US2262519A - Evaporator - Google Patents

Description

Patented-Nov. 11, 1941- EVAPORATOB Joe E. Talton, Crossett, Ark., 'asslgnor to Crossett Paper Arkansas.

Mills, Crossett, Ark., a corporation of 'Application March 21, 1940, serial No. 325,124

' 1 claim. (ci. 15s-2o) This invention relates to a new and improved type of evaporator for concen rating liquids and more particularly to a new multiple effect evaporator. One of the objects of the invention` is to provide an evaporator or evaporator system forconcentrating liquidsl which is much more eilicient than evaporators or. evaporator systems now in use. l Still another object of the invention is to provide a new and improved method and apparatus for the evaporation and concentration of liquors employed in the paper industry for digesting or otherwise treating paper pulp;

Still another object of this invention is to provide a new and improved method and apparatus of the character described havingan increased capacity.

Other featuresand advantages of the invention will be apparent by reference to the following description in conjunction with the accompanying drawing in which:

Figure 1 represents tiple eifect evaporator as invention;

Figure 2 is a repre grammatically partly in section and with parts broken away of a single evaporator body or unit.

The evaporator shown in Figure 1 is a five body, long tuba-vertical, film type, quadruple effect evaporator.

diagrammatically a mulillustrating the present The evaporator is arranged to operate as a quadruple effect with two bodies in the first eiect and one body in each of the second, third and fourth effects.

As shown in Figure 2, each evaporator body consists of a bundle of long, small diameter tubes I8 surrounded by a steam chest l2. The bottom of the tube bundle is connected to a cone I4v through which the liquor enters the tubes and the tops ofthe tubes I6 connect into a large vapor chest or dome I8. yAny condensation from steam chest I2 is withdrawn through a conduit or pipe 28. Steam enters steam chest I2 through a conduit or pipe 22. The condensate is withdrawn from the vapor chest or dome I8 through a conduit 24. Vapors are withdrawn and improved type of entation somewhat diafrom the chest or dome. I8 through a conduit 28.

It will be understood that any other suitable arrangement of conduits may be employed to' acccmplish the same result, The invention herein does not reside specifically in the construction of the evaporator body and the general details thereof are given merely for convenience in describing thel evaporator system.

Referring again to Figure 1 the four effects of vI--A and evaporator body the evaporatorA are indicated generally by the numerals I, II, III and IV. -Each effect except effect I comprises one evaporatorbody of the type previously described. Effect I comprises two evaporator bodies illustrated as evaporator body I--B. Starting with eii'ect I the domes of evaporator bodies I-A and I--B are connectedto the steam chest of II effect through catch- alls 28 and 38, respectively.y The conduit 32 connects the dome of evaporator body I--A to catch-all 28. Catch-all 28 in turn is connected as shown by a conduit 34 to con leads to the steam chest of4 38 connects the dome of' I-B with catch-all 38 which in duit 36 which in turn `A conduit evaporator body turn is connected to the steam chest of II effect through circuit 36. r

'I'he dome of II effect is connected to the steam chest of III effect through a conduit 48, a catch'- all 42 and a conduit 44. The dome of III effect is connected to the steam chest of IV effect through a conduit 4B, a catch-all 48 and a conduit 58. The dome of IV effect is connected to barometrlccondenser 52 through a'conduit 54, a catch-all 56 and a conduit 58. Connected in this manner the steam chest of'II effect acts as a condenser for the vapor from the domes of evaporator bodies I-A and I-B. The steam the barometric condenser 52. The,.catchalls 28,

38, 42, 48 and 56 'serve to remove any water condensing from the vapor.

- With this condenser action on each effect the dome pressures preferably range from approximately 14 lbs. per square inch in evaporator bodies I.-A and I--B to 3 lbs. per square inch in II effect, to 10 inches of vacuum in III effect to 24.5 inches of vacuum words, water is evaporated from the liquor in each eifect at t e temperature corresponding to the dome pressure .in that effect. Live steam is preferably added to the steam chest of evaporator bodies. I-A and I-B at from 32 vto 40 lbs.v per Asquare inch pressure. The difference in temperature of the steam in the chest and the liquor in the ,tube causesheat to flow from the steam chest through the tube walls to the liquor lwhere it vaporizes or boils oli. water from theliquor. The water vapor from the domes of evaporator bodies I-A and I-B is carried to the chest of II effect where it gives up heat to vaporize more water at ka lower pressure and temperature. This in IV effect. In other continues in progression to the last eiiect where vapor is drawn o to the barometric condenser.

The now of the liquor through the. evaporator unit is in the opposite direction to that of the steam or vapor. 'I'he weak liquor feed is divided to the III and IV eects. The division of the feed liquor vto these two effects is controlled by the operator with IV eiect usually taking more than III effect. The feed liquor passes up through the tubes i of III and IV effects where some water is evaporated from the liquor. The water vapor, being lighter than the liquor, leaves at the top of the dome in each effect while the liquor flows from the bottom ofthe dome to pumps and is pumped to II eect.

As illustrated in the diagrammatic drawing of Figure l, the weak black liquor is pumped by a pump 60 through a line 62 which carries it to a line S4 entering the tubes l0 of 1V e'ect and controlled by a valve 60 and also to a line 08 entering the tubes i0 of III eiect and controlled by a valve l0. A

As previously explained, the weak liquor vaporizes in the tubes of III and IV effects, the vapor passing to the domes i8 of each. The partially removed by passing the condensate through trap |28 from which the vapors can escape through conduit |34 to conduit 50.

The condensateefrom the steam chest of II effect passes through conduit |40 to a trap |42 where any vapors can escape through conduit |44 to conduit 44. From trap |42 the condensate passes through conduit |46 to conduit |38.

The condensate from the steam chest of evaporator body I-B passes through a .conduit |48 and merges with the condensate from evaporator body I--A which is carried from the steam chest of evaporator I-A through conduit |50. The total condensate is then pumped by means of pumps |52 and |54 through a conduit |56 to storconcentrated liquid in'each dome is withdrawn through suitable conduits. Thus, as shown, a conduit l2 withdraws the residual liquid from the` dome of IV eect, conduit 'l2 being controlled by a valve 74 and merging with a conduit 16 from trap 56. The entire residual liquid is -pumped by means of a pump 'i8 through conduit 80 to the tubes I0 of II effect where further evaporation takes place.

'I'he partially concentrated liquid from the dome of III eiect passes through conduit 82 and merges with the liquid from trap 48 delivered through conduit 84. The total feed liquor is then pumped by means of pump 86 through conduit 80 to conduit 00 where it merges with the feed liquor from IV eiect and is delivered to the tubes t0 of II eiiect.

The residual liquid from the dome of II effect passes through conduit 90 and merges with the liquid from conduit et delivered from trap 42.

The total liquid is then pumped by'means of a pump 94 through a` conduit 06 to the tubes I0 of evaporator body- I-B. The liquid from the dome of evaporator body I-B passes through conduit 98 and merges with the liquid from trap delivered through conduit |00. The total remaining liquid is then .pumped by means of a pump |02 through a conduit |04 to the tubes I0 of evaporator body I-A. 'I'he liquid from the dome of evaporator body I-A passes through conduit |06 and merges with the liquid passing through conduit |08 delivered from trap 28. This residual liquid has now attained its desired state of concentration.

The concentrated liquid is passed through a age or it may be used for any other suitable purpose. AThe ow -of the condensate from evaporator bodies I-B and I-A is controlled by valves |58 and |60. l

In order to facilitate the description the conduits or pipes containing the liquid which isbeing concentrated by the evaporator are shown in heavy lines. The condensate conduits are shown in dotted lines and the steam or vapor conduitsare shown by double lines in the drawing.

A particular feature of the present-invention is the use of a reux system shown by the conduit R controlled by the valve V and connecting conduits 68 and 80. The use of this reux system causes a part of the liquid entering II eifect to be diverted and. injected into the feed liquor entering III eiect. This reflux system only re'- quires a line connecting the feed line entering the .bottom of II eiect with the feed line entering the liquor discharge oat control H0 and a conduit H2 to a flash tank l I4 where any residual vapors are removed and passed to the dome of II effect evaporator body through conduit IIS. The ash tank ||4 serves to remove more vapor and lower the temperature of the liquor from I-A before discharging to the storage tank. From ash tank H4 the concentrated liquid passes through conduit H8 and is pumped by means of pump |20 through conduit |22 to any suitable storage.

During the course of the operations just described condensation is taking place in each of the steam chests of each of the evaporator bodies. As illustrated in Figure 1, the condensate from the steam chest of IV eiectis Withdrawn bottom of III effect. The higher pressure carried in II e'ect causes the reux to iiow back to III effect Without the aid of a pump. This very simple expedient has provided greatly improved results. For example, in one installation by employing a reux system of the character just described it was possible to increase the overall coeflicient of heat transfer of the evaporator by about 26% and this meant an increase in capacity of about 19%.

While the invention is not limited to any theory it is believed that the enormous improvement resulting from the invention may be attributed to the following factors. The performance of an evaporator of this type is dependent upon the flow of heat from .the steam tube Walls to the liquor (as explained in connection with Figure 2). The ow of heat under these conditions is controlled by three factors,`

namely, (l) the area of heating surface; (2) the total Working temperature drop; and (3) the coefficient of heat transfer. The heating surface is a xed quantity and in a typical evaporator body may be, for example, 3600 sq. ft. The total work ing temperature drop is trolled by the condenser capacity and the maximum steam pressure. To alter eitherl of these in order to secure increased capacity is an expensive undertaking.

The third factor, namely, the coeicient of heat transfer is in turn governed by three other factors, namely, (1) the` steam lm resistance bein the chest throughv theA also xed, being conmeurthe steam andthe metal was; (2) the resistance of the metal itself; and V(3) the liquor nlm resistance between the metaLwall and the,

liquid which is being concentrated. Since the liquor nlm resistance is by far the largest of the three', the iirst two factors may be considered negligible.

' The liquor nlm resistance is in turn dependent.

upon two factors, namely, (1) .theviscosity of the liquor at the prevailing concentration and temperature; and -(2) the turbulence or velocity of the liquid at the'wall of the tube.. In other words, the greater the turbulence or velocitythe greater is the heat transfer. Thus it has been found in accordance with the present invention that the heat transfer in III effect can be almost doubled by taking a part of the liquor thatwas entering II effect and injecting it into the feed liquor entering III eifect.

This reflux liquor is a mixture of the liquor leaving III and IV effectsand is at a lowertemperature and a higher concentration than the feed liquor to which it is added. The reilux'preferably amounts to approximately 23% fed to the'unit. The increased volume of liquor passing Athrough III effect due to the reflux would increase the velocity in the tube and thus increase the heat transfer. The higher concentra-A tion and lower temperature, of the reflux, however, would make the liquor more viscous and tends to offset the heat transfer gained by the increased velocity. Hence, the difference in temperature .and concentration of the reflux and the feed liquor increase in heat transfer gained by the use of the reflux. A

The advantages of the invention will be further illustrated but it will be understood that the invention is not limited by the following examples, showing comparative test runs with and without the use of the reflux togetherwith heat balances calculated and tabulated to show the conditions lin each body of the evaporator unit when operating with and without the reflux.

of the total' probably results in a different andy more Aturbulent type of flow which gives the large' The results for test No. I were as follows:

Tssr No. I Reflux not being used Total Evspcrator eect I L I B II III IV oven Steamchestpressure..- 136 130111.15 |25 I14.5 |30 Steam temp., F 282 274 v243 2m 1m 274 Temp. drop A T. F.-.- 24 Z) 16 34 38 132' Liquortempersturc F. 258 254 227 186 142 Boiling poincrlse, di--- 1s o 1 s 5 4o. Vapor temperature. F. 245 245 220 180 137 Vaporpressure x12.51125 I2.5 14.5 l24.5 *24.5 Latent heat. B. LiL/# 948 948 967 91X) 1015 1015 Eva ration, #Ihr 6,040 19,150 19.800 209m 25,900 92,300 Disc srgeconceut., pereen T. S..." 55.5- 47.3 324 25.3 211.7 15.4-55.5 Heatingsurtace, sq. it.- 3600 3500 3600 3600 3600 18.000 Coeiilcient-u 65 272 414 165 -164 194 Liquor vel.,in tubes A ft./Sec- 0144 .022 .031 .0232 .032 .032

Vapor vel. vapor heads,

(msec .0503 1.12 1.62 3.71 10.23 10.23 Vapor vel. vapor lines,

t-.lsec 14.3 36.8 48.2 92 5 104.0 164.0

' Pounds. Inches o! vacuum.

. The evaporator unit upon which the vreflux was developed and tested was a Swenson 5-body, long tube, vertical, nlm type quadruple effect evaporator designed and guaranteed to concentrate 133,000 lbs. of kraft rnill black liquor hourly, containing 16% total solids at 160 F., to 38,600 lbs. of concentrated liquorhourly discharged from I-A eiect at total solids and 220 F. The evaporation rate under these conditions is 94,400 lbs. per hour. 'I'he steam- -required for .the above performance was not to exceed 30,500 lbs. per hour. 'I'he evaporative economy'for the above conditions is 3.09 lbs. of ,evaporation per pound of steam.

The evaporator was arranged to operate as a quadruple eiect in the manner already described and as illustrated in Figure 1.

'Iest No. I was made without the-reflux and 4with the evaporator operating close to capacity. Test No. II was made using the reilux and with vthe evaporator again operating close to capacity. Other tests were made under varying conditions of load but as theresults confirmed with heat balances shown inthe tests herein described, the other tests are not tabulated. The steam and vapor pressures were calculated from the pres- Feed i uorto and IV@ 198 F. and 15.4% T. S.

3600x132 Total evaporation=92,300 #/hr. Total steam supplies=27,500 #/hr. 92,300:27,500=3.35# evaporation per pound of steam.

The-test data for T est No. II is as follows:

' Tssr No. II v (Rejlu being used) Evaporator enea I-.i I-B n m 1v o'grfl Steam chest preSS11r0. l 40 l 32 l 13 l 3 i 10 l 32 Steam temperature. F. 287 277 246 222 192 277 Temp. drop, A T, F. 24 20 18 24 47 132 Liquor temperature F 262 257 228 198 145 Boiling point rise, l-... 14 9 c s s 443 Vapor temperature, F. 248 248 222 192 137 137 Vapor pressure.; 114 114 I3 I 10 l 24.5 l 24.5

. Latent heat, B. t. u. 947 947 967 982 1015 1015 Eva eration, #Ihr 7, 800 24, 200 26, 350 26,900 32, 700 118, 735

Disc arge consent.. per

een T. 55.7 47.9 33.4 26.0 24.1 161=56-6 Heating surface. sq. f t. 3600 3600 3600 3600 3600 18, 000 Coeiilcient =U 8 343 467 313 167 247 Liquor vel. in tubes,

ft./sec .0193 .029 .0422 .0456 .042 .0456 Vapor vel.vapor heads,

sec 0. 62 1. 34 2. 10 3.74 12.95 12.95 Vapor vcl.vapor lines 'ft./sec 17. 6 44. 2 62. 5 93.3 207.5 207. 5

l Pounds. l Inches.

Feed liquor to III and IV effects '200 F. and 16.1%

. P. M. feed to IV effect.

126 G. P. M. feed to III eifect.

Total heat transfer in all eifects=117,965,000 B. 'I'. U./hr.

Overall colicient of heat transfer=117,967,000247 B. T.

U./sq. ft./ F./hr.

' it to do more work.

sures. The liquor temperatures and concentrations were measured directly.

The reflux system Just describedlis not a recirculating system as 'it takes a mixture of the `feed liquor entering III effect. The

. The reflux apparently increases the heat trans` -fer coefcient in III effect by increasing the velocity through the tubes. The velocity is apparently increased for two reasons, namely, (l) the reflux increases the velocity by increasing the quantity of liquor passing through the tubes. This is accomplished without increasing the evaporation per unit of feed enough to unbalance the evaporators, as there is very little evaporation from the reflux liquor; (2) the reflux probably changes the type of ilow through the tubes. In

numerous tests `it has been found that there are threedistinct types of ow in evaporator' tubes, as follows: (a) The spurt type flow; (b) The spurt and spray type ow; and (c) The spray type flow.

The spurt type ow'is the same type of ow that occurs in a glass of water full of ai'r pockets. It consists of a bubble of liquor then a bubble of air. This type of ilow gives the highest coeflicient of heat transfer because it has the highest velocity for a given rate ofiiow.

The spurt and spray type flow is similar to the spurt type of flow except that the air pockets are carrying a spray of the liquid in between the bubbles of liquid. 'The coefficient for this type of ow is not as high as for the spurt type because the velocity per unit of volume is not so great. The spray type of flow is similar to the flow obtained by pointing a water hose straight up in the air. 'Ihis type of flow has the lowest velocity per unit volume and therefore the smallest coemcient of heat transfer.

`fthe flow tothe spurt type further increasing the velocity over the added velocity vdue to the added volume of the reflux and thus increasing v the coeillcient of heat transfer. As previously indicated, while the invention is not limited to any theory, it is believed that this is the explanation of the large increase in the heat trans,-

fer coefficient for III effect.' If the type of flow in the tubes did not change, the low temperature and `higher concentration of the reflux liquor would increase the viscosity of the liquor in the tubes and tend to offset the'heat transfer gained by the increased velocity due to the greater volume. Regardless of whether ornotl this is the correct explanation the use of the reflux has resulted in an increase in capacity in one installation, as already stated, of approximately' 19%. It will be recognized that this is a tremendous increase in an evaporator system of this type.

The invention herein described is particularly applicable to the concentration of liquids containing solid materials therein and especially of the weak black liquors which result from the kraft or sulfate process of preparing paper pulp. For reasons of economy it is essential that the weak black liquor be concentrated in order that it may be used again.

It will be understood, however, that the in- I vvention is applicable to the concentration of other liquids or of other mixtures of liquids and solids. 'Likewise many variations may be made in the type of apparatus or equipment employed without departing from the invention..

The ow in IV effect is apparently the spray n type since a large part of the evaporation oc' Having thus 'described the invention, what I claim as new and desire to secure by Letters Patent of the United States is:

In a multiple effect evaporator for concentrating liquids, a plurality of evaporator tube 'units comprising flve units maintained at successively higher temperatures and pressures, means for feeding a liquid to be concentrated into the fourth andflfth units maintained at lower temperatures and ,pressures than the other three units, means for successively passing partially concentrated liquor from the fourth and iifth units to the third, second and first units, and means for returning a portion of the partially concentrated liquor from the feed liquor to the thirdunit to the feed liquor to the fourth unit.

JOE E. TALTON.

cERTIFICATE oF CORRECTION. Patqnt No; '2,262,519'. November 11, 19m.

' JOE E. 'TALToN.

It .is'xnheeby certified that error appears in the printed specifi catin ofthe-above mnbered patent requiringcrrectionaLsfollows: Page-5, second'column, l-ine` in" 'the Vtrafnllvf :second column thereof 'ior "8U *read "8g-1;' and that the said Lamers Patent should be read with-"thiscorrectiontherein that the'seme'hxay'conform to'the record of the case the Patent offie." e

Signed and sealledizhis- 25rd day` 'of De'cembe-r, .`D.

Henry Van Ardale, (Saai) .Acting Commissioner of"'Patents. v

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