerably above the point of connection of conduit l2. Conduit I3 is adapted for connection with the inlet side of a blower or like draft means.

The upper portion Nb of the desiccating chamber is substantially cylindrical in form, and receives hot drying gas from the ducts l6, l1, l8 and I9 (Figure 2). As will be presently explained these ducts are arranged in a particular manner. Disposed concentric with the axis II, and likewise in the upper part of the desiccating chamber, there is an atomizing device 2|, which may be of the centrifugal type. The details of this device have not been illustrated, because it may be made according to any one of a number of prior art atomizing units. Good results have been secured by utilizing the improved centrifugal atomizing device disclosed and claimed in copending application Serial No. 626,219, filed November 2, 1945/ and now matured into Patent Number 2,473,035.

In addition to the hot air ducts'li, l1, l8 and I9 it is desirable to provide for the introduction of additional hot drying gas into a zone immediately surrounding the atomizing device 2|. Thus a hot air inlet opening 22 is illustrated in Figure 1, which is circular in contour and which is concentric with the axis This opening connects.

with the chamber 23, which in jturn'con'nects with the hot air supply conduit 2d.

With respect to the positioning of the ducts l6, ll, I8 and 15, it will be noted from Figure 2 that they are not tangential with respect to the adjacent peripheral wall of the desiccating chamber. Each duct is directed generally toward the region of the discharge end of the next adjacent duct. As illustrated in Figure 1, taken together with Figure 2, the axes of these ducts are disposed in a common plane, and this plane is likewise the general plane in which the atomizing device 2! is located. It will be evident that with the arrangement just described the projected axes 26, 21, 28 and 29 form substantially a square as illustrated in Figure 2. Likewise the angle of each axis, with respect to an axis which would be truly tangential to the adjacent peripheral walls of the desiccating chamber (angle a, Figure 2) is in this instance about 45. The discharge from each conduit therefore does not follow and is not guided by the adjacent peripheral chamber walls, but follows the line of the projected axes 26, 21, 28 and 29, or in other words it follows a generally linear path extending between the discharge ends of two adjacent conduits.

Operation of the apparatus described above, and the'carrying out of the present method, can be described as follows: Assuming that a liquid material containing solids in suspension, such as milk concentrated to from 40 to 60% solids is being supplied to the atomizing device 2|, this material is discharged outwardly from the atomizer at relatively high velocity, and the initial direction of flight is horizontal and toward the surrounding peripheral walls. Conduits l6, ll, l8 and i9, together with conduit 24, are connected to a source of drying gas, such as a blower and heater serving to deliver hot air at a suitable temperature, and with substantially equal distribution through the conduits IS, IT, I8 and I9. Due to the cross-flow action of hot drying air being delivered downwardly from the opening 22 about the atomizing device, some of the atomized particles, are immediately directed downwardly, while other particles continue a substantially horizontal path of movement. The latter particles continue outwardly until acted upon by the hot air jetting from the ducts l6, l1, l8 and 19. These hot air currents are of suflicient velocity to deflect such particles and carry them along in suspension, thus preventing the particles from continuing their movement until contacting the surrounding peripheral walls. After being acted upon by the hot air blasts from the ducts l6. l1, I8 and i9, atomized particles progress downwardly into the main part of the desiccating chamber, thus partaking of a sustained free flight the same as the atomized particles which were initially deflected downwardly. There is a general swirling or cyclonic movement of gas and suspended particles in the desiccating chamher, with the result that there is a centrifugal separating action, which causes the dried particles to progress downwardly about the conduit [3, to be finally removed through conduit I2. Spent gas is continuously exhausted through the conduit B. When viewed from the side as in Figure l, the path of movement of atomized particles leaving the atomizing device 2|, is either directed outwardly until acted upon by hot air discharging from ducts l 6, l1, l8 and I9, and then downwardly in a spiral path in the main part of the desiccating chamber, or generally outwardly and downwardly from the atomizing device, with spiral movement until the particle is finally collected in the lower part of the chamber.

The desiccating method and apparatus described above has many novel characteristics. Assumingthat the atomizer produces particles which vary as to size, such particles can be dried more uniformly, to produce a flnal product having a free moisture content which is relatively uniformly distributed through both the coarse and finer particles. The coarse or larger atomizer particles tend to be discharged directly outwardly from the atomizer 2|, to be acted upon and entrained by the jets of hot air discharging from the hot air inlets IE to l9 inclusive, and to be thereafter caused to swirl about the axis of the chamber and to progress downwardly into the region extending generally below the atomizer and surrounding the central axis. The finer atomized particles tend to be deflected downwardly by hot air entering through the inlet passage 22, and to likewise be suspended in swirling currents in the desiccating chamber, without first being acted upon by the free discharge issuing from the conduits |6l9. The larger or coarser particles are therefor caused to traverse a longer path of travel to thereby reduce the moisture content to a value consistent with the reduction in free moisture content of the finer particles. Thus a superior powdered product can be produced consist g of both coarse and fine particles, with a rel tively uniform distribution of residual moisture content.

The feature just described can be utilizedto advantage in the drying of materials where it is desired to provide a specified amount of residual free moisture content in the final material. For example the drying of lacteal material containing substantial amounts of lactose (e. g. whey), it desirable to producea" final product by a spray drying operation which has a sufficient amount of moisture to provide for water of hydration of the lactose present, together with an additional amo unt of free moisture. In the past it has been customary to provide for a substantial free moisture content in the final product, such as say from 8 to 20% in order to insure ample water of hydration in both the coarse and finer particles, and this practice has necessitated the use of secondary drying after the initial primary spray drying operation. With the present apparatus and method the free moisture content, over and above the water of hydration required, can be kept to a relatively low value, such as from 2 to 5%, with a uniform distribution of moisture in both the fine and coarse particles, thus obviating secondary drying.

As previously stated it has been customary practice in the past to operate atomizers for spray driers in such a manner as to atomize the bulk of the material to relatively small particle size. This has required use of relatively high rotative speeds for centrifugal atomizers, or relatively high fluid pressures for the stationary type of atomizers, both of which are objectionable. With the present invention it is possible to depart from such conventional prior practice. Thus it is possible to operate the atomizing devices at lower speeds or pressures, in order to produce a larger percentage of coarser particles, and such coarser particles can be adequately dried to a desired moisture content by the use of our apparatus and method. In prac-, tice this feature makes it possible to reduce the the emphasis upon production of relatively small sized atomized particles, and the bulk of the atomized particles produced by the atomizing device may be relatively coarse. In the production of many commercial products this feature is highly desirable, because it enables the production of final products of a coarser grain. In addition operation of the apparatus in this manner serves to effectively reduce power consumption and maintenance costs of the atomizing devices.

cross-sectional contour.

' Previous reference has been made to the fact that in conventional spray drying equipment the incoming hot gas is guided by the adjacent walls of the desiccating chamber. With the present invention the incoming hot gas is no longer directly guided by adjacent walls of the chamber,'and fiie of'tlie' walls constituting the upper chamber pa t emain at a relatively low temperature. This *because incoming gases, before contacting these walls, have been cooled by evaporation of moisture from the atomized material. Because of this feature any material which comes into direct contact with the walls of the chamber portion 10b, are not burned or injured by excessive heating.

The jets or streams of hot gas discharging from the conduits Hi to I9 afford a greater degree of turbulence within the main part of the drying chamber. This is because the incoming streams of gas initially have substantially linear flow, and thereafter swirl generally about the axis of the chamber, and within the main portion of the drying zone. Greater turbulence tends to accelerate evolution of moisture from the atomized particles, thus enabling a corresponding increase in the capacity of the equipment.

It will be evident that the invention is not limited to the use of four hot an inlet ducts. Thus it is possible for example to use five or six duets, with the distribution such that they all discharge into the desiccating chamber on axes located in a common plane with the general plane of the atomizing device, and with each axis being directed generally toward the discharge opening of an adjacent duct. It will be evident that the axis 21 of each duct need not be directed precisely toward the outlet of the adjacent duct. The general direction is sufficient provided the air is discharged from each duct with sufficient velocity and such direction as to sweep directly over a generally linear path to intermingle with air discharged from the next duct. Thus the theoretical discharge axes 26, 21, 28, 29 may each bedirected to intersect the adjacent axis at a point somewhat beyond the outlet end of the latter.

In that embodiment of the invention illustrated in Figures 3 and 4, the desiccating chamber 3| is likewise formed symmetrical about the vertical axis 32, and is annular in transverse The lower portion 3Ia of the chamber is conical shaped to aid in centrifugal separation of dried powder. The dried powder in thisinstance is removed through the lower outlet conduit 33. The upper portion 3Ib of the desiccating chamber is cylindrical, and connects with the hot air ducts 36, 31, 38 and 39 (Figure 4). These ducts all connect with a common hot air supply conduit 4|, which in turn connects to the discharge side of a blower, through the air heater 42. Spent drying air is removed through the exhaust conduit 43, which connects with the cyclone separator 44. The upper open end 46 of the conduit 43 is located concentric with the xis 32, and intermediate the upper and lower ends of the desiccating chamber. The centrifugal atomizing device 41 is located above the openend 46 of the exhaust conduit, and is in the same general plane as the ducts 36, 31, 38 and 39. A hot air inlet opening 48 is located adjacent and above the atomizing device 41, and it receives hot air from the chamber 49, and conduit 5|. This conduit can connect with the outlet side of the same heater 42. An additional conduit 52 can be provided for introducing supplemental drying air, and it is shown communicating tangentially with the chamber at a level below the end 46 of the exhaust conduit 43.

The apparatus of Figures 3 and 4 possesses the same features as the previously described apparatus. The atomized particles delivered by the device 41 will be prevented from directly impacting the surrounding peripheral walls of the desiccating chamber, and the atomized particles will take a free flight in which they swirl about the axis 32, and gradually progress downwardly until separated out in the lower end oi the chamber. The spent drying air removed through conduit 43 may carry with it a certain amount of classified dried material, and this is removed in the cyclone separator 44.

We claim:

1. In a spray drying method making use of a vertical drying zone, the steps of introducing atomized particles of material to be dried into the upper portion of said zone adjacent the vertical axis thereof and projecting said particles generally horizontally from said axis, continuously introducing hot drying gas into the upper portion of the zone to maintain a diving atmosphere within the zone for drying the atomized particles, thehot drying gas being introduced as free discharging jets of gas distributed about the horizontal region in which the atomized particles are introduced, said jets being on substantially linear axes which successively intersect and which are in a conmion plane at right angles to the said vertical axis of the zone and substantially coincident with the horizontal plane in which the atomized particles are introduced and projected, said jets being localized whereby the direct discharge of the same into the zone is confined to the proximity of the upper and lower sides of said plane.

2. In a spray drying method making use of a vertical drying zone, the steps of introducing atomized particles of material to be dried into the upper portion of said zone adjacent the vertical axis thereof and projecting said particles generally horizontally from said axis, continuously introducing hot drying gas into the upper portion of the zone to maintain a drying atmosphere within the zone for drying the atomized particles, the hot drying gas being introduced as free discharging jets of gas distributed about the horizontal region in which the atomized particles are introduced, said jets being on substantially linear axes which successively intersect and which are in a common plane at right angles to the said vertical axis of the zone and substantially coincident with the horizontal plane in which the atomized particles are introduced and projected,

said jets being localized whereby the direct discharge of the same into the zone is confined as to the proximity of the upper and lower sides of sai plane, introducing additional hot drying gas into a region surrounding the region into which the atomized particles are introduced and in a downward direction, to thereby deflect the smaller I sized particles downwardly into the main portion of the chamber, while permitting coarser particles to be discharged laterally into said jets of hot drying gas.

3. In a spray drier, a desiccating chamber formed symmetrical about a vertical axis, the lower portion of said chamber being provided with means for removal of powder and drying gas. a centrifugal atomizing device disposed in the upper portion of the chamber concentric with said axis and arranged to project particles outwardly in a generally horizontal plane, a plurality of ducts adapted to receive hot primary drying gas and communicating with the upper partof the chamber through the peripheral wall of the same, said ducts being spaced circumferentially at regularly spaced intervals and being disposed to discharge on axes which are substantially in a horizontal plane coincident with the plane in which said atomizing device project particles, the axis of each duct being directed chordally of said chamber and intersecting the axis of a jet issuing from an adjacent duct and all of said ducts discharging in-a zone which'is confined to the proximity of said common plane, and means .a

for introducing supplemental gas into said chamber in a region generally surrounding the centriiugal atomizing device and downwardly with respect to said axis, and serving to receive and entrain fine particles of material discharged laterally of the device and to cause such material to progress into the lower portion of the desiccating chamber.

DAVID D. PEEBLES.

REGINALD E. MEADE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,107,784 Gray Aug. 18, 1914 2,081,909 Bowen June 1, 1937 2,151,079 Bowen -i- Mar. 21, 1939 2,184,314 Peebles Dec. 26, 1939 2,473,035 Meade et al June 14, 1949 FOREIGN PATENTS Number Country Date 317,166 Great Britain Aug. 15, 1929

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