STIRRINGER FOR EVAPORATION CRYSTALLIZER

Description

[0001] This invention relates to improved stirrers suitable for evaporative crystallizers, particularly for the crystallization of sucrose from sugar beet syrup. The stirrers have one or more additional actively flow-directing means.

[0002] Evaporative crystallizers are used to obtain crystals from solutions or suspensions by removing the solvent, especially water. Reducing the solvent content causes the crystallizable substances contained in the solution or suspension to crystallize. To expel the solvent, heat is applied to the solution or suspension in the crystallizer vessel via heat exchangers, causing the solvent to evaporate. The solvent is removed as volatile vapor or mist in the so-called vapor chamber at the top of the vessel.

[0003] Known systems for discontinuous evaporative crystallization, in particular, usually have a vertically arranged container (tank). A heating element or heat exchanger is located at the bottom of the container. The heat exchanger heats the solution or suspension introduced into the container. The solution is also called syrup, and the crystal-containing suspension is called magma.

[0004] The state of the art in this context is an agitator consisting of a drive, a stirrer shaft, and a stirrer, which mixes the solution or suspension and essentially fulfills two functions: the solution or suspension is pumped through the heat exchanger (heating chamber) to achieve good heating and efficient expulsion of the solvent gas throughout the entire filling, and the suspension is dispersed to achieve the most uniform and efficient crystallization possible and, in particular, to prevent the formation of unwanted crystal aggregates. The stirrer is typically located in the so-called "dispersion zone," in the lower section of the vessel, particularly within a draft tube or central tube.There, a primary mixing zone forms as the stirrer rotates, creating a primarily downward flow to ultimately support the movement of the medium through the heating chamber(s), which is driven by convection and vapor bubble formation. It is advantageous if the medium in the vessel, i.e., the solution or suspension, forms a flow that is as circular as possible, and especially if the entire medium is repeatedly passed through the heating chamber of the reactor.

[0005] US 2010 / 305367 A1 discloses a reactor with a stirrer having a vertical shaft and propeller. PL 159 330 B1 discloses a high-temperature crystallizer with a vertical shaft and propeller. CN 103 263 864 A also discloses a reactor with a stirrer having a vertical shaft and propeller. US 3 395 897 A discloses a reaction vessel with a stirring mechanism having an inclined shaft. JP 2002 282667 A discloses a crystallization vessel with a mixing device having a vertical shaft and propeller. US 5 223 040 A discloses the production of sugar crystals from syrup in a batchwise apparatus. US 328 585 A discloses the use of screws for mixing.

[0006] Crystallizers of this type have been known for a long time. However, recent studies have shown that the resulting circular flow of the medium, i.e., the solution or suspension, especially the magma, during the various phases of the evaporative crystallization process in the vessel requires improvement. Turbulence occurs, particularly at high fill levels in the vessel, especially in the area of ​​the stirrer shaft, which disrupts or, in extreme cases, prevents the desired circular net flow, which is characterized by the medium rising in the wall zone of the vessel and the medium descending in the central area, in the area of ​​the stirrer shaft. This reduces the efficiency of evaporative crystallization; in particular, the dispersion or overall mixing of the medium in the stirred reactor, i.e., in particular, of a crystal suspension (magma) in an evaporative crystallizer, requires improvement.

[0007] Another problem is that in stirred reactors of this type, and particularly in evaporative crystallizers, the stirrer is guided in a so-called central tube in order to enable or improve the flow-guiding or pumping effect of the stirrer. A central tube usually completely encloses the stirrer and includes a lower opening through which the pumped medium exits and an upper opening through which the medium enters the central tube. To enable the pumping effect, the fill level of the medium, i.e. solution or suspension, in the container of the evaporative crystallizer must always be above the upper opening of the central tube, and to a sufficient extent to allow a sufficiently large amount of solution or suspension to flow into the upper opening of the central tube to enable the desired circular flow.The disadvantage is that the evaporative crystallizer must always be filled to a certain degree to establish and maintain the desired circular or toroidal flow. However, the filling level of the vessel fluctuates, especially during discontinuous operation. If this level falls below the level of the upper opening of the central tube, conventional stirrers with a central tube become ineffective, and the circular flow ceases.

[0008] The invention is based on the problem of overcoming these disadvantages of known stirred reactors and in particular of crystallizers and, above all, of achieving improved mixing and dispersion of the medium in the reactor vessel with little energy input, in particular in discontinuous evaporative crystallization.

[0009] The invention solves the technical problem by an evaporative crystallizer according to claim 1.

[0010] The invention solves the technical problem by an evaporative crystallizer comprising: a vertical reactor vessel; a heating chamber arranged in the lower region of the vessel; and a stirrer for circulating the solution or suspension to be crystallized in the vessel and feeding it into the heating chamber, with a shaft suspended vertically from above and a main propeller connected directly to the lower end of the shaft and arranged in a primary mixing zone of the vessel, wherein the stirrer on this shaft additionally has at least one further means for actively directing the flow of the solution or suspension in addition to the main propeller, wherein either the means for actively directing the flow is designed as a propeller whose diameter is less than 50% of the diameter of the main propeller, wherein a plurality of propellers are arranged spaced apart from one another along the shaft, or wherein the means for actively directing the flow is designed as a screw running along the shaft above the main propeller, whose diameter is a maximum of 50% of the diameter of the main propeller, wherein fixed baffles are arranged adjacent to the main propeller.

[0011] Disclosed is a stirrer for a stirred reactor, and in particular for a crystallizer, wherein at least one additional active flow conduction means is provided on or in the immediate area of ​​the shaft of the stirrer, which means causes an active flow conduction of the medium, that is to say the solution or suspension in the reactor vessel, particularly directly in the vicinity of the shaft, downwards, that is to say towards the main propeller of the stirrer.

[0012] Accordingly, a stirrer suitable for circulating a medium, i.e., a solution or suspension, in this container is disclosed. The stirrer has a shaft and at least one main propeller connected to the shaft for circulating the medium. The main propeller is arranged in a primary mixing zone of the container. According to the invention, the stirrer is characterized in that at least one further mechanical means for actively directing the flow of the medium, i.e., the solution or suspension, is additionally present on or immediately adjacent to its shaft and upstream of the main propeller (with respect to the main flow direction of the circulated medium).

[0013] Preferably, the at least one means is arranged immediately adjacent to the primary mixing zone.

[0014] Preferably, several such means are arranged there along the shaft, preferably at regular intervals from one another.

[0015] The means is preferably arranged directly on the shaft itself. Alternatively, it is arranged, preferably directly, adjacent to the shaft and, in particular, has an axis of rotation substantially parallel to the shaft.

[0016] The stirrer consists of a rotatable shaft arranged essentially centrally in the reactor vessel in the longitudinal direction, i.e. usually vertically, and a propeller, i.e. a blade stirrer, mechanically rigidly connected to it. The propeller is located in the dispersion zone and is driven by the shaft. It is intended to create a circular, particularly toroidal, flow of the medium in the vessel. In connection with its application in crystallization, the solution (syrup) and the viscous, crystal-containing suspension (magma) later formed during the crystallization process sink due to gravity in the central region of the vessel, i.e., in the area of ​​the stirrer shaft, into the primary mixing zone of the main propeller, which essentially pumps downwards. There, the medium is captured by the propeller and subsequently pumped through the heating chamber (heat exchanger), which is usually arranged in the wall area of ​​the vessel.

[0017] The invention therefore provides for the arrangement of active flow guidance means, i.e., several small stirring elements, on the stirrer, in particular at least immediately above the main propeller, i.e., the blade stirrer, i.e., in the upper region along the propeller shaft, in particular mechanically connected thereto. This advantageously significantly assists the downward movement of the solution or suspension along the stirrer shaft in order to feed it to the main propeller in the vicinity of the shaft. According to the invention, this avoids otherwise undesirable diffuse cross-flows in the region above the heating chamber. In particular, in the final phases of evaporative crystallization, i.e., in the final cooking sections, the effect of the stirrer can be extended to the entire volume of the cooking apparatus.

[0018] Advantageously, this type of active flow guidance according to the invention eliminates the need for a conventional flow-guiding central tube on the stirrer. The technical teaching according to the invention is therefore particularly advantageous for use in discontinuous evaporative crystallization, since the effect of the stirrer is improved regardless of the fill level of the vessel.

[0019] The stirrer therefore fulfills two tasks to a special degree: (1) the efficient dispersion of a crystal suspension (magma) and (2) its efficient mixing during the evaporative crystallization process. In this application of the stirred reactor as a discontinuously operated cooking apparatus, these two tasks are of different importance over the course of the cooking process: Dispersion is particularly important in the first cooking phase, especially for crystal sizes below 100 µm. There, the main aim is to distribute the seed medium, which is introduced into the solution (syrup) to trigger crystallization, and to prevent the still small crystals from aggregating. The mechanical shear effect and a high specific power input into the crystal suspension are particularly important here. Mixing is particularly important in the so-called boiling and boiling-down phase of the crystal suspension.This optimizes heat transfer from the heating elements to the suspension. This is particularly important when using low-energy heating vapors. Efficient mixing avoids local variations in the magma's composition, particularly differences in concentration, temperature, or crystal distribution.

[0020] In one embodiment of the invention, the at least one active flow-conducting means itself is designed in the form of an impeller or propeller of the blade agitator. However, this is significantly smaller than the preferably single main propeller. According to the invention, the active flow conduction of the medium, i.e., the solution or suspension, remains directly limited to the area around the shaft of the main propeller, i.e., in particular, to the primary mixing zone. Surprisingly, without wishing to be bound by theory, a highly efficient improvement in the overall effect of the agitator can be achieved precisely through the active flow conduction in the immediate area around the shaft of the agitator, which is also independent of the fill level of the container.

[0021] According to the invention, the means for active flow guidance is designed in the form of a propeller, the diameter of which is preferably less than 50% of the diameter of the main propeller. The diameter of this flow guidance means in the form of an impeller or propeller is preferably one-tenth (10%) to one-half (50%), preferably 15% to 30% of the diameter of the main propeller or less.

[0022] The angle of attack of the active flow guide device according to the invention, designed as a propeller, is preferably a maximum of 45° relative to the horizontal (perpendicular to the axis of rotation), but preferably from 10 to 30° relative to the horizontal. According to the invention, the number of blades is preferably at least two, but preferably from four to six.

[0023] In In an alternative or additional embodiment of the invention, the at least one flow-guiding means is designed in the form of a single-start or multi-start transport screw (worm) and is preferably dimensioned such that the flow-guiding effect of this screw is also limited to the immediate vicinity of the shaft of the main propeller, that is to say in particular to the primary mixing zone.

[0024] According to the invention, the diameter of the screw is a maximum of 50% of the diameter of the main propeller. The diameter of the screw is preferably at least 5% to a maximum of 50% of the diameter of the main propeller, preferably from 15 to 30% of the diameter. The angle of attack of the screw blades is preferably a maximum of 45° relative to the horizontal (perpendicular to the rotation axis), but preferably from 10° to 30°.

[0025] In an embodiment of the invention not according to the invention, the diameter of the means for active flow guidance is 50% to 80% of the diameter of the main propeller.

[0026] Fundamentally, the invention provides that, regardless of the specific design of the flow-guiding means(s) on the shaft, the active flow guidance effected is limited to the immediate area around the agitator shaft. An additional large propeller on the shaft, similarly dimensioned to the main propeller, is therefore excluded according to the invention as an additional active flow-guiding means of the agitator.

[0027] According to the invention, the actively flow-guiding means is designed to support the flow guidance of the solution or suspension in which they are immersed, following the primary net flow of the circular flow of the medium towards the main propeller. To this end, the means for actively flow-guiding is located upstream of the shaft and preferably in the immediate vicinity of the main propeller, i.e., particularly close to the primary mixing zone. The flow-guiding effect of the active means according to the invention on the medium overlaps with the mixing zone of the main propeller. Medium directed towards the main propeller in the flow direction is thus transferred, so to speak, into the primary mixing zone.

[0028] Preferably, the distance between an active flow guide configured as a propeller and the main propeller is at most approximately 1.5 times the diameter of the active flow guide and preferably corresponds exactly to the diameter of the active flow guide. If the active flow guide according to the invention is configured as a single- or multi-threaded screw in the alternative embodiment, the distance between the start of the screw at the agitator shaft and the main propeller is at most approximately the diameter of the screw, but preferably the distance is only approximately 10% of the diameter. Particularly preferably, the screw begins directly at the main propeller.

[0029] Preferably, several, i.e., at least two or three, such flow-conducting means are formed on and around the agitator shaft and are driven by it. These are arranged at a distance from one another along the shaft, preferably at regular intervals. Depending on the fill level in the container, more or fewer of the flow-conducting means arranged on the agitator shaft engage with the solution or suspension to effect active flow conduction. Flow-conducting means located above the current level of the solution or suspension in the container, i.e., in the vapor space, run empty; only those immersed in the solution or suspension effect active flow conduction.

[0030] In particular, it is provided that the actively flow-directing means are spaced apart on the agitator shaft such that their respective effective areas of the flow guide of the suspension overlap. The medium directed in the flow direction toward the main propeller, i.e., along the agitator shaft, is thus transferred, in a manner of a bucket chain, from one actively flow-directing means to the next and finally into the primary mixing zone of the main propeller. It is preferably provided that the actively flow-directing means are arranged at a distance from one another that is at most approximately 1.5 times their diameter; preferably, the distance between the actively flow-directing means corresponds to their diameter or the distance is less.

[0031] In In a preferred embodiment, the means are regularly spaced from one another. Alternatively, the spacing of the means is preferably increased with increasing distance from the main propeller or from the primary mixing zone. This advantageously improves the flow direction toward the main propeller. Without wishing to be bound by theory, the volume flow of the additional active flow line according to the invention can be reduced with increasing distance from the main propeller or from the primary mixing zone and adapted to the local compensating flows, which supports the formation of the desired toroidal circular flow in the reactor vessel.

[0032] Alternatively or additionally, the active flow line can be adjusted with increasing distance from the main propeller or the primary mixing zone by individually selecting the mechanical pumping effect of the medium itself, in particular the angle of attack of the propeller blades or the screw flights and / or the diameter of the medium, depending on its distance from the main propeller or mixing zone. This adjustment of the active flow line with increasing distance from the main propeller preferably consists in a reduction of the pumping effect with increasing distance from the main propeller. In In an alternative embodiment, depending on the geometric design or orientation of the vessel and the specific local flow conditions, the pumping effect of the respective means increases with distance from the main propeller. A preferred variant is one in which the individual pumping power of the individual actively flow-directing means decreases with distance from the main propeller, but increases again for the "upper" means adjacent to the vapor chamber. It is specifically intended to adapt the profile of the pumping effect of the actively flow-directing means arranged along the agitator shaft specifically to the design or orientation of the vessel and / or to the type of operation, in particular to discontinuous operation, and / or to the rheological properties of the medium / magma. This adaptation occurs once during the planning and manufacture of the agitator shaft.Alternatively or additionally, means are provided to specifically adjust the pumping action of the active flow-guiding means during operation. This can be achieved through known adjustment mechanisms (see below) or through replaceable elements on the active flow-guiding means, e.g., the propeller blades.

[0033] The at least one actively flow-guiding means is mechanically connected directly or indirectly to the shaft rotating in the operating state in order to achieve the active flow guidance through the means or means, wherein the specific energy input at the shaft is distributed to the main propeller and the further means for active flow guidance according to the invention.

[0034] In particular, the actively flow-guiding means is arranged directly on the main propeller shaft and mechanically connected to it, allowing the means to be driven directly by the shaft. The mechanical connection is preferably rigid.

[0035] Alternatively, the drive is provided via known gears such as planetary gears and / or via coaxial multi-part shafts.

[0036] In In one embodiment of the invention, the specific energy input for the flow-guiding effect of the means is controllable, in particular by variability of the angle of attack of the stirring blade of a means designed as a propeller and / or by variability of the mechanical transmission of the drive of the means, especially in connection with designs with planetary gears and / or coaxial multi-part stirrer shafts.

[0037] Alternatively, the actively flow-guiding means have their own shaft with completely separate mechanics, so that the energy input can be controlled separately.

[0038] The active flow control device surprisingly causes the otherwise undesirable turbulent flow at the propeller shaft to collapse. It creates a targeted downward flow in the vicinity of the propeller shaft without requiring significantly more mechanical work and thus higher specific energy than would be the case, for example, with a second main propeller on the shaft, dimensioned similarly to the first.It has been shown that, particularly through small-sized and repetitively implemented actively flow-guiding structures along the propeller shaft, such as short propeller blades or interrupted screw flights, highly efficient flow guidance at the shaft and, as a result, stable toroidal flow of the solution and suspension through the vessel can be achieved. With little additional mechanical effort to drive the agitator, the pumping action of the main propeller and thus the efficiency of the entire process, in particular evaporative crystallization, is improved. Above all, the dispersion of the resulting crystal-containing suspension is improved. This leads to a reduction in the specific total energy input into the crystallizer while maintaining the same mixing quality, or to an improvement in the mixing quality while maintaining the specific total energy input into the crystallizer.

[0039] Also disclosed is a stirred reactor, but especially a crystallizer, especially for discontinuous evaporative crystallization, which contains the improved stirrer described here.

[0040] Also disclosed is the teaching of using at least one actively flow-guiding means, as characterized herein, on a shaft of a crystallizer stirrer to improve its mixing performance. A particular application is in an evaporative crystallizer to maintain the stirring performance of the stirrer during discontinuous evaporative crystallization, even with varying fill levels of the crystallizer vessel.

[0041] The invention is described in more detail by the following figure, without this being understood as limiting:

[0042] Figur 1shows a schematic representation of a generic evaporative crystallizer with a vertical reactor vessel 100. This contains a solution or suspension 140 in the lower region, from which the crystal product is to be obtained. At the upper end is the vapor chamber 125 with a vapor outlet. Flow-through heating elements (heat exchangers) 120 are arranged in the lower region of the vessel. A stirrer shaft 150 is suspended essentially vertically (perpendicularly) from above into the vessel 100. In a primary mixing zone 110, here directly at the lower end of the shaft, a main propeller, i.e., a blade stirrer 160, is arranged, which, upon rotation of the drive shaft 150, primarily pumps the solution or suspension in the vessel downwards and toward the vessel bottom. The downward flow 144 of the solution or suspension is directed into the heating chambers 120 by the bottom-side baffle plate 130 on the vessel 100.Due to the active pumping action and supported by a reduction in density due to the heating of the solution or suspension in the heating chamber 120 as well as by the formation of vapor bubbles, the solution or suspension leaves the heating chamber essentially vertically upwards 145. The pumping action of the main propeller 160 is supported by stationary baffle plates 170 arranged adjacent thereto. According to the invention, actively flow-guiding means 180 are provided along the drive shaft 150 above the main propeller 160, which in the immediate vicinity of the drive shaft 150 effect an active flow 147 of the solution or suspension running downwards in the direction of the drive shaft towards the main propeller 160. This efficiently supports the action of the agitator. A stable circular flow 144, 145, 146, 147 is formed via the compensating flow 146.Depending on the fill level 141, 142, or 143 of the solution or suspension 140 in the reactor vessel 100, more or fewer sections or units of the flow guide means 180 engage with it. The specific total energy input to the shaft 150 is thus always adapted to the fill level 141, 142, or 143.

Claims

1. A crystallizing evaporator including: - a vertical reactor vessel (100); - a heating chamber (120) arranged in the lower region of the vessel (100); and - a stirrer for circulating a solution or suspension (140) to be crystallized in the vessel (100) and directing it into the heating chamber (120), having a shaft (150) suspended vertically from the top and a main propeller (160) connected directly to the lower end of the shaft (150) and arranged in a primary mixing zone (110) of the vessel (100), wherein the stirrer further has on this shaft (150) and upstream of the main propeller (160) at least one further means (180) for active flow conducting of the solution or suspension (140), wherein either the means (180) for active flow conducting is each designed as a propeller, whose diameter is less than 50% of the diameter of the main propeller (160), wherein a plurality of propellers (182) are arranged at a distance from each other along the shaft (150), or wherein the means (180) for active flow conducting is designed as a screw running along the shaft (150) above the main propeller (160), which diameter is at most 50% of the diameter of the main propeller (160), characterised in that fixed baffle plates (170) are arranged adjacent to the main propeller (160).

2. The crystallizing evaporator of claim 1, wherein the at least one means (180) is arranged directly adjacent to the primary mixing zone (110).

3. Use of a crystallizing evaporator characterised in one of claims 1 and 2 in discontinuous operation for improving the mixing capacity thereof.

4. Use of a crystallizing evaporator characterised in one of claims 1 and 2 for maintaining the mixing capacity of the stirrer during a discontinuous evaporation crystallization at varying levels of filling of the crystalliser vessel.