Patent Publication Number: US-2021187433-A1

Title: System and process for recovering valuables from vent gas in polyolefin production

Description:
FIELD OF THE INVENTION 
     The present disclosure relates to chemical industry, and particularly to a system and a process for recovering valuables from vent gas in polyolefin production. 
     BACKGROUND OF THE INVENTION 
     In monomer purification, polymerization reaction, and degassing of polyolefin resin steps during polyolefin production process, vent gas which comprises a large amount of olefin monomer will be discharged, such as light components stream coming from a top of a degassing bin in monomer purification step, reaction purge gas for controlling inert gas content in polymerization reaction step, and vent gas of degassing bin which is produced after a mixed gas of nitrogen and steam is fed into a degassing bin from a bottom thereof to remove hydrocarbons and make residual catalyst deactivation, in which the content of the vent gas of degassing bin is the largest. All these gases are called as vent gas in polyolefin production, main components of which are nitrogen and olefin monomer, such as propylene, ethylene, butylene and other hydrocarbons, with a certain amount of water vapor. Since hydrocarbons and nitrogen in the vent gas in polyolefin production have very high values, recovery of vent gas is an important step in polyolefin production process. At present, compression, condensation coupling membrane separation process becomes a typical operation during recovery of vent gas in polyolefin production. Since a content of hydrocarbon in the vent gas is relatively low, generally in a range from 7% (V/V) to 40% (V/V), and a critical temperature of hydrocarbon is high which is not easy to condensate, a temperature in a condensation step is generally below 0° C. In order to avoid ice blockage problem in the condensation step, water in the vent gas should be removed first before low temperature condensation step. Therefore, a drying step is necessary. 
     During practical use, an adsorption bed containing solid desiccant is mostly used for removing water. When the desiccant is saturated after adsorption, the desiccant is regenerated by a heated regeneration gas. The desiccant is recovered and reused; the vent gas generated is discharged to a flare. At present, two-bed drying process is mainly used, in which one bed is in an adsorption process, and the other bed is in regeneration process, heating or cooling. The regeneration gas used in regeneration process is generally the vent gas produced by the subsequent membrane separation process, or the recovered nitrogen/fresh nitrogen. The amount of regeneration gas is generally 40 wt % to 100 wt % of the nitrogen in the original vent gas. Therefore, a large part or all of the nitrogen recovered in the subsequent membrane separation process is used for the regeneration process, resulting in a rather low recovery of nitrogen. Consequently, the nitrogen consumption of polyolefin plant is far higher than expected. Meanwhile, there are a pressure increasing step and a pressure decreasing step during a switching procedure of two beds between the adsorption process and the regeneration process, and thus there is an operation problem of large pressure fluctuation. 
     Therefore, in a process and a system for recovering valuables from vent gas in polyolefin production in the prior art, hydrocarbons and nitrogen cannot be effectively recovered and reused simultaneously. 
     SUMMARY OF THE INVENTION 
     In order to solve the aforesaid technical problem in the prior art, the present disclosure provides a system and a process for recovering valuables from vent gas in polyolefin production to realize high efficient recovery and reuse of hydrocarbons and nitrogen from vent gas by combination of a compression device, a drying device in which three adsorption beds are provided, a condensation and separation device, and a membrane separation device. 
     The present disclosure provides a system for recovering valuables from vent gas in polyolefin production, and the system comprises: 
     a compression device, configured to perform compressing, cooling and separating treatments on vent gas in polyolefin production so as to output a condensed liquid and a normal temperature compressed gas stream; 
     a drying device, connected to the compression device and configured to perform dehydration treatment on the normal temperature compressed gas stream that is output by the compression device so as to output a dry gas stream; 
     a condensation and separation device, connected to the drying device and configured to perform cooling and separating treatments on the dry gas stream that is output by the drying device so as to output recovered hydrocarbon and nitrogen stream (hydrocarbon-depleted); and 
     a membrane separation device, connected to the condensation and separation device and configured to perform separating treatment on the nitrogen stream (hydrocarbon-depleted) that is output by the condensation and separation device so as to output a hydrocarbon-enriched gas stream and a nitrogen-enriched gas stream; 
     wherein the drying device comprises a first adsorption bed and a second adsorption bed which are in parallel connection with each other and in which a desiccant is provided, a third adsorption bed which is in communication with the first adsorption bed and the second adsorption bed respectively and in which a desiccant is provided, a regeneration gas heater which is in communication with the first adsorption bed and the second adsorption bed respectively, a regeneration gas-liquid separator which is in communication with the first adsorption bed and the second adsorption bed respectively, and a regeneration gas cooler which is in communication with the first adsorption bed and the second adsorption bed respectively, the third adsorption bed being in communication with the regeneration gas heater, and the regeneration gas-liquid separator being in communication with the regeneration gas cooler; 
     wherein the first adsorption bed and the second adsorption bed are configured to be in an adsorption process and a regeneration process alternately, and the third adsorption bed is configured to be in an auxiliary regeneration process; and 
     wherein a flow regulation valve is provided on a pipeline to which the first adsorption bed and the second adsorption bed are in parallel connection with each other and is configured to separate the normal temperature compressed gas stream into a first gas stream and a second gas stream, and the second gas stream directly serves as a regeneration gas stream to regenerate saturated desiccant. 
     According to some specific embodiments, the regeneration process comprises a heating process and a cooling process. 
     According to some specific embodiments, in the heating process, the second gas stream is treated by the third adsorption bed, the regeneration gas heater, an adsorption bed in the regeneration process, the regeneration gas cooler, and the regeneration gas-liquid separator in sequence, and then treated by an adsorption bed in the adsorption process mixed with the first gas stream so as to output the dry gas stream. 
     According to some specific embodiments, in the cooling process, the second gas stream is treated by an adsorption bed in the regeneration process, the regeneration gas heater, the third adsorption bed, the regeneration gas cooler, and the regeneration gas-liquid separator in sequence, and then treated by an adsorption bed in the adsorption process mixed with the first gas stream so as to output the dry gas stream. 
     According to some specific embodiments, a ratio of the second gas stream to the normal temperature compressed gas stream is in a range from 10 wt % to 30 wt %. 
     According to some specific embodiments, the desiccant is one or more selected from a group consisting of activated alumina, silica gel, and molecular sieve. 
     According to some specific embodiments, along a flow direction of the vent gas in polyolefin production, the compression device comprises a compressor to perform compressing treatment on the vent gas in polyolefin production so as to meet requirements for operation pressures in condensation separation and membrane separation, a first heat exchanger to perform cooling treatment on the vent gas in polyolefin production after compressing treatment, and a first gas-liquid separator to perform gas-liquid separating treatment on the vent gas in polyolefin production after cooling treatment in sequence. 
     According to some specific embodiments, along a flow direction of the dry gas stream, the condensation and separation device comprises a second heat exchanger to perform cooling treatment on the dry gas stream and a second gas-liquid separator to perform gas-liquid separating treatment on the dry gas stream after cooling treatment in sequence. 
     According to some specific embodiments, liquid hydrocarbon flowing from a bottom of the second gas-liquid separator passes through a throttle expansion valve and provides cold energy to the second heat exchanger, and recovered hydrocarbon is output by the second heat exchanger. Noncondensable gas flowing from a top of the second gas-liquid separator provides cold energy to the second heat exchanger, and nitrogen stream (hydrocarbon-depleted) is output by the second heat exchanger. 
     According to some specific embodiments, the condensation and separation device further comprises external liquid hydrocarbon which provides cold energy to the second heat exchanger, and then recovered hydrocarbon is output by the second heat exchanger. 
     According to some specific embodiments, the condensation and separation device further comprises a refrigeration unit which is configured to provide cold energy to the second heat exchanger. 
     According to some specific embodiments, a heat exchanger is one or more selected from a group consisting of a shell-tube heat exchanger, a plate-fin heat exchanger, and a coil-wound heat exchanger. 
     According to some specific embodiments, the membrane separation device comprises at least one membrane separator in which a membrane module for gas separation is provided. 
     According to some specific embodiments, when the membrane separation device comprises at least two membrane separators in each of which a membrane module for gas separation is provided, the membrane separators are in series connection to each other along a flow direction of the nitrogen stream (hydrocarbon-depleted) in sequence. The hydrocarbon-enriched gas stream is output from a membrane permeate side of a first membrane separator, and the nitrogen-enriched gas stream is output from a membrane residual side of a last membrane separator. 
     According to some specific embodiments, the membrane module for gas separation comprises a hydrocarbon selective membrane module and/or a hydrogen selective membrane module. 
     According to some specific embodiments, the hydrocarbon selective membrane module comprises a hydrocarbon selective membrane, and the hydrocarbon selective membrane is one kind of membrane which is more permeable to hydrocarbon components than to H 2  and N 2 . The hydrocarbon selective membrane is preferably a rubbery polymer membrane, such as an organic siloxane polymer membrane. 
     According to some specific embodiments, the hydrogen selective membrane module comprises a hydrogen selective membrane, and the hydrogen selective membrane is one kind of membrane which is more permeable to H 2  than to N 2  and hydrocarbon components. The hydrogen selective membrane is preferably a glassy polymer membrane, such as polyimide membrane, polyaramide membrane, and polysulfone membrane. 
     The hydrocarbon selective membrane module and the hydrogen selective membrane module is one or more selected from a group consisting of a spiral-wound membrane module, a plate-frame membrane module, and a hollow fiber membrane module. 
     According to some specific embodiments, the hydrocarbon-enriched gas stream that is output by the membrane separation device is returned to an inlet of the compressor of the compression device. 
     According to some specific embodiments, the nitrogen-enriched gas stream that is output by the membrane separation device is returned to a degassing bin of polyolefin plant. 
     The present disclosure further provides a process for recovering valuables from vent gas in polyolefin production, and the process comprises steps of: 
     performing, in a compression device, compressing, cooling and separating treatments on vent gas in polyolefin production so as to output a condensed liquid and a normal temperature compressed gas stream; 
     performing, in a drying device that is connected to the compression device, dehydration treatment on the normal temperature compressed gas stream that is output by the compression device so as to output a dry gas stream; 
     performing, in a condensation and separation device that is connected to the drying device, cooling and separating treatments on the dry gas stream that is output by the drying device so as to output recovered hydrocarbon and nitrogen stream (hydrocarbon-depleted); and 
     performing, in a membrane separation device that is connected to the condensation and separation device, separating treatment on the nitrogen stream (hydrocarbon-depleted) that is output by the condensation and separation device so as to output a hydrocarbon-enriched gas stream and a nitrogen-enriched gas stream; 
     wherein the drying device comprises a first adsorption bed and a second adsorption bed which are in parallel connection with each other and in which a desiccant is provided, a third adsorption bed which is in communication with the first adsorption bed and the second adsorption bed respectively and in which a desiccant is provided, a regeneration gas heater which is in communication with the first adsorption bed and the second adsorption bed respectively, a regeneration gas-liquid separator which is in communication with the first adsorption bed and the second adsorption bed respectively, and a regeneration gas cooler which is in communication with the first adsorption bed and the second adsorption bed respectively, the third adsorption bed being in communication with the regeneration gas heater, and the regeneration gas-liquid separator being in communication with the regeneration gas cooler; 
     wherein the first adsorption bed and the second adsorption bed are configured to be in an adsorption process and a regeneration process alternately, and the third adsorption bed is configured to be in an auxiliary regeneration process; and 
     wherein a flow regulation valve is provided on a pipeline to which the first adsorption bed and the second adsorption bed are in parallel connection with each other and is configured to separate the normal temperature compressed gas stream into a first gas stream and a second gas stream, and the second gas stream directly serves as a regeneration gas stream to regenerate saturated desiccant. 
     According to some specific embodiments, the regeneration process comprises a heating process and a cooling process. 
     According to some specific embodiments, in the heating process, the second gas stream is treated by the third adsorption bed, the regeneration gas heater, an adsorption bed in the regeneration process, the regeneration gas cooler, and the regeneration gas-liquid separator in sequence, and then treated by an adsorption bed in the adsorption process mixed with the first gas stream so as to output the dry gas stream. 
     According to some specific embodiments, in the cooling process, the second gas stream is treated by an adsorption bed in the regeneration process, the regeneration gas heater, the third adsorption bed, the regeneration gas cooler, and the regeneration gas-liquid separator in sequence, and then treated by an adsorption bed in the adsorption process mixed with the first gas stream so as to output the dry gas stream. 
     According to some specific embodiments, a ratio of the second gas stream to the normal temperature compressed gas stream is in a range from 10 wt % to 30 wt %. 
     According to some specific embodiments, the desiccant is one or more selected from a group consisting of activated alumina, silica gel, and molecular sieve. 
     According to some specific embodiments, along a flow direction of the vent gas in polyolefin production, the compression device comprises a compressor to perform compressing treatment on the vent gas in polyolefin production so as to meet requirements for operation pressures in condensation separation and membrane separation, a first heat exchanger to perform cooling treatment on the vent gas in polyolefin production after compressing treatment, and a first gas-liquid separator to perform gas-liquid separating treatment on the vent gas in polyolefin production after cooling treatment in sequence. 
     According to some specific embodiments, along a flow direction of the dry gas stream, the condensation and separation device comprises a second heat exchanger to perform cooling treatment on the dry gas stream and a second gas-liquid separator to perform gas-liquid separating treatment on the dry gas stream after cooling treatment in sequence. 
     According to some specific embodiments, liquid hydrocarbon flowing from a bottom of the second gas-liquid separator passes through a throttle expansion valve and provides cold energy to the second heat exchanger, and recovered hydrocarbon is output by the second heat exchanger. Noncondensable gas flowing from a top of the second gas-liquid separator provides cold energy to the second heat exchanger, and nitrogen stream (hydrocarbon-depleted) is output by the second heat exchanger. 
     According to some specific embodiments, the condensation and separation device further comprises external liquid hydrocarbon which provides cold energy to the second heat exchanger, and then recovered hydrocarbon is output by the second heat exchanger. 
     According to some specific embodiments, the condensation and separation device further comprises a refrigeration unit which is configured to provide cold energy to the second heat exchanger. 
     According to some specific embodiments, a heat exchanger is one or more selected from a group consisting of a shell-tube heat exchanger, a plate-fin heat exchanger, and a coil-wound heat exchanger. 
     According to some specific embodiments, the membrane separation device comprises at least one membrane separator in which a membrane module for gas separation is provided. 
     According to some specific embodiments, when the membrane separation device comprises at least two membrane separators in each of which a membrane module for gas separation is provided, the membrane separators are in series connection to each other along a flow direction of the nitrogen stream (hydrocarbon-depleted) in sequence. The hydrocarbon-enriched gas stream is output from a membrane permeate side of a first membrane separator, and the nitrogen-enriched gas stream is output from a membrane residual side of a last membrane separator. 
     According to some specific embodiments, the membrane module for gas separation comprises a hydrocarbon selective membrane module and/or a hydrogen selective membrane module. 
     According to some specific embodiments, the hydrocarbon selective membrane module comprises a hydrocarbon selective membrane, and the hydrocarbon selective membrane is one kind of membrane which is more permeable to hydrocarbon components than to H 2  and N 2 . The hydrocarbon selective membrane is preferably a rubbery polymer membrane, such as an organic siloxane polymer membrane. 
     According to some specific embodiments, the hydrogen selective membrane module comprises a hydrogen selective membrane, and the hydrogen selective membrane is one kind of membrane which is more permeable to H 2  than to N 2  and hydrocarbon components. The hydrogen selective membrane is preferably a glassy polymer membrane, such as polyimide membrane, polyaramide membrane, and polysulfone membrane. 
     The hydrocarbon selective membrane module and the hydrogen selective membrane module is one or more selected from a group consisting of a spiral-wound membrane module, a plate-frame membrane module, and a hollow fiber membrane module. 
     According to some specific embodiments, the hydrocarbon-enriched gas stream that is output by the membrane separation device is returned to an inlet of the compressor of the compression device. 
     According to some specific embodiments, the nitrogen-enriched gas stream that is output by the membrane separation device is returned to a degassing bin of polyolefin plant. 
     According to the present disclosure, the term “nitrogen stream (hydrocarbon-depleted)” refers to a gas stream in which a content of hydrocarbon in the nitrogen stream is less than 15% (V/V). 
     According to the present disclosure, the term “hydrocarbon-enriched gas stream” refers to a gas stream in which more than 25% (V/V) is hydrocarbon. 
     According to the present disclosure, the term “nitrogen-enriched gas stream” refers to a gas stream in which more than 98% (V/V) is nitrogen. 
     According to the present disclosure, the term “normal temperature” refers to a temperature in a range from 5° C. to 40° C. 
     According to the present disclosure, the system and the process for recovering valuables from vent gas in polyolefin production can be used for recovering valuables from vent gas in gas-phase olefin polymerization process. The following beneficial effects can be brought about by the present disclosure. 
     1. By using the drying device in which three adsorption beds are provided, one part of the normal temperature compressed gas stream output by the compression device directly serves as the regeneration gas, and it is unnecessary for the external supply of regeneration gas, whereby the actual recovery of nitrogen can be effectively improved. 
     2. Heat recovery is considered during the drying step, and thus the energy consumption of the regeneration process can be reduced. 
     3. The hydrocarbon and nitrogen in the vent gas in polyolefin production can be effectively recovered. The recovery of hydrocarbon is 98% or above; the recovery of nitrogen is 65% or above; and the purity of nitrogen is 98% (V/V) or above. 
     4. The system can operate stably and reliably and has a low requirement for utility as well as a wider application scope. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will be further illustrated hereinafter with reference to the drawings. In the drawings, the same components are represented by the same reference signs. 
         FIG. 1  schematically shows a structure of an embodiment of a system for recovering valuables from vent gas in polyolefin production according to the present disclosure; 
         FIG. 2  schematically shows specific structures of two embodiments of a drying device of the system as shown in  FIG. 1  according to the present disclosure; and 
         FIG. 3  schematically shows specific structures of three embodiments of a membrane separation device of the system as shown in  FIG. 1  according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present disclosure will be illustrated in detail hereinafter with reference to the specific embodiments and the drawings. It should be noted that, the terms “first”, “second” and “third” used herein are only for distinguishing the same kinds of devices or substances, but it does not mean difference therebetween in sequence or importance. 
       FIG. 1  schematically shows a structure of an embodiment of a system for recovering valuables from vent gas in polyolefin production according to the present disclosure. As shown in  FIG. 1 , the system for recovering valuables from vent gas in polyolefin production comprises a compression device  100 , a drying device  200 , a condensation and separation device  300  and a membrane separation device  400  that are connected to each other in sequence. 
     Along a flow direction of the vent gas  10  in polyolefin production, the compression device  100  comprises a vent gas compressor  110 , a cooling water cooler  120  and a first gas-liquid separator  130 . The vent gas  10  generated in polyolefin production is introduced to an inlet of the vent gas compressor  110  through a pipeline. In the vent gas compressor  110 , a pressure of the vent gas is increased to 0.6 MPa to 3.0 MPa so as to meet requirements for operation pressures in following condensation separation and membrane separation steps. The compressed vent gas  11  is transmitted to the cooling water cooler  120  through a pipeline. In the cooling water cooler  120 , the compressed vent gas  11  is cooled to normal temperature. Vent gas  12  that is cooled to normal temperature is transmitted to the first gas-liquid separator  130  through a pipeline. In the first gas-liquid separator  130 , gas-liquid separation is performed on the vent gas  12  that is cooled to normal temperature. A condensed liquid  20  is obtained at a bottom of the first gas-liquid separator  130  and is output from the compression device  100  through a pipeline. A normal temperature compressed gas stream  13  is obtained at a top of the first gas-liquid separator  130  and is transmitted to the drying device  200  through a pipeline. 
     The drying device  200  comprises a first adsorption bed  210  and a second adsorption bed  220  which are in parallel connection with each other and in which a desiccant is provided, a third adsorption bed  230  which is in communication with the first adsorption bed  210  and the second adsorption bed  220  respectively and in which a desiccant is provided, a regeneration gas heater  240  which is in communication with the first adsorption bed  210  and the second adsorption bed  220  respectively, a regeneration gas-liquid separator  250  which is in communication with the first adsorption bed  210  and the second adsorption bed  220  respectively, and a regeneration gas cooler  260  which is in communication with the first adsorption bed  210  and the second adsorption bed  220  respectively. The third adsorption bed  230  is in communication with the regeneration gas heater  240 , and the regeneration gas-liquid separator  250  is in communication with the regeneration gas cooler  260 , as shown in  FIG. 2 . 
     As shown in  FIG. 2 , the first adsorption bed  210  and the second adsorption bed  220  are configured to be in an adsorption process and a regeneration process alternately, and the third adsorption bed  230  is configured to be in an auxiliary regeneration process. 
     A flow regulation valve  201  is provided on a pipeline to which the first adsorption bed  210  and the second adsorption bed  220  are in parallel connection with each other and is configured to separate the normal temperature compressed gas stream  13  which enters the drying device  200  into a first gas stream  13 - 1  and a second gas stream  13 - 2 , and the second gas stream  13 - 2  directly serves as a regeneration gas stream to regenerate saturated desiccant. 
     It is assumed that the first adsorption bed  210  is in the adsorption process, while the second adsorption bed  220  has reached adsorption saturation and is in the regeneration process. 
     The first gas stream  13 - 1  directly enters into the first adsorption bed  210 , and the water therein is adsorbed by molecular sieve desiccant filled in the adsorption bed. After dew point of the gas stream dropping to 0° C. to −60° C., a dry gas stream  14  obtained therein is output by the drying device  200  to the condensation and separation device  300  through a pipeline. 
     When the second adsorption bed  220  is in a heating process, as shown in  FIG. 2( a ) , the second gas stream  13 - 2  enters into the third adsorption bed  230  to remove water, and then enters into the regeneration gas heater  240 . The gas, after temperature rising, enters into the second adsorption bed  220  for heating process. A temperature of the molecular sieve desiccant in the second adsorption bed  220  rises, and the water adsorbed therein is desorbed. The regenerated gas is cooled by the regeneration gas cooler  260  to normal temperature, and then enters into the regeneration gas-liquid separator  250  for gas-liquid separation. A gas stream output from a top of the regeneration gas-liquid separator  250  and the first gas stream  13 - 1  are mixed together and enter into the first adsorption bed  210  to be dried. A dry gas stream  14  obtained therein is output by the drying device  200  to the condensation and separation device  300  through a pipeline, and water  21  obtained at a bottom of the regeneration gas-liquid separator  250  is output by the drying device  200  through a pipeline. 
     After the heating process of the second adsorption bed  220  comes to an end, it enters a cooling process. When the second adsorption bed  220  is in the cooling process, as shown in  FIG. 2( b ) , the second gas stream  13 - 2  enters into the second adsorption bed  220  to reduce a temperature thereof, and a cooling gas is output by the second adsorption bed  220 . The cooling gas enters into the regeneration gas heater  240 , and then enters into the third adsorption bed  230  for heating process after temperature rising. A temperature of the molecular sieve desiccant in the third adsorption bed  230  rises, and the water adsorbed therein is desorbed. The regenerated gas is cooled by the regeneration gas cooler  260  to normal temperature, and then enters into the regeneration gas-liquid separator  250  for gas-liquid separation. A gas stream output from a top of the regeneration gas-liquid separator  250  and the first gas stream  13 - 1  are mixed together and enter into the first adsorption bed  210  to be dried. A dry gas stream  14  obtained therein is output by the drying device  200  to the condensation and separation device  300  through a pipeline, and water  21  obtained at a bottom of the regeneration gas-liquid separator  250  is output by the drying device  200  through a pipeline. 
     When the first adsorption bed  210  is saturated after adsorption, the valves in pipeline are switched so that the second adsorption bed  220  is in the adsorption process and the first adsorption bed  210  is in the regeneration process. 
     The status of the three adsorption beds of the drying device according to the present disclosure is shown in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 status of the three adsorption beds of the drying device 
               
            
           
           
               
               
               
            
               
                   
                 Time 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 4.5 h 
                 3.5 h 
                 4.5 h 
                 3.5 h 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 The first 
                 A 
                 A 
                 H 
                 C 
               
               
                   
                 adsorption bed 
               
               
                   
                 The second 
                 H 
                 C 
                 A 
                 A 
               
               
                   
                 adsorption bed 
               
               
                   
                 The third 
                 C 
                 H 
                 C 
                 H 
               
               
                   
                 adsorption bed 
               
               
                   
                   
               
            
           
         
       
     
     As shown in Table 1, A means adsorption, H means heating, and C means cooling. 
     It can be seen from Table 1 that, the first adsorption bed  210  and the second adsorption bed  220  are in the adsorption process and the regeneration process alternately, and the third adsorption bed  230  is always in heating or cooling auxiliary regeneration process. The regeneration gas heater  240  is always in work. The regeneration gas (i.e., the second gas stream  13 - 2 ) absorbs heat from the adsorption bed in the cooling process, and temperature of the regeneration gas is higher than normal temperature, whereby energy consumption of the regeneration gas heater  240  can be reduced. 
     As shown in  FIG. 1 , along a flow direction of the dry gas stream  14 , the condensation and separation device  300  comprises a second heat exchanger  310  and a second gas-liquid separator  320 . 
     The dry gas stream  14  output by the drying device  200  first enters into the second heat exchanger  310 , which is a multi-stream heat exchanger. In the second heat exchanger  310 , after the temperature of the dry gas stream  14  is reduced below a hydrocarbon dew point, the dry gas stream  14  enters into the second gas-liquid separator  320  for gas-liquid separation. Liquid hydrocarbon obtained at a bottom of the second gas-liquid separator  320  passes through a throttle expansion valve to reduce the temperature and pressure thereof, and is returned to the second heat exchanger  310  to exchange heat with hot medium in the second heat exchanger  310  and provide cold energy for the hot medium. When the cold energy is not enough, external liquid hydrocarbon  23  performs refrigeration through the throttle expansion valve and provides cold energy for the second heat exchanger  310  so as to ensure that expected condensation temperature is reached. After the liquid hydrocarbon is cooled in the second heat exchanger  310 , recovered hydrocarbon  22  is obtained and is output by the condensation and separation device  300 . Noncondensable gas obtained at a top of the second gas-liquid separator  320  is returned to the second heat exchanger  310  to exchange heat with the hot medium in the second heat exchanger  310  and provide cold energy for the hot medium. Nitrogen stream (hydrocarbon-depleted)  15  is output by the second heat exchanger  310  and enters into the membrane separation device  400 . According to the present disclosure, the condensation and separation device  300  can also be provided with an independent refrigeration unit (not shown in  FIG. 1 ) to provide cold energy for condensation of hydrocarbon in the second heat exchanger  310 . In this case, the supplement of external liquid hydrocarbon is unnecessary. 
       FIG. 3  schematically shows specific structures of three embodiments of the membrane separation device  400  of the system as shown in  FIG. 1  according to the present disclosure. As shown in  FIG. 3 , the membrane separation device  400  comprises at least one membrane separator in which a membrane module for gas separation is provided. 
     When the membrane separation device comprises only one membrane separator (i.e., a first membrane separator  410 ), as shown in  FIG. 3( a ) , after the nitrogen stream (hydrocarbon-depleted)  15  is treated by the first membrane separator  410 , the hydrocarbon-enriched gas stream  17  is output from permeate side of the first membrane separator  410 , and the nitrogen-enriched gas stream  16  is output from residual side of the first membrane separator  410 . 
     When the membrane separation device comprises two or more membrane separators (i.e., a first membrane separator  410 , a second membrane separator  420 , and a third membrane separator  430 ), as shown in  FIG. 3( b )  and  FIG. 3( c ) , the membrane separators are in series connection to each other along a flow direction of the nitrogen stream (hydrocarbon-depleted) in sequence. The hydrocarbon-enriched gas stream  17  is output from permeate side of a first membrane separator, and the nitrogen-enriched gas stream  16  is output from residual side of a last membrane separator. 
     The hydrocarbon-enriched gas stream  17  that is output by the membrane separation device is returned to an inlet of the vent gas compressor  110  of the compression device  100 , and the nitrogen-enriched gas stream  16  that is output by the membrane separation device is returned to a degassing bin of polyolefin plant to be recovered and reused. 
     According to the present disclosure, recovery of nitrogen can be calculated according to following formula: recovery of nitrogen=S 16 /S 10 ×100%, wherein S 16  is a mass flow of nitrogen in recovered nitrogen stream with a unit of kg/hr, and S 10  is a mass flow of nitrogen in vent gas with a unit of kg/hr. 
     According to the present disclosure, recovery of propylene can be calculated according to following formula: recovery of propylene=[1−S 16 /S 10 ]×100% (when stream discharged to flare is not contained), or recovery of propylene=[1−(S 16 +S 18 )/S 10 ]×100% (when stream discharged to flare is contained), wherein S 16  is a mass flow of propylene in recovered nitrogen stream with a unit of kg/hr, S 18  is a mass flow of propylene in the stream discharged to flare with a unit of kg/hr, and S 10  is a mass flow of propylene in original vent gas with a unit of kg/hr. 
     Example 1 
     The system for recovering valuables from vent gas in polyolefin production as shown in  FIG. 1  is used for treating 300,000 tons of vent gas generated in polypropylene plant to recover hydrocarbon and nitrogen. 
     A pressure of the vent gas in polyolefin production is 0.01 MPa, a temperature is 50° C., a volume flow is 1100 Nm 3 /hr, and components thereof are shown in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Components of the vent gas of example 1 
               
            
           
           
               
               
               
            
               
                   
                 Components 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Propylene 
                 Propane 
                 Water 
                 Nitrogen 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Content % 
                 25.34 
                 4.03 
                 1.40 
                 69.23 
               
               
                   
                 (V/V) 
               
               
                   
                   
               
            
           
         
       
     
     The vent gas  10  first enters into the compression device  100 . In the compression device  100 , the pressure of the vent gas  10  is increased by the vent gas compressor  110  to 2.2 MPa. The compressed vent gas  11  enters into the cooling water cooler  120  to be cooled to 40° C. The cooled vent gas  12  enters into the first gas-liquid separator  130  for gas-liquid separation. The condensed liquid  20  obtained at a bottom of the first gas-liquid separator  130  is output therefrom, and the normal temperature compressed gas stream  13  obtained at a top thereof enters into the drying device  200  through a pipeline. As shown in  FIG. 2( a ) , the drying device  200  comprises three adsorption beds in each of which a molecular sieve desiccant is provided. The first adsorption bed  210  is in the adsorption process; the second adsorption bed  220  is in the heating process; and the third adsorption bed  230  is in the cooling process. The second gas stream  13 - 2  (i.e., 20 wt % of the normal temperature compressed gas stream  13 ) serves as the regeneration gas through the flow regulation valve  201 . The regeneration gas  13 - 2  enters into the third adsorption bed  230  to remove water, and then enters into the regeneration gas heater  240 . The gas, after temperature rising to 220° C., enters into the second adsorption bed  220  for heating process. A temperature of the molecular sieve desiccant in the second adsorption bed  220  rises, and the water adsorbed therein is desorbed. The regenerated gas is cooled by the regeneration gas cooler  260  to normal temperature, and then enters into the regeneration gas-liquid separator  250  for gas-liquid separation. The condensed water  21  obtained at a bottom of the regeneration gas-liquid separator  250  is output through a pipeline, and a gas stream obtained at a top of the regeneration gas-liquid separator  250  and the first gas stream  13 - 1  are mixed together and enter into the first adsorption bed  210  to be dried. 
     A content of H 2 O in the vent gas is reduced below 1 ppmv by the adsorption beds to avoid ice blockage in the following condensation and separation steps. The dry gas stream  14  enters into the condensation and separation device  300 . The dry gas stream  14  first enters into the second heat exchanger  310  (a multi-stream heat exchanger), and the temperature of the dry gas stream  14  is reduced below −20° C. Then, the dry gas stream  14  enters into the second gas-liquid separator  320  for gas-liquid separation. Liquid hydrocarbon is obtained at a bottom of the second gas-liquid separator  320 . The liquid hydrocarbon passes through a throttle expansion valve to reduce the temperature and pressure thereof, and is returned to the second heat exchanger  310  to exchange heat with hot medium and provide cold energy for the hot medium. When the cold energy is not enough, external liquid hydrocarbon  23  performs expansion refrigeration through the throttle expansion valve so as to ensure that expected condensation temperature is reached. The liquid hydrocarbon is changed into recovered hydrocarbon  22  by the second heat exchanger  310  and is output. Noncondensable gas obtained at a top of the second gas-liquid separator  320  is returned to the second heat exchanger  310  to exchange heat with the hot medium and provide cold energy for the hot medium. Nitrogen stream (hydrocarbon-depleted)  15  is output by the second heat exchanger  310  and enters into the membrane separation device  400  through a pipeline. 
     As shown in  FIG. 3( a ) , the membrane separation device  400  comprises the first membrane separator  410  in which a hydrocarbon selective membrane module is provided. The membrane material used is polyorganosiloxane. The nitrogen stream (hydrocarbon-depleted)  15  passes through the separation membrane, and enriched propylene stream  17  is obtained at a membrane permeate side and returned to the inlet of the vent gas compressor  110 . Then, propylene is further recovered through compression and condensation steps. The nitrogen-enriched gas stream  16 , in which the purity of nitrogen is larger than 98.5% (V/V), is obtained at a membrane residual side and returned to the degassing bin of the polypropylene plant to be reused. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Material balance table of example 1 
               
            
           
           
               
               
            
               
                   
                 Serial number 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 10 
                 16 
                 22 
                 21 
                 23 
               
            
           
           
               
               
            
               
                   
                 Stream name 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                 Recovered 
                 Recovered 
                 Removed 
                 Supplement 
               
               
                   
                 Vent gas 
                 nitrogen 
                 propylene 
                 water 
                 propylene coolant 
               
            
           
           
               
               
            
               
                   
                 Phase state 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Vapor-phase 
                 Vapor-phase 
                 Vapor-phase 
                 Liquid-phase 
                 Liquid-phase 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Volume flow 
                 1100.00 
                 756.67 
                 429.83 
                 15.40 
                 101.90 
               
               
                 (Nm 3 /hr) 
               
               
                 Temperature (° C.) 
                 50.00 
                 21.00 
                 20.00 
                 40.00 
                 40.00 
               
               
                 Pressure (MPa) 
                 0.01 
                 1.90 
                 0.15 
                 2.10 
                 1.70 
               
               
                 Molar percentage 
               
               
                 (%) 
               
               
                 Propylene 
                 25.34 
                 0.71 
                 87.21 
                 0.00 
                 99.60 
               
               
                 Propane 
                 4.03 
                 0.10 
                 10.23 
                 0.00 
                 0.40 
               
               
                 Nitrogen 
                 69.23 
                 99.19 
                 2.56 
                 0.00 
                 0.00 
               
               
                 Water 
                 1.40 
                 0.00 
                 0.00 
                 100.00 
                 0.00 
               
               
                 Components 
               
               
                 mass flow (kg/hr) 
               
               
                 Propylene 
                 516.47 
                 9.96 
                 694.56 
                 0.00 
                 188.05 
               
               
                 Propane 
                 86.07 
                 1.46 
                 85.40 
                 0.00 
                 0.79 
               
               
                 Nitrogen 
                 939.33 
                 925.78 
                 13.55 
                 0.00 
                 0.00 
               
               
                 Water 
                 12.22 
                 0.00 
                 0.00 
                 12.22 
                 0.00 
               
               
                   
               
            
           
         
       
     
     Based on the material balance data as shown in Table 3, it can be obtained after calculation that, the recovery of propylene of the system is 98.07%, and the recovery of nitrogen is 98.56% with the purity of 99% (V/V) or above. If traditional two-bed dehydration process is used, about 750 kg/hr regeneration nitrogen needs to be consumed according to the present 12.22 kg/hr dehydration load. In this case, the actual recovery of nitrogen is about 18.7%, which is much lower than the recovery of nitrogen of the system disclosed herein. 
     Example 2 
     The system for recovering valuables from vent gas in polyolefin production as shown in  FIG. 1  is used for treating 350,000 tons of vent gas generated in polypropylene plant to recover hydrocarbon and nitrogen. 
     A pressure of the vent gas in polyolefin production is 0.01 MPa, a temperature is 40° C., a volume flow is 1250 Nm 3 /hr, and components thereof are shown in Table 4. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Components of the vent gas of example 2 
               
            
           
           
               
               
            
               
                   
                 Components 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Propylene 
                 Propane 
                 Ethylene 
                 Water 
                 Nitrogen 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Content % 
                 27.16 
                 5.77 
                 2.79 
                 1.32 
                 62.96 
               
               
                 (V/V) 
               
               
                   
               
            
           
         
       
     
     The vent gas  10  first enters into the compression device  100 . In the compression device  100 , the pressure of the vent gas  10  is increased by the vent gas compressor  110  to 2.0 MPa. The compressed vent gas  11  enters into the cooling water cooler  120  to be cooled to 40° C. The cooled vent gas  12  enters into the first gas-liquid separator  130  for gas-liquid separation. The condensed liquid  20  obtained at a bottom of the first gas-liquid separator  130  is output therefrom, and the normal temperature compressed gas stream  13  obtained at a top thereof enters into the drying device  200  through a pipeline. As shown in  FIG. 2( a ) , the drying device  200  comprises three adsorption beds in each of which a molecular sieve desiccant is provided. The first adsorption bed  210  is in the adsorption process; the second adsorption bed  220  is in the heating process; and the third adsorption bed  230  is in the cooling process. The second gas stream  13 - 2  (i.e., 22 wt % of the normal temperature compressed gas stream  13 ) serves as the regeneration gas through the flow regulation valve  201 . The regeneration gas  13 - 2  enters into the third adsorption bed  230  to remove water, and then enters into the regeneration gas heater  240 . The gas, after temperature rising to 220° C., enters into the second adsorption bed  220  for heating process. A temperature of the molecular sieve desiccant in the second adsorption bed  220  rises, and the water adsorbed therein is desorbed. The regenerated gas is cooled by the regeneration gas cooler  260  to normal temperature, and then enters into the regeneration gas-liquid separator  250  for gas-liquid separation. The condensed water  21  obtained at a bottom of the regeneration gas-liquid separator  250  is output through a pipeline, and a gas stream obtained at a top of the regeneration gas-liquid separator  250  and the first gas stream  13 - 1  are mixed together and enter into the first adsorption bed  210  to be dried. 
     A content of H 2 O in the vent gas is reduced below 1 ppmv by the adsorption beds to avoid ice blockage in the following condensation and separation steps. The dry gas stream  14  enters into the condensation and separation device  300 . The dry gas stream  14  first enters into the second heat exchanger  310 , and the temperature of the dry gas stream  14  is reduced below −22° C. Then, the dry gas stream  14  enters into the second gas-liquid separator  320  for gas-liquid separation. Liquid hydrocarbon is obtained at a bottom of the second gas-liquid separator  320 . The liquid hydrocarbon passes through a throttle expansion valve to reduce the temperature and pressure thereof, and is returned to the second heat exchanger  310  to exchange heat with hot medium and provide cold energy for the hot medium. When the cold energy is not enough, external liquid hydrocarbon  23  performs expansion refrigeration through the throttle expansion valve so as to ensure that expected condensation temperature is reached. The liquid hydrocarbon is changed into recovered hydrocarbon  22  by the second heat exchanger  310  and is output. Noncondensable gas obtained at a top of the second gas-liquid separator  320  is returned to the second heat exchanger  310  to exchange heat with the hot medium and provide cold energy for the hot medium. Nitrogen stream (hydrocarbon-depleted)  15  is output by the second heat exchanger  310  and enters into the membrane separation device  400  through a pipeline. 
     As shown in  FIG. 3( b ) , the membrane separation device  400  comprises the first membrane separator  410  and the second membrane separator  420  in each of which a hydrocarbon selective membrane module is provided. The membrane material used is polyorganosiloxane. The nitrogen stream (hydrocarbon-depleted)  15  passes through the separation membrane, and enriched propylene stream  17  is obtained at a membrane permeate side of the first membrane separator  410  and returned to the inlet of the vent gas compressor  110 . Then, propylene is further recovered through compression and condensation steps. The gas stream obtained at a membrane residual side of the first membrane separator  410  enters into the second membrane separator  420 , and light hydrocarbon components therein, such as methane and ethylene, are separated from nitrogen. The enriched light hydrocarbon components stream  18  is obtained at a membrane permeate side of the second membrane separator  420  and is discharged to a flare. The nitrogen-enriched gas stream  16 , in which the purity of nitrogen is larger than 98.5% (V/V), is obtained at a membrane residual side of the second membrane separator  420  and returned to the degassing bin of the polypropylene plant to be reused. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Material balance table of example 2 
               
            
           
           
               
               
            
               
                   
                 Serial number 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 10 
                 16 
                 18 
                 22 
                 21 
                 23 
               
            
           
           
               
               
            
               
                   
                 Stream name 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Vent 
                 Recovered 
                 Components discharged 
                 Recovered 
                 Removed 
                 Supplement 
               
               
                   
                 gas 
                 nitrogen 
                 to flare 
                 propylene 
                 water 
                 propylene coolant 
               
            
           
           
               
               
            
               
                   
                 Phase state 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Vapor-phase 
                 Vapor-phase 
                 Vapor-phase 
                 Vapor-phase 
                 Liquid-phase 
                 Liquid-phase 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Volume flow 
                 1250.00 
                 666.47 
                 122.09 
                 568.99 
                 16.50 
                 124.11 
               
               
                 (Nm 3 /hr) 
               
               
                 Temperature (° C.) 
                 40.00 
                 19.99 
                 19.00 
                 20.00 
                 40.00 
                 42.83 
               
               
                 Pressure (MPa) 
                 0.01 
                 1.62 
                 0.05 
                 0.15 
                 0.20 
                 1.66 
               
               
                 Molar percentage 
               
               
                 (%) 
               
               
                 Propylene 
                 27.16 
                 0.41 
                 3.16 
                 80.23 
                 0.00 
                 99.60 
               
               
                 Propane 
                 5.77 
                 0.08 
                 0.58 
                 12.55 
                 0.00 
                 0.40 
               
               
                 Ethylene 
                 2.79 
                 0.44 
                 2.74 
                 5.01 
                 0.00 
                 0.00 
               
               
                 Nitrogen 
                 62.96 
                 99.07 
                 93.51 
                 2.21 
                 0.00 
                 0.00 
               
               
                 Water 
                 1.32 
                 0.00 
                 0.00 
                 0.00 
                 100.00 
                 0.00 
               
               
                 Components mass 
               
               
                 flow (kg/hr) 
               
               
                 Propylene 
                 629.05 
                 5.08 
                 7.15 
                 845.84 
                 0.00 
                 229.03 
               
               
                 Propane 
                 140.04 
                 0.98 
                 1.39 
                 138.63 
                 0.00 
                 0.96 
               
               
                 Ethylene 
                 43.08 
                 3.65 
                 4.13 
                 35.21 
                 0.00 
                 0.00 
               
               
                 Nitrogen 
                 970.75 
                 814.42 
                 140.82 
                 15.52 
                 0.00 
                 0.00 
               
               
                 Water 
                 13.09 
                 0.00 
                 0.00 
                 0.00 
                 13.09 
                 0.00 
               
               
                   
               
            
           
         
       
     
     Based on the material balance data as shown in Table 5, it can be obtained after calculation that, the recovery of propylene of the system is 98.05%, and the recovery of nitrogen is 83.9% with the purity of 99% (V/V) or above. If traditional two-bed dehydration process is used, about 800 kg/hr regeneration nitrogen needs to be consumed according to the present 13.09 kg/hr dehydration load. In this case, the actual recovery of nitrogen is about 17.6%, which is much lower than the recovery of nitrogen of the system disclosed herein. 
     Example 3 
     The system for recovering valuables from vent gas in polyolefin production as shown in  FIG. 1  is used for treating 350,000 tons of vent gas generated in polypropylene plant to recover hydrocarbon and nitrogen. 
     A pressure of the vent gas in polyolefin production is 0.01 MPa, a temperature is 40° C., a volume flow is 1380 Nm 3 /hr, and components thereof are shown in Table 6. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Components of the vent gas of example 3 
               
            
           
           
               
               
            
               
                   
                 Components 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Propylene 
                 Propane 
                 Ethylene 
                 Ethane 
                 Hydrogen 
                 Water 
                 Nitrogen 
                 Methane 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Content % 
                 30.54 
                 2.74 
                 2.32 
                 0.07 
                 1.45 
                 1.5 
                 61.23 
                 0.15 
               
               
                 (V/V) 
               
               
                   
               
            
           
         
       
     
     The vent gas  10  first enters into the compression device  100 . In the compression device  100 , the pressure of the vent gas  10  is increased by the vent gas compressor  110  to 2.5 MPa. The compressed vent gas  11  enters into the cooling water cooler  120  to be cooled to 40° C. The cooled vent gas  12  enters into the first gas-liquid separator  130  for gas-liquid separation. The condensed liquid  20  obtained at a bottom of the first gas-liquid separator  130  is output therefrom, and the normal temperature compressed gas stream  13  obtained at a top thereof enters into the drying device  200  through a pipeline. As shown in  FIG. 2( a ) , the drying device  200  comprises three adsorption beds in each of which a molecular sieve desiccant is provided. The first adsorption bed  210  is in the adsorption process; the second adsorption bed  220  is in the heating process; and the third adsorption bed  230  is in the cooling process. The second gas stream  13 - 2  (i.e., 25 wt % of the normal temperature compressed gas stream  13 ) serves as the regeneration gas through the flow regulation valve  201 . The regeneration gas  13 - 2  enters into the third adsorption bed  230  to remove water, and then enters into the regeneration gas heater  240 . The gas, after temperature rising to 220° C., enters into the second adsorption bed  220  for heating process. A temperature of the molecular sieve desiccant in the second adsorption bed  220  rises, and the water adsorbed therein is desorbed. The regenerated gas is cooled by the regeneration gas cooler  260  to normal temperature, and then enters into the regeneration gas-liquid separator  250  for gas-liquid separation. The condensed water  21  obtained at a bottom of the regeneration gas-liquid separator  250  is output through a pipeline, and a gas stream obtained at a top of the regeneration gas-liquid separator  250  and the first gas stream  13 - 1  are mixed together and enter into the first adsorption bed  210  to be dried. 
     A content of H 2 O in the vent gas is reduced below 1 ppmv by the adsorption beds to avoid ice blockage in the following condensation and separation steps. The dry gas stream  14  enters into the condensation and separation device  300 . The dry gas stream  14  first enters into the second heat exchanger  310 , and the temperature of the dry gas stream  14  is reduced below −21° C. Then, the dry gas stream  14  enters into the second gas-liquid separator  320  for gas-liquid separation. Liquid hydrocarbon is obtained at a bottom of the second gas-liquid separator  320 . The liquid hydrocarbon passes through a throttle expansion valve to reduce the temperature and pressure thereof, and is returned to the second heat exchanger  310  to exchange heat with hot medium and provide cold energy for the hot medium. When the cold energy is not enough, external liquid hydrocarbon  23  performs expansion refrigeration through the throttle expansion valve so as to ensure that expected condensation temperature is reached. The liquid hydrocarbon is changed into recovered hydrocarbon  22  by the second heat exchanger  310  and is output. Noncondensable gas obtained at a top of the second gas-liquid separator  320  is returned to the second heat exchanger  310  to exchange heat with the hot medium and provide cold energy for the hot medium. Nitrogen stream (hydrocarbon-depleted)  15  is output by the second heat exchanger  310  and enters into the membrane separation device  400  through a pipeline. 
     As shown in  FIG. 3( c ) , the membrane separation device  400  comprises the first membrane separator  410 , the second membrane separator  420 , and the third membrane separator  430 . A hydrocarbon selective membrane module is provided in the first membrane separator  410  and the second membrane separator  420 , and the membrane material used is polyorganosiloxane. A hydrogen selective membrane module is provided in the third membrane separator  430 , and the membrane material used is polyimide. The nitrogen stream (hydrocarbon-depleted)  15  passes through the separation membrane, and enriched propylene stream  17  is obtained at a membrane permeate side of the first membrane separator  410  and returned to the inlet of the vent gas compressor  110 . Then, propylene is further recovered through compression and condensation steps. The gas stream obtained at a membrane residual side of the first membrane separator  410  enters into the second membrane separator  420 , and light hydrocarbon components therein, such as methane, ethylene and ethane, are separated from nitrogen. The gas stream obtained at a membrane residual side of the second membrane separator  420  enters into the third membrane separator  430 , and hydrogen is further separated from nitrogen. The hydrogen-enriched stream obtained at a membrane permeate side of the third membrane separator  430  and the light hydrocarbon-enriched stream obtained at a membrane permeate side of the second membrane separator  420  are mixed as a gas stream  18  and discharged to a flare. The nitrogen-enriched gas stream  16 , in which the purity of nitrogen is larger than 98.5% (V/V), is obtained at a membrane residual side of the third membrane separator  430  and returned to the degassing bin of the polypropylene plant to be reused. 
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 Material balance table of example 3 
               
            
           
           
               
               
            
               
                   
                 Serial number 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 10 
                 16 
                 18 
                 22 
                 21 
                 23 
               
            
           
           
               
               
            
               
                   
                 Stream name 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Vent 
                 Recovered 
                 Components discharged 
                 Recovered 
                 Removed 
                 Supplement 
               
               
                   
                 gas 
                 nitrogen 
                 to flare 
                 propylene 
                 water 
                 propylene coolant 
               
            
           
           
               
               
            
               
                   
                 Phase state 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Vapor-phase 
                 Vapor-phase 
                 Vapor-phase 
                 Vapor-phase 
                 Liquid-phase 
                 Liquid-phase 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Volume flow 
                 1380.00 
                 564.47 
                 298.42 
                 620.20 
                 20.70 
                 123.81 
               
               
                 (Nm 3 /hr) 
               
               
                 Temperature (° C.) 
                 40.00 
                 16.19 
                 19.00 
                 20.00 
                 40.00 
                 42.83 
               
               
                 Pressure (MPa) 
                 0.01 
                 1.88 
                 0.05 
                 0.15 
                 0.20 
                 1.66 
               
               
                 Molar percentage 
               
               
                 (%) 
               
               
                 Propylene 
                 30.54 
                 0.28 
                 1.73 
                 86.75 
                 0.00 
                 99.60 
               
               
                 Propane 
                 2.74 
                 0.02 
                 0.14 
                 6.09 
                 0.00 
                 0.40 
               
               
                 Ethylene 
                 2.32 
                 0.24 
                 1.22 
                 4.35 
                 0.00 
                 0.00 
               
               
                 Ethane 
                 0.07 
                 0.01 
                 0.02 
                 0.14 
                 0.00 
                 0.00 
               
               
                 Hydrogen 
                 1.45 
                 0.29 
                 6.05 
                 0.05 
                 0.00 
                 0.00 
               
               
                 Nitrogen 
                 61.23 
                 99.02 
                 90.53 
                 2.56 
                 0.00 
                 0.00 
               
               
                 Water 
                 1.50 
                 0.00 
                 0.00 
                 0.00 
                 100.00 
                 0.00 
               
               
                 Methane 
                 0.15 
                 0.13 
                 0.31 
                 0.07 
                 0.00 
                 0.00 
               
               
                 Components mass 
               
               
                 flow (kg/hr) 
               
               
                 Propylene 
                 780.90 
                 2.96 
                 9.57 
                 996.86 
                 0.00 
                 228.49 
               
               
                 Propane 
                 73.42 
                 0.26 
                 0.79 
                 73.33 
                 0.00 
                 0.96 
               
               
                 Ethylene 
                 39.55 
                 1.70 
                 4.50 
                 33.32 
                 0.00 
                 0.00 
               
               
                 Ethane 
                 1.28 
                 0.04 
                 0.10 
                 1.14 
                 0.00 
                 0.00 
               
               
                 Hydrogen 
                 1.78 
                 0.15 
                 1.60 
                 0.03 
                 0.00 
                 0.00 
               
               
                 Nitrogen 
                 1042.26 
                 689.45 
                 333.21 
                 19.60 
                 0.00 
                 0.00 
               
               
                 Water 
                 16.42 
                 0.00 
                 0.00 
                 0.00 
                 16.42 
                 0.00 
               
               
                 Methane 
                 1.46 
                 0.51 
                 0.66 
                 0.29 
                 0.00 
                 0.00 
               
               
                   
               
            
           
         
       
     
     Based on the material balance data as shown in Table 7, it can be obtained after calculation that, the recovery of propylene of the system is 98.4%, and the recovery of nitrogen is 66.15% with the purity of 99% (V/V) or above. If traditional two-bed dehydration process is used, about 900 kg/hr regeneration nitrogen needs to be consumed according to the present 16.42 kg/hr dehydration load. In this case, the actual recovery of nitrogen is about 13.65%, which is much lower than the recovery of nitrogen of the system disclosed herein. 
     It can be seen from the above embodiments that, the system and process provided herein can be used for better recovering propylene and nitrogen from vent gas in polyolefin production. The recovery of propylene in the vent gas is higher than 98%; the purity of nitrogen is higher than 98.5% (V/V); and the recovery of nitrogen is higher than 65%. In addition, the system and the process provided herein have the advantages of low investment cost and easy operation. 
     It should be noted that, the above embodiments are only used for illustrating, rather than restricting the present disclosure. The present disclosure is illustrated hereinabove with reference to typical embodiments, but is can be understood that, the words used herein are descriptive and illustrative ones, rather than restrictive ones. The present disclosure can be amended in the scope of the claims and without departing from the scope and spirit thereof. The present disclosure is described involving specific methods, materials and embodiments, but it does not mean that the present disclosure is limited to the specific embodiments disclosed herein. On the contrary, the present disclosure can be expanded to other methods and applications with the same functions.