Two-stage negative pressure adiabatic dehydrogenation system for preparing styrene

Abstract

This invention discloses a two-stage negative pressure adiabatic dehydrogenation system for styrene production, comprising a pre-separation tower and an ethylbenzene / styrene tower. The bottom of the pre-separation tower is connected to a heat exchanger and the ethylbenzene / styrene tower sequentially via pipelines. The top of the ethylbenzene / styrene tower is connected to a first-stage condenser and a falling film reboiler via branch pipelines. After heat exchange in the first-stage condenser and the falling film reboiler, the gas phase of the material at the top of the ethylbenzene / styrene tower merges and enters the second-stage condenser. A branch pipeline connects the bottom of the pre-separation tower to the heat exchanger and is connected to the falling film reboiler, allowing the material in the bottom of the pre-separation tower to be returned after heat exchange. The heat source for the heat exchanger is the dehydrogenated tail gas from the outlet of the tail gas compressor after temperature enhancement. The tail gas outlet of the heat exchanger is also connected to the feed heat exchanger of the pre-separation tower. Taking a 400,000-ton / year styrene plant as an example, the energy-saving solution provided by this invention can reduce the overall energy consumption by at least 11.364 kg of standard oil per ton of styrene.

Description

Technical Field

[0001] This invention relates to the field of dehydrogenation to styrene technology, and more particularly to a two-stage negative pressure adiabatic dehydrogenation to styrene energy-saving system. Background Technology

[0002] The two-stage negative pressure adiabatic dehydrogenation process for styrene production, independently developed by Sinopec and featuring an intermediate heat exchanger, has been successfully applied to multiple styrene plants. This process consists of ethylbenzene evaporation and dehydrogenation, oil-water separation and condensate recovery, tail gas compression and absorption, dehydrogenation liquid pre-separation, ethylbenzene / styrene separation, styrene distillation, and a polymerization inhibitor system. The process flow for the dehydrogenation liquid pre-separation and ethylbenzene / styrene separation sections is as follows: Figure 2 As shown:

[0003] The ethylbenzene / styrene column C-2 is a fractionating column operating under negative pressure. The ethylbenzene / styrene mixture from the bottom of the pre-fractionation column C-1 is separated in the C-2 column. The top portion of the ethylbenzene is recycled and returned to the dehydrogenation reactor. The bottom portion, a styrene / tar mixture, is fed back to the styrene column for further fractionation to obtain styrene product. The top of the ethylbenzene / styrene column C-2 employs a three-stage condenser. The first stage, E-5, evaporates to produce an azeotropic mixture of ethylbenzene and water, which serves as feed to the dehydrogenation reactor. The second stage, E-6, and the third stage, E-7, use recycled water and chilled water, respectively, to condense the top material. The reboilers E-1 and E-8 of both columns are heated by steam.

[0004] Based on past project experience and feedback from actual on-site operations, the heat carried by the material at the top of column C-2, after being used for the azeotropic evaporation of ethylbenzene and water in the top condenser E-5 (first stage), leaves more heat remaining than expected. This results in the subsequent tail gas condenser E-6 (second stage) and tail gas subcooler E-7 (third stage) consuming a large amount of circulating water and chilled water, leading to energy waste. Therefore, it is necessary to propose an energy-saving optimization scheme for the styrene plant process flow, focusing on the rational utilization of the heat carried by the material at the top of the column. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by providing a two-stage negative pressure adiabatic dehydrogenation system for styrene production.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] This invention provides a two-stage negative pressure adiabatic dehydrogenation system for styrene production, comprising a pre-separation tower and an ethylbenzene / styrene tower. The bottom of the pre-separation tower is connected to a heat exchanger and the ethylbenzene / styrene tower sequentially via pipelines. The top of the ethylbenzene / styrene tower is connected to a first-stage condenser and a falling film reboiler via branch pipelines. After heat exchange in the first-stage condenser and the falling film reboiler, the gas phase of the material at the top of the ethylbenzene / styrene tower merges and enters the second-stage condenser. A branch pipeline is provided on the pipeline connecting the bottom of the pre-separation tower and the heat exchanger to the falling film reboiler, and the material in the bottom of the pre-separation tower after heat exchange is sent back to the bottom of the pre-separation tower. The heat source of the heat exchanger is the dehydrogenated tail gas from the outlet of the tail gas compressor, and the tail gas outlet of the heat exchanger is also connected to the feed heat exchanger of the pre-separation tower.

[0008] Furthermore, increasing the vacuum level of the pre-separation column reduces the bottom temperature of the column, thereby increasing the heat exchange temperature difference between it and the material at the top of the ethylbenzene / styrene column.

[0009] Furthermore, the liquid phase of the ethylbenzene / styrene tower top material after heat exchange in the falling film reboiler first enters the third buffer tank, and then is sent to the second buffer tank.

[0010] Furthermore, the top of the ethylbenzene / styrene tower adopts a three-stage condenser. The first-stage condenser is used to evaporate and produce an azeotropic mixture of ethylbenzene and water, which serves as the feed for the dehydrogenation reactor. The second-stage and third-stage condensers use circulating water and chilled water, respectively, to condense the top material. The resulting recycled ethylbenzene first enters the second buffer tank.

[0011] Furthermore, both the reboiler of the pre-separation column and the reboiler of the ethylbenzene / styrene column are heated by steam.

[0012] Furthermore, the bottom of the ethylbenzene / styrene tower contains a styrene / tar mixture, which is fed into the styrene tower for further fractionation to obtain the styrene product.

[0013] Furthermore, the top material of the pre-separation tower is condensed sequentially by a circulating water condenser and a chilled water condenser, and the condensed liquid phase enters the first buffer tank.

[0014] Furthermore, a portion of the liquid phase in the first buffer tank is refluxed to the pre-separation tower, and a portion is sent out as a byproduct benzene / toluene.

[0015] Furthermore, the temperature of the dehydrogenated tail gas at the outlet of the tail gas compressor is increased, and the high-temperature tail gas is sequentially fed into the heat exchanger at the feed of the ethylbenzene / styrene tower and the heat exchanger at the feed of the pre-separation tower to heat the feed of the ethylbenzene / styrene tower and the pre-separation tower.

[0016] Compared with the prior art, the present invention has the following advantages:

[0017] This invention adds a falling film reboiler E-9 between the pre-separation tower and the ethylbenzene / styrene tower, using the heat from the top material of the ethylbenzene / styrene tower C-2 to heat the bottom of the pre-separation tower C-1, reducing the steam consumption of the reboiler in the bottom of the C-1 tower; at the same time, it reduces the pressure of the ethylbenzene / styrene tower C-1, thereby reducing the bottom temperature of the ethylbenzene / styrene tower C-1 and increasing the heat exchange temperature difference of the falling film reboiler E-9.

[0018] This invention reduces the amount of cooling water used in the exhaust gas compressor and increases the temperature of the exhaust gas at the compressor outlet. It adds heat exchanger E-10 and pre-separation tower feed heat exchanger E-11, utilizing the high-temperature exhaust gas to heat the C-1 and C-2 feeds, eliminating the need for the original C-1 feed preheater. Taking a 400,000-ton / year styrene plant as an example, the energy-saving solution provided by this invention can reduce overall energy consumption by at least 11.364 kg of standard oil per ton of styrene. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the two-stage negative pressure adiabatic dehydrogenation styrene production energy-saving system of the present invention;

[0020] Figure 2 This is a schematic diagram of an existing two-stage negative pressure adiabatic dehydrogenation system for styrene production, including pre-separation of the dehydrogenation liquid and ethylbenzene / styrene separation. Detailed Implementation

[0021] Compared to traditional two-stage negative pressure adiabatic dehydrogenation to styrene dehydrogenation liquid pre-separation and ethylbenzene / styrene separation systems (such as...) Figure 2 As shown), in order to reduce energy consumption, the present invention addresses the following... Figure 2 The system shown is optimized:

[0022] To fully utilize the heat from the top material of the ethylbenzene / styrene tower C-2, a falling film reboiler E-9 was added to the optimized process. This allows a portion of the top material from the ethylbenzene / styrene tower C-2 to supply heat to the pre-separation tower C-1, recovering the heat from the top of the ethylbenzene / styrene tower C-2 while also reducing the steam consumption of the reboiler E-1 in the bottom of the ethylbenzene / styrene tower C-1.

[0023] To increase the heat exchange temperature difference of the falling film reboiler E-9, improve the vacuum level of the pre-separation column C-1, and reduce the column bottom temperature, the heat exchange temperature difference is increased by about 10℃. However, at the same time, the decrease in the bottom temperature of the pre-separation column C-1 will affect the feed temperature of the ethylbenzene / styrene column C-2. Therefore, it is necessary to add heat exchanger E-10 to preheat this feed stream and maintain the feed temperature of the ethylbenzene / styrene column C-2 unchanged.

[0024] When searching for a heat source for heat exchanger E-10, to further save energy, it was considered to reduce the amount of cooling water injected into the tail gas compressor K-1, thereby increasing the outlet tail gas temperature by approximately 80°C. Utilizing the high-temperature tail gas to heat the feed to the ethylbenzene / styrene tower C-2 (heat exchanger E-10) would result in more efficient energy utilization. However, calculations using this scheme revealed that heat exchanger E-10 failed to fully utilize the heat from the compressor outlet tail gas. Therefore, an additional pre-separation tower feed heat exchanger E-11 was added at the tail gas outlet of E-10 to heat the pre-separation tower feed, replacing the original pre-separation tower feed preheater E-4 which used steam as a heat source, thus saving steam consumption. After two stages of heat exchange, the tail gas compressor outlet gas phase temperature is lower than the original tail gas compressor outlet temperature, further reducing the circulating cooling water consumption of the tail gas cooler E-12.

[0025] The present invention will now be described in detail with reference to specific embodiments and accompanying drawings to enable a better understanding of the invention. However, the following embodiments do not limit the scope of the invention.

[0026] Example 1

[0027] like Figure 1 As shown, this embodiment provides a two-stage negative pressure adiabatic dehydrogenation styrene production energy-saving system, including a pre-separation tower C-1 and an ethylbenzene / styrene tower C-2. The bottom of the pre-separation tower C-1 is connected to the heat exchanger E-10 and the ethylbenzene / styrene tower C-2 in sequence via pipelines. The top of the ethylbenzene / styrene tower C-2 is connected to the first-stage condenser E-5 and the falling film reboiler E-9 via branch pipelines. After heat exchange in the first-stage condenser E-5 and the falling film reboiler E-9, the gas phase of the material at the top of the ethylbenzene / styrene tower merges and enters the second-stage condenser E-6. A branch pipeline is provided on the pipeline connecting the bottom of the pre-separation tower C-1 and the heat exchanger E-10 to the falling film reboiler E-9. The material in the bottom of the pre-separation tower after heat exchange is sent back to the bottom of the pre-separation tower C-1.

[0028] The heat source for heat exchanger E-10 is the dehydrogenated tail gas from the outlet of the tail gas compressor. The tail gas outlet of heat exchanger E-10 is also connected to the pre-separation tower feed heat exchanger E-11.

[0029] Increasing the vacuum level of pre-separation column C-1 reduces the column bottom temperature and increases the heat exchange temperature difference between it and the top material of ethylbenzene / styrene column C-2.

[0030] After passing through the falling film reboiler E-9, the liquid phase of the ethylbenzene / styrene tower top material first enters the third buffer tank D-3, and then is sent to the second buffer tank D-2.

[0031] The top of the ethylbenzene / styrene tower C-2 adopts a three-stage condensation. The first-stage condenser E-5 is used to evaporate the azeotropic mixture of ethylbenzene and water, which is used as feed for the dehydrogenation reactor. The second-stage condenser E-6 and the third-stage condenser E-7 use circulating water and chilled water to condense the top material of the tower, respectively. The resulting recycled ethylbenzene first enters the second buffer tank D-2.

[0032] The bottom of the C-2 column of the above-mentioned ethylbenzene / styrene column contains a styrene / tar mixture, which is fed into the styrene column for further fractionation to obtain styrene product.

[0033] The material at the top of the pre-separation tower C-1 is condensed sequentially by circulating water condenser E-2 and chilled water condenser E-3. The condensed liquid phase enters the first buffer tank D-1. Part of the liquid phase in the first buffer tank D-1 is returned to the pre-separation tower C-1, and part is sent out as a by-product benzene / toluene.

[0034] Increase the temperature of the dehydrogenated tail gas at the outlet of the tail gas compressor K-1, and use the high-temperature tail gas to sequentially heat the feed heat exchanger E-10 of the ethylbenzene / styrene tower and the feed heat exchanger E-11 of the pre-separation tower to heat the feed of the ethylbenzene / styrene tower C-2 and the pre-separation tower C-1.

[0035] As a preferred example, the reboiler E-1 of the pre-separation tower C-1 and the reboiler E-8 of the ethylbenzene / styrene tower C-2 are both heated by steam.

[0036] Example 2

[0037] Taking a 400,000-ton / year styrene plant as an example, the changes in utility consumption and overall energy consumption involved in process optimization are shown in Table 1.

[0038] Table 1. Changes in utilities consumption and overall energy consumption before and after process optimization.

[0039]

[0040] In summary, the optimized process reduced the overall energy consumption of the styrene unit by 11.364 kg of standard oil per t of styrene.

[0041] The above description is merely a preferred embodiment of the present invention and does not limit the implementation and protection scope of the present invention. Those skilled in the art should realize that any equivalent substitutions and obvious changes made based on the content of the present invention specification and drawings should be included within the protection scope of the present invention.

Claims

1. A two-stage negative pressure adiabatic dehydrogenation system for producing styrene, comprising a pre-fractionator (C-1) and an ethylbenzene / styrene column (C-2), characterized in that, The pre-fractionator (C-1) is connected in sequence with a heat exchanger (E-10), the ethylbenzene / styrene column (C-2) through a pipeline; the top of the ethylbenzene / styrene column (C-2) is connected with a first-stage condenser (E-5) and a falling-film reboiler (E-9) through branch pipelines, respectively; the ethylbenzene / styrene column top material after heat exchange through the first-stage condenser (E-5) and the falling-film reboiler (E-9) is merged and then enters a second-stage condenser (E-6); a branch pipeline is arranged on the pipeline connecting the pre-fractionator (C-1) column still with the heat exchanger (E-10) and is connected with the falling-film reboiler (E-9); the pre-fractionator column still material after heat exchange is sent back to the pre-fractionator (C-1) column still; the heat source of the heat exchanger (E-10) is the dehydrogenation tail gas at the outlet of the tail gas compressor; the tail gas outlet of the heat exchanger (E-10) is further connected with a pre-fractionator feed heat exchanger (E-11); The top of the ethylbenzene / styrene column (C-2) is condensed in three stages; the first-stage condenser (E-5) is used to evaporate the azeotrope of ethylbenzene and water to generate the azeotrope as the feed of the dehydrogenation reactor; the second-stage condenser (E-6) and the third-stage condenser (E-7) are used to condense the column top material with circulating water and chilled water, respectively; the obtained circulating ethylbenzene first enters a second buffer tank (D-2); The temperature of the dehydrogenation tail gas at the outlet of the tail gas compressor (K-1) is increased; the high-temperature tail gas is sent in sequence to the heat exchanger (E-10) at the ethylbenzene / styrene column feed and the pre-fractionator feed heat exchanger (E-11) to heat the feeds of the ethylbenzene / styrene column (C-2) and the pre-fractionator (C-1).

2. The two-stage negative pressure adiabatic dehydrogenation system for preparing styrene according to claim 1, characterized in that, The vacuum degree of the pre-fractionator (C-1) is increased to reduce the column still temperature and increase the heat exchange temperature difference with the column top material of the ethylbenzene / styrene column (C-2).

3. The two-stage negative pressure adiabatic dehydrogenation system for preparing styrene according to claim 1, characterized in that, The ethylbenzene / styrene column top material liquid phase after heat exchange through the falling-film reboiler (E-9) first enters a third buffer tank (D-3) and then is sent to a second buffer tank (D-2).

4. The two-stage negative pressure adiabatic dehydrogenation system for preparing styrene according to claim 1, characterized in that, The column still reboiler (E-1) of the pre-fractionator (C-1) and the column still reboiler (E-8) of the ethylbenzene / styrene column (C-2) are both steam heated.

5. The two-stage negative pressure adiabatic dehydrogenation system for preparing styrene according to claim 1, characterized in that, The column still of the ethylbenzene / styrene column (C-2) is a styrene / tar mixture which is sent to a styrene column for further fractionation to obtain styrene products.

6. The two-stage negative pressure adiabatic dehydrogenation system for preparing styrene according to claim 1, characterized in that, The column top material of the pre-fractionator (C-1) is condensed in sequence with a circulating water condenser (E-2) and a chilled water condenser (E-3); the condensed liquid phase enters a first buffer tank (D-1).

7. The two-stage negative pressure adiabatic dehydrogenation system for preparing styrene according to claim 6, characterized in that, Part of the liquid phase in the first buffer tank (D-1) is refluxed to the pre-fractionator (C-1) and part is sent out as by-product benzene / toluene.

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