Patent Application
20110094264
The present invention relates to a process for the removal of acidic contaminants from a gas stream comprising hydrocarbons and acidic contaminants. The invention especially relates to a process in which carbon dioxide and hydrogen sulphide are removed from natural gas that contains hydrocarbons and acidic contaminants.
Such a process is known from WO-A 2004/070297. This document discloses a process in which a natural gas stream comprising hydrocarbons and acidic contaminants is first cooled in a first vessel to remove water from the natural gas, and subsequently the natural gas is further cooled in a second vessel to solidify acidic contaminants or dissolve such contaminants in a liquid, which contaminants are removed so that a purified natural gas is recovered. In the specification it is acknowledged that solid acidic contaminants may block the outlet of the second vessel. To prevent solid acidic contaminants from blocking such outlet a warm liquid comprising natural gas condensates may be introduced into the lower part of the vessel so that at least part of the solid acidic contaminants melts.
In WO-A 2007/030888 a similar process for the removal of acidic contaminants from natural gas is described. In this process the formed solid acidic contaminants are heated to a temperature above the melting point temperature of the contaminants by means of a heat exchanger in the form of a bundle coil. The fluid that is passed through the bundle coil can be the natural gas or any other process stream. Alternatively, a liquid process stream derived from another part of the process can be mixed with the solid acidic contaminants to melt these contaminants. The addition of a relatively warm stream to the solid acidic contaminants has the advantage that it provides a more efficient and direct heat transfer than the indirect heat exchange via a bundle coil. However, by adding either a condensate stream or another process stream to the solid acidic contaminants, it may become necessary to separate these from the contaminants since otherwise a significant loss of valuable hydrocarbons could be incurred. Such separation needlessly complicates the process. The present invention has as objective to avoid such complications.
Accordingly, the invention provides a process for producing purified hydrocarbon gas from a gas stream comprising hydrocarbons and acidic contaminants, which process comprises the steps:
In the present process the reheated recycle stream is recycled to the vessel, to provide heat to the solid and optionally liquid contaminants to melt at least part of the solid acidic contaminants. In this way the benefits of direct heat exchange are obtained whilst no alien species are introduced into the mixture. Further, the process can be carried out also when no condensates are available. Moreover, the present process avoids the need to provide for a complex heat exchanger in the lower part of he vessel.
The gas stream can be any stream of gas that comprises acidic contaminants and hydrocarbons. In particular the process according to the present invention can be applied to a natural gas stream, i.e., a gas stream that contains significant amounts of methane and that has been produced from a subsurface reservoir. It includes a methane natural gas stream, an associated gas stream or a coal bed methane stream. The amount of the hydrocarbon fraction in such a gas stream is suitably from 10 to 85 mol % of the gas stream, preferably from 25 to 80 mol %. Especially, the hydrocarbon fraction of the natural gas stream comprises at least 75 mol % of methane, preferably 90 mol %. The hydrocarbon fraction in the natural gas stream may suitably contain from 0 to 20 mol %, suitably from 0.1 to 10 mol %, of C2-C6 compounds. The gas stream may also comprise up to 20 mol %, suitably from 0.1 to 10 mol % of nitrogen, based on the total gas stream.
Gas streams, such as natural gas streams, may become available at a temperature of from −5 to 150° C. and a pressure of from 10 to 700 bar, suitably from 20 to 200 bar. In the process of the present invention the gas stream comprises suitably hydrogen sulphide and/or carbon dioxide as acidic contaminants. It is observed that also minor amounts of other contaminants may be present, e.g. carbon oxysulphide, mercaptans, alkyl sulphides and aromatic sulphur-containing compounds. The major part of these components will also be removed in the process of the present invention.
The amount of hydrogen sulphide in the gas stream containing methane is suitably in the range of from 5 to 40 volume % of the gas stream, preferably from 20 to 35 volume % and/or the amount of carbon dioxide is in the range of from 10 to 90 vol %, preferably from 20 to 75 vol %, based on the total gas stream. It is observed that the present process is especially suitable for gas streams comprising large amounts of contaminants, e.g. 10 vol % or more, suitably between 15 and 90 vol %.
Gas stream containing the large amounts of contaminants as described above cannot be processed using conventional techniques as amine extraction techniques as they will become extremely expensive, especially due to the large amounts of heat needed for the regeneration of loaded amine solvent.
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 natural 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 for drying natural gas are also possible. Other dehydration processes include treatment with molecular sieves or drying processes with glycol or methanol. Suitably, water is removed until the amount of water in the natural gas stream comprises at most 50 ppmw, preferably at most 20 ppmw, more preferably at most 1 ppmw of water, based on the total natural gas stream.
As indicated above, acidic contaminants that are usually present in natural gas streams include hydrogen sulphide and carbon dioxide. It is also possible that the natural gas stream contains other components, including ethane, propane and hydrocarbons with four or more carbon atoms, even after an optional earlier recovery of condensates. It will be appreciated that when a portion of acidic contaminants, e.g., carbon dioxide, solidifies in the cooling stage, other components, e.g., hydrogen sulphide and hydrocarbons other than methane, may liquefy. The liquid components are suitably removed together with the solid acidic contaminants from the vapour.
In a first step of the present process the gas stream is cooled. The cooling may be effected by any known method, such as indirect heat exchange and expansion. Alternatively, a direct heat exchange, e.g., by spraying with a cold liquid, as shown in WO-A 2004/070297, is also possible. The skilled person will appreciate that expansion causes a lowering of temperature, so that cooling may be achieved by expansion and adapting pressure. Preferably the expansion is done by isenthalpic expansion, preferably isenthalpic expansion over an orifice or a valve, especially a Joule-Thomson valve or a series of Joule-Thomson valves. In another preferred embodiment the expansion is done by nearly isentropic expansion, especially by means of an expander, preferably a turbo expander, or a laval nozzle. The cooling may be conducted in several steps. It is preferred that the gas stream is subjected to heat exchange with one or more other cold process streams or external streams. Cold external streams may be suitably streams from an LNG (liquefied natural gas) plant, such a cold LNG stream or a refrigerant stream, or from an air separation unit. One suitable stream comprises the purified hydrocarbon gas. At such cooling hydrocarbons may condense and such liquid condensate may be recovered before the gas stream is cooled further to the temperature at which acidic contaminants solidify. Preferably, the cooling stage of the natural gas stream comprises one or more expansion steps. For this purpose conventional equipment may be used. Conventional equipment includes turbo-expanders, so-called Joule-Thomson valves and venturi tubes. It is preferred to at least partly cool the gas stream over a turbo-expander, releasing energy. One advantageous effect of using the turbo-expander is that the energy that is released in the turbo-expander can suitably be used for compressing at least part of the purified hydrocarbon gas. Since the stream of the purified hydrocarbon gas is smaller than the gas stream now that acidic contaminants have been removed, the energy is suitably such that the purified hydrocarbon gas may be compressed to an elevated pressure that makes it suitable for transport in a pipeline.
The cooling steps eventually lead to the desired temperature at which acidic contaminants solidify. However, since the natural gas stream also may comprise hydrocarbons other than methane it is preferred to cool the natural gas stream, suitably by expansion, to a temperature below the dew point of propane. In this way the vaporous natural gas stream will develop liquid hydrocarbons, including propane, which can subsequently be recovered easily from the vapour.
It is preferred to achieve the cooling in several steps, e.g., by indirect heat exchange and/or expansion. It is also possible to solidify by spraying with a cold liquid, as shown in WO-A 2004/070297. Suitably, solid acidic contaminants are obtained in a final expansion step. The final expansion step is preferably achieved over a Joule-Thomson valve. Therefore, preferably, in a first step, which may be achieved by various intermediate steps and various methods, the gas stream is cooled to a temperature ranging from 1 to 40° C. above the freeze out temperature of the first acidic contaminant to freeze out, the freeze out temperature being the temperature at which solid contaminants are formed. Preferably, the cooling is effected till from 2 to 10° C. above the freeze out temperature. In a final step the gas stream is preferably cooled to the temperature at which a mixture of solid and/or liquid acidic contaminants and a vapour comprising gaseous hydrocarbons are formed by expansion over a valve. Preferably, the gas stream is partly or completely liquid before being expanded over the valve, and solid contaminants are formed upon expansion. This ensures better separation performance in the vessel. Suitably, the gas stream is expanded from a pressure ranging from 40 to 200 bar to a pressure of 10 to 40 bar. Expansion over this pressure range suitably causes that solid acidic contaminants are formed. It will be appreciated by the person skilled in the art that at the formation of solid acidic contaminants also liquid acidic contaminants may be formed and/or hydrocarbons may condense. These liquid components are suitably separated together with the solid acidic contaminants.
The solidification of acidic contaminants may take place very rapidly, especially upon expansion over a valve, thereby forming the mixture comprising solid and optionally liquid acidic contaminants and a vapour comprising gaseous hydrocarbons. To facilitate the separation the mixture is passed into a vessel wherein the separation between solid acidic contaminants and vapour takes place. By gravity the solid acidic contaminants, and any liquid that is formed, drop to the bottom of the vessel. After such separation the solid acidic contaminants can be removed from the process.
After separation of solid and optionally liquid acidic contaminants, the purified hydrocarbon gas that is being recovered after the separation step can be used as product. The recovered purified hydrocarbon gas may also be subjected to further treatment and/or purification. For instance, the purified hydrocarbon gas may be subjected to fractionation. In the event that the purified hydrocarbon gas is natural gas intended for pipeline transportation or for producing liquefied natural gas (LNG),in order to reach pipeline specifications or LNG specifications the purified natural gas may further purified. Further purification can for example be done in an additional cryogenic distillation column, suitably with a bottom temperature between −30 and 10° C., preferably between −10 and 5° C. A reboiler may be present to supply heat to the column. Suitably the top temperature column is between −110 and −80° C., preferably between −100 and −90° C. In the top of the cryogenic distillation column a condenser may be present, to provide reflux and a liquefied (LNG) product.
As an alternative, further purification may be accomplished by absorption with a suitable absorption liquid. Suitable absorbing liquids may comprise chemical solvents or physical solvents or mixtures thereof.
A preferred absorbing liquid comprises a chemical solvent and/or a physical solvent, suitably as an aqueous solution.
Suitable chemical solvents are primary, secondary and/or tertiary amines, including sterically hindered amines.
A preferred chemical solvent comprises a secondary or tertiary amine, preferably an amine compound derived from ethanolamine, more especially DIPA, DEA, MMEA (monomethyl-ethanolamine), MDEA (methyldiethanolamine) TEA (triethanolamine), or DEMEA (diethyl-monoethanolamine), preferably DIPA or MDEA. It is believed that these chemical solvents react with acidic compounds such as CO2 and H2S.
Suitable physical solvents include tetramethylene sulphone (sulpholane) and derivatives, amides of aliphatic carboxylic acids, N-alkyl pyrrolidone, in particular N-methyl pyrrolidine, N-alkyl piperidones, in particular N-methyl piperidone, methanol, ethanol, ethylene glycol, polyethylene glycols, mono- or di(C1-C4)alkyl ethers of ethylene glycol or polyethylene glycols, suitably having a molecular weight from 50 to 800, and mixtures thereof. The preferred physical solvent is sulfolane. It is believed that CO2 and/or H2S are taken up in the physical solvent and thereby removed.
Other treatments may include a further compression, when the purified hydrocarbon gas is wanted at a higher pressure. If the amounts of acidic contaminants in the purified hydrocarbon gas are undesirably high, the purified hydrocarbon gas may be subjected to one or more repetitions of the present process.
In the event that the hydrocarbon gas is natural gas, the purified natural gas can be processed further in known manners, for example by catalytic or non-catalytic combustion to produce synthesis gas, to generate electricity, heat or power, or for the production of liquefied natural gas (LNG), or for residential use. It is an advantage of the present process enables purification of natural gas comprising substantial amounts of acidic contaminants, resulting in purified natural gas comprising low levels of contaminants, especially of sulphur contaminants. The production of LNG from such natural gas, which would be very difficult if not impossible by conventional processes, is made possible. Thus, the invention also provides LNG obtained from liquefying purified natural gas obtained by the process. The LNG thus-obtained typically has very low concentrations of contaminants other than natural gas.
Since it is easier to transport liquids than to transport solids, it is preferred to melt at least partly the solid acidic contaminants. Therefore, it has been proposed to heat at least a part of the solid acidic contaminants to cause melting, thereby yielding the heated contaminant-rich stream that is withdrawn from the vessel bottom, suitably by pumping.
According to the present process at least a part of the heated contaminant-rich stream is reheated to yield a reheated recycle stream. The recycle of part of the reheated recycle stream is meant to melt at least part of the solid acidic contaminants in the vessel so that blocking is prevented and removal of the acidic contaminants is facilitated. Preferably, the heat that is being provided by the recycled reheated recycle stream is such that it causes the melting of all solid acidic contaminants. The skilled person may achieve this by selecting the desired temperature of the reheated recycle stream and/or the amount of reheated recycle stream. Therefore, the part of the contaminant-rich stream that is reheated to form the reheated recycle stream is preferably heated to form a liquid stream, more preferably without any solid acidic contaminant. Suitably the heating up is done to a temperature well above the melting point of the solid acidic contaminants, such as at least 5° C. above the highest melting point. The heat of the relatively warm liquid will melt at least part of the solid acidic contaminants in the vessel. It is even more preferred that the part of the heated contaminant-rich stream that is reheated to form the reheated recycle stream is heated to such a temperature that the stream becomes at least partly vaporous. Not only will more energy be recycled to the vessel so that the melting of solid acidic contaminants is conducted more smoothly, but also any light hydrocarbon that may be entrained in the heated contaminant-rich stream will be freed up and can be included in the purified hydrocarbon gas that is withdrawn from the vessel. In this way the recovery of purified hydrocarbon gas is enhanced.
In order to improve the heat transfer between the warm fluid, i.e. either liquid or vaporous, and the cold solid and optionally liquid contaminants, the vessel is preferably provided with internals. These internals will increase the contacting surface between cold solids and warm fluid as well as provide residence time to the components in the vessel so that acidic contaminants may condense and/or solidify and liquid hydrocarbons may be evaporated. The skilled person may select the internals from a variety of possibilities. Very suitable are sieve plates, perforated plates or bubble trays. Their construction is relatively easy in the cryogenic environment of the vessel, whereas the contacting performance is very good.
An even more preferred embodiment comprises a vessel that has been provided with at least one deflecting means that has been arranged in the interior of the vessel. Downwards-falling solid and liquid acidic contaminants are distributed more homogeneously over the cross-section of the vessel, thereby improving the separation between solid and acidic contaminants on the one hand and the gaseous hydrocarbons on the other. The shape of the deflecting means can be selected from a variety of shapes; the deflecting means may, e.g., be square, circular or of a ring-shape. Preferably the deflecting means has downwards-directed slopes to avoid build up of solid material on the deflecting means. A very suitable shape is a cone or a combination of a cone and an inverted cone. Whereas the cone ensures a smooth distribution of solid and liquid material, the inverted cone provides for a suitable passage for upwards-flowing gases. The deflecting means suitably covers from 5 to 75% of the cross-section of the vessel.
It is evident that the reheating of part of the heated contaminant-rich stream requires energy. In a preferred embodiment a part of the heated contaminant-rich stream is separated and this part of the heated contaminant-rich stream is reheated to form the reheated recycle stream. In this way only a portion of the heated contaminant-rich stream requires to be heated up. The size of the part of the heated contaminant-rich stream that is separated can be selected by the skilled person depending on conditions such as the temperature of the reheated recycle stream and the amount and nature of the solid acidic contaminants. Suitably, the part of the heated contaminant-rich stream that is separated is selected such that the reflux ratio ranges from 0.5 to 10. This embodiment is especially advantageous when the part of the heated contaminant-rich stream is heated up to form a vaporous recycle stream.
In another embodiment of the present invention substantially the entire heated contaminant-rich stream that is withdrawn from the vessel is reheated to form the reheated recycle stream, and a part of the thus obtained reheated recycle stream is recycled to the mixture. This embodiment is especially useful when the heated contaminant-rich stream is heated up to a liquid. A part of the reheated recycle stream is recycled, whereas the other part is withdrawn, optionally after recovery of entrained hydrocarbons. The size of the part that is being recycled can be determined by the skilled person, based on the conditions, also indicated above. Suitably, the part of the reheated recycle stream that is recycled to the mixture is selected such that the reflux ratio ranges from 0.5 to 10.
The way in which the contaminant-rich stream is reheated can be done in any feasible way. External, e.g., electrical, heaters are possible. However, preferably, the reheating of at least part of the contaminant-rich stream is conducted via heat exchange. Any process stream with a sufficiently higher temperature can be used for this. This includes any condensate stream or hydrate-stream. Preferably, the heat exchange is conducted with at least part of the gas stream. Alternatively, a warm fluid may be added to the contaminant-rich stream. Suitable warm fluids include a natural gas condensate.
In the event that the contaminant-rich stream mainly comprises carbon dioxide and is therefore a CO2-rich stream, preferably CO2-rich stream is further pressurised and injected into a subterranean formation, preferably for use in enhanced oil recovery or for storage into an aquifer reservoir or for storage into an empty oil reservoir. It is an advantage that a liquid CO2-rich stream is obtained, as this liquid stream requires less compression equipment to be injected into a subterranean formation.
The process will be explained in more detail by means of the following figures.
In
A natural gas stream is introduced via a line 101 into a dehydrating unit 118. In the dehydration unit 118 water is being removed from the natural gas stream, e.g., by means of molecular sieves. The water is eventually removed via a line 102. The dehydrated natural gas is passed via a line 103 to a turbo-expander 119 where it is cooled, and subsequently forwarded via a line 104. The natural gas in line 104 is cooled further via a heat exchanger 122. Subsequently, the natural gas stream is passed via a line 105 for further heat exchange. To recover as much energy as possible the natural gas stream may be passed to a heat exchanger 124 wherein it exchanges heat with purified hydrocarbon gas and heated contaminant-rich stream. If desired further heat exchange may optionally be established in heat exchanger 125. Via a line 106 and la line 107 the further cooled natural gas stream is passed to a Joule-Thomson valve 126 where it is cooled to a temperature at which acidic contaminants solidify so that a mixture of solid acidic contaminants and vaporous hydrocarbons enter vessel 120 via a line 108. Alternatively, a venturi tube may be used instead of a Joule-Thomson valve. There separation takes place so that purified hydrocarbon gas is withdrawn at the top via a line 109. The figure shows that short line 108 connects the Joule Thomson valve 130 with the vessel 120. This line is typically short so that blocking of the line by solids is prevented. It is also possible to do away with the line altogether and connect the Joule Thomson valve directly to the wall of vessel 120.
The solid and optionally liquid acidic contaminants and optionally liquid hydrocarbons are falling down, preferably along a deflecting means (not shown), towards the bottom of vessel 120 where they are collected and heated by means of a warm reheated recycle stream entering the vessel 120 via a line 117, thereby melting solid acidic contaminants. The thus obtained heated contaminant-rich stream is withdrawn from the vessel via a line 112 which is pumped further using pump 121. The heated contaminant-rich stream is divided into a part that is withdrawn via a line 113 and a part that is forwarded to the heat exchanger 122 via a line 115. In heat exchanger 122 the part of the heated contaminant-rich stream is reheated by means of heat exchange with the natural gas stream provided via line 104, to form a reheated recycle stream. The reheated recycle stream is forwarded via line 116 to a valve 123 which controls the flow of the reheated recycle stream. Via the line 117, which may be provided with a nozzle (not shown) the reheated recycle stream is introduced into vessel 120.
The line 113 with the heated contaminant-rich stream leads the molten contaminants to the heat exchanger 124, and subsequently, the contaminants are withdrawn via line 114. In heat exchanger 124 the molten contaminants in line 113 and cold purified hydrocarbon gas in line 109 are subjected to heat exchange with the natural gas stream in line 105. The streams are shown in co-current fashion. It is evident to the skilled person that the streams may also be arranged in a counter-current way, e.g., the relatively warm natural gas stream is counter-current with the two other streams. It will be appreciated that it is also feasible to use only one of the other streams or use a stream from another process, such as a stream from an LNG plant and/or an air separation plant.
From the heat exchanger 124 the purified hydrocarbon gas is passed via a line 110 to a compressor 127. The compression energy for compressor 127 is suitably provided by the expander 119. The compressed gas may be recovered as product in line 111 or used for further treatment.
| Number | Date | Country | Kind |
|---|---|---|---|
| 08157282.8 | May 2008 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP09/56539 | 5/28/2009 | WO | 00 | 11/30/2010 |
