National Sugar Institute, Kanpur, India

Transcription

1 ANIL. C. RAHA I effect on rate of crystallization. However, as you go up beyond these concentrations, you begin to produce some retardation of crystallization due, we believe, simply to interference of inclusion on the surface of the crystals. However, at these concentrations, the effect is very small. MR. WALKE~: I think that actually our data would show that they were trending in the direction which you just mentioned. Even at the highest levels that we have measured, they are dropping off shghtly, but we don't Bnow enough about the precision of the results to see where the significant difference in delta-v is., FORMATION OF FALSE GRAINS IN THE CRYSTALLISATION PROCESS OF SUGAR AND ITS EFFECT ANIL. C. RAHA National Sugar Institute, Kanpur, India The crystallisation process in the sugar industry consists essentially of two stages, the first of which is the formation of small particles or nuclei. Since crystallisation is a batch process, care is taken to mabe sure that once the nuclei have been formed and the growth period has started, no further nucleation is induced. If fresh nucleation occurs at a later stage, the original crystals can not grow to the desired size. The fine crystals formed at a considerably later stage than the original crystals, are known as false grains. Since false grains can cause serious trouble in the operation of the sugar factory, information concerning the factors which influence their formation is of considerable importance. The prevailing rate of crystal growth in a massecuite, the surface area available for deposition, and the degree of supersaturation in the mother liquor, are the three main factors which determine whether false grains would appear in a massecuite at a particular point. If these factors can be controlled, the formation of false grains can be prevented. The degree of supersaturation in a mother liquor, due to the presence of many non-sugars in the factory syrup, is an unknown factor; the rate of crystal growth, which depends on so many unknown factors, is also unknown, and though the surface area provided at the start of the boiling for deposition of sugar is under the control of the operator, there is no simple and direct method by which the rate of deposition of sugar can be determined for the conditions existing in the pans and crystallisers. So the actual stage at which the false grain would appear in a massecuite cannot be predicted precisely. Theoretical considerations and experimental results obtained from the studies confined to solution of pure sucrose, cannot be applied. GEERLIGS~ and SJILMAN~ have studied the influence of various non-sugars and reducing sugars on the solubility of sucrose in water and its ability to crystallise, but these observations were made on simple systems with 2 or 3 components and hence cannot be directly applied to cane molasses which is a complex of many components. Presence of certain substances even in very small amounts, e.g. of amino acids such as &spartic acid and tyrosine3, can exercise such a controlling influence on the crystallisation of sucrose, particularly when they are present as fifth or sixth components in a complex system, that the results obtained on a simple system of 3 or 4 components will not be of much value in controlling a multi-component system. So it was thought that if the rate of crystal growth in an actual factory syrup and in molasses were measured from time to time, it might be possible to obtain relevant information on the conditions which promote formation of false grains in a massecuite.

2 278 FACTORY The work was carried out under actual factory conditions, and in spite of all the care talcen in the mechanical manipulation, it left much to be desired. sarr mo. INVESTIGATIONAL PROCEDURE After a drink was taken in the pan and steady boiling conditions were attained in a commercial sugar pan, samples (weighing about 50 g) of massecuites were withdrawn at five ~llinutes intervals and the sugar crystals were washed with alcohol, dried and weighed on a sensitive balance. The weight of the sample obtained was divided by the number of crystals present. This gave the average weight of the crystals in the sample and it was assumed that this average weight was the average ow (F: co: gr th weight of the crystals in the pan at the moment when the sample 'was withdrawn. This was repeated 3 times for each pan under investigation, at the early part of boiling, about midway, and again near the end of boiling. During each trial, boiling was continued until false grains were found. The false grains were then dissolved by adding syrup and the boiling was continued for the subsequent trial. The work was carried on throughout one whole season lasting from the first week of December until the last week of May. RESULTS The results obtained are shown in Figs. I, 2 and 3. The average weight of the crystals so obtained was plotted against the time interval. In case of C massecuites, the direct measurement of the weight of crystals was not possible; so the purity of the s,

3 ANIL. C. RAHA sample withdrawn was larger than in the cases of A & B massecuites. Brixs of the molasses of the,sainples of massecuite were also determined by refractometer. DISCUSSION In case of A massecuites (Fig. Ia and ~ b) the crystals growth continued throughout the boiling until the false grains appeared, whereas in the case of C massecuites (Fig. 3b), if the fall in purity of the mother liquor can be taben as an indication of the continuance'of crystal growth, crystals cease growing some time before the false grains appear. This cessation of crystal growth occurs only during the latter part of the boiling when two or three drinks of B heavy molasses have been added to the pan TIME INTERVAL IN MINUTES TIME INTERVAL IN MINUTES Fig. 2. (a) growth of crystal in B massecuite during early part of boiling (Purity of Mother liquor 72-70). (b) growth of crystal in B massecuite during latter part of boiling (Punty of Mbther liquor 57-55). and the purity of the mother liquor has fallen below 50. In the early part of the boiling (Fig. ga) there is a regular drop in purity with time in mother liquor. In certain cases it has been observed that the rate of purity drop decreases when the C light molasses is added to the pan. In a few cases of B massecuite (Purity 70-72) a similar decrease in crystal growth is marked near the striking point of the boiling (Fig, zb). This indicates that with the increase in concentration of non-sugars, there is a definite depression in crystal growth. It is probable that some of the non-sugars which are capillaryactive are adsorbed on the surface of the crystal and thus inhibit the crystal growth. As the crystal ceases to grow while the boiling is continued, the supersaturation increases to the limit of the labile zone and the false grains appear. Sometimes under these circumstances, false grains do not appear in the pan but come out i11 the crystalliser where supersaturation increases further due to cooling. In the cases where false grains appear in the massecuite in spite of steady crystal growth (e.g. in A massecuite and the early part of B & C massecuites) surface area and factors such as circulation and viscosity are the deciding factors rather than the crystal growth. It has been found that when the purity of the mother liquor in cases of C massecuites is not falling during the boiling, a drink of hot water causes the purity of the mother liquor to fall again for some time. 1 The addition of hot water dilutes the mother liquor, decreases its viscosity and

4 280 FACTORY increases the circulation of the massecuite. Consequently, there is more relative velocity between the crystal and mother liquor, and more rubbing effect between the crystal faces. All these decrease the effective thickness,of the diffusion layer and allow the crystal to grow a little faster. Once the thickness of the diffusion layer decreases, the absorbed layer returns to the bull< of the diluted mother liquor and the crystals can grow again. It is difficult under factory conditions to isolate and measure the individual effects. As an indirect proof of the adsorption of capillary-active substances on the surface of the crystal, a series of experiments in boiling of C massecuites with B heavy and C light molasses of low surface tension, showed that the crystal growth ceases in mother liquors of lower consistency than when it is boiled with molasses of higher surface tension. It may be recalled that the presence of capillary-active TIME INTERVAL IN MINUTES TIME INTERVAL IN MINUTES Fig. 3 (a) Drop in Purity of Nother liquor in C massecuite during early part of boiling (Purity of Mother liquor (70-66). Drop in Purity of Mother liquor in C massecuite during latter part. of boiling (Purity of Mother liquor (48-45). substances decrease the surface tension of a liquor. It is yet to be determined what the nature of these capillary-active substances is and if any critical value of surface tension can be prescribed for boiling molasses and syrups without producing false grains. PRACTICAL EFFECTS The effect of false grains depends upon the nature of the massecuite and the stage at which the false grains are formed. Purging difficulties, fall in purity of the product, and loss of sugar in the final.molasses are consequences. Pu~ging dificulties The rate of flow of mother liquor through the capillary tubes of the sugar wall in a centrifugal machine streamlined and can be expressed by POISEUILLE's~ equation where D is the diameter of the capillary tube P is the pressure drop through the tube,u is the viscosity L is the length of the capillary tube.

5 ANIL. C. RAHA If the crystals are not of uniform size, the voids between them become less, the ameter of the capillaries on the sugar wall decreases making the path of the fluid ore tortuous;'hence the flow rate of the mother liquor through the sugar wall deeases. Consequently, more purging time is required. The time cycles of various massec~ite~ were noted and it was found that the curing times for all massecuites containing false grains were higher than for those without false grains. Diameter of the capillary being 4th power, its effect on the curing time is very pronounced. In cases of "C" massecuites where the viscosity of the mother liquor is much higher - than that of A & B massecuites, the effect of this irregular grain size will be felt more. From Poiseuille's equation, the viscosity is in the denominator; so with materials of higher viscosities, it is still more essential that the sizes should be very uniform. Fall in. Purities Although a crystal itself is primarily pure, it retains less pure mother liquor when removed from the magma, and the adhering mother liquor will carry its share of the impurities present in it. If the crystals contain false grains, and there are purging difficulties, the amount of mother liquor retained by the crystals will be higher than it would be if the crystals were free from false grains. The usual practice in some sugar factories is, of course, double purging, and the sugar is also washed in the centrifugals with hot water. Nevertheless the retained mother liquor in the first purging will lower the purity of the magma and the effect, though minimised, will still continue, and this increases the circulation of molasses. It should also be remembered that only crystals of uniform size offer satisfactory washability. The other bad effects of non-uniform crystals are high steam consumption in the centrifugal machines and high power consumption for driving them. Non-uniform crystals tend to hold more moisture, and have a tendency to cake and poor keeping quality. I Loss of S~gav Besides creating purging difficulties if these false grains are too small to be retained by the net of the centrifugal machines, a percentage of the false grains will pass out of the machines with the mother liquor and in the case of final massecuite, cause loss of sugar. With A & B massecuites where the respective molasses is returned to the process, no such loss will occur, but the crystals will settle down in the molasses tank and will require frequent cleaning. In the case of final boiling, if the crystals and mother liquor are not brought near the equilibrium saturation point corresponding to the lowest possible vacnum in the pan, by addition of water, there is danger of false grain forming in the crystallizer due to cooling when the massecuite is dropped. If the false grains formed at this stage are many, they might not grow sufficiently to be easily detected by the naked eye, and will flow out with the molasses without causing much purging difficulty, but causing ahigh loss of sugar in the molasses without arousing any suspicion regarding the cause of it. REFERENCES GEERLIG, P. H. C., Cane Sugav and its nzanufacture, p Norman Rodgers, London. HONIG, P., Princzples of Sugar Technology, p Elsevier, Amsterdam. KELLY, F. H. C., Phase rule considerations in the crystallisation of sucrose. PYOC. Queensland Soc. Sugar Cane Technologists., 22 : 265. SJILMAN, C., Practical exhaustibility of Java molasses. Intern. Sugar J., 36 : 437.

6 282 FACTORY ACKNOWLEDGEMENTS The author wishes to thank MR. S. N. GUNDU RAO Director, National Sugar Institute for his interest in this work and DR. N. A< RAMAIAH for his valuable suggestions. ZATION, L is a wellconditions In that the is regarded mtly result nges could (position of j inclusions ken crystal- ]articles or tlp and inwell as in- the relative tions in the rrelocities of eristic final 7H20) lead dnsists of a )ure sucrose 9, and [III]. ~m solutions mpurities. ' lhder which ystal. These,f crystal or' urn into the [thors4. se in superhe difference 1 covering of n there isla 1 to the sur-