FR2647812A1 - METHOD FOR IMPROVING THE SUGAR EXTRACTION RATE FROM A THIRD JET SYRUP, BY REDUCING THE END OF CRYSTALLIZATION TEMPERATURE - Google Patents

Abstract

The process is an improvement of the so-called pre-turbining process where a massecuite 1 supersaturated with sugar is seeded with crystallization seeds, at the end of desursaturation, part of the cooked mass is removed, the sugar is extracted therefrom by centrifugation 20 and the particles are returned. mother liquors to a mixer 4 to join them to the second part 41 of cooked mass, before leaving it to cool in a mixer 5. Until now, the mother liquors separated from the first part of cooked mass were diluted by the sugar rinses and the greater part of the gain obtained by prolonging the mixing was thus lost to a temperature below 50 degree (s) C. According to the invention, the mother liquors are sent to a buffer tank 30, from where they are taken to be sent to a temperature exchanger 31, then sprayed into a chamber 32 under vacuum, to be concentrated and to find the temperature and concentration. at the crystallizer outlet 1.

Description

"Process for improving the sugar extraction rate from a syrup

  third jet, by reducing the temperature at the end of crystallization "The invention relates to a process for increasing the yield of sugar extraction from a syrup of third jet, by reducing the temperature at the end of crystallization before elimination of molasses by centrifugation, according to which the syrup brought to supersaturation is seeded in crystallization seeds at a temperature at the start of crystallization in order to obtain a so-called mass-cooked mixture composed of sugar crystals in a mother liquor in flight with desaturation, this mass is divided into two parts cooked to take a first part where the mother water is separated by centrifugation from the sugar crystals which are rinsed with water, the rinsing being joined to the mother water which it dilutes, the mixture is heated diluted mother liquor and it is joined to the second part of the cooked mass, the crystal content of which is reduced, and this second part of cooked mass with reduced crystal content in the m is allowed to cool alaxating to a final crystallization temperature, after which the crystallized sugar is separated

  molasses by conventional centrifugation.

  The classic operation, as it is constantly practiced on first and second jet syrups, and frequently on third jet syrups, consists in sowing the syrup supersaturated with crystalline germs, to allow the cooked mass thus cooled to cool. kneading it to a temperature at the end of crystallization, and separating by centrifugation the mother liquors which, for the syrups of first and second jets, will form, suitably reheated and concentrated, syrups of second and third Jets, and which, for the third spray syrup, constitute the molasses, in which the solutes, mineral salts and organic compounds inhibit the

crystallization of the remaining sugar.

  The temperature at the end of crystallization, and therefore the sugar content of the mother liquors, is governed by the macroscopic viscosity of the mixture of sugar crystals / mother liquors. If the end of crystallization temperature is too low, the mixing energy becomes excessive and compromises the effectiveness of this mixing, which is essential for the crystals to be able to develop in contact with the mother liquors. While the end of crystallization temperature can be relatively high for the first and second jet cooked masses without major drawbacks, the sugar remaining in the mother liquors being recovered the following cooked, it would be advantageous to operate with an end temperature of crystallization as low as possible on the syrups of third jet, the sugar entrained by molasses being practically not recoverable economically under

form of sucrose.

  So we tried, during the crystallization of the baked mass, to extract a part of the crystallized sugar before proceeding to cooling during mixing, to avoid overloading the mixer and rendering the mixing ineffective;

  the process has been defined above.

  However, the attempts made in this direction have proved disappointing, since the mother liquors separated from the first part of the cooked mass, and diluted by the rinses

  subsequent, are in a state of undersaturation in sugar.

  By adding them to the second part of the baked mass, partial sugar is already dissolved in this second part. The gain in yield obtained by lowering the end-of-crystallization temperature is offset by the dilution of the mother liquors, and the overall gain allowed by the process, commonly known as pre-turbination, is

low.

  In addition, centrifugal spinning causes the formation of an air emulsion in the mother liquors, all the more so as the impurities contained in the mother liquors, close to molasses, have surface-active properties which favor the

foaming.

  Conventionally, the mother liquors are demulsified and reheated by injecting steam; vapor condensation and

  the rise in temperature combine to accentuate the

  relative saturation, i.e. the ratio between the actual sugar content and the saturation content at the syrup temperature. We tried to concentrate the mother liquors from the first spin to bring them to the concentration and temperature of the liquid phase of the second part of the baked mass before joining them. This concentration operation must not lead to appreciable losses of sucrose, that is to say must be rapid and carried out at too high a temperature; it is therefore necessary to operate by boiling under reduced pressure. The tests carried out in a vacuum chamber equipped with an exchanger bundle supplied with steam caused such foaming

  that the boiling could not be started.

  To overcome these difficulties, the inventlon proposes a method for increasing the sugar extraction yield from a third-jet syrup, by reducing the temperature at the end of crystallization before elimination of the molasses by centrifugation according to which seeds of crystallization seeds are sown. the syrup brought to supersaturation at a temperature of start of crystallization to obtain a so-called massed mixture composed of sugar crystals in a mother liquor in flight of desesaturation, this mass is divided into two parts to take a first part where the the mother water is separated by centrifugation from the sugar crystals which are rinsed with water, the rinsing being joined to the mother water which it dilutes, the diluted mother water is heated and it is joined to the second part of baked mass, whose crystal content is reduced, and this second part of baked mass with reduced crystal content is allowed to cool by kneading it to a final temperature of cr lstallisatlon, after which the crystallized sugar is separated from the molasses by conventional centrifugation, a process characterized in that, after reheating the diluted mother liquor, it is concentrated, by spraying it into a chamber in a maintained vacuum while compensating for the thermal evaporation losses, so as to at least substantially re-establish the temperature and concentration of the mother water in the starting cooked mass,

  before joining it to the second part of the cooked mass.

  Thus, the heating of the mother liquors is carried out under a pressure high enough to be relatively far from the boiling, so that the foaming in contact with the heat exchanger is not annoying. And spraying under vacuum leads to partial evaporation and almost instant deaeration of the mother liquors. As a corollary, the mother water mass during evaporation and deaeration is very small; foaming

is weak and has no drawbacks.

  Preferably, the mother liquor is overheated relative to the temperature at the start of crystallization before being sprayed into the vacuum enclosure, to compensate for the cooling due to evaporation. This superheat temperature is suitably 80 to 12 "C higher than that

at the start of crystallization.

  It is also appropriate that the absolute pressure in the vacuum enclosure is between 20 and kPa (0.2 to 0.3 bar) corresponding to a temperature

  of boiling water in a range centered on 65 "C.

  In preferred arrangement, the diluted mother liquor is heated and concentrated in a closed cycle, by drawing from this cycle the mother water at substantially the same temperature and concentration as the mother water in the cult starting mass, while the 'we compensate for the withdrawal by the addition of diluted centrifuged mother liquor from the first part of cult mass. There is thus a buffer of mother liquor in concentration and

  reheating, in particular to mitigate the surges of operation.

  Additional features and benefits

  inventlon will also emerge from the description which

  will follow, by way of example, with reference to the appended drawings in which: FIG. 1 represents the diagram for extracting sugar from a syrup of third jet, with a pre-turbination according to the invention

  Figure 2 is a diagram of the heating stage-

  concentration of mother liquors from pre-turbines.

  According to the mode of execution of the treatment of third-jet syrups represented in FIG. 1, the installation comprises a stage 1 of cooked, where the third-jet syrup is concentrated to be in a state of supersaturation for the sucrose, then seeded with germs. crystallization, in order to crystallize the excess dissolved sucrose on saturation. The mixture of crystals in the process of magnification and mother liquors in the process of desaturation is extracted by a pump 10 and returned to a line 11 which serves on the one hand a pre-purging stage 2 and on the other hand a crystallizer or kneader 5 through a mixer 4. In the mixer 5, the cooked mass is stirred while its temperature drops to a temperature at the end of crystallization, where the cooked mass is a mixture of sugar crystals and mother liquors unsaturated in sucrose at the end of crystallization temperature, and highly charged with various impurities, mineral salts and organic impurities, generally called molasses. At this stage the cult mass is transferred by a pump 50 to a turblnage stage 6, where the sugar is separated from the molasses in a centrifuge 60, the rinsed sugar leaving at 61, while the molasses and

  rinses are eliminated in 62 as a by-product.

  On the pre-turbining stage 2, a first part of the cooked mass is poured into a centrifuge 20 (called, in terms of profession, turbine), where the sugar is separated from the mother liquors which leave at 22, then rinsed, the rinses also being sent to exit 22, while the sugar

  crystallized is collected at exit 21.

  The mother liquors and rinses leaving the centrifuge

  via exit 22 cross a concentration floor

  reheating 3 to be sent to the input 40 of the mixer 4, and to dilute the second part of cult mass arriving

  directly from line 11 on entrance 41.

  Thus the baked mass arriving at the mixer 5 through the inlet 42 contains approximately the entire volume of mother liquor sent to the line 11, but an amount of crystallized sugar reduced by what has been removed in the centrifuge 20 and recovered at the exit 21. We have commented, in the description of the state of the art, the advantage of bringing to the end of crystallization in the mixer 5, a cult mass depleted in crystallized sugar, in order to be able to lower the end of crystallization temperature without overloading the mixer and without making it difficult to separate the molasses in the

centrifuge 60.

  We have also commented, in the description of the state of the art, the difficulties encountered in re-injecting, at the inlet 40 of the mixer 4, mother liquors sufficiently concentrated in sucrose so as not to lose again, by diluting the molasses, the more of the sucrose than could be expected from the lowering of the end-of-crystallization temperature, and the concomitant lowering of the

sucrose saturation content.

  By explaining the means of the invention, it has been brought out that the process of concentration and reheating carried out in stage 3 made it possible to bring to the inlet 40 of the mixer 4 mother liquors whose composition and temperature corresponded very substantially to those of the mother liquors before their separation of the sugar in the centrifuge 20, whereby it was possible to recover all the sugar capable of crystallizing thanks to the lowering

  the end of crystallization temperature.

  As best seen in FIG. 2, the reheating and concentration circuit essentially comprises a sealed buffer tank 30, a temperature exchanger 31 with a bundle of tubes supplied with steam, and a spray vaporization box 32, capable of '' be placed under vacuum and connected for this purpose to a vacuum pipe 34. The buffer tank 30 is arranged at a level lower than that of the vaporization box 32, so that the contents of this box can return to the buffer tank, despite the depression prevailing in the box 32. The buffer tank 30 is divided into two parts 35 and 36 by a weir partition 30c with a horizontal crest. Part 36, assigned to the outlet, is supplied with excess mother liquor in part 35, which overflows over the crest of the partition 30c; this part 36, of relatively small volume, is equipped with a recovery pump 30d to send the mother liquors, after concentration and

  reheating, in mixer 4 (figure 1).

  The part 35, which occupies the major part of the tank 30, is supplied by the line 22 which comes from the ego ts of the centrifuge 20 (FIG. 1), this line 22 extending, in the tank 30, by a supply valve 30b which opens clearly below the

  mother liquor level, fixed by the partition crest 30c.

  In addition, a circulation pump 30d draws from the lower part of the major part 35 of the tank 30 to discharge the mother liquors into the exchanger 31 at the inlet 31a, the mother liquors emerging through the outlet 31b, after having passed a bundle of steam tubes supplied with steam via the inlet 31c, this steam leaving in 31d in the plus state or

less condensed.

  The mother liquors heated in the exchanger 31 are sent to spray nozzles 32a in the box.

evaporator 32.

  This box 32 is composed of two superimposed cylindrical parts, the upper part, of larger diameter forming a vapor buffer, while the lower part

  receptacle form for mother liquors, up to level 32f.

  The spray nozzles 32a are arranged towards the top of

the bottom part.

  The upper part is dome-shaped, at the top of which a drain pipe 32b is stitched, connected to a vacuum service pipe 34, through a valve

  32_ controlled by the vacuum in the box 32.

  It will be noted that the upper part is provided with a nozzle '32d for the introduction of hot water, with a valve 32e controlled by the temperature in the upper part of the box, this arrival of hot water being provided to avoid

  supersaturation of mother liquors, in particular.

  The lower part of the box 32 ends in a cone from where a pipe 33, which ends in the tank 30 to a supply rod 30a which plunges into the major part 35 of the tank 30 as close to the bottom. It will be understood that the height of column h between the level of the crest of the partition 30c corresponds, taking into account the density of the mother liquors at the outlet of the caisson 32, a function of the concentration and of the temperature, unlike the

  pressures prevailing in the tank 30 and in the box 32.

  A circulation in a closed circuit is thus obtained between the major part 35 of the buffer tank 30 and the box 32, the mother liquors, withdrawn by the pump 30_, passing through the exchanger 31 to be sprayed into the box 32, from which they return by gravity to the tank 30 through the pipe 33. This closed circuit circulation operates in parallel with the circulation from the

  line 22 coming from the turbine 20 to the mixer 4.

  It will be noted that the heating of the mother liquors in the exchanger 31 occurs upstream of the spraying in the box 32, so that the pressure in the exchanger can be sufficient to avoid the formation of foam. Furthermore, the spraying by the nozzles 32a can impart to the mother liquors, at the outlet of the nozzles, sufficient speeds to overcome the internal cohesion forces of the mother liquors, despite the presence of surfactants, and obtain a fine spray and not the development of an agglomerate of air bubbles, so that the evaporation is of the so-called type:

flash evaporation.

  It will be noted that the quantity of water evaporated is a direct function, per unit of flow rate, of the temperature difference of the mother liquors between their spraying by the nozzles 32a and their collection at the bottom of the box 32. The concentration can thus be adjusted mother liquors in the return pipe 33, measured by a sensor 31f by adjusting the quantity of steam admitted into the exchanger 31, by the intermediary of the slaved valve 31e, controlled by

the sensor 311.

  To give an example of operation, the mother liquors from the ego ts of the centrifuge 20 arrived at a flow rate of 10 tonnes per hour, at a temperature of 80 "C and

a concentration of 89 Brlx.

  It is recalled that the Brlx is a unit conventionally used in sweets to define the content of dry matter, sugar or impurities, of a sugar syrup, by measurement of the density with the Brlx hydrometer. The Brix degree of a syrup in measure expresses the percentage sugar content by weight of a pure syrup which has, at 15 "C, the same density as

  syrup under the measurement conditions.

  The stage 3 of concentratlon-reheating is adjusted to restore, to the mixer 4, a flow rate of substantially 10

  tonnes / hour of mother liquor at 82 "C and 92 Brlx.

  In operating mode, the contents of the tank have, on the surface, a temperature of 82 C and a Brix degree of 92, corresponding of course to the temperature and concentration of the mother liquors sent to the mixer 4. But in the background the ego contributions t of turbine 20 reduce the concentration and temperature. The pump 30_ provides a circulation rate of 30 tonnes / hour. Given the open circuit circulation rate of 10 tonnes / hour, the mother liquors taken from the bottom of the major part 35 of the reservoir

  have a temperature of 81C and a Brix of 91.

  At the outlet 31b of the exchanger 31, the mother liquors reach a temperature of 930C, while neither the flow of

  t / h, nor the concentration of 91 degrees Brlx have not changed.

  Leaving the nozzles 32a of the box 32 in the form of a mist of droplets, the mother liquors undergo partial evaporation in the atmosphere in depression. Their temperature drops from 93 to 82 C, while their concentration goes from 91 to 92 Brix. It is understood that the lowering of temperature by 11 C corresponds to the heat of vaporization of the water. It will also be noted that if, in all rigor, the mass of mother liquors which returns from the box 32 to the reservoir 30 by the pipe 33 is less by the mass of evaporated water than that which is introduced by the nozzles 32a, this mass d evaporated water corresponds to approximately 1% of the mass of circulating mother liquors, and can be

  neglected in the calculation of flow rates.

  As already mentioned, the return of the mother liquors from the box-32 to the buffer tank 30 via the pipe 33 takes place by gravity, acting on the difference in level between the actual surface of the mother liquors at the bottom of the box 32 and the equilibrium level 321, the height of column h of the mother liquors (at 92 Brix and 82 C) between the level 321 and the level 30f in the major part 35 of the reservoir (defined by the wall crest 30c) balancing the depression in box 32. This depression corresponds to the saturated water vapor pressure at 65 C, adjusted by

the servo 32c.

  The pipe 33 opens near the bottom of the tank, in the major part 35, so as not to disturb the gathering of the mother liquors heated and concentrated on the surface. These mother liquors pass into part 36, overflowing above the crest of the wall 30c, to be

  taken up by pump 30e and returned to mixer 4.

  Taking into account that the quantity of water evaporated in the box 32 is of the order of a percent relative to the closed circuit flow rate of the mother liquors in concentration, and that this flow rate is approximately triple the flow rate arriving through the pipe 22, the difference in flow rates between the inlet and the outlet of the heating-concentratlon assembly remains negligible, under the aspect of engineering projects. But this difference corresponds to the concentration of the mother liquors, and is responsible for the production gains, given that the practice of pre-coating makes it possible to lower the temperature of end of crystallization below 50 C in

  keeping the efficiency of the mixer 5.

  It will be noted that, with respect to the example which has just been described, temperature dispersions of + 2 C are admissible; the concentration dispersions can reach + 1 Brix in the starting mother liquors, and + 2 Brlx in the mother liquors added to the second part of the cooked mass

in the mixer 4.

  Tests have shown that the process according to the invention made it possible to reduce the quantity of sugar extracted from 100 kg of cooked mass, from 48.03 kg to 52.64 kg.

  a gain of 9.6% over previous productions.

  Thus, during a campaign in which 23,500 tonnes of refined brown sugar will be produced, the process of the invention will have increased production by approximately 2,100 tonnes; the ton of sugar, at this stage, has a value of around

400 francs.

  Of course, the invention is not limited to the example described, but embraces all its variants

  of execution within the framework of the claims.

Claims (8)

  1. Process for increasing the sugar extraction yield from a third-jet syrup, by reducing the temperature at the end of crystallization before elimination of the molasses by centrifugation, according to which the syrup brought to crystallization seeds is seeded (1}) supersaturation at a temperature at the start of crystallization to obtain a so-called cult mass mixture composed of sugar crystals in a mother liquor in the process of desaturation, we divide (111 into two parts this cooked mass to take a first part where we separate, by centrifugation (2), the mother water of the sugar crystals which is rinsed with water, the rlnçat being joined (22) to the mother water which it dilutes, the mother water (3) is heated diluted and joined (4) to the second part of baked mass, whose crystal content is reduced, and this second part of baked mass with reduced crystal content is allowed to cool by kneading it (5) to a temperature final crystallization, after s which separates the crystallized sugar from molasses by conventional centrifugation (6), a process characterized in that, after reheating (31) of the diluted mother liquor, it is concentrated by spraying it into an enclosure (32) in maintained vacuum while that the thermal evaporation losses are compensated, so as to at least substantially restore the temperature and concentration of the mother liquor in the cult starting mass, before the   join (4) to the second part of the cult mass.   2. Method according to claim 1, characterized in that the mother water is superheated (31) relative to the temperature at the start of crystallization before the   spray in the enclosure (32) in maintained vacuum.   3. Method according to claim 2, characterized in that the superheating temperature is greater than 80 to   12 "C at that of the start of crystallization.   4. Method according to any one of claims 1   to 3, characterized in that the absolute pressure in   the depression enclosure is between 20 and 30 kPa.   5. Method according to any one of claims 1   to 4, characterized in that the diluted mother liquor is heated and concentrated in a closed cycle, by drawing (30e) from this cycle the mother liquor at substantially the same temperature and concentration as the mother liquor in the cult mass (1) at the start, while compensating for the withdrawal by supplying diluted mother liquor (22) centrifuged from the first part of cooked mass.   6. Method according to any one of claims 1   to 5, characterized in that, in the mother liquor of the starting cooked mass, the temperature is between 78 and 82 C and   the dry matter content between 88 and 90 Brix.   7. Method according to claim 6, characterized in that in the mother liquor joined to the second part of the baked mass, the temperature is between 80 "and 84" C and the   dry matter content between 90 and 94 Brlx.   8. Method according to any one of claims 1   to 7, characterized in that the final temperature of   crystallization is less than 50 "C.