A purification plant comprising distillation and crystallization units with improved energy efficiency

Abstract

The present invention relates to a plant for purifying a crude composition by distillation and crystallization, wherein the plant comprises at least one distillation column being connected with an overhead or side condenser, at least one crystallizer, at least one heat exchanger, at least one heat pump and optionally an energy storage system, wherein at least one heat pump is embodied to operate between the overhead or side condenser and the energy storage system, if present, or, if the energy storage system is not present, at least one crystallizer, and wherein at least one heat exchanger is embodied to operate between the overhead or side condenser of at least one distillation column and the energy storage system, if present, or, if the energy storage system is not present, at least one crystallizer.

Description

[0001] Sulzer Management AG S13810PEP - Pl / Fa

[0002] A purification plant comprising distillation and crystallization units with improved energy efficiency

[0003] The present invention relates to a plant for purifying a crude composition by distillation and crystallization as well as to a process for purifying a crude composition by distillation and 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. Frequently, several of the above-mentioned purification processes are combined, following cost optimization. More specifically, often first a purification comprising one or more distillation steps is performed, before the pre-purified composition is subsequently purified in one or more crystallization steps. Distillation is a continuous purification technique, 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, are condensed 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. Hence, if the purification requires to separate from the mixture compounds having different boiling points as well as compounds having similar boiling points, after the distillation a further purification is required, which allows to separate different compounds having similar boiling points, such as crystallization.

[0005] Crystallization is an 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 10% 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, but is also often use for purifying less pure composition, such as those containing the target compound in an amount of as low as 30% by weight. That is the reason, why crystallization is usually not performed before a distillation, but usually after a distillation. 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 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. After completion of the crystallization, 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.

[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 is filled into the sump area of the falling film crystallizer, before the crystallization process is started and 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 cooled hollow tubes and allowed to fall down as falling film on the inner surfaces of the cooled hollow tubes back to the sump of the falling film crystallizer. Crystal layers being enriched in the target compound are deposited on the cooled inner wall surfaces of the hollow tubes, 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 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 crys- tallization 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. 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 or tubes. 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 or tubes is terminated and the plates or tubes 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.

[0007] All in all, while distillation is a continuous process, 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. Therefore, the distillation and crystallization processes have usually separate energy systems, which are not integrated with each other. It has been already suggested to connect the energy systems of both processes, but these solutions are usually not very efficient. This is, among others, due to the facts that distillation is a continuous process, whereas layer crystallization is a batch process, that the heating as well as cooling needs of distillation and crystallization require different temperature levels because of the properties of the product and the specific aspects of each technology usually requiring the use of different energy sources or temperature levels for the cooling and the heating of each technology and that the temperatures are essentially stable during the distillation, but variable over a wide range during the crystallization. In view of this, the object underlying the present invention is to provide a plant for purifying a crude composition by distillation and crystallization, in which the energy systems of one or more distillation columns and of one or more crystallizers are integrated with each other so as to allow to operate the plant with minimal operational costs, and which is further flexible so that an energy efficient operation of the plant may be obtained for a plurality of possible applications or independently from the cooling and heating loads required for the one or more distillation columns and one or more crystallizers, respectively, as well as to provide a respective process for purifying a crude composition.

[0008] In accordance with the present invention, this object is satisfied by providing a plant for purifying a crude composition by distillation and crystallization, wherein the plant comprises at least one distillation column being connected with an overhead or side condenser, at least one crystallizer, at least one heat exchanger, at least one heat pump and optionally an energy storage system, wherein at least one heat pump is embodied to operate between the overhead or side condenser and the energy storage system, if present, or, if the energy storage system is not present, at least one crystallizer, and wherein at least one heat exchanger is embodied to operate between the overhead or side condenser of at least one distillation column and the energy storage system, if present, or, if the energy storage system is not present, at least one crystallizer.

[0009] By providing both, at least one heat exchanger and at least one heat pump, each of which being embodied to operate between the overhead or side condenser of at least one distillation column and the energy storage system, if present, or, if not present, the crystallizer, the heat of a process stream of the distillation column, such as of the overhead stream being connected with the distillation column, may be exploited to provide heating energy in the crystallizer for sweating and melting and, in turn, the crystallizer may be exploited to provide cooling energy for con- densation in the overhead or side condenser being connected with the distillation column. This allows to drastically reduce the operational costs during the operation of the plant In addition, by providing both, at least one heat exchanger as well as at least one heat pump, each of which being embodied to operate between the overhead or side condenser of at least one distillation column and the energy storage system, if present, or, if the energy storage system is not present, at least one crystallizer, the improved energy efficiency is obtained for a plurality of possible applications or, in other words, essentially independently from the cooling and heating loads required for the one or more distillation columns and one or more crystallizers, respectively. This is due to the fact that on account of the presence of at least one heat exchanger as well as of at least one heat pump, each of which being embodied to operate between the overhead or side condenser of at least one distillation column and the energy storage system, if present, or, if the energy storage system is not present, the plant may be operated by only using the at least one heat exchanger, or, by only using the at least one heat pump or by using both, the at least one heat exchanger as well as the at least one heat pump, depending on the cooling and heating loads required for the one or more distillation columns and one or more crystallizers, respectively. If, for instance, the cooling within the overhead or side condenser, with which the at least one distillation column is connected, occurs at a higher temperature than the product melting temperature to which the crystal layers have to be heated during the melting phase of the crystallization within the at least one crystallizer, only the at least one heat exchanger needs to be operated during the process in order to transfer as much as possible energy from the cooling agent used on the utility side of the overhead or side condenser to the crystallizer for sweating and melting the crystal layers during the sweating and melting phases. However, if the cooling within the overhead or side condenser, with which the at least one distillation column is connected, occurs at a lower temperature, namely at a temperature being between the temperature of the mother liquid during the crystallization and the product melting temperature to which the crystal layers have to be heated during the melting phase of the crystal- lization within the at least one crystallizer, in addition to the at least one heat exchanger also the at least one heat pump needs to be operated in order to provide additional energy required for the sweating and melting phases during the operation of the crystallizer. All in all, the plant for purifying a crude composition by distillation and crystallization in accordance with the present invention may be operated with minimal operational costs and is further flexible so that an energy efficient operation may be obtained for a plurality of possible applications or independently from the cooling and heating loads required for the one or more distillation columns and one or more crystallizers, respectively.

[0010] In accordance with the present invention, the plant comprises at least one heat exchanger as well as at least one heat pump, each of which being embodied to operate between the overhead or side condenser and the energy storage system, if present, or, if the energy storage system is not present, at least one crystallizer. This means in accordance with the present invention that the heat exchanger as well as the heat pump are able to transfer energy or heat, respectively, from the overhead or side condenser to the energy storage system, if present, or, if not present, to the crystallizer. More specifically, the heat exchanger as well as the heat pump are able to transfer energy or heat, respectively, from the cooling agent flowing through the utility side of the overhead or side condenser to a heat transfer medium flowing into the energy storage system, if present, or, if not present, into the crystallizer.

[0011] 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, i.e. in the present case with the overhead or side condenser, is also called process side of the heat pump’s evaporator, whereas the side of the heat pump being in contact with the hot sink, i.e. in the present case with the energy storage system or crystallizer, is also called utility side of the heat pump’s conden- ser. 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.

[0012] As mentioned above, the plant may or may not comprise an energy storage system. For example, if the plant comprises a plurality of crystallizers, in particular at least three or even at least five crystallizers, such as ten crystallizers, and the crystallizers are operated out-of-phase, an energy storage system is not required, but possible. Out-of-phase operation means herein that the different crystallizers operate at the same time in a different crystallization phase so that in fact the sum of crystallizers work semi-continuous. For instance, when at the same time one or more crystallizers operate in the filling and precooling phase, one or more other crystallizers operate in the crystallization phase and / one or more crystallizers operate in the sweating phase and / one or more crystallizers operate in the melting phase and / or one or more crystallizers operate in the product withdrawal phase, an energy storage system is not required, but possible. However, if the plant comprises less crystallizers or the crystallizers are not operated out-of-phase, it is preferred that the plant comprises an energy storage system, in order to temporarily store heat transfer medium, wherein energy or heat, respectively, is continuously transferred from the overhead or side condenser via the heat exchanger and / or heat pump to the heat transfer medium, whereas energy or heat, respectively, is transferred from the heat transfer medium to the crystallizers.

[0013] Preferably, the energy storage system comprises at least one buffer vessel, which is filled during the operation with a heat transfer medium having a different tern- perature than the 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. Good results are in particular obtained, when the energy storage system comprises two to ten buffer vessels, more preferably two to four buffer vessels and most more preferably three buffer vessels.

[0014] Moreover, the energy storage system is preferably connected via at least one line with the at least one heat exchanger and is connected via at least one line with the at least one heat pump and is connected via at least one line with the at least one crystallizer. During the operation of the plant, heat transfer medium is flowing through the aforementioned lines between the heat exchanger and heat pump via the energy storage system to the at least only crystallizer and from the at least only crystallizer via the energy storage system to the heat exchanger and heat pump and thereby transfers heat from the heat exchanger and heat pump on the one hand to the at least only crystallizer on the other hand.

[0015] In accordance with the present invention, the plant comprises at least one heat exchanger as well as at least one heat pump. In principle, the at least one heat exchanger and the at least one heat pump may be arranged in parallel to each other, i.e. the at least one heat exchanger may be arranged upstream or downstream of the at least one heat pump. However, it is preferred that the at least one heat exchanger and the at least one heat pump are arranged parallel to each other.

[0016] The present invention is not particularly limited concerning the kind of the at least one heat pump. Good results are in particular obtained, when the at least one heat pump is 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 distillation or crystallization process, i.e. a heat pump, in which no fraction or composition, respectively, obtained in the distillation or crystallization process, such as the overhead vapor composition of the distillation column, 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.

[0017] Preferably, the evaporator side of the heat pump, which is also called process side of the heat pump’s evaporator, is in contact with the cold sink, i.e. the overhead or side condenser, whereas the condenser side of the heat pump, which is also called utility side of the heat pump’s condenser, is in contact with the hot sink, i.e. in the present case with the energy storage system or crystallizer, respectively. 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. cooling agent flowing through the utility side of the overhead or side condenser), 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. 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.

[0018] Suitable refrigerants for the operation of the heat pump are R-1233zd, R-1233ze, HFO-1234yf, carbon dioxide (R744), ammonia (R717), water and others. Alternatively, even if less preferred, an absorption heat pump may be used as at least one 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.

[0019] The present invention is not particularly limited concerning the kind of the at least one heat exchanger. Preferably, the at least one heat exchanger is selected from the group consisting of shell and tube heat exchangers, plate and frame evaporators, plate evaporators, falling film evaporators, forced circulation evaporators, kettle evaporators and internal evaporators. 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.

[0020] 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 in the same plant. Particularly preferably, the plant comprises at least one layer crystallizer and most preferably at least one static crystallizer or at least one falling film crystallizer.

[0021] In accordance with the present invention, the plant comprises an overhead or side condenser, which is connected with at least one distillation column. The prefixes “overhead” or “side” condenser are only used to easily distinguish the condenser being connected with the distillation column from the condenser being part of the heat pump. Both condensers, i.e. that being connected with the distillation column as well as that being part of the heat pump, may be the same, i.e. both are preferably selected from the group consisting of shell and tube heat exchangers, plate and frame evaporators, plate evaporators, falling film evaporators, forced circulation evaporators, kettle evaporators and internal evaporators. Overhead condenser means a condenser being connected with the overhead or overhead outlet of the distillation column, with which it is connected, whereas side condenser means a condenser being connected with the side or an optional side outlet of the distillation column. The condenser may be directly connected with the respective overhead or side outlet of the distillation column or may be connected via a line with the respective overhead or side outlet of the distillation column.

[0022] More preferably, the overhead or side condenser comprises a process side and a utility side, wherein the utility side comprises an inlet line for cooling agent and an outlet line for cooling agent, wherein the outlet line for cooling agent splits into a first connection line being connected with an inlet of the at least one heat exchanger and into a second connection line being connected with an inlet of the at least one heat pump. In turn, the process side of the overhead or side condenser preferably comprises an inlet line being connected with an overhead outlet or side outlet, respectively, of the at least one distillation column and an outlet line splitting into a recycle line leading back into the at least one distillation column and into an overhead or side composition removal line, respectively. In other words, the process side of the overhead or side condenser is the side, through which the overhead or side composition of the distillation column, respectively, flows, whereas the utility side of the overhead or side condenser is the side, through which the cooling agent flows. Cooling agent means the same as refrigerant as well as heat transfer medium. However, for the ease of distinguishing the fluid used in the heat pump, that used in the side or overhead condenser being connected with the distil- lation column and that used between the heat exchanger and heat pump on the one hand and preferably via the energy storage system the at least one crystallizer on the other hand, the fluid used in the heat pump is called refrigerant, the fluid used in the overhead or side condenser is called cooling agent and the fluid used between the heat exchanger and heat pump on the one hand and the at least one crystallizer on the other hand preferably via the optional energy storage system is called heat transfer medium.

[0023] Suitable cooling agents are and suitable heat transfer media are water, mixtures of water and glycol, thermal oil and others.

[0024] The at least one distillation column may be any known distillation column. For instance, the distillation column may only comprise an overhead outlet and a bottom outlet or, in addition thereto, one or more side outlets. Furthermore, the distillation column may not comprise any dividing wall therein, or it comprise one or more dividing walls. For example, the distillation column may be a top divided wall distillation column, a middle divided wall distillation column or a bottom divided wall distillation column.

[0025] In a further development of the idea of the present invention, it is suggested that the first connection line comprises a valve and the second connection line comprises a valve. This allows to selectively allow cooling agent exiting the utility side of the overhead or side condenser to flow only through the heat exchanger or only through the heat pump, by opening one of the valves and by closing the other. Moreover, it allows cooling agent exiting the utility side of the overhead or side condenser to flow through the heat exchanger as well as through the heat pump, if both valves are partially or completely open, wherein the flow rate of the cooling agent through the heat exchanger and through the heat pump may be the same or different, depending on the degree of opening of both valves. In accordance with a particularly preferred embodiment of the present invention, the at least one heat exchanger comprises a process side and a utility side, wherein the process side comprises the inlet being connected with the first connection line and an outlet line being connected with the inlet line for cooling agent of the overhead or side condenser, whereas the utility side comprises an inlet line and an outlet line both being connected with the energy storage system, if present, or, if the energy storage system is not present, with the at least one crystallizer of the plant

[0026] As set out above, it is preferred that the at least one heat pump comprises an evaporator, a condenser, an expansion valve and a compressor. Preferably, the evaporator of the at least one heat pump comprises a process side and a utility side, wherein the process side of the evaporator comprises an inlet being connected with the second connection line and an outlet line being connected with the inlet line for cooling agent of the overhead or side condenser, 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 of the heat pump, wherein the compressor comprises an outlet line being connected with an inlet of the condenser of the at least one heat pump and the expansion valve comprises an inlet line being connected with an outlet of the condenser of the at least one heat pump.

[0027] Moreover, it is preferred in the aforementioned embodiment that the condenser of the at least one heat pump comprises a process side and a utility side, wherein the process side of the condenser comprises an inlet being connected with the outlet line of the compressor and comprises an outlet being connected via a line with an inlet of the expansion valve, whereas the utility side of the condenser comprises an inlet line and an outlet line both being connected with the energy storage system, if present, or, if the energy storage system is not present, with the at least one crystallizer of the plant. In accordance with a further particularly preferred embodiment of the present invention, the plant comprises an energy storage system comprising at least one buffer vessel, wherein the at least one buffer vessel comprises an outlet line being connected with the at least one heat pump, an outlet line being connected with the at least one heat exchanger, an inlet line being connected with the at least one heat pump, an inlet line being connected with the at least one heat exchanger, at least one outlet line being connected with the at least one crystallizer as well as at least one inlet line being connected with the at least one crystallizer. The outlet and inlet lines connected with the at least one crystallizer may be equipped each with a valve.

[0028] Good results are particularly achieved in the aforementioned embodiment, if the energy storage system comprises two to ten, preferably two to four and more preferably three buffer vessels, wherein each of the buffer vessels comprises one outlet line being connected with the at least one crystallizer as well as one inlet line being connected with the at least one crystallizer, wherein one buffer vessel comprises an outlet line being connected with the at least one heat pump as well as an inlet line being connected with the at least one heat pump and another buffer vessel comprises an outlet line being connected with the at least one heat exchanger as well as an inlet line being connected with the at least one heat exchanger. The advantage of using more than one buffer vessel is to buffer or store more efficiently the heat transfer fluid at a desired temperature level by buffering the variable temperature medium flowing to a process requiring constant temperature. Moreover, the heat exchanger and the heat pump are connected from the other side directly or indirectly to the utility side or energy side of the distillation column, respectively. Again, the outlet and inlet lines connected with the at least one crystallizer may be equipped each with a valve. In a further development of the idea of the present invention, it is proposed that the outlet line of the one buffer vessel of the aforementioned embodiment (i.e. the buffer vessel being connected via an inlet line as well as via an outlet line with the at least one heat pump) is connected with the inlet line of the utility side of the condenser of the at least one heat pump and the inlet line of the one buffer vessel is connected with the outlet line of the utility side of the condenser of the at least one heat pump, whereas the outlet line of the another buffer vessel (i.e. the buffer vessel being connected via an inlet line as well as via an outlet line with the at least one heat exchanger) is connected with the inlet line of the utility side of the at least one heat exchanger and the inlet line of the another buffer vessel is connected with the outlet line of the utility side of the at least one heat exchanger.

[0029] The plant in accordance with the present invention may comprise in addition to the at least one crystallizer at least one further heat consuming device being coupled with at least one heat exchanger and / or at least one heat pump. This allows to exploit the heat of the overhead or side composition of the at least one distillation column for transferring energy or heat, respectively, to the at least one crystallizer, but also for transferring energy or heat, respectively, to the at least one further heat consuming device. On account thereof, it is preferred in this embodiment that at least one heat exchanger and / or at least one heat pump of the plant is embodied to operate between the overhead or side condenser and the heat consuming device. For instance, the plant comprises two heat exchangers and two heat pumps, wherein one heat exchanger as well as one heat pump are each embodied to operate between the overhead or side condenser and the energy storage system, if present, or, if the energy storage system is not present, at least one crystallizer, whereas the other heat exchanger and the other heat pump are each embodied to operate between the overhead or side condenser and the at least one further heat consuming device. Alternatively, it is possible that one heat exchanger and one heat pump are embodied to operate between the overhead or side condenser and the energy storage system, if present, or, if the energy storage system is not present, at least one crystallizer as well as between overhead or side condenser and the at least one further heat consuming device. Still alternatively, the plant may comprise two heat exchanger, one being embodied to operate between the overhead or side condenser and the energy storage system, if present, or, if the energy storage system is not present, at least one crystallizer and the other being embodied to operate between the overhead or side condenser and the at least one further heat consuming device, as well as one heat pump comprising two condensers or as condenser two heat exchangers in series, one condenser providing heat to the crystallization and the other condenser providing heat to the at least one further heat consuming device.

[0030] In accordance with a further aspect, the present invention relates to a process for purifying a crude composition by distillation and crystallization, wherein the process is performed in an above described plant.

[0031] During the process, heat of the cooling agent flowing through the utility side of the overhead or side condenser being connected with the at least one distillation column is transferred via the at least one heat exchanger and / or the at least one heat pump to the heat transfer medium flowing between the utility sides of the heat exchanger and heat pump on the one hand preferably via the preferred energy storage system to the at least one crystallizer on the other hand and vice versa. More specifically, if the temperature of the cooling agent flowing through the utility side of the overhead or side condenser is higher than the temperature of the mother liquid during the crystallization performed in the at least one crystallizer, heat of the cooling agent flowing through the utility side of the overhead or side condenser being connected with the at least one distillation column may be transferred via the at least one heat exchanger optionally via the energy storage system by the heat transfer medium to the at least one crystallizer. If the temperature of the cooling agent flowing through the utility side of the overhead or side condenser is even higher than the melting temperature used to melt the crystals during the melting phase of the crystallization performed in the at least one crystallizer, then the heat being transferred from the cooling agent flowing through the utility side of the overhead or side condenser via the heat exchanger and via the heat transfer medium to the at least one crystallizer is enough to cover fully or at least partially the heating need of all phases of the crystallization, i.e. in particular of the sweating and melting phases. However, if the temperature of the cooling agent flowing through the utility side of the overhead or side condenser is lower than the melting temperature used to melt the crystals during the melting phase of the crystallization performed in the at least one crystallizer, but higher than the temperature of the mother liquid during the crystallization performed in the at least one crystallizer, additional heat is needed at least for the melting phase of the crystallization, wherein this additional heat is supplied to the heat transfer medium by also operating the at least one heat pump. In other words, if the temperature of the cooling agent flowing through the utility side of the overhead or side condenser is higher than the melting temperature used to melt the crystals during the melting phase of the crystallization performed in the at least one crystallizer, then it is sufficient to operate during the process only the at least one heat exchanger, whereas it is necessary to also operate during the process the at least one heat pump, if the temperature of the cooling agent flowing through the utility side of the overhead or side condenser is lower than the melting temperature used to melt the crystals during the melting phase of the crystallization performed in the at least one crystallizer, but higher than the temperature of the mother liquid during the crystallization performed in the at least one crystallizer. When the cooling agent temperature does not cover all of the heating temperature range for the crystallization, it is preferred to supply the cooling agent to the heat pump’s evaporator where the cooling agent is cooled and adjusted to the necessary heat for the evaporator of the heat pump. On the other side of the heat pump, the heat transfer fluid used for crystallization heating is heated on the condenser of the heat pump to a temperature level higher than the temperature level possible to reach by the direct heat exchanger and higher than the temperature of the cooling agent itself. This allows to provide at the same time heating for crystallization and cooling for distillation with a good efficiency of the heat pump.

[0032] In view of this, it is preferred that at least one heat exchanger operates during the process between the overhead or side condenser and the energy storage system, if present, or, if the energy storage system is not present, at least one crystallizer if the temperature, at which the overhead or side condenser is operated (i.e., with which the cooling within the overhead or side condenser is effected), is higher than the temperature of the mother liquid during the crystallization performed in the at least one crystallizer, whereas the at least one heat pump preferably only operates between the overhead or side condenser and the energy storage system, if present, or, if the energy storage system is not present, at least one crystallizer if the temperature, at which the overhead or side condenser is operated, is lower than the melting temperature used to melt the crystals during the melting phase of the crystallization performed in the at least one crystallizer. If the temperature, at which the overhead or side condenser is operated is lower than the temperature of the mother liquid during the crystallization performed in the at least one crystallizer, only the at least one heat pump is operated, but no heat exchanger.

[0033] In a further development of the idea of the present invention, it is proposed that the overhead or side condenser of the plant, in which the process is performed, comprises a process side and a utility side, wherein the utility side comprises an inlet line for cooling agent and an outlet line for cooling agent through which during the process a cooling agent flows, wherein the outlet line for cooling agent splits into a first connection line being connected with an inlet of the at least one heat exchanger and into a second connection line being connected with an inlet of the at least one heat pump, wherein the first connection line comprises a valve and is connected with an inlet of at least one heat exchanger, wherein the second connection line comprises a valve and is connected with an inlet of at least one heat pump, wherein the valve of the first connection line is completely or at least partial- ly opened if the temperature of the cooling agent flowing through the outlet line of the utility side of the overhead or side condenser is higher than the temperature of the mother liquid during the crystallization performed in the at least one crystallizer, and wherein the valve of the second connection line is preferably only completely or at least partially opened if the temperature of the cooling agent flowing through the outlet line of the utility side of the overhead or side condenser is lower than the melting temperature used to melt the crystals during the melting phase of the crystallization performed in the at least one crystallizer.

[0034] Furthermore, it is preferred that a discretization of the temperature is done to ensure a proper temperature management during the process. More specifically, if both, the heat exchanger and the heat pump are operated, the temperature of the cooling agent flowing through the utility side of the overhead or side condenser being connected with the at least one distillation column is higher than the temperature of the heat transfer medium flowing through the at least one heat exchanger and lower than the temperature of the heat transfer medium flowing through the at least one heat pump. Accordingly, the temperature of the heat transfer medium flowing through the at least one heat pump is higher than the transfer medium flowing through the at least one heat exchanger. At least the transfer medium flowing through the at least one heat pump is adjusted to be higher than the melting temperature used to melt the crystals during the melting phase of the crystallization performed in the at least one crystallizer. For technology reasons or for the efficiency of the heat pump reason, the temperature of the heat transfer fluid returned from the heat pump may be somewhere between the temperature of the mother liquor and the melting temperature of the crystals. Of course, the temperature of the heat transfer medium entering the at least one crystallizer is higher than the temperature of the heat transfer medium exiting the at least one crystallizer and the temperature of the cooling agent exiting the utility side of the overhead or side condenser is higher than the temperature of the cooling agent entering the utility side of the overhead or side condenser. In turn, the temperature of the heat transfer medium exiting the utility side of the at least one heat exchanger is lower than the temperature of the heat transfer medium exiting the utility side of the at least one heat pump, if operated during the process.

[0035] The cooling agent, the refrigerant as well as the heat transfer medium used in the process are preferably as described above for the plant.

[0036] Subsequently, the present invention is described by means of an illustrative, but not limiting figure, wherein:

[0037] Fig. 1 is a schematic view of a plant for purifying a crude composition by distillation and crystallization in accordance with one embodiment of the present invention.

[0038] The plant 10 shown in figure 1 comprises a distillation column 12, which is connected with an overhead condenser 14, and further comprises a crystallizer 16 as well as between the distillation column 12 and the crystallizer 16 a heat exchanger 18, a heat pump 20 and an energy storage system 22 comprising three buffer vessels 24, 24’, 24”. The overhead condenser 14 is schematically shown in figure 1 in the upper right corner of the distillation column 12 as part of the distillation column 14, in order to keep the figure as simple as possible. However, as common the overhead condenser 14 is not part of the distillation column 14, but a separate device. More specifically, as not shown in figure 1 the distillation column 12 comprises an overhead outlet line, which is connected with an inlet of the process side of the overhead condenser 14, which in turn comprises an outlet line, which splits into a return line leading back into the distillation column 14 and into a withdrawal line for overhead composition. Both, the heat exchanger 18 as well as the heat pump 20 are embodied to operate between the overhead condenser 14 and the energy storage system 22. More specifically, the utility side of the overhead condenser 14 comprises an inlet line 26 for cooling agent and an outlet line 28 for cooling agent, wherein the outlet line 28 for cooling agent splits into a first connection line 30 being connected with an inlet of the heat exchanger 18 and into a second connection line 32 being connected with an inlet of the heat pump 20. Both, the first connection line 30 as well as the second connection line 32 each comprise a valve 34, 34’. The heat exchanger 18 comprises a process side 36 and a utility side 38, wherein the process side 36 comprises the inlet being connected with the first connection line 30 and an outlet line 40 being connected with the inlet line 26 for cooling agent of the overhead condenser 14. In turn, the utility side 38 of the heat exchanger 14 comprises an inlet line 42 and an outlet line 44, both being connected with the third buffer vessel 24” of the energy storage system 22.

[0039] Furthermore, the heat pump 20 comprises an evaporator 46, a condenser 48, an expansion valve 50 and a compressor 52. More specifically, the evaporator 46 of the heat pump 20 comprises a process side 54 and a utility side 56, wherein the process side 54 comprises the inlet being connected with the second connection line 32 and further comprises an outlet line 58 being connected with the inlet line 26 for cooling agent of the overhead condenser 14. In turn, the utility side 56 of the evaporator 46 of the heat pump 20 comprises an inlet line 60, which is connected with the expansion valve 50, as well as an outlet line 62, which is connected with the compressor 52, wherein the compressor 52 comprises an outlet line 64, which is connected with an inlet of the condenser 48, and wherein the expansion valve 50 is connected with a line 66 being connected with an outlet of the condenser 48. More specifically, the condenser 48 comprises a process side 68 and a utility side 70, wherein the process side 68 comprises the inlet being connected with the outlet line 64 of the compressor 52 and further comprises the outlet being connected via the line 66 with the expansion valve 50. In turn, the utility side 70 of the condenser 48 comprises an inlet line 72 and an outlet line 74, both being connected with the first buffer vessel 24 of the energy storage system 22. Each of the buffer vessels 24, 24’, 24” further comprises an outlet line 76, 76’, 76”, each of which being provided with a valve 78, 78’, 78”, wherein the outlet lines 76, 76’, 76” combine to a central inlet line 80 leading into the crystallizer 16. In addition, the crystallizer 16 comprises an outlet line 82, which splits into the three inlet lines 84, 84’, 84”, each of which being provided with a valve 86, 86’, 86” and each of which leading into one of the buffer vessel 24, 24’, 24”.

[0040] During the operation of the plant 10, crude composition to be purified is distilled in the distillation column 12 and is separated therein into an overhead composition and into a bottom composition. While the bottom composition is withdrawn from a bottom outlet from the distillation column 12, the overhead composition is withdrawn from an overhead outlet of the distillation column 12 and is led into the process side of the overhead condenser 14. Cooling agent being led into the utility side of the overhead condenser 14 through the inlet line 26 flows through the overhead condenser 14 and absorbs heat from the hotter overhead composition. The heated cooling agent then flows, when valve 34 is open, through lines 28, 30 into the process side 36 of the heat exchanger 18 and, when valve 34’ is open, also through lines 28, 32 into the process side 54 of the evaporator 46 heat pump 20. While heat transfer medium being colder than the cooling agent and coming from the third buffer vessel 24” is led through the utility side 38 of the heat exchanger 38, refrigerant being colder than the cooling agent flows through the utility side 54 of the evaporator 46 of the heat pump 20. Hence, the cooling agent transfers heat in the evaporator 46 to the refrigerant and in the heat exchanger 18 to the heat transfer medium, before the thereby cooled cooling agent flows via lines 48, 40, 26 back to the utility side of the overhead condenser 14. The refrigerant exiting the utility side of the evaporator 46 flows through the compressor 52, in which it is compressed and thereby heated, through the process side 68 of the condenser 48 of the heat pump 20, from there to the expansion valve 50, in which it is expanded, and from there back to the utility side 56 of the condenser 46 of the heat pump 20. Heat transfer medium coming from the first buffer vessel 24 flows through the utility side 70 of the condenser 48 of the heat pump 20, wherein the heat transfer medium is colder than the refrigerant flowing through the process side 68 of the condenser 48. Accordingly, heat is transferred within the condenser 48 from the refrigerant to the heat transfer medium. The heat transfer medium being heated by the heat pump 20 flows via line 74 back into the first buffer vessel 24 of the energy storage system 22, whereas the heat transfer medium being heated in the heat exchanger 18 flows via line 44 back into the third buffer vessel 24” of the energy storage system 22. In this application, in which both valves 34, 34’ are open, the temperature of the cooling agent flowing through the utility side of the overhead condenser 14 being connected with the distillation column 12 is lower than the melting temperature used to melt the crystals during the melting phase of the crystallization performed in the crystallizer 16, but higher than the temperature of the mother liquid during the crystallization. In addition, the temperature of the cooling agent flowing through the utility side of the overhead condenser 14 being connected with the distillation column 12 is higher than the temperature of the heat transfer medium being heated by the heat exchanger 18 and being temporarily stored in the third buffer tank 24”, but lower than the temperature of the heat transfer medium being heated by the heat pump 20 and being temporarily stored in the first buffer tank 24. Hence, the temperature of the heat transfer medium in the first buffer vessel 24 is higher than the temperature of the heat transfer medium in the third buffer vessel 24”, which is higher than the temperature of the heat transfer medium in the second buffer vessel 24’, which is not heated. More precisely, the temperature of the heat transfer medium being contained in the first buffer vessel 24 is adjusted to be higher than the melting temperature used to melt the crystals during the melting phase of the crystallization performed in the crystallizer 16, whereas the temperature of the heat transfer medium being contained in the third buffer vessel 24” is adjusted to be between the temperature of the mother liquid during the crystallization and the melting temperature used to melt the crystals during the melting phase of the crystallization performed in the crystallizer 16. In turn, the temperature of the heat transfer medium being contained in the second buffer vessel 24’ is adjusted to be below the temperature of the mother liquid during the crystallization. In this case, an additional external heating source is used directly via heat exchanger or indirectly via an additional buffer vessel to provide the remaining heating energy needed at a temperature higher than the melting temperature of the crystals. Alternatively, the temperature of the heat transfer medium being contained in the first buffer vessel 24 may be higher than the temperature of the heat transfer medium being contained in the third buffer vessel 24" but lower than the melting temperature, if the temperature is not reached because of the technological limitation of heat pump 20 or in order to keep a high efficiency of the heat pump 20. During the cooling phase of the crystallization, heat transfer medium is circulated from the second buffer vessel 24’ through lines 76’, 80, 82, 84’ through the crystallizer 16 back to the second buffer vessel 24, whereas during the sweating phase heat transfer medium is circulated from the third buffer vessel 24 through lines 76”, 80, 82, 84” through the crystallizer 16 back to the third buffer vessel 24” and during the melting phase heat transfer medium is circulated from the first buffer vessel 24 through lines 76, 80, 82, 84 through the crystallizer 16 back to the first buffer vessel 24.

[0041] When the temperature of the cooling agent flowing through the utility side of the overhead condenser 14 being connected with the distillation column 12 is higher than the melting temperature used to melt the crystals during the melting phase of the crystallization performed in the crystallizer 16, the heat pump 20 needs not to be operated, i.e. the valve 34’ may be closed.

[0042] Subsequently, the present invention is described by means of illustrative, but not limiting examples.

[0043] Example 1 and Comparative Example 1 In example 1 , a crude composition containing as target compound 99.63% L- lactide and 91.97% meso-lactide has been purified in a plant being similar to that shown in figure 1 but comprising four buffer vessels and two distillation columns, wherein from the second distillation column L-lactide product was condensed on a side condenser and meso-lactide product was condensed on a hammer or head condenser. Moreover, the plant comprises as crystallizer a combination of a falling film crystallizer in order to further purify the L-lactide and of two static crystallizers in order to further purify the meso-lactide. More specifically, a crude composition containing 84.61% and 9.00% by weight of respectively target compounds L- lactide and meso-lactide and in sum 6.39% by weight the rest of impurities, which contained water, lactic acid, oligomers and others, was distilled at a temperature of 160°C and at a pressure of 0.018 mbar(a) in the first distillation column and at a temperature of 161 °C and at a pressure of 0.028 mbar(a) in the second distillation column so as to obtain in the second distillation column an overhead composition containing 78% by weight of meso-lactide, on the side condenser a composition containing 95.75% by weight of L-lactide and in the first distillation column a bottom composition containing 57.78% by weight of L-lactide and 1.94% of mesolactide. For the side condensers of both distillation columns temper water was used as the cooling agent entering at 90°C and leaving at 95°C with a flow rate of 234'520 for first distillation column and 201'294 for the second distillation column. From the five buffer vessels, two of which were used for cooling one cooled with cooling water and the other cooled with chilled water, whereas to other buffer vessels were used for heating, one of which being regenerated by connection via the heat exchanger 18 connected to the cooling agent coming from the side condenser through valve 34, while the other buffer vessel was regenerated via the heat pump 20 connected to the cooling agent coming from the side condenser through valve 34'. Valve 34 was open and fluid agent flowed with a flowing rate of 143'981 kg / h through the process side 36 of the heat exchanger 18 and with a flowing rate of 290'105 kg / h through the process side 54 of the heat pump 20. While HCFO-R1233ZD(E) was used as refrigerant in the heat pump 20, wa- ter / monopropylene glycol (50t% by weight / 50% by weight) was used as the heat transfer medium in all four buffer vessels. The heat transfer medium in the first and third buffer vessels had a temperature T4 of 90°C and of 120°C, respectively, whereas the heat transfer medium in the two other buffer vessels had a temperature of 5°C and of 40°C, respectively. The falling film crystallizer was cooled during the crystallization phase to a temperature T5 of 45°C in lowest stage and the two static crystallizers were cooled to 7°C in lowest stage, heated during the sweating steps from T6 of 70°C to T6 103°C for the falling film crystallizer and from a temperature T6 of 5°C to a temperature T6 of 52°C for the static crystallizers and during the melting phase to a temperature T7 of 115°C for the falling film crystallizer and of 65°C for the static crystallizers.

[0044] As comparative example 1 , the same process was performed except that the plant did neither comprised a heat exchanger nor a heat pump and that the plant comprised three buffer vessels, from which two were used for cooling and one was used for heating at 120°C.

[0045] The process of the example 1 allowed in comparison to the comparative example 1 to reduce the crystallization heating by 100% (without accounting for the electrical consumption of the heating pump) or 67% (taking into consideration the electrical consumption equivalent of the heat pump) and the distillation cooling by 48%, which was the result of a reduction of the crystallization heating by 36.1% and of the distillation cooling by 22.2% on account of the use of the heat exchanger 18 and the reduction of the crystallization heating by 30.9% and of the distillation cooling by 25.8% on account of the use of the heat pump 20.

[0046] Example 2 and Comparative Example 2

[0047] In example 2, a crude composition containing as target compound paraphenylenediamine and a crude composition containing as target compound meta- phenylenediamine have been purified in a plant being similar to that shown in figure 1 but comprising various distillation columns. Furthermore, the plant comprised as crystallizer three static crystallizers. More specifically, a crude composition containing 24.68% by weight of target compound para-phenylenediamine and in sum 75.32% by weight of impurities or co-products, which contained water, 2- methoxyethanol, ortho-phenylenediamine and others, and a crude composition containing 21.18% by weight of target compound meta-phenylenediamine, 2.74% by weight of target compound ortho-phenylenediamine, 0.57% by weight of target compound meta-phenylenediamine and in sum 75.51% by weight of impurities or co-products, which contained water, aniline meta-dinitrobenzene and others, were distilled at a temperature of 180°C and at a pressure of -0.952 bar(g) for meta- phenylenediamine, at a temperature of 185°C and at a pressure of -0.952 bar(g) for the para-phenylenediamine and a temperature of 169°C and at a pressure of - 1.005 bar(g) for ortho-Phenylenediamine which is a co-product so as to obtain an overhead composition and a bottom composition. The overhead composition having a temperature of 129.7°C was led through the process side of the overhead condenser 14, while water as cooling agent with a temperature T1 of 100.4°C was led with a flowing rate of 280'000 kg / h through the utility side of the overhead condenser 1 and had in line 28 after leaving the overhead condenser 14 a temperature T2 of 105.4°C. Valves 34 and 34’ were open and fluid agent flowed with a flowing rate of 4T151 kg / h through the process side 36 of the heat exchanger 18 and with a flowing rate of 45'266 kg / h through the process side 54 of the heat pump 20. While R-600 was used as the refrigerant used in the heat pump 20, thermal oil was used as the heat transfer medium in all three buffer vessels 24, 24”. The heat transfer medium in the third buffer vessel 24” had a temperature T3 of 155°C, whereas the heat transfer medium in the first buffer vessel 24 had a temperature T4 of 90°C. The crystallizers were three static crystallizers being cooled during the crystallization phase to a temperature T5 of 128°C for para- phenylenediamine 61 °C for meta-phenylenediamine, heated during the sweating steps to a temperature T6 of 142°C for para-Phenylenediamine 65°C and for me- ta-phenylenediamine and during the melting phase to a temperature T7 of 150°C for para-phenylenediamine and 80°C for meta-phenylenediamine.

[0048] In summary, various distillation columns were with a head condenser. These distillation columns received feed for meta-phenylenediamine production and feed for para-phenylenediamine production. In addition, these distillation columns produced ortho-phenylenediamine and recovered the solvent. In the crystallization section, some crystallizers purified the meta-phenylenediamine and other crystallizers purified the para-phenylenediamine. All crystallizers had the same energy system. A part of the cooling agent flow going from the head condensers used to condensate the ortho-phenylenediamine was going to the heat exchanger 18 through valve 34 and another part was going to the heat pump 20 through valve 34'.

[0049] As comparative example 2, the same process was performed except that the plant did neither comprised a heat exchanger nor a heat pump and that the plant comprised no buffer vessel.

[0050] The process of the example 2 allowed in comparison to the comparative example 2 to reduce the crystallization heating by 100% (without accounting for the electrical consumption of the heating pump) or 56% (taking into consideration the electrical consumption equivalent of the heat pump) and the distillation cooling by 10%, which was the result of a reduction of the crystallization heating by 34% and of the distillation cooling by 4.8% on account of the use of the heat exchanger 18 and the reduction of the crystallization heating by 22% and of the distillation cooling by 5.2% on account of the use of the heat pump 20.

[0051] Example 3 and Comparative Example 3 In example 3, the feed was led to crystallization and the product of the crystallization was further processed by distillation that produced two final co-products. More specifically, in example 3, a crude composition containing as target compounds 96.0% of anthracene and 98.5% of carbazole has been purified in a plant being similar to that shown in figure 1 , but which contained 4 buffer vessels and in which the crude composition was first fed into the crystallizer. More specifically, a crude composition containing 7.4% anthracene by weight and 3.3% carbazole by weight of target compound and in sum 89.3% by weight of impurities, which contained naphthalene, dibenzofuran, fluorene, phenanthrene, pyrene and others, was purified by crystallization before getting distilled at a temperature of 285°C and at a pressure of 15 kPa so as to obtain an overhead composition and a bottom composition. The overhead composition having a temperature of 253°C was led through the process side of the overhead condenser 14, while thermal oil Shell S5T as cooling agent with a temperature T 1 of 220°C was led with a flowing rate of 181'583 kg / h through the utility side of the overhead condenser 1 and had in line 28 after leaving the overhead condenser 14 a temperature T2 of 230°C. Valve 34 only was open and fluid agent flowed with a flowing rate of 181'583 kg / h through the process side 36 of the heat exchanger 18 and with a flowing rate of 0 kg / h through the process side 54 of the heat pump 20. Shell thermal oil S4T was used as the heat transfer medium in all four buffer vessels. The heat transfer medium in the third buffer vessel had a temperature T3 of 200°C, whereas the heat transfer medium in the first buffer vessel had a temperature T4'_1 of 20°C, the heat transfer medium in the second buffer vessel had a temperature T4'_2 of 40°C and the heat transfer medium in the fourth buffer vessel had a temperature of 270°C. The crystallizers were 12 static crystallizers being cooled during the crystallization phase to a temperature T5 of 25°C, heated during the sweating steps to a temperature T6 of 219°C and during the melting phase to a temperature T7 of 250°C.

[0052] In summary, one distillation column with a head condenser and with a side condenser was used. All the cooling agent flow went from the head condenser being connected to the heat exchanger 18 through valve 34. It is 100% connected to the exchanger because the heating energy in the cooling agent is lower than the heating requirement of the crystallizers at this temperature. As comparative example 2, the same process was performed except that the plant did neither comprised a heat exchanger nor a heat pump and that the plant comprised three buffer vessels, from which two were used for cooling and one was used for heating at 270°C. The process of the example 3 allowed in comparison to the comparative example 3 to reduce the crystallization heating by 20% and the distillation cooling by 93%, which was the result of a reduction of the crystallization heating by 20% and of the distillation cooling by 93% on account of the use of the heat exchanger 18.

[0053] Reference numerals

[0054] 10 Plant

[0055] 12 Distillation column

[0056] 14 Overhead condenser

[0057] 16 Crystallizer

[0058] 18 Heat exchanger

[0059] 20 Heat pump

[0060] 22 Energy storage system

[0061] 24, 24’, 24” Buffer vessel of the energy storage system

[0062] 26 Inlet line for cooling agent

[0063] 28 Outlet line for cooling agent

[0064] 30 First connection line

[0065] 32 Second connection line

[0066] 34, 34’ Valve

[0067] 36 Process side of the heat exchanger

[0068] 38 Utility side of the heat exchanger

[0069] 40 Outlet line of the process side of the heat exchanger

[0070] 42 Inlet line of the utility side of the heat exchanger / Outlet line of a buffer vessel

[0071] 44 Outlet line of the utility side of the heat exchanger / Inlet line of a buffer vessel

[0072] 46 Evaporator of the heat pump

[0073] 48 Condenser of the heat pump

[0074] 50 Expansion valve of the heat pump

[0075] 52 Compressor of the heat pump

[0076] 54 Process side of the evaporator of the heat pump

[0077] 56 Utility side of the evaporator of the heat pump 58 Outlet line of the process side of the evaporator of the heat pump

[0078] 60 Inlet line of the utility side of the evaporator of the heat pump

[0079] 62 Outlet line of the utility side of the evaporator of the heat pump

[0080] 64 Outlet line of the compressor 66 Line from the condenser to the expansion valve

[0081] 68 Process side of the condenser of the heat pump

[0082] 70 Utility side of the condenser of the heat pump

[0083] 72 Inlet line of the utility side of the condenser of the heat pump / Outlet line of a buffer vessel 74 Outlet line of the utility side of the condenser of the heat pump /

[0084] Inlet line of a buffer vessel

[0085] 76, 76’, 76” Outlet line of buffer vessel

[0086] 78, 78’, 78” Valve

[0087] 80 Central inlet line to the crystallizer 82 Outlet line of the crystallizer

[0088] 84, 84’, 84” Inlet line of the buffer vessel

[0089] 86, 86’, 86” Valve

Claims

Sulzer Management AG S13810PEP - Pl / FaClaims:1 . A plant (10) for purifying a crude composition by distillation and crystallization, wherein the plant comprises at least one distillation column (12) being connected with an overhead or side condenser (14), at least one crystallizer (16), at least one heat exchanger (18), at least one heat pump (20) and optionally an energy storage system (22), wherein at least one heat pump (20) is embodied to operate between the overhead or side condenser (14) and the energy storage system (22), if present, for heat exchange or, if the energy storage system (22) is not present, at least one crystallizer (16) for heat exchange, and wherein at least one heat exchanger (18) is embodied to operate between the overhead or side condenser (14) of at least one distillation column (12) and the energy storage system (22), if present, for heat exchange or, if the energy storage system (22) is not present, at least one crystallizer (16) for heat exchange.

2. The plant (10) in accordance with claim 1 , which comprises an energy storage system (22), wherein the energy storage system (22) comprises at least one buffer vessel (24, 24’, 24”), preferably two to ten buffer vessels (24, 24’, 24”), more preferably two to four buffer vessels (24, 24’, 24”) and most more preferably three buffer vessels (24, 24’, 24”), and wherein the energy storage system (22) is connected via at least one line (42, 44) with the at least one heat exchanger (18), is connected via at least one line (72, 74) with the at least one heat pump (20) and is connected via at least one line (76, 76’, 76”, 80, 82, 84, 84’, 84”) with the at least one crystallizer (16).

3. The plant (10) in accordance with claim 1 or 2, wherein the at least one heat pump (20) is arranged in parallel to the at least one heat exchanger (18).

4. The plant (10) in accordance with any of the preceding claims, wherein the at least one heat pump (20) is a mechanical heat pump (20), which comprises an evaporator (46), a condenser (48), an expansion valve (50) and a compressor (52).

5. The plant (10) in accordance with any of the preceding claims, wherein the at least one heat exchanger (18) is selected from the group consisting of shell and tube heat exchangers and plate and frame evaporator.

6. The plant (10) in accordance with any of the preceding claims, wherein the at least one crystallizer (16) is at least one layer crystallizer and preferably at least one static crystallizer, at least one falling film crystallizer or a combination of at least one static crystallizer and at least one falling film crystallizer.

7. The plant (10) in accordance with any of the preceding claims, wherein the overhead or side condenser (14) comprises a process side and a utility side, wherein the utility side comprises an inlet line (26) for cooling agent and an outlet line (28) for cooling agent, wherein the outlet line (28) for cooling agent splits into a first connection line (30) being connected with an inlet of the at least one heat exchanger (18) and into a second connection line (32) being connected with an inlet of the at least one heat pump (20), wherein preferably the first connection line (30) comprises a valve (34) and the second connection line (32) comprises a valve (34’).

8. The plant (10) in accordance with claim 7, wherein the at least one heat exchanger (18) comprises a process side (36) and a utility side (38), where-in the process side (36) comprises the inlet being connected with the first connection line (30) and an outlet line (40) being connected with the inlet line (26) for cooling agent of the overhead or side condenser (14), whereas the utility side (38) comprises an inlet line (42) and an outlet line (44) both being connected with the energy storage system (22), if present, or, if the energy storage system (22) is not present, with the at least one crystallizer (16).

9. The plant (10) in accordance with claims 7 or 8, wherein the at least one heat pump (20) comprises an evaporator (46), a condenser (48), an expansion valve (50) and a compressor (52).

10. The plant (10) in accordance with claim 9, wherein the evaporator (46) of the at least one heat pump (20) comprises a process side (54) and a utility side (56), wherein the process side (54) comprises the inlet being connected with the second connection line (32) and an outlet line (58) being connected with the inlet line (26) for cooling agent of the overhead or side condenser (14), whereas the utility side (56) comprises an inlet line (60) being connected with the expansion valve (50) as well as an outlet line (62) being connected with the compressor (52), wherein the compressor (52) comprises an outlet line (64) being connected with an inlet of the condenser (48) of the at least one heat pump (20), and wherein the expansion valve (50) comprises a line (66) being connected with an outlet of the condenser (48) of the at least one heat pump (20).

11. The plant (10) in accordance with claim 10, wherein the condenser (48) of the at least one heat pump (20) comprises a process side (68) and a utility side (70), wherein the process side (68) comprises the inlet being connected with the outlet line (64) of the compressor (52) and comprises the outlet being connected via the line (66) with the expansion valve (50), whereasthe utility side (70) comprises an inlet line (72) and an outlet line (74) both being connected with the energy storage system (22), if present, or, if the energy storage system (22) is not present, with the at least one crystallizer (16).

12. The plant (10) in accordance with any of the preceding claims, which comprises an energy storage system (22) comprising at least one buffer vessel (24, 24’, 24”), wherein the at least one buffer vessel (24, 24’, 24”) comprises an outlet line (72) being connected with the at least one heat pump (20), an outlet line (42) being connected with the at least one heat exchanger (18), an inlet line (74) being connected with the at least one heat pump (20), an inlet line (44) being connected with the at least one heat exchanger (18), at least one outlet line (76, 76’, 76”) being connected with the at least one crystallizer (16) as well as at least one inlet line (84, 84’, 84”) being connected with the at least one crystallizer (16).

13. The plant (10) in accordance with any of the preceding claims, which comprises in addition to the at least one crystallizer (16) at least one further heat consuming device, wherein the at least one further heat consuming device is a reactor, preferably a lactide synthesis reactor, wherein at least one heat exchanger (18) and / or at least one heat pump (20) is embodied to operate between the overhead or side condenser (14) and the heat consuming device.

14. A process for purifying a crude composition by distillation and 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 at least one heat exchanger (18) operates between the overhead or side condenser (14) andthe energy storage system (22), if present, or, if the energy storage system (22) is not present, at least one crystallizer (16) if the temperature, at which the overhead or side condenser (14) is operated, is higher than the temperature of the mother liquid during the crystallization performed in the at least one crystallizer (16), whereas the at least one heat pump (20) preferably only operates between the overhead or side condenser (14) and the energy storage system (22), if present, or, if the energy storage system (22) is not present, at least one crystallizer (16) if the temperature, at which the overhead or side condenser (14) is operated, is lower than the melting tempera- ture used to melt the crystals during the melting phase of the crystallization performed in the at least one crystallizer (16).

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