WO2019071869A1 - 一种采用阶式制冷的天然气乙烷回收装置及方法 - Google Patents

一种采用阶式制冷的天然气乙烷回收装置及方法 Download PDF

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Publication number
WO2019071869A1
WO2019071869A1 PCT/CN2018/071695 CN2018071695W WO2019071869A1 WO 2019071869 A1 WO2019071869 A1 WO 2019071869A1 CN 2018071695 W CN2018071695 W CN 2018071695W WO 2019071869 A1 WO2019071869 A1 WO 2019071869A1
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Prior art keywords
propane
ethylene
propylene
heat exchange
exchange system
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PCT/CN2018/071695
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English (en)
French (fr)
Inventor
蒲黎明
周璇
王长锋
王科
陈运强
李莹珂
田静
汤晓勇
郑春来
龙海洋
缪晖
陆永康
李娜
郭成华
张庆林
胡益武
秦兴述
王刚
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中国石油工程建设有限公司
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Priority to RU2018143421A priority Critical patent/RU2708667C1/ru
Publication of WO2019071869A1 publication Critical patent/WO2019071869A1/zh

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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0204—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the feed stream
    • F25J3/0209—Natural gas or substitute natural gas
    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C7/00—Purification; Separation; Use of additives
    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C7/00—Purification; Separation; Use of additives
    • C07C7/04—Purification; Separation; Use of additives by distillation
    • C—CHEMISTRY; METALLURGY
    • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
    • C10G—CRACKING HYDROCARBON OILS; PRODUCTION OF LIQUID HYDROCARBON MIXTURES, e.g. BY DESTRUCTIVE HYDROGENATION, OLIGOMERISATION, POLYMERISATION; RECOVERY OF HYDROCARBON OILS FROM OIL-SHALE, OIL-SAND, OR GASES; REFINING MIXTURES MAINLY CONSISTING OF HYDROCARBONS; REFORMING OF NAPHTHA; MINERAL WAXES
    • C10G5/00—Recovery of liquid hydrocarbon mixtures from gases, e.g. natural gas
    • C10G5/06—Recovery of liquid hydrocarbon mixtures from gases, e.g. natural gas by cooling or compressing
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0228—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream
    • F25J3/0233—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream separation of CnHm with 1 carbon atom or more
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0228—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream
    • F25J3/0238—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream separation of CnHm with 2 carbon atoms or more
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0228—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream
    • F25J3/0242—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream separation of CnHm with 3 carbon atoms or more
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2200/00—Processes or apparatus using separation by rectification
    • F25J2200/04—Processes or apparatus using separation by rectification in a dual pressure main column system
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2200/00—Processes or apparatus using separation by rectification
    • F25J2200/74—Refluxing the column with at least a part of the partially condensed overhead gas
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2200/00—Processes or apparatus using separation by rectification
    • F25J2200/76—Refluxing the column with condensed overhead gas being cycled in a quasi-closed loop refrigeration cycle
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2205/00—Processes or apparatus using other separation and/or other processing means
    • F25J2205/02—Processes or apparatus using other separation and/or other processing means using simple phase separation in a vessel or drum
    • F25J2205/04—Processes or apparatus using other separation and/or other processing means using simple phase separation in a vessel or drum in the feed line, i.e. upstream of the fractionation step
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2215/00—Processes characterised by the type or other details of the product stream
    • F25J2215/04—Recovery of liquid products
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2215/00—Processes characterised by the type or other details of the product stream
    • F25J2215/62—Ethane or ethylene
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2240/00—Processes or apparatus involving steps for expanding of process streams
    • F25J2240/02—Expansion of a process fluid in a work-extracting turbine (i.e. isentropic expansion), e.g. of the feed stream
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2270/00—Refrigeration techniques used
    • F25J2270/12—External refrigeration with liquid vaporising loop
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2270/00—Refrigeration techniques used
    • F25J2270/60—Closed external refrigeration cycle with single component refrigerant [SCR], e.g. C1-, C2- or C3-hydrocarbons

Definitions

  • the invention relates to the field of light hydrocarbon recovery, in particular to the field of ethane recovery, in particular to a process device and a method for recovering ethane in natural gas by using cascade refrigeration, which is suitable for various raw material gas pressures, and for the raw material gas of the ethane recovery plant.
  • the temperament condition with a pressure less than 5 MPa has the advantages of high ethane recovery rate, investment saving and energy consumption reduction.
  • Ethane is a high-quality raw material for quenching and thermal cracking of ethylene. It has high olefin yield, few by-products, simple process, corresponding investment reduction, short construction period and fast return on investment. It has been obtained in the Middle East and the United States. Utilizing a large number of countries and regions where their oil reserves are relatively poor, ethylene imports are rising year by year. Under the stimulation of low cost and high profits, the demand for lighter olefin raw materials is becoming stronger, and the natural gas exported from the products contains a large amount of B. Alkanes, the current utilization rate is low, according to the latest demand, in order to more efficiently use natural gas, diversify natural gas products, and vigorously develop ethane recovery technology is of great significance.
  • the pre-cooling temperature of the feed gas is relatively low, mainly relying on expansion refrigeration to provide cooling capacity.
  • the feed gas pressure is not too high and cannot provide sufficient cooling capacity, the ethane yield is relatively low.
  • the "stage cooling + expander refrigeration process” can provide low pre-cooling temperature and increase ethane yield. It is suitable for various raw material gas pressures. It can solve high investment when the raw material gas pressure is not high. A problem that is expensive and complicated to operate.
  • the present invention provides a natural gas ethane recovery device and method using a cascade refrigeration, which has low energy consumption, easy driving, low power of a single compressor, and low investment and operation. advantage.
  • the technical scheme adopted by the invention is: a natural gas ethane recovery device adopting a cascade refrigeration, comprising a cold box, a cryogenic separator, a demethanizer, a deethanizer, a propane heat exchange system and an ethylene heat exchange system, wherein The natural gas outlet of the cold box is connected with the cryogenic separator, and the liquid phase outlet of the cryogenic separator is sequentially exchanged with the demethanizer, the deethanizer, the deethanizer overhead condenser, the deethanizer overhead reflux tank, and the propane heat exchange.
  • the system and the ethylene heat exchange system are connected; the gas phase of the top of the demethanizer is connected to the cold box, the expander and the product gas compressor; the natural gas outlet of the ethylene heat exchange system is connected to the top inlet of the demethanizer; the propane compressor and the propane are condensed And propane heat exchange system cycle connection; ethylene compressor, propane heat exchange system, ethylene heat exchange system cycle connection.
  • the invention also provides a method for recovering natural gas ethane using cascade refrigeration, comprising the following contents:
  • the dry ethane-containing natural gas of 3.0 ⁇ 7.0MPa.g is pre-cooled to -35 ⁇ -75°C in a cold box and then enters the low-temperature separator for gas-liquid separation. Most of the separated gas phase is expanded by the expander to a pressure of 2.0-4.0. After MPa.g, it enters the demethanizer tower, and a small part of the separated gas phase enters the cold box and condenses to -80 ⁇ -95°C to form a liquid and then enters the upper part of the demethanizer; the liquid phase separated by the low temperature separator is throttled to 2.2 ⁇ 4.0 After MPa.g, enter the demethanizer; the pressure of the demethanizer is controlled at 1.8-3.8MPa.g;
  • the pressure of the deethanizer is controlled at 1.6-3.5 MPa.g.
  • the gas phase at the top of the tower is passed through the top of the deethanizer.
  • the condenser is condensed to -30 to 5 ° C, and then separated by a de-ethanizer overhead reflux tank.
  • the obtained liquid is sent to the top of the deethanizer column as a reflux liquid through a deethanizer reflux pump, and the deethanizer column is topped.
  • the gas phase separated by the reflux tank is an ethane product and enters the propane heat exchange system;
  • the gas phase from the top of the demethanizer tower is exchanged to a temperature of 10 to 35 ° C through a cold box, and then passed through the coaxial pressurizing end of the expander and the product gas compressor to pressurize most of the natural gas as a product, and the remaining part enters.
  • Propane heat exchange system
  • the propane heat exchange system provides cooling for the liquefaction and supercooling of the product:
  • the propane propeller pressurized to 1300 ⁇ 1700 kPa.g by a propane compressor is condensed by a propane condenser and cooled to 30 ⁇ 50 ° C before entering the propane heat exchange system.
  • the propane heat exchange system condenses and cools the gas phase ethane product to -34 ⁇ 37 ° C, the refluxed product natural gas is pre-cooled to -34 ⁇ -37 ° C, and the refrigerant ethylene is condensed and cooled to -34 ⁇ -37 ° C;
  • the ethylene heat exchange system provides cooling for the gas liquefaction and supercooling of the reflux product:
  • the refrigerant ethylene pressurized by the ethylene compressor to 1600-2400 kPa.g is condensed by the ethylene heat exchange system and cooled to -36 to -34 ° C, then enters the ethylene heat exchange system, and the ethylene heat exchange system condenses the ethane product. Cool to -90 to -85 ° C, liquefy the refluxed product natural gas and subcool to -95 to -90 ° C.
  • the positive effect of the invention is: suitable for various raw material gas pressure conditions in the ethane recovery plant, and the multi-stage throttling of the single component refrigerant for the ethane recovery plant with a pressure less than 5 MPa
  • the refrigeration efficiency is improved, and the refrigerant compressor load of the refrigeration part of the ethane recovery unit is reduced to some extent.
  • the utility model has the advantages of simple design, strong adaptability to change of the composition of the raw material gas, low power of the single compressor, low investment, low energy consumption and simple and flexible operation.
  • Figure 1 is a flow chart of the process principle of the device.
  • a natural gas ethane recovery device adopting the cascade refrigeration comprises: a cold box 1, a low temperature separator 2, an expander 3, a demethanizer 4, a demethanizer bottom reboiler 5, Ethane column 6, deethanizer overhead condenser 7, deethanizer overhead reflux tank 8, deethanizer overhead reflux pump 9, deethanizer bottom reboiler 10, product gas compressor 11
  • a natural gas ethane recovery device using cascade refrigeration is mainly divided into three parts: a rectification section, a propane heat exchange system and an ethylene heat exchange system.
  • Rectification section This section separates the raw gas natural gas into ethane products, C 3 + and natural gas.
  • the cold box 1 is a place where the raw material gas is precooled and the low temperature gas is subcooled in the ethane recovery unit, and the cold box 1 is sequentially connected to the low temperature separator 2, the expander 3, the demethanizer 4, and the product gas compressor 11.
  • the liquid phase at the outlet of the cryogenic separator 2 is directly connected to the demethanizer 4; the liquid phase at the outlet of the demethanizer 4 is directly connected to the deethanizer 6; the deethanizer 6 and the deethanizer overhead condenser 7
  • An alkane top reflux tank 8 and a deethanizer overhead reflux pump 9 are sequentially connected; an outlet of the deethanization overhead reflux pump 9 is connected to the top outlet of the deethanizer 6; a demethanizer bottom is provided at the bottom of the demethanizer 4
  • the reboiler 5 is for supplying heat to the bottom of the demethanizer 4; the bottom of the deethanizer 6 is provided with a deethanizer bottom reboiler 10 for supplying heat to the bottom of the deethanizer 6.
  • propane heat exchange system This part mainly provides cooling capacity for product liquefaction and supercooling.
  • the gas phase at the outlet of the de-ethanizer overhead reflux tank 8 is an ethane product which is connected to the propane heat exchange system 14 and the ethylene heat exchange system 16 to be condensed into a liquid and then sent to an ethane storage tank.
  • the outlet gas of the propane compressor 12 is sequentially connected to the propane condenser 13 and the propane heat exchange system 14 to form a propane refrigeration system cycle.
  • ethylene heat exchange system This part mainly provides cooling capacity for gas liquefaction and supercooling of reflux products.
  • the gas phase at the outlet of the ethylene compressor 15 is in turn connected to the propane heat exchange system 14 and the ethylene heat exchange system 16 to form an ethylene refrigeration system cycle.
  • the present invention also provides a method for recovering ethane from natural gas using cascade refrigeration, comprising the steps of:
  • the dry ethane-containing natural gas of 3.0 to 7.0 MPa.g is connected to the cold box 1 and is pre-cooled by the cold box 1 to about -35 to -75 ° C, and most of the gas phase separated by the low temperature separator 2 (about 60 to 80) Entering the expander 3, the gas pressure after expansion reaches 2.0 to 4.0 MPa.g, and then directly enters the demethanizer 4, and the remaining part of the gas phase continues to enter the cold box 1 and further condenses to -80 to -95 ° C to form a liquid. Enter the upper part of the demethanizer 4.
  • the liquid phase separated by the cryogenic separator 2 is throttled to 2.2 to 4.0 MPa.g and directly enters the demethanizer 4.
  • the pressure of the demethanizer is controlled at 1.8 to 3.8 MPa.g.
  • the gas phase at the top of the tower is The deethanization overhead condenser 7 is connected, and the liquid which is partially condensed to about -30 to 5 ° C and then separated by the deethanizer overhead reflux tank 8 is sent to the deethanizer through the deethanizer overhead reflux pump 9 .
  • the top of the column 6 is used as its reflux liquid, and the gas phase separated by the deethanizer overhead reflux tank 8 is an ethane product.
  • the gas phase from the top of the demethanizer 4 is exchanged to the temperature of 10 to 35 ° C through the cold box 1, and then passed through the coaxial pressurizing end of the expander 3 and the product gas compressor 11 after pressurization 80% to 90% as the product natural gas.
  • Lose. 2 propane refrigeration part
  • the propane heat exchange system 14 includes a high pressure propane evaporator, a medium pressure propane evaporator, and a low pressure propane evaporator, in which the gas phase ethane product is condensed and cooled to -34 to -37 ° C, and the refluxed product natural gas is precooled to -34 to -37 ° C, and the refrigerant ethylene was condensed and cooled to -34 to -37 °C.
  • the refrigerant propane pressurized by the propane compressor 12 to 1300 to 1700 kPa.g is condensed by the propane condenser 13 and cooled to 30 to 50 ° C, and then introduced into the propane heat exchange system 14 to be discharged by high pressure propane by throttling to 15 to 23 ° C.
  • the unit provides cooling capacity, pre-cools the refluxed product natural gas and refrigerant ethylene to 18 to 26 ° C and enters the medium pressure propane evaporator.
  • the vapor phase propane in the high pressure propane evaporator returns to the tertiary boost inlet of the propane compressor 12, and the separated liquid propane is continuously throttled to -15 to -5 ° C to enter the medium pressure propane evaporator to provide cooling capacity.
  • the ethane product, the refluxed product natural gas, and the refrigerant ethylene are cooled to -12 to -2 ° C and then passed to a low pressure propane evaporator.
  • the vapor phase propane in the medium pressure propane evaporator returns to the secondary pressurization inlet of the propane compressor, and the separated liquid propane is further throttled to -39 to -37 ° C into the low pressure propane evaporator to provide cooling capacity.
  • the ethane product, the refluxed product natural gas and the refrigerant ethylene are cooled to -36 to -34 ° C into the ethylene heat exchange system.
  • the gas phase separated by the low pressure propane evaporator enters the primary pressurized inlet of the propane compressor 12.
  • a propane refrigeration cycle is formed.
  • the ethylene heat exchange system 16 comprises a high pressure ethylene evaporator, a medium pressure ethylene evaporator and a low pressure ethylene evaporator, in which the condensed ethane product is subcooled to -90 to -85 ° C, and the refluxed product natural gas is liquefied. Cold to -95 ⁇ -90 °C.
  • the refrigerant ethylene pressurized by the ethylene compressor 15 to 1600-2400 kPa.g is sequentially condensed by the ethylene heat exchange system and cooled to -36 to -34 ° C, and then supplied to the high pressure ethylene evaporator by throttling to -60 to -50 ° C.
  • the gas phase ethylene in the high-pressure ethylene evaporator is returned to the three-stage pressurized inlet of the ethylene compressor 15, and the separated liquid ethylene is further throttled to -81 to -73 ° C to enter the medium-pressure ethylene evaporator to provide cooling capacity.
  • the refluxed product natural gas and ethane were cooled to -78 to -70 ° C and then passed to a low pressure ethylene evaporator.
  • the vapor phase ethylene in the medium pressure ethylene evaporator is returned to the secondary pressurized inlet of the ethylene compressor 15, and the separated liquid ethylene is further throttled to -98 to -93 ° C to enter the low pressure ethylene evaporator to provide cooling capacity.
  • the refluxed product natural gas is cooled to -95 to -90 ° C and then to the top of the demethanizer 4 as its reflux liquid, and the ethane is cooled to -90 to -85 ° C and then sent to an ethane storage tank for storage.
  • the gas phase separated by the low pressure ethylene evaporator enters the primary pressurized inlet of the ethylene compressor 15. An ethylene refrigeration cycle is formed.
  • the propane refrigeration portion can be cooled by propylene, and the ethylene refrigeration portion can be cooled by ethane.
  • the demethanizer and the deethanizer are respectively provided with reboilers.
  • the demethanizer is a plate column or a combination of a filler and a plate
  • the deethanizer may be a plate column or a packed column.
  • the propane heat exchange system may also be a single-stage heat exchange according to the pressure of the ethane product, or may be a two-stage heat exchange or a four-stage heat exchange, and the corresponding propane compressor inlet may be a first-stage pressurization, It can be two-stage boost or four-stage boost.
  • the propane refrigerant may also be propylene.
  • the propane condenser 13 is condensed by circulating water depending on the outlet pressure of the propane compressor, and may be condensed by an air cooler.
  • the ethylene heat exchange system may be a two-stage heat exchange or a four-stage heat exchange according to the pressure of the refluxed product natural gas, and the corresponding ethylene compressor inlet may be a two-stage pressurization or a four-stage pressurization.
  • the ethylene refrigerant can be exchanged for ethane depending on the requirements of the ethane storage conditions and the natural gas refrigeration temperature of the refluxed product.
  • the liquid phase derived from the bottom of the deethanizer is C 3 +, which can be directly sold, and can be determined according to the needs of the user whether or not the propane, butane (LPG) product is obtained by fractional distillation of the depropanizer or debutanizer, and is stable and light. Hydrocarbon products are sold outside to achieve product diversification.
  • LPG propane, butane

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Abstract

提供了一种采用阶式制冷的天然气乙烷回收装置及方法。回收装置包括：冷箱（1）、低温分离器（2）、脱甲烷塔（4）、脱乙烷塔（6）、丙烷换热系统（14）和乙烯换热系统（16）。冷箱（1）的天然气出口与低温分离器（2）连接，低温分离器（2）的液相出口依次与脱甲烷塔（4）、脱乙烷塔（6）、脱乙烷塔顶冷凝器（7）、脱乙烷塔塔顶回流罐（8）、丙烷换热系统（14）、乙烯换热系统（16）相连；脱甲烷塔（4）的塔顶气相依次与冷箱（1）、膨胀机（3）和产品气压缩机（11）相连。该回收装置在一定程度上减小了乙烷回收装置制冷部分的制冷剂压缩机负荷，对原料气组成变化适应性强，具有能耗低、单台压缩机功率较低、成本低和操作简单的优点。

Description

一种采用阶式制冷的天然气乙烷回收装置及方法 技术领域
本发明涉及轻烃回收领域，尤其是乙烷回收领域，具体为一种采用阶式制冷在天然气回收乙烷的工艺装置及方法，适用于各种原料气压力，针对乙烷回收工厂的原料气压力小于5MPa的气质条件，既具有较高的乙烷回收率，又有节省投资和降低能耗的优点。
背景技术
乙烷是优质的急冷热裂解制乙烯的原料，具有烯烃收率高，副产品很少，工艺较简单，相应的投资减少，建设周期短，投资回报快等诸多优势，在中东和美国得到了大量利用，对于自身石油储量较为贫瘠的国家和地区，乙烯进口量逐年攀升，在低成本高利润的刺激下，烯烃原料轻质化的需求越发强烈，而外输的产品天然气中含有大量的乙烷，目前利用率较低，根据最新的需求，为更高效地利用天然气，实现天然气产品多元化，大力发展乙烷回收技术具有重要意义。
目前天然气乙烷回收工厂主要多数采用“丙烷/丙烯预冷+膨胀机制冷”工艺。因为采用单组分制冷剂预冷，原料气预冷温度较低，主要依靠膨胀制冷提供冷量，当原料气压力不太高，不能提供足够的冷量时，乙烷收率则相对较低，不够经济。而“阶式制冷+膨胀机制冷工艺”既可以提供较低的预冷温度又可提高乙烷收率，适用于各种原料气压力，针对原料气压力不高时，可以解决投资高，能耗大，操作复杂的问题。
发明内容
为了克服现有技术的缺点，本发明提供了一种采用阶式制冷的天然气乙烷回收装置及方法，具有能耗低、开车容易，单台压缩机功率较低，降低投资和 操作简单灵活的优点。
本发明所采用的技术方案是：一种采用阶式制冷的天然气乙烷回收装置，包括冷箱、低温分离器、脱甲烷塔、脱乙烷塔、丙烷换热系统和乙烯换热系统，其中：冷箱的天然气出口与低温分离器连接，低温分离器的液相出口依次与脱甲烷塔、脱乙烷塔、脱乙烷塔顶冷凝器、脱乙烷塔塔顶回流罐、丙烷换热系统、乙烯换热系统相连；脱甲烷塔的塔顶气相依次与冷箱、膨胀机和产品气压缩机相连；乙烯换热系统的天然气出口与脱甲烷塔顶部入口相连；丙烷压缩机、丙烷冷凝器、丙烷换热系统循环连接；乙烯压缩机、丙烷换热系统、乙烯换热系统循环连接。
本发明还提供了一种采用阶式制冷的天然气乙烷回收方法，包括如下内容：
一、将原料气天然气进行精馏分离为乙烷产品、C 3+和产品天然气：
3.0～7.0MPa.g的干燥含乙烷天然气经过冷箱预冷至-35～-75℃后进入低温分离器进行气液分离，分离出的气相大部分经过膨胀机膨胀至压力为2.0～4.0MPa.g后进入脱甲烷塔，分离出的少部分气相进入冷箱冷凝至-80～-95℃成液体后再进入脱甲烷塔上部；低温分离器分离出的液相节流至2.2～4.0MPa.g后进入脱甲烷塔；脱甲烷塔的压力控制在1.8～3.8MPa.g；
脱甲烷塔塔底出来的C 2+进入脱乙烷塔，脱乙烷塔的压力控制在1.6～3.5MPa.g，经过脱乙烷塔分馏后，塔顶出来的气相经脱乙烷塔顶冷凝器冷凝至-30～5℃后经脱乙烷塔塔顶回流罐分离，得到的液体经脱乙烷塔回流泵输送至脱乙烷塔塔顶作为其回流液，脱乙烷塔塔顶回流罐分离得到的气相为乙烷产品，进入丙烷换热系统；
脱甲烷塔塔顶出来的气相通过冷箱换热至10～35℃后，再依次通过膨胀机 同轴增压端和产品气压缩机增压后大部分作为产品天然气外输，其余少部分进入丙烷换热系统；
二、丙烷换热系统为产品乙烷液化及过冷提供冷量：
经丙烷压缩机增压至1300～1700kPa.g的制冷剂丙烷经过丙烷冷凝器冷凝并冷却至30～50℃后进入丙烷换热系统，丙烷换热系统将气相乙烷产品冷凝并冷却至-34～-37℃，将回流的产品天然气预冷至-34～-37℃，并且将制冷剂乙烯冷凝并冷却至-34～-37℃；
三、乙烯换热系统为回流的产品气液化及过冷提供冷量：
经乙烯压缩机增压至1600～2400kPa.g的制冷剂乙烯依次通过乙烯换热系统冷凝并冷却至-36～-34℃后进入乙烯换热系统，乙烯换热系统将冷凝的乙烷产品过冷至-90～-85℃，将回流的产品天然气液化并过冷至-95～-90℃。
与现有技术相比，本发明的积极效果是：适用于乙烷回收工厂各种原料气压力条件，针对压力小于5MPa的乙烷回收工厂，本发明可将单组分制冷剂多级节流，提高制冷效率，在一定程度上减小了乙烷回收装置制冷部分的制冷剂压缩机负荷。具有设计简单，对原料气组成变化适应性强，单台压缩机功率较低，降低投资、能耗低和操作简单灵活的优点。
附图说明
本发明将通过例子并参照附图的方式说明，其中：
图1是本装置的工艺原理流程图。
具体实施方式
一种采用阶式制冷的天然气乙烷回收装置，如图1所示，包括：冷箱1、低温分离器2、膨胀机3、脱甲烷塔4、脱甲烷塔塔底重沸器5、脱乙烷塔6、脱乙烷塔顶冷凝器7、脱乙烷塔塔顶回流罐8、脱乙烷塔塔顶回流泵9、脱乙烷 塔塔底重沸器10、产品气压缩机11、丙烷压缩机12、丙烷冷凝器13、丙烷换热系统14、乙烯压缩机15、乙烯换热系统16。一种采用阶式制冷的天然气乙烷回收装置主要分为三部分，分别为：精馏部分，丙烷换热系统和乙烯换热系统。
一、精馏部分：该部分将原料气天然气进行精馏分离为乙烷产品，C 3+和产品天然气。
冷箱1是乙烷回收装置原料气预冷和低温气过冷的场所，冷箱1依次与低温分离器2、膨胀机3、脱甲烷塔4和产品气压缩机11连接。
低温分离器2出口的液相与脱甲烷塔4直接相连；脱甲烷塔4出口的液相与脱乙烷塔6直接相连；脱乙烷塔6与脱乙烷塔顶冷凝器7、脱乙烷塔顶回流罐8、脱乙烷塔顶回流泵9依次相连；脱乙烷塔顶回流泵9的出口与脱乙烷塔6塔顶入口相连；脱甲烷塔4底部设有脱甲烷塔底重沸器5，用于为脱甲烷塔4底部提供热量；脱乙烷塔6底部设有脱乙烷塔底重沸器10，用于为脱乙烷塔6底部提供热量。
二、丙烷换热系统：该部分主要为产品乙烷液化及过冷提供冷量。
脱乙烷塔塔顶回流罐8出口的气相为乙烷产品，与丙烷换热系统14和乙烯换热系统16相连冷凝为液体后输送至乙烷储罐。
丙烷压缩机12出口气体依次与丙烷冷凝器13、丙烷换热系统14相连形成丙烷制冷系统循环。
三、乙烯换热系统：该部分主要为回流的产品气液化及过冷提供冷量。
产品气压缩机11出口大部分直接外输，剩余部分需要冷凝为脱甲烷塔4提供冷量，因此依次与丙烷换热系统14和乙烯换热系统16相连，冷凝为液体后再与脱甲烷塔4相连。
乙烯压缩机15出口的气相依次与丙烷换热系统14和乙烯换热系统16相连形成乙烯制冷系统循环。
本发明还提供了一种采用阶式制冷从天然气中回收乙烷的方法，包括如下步骤：
1、精馏部分
3.0～7.0MPa.g的干燥含乙烷天然气与冷箱1相连，经过冷箱1预冷至-35～-75℃左右后，经过低温分离器2分离出的气相大部分(约60～80％左右)进入膨胀机3，膨胀后的气相压力至2.0～4.0MPa.g后直接进入脱甲烷塔4，剩余少部分气相继续进入冷箱1进一步冷凝至-80～-95℃成液体后再进入脱甲烷塔4上部。低温分离器2分离出的液相则节流至2.2～4.0MPa.g后直接进入脱甲烷塔4。脱甲烷塔的压力控制在1.8～3.8MPa.g。
脱甲烷塔4塔底出来的C 2+直接进入脱乙烷塔6，脱乙烷塔6的压力控制在1.6～3.5MPa.g，经过脱乙烷塔6分馏后，塔顶出来的气相与脱乙烷塔顶冷凝器7相连，部分冷凝至-30～5℃左右后通过脱乙烷塔塔顶回流罐8分离得到的液体则通过脱乙烷塔塔顶回流泵9输送至脱乙烷塔6塔顶作为其回流液，脱乙烷塔塔顶回流罐8分离得到的气相则为乙烷产品。
脱甲烷塔4塔顶出来的气相通过冷箱1换热至10～35℃后，再依次通过膨胀机3同轴增压端和产品气压缩机11增压后80％～90％作为产品天然气外输。2、丙烷制冷部分
丙烷换热系统14包括高压丙烷蒸发器、中压丙烷蒸发器和低压丙烷蒸发器，在该系统内将气相乙烷产品冷凝并冷却至-34～-37℃，将回流的产品天然气预冷至-34～-37℃，并且将制冷剂乙烯冷凝并冷却至-34～-37℃。
经丙烷压缩机12增压至1300～1700kPa.g的制冷剂丙烷经过丙烷冷凝器 13冷凝并冷却至30～50℃后进入丙烷换热系统14，通过节流至15～23℃为高压丙烷蒸发器提供冷量，将回流的产品天然气和制冷剂乙烯预冷至18～26℃后进入中压丙烷蒸发器。高压丙烷蒸发器中的气相丙烷返回丙烷压缩机12的三级增压入口，分离出的液体丙烷则被继续节流至-15～-5℃进入中压丙烷蒸发器，为其提供冷量，将乙烷产品、回流的产品天然气和制冷剂乙烯冷却至-12～-2℃后进入低压丙烷蒸发器。中压丙烷蒸发器中的气相丙烷返回丙烷压缩机的二级增压入口，分离出的液体丙烷则被继续节流至-39～-37℃进入低压丙烷蒸发器，为其提供冷量，将乙烷产品、回流的产品天然气和制冷剂乙烯冷却至-36～-34℃进入乙烯换热系统。低压丙烷蒸发器分离出来的气相进入丙烷压缩机12的一级增压入口。形成丙烷制冷循环。
3、乙烯制冷
乙烯换热系统16包括高压乙烯蒸发器、中压乙烯蒸发器和低压乙烯蒸发器，在该系统内将冷凝的乙烷产品过冷至-90～-85℃，将回流的产品天然气液化并过冷至-95～-90℃。
经乙烯压缩机15增压至1600～2400kPa.g的制冷剂乙烯依次通过乙烯换热系统冷凝并冷却至-36～-34℃后通过节流至-60～-50℃为高压乙烯蒸发器提供冷量，将回流的产品天然气和乙烷预冷却-57～47℃后进入中压乙烯蒸发器。高压乙烯蒸发器中的气相乙烯返回乙烯压缩机15的三级增压入口，分离出的液体乙烯则被继续节流至-81～-73℃进入中压乙烯蒸发器，为其提供冷量，将回流的产品天然气和乙烷冷却至-78～-70℃后进入低压乙烯蒸发器。中压乙烯蒸发器中的气相乙烯返回乙烯压缩机15的二级增压入口，分离出的液体乙烯则被继续节流至-98～-93℃进入低压乙烯蒸发器，为其提供冷量，将回流的产品天然气冷却至-95～-90℃后至脱甲烷塔4顶部作为其回流液，乙烷冷却至 -90～-85℃后输送至乙烷储罐储存。低压乙烯蒸发器分离出来的气相进入乙烯压缩机15的一级增压入口。形成乙烯制冷循环。
本发明中丙烷制冷部分可采用丙烯制冷，乙烯制冷部分可采用乙烷制冷。
本发明中脱甲烷塔和脱乙烷塔分别设有重沸器。
本发明中脱甲烷塔是板式塔也可以是填料和板式相结合的形式，脱乙烷塔可以是板式塔也可以是填料塔。
本发明中，丙烷换热系统根据乙烷产品的压力，还可以是单级换热，也可以是两级换热或者四级换热，相应的丙烷压缩机入口可以是一级增压，也可以是两级增压或者是四级增压。
本发明中，丙烷冷剂还可以是丙烯。
本发明中，根据丙烷压缩机出口压力的不同，丙烷冷凝器13是循环水冷凝，也可以采用空冷器冷凝。
本发明中，乙烯换热系统根据回流的产品天然气的压力，可以是两级换热或者四级换热，相应的乙烯压缩机入口可以是两级增压或者是四级增压。
本发明中，根据乙烷储存条件的要求和回流的产品天然气制冷温度的不同，乙烯冷剂还可以换成乙烷。
本发明中脱乙烷塔底出来的液相为C 3+，可以直接外售，可根据用户需求决定是否通过脱丙烷塔、脱丁烷塔分馏得到丙烷、丁烷(LPG)产品和稳定轻烃产品外售，实现产品多元化。

Claims (9)

  1. 一种采用阶式制冷的天然气乙烷回收装置，其特征在于：包括冷箱、低温分离器、脱甲烷塔、脱乙烷塔、丙烷或丙烯换热系统和乙烯换热系统，其中：冷箱的天然气出口与低温分离器连接，低温分离器的液相出口依次与脱甲烷塔、脱乙烷塔、脱乙烷塔顶冷凝器、脱乙烷塔塔顶回流罐、丙烷或丙烯换热系统、乙烯换热系统相连；脱甲烷塔的塔顶气相依次与冷箱、膨胀机和产品气压缩机相连；乙烯换热系统的天然气出口与脱甲烷塔顶部入口相连；丙烷或丙烯压缩机、丙烷或丙烯冷凝器、丙烷或丙烯换热系统循环连接；乙烯压缩机、丙烷或丙烯换热系统、乙烯换热系统循环连接。
  2. 根据权利要求1所述的一种采用阶式制冷的天然气乙烷回收装置，其特征在于：低温分离器的气相出口分成两路，一路依次与膨胀机、脱甲烷塔连接，另一路依次与冷箱、脱甲烷塔上部进口连接。
  3. 根据权利要求1所述的一种采用阶式制冷的天然气乙烷回收装置，其特征在于：脱乙烷塔塔顶回流罐的液相出口依次与脱乙烷塔塔顶回流泵、脱乙烷塔塔顶回流液进口连接。
  4. 根据权利要求1所述的一种采用阶式制冷的天然气乙烷回收装置，其特征在于：所述脱甲烷塔和脱乙烷塔塔底均设有重沸器。
  5. 根据权利要求1所述的一种采用阶式制冷的天然气乙烷回收装置，其特征在于：所述脱甲烷塔为板式塔或填料和板式相结合的形式，脱乙烷塔为板式塔或填料塔。
  6. 一种采用阶式制冷的天然气乙烷回收方法，其特征在于：包括如下内容：
    一、将原料气天然气进行精馏分离为乙烷产品、C 3+和产品天然气：
    3.0～7.0MPa.g的干燥含乙烷天然气经过冷箱预冷至-35～-75℃后进入低 温分离器进行气液分离，分离出的气相大部分经过膨胀机膨胀至2.0～4.0MPa.g后进入脱甲烷塔，分离出的少部分气相进入冷箱冷凝至-80～-95℃成液体后再进入脱甲烷塔上部；低温分离器分离出的液相节流至2.2～4.0MPa.g后进入脱甲烷塔；脱甲烷塔的压力控制在1.8～3.8MPa.g；
    脱甲烷塔塔底出来的C 2+进入脱乙烷塔，脱乙烷塔的压力控制在1.6～3.5MPa.g，经过脱乙烷塔分馏后，塔顶出来的气相经脱乙烷塔顶冷凝器冷凝至-30～5℃后经脱乙烷塔塔顶回流罐分离，得到的液体经脱乙烷塔回流泵输送至脱乙烷塔塔顶作为其回流液，脱乙烷塔塔顶回流罐分离得到的气相为乙烷产品，进入丙烷或丙烯换热系统；
    脱甲烷塔塔顶出来的气相通过冷箱换热至10～35℃后，再依次通过膨胀机同轴增压端和产品气压缩机增压后大部分作为产品天然气外输，其余少部分进入丙烷或丙烯换热系统；
    二、丙烷或丙烯换热系统为产品乙烷液化及过冷提供冷量：
    经丙烷或丙烯压缩机增压至1300～1700kPa.g的制冷剂丙烷或丙烯经过丙烷或丙烯冷凝器冷凝并冷却至30～50℃后进入丙烷或丙烯换热系统，丙烷或丙烯换热系统将气相乙烷产品冷凝并冷却至-34～-37℃，将回流的产品天然气预冷至-34～-37℃，并且将制冷剂乙烯冷凝并冷却至-34～-37℃；
    三、乙烯换热系统为乙烷产品过冷和回流的产品气液化及过冷提供冷量：
    经乙烯压缩机增压至1600～2400kPa.g的制冷剂乙烯依次通过乙烯换热系统冷凝并冷却至-36～-34℃后进入乙烯换热系统，乙烯换热系统将冷凝的乙烷产品过冷至-90～-85℃，将回流的产品天然气液化并过冷至-95～-90℃。
  7. 根据权利要求6所述的一种采用阶式制冷的天然气乙烷回收方法，其特征在于：所述丙烷或丙烯换热系统包括高压丙烷或丙烯蒸发器、中压丙烷或 丙烯蒸发器和低压丙烷或丙烯蒸发器；进入丙烷或丙烯换热系统的制冷剂丙烷或丙烯节流至15～23℃为高压丙烷或丙烯蒸发器提供冷量，将回流的产品天然气和制冷剂乙烯预冷至18～26℃后进入中压丙烷或丙烯蒸发器；高压丙烷或丙烯蒸发器中的气相丙烷或丙烯返回丙烷或丙烯压缩机的三级增压入口，分离出的液体丙烷或丙烯则被继续节流至-15～-5℃后进入中压丙烷或丙烯蒸发器，将乙烷产品、回流的产品天然气和制冷剂乙烯冷却至-12～-2℃后进入低压丙烷或丙烯蒸发器；中压丙烷或丙烯蒸发器中的气相丙烷或丙烯返回丙烷或丙烯压缩机的二级增压入口，分离出的液体丙烷或丙烯节流至-39～-37℃后进入低压丙烷或丙烯蒸发器，将乙烷产品、回流的产品天然气和制冷剂乙烯冷却至-36～-34℃后进入乙烯换热系统；低压丙烷或丙烯蒸发器分离出来的气相进入丙烷或丙烯压缩机的一级增压入口，形成丙烷或丙烯制冷循环。
  8. 根据权利要求6所述的一种采用阶式制冷的天然气乙烷回收方法，其特征在于：所述乙烯换热系统包括高压乙烯蒸发器、中压乙烯蒸发器和低压乙烯蒸发器；进入乙烯换热系统的制冷剂乙烯节流至-60～-50℃为高压乙烯蒸发器提供冷量，将回流的产品天然气和乙烷预冷却至-57～47℃后进入中压乙烯蒸发器；高压乙烯蒸发器中的气相乙烯返回乙烯压缩机的三级增压入口，分离出的液体乙烯节流至-81～-73℃后进入中压乙烯蒸发器，将回流的产品天然气和乙烷冷却至-78～-70℃后进入低压乙烯蒸发器；中压乙烯蒸发器中的气相乙烯返回乙烯压缩机的二级增压入口，分离出的液体乙烯节流至-98～-93℃后进入低压乙烯蒸发器，将回流的产品天然气冷却至-95～-90℃后至脱甲烷塔顶部作为其回流液，乙烷冷却至-90～-85℃后输送至乙烷储罐储存；低压乙烯蒸发器分离出来的气相进入乙烯压缩机的一级增压入口，形成乙烯制冷循环。
  9. 根据权利要求6所述的一种采用阶式制冷的天然气乙烷回收方法，其 特征在于：所述丙烷或丙烯冷凝器采用循环水冷凝或空冷器冷凝。
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