OEI LARS ERIK (NO)
| SE449740B | 1987-05-18 | |||
| DE1592352A1 | 1970-12-17 | |||
| FR2418770A1 | 1979-09-28 | |||
| US4549890A | 1985-10-29 | |||
| DE1792285A1 | 1971-10-28 | |||
| EP0119001A2 | 1984-09-19 |
| 1. | Method of final purification of ammonia synthesis gas subsequent to prepurification 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 prepurified 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 prepurified 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 prepurified 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 prepurified gas and then mixed therewith ahead of said heat exchanging step. |
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•
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 CH 4 , Ar and simultaneously removal of excess N 2 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, 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.
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 H 2 /N 2 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 CH 4 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 CH 4 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 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. 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 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.
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
H 2 5359.1 60.8 5298.3 1.14
N 2 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
N 2 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 /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.
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:
The H /N ratio of purified synthesis gas (2) was 2.9937.
96.76% of the CH 4 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 CH 4 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 CH 4 /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 CH 4 and H 2 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 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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