FR2913468A1 - Gas mixture compressing method for e.g. steam reforming unit, involves drying gas mixture before compression to eliminate water molecules and obtain dry mix for specific value, where one pressure lies between other two pressures - Google Patents

Abstract

The method involves compressing a gas mixture (1) at low pressure (PL) to high pressure (PH) by a compression unit (32). Pressure difference is such that compression of the mixture of pressure (P(i-1)) with pressure (Pi) in successive compression steps (Ei), where i varies from 1 to N, N is greater than 1, PO=PL and PN=PH. The mixture attains its dew point for a pressure (PR) which lies between the pressures (P1, PH). The mixture is dried before compression to eliminate water molecules and obtain dry mix by a dryer (34) for i, where the pressure (PR) lies between the pressures (P(i-1), Pi). The gas mixture contains carbon-di-oxide and water. An independent claim is also included for a device for compressing a gas mixture available at a low pressure to high pressure.

Description

The present invention relates to a method for compressing a gas

  containing at least carbon dioxide and water vapor. Many industries produce gaseous mixtures containing carbon dioxide (CO2) and water as by-products. Such mixtures are notably derived from synthesis gas generation units such as steam reforming units (SMR) or partial oxidation units (PDX), but this also concerns waste gases (off gas) of iron and steel industry for example. In general, these mixtures are available at low pressure, so it is often necessary to compress them, whether for storing or using carbon dioxide, or for various other uses. When it is desired to compress a wet gas containing 002, it is necessary to take into account the problem of corrosion related to the simultaneous presence of 002 and water which, under particular conditions of pressure and temperature, then that the dew point (dew point in English) will be reached, generate carbonic acid which will corrode the oxidizable elements in contact with the mixture. The solution usually adopted to eliminate the risk of corrosion of the material in contact with the corrosive fluid is to use a compression machine whose parts in contact with the fluid to be compressed must be provided in stainless steel to prevent corrosion due to the presence of wet carbon dioxide. In the case of a multistage compression machine, it is thus in particular the mechanical parts of the compression stages in contact with the gas to be compressed under conditions (of pressure and temperature) such that it is likely to condense which must be provided in stainless steel. Another solution is to dry the gas to be compressed upstream of the compression machine, or upstream (or during) of the gas-generating process containing 002 to be compressed. The mechanical parts of the compression machine in contact with the gas can then be made of carbon steel. However, both of the above solutions involve costly investments. In particular, in order to implement the first of these solutions, it will be necessary to invest in a particularly expensive compression machine because of the presence of the stainless steel parts. The investment in a dryer capable of drying a gas upstream of the compression, as required by the second solution, is lower, but still remains important.

  The object of the present invention is to provide a less expensive solution to the problem of drying a moist gas mixture containing 002 which must be compressed to a pressure much higher than that at which it is available, so that is necessary to provide a multi-stage compression machine.

  The invention then involves installing dryers on the wet gas at one of the inter stages of the compression machine at a point where the wet gas has not reached its dew point. The solution according to the invention thus makes it possible, by judiciously placing the drying step with respect to the compression steps, to use dryers of a size smaller than that of the prior art dryers and to compress only mixtures gas that does not condense during compression. The invention can thus use, for all the compression means that will be used, means whose mechanical parts are carbon steel. For this purpose, the invention relates to a method of compressing a gas mixture, available at a low pressure PL, up to a high pressure PH, a process comprising the steps of providing the mixture to be compressed, and compression of said mixture, and wherein: - the mixture to be compressed contains at least carbon dioxide and water, - the pressure difference PH - PL is such that the process comprises N successive stages Ei, i ranging from 1 to N of compressing the gaseous mixture of the pressure P 1 at the pressure P 1, with N> 1, Po = PL and PN = PH, the mixture to be compressed reaches its dew point for a pressure PR such that such as P1 <PR <PH, characterized in that for i such that P; _1 <PR <P ;, the gas mixture is subjected before the compression step Ei, to a drying step for the removal of the molecules of water and obtaining a dry mix. Preferably, the drying step is carried out immediately before the compression step during which the dew point must be reached; since the dryers are placed immediately at the entrance of the stage in which the gas is likely to condense, this makes it possible to minimize the volume of gas to be considered and the size of the driers while preserving the material from corrosion. The driers may be horizontal or vertical type with gas flow from bottom to top or vice versa.

  In a preferred embodiment, they are regenerated using a fraction of the dried mixture as a regeneration gas. In another embodiment, the dryers are regenerated with dried nitrogen that may be provided by a network.

  One could also use any other type of dry gas available, for example CO2, or H2. According to another object of the invention, this also relates to a device for compressing a gas mixture available at a low pressure PL, up to a high pressure PH comprising: a source of the mixture to be compressed containing at less carbon dioxide and water, - a compression means comprising N successive compression modules Ci for the compression of the gas mixture, with N> 1, - a drying means for the removal of water molecules and obtaining a dry mixture, conduits connecting the source of the mixture to be compressed at the inlet of the compression means, and driving means connected to the inlet and outlet of the drying means, characterized in that that the conduit means connected at one end to the inlet of the drying means is connected at its other end to a compression module Ci-1, and the conduit means connected at one end to the outlet of the drying means is connected by its other extremity the compression module Ci, Ci-1 succeeding module in the direction of increasing pressures, for i such that P; _1 <PR <P ;. A compression module is an assembly comprising (in this order) a compression stage, a cooling means and a liquid separator. The invention is now described with reference to the appended figures 1 to 3 which show in FIGS. 1 and 2 two simplified diagrams of a device for compressing a gas mixture containing at least carbon dioxide and steam. water, according to the prior art, and in Figure 3 a simplified diagram of a device for compressing a gas mixture containing at least carbon dioxide and water vapor according to the invention. In these 3 figures, the compression devices have 3 stages of compression. The gaseous mixture to be treated is supposed to have a composition such that its dew point is reached during the passage in the third compression module. The simplified diagram of FIG. 1 essentially presents a compression device according to the prior art comprising a feed in one low pressure gas mixture, feed referenced 1, a compression unit 2 comprising three compression modules Ci, each corresponding to a compression step Ei, and referenced 2c1, 2c2, 2c3 for the production of a compressed gas 3, and a drying unit 4. The simplified diagram of FIG. 2 essentially presents another compression device according to the prior art comprising a supply of the low-pressure gas mixture referenced 1, a dryer 21, a compression unit 22 containing three modules of compression Ci referenced 22cl, 22C2, 22c3 for the production of a dry compressed gas 23. The simplified diagram of the FIG. 3 essentially presents a supply of the low-pressure gas mixture referenced 1, a compression unit 32 containing three compression modules Ci referenced 32c1, 32c2, 32c3 for the production of a compressed gas 33 and a dryer 34 arranged between the stages 32C2 and 32C3 of the compression unit. The device of Figure 1 operates in the following manner. The device is supplied with 1 in a low pressure wet gas mixture containing CO2 and water. The composition of the mixture is such that the dew point of the mixture is reached during the passage through the compression module 2C3. The wet gas mixture successively passes through the modules 2c1, 2C2 and 2C3 constituting the compression unit 2. compression, the mixture reaches its dew point; in order to avoid the corrosion of the various pieces of equipment in contact with the acid mixture CO2 + H2O, these different parts are designed in stainless steel, that is to say at least the 2C3 module. . At the outlet of the module 2C3, a moist compressed gas 3 containing CO 2 is recovered as well as water vapor, which is generally saturated, which is subsequently removed in the dryer 4. The device of FIG. the following way. The device is supplied with 1 in a low pressure wet gas mixture containing CO2 and water. The wet gaseous mixture passes into dryers 21 where it is stripped of its water to produce a gaseous mixture of dry low pressure containing CO2 which passes successively through the modules 22c1, 22c2, 22c3 constituting the compression unit 22. this compression, the mixture is dry, the various pieces of equipment intended to be in contact with the mixture are therefore made of carbon steel. At the outlet of the module 22C3, a compressed and dry gas 23 containing CO 2 optionally accompanied by traces of H2, CO, CH4, Ar and / or N2 is recovered. The device of FIG. 3 operates in the following manner.

  The device is supplied with 1 in a low pressure wet gas mixture containing CO2 and water. The wet gas mixture feeds the compression unit 32, where it is compressed successively in the modules 32c1, 32C2. Given its composition, the mixture does not reach its dew point during these first two compressions; the parts of the first two stages in contact with the compression mixture containing the water vapor are therefore carbon steel. The partially compressed mixture from the module 32C2 passes into the dryers 34. The volume of gas to be dried being lower due to compression, the size of the driers 34 is smaller than that of the prior art device dryers (FIG. 2). The dry and partially compressed mixture exiting the driers 34 then passes into the compression module 32C3. During this compression, the mixture being dry, the different pieces of equipment of the compression module 32C3 intended to be in contact with the mixture are also made of carbon steel. At the outlet of the module 32C3, a compressed and dry gas 33 containing CO2 is recovered, possibly accompanied by traces of H2, CO, CH4, Ar, and / or N2.

  The compression process according to the invention finds its application for the compression of gaseous mixtures of different origins. The process of the invention is thus particularly suitable for the compression of waste gas of adsorption unit with pressure modulation (PSA). as well as the permeate compression of a membrane separation unit in particular.

  The method of the invention is particularly advantageous when it is desired, for example, to upgrade a waste gas from a hydrogen separation unit of the pressure swing adsorption type (PSA). The CO2 content of said waste is typically of the order of 30 to 50%. The residual PSA being available at a pressure of the order of 1.3 bar absolute, it must be compressed for recovery. Thus, when it is desired to extract by liquefaction the carbon dioxide contained in the waste, it is necessary to compress it to a pressure of the order of 25 to 60 bar absolute. This is achieved by using 4 to 6 compression modules with drying at one of the inter stages.

  When it is desired to upgrade the PSA waste by recycling it to one or more points of the synthesis gas generating unit supplying the separation unit, or upstream of the PSA unit itself, it will be in this case too, it is necessary to compress the waste gas. Compression up to a pressure of the order of 25 to 45 bar absolute in this case then requires 4 to 5 stages of compression with drying at one of the inter stages. The method of the invention allows in the same way to enhance the permeate of a membrane by recycling it to one or more points of a synthesis gas generation unit; available at a pressure of between 1 and 25 bar absolute, the permeate must be compressed to a pressure of the order of 25 to 45 bar absolute; this compression also requires 4 to 5 compression stages with drying at one of the inter stages. The process of the invention is also particularly suitable when it is desired to compress wet 002 extracted from a natural source in order to liquefy it or to recover it in a synthesis gas generation unit.

Claims (5)

  1. A method of compressing a mixture of available gas at a low pressure PL, up to a high pressure PH, comprising the steps of providing the mixture to be compressed, and compressing said mixture, and wherein: - the mixture to be compressed contains at least carbon dioxide and water, - the pressure difference PH - PL is such that the process comprises N successive stages Ei, i varying from 1 to N, compressing the gaseous mixture of the pressure At the pressure P 1, with N> 1, Po = PL and PN = PH, the mixture to be compressed reaches its dew point for a pressure PR such that P1 <PR <PH, characterized in that for i such that P; _1 <PR <P ;, the gas mixture is subjected before the compression step Ei, to a drying step for the removal of water molecules and obtaining a dry mixture.   2. Method according to claim 1 wherein the drying step is carried out immediately before the compression step during which the dew point must be reached.   3. Method according to one of claims 1 or 2 wherein the driers are regenerated using a fraction of the dried mixture as a regeneration gas. Process according to one of claims 1 or 2 wherein the driers are regenerated using dried nitrogen from a network. 5. Device for compressing a mixture of available gas at a low pressure PL, up to a high pressure PH comprising: a source of the mixture to be compressed containing at least carbon dioxide and water; compression means comprising N successive compression modules Ci for the compression of the gas mixture, with N> 1, - a drying means for the removal of the water molecules and obtaining a dry mixture, - ducts connecting the source of the mixture to be compressed at the inlet of the compression means, as well as the driving means connected to the inlet and the outlet of the drying means, characterized in that the driving means connected at one end to the The inlet of the drying means is connected at its other end to a compression module Ci-1, and the conduit means connected at one end to the outlet of the drying means is connected by its other end to the compression module Ci, succeeding the module Ci-1 in the direction of the increasing pressures, for i such that P; _1 <PR <P ;.