STREAM

The present invention relates to a process for removing gaseous contaminants from a feed stream which comprises a gaseous product and gaseous contaminants.

Gas streams produced from subsurface reservoirs such as natural gas, associated gas and coal bed gas methane, or from (partial) oxidation processes, usually contain in addition to the gaseous product concerned such as methane, hydrogen and/or nitrogen contaminants 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 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. 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 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 which is based on a natural gas loop 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, and an internal natural gas loop is used in the interstage cooler. 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.

In WO 2007/030888, a process is described for removing gaseous contaminants from a gas stream by cooling the contaminated gas stream to form a slurry of solid sour species and a gaseous stream containing gaseous sour species, and separating the gaseous stream containing gaseous sour species. The gaseous stream is treated with a liquid solvent to form a dehydrated sweetened gas stream. In the process described in WO 2007/030888, at least two steps are needed to remove sour species, one of the steps involving the use of expensive and energy-consuming solvents .

Thus, there is still room for improvement in terms of efficient removal of gaseous contaminants.

It has now been found that from a feed stream comprising a gaseous product and gaseous contaminants such as carbon dioxide and hydrogen sulphide can very effectively be removed, when use is made of a particular sequence of cooling steps followed by a cryogenic separation step. Moreover, by using the specific sequence of cooling steps the energy efficiency of the overall processing can be improved.

Accordingly, the present invention provides a process for removing gaseous contaminants from a feed stream which comprises a gaseous product and gaseous contaminants, which feed stream further comprises a liquid phase comprising liquid phase contaminant, the process comprising:

1) providing the feed stream;

2) cooling the feed stream to a temperature at which liquid phase contaminant is formed as well as a gaseous phase rich in gaseous product; 3) cooling the stream as obtained in step 2) further to a temperature at which a slurry is formed which comprises solid contaminant, liquid phase contaminant and a gaseous phase rich in gaseous product;

4) introducing the stream obtained in step 3) into a gas/liquid/solids separation device; and 5) removing from the gas/liquid/solids separation device a gas stream rich in gaseous product and a stream which comprises solid contaminant and/or liquid phase contaminant .

It has been found that by lowering the temperature of the feed stream, thus causing the majority of the contaminants to liquefy, removal of contaminants can be achieved in a more controlled manner, resulting in a deeper removal of these contaminants. The gas stream rich in gaseous product obtained contains very low amounts of contaminants. The gas stream obtained in accordance with the present process contains very low amounts of contaminants .

Suitably, the feed stream comprises a gaseous phase comprising the gaseous product and gaseous contaminants, and a liquid phase comprising liquid phase contaminant.

Suitably, the gaseous contaminants, solid contaminant and liquid phase contaminant are carbon dioxide and/or hydrogen sulphide.

The stream as obtained in step 2) has preferably a temperature between 1 and 40 °C, more preferably between 2 and 20 °C above the temperature at which carbon dioxide freezes from the feed stream.

Preferably, in step 2) of the present process the feed stream is cooled to a temperature between 5 and -70°C, more preferably between -25 and -50°C, and the pressure of the stream so obtained is preferably between 5 and 90 bara, more preferably between 40 and 60 bara.

In one preferred embodiment, in step 2) the stream is cooled by means of direct contact with a cooling medium, preferably a fluid comprising hydrocarbons. In this preferred embodiment, the cooling in step 3) is preferably done by isenthalpic expansion, more preferably by isenthalpic expansion over an orifice or a valve, especially a Joule-Thomson valve, or by nearly isentropic expansion, especially by means of an expander, preferably a turbo expander or a laval nozzle.

In an alternative embodiment, the cooling in step 2) is done by isenthalpic expansion, preferably isenthalpic expansion over an orifice or a valve, especially a Joule- Thomson valve, or by nearly isentropic expansion, especially by means of an expander, preferably a turbo expander or a laval nozzle. In this alternative embodiment, preferably in step 3) the stream is cooled by means of direct contact with a cooling medium, preferably a fluid comprising hydrocarbons. In this alternative embodiment, the cooling in step 3) can also be done by means of isobaric cooling, more preferably isobaric cooling using one or more heat exchangers .

In step 3) the stream is preferably cooled to a temperature which is between 10 and 50 °C lower than the temperature to which the feed stream is cooled in step 2) .

Preferably, in step 3) the stream is cooled to a temperature between -50 and -110 ⁰ C, more preferably between -60 and -105 ⁰ C, and the pressure of the stream so obtained is preferably between 5 and 40 bara, more preferably between 5 and 30 bara. Preferably, the feed stream to be provided in step 1) of the present invention has a temperature between -20 and -55 °C, more preferably between -25 and -45 °C, and the pressure of the feed stream is preferably between 30 and 70 bara, more preferably between 40 and 60 bara.

Suitably, the feed stream provided in step 1) of the present process is obtained by cooling a feed gas stream which comprises the gaseous product and gaseous contaminants by means of nearly isentropic expansion, especially by means of an expander, preferably a turbo expander or a laval nozzle.

Preferably, the feed gas stream is cooled to a temperature at which liquid phase contaminant is formed as well as a gaseous phase rich in gaseous product. In another embodiment of the present invention the feed stream provided in step 1) is obtained by cooling a feed gas stream which comprises the gaseous product and gaseous contaminants in a step a) by means of isenthalpic expansion, preferably isenthalpic expansion over an orifice or a valve, especially a Joule-Thomson valve, or by nearly isentropic expansion, especially by means of an expander, preferably a turbo expander or a laval nozzle, to a first temperature, and cooling the stream so obtained in a step b) to a second temperature which is lower than the first temperature.

Preferably, the feed gas stream is cooled in step b) to a temperature at which liquid phase contaminant is formed as well as a gaseous phase rich in gaseous product. Preferably, the cooling in step b) is carried out by means of isobaric cooling, preferably isobaric cooling using one or more heat exchangers. More preferably, in step 3) of the present process the stream is cooled by means of direct contact with a cooling medium, preferably a fluid comprising hydrocarbons.

Suitably, in the embodiment of the present invention as described hereanabove, the first temperature in step a) is between -20 and 15°C, preferably between -10 and 10 ⁰ C, and the pressure of the stream so obtained is preferably between 30 and 70 bara, more preferably between 40 and 60 bara . Preferably, the second temperature in step b) is between -20 and -55 °C, more preferably between -25 and -45 °C, and the pressure of the stream so obtained is preferably between 30 and 70 bara, more preferably 40 and 60 bara. Suitably, the feed stream to be provided in step 1) can be derived from a feed gas stream comprising a natural gas and which comprises between 10 and 90 vol% of carbon dioxide, between 0.1 and 60 vol% of hydrogen sulphide, and between 20 and 80 vol% of methane. Preferably, the feed stream to be provided in step 1) is a dehydrated feed stream. In the context of the present invention a "dehydrated feed stream" is defined as a feed stream from which water is removed until the amount of water in the feed stream is less than 20 ppmv, preferably less than 5 ppmv, more preferably less than 1 ppmv.

Preferably, the feed gas stream gas has a pressure between 110-150 bara. More preferably, the feed gas stream is a dehydrated feed gas stream having a pressure between 110-150 bara. Most preferably, the feed gas stream is a dehydrated natural gas having a pressure between 110-150 bara . As will be clear from the above there exist a number of ways to obtain the liquid phase containing feed stream provided in step 1) from a feed gas stream, for instance, a feed gas stream from a subsurface reservoir. 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 liquefying the purified feed gas stream by methods known in the art.

The invention will be further illustrated by means of Figures 1 and 2. In Figure 1, a dehydrated natural gas having a temperature of 40 °C and a pressure of 146 bara is passed via a conduit 1 through a turbo expander 2, wherein nearly isentropic expansion is applied, whereby a stream is obtained comprising liquid phase comprising as liquid phase contaminants carbon dioxide and hydrogen sulphide and a gaseous phase comprising as gaseous contaminants carbon dioxide and hydrogen sulphide. The stream so obtained has a temperature of -30 °C and a pressure of 52 bara. Subsequently, the stream so obtained is passed via a conduit 3 to a valve 4, wherein isenthalpic expansion is applied, whereby a stream is obtained having a temperature of -64 °C and a pressure of 25 bara. The stream so obtained is then introduced via conduit 5 into a device 6 wherein the stream is cooled by means of direct contact with a cooling medium, preferably a fluid comprising hydrocarbons, to obtain a slurry having a temparture of -80 °C and a pressure of 25 bara. The slurry comprises solid carbon dioxide and hydrogen sulphide contaminants, liquid phase contaminant in the form of carbon dioxide and hydrogen sulphide, and a gaseous phase rich in methane. Subsequently, the slurry as introduced via a conduit 7 into a gas/liquid/solids separation device 8 from which a gaseous product rich in methane is removed via outlet means 9 and a stream comprising solid and/or liquid comprising carbon dioxide and hydrogen sulphide is removed via an outlet means 10.

In Figure 2, a dehydrated natural gas having a temperature of 40 °C and a pressure of 146 bara is passed via a conduit 1 through a valve 2, wherein isenthalpic expansion is applied, whereby a stream is obtained having a temperature of 2 °C and a pressure of 52 bara.

Subsequently, the stream so obtained is passed via a conduit 3 to a heat exchanger 4 wherein the stream is subjected to an isobaric cooling step to obtain a stream having a temperature of -30 °C and a pressure of 52 bara. Said stream comprises a liquid phase comprising as liquid phase contaminants carbon dioxide and hydrogen sulphide and a gaseous phase which comprises as gaseous contaminants carbon dioxide and hydrogen sulphide. The stream so obtained is then introduced via conduit 5 into a device 6 wherein the stream is cooled by means of direct contact with a cooling medium, preferably a fluid comprising hydrocarbons, to obtain a slurry having a temparture of - 80 °C and a pressure of 25 bara. The slurry comprises solid carbon dioxide and hydrogen sulphide contaminants, liquid phase contaminant in the form of carbon dioxide and hydrogen sulphide, and a gaseous phase rich in methane. Subsequently, the slurry as introduced via a conduit 7 into a gas/liquid/solids separation device 8from which a gaseous product rich in methane is removed via outlet means 9 and a stream comprising solid and/or liquid comprising carbon dioxide and hydrogen sulphide is removed via an outlet means 10.

The invention will also be illustrated using the following non-limiting examples. Example 1. The temperature of a gas stream comprising 21 volume% of carbon dioxide, 76 volume% of methane, 0.2 volume% of propane and 0.1 volume% of hydrogen sulphide and having a pressure of 80 bara is lowered to - 40 ⁰ C. The resulting feed stream is cooled by passing the stream through a Joule-Thomson valve, thereby lowering the pressure to 10 bara and the temperature to -79 ⁰ C. A slurry comprising solid carbon dioxide, a liquid phase comprising carbon dioxide and a gseous phase comprising methane is obtained. This slurry is introduced into a gas/liquids/solids separating device and a stream rich in gaseous product is removed from the separating device. The stream rich in gasesous product comprises 11.4 volume % of carbon dioxide.

Example 2. The temperature of a gas stream comprising 21 volume% of carbon dioxide, 76 volume% of methane, 0.2 volume% of propane and 0.1 volume% of hydrogen sulphide and having a pressure of 80 bara is lowered to - 67 ⁰ C to obtain a feed stream containing a liquid phase, in which liquid phase a substantial amount of liquid carbon dioxide is present. The resulting stream is cooled by passing the stream through a Joule-Thomson valve, thereby lowering the pressure to 10 bara and the temperature to -85 ⁰ C. A slurry comprising solid carbon dioxide, a liquid phase comprising carbon dioxide and a gseous phase comprising methane is obtained. This slurry is introduced into a gas/liquids/solids separating device and a stream rich in gaseous product is removed from the separating device. The stream rich in gasesous product only comprises 3.5 volume % of carbon dioxide .

The examples show that the process according to the invention, using a feed stream that comprises a liquid phase comprising contaminants, results in a gaseous product with a considerably lower amount of carbon dioxide.