GB2206293A - Manufacture of sugar by crystallisation by cooling - Google Patents

Description

I" 2 2. 0 0251D 1 M&C FOLIO: 230P56567 MANUFACTURE OF SUGAR BY

CRYSTALLIZATION BY COOLING OF SUGAR SOLUTIONS WANGDOC: 10990 This invention relates to a sugar manufacturing process involving crystallisation by cooling.

In the manufacture of sugar the crystallisation stage is crucial to the production of sugar which is not undesirably discoloured. The more extreme the temperature, supersaturation and solution viscosity the more marked are the inclusions of discolouring matter and ash.

Known crystallisation processes do not yield in practice a sufficiently white sugar and although suggestions have been made concerning crystallisation by cooling (De Vries, Accinelli) no industrially viable procedure has yet been developed.

The present invention sets out to provide such a process.

The invention consists in a cooling crystallisation process for manufacturing sugar, comprising in sequence:

2 a) rapidly concentrating a starting syrup of BRIX value Bx between 65 and 75. and purity quotient Qz between 85 and 95 up to saturation at a temperature between 75 0 and 100 0 C; b) mixing crystallisation nuclei with the concentrated syrup; c) gradually cooling the syrup to promote growth of sugar crystals on the nuclei; and d) centrifuging off the crystals formed by cooling.

Vacuum heat evaporators are preferably used to concentrate the starting syrup, so that it remains only briefly at high temperatures, and hence discolours to a minimal extent.

The crystallisation nuclei may be present in a footing to which the concentrated syrup is added e.g. at a footing: syrup volume ratio of 1:8 to 1:12, preferably 1:9 to 1:11. Alternatively, the crystallisation nuclei can be added as a slurry of powdered sugar. Cooling preferably takes place at a temperature gradient of about 10 0 C (e.g. 50-15OC) per hour and for over about four hours (e.g. 2 to 6 hours), to terminate at 30 0- 40 0 C.

3 Most particularly, however, the invention provides a cooling crystallisation process for manufacturing sugar in which the sequence of stages defined above is repeated two or more times, and modified in that the starting syrup for second or subsequent repetitions is the mother liquor remaining after centrifuging of crystals from the next earlier such procedure.

Conveniently, the mother liquor from the final repetition of the four stages is subject to conventional crystallisation to produce sugar and molasses, and the sugar produced from such conventional crystallisation is incorporated into a starting syrup for one of the four-stage procedures.

The invention will be further described with reference to the accompanying drawings in which:- Figure 1 is a diagram of an experimental device by means of which the invention can be practiced on a laboratory scale; Figure 2 is a block diagram of a working process of the invention; Figure 3 is similar to Figure 2 but modified in operational features; and 4 Figure 4 is again similar to figure 2, but differently modified in operational features.

For the purpose of laboratory-scale tests, the simpl device shown in figure 1 was used.

e The abbreviations used below in the description have the following meanings:-

11Brixii is the dry-substance contents of the solution; lIQzl' is the quotient of purity, equal to the ratio of the sugar substance contained in the solution to the total dry substance (sugar and nonsugar) x 100; ,CS" is saturation coefficient of the impure solution as compared to the pure solution; "BETA" is the supersaturation coefficient; 11C.V.'I is the variability coefficient; 11M.A.11 is the mean opening.

z First crystallisation A first series of tests was carried out on portions of standard factory- produced syrup having Brix = 72 and Qz = 91. which was first concentrated beforehand up to Brix = 81 and kept at a temperature of 80 0 in storage flask A. In these conditions. considering the CS of 1.05, the syrup is saturated.

Part of the same standard factory syrup was concentrated in flask B until it reached Brix = 84 at a temperature of 80 0 C. Assuming a CS of 1.05. in these conditions the syrup has a supersaturation coefficient BETA = 1. 23. A small quantity of the seeding solution normally used for industrial production (one or two drops) was then added to crystallisation vessel B. Once the formation of the grains of sugar had been ascertained, slow cooling was started, with slow addition at the same time of syrup from A to B by means of a suction device. The ratio of the quantity of footing initially added to B to the feed syrup was 1:9.

Feeding was terminated after about 40 minutes and cooling after a further four hours. At the end of the crystallisation. the massecuite was centrifugated using a laboratory-scale basket-centrifuge. and the sugar was 6 carefully washed in order to remove completely the layer of adhering syrup. The syrup obtained had a Brix of 74, confirming the theoretical yield discussed below.

For comparison purposes, samples of sugar obtained by centrifugation of factory-produced massecuite made from the standard syrup were taken, and were washed in the same manner used for the samples of sugar obtained by cooling as above.

The comparision between the characteristics of average samples of both types of sugar is shown in Table 1. The difference between the two types of sugar in colour and grain size were noticeable by visual inspection.

Second Crystallisation The drained syrup obtained from this first crystallisation with Brix = 74 at 35 0 C and Qz = 86 still contained a considerable quantity of potentially crystallizable saccharose.

Further crystallisation could be achieved by bringing said drained syrup from the first crystallisation to saturation at a high temperature (for example 80 0 C), rapid evaporation and repeating the cooling crystallisation as described above. In order to effect this on a laboratory scale it was however necessary to 7 use mixtures of standard syrups and factory-produced green syrups so as to obtain a sample of syrup reproducing the syrup to be used in practice for the second cooling crystallisation.

Using the same device as shown in figure 1 and preparing the footing in exactly the same way as with the first crystallisation, the second cooling crystallisation was carried out to obtain a drained syrup having Brix = 76 and Qz of about 80. being effectively saturated at a temperature of 35 0 C.

Third Crystallisation In order to have a sufficiently exhausted drained syrup to proceed with traditional low-product boiling, it was first necessary to carry out a third crystallisation. repeating the process as above, that is to say saturating again by rapid evaporation the drained syrup at 80 0 C and carrying out another cooling crystallisation. In this case again, in order to produce a sufficient quantity of the solution to be crystallised, it was necessary to prepare the mixture using factory-produced syrups and molasses.

The third crystallisation was carried out using footing prepared in the same way and maintaining. as in the 8 previous two cases. the 1:9 ratio of footing/feed/syrup. This third stage of cooling crystallisation was carried out to obtain a syrup with a Brix of about 78.5 and a Qz of about 73. With this type of draining syrup it is then possible to proceed with the final exhaustion to molasses using a traditional type of boiling.

The analytical characteristics of the three types of sugar produced are shown together in Table 2.

The colour and the ash content of the sugar obtained from the second and especially from the third crystallisation are in fact inferior to those of the first crystallisation stage because the feed syrup mixtures were factory-produced draining syrups which had previously undergone boiling at a high temperature. if syrups obtained from earlier cooling crystallisations were available, the ash content and the colour especially of crystals originating from the third crystallisation would be considerably lower. In three-stage crystallisation, the deepening of the colour during the three stages is very limited since the syrups do not remain at high temperatures for very long, unlike the syrups in traditional boiling.

Comparison of the C.V. values of the three sugars with the C.V. of factory-produced raw sugar indicates the 9 uniform size of the crystals obtained. Also, such crystals (obtained by cooling) are especially shiny because they have not undergone any particular thermal shock.

on the basis of the results obtained in the laboratory tests described above, a three-stage cooling crystallisation process was set up, as shown in Figure 2. On examining the Brix and Qz values of the various syrups it is seen that the crystallisation yields obtained make it impossible to exhaust the various syrups to a very satisfactory degree.

The crystallisation yield in the three stages varies between 37 and 38%, thus providing very fluid massecuites to make centrifugation and washing easy.

The total yield in crystals is of about 73% of the initial saccharose. The distribution of the percentage of crystals between the three stages is 45.6% for the first stage, 32.3% for the second and 22.1% for the third.

In the diagram of Fig. 2. neither the use of the sugar obtained by crystallisation of the low product nor the refining drain-off deriving from the centrifugal washing of the crystals have been considered.

in the diagram of Figure 3 the refining of the first-product sugar, its dissolving and addition to the second syrup, and the dissolving of 8% of the crystals during centrifugal washing is taken into account. These differences entail a different quantity of sugar to be crystallised in the various stages. and this requires different temperature distributions for the cooling.

In the diagram of Figure 4, the low-product sugar is sent after refining to the last cooling crystallisation stage, again with a dissolving of 8% of the crystals during centrifugal washing. The temperature distribution among the various stages is different again in order to achieve the best possible degree of exhaustion.

A comprehensive examination of the three diagrams shows that depending on the type of solution chosen, it is possible to achieve the planned degree of exhaustion in the three crystallisation stages based on temperature distribution.

Table 3 shows a smaller quantity of sugar dissolved during centrifugation (5%), a Qz of the thick juice of 90 instead of 91. and a hypothetical crystallisation diagram in four stages rather than three.

11 In order to evaluate the whole cooling crystallisation process from the point of view of energy consumption and plant engineering, the arrangement of Figure 3 was compared with the arrangement of a refinery and with that of a free-boiling white sugar process. The data obtained. and set forth in Table 4 shows that the energy balance is clearly in favour of cooling crystallisation as compared to a refinery cycle (energy consumption is more than halved). The comparison with a free-boiling white-sugar arrangement shows comparable energy consumption levels, but the sugar produced is of a much poorer quality from tree-boiling. The total evaporating surface area is about four times smaller for cooling crystallisation than for the traditional method. The volume of the crystallisation vessels required for cooling crystallisation is about 1.5 times that for traditional crystallisation.

The advantages of the invention specifically described are (a) as to starting material; thick juice can be made directly into commercial grade granulated sugar (b) as to procedures; the crystallisation times need not exceed four hours, the temperature drop is variable in a range between say 80 0 and 35 0 to accomodate 12 alterations in feed and procedures. the energy requirements are less, and the evaporating surface is greatly reduced with a manageable increase in crystallisation volume (c) As to product handling; the sugar is easily centrifugable and washable because of the fluidity of the mother liquor and the syrups do not discolour to the same extent as in the traditional processes (d) as to the final product, the sugar is white. shiny and of good M.A. and C.V. values.

2206293D T A B L E.1 First-product 1st cooling factory sugar crystallization Colour in solution (MEC points) 19.3 5.3 Ash (MEC points) 27.8 8.3 Farbtype (MEC points) 11 4 Total MEC poipts 58.1 17.3 M.A. 0.57 0.61 C.V. 40 27 Comparison between characteristics of sugar obtained by cooling (Ist stage) and first-product factory manufactured sugar.

T A B L E 2 '22' 0 (':) 2 9 3 p Ist cooling 2nd cooling 3rd cooling crystalliz. crystalliz, crystalliz.

Colour in solu- tion (MEC points) 5.3 5.5 -10.5 Ash (MEC points) 8.3 8.9.17.7 Farbtype (MEC points) 4 4 8 Total MEC points 17.3 18.4 36.2 M.A. 0.61 0.56 0.56 C.V. 27 27 29 Analytical characteristics of the three types of sugar produced on a laboratory scale in the three cooling crystallization stages.

Artificial draining syrup obtained by mixing thick juice and factoryproduced greeri syrup.

Artificial draining syrup obtained by mixing thick juice with factory produced molasses.

1 1 1 T A B L E 3 1 ul 1 Th i ck Temperature in degrees C 0z 1st crystal. 2nd crystal. 3rd cry;,al. 4th crystal.

beg. end beg. end beg. end beg. end Reffielting of raw su gar to thick juice 91 80 35 80 35 80 35 Refining of raw su gar, last stage, sol. 5% xx to white 91 80 35 80 35 10.1 35 As above but with sol. 8% XX 91 85 35 85 35 96 35 - - As above but in 4 stages 91 80 40 80 40 80 40 81 35 1 91 80 40 80 40 80 40 92 40 Refining of raw su gar, remelting to thick juice, sol.

5% xx to white 91 80 35 80 35 87 35 As above but with sol. 8% xx 91 85 35 85 35 88 35 - - As above but in 4 stages 9.1 80 40 80 40 80 40 80 5.1 As above but in 3 stages, 0z d. 90 90 85 35 85 35 72 35 -- - 85 35 85 35 85 52 Various possibilities for implementing cooling crystallization.

T A B L E 4 2 61 t.") 2r- 9 3 Crystalli- Traditional Traditional zation by refinery free-boil.

cooling cycle crys- white-sugar tallization crystalliz.

Total evaporating surface (sq.m.) 700 2700 2600 Crystallizing ves sels requ.(cu.m. 450 300 250 No. centrifuges needed (1000 kg each 10 12 10 Steam consumed (in t/d) for:

a) Water to be evaporated 508.9 1408.7 60.1.1 b) Heating 113.5 - Comparison between cooling crystallization, a refinery cycle and a freeboiling white-sugar process. Production of 1000 t/d sugar.

Assuming that the draining syrups are heated by steaffl only; they could be heated at least partially using hot recirculation water.

2' rI, 0, -,./- o 2 9 IST 17

Claims (11)

1. A cooling crystallisation process for manufacturing sugar, comprising in sequence:

(a) rapidly concentrating a starting syrup of Brix value Bx between 65 and 75, and purity quotient Qz between 85 and 95 up to saturation at a temperature between 75 0 and 100 0 C; (b) mixing crystallisation nuclei with the concentrated syrup; (c) gradually cooling the syrup to promote growth of sugar crystals on the nuclei; and (d) centrifuging off the crystals formed by cooling.

2. A process as claimed in Claim 1 in which vacuum heat evaporators are used to concentrate the starting syrup.

3. A process as claimed in Claim 1 or 2 in which the crystallisation nuclei are present in a footing to which the concentrated syrup is added.

is

4. A process as claimed in Claim 3 in which the volume ratio of the footing to the syrup is between 1:8 and 1:12.

5. A process as claimed in Claim 4 in which the said ratio is between 1:9 and 1:11.

6. A process as claimed in Claim 1 or 2 in which the crystallisation nuclei are added as a slurry of sugar powder.

7. A process as claimed in any one proceding claim in which cooling takes place at a temperature gradient of about 100C per hour and is terminated when the temperature of the supersaturated solution achieved is 0 0 C.

8. A process as claimed in any one preceding claim in which cooling takes place over about four hours and is terminated when the temperature of the supersaturated solution achieved is 30 0- 40 0 C.

9. A process as claimed in Claim 1 and substantially as herein described.

19

10. A cooling crystallisation process for manufacturing sugar in which the sequence of stages as claimed in any one preceding claim is repeated two or more times, and modified in that the starting syrup for second or subsequent repetitions is the mother liquor remaining after centrifuging of crystals from the next earlier such procedure.

11. A cooling crystallisation process as claimed in Claim 10 in which the mother liquor from the final repetition of the four stages is subject to conventional crystallisation to produce sugar and molasses, and in which the sugar produced from such conventional crystallisation is incorporated into a starting syrup for one of the four-stage procedures.

Published 19B93 at The Patent Office. State House. 6671 High Ho'born. London WC1R 4TP- Further copies may be obtained from The Patent Office, Sales Braneb. St Ma:'Y Cray. Orpingon. Aen: BR5 25RD Printed by Multiplex tecb--1qis ltd. St Mary Cray. Kent Con. 1 87