A combined falling film crystallizer and evaporator

Abstract

The present invention relates to a falling film crystallization plant for purifying a crude composition by crystallization, wherein the falling film crystallization plant comprises a falling film crystallizer, which comprises a feed inlet line for crude composition, a melt outlet line as well as a melt inlet line both being connected with each other via a recirculation line, a plurality of hollow tubes, an inlet line for heat transfer medium and an outlet line for heat transfer medium, wherein the falling film crystallization plant further comprises at least two parallel heat exchangers, wherein the inlet line for heat transfer medium and the outlet line for heat transfer medium are connected with each other via the at least two parallel heat exchangers, and wherein the falling film crystallization plant further comprises a vacuum unit being connected with the falling film crystallizer.

Description

[0001] A combined falling film crystallizer and evaporator

[0002] The present invention relates to a falling film crystallization plant for purifying a crude composition by crystallization.

[0003] Typically, the raw products obtained from chemical synthesis processes are mixtures of a target compound and impurities, the latter being undesirable by-product components. Also typically, the contents of impurities are higher than desired in the raw products. Consequently, the raw products coming from chemical reactors, which have been synthesized in a conventional manner or by recycling end products manufactured in the past to recover the raw product, need to be purified by removing proper quantities of the impurities from the raw products by suitable separation techniques in order to obtain desired final purities of the target compound. For example, the catalytic oxidation of propylene in the gas phase using an oxygen-containing gas leads to a crude composition containing acrylic acid as target compound together with impurities, such as acetic acid, propionic acid, maleic acid, maleic anhydride, acrolein, furfural, benzaldehyde, phenothiazine and protoanemonin. A plurality of separation, i.e. purification methods, are known to purify such crude compositions, such as distillation, crystallization, extraction, absorption, adsorption, ion exchange chromatography and others. Frequently, several of the above-mentioned purification processes are combined, following cost optimization.

[0004] A common purification technique is distillation, which allows to separate organic compounds based on their different boiling points. During the distillation, the mixture to be separated is heated, which causes that compounds with lower boiling points mainly evaporate and rise, highly concentrated in the vapor stream, to the top of the distillation column, where they are withdrawn as overhead stream, con- densed and are then further processed according to the need. In contrast thereto, compounds with higher boiling points mainly remain as liquid and highly concentrated in the liquid stream, flow down to the bottom of the distillation column, where they are removed as bottom stream and are then further processed according to the need. However, distillation is not applicable to separate different compounds having similar boiling points.

[0005] Crystallization is another important industrial process for separating and purifying a compound from a mixture, in which the target compound to be purified is contained in a high, medium or even low concentration of as low as about 70% by weight. Typically, the separation efficiency of crystallization significantly increases with higher purities of the product being crystallized. For this reason, crystallization is often used to purify compositions having a high concentration of the target compound, such as close to 100% by weight, for example 99.99% by weight. Crystallization separates organic compounds mainly based on their different melting points. More specifically, the target compound is strongly concentrated in the solid crystals, whereas the impurities concentrate in the residual melt, which is then separated from the crystal mass after the end of the crystallization phase. Generally, crystallization processes are subdivided in layer crystallization and in suspension crystallization. In the suspension melt crystallization a melt including the target compound and at least one different compound is cooled in a vessel so that crystals are formed in a suspension of crystalline particles, in which the target compound is enriched, and which are dispersed in the melt depleted of the target compound. After completion of the crystallization, the crystals are separated from the melt, e.g. by filters, centrifuges or other equipment for solid / liquid separation, and, if necessary, further purified, such as in a second crystallization stage or by means of another suitable purification method. In contrast to this, in a layer melt crystallization crystals grow in form of crystal layers on a cooled wall surface, wherein the generated crystallization heat is conveyed through the crystal layers to the cooling medium flowing on the other side if the cooled wall. Due to this, the crystal layers are in a layer melt crystallization cooler than the melt. More precisely, the temperature at the surface of the crystal layer being in contact with the melt is almost equal to the melt temperature, but the opposite surface of the crystal layer being attached to the cooled wall is close to the cooling medium temperature, which is lower than the melt temperature. The resulting temperature gradient from the melt across the crystal layer to the cooling medium is the driving force for the flow of solidification heat from the crystallization front to the cooling medium. On the contrary, the temperature of crystals in a suspension is substantially equal to the melt temperature. Layer crystallization processes are of particular industrial importance. At present, two general kinds of layer crystallization are known, namely static crystallization and dynamic crystallization.

[0006] A prominent dynamic crystallization technique is falling film crystallization, which is performed in a falling film crystallizer, which is a crystallization column, which comprises in the shell a bundle of hollow tubes being arranged at least substantially vertically and extending from the upper part of the falling film crystallizer into the bottom area of the falling film crystallizer. In the bottom area a sump is placed within the crystallizer column directly below the lower end of the tube bundle. Liquid feed crude composition including the target compound in a certain concentration together with one or more other (undesired) compounds is filled into the sump area of the falling film crystallizer, before the crystallization process is started. During the crystallization process, a portion of this liquid composition is pumped by means of one or more pumps continuously from the sump area of the falling film crystallizer to the upper part of the falling film crystallizer and is introduced into the upper ends of the hollow tubes and allowed to fall down as falling film on the inner surfaces of the hollow tubes back to the sump of the falling film crystallizer. Concurrently, the outer walls of the hollow tubes are cooled to a temperature below the freezing temperature of the liquid composition by allowing a cold heat transfer medium to flow as falling film down on the outer surfaces of the hollow tubes so that crystal layers enriched in the target compound are deposited on the cooled inner wall surfaces of the hollow tubes. As a consequence of the deposition of crystal layers of the target compound on the cooled inner wall surfaces of the hollow tubes leading to a depletion of this compound in the liquid crude composition, a mother liquid, which is the residual melt, is formed from the liquid feed crude composition. Accordingly, the mother liquid has a lower concentration of the target compound than the liquid feed crude composition, but is concentrated concerning the impurity compounds. The circulation of the mother liquid is conducted as long as necessary to crystallize the desired amount of target compound from the mother liquid and to deposit it as crystal layers on the inner wall surfaces of the hollow tubes. After completion of the crystallization, the mother liquid is completely removed from the falling film crystallizer, the crystallization layers deposited on the inner wall surfaces of the hollow tubes are molten and then removed from the falling film crystallizer in order to obtain the separated and purified target compound. Optionally, in order to increase the purity of the target compound the crystallization layers may be sweated by gently heating them to a temperature being close to the melting point of the purified compound in order to reject rests of the mother liquid from the pores of the crystalline layers, before completely melting the purified crystal mass. If the purity of the obtained product composition is not high enough, the product composition may be subjected to one or more subsequent falling film crystallization stages so as to increase the purity degree. All in all, layer crystallization and in particular falling film crystallization is a batch process, which comprises a filling and precooling phase, a subsequent crystallization phase, optionally one or more subsequent sweating phases and a melting phase, before the purified product is withdrawn in a product withdrawal phase from the falling film crystallizer. Most of the commercially operated crystallization processes need more than one stage to obtain the specified purity of the target compound. In such a multistage operation mode, the molten crystals of a stage are recrystallized in subsequent stages until in a certain stage of such stage sequence, being then the last stage of the sequence, the desired purity degree has been reached. The more stages are necessary in a stage sequence of a commercially operated crystalliza- tion process, the larger crystallizers must be employed for a specified production capacity and the higher energy consumption results per ton of purified target compound.

[0007] In various cases, the crude composition feed contains low boiling components, which have not been removed from the crude composition feed to a sufficient extent by a previous distillation. In such cases, a pretreatment of the crude composition feed by evaporation equipment, like falling-film evaporation, would contribute to reduction of the crystallization effort and so reduction of the crystallization equipment sizes. However, the cost savings in crystallization equipment would be more than outweighed by the additional cost of evaporator equipment. In addition, due to the size of commercially available falling film evaporators, several such devices would be required given the typical production capacity of the crystallization plants.

[0008] In view of this, the object underlying the present invention is to provide a falling film crystallization plant and a falling film crystallization process for purifying a crude composition by crystallization, which is more efficient especially in removing low-boiling components and which is characterized by lower overall costs than the known falling film crystallization plants and processes, respectively.

[0009] In accordance with the present invention, this object is satisfied by providing a falling film crystallization plant for purifying a crude composition by crystallization, wherein the falling film crystallization plant comprises a falling film crystallizer, which comprises a feed inlet line for crude composition, a melt outlet line as well as a melt inlet line both being connected with each other via a recirculation line, a plurality of hollow tubes, an inlet line for heat transfer medium and an outlet line for heat transfer medium, wherein the falling film crystallization plant further comprises at least two parallel heat exchangers, wherein the inlet line for heat transfer medium and the outlet line for heat transfer medium are connected with each other via the at least two parallel heat exchangers, and wherein the falling film crystallization plant further comprises a vacuum unit being connected with the falling film crystallizer.

[0010] Thereby, the operational costs of falling film crystallization are significantly reduced. In a common falling film crystallization process, the falling film crystallizer is operated under slight overpressure using a protective gas, such as nitrogen, which is supplied from a vent unit into the falling film crystallizer so that no oxygen is present in the falling film crystallizer and so that no oxygen may enter into the falling film crystallizer in the case of a leakage or the like so as to reliably avoid any oxidization reaction or corrosion within the falling film crystallizer. In contrast thereto, the falling film crystallizer of the falling film crystallization plant in accordance with the present invention comprises a vacuum unit being connected with the falling film crystallizer, which allows to perform the complete crystallization process or one or more single phases of the crystallization process under vacuum. For instance, if vacuum is applied to the falling film crystallizer during the sweating phase and / or melting phase, impurities with a comparable low boiling point being present in the pores of the crystals evaporate and are withdrawn from the falling film crystallizer via the vacuum unit. The same occurs, even if in a lower extent, if vacuum is applied during the crystallization phase. If vacuum is applied during the filling and an optional, but preferred preheating phase, as suggested for the present invention, with delayed precooling, impurities with a comparable low boiling point are evaporated directly from the crude composition being present in the sump of the falling film crystallizer and as a falling film on the inside walls of the crystallization tubes are withdrawn via the vacuum unit from the falling film crystallizer. Since the crystallization is performed in accordance with the present patent invention as melt crystallization, the crude composition being filled into the sump of the falling film crystallizer during the filling and the optional, but preferred preheating phase is usually a melt of a crude composition. By removing the respective impurities by evaporation, less impurities have to be removed by crystallization and / or sweating so that the crystallization time and / or the number of sweating phases and / or the number of subsequent falling film crystallization stages are reduced, which leads to a decrease of the operational costs based on a given amount of target compound with a predetermined purity degree. Thus, in principle the falling film crystallization plant in accordance with the present invention combines falling film evaporation and falling film crystallization in one plant and one process, respectively. In difference to a common falling film crystallizer, the falling film crystallizer of the falling film crystallization plant in accordance with the present invention comprises a vacuum unit, and in difference to a common falling film evaporator, the falling film crystallizer of the falling film crystallization plant in accordance with the present invention comprises two parallel heat exchangers, one for cooling the melt during the crystallization phase and one for heating the falling film crystallizer during the sweating and melting phases. Even if the falling film crystallizer of the falling film crystallization plant in accordance with the present invention is a combination of a falling film crystallizer and a falling film evaporator, it is subsequently denoted for the ease of formulation as falling film crystallizer.

[0011] The term crude composition as used herein refers to a mixture of typically organic components, usually coming from a chemical reactor directly or, in majority of cases, after being pre-purified by other processes, like distillation, as a raw product. However, the crude composition may also be a fraction of e.g. coal tar distillation, like raw naphthalene or raw anthracene fractions. High purity naphthalene and anthracene are used among others for the manufacture of special plastics, dye pigments, specific drugs and others. Also, specific inorganic compounds may be purified using the falling film crystallization, such as for example hydrazine, which is purified to obtain an ultra-high purity of above 99.99% by weight for use in spacecraft auxiliary engines.

[0012] In accordance with the present invention, the falling film crystallization plant comprises a vacuum unit being connected with the falling film crystallizer so as to allow to generate sub-atmospheric pressure within the falling film crystallizer during its operation. Preferably, the vacuum unit, which comprises a vacuum source, such as a vacuum pump, is connected with a line or vacuum line, respectively, with the falling film crystallizer.

[0013] Furthermore, the falling film crystallizer comprises a melt outlet line as well as a melt inlet line both being connected with each other via a recirculation line. At the beginning of the process, i.e. before starting the crystallization phase, the melt being transported through the recirculation line from the melt outlet to the melt inlet is the melt of the crude composition, whereas the melt being transported through the recirculation line after the start of the crystallization is the mother liquid, which is formed as a consequence of the deposition of crystals of the target compound on the cooled inner wall surfaces of the hollow tubes leading to a depletion of this compound in the melt of crude composition. In order to transport the melt through the recirculation line, preferably a pump is arranged within the recirculation line.

[0014] In addition, the falling film crystallizer comprises a plurality of hollow tubes. The number of hollow tubes depends on the size of the falling film crystallizer and on the desired production capacity as well as on the purification effort, i.e. number of stages, for which the crystallization is repeated until the desired purity is reached.

[0015] Preferably, the falling film crystallizer as well as the plurality of hollow tubes within the falling film crystallizer are at least substantially vertically arranged, wherein at least substantially vertically arranged means that the angles between the length axes of the falling film crystallizer and of the plurality of hollow tubes and the vertical direction are at most 10°, preferably at most 5°, more preferably at most 1 .5° and most preferably 0°.

[0016] Good results are in particular obtained, when each of the plurality of hollow tubes extends from the upper portion to the lower portion of the falling film crystallizer. Preferably, each of the plurality of hollow tubes extends, seen from the top to the bottom of the falling film crystallizer, from a point being located at 0 to 30% and preferably 0 to 10% of the distance from the top to the bottom of the falling film crystallizer to a point being located at 60 to 80% and preferably 70 to 80% of the distance from the top to the bottom of the falling film crystallizer so that below the lower ends of the plurality of hollow tubes, a sump portion is located, in which no hollow tubes are arranged.

[0017] In accordance with an alternative embodiment of the present invention, each of the plurality of hollow tubes extends, seen from the top to the bottom of the falling film crystallizer, from a point being located at 0 to 10% and preferably 0 to 5% of the distance from the top to the bottom of the falling film crystallizer to the bottom of the falling film crystallizer, wherein a separate vessel is placed directly below the falling film crystallizer and is connected with the falling film crystallizer by a product drain line extending from the bottom of the falling film crystallizer to the top of the a separate vessel.

[0018] Furthermore, it is preferred that the melt inlet line is connected with the top of the falling film crystallizer and the melt outlet line is connected with the bottom of the falling film crystallizer.

[0019] Each of the hollow tubes comprises an interior cavity, which is bordered at its peripheral area by the inner wall of the respective hollow tube and which is open at its top and open at its bottom. Even if the cross-sectional form of the hollow tubes may be square, rectangular, polygonal or elliptic, it is preferred that the cross- sectional form of the hollow tubes is circular, i.e. that the hollow tubes have three- dimensionally the form of a hollow cylinder. While the heat transfer medium flows during the operation of the falling film crystallizer around the outer walls of the hollow tubes, the melt flows through the interior cavities of the hollow tubes and more specifically as thin films on the surfaces of the inner walls of the hollow tubes downwardly so as to allow that during the crystallization phase crystals form on the surfaces of the cooled inner walls of the hollow tubes. One open end of each hollow tube and preferably the upper open end of each hollow tube is thus the inlet for melt of the hollow tube, while the opposite and preferably lower open end of each hollow tube is the outlet for melt of the hollow tube.

[0020] Therefore, it is preferred that each upper end of the interior cavities of the plurality of hollow tubes is directly or indirectly connected with the melt inlet line of the falling film crystallizer and each lower end of the interior cavities of the plurality of hollow tubes is directly or indirectly connected with the melt outlet line of the falling film crystallizer, so that melt may flow from the melt inlet line through the interior cavity of each hollow tube to the melt outlet line of the falling film crystallizer, before it is recirculated during the crystallization phase through the recirculation line. More preferably, the lower end of each of the hollow tubes is indirectly connected with the melt outlet line of the falling film crystallizer, namely the lower end of each of the hollow tubes is connected with the sump portion of the falling film crystallizer, which in turn is connected at its opposite end with the melt outlet line of the falling film crystallizer. In turn, a distributor may be arranged between the melt inlet line of the falling film crystallizer and the upper end of the interior cavities of the plurality of hollow tubes so as to uniformly distribute the melt into the single interior cavities of the plurality of hollow tubes. Preferably, the distributor for the melt is designed so that an individual thermal expansion of the individual tubes is possible. Examples for such a design are for instance described in CH478392 and CH478393.

[0021] In a further development of the idea of the present invention, it is suggested that the feed inlet line for crude composition is connected with the sump portion of the falling film crystallizer. In order to avoid that the recirculated melt is contaminated with the heat transfer medium, it is important that the portions of the falling film crystallizer, through which the heat transfer medium flows, are fluid-tightly separated from the portions of the falling film crystallizer, through which the melt flows. For this purpose, it is preferred that separation means are arranged in the falling film crystallizer which fluid-tightly separate the hollow space formed between the outer peripheral walls of the plurality of hollow tubes from the interior cavities of each hollow tube so that no heat transfer medium may flow from the hollow space formed between the outer peripheral walls of the plurality of hollow tubes into any of the cavities of the plurality of hollow tubes, no melt may flow from any of the interior cavities of the plurality of hollow tubes into the hollow space formed between the outer peripheral walls of the plurality of hollow tubes and no melt may flow from outside through the separation means into the hollow space formed between the outer peripheral walls of the plurality of hollow tubes.

[0022] Preferably, the falling film crystallizer as well as the plurality of hollow tubes are at least substantially vertically arranged, wherein as separation means an upper perforated plate and a lower perforated plate are arranged within the falling film crystallizer, wherein each perforation of the upper perforated plate fluid-tightly surrounds a hollow tube and each perforation of the lower perforated plate fluid- tightly surrounds a hollow tube so that a hollow space is formed between the outer peripheral walls of the plurality of hollow tubes and between the upper perforated plate and the lower perforated plate, wherein the hollow space is bordered by the non-perforated portions of the upper and lower perforated plates and the peripheral inner wall of the falling film crystallizer. Thus, preferably each hollow tube is surrounded at its upper end by the upper perforated plate and at its lower end by the lower perforated plate. When the hollow tubes comprise a lower portion with a first diameter and an upper portion with a second, reduced diameter, each perforation of the upper perforated plate fluid-tightly surrounds the upper portion of a hollow tube and each perforation of the lower perforated plate fluid-tightly sur- rounds the lower portion of a hollow tube. If the connection of the tubes is like in above-mentioned CH478392 and / or CH478393, the fluid-tightness is provided by distributors and it does not need to be provided by the upper plate perforations.

[0023] In order to allow the heat transfer medium to enter and leave the hollow space forming a heat transfer medium compartment of the falling film crystallizer, the inlet line for heat transfer medium as well as the outlet line for heat transfer medium are preferably connected with the hollow space being formed between the outer peripheral walls of the plurality of hollow tubes and being bordered by the upper and lower perforated plates.

[0024] In accordance with the present invention, the inlet line for heat transfer medium and the outlet line for heat transfer medium are connected with each other via the at least two parallel heat exchangers of the falling film crystallization plant. In accordance with a particular preferred embodiment of the present invention, the outlet line for heat transfer medium splits into a first branch line and into a second branch line. While the first branch line is connected with a first valve, downstream of the first valve with a first of the at least two parallel heat exchangers and downstream of the first heat exchanger with the inlet line for heat transfer medium, the second branch line is connected with a second valve, downstream of the second valve with a second of the at least two parallel heat exchangers and downstream of the second heat exchanger with the inlet line for heat transfer medium. In addition, it is preferred that the inlet line for heat transfer medium and / or the outlet line for heat transfer medium is connected with a pump in order to allow to pump the heat transfer medium through the respective lines. This arrangement allows by opening the first valve and by closing the second valve to selectively transport the heat transfer medium through the first heat exchanger and by closing the first valve and by opening the second valve to selectively transport the heat transfer medium through the second heat exchanger. For instance, the first heat exchanger is operated with a cooling agent and the second heat exchanger is operated with a heating agent, wherein during the crystallization phase the first valve is open and the second valve is closed so that the heat transfer medium is selectively transported through the first heat exchanger and cooled therein so as to then cool the walls of the hollow tubes, whereas during the optional preheating phase as well as the sweating and melt phases the first valve is closed and the second valve is open so that the heat transfer medium is selectively transported through the second heat exchanger and heated therein so as to then heat the walls of the hollow tubes.

[0025] Good results are in particular obtained, when at least one of the at least two parallel heat exchangers and preferably each of the at least two parallel heat exchangers is a shell and tube or a plate and frame type heat exchanger. For instance, one of the shell side or of the tube side of the first heat exchanger is connected with the first branch line and the other of the shell side or tube side of the first heat exchanger is connected with an inlet line and outlet line for a cooling agent, whereas one of the shell side or of the tube side of the second exchanger is connected with the second branch line and the other of the shell side or tube side of the second heat exchanger is connected with an inlet line and outlet line for a heating agent, such as steam, tempered water, hot thermal oil or the like.

[0026] In a further development of the idea of the present invention it is suggested that between at least one of the at least two heat exchangers and the falling film crystallizer a process energy buffering device is arranged, wherein the energy buffering device is connected with the inlet line for heat transfer medium to the falling film crystallizer as well as with the outlet line for heat transfer medium leading from the falling film crystallizer, and wherein the buffering device is connected with separate inlet and outlet lines leading to and from the corresponding heat exchanger. The process energy buffering device allows to keep the thermal duty of the heat exchangers almost constant, while the cooling and heating duties requested during the different phases of a crystallization stage are strongly variable. In accordance with the present invention, the falling film crystallization plant comprises a vacuum unit, which is connected with the falling film crystallizer. In the simplest embodiment, the vacuum unit consists of a vacuum source, such as preferably a vacuum pump, which is connected with the falling film crystallizer via a vacuum line. However it is preferred that the vacuum unit comprises other elements in addition to the vacuum source.

[0027] More specifically, it is preferred that a valve is arranged in the vacuum line between the falling film crystallizer and the vacuum source so that the vacuum line can be opened and closed so as to allow to apply the vacuum during single phases of the operation of the falling film crystallization plant, while not at other phases. For instance, the valve of the vacuum line is opened and thus vacuum is applied to the falling film crystallizer during the sweating and melting phases, but the valve of the vacuum line is closed during the filling and precooling phase, during the crystallization phase and during the product withdrawal phase. Alternatively, the valve of the vacuum line is opened and thus vacuum is applied to the falling film crystallizer during all of the filling and precooling phase, the crystallization phase, the sweating phase(s) and the melting phase, but not during the product withdrawal phase. Still alternatively, the valve of the vacuum line is opened and thus vacuum is applied to the falling film crystallizer during the filling and precooling phase and during the crystallization phase, but not during the sweating phase(s), the melting phase and the product withdrawal phase.

[0028] In accordance with a further preferred embodiment of the present invention, the present invention, the vacuum unit of the falling film crystallization plant comprises a condenser. Thereby, at least a portion of the impurities with a low boiling point being withdrawn from the falling film crystallizer during the periods, during which vacuum is applied to the falling film crystallizer, is condensed and may be withdrawn from the crystallization unit, whereas only a minor amount of the impurities is withdrawn from the falling film crystallization plant as off-gas. Good results are in particular obtained, when the condenser is a shell and tube condenser, wherein one of the shell side or of the tube side of the shell and tube condenser is connected with the vacuum line and the other is connected with an inlet line and outlet line for a cooling agent.

[0029] Preferably, the condenser is arranged in the vacuum line between the valve and the vacuum source.

[0030] In a further development of the idea of the present invention it is proposed that the falling film crystallization plant further comprises a vent unit, which is connected with the falling film crystallizer via a vent line, in which a valve is arranged. The presence of a vent unit allows to vent the falling film crystallization plant and in particular the falling film crystallizer with blanketing gas or protective gas, respectively, such as with nitrogen, under slight overpressure during the periods, in which no vacuum is applied to the falling film crystallization plant and in particular the falling film crystallizer. In other words, the valve of the vent unit is only opened, when the valve of the vacuum unit is closed and the valve of the vacuum unit is only opened, when the valve of the vent unit is closed. For instance, the valve of the vacuum unit is opened and thus vacuum is applied to the falling film crystallizer, whereas the valve of the vent unit is closed during the sweating and melting phases, whereas the valve of the vacuum unit is closed and the valve of the vent unit is opened and thus the falling film crystallizer is vented during the filling and the optional, but preferred preheating phase, during the crystallization phase and during the product withdrawal phase. Alternatively, the valve of the vacuum unit is opened and thus vacuum is applied to the falling film crystallizer, whereas the valve of the vent unit is closed during all of the filling and the optional, but preferred preheating phase, the crystallization phase, the sweating phase(s) and the melting phase, whereas the valve of the vacuum unit is closed and the valve of the vent unit is opened and thus the falling film crystallizer is vented during the product withdrawal phase. Still alternatively, the valve of the vacuum unit is opened and thus vacuum is applied to the falling film crystallizer, whereas the valve of the vent unit is closed during the filling and the optional, but preferred preheating phase and during the crystallization phase, whereas the valve of the vacuum unit is closed and the valve of the vent unit is opened and thus the falling film crystallizer is vented during the sweating phase(s), the melting phase and the product withdrawal phase.

[0031] In accordance with a further preferred embodiment of the present invention, the falling film crystallizer comprises a connection line, which splits into the vacuum line and into the vent line.

[0032] In a further development of the idea of the present invention it is suggested that the vent line is connected downstream of the valve of the vent line with a pressure balancing line, downstream thereof via a further valve with a blanketing gas supply line and downstream thereof via a further valve with a blanketing off-gas line.

[0033] In accordance with a further particularly preferred embodiment of the present invention, the falling film crystallizer further comprises a stripping agent inlet line, which is preferably connected with the recirculation line or with the sump portion of the falling film crystallizer. This allows to add a stripping agent, such as water, to the melt being recirculated through the recirculation line, which allows to improve the evaporation selectivity of the impurities with low boiling point during the periods, during which vacuum is applied to the falling film crystallization plant and in particular to the falling film crystallizer.

[0034] Moreover, the falling film crystallization plant preferably further comprises a discharge line for discharging the purified product from the falling film crystallization plant during the product withdrawal phase. Good results are in particular obtained, when the discharge line being equipped with a valve is connected with the melt outlet line, preferably upstream of a pump being located in the recirculation line, so that the valve allows to open the discharge line during the product withdrawal phase and to close it during all other phases.

[0035] In a further development of the idea of the present invention, it is proposed that the plants further comprises a bypass line, which branches off from the outlet line for heat transfer medium and is connected with the inlet line for heat transfer medium at a location downstream of the at least two parallel heat exchangers, thereby allowing during the operation of the plant that heat transfer medium bypasses the at least two parallel heat exchangers. Preferably, a valve is arranged in the bypass line just upstream of the location at which the bypass line is connected with the inlet line for heat transfer medium. Likewise thereto, preferably a valve is arranged in the inlet line for heat transfer medium just upstream of the location at which the first and second branch lines are connected with the inlet line. Furthermore, it is preferred that the plant comprises a temperature measurement instrument being arranged downstream of the location at which the bypass line is connected with the inlet line for heat transfer medium. The temperature instrument and both valves and are members of a temperature control loop, in which the positions of both valves are commanded according to the difference between the desired and the measured temperatures. Both valves are reverse moving, i.e. if one of the valves increases its opening degree, the other valve reduces it and vice versa. Moreover, both control valves are preferably operated in such a manner that the joint heat carrier medium flow rate remains constant at all positions of the valves. By this arrangement, the temperature of the heat transfer medium flowing via the inlet line for heat transfer medium to the falling film crystallizer can be adjusted by adjusting the split ratio between the flow through the at least two parallel heat exchangers and through the bypass line. Consequently, the temperature control loop enables a more precise control of the process temperature, i.e. the temperature of the heat transfer medium that flows via the inlet line for heat transfer medium to the head of the falling film crystallizer. In accordance with an alternative particular preferred embodiment of the present invention, the falling film crystallization plant does not comprise a vent unit. It is preferred in this embodiment that the recirculation line being equipped with a pump further comprises a valve downstream of the pump and that the discharge line is connected with the recirculation line at a location downstream of the pump, but upstream of the valve of the recirculation line. By opening the valve of the discharge line, by running the pump and by closing the valve of the recirculation line during the product withdrawal phase the melt of target compound may be completely discharged from the falling film crystallization plant until the sump portion of the falling film crystallizer is empty. Afterwards, the pump is stopped, the valve of the discharge line is closed and the valve of the recirculation line is opened. This embodiment without vent unit has the advantage that the vacuum in the falling film crystallizer is permanently maintained, which means that the blanketing gas consumption and the size of the vacuum unit can be significantly reduced.

[0036] In an alternative variant of the embodiment of the present invention, in which the falling film crystallization plant does not comprise a vent unit, it is preferred that the discharge line being equipped with a valve is connected with the melt outlet line of the falling film crystallizer and preferably is arranged upstream of a pump being located in the recirculation line, wherein the plant further comprises a measuring vessel, which is connected with the discharge line and which is equipped with at least one instrument for the measurement of the product melt mass drained into the measuring vessel. Moreover, the measuring vessel is preferably connected with a product outlet line, which is equipped with a pump and a valve, and the measuring vessel is further connected via a connection line with the vacuum line of the vacuum unit. This variant allows to completely discharge the melt of target compound from the falling film crystallization plant until the sump portion of the falling film crystallizer is empty, by running the pump, by opening the valve of the discharge line and by opening the valve of the product outlet line during the product withdrawal phase. Also, this variant of the embodiment without vent unit has the advantage that the vacuum in the falling film crystallizer is permanently maintained, which means that the blanketing gas consumption and the size of the vacuum unit can be significantly reduced. In a further advantage, this variant allows a more precise separation between different liquid product fractions being drained out of the falling film crystallizer. This is all the more important the higher the desired final purity of the final target compound is.

[0037] In accordance with a further aspect, the present invention relates to a process for purifying a crude composition by crystallization and preferably melt crystallization, wherein the process is performed in an aforementioned falling film crystallization plant.

[0038] In accordance with a particularly preferred embodiment of the present invention, the process comprises in each crystallization stage not only the phases of cooling and crystallizing the crude composition, of sweating and thereafter of melting the crystals as well as withdrawing the resulting melt from the crystallizer as in the prior art processes, but at least at the beginning of the first crystallization stage and more preferably at the beginning of each crystallization stage a preheating phase, which is preferably performed under vacuum. The preheating phase allows to remove low boiling impurities from the composition to be crystallized and thus increases the purification efficiency.

[0039] Thus, preferably the process comprises, in series, a filling and preheating phase of feeding crude composition into the falling film crystallizer and preheating it, a subsequent crystallization phase of leading melt as films over the surfaces of the inner walls of the plurality of hollow tubes so as to form crystals of the target compound, one or more subsequent sweating phases of heating the crystals to a temperature being close to the melting point of the target compound, a subsequent melting phase of melting the crystals so as to obtain a melt of the target compound and a final product withdrawal phase of withdrawing the melt of target compound from the falling film crystallization plant.

[0040] Vacuum may be applied to the falling film crystallization plant and in particular the falling film crystallizer during all process phases or during only one or some of the process phases. Good results are in particular obtained, when vacuum is applied to the falling film crystallization plant: i) during the melting phase, ii) during the one or more sweating phases and during the melting phase, iii) during the crystallization phase, iv) during filling and preheating phase and during the melting phase, or v) during filling and preheating phase, during the crystallization phase, during the one or more sweating phases and during the melting phase.

[0041] In accordance with a further particular preferred embodiment of the present invention, a stripping agent, like water or any other suitable compound, is added before or during the crystallization phase to the melt so that the melt has a content of stripping agent of 1 to 100,000 ppm based on 100% by weight of the melt. Preferably, the stripping agent is added to the melt, before the crystallization phase or at the beginning of the crystallization phase at the latest. Even if the mother liquid comprises in this embodiment up to 100,000 ppm or 10% by weight, respectively, of a stripping agent, which may be a solvent, the respective crystallization is still considered to be a melt crystallization. Solvent crystallization means in accordance with the present invention a crystallization from a liquid composition, which comprises at least 50% by weight of solvent(s).

[0042] In a further development of the idea of the present invention it is proposed that during the crystallization phase the heat transfer medium is led through one of the at least two parallel heat exchangers being operated with a cooling agent, where- as the heat transfer medium is led during the one or more sweating phases and during the melting phase through another of the at least two parallel heat exchangers being operated with a heating agent.

[0043] Subsequently, the present invention is described by means of illustrative, but not limiting figures, wherein:

[0044] Fig. 1 is a schematic view of a falling film crystallization plant for purifying a crude composition by crystallization in accordance with one embodiment of the present invention.

[0045] Fig. 2 is a schematic view of a falling film crystallization plant for purifying a crude composition by crystallization in accordance with another embodiment of the present invention.

[0046] Fig. 3 is a schematic view of a falling film crystallization plant for purifying a crude composition by crystallization in accordance with still another embodiment of the present invention.

[0047] Fig. 4 is a schematic view of a falling film crystallization plant for purifying a crude composition by crystallization in accordance with still another embodiment of the present invention.

[0048] Fig. 5 is a schematic view of a falling film crystallization plant for purifying a crude composition by crystallization in accordance with still another embodiment of the present invention.

[0049] The falling film crystallization plant 10 for purifying a crude composition by crystallization shown in figure 1 comprises a falling film crystallizer 12, two parallel heat exchangers 14, 16, a vacuum unit 18 and a vent unit 20. The falling film crystal I iz- er 12 comprises a plurality of hollow tubes 22, from which in the schematic figure 1 only one is shown. Furthermore, the falling film crystallizer 12 comprises a feed inlet line 24 for crude composition comprising a feed valve 25, wherein the fed inlet line 24 leads into the falling film crystallizer 12 at a location below the plurality of hollow tubes 22, a connection line 26, at the bottom a melt outlet line 28, at the top a melt inlet line 30, in the upper portion an inlet line 32 for heat transfer medium and in the lower portion an outlet line 34 for heat transfer medium.

[0050] The falling film crystallizer 12 as well as the plurality of hollow tubes 22 are at least vertically arranged, wherein the falling film crystallizer 12 further comprises an upper perforated plate 36 and a lower perforated plate 38. Both, the upper perforated plate 36 as well as the lower perforated plate 38 are functioning as separation means. More specifically, the upper perforated plate 36 and the lower perforated plate 38 are arranged within the falling film crystallizer 12 so that each perforation or opening, respectively, of the upper perforated plate 36 fluid-tightly surrounds a hollow tube 22 and each perforation or opening, respectively, of the lower perforated plate 38 fluid-tightly surrounds a hollow tube 22 so that each hollow tube 22 is surrounded on its upper end by the upper perforated plate 36 and on its lower end by the lower perforated plate 38. Thereby, a hollow space 40 is formed between the outer peripheral walls 42 of the plurality of hollow tubes 22 and between the upper perforated plate 36 and the lower perforated plate 38, wherein the hollow space 40 is bordered by the non-perforated portions of the upper plate 36 and of the lower perforated plate 38 and by the peripheral inner wall 44 of the falling film crystallizer 12. Thus, the hollow space 40 is fluid-tightly separated from the interior cavities 46 of each hollow tube 22. While the inlet line for heat transfer medium 32 as well as the outlet line 34 for heat transfer medium are connected with the hollow space 40 being formed between the outer peripheral inner walls 44 of the plurality of hollow tubes 22, the melt inlet line 30 being connected with the top of the falling film crystallizer 12 is connected with each upper end of the interior cavities 46 of the plurality of hollow tubes 22 and each lower end of the interior cavities of the plurality of hollow tubes is connected with the lower portion of the falling film crystallizer 12. Consequently, the hollow space 40 being formed between the outer peripheral walls 42 of the plurality of hollow tubes 22 and between the upper perforated plate 36 and the lower perforated plate 38 functions as heat transfer compartment, in which the heat transfer medium flows as film on the surfaces of the outer walls of the hollow tubes. In contrast thereto, the interior cavities 46 of the hollow tubes 22, which are fluid-tightly separated from the hollow space 40, functions as crystallization compartments allowing the melt to flow from the melt inlet line 30 on the surfaces of the inner walls 47 through the interior cavities 46 of each hollow tube 22 to the lower portion of the falling film crystallizer 12 and from there to the melt outlet line 28, thereby allowing crystals to be formed on the surfaces of the inner walls 47 of the plurality of hollow tubes 22.

[0051] The feed inlet line 24 for crude composition is connected with the sump portion 48 of the falling film crystallizer 12.

[0052] Moreover, the outlet line 34 for heat transfer medium splits into a first branch line 50 and into a second branch line 52. While the first branch line 50 is connected with a first valve 54, downstream of the first valve 54 with the first heat exchanger 14 and downstream of the first heat exchanger 14 with the inlet line 32 for heat transfer medium, the second branch line 52 is connected with a second valve 56, downstream of the second valve 56 with the second heat exchanger 16 and downstream of the second heat exchanger 16 with the inlet line 32 for heat transfer medium. In order to transport the heat transfer medium through the lines, a pump 58 is provided in the outlet line 34 for heat transfer medium. Thus, if the first valve 54 is closed and the second valve 56 is open, the heat transfer medium flows via the outlet line 34 for heat transfer medium, via the second branch line 52, via the second heat exchanger 16 and via the inlet line 32 for heat transfer medium into the hollow space 40 of the falling film crystallizer, whereas the heat transfer medium flows via the outlet line 34 for heat transfer medium, via the first branch line 50, via the first heat exchanger 14 and via the inlet line 32 for heat transfer medium into the hollow space 40 of the falling film crystallizer, if the first valve 54 is open and the second valve 56 is closed. Thereby, it is possible to efficiently and fast change the temperature of the heat transfer medium, when the first heat exchanger 14 and the second heat exchanger 16 are operated with different media, such as the first heat exchanger 14 with cooling medium for the crystallization phase and the second heat exchanger 16 with heating medium for the sweating and melting phases.

[0053] The melt outlet line 28 splits into a discharge line 60, which may be opened or closed via the valve 62, and into a recirculation line 64, which connects the melt outlet line 28 with the melt inlet line 30. This allows by appropriately controlling, i.e. opening or closing, the valve 62 to either circulate mother liquid during the crystallization phase via the recirculation line 64 between the melt outlet line 28 and the melt inlet line 30 or to discharge melt during the sweating and melting phases or to discharge the product composition during the product withdrawal phase. In order to transport the fluid through the respective lines, a pump 66 is arranged in the recirculation line 64.

[0054] In turn, the connection line 26 splits into a vacuum line 68 leading into the vacuum unit 18 and into a vent line 70 leading into the vent unit 20. Both, the vacuum line 68 as well as the vent line 70 are provided with a valve 72, 74, which allow to selectively open the respective lines 68, 70. Thus, if the falling film crystallization plant 10 shall be vented, the valve 72 of the vacuum unit 18 will be closed and the valve 74 of the vent unit 20 will be opened, whereas the valve 72 of the vacuum unit 18 will be opened and the valve 74 of the vent unit 20 will be closed, if the falling film crystallizer plant 12 shall be set under sub-atmospheric pressure or vacuum, respectively. More specifically, the vacuum unit 18 comprises downstream of the valve 72 a condenser 76, which comprises a discharge line 78 for condensed liquid and an off-gas line 80 leading to the vacuum source 82, which is a vacuum pump. In turn, the vent unit 20 comprises downstream of the valve 74 a pressure balancing line 84, downstream thereof being connected therewith via a further valve 86 a blanketing gas supply line 88 and downstream thereof being connected therewith via a further valve 90 a blanketing off-gas line 92.

[0055] During the batchwise operation of the falling film crystallization plant 10 according to one exemplary embodiment, the following phases are performed in series, namely a filling and preheating phase, thereafter a crystallization phase, thereafter one sweating phase, thereafter a melting phase and thereafter finally a product withdrawal phase. More specifically, at the beginning of the operation of the falling film crystallization plant 10 melt of crude composition 94 is fed via the feed inlet line 24 into the sump portion 48 of the falling film crystallizer 12. During the feeding of the melt of crude composition 94 into the sump portion 48 of the falling film crystallizer 12, the feed valve 25 is open, whereas the feed valve 25 is closed during all other phases of the crystallization stage. When a certain level of the crude composition 94 is reached in the sump portion 48 of the falling film crystallizer 12, the pump 66 is started and thereby the crude composition 94 is recirculated via the recirculation line 64 into the interior cavities 46 of the hollow tubes 22. The valve 72 of the vacuum unit 18 is opened and the valve 74 of the vent unit 20 is closed so that sub-atmospheric pressure generated by the vacuum pump 82 is applied to the falling film crystallization plant 10 and in particular to the falling film crystallizer 12. On account of the applied vacuum, the optional preheating phase and the circulation via pump 58, a portion of the impurities with a low boiling point being contained in the crude composition 94 is evaporated and withdrawn via the connection line 26 and vacuum line 68 from the falling film crystallization plant 10. Furthermore, for initiating the crystallization phase the pump 58 is started, the first valve 54 is opened, whereas the second valve 56 is closed, and the first heat exchanger 14 is operated with a cooling agent, so that thereby the heat transfer medium is cooled in the first heat exchanger 14 and recirculated via inlet 32 into the hollow space 40 of the falling film crystallizer 12 so as to cool the outer peripheral walls 42 of the hollow tubes 22. During the crystallization phase, crystals of the target compound to be purified with traces of impurities are deposited on the surfaces of the inner walls 47 of the hollow tubes 22, wherein a portion of impurities with a low boiling point being contained in the crystals and in particular in the pores of the crystals is evaporated due to the applied vacuum and is withdrawn from the falling film crystallization plant 10 via the connection line 26 and vacuum line 68. As a consequence of the deposition of crystals of the target compound on the cooled inner wall 47 surfaces of the hollow tubes 22 leading to a depletion of this compound in the crude composition 94, a mother liquid 94 is formed from the crude composition 94, which has a lower concentration of the target compound than the crude composition 94. The mother liquid 94 is recirculated during the whole crystallization phase via the recirculation line 64.

[0056] When a predetermined amount of crystals is obtained, the pump 66 is stopped so as to terminate the recirculation of the mother liquid 94. Moreover, the valve 72 of the vacuum unit 18 is closed and the valves 74, 86 and 90 of the vent unit 20 are opened so as to vent the falling film crystallization plant 10 and in particular the falling film crystallizer 12. Thereafter the valve 62 is opened and the mother liquid is withdrawn via the discharge line 60. After completion of the withdrawal of the mother liquid 94, the valve 62 is closed, the valves 74, 86 and 90 of the vent unit 20 are closed and the valve 72 of the vacuum unit 18 is opened so as to again apply vacuum to the falling film crystallization plant 10 and in particular the falling film crystallizer 12. In addition, for initiating the sweating phase the first valve 54 is closed, the second valve 56 is opened and the second heat exchanger 16 is operated with a heating agent so that thereby the heat transfer medium is heated in the second heat exchanger 16 and led via the inlet line 32 into the hollow space 40 of the falling film crystallizer 12 so as to heat the outer peripheral walls 42 of the hollow tubes 22 so as to heat the crystals being deposited on the inner walls 47 of the hollow tubes 22 to a temperature being close to the melting point of the target compound in order to partially melt the crystals. On account of the applied vacuum, a portion of the impurities with a low boiling point being contained in the crystals and in particular in the pores of the crystals is evaporated and withdrawn via the connection line 26 and vacuum line 68 from the falling film crystallization plant 10. When a certain amount of melt is obtained, the valve 72 of the vacuum unit 18 is closed and the valves 74, 86 and 90 of the vent unit 20 are opened so as to vent the falling film crystallization plant 10 and in particular the falling film crystallizer 12, whereafter the valve 62 is opened and the obtained melt 94 is withdrawn via the discharge line 60. After completion of the withdrawal of the obtained melt 94, the valve 62 is closed, the valves 74, 86 and 90 of the vent unit 20 are closed and the valve 72 of the vacuum unit 18 is opened so as to again apply vacuum to the falling film crystallization plant 10 and in particular the falling film crystallizer 12.

[0057] Thereafter, the temperature of the heating agent led through the second heat exchanger 16 is increased so as to increase the temperature of the crystals being deposited on the inner walls 47 of the hollow tubes 22 so as to completely melt the crystals. During this melting phase a portion of impurities with a low boiling point being contained in the crystals and in particular in the pores of the crystals is evaporated due to the applied vacuum and is withdrawn from the falling film crystallization plant 10 via the connection line 26 and vacuum line 68. After completion of the melting phase, the pump 58 is stopped, the valve 72 of the vacuum unit 18 is closed and the valves 74, 86 and 90 of the vent unit 20 are opened so as to initiate the product withdrawal phase and to provide a slight overpressure to the falling film crystallization plant 10 and in particular to the falling film crystallizer 12. Then, the valve 62 is opened and the purified target compound or product, respec- tively, is withdrawn from the falling film crystallization plant 10 via the discharge line 60.

[0058] The plant 10 shown in figure 2 differs from that of figure 1 in that it has not vent unit 20. Furthermore, the discharge line 60 with the valve 62 is not arranged upstream, but downstream of the pump 66. In addition, a valve 96 is arranged in the recirculation line 64 downstream of the pump 66 and downstream of the branching off site of the discharge line 60. When operating the plant 10, during the product withdrawal phase of withdrawing the melt of target compound from the falling film crystallization plant 10 the valve 96 is closed and the valve 62 is open, until the melt is completely discharged from the sump portion 48 of the falling film crystallizer 12. Afterwards, the pump 66 is stopped, the valve 62 is closed and the valve 96 is opened. In this embodiment of the present invention, no vent unit 20 is required. This has the advantage that the vacuum in the falling film crystallizer 12 is permanently maintained, which means that the blanketing gas consumption and the size of the vacuum unit 18 can be significantly reduced.

[0059] The plant 10 shown in figure 3 differs from that of figure 1 in that it has not vent unit 20. Furthermore, the plant 10 comprises a measuring vessel 98, which is connected with the discharge line 60 and which is equipped with suitable instruments (not shown) for the measurement of the product melt mass drained into the measuring vessel 98. Moreover, the measuring vessel 98 is connected with a product outlet line 100 which is equipped with a pump 102 and a valve 104. In addition, the measuring vessel 98 is connected via a connection line 106 with the vacuum line 68 of the vacuum unit 18. When operating the plant 10, during the product withdrawal phase of withdrawing the melt of target compound from the falling film crystallization plant 10 the pump is working and the valve 104 is open, until the melt is completely discharged from the sump portion 48 of the falling film crystallizer 12. Afterwards, the pump 102 is stopped and the valve 104 is closed. Also in this embodiment, no vent unit 20 is required. This has the advantage that the vacuum in the falling film crystallizer 12 is permanently maintained, which means that the blanketing gas consumption and the size of the vacuum unit 18 can be significantly reduced. In a further advantage, the embodiment of figure 3 allows a more precise separation between different liquid product fractions being drained out of the falling film crystallizer 12. This is all the more important the higher the desired final purity of the final target compound is.

[0060] The plants 10 shown in figures 4 and 5 differ from that of figure 1 in that they comprises a bypass line 108, which branches off from the outlet line 34 for heat transfer medium and is connected with the inlet line 32 for heat transfer medium at a location downstream of the heat exchangers 14 and 16, thereby allowing heat transfer medium to bypass the heat exchangers 14 and 16. A valve 110 is arranged in the bypass line 108 just upstream of the location at which the bypass line 108 is connected with the inlet line 32 for heat transfer medium. Likewise thereto, a valve 112 is arranged in the inlet line 32 for heat transfer medium just upstream of the location at which the first and second branch lines 50 and 52 are connected with the inlet line 32. Furthermore, the plant comprises a temperature measurement instrument 114 being arranged downstream of the location at which the bypass line 108 is connected with the inlet line 32 for heat transfer medium. The temperature instrument 114 and both valves 110 and 112 are members of a temperature control loop 116, in which the positions of both valves 110 and 112 are commanded according to the difference between the desired and the measured temperatures. Both valves 110 and 112 are reverse moving, i.e. if the valve 112 increases its opening degree, the valve 110 reduces it and vice versa. Moreover, both control valves 110 and 112 are operated in such a manner that the joint heat carrier medium flow rate remains constant at all positions of the valves 110 and 112. By this arrangement, the temperature of the heat transfer medium flowing via line 32 to the falling film crystallizer 12 can be adjusted by adjusting the split ratio between the flow through the heat exchangers 14 and 16 and through the bypass line 108. Consequently, the temperature control loop 116 enables a more precise control of the process temperature, i.e. the temperature of the heat transfer medium that flows via line 32 to the head of the falling film crystallizer 12.

[0061] Subsequently, the present invention is described by means of an illustrative, but not limiting example and comparable example.

[0062] Example and Comparable Example

[0063] The purification of a crude composition containing as target compound bi- sphenol-A and as impurity phenol using a falling film crystallizer plant in accordance with the present invention (example) as well as using a falling film crystallizer plant according to the prior art (comparative example) have been calculated using data from pilot trials for the crystallization and using simulation by commercially available software for the evaporation.

[0064] The obtained results are summarized in the below table showing a comparison of the purification progress in four stages using the conventional crystallization process (comparative example) and the process including evaporation (example).

[0065] Table: Examples comparison: Purification with (example) and without evaporation (comparative example); the numbers show target component (BPA) and impurities contents in % by weight.

[0066] The data show that the targeted phenol content of below 5 ppm has been obtained after four crystallization stages in the operation of the comparative example without evaporation, whereas the same depletion effect has already been achieved after stage 3 of the crystallization of the example, i.e. if using the evaporation. Therefore, a significant smaller falling film crystallization plant in accordance with the present invention achieves the same production capacity than the falling film crystallization plant of the prior art. In addition, specific cooling / heating energy consumption per ton of final product as well as specific electricity consumption per ton of final product for all pumps may be significantly reduced by reducing the number of necessary stages, in this example from four to three, if employing a falling film crystallization plant according to this invention.

[0067] Reference numerals

[0068] 10 Falling film crystallization plant

[0069] 12 Falling film crystallizer

[0070] 14 First of two parallel heat exchangers

[0071] 16 Second of two parallel heat exchangers

[0072] 18 Vacuum unit

[0073] 20 Vent unit

[0074] 22 Hollow tube

[0075] 24 Feed inlet line

[0076] 25 Feed valve

[0077] 26 Connection line

[0078] 28 Melt outlet line

[0079] 30 Melt inlet line

[0080] 32 Inlet line for heat transfer medium

[0081] 34 Outlet line for heat transfer medium

[0082] 36 Upper perforated plate

[0083] 38 Lower perforated plate

[0084] 40 Hollow space

[0085] 42 Outer peripheral wall of a hollow tube

[0086] 44 Peripheral inner wall of the falling film crystallizer

[0087] 46 Interior cavity of a hollow tube

[0088] 47 Inner wall of a hollow tube

[0089] 48 Sump portion of falling film crystallizer

[0090] 50 First branch line

[0091] 52 Second branch line

[0092] 54 First valve

[0093] 56 Second valve 58 Pump

[0094] 60 Discharge line

[0095] 62 Valve

[0096] 64 Recirculation line

[0097] 66 Pump

[0098] 68 Vacuum line

[0099] 70 Vent line

[0100] 72 Valve

[0101] 74 Valve

[0102] 76 Condenser

[0103] 78 Discharge line

[0104] 80 Off-gas line

[0105] 82 Vacuum source / vacuum pump

[0106] 84 Pressure balancing line

[0107] 86 Valve

[0108] 88 Blanketing gas supply line

[0109] 90 Valve

[0110] 92 Blanketing off-gas line

[0111] 94 Crude composition I mother liquid I melt

[0112] 96 Valve

[0113] 98 Measuring vessel

[0114] 100 Product outlet line

[0115] 102 Pump

[0116] 104 Valve

[0117] 106 Connection line

[0118] 108 Bypass line

[0119] 110 Valve

[0120] 112 Valve

[0121] 114 Temperature measurement instrument

[0122] 116 Temperature control loop

Claims

Claims:1 . A falling film crystallization plant for purifying a crude composition by crystallization, wherein the falling film crystallization plant comprises a falling film crystallizer, which comprises a feed inlet line for crude composition, a melt outlet line as well as a melt inlet line both being connected with each other via a recirculation line, a plurality of hollow tubes, an inlet line for heat transfer medium and an outlet line for heat transfer medium, wherein the falling film crystallization plant further comprises at least two parallel heat exchangers, wherein the inlet line for heat transfer medium and the outlet line for heat transfer medium are connected with each other via the at least two parallel heat exchangers, and wherein the falling film crystallization plant further comprises a vacuum unit being connected with the falling film crystallizer.

2. The falling film crystallization plant in accordance with claim 1 , wherein the falling film crystallizer as well as the plurality of hollow tubes within the falling film crystallizer are at least substantially vertically arranged, wherein at least substantially vertically arranged means that the angles between the length axes of the falling film crystallizer and of the plurality of hollow tubes and the vertical direction are at most 10°, preferably at most 5°, more preferably at most 1 .5° and most preferably 0°.

3. The falling film crystallization plant in accordance with claim 1 or 2, wherein each of the plurality of hollow tubes extends, seen from the top to the bottom of the falling film crystallizer, from a point being located at 0 to 30% and preferably 0 to 10% of the distance from the top to the bottom of the fallingfilm crystallizer to a point being located at 60 to 80% and preferably 70 to 80% of the distance from the top to the bottom of the falling film crystallizer so that below the lower ends of the plurality of hollow tubes a sump portion extends, in which no hollow tubes are arranged.

4. The falling film crystallization plant in accordance with any of the preceding claims, wherein the melt inlet line is connected with the top of the falling film crystallizer and the melt outlet line is connected with the bottom of the falling film crystallizer, wherein each upper end of the interior cavities of the plurality of hollow tubes is connected directly or indirectly with the melt inlet line and each lower end of the interior cavities of the plurality of hollow tubes is directly or indirectly connected with the melt outlet line, so that melt may flow from the melt inlet line through the interior cavity of each hollow tube to the melt outlet line.

5. The falling film crystallization plant in accordance with claim 3, wherein the feed inlet line for crude composition is connected with the sump portion of the falling film crystallizer.

6. The falling film crystallization plant in accordance with any of the preceding claims, wherein separation means are arranged in the falling film crystallizer which fluid-tightly separate the hollow space formed between the outer peripheral walls of the plurality of hollow tubes from the interior cavities of each hollow tube so that no heat transfer medium may flow from the hollow space formed between the outer peripheral walls of the plurality of hollow tubes into any of the cavities of the plurality of hollow tubes and no melt may flow from any of the interior cavities of the plurality of hollow tubes into the hollow space formed between the outer peripheral walls of the plurality of hollow tube, wherein preferably the falling film crystallizer as well as the plurality of hollow tubes are at least substantially vertically arranged, where-in as separation means an upper perforated plate and a lower perforated plate are arranged within the falling film crystallizer, wherein each perforation of the upper perforated plate fluid-tightly surrounds a hollow tube and each perforation of the lower perforated plate fluid-tightly surrounds a hollow tube so that a hollow space is formed between the outer peripheral walls of the plurality of hollow tubes and between the upper perforated plate and the lower perforated plate, wherein the hollow space is bordered by the non-perforated portions of the upper and lower perforated plates and the peripheral inner wall of the falling film crystallizer, wherein preferably the inlet line for heat transfer medium as well as the outlet line for heat transfer medium are connected with the hollow space being formed between the outer peripheral walls of the plurality of hollow tubes.

7. The falling film crystallization plant in accordance with any of the preceding claims, wherein the outlet line for heat transfer medium splits into a first branch line and into a second branch line, wherein the first branch line is connected with a first valve, downstream of the first valve with a first of the at least two parallel heat exchangers and downstream of the first heat exchanger with the inlet line for heat transfer medium, whereas the second branch line is connected with a second valve, downstream of the second valve with a second of the at least two parallel heat exchangers and downstream of the second heat exchanger with the inlet line for heat transfer medium.

8. The falling film crystallization plant in accordance with any of the preceding claims, wherein the vacuum unit comprises a vacuum source and preferably a vacuum pump, which is connected with the falling film crystallizer via a vacuum line.

9. The falling film crystallization plant in accordance with claim 8, wherein a valve is arranged in the vacuum line between the falling film crystallizer and the vacuum source.

10. The falling film crystallization plant in accordance with any of the preceding claims, wherein the vacuum unit comprises a condenser and preferably a shell and tube condenser, wherein preferably the condenser is arranged in the vacuum line between the valve and the vacuum source.11 . The falling film crystallization plant in accordance with any of the preceding claims, wherein the falling film crystallization plant further comprises a vent unit, which is connected with the falling film crystallizer via a vent line, in which a valve is arranged.

12. The falling film crystallization plant in accordance with claims 8 and 11 , wherein the falling film crystallizer comprises a connection line, which splits into the vacuum line and into the vent line.

13. The falling film crystallization plant in accordance with any of claims 1 to 10, wherein the falling film crystallization plant does not comprise a vent unit, but wherein: i) the recirculation line being equipped with a pump further comprises a valve downstream of the pump and a discharge line, which comprises a valve, wherein the discharge line is connected with the recirculation line at a location downstream of the pump, but upstream of the valve of the recirculation line, or ii) a discharge line, which comprises a valve, is connected with the melt outlet line of the falling film crystallizer, wherein the plant further comprises a measuring vessel, which is connected with the discharge line and which is equipped with at least one instrument for the measure-merit of the product melt mass drained into the measuring vessel, wherein the measuring vessel is connected with a product outlet line, which is equipped with a pump and a valve, and the measuring vessel is further connected via a connection line with a vacuum line of the vacuum unit.

14. A process for purifying a crude composition by crystallization and preferably melt crystallization, wherein the process is performed in a falling film crystallization plant in accordance with any of the preceding claims.

15. The process in accordance with claim 14, which comprises, in series, a filling and preheating phase of feeding crude composition into the falling film crystallizer and preheating it, a subsequent crystallization phase of leading melt as films over the surfaces of the inner walls of the plurality of hollow tubes so as to form crystals of the target compound, one or more subsequent sweating phases of heating the crystals to a temperature being close to the melting point of the target compound, a subsequent melting phase of melting the crystals so as to obtain a melt of the target compound and a final product withdrawal phase of withdrawing the melt of target compound from the falling film crystallization plant, wherein vacuum is applied to the falling film crystallization plant: i) during the melting phase, ii) during the one or more sweating phases and during the melting phase, iii) during the crystallization phase, iv) during filling and preheating phase and during the melting phase, or v) during filling and preheating phase, during the crystallization phase, during the one or more sweating phases and during the melting phase.

Images