Method and apparatus for separating, by cryogenic distillation, a mixture of hydrogen, propane and / or propylene and at least one other hydrocarbon

Abstract

The invention relates to a process for separating a feed gas mixture comprising hydrogen, propane and / or propylene, and a hydrocarbon. It comprises cooling the feed gas mixture (101) in a first heat exchanger (HEX 2) to produce a cooled feed gas mixture (102), distilling (T101) the cooled feed mixture to produce a hydrogen-enriched overhead gas (106) depleted in propane and / or propylene, the overhead gas being partially condensed, without expansion, forming a hydrogen-enriched gas (113, 114, 115) at a pressure preferably of at least 15 bar. Abstract figure: Fig. 1

Description

Title of the invention: Process and apparatus for separation by cryogenic distillation of a mixture of hydrogen, propane and / or propylene and at least one other hydrocarbon

[0001] The present invention relates to a process for the separation by cryogenic distillation of a mixture of hydrogen, propane and / or propylene and at least one other hydrocarbon, in particular a residual gas from a dehydrogenation of propane.

[0002] Dehydrogenation is a chemical reaction often requiring the use of catalysts, and under defined temperature and pressure conditions, which corresponds to the loss of hydrogen atoms. The function of a dehydrogenation process, when used in the processing of hydrocarbons, is to convert paraffinic hydrocarbons into their corresponding olefins.

[0003] Thus, in a propane dehydrogenation process (or in English acronym PDH (Propane DeHydrogenation)), propane is converted into propylene:

[0004] [Cheml] C3H8 ^C3H6+H2

[0005] Propane dehydrogenation (PDH) is a step in the process of producing propylene from propane. This PDH step is vital for the petrochemical industry: propylene is the second most important commodity for the petrochemical industry after ethylene. Propylene is the raw material used for the manufacture of polypropylene plastic, a common component mainly used in the textile and automotive industries, for the manufacture of plastic packaging films as well as many other products.

[0006] The waste gas from the reactor where the propane dehydrogenation process takes place contains hydrogen, propane and / or propylene as well as lower hydrocarbons, such as ethane, ethylene, methane ... A cryogenic process is used to condense the C3 + components into a liquid phase product. The residual vapor stream must be free of the C3 components to minimize their losses.

[0007] Prior to the cryogenic step, the waste gas is compressed and dried to remove water and CO2, which could cause operational problems.

[0008] Several existing patents describe the separation of hydrocarbons by cryogenics in a series of process steps including cooling, phase separation and expansion. Refrigeration is generally provided by indirect heat exchange through expanded light products to save energy and power consumption. Accordingly, the light products are expanded to provide cold and are therefore produced at low pressure. When pure H2 at high pressure must be valued from such a cryogenic scheme, the expanded light products are compressed upstream of the inlet of a PSA by a dedicated machine.

[0009] It is desirable to reduce the overall production cost for a cryogenic separation process and the apparatus used to carry out the process when H2 contained in the light products is to be purified in a PSA and produced at high pressure.

[0010] US6333445 describes the expansion of lightweight products with energy reduction consumed using the frigories of the feed liquid.

[0011] US2011041550 shows another diagram: the gas separation unit receives the gas re PDH siduary, after cooling, compression and removal of CO2, and divides it into: - propane and propylene which are fed to the C2 column to obtain high purity, then the C3 separator - light products (H2 + CH4 + C2) which are partially recycled from the top of the deethanizer to the inlet of the gas separation unit.

[0012] The light products are expanded in two turbines to produce frigories for the condensation of C3. Additional refrigeration is provided by the ammonia absorption refrigeration unit. This refrigeration system is implemented when intermediate level heat (150 °C) is available in excess, which is the case in a PDH unit.

[0013] The disadvantages of this method are: - hydrogen recovered at low pressure - recompression for PSA purification is not cost-effective. - ammonia is used as a liquid refrigerant in self-contained chillers.

[0014] Other documents highlight the use of liquid paraffin feed to the dehydrogenation unit (US6333445, US2018335254) or a gaseous refrigerant, typically nitrogen, methane, or air (US6266977) as the refrigerant.

[0015] The present invention relates to a cryogenic distillation process for separating a gas mixture containing hydrogen, propane and / or propylene as well as at least one hydrocarbon which is ethane, ethylene or methane.

[0016] The gas mixture may come from a propane dehydrogenation unit and may be compressed upstream of the cryogenic separation. It may also be dried and / or purified of CO2 upstream of the cryogenic separation.

[0017] The separation produces a hydrogen-enriched gas at a high pressure, greater than 15 bars. This gas is preferably sent to an adsorption unit to produce a hydrogen-rich gas.

[0018] Unlike the traditional scheme, the frigories required for the process are not provided by expansion of the hydrogen-enriched gases, which are instead kept under pressure in order to supply a pressure swing adsorption (PSA) unit.

[0019] The frigories are supplied by two independent refrigeration loops:

[0020] A loop whose refrigerant is liquid ammonia

[0021] A loop whose refrigerant is a mixed liquid refrigerant

[0022] A single distillation column is used to extract the C3+ at the bottom and the gases enriched in light component(s) at the top. The gases are partially condensed in at least two stages in series at at least two different temperatures in one or more heat exchangers.

[0023] According to an object of the invention, there is provided a process for separating a feed gas mixture comprising hydrogen, propane and / or propylene as well as one or more hydrocarbons, for example methane and / or ethane and / or ethylene, which process comprises: - a. A step of cooling the feed gas mixture in a first multi-fluid heat exchanger to produce a cooled feed gas mixture - b. A step of distilling the cooled feed mixture to produce a hydrogen-enriched overhead gas depleted in propane and / or propylene and a hydrogen-depleted liquid enriched in propane and / or propylene - c. The overhead gas is partially condensed by at least one cooling and partial condensation step, without having been expanded, forming a hydrogen-enriched gas at a pressure preferably of at least 15 bars and - d. cold for step a) is provided by indirect heat exchange in the first multi-fluid heat exchanger by at least one first refrigerant circulating in a closed loop.

[0024] According to other optional aspects: - the overhead gas is partially condensed, without having been expanded, in the first heat exchanger to form a two-phase fluid and the two-phase fluid is separated in a phase separator, forming a gas which is partially condensed, without having been expanded, in a second heat exchanger to form a two-phase fluid, which is separated to form the hydrogen-enriched gas. - the two-phase fluid is separated in a second phase separator to form the hydrogen-enriched gas. - liquid from the second phase separator is sent to the top of the column. - the liquid from the second separator is pressurized in a pump upstream of the column. - the hydrogen-enriched gas is separated by pressure swing adsorption to form a hydrogen-rich gas. - the first refrigerant is a mixture of at least two hydrocarbons which provides cold to the first and possibly to the second multi-fluid heat exchanger by circulating in a closed loop. - the first refrigerant comprises at least i) at least 50 mol% composed of ethane and / or ethylene and ii) propane and / or propylene. - a second refrigerant provides cold to the first heat exchanger and to the closed loop of the first refrigerant. - the first and / or second heat exchanger is made of stainless steel. - the temperature difference in the second heat exchanger is su above 20°C. - the first and / or second refrigerant does not contain water. - the second refrigerant is at least partially composed of ammonia in liquid form. - the feed flow contains between 70 and 95% mol ethane and / or ethylene and / or between 5 and 30% mol propane and / or propylene. - the liquid from the column tank is sent to a deethanization process. - liquid from the phase separator is sent to the column at a level at least one theoretical plate below the column head. - hydrogen-enriched gas contains at least 80 mol% hydrogen - the gas enriched in hydrogen and containing at least 80% mol of hydrogen is sent directly to a hydrogen liquefier, preferably without having been warmed to room temperature, or even preferably without having been warmed. - the hydrogen-enriched gas has a concentration preferably greater than 85% mol - the hydrogen-enriched gas is sent to the liquefier at a temperature below -70°C, or even below -85°C

[0025] According to another object of the invention, there is provided an apparatus for separating a feed gas mixture comprising hydrogen, propane and / or propylene as well as one or more hydrocarbons, for example methane and / or ethane and / or ethylene, comprising: - a) a first multi-fluid heat exchanger for cooling feed gas mixture to produce a cooled feed gas mixture - b) a distillation column for carrying out a distillation step of the cooled feed mixture, the column being connected to the first exchanger heat exchanger for sending the cooled feed gas mixture from the first exchanger to the column and comprising a conduit for discharging a hydrogen-enriched overhead gas depleted in propane and / or propylene from the column and a conduit for discharging a hydrogen-depleted liquid enriched in propane and / or propylene from the column - c) means for partially condensing the head gas connected to the head of the column, without the head gas passing through an expansion means, forming a hydrogen-enriched gas and - d) a closed loop in which at least a first refrigerant circulates, connected to the first heat exchanger to provide cold to the first multi-fluid heat exchanger.

[0026] According to another object of the invention, there is provided a separation apparatus as described above integrated into a hydrogen liquefier and comprising means for sending the hydrogen-enriched gas from the separation apparatus to the liquefier.

[0027] According to another object of the invention, there is provided a process for liquefying hydrogen supplied with a hydrogen-rich gas, containing at least 80 mol% of hydrogen, produced by cryogenic separation of a feed gas mixture comprising hydrogen, propane and / or propylene as well as one or more hydrocarbons.

[0028] Preferably the hydrogen-rich gas is produced by cryogenic separation at a temperature below 0°C and is sent at this temperature to liquefaction.

[0029] The invention will be described in more detail with reference to the figures:

[0030] [Fig-1] illustrates a method according to the invention and;

[0031] [Fig.2] illustrates a variant of the method according to the invention.

[0032] In [Fig.l], a feed gas 101 contains hydrogen, propane and / or propylene as well as at least one hydrocarbon which is ethane. The gas may also contain ethylene and / or methane.

[0033] The gas 101 at [10 bar; 70 bar] is cooled from ambient temperature to [0°C; -20°C] in a first indirect stainless steel heat exchanger HEX2 forming a fluid 102. Partially condensed, the stream 102 enters a phase separator D1010 and the vapor 104 from the separator is fed to the bottom of a distillation column T101 while the liquid 103 from the separator is collected and sent to deethanization. In the lower part of the distillation column T101, a bottom liquid is produced which is enriched in propane and / or propylene, in at least one component heavier than propane if present and which contains a part of C2 which can be removed by deethanization. The bottom liquid 105 is produced at [0 ° C; -200 ° C]. The gaseous product 106, enriched in hydrogen, leaves at the top of column T101, is cooled to [-20 ° C; -55 ° C] in the first indirect heat exchanger HEX2 to partially condense it. The partially condensed flow is sent to a first phase separator D102 whose liquid 108 is returned to column T101 at a level of a few theoretical plates below the column head, and pumped if necessary by pump P101. The vapor 109 from phase separator DI02 is further cooled in a second multi-fluid heat exchanger HEX1, which can be made of aluminum since no corrosive components are present. The two-phase fluid 110 thus formed is sent to another phase separator D103 producing a liquid phase 111 and a hydrogen-enriched gas 113.Cooled to [-70; -95] °C and partially condensed, the liquid phase 111 is sent to the top of the column T101, above the arrival point of the liquid 108, while the vapor 113, free of C3 +, enriched in hydrogen, is used as a cold source both in the second multi-fluid heat exchanger called "cold" HEX1 and in the first multi-fluid heat exchanger called "hot" HEX2 where it heats up. The vapor heated in the exchangers HEX1, HEX2 forms the gas 115, 116 at at least 15 bars and is sent to a PSA unit without any compression necessary.

[0034] The PSA unit separates the gas 116 by pressure swing adsorption producing a hydrogen-rich flow 118 and a hydrogen-depleted flow 117.

[0035] Part of the cold is provided by a stream of liquid ammonia, produced by adsorption in water in a closed cycle. This process requires a lot of heat and uses the residual heat from the PDH process. The water content is low but significant, which makes it incompatible with aluminum for corrosion problems. In this particular scheme, ammonia provides refrigeration at two temperature levels [-30 ° C; -60 ° C] (LLP NH3) and [-5 ° C; -20 ° C] (LP NH3).

[0036] The remainder of the cold is provided by a mixed hydrocarbon refrigerant with the following typical composition; mol% C2 [70; 95], mol% C3 with a cycle pressure of [5 bar; 30 bar]. The mixed refrigerant comprises at least two hydrocarbons of which i) at least 50 mol% is composed of ethane and / or ethylene and ii) propane and / or propylene.

[0037] After being compressed into K201 at several bars [5bar; 30bar], the MR 201 is condensed by air E201 and ammonia E202 / E203 to form MR 204 which is subcooled in the heat exchanger HEX1. Subcooled to [-70°C; -95°C], the MR 205 is expanded in a valve to [1; 5atm] to form the two-phase fluid 206. The fluid 206 is separated in a phase separator D201 to produce a gas 209 and a liquid 207. The liquid 207 provides cold in the first heat exchanger HEX1 where it vaporizes. The vaporized liquid 208 is mixed with the vapor 209, the flow formed by mixing 208 and 209 is divided into two and partly heated in the second heat exchanger HEX2 before compression to avoid material problems of equipment forming the gas 211. The heated gas 211 and the rest of the flow 208+209 are mixed to power the MR K201 cycle compressor.

[0038] The first and second heat exchangers HEX1 and HEX2 may constitute a single multi-fluid heat exchanger made of stainless steel, integrating or not the coolers E202 / E203 used for MR condensation.

[0039] The first and second heat exchangers HEX1 and HEX2 may constitute a single multi-fluid aluminum heat exchanger, integrating or not the E202 / E203 coolers used for MR condensation if the ammonia refrigerant is dried. The wet ammonia may be dried by gas phase adsorption or liquid phase adsorption with temperature variation. Several dry gases are available on the plan to allow the regeneration of the beds.

[0040] While in [Fig.l] all the gas 113, 114, 115 is sent to the PSA, in [Fig.2], a part 119 of the gas 114 is expanded in a turbine K101 forming the expanded gas 120 which is sent reheated into the exchangers HEX2, HEX1 to serve as fuel.

[0041] In both cases, the hydrogen 113 may have a concentration of at least 80 mol% hydrogen, preferably greater than 85 mol%, and be at a temperature below -70°C, or even below -85°C, in which case it is advantageous to send it directly to a hydrogen liquefier, preferably without heating it and without passing through the PSA. In this case, a purification step must be provided upstream of the liquefier to remove impurities capable of freezing in the liquefier, which may be at least one of the following impurities: air or carbon dioxide from leaks, nitrogen, ammonia,

[0042] In this case, it is possible to operate the process with less refrigeration, for example by eliminating at least one refrigeration cycle.

[0043] At least part of the refrigeration for the described processes may come from: - At least one absorption refrigeration cycle (vacuum or not) and / or - At least one SCR and / or - At least one MCR.

[0044] A stainless steel multi-fluid heat exchanger also allows a large temperature spread for the configuration (AT>20°C) for the HEX2 and / or HEX1 exchanger.

[0045] The mixed refrigerant 208, 209, 211 of the examples is hydrocarbon. It contains at least two hydrocarbons. One of them is ethane and / or ethylene with %mol>50. The second hydrocarbon is propene and / or propylene. The mixed hydrocarbon refrigerant makes it possible to achieve the lowest condensation level (T°<-40°C). The mixed refrigerant is condensed by the first refrigerant (ammonia), then subcooled before being expanded. The outlet pressure of the compressor K101 Pcomp is [5;30bar] for a mixed refrigerant with %molC2 [60;95], %molC3 [5;40].

Claims

Claims

1. A method for separating a feed gas mixture comprising hydrogen, propane and / or propylene and one or more hydrocarbons, for example methane and / or ethane and / or ethylene, which method comprises: - a. A step of cooling the feed gas mixture (101) in a first multi-fluid heat exchanger (HEX 2) to produce a cooled feed gas mixture (102) - b. A step of distilling (T101) the cooled feed mixture to produce a hydrogen-enriched overhead gas (106) depleted in propane and / or propylene and a hydrogen-depleted liquid (103, 105) enriched in propane and / or propylene - c. The overhead gas is partially condensed by at least one cooling and partial condensation step, without having been expanded, forming a hydrogen-enriched gas (113, 114, 115) at a pressure preferably of at least 15 bars and - d.cold for step a) is provided by indirect heat exchange in the first multi-fluid heat exchanger by at least one first refrigerant circulating in a closed loop (211, K201, 204, 208, 209):.

2. A method according to claim 1 wherein the overhead gas (106) is partially condensed, without having been expanded, in the first heat exchanger (HEX 2) to form a two-phase fluid (107) and the two-phase fluid is separated in a phase separator (D102), forming a gas (109) which is partially condensed (110), without having been expanded, in a second heat exchanger (HEX 1) to form a two-phase fluid, which is separated to form the hydrogen-enriched gas (113, 114, 115).

3. A method according to claim 2 wherein the two-phase fluid is separated in a second phase separator (D103) to form the hydrogen-enriched gas (113, 114, 115).

4. Method according to claim 3 in which liquid (111) from the second phase separator (D103) is sent to the top of the column (T101).

5. Method according to claim 3 or 4 in which the liquid (111) of the second separator (D103) is pressurized in a pump (P102) upstream of the column (T 101).

6. A method according to one of claims 1 to 5 wherein the hydrogen-enriched gas (113, 114, 115) is separated by pressure swing adsorption (PSA) to form a hydrogen-rich gas (118).

7. Method according to one of the preceding claims in which the first refrigerant (211, 208, 209) is a mixture of at least two hydrocarbons which provides cold to the first and possibly to the second multi-fluid heat exchanger (HEX2, HEX1) by circulating therein in a closed loop.

8. The method of claim 7 wherein the first refrigerant (208, 209, 211) comprises at least i) at least 50 mol% ethane and / or ethylene and ii) propane and / or propylene.

9. A method according to any preceding claim wherein a second refrigerant provides cold to the first heat exchanger (HEX2) and to the closed loop of the first refrigerant (208, 209, 211).

10. Method according to one of the preceding claims in which the first and / or the second heat exchanger (HEX2, HEX1) is made of stainless steel.

11. A method according to claim 2 or a claim dependent on claim 2 wherein the temperature difference in the second heat exchanger (HEX1) is greater than 20°C.

12. Method according to one of the preceding claims in which the first and / or the second refrigerant (208, 209, 211) does not contain water.

13. Method according to one of the preceding claims in which the second refrigerant is at least partially composed of ammonia in liquid form.

14. Method according to one of the preceding claims in which the feed flow (101) contains between 70 and 95 mol% ethane and / or ethylene and / or between 5 and 30 mol% propane and / or propylene.

15. Method according to one of the preceding claims in which the tank liquid (105) of the column (T101) is sent to a deethanization process.

16. A method according to claim 2 or a claim dependent on claim 2 wherein liquid (108) from the phase separator (DI02) is sent to the column (T 101) at a level at least one theoretical plate below the head of the column.

17. A method according to any preceding claim wherein the hydrogen-enriched gas (113, 114, 115) preferably contains at least 80 mol% hydrogen and is sent directly to a hydrogen liquefier, preferably without having been heated to room temperature.

18. Apparatus for separating a feed gas mixture (101) comprising hydrogen, propane and / or propylene and one or more hydrocarbons, for example methane and / or ethane and / or ethylene, comprising: - a. a first multi-fluid heat exchanger (HEX 2) for cooling feed gas mixture to produce a cooled feed gas mixture (102) - b. a distillation column (T101) for carrying out a distillation step of the cooled feed mixture, the column being connected to the first heat exchanger for sending the cooled feed gas mixture from the first exchanger to the column and comprising a pipe for removing a hydrogen-enriched overhead gas (106) depleted in propane and / or propylene from the column and a pipe (105) for removing a hydrogen-depleted liquid enriched in propane and / or propylene from the column - c. means (HEX2, HEX1) for partially condensing the head gas connected to the head of the column, without the head gas passing through an expansion means, forming a hydrogen-enriched gas (113) and - d. a closed loop in which at least a first refrigerant (208, 209, 211) circulates, connected to the first heat exchanger to provide cold to the first multi-fluid heat exchanger.

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