A bypass isolation and sectional switching control method and system for a driving stage

Abstract

This invention relates to a bypass isolation and segmented switching control method and system during the start-up phase, particularly in the field of maleic anhydride production and recovery process control technology. The invention proposes that, during the initial start-up phase, the process gas completely bypasses the absorber tower via a bypass; simultaneously, it monitors dust reduction criteria based on filter unit pressure difference changes, safe temperature margin criteria calculated based on dew point, and safe flammable concentration criteria in the bypass gas. Only when all criteria are met are the process gas segmented into the absorber tower in a stepwise manner (30% → 60% → 100%), and the system stability must be verified after each step switch; otherwise, a reversal or downgrade is implemented. The invention also discloses a system for implementing this method. This method can prevent the impact of unstable process gas on the absorption system from the source, significantly suppress solvent emulsification, reduce solvent loss, and improve the start-up success rate.

Description

Technical Field

[0001] This invention relates to the field of maleic anhydride production and recycling process control technology, and particularly to a bypass isolation and segmented switching control method and system during the start-up phase. Background Technology

[0002] Maleic anhydride is typically produced by oxidizing hydrocarbon feedstocks on a vanadium phosphate oxide catalyst. The reaction tail gas is cooled and then enters an absorption tower where organic solvents such as DBP absorb the maleic anhydride, which is then recovered through subsequent desorption / refining. During industrial operation, solvent emulsification frequently occurs during the start-up phase (especially initial start-up or after catalyst replacement): this leads to difficulties in water washing / centrifugal phase separation, increased water content in the solvent, and increased oil content in the wastewater, resulting in significant solvent loss, increased wastewater treatment load, and potential long-term instability of the recovery system.

[0003] The mechanism of this type of emulsification is usually related to the "worst window" during the start-up phase: (1) When the space velocity / load gradually increases, the temperature of the catalyst bed is high and fluctuates, and the reaction tail gas is more likely to carry catalyst dust; dust can act as an emulsification stabilizing factor, but may also aggravate solvent deterioration; (2) The start-up personnel, equipment and parameters have not yet been broken in, and the dew point control of the tail gas cooling and absorption system is unstable, which can easily lead to water accumulation in the absorption tower / loop; (3) After water enters the solvent loop, it can easily trigger maleic anhydride hydrolysis, solvent hydrolysis / pyrolysis and generate interfacial active byproducts such as acids, monoesters, polymers / colloids, forming stable emulsions that last for a long time.

[0004] Existing technologies mostly employ measures such as filtration, dehydration, sewage discharge, or post-treatment demulsification, but it is still difficult to avoid the worst operating window of "high dust, high dew point risk, and large parameter fluctuations" from the source, resulting in widespread emulsification problems in different devices and significant economic losses. Summary of the Invention

[0005] (1) Technical solution This invention provides a bypass isolation and segmented switching control method during the start-up phase, applied to a maleic anhydride production unit. The unit includes an oxidation reactor, a tail gas cooling unit, a tail gas precision filtration unit, an absorption tower, and a tail gas incineration or treatment unit. The method comprises the following steps: (1) Bypass isolation stage: During the initial start-up and transition stage of air speed / load increase, the cooled reaction exhaust gas is bypassed through the absorption tower via the bypass pipeline and directly sent to the exhaust gas incineration or exhaust gas treatment unit. (2) Dust reduction criterion establishment stage: During step (1), the change trend of the pressure difference ΔP of the tail gas precision filter unit is used to characterize the dust entrainment intensity of the tail gas, and the bypass isolation is maintained before the dust entrainment intensity meets the reduction criterion. (3) Dew point margin criterion and safety criterion control stage: Starting from step (1), the following control is continuously implemented throughout the entire start-up phase: calculate the water dew point temperature T based on the exhaust gas water content. d Control the inlet gas temperature T of the absorption tower in Temperature T at the top of the absorption tower top and the temperature T of the lean solvent entering the tower solv All satisfy T in T top T solv ≥ T d + ΔT, while controlling the volume fraction of combustibles in the bypass exhaust gas to not exceed 0.6%; (4) Admission determination and segmented switching stage: When the preset admission conditions are met, the reaction tail gas is switched from the bypass to the absorption tower in stages according to the step ratio. The step ratio is 30%→60%→100%. After each step switch, a preset stable time is maintained and the stability criterion is verified. If the stability criterion is met, the next step is entered. If the stability criterion is not met, the current step is maintained, the switching ratio is reduced, or the bypass is returned. (5) Controlled operation stage: During the segmented switching and normal absorption stages, measures such as controlling the pressure drop of the absorption tower, solvent water content, solvent filtration / dehydration / discharge and oil-water separation load are used to suppress the hydrolysis of maleic anhydride, solvent hydrolysis / pyrolysis and the generation and accumulation of interfacial active byproducts, thereby reducing emulsification and solvent loss in the water washing / centrifugal phase separation process.

[0006] Preferably, the dust reduction criterion in step (2) includes at least one or a combination of the following: (a) Within the preset observation window, the frequency of events in which the pressure difference ΔP of the precision filter unit exceeds the preset incremental threshold is lower than the first frequency threshold; (b) The rate of increase of ΔP is lower than the first rate threshold; (c) The rate of increase of ΔP decreases by more than a preset proportional threshold compared to the peak value at the beginning of operation and remains stable for a period of time.

[0007] Preferably, the observation window is 10 to 60 minutes, and the stabilization time is 10 to 120 minutes.

[0008] Preferably, the temperature margin ΔT in step (3) is ≥3℃.

[0009] Preferably, the volume fraction of combustibles in the exhaust gas entering the exhaust gas incineration or exhaust gas treatment unit during the bypass stage and the segmented switching stage does not exceed 0.6%; when the combustibles exceed 0.6%, an interlocking action is triggered to maintain the bypass and perform load reduction, dilution, or enhanced cooling.

[0010] Preferably, the stability criterion after each step switch in step (4) includes at least one of the following or a combination thereof: (a) The pressure drop in the absorption tower is within the allowable range and there is no abnormal upward trend; (b) The water content in the solvent circulation loop does not exceed the set upper limit or its rate of increase is lower than the threshold; (c) The water content in the oil and the oil content in the water after oil-water separation are within a controllable range; (d) The frequency of ΔP events or the upward trend of ΔP in the precision filter unit remains within the allowable range; (e) Satisfy the dew point margin described in step (3).

[0011] Preferably, the admission criteria in step (4) include at least one of the following: (a) The airspeed or device load reaches 30% to 70% of the design value and remains stable for a preset time; (b) The volume fraction of maleic anhydride in the tail gas to be introduced into the absorption tower shall not exceed 0.5% to 1.0%.

[0012] Preferably, the temperature margin ΔT is ≥5℃, the space velocity or device load reaches 45% to 55% of the design value, and the volume fraction of maleic anhydride in the tail gas to be introduced into the absorption tower is not higher than 0.8%.

[0013] Preferably, the organic solvent is a dialkyl phthalate solvent, more preferably dibutyl phthalate (DBP), or a high-boiling-point ester solvent with similar absorption properties.

[0014] Preferably, a bypass isolation and segmented switching control system during the driving phase includes: Process piping module: includes a three-way switching valve installed before the inlet of the absorption tower, and a bypass pipeline connected to the three-way switching valve for guiding the exhaust gas to the subsequent treatment unit; Parameter monitoring module: includes at least a differential pressure transmitter for measuring the pressure difference ΔP before and after the tail gas precision filter unit, a water content analyzer for measuring the water content in the tail gas, a dew point calculation unit, and a combustible gas analyzer for measuring the concentration of combustibles in the process gas. The controller is configured to perform bypass isolation during the initial stage of start-up and, based on the frequency of ΔP events or the upward trend of ΔP, the dew point margin and the upper limit of combustibles, output switching commands in a step ratio of 30%→60%→100% to gradually introduce exhaust gas into the absorption tower. If the stability criteria are not met, it will perform interlocking actions to maintain, reduce the bypass level or reverse the bypass.

[0015] This invention employs a "four-stage control strategy": Phase A: Bypass Isolation Phase. After cooling, the reaction tail gas does not enter the absorption tower but is directly sent to the tail gas incineration / treatment unit via a bypass. The bypass phase preheats and circulates the absorption system to prevent the "cold tower / wet tower" from directly receiving the tail gas.

[0016] Phase B: Criterion Monitoring and Access Determination Phase. The frequency of events related to the differential pressure ΔP of the precision filter or the upward trend of ΔP is used to characterize the dust entrainment intensity, requiring it to reach a stable downward trend; the dew point Td is calculated based on the water content of the exhaust gas, requiring Tin, Ttop, and Tsolv ≥ Td + ΔT (preferably ΔT ≥ 5℃); simultaneously, the volume fraction of combustibles in the bypass exhaust gas is required to be ≤0.6%.

[0017] Phase C: Segmented Switching Phase (30%→60%→100%). After meeting the access criteria, the tail gas is introduced into the absorption tower in stepwise proportions of 30%→60%→100%. Each step is maintained for a stable period and the stability criteria (tower pressure drop, solvent water content trend, oil-water separation index, ΔP trend and dew point margin, etc.) are verified. If the criteria are not met, the absorption tower is maintained, the step is downgraded, or the bypass is reversed.

[0018] Phase D: Controlled operation phase. After complete entry into the tower, the solvent system continues to be kept clean through dew point balance, solvent water content, filtration, and sludge discharge; if necessary, the oil-water separation process is activated to reduce entrainment and minimize solvent loss.

[0019] (2) Beneficial effects The present invention proposes a bypass isolation and segmented switching control method and system for the driving phase, which has the following advantages compared with the prior art: (1) The rationality of using the frequency / trend of the ΔP event of the precision filter as the dust criterion: During the start-up phase, the amount of dust entrained in the exhaust gas changes rapidly and has large transient fluctuations. Direct online dust measurement is costly and complex to maintain. The precision filter is sensitive to particulate load, and its pressure difference ΔP increases with the increase of dust collection. The frequency or rate of increase of the ΔP event can be used as an engineering feasibility indicator of the dust load decline trend. It is suitable for determining the moment when "dust has significantly decreased and tends to stabilize" during the start-up phase.

[0020] (2) Reasonableness of dew point temperature margin preferably ≥5℃: During the start-up phase, there are large fluctuations in cooling, ambient temperature and water content. If it is only slightly higher than the dew point, local cold spots and short-term fluctuations may still cross the dew point to form condensate and be absorbed by the solvent. Setting the temperature margin to ≥5℃ can significantly improve the resistance to fluctuations, make the transition period safer and reduce the probability of solvent water absorption and hydrolysis chain triggering.

[0021] (3) The rationality of using combustible material ≤0.6% as the safety limit for bypass: In the maleic anhydride industry, combustible material control at the reactor outlet is often used to avoid afterburning / safety risks; when bypassing to the incineration / treatment unit, setting the combustible material volume fraction ≤0.6% can be used as a safety boundary during the start-up phase. When the limit is exceeded, risk control can be achieved by maintaining the bypass and interlocking to reduce load, increase dilution or strengthen cooling.

[0022] (4) The rationality of the 30%→60%→100% step switching: Step switching can avoid hard switching that would cause a sudden increase in the transient load of the absorption tower, water absorption and dust input; the 30% stage is used for "exploratory controlled introduction", the 60% stage is used for "expanded introduction and verification of the stability of the tower and solvent system", and the 100% stage completes the full transition, forming a closed-loop strategy that can be reversed and verified. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the maleic anhydride recovery section bypass pipeline of the present invention; Figure 2 This is a control logic block diagram of the present invention. Detailed Implementation Example 1

[0024] Bypass isolation + 30 / 60 / 100 segmented switching (1) During the initial start-up, the exhaust gas is cooled and then introduced into the bypass to the incineration / treatment unit; the volume fraction of combustibles in the bypass exhaust gas is monitored in real time to ensure that it is ≤0.6%.

[0025] (2) Monitor the precision filter ΔP, count the frequency of ΔP events (e.g., the number of times ΔP exceeds the preset incremental threshold per unit time) and calculate the rate of increase of ΔP; when the event frequency decreases and remains within the allowable range within the preset stable time, it is determined that the dust entrainment has entered the declining and stable stage.

[0026] (3) Monitor the water content of the exhaust gas and calculate the dew point Td, and control the exhaust gas temperature Tin at the inlet of the absorption tower and the temperature at the key point at the top of the tower T. top and the temperature of lean solvent T solv Satisfying ≥ T d +ΔT, preferably ΔT≥5℃.

[0027] (4) After simultaneously satisfying the dust reduction criterion, dew point residual criterion and bypass safety criterion, perform a 30% switch: 30% of the tail gas enters the absorption tower and 70% is still bypassed; maintain the stability time and verify the stability criteria (tower pressure drop, solvent water content trend, oil-water separation index, etc.).

[0028] (5) After the stability criterion is met in the 30% stage, the 60% switch is executed: 60% enters the tower; and the same is true for maintaining and verifying.

[0029] (6) After the stability criterion is met at 60% stage, a 100% switch is performed: all inputs are removed from the bypass. If solvent water content or phase separation indicators deteriorate at any stage, the current step is maintained or the process is reversed to the previous step / bypass and corrective measures are taken.

[0030] Comparison of proportions and expected effects (based on historical statistics and expected control targets) Note: The following comparative data are the historical statistical intervals of existing similar devices and the expected improvement targets of using the control logic of this invention under the same process boundaries, used to illustrate the technical effects and the rationality of the threshold.

[0031] Comparative Example A (Traditional driving: no bypass, exhaust gases directly enter the absorption tower) - Historical performance statistics: Peak solvent water content: exceeding 0.5%.

[0032] Water content in the oil after centrifugation / phase separation: exceeds 3%.

[0033] Solvent loss: 18 kg solvent / ton maleic anhydride.

[0034] Emulsification duration: Severe emulsification with a cycle exceeding 30 days, accompanied by increased oil content in wastewater and a significant increase in wastewater treatment load.

[0035] Implementation Expected B (using bypass isolation of this invention + 30 / 60 / 100 segmented switching) - Expected Control Objective: Solvent water content control: below 0.15%.

[0036] Oil water content: less than 1.5%.

[0037] Solvent loss: less than 9 kg solvent / ton maleic anhydride.

[0038] Emulsification level: Slight emulsification and complete restoration to normal phase separation in about 1 week.

[0039] As can be seen from the above comparison, by isolating the absorption system during the worst-case driving window and gradually admitting it under the stability criterion, the present invention can reduce the accumulation of interfacial active byproducts caused by solvent water absorption and dust input, thereby effectively reducing the emulsion strength and duration and significantly reducing solvent loss.

[0040] The embodiments described above are merely preferred embodiments of the present invention, and are described in a relatively specific and detailed manner. However, the present invention is not limited to these embodiments. It should be noted that for those skilled in the art, any modifications made without departing from the spirit of the present invention fall within the protection scope of the present invention. Therefore, the protection scope of this patent should be determined by the appended claims.

Claims

1. A bypass isolation and segmented switching control method during the start-up phase, applied to a maleic anhydride production unit, the unit comprising an oxidation reactor, a tail gas cooling unit, a tail gas precision filtration unit, an absorption tower, and a tail gas incineration or treatment unit, characterized in that, Includes the following steps: (1) Bypass isolation stage: During the initial start-up and transition stage of air speed / load increase, the cooled reaction exhaust gas is bypassed through the absorption tower via the bypass pipeline and directly sent to the exhaust gas incineration or exhaust gas treatment unit. (2) Dust reduction criterion establishment stage: During step (1), the change trend of the pressure difference ΔP of the tail gas precision filter unit is used to characterize the dust entrainment intensity of the tail gas, and the bypass isolation is maintained before the dust entrainment intensity meets the reduction criterion. (3) Dew point margin criterion and safety criterion control stage: Starting from step (1), the following control is continuously implemented throughout the entire start-up phase: calculate the water dew point temperature T based on the exhaust gas water content. d Control the inlet gas temperature T of the absorption tower in Temperature T at the top of the absorption tower top and the temperature T of the lean solvent entering the tower solv All satisfy T in T top T solv ≥ T d + ΔT, while controlling the volume fraction of combustibles in the bypass exhaust gas to not exceed 0.6%; (4) Admission determination and segmented switching stage: When the preset admission conditions are met, the reaction tail gas is switched from the bypass to the absorption tower in stages according to the step ratio. The step ratio is 30%→60%→100%. After each step switch, a preset stable time is maintained and the stability criterion is verified. If the stability criterion is met, the next step is entered. If the stability criterion is not met, the current step is maintained, the switching ratio is reduced, or the bypass is returned. (5) Controlled operation stage: During the segmented switching and normal absorption stages, measures such as controlling the pressure drop of the absorption tower, solvent water content, solvent filtration / dehydration / discharge and oil-water separation load are used to suppress the hydrolysis of maleic anhydride, solvent hydrolysis / pyrolysis and the generation and accumulation of interfacial active byproducts, thereby reducing emulsification and solvent loss in the water washing / centrifugal phase separation process.

2. The bypass isolation and segmented switching control method during the driving phase according to claim 1, characterized in that, The dust reduction criterion in step (2) includes at least one of the following or a combination thereof: (a) Within the preset observation window, the frequency of events in which the pressure difference ΔP of the precision filter unit exceeds the preset incremental threshold is lower than the first frequency threshold; (b) The rate of increase of ΔP is lower than the first rate threshold; (c) The rate of increase of ΔP decreases by more than a preset proportional threshold compared to the peak value at the beginning of operation and remains stable for a period of time.

3. The bypass isolation and segmented switching control method during the driving phase according to claim 2, characterized in that, The observation window is 10–60 minutes, and the stabilization time is 10–120 minutes.

4. The bypass isolation and segmented switching control method during the driving phase according to claim 1, characterized in that, The temperature margin ΔT mentioned in step (3) is ≥3℃.

5. The bypass isolation and segmented switching control method during the driving phase according to claim 1, characterized in that, During the bypass phase and the segmented switching phase, the volume fraction of combustibles in the exhaust gas entering the exhaust gas incineration or exhaust gas treatment unit shall not exceed 0.6%; when the combustibles exceed 0.6%, the bypass is maintained and interlocking actions such as load reduction, dilution increase or enhanced cooling are executed.

6. The bypass isolation and segmented switching control method during the driving phase according to claim 1, characterized in that, The stability criterion after each step switch in step (4) includes at least one of the following or a combination thereof: (a) The pressure drop in the absorption tower is within the allowable range and there is no abnormal upward trend; (b) The water content in the solvent circulation loop does not exceed the set upper limit or its rate of increase is lower than the threshold; (c) The water content in the oil and the oil content in the water after oil-water separation are within a controllable range; (d) The frequency of ΔP events or the upward trend of ΔP in the precision filter unit remains within the allowable range; (e) Satisfy the dew point margin described in step (3).

7. The bypass isolation and segmented switching control method during the driving phase according to claim 4, characterized in that, The admission criteria in step (4) include at least one of the following: (a) The air speed or device load reaches 30% to 70% of the design value and remains stable for a preset time; (b) The volume fraction of maleic anhydride in the tail gas to be introduced into the absorption tower shall not exceed 0.5% to 1.0%.

8. The bypass isolation and segmented switching control method during the driving phase according to claim 7, characterized in that, The temperature margin ΔT is ≥5℃, the space velocity or device load reaches 45% to 55% of the design value, and the maleic anhydride volume fraction in the tail gas to be introduced into the absorption tower is not higher than 0.8%.

9. The bypass isolation and segmented switching control method during the driving phase according to claim 1, characterized in that, The organic solvent is a dialkyl phthalate solvent.

10. A bypass isolation and segmented switching control system during the driving phase, characterized in that, include: Process piping module: includes a three-way switching valve installed before the inlet of the absorption tower, and a bypass pipeline connected to the three-way switching valve for guiding the exhaust gas to the subsequent treatment unit; Parameter monitoring module: includes at least a differential pressure transmitter for measuring the pressure difference ΔP before and after the tail gas precision filter unit, a water content analyzer for measuring the water content in the tail gas, a dew point calculation unit, and a combustible gas analyzer for measuring the concentration of combustibles in the process gas. The controller is configured to perform bypass isolation during the initial stage of start-up and, based on the frequency of ΔP events or the upward trend of ΔP, the dew point margin and the upper limit of combustibles, output switching commands in a step ratio of 30%→60%→100% to gradually introduce exhaust gas into the absorption tower. If the stability criteria are not met, it will perform interlocking actions to maintain, reduce the bypass level or reverse the bypass.

Images