A system and method for treating a polyester process off-gas

Abstract

This invention proposes a system and method for treating polyester process tail gas, belonging to the field of tail gas treatment technology. It includes a condensate circulation pump, a condensate cooler, a jet pump, a condensate recovery tank, a raw wastewater tank, an electromagnetic vent valve, a vent flame arrester, a pre-flame arrester, a Roots blower, a post-flame arrester, an electromagnetic shut-off valve, a furnace-front flame arrester, and a raw heat medium furnace. The jet pump is placed in the condensate recovery tank. The condensate cooler is connected to a cooling water inlet and outlet, and its other end is connected to the jet pump. The condensate circulation pump is positioned between the condensate cooler and the condensate recovery tank. The top discharge port of the condensate recovery tank is connected to a three-way pipe. One pipe connects sequentially to the pre-flame arrester, the Roots blower, the post-flame arrester, the electromagnetic shut-off valve, the furnace-front flame arrester, and the raw heat medium furnace. The other pipe connects to the electromagnetic vent valve and the vent flame arrester, and is open to the atmosphere. This system enables low-cost, safe, environmentally friendly, and energy-saving treatment of tail gas generated during polyester production under various pressure conditions.

Description

Technical Field

[0001] This invention relates to the field of exhaust gas treatment technology, and specifically to a system and method for treating exhaust gas from polyester processes. Background Technology

[0002] During the production process of polyester manufacturing plants, the esterification side reaction results in the presence of volatile pollutants such as acetaldehyde in the esterification process tail gas and wastewater. This pollutes the on-site environment and adversely affects the physical and mental health of employees. Therefore, with continuous technological advancements, the process tail gas and wastewater generated by polyester manufacturing plants have gradually shifted from direct discharge to centralized collection and treatment.

[0003] Patent CN202229196U discloses a safe incineration system for polyester waste gas, comprising a waste gas collection pipe. The waste gas collection pipe is sequentially connected to a fan, a first solenoid valve, a ball valve, and a second solenoid valve via pipelines. The second solenoid valve is connected to a heat transfer fluid furnace and is connected to a safety interlock control circuit via wiring. While this patent can be used for the safe incineration of polyester waste gas, it is limited to the treatment of fugitive emissions and cannot be used for the treatment of pressurized exhaust gases such as those from esterification processes.

[0004] Patent CN202440456U discloses a device for treating tail gas during polyester or copolyester production. This device includes a water jet pump, a tail gas discharge main pipe or tail gas buffer tank, a spray absorption tower, a water circulation system, and an automatic discharge system for tetrahydrofuran recovery liquid. The water circulation system includes a circulating water storage tank, a circulating water pump, and a heat exchanger, all connected sequentially. Pipelines, after passing through the heat exchanger, connect to the top of the spray absorption tower and the water jet pump. The top of the circulating water storage tank has a tail gas pipeline connected to the lower part of the packing layer of the spray absorption tower, and the circulating water storage tank is connected to a fresh water pipeline. The water jet pump is connected to the circulating water storage tank via a tailpipe. The tail gas buffer tank is connected to the spray absorption tower. While this patent can be used for tail gas treatment during polyester or copolyester production, it is limited to treating tail gas containing tetrahydrofuran in fugitive emissions and is not suitable for tail gas treatment with pressures above 20 kPa, such as tail gas from esterification processes. Furthermore, it has high construction costs.

[0005] Patent CN101576097 discloses a method and system for treating process tail gas (containing acetaldehyde, abbreviated as EA) from a polyester (PET) plant using a jet ejector. This method is used to treat organized emissions of process tail gas from polyester plants. The method employs a jet ejector to transport the process tail gas. The collected tail gas is connected to the jet ejector's inlet, and the tail gas is drawn into the jet stream of the ejector and sent to an incinerator for incineration. The system includes a jet ejector and an incinerator, using compressed air or pressurized steam as the jet source. The pressure at the tail gas inlet is stabilized by controlling the jet medium flow rate, thus allowing the jet ejector's delivery rate to change with the tail gas volume. While this patent can be used to treat tail gas from polyester plants, it lacks a safety design for the incinerator and requires compressed air or pressurized steam as power, resulting in high energy consumption and potential safety hazards.

[0006] In summary, existing polyester plant tail gas treatment technologies have drawbacks such as narrow applicability, insufficient safety design leading to safety hazards, or high construction costs. Summary of the Invention

[0007] The purpose of this invention is to propose a treatment system and method for polyester process tail gas. The treatment system for polyester process tail gas can rely on existing facilities such as heat transfer furnaces in the original production unit to recover and utilize substances such as acetaldehyde in the exhaust tail gas, eliminating the need for investment in the construction of an acetaldehyde stripping and distillation recovery system, and achieving low-cost, safe, environmentally friendly, and energy-saving treatment of tail gas generated during polyester production under various pressure conditions.

[0008] The technical solution of this invention is implemented as follows:

[0009] This invention provides a treatment system for polyester process tail gas. The system includes a condensate circulation pump, a condensate cooler, a jet pump, a condensate recovery tank, a raw wastewater tank, an electromagnetic vent valve, a vent flame arrester, a front flame arrester, a Roots blower, a rear flame arrester, an electromagnetic shut-off valve, a furnace front flame arrester, and a raw heat medium furnace. The jet pump is placed in the condensate recovery tank. The condensate cooler is connected to a cooling water inlet and a cooling water outlet, and its other end is connected to the jet pump. The condensate circulation pump is located between the condensate cooler and the condensate recovery tank. The discharge port at the top of the condensate recovery tank is connected to a three-way pipe. One pipe is connected sequentially to the front flame arrester, the Roots blower, the rear flame arrester, the electromagnetic shut-off valve, the furnace front flame arrester, and the raw heat medium furnace. The other pipe is connected to the electromagnetic vent valve and the vent flame arrester and is open to the atmosphere.

[0010] As a further improvement of the present invention, the opening and closing states of the electromagnetic vent valve and the electromagnetic shut-off valve are interlocked with the operating states of the Roots blower and the original heat medium furnace for safety control. Under normal operating conditions, the electromagnetic vent valve is in the closed state and the electromagnetic shut-off valve is in the open state; when the Roots blower or the original heat medium furnace is operating abnormally or stops, the electromagnetic vent valve automatically opens and the electromagnetic shut-off valve automatically closes.

[0011] As a further improvement of the present invention, the interlocking control adopts one of PLC or DCS controller.

[0012] As a further improvement of the present invention, the rotational speed of the Roots blower is controlled by connecting it in series with the pressure indication signal before the Roots blower inlet, and the pressure control range before the Roots blower inlet is gauge pressure -0.05kPa to -5.0kPa.

[0013] As a further improvement of the present invention, the pressure series control adopts one of PLC or DCS controller.

[0014] Furthermore, the opening and closing states of the electromagnetic vent valve and electromagnetic shut-off valve are interlocked with the operating signals of the Roots blower and the original heat medium furnace for safety control. When the operating signals of the Roots blower and the original heat medium furnace are normal, the electromagnetic vent valve is in the closed state and the electromagnetic shut-off valve is in the open state. When any operating signal of the Roots blower or the original heat medium furnace stops or malfunctions, the electromagnetic vent valve automatically opens and the electromagnetic shut-off valve automatically closes. The interlock control adopts one of PLC or DCS controllers. The electromagnetic vent valve and electromagnetic shut-off valve use 0.3MPa instrument air as the gas source. When the instrument air supply or power is interrupted, the electromagnetic vent valve automatically opens and the electromagnetic shut-off valve automatically closes to prevent combustible gas from entering the original heat medium furnace when it stops operating, thereby ensuring operational safety.

[0015] As a further improvement of the present invention, the top of the condensate recovery tank is provided with an industrial water pipe and a valve for replenishing the condensate recovery tank with industrial water.

[0016] Furthermore, the condensate recovery tank is equipped with one or both of a magnetic float level gauge and a differential pressure level gauge.

[0017] As a further improvement of the present invention, the condensate recovery tank is connected to the original wastewater tank through a side overflow pipe and a top connecting pipe, and the bottom of the condensate recovery tank is installed at a height higher than the top of the original wastewater tank.

[0018] Furthermore, the condensate recovery tank is connected to the original wastewater tank via a side overflow pipe and a top connecting pipe, respectively. The bottom of the condensate recovery tank is installed at a height higher than the top of the original wastewater tank. The condensate recovery tank has a height of 2.0–4.0m and a diameter of 1.0–2.5m. The side overflow pipe has a height of 1.4–3.4m, and the height of the side overflow pipe is ≥0.5m from the top of the condensate recovery tank. After continuously absorbing the esterification process tail gas sprayed by the jet pump, the liquid level in the condensate recovery tank continuously rises. Once the condensate level is higher than the height of the side overflow pipe, the condensate automatically overflows into the original wastewater tank.

[0019] Furthermore, the original wastewater pool is the original wastewater collection pool of the polyester production unit, and the top of the wastewater pool is covered and sealed; the top of the original wastewater pool is equipped with pipes that connect to the original vacuum pump exhaust port and the roof unorganized exhaust port of the polyester production unit, and each exhaust outlet is equipped with a flame arrester and a shut-off valve on the pipes connecting to the top of the wastewater pool; the original wastewater pool of the polyester production unit is used to collect esterification wastewater and temporary production wastewater, and is equipped with a wastewater pump to send the wastewater to the downstream sewage treatment system for further treatment.

[0020] Furthermore, the condensate cooler is one of a plate heat exchanger or a tubular heat exchanger, and uses one of circulating cooling water or circulating chilled water to cool the condensate in the condensate recovery tank. The condensate temperature is controlled within the range of 21 to 70°C.

[0021] Furthermore, the Roots blower is an explosion-proof device, and a one-way valve is installed on the outlet pipe of the Roots blower; between the Roots blower and the original heat medium furnace connecting pipe, two flame arresters, a rear flame arrester and a furnace front flame arrester, are installed to prevent the flue gas from the original heat medium furnace from flowing back into the closed pipe and causing a flash explosion or combustion hazard.

[0022] Furthermore, the top of the condensate recovery tank is equipped with an industrial water pipe and valve, which are used to replenish the condensate recovery tank with industrial water when the liquid level in the condensate recovery tank is too low.

[0023] This invention further protects a method for treating polyester process tail gas, which uses the above-described polyester process tail gas treatment system. The process tail gas enters the jet pump through the discharge pipe, and after being sprayed and absorbed by the jet pump, it is collected with the volatile tail gas from the original wastewater tank. A certain negative pressure is generated by the Roots blower to send the tail gas with the easily volatile components as auxiliary fuel into the original heat medium furnace for incineration.

[0024] As a further improvement of the present invention, the process tail gas is generated by the esterification process tower of the polyester production unit, and the gauge pressure of the esterification process tail gas is ≥20kPa.

[0025] The present invention has the following beneficial effects:

[0026] (1) The present invention treats the pressurized process tail gas generated by the esterification process tower of the polyester production unit by spraying and absorbing it with the jet pump, and then collects it with the original vacuum pump tail gas connected to the original wastewater pool, the roof unorganized emission tail gas and other atmospheric pressure process tail gas. The tail gas containing acetaldehyde and other volatile components is drawn into the original heat medium furnace for combustion as auxiliary fuel. This invention realizes the treatment of tail gas generated in the polyester production process under various pressure conditions and has the advantage of wide applicability.

[0027] (2) The present invention uses a Roots blower to generate a certain negative pressure to treat volatile components such as acetaldehyde in process tail gas and esterification wastewater. This can save the investment and construction of systems such as acetaldehyde stripping and distillation recovery, and has the advantages of low investment cost, energy saving and good operating economy.

[0028] (3) The present invention is equipped with flame arresters before and after the Roots blower, after the electromagnetic vent valve, and before the original heat medium furnace. The opening and closing states of the electromagnetic vent valve and the electromagnetic shut-off valve are interlocked with the operating states of the Roots blower and the original heat medium furnace for safety control, which can effectively ensure the safe operation of the polyester process tail gas treatment system and has the advantage of high safety performance. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a schematic diagram of a polyester process tail gas treatment system according to the present invention.

[0031] Among them, 1. Condensate circulation pump, 2. Condensate cooler, 3. Jet pump, 4. Condensate recovery tank, 5. Original wastewater pool, 6. Electromagnetic vent valve, 7. Vent flame arrester, 8. Front flame arrester, 9. Roots blower, 10. Rear flame arrester, 11. Electromagnetic shut-off valve, 12. Furnace front flame arrester, and 13. Original heat medium furnace. Detailed Implementation

[0032] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] Taking the production process of polyethylene terephthalate (PET) as an example, the common production processes are currently divided into three-reactor and five-reactor processes. The three-reactor process includes three reactors: an esterification reactor, a prepolymerization reactor, and a final polymerization reactor. The esterification process exhaust gas is generated by the esterification reactor, and its pressure is generally above 50 kPa (gauge pressure). Other atmospheric pressure exhaust gases are generated by a vacuum pump, an ethylene glycol hot well, a wastewater tank, and a material preparation tank. The five-reactor process includes five reactors: a first esterification reactor, a second esterification reactor, a first prepolymerization reactor, a second prepolymerization reactor, and a final polymerization reactor. The esterification process exhaust gas generated by the first esterification reactor generally has a pressure above 40 kPa (gauge pressure), and is a pressurized process exhaust gas. Other atmospheric pressure exhaust gases are generated by the second esterification reactor, a vacuum pump, an ethylene glycol hot well, a wastewater tank, and a material preparation tank.

[0034] Example 1

[0035] This embodiment provides a treatment system for polyester process tail gas, such as... Figure 1 As shown, the system includes a condensate circulation pump 1, a condensate cooler 2, a jet pump 3, a condensate recovery tank 4, a raw wastewater tank 5, an electromagnetic vent valve 6, a vent flame arrester 7, a front flame arrester 8, a Roots blower 9, a rear flame arrester 10, an electromagnetic shut-off valve 11, a furnace front flame arrester 12, and a raw heat medium furnace 13. The jet pump 3 is placed in the condensate recovery tank 4. The condensate cooler 2 is connected to the cooling water inlet and outlet, and the other end is connected to the jet pump 3. The condensate circulation pump 1 is located between the condensate cooler 2 and the condensate recovery tank 4. The discharge port at the top of the condensate recovery tank 4 is connected to a three-way pipe. One pipe is connected in sequence to the front flame arrester 8, the Roots blower 9, the rear flame arrester 10, the electromagnetic shut-off valve 11, the furnace front flame arrester 12, and the raw heat medium furnace 13. The other pipe is connected to the electromagnetic vent valve 6 and the vent flame arrester 7 and is open to the atmosphere.

[0036] For the three-reactor process, during operation, the condensate in the condensate recovery tank 4 is transported to the condensate cooler 2 for cooling by the condensate circulation pump 1, and then enters the jet pump 3. After spraying and cooling the pressurized tail gas A (pressurized process tail gas generated by the esterification process tower, with a gauge pressure of 70±5kPa), it is circulated back to the condensate recovery tank 4. Under its own pressure, the pressurized tail gas A enters the jet pump 3 from the process tail gas outlet of the esterification reactor.

[0037] The condensate cooler 2 uses circulating cooling water for cooling, and controls the temperature range of the condensate in the condensate recovery tank 4 to be within 40±5℃.

[0038] The condensate recovery tank 4 has a height of 2.5m and a diameter of 2.0m. The side overflow pipe is 1.8m high and is connected to the top of the original wastewater tank 5. After the condensate recovery tank 4 continuously absorbs the esterification process tail gas sprayed by the jet pump 3, the liquid level continuously rises. When the condensate liquid level is higher than the height of the side overflow pipe, the condensate automatically overflows into the original wastewater tank 5.

[0039] The condensate recovery tank 4 is equipped with a local magnetic level gauge and a remote differential pressure level gauge, with a level gauge range of 2.0m.

[0040] The top of the condensate recovery tank 4 is equipped with an industrial water pipe and valve, which are used to replenish the condensate recovery tank with industrial water when the liquid level in the condensate recovery tank 4 is too low, so as to prevent the condensate circulation pump 1 from being damaged by dry running.

[0041] The atmospheric pressure exhaust gas B comes from the exhaust gas emitted by the vacuum pump, ethylene glycol hot well, wastewater pool, material preparation tank, etc. Under the suction action of the Roots blower 9, it passes through the flame arrester and is collected in the original wastewater pool 5.

[0042] The exhaust gas from the condensate recovery tank 4 and the original wastewater pool 5 is collected under the negative pressure generated by the Roots blower 9 and then sequentially passes through the front flame arrester 8, the Roots blower 9, the rear flame arrester 10, the electromagnetic shut-off valve 11, and the furnace front flame arrester 12. It is then sent as auxiliary fuel into the original heat medium furnace 13 for incineration, thereby achieving the treatment of exhaust gas containing volatile components such as acetaldehyde and the recovery and utilization of energy.

[0043] The opening and closing states of the electromagnetic vent valve 6 and electromagnetic shut-off valve 11 are interlocked with the operating signals of the Roots blower 9 and the original heat medium furnace 13 for safety control. When the operating signals of the Roots blower 9 and the original heat medium furnace 13 are normal, the electromagnetic vent valve 6 is in the closed state, and the electromagnetic shut-off valve 11 is in the open state. When either the Roots blower 9 or the original heat medium furnace 13 experiences a stop or fault signal, the electromagnetic vent valve 6 automatically opens, and the electromagnetic shut-off valve 11 automatically closes. The interlock control is implemented using a DCS system. The electromagnetic vent valve 6 and electromagnetic shut-off valve 11 use 0.3MPa instrument air as their gas source. When the instrument air supply or power supply is interrupted, the electromagnetic vent valve 6 automatically opens, and the electromagnetic shut-off valve 11 automatically closes, preventing combustible gas from entering the original heat medium furnace 13 when it is not in operation, thus ensuring operational safety.

[0044] When the electromagnetic vent valve 6 is in the open state and the electromagnetic shut-off valve 11 is in the closed state, the exhaust gas in the condensate recovery tank and the 5-original wastewater pool is connected to the atmosphere after passing through the electromagnetic vent valve 6 and the vent flame arrester 7 in sequence, to prevent pressure buildup in the condensate recovery tank and the 5-original wastewater pool.

[0045] The rotational speed of the Roots blower 9 is controlled by a series connection with the pressure indication signal before the inlet of the Roots blower 9. The set pressure control range before the inlet of the Roots blower 9 is gauge pressure -0.05kPa to -3.0kPa, and the pressure series control adopts DCS control.

[0046] The bypass pipeline at the outlet of the Roots blower 9 is equipped with a safety relief valve. The outlet of the safety relief valve is connected to the pipeline between the electromagnetic vent valve 6 and the vent flame arrester 7. The opening pressure of the safety relief valve is set to 20 kPa. When the outlet pressure of the Roots blower 9 is higher than 20 kPa, the safety relief valve opens, and the exhaust gas at the outlet of the Roots blower 9 passes through the vent flame arrester 7 and is discharged into the atmosphere to protect the Roots blower 9 and prevent overpressure damage to the Roots blower 9.

[0047] The Roots blower 9 is an explosion-proof device, and a one-way valve is installed on the outlet pipe of the Roots blower 9.

[0048] Example 2

[0049] This embodiment provides a treatment system for polyester process tail gas, such as... Figure 1 As shown, for the five-reactor process, during operation, pressurized tail gas A comes from the process tail gas generated by the first esterification reactor process tower; atmospheric tail gas B comes from the process tail gas generated by the second esterification reactor process tower, as well as the tail gas from the original vacuum pump, ethylene glycol hot well, wastewater pool, material preparation tank, and fugitive emissions from the roof; except for the different sources of the above process tail gases, the rest of the process flow and working principle of this embodiment are the same as those of Example 1.

[0050] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A treatment system for polyester process tail gas, characterized in that, The system includes a condensate circulation pump, a condensate cooler, a jet pump, a condensate recovery tank, a raw wastewater pool, an electromagnetic vent valve, a vent flame arrester, a front flame arrester, a Roots blower, a rear flame arrester, an electromagnetic shut-off valve, a furnace front flame arrester, and the original heat medium furnace. The jet pump is placed in the condensate recovery tank. The condensate cooler is connected to the cooling water inlet and outlet, and the other end is connected to the jet pump. The condensate circulation pump is located between the condensate cooler and the condensate recovery tank. The discharge port at the top of the condensate recovery tank is connected to a three-way pipe. One pipe is connected in sequence to the front flame arrester, the Roots blower, the rear flame arrester, the electromagnetic shut-off valve, the furnace front flame arrester, and the original heat medium furnace. The other pipe is connected to the electromagnetic vent valve and the vent flame arrester and is open to the atmosphere. The rotational speed of the Roots blower is controlled by connecting it in series with the pressure indication signal before the Roots blower inlet. The pressure control range before the Roots blower inlet is gauge pressure -0.05 kPa to -5.0 kPa. The condensate recovery tank is connected to the original wastewater tank through a side overflow pipe and a top connecting pipe, and the bottom of the condensate recovery tank is installed at a height higher than the top of the original wastewater tank. The process tail gas is generated by the esterification process tower of the polyester production unit, and the gauge pressure of the esterification process tail gas is ≥20kPa.

2. The polyester process tail gas treatment system according to claim 1, characterized in that, The opening and closing states of the electromagnetic vent valve and electromagnetic shut-off valve are interlocked with the operating states of the Roots blower and the original heat medium furnace for safety control. Under normal operating conditions, the electromagnetic vent valve is in the closed state and the electromagnetic shut-off valve is in the open state. When the Roots blower or the original heat medium furnace is operating abnormally or stops, the electromagnetic vent valve automatically opens and the electromagnetic shut-off valve automatically closes.

3. The polyester process tail gas treatment system according to claim 2, characterized in that, The interlocking control uses either a PLC or a DCS controller.

4. The polyester process tail gas treatment system according to claim 1, characterized in that, The pressure series control uses either a PLC or a DCS controller.

5. The polyester process tail gas treatment system according to claim 1, characterized in that, The top of the condensate recovery tank is equipped with industrial water pipes and valves for replenishing industrial water to the condensate recovery tank.

6. The polyester process tail gas treatment system according to claim 1, characterized in that, The original wastewater pool was the original wastewater collection pool of the polyester production unit, and the top of the wastewater pool was covered and sealed; the top of the original wastewater pool was equipped with a pipe connected to the original vacuum pump exhaust port and the roof unorganized exhaust port of the polyester production unit.

7. A method for treating tail gas from a polyester process, characterized in that, The process tail gas of polyester is treated using the treatment system described in any one of claims 1-6. The process tail gas enters the jet pump through the discharge pipe. After being sprayed and absorbed by the jet pump, it is collected with the volatile tail gas from the original wastewater tank. A certain negative pressure is generated by the Roots blower to send the tail gas of the easily volatile components as auxiliary fuel into the original heat medium furnace for incineration.

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