Patent Application
20160054054
This invention relates to processes and apparatus for low temperature separation of nitrogen from a gaseous mixture comprising nitrogen, hydrocarbons and at least 0.005 mol % carbon dioxide; in particular, a process and apparatus in which a heat pump provides refrigeration to one or more stages of the separation process.
Nitrogen is inert and does not burn, so natural gas containing more than several per cent nitrogen typically needs to be processed to remove nitrogen to meet sales gas specifications. This is normally performed by low temperature fractionation, with the exception of small scale facilities for which other technologies, such as membrane separation, may be appropriate, see for example Finn, A. J., “Rejection Strategies”, Hydrocarbon Engineering, vol. 12, no. 10, page 49, 2007.
Low temperature fractionation presents an energy efficient method for separation of nitrogen from gaseous hydrocarbon streams, particularly gaseous hydrocarbon streams wherein the hydrocarbons comprise predominantly methane, such as natural gas. Such techniques provide nitrogen streams of high purity, thereby maximizing hydrocarbon recovery and, where the nitrogen is vented to atmosphere, minimizing environmental impact.
However, the carbon dioxide content of such gases can have a significant impact on the processing requirements for nitrogen rejection. By way of example, the carbon dioxide content of natural gas varies widely, but carbon dioxide is commonly present at levels ranging from several hundred ppm to several percent. In particular, carbon dioxide solidifies at the temperatures employed in cryogenic nitrogen rejection units, and the carbon dioxide content of the feed gas is therefore limited to that which is soluble in the methane rich streams within the process, i.e. generally 0.3 to 0.4 mol % at most, or much lower if a pre-separation step is not also included. At higher feed gas carbon dioxide content, there is a need to remove this component to low enough levels to ensure that freezing cannot occur, as this will cause the blockage of process equipment and the consequent need for thawing. Furthermore, carbon dioxide has no calorific value, and is an additional “inert” component in the natural gas feed stream. Removal of carbon dioxide may therefore be necessary to meet the sales gas specifications, although this is not so stringent as the levels required for cryogenic processing. For example, a carbon dioxide content of several per cent may be tolerable in sales gas if it is the only inert. If nitrogen is present, then carbon dioxide will be removed in preference to removing nitrogen, if possible, due to the lower processing cost for carbon dioxide removal.
Where carbon dioxide solubility levels within the nitrogen removal unit would be exceeded, typically with feed gas of above 0.005 mol %, or around 0.3 mol % carbon dioxide content if a pre-separation step is to be included, removal of carbon dioxide is typically achieved upstream of the nitrogen rejection process, for example by using a chemical solvent to remove carbon dioxide in an Acid Gas Removal Unit (AGRU). There are a number of alternative process options for carbon dioxide removal, but all have considerable costs and impact on plant reliability and operation. Thus, there is great interest in avoiding the need for, or reducing the cost of, carbon dioxide removal.
Where feed gas nitrogen content is lower than approximately 35 mol %, typically 5 to 25 mol %, a fractionation column is commonly employed for “pre-separation” of the feed stream. A suitable fractionation column for pre-separation of a hydrocarbon feed gas is described in GB 2456691. The pre-separation fractionation column produces a bottom product with nitrogen content lower than that of the feed, and an overhead stream enriched in nitrogen which is suitable for further processing.
While the primary purpose of the pre-separation fractionation column is to recover a portion of the feed gas hydrocarbons at relatively high temperatures and pressures (and partially devoid of nitrogen), it is also effective in removing carbon dioxide from the feed gas, thereby avoiding this component passing with the overhead stream to low temperature parts of the process, where it has limited solubility. However, whilst the pre-separation column therefore provides some tolerance to carbon dioxide, carbon dioxide is concentrated in the bottom product from the column, and this product is conventionally reduced in pressure and evaporated to provide refrigeration in the temperature range −80 to −120° C. for cooling feed to the fractionation column and condensing the overhead vapour. Thus, in order to avoid problems arising due to carbon dioxide solidification during this further processing step, feed gas carbon dioxide contents of typically less than 0.3 mol % are necessary. Moreover, if there is no need for a condenser, for example as found in the process disclosed in GB 2456691, then using the condenser as a way of increasing carbon dioxide solubility is not necessarily optimal.
An example of a conventional cryogenic separation process is shown in
Carbon dioxide present in the feed gas (100) is concentrated in the hydrocarbon liquid stream (220) obtained from the bottom of the fractionation column (180). This carbon dioxide containing stream provides refrigeration for feed gas cooling and operation of the pre-separation column (180), but is not subjected to the very low temperatures required in downstream processing for the final separation of nitrogen in the downstream nitrogen rejection unit (NRU) (215).
Carbon dioxide solubility limitations at the low temperatures needed for processing the overhead stream (185) from the fractionation column (180) require an overhead condenser and rectification section (200) above the column feed. The operating temperature of the overhead condenser system (200) is typically in the range of −110 to −120° C.
A portion (221) of the liquid stream (220) obtained from the bottom of the fractionation column (180) is used to provide cooling in heat exchanger (190) for the overhead stream (185). However, solidification of carbon dioxide can occur in the stream (221) due to the pressure let down (and therefore temperature reduction) required to provide evaporation of the carbon dioxide containing liquid hydrocarbon stream (221) from the fractionation column (180) to provide sufficient refrigeration. To avoid such solidification, the maximum feed gas carbon dioxide concentration in the process illustrated in
Separating carbon dioxide from methane by cryogenic distillation at warmer temperatures, to avoid/minimize carbon dioxide passing to the nitrogen pre-separation section, is difficult due to the relatively close volatility of these components. Thus, where feed gas carbon dioxide concentration exceeds the above limit, it is generally performed upstream of the low temperature separation process, for example in an AGRU.
The present invention aims to provide a process and apparatus for the separation of a gaseous mixture comprising nitrogen gas and hydrocarbons which may operate effectively despite the presence of relatively high levels of carbon dioxide in the gaseous mixture.
There is provided a process for the separation of a gaseous feed comprising a mixture of hydrocarbons, nitrogen gas and at least 0.005 mol % carbon dioxide, the process comprising:
The heat pump used in the process of the invention provides refrigeration at relatively low temperatures, thereby replacing refrigeration provided by evaporating hydrocarbon streams containing the carbon dioxide separated from the feed gas, or allowing such evaporation to be done at relatively high temperature levels, and thereby increasing tolerance to carbon dioxide in the feed gas. Thus, the use of a heat pump to provide refrigeration to one or more stages of the separation process gives the potential to reduce, or eliminate, the requirement for carbon dioxide removal upstream of the cryogenic plant, for example in an AGRU, due to increased tolerance to carbon dioxide in the feed gas, significantly reducing the cost of the process. Furthermore, reduced expansion of the streams produced in the process also reduces the need for recompression, and therefore reduces the sales gas compression duty.
A heat pump is a system or device by which energy is transferred from a heat source to a heat sink against a temperature gradient. In particular, a heat pump system for use in the present invention is a system in which a refrigerant fluid is compressed and cooled, and subsequently expanded at one or more pressures below the condensing pressure, so that at least a part of the fluid is evaporated, and is then used in heat exchange to cool the gaseous feed and/or one or more streams generated by the separation process to provide refrigeration thereto. The cooled and expanded heat pump refrigerant fluid is at least partially condensed and heated in the heat exchange, and at least part of this heated refrigerant is recycled through the heat pump system.
Expansion of the heat pump refrigerant fluid may be carried out in any conventional manner, for example by use of expansion valves or one or more liquid or two-phase expansion turbines.
Compression of the heat pump refrigerant fluid may be carried out in any conventional manner, for example by use of a multi-stage compressor, particularly a multi-stage compressor incorporating inter-cooling and/or after-cooling.
The heat pump system may conveniently provide cooling to a number of stages of the separation process, and preferably provides cooling to at least one or more streams, more preferably two or more streams, generated in the separation process, and optionally also the gaseous feed.
The gaseous feed preferably comprises methane. For example, the gaseous feed may comprise or consist of natural gas. In preferred embodiments, the gaseous feed comprises up to 40 mol % nitrogen, more preferably up to 30 mol % nitrogen, and still more preferably up to 25 mol % nitrogen. In preferred embodiments, the gaseous feed comprises at least 1 mol % nitrogen, more preferably at least 2 mol % nitrogen, and still more preferably at least 5 mol % nitrogen. For example, particularly suitable gaseous feeds for use in the present invention may comprise from 1 to 40 mol % nitrogen, such as from 2 to 30 mol % nitrogen or from 5 to 25 mol % nitrogen. The gaseous feed may further comprise other inert gases, such as helium, or components such as ethane or propane.
The gaseous feed further comprises at least 0.005 mol % carbon dioxide, preferably from 0.01 mol % to 4.0 mol % carbon dioxide, more preferably from 0.2 mol % to 3.0 mol % carbon dioxide, even more preferably from 0.3 mol % to 2 mol % carbon dioxide.
The gaseous feed may be treated at any suitable combination of temperature and pressure. For example, at the beginning of the process the gaseous feed may be at a temperature of from −100° C. to 50° C., such as from 20° C. to 40° C., and at a pressure of from 1.0 MPa to 10 MPa, such as from 2 MPa to 6 MPa.
Any fluid effective over the temperature range of interest may be used as the heat pump refrigerant; for example one or more components derived from natural gas, such as nitrogen, methane, ethane or propane.
Preferably the heat pump refrigerant fluid contains no, or only very low amounts of, components that freeze at temperatures and conditions likely to be encountered in the process of the invention. In particular, the heat pump refrigerant fluid preferably comprises less than 0.02 mol % carbon dioxide, more preferably less than 0.01 mol % carbon dioxide.
In order to provide sufficient cooling to the process of the invention, the heat pump refrigerant is preferably at a temperature of −100° C. or lower after expansion in at least one stage of the process, more preferably −105° C. or lower, even more preferably −110° C. or lower.
Preferably, the heat pump refrigerant fluid is sub-cooled prior to expansion to reduce evolution of flash vapour. In particular, the heat pump refrigerant fluid may be sub-cooled in heat exchange with one or more cold streams generated in the process of the invention.
In a preferred embodiment, the process of the invention comprises an additional, intermediate, stage, in which the cooled and at least partly condensed gaseous feed is separated into a first intermediate stream with a nitrogen content lower than that of the gaseous feed and a second intermediate stream with a nitrogen content higher than that of the gaseous feed.
The cooled and at least partially condensed gaseous feed may be separated into the first intermediate stream and a second intermediate stream in any conventional apparatus, such as a separator or column. In a particularly preferred embodiment, this is carried out in a fractionation column, particularly a fractionation column including a reboil heat exchanger. The operating pressure for the fractionation column will typically be in the range of from 2.0 MPa to 4.0 MPa, and the gaseous feed is supplied at a pressure above, and preferably significantly above, the operating pressure of the fractionation column. Accordingly, the gaseous feed may be supplied at a pressure of at least 0.2 MPa above, more preferably at least 0.5 MPa above, and most preferably at least 1.0 MPa above the operating pressure of the fractionation column. The gaseous feed may be supplied to the fractionation column at a temperature of from −80° C. to −125° C., such as from −90° C. to −110° C.
In the particularly preferred embodiment, the process may further comprise reducing the levels of components having limited solubility at lower operating temperatures in the second intermediate stream. In particular, the levels of carbon dioxide in the second intermediate stream is preferably reduced to less than 0.01 mol %, more preferably less than 0.005 mol %. This reduction may be carried out in any conventional apparatus, for example by use of an overhead condenser and reflux drum above the column feed to effect rectification. In the particularly preferred embodiment, the first intermediate stream may contain substantially all of the carbon dioxide removed from the gaseous feed. Optionally this stream may be further processed in order to provide cooling to at least one stage of the process. For example, at least a portion of the first intermediate stream may be expanded to provide additional cooling for the gaseous feed, but this expansion/evaporation is carried out at a pressure and temperature selected to avoid solidification of carbon dioxide in the stream.
In the embodiment of the process in which the cooled and at least partly condensed gaseous feed is separated into a first intermediate stream and a second intermediate stream, at least part of the cooling may be provided by heat exchange with one or more cold streams generated in the process of the invention and/or heat exchange with the cooled heat pump refrigerant fluid.
Preferably, the gaseous feed undergoes one or more stages of partial condensation and vapour/liquid separation before separation into the first and second intermediate streams. This partial condensation and vapour/liquid separation may be carried out in any suitable apparatus, such as pressure reduction valve and a separator drum.
Preferably, in the embodiment of the process in which the cooled and at least partly condensed gaseous feed is separated into a first intermediate stream and a second intermediate stream, the second intermediate stream is subjected to downstream separation to produce a hydrocarbon rich stream very low in nitrogen and the nitrogen rich reject stream low in hydrocarbons. In this embodiment, the second intermediate stream is further cooled. Energy efficient cooling for this step may be obtained by heat exchange with at least a portion of the hydrocarbon rich stream very low in nitrogen obtained from the downstream separation and/or at least a portion of the nitrogen rich reject stream low in hydrocarbons obtained from the downstream separation.
The downstream separation may be effected according to any suitable procedure known in the art for the separation of nitrogen from a gaseous mixture comprising nitrogen and hydrocarbons. Suitable procedures include low temperature single column processes or low temperature double column processes. For instance, a suitable low temperature single column process may comprise one or more condensers. A suitable low temperature double column process may comprise final separation in a column operating near atmospheric pressure, and a high pressure column producing methane enriched and nitrogen rich streams which are fed to the low pressure column.
In an embodiment of the present invention, the heat pump system operates in a closed cycle, so that substantially all of the heated refrigerant fluid is recycled through the heat pump system. In this embodiment, any suitable fluid may be used as the heat pump refrigerant, for example components derived from natural gas such as nitrogen, methane, ethane or propane.
Alternatively, the heat pump refrigerant fluid may comprise at least a portion of one or more streams obtained from the gaseous feed in the separation process, wherein the stream comprises less than 0.02 mol % carbon dioxide, i.e. the heat pump operates in an open cycle with compression and recycle of at least a portion of one or more the streams produced in the process. In this embodiment, the heat pump refrigerant fluid may comprise at least a portion of the second intermediate stream, and most preferably at least a portion of the hydrocarbon stream very low in nitrogen obtained by downstream separation of the second intermediate stream. In this preferred embodiment, a portion of the hydrocarbon stream very low in nitrogen produced by downstream separation of the second intermediate stream is used as the heat pump refrigerant fluid, and the remainder of the stream is removed as a product stream. At least part of the heated refrigerant, obtained following heat exchange between the cooled and expanded heat pump refrigerant fluid and the gaseous feed and/or one or more streams generated by the separation process, is recycled through the heat pump system. Preferably therefore, the heat pump refrigerant consists essentially of a portion of the hydrocarbon rich stream very low in nitrogen obtained by downstream separation of the second intermediate stream, together with recycled refrigerant fluid.
Before removal from the system, the hydrocarbon rich stream very low in nitrogen obtained by downstream separation of the second intermediate stream may be blended with streams high in carbon dioxide, such as the first intermediate stream, to balance the carbon dioxide content of the streams removed from the system.
It will be appreciated by the skilled person that the residual nitrogen content of the hydrocarbon rich product stream low in nitrogen, and the nitrogen content of the nitrogen rich reject stream low in hydrocarbons, are dependent on the composition of the gaseous feed. However, the hydrocarbon rich product stream low in nitrogen may comprise less than 5 mol % nitrogen gas, for example less than 3 mol % nitrogen gas, such as less than 2 mol % nitrogen gas. The nitrogen rich reject stream low in hydrocarbons may comprise more than 95% nitrogen gas, for example more than 99 mol % nitrogen gas.
In a further aspect, the present invention provides an apparatus for the separation of a gaseous feed comprising a mixture of hydrocarbons, nitrogen gas and at least 0.005 mol % carbon dioxide, the apparatus comprising:
The heat pump system used in the apparatus of the present invention may comprise any conventional components used in heat pump systems. For example, the means to expand the heat pump refrigerant fluid may include components such as expansion valves, or preferably, one or more liquid or two-phase expansion turbines. Advantageously, the use of liquid or two-phase expansion turbines allows energy to be recovered from the system, further improving the efficiency of the separation.
The means to compress the heat pump refrigerant preferably comprises a multi-stage compressor, most preferably incorporating inter-cooling and/or after-cooling.
In a particular embodiment, the apparatus of the invention comprises means for separating the gaseous feed into a first intermediate stream with a nitrogen content lower than that of the gaseous feed, and a second intermediate stream with a nitrogen content higher than that of the gaseous feed. This apparatus may comprise any conventional separation components, but preferably comprises a fractionation column.
In a preferred embodiment, the fractionation column includes a reboil heat exchanger, which enables reboil in the fractionation column to be provided in an energy efficient way during cooling of the gaseous feed. The reboil heat exchanger may be submerged in liquid in the sump of the fractionation column, or alternatively boiling liquid at the bottom of the fractionation column may be piped to the reboil heat exchanger from a bottom tray or packed section of the fractionation column.
In a preferred arrangement, the fractionation column further incorporates an overhead condenser and a reflux drum above the column feed. This apparatus may be used in order to reduce the level of components having limited solubility at lower operating temperatures, particularly such as carbon dioxide, in the second intermediate stream.
Preferably, the apparatus comprises means for partial condensation and vapour/liquid separation upstream of the fractionation column. These means may include one or more heat exchangers and/or expansion valves.
Heat exchangers used in the apparatus of the invention include multistream heat exchangers which combine a number of heat exchange duties into a single heat exchange unit. For example, heat exchangers may be multistream plate-fin type heat exchangers, such as multistream brazed aluminium plate-fin type heat exchangers.
Preferably, the apparatus further comprises means to process the second intermediate stream to produce a hydrocarbon rich stream very low in nitrogen and the nitrogen rich reject stream low in hydrocarbons. Any suitable means may be used, such as typical nitrogen rejection units, including single column and double column units.
The apparatus of the invention may be arranged so that the heat pump system forms an essentially closed cycle, so that substantially all of the heated refrigerant fluid produced by heat exchange with one or more of the gaseous feed and/or streams generated by the separation process is recycled through the heat pump system. Alternatively, the apparatus may be arranged so that the heat pump refrigerant fluid comprises at least a portion of one or more streams obtained from the gaseous feed in the separation process comprising less than 0.02 mol % carbon dioxide. For example, the apparatus may be arranged so that the heat pump refrigerant fluid comprises at least a portion of the second intermediate stream and/or a portion of the hydrocarbon rich stream very low in nitrogen obtained by downstream processing of the second intermediate stream.
The invention will now be described in greater detail with reference to a preferred embodiment of the invention and with the aid of the accompanying figures in which:
The apparatus also comprises a heat pump system, which incorporates the heat exchangers (105, 125) and overhead condenser (190), together with pressure reduction valves (430, 460) and a multistage compressor with inter-cooling and after-cooling (335, 345, 355, 365, 380, 390).
Dry feed gas (100) enters the process, for example at a temperature of from 10° C. to 50° C. and a pressure of from 1.0 MPa to 10 MPa; and is cooled against returning refrigerant and product streams in the primary heat exchanger (105). The partially cooled feed stream (110) is cooled further in heat exchanger (115), where it provides heat for reboil of the fractionation column (180), and then in the secondary feed gas cooler (125). The cooled, and at least partially condensed stream (130), is let down in pressure across a valve (150) and routed to the fractionation column (180) as a liquid or two-phase stream (155), for example at a temperature of from −80° C. to −125° C. and a pressure of from 2.0 MPa to 4 MPa.
The pre-separation column (180), with associated reboiler (115) and overhead condenser (190), produces a methane rich liquid stream (220), low in nitrogen and containing the majority of carbon dioxide from the feed gas, and a nitrogen rich vapour stream (205), which is essentially free of carbon dioxide. The condenser (190), cools the methane-rich stream to a temperature of −100° C. or lower, for example from −110° C. to −120° C. A reflux stream (210) is returned to the column following partial condensation of the overhead stream (185) in the overhead condenser (190), and separation of the vapour (205) and liquid (210) phases in the separator (200). Refrigeration for the overhead condenser (190) is principally provided by a heat pump refrigerant stream (435), together with returning product (290) and reject nitrogen (270) streams from the downstream nitrogen rejection unit (NRU) (215).
The nitrogen rich top product from the pre-separation column (205) is routed to the downstream nitrogen rejection unit (NRU) (215), where the separation of nitrogen from methane is completed. There are various process options, but this may typically be based on a double column process, enabling high methane recovery in a self-refrigerated process. This low temperature unit produces a high purity reject nitrogen stream (290) at a pressure typically just above atmospheric pressure, and a high purity methane product stream (270) which is pumped from the NRU (215) at low pressure.
The nitrogen stream (290) is rewarmed in heat exchange with warm streams in heat exchangers (190, 125 and 105), and is typically rejected to atmosphere. The low pressure methane stream (270) is also evaporated and rewarmed in heat exchangers (190, 125 and 105), and a portion of this stream from the NRU (270), with very low carbon dioxide content, such as less than 0.02 mol %, for example less than 0.01 mol %, is used in a heat pump refrigeration cycle to provide the balance of cooling duty for the process.
The re-warmed methane product stream from the NRU (285) is compressed in the multistage compressor with inter-cooling and after-cooling (335, 345, 355, 365, 380, 390), typically against ambient air or cooling water. A portion of the compressed product gas stream (400) from the NRU (215) is combined with the product stream (330) separated in the fractionation column (180) to make up the overall sales gas stream (480).
A portion (405) of the high pressure gas stream (400) is recycled to heat exchangers (105 and 125) where it is cooled and condensed against returning streams, primarily the evaporating product from the pre-separation column (235 and/or 260).
A portion (420) of the condensed refrigerant stream (415) is further cooled in heat exchanger (190), giving a sub-cooled liquid methane stream (425) which is reduced in pressure across a valve (430). The resulting low temperature stream (435), having a temperature of, for example −100° C. or lower, −110° C. or lower, or −120° C. or lower, is used to provide the majority of the refrigeration in overhead condenser (190). Further evaporation and re-warming of the refrigerant stream contributes towards cooling of the feed/refrigerant in heat exchangers (125 and 105). The re-warmed stream (450) is re-introduced at an intermediate stage in the low carbon dioxide gas compression train (355, 365, 380, 390).
A further portion (455) of the condensed refrigerant stream (415) is reduced in pressure across a valve (460). The resulting low temperature stream (465) is used to provide the balance of cold duty required to produce a cooled and at least partially condensed feed stream (130) and a condensed high pressure heat pump refrigerant fluid (415). Further evaporation and re-warming of the refrigerant stream contributes towards cooling of the feed/refrigerant in heat exchanger (105). The re-warmed stream (475) is introduced at an intermediate stage in the compression train (380, 390).
The liquid stream (220) from the fractionation column (180) is evaporated and re-warmed in heat exchangers (125 and 105). The evaporation pressure of this stream is maximised to avoid low temperatures, and maintain solubility of carbon dioxide in the liquid hydrocarbons in this stream to avoid solidification. The re-warmed stream (245) is routed to a separate compression train (310, 320). Optionally, as a function of feed gas pressure and nitrogen content, it may be possible to pump part or all of the liquid stream (220) with a portion (250) being evaporated and re-warmed at elevated pressure in heat exchanger (105) to further reduce compression requirements in the high carbon dioxide compressor (310).
Table 1 shows typical operating parameters for the apparatus of this invention shown in
Predicted carbon dioxide concentration solubility is presented for key process streams in
| Number | Date | Country | Kind |
|---|---|---|---|
| 1306342.5 | Apr 2013 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2014/050910 | 3/24/2014 | WO | 00 |
