Solubility of Sucrose in Beet House Sirups R. J. BROWN AND A. R. NEES T h e Great Western Sugar Company, Denver, Colo.
A
The solubilities of sucrose in a variety of sirups employed were the final m ~ t h e r PRELIMINARY paper handled in the beet sugar process are presented. l i q u o r s (low g r e e n ) from a describing the method number of beet sugar factories. employed to obtain the The jield investigated covers the whole range The sirups were carefully anasolubility data tabulated here, employed in factory operations. Variations in and giving r e s u l t s of a deterlyzed to determine the percentr m i n a t i o n on one s i r u p , was solubility at constant temperature and purity are age sugar and percentage water. observed, and the factors known to affect these They Were then made to the depresented earlier ($), sired ratio of impurity to water The following presents and disvariationsare discussed. by dilution with water or by cusses the solubility of sucrose in a wide variety of sirups, all dee v a p o r a t i o n and were then terminations having been made by the above mentioned saturated, sampled, and analyzed. method. The field investigated consists of sirups obtained Water, or dry substance, was determined by the method from beets grown in the Rocky Mountain territory during a referred to above (2). Sucrose was determined by the number of seasons. The sirups are those from Steffen and invertase-melibiase inversion method of Paine and Balch (6)) non-Steffen houses (that is, from factories employing trhe cal- modified to meet present requirements. Hydrolysis of cium saccharate process for the recovery of sugar from mo- raffinose by enzymes was checked by a fermentation method, lasses, and from factories employing no molasses-desugarizing and all sources of error other than the possibility of a slight process); sirups from a process employing beet molasses only one resulting from the influence of salts on the polarization of as source of sugar; sirups prepared from calcium saccharate sucrose and invert sugar, were reduced to a minimum. alone, in which the only impurities were those precipitated Waste water sirup was analyzed, as were the beet house from beet molasses by lime; and sirups prepared from waste mother liquors, water from the calcium saccharate process, in which the Calcium saccharate sirup was analyzed likewise, but beimpurities were those not precipitated by lime. In addition, cause of the high raffinose content of this sirup the water one sirup from California was studied, which consisted of determinations were inaccurate, since the importance of this a mixture of California beet and Hawaiian cane molasses. sirup is slight, no particular effort to obtain accuracy was made. EXPERIMENTAL PROCEDURE Mother liquor from the molasses-desugarizing process had In brief, the method employed consisted in the saturation to be analyzed differently. This sirup is high in raffinose and of a carefully analyzed sirup with sugar, under conditions cannot be dried a t 90" C. without decomposition. Furtherso controlled that no change in the impurity : water ratio of more, i t cannot be dried at 70" C. under vacuum in one week. the batch under investigation occurred, followed by a de- Therefore, refractometer dry substance was taken rn the true termination of the percentage dry substance of the saturated dry substance. As a result of a study of refractometer sirup. During the first stages of the work, solubility de- readings on this type of sirup, it was concluded that the terminations were made a t five temperatures (40°,50", 60", refractometer reading showed the most constant agreement 70", and 80" C.) and at a number of ratios of impurity to with true dry substance and was, therefore, probably most water. The regularity of the solubility curves prepared from nearly correct, if the refractometer readings were made on the the data so obtained and the similarity of curves showing the sirup diluted to approximately 40 per cent dry substance. solubility of sugar in various sirups made it unnecessary to de- All refractometer readings were therefore made a t about 40 termine solubilities a t per c e n t d r y s u b a l a r g e n u m b e r of stance, and the reading calculated back to points on later sirups. original sirup. Owing The h a 1 portion of the investigation conto the high raffinose sisted in determining content of this type of the solubility of sugar sirup, the invertasein sirups a t two ratios melibiase hydrolysis of impurity to water cannot be employed a t 40°,60°, and 80" C. with satisfactory reT h i s r e d u c t i o n in sults on thelow punty number of determinaproducts. Hydrolytions made on an insis of the melibiose d i v i d u a l sirup perfraction of raffiose mitted the investigaby melibiase does not tion of a large number proceed t o compleof d i f f e r e n t sirups tion. It was f o u n d without serious loss that raffiose could be in a c c u r a c y of the determined with solubility curves. accuracy by a bottomIn the m a j o r i t y yeast extract hydrolyFIGURE1. CONCENTRATION OF SATURATED BEETSUGARSIRUPS of t e s t s the s i r u p s sis of the residue re(40' C. solubility isotherms) 555
INDUSTRIAL AND ENGISEERING
556
maining after complete fermentation of the sirup by pure top yeast, followed by distillation of the alcohol. Knowing the quantity of raffinose present, sucrose was determined by use of direct polarization, top yeast extract, invert polarization, and raffinose content. The data showing parts of dry substance per 100 parts of water in the saturated solutions a t various ratios of impurity t o water are given in Table I. I n this form the data have no particular significance, but it is the simplest manner of expressing the results from which all desired information may be gained. Because of the high rate of decomposition of sugar at high temperatures, and the low rate of dissolution and crystallization a t low temperatures, only the range from 40' to 80" C. was studied. If the points representing the solubility of sugar in sirups a t constant ratio of impurity to water and various temperatures are plotted on a graph, the curve drawn through these points may be extrapolated to higher and lower temperatures with a satisfactory degree of accuracy. TABLEI. CONCENTRATION OF SATURATED SUGARSIRUPSAT VARIOUS RATIOSOF IMPURITY TO WATER TEXP.
PARTSDRYSUBSTANCE PER 100 P A R T S WATERI N S A T U R A TSOLUTION ~D
c. Impurities/water: 40 60 80
SIRUP 1
0.63 297 357 445
0.22 248 309 390
1.09 365 427 525
SIRUP 2
Impurities/water: 40 50 60 70 80
0.36 0.46 0.59 0.75 0.88 310 324 264 276 291 299 314 333 348 288 331 346 366 383 320 369 385 407 424 358 418 434 454 471 405
Impurities/water: 40 60 80
0.92 336 389 475
Impurities/water: 40 60 80
1.02 335 396 477
SIRUP 8
1.47 408 478 572
1.48 404 465 558
80
1.00 340 396 486 1.00 341 397 485
1.00 338 395 485
SIRUP 14
1.38 375 432 530
1.94 497 547 645
1.00 347 399 495
0.83 296 351 431
1.81 440 487 576
Impurities/water: 40 60 80
0.65
1.09
1.48
1.86
333 413
367 449
403 483
448 522
0.99 327 380 465
...
...
SIRUP 22
Impurities/water: 0.15 0.30 252 40 243 269 278 50 308 60 298 346 70 335 80 381 391 a According t o Herzfeld (3).
1.18 348 399 488 1.72 446 494 591
SIRUP 21
20-
-SIRUP
1.96 513 569 669
SIRUP 19
1.65 393 447 532
1.00 326 379 469
2.05 478 518 611
1.80 463 522 612
BIRUP 10
SIRUP 16
1.00 324 378 463
SIRUP 18
BIRUP 17
1.95 497 548 649
SIRUP 12
2.10 517 574 669
Impurities/water: 40 60 80
...
1.50 416 473 562
SIRUP 11
0.98 335 393 481
1.89 497 551 646
2.54 586 661 738
SIRCP 9
1.79 461 521 616
1.01 340 397 486
BIRUP 13
Impurities/water: 40 60 80
1.00 339 394 483
...
SIRUP 10
Impurities/water: 40 60
1.77 447 508 598
SIRUP 8
SIRUP 7
1.38 398 424 459 505 554
SIRUP \
J
0.50 276 331
1.95 475 538 633
1 16 362 388 423 463 517 - 4
-SIRUP
1.97 503 567 669
BIRUP 6
Impurities/water: 40 60 80
1.00 339 364 399 441 488
...
.
0.20 240 297 380
0.45 0.60 0.76 0.92 262 274 286 298 324 288 299 311 341 353 318 330 367 381 392 356 401 412 426 438
0.35
., ,
294 385
P U R E SUGAR'
0.00 238 260 287 321 362
If the two coordinates are used to express parts impurities per 100 parts water in solution, and parts dry substance per 100 parts water in the saturated solution, solubility isotherms may be prepared from the previously described set of curves. Straight lines may be drawn across this latter graph to express constant purity, and, from the intersection of these lines with the solubility isotherms, points may be plotted to express concentrations of the saturated solution at constant purity
CHEMISTRY
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over the whole temperature range. Astudy of the data given in Table I will reveal that it is impractical to attempt to present all the results in graphic form, because of the great number of graphs required for complete presentation.
DISCUSSION OF RESULTS While it was realized a t the inception of the work that the solubility of sugar in sirups of the same purity would show some variation, it was hoped that in definite types of sirups the variation in solubility would be sufficiently small to permit employment of standard solubility data in pan and crystallizer control. Such has not proved to be the case, Although certain types of impurities show definite trends in affecting the solubility of sucrose, the knowledge of the composition of these types is not sufficient to determine their effects with a high degree of accuracy. Examination of the various types of sirups shows that the solubility of sugar is highest in those sirups prepared from Steffen waste water, lower in those prepared from nonSteffen molasses, still lower in those prepared from Steffen molasses, and least in those prepared from mother liquor from the molasses desugarizing process. In any one season this relationship holds definitely, but in different seasons the ratios of the different impurities vary sufficiently to cause the solubilities in all classes to move up or down, resulting in overlapping of the different types of sirups when all types for a number of seasons are plotted together. The solubility of sugar is controlled by the temperature, the ratio of water to total impurities, and the composition of the impurities. The temperature coefficient of solubility is dependent upon the composition of impurities. At one temperature two sirups of differing impurity composition may show one solubility relationship and a reverse relationship a t another temperature. The all-important factor is the composition of the impurities. The analyses of the impurity components of the various sirups employed in this investigation are given, so far as determined, in Table 11. It is well recognized that this type of analysis is not highly informative. For instance, the nitrogen content represents, besides an insignificant quantity of ammonia and amines, compounds ranging from nitrates to amino acids. One per cent nitrogen represents a quantity of nitrogenous impurities ranging from 4 to 10 per cent. Invert sugar determinations show the quantity of invert sugar present in the sirup when analyzed, but they do not suggest the quantity of sugar which may have been inverted in the process of making sugar from the sirup, since the rate of destruction of invert sugar in low purity products in factory operations is comparatively rapid (1). This factor is of importance, since invert sugar and some of its decomposition products are able to replace sucrose in solution. The value given for raffinose is correct, but raffinose is always accompanied by a varying quantity of some unknown carbohydrate, and both of these products replace sucrose in solution. The inorganic constituents of the impurities are important factors in increasing the solubility of sucrose, but the ash analysis does not picture accurately the inorganic constituents as they exist in the sirups. Further, the effects of these constituents are not simple functions of their respective concentrations. For the purpose of comparing the concentrations of the different saturated sirups, Figure 1 is given. The various curves show the concentrations of the saturated skups a t constant temperature (40' C.) and a t varying ratio of impurity to water. Although the concentrrttion of the Raturated sirup increases as the concentration of the impurities increases, this rate of increase in saturation concentration varies greatly with different sirups, The sirup showing the lowest concentration a t all purities is No. 22 from the molasses-desugarizing process; next are sirups 14 to 19, all from factories employing
I N D US TR I A L A N D E N G I N E E R I N G C H E M I S T R Y
May, 1933
the Steffen process; next are the sirups from the non-Steffen houses ( t h e s e two g r o u p s overlap slightly) ; and showing greatest solubility is the sirup prep a r e d f r o m Steffen waste water impurities (No. 1). A comparison of these curves with the impurity analyses shows t r e n d s toward low solutlility when raffinose is h i g h a n d when ash is low, and vice versa. However, closely adjacent curves show, a t times, the reverse relationship. While mathematical e x p r e s s i o n s for the effect of the v a r i o u s i m p u r i t i e s have not been found, the following discussion of effects of s p e c i f i c impurities will help e x p l a i n the apparent irregularities in results.
0
557
Figure 2, which is similar to Figure 1 except that sucrose concentration rather than dry substance concentration of the saturated solution is given. Starting with pure sugar solutions, the solubility of sucrose drops to a minimum with increasing impurity content, and then begins to increase until it finally is above the concentration of sugar in water alone.
b
Port4 \5ucjar per1 100 Pdrts W d t e r 2 50 300 3 0
FIGURE2. SOLUBILITY OF SUGARIN SATURATED BEETSUGARSIRUPS (40' C. solubility isotherms)
The ash analyses show the ash to be made up primarily of chlorides, sulfates, and carbonates of sodium and potassium. EFFECTS OF SPECIFIC IMPURITIES Since no appreciable amounts of carbonates exist in the original sirup, the carbonate content represents nitrates and T h e carbohydrate organic acids. The investigators mentioned by Lippmann (4) impurities appear to re- class chlorides and nitrates as molasses formers, and find place sucrose. This is potassium to be more effective in increasing the solubility than shown by the results sodium. Tables I and I1 show a trend toward high solubility on sirup 21 (from cal- when nitrates, chlorides, and potash are high. An extensive study of the various solubility determinations cium saccharate sirup and containing negli- and analytical data in the present investigation has indicated gible quantities of ni- the existence of a rather complex relation between the controgen and ash), and on centration of sucrose, raffinose, and related carbohydrates in sirup 20 (from the beet the saturated solution on the one hand, and the concentration and cane molasses mix- of nitrates, chlorides, sulfates, potash, and soda on the other. ture). If the i n v e r t Thus, it appears that the total carbohydrate concentration a t sugar content of this saturation, in low purity solutions, is increased greatly by sirup is calculated as nitrates and appreciably by chlorides. Also, the solubility is sucrose, the solubility increased by an increased ratio of potash to soda, whereas shown agrees well with sulfates appear to act as a negative factor by rendering inthe solubility of the active an equimolecular quantity of potash. No attempt has been made to correlate solubility a t high punties with the non-Steffen sirups. Numerous investiga- impurity constituents. The practical value of the indicated relationship is questiontors have shown that inorganic s a l t s when able, since such a detailed knowledge of the impurities is p r e s e n t in sufficient required before an estimate of the solubility can be made. quantity i n c r e a s e t h e The observation has been made that a high ash content is solubility of sucrose in generally accompanied by a high solubility, but this agreesolution (4). It has ment is not constant, because, although high ash content been noticed that these generally is associated with high chlorine and nitrogen pencompounds may dis- toxide, it is also associated with a decrease in the ratio of place sugar from solu- potassium oxide to sodium oxide. Obviously the solubility of sugar in saturated sirups varies tion when p r e s e n t in small amounts and in- with sirups from beets of different districts and seasons. In crease the s ol u b i l i t y some districts the ratio of potassium oxide to sodium oxide is of s u c r o s e w h e n well above that in others, year after year. The raffinose p r e s e n t i n l a r g e r content also varies in different districts. The nitrate content, quantities. This effect which is related inversely to the degree of ripeness of the beets m a y be n o t e d in a t time of harvest, is a highly variable factor. I n good grow-
558
11DlJST11IA1, A N D E N G I N E E l l I h G CIIEMI
ing seasons and under advanced agricultural practices the quantity of nitrates in the beets a t harvest time is reduced to a minimum. The high desirability of low nitrates is appreciated when i t is observed that low nitrates increase the recovery of sugar in the manufacturing processes by lowered quantity of molasses produced and lowercd percentage of sugar in that molasses. Since the composition of tlie impurities varies greatly, and since effects of various impurities on the solubility of sugar are markedly different, it seems strange offlmnd that a final molasses of about ti0 per cent purity is invariably produced. One factor wbich has been greatly overrated is the total nitrogen content of the molasses. On a dry substance basis, beet molasses contains abont ti0 per cent sugar and 2 per cent nitrogen. This fairly constant ratio has led sorue to the conclusion that 1 part of nitrogen holds 30 parts of sugar in solution. That the nitrogen in molasses has no greater inflnence than other constituents of tlie impurities is indicated by the results of Vyskocil (0)w-ho extracted 73 per cent of the nitrogen (betain almost quantitatively) from molasses by means of phenol. The extract was 32 per cent purity, and the residue, containing only 27 per cent of the nitrogen in the original molasses, was about 74 per cent purity. The residue was evaporated and allowed to crystallize. Sucrose crystnls were obtained and the mother liquor showed about the same purity aa the original molasses had shown, in spite of the fact that the percentage nitrogen on impurities in the latter liquor was only half that in the original molasses. About the only conclusion to be reached is that practically all the impurities in the sirups have melassigenic properties, and, if one is removed, the result is only to increase the concentration of those remaining. Of the inorganic compounds there appears to be a natural tendency to maintain a balance during growth of the beet which keeps a degree of constancy in solubility. If growing conditions cause large quantities of chlorides and nitrates to exist in the juice, this condition is generally balanced to some extent by an increase in the sodium content. Of the organic impurities, the variations do not appear to be great enough to produce great effects. Ra5nose might be exceedingly effective, but its variations are not sufficiently great to have important effects on the purity of the final molasses. Organic nitrogenous compounds do not seem to be as important as some other impurities, and their quantity is rather constant. APPLICATIONS '1'0 FAmonY OPERATIONS
It is apparently correct to conclude that no ultimate purity of molasses exists. That is, from any given sirup more sugar may be crystallized by employing proper procedure, even though from the shape of the solubility isotherms it may be inferred that, as the ratio of impurities to water is increased at
Vof. 25, No 5
constant temperature, the solubility relations become such that constant purity is reached. But it may be observed that at the ratios of impurity to water where constant purity is apparently reached a t a given temperature, the purity of the saturated sirup is lower at any lower temperature. Lowered molasses purities are to be sought, therefore, by means of lowered crystallizer teinperatmss, as well as by increased conccntratioiis. This makes the problem largely mechanical because the enormous increases in viscosity of the massecuite accompanying higher concentrations and lowered temperatures grcatly increase the strain on the stirring mechanism in the crystallizers. A study of the solubility curves which may be prepared from the data given will show that. under certain conditions of raw pan work it may be advisable to add water to the massecuite during the latter part of the crystallizer period in order to prevent viscosity from reaching unreasonable values, while still maintaining a proper degree of supersaturation. These occasions arise when raw massecuite is dropped to the crystallizer at relatively high temperatures. I n this case the concentration must be high at the time the pan is dropped if valuable crystallizer time is not to be wasted during the early stages of cooling; yet this concentration is such that viscosity becomes so high that crystallization may be retarded during the la.ter cooling stages if the concentration is not reduced somewhat. The solubility of sugar in different sirups varies sufficiently so that the data on any one composition of sirup cannot be used for accurate control on other sirups. And if such control is desired, it is necessary to make a few determinations of solubility on the sirups actually being worked; use may bt? made of the data given herein as guides for purposes of interpolation and extrapolation, .%CKNoWLEDQMEWT
The writers wish to acknowledge their indebtedness to
J. E. Sharp, who made the solubility determinations during the early part of the work, and to J. B. Snider, now with the Bureau of Chemistry and Soils, who did the same during the latter portion and who, in addition, performed the greater part of the analytical work. LITERATURE CITED Brown and Dahlberg, IND. ENa. CEEX.. 21, 282 (1929). B m m . Sharp, snd Dshlberg, Ibid.. 20, 1230 (1928). Bmme. "Hsndbook of Sugar Analysis," P. 649, Wiley, 1922. Lippmann,"ChemiaderZuokers~teo,"3rded..Vol. 11. pp. 1151-7. Friedrich Viewel: and S o h . Braunschweieig. 1904. (5) Peine and Bsleh, IND. Exa. CHEM.. 17, 240 (1925). (6) Vyskocil, 2. Zudcerind. Eechoduoak. Rep., 52, 77-98 (1927-8). (I) (2) (3) (4)
Kncsrvlro Ssptember 12. 1932. Prassnted before the Diviaion of Sugar Chemistry at the 84th Meeting of the Amsriosn Chamical Society, DBOVQI. Coio., August 22 to 28, 1932.
