JPH035373B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for distilling a styrene-containing mixture containing components with boiling points close to each other, such as styrene and ethylbenzene. [Prior Art] Styrenes such as styrene, vinyltoluenes, and α-methylstyrene are produced by dehydrogenating corresponding ethylbenzenes such as ethylbenzene, ethyltoluenes, and kyumene. This dehydrogenated oil is mainly composed of produced styrenes and unreacted ethylbenzenes, but also contains small amounts of light components such as benzene and toluene, and heavy components where styrenes are partially polymerized. Therefore, in the production of styrenes, it is necessary to separate and remove the light and heavy components from the dehydrogenated oil, and a distillation separation method using multiple distillation columns is adopted as a method for this purpose. ing. By the way, the boiling points of styrene and ethylbenzene are close to each other. For example, in the case of styrene and ethylbenzene, the boiling point of the former is 145.2°C under atmospheric pressure, and the boiling point of the latter is 145.2°C.
136.2℃, and the boiling point difference between them is only 9.0℃. Therefore, in order to separate styrenes and ethylbenzenes in a distillation column, high distillation efficiency is required, and it is necessary to increase the number of plates and the reflux ratio of the distillation column. For this reason, if the distillation column used for distilling styrenes is a tray column, it is common to have 60 to 100 trays installed, and the reflux ratio is also 10. Generally, the pressure difference between the top and bottom of the column is large, ranging from 90 to 250 mmHg. Therefore, in such a distillation column, there is a problem that a large amount of energy needs to be supplied as a heat source. Further, as a method for reducing the energy used in a distillation column, a distillation method called a heat pump method is known (for example, Japanese Patent Laid-Open No. 111466/1983). In this distillation method, the steam distilled from the top of the tower is adiabatically compressed to raise its temperature, and this is used as a heat source for the reboiler, but the temperature difference between the top temperature and the bottom temperature in the distillation tower This method cannot be used when the compression ratio becomes large, and it was thought that this method could not be used for the distillation of styrenes. [Problems to be Solved by the Invention] The present invention was devised in view of this point of view, and its purpose is to provide a styrene product that consumes less energy and can perform stable distillation operations. The objective is to provide a similar distillation method. [Means for Solving the Problems] That is, the present invention provides a charging system operated in a reduced pressure system when distilling dehydrogenated oil obtained by dehydrogenating ethylbenzenes and separating styrenes and ethylbenzenes. The above-mentioned dehydrogenated oil is introduced into the middle part of a column-type distillation column, and low-boiling point vapor mainly containing ethylbenzenes is extracted from the top of the column, and high-boiling point vapor mainly containing styrene is extracted from the bottom of the column. The component liquid is extracted, and part or all of the low boiling point distillate vapor is led to a compressor and adiabatically compressed to raise the temperature, and then led to the reboiler of the distillation column, where part or all of it is condensed to produce the above-mentioned reboiler. This is a method for distilling styrenes, in which the condensed low-boiling fraction or a part of the uncondensed vapor is returned to the distillation column circulation line, and the remainder is discharged from the distillation column. The dehydrogenated oil to which the method of the present invention is applied is obtained by dehydrogenating ethylbenzenes, and is mainly composed of produced styrenes and unreacted ethylbenzenes, and in some cases may contain a small amount of benzene. , light components such as toluene, heavy components partially polymerized with styrene, polymerization inhibitors added to suppress polymerization during distillation separation process, moisture, air, nitrogen, etc. The styrenes include styrene, vinyltoluenes, α-methylstyrene, and the like, and the ethylbenzenes include ethylbenzene, ethyltoluenes, isopropylbenzene, and the like. For example, the structure of the distillation column for distilling the above-mentioned dehydrogenated oil is such that the first column separates light components with boiling points lower than ethylbenzenes, and then the second column separates ethylbenzenes, styrenes, and higher boiling points. A type that separates heavy components and further separates styrenes and heavy components in a third column,
Alternatively, ethylbenzenes and light components with lower boiling points are extracted from the top of the first column, and
Styrenes and higher-boiling-point heavy components are extracted and separated from the bottom of the column, and then the top distillate and the bottom distillate are charged into separate second columns, and the above-mentioned top distillate is extracted. There is a type in which the liquid is separated into light components and ethylbenzenes, and the bottom distillate is separated into styrenes and heavy components. The present invention can be applied to any of the above-mentioned types, and the distillation column to which the present invention is applied is the second column in the former case, and the first column in the latter case. It is a tower. Therefore,
The dehydrogenated oil to which the present invention is applied may be one that has just been dehydrogenated from ethylbenzenes, or one that has previously separated and removed light components such as benzene and toluene, which have a lower boiling point than ethylbenzenes. It's okay. Further, the distillation column to which the present invention is applied is a packed column type distillation column operated in a reduced pressure system. The packing to be filled in this distillation column is preferably a regular packing, particularly one having a differential pressure per theoretical plate number of 1.5 mmHg or less, preferably 0.8 mmHg or less.
By using such a distillation column, the distillation efficiency is excellent and the reflux ratio can also be reduced. Therefore, the differential pressure in the distillation column is also as small as 100 mmHg or less, preferably 70 mmHg or less, and as a result, the bottom temperature can be lowered, and the temperature difference between the top and bottom of the column can be reduced. In the above-mentioned distillation column, a low-boiling fraction vapor mainly composed of ethylbenzenes is extracted from the top of the column, and a high-boiling component liquid mainly composed of styrene is extracted from the bottom of the column. Part or all of the low-boiling fraction vapor extracted from the top of the distillation column is led to a compressor, where it is adiabatically compressed to raise its temperature. The steam is led to the reboiler of the distillation column, where the majority of the steam is condensed, leaving some or all of the uncondensed steam, and is used as a heating source for the reboiler. In particular, if it does not contain light components such as benzene and toluene, which have a lower boiling point than ethylbenzenes, it is preferable to condense all of the components, and if it does contain light components, it is preferable to leave uncondensed vapor. Here, the proportion of low-boiling distillate vapor led to the compressor is determined by the composition of the dehydrogenated oil charged into the distillation column, the composition of the low-boiling distillate vapor extracted from the top of the distillation column, and the composition of the low-boiling distillate vapor extracted from the top of the distillation column. It is determined by considering the amount of steam, the performance of the compressor, the amount of heat added by this compressor, the amount of heat required by the reboiler of the distillation tower, etc. In addition, a portion of the low-boiling fraction and uncondensed steam condensed in this reboiler is returned to the distillation tower circulation line, and the remainder is discharged from the distillation tower. [Function] Hereinafter, an example of the present invention will be described in detail according to a flow sheet shown in the drawings. In FIG. 1, dehydrogenated oil is charged from a charging line 1 to the middle part of a packed column-type distillation column 2 that is operated in a reduced pressure system, and is distilled under reduced pressure in this distillation column 2. Low boiling point distillate vapor mainly composed of ethylbenzenes is extracted from line 3, and
A high-boiling component liquid mainly containing styrene is extracted from line 4 at the bottom of the column. The low boiling point distillate vapor extracted from the line 3 is distributed to line 6 on the condenser 5 side and line 8 on the compressor 7 side as necessary, and the low boiling point distillate vapor is distributed to the line 6 side. After the vapor is cooled in the condenser 5, it is once charged into the overflow drum 9, and a portion of the low-boiling fraction that exits the overflow drum 9 is refluxed to the upper part of the distillation column 2 through the line 10. At the same time, the remaining portion is extracted from the line 11 to the outside. Furthermore, the low boiling point distillate vapor distributed to the line 8 side is heated by adiabatic compression in the compressor 7, and then charged into the reboiler 13 via the line 12, which heats the dehydrogenated oil introduced into the distillation column 2. used as a source. The distribution of the low boiling point vapor to the line 6 on the condenser 5 side and the line 8 on the compressor 7 side is mainly to control the amount of low boiling point vapor introduced into the compressor 7 to a set value. The control method is, for example, by storing a condensate of low boiling point vapor in the condenser 5 and controlling the liquid level with a liquid level gauge and a control valve at the condensate outlet. Any method can be adopted, such as a method of controlling by increasing or decreasing the heat transfer surface of the condenser 5 or a method of providing direct control by providing a control valve on the inlet side of the condenser 5 in the line 6. Furthermore, if the low boiling point distillate vapor introduced into the compressor 7 is saturated vapor, when it is adiabatically compressed by the compressor 7, it will condense inside the compressor 7 or at its outlet line, and the vapor will become liquid inside the compressor 7. Droplets may cause impeller trouble or polymerization of styrene monomer, resulting in great damage, so preferably a preheater 14 is provided in the line 8 to preheat the low boiling point distillate vapor introduced into the compressor 7. It is better to raise the temperature. The low-boiling fraction steam was introduced into the compressor 7 through the line 8, heated by adiabatic compression in the compressor 7, and then charged into the reboiler 13 via the line 12. The low-boiling fraction vapor was introduced into the distillation column 2. After being used as a heating source for dehydrogenated oil, it is charged into a gas-liquid separation tank 16 through a line 15, and in this gas-liquid separation tank 16, the low-boiling fraction condensed in the reboiler 13 and uncondensed low-boiling fraction steam are It is separated into At this time, preferably, in order to reduce the pressure loss on the heating side in the reboiler 13 and improve heat transfer efficiency, the line from the reboiler 13 to the gas-liquid separation tank 16 is preferably divided into three. In other words, the first line is a line that causes condensate to flow into the gas-liquid separation tank 16, the second line is a line that causes uncondensed low-boiling fraction vapor to flow into the gas-liquid separation tank 16, and the third line
The line is a line that allows inert gas such as air or nitrogen to flow into the gas-liquid separation tank 16 from the upper part of the reboiler 13. The low-boiling fraction condensed in the reboiler 13 is a component with a relatively high boiling point among the low-boiling fraction vapors and is mainly composed of ethylbenzenes. The low-boiling distillate vapor is a component with a relatively low boiling point among the low-boiling distillate vapors, and is mainly composed of benzene, toluene, water, air, nitrogen, etc. In this flow sheet, the condensed low-boiling fraction extracted from the bottom of the gas-liquid separation tank 16 is introduced into the preheater 14 through the line 17, and in this preheater 14, the low-boiling fraction is introduced into the compressor 7. After exchanging heat with the boiling point distillate vapor and being used as a heat source for preheating the low boiling point vapor, it joins the line 6 from the line 18. In addition, the above gas-liquid separation tank 1
Uncondensed low-boiling distillate vapor extracted from the upper part of line 6 flows through line 19 to join line 6. Therefore, the reboiler 13 is controlled so as not to completely condense the low boiling point steam. By controlling the reboiler 13 so as not to completely condense the low boiling point distillate steam, components with relatively high boiling points in the low boiling point steam can be selectively condensed. In addition to increasing the condensation temperature and increasing the amount of heat transfer,
The compression ratio of the compressor 7 can be reduced to reduce running costs and equipment costs, and furthermore,
The temperature of the low-boiling fraction condensed in the reboiler 13 can be raised to a temperature necessary and sufficient to heat the low-boiling fraction steam in the preheater 14. The advantage is that there is no need to introduce Incidentally, the low boiling point distillate vapor on the inlet side of the compressor 7 may be partially condensed due to natural cooling from the outside, but this condensed liquid is not processed by the low boiling point vapor in the preheater 14 having a low pressure drop. Because the flow rate of
It is better to prevent intrusion into the Furthermore, a change in the conditions of the low boiling point vapor introduced into the compressor 7 immediately leads to a change in the conditions of the low boiling point vapor coming out of the compressor 7, which in turn leads to a change in the conditions of the low boiling point vapor introduced into the compressor 7.
In order to operate this distillation system stably, the pressure at the top of the distillation column 2, the flow rate of low-boiling distillate vapor to the compressor 2, and its temperature must be adjusted. Preferably, the conditions of the low-boiling distillate vapor introduced into the compressor 7 are controlled and kept constant. There are various ways to drive the compressor 7, such as a motor alone, a combination of a motor and a steam turbine, or a steam turbine alone, but in order to stabilize the distillation system, the compressor 7 is driven with precision. It is preferable to provide a rotation speed control mechanism. As this rotational speed control mechanism, a system such as a VVVF mechanism or a VV mechanism can be adopted, and a feedback mechanism is incorporated in response to voltage and frequency fluctuations of the power supply to control the rotational speed, that is, the capacity fluctuation.
Further, when steam is used as a driving source, it is preferable to control fluctuations in steam pressure using a feedback control mechanism or the like to minimize fluctuations in rotational speed. Furthermore, in order to stabilize the amount of heat supplied to the reboiler 13, a control valve that requires precision is provided in the bypass line from the compressor 7 to the condenser 5 to absorb fluctuations in heat capacity that cannot be controlled by the compressor 7. You can also do it. By the way, in order to operate the distillation system stably, it is particularly important to stabilize the amount of heat supplied to the reboiler 13. For this purpose, as mentioned above, it is of course necessary to accurately control the rotation speed of the drive machine of the compressor 7, but in addition to this, it is also necessary to control the condensation pressure of the low boiling point distillate steam in the reboiler 13. It is preferable to control the gas-liquid separation tank 1 through the reboiler 13 and to achieve this purpose.
It is preferable to control the pressure of the uncondensed low-boiling fraction vapor charged in the gas-liquid separation tank 16 and separated in the gas-liquid separation tank 16. In this flow sheet, reboiler 1
The low boiling point fraction condensed in step 3 is transferred to the gas-liquid separation tank 16.
The preheater 14 is controlled by a liquid level gauge 20 provided in the line 17 and a control valve 21 provided in the line 17.
In addition, uncondensed low-boiling distillate vapor that passes through the reboiler 13 without being condensed and is separated in the gas-liquid separation tank 16,
In order to control the condensing pressure in the reboiler 13 and the amount of heat supplied to the reboiler 13,
It is controlled by a pressure gauge 22 provided in the gas-liquid separation tank 16 and a control valve 23 provided in the line 19 to merge into the line 6. In this way, by controlling the condensation pressure of the low-boiling fraction vapor in the reboiler 13 by controlling the pressure of the uncondensed low-boiling fraction vapor in the gas-liquid separation tank 16, the condensation pressure in the reboiler 13 can be controlled. It can be controlled arbitrarily, and the amount of heat transferred in the reboiler 13 can also be controlled. Note that when the distillation system starts operating or when there is little uncondensed low-boiling distillate vapor in the gas-liquid separation tank 16, the condensation pressure of the low-boiling distillate vapor in the gas-liquid separation tank 16 may be controlled. In order to be able to carry out the process smoothly, the inside of the gas-liquid separation tank 16 or this gas-liquid separation tank 1 is
Inert gas such as nitrogen gas may also be introduced between the control valve 6 and the control valve 23. In addition, in cases where the reboiler 13 alone is insufficient as a heat source for the distillation column 2, or in order to supply the necessary amount of heat at startup, an auxiliary reboiler 2 is installed in the distillation column 2.
4, so that the amount of heat can be replenished by high-pressure steam or other heating source. [Example] According to the distillation system of the flow sheet shown in Fig. 1,
The dehydrogenated oil obtained by dehydrogenating ethylbenzene was distilled. In this distillation system, the distillation column 2 is packed with regular packing (product name: Merapakku manufactured by Sumitomo Heavy Industries, Ltd.).
A packed column was used, and the pressure difference between the top and bottom of the column was controlled to be 70 mmHg or less. This distillation system was operated by constantly supplying dehydrogenated oil and extracting 40 parts of low-boiling component liquid from line 11 and 60 parts of high-boiling component liquid from line 4. The flow rates and temperatures at each point A, B, C, D, and E shown in the flow sheet above were controlled to the values shown in Table 2. The compositions of the dehydrogenated oil introduced into the distillation system, the top distillate of distillation column 2, and the bottom extract of distillation column 2 were as shown in Table 1. In this example,
It is possible to distill styrene in an extremely stable manner, and the energy consumed by the compressor 7 is on average about 30% of that when using only pressure steam, resulting in significant energy savings. found.

【table】

〔Effect of the invention〕

According to the present invention, the heat pump method is adopted for the distillation of styrenes, for which conventional heat pump methods could not be used as a heat source, and it is possible to perform stable distillation of styrenes with low energy consumption.

[Brief explanation of drawings]

FIG. 1 is a flow sheet showing a method for distilling styrenes according to an embodiment of the present invention.

Claims (1)

[Claims] 1. When distilling the dehydrogenated oil obtained by dehydrogenating ethylbenzenes and separating styrene and ethylbenzenes, the dehydrogenated oil is introduced into the middle part of a packed column type distillation column operated in a reduced pressure system. ,
A low boiling point distillate vapor mainly composed of ethylbenzenes is extracted from the top of the distillation column, and a high boiling point liquid mainly containing styrene is extracted from the bottom of the column. is introduced into a compressor and heated by adiabatic compression, then introduced into a reboiler of a distillation column, and used as a heating source for the reboiler by condensing part or all of it,
A method for distilling styrenes, characterized in that the condensed low-boiling fraction or a part of the uncondensed vapor is then returned to a distillation column circulation line, and the remainder is discharged from the distillation column.