JP4845173B2 - Method and apparatus for adiabatic cooling crystallization of organic compounds - Google Patents

Description

  The present invention relates to an adiabatic cooling crystallization method and apparatus for organic compounds. In particular, the present invention relates to a method and apparatus suitable for obtaining paraxylene crystals.

  Separation and purification of a certain isomer mixture is difficult by distillation operation because the boiling points of the components constituting the mixture are close to each other. However, since melting points often differ greatly due to differences in molecular structure, separation by crystallization operation is often effective.

  There are extraction crystallization and addition compound crystallization in which a solvent (extractant, additive) is added as a third component to the eutectic two-component system or the eutectic multi-component system, but it is difficult to recover the solvent.

  In this regard, when a liquefied gas component is used as the refrigerant, it is an effective method because it can be easily recovered.

  The inventor of the present invention uses mixed xylene (m-xylene + o-xylene + ethylbenzene + p-xylene system), which is a raw material in typical p-xylene production in petrochemical industrial processes, and mixed xylene (m-xylene) after isomerization reaction. + O-xylene + p-xylene system) and other eutectic multicomponent systems were found to be effective to perform crystallization operation using propane (or propylene, ethylene, carbon dioxide, ammonia, etc.) as a refrigerant. .

  In this case, although it is possible to perform the crystallization operation in a jacket type crystallization tank, the p-xylene in the eutectic multicomponent system needs to be cooled to about -30 ° C to -60 ° C for crystallization. For this reason, a refrigeration system is required in which the crystallization tank is equipped with a cooling surface scraping mechanism, the refrigerant from the jacket is compressed by a compressor, condensed, for example, under a high pressure of 20 atm, and circulated through the jacket. .

  This not only increases the power cost of the compressor but also increases the equipment cost and maintenance cost because the crystallization tank must be equipped with a cooling surface scraping mechanism that is complicated and requires frequent maintenance.

Then, although the form using a heat pump is also considered (patent document 1), it is hard to say that it is a system which always corresponds to the device cost which constitutes a heat pump.
JP-A-4-327542

  Accordingly, a main object of the present invention is to provide an adiabatic cooling crystallization method and apparatus for organic compounds that can reduce operating costs (including maintenance costs) and equipment costs.

  Another object is to provide a method and apparatus suitable for crystallization of p-xylene.

The present invention that has solved the above problems is as follows.
<Invention of Claim 1>
Perform adiabatic cooling and evaporation of propane for mixed xylene containing paraxylene, which contains propane in the crystallization tank,
The crystal slurry generated by this operation is extracted from the crystal tank,
The evaporated propane vapor is pressurized by a compressor and led to an absorption condenser,
The operation pressure of the crystallization tank is close to normal pressure or at most 4 atmospheres, and the pressurization by the compressor is less than or equal to the operation pressure of the crystallization tank,
In the absorption condenser, the mixed xylene containing para-xylene and the pressurized propane vapor are brought into contact with each other and cooled by a cooling pipe through which cooling water is passed to condense the propane vapor, and the absorption condensate is obtained. Producing and leading this absorption condensate to the crystallization vessel,
In the evaporator installed before introducing the condensate into the crystallization tank, adiabatic cooling and evaporation operation of the required amount of the propane in advance is performed to prevent rapid adiabatic cooling in the next crystallization tank and The mother liquor containing crystals of
The evaporation vapor in the evaporator is pressurized by the compressor above the operating pressure of the crystallization tank and led to the absorption condenser together with the evaporation vapor from the crystallization tank,
An adiabatic cooling crystallization method for an organic compound, characterized in that para-xylene crystals are obtained from a crystal slurry purified in the crystallization tank .

(Function and effect)
When the refrigerant is subjected to adiabatic cooling and evaporation of the mixed solution of the target organic compound containing the refrigerant in the crystallization tank, the crystallization heat is substantially taken away by evaporation of only the refrigerant liquid component, and crystals are deposited. To do. The evaporation vapor is pressurized by the compressor to a pressure higher than the operating pressure of the crystal tank and led to an absorption condenser for condensation. The reason why the evaporation vapor is pressurized to a pressure higher than the operation pressure of the crystallization tank by the compressor is to secure a temperature difference for condensation by pressurization by the compressor as in a general refrigeration cycle. In the absorption condenser, since it comes into contact with a mixed solution of an organic compound having a low vapor pressure, the boiling point rises and the temperature at which absorption and condensation can be increased. Therefore, it is sufficient that the degree of pressurization required is small and the input energy required for condensation is small.

The condensate in the absorption condenser can be continuously crystallized by introducing it into the crystal tank. Taking crystallization of p-xylene as an example, propane is used as a refrigerant, and in a crystallization tank, for example, normal pressure, and in an absorption condenser, for example, about 8 atm due to pressurization by a compressor.
As for the operation pressure (evaporation pressure) of the crystal tank, it is desirable to operate near normal pressure, considering the pressure resistance required for the crystal tank and the separation method and apparatus of the produced crystal, and at most 4 atm. Is desirable.

  The crystal slurry generated in the crystallization tank is extracted and separated into a crystal component and a mother liquor by solid-liquid separation means, and the crystal component is purified as it is by a purification means if necessary to be commercialized.

  According to such an operation, a crystallization operation can be performed without using the crystallization tank as a pressure vessel. In addition, since the compressor and the absorption condenser are at a minimum, it is not necessary to use an expensive heat pump equipment configuration as in the prior art, so that it is economical in terms of the entire system and operating costs. is there.

  On the other hand, in the present invention, in the evaporator installed before the condensate is led to the crystallization tank, the required amount of the refrigerant is adiabatically cooled and evaporated in advance. The pressure is applied to the absorption condenser together with the evaporation vapor from the crystal tank.

  In the above-described configuration, the absorption condensate pressurized by the compressor is guided to the crystallization tank. Therefore, when the condensate in this absorption condenser is introduced directly into the crystallization tank, when it is introduced into the crystallization tank, crystallization immediately occurs simultaneously with severe foaming due to a large pressure drop, making it difficult to obtain large crystals. In the crystallization separation process, the size of the produced crystal grains has a great merit in producing the final product with high purity. This phenomenon is also affected by the temperature difference between the temperature of the condensate in the absorption condenser and the temperature at which crystals form, and the difference in concentration between the refrigerant in the condensate at the absorption condenser and the refrigerant concentration in the crystal tank in which crystals form. It has been found that

  In the crystallization tank, it is desirable that the mother liquor containing the entire crystal in the crystallization tank is as uniformly supersaturated as possible and is circulated well from the lower part to the upper part. Therefore, the condensate pressurized by the compressor is directly guided to the crystal tank, and if evaporation occurs immediately, only the supply liquid is cooled violently, so this is reduced as much as possible and is already in the crystal tank. Prompt mixing with the mother liquor is desirable to obtain large crystals. Therefore, it is desirable to perform adiabatic cooling and evaporation operation of the required amount of the refrigerant in advance in an evaporator before introducing the condensate into the crystallization tank, in order to obtain a large crystal. Temperature difference and refrigerant concentration difference) can be eliminated. This method is a technique that should be referred to as prior (adiabatic self) evaporation (pre-flush). Also, according to this method, since the refrigerant is adiabatically cooled and evaporated in advance in the evaporator, the evaporation load in the crystallization tank can be reduced, the apparatus can be designed compactly, and the equipment cost can be reduced. At the same time, it acts to buffer pressure fluctuations caused by rapid evaporation in the crystal can, thereby facilitating control of the system.

<Invention of Claim 2>
The adiabatic cooling crystallization method for an organic compound according to claim 1, wherein the crystal slurry extracted from the crystallization tank is subjected to solid-liquid separation, and the separated mother liquor is returned to the absorption condenser.

(Function and effect)
In the absorption condenser, a mixed solution of organic compounds is supplied to make contact with the pressurized evaporation vapor, and the crystal slurry extracted from the crystallization tank is subjected to solid-liquid separation, and the separated mother liquor is returned to the absorption condenser. By doing so, contact with the pressure evaporation vapor can be achieved. Which form is adopted can be selected depending on the type of the target organic compound, the concentration of the target organic compound in the mixed solution, the operating conditions, and the like. Here, the facilities and processes for solid-liquid separation are not limited, and examples thereof include a centrifugal separator, a filter, a melt purification tower, a piston type or screw type washing tower, and the like.

<Invention of Claim 3 >
The adiabatic cooling crystallization method of an organic compound according to claim 1, wherein the refrigerant concentration of the absorption condensate led from the absorption condenser to the crystallization tank is 1 to 70%.

(Function and effect)
As the refrigerant concentration in the absorption condensate increases, the crystallization point decreases and the vapor pressure also decreases. When the refrigerant concentration of the absorption condensate becomes thin, the vapor pressure decreases due to the partial pressure. Therefore, there is a maximum point of vapor pressure. If the refrigerant concentration of the absorption condensate is 1 to 70%, operation near the highest point of the vapor pressure is possible.

  Summarizing the effects described in the above operational effect column, cooling (freezing) can be performed without installing an apparatus for scraping off crystals deposited on the cooling surface, which is inevitable in the cooling crystallization facility, and the cooling. The required amount of utility energy for (freezing) can be reduced, thereby reducing operating costs and equipment costs. Furthermore, it becomes suitable for crystallization of p-xylene.

  In addition, the evaporator installed between the absorption condenser and the crystallization tank is subjected to adiabatic cooling and evaporation of the required amount of refrigerant in advance, and the evaporation vapor in the evaporator is pressurized by the compressor above the operating pressure of the crystallization tank. And lead to the absorption condenser together with the evaporated vapor from the crystallizer. As a result, it is possible to reduce the evaporation load in the crystal tank and to obtain a large crystal.

Hereinafter, the present invention will be described in further detail with reference to embodiments.
<First Embodiment>
FIG. 1 shows a basic embodiment, which includes an absorption condenser 10, a crystal tank 20, a compressor 30, and a solid-liquid separation means 40.

  A mixed solution 1 of a target organic compound containing a refrigerant (a target liquid for crystallization operation, for example, a liquid of a eutectic multicomponent mixture containing p-xylene and its isomer) is led to an absorption condenser 10, where Refrigerant vapor (for example, propane) is absorbed and condensed to form a homogeneous refrigerant mixture.

  The homogeneous refrigerant mixture from the absorption condenser 10 is guided to the evaporator 50 by the pipe 61A. In this evaporator 50, adiabatic cooling and evaporation operation of a required amount of refrigerant is performed in advance.

  The refrigerant mixed liquid from the evaporator 50 is introduced into the crystallization tank 20 through the pipe 61, and the refrigerant is subjected to adiabatic cooling and evaporation of the condensate containing the refrigerant in the crystallization tank 20.

  The crystal slurry generated by this operation is extracted from the crystal tank 20 by a pump 62 and separated into a crystal fraction stream Cr and a mother liquor stream Mo by a solid-liquid separation means 40 such as a centrifugal separator or a liquid cyclone.

The evaporation vapor in the crystallization tank 20 passes through the pipe line 63, is pressurized to a pressure higher than the operating pressure of the crystallization tank 20 by the compressor 30, and is led to the absorption condenser 10. The evaporation vapor in the evaporator 50 is also pressurized through the pipe 61B to a pressure higher than the operating pressure of the crystallization tank 20 by the compressor 30 and led to the absorption condenser 10.

  The absorption condenser 10 has a cooling medium 2 (for example, cooling water tower cooling water, refrigerator brine, etc.) while bringing the mixed solution of organic compounds (mixed liquid 1) and the pressurized vaporized vapor into contact with each other. Cooling by cold heat is performed for absorption condensation, and this absorption condensate is guided to the crystal tank 20 through the evaporator 50.

  When the adiabatic cooling and evaporating operation of the refrigerant liquid component is performed on the target organic compound mixed liquid containing the refrigerant liquid component in the crystallization tank 20, the heat of crystallization is taken away with the evaporation of the refrigerant liquid component, and crystals are deposited. . The evaporating vapor is pressurized by the compressor 30 to a pressure higher than the operating pressure of the crystal tank 20 and led to the absorption condenser 10 for absorption condensation.

  The reason why the compressor (pressurizer) 30 is provided to pressurize the evaporation vapor is that the temperatures of the crystal tank 20 and the absorption condenser 10 for recondensing the refrigerant at a temperature much higher than the operating temperature of the crystal tank 20. The difference is secured by pressurizing with a compressor (pressurizer) 30. Taking crystallization of p-xylene as an example, propane is used as a refrigerant, the crystallization tank 20 is, for example, normal pressure, and the absorption condenser 10 is, for example, about 8 atm due to pressurization by the compressor 30. .

  Since the absorption condenser 10 is in contact with a solution of an organic compound having a high boiling point, the boiling point rises and the temperature at which absorption and condensation can be increased. Therefore, a small amount of external input energy necessary for absorption condensation is sufficient.

  The crystal slurry produced in the crystallization tank 20 is extracted and separated by the crystal fraction stream Cr and the mother liquor stream Mo by solid-liquid separation means, and the crystal fraction stream Cr is purified as it is, if necessary, by purification means and washing means as described later. The product is then commercialized with increased purity.

  According to this operation, the crystallization operation can be performed without using the crystallization tank 20 as a high pressure resistant container. Since the crystal tank 20, the compressor 30, and the absorption condenser 10 are sufficient at the minimum, it is not necessary to have an expensive heat pump equipment configuration as in the prior art. Is economical in terms of

<Second Embodiment>
FIG. 2 shows a second embodiment, in which the crystal slurry extracted from the crystallization tank 20 is solid-liquid separated by the solid-liquid separation means 40, and the separated mother liquor stream Mo is passed through the pipe 64 to the absorption condenser 10. To be returned to. The refrigerant discharged from the system in a state dissolved in the filtrate in the solid-liquid separation means 40 can be supplied to the suction section of the compressor 30 as a recovery by a subsequent distillation tower or as a make-up (FIG. 1 also). reference).
The original organic compound mixed solution 1 may be directly supplied to the crystallization tank 20.

<Description of crystallization method>
The crystallization method will be explained using an example of the benzene-cyclohexane system.
In a common production in the chemical industry, cyclohexane is produced by hydrogenating benzene.
C ₆ H ₆ + 3H ₂ → C ₆ H ₁₂
In this hydrogenation reaction, the following impurities are generated by side reactions.
Methylcyclopentane n-hexane n-pentane methylcyclohexane Furthermore, toluene and paraffin contained in the raw material benzene are included.

  In such a case, the most difficult point in obtaining high-purity cyclohexane is that when unreacted benzene is contained, it is almost impossible to separate by distillation. The boiling point of benzene at normal pressure is 80.75 ° C., and the boiling point of cyclohexane is 80.16 ° C. The difference is only 0.59 ° C. Furthermore, there is a minimum azeotropic point (77.62 ° C.) in the vicinity of 54%.

  On the other hand, as can be seen from the solid-liquid equilibrium diagram in the propane-benzene-cyclohexane eutectic composition shown in FIG. 3, when obtaining high-purity cyclohexane, a method of crystallization and separation and purification can be employed. And a small amount of impurities such as methylcyclopentane contained together can be removed at the same time.

  That is, on the phase diagram, a solid-liquid equilibrium line of a binary system of cyclohexane and benzene is obtained. At this time, the inclusion of a trace amount of impurities is only slightly lowered the crystallization point curve, and there is no substantial difference. Here, when the cyclohexane-rich mixed raw material is cooled to crystallize cyclohexane, it hits the solid-liquid line on the left side and starts crystallization. Next, in the propane adiabatic cooling method, when the supply liquid and propane are mixed, the pressure is released, and cooling is started, the ternary solid-liquid equilibrium line to which propane is added (although indicated as a composition with propane removed) When hitting and cooling to near the eutectic point, cyclohexane crystals crystallize and separate from the mother liquor.

  Such an operation is continuously performed under the equipment configuration according to the present invention. The mother liquor separated from the benzene crystals is depropanated, mixed with the raw material, and fed back. The mixed liquid system of benzene and cyclohexane is a eutectic system in the entire concentration range. The pure benzene has a crystallization point of 5.5 ° C, and cyclohexane has a crystallization point of 5.7 ° C.

  From this explanation of the principle, it will be clear that cyclohexane crystals can be obtained from a eutectic composition of the propane-benzene-cyclohexane system. In the separation, it will be apparent that the method of the present invention is an inexpensive crystallization process.

<Other explanation>
The above example is an example in which there is one crystallization tank, but the present invention is also directed to a configuration including a plurality of crystallization tanks. In the equipment provided with a plurality of crystal tanks, the crystal slurry in the preceding crystal tank is guided to the subsequent crystal tank to further crystallize.

  In this embodiment, the compressor is provided with a single presser than the case where it is attached to the crystal tank of each stage, and the evaporation vapor from the crystal tank of each stage is guided to the compressor and pressurized, and then the final stage. A structure that leads to absorption condensation attached to the crystal tank is desirable. Further, it is desirable to provide the evaporator in at least the first crystal tank. Of course, each evaporator can be attached to each crystal tank.

Hereinafter, effects of the present invention will be clarified by showing examples.
Example 1
In the examples, crystallization was performed by the process shown in FIG. The absorption condenser 10 is a horizontal tube type, and a centrifuge 40 is used as a solid-liquid separator.

  A mixed xylene raw material having a paraxylene concentration of 80 to 90% and a normal temperature is supplied to the absorption condenser 10 at a rate of 15 to 25 kg / hr and is pressurized from the crystallization tank 20 through the compressor 30 to 0.2 to 0.7 MPa. The mixture was condensed at about 30 ° C. while being contact-mixed with the vapor. The obtained condensate is a mixed xylene solution of propane having a propane concentration of 10 to 30%. This is preflashed in a preflash tank (evaporator) 50 under a pressure of 130 to 350 kPa, and the liquid temperature is set to 0 to 0. Cooled to 15 ° C. The vapor generated by the preflash was pressurized to 0.2 to 0.7 MPa through the compressor 30 and contacted and mixed with the absorbing solution in the absorption condenser 10. The liquid after the preflash was led to a crystallization tank 20 operated at −10 to 0 ° C. under normal pressure for crystallization. The paraxylene crystal slurry obtained by crystallization was supplied from the crystal tank 20 to the centrifuge 40. As a result, 4-7 kg / hr xylene crystals could be obtained. The filtrate, which is a mixed xylene solution of propane, was discharged out of the system, and the propane that was discharged out of the system in a state dissolved in the filtrate was supplied as a make-up to the suction section of the pressurizer. The paraxylene concentration in the filtrate was 60-80%.

It is a flow sheet of a basic embodiment. It is a flow sheet of other embodiments. It is a solid-liquid equilibrium diagram in the eutectic composition of a propane-benzene-cyclohexane system.

  DESCRIPTION OF SYMBOLS 1 ... Stock solution, 2 ... Cooling medium, 10 ... Absorption condenser, 20 ... Crystal tank, 30 ... Compressor, 40 ... Solid-liquid separation means, Cr ... Crystal component (flow), Mo ... Mother liquor (flow).

Claims (3)

Perform adiabatic cooling and evaporation of propane for mixed xylene containing paraxylene, which contains propane in the crystallization tank,
The crystal slurry generated by this operation is extracted from the crystal tank,
The evaporated propane vapor is pressurized by a compressor and led to an absorption condenser,
The operation pressure of the crystallization tank is close to normal pressure or at most 4 atmospheres, and the pressurization by the compressor is less than or equal to the operation pressure of the crystallization tank,
In the absorption condenser, the mixed xylene containing para-xylene and the pressurized propane vapor are brought into contact with each other and cooled by a cooling pipe through which cooling water is passed to condense the propane vapor, and the absorption condensate is obtained. Producing and leading this absorption condensate to the crystallization vessel,
In the evaporator installed before introducing the condensate into the crystallization tank, adiabatic cooling and evaporation operation of the required amount of the propane in advance is performed to prevent rapid adiabatic cooling in the next crystallization tank and The mother liquor containing crystals of
The evaporation vapor in the evaporator is pressurized by the compressor above the operating pressure of the crystallization tank and led to the absorption condenser together with the evaporation vapor from the crystallization tank,
An adiabatic cooling crystallization method for an organic compound, characterized in that para-xylene crystals are obtained from a crystal slurry purified in the crystallization tank .   The adiabatic cooling crystallization method for an organic compound according to claim 1, wherein the crystal slurry extracted from the crystallization tank is subjected to solid-liquid separation, and the separated mother liquor is returned to the absorption condenser.   The adiabatic cooling crystallization method of an organic compound according to claim 1, wherein the refrigerant concentration of the absorption condensate from the absorption condenser is 1 to 70%.

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