GB2521177A

GB2521177A - Process and apparatus for separation of carbon dioxide and hydrocarbons

Info

Publication number: GB2521177A
Application number: GB1321942.3A
Authority: GB
Inventors: Terence Ronald Tomlinson; Adrian Joseph Finn; Muneeb Nawaz
Original assignee: Costain Oil Gas and Process Ltd
Current assignee: Costain Oil Gas and Process Ltd
Priority date: 2013-12-11
Filing date: 2013-12-11
Publication date: 2015-06-17
Anticipated expiration: 2033-12-11
Also published as: GB2521177B; GB201321942D0

Abstract

A process and apparatus for the separation of carbon dioxide (CO2) from a gaseous feed comprising methane and carbon dioxide wherein the apparatus comprises a first fractionation column 210 including a first reboil heat exchanger 115 and a second fractionation column 255 including a second reboil heat exchanger 125. A means is provided for cooling and partially condensing the gaseous feed in heat exchange with boiling liquid in the first reboil heat exchanger, and further cooling and partially condensing the cooled and partially condensed gaseous feed in heat exchange with boiling liquid in the second reboil heat exchanger. A means is provided for feeding at least a portion of the further cooled and partially condensed gaseous feed to the first fractionation column, for separation into a first bottom stream 220 having a carbon dioxide concentration higher than that of the gaseous feed and a first overhead stream 225 having a methane concentration higher than that of the gaseous feed. A means is provided for feeding at least a portion of the first overhead stream to the second fractionation column, for separation into a second bottom stream 260 having a carbon dioxide concentration higher than that of the first overhead stream and a second overhead stream 285 having a methane concentration higher than that of the first overhead stream. The process and apparatus is useful for the separation of gaseous mixtures comprising methane and at least 10 mol% carbon dioxide, is effective over wide ranges of carbon dioxide and hydrocarbon content, and requires reduced refrigeration and power consumption.

Description

PROCESS AND APPARATUS FOR SEPARATION OF CARBON DIOXIDE AND

HYDROCARBONS

This invention relates to processes and apparatus for the separation of carbon dioxide from a gaseous mixture comprising methane and at least 10 mol% carbon dioxide; in particular, a process and apparatus in which relatively pure methane is obtained from a mixture comprising methane, at least 10 mol% carbon dioxide and one or more natural gas liquids, such as a stream obtained from an enhanced oil recovery process using injected high pressure carbon dioxide.

Carbon dioxide has been used extensively to increase the recovery of oil from depleted oil fields, based on the availability of naturally occurring carbon dioxide. The use of carbon dioxide captured from flue gas has also been developed, for example using carbon dioxide produced from coal gasification. Carbon dioxide is miscible with crude oil and can increase oil production by over 100% of pre-flood rates. Between 3 and 15 Mscf (80 to 450 m3) of carbon dioxide is required per barrel of recovered oil.

Additionally, carbon dioxide injection into depleted oil fields is increasingly seen as an attractive method of storing carbon dioxide and reducing emission of greenhouse gases to the atmosphere, and this also provides revenue from the enhanced oil recovery (EOR). The technique is particularly appropriate where oil fields have low recovery rates and are located close to sources of carbon dioxide (to minimise transportation costs) and/or where carbon dioxide emissions into the atmosphere incur a significant cost penalty; see Journal of Petroleum Technology; January 2012; page 70.

The injected carbon dioxide ultimately breaks through with the produced oil and associated gas, and therefore the proportion of carbon dioxide present in natural gas produced using enhanced oil recovery techniques varies widely and may change very quickly; i.e. from very low levels of carbon dioxide (less than 10 mol%) at the start of the recovery process to levels as high as 90 mol% at the end of the recovery process.

Using carbon dioxide for enhanced oil recovery also leads to the presence of hydrocarbons that are heavier than methane in the recovered gas. In particular, so-called natural gas liquids (NGL5) may be present in significant quantities, and the proportions of these materials also increase over the recovery period, so that eventually over 10% of crude production can be present in the gas phase with the carbon dioxide.

Thus, enhanced oil recovery using carbon dioxide leads to the production of a gaseous mixture comprising methane and varying amounts of carbon dioxide and NGLs, with all three components being potentially valuable if separated to a sufficient degree of purity.

For example, in order to meet sales gas' specifications, the methane component of the mixture should be separated sufficiently to meet natural gas transmission, hydrocarbon dewpoint and heating value specifications, and the level of carbon dioxide contained in the methane should be 2 mol% or less. Additionally, the carbon dioxide component of the mixture may be used in further enhanced oil recovery. However, in order to be useful in such a process it must be purified sufficiently to exceed the minimum miscibility pressure (MMP) of the oil field for recycle of the carbon dioxide, and should also be within agreed transportation limits. A typical specification is 3 mol% maximum of total nitrogen, methane and ethane, as these components increase MMP; see Johnson, J.E.

and Walter, F.B.; Hydrocarbon Processing; October 1985; page 62. Finally, natural gas liquids are also valuable products when separated from the carbon dioxide and methane components of the mixture.

Various means are known for separating carbon dioxide and/or NGLs from gaseous methane mixtures, including the use of various solvent and/or semi-permeable membranes techniques. However, such processes are not suitable for use with gaseous mixtures comprising large volumes of carbon dioxide, or for producing relatively high pressure products. Alternative processes include cryogenic distillation separation techniques; see, for example, US 4,318.723 and 4,451,274, which both disclose cryogenic distillation separation methods in which a solvent is used to prevent the freezing of carbon dioxide during separation of the methane and carbon dioxide in a cryogenic demethanizer column.

Further details of cryogenic separation procedures are given in Section 16 of the GPSA Engineering Data Book; 13th Edition volume II; in particular, page 16-48 discusses a four column separation process, in which methane is removed from the feed gas in a first column, with carbon dioxide and methane being removed in the overhead stream, before being compressed for subsequent carbon dioxide recovery and demethanization stages.

This process incorporates a large degree of carbon dioxide recycle, together with hydrocarbon solvent, which, in the case of high carbon dioxide content feed, will lead to significant energy consumption for both the reboiling of fractionation columns and to supply process refrigeration.

The present invention aims to provide a process and apparatus for the separation of gaseous mixtures comprising methane and at least 10 mol% carbon dioxide, and optionally hydrocarbons heavier than the methane, which is effective over very wide ranges of carbon dioxide and hydrocarbon content, and which requires reduced refrigeration and power consumption.

There is provided a process for the separation of carbon dioxide from a gaseous feed comprising methane and at least 10 mol% carbon dioxide, the process comprising: (i) cooling and at least partially condensing the gaseous feed in heat exchange with boiling liquid at the bottom of a first fractionation column in a first reboil heat exchanger; (ii) further cooling and at least partially condensing the cooled and at least partially condensed gaseous feed in heat exchange with boiling liquid at the bottom of a second fractionation column in a second reboil heat exchanger; (iii) feeding at least a portion of the further cooled and at least partially condensed gaseous feed to the first fractionation column, and separating it therein into a first bottom stream having a carbon dioxide concentration higher than that of the gaseous feed and a first overhead stream having a methane concentration higher than that of the gaseous feed; and (iv) feeding at least a portion of the first overhead stream to the second fractionation column, and separating it therein into a second bottom stream having a carbon dioxide concentration higher than that of the first overhead stream and a second overhead stream having a methane concentration higher than that of the first overhead stream.

The process of the present invention uses the gaseous feed to provide heat to the first and second fractionation columns by heat exchange with boiling liquid at the bottom of the columns in reboil heat exchangers, which consequently also cools and at least partially condenses the gaseous feed, and this reduces the refrigeration requirement of the process. Additionally, by carrying out a first separation step removing a large proportion of the carbon dioxide, and any hydrocarbons heavier than methane present in the gaseous feed, in the first fractionation column, followed by a subsequent second methane fractionation step in the second fractionation column, the refrigeration requirement, and therefore power consumption, of the process is again reduced, whilst a high degree of carbon dioxide removal from methane is achieved.

The process of the present invention is suitable for the separation of carbon dioxide from gaseous feeds comprising methane and at least 10 mol% carbon dioxide obtained from any source. However, the process of the present invention is particularly suitable for separating carbon dioxide from gaseous feeds further comprising one or more natural gas liquids (NGL5), for example feed streams obtained in an enhanced oil recovery (EOR) process.

Where the gaseous feed used in the process of the present invention further comprises one or more NGLs, the first bottom stream produced in the first fractionation column will have an NGL concentration higher than that of the gaseous feed, and the first overhead stream will have an NGL concentration lower than that of the gaseous feed. Additionally, the second bottom stream will have an NGL concentration higher than that of the first overhead stream, and the second overhead stream will have an NGL concentration lower than that of the first overhead stream.

Natural gas liquids (NGLs) refers to hydrocarbons that are heavier than methane and which are often found in natural gas. In particular, NGLs generally comprise one or more of ethane, propane, butane, pentane and n-hexane. The process of the present invention is particularly suitable for separating carbon dioxide from a gaseous feed comprising methane, at least 10 mol% carbon dioxide and one or more NGLs, wherein the total concentration of the one or more NGLs in the gaseous feed is at least I mol%, but it is also suitable for use with gaseous feeds comprising much higher NGL concentrations, such as at least 5 mol%, at least 10 mol%, at least 15 mol% and up to 20 mol%.

By use of the process of the present invention, the total concentration of the one or more NGLs in the second overhead stream may be reduced to less than 1 mol%, such as less than 0.5 mol%, for example 0.3 mol%, or 0.2 mol%.

In a preferred embodiment, the process of the present invention further comprises feeding one or more natural gas liquids (NGLs) to the second fractionation column.

Adding one or more NGLs to the second fractionation column increases the solubility of carbon dioxide in the liquid phase, thereby helping to prevent solidification of carbon dioxide at reduced operation temperatures and improving the purity of the methane separated in the fractionation column. The one or more NGLs fed to the second fractionation column is suitably added to the column towards the top thereof, but it may also be incorporated into one or more feed streams to the column, such as the feed from the first fractionation column or a recycle overhead stream, if present.

The one or more NGLs fed to the second fractionation column may be obtained from any suitable source, including external sources, or from upstream gaseous feed partial condensation and/or separation from the first and/or second bottom stream.

In a particular embodiment, the one or more NGLs fed to the second overhead stream comprises propane, optionally in combination with at least one of ethane, butane and pentane. For example, the NGL feed to the second fractionation column may comprise 80% propane, 10% ethane, 5% butane and 5% pentane.

The gaseous feed may be used 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 15°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 4 MPa to 8 MPa, for example 7 MPa.

The process of the present invention may be carried out under any suitable process conditions. For example, the first and second fractionation columns may be operated at any conditions suitable for the separation of the streams fed thereto. Suitably, the second fractionation column is operated at a lower temperature than the first fractionation column, to improve the efficiency of the separation of carbon dioxide from methane compared to the first fractionation column. The overall pressure range of the process may be from 0.5 MPa to 10 MPa, such as 1 MPa to 5 MPa. For example, the first fractionation column may conveniently be operated at pressure of 4 MPa and the second distillation column may be conveniently operated at a pressure of 3.7 MPa.

The process of the present invention is suitable for the separation of carbon dioxide from gaseous feeds comprising methane and any amount of carbon dioxide from 10 mol% or higher. In particular, the process of the present invention is particularly suitable for use with gaseous feeds comprising at least 20 mol% carbon dioxide, such as at least 40 mol% carbon dioxide or at least 50 mol% carbon dioxide, including, for example, 20 mol%, 50 mol% and 70 mol%.

In the process of the present invention, the first bottom stream produced in the first fractionation column has a carbon dioxide concentration higher than that of the gaseous feed and, where one or more NGLs are present in the gaseous feed, the first bottom stream also has an NGL concentration higher than that of the gaseous feed. Similaily, the second bottom stream produced in the second fractionation column has a carbon dioxide concentration higher than that of the first overhead stream and, where one or more NGLs are present in the gaseous feed, the second bottom stream also has an NGL concentration higher than that of the first overhead stream. The bottom streams from both the first and second fractionation columns will also have greatly reduced concentrations of methane compared to the gaseous feed, and will both generally comprise less than 1 mol% methane.

Where the bottom streams from the first and second fractionation columns comprise one or more NGLs, they may be further processed to produce a carbon dioxide stream and a NGL stream, both of which will have minimal methane content. Any suitable processes may be used to separate the carbon dioxide and NGLs, such as fractionation. The bottom stream from the second fractionation column may be processed using the same separation system as the bottom stream from the first fractionation column or, alternatively, two separate separation processes may be employed.

The first overhead stream obtained from the first fractionation column will have a methane concentration higher than that of the gaseous feed, and will generally also have a carbon dioxide concentration lower than that of the gaseous feed. Preferably, the concentration of carbon dioxide in the first overhead stream is 20 mol% or less.

The first overhead stream obtained from the second fractionation column has a methane concentration higher than that of the first overhead stream, and therefore also higher than the methane concentration of the gaseous feed. Additionally, the second overhead stream will have a carbon dioxide concentration lower than that of the first overhead stream, and therefore also lower than the carbon dioxide concentration of the gaseous feed. Preferably, the concentration of carbon dioxide in the second overhead stream is 3 mol% or less, such as 2.5 mol%. Preferably, the methane concentration of the second overhead stream is at least 95 mol%, such as at least 97 mol%.

Cooling and at least partially condensing the gaseous feed in heat exchange with boiling liquid at the bottom of a first fractionation column in a first reboil heat exchanger, and further cooling and at least partially condensing the feed in heat exchange with boiling liquid at the bottom of a second fractionation column in a second reboil heat exchanger results in a cooled and at least partially condensed gaseous feed, the temperature and degree of condensation depending upon a number of factors, including the starting temperature of the gaseous feed, the operation temperature of the first and second fractionation column and the relative proportions of the various components in the gaseous feed. For example, in compositions comprising a relatively high proportion of methane, such as 70 mol%, and a relatively low proportion of carbon dioxide, such as 20 mol%, relatively little condensation will take place. Alternatively, where the gaseous feeds comprises a relatively low proportion of methane, such as 15 mol%, and a relatively high proportion of carbon dioxide, such as 70 mol%, the gaseous feed may be substantially entirely condensed. For intermediate compositions of the gaseous feed, intermediate degrees of condensation will be produced.

In a first preferred embodiment of the present invention, where cooling of the gaseous feed in the first and second reboil heat exchangers does not result in substantially full condensation of the gaseous feed, the process further comprises separating the further cooled and at least partially condensed gaseous feed obtained in step (U) into a first liquid portion and a first vapour portion, and feeding at least a portion of the first liquid portion to the first fractionation column in step (Hi). Optionally, in this embodiment the cooled and at least partially condensed feed stream obtained from the first and second reboil heat exchangers may be further cooled and at least partially condensed before separation into the first liquid portion and the first vapour portion. Such additional cooling may be by any means, including heat exchange with one or more streams produced in the separation process, heat exchange with a side stream from at least one of the fractionation columns and/or external refrigeration, such as by evaporation of propane.

The first liquid portion may optionally be expanded and further cooled before being fed to the first fractionation column.

The first vapour portion may optionally be further cooled and at least partially condensed, for example by external refrigeration, before being separated into a second liquid portion and a second vapour portion, with the second vapour portion being fed to the first fractionation column. Optionally, the further cooled and at least partially condensed first vapour portion may be expanded before being separated into the second liquid portion and second vapour portion.

Optionally, at least a portion of the second vapour portion may be fed to the second fractionation column, optionally after undergoing work expansion or further cooling.

The first preferred embodiment is particularly suitable for use with gaseous feeds comprising less than 50 mol% carbon dioxide, such as less than 40 mol%, for example mol%.

In a second preferred embodiment of the present invention, where cooling the gaseous feed in heat exchange with boiling liquid in the bottom of the first and second fractionation columns in the first and second reboil heat exchangers results in significant or substantially complete condensation of the gaseous feed, substantially all of the further cooled and at least partially condensed gaseous feed obtained in step (ii) may be fed to the first fractionation column in step (Hi). This embodiment is particularly suitable for use with gaseous feeds comprising greater than 40 mol% carbon dioxide, for example 50 mol% or 70 mol%.

Optionally, in the second preferred embodiment the further cooled and at least partially condensed gaseous feed obtained in step (U) may be further cooled to ensure it is substantially fully condensed before it is fed to the first fractionation column in step (Di).

Such additional cooling may be provided by any suitable means, such as heat exchange with one or more streams produced in the separation process, heat exchange with a side stream from at least one of the fractionation columns, or external refrigeration, such as by the evaporation of propane.

In the second preferred embodiment of the invention, the substantially fully condensed gaseous feed may optionally be expanded and further cooled before being fed to the first fractionation column in step (iii).

Additional refrigeration may be provided at one or more stages of the separation process of the present invention. Such additional refrigeration may be provided by heat exchange with one or more streams produced in the process and/or by external sources of refrigeration, such as the evaporation of propane and/or ethane.

The second overhead stream produced in the process of the present invention may be warmed before removal from the process as sales gas. Such warming may be provided by heat exchange with the feed stream to the process and/or one or more streams produced during the separation process. Additionally or alternatively, the second overhead stream may be compressed before removal from the process as sales gas.

In a further aspect, the present invention provides an apparatus for the separation of carbon dioxide from a gaseous feed comprising methane and at least 10 mol% carbon dioxide, the apparatus comprising: (i) a first fractionation column including a first reboil heat exchanger; -10- (U) a second fractionation column including a second reboil heat exchanger; (Ui) means for cooling and at least partially condensing the gaseous feed in heat exchange with boiling liquid at the bottom of the first fractionation column in the first reboil heat exchanger, and further cooling and at least partially condensing the cooled and at least partially condensed gaseous feed in heat exchange with boiling liquid at the bottom of the second fractionation column in the second reboil heat exchanger; (iv) meansfor feeding at least a portion of the further cooled and at least partially condensed gaseous feed to the first fractionation column, for separation therein into a first bottom stream having a carbon dioxide concentration higher than that of the gaseous feed and a first overhead stream having a methane concentration higher than that of the gaseous feed; and (v) means for feeding at least a portion of the first overhead stream to the second fractionation column, for separation therein into a second bottom stream having a carbon dioxide concentration higher than that of the first overhead stream and a second overhead stream having a methane concentration higher than that of the first overhead stream.

The first and second fractionation columns incorporated in the apparatus of the present invention may be any conventional fractionation column suitable for the separation of methane and carbon dioxide. Each 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 exchangers may be submerged in liquid in the sumps of the fractionation columns, or alternatively boiling liquid at the bottom of each fractionation column may be supplied to the reboil heat exchangers from bottom trays or packed sections of the fractionation columns.

Optionally, the apparatus of the present invention further comprises means for feeding one or more natural gas liquids (NGL5) to the second fractionation column. This may include means for feeding the one or more NGLs to the top of the second fractionation column, or to incorporate the one or more NGLs in a feed stream to the fractionation column, such as the feed from the first fractionation column or a recycle overhead -11 -stream, if present.

In a first preferred embodiment, the apparatus of the present invention may further comprise means to separate the further cooled and at least partially condensed gaseous feed obtained from the second reboil heat exchanger into a first liquid portion and a first vapour portion, and means for feeding at least a portion of the first liquid portion to the first fractionation column. Such separation means may include any conventional vapour/liquid separation apparatus, such as a knock-out drum.

The apparatus of the present invention may further comprise means to further cool the further cooled and at least partially condensed gaseous feed obtained from the second reboil heat exchanger before it is separated into the first liquid portion and the first vapour portion. Such means may comprise any suitable cooling apparatus including heat exchangers and/or external refrigeration systems.

The apparatus of the present invention may further comprise means to expand and further cool the first liquid portion before it is fed to the first fractionation column. Such means may include any conventional expansion apparatus, for example one or more expansion valves or one or more liquid or two-phase expansion turbines.

The apparatus of the present invention may further comprise means to further cool and at least partially condense the first vapour portion, such as external refrigeration, means to separate the further cooled and at least partially condensed first vapour portion into a second liquid portion and a second vapour portion, such as one or more knock-out drums, and means for feeding at least a portion of the second liquid portion to the first fractionation column. The apparatus may additionally comprise means for expanding the cooled and at least partially condensed first vapour portion before it is separated into the second liquid portion and the second vapour portion, for example one or more expansion valves or liquid or two-phase expansion turbines.

The apparatus of the present invention may further comprise means for feeding at least a portion of the second vapour portion to the second fractionation column.

-12 -In a second preferred embodiment, the apparatus of the present invention may further comprise means to further cool and substantially fully condense the further cooled and at least partially condensed gaseous feed obtained from the second reboil heat exchanger before it is fed to the first fractionation column. Such means may include heat exchange and/or external refrigeration. In this embodiment the apparatus may optionally further comprise means to expand and further cool the substantially condensed gaseous feed before it is fed to the first fractionation column. Such means may include one or more expansion valves, or liquid or two-phase expansion turbines.

Optionally, the first fractionation column may incorporate an overhead condenser and vapour/liquid separator above the column feed. Additionally or alternatively, the second fractionation column may incorporate an overhead condenser and vapour/liquid separator above the column feed. Any suitable condenser and vapour/liquid separator may be used, such as a reflux drum.

Where the first and/or second fractionation column incorporate an overhead condenser and vapour/liquid separator above the column feed, the liquid stream produced in the vapour/liquid separator is returned to the fractionation column as a reflux stream, whilst the vapour fraction separated in the vapour/liquid separator forms the overhead stream.

Optionally, the apparatus of the present invention further comprises means for providing additional refrigeration at one or more stages of the separation process by heat exchange with one or more streams produced in the process. Heat exchangers used in the apparatus of the present invention may include multi-stream heat exchangers which combine a number of heat exchange duties into a single heat exchange unit. For example, heat exchangers may be multi-stream heat exchangers, such as multi-stream brazed aluminium plate-fin heat exchangers.

Additional or alternatively, the apparatus of the present invention may further comprise means for providing additional refrigeration at one or more stages of the separation process by the evaporation of propane and/or ethane. Typically, propane may be evaporated at three levels; i.e. 7°C, -20°C and -40°C, and ethane may be evaporated at -13-two levels; i.e. -60°C and -90°C. Propane condensing at 40°C may also be incorporated in the process and apparatus of the present invention.

In order to meet sales gas specifications, the apparatus of the present invention may further comprise means to warm the second overhead stream in heat exchange with the feed stream to the process and/or one or more streams produced during the separation process. Additionally or alternatively, the apparatus of the present invention may further comprise means to compress the second overhead stream and, optionally, to further cool the compressed overhead stream.

The invention will now be described in greater detail with reference to preferred embodiments of the invention and with the aid of the accompanying figures in which: Figure 1 shows a first embodiment of a carbon dioxide separation process of the present invention; and Figure 2 shows a second embodiment of a carbon dioxide separation process of the present invention.

Figure 1 shows a first embodiment of a separation apparatus and process in accordance with the present invention. The apparatus comprises a first fractionation column (210) and a second fractionation column (255). The first fractionation column (210) includes a first reboil heat exchanger (115) and the second fractionation column (255) includes a second reboil heat exchanger (125). Boiling liquid at the bottom of each of the fractionation columns (210, 255) may either be piped to the associated reboil heat exchanger (115, 125) from a bottom tray or packed section of the fractionation column (210, 255), or the reboil heat exchangers (115, 125) may be submerged in boiling liquid in the sump of the fractionation columns (210, 255). The apparatus also comprises a first vapour/liquid separator (150) and a second vapour/liquid separator (190). Means for heating and cooling the various product and feed streams is provided by heat exchangers (105, 135, 288) and pressure reduction valves (165, 180). The apparatus further comprises a compressor (315) and a cooler (320).

Dried feed gas (100) enters the process, for example at a temperature of 37°C and a pressure of 7 MPa, and is cooled in the first heat exchanger (105) against returning sales -14-gas (300). The cooled feed stream (110) is further cooled in the reboil heat exchangers (115, 125), where it provides reboil heat for the first fractionation column (210) and the second fractionation column (255). The cooled feed (130) is then routed to the second heat exchanger (135), where it is cooled against a side stream (140) from the first fractionation column (115), returning sales gas (295) and mechanical refrigeration, typically using propane at 7°C and -20°C. The cooled and at least partially condensed feed (145) is separated in the first vapour/liquid separator (150), and the liquid stream (155) is reduced in pressure in a valve (165) and sent to the pre-separation (first fractionation) column (210). The vapour stream (160) from the first separator (150) is cooled and partially condensed against mechanical refrigeration (175), typically using propane evapouration at -40°C, let down in pressure in a valve (180) and routed to the second vapour/liquid separator (190). The liquid stream (215) from the second vapour/liquid separator (190) is fed to the top of the first fractionation column (210) as reflux, while the vapour stream (205) is cooled in the third heat exchanger (288) against returning sales gas (285), and the cooled vapour stream (206) is fed to the upper section of the second fractionation column (255).

The first fractionation column (210) with associated reboiler (115), which provides the heat for distillation, removes a significant proportion of the carbon dioxide in the feed (100) and produces an overhead vapour product (225) containing, at most, about 20 mol% C02, and a bottom liquid product (220) enriched in carbon dioxide and NGLs (consisting of ethane, propane, and heavier hydrocarbons present in the feed gas).

The vapour stream (225) from the top of the first fractionation column (210) is routed to a downstream carbon dioxide rejection (second fractionation) column (255). This column may involve a hydrocarbon wash stream (290), which typically consists of natural gas liquids. The NGL stream is added to the top section of the second fractionation column (255), for example in the top tray of the column, and helps to avoid carbon dioxide freezing. A bottom stream (260) is produced in the second fractionation column (255), comprising the bulk of the carbon dioxide and NGLs fed to the column (255). An overhead methane-rich vapour stream (285), typically comprising less than 3 mol% carbon dioxide, is removed from the top of the second fractionation column (255). -15-

The methane product stream (285) is rewarmed in the heat exchangers (288, 135, 105) in heat exchange with the feed stream (100), cooled feed stream (130) and the vapour stream (205) from the second vapour/liquid separator (190), before being sent to the compressor (315) and cooler (320).

The process and apparatus illustrated in Figure 1 is particularly suitable for use with gaseous feeds comprising less than 50 mol% carbon dioxide, such as 40 mol% carbon dioxide or 20 mol% carbon dioxide.

Figure 2 shows a separation apparatus in accordance with a second embodiment of the invention. Where components and feed streams shown in Figure 2 are the same as those shown in Figure 1, they are numbered identically. The apparatus comprises a first fractionation column (210), incorporating a first reboil heat exchanger (115), and a second fractionation column (255), incorporating second reboil heat exchanger (125).

The first fractionation column (210) also incorporates an overhead condenser (230) and a separator (240). Similarly, the second fractionation column (255) incorporates an overhead condenser (270) and a separator (275). The apparatus further comprises heat exchangers (105, 135, 230), a pressure reduction valve (165), a compressor (315) and a cooler (320).

Dehydrated feed gas (100) enters the process and is cooled in the first heat exchanger (105) against returning sales gas (300). The cooled feed stream (110) is further cooled in the reboil heat exchangers (115, 125), where it provides reboil heat for the first and second fractionation columns (210, 255). The cooled feed (130) is then routed to the second heat exchanger (135), where it is cooled against a side stream (140) from the first fractionation column (210), returning sales gas (295) and mechanical refrigeration, typically using propane, as described with respect to Figure 1. The cooled and substantially fully condensed feed (145) is reduced in pressure in pressure reduction valve (165), and the expanded stream (170) is sent to the pre-separation (first fractionation) column (210).

The first fractionation column (210), with associated reboiler (115), which provides the heat for distillation, together with the overhead condenser (230), removes the bulk of the -16-carbon dioxide (greater than 80% recovery) in the feed, and produces an overhead vapour product (250) containing, at most, about 20% carbon dioxide, and a bottom liquid product (220) enriched in carbon dioxide and any natural gas liquids contained in the feed (generally consisting of ethane, propane and heavier hydrocarbons). The overhead vapour stream (215) from the first fractionation column (210) is partially condensed in the overhead condenser (230), followed by separation of the vapour (250) and liquid (245) phases in the separator (240). The liquid stream (245) is returned to the top of the first fractionation column (210) as reflux. Refrigeration for the overhead condenser (230) is provided by mechanical refrigeration, typically by evaporation of ethane at -60°C, and the returning vapour stream (285) from the second fractionation column (255).

The methane-rich vapour stream (250) from the first fractionation column (210) is fed to the downstream carbon dioxide rejection (first separation) column (255). This column may incorporate a hydrocarbon wash stream (290), as discussed with respect to Figure 1. The NGL stream (290) is added to the top section of the second fractionation column (255), for example in the top tray of the column, or in the condenser (270), and helps to avoid carbon dioxide freezing.

The overhead stream from the second fractionation column (265) is partially condensed in the overhead condenser (270), which employs ethane refrigeration at -90°C, to produce a partially condensed stream, which is then separated in the separator (275) to produce a reflux stream (280) and a methane-rich stream (285), with less than 3 mol% carbon dioxide. A bottom stream (260), with the remaining carbon dioxide and NGLs is removed from the bottom of the second fractionation column (255).

The methane product stream (285) from the second fractionation column (255) is rewarmed in heat exchangers (230, 135, 105) before being sent to compressor (315) and cooler (320).

The process and apparatus illustrated in Figure 2 is particularly suitable for use with gaseous feeds comprising greater than 40 mol% carbon dioxide, for example 50 mol% or 70 mol%. -17-

EXAM P LES

Example I

Table 1 shows typical operating parameters for the apparatus of the present invention shown in Figure 1, when used to separate a gaseous feed comprising 70 mol% methane, 20 mol% carbon dioxide, 5 mol% ethane, 2.5 mol% propane, 1.5 mol% iso-butane, 0.5 mol% iso-pentane and 0.5 mol% n-hexane. The gaseous feed is initially at a temperature of 37°C and a pressure of 7 MPa(a). The compositions of the feed stream and the various streams produced in the process are shown using numbering corresponding to that employed in Figure 1.

Table I

Stream Number 100 130 140 145 155 160 Vapour Fraction 1.00 0.99 0.00 0.93 0.00 1.00 Temperature (°C) 37.00 2.00 -39.27 -17.00 -17.00 -17.00 Pressure (kPa) 7000 7000 4012 7000 7000 7000 Mass Flow (kg/h) 302595 302595 36210 302595 31247 271349 Molar Flow Methane (kgmolelh) 8716 8716 296 8716 350 8366 CO2 (kgmole/h) 2490 2490 413 2490 209 2282 Ethane (kgmolelh) 623 623 116 623 75 548 Propane (kgmole/h) 311 311 83 311 76 235 i-Butane (kgmole/h) 187 187 54 187 69 118 i-Pentane(kgmole/h)62 62 19 62 36 26 n-Hexane(kgmole/h)62 62 19 62 49 13 Total (kgmole/h) 12452 12452 1000 12452 864 11588 Stream Number 170 185 205 206 215 220 Vapour Fraction 0.29 0.75 1.00 0.98 0.00 0.00 Temperature (°C) -31.88 -54.94 -54.94 -58.00 -54.94 16.14 Pressure (kPa) 4020 4020 4020 4020 4020 4020 Mass Flow (kg/h) 31247 271349 181080 181080 90269 82202 -18-Molar Flow Methane (kgmole/h) 350 8366 7153 7153 1214 9 CO2 (kgmole/h) 209 2282 1263 1263 1019 954 Ethane (kgmole/h) 75 548 267 267 281 298 Propane (kgmole/h) 76 235 47 47 188 254 i-Butane (kgmole/h) 69 118 10 10 107 174 i-Pentane(kgmole/h) 36 26 1 1 26 61 n-Hexane(kgmole/h)49 13 0 0 13 62 Total (kgmole/h) 864 11588 8741 8741 2848 1813 Stream Number 225 260 285 287 290 300 VapourFraction 1.00 0.00 1.00 1.00 0.00 1.00 Temperature (°C) -55.1 7.12 -86.06 -82.09 -60.0 1.00 Pressure (kPa) 4000 3720 3700 3700 4300 3700 Mass Flow (kg/h) 39317 92761 150035 150035 22399 150035 Molar Flow Methane(kgmole/h) 1554 11 8696 8696 0 3696 CO2 (kgmole/h) 274 1313 224 224 0 24 Ethane (kgmole/h) 58 357 17 17 50 17 Propane (kgmolelh) 10 454 4 4 400 i-Butane (kgmole/h) 2 37 0 0 25) i-Pentane(kgmole/h) 0 26 0 0 25) n-Hexane(kgmolelh) 0 0 0 0 0) Total (kgmole/h) 1899 2198 8941 8941 500 3941 Stream Number 310 325 Vapour Fraction 1.00 1.00 Temperature (°C) 1.69 40.00 Pressure (kPa) 3700 7000 Mass Flow (kg/h) 150035 150035 Molar Flow Methane (kgmolelh) 8696 8696 CO2 (kgmole/h) 224 224 -19-Ethane (kgmole/h) 17 17 Propane (kgmole/h) 4 4 i-Butane (kgmole/h) 0 0 i-Pentane(kgmole/h) 0 0 n-Hexane(kg mole/h) 0 0 Total (kgmole/h) 8941 8941 Residue Gas Compression Power 4692 kW at 75% efficiency Mechanical refrigeration is based on using two levels for Ethane refrigeration at -90 C and -60 C and three levels of Propane refrigeration at -40 C, -20 C, and 7 C. Single stage compressors with 75% efficiency are used for these levels as given in Table 2.

Table 2

Refrigeration level Compressor Power (MW) Ethane at -90 C 5.71 Propane at -40 C 2.33 Propane at -20 C 3.51 Propane at7C 5.16 Total Refrigeration Power 16.71

EXAMPLE 2

Table 3 shows typical operating parameters for the apparatus of the present invention shown in Figure 2, when used to separate a feed stream comprising 30 mol% methane, 50 mol% carbon dioxide, 10 mol% ethane, 5 mol% propane, 3 mol% iso-butane, 1 mol% iso-pentane and 1 mol% n-hexane. The feed is initially at 37°C and 7 MPa(a).

Table 3

Stream Number 100 130 140 145 155 170 Vapour Fraction 1.00 0.35 0.00 0.00 0.00.24 Temperature (°c) 37.0 3.0 -20.0 -17.0 -17.0 29.6 -20 -Pressure (kPa) 7000 7000 4511 7000 7000 4020 Mass Flow (kg/h) 440252 440252 76668 440252 440252 440252 Molar Flow Methane (kgmole/h) 3736 3736 376 3736 3736 3736 CO2 (kgmole/h) 6226 6226 1192 6226 6226 3226 Ethane (kgmole/h) 1245 1245 222 1245 1245 1245 Propane (kgmolelh) 623 623 109 623 623 523 i-Butane (kgmole/h) 374 374 62 374 374 374 i-Pentane(kgmole/h) 125 125 20 125 125 125 n-Hexane(kgmolelh) 125 125 19 125 125 125 Total (kgmolelh) 12452 12452 2000 12452 12452 12452 Stream Number 220 225 245 250 260 265 Vapour Fraction 0.00 1.00).00 1.00).00 1.00 Temperature (°C) 8.5 -36.0 51.0 51.0 7.1 81.5 Pressure (kPa) 4020 4000 1000 1000 3720 3700 Mass Flow (kg/h) 329733 193436 2963 110474 34640 157967 Molar Flow Methane (kgmole/h) 37 4404 105 3699 3 3509 CO2 (kgmole/h) 5237 2433 1445 388 393 431 Ethane (kgmole/h) 993 514 162 252 284 32 Propane (kgmole!h) 621 6 I 1 320 i-Butane (kgmolelh) 373 0) ) 20 i-Pentane(kgmole/h) 124 0) ) 20) n-Hexane(kgmole/h) 124 0) ) ) ) Total (kgmole/h) 7510 7357 2416 1940 1544)022 Stream Number 280 285 290 295 300 310 Vapour Fraction 0.00 1.00).00 1.00 1.00 1.00 Temperature (°C) -86.2 -86.2 60.0 45.0).0 30.0 Pressure (kPa) 3700 3700 3720 3700 3700 3699.9 Mass Flow (kg/h) 94186 63782 17919 33782 33782 33782 Molar Flow Methane(kgmole/h) 4815 3693) 3693 3693 3693 -21 -CO2 (kgmole/h) 336 95) 35 35 Ethane (kgmole/h) 71 12 10 12 12 12 Propane (kgmole/h) 320 i-Butane (kgmole/h) 0 0 i-Pentane(kgmole/h) 0 0 0) ) ) n-Hexane(kg mole/h) 0 0 Total (kgmole/h) 5223 3800 100 3800 3800 3800 Stream Number 325 Vapour Fraction 1.00 Temperature (°C) 40.0 Pressure (kFa) 7000 Mass Flow (kg/h) 63782 Molar Flow Methane (kgmole/h) 3693 CO2 (kgmole/h) 95 Ethane (kgmole/h) 12 Propane (kgmole/h) i-Butane (kgmole/h) 0 i-Pentane(kgmole/h) 0 n-Hexane(kg mole/h) 0 Total (kgmole/h) 3800 Residue Gas Compression Power 2265 kW at 75% efficiency Mechanical refrigeration is based on using two levels for Ethane refrigeration at -90 C and -60 C and three levels of Propane retrigeration at -40 C, -20 C, and 7 C. Single stage compressors with 75% efficiency are used for these levels as given in Table 4.

Table 4

Refrigeration level Compressor Power (MW) Ethane at -90 C 3.26 -22 -Ethane at -60 C 0.77 Propane at -40 C 2.10 Propane at -20 C 3.16 Propane at 7 C 5.24 Total Refrigeration Power 14.53

EXAMPLE 3

Table 5 shows typical operating parameters for the apparatus of the present invention shown in Figure 2, when used to separate a gaseous feed comprising 15 mol% methane, mol% carbon dioxide, 6 mol% methane, 4 mol% propane, 3 mol% iso-butane, 1 mol% iso-pentane and 1 mol% n-hexane. The feed is initially at 37°C and 7 MPa(a).

Table 5

Stream Number 100 130 145 155 170 20 Vapour Fraction 1 0 0 0).03).00 Temperature (°C) 37.0 7.9 -17.0 -17.0 20.2 7.0 Pressure (kPa) 7000 7000 7000 7000 W20 1020 Mass Flow (kg/h) 499419 499419 499419 499419 199419 145572 Molar Flow Methane (kgmole/h) 1868 1868 1868 1868 1868 50 CO2 (kgmole/h) 8716 8716 8716 8716 3716 3233 Ethane (kgmole/h) 747 747 747 747 147 333 Propane (kgmole/h) 498 498 498 498 198 198 i-Butane (kgmole/h) 374 374 374 374 374 374 i-Pentane(kgmole/h) 125 125 125 125 125 125 n-Hexane(kgmole/h) 125 125 125 125 125 125 Total (kgmole/h) 12452 12452 12452 12452 12452 10037 Stream Number 225 245 50 60 65 80 Vapour Fraction 1 J 1).0000 1 Temperature (°C) -29.2 51.4 51.4 15.0 71.4 85.4 Pressure (kPa) 4000 4000 1000 3720 3700 3700 -23 -Mass Flow (kg/h) 138149 34302 53848 W415 57664 36312 Molar Flow Methane (kgmole/h) 2503 386 1818 5 3594 1781 002 (kgmole/h) 1986 1503 183 36 179 132 Ethane (kgmole/h) 348 234 114 149 9 Propane (kgmolelh) 2 2) 319 ?8 U i-Butane (kgmolelh) 0 3) ?0 1 1 i-Pentane(kgmolelh) 0 3) ?0) ) n-Hexane(kgmole/h) 0 3) ) ) ) Total (kgmolelh) 4840 2425 MiS 349 3830 1964 Stream Number 285 290 295 300 310 325 Vapour Fraction 1 0 1 1 1 1 Temperature (°C) -85.4 60.0 38.0 3.0 32.0 10.0 Pressure (kFa) 3700 t000 3700 3700 3700 1000 Mass Flow (kg/h) 31352 17919 31352 31352 31352 31352 Molar Flow Methane(kgmole/h) 1813 0 1813 1813 1813 1813 002 (kgmole/h) 47 0 17 7 47 17 Ethane (kgmole/h) 5 0 5 5 5 5 Propane (kgmolelh) 2 320 2 2 i-Butane (kgmole/h) 0 20) ) 3 i-Pentane(kgmole/h) 0 20) ) 3 n-Hexane(kgmole/h) 0 0) ) 3 Total (kgmole/h) 1866 00 1866 1866 1866 1866 Residue Gas Compression Power 1120 kW at 75% efficiency Mechanical refrigeration is based on using two levels for Ethane refrigeration at -90 C and -60 C and three levels of Propane refrigeration at -40 C, -20 C, and 7 C. Single stage compressors with 75% efficiency are used for these levels as given in Table 6.

-24 -

Table 6

Refrigeration level Compressor Power (MW) Ethane at -90 C 1.62 Ethane at -60 C 0.96 Propane at -40 C 1.60 Propane at -20 C 3.82 Propane at 7 C 5.92 Total Refrigeration Power 13.92

Claims

-25 -CLAIMS1. A process for the separation of carbon dioxide from a gaseous feed comprising methane and at least 10 mol% carbon dioxide, the process comprising: (i) cooling and at least partially condensing the gaseous feed in heat exchange with boiling liquid at the bottom of a first fractionation column in a first reboil heat exchanger; (U) further cooling and at least partially condensing the cooled and at least partially condensed gaseous feed in heat exchange with boiling liquid at the bottom of a second fractionation column in a second reboil heat exchanger; (Ui) feeding at least a portion of the further cooled and at least partially condensed gaseous feed to the first fractionation column, and separating it therein into a first bottom stream having a carbon dioxide concentration higher than that of the gaseous feed and a first overhead stream having a methane concentration higher than that of the gaseous feed; and (iv) feeding at least a portion of the first overhead stream to the second fractionation column, and separating it therein into a second bottom stream having a carbon dioxide concentration higher than that of the first overhead stream and a second overhead stream having a methane concentration higher than that of the first overhead stream.
2. A process according to claim 1, wherein the gaseous feed further comprises one or more natural gas liquids (NOL5); and wherein the first bottom stream has an NGL concentration higher than that of the gaseous feed, and the first overhead stream has an NGL concentration lower than that of the gaseous feed; and further wherein the second bottom stream has an NGL concentration higher than that of the first overhead stream, and the second overhead stream has an NGL concentration lower than that of the first overhead stream.
3. A process according to claim 2, wherein the one or more natural gas liquids comprise one or more of ethane, propane, butane, pentane and n-hexane.

-26 -
4. A process according to claim 2 or claim 3, wherein the total concentration of the one or more NOLs in the gaseous feed is at least 1 mol%.
5. A process according to any of claims 2 to 4, wherein the total concentration of the one or more NGLs in the second overhead stream is less than 1 mol%.
6. A process according to any preceding claim, further comprising feeding one or more natural gas liquids (NGL5) to the second fractionation column.
7. A process according to claim 6, wherein the one or more NGLs comprises propane.
8. A process according to claim 7, wherein the one or more NGLs further comprise at least one of ethane, butane and pentane.
9. A process according to any preceding claim, wherein the gaseous feed comprises at least 20 mol% carbon dioxide.
10. A process according to any preceding claim, wherein the gaseous feed comprises at least 50 mol% carbon dioxide.
11. A process according to any preceding claim, comprising separating the further cooled and at least partially condensed gaseous feed obtained in step (U) into a first liquid portion and a first vapour portion, and feeding at least a portion of the first liquid portion to the first fractionation column in step (Hi).
12. A process according to claim 11, comprising further cooling the further cooled and at least partially condensed gaseous feed obtained in step (ii) before separation into the first liquid portion and the first vapour portion.
13. A process according to claim 11 or claim 12, wherein the first liquid portion is expanded and further cooled before being fed to the first fractionation column.

-27 -
14. A process according to any of claims 11 to 13, comprising further cooling and at least partially condensing the first vapour portion, separating the further cooled and at least partially condensed first vapour portion into a second liquid portion and a second vapour portion, and feeding at least a portion of the second liquid portion to the first fractionation column in step (H).
15. A process according to claim 14, further comprising feeding at least a portion of the second vapour portion to the second fractionation column.
16. A process according to any of claims Ito 10, comprising feeding substantially all of the further cooled and at least partially condensed gaseous feed obtained in step (U) to the first fractionation column in step (H).
17. A process according to claim 16, comprising further cooling and substantially fully condensing the further cooled and at least partially condensed gaseous feed obtained in step (ii) before feeding it to the first fractionation column in step (iii).
18. A process according to claim 17, wherein the substantially fully condensed gaseous feed is expanded and further cooled before being fed to the first fractionation column in step (iii).
19. A process according to any preceding claim, wherein the concentration of carbon dioxide in the first overhead stream is 20 mol% or less.
20. A process according to any preceding claim, wherein the concentration of carbon dioxide in the second overhead stream is 3 mol% or less.
21. A process according to any preceding claim, wherein additional refrigeration is provided at one or more stages of the separation process by heat exchange with one or more streams produced in the process.
22. A process according to any preceding claim, wherein additional refrigeration is provided at one or more stages of the separation process by the evaporation of propane -28 -and/or ethane.
23. A process according to any preceding claim, further comprising warming the second overhead stream in heat exchange with the feed stream and/or one or more streams produced during the separation process.
24. A process according to any preceding claim, further comprising compressing the second overhead stream.
25. An apparatus for the separation of carbon dioxide from a gaseous feed comprising methane and at least 10 mol% carbon dioxide, the apparatus comprising: (i) a first fractionation column including a first reboil heat exchanger; (U) a second fractionation column including a second reboil heat exchanger; (Ui) means for cooling and at least partially condensing the gaseous feed in heat exchange with boiling liquid at the bottom of the first fractionation column in the first reboil heat exchanger, and further cooling and at least partially condensing the cooled and at least partially condensed gaseous feed in heat exchange with boiling liquid at the bottom of the second fractionation column in the second reboil heat exchanger; (iv) meansfor feeding at least a portion of the further cooled and at least partially condensed gaseous feed to the first fractionation column, for separation therein into a first bottom stream having a carbon dioxide concentration higher than that of the gaseous feed and a first overhead stream having a methane concentration higher than that of the gaseous feed; and (v) means for feeding at least a portion of the first overhead stream to the second fractionation column, for separation therein into a second bottom stream having a carbon dioxide concentration higher than that of the first overhead stream and a second overhead stream having a methane concentration higher than that of the first overhead stream.
26. An apparatus according to claim 25, further comprising means for feeding one or more natural gas liquids to the second fractionation column.

-29 -
27. An apparatus according to claim 25 or claim 26, further comprising means to separate the further cooled and at least partially condensed gaseous feed obtained from the second reboil heat exchanger into a first liquid portion and a first vapour portion, and means for feeding at least a portion of the first liquid portion to the first fractionation column.
28. An apparatus according to claim 27, further comprising means to further cool the further cooled and at least partially condensed gaseous feed obtained from the second reboil heat exchanger before it is separated into the first liquid portion and the first vapour portion.
29. An apparatus according to claim 27 or claim 28, further comprising means to expand and further cool the first liquid portion before it is fed to the first fractionation column.
30. An apparatus according to any of claims 27 to 29, further comprising means to further cool and at least partially condense the first vapour portion, means for separating the further cooled and at least partially condensed first vapour portion into a second liquid portion and a second vapour portion, and means for feeding at least a portion of the second liquid portion to the first fractionation column.
31. An apparatus according to claim 30, further comprising means for feeding at least a portion of the second vapour portion to the second fractionation column.
32. An apparatus according to claim 25 or claim 26, further comprising means to further cool and substantially fully condense the further cooled and at least partially condensed gaseous feed obtained from the second reboil heat exchanger before it is fed to the first fractionation column.
33. An apparatus according to claim 32, further comprising means to expand and further cool the substantially fully condensed gaseous feed before it is fed to the first fractionation column.

-30 -
34. An apparatus according to any of claims 25 to 33, wherein the first fractionation column incorporates an overhead condenser and vapour/liquid separator above the column feed.
35. An apparatus according to any of claims 25 to 34, wherein the second fractionation column incorporates an overhead condenser and vapour/liquid separator above the column feed.
36. An apparatus according to any of claims 25 to 35, further comprising means for providing additional refrigeration at one or more stages of the separation process by heat exchange with one or more streams produced in the process.
37. An apparatus according to any of claims 25 to 36, further comprising means for providing additional refrigeration at one or more stages of the separation process by the evaporation of propane and/or ethane.
38. An apparatus according to any of claims 25 to 37, further comprising means to warm the second overhead stream in heat exchange with the feed stream and/or one or more streams produced during the separation process.
39. An apparatus according to any of claims 25 to 38, further comprising means to compress the second overhead stream.