Automatic control method for isocyanate distillation column gas phase extraction and prepared isocyanate

Abstract

The application provides an automatic control method for gas phase extraction of an isocyanate rectification tower and prepared isocyanate, and the automatic control method is characterized in that an equalizing pressure regulating valve is arranged on an equalizing pressure pipeline connected with a vacuum system of a heat exchanger for side line extraction of the isocyanate rectification tower; an inert gas supplement line is arranged on the equalizing pressure pipeline between the heat exchanger and the equalizing pressure regulating valve, and an inert gas flow regulating valve is arranged on the inert gas supplement line; the method builds a single loop for controlling the liquid level of the heat exchanger through the opening degree of the equalizing pressure regulating valve and the opening degree of the inert gas flow regulating valve, and switches the two control loops through a logic controller. The method of the application replaces the control of the flow of the side line extraction of the rectification tower by a traditional butterfly valve, realizes the stable control and rapid response of the flow of the side line extraction of the rectification tower and the liquid level of the heat exchanger, and completely solves the problems of oxidation and coking of the liquid accumulated on the back of the valve plate of the butterfly valve, the color number rising of the product, the jamming of the valve of the equalizing pressure pipeline, the blockage of the pipeline and the corrosion and leakage problems.

Description

Technical Field

[0001] This invention relates to the technical field of isocyanates, and more specifically to an automatic control method for the vapor phase extraction of a distillation column. Background Technology

[0002] Isocyanates are an important raw material for the preparation of polyurethane materials. The mainstream process at present is the phosgenation process. Its industrial production process is as follows: First, the amine corresponding to the isocyanate is prepared, and then crude isocyanate is prepared through phosgenation reaction. Crude isocyanate includes a variety of isomers and polycyclic heavy components. Then, it is purified by distillation to obtain products with different compositions and contents of specific isomers.

[0003] Separating isocyanate isomers via distillation columns is a well-known industrial method. Distillation columns are often side-stream or partitioned-wall columns to achieve the goal of obtaining products with multiple isomer contents from a single column, while also saving energy. For example:

[0004] Patent CN101003497A discloses a distillation method for a mixture of diphenylmethane diisocyanates. The raw material mixture of diphenylmethane diisocyanates is added to a distillation column, and two streams are collected from the side stream, each containing 50-60 wt% 2,4-MDI, 40-50 wt% 4,4-MDI, and less than 1.5 wt% 2,2-MDI, and the other containing less than 1 wt% 2,4-MDI and more than 99 wt% 4,4-MDI. This method can reduce equipment investment and operating costs.

[0005] Patent CN1810776A discloses a distillation method for a mixture of isomers of diphenylmethane diisocyanate, which separates the MDI bicyclic mixture by means of a partition wall column, wherein the side stream of the main fractionation zone in the separation wall region yields an isocyanate product with a very low 2,2-MDI content.

[0006] Patent CN103313967A discloses a method for purifying a mixture of MDI isomers by distillation. The method involves collecting a stream with a high 4,4-MDI content from the side stream of the distillation column. It specifically points out that in order to reduce the content of dimer (ureidone) in the isocyanate product, the product must be collected in gaseous form and then rapidly cooled to 20-60°C within a maximum of 5 seconds to improve the storage stability of the product.

[0007] Since isocyanates can polymerize into a large number of dimers (ureidone) in a short time when they exist in liquid form at high temperatures, the solubility of the dimers decreases after cooling, which will cause precipitation and make the product turbid or significantly shorten the shelf life. Therefore, the side stream of the distillation column must be directly collected in gaseous form and cooled rapidly in the heat exchanger.

[0008] To maintain stable operation of the distillation column, the vapor phase outflow rate must be controlled. The most common control method is to install a valve on the vapor phase outflow pipeline between the distillation column and the side-stream heat exchanger. However, due to the large volumetric flow rate of the stream under vacuum conditions and the large diameter of the outflow pipeline (usually greater than 1m), butterfly valves are the most commonly used large-diameter regulating valves. Patent CN111848455A discloses an automatic control method for the vapor phase outflow rate of an isocyanate distillation column. This invention points out that in actual operation, a large amount of reddish-brown solid coking material accumulates on the back of the butterfly valve plate (near the heat exchanger side) over time, causing slow valve opening and closing, jamming, and an increase in product color grade, affecting the long-term stable operation of the unit. Therefore, this invention uses dual control of a large butterfly valve on the vapor phase pipeline and a regulating valve on the equalizing pipeline to control the vapor phase outflow rate while maximizing the butterfly valve opening and minimizing the frequency of butterfly valve operation, thus alleviating a series of problems caused by butterfly valves.

[0009] The flow rate of the isocyanate distillation column's side-stream vapor phase can be controlled collaboratively by the vapor phase pipeline butterfly valve and the equalization pipeline regulating valve. However, this invention reveals that when the side-stream flow rate is significantly adjusted, the liquid level in the side-stream heat exchanger fluctuates greatly, and the butterfly valve still requires frequent operation. The problem of liquid accumulation, oxidation, and coking on the back of the butterfly valve plate, leading to an increase in color grade, persists. Furthermore, to prevent isocyanate crystallization from the side-stream, the cooling medium temperature of the side-stream heat exchanger should not be too low, causing light component impurities such as hydrogen chloride, as well as some isocyanate, to enter the equalization pipeline. This condenses and adheres to the pipes and valves of the equalization pipeline, leading to valve jamming and pipe blockage over long-term operation. Simultaneously, due to the high acid content, corrosion leaks may also occur. Based on the operation of existing equipment, the total frequency of valve jamming, pipe blockage, and corrosion leaks in the equalization pipeline exceeds 60 times per year. Existing technologies do not address this problem or provide corresponding solutions. Summary of the Invention

[0010] To overcome the shortcomings of existing technologies, this invention provides an automatic control method for the gas phase extraction of isocyanate distillation column and the prepared isocyanate, which replaces the traditional butterfly valve for controlling the side stream extraction flow of distillation column, realizes stable control and rapid response of the side stream extraction flow of distillation column and the liquid level of heat exchanger, and completely solves the problem of product color increase caused by liquid accumulation, oxidation and coking on the back of butterfly valve plate, as well as the problems of valve jamming, pipeline blockage and corrosion leakage in pressure equalization pipeline.

[0011] To achieve the above objectives, the present invention adopts the following technical solution:

[0012] The present invention provides an automatic control method for the vapor phase extraction of an isocyanate distillation column in a first aspect. The automatic control method involves installing a pressure equalization regulating valve on the pressure equalization pipeline connecting the heat exchanger and the vacuum system at the side extraction point of the isocyanate distillation column; an inert gas replenishment line is installed on the pressure equalization pipeline between the heat exchanger and the pressure equalization regulating valve; and an inert gas flow regulating valve is installed on the inert gas replenishment line.

[0013] The automatic control method establishes a single loop to control the liquid level of the heat exchanger by adjusting the opening of the equalizing pressure regulating valve and the opening of the inert gas flow regulating valve, and switches between the two control loops by a logic controller.

[0014] Set the lower limit VL and upper limit VH of the opening of the equalizing pressure regulating valve with regulating capability, and set the lower limit LL and upper limit LH of the safe liquid level of the heat exchanger.

[0015] Set the initial opening of the inert gas regulating valve, and use the opening of the equalizing pressure regulating valve to control the heat exchanger liquid level, so that the opening of the equalizing pressure regulating valve is within the range of VL-VH and the heat exchanger liquid level is within the range of LL-LH; when the opening of the equalizing pressure regulating valve is lower than the lower limit of opening VL or higher than the upper limit of opening VH, or when the liquid level of the heat exchanger is lower than the lower safety limit LL or higher than the upper safety limit LH, switch to single-loop control of the heat exchanger liquid level by controlling the opening of the inert gas flow regulating valve; when the heat exchanger liquid level is controlled within the range of LL-LH and the opening of the equalizing pressure regulating valve returns to VL-VH, switch back to single-loop control of the equalizing pressure regulating valve opening and the heat exchanger liquid level.

[0016] In some specific embodiments, the lower limit of the opening of the pressure equalization regulating valve, VL, is 1-40%, for example, 5% or 35%; preferably 10-30%, for example, 10% or 20%; and the upper limit of the opening of the pressure equalization regulating valve, VH, is 60-99%, for example, 65% or 95%; preferably 70-90%, for example, 80% or 85%.

[0017] In some specific embodiments, the lower limit LL of the heat exchanger liquid level is 1-30%, for example, 5%, 25%; preferably 10-20%, for example, 15%, 18%; and the upper limit LH of the heat exchanger liquid level is 30-60%, for example, 35%, 55%; preferably 40-50%, for example, 45%, 47%.

[0018] In a specific embodiment of the automatic control method of the present invention, the initial opening degree of the inert gas flow regulating valve is set to 10-40%, for example, 15% or 35%; preferably 20-30%, for example, 25%.

[0019] In some specific embodiments, the inert gas supply line is used to supply inert gas to the equalizing pipeline, wherein the inert gas is nitrogen and / or carbon dioxide, preferably nitrogen; the pressure of the inert gas is 0.01-0.4 MPaG, preferably 0.1-0.2 MPaG; and the diameter of the inert gas flow regulating valve is DN6-30, preferably DN15-20.

[0020] In a specific embodiment of the automatic control method of the present invention, at least one side stream of the isocyanate distillation column is extracted in gaseous form. After the gaseous phase extracted from the side stream is heated by a heat exchanger, condensate and non-condensable gas are obtained. The condensate is extracted from the bottom of the heat exchanger, and the non-condensable gas is transported to the vacuum system through a pressure equalization pipeline.

[0021] In some specific embodiments, the feed location of the isocyanate distillation column is the bottom, middle or top of the column, preferably the middle of the isocyanate distillation column.

[0022] In some specific embodiments, the feed to the isocyanate distillation column is an isocyanate prepared by liquid-phase phosgene or gas-phase phosgene; the isocyanate may be specifically selected from diphenylmethane diisocyanate, hexamethylene diisocyanate, toluene diisocyanate, xylene diisocyanate, hexamethylene diisocyanate, dicyclohexylmethane diisocyanate or isophorone diisocyanate; preferably diphenylmethane diisocyanate.

[0023] The control method of the present invention can control the total frequency of jamming of the equalizing regulating valve, blockage of the equalizing pipeline and corrosion leakage on the side line of the isocyanate distillation column to no more than 2 times / year, preferably no more than 1 time / year.

[0024] In a second aspect, the present invention provides an isocyanate prepared by the above-described automatic control method, wherein the color number of the obtained isocyanate product is <30 Hazen (platinum-cobalt color number), for example, 10 Hazen or 5 Hazen.

[0025] In some specific embodiments, the coking content in the isocyanate is <10ppm, for example, 5ppm or 1ppm.

[0026] The above technical solution achieves the following technical effects:

[0027] The automatic control method provided by this invention replaces the traditional butterfly valve for controlling the produced flow rate, completely solving the problem of product color grade increase caused by liquid accumulation, oxidation, and coking on the back of the butterfly valve plate. This invention adds inert gas to the pressure equalization pipeline between the distillation column side-line heat exchanger and the pressure equalization regulating valve. Through a dual control logic that switches between the inert gas flow regulating valve and the pressure equalization regulating valve, it effectively alleviates the problems of valve jamming, pipeline blockage, and corrosion leakage on the pressure equalization pipeline, achieving stable control and rapid response of the gas phase produced flow rate and heat exchanger liquid level.

[0028] The control method of the present invention is simple and easy to operate. It requires little technical modification to existing devices, requires little investment, and can also extend the operating cycle of the device. Attached Figure Description

[0029] Figure 1 This is the automatic control logic diagram for the vapor phase extraction of the isocyanate distillation column of the present invention;

[0030] Figure 2 This is a specific embodiment of the isocyanate distillation column vapor phase recovery process of the present invention;

[0031] Figure 3 This describes the changes in the flow rate from side line 2 and the fluctuations in the liquid level of the heat exchanger in Example 1.

[0032] Figure 4 This describes the changes in the flow rate from side line 2 and the fluctuations in the liquid level of the heat exchanger in Example 2.

[0033] Figure 5 To compare the changes in the flow rate of side line 2 and the fluctuations in the liquid level of the heat exchanger in Example 1;

[0034] Figure 6 This is the automatic control logic for switching between butterfly valve control and pressure equalization regulating valve control in Comparative Example 2;

[0035] Figure 7 To compare the changes in the flow rate of side line 2 and the fluctuations in the liquid level of the heat exchanger in Example 2;

[0036] Among them, 1. heat exchanger, 2. inert gas flow regulating valve, 3. pressure equalization regulating valve, and 4. isocyanate product collection pump. Detailed Implementation

[0037] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0038] In the following examples, the raw material (distillation column feed) is a mixture of diphenylmethane diisocyanate, which comes from Wanhua Chemical. It is a crude bicyclic intermediate obtained by evaporation and pre-separation of crude MDI after the polyamine photochemical process (formaldehyde and aniline undergo condensation and transposition reaction under acid catalysis to obtain polymethylene polyphenyl polyamine (DAM), DAM reacts with phosgene to obtain crude M, and crude M is pre-separated by evaporation to obtain a mixture of PM and diphenylmethane diisocyanate isomers, referred to as crude bicyclic M).

[0039] Example 1 The main components of the raw material are 7-9% 2,4-MDI, 90.5-92% 4,4-MDI, and 0.2-1.2% 2,2-MDI;

[0040] Example 2 The main components of the raw materials are 0.5-2% 2,2-MDI, 49-74% 2,4-MDI, and 25-50% 4,4-MDI.

[0041] Example 1

[0042] Adopting such Figure 2 The process flow shown involves the distillation of a mixture of diphenylmethane diisocyanate using a multi-side-stream distillation column (top pressure 100-300 Pa, bottom pressure 800-1000 Pa). There is one feed and four outflows. From top to bottom, the four outflows are: top fraction (removed from the vacuum system), side stream 1, side stream 2, and bottom residue. Side streams 1 and 2 are the products, with side stream 1 being the liquid phase, composed of 50-60% 2,4-MDI. The MDI-50 series products, containing 8-48% 4,4-MDI and 1-5% 2,2-MDI, are used in subsequent purification processes and are not directly used as products. Side stream 2 is a gas phase product that directly yields the MDI-100 product, which contains 0-1% 2,4-MDI and 99-100% 4,4-MDI. The pressure at the outlet of side stream 2 is 500-600 Pa, the bottom temperature is 210℃, and the outlet temperature is 200℃.

[0043] like Figure 2 As shown, side line 2 is a gas phase extraction pipeline. A pressure equalization pipeline connected to the vacuum system is installed at the bottom of heat exchanger 1. A DN100 pressure equalization regulating valve 3 is installed on the pressure equalization pipeline. An inert gas replenishment pipeline is installed on the pressure equalization pipeline between heat exchanger 1 and pressure equalization regulating valve 3 on the side line. An inert gas flow regulating valve 2 is installed on the inert gas replenishment pipeline. The liquid phase isocyanate product at the bottom of heat exchanger 1 is extracted by isocyanate product extraction pump 4.

[0044] use Figure 1The automatic control method shown controls the above process flow. First, the initial opening of the inert gas flow regulating valve 2 is set to 25%. Then, the opening of the equalizing pressure regulating valve 3 is used to control the liquid level in the heat exchanger 1 in a single-loop control manner. The opening of the equalizing pressure regulating valve 3 adjusts the liquid level in the heat exchanger 1, ensuring that the opening of the equalizing pressure regulating valve 3 is between 20-80% (i.e., the equalizing pressure regulating valve 3 has regulating capability within this range), and the liquid level in the heat exchanger 1 is between 15-45%. However, when the opening of the equalizing pressure regulating valve V is lower than the lower limit VL (20%) or higher than the upper limit VH (80%), or when the liquid level L in the heat exchanger is lower than the lower safety limit LL (15%) or higher than the upper safety limit LH (45%), the liquid level in the heat exchanger 1 will be controlled according to the following conditions: When the pressure equalization valve L exceeds the regulating capacity of the equalization valve 3, the system switches to single-loop control of the heat exchanger 1 level by controlling the opening of the inert gas flow regulating valve 2. The opening of the inert gas flow regulating valve 2 regulates the heat exchanger 1 level. When the heat exchanger level L is stably controlled within the range of 15-45%, and the opening of the equalization valve V recovers to between 20-80%, the system switches back to single-loop control of the equalization valve 3 opening and the heat exchanger 1 level. The switching logic is as follows: Figure 1 As shown.

[0045] Using the control methods described above, the changes in the outflow rate of side line 2 and the fluctuations in the heat exchanger liquid level are as follows: Figure 3 As shown, the change in the output flow rate has little effect on the liquid level of the heat exchanger. The maximum fluctuation range under the output flow rate of 3000 kg / h is about ±0.5%. When the output flow rate is stable, the fluctuation of the liquid level of the heat exchanger is also very small, with a fluctuation range of about ±0.1%.

[0046] The product index analysis of samples taken from side line 2 is shown in Table 1 below (one sample was taken every 30 days):

[0047] Table 1

[0048]

[0049] Using the above control methods, after one year of operation, the color number of the isocyanate product remained at 5 Hazen, no coking content was detected in the product, and the total frequency of pressure equalization valve jamming, pressure equalization tube blockage, and pressure equalization tube corrosion leakage was 2 times.

[0050] Example 2

[0051] Adopting such Figure 2The schematic diagram of the vapor phase recovery process shown illustrates the distillation of a diphenylmethane diisocyanate mixture using a multi-side-stream recovery distillation column (top pressure 100-300 Pa, bottom pressure 400-900 Pa). There is one feed stream and four recovery streams. From top to bottom, the four recovery streams are: top fraction (removed from the vacuum system), side stream 1, side stream 2, and bottom residue. Side streams 1 and 2 are the products. Side stream 1 is the light component, composed of 10-40% 2,2-MDI, 50-70% 2,4-MDI, and 5-25% 4,4-MDI. Side stream 2 is the vapor phase recovery, directly yielding MDI-50, composed of 0-1% 2,2-MDI, 50-60% 2,4-MDI, and 39.5-49.5% 4,4-MDI. The pressure at the outlet of side line 2 is 300-600 Pa, the temperature at the bottom of the tower is 200℃, and the temperature at the outlet of side line 2 is 190℃.

[0052] like Figure 2 As shown, side line 2 is the gas phase extraction pipeline. A pressure equalization pipeline connecting to the vacuum system is installed at the bottom of heat exchanger 1. A DN100 pressure equalization regulating valve 3 is installed on the pressure equalization pipeline. An inert gas replenishment pipeline is installed on the pressure equalization pipeline between heat exchanger 1 and pressure equalization regulating valve 3. An inert gas flow regulating valve 2 is installed on the inert gas replenishment pipeline. The isocyanate product at the bottom of heat exchanger 2 is extracted via isocyanate product extraction pump 4.

[0053] use Figure 1 The automatic control method shown controls the above process flow. The initial opening of the inert gas flow regulating valve 2 is set to 25%, and the opening of the equalizing pressure regulating valve 3 is used to control the liquid level of the heat exchanger 1 in a single-loop control manner. The opening of the equalizing pressure regulating valve 3 adjusts the liquid level of the heat exchanger 1, keeping the opening of the equalizing pressure regulating valve 3 between 20-80% (i.e., the equalizing pressure regulating valve 3 has regulating capability within this range), and the liquid level of the heat exchanger 1 between 15-45%. However, when the opening of the equalizing pressure regulating valve V is lower than the lower limit VL (20%) or higher than the upper limit VH (80%), or when the liquid level L of the heat exchanger is lower than the lower safety limit LL (15%) or higher than the upper safety limit LH (45%), the liquid level will be controlled according to the following conditions: When the pressure equalization valve L exceeds the regulating capacity of the equalization valve 3, the system switches to single-loop control of the heat exchanger 1 liquid level by controlling the opening of the inert gas flow regulating valve 2. The opening of the inert gas flow regulating valve 2 regulates the heat exchanger 1 liquid level. When the heat exchanger liquid level L is stably controlled within the range of 15-45%, and the opening of the equalization valve V recovers to between 20-80%, the system switches back to single-loop control of the equalization valve 3 opening and the heat exchanger 1 liquid level. The switching logic is as follows: Figure 1 As shown.

[0054] Using the control methods described above, the changes in the outflow rate of side line 2 and the fluctuations in the heat exchanger liquid level are as follows: Figure 4As shown, the change in the output flow rate has little effect on the heat exchanger liquid level. The maximum fluctuation range under the output flow rate of 2000 kg / h is about ±1.7%. When the output flow rate is stable, the fluctuation range of the heat exchanger liquid level is also very small, with a fluctuation range of about ±0.7%.

[0055] The product index analysis of samples taken from side line 2 is shown in Table 2 below (one sample was taken every 30 days):

[0056] Table 2

[0057]

[0058]

[0059] Using the above control methods, after one year of operation, the product color number remained at 5 Hazen, no coking content was detected in the product, and the total frequency of pressure equalization valve jamming, pressure equalization tube blockage, and pressure equalization tube corrosion leakage was 1 time.

[0060] Comparative Example 1

[0061] Diphenylmethane diisocyanate mixtures are distilled using a multi-side-stream distillation column (top pressure 100-300 Pa, bottom pressure 800-1000 Pa). One feed stream and four side streams are used. The feed is a mixture of MDI isomers, consisting of 7-9% 2,4-MDI, 90.5-92% 4,4-MDI, and 0.2-1.2% 2,2-MDI. The four side streams, from top to bottom, are: top fraction (to the vacuum system), side stream 1, and side stream 2. The distillation column contains two side streams: side stream 1 and side stream 2. Side stream 1 is the liquid phase, containing 50-60% 2,4-MDI, 38-48% 4,4-MDI, and 1-5% 2,2-MDI (MDI-50 series), which is used in subsequent purification processes and is not directly used as a product. Side stream 2 is the vapor phase, directly yielding MDI-100, containing 0-1% 2,4-MDI and 99-100% 4,4-MDI. The pressure at the outlet of side stream 2 is 500-600 Pa, the bottom temperature is 210℃, and the outlet temperature is 200℃.

[0062] Side line 2 is the gas phase extraction pipeline. A large-diameter butterfly valve of DN1400 is installed between the isocyanate distillation column and the heat exchanger. A pressure equalization pipeline connected to the vacuum system is installed at the bottom of the heat exchanger. The method to control the extraction flow rate is to establish a single-loop control between the butterfly valve and the heat exchanger liquid level. The liquid level of the heat exchanger is stabilized by controlling the opening degree of the butterfly valve.

[0063] Using this method, the changes in flow rate from side line 2 and the fluctuations in the liquid level of the heat exchanger are as follows: Figure 5As shown, changes in the output flow rate have a significant impact on the heat exchanger level. With an output flow rate of 3000 kg / h, the maximum fluctuation range is approximately ±26.5%. Furthermore, the heat exchanger level takes a long time to stabilize, and even when the output flow rate is stable, the heat exchanger level fluctuates considerably, with a fluctuation range of approximately ±6%.

[0064] The product index analysis for sideline 2 is shown in Table 3 (one sample is taken every 30 days):

[0065] Table 3

[0066]

[0067] Due to the increase in product color and coking content, as well as the increased frequency of pressure equalization pipe blockage, corrosion and leakage, the isocyanate distillation tower had to be shut down for maintenance after only 6 months of operation. The total frequency of pressure equalization pipeline abnormalities in the six months was 31 times.

[0068] Comparative Example 2

[0069] Diphenylmethane diisocyanate mixtures are distilled using a multi-side-stream distillation column (top pressure 100-300 Pa, bottom pressure 800-1000 Pa). One feed stream and four side streams are used. The feed is a mixture of MDI isomers, consisting of 7-9% 2,4-MDI, 90.5-92% 4,4-MDI, and 0.2-1.2% 2,2-MDI. The four side streams, from top to bottom, are: top fraction (to the vacuum system), side stream 1, and side stream 2. The distillation column contains liquid and residual liquid. Side stream 1 and side stream 2 are the products. Side stream 1 is the liquid phase, containing 50-60% 2,4-MDI, 38-48% 4,4-MDI, and 1-5% 2,2-MDI, forming the MDI-50 series product. This product is used in subsequent purification processes and is not directly used as a product. Side stream 2 is the vapor phase, directly yielding the MDI-100 product, which contains 0-1% 2,4-MDI and 99-100% 4,4-MDI. The pressure at the outlet of side stream 2 is 500-600 Pa, the bottom temperature is 200℃, and the outlet temperature is 195℃.

[0070] Side line 2 is the gas phase extraction pipeline. A large-diameter butterfly valve with a diameter of DN1400 is installed between the distillation column and the heat exchanger. A pressure equalization pipeline connected to the vacuum system is installed at the bottom of the heat exchanger, and a pressure equalization regulating valve with a diameter of DN100 is installed on the pressure equalization pipeline.

[0071] Using the pressure difference (PDI) between the pressure before the equalizing regulating valve and the pressure before the butterfly valve, a cascade control loop (PDIC) is constructed between the PDI and the heat exchanger liquid level (L). The main loop controls the heat exchanger liquid level (L) using the PDI, and the secondary loop controls the PDI using the opening degree of the equalizing regulating valve. Additionally, a single-loop control loop (LIC) is established to control the heat exchanger liquid level (L) using the butterfly valve opening degree. The two control loops are switched via a logic controller, with the switching logic as follows: Figure 6 As shown, the initial opening of the butterfly valve is set to 60%. The cascade control PDIC of differential pressure and heat exchanger liquid level is activated. The heat exchanger liquid level L is controlled by the differential pressure PDI (pressure equalization regulating valve). The lower limit of differential pressure PL is set to 50 Pa, the upper limit of differential pressure PH is set to 450 Pa, the lower safety limit of heat exchanger liquid level LL is set to 10%, and the upper safety limit LH is set to 30%. When the differential pressure PDI deviates from 50-450 Pa or the liquid level deviates from the range of 10-30%, the system switches to the single loop LIC of butterfly valve opening control to control the heat exchanger liquid level. The liquid level L is controlled by adjusting the butterfly valve opening. When the liquid level L returns to between 10-30% and the differential pressure PDI returns to 50-450 Pa, the system switches back to the cascade control loop PDIC of differential pressure and heat exchanger liquid level.

[0072] Using the above control methods, the changes in the flow rate of side line 2 and the fluctuations in the heat exchanger liquid level are as follows: Figure 7 As shown, changes in the output flow rate have a significant impact on the heat exchanger level. With an output flow rate of 3000 kg / h, the maximum fluctuation range is approximately ±18%. Even when the output flow rate is stable, the heat exchanger level fluctuates considerably, with a fluctuation range of approximately ±4.5%.

[0073] The product index analysis for sideline 2 is shown in Table 4 (one sample is taken every 30 days):

[0074] Table 4

[0075]

[0076]

[0077] Using the above control methods, after one year, the product color number increased to 30 Hazen, the coking content increased to 15 ppm, and the total frequency of pressure equalization valve jamming, pressure equalization pipe blockage, and pressure equalization pipe corrosion leakage reached as high as 170 times.

Claims

1. An automatic control method for a gas phase take-off of an isocyanate rectification column, characterized in that, The automatic control method is to set a pressure equalization regulating valve on a pressure equalization pipeline connected between a heat exchanger tapping from a side line of an isocyanate rectifying tower and a vacuum system; an inert gas supplement line is set on the pressure equalization pipeline between the heat exchanger and the pressure equalization regulating valve, and an inert gas flow regulating valve is set on the inert gas supplement line; The automatic control method sets up a single loop of the pressure equalization regulating valve opening degree and the inert gas flow regulating valve opening degree to control the liquid level of the heat exchanger, and switches the two control loops through a logic controller; The pressure equalization regulating valve has a lower limit VL and an upper limit VH of the regulating capacity opening degree, and the heat exchanger liquid level has a safety lower limit LL and a safety upper limit LH; The initial opening degree of the inert gas regulating valve is set, the pressure equalization regulating valve opening degree is used to control the liquid level of the heat exchanger, the opening degree of the pressure equalization regulating valve is within the range of VL-VH, and the liquid level of the heat exchanger is within the range of LL-LH; when the opening degree of the pressure equalization regulating valve is lower than the lower limit VL or higher than the upper limit VH, or when the liquid level of the heat exchanger is lower than the safety lower limit LL or higher than the safety upper limit LH, the single loop control of the inert gas flow regulating valve opening degree to control the liquid level of the heat exchanger is switched; when the liquid level of the heat exchanger is controlled within the range of LL-LH and the opening degree of the pressure equalization regulating valve returns to VL-VH, the single loop control of the pressure equalization regulating valve opening degree and the heat exchanger liquid level is switched again.

2. The automatic control method according to claim 1, characterized by, The lower limit VL of the opening degree of the pressure equalization regulating valve is 1-40%; the upper limit VH of the opening degree of the pressure equalization regulating valve is 60-99%; The lower limit LL of the liquid level of the heat exchanger is 1-30%; the upper limit LH of the liquid level of the heat exchanger is 30-60%.

3. The automatic control method according to claim 2, wherein The lower limit VL of the opening degree of the pressure equalization regulating valve is 10-30%.

4. The automatic control method according to claim 2, wherein The upper limit VH of the opening degree of the pressure equalization regulating valve is 70-90%.

5. The automatic control method according to claim 2, wherein The lower limit LL of the liquid level of the heat exchanger is 10-20%.

6. The automatic control method according to claim 2, wherein The upper limit LH of the liquid level of the heat exchanger is 40-50%.

7. The automatic control method according to claim 2, wherein The initial opening degree of the inert gas flow regulating valve is set to 10-40%.

8. The automatic control method according to claim 7, wherein The initial opening degree of the inert gas flow regulating valve is set to 20-30%.

9. The automatic control method according to any one of claims 1 to 8, characterized by, The inert gas supplement line is used to transport inert gas into the pressure equalization pipeline, wherein the inert gas is selected from nitrogen and / or carbon dioxide; The pressure of the inert gas is 0.01-0.4 MPaG; the diameter of the inert gas flow regulating valve is DN6-30.

10. The automatic control method according to claim 9, wherein The inert gas is selected from nitrogen.

11. The automatic control method according to claim 9, wherein The pressure of the inert gas is 0.1-0.2 MPaG.

12. The automatic control method according to claim 9, wherein The diameter of the inert gas flow regulating valve is DN15-20.

13. The automatic control method according to any one of claims 1 to 8, 10 to 12, characterized by, The isocyanate rectifying tower has at least one side line to tap in a gas phase form; The gas phase after tapping from the side line is heat-exchanged in the heat exchanger to obtain condensate and non-condensable gas, the condensate is tapped from the bottom of the heat exchanger, and the non-condensable gas is transported to the vacuum system through the pressure equalization pipeline.

14. The automatic control method according to claim 13, wherein The feed position of the isocyanate rectifying tower is the tower bottom, the middle of the tower or the tower top.

15. The automatic control method according to claim 14, wherein The feed of the isocyanate rectifying tower is isocyanate prepared by using a liquid phase phosgene method or a gas phase phosgene method; 16. The automatic control method according to claim 13, wherein The feed of the isocyanate rectifying tower is isocyanate prepared by using a liquid phase phosgene method or a gas phase phosgene method; The isocyanate is selected from the group consisting of diphenylmethane diisocyanate, hexamethylene diisocyanate, toluene diisocyanate, xylylene diisocyanate, hexane diisocyanate, dicyclohexylmethane diisocyanate, or isophorone diisocyanate.

17. The method of automatically controlling according to claim 16, wherein, The isocyanate is selected from the group consisting of diphenylmethane diisocyanate.

18. The automatic control method according to any one of claims 1 to 8, 14 to 17, characterized by, The total frequency of sticking of the equalization regulating valve on the side line of the isocyanate rectification tower, blockage of the equalization pipeline, and corrosion leakage is not more than 2 times / year.

19. The method of automatically controlling according to claim 18, wherein, The total frequency of sticking of the equalization regulating valve on the side line of the isocyanate rectification tower, blockage of the equalization pipeline, and corrosion leakage is not more than 1 times / year.

Images