MODERNBEET SUGARFACTORY OF SPRECKELS SUGARCOMPANY AT WOODLAND, CALIF.
Low-Purity Beet Sugar Factory Materials Rates of Crystallization in Crystallizer Fillmass R. A. McGINNIS, SOMERS IMOORE, JR., AND P. W. ALSTON Spreckels Sugar Company, Woodland, Calif.
I
Data on rates of crystallization at various crystallizer fillmass are desirN T H E manufacture of beet sugar, the extracted, defetemperatures and supersaturations are able to determine optimum timecated, and 'Oncentrated desirable for control of the rapid crystalable temperature work has curves. been published Considerbeet juices are normally subjected to three vacuum pan lizers now coming into use for the on rates of crystallization of crystallizations. The third cryslization of low raw, beet sugar fillmass. sucrose, but the great bulk has tallization is carried out with These rates have been determined for a dealt with solutions of pure difficulty, the rates of crystallifillmass of typical composition^ The resucrose. Rates are so strongly being retarded sults show a rather critical optimum superas a result of the high conphysical affected by conditions impurities of crystalliand the centration of nonsugars, These saturation (1.5) at low temperatures. zation that these data are of rates are so lev-that it is little value in practical operanot practical to carry crystallization to completion in the tion. An outstanding exception is the work of Nees and Hungerford (9) who determined rates for the purity range vacuum pans. After pan boiling, common practice is to cool the fillmass (mass of crystals and mother liquor) in waterencountered in low raw (third) pan boiling. The present injacketed crystallizers. Crystallbation for several days is vestigation reports rates for a typical fillmass in the green frequently involved in the usual type of crystallizer, and the sirup (mother liquor) purity range encountered in the crystallizers. use of crystallizers employing more rapid cooling has become common in recent years. Most of them have cooling surfaces General Method inside, and are typified by the Blanchard, Lafeuille, and Werkspoor apparatus. Two Werkspoor crystallizers are used Rates of crystallization are dependent on (a) the nature and amount of nonsugars, (b) the total area of crystal surface, a t the Woodland factory. The Woodland factory includes a ( c ) the viscosity of the green sirup, (d] the amount of stirring, Steffen unit. In the newer types the rate of fillmass cooling is capable of ( e ) the supersaturation of the green sirup, and (f) the temperacontrol, and data on effective rates of crystallization in ture. Since the first four factors are fixed when the fillmass 171
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 34, No. 2
WERKSPOOR CRYSTALLIZER
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resentative type of nonsugars. This molasses had the following composition: Apparent purity coefficient Refraotbmeter dry substance Per cent invert Per cent raffinose
60.0 85.5 1.2 1.0
Water and sugar were added t o the molasses, and a green sirup of the required composition boiled up in the small vacuum pan. For crystals, granulated sugar of reasonably constant size distribution was used.
enters the crystallizer, it was desired to find the rates for varying supersaturation and temperature. I n the test work the first four variables were fixed a t what were considered characteristic values. A pilot crystallizer closely resembling that of Xees and Hungerford was used. The temperature of the fillmass was positively controlled through the temperature of the water circulated through the jacket. A recording thermometer bulb was immersed in the fillmass; its accuracy was 1 0 . 5 " C. The crystallizer rotor was normally turned a t 0.33 r. p. m., but was speeded up to 60 r. p. m. for mixing materials. A small vacuum pan was used in preparing the green sirups, The standard fillmass had the following analysis: Refractometer dry substance True dry substance Apparent purity coefficient True purity
92.7 90.3 76.7 79.0
The a m a s s in practice is dropped from the vacuum pans into open mixers where it remains for 2 hours, entering the crystallizers in the following condition: Green sirup Ap arent purity coefficient Regactometer dry substance True purity True dry substance Temperature, C. Per cent crystals
62.5 88.5 64.5 86.0 64 35.0 0.E
I n order to have a uniform material for all tests, a synthetic fillmass was used. A green sirup of the above analysis was made up by enriching an isolated stock of molasses considered to have a rep-
0.4 SUPERBATURATION OF GREEN 51 RUP
FOR STANDARD Low RAWFILLMASS AT VARIOUS FIGURE 1. RATESOF CRYSTALLIZATION SUPERSATURATIONS AND TEMPERATURES
INDUSTRIAL AND ENGINEERING CHEMISTRY
February, 1942
Since the prepared sirup was often stored at room temperature for several days before use, it was necessary to dissolve any spontaneously formed crystals before mixing. This was done by weighing the sirup into the crystallizer and holding i t at 80" C. for 8 hours. The refractometer dry substance was then adjusted, the temperature dropped to 68", and the proper amount of sugar crystals added and mixed thoroughly and quickly in less than 5 minutes. The fillmass temperature was then 64". During the test period the operator kept the green sirup a t the desired constant supersaturation by temperature control. This was made possible by green sirup analyses run at 20-minute intervals. The green sirup was separated from the crystals by spinning in a small centrifugal basket lined with standard centrifugal screen. After a little experience it was found possible to keep the supersaturation constant to * 0.05. Since previous tests had shown the solubility tables of Grut (1) to be valid for this material, they were used in calculating supersaturations. Of the many methods of calculating supersaturation coefficients, the following is most common and was used in this work: supersaturation
=
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An examination of the graphed surface shows that rates of crystallization are fastest a t the high temperatures. As crystallization proceeds, however, it is necessary to go to high supersaturations to maintain high rates. The data show clearly that a t the lower temperatures the supersaturation for optimum rate is near 1.49. At higher supersaturations the rates drop rapidly, probably as a result of the increased viscosity of the green sirup and the formation of new nuclei, or false grain. This fine grain readily passes through the centrifugal basket screen, increasing the purity of the molasses and decreasing the effective crystallization. This fine grain also tends to clog the screen openings and the voids in the sugar crystal layer, making good purging impossible.
sugar/water (sugar/water) a t satn., at given temp.
A series of twenty-five 14-hour crystallization runs were made, each at a constant supersaturation; analytical data were taken on the green sirups a t frequent intervals. The percentage of crystals present a t these times was calculated from the analyses. By plotting the percentages against time, a series of curves were obtained which were fitted with mathematical equations (generally parabolic). The first derivatives of these equations gave the rates of crystallization a t any time and corresponding temperature. The rates were corrected to constant crystal surface in a manner similar to that of Nees and Hungerford. The final rates obtained were in units of grams of sugar crystallized per hour per square meter of crystal surface.
F I Q U R2. ~ TIME-TEMPERATUR~ CURVEFOR STANDARD Low RAWFILLMASS IN WERKSPOOR WITH SUPERSATURATION AT THE CRYSTALLIZER, OPTIMUM POINT THROUGHOUT
Further tests of the same nature on fillmasses with refractometer dry substance lower than 92.7 showed that here the rates of crystallization are slower at comparable supersaturations, and that to obtain high total crystallization, impracInterpretation of Results tically low temperatures are required. A refractometer dry substance appreciably higher than 92.7 results in a fillmass The results are given in Figure 1 and Table I. Each conthat is difficult to handle on account of its high apparent visstant supersaturation curve is the average of three to five cosity. I n the practical application of these results it is imruns. Since the refractometer dry substance and apparent possible to regulate the cooling so that the optimum superpurity coefficient of the fillmass are set, and since the solusaturation is maintained a t all times. Owing to changes in bility tables of Grut were found valid in this case, the complete amount and nature of nonsugars, the solubility of sucrose composition of the fillmass is defined for any point on the and consequently the supersaturations deviate frequently graphed surface; this includes the purity and refractometer and sometimes widely. From the results of test work with a dry substance of the green sirup and the percentage crystals, Saturascope, however, the arrangement of cooling elements, if the reasonable assumption is made that all of the nonsugars cooling water flow, etc., may be set so that for average maand water are in the sirup phase. terials the supersaturation will be close to and on the safe side of the optimum. Figure 2 shows a time-temperature TABLEI. RATEOF CRYSTALLIZATION FOR STANDARD Low RAW curve for the standard fillmass which maintains the superAT VARIOUSSUPERSATURATIONS AND TEMPERATURES saturation of the green sirup a t the optimum point. The FILLMASS work showed that temperatures may be advantageously Crystallization Rate (G.,/Sq. M./Hr.) at BupersatuTemperaration of: dropped as low as 32', with rates of crystallization still fast ture, ' C. 1.22 1.28 1.39 1.49 1.51 1.59 enough to be appreciable for short-term crystallization. At 65 2.10 ... ... ... ... ... Woodland it has been found that a 13.5-hour period crystal64 1.50 ... ... ... ... ... . .. 63 2.35 ... ... ... ... lizes practically all the readily available sugar. Following the 62 i.ii 2.15 ... ... ... ... crystallizing period, reheating is, in general, required to make 1.02 1.82 60 ... 2.49 ... ... 2.46 0.88 1.53 58 ... ... ... effkcient purging possible a t the centrifugal station. No sugar 1.34 56 ... 2.43 .76 ... ... 54 2.01 2.40 .65 1.19 ... is dissolved, provided the reheating is slow ( 5 hours a t Wood... 1.66 2.38 .54 1.04 52 ... ... land) and the saturation temperature of the green sirup is not .42 50 2.35 1.49 2.35 0.89 ... 48 ... .73 1.36 2.32 1.61 1.93 exceeded. 46 .58 ... 1.24 1.57 2.29 1.32 44 42 40 38 36
34 88
... ... ... ... ... ...
...
.44 ... ... ... ... ... ...
1.13 1.04 0.97 * 89 .83
... ...
2.26 2.24 2.21 2.18 2.14
2.11 2.09
1.29 1.09 0.90 .76 .64 .55
...
1.06 0.84 .66 * 53 .43
... ...
Literature Cited (1) Grut, E. W., Quoted in "Six Years Experience with Werkspoor Rapid Crystallizers", Amsterdam, Werkspoor N. V., 1939. (2) Nees, A. R., and Hungerford, E. H., IND.ENG. C H ~ M .28, ,
893-7 (1936).
