JP2005224695A - Control system of distillation column - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To control a distillation column 1, which receives the continuous supply of a raw material to distill the raw material, to always stably operating the same so as to develop the maximum capacity thereof while preventing the occurrence of a flooding phenomenon without bringing about an increase in cost or the deterioration of maintenance properties in the case of continuously changing in the operation condition of the column. <P>SOLUTION: The control system of the distillation column 1 is equipped with a pressure acquiring means 6-1 for acquiring the pressure of the distillation column 1, a differential pressure acquiring means 6-2 for acquiring the differential pressure of the distillation column 1, a flooding ratio calculating means 6-3 for calculating the flooding ratio of the distillation column 1 on the basis of the acquired differential pressure or pressure of the distillation column 1 and a load regulating means 6-4 for adjusting the load of the distillation column 1 so that the calculated value of the flooding ratio becomes the value equal to or less than the value of the maximum flooding ratio. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a control system for a distillation column. More specifically, the present invention relates to a system for controlling a distillation column that continuously feeds raw materials and distills the raw materials while preventing the occurrence of flooding.

  Conventionally, in various chemical plants, a distillation column (rectifying column) is used in each step in order to purify a target product by distilling a raw material or a decomposition product thereof.

Hereinafter, the prior art as the background of the present invention will be described by taking an ethylene production plant which is a kind of chemical plant as an example. However, the scope of application of the present invention is not limited to this example.
As an example of a typical configuration of an ethylene production plant, a process of thermally decomposing raw materials such as naphtha in a cracking furnace (decomposition process), a process of quenching and rectifying the obtained cracked gas (quenching process), A process of compressing the rapidly cooled and rectified cracked gas (compression process) and a process of separating and purifying the compressed cracked gas into its components to obtain ethylene, propylene, etc. as main components (purification process) Configuration.

Control of the operation of the distillation column is required at various points in the series of steps.
Hereinafter, the control of the operation of the distillation tower will be described using a distillation facility used in a purification process of a general ethylene plant as an example.

  FIG. 2 is a diagram schematically illustrating an example of a configuration of a distillation facility used for separating ethylene and ethane in a refining process of a general ethylene plant. The distillation facility shown in FIG. 2 is operated continuously, and includes an ethylene tower (distillation tower) 1, a condenser 2, a drum 3, and a reboiler 4.

  The ethylene column 1 is a distillation column for separating ethylene and ethane. Supply (feed) the mixture a (mainly composed of ethylene and a small amount of light fractions such as ethane and hydrogen, etc.) obtained in the upstream process, and supply ethylene b from the top or middle stage of the tower Ethane c is recovered from the bottom.

  The condenser 2 cools the gas at the top of the ethylene tower 1 by heat exchange and condenses a part or all of it. Propylene is used as the refrigerant B for heat exchange in the condenser 2.

  The drum 3 collects the liquid condensed by the condenser 2 and separates it from a gas containing a light fraction. The liquid in the drum 3 is refluxed into the distillation column 1, and the gas in the drum 3 is appropriately extracted outside.

  In addition, a part of the liquid extracted from the bottom of the distillation column 1 is introduced into the reboiler 4, heated, and refluxed into the distillation column 1.

  Further, the distillation equipment shown in FIG. 2 is provided with valves 51 to 55 for flow rate adjustment. The valve 51 is for adjusting the amount of the mixture a of ethane and ethylene fed to the distillation column 1. The valve 52 adjusts the flow rate of the refrigerant in the capacitor 2. The valve 53 is for adjusting the flow rate of the liquid refluxed from the drum 3 into the distillation column 1. The valve 54 is for adjusting the amount of ethylene b recovered from the middle column of the distillation column 1. The valve 55 is for adjusting the amount of ethane c recovered from the bottom of the distillation column 1. The valve 56 adjusts the amount of gas extracted from the drum 3.

  Various methods are known for controlling the operation of the distillation column. For example, particularly at the top of the column, there are a control for making the reflux amount constant, a control for making the reflux ratio constant, a control for making the specific component concentration at the top of the tower constant, and the like. As for the bottom of the column, there are a control for keeping the temperature of a specific stage constant, a control for keeping the amount of extraction at the bottom of the column constant, a control for keeping a specific component of the bottom extraction, for example (Non-Patent Document 1).

  As another method, model predictive control that controls process prediction by modeling process behavior, multivariable control that controls many control variables with many manipulated variables at the same time, multivariable model predictive control that combines these Etc. In these controls, it is necessary to set various control targets of the distillation column to specific values set in advance or values within a specific range.

In order to operate the distillation column efficiently, it is desirable to always operate the distillation column at its maximum capacity. Therefore, the control target is preferably set to a value corresponding to the maximum capacity of the distillation column.
However, when the driving situation changes continuously and the control target changes accordingly, the problem is how to determine the control target. In this regard, there is a method in which an operator determines a control target while observing a change in operating conditions, and manually changes this, but in such a method, the load on the operator increases and an adjustment delay occurs, resulting in energy loss. And product loss occurs. In addition, there is a method of setting a value having a margin on the safe and stable side as a control target. However, in such a method, energy loss and product loss also occur due to the margin.
In this way, when operating at the maximum capacity of the distillation column, if the maximum value of the capacity changes continuously, the conventional operation method can be operated below the original maximum capacity due to adjustment delay and adjustment margin. I must.

  When the operation status of the distillation tower exceeds the capacity of the distillation tower, various abnormal phenomena occur depending on the portion exceeding the capacity (Non-Patent Document 2). Among these abnormal phenomena, the one that appears most frequently and makes it impossible to continue driving is flooding. Flooding is a phenomenon in which the liquid that should flow down through the downcomer overflows from the top of the tower.

  Therefore, there has been a demand for stable operation of the distillation column at its maximum capacity without causing flooding. For this reason, the “flooding ratio”, which is a value representing the degree of flooding in the distillation column, is used to determine the maximum flooding ratio value (maximum flooding ratio) that can maintain product quality, and flooding in the tower It is required to adjust the feed amount to the distillation column so that the ratio is equal to or less than the maximum flooding ratio.

Here, it is known that the flooding phenomenon is closely related to the pressure loss in the distillation column (Non-patent Document 3). Therefore, the flooding ratio in the column can be grasped by measuring the pressure loss by changing the amount of vapor or liquid in the distillation column.
It is also known that the flooding phenomenon varies depending on the density of liquid and gas in the tower (Non-patent Document 3). As the distillation column pressure changes, the flooding rate also changes.

Patent Document 1 is based on such knowledge, and by controlling the differential pressure of the distillation column to be constant under certain operating conditions, the flooding rate in the column is equal to or less than the maximum flooding rate. The technique of adjusting to become is disclosed.
However, this technology is based on the assumption that the operating conditions are constant. When the operating conditions change continuously, it is necessary to obtain the flooding ratio in real time from the operating conditions, which complicates the mechanism. As a result, there is a problem that costs are increased and maintenance is deteriorated.

Further, in Patent Document 2, when the operating condition changes, an appropriate level value is selected according to the operating state from a plurality of predetermined level values of the gas in the distillation column, and the level value is used. The technique of controlling is disclosed.
However, in this technique, since the level value is a predetermined value, the control target value changes only discontinuously, the calculation of the gas amount in the tower and the selection of the optimum gas amount level value are complicated. There are issues such as.

"PLANTWIDE PROCESS CONTROL", published by McGrawHill, William L. Luyben, Bjorn D. Tires, Michael L. Luyben, 1999 "Distillation Engineering Handbook", published by Asakura Shoten, Mitsuho Hirata, Masahiro Semi, 4th edition, 1974 "Distillation engineering" Kodansha Scientific, written by Shuzo Oe, 1990 JP 2000-176202 A JP 2001-259301 A

  The present invention has been made in view of the above-mentioned problems, and its purpose is to prevent flooding without causing a rise in cost and deterioration of maintenance when the operating conditions of the distillation column are continuously changed. The object of the present invention is to provide a distillation column control system capable of stably operating the distillation column at its maximum capacity at all times while preventing the occurrence.

  As a result of intensive studies to solve the above problems, the present inventors have found that the degree of flooding under various operating conditions can be expressed by a very simple formula based on the operation measurement data related to the target distillation column. It was. Furthermore, by controlling the distillation column using the equation, even in a situation where the operating conditions are continuously changed, the occurrence of flooding phenomenon is prevented without incurring an increase in equipment cost and deterioration of maintenance. It has become possible to always operate the distillation column stably at its maximum capacity, and found that the above-mentioned problems can be effectively solved, thereby completing the present invention.

  That is, the gist of the present invention is a system for controlling a distillation column that continuously feeds raw materials and distills the raw material while preventing the occurrence of flooding, and obtains the pressure of the distillation column. The pressure acquisition means, the differential pressure acquisition means for acquiring the differential pressure between the upper and lower pressures in the distillation column, and the flooding rate of the distillation column is calculated based on the acquired differential pressure and pressure of the distillation tower. A distillation column, comprising: a flooding rate calculating unit that adjusts the load of the distillation column so that the calculated flooding rate is equal to or less than a value of the maximum flooding rate. Exist in the control system.

  According to the present invention, in a distillation column in which raw materials are continuously supplied and distilled, a flooding phenomenon occurs without causing an increase in cost or deterioration of maintenance when operating conditions continuously change. It is possible to always operate the distillation column stably at its maximum capacity while preventing the above.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the following embodiment is a representative example, and the present invention is not limited to this, and can be arbitrarily modified without departing from the scope of the invention.

  FIG. 1 is a diagram schematically illustrating a configuration of a distillation facility to which a control system according to an embodiment of the present invention is applied. The basic configuration of the distillation equipment shown in FIG. 1 is the same as that of the conventional distillation equipment shown in FIG. That is, the distillation facility shown in FIG. 1 includes a distillation column 1 that continuously feeds raw materials and distills them. In addition, about the component similar to each component shown in FIG. 2, it has shown with the same code | symbol.

  1 is provided with a control system 6 for operating the distillation column (ethylene column) 1 while preventing the occurrence of flooding. The control system 6 includes a pressure acquisition unit 6-1, a differential pressure acquisition unit 6-2, a flooding rate calculation unit 6-3, and a load adjustment unit 6-4.

  The pressure acquisition means 6-1 acquires the pressure inside the distillation column 1. The pressure acquisition site | part of the distillation column 1 is arbitrary, You may acquire from any site | parts, such as the upper stage in a tower, a middle stage, and a lower stage. Moreover, even if it does not acquire the tower pressure of the distillation column 1 directly, the pressure correlated with the tower pressure may be acquired and used as the tower pressure. Specifically, since the pressure in the distillation column 1 substantially coincides with the pressure in the drum 3, the pressure acquisition means 6-1 in FIG. 1 is provided in the drum 3, and the pressure in the drum 3 is continuously or intermittently provided. This is measured and used as the pressure value of the distillation column 1. However, the pressure acquisition means 6-1 may be provided in the distillation column 1 and the pressure in the column of the distillation column 1 may be directly measured.

The differential pressure acquisition means 6-2 acquires the differential pressure of the distillation column 1. The differential pressure in the distillation column 1 refers to the pressure difference between two parts inside the distillation column. Specifically, the differential pressure acquisition means 6-2 is provided in the distillation column 1 and continuously calculates a pressure difference (differential pressure) between two predetermined sites (differential pressure acquisition sites) in the column of the distillation column 1. Or it measures intermittently.
The differential pressure acquisition site needs to be selected so as to include a location where flooding occurs in the tower. Therefore, when the flooding occurrence location cannot be specified, the differential pressure from the tower top to the tower bottom (that is, the differential pressure of the entire tower) is acquired. However, since the sensitivity to flooding is higher when the interval between two differential pressure acquisition parts is narrower, if the flooding occurrence part can be specified in a certain range (a range narrower than the entire tower), the two parts including that range It is preferred to measure the differential pressure between them (ie, the partial differential pressure in the column). For example, when the flooding occurrence point exists in the range from the top of the column to the feed stage of the raw material, it is preferable to measure the differential pressure from the top of the column to the feed stage of the raw material. When it exists in the range to the tower bottom, it is preferable to measure the differential pressure from the feed stage of the raw material to the tower bottom. However, if the operating conditions of the distillation column fluctuate, the flooding occurrence location also changes, and the flooding occurrence location may deviate from the differential pressure acquisition location. Therefore, care must be taken such as maintaining the operating conditions constant during the execution of the control, and setting the differential pressure acquisition part with a margin in anticipation of fluctuations in the flooding occurrence part due to fluctuations in the operating conditions.

  Basically, it is necessary to acquire the pressure and the differential pressure for the same part without changing the pressure acquisition part and the differential pressure acquisition part during the control. However, by preparing a plurality of flooding rate calculation functions (functions expressed by Equation (1)) described later according to a plurality of pressure acquisition sites and a plurality of sets of differential pressure acquisition sites, It is also possible to change the pressure acquisition site.

  The flooding rate calculation means 6-3 is based on the pressure of the distillation column 1 acquired by the pressure acquisition means 6-1 and the differential pressure of the distillation column 1 acquired by the differential pressure acquisition means 6-2. The flooding rate is calculated.

  Specifically, the flooding rate calculation means 6-3 receives information about the pressure and the differential pressure of the distillation column 1 from the pressure acquisition means 6-1 and the differential pressure acquisition means 6-2, respectively. . Based on the pressure and differential pressure of the distillation column 1, a “flooding rate” that is an index representing the degree of flooding in the distillation column 1 is calculated.

  Conventionally, the “flooding ratio” that has been used as an indicator of the degree of flooding is a value defined in the tray design of a distillation column and is a very complicated formula. It is difficult to perform control with high accuracy. In addition, in order to accurately determine the “flooding ratio”, it is necessary to grasp the internal situation of the distillation column, but it is difficult to grasp the internal situation of the distillation column during actual operation, and verification is also performed accurately. It is difficult.

  On the other hand, the present inventors use the pressure and differential pressure of the distillation column 1 which are parameters that are easy to grasp during actual operation to calculate the degree of flooding in the actual operation range of the distillation column 1 using a statistical method. It was found that it can be approximated by and defined as “flooding rate”.

  As a function for calculating the flooding rate of the distillation column 1, an appropriate function can be selected according to conditions such as the processing capacity of the computer to be used.

Specifically, when the processing capability of the computer is sufficiently high, it is preferable to use a nonlinear function represented by a multi-degree polynomial from the viewpoint of accurately expressing the degree of flooding.
However, when there is a limit to the processing capability of the computer, using a very complicated function places a load on the computer, which is not preferable in terms of control. In addition, it is not preferable to use a complicated function from the viewpoint of reducing the labor for obtaining the function. Therefore, in such a case, it is preferable to use a function represented by a relatively low-order polynomial, for example, a function represented by a first-order polynomial.

An example of a function represented by a first order polynomial is a function represented by the following mathematical formula (1).
Flooding rate = a × ΔP + b × P + C (1)

  In Equation (1), a, b, and c are coefficients derived from the plant operation data by a statistical method, and are different values depending on the structure of the distillation column 1 (particularly, the configuration of the tray), operation conditions, and the like. In the present invention, prior to the operation of the present control system 6, these coefficients a, b, c are determined in advance to create the mathematical formula (1), which is stored in the flooding rate calculating means 6-3.

Then, the flooding rate calculating means 6-3 calculates the differential pressure ΔP of the distillation column 1 input from the differential pressure acquisition means 6-2 and the pressure P of the distillation column 1 input from the differential pressure acquisition means 6-2. make use of,
The flooding rate of the distillation column 1 is calculated by this mathematical formula (1).

  In addition, when changing a pressure acquisition part and a differential pressure acquisition part during control as mentioned above, a plurality of mathematical formulas (1) corresponding to a plurality of pressure acquisition parts and a plurality of sets of differential pressure acquisition parts are created, This is stored in the flooding rate calculation means 6-3. And according to the change of a pressure acquisition site | part or a differential pressure acquisition site | part, appropriate numerical formula (1) is selected and a flooding rate is calculated. This makes it possible to change the pressure acquisition site and the differential pressure acquisition site during the control as required for the maintenance and operating conditions of the distillation column.

The formula (1) can be specifically created by the following method.
Prior to the operation of the control system 6, the distillation column 1 is actually operated in the vicinity of its maximum capacity in advance, and the load on the distillation column 1 is gradually increased while monitoring the operation state of the distillation column 1. At this time, the differential pressure in the distillation column 1 is continuously measured.
In addition, if there is a performance curve for the tray, the operation of the tower near the flooding rate can be estimated from the flow rate of liquid and gas passing through the tray. It can also be indexed and reflected in actual driving.

  By collecting similar data under various operating conditions (for example, when the pressure conditions are different, etc.) and performing statistical analysis on these data, the above-described coefficients a, b, c are determined. (1) can be created.

  Here, when there is an operating condition that may change particularly in the target distillation column, more accurate control is possible if this operating condition is also used as a variable. For example, if the distillation column frequently changes the composition of the feed to the distillation column, it is preferable to use the feed composition as one of the variables in the formula for calculating the flooding rate.

  In data collection, formula (1) can be created by performing statistical analysis using data if there is enough data available in the past, even if no tests are performed. You can also. Further, data calculated by a simulation using a computer may be used.

  The load adjusting unit 6-4 adjusts the load of the distillation column 1 so that the flooding rate calculated by the flooding rate calculating unit 6-3 is equal to or less than the value of the maximum flooding rate. is there.

  Here, the “maximum flooding rate” means a value of the maximum flooding rate that can maintain the quality of the product. As described above, it is known that the flooding phenomenon is closely related to the pressure loss in the distillation column 1 and appears as a phenomenon in which the differential pressure in the distillation column 1 increases rapidly. Therefore, in the present invention, the point at which the differential pressure suddenly increases is determined as “maximum flooding rate” from the differential pressure increase data with respect to the load on the distillation column 1, and this is defined as the maximum capacity of the distillation column 1. Prior to the operation of the control system 6, the maximum flooding rate is determined in advance based on the above formula (1), and this is set in the load adjusting means 6-4.

  Specifically, the load adjusting unit 6-4 receives the calculated value of the flooding rate from the flooding rate calculating unit 6-3, and is connected to the valve 51 and fed to the distillation column 1 with a mixture of ethane and ethylene. The feed amount of a can be adjusted. Then, the feed amount of the mixture a to the distillation column 1 is adjusted so that the flooding rate of the distillation column 1 is equal to or less than the value of the maximum flooding rate. Note that the feed amount of the mixture a to the distillation column 1 is not adjusted directly through the valve 51 in this way, but the feed amount of the mixture a to the distillation column 1 is adjusted by adjusting the feed amount of the raw material upstream of the plant. May be adjusted indirectly.

  As described above, the control system 6 according to the present embodiment represents the degree of flooding of the distillation column with an index (flooding rate) consisting of a very simple formula, and the operation measurement data of the distillation column 1 under various operating conditions. Based on this, the value of the maximum flooding rate capable of maintaining the quality of the product is obtained, and the load of the distillation column 1 is set so that the flooding rate of the distillation column 1 is equal to or less than this maximum flooding rate value. Control is performed. This makes it possible to easily obtain the maximum flooding rate in real time without causing a rise in cost and deterioration of maintenance, even in a situation where operating conditions continuously change, while preventing the occurrence of flooding phenomenon. It is possible to always operate the tower 1 stably at its maximum capacity.

  In addition, in the control system 6 of FIG. 1, the load adjustment means 6-4 is comprised so that the load of the distillation column 1 may be adjusted by adjusting the feed amount of the raw material to the distillation column 1. FIG. Such a configuration is most effective, but the method for adjusting the load of the distillation column 1 is not limited to this. In particular, there are many cases where the feed amount of the raw material cannot be easily changed, and there is a case where a large delay occurs in the change. In such a case, for example, the load adjusting means 6-4 may be configured to adjust the load of the distillation column 1 by adjusting the pressure of the distillation column 1 through the valves 52 to 56 and the like. .

The control system of the present invention can be applied to control of a general distillation column. The control to be applied may be simple set point control, or may be more complex and advanced control such as model predictive control, multivariable control, multivariable model predictive control.
For example, when multi-variable control is performed, specifically, values such as the flow rate in the valves 51 to 54, the temperature in the distillation column 1 and the flow rate of the heating medium in the reboiler 4 are used as operating variables and are recovered from the distillation column 1. The composition of the product (ethylene b and ethane c in FIG. 1) is set as a control variable, and multi-variable model predictive control is performed, so that the flooding rate of the distillation column 1 is equal to the value of the maximum flooding rate or It becomes possible to stably control the load of the distillation column 1 so as to be below this value.

  The control system of the present invention can be easily realized by using a normal computer system (for example, various network servers, workstations, personal computers, etc.) provided with a CPU, RAM, hard disk, various peripheral devices, and the like. it can. In such a computer system, the CPU executes various arithmetic processes according to a computer program stored in a hard disk or the like, and data necessary for the various arithmetic processes of the CPU is appropriately stored and updated in the RAM. The computer system further includes various devices (input devices such as a keyboard and a mouse, output devices such as a display and a printer, a storage medium reading / writing device such as a CD-ROM drive and a floppy disk drive, communication, etc. A network interface or the like) is built-in or connected, and is controlled by CPU arithmetic processing. That is, the above-described means 6-1 to 6-4 are realized by the functions of the CPU, RAM, hard disk, and various peripheral devices.

  Functions corresponding to the respective means 6-1 to 6-4 of the control system of the present invention are normally obtained by reading a computer program recorded on the hard disk into the RAM, starting the computer program and executing it by the CPU. It is realized as an operation of the CPU or as a cooperative operation between the CPU and the RAM, the hard disk, various peripheral devices, and the like. Here, this computer program is recorded on various computer-readable storage media such as CD-ROM, DVD-ROM, MO disk, etc., and through a storage medium reader provided in the computer system as needed. These are preferably used directly or installed on a hard disk. Alternatively, the computer program is recorded in an external storage area (another computer or the like) that can be accessed from the computer system through the information communication network, and if necessary, it is directly stored in the external storage area through the information communication network. Or, it is preferable to install and use it on a hard disk.

  According to the present invention, in the distillation column in which the raw material is continuously supplied and distilled, the flooding phenomenon can be prevented without causing an increase in equipment cost or deterioration of maintenance when the operating conditions are continuously changed. It is possible to always stably operate at the maximum capacity while preventing the occurrence. Therefore, the distillation tower having such a configuration is used, and it can be used extremely highly in fields where stable and efficient control is always required, for example, various chemical plants represented by ethylene purification plants. It is thought that it has nature.

It is a figure which shows typically the structure of the distillation installation to which the control system which concerns on one Embodiment of this invention is applied. It is a figure which shows typically an example of a structure of a general distillation installation.

Explanation of symbols

1 Distillation tower (ethylene tower)
2 condenser 3 drum (gas-liquid separator)
4 Reboiler 51 to 56 Valve 6 Distillation Tower Control System 6-1 Pressure Acquisition Unit 6-2 Differential Pressure Acquisition Unit 6-3 Flooding Ratio Calculation Unit 6-4 Load Control Unit

Claims (3)

A system for controlling a distillation column that continuously feeds raw materials and distills the raw material while preventing operation of flooding,
Pressure acquisition means for acquiring the pressure of the distillation column;
Differential pressure acquisition means for acquiring the differential pressure of the distillation column;
A flooding rate calculating means for calculating a flooding rate of the distillation column based on the obtained differential pressure and pressure of the distillation column;
A distillation column control system comprising load adjusting means for adjusting a load of the distillation column so that the calculated flooding rate is equal to or less than a value of the maximum flooding rate. 2. The distillation column control system according to claim 1, wherein the load adjusting means adjusts the load of the distillation column by adjusting the amount of the raw material supplied to the distillation column. The distillation column control system according to claim 1, wherein the load adjusting means adjusts the load of the distillation column by adjusting the pressure of the distillation column.

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