Crystallization system and crystallization method

Abstract

The present invention suitably crystallizes an object to be separated. The present invention comprises: an adjustment unit which reduces the pressure of a solution that has an object to be separated dissolved therein; a crystallization tank in which is formed an opening that the adjustment unit is connected to, into which the solution that has been reduce in pressured by the adjustment unit is introduced from the opening, and inside which the object to be separated is crystallized from the solution; and a stirring unit which is provided in the crystallization tank, which stirs the inside of the crystallization tank, and which generates a downward flow in the solution, wherein a stirring blade of the stirring unit that stirs the solution is positioned lower than the opening in the vertical direction.

Description

Crystallization system and crystallization method

[0001] The present disclosure relates to a crystallization system and a crystallization method.

[0002] For example, crystallization may be performed to recycle polyester. Patent Document 1 describes a crystallization tank to which a pressure reducing valve is connected, and the pressure reducing valve is directly connected to the crystallization tank.

[0003] Japanese Patent Application Publication No. 8-89706

[0004] In the technology of Patent Document 1, there is a possibility that crystals precipitated by crystallization may accumulate at the outlet of the pressure reducing valve, causing the pressure reducing valve to become clogged, which may prevent the separation target from being properly crystallized.

[0005] The present disclosure is intended to solve the above-mentioned problems, and aims to provide a crystallization system and a crystallization method that can appropriately crystallize a separation target.

[0006] In order to solve the above-mentioned problems and achieve the object, the crystallization system according to the present disclosure comprises an adjustment unit that reduces the pressure of a solution in which a substance to be separated is dissolved; a crystallization tank that is formed with an opening connected to the adjustment unit and into which the solution reduced in pressure by the adjustment unit is introduced through the opening and in which the substance to be separated is crystallized from the solution; and an agitation unit that is provided within the crystallization tank and that agitates the solution within the crystallization tank to generate a downward flow of the solution, and the agitation blades of the agitation unit that agitate the solution are positioned vertically below the opening.

[0007] In order to solve the above-mentioned problems and achieve the object, the crystallization method according to the present disclosure is a crystallization method for a crystallization system having an adjustment unit that depressurizes a liquid, a crystallization tank having an opening connected to the adjustment unit, and an agitation unit that is provided in the crystallization tank and has an agitation blade that agitates the liquid and is positioned vertically below the opening, the method comprising the steps of: supplying a solution in which a separation target is dissolved to the adjustment unit, causing the adjustment unit to depressurize the solution and introduce it into the crystallization tank through the opening, and crystallizing the separation target from the solution in the crystallization tank; and rotating the agitation unit with the depressurized solution introduced into the crystallization tank to stir the inside of the crystallization tank and generate a downward flow of the solution.

[0008] According to the present disclosure, the object to be separated can be appropriately crystallized.

[0009] Fig. 1 is a schematic diagram of a polyester recycling process in this embodiment. Fig. 2 is a schematic diagram of a separation system according to the first embodiment. Fig. 3 is a schematic side view of a crystallization system. Fig. 4 is a schematic top view of a crystallization system. Fig. 5 is a schematic top view showing the internal structure of a crystallization section. Fig. 6 is a schematic top view of a crystallization section showing the arrangement of different numbers of baffles.

[0010] Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Note that the present invention is not limited to these embodiments, and when there are multiple embodiments, the present invention also includes combinations of the embodiments.

[0011] (First embodiment) (Recycling process) FIG. 1 is a schematic diagram of a polyester recycling process in this embodiment. In this embodiment, a polyester raw material Pm is depolymerized to form monomers, and the monomers are then polymerized again to recycle (regenerate) the polyester raw material Pm. Specifically, as shown in FIG. 1 , the polyester raw material Pm is flaked (step S100), the solution is mixed with a reaction solvent M and depolymerized (step S102), the depolymerized polyester monomers are purified (separated) to produce a carboxylic acid-derived monomer D and an alcohol component monomer E (step S104), the monomer D is hydrolyzed to separate the reaction solvent M (step S106), and the monomer F produced by hydrolysis of the monomer D is polymerized with the monomer E (step S108), thereby regenerating the polyester raw material Pm. In addition, in the recycling process employing the separation system 1 of this embodiment, the flaking step S100 may be omitted, or only the process of recovering monomers D and E shown in step S104 and monomer F shown in step S106 may be performed without performing the repolymerization process as in step S108.

[0012] (Polyester Raw Material) In this embodiment, the polyester raw material Pm to be depolymerized is a substance containing polyester. The polyester raw material Pm is not particularly limited, but examples include waste products such as polyethylene terephthalate (PET), polyethylene butylene terephthalate (PEBT), polybutylene terephthalate (PBT), polycyclohexane dimethyl terephthalate (PCT), polyethylene naphthalate (PEN), polybutylene naphthalate (PBN), and polycarbonate (PC). The polyester raw material Pm is not limited to those containing only polyester components, but also includes components other than polyester components. Examples of components other than polyester contained in the polyester raw material Pm include plastics other than polyester, such as polyethylene, polystyrene, polypropylene, and polyvinyl chloride, metals, dyes, pigments, and polymerization catalysts. Examples of polyester raw material Pm include clothing in which polyester and other components are knitted into fibers. Hereinafter, components other than polyester contained in the polyester raw material Pm are referred to as impurities.

[0013] (Reaction Solvent) The reaction solvent M is a solvent that reacts with the polyester to depolymerize the polyester. The reaction solvent M may be, for example, at least one of methanol, ethanol, water, and ethylene glycol.

[0014] (Carboxylic acid-derived monomer) The carboxylic acid-derived monomer D is a monomer having a carboxyl group produced by depolymerization of a polyester. The monomer D may be, for example, dimethyl carboxylate or diethyl carboxylate. Furthermore, the monomer D is preferably a terephthalic acid monomer, for example, dimethyl terephthalate (DMT).

[0015] (Alcohol Component Monomer) The alcohol component monomer E is an alcohol component monomer produced by a depolymerization reaction of the polyester. The monomer E may be, for example, a dihydroxy compound (dihydric alcohol), or more specifically, ethylene glycol (EG).

[0016] In the following, an example will be described in which the polyester is PET, the reaction solvent M is methanol, the monomer D is DMT, and the monomer E is EG.

[0017] (Separation System) Fig. 2 is a schematic diagram of a separation system according to the first embodiment. The separation system 1 according to the first embodiment is a system that monomerizes polyester contained in a polyester raw material Pm to produce monomers D and E. As shown in Fig. 2, the separation system 1 includes a raw material storage section 10, a dissolving section 12, a solid-liquid separation section 13, a solvent storage section 14, a reaction section 16, a separation section 18, a control section 30, a temporary storage section 70, and a crystallization system 80.

[0018] Hereinafter, the Z direction is the vertical direction (up-down direction). The vertically upward direction of the Z direction is referred to as the Z1 direction, and the vertically downward direction of the Z direction is referred to as the Z2 direction.

[0019] (Raw Material Storage Section) The raw material storage section 10 is a tank into which the polyester raw material Pm is introduced and stored. In this embodiment, the raw material storage section 10 stores flaked polyester raw material Pm, but the shape and size of the polyester raw material Pm may be arbitrary. The raw material storage section 10 is connected to the dissolving section 12 via an inlet pipe 10a. The polyester raw material Pm in the raw material storage section 10 is supplied to the dissolving section 12 through the inlet pipe 10a. The inlet pipe 10a is provided with an adjustment section 10b that adjusts the amount of polyester raw material Pm supplied from the raw material storage section 10 to the dissolving section 12. The adjustment section 10b is, for example, an on-off valve. When in an open state, the adjustment section 10b allows the polyester raw material Pm in the raw material storage section 10 to be supplied to the dissolving section 12, and when in a closed state, it stops the supply of the polyester raw material Pm in the raw material storage section 10 to the dissolving section 12. However, the adjusting section 10b is not limited to being an on-off valve, and may be any mechanism capable of adjusting the supply of the polyester raw material Pm to the dissolving section 12. Furthermore, the polyester raw material Pm may be supplied directly to the dissolving section 12 without passing through the raw material storage section 10, the introduction pipe 10a, and the adjusting section 10b.

[0020] (Dissolving section) The dissolving section 12 is a tank in which the dissolving liquid Pd is stored. The dissolving liquid Pd is a solution produced by mixing the polyester raw material Pm with the monomer D. Here, the polyester component contained in the polyester raw material Pm dissolves in the monomer D, but impurities, which are components other than the polyester contained in the polyester raw material Pm, remain without dissolving in the monomer D. Therefore, it can be said that the dissolving liquid Pd contains a polyester solution P in which the polyester contained in the polyester raw material Pm is dissolved in the monomer D, and impurities contained in the polyester raw material Pm.

[0021] Monomer D and polyester raw material Pm are supplied to the dissolution section 12. In the dissolution section 12, the polyester contained in the polyester raw material Pm dissolves in the monomer D, while impurities remain undissolved in the monomer D, thereby producing a polyester solution P and a solution Pd containing impurities. By dissolving the polyester in the monomer D in this manner, the viscosity can be reduced and the fluidity can be improved, allowing the polyester to be easily introduced into the reaction section 16. Note that the polyester solution P is not limited to one in which the entire amount of polyester is dissolved in the monomer D; at least a portion of the polyester may be insoluble in the monomer D. Furthermore, if there is a component soluble in the monomer D among the components other than the polyester contained in the polyester raw material Pm, the polyester solution P may also contain that component dissolved in the monomer D.

[0022] The dissolving section 12 is connected to a first reaction section 16A (described later) via an inlet pipe 12a. The dissolving solution Pd in ​​the dissolving section 12 is supplied to the first reaction section 16A through the inlet pipe 12a. The inlet pipe 12a is also provided with a supply section 12a1. The supply section 12a1 is a mechanism for supplying the polyester solution P in the dissolving section 12 to the first reaction section 16A, and is a pump in this embodiment.

[0023] In this embodiment, the dissolving section 12 is provided with a heating section 12A. The heating section 12A heats the interior of the dissolving section 12, thereby heating the monomer D and polyester raw material Pm supplied to the dissolving section 12 to a predetermined temperature. The predetermined temperature is a temperature at which the polyester can be dissolved in the monomer D. By heating at this predetermined temperature, the polyester contained in the polyester raw material Pm can be properly dissolved in the monomer D. The predetermined temperature is preferably 140°C or higher and 300°C or lower, more preferably 160°C or higher and 280°C or lower, and even more preferably 190°C or higher and 250°C or lower. Note that the impurities also include components that melt when heated to a predetermined temperature (a temperature at which the polyester can be dissolved in the monomer D). Therefore, if the impurities include a component that melts when heated to a predetermined temperature, the impurities will be contained in the solution Pd in ​​a partially melted state. In this embodiment, the heating section 12A is provided in the dissolving section 12, but the location of the heating section 12A is not limited thereto and may be any location.

[0024] (Solvent reservoir) The solvent reservoir 14 is a tank into which the reaction solvent M is introduced and where the reaction solvent M is stored. The solvent reservoir 14 is connected to the reaction section 16 via an inlet pipe 14a. The reaction solvent M in the solvent reservoir 14 is supplied to the reaction section 16 through the inlet pipe 14a. More specifically, the inlet pipe 14a is provided with a heating and pressurizing section 14b that pressurizes and heats the reaction solvent M. The heating and pressurizing section 14b pressurizes and heats the reaction solvent M, thereby bringing the reaction solvent M into a supercritical state or a subcritical state (pressurized gas or pressurized liquid). The reaction section 16 is supplied with the reaction solvent M in a supercritical state or a subcritical state (pressurized gas or pressurized liquid).

[0025] (Reaction Section) The reaction section 16 is a container into which the solution Pd and the reaction solvent M are introduced to depolymerize the polyester in the solution Pd. The reaction section 16 includes a first reaction section 16A and a second reaction section 16B.

[0026] (First Reaction Section) The first reaction section 16A is formed within the reaction section 16. In this embodiment, the first reaction section 16A can be said to be a portion of the reaction section 16 that is filled with a filler. The first reaction section 16A can be made of a known filler used in gas-liquid or liquid-liquid contactors, such as fillers similar to those used in contactors that bring heavy oil and water into contact to extract active ingredients. Specific examples of fillers include pipes made of stainless steel or the like, Raschig rings, Berl saddles, terrarettes, balls, and the like.

[0027] An inlet pipe 12a is connected to the first reaction unit 16A. More specifically, an inlet 16C, which is an opening of the inlet pipe 12a through which the dissolution liquid Pd from the dissolution unit 12 is introduced, is connected to the first reaction unit 16A. The inlet 16C is connected to a surface 16A1 on the first direction D1 side of the first reaction unit 16A. The inlet pipe 12a is connected to the surface 16A1 so that the inlet 16C opens toward a second direction D2, which is opposite to the first direction D1. In this embodiment, the inlet 16C opening toward the second direction D2 is connected to the surface 16A1 of the first reaction unit 16A, but this is not limiting. For example, the inlet 16C does not have to be directly connected to the first reaction unit 16A, and the inlet 16C opening toward the second direction D2 may be connected to a position closer to the first direction D1 side of the surface 16A1 of the first reaction unit 16A within the reaction unit 16.

[0028] An inlet pipe 14a is connected to the reaction section 16. More specifically, an inlet 16D, which is an opening of the inlet pipe 14a through which the reaction solvent M is introduced from the solvent reservoir 14, is connected to the reaction section 16. The inlet 16D is connected closer to the second direction D2 than the surface 16A2 on the second direction D2 side of the first reaction section 16A. The inlet pipe 14a is connected closer to the second direction D2 than the surface 16A2 so that the inlet 16D opens toward the first direction D1 or from the side toward the center. In this embodiment, the inlet 16D, which opens toward the first direction D1 or from the side toward the center, is connected closer to the second direction D2 than the surface 16A2 of the first reaction section 16A. However, this is not limited thereto. For example, the inlet 16D may be directly connected to the first reaction section 16A or may be connected to the surface 16A2 of the first reaction section 16A.

[0029] In this embodiment, the inlet 16C through which the solution Pd is introduced opens in the second direction D2, and the inlet 16D through which the reaction solvent M is introduced opens in the first direction D1 or from the side toward the center. Therefore, the solution Pd and the reaction solvent M are introduced into the first reaction section 16A in directions facing each other.

[0030] The solution Pd introduced into the first reaction zone 16A from the inlet 16C moves in the second direction D2 on the surface of the filler in the first reaction zone 16A. Meanwhile, the reaction solvent M in a supercritical or subcritical state (pressurized gas or pressurized liquid) introduced from the inlet 16D moves in the first reaction zone 16A in the first direction D1. In the first reaction zone 16A, the reaction solvent M in a supercritical or subcritical state (pressurized gas or pressurized liquid) comes into contact with the solution Pd. The polyester in the solution Pd is depolymerized (reduced in molecular weight) by the reaction solvent M, and the depolymerized polyester is extracted into the reaction solvent M in a supercritical or subcritical state (pressurized gas or pressurized liquid). Hereinafter, the polyester depolymerized in the first reaction section 16A will be referred to as a first depolymerized polyester P1, and the mixture of the first depolymerized polyester P1 and the reaction solvent M (the reaction solvent M from which the first depolymerized polyester P1 has been extracted) will be referred to as a first solvent M1. The first solvent M1 containing the first depolymerized polyester P1 proceeds in the first direction D1 through the first reaction section 16A and is discharged to the first direction D1 side of the first reaction section 16A.

[0031] The first depolymerized polyester P1 includes monomers D and E produced by depolymerizing the polyester in the solution Pd, monomer D originally mixed in the solution Pd, and oligomers produced by depolymerizing the polyester. The oligomers referred to here refer to carboxylic acid-derived or alcohol-derived oligomers (carboxylic acid-derived or alcohol-derived oligomers with smaller molecular weights than polyester) that are not monomerized but are depolymerized from the polyester. Furthermore, oligomers contained in the residual substance R in the polyester solution P are also depolymerized by the reaction solvent M. Therefore, the first depolymerized polyester P1 also includes the depolymerized residual substance R. The depolymerized residual substance R includes oligomers contained in the residual substance that have been depolymerized, as well as monomers D and E formed by depolymerizing the oligomers contained in the residual substance.

[0032] (Second Reaction Unit) The second reaction unit 16B is formed within the reaction unit 16, and is formed at a location where the first solvent M1 is discharged from the first reaction unit 16A. In this embodiment, the first solvent M1 is discharged in the first direction D1, and therefore the second reaction unit 16B can be said to be a space formed on the first direction D1 side of the first reaction unit 16A.

[0033] In the second reaction zone 16B, the first depolymerized polyester P1 contained in the first solvent M1 is further depolymerized (reduced in molecular weight) by the reaction solvent M contained in the first solvent M1. Hereinafter, the first depolymerized polyester P1 further depolymerized in the second reaction zone 16B will be referred to as the second depolymerized polyester P2, and the mixture of the second depolymerized polyester P2 and the reaction solvent M (the reaction solvent M in which the second depolymerized polyester P2 is dissolved) will be referred to as the second solvent M2. An outlet pipe 16a is connected to the second reaction zone 16B. More specifically, an outlet 16E, which is an opening of the outlet pipe 16a through which the second solvent M2 from the second reaction zone 16B is discharged, is connected to the second reaction zone 16B. The second solvent M2 containing the second depolymerized polyester P2 in the second reaction zone 16B is discharged from the outlet 16E through the outlet pipe 16a to the outside of the second reaction zone 16B.

[0034] The second depolymerized polyester P2 contains the monomers D and E in the first depolymerized polyester P1, the monomers D and E produced by depolymerizing the oligomers in the first depolymerized polyester P1, and the oligomers produced by depolymerizing the first depolymerized polyester P1.

[0035] A discharge pipe 16b is connected to the bottom of the reaction zone 16. More specifically, a discharge port 16F, which is an opening of the discharge pipe 16b through which non-extractable matter (described below) in the reaction zone 16 is discharged, is connected to the bottom of the reaction zone 16. The non-extractable matter includes impurities such as metal compounds that were not extracted into the reaction solvent M and residues of undecomposed polyester that were not extracted into the reaction solvent M. That is, the non-extractable matter at the bottom of the reaction zone 16 is discharged from the discharge port 16F through the discharge pipe 16b to the outside of the reaction zone 16. The non-extractable matter discharged from the discharge port 16F can be said to be components of the polyester solution P that were not led to the separation zone 18 as the second solvent M2 (reaction solvent M in which the second depolymerized polyester P2 is dissolved) but remained in the first reaction zone 16A and the second reaction zone 16B.

[0036] The reaction section 16 may also be provided with a heating section that heats the interior of the reaction section 16 and a pressurizing section that maintains the pressure inside the reaction section 16 at a predetermined value or higher. The temperature inside the reaction section 16 is preferably 250°C or higher and 400°C or lower, and more preferably 250°C or higher and 350°C or lower. The pressure inside the reaction section 16 is preferably 1 MPa or higher and 30 MPa or lower, and more preferably 6 MPa or higher and 25 MPa or lower. The pressurizing section and the heating section may be controlled by the control section 30.

[0037] (Separation Section) The separation section 18 is introduced with a second solvent M2 containing a second depolymerized polyester P2, and separates the second solvent M2 into a reaction solvent M, a monomer D derived from a carboxylic acid contained in the second depolymerized polyester P2, a monomer E of an alcohol component contained in the second depolymerized polyester P2, and a residual substance R. The separation section 18 distills the second solvent M2 to separate it into the monomer D, the monomer E, and the residual substance R. The residual substance R is a component of the second solvent M2 other than the reaction solvent M, the monomer D, and the monomer E, and includes oligomers.

[0038] In this embodiment, the separation section 18 has a first separation section 18A, a second separation section 18B, and a third separation section 18C.

[0039] The first separation section 18A is a separation column connected to the outlet pipe 16a. A second solvent M2 containing the second depolymerized polyester P2 is introduced into the first separation section 18A via the outlet pipe 16a. The first separation section 18A separates the second solvent M2 into a low-boiling component and a high-boiling component having a higher boiling point than the low-boiling component. For example, in the first separation section 18A, the second solvent M2 may be heated to a predetermined temperature, with the gaseous component being the low-boiling component and the liquid component being the high-boiling component. The first separation section 18A is connected to outlet pipes 18Aa and 18Ab. The low-boiling component is discharged from the outlet pipe 18Aa, and the high-boiling component is discharged from the outlet pipe 18Ab.

[0040] The second separation section 18B is a separation column connected to the first separation section 18A via an outlet pipe 18Aa. Low-boiling point components are introduced into the second separation section 18B via the outlet pipe 18Aa. The second separation section 18B separates the low-boiling point components into reaction solvent M and monomer E. Outlet pipes 18Ba and 18Bb are connected to the second separation section 18B. The reaction solvent M is discharged from the outlet pipe 18Ba, and the monomer E is discharged from the outlet pipe 18Bb. The outlet pipe 18Ba is connected to the second separation section 18B and the solvent reservoir 14. Therefore, the reaction solvent M discharged from the second separation section 18B is returned to the solvent reservoir 14 and reused for depolymerization of polyester.

[0041] The third separation section 18C is a separation column connected to the first separation section 18A via the outlet pipe 18Ab. High-boiling components are introduced into the third separation section 18C via the outlet pipe 18Ab. The third separation section 18C further separates the high-boiling components into high-boiling residual material R, low-boiling components containing reaction solvent M and monomer E, and monomer D. The third separation section 18C is connected to outlet pipes 18Ca, 18Cb, and 18Cc. The outlet pipe 18Ca is connected to the second separation section 18B. The low-boiling components separated in the third separation section 18C are discharged to the second separation section 18B via the outlet pipe 18Ca. The monomer D separated in the third separation section 18C is discharged from the outlet pipe 18Cb and introduced into the temporary storage section 70. The residual material R separated in the third separation section 18C is discharged from the outlet pipe 18Cc.

[0042] An inlet pipe 18Cd is connected to the third separation section 18C. The inlet pipe 18Cd is also connected to the dissolving section 12 and introduces the monomer D discharged from the third separation section 18C into the dissolving section 12. In the example shown in FIG. 2 , the inlet pipe 18Cd branches off from the outlet pipe 18Cb. The inlet pipe 18Cd is provided with an adjustment section 18Ce that adjusts the amount of monomer D supplied from the third separation section 18C to the dissolving section 12. The adjustment section 18Ce is, for example, an on-off valve that, when open, allows the monomer D to be supplied to the dissolving section 12 and, when closed, stops the supply of the monomer D to the dissolving section 12. However, the adjustment section 18Ce is not limited to an on-off valve and may be any mechanism capable of adjusting the supply of monomer D to the dissolving section 12. In this embodiment, the adjustment section 18Ce is provided at the point where the inlet pipe 18Cd branches off from the outlet pipe 18Cb, but the adjustment section 18Ce may be provided at any position. Furthermore, the inlet pipe 18Cd does not have to be connected to the outlet pipe 18Cb, and may be directly connected to the third separation section 18 C. Alternatively, for example, the outlet pipe 18Cb may be provided with a reservoir (tank) for storing the monomer D, and the inlet pipe 18Cd may be connected to the reservoir.

[0043] An inlet pipe 18Cf is connected to the third separation section 18C. The inlet pipe 18Cf is also connected to the dissolving section 12 and introduces the residual material R discharged from the third separation section 18C into the dissolving section 12. In the example shown in FIG. 2, the inlet pipe 18Cf branches off from the outlet pipe 18Cc. The inlet pipe 18Cf is provided with an adjustment section 18Cg that adjusts the amount of residual material R supplied from the third separation section 18C to the dissolving section 12. The adjustment section 18Cg is, for example, an on-off valve. When open, the adjustment section 18Cg allows the residual material R to be supplied to the dissolving section 12, and when closed, the adjustment section 18Cg stops the supply of the residual material R to the dissolving section 12. However, the adjustment section 18Cg is not limited to an on-off valve and may be any mechanism capable of adjusting the supply of the residual material R to the dissolving section 12. In this embodiment, the adjustment section 18Cg is provided at the point where the inlet pipe 18Cf branches off from the outlet pipe 18Cc, but the adjustment section 18Cg may be provided at any location. Furthermore, the inlet pipe 18Cf does not have to be connected to the outlet pipe 18Cc, and may be directly connected to the third separation section 18C.

[0044] For example, the discharge pipe 18Cc may be provided with a reservoir (tank) for storing the residual substance R, and the introduction pipe 18Cf may be connected to the reservoir. The introduction pipe 18Cf may also be provided with a filter that allows oligomers in the residual substance R to pass through while collecting foreign matter in the residual substance R.

[0045] (Temporary Storage Section) The temporary storage section 70 is, for example, a tank, and temporarily stores the monomer D supplied from the third separation section 18C. The temporary storage section 70 is connected to the third separation section 18C via an outlet pipe 18Cb. The temporary storage section 70 is connected to the dissolution tank 82 via an outlet pipe 72. The temporary storage section 70 supplies the temporarily stored monomer D to the dissolution tank 82 of the crystallization system 80.

[0046] (Crystallization System) The crystallization system 80 crystallizes the monomer D from a solution containing the monomer D dissolved in the separation unit 18. The monomer D separated in the separation unit 18 may contain impurities. The crystallization system 80 crystallizes the highly pure monomer D from the solution containing the monomer D, removing the impurities. Examples of impurities include isomers of the monomer D that were not completely separated by distillation in the third separation unit 18C. For example, when the monomer D is DMT, the isomer of DMT is DMI (dimethyl isophthalate) derived from the copolymer IPA (isophthalic acid). DMI has a boiling point close to that of DMT, making it difficult to separate by distillation, but it can be separated by crystallization. That is, in this embodiment, highly pure monomer D can be extracted by crystallizing the monomer D (DMT) without crystallizing the DMI. Hereinafter, the highly pure monomer D extracted by crystallization will be referred to as monomer HD, as appropriate.

[0047] Furthermore, in this embodiment, the crystallization system 80 hydrolyzes the crystallized monomer HD to produce PTA (high-purity terephthalic acid). However, the crystallization system 80 is not limited to performing the process of obtaining PTA from the crystallized monomer HD, and may perform only the process of crystallizing the monomer HD.

[0048] Furthermore, although the crystallization system 80 of this embodiment is provided in the separation system 1 and used to crystallize the monomer HD, the use of the crystallization system 80 is not limited thereto. The crystallization system 80 may be a system that separates, by crystallization, any component to be separated, which is dissolved in a solution of any component.

[0049] The crystallization system 80 according to this embodiment will be specifically described below. Fig. 3 is a schematic side view of the crystallization system. Fig. 4 is a schematic top view of the crystallization system. Fig. 5 is a schematic top view showing the internal structure of the crystallization section. Fig. 6 is a schematic top view of the crystallization section showing the arrangement of different numbers of baffles.

[0050] The crystallization system 80 includes a dissolution tank 82, an adjustment section 86, a crystallization tank 90, an agitation section 100, a solid-liquid separation section 120, a melting tank 130, a temporary melt storage tank 140, and a hydrolysis separation reaction section 150. In this embodiment, the crystallization system 80 is connected downstream of the discharge pipe 72, and extracts, by crystallization, a monomer HD from a solution containing dissolved monomer D that flows through the discharge pipe 72.

[0051] (Dissolution Tank) The dissolution tank 82 is a tank that stores a solution L in which the monomer D is dissolved. The dissolution tank 82 is connected to the temporary storage section 70 via the outlet pipe 72. The monomer D containing impurities separated in the third separation section 18C is introduced from the temporary storage section 70 into the dissolution tank 82. A solvent that dissolves the monomer D is also introduced into the dissolution tank 82. As a result, the monomer D is dissolved in the solvent in the dissolution tank 82 and stored as a solution L. Note that any liquid may be used as the solvent, but methanol is used in this embodiment.

[0052] The dissolution tank 82 is connected to an adjustment unit 86 and a crystallization tank 90 via an inlet pipe 84. The solution L stored in the dissolution tank 82 is introduced into the crystallization tank 90 via the inlet pipe 84 and the adjustment unit 86. The dissolution tank 82 is located in the Z1 direction (vertically higher) than the crystallization tank 90, which will be described later. Furthermore, the bottom surface of the dissolution tank 82 on the Z2 direction side is located in the Z1 direction above an opening 90c formed in the crystallization tank 90. ​​However, the positional relationship between the dissolution tank 82 and the crystallization tank 90 in the Z direction is not limited to this and may be arbitrary.

[0053] (Inlet Pipe) The inlet pipe 84 is a pipe connecting the dissolution tank 82 and the crystallization tank 90. ​​In this embodiment, as described below, the inlet pipe 84 is connected to the adjustment unit 86, which is connected to the opening 90c of the crystallization tank 90. ​​Therefore, it can be said that the inlet pipe 84 is connected to the crystallization tank 90 (opening 90c) via the adjustment unit 86. The inlet pipe 84 extends in the Z2 direction from the point where it is connected to the dissolution tank 82 to the point where it is connected to the crystallization tank 90 (the adjustment unit 86 in this example). In other words, in the section from the point where it is connected to the dissolution tank 82 to the point where it is connected to the crystallization tank 90 (the adjustment unit 86 in this example), the inlet pipe 84 extends in the Z2 direction rather than in the Z1 direction as it approaches the point where it is connected to the crystallization tank 90 (the adjustment unit 86 in this example). Note that, although the inlet pipe 84 extends along the Z2 direction in the example of FIG. 3 , this is not limiting and it may extend at an angle relative to the Z2 direction.

[0054] The solution L in the dissolution tank 82 flows through the inlet pipe 84 and the adjustment section 86 into the crystallization tank 90 .

[0055] (Adjustment Unit) The adjustment unit 86 is a device that reduces the pressure of the solution L. The adjustment unit 86 is connected to the inlet pipe 84 and the crystallization tank 90. ​​The adjustment unit 86 reduces the pressure of the solution L introduced from the dissolution tank 82 via the inlet pipe 84, and introduces the solution L in a reduced pressure state into the crystallization tank 90.

[0056] In this embodiment, the adjustment unit 86 is a pressure reducing valve. The adjustment unit 86 may have any structure as long as it is a pressure reducing valve, but in this embodiment, it is an angle valve. By using the adjustment unit 86 as an angle valve, it is possible to reduce the pressure efficiently. As shown in FIG. 4, the adjustment unit 86 in this embodiment includes a valve portion 86a, an adjustment handle 86b, and a pipe portion 86c.

[0057] The pipe portion 86c is a tubular member having a flow path formed therein, and one end 86d is connected to the introduction pipe 84. In this embodiment, the end 86d has a shape in which a flange is formed around the periphery of an opening, and the end 84a of the introduction pipe 84 on the side connected to the adjustment portion 86 also has a shape in which a flange is formed around the periphery of an opening. In this embodiment, a plurality of fastening holes (not shown) pass through the flanges of the ends 86d and 84a, and the end 86d and the end 84a are connected by fastening with fastening members 85 inserted into the fastening holes.

[0058] The other end 86e of the pipe portion 86c is connected to the crystallization tank 90. ​​In this embodiment, the end 86e has a shape in which a flange is formed around the opening. The wall surface of the crystallization tank 90 is formed with a recess 90a and an opening 90c, which is formed at a position overlapping the recess 90a and connects the inside and outside of the crystallization tank 90. ​​In this embodiment, the pipe portion 86c is directly connected to the crystallization tank 90 by fastening the end 86e of the pipe portion 86c with a fastening member 87 while the end 86e is inserted into the recess 90a of the crystallization tank 90.

[0059] As described above, since the adjustment unit 86 in this embodiment is an angle valve, the flow path on the end 86d side of the pipe portion 86c intersects with the flow path on the end 86e side of the pipe portion 86c. In this embodiment, the flow path on the end 86d side of the pipe portion 86c intersects with the flow path on the end 86e side in the horizontal direction. The horizontal direction is a direction perpendicular to the Z direction (vertical direction).

[0060] The valve portion 86a is a valve provided in the flow path inside the pipe portion 86c. The adjustment handle 86b is attached to the valve portion 86a and is a mechanism for adjusting the opening degree of the flow path inside the pipe portion 86c by the valve portion 86a. The adjustment portion 86 adjusts the opening and closing of the valve inside the valve portion 86a by rotating the adjustment handle 86b, thereby adjusting the flow rate and pressure of the solution liquid L.

[0061] The solution L introduced into the adjusting section 86 is introduced into the crystallization tank 90 in a state where the pressure of the solution L has been reduced by the adjusting section 86. Since the temperature of the reduced-pressure solution L is reduced, the monomer HD to be separated is crystallized from the reduced-pressure solution L.

[0062] In this embodiment, the adjustment unit 86 is directly connected to the crystallization tank 90, but this is not limited thereto. A pipe connecting the adjustment unit 86 and the opening 90c of the crystallization tank 90 may be provided between the adjustment unit 86 and the opening 90c.

[0063] (Crystallization Tank) The crystallization tank 90 is a tank into which the solution L decompressed by the adjustment unit 86 is introduced and into which the separation target (here, the monomer HD) is crystallized from the solution L. As described above, the crystallization tank 90 is formed with an opening 90c connected to the adjustment unit 86, and the solution L decompressed by the adjustment unit 86 is introduced through the opening 90c. In this way, the decompressed solution L is introduced into the crystallization tank 90, and the solution L is stored in a decompressed state. In other words, the crystallization tank 90 can be said to be a flash-type crystallization tank in which the internal pressure is reduced below the external pressure.

[0064] The crystallization tank 90 is, for example, a tank, and may have any shape. In this embodiment, the crystallization tank 90 has an upper surface 90a1, a bottom surface 90a2, and a side wall 90b.

[0065] The sidewall 90b is a wall surface of the crystallization tank 90, the top surface 90a1 is a member covering the end of the sidewall 90b on the Z1 direction side, and the bottom surface 90a2 is a member covering the end of the sidewall 90b on the Z2 direction side. In this embodiment, the sidewall 90b is cylindrical, and the top surface 90a1 and the bottom surface 90a2 are hemispherical, but their shapes are not limited to this and may be any shape. In this embodiment, the recess 90a and the opening 90c are formed in the sidewall 90b. In this embodiment, a through-hole is formed in the top surface 90a1, through which the stirring unit 100 (described later) passes. In addition, a discharge pipe 121 for discharging the slurry S is connected to the bottom surface 90a2. The slurry S is a slurry containing the crystallized monomer HD and the solution L from which the monomer HD has been removed. However, the locations of the recess 90a and opening 90c, the through hole through which the stirring part 100 passes, and the discharge pipe 121 are not limited to this and may be arbitrary, for example, the discharge pipe 121 may be provided on the side wall 90b.

[0066] A baffle 110 is provided inside the crystallization tank 90. ​​The baffle 110 is a member that rectifies the flow of the solution L inside the crystallization tank 90. ​​The shape, attachment position, and number of the baffle 110 are arbitrary, but in this embodiment, the baffle 110 is provided on the inner wall surface of the crystallization tank 90 (side wall 90b), and more specifically, is a plate-shaped member that extends in the Z direction on the inner wall surface of the side wall 90b.

[0067] In this embodiment, a plurality of baffles 110 are provided on the inner wall surface of the side wall 90b in the circumferential direction, with the central axis of the crystallization tank 90 aligned in the Z direction as the axial direction. In this embodiment, the baffles 110 are arranged at equal intervals in the circumferential direction as viewed from the Z direction. For example, as shown in FIG. 5 , when there are three baffles 110, the three baffles 110 are arranged at 120° intervals. Also, as shown in FIG. 6 , when there are two baffles 110, the two baffles 110 are arranged facing each other, in other words, at 180° intervals. When multiple baffles 110 are provided, the opening 90c is provided at the center of two adjacent baffles 110 in the circumferential direction. That is, for example, in the example shown in FIG. 7 , the baffles 110 are adjacent to each other at 180° intervals, and therefore the opening 90c is provided adjacent to each baffle 110 at 90° intervals.

[0068] However, the baffle 110 is not an essential component and does not have to be provided in the crystallization tank 90.

[0069] (Agitation Unit) The agitation unit 100 is a device that is provided in the crystallization tank 90 and agitates the inside of the crystallization tank 90. ​​The agitation unit 100 has a rotation shaft 104 and an agitation blade .

[0070] The rotating shaft 104 is a shaft-shaped member rotatably inserted into the crystallization tank 90. ​​The stirring blade 106 is a blade provided at the tip of the rotating shaft 104 on the Z2 direction side. The stirring blade 106 may have a shape in which multiple blades are arranged circumferentially, for example. The stirring blade 106 is located on the Z2 direction side of the opening 90c inside the crystallization tank 90. ​​In this embodiment, the stirring blade 106 is located on the Z2 direction side of the adjustment unit 86. The stirring blade 106 is located on the Z1 direction side of the bottom surface 90a2 of the crystallization tank 90, facing the bottom surface 90a2 (i.e., overlapping the bottom surface 90a2 when viewed from the Z direction). The stirring unit 100 is preferably provided at the center of the crystallization tank 90 when viewed from the Z direction. That is, it is preferable that the central axis of the stirring unit 100 and the central axis of the crystallization tank 90 coincide with each other.

[0071] The agitator 100 rotates by driving a drive unit 102 (e.g., a motor) to agitate the crystallization tank 90. ​​The agitator 100 is configured to generate a downward flow Df within the crystallization tank 90 as it rotates. The downward flow Df refers to a water flow from the tip of the agitator blade 106 in the Z2 direction. For example, the rotation direction of the agitator 100 and the orientation of the agitator blade 106 are configured to generate the downward flow Df. Therefore, when the agitator 100 rotates within the crystallization tank 90 containing the decompressed solution L, as shown in FIG. 3 , the solution L flows from the tip of the agitator blade 106 toward the Z2 direction along the downward flow Df and reaches the bottom surface 90a2. After reaching the bottom surface 90a2, the solution L flows radially outward along the bottom surface 90a2 and then flows as an upward flow Uf along the sidewall 90b toward the Z1 direction. That is, in this embodiment, the dissolved liquid L in the crystallization tank 90 flows in the Z2 direction according to the downward flow Df in the region on the Z2 side of the agitator 106, and flows in the Z1 direction according to the upward flow Uf in the region on the Z1 side of the agitator 106 near the side wall 90b.

[0072] (Crystallization of Monomer HD) With the above-described configuration, the crystallization system 80 crystallizes the monomer HD from the solution L in the crystallization tank 90. ​​That is, when the solution L in the dissolution tank 82 is supplied to the adjustment unit 86 via the inlet pipe 84, the solution L is depressurized by the adjustment unit 86. The solution L depressurized by the adjustment unit 86 is introduced into the crystallization tank 90 through the opening 90c. Because the temperature of the depressurized solution L decreases, the monomer HD is crystallized in the crystallization tank 90 and stored as a slurry S. Furthermore, the inside of the crystallization tank 90 is stirred by the stirring unit 100, which promotes the crystallization of the monomer HD in the crystallization tank 90.

[0073] Here, since the pressure of the solution L is reduced immediately after passing through the adjusting section 86, the monomer HD may be crystallized at the opening 90c, which is the inlet for introducing the solution L into the crystallization tank 90. ​​In this case, the monomer HD, which is a solid component, may accumulate at the opening 90c and block at least a portion of the opening 90c. This may prevent the solution L from being properly introduced into the crystallization tank 90, making it impossible to properly obtain the monomer HD to be separated.

[0074] In contrast, in this embodiment, the stirring unit 100 is configured to generate a downward flow Df, while the stirring blade 106 is positioned closer to the Z2 side than the opening 90c. This generates an upward flow Uf in the region where the opening 90c is formed, allowing the monomer HD deposited or flowing within the opening 90c to be introduced into the crystallization tank 90 by the upward flow Uf, thereby preventing clogging of the opening 90c. Furthermore, because the monomer HD deposits on the edge of the opening 90c on the Z2 side, the upward flow Uf flowing in the Z1 direction lifts the deposited monomer HD upward and allows it to be properly removed. In this embodiment, the adjustment unit 86 is directly connected to the opening 90c, thereby shortening the flow path from the adjustment unit 86 to the crystallization tank 90 and more properly preventing clogging by the monomer HD. Furthermore, in this embodiment, a baffle 110 is provided inside the crystallization tank 90, rectifying the upward flow Uf and more properly removing the monomer HD.

[0075] Furthermore, if the head pressure increases the pressure loss of the solution L in the inlet pipe 84 connecting the dissolution tank 82 and the crystallization tank 90, there is a risk that bubbles will be generated from the solution L due to reduced pressure. If bubbles are generated, vapor lock will occur in the adjustment unit 86, making it impossible to properly introduce the solution L into the crystallization tank 90 and preventing proper crystallization of the monomer HD. In contrast, in this embodiment, the dissolution tank 82 is disposed on the Z1 direction side of the crystallization tank 90. ​​This suppresses the pressure loss of the solution L due to the head pressure, suppresses the occurrence of vapor lock due to bubbles, and enables proper crystallization of the monomer HD.

[0076] (Production of PTA) Next, a method for producing PTA from monomer HD using the crystallization system 80 will be described. The crystallization system 80 produces PTA using a solid-liquid separation section 120, a melting tank 130, a temporary melt storage tank 140, and a hydrolysis separation reaction section 150.

[0077] The solid-liquid separation section 120 is connected to the crystallization tank 90 via a discharge pipe 121. The slurry S (solution L in which the monomer HD has been crystallized) produced in the crystallization tank 90 is introduced into the solid-liquid separation section 120 from the crystallization tank 90 via the discharge pipe 121. The solid-liquid separation section 120 separates the introduced slurry S into solid and liquid components. The solid-liquid separation section 120 separates the slurry S into solid components to be separated and liquid components. In this embodiment, the solid-liquid separation section 120 separates the slurry S into the monomer HD and the solution L from which the monomer HD has been removed. The solution L from which the monomer HD has been removed is discharged from a discharge pipe 123 connected to the solid-liquid separation section 120 and treated. The solid-liquid separation unit 120 may be, for example, a centrifuge, which separates the separation target from the solution L by centrifuging, stirring the interior of the solid-liquid separation unit 120 around a predetermined axis and moving the separation target radially outward from the rotating shaft. The solid-liquid separation unit 120 is not limited to centrifugal separation as long as it can separate the separation target from the solution L. For example, the solid-liquid separation unit 120 may separate the separation target from the solution L using a filter.

[0078] The highly pure monomer HD obtained by solid-liquid separation from the slurry S is introduced into the melting tank 130 through an outlet pipe 122 connected to the solid-liquid separation section 120. The melting tank 130 is a tank that heats and melts the introduced monomer HD. The monomer HD melted in the melting tank 130 is introduced into a temporary melt storage tank 140 where it is temporarily stored, and then introduced from the temporary melt storage tank 140 into the hydrolysis separation reaction section 150. The hydrolysis separation reaction section 150 is a tank that adds water to the introduced monomer HD to hydrolyze the monomer HD. The hydrolysis separation reaction section 150 hydrolyzes the monomer HD to produce PTA. The PTA then undergoes a crystallization process and a drying process (not shown) before being stored in a hopper.

[0079] (Control Unit) The control unit 30 is a control device that controls the separation system 1. The control unit 30 controls the adjustment unit 10b to control the amount of polyester raw material Pm supplied from the raw material storage unit 10 to the dissolution unit 12. Furthermore, if the solid-liquid separation unit 13 is equipped with a drive unit, the control unit 30 controls the operation of the solid-liquid separation unit 13. The control unit 30 controls the supply unit 12a1 to control the amount of solution Pd supplied from the dissolution unit 12 to the reaction unit 16. The control unit 30 controls the heating and pressurization unit 14b to bring the reaction solvent M to a supercritical state or subcritical state (pressurized gas or pressurized liquid) and controls the amount of reaction solvent M in the supercritical state or subcritical state (pressurized gas or pressurized liquid) supplied to the reaction unit 16. The control unit 30 controls the adjustment unit 18Ce to control the amount of monomer D supplied to the dissolution unit 12. The control unit 30 controls the adjustment unit 18Cg to control the amount of residual material R supplied to the dissolution unit 12. The control unit 30 adjusts the adjustment unit 86 to reduce the pressure of the solution L while controlling the flow rate of the solution L into the crystallization tank 90. ​​The control unit 30 controls the drive unit 102 to control the rotation of the stirring unit 100.

[0080] In this embodiment, the control unit 30 is a computer, and includes a processor including an arithmetic circuit such as a CPU (Central Processing Unit), and a storage unit that stores various information such as the contents of calculations performed by the processor and programs. The control unit 30 executes control of the separation system 1 by reading out programs from the storage unit.

[0081] However, the separation system 1 is not limited to being automatically controlled by the control unit 30, and for example, at least a part of the processing may be controlled by the operation of an operator.

[0082] (Operation of Separation System) Next, the operation of the separation system 1 will be described. The control unit 30 controls the adjustment units 10b and 18Ce to introduce the polyester raw material Pm and the monomer D into the dissolution unit 12, and mixes the polyester raw material Pm and the monomer D in the dissolution unit 12 to generate a solution Pd. The control unit 30 controls the supply unit 12a1 to introduce the solution Pd generated in the dissolution unit 12 into the first reaction unit 16A.

[0083] The control unit 30 controls the heating and pressurizing unit 14b to supply the reaction solvent M in a supercritical state or a subcritical state (pressurized gas or pressurized liquid) to the reaction unit 16. The control unit 30 preferably sets the reaction solvent M at 250° C. or higher and 400° C. or lower, and more preferably at 250° C. or higher and 350° C. or lower. The control unit 30 preferably sets the reaction solvent M at 1 MPa or higher and 30 MPa or lower, and more preferably at 6 MPa or higher and 25 MPa or lower.

[0084] In this manner, by supplying the solution Pd and the reaction solvent M to the reaction section 16, the polyester contained in the solution Pd is depolymerized in the first reaction section 16A to produce a first depolymerized polyester P1. Then, in the second reaction section 16B, the first depolymerized polyester P1 is further depolymerized to produce a second solvent M2, which is a mixture of a second depolymerized polyester P2 and the reaction solvent M. The second solvent M2 is separated into the reaction solvent M, monomer D, monomer E, and residual substances in the first separation section 18A, the second separation section 18B, and the third separation section 18C.

[0085] The control unit 30 controls the adjustment unit 86 to supply the solution L to the adjustment unit 86, depressurize the solution L using the adjustment unit 86, and introduce the depressurized solution L into the crystallization tank 90 through the opening 90c. Because the solution L introduced into the crystallization tank 90 is depressurized, the monomer HD to be separated is crystallized from the solution L in the crystallization tank 90. ​​The control unit 30 also controls the rotation of the stirring unit 100. This stirs the inside of the crystallization tank 90, generating a downward flow Df of the solution L in the crystallization tank 90. ​​This generates an upward flow Uf near the opening 90c, making it possible to prevent the monomer HD from accumulating at the opening 90c.

[0086] (Effects of the Present Disclosure) The crystallization system according to the first aspect of the present disclosure comprises an adjustment unit 86 that reduces the pressure of a solution L in which a material to be separated is dissolved, a crystallization tank 90 that has an opening 90c connected to the adjustment unit 86, into which the solution L reduced in pressure by the adjustment unit 86 is introduced through the opening 90c, and in which the material to be separated is crystallized from the solution L, and an agitation unit 100 that is provided within the crystallization tank 90 and that agitates the inside of the crystallization tank 90 to generate a downward flow Df of the solution L, and an agitation blade 106 of the agitation unit 100 that agitates the solution L is positioned vertically below the opening 90c.

[0087] This allows the stirring blades to generate an upward flow, which lifts and removes the monomer crystals that have accumulated at the opening, allowing the material to be separated to be properly crystallized.

[0088] The crystallization system according to the second aspect of the present disclosure is the crystallization system according to the first aspect, in which the adjustment unit 86 is directly connected to the opening 90c, thereby shortening the flow path between the adjustment unit and the crystallization tank and suppressing clogging of the opening by the monomer.

[0089] A crystallization system according to a third aspect of the present disclosure is the crystallization system according to the first or second aspect, further comprising a baffle 110 provided inside the crystallization tank 90 to straighten the flow of the solution L in the crystallization tank 90. ​​This can assist the flow of the solution L, allowing the material to be separated to be properly crystallized.

[0090] A crystallization system according to a fourth aspect of the present disclosure is the crystallization system according to the third aspect, wherein the baffle 110 is a plate member extending vertically on a wall surface that is not surrounded by the crystallization tank 90. ​​This can assist the flow of the solution, thereby enabling the material to be separated to be properly crystallized.

[0091] A crystallization system according to a fifth aspect of the present disclosure is the crystallization system according to any one of the first to fourth aspects, wherein the adjustment unit 86 is an angle valve. This allows the solution to be depressurized efficiently because the solution is depressurized by the valve itself and by flowing through the flow path of the angle.

[0092] A crystallization system according to a sixth aspect of the present disclosure is the crystallization system according to any one of the first to fifth aspects, further comprising a dissolution tank 82 for storing a solution L and an inlet pipe 84 connecting the dissolution tank 82 to an opening 90c, the dissolution tank 82 being positioned vertically above the crystallization tank 90. ​​This allows the solution L to flow from the dissolution tank to the crystallization tank without resisting gravity. This prevents an increase in pressure loss due to head pressure. Furthermore, this also eliminates vapor lock caused by bubbles generated in the adjustment section, leading to proper crystallization of the material to be separated.

[0093] A crystallization system according to a seventh aspect of the present disclosure is the crystallization system according to any one of the first to sixth aspects, in which the separation target is dimethyl terephthalate, and therefore dimethyl terephthalate (DMT) can be crystallized.

[0094] A crystallization method according to an eighth aspect of the present disclosure is a crystallization method for a crystallization system having an adjustment unit 86 that decompresses the liquid, a crystallization tank 90 formed with an opening 90c connected to the adjustment unit 86, and an agitation unit 100 that is provided within the crystallization tank 90 and has an agitation blade 106 that agitates the liquid located vertically below the opening 90c, the method including the steps of: supplying a solution L containing a substance to be separated dissolved to the adjustment unit 86, causing the adjustment unit 86 to decompress the solution L and introduce it into the crystallization tank 90 through the opening 90c, thereby crystallizing the substance to be separated from the solution L in the crystallization tank 90; and rotating the agitation unit 100 with the decompressed solution L introduced into the crystallization tank 90 to stir the inside of the crystallization tank 90 and generate a downward flow Df of the solution L.

[0095] This allows the stirring blades to generate an upward flow, which lifts and removes the monomer crystals that have accumulated at the opening, allowing the material to be separated to be properly crystallized.

[0096] Although the embodiments of the present invention have been described above, the embodiments are not limited to the contents of these embodiments. Furthermore, the above-described components include those that can be easily imagined by a person skilled in the art, those that are substantially the same, and those that are within the scope of what is called equivalents. Furthermore, the above-described components can be combined as appropriate. Furthermore, various omissions, substitutions, or modifications of the components can be made without departing from the spirit of the above-described embodiments.

[0097] REFERENCE SIGNS LIST 1 Separation system 12 Dissolution section 13 Solid-liquid separation section 14 Solvent storage section 16 Reaction section 18 Separation section 30 Control section 80 Crystallization system 82 Dissolution tank 86 Adjustment section 90 Crystallization tank 100 Stirring section 110 Baffle D, HD, E Monomer M Reaction solvent P Polyester solution Pd, L Dissolution liquid Pm Polyester raw material R Residual substance

Claims

1. A crystallization system comprising: an adjustment unit for depressurizing a solution in which a separation target is dissolved; a crystallization tank having an opening connected to the adjustment unit, into which the solution depressurized by the adjustment unit is introduced from the opening, and in which the separation target crystallizes from the solution; and a stirring unit provided in the crystallization tank for stirring the inside of the crystallization tank to generate a downward flow of the solution, wherein a stirring blade of the stirring unit for stirring the solution is located vertically below the opening.

2. The crystallization system according to claim 1, wherein the adjustment unit is directly connected to the opening.

3. The crystallization system according to claim 1 or 2, further comprising a baffle provided inside the crystallization tank for rectifying the flow of the solution in the crystallization tank.

4. The crystallization system according to claim 3, wherein the baffle is a plate member extending in the vertical direction on the inner wall surface of the crystallization tank.

5. The crystallization system according to claim 1 or 2, wherein the adjustment unit is an angle valve.

6. The crystallization system according to claim 1 or 2, further comprising a dissolution tank for storing the solution and an introduction pipe connecting the dissolution tank and the opening, wherein the dissolution tank is located vertically above the crystallization tank.

7. The crystallization system according to claim 1 or 2, wherein the separation target is dimethyl terephthalate.

8. A crystallization method for a crystallization system comprising: an adjustment unit for depressurizing a liquid; a crystallization tank having an opening connected to the adjustment unit; and a stirring unit provided in the crystallization tank, wherein a stirring blade for stirring the liquid is located vertically below the opening, the method comprising: supplying a solution in which a separation target is dissolved to the adjustment unit to depressurize the solution by the adjustment unit and introduce it into the crystallization tank from the opening, and crystallizing the separation target from the solution in the crystallization tank; and rotating the stirring unit with the depressurized solution introduced into the crystallization tank to stir the inside of the crystallization tank and generate a downward flow of the solution.

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