CN113354502A - Method for separating low-carbon hydrocarbon from unsaturated dry gas by combined absorption and recovery - Google Patents
Method for separating low-carbon hydrocarbon from unsaturated dry gas by combined absorption and recovery Download PDFInfo
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Abstract
The invention provides a method for separating low-carbon hydrocarbons from unsaturated dry gas by combined absorption and recovery, which comprises the following steps: s1, gradually compressing unsaturated dry gas in a refinery to 1-2 MPag by a compressor, cooling to 25-35 ℃ by a cooler, and then feeding into a high-pressure depropanizing tower; the gas phase at the top of the high-pressure depropanizing tower is cooled to-15 to-35 ℃ step by step and then enters an absorption tower; s2, the absorption tower adopts liquid phase propylene or C3 fraction rich in propylene as an absorbent, and the absorbent enters the absorption tower from the top of the tower to absorb components of more than C2 and C2 in unsaturated dry gas; the invention takes propylene or C3 fraction rich in propylene as an absorbent, takes toluene or xylene and industrial hexane as a reabsorber, and carries out absorption and separation on unsaturated dry gas from refinery devices such as catalytic cracking and catalytic cracking under the condition of intercooling, the recovery rate of ethylene and propylene can reach more than 99 percent, meanwhile, the circulation amount and the loss amount of the absorbent are relatively small, and the energy consumption of the device is low.
Description
Technical Field
The invention relates to a method for separating low-carbon hydrocarbons from unsaturated dry gas, in particular to a method for separating low-carbon hydrocarbons from unsaturated dry gas by combined absorption and recovery.
Background
The unsaturated dry gas of the refinery mainly comes from refinery devices such as catalytic cracking, catalytic cracking and the like, and the components mainly comprise hydrogen, methane, ethylene, ethane, propylene, propane and the like. Wherein, ethane and propane can be used as raw materials of an ethylene cracking device for producing ethylene and propylene; ethylene and propylene have higher economic value and can be used as raw materials of downstream chemical devices.
At present, the method for separating ethylene and propylene from unsaturated dry gas in a refinery mainly comprises the traditional absorption stabilization separation method, a cryogenic separation method, an intermediate cold oil absorption method, a complex separation method, a pressure swing adsorption method, a shallow cold oil absorption method and the like, and various methods have respective characteristics. The cryogenic separation method has high ethylene recovery rate and mature process, but has large investment and higher energy consumption for recovering the dilute ethylene; the complex separation method has strict requirements on impurities in raw materials, has higher pretreatment cost and needs a special complexing agent; the pressure swing adsorption has low energy consumption, simple operation, large occupied area, poor absorption effect and low ethylene recovery rate.
The unsaturated dry gas in the refinery adopts a traditional absorption stable separation method, the ethylene content in the separated dry gas is about 30-50 mol%, and the ethane content is about 20-30 mol%; the liquefied gas contains about 45-50 mol% of propylene and about 5-10 mol% of propane. Because the ethylene is difficult to directly recover, the ethylene in the dry gas can be used as the raw material for preparing the ethylbenzene by the dilute ethylene, and the ethylene in the dry gas is not recovered as the fuel gas of a refinery in general; the propylene in the liquefied gas needs to be further refined by gas fractionation and purification to obtain polymerization-grade propylene. The whole separation process is longer, the occupied area is large, and the production cost is higher.
Patent publication No. CN101063048A proposes a method for separating refinery catalytic dry gas by adopting an intercooled oil absorption method, which comprises the steps of compression, impurity removal, drying, absorption, desorption, cold recovery, rough separation and the like. The liquefied gas is used as an absorbent, the pressure is 2.5-3.5 MPag, the absorption temperature is-25 ℃ to-35 ℃, the absorption temperature is lower, the equipment investment and the energy consumption are higher, the highest ethylene recovery rate can reach 98.8%, the propylene recovery rate is 96.4%, the recovery rate is relatively lower, and the purity of ethylene and propylene products is not high. The middle cooling oil absorption method has strong adaptability, but the defects of low ethylene recovery rate, large circulation amount and loss of the absorbent, high energy consumption, dry gas and the like exist by adopting liquefied gas as the absorbent.
The patent publication No. CN101759516A proposes a method for separating refinery catalytic dry gas by oil absorption, which adopts pentaalkane-containing carbon five fraction as absorbent, absorbs C2 fraction and heavier components in the catalytic dry gas after compression and cooling in a main absorption tower, the bottom stream of the main absorption tower is sent to a desorption tower, and the top of the desorption tower obtains recovered C2 concentrated gas. The pressure of the absorption tower is about 3.5-5.5 MPag, the temperature of the top of the absorption tower is about 35-45 ℃, and the temperature of the bottom of the absorption tower is 95-115 ℃. The temperature of the top of the desorption tower is 35-45 ℃, and the temperature of the bottom of the desorption tower is 145-165 ℃. The method is provided with a reabsorption tower, uses crude gasoline as an absorbent to absorb carbon five absorbent carried out from gas at the top of a main absorption tower, and rich absorption oil enters a stabilization tower. The method has the advantages of low ethylene recovery rate of about 89.2 percent, high tower bottom temperature of the absorption tower and the desorption tower, relatively high energy consumption and gasoline component contained in the tower top gas of the reabsorption tower.
The patent publication No. CN101812322A provides a method for separating refinery catalytic dry gas by adopting oil absorption, which adopts four carbon fractions as an absorbent for shallow cold absorption, the absorption pressure is 3.5-5.5 MPag, the absorption temperature is 5-20 ℃, an expander and a cold box are adopted for recovering cold, the outlet pressure of the expander is 1.0-2.5 MPag, and the temperature is-40 to-20 ℃, the method can ensure that the recovery rate of ethylene reaches 93 percent, the loss rate of ethylene is also higher, and methane-hydrogen tail gas still carries more than four carbon components.
In conclusion, the existing process for recovering C2 and C3 from unsaturated dry gas in a refinery has the problems of high energy consumption, large equipment scale, more low-temperature equipment, high investment, low ethylene recovery rate, dry gas and the like.
Disclosure of Invention
The invention aims to provide a method for separating low-carbon hydrocarbons from unsaturated dry gas by combined absorption and recovery, so as to solve the problems in the background technology.
In order to achieve the purpose, the invention provides the following technical scheme:
a method for separating low carbon hydrocarbon in unsaturated dry gas by combined absorption and recovery comprises the following steps:
s1, gradually compressing unsaturated dry gas in a refinery to 1-2 MPag by a compressor, cooling to 25-35 ℃ by a cooler, and then feeding into a high-pressure depropanizing tower;
after the gas phase at the top of the high-pressure depropanizing tower is treated by impurity removal and drying equipment, the gas phase is gradually cooled to-15 to-35 ℃ and then enters an absorption tower; the liquid phase at the bottom of the high-pressure depropanizing tower enters the middle upper part of the low-pressure depropanizing tower;
the gas phase at the top of the low-pressure depropanizing tower returns to the high-pressure depropanizing tower after being condensed, and the C4+ liquid phase at the bottom of the low-pressure depropanizing tower is sent to a stabilizing tower for continuous separation;
s2, the absorption tower adopts liquid phase propylene or C3 fraction rich in propylene as an absorbent, the absorbent enters the absorption tower from the tower top to absorb components of C2 and above C2 in unsaturated dry gas, the tower bottom material of the absorption tower is sent to a deethanizer, and the tower top gas phase of the absorption tower enters a reabsorption tower;
s3, adopting liquid-phase methylbenzene or dimethylbenzene and industrial hexane as a reabsorber by the reabsorber to recover C2 and C3 components carried out by a gas phase at the top of the absorption tower; the gas at the top of the reabsorption tower is used as dry gas and is merged into a fuel gas pipe network of a refinery, and the liquid phase at the bottom of the reabsorption tower is sent into a re-desorption tower;
s4, condensing the gas phase at the top of the re-desorption tower to obtain a liquid phase, pressurizing and cooling the liquid phase by a pump, returning the liquid phase to the absorption tower, and taking the liquid phase at the bottom of the re-desorption tower as a re-absorbent to circulate and return to the top of the re-absorption tower;
s5, feeding the mixed components of ethane and ethylene obtained from the top of the deethanizer into an ethylene rectifying tower for further separation; one part of the C3 component at the bottom of the deethanizer is used as a circulating absorbent to be pressurized and cooled and then returns to the top of the absorption tower, and the other part enters a propylene rectifying tower to be continuously separated;
s6, separating the top of the ethylene rectifying tower to obtain liquid-phase polymerization-grade ethylene, and obtaining high-purity ethane at the bottom of the tower;
s7, separating the top or top lateral line of the propylene rectifying tower to obtain liquid-phase polymerization-grade propylene, and obtaining high-purity propane at the bottom of the tower.
As a further scheme of the invention: in step S1, compressing unsaturated dry gas of the refinery stage by 3-4 stages; the impurity removal comprises amine washing, alkali washing, hydrogenation and oxygen removal, acetylene, propyne and propadiene.
As a further scheme of the invention: 15-60 layers of theoretical plates of the high-pressure depropanizing tower and 5-40 layers of feeding positions, wherein the pressure at the top of the tower is 1-2 MPag, the temperature at the top of the tower is 0-40 ℃, and the temperature at the bottom of the tower is 60-95 ℃; the high pressure depropanizer is provided with a plurality of mid-section refluxes.
As a further scheme of the invention: the theoretical plates of the low-pressure depropanizing tower are 15-60 layers, the pressure at the top of the tower is 0.5-0.8 MPag, the temperature at the top of the tower is 10-30 ℃, and the temperature at the bottom of the tower is 70-95 ℃; the low pressure depropanizer is provided with a plurality of mid-section refluxes.
As a further scheme of the invention: 20-80 layers of theoretical plates of the absorption tower, 10-45 layers of unsaturated dry gas feeding positions, 3-4.5 MPag of tower top pressure, -35-10 ℃ of tower top temperature and 45-80 ℃ of tower bottom temperature; the absorber column is provided with a plurality of mid-section refluxes.
As a further scheme of the invention: 15-70 layers of theoretical plates of the reabsorption tower, 5-50 layers of feeding positions, 1.5-2.5 MPag of tower top pressure and 0-15 ℃ of tower top temperature; the reabsorption tower is provided with a plurality of mid-section refluxes.
As a further scheme of the invention: the re-desorption tower comprises 15-60 layers of theoretical plates and 5-45 layers of feeding positions, the pressure at the top of the tower is 0.6-1 MPag, the temperature at the top of the tower is 10-25 ℃, and the temperature at the bottom of the tower is 180-230 ℃.
As a further scheme of the invention: the deethanizer comprises 20-80 layers of theoretical plates and 10-40 layers of feeding positions, wherein the pressure at the top of the deethanizer is 2.0-3.0 MPag, the temperature at the top of the deethanizer is-15-0 ℃, and the temperature at the bottom of the deethanizer is 50-85 ℃; the top of the deethanizer adopts propylene at 0-7 ℃ or-24 ℃ as a refrigerant, and the deethanizer is provided with a plurality of middle-section refluxes.
As a further scheme of the invention: 40-100 layers of theoretical plates of the ethylene rectifying tower, 10-80 layers of feeding positions, 1.9-3 MPag of tower top pressure, -30-0 ℃ of tower top temperature and-5-15 ℃ of tower bottom temperature; the top of the ethylene rectifying tower adopts propylene at the temperature of minus 24 ℃ or minus 40 ℃ as a refrigerant.
As a further scheme of the invention: the propylene rectifying tower comprises 100-280 layers of theoretical plates, 60-180 layers of feeding positions, 0.8-2 MPag of tower top pressure, 35-50 ℃ of tower top temperature and 45-65 ℃ of tower bottom temperature.
Compared with the prior art, the invention has the beneficial effects that:
1. the invention takes propylene or C3 fraction rich in propylene as an absorbent, takes toluene or xylene and industrial hexane as a reabsorber, and carries out absorption and separation on unsaturated dry gas from refinery devices such as catalytic cracking and catalytic cracking under the condition of intercooling (-15 to-35 ℃), the recovery rate of ethylene and propylene can reach more than 99 percent, meanwhile, the circulation amount and the loss amount of the absorbent are relatively small, and the energy consumption of the device is lower. The method improves the recovery rate of ethylene, ethane, propylene and propane, and solves the problems of large absorbent consumption, high energy consumption, large scale of towers and other equipment, large investment, low recovery rate, no dry delivery of dry gas and the like in the separation process of unsaturated dry gas C1, C2 and C3 in the existing refinery.
2. The invention takes propylene or C3 fraction rich in propylene as absorbent, has better effect of absorbing ethylene and small dosage of the absorbent. And the propylene or the C3 fraction rich in propylene is produced by the device, so that the operation cost is reduced. Meanwhile, the absorbent is absorbed under the intercooling condition, the consumption of the absorbent is small, the scale of the tower and other equipment is small, and the investment is low.
3. The invention takes toluene (or dimethylbenzene and industrial hexane) as a reabsorber, is a product produced by a refinery and reduces the operation cost. And the absorption and desorption effects are good, the C3 component carried by the methane fuel gas is less, the recycling amount of the reabsorber is small, and the carrying amount of the heavy component in the dry gas is not more than 0.2 mol%.
4. The invention does not use equipment such as a cold box, an ethylene machine and the like, has low refrigeration load, greatly reduces the use of low-temperature materials and cold insulation materials, and has low device investment.
5. The tower bottoms of the absorption tower, the deethanizer and the depropanizer have lower temperature, and the reboiler heat source at the tower bottom can adopt hot water, low-temperature process material waste heat and the like, so the energy consumption is lower.
6. The tower bottoms of the high-pressure depropanizing tower and the low-pressure depropanizing tower have lower temperature (not more than 85 ℃), and a reboiler heat source at the tower bottom can adopt hot water to reduce the polymerization of unsaturated olefin in a reboiler at the tower bottom.
Drawings
FIG. 1 is a schematic structural diagram of a method for separating low carbon hydrocarbons from unsaturated dry gas by combined absorption and recovery.
In the figure: 1. a compressor; 2. a cooler; 3. a high pressure depropanizer; 4. impurity removal and drying equipment; 5. an absorption tower; 6. a low pressure depropanizer; 7. a deethanizer; 8. a reabsorption tower; 9. then desorbing the tower; 10. an ethylene rectification column; 11. a propylene rectifying tower.
Detailed Description
The technical solution of the present patent will be described in further detail with reference to the following embodiments.
Referring to fig. 1, a method for separating low carbon hydrocarbons from unsaturated dry gas by combined absorption and recovery includes the following steps:
s1, gradually compressing unsaturated dry gas in a refinery to 1-2 MPag by a compressor 1, cooling to 25-35 ℃ by a cooler 2, and then feeding into a high-pressure depropanizing tower 3; the components of the gas phase C3 and C3 at the top of the high-pressure depropanizing tower 3 are treated by an impurity removing and drying device 4, and then are cooled step by step to-15 to-35 ℃ and enter an absorption tower 5; the liquid phase at the bottom of the high-pressure depropanizing tower 3 enters the middle upper part of a low-pressure depropanizing tower 6; the impurity removal comprises amine washing, alkali washing, hydrogenation and removal of oxygen, acetylene, propyne, propadiene and other impurities. The gas phase at the top of the low-pressure depropanizing tower 6 is condensed and then returns to the high-pressure depropanizing tower 3, and the C4+ liquid phase at the bottom of the low-pressure depropanizing tower 6 is sent to a stabilizing tower for continuous separation;
in step S1, the unsaturated dry gas in the refinery is compressed by 3-4 stages.
S2, the absorption tower 5 adopts liquid-phase propylene or C3 fraction rich in propylene as an absorbent, the absorbent enters the absorption tower 5 from the tower top to absorb components of C2 and above C2 in unsaturated dry gas, the tower bottom material of the absorption tower 5 is sent to a deethanizer 7, and the tower top gas phase of the absorption tower 5 enters a reabsorption tower 8;
s3, adopting liquid-phase toluene or xylene and industrial hexane as a reabsorber by the reabsorber 8, and recovering C2 and C3 components carried out from the gas phase at the top of the absorption tower 5; the gas at the top of the reabsorption tower 8 is used as dry gas and is merged into a fuel gas pipe network of a refinery, and the liquid phase at the bottom of the reabsorption tower 8 is sent into a re-desorption tower 9;
s4, condensing the gas phase at the top of the re-desorption tower 9 to obtain a liquid phase, pressurizing and cooling the liquid phase by a pump, returning the liquid phase to the absorption tower 5, and circulating the liquid phase at the bottom of the re-desorption tower 9 serving as a re-absorbent to return to the top of the re-absorption tower 8;
s5, feeding the ethane and ethylene mixed component obtained from the top of the deethanizer 7 into an ethylene rectifying tower 10 for further separation; a part of the C3 component at the bottom of the deethanizer 7 is used as a circulating absorbent to be pressurized and cooled and then returns to the top of the absorption tower 5, and the other part enters the propylene rectifying tower 11 to be continuously separated;
s6, separating the top of the ethylene rectifying tower 10 to obtain liquid-phase polymerization-grade ethylene, and obtaining high-purity ethane at the bottom of the tower;
s7, separating the top or top side line of the propylene rectifying tower 11 to obtain liquid-phase polymerization-grade propylene, and obtaining high-purity propane at the bottom of the tower.
Further, the high-pressure depropanizer 3 has 15-60 layers of theoretical plates and 5-40 layers of feeding positions, the pressure at the top of the tower is 1-2 MPag, the temperature at the top of the tower is 0-40 ℃, and the temperature at the bottom of the tower is 60-95 ℃; the high pressure depropanizer 3 is provided with a plurality of mid-section refluxes.
Further, the low-pressure depropanizing tower 6 has 15-60 layers of theoretical plates, the pressure at the top of the tower is 0.5-0.8 MPag, the temperature at the top of the tower is 10-30 ℃, and the temperature at the bottom of the tower is 70-95 ℃; the low pressure depropanizer 6 is provided with a plurality of mid-section refluxes.
Further, the absorption tower 5 comprises 20-80 layers of theoretical plates and 10-45 layers of unsaturated dry gas feeding positions, the pressure at the top of the tower is 3-4.5 MPag, the temperature at the top of the tower is-35-10 ℃, and the temperature at the bottom of the tower is 45-80 ℃; the absorption tower 5 is provided with a plurality of middle-section reflux streams.
Further, the reabsorption tower 8 comprises 15-70 layers of theoretical plates and 5-50 layers of feeding positions, the tower top pressure is 1.5-2.5 MPag, and the tower top temperature is 0-15 ℃; the reabsorption column 8 is provided with a plurality of mid-section reflux streams.
Further, the re-desorption tower 9 comprises 15-60 layers of theoretical plates and 5-45 layers of feeding positions, the pressure at the top of the tower is 0.6-1 MPag, the temperature at the top of the tower is 10-25 ℃, and the temperature at the bottom of the tower is 180-230 ℃.
Further, the deethanizer 7 has 20-80 layers of theoretical plates and 10-40 layers of feeding positions, the pressure at the top of the tower is 2.0-3.0 MPag, the temperature at the top of the tower is-15-0 ℃, and the temperature at the bottom of the tower is 50-85 ℃; the top of the deethanizer 7 adopts propylene at 0 to minus 7 to minus 24 ℃ as a refrigerant, and the deethanizer 7 is provided with a plurality of middle-section refluxes.
Further, the ethylene rectifying tower 10 comprises 40-100 layers of theoretical plates and 10-80 layers of feeding positions, the pressure at the top of the tower is 1.9-3 MPag, the temperature at the top of the tower is-30-0 ℃, and the temperature at the bottom of the tower is-5-15 ℃; the top of the ethylene rectifying tower 10 adopts propylene with the temperature of minus 24 ℃ or minus 40 ℃ as a refrigerant.
Further, the propylene rectifying tower 11 comprises 100-280 layers of theoretical plates and 60-180 layers of feeding positions, the pressure at the top of the tower is 0.8-2 MPag, the temperature at the top of the tower is 35-50 ℃, and the temperature at the bottom of the tower is 45-65 ℃.
In this example, the composition of the unsaturated dry gas, the absorbent and the reabsorber is shown in the following table:
the unsaturated dry gas from the refinery enters a gas compressor 1 at a pressure of 0.06MPag, and is compressed in three stages to increase the pressure to 1.75 MPag. The pressurized unsaturated dry gas is cooled to about 30 ℃ by a cooler 2 and enters a high-pressure depropanizing tower 3. The number of theoretical plates of the high-pressure depropanizer 3 is preferably 20 layers, the operating pressure is 1.45MPag, the tower top temperature is 7 ℃, the tower bottom temperature is 75 ℃, and a plurality of middle-section refluxes are arranged. The material at the bottom of the high-pressure depropanizing tower 3 is sent into a low-pressure depropanizing tower 6, C3 and lighter components are discharged from the top of the tower, after impurity removal, drying and compression, the pressure is increased to 3.8MPag, and then the material is sent into an absorption tower 5 after being cooled to-20 ℃. The number of theoretical plates of the low-pressure depropanizer 6 is preferably 25, the operating pressure is 0.58MPag, the tower top temperature is 23.7 ℃, the tower bottom temperature is 82 ℃, and a plurality of middle-section refluxes are arranged. The liquid phase at the top of the low-pressure depropanizing tower 6 returns to the high-pressure depropanizing tower 3, and the product at the bottom of the tower is sent out of the device for continuous separation. The absorption tower 5 takes rich propylene as an absorbent, enters from the top of the tower and absorbs components above C2 in the gas. The number of theoretical plates of the absorption tower 5 is preferably 50 layers, the operating pressure is 3.7MPag, the tower top temperature is-24 ℃, the tower bottom temperature is 56.5 ℃, and a plurality of middle-section reflux is arranged.
The gas at the top of the absorption tower 5 enters a reabsorption tower 8, the reabsorption tower 8 takes toluene as a reabsorber, the top of the reabsorption tower 8 is separated to obtain methane fuel gas, and the material at the bottom of the absorption tower 5 is sent to a deethanizer 7 for treatment. The reabsorption tower 8 preferably has 20 theoretical plates, an operating pressure of 2.0MPag, a tower top temperature of about 7 ℃, a plurality of middle reflux stages, and a bottom product of the reabsorption tower 8 fed to the reabsorption tower 9. The number of theoretical plates of the re-desorption tower 9 is preferably 20, the operating pressure is 0.8MPag, the temperature at the top of the tower is about 13.1 ℃, the temperature at the bottom of the tower is 213.6 ℃, the condensate rich in propylene and propane obtained by separation at the top of the re-desorption tower 9 is pressurized by a pump and then returns to the absorption tower 5, and the toluene at the bottom of the re-desorption tower 9 is used as a reabsorber to be subjected to heat exchange and then circularly returns to the top of the re-absorption tower 8. The number of theoretical plates in the deethanizer 7 is preferably 50, the operating pressure is 2.5MPag, the overhead temperature is about-11.6 ℃, the bottom temperature is 63.9 ℃, and multiple mid-section reflux streams are provided. Sending the C2 component at the tower top of the deethanizer 7 to an ethylene rectifying tower 10, and separating to obtain liquid-phase polymerization-grade ethylene and high-purity ethane; part of the C3 component at the bottom of the deethanizer 7 is returned to the absorption tower 5 as a circulating absorbent, the rest is sent to the propylene rectifying tower 11, and liquid-phase polymerization-grade propylene and high-purity propane are extracted from the side line at the top of the tower.
The isolated products are shown in the following table:
in this example, the ethylene recovery rate was 99.96%, the ethane recovery rate was 99.86%, the propylene recovery rate was 99.90%, the propane recovery rate was 98.75%, and the entrainment of heavies in the methane fuel gas was less than 0.2 mol%.
The working principle of the invention is as follows: in the invention, the content of C2 and C3 in the unsaturated dry gas of the refinery is about 30-50 mol%, and the rest is hydrogen, methane, water, impurities, components with more than C4 and the like. The invention has special limitation to the absorbent and the reabsorber, the absorbent is propylene or C3 fraction rich in propylene, and the reabsorber is toluene or xylene, industrial hexane and other components.
The specific process flow is as follows: unsaturated dry gas from a refinery enters a gas compressor 1, is compressed to about 1.7MPag through three-stage or four-stage compression, enters a high-pressure depropanizer 3 at the temperature of about 30 ℃, and enters 5-40 layers at the feeding position. The pressure at the top of the high-pressure depropanizing tower 3 is 1-2 MPag, the temperature is 0-40 ℃, the temperature at the bottom of the tower is 60-95 ℃, and the theoretical plates are 15-60 layers; after gas-phase products at the top of the high-pressure depropanizing tower 3 are subjected to desulfurization, decarburization and drying and other impurities (such as hydrodeoxygenation, acetylene, propyne, propadiene and the like) are removed, the gas-phase products enter a compressor to be further compressed to 3.5-4.5 MPag, and the gas-phase products are gradually cooled to-15 to-35 ℃ and enter an absorption tower 5; the liquid phase at the bottom of the high-pressure depropanizing tower 3 enters a low-pressure depropanizing tower 6, the theoretical plates of the low-pressure depropanizing tower 6 are 15-60 layers, the tower top pressure is 0.5-0.8 MPag, the tower top temperature is 10-30 ℃, the tower bottom temperature is 70-95 ℃, the gas phase at the top of the low-pressure depropanizing tower 6 is condensed into a liquid phase and returns to the high-pressure depropanizing tower 3, and the C4+ liquid phase at the bottom of the tower enters a subsequent device for treatment.
The absorption tower 5 takes propylene or C3 fraction rich in propylene as an absorbent, the top pressure of the absorption tower 5 is 3-4.5 MPag, the tower top temperature is-35-10 ℃, the tower bottom temperature is 45-80 ℃, the theoretical plates are 20-80 layers, and the feeding positions are 10-45 layers; the gas phase at the top of the absorption tower 5 enters a reabsorption tower 8, the theoretical plates of the reabsorption tower 8 are 15-70 layers, the feeding position is 5-50 layers, the pressure at the top of the tower is 1.5-2.5 MPag, and the temperature at the top of the tower is 0-15 ℃; the rich absorption liquid at the bottom of the absorption tower 5 enters a deethanizer 7 for separation. The reabsorption tower 8 adopts toluene as a reabsorption agent to absorb part of components C2 and C3 which are carried by the non-condensed steam at the top of the absorption tower 5, methane-hydrogen fuel gas (dry gas) obtained at the top of the reabsorption tower 8 enters a fuel gas pipe network of a refinery, and liquid phase at the bottom of the reabsorption tower 8 is sent to a reabsorption tower 9. And 15-60 layers of theoretical plates of the re-desorption tower 9 and 5-45 layers of feeding positions, wherein the pressure at the top of the tower is 0.6-1 MPag, the temperature at the top of the tower is 10-25 ℃, the temperature at the bottom of the tower is 180-230 ℃, a condensate rich in propylene and propane and obtained by separation at the top of the re-desorption tower 9 is pressurized by a pump and then returns to the absorption tower 5, and a liquid phase at the bottom of the re-desorption tower 9 is used as a reabsorber to circulate and return to the top of the reabsorber 8. The pressure at the top of the deethanizer 7 is 2.0-3.0 MPag, the temperature at the top of the deethanizer 7 is-15-10 ℃, propylene at 0-7 ℃ or-24 ℃ is used as a refrigerant at the top of the deethanizer 7, the temperature at the bottom of the deethanizer 7 is 50-85 ℃, the theoretical plates are 20-80 layers, and the feeding positions are 10-40 layers; the gas phase at the top of the deethanizer 7 enters an ethylene rectifying tower 10 after being cooled, one part of the C3 component at the bottom of the tower is cooled to-20 ℃ by steps through circulating water and refrigerant water and propylene at-24 ℃ and is used as a circulating absorbent to return to an absorption tower 5, and the other part enters a propylene rectifying tower 11 for further separation. The pressure at the top of the ethylene rectifying tower 10 is 1.9-3 MPag, the temperature at the top of the tower is-30-0 ℃, the temperature at the bottom of the tower is-5-15 ℃, the number of theoretical plates is 40-100, and the number of feeding positions is 10-80; the top of the ethylene rectifying tower 10 obtains a polymer grade ethylene product with the purity of more than 99.95mol percent, and the bottom of the ethylene rectifying tower obtains a high-purity ethane product with the purity of more than 99.5mol percent. Rectifying the propylene, wherein the pressure at the top of the tower is 0.8-2 MPag, the temperature at the top of the tower is 35-50 ℃, the temperature at the bottom of the tower is 45-65 ℃, the theoretical plates are 100-280 layers, and the feeding position is 60-180 layers; obtaining a polymer-grade propylene product with the purity of more than 99.6 mol% at the top of the propylene rectifying tower 11, wherein most of the product is taken as a product and sent out of a device, and a small amount of the product is taken as a supplementary absorbent; high-purity propane is obtained at the bottom of the tower, the purity is more than 99 mol%, and the propane is taken as a product and sent out of the device.
Although the preferred embodiments of the present patent have been described in detail, the present patent is not limited to the above embodiments, and various changes and modifications can be made without departing from the spirit and scope of the present patent within the knowledge of those skilled in the art.
Claims (10)
1. A method for separating low-carbon hydrocarbon in unsaturated dry gas by combined absorption and recovery is characterized by comprising the following steps:
s1, gradually compressing unsaturated dry gas in a refinery to 1-2 MPag by a compressor, cooling to 25-35 ℃ by a cooler, and then feeding into a high-pressure depropanizing tower;
after the gas phase at the top of the high-pressure depropanizing tower is treated by impurity removal and drying equipment, the gas phase is gradually cooled to-15 to-35 ℃ and then enters an absorption tower; the liquid phase at the bottom of the high-pressure depropanizing tower enters the middle upper part of the low-pressure depropanizing tower;
the gas phase at the top of the low-pressure depropanizing tower returns to the high-pressure depropanizing tower after being condensed, and the C4+ liquid phase at the bottom of the low-pressure depropanizing tower is sent to a stabilizing tower for continuous separation;
s2, the absorption tower adopts liquid phase propylene or C3 fraction rich in propylene as an absorbent, the absorbent enters the absorption tower from the tower top to absorb components of C2 and above C2 in unsaturated dry gas, the tower bottom material of the absorption tower is sent to a deethanizer, and the tower top gas phase of the absorption tower enters a reabsorption tower;
s3, adopting liquid-phase methylbenzene or dimethylbenzene and industrial hexane as a reabsorber by the reabsorber to recover C2 and C3 components carried out by a gas phase at the top of the absorption tower; the gas at the top of the reabsorption tower is used as dry gas and is merged into a fuel gas pipe network of a refinery, and the liquid phase at the bottom of the reabsorption tower is sent into a re-desorption tower;
s4, condensing the gas phase at the top of the re-desorption tower to obtain a liquid phase, pressurizing and cooling the liquid phase by a pump, returning the liquid phase to the absorption tower, and taking the liquid phase at the bottom of the re-desorption tower as a re-absorbent to circulate and return to the top of the re-absorption tower;
s5, feeding the mixed components of ethane and ethylene obtained from the top of the deethanizer into an ethylene rectifying tower for further separation; one part of the C3 component at the bottom of the deethanizer is used as a circulating absorbent to be pressurized and cooled and then returns to the top of the absorption tower, and the other part enters a propylene rectifying tower to be continuously separated;
s6, separating the top of the ethylene rectifying tower to obtain liquid-phase polymerization-grade ethylene, and obtaining high-purity ethane at the bottom of the tower;
s7, separating the top or top lateral line of the propylene rectifying tower to obtain liquid-phase polymerization-grade propylene, and obtaining high-purity propane at the bottom of the tower.
2. The method for combined absorption, recovery and separation of low carbon hydrocarbons in unsaturated dry gas according to claim 1, wherein in step S1, the unsaturated dry gas of refinery is compressed by 3-4 stages; the impurity removal comprises amine washing, alkali washing, hydrogenation and oxygen removal, acetylene, propyne and propadiene.
3. The method for combined absorption, recovery and separation of low carbon hydrocarbons in unsaturated dry gas according to claim 1, wherein the high pressure depropanizer has 15-60 theoretical plates, 5-40 feeding positions, 1-2 MPag overhead pressure, 0-40 ℃ overhead temperature and 60-95 ℃ bottom temperature; the high pressure depropanizer is provided with a plurality of mid-section refluxes.
4. The method for combined absorption, recovery and separation of low-carbon hydrocarbons in unsaturated dry gas according to claim 1, wherein the theoretical plates of the low-pressure depropanizer are 15-60 layers, the pressure at the top of the tower is 0.5-0.8 MPag, the temperature at the top of the tower is 10-30 ℃, and the temperature at the bottom of the tower is 70-95 ℃; the low pressure depropanizer is provided with a plurality of mid-section refluxes.
5. The method for combined absorption, recovery and separation of low carbon hydrocarbons in unsaturated dry gas according to claim 1, wherein the absorption tower has 20-80 layers of theoretical plates, 10-45 layers of unsaturated dry gas feeding positions, 3-4.5 MPag overhead pressure, -35-10 ℃ overhead temperature and 45-80 ℃ bottom temperature; the absorption tower is provided with a plurality of middle reflux sections.
6. The method for combined absorption, recovery and separation of low carbon hydrocarbons in unsaturated dry gas according to claim 1, wherein the reabsorption tower has 15-70 layers of theoretical plates, 5-50 layers of feeding positions, 1.5-2.5 MPag of overhead pressure and 0-15 ℃ of overhead temperature; the reabsorption tower is provided with a plurality of mid-section refluxes.
7. The method for combined absorption, recovery and separation of low carbon hydrocarbons in unsaturated dry gas according to claim 1, wherein the re-desorption tower has 15-60 layers of theoretical plates, 5-45 layers of feeding positions, 0.6-1 MPag of overhead pressure, 10-25 ℃ of overhead temperature and 180-230 ℃ of bottom temperature.
8. The method for combined absorption, recovery and separation of low carbon hydrocarbons in unsaturated dry gas according to claim 1, wherein the deethanizer has 20-80 layers of theoretical plates and 10-40 layers of feeding positions, the pressure at the top of the deethanizer is 2.0-3.0 MPag, the temperature at the top of the deethanizer is-15-0 ℃, and the temperature at the bottom of the deethanizer is 50-85 ℃; the top of the deethanizer adopts propylene at 0-7 ℃ or-24 ℃ as a refrigerant, and the deethanizer is provided with a plurality of middle-section refluxes.
9. The method for combined absorption, recovery and separation of low carbon hydrocarbons in unsaturated dry gas according to claim 1, wherein the ethylene rectification tower has 40-100 layers of theoretical plates, 10-80 layers of feeding positions, 1.9-3 MPag of tower top pressure, 30-0 ℃ of tower top temperature and 5-15 ℃ of tower bottom temperature; the top of the ethylene rectifying tower adopts propylene at the temperature of minus 24 ℃ or minus 40 ℃ as a refrigerant.
10. The method for combined absorption, recovery and separation of low carbon hydrocarbons in unsaturated dry gas as claimed in claim 1, wherein the propylene rectification tower has 100-280 theoretical plates, 60-180 feeding positions, 0.8-2 MPag overhead pressure, 35-50 ℃ overhead temperature and 45-65 ℃ bottom temperature.
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