A purification plant comprising a crystallization unit with improved energy efficiency

Abstract

The present invention relates to a plant for purifying a crude composition by crystallization, wherein the plant comprises at least one crystallizer, at least one buffer vessel and at least two heat pumps, wherein each of at least two heat pumps is connected with at least one buffer vessel, as well as to a process for purifying a crude composition by crystallization being performed in such a plant.

Description

[0001] Sulzer Management AG S13808P

[0002] A purification plant comprising a crystallization unit with improved energy efficiency

[0003] The present invention relates to a plant for purifying a crude composition by crystallization as well as to a process for purifying a crude composition by crystallization.

[0004] 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.

[0005] Crystallization is an important industrial process for separating and purifying a compound from a mixture and this technique 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 being also called mother liquid, 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 mother liquid being depleted of the target compound. The crystals are separated from the melt, e.g. by filters, centrifuges or other equipment for solid / liquid separation. At present, two general kinds of layer crystallization are known, namely static crystallization and dynamic crystallization. A prominent dynamic crystallization technique is falling film crystallization, which is performed in a falling film crystallizer, in which liquid feed crude composition is circulated by pumps over cooled surfaces. Crystal layers being enriched in the target compound are deposited on the cooled surfaces, whereas the mother liquid being enriched in the impurities is formed from the liquid feed crude composition. After completion of the crystallization, the mother liquid is completely removed from the falling film crystallizer, the crystallization layers deposited on the surfaces are molten by heating the surfaces 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 layers. In contrast to falling film crystallization, during a static crystallization, the liquid phase is not moved. More specifically, a typical static crystallizer comprises a plurality of walls, such as plates, or tubes or finned tubes, which immerse into the liquid feed crude composition or later mother liquid, and which can be cooled and heated by circulating a heat transfer medium through the interior of the plates. As in the falling film crystallization, after completion of the crystallization the mother liquid is completely removed from the crystallization vessel, the cooling of the plates is terminated and the plates are heated so as to sweat the crystal layers and then further heated to a temperature above the melting point of the target compound so as to melt the crystal layers, before removing the melt from the crystallization vessel.

[0006] All in all, layer crystallization is a batch process, which comprises a filling and precooling phase, a subsequent crystallization phase, optionally one or more subsequent sweating phases, a melting phase and a product withdrawal phase of withdrawing the purified product from the falling film crystallizer. Hence, during the crystallization surfaces of the crystallizer, on which the crystals shall be formed, are repeatedly subjected to cooling phases and heating phases, wherein the required cooling and heating is performed by circulating appropriately tempered heat transfer medium through a cooling / heating circuit of the crystallizer so that the heat transfer medium comes in indirect contact with the surface, on which the crystals shall be formed. Typically, the energy system of a falling film crystallizer or static crystallizer or any combination of falling film and static crystallizers comprises for this purpose a heat transfer medium circulation pump and one or more heat transfer medium buffer vessels being filled with heat transfer medium and being connected to one or more heat exchangers for heating or cooling heat transfer medium, or comprises alternatively one or more heat exchangers connected to the heat transfer medium circulation pump without the use of heat transfer medium buffer vessels. In case of using heat transfer medium buffer vessels, the crystallizer is connected, when the crystallizer is in a cooling phase, via a series of pipes and valves to one or more cold buffer vessels containing cold heat transfer medium having a temperature below the crystallization temperature. More specifically, cold heat transfer medium is pumped from the respective buffer vessel through the cooling / heating circuit of the crystallizer, whereby the cold heat transfer medium is heated and then returned to the respective buffer vessel. However, when the crystallizer is in a heating phase, it is connected via a series of pipes and valves to one or more hot buffer vessels containing hot heat transfer medium having a tempera- ture being high enough to perform the sweating and / or melting phase. More specifically, hot heat transfer medium is pumped from the respective buffer vessel through the cooling / heating circuit of the crystallizer, whereby the hot heat transfer medium is cooled and then returned to the respective buffer vessel. By using a set of temperature control valves, which are controlled by a temperature controller, in each cooling and heating phase heat transfer medium with the correct temperature as required by the respective process phase may be pumped through the cooling / heating circuit of the crystallizer, namely by pumping in each phase heat transfer medium from an appropriately tempered buffer vessel or by pumping a mixture of heat transfer media from two or more buffer vessels containing heat transfer media having different temperatures through the cooling / heating circuit of the crystallizer or by pumping a mixture of heat transfer media from one buffer vessel and recycled heat transfer media from the crystallizer through the cooling / heating circuit of the crystallizer. As set out above, cold heat transfer medium used to cool the surfaces of the crystallizer, on which crystals shall deposit, is heated during the flow through the cooling / heating circuit of the crystallizer, whereas hot heat transfer medium used to heat the surfaces of the crystallizer, on which crystals shall deposit, is cooled during the flow through the cooling / heating circuit of the crystallizer. Hence, the cold heat transfer medium returning from the crystallizer needs to be re-cooled and the hot heat transfer medium returning from the crystallizer needs to be re-heated so as to maintain a stable temperature within each of the buffer vessels. This re-cooling and re-heating of the heat transfer media is an energy consuming process.

[0007] In view of this, the object underlying the present invention is to provide a plant for purifying a crude composition by crystallization, which may be operated with minimal operational costs, as well as to provide a respective process for purifying a crude composition. In accordance with the present invention, this object is satisfied by providing a plant for purifying a crude composition by crystallization, wherein the plant comprises at least one crystallizer, at least one buffer vessel and at least two heat pumps, wherein each of at least two heat pumps is connected with at least one buffer vessel.

[0008] By providing at least two heat pumps, the operational costs during the operation of the plant may be drastically reduced. This is among others due to the fact that each heat pump has two sides, namely a process side and a utility side, both having different temperatures so that, if both heat pumps are operated at different temperatures, the two heat pumps have in sum four sides having each different temperatures so that thereby four different buffer vessels or four different sections of one buffer vessel may be tempered to an appropriate temperature. Thereby, with comparable low capital expenditures an efficient energy system is provided, which covers the complete or at least a major part of the temperature range required by the at least one crystallizer. Since a heat pump merely requires the electrical energy required for the operation of the heat pump, which is much lower than the process energies, the plant in accordance with the present invention may be operated with minimal operational costs for purifying a crude composition by crystallization. For instance, the main energy consumption of a mechanical heat pump based on the compression principle is the electricity consumed by the heat pump compressor. A further advantage of the plant in accordance with the present invention is that it allows to electrify the i) heating or ii) the cooling or iii) the heating and cooling or iv) the partial heating and / or cooling of the process, which significantly reduces the carbon dioxide footprint.

[0009] In accordance with the present invention, the plant comprises at least one crystallizer, at least one buffer vessel and at least two heat pumps, wherein each of at least two heat pumps is connected with at least one buffer vessel. A heat pump being connected with a buffer vessel means in accordance with the present inven- tion that the heat pump and the buffer vessel are connected for instance by means of a piping so that heat transfer medium being contained in the buffer vessel may flow or may be pumped, respectively, through a side, i.e. the process side or the utility side, of the evaporator or of the condenser of the heat pump of the heat pump and then back into the respective buffer vessel so as to adjust the temperature of the heat transfer medium within the buffer vessel as desired.

[0010] Furthermore, a heat pump means in accordance with the present invention any device, which transfers heat from a cold heat sink to a hot heat sink and which comprises a refrigerant circuit. The side of the heat pump being in contact with the cold sink is also called process side of the evaporator of the heat pump, whereas the side of the heat pump being in contact with the hot sink is also called utility side of the condenser of the heat pump. In contrast to this, a heat exchanger means in accordance with the present invention any device, which does not comprise a refrigerant circuit and which passively transfers heat from a hot heat sink to a cold heat sink, such as form a hot fluid to a colder fluid, wherein the hot fluid and the colder fluid are separated from each other in the heat exchanger for instance by a separating wall. The side of the heat exchanger, through which the hot fluid flows, is also called process side of the heat exchanger, whereas the other side of the heat exchanger through which the colder fluid flows, is also called utility side of the heat exchanger.

[0011] In accordance with a particular preferred embodiment of the present invention, at least two heat pumps of the plant are connected with each other. Connected with each other means in this connection that the heat pumps are directly or indirectly connected. A direct connection may be realized by connecting two heat pumps with each other by a piping, for instance by a line connected one side, i.e. the process side or the utility side, of one heat pump with one side, i.e. the process side or the utility side, of another heat pump. For instance, the process side one heat pump is directly connected by a line with the utility side of another heat pump, wherein the process side of the one heat pump and the utility side of the other heat pump are further connected with the same or with a different buffer vessel. Preferably, the at least two heat pumps are indirectly connected with each other, more preferably by a heat exchanger being arranged between two heat pumps. For example, the process side of one heat pump is connected with the utility side of another heat pump indirectly via a heat exchanger, wherein the process side of the one heat pump and the utility side of the other heat pump are connected with the same or with a different buffer vessel. More specifically, two heat pumps may be indirectly connected with each other by a heat exchanger, wherein the process side of the heat exchanger comprises an inlet being connected via a line with an outlet of the utility side of one heat pump and the process side of the heat exchanger further comprises an outlet being connected via a line provided with an expansion valve with an inlet of the process side of the same heat pump, whereas the utility side of the heat exchanger comprises an inlet being connected via a line with the outlet of the process side of another heat pump and the utility side of the heat exchanger further comprises an outlet being connected via a line provided with a compressor with the inlet of the utility side of the same heat pump. If the plant comprises three or more heat pumps, two or more or even all of them may be connected with each other, hence forming a network of heat pumps. If the plant comprises for instance three heat pumps, a first heat pump may be connected via a heat exchanger as described above with a second heat pump, whereas the second heat pump may be connected via another heat exchanger with a third heat pump.

[0012] The present invention is not particularly limited concerning the kind of the at least two heat pumps. Good results are in particular obtained, when at least one or all of the at least two heat pumps is / are a mechanical heat pump. In accordance with the present invention, the term heat pump means a heat pump, in which a refrigerant is used, which does not derive from the crystallization process, i.e. a heat pump, in which no fraction or composition, respectively, obtained in the crystalliza- tion process, such as the mother liquid of the crystallizer, is used as refrigerant. Mechanical heat pump means in this connection any heat pump, which operates so that the refrigerant undergoes periodically phase changes, namely an evaporation and a condensation so as to absorb and release heat between the process side and the utility side of the heat pump. Hence, it is preferred that the at least one heat pump comprises an evaporator, a condenser, an expansion valve and a compressor. More specifically, preferably an outlet of the evaporator is connected via a line with the inlet of the compressor, which further comprises on its opposite side an outlet line being connected with an inlet of the condenser. In turn, preferably the condenser comprises an outlet line, which is connected with an inlet of the expansion valve, which comprises on its opposite side an outlet line being connected with an inlet of the evaporator.

[0013] Furthermore it is preferred that the evaporator side of the heat pump, i.e. the process side of the heat pump, is in contact with the cold sink, such as a line through which cold heat transfer medium having a temperature being higher than the evaporation temperature of the refrigerant at the evaporator pressure or compressor suction pressure, respectively, flows, whereas the condenser side of the heat pump, i.e. the utility side of the heat pump, is in contact with the hot sink, such as a line through which hot heat transfer medium having a temperature being lower than the condensation temperature of the refrigerant at the condenser pressure or compressor discharge pressure, respectively, flows. During the operation of the heat pump, refrigerant flows through the evaporator and is evaporated therein by absorbing heat from the cold sink (i.e. cold heat transfer medium), whereafter the refrigerant flows through the compressor, in which it is compressed and heated thereby, before the hot refrigerant flows through the condenser of the heat pump, where it transfers heat to the heat sink (i.e. hot heat transfer medium). The refrigerant then leaves the condenser, flows through the expansion valve, where it is expanded and thereby further cooled, before it again enters the evaporator. Suitable refrigerants for the operation of each of the at least two heat pumps are R-744, R-290, R-717, R-410A, R-1234yf, R-1234ze, R-600, R-23, R-170 and others.

[0014] Alternatively, even if less preferred, at least one or all of the at least two heat pumps may be an absorption heat pump. In an absorption heat pump, a secondary fluid is introduced into the system to replace mechanical compression by absorbing evaporated refrigerant. After exiting the evaporator, the refrigerant is mixed with the secondary fluid, yielding a two-component multi-phase mixture. This mixture is then pumped to a generator where the two fluids separate when being subjected to high-temperature heat input, wherein the refrigerant enters the condenser while the absorbent flows back to the absorber, hence completing the cycle. Suitable refrigerants for the operation of the heat pump of the absorbent type are mixtures of ammonia and water, mixtures of water and lithium bromide and others.

[0015] Both aforementioned embodiments may be combined so that the plant may comprise at least one mechanical heat pump as well as at least one absorption heat pump.

[0016] It is particularly preferred that each of the at least two heat pumps is a mechanical heat pump, which comprises an evaporator, a condenser, an expansion valve and a compressor. Again, it is preferred that an outlet of the evaporator is connected via a line with the inlet of the compressor, which further comprises on its opposite side an outlet line being connected with an inlet of the condenser, wherein the condenser comprises an outlet line, which is connected with an inlet of the expansion valve, which comprises on its opposite side an outlet line being connected with an inlet of the evaporator. Each heat pump may comprise instead of one condenser two condensers arranged in series, wherein the two condensers are connected via a line from one side, and / or each heat pump may comprise instead of one evaporator two evaporators arranged in series, wherein the two evaporators are connected via a line from one side. Moreover, the two condensers or two evaporators, respectively, may be connected from the other side or not, since they provide heating or cooling to two different heating energy consumers for the condensers or cooling energy consumers for the evaporators.

[0017] In accordance with the present invention, the plant comprises at least one buffer vessel, which is filled during the operation with a heat transfer medium having a different temperature than ambient temperature. The at least one buffer vessel may be one or more vertical vessels and / or one or more horizontal vessels, wherein one or more vertical vessels are preferred.

[0018] Suitable heat transfer media are water, mixtures of water and glycol, thermal oil and others.

[0019] The present invention is not particularly restricted concerning the number of buffer vessels being comprised in the plant. For instance, the plant may comprise one large buffer vessel, which is preferably a vertical buffer vessel comprising - seen from the bottom to the top of the vertical buffer vessel - heat transfer medium having different temperatures. For instance, the temperature from the heat transfer medium may increase from the bottom to the top of the buffer vessel so that a temperature gradient is formed in the vertical direction. The gradient may be continuous or stepwise. More specifically, the heat transfer medium contained at the bottom of the buffer vessel has the lowest temperature, wherein the temperature of the heat transfer medium increases with increasing height of the buffer vessel so that the heat transfer medium contained at the top of the buffer vessel has the highest temperature. The one buffer vessel preferably contains a first set of a plurality of outlet and inlet lines being arranged along the vertical axis of the buffer vessel, wherein the outlet lines of the first set are connected with an inlet of the at least one crystallizer, whereas the inlet lines of the buffer vessel are connected with an outlet of the at least one crystallizer. Moreover, the one buffer vessel preferably contains a second set of a plurality of outlet and inlet lines being arranged along the vertical axis of the buffer vessel, wherein each of the outlet lines of the second set is connected with one of the two sides, i.e. with the process side or with the utility side, of one of the at least two heat pumps. This allows to circulate heat transfer medium with an appropriate temperature for the respective crystallization process phase between the respective axial section of the buffer vessel, which contains heat transfer medium with the appropriate temperature, and the crystallizer in order to temper the surfaces of the at least one crystallizer, on which the crystal layers shall deposit, as well as to circulate heat transfer medium of the single axial sections of the buffer vessel between the respective axial section of the buffer vessel and a side of one of the at least two heat pumps in order to temper the re-cool or re-heat, respectively the heat transfer medium to the temperature required in the respective axial section of the buffer vessel. For instance, if the crystallizer is operated in the heating phase, such as in the pre-heating, sweating or melting phase, hot heat transfer medium from the upper part of the buffer vessel is circulated between the buffer vessel and the crystallizer through an inlet line and outlet line of the first set of a plurality of outlet and inlet lines, wherein the heat transfer medium from the same upper part of the buffer vessel is circulated between the buffer vessel and a side of one of the heat pumps through an inlet line and outlet line of the second set of a plurality of outlet and inlet lines to thereby reheat the heat transfer medium to a pre-determined temperature. In turn, if the crystallizer is operated in the crystallization phase, cold heat transfer medium from a lower part of the buffer vessel is circulated between the buffer vessel and the crystallizer through an inlet line and outlet line of the first set of a plurality of outlet and inlet lines, wherein the cold transfer medium from the same lower part of the buffer vessel is circulated between the buffer vessel and a side of one of the heat pumps through an inlet line and outlet line of the second set of a plurality of outlet and inlet lines to thereby re-cool the heat transfer medium to a pre-determined temperature. In a further development of the idea of the present invention, it is suggested that the plant comprises at least one and preferably more than one buffer vessel, wherein each of at least two heat pumps is connected with at least one buffer vessel and preferably with two different buffer vessels, wherein at least one heat pump is connected with one or two other buffer vessels than another heat pump. For instance, the plant comprises at least three buffer vessels, wherein each of the at least three heat pumps is connected with at least one buffer vessel, wherein each of at least three heat pumps is connected with another buffer vessel than any of the other heat pumps. During the operation of the plant, each of the buffer vessels preferably contains heat transfer medium having a different temperature than the heat transfer media contained in any of the other buffer vessels.

[0020] In accordance with a particular preferred embodiment of the present invention, the plant comprises at least four buffer vessels. Again, it is preferred that each of the buffer vessels contains heat transfer medium having a different temperature than the heat transfer media contained in any of the other buffer vessels during the operation of the plant. Good results are in particular obtained, when the plant comprises four to ten buffer vessels, more preferably four to eight buffer vessels, still more preferably five to seven buffer vessels and most preferably six buffer vessels, wherein a first buffer vessel is connected with the process side of a first heat pump, a second buffer vessel is connected with the process side of a second heat pump, a third buffer vessel is connected with the utility side of the first heat pump and a fourth buffer vessel is connected with the utility side of the second heat pump. More specifically, the first buffer vessel is connected via an outlet line with an inlet of the process side of the first heat pump, which comprises an outlet line being connected with an inlet of the same buffer vessel, whereas the second buffer vessel is connected via an outlet line with an inlet of the process side of the second heat pump, which comprises an outlet line being connected with an inlet of the same buffer vessel, wherein the third buffer vessel is connected via an outlet line with an inlet of the utility side of the first heat pump, which comprises an outlet line being connected with an inlet of the same buffer vessel, and wherein the fourth buffer vessel is connected via an outlet line with an inlet of the utility side of the second heat pump, which comprises an outlet line being connected with an inlet of the same buffer vessel.

[0021] Furthermore, it is preferred that in the aforementioned embodiment each of the at least two heat pumps is a mechanical heat pump, which comprises an evaporator, a condenser, an expansion valve and a compressor, wherein the evaporator of the first heat pump comprises a process side and a utility side and the condenser of the first heat pump comprises a process side and a utility side, wherein the process side of the evaporator comprises an inlet line being connected with the first buffer vessel and an outlet line being connected with the first buffer vessel, whereas the utility side of the evaporator comprises an inlet line being connected with the expansion valve as well as an outlet line being connected with the compressor, wherein the compressor comprises an outlet line being connected with an inlet of the process side of the condenser, wherein the expansion valve comprises a line being connected with an outlet of the process side of the condenser. Furthermore, it is preferred that the utility side of the condenser of the first heat pump comprises an inlet line being connected with the third buffer vessel and an outlet line being connected with the third buffer vessel.

[0022] Good results are in particular obtained with the aforementioned embodiment, when the evaporator of the second heat pump comprises a process side and a utility side and the condenser of the second heat pump comprises a process side and a utility side, wherein the process side of the evaporator comprises an inlet line being connected with the second buffer vessel and an outlet line being connected with the second buffer vessel, whereas the utility side of the evaporator comprises an inlet line being connected with the expansion valve as well as an outlet line being connected with the compressor, wherein the compressor com- prises an outlet line being connected with an inlet of the process side of the condenser, wherein the expansion valve comprises a line being connected with an outlet of the process side of the condenser. Furthermore, it is preferred that the utility side of the condenser of the second heat pump comprises an inlet line being connected with the fourth buffer vessel and an outlet line being connected with the fourth buffer vessel.

[0023] It is further preferred that in the aforementioned embodiment that the two heat pumps are connected with each other. More preferably, a heat exchanger is arranged between the outlet line of the process side of the condenser of the first heat pump and the expansion valve, which comprises a process side and a utility side, wherein the process side of the heat exchanger comprises an inlet being connected with the outlet line of the process side of the condenser of the first heat pump and comprises an outlet being connected with the line being connected with an inlet of the expansion valve of the first heat pump, whereas the utility side of the heat exchanger comprises an inlet being connected with the outlet line of the process side of the evaporator of the second heat pump and comprises an outlet being connected with an inlet of the compressor of the second heat pump. The present invention is not particularly limited concerning the kind of the heat exchanger. Preferably, the heat exchanger is selected from the group consisting of shell and tube heat exchangers, plate and frame heat exchangers, plate evaporators, falling film evaporators, forced circulation evaporators, kettle evaporators and internal evaporators. Again, the side of the heat exchanger, through which the hot fluid flows, is called primary side or process side of the heat exchanger, whereas the side of the heat exchanger, through which the colder fluid flows, is called secondary side or utility side of the heat exchanger.

[0024] In a further development of the idea of the present invention, it is proposed that in the aforementioned embodiment a bypass line being provided with a valve is arranged between the outlet line of the utility side of the evaporator of the second heat pump and the line being connected with the inlet of the compressor of the second heat pump, wherein the bypass line directly connects the outlet line of the evaporator and the line being connected with the inlet of the compressor of the second heat pump without being connected with the heat exchanger. When the valve is closed, refrigerant of the second heat pump only flows from the evaporator of the second heat pump through the outlet line of the evaporator into the process side of the heat exchanger and from there via a line to the compressor of the second heat pump, whereas, when the valve is open, also a portion of the refrigerant flows from the evaporator of the second heat pump through the bypass line directly into the compressor of the second heat pump without flowing through the heat exchanger. Preferably, an additional valve is provided between the outlet of the heat exchanger and the line being connected with the inlet of the compressor of the second heat pump so that by closing this valve it may be obtained that all of the refrigerant flows from the evaporator of the second heat pump through the bypass line directly into the compressor of the second heat pump without flowing through the heat exchanger.

[0025] The plant may also comprise three or more heat pumps, wherein each heat pump is embodied as outlined above, wherein each two heat pumps may be connected with each other by means of a heat exchanger as described above for the embodiment of the plant comprising two heat pumps. If the plant of this embodiment comprises three heat pumps, the three heat pumps may temper six buffer vessels, namely the process side of a first heat pump may temper a first buffer vessel, the process side of a second heat pump may temper a second buffer vessel, the process side of a third heat pump may temper a third buffer vessel, the utility side of the first heat pump may temper a fourth buffer vessel, the utility side of the second heat pump may temper a fifth buffer vessel and the utility side of the third heat pump may temper a sixth buffer vessel. Furthermore, it is possible that the plant comprises in addition to the one or more buffer vessels being connected with a heat pump, a buffer vessel being not connected with a heat pump, but being connected with a cooler, preferably a heat exchanger being connected with an external cooling source, and / or a further buffer vessel being not connected with a heat pump, but being connected with a heater, preferably a heat exchanger being connected with an external heating source. In this embodiment, for instance four buffer vessels are tempered by two heat pumps as described above, whereas a fifth buffer vessel is tempered by the external cooling source and a sixth buffer vessel is tempered by the external heating source. For example, the process side of the heat exchanger is connected via an inlet line with the external heating source and via an outlet line with a hot energy return, which may be a vessel or a line leading into a heat exchanger of another plant or the like, whereas the utility side of the heat exchanger is connected with a circulation line leading from the sixth buffer vessel through the utility side of the heat exchanger and from there back into the sixth buffer vessel. Likewise, the utility side of the other heat exchanger may be connected via an inlet line with the external cooling source and via an outlet line with a cold energy return, which may be a vessel or a line leading into a heat exchanger of another plant or the like, whereas the process side of the heat exchanger is connected with a circulation line leading from the fifth buffer vessel through the process side of the heat exchanger and from there back into the fifth buffer vessel. Also in this embodiment, the heat exchangers may be selected from the group consisting of shell and tube heat exchangers, plate and frame heat exchangers, plate evaporators, falling film evaporators, forced circulation evaporators, kettle evaporators and internal evaporators.

[0026] Also concerning the kind of the at least one crystallizer, the present invention is not particularly limited. Thus, the at least one crystallizer may be any kind of suspension crystallizer, static crystallizer and falling film crystallizer. Also combinations thereof are possible, such as a combination of one or more static crystallizers and one or more falling film crystallizers, a combination of one or more suspension crystallizers and one or more static crystallizers or a combination of one or more suspension crystallizers and one or more falling film crystallizers. Particularly preferably, the plant comprises at least one layer crystallizer and most preferably at least one static crystallizer and / or at least one falling film crystallizer.

[0027] As set out above, it is preferred that at least one and preferably each of the at least one buffer vessel comprises at least one outlet line being connected with an inlet line of the at least one crystallizer as well as at least one inlet line being connected with an outlet line of the at least one crystallizer.

[0028] In accordance with another aspect, the present invention refers to a process for purifying a crude composition by crystallization, wherein the process is performed in an above-described plant.

[0029] Preferably, each of the at least one buffer vessel is filled with a heat transfer medium, wherein at least a portion of the heat transfer medium is kept below the product melting temperature or below the crystallization temperature and at least another portion of the heat transfer medium is kept above the product melting temperature.

[0030] During the process, for each of the phases of the crystallization an appropriately tempered heat transfer medium is circulated from the respective buffer vessel or respective axial section thereof, which contains heat transfer medium with the temperature required for the respective crystallization phase, through the cool- ing / heating circuit of the crystallizer back into the respective buffer vessel or respective axial section thereof so as to temper the surfaces, on which the crystal layers shall deposit, to the appropriate temperature. If the heat transfer medium is used to cool the surface of the crystallizer it is heated and needs to be re-cooled, whereas if the heat transfer medium is used to heat the surface of the crystallizer it is cooled and needs to be re-heated. For this purpose, the heat transfer medium of the respective buffer vessel is re-cooled or re-heated by circulating the heat transfer medium of at least some of the buffer vessels or of the axial sections of a buffer vessel from the buffer vessel or respective axial section thereof through a side of one of the at least two heat pumps back into the respective buffer vessel or respective axial section thereof.

[0031] The refrigerant as well as the heat transfer medium used in the process are preferably as described above for the plant.

[0032] Subsequently, the present invention is described by means of illustrative, but not limiting figures, wherein:

[0033] Fig. 1 is a schematic view of a plant for purifying a crude composition by crystallization in accordance with one embodiment of the present invention.

[0034] Fig. 2 is a schematic view of a plant for purifying a crude composition by crystallization in accordance with another embodiment of the present invention.

[0035] The plant 10 shown in figure 1 comprises a static crystallizer 12, six buffer vessels 14, 14’, 14”, 14”’, 14’v, 14V, two heat pumps 16, 16’, two heat exchangers 18, 20, a cold energy supply 21 , a cold energy return 22, a hot energy supply 24 as well as a hot energy return 26. Each of both heat pumps 16,16’ comprises an evaporator 28, 28’, a compressor 30, 30’, a condenser 32, 32’ and an expansion valve 34, 34’. Instead of a static crystallizer, also a falling film crystallizer, a combination of static crystallizers, a combination of falling film crystallizers or any combination of static and falling film crystallizers may be used. While each of the evaporators 28, 28’ comprises a process side 36, 36’ and a utility side 38, 38’, each of the condensers 32, 32’ comprises a process side 40, 40’ and a utility side 42, 42’. More specifically, the process side 36 of the evaporator 28 of the first heat pump 16 comprises an inlet line 44 being connected with the buffer vessel 14’ and an outlet line 46 being connected with the buffer vessel 14’, whereas the utility side 38 of the evaporator 28 of the first heat pump 16 comprises an inlet line 50 being connected with the expansion valve 34 as well as an outlet line 52 being connected with the compressor 30. In turn, the compressor 30 comprises an outlet line 54 being connected with an inlet of the process side 40 of the condenser 32 of the first heat pump 16, whereas the expansion valve 34 comprises a line 56 being indirectly connected with an outlet of the process side 40 of the condenser 32. Furthermore, the utility side 42 of the condenser 32 of the first heat pump 16 comprises an inlet line 58 being connected with the buffer vessel 14’” and an outlet line 60 being connected with the buffer vessel 14’”. Between the line 56 and the outlet of the outlet of the process side 40 of the condenser 32 of the first heat pump 16 a heat exchanger 62 is arranged, which comprises a process side 64 and a utility side 66. The process side 64 of the heat exchanger 62 comprises an inlet line 68 being connected with the outlet of the process side 40 of the condenser 32 of the first heat pump 16 and comprises an outlet being connected with the line 56 being connected with the inlet of the expansion valve 34 of the first heat pump 16.

[0036] The second heat pump 16’ is embodied similar to the first heat pump 16. More specifically, the process side 36’ of the evaporator 28’ of the second heat pump 16’ comprises an inlet line 70 being connected with the buffer vessel 14” and an outlet line 72 being connected with the buffer vessel 14”, whereas the utility side 38’ of the evaporator 28’ of the second heat pump 16’ comprises an inlet line 50’ being connected with the expansion valve 34’ as well as an outlet line 52’ being connected with the compressor 30’. More specifically, the outlet line 52’ is connected with an inlet of the utility side 66 of the heat exchanger 62, wherein the utility side 66 of the heat exchanger 62 further comprises an outlet line 74, which is connected with an inlet of the compressor 30’ of the second heat pump 16’. A valve 77 is arranged in the outlet line 74 downstream of the heat exchanger 62 and upstream of the compressor 30’. In addition, a bypass line 76 being provided with a valve 78 is arranged upstream of the heat exchanger 62, wherein the bypass line 76 connects the outlet line 52’ and the outlet line 74 at a point being downstream of valve 77 so as to directly connect the utility side 38’ of the evaporator 28’ with the inlet of the compressor 30’ of the second heat pump 16’. In turn, the compressor 30’ of the second heat pump 16’ comprises an outlet line 54’ being connected with an inlet of the process side 40’ of the condenser 32’ of the second heat pump 16’, whereas the expansion valve 34’ comprises a line 56’ being connected with an outlet of the process side 40’ of the condenser 32’. Furthermore, the utility side 42’ of the condenser 32’ of the second heat pump 16’ comprises an inlet line 80 being connected with the buffer vessel 14’vand an outlet line 82 being connected with the buffer vessel 14’v.

[0037] Moreover, the buffer vessel 14 comprises a circulation line 84, 84’ leading through one side of the heat exchanger 20, wherein the cold energy supply 21 and the cold energy return 22 are connected via a line 86, 86’ leading through the other side of the heat exchanger 20. Likewise thereto, the buffer vessel 14vcomprises a circulation line 88, 88’ leading through one side of the heat exchanger 18, wherein the hot energy supply 24 and the hot energy return 26 are connected via a line 90, 90’ leading through the other side of the heat exchanger 18. Each of the buffer vessels 14, 14’, 14”, 14’”, 14’v, 14vfurther comprises an outlet line 92, 92’, 92”, 92’”, 92’v, 92v, each of which being provided with a valve 94, 94’, 94”, 94”’,94’v, 94v, wherein the outlet lines 92, 92’, 92”, 92’”, 92’v,92vcombine to a central inlet line 96 leading into the crystallizer 12. In addition, the crystallizer 12 comprises an outlet line 98, which splits into the six inlet lines 100, 100’, 100”, 100’”, 100’v, 100v, each of which being provided with a valve 102, 102’, 102”, 102’”, 102’v, 102vand each of which leading into one of the buffer vessel 14, 14’, 14”, 14’”, 14’v, 14v. During the operation of the plant 10, crude composition to be purified is subjected to a crystallization in the crystallizer 12 and is subjected repeatedly to the crystallization phases, in this series, of a filling and precooling phase at a temperature T1 , a subsequent crystallization phase during which the mother liquid is cooled to a temperature T2, one or more subsequent sweating phases during which the crystal layers are heated to a temperature T3 and a melting phase during which the crystal layers are heated to a temperature T4, wherein T4>T3>T1 >T2. Each of the buffer vessels 14, 14’, 14”, 14”’, 14’v, 14vis filled with heat transfer medium, wherein the temperature of the heat transfer medium stepwise increases from the buffer vessel 14 to the buffer vessel 14V, i.e. the temperature of the heat transfer medium contained in the buffer vessel 14 is the coldest, whereas the temperature of the heat transfer medium contained in the buffer vessel 14’ is higher, wherein the temperature of the heat transfer medium contained in the buffer vessel 14” is even higher than that contained in the buffer vessel 14’ and so on so that the temperature of the heat transfer medium contained in the buffer vessel 14vis the highest. For instance, the temperatures of the heat transfer media contained in the buffer vessels 14, 14’, 14” is lower than the temperatures of the heat transfer media contained in the buffer vessels 14’”, 14’v, 14vand are suitable for cooling purpose and for crystallization. During each crystallization phase, the crystallizer 12 is either cooled or heated by a heat transfer medium, which is pumped from one of the buffer vessels 14, 14’, 14”, 14’”, 14’v, 14vthrough one of the outlet lines 92, 92’, 92”, 92’”, 92’v, 92vand the central inlet line 96 into the crystallizer 12, therein through the cooling / heating circuit of the crystallizer 12 and from there via outlet line 98 and one of the inlet lines 100, 100’, 100”, 100’”, 100’v, 100vback into the respective buffer vessels 14, 14’, 14”, 14’”, 14’v, 14v. Likewise, a mixture of heat transfer media deriving from two or more of the buffer vessels 14, 14’, 14”, 14’”, 14’v, 14vmay be pumped from the respective buffer vessels 14, 14’, 14”, 14’”, 14’v, 14vand the associated outlet lines 92, 92’, 92”, 92’”, 92’v, 92vand the central inlet line 96 into the crystallizer 12, therein through the cooling / heating circuit of the crystallizer 12 and from there via outlet line 98 and the associated inlet lines 100, 100’, 100”, 100”’, 100’v, 100vback into the respective buffer vessels 14, 14’, 14”, 14’”, 14’v, 14v. More specifically, for the heating phase including the sweating and melting steps heat transfer medium from the buffer vessel 14’”, 14’vor 14vor a mixture of heat transfer media from the buffer vessels 14’”, 14’vand 14vis pumped through the cooling / heating circuit of the crystallizer 12, whereas the cooling phase, during which the temperature of the mother liquid is continuously cooled down to the crystallization temperature, is initiated by pumping first heat transfer media from the buffer vessel 14”, by then pumping heat transfer media from the buffer vessel 14’ and by then pumping heat transfer media from the buffer vessel 14 through the cooling / heating circuit of the crystallizer 12. Thereby, virtually any desired temperature profile may be adjusted during the crystallization.

[0038] The temperatures of the heat transfer media in the single buffer vessels 14, 14’, 14”, 14’”, 14’v, 14vare adjusted by the cold energy supply 21 , the two heat pumps 16, 16’ and the hot energy supply 24. More specifically, the heat transfer medium contained in the buffer vessel 14, which is heated during its use as cooling agent in the crystallizer 12, is re-cooled in the heat exchanger 20 by heat transfer from the cooling agent flowing from the cold energy supply 21 through line 86 into the heat exchanger 20 and through line 86’ from the heat exchanger 20 to the cold energy return 22. In turn, the heat transfer medium contained in the buffer vessel 14’, which is heated during its use as cooling agent in the crystallizer 12, is recooled in the process side 36 of the evaporator 28 of the first heat pump 16 by refrigerant flowing through the utility side 38 of the evaporator 28 of the first heat pump 16, whereas the heat transfer medium contained in the buffer vessel 14”, which is heated during its use as cooling agent in the crystallizer 12, is re-cooled in the process side 36’ of the evaporator 28’ of the second heat pump 16’ by refrigerant flowing through the utility side 38’ of the evaporator 28’ of the second heat pump 16’. In contrast thereto, the heat transfer medium contained in the buffer vessel 14’”, which is cooled during its use as heating agent in the crystallizer 12, is re-heated in the utility side 42 of the condenser 32 of the first heat pump 16 by refrigerant flowing through the process side 40 of the condenser 32 of the first heat pump 16, whereas the heat transfer medium contained in the buffer vessel 14’v, which is cooled during its use as heating agent in the crystallizer 12, is re-heated in the utility side 42’ of the condenser 32’ of the second heat pump 16’ by refrigerant flowing through the process side 40’ of the condenser 32’ of the second heat pump 16’. Finally, the heat transfer medium contained in the buffer vessel 14v, which is cooled during its use as heating agent in the crystallizer 12, is re-heated in the heat exchanger 20 by heat transfer from the heating agent flowing from the hot energy supply 24 through line 90 into the heat exchanger 90 and through line 90’ from the heat exchanger 20 to the hot energy return 26.

[0039] As set out above, refrigerant flows through both heat pumps 16, 16’. More specifically, in the first heat pump 16 refrigerant exiting the utility side 38 of the evaporator 28 flows through the compressor 30, in which it is compressed and thereby heated, through the process side 40 of the condenser 32 of the first heat pump 16, from there to the expansion valve 34, in which it is expanded, and from there back to the utility side 38 of the evaporator 28 of the first heat pump 16. Likewise, in the second heat pump 16’ refrigerant exiting the utility side 38’ of the evaporator 28’ flows through the compressor 30’, in which it is compressed and thereby heated, through the process side 40’ of the condenser 32’ of the second heat pump 16’, from there to the expansion valve 34’, in which it is expanded, and from there back to the utility side 38’ of the evaporator 28’ of the second heat pump 16’.

[0040] The plant of figure 2 corresponds to that of figure 1 except that instead of six different buffer vessels 14, 14’, 14”, 14’”, 14’v, 14vone buffer vessel 14 is provided in which the heat transfer medium contained therein has a temperature gradient of, from the bottom to the top of the buffer vessel 14, cold to hot.

[0041] Subsequently, the present invention is described by means of illustrative, but not limiting examples. Example 1 and Comparative Example 1

[0042] In example 1 , a crude composition containing as target compound naphthalene has been purified in a plant in accordance with figure 1 . More specifically, a crude composition containing 97.6% by weight of target compound and in sum 2.4% by weight of impurities, which contained thionaphthene, indene, xylenols, quinoline and others, was crystallized in a falling film crystallizer 12 with a crystallization temperature of 59°C for the lowest stage, a sweating temperature of 80°C for the highest stage and a melting temperature of 95°C for the highest stage each at ambient pressure. The temperature of the heat transfer medium contained in the buffer vessel 14 was adjusted to 40°C, whereas the temperature of the heat transfer medium contained in the buffer vessel 14’ was adjusted to 45°C, the temperature of the heat transfer medium contained in the buffer vessel 14” was adjusted to 60°C, the temperature of the heat transfer medium contained in the buffer vessel 14”’ was adjusted to 80°C and the temperature of the heat transfer medium contained in the buffer vessel 14’vwas adjusted to 105°C.

[0043] During the cooling phase, the buffer vessel 14" was used during precooling, then - when the temperature of the buffer vessel 14" was not enough - the buffer vessel 14' was used to continue the precooling and to cover a part of the crystallization. Once the temperature of the buffer vessel 14' was not enough low to finish the crystallization phase, the buffer vessel 14 was used to finish the crystallization. In turn, during the heating phase the buffer vessel 14'" was used for preheating and for a part of the sweating phase if not all the sweating depending on the quality of the feed of the performed crystallization stage. Then, once the temperatures of the buffer vessel 14'" was not enough, the buffer vessel 14’v was used to finish the sweating and to perform preheat in preparation for melting and the melting phase. As comparative example 1 , the same process was performed except that all buffer vessels were omitted and that the heat pumps were omitted.

[0044] The process of the example 1 allowed in comparison to the comparative example 1 to reduce the crystallization heating by 61 % and the crystallization cooling by 69%.

[0045] Example 2 and Comparative Example 2

[0046] In example 2, a crude composition containing as target compound anthracene and carbazole has been purified in a plant being similar to that shown in figure 1 , but which comprised seven buffer vessels instead of six, namely an additional buffer vessel (designated below as additional buffer vessel) between the buffer vessels 14’ and 14”. More specifically, a crude composition containing 7.4% by weight of target compound anthracene and 3.3% by weight of target compound carbazole and in sum 89.3% by weight of impurities, which contained naphthalene, dibenzofuran, fluorene, phenanthrene, pyrene and others, was crystallized in the twelve static crystallizers 12 with a crystallization temperature of 25°C for the lowest stage, a sweating temperature of 220°C for the highest stage and a melting temperature of 250°C each at ambient pressure. The temperature of the heat transfer medium contained in the buffer vessel 14 was adjusted to 20°C, whereas the temperature of the heat transfer medium contained in the buffer vessel 14’ was adjusted to 26°C, the temperature of the heat transfer medium contained in the additional buffer vessel was adjusted to 38°C, the temperature of the heat transfer medium contained in the buffer vessel 14” was adjusted to 70°C, the temperature of the heat transfer medium contained in the buffer vessel 14’” was adjusted to 80°C, the temperature of the heat transfer medium contained in the buffer vessel 14’vwas adjusted to 140°C and the temperature of the heat transfer medium contained in the buffer vessel 14vwas adjusted to 265°C. As comparative example 2, the same process was performed except that the heat pumps were omitted and that only three buffer vessels were used, namely two buffer vessels for cooling at 20°C and 38°C and one buffer vessel for heating at 265°C.

[0047] The process of the example 2 allowed in comparison to the comparative example 2 to reduce the crystallization heating by 22% and the crystallization cooling by 39%.

[0048] Example 3 and Comparative Example 3

[0049] In example 3, a crude composition containing as target compound methyl methacrylate has been purified in a plant being similar to that shown in figure 1 , but which comprised three buffer vessels instead of six. More specifically, a crude composition containing 98.5% by weight of target compound and in sum 99.8% by weight of impurities, which contained propene, methanol, methylpropionate-MeP, methylbutyrate and others, was crystallized in a static crystallizer 12 with a crystallization temperature of -53°C for the lowest stage, a sweating temperature of -46°C for the highest stage and a melting temperature of -30°C for the highest stage each at ambient pressure. The temperature of the heat transfer medium contained in the buffer vessel 14 was adjusted to -60°C, whereas the temperature of the heat transfer medium contained in the buffer vessel 14’ was adjusted to -30°C, the temperature of the heat transfer medium contained in the buffer vessel 14” was adjusted to 25°C.

[0050] During the cooling phase, the buffer vessel 14 was used during precooling and crystallization, whereas during the heating phase the buffer vessel 14' was used to preheat, for sweating and part of the melting phase if not all the melting depending on the quality of the feed of the performed crystallization stage. Then, once the temperatures of buffer vessel 14' was not enough, the buffer vessel 14” was used to finish the melting.

[0051] As comparative example 3, the same process was performed except that the buff- er vessel 14' was omitted and that as heat pump configuration a classical cascade was used.

[0052] The process of the example 3 allowed in comparison to the comparative example 3 to reduce the crystallization heating by 37% and the-crystallization cooling by 22%.

[0053] Reference numerals

[0054] 10 Plant

[0055] 12 Crystallizer

[0056] 14,14’,14”,14’”,14’v,14vBuffer vessel

[0057] 16,16’ Heat pump

[0058] 18 Heat exchanger

[0059] 20 Heat exchanger

[0060] 21 Cold energy supply

[0061] 22 Cold energy return

[0062] 24 Hot energy supply

[0063] 26 Hot energy return

[0064] 28, 28’ Evaporator

[0065] 30,30’ Compressor

[0066] 32,32’ Condenser

[0067] 34,34’ Expansion valve

[0068] 36,36’ Process side of evaporator

[0069] 38,38’ Utility side of evaporator

[0070] 40,40’ Process side of condenser

[0071] 42,42’ Utility side of condenser

[0072] 44 Inlet line

[0073] 46 Outlet line

[0074] 50,50’ Inlet line

[0075] 52,52’ Outlet line

[0076] 54,54’ Outlet line

[0077] 56,56’ Line

[0078] 58 Inlet line

[0079] 60 Outlet line Heat exchanger

[0080] Process side of heat exchanger

[0081] Utility side of heat exchanger

[0082] Inlet line

[0083] Inlet line

[0084] Outlet line

[0085] Outlet line

[0086] Bypass line

[0087] Valve

[0088] Valve

[0089] Inlet line

[0090] Outlet line ,84’ Circulation line ,86’ Line ,88’ Circulation line ,90’ Line ,92’ ,92” ,92’” ,92’v,92vOutlet line ,94’ ,94” ,94’” ,94’v,94vValve

[0091] Central inlet line Outlet line 0, 100’, 100”, 100”’, 100’v,100vInlet lines 2, 102’, 102”, 102’”, 102’v, 102vValve

Claims

Claims:1 . A plant (10) for purifying a crude composition by crystallization, wherein the plant comprises at least one crystallizer (12), at least one buffer vessel (14, 14’, 14”, 14”’, 14’v, 14v) and at least two heat pumps (16, 16’), wherein each of at least two heat pumps (16, 16’) is connected with at least one buffer vessel (14, 14’, 14”, 14’”, 14’v, 14v).

2. The plant (10) in accordance with claim 1 , wherein at least two heat pumps (16, 16’) are connected with each other.

3. The plant (10) in accordance with claim 2, wherein the process side (28’) of one heat pump (16’) is directly or indirectly via a heat exchanger connected with the utility side (32) of another heat pump (16), wherein the process side (28) of the one heat pump (16) and the utility side (28’) of the other heat pump (16’) are connected with the same or with a different buffer vessel (14, 14’, 14”, 14’”, 14’v, 14v).

4. The plant (10) in accordance with any of the preceding claims, wherein each of the at least two heat pumps (16, 16’) is a mechanical heat pump (16, 16’), which comprises an evaporator (28, 28’), a condenser (32, 32’), an expansion valve (34, 34’) and a compressor (30, 30’).

5. The plant (10) in accordance with any of the preceding claims, which comprises at least two and preferably at least three buffer vessels (14, 14’, 14”, 14’”, 14’v, 14V), wherein each of at least two heat pumps (16, 16’) is connected with at least one buffer vessel (14, 14’, 14”, 14’”, 14’v, 14v) and preferably with two different buffer vessels (14, 14’, 14”, 14’”, 14’v, 14v), wherein at least one heat pump (16) is connected with one or two other buffer vessels (14’, 14’”) than another heat pump (16’).

6. The plant (10) in accordance with claim 5, which comprises at least four buffer vessels (14, 14’, 14”, 14”’, 14’v, 14v), preferably four to ten buffer vessels (14, 14’, 14”, 14’”, 14’v, 14v), more preferably four to eight buffer vessels (14, 14’, 14”, 14’”, 14’v, 14v), still more preferably five to seven buffer vessels (14, 14’, 14”, 14’”, 14’v, 14v) and most preferably six buffer vessels (14, 14’, 14”, 14’”, 14’v, 14v), wherein a first buffer vessel (14’) is connected with the process side (28) of a first heat pump (16), a second buffer vessel (14”) is connected with the process side (28’) of a second heat pump (16’), a third buffer vessel (14’”) is connected with the utility side (42) of the first heat pump (16) and a fourth buffer vessel (14’v) is connected with the utility side (42’) of the second heat pump (16’).

7. The plant (10) in accordance with claim 6, wherein each of the at least two heat pumps (16, 16’) is a mechanical heat pump (16, 16’), which comprises an evaporator (28, 28’), a condenser (32, 32’), an expansion valve (34, 24’) and a compressor (30, 30’), wherein the evaporator (28, 28’) of the first heat pump (16) comprises a process side (36) and a utility side (38) and the condenser (32) of the first heat pump (16) comprises a process side (40) and a utility side (42), wherein the process side (36) of the evaporator (28) comprises an inlet line (44) being connected with the first buffer vessel (14’) and an outlet line (46) being connected with the first buffer vessel (14’), whereas the utility side (38) of the evaporator (28) comprises an inlet line (50) being connected with the expansion valve (34) as well as an outlet line (52) being connected with the compressor (30), wherein the compressor (30) comprises an outlet line (54) being connected with an inlet of the process side (40) of the condenser (32), wherein the expansion valve (34) comprises a line (56) being connected with an outlet of the process side (40) of the condenser (32), wherein the utility side (42) of the condenser (32) compris-es an inlet line(58) being connected with the third buffer vessel (14”’) and an outlet line (60) being connected with the third buffer vessel (14”’).

8. The plant (10) in accordance with claim 7, wherein the evaporator (28’) of the second heat pump (16’) comprises a process side (36’) and a utility side (38’) and the condenser (32’) of the second heat pump (16’) comprises a process side (40’) and a utility side (42’), wherein the process side (36’) of the evaporator (28’) comprises an inlet line(50’) being connected with the second buffer vessel (14”) and an outlet line (52’) being connected with the second buffer vessel (16’), whereas the utility side (38’) of the evaporator (28’) comprises an inlet line (50’) being connected with the expansion valve (34’) as well as an outlet line (52’) being connected with the compressor (30’), wherein the compressor (30’) comprises an outlet line (54’) being connected with an inlet of the process side (40’) of the condenser (32’), wherein the expansion valve (34’) comprises a line (56’) being connected with an outlet of the process side (40’) of the condenser (32’), wherein the utility side (42’) of the condenser (32’) comprises an inlet line (80) being connected with the fourth buffer vessel (14’v) and an outlet line (82) being connected with the fourth buffer vessel (14’v).

9. The plant (10) in accordance with claim 8, wherein between the outlet line (68) of the process side (40) of the condenser (32) of the first heat pump (16) and the expansion valve (34) a heat exchanger (62) is arranged, which comprises a process side (64) and a utility side (66), wherein the process side (64) of the heat exchanger (62) comprises an inlet being connected with the outlet line (68) of the process side (40) of the condenser (32) of the first heat pump (16) and comprises an outlet being connected with the line (56) being connected with an inlet of the expansion valve (34) of the first heat pump (16), whereas the utility side (66) of the heat exchanger (62) comprises an inlet being connected with the outlet line (52’) of the utilityside (38’) of the evaporator (28’) of the second heat pump (16’) and comprises an outlet being connected with an inlet of the compressor (30’) of the second heat pump (16’).

10. The plant (10) in accordance with claim 9, wherein a bypass line (76) being provided with a valve (78) is arranged between the outlet line (52’) of the utility side (38’) of the evaporator (28’) of the second heat pump (16’) and the line (74) being connected with the inlet of the compressor (30’) of the second heat pump (16’), wherein the bypass line (76) directly connects the outlet line (52’) of the evaporator (28’) and the line (74) being connected with the inlet of the compressor (30’) of the second heat pump (16’) without being connected with the heat exchanger (62), wherein preferably an additional valve (77) is provided between the outlet of the heat exchanger (62)) and the line (74) being connected with the inlet of the compressor (30’) of the second heat pump (16’).11 . The plant (10) in accordance with any of any of the preceding claims, which comprises a buffer vessel (14) being not connected with a heat pump (16, 16’), but being connected with a cooler, preferably a heat exchanger (20) being connected with an external cooling source (21 ), and / or a further buffer vessel (14v) being not connected with a heat pump (16, 16’), but being connected with a heater, preferably a heat exchanger (18) being connected with an external heating source (24).

12. The plant (10) in accordance with any of the preceding claims, wherein the at least one crystallizer (12) is at least one layer crystallizer (12) and preferably at least one static crystallizer (12) and / or at least one falling film crystallizer.

13. The plant (10) in accordance with any of the preceding claims, wherein at least one and preferably each of the at least one buffer vessel (14, 14’, 14”, 14”’, 14’v, 14v) comprises at least one outlet line (92, 92’, 92”, 92’”, 92’v,92v) being connected with an inlet line (96) of the at least one crystallizer (12) as well as at least one inlet line (100, 100’, 100”, 100’”, 100’v, 100v) being connected with an outlet line (98) of the at least one crystallizer (12).

14. A process for purifying a crude composition by crystallization, wherein the process is performed in a plant (10) in accordance with any of the preceding claims.

15. The process in accordance with claim 14, wherein each of the at least one buffer vessel (14, 14’, 14”, 14’”, 14’v, 14v) is filled with a heat transfer medium, wherein at least a portion of the heat transfer medium is kept below the product melting temperature or below the crystallization temperature and at least another portion of the heat transfer medium is kept above the product melting temperature.

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