US2883273A - Crystallization - Google Patents

Description

April 21, 1959 driginal Filed March 4, .1954

w. c; SAEMAN CRYSTALLIZATION 2" Sheets-Sheet 1 INVENTOR. WALTER C. SAEMAN April 21, 1959 w. c. 'SAEMAN 2,883,273 r v CRYSTALLIZATION v 1 Original Filed Ma ch 4, i954 sheds-Sheet 2 FIG-"2' v IINVENTOR. WALTER c. SAEMAN BY A zsrm United States Patent 3 CRYSTALLIZATION walter tCmSaeman, Orange, .Conu las'signortto .Dlin sMathieson "Chemical Corporation, .a ,.corporation ,of Mirginia "original 'application March 4, 1954, Serial Nos 414,192,

noWPatent No.2,827-,366, dated March 18,1958. Di- .v ided and this application February 14, 1957, 'Serial "No-"643,907

.lihistinvention relates. to ancrystallization. process and lapparatus ,for producing; large. crystalline material from cturbulent ,suspensions .under controlled conditions of operation. ,More particularly .thisinven'tion relatesto crystallization process and apparatus inwhich a control exerted. overrthe'existence time and number of fine acrystalstin v,the.turbulent, suspension of the process j in isuch manner that an zunexpectedly'high yield oflarge tptoductscrystalsgis K produced. Furthermore, the control .Hisaso. formulated as torenable the economical reuse of vonattercontained]inftheuridesired1fine crystals with "a iiminimumguiib'alance of the optimum crystallizing con- ;iiditions that exist.

j Hereto'fore, standard commercial procedures of 'the tturbulent type ifor obtaining product crystals of jlarge g size involved only a means for growing "and selectively removing crystals after theyhave-reached-a certain size. "-They in effect were only procedures in which a control wastexerted over the selective -removal, --no importance being given: tothe eflect of the number of 'fine crystals "existing in "the' turbulent -suspension uponnwhich growth of material will occur. In so failing to exert acontrol g over'i'the "number of-fine crystals eiiisting in the suspenrsion, an economical and 'optimum -production of coarse -crystalline-material could not result.

""r'rly one*disclosure found which at-temptstowefiect 40 Lfsuchj a control of a turbulent types crystallization process 'TWQSUTS. lg-845,142. *But the-process and apparatus xdisclosed in "this" patent has 1 been found to' be impractical. for commercialoperation. For example,:its.useuof a z screen to separate out unwanted seed crystals. isattended with frequent clogging and "crust-mg. Also its ruse gof on aaux iliary stream for performing three l independent itunctionsi it /impractical: because it. combinesrthreestteams which require :lthree trdifierent Fflow lv'elocities; :;namely, (1 )1 the: stream :which withdraws excess seed crystals from tithe QSHSPEIISiOI! requires talihigh :diow rate I so that there his promptremoval :of ,the excess, seed crystals in as. small a size as possible, preferablytwhenithey; canwsti-llzbel con- ;sidered muclei ;:crysta1s,t(j2) sthetfiow rate .through the :dissolving .tank must besrelat-ively low to permit crystalr-lizedxmaterial in the; tankto be. dissolved with. a minimum :1amount.ofcoverheating or overdilution so thattupon return.

[the solution.,to- ..the. crystal growth circuit, there lwill est-,no unbalanceofg the optimum .crystallizing conditions .-existi.n g therein, sand (3) the ,fiow .rate through ,the celutriationcolumn for the selective. removal of ,product .crystals a must also 1 be independently controlled .so that -;-an1.eflicient regulation of .the. removal of the 3 product crystal soflcertain sizeswis effected. It. has been .found that thisgflowi rate. should be "intermediate the rates dis- ,cussed rabove.

An; object ,of. this, invention, therefore, is a :ycrystallizaotion processtandflapparatus .which efiiciently exerts a control over the existence. of .finehcrystals present in. .the turbulent suspension so as u to prevent growth upon an tunduly large anumber. ofhsuchcrystals. Another object Qof .thisainvention. is. a crystallization ,process and apparatus which economically and continuously utilizes the timaterial contained-in suchl undesired :-1ine crystals for .iifurther crystal,growthupon other fdesired finecrystals. ,A gstill'further object of this. invention is a crystallization .process and apparatus; ofqtheturbulent,susp ns omiyp :which will .operate. continuouslyand which will produce -1. I1expectedlyllarge crystals. Other .objects will become apparent; to those skilled in the .art upon reading \the Ifollowing disclosure.

qfienerally; speaking. in: accordance with: this invention, the foregoing objects as well as otherstwhichwill become :apparent from therfollowing description are. accomplished by providing an apparatus. and method fornsubstantial ly .;immediately removing undesired fine crystals ."from the turbulent suspension. circulatingin the crystallizinggbath.

'-T,his.is accomplished byt, elu;triating the undesiredlfine crystals from. the suspension. To. .QQnserve ,the material ccontained intthese eXIractedundesiredfine crystals,gthe scrystals may :be redissolved :in .a separate quiescent ,chamberreither .byth-eat, byndding additional solvent,

..or by .other solvating means, Q and the resulting solution returned to.t he .main bodyofthe liquid preferably at a tflow irate, temperature and concentration substantially equalling .the conditions existing [in the main body,

thereby avoiding .any .abrupt changesj in the overall Theqaccompanying drawings which show a certain method of carrying out the invention are not toibe conasidered restrictive and are [shown for illustrative purposes onlyjin which:

Figure 1..is one embodiment of anevapOratiomtype .crystallizer used in "this linvention having an outside .circulat rysvst m,

'Figure'Zis another embodiment of an evaporation-type crystallizer usedin this invention l aving an"inside circulatory system.

jInFigures 1 and 2,the main crystallization process occurs in crystallizer tankl. A body of saturatedor supersaturated liquid 2, containing suspended crystalline material contained withimthis crystallizer tank, is constantly being circulated as more fully described hereinafter andislibjected either toconcentration by an evaporationprocess using "diminished pressure and heat Chins trated in 'the diagrams) or else to cooling so that a depositionofcrystalline material results.

Where the crystallization is effected by an evaporation process (illustrated in the drawings) "the diminished pressure 1 is prefera-bly maintained 1 partly by a vacuum line ii and partly by the use-of asteamejectorl,which 8. The suspension is preferably kepttheated byeusing gonly a hot feed (9 intFigure 51,;and21lbintlfiigure Q) ,301' uelsel byi using heating elementsinithetvarious inlet and outlet pipes ehereinafiter l enumerated. t Other v diminished pressure-maintaining and heat-maintaining .means mobvitouslyimayralso beuse'd. tThe vapor .6,"thatistsweptout rbyi the steammejectonis *CDIldCnSGdbYrthB :usero'f cold ccondenserispray water on similar spray condensing media fintroduced into ..the sbarornetric condenser. at inlet 11 2 andtrexpelled through outlet 12. Alternatively a, surface qcond nser canrualso be; used in 1 .which u case. the condensed papers areutaken ,ofi sin a .;separate soutlet. @Such coni, dente wap r @maytbe reused a in i des e ei he a .a solvent tennis-solving unde ired se c ystal e v.f "dissolving rah/crystalline. matter.

..Where the cry all ation. is. efie ted. byta cool n Pr e ess the crystallizer tank need not be equipped with a pressure diminishing means. herefore; the crystallizer [tankfshown in either'Figurel or 2,mayrbe closedratits uppermost region. The cooling is effected preferably by the use of a cold inlet solution coupled with cooling means contained in the various inlet and outlet pipes hereinafter enumerated.

In the embodiment shown in Figure l, the suspension 2 is circulated from the upper chamber of the crystallizer tank down through a conical false bottom 13, attached at an intermediate point of the inner walls of the tank, and into the lower chamber which has a funnelshaped bottom. A crystal suspension throttling valve 14, varies the size of the funnel-shaped opening 15, for the purpose of regulating the rate of flow through side outlet 16 by its relationship to the main circulating flow. Sight glasses 17, permit observation of this flow through the bottom outlet thereby facilitating adjustment of the throttling valve. With the valve wide open the flow to the circulating pump will pass essentially all through the bottom outlet in view of the greater density of suspension over that of solution. As the valve is closed a greater portion of the circulating stream will emerge through the side outlet 16, and reach the pump via settling tank 18. In practice it is generally found that the valve should be maintained intermediate its full open position and its closed position, its exact position depending upon the size product crystal desired. In this manner, undesired fine crystals are continuously removed from the turbulent suspension as the crystallization process proceeds. They are redissolved in the settling tank and the solution recirculated to the main circulatory body at a rate insulticient to upset the optimum crystallizing conditions existing in the main circulatory system.

The downward flowing suspension is circulated through conduit 19, through pump, 20, and then upward through conduit 21, back into the crystallizer tank 1. The pump is operated by motor 22. Other similar flow inducing devices such as a motor driven propeller blade obviously may also be used.

Into the circulatory fluid system, feed solution is introduced at inlet 9, either to make up for the evaporation losses through steam ejector 4 when an evaporation process is being used or else to make up the losses due to the intermittent opening of product crystal outlet 23. The feed solution is introduced at such rate and under such temperature conditions that the suspension will be induced to remain in a condition substantially matching the crystallizing conditions existing in the tank. Product crystals are removed from the crystallizer through product outlet 23 containing valve 24 for intermittent operation either manually or automatically.

The space beneath the conical false-bottom 11, comliquor is thereby returned to the main suspension by conduit 27 to again take part in the crystallizing process thereby avoiding loss of solid matter to fine crystal formation. Several valves 28 (only one shown) contained in conduits 27 (also only one shown), are used to place these tanks on and off stream as the need arises. The undesired fine crystals obviously may also be dissolved in the mother liquor by other means such as by the addition of solvent to the settling tank while it is ofi stream. This off stream-on stream technique avoids the undue upsetting of the crystallizing condition that would occur if only one settling tank were held on stream at all times.

In the embodiment shown in Figure 2, the suspension 2, is circulated upwardly through tubular conduit 31, which is located axially within the crystallizer tank 1, and held in place by several supporting members 32 (only two shown) and by a vertically adjustable bracket 33. Depending upon the needs of the process, this tubular conduit can be raised or lowered therebyregulating opening 35 by means of screwcrank 34, located outside of the crystallizer tank at the upper end of adjustable bracket 33. When the upward flowing suspension reaches the top it flows over the edges of the tubular conduit and then downward through a conical false-bottom 36, located intermediate the crystallizer tank into the lowermost funnel-shaped chamber of the tank. After passing through controllable opening 35, it again flows upwardly through axially located conduit 31, thereby causing an inner" circulatory system. To efiect the circulation, an upward directing propeller 37, axially located in conduit 31 is driven by propeller shaft 38,, and motor assembly municates with a side outlet 16, so that the undesired fine crystals may be removed by an elutriation method,

the size being determined by the rate of upward flow of the elutriating zone 25, that exists immediately underneath the conical false-bottom 13, and the walls of the crystallizer tank 1. By the term elutriation is meant the separation of fine particles from coarse particles by passing an upward flow of fluid through said mixture at a rate that induces the smaller size particles to be in a more upward position of the flow than the coarse particles. Separating said zones will cause a separation of the fine particles from the coarse particles. The undesired fine crystals taken ofi by this elutriation are deposited in one or more fine crystal settling tanks 18 (only one of which is shown) wherein the crystals settle to the bottom and the mother liquor is then returned to the main circulatory system. More than one settling tank is desired so that while one settling tank is on stream" for the purpose of settling out undesired fine crystals the other tank or tanks containing fine crystals and mother liquor is preferably being heated to a slight extent by steam coils 26, so that small crystals will be redissolved in the mother liquor. When redissolved the tank is again placed on stream," manually or automatically, and the 39. Other means for effecting the circulatory system obviously may also be used without departing from the spirit and scope of this invention.

Undesired fine crystals are removed from this inner circulatory system by elutriating the undesired fine crystals upwardly between the conical false bottom 36, and the walls of the crystallization tank 1, and then into either of the fine crystal outlet conduits 40, depending upon which is on stream as more fully described below. With conduit 31 raised to allow the free flow of suspension through opening 35, the flow through fine crystal outlet 40, will be small and only very fine crystals will be carried into the settling tank 41. As' conduit 31 is lowered, the proportion of circulating suspension through the fine crystal settling tank 41, increases thereby carry: ing more fine crystals of increasing size out of the crystal suspension. In practice, it is generally found that the tubular conduit should be maintained intermediate its lowermost position and its uppermost position, its exact position depending upon the size of product crystal desired. In this manner, undesired fine crystals are continuously removed from the turbulent suspension'as the crystallization process proceeds.

The solution containing the unwanted fine crystals enters the fine crystal setting tank 41, wherein the crystals settle to the bottom while the mother liquor leaves the tank to return to the main circulating body through conduit 42. When the fine crystal settling tank contains sufiicient fine crystalline matter, it is placed off stream by closing valve 43 either manually or automatically. The fine crystals are then redissolved preferably by heating the contents of the tank with the use of heating coils 44, contained in the base of the tank. Other means of dissolving fine crystals, such as by adding additional solvent may be employed also. When the undesired fine crystals are redissolved, the tank is again placed on stream by manual or automatic operation sothat the saturated solution containing dissolved crystalline matter might be re-used most economically.

Produce crystalline matter is removed from the crystallizer tank by means of product outlet' 45, which is opened intermittently with the use of valve 46, depending upon the size of crystallineimatter desired. Thiswopening of valve may be automatically ,performedto effect a totally continuous process. Use of sight,glasses 47, contained in the funnel-shapedportionof the crystallizer tank enables the determination of this desired. size.

.The apparatus described above is used in accordance with the following procedure. A heated solution or suspension of material is introduced into the crystallizer tank at the start of the process and whenever the liquid level is deplenished beyond operability by evaporative losses or by product crystal. removed by .means of. feed inlets 9 and 10. .If the heat introduced with the solution is insufficient for evaporating. the solvent. from the solution, auxiliary sources .of'heat connected iwith the apparatus are used.

It is to be again noted here .that the apparatus described above maybe modified for a crystallization process by cooling. In suchevent, the. solution is preferablyintroduced in a cooled state and subsequently further cooledby cooling means located in the inlet and .outlet conduits of the apparatus or else by, acoolingjacket.

"The solution is caused to circulate through the"outside or. the inside type ,of circulatory system depending. upon the apparatus used, the velocity being regulated for the most efficient crystallizing conditions,by means of the appropriate regulating devices shown'in the drawings. ;Such velocity. regulationisinfiuenced by the following factors; namely, strong agitation aggravates the formation of excess nucleiwhile weak agita- Ltion allows .an excessive .accumulation of crystals to LSBttlCOllt on the.bottom.therebywobstructing Ithe circunlation of. suspension.

As the suspension circulates, crystals are either deposited out of solution, or are in the growing stage in which more crystalline matter deposits upon the surface of existing crystals. For the purpose of definition the following terms are used in referring to the various crystals existing in the suspension; namely crystal nuclei are those bodies which have just been deposited out of solution, seed or fine crystal are either crystal nuclei or crystal nuclei having additional matter deposited upon their surface by reason of their relatively short existence in the suspension, and product crystals are those crystals which have grown to the size desired as the end product. In stages of growth the above crystal definitions fall into the following order: First, there comes into being a crystal nucleus. It then grows until it is a fine crystal. If this crystal is allowed to remain in the suspension and grow still further it is termed a seed crystal. This seed crystal eventually becomes a product crystal, the end result of the process.

During the crystal deposition undesired fine crystals are constantly removed by an elutriating stream located below the conical false-bottom contained in the crystallizer tank. These crystals are transported to a fine crystal settling tank where they settle out while the mother liquor is returned to the main circulatory system. The tank, when sufficiently filled with deposited fine crystals is preferably placed oflf stream and then the settled crystals are redissolved, either by the use of heat, by the use of additional solvent, or other similar solvating means. The resulting solution is then returned to the main circulating stream so that there is an efficient use of all crystalline matter.

As the crystallization process proceeds, the product crystals that are produced, are removed intermittently either manually or automatically by opening the product outlet. The product crystals are filtered off and the remaining suspension may either be discarded or returned to the crystallizer tank. An explanation for the success of this invention follows hereinafter, but it should be noted that this discussion is only a discussion of what is believed to be correct. Other more plausible explanations may be later discovered and therefore this inventionshould not be, limited bythe herein offeredexplanation.

. According to angarti'cle. in"chemicalllEngineerlstHand- 'book,third edition, pagesl058 and 1059 (1950),,Mc-

Graw-Hill Book Co., New York,;:N..Y-,., lthesize .of, product crystals from .a continuous crystallization appears ,to lbe dependent uponat least thefollowing'four principles:

.(I) "The rate of .crystalgrowthunder turbulent .conditions. is directly proportional to the..;supersaturationtof the solution.

(2) The: rate of .crystal growth is proportional to lithe area of crystal surface exposed to the solution.

(3) Eachfaceof a crystal is characterized by a rate of growth coetficient which .determines .the geometric shape of the crystal. Cor'ollarics of this .are that ithe geometric shape of a crystal essentially ;.remains ;;un-

changed asthe crystal growsqandihatfthe .intei'facial' .;.angles of .a. crystalremain .constant.

W-I1L f"W=anL where:

.rW: cumulative .weight mar-constant From this relationship one can see that the cumulative weight of crystals up to a given size varies as the fourth power of the crystal size. Using this fourth power relationship, the cumulative weight of relatively small seed crystals in a turbulent suspension is found to be generally negligible as compared to the weight of the product crystals. For example, crystals smaller than half size constitute only & or A of the total weight of crystals in suspension. It can, therefore, be seen that heretofore used commercial processes were too concerned with selective separation rather than other factors. The minor proportion of undersized crystals is actually such a minor proportion of the total that they can generally be disregarded. Items of more importance however would appear to reside in the n, the seed rate of the relationship. The seed rate it must be controlled to realize the desired crystal size L in the weight of product W; for example, if n is very large for a given weight of product the size L will be comparatively small, whereas a low seed rate It will yield comparatively larger crystals for the same weight of product. Therefore, complete size control in a crystallizer cannot be effected without controlling the seed rate n and if no means are provided for doing this the average crystal size L will correspond to the natural seed rate of the system. a

In accordance with this invention it has been found that in turbulent suspension type crystallizations removing undesired fine crystals substantially as fast as they are formed eifects the seed rate, 12, in such manner that the size L is proportionally greater than heretofore obtained. The only patent that discloses such a technique is US. 1,845,742 but in view of the discussion given previously of the manner of operation used, it can be seen that commercial exploitation of this patent was impossible. The process herein disclosed is, however, readily adaptable to commercial operation in view of its novel mode of operation and its novel apparatus.

Among the crystals which may be readily produced by the herein disclosed process and apparatus the following are typical: Ammonium nitrate, ammonium sulfate, sodium sulfate, sodium chloride, potassium sulfate, and other similar-type crystalline-producing compounds. However,

the invention is obviously not to be limited by a recitation of crystalline-producing material for its breadth rests upon the method and apparatus rather than the specific crystal being produced.

It is to be noted that in the operation of this invention the control over the seed rate may be varied. Should extremely large crystals be desired the seed rate should be kept at a low rate by increasing the rate of destruction of fine crystals. The throttling valve of Figure 1 or the suspended conduit of Figure 2 in this case should be kept so that its opening is small. If small product crystals are desired the fine crystal destroying rate should be kept at a minimum. The opening of the crystal growth inducing circuit (#15 of Figure 1 and 35 of Figure 2) should be large.

While a detailed description of the invention has been provided, it is realized that those skilled in the art may make modifications in and adaptations of the process and apparatus described above without departing from the spirit and scope of. this invention. It is, therefore, to be specifically understood that such obvious modifications are considered within the scope of the herein described process and apparatus.

This application is a division of application Serial No. 414,192 filed March 4, 1954, now Patent No. 2,827,366.

The invention having thus been described, what is claimed and desired to be secured by Letters Patent is:

1. In a crystallization process occurring in a single, vertical reaction vessel wherein there is a suspension of product size and fine crystals normally mixed-together in a turbulent solution, a. method of eliminating exexcessive fine crystals from said' mixed suspension comprising the steps of establishing an internal quiescent zone within said suspension, said zone falling within the intermediate region of the vessel, directing said fine crystals out of the mixed suspension towards said quiescent zone, withdrawing said fine crystals together with solution from the zone, eliminating the withdrawn fine crystals from the solution and returning the solution, substantially free of fine crystals, to the main body of the suspension.

2. In a crystallization process occurring in a single vertical reaction vessel wherein there is a suspension of product size and fine crystals normally mixed together in a turbulent solution, a method of eliminating excessive fine crystals from said mixed suspension comprising the steps of establishing an internal quiescent zone within said suspension, said zone falling within the intermediate region of the vessel, establishing a distinct current within said suspension, directing said fine crystals out of the mixed suspension towards said quiescent zone, utilizing said current as a conveyor, withdrawing said fine crystals together with solution from the zone, eliminating the withdrawn fine crystals from the solution, returning clear solution substantially free of fine crystals to the main body of the suspension and dissolving said fines and returning the resulting solution to the main body of the suspension.

References Cited in the file of this patent UNITED STATES PATENTS 1,478,337 Isaachsen Dec. 18, 1923 2,631,926 Eckstrom Mar. 17, 1953 2,737,451 Saeman Mar. 6, 1956

Claims (1)

1. IN A CRYSTALLIZATION PROCESS OCCURING IN A SINGLE, VERTICAL REACTION VESSEL WHEREIN THERE IS A SUSPENSION OF PRODUCT SIZE AND FINE CRYSTALS NORMALLY MIXED TOGETHER IN A TURBULENT SOLUTION, A METHOD OF ELIMINATING EXEXCESSIVE FINE CRYSTALS FROM SAID MIXED SUSPENSION COMPRISING THE STEPS OF ESTABLISHING AN INTERNAL QUIESCENT ZONE WITHIN SAID SUSPENSION, SAID ZONE FALLING WITHIN THE INTERMEDIATE REGION OF THE VESSEL, DIRECTING SAID FINE CRYSTALS OUT OF THE MIXED SUSPENSION TOWARDS SAID QUIESCENT

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