GB2150855A

GB2150855A - Acid gas removal with power recovery

Info

Publication number: GB2150855A
Application number: GB08327728A
Authority: GB
Inventors: Christopher Leslie Winter
Original assignee: Humphreys and Glasgow Ltd
Current assignee: Humphreys and Glasgow Ltd
Priority date: 1983-10-17
Filing date: 1983-10-17
Publication date: 1985-07-10
Also published as: GB8327728D0

Abstract

A process for retaining a significant part of the partial pressure of acid gas after stripping it from a mixture of gases by utilising a stripping gas at a total pressure greater than 50% of the original partial pressure of the acid gas.

Description

SPECIFICATION Acid gas removal with power recovery Present state of the art processes for removing acid gases may be divided into two categories; physical solvent and thermal regeneration. This application is concerned with physical solvent removal processes.

In physical solvent systems a feed gas containing the acid gas to be removed is contacted in an absorber with the physical solvent into which the acid dissolves. The solution containing the dissolved acid gas is removed from the bottom of the absorber, let down in pressure in one or more stages, sometimes with power recovery and sometimes recycling flashed gas streams to the feed, and then passed to a stripper wherein the residual acid gas is stripped out of the physical solvent.

Some designs are quite complex, such as those wherein acid gases are separated from each other.

(E.g. the separation of H2S from CO2).

Normally the evolution of the acid gas from the physical solvent takes place at a low pressure, sometimes utilising a small amount of stripping gas.

It would often be an advantage if the acid gas could retain most of its partial pressure and this invention describes a process and a device whereby this advantage may be obtained. The advantage is that if the acid gas is required at pressure, for instance to expand it to produce cold or power, the pressure of the separated acid gas is maximised.

In this invention, the acid gas is removed from the feed in the manner described above and then the physical solvent may be changed to another pressure. The new pressure would be equal to the sum of a pressure which is equal to the partial pressure of the acid gas in the feed stream less the approaches to equilibrium in the stripper and absorber and any intervening equipment, and the partial pressure of a stripping gas.

The new pressure may be above or below the absorber pressure. The stripping gas enters the bottom of the stripper. The higher the stripping pressure, the greater the flow of stripping gas required.

By such means the flow of stripping gas can be augmented by the flow of acid gas at the pressure of the stripping gas which may be above the partial pressure of the stripping gases.

The acid gas removed by the stripping gas may, if desired, be heated and expanded to gain power and, in certain circumstances, refrigeration, both of which are very useful.

Nitrogen obtained at pressure from an air separation unit or purge streams such as those obtained from methanol or ammonia synthesis loops can be used as stripping gases. Alternatively air may be used as the stripping gas, particularly as the combined airlacid gas may then be fed to a gas turbine wherein fuel is added to the stream and the combusted stream expanded to gain power.

A further advantage of this system is that the difference in pressure between the absorber and stripper may be considerably less than that normally obtaining in present state of the art and this means that the power required by the solvent pumps pumping the lean liquor from the stripper pressure to the absorber pressure may be considerably reduced. This in turn means that the temperature rise across the pumps is reduced 8 an advantage when dealing with low temperature physical solvent removal processes in that the amount of refrigeration required to maintain the low temperature is reduced.

Accordingly this invention seeks to provide a method of removing an acid gas from a feed stream containing the same wherein the partial pressure of substantially all the separated acid gas is significantly higher than conventional means would obtain from the said stream, and wherein the total pressure of the stream containing the separated acid gas is at least 50% of the partial pressure of the acid gas in the feed stream.

Figure 1 shows the process. The feed stream (1 ) containing the acid gas passes into an absorber (2) at the top of which the stream of lean physical solvent (3) enters and by means of contact devices picks up the acid gas from the feed. Sweet gas (4) leaves the top of the absorber and a rich solvent stream (5) leaves the bottom of the absorber. The rich solvent stream is then shown passing through a liquid turbine (6) although this is optional and it is possible that the pressure of the physical solvent may even be increased, alternatively it may also pass through a number of pressure let down stages, not shown, from some of which the flashed gas may be re-circulated to the feed or used elsewhere according to the process needs.Ultimately the rich gas stream (7) passes to the top of a stripper (8) which, as stated above, may operate at a pressure above or below the pressure of the absorber. Entering the bottom of the stripper is the stripping gas (9) which strips the acid gas from the incoming rich solution, usually using contact devices, and leaves the top of the stripper (8) as stream (10) which may than be heated or cooled and expanded according to the needs of the process.

Figure 11 shows a flowsheet incorporating the invention into a typical ammonia plant although of course air separation units (14) are presently more associated with partial oxidation needs. The invention may also be incorporated into methanol plants wherein the invention would be used to remove carbon dioxide surplus to the requirements of the process. Figure 2 shows natural gas feed (20) being fed to a reformer (11) and thence into a secondary reformer (12) wherein it joins with oxygen nitrogen stream (13) produced in a cheap air separation unit (14). The stream leaving the secondary reformer (15) may be shifted (16) and then may be compressed (17) before being fed to the absorber itself which is the absorber (2) referred to in the first part of this specification. Also shown on Figure ll is means for heating the stream (10) that is the stripping gas plus the acid gas removed and a gas turbine (18) for extracting power from the hot gases. The substantially acid-gas-free stream (19) may be further purified by e.g. methanation, or cryogenic means before passing to an ammonia synthesis loop.

Claims

1. A process for recovering an acid gas from a stream containing the same having the following characteristics: a. absorption of the acid gas by means of physical solvent b. removal of the acid gas-carrying physical solvent from contact with the original acid gascontaining stream c. using a stripping gas desorbtion of the acid gas at a pressure which is greater than 50% of the partial pressure of the acid gas in the original gas stream.

2. A process as claimed in claim 1 wherein the stripping gas is substantially nitrogen from an air separation unit, or the purge stream from a methanol or ammonia synthesis loop or air.

3. Products resulting from processes using the processes claimed in claims 1 and 2.