I
D. R. ASHER Physical Research Laboratory, Dow Chemical Co., Midland, Mich.
Sugar Purification
THE
possible application of ion exclusion to the demineralization of sugar solutions was suggested and dismissed in the original article on solute separations by this method ( 4 ) . Subsequent experimentation and evaluation of the method suggested that this system might well be adapted, under different conditions. to purification by ion exclusion. I t has been shown previously (I) that solute separations by ion exclusion are entirely due to physical characteristics of the resin, as no effective exchange of ions takes place and no chemical regeneration is required. The advantages of ion exclusion treatment of some systems, particularly those containing concentrations of ionic material greater than 0.1 to l.OYc, becomes readily apparent. I n the treatment of such solutions by ion exchange, frequent regeneration of the resin is necessary and a large portion of the cost of the process then becomes that due to chemical regeneration. In ion exclusion only a solvent rinse, normally Lvater? is required. Also, there are systems where normal demineralization procedure must be altered. This is true of sugar solutions where a method of reverse demineralization is used to prevent acidification of the feed material. Wheaton and Bauman ( 3 )have shobvn that the differential in the K , value of two solutes should indicate the degree of separation possible by ion exclusion column chromatography. The K d value, which is the inside-outside concentration ratio of a solute in contact with a n ion exchange resin, is normally of the order of 0.1 for highly ionized solutes such as sodium chloride, hydrochloric acid, and the like, in dilute solutions. This can be explained on the basis of the Donnan membrane theory, if one assumes that the resin shell is a membrane and that the ionic material inside the resin is in true solution. However, the same concentration ratio is found to have values from 0.2 to 1.2 and greater (2) for essentially nonionized substances. The value of K d is dependent upon resin type, ionic form, and. in many cases, upon concentration of the solutes in contact Ivith the resin. T h e rather small difference between the K d values for sodium chloride and xylose ( K , = 0.1 and 0.56, respectively, a t 5% concentration) ( 3 ) suggests that the separation found would be considerably less than for systems where the solute K , difference is large and the separation apparent.
by Ion Exclusion T h e degree - of separation obtained is dependent upon flow rate, resin particle size, feed volume, and percentage of divinylbenzene in the case of Dowex resins (styrene-divinylbenzene copolymer matrix) ( 7 ) . The effect of some of these variables upon the separation of saltsugar solutions by ion exclusion has been studied in a n attempt to determine optimum conditions.
of cross-linking material determines to a n extent the physical properties of the final resin. A lower percentage of divinylbenzene gives faster diffusion rates and a higher moisture content? which results in the later appearance of the nonionic fraction in the effluent. This is seen to be true as shown by Figure 1, which compares the two resins, Dowex 50 X-2 and X-8. The feed material for these separations was a 5yo sodium chloride--lO~odextrose solution. The feed volume was 10 ml.. and the separations \vere made at room temperature (26 f 1 C.). Separations under similar conditions were made using an intermediate cross-linked resin. The general trend was found to be one of less pronounced separation with higher cross linkage. Temperature. Several duplicate separations were made a t 25' C. and a t 80' C. At the higher temperature a better separation is to be expected because viscosities are lower and conditions are nearer equilibrium (because of faster diffusion rates). The feed material
Procedure I n most cases. the resin was converted and used in an ionic form common with the ionic component of the feed material. The column used for most of the separations was of borosilicate glass, 1 inch in inside diameter and 5 feet in length, water jacketed, and connected to a constant temperature bath. T h e resin bed was 36 inches in height (460 cc. of resin). I n all cases, 50- to 100-mesh resin was used and a n effluent flow rate of 5 ml per minute (qallons per minute per square foot) maintained The effluknt was obtained as a series of 2 5 - m l . f r a c t i o n s which were analyzed for chloride by potentiometric silver nitrate titrations when sodium chloride was used as the ionic solute, and by refractive index difference to obtain a value for sugar concentration. I n each series of experiments, conditions other than the variable studied were maintained constant. Prior to each separation the resin column was backwashed with water and allo\ved to settle; this maintained a constant bed volume and prevented channeling.
Effect of Variables Cross Linkage. In t h e p r e p a r a t i o n of Dowex resins (Dow Chemical Co.) the X number refers to the per cent of divinylbenzene used in the copolymer bead preparation. This percentage
150
200
250 TOTAL
Figure 1.
300 EFFLUENT
350
400 450 (ML.)
500
VOLUME
Effect of cross linkag
Feed: 10 rnl. of 570 sodium chloride-1 0% dextrose Temperature, 25' C. A. Dowex 50 X - 2 8. Dowex 5 0 X - 8
VOL. 48, NO. 9
SEPTEMBER 1956
1465
!
1':
:I
I
Ii 1.333
I
i
1332-
I
1331, ID0
2 00
EFFLUENT
3 00 VOLUME I M L )
Figure 2. Effect of temperature Feed: 10 ml. of 5% sodium chloride-1 0% dextrose Resin: Dowex 50 X-4
25' C. 80" c.
\vas again a 5y0 sodium chloride-lO% dextrose solution and the feed volume was 10 ml. As shown in Figure 2 both the salt and sugar fractions peak at higher concentrations a t 80" C ; thus the separation is more complete at the higher temperature. Figure 2 was obtained using a 4% cross-linked resin; similar results were found using resins of different cross linkage. Feed Concentration. Three different feed solutions were eluted through identical columns of Dowex 50 X-4 a t 80" C. One feed contained 5% sodium chloride-lOyo dextrose. The second was 570 sodium chloride-20% dextrose, and the third, 2% sodium chloride-20% dextrose. The feed volume was again 10 ml. of solution. A complete separation was obtained in all cases. the effluent peak concentrations being a function of the feed concentration. I t was noted that, although the nonionic concentration seemed to have little effect on the ionic wave, a n increase in the ionic concentration tended to delay the appearance of the nonionic fraction. This effect has been noted previously ( 7 ) and is due to an increase in the K d value for sugar with increased ionic concentration. Resin Type. Separations of identical feeds were made on three resins : Dowex: 50j a strongly acid cation exchange resin; Dowex 1, a strongly basic anion exchange resin; and Dowex 3> a weakly basic anion exchange resin. The separations obtained with Dowex 50 X-4 in the sodium form and Dowex 1 X-4 in the chloride form were essentially identical; however, with Dowex 3 X-4 in the free base form very little separation was obtained. This was not unexpected, however, because Dowex 3 in the free base form is not highly ionized as are the strongly basic and acidic resins. The recovery of the sugar fraction, of the order
1466
100
4 GO
e00 300 E F F L U E b T VOLUME i M L i
E F F L U E N T VOLUME
(ML)
400
Figlrre 3. Effect of feed material composition Figure 4. Effect of increased feed volume Resin: Dowex 50 X-4 Temperature: 80" C. Feed: 1 0 ml. 5% sodium chloride-1 0% xylose 1 % potassium o c o n i t o t e - I ~ % sucrose
-.-.
of 95 to 100cc for the Doicex 50 and Dowex 1. was very poor in the case of Dowex 3. It is known that sugar cannot be eluted through strongly basic Dowex I in the hydroxide form because sugar is actually sorbed by the resin. Partial sorption in the case of Dolvex 3 !could explain the poor recovery. Separation of Other Salt-Sugar Feed Solutions. To determine if other sugars would separate equally well, feed solutions containing 5% sodium chloride10% D-xylose and 1% potassium aconitate-1070 sucrose were prepared and eluted through Dowex 50 X-4 at 80" C. These tbvo separations are shoivn in Figure 3. Comparison with Figure 2 shows that the separations are complete and equally as good as those obtained using sodium chloride and dextrose. T o determine if a larger feed volume would appreciably alter the separation obtained, 46 ml. (10% of the resin bed volume) of the lyOpotassium aconitate10% sucrose solution was eluted through Dowex 50 X-4, potassium form. .4s shown in Figure 4, the separation is still essentially complete. I n actual practice it would be possible to introduce an even larger quantity of feed and, by the method of recycle described by Simpson and Bauman (7): to obtain a sugar fraction concentration near that of feed concentration. This ~vouldbe accomplished by taking a series of cuts through the cross-contaminated portion of effluent to be fed back with new feed for the subsequent cycle through the column. The contaminated recycle cuts wzould be fed to the column in the order of elution, with the new feed being inserted a t the point of maximum cross contamination. The possible use of ion exclusion for commercial sugar refining is again being suggested. Further evaluation of this method to determine maximum obtain-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Resin: Dowex 50 X-4, potassium form Temperature: 25' C. Feed: 4 6 ml. of 1% potassium aconitote10% sucrose
able product concentrations, puri[y and productivity, and use of' crude sugar juices will be required and is being contemplated. A side light of this method \could be the recovery of the ionic portion of the sugar juices containing the organic acids and salts present. Methods could presumably be instituted t o separate and recover the aconitic and amino acids. .4 single run was made to deterrriine if decolorization of sugars \vould be accomplished incidental to regular demineralization. Ten milliliters of a 30c,> aqueous solution of dark molasses \vas fed through a 100-cc. bed of DoLcex 50 X-4. sodium form. contained in a 100-m!. buret. The bulk of the color \cas eluted with the ionic material, separate from the sugar-containing fraction of effluent; similar results Mere found using a dilute solution of a crude cane sugar. This !could suggest another advantaTe to this method as well as those previousl:. cited-the elimination of required chemical regenerants. decreased inversion of the disaccharides. and recovery of the ionic materials present. Acknowledgment
The author mishes to acknotiledge the helpful guidance of Robert M. Wheaton throughout the course of this investigation. Literature Cited ( 1 ) Simpson, D. \V.; Bauman, W. C . . I K D . ENG.CHEM.46, 1958 (1954). ( 2 ) Simpson, D. W., Wheaton, R. M., Chem. Eng. Progr. 50, 45 (1954). ( 3 ) Wheaton, R. M . , Bauman, Lk'. C., Ann. AY,I'.A cad. Sci.57, 159 (1953). ( 4 ) Wheaton. R. h l . , Bauman, W. C., IND. ENG.CHEM. 45, 228 (1953). RECEIVED for review September 14, 1955 ACCEPTEDMarch 23. 1956
