Abstract:
The provided is a process for separating by absorption the pyrolysis gas from preparation of lower carbon olefins, wherein a primary absorbent and a secondary absorbent are introduced into the demethanizer to separate by absorption the feedstock of the demethanizer through countercurrent contact therewith at a moderate temperature and pressure, thereby to obtain a top fraction primarily comprising hydrogen and methane and a bottom fraction primarily comprising the absorbents and C2+ fraction, wherein the primary absorbent essentially is a mixed Cn or Cn+ fraction, the secondary absorbent essentially is a Cn′ alkane fraction or mixed Cn′ or Cn′+ fraction, and wherein n and n′ are independently 3, 4 or 5 with the proviso when the secondary absorbent is a mixed fraction, n′ is not 3.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the separation and purification of lower carbon olefins such as ethylene and/or propylene, particularly to a process for separating by absorption the pyrolysis gas from preparation of lower carbon olefins such as ethylene and/or propylene. 
     BACKGROUND OF THE INVENTION 
     As important basic petrochemical feedstock, lower carbon olefins such as ethylene and/or propylene have attracted a lot of attention from research and development teams to their preparation as well as subsequent separation and purification. In the past lower carbon olefins such as ethylene and/or propylene were primarily prepared by pyrolysis of petroleum hydrocarbon fractions such as naphtha and light diesel, however, in recent years a process for preparing olefins by pyrolysis of oxygenates had been developed due to the gradual short supply of crude oil. 
     No matter the pyrolysis is of petroleum hydrocarbons or of oxygenates, the resultant pyrolysis gas is always a mixture of complicated ingredients and depending on the process conditions generally comprises lower carbon olefins such as C2-C4 olefins at relative large amounts, also some non-olefin byproducts such as hydrogen, C1-C6 alkanes and little alkyne as well as in the case of pyrolysis of oxygenates some unreacted oxygenates such as alcohol and/or ether etc. Thus, a complicated separation and purification process is necessary to separate and purify such a complicated pyrolysis gas to obtain lower carbon olefins such as ethylene and/or propylene of polymerization grade. 
     The pyrolysis gas from preparation of lower carbon olefins is generally subjected to a cryogenic separation process, which typically covers three separation schemes, i.e. sequential scheme removing methane firstly, front end deethanizer scheme removing C2 and the lower fractions firstly, and front end depropanizer scheme removing C3 and the lower fractions firstly. In these separation schemes, the pyrolysis gas is generally pretreated, e.g. cooled, compressed, removed of impurities and dried as well as optionally finished, and then further treated to obtain lower carbon olefins of polymerization grade finally. In these separation schemes, when separating methane and hydrogen from C2+ fractions, a cryogenic separation process with high investment cost and energy consumption is necessary. In order to overcome the disadvantages of the cryogenic separation process, newly proposed is a process for separating by absorption the pyrolysis gas from preparation of lower carbon olefins, i.e. separating methane and hydrogen by absorbing C2+ fractions with an absorbent at moderate temperature and pressure. 
     In the absorption process, mixed hydrocarbons or pure hydrocarbon are generally used as the absorbents to separate methane and hydrogen from C2+ fractions at reasonable operating conditions and minimize the loss of targeted products such as ethylene and/or propylene as possible as can. In order to minimize the concentration of targeted products such as ethylene and/or propylene at the overhead of the absorption column, some measures such as circulating a lot of absorbent or decreasing the temperature of the absorbent are used to increase the absorption capacity, however, all these measures are with high energy consumptions. Thus, a compromise is necessary between minimizing the loss of targeted products such as ethylene and/or propylene and the energy consumption during the process. 
     Thus, in the art it is still needed to further improve the yield of targeted products such as ethylene and/or propylene and decrease the energy consumption during the separation and purification of the pyrolysis gas from preparation of lower carbon olefins. 
     SUMMARY OF THE INVENTION 
     Based on the composition of the pyrolysis gas from preparation of lower carbon olefins, the present invention further improve the separation of the pyrolysis gas, wherein composite absorbents are used in the demethanizer to separate methane and hydrogen from C2+ fractions, specifically, a mixed hydrocarbon fraction is used as a primary absorbent and a pure hydrocarbon or mixed hydrocarbon fraction is used as a secondary absorbent, so that to obtain lower carbon olefins such as ethylene and/or propylene of polymerization grade with significantly reduced cooling capacity. 
     Specifically, the present invention provides a process for separating by absorption the pyrolysis gas from preparation of lower carbon olefins, wherein a primary absorbent and a secondary absorbent are introduced into the demethanizer to separate by absorption the feedstock fed to the demethanizer through countercurrent contact therewith at a moderate temperature and pressure, thereby to obtain a top fraction primarily comprising hydrogen and methane and a bottom fraction primarily comprising the absorbents and C2+ fraction, wherein the primary absorbent essentially is a mixed Cn or Cn+ fraction, the secondary absorbent essentially is a Cn′ alkane fraction or mixed Cn′ or Cn′+ fraction, and wherein n and n′ are independently 3, 4 or 5 with the proviso when the secondary absorbent is a mixed fraction, n′ is not 3. 
     According to the process of the present invention, wherein into the demethanizer the feedstock is introduced at the middle or the bottom, the primary absorbent is introduced at the middle, the secondary absorbent is introduced at the top, and in the demethanizer the temperature is above −45            and the pressure is of 1.5-3.5 MPaG.
     According to the process of the present invention, wherein the primary absorbent is preferably introduced into the demethanizer at the middle and the bottom simultaneously with a mass flowrate ratio generally in the range of 1.0-15, preferably in the range of 1.2-10, more preferably in the range of 1.5-8. That is to say, according to the process of the present invention, wherein the primary absorbent may be introduced into the demethanizer at different locations proportionally to absorb C2+ fraction from the lower carbon hydrocarbon mixture gradually, thereby to separate more thoroughly. According to the process of the present invention, wherein the primary absorbent and the feedstock are introduced into the demethanizer at a total mass flowrate ratio in the range of 0.03-4, preferably in the range of 0.05-2.5, more preferably in the range of 0.1-1, and the primary absorbent and the secondary absorbent are introduced into the demethanizer at a total flowrate ratio in the range of 10-1.05, preferably in the range of 8-1.1, more preferably in the range of 6-1.2 
     According to the process of the present invention, wherein the primary absorbent and the secondary absorbent may be combined in many ways, e.g. the primary absorbent may essentially be mixed C3, C4 or C5 fraction, or may essentially be mixed C3+, C4+ or C5+ fraction, and the secondary absorbent may essentially be C3, C4 or C5 alkane fraction, or may essentially be mixed C4 or C5 fraction, and also may essentially be mixed C4+ or C5+ fraction, wherein the absorbents may be preferably mixed C3 fraction or mixed C3+ fraction and C3 alkane fraction in combination. 
     Herein, it is noted that “mixed fraction” means the fraction primarily comprises alkanes and olefins with some impurities such as alkynes and cyclic hydrocarbons, e.g. mixed C3 fraction primarily comprises C3 alkane and C3 olefin, and mixed C3+ fraction primarily comprises C3+ alkanes and C3+ olefins, and so on, and “alkane fraction” means the fraction essentially is alkanes with some impurities such as olefins, alkynes and cyclic hydrocarbons, e.g. C3 alkane fraction essentially is C3 alkane, and C3+ alkane fraction essentially is C3+ alkanes, and so on. 
     Furthermore, both the primary absorbent and the secondary absorbent may be from external sources, however, they are preferably from the pyrolysis gas separation scheme per se, that is to say, both the primary absorbent and the secondary absorbent are preferably supplied by the separation scheme per se. According to the process of the present invention, wherein a specified mixed fraction is used as the primary absorbent in the demethanizer to absorb most of C2+ fraction, then subsequently only the C2+ fraction and the absorbents from the bottom of the demethanizer need to be further separated from each other with less energy consumption; and a specified alkane fraction or mixed fraction is used as the secondary absorbent to be introduced at the top of the demethanizer to further absorb C2+ fraction, so that the top fraction of the demethanizer has a smaller concentration of the targeted olefins such as ethylene and/or propylene; furthermore, it is better that the mixed fraction as the secondary absorbent comprises no or as less as possible of the targeted olefins such as ethylene and/or propylene, so that to further minimize the loss of the targeted olefins due to entrainment or the like; at the same time, the secondary absorbent is used at a relative small amount, thus having little influence to the subsequent separation load. 
     According to the process of the present invention, the pyrolysis gas from preparation of lower carbon olefins may be separated in various schemes in the art. The pyrolysis gas may be pretreated and optionally finished and then directly fed into the demethanizer, i.e. it is separated in a sequential scheme; or the pyrolysis gas may be pretreated, suitably split and optionally finished and then fed into the demethanizer, i.e. it is separated in a front end depropanizer scheme or front end deethanizer scheme. During the process, C2, C3 and C4 fractions etc. are split out gradually and optionally finished respectively, thereby to obtain the lower carbon olefins such as ethylene and/or propylene of polymerization grade. 
     Thus, according to the process of the present invention, in addition to demethanizer, the separation process may further comprise compressor, finishing system, deethanizer, depropanizer, debutanizer as well as ethylene distillation column and propylene distillation column etc. 
     Specifically, according to the process of the present invention, the pyrolysis gas may be separated in a sequential scheme, wherein the pyrolysis gas is compressed and optionally finished and fed into the demethanizer. In such a case, a portion of the mixed C3 fraction derived from the top of the depropanizer may be used as the primary absorbent, and a portion of the C3 alkane fraction derived from the bottom of the propylene distillation column may be used as the secondary absorbent; or a portion of the mixed C3 fraction derived from the top of the depropanizer may be used as the primary absorbent, and a portion of the mixed C4+ fraction derived from the bottom of the depropanizer may be used as the secondary absorbent; or a portion of the mixed C3+ fraction derived from the bottom of the deethanizer may be used as the primary absorbent, and a portion of the mixed C4+ fraction derived from the bottom of the depropanizer may be used as the secondary absorbent; or a portion of the mixed C3+ fraction derived from the bottom of the deethanizer may be used as the primary absorbent, and a portion of the mixed C4 fraction derived from the top of the debutanizer may be used as the secondary absorbent; or a portion of the mixed C4+ fraction derived at the bottom of the depropanizer may be used as the primary absorbent, and a portion of the mixed C4 fraction derived from the top of the debutanizer may be used as the secondary absorbent. 
     Specifically, according to the process of the present invention, the pyrolysis gas may also be separated in a front end depropanizer scheme, wherein a single depropanizer may be used, or a high pressure depropanizer and a low pressure depropanizer may be used in combination. 
     When a single depropanizer is used in the front end depropanizer scheme, the pyrolysis gas is compressed and then introduced into the depropanizer, from which the top fraction is optionally finished and then fed into the demethanizer and the bottom fraction is fed into the debutanizer, wherein a portion of the mixed C3 fraction derived from the bottom of the deethanizer may be used as the primary absorbent, and a portion of the C3 alkane fraction derived from the bottom of the propylene distillation column may be used as the secondary absorbent. 
     When a high pressure depropanizer and a low pressure depropanizer is used in combination in the front end depropanizer scheme, the pyrolysis gas is compressed and then fed into the high pressure depropanizer, from which the top fraction is optionally finished and then fed into the demethanizer and the bottom fraction is fed into the low pressure depropanizer, from which the top fraction is back to the high pressure depropanizer and the bottom fraction is fed into the debutanizer, wherein a portion of the mixed C3 fraction derived from the bottom of the deethanizer may be used as the primary absorbent, and a portion of the C3 alkane fraction derived from the bottom of the propylene distillation column may be used as the secondary absorbent; and herein, a portion or all of the top fraction of the low pressure depropanizer may also be used as the primary absorbent, and in this case from the low pressure depropanizer the remaining portion of the top fraction, if any, is back to the high pressure depropanizer and the bottom fraction is fed into the debutanizer. 
     Specifically, according to the process of the present invention, the pyrolysis gas may also be separated in a front end deethanizer scheme, which generally comprises two deethanizers, i.e. a first deethanizer and a second deethanizer, wherein the pyrolysis gas is compressed and optionally finished and then fed into the first deethanizer, from which the top fraction is fed into the demethanizer and the bottom fraction is fed into the depropanizer, and the bottom fraction of the demethanizer is fed into the second deethanizer. 
     More specifically, in the front end deethanizer scheme, a portion of the mixed C3 fraction derived from the top of the depropanizer may be used as the primary absorbent, and a portion of the C3 alkane fraction derived from the bottom of the propylene distillation column may be used as the secondary absorbent; or a portion of the mixed C3+ fraction derived from the bottom of the first deethanizer may be used as the primary absorbent, and a portion of the mixed C4 fraction derived from the top of the debutanizer may be used as the secondary absorbent; or a portion of the mixed C3+ fraction derived from the bottom of the first deethanizer and/or the bottom of the second deethanizer may be used as the primary absorbent, and a portion of the mixed C4+ fraction derived from the bottom of the depropanizer may be used as the secondary absorbent; or both the primary absorbent and the secondary absorbent may be the mixed C4+ fraction derived from the bottom of the depropanizer; or both the primary absorbent and the secondary absorbent may be the mixed C4 fraction derived from the bottom of the second deethanizer and the top of the debutanizer. 
     Based on the technical solution of the process of the present invention and various embodiments thereof, it can be known that the process of the present invention can be easily incorporated into the prior art without too much changes or modifications to the old separation schemes. Thus, the process of the present invention can be used in the prior art to reach the corresponding technical improvements very well. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Now, the demethanizer and several typical embodiments of the process of the present invention are further illustrated with reference to the drawings, herein all the embodiments are not intended to limit the scope of the present invention. 
       In the drawings: 
         FIG. 1  is a schematic representative of the demethanizer in the process of the present invention; 
         FIG. 2  is an embodiment of the process of the present invention, wherein the pyrolysis gas is separated in a sequential scheme, wherein the pyrolysis gas is compressed and fed into the demethanizer, wherein the primary absorbent is the mixed C3+ fraction from the bottom of the deethanizer, and the secondary absorbent is the mixed C4+ fraction from the bottom of the depropanizer; 
         FIG. 3  is another embodiment of the process of the present invention, wherein the pyrolysis gas is separated in a sequential scheme, wherein the pyrolysis gas is compressed and fed into the demethanizer, wherein the primary absorbent is the mixed C3 fraction from the top of the depropanizer, and the secondary absorbent is the mixed C4+ fraction from the bottom of the depropanizer; 
         FIG. 4  is another embodiment of the process of the present invention, wherein the pyrolysis gas is separated in a front end depropanizer scheme, wherein a high pressure depropanizer and a low pressure depropanizer is used in combination, and the pyrolysis gas is compressed and fed into the high pressure depropanizer, from which the top fraction is compressed and fed into the demethanizer, wherein the primary absorbent is the mixed C3 fraction from the bottom of the deethanizer, and the secondary absorbent is the C3 alkane fraction from the bottom of the propylene distillation column; 
         FIG. 5  is another embodiment of the process of the present invention, wherein the pyrolysis gas is separated in a front end deethanizer scheme comprising a first deethanizer and a second deethanizer, wherein the pyrolysis gas is compressed and fed into the first deethanizer, from which the top fraction is fed into the demethanizer, from which the bottom fraction is fed into the second deethanizer, wherein the primary absorbent is the mixed C3+ fraction from the bottom of the first deethanizer and the bottom of the second deethanizer, and the secondary absorbent is the mixed C4+ fraction from the bottom of the depropanizer; 
         FIG. 6  is another embodiment of the process of the present invention, wherein the pyrolysis gas is separated in a front end deethanizer scheme comprising a first deethanizer and a second deethanizer, wherein the pyrolysis gas is compressed and fed into the first deethanizer, from which the top fraction is fed into the demethanizer, from which the bottom fraction is fed into the second deethanizer, wherein both the primary absorbent and the secondary absorbent are the mixed C4 fraction derived from the bottom of the second deethanizer and the top of the debutanizer, and wherein the mixed C4 fraction derived from the top of the debutanizer is introduced into the line for the secondary absorbent. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Now, several typical embodiments of the process of the present invention are further illustrated in details with reference to the drawings. 
     Firstly, the demethanizer in the process of the present invention is described with reference to  FIG. 1 . In  FIG. 1  the depicted is a schematic representative of demethanizer T 1  in the process of the present invention, wherein into the demethanizer feedstock  11  is introduced at the middle, primary absorbent  14  is introduced at the middle or at both the middle and the bottom proportionally (as shown by the dotted line), secondary absorbent  13  is introduced at the top, and then top fraction  12  primarily comprising hydrogen and methane and bottom fraction  15  primarily comprising the absorbents and C2+ fraction are obtained. Now, the cases wherein the pyrolysis gas from preparation of lower carbon olefins is separated in a sequential scheme are described with reference to  FIG. 2  and  FIG. 3 . 
     Referring to the scheme shown in  FIG. 2 , pyrolysis gas  10  from preparation of lower carbon olefins is compressed and introduced as feedstock  11  into demethanizer T 1 , primary absorbent  14  is the mixed C3+ fraction from the bottom of deethanizer T 2 , which is cooled and introduced into the middle of demethanizer T 1 , and secondary absorbent  13  is the mixed C4+ fraction from the bottom of depropanizer T 3 , which is cooled and introduced into the top of demethanizer T 1 ; the primary absorbent and the secondary absorbent together absorb C2+ fraction from feedstock  11  in demethanizer T 1  to obtain top fraction  12  primarily comprising methane and hydrogen, which is used as fuel gas after the cooling capacity being recovered therefrom, and bottom fraction  15  primarily comprising the absorbents and C2+ fraction, which is introduced into deethanizer T 2 ; from deethanizer T 2  the top fraction is introduced into the ethylene distillation column and the bottom fraction is introduced into depropanizer T 3 ; from the depropanizer T 3  the top fraction is introduced into the propylene distillation column and the remaining portion of the bottom fraction is introduced into the debutanizer. 
     And, referring to the scheme shown in  FIG. 3 , pyrolysis gas  10  from preparation of lower carbon olefins is compressed and introduced as feedstock  11  into demethanizer T 1 , primary absorbent  14  is the mixed C3 fraction from the top of depropanizer T 3 , which is cooled and introduced into the middle of demethanizer T 1 , and secondary absorbent  13  is the mixed C4+ fraction from the bottom of depropanizer T 3 , which is cooled and introduced into the top of demethanizer T 1 ; the primary absorbent and the secondary absorbent together absorb C2+ fraction from feedstock  11  in demethanizer T 1  to obtain top fraction  12  primarily comprising methane and hydrogen, which is used as fuel gas after the cooling capacity being recovered therefrom, and bottom fraction  15  primarily comprising the absorbents and C2+ fraction, which is introduced into deethanizer T 2 ; from deethanizer T 2  the top fraction is introduced into the ethylene distillation column and the bottom fraction is introduced into depropanizer T 3 ; from depropanizer T 3  the remaining portion of the top fraction is introduced into the propylene distillation column and the remaining portion of the bottom fraction is introduced into the debutanizer. 
     Furthermore, the cases wherein the pyrolysis gas from preparation of lower carbon olefins is separated in a front end depropanizer scheme are described with reference to  FIG. 4 . 
     Referring to the scheme shown in  FIG. 4 , pyrolysis gas  10  from preparation of lower carbon olefins is compressed and introduced into high pressure depropanizer T 31  to be split to obtain a top fraction primarily comprising C3 and the lower fractions, which is compressed and introduced as feedstock  11  into demethanizer T 1 , and a bottom fraction, which is introduced into low pressure depropanizer T 32  to be split furthermore; from low pressure depropanizer T 32  the top fraction i.e. the mixed C3 fraction is back to the top of high pressure depropanizer T 31  and the bottom fraction is introduced into the debutanizer; primary absorbent  14  is the bottom fraction, i.e. the mixed C3 fraction from deethanizer T 2 , which is compressed and introduced into the middle of demethanizer T 1 , and secondary absorbent  13  is the C3 alkane fraction, i.e. propane fraction from the bottom of propylene distillation column T 3 ′, which is cooled and introduced into the top of demethanizer T 1 ; the primary absorbent and the secondary absorbent together absorb C2+ fraction from feedstock  11  in demethanizer T 1  to obtain top fraction  12  primarily comprising methane and hydrogen, which is used as fuel gas after the cooling capacity being recovered therefrom, and bottom fraction  15  primarily comprising the absorbents and C2+ fraction, which is introduced into deethanizer T 2 ; from deethanizer T 2  the top fraction is introduced into the ethylene distillation column and the bottom fraction is introduced into propylene distillation column T 3 ′; from propylene distillation column T 3 ′ the top fraction, i.e. propylene fraction is withdrawn from the scheme as product and the bottom fraction, i.e. the remaining portion of the propane fraction is withdrawn from the scheme as byproduct. 
     Furthermore, the cases wherein the pyrolysis gas from preparation of lower carbon olefins is separated in a front end deethanizer scheme are described with reference to  FIG. 5  and  FIG. 6 . 
     Referring to the scheme shown in  FIG. 5 , pyrolysis gas  10  from preparation of lower carbon olefins is compressed and introduced into first deethanizer T 21 , from which the top fraction is introduced as feedstock  11  into demethanizer T 1 ; primary absorbent  14  is the mixed C3+ fraction derived from the bottom of first deethanizer T 21  and the bottom of second deethanizer T 22 , which is cooled and introduced into the middle of demethanizer T 1 , and secondary absorbent  13  is the mixed C4+ fraction from the bottom of depropanizer T 3 , which is cooled and introduced into the top of demethanizer T 1 ; the primary absorbent and the secondary absorbent together absorb C2 fraction from feedstock  11  in demethanizer T 1  to obtain top fraction  12  primarily comprising methane and hydrogen, which is used as fuel gas after the cooling capacity being recovered therefrom, and bottom fraction  15  primarily comprising the absorbents and C2 fraction, which is introduced into second deethanizer T 22 ; from second deethanizer T 22  the top fraction is introduced into the ethylene distillation column; the remaining portion of the bottom fraction from first deethanizer T 21  and the remaining portion of the bottom fraction from second deethanizer T 22  are introduced into depropanizer T 3 ; from depropanizer T 3  the top fraction is introduced into the propylene distillation column and the remaining portion of the bottom fraction is introduced into the debutanizer. 
     And, referring to the scheme shown in  FIG. 6 , pyrolysis gas  10  from preparation of lower carbon olefins is compressed and introduced into first deethanizer T 21 , from which the top fraction is introduced as feedstock  11  into demethanizer T 1  and the bottom fraction is introduced into depropanizer T 3  to be further split; both primary absorbent  14  and secondary absorbent  13  are the mixed C4 fraction, which is derived from the bottom of the second deethanizer and the top of the debutanizer, and introduced into the middle and the top of demethanizer T 1  after being cooled respectively, wherein the mixed C4 fraction from the top of the debutanizer is introduced into the line for the secondary absorbent to the top of demethanizer T 1 ; the primary absorbent and the secondary absorbent together absorb C2 fraction from feedstock  11  in demethanizer T 1  to obtain top fraction  12  primarily comprising methane and hydrogen, which is used as fuel gas after the cooling capacity being recovered therefrom, and bottom fraction  15  primarily comprising the absorbents and C2 fraction, which is introduced into second deethanizer T 22 ; from second deethanizer T 22  the top fraction is introduced into the ethylene distillation column; from depropanizer T 3  the top fraction is introduced into the propylene distillation column and the bottom fraction is introduced into the debutanizer; from the debutanizer the bottom fraction is introduced into the subsequent process or withdrawn from the scheme as byproduct. Furthermore, in addition to being used as the primary and secondary absorbents, the remaining portion of the bottom fraction of second deethanizer T 22  and the remaining portion of the top fraction of debutanizer T 4  are introduced into the subsequent process or withdrawn from the scheme as byproducts. 
     Now, the present invention is further illustrated by the following examples, which are not intended to limit the scope of the present invention. 
     EXAMPLES 
     Example 1 
     This example is provided regarding the cases wherein the pyrolysis gas from preparation of lower carbon olefins is separated in the sequential scheme as shown in  FIG. 2 . The operation parameters for effecting the process are listed in Table 1, and the calculated results are shown in Table 2. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 The operation parameters for the demethanizer in example 1 
               
             
          
           
               
                 Item 
                 Unit 
                 Value 
               
               
                   
               
               
                 Feed pressure of demethanizer 
                 MPaG 
                 3.1 
               
               
                 Top pressure of demethanizer T1 
                 MPaG 
                 2.6 
               
               
                 Temperature of demethanizer T1 (Top/Bottom) 
                 
                           
                 
                 6/23 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 The results of the simulation calculation for the scheme in example1 
               
             
          
           
               
                   
                 Stream No. 
               
             
          
           
               
                   
                 10 
                 11 
                 12 
                 13 
                 14 
                 15 
               
               
                   
                   
               
             
          
           
               
                 Temperature            
                 40 
                 32 
                 −32.6 
                 −10 
                 75.8 
                 22.7 
               
               
                 Pressure MPaG 
                 0.03 
                 3.13 
                 2.615 
                 2.955 
                 2.763 
                 2.665 
               
               
                 Flowrate kg/hr 
                 54141.52 
                 52061.597 
                 855.981 
                 3570.752 
                 14194.084 
                 68970.451 
               
               
                 Molar composition 
               
               
                 H2O 
                 0.020744 
                 0.019697 
                 0.398293 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 CH4 
                 0.025334 
                 0.027118 
                 0.548258 
                 0.000000 
                 0.000000 
                 0.000003 
               
               
                 C2H4 
                 0.470997 
                 0.502521 
                 0.004823 
                 0.000000 
                 0.000024 
                 0.420781 
               
               
                 C2H6 
                 0.010853 
                 0.013129 
                 0.000172 
                 0.000000 
                 0.000357 
                 0.011052 
               
               
                 C3H6 
                 0.310396 
                 0.331159 
                 0.008995 
                 0.005102 
                 0.696495 
                 0.395935 
               
               
                 C3H8 
                 0.024604 
                 0.027332 
                 0.000694 
                 0.001811 
                 0.057643 
                 0.032754 
               
               
                 1,3-C4H6 
                 0.000730 
                 0.000781 
                 0.000186 
                 0.010065 
                 0.002468 
                 0.001402 
               
               
                 C4H8 
                 0.053305 
                 0.056948 
                 0.010925 
                 0.726583 
                 0.179559 
                 0.102019 
               
               
                 i-C4H10 
                 0.000076 
                 0.000082 
                 0.000030 
                 0.001009 
                 0.000253 
                 0.000144 
               
               
                 n-C4H10 
                 0.002353 
                 0.002519 
                 0.000459 
                 0.032268 
                 0.007955 
                 0.004520 
               
               
                 C5 
                 0.011768 
                 0.011766 
                 0.000486 
                 0.152975 
                 0.037523 
                 0.021319 
               
               
                 C6 
                 0.005211 
                 0.005577 
                 0.000004 
                 0.069310 
                 0.017542 
                 0.009967 
               
               
                 CO 
                 0.000395 
                 0.000414 
                 0.008368 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 CO2 
                 0.000199 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 CH4O 
                 0.002438 
                 0.000002 
                 0.000000 
                 0.000031 
                 0.000007 
                 0.000004 
               
               
                 C2H6O 
                 0.002486 
                 0.000051 
                 0.000009 
                 0.000137 
                 0.000117 
                 0.000067 
               
               
                 H2O 
                 0.057250 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 O2 
                 0.000002 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 N2 
                 0.000859 
                 0.000905 
                 0.018291 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                   
               
             
          
         
       
     
     As known from the results shown in Table 2, when the process of the present invention is effected according to the scheme shown in  FIG. 2 , at the overhead of the absorption column the ethylene concentration is of 0.48% and the propylene concentration is of 0.9%, that is to say, relative to the ethylene and propylene in the fed pyrolysis gas, at the top of the demethanizer the loss rates for ethylene and propylene are of 0.05% and 0.13% respectively. Thus, when being effected according to the scheme shown in  FIG. 2 , the process of the present invention reaches excellent technical effects. 
     Example 2 
     This example is provided regarding the cases wherein the pyrolysis gas from preparation of lower carbon olefins is separated in the sequential scheme as shown in  FIG. 3 . The operation parameters for effecting the process are listed in Table 3, and the calculated results are shown in Table 4. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 The operation parameters for the demethanizer in example 2 
               
             
          
           
               
                 Item 
                 Unit 
                 Value 
               
               
                   
               
               
                 Feed pressure of demethanizer 
                 MPaG 
                 3.1 
               
               
                 Top pressure of demethanizer T1 
                 MPaG 
                 2.6 
               
               
                 Temperature of demethanizer T1 (Top/Bottom) 
                 
                           
                 
                 −2/21 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 The results of the simulation calculation for the scheme in example 2 
               
             
          
           
               
                   
                 Stream No. 
               
             
          
           
               
                   
                 10 
                 11 
                 12 
                 13 
                 14 
                 15 
               
               
                   
               
             
          
           
               
                 Temperature            
                 40 
                 32 
                 −28.6 
                 −10 
                 40 
                 21.3 
               
               
                 Pressure MPaG 
                 0.03 
                 3.13 
                 2.615 
                 2.955 
                 2.89 
                 2.665 
               
               
                 Flowrate kg/hr 
                 54141.52 
                 52061.597 
                 845.931 
                 5578.453 
                 9680.951 
                 66475.069 
               
               
                 Molar composition 
               
               
                 H2O 
                 0.020744 
                 0.019697 
                 0.400332 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 CH4 
                 0.025334 
                 0.027118 
                 0.551066 
                 0.000000 
                 0.000000 
                 0.000004 
               
               
                 C2H4 
                 0.470997 
                 0.502521 
                 0.002856 
                 0.000000 
                 0.000024 
                 0.43082 
               
               
                 C2H6 
                 0.010853 
                 0.013129 
                 0.000085 
                 0.000000 
                 0.00048 
                 0.011319 
               
               
                 C3H6 
                 0.310396 
                 0.331159 
                 0.002943 
                 0.005099 
                 0.923094 
                 0.405547 
               
               
                 C3H8 
                 0.024604 
                 0.027332 
                 0.000384 
                 0.001814 
                 0.075913 
                 0.033504 
               
               
                 1,3-C4H6 
                 0.00073 
                 0.000781 
                 0.00023 
                 0.009919 
                 0.000011 
                 0.001188 
               
               
                 C4H8 
                 0.053305 
                 0.056948 
                 0.013961 
                 0.726351 
                 0.00036 
                 0.086871 
               
               
                 i-C4H10 
                 0.000076 
                 0.000082 
                 0.000036 
                 0.001015 
                 0.000005 
                 0.000123 
               
               
                 n-C4H10 
                 0.002353 
                 0.002519 
                 0.000599 
                 0.032215 
                 0.000001 
                 0.003846 
               
               
                 C5 
                 0.011768 
                 0.011766 
                 0.000694 
                 0.151471 
                 0.000000 
                 0.018106 
               
               
                 C6 
                 0.005211 
                 0.005577 
                 0.000008 
                 0.071954 
                 0.000000 
                 0.008604 
               
               
                 CO 
                 0.000395 
                 0.000414 
                 0.008411 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 CO2 
                 0.000199 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 CH4O 
                 0.002438 
                 0.000002 
                 0.000000 
                 0.00003 
                 0.000000 
                 0.000004 
               
               
                 C2H6O 
                 0.002486 
                 0.000051 
                 0.00001 
                 0.00013 
                 0.000111 
                 0.000065 
               
               
                 H2O 
                 0.05725 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 O2 
                 0.000002 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 N2 
                 0.000859 
                 0.000905 
                 0.018385 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                   
               
             
          
         
       
     
     As known from the results shown in Table 4, when the process of the present invention is effected according to the scheme shown in  FIG. 3 , at the overhead of the absorption column the ethylene concentration is of 0.29% and the propylene concentration is of 0.29%, that is to say, relative to the ethylene and propylene in the fed pyrolysis gas, at the top of the demethanizer the loss rates for ethylene and propylene are of 0.03% and 0.04% respectively. Thus, when being effected according to the scheme shown in  FIG. 3 , the process of the present invention also reaches excellent technical effects. 
     Example 3 
     This example is provided regarding the cases wherein the pyrolysis gas from preparation of lower carbon olefins is separated in the front end depropanizer scheme as shown in  FIG. 4 . The operation parameters for effecting the process are listed in Table 5, and the calculated results are shown in Table 6. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 The operation parameters for the demethanizer in example 3 
               
             
          
           
               
                 Item 
                 Unit 
                 Value 
               
               
                   
               
               
                 Feed pressure of demethanizer 
                 MPaG 
                 3.1 
               
               
                 Top pressure of demethanizer T1 
                 MPaG 
                 2.6 
               
               
                 Temperature of demethanizer T1 (Top/Bottom) 
                 
                           
                 
                 −10/19 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 The results of the simulation calculation for the scheme in example 3 
               
             
          
           
               
                   
                 Stream No. 
               
             
          
           
               
                   
                 10 
                 11 
                 12 
                 13 
                 14 
                 15 
               
               
                   
                   
               
             
          
           
               
                 Temperature            
                 40 
                 55.5 
                 −24 
                 −21 
                 62.8 
                 19.2 
               
               
                 Pressure MPaG 
                 0.03 
                 3.13 
                 2.615 
                 2.955 
                 2.763 
                 2.665 
               
               
                 Flowrate kg/hr 
                 54141.52 
                 45043.202 
                 1142.131 
                 4418.281 
                 14351.4 
                 62671.357 
               
               
                 Molar composition 
               
               
                 H2O 
                 0.020744 
                 0.021363 
                 0.365577 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 CH4 
                 0.025334 
                 0.029409 
                 0.503202 
                 0.000000 
                 0.000000 
                 0.000004 
               
               
                 C2H4 
                 0.470997 
                 0.544987 
                 0.000143 
                 0.000000 
                 0.000037 
                 0.432746 
               
               
                 C2H6 
                 0.010853 
                 0.014239 
                 0.000034 
                 0.000000 
                 0.000363 
                 0.011375 
               
               
                 C3H6 
                 0.310396 
                 0.358828 
                 0.008758 
                 0.052366 
                 0.795238 
                 0.442266 
               
               
                 C3H8 
                 0.024604 
                 0.029473 
                 0.097473 
                 0.937192 
                 0.202141 
                 0.112374 
               
               
                 1,3-C4H6 
                 0.00073 
                 0.000006 
                 0.000002 
                 0.000123 
                 0.000033 
                 0.000018 
               
               
                 C4H8 
                 0.053305 
                 0.000221 
                 0.000063 
                 0.004479 
                 0.001193 
                 0.000662 
               
               
                 i-C4H10 
                 0.000076 
                 0.000004 
                 0.000001 
                 0.000067 
                 0.00002 
                 0.000011 
               
               
                 n-C4H10 
                 0.002353 
                 0.000003 
                 0.000000 
                 0.000036 
                 0.000011 
                 0.000006 
               
               
                 C5 
                 0.011768 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 C6 
                 0.005211 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 CO 
                 0.000395 
                 0.000449 
                 0.00768 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 CO2 
                 0.000199 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 CH4O 
                 0.002438 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 C2H6O 
                 0.002486 
                 0.000038 
                 0.000243 
                 0.005009 
                 0.000854 
                 0.000475 
               
               
                 H2O 
                 0.05725 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 O2 
                 0.000002 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 N2 
                 0.000859 
                 0.000981 
                 0.016788 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                   
               
             
          
         
       
     
     As known from the results shown in Table 6, when the process of the present invention is effected according to the scheme shown in  FIG. 4 , at the overhead of the absorption column the ethylene concentration is of 0.01% and the propylene concentration is of 0.87%, that is to say, relative to the ethylene and propylene in the fed pyrolysis gas, at the top of the demethanizer the loss rates for ethylene and propylene are of 0.002% and 0.14% respectively. Thus, when being effected according to the scheme shown in  FIG. 4 , the process of the present invention also reaches excellent technical effects. 
     Example 4 
     This example is provided regarding the cases wherein the pyrolysis gas from preparation of lower carbon olefins is separated in the front end deethanizer scheme as shown in  FIG. 5 . The operation parameters for effecting the process are listed in Table 7, and the calculated results are shown in Table 8. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 The operation parameters for the demethanizer in example 4 
               
             
          
           
               
                 Item 
                 Unit 
                 Value 
               
               
                   
               
               
                 Feed pressure of demethanizer 
                 MPaG 
                 3.1 
               
               
                 Top pressure of demethanizer T1 
                 MPaG 
                 2.6 
               
               
                 Temperature of demethanizer T1 (Top/Bottom) 
                 
                           
                 
                 −5/18 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 8 
               
             
             
               
                   
               
               
                 The results of the simulation calculation for the scheme in example 4 
               
             
          
           
               
                   
                 Stream No. 
               
             
          
           
               
                   
                 10 
                 11 
                 12 
                 13 
                 14 
                 15 
               
               
                   
                   
               
             
          
           
               
                 Temperature            
                 40 
                 32 
                 −18.5 
                 −10 
                 83.4 
                 18.3 
               
               
                 Pressure MPaG 
                 0.03 
                 3.13 
                 2.615 
                 2.955 
                 2.763 
                 2.665 
               
               
                 Flowrate kg/hr 
                 54141.52 
                 52061.597 
                 879.41 
                 15127.658 
                 24047.438 
                 60718.006 
               
               
                 Molar composition 
               
               
                 H2O 
                 0.020744 
                 0.019697 
                 0.397212 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 CH4 
                 0.025334 
                 0.027118 
                 0.546013 
                 0.000000 
                 0.000000 
                 0.000041 
               
               
                 C2H4 
                 0.470997 
                 0.502521 
                 0.00481 
                 0.000000 
                 0.00006 
                 0.49868 
               
               
                 C2H6 
                 0.010853 
                 0.013129 
                 0.000012 
                 0.000000 
                 0.000521 
                 0.012973 
               
               
                 C3H6 
                 0.310396 
                 0.331159 
                 0.00109 
                 0.005168 
                 0.549859 
                 0.178334 
               
               
                 C3H8 
                 0.024604 
                 0.027332 
                 0.000298 
                 0.001776 
                 0.045771 
                 0.015046 
               
               
                 1,3-C4H6 
                 0.00073 
                 0.000781 
                 0.000346 
                 0.010129 
                 0.004091 
                 0.002994 
               
               
                 C4H8 
                 0.053305 
                 0.056948 
                 0.021206 
                 0.726629 
                 0.29528 
                 0.215564 
               
               
                 i-C4H10 
                 0.000076 
                 0.000082 
                 0.000049 
                 0.001007 
                 0.000412 
                 0.000299 
               
               
                 n-C4H10 
                 0.002353 
                 0.002519 
                 0.000929 
                 0.032264 
                 0.013096 
                 0.009567 
               
               
                 C5 
                 0.011768 
                 0.011766 
                 0.001405 
                 0.153648 
                 0.062235 
                 0.045668 
               
               
                 C6 
                 0.005211 
                 0.005577 
                 0.000021 
                 0.068416 
                 0.028321 
                 0.020561 
               
               
                 CO 
                 0.000395 
                 0.000414 
                 0.008345 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 CO2 
                 0.000199 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 CH4O 
                 0.002438 
                 0.000002 
                 0.000000 
                 0.000031 
                 0.000012 
                 0.000009 
               
               
                 C2H6O 
                 0.002486 
                 0.000051 
                 0.000009 
                 0.000107 
                 0.000113 
                 0.000054 
               
               
                 H2O 
                 0.05725 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 O2 
                 0.000002 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 N2 
                 0.000859 
                 0.000905 
                 0.018242 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                   
               
             
          
         
       
     
     As known from the results shown in Table 8, when the process of the present invention is effected according to the scheme shown in  FIG. 5 , at the overhead of the absorption column the ethylene concentration is of 0.48% and the propylene concentration is of 0.11%, that is to say, relative to the ethylene and propylene in the fed pyrolysis gas, at the top of the demethanizer the loss rates for ethylene and propylene are of 0.05% and 0.02% respectively. Thus, when being effected according to the scheme shown in  FIG. 5 , the process of the present invention also reaches excellent technical effects. 
     Example 5 
     This example is provided regarding the cases wherein the pyrolysis gas from preparation of lower carbon olefins is separated in the front end deethanizer scheme as shown in  FIG. 6 . The operation parameters for effecting the process are listed in Table 9, and the calculated results are shown in Table 10. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 9 
               
             
             
               
                   
               
               
                 The operation parameters for the demethanizer in example 5 
               
             
          
           
               
                 Item 
                 Unit 
                 Value 
               
               
                   
               
               
                 Feed pressure of demethanizer 
                 MPaG 
                 3.1 
               
               
                 Top pressure of demethanizer T1 
                 MPaG 
                 2.6 
               
               
                 Temperature of demethanizer T1 (Top/Bottom) 
                 
                           
                 
                 −14/17 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 10 
               
             
             
               
                   
               
               
                 The results of the simulation calculation for the scheme in example 5 
               
             
          
           
               
                   
                 Stream No. 
               
             
          
           
               
                   
                 10 
                 11 
                 12 
                 13 
                 14 
                 15 
               
               
                   
                   
               
             
          
           
               
                 Temperature            
                 40 
                 32 
                 −27.8 
                 −20 
                 40 
                 17.1 
               
               
                 Pressure MPaG 
                 0.03 
                 3.13 
                 2.615 
                 2.955 
                 2.743 
                 2.665 
               
               
                 Flowrate kg/hr 
                 54141.52 
                 52061.597 
                 853.461 
                 10181.999 
                 24756.056 
                 56506.913 
               
               
                 Molar composition 
               
               
                 H2O 
                 0.020744 
                 0.019697 
                 0.399288 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 CH4 
                 0.025334 
                 0.027118 
                 0.548868 
                 0.000000 
                 0.000000 
                 0.000045 
               
               
                 C2H4 
                 0.470997 
                 0.502521 
                 0.005987 
                 0.000016 
                 0.000016 
                 0.539403 
               
               
                 C2H6 
                 0.010853 
                 0.013129 
                 0.000393 
                 0.000594 
                 0.000595 
                 0.014098 
               
               
                 C3H6 
                 0.310396 
                 0.331159 
                 0.000843 
                 0.00575 
                 0.005063 
                 0.002501 
               
               
                 C3H8 
                 0.024604 
                 0.027332 
                 0.000255 
                 0.002087 
                 0.001839 
                 0.000843 
               
               
                 1,3-C4H6 
                 0.00073 
                 0.000781 
                 0.000268 
                 0.012757 
                 0.012754 
                 0.005694 
               
               
                 C4H8 
                 0.053305 
                 0.056948 
                 0.016573 
                 0.931935 
                 0.932823 
                 0.416472 
               
               
                 i-C4H10 
                 0.000076 
                 0.000082 
                 0.000041 
                 0.001306 
                 0.001301 
                 0.000581 
               
               
                 n-C4H10 
                 0.002353 
                 0.002519 
                 0.000727 
                 0.041318 
                 0.041351 
                 0.018463 
               
               
                 C5 
                 0.011768 
                 0.011766 
                 0.00002 
                 0.004021 
                 0.004044 
                 0.001806 
               
               
                 C6 
                 0.005211 
                 0.005577 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 CO 
                 0.000395 
                 0.000414 
                 0.008389 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 CO2 
                 0.000199 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 CH4O 
                 0.002438 
                 0.000002 
                 0.000000 
                 0.000038 
                 0.000038 
                 0.000017 
               
               
                 C2H6O 
                 0.002486 
                 0.000051 
                 0.00001 
                 0.000178 
                 0.000175 
                 0.000078 
               
               
                 H2O 
                 0.05725 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 O2 
                 0.000002 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                 N2 
                 0.000859 
                 0.000905 
                 0.018337 
                 0.000000 
                 0.000000 
                 0.000000 
               
               
                   
               
             
          
         
       
     
     As known from the results shown in Table 10, when the process of the present invention is effected according to the scheme shown in  FIG. 6 , at the overhead of the absorption column the ethylene concentration is of 0.60% and the propylene concentration is of 0.08%, that is to say, relative to the ethylene and propylene in the fed pyrolysis gas, at the top of the demethanizer the loss rates for ethylene and propylene are of 0.06% and 0.013% respectively. Thus, when being effected according to the scheme shown in  FIG. 6 , the process of the present invention also reaches excellent technical effects.