DE1230407B - Process for separating ammonia from the circulating gas of the ammonia synthesis - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

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German class: 12 k -1/04

1230407
B 78225IV a / 12 k
22nd August 1964
December 15, 1966

The pursuit of greater economic efficiency in ammonia synthesis leads to extensive simplifications in the individual procedural stages. The focus is on in particular the compression and purification process of the synthesis gas.

If turbo compressors are used instead of piston compressors, the compression process becomes simplified. In a similar way, the methanation process occurs to simplify the gas cleaning the place of the usual cleaning procedures.

The use of turbo compressors assumes that the last compression stage in With regard to a still sufficient efficiency of the compressor a correspondingly large amount of gas for Available. This requirement leads to unusually large compressor units, apparatus and amounts of synthesis gas. The properties of the turbo compressors are currently at the correspondingly large Gas quantities to final pressures of about 200 atü.

In the methanation process, the inert gas content of the synthesis gas is much higher than in the otherwise usual Cleaning process. This high inert gas content, together with the low pressure (200 atü and including) to relatively large amounts of residual gas taken from the circulating gas of the ammonia synthesis Need to become.

Residual gas always means a loss of valuable synthesis gas. The residual gas is used for heating or generation of steam, electrical or compression energy used for the corresponding process stages in the generation of synthesis gas, there is no loss of gas in terms of quantity, but in the case of the direct expansion, there is the one in the Compression energy contained in residual gas is lost.

It is known that the energy contained in compressed gases in the case of work-performing relaxation in Turbines or piston engines can be used to a large extent to generate cold and to generate electricity To gain energy.

It was the object of the present invention to provide a method for separating ammonia from the Elaborate cycle gas of an ammonia synthesis, with part of the cycle gas withdrawn as residual gas is, the existing pressure energy in the residual gas but also expediently exploited and the process can largely be supplied again. This is achieved according to the invention in that the dried and residual gas largely freed from ammonia in at least one stage under power external work relaxed and after the resulting cooling in heat exchange with at least

Process for the separation of ammonia from the ammonia synthesis cycle gas

Applicant:

Aniline & Soda Factory in Baden

Aktiengesellschaft, Ludwigshafen / Rhein

Named as inventor:

Dr. Walter Koelsch,

Dipl.-Ing. Günther Schulze, Ludwigshafen / Rhine

a part of the precooled cycle gas is brought, one of the cycle gas being used for cooling corresponding amount of ammonia is deposited in liquid form.

Appropriately, the entire cycle gas before cooling with the relaxed residual gas is partly with the all of the cold cycle gas largely freed from ammonia, partly with all of the separated gas liquid ammonia and also in a Veras evaporative cooler with evaporating, separately supplied Ammonia cooled. The cycle gas is either before or after cooling with the relaxed residual gas cooled in the evaporative cooler.

The expanded residual gas is suitably passed through a heat exchanger and brought here by still under synthesis pressure residual gas in heat exchange, wherein it cools this immediately before its drying is not lower than ⁰ ° C, preferably ⁰ +2 C.

With the known ammonia synthesis processes, which work at a pressure of about 350 atmospheres, this is Circulating gas cooled to that formed in the reaction furnace To be able to separate ammonia. One stage of this cooling is generally operated with cooling water. The circulating gas is then mixed with evaporating liquid ammonia in the evaporation cooler cooled further. This vaporized portion of the liquid ammonia goes to the in liquid form the amount of ammonia produced is lost. Should the precipitated ammonia be processed further in liquid form the ammonia evaporated in the evaporative cooler must be by compression and Condensation will be deposited.

According to the method according to the invention, part of the necessary cooling with cold is provided by work expansion of the residual gas in each

609 747/288

Ammonia synthesis with a high inert gas content inevitable accrues, is generated, carried out. To the successive heat exchangers for the respective To be able to perform expansion stages of the residual gas as small as possible, the partial flow of the Circulating gas, which is passed through these heat exchangers, can be kept small. This is because of this achieved that the cold cycle gas this way after separation of the liquid ammonia with the cold Recycle gas from the evaporative cooler is combined and this collected amount of cold recycle gas for pre-cooling behind the water cooler in heat exchange with the remaining part of the cycle gas, which in the Evaporative cooler is cooled with evaporating ammonia, is brought.

In the same way, the two subsets of the precipitated ammonia are combined and likewise for pre-cooling behind the water cooler with the remaining partial flow of the circulating gas in heat exchange brought. The cold from the combined circulating gas stream is just like the cold from the collected liquid ammonia either in the same heat exchanger with several gas paths or in several recovered heat exchangers arranged side by side.

In the drawings, the method according to the invention is illustrated by means of three examples, and although shows

F i g. 1 the division of the circulating gas stream into 3 parts I, II, III,

F i g. 2 the arrangement of the evaporative cooler in front of the expansion turbines and
'F i g. 3 the arrangement of the evaporative cooler behind the expansion turbines.

example 1

^{120,000 Nm 3} / h of synthesis gas at 160 ata are fed into the cycle gas of an ammonia synthesis plant operating in a known manner. The main components of the synthesis gas are hydrogen and nitrogen in a ratio of 3: 1, as well as 1% methane and 0.4% argon. ^{530,000 Nm 3} / h of cycle gas with a content of 12% ammonia, 4.6% methane and 1.8% argon leave the reaction furnace. The cycle gas enters in a conduit 1 into a water-cooled by +8 ⁰ C precooler. 2 and is cooled here. The cycle gas is divided behind the pre-cooler 2! Part (I) is fed to a heat exchanger 5 via a valve 4 and is precooled here with the cold liquid ammonia. The other part of the cycle gas (part II) is fed to a heat exchanger via a valve 6? fed and cooled here with cold cycle gas.

Behind the heat exchangers 5 and 7, the two partial flows I and II of the cycle gas are combined and cooled to -12 ° C. in an evaporative cooler 8. The evaporation cooler 8 is via a line 9 liquid ammonia supplied. The ammonia evaporates in it. The resulting ammonia gas leaves the evaporation cooler 8 via a line 10. In a container 11 is the from the cycle gas Cooling collected liquid ammonia precipitated. It is continuous via a valve 12 deducted. The cycle gas leaves the container 11 via a line 13. It contains 3% ammonia, 5% Methane and 2% argon.

The third and last part (III) of the cycle gas. Will taken from line 3 via a valve 14 and in three heat exchangers 15 arranged one behind the other, 16 and 17 cooled to -12 ° C. The liquid ammonia separated during this cooling is in collected in a container 18 and continuously withdrawn via a valve 19. The cycle gas leaves with a content of 3% ammonia, 5% methane and 2% argon the container 18 via a line 20, is combined with the circulating gas stream (I and II) from line 13 and passes through a line 21 into the heat exchanger 7. Here it cools the incoming cycle gas (part II) and leaves this apparatus via a line 22. It is then circulated back to the ammonia synthesis furnace in a known manner. The liquid ammonia withdrawn via the valves 12 and 19 is combined and passes through a Line 23 to the heat exchanger 5, in which it cools the cycle gas (part I). It leaves this apparatus liquid via a line 24 and is fed to a collecting vessel for liquid ammonia.

26,000 Nm ³ / h of residual gas are withdrawn from line 22 via line 25, freed from ammonia by washing with water and dried in an adsorption system 31 with silica gel. Before entering into the system 31, the residual gas flows through a heat exchanger 30 where it is cooled from the facility under a pressure of 1.5 ata leaving residual gas to +2 ⁰ C. The dried residual gas then enters an expansion turbine 26 under a pressure of 151 ata. It leaves them with a pressure of 30 ata. The in this work-performing development

• Voltage cooled residual gas cools the counter-flowing cycle gas in the heat exchanger 15 (Part III) away. The residual gas is heated up again. It is again expanded to 6 ata in an expansion turbine 27 and then cools the cycle gas ΠΙ in the heat exchanger 16. After heating the residual gas is finally expanded to 1.5 ata in a turbine 28 and to cool the circulating gas fed to the heat exchanger 17, whereupon the part III of the cycle gas which is under synthesis pressure -12 ° C cools down and leaves the heat exchanger on the warm side at + 2 ° C. The relaxed residual gas is through line 29 the heat exchanger 30 and then the power gas network for energy generation forwarded.

With this arrangement, the amount of circulating gas which reaches the heat exchangers 15, 16 and 17 via the valve 14 is adjusted so that the cold generated in the turbines 26, 27 and 28 reduces the circulating gas III at the outlet from the heat exchanger 17 to -12 ° C cools down. The circulating gas quantities I and II are distributed via the valves 4 and 6 in such a way that the amount of liquid ammonia required in the evaporation cooler 8, through the evaporation of which the circulating gas is cooled to -12 ° C., is as small as possible. This amount of ammonia here is 14.21 NH ₃ / h. In this case, 18% of the circulating gas quantities from line 3 pass through valve 14, while the remaining 82% flow through valves 4 and 6.

To determine how big the saving is

■ liquid ammonia in the arrangement according to the invention compared to the known arrangement that works without expansion turbines, the valve 14 was closed and all the cycle gas is off Line 3 passed through valves 4 and 6. The distribution in these valves was again carried out in such a way that that in the evaporative cooler 8, the cycle gas with the smallest amount of liquid ammonia

-12 ° C was cooled. For this, an amount of ammonia of 20.21 NH ₃ / h had to be evaporated in the evaporation cooler 8. Accordingly, under the specified operating conditions, 30% liquid ammonia can be saved if the cold produced during the work-related expansion of the residual gas is used to cool a partial flow of the circulating gas.

Since one fifth (26,000 Nm ³ / h) of the synthesis gas used can be lost as residual gas during operation for reasons of economy, a system that works without the inventive arrangement for utilizing the residual gas for cooling will be much smaller take off residual gas. The associated worsening of the ammonia formation reaction can be seen from the following comparative experiment:

Under the same reaction conditions as in Example 1, only 8,000 Nm ³ / h of residual gas were withdrawn and released directly. Here, the 530,000 Nm ³ / h of cycle gas at the inlet to the cold water cooler only contained 11.4% NH ₃ . _{When they were cooled to -12 ° C., 20.21 NH 3} / h had to be evaporated, just as in the part of the example in which there was no expansion cooling. The amount of ammonia produced decreased to 34.91 NH ₃ / h. Only 101,000 Nm ³ / h of synthesis gas could be processed in the plant. The residual gas contained 3% NH ₃ , 12.5% methane and 4.9% argon.

Two further possible uses of the invention are described with reference to FIGS. 2 and 3:

As in Example 1, the cycle gas occurs both in FIG. 2 as in FIG. 3 via line 1 into the with cooling water fed precooler 2 and leaves it via line 3. Part of the cycle gas (I) passes through Valve 4 in the operated with cold, liquid ammonia heat exchanger 5, while the rest Part II of the cycle gas is passed through valve 6 through heat exchanger 7 and in this from cold Circulating gas is cooled. The two circulating gas flows are behind the heat exchangers 5 and 7 reunited with each other.

In the arrangement according to FIG. 2, the combined cycle gas enters the evaporation cooler 8. This is charged with liquid ammonia under slight excess pressure through line 9. The vaporized ammonia emerges via line 10. The cycle gas is cooled further in the three heat exchangers 15, 16 and 17 arranged one behind the other. The liquid ammonia collects in the container 11; it is withdrawn via valve 12 and passed through line 23 into heat exchanger 5 for precooling part I of the circulating gas. It leaves this through line 24. The _{circulating gas, which has largely been freed of NH 3} , exits the container 11 through line 13, cools part II of the entering circulating gas in the heat exchanger 7 and is circulated back to the synthesis furnace via line 22 in a known manner. The residual gas, washed free of ammonia, enters via line 25 under the pressure of the cycle gas of about 150 ata. It is cooled in heat exchanger 30 by the outgoing cold and relaxing residual gas to +2 ⁰ C and freed in the drying plant 31 in a known manner of water. In the expansion turbine 26, it is expanded from 151 to 30 ata, in the expansion turbine 27 and 30 to 6 ata and in the expansion turbine 28 from 6 to 1.5 ata while performing work. The residual gas cools down in each of these three expansion stages. The cold residual gas

ίο is passed through the heat exchangers 15, 16 and 17 and cools the cycle gas in this. The expanded to 1.5 ata residual gas passes via line 29 into the heat exchanger 30, cooled in this, the residual gas still under synthesis pressure to +2 ⁰ C, and is then fed into the general power network the gas for power generation.

When working according to FIG. 3 is the combined cycle gas downstream of the heat exchangers 5 and 7 in the heat exchangers 15, 16 and 17 by the expanded in the expansion turbines 26, 27 and 28 Residual gas cooled. The cycle gas only then reaches the evaporative cooler 8 and becomes cooled here by the evaporating ammonia. All other operations are in accordance with the working method F i g. 3 the same as in the operation according to FIG. 2.

Claims (4)

Patent claims: 1. A method for separating ammonia from the cycle gas of ammonia synthesis, wherein part of the cycle gas is withdrawn as residual gas, characterized in that the dried and residual gas largely freed from ammonia in at least one stage under power external work relaxes and after the resulting cooling in heat exchange with at least part of the precooled cycle gas is brought, one of the cycle gas Cooling corresponding amount of ammonia is deposited in liquid form. 2. The method according to claim 1, characterized in that the entire cycle gas before cooling with the relaxed residual gas partly with the whole of the ammonia largely freed cold cycle gas, partly with all the separated liquid ammonia and also in one Evaporative cooler is cooled with evaporating, separately supplied ammonia. 3. The method according to claim 2, characterized in that the cycle gas after cooling with the relaxed residual gas in the evaporative cooler is cooled. 4. The method according to claim 1, characterized in that the relaxed residual gas is passed through a heat exchanger and is brought into heat exchange here with the residual gas still under synthesis pressure, this being no lower than O ⁰ C, preferably +2, immediately before it is dried ⁰ C, cools. 1 sheet of drawings 609 747/288 12.66 © Bundesdruckerei Berlin