JPH0568283B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for distilling a multi-component mixture, and is particularly applicable to cases where the component composition of the feedstock supplied to a distillation column varies. The present invention relates to a distillation method.

[Conventional technology]

Generally, a distillation method is employed as a means for separating and purifying multi-component mixtures having different boiling points.

For example, crude styrene obtained by dehydrogenating ethylbenzene contains impurities such as unreacted ethylbenzene and benzene, toluene, and α-methylstyrene produced by side reactions, in addition to styrene, which is a reaction product. Distillation is used to produce high-purity styrene from this crude styrene.

By the way, there is no problem if the component composition of the feed liquid supplied to the distillation column is always constant, but in reaction mixtures obtained by various reactions,
The component composition changes depending on the reaction conditions and the reaction mixture over time, and this change in the component composition may affect the distillation conditions, making it impossible to perform a stable distillation operation.

For example, in the case of the above crude styrene,
Catalyst performance and changes over time during the dehydrogenation reaction of ethylbenzene, types and amounts of impurities contained in the raw material ethylbenzene, reaction conditions such as reaction temperature and reaction pressure, and the amount of styrene used in the dehydrogenation reaction. The composition of crude steam, which is a reaction product, changes due to the influence of impurities in the steam.

In addition, styrene production plants also require periodic repairs once a year for equipment maintenance, catalyst replacement, or statutory inspections, and during these periodic repairs, the operation of the distillation tower is stopped, but then it is restarted. When starting the dehydrogenation reaction catalyst system, the method usually adopted is to add an inert gas with a low dew point such as nitrogen gas to the catalyst layer, gradually raise the temperature, and then introduce ethylbenzene. ,
In the initial stage of startup, the reaction rate is gradually increased to protect the catalyst, so the styrene concentration in the reaction product, crude styrene, is low, and the composition of the crude styrene is large compared to when the distillation tower is normally operated. Differences arise. Also, even during normal operation,
Water contamination from the intermediate tank between the reaction process and the distillation process may occur, or the performance of the reaction system may deteriorate, making it necessary to change conditions, and as a result, the composition of crude styrene, the reaction product, changes. There are things to do.

On the other hand, in crude styrene distillation towers, even in systems where light components such as benzene and toluene and ethylbenzenes are separated in the first distillation tower, benzene, toluene, etc.
Even when separating ethylbenzenes from styrene and heavy components, if there are variations in the composition of crude styrene as described above, the response of the distillation column will be slow and useful styrene will be entrained in the distillation column distillate. This not only increases the utility load in the ethylbenzene recovery system, but also entrains ethylbenzenes into the bottom liquid of the distillation column, degrading the quality of the styrene product.

[Problem that the invention seeks to solve]

In this way, when there is a change in the component composition of the feed liquid supplied to the distillation tower, the operating personnel usually take action by observing the temperature inside the distillation tower, but it is difficult to constantly monitor the temperature. In addition, to deal with this, a thermometer is installed at a point in the distillation column where the component composition fluctuates frequently, such as a pinch point, and the top reflux rate of the distillation column is controlled by information from this thermometer and liquid level control in the reflux tank. Alternatively, it is also possible to cascade control the distillate flow rate and/or the heat transfer amount of the reboiler heating medium controlled by flow rate, pressure, etc., and thereby suppress the amount of styrene entrained in the distillate as much as possible. However, in this method, the response of the distillation column to changes in the component composition of the feed liquid is slow, and there is a problem that the distillation column cannot sufficiently follow changes in the component composition of the crude styrene.

[Means to solve the problem]

Therefore, the present invention was devised in view of this point of view, and is designed to reduce the temperature of low-boiling fractions so that the temperature inside the distillation column at the point of operation in distilling a multi-component mixture becomes the set temperature. When controlling the withdrawal amount and/or the heat transfer amount of the reboiler heating medium in a cascade manner, multi-component steam is used as the reboiler heating medium, and the composition of the multi-component mixture supplied to the distillation column is continuously analyzed. By performing feed-forward control calculations on the analysis results and the temperature inside the distillation column, and using the output signal to automatically adjust the set pressure inside the gas-liquid separation tank, which determines the condensation temperature of the reboiler heating medium, it is possible to adapt to fluctuations in raw material composition. This is a method for distilling multi-component mixtures that can create optimal operating conditions for the corresponding distillation column.

The method of the present invention can be applied to any multi-component mixture as long as it can be separated and purified by distillation, but the method of the present invention is most applicable. Hydrocarbon mixtures exhibit this effect, particularly crude styrene obtained by dehydrogenating ethylbenzene in the styrene manufacturing process and purified by distillation.

There are no particular restrictions on the configuration type of the distillation column for distilling the above multi-component mixture.
It may be of a type in which the target component is taken out in high purity in the first column.
Light components with a boiling point lower than ethylbenzene, such as benzene and toluene, are separated in a column, then ethylbenzenes, styrenes, and heavy components with a boiling point higher than that are separated in a second column, and then styrenes are separated in a third column. In the first column, ethylbenzenes and light components with a boiling point lower than this are separated from the top of the first column, and styrene and heavy components with a boiling point higher than this are separated from the bottom of the column. The components are separated, and then the top low-boiling fraction and the bottom high-boiling fraction are charged into separate second columns, and the top low-boiling fraction is separated into ethylbenzenes and light components, Further, for the high-boiling fraction at the bottom of the column, a type that separates styrenes and heavy components can be adopted. Further, there is no particular restriction on the type of distillation column to which the present invention is applied, and the present invention can be applied to conventionally known types such as plate column type and packed column type.

When controlling the distillation column by the method of the present invention, the installation point of the thermometer installed to detect the temperature inside the distillation column is the point of operation, and the multi-component system subjected to distillation is It is determined as appropriate depending on the type of mixture, and may be located at any position in the distillation column.

In addition, when controlling the distillation column by the method of the present invention, the amount of low-boiling fraction extracted and/or the amount of heat transferred from the reboiler heating medium is adjusted as described above so that the temperature inside the distillation column is controlled to the set temperature. Cascade control with distillation tower internal temperature. The information to be controlled in cascade with the temperature inside the distillation column may be either the extraction amount of the low boiling point fraction or the amount of heat transferred to the reboiler heating medium, but it is preferable to perform cascade control on both of them. .

The flowmeter that detects the amount of low-boiling fraction withdrawn from the distillation column and the control valve that controls this amount of withdrawal are the boiling point distillate extraction line;
Preferably, it is provided in a line for withdrawing the low boiling point fraction from the reflux drum, which is provided in the circulation line for the low boiling point vapor. In addition, as means for detecting and controlling the amount of heat transfer of the reboiler heating medium, there is a pressure gauge that detects the pressure in the gas-liquid separation tank of the heating medium installed at the reboiler outlet, and a pressure gauge that extracts the vaporized steam from this gas-liquid separation tank. This can be done in combination with a control valve provided in the extraction line.

The reboiler heating medium used in the present invention is a multi-component steam consisting of at least two or more components having different boiling points. The reboiler heating medium may be one that condenses under a higher temperature condition than the bottom liquid of the distillation column by pressure control, and conventionally known ones can be used in combination of two or more types, but the method of the present invention When the multi-component mixture subjected to distillation is crude styrene, it is a low-boiling distillate vapor distilled from the top of the distillation column and composed of multiple components. It is preferable to heat the steam by adiabatically compressing it with a compressor.

Furthermore, in the present invention, the composition of the multi-component mixture supplied to the distillation column is continuously analyzed, the composition analysis result and the temperature inside the distillation column are subjected to a feed forward control calculation, and the output signal is Automatically adjusts the set pressure of the reboiler heating medium in the gas-liquid separation tank.

[Effect]

Hereinafter, an example in which the present invention is applied to crude styrene obtained by dehydrogenating ethylbenzene as a multicomponent mixture will be described in detail with reference to a flow sheet shown in the drawings.

In Figure 1, crude styrene is placed in charging line 1.
The distillation column 2 is charged into the middle of a packed column-type distillation column 2 operated in a reduced pressure system, and is distilled in the distillation column 2. From the top line 3 at the top of the column, low boiling point distillate vapor mainly containing ethylbenzenes is released. is extracted, and a high-boiling component liquid mainly containing styrene is extracted from the bottom line 4 at the bottom of the column.

The low boiling point vapor extracted from the top line 3 is distributed to line 6 on the condenser 5 side and line 8 on the compressor 7 side, and the low boiling point vapor distributed to the above line 6 side is The low-boiling fraction liquid is cooled in the condenser 5 and once charged into the reflux drum 9, and a portion of the low-boiling fraction liquid exiting the reflux drum 9 is refluxed to the upper part of the distillation column 2 through the reflux line 10.
The remaining portion is extracted from the extraction line 11 to the outside.

On the other hand, the low boiling point distillate vapor distributed to the line 8 side is heated by adiabatic compression in the compressor 7, and then charged to the reboiler 13 via the line 12, where it is used to convert crude styrene into the distillation column 2. Used as a reboiler heating medium for heating.

The above-mentioned distribution of the low-boiling distillate vapor to the line 6 and line 8 is performed mainly to control the amount of low-boiling distillate vapor introduced into the compressor 7 to a set value, and the control method is as,
For example, by storing a condensate of low boiling point distillate vapor in the condenser 5 and controlling the liquid level with a liquid level gauge and a control valve at the condensate outlet, the heat transfer surface of the condenser 5 can be increased or decreased. method and
Any method can be adopted, such as a method of providing a control valve on the inlet side of the condenser 5 of the line 6 to directly control it.

Furthermore, if the low boiling point distillate vapor introduced into the compressor 7 is saturated vapor, when the compressor 7 performs adiabatic compression, it will condense within the compressor 7 or at its outlet line, and droplets will form inside the compressor 7. This may cause impeller trouble or polymerization of styrene monomer, resulting in great damage. Therefore, preferably, a preheater 14 is provided in the line 8 to preheat the low boiling point distillate vapor introduced into the compressor 7. and raise its temperature.

The low boiling point distillate vapor charged into the reboiler 13 through the line 12 is used as a heating medium for the distillation column 2, and then charged into the gas-liquid separation tank 16 from the line 15, and this vapor-liquid separation tank At 16, the low boiling point fraction condensed in the reboiler 13 and the uncondensed low boiling point steam are separated. In this case, it is preferable to provide the line 15 from the reboiler 13 to the gas-liquid separation tank 16 in three parts in order to reduce pressure loss on the heating side in the reboiler 13 and improve heat transfer efficiency. . That is, the first line is a line for flowing condensed liquid into the gas-liquid separation tank 16, and the second line is a line for flowing uncondensed low-boiling distillate vapor into the gas-liquid separation tank 16. The third line is a line for flowing inert gas such as air or nitrogen from the upper part of the reboiler 13 into the gas-liquid separation tank 16. This reboiler 13
The low boiling point fraction that is condensed in
The uncondensed low-boiling distillate vapor that passes through without being condensed is a relatively low-boiling component of the low-boiling distillate vapor, which is mainly composed of benzene, toluene, water, air, nitrogen, etc. .

The uncondensed low-boiling fraction vapor separated in the gas-liquid separation tank 16 is extracted from the line 17 and joins the line 6 leading to the condenser 5. The fraction is withdrawn from line 18 and introduced into preheater 14 where it is used as a heating source for the low boiling distillate vapor introduced into compressor 7 before joining line 6 through line 19.

In FIG. 1, reference numeral 20 is a reboiler used when starting up the distillation column 2, and steam is introduced into the reboiler 20 from a line 21. Further, a line 22 communicating with the line 6 from the outlet side of the compressor 7 is also a bypass line used during startup of the distillation column 2 until the distillation system is stabilized or for warming up the compressor 7.

In this flow sheet, a gas chromatograph 23 for continuous analysis is provided in the charging line 1 to continuously analyze the component composition of crude styrene charged into the distillation column 2 through the charging line 1, A thermometer 24 for detecting the internal temperature of the distillation column 2 is provided at an intermediate position of the distillation column 2, and the reflux drum 9 is discharged through the withdrawal line 11 from the reflux drum 9. A flow meter 25 for detecting the flow rate of the low boiling point fraction and a control valve 26 for controlling the flow rate of the low boiling point fraction are provided. A line 1 is provided with a pressure gauge 27 for detecting the pressure of the low boiling point distillate vapor and extracts the uncondensed low boiling point vapor in the gas-liquid separation tank 16 to join the line 6.
7 is provided with a control valve 28 for controlling the pressure of the uncondensed low boiling point distillate vapor.

The distillation tower 2 detected by the thermometer 24
The internal temperature, the extraction amount of the low-boiling fraction detected by the flow meter 25, and the pressure of the uncondensed low-boiling fraction vapor in the gas-liquid separation tank 16 detected by the pressure gauge 27 are controlled in cascade, respectively. At the same time, the analysis results of the continuous analysis gas chromatograph 23 and the distillation column 2 detected by the thermometer 24 are
The internal temperature is calculated by feed forward control, and the set pressure of the low boiling point distillate vapor in the gas-liquid separation tank 16 is controlled based on the output signal. The arithmetic expression for the food forward control described above is determined as appropriate depending on the type of the multicomponent mixture supplied from the charging line 1 into the distillation column 2.

In this flow sheet, since the multi-component mixture supplied from the charging line 1 is crude styrene, for example, if the low boiling point fraction increases, the analysis results and the temperature inside the distillation column 2 will differ. Based on this, a feedforward operation is performed so that the temperature inside the distillation column 2 is controlled to a set value, and the output signal is used to control the control valve 26 so as to increase the amount of low boiling point distillate extracted, and also to control the gas-liquid separation tank. The control valve 28 is controlled so as to keep the set pressure of the low boiling point distillate vapor in the distillation column 2 constant. Immediately after entering the reboiler 13, the condensation temperature of the low-boiling fraction steam is increased, and the amount of heat transfer of the low-boiling fraction steam flowing through the reboiler 13 is increased. In addition,
On the other hand, when the low boiling point fraction decreases, it goes without saying that the completely opposite control is performed as in the case where the low boiling point fraction increases.

〔Example〕

According to the distillation system of the flow sheet shown in Figure 1,
Crude styrene obtained by dehydrogenating ethylbenzene was distilled. In this distillation system, a packed column filled with a regular packing (product name: Merapack, manufactured by Sumitomo Heavy Industries, Ltd.) was used as the distillation column 2, and the differential pressure between the top and bottom of the column was controlled to 70 mmHg or less. This distillation system was operated by constantly supplying crude styrene.

Figures 2 and 3 show the fluctuations in the composition of crude styrene supplied from the charging line 1 to the distillation column 2 and the pressure fluctuations in the gas-liquid separation tank of the low-boiling fraction supplied to the reboiler as a heat transfer medium, respectively. It is shown in Figure 3.

In this embodiment, as shown in FIG.
Despite fluctuations in the composition of the crude styrene, the distillation system was able to operate stably, and the composition of the high-boiling bottom component liquid, which is mainly styrene, was stabilized.

〔Effect of the invention〕

According to the method of the present invention, it is possible to automatically create optimal operating conditions for the distillation column in response to fluctuations in the raw material composition, thereby preventing an increase in the utility load in the low-boiling fraction recovery system. In addition, the distillation system can be operated stably without causing low-boiling fractions to be entrained in the bottom liquid of the distillation tower and deteriorating the quality of the product.

[Brief explanation of drawings]

FIG. 1 is a flow sheet showing a method for distilling a multicomponent mixture according to an example of the implementation of the present invention. FIG. 2 is a graph showing fluctuations in the component composition of crude styrene.
The figure is a graph showing pressure fluctuations of a low boiling point fraction in a gas-liquid separation tank. Code explanation, 2... Distillation column, 7... Compressor, 13
... Reboiler, 16 ... Gas-liquid separation tank.

Claims (1)

[Claims] 1. In the distillation of a multi-component mixture, the amount of low-boiling fraction extracted and/or the amount of heat transferred from the reboiler heating medium is controlled so that the temperature inside the distillation column at the point of operation becomes the set temperature. When performing cascade control,
Using multi-component steam as the reboiler heating medium and continuously analyzing the composition of the multi-component mixture supplied to the distillation column, and performing feedforward control calculations on the composition analysis results and the temperature inside the distillation column; A method for distilling a multi-component mixture, characterized in that the set pressure in a gas-liquid separation tank of a heating medium installed at the outlet of the reboiler is automatically adjusted based on the output signal. 2. Distillation of a multi-component mixture according to claim 1, wherein the reboiler heating medium is one in which the temperature is raised by adiabatically compressing low-boiling point multi-component vapor distilled from the top of a distillation column using a compressor. Method. 3. The method for distilling a multi-component mixture according to claim 1 or 2, wherein the multi-component mixture is a hydrocarbon mixture. 4. Claim 3: The hydrocarbon mixture is crude styrene obtained by dehydrogenating ethylbenzene, and the low-boiling multi-component vapor is a low-boiling hydrocarbon vapor mainly composed of ethylbenzene with a styrene content of 5% by weight or less. A method for distilling the described multicomponent mixture.