EP0370365A2 - Process and apparatus for continuous sugar crystallisation - Google Patents

Abstract

A process and an apparatus for continuous sugar crystallisation with horizontally or vertically arranged crystallisation chambers which are connected to one another by pipelines and have in their interiors baffle-like guide plates and in which different vacua are generated, are described. The guide plates form meandering flow paths appropriate for the temperature or the vacuum. The guide plates have a diametral or chordal arrangement or are concentric cylinders or truncated cones.

Description

The present invention relates to a method and an apparatus for continuous sugar crystallization.

Attempts to continuously crystallize sucrose - hereinafter referred to as sugar - have been carried out since the beginning of this century. In recent decades, new tests and the subsequent technical developments have been intensified (Zuckerind. 107, 1982, No. 5, page 401 ff.). Despite the known advantages of continuous operation such. B. Reduction of the driving temperature gradient (temperature difference between heating steam and magma), uniform decrease in feed solution and heating steam, fixed-value control instead of time-dependent control programs, better adaptation of the heating chamber and stirrer to the respective process conditions and cheaper designs of the corresponding devices, so far she has not achieved the major breakthrough .

There are a number of reasons for this:
- The quality of the sugar produced does not meet the requirements, in particular with regard to the desired uniformity of crystal sizes in the end product and with regard to the exclusion of crystal agglomerates and aggregates. Improvements are also to be striven for with regard to the quality characteristics of the sugar "color" and "ash".
- Occurring incrustations in the continuous cooking apparatus shorten the so-called "travel time" after which they have to be taken out of operation or cleaned completely or partially.
- The well-known - especially the continuous evaporation crystallizers - are in operation and very expensive to manufacture and can be improved with regard to energy consumption.
- The production of crystal nuclei or crystal seeds has so far been carried out with considerable effort and mainly discontinuously in a separate part of the process.

The known continuous sugar crystallizers are essentially evaporative crystallizers, which can be assigned to the "mixed single apparatus" or "stirred tank cascade" apparatus systems. With both systems there is no defined residence time of the crystals in the individual stirring chambers and therefore no uniform crystal sizes in the product. In addition, local undersaturation or oversaturation is easily given in the evaporative crystallizers because of the locally concentrated heat transfer on the heating surfaces and the associated locally concentrated evaporation. The risk of dissolution or the formation of fine grain is great.

Approaches to overcoming or mitigating these disadvantages are in the apparatus types "continuous apparatus" - comparable to a flow tube - to find. These suggestions could not prevail in practice, since neither the working methods nor the operating results satisfied. The reasons lie in the procedure as well as in the equipment. You are e.g. T. also only been recognized by recent research.

The present invention has for its object to use a method that avoids these disadvantages. The invention is based on the known methods of continuous vacuum crystallization using a continuous apparatus (tube).

This object is achieved in that a sugar solution saturated at higher temperatures (80-120 ° C.) pulsates in cocurrent - with no mixing taking place in any process stage - is continuously cooled by flash evaporation.

This cooling is carried out at pressures of approximately 0.9 to 0.03 bar in such a way that optimal supersaturation and thus an optimal crystallization rate is ensured while avoiding secondary nucleation.

It is advantageous if the last part of the crystallization takes place as far as possible in the lower temperature range (70-40 ° C), since here the best material qualities, e.g. B. in terms of "ash" and "color" can be achieved.

Achievable results with regard to crystallization performance are to be shown using two examples. If a sugar solution saturated at 90 ° C. with a purity of 93 (93% sugar in the dry substance) is subjected to the continuous crystallization according to the present method, then at 40 ° C. a magma with a crystal content of approx. 50% results with a sugar yield of approx. 60%, the syrup being approx. 84% pure.

If the input sugar solution is saturated at 105 ° C, then at 40 ° C this results in a magma with a crystal content of approx. 65% with a yield of approx. 80%. The syrup is about 75% pure.

In order to maintain favorable flow properties of the magma at the higher crystal contents, syrup separated from crystals is recycled in a known manner.

To form crystal nuclei, a very small amount of magma with newly formed very small crystals or crystal nuclei is returned to the appropriately supersaturated sugar solution and mixed intensively with it using a static tube mixer. The returned crystals or crystal nuclei are supposed to induce secondary nucleation to the desired extent as seed or excitation crystals. In order to control this, the supersaturation is increased by cooling and thus the nucleation is increased, while it is subsequently reduced or broken off by appropriate heating. Crystal nucleation can e.g. B. be monitored using turbidity measurements. It is also possible, to induce crystal nucleation by shock (blowing in air, evenly or intermittently) or by means of slurry (mixture of ground sugar with dispersing liquid) in a static tube mixer instead of the recirculated magma according to the classic methods for discontinuous boiling.

The conditioning of the thick juice obtained after the evaporation station (concentrated sugar solution) or of the mother syrup obtained for the crystallization in the individual station stages takes place in an evaporation plant in known designs. B. in a flash evaporation in which the pumped sugar juice is heated by plate warmer. In the sugar solution mentioned at 90 ° C mentioned in the previous example, an evaporation of z. B. 72% dry matter after the evaporation station to 82% dry matter and the saturated sugar solution at 105 ° C an evaporation of z. B. 72% dry substance to 85% dry substance required.

If desired, a known type of decanter can be installed downstream of the crystallization apparatus, the decanter being returned and the magma enriched with crystals being fed to the centrifuges.

• Fig. 1 eine schematische Darstellung der Vor­richtung und ein Fließschema des Magmas bei der kontinuierlichen Kristallisation,
• Fig. 2a + 2b Querschnitte durch eine der Kammern der Kristallisationsvorrichtung,
• Fig. 3 zwei Kristallisationskammern in einer anderen Ausführung, in der die Kammern vertikal übereinander angeordnet sind,
• Fig. 4 einen Kristallisationsturm mit vier übereinander angeordneten Kammern,
• Fig. 5 einen Querschnitt durch eine solche Kammer in der links die Leitbleche konzentrisch also in Form von Zylindern ausgebildet sind und in der rechten Hälfte der Figur sind Bleche sehnenartig angeordet,
• Fig. 6a + 6b eine Darstellung ähnlich der Fig. 2a + 2b bei horizontal übereinander angeordneten Kristallisationskammern und
• Fig. 7 + 8 eine weitere Ausgestaltung der Entspannungs­kammern.

The invention will now be explained in more detail with reference to the accompanying drawing. They represent:

• 1 is a schematic representation of the device and a flow diagram of the magma in the continuous crystallization,
• 2a + 2b cross sections through one of the chambers of the crystallization device,
• 3 two crystallization chambers in another embodiment, in which the chambers are arranged vertically one above the other,
• 4 shows a crystallization tower with four chambers arranged one above the other,
• 5 shows a cross section through such a chamber in which the guide plates are formed concentrically, that is to say in the form of cylinders, on the left and plates are arranged in a tendon-like manner in the right half of the figure,
• Fig. 6a + 6b a representation similar to Fig. 2a + 2b with horizontally superposed crystallization chambers and
• Fig. 7 + 8 a further embodiment of the relaxation chambers.

In the embodiment according to FIG. 1, a horizontally lying cylindrical vessel 6 is provided, which is divided into several chambers 7, the individual chambers being delimited against one another by vertical walls 8.

The vertical version with an upright cylindrical vessel 10 is shown in FIGS. 3 and 4, in which the individual chambers 10 are delimited against one another by horizontal floors 11 or ceilings 12. If the chambers are directly on top of each other, the ceiling of one chamber forms the floor for the chamber above. If the chambers are separated by spaces 13 on the vertical axis, each chamber has its own floor and ceiling.

Baffles are installed in the individual chambers in the horizontal and vertical versions. One group 14 of guide vanes forms several compartments 15 within a chamber by sealingly connecting them to the bottom and forming an overflow weir at the top. The overflow weirs of a chamber result in a cascade, the step height of which results from the flow properties of the respective magma state or the desired relaxation time.

The second group of guide plates 16 is inserted into the compartments of the cascade plates from above. The baffles have an opening 17 for underflow at the bottom and are raised higher as a splash guard, each with sufficient passages for the vapor streams 18.

In the horizontal and vertical versions, the baffles are installed as parallel vertical plates. In the vertical version, the use of concentric cylinders 19 is also possible, and truncated cones tapering downwards can also be used.

Both groups of baffles can have dents or bulges 20 which cause a cross-sectional constriction or widening and thus lead to an increase in the pulsation of the magma flow.

To transfer the magma from one chamber to the next, an outer tube 21 with a control element is provided at the bottom.

On the inlet side of the cylindrical crystallization apparatus there are several static tube mixers 23 connected in series and each provided with an inlet nozzle 22, which are equipped with cooling jacket 24 and heating jacket 25. With the help of the nozzle mentioned, it should be ensured that, due to a high inflow velocity, the nucleation can only take place behind the inlet, as seen in the direction of flow. Instead of static tube mixers, dynamic tube mixers are also possible - less cheaply.

By means of controllable pumps 26, 27, the thick juice is fed through the static tube mixers and then to the crystallization apparatus 28 as newly formed magma and at the end of the crystallizer against the prevailing vacuum and pumped to the centrifuges 29. Both pumps are controlled according to the level 30, 31 in front of them.

With a further controllable pump 32, a small amount of the newly created magma is withdrawn 33 behind the first or second static tube mixer and returned to the first static tube mixer.

All compartments have idle lines at the lowest point, which open into a manifold. Steam and water connections are provided in all compartments at the top and bottom for malfunctions.

The vapor is removed from the top of each chamber to the vacuum pump 34. Control elements are also built into the vapor outlets, with which the vacuum desired in the respective chamber and thus the desired magma temperature are set.

The cooling of the magma by relaxation takes place essentially when the weirs overflow in the area of the vapor space. The intensity of relaxation as the magma rises in the upper part of each compartment gradually increases as a result of decreasing fluid pressure and gradually subsides in the descending part. The supersaturation caused by cooling and evaporation enables the crystals to grow evenly especially in the lower part of the chamber. With this method, there can be no local overheating or evaporation concentrations on the heating surfaces or local supercooling on cooling surfaces - effects in other systems. This prevents unwanted oversaturation or undersaturation and eliminates the risk of secondary nucleation or redissolving. The conditions for the desired constant supersaturation during the entire crystallization process are therefore almost optimal.

The pulsation in the flow of the magma is achieved by changes in cross-section in the compartments:
- With the horizontal solution due to the outer boundary as a segment of a circle for each compartment. With the flow from top to bottom, the cross-section becomes narrower and vice versa, from bottom to top it gets wider. When flowing around the baffles at the top and bottom, there is also a pulse-like effect.
- With the vertical solution, the conditions are corresponding.
- In addition, in both versions, any desired pulsation of the flow is achieved by installing dents or similar bulges in the guide plates in the appropriate size and number.

As a result of the shear effect, the pulsation causes the layers of liquid to shift relative to one another. As a result, the crystals in the magma are always supersaturated The syrup is poured on and the diffusion resistance on the crystal surface is reduced analogously to an agitator effect. In addition, the sinking of the crystals - especially on the second half of the way through the crystallizer - causes a relative movement to the syrup and thus a further reduction in the diffusion resistance.

Due to the constant up and down movements in the chambers, no crystal concentrations or deposits can occur. All walls are flooded or overflowed by the magma without local over- or under-saturation. With the vertical baffles there are no horizontal surfaces at the top and bottom, but only thin edges in the relatively thin plate thickness. Deposits and incrustations are largely - if not completely - avoided.

The main advantage of this procedure is that
- In contrast to the known systems of the stirred tank cascade - due to the direct current without mixing, all crystals have uniform residence times under the same conditions. This results in a largely uniform crystal size with a very narrow particle size distribution. That is a goal of all progressive efforts in sugar crystallization.

Another goal is to avoid the formation of crystal agglomerates and aggregates to produce well-formed single crystals. In the present case this is already taken into account during crystal nucleation. By recycling extremely small crystals for vaccination, they are only slightly larger than the secondary crystal nuclei induced by them.

The high shear forces in the static tube mixer prevent agglomerates and aggregates from the very beginning of crystallization. In the further course of crystallization, the shear forces of the pulsating magma act in the same way.

The operation of the apparatus described is much easier compared to the known devices. There are no agitators and no heating chambers. The entire process sequence within the apparatus described is only controlled with a vapor pressure and level control for each chamber.

The evaporation of the thin sugar juice to the thick juice prior to the crystallization work can be carried out more uniformly, since it is no longer dependent on the steam being taken off by the crystallization system.

The mechanical production of the crystallization apparatus described is less complex than the known constructions, since it basically consists of a cylindrical vessel with built-in vertical walls.

By eliminating the agitators and the heating chambers, there is considerable savings in electrical and thermal energy.

A further embodiment of the chambers, which is extremely advantageous for the crystallization process, is shown in FIGS. 7 and 8, in that the relaxation rate is reduced predominantly in the first compartment 15 of each chamber 7 or 10, as a result of which the upward flow rate is greatly reduced. This invention is achieved in that the first compartment of the respective chamber is enlarged at the expense of the following compartments to such an extent that this effect occurs. There are then at least two compartments 15 in each chamber (FIGS. 7 and 8).

In order to channel the upward flow if necessary, baffles 16 'can be installed which communicate the flow space through openings in the lower part. The baffles 16 'can also be carried out to the bottom, in which case the magma is inserted between two baffles at 28'. Even with the baffles open at the bottom or in the event that there are no baffles at all, the magma is supplied at several points in the bottom of the chambers for better distribution in the lower region. The pulsating movement is essentially caused by the resulting vapor bubbles in the enlarged sections. But you can also by Shape of the baffles and in the transition areas from one chamber to another are reinforced by the type of flow. The advantage of this configuration is a very slow relaxation of the magma and associated gradual cooling and concentration as a result of water evaporation from the magma. These improved conditions for crystallization - namely the exact maintenance of the desired supersaturation - results in a significantly improved crystal quality.

• 1) Bessere Qualitäten des erzeugten Zuckers sowohl hinsichtlich der Gleichmäßigkeit der Kristallgrößen und der Kristallform bei Vermeidung von Agglomeraten und Aggregaten wie auch hinsichtlich anderer Qualitätsmerkmale wie z. B. "Farbe" und "Asche".
• 2) Vereinfachter Betrieb und vereinfachte Prozeßregelung.
• 3) Geringerer Verbrauch an elektrischer und thermischer Energie, wobei letztere auch Verbesserungen bei der vor­geschlagenen Verdampfstation bewirkt.
• 4) Reduzierung bzw. vollständige Vermeidung von Inkrustationen.
• 5) Weniger aufwendige und damit preiswertere maschinentech­nische Herstellung des Kristallisationsapparates.

The following advantages thus result from the method and the device according to the present invention:

• 1) Better qualities of the sugar produced both with regard to the uniformity of the crystal sizes and the crystal shape while avoiding agglomerates and aggregates as well as with regard to other quality features such as B. "Color" and "Ash".
• 2) Simplified operation and simplified process control.
• 3) Lower consumption of electrical and thermal energy, the latter also bringing about improvements in the proposed evaporation station.
• 4) Reduction or complete avoidance of incrustations.
• 5) Less complex and therefore less expensive mechanical production of the crystallization apparatus.

Claims (9)

1. A process for continuous sugar crystallization by means of a continuous apparatus, characterized in that a sugar solution saturated at higher temperatures (80-120 ° C) pulsates in cocurrent - with no mixing taking place in any process stage - is continuously cooled by flash evaporation. 2. The method according to claim 1, characterized in that for nucleation a small amount of magma with newly formed very small crystals or crystal nuclei is returned to the appropriately supersaturated sugar solution and mixed intensively with this with static tube mixers. 3. A device for performing the method according to claim 1 and 2, characterized in that a lying (Fig. 1) or standing (Fig. 3 and 4) cylindrical vessel is divided into several chambers (7 and 10) and the individual chambers (7) in the horizontal version by vertical walls (8) and in the vertical version by horizontal floors (11) or ceilings (12) are limited to each other and the individual chambers of both versions contain baffles (14, 16). 4. The device according to claim 3, characterized in that the one baffles (14) within a chamber (7 or 10) form several departments by being tightly connected to the bottom or wall and form an overflow weir, which in turn is a cascade form, the step heights of which depend on the flow properties of the respective magma state and the desired relaxation time, while the second group of guide plates (16) is inserted from above between the cascade plates (14), so that there are meandering flow paths and above passages for the vapor flows and that the baffles run parallel or curved parallel to one another in the horizontal and also in the vertical configuration and have a flat, cylindrical or tapered frustoconical shape. 5. The device according to claim 4, characterized in that the guide plates have dents or similar bulges (20). 6. The device according to claim 3, characterized in that an external tube (21) with control element is provided for transferring the magma from one chamber to the next below. 7. The device for carrying out the method according to claim 1, characterized in that one or more static tube mixers (23) with cooling or heating jackets (24, 25) are connected upstream of the horizontal and standing crystallization apparatus. 8. The method according to claim 1 to 7, characterized in that instead of the recirculated magma, slurry or even or intermittent air is introduced into the static tube mixer. 9. The device according to claim 1 to 3, characterized in that the first compartment (15 ') of the respective chamber (7, 10) is enlarged at the expense of the following compartments to such an extent that at most only two compartments are left in one chamber, whereby If necessary, baffles (16 ') are installed in each compartment, which are led to the bottom or have openings and the magma is fed in the first case between the baffles (14') and otherwise at several points (28 ') in the floor.

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