EP0370365B1 - Process and apparatus for continuous sugar crystallisation - Google Patents

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 .

• Die Qualität des erzeugten Zuckers entspricht nicht den gestellten Anforderungen, insbesondere hinsichtlich der gewünschten Gleichmäßigkeit der Kristallgrößen im Endprodukt sowie hinsichtlich des Ausschlußes von Kristallagglomeraten und -aggregaten. Auch hinsichtlich der Qualitätsmerkmale des Zuckers "Farbe" und "Asche" sind Verbesserungen anzustreben.
• Auftretende Inkrustationen in den kontinuierlichen Kochapparaten verkürzen die sog. "Reisezeit", nach der sie ganz oder teilweise außerbetrieb gesetzt und gereinigt werden müssen.
• Die bekannten - insbesondere die kontinuierlichen Verdampfungskristallisatoren - sind inbetrieb und in der Herstellung recht aufwendig und hinsichtlich des Energieverbrauchs verbesserungsfähig.
• Die Herstellung der Kristallkeime bzw. der Kristallsaat erfolgt bisher mit beträchtlichem Aufwand und hauptsächlich diskontinuierlich in einem abgetrennten Verfahrensteil.

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 completely or partially taken out of operation and cleaned.
• The known ones - in particular the continuous evaporation crystallizers - are in operation and quite complex to manufacture and can be improved with regard to energy consumption.
• Up to now, the production of the crystal nuclei or the crystal seed has been carried out with considerable effort and mainly batchwise 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. In 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 formation of fine grains is great (DE-B-1 188 518, DE-A-2 627 825 and US-A-3 627 582).

Approaches to overcoming or reducing 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 implementation of the apparatus. You are e.g. T. also only been recognized by recent research.

The object of the present invention is to use a method which 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 continuous crystallization according to the present method, then a magma with a crystal content of approx. 50% with a sugar yield of approx. 50% results at 40 ° C. 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 has a purity of approx. 75%.

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 static tube mixers. The returned crystals or crystal nuclei are supposed to induce secondary nucleation to the desired extent as seed or excitation crystals. 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 in the individual station stages for crystallization 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 case of the sugar solution mentioned at 90 ° C. in the example above, an evaporation of e.g. 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 matter to 85% dry matter required.

If desired, a known type of decanter can be installed downstream of the crystallizer, 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:

Fig. 1

1 shows 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,

Fig. 3

two crystallization chambers in another embodiment, in which the chambers are arranged vertically one above the other,

Fig. 4

a crystallization tower with four chambers arranged one above the other,

Fig. 5

3 shows a cross section through such a chamber in which the guide plates are concentrically formed on the left, that is to say in the form of cylinders, and in the right half of the figure, plates are arranged in a string-like manner,

6a + 6b

a representation similar to FIGS. 2a + 2b with horizontally arranged 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 from one another by vertical walls 8.

The vertical design with a standing 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 for the horizontal and vertical versions. One group 14 of guide vanes forms a plurality of compartments 15 within a chamber in that they are sealingly connected to the bottom at the bottom and form an overflow weir at the top. The overflow weirs of a chamber result in a cascade, the height of which steps result 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 at the bottom for underflow 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 removed 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 devices 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.

• Bei der waagerechten Lösung durch die äußere Begrenzung als Kreissegment bei jedem Abteil. Beim Strom von oben nach unten wird der Querschnitt immer enger, umgekehrt von unten nach oben immer weiter. Beim Umströmen der Leitbleche oben und unten entsteht ebenfalls ein pulsartiger Effekt.
• Bei der senkrechten Lösung sind die Verhältnisse entsprechend.
• Zusätzlich wird bei beiden Ausführungen durch den Einbau von Dellen oder ähnlichen Ausbuchtungen in die Leitbleche in der entsprechenden Größe und Anzahl jede gewünschte Pulsation der Strömung erreicht.

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 ever narrower, conversely from bottom to top, it becomes wider and wider. When flowing around the baffles at the top and bottom, there is also a pulse-like effect.
• The situation is the same for the vertical solution.
• 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 always become 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 most important advantage of this process is that - in contrast to the known systems of the stirred tank cascade - all crystals have uniform residence times under the same conditions due to the direct current without mixing. 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 into the thick juice prior to the crystallization work can take place more uniformly since it no longer depends 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 primarily 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 to the extent that the following compartments at the expense of the following compartments. 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 in the enlarged sections is essentially caused by the resulting vapor bubbles. 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 due to the evaporation of water 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. Process for the continuous crystallisation of sugar using a continuous flow apparatus, characterised in that a sugar solution that is saturated at relatively high temperatures, for example from 80° to 120 °C, is continuously cooled by flash evaporation in pulses in cocurrent flow, there being no interblending at any process stage.
2. Process according to claim 1, characterised in that for the formation of seed crystals a small amount of magma containing newly formed very small crystals or seed crystals is fed back into the suitably supersaturated sugar solution and is mixed intensively therewith by means of static tube mixers.
3. Apparatus for carrying out the process according to claims 1 and 2, characterised in that a horizontal (Fig. 1) or upright (Figs. 3 and 4) cylindrical vessel is divided into a plurality of chambers (7 and 10, respectively) and the individual chambers (7) are separated from one another by vertical walls (8) in the horizontal design and by horizontal floors (11) and ceilings (12) in the upright design, and the individual chambers of both designs contain guide plates (14, 16).
4. Apparatus according to claim 3, characterised in that one set of guide plates (14) forms a plurality of compartments within a chamber (7 or 10) as a result of the guide plates being joined at the bottom tightly to the floor or the wall and forming an overflow weir, the overflow weirs in turn forming a cascade the step heights of which are determined according to the flow characteristics of the magma in question and the desired flash time, while the second group of guide plates (16) extends downwards between the cascade plates (14) so that meandering flow paths are formed and, at the top, apertures for the currents of exhaust vapours are formed, and in the case of both the horizontal and the vertical designs the guide plates extend parallel to one another or are curved and parallel to one another and have a flat, cylindrical or tapered frustoconical form.
5. Apparatus according to claim 4, characterised in that the guide plates have depressions or similar indentations (20).
6. Apparatus according to claim 3, characterised in that an external pipe (21) having a regulating device is provided at the bottom for conveying the magma from one chamber to the next.
7. Apparatus for carrying out the process according to claim 1, characterised in that one or more static tube mixers (23) having cooling and heating jackets (24, 25) are arranged upstream of the horizontal or the upright crystallization apparatus.
8. Process according to claim 2, characterised in that instead of the returned magma, a slurry is introduced or air is uniformly or intermittently introduced into the static tube mixer.
9. Apparatus according to claim 3, characterised in that the first compartment (15') of each chamber (7, 10) is enlarged at the expense of the subsequent compartments to the extent that at most only two compartments are present in a chamber, there being installed, if necessary, in each compartment guide plates (16') extending as far as the floor or having openings, and the magma is supplied in the former case between the guide plates (14') and otherwise at a plurality of locations (28') in the floor.

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