COOLING CRYSTALLISATOR AND METHOD FOR SUGAR CRYSTALLISATION

Description

[0001] The invention relates to a cooling crystallizer for sucrose magma in a vertically oriented container, which has a top inlet for supplying magma and a bottom outlet for removing magma, with several cooling blocks spaced vertically apart from one another, wherein the cooling blocks are permeated by a heat transfer fluid and coupled to a heat exchanger to dissipate heat from the magma. The invention also relates to a method for sugar crystallization and cooling of sugar magma in a cooling crystallizer.

[0002] Sugar crystallization is a crucial process engineering step in obtaining sucrose dissolved in thick juice. During a single crystallization step, the technically achievable desugaring of a solution is limited by the crystal content in the magma. Therefore, several crystallization stages are necessary. The physical process of crystallization allows for excellent separation of the sugar from non-sugar components. This requires a well-controlled crystallization process that produces a crystallizate with a low aggregate content and minimal false grains. For a very long time, evaporative crystallization of sugar was carried out almost exclusively in batch-operated apparatus.Improvements in both sugar quality and energy consumption during crystallization were achieved through the use of mechanical agitators in such batch evaporative crystallizers. Continuous evaporative crystallizers are now commonplace to ensure consistent magma production.

[0003] After evaporative crystallization, in which the majority of the sugar crystal mass is produced, the molten molasses is cooled. The aim here is to extract as much sucrose as possible from the mother syrup by further crystallizing the existing crystals. This is the final desugaring stage in sugar production. Errors in the cooling process have irreversible effects on sugar losses in the molasses, making cooling crystallization of paramount importance. After cooling, the mother solution is separated from the crystals in a centrifuge. It is crucial to ensure that the crystals do not fall below a certain size, otherwise they will pass through the centrifuge sieve with the mother solution and be lost as sugar crystals, thus reducing the sugar yield.Therefore, during cooling, one goal is to prevent the formation of new crystals and ensure that crystallization occurs only on existing crystals. The formation of new crystals is prevented by ensuring that certain supersaturation levels are not exceeded. Furthermore, during sugar crystallization, it is important to note that continued crystal growth reduces the supersaturation in the mother solution, ultimately preventing further crystallization. Only by lowering the temperature of the magma can the supersaturation be increased again to the desired level.

[0004] Basically, the cooling of the magma can take place in open containers without insulation or in containers with built-in cooling pipes or cooling surfaces.

[0005] From GB 2053019A, a heat exchanger for the crystallization of suspensions is known, in which several vertically spaced cooling elements are arranged in a cylindrical body, through which a heat exchanger fluid is pumped. The cooling elements are raised and lowered by means of two hydraulic pistons.

[0006] DE 35 17 511 C2 relates to a cooling crystallization tower for sugar magma, comprising a vertical, cylindrical vessel with a top inlet and a bottom outlet, and tiered shelves dividing the vessel into chambers, which have a downwardly tapered conical shape. A cylinder is arranged in each chamber, forming a closable through-opening with the shelf. Furthermore, each chamber contains a heat exchanger through which a cooling medium flows, and vertically oscillating circulation elements.

[0007] US Patent 8,475,597 B2 relates to a method and apparatus for sugar crystallization by controlled cooling of saturated sugar solution in a crystallizer with a cylindrical housing. The housing has an inlet for a saturated sugar solution at its top and an outlet for a crystallized mass at its bottom. A plurality of heat exchangers are arranged in different planes transverse to the housing's longitudinal axis. Each heat exchanger defines a specific crystallization stage of the mass within the housing, which is moved from top to bottom. The heat exchangers are in the form of a spiral or a coil and each is provided with a connection for heating or cooling water. From an outlet, the heat exchange medium is directed into the heat exchanger immediately above it, up to the uppermost heat exchanger, from which the heat exchange medium is discharged for processing.Each heat exchanger, except for the top and bottom ones, has an inlet connected to an external heat exchanger, ensuring that the magma temperature is maintained within a predetermined range at each crystallization stage. Temperature sensors are integrated into the inside of the housing and connected to an electronic control module. This module opens or closes valves based on the temperature signal to regulate the flow rates and temperatures of the heat exchange medium. The heat transfer fluid can be colder or warmer than the heat exchange medium at the valve.

[0008] Furthermore, a cooling crystallizer is known from the publication "technik-programm" of Braunschweigische Maschinenbauanstalt AG. This crystallizer consists of standardized cooling block elements in which the cooling medium is forcibly guided through pipes from bottom to top via a vertical cooling cylinder. Within the cylinder, magma is moved downwards by gravity. The cooling block elements oscillate vertically and are arranged in two circuits, allowing either one or both circuits to be operated simultaneously. Should one circuit fail, the cooling crystallizer can continue to operate. The two circuits are hydraulically connected in series.

[0009] Between two cooling blocks of one cooling block circuit, a cooling block of the other cooling block circuit is arranged, with the exception of the upper and lower cooling blocks.

[0010] The article by Michael Getaz et al "Recent developments in vertical cooling crystalliser design", November 1, 2011, pages 1 to 8, XP055757943 concerns developments in the design of vertical cooling crystallisers with regard to vessel heights and vessel diameters, the design of the cooling surfaces, the design of the stirring elements and their drives, and the coolant routing.

[0011] DE 27 43 671 A1 relates to a crystallizer comprising an elongated container with cooling elements arranged therein and a device that causes relative motion between the liquid to be crystallized and the cooling elements. The elongated container has at least one bundle of stationary cooling tubes arranged at a distance from one another, extending longitudinally along the container. The container includes at least one vane extending along a shaft and attached to it. The shaft extends along the container and is connected to a device for causing an oscillating motion of the shaft. The coolant in the cooling tubes and the liquid to be crystallized in the container flow through the crystallizer in the same direction.

[0012] The article by Mario Llanao-Restrepo "Modeling and simulation of vertical continuous cooling crystallizers for the sugar industry", Industrial & Engineering Chemistry Research, Vol. 44, Number 24, November 1, 2005, pages 9244 to 9263, XP 05575 7940, ISSN: 0888-5885, concerns the modeling and simulation of continuously operated, vertical cooling crystallizers for the sugar industry, in which the aforementioned cooling crystallizer of Braunschweigische Maschinenbauanstalt AG is also described.

[0013] The object of the present invention is to provide a cooling crystallizer and a method for sugar crystallization with which an improved yield of sugar crystals from a magma can be achieved with the least possible equipment effort.

[0014] According to the invention, this problem is solved by a device having the features of the main claim and a method having the features of the dependent claim. Advantageous embodiments and further developments of the invention are disclosed in the dependent claims, the description, and the figures.

[0015] The cooling crystallizer for sucrose magma, comprising a vertically oriented vessel with a top inlet for magma intake and a bottom outlet for magma discharge, and several vertically spaced cooling blocks through which a heat transfer fluid flows and which are coupled to a heat exchanger to dissipate heat from the magma, is designed so that several cooling blocks are grouped into cooling packages, and these cooling packages are configured as separate cooling circuits with separate heat exchangers. This design of the cooling crystallizer with multiple cooling packages consisting of several cooling blocks and their thermal and hydraulic separation makes it possible to set different cooling water volumes as well as different temperature differentials between the magma and the heat exchangers or the respective heat transfer medium.This makes it possible to adjust the temperature in specific areas according to the respective saturation level of the magma and the desugaring level of the mother solution. Hydraulic decoupling allows for optimal adjustment of the required amount of heat transfer fluid, such as water, ensuring optimal crystallization conditions at all times during the cooling process. This allows for a higher magma throughput with the same equipment size, or conversely, a smaller equipment size for a given throughput. Even at higher throughputs, the pressure drop in the heat transfer fluid line no longer becomes a limiting factor.

[0016] In addition to the adjustability of the flow rates through hydraulic decoupling, the thermal decoupling with the separate heat exchangers makes it possible to supply the cooling packages with heat transfer fluid at different temperatures, so that the temperature differences over the contact length of the magma with the respective cooling package can be optimally adapted to the respective saturation level of the magma.

[0017] Further development provides that the cooling packages are designed to be vertically separated from each other and arranged inside the container, so that an adapted temperature difference can be set over the transport path of the magma within the container through spatial separation and vertical spacing.

[0018] The temperature difference between the magma and each cooling pack is preferably set to decrease from top to bottom. The uppermost cooling pack thus has the greatest temperature difference between the magma and the respective cooling block or heat transfer fluid, while the lowermost cooling pack has the smallest. This takes into account the different saturation levels and the varying crystallization rates depending on the temperature. In cases of advanced crystal growth at low magma temperatures, a slow cooling rate can thus be implemented.

[0019] The flow rate of the heat transfer fluid through the cooling packs is advantageously adjustable, with the required flow rate being set based on sensor data or state variables. This makes it possible to control or regulate sugar crystallization within the cooling crystallizer. Similarly, the inlet temperature of the heat transfer fluid to each cooling pack is advantageously adjustable separately from the inlet temperature to the other cooling packs. Since the maximum possible crystal growth rate decreases with decreasing magma purity, it is advantageous if the temperature difference decreases from top to bottom along the path through the container, thus ensuring a small temperature difference between the magma and the heat transfer fluid at low magma temperatures.Towards the end of the cooling process, the temperature difference between the heat transfer fluid and the magma should be adjusted so that the supersaturation within the mother solution allows the maximum possible crystal growth rate.

[0020] The cooling blocks, and thus also the cooling packages, can be arranged vertically within the container and coupled to a drive system, allowing them to be moved within the container, either together or individually. For this purpose, the cooling packages can be connected to lifting tubes driven by hydraulic cylinders that travel a predetermined distance vertically. Cooling water is conveyed through the lifting tubes to the cooling pipes in the respective cooling packages or blocks and then discharged again. The vertical movement and mobility of the cooling blocks and packages within the magma initially results in improved temperature distribution and more uniform cooling. Furthermore, the movement of the cooling packages within the magma creates a cleaning effect on the cooling blocks, preventing sugar crystals from adhering to the cooling pipes of the cooling packages or blocks and removing them from the cooling process.The purely vertical movement of the cooling parcels does create relative motion between them and the magma, but it results in only minimal mixing of the magma. This oscillating vertical movement prevents large temperature fluctuations within the magma and thus the formation of fine crystals. However, a continuous temperature decrease is still ensured by this vertical movement.

[0021] In addition to the thermal and hydraulic separation of the cooling packages, it is advantageous to install a magma temperature sensor in the area between two cooling packages and / or two cooling blocks to obtain information about the crystallization process based on the magma temperature. Magma flow rates, heat transfer fluid flow rates, and the respective temperatures can be adjusted based on the sensor data to achieve optimal sugar yield. It is particularly advantageous to position a temperature sensor at the interface between two cooling packages to adjust the relevant parameters in specific areas. Grouping the cooling blocks into cooling packages facilitates parameter adjustments to the respective crystallization progress without excessively increasing the system's complexity.

[0022] A further development of the invention provides that at least one optical sensor is arranged in the container and / or a pipeline to detect crystal formation. The sensor data from the optical sensor can be used alone or together with other sensor data, for example, data from the temperature sensor(s), to control or regulate sugar crystallization within the cooling crystallizer. This can be achieved, for example, by changing the flow rates of magma and / or heat transfer fluid, changing the temperatures, or by other changes to operating parameters, depending on whether or not new sugar crystals have formed. If, for example, unwanted new crystals are detected in the crystallizer using an optical sensor, they can be dissolved again in a downstream mixer, for example, by adding a hot medium, such as hot molasses.Additionally, other parameters can be adjusted to prevent the formation of new crystals.

[0023] The cooling crystallizer with two cooling circuits is specifically designed for processing sucrose magma; however, it is also possible to process fructose or glucose magma with such a cooling crystallizer. The control and / or regulation of the crystallization process is particularly simplified when a sensor for detecting crystal formation, e.g., an optical sensor, is integrated.

[0024] The process for sugar crystallization and cooling of sugar magma in a cooling crystallizer, as described above, involves establishing a decreasing temperature difference between the cooling blocks and the magma in the direction of flow from top to bottom. The temperature difference is advantageously set individually for each cooling block, with the temperature difference being greatest in the uppermost cooling block and smallest in the lowermost. The driving temperature difference at the beginning of the magma cooling process is preferably greatest in the region of the upper cooling block, as this is where the mother syrup is of the highest purity, thus enabling a high rate of crystal growth. At the end of the cooling crystallization process, the mother syrup has a comparatively low purity, making slow cooling and a small temperature difference advantageous from a process engineering perspective.The process is particularly applicable to sucrose magma, but can also be used for glucose or fructose magma.

[0025] The temperature difference between the heat transfer fluid and the magma at the uppermost cooling package is set between 15 K and 20 K, while the temperature difference in a cooling package arranged vertically below it is set between 8 K and 12 K.

[0026] Different magma supersaturations can be set in each section of the cooling packs, achieved through temperature control and adjusting the magma flow rate through the container. The magma flow rate can be varied, for example, by opening or closing a port, slide, or valve at the bottom outlet.

[0027] To ensure uniform cooling, the heat transfer fluid is advantageously passed through the cooling blocks in counterflow, so that a uniform temperature difference can be achieved along the length of the cooling package, from the inlet to the outlet of the respective cooling blocks and cooling packs. This makes it possible, for example, to keep the temperature difference between the magma and the cooling blocks within a cooling pack constant.

[0028] In another embodiment, the previously described temperature difference between the magma and the cooling blocks in a cooling package can be varied or variably adjusted and, for example, decrease over the residence time of the magma in the cooling crystallizer. This allows, for example, a more uniform adjustment of the magma supersaturation and an increase in crystal growth.

[0029] Exemplary embodiments of the invention are explained in more detail below with reference to the accompanying figures. These show: Figure 1 – a schematic representation of a cooling crystallizer in partial sectional view; Figure 2 – a schematic representation of a cooling block; Figure 3 – a circuit diagram of a cooling crystallizer; Figure 4a – the temperature profile with constant temperature differences over the residence time of the magma in the cooling crystallizer; Figure 4b – the temperature profile with varying temperature differences over the residence time of the magma in the cooling crystallizer; Figure 5a – the supersaturation and crystal growth with constant temperature differences over the residence time of the magma in the cooling crystallizer; and Figure 5b – the supersaturation and crystal growth with varying temperature differences over the residence time of the magma in the cooling crystallizer.

[0030] In the Figure 1A schematic cross-sectional view shows a cooling crystallizer 2.0, which has a vertically oriented container 2.1 with a top inlet 2.2 and a bottom outlet 2.3. Magma from the precursors of sugar crystallization is introduced into the container 2.1 through the inlet 2.2. These precursors include, for example, the initial crystal base formation, white sugar production, raw sugar production, and the production of the final sugar. The final sugar is prepared, for example, in an evaporation crystallization tower, where the purity of the mother syrup is reduced and a major portion of the final sugar crystal mass is produced. Subsequently, the magma is cooled in the cooling crystallizer 2.0. The aim is to remove as much sucrose as possible from the mother syrup by increasing the size of the existing crystals.This is the final desugaring stage of the mother syrup, which can be carried out continuously using the cooling crystallizer 2.0. The vessel 2.1 is completely filled with magma from the evaporation crystallization tower via the inlet 2.2. A distributor is driven by a motor 6.2, which rotates and evenly distributes the magma from the inlet 2.2 over the entire surface of the magma within the vessel 2.1. Within the vessel 2.1, which can have an operating volume of several hundred cubic meters, cooling blocks 5.0 are arranged vertically, spaced apart from one another, in levels to cool the warm magma from the evaporation crystallization tower. Cooling water is pumped through the cooling blocks 5.0, which are preferably arranged at uniform intervals, via a water inlet 2.4. The cooling water acts as a heat transfer fluid, flowing countercurrently to the flow of magma through the cooling crystallizer 2.0.0, meaning it is directed from bottom to top. The cold cooling water is thus first pumped through the lowest cooling block 5.0 and from there upwards through the individual cooling blocks 5.0 to the top water outlet 2.5. From there, the heated heat transfer fluid or cooling water is processed, in particular cooled.

[0031] The entire cooling system, including the cooling blocks 5.0 and the cooling water lines (designed as pipes), oscillates vertically. This movement is driven by actuators 6.1, preferably in the form of hydraulic cylinders. The hydraulic cylinders 6.1 are preferably arranged symmetrically on the lid of the cooling crystallizer 2.0. The raising and lowering of the cooling blocks 5.0, the design of which will be explained below, provides a good self-cleaning effect, preventing potential encrustation on the cooling surfaces. This allows even highly viscous magmas to be processed without difficulty.

[0032] The vertical orientation of vessel 2.1 results in a small footprint. Furthermore, the modular design of vessel 2.1 allows for easy adaptation of the system to different throughput rates. Due to the uniform relative movement of the magma to the cooling blocks, a consistent and efficient heat transfer between the magma and the cooling medium or heat transfer fluid, particularly cooling water, is ensured. The outlet temperature at the bottom outlet 2.3 can be precisely controlled. The typical inlet temperature is between 60 °C and 85 °C, depending on the upstream process, while the outlet temperature is usually around 40 °C.

[0033] In the Figure 2An exemplary cooling block 5.0 is shown, constructed from straight pipe sections. The pipes are arranged hexagonally and guided in a spiral shape in two or more vertical planes. The pipes themselves are arranged in a substantially circular frame (not shown) and mounted on radially outward-projecting supports, which can be raised or lowered vertically via lifting devices (not shown). The cooling water or heat transfer fluid is supplied at a cooling block inlet 5.0.2 on the bottom. In the illustrated embodiment, the cooling block inlet 5.0.2 is arranged on an outer pipe. From there, the cooling fluid is pumped inwards through the pipes in a spiral pattern and guided through an inner passage into the next cooling pipe plane arranged vertically above, in which the cooling water is pumped from the inside out through the pipes. From the cooling block outlet 5.0.2 on the top...3 The heat transfer fluid or cooling water is then directed into a cooling block 5.0 arranged above it.

[0034] In the Figure 3A circuit diagram shows a cooling crystallizer 2.0 with its essential components. Magma to be cooled is pumped from the evaporation crystallizer to the magma inlet 2.2 at the top via a magma pump 1.0. The magma to be cooled is then evenly distributed across the surface of the magma already inside the container 2.1 via a distributor (not shown) driven by the motor 6.2. A level sensor can be coupled to a controller (not shown) to ensure a uniform fill level in the container 2.1. The supply via the magma pump 1.0 corresponds to the discharge of the fully crystallized and cooled magma through the outlet 2.3 at the bottom of the container 2.1. From the outlet 2.3, the cooled magma is fed to further processing, for example, a suitable centrifuge. This is done via a magma pump 4.0, which is connected to a molasses-magma mixer 3.0 can be prior.

[0035] Several cooling blocks 5.0 are arranged inside container 2.1. In the diagram of the Figure 3 These are depicted as zigzag lines; in particular, they have a shape similar to that found in the Figure 2 has been described. Deviating shapes, for example a true spiral shape or a different number of corners in a polygonal structure, are possible, as are variations regarding the pipe planes per cooling block 5.0. The cooling blocks 5.0 in the illustrated embodiment of the Figure 3The cooling elements are grouped into two cooling packages 5.1, 5.2. The first cooling package 5.1 is located below the second cooling package 5.2. The terms "above" and "below" refer to a vertical orientation or the direction of gravity. Each cooling package 5.1, 5.2 has its own cooling water inlet 5.1.2, 5.2.2 and its own cooling water outlet 5.1.3, 5.2.3, from which the cooling water or heat transfer fluid is discharged from the container 2.1. The cooling water outlet 5.1.3 of the first cooling package 5.1 is located below or at the same level as the cooling water inlet 5.2.2 of the cooling package above it, in the illustrated embodiment of the second cooling package 5.2. In addition to the illustrated embodiment with two cooling packages 5.1, 5.2, three or more cooling packages can also be arranged within the container 2.1. Each cooling package 5.1, 5.2 is supplied with cooling water via its own cooling water pump 2.1.1, 2.2.1.Valves in the supply lines regulate the amount of cooling water supplied. From the respective cooling water outlet 5.1.3, 5.2.3, the heated cooling water is fed to a separate heat exchanger 2.1.2, 2.2.2. Each cooling circuit also has a separate expansion tank 2.1.3, 2.2.3 from which cooling water is drawn. Each cooling package 5.1, 5.2 thus has an independent cooling circuit with its own cooling water pump 2.1.1, 2.2.1, its own heat exchanger 2.1.2, 2.2.2, and its own expansion tank 2.1.3, 2.2.3, so that both cooling packages 5.1, 5.2 are thermally and hydraulically separated from each other. Both cooling packages 5.1, 5.2 can be raised and, if necessary, lowered together via the hydraulic cylinders 6.1 (not shown). The lowering can also occur via gravity. The lowering of the cooling packages 5.1, 5.2 within the magma occurs faster than the lowering rate of the magma within the container.The rate at which the magma settles within the vessel 2.1 is determined by a valve (not described in detail) or a control slide at the outlet 2.3, or by the pump 4.0. While it is also possible for the individual cooling units 5.1 and 5.2 to move independently, either upwards or downwards, it is advantageous from a process engineering perspective to minimize the mixing of magma at different temperatures. This prevents disruption to the crystallization process and avoids the formation of new, undesirable crystals. The formation of new, undesirable crystals is preferably detected by one or more optical sensors located at suitable points in the vessel 2.1 and / or adjacent pipelines. Such sensors are suitable for implementing appropriate control mechanisms to optimize operation without the formation of new crystals.Undesired new crystals formed in the crystallizer can be detected using an optical sensor and dissolved again in a downstream mixer, such as the molasses-magma mixer 3.0, by adding a hot medium, such as hot molasses, in a control loop.

[0036] A temperature sensor 5.3 is located between the two cooling packages 5.1 and 5.2. This sensor measures the transition temperature of the magma between the two cooling packages. The temperature sensor 5.3 is coupled to a control unit (not shown), which is also coupled to the pumps 2.1.1 and 2.2.1. The temperature of the cooling fluid or heat transfer medium is also monitored to take the supply temperature of the heat transfer fluid into account and to be able to change it if necessary. By changing the supply temperature and / or the quantity of heat transfer fluid or cooling water, it is possible to set different temperature differences between the magma inside the vessel 2.1 and the cooling packages 5.1 and 5.2, or the cooling water. The vertically separated arrangement of the cooling packages 5.1 and 5.2 makes it possible to adjust the temperature differences depending on the temperature of the magma.The crystallization rate of the magma changes with its purity and temperature. If cooling water from only one cooling circuit is pumped counterflow through the cooling blocks 5.0 over the entire height of the cooling crystallizer 2.0, the temperature difference can only be adjusted at the lower cooling fluid inlet 5.1.2, 5.2.2. The temperature difference between the cooling packages 5.1, 5.2 and the magma in the vessel 2.1 can be influenced by changing the flow rates of the heat transfer fluids. However, the amount of heat transfer fluid cannot be increased arbitrarily, as the pressure drop in the heat transfer fluid line increases relative to the velocity of the heat transfer fluid in the pipes of the cooling blocks 5.0, so that a technical limit of approximately 10 bar cannot be exceeded. This problem can be solved with the hydraulically and thermally decoupled cooling packages 5.1, 5.2.2.The need for two vertically separated cooling circuits is avoided, as the pressure loss is almost halved with two hydraulically separated, equally sized cooling circuits. Furthermore, with the two or more cooling packages, it is possible to achieve optimal process conditions at all times. Surprisingly, it has been found that with the previously described concept for vessels up to several hundred cubic meters in size and the unknown flow behavior of the magma in the very tall and slender vessels 2.1, an intensification of the cooling capacity with a corresponding reduction in the number of cooling blocks can be achieved compared to the prior art. This allows for a reduction in the size of the vessel 2.1. Alternatively, the magma throughput can be increased while maintaining the same size of the cooling crystallizer.It is also possible to build cooling crystallizers modularly and to adapt the cooling conditions to the residence times in the container, whereby the pressure loss in the heat transfer fluid line is only a subordinate criterion.

[0037] In the Figure 4aThe temperature profile of magma over its residence time in a two-stage cooling crystallizer with constant temperature differences is shown schematically. The magma temperature is represented by the upper curve, and the temperature of the cooling water (heat transfer fluid) by the two lower straight lines. The magma inlet temperature is approximately 75 °C to 80 °C. Over a mean residence time of approximately 27 hours, the magma is cooled to about 42 °C. The residence time is the mean time the magma or crystal suspension spends in the cooling crystallizer; the temperature is measured at the point where the magma or crystal suspension has reached this mean residence time. The outlet temperature of the cooling water (heat transfer fluid) from the lower cooling pack 5.1 is approximately 48 °C, and the outlet temperature of the second, upper cooling pack 5.2 is approximately 55 °C.The inlet temperature of the heat transfer fluid in the upper cooling package 5.2 is 40 °C, and the inlet temperature of the heat transfer fluid in the lower, first cooling package 5.1 is 34 °C. Due to the counterflow flow, the magma temperature decreases and the temperature of the heat transfer fluid increases as it flows through the vessel 2.1. Figure 4aIt can be seen that the temperature difference between the magma and the heat transfer fluid differs for the two cooling packages 5.1 and 5.2. For the lower cooling package 5.1, the temperature difference is approximately 8 K and remains essentially constant over the entire contact length or duration of the magma with the lower cooling package 5.1. The temperature difference between the magma temperature and the temperature of the heat transfer fluid in the upper, second cooling package 5.2 is approximately 18 K, but can also be adjusted, for example, to between 12 K and 15 K. Here, too, the temperature difference remains constant over the entire residence time or contact distance.

[0038] In the Figure 5aThe solid line represents the supersaturation YÜ of the magma, while the dotted line represents the crystal growth ΔmK over the residence time of the magma at constant temperature differences. During cooling and crystallization in the upper section of the cooling crystallizer 2.0, the supersaturation increases linearly from approximately 1.18 to approximately 1.22. Supersaturation is a dimensionless concentration difference and is defined as the concentration of the dissolved substance to be crystallized in the liquid phase of the crystal suspension relative to the equilibrium concentration. Crystal growth is the amount of the crystallizing component that has grown from the liquid phase onto the existing crystals. Crystal growth is plotted against the height of the crystallizer; in this case, one height is the distance between two cooling blocks with their corresponding magma volume.The supersaturation decreases continuously after approximately eight hours or at the transition between the upper cooling package 5.2 and the lower cooling package 5.1, ultimately reaching a value of 1.149. This is due to the increasing crystallization, such that the crystal suspension exhibits a progressively lower sucrose content with increasing cooling and residence time. The crystal growth ΔmK in the upper cooling package 5.2 is initially approximately 0.31 and decreases to a value of 0.11 with increasing residence time after contact with the lower, first cooling package 5.1, thus slowing down crystal growth.

[0039] In the Figure 4bThe temperature profile of a two-stage cooling crystallizer with varying temperature differences is schematically depicted over the residence time of the magma. The magma temperature is shown in the upper curve, and the temperature of the cooling water (heat transfer fluid) is shown in the two lower curves. The magma inlet temperature is approximately 75 °C to 80 °C. Over an average residence time of approximately 27 hours, the magma is cooled to approximately 42 °C. The outlet temperature of the cooling water (heat transfer fluid) from the lower cooling package 5.1 is approximately 45 °C, and the outlet temperature of the second, upper cooling package 5.2 is approximately 48 °C. The inlet temperature of the heat transfer fluid in the upper cooling package 5.2 is 44 °C, and the inlet temperature of the heat transfer fluid in the lower, first cooling package 5.1 is 35 °C. Figure 4bIt can be seen that the temperature difference between the magma and the heat transfer fluid differs for the two cooling packages 5.1 and 5.2 and changes during the residence time. For the lower cooling package 5.1, the temperature difference is between approximately 7 K at the inlet 5.1.2 and approximately 11 K at the outlet 5.1.3. The temperature difference between the magma temperature and the temperature of the upper, second cooling package 5.2 is between approximately 15 K at the inlet 5.2.2 and approximately 24 K at the outlet 5.2.3.

[0040] In the Figure 5bThe solid line represents the magma supersaturation YÜ, while the dotted line represents the crystal growth ΔmK over the magma's residence time with varying temperature differences. During cooling and crystallization in the upper section of the cooling crystallizer 2.0, the supersaturation increases degressively from approximately 1.18 to approximately 1.22. After about eight hours, or at the transition between the upper cooling package 5.2 and the lower cooling package 5.1, the supersaturation decreases continuously, ultimately reaching a value of 1.148. The crystal growth ΔmK in the upper cooling package 5.2 is approximately 0.31 and decreases to 0.11 with increasing residence time after contact with the lower, first cooling package 5.1, thus slowing down crystal growth.

Claims

1. A cooling crystallizer (2.0) for sucrose magma in a vertically oriented container (2.1), which has an inlet (2.2) at the top for feeding in magma and an outlet (2.3) at the bottom for discharging magma, having a plurality of cooling blocks (5.0) which are vertically spaced apart from one another, wherein a heat transfer fluid flows through the cooling blocks (5.0) and the cooling blocks are coupled with a heat exchanger (2.1.2; 2.2.2) in order to dissipate heat from the magma, characterized in that a plurality of cooling blocks (5.0) are combined to form a cooling pack (5.1; 5.2) and at least two cooling packs (5.1; 5.2) are configured as separate cooling circuits with separate heat exchangers (2.1.2; 2.2.2) and are thermally and hydraulically separate from one another.

2. The cooling crystallizer as claimed in claim 1, characterized in that the cooling packs (5.1; 5.2) are configured so as to be vertically separate.

3. The cooling crystallizer as claimed in claim 1 or 2, characterized in that the temperature difference between the magma and the respective cooling pack (5.1; 5.2) is adjusted so that it decreases from top to bottom.

4. The cooling crystallizer as claimed in one of the preceding claims, characterized in that the flow rates of the heat transfer fluid flowing through the cooling packs (5.1; 5.2) can be adjusted separately.

5. The cooling crystallizer as claimed in one of the preceding claims, characterized in that the inlet temperature of the heat transfer fluid into the respective cooling pack (5.1; 5.2) can be adjusted separately.

6. The cooling crystallizer as claimed in one of the preceding claims, characterized in that the cooling blocks (5.0) are arranged in the container (2.1) so as to be vertically displaceable and are coupled with a drive (6.1).

7. The cooling crystallizer as claimed in one of the preceding claims, characterized in that a temperature sensor (5.3) is arranged in the region between two cooling packs (5.1; 5.2) and / or cooling blocks (5.0).

8. The cooling crystallizer as claimed in one of the preceding claims, characterized in that at least one optical sensor is arranged in the container (2.1) and / or in a pipeline for detecting the formation of new crystals.

9. A method for sugar crystallization and cooling of sugar magma in a cooling crystallizer as claimed in one of the claims 1 to 8, characterized in that a decreasing temperature difference between the cooling blocks (5.0) and the magma is adjusted from top to bottom by adjusting a temperature difference of between 15 K and 20 K in the uppermost cooling pack (5.2) and adjusting a temperature difference of between 8 K and 12 K in a cooling pack (5.1) arranged vertically beneath.

10. The method as claimed in claim 9, characterized in that different supersaturations of the magma are adjusted for each cooling pack (5.1; 5.2).

11. The method as claimed in one of claims 9 to 10, characterized in that the heat transfer fluid is guided countercurrently through the cooling blocks (5.0).

12. The method as claimed in one of claims 9 to 11, characterized in that the temperature difference between the magma and the cooling blocks (5.0) in a cooling pack (5.1; 5.2) is kept constant or is variably adjusted.

13. The method as claimed in one of claims 9 to 12, characterized in that the formation of new crystals is detected by at least one optical sensor in the container (2.1) and / or in a pipeline and the new crystals are removed and / or operating parameters are changed on the basis of the sensor data.