US20100107687A1

US20100107687A1 - Process for removing gaseous contaminants from a feed gas stream comprising methane and gaseous contaminants

Info

Publication number: US20100107687A1
Application number: US12/614,027
Authority: US
Inventors: Diki Andrian; Rick Van Der Vaart
Original assignee: Individual
Current assignee: Shell USA Inc
Priority date: 2008-11-06
Filing date: 2009-11-06
Publication date: 2010-05-06

Abstract

A process for removing gaseous contaminants from a feed gas stream that comprises a gaseous product and gaseous contaminants, comprising: providing the feed gas stream, cooling the feed gas stream to a temperature at which liquid phase contaminant is formed as well as a gaseous phase rich in gaseous product, separating the two phases by means of a gas/liquid separator, and introducing the gaseous phase rich in gaseous product into a cryogenic separation device that comprises a freezing zone and a distillation zone positioned below the freezing zone, and removing from the cryogenic separation device a bottom stream rich in liquid phase contaminant and lean in gaseous product, and a top stream rich in gaseous product and lean in gaseous contaminant. The invention further includes a device for carrying out the present process, the purified gas stream, and a process for liquefying a feed gas stream.

Description

RELATED CASES

This case claims priority to European application 08168502.6, filed 6 Nov. 9, 2008, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a process for the removal of gaseous contaminants from a feed gas stream which comprises a gaseous product and gaseous contaminants, in particular the removal of gaseous contaminants such as carbon dioxide and/or hydrogen sulphide from a natural gas or a gas stream from partial or complete oxidation processes, like syngas or flue gas.

BACKGROUND OF THE INVENTION

Gas streams produced from subsurface reservoirs such as natural gas, associated gas and coal bed methane or from (partial)oxidation processes, usually contain in addition to the gaseous product concerned such as methane, hydrogen and/or nitrogen contaminants such as carbon dioxide, hydrogen sulphide, carbon oxysulphide, mercaptans, sulphides and aromatic sulphur containing compounds in varying amounts. For most of the applications of these gas streams, the contaminants need 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 into the ppm level, e.g. LNG applications, or down to 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.
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 adsorption processes using e.g. 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 2006/087332, a method has been described for removing contaminating gaseous components, such as carbon dioxide and hydrogen sulphide, from a natural gas stream. In this method a contaminated natural gas stream is cooled in a first expander to obtain an expanded gas stream having a temperature and pressure at which the dewpointing conditions of the phases containing a preponderance of contaminating components, such a carbon dioxide and/or hydrogen sulphide are achieved. The expanded gas stream is then supplied to a first segmented centrifugal separator to establish the separation of a contaminants-enriched liquid phase and a contaminants-depleted gaseous phase. The contaminants-depleted gaseous phase is then passed via a recompressor, an interstage cooler, and a second expander into a second centrifugal separator. The interstage cooler and the second expander are used to cool the contaminants-depleted gaseous phase to such an extent that again a contaminants-enriched liquid phase and a further contaminates-depleted gaseous phase are obtained which are subsequently separated from each other by means of the second centrifugal separator. In such a method energy recovered from the first expansion step is used in the compression step, air, water and/or and an internal natural gas loop is used in the interstage cooler.
A disadvantage of this known method is that there is still room for improving the removal of the gaseous contaminants from the feed gas stream, ensuring that levels can be reached that are specified for pipeline transport of the feed gas stream or the production of liquefied natural gas. Moreover, the use of a recompressor, interstage cooler and an expander between the two centrifugal separators affects the energy efficiency of the separation process, which energy efficiency is a measure of the fuel gas consumption and the hydrocarbon loss in the liquid phase contaminant streams during the process.

SUMMARY OF THE INVENTION

It has now been found that in an integrated process for removing gaseous contaminants from gas streams that contain relatively large amount of gaseous contaminants the removal of gaseous contaminants can be improved, as well as the energy efficiency of the overall processing when after a gas/liquid separation the contaminants-depleted gaseous phase is introduced into a cryogenic separation device wherein use is made of a distillation zone in combination with a freezing zone.
Thus, the present invention concerns a process for removing gaseous contaminants from a feed gas stream which comprises a gaseous product and gaseous contaminants, the process comprising:
1) providing the feed gas stream;
2) cooling the feed gas stream to a temperature at which liquid phase contaminant is formed as well as a gaseous phase rich in gaseous product;
3) separating the two phases as obtained in step 2) by means of a gas/liquid separator; and
4) introducing the gaseous phase rich in gaseous product as obtained in step 3) into a cryogenic separation device which comprises a freezing zone and a distillation zone which is positioned below the freezing zone; and
5) removing from the cryogenic separation device a bottom stream rich in liquid phase contaminant and lean in gaseous product, and a top stream rich in gaseous product and lean in gaseous contaminant.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the invention, reference will be made to the accompany Figures, in which:

FIG. 1 is a schematic diagram illustrating one embodiment of the invention;

FIG. 2 is a schematic diagram illustrating one component of the embodiment of FIG. 1;

FIG. 3 a schematic diagram illustrating another component of the embodiment of FIG. 1;

FIG. 4 a schematic diagram illustrating an alternative embodiment of the component shown in FIG. 3; and

FIG. 5 a schematic diagram illustrating still another component of the embodiment of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially to FIG. 1, natural gas via a conduit 1 is passed through an expansion means 2, whereby a stream is obtained comprising liquid phase contaminant and a methane enriched gaseous phase. The stream flows via a conduit 3 into a gas/liquid separator 4 wherein the two phases are separated from each other. The liquid phase contaminant is recovered via a conduit 5, whereas the methane enriched gaseous phase is passed via a conduit 6 into a heat exchanger 7. In heat exchanger 7 ethane is used as an external refrigerant whereby the ethane is cooled by means of an ethane/propane cascade 8 as depicted in more detail in FIG. 2. The cooling in heat exchanger 7 is such that liquid phase contaminant and a methane enriched gaseous phase are formed. The stream which comprises these two phases is then passed via a conduit 9 into a cryogenic separation vessel 10, which comprises a first distillation zone 11, a controlled freezing zone 12 and a second distillation zone 13. From the cryogenic separation vessel 10 a further enriched methane enriched gaseous phase is recovered via a conduit 15 and liquid phase contaminant is recovered via a conduit 14.
In FIG. 2 a suitable heat exchanger 7 is shown which is based on an ethane/propane cascade which comprises an ethane loop and a propane loop. In the ethane loop an ethane stream is passed via a conduit 16 into an expander 17 (e.g. a turbine expander or a Joule-Thomson valve), and the cooled ethane stream so obtained is passed via a conduit 18 into the heat exchanger 7. A stream of warm ethane is then passed from the heat exchanger 7 to a recompressor 20 via a conduit 19 to increase the pressure of the ethane stream. The compressed stream of ethane obtained from recompressor 20 is then passed via a conduit 21 into the heat exchanger 22 wherein the ethane stream is cooled and at least partly condensed. Via the conduit 16 the ethane stream is then recycled to the expander 17. In the propane loop a propane stream is passed via a conduit 23 into an expander 24 (e.g. a turbine expander or a Joule-Thomson valve), and the cooled propane stream so obtained is passed via a conduit 25 into the heat exchanger 22 of the ethane loop. A stream of warm propane is then passed from the heat exchanger 22 via a conduit 26 into a recompressor 27 to increase the pressure of the propane stream. The compressed stream of propane obtained from recompressor 27 is then passed via a conduit 28 into a heat exchanger 29 wherein the propane stream is cooled and at least partly condensed by means of water or air. Via the conduit 23 the propane stream is then recycled to the expander 24.
In FIG. 3 a preferred gas/liquid separator is shown for carrying out step 3) of the present process. The stream comprising liquid phase contaminant and a methane enriched gaseous phase is passed via the conduit 3 into the gas/liquid separator 4 via supply and distribution assembly 30. Most of the liquid will flow down to the lower end of the separator and leave the separator via the liquid outlet 5. The gaseous stream comprising larger and smaller droplets will flow upwards via liquid coalescer 31, centrifugal separator 32 and a second liquid coalescer 33 to the top of the separator vessel, and leave the separator vessel via gas outlet 6.
In FIG. 4 another preferred gas/liquid separator is shown for carrying out step 3) of the present process. The stream comprising liquid phase contaminant and a methane enriched gaseous phase is passed via the conduit 3 to a gas inlet 34 in a housing 35 of the gas/liquid separator 4. The housing 35 further comprises a separating body 36 which shows a large number of ducts 37 which are arranged around a shaft 38, which provides an axis of rotation. Separating body 36 has been composed of six discs 36 a, 36 b, 63 c, 36 d, 36 e and 36 f that have been combined by welding or gluing. In the rotating separating body droplets of carbon dioxide and/or hydrogen sulphide are separated from the natural gas. The separated contaminants are discharged from the housing via a contaminants outlet 38 which has been arranged downstream of the separating body 36, and via a discharge conduit 5. Purified natural gas leaves housing 35 via the gas outlet 6 arranged at the opposite end of the housing 35.
In FIG. 5 a preferred cryogenic separation device has been shown for carrying out step 4) of the present invention. Such a device has, for instance been described in U.S. Pat. No. 4,533,372. The stream that has been cooled by the heat exchanger 7 is introduced via the conduit 9 into the first distillation zone 11 of the cryogenic separation vessel 10. The cryogenic separation vessel 10 is divided into three distinct sections, viz. the first distillation zone 11, the controlled freezing zone 12, and the second distillation zone 13. The stream that has been introduced into the first distillation zone 11 will be subjected to a distillation treatment. The internals of the first distillation zone 11 may include suitable trays, downcomers, and weirs, which are suitable for separating a methane enriched gaseous phase from liquid phase contaminant. Liquid phase contaminant is withdrawn from the bottom of the first distillation zone 11 via the conduit 14. The withdrawn liquid phase contaminant can be heated in a reboiler 40, and a portion can be returned to the tower as reboiled liquid phase contaminant via a conduit 41. The remainder of the reboiled liquid phase contaminant is withdrawn as a product via a conduit 42. In the first distillation zone 11 a methane enriched gaseous phase can leave this distillation zone and enter the controlled freezing zone 12 via a chimney tray 43. In controlled freezing zone 12, the methane enriched gaseous phase will contact a spray of liquid phase contaminant emanating from nozzles or spray jet assemblies 44. The methane enriched gaseous phase then continues to flow up through the second distillation zone 13. A methane enriched gaseous phase is withdrawn via the conduit 15, partially condensed in a reflux condenser 45 and separated into liquid phase contaminant and a methane enriched gaseous phase in a reflux drum 46. Liquid phase contaminant from reflux drum 46 is returned to the cryogenic separation device via a conduit 47, whereas a methane enriched gaseous phase product can be recovered via a conduit 48. Liquid phase contaminant is drawn from the bottom tray 48 of the second distillation zone 13 via a conduit 49. Liquid phase contaminant is accumulated in a vessel 50 and returned to the controlled freezing zone via a conduit 51, a pump 52, a conduit 53 and nozzles or spray jet assemblies 44.
Suitably, the feed gas stream is a natural gas stream in which the gaseous contaminants are carbon dioxide and/or hydrogen sulphide, or it is a gas stream from a (partial) oxidation process which comprises carbon dioxide as the gaseous contaminant. It has been found that the process is especially suitable for removal of hydrogen sulphide. The natural gas stream suitably comprises between 0.1 and 60 vol % of hydrogen sulphide, preferably between 20 and 40 vol % of hydrogen sulphide. The natural gas stream suitably comprises between 1 and 90 vol % of carbon dioxide, preferably between 5 and 80 vol % of carbon dioxide.
The feed gas stream to be used in accordance with the present invention comprises between 20 and 80 vol % of methane.
Suitably, the feed gas stream in step 1) has a temperature between ±20 and 150° C., preferably between −10 and 70° C., and a pressure between 10 and 150 bara, preferably between 80 and 120 bara.
The raw feed gas stream may be pre-treated to partially or completely remove water and optionally some heavy hydrocarbons. This can be for instance done by means of a pre-cooling cycle, against an external cooling loop or a cold internal process stream. Water may also be removed by means of a pre-treatment with molecular sieves, e.g. zeolites, or silica gel or alumina oxide or other drying agents such as glycol, MEG, DEG or TEG, or glycerol. The amount of water in the feed gas stream is suitably less than 1 vol %, preferably less than 0.1 vol %, more preferably less than 0.0001 vol %.
The cooling in step 2) of the feed gas stream may be done by methods known in the art. For instance, cooling may be done against internal or an external cooling fluid. In the case that the pressure of the feed gas is sufficiently high, cooling may be obtained by expansion of the feed gas stream. Combinations may also be possible. A suitable method to cool the feed gas stream is by nearly isentropic expansion, especially by means of an expander, preferably a turbo expander or laval nozzle. Another suitable method is to cool the feed gas stream by isenthalpic expansion, preferably isenthalpic expansion over an orifice or a valve, especially over a Joule-Thomson valve.
In a preferred embodiment the feed gas stream is pre-cooled before expansion. This may be done against an external cooling loop or against a cold internal process stream, e.g. liquid acidic contaminant. Preferably the gas stream is pre-cooled before expansion to a temperature between 15 and −35° C., preferably between 10 and −20° C. Especially when the feed gas stream has been compressed, the temperature of the feed gas stream may be between 100 and 150° C. In that case air or water cooling may be used to decrease the temperature first, optionally followed by further cooling.
Another suitable cooling method is heat exchange against a cold fluidum, especially an external refrigerant, e.g. a propane cycle, an ethane/propane cascade or a mixed refrigerant cycle, optionally in combination with an internal process loop, suitably a contaminants stream (liquid or slurry), a cold methane enriched stream or washing fluid.
Suitably the feed gas stream is cooled in step 2) to a first temperature between −30 and −80° C., preferably between −40 and −65° C. At these temperatures liquid phase contaminant will be formed.
In the present invention both liquid phase contaminant and gaseous contaminant will comprise hydrogen sulphide and carbon dioxide, whereas solid contaminant will usually mainly contain carbon dioxide.
In accordance with the present invention the gaseous phase rich in gaseous product as obtained in step 3) is introduced in step 4) into a cryogenic separation device which comprises a freezing zone and a distillation zone which is positioned below the freezing zone.
Preferably, use is made of a so-called controlled freezing zone (CFZ). Such a freezing zone is designed to control the formation and melting of solid contaminant and to prevent the introduction of solid contaminant into the distillation zone.
Step 4) of the process according to the present invention can suitably be carried out as follows:
a) the methane enriched gaseous phase is introduced in the distillation zone of the cryogenic separation device for forming liquid phase contaminant and a gaseous feed stream rich in gaseous product for the freezing zone;
b) the gaseous feed stream rich in gaseous product so obtained is introduced into the freezing zone;
c) the gaseous phase rich in gaseous product is contacted in the freezing zone with a cold stream for forming solid contaminant and a gaseous phase rich in gaseous product;
d) the solid contaminant obtained in step c) is melted and a stream of melted solid contaminant is introduced into the distillation zone; and
e) at least part of the gaseous phase rich in gaseous product obtained in step c) is condensed to form liquid phase contaminant.
In a preferred embodiment of the present invention at least part of the liquid phase contaminant formed in step e) is used as the stream of liquid phase contaminant in step c).
In another preferred embodiment of the present invention at least part of the bottom stream rich in liquid phase contaminant as obtained in step 5) is returned to the distillation zone.
The cryogenic separation section suitably comprises a single vertical vessel having the distillation zone in its lower section and the freezing zone in an upper section.
Suitably, step 4) of the present invention is carried out as follows:
(a) maintaining the distillation zone engineered to produce a bottom stream rich in liquid phase contaminant and lean in gaseous product and a gaseous feed stream for the freezing zone which stream is rich in gaseous, and wherein the distillation zone is operated at a temperature and pressure at which substantially no solid contaminant is formed within the distillation zone;
(b) maintaining the freezing zone engineered to contact the gaseous feed stream for the freezing zone which stream is rich in gaseous product with a stream of a cold liquid at a temperature and pressure whereby solid contaminant and a gaseous phase rich in gaseous product are formed in the freezing zone;
(c) introducing the gaseous phase rich in gaseous product obtained in step 3) into the distillation zone,
(d) producing the liquid phase contaminant and the gaseous feed stream for the freezing zone which stream is rich in gaseous product;
(e) introducing the gaseous feed stream for the freezing zone which is rich in gaseous product into the freezing zone;
(f) contacting in the freezing zone the gaseous feed stream for the freezing zone which is rich in gaseous product with the stream of cold liquid;
(g) forming in the freezing zone solid contaminant and a gaseous phase further enriched in gaseous product;
(h) melting the solid contaminant and introducing the liquid stream containing the melted solid contaminant into the distillation zone;
(i) condensing at least a portion of the gaseous phase further enriched in gaseous product and forming the stream of cold liquid with at least a portion of the condensed gaseous phase further enriched in gaseous product; and
(j) recovering at least a portion of the remainder of the gaseous phase further enriched in gaseous product as a gaseous product stream.
Suitably, the solid contaminant is melted in the freezing zone by adding heat. Preferably, the heat is added through indirect heat exchanger means placed within the freezing zone or it is added through electrical heating means placed within the freezing zone or via direct heat exchange from, e.g. a condensing vapor.
Suitably, the stream of cold liquid is introduced through spray means placed within the freezing zone. Preferably, such spray means comprise one or more separate spray nozzle assemblies through which the stream of cold liquid can be pumped. The cold liquid may have been subcooled by heat exchange prior to introducing into the freezing zone.
The cryogenic separation section suitably comprises a single vertical vessel having the first distillation zone in its lower portion and the freezing zone in an upper portion.
Preferably, the condensation of at least a portion of the methane enriched gaseous phase is carried out in a second distillation zone which is positioned above the freezing zone.
It should be noted, however, that such a second distillation zone is not required for carrying out the present invention.
Hence, in a suitable embodiment of the present invention the cryogenic separation device comprises a single vertical vessel having a first distillation zone in a lower part, a freezing zone in an intermediate part, and a second distillation zone in an upper part.
The cryogenic separation device to be used in accordance with the present invention suitably comprises a first lower distillation zone having an upper end and a lower end and containing gas-liquid contact means, outlet means in the lower end of the lower distillation zone suitable for allowing liquid phase contaminant to exit the distillation zone, means for allowing reboiled liquid phase contaminant to enter the lower end, means for allowing liquid phase contaminant to enter the upper end of the lower distillation zone from the freezing zone, and means for allowing a gaseous phase rich in gaseous product to exit the first lower distillation zone into the freezing zone while maintaining a liquid level within a lower end of the freezing zone, whereby the freezing zone is engineered to contact a gaseous phase rich in gaseous product from the first lower distillation zone with a stream of cold liquid to produce solid contaminant as well as a gaseous phase rich in gaseous product, the freezing zone having an upper end and a lower end and containing spray means suitable for introducing a stream of cold liquid into the freezing zone in a spray, and means for allowing the gaseous phase rich in gaseous product to exit the upper end of the freezing zone.
Suitably, the means for allowing the gaseous phase rich in gaseous product to exit the first lower distillation zone into the freezing zone comprise a chimney tray.
Suitably, the cryogenic separation device also comprises heating means situated in the vicinity of the means for allowing the gaseous phase rich in gaseous product to exit the first lower distillation zone which heating means are suitable for melting frozen solid contaminant which may be produced in the freezing section.
The spray means to be used in accordance with the present invention comprise one or more levels of spray assemblies.
The cryogenic separation device suitably comprises a second upper distillation zone having an upper end and a lower end and containing gas-liquid contact means, inlet means in the upper end of the second upper distillation zone for allowing reflux liquids to contact the gas-liquid contact means in the second upper distillation zone, means in the lower end of the second distillation zone for collecting liquid phase contaminant and allowing the liquid phase contaminant to exit the second distillation zone, and means for allowing a gaseous phase rich in gaseous product to enter the upper distillation zone from the freezing zone.
Suitably, the means for allowing the gaseous phase rich in gaseous product to enter the second distillation zone from the freezing zone comprise a chimney tray.
Suitably, the gas-liquid contact means in the second distillation zone are distillation trays.
Suitably, the cryogenic separation section also includes a reboiler adapted to heat liquid phase contaminant exiting the first lower distillation zone, whereby at least a fraction of the reboiled liquid phase contaminant is returned to enter the lower end of the first lower distillation zone.
Preferred cryogenic separation devices to be used in accordance with the process of the present invention have, for instance, been described in U.S. Pat. No. 4,533,372; U.S. Pat. No. 4,923,493; U.S. Pat. No. 5,062,270; U.S. Pat. No. 5,265,428; U.S. Pat. No. 5,956,971; U.S. Pat. No. 6,053,007 and U.S. Pat. No. 5,120,338, which documents are herewith incorporated by reference.
In the present process the gaseous phase rich in gaseous product can suitably be recompressed in one or more compression steps before it is introduced into the cryogenic separation device in step 4).
Suitably, energy that is recovered in step 2) can be used for such one or more compression steps.
In a preferred embodiment of the present invention the gaseous phase rich in gaseous product as obtained in step 3) is cooled in a cooling step to a temperature at which liquid phase contaminant is formed as well as a gaseous phase rich in gaseous product, which gaseous phase rich in gaseous product is then introduced into the cryogenic separation device in step 4).
In another preferred embodiment of the present invention the recompressed gaseous phase rich in gaseous product as obtained in the one or more compression steps is cooled in a cooling step to a temperature at which liquid phase contaminant is formed as well as a gaseous phase rich in gaseous product, after which the gaseous stream rich in gaseous product so obtained is introduced into the cryogenic separation device in step 4).
The cooling of the gaseous phase rich in gaseous product between steps 3) and 4) can be carried out by means of an internal process stream such as a stream of liquid phase contaminant which is separated from the gaseous phase rich on gaseous product in step 3).
In accordance with the present invention the cooling of the gaseous phase rich in gaseous product between steps 3) and 4) can suitably at least partly be done by means of an external refrigerant.
Preferably, the external refrigerant to be used in step 4) has a higher molecular weight than the gaseous phase rich in gaseous product to be cooled. Suitable examples of such cooling medium include ethane, propane and butane. Preferably, the cooling medium comprises ethane and/or propane.
More preferably, the external refrigerant to be used comprises a propane cycle, an ethane/propane mixed refrigerant or an ethane/propane cascade. Such an ethane/propane cascade is described in more detail hereinbelow.
The cooling between steps 3) and 4) as described herebefore can suitably partly be done by means of an external refrigerant and partly by means of an internal process stream.
In another embodiment of the present invention the gaseous phase rich in gaseous product as obtained in step 3) is recompressed in one or more compression steps before it is introduced in the cryogenic separation device in step 4).
In yet another embodiment of the present invention, the gaseous phase rich in gaseous product as obtained in step 3) is firstly recompressed in one or more compression steps, than cooled between steps 3) and 4) as described herein, and the methane enriched gaseous phase so obtained is introduced into the cryogenic separation device in step 4).
The cooling between steps 3) and 4) as described herebefore is suitably carried out at a temperature between −50 and −90° C., preferably at a temperature between −30 and −70° C., and at a pressure which is between 20 and 80 bara, preferably a pressure between 30 and 60 bara.
Suitably, such an interstage cooler will be based on a internal process stream.
In the one or more compression steps suitably energy is used that is recovered in step 2).
In yet another embodiment of the present invention the gaseous phase rich in gaseous product as obtained in step 3) is again subjected to a step 2) and subsequently to a step 3) before it is introduced in the cryogenic separation section in step 4). In this case the gas stream is subsequently subjected to a total number of combinations of subsequent recompression, cooling and separation steps. Suitably, the sequence of steps 2) and 3) can be repeated or three times before the gaseous phase rich in gaseous product thus obtained is introdiced into the cryogenic separation device in step 4).
In this way a further enriched gaseous product-containing gaseous phase can be obtained containing a low level of gaseous contaminants.
The gas stream, and in particular natural gas streams produced from a subsurface formation, may typically contain water. In order to prevent the formation of gas hydrates in the present process, at least part of the water is suitably removed. Therefore, the gas stream that is used in the present process has preferably been dehydrated. This can be done by conventional processes. A suitable process is the one described in WO-A 2004/070297. Other processes for forming methane hydrates or drying natural gas are also possible. Other dehydration processes are also possible, including treatment with molecular sieves or drying processes with glycol or methanol. Suitably, water is removed until the amount of water in the gas stream comprises at most 50 ppmw, preferably at most 20 ppmw, more preferably at most 1 ppmw of water, based on the total gas stream.
The hydrocarbon gas that is obtained in step 5) can be used as product. It is also possible that it is desirable to subject the recovered sweet hydrocarbon gas after step 5) to further treatment and/or purification. For instance, the sweet hydrocarbon gas may be subjected to fractionation. Further purification may be accomplished by absorption with an alkanolamine fluid, optionally in combination with a sulphone, such as tetramethylene sulphone (sulpholane), with N-methyl pyrrolidone, or with methanol. Other treatments may include a further compression, when the sweet gas is wanted at a higher pressure.
In the process according to the present invention a variety of gas/liquid separators can suitably be used in step 3), such as, for instance, rotating centrifuges or cyclones.
Suitable gas/liquid separators to be used in accordance with the present invention have, for instance, been described in WO 2008/082291, WO 2006/087332, WO 2005/118110, WO 97/44117, WO 2007/097621 and WO 94/23823, which documents are hereby incorporated by reference.
In a preferred embodiment of the present invention, the gas/liquid separator vessel in step 3) comprises a gas/liquid inlet at an intermediate level, a liquid outlet arranged below the gas/liquid inlet and a gas outlet arranged above the gas/liquid inlet, in which vessel a normally horizontal coalescer is present above the gas/liquid inlet and over the whole cross-section of the vessel and in which vessel a centrifugal liquid separator is arranged above the coalescer and over the whole cross-section of the vessel, the liquid separator comprising one or more swirl tubes.
When using a vertical gas/liquid separator vessel, the process only needs a relatively small area.
According to a preferred embodiment, the gas/liquid inlet comprises an admittance with a supply and distribution assembly extending horizontally in the separator vessel. In its most simple form, the inlet is a simple pipe, having a closed end and a number of perforations evenly distributed over the length of the pipe. Optionally, the pipe may have a tapered or conical shape. One or more cross pipes may be present to create a grid system to distribute the gas-liquid mixture more evenly over the cross-section of the vessel. Preferably, the assembly includes a chamber, e.g. a longitudinal box-like structure, connected to the gas inlet and having at least one open vertical side with a grid of guide vanes disposed one behind each other, seen in the direction of the flow. By means of this supply and distribution assembly, the gas is evenly distributed by the guide vanes over the cross-section of the column, which brings about an additional improvement of the liquid separation in the coalescer/centrifugal separator combination. A further advantage is that the supply and distribution assembly separates from the gas any waves of liquid which may suddenly occur in the gas stream, the separation being effected by the liquid colliding with the guide vanes and falling down inside the column. Suitably, the box structure narrows down in the direction of the flow. After having been distributed by the vanes over the column cross-section, the gas flows up to the coalescer.
In a preferred embodiment the longitudinal chamber has two open vertical sides with a grid of guide vanes.
Suitable gas/liquid inlets are those described in e.g. GB 1,119,699, U.S. Pat. No. 6,942,720, EP 195,464, U.S. Pat. No. 6,386,520 and U.S. Pat. No. 6,537,458. A suitable, commercially available gas/liquid inlet is a Schoepentoeter.
There are numerous horizontal coalescers available, especially for vertical columns. A well-known example of a mist eliminator is the demister mat. All of these are relatively tenuous (large permeability) and have a relatively large specific (internal) surface area. Their operation is based on drop capture by collision of drops with internal surfaces, followed by drop growth on these surfaces, and finally by removal of the grown drop either by the gas or by gravity.
The horizontal coalescer can have many forms which are known per se and may, for example, consist of a bed of layers of gauze, especially metal or non-metal gauze, e.g. organic polymer gauze, or a layer of vanes or a layer of structured packing. Also unstructured packings can be used and also one or more trays may be present. All these sorts of coalescers have the advantage of being commercially available and operating efficiently in the column according to the invention. See also Perry's Chemical Engineers' Handbook, Sixth edition, especially Chapter 18. See also EP 195464.
Through the use of these three stages of coalescence and separation, a high separation efficiency is achieved.
The centrifugal liquid separator in one of its most simple forms may comprise a horizontal plate and one or more vertical swirl tubes extending downwardly from the plate, each swirl tube having one or more liquid outlets below the horizontal plate at the upper end of the swirl tube. In another form, the centrifugal liquid separator comprises one or more vertical swirl tubes extending upwardly from the plate, each swirl tube having one or more liquid outlets at the upper end. The plate is provided with a downcomer, preferably a downcomer that extends to the lower end of the separator vessel.
In a preferred embodiment of the invention, the centrifugal liquid separator comprises two horizontal trays between which vertical open-ended swirl tubes extend, each from an opening in the lower tray to some distance below a coaxial opening in the upper tray, means for the discharge of secondary gas and of liquid from the space between the trays outside the swirl tubes, and means provided in the lower part of the swirl tubes to impart to the gas/liquid a rotary movement around the vertical axis.
The liquid separator is also preferably provided with vertical tube pieces which project down from the coaxial openings in the upper tray into the swirl tubes and have a smaller diameter than these latter. This arrangement enhances the separation between primary gas on the one hand and secondary gas and liquid on the other hand, since these latter cannot get from the swirl tubes into the openings in the upper tray for primary gas.
According to a preferred embodiment, the means for discharging the secondary gas from the space between the trays consist of vertical tubelets through the upper tray, and the means for discharging liquid from the space between the trays consist of one or more vertical discharge pipes which extend from this space to the bottom of the column. This arrangement has the advantage that the secondary gas, after having been separated from liquid in the said space between the trays, is immediately returned to the primary gas, and the liquid is added to the liquid at the bottom of the column after coming from the coalescer, so that the secondary gas and the liquid removed in the centrifugal separator do not require separate treatment.
In order to improve even further the liquid separation in the centrifugal separator, openings are preferably provided in accordance with the invention at the top of the swirl tubes for discharging liquid to the space between the trays outside the swirl tubes. This has the advantage that less secondary gas is carried to the space between the trays. A suitable, commercially available centrifugal separator is a Shell Swirltube deck.
In a preferred embodiment, the separation vessel comprises a second normally horizontal liquid coalescer above the centrifugal liquid separator and over the whole cross-section of the vessel. This has the advantage that any droplets still present in the gas stream are removed. See for a further description hereinabove. Preferably, the second coalescer is a bed of one or more layers of gauze, especially metal or non-metal gauze, e.g. organic polymer gauze. In another preferred embodiment, the second normally horizontal liquid coalescer is situated above the secondary gas outlets, for instance in the way as described in EP 83811, especially as depicted in FIG. 4.
In another preferred embodiment of the present invention the gas/liquid separator in step 3) comprises a centrifugal separator which comprises a bundle of parallel channels that are arranged within a spinning tube parallel to an axis of rotation of the spinning tube.
Suitably, the centrifugal separator is spinned by introducing a swirling gas stream into the spinning tube.
Preferably, such a centrifugal separator to be used in accordance with the present invention comprises a housing with a gas inlet for contaminated gas at one end of the vessel, a separating body, a gas outlet for purified gas at the opposite end of the housing and a contaminants outlet downstream of the separating body or upstream or downstream of the separating body, wherein the separating body comprises a plurality of ducts over a part of the length of the axis of the housing, which ducts have been arranged around a central axis of rotation, in which apparatus the separating body has been composed of a plurality of perforated discs wherein the perforations of the discs form the ducts.
It will be appreciated that the discs can be easily created by drilling or cutting a plurality of perforations into the relatively thin discs. By attaching several discs together these discs form a separating body. By aligning the perforations ducts are obtained.
It is now also very easy to attach the discs such that the perforations are not completely aligned. By varying the number and nature of the non-alignment of the perforations the resulting ducts can be given any desired shape. In such cases not only ducts are obtainable that are not completely parallel to the central axis of rotation, but also ducts that form a helix shape around the axis of rotation. So, in this way very easily the preferred embodiment of having non-parallel ducts can be obtained. Hence it is preferred that the perforations of the discs have been arranged such that the ducts are not parallel to the central axis of rotation or form a helix shape around the axis of rotation.
Further, it will be appreciated that it is relatively easy to increase or decrease the diameter of the perforations. Thereby the skilled person has an easy manner at his disposal to adapt the (hydraulic) diameter of the ducts, and thereby the Reynolds number, so that he can easy ascertain that the flow in the ducts is laminar or turbulent, just as he pleases. The use of these discs also enables the skilled person to vary the diameter of the duct along the axis of the housing. The varying diameter can be selected such that the separated liquid or solid contaminants that are collected against the wall of the duct will not clog up the duct completely, which would hamper the operation of the apparatus.
The skilled person is also now enabled to maximise the porosity of the separating body. The easy construction of the discs allows the skilled person to meticulously provide the disc with as many perforations as he likes. He may also select the shape of the perforations. These may have a circular cross-section, but also square, pentagon, hexagon, octagon or oval cross-sections are possible. He may therefore minimise the wall thickness of the separating body and the wall thicknesses of the ducts. He is able to select the wall thicknesses and the shape of the ducts such that the surface area that is contributed to the cross-section of the separating body by the walls is minimal. That means that the pressure drop over the separating body can be minimised.
The apparatus can have a small or large number of ducts. Just as explained in the prior art apparatuses the number of ducts suitably ranges from 100 to 1,000,000, preferably from 500 to 500,000. The diameter of the cross-section of the ducts can be varied in accordance with the amount of gas and amounts and nature, e.g., droplet size distribution, of contaminants and the desired contaminants removal efficiency. Suitably, the diameter is from 0.05 to 50 mm, preferably from 0.1 to 20 mm, and more preferably from 0.1 to 5 mm. By diameter is understood twice the radius in case of circular cross-sections or the largest diagonal in case of any other shape.
The size of the apparatus and in particular of the separating body may vary in accordance with the amount of gas to be treated. In EP-B 286 160 it is indicated that separating bodies with a peripheral diameter of 1 m and an axial length of 1.5 m are feasible. The separating body according to the present invention may suitably have a radial length ranging from 0.1 to 5 m, preferably from 0.2 to 2 m. The axial length ranges conveniently from 0.1 to 10 m, preferably, from 0.2 to 5 m.
The number of discs may also vary over a large number. It is possible to have only two discs if a simple separation is needed and/or when the perforations can be easily made. Other considerations may be whether parallel ducts are desired, or whether a uniform diameter is wanted. Suitably the number of discs varies from 3 to 1000, preferably from 4 to 500, more preferably from 4 to 40. When more discs, are used the skilled person will find it easier to gradually vary the diameter of the ducts and/or to construct non-parallel ducts. Moreover, by increasing or decreasing the number of discs the skilled person may vary the duct length. So, when the conditions or the composition of the gas changes, the skilled person may adapt the duct length easily to provide the most optimal conditions for the apparatus of the present invention. The size of the discs is selected such that the radial diameter suitably ranges from 0.1 to 5 m, preferably from 0.2 to 2 m. The axial length of the discs may be varied in accordance with construction possibilities, desire for varying the shape etc. Suitably, the axial length of each disc ranges from 0.001 to 0.5 m, preferably from 0.002 to 0.2 m, more preferably from 0.005 to 0.1 m.
Although the discs may be manufactured from a variety of materials, including paper, cardboard, and foil, it is preferred to manufacture the discs from metal or ceramics. Metals discs have the advantage that they can be easily perforated and be combined to firm sturdy separating bodies. Dependent on the material that needs to be purified a suitable metal can be selected. For some applications carbon steel is suitable whereas for other applications, in particular when corrosive materials are to be separated, stainless steel may be preferred. Ceramics have the advantage that they can be extruded into the desired form such as in honeycomb structures with protruding ducts.
Typically, the ceramics precursor material is chosen to form a dense or low-porosity ceramic. Thereby the solid or liquid contaminants are forced to flow along the wall of the ducts and not, or hardly, through the ceramic material of the walls. Examples of ceramic materials are silica, alumina, zirconia, optionally with different types and concentrations of modifiers to adapt its physical and/or chemical properties to the gas and the contaminants.
The discs may be combined to a separating body in a variety of ways. The skilled person will appreciate that such may depend on the material from which the discs have been manufactured. A convenient manner is to attach the discs to a shaft that provides the axis of rotation. Suitable ways of combining the discs include clamping the discs together, but also gluing them or welding them together can be done. Alternatively, the discs may be stacked in a cylindrical sleeve. This sleeve may also at least partly replace the shaft. This could be convenient for extruded discs since no central opening for the shaft would be required. It is preferred to have metal discs that are welded together.
The gas/liquid separator in step 3) comprises:
a) a housing comprising a first, second and third separation section for separating liquid from the mixture, wherein the second separation section is arranged below the first separation section and above the third separation section, the respective separation sections are in communication with each other, and the second separation section comprises a rotating coalescer element;
b) tangentially arranged inlet means to introduce the mixture into the first separation section;
c) means to remove liquid from the first separation section;
d) means to remove liquid from the third separation section; and
e) means to remove a gaseous stream, lean in liquid, from the third separation section.
In a preferred embodiment of the invention, the methane enriched gaseous phase obtained in accordance with the present invention is further purified, e.g. by extraction of remaining acidic components with a chemical solvent, e.g. an aqueous amine solution, especially aqueous ethanolamines, such as DIPA, DMA, MDEA, etc., or with a physical solvent, e.g. cold methanol, DEPG, NMP, etc.
The contaminated gas stream is continuously provided, continuously cooled and continuously separated.
The present invention also relates to a device (plant) for carrying out the process as described above, as well as the purified gas stream obtained by the present process. In addition, the present invention concerns a process for liquefying a feed gas stream comprising purifying the feed gas stream by means of the present process, followed by liquifying the feed gas stream by methods known in the art.

Claims

1. A process for removing gaseous contaminants from a feed gas stream that comprises a gaseous product and gaseous contaminants, the process comprising:

1) providing the feed gas stream;

2) cooling the feed gas stream to a temperature at which liquid phase contaminant is formed as well as a gaseous phase rich in gaseous product;

3) separating the two phases as obtained in step 2) by means of a gas/liquid separator; and

4) introducing the methane enriched gaseous phase as obtained in step 3) into a cryogenic separation device which comprises a freezing zone and a distillation zone which is positioned below the freezing zone; and

5) removing from the cryogenic separation section a bottom stream rich in liquid phase contaminant and lean in gaseous product, and a top stream rich in gaseous product and lean in gaseous contaminant.

2. The process according claim 1 wherein the gas/liquid separator in step 3) comprises a gas/liquid inlet at an intermediate level, a liquid outlet arranged below the gas/liquid inlet and a gas outlet arranged above the gas/liquid inlet, in which vessel a normally horizontal coalescer is present above the gas/liquid inlet and over the whole cross-section of the vessel and in which vessel a centrifugal liquid separator is arranged above the coalescer and over the whole cross-section of the vessel, the liquid separator comprising one or more swirl tubes.

3. The process according to claim 1 wherein the gas/liquid separator in step 3) comprises a centrifugal separator which comprises a bundle of parallel channels that are arranged within a spinning tube parallel to an axis of rotation of the spinning tube.

4. The process according claim 3 wherein the gas/liquid separator in step 3) comprises:

a) a housing comprising a first, second and third separation section for separating liquid from the mixture, wherein the second separation section is arranged below the first separation section and above the third separation section, the respective separation sections are in communication with each other, and the second separation section comprises a rotating coalescer element;

b) tangentially arranged inlet means to introduce the mixture into the first separation section;

c) means to remove liquid from the first separation section;

d) means to remove liquid from the third separation section; and

e) means to remove a gaseous stream, lean in liquid, from the third separation section.

5. The process according to claim 1 wherein the gas/liquid separator in step 3) comprises a housing with a gas inlet for contaminated gas at one end of the vessel, a separating body, a gas outlet for purified gas at the opposite end of the housing and a contaminants outlet downstream of the separating body or upstream and downstream of the separating body, wherein the separating body comprises a plurality of ducts over a part of the length of the axis of the housing, which ducts have been arranged around a central axis of rotation, in which apparatus the separating body has been composed of a plurality of perforated discs wherein the perforations of the discs form the ducts.

6. The process according to claim 1 wherein the gaseous contaminants are carbon dioxide, and/or hydrogen sulphide, wherein the carbon dioxide, if present, is present in the range of from 1 to 90 vol %, based on the total feed gas stream, and wherein which the hydrogen sulphide, if present, is present stream in the range of from 0.1 to 60 vol % based on the total feed gas stream.

7. The process according to claim 1 wherein the feed gas stream is a natural gas which comprises between 20 and 80 vol % of methane.

8. The process according to claim 1 wherein the feed gas stream in step 1) has a temperature between −20 and 150° C. and a pressure between 10 and 150 bara.

9. The process according to claim 1 wherein the cooling in step 2) is performed using at least one technique selected from the group consisting of: expansion over an orifice or a valve; an expander; a turbo expander; and a laval nozzle, and wherein the feed gas stream is pre-cooled to a temperature between 15 and −35° C.

10. The process according to claim 1 wherein the feed gas stream is cooled in step 2) to a temperature between −30 and −80° C.

11. The process according to claim 1 wherein the freezing zone in step 4) is designed to control the formation and melting of solid contaminant and to prevent the introduction of solid contaminant into the distillation zone.

12. The process according to claim 1 wherein step 4) is carried out as follows:

a) the gaseous phase rich in gaseous product is cooled to a temperature above the freeze out temperature of any contaminant present in the feed gas stream to obtain a cooled gaseous phase rich in gaseous product,

b) the cooled gaseous phase rich in gaseous product as obtained in step a) is introduced into the distillation zone of the cryogenic separation device;

c) gaseous phase rich in gaseous product inside the top section of the distillation is introduced into the freezing zone;

d) the gaseous phase rich in gaseous product that is introduced in the freezing zone is contacted in the freezing zone with a stream of cold liquid at a temperature lower than the freeze out temperature of any contaminant present in the gaseous phase rich in product, for forming solid contaminant and a further gaseous product enriched gaseous phase;

e) the solid contaminant obtained in step d) is melted and a stream of melted solid contaminant is introduced into the distillation zone; and

at least part of the further gaseous product enriched gaseous phase obtained in step d) is condensed to form liquid phase.

13. The process according to claim 1 wherein the gaseous phase rich in gaseous product obtained in step 3) is cooled in a cooling step to a temperature at which at least part of the gaseous phase rich in gaseous product is condensed, and the fluid so obtained is introduced into the cryogenic separation device in step 4).

14. The process according to claim 1 wherein the gaseous phase rich in gaseous product obtained in step 3) is recompressed in one or more compression steps and the recompressed gaseous phase rich in gaseous product so obtained is cooled by means of expansion to a temperature above the freeze out temperature of any contaminant present in the feed gas stream before it is introduced into the cryogenic separation device in step 4), wherein the cooling between steps 3) and 4) is at least partly be done by means of an external refrigerant.

15. The process according to claim 14 wherein the external refrigerant has a higher molecular weight than the gaseous phase rich in gaseous product to be cooled.

16. The process according to claim 14 wherein the external refrigerant comprises a propane cycle, an ethane/propane mixed refrigerant or an ethane/propane cascade.