IE56053B1 - Process and device for continuous crystallisation of a massecuite - Google Patents

Abstract

The cooked mass (7) is subjected to a continuous vacuum kneading in an apparatus (8) allowing to carry out such treatment in at least two successive steps. The crystals of the mother liquor are then separated from the crystallization jet and the poor sewer is then recycled during the vacuum malaxing steps. The enlargement of the crystals for a first refined jet exceeds 60 %. Application to the sugar industry. [FR2534595A1]

Description

The present invention relates to a process and a device for continuous mixing of a massecuite obtained particularly during the manufacture of sugar, it being understood that sugar1* is understood to mean sucrose.

The processes for the extraction of sugar rely, in general, on two kinds of factories: factories for manufacture as such (sugar mills) and factories for after-treatment (refineries) in which the sugars are refined, filtered crystallised and formed.

The sugar trills have equipment which is adapted to the raw material employed which is sugar beet or cane. Thus, sugar mills have specific equipment whilst the refineries process raw sugars, whether they be from cane or from sugar beet.

When the raw material is sugar beet, a diffusion process is employed to obtain a juice which will have collected the sugar contained in the sugar beet, which is first cut up into thin strips or cossettes.

In general terms, the diffusers are devices in which water is circulated countercurrentwise. The juices which are obtained contain approximately 11 to 12X of impurities, calculated on the dry materials» A treatment with lime followed by a carbonation and a separation by filtering or decanting permits a satisfactory purification from organic non-sugars.

When the raw material is cane, the operation involves crushing and pressing in mills to extract the juice, rarely involving diffusion* Since it does not contftiin the 9 0(31(2 impurities es the sugor beet .juice, the cane juice undergoes ® different purification and the corbonaition stage is eliminated» On the other hand, insofar es the concentration and crystallisation stages are concerned, the required operations are comparable in both kinds of sugar miiu The present invention relates more particularly to the stages permitting the sugar to be crystallised and applies to all the sugar juices, whatever the raw material employed* The crystallisation at the mill (for cane or beet) and at the refinery (for cane or beet) Is aimed at extracting in a crystallised form, with as high a yield as posw sible, the sugar dissolved in the syrup and thus to separate it from the soluble impurities which accompany it0 The rate of crystallisation depends principally on the following interr*related parameters: supersaturation, viscosity, temperature, internal agitation, purity and pHa French Patent &loD 1,520,738, describes a process for the crystallisation of sugar mill syrup according to which concentrated sugared juices wherein a fraction of the sugar is crystallised are introduced into an evaporator* The raassecuite obtained is mixed for a time at an elevated temperature end then is screened in a centrifugal filter where the crystals are separated from the syrupy The mother syrup is then mixed with crystals and then undergoes a second mining in order to induce the crysta 11 isation of the sugar contained in the said syrup* In this process, the fact that the operation takes place at an elevated temperature, which is essential on account of its characteristics, does not permit a sufficiently high degree of crystallisation to be reached. Moreover, the consumption of energy is high.

French Patent No. 2,064,277 describes a process for producing crystals continuously according to which a sugar syrup, in the presence of previously added crystalline seeds, is concentrated under vacuum in a crystalliser having several successive compartments. This process requires a relatively high temperature, which, on the one hand, increases the energy cost and, on the other hand, reduces the crystallisation yield.

French Patent 1,368,118 describes a crystallisation process consisting in removing a part of the massecuite at the outlet of the cooling apparatus, subjecting said part to a centrifugal turbine action operation and reinjecting the mother liquor thereby obtained at the level of one of the crystallisation steps. This process necessitates, on the one hand, a supplementary treatment of centrifugal turbine action and, on the other hand, results in a loss of energy to the detriment of the crystallisation. Furthermore, on setting aside a large part of the massecuite one reduces the surface of crystallisation of the remaining massecuite which Is directed towards the mixer. In this way, the reinjected mother liquor crystallises spontaneously on cooling, giving rise to small-sized extraneous particles which pass across the strainer of centrifuges, which further reduces the extraction of crystals at the jet assembly.

The crystallisation process according to the invention makes it possible to obtain immediately after the end of the mixing of the massecuite a much more substantial quantity of crystallised sugar than when the processes which are known at present are employed. In fact, the process aims at a maximum lowering of the temperature of the massecuite to increase the crystallisation.

According to the invention there is provided a process for the crystallisation of a sugar syrup comprising subjecting the syrup to a continuous or non-continuous boiling so as to obtain a massecuite, subjecting the massecuite to one or more stages of continuous mixing under vacuum, and separating the crystals from the mother liquor of the crystallisation stage in question, at least a part of the mother liquor being recycled during the stage or stages of mixing under vacuum.

Another subject of the invention is a device for mixing a nnssecuite, comprising one or more cylindrical mixers,means for entry and exit of the massecuite, at least one vacuum takeoff, means for entry of mother liquor connected to an exit for the centrifugal running from a centrifuging device, the exit for the massecuite and the means for entry for mother liquor being connected, respectively, one to the entry for the massecuite and the other to an exit for the mother liquor from the same centrifuging device.

Preferably, only the part called ’’centrifugal runnings" is recycled during the vacuu® mixing stages® The part called wash syrup is reinjected, for example, at the boiling stage. The terras centrifugal runnings and wash syrup" are defined according to J» DU00UR6, Sucreries de betteraves (Sugar manufacture fro© sugar beet) 1952«, Also preferably, the recycled part of the mother liquor is reheated and de-etauLsified before the said recyclinge Although the viscosity of the massecuite increases, the lowering in temperature is wade possible by the vacuum self^evaporation, in a continuous process, of the water contained in the massecuite and by the recycling of all or a part of the mother Liquor, which thereby produces a violent and continual agitation* This agitation is all the more efficient because it tabes place in a regular manner in a thin and uniform layer of the massecuite® Xt is advantageous that the continuous vacuum fixing comprises several successive stages® In the case of two stages, the first is operated under a vacuum between 82o6 ft Pis and 88.0 ftPa, the second under a vacuum between 88„0 ftPa and 9d®0 ftps.

The Biessscuit© must heiv® ®n initial temperature of approximately 80°C and, at the outlet frots the continuous vacuum mining its temperature should have decreased to a value*bet® ween »0°C end 5O°C0 The crystallised nassecuite is then filtered (spinning): the crystals are separated from the mother liquor and the centrifugal runnings frois the crystal li sation stage in question is recycled, for the most part, in the region of the continuous vacuum mixer or mixersa The recycled cen™ trifugel runnings maintain a sufficient fluidity so that the massecuite does not set and that the mobility of the crystals allows them to move» Preferably, the means for entry of the centrifugal runnings consist of one or more pipes situated in the lower part of the cylinder» Preferably, these pipes arrive at the cylinder tangentially. Preferably, their number is between two and ten» The process and the device according to the inven~ tion will be better understood by virtue of the descrip20 tion of Figures 1, 2, 3, 4, 5 and 6, and by virtue of an example of embodiment» Figure 1 shows a flow-sheet of an embodiment of the process according to which, after continuous boiling, the continuous vacuum mixing is carried out in a device consisting of a single miner» Figure 2 shows ti flow-sheet of a preferred embodiment of the process according to which, after continuous boiling, the continuous vacuum mixing is carried out in a device consisting of two separate mixers» Figure 3 shows the device for carrying out th® process 11tustrated In Figure h Figure 4 shows the device for carrying out the process 11 Lustrated In Figure 2« Figure 5 is a transverse view In a cross-section which Is broken 1n β parallel plane of a vacuum wixer 11« lustrated in Figure 4* Figure 6 is a view of the same mixer in profile a long A„A„ According to Figures 1, 2, 3 and 4, a fraction 1a, 21a of the sugar syrup 1, 21 is directed to a vertical device 2» 22 to form the seed. The device has a volume of 300 hectolitres and comprises a calandria, with tubes supported by two plates of a frustoconical shape, sloping towards the central well equipped with a propeller stirrer.

The feet 3, 23 of the vertical device 2, 22 supply the magma mixer 4Z 24.

The magma mixer 1s a completely closed cylindri cal horizontal capacity, fitted with an agitator consisting of a helix supported by a longitudinal shaft turning at 1 rpm and equipped with a double jacket for the circulation of hot water st 80°C enabling the whole to be controlled at constant temperature; this magma mixer acts only as buffer storage* The magma 5, 25 travelling from the mixer 4, 24 and the fraction 1b, 21b of the sugar syrup supply the continuous boiling device 6, 26w The continuous boiling device 6, 26 consists of a cylindrical horizontal vessel, made of steel, inside which supplied by & bundle of longitudinal tubes of stainless steely airranged in layers. Th® lower part of the vessel is provided with & double jacket in which cir* eulaite the vapours which sire not condensed in the bundle.

Ste&w is injected et a certain flow-rate into the lower part of the device for boiling, to provide agitation of the massecuite® The device is divided into corapertments by transverse partitions end a longitudinal partition at the base of which Sin orifice permits the forward movement of the masse cuite. The first cospartaent is supplied with the magma, the following ones with the massecuite originating from the preceding compertraentD Each compartment is also provided with a supply of syrup 1b, 21b which sprinkles through turning pipes the emerging walls of the massecuite.

The massecuite arriving at the last compartment is e x t r a c ted at its base by a variable-speed pump. The ci rcul&t i on and the flow-r&te of the fluids in the device are controlled by control chains for vacuum, steam pressures, agitation, density and massecuite level.

As an example, the main characteristics of a de® vice for continuous boiling (constructed by Mvos^Cei IBabcock) are as follows? Overall length 9.0 m internal length ?.& m Overall width 3.5 m Overall height 4.1 m internal diameter of the shell 3.1 m Volume occupied by the raasseculte 32 ra3 Tot til heating « u r f a C e 32ft ®2 Stainless tubes^ length 7.5 η Total number of tubes 404 Test plate of the bundle 2 bars Empty weight 32 tonnes Operating weight 77 tonnes dumber of compartments 10 The operating conditions of the device described above are as follows; Seed magma 3, 23: Br 1 x 8& « 38° Purity 99.0 " 99e5 Rate 6 to 7 tonnes/hour Content of crystals 30 to 40S Average Opening of the crystals 0„20 to 0.25 mm Feed liquor 1b, 21bi Br 1 x 08 " 70° Purity 99.0 - 99,5 Rate 26 to 30 tonnes/hour Masseculte 7, 27s Britt 90 - 91° Purity 99.0 - 99e5 Rate 26 to 30 tonnes/hour Content of crystals 50 to 55® Average Opening of the crystals 0=,5 to Οθό mm (Orin b^ing defined as the value of the ratio of the weight of dry watter/total weight of the syrup)? The wassecuite 7, 27 is moved towards & device for continuous wining under vacuum 8, 23. cording to an effibodifeent of the invention Figures 1 and 3 * this device consists of a Miner 3 coa^ prising fsn hermetic, thermally insulated, horizontal shell 9, fitted with a longitudinal shaft 10 supporting a helix 11 and divided into two-cosapsrtwsnts 12, 13 by a leaktlght wall 14 which is equipped in its low part with sn orifice 15 permitting the transfer of the masse10 cuite from an entry compartment to the exit compartment, each compartment being provided with a tubing 35 permitting a connection to a source of vacuu©. The entry compartment snd the exit compartment are respectively provi~ ded with tubings for the entry 16a and exit 16b of the massecuite, these tubings being of small cross-section and situated in the low part of each compartment» As an example, the first compartment is subjected to a vacuum of 82.2 HPa, and the second to a vacuum of 89 o 4 MPa. The delivery and the discharge of the massecuite take place through the low pert of the mixer with the eid, respectively, of a variable-speed positive displacement vacuum pump - which can be the extraction pump for the continuous boiling d - and a variable speed positive dis® placement vacuum extraction puisp, As the growth of the crystals takes place, fluidification of the msssecuite is produced with the centrifugal running 18 obtained when the crystallised massecuit? is filtered (centrifuging stage 40 permitting the crystallised sugar 41 to be 1$olated)o This centrifugal running is delivered to ths bottom part of each of the compartments 12, 13 by three? tangential tubings 19, the flow-rates therein being controlled* According to β preferred embodiment of the invent tion - Figures 2, 4, 5 and ό - the two successive mixing stages are carried out in two continuous vacuum fixers 28, 29, with a single compartment, and mounted in series» The crystallised massecuite 27 leaving the first continuous vacuum mixer 28 is sent to the second mixer 29„ Each mixer 28, 29 consists of a horizontal cylin~ der 50 fitted with a low-power internal movement scraping the walls and thus preventing the build-ups of sugar* It also comprises tubings for the entry 31a and exit 31b of the massecuite, of a small cross-section and situated in its lower part» The massecuite level is maintained essentially in the plane of the diameter, so as to offer the maximum surface area for the evaporations The fraction called ’’centrifugal runnings" 39, originating from the massecuite after filtration 40, reheated 42 and de-emu Isified 43 beforehand, is injected 211a, 211b into the mixers 28, 29 in four places through horizon tai deliveries 32 distributed along the lower generatrix 33« The fraction called wash syrup" 44 is recycled with the fraction 21Ρθ Each mixer 28, 29 forms a stage which is placed under a specified vacuum which corresponds to the required massecuite temperature., The stagewiso change in the vacuum allows the lower limit of tempera*3 ture to be reduced, whilst avoiding the spontaneous for® raation of false grains.

The whole system Is continuously fed by a variable-speed positive displacement vacuum pump 34 from the outlet of & continuous boiling or of a charge mixer of a non^contIn® uoug boiling» Each stage is connected 35 to a high^vacuum station 37, the vacuum being regulated by means of an automatic valve 36, After a residence time of 90 minutes, the cooled masse*3* cult© is continuously extracted by a variable^speed posi~ tive displacement vacuum pump 38.

The speeds of the esassecuite pumps are controlled by level regulators ? The vacua are regulated, at displayed set-points, by automatic valves.

The rate of delivery of running to each stage is regulated proportionally to the flow*rate of massecuite and corrected in the last stage depending on the exit 9rixo E«amp Is 1* The characteristics of a device for continuous mixing under vacuum, formed by two separate continuous vacuum minors are as follows: Winer sise Overall length 25 Internal length Overall height Internal diameter of the shell Total volume Working volume F i rst Hix*r 3igg F i rst Second Height empty 8*3 t 12*9 t Hovewent wot or power 2*2 kw 4 kW V ei c_uui w 31 a tion: Vacuum pump giving 1,000 ®^/h at 96*0 HP a with 40 kU installed power* Operation conditions for a first stage of refinings Vacuum: first stage 85.3 RPa second stage 95„3 RPa Hassecuite temperatures: first stage entry 82°C first stage exit 60°C second stage exit 4Q°C Characteristics of t h e taas se cu i t e _ent e r i ng: Brix 90*72° Rate 16*23 tonnes/hour Crystals 48«68%/massecuite 53.66%/dry matter Characteristics of the^ massecuite leavjn_g: Brix Rate Crystals Flowrates of recycled runnings: Dimensions of the crystals: 86*76° 26*24 tonnes/hour 49*60%/massecui to 57*17%/dry matter 11«08 tonnes/hour Entry average opening 0*50 mm veriation coefficient 30 Exit average opening 0*60 mm 1$ variation coefficient 27 Weight of- crysta las Entry 7,90 tonnes/hour Exit 13.01 tonnes/hour ©rowth 1.65 By comparison to a conventional process comprising boilings heated with stee&i, the process according to the invention permits approximately 60& of the steam consumption to be saved.

Xn the example described, a significant growth of the crystals is observed, in excess of 60 for a first stags of refining.

The essential difference between the continuous vacuum mixer and the other types of crystalliser is that the cooling does not take place by an exchange with a fluid but by self-evaporation. The continuity of the operation favours the uniformity of the final product, which is obtained by maintaining the value of all the parameters of the control systems® The flash" which is produced at the delivery of the massecuite and the running which has previously been reheated and de-emulsified creates a turbulent regime which facilitates the material transfers from the fluid to the crystal® The fact that the running is reheated which introduces heat into the system - results in an additional crystallisation of this running by sclf-evepo® ration· depending on the conditions of operation of the continuous vacuum mixing plant, the growth coefficient of the crystals is between 1e30 and 1β80» The crystallisation process according to the invention can be called a cold crystallisation process; it eliminates all phono sene of recoloration of the raassecuite, thereby reducing the quantity of water required in the washing stage, an operation which is always accompanied by a redissolution of the sugar crystals» Thus, the process and the device for crystallisa^ tion of a sugar syrup according to the invention permit the extraction to be raised to a level which has never been attained, whatever the technique employed» Furthermore, they confer on the mixing all the advantages of a continuous process, namely: steady running - hence a better quality of the product obtained , a reduction in the sise of the? plant, o simplification of the control systems and of automation, and a very marked improvement in the operating cost of the plant.

Claims (26)

CLAIMS:

1. A process for the crystallisation of a sugar syrup comprising subjecting the syrup to a continuous or noncontinuous boiling so as to obtain a massecuite, subjecting the massecuite to one or more stages of continuous mixing under vacuum, and separating the crystals from the mother liquor of the crystallisation stage in question, at least a part of the mother liquor being recycled during the stage or stages of mixing under vacuum.

2. A process for the crystallisation of a sugar syrup according to claim 1, wherein the crystals are separated from the mother liquor by spinning.

3. A process for the crystallisation of a sugar syrup according to claim 1, wherein the mother liquor is recycled into the lower part of the mixer or mixers forming said stage or stages of mixing under vacuum.

4. A process for the crystallisation of a sugar syrup according to claim 1 or 2, wherein the recycled fraction of the mother liquor is the fraction called centrifugal running.

5. A process for the crystallisation of a sugar syrup according to any one of claims 1 to 4, wherein the recycled fraction of the mother liquor is reheated and de-emulsified before the said recycling.

6. A process according to claim 1, wherein the continuous mixing of the massecuite under vacuum comprises several successive stages.

7. A process according to claim 6, wherein the continuous mixing of the massecuite under vacuum comprises two successive stages.

8. A process according to claim 7, wherein the first stage of mixing is conducted under a vacuum of between 82.6 and 88.0 MPa.

9. A process according to claim 7 or 8, wherein the second stage of mixing is conducted under a vacuum of between 88.0 and 96.0 MPa.

10. A process according to any one of the preceding claims, wherein the temperature of the massecuite is in the region of 80°C.

11. A process according to claim 10, wherein the temperature of the crystallised syrup is between 40°C and 50°C at the end of the (last) stage of mixing under vacuum.

12. A process according to any one of the preceding claims, wherein, between the end of the continuous or non-continuous boiling stage and the end of the continuous mixing under vacuum, the ratio weight of crystals discharged/weight of crystals entered is between 1.30 and 1.80.

13. A process according to claim 7, wherein the two successive stages of continuous mixing under vacuum are carried out in a single mixer.

14. A process according to claim 6, wherein each stage of continuous mixing under vacuum is carried out in a separate mixer.

15. A process according to one of the preceding claims, wherein the level of massecuite is maintained essentially in the plane of the diameter of each continuous vacuum mixer.

16. A device for mixing a massecuite, comprising one or more cylindrical mixers, means for entry and exit of the massecuite, at least one vacuum takeoff, means for entry of mother liquor connected to an exit for the centrifugal is running from a centrifuging device, the exit for the massecuite and the means for entry for mother liquor being connected, respectively, one to the entry for the massecuite and the other to an exit for the mother liquor from the same 5 centrifuging device.

17. A device for mixing according to claim 16, wherein the means for entry consists of one or more tubings situated in the lower part of the cylinder.

18. A device for mixing according to claim 17, wherein 10 the tubings come to the cylinder tangentially.

19. A device for mixing according to one of the preceding claims 16 to 18, wherein each mixer comprises between two and ten tubings.

20. A device for mixing according to one of claims 16 to 15 19 consisting of a horizontal cylindrical mixer comprising a hermetic, thermally insulated shell, fitted with a longitudinal shaft supporting a helix and divided into two compartments by a leak-tight wall which is equipped in its low part with an orifice permitting the transfer of the 20 massecuite from a discharge compartment to a delivery compartment, each compartment being provided with a tubing permitting a connection to a source of vacuum, each compartment being provided with tubing forming said means for entry of a mother liquor. 25

21. A device for mixing according to claim 20, wherein each compartment is provided with three tangential tubings.

22. A device for mixing according to one of claims 16 to 19 consisting of two horizontal cylindrical mixers arranged in series fitted with a low-power internal movement, each 30 mixer being fitted with tubing forming said means for entry of a mother liquor.

23. A device according to claim 20, wherein the compartments for delivery and for discharge of the massecuite are provided with tubings for entry and exit of massecuite with a small cross-section and situated in their low part.

24. A process for the crystallisation of a sugar syrup as claimed in claim l, substantially as hereinbefore 5 described with reference to and as illustrated in the accompanying drawings.

25. Sugar syrup whenever crystallised by a process claimed in any one of claims 1 to 15 and 24.

26. A device for mixing a massecuite as claimed in 10 claim 16, substantially as hereinbefore described with particular reference to and as illustrated in Figs. 3-6 of the accompanying drawings.