Patented Apr. 6, 1943 2,315,699"

UNITED STATES PATENT OFFlCE 2,315,699

CRYSTALLIZATION OF sormrror. Rudolph Max Goepp, In, New Castle, Del., as

signor to Atlas Powder Company, Wilmington, Del., a corporation of Delaware No Drawing. Application July 27, 1939,

Serial No. 286,893 5 Claims.

The present invention relates to a method for crystallizing sorbitol. More particularly the in vention is concerned with the recovery of sor bitol crystals from the technical process liquors obtained by the catalytic hydrogenation or by the electrolytic reduction of monosaccharides and the like. The principal object of the invention is to

control the crystallization of sorbitol from these process liquors so as to produce a pure uniform quality product which is dry and free ?owing. The method comprises ?rst initiating crystal

growth in the liquors; then controlling the progress of crystallization to produce the maxi mum quantity of crystals of the largest possible size and with the greatest possible amount of freedom from one another so as to give a readily separable mass of crystals; and lastly, separating the crystals so produced from the solvent or mother liquor. An object of the invention is to prepare 'seed

crystals upon which the sorbitol will later be deposited. A further object is to prepare a magma or

mass of seed crystals in syrup starting from dry, solid sorbitol crystals and syrup by intimate mix ing of the crystals and syrup accompanied by violent agitation to break up the crystals into a large number of smaller ones. Another object is to work up an initial batch

of magma by repeated dilutions and violent in termixings, until the necessary volume is at tained. Another object is the production of a magma

consisting of crystals and mother liquor which can be separated by ?ltration on a ?lter press or in a centrifuge and freed of most of the mother liquor (which contains most of the im purities) by washing. A particular object is to control the crystallization as to crystal size and as to the amount of solid phase in the crystal lizing magma. The invention embraces the method of growing crystals from either aqueous or alcoholic solutions. Another object is to provide a large mass of

seed crystals and to keep the syrup and seed in constant slow motion so that crystal growth will consist largely in increasing the size and thick ness of the seed crystals and so that only a minor part of the crystallization will arise from spon~ taneous nucleation. In this connection the method contemplates, in one aspect, a semi continuous procedure in which a large portion of a crystallized mass of syrup is left to seed the next portion of uncrystallized syrup, only a .

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(Cl. 260-637) part of the crystallized mass being withdrawn at a time for separation. One of the objects of the invention is to grow

the crystals isothermally at a moderate eco nomical temperature. The method may be car ried out as a batch procedure or it may be oper ated as a semi-continuous or continuous crystal lization. Another object is to decrease false grain, re

fuse solid phase and increase the dissolved phase by recycling, that is, withdrawing portions of the crystallizing mass, heating to melt and dissolve solids and returning to the mass from which it was withdrawn. A further object consists in separating the

crystals from the mother liquor by ?ltration which removes both the mother liquor and the dissolved impurities. As part of the separation of the crystals from

the liquor, the cake obtained by ?ltering is dried to remove remaining traces of solvent.

Finally, the dried crystals can be ground or otherwise prepared for use. A further object of the invention is the con

version of puri?ed sorbitol dissolved in volatile solvents to a dry free-running powder without recourse to spray drying.

Still another object is the stripping of sorbitol from the process liquors from monosaccharide reductions to produce liquors containing mix tures of sorbitol and related polyhydroxylic compounds, which mixtures are suitable as an ingredient in the preparation of mixtures of polyhydroxylic compounds distinguished by their high solubility in water, resistance to crystalliza tion therefrom and suitability for conditioning purposes and'other uses. The above and other objects will be better un

derstood by reference to the following detailed description of the preferred embodiments of the invention. '

In the following description several terms are repeatedly employed and it is here deemed ad visable to set out de?nitely the meaning of these terms as employd in this speci?cation and the claims appended hereto. Magma refers to the mixture of crystals and

liquor. Concentration refers to the fraction by weight

of total solids in the magma, whether in crystal form or in solution. Concentration may be de termined by known direct methods such as dry ing on sand or ?lter paper or, more conveniently, a secondary standard may be employed, e. g. re fractive index which may be converted quickly to

2 concentration by reading the corresponding value on a standardized refractive index-concentration curve. For the satisfactory working of the proc ess it is necessary to know only approximately the purity and absolute concentration of the syrup. Analytical errors in determining these factors can readily be compensated by making slight changes in the liquid content of the re placement. The liquid content is determined by a reproducible, if not absolutely accurate, test such as refractive index. For methanolic mag mas the concentration can be controlled syntheti- cally, that is, in the mixing ofithe magma, o1? analytically by a combined refractive index-den sity-composition chart.

Total solids includes all constituents of the magma or solution other than the solvent used in the process. This is essentially the non-vola tile portion.

False grain refers to crystals of sorbitol dis tinctly smaller than those of maximum size ob tainable under normal conditions, and which small crystals result from the breaking of larger crystals or from spontaneously produced nuclei. Overload is a measure of supersaturation of a

solution and is de?ned as the theoretical yield of solids, based on original total dissolved solids, ob tained in passing from a higher concentration S1 to a lower concentration S2. It is defined mathematically as

where S1 is the initial concentration of the solu tion being measured and S2 is the concentration of a saturated solution at the temperature under consideration.

Purgeability is the ease and completeness of removal of mother liquor from a ?lter cake, and by extension, the potential ?lterability of crys tals in a magma.

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Purity is the percentage of sorbitol in the total > solids. ~ Percentage separated solids as used herein, is

the percentage of crystals separated, based on total solids. This is mathematically equal to

Cm- Cl 100 X ~-Cm(1 _ of)

where Cm=concentration of total solids of the magma

and Cr =concentration of total solids of the ?ltrate

after removing the crystals from the magma.

When crystallization of a given solution has been carried to true equilibrium the percentage separated solids has the same value as the over~ load.

Pyridine number (P. N.) as used herein is an index of sorbitol content of sorbitol-containing material. This index is determined by crystalliz ing sorbitol from sorbitol-containing products in the form of a sorbitol-pyridine complex, ?ltering the crystalline complex, adding water to it to de compose the complex into pyridine and sorbitol, driving off the pyridine by vacuum distillation with water, dehydrating the sorbitol residue and weighing it as sorbitol. The procedure is speci?c for sorbitol since no other polyhydric material, such as sugar, mannitol, etc., exhibits the same behavior with pyridine. The pyridine number is the weight of sorbitol crystallized from anhy drous pyridine as above multiplied by 100, and divided by the weight of the sample (ash, mois

2,315,699 ture and sugar free). The pyridine number for pure sorbitol is about 95. The preparation of the sorbitol pyridine complex and its treatment to free sorbitol therefrom is described by Strain in J. Am. Chem. Soc. vol. 56, page 1757 (1934). The pyridine number of a sorbitol-containing product is an index of its crystallizing tendency from relatively highly concentrated aqueous so~ lutions. The higher the pyridine number the greater the crystallizing tendency. The greater the complexity of the sorbitol-containing prod not the less its crystallizing tendency and vice versa.

Replacement is the fresh liquor added to re place the vvithdrawal, in the process of continu ous or semi-continuous crystallization. Withdrawal is the removal of a portion of the

crystallizing batch for further processing, and also the portion withdrawn.

Sorbitol has been one of the most dif?cultly crystallizable of the hexahydrc alcohols. Com mercial sorbitol produced by catalytic hydro genation or electrolytic reduction of monosac charides has generally been marketed in the form of 50% aqueous solutions or lumps of solidi?ed semi-translucent gel containing ap proximately 3 to 5% of water, or as a spray dried powder containing all impurities originally present. So far as is known, no one has yet disclosed a practicable process for the large scale crystallization of impure sorbitol before the present invention.

It is known that, under certain hitherto im perfectly understood conditions, sorbitol crystal lizes from certain types of concentrated aqueous solution, particularly extracted sorb apple juice, as long needles, which may be freed of mother liquor by trituration with alcohol, and then re crystallized, in small yield, from ethyl alcohol. Even a few percent of impurities are su?cient to change the crystallizing behavior of sorbitol profoundly, so that, instead of separating from ethyl alcohol as individual crystals, it comes out as small droplets of sorbitol-rich solution. The droplets then crystallize very imperfectly to give clusters of microcrystals enclosing considerable mother liquor. If methyl alcohol is used as the solvent, pronounced supersaturation occurs, and separation, when ?nally induced by the custom ary methods of seeding and scratching, is in the form of gelatinous masses. Similarly, if water is used as the solvent, a supersaturated solution of pure or impure sorbitol, at a concentration of 85% or more, at 25 C., either sets up to a stiff, translucent gel, or, if the concentration is not too great, a few very ?ne crystals are produced spontaneously, which grow for a time, and then, sooner or later are engulfed in a solid, virtually un?lterable mass of very small crystals, sur rounded by gelatinized syrup. The unsatisfactory crystallizing behavior of

impure sorbitol is re?ected in the various meth ods hitherto proposed for purifying it, such as preparation of the benzal derivative and regen eration therefrom, regeneration from the read ily crystallized hexa-acetate by saponi?cation, or from the well-crystallized pyridine-sorbitol complex. All these methods are slow, require expensive reagents, give low yields and are un suited for the large scale economical puri?ca tion and preparation of crystalline sorbitol. The crystallization behavior of sorbitol 'in

impure solution is due, as I have found, to the natural tendency of sorbitol to crystallize in rela tively long needles. This characteristic crystal

2,315,699 habit is, in turn, due to the fact that the ends of the crystals grow much faster than the sides. As a result, supersaturated aqueous solutions of sorbitol somewhat resemble solutions of starch or gelatin, in their consistency when gelled. The gelatinization is in all probability due to the spontaneous formation of numerous nuclei, which then proceed to grow almost entirely along one dimension, forming, in effect, tiny ?bers. These ?brous crystals grow very rapidly through the solution, twisting and intertwining, thus forming a network of interlaced ?bers which soon immobilize the interstitial liquor. In im pure solutions, growth slows down due to ad sorption of impurities on the crystal faces, so that crystallization soon comes to a halt, due to coating of all faces of the crystals with im purities, with only a few percent of crystals separated. As a matter of experimental fact, the separation of less than 3% of ?brous crystals can immobilize an impure aqueous sorbitol syrup of 75% concentration at 25 C.

If the spontaneously crystallizing magma is stirred, the effect is to accelerate very much the rate of gelatinization without changing its character. This is due to the breakage of the very slender and fragile sorbitol crystal by the internal friction of the solution, thus increasing the number of active end surfaces, and hence the number of crystals produced. The present invention in its general form,

comprises the crystallization of sorbitol, in mo tion, from solutions in volatile solvents, with careful control of overload, and in the presence of sufficient seed crystals, the amount of seed crystals present being su?icient to prevent gelatinization, but not enough to increase the viscosity of the solution unduly, or to yield ex cessive false grain. The invention produces crystals which are well formed, relatively strong straight needles, not interlaced, and readily purgeable of the mother liquor. The magma thus obtained may be freed of most of the ,mother liquor by ?ltration, and the product con verted to dry, substantially moisture-free solid crystalline sorbitol by drying in thin layers or in a ?nely divided condition.

I. CRYSTALLIZATION FROM AQUEOUS SOLUTION

It has been found that sorbitol crystals can be satisfactorily grown from water solutions. As a starting material any suitable source of sorbitol may be used, such as the syrup ob tained by the electrolysis or hydrogenation of glucose, fructose. invert sugar, inverted lactose, or by the extraction of toyon or mountain ash berries with alcohol. Preferably the starting material should have a P. N. of at least 65-70 with an ash content preferably not exceeding 2%, although it is possible to conduct the proc ess with syrups of diiTerent purities. Thus a syrup of P. N. 50 can be used but the rate of crystal growth is slow and the product still rela tively impure. To get proper growth from a syrup of low

purity. the overload must be increased and this increases the viscosity. A syrup of medium purity, such as P. N. 77, crystallizes much faster at a given overload than a material of P. N. 65. The growth rate may be adjusted by changing the overload as described below. Since the proc ess accomplishes only a relative puri?cation, the ash and reducing sugar of the ?nal product are

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best controlled by keeping the ash and sugar of 75

3 the starting syrup below the necessary limits. The reducing sugar in the crystals after the crys tallization is generally about 0.3 of the reducing sugar concentration of the starting syrup (dry basis) and the ash content of the crystals is about 0.4 that of the starting syrup (dry basis). Also, if the starting material is low in ash and reduc ing sugar then, providing the original P. N. is high enough (preferably above '79), two crops of sorbitol can be taken out both of satisfactory ash and sugar content. The preferred syrup for isothermal crystalliza

tion at 25 0. should have a concentration of about 75-82% of total solids depending on purity. Such a syrup can be added to a heel of 6090% of the original or a previous days batch, which heel should contain for seed a high percentage, say 10 to 13 % separated sorbitol crystals based on the total sorbitol in the heel. In the course of about 24 hours with constant temperature conditions and stirring, the separated sorbitol crystal con~ tent should be about 10 to 13% and a withdrawal of 10-40% may be made if the semi-continuous process is followed. While such a syrup can be made to grow 10 to 13% of crystals at 25 C. the viscosity of the solution. is so high that the crys~ tals cannot be purged satisfactorily and it is nec essary to dilute the magma before ?ltering it, as described hereinafter. This dilution is preferably made with methanol, although ethanol may also be used. The alcohol reduces the viscosity and hence increases the ?lterability of the sorbitol. Without the use of an alcohol diluent the ?lter cake from a straight aqueous magma contains a very high percentage of mother liquor. e. syrup of P. N. 65-70 purged with an alcohol, a yield averaging 25 of a product which. averages close to P. N. 85 is obtained. The syrup remain~ ing after the separation of crystals from the magma has a P. N. of about 60. In preparing a new crystallizinor run an initial

body of seed crystals is required and this may be prepared as follows: A small quantity of pure dry sorbitol, prepared

for instance by the method of Strain, J. A. C. S. 56, 3.757. is first taken and ground in a mortar. or stirred with an egg beater or chopping type of stirrer, with about ten times its weight of slightly supersaturated sorbitol solution which may be made from the pure crystals or may be a less pure technical syrup. Working at 25" 0., at which temperature pure sorbitol is saturated at concentration, a feed of 75% concentration is used corresponding to an overload of about 12?; as determined by the formula forth. above. Material of S?-SGit purity such may be ob tained by the hydrogenaticn of glucose or invert sugar, would require a 76-78% solids con_ centration, while a still less pure material would need a total solids concentration up to to 82 7?. The strength of feed to be used be calculated from the overload formula after the saturation concentration of the feed with respect to pure sorbitol has been determined at the oper ' g temperature. The overload at any ccncen~ tration will vary with the purity of the syrup so that the saturation or equilibrium of the particular syrup to be used as is d mus?r

be determined with respect to pure sorbitcl. This saturation concentration may be determined by ?nding the concentration of the syrup in which pure sorbitol crystals will neither dissolve grow larger.

After grinding'or chopping minutes, vigorously for the seed takes hold, and the magma be:

4 comes appreciably thicker than the original syrup. It is then diluted with one to three times its total weight of fresh syrup and stirred vigorously, pref erably with a chopping stirrer, for 15-20 minutes further, when the magma is again diluted with a further one to three times its weight of addi tional syrup, and the stirring continued. In this way, starting with a few grains of pure sorbitol, a batch of any desired size can be built up by successive additions of supersaturated syrup to the magma of chopped-up sorbitol crystals in the proportion of one to three times the weight of syrup to the weight of magma. With each in crease in size of the batch, unless the eiliciency of stirring is increased proportionally, the time required to thicken sufficiently before the next addition becomes successively longer and longer. During this procedure tiny fragile crystals

have been produced which tend to grow into the undesirable interlacing ?brous type. The vigor ous chopping has broken up the free crystals and clots or" intertwined crystals into numerous short fragments to give a large amount of fast-grow ing, end-face crystal area. Agitation disperses these fragments throughout the solution thus . adequately seeding it and preventing spontaneous nucleation. This solution tends to become im mobile, however. Due to the elongated shape and very small size of the crystals, a 76% sorbi tol solution can tolerate only about 3% or" such ;. solid material without becoming stiff and buttery. The ?ne needle crystals at this point have a length of about 0.03 mm. and a transverse di mension of about 0.0001 mm. The solution is therefore thinned out from time to time when ~~ necessary to prevent immobilization. The crys tals are by this means allowed to grow sepa rately without appreciable entanglement until appreciable lateral growth is attained. Instead of dilution, the volume of the batch

can be kept constant and a fraction of the batch withdrawn, heated sufficiently to dissolve the crystals and, after cooling to the temperature of the batch, returned to the main portion. This in effect reduces the number of crystals and accomplishes the same eiiect as dilution, namely, the production of a new mixture containing chopped crystals from the preceding solution and overloaded sorbitol solution, the proportion of crystals in said new mixture being less than the I proportion of crystals in the preceding chopped mixture. A point is ultimately reached where the

magma can be allowed to crystallize with only gentle agitation, just enough to bring the crystals into contact with fresh liquor. The reason for this is that the crystals have now sui?cient thickness and breadth to grow as separate, free ?owing needles instead of ?bers which bend and interlace. This point has been reached when the magma will tolerate about 8 to 10% separated solids and the preparation of the seed crystals is now completed.

This magma has present therein a sorbitol solution having an overload sumcient to permit crystallization and, in addition, crystals of sur? cient size which are present in su?icient quan tity so that additional crystallization may be limited to the formation of well-formed, indi vidual crystals without further formation of ?brous crystals. In accordance with an em bodiment of the present invention, this magma is slowly moved in contact with the crystals, care being taken to maintain the relationship of the overload of the sorbitol solution to the quantity

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2,315,699 of well-formed crystals, thereby to limit further separation of solids substantially to the forma tion of additional well-formed crystals and to the building up of the size of the crystals. In this way, the crystals can be brought up

to an average maximum length of about 0.15 mm, and a thickness of .002 mm. Any further at tempt to increase the size of the crystal results in breakage, with formation of shorter frag ments which grow out again rapidly to the limit ing size.

In operating a batch method the magma pro duced by the above steps is separated all at once according to the procedure described herein after. The separation should be made when the percentage separated solids has preferably reached a value of 10-13% for an aqueous magma at 25 C. With further increase of separated solids, the

crystals tend to break up into smaller crystals which impair purgeability. The viscosity of the magma also becomes higher and at say 30% sep arated solids the magma is very difficult to stir and purgeability of the crystals is so poor that operation at such conditions is impracticable. At temperatures above 25 C. the viscosities

of the magmas will be higher than at 25 be cause, although increasing the temperature lowers the viscosity of a solution of a given con centration, the increase in solubility at the higher temperatures demands a higher concentration (which increases the viscosity) to produce an equivalent overload. Similarly at temperatures below 25 C. viscosities will be lower. There fore, when operating at temperatures other than 25 C. the preferred values for percent separated solids will be somewhat greater or less depend ing on the temperature employed.

Obviously it is preferable not to have to repeat this procedure in building up the seed crystals very frequently, and hence a semi-continuous or continuous process is preferable. These latter processes involve the separation of only a portion of the magma at a time so that a substantial part of the magma remains to furnish seed crys tals for the crystallization of further syrup. The growing of the crystals can take place in

any suitable vessel at constant temperature and with constant slow stirring. In order to increase the size of the seed crystals, these crystals must be contacted with large quantities of the mother liquor and therefore constant stirring is needed. At the same time the stirring produces breakage of some of the crystals thereby furnishing fresh growing surfaces. The rate of stirring is im portant in that the breakage of large crystals must not be excessive-a. large number of chips and small crystals leading to excessive false grain, raised viscosities, and di?cult purgeability. In place of creating motion of the magma by in ternal stirring, other means may be employed such as rocking or tumbling the whole vessel in which crystallization is taking place. It has been found that such other ways of moving the liquid produce breakage of crystals also, so that pre~ sumably the internal friction of the magma in motion is a large factor in crystal breakage.

In general, the overload of a solution increases as the temperature is lowered and this fact is frequently employed in crystallizing where suiii cient supersaturation is not available at given temperature. However, it is diiiicult to control supersaturation induced by cooling solution and since sorbitol solutions in either water or alcohols may be readily obtained in suihciently

2,815,699 supersaturated conditions even at room tempera ture to give satisfactory overloads, it is prefer able and much simpler to conduct the crystalliza tion at one temperature. In general it has been found convenient to conduct the crystallization at a temperature of 1540 C. and preferably 25 is selected as a mean of indoor temperature which can be maintained with a- minimum of heating or cooling of the solution at any time. With relatively impure syrups the viscosity overload relationship favors operations at lower temperatures. According to the preferred method, a portion

of the magma prepared as previously described, is withdrawn for sparation and this portion is replaced with an equal quantity of fresh super saturated syrup. In practice the process is car ried on semi-continuously by the periodic re moval of a portion for separation and its re placement by an equal quantity of fresh syrup. The withdrawal of a portion of the magma at a time when the growth of well-formed, readily purgeable crystals has reached a maximum, and the replacement with sorbitol syrup results in a decrease in the proportion of crystals in the magma and an increase in the percentage of the sorbitol in solution. The minor amount of breakage caused by the slow agitation is suffi cient to provide fresh crystal surface for the promotion of crystal growth. This crystal growth is thereby controlled so that it tends to take place on the small broken crystals thereby preventing the accumulation of a sui?ciently large amount of small crystals which would render the magma di?icult to purge, and further preventing spon taneous crystallization in amounts sufficient to immobilize the magma.

Since crystallization is preferably isothermal the variable elements are only concentration of replacement syrup, agitation, and amount and form of seed used. The amount of seed can be controlled by the frequency and depth (quantity) of withdrawal, the amount of seed being what is left in the crystallizer after a withdrawal is made. Amount of seed and also the form thereof can be varied by recycling as before explained. The replacement syrup has preferably 75-82%

total solids depending partly upon purity and partly on the operating conditions in the crystal lizer. If a test of the total solids of the magma in the crystallizer shows that it is not holding its own against periodic withdrawal and replace ment, the concentration of the replacement is increased until the magma reaches equilibrium. Conversely, too high an actual overload leading to undue false grain and lessened purgeability is corrected by lowering the concentration of the replacement. Another factor to be considered in the choice of concentration is the viscosity of the magma, since too high an overload makes the magma di?icultly stirrable and. also leads to false grain formation. The frequency and depth of withdrawal are

varied in accordance with the purity of the start ing material since rate of crystal growth is de pendent on purity, all other conditions being equal. With a syrup of P. N. '70 crystallized at 78% concentration at 25 C. a satisfactory growth is maintained when 10% is removed every 12 hours. On processing with methanol as de scribed below, a 22% yield is had. Less pure syrups crystallize more sluggishly so that longer growth periods are necessary. Purer materials make it possible to withdraw larger amounts for separation, If withdrawals are made more fre

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quently, a high overload can be used and a greater capacity achieved Without incurring the risk of undue thickening and false grain forma tion. The semi-continuous withdrawal and replace

ment process is much better adapted to com mercial operation than the batch process. The equipment required for semi-continuous crystal lization is a slowly stirred crystallizer which can be maintained at a constant temperature, a mix ing tank, a centrifuge or ?lter press, and a dryer. In this form of the invention there is no need for continued use of small scale equipment as in the case of the batch process where the crys tals must be built up from small quantities of material. In the semi-continuous process the small depth of withdrawal and replacement make for uniformity and homogeneity of the product despite fairly wide fluctuations in the purity of the feed. The control of the process is very sim ple, requiring only slight adjustment from time to time in the concentration of the feed syrup to raise or lower the overload to compensate for changes in purity. These advantages more than outweigh the disadvantages of getting a daily yield of only a small fraction of the solids in process. The semi-continuous, and also the continuous,

processes of this invention provide means for crystallizing sorbitol at the maximum obtainable rate of growth compatible with production of purgeable crystals. The attenuated growing habit of the sorbitol crystals is such that in a moving viscous solution breakage occurs at an average length of 0.15 mm. The distribution of crystal sizes, therefore, varies, the majority of crystals being between 0.05 and .15 mm. in length and about 0.00'10.002 mm. in thickness and width. There is a slight amount of false grain and there are also a very few crystals substan tially thicker and a little longer than the others. At this particle size and distribution and under the preferred operating conditions, the crystals can increase in total amount from an initial 9% (obtained from the seed) to 13% within 24 hours. If the number of 0.15 mm. sorbitol crystals is in creased much above 13% there is a tendency to increase the rate of breakage and false grain for mation with attendant impairment of purge iability. Hence, with aqueous magmas at 25 C. it is preferred to operate with not more than about 13% separated solids present in the magma.

Instead of growing the crystals in a semi-con ' tinuous manner and periodically removing por tions of the magma, the method can be operated continuously with high concentration syrup, say 80% total solids, feeding and withdrawing at con stant rates. Although the entire contents of the crystal

lizer in a batch process, or the withdrawal in the semi-continuous or continuous process, may be ?ltered directly in a press if so desired, the re moval of the viscous aqueous mother liquor from the crystals is not complete and the puri?cation achieved is not as good as may be desired for some purposes. Accordingly, it has been found advan tageous in most cases to thin the viscous aqueous mother liquor with a quantity of alcohol, either ethyl or methyl. The quantity and temperature of alcohol used is such as to reduce the viscosity of the magma substantially but not enough to diminish the overload or to cause the formation of two liquid phases. The ethyl alcohol maybe used in the form of a mixture with methyl alco

hol such as the denatured alcohol formula No. l. Ethanol is much less satisfactory than methanol due to formation of two liquid phases at certain concentrations, that is to say, ethanol and aque ous sorbitol solutions are not miscible in all con centrations, particularly those obtained momen tarily when stirring the ethanol into the magma. There appear two solutions of different densi ties, the heavier rich in sorbitol containing a small amount of alcohol and water, the other consisting mainly of alcohol containing sorbitol and water. Droplets of this heavier solution in contact with the sorbitol crystals agglomerate them into a sticky mass which is very difficult to ?lter. For this reason, ethanol gives a product of lower purity which is much harder to dry than the methanol purged crystals. However, it is to be understood that ethanol can be used to dilute an aqueous magma prior to purging if so desired. The dilution is preferably made with the alco

hol, and water ifenecessary, to give a mixture whose composition is 35% total solids in 52% alcohol and 13% water. It is essential to mix the alcohol and the magma thoroughly so that the dilution is complete and the resulting mother liquor is a homogeneous alcohol solution. Vigor ous stirring for several minutes is therefore used to insure this result. The diluted magma is thereupon ?ltered in a

centrifuge or other suitable ?ltering device. In the interest of uniformity it is desirable to main tain constant ?ltering termperatures. The ?lter cake thus obtained may be dried readily either in a vacuum dryer or in air of humidity less than 50%. Vifhere atmospheric humidities above 50% relative humidity are encountered, an enclosed solvent centrifuge or a ?lter press are preferably used. Where heat is employed in the drying, care must be taken that the temperature is not high enough to melt the crystals in the ?lter cake. The dried product is a soft, readily comminuted mass of very ?ne crystals having an impurities content of less than 35% of that of the starting syrup. The dilution and ?ltration are preferably con

ducted at the same temperature as the crystal lization,

Dilution with alcohol is desirable to get good puri?cation and rapid drying of the ?lter cake. if after diluting the magma with alcohol any considerable period of time is allowed to elapse before ?ltering, considerable further crystalliza tion takes place in the alcoholic magma. Hence, the alcoholic magma can be used also as a growth medium for a crystallizing process. As will be seen below, even better crystallization results are obtained where the crystals are grown in an alco holic, preferably methanolic, magma. Where it is desired to obtain a product of

greater purity than the starting solution it is necessary to remove the mother liquor as a liq uid to carry off the dissolved impurities. How ever, when it is desired only to convert sorbitol syrup to crystalline form without puri?cation, then centrifuging and the alcohol dilution re quired for the latter can both be dispensed with and the liquor from a water crystallization dried down to solid crystalline sorbitol. It has been found that un?ltered aqueous magmas of high purity will dry if they contain originally even as low as 10% separated crystals, 20% volatiles as water, and 70% uncrystallized or dissolved solids, providing that the magmas are given a large sur face area-tofacilitate evaporation and permit dif

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2,315,699 fusion of water vapor from the interior of the crystallizing mass. The method of getting free running crystals

from magmas of relatively high purity has been denominated magmatic drying and is similar to the drying of damp ?lter cakes, the principal difference being in the relative proportions of crystallized and uncrystallized solids at the start of the drying process because no ?ltration step is interposed where the magma is dried directly. For example, a soggy ?lter cake, which may con tain 40% crystalline or separated solids, 48% dissolved solids and 12% volatiles can be dried readily under vacuum or in dry air if the overall purity is 80% or better. Likewise, an un?ltered magma of like purity, but having a greater per centage of dissolved solids can be dried if spread out into a thin layer or otherwise given a large surface area and subjected to drying tempera tures or placed in dry air. The temperature used at the start in the mag

matic drying should not exceed that used for growing crystals in the magma, otherwise seed crystals will be dissolved and gelled portions produced in the cake, which lose water only very slowly and are very difficult to comminute. As drying proceeds, the temperature may be raised. Accordingly, the temperature used for growing magmas for magmatic drying should be as high as possible so that the Volatile solvent can be re moved as rapidly as possible without dissolving any crystals in the mother liquor. The apparatus and procedure used for grow

ing the crystals in a magma to be separated by magmatic drying are similar to those used in the preparation of the magma for separation by ?l tration. In preparing magma for magmatic dry ing, however, the process should be started with a cold, aqueous magma at 25 C. and the temper ature gradually raised to LO-45 C., the temper ature and concentration being controlled throughout so that the magma is at all times supersaturated with respect to sorbitol. The preparation of a magma for magmatic drying can be advantageously conducted in a vacuum pan which can be operated as a continuous crys tallizer, using periodic withdrawal and replace ment. When using a vacuum pan, agitation can be provided by bubbles, so that mechanical stirring is not necessary, but where this is done, the feed should not be supersaturated, but should be saturated or even less, i. e., it should contain excess water, whose removal under vacuum sup plies the bubbles necessary for agitation. This magmatic process is particularly adapted

to the preparation of crystals from aqueous mag mas due to the high concentration of sorbitol in saturated aqueous solutions. At 35 C. an 87.5% solution of sorbitol is somewhat super saturated and has only 12.5% of volatile sub stances to be removed by drying or evaporation.

he rate at which drying should be done de pends primarily on the crystallizing power and hence the purity of the syrup. If the evapora tion of water is pushed too rapidly, the super saturation will increase beyond the point Where the seed crystals can control, so that gelatini'za tion will take place due to the formation of the ?brous type of crystais. The rate of agitation used in preparing a mag

ma for magmatic drying may conveniently be greater than that for crystallizing a magma for subsequent ?ltration, in order to break up crys tals. Since purgeability is no object in magmatic drying, andsince. it. is desirable to have crystalli

2,315,699 zation proceed as rapidly as possible to facilitate quick drying, the maximum number of well formed crystals should be produced in the mag ma, regardless of size and purgeability. This is best accomplished by vigorous agitation of a rather thick magma, which produces numerous short fragments of well formed crystals. These provide sufficient surface for crystallization dur ing the magmatic drying and prevent the spon taneous nucleation which is associated with gel atinization caused by ?brous crystals. By whichever steps the sorbitol is reduced to

crystalline form the dried material may be comminuted or otherwise prepared in usable form. EXAMPLES or (lRrs'rsLLIzArios lluoir. Aqueous

SOLUTION Example 1

50 g. of crystalline sorbitol, either pure or obtained from a previous crystallization from impure syrup, and 300 g. of an 80% aqueous so lution of sorbitol syrup, P. N. 65, are stirred at 25 C. with an egg beating type chopping stirrer until the viscosity begins to increase, which re quires 28-30 minutes. 1000 g. of fresh solution are then added and the chopping process contin~ ued. Thereafter a further quantity of 3000 g.

fesh solution is added and the chopping and . ng again repeated. After this dilution the batch is transferred to a ?ve-gallon vertically s! and crystallizer, thermostated at 25 C. and surreal at about 5 R. P. M. After 12 hours the resulting magma is diluted with a further quan tity of 8000 g. of fresh solution, allowed to crys- r tallize for 12 hours and then the crystallizer is ?lled up to a total of 30 kg.

After 24 hours the magma contains about 13% separated solids as determined by a ?ltration test at 25 C. and a 3 kg. withdrawal is taken and beaten at 25 C. with 3000 cc. of absolute meth anol using a high speed turbine stirrer and add ing the alcohol slowly over a period of five min utes to insure uniformity. The thin aqueous al coholic magma is then centrifuged and dried par tially by spinning for an hour, although the bulk of the ?ltration is completed in 15 minutes. The centrifuge cake is porous and dries readily to a light friable microcrystalline mass. The yield is 27% of original solids, the purity is about P. N. . 85. Since the yield of dry product usually ex ceeds 25% and the separated solids before stir ring up are only 1013%, a substantial part of the crystallizing process takes place during the stirring up, centrifuging and drying, the addi tional solids coming from the mother liquor.

Example 3 A 3 kg. withdrawal of magma from the batch

of Example 1 is replaced with 3 kg. of fresh solu tion and crystallization allowed to proceed 24 hours with constant stirring and at constant tem perature. At the end of this period the sep arated solids have returned to their original level of 13%. A 3 kg. or 10% withdrawal is taken, stirred up with 3 liters of absolute methanol, cen trifuged and dried, giving a 28% yield of dried product, P. N. 85. . ' ' Example 3

A magma is built up at 25 C. in a 30 kg. crys tallizer as described in Example 1, using a sorbitol syrupof P. N. 68 at a concentration of total solids of 79.5%. When 13% separated solids are reached, as withdrawals are taken, proceeding as in Example 1, replacements are made with a

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7 purer syrup, P. N. 76, and having 77.5% total solids concentration. After 27 daily withdrawals and replacements with the syrup of the above composition, the magma is of 77.5% total solids concentration and substantially P. N. '76, the yield from the 28th withdrawal is 25% and the ash and reducing sugar impurities of the product 0.3 those of the original, while the purity of the product is increased to P. N. 86.

Example 4 150 lbs. of sorbitol syrup of P. N. 73, having a

concentration of 78.0% was introduced into a crystallizing vessel capable of providing slow and continuous agitation, which contained 350 lbs. of crystalline sorbitol magma of the same percent age of total solids as the fresh syrup. The amount of crystals in the heel was 13.6% of the sorbitol in the crystallizer before the addition of the fresh syrup and was 9.5% after the addition. The amount of the heel or seed magma was 70% of the total batch after the addition of the fresh syrup.

After stirring at 24 C. for 2-: hours the amount of crystals had built up to 14.9% of the total sorbitol. 150 lbs. of magma or 30% of the con tents of the crystallizer was thereupon withdrawn and the crystallizer again ?lled up with fresh syrup. The portion of the magma withdrawn was

stirred up with methanol and water to give a mix ture containing 42.8% sorbitol, the solvent being aqueous methanol 77% strength by weight. This diluted mixture was then ?ltered on a centrifuge and spun for one hour. The wet cake contained 73.3% solids and after drying in a vacuum drier yielded 32.8 lbs. of a dry, white, fluffy, friable powder representing a yield of 27.4% dry basis. The dry product contained 27% of the ash in the original syrup and 31% of the original reducing sugar. The dry product had a P. N. of 87 .8. II. CRYSTALLIZATION FROIVI ALCGHOLIC

SCLUTIONS In place of growing the crystals in water solu

tions and thereafter diluting with an alcohol to permit separation of the crystals from the moth er liquor, it has been found advantageous to con duct both the crystal growth and the separation in alcoholic solution. As solvent, either methanol or ethanol may be used, but methanol is preferred because of the fact that ethanol produces double layer or two liquid phase formation in many cases. Puri?cation is better with methanol than with ethanol. On the other hand, ethanol is some what cheaper than methanol. Compared to the water solutions, the alcoholic

solutions have much lower viscosities for the same overload and therefore the alcoholic solutions are more readily handled. High overloads are pos sible in alcoholic solutions at lower total solids concentrations than are possible in aqueous solu tions giving the same overloads. Hence, more cli lute sources of sorbitol may be employed where the crystal growth is to take place in alcoholic so lution. The crystals may be grown in a water alcohol magma by starting with a supersaturated alcoholic solution of sorbitol, withdrawing a por tion of the magma and replacing it with solution. The composition of the magma may be adjusted to give supersat rations and growth rates com patible with a suitable withdrawal schedule. The use of aqueous alcohol as a solvent carries

with it several advantages. The viscosity of the 'saturated, or moderately supersaturated, solu tions of impure sorbitol in strong alcohol is much

8 lower than that of the corresponding aqueous so lution, thus enabling a higher overload to be car ried by the magma without increasing the vis cosity unduly.

Also, with alcohol as solvent much more dilute solutions or sorbitol can be used advantageously so that a higher yield in proportion to total dis solved solids can be obtained from an alcoholic magma. Alcohol solutions of low purity can also be worked economically for puri?ed sorbitol. Be~ cause of low viscosity the replacement can be mixed into the batch much more quickly without excessive power requirements and Without undue crystal breakage and false grain formation.

Also, much higher overloads can be used in alcohol than in water without is r of gelatiniza tion, enabling faster crystallization and higher capacity for crystallizing equipment without sac ri?cing purgeability and product quality unduly. By comparison, the aqueous solutions must be kept at relatively low overloads in order to re tain the purgeablity of the crystals and there fore crystallization is slower. In the operation of an alcoholic crystallizing

method the equipment and procedure is modi fled in accordance with known practice in other chemical arts dealing with volatile and relatively expensive solvents so that the solvent content of the magma is kept constant despite evapora tion and so that substantially all or" the solvent is kept in process and not lost. . Where the solvent is dilute aqueous alcohol,

high sorbitol concentrations are necessary to produce the necessary supersaturation. On the other hand, where strong alcohol is used, the sorbitol concentration for the necessary super saturation is considerably less. In this latter case the volume of solution per unit quantity of product obtained is larger than such volume per unit of product when dilute aqueous solu tions are employed. The optimum composition of solution is determined by balancing the cost of large equipment with the cost of recovering the sorbitol from the ?ltrate since in the former case where dilute aqueous alcohol is used a con siderable quantity of sorbitol passes througl'i the process unused and must be recovered from the ?ltrate. It has been found preferable, in gen eral, to employ aqueous alcohol of 93% strength by weight, To prepare the alcoholic for crystal

growth, a sorbitol syrup or the wet cake from a previous ?ltration, or dry sorbitol, is mixed with alcohol and water to prepare a syrup of the ole- sired composition. This sorbltol concentration is adjusted to produce supersaturation or over load in amount depending on the purity (P. N.) of the corbitol used so that the magma at the time of ?ltration will not be too thick. to handle in pumps, pipes, ?ltering equipment, etc. in the alcoholic crystallization, in contrast to the aqueous process where resistance to ?ow is due to viscosity, the alcoholic magmas are slightly thixotropic in that they set slightly when not stirred and do not flow easily under low head. It has been found experimentally that alca holic sorbitol magma containing not more than 8% solid phase (wet basis) is as thick: can be conveniently handled. lviagmas containing more than this amount of solid phase tend to clog in constrictions, pipe bends, etc. This syrup is seeded with a suitable seed such as the heel from a previous batch so that the actual crystal content is equal to at least 1% or the total sorbitol in the batch. If the heel from a

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2,315,699 previous batch is not available, a seed magma may be used which is built up as described above with reference to crystallization from aqueous solution and the magma thus grown may be'diluted with alcohol before addition to the syrup to bring it to equal alcoholic content. The seeded magma is maintained in a crystallizer, generally at constant temperature. and stirred continuously for the period necessary to produce the maximum crystal growth. As a rule the min imum period for this growth is 24: hours and at the end of this time a withdrawal can be made and the crystals seperaied from the mother liquor. As in the case of the aqueous crystalliza tion care is taken to maintain the relationship of the overload of the sorbitol solution to the quantity of well-formed crystals thereby to limit further separation of solids substantially to the formation of additional well-formed crys tals and the building up of the size of the crys tals to an average length of 0.05 to 0.15 mm. and an average thickness of 0.001 to 0.002 mm. In alcoholic solution the withdrawal can amount to about 50% of the contents or" the crystalllzer,

- where the process is operated semi-continuous ly. The depth and frequency of withdrawals and replacements depend on the rate at which crys tal growth proceeds and the form of crystals produced. By observing the progress of the crys tal content of the magma the operator will know whether to increase the depth of frequency of withdrawal or to decrease one or both. Thus. if the magma is losing in crystal content or if a large amount of false grain and undersize crystals appear, the depth of withdrawal can be decreased so as to leave a larger heel to seed the replacement. The frequency of withdrawal may be cut down to give the crystals more grow ing time to reach normal size, Excessive false grain can be remedied by lowering the actual number of crystals in the magma by either dilut ing the magma with a fresh quantity of feed so lution or by withdrawingr a portion of the magma, heating the withdrawal suiliciently to dissolve the crystals therein and returning the withdrawal to the magma from which it was taken. This latter alternative in effect makes a new mixture of the in which the proportion of crys tals to dissolved solids is lowered. Loss of crys tal content may also be overcome in many cases by increasing the overload of the replacement syrup. Conversely, if the magma appears to show an increase in normal crystal content, larger withdrawals can be made and/or fre quency of withdrawal increased until the maxi mum yield is being taken, that is until further increases produce injury either to the quantity of crystals or to the form thereof. The magma being already diluted with alcohol,

it is only necessary to withdraw the desired por tion and ?lter the same. The ?ltration can therefore be carried out without the intermediate steps of diluting and mixing, which steps are usually desirable Where the crystal growth took place in aqueous solution. The ?ltering can be performed in the same manner as described in connection with aqueous magmas.

It has been found advantageous to the purity of the product, although not to the yield, to wash the ?lter cake with a small quantity or alcohol. This is equally true whether the crystals were grown in aqueous or alcoholic solution. Substan tial increases in the P. N. of the product follow an alcohol wash of the ?lter cake. The wash is used, as, say, 4 to 5% of the quantity of magma ?ltered

2,315,699 although the quantity will be regulated by con-' sidering; the results obtained by washing with various sized portions. The ?lter cake separated from alcoholic mother

liquor is readily dried to a white friable mass. EXAMPLES OF CRYSTALLIZATION FROM ALcoHoLIo

SOLUTIONS Example 5

7000 g. of a 25% solution by weight of P. N. 60 sorbitol in 90% by weight methanol, is seeded with a portion of aqueous magma containing suflicient crystals to give a total crystal con-tent of 1% in the methanol solution. This seeded solution is maintained at 25 in a crystallizer and is stirred continuously. The solution is allowed to remain in the crystallizer for 3 days, after which time the ?ltration and the recovery of a 10% withdrawal show a 22% yield, a value which is not sensibly increased after several more days crystallization without withdrawal. The with drawal is replaced in each instance by an equal quantity of solution of the same strength and purity. Thereafter ?ve daily withdrawals are made averaging 22% in yield. .The percentage of residual sugar and ash in the dried product are about 35% those of the starting material, the P. N. of the product being about 83.

Example 6 "7000 g. of an alcoholic magma arefmade up from? an aqueous magma obtained as described in Example 1. The alcohol used is specially. de-. natured formula No. 1 ethyl alcohol which 'is' substantially 4.5% methanol and 95% ethanol. The alcoholic magma as made up has a 51 % con-. c'e'ntration of sorbitol of P. N. 65 in 66% alcohol and the separated solids are 6-7 %. After stirring 24 hours at 25 C. in a crystallizer a 44.5% yield is obtained on withdrawal. Two recyclings are then made at 24 hour intervals, the ?rst 90%, the second 50% of the magma. 24 hours after the last recycling, a 10% withdrawal is made. A 33% yield is obtained. The P. N. is 718 and the ash and reducing sugar are half those of the original. The i?ltered cake can be dried to a friable, semi crystalline solid. ' . ' '

- .1 Example 7

A semi-continuous process was operated in which sorbitol wascrystallized from an 18% solu tion in 90 %' methyl alcohol in a 45 gallon hori zontal aluminum crystallizer. The P. N. of the original sorbitol was 80.5. Every 24 hours 45 to 50% of the contents of the crystallizer (135-170 lbs.) was removed and ?ltered. Fresh sorbitol solution was added to the large heel in the crystal lizer and after 24 hours another 50% withdrawal and replacement were made. Twelve with drawals were made in all. The magma withdrawal was centrifuged and

washed with 90% methyl alcohol by weight in an amount equal to 4.6% of the weight of the magma ?ltered. The wet cake contained an average of 57% solids. After drying in a vacuum dryer, a white friable product was obtained correspond ing to an average yield of 43.5%. This material was easily pulverized and screened. It had an average P. N. of 90 indicating a purity of about 94% and contained .32 and .14 respectively of the percentage of ash and reducing sugar present in the original sorbitol.

Example 8 266 lbs. of crystalline sorbitol as_a methanol

wet cake from a previous crystallization were

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9., mixed with 91.5% methyl alcohol and heated to dissolve the sorbitol. After cooling to 66 F. the batch was seeded with 13.5 lbs. of wet press cake, stirred up to a cream with ?ve gallons of the al

; coholic sorbitol solution. The mixture now had the composition 15% sorbitol, 8.3% water vand 76.7% methanol. Of the sorbitol, about 2% of it was in the form of individual seed crystals dis persed throughout the mixture. After stirring for 39 hours, during which time

the temperature was gradually raised to 75 F., the mixture was ready to ?lter. Filtration on a frame press at 40 lbs. per sq. inch pressure yielded a press cake containing 35.9% solids which on drying gave a white, friable product weighing 153 lbs, corresponding to a 56.5% yield. The re covered crystals had a P. N. of 90.3. The method of the present invention produces

from a sorbitol syrup of intermediate purity and relatively low value, two products each of which is more valuble than the starting syrup. A crys talline product of improved purity and in dry form is one of the products. The other product is a syrup of lower sorbitol content which is of

, value in humectant and conditioning uses. Par ticularly the syrups of low sorbitol content are of value because they are non-crystallizing. The puri?cation of the syrup is a relative one, as seen from the examples, and the quality of the crystal line product obtained by one crystallization de pends upon the quality of the starting syrup. The crystalline product will contain a de?nite fraction of the ash and reducing sugar content found in the starting syrup. Likewise, the P. N. of the product will be dependent upon the P. N. of the-starting syrup. Where the starting syrup is of intermediate or low purity, it is necessary to recrystallize the product to obtain high purity crystals. For many purposes the magmatic drying of an

aqueous magma produces satisfactory products. This particular type of drying can be accom plishedwhere the starting syrup is of relatively high purity. Where this condition is met, a prod

_, not of the magmatic drying is a crystalline mass capable of reduction to a powder, as distinguished from the glasses normally produced by the evapo ration of sorbitol solutions.

_. Finally,it is to be understood that the descrip tion and instructions set forth above are for the purpose of guiding those skilled in this art in the crystallization of sorbitol. The adjustment of concentrations, temperatures, rates of stirring and the like will be determined by the conditions of operation under which the crystallization must be conducted. The examples are illustra tive of some of the possible ways of controlling the crystallization and are meant to be illustrative only and not limiting.

I claim: 1. The method of preparing crystalline sorbitol

from an overloaded impure feed solution of sor bitol in water, said sorbitol being produced by the hydrogenation of a sugar and having a pyridine number not less than 50, which comprises slowly mixing said solution at approximately constant temperature with a seed magma consisting of a water solution saturated with respect to sorbitol and containing about 10 to 13% separated solids as well-formed sorbitol seed crystals, said seed crystals having substantial thickness and strength such that a gently stirred overloaded sorbitol solution in water at 25 C. and contain ing at least 8% of such crystals as separated solids remains mobile, said seed magma being

16 present in an amount of at least 60% of the total mixture; continuing said mixing until the amount of well-formed sorbitol crystals in the total mix ture increases to 10 to 13%; withdrawing a por tion not exceeding 40% of the said total mix ture; separating the crystals in the withdrawn portion from the mother liquor; replacing. the withdrawn portion. with an equal quantity of fresh overloaded impure feed solution of sorbitol in water; and repeating the steps of mixing, with drawing, separating and replacing to continue the operation of the method.

2. The method of preparing crystalline sorbitol from an overloaded impure aqueous feed solution containing about 75~82% sorbitol, said sorbitol being produced by the hydrogenation of a sugar and having a pyridine number not less than 65, which comprises slowly mixing said solution at approximately 25 C. with a seed magma con sisting of a water solution saturated with respect to sorbitol and containing about 10 to 13% sepa rated solids as well-formed sorbitol seed crystals, said seed crystals having substantial thickness and strength such that a gently stirred overloaded sorbitol solution in water at 25 C. and contain ing at least 8% of such crystals as separated solids remains mobile, said seed magma being present in an amount of from 60 to 90% of the'total mix ture; continuing said mixing until the amount of well-formed sorbitol crystalsin the total mixture increases to 10 to 13%; withdrawing a portion of from 10 to 40% of the said total mixture; sepa rating the crystals in the withdrawn portion from the mother liquor; replacing the withdrawn por tion with an equal quantity of fresh overloaded impure aqueous feed solution of sorbitol; and repeating the steps of mixing, withdrawing, sepa rating, and replacing to operate themethod semi continuously. -

3. The method of preparing crystalline sorbi tol from an overloaded impure feed solution of sorbitol in water, said sorbitol being produced by the hydrogenation of a sugar and having a pyri dine number not less than 50, which comprises slowly mixing said solution at approximately con stant temperature with a seed magma consisting of a water solution saturated with respect to sorbitol and containing about 10 to 13% sepa rated solids as well-formed sorbitol seed crystals, said seed crystals having substantial thickness and strength such that a gently stirred overload

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2,315,699 ed sorbitol solution in water at 25 C. and cone taining at least 8% of such crystals as: separated solids remains mobile, said seed magma being present in an amount of at least 60 %. of the total mixture; continuing said mixing until the amount of well-formed sorbitol crystals in the total mix ture increases to 10 to 13% ; Withdrawing a por tion not exceeding 40% of the said total mixture; diluting the withdrawn portion with a monohy dric alcohol having not more than 2 carbon atoms in a quantity to reduce substantially the viscosity of the said withdrawn portion; ?ltering the with drawn portion to separate the sorbitol crystals from the mother liquor and diluting alcohol; replacing the withdrawn portion with an equal quantity of fresh overloaded impure feed solution of sorbitol in water; and repeating the steps of mixing, withdrawing, separating, and replacing to continue the operation of the method.

a. The method of preparing crystalline sorbitol as in claim 5 wherein the monohydric alcohol used to dilute the withdrawn portion of the said total mixture is methanol.

5. The method of producing sorbitol seed crystals which comprises mixing pure dry sorbitol with an overloaded water solution of sorbitol with a pyridine number at least 65, the sorbitol content of said solution being at least 75% and sufiicient to constitute a substantial overload with respect to sorbitol; violently agitating said mixture to chop and break up the sorbitol crystals to give a large amount of fast-growing end-face crystal area, until the mixture becomes substantially thickened; thinning the thickened mixture by dilution with several times its weight of fresh overloaded water solution of sorbitol to prevent immobilization of the said thickened mixture; violently agitating the thinned mixture to chop and break up the sorbitol crystals and to disperse them throughout the mixture, until the diluted mixture becomes substantially thickened; and thereafter alternately thinning the mixture to prevent immobilization thereof and violently agitating the thinned mixture until the chopped sorbitol crystals grow into strong well-formed seed crystals and the ?nal mixture of seed crystals and overloaded water solution of sorbitol con tains at least 8% of seed crystals as separated solids and is of mobile consistency. '

RUDOLPH MAX GOEPP, JR.

CERTIFICATE OF CORRECTION. Patent No. 2,515,699. April 6, 1915.

RUDOLPH MAX GOEPP, JR.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 1, sec ond colunm, line 9-10, for "refuse" read -reduce-; page 5, first column, line 15, for "sparation" read -separation--; page 8, first column; linen /

"58, for "corbitol" read sorbitol--; page 10, second column, line 21, for the claim reference numeral "5" read 5"; and that the said Letters Pat- """f" ent should be read with this correction therein that the same may conform to the record of the case in the Patent Office. Signed and sealed this 6th day of July, A. D. 19%.

Henry Van Arsdale, (Seal) _ Acting Commissioner of Patents.