WO2008152030A1

WO2008152030A1 - Process for the purification of methane containing streams by cooling and extraction

Info

Publication number: WO2008152030A1
Application number: PCT/EP2008/057215
Authority: WO
Inventors: Henricus Abraham Geers; Adriaan Pieter Houtekamer; Rick Van Der Vaart
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2007-06-12
Filing date: 2008-06-10
Publication date: 2008-12-18
Also published as: AU2008263948B2; AU2008263948A1

Abstract

The invention concerns a process for removing gaseous contaminants from a gas stream (201) containing methane and gaseous contaminants, the process comprising cooling the gas stream in two stages (202, 203, 204, 205) to a temperature at which gaseous contaminants are present in the solid or solid and liquid phase while the methane still being present in the gaseous phase, separating solid or solid and liquid contaminants (211) from a remaining gas stream, followed by the further extraction (209) of contaminants from the remaining gas stream by extraction with cold methanol (216). The process is carried out by using two cooling stages, in the first stage the gas stream comprising methane is cooled by heat exchange until all components are liquid, followed by a second cooling stage (205) involving an expansion step in which liquid methane is converted into gaseous methane.

Description

PROCESS FOR THE PURIFICATION OF METHANE CONTAINING STREAMS BY COOLING AND

The present invention concerns a process for the removal of gaseous contaminants from a methane containing gas stream, especially for the removal of sour contaminants as carbon dioxide and hydrogen sulphide from natural gas. The invention comprises a two step process in which in the first step a major amount of the contaminants is removed in a process carried out at sub- ambient temperature and in the second step a further amount of the contaminants is removed by means of extraction with a physical solvent, preferably also carried out at sub-ambient temperature. The invention especially comprises a sub-ambient temperature first step comprising a two stage cooling process of the gas stream in which the gas stream is first cooled by means of heat exchange, followed by an expansion step. In this way gaseous contaminants are converted into solids or solids and liquids, while the methane is mainly present in the gaseous phase, making it possible to perform a bulk separation of the methane and the contaminants. In the extraction step a physical solvent, especially methanol or glycol, is used for the further removal of the gaseous contaminants. In a preferred embodiment, the two steps are thermally integrated.

Methane comprising gas streams produced from subsurface reservoirs, especially natural gas, associated gas and coal bed methane, usually contain contaminants as carbon dioxide, hydrogen sulphide, carbon oxysulphide, mercaptans, sulfides and aromatic sulphur containing compounds in varying amounts. For most of the applications of these gas streams, the contaminants needs to be removed, either partly or almost completely, depending on the specific contaminant and/or the use. Often, the sulphur compounds need to be removed into the ppm level, carbon dioxide sometimes up till ppm level, e.g. LNG applications, or up till 2 or 3 vol. percent, e.g. for use as heating gas. Higher hydrocarbons may be present, which, depending on the use, may be recovered. In addition to the above-mentioned contaminants, methane comprising gas streams from underground reservoirs also contain water. For many applications the water needs to be removed, e.g. to prevent the formation of gas hydrates. Methods for the dehydration of methane comprising gas stream are known in the art. These methods include absorption of water in glycol, or adsorption of the water using solids such as aluminium oxide, silica gels, silica-alumina gels and molecular sieves.

Processes for the removal of carbon dioxide and sulphur compounds are know in the art. These processes include absorption processes using e.g. aqueous amine solutions or molecular sieves. These processes are especially suitable for the removal of contaminants, especially carbon dioxide and hydrogen sulphide, that are present in relatively low amounts, e.g. up till several vol%.

In WO 2004/070297 a process is described in which water and/or sour species (especially carbon dioxide and hydrogen sulphide) are removed by expansion in two stages of a pressurized methane comprising gas stream. The process is carried out in such a way that due to the temperature decrease in the first step gas hydrates are formed, followed by removal of the gas hydrates, and after further cooling by means of expansion in the second step solid or liquid sour contaminants are formed, followed by removal of the solid or liquid components.

A disadvantage of the known process is that the methane containing stream still contains a relatively large amount of contaminants, especially sour contaminants. In general, the process will remove up till 80 or perhaps up till 90 vol% of the sour components, which, however, for quite some applications will be insufficient. The remaining sour components may be removed by conventional processes, e.g. amine extraction.

It has now been found that in an integrated process using bulk removal of contaminants, especially sour contaminants, followed by further purification using cold methanol, it is possible to remove all contaminants up till a very low level.

Thus, the present application concerns a process for removing gaseous contaminants from a gas stream containing methane and gaseous contaminants, the process comprising cooling the gas stream in two stages to a temperature at which gaseous contaminants are present in the solid or solid and liquid phase while methane still being present in the gaseous phase, separating liquid or solid contaminants from a remaining gas stream, followed by further extraction of contaminants from the remaining gas stream by extraction with cold methanol.

In the invention, the process as described above is carried by cooling of the gas stream in two stages. The first stage comprises one or more heat exchange steps. In this heat exchange steps the gas containing methane and gaseous contaminants is heat exchanged against streams that are at a lower temperature that the gas stream itself. In a preferred embodiment the heat exchange is carried out against cold streams produced in the process. Also external cold streams may be used, for instance cold fluidum streams produced in an LNG process or air separation process. The second cooling stage is carried out by means of expansion. In this step no external source is used, but the cold is created in the expansion process. Any expansion process may be used, e.g. using an expander, an orifice or a venturi, but preferably a valve is used.

In order to obtain an optimum efficiency, the first cooling stage is especially carried out in such a way that all components are in the liquid phase after the first cooling stage. In the second stage the pressure relief is such that at least a part of the components in the liquid phase are transferred to the gaseous phase. Due to the temperature decrease a substantial part of one or more of the contaminants will transfer to the solid state. Preferably the main part of the methane will be converted to the gaseous phase, e.g. at least 90 wt%, preferably at least 95 wt%. Preferably most of the contaminants, e.g. at least 60 wt%, preferably at least 75 wt% will remain in the solid/liquid stage. Suitably, the gas stream containing methane in the first cooling step is cooled to a temperature between 1 and 40 ⁰C, preferably between 2 and 10 ⁰C, above the freeze out temperature of one of the contaminants, the freeze out temperature being the temperature at which solid contaminants are formed.

In the process of the invention the gaseous contaminants are suitably sour contaminants, especially hydrogen sulphide and/or carbon dioxide. It is observed that also minor amounts of other contaminants may be present, e.g. carbon oxysulphide, mercaptans, sulfides and aromatic sulphur containing compounds. The major part of these components will also be removed in the process of the present invention.

The amount of hydrogen sulphide in the gas stream containing methane is suitably between 5 and 40 vol % of the gas stream, preferably 20 - 35 and/or the amount of carbon dioxide is between 10 and 90 vol%, preferably 20 - 75 vol%, of the gas stream. It is observed that the present process is especially suitable for gas streams comprising large amounts of sour contaminants, e.g. 10 vol% or more, suitably between 15 and 90 vol%. In principle it would be possible to use conventional techniques as amine extraction, however, with the large amounts of sour contaminants as described above, these techniques will become extremely expensive, especially due to the large amounts of heat needed for the regeneration of loaded amine solvent.

The gas stream containing methane and contaminants may be any stream comprising these constituents. Suitable, the gas stream is a natural gas stream, an associated gas stream, a coal bed methane stream or a refinery stream. The amount of the hydrocarbon fraction in such a gas stream is suitably between 10 and 85 vol % of the gas stream, preferably between 25 and 80 vol%. Especially the hydrocarbon fraction of the gas stream comprises at least 75 vol % of methane, preferably 90 vol%, of the total gas stream. The hydrocarbon fraction in the gas stream comprising methane suitably contains 0 to 20 vol %, preferably 0.1 to 10 vol%, of C2-C5 compounds or in which the gas stream containing methane comprises up till 20 vol% of nitrogen, preferably between 0.1 and 10 vol%.

In almost all cases any methane containing stream produced from a subsurface formation will contain water. In order to prevent the formation of gas hydrates in the present process, part of the water will be removed. This can be done by conventional processes. Suitably, water is removed until the amount of water in the gas stream is less than 20 ppmv, preferably less than 5 ppmv, more preferably less than 1 ppmv.

The mixture obtained after the final cooling stage may be introduced in a separation vessel. The liquid/solid stage material, i.e. the main part of the contaminants, will be separated by gravitational forces to the lower part of the separation vessel, while the gaseous phase, i.e. the main part of the methane, will be present in the upper part of the separation vessel, from where it can be removed by one or more outlet pipes. Separation may also be obtained by the use of centrifugal forces, e.g. by using a cyclone.

After the first separation step, the remaining gas stream is extracted with cold methanol. This may be done in a separate vessel, but the extraction process is preferably integrated with the first separation step. In a suitable integrated arrangement the extraction is carried out in a vessel on top of the first separation vessel. The extraction may be carried out in the second vessel only, but in a further integrated version, the extraction is partly carried out in the top vessel, and partly in the first separation vessel. In that way an optimum structural integration is possible, while also an optimum temperature integration is possible.

In one embodiment of the invention the separation of the liquid/solid contaminants from the remaining gas stream is carried out separately from the cold methanol extraction process. In another embodiment of the invention the separation of the liquid/solid contaminants from the remaining gas stream is carried out together with a part of the cold methanol extraction.

In the case that the separation of the liquid/solid contaminants from the remaining gas stream is carried out separately from the cold methanol extraction process, this is be done suitably in one reactor shell, in which the separation takes place in the lower part of the shell and the extraction takes place in the upper part of the shell. Methanol is introduced in the top of the unit. Suitably a packing is present in the top part of the unit, e.g. a packed bed of particles (e.g. saddles or rings), or structured packing are present, similar to structured packings as used in distillation columns. The two parts are suitably separated by one or more trays, e.g. a chimney tray. In the case that the separation of the liquid/solid contaminants from the remaining gas stream is carried out together with a part of the cold methanol extraction, a unit similar to the unit described above can be used, but without a tray can be used. The loaden methanol stream obtained after the extraction step is preferably regenerated and recirculated to the extraction step. Regeneration is suitably done by temperature increase and/or pressure decrease, preferably both. The gaseous phase obtained during regeneration (mainly carbon dioxide and/or hydrogen sulphide) may be used for suitable purposes. For instance, it can be used for enhanced oil recovery and or of carbon dioxide and/or sulphur sequestration.

In the case that the separation of the liquid/solid contaminants from the remaining gas stream is carried out separately from the cold methanol extraction process, a stream of mainly carbon dioxide and/or hydrogen sulphide is obtained after the separation step, which stream, after some heating, may be converted into a fully liquid stream that can be used, optionally after pressurization, especially for enhanced oil recovery. It is observed that the cold present in this liquid stream may be used for heat exchange with the incoming feed gas stream.

The regeneration of the loaden methanol stream is suitably done by thermal regeneration, for instance at temperatures between -35 ⁰C and 50 ⁰C. In the case that a flash step is present, the regeneration is suitably done at a temperature between -35 and -15 ⁰C and a pressure between 1 and 10 bara, preferably 1 and 5 bara. When no intermediate flash step is used, the regeneration temperature is suitably between 5 and 50 ⁰C and a pressure between 5 and 40 bara, preferably between 10 and 25 bara. In a preferred embodiment the regeneration comprises a flash step between the extraction step and the thermal regeneration step. In the flash step, in which the pressure is decreased by e.g. between 5 and 40 bar, a stream containing a relatively high amount of hydrocarbons is obtained (especially methane/ethane) that can be used as fuel gas for e.g. the generation of energy. In another embodiment the flash gas stream is recirculated to the separation and/or extraction stage . In that case the higher hydrocarbons ultimately will end- up mainly in the purified methane stream.

The process of the present invention will remove a very substantial part of the contaminants pre sent in the feed gas stream. Suitably, between 80 and 99 vol% of the sour species are removed in the total process, preferably between 90 and 98 vol %.

The methanol extraction stream may be used in combination with another physical solvent, preferably the physical solvent being present in an amount up till 40 w%, preferably up till 20 wt%, of the total stream. This physical solvent is suitably a C2-C5 alcohol or a

C2-C5 di-alcohol, suitably ethanol or ethylene glycol.

Also sulfolane, N-methyl pyrrolidone, DMSO, dimethyl sulphone and other polar solvents may be used.

The feed gas stream comprising methane to be used in the present invention suitably has a temperature between -5 and 50 ⁰C, preferably between 0 and 40 ⁰C and a pressure between 20 and 120 bara, preferably between 35 and 70 bara. The feed gas stream comprising methane is suitably cooled in two stages to a temperature between -45 ⁰C and -100 ⁰C, preferably between -55 and -80 ⁰C, and a pressure between 5 and 40 bara, preferably between 10 and 25 bara. The extraction with cold methanol is suitably carried out at a temperature between -45 ⁰C and -90 ⁰C and a pressure between 5 and 40 bara, preferably between 10 and 25 bara. The combination of the cryogenic separation step and the methanol extraction step in the invention makes it possible to use a relatively very low temperature in the methanol extraction stage, as the gas stream to be extracted is available at a very low temperature. This results in an optimum thermal integration of the process. Suitably the pressure drop in the expansion step is between 15 and 80 bar, preferably between 25 and 45 bar. In order to further improve the thermal integration of the process, the purified methane stream after the methanol extraction is heat exchanged with the feed gas stream containing methane and gaseous contaminants.

In the process of the invention, in the first (cooling) step between 30 and 90 vol% of the sour species are removed, preferably between 40 and 80 vol%. The invention is illustrated by Figures 1 and 2.

In Figure 1 a typical scheme is described for a plant to be used for the process of the present invention. A feedgas (1) comprising methane as main hydrocarbon component and further comprising substantial amounts of CO2 and H2S (as high as 75 and 35 vol%, respectively), higher hydrocarbons (up till 10 vol%), nitrogen (up till 5 vol%) and water (less than 1 ppmv) is introduced via heat exchangers (2), (3), (4) and (5) and via expansion valve (6) into separator/extractor column (7). Separator/extractor column (7) comprises separation compartment (8), extraction compartment (9), chimney tray (10) and mass transfer device, for example a structured packing (11). The feed gas (1), after cooling by means of heat exchange and expansion over a valve will enter separation compartment (8) as a gas/liquid/solid mixture, the gas comprising methane and significantly lower amounts of hydrogen sulphide and carbon dioxide (e.g. between 1 and 40 vol% ), the solid/liquid being mainly (i.e. more than 95 wt%) hydrogen sulphide and carbon dioxide. Most of the ethane will be present in the methane stream, most of the C3+ components will be present in the solid/liquid phase. Gravitational separation will result in a slurry in the bottom fraction of (8) . The gas phase will leave (8) via openings in the chimney tray (10) and will enter extraction compartment (9). Methanol is introduced via methanol distribution means (12) into the extraction compartment. On its way down, the methanol will extract the hydrogen sulphide out of the gas stream (e.g. up till a level between 50 and 100 ppmv) as well as the carbon dioxide (e.g. up till a level of 2 - 3 vol%) . The slurry/liquid stream (13) leaves the separator extractor at the bottom section of - li ¬

the column. After pressurizing this liquid stream leaves the process via heat exchanger (2) . In a preferred embodiment this stream is used for enhanced oil recovery. The purified methane stream (14) leaves the process via heat exchanger (4). It can e.g. be sold as sales gas. The loaded methanol stream (15) leaves the extractor compartment form the chimney tray (10) and is introduced in flash vessel (16) . Flash gas (17), comprising most of the co-absorbed methane and ethane, is introduced into the separation compartment of (7) . In another embodiment the flashed gas stream may be used as fuel gas . The flashed solvent (18) goes via heat exchanger (3) to methanol regeneration vessel (19). Regenerated methanol (20) goes via heat exchanger 21 to the extraction compartment (9). Deabsorbed carbon dioxide and hydrogen sulphide leaves the process via (22) . This stream may be liquefied and re-pressurized and used for enhanced oil recovery. In separation compartment (8) heating means may be present to convert the carbon dioxide/hydrogen sulphide slurry into the liquid phase. Preferably the streams 1 (between heat exchangers (4) and (5)) and/or (20) (after regeneration) are used as heating liquid in the heating means (23).

In Figure 2 another scheme is described for a plant to be used for the process of the present invention. A feedgas (201) comprising methane as main hydrocarbon component and further comprising substantial amounts of CC>2 and H2S (e.g. 75 respectively 35 vol%), higher hydrocarbons (e.g. up till 10 vol%), nitrogen (up till 5 vol%) and water (less than 1 ppmv) is introduced via heat exchangers (202), ( 203) and (204) and via expansion valve (205) into separator/extractor column (206) . Separator/extractor column (206) comprises separation/extraction compartment (207), extraction compartment (208) and mass transfer device, e.g. structured packing (209). The feed gas (201), after cooling by means of heat exchange and expansion over a valve will enter separation/extraction compartment (207) as a gas/liquid/solid mixture, the gas comprising methane and lower amounts of hydrogen sulphide and carbon dioxide (e.g. between 1 and 40 vol% each), the solid/liquid being mainly (i.e. more than 95 wt%) hydrogen sulphide and carbon dioxide. Most of the ethane will be present in the methane stream, most of the C3+ components will be present in the solid/liquid phase. Gravitational separation will result in a slurry in the bottom fraction of (207). The gas phase enters extraction compartment (208). Methanol is introduced via methanol distribution means (210) into the extraction compartment. On its way down, the methanol will extract the hydrogen sulphide out of the gas stream (e.g. up till a level between 50 and 100 ppmv) as well as the carbon dioxide (e.g. up till a level of 2 - 3 vol%). The purified methane stream (219) leaves the process via heat exchanger (218) . It can e.g. be sold as sales gas. The loaded methanol stream (211) leaves the separation/extraction compartment from the bottom end and is introduced in flash vessel (212) . Flash gas (213), comprising most of the co-absorbed methane and ethane may leave the process (via heat exchanger (203)) for use e.g. as fuel gas. In another embodiment, it is introduced into the separation/extraction compartment (207) in order to maximize the hydrocarbon yield. The flashed solvent (214) goes via heat exchanger (202) to methanol regeneration vessel (215) . Regenerated methanol (216) goes via heat exchanger (218) to the extraction compartment (208). If desired, a further heat exchanger (against an external stream, e.g. an LNG stream) may be used to cool the regenerated methanol stream. Deabsorbed carbon dioxide and hydrogen sulphide leaves the process via (217) . This stream may be liquefied and re- pressurized and used for enhanced oil recovery. In separation/extraction compartment (207) heating means (220) may be present to convert the carbon dioxide/hydrogen sulphide slurry into the liquid phase. Preferably the streams (201) (between heat exchangers (203) and (205)) and/or (216) are used as heating liquid in the heating means (23) .

Claims

C L A I M S

1. A process for removing gaseous contaminants from a gas stream containing methane and gaseous contaminants, the process comprising cooling the gas stream in two stages to a temperature at which gaseous contaminants are present in the solid or solid and liquid phase while methane still being present in the gaseous phase, separating solid or solid and liquid contaminants from a remaining gas stream, followed by the further extraction of contaminants from the remaining gas stream by extraction with cold methanol.

2. A process according to claim 1, in which the separation of the liquid/solid contaminants from the remaining gas stream is carried out together with a part of the cold methanol extraction.

3. A process according to claim 1 or 2, in which the loaded methanol stream is thermally regenerated, preferably at a pressure between 1 and 40 bara, and the regenerated methanol is recirculated to the extraction step .

4. A process according to claim 3, in which the regeneration comprises a flash step between the extraction step and the thermal regeneration step and the gas stream obtained in the flash step is introduced into the solid/liquid contaminant separation stage or is used as a fuel gas.

5. A process according to claim 1, in which the first cooling stage is carried out in such a way that the gas mixture is completely liquefied.

6. A process according to claim 1 or 2, in which the gas stream containing methane in the first cooling stage is cooled to a temperature between 1 and 40 ⁰C, preferably between 2 and 10 ⁰C, above the freeze out temperature of one of the contaminants, the freeze out temperature being the temperature at which solid contaminants are formed.

7. A process according to any of claims 1 to 3, in which the second cooling stage is carried out by means of expansion, preferably in which the expansion is an expansion over a valve.

8. A process according to any of claims 1 to 4, in which the gaseous contaminants are sour contaminants, especially hydrogen sulphide and/or carbon dioxide.

9. A process according to any of claims 1 to 8, in which the gas stream comprising methane is cooled in two stages to a temperature between -45 ⁰C and -100 ⁰C, preferably between -55 and -80 ⁰C, and a pressure between 5 and 40 bara, preferably between 10 and 25 bara and the extraction with cold methanol is carried out at a temperature between -45 ⁰C and -90 ⁰C and a pressure between 5 and 40 bara, preferably between 10 and 25 bara.

10. A process according to any of the preceding claims in which the pressure drop in the expansion step is between 15 and 80 bar, preferably between 25 and 45 bar.