US2330221A - Method and apparatus for concentrating liquids which upon concentration deposit crystals and leave a residual concentrated liquor - Google Patents

Description

sept. 2s, 1943.

M. J.. KERMER METHOD AND APPARATUS FOR CONCENTRATING LIQUIDE WHICH UBON CONCENTRATION DEPOSIT CRYSTALS AND LEAVE A RESIDUAL CONGENTRATED LIQUOR 2 Sheets-Sheet l 'Filed Oct: 4, 1957 kuwkkh L walk l ...e NS

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@5W AnoRNEYsS Sept. 28, Y M.- J. KERMER METHOD AND APPARATUS FR CONCENTRATING LIQUIDS WHICH UPON CONCENTRATION DEPOSIT CRYSTALS AND LEAVE A RESIDUAL CONGENTRATED LIQUOR Filed OCT.. 4, 1937 2 Sheets-Sheet 2" Patented Sept. 28, 1943.

METHOD AND MPARATUS FOR CONCEN- TRATING LIQUIDS WHICH UPON CONCEN- TRATION DEPOSIT CRYSTALS AND LEAVE A RESIDUAL CONCENTRATED LIQUOR Martin J. Kenner. ulralo, N. Y., assigne;- to Buffalo Foundry & Machine Co., Buialo, N. Y.,

a corporation of New York Application October 4, 193-7, Serial No. 167,230

(Cl. zs-zsm 17 claims.

This invention relates to a method and apparatusl for concentrating liquids which upon concentration deposit crystals and leave a residual concentrated liquor. More particularly, the invention relates to the separation from each other of chemically andV physicallyvdiierent components which are carried in substantial proportions in solution in a common solvent, one of which components crystallizes out in said solution, and the other component does not, when said solution reaches at least a particular concentration of said one component. Such liquids are, for example, elctrolytic caustic soda which on withdrawal from the electrolyticcells contains a large amount of unconverted sodium chloride or other salt; chemical caustic soda containing sodium sulfate and sodium carbonate; brines containing sodium chloride, calcium chloride and magnesium salts and glycerine containing sodium chloride or other salt. The invention is more specifically described with reference to electrolytlo caustic soda which because of its caustic character is dicult to handle.

In themanufacture of electrolytic caustic soda, the cells are 4charged with sodiumA chloride brine -which is partly converted through electrolysis into caustic soda, the by-products being hydrogen and chlorine. In commercial practice the average As now practiced, the salt precipitated in each effect is collected as a sludge in a salt receiver which is connected by a valve to the cone bottom of each eiect. When the receivers become filled vwith salt slurry they are disconnected from the evaporators by closing the valves between the cone bottoms of the eiects and the salt receivers. Each receiver also has a cone bottom with a valve which in turn is connected to a slurry pump which discharges the entire contents of each salt receiver into one main receiving tank in which all the salt slurry is collected from all the eects. In this tank the salt precipitates and the clear liquor is used to rell the empty salt receivers.

The clear liquor so removed is,of course, a mix.

.ture of the different concentrations from the several effects. Therefore this removal of the slurry not only disturbs the operationoi the apparatus but the'return of the clear liquor disturbs the balance of the system because the liquor in the several eiectsi is at dierent degrees of concentration whereas` the returned liquor is a mixture of the liquor from all of the effects and cell produces approximately a 9% caustic soda solution with a 50% decomposition factor resulting in about 13% sodium chloride in the cell liquor withdrawn. To separate the salt from the caustic it is lnecessary to concentrate the cell liquor to crystallize most of the salt after which the salt is removed, washedand reused, it being important that the brine fed to the cells be as free from impurities .as possible.

It is now the common practice in the industry to concentrate cell liquor to approximately' 30% caustic soda in a triple eiect evaporator and then to approximately 50% in a single elect evaporator. At times a double or triple effect evaporator is used lfor concentrating the cell liquor to 50%, depending upon the steam pressure available and quantity to be handled. It

frequently happens that there is an insuilicient supply of low. pressure -steam for the triple or' double eiect evaporatorwhich receives the cell its concentration diiierent from any one of the elects. In addition the separated salt is usually first washed with the cell liquor and since thev cell liquor is not saturated with salt part of the removed salt so washed is redissolved.

One of the objects of the invention is to proi vide an automatic, continuous and balanced method of concentrating liquor which upon concentration deposits crystals and leaves a residual several effects in the unsaturated cell liquor byV of washinglthe salt with the saturated liquor has greatly increased thel cost of the apparatus and it is therefore highly desirable to reduce the size of such apparatus or increase the capacity of existing equipment without increasing its can also -by installing a setsof salt separators.

in the evaporator system. This washing method be carried out in existing installations Another object is to provide such a continuous method of washing the salt which reduces the salt losses as compared with present processes.

Another obiectis to provide 'such a process in which the liquor is substantially saturated in a pre-evaporator and used for washing all oil the salt.

Another object isto eiect a large saving in steam by the use of thepre-evaporator, by the continuous countercurrent washing of the salt and by the use of heat interchangers as hereinafter set forth, the amount of steam required being reduced from the usual consumption of about 4l pounds for each hundred pounds of cell liquor treated to about 29 pounds in a triple eect evaporator and finishing pan.

Another object of the inventionis to provide such method and apparatus which are particularly applicable to the continuous separation from each other of two chemically and physically dif;- ferent components carried in substantial proportions in solution in a common solvent, one of which components crystallizes out in said solution, and the other component does not, when said solution reaches atleast a particular concentration of said one component.

Another object of the invention is to provide such a process and apparatus in which the residual solution is removed in a substantially crystal free condition and to permit the progress of the solutionthrough the lseveral effects with a minimum disturbance with the separationfof the crystals from the solution and with the maximum i opportunity for the crystals to separate out and be locally segregated.

4Another object of the invention is to provide such a method and apparatus in which the liquor is reduced in temperature between those stages where a lower temperature is desirable for washing the crystals and for obtaining the maximum economy and in which the cooling is'eiectcd without heat loss.

In the accompanying drawings: y

Fig. l is a diagrammatic representation of the .preferred form of apparatus for carrying out my improved process. I

Fig. 2 is a view similar to Fig. l, showing a modified arrangement oi' the multiple eect evaporators. Fig. 3 is a diagrammatic representation of a modified form of crystal separator.

As best shown in Fig. 1 the letter 4 represents a vapor separator 20, the vapors passing upwardly through a vapor outlet conduit l2l and the separated liquor returned through a return pipe 22.

A pressure responsive instrument 23 of any suitable construction is placed in the vapor outlet line 2I and is responsive t'o the pressure therein. This instrument 23 controls, in any suitable manner, a'valve 24 which -in turn controls the amount of steam admitted .to the pre-evaporator A. By the setting of the instrument 23 or the valve 24 the pre-evaporator can be set to maintain, within very close limits, any desired vapor outlet pressure.

In the following description it will be assumed that it is desired to evaporate and remove the salt from the liquor withdrawn from electrolytic cells producing caustic soda. This liquor is withyfloat valve 38.

drawn from the cells through a cell liquor supplyline 28 by a pump 21 which delivers it to a heater 28. The cell liquor passes through the tubes of this heater and through a line 29 to a heat exchanger 30. The cell liquor passes through the i tubes of this heat exchanger 30 and through a line3| to a second heat exchanger I2. The cell liquor passes through the tubes of this heat exchanger 32 and through a lin'e I3 to a heater 34.

On passing through the tubes of this heater 34 Y float valve 38 controls, in any suitable manner,

a valve 40 in the line 31, this valve thereby controlling the amount of preheated cell liquor'admitted'to the pre-evaporator in response to the The heater 34 .is supplied with steam from the branch steam line I4 under control of a valve and the condensate is with, drawn from ,this heater through a line 46 which conducts the condensed water to the heat exchanger 30. The heat exchanger 32 is heated by the preconcentrated liquor withdrawn from the pre-evaporator A through a line 41, this being under control of a valve 48. This liquor passes around the outside of the tubes of the heat exchanger 32 in the heat exchanging relation to the original solution or feed liquor and into a pipe 49 which conducts the [cooled preconcentrated liquor tothe bottom of a slurry tank 50. 'Ihe heat exchanger 35 is heated by the condensate of the steam supplied4 to the pre-evaporator A andthe heater 34. this condensate passing out through a condensate line 5I and around the outside oi the tubes of the` heat exchanger 3B to a boiler return line 52. The heater 28 is heated by vapor' from the apparatus, as hereinafter described. It will be understood that the system oi cell liquor preheaters shown isset forth purely as an example of how savings can be elected and the number, use and arrangement oi' such preheaters will depend upon the conditions en- ,c

countered in each installation such as the steam pressure available, the degree of concentration of the feed liquor, the degree of concentration desired, etc.

The vapor from the liquor evaporated in the pre-evaporator or pre-concentrator A and passing outv through the outlet pipe 2i is conducted to a line 55 leading to the unit or first effect B of a multiple unit or multiple effect evaporator, the other units or effects of which are designated ,at C and D respectively. The vapor supplied to the ,ilrst unit or eect B is under control of a valve .55. The several units or effects B, C and D are shown as being identical in construction and therefore' a description o! one will be deemed to apply to all.

Upper and lower tube- Each of the effects 'can be of any usual and 1 well known construction to provide an vaporatber 60 and are connected by a large central tube 61 and a plurality of smaller tubes 68A. The vapor from the branch line 55 is conducted to the steam chamber around the tubes 61 and 68, the condensate being withdrawn in any suitable manner. wardly through the small tubes 68 and downwardly through the large central tube 61, the vapors passing out through a central outlet provided in the dome 6 I.

The salt or crystal separator 64 consists of an Upper cylindrical portion 10 covered by a dome 1| and a lower conical part 12. Immediately un- The liquor in each efiect circulates upnected by a recirculation line 94 with the second effect C under control of a valve 95. this valve serving to regulatethe rate of discharge of this effect C.

Theliquorv from the first eiect B is withdrawnl is provided. .As with the other eects a part of der the dome 1| is arranged a bale 13 and above Y the salt crystals to the bottom of the conical portion 12 from which the crystalline salt, in the form of a sludge, is removed. An individual salt or crystal separator is provided into which each .of the effects solely discharges,y and it will be seen that each separator has an outlet for liquid in an upper zone which is spaced materially in a horizontal direction from the inlet from its related eifect and has an opening in its bottom zone for the discharge of crystals settling out therein.

The vapors generated in the first effect B are conducted through a. line 15 to the steam chamber of the second eiect C and the vapors from the second effect C are conducted through a vapor conduit 16 to the steam chest of the third effect D. The vapors generated in the third effect D are conducted, through a conduitl 11, usually to a condenser, this efect being usually maintained under a. vacuum. i

The slurry tank 50 is shown as having a conical bottom leading to an outlet 8|; The preconcentrated liquor from the line 49 overflows in the slurry tank through an outlet line 32 which discharges into an evaporator storage tank 83. The preconcentrated liquor is withdrawn from this tank by a pump 84 and conducted through a line 85 to the third eilect D undercontrol of a valve 66. In this third effect D the liquor is concentrated and in this concentration an amount of salt crystallizes out of solution and is segregated in the salt separator 64 arranged below this effect D. The liquor from the third effect D is withdrawn from the dome of this salt4 separator by a pumpl 81 and is conducted to the second effect C under control of a valve 80. 'Ihe outlet line from the pump 81 Vis also connected by a branch 89 with the third effect D under control of a valve 93. By this means any desired proportion of the liquor from the effect D can be recirculated through this effect and the amount of liquor circulating through the separator be maintained constant regardless of the demand for liquor of the effect C. It willbe understood that a certain amount of seed crystals maybe carried through the system, these seed crystals serving to facilitate the precipitation of the salt in the evaporating chamber in crystalline form. l

The liquor from the second effect C is with-y drawn from the dome of the salt separator arranged thereunder b`y a pump 9| which 'delivers it through a line 92 to the flrst effect B under control of a valve 33. The line 92 is also concomprises a large cylindrical body |00 vhaving a;

vapor dome |0| and a conical bottom |02 terminating` in a discharge spout which leads into a salt separator |03. The steam tube assembly within this finishing panconsists of alarge central tube |04 and a plurality* of smaller tubes |05 through which the liquor is circulated. The steam chamber around the tubes |04 and |05 is supplied, under control of a valve |06, with the vapors generated in the l pre-evaporator A through the pre-evaporator outletline 2| which connects with the inlet to the steam chest ofthe nnis'hing pan.

The vapor outlet line |01 from the finishin pan E or the vapor outlet line 11 from the lthird effect D can be arranged to conduct the vapor tothe heater 28 and thence to a barometric condenser or other means for withdrawing the vapors and maintaining the finishing pan E or the third eiect under vacuum.

The crystal bearing liquor entering the salt separator |03v of the nishing pan is forced to flowvat slow velocity horizontally to the perimeter of a baffle ||0 in the top of this salt separator and because of this reduced velocity most of the crystals which are suspended in this liquor drop to the conical bottom and are removed as a sludge by a pump l||2 under control of a valve H3. The practically crystal free liquor flowing around the upper side` of the baille. I0 is `liquor discharge line I I6 under control of a valve H1. To permit of adjusting the rate of flow through the separator |03 v,under the finishing pan a, branch line ||8 connects the outlet of the pump l5 with the finishing pan under control of a valve H9.

The sludge from each of the salt separators is4 washed with the salt-saturated liquor of the next evaporator, this salt being progressively washed in liquor having a smaller amount of caustic soda so that when it is subjected to the final washing the amount of sodium hydroxide in the salt is very small and can be readily removed. For this purpose the sludge is removed from the bottom of the salt separator under the rst efl'ect Bby a pump |25 through a valve |26 and is discharged through lines |21 and |23, under control of a valve |29, into the conical -lower part of the second effect C. l

The sludge from the separator under this second effect C is withdrawn by a pump |30 through a valve |3| and is discharged through lines |32 and |33 into the conical lowerpart'of the third effect D under control of a valve |34.

The sludge from the third eiect D is withdrawn by apump |35 through alvalve |36 and discharged through a line |31 into the sludge tank 50 under control of a valve |38.

At the end of the weeks operation it is necessary to drain the contents of each of the eects and also the salt separators into the sludge tank 50. For this purpose and also to permit of cutting either ot the efl'ects C or D out of operation, the sludge discharge line |21 from the ilrst effect B is connected by a. by-pass valve |49 with the discharge line |32 from the second eii'ect C and likewise the discharge line |32 from this second efl'ect C is connected by a by-pass valve -|4| with the sludge discharge line |31 oi' the third ei.'- fect D.

The slurry withdrawn from the separator |03 under the finishing pan E by the pump ||2 is preferably discharged into a. line |45 and then into a line |49'under control of a valve |41.A The with 'existing installations it freqnuy happens that the flow of materials through the several effects is concurrent to the ilow of steam.-

When such a situation is encountered the invention can be carried out as illustrated in Fig. 2.

'I'he construction shown in Fig. 2 is identical with the construction shown in Fig. 1, the vapor from the pre-evaporator A being supplied to a line |46 leads to the conical bottom of the rst eiect B and hence it will be seen that the slurry from the finishing pan E will be progressively washeddn the three effects B. C and D before being discharged into the slurry tank 50 for nal washing. The amount of salt crystalllzing in the finishing pan E is, however, very small in amount and it is practicable to conduct the slurry ilrst effect F and the vapors from this effect supplied toa second eilect G which in turn supplies vapors for the operation ofi` a third eect H. Instead of supplying the pre-evaporated liquor from the evaporator storage tank 93 to the third eiiect, as in the preferred construction, the pump 94 delivers this pre-evaporated liquor through a valved line I 91 to the iirst eiiect F. The clear liquor from this iirst effect F is withdrawn by a upon the setting of valves |99 and |19 in the 'branch outlet lines from the discharge of the from the ilnishing pan through a line |43 and I valve |49 directly to the slurry tankV 50, should this be desired. f

' The salt settling in the bottomoi the sludge tank .E0 passes out through the outlet line 8| into a rotary or other lter |55 in which the salt is rst washed with pre-evaporated liquor supplied from a line |56 connecting with the line 49 and then with brine supplied by a line' |51. The salt is discharged from the lter into a conveyer |58 and the mother liquor and the wash from the iilter passes into a separator I 59 having a stand pipe i 60 which discharges into the evaporator storage tank 83, a vacuum being maintained in the chamber |59 by a vacuum purupgli. II'he amount of the brine used in the meer is; of course, very small because of the countercurrent washing of the salt crystals in the evaporators.

If for any reason it becomes impossible for the pre-evaporator A to deliver vapor atsuiilciently high pressure for the operation of the' effects B, C and D` and the nishing pan E, means same places as the removed salt receivers'and it may be desirable to employ the modied form of the salt separator shown in Fig. 3. For reasons of economy or head room requirements,'the arrangement shown in Fig. 3 can -also be employed in original installations. Under these circum- W stances, asFl'ndicated in Fig. 3, the outlet 63 from the bottom of each oi.' the eects can be connected to av pump |65 which discharges into a line |66 extending downwardly through the small dome 14 of a salt separator 64, this salt separator being arrangedalongside the corresponding effect instead of directly under, as illustratedin Fig. l. In other respects the' installation could follow the form of the invention shown in Fig. 1.

' are provided for admitting steam from the high pump |69. Similarly, the clear liquor from the second effect G is delivered by a pump |1| past 'a valve |12 to the third eil'ect H or, by opening the valve |13, a part of this liquor is recirculated through the second efl'ect G. The clear liquor from the third efiect H is shown as being delivered by a pump`|14 under control oi a valve |16 to the inishing pan E. In other respects the installation shown is identical with that shown in Fig. 1, the slurry from the finishing pan being discharged into the cone bottom of the eect H or into the slurry tank 50. It will also be understood that with the arrangement-shown in Fig. 2. countercurrent washing of the salt slurry is effected by passing the slurry progressively through the eiiects H, G and F.

Operation f The original sommen such as com een nqur which contains as its main constituentsfcaustic,

soda, sodium chloride, and water as a common solvent of these two chemically and physically different components, and which iii-unsaturated in regard to the first two components, is pumped from the yard storage tanks by the pump 21, through the heater 28. In this heater, the incoming cell liquor is heated by the vapors generated in the nishing pan E and then passes through charged from the pre-evaporator at a tempera ture close to the boiling point of this liquor under the pressure maintained in the pre-evaporator. I'he cell liquor is then passed through the preheater 34 in which the temperature of the cell liquor is raised in proximity oi the temperature maintained in the pre-evaporator itself.

The advantage of preheating the cell liquor close to the boiling point before it/is permitted -permits the use oi' to enter the pre-evaporator is that otherwise extra steam must be provided i'or preheating and a certain amount provided in the pre-evaporatorv for this purpose. which would be less economical, in particular in construction of expensive metals. To accomplish this part of the process in the steps indicated a minimum amount of steam in preheater 34, provides for cooling of the steam o i heating surface has to beV The stream of cell liquor at approximately its boiling point is then introduced into the preevaporator A, in` which this liquor is preferably concentrated near its saturation point in regard to sodium chloride at the temperature at which it leaves preheater 32 in order that no crystals of sodium chloride may separate until the preconcentrated liquor has passed this point, although a slightly higher concentration and the separation of a small amount of crystals, either temporarily or continuously, does little harm unless it is carried to the extent that the salt crystals will cause incrustation of the heating surfaces or clogging of the pipe lines.

In order to operate the pre-evaporator under most favorable conditions with the leas't amount A of supervision and trouble, the controlling factors which govern operation, such as liquor level, steam and vapor pressure, maybe automatically controlled by suitable instruments such as oat valve 38 and pressure control 23 which automatically regulates the steam inlet valve 24 in response to any'variation in the steam outlet linel 2l and thereby maintains a uniform pressurein this lineby governing the amount of high pressure steam owlng to the pre-evaporator.

By such means the pressure of the vapors generated in the pre-evaporator can be maintained at any desired point, usually substantially above atmospheric pressure. generated and the corresponding degree of concentration of the cell liquor obtained in the pre` evaporator can be established hin accordance with local plant conditions and canibe varied as need- Y. ed Within the limits of vapor pressure and steam pressure available. The pressure under'which the vapors are maintained also can be varied The amount of vapors' sively through the three eisectsand the. temperature dropping correspondingly.

In the pre-evaporator the cell liquor is reduced to a point where it is nearly saturated with salt unless requirements of low pressure steam are less than necessary to concentrate the liquor to this point. The progressively moving solution of preconcentrated liquor after leaving the preevaporator and cooling in the heat exchanger 32,

enters the slurry tank 50 where it is used for Washing the salt which has crystallized out in the multiple effect evaporator and nishingpan and is thence conducted to the evaporator storage tank 83. From. this tank the pre-evaporated liquor is pumped by pump 84 into the third eiect D, in which effect a certain, amount of wate`r is evaporatedfrom the portion of the stream passing therethrough and in which a certain amount of salt is` precipitated which settles in the salt separator connected thereto. A certain amount of liquor is circulated through the salt separator by means of pump Bl sumcient to maintain the desired circulation in this separator and allow all or most of the salt to settle out. Part ofV this circulated liquor is conducted to the subsequent evaporating unit or second eiect C and the balance is returned to eect D. The same procedure takes place in thesecond-eect C and in the lrst effect B, from which a regulated amount ofsettled liquor is carried to the nishing pan E, if provided, or to a storage tank for residual liquor in case a finishing pan does not form part of the installation.

As evaporation takes place in the transfer of the stream of .solution passing from stage to stage progressively through the4 several evaporating chambers, the progressive concentration within certain practical limits as desired and as is 'most economical in connection with the pressure of any low pressure steam available from other sources, the final concentration of the caustic liquor, the number of eiect's of the multiple eiiect evaporator andthe provision or omission of a separate nishing pan as part of the concentration system.

In view of the increasing concentration of the residual component in the concentrated liquor in the several effects the boiling point in each effect is considerably abovethat of water under the same pressure, particularly in case of caustic soda. In order to maintain good circulation of the liquor in each effect and substantial temperature differences between vapor in the steam space and liquor in the evaporating chamber, the last effect and finishing pan generally are placed under a subatmospheric -pressure or vacuum `and the pressure of the steam in the rst eiect and in the steam space of the finishing pan, if pro vided, as vvell as that of the vapors in the preevaporator preferably -are kept considerably above atmospheric pressure. The multiple eiect utilized to heat the third unit or effect D, the

vapors from the third effect and from the iinishing pan being condensed in the same or in separate condensers; the pressure dropping. progresstages of the residual component in the liquor increases from one effect to the next and -from the iirst effect to the'nishing pan. The amount of low pressure steam used in the first eifect of the multiple eilect evaporator and the amount` of water evaporated in each eiiect are determined by the total amount of water evaporated from the preconcentrated liquor on its way through the multiple eiect evaporator, the initial and nal concentration of the liquor and the initial,

and -flnal pressure maintained, so that these amounts and the temperatures maintained in each eiect are balanced in such a way that the heat given up by the steam or`vapor in condens" ing in the steam chamber in each effect is the same as the amount of heat taken up by the liquor circulating through the tubes of the steam chamber where the heat is used for concentrating the liquor,rand other purposes connected with the evaporator process. In the nishing pan, low pressure steam is used directly for the` nal concentration ofthe liquor,l the amount of which is determined by the concentrations, pressures, and temperatures maintained. From the iinishng panl E the liquid is continuously removed through the liquor discharge line I I6 as one nal product. l.

Each eect of the multiple effect evaporator -therefore is operated atadiierent pressure and temperature which drop from the rst to the second and to the third effect,\while the progressive concentrations of the liquor in regard to-the residual component 'increases in sequence from the third to the second to the rst effect and then to the finishing pan. As this concentration increases, thesolubility of the salt decreases and crystallization of this salt takes place in each effect and in the nishing pan, the amount o! salt which separates in each eilect being determined by the saturation point of the salt in each one oi' the eiiects at the temperature maintained therein. The nishing pan may be omitted; and the liquor raised ,to the ilnal concentration in the multiple effect evaporator, which may be a triple or double etlect evaporator as governed by practical considerations.

The suspended salt crystals segregated in the rst effect B which are in contact with liquor of highest concentration of the residual component i in the multiple eilect evaporator are periodically or continuously removed from the salt separator connected thereto and are added to the solution being circulated through the evaporating chamber of the second eil'ect C and its connected separator,-this being a more dilute solution, but saturated with regard to the crystallizing component. In this e'ect the more concentrated solution adhering to the crystals is removed and replaced by the more dilute liquor in eiect C without dissolving any oi' the crystallizing component. These crystals together with those Acrystallizing out in eiIect C are locally segregated, removed and carried in succession in the same way to the third effect D in which they are washed by a still more dilute solution, but still saturated with regard to the crystallizing component.

The slurry from all three eiects ispumped into the slurry tank U. where it is washed by the preconcentrated liquor from the pre-evaporator A. It will be therefore most advantageous, as far as washing is concerned. if this preconcentrated liquor be as nearly saturated in regard to the crystallizingcomponent as possible so that the least amount of crystals will be redissolved. It is evident that this method is a great advance over the use of cell liquor for washingthe slurry, because this cell liquor is not saturated in regard to the crystallizing component and therefore will redissolve a certain amount of the crystals, which then will enter the multiple effect evaporator and increase the amount of crystalsto be separated therein. The method of progressive washing also is a great improvement over the method of simultaneous removal of the salt from all effects and washing the combined salt -with cell liquor because the concentration of the liquor, removed with the salt, in regard to caustic soda then is much greater and more cell liquor has to be used for washing purposes.

In the progressive method. the greater part of the washing has been effected by the time the salt is ready for ilnal washing on the iilter by means oi' brine.

The crystals removed from the separator |03 connected to the nishing pan E in the same way can be pumped by the pump H2 to the ilrst eil'ect B and carried through the other effects. These crystals generally are much finer and in their passage through the multiple effect are afforded an opportunity to grow in size. 0n account of its smaller quantity, however, itV may be preferred to pump these crystals directly to the slurry tank I0 without greatly affecting the economy of the process.

In the arrangement shown in Fig.2, the liquor enters the multiple effect evaporator in the first eilect and flows through the second into the third eiIect. In this case the crystals areY carried from the separator connected to the third eilect I-I intothe second effect G and then into the rst efl'ect F. 'l'he adhering liquor in this form of the invention is carried into an eiIect operating at higher temperature, which will heat this adhering liquor, and may make it slightly unsaturated in regard to the crystallizing component whereby a small amount of the crystals may be'redissolved. This arrangement therefore may be slightly less efficient than the arrangement shown in Fig. 1. Cooling of the preconcentrated liquor in this case is less desirable because this preconcentrated liquor is fed to the rst effect F and if cooled too much will'have'to be reheated to the boiling point in this eiect.

In the absence of the pre-evaporator concentration to the salt saturation point will take place in the effect to which the cell liquor is fed. The same progressive method of Washing can be followed except that cell liquor is used `for washing in the slurry tank, if desired.

An important feature of my invention consists in effecting a large economy in steam consump tion in concentrating, for example, caustic soda and removing' the salt therefrom. The following lwill serve as an example of the saving in steam consumption obtained by the installation of a i pre-evaporator and system of heat interchangers -in -connection with a triple effect evaporator and iinishing pan in the concentration of electrolytic caustic soda as described and illustrated herein. 4The composition of the liquor at each stage of the process will be approximately as follows:

For every lbs. Aof cell liquor of the above composition the quantity of material leaving each stage at the above composition and the amount of water evaporated in each stage will be:

In the In the In the pre-evaptripla effect finishing orator evaporator pan Pounds Pounds Pounds LiQlIOi' 75. 5 30. 0 18. 4 9. 0 9. 0 9. 0 13.15 1. 9 o. 4 Water in liquor... i 53. 35 19. l 9. 0 Water evaporated 24. 5 34. 25 la l To accomplish this amount of evaporation the' approximate amount oi' low pressure steam of 30 lbs. gage pressure required when operating under 26 inches vacuum will be:

In the In the triple eiiect ilnishing evaporator pan Pounds Pounds Low pressure steam 13. 7 l0. 8 Vapor to be condensed in condenser. 10. 9 10. 1

of 58.75 lbs. has to be evaporated which requires approximately 23.5 lbs. of low pressure steam. Including the 10.8 lbs. of steam for the finishing pan, total steam consumption in this case will be approximately 34.3 lbs. Saving in steam by the installation of the pre-evaporator therefore will be 34.3-25.8=8.5. lbs. for very 100 lbs. of cell liquor. In addition only 21 lbs. of vapor will have to be condensed in place of 28.9 lbs. t

An additional savinglin steam and cooling water is obtained by the system of heaters provided for preheating the cell liquor to the boiling point. First of all the cell liquor which is supplied from storage at about 70 F. is heated in av heater 28 by the vapors from the nishing pan which have a temperature of about 125 F. under 26 inches 4vacuum and will heat the cell liquor to about 122 F. whereby about 4.5 lbs. of the vapor will be condensed. Hereby a further reduction-in the amount of vapor to be condensed will be obtained from 21 lbs. vto 16.5 lbs. per 100 lbs. of cell liquor.

ing in steam consumption because heretofore the crystal slurry from the several effects has been withdrawn periodically and transferred to a common slurry tank, from which the settled clear liquor is returned to the salt receivers connected to the effects, which procedure results in a loss of temperature and heat. On the filter the crystals are washed with cell liquor and brine in considerably larger amount than in the countercurrent system. The increase in ,eiiiciency produced by countercurrent washing results in a saving in steam which is appreciable.

From the foregoing it is apparent that the use lof the pre-evaporator permits greater steam mre,

Next the cell liquor at about 122.0 F. will be passed through heat exchanger 30 where it will be heated to about 165 F. by the steam conl densate from, the pre-evaporator and the preheater 36, whichrcondensate thereby is cooled from about 338 F. to about 208 F.

From the heat exchanger 30 the cell liquor at about 165 F. is passed through the heat exchanger 32 in which it is heated to about 256 F. by the pre-evaporated liquor from the preevaporator which thereby is cooled from about 295 F. to about 175 F. At this temperature the pre-evaporated liquor is passed through the` slurry tank into the last eect of the triple effect evaporator, where the boiling point under 26 inches vacuum is about 155 F.

Finally the cell liquor is heated in the preheater 3 0@ from about 256 F. to about 295 F., \the boiling point of the liquor in the pre-evapvoi'ator.

for this purpose, about 4.0 lbs. of which are required, which is the only .steam used for heating the cell liquor.

Without the pre-evaporator the cell liquor enters the third eect of the triple effect evaporator, inwhich in this case the liquor boils at a temperature of about 145 F. under 26- inches vacuum. To heat the 100 lbs. of cell liquor from '70 to 145 F. requires about 7.3 lbs. of low pressure steam at 30 lbs. gage pressure.

The provision of the system of heat interchangers and heater in connection with the preevaporator therefore results in a further saving of '7.3-4.0=3.3 lbs. of steam per 100 lbs. of cell liquor.` Total steam saving amounts to 11.8 lbs. or 28.4%.

For a plant handling 1,000,000 lbs. of cell liquor per day and operating`350 days per year steam saving alone will amount to 118,000 lbs. per day v which at 40 cents per 10000 lbs. will have a value In existing installations consisting of a triple eiect evaporator and finishing pan the amount of water evaporated per 100 lbs. of cell liquor is 58.75 lbs. in thetriple evaporator. By installing a pre-evaporator this amount is reduced to 34.25 lbs. and the same evaporator would be able to handle almost 72% more liquor except that capacity per square foot heating surface is somewhat reduced by increasing the concentration of the liquor in regard to caustic soda in the several effects.

The introduction of the countercurrent washing' system of the crystals also results in a sav- Steam at 100 lbs. gage pressure is used economy and the use of smaller triple effect units for the same capacity. Furthermore,` the preevaporator can be added to existing installations in order to increase capacity without proportionally increasing the ste'am consumption. 'Furtherthe countercurrent washing of the salt provides an automatic continuous and gradual method of washing the salt with saturated liquor in the evaporator itself and can be applied to existing installations by installing a set of continuoussalt separators as shown. It'will further be seen that the pre-evaporator is of value in combination with the method of salt Washing in that the pre-evaporator produces a liquor substantially saturated with regard to salt for final washing of the salt while the cell liquor heretofore used necessarily has a tendency to dissolve a lsubstantial amount of the crystalline salt.

I claim as my invention:

1. The continuous process of concentrating liquors of mixed components which upon concentration deposit crystals of one component and leave a liquor containing a residue of another component, which process comprises introducing the feed liquor to a preconcentrating chamber having a heating surface, applying said feed liquor to said heating surface, maintaining said heating surface at high temperature to preconcentrate said feed liquor, withdrawing said preconcentrated liquor in a condition in which no substantial amount of the crystallizing component has yet been precipitated therefrom, passing said preconcentrated liquor successively through a system including a series of evaporating chambers, heating the liquor in one of said evaporating chambers by heat exchange with the vapors generated in said preconcentrating chamber, heating the liquor in the other of said evaporating chambers by heat exchange with the vapors generated in said first evaporating chamber, said liquor being further concentrated in said series of evaporating chambers' and its crystallizing component crystallized, separating a slurry g of the crystals from said concentrated liquor,

liquors of mixed components 'which upon con- Y centration deposit crystals of one component and leave a liquor containinga residue of another component, which process comprises introducing 2. The continuous process of concentrating` the feed liquor to a preconcentrating chamber.V applying heat at high temperature to said liquorI while in said chamber 'to preconcentrate the same, withdrawing said preconcentrated liquor in a liquid condition in which no substantial concentration of the liquor, passing the preconcentrated liquor through a system including an evaporating chamber, applying heat at lower temperature to said liquor in said evaporating chamber to continue the concentration of said liquor to said final concentration and crystallize said crystallizing component, withdrawing a\ slurry of the4 crystals from said concentrated liquor en d subsequently' continuoimlyre'moving said concentrated liquor from the system.

3. The continuous process4 of concentrating 20 liquors of mixed components which upon concentration deposit crystals of one component and leave a liquor containing a resdue-of another component, which process comprises introducing the feed liquors to a preconcentrating chamber.

applying heat to said liquor while in said chamber to preconcentrate the same, withdrawing said preconcentrated liquor in a substantially crystalfree condition, passing said preconcentrated liquor in a substantially crystal-free condition through one evaporating chamber and then through the other of a series of evaporating chambers, heating the liquor in said other evaporating chamber by heat exchange with the vapors `generated in said preconcentrating chamber, heating the -liquor in said one evaporating chamber by heat exchange with the vapors gen-P erated in said other evaporating chamber, said liquor being further concentrated in said series of evaporating chambers and its crystallizing component crystallized, separately withdrawing Aa slurry of the crystals from said other evaporating chamber, introducing said slurry of withdrawn crystals into said one evaporating chamber whereby the crystals thereof are progressively washed in counterflow relation to the liquor and in a solution which is increasingly dilute with regard to the residual component, and continuously removing theconcentrated substantially crystal-free liquor from said other of said evaporating chambers.

4. I'he continuous process of concentrating liquors of mixed components which upon concentration deposit crystals of one component and leave a liquor containing a residue of another component, which process comprises introducing the feed liquor to a preconcentrating chamber, applying heat to said liquorwhile in said chamber to preconcentrate the same, withdrawing said preconcentrated liquor in a liquid substantially 50 crystal-'free condition, passing said preconcentrated liquor in a substantially crystal-free conditionI successively through one evaporating chamber, an intermediate evaporating chamber and a final evaporating chamber, heating the liquor in said nal evaporating chamber by heat exchange with the vapors generated in said preconcentrating chamber, heating the liquor in said intermediate evaporating chamber by heat exchange 'with the vapors generated in said iinal 70 evaporating chamber, heating the liquor in said one'evaporating chamber by heat exchange with the vapors generated in said intermediate evaporating chamber, said liquor being further concentrated in said series of evaporating chambers 7s evaporating chamber whereby the crystals thereof are washed in a solution which is more dilute with regard to the residual component, withdrawing the slurry of A crystals from said one evaporating chamber and washing said slurry of last withdrawn crystals in the preconcentrated liquor from said preconcentrating chamber, and continuously removing the concentrated substantially crystal-free liquor from said iinal evaporating chamber.

5. The continuous process of concentrating liquors of mixed components which upon concentration deposit crystals of one component and leave a liquor containing a residue of another component, which process comprises introducing the feed liquor to a preconcentrating chamber, applying heat to said liquor while in said chamber to preconcentrate the same, withdrawing said preconcentrated liquor in a substantially crystalfree condition, passing said preconcentrated liquor through one evaporating chamber and then through the other of a series of evaporating chambers, heating the liquor in said other evaporating chamber by heat exchange with the vapors generated in said preconcentrating chamber, heating the liquor in said one evaporating chamber by heat exchange with the vapors `generated Vin said`other evaporating chamber. said liquor being further concentrated in said series of evaporating chambers and its crystallizing component crystallized, withdrawing a slurry of the crystals from said other evaporating chamber, introducing said slurry of withdrawn crystals into said one evaporating chamber whereby the crystals thereof are progressively washed in counterow relation to Kthe liquor and in a solution which is increasingly dilute with regard tothe residual component, separately withdrawing a slurry of the crystals from said one evaporating chamber, washing the crystals thereof so withdrawn in the preconcentrated liquor from said preconcentrating chamber and continuously removing the concentrated substantially crystal-free liquor from said other of said evaporating chambers.

6. In the approximately continuous process ot separating from .each other, two chemically and physically different components carried in substantial proportions in solution in a common solvent, one of which components crystallizes out in said solution, andthe other component does not,

-when said solution reaches at least a particular concentration of said one component, by passing all of said solution progressively and continuously through a plurality of concentration stages, that improvement which comprises locally separating and segregating from each other, in one zone au of said sermon from stage to stage, until the last stage is reached, discharging such separated substantially crystal-free mother liquor from said last stage as a concentration of said other component in solution and as one iinal product of the process, to aid the continuous movement of said solution through said stages, locally removing the segregated slurry crystallized component from each of said stages, delivering the slurry of crystallized component removed from each of said stages after that stage into which the solution is first passed, into the solution of a preceding stage in said progression of movement of said solution, and separating the crystals from the removed slurry of crystallized component from said rst stage as another final product of the process.

7. In the approximately continuous process of separating from each other, two chemically and physically differentycomponents carried in substantial proportions in solution in a common solvent, one of which 'components crystallizes out in said solution, and the other component does not,

9. In the process or separating a crystallizable substance from a solution of that substance in which a stream of the solution is passed progreswhen said solution reaches at least a particular concentration of said one component, by passing all of said solution progressively and continuously through a plurality of concentration stages, that improvement which comprises locally collecting a slurry ofthe crystals of one component in the solution While in each of said stages, removing relatively crystal free solution from each of said stages, beginning withrthe rst stage into which thesolution is introduced, and delivering it to the solution in the next stage to produce progressive movement of all of said solutionafrom stage to' stage, until the last stage is reached, discharging such separated substantially crystalfree mother liquor from said last stage as a concentration of said other component in solution and as one iinal product of the process, to aid the continuous movement of said solution through said stages, and locally removing the segregated slurryl of crystallized component from each of said stages.

8. In the improved process of separating a crys;

tallizable substance from a solution of that substance, in which the solution is passed progressively through a plurality of eiects of a multiple effect' evaporator while the solution is progressively concentrated, that improvement which comprises locally separating and segregating from each other, in one zone of each of said effects, a slurry of crystals of said one component and a substantially crystal-free mother liquor, removing saidsubstantially crystal-free mother liquor from each of said effects, beginning with the rst effect into which the solution is introduced and delivering it to the solution in the next eiect at a point remote from the said zone of local sagregation o f .s`lurry in that next leiect to produce progressive movement of all of said solution from eect to effect, until the last effect in said progressive movement is reached, removing said segregated mother liquor from the solution contained in said last effect to promote the progressive movement of said solution through said effects, separately removing the segregated slurry of crystallized substance from each of said ef"- fects following that effect into which the original solution is first introduced, and adding it to the solution of a preceding effect in said progression of movement of said original solution at a zone remote from said zone of segregation of saidslurry, and removing entirely from the progressively moving solution the segregated slurry of crystals in that one of said effects into which said original 'solution is rst introduced.

l lution before it sively through a plurality of evaporator umts of a multiple unit evaporator in which the solution is progressively concentrated, that improvement which comprises preconcentrating said solution in said stream in one stage of its transfer to said evaporator and to a temperature substantially higher than the temperature of the solution in the rst unit of said evaporator through which the solution passes, heating the solution in the` first unit of said evaporators through which said solution passes by heat exchange with the vapors generated from said solution while in a subsesively through a plurality of evaporator units of a multiple unit evaporator in which the solution is .progressively concentrated, that improvement which comprises preconcentrating said solution` in said stream in one stage of its transfer to said evaporator and to a temperature substantially higher thanthe temperature of the solution in the first unit of said evaporator throughwhich the :solution passes, heating the portion of the solution inthe iirst unit of said evaporator` through which said solution passes under subatmospheric pressure by heat exchange with the vapors generatedv from said solution while in a subsequent unit in the progression through which said solution passes, heating the solution in said subsequent unit by heat exchange with the vapors generated from said solution during said preconcentration thereof, and cooling said preconcentrated solution before it enters lsaid rst unit.

1l. In the process of separating a crystallizable substance from a solution of that substance in which a stream of the solution is passed progressively through a plurality of evaporator units of a multiple unit evaporator in which units the solution is progressively concentrated sufciently to cause formation of crystals in said units, that .improvement which comprises preconcentrating in one stage of itsl said solution in said stream transfer to said evaporator and to a temperature substantially higher than the temperature of the solution in theV first unit of said evaporator through which the solution passes, heating the solution in the rst unit of said evaporator through which said solution passies by heat exchange with the vapors generated from said solution in a subsequent unit in the progression through which said solution passes sufilciently to cause concentration of the solution and formation of crystals, heating the solution -in said subsequent units Vto similarly cause concentration of the solution and formation oi crystals-effecting a heatexchange between the portion of said stream of lsaid solution before said preconcentrathe Aformed crystals-from the mother liquor, in said units and using said cooled and preconcentrated solution to wash said crystals 'separated from said enters said. first unit, 'separating solution as a result of solution concentration in said units.

12. In the process of separating a crystallizable substance from a solution of that substance in which a stream of the solution is passed progressively through a. plurality of evaporator units of a multiple unit evaporator in -which units the solution is progressively concentrated sumciently to cause formation' of crystals in said units, that improvement which comprises preconcentrating said solution in said stream in one stage of its transfer to said evaporator and to a. temperature substantially higher than thetemperature of the solution in the rst unit of said evaporator through which the solution passes, heating the solution in the i'lrst unit of said evaporators through which said solution passes by heat exchange with the vapors generated from said solution while in a subsequent unit in the progression through which said solution passes sumciently to cause concentration of the solution and formation of crystals, heating the solution in said subsequent unit by heat exchange with the vapors generated from said solution during said preconcentration thereof to similarly cause concentration of the solution and formation of crystals, coolingthe preconcentrated solution before it enters said first unit, separating the formed crystals from the mother liquor in said units and using said cooled and preconcentrated solution to wash said crystals separated from said solution as a result of solution concentration in said units.

13. In the process of separating a'crystallizable substance from a solution of that substance in which a stream of the solution is passed progressively through a plurality of at least three evaporator units of a multiple unit evaporator in which the solution is progressively concentrated, that improvement which comprises preconcentrating said solution Ain said stream in one stage of its transfer to said evaporator, heating the solution in each unit of said evaporator through which the solution passes by heat exchange with the vapors generated from said solution in the next subsequent unit in the progression through which said solution passes, until the last of said units through which said solution paases is reached, heating said last of said units by heat exchange with the vapors generated from said solution during said preconcentration thereof, cooling said preconcentrated solution before it passes to said evaporator and subjecting the solution in the first of said units through which the solution passes to subatmospheric pressure.

14. In the process of separating a crystallzable substance from a solution of that substance in which a stream of the solution'is passed progressively through a plurality of evaporator units of a multiple unit evaporator in which the solution is progressively concentrated and crystals formed, that improvement which comprises preconcentrating said solution in said stream in one stage of its transfer to said evaporator, heating the solution in the 'first' unit of said evaporators through' which said solution passes by heat exchange with the vapors generated from said solution while in a subsequent unit in the progression through which said solution passes, heating the solution in said subsequent unit, cooling the preconcentrated solution before it passes to said rst unit, separating a slurry ofAsaid crystals from the solution in said units and washing the slurry of crystals so separated from said solution as a result of solution concentration in said umts with said cooled and preconcentrated solution.

15. In the process of separating a crystallizable substance from a solution of that substance in which a stream of the solution is passed progressively through a plurality of evaporator units of a multiple unit evaporator in which the solution is progressively concentrated and crystals formed, that improvement which comprises preconcentrating said solution in said stream in one stage of its transfer to said evaporator, heating the solution in each unit of said evaporator, cooling the -preconcentrated solution before it passes to the rst of said units. separating a slurry of said crystals from the solution in said units and washing the slurry of crystals so separated from said solution as a result of solution concentration in said units with said cooled and preconcentrated solution.

16. In the approximately continuous process of separating from each other, two chemically and physically different components carried in substantial proportions in solution in a common solvent', one of which components crystallizes out in said solution, and the other component does not, when said solution reaches at least a particular concentration of said one component, by passing all of said solution progressively and continuously through a plurality of concentrators each heated to provide a concentration stage and having a zone in which crystals are formed and each having a crystal separator with a separating element forming a lower zone of slurry separation and an upper zone of substantially crystalfree mother liquor, that improvement which comprises introducing, in each of said stages, the crystal bearing liquor from said crystal forming zone into said lower zone of slurry separation below said separating element to separate and segregate a slurry of crystals of said one component,

removing the separated substantially crystalfree mother liquor from said upper zone above said separating element in each of said stages, beginning with the first stage into which the solution is introduced, and delivering it to the crystal forming zone in the next stage to produce progressive movement of all of said solution from stage to stage, until the last stage is reached, discharging such separated substantially crystalfree mother liquor from the upper zone of substantially crystal-free mother liquor of said last stage as one final product of the process, to aid the continuous movement of said solution through said stages, and removing the segregated slurry of crystallized component from said lower zone of slurry separation of each of said stages.

17. In improved apparatus for separating from each other chemically and physically different components carried in substantial proportions in solution in a common solvent, one of which components crystallizes out in said solution, and the other component does not, when said solution reaches at least a particular concentration of said one component, the combination which comprises a plurality of evaporating chambers each including means for heating and evaporating the solution therein and each having in its bottom 4zone of crystal formationan individual crystal zone of crystal formation is conveyed to the said bottom zone of slurry separation of its related separator, a second conduit connected to the top portion of each separator at a point horizontally remote from the opening into that separator of moval of all of the slurry of crystalssettling out therein, means for introducing into the first of said chambers said solution carrying said componnts, the second conduit from the separator related to said rst chamber opening into a second 4of said chambers to cause a progressive movement of solution from chamber to chamber, the second conduit from the separator related to said second chamber opening into a third of said chambers to continue the progressive movement of solution from chamber to chamber, a conduit for receiving substantially all of the slurry from said 'discharge passage in the bottom zone of slurry separation of each separator to the next chamber in said progression of movement butin a direction opposed to said progression of movement of said solution until said rst chamber is reached, and means for receiving, as one product, all of the slurry of crystals from the bottom zone of slurry separation of the separator re- 15 lated to said rst chamber.

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