US2165644A - Method of absorbing and condensing digestion gases and vapors - Google Patents

Description

July 11 1939- A. D. MERRILL E-r AL 2,165,644

METHOD oF ABsoRBING AND coNDENsING DIGEsTIoN GASES AND vAPoRs Filed June 26, 1935 2 Sheets-Sheet l Een l July 11;'1939- A. D. MERRILL E1- AL 72,165,544

METHOD OF ABSORBING AND CONDNSING DIGESTION lGASES ND VAPORS Filed June 26, 1935 2 Sheets-Sheet 2 Wwf/jf Patented July 11, 1939 UNITED STATES METHOD F ABSORBING AND CONDENSING DIGES-TION GASES AND VAPORS Albert D. Merrill and Thomas L. Dunbar, Watertown, N. Y., assignors to Chemipulp Process Inc., Watertown, N. Y., a corporation of New York Application June 26, 1935, Serial No 28,580

17 claims.

This invention relates to improvements in the digestion of fibrous material, and more particularly to an improved method of and apparatus for absorbing and condensing gases and vapors evolved during the digestion treatment.

It has been proposed heretofore to utilize a flowing stream of acid to withdraw and condense gases generated or evolved in a pulp digester as well as from other units of a digestion system. A

l0 typical example of such use is the employment of an eductor for withdrawing gases and vapors from the vapor space of a digester. In such prior use relatively cold acid Was pumped, at relatively high velocity, through the nozzle of an eductor and this stream utilized to induce a flow of gas from the digester. Such eductors usually comprised a single gas inlet and a single nozzle with a valve means associated with the nozzle to control the flow of liquor therethrough.

To date but little attention has been paid to the proper design of such eductors. Ifhese units, however, are very important in respect of the efficient operation of a pulp digestion system since they perform a number of important functions.

They serve in the rst instance as a pressure con- Ytrol means to adjust and/or maintain a desirable loW pressure in the digester becauseof the A fact that this pressure is due substantially completely to condensible gases (SO2) and vapors (H20) densation therefore largely controlsthe pressure in the digester. Secondly such eductors serve as a combined heat exchanger and mixer inasmuch as they provide a zone in which cold acid is caused to condense and absorb hotter gases and vapors, thus uniformly increasing the temperature and concentration of the acid.

As a result of considerable experimentation We have found that ifn eductor is properly designed' 40 its eiliciency can be very greatly increased. The novel construction and operatinlof the improved eductor, tobe described, is based upon a recognition of factors, obtaining in this art, which heretofore were either unappreciated or were disregarded.

' An object of the invention therefore is to providean improved method of gontrolling pressure in a pulp digester.

Another object is to provide a novel method of withdrawing and condensing condensible gases' and vapors evolved .in a pulp unit.

A Afurther object is to condense condensible gases and vapors Yin a. pulp digestion unit by utilizing a plurality of streams of absorbing fluid.

Another object is to provide an eductor of im- The quantity or degreegof .such con-l (ol. 92-7) t proved design by reason of which optimum induced flow of gases and vapors is secured.

Yet another object is to provide a novel method of controlling the quantity of gas flowing through anieductor.

A still further object is to devise means for in' suring optimum stream line flow of gases through an eductor.

An additional object is to devise an eductor in which a plurality of zones are provided for the v 10 direct contact of liquid and gases and/or vapors. With these and other equally important'and related objects in view the invention comprehends the concept of providing an eductor of special design in which the size of the liquor inlet 15 pipe is specially correlated with respect to the position of the gas inlet so as to achieve improved results and in which improved adjustable control and optimum absorption are securedby utilizing a split or plural stream of the absorbing 2,0 liquid. By reason of the provisiony for adjustable`- control the eductor may be substantially standardized so as to fulfilll the requirements of plant units of varying capacity.

In order to enable a more ready comprehen# t2,5 I

sion of the underlying principles of the invention preferred illustrative embodiments are shown in the accompanying drawings, in which- Figure 1 is a diagrammatic illustration of a hot acid/ unit embodying the improved-operation andl 30 structure.

Fig. 2 is an enlarged longitudinal section of one modification of the improved eductor. l Fig. 3 is a cross section taken on line 3-3 of Fig. 2. f1 3 5 Fig. 4 is a central longitudinal section of another type of eductor. v

Fig. 5 is a detail of the eductor shown in Fig. 4. Fig. 6 is a diagrammatic illustration of the type of flow of the fluids through the eductor. .540

As shown in the drawings and as is now known in the art, the major elements of the apparatus include the acid storage tank I, hot acid accumulator 2 and one or more digesters 3. Cold acid may be pumped from the acid storage tank by 45 means of pump 4 and forced through line 5 and check valve 5 to the eductor l0, through main eductor branch-6, controlled by valve 6 and optionally through branch 'l controlled by valve 1' in a manner more particularly to be described. 50 Cold acid from branch 6 passes at relatively high velocity through nozzle 8 and induces a flow of` gasand/or vapor through the relief header 9. The cold acid .condenses and absorbs the hot relief gases and vapors and is correspondingly 55 and proportionately preheated. The combined uid stream then passes downwardly through drop leg II to second eductor I2. Here the fluids are contacted with a stream of acid recycled to and from the accumulator whereby further condensation and absorption is insured and the pressure in line II correspondingly reduced. Such cooling and absorbing stream is withdrawn from the accumulator through line I3, pump I4 andV nozzle I5, thence through valve I6 back to the accumulator. The flow of liquid through the recycle line may be controlled by proper adjustment of valve I5'. As will be appreciated, since eductors I and I2 are in series, maximum condensation and absorption, and hence maximum reduction in pressure in header 9, is secured, while' at the same time the make-up acid entering through line is preheated.

In the operation of this type of plant, as is now known to those skilled in the art, the digester is first filled with chips by removing the manhole cover 20 and feeding the chips in by gravity or by means of a chip packing device. After the digester has been filled with chips the valve 2l' in the top relief line 2l is opened, valve 22' in side relief line 22 and valve 23 in high pressure header 9 are closed. Valve 24 in the low pressure relief branch 26 is closed and air vent valve 25 is opened. Hot acid is; then withdrawn from the accumulator 2 through the line 30 and forced by the digester-'filling pump 3l through header 32 into the branch line 33. As the hot acid enters the digester the air contained therein is correspondingly displaced and is discharged through the open valves 2| and 25 to the atmosphere.- lValves 2|' and 25' maybe kept open until the digester is completely lled with liquor or until gas appears at the vent valve 25'. When such gas Vappears valve 25 may be closed and valve 24 opened, valve 2l renaining open so as -to pass such gas through the relief header 26 and thence through drop leg 2l to the acid storage tank. When the digester is completely filled valves 24' and 2|. are closed.

Thereupon, that is to say when the digester is charged with chips and is filled or substantially filled with liquor, operation of the-pump 3| may be continued so as to force in addi-l tional hot acid and build up any desired hydrostatic pressure within'the digester. In the usual `run pump 3| is continued in operation until pressure of the order of from fifty to ninety pounds per square inch is developed in the digester. Thereafter ow of acid from the accumulator to the digester may be discontinued either by stopping the pump or closing a valve in the branch 33.

Then, if desired, in order to insure a thorough penetration of the chips with the hot acid and to equalize the temperature throughout the digester, the digester recirculation circuit may be operated. This may be done,'as will be noted, by opening valve 40', operating pump 4I, and thus withdrawing acid from the upper portion of the digester through line 40 and discharging it into the lower portion of the digester through line 42. This recycling ismaintained until the desired conditions obtain, such conditions being a thorough penetration or saturation of the chips with acid at a substantially uniform and equalized temperature throughout the digester. Such precirculation-soaking period may be maintainedA for a period of from one-half to two hours.

At the end of the soaking period the chips within the digesterare uniformly penetrated with hot acid at a temperature of from 75 C. more or less to about 109 C, Then, as more particularly explained in copending application of Thomas L. Dunbar, Serial No. 758,314, the liquid level within the digester is lowered` by drawing off relief liquor through side relief line 22 and passing it by way of relief header 9, eductor I0 and drop leg II to the accumulator. Upon the lowering ofthe liquid level in the digester there is an abrupt drop in pressure from the existing pressure of 50 to 90 lbs. down to 30 lbs. more or less. This drop in pressure, as will be understood, permits the use of low pressure steam for bringing the mass up to-cooking temperatures. Upon lowering of the liquid level to the desired extent steam is then admitted to the digester through steam lines (not shown) connected to the bottom cone of the digester.

This admission of steam soon builds up the pressure within the digester and the temperature correspondingly increases at a rapid rate. When the pressure has been raised to the desired operating level it may be maintained at this value by opening valves 2| and 23'. During the cooking operation the liquid level rises due to the condensation of steam. Some time after the initiation of the cook the top relief valve 2l may be closed and the side relief line 22 opened so as to maintain the proper liquid level. Thus in normal operation the side relief line maybe opened at about the fourth or sixth hour of the cook. In the manner well known to those skilled in the art, by the proper manipulation of the valves 2| and 22 the desired pressure and liquid level may be maintained.

If desired, during the actual cooking or digestion process liquor may be recycled to andfrom the digester by operating the digester recycle passed through line 26 and eductor fitting 28 through the drop leg 2 1 into the acid storage tank. The eductor 28 may, if desired, be of substantially the same type as eductor I0 or any other preferred design.

As explained in copending application Serial No. 758,314, the pressure conditions obtaining inthe system, and particularly in the accumulator, may be accurately controlled withina relatively narrow range. means of the vent line 50 connected at one end through the valve 50 to the-vapor space of the accumulator and at the other through a fitting 5I to the drop leg 2l. The valve 50' is preferably a pressure regulating valve adjusted to a predetermined setting so that when the pressure in the accumulator exceeds this point gases This may be achieved by accumulator. The valve 5I', like valve 50', is a pressure regulating valve. This may be set at any desired pressure differential of from the order of 1 to 4 or more pounds. In thesecircumstances when the pressure in the relief line 9 exceeds by this amount the pressure in the accumulator, the valve 54 automaticallyoperates and fluids are -by-passed around eductor i and vintroduced directly into the accumulator. v

The digester and accumulator are provided, as is known by those skilled in the art, with pressure, temperature and liquid level' indicating and/or recording devices.

This type of operation is greatly improved by utilizing the novel eductor. As shown in Figs. 1

and 2 the stream of cold acid forced through line 5 may be split into. streams, one passing through line 6 and the other passing through line. 1. The acid entering through line 6 passes downwardly through the central pipe or channel 60 and is .ejected through the nozzle 8. Acid flowing from line 1 enters the eductor 'at a point approximately opposite the entrance of the relief fluids, that is to say in thearea of the outlet of header 9.

In the preferred operation acid is forced through lines 5 and 6 so that a relatively high pressure obtains at the nozzle 8. Operations have been conducted in which the pressure at this `point is about 110 lbs. per sq. in. gauge. The size of the pipe 60 and the nozzle 8 is specially de signed so that for normal operation of the plant the control valve 6,' is completely opened. The nozzle thus discharges the required amount of liquor without utilizing the auxiliary stream passing through line 1; If for any reason the plant capacity is increased over the normal capacity,

the additional liquor requirements are satisfied by opening valve 1 and passing the additional stream intothe eductor. In these circumstances the cold acid flowing through line 1 immediately contacts the gases entering from header 9 and eil'ectsja'condensation and reduction of pressure at this zone. The liquor flowing through the nozzle 8 at relatively high velocity establishes a zone of' correspondingly reduced pressure which acts to induce 'a flow of fluids downwardly through the eductor. Y

In the preferred type of operation an auxiliary condensing stream for the low pressure gases is also provided. As shown, a line 10, controlled by valve 10', is connected to the cold acid line 5. Such line communicates with the upper end of the drop leg 21 through a suitable eductor type of fitting 28. Cold acid flowing through the line 10 thus contacts with low pressure gases flowing in through header 26 and cools, condenses and absorbs such gases. This 'condensation and absorption reduces the pressure in line 26, causing a rapid ilow of fluids therethrough.

Thecold acid pump 4 is so designed that it will maintain a pressure at the vnozzle 8 of the order of 110 lbs. and will deliver 50% more liquor at that pressure than is actually required for a given operation.

As .shown in'Fig. 4, more rapid contact and,

consequent condensation of gases may be obtained by feeding the relief fluids into the eductor in-a plurality of streams. I'hus the tubular the same zone as the liquid inlet 63, which latter is connected to the auxiliary-cooling acid line 1. As in the eductor shown in Fig. 2, the main conare intimately contacted with densing stream of acid enters the eductor through the central tube 60 andextends downwardly, below the gas inlet 6|-62, a predetermined dis tance, as will be more fully explained. In the op# eration of this type of eductor' streams of gases 5 entering the inlet 6i`62 respectively mutually impinge in the central area of the educto'rand at v that point, of high turbulence, may be'contacted with a cooling stream of acid admittedthrough branch line 1. The acid flowing downwardly 1o through nozzle 8 establishes a reduced pressure at this zone, ,which pressure sucks or aspirates fluids from the upper section of the eductor.l InA the area about nozzle 8, therefore, the relief fluids cold acid flowing at relatively high velocity.

In this type of eductor, therefore, there is pro*- vided two condensing and/or absorbing zones. The first or main zone in the area about nozzle .8' operates continuously and fullls the normal 4re- 20 quirements of a given installation. A supplemental condensing zone is provided, as explained, in

. the upper portion of theeductor and vcomes into play when cold acid is admitted through the separate line 1. The specific mechanical construction of -the eductor may of course be widely varied. To facilitate assemblage and servicing, that in the drawings is preferred. Thus a complete'unit may comprise a fitting 80 and the tube lill. Thisfiltting 30 may be a casting of a suitable corrosion `resistant metal or alloy and ls provided with the terminal flanges 8| and 82. 'Such fitting is formed with the gas inlet (or inlets) 83 which is provided with the peripheral flange 84. The flanges are tapped in the usual manner to provide for attachment to a corresponding coupling flangeon the header 9. The eductor tting is also formed with the liquor rinlet pipe section 85, which is suitably formed to provide for tight attachment to the acid line 1.' o

- The tube preferably constitutesla separate member. This is formed at its `upper end with anr integral lateral flange, of greater diameter than the bore of fitting '80, so as to seat upon the upper flange 8|. The tube is provided at suitable 5 points with spacer lugs 90 (see Fig. 3). Such lugs are preferably of stream line form in cross section so as to diminish resistance to iiow.

The upper end of the drop leg Il may be formed with a flange I I' which is tightly held between the bottom flange 82 of the'tting and the coupling ring ill. It will be understood of course that/interposed 'between the respectively cooperating flanges orabutting surfaces' are suitably corrosion resistant packlngs -or glands.

` It is particularly to be observed that in a pre ferred structure the liquid-inlet 85 extends at an angle to the.central axis of the tube and to the axis of the header 9. In other words, liquor entering-through the line 1 and pipe -section I5 o impinges the entering stream of gas tangentially, thus denect'ing the flow of gas downwardly and centrally of the fitting and Vdrop leg.

As a result of considerable experimentation we have found that in order to secure the best results in this type of operation, that is-to say in the utilization of an eductor, the length 'of the pipe 60, or` its equivalent. is very important and substantially critical. Specifically we have found that in order'to obtain optimum results the length 70 of the tubetil, below the gas inlet, should be about etl ft. (seerzFig. 6). vWe have furtherfound, paradoxically, that variations in the diameter of the pipe II do not substantially affect this factor. In other words, we have found as a mat-MA 45 burble angle.

50 cial relationship existing 5 that the velocity of the eillux stream from 'the nozzle 8 will vary with the diameter of this nozzle and as a corollary the pressure in the zone of the nozzle will correspondingly inversely vary, that the requisite length of the pipe section would be commensurately affected. It is probable that within a certain degree this is true but, as noted above, we have found that for usual diameters of the nozzle 8, the best results can only "be achieved by having the pipe section 60 of a ,length of the order of 4 ft. below the gas inlet.

A typical eductor which has successfully been employed comprises'one having an 8 inch bore, a central liquor pipe (60) of 3 inch diameter, and 1 l@ inch nozzle, and a gas inlet of 8 inch diameter. While, attempting no accurate rationale of this result, it would appear that the peculiar 'effectiveness of this particular length is due to stream 'line flow, as diagrammatically. illustrated in Fig. 6. During operation gas enters the eductor tting from the line 9 at a pressure of the order of 70 lbs.'or less.` As pointed out above, the fluid pressure of the entering liquid, at the nozzle 8, is of the order of 110 lbs. per sq. in. The gas entering the eductor fitting is deflected substantially at right angles and, due to its elasticity, is in a turbulent or burbling condition for a considerable period of time, despite the rapid flow of liquor` from the discharge nozzle 8. It would appear then that the effectiveness of this particular pipe length is due to the fact that it insures an egrtended length of travel, in which length the gases and /or vapors are extenuated from-a turbulent, swirling or burbling A. condition tonne of stream line or filament iiow. In other words we t believe the situation here is analogous to that obtaining to aerodynamics where, to insure the maximum lift, that is to say the greatest continuity of airflow, the alrfoil must be maintained below what is known in that art as the critical or At any rate we nd as amat ter of actual fact that if the conditions outlined above are fulfilled, improved operations4 are secured.

We have found that in this art there is a spebetween the character and /orcondition ofthe condensable medium and the length of its travel under induced iiow. -We have found further that in a typical pulp plant optimum results in respect of pressure control and condensation may be achieved `by utilizing a relatively long length of pipe section 60, or, stated differently, by displacing the liquid eillux 'some considerable distance from the gas inlet, such distance being suiiicient to insurev extenuation of the relatively elastic gaseous orvaporous medium from a swirling or turbulent motion into asubstantially stream line flow. In these circumstances liquid discharges from nozzle 8 into a substantially parallel encasing stream of gases and vapors, thus insuring optimum induction effect, condensation and absorption.

i The novel eductor has been described particularly with respect to its utilization in the high pressure relief line. It will be manifest, however,

that this improved structure may be employed at any other point in the pulp plant or in any other art or circumstances where the same results are l desired and comparable conditions obtain.-

Whilea preferred structure has been described,

it is`to be understood that this is vgiven for the purpose of explaining the principle of operation of the device. The invention is considered to residein the several broad concepts defined, such as the utilization of a plurality of streams of condensing medium and the principle of optimum displacement of the nozzle from the gas inlet.

We claim:

l1. In a cellulosic pulp digesting process that method of recovering the chemical and thermal values of evolved gases which comprises admitting such gases'to an eductor and contacting the gas therein with a plurality of streams of condensing liquid and utilizing the resultant liquid for digesting cellulosic material.

2. In a cellulosic pulp digestion process that method of recovering the chemical and thermal values of evolved gases and vapors, which comprises admitting such gases to an eductor zone and contacting the gas therein with a plurality of streams of cellulosic pulp digestion liquor at different sections of the zone.

3. That method of inducing a flow of gas from a digestion zone of a cellulosic pulp digestion process in which gas is evolved which comprises admitting such gas to an eductor, contacting such gas immediately with a stream of a relatively cool cellulosic pulp Vdigestion liquor,fpassing the combined stream through a predetermined travel and then contactingA such combined stream with a separate stream of cellulosic pulp digestion liquor.

4- That method of inducing a flow of gas from a pulp digester of a cellulosic pulp digesting process which comprises connecting the vapor space of the digester through a conduit to an eductor, admitting a stream of relatively cool cellulosic pulp digestion liquor to the eductor adjacent the area of admission of the gas, and admitting another stream of suchdigestion liquor to the eductor at a point displaced from the inlet of the gas,

5. In a cellulosic pulp digestion process that method of recovering the chemical and thermal Avalues of evolved gases and vapors which com- '7. vA method of reducing the pressure in a pulp.

digester and recovering the ohemicaland thermal units of evolved gases and vapors which comprises lconnecting-the vapor space of the digester through a conduit to an eductor, admitting a stream of cooling and absorbing liquid to the eductor adjacent the inlet of the gases and vapors and in a direction tangential to the entering stream of gas, admitting a second stream of cooling liquid to the eductor at a point displaced from the gas inlet and in a direction'substantially parallel to the iiow of gas at such point.

8. A method of withdrawing gases from a cellulosic pulping operating plant unit in which such tor at a point suiciently displaced from the gas inlet that at such point the gases have assumed a non-turbulent, substantially stream line ow, and then utilizing the resulting liquor for the digestion of cellulosic material.

9. A pulp digestion process comprising cooking a mass of fibrous material in a digester with acid liquor, withdrawing gases and vapors from the vapor space of the digester and passing such gases and vapors to an eductor, contacting the gases in the eductor with plural streams of relatively cool acid liquor and discharging fluids from the eductor to an accumulator.

l0. A method of reducing the pressure in a pulp digester in which cellulosic material is pulped, and recuperating the thermal and chemical values of evolved gases which comprises, admitting gases evolved in the digester to an eductor in a plurality of streams and contacting such gas in the eductor with a stream of relatively cool cellulosic digestion liquor, and utilizing the resulting liquor for digestion of cellulosic material.

11. A method of reducing the pressure in a pulp digester in 'which cellulosic material is pulped, and recuperating the thermal and chemical values of evolved gases which comprises, yadmitting gases evolved inthe digester to an eductor in a plurality of mutually impinging streams and contacting the combined stream of gas in the eductor with a stream of relatively cool cellulosic digestion liquor, and utilizing the resulting liquor for digestion of cellulosic material.

12. In a cellulosic pulp digesting process that method of recovering chemical and thermal values of evolved gases which comprises, admitting such gases in a plurality of streams to an educ# tor and contacting the gas in the eductor with a plurality of differential velocity streams of relatively cool cellulosic digestion liquor, and utilizing the resulting liquor for digestion of cellulosic material.

13. In a cellulosic pulp digesting process that method of recovering the chemical and Athermal values of evolved gases which comprises admitting such gases in a plurality of mutually impinging streams to anA eductor, admitting a stream of relatively cool cellulosic digestion liquor to the eductor in the zone of impingement and admittingj a second stream of similar relatively cool di..v gestion liquor at a point displaced from said zone of impingement and utilizing the resulting liquor for the digestion of cellulosic material in the process.

14. A method of digesting pulp which comprises cooking a mass of fibrous material with acid liquor in a digester, withdrawing gases and vapors evolved in the digester andmassing them through a conduit to an eductor; forcing a, stream of relatively cool acid liquor from a con-v tainer toward' the eductor, splitting such stream of cooler acid and optionally admitting the split streams to the eductor at a plurality-of points therein; passing low pressure gasesfrom the digester through a second conduit and contacting such gases with a split stream of acid derived .l

from the said first acid stream.

15. A pulp digestion process comprising cooking a massof brous material in a digester with a digesting liquor comprised of a volatile component, withdrawing volatile components from the vapor space of the digester and passing such volatilesto an eductor, contacting the volatiles in the eductor with plural streams of relatively cool digestion liquor and discharging fluids from the eductor to an accumulator.

16. In a cellulosic pulp digesting process, that method of recovering the chemical and thermal valuesrof evolved gases which comprises admitting such gases in a plurality of mutually impinging streams to an eductor, preliminarily condensing the gases adjacent the point of admission by contacting said gases with a stream of relatively cool cellulosic digestion liquor and further condensing said gas by admitting a stream of relatively cool cellulosic digestion liquor to the eductor at a point displaced from said zone of impingement and utilizing the resulting liquor after such contact asa cellulosic digestion reagent.

17. In al cellulosic pulp digesting process, that method of recovering the chemical and thernial values of evolved gases which comprises admitting such gases to an eductor and connected drop leg, and contacting and condensing the gases therein with a plurality of streams of relatively cool cellulosic digestion liquor, said plural streams respectively contacting the gases at dierent points along the line of ow of the gases in the eductor and then utilizing the condensing( liquid as a digestion reagent in the pulping process.

' ALBERT D. MERRILL. THOMAS L. DUNBAR.

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