WO1992012927A1 - Method for purification of synthesis gas - Google Patents

Method for purification of synthesis gas Download PDF

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Publication number
WO1992012927A1
WO1992012927A1 PCT/NO1992/000015 NO9200015W WO9212927A1 WO 1992012927 A1 WO1992012927 A1 WO 1992012927A1 NO 9200015 W NO9200015 W NO 9200015W WO 9212927 A1 WO9212927 A1 WO 9212927A1
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Prior art keywords
gas
vent
synthesis gas
purified
distillation column
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PCT/NO1992/000015
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French (fr)
Inventor
Dag Arne Eimer
Lars Erik ØI
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Norsk Hydro A.S
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Publication of WO1992012927A1 publication Critical patent/WO1992012927A1/en

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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
    • F25J3/0228—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
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B3/00—Hydrogen; Gaseous mixtures containing hydrogen; Separation of hydrogen from mixtures containing it; Purification of hydrogen
    • C01B3/02—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen
    • C01B3/025—Preparation or purification of gas mixtures for ammonia synthesis
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B3/00—Hydrogen; Gaseous mixtures containing hydrogen; Separation of hydrogen from mixtures containing it; Purification of hydrogen
    • C01B3/50—Separation of hydrogen or hydrogen containing gases from gaseous mixtures, e.g. purification
    • 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
    • F25J3/0219—Refinery gas, cracking gas, coke oven gas, gaseous mixtures containing aliphatic unsaturated CnHm or gaseous mixtures of undefined nature
    • 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
    • F25J3/0276—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 H2/N2 mixtures, i.e. of ammonia synthesis gas
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/04—Integrated processes for the production of hydrogen or synthesis gas containing a purification step for the hydrogen or the synthesis gas
    • C01B2203/0465—Composition of the impurity
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/04—Integrated processes for the production of hydrogen or synthesis gas containing a purification step for the hydrogen or the synthesis gas
    • C01B2203/0465—Composition of the impurity
    • C01B2203/047—Composition of the impurity the impurity being carbon monoxide
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/04—Integrated processes for the production of hydrogen or synthesis gas containing a purification step for the hydrogen or the synthesis gas
    • C01B2203/0465—Composition of the impurity
    • C01B2203/0475—Composition of the impurity the impurity being carbon dioxide
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/04—Integrated processes for the production of hydrogen or synthesis gas containing a purification step for the hydrogen or the synthesis gas
    • C01B2203/0465—Composition of the impurity
    • C01B2203/048—Composition of the impurity the impurity being an organic compound
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/04—Integrated processes for the production of hydrogen or synthesis gas containing a purification step for the hydrogen or the synthesis gas
    • C01B2203/0465—Composition of the impurity
    • C01B2203/0495—Composition of the impurity the impurity being water
    • 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
    • F25J2200/02—Processes or apparatus using separation by rectification in a single 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
    • F25J2200/04—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
    • F25J2210/00—Processes characterised by the type or other details of the feed stream
    • F25J2210/20—H2/N2 mixture, i.e. synthesis gas for or purge gas from ammonia synthesis
    • 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
    • F25J2270/04—Internal refrigeration with work-producing gas expansion loop

Definitions

  • the present invention relates to final purification of ammonia synthesis gas, subsequent to removal of CO, CO2 and H2O and prior to the ammonia synthesis by applying a cryogenic unit for removal of CH 4 , Ar and excess N2•
  • the object of the present invention was to develop a final purification process producing an ammonia synthesis gas substan ⁇ tially free of CO, CH 4 and excess N 2 without significant pressure loss and being more energy efficient than known processes.
  • a further object was to recover CH4 from the gas in order to improve the overall economy of the process and avoid discharge of CH 4 to the environment.
  • a major disadvantage of a known final purifica ⁇ tion process is the pressure drop through the cryogenic unit.
  • the inventors therefore investigated various alternatives of the process streams through the said unit in order to minimize the pressure drop. It was then found that it was not necessary to reduce the pressure of the incoming gas before it was supplied to the separation unit. If the incoming gas containing the undesired gas components was simply cooled from the process temperature of the preceding unit, for instance a methanation unit, ahead of the cryogenic unit to saturation temperature and supplied to a distillation column at about preceding process pressure, required removal of undesired components could still be obtained. It was further found that more of the inherent energy could be recovered by carrying out the decompression of the vent gas to about atmospheric pressure through an expander.
  • Fig. 1 shows a conventional process utilizing a cryogenic unit for final purification of synthesis gas.
  • Fig. 2 shows a process according to the invention.
  • Fig. 3 shows a process according to the invention comprising recovery of CH 4 from the vent gas.
  • Fig. 1 pre-purified synthesis gas from the methanation step is fed to a cryogenic unit as process stream 1 to a heat exchanger unit 7 with intermediate decompression in a turbine 4.
  • the cooled gas 9 is fed to a distillation column 5 where CH4, Ar and excess N are removed from the synthesis gas which leaves the top of the column 5 through pipe 11.
  • the final purified synthesis gas is then heated in a heat exchanger 7 and fed to an ammonia synthesis unit (not shown) through conduit 2.
  • the removed gas components leave the bottom of the column 5 through pipe 12 and further through a pressure relief valve 6.
  • This vent gas is then heated at the upper part of the column 5 which it leaves through conduit 10.
  • the vent gas is further heated in the heat exchanger 7 and is discharged to the atmosphere through conduit 3.
  • Fig. 1 pre-purified synthesis gas from the methanation step is fed to a cryogenic unit as process stream 1 to a heat exchanger unit 7 with intermediate decompression in a turbine 4.
  • the cooled gas 9 is fed to
  • FIG. 3 a process according to the invention comprising CH 4 recovery is shown.
  • Incoming gas 1, possibly mixed with recovered H2 is again simply cooled in a heat exchanger 7 and fed directly to column 5 through conduit 9.
  • the bottom fraction from column 5 is first expanded over valve 6 and then heat exchanged at the top of column 5 and then fed to a second distillation column 8 through conduit 13.
  • Recovered CH 4 leaves the bottom of column 8 through conduit 14 and is passed through heat exchanger 7 before it is returned to the processes ahead of the cryogenic unit for further conversion or combustion.
  • the hydrogen recovery unit (not shown) can be placed in stream 12 between valve 6 and the heat exchanger on top of column 5.
  • This example shows final purification of synthesis gas according to a conventional process as shown in Fig. 1 to which reference here is made.
  • Pre-purified gas 1 is fed to the cryogenic unit from which purified gas 2 leaves.
  • the composition of the various gas streams is stated in Kmol/h.
  • the H2/N2 ratio of purified synthesis gas (2) was 2.9977.
  • the distillation column 5 was run at 24.8 bar and the incoming gas 1 had a pressure of 27.8 bar being reduced to 24.8 bar over the turbine 4.
  • the vent gas leaving the distillation column 5 was reduced to 3 bar over the reduction valve 6.
  • the total pressure drop through the cryogenic unit was 3.4 bar, and this corresponds to approximately 0.7 MW.
  • the H /N 2 ratio of purified synthesis gas (2) was 2.9916, Incoming gas (1) had a pressure of 27.5 bar, and the distillation column 5 was run at 27 bar. The pressure in the vent gas (3) leaving the distillation column 5 was reduced from 27 bar to 3.6 bar of the reduction valve 6 and further down to 2 bar over the turbine 4.
  • the pressure drop in the cryogenic unit was 0.5 bar.
  • This example shows a process according to the invention compris ⁇ ing recovery of CH as shown in Fig. 3.
  • composition in Kmol/h for the various gas streams was:
  • the H /N ratio of purified synthesis gas (2) was 2.9937.
  • the effect with regard to removal of CH4, Ar and excess N2 from the synthesis gas is substantially the same for the conventional process and the process according to the invention.
  • the invention also includes recovery of more than 96% of the CH 4 content of the pre- purified gas as shown in Example 3.
  • a substantial amount of the hydrogen content in the vent gas can be removed by means of minor investments as indicated for the process according to Fig. 3.
  • the present invention thus gives a process that maintains the advantages of the known process, and in addition to that a substantial recovery of energy is obtained.
  • the pressure drop is reduced substantially, representing savings of at least 0.7 MW having a value of approximately 2 mill. NOK/year.
  • the present process is especially advantageous, particularly when practised according to claim 3, i.e. recovery of both CH 4 and H 2 from the vent gas.
  • the method according to the present invention is especially advantageous in combination with low pressure ammonia processes where the ammonia synthesis can be performed at substantially the same pressure as the methanation, as no recompression of the synthesis gas is necessary after the final purification step.

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  • Chemical & Material Sciences (AREA)
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Abstract

The present invention relates to a method of purification of ammonia synthesis gas subsequent to pre-purification. The pre-purified gas from a preceding unit is cooled from its process temperature to saturation temperature level and fed to a distillation column for separation of the gas into purified synthesis gas and vent gas containing the removed CH4, Ar and excess N2. The separation is performed at substantially the same pressure as in the preceding unit. The vent gas is heated to the final vent temperature with intermediate pressure release through a pressure relief valve and at least one expansion turbine. The purified synthesis gas is heat exchanged with the pre-purified synthesis gas. Most of the CH4 content of the vent gas can be removed in a second distillation column prior to final pressure release. H2 can be removed from the vent gas prior to CH4 removal.

Description

Method for purification of synthesis gas
The present invention relates to final purification of ammonia synthesis gas, subsequent to removal of CO, CO2 and H2O and prior to the ammonia synthesis by applying a cryogenic unit for removal of CH4, Ar and excess N2•
In connection with ammonia production it is important to purify the synthesis gas and adjust the relative amounts of nitrogen and hydrogen before the gas is supplied to the ammonia synthesis. Removal of CO, CO2 and H2O can be performed relatively easily. A high degree of removal of CH4, Ar and simultaneously removal of excess N2 has been difficult to obtain economically. Especially for low pressure synthesis this latter purification is of great importance.
From US patent No. 3,442,613 it is known a hydrocarbon reforming process for production of synthesis gas, comprising a cryogenic unit for final purification of the synthesis gas. The final purification with regard to removal of undesired components is acceptable, but this is obtained at the cost of a high pressure drop and loss of energy from the gas.
The object of the present invention was to develop a final purification process producing an ammonia synthesis gas substan¬ tially free of CO, CH4 and excess N2 without significant pressure loss and being more energy efficient than known processes. A further object was to recover CH4 from the gas in order to improve the overall economy of the process and avoid discharge of CH4 to the environment.
Having considered all the available processes and the various process combinations for final purification of the gas mixture in question, the inventors decided to further investigate utiliza¬ tion of cryogenic units. The reason for this was that it had been found that a most pure synthesis gas having the correct H2/N2 ratio could be produced by means of such cryogenic units. The question was whether the process itself could be altered and made more energy efficient without losing effect with regard to purification of the final synthesis gas.
As stated above, a major disadvantage of a known final purifica¬ tion process is the pressure drop through the cryogenic unit. The inventors therefore investigated various alternatives of the process streams through the said unit in order to minimize the pressure drop. It was then found that it was not necessary to reduce the pressure of the incoming gas before it was supplied to the separation unit. If the incoming gas containing the undesired gas components was simply cooled from the process temperature of the preceding unit, for instance a methanation unit, ahead of the cryogenic unit to saturation temperature and supplied to a distillation column at about preceding process pressure, required removal of undesired components could still be obtained. It was further found that more of the inherent energy could be recovered by carrying out the decompression of the vent gas to about atmospheric pressure through an expander. Within this concept it was also possible to recover most of the CH4 from the vent gas by simple distillation without further use of external energy. The cost of such CH4 recovery could be justified not only from its value, but also from an environmental point of view. Discharge of hydrocarbons into the atmosphere from petrochemical plants is becoming increasingly undesired. Hydrogen can also be recovered economically within this concept with small investment in an extra unit.
The scope of the invention and its special features are as defined in the attached claims.
The invention and its advantages will be further explained in the following description of the drawings and the examples.
Fig. 1 shows a conventional process utilizing a cryogenic unit for final purification of synthesis gas.
Fig. 2 shows a process according to the invention.
Fig. 3 shows a process according to the invention comprising recovery of CH4 from the vent gas.
In Fig. 1 pre-purified synthesis gas from the methanation step is fed to a cryogenic unit as process stream 1 to a heat exchanger unit 7 with intermediate decompression in a turbine 4. The cooled gas 9 is fed to a distillation column 5 where CH4, Ar and excess N are removed from the synthesis gas which leaves the top of the column 5 through pipe 11. The final purified synthesis gas is then heated in a heat exchanger 7 and fed to an ammonia synthesis unit (not shown) through conduit 2. The removed gas components leave the bottom of the column 5 through pipe 12 and further through a pressure relief valve 6. This vent gas is then heated at the upper part of the column 5 which it leaves through conduit 10. The vent gas is further heated in the heat exchanger 7 and is discharged to the atmosphere through conduit 3. In Fig. 2 a process according to the invention is shown where the pre-purified gas 1 is fed directly through the heat exchanger 7 and through conduit 9 to the distillation column 5. According to this process the removed components, i.e. the vent gas 10 from the column 5, are heated in heat exchanger 1 , but in between two heat exchanging steps its pressure is reduced in a turbine expander 4.
In Fig. 3 a process according to the invention comprising CH4 recovery is shown. Incoming gas 1, possibly mixed with recovered H2, is again simply cooled in a heat exchanger 7 and fed directly to column 5 through conduit 9. In this case the bottom fraction from column 5 is first expanded over valve 6 and then heat exchanged at the top of column 5 and then fed to a second distillation column 8 through conduit 13. Recovered CH4 leaves the bottom of column 8 through conduit 14 and is passed through heat exchanger 7 before it is returned to the processes ahead of the cryogenic unit for further conversion or combustion.
The hydrogen recovery unit (not shown) can be placed in stream 12 between valve 6 and the heat exchanger on top of column 5.
The top fraction of column 8 leaves through conduit 16 and its pressure is reduced in turbine 4, wherefrom it is fed through conduit 17 back to column 8 for being heat exchanged before the vent gas 18 is finally heated in the heat exchanger 7 and discharged from the cryogenic unit through conduit 3.
Example 1
This example shows final purification of synthesis gas according to a conventional process as shown in Fig. 1 to which reference here is made. Pre-purified gas 1 is fed to the cryogenic unit from which purified gas 2 leaves. The composition of the various gas streams is stated in Kmol/h.
Comp. Incoming Vent Purified % gas (1) gas (3) gas (2) Removed
H2 5359.1 60.8 5298.3 1.14
N2 2725.6 955.2 1770.4 35.05 Ar 39.9 27.0 12.9 67.9
CHz 197.7 196.6 1.14 99.49
The H2/N2 ratio of purified synthesis gas (2) was 2.9977.
The distillation column 5 was run at 24.8 bar and the incoming gas 1 had a pressure of 27.8 bar being reduced to 24.8 bar over the turbine 4. The vent gas leaving the distillation column 5 was reduced to 3 bar over the reduction valve 6. The total pressure drop through the cryogenic unit was 3.4 bar, and this corresponds to approximately 0.7 MW.
Example 2
This example shows a process according to the invention as shown in Fig. 2. Incoming gas (1), vent gas (3) and purified gas (2) had the following composition in Kmol/h:
Comp. Incoming Vent Purified gas (1) gas (3) gas (2) Removed
K2 5359.1 70.1 5289.0 1.31
N2 2725.6 957.4 1768.2 35.13 Ar 39.9 25.1 14.8 62.8
CHz 197.7 196.9 0.80 99.58
The H /N2 ratio of purified synthesis gas (2) was 2.9916, Incoming gas (1) had a pressure of 27.5 bar, and the distillation column 5 was run at 27 bar. The pressure in the vent gas (3) leaving the distillation column 5 was reduced from 27 bar to 3.6 bar of the reduction valve 6 and further down to 2 bar over the turbine 4.
The pressure drop in the cryogenic unit was 0.5 bar.
Example 3
This example shows a process according to the invention compris¬ ing recovery of CH as shown in Fig. 3.
The composition in Kmol/h for the various gas streams was:
Figure imgf000008_0001
The H /N ratio of purified synthesis gas (2) was 2.9937.
96.76% of the CH4 was recovered from the vent gas.
As can be seen from the examples, the effect with regard to removal of CH4, Ar and excess N2 from the synthesis gas is substantially the same for the conventional process and the process according to the invention. However, the invention also includes recovery of more than 96% of the CH4 content of the pre- purified gas as shown in Example 3. Also a substantial amount of the hydrogen content in the vent gas can be removed by means of minor investments as indicated for the process according to Fig. 3. The present invention thus gives a process that maintains the advantages of the known process, and in addition to that a substantial recovery of energy is obtained. By choosing the presently defined process for the cryogenic unit, the pressure drop is reduced substantially, representing savings of at least 0.7 MW having a value of approximately 2 mill. NOK/year. Recovery of for instance 187 Kmol CH4/h (Example 3), i.e. about 24000 ton/year CH4, as fuel gas, corresponds to a value of approxi¬ mately 10 mill. NOK per year. By expanding only a fraction of the gas, and at a lower total pressure in an expansion turbine, reduced turbine costs will be obtained.
The possibility within the same concept to recover CH4 and H2 from the vent gas further improves the economics of the process, which can utilize conventional units like heat exchangers, distillation columns and expanders, but combines these units in a special way and runs these at especially chosen operating conditions.
If for any reason it is found advantageous to allow a higher content of CH4 from the reformer section, the present process is especially advantageous, particularly when practised according to claim 3, i.e. recovery of both CH4 and H2 from the vent gas.
The method according to the present invention is especially advantageous in combination with low pressure ammonia processes where the ammonia synthesis can be performed at substantially the same pressure as the methanation, as no recompression of the synthesis gas is necessary after the final purification step.

Claims

Claims
Method of final purification of ammonia synthesis gas subsequent to pre-purification by removal of CO, C02 and H2O, comprising application of a cryogenic unit for removal of CH4, Ar and excess N2' c h a r a c t e r i z e d i n t h a t the pre-purified gas from a preceding unit is cooled from process temperature in said unit to saturation temperature level and fed directly to a distillation column for separation of the gas into a purified synthesis gas and a vent gas containing the removed
CH4, Ar and excess N2, and that the separation is performed at substantially the same pressure as in the preceding unit and that the vent gas is heated to final vent temperature with intermediate pressure release through a pressure relief valve and at least one expansion turbine and that the purified synthesis gas is heat exchanged with the pre-purified synthesis gas.
Method according to claim 1, c h a r a c t e r i z e d i n t h a t most of the CH4 content of the vent gas is removed in a second distillation column prior to final pressure release in the expansion turbine and the reflux for the second distillation column being provided by partial condensation by heat exchange with the turbine outlet followed by heating this heat ex¬ changed turbine outlet, the vent gas, and the finally purified synthesis gas against the pre-purified synthesis gas entering the process. Method according to claim 1, c h a r a c t e r i z e d i n t h a t both CH4 and H2 are removed from the vent gas and the hydrogen is removed from the vent gas prior to the CH4 removal and removed as a separate process stream to be heat exchanged with the pre-purified gas and then mixed therewith ahead of said heat exchanging step.
PCT/NO1992/000015 1991-01-23 1992-01-22 Method for purification of synthesis gas WO1992012927A1 (en)

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NO910272A NO171966C (en) 1991-01-23 1991-01-23 PROCEDURE FOR PURIFICATION OF SYNTHESIC GAS FOR AMMONIA PRODUCTION
NO910272 1991-01-23

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US6620399B1 (en) 2000-04-10 2003-09-16 Robert Rudolf Josef Jungerhans Purification of gases, in synthesis gas production process
CN101102965B (en) * 2004-07-29 2012-05-30 弗劳尔科技公司 Improved ammonia plant
CN103641069A (en) * 2013-11-25 2014-03-19 张周卫 Low-temperature flash evaporation gas-liquid separator for waste nitrogen
WO2018057299A1 (en) * 2016-09-21 2018-03-29 Praxair Technology, Inc. System and method for cryogenic purification of a feed stream comprising hydrogen, methane, nitrogen and argon
US10088229B2 (en) 2016-09-21 2018-10-02 Praxair Technology, Inc. System and method for cryogenic purification of a feed stream comprising hydrogen, methane, nitrogen and argon
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US6620399B1 (en) 2000-04-10 2003-09-16 Robert Rudolf Josef Jungerhans Purification of gases, in synthesis gas production process
CN101102965B (en) * 2004-07-29 2012-05-30 弗劳尔科技公司 Improved ammonia plant
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CN103641069B (en) * 2013-11-25 2015-07-01 张周卫 Low-temperature flash evaporation gas-liquid separator for waste nitrogen
WO2018057299A1 (en) * 2016-09-21 2018-03-29 Praxair Technology, Inc. System and method for cryogenic purification of a feed stream comprising hydrogen, methane, nitrogen and argon
US10024595B2 (en) 2016-09-21 2018-07-17 Praxair Technology, Inc. System and method for cryogenic purification of a feed stream comprising hydrogen, methane, nitrogen and argon
US10088229B2 (en) 2016-09-21 2018-10-02 Praxair Technology, Inc. System and method for cryogenic purification of a feed stream comprising hydrogen, methane, nitrogen and argon
US10295251B2 (en) 2016-09-21 2019-05-21 Praxair Technology, Inc. System and method for cryogenic purification of a feed stream comprising hydrogen, methane, nitrogen and argon
EP3640570A3 (en) * 2016-09-21 2020-07-29 Praxair Technology, Inc. Method for cryogenic purification of a feed stream comprising hydrogen, methane, nitrogen and argon

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NO171966C (en) 1993-05-26
NO910272D0 (en) 1991-01-23
NO171966B (en) 1993-02-15
NO910272L (en) 1992-07-24
AU1187192A (en) 1992-08-27

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