The invention relates to a process for the removal of contaminants from a feed stream comprising 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 desired gaseous product, also contaminants such as 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, these 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 known 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 liquid phase containing feed stream meeting particular temperature and pressure requirements 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 process for removing contaminant from a feed stream having a temperature of less than -40 °C and a pressure of less than 110 bara, which feed stream comprises a liquid phase or a liquid phase and a gaseous phase, wherein the liquid phase comprises liquid phase contaminant and the gaseous phase, if present, comprises a gaseous product and gaseous contaminants, the process comprising: 1) providing the feed stream;

2) lowering the temperature and optionally the pressure of the feed stream to obtain a stream having a temperature between -50 and -85 ⁰ C and optionally a pressure between 30 and 80 bara; 3) cooling the stream as obtained in step 2) further to a temperature at which a slurry is formed which comprises a gaseous phase rich in gaseous product, solid phase contaminant and optionally liquid phase contaminant; 4) introducing the slurry obtained in step 3) into a gas/liquid/solids separation device; and 5) removing from the separation device a gas stream rich in gaseous product and a stream comprising solid contaminant and optionally comprising liquid phase contaminant, wherein the solid contaminant, liquid phase contaminant and gaseous contaminants are carbon dioxide and/or hydrogen sulphide. It has been found that by lowering the temperature of a liquid phase containing 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 feed stream provided in step 1) has a temperature of less than -40 °C and a pressure of less than 110 bara. The temperature of the stream as obtained in step 2) preferably is between 1 and 40 °C above the temperature at which carbon dioxide freezes from the feed stream, more preferably between 2 and 20 °C above the temperature at which carbon dioxide freezes from the feed stream. Thus, in step 2) of the process the temperature and optionally the pressure of the feed stream are lowered to obtain a stream having a temperature between -50 and -85 ⁰ C and optionally a pressure between 30 and 80 bara.

As a result, the feed stream obtained after step 2 comprises a substantial amount of liquid phase, the liquid phase comprising contaminants as well as geasous products. Preferably, the feed stream obtained in step 2) comprises more than 80 volume %, more preferably more than 90 volume % of liquid phase. Most preferably, the feed stream obtained in step 2) consists entirely of the liquid phase. Because the feed stream obtained after step 2 comprises a substantial amount fo liquid phase, step 3 can be performed in a more controlled and in a more efficient way.

Preferably, in step 3) of the present process the stream as obtained in step 2) is cooled to a temperature between -75 and -110 ⁰ C, more preferably between -80 and - 105 ⁰ C. The cooling in step 3) is preferably done by isenthalpic expansion, more preferably isenthalpic expansion over an orifice or a valve, especially a Joule- Thomson valve. Preferably, the liquid phase containing 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 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.

Suitably, the feed stream provided in step 1) is obtained by cooling a feed gas stream which comprises the gaseous product and gaseous contaminants by nearly isentropic expansion, especially by means of an expander, preferably a turbo expander or a laval nozzle; by isenthalpic expansion, preferably isenthalpic expansion over an orifice or a valve, especially a Joule-Thomson valve; and/or by isobaric cooling, preferably isobaric cooling using one or more heat exchangers. In a preferred embodiment of the present invention, the feed stream provided in step 1) is obtained by cooling a feed gas stream in a step a) by nearly isentropic expansion, especially by means of an expander, preferably a turbo expander or a laval nozzle, and/or by isenthalpic expansion, preferably isenthalpic expansion over an orifice or a valve, especially a Joule-Thomson valve, to a first temperature, and cooling the stream so obtained in a step b) by isobaric cooling, preferably isobaric cooling over one or more heat exchangers, to a second temperature which is lower than the first temperature.

In another preferred embodiment of the present invention, the feed stream provided in step 1) is obtained by cooling a feed gas stream in a step a) by isobaric cooling, preferably isobaric cooling over one or more heat exchangers, to a first temperature, and cooling the stream so obtained in a step b) by nearly isentropic expansion, especially by means of an expander, preferably a turbo expander or a laval nozzle, and/or by isenthalpic expansion, preferably isenthalpic expansion over an orifice or a valve, especially a Joule-Thomson valve, to a second temperature which is lower than the first temperature.

In these preferred embodiments, the first temperature in step a) is preferably between -75 and 15 °C, and the pressure of the stream so obtained is preferably between 40 and 150 bara.

Preferably, at the first temperature liquid phase contaminant is formed as well as a gaseous phase rich in gaseous product. Suitably, the feed gas stream is a natural gas and 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 gas stream has a pressure lower than 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 following modes of operation constitute suitable embodiments to obtain the liquid phase containing feed stream as provided in step 1) from a feed gas stream. These embodiments illustrate, but do not limit in any way, the present invention . A. A dehydrated natural gas stream having a pressure between 110-150 bara is cooled in a step a) by means of isenthalpic expansion, preferably isenthalpic expansion over a valve, especially a Joule-Thomson valve, to a temperature between -15 and 15 °C, whereby the stream so obtained has a pressure between 40 and 80 bara. The stream so obtained is then subjected to a single isobaric cooling step or a sequence of isobaric cooling steps. In the isobaric coolings step(s) use can be made of heat exchangers. If use is made of a single isobaric cooling step, the stream having temperature between -15 and 15 °C and a pressure between 40 and 80 bara is preferably cooled by means of a heat exchanger to obtain a stream having a temperature between -85 and -50 °C and a pressure between 40 and 80 bara. In case, two or more isobaric coolings steps are used, the following embodiments are possible. The stream as obtained in step a) and having a temperature between -15 and 15 °C and a pressure between 40 and 80 bara is first preferably cooled by means of a first heat exchanger to obtain a stream having a temperature between - 40 and -25 °C and a pressure between 40 and 80 bara. Subsequently, the stream so obtained is cooled, preferably by means of a second heat exchanger, to obtain a stream having a temperature between -60 and -40 °C and a pressure between 40 and 80 bara. The stream so obtained is then cooled, preferably, by means of a third heat exchanger to obtain a stream having a temperature between -85 and -50 °C and a pressure between 40 and 80 bara. Alternatively, the stream as obtained in step a) and having temperature between -15 and 15 °C and a pressure between 40 and 80 bara is first cooled, preferably by means of a first heat exchanger, to obtain a stream having a temperature between -60 and -40 °C and a pressure between 40 and 80 bara.

Subsequently, the stream so obtained is preferably cooled by means of a second heat exchanger to obtain a stream having a temperature between -85 and -50 °C and a pressure between 40 and 80 bara. B. A dehydrated natural gas stream having a pressure between 110-150 bara is cooled in a step a) by nearly isentropic expansion, especially by means of an expander, preferably a turbo expander, to obtain a stream having a temperature between -40 and -25 °C and a pressure between 40 and 80 bara. The stream so obtained can subsequently cooled by means of one or more isobaric cooling steps. The feed stream obtained in step a) is firstly cooled, preferably by means of a first heat exchanger, to obtain a stream having a temperature between -60 and -40 °C and a pressure between 40 and 80 bara. Subsequently, the stream so obtained is preferably cooled by means of a second heat exchanger to obtain a stream having a temperature between -85 and -60 °C and a pressure between 40 and 80 bara. Alternatively, the stream as obtained in step a) and having temperature between -40 and -25 °C and a pressure between 40 and 80 bara is cooled in a single isobaric cooling step, wherein preferably use is made of a heat exchanger, to obtain directly a stream having a temperature between -85 and -60 °C and a pressure between 40 and 80 bara. C. A dehydrated natural gas stream having a pressure between 110-150 bara is isobarically cooled, preferably, by means of a first heat exchanger to obtain a feed stream having a temperature between -20 and 10 °C and a pressure between 110 and 150 bara. The feed stream so obtained is then subjected to a second isobaric cooling step wherein preferably use is made of a second heat exchanger to obtain a stream having a temperature between -75 and -60 °C and a pressure between 110 and 150 bara. The stream obtained in this second isobaric cooling step can subsequently be subjected to one or more expansion steps to obtain a stream having a temperature between -85 and -60 °C and a pressure between 40 and 80 bara. The stream obtained in the second isobaric cooling step can be expanded in a single expansion step to obtain a stream having a temperature between -85 and -60 °C and a pressure between 40 and 80 bara. Preferably, in such an expansion step use is made of nearly isentropic expansion or isenthalpic expansion, whereby use can be made of a valve or a turbo expander respectively. Alternatively, the stream obtained in the second isobaric cooling step is firstly subjected to a first expansion step to obtain a stream having a temperature between -75 and - 60 °C and a pressure between 70 and 100 bara, whereafter the stream so obtained is subjected to a second expansion step to obtain a stream having a temperature between -85 and -60 °C and a pressure between 40 and 80 bara. In both the first and second expansion step use can be made of nearly isentropic expansion or isentropic expansion, or in the first expansion step use is made of nearly isentropic expansion and in the second expansion step use is made of isenthalpic expansion or vise versa. Preferably, in the isenthapic and nearly isentropic expansions, valves or turbo expanders are respectively used.

D. A dehydrated natural gas stream having a pressure between 110-150 bara is isobarically cooled, preferably by means of a first heat exchanger, to obtain a feed stream having a temperature between -20 and 10 °C and a pressure between 110 and 150 bara. The feed stream so obtained is then subjected to one or more expansion steps to obtain a stream having a tempereature between -60 and -40 °C and a pressure between 40 and 80 bara. The feed stream so obtained is then subjected to an isobaric cooling step wherein preferably use is made of a heat exchanger to obtain a stream having a temperature between -85 and -60 °C and a pressure between 40 and 80 bara. The feed stream having a temperature between -20 and 10 °C and a pressure between 110 and 150 bara can be subjected to a single expansion step using nearly isentropic or isenthalpic expansion, wherein preferably use is made of a valve or a turbo expander respectively, to obtain a stream having a temperature between between -60 and -40 °C and a pressure between 40 and 80 bara. Alternatively, the feed stream having a temperature between -20 and 10 °C and a pressure between 110 and 150 bara can be subjected to two separate expansion steps. In a first expansion step using nearly isentropic or isenthalpic expansion, a stream is obtained having a temperature between between -40 and -15 °C and a pressure and a pressure between 70 and 100 bara. In such isentropic expansion preferably use is made of a turbo expander, whereas in such isenthalpic expansion preferably use is made of a valve. Subsequently, the stream so obtained can be subjected to a second expansion step using nearly isentropic or isenthalpic expansion, wherein preferably use is made of a valve or a turbo expander respectively, to obtain a stream having a temperature between -60 and -40 °C and a pressure between 40 and 80 bara. Subsequently, the isobaric cooling step will be carried out to obtain the stream having a temperature between -85 and -60 °C and a pressure between 40 and 80 bara .

E. A dehydrated natural gas stream having a pressure between 110-150 bara is isobarically cooled by means of a first heat exchanger to obtain a feed stream having a temperature between -20 and 10 °C and a pressure between 110 and 150 bara. The feed stream so obtained is then subjected to an expansion step using nearly isentropic or isenthalpic expansion, wherein preferably use is made of a valve or a turbo expander respectively, to obtain a feed stream having a temperature between a temperature between between -40 and -15 °C and a pressure between 70 and 100 bara. Subsequently, the stream so obtaine can be subjected to a isobaric cooling step wherein preferably use is made of a heat exchanger to obtain a stream having a temperature between -75 and -60 °C and a pressure between 70 and 100 bara. The feed stream so obtained is then subjected to an expansion step using nearly isentropic or isenthalpic expansion, wherein preferably use is made of a valve or a turbo expander respectively, to obtain a stream having a temperature between -85 and -60 °C and a pressure between 40 and 80 bara.

The skilled person will understand that a wide variety of sequences of isobaric, isentropic and isenthalpic cooling steps can be used to obtain from a preferably dehydrated feed gas stream the liquid phase containing feed stream to be provided in step 1) of the process according to the present invention.

The 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 Figure 1. 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 -32 °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. The stream so obtained has a temperature of -70 °C and a pressure of 52 bara. Subsequently, the stream so obtained is then passed via a conduit 5 through a valve 6 which involves isentropic cooling to obtain a slurry having a temperature of -104 °C and a pressure of 10 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.

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 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. 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 results in a gaseous product with a considerably lower amount of carbon dioxide. In the preferred temperature range of -60 to -80 ⁰ C, the amount of carbon dioxide is even substantially lower.