JPH0730998B2 - Method for recovering argon from ammonia synthesis purge gas - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for separating and recovering argon contained in an ammonia synthesis purge gas by a cryogenic separation method.

“Prior Art” Conventionally, in an ammonia synthesis reaction using a steam reforming gas of naphtha as a raw material, undecomposed methane and argon contained in the raw material air are accumulated in the synthesis system to hinder the reaction. Is released to the outside of the system as a purge gas together with unreacted hydrogen.

By the way, the above-mentioned purge gas usually contains hydrogen, nitrogen, argon, and methane. As a method for recovering useful argon from such a purge gas, there is a cryogenic separation method as shown in FIG. 2 (flow chart). Are known.

In order to recover argon by this cryogenic separation method, first, a raw material gas A containing hydrogen, nitrogen, argon, and methane is sequentially introduced into the first heat exchanger 1 and the second heat exchanger 2, and the inside of these exchangers is introduced. The raw material gas A is cooled to a predetermined temperature by exchanging heat with a separated product gas, which will be described later, to liquefy nitrogen, argon and methane in the raw material gas A, and the partially liquefied raw material gas. The gas-liquid separator 3 separates A into a mixed liquid B obtained by liquefying the above components and a hydrogen gas C. Next, the mixed solution B is introduced into the flash tank 4 and decompressed to separate the flash gas D composed of a minute amount of hydrogen gas or the like dissolved in the mixed solution B, and the flash gas D from a line L ₂ to be described later is separated. It joins with the effluent of the methane tower and is taken out as fuel gas after heating. Then, the mixed liquid B derived from the bottom of the flash tank 4 is introduced into the methane tower 5,
By heating and rectifying, the bottoms E containing methane having a high boiling point as a main component and the distillate F containing nitrogen and argon as main components
And separate. Here, a reboiler 6 is provided at the bottom of the methane tower 5, and the reboiler 6 is heated by high-pressure nitrogen sent from a high-pressure circulating compressor described later through a line L ₁ to refine the mixed solution B. Stay. Also,
A condenser 7 is provided at the top of the methane tower 5, and the condenser 7 cools the components vaporized by the reboiler 6 in the mixed solution B to condense a part of the components and condense the bottom of the methane tower 5. It has been refluxed. The bottom liquid E above is a line L ₂ from the bottom of the methane tower 5.
After passing through the first heat exchanger 1 to exchange heat with the raw material gas A, it is discharged to the outside of the system to become fuel gas.

Then, the distillate F having passed through the condenser 7 is introduced into the argon tower 8 and rectified to separate and collect argon. Here, a reboiler 9 is provided at the bottom of the argon tower 8, and this reboiler 9 uses the low-pressure nitrogen or the like sent from a low-pressure circulating nitrogen compressor described later via a line L ₃ as a heat source to generate the distillate F. Is rectifying. The high-purity liquefied argon G rectified and separated in the argon tower 8 is discharged from the argon tower 8 to be a product. On the other hand, the nitrogen gas H separated from argon and containing a trace amount of hydrogen, argon, etc. is discharged from the top of the argon tower 8, and then the subcooler 10, the circulating second heat exchanger 11 and the circulating first heat exchanger are used. 12 are sequentially introduced, and the temperature is raised by exchanging heat with each. Further, this nitrogen gas G is introduced into the low pressure circulation compressor 13 and compressed to 5 to 7 kg / cm ² G,
Part H ₁ is circulated again via line L ₄ First heat exchanger 12
Is introduced to heat exchange with the above-mentioned nitrogen gas H to lower the temperature. Then, a part H ₁ of the above-mentioned nitrogen gas H is further branched, and a part H ₁₁ thereof passes through the line L ₅ and the expansion turbine 1
It is introduced into 4 to expand and reduce the pressure to lower the temperature, and the expansion turbine 14 is used for the refrigeration necessary for cooling the argon recovery system shown by the alternate long and short dash line in FIG. The remaining H ₁₂ of the gas H ₁ that has passed through the line L ₄ of the nitrogen gas H excluding the above H ₁₁ is again introduced into the second circulation heat exchanger 11 to be heat-exchanged to lower the temperature, and then the line L ₃ It is introduced into the reboiler 9 of the argon tower 8 via the and becomes a heat source of the reboiler 9.
On the other hand, the nitrogen gas H discharged from the low-pressure circulation compressor 13
Of the above, the remaining H ₂ excluding the above H ₁ is the high pressure circulating nitrogen compressor.
After being introduced into 15 and pressurized to 24-35 Kg / cm ² G, it is introduced into the first circulation heat exchanger 12 and the temperature is lowered in the same manner as described above. Then, this high-pressure nitrogen H ₂ is passed through the line L ₁ to the methane column 5
The reboiler 6 is introduced into the reboiler 6 to serve as a heating source for the reboiler 6, and heat is exchanged there to completely liquefy itself. Further, this liquefied nitrogen H ₂ is decompressed and introduced into the reboiler 9 of the argon tower 8, where it joins the above-mentioned nitrogen H ₁₂ to become nitrogen H ₃ , which is heat-exchanged in the reboiler 9. After this nitrogen H ₃ is discharged from the reboiler 9, a part H _{31 of the} nitrogen H ₃ is introduced into the argon column 8 through the supercooler 10 and becomes a reflux liquid. Another part of H ₃₂ is the methane tower 5
Is introduced into the condenser 7 as a refrigerant. Further, the remaining H ₃₃ of the liquefied nitrogen H ₃ is introduced into the second heat exchanger 2 to adjust the temperature distribution of the heat exchanger 2, and then the first heat exchanger 1
After being introduced into the system and undergoing heat exchange, it is discharged outside the system.

"Problems to be Solved by the Invention" By the way, in the above-mentioned argon recovery method, a compressor for pressurizing the treatment fluid according to the operating conditions of the apparatus is used. The power cost due to use occupies most of the power cost of the entire equipment, and this hinders the cost reduction of the recovered argon, so that the cost of the recovered argon cannot be easily reduced.

[Means for Solving Problems] Therefore, in the argon recovery method of the present invention, the collection of bottoms from the bottom of the methane column is changed to gas collection to form a methane cycle, and the cycle methane is converted to the same methane column. In the column bottom mixture in, the gas obtained from the flash tank was added to a part of the bottom gas used as this heating source to form a mixture, and this mixture was nitrogen distillate from the top of the argon column. Is heated and compressed to lower the temperature and then subjected to heat exchange with a low-pressure circulating nitrogen gas used as a heat source and a reflux liquid for the argon column reboiler and a refrigerant for the methane column condenser, or a nitrogen gas obtained from the argon column in this mixture. Of at least one of the hydrogen gas obtained from the gas-liquid separator and subjecting this to heat exchange with the above-mentioned low-pressure circulating nitrogen gas, high-pressure circulating nitrogen The use of compressors was discontinued and this solved the above problems.

[Examples] Hereinafter, the method for recovering argon according to the present invention will be described in detail with reference to the drawings.

FIG. 1 is a flow chart for explaining an example of the argon recovery method of the present invention. In this figure, the same components as those shown in FIG. 2 are designated by the same reference numerals, and the description thereof will be omitted. The difference between the method for recovering argon shown in FIG. 1 and the method shown in FIG. 2 is that the bottom product E from the methane tower 5 is changed from liquid sampling to gas sampling to form a methane gas cycle. Cycle methane to the same methane tower 5
Used as a heating source for the reboiler 6 of the flash tank, and the flash tank 4 for the cycle methane E provided as the heating source.
The point is that flash gas D or the like discharged from the tank is mixed and used as the refrigerant of the circulation second heat exchanger 11. The reason why the methane column bottoms is taken from the liquid instead of the gas is that the heat exchange temperature difference between the first heat exchanger 1 and the circulating second heat exchanger 11 cannot be obtained only by constructing the methane cycle.

In order to recover argon by the method shown in FIG. 1, the raw material gas A is sequentially introduced into the first heat exchanger 1 and the second heat exchanger 2 as in the conventional method shown in FIG. The hydrogen gas C and the flash gas D are separated and removed by sequentially introducing them into the liquid separator 3 and the flash tank 4. Next, the mixed liquid B derived from the flash tank 4 is introduced into the methane column 5, and rectified to separate into a bottom fraction E and a bottom fraction F. Then, the bottom fraction E (generally 99% or more of methane and 1% or less of argon) derived from the methane tower 5 is withdrawn in the form of gas, introduced into the first heat exchanger 1 through a line L ₁₀ , and heated and vaporized. It is introduced into the methane compressor 16. Here, the reboiler 6 for heating the mixed liquid B in the methane tower 5 to generate rising gas has a methane compressor 16 of about 1.0 kg / cm as a heating medium.
Circulating ethane I compressed to ² G is used. This circulated methane I is the above-mentioned bottom gas (bottom fraction) E introduced into the methane compressor 16 and compressed, and is introduced into the first heat exchanger 1 through line L ₁₁ and cooled. Then, it is introduced into the reboiler 6. In this case, the above circulating methane I is
It is cooled to a temperature close to the temperature at which it condenses in the first heat exchanger 1 (about −153 ° C.), and when it is introduced into the reboiler 6 and heat-exchanged, it condenses to become completely liquefied methane. In the methane column, 99% or more of the liquid is distilled out at the bottom by rectification, and conventionally, medium pressure nitrogen was used to boil this. In this case, a temperature difference of 2 ° C. was added to the saturation temperature of −157 ° C. at 0.5 Kg / cm ² G of methane to give −155 ° C., and the pressure of the cycle nitrogen gas to be condensed required a minimum of 23 Kg / cm ² G. On the other hand, methane has a saturation pressure of 0.7 Kg / cm ² G at -155 ° C.
Therefore, the heat exchange in the methane tower 5 is performed at the above pressure of 1K.
The latent heat exchange consisting of the condensation of the circulating methane I of g / cm ² G and the partial vaporization of the gas-liquid mixture B is carried out, whereby the rectification operation in the methane column 5 is carried out without any trouble. Furthermore, in this case, the latent heat lost when the high-pressure nitrogen is condensed in the conventional method is 20 to ³⁰ kcal / Nm ³ , whereas this circulating methane I is 84 kcal / Nm ³ , so the gas-liquid mixture is very efficient. B can be heated, and thus the amount of heat medium introduced into the reboiler 6 can be greatly reduced. Further, the methane compressor 16 described above has a suction pressure of about 0.
1Kg / cm ² G, and the like be operated at a discharge pressure of about 1.0Kg / cm ² G, and has a significantly low in power requirements as compared to the high pressure circulation nitrogen compressor used in the conventional method.

The circulating methane I derived from the reboiler 6 branches in two directions, part of which is line L ₁₂ and line L via valve V _1.
It reaches ₁₀ and is mixed and mixed with the above gaseous bottom fraction E. Based on the above configuration, the circulating methane I is methane compressor 16, line L ₁₁ , reboiler 6, line L ₁₂ , valve
A methane cycle consisting of V ₁ and line L ₁₀ is circulated to repeat heat exchange or expansion / compression. Here, the valve V ₁ adjusts the circulation amount of the methane cycle by determining the inflow amount of the circulation methane I derived from the reboiler 6 into the methane cycle.

The branched remaining portion of the circulating methane I derived from the reboiler 6 joins and is mixed with the flash gas D discharged from the flash tank 4 in the line L ₁₃ to become the refrigerant J,
After being sequentially introduced into the second circulation heat exchanger 11 and the first heat exchanger 1 for heat exchange, they are discharged as a fuel gas to the outside of the system. In this case, by mixing the flash gas D into the circulating methane I, the partial pressure of methane of the mixed refrigerant J is lowered, so that the vaporization temperature of the refrigerant J is lowered. Further, the hydrogen gas C discharged from the gas-liquid separator 3 may be mixed into the refrigerant J through the valve V ₂ , and in that case also the refrigerant J
The vaporization temperature of can be lowered.

After that, the overhead fraction F of the methane tower 5 is introduced into the middle part of the argon tower 8 and rectified to obtain highly pure liquefied argon G.
Is separated, and this is discharged from the bottom of the argon column 8 through the line L ₁₄ to the outside of the system and recovered. On the other hand, the nitrogen gas H separated from argon is sequentially introduced into the subcooler 10, the second circulation heat exchanger 11 and the first circulation heat exchanger 12 through the line L ₁₅ to raise the temperature and further circulate at a low pressure. Pressure 5 introduced into compressor 13
Compressed to ~ 9Kg / cm ² G. Here, a part of this nitrogen gas H is caused to flow into the line L ₁₃ via the valve V ₃ to generate the refrigerant J.
May be mixed with the refrigerant J, and in that case, the refrigerant J
The vaporization temperature of can be lowered.

The nitrogen gas H compressed by the low-pressure circulation compressor 13 passes through the circulation first heat exchanger 12 again to lower the temperature, then a part thereof is introduced into the expansion turbine 14, and the rest circulates through the line L _16. It passes through the second heat exchanger 11, is decompressed by the valve V _4, and is introduced into the reboiler 9 of the argon tower 8. Further, the nitrogen H introduced into the reboiler 9 is heat-exchanged and completely liquefied, and then heat-exchanged in the subcooler 10 as in the conventional method shown in FIG. It is used as the refrigerant of No. 7 and the reflux liquid of the argon tower 8 or as the refrigerant of the first heat exchanger 1 and the second heat exchanger 2, and is discharged out of the system.

In addition, as described above, the refrigerant J obtained by adding the flash gas D from the flash tank 4 to the bottom gas E from the methane tower 5 is the hydrogen gas C from the gas-liquid separator 3 and the nitrogen from the argon tower 8 as described above. One or both of the gases H can be added, and their selection is appropriately determined depending on the operating state of the argon recovery system or the recovery amount (rate) of the recovered gas.

Since methane was used as the methane tower reboil source in the method of the present invention, it is necessary to generate liquefied nitrogen to be sent to the methane tower condenser 7 at a place other than the reboiler. Therefore, the methane-rich mixed refrigerant J and the low pressure circulating nitrogen are circulated. Liquefied nitrogen was obtained by exchanging heat with the second heat exchanger 11. Therefore, maintaining the temperature difference required for heat exchange between the liquid evaporation temperature of the methane-rich mixed liquid and the liquefaction temperature of the circulating low-pressure nitrogen is an important point in process operation. That is, if the pressure of the circulating low-pressure nitrogen is kept low, it is necessary to increase the mixing of hydrogen gas from the valve V ₂ or the mixing of nitrogen gas from the valve V _{3 in} order to lower the evaporation temperature of the methane-rich refrigerant liquid. On the other hand, if the evaporation temperature of the methane-rich refrigerant liquid is kept constant, it is necessary to raise the pressure of the circulating low-pressure nitrogen. An increase in circulating nitrogen pressure is not preferable because it increases power.
On the contrary, mixing of hydrogen and nitrogen into the cycle methane reduces the yield of each gas. The optimum pressure for circulating low-pressure nitrogen is 8 kg / cm ² G when considering these factors.

In such an argon recovery method, a methane cycle is formed and the bottom gas E from the methane tower 5 is used as a heating source for the bottom mixed liquid B in the methane tower 5, and so on. High-pressure circulating nitrogen compressor with high required power shown in
A methane compressor 16 having a low required power can be used in place of 15, so that the cost of power can be significantly reduced, and thus the cost of recovered argon can be reduced. Also, since the above compressor 15 is not used, this compressor 15
It is possible to prevent interruption of the operation of the argon recovery system due to inspections, etc., and thereby to improve the operation efficiency of this argon recovery system. Furthermore, since the compressor 15 is not used, the facility cost of the argon recovery system can be significantly reduced.

"Effects of the Invention" As described above, the method for recovering argon of the present invention provides the bottom gas from the methane column as a heating source for the mixture in the methane column, and uses the cycle methane of the cycle methane provided as the heating source. Add the gas obtained from the flash tank to a part of the mixture to make a mixture.Then, the nitrogen, which is the distillate of the argon tower, is heated and then compressed to lower the temperature, and the heat source and reflux liquid of the argon tower reboiler and the refrigerant of the methane tower condenser are used. Or at least one of nitrogen gas obtained from an argon column or hydrogen gas obtained from a gas-liquid separator is added to this mixture, and this is mixed with the above-mentioned low-pressure circulating nitrogen gas. By using it for heat exchange, the use of the high-pressure circulating nitrogen compressor in the conventional method has been discontinued. The cost of the argon recovery system can be reduced, the operating efficiency of the argon recovery system can be improved, and the facility cost of the argon recovery system can be significantly reduced. Furthermore, by using cycle methane as the heating source of the methane tower reboiler, the amount of heating can be easily controlled and the operation control of the device can be facilitated.

[Brief description of drawings]

FIG. 1 is a flow chart for explaining the argon recovery method of the present invention, and FIG. 2 is a flow chart for explaining the conventional argon recovery method. 1 ... First heat exchanger, 2 ... Second heat exchanger, 3 ... Gas-liquid separator, 4 ... Flash tank, 5 ... Methane tower, 6 ... Reboiler, 7 ... Condenser, 8 ... … Argon tower, 9 …… Reboiler, 11 …… Circulating second heat exchanger, 12 …… Circulating first heat exchanger, 13 …… Low pressure circulating nitrogen compressor, 14 …… Expansion turbine, 16 …… Methane compressor

Claims (3)

[Claims] 1. A raw material gas containing nitrogen, hydrogen, methane, and argon is cooled and liquefied nitrogen, liquefied methane, and
Separated into a mixed liquid containing liquefied argon as a main component and hydrogen gas, then decompressing the mixed liquid in a flash tank to partially evaporate it, and then introducing the mixed liquid derived from the flash tank into a methane column. Separate the distillate containing nitrogen and argon as the main components and the bottom product containing liquefied methane as the main component, and then introduce the distillate derived from the methane tower into the argon tower to separate and recover the argon. In the recovery method, a bottom fraction is extracted in a gaseous state from the lower part of the methane column, heated, pressurized and then cooled again, and introduced into the methane column reboiler to serve as a heating source. How to recover argon. 2. A raw material gas containing nitrogen, hydrogen, methane and argon is cooled and liquefied nitrogen, liquefied methane,
Separated into a mixed liquid containing liquefied argon as a main component and hydrogen gas, then decompressing the mixed liquid in a flash tank to partially evaporate it, and then introducing the mixed liquid derived from the flash tank into a methane column. Separate the distillate containing nitrogen and argon as the main components and the bottom product containing liquefied methane as the main component, and then introduce the distillate derived from the methane tower into the argon tower to separate and recover the argon. In the recovery method, a bottom fraction is gaseously extracted from the lower part of the methane column, heated, pressurized and then cooled again, introduced into the methane column reboiler and supplied to a heating source, and a liquid derived from the methane column reboiler. And one of them is returned to the temperature raising and pressurizing step through a valve and the other is mixed with the evaporative gas from the flash tank to form a gas-liquid mixed flow, and the nitrogen discharged from the top of the argon column is discharged. A method for recovering argon from an ammonia synthesis purge gas, characterized in that it is heat-exchanged with low-pressure circulating nitrogen. 3. The liquid discharged from the methane tower reboiler is branched, and the vaporized gas from the flash tank is mixed into one of the flows, and a part of the hydrogen gas discharged from the gas-liquid separator or the argon gas is further mixed. At least one of nitrogen gas derived from the tower top is decompressed and added, and the obtained gas-liquid mixed flow is subjected to heat exchange with the low pressure circulating nitrogen to cool the low pressure circulating nitrogen gas. The method for recovering argon from the ammonia synthesis purge gas according to claim 2.