Amine recovery monitoring method and system for polyurethane synthesis off-gas

Abstract

The application relates to the technical field of gas separation, in particular to an amine recovery monitoring method and system for polyurethane synthesis tail gas. The method comprises the following steps: obtaining a triethylamine solution through preliminary separation; collecting parameter values of different parameters through distillation of the triethylamine solution; obtaining a parameter sequence in a preset time period; decomposing the parameter sequence to obtain a period sequence and a trend sequence; obtaining period significant values of different parameters based on the period sequence, and obtaining fluctuation consistency coefficients based on the trend sequence; obtaining a distillation interference coefficient after weighting; obtaining pure triethylamine based on the distillation interference coefficient of the control system; collecting attribute values of different attributes, constructing a change characteristic value sequence through the difference between data values and predicted values of the attributes in a sliding window, obtaining a dehydration steady-state index based on sequence similarity and vapor pressure difference; and realizing recovery and monitoring of the triethylamine based on the dehydration steady-state index. The application helps to improve the dehydration efficiency of triethylamine and further improve the purity of the recovered triethylamine.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of gas separation, in particular to an amine recovery monitoring method and system for polyurethane synthesis tail gas. BACKGROUND

[0002] Polyurethane is a high molecular material formed by polyisocyanate and polyol through condensation polymerization, which has excellent mechanical properties, wear resistance, oil resistance and plasticity. In the production process of polyurethane, amine substances are often used as catalysts, which mainly accelerate the reaction between isocyanate and polyol or water, thereby promoting the foaming and gelation process. For example, amine catalysts such as triethylamine can significantly improve the reaction rate and adjust the formation speed of foam and product quality.

[0003] However, amine substances are volatile, and part of the amine substances will be discharged with the tail gas during production. If directly discharged, it will cause environmental pollution. Recycling amine substances in the tail gas can not only reduce pollution, but also reuse the recycled amine in the production process, reduce production cost and improve resource utilization rate. The recovery of amine needs to go through separation and purification steps, and the accuracy of distillation and membrane dehydration process control in the purification step is crucial. In actual operation, the composition of raw materials and the state of recovery equipment may change, thereby affecting the purity of the recovered amine substances. Different temperature changes will lead to low separation purity of amine substances; the existing amine recovery method does not consider the comprehensive influence of the above factors, and it is difficult to quickly adapt to these dynamic changes in the monitoring process, resulting in low purity of the recovered amine substances. SUMMARY

[0004] In order to solve the technical problem of low separation purity of amine substances, the present application provides an amine recovery monitoring method and system for polyurethane synthesis tail gas, and the technical solution is as follows:

[0005] In the first aspect, the present application provides an amine recovery monitoring method for polyurethane synthesis tail gas, which comprises the following steps:

[0006] The polyurethane synthesis tail gas is preliminarily separated to obtain a triethylamine solution;

[0007] The triethylamine solution is added to other solutions for distillation, and the parameter values of different parameters at each time are collected; the parameters include the top temperature, the bottom temperature, the condenser flow and the triethylamine concentration;

[0008] The specific steps of distillation are as follows:

[0009] An adjustment time period is set, and the parameter sequence of each parameter is obtained;

[0010] The decomposition parameter sequence obtains a period sequence and a trend sequence; a period significance value of the parameter sequence is obtained based on fluctuation of peak value difference in the period sequence;

[0011] A fluctuation consistency coefficient between two parameters is obtained according to a correlation coefficient of the trend sequence between the two parameters; a distillation disturbance coefficient of an adjustment time period is obtained by weighting the fluctuation consistency coefficient between the top temperature and the rest parameters with the period significance value of the top temperature and the rest parameters as the weight;

[0012] The distillation disturbance coefficient is used for optimizing the control parameters, and pure triethylamine is obtained by processing the next time period;

[0013] The specific steps of membrane dehydration are as follows:

[0014] The pure triethylamine is added to the preheater and the evaporator; attribute values of different attributes at a preset time are collected, including the evaporator temperature, and the steam pressures on both sides of the collection membrane;

[0015] A sliding window is set, a change characteristic value is obtained based on the difference between the data value of each attribute in the sliding window and the obtained predicted value, and a change characteristic value sequence is constructed; a dehydration steady-state index is obtained based on the similarity of the change characteristic value sequence and the difference between the steam pressures on both sides of the membrane;

[0016] Triethylamine is recovered and monitored based on comparison of the dehydration steady-state index and a membrane dehydration abnormal threshold value.

[0017] In the above scheme, the present application proposes an amine recovery and monitoring method and system for polyurethane synthesis tail gas, which includes three steps of triethylamine separation, acid-base conversion and distillation, and membrane dehydration. The distillation step is controlled and monitored in view of the influences of unstable feeding rate, heating power and cooling system, and the influences of the evaporator and membrane pollution factors in the membrane dehydration step. The distillation disturbance coefficient of the distillation state is calculated by analyzing the correlation characteristics between the top temperature in the distillation kettle and the bottom temperature, the condenser flow, and the triethylamine concentration at the top of the rectifying column, and the periodic fluctuation characteristics of the above data. The reflux flow of the rectifying column is optimized and adjusted based on the value, which has the advantage of being able to reduce the influences of raw material composition change and unstable cooling system, and obtain relatively pure triethylamine. In the membrane dehydration step, the dehydration steady-state index is calculated by analyzing the synchronization characteristics of the evaporator temperature and the steam pressures on both sides of the membrane and the fluctuation degree of the partial pressure difference, and then the triethylamine dehydration step is monitored and evaluated, which has the advantage of being helpful to improve the efficiency of triethylamine dehydration and further improve the purity of recovered triethylamine.

[0018] In one embodiment, the method of preliminarily separating the polyurethane synthesis tail gas to obtain a triethylamine solution is as follows:

[0019] The triethylamine is converted from gaseous state to liquid state by condensation method, and then the amine in the tail gas is separated; wherein two condensers are placed, and the tail gas passes through the two condensers in turn, the temperature of the first condenser needs to be greater than 89.5 degrees Celsius and less than 200 degrees Celsius, and the temperature of the second condenser needs to be less than 89.5 degrees Celsius, and the liquid after passing through the two condensers is the triethylamine solution after primary separation.

[0020] In one embodiment, the method for obtaining the period significant value of the parameter sequence based on the fluctuation of the peak difference in the period sequence is:

[0021] The obtained period sequence is processed by using a polynomial fitting technique, and then the peaks are extracted from the fitting curve through derivative analysis; the time values corresponding to all the peak points are arranged in ascending order as a peak time sequence, and the standard deviation of the first difference sequence of the obtained peak time sequence is calculated; the reciprocal of the standard deviation is taken as the period significant value of the parameter sequence.

[0022] In one embodiment, the method for obtaining the fluctuation consistency coefficient between two parameters according to the correlation coefficient of the trend sequence between the two parameters is:

[0023] The Spearman correlation coefficient of the trend sequence between the two parameters is calculated, and the sum of the Spearman correlation coefficient and 1 is taken as the fluctuation consistency coefficient.

[0024] In one embodiment, the method for obtaining the distillation disturbance coefficient of the adjustment time period by weighting the fluctuation consistency coefficient between the overhead temperature and the remaining parameters according to the period significant value of the overhead temperature and the period significant value of the remaining parameters is:

[0025] Let the sum of the period significant value of the overhead temperature and the period significant value of any one of the remaining parameters divided by the sum of the period significant values of all parameters be the influence weight of any one parameter;

[0026] The distillation disturbance coefficient is positively correlated with the influence weight, and negatively correlated with the fluctuation consistency coefficient.

[0027] In one embodiment, the method for optimizing the control parameters based on the distillation disturbance coefficient is:

[0028] The proportional term parameter optimization formula is: ; wherein is the optimized proportional term parameter, is a preset adjustment parameter, represents a logarithmic function with base 10, represents the distillation disturbance coefficient.

[0029] In one embodiment, the method for obtaining the change characteristic value based on the difference between the data value of each attribute in the sliding window and the obtained predicted value is as follows:

[0030] The data value of the attribute in the input sliding window is input, and a predicted value is output by a one-time exponential smoothing algorithm;

[0031] The accumulated sum of the difference between the predicted value and each data value in the sliding window is calculated, and the accumulated sum is taken as the change characteristic value of the attribute;

[0032] A change characteristic value is calculated for each sliding of the sliding window, and the change characteristic values of all sliding windows corresponding to each attribute are sorted by time to obtain the change characteristic value sequence of the attribute.

[0033] In one embodiment, the method for obtaining the dehydration steady-state index based on the similarity of the change characteristic value sequence and the difference between the vapor pressures on both sides of the membrane is as follows:

[0034] The SBD distance corresponding to all combinations of the change characteristic sequences is calculated; the inverse of the average of all SBD distances is taken as the synchronization coefficient of temperature and vapor pressure;

[0035] The difference between the vapor pressure data on both sides of the membrane at the same time is calculated, and the ratio of the synchronization coefficient to the standard deviation of all the differences is taken as the dehydration steady-state index.

[0036] In one embodiment, the method for realizing the recovery and monitoring of triethylamine based on the comparison between the dehydration steady-state index and the membrane dehydration abnormal threshold is as follows:

[0037] After the dehydration steady-state index is normalized, if the normalized value is less than the preset membrane dehydration abnormal threshold, the efficiency is low, and the vapor pressure needs to be adjusted; if S is greater than or equal to 0.6, the membrane dehydration process efficiency is good.

[0038] In a second aspect, the embodiments of the present application also provide an amine recovery monitoring system for polyurethane synthesis tail gas, which comprises a memory, a processor, and a computer program stored in the memory and running on the processor, and the processor implements the steps of the amine recovery monitoring method for polyurethane synthesis tail gas as described in any one of the above aspects when executing the computer program.

[0039] The present application has the following beneficial effects:

[0040] The application provides an amine recovery monitoring method and system for polyurethane synthesis tail gas, which comprises three steps of triethylamine separation, acid-base conversion and distillation, and membrane dehydration. The distillation step is affected by the factors of unstable feed rate, heating power and cooling system, and the evaporation and membrane pollution factors in the membrane dehydration step are monitored. By analyzing the correlation between the top temperature and the bottom temperature in the distillation kettle, the condenser flow and the triethylamine concentration at the top of the rectifying column, and the periodic fluctuation characteristics of the above data, the distillation disturbance coefficient of the distillation state is calculated. Based on the value, the reflux of the rectifying column is optimized and adjusted, which has the advantages of reducing the influence of raw material composition change and unstable cooling system, and obtaining relatively pure triethylamine; in the membrane dehydration step, by analyzing the synchronous characteristics of the evaporation temperature and the vapor pressure on both sides of the membrane and the fluctuation degree of the partial pressure difference, the dehydration steady state index is calculated, and then the triethylamine dehydration step is monitored and evaluated, which has the advantages of improving the efficiency of triethylamine dehydration and further improving the purity of recovered triethylamine. BRIEF DESCRIPTION OF DRAWINGS

[0041] In order to more clearly illustrate the technical solutions and advantages of the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and those skilled in the art can also obtain other drawings according to these drawings without creative labor.

[0042] Figure 1 A flow chart of an amine recovery monitoring method for polyurethane synthesis tail gas provided by an embodiment of the present application;

[0043] Figure 2 A method flow chart for the distillation step;

[0044] Figure 3 A method flow chart for the membrane dehydration step. DETAILED DESCRIPTION

[0045] In order to further illustrate the technical means and effects adopted by the present application to achieve the predetermined invention purpose, the specific embodiments, structures, features and effects of the amine recovery monitoring method and system for polyurethane synthesis tail gas according to the present application are described in detail as follows. In the following description, different "one embodiment" or "another embodiment" do not necessarily refer to the same embodiment. In addition, the specific features, structures or characteristics in one or more embodiments can be combined in any suitable form.

[0046] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present application belongs.

[0047] An amine recovery monitoring method and system embodiment for polyurethane synthesis tail gas:

[0048] The specific scheme of the amine recovery monitoring method for polyurethane synthesis tail gas provided by the present application will be specifically described below with reference to the accompanying drawings.

[0049] Please refer to Figure 1 , which shows a flow chart of an amine recovery monitoring method for polyurethane synthesis tail gas provided by an embodiment of the present application, which comprises the following steps:

[0050] Step S1, the polyurethane synthesis tail gas is preliminarily separated to obtain a triethylamine solution.

[0051] The synthesis reaction process of polyurethane is the main source of polyurethane tail gas in rubber and plastic plants. When isocyanate and polyol reactants are polymerized, due to incomplete reaction or under high temperature, stirring and other operating conditions, part of the raw materials and intermediate products will volatilize into the air to form tail gas. These tail gases contain a variety of organic compounds, among which volatile organic compounds are the main components, including but not limited to toluene diisocyanate (TDI), diphenyl methane diisocyanate (MDI) and other isocyanate substances, in addition, amine catalysts may also be contained in the polyurethane synthesis tail gas. The types of these catalysts include but are not limited to bis(2-dimethylaminoethyl) ether, dimethylcyclohexylamine, triethylamine, and the polyurethane production of the present application uses triethylamine as a catalyst, so the triethylamine in the synthesis tail gas is separated and recovered.

[0052] Toluene diisocyanate, diphenyl methane diisocyanate and other substances in the tail gas have high boiling point characteristics, usually above above, while the boiling point of triethylamine is about Therefore, by taking advantage of the low boiling point characteristics of triethylamine, the triethylamine is converted from gas to liquid by condensation method, and then separated from the amine in the tail gas. In the present application, the mixed tail gas generated in the polyurethane production is sequentially passed through two condensers, the temperature of the first condenser is controlled at above, in the present embodiment, it is set to At this time, toluene diisocyanate, diphenyl methane, diisocyanate and other substances will be converted into liquid first, while triethylamine will still be in gas state, then flow through the second condenser, and the temperature of the second condenser is controlled at below, the triethylamine is converted into liquid, thereby realizing the separation of triethylamine.

[0053] At this point, the triethylamine solution after the initial separation is obtained.

[0054] Step S2, the triethylamine solution is added to other solutions and distilled, and the parameter values of different parameters at each time are collected.

[0055] The separated triethylamine solution obtained by the above steps can contain other organic compound impurities such as alcohols and acids. In order to extract and recover the triethylamine, the application removes the impurities in the triethylamine solution by acid-base conversion method.

[0056] First, hydrochloric acid solution is added to the triethylamine solution to convert the amine into ammonium salt dissolved in the aqueous phase, and the aqueous phase is collected by oil-water separation. Then, sodium hydroxide solution is added to the obtained aqueous phase to convert the ammonium salt back to amine. After acid-base conversion, although most of the impurities have been removed, the solution can still contain a small amount of water and other volatile impurities. The application uses distillation method for further separation and purification. By heating, the triethylamine is evaporated, and by condensation collection, these impurities can be further removed, and the purity of the triethylamine can be improved.

[0057] Further, the triethylamine solution after acid-base treatment is then added to the distillation kettle for heating and vaporization. The distillation kettle is connected to the rectifying column. After stable operation under total reflux, the triethylamine vapor containing water is collected at the top of the rectifying column. After condensation by the distillation condenser, part of the reflux is collected at the top of the rectifying column, and part is collected to the distillation receiving tank. During each distillation process, due to the fluctuation of impurity content in the raw material, and the dynamic change characteristics of the operating parameters of the key equipment such as the distillation kettle, condenser and rectifying column, the above factors will have a significant impact on the extraction purity of triethylamine, and the reflux ratio can better control the product quality. In order to improve the purity of the product, the operating parameters of the distillation system need to be monitored in real time, and then the reflux amount of the distillation process is accurately controlled to realize the efficient separation and purification of triethylamine.

[0058] The temperature state in the distillation kettle and the load of the condenser are important factors affecting the extraction purity. Therefore, temperature sensors are installed at the top and bottom of the distillation kettle to collect the kettle top temperature data and kettle bottom temperature data. The load of the condenser is monitored by the flow rate of the condensate, and a flow sensor is installed inside the condenser to collect the flow rate data of the condensate. In addition, the purity of the triethylamine vapor at the top of the rectifying column can more directly reflect the current separation and extraction effect, so a fixed triethylamine gas detector is installed at the top of the rectifying column to collect the triethylamine concentration data. The time interval for collecting data by the above sensors is set to 1 second. Thus, the kettle top temperature data, kettle bottom temperature data, condenser flow data and column top triethylamine concentration data at each time are obtained, and the kettle top temperature, kettle bottom temperature, condenser flow and triethylamine concentration at each time are denoted as parameters.

[0059] Thus, the kettle top temperature, kettle bottom temperature, condenser flow and triethylamine concentration at each time are obtained.

[0060] Step S3, obtain the parameter sequence in the preset time period; decompose the parameter sequence to obtain the period sequence and the trend sequence; obtain the period significant value of different parameters based on the period sequence, and obtain the fluctuation consistency coefficient based on the trend sequence; obtain the distillation interference coefficient after weighting; and obtain pure triethylamine based on the distillation interference coefficient.

[0061] The top temperature, the bottom temperature, the condenser flow and the triethylamine concentration at each moment are obtained through the above steps. The top temperature and the bottom temperature are easily affected by the feed rate, the heating power and the cooling system, and the two respectively reflect the gas phase temperature state at the top position and the liquid phase temperature state at the bottom position in the distillation process.

[0062] The top temperature and the bottom temperature are easily affected by the feed rate, the heating power and the cooling system, and the two respectively reflect the gas phase temperature state at the top position and the liquid phase temperature state at the bottom position in the distillation process.

[0063] Through the above analysis, it can be known that, in a good distillation state, the top temperature data and the bottom temperature data, the condenser flow data and the triethylamine concentration data show a positive correlation. The fluctuation stability of each data will affect the final separation purity.

[0064] S3.1, set an adjustment time period, and obtain the parameter sequence of each parameter in the adjustment time period.

[0065] Since the separation purity of triethylamine is affected by many factors, if the control parameters remain unchanged, the separation purity will be greatly affected. A time period with a preset time length is set, and for each time period, an adjacent time period before it is recorded as an adjustment time period. The adjustment time period is analyzed, the proportional parameter in the control algorithm is adjusted, and the purity of triethylamine is improved. In this embodiment, the length of each time period is 8 minutes. That is, the current time period is adjusted by analyzing the previous time period.

[0066] Firstly, when the feed rate and heating power are unstable, it is easy to cause periodic fluctuations in the temperature of the distillation kettle. The more significant the periodic fluctuations, the more obvious the temperature fluctuations at the top and bottom of the kettle, and the more likely it is to affect the separation and extraction effect.

[0067] Further, all kettle top temperature, kettle bottom temperature, condenser flow and tower top triethylamine concentration in the adjustment period are respectively constituted into four sequences, which are recorded as kettle top temperature sequence, kettle bottom temperature sequence, condenser flow sequence and tower top triethylamine concentration sequence; the above four sequences are uniformly recorded as parameter sequence.

[0068] S3.2, decompose the parameter sequence to obtain the period sequence and the trend sequence; obtain the period significance value of the parameter sequence based on the fluctuation of the peak difference in the period sequence.

[0069] In order to analyze the periodic fluctuation characteristics, for any one parameter sequence, the STL (Seasonal and Trend decomposition using Loess) sequence decomposition algorithm is used to perform sequence decomposition on the parameter sequence, and the period sequence, trend sequence and residual sequence of the parameter sequence are output.

[0070] The obtained period sequence can better reflect the period characteristics of the data itself. When the period characteristics are more significant, the distance between each fluctuation peak in the obtained period sequence is more regular. Therefore, the obtained period sequence is processed by using polynomial fitting technology, and then the peak value is extracted from the fitting curve by derivative analysis. The purpose of fitting processing is to avoid identifying local jitter in the sequence as a peak value. All peak time values corresponding to the peak time sequence are arranged in order from small to large, and the standard deviation of the first difference sequence of the obtained peak time sequence is calculated. The smaller the standard deviation, the more regular the distribution of the peaks, and the more significant the periodic fluctuation of the corresponding parameter sequence. The reciprocal of the standard deviation is taken as the period significance value of the parameter sequence.

[0071] The greater the influence of kettle top temperature fluctuation on the separation and purification effect, the greater the corresponding period significance value; and the temperature of the distillation kettle will affect the flow of the condensate and the triethylamine concentration at the top of the rectifying column, so that the condenser flow data and the triethylamine concentration data also have certain periodic fluctuation characteristics, thereby affecting the separation purity.

[0072] Step S3.3, obtain the fluctuation consistency coefficient between two parameters according to the correlation coefficient of the trend sequence of the two parameters; weight the fluctuation consistency coefficient between the kettle top temperature and the remaining parameters according to the period significance value of the kettle top temperature and the remaining parameters to obtain the distillation disturbance coefficient of the adjustment period.

[0073] After the period significance of each parameter sequence is obtained through the above steps, the correlation characteristics between different parameters are further analyzed. Since the trend sequences of different parameter sequences are obtained through the STL algorithm in the above steps, the data in the trend sequences reflect the overall change direction in a long time, and the correlation between the parameters has a certain time delay. Therefore, by comparing the correlation characteristics between the trend sequences, the distillation state of the current period can be better evaluated.

[0074] The better the separation and purification effect during distillation, the more obvious the positive correlation between the overhead temperature and other data, and vice versa, the more obvious the corresponding negative correlation characteristics. Therefore, the correlation coefficient of the trend sequence corresponding to the overhead temperature sequence and the rest of the parameter sequence is calculated. In this embodiment, the overhead temperature and the condenser flow data are taken as examples, the Spearman correlation coefficient of the trend sequences corresponding to the two is calculated, and the value range is [-1, 1]. The sum of the Spearman correlation coefficient and 1 is taken as the fluctuation consistency coefficient. The smaller the fluctuation consistency coefficient, the worse the degree of cooperation between the overhead temperature and the condenser state in the period, and the more unfavorable for separation and purification. The same as the above steps, the fluctuation consistency coefficients of the overhead temperature and the bottom temperature, and the triethylamine concentration can be obtained respectively.

[0075] Since the period significance value reflects the stability characteristics of each device operation, and the stability of different device parameters is inconsistent, it will have different degrees of influence on the separation and purification effect of the distillation process. In this application, the bottom temperature, the condenser flow and the triethylamine concentration are all affected by the overhead temperature. Therefore, the influence weight of each parameter except the overhead temperature is obtained based on the period significance value of the bottom temperature, the condenser flow and the triethylamine concentration.

[0076] Preferably, in this embodiment, the expression of the influence weight of the bottom temperature, the condenser flow and the triethylamine concentration is:

[0077] The period significance value of the overhead temperature is represented by The period significance value of the bottom temperature is represented by The period significance value of the condenser flow is represented by The period significance value of the triethylamine concentration is represented by The sum of the period significance values of all parameters is represented by The influence weight of the bottom temperature is represented by The influence weight of the condenser flow is represented by The influence weight of the triethylamine concentration is represented by

[0078] The distillation interference coefficient is obtained according to the influence weight of each parameter except the overhead temperature and the fluctuation consistency coefficient of the overhead temperature and the rest of the parameters.​​​

[0079] The distillation interference coefficient is positively correlated with the influence weight and negatively correlated with the fluctuation consistency coefficient.

[0080] It should be noted that the positive correlation means that when one variable increases, the other variable also increases, and the two variables change in the same direction. When one variable changes from large to small or from small to large, the other variable also changes from large to small or from small to large. The specific relationship is determined by actual application, and the present application does not make special limitations.

[0081] It should be noted that the negative correlation means that when one variable increases, the other variable decreases, and the two variables change in opposite directions. When one variable changes from large to small or from small to large, the other variable also changes from small to large or from large to small. The specific relationship is determined by actual application, and the present application does not make special limitations.

[0082] Preferably, in the present embodiment, the expression of the distillation interference coefficient is:

[0083] represents the influence weight of the bottom temperature, represents the influence weight of the condenser flow, represents the influence weight of the triethylamine concentration, represents the fluctuation consistency coefficient of the top temperature and the bottom temperature, represents the fluctuation consistency coefficient of the top temperature and the condenser flow, represents the fluctuation consistency coefficient of the top temperature and the triethylamine concentration, represents the distillation interference coefficient; is a very small constant, which is 0.01, and the purpose is to prevent the denominator from being 0;

[0084] Thus, the distillation interference coefficient of the adjustment time period is obtained. The larger the distillation interference coefficient, the greater the interference from equipment operation and feed during the distillation process.

[0085] Thus, the distillation interference coefficient of the adjustment time period is obtained.

[0086] Step S3.4, optimizing the control parameters based on the distillation interference coefficient and processing the next time period to obtain pure triethylamine.

[0087] The distillation interference coefficient of the adjustment time period is obtained through the above steps. In order to reduce the influence of the distillation interference coefficient, the present application controls the reflux flow of the rectifying column through the regulating valve, thereby improving the separation purity. The opening degree of the regulating valve is controlled by the PID algorithm. And the reflux flow of the current period is adjusted according to the distillation state of the last period. Specifically, if the last period is calculated ​The greater the value, the greater the interference suffered by the distillation process, and the corresponding proportional term parameter in the PID algorithm is increased to improve the response rate of the system; otherwise, the corresponding proportional term can be reduced to avoid overshoot.

[0088] The initial value of the proportional term parameter in the PID is set to 1. The specific proportional term parameter optimization formula is: . Wherein is the optimized proportional term parameter, is the adjustment parameter, and its value range is set to [1, 3] according to the experience value, represents the logarithmic function with base 10.

[0089] After adjusting the PID control parameters, the current reflux flow and the expected adjusted reflux flow are input into the PID control system, and a more pure triethylamine is obtained after processing by the system. The specific implementation flowchart of the distillation step is shown in Figure 2 .

[0090] At this point, the distillation system is accurately controlled, and efficient separation and purification of triethylamine is achieved.

[0091] Step S4, collect attribute values of different attributes, construct a change feature value sequence through the difference between the data values and the predicted values of the attributes in the sliding window, and obtain a dehydration steady-state index based on the sequence similarity and the vapor pressure difference; based on the dehydration steady-state index, the recovery and monitoring of triethylamine are realized.

[0092] S4.1, add pure triethylamine to the preheater and evaporator; collect attribute values of different attributes at a predetermined time, including the temperature of the evaporator and the vapor pressures on both sides of the collection membrane.

[0093] Although distillation purification can remove most impurities and water, the liquid triethylamine obtained may still contain a small amount of water. Triethylamine is very sensitive to water, and trace amounts of water can affect its purity and subsequent performance, which cannot meet the requirements of industrial applications. Therefore, it is necessary to perform a dehydration treatment. The effect of distillation purification directly affects the efficiency and cost of the dehydration treatment. The lower the water content after distillation, the higher the efficiency of the dehydration treatment, and the higher the accuracy in amine recovery monitoring.

[0094] The liquid triethylamine obtained in the previous step is fed into the preheater and evaporator through the raw material pump, reaches a certain temperature, and then enters the membrane separation unit in the form of steam. The water and a small amount of organic matter in the raw material are permeated from the upstream side of the membrane to the downstream side of the membrane through the membrane assembly. The last stage on the upstream side of the membrane obtains the finished product, and the downstream side of the membrane uses vacuum pumping and condensation to form a vapor pressure difference between the components on both sides of the upstream and downstream sides. The permeate vapor is pumped into the condenser under the action of the vacuum unit, and the condensed permeate is pumped out for further treatment.

[0095] During the membrane dehydration process, the dehydration performance can be affected by unstable heating power of the evaporator or membrane fouling and clogging factors. Changes in evaporator temperature and membrane pressure difference on both upstream and downstream sides are key factors affecting the quality of amine recovery. By monitoring the two parameters in real time, it is helpful to ensure efficient dehydration of triethylamine and product quality. The evaporator temperature and membrane pressure difference need to be maintained within the standard range. If the evaporator temperature is too high, the vapor pressure on the upstream side of the membrane will be high, and when the permeation efficiency of the membrane is affected, the vapor pressure on the downstream side of the membrane will be unstable. This will cause the vapor partial pressure difference on both sides of the membrane to change unstably, and the partial pressure difference is the key driving force for membrane dehydration. The vacuum degree on the downstream side needs to be precisely controlled to ensure a sufficient and stable partial pressure difference, thereby improving the dehydration efficiency.

[0096] Therefore, the present application monitors the evaporator temperature, vapor pressure on both upstream and downstream sides of the membrane during the dehydration process. The evaporator temperature data is collected by a temperature sensor, and the vapor pressure data on both sides of the membrane is collected by a pressure sensor. The collection time interval of each data is 1 second. The evaporator temperature and the vapor pressure on both sides of the membrane are obtained.

[0097] S4.2, a sliding window is set, a change feature value is obtained based on the difference between the data value of each attribute in the sliding window and the obtained predicted value, and a change feature value sequence is constructed; a dehydration steady-state index is obtained based on the similarity of the change feature value sequence and the difference in vapor pressure on both sides of the membrane.

[0098] Since the temperature rise will cause the vapor pressure on the upstream side to increase rapidly, in order to analyze the changes and relationship characteristics of temperature and pressure in the short term, a sliding window is set, and the sliding step is set to 1; in this embodiment, the value of the sliding window is 7; and since the changes in temperature and vapor pressure are relatively rapid, in order to facilitate real-time optimization and adjustment, the length of the adjustment interval of membrane dehydration is set to 1 minute, and the sliding window slides in this adjustment interval length.

[0099] An increase in evaporator temperature will cause the vapor pressure on the upstream side of the membrane to increase, and in order to maintain the stability of the partial pressure difference, the vapor pressure on the downstream side of the membrane needs to be increased accordingly. Therefore, the change difference characteristics between temperature and pressure are further analyzed.

[0100] The collected evaporator temperature and the vapor pressure on both sides of the membrane are denoted as attributes. For each attribute, the data values of the attribute in the sliding window are input, and the predicted value is output by the exponential smoothing algorithm. Then the cumulative sum of the difference between the predicted value and each data value in the sliding window is calculated, and the cumulative sum is taken as the change feature value of the attribute. The size of the obtained change feature value reflects the change direction and degree of the data in the sliding window. Therefore, the corresponding change feature value at the corresponding time can be calculated during the window sliding process, and the change feature sequence of each attribute in the adjustment interval is obtained by arranging in ascending order of time.

[0101] Further, during the membrane dehydration operation, the vapor pressure on the downstream side of the membrane needs to change synchronously with the vapor pressure on the upstream side of the membrane, and the vapor pressure on the upstream side of the membrane has a strong correlation with the evaporator temperature. Therefore, to analyze the current dehydration state, first, the SBD (Shape Based Distance) distances corresponding to all change characteristic sequences in the case of two-by-two combination are calculated respectively. The inverse of the average of all the SBD distances is taken as the synchronization coefficient of the temperature and the vapor pressure, and the larger the obtained synchronization coefficient is, the more synchronous the changes between the evaporator temperature and the vapor pressure are.

[0102] In addition, in combination with the fluctuation degree characteristic of the pressure difference between the two sides of the membrane, the efficiency of the membrane dehydration process is further analyzed. The difference values of the vapor pressure data on the two sides at the same time are calculated respectively, and the ratio of the synchronization coefficient to the standard deviation of all the difference values is taken as the dehydration steady-state index, and the larger the dehydration steady-state index is, the better the dehydration state under the current regulation interval time is. The corresponding dehydration steady-state index is calculated for each regulation interval.

[0103] S4.3, based on the comparison of the dehydration steady-state index and the membrane dehydration abnormal threshold, the recovery and monitoring of triethylamine are realized.

[0104] The dehydration steady-state index of each regulation interval is calculated according to the above steps. In order to perform quantitative evaluation, the softmax function is used to normalize the steady-state coefficient, and the result is recorded as S. The membrane dehydration abnormal threshold is set to 0.6.

[0105] If S is less than 0.6, it is considered that the efficiency of the membrane dehydration process during the recovery of amine is low, and the vapor pressure state on the downstream side of the membrane needs to be adjusted in the next regulation interval to ensure that the partial pressure difference between the two sides of the membrane is within the target range. Specifically, if the vapor pressure on the upstream side of the membrane increases, the suction power of the vacuum unit on the downstream side of the membrane is reduced to increase the vapor pressure on the downstream side; otherwise, the suction power of the vacuum unit on the downstream side of the membrane is increased to reduce the vapor pressure on the downstream side, and the increased or reduced pressure value is the same as the change amount of the vapor pressure on the upstream side of the membrane.

[0106] If S is greater than or equal to 0.6, it is considered that the efficiency of the membrane dehydration process is good. The implementation flowchart of the membrane dehydration step is as shown in Figure 3 .

[0107] Through the above steps, the recovery and monitoring of triethylamine are realized.

[0108] Based on the same inventive concept as the above method, the embodiments of the present application also provide an amine recovery monitoring system for polyurethane synthesis tail gas, which comprises a memory, a processor and a computer program stored in the memory and running on the processor, and the processor implements the steps of any one of the methods in the above-mentioned amine recovery monitoring method for polyurethane synthesis tail gas.

[0109] It should be noted that the above-described embodiments are merely used to illustrate the technical solutions of the present application, but not limit the technical solutions; even though the above-mentioned embodiments are described in detail, those skilled in the art should understand that the technical solutions recorded in the above-mentioned embodiments can be modified, or some technical features can be replaced equivalently; and these modifications or replacements do not make the technical solutions deviate from the scope of the technical solutions of the embodiments of the present application, and should be included in the protection scope of the present application.

[0110] Each of the embodiments in the specification is described in a progressive manner, and the same or similar parts between the embodiments can be referred to each other, and each of the embodiments mainly describes the difference from other embodiments.

Claims

1. A method for monitoring amine recovery for polyurethane synthesis off-gas, characterized in that, The method comprises the following steps: The polyurethane synthesis tail gas is preliminarily separated to obtain a triethylamine solution; The triethylamine solution is added to other solutions for distillation, and the parameter values of different parameters at each time are collected; the parameters include the top temperature, the bottom temperature, the condenser flow and the triethylamine concentration; The specific steps of distillation are as follows: An adjustment time period is set, and the parameter sequence of each parameter in the adjustment time period is obtained; The periodic sequence and the trend sequence are obtained by decomposing the parameter sequence; the periodic significant value of the parameter sequence is obtained based on the fluctuation of the peak value difference in the periodic sequence; The fluctuation consistency coefficient between two parameters is obtained according to the correlation coefficient of the trend sequences between the two parameters; the distillation disturbance coefficient of the adjustment time period is obtained by weighting the fluctuation consistency coefficients between the top temperature and the other parameters with the periodic significant values of the top temperature and the other parameters as weights. Based on the distillation disturbance coefficient, the control parameters are optimized, and the pure triethylamine is obtained by processing the next time period. The specific steps of membrane dehydration are as follows: The pure triethylamine is added to the preheater and the evaporator; the attribute values of different attributes at a preset time are collected, including the evaporator temperature and the vapor pressures on both sides of the collection membrane; A sliding window is set, the change characteristic value is obtained based on the difference between the data value of each attribute in the sliding window and the obtained predicted value, and the change characteristic value sequence is constructed; the dehydration steady-state index is obtained based on the similarity of the change characteristic value sequence and the difference between the vapor pressures on both sides of the membrane. Based on the comparison between the dehydration steady-state index and the membrane dehydration abnormal threshold value, the triethylamine is recovered and monitored, specifically: after the dehydration steady-state index is normalized, if the normalized value is less than the preset membrane dehydration abnormal threshold value, the efficiency is low, and the vapor pressure needs to be adjusted; if the normalized value is greater than or equal to 0.6, the membrane dehydration process is efficient.

2. A method of amine recovery monitoring for polyurethane synthesis off-gas according to claim 1, characterized in that, The method for preliminarily separating the polyurethane synthesis tail gas to obtain a triethylamine solution is as follows: The triethylamine is converted from gas to liquid by condensation method, and then separated from the amine in the tail gas; two condensers are placed, so that the tail gas passes through the two condensers in turn; the temperature of the first condenser needs to be greater than 89.5 degrees Celsius and less than 200 degrees Celsius, and the temperature of the second condenser needs to be less than 89.5 degrees Celsius; the liquid after passing through the two condensers is the triethylamine solution after preliminary separation.

3. A method of amine recovery monitoring for polyurethane synthesis off-gas according to claim 1, characterized in that, The method for obtaining the periodic significant value of the parameter sequence based on the fluctuation of the peak value difference in the periodic sequence is as follows: The obtained periodic sequence is processed by using polynomial fitting technology, and then the peak value is extracted from the fitted curve by derivative analysis; the time values corresponding to all peak points are arranged in ascending order as the peak time sequence, and the standard deviation of the first difference sequence of the obtained peak time sequence is calculated; the reciprocal of the standard deviation is taken as the periodic significant value of the parameter sequence.

4. A method of amine recovery monitoring for polyurethane synthesis off-gas according to claim 1, characterized in that, The method for obtaining the fluctuation consistency coefficient between two parameters according to the correlation coefficient of the trend sequences between the two parameters is as follows: The Spearman correlation coefficient of the trend sequences between the two parameters is calculated, and the sum of the Spearman correlation coefficient and 1 is taken as the fluctuation consistency coefficient.

5. A method of amine recovery monitoring for polyurethane synthesis off-gas according to claim 1, characterized in that, The method for obtaining the distillation disturbance coefficient of the adjustment time period by weighting the fluctuation consistency coefficients between the top temperature and the other parameters with the periodic significant values of the top temperature and the other parameters as weights is as follows: The sum of the period significant value of the kettle top temperature and the period significant value of any one of the other parameters is divided by the sum of the period significant values of all parameters to obtain the influence weight of any one parameter; The distillation interference coefficient is positively correlated with the influence weight and negatively correlated with the fluctuation consistency coefficient.

6. A method of amine recovery monitoring for polyurethane synthesis off-gas according to claim 1, characterized in that, The method for optimizing the control parameters based on the distillation interference coefficient is: The proportional term parameter optimization formula is: ; wherein is the optimized proportional term parameter, is a preset adjustment parameter, represents a logarithmic function with base 10, represents a distillation interference coefficient.

7. A method of amine recovery monitoring for polyurethane synthesis off-gas according to claim 1, characterized in that, The method for obtaining the change characteristic value based on the difference between the data value of each attribute in the sliding window and the obtained predicted value, and constructing the change characteristic value sequence is: The data value of the attribute in the input sliding window is input, and the predicted value is output through the exponential smoothing algorithm once; The accumulated sum of the difference between the predicted value and each data value in the sliding window is calculated, and the accumulated sum is taken as the change characteristic value of the attribute; A change characteristic value is calculated for each sliding of the sliding window, and the change characteristic values of all the sliding windows corresponding to each attribute are sorted according to time to obtain the change characteristic value sequence of the attribute.

8. A method of amine recovery monitoring for polyurethane synthesis off-gas according to claim 1, characterized in that, The method for obtaining the dehydration steady-state index based on the similarity of the change characteristic value sequence and the difference between the steam pressures on both sides of the membrane is: The SBD distance corresponding to all the change characteristic sequences in the case of two-by-two combination is calculated, the reciprocal of the mean value of all the SBD distances is taken as the synchronization coefficient of temperature and steam pressure, and the difference between the steam pressure data on both sides of the membrane at the same time is calculated. The ratio of the synchronization coefficient to the standard deviation of all the difference values is taken as the dehydration steady-state index.

9. An amine recovery monitoring system for polyurethane synthesis offgas, comprising a memory, a processor, and a computer program stored in the memory and running on the processor, characterized in that, The processor executes the computer program to realize the steps of the amine recovery monitoring method for polyurethane synthesis tail gas in any one of claims 1-8.

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