BHT (butylated hydroxytoluene) crystallization solvent recovery method

Abstract

The invention belongs to the technical field of chemical product separation, and discloses a BHT (butylated hydroxytoluene) crystallization solvent recovery method. Comprising the following steps: solving the problem of recovering a BHT crystallization solvent, and efficiently recovering a low-boiling-point solvent contained in a high-boiling-point substance by adopting a rectification and gas stripping combined process, so that not only can the low-boiling-point solvent be completely removed, but also the operation temperature is very low, and decomposition and discoloration of BHT cannot be caused.

Description

Technical Field

[0001] The present invention belongs to the technical field of separation of chemical products, and particularly relates to a method for recovering the crystallization solvent of BHT. Background Art

[0002] 2,6-Di-tert-butyl-4-methylphenol, also known as BHT (2,6-Di-tert-butyl-4-methylphenol) or antioxidant 264, is an antioxidant with excellent performance. Due to its good antioxidant effect, low price, stability, safety, and easy solution of environmental pollution problems, it is widely used in the fields of food preservation, plastic processing, and synthetic rubber manufacturing, and is the most demanded type of phenolic antioxidant in the market. Currently, the production of BHT generally uses p-cresol or a mixture of m- and p-cresol as raw materials. First, 2,6-di-tert-butyl-4-methylphenol is obtained through an alkylation reaction with isobutene, and then a crude BHT with a higher purity is obtained through rectification separation. Further crystallization purification steps are carried out to obtain high-purity BHT crystals.

[0003] The crystallization of BHT is generally carried out in a solvent. The solvents used are mainly methanol or ethanol. By cooling crystallization, BHT is precipitated from the solution in the form of crystals, and then the BHT finished product is obtained through filtration and drying steps. After crystallization, the mother liquor recovers and recycles the solvent through rectification. The BHT with a lower purity at the bottom of the kettle after removing the solvent can be returned to the previous process section for further purification by rectification means. It should be noted that during the solvent recovery process, if the low-boiling solvent is not removed completely, it will not only have a great negative impact on the previous rectification section of BHT, but also bring the low-boiling solvent into the reaction system, resulting in an increase in side reactions. Due to the significant boiling point difference between the solvent and BHT, it is difficult to remove the solvent completely (the solvent content is less than 0.1%). If a high degree of solvent removal is pursued, the kettle temperature needs to be significantly increased to above 240°C. This will not only easily cause thermal decomposition of BHT, reduce the product yield, but also make the color of the material after removing the solvent darker. Therefore, it is particularly important to explore a more economical and efficient solvent recovery strategy.

[0004] The prior art CN114213218A discloses a method for producing BHT. Specifically, it discloses that cresol undergoes an alkylation reaction with isobutene in the presence of an acidic catalyst. The reaction product is neutralized and washed with water to obtain an alkylated solution, and then the BHT finished product is refined and purified by the melt crystallization method. The purification process includes the following steps: preheating the alkylated solution to a molten state; slowly controlling the cooling and crystallization rate, cooling to a set temperature within a set time to complete crystallization; discharging the crystallization mother liquor, collecting the primary crystallization mother liquor and sending it to the rectification and recovery product unit for separation, recovery and reuse; slowly heating to a set temperature within a set time to melt a part of the crystals attached to the surface of the crystallizer, and discharging them together with a thin layer of crystallization mother liquor adhered to the crystal surface. The collected primary sweating mother liquor is used for the feed of the primary crystallization; increasing the temperature of the heating control system to heat and melt all the crystals attached to the surface of the crystallizer, and collecting the primary crystallization product. However, the method it adopts is the melt crystallization method and does not involve solvent recovery. Summary of the Invention

[0005] In order to overcome the deficiencies of the prior art, the present invention provides a method for recovering the crystallization solvent of BHT, which solves the problem of recovering the crystallization solvent of BHT. The combined process of rectification + stripping can efficiently recover the low-boiling solvent contained in the high-boiling substances. Not only can the low-boiling solvent be completely removed, but also the operating temperature is very low, and it will not cause the decomposition and discoloration of BHT.

[0006] The above object of the present invention is achieved by the following technical solutions: A method for recovering the crystallization solvent of BHT, the steps are as follows:

[0007] The filtered mother liquor after crystallization and the pressurized condensate from the top of the solvent stripping tower are mixed by a liquid mixer and then sent to a solvent removal tower for rectification. The refined solvent is taken out from the top of the solvent removal tower and returned to the crystallization section for reuse. The crude BHT solution containing a small amount of solvent taken out from the bottom of the solvent removal tower is pressurized by a pump at the bottom of the solvent removal tower and fed into the solvent stripping tower from the upper part. The circulating nitrogen gas condensed at the top of the solvent stripping tower is boosted into compressed gas by a circulating gas compressor, and the compressed gas is preheated by a circulating gas heater to form the inlet gas of the solvent stripping tower and is fed into the bottom of the solvent stripping tower. The crude BHT liquid after removing the solvent is taken out from the bottom of the solvent stripping tower and returned to the BHT rectification section for re-refining. The circulating gas entraining the solvent is condensed by a condenser at the top, and the condensate from the top of the solvent stripping tower is pressurized by a pump at the top of the solvent stripping tower and mixed with the filtered mother liquor and sent to the solvent removal tower for rectification. The gas phase condensed at the top of the solvent stripping tower is boosted by a circulating gas compressor and returned to the solvent stripping tower as circulating gas.

[0008] Further, the solvent includes: methanol, ethanol, methanol aqueous solution with a concentration of more than 95%, ethanol aqueous solution with a concentration of more than 95%.

[0009] Further, the solvent mass fraction in the bottom material of the solvent removal tower is 1-5%, the phenol mass fraction in the refined solvent drawn from the top of the solvent removal tower is less than 0.1%, and the operating pressure is atmospheric pressure.

[0010] Further, the solvent mass fraction in the crude BHT liquid after the solvent is removed by the solvent stripping tower is less than 0.1%, and the operating pressure is atmospheric pressure.

[0011] Further, the circulating gas is nitrogen, the ratio of the circulating gas flow rate to the volume flow rate of the liquid drawn from the bottom of the solvent removal tower is 200-2000, and the preheating temperature of the circulating gas is 80-150°C.

[0012] Further, the top condensation temperature of the solvent stripping tower is 2-10°C.

[0013] Another object of the present invention is to protect the system used in the above BHT crystallization solvent recovery method, wherein the liquid mixer is connected to the solvent removal tower, a solvent removal tower bottom extraction pump is connected to the bottom of the solvent removal tower, the solvent removal tower bottom extraction pump is further connected to the solvent stripping tower, the top of the solvent stripping tower is connected to a circulating gas compressor, the circulating gas compressor is further connected to a circulating gas heater, the circulating gas heater is further connected back to the solvent stripping tower, the top of the solvent stripping tower is connected to a solvent stripping tower top extraction pump, the solvent stripping tower top extraction pump is further connected to the liquid mixer, the liquid mixer is also provided with a filtered mother liquor inlet, the bottom of the solvent stripping tower is provided with a solvent stripping tower bottom extraction liquid outlet, and the top of the solvent removal tower is provided with a solvent removal tower top refined solvent outlet.

[0014] The beneficial effects of the present invention compared with the prior art are as follows: The combined process of rectification + stripping can efficiently recover the low-boiling solvent contained in the high-boiling substance. Not only can the low-boiling solvent be removed completely, but also the operating temperature is very low, and it will not cause the decomposition and discoloration of BHT. The solvent recovery temperature is low, and the solvent removal rate in the mother liquor is high. Description of the Drawings

[0015] The present invention will be further described below in conjunction with the drawings and specific embodiments

[0016] Figure 1 It is a schematic structural diagram of the system used in the BHT crystallization solvent recovery method of the present invention.

[0017] In the figure, S1. Filtered mother liquor; S2. Feed to the solvent stripping column; S3. Fine solvent withdrawn from the top of the solvent stripping column; S4. Crude BHT solution; S5. Feed to the solvent stripping column; S6. Crude BHT liquid after solvent removal; S7. Condensate from the top of the solvent stripping column; S8. Condensate from the top of the solvent stripping column after pressurization; S9. Recirculating gas after condensation; S10. Compressed gas; S11. Gas inlet to the solvent stripping column; M1. Liquid mixer; T1. Solvent removal column; T2. Solvent stripping column; P1. Pump for withdrawing from the bottom of the solvent removal column; P2. Pump for withdrawing from the top of the solvent stripping column; C1. Recirculating gas compressor; E1. Recirculating gas heater. Detailed implementation mode

[0018] To more clearly elaborate the purpose, technical solution and advantages of the present disclosure, the embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the following description of the embodiments is intended to explain and illustrate the general concept of the present invention, and should not be construed as a limitation of the present invention. In the specification and drawings, the same or similar reference numerals refer to the same or similar components or elements. For clarity, the drawings are not necessarily drawn to scale, and some well-known components and structures may be omitted in the drawings. Unless otherwise specified, the experimental methods used in the present invention are all conventional methods, and the experimental equipment, materials, reagents, etc. used can be obtained from commercial channels.

[0019] In the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention. In addition, the terms "first", "second", "third" are only used for descriptive purposes and should not be construed as indicating or implying relative importance.

[0020] In the description of the present invention, it should be noted that unless otherwise clearly specified and defined, the terms "installed", "connected", "connected" should be understood in a broad sense. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0021] In addition, the technical features involved in different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0022] Embodiment 1

[0023] Methanol is used as the crystallization solvent for BHT. The mother liquor after crystallization filtration and the methanol discharged from the solvent stripping tower T2 are mixed in a mixer and then sent to the solvent removal tower T1 for rectification. The refined methanol with a purity greater than 99.9% is taken out from the top of the tower and returned to the crystallization section for reuse. The temperature at the bottom of tower T1 is controlled at 100 °C. At this time, the bottom discharge liquid contains about 4% methanol. After being pressurized by pump P1, it is fed into the stripping tower T2 from the upper part. The recycled nitrogen is pressurized by a compressor and then preheated to 120 °C by a heater and fed into the bottom of T2. The ratio of the recycled gas flow rate to the liquid-phase feed volume flow rate is controlled at 1200, and the methanol content in the bottom discharge of T2 is 0.07%.

[0024] Example 2

[0025] Methanol is used as the crystallization solvent for BHT. The mother liquor after crystallization filtration and the methanol discharged from the solvent stripping tower T2 are mixed in a mixer and then sent to the solvent removal tower T1 for rectification. The refined methanol with a purity greater than 99.9% is taken out from the top of the tower and returned to the crystallization section for reuse. The temperature at the bottom of tower T1 is controlled at 115 °C. At this time, the bottom discharge liquid contains about 2% methanol. After being pressurized by pump P1, it is fed into the stripping tower T2 from the upper part. The recycled nitrogen is pressurized by a compressor and then preheated to 100 °C by a heater and fed into the bottom of T2. The ratio of the recycled gas flow rate to the liquid-phase feed volume flow rate is controlled at 1000, and the methanol content in the bottom discharge of T2 is 0.08%.

[0026] Example 3

[0027] Ethanol is used as the crystallization solvent for BHT. The mother liquor after crystallization filtration and the ethanol discharged from the solvent stripping tower T2 are mixed in a mixer and then sent to the solvent removal tower T1 for rectification. The refined ethanol with a purity greater than 99.9% is taken out from the top of the tower and returned to the crystallization section for reuse. The temperature at the bottom of tower T1 is controlled at 138 °C. At this time, the bottom discharge liquid contains about 3% ethanol. After being pressurized by pump P1, it is fed into the stripping tower T2 from the upper part. The recycled nitrogen is pressurized by a compressor and then preheated to 110 °C by a heater and fed into the bottom of T2. The ratio of the recycled gas flow rate to the liquid-phase feed volume flow rate is controlled at 1500, and the ethanol content in the bottom discharge of T2 is 0.05%.

[0028] Example 4

[0029] 95% ethanol is used as the crystallization solvent for BHT. The mother liquor after crystallization filtration and the ethanol discharged from the solvent stripping tower T2 are mixed in a mixer and then sent to the solvent removal tower T1 for rectification. The refined ethanol with a purity greater than 99.9% is taken out from the top of the tower and returned to the crystallization section for reuse. The temperature at the bottom of tower T1 is controlled at 130 °C. At this time, the bottom discharge liquid contains about 3.7% ethanol. After being pressurized by pump P1, it is fed into the stripping tower T2 from the upper part. The recycled nitrogen is pressurized by a compressor and then preheated to 120 °C by a heater and fed into the bottom of T2. The ratio of the recycled gas flow rate to the liquid-phase feed volume flow rate is controlled at 1300, and the ethanol content in the bottom discharge of T2 is 0.06%.

[0030] The above-described embodiments are only the preferred embodiments of the present invention, rather than all the feasible embodiments of the present invention. For those of ordinary skill in the art, any obvious changes made without departing from the principle and spirit of the present invention should be considered to be included within the scope of protection of the claims of the present invention.

Claims

1. A BHT crystallization solvent recovery method, characterized in that: The steps are as follows: the filtered mother liquor (S1) after crystallization filtration and the pressurized solvent stripping tower top condensate (S8) removed from the top of the solvent stripping tower (T2) are mixed by a liquid mixer (M1) and then sent to the desolventizing tower (T1) for rectification; the refined solvent (S3) is collected from the top of the desolventizing tower and returned to the crystallization section for reuse; the crude BHT solution (S6) containing a small amount of solvent is collected from the bottom of the desolventizing tower (T1) and is pressurized by a desolventizing tower bottom collection pump (P1) and then sent to the solvent stripping tower (T2) from the top; the circulating nitrogen condensed at the top of the solvent stripping tower (T2) is pressurized into compressed gas (S10) by a circulating gas compressor (C1); the compressed gas (S 10) It is then preheated by a circulating gas heater (E1) to become the solvent stripping tower inlet gas (S11), and fed into the bottom of the solvent stripping tower (T2). The crude BHT liquid (S6) after the solvent is removed is extracted from the bottom of the solvent stripping tower (T2) and returned to the BHT distillation section for refining. The circulating gas with the solvent is condensed from the top of the tower by a condenser, and the condensate (S7) at the top of the solvent stripping tower is pressurized by a solvent stripping tower top extraction pump (P2) and mixed with the filtered mother liquor (S1) and fed into the desolventizing tower (T1) for distillation. The gas phase condensed at the top of the solvent stripping tower (T2) is pressurized by a circulating gas compressor (C1) and returned to the solvent stripping tower (T2) as circulating gas.

2. The BHT crystallization solvent recovery method according to claim 1, characterized in that: The solvent includes: methanol, ethanol, methanol aqueous solution with a concentration of more than 95%, and ethanol aqueous solution with a concentration of more than 95%.

3. The BHT crystallization solvent recovery method according to claim 1, characterized in that: The mass fraction of solvent in the bottom material of the desolventizing tower (T1) is 1-5%, the mass fraction of phenol in the refined solvent (S3) extracted from the top of the desolventizing tower is less than 0.1%, and the operating pressure is normal pressure.

4. The BHT crystallization solvent recovery method according to claim 1, characterized in that: The solvent stripping tower (T2) removes the solvent, and the solvent mass fraction in the crude BHT liquid (S4) is less than 0.1%, and the operating pressure is normal pressure.

5. The BHT crystallization solvent recovery method according to claim 1, characterized in that: The circulating gas is nitrogen, the ratio of the circulating gas flow rate to the volume flow rate of the produced liquid at the bottom of the desolventizing tower is 200-2000, and the circulating gas preheating temperature is 80-150°C.

6. The BHT crystallization solvent recovery method according to claim 1, characterized in that: The top condensation temperature of the solvent stripping tower (T2) is 2-10°C.

7. The system used in the BHT crystallization solvent recovery method according to claim 1, characterized in that: The liquid mixer (M1) is connected to the desolventizing tower (T1), the bottom of the desolventizing tower (T1) is connected to a desolventizing tower bottom extraction pump (P1), the desolventizing tower bottom extraction pump (P1) is further connected to a solvent stripping tower (T2), the top of the solvent stripping tower (T2) is connected to a circulating gas compressor (C1), the circulating gas compressor (C1) is further connected to a circulating gas heater (E1), the circulating gas heater (E1) is further connected back to the solvent stripping tower (T2), the top of the solvent stripping tower (T2) is connected to a solvent stripping tower top extraction pump (P2), the solvent stripping tower top extraction pump (P2) is further connected to the liquid mixer (M1), the liquid mixer (M1) is also provided with a filtered mother liquor inlet, the bottom of the solvent stripping tower (T2) is provided with a solvent stripping tower bottom extraction liquid outlet, and the top of the desolventizing tower (T1) is provided with a desolventizing tower top extraction refined solvent outlet.

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