US2403211A - Agitator for cascade evaporators - Google Patents

Description

July 2, l946.- c.. R. P. CASH 2,403,211

AGI-TATOR FOR Y CASCADE EVAPORATORS Fired Nov. 21, 1942 2 sheets-shea 1 'mmf ,l i INVENToR. (Aarle: H12/zizi )perfil/a! ds NMFA/fr Patented July 2, 1946 AGITATOR FOR CASCADE EVAPORATORS Charles Richard Percival Cash, Canton, N. C.

Application November 21, 1942, Serial No. 466,513

1 2 Claims. 1

(Cl. g- 10) This invention relates primarily to apparatus 1 for recovering chemicals from black liquor obtained in thev preparation of rwood pulp by the sulphate or caustic soda process. Y

The paper pulp is made in the sulphate process by treating chips of wood with sodium hydrates or sodium sulphite. After the Vaction of the chemicals on the wood has been completed in the digester, the material is washed and the wash liquor, usually termed black liquor, contains substances which should be recovered for reasons of economy. In the'usual process the black liquor solution is carried through a, multiple effect evaporator and thence through a -so-called cascade evaporator within vwhich a further portion of until the liquor is of a relatively heavy density. The cascade evaporator comprises a plurality of rotating baille wheels mounted within a chamber. The bottom of this chamber contains a pool of liquor into which the wheels having baflle plates dip as they rotate.` Hot gases ilow through the top of the chamber and over the wheels to evaporate moisture from the liquor which has adhered to the baiile plates. By thus evaporating water from the liquor wetting the wheels the density of the pool of liquor is increased.

It has been found that in the spaces between adjacent groups of wheels and the wheels and the corners of the chamber there is substantially no agitation of the liquor and consequently lumps congeal and form into piles there. These piles must periodically be removed or they will seriously interfere with the rotation of the wheels.

It is an object of this invention to provide means for continuously agitating and moving the liquor in currents within the cascade evaporator thereby preventing lump accumulations in piles and assuring the continuity of operation of the evaporator.

In the accompanying drawings: Figure 1 is a sectional elevation on line I in Figure 2 of a cascade evaporator embodying the invention. s Figure 2 is a plan view corresponding to Figure 1.

Fig. 1. The chamber Ill has an inlet 20 and an outlet 2| which connect the ilue gas outlet of a sulphate furnace waste heat boiler and to a stack or induced draft fan, respectively. The flue gases from the waste heat boiler flow through the top portion of the casing. Each baille wheel is separately rotated through a pulley 22 connected by a belt or chain 23 with a pulley 24 driven from a motor 25 through reduction gears 26.

The gases Afrom the recovery furnace and thewasteheat boiler contain quantities of sodium sulphate and sodium carbonate. When these gases impinge on the wetted bailles. of the 'evap' orator the chemicals adhere tothe baiiies'and are thus scrubbed out of the gases; Should: stratified the water is evaporated by means of hot gases f scrubbing 000111',V lumps 'are' apttO form. Furthermore, the liquor in the evaporator is generally at a temperature not far above Vthe point.. I

where congealing` ofthe liquor can occur'. 't Should there be any regions in the quiescent corners or recesses of the evaporator where cooling may take place, small quantities of liquor will congeal and form lumps. Liquor splashed on the walls of the Y Vevaporator forms a solidied coating and at ir- Unless these accumulations are removed they grow into piles until these may seriously interfere' with the operation of the bafe wheels. AHeretofore these piles have been raked out periodically n by hand, which cleaningA interrupted the con- The cascade evaporator comprises a closed tank IIJ within which are four groups of bailles I2, I 3, I4 and I5. Since the construction of cascade evaporators is generally known it need only be mentioned that the individual baiiles I6 (Fig. 1) of each group I2 to I5 are attached at their ends to wheels or disks I1 each rotating with a. shaft I8. In the bottom of the tank is a pool of liquor generally maintained at the level indicated in tinuity'of operation of the evaporator. With the increased capacity of recovery units, lump re-y moval thus became a serious interruption.

In the past the baille wheels of the evaporators all revolved in a clockwise direction,'as viewed in Fig. 1. As a consequence the uid circulated along both side walls of the tank in the same direction, the two vstreams mingling to flow back through the center of the tank from one end to the other. According to the invention two of the groups of wheels are rotated in reverse direction with respect to the other two. Referring to Fig. 2, wheels I2 and I3 have their driving connections arranged for counterclockwiserotation while wheels I4 and I5 are revolved clockwise. The liquor in the bottom of the tank is then advanced `by wheels I2 and I3 toward the right and by wheels I4 and l5 toward Athe left. 'I'here will also be cross ow of the ,liquor at the ends of the tank from the wheel I4 toward the wheel I2 and from the wheel I3 toward the wheel i where the reverse movement affords. ample circulation.

afmerenl n I 5. l In this way theliquor is continuously moved in a current running counterclockwise raround l the tank, as viewed in Fig. 2. To further agitate the liquor in the quiescent corners yand to aid this current helical screws 30 i and 3| and paddle wheels 32 are provided. Screw l 30 is located below'and alongside baille wheels I2,

I4 in the left hand lower corner of the tank and is rotated by pulley 33 and belt 34 from the shaft y 6 of the baille wheel I2 so that it turns counter- 1 clockwise. e l 35 and belt 36 and pulley 3l from the shaft of l Paddle wheel 32 is driven by pulley baie wheel I3 and also turns fcounterclockwise.

l The screw conveyor 32 is driven by gears 31 from one set of reduction gears 26 so as to 'turn counterclockwise. As mentioned the screws 33 and 3| and paddle wheel 32 are sov placed as to be where I lthe lump piles accumulate. The helixes of thel screws 30 and 3| are so arranged that as they .turn they move'the solid material in the direction of the liquor current. In this way the piles are wheels rotatable on aligned Vaxes extending transy versely of said tank adjacent either end thereof; means for rotating the baille wheel of each pair that is located adjacent the front wall of the tank in one direction and the other wheel of each pair that is located adjacent the rear wall of the tank in the opposite direction for causing circulation of the liquid in a definite path around the tank; and rotatable agtators disposed below the liquid level on axes parallel to said kbaffle Wheels and Y in locations between either end of the tank and prevented from forming and the material is given ageneral movement in the direction of the liquor current. This keeps the particles in notation and the generalcurrent flow tends to maintain a `uni- Y form Ytemperature ofthe liquor in all 4parts'o-f theY tankiand. thereby avoids congealing and lump deposits which voccurred heretofore. In place of the spiral agitators 30, 3 I ipaddle .wheels like that designated 32may be employedin installations What I claim is:r

` 1."In 'anevaporator having a tank containing a pool of black liquor Vor the like, a gas inlet and I outletV at opposite ends Yof Isaid f tank above the levelof liquid in saidpool for circulating'gas longitudinally through the tankg" pairs of baille of the baffle wheels the baille wheels adjacent thereto for preventing deposit of material solidified from said pool and moving it away` from said locations.

2. In' an evaporator having a tank containing a pool of black liquor or the like, a gas inlet and each pair that is located adjacent the rear wallV of the tank inthe opposite direction for causing circulation of the liquid in a deniste path around the tank; and rotatable agitators disposed below the liquidlevel on axes parallel to said baille wheelsand in locations between either end of the tank and the bale wheels adjacent thereto for breaking up material solidiiied from said pool and means forY rotating said'agitators soy as to cause them to move saidV material in the same direction as that of the uid circulation in said tank. 1'

CHARLES RICHARD PERCIVAL CASH;

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