GB2271628A

GB2271628A - Hydrocarbon recovery process

Info

Publication number: GB2271628A
Application number: GB9221160A
Authority: GB
Inventors: Adrian Joseph Finn
Original assignee: Costain Oil Gas and Process Ltd
Current assignee: Costain Oil Gas and Process Ltd
Priority date: 1992-10-08
Filing date: 1992-10-08
Publication date: 1994-04-20
Anticipated expiration: 2012-10-08
Also published as: GB9221160D0; GB2271628B

Abstract

A method of separating and recovering heavier hydrocarbons from gaseous hydrocarbon feed 110 involves cooling and partially condensing the feed in counter current indirect heat exchanger 111, separating the feed into condensate 118 and a first uncondensed vapour stream 114 in a separator 113 and fractionating in column 121 the condensate which has been increased in pressure by pump 119, to form a liquid stream 123 rich in said heavier hydrocarbons and an overhead vapour stream 122, the overhead vapour stream 122 being cooled and at least partially condensed in the exchanger 111, expanded to a lower pressure by valve 125 and subsequently evaporated in exchanger 111 to provide refrigeration for the process, the partial condensation of the overhead vapour stream being effected in countercurrent heat exchange relationship with the subsequent evaporation thereof. The expanded at least partially condensed vapour stream 126 is preferably combined with at least part of the first uncondensed vapour stream 114, 117 prior to evaporation. Additional mechanical refrigeration 129 may be provided for exchanger 111. The uncondensed vapour stream may be subjected to rectification in a reflux exchanger (14 - 16, Figure 2). <IMAGE>

Description

HYDROCARBON RECOVERY PROCESS This invention relates to an improved method for the separation of heavier hydrocarbons from a gaseous feed.

Typical processes to which the method of the invention may be applied include the recovery of natural gas liquids (NGL), i.e. C2 and heavier hydrocarbons, from natural gas and the recovery of liquefied petroleum gas (LPG), i.e. C3 and heavier hydrocarbons, from refinery off-gases.

Many processes are known for recovering heavier hydrocarbons from such gaseous feed streams in which the feed stream is converted into a residual gas containing the more volatile components and a liquid stream containing a high proportion of the desired components.

This conversion is conventially achieved by reducing the temperature of the gaseous feed to partially condense it and then separating it into vapour and liquid portions. At least one liquid portion, which comprises the majority of the desired components, is passed to a fractionation column which further concentrates the desired components in a liquid stream which is recovered from the fractionation column as the bottoms product and can be subjected to further treatment if required.

The partial condensation of the gaseous feed is commonly achieved by heat exchange with cold process streams which are consequently reheated. Alternatively, partial condensation may be achieved by passing the feed gas through one or more side reboilers associated with the fractionation column.

In order to recover hydrocarbons from high pressure feed gases such as the recovery of natural gas liquid from natural gas, it is necessary to reduce the operating pressure to below the cricondenbar of the residual gas in order to effect the separation of the gas. In this case, work-expansion of the feed gas, or that part of the feed gas remaining after partial condensation, may be used to provide refrigeration.

To recover hydrocarbons from medium to low pressure feed gases such as the recovery of LPG from refinery off-gas, the partial condensation and fractionation steps are carried out at feed gas pressure and the work-expansion of product gases may be used to provide refrigeration.

Additional refrigeration may be provided by a mechanical refrigeration cycle, work expansion of one or more process gas streams, or a combination thereof.

Where the majority of the refrigeration is supplied by mechanical refrigeration it is necesary to cool the feed to such low temperatures in order to achieve high recovery of the desired hydrocarbon that a large proportion of the lighter hydrocarbons are condensed thereby increasing the heat load of the system.

Cooling the feed to such low temperatures increases the power consumption of the refrigeration cycle which has a major impact on plant capital and operating costs.

For processes which use work-expansion of residual gas(es) to supply refrigeration, the pressure ratio across the work expansion step is essentially fixed and therefore increased hydrocarbon recovery has conventionally been achieved by reducing the temperature. This requires additional mechanical refrigeration which again results in increased overall cost.

For processes wherein all or the majority of the refrigeration is supplied by work-expansion of feed gas, the need to cool the feed to low enough temperatures to achieve high hydrocarbon recovery means that the pressure ratio across the work expansion step is large. As it is conventional for residual gas to be recompressed to form sales gas the power consumption of the process increases sharply as hydrocarbon recovery increases. This sales gas compression often has a major impact on both plant capital and operating costs and should therefore be reduced and if possible avoided.

Numerous processes have been suggested to increase efficiency and to reduce plant capital and operating costs in recovery processes of this type. In such processes it is common to increase heat exchange or 'integration' between process streams so as to more effectively utilise the refrigeration available from cold process streams. Examples of such processes are described in US 4171964, US 4140504, US 4690702, US 4707170, US 4921514, US 4714487, US 4157904 and EP 0126309. More, efficient heat exchange between process streams is achieved if the combined heating curve of the cold process streams matches the combined cooling curve of the warm process streams.

Some of the processes listed above include rectification of the feed upstream of the main fractionation column. This rectification step may be achieved by a column or by a reflux exchanger (dephlegmator).

The procedure described in GB 2146751, Figure 5, is typical of a process in which residual vapour from the vapour/liquid separator is subjected to rectification in a reflux exchanger. The rectified vapour is warmed in the reflux exchanger, expanded and rewarmed. In this procedure, however, the overhead from the fractionator is returned to the feed and is not combined with the residual vapour.

In EP-0148070, wherein the feed stream is cooled and partially condensed to form a condensate and a residual vapour and the condensate is then fractionated in a column, refrigeration is provided from three sources, namely work expansion of residual vapour, expanded overhead vapour from the column and sub-cooled and expanded liquid collected from the top of the column, these three systems being combined prior to being used to cool the feed.

In the process of Figure 2 of US 4002042, which includes a rectification step upstream of the fractionator, overhead vapour from the fractionator is cooled and partially condensed, and refrigeration is provided in part by evaporation of liquid which has been derived from the condensate, which is sub-cooled and then expanded. and in part by evaporation of uncondensed overhead vapour from the fractionator which has been cooled and partially condensed and then let down in pressure.

We have now found an improved method of recovering heavier hydrocarbons from a gaseous feed which exhibits increased efficiency and reduced plant capital and operating costs.

Thus in accordance with the present invention there is provided a method of separating and recovering heavier hydrocarbons from a gaseous hydrocarbon feed by cooling and partially condensing the feed, separating the feed into condensate and a first uncondensed vapour stream and fractionating the condensate to form a liquid stream rich in said heavier hydrocarbons and an overhead vapour stream, in which said overhead vapour stream is cooled and at least partially condensed, expanded to a lower pressure and subsequently evaporated to provide refrigeration for the process, characterised in that said at least partial condensation of the column overheads is effected in counter-current heat exchange relationship with the subsequent evaporation thereof. The expanded at least partially condensed vapour stream is preferably combined with at least part of said first uncondensed vapour stream prior to said evaporation.

Conducting the at least partial condensation of the column overheads in counter-current heat exchange with the subsequent evaporation thereof results in the heating and cooling curves being better matched, thus increasing the efficiency of the process. The efficiency may be further increased by combining the at least partially condensed vapour stream with at least part of the first uncondensed vapour stream prior to evaporation in the heat-exchanger as the heating and cooling curves become even more closely matched.

In one embodiment of the present invention, which is particularly suitable where the hydrocarbon separation and recovery occurs at essentially feed gas pressure and refrigeration for the process is provided by mechanical refrigeration, residue gas work-expansion or both, e.g as in the recovery of C 3+ components from refinery off-gas streams, the overhead vapour stream is cooled co-currently with the gaseous feed and the first uncondensed vapour stream is work expanded prior to its combination with the at least partially condensed and expanded overhead vapour stream. The feed is cooled by heat exchange with the combined stream.

In another embodiment of the present invention which is suitable for the procedure in which the first uncondensed vapour stream is subjected to rectification preferably in a refluxing exchanger, the at least partially condensed and expanded overhead vapour stream is combined with residual vapour from the rectification step. The combined stream provides refrigeration for the rectification step and is then work-expanded. The work expanded stream provides refrigeration for both the rectification and the cooling of the feed stream.

The present invention enables 95% or more of the desired hydrocarbon component to be recovered from the feed gas in a manner which is efficient in both capital and operating costs.

This invention is now described in greater detail with reference to two embodiments and with the aid of accompanying drawings in which: Figure 1 is a flow diagram of a hydrocarbon recovery plant according to one aspect of the present invention; and Figure 2 is a flow diagram of a hydrocarbon recovery plant according to another aspect of the present invention in which the first uncondensed vapour is subjected to rectification.

The embodiment of the present invention described in Figure 1 is suitable for application to the type of process in which the hydrocarbon separation and recovery step operates at essentially feed gas pressure.

Typically this process would apply, for example, to recovery of C3 and heavier hydrocarbon components from refinery off-gas streams. It may also be used for the recovery of natural gas liquids ie C2 and heavier hydrocarbons, from natural gas.

Referring to Figure 1, the gaseous feed stream is passed through line 110 and is coooled and partially condensed by indirect counter-current heat exchange in a multi-stream heat exchanger 111, conventionally a plate-fin exchanger. The resultant two-phase mixture is passed through line 112 to a vapour-liquid separator 113 in which the uncondensed vapour and condensed liquid are separated. The uncondensed vapour is recovered in line 114 and is passed back to the multi-stream heat exchanger 111 in which it is reheated against feed thus cooling and condensing the feed. The resultant warmed vapour is recovered in line 115 and is passed to an expander 116. The work expanded, low temperature gas leaving the expander is recovered in line 117.

The condensed liquid from the vapour-liquid seperator 113 is recovered in line 118 and is increased in pressure by pump 119. The resultant stream is passed via line 120 to the fractionator, 121.

Optionally 10-50 of the pumped liquid in line 120 may be routed via line 131 to the heat exchanger 111 where it is passed in counter-current to the feed stream to provide additional refrigeration and thereby improve separation, reduction of mechanical refrigeration requirement or both. The warmed stream in 131 is fed to the fractionating column 121 at a point lower than line 120. Whilst introducing the warmer stream 131 into the column 121 has the disadvantage that the amount of external refrigeration required by the column is increased, this is substantially outweighed by the reduction in refrigeration needed to cool and condense the feed gas. The disadvantage is further mitigated since the fractionation column reboiler load would be reduced.

In the fractionator the feed stream is fractionated into a vapour overhead stream, which is removed in line 122 and a liquid bottoms stream, which is removed in line 123 and which contains a high proportion of the desired hydrocarbon component and forms the liquid product stream of the process. The overhead vapour stream 122 is cooled and condensed in the multi-stream heat exchanger 111 co-currently with condensing feed. The resultant cooled stream 124 is let-down in pressure across a valve 125 to the same pressure as the expander outlet gas 117. The resultant expanded stream 126 is at a lower temperature than the feed leaving the multi-stream exchanger and it is combined with the expander outlet gas 117.The combined stream 127 is reheated in the multi-stream exchanger 111 to provide refrigeration via indirect heat exchange with the condensing feed and column overhead vapour streams.

The reheated gas is recovered in line 128 as residual gas.

External refrigeration from a mechanical system is provided by evaporating refrigerant 129 in the multi-stream exchanger 111 and withdrawing the resultant refrigerant in line 130.

With this scheme the feed gas can reach a colder temperature than is conventional. The reason for this is that the mixed stream 127 evaporates at relatively cold temperatures which widens the cold end temperature difference and thereby allows the feed gas to reach colder temperatures. Higher recoveries can be achieved without the need for low-level mechanical refrigeration.

The embodiment of the present invention described in Figure 2 is also suitable for application to, for example, recovery of C3 and heavier hydrocarbon components, from refinery off-gas streams and to the recovery of natural gas liquids, ie. C2 and heavier hydrocarbons, from natural gas.

In Figure 2, the gaseous feed stream in line 10 is cooled and partially condensed by indirect counter-current heat exchange in a multi-stream heat exchanger (main exchanger), 11 which is conventionally a plate-fin exchanger. The resultant two phase mixture is passed through line 12 to a vapour liquid separator 13 in which the uncondensed vapour and condensed liquid are separated.

The uncondensed vapour is recovered in line 14 and is passed to a refluxing exchanger 15, which is also conventionally a plate-fin exchanger. The uncondensed gas passes upwards in the passages of the refluxing exchanger 15 where it is further cooled by indirect heat exchange which causes further condensation. The condensed liquid so formed descends in line 16 counter-current to and in intimate contact with the rising gas, and is returned to the vapour-liquid separator 13 where it mixes with the condensed liquid therein. The vapour recovered at the top of the refluxing exchanger in line 17 is lean in the desired hydrocarbon component and is rich in lighter components.

The condensed liquid from the vapour-liquid separator 13 is passed through line 18 to a pump 19 in which the pressure of the liquid is increased. It is then passed to the fractionating column 21 via line 20.

As in Figure 1, 10-50% of the pumped liquid in line 20 may be optionally routed via line 22 to the main heat exchanger 11 where it is passed in counter-current to the feed stream to provide additional refrigeration and thereby improve separation of the condensate and/or reduce the need for additional mechanical refrigeration. The warmed stream 22 is fed to the fractionating column 21 at a point lower than that at which the liquid feed is introduced.

In the column, the condensed liquid is fractionated into a vapour overhead stream which is removed in line 23 and a liquid bottoms stream which is removed in line 24. The liquid bottoms stream 24 contains a high proportion of the desired hydrocarbon component and forms the liquid product stream of the process.

The overhead vapour stream 23 is cooled and condensed in the refluxing exchanger 15 via indirect heat exchange. The resultant condensate 25 is let-down in pressure across a valve 26 such that the expanded stream 27 is at a lower temperature than the refluxed vapour 17 leaving the refluxing exchanger but the two streams are at the same pressure. The combined stream 28 is evaporated in the refluxing exchanger 15 to provide refrigeration via indirect heat exchange with the condensing streams. The reheated gas is recovered in line 29 and is then passed to an expander 30. The work expanded, low-temperature stream 31 leaving the expander is then reheated in the refluxing exchanger 15 to provide additional refrigeration. The resultant gas is recovered in line 32 and is passed to the main exchanger 11 where it provides refrigeration to cool the feed gas 10.The resultant gas is recovered in line 33 as residual gas.

The main exchanger 11 is provided with additional external refrigeration from a mechanical system in which evaporating refrigerant is introduced in line 34 and withdrawn in line 35.

The fractionation column is also provided with external refrigeration to further cool and condense any desired hydrocarbons remaining in the vapour overheads stream and a heating means at its base.

With this method, the refluxed outlet vapour can reach colder temperatures than is conventional. The reason for this is that the mixed stream 28 evaporates at relatively cold temperatures which widens the cold end temperature difference and thereby allows the refluxed vapour to reach colder temperatures. Higher recoveries can be achieved without the need for low-level mechanical refrigeration.

Where the column overheads are completely liquified by being cooled in the reflux exchanger, let-down in pressure and mixed with refluxed vapour, the mixed stream may evaporate at sufficiently cold temperatures to remove the need for mechanical refrigeration in the reflux exchanger.

However, should even lower temperatures be required, mechanical refrigeration may be added to further increase hydrocarbon recovery.

Whilst the scheme as illustrated shows the column overheads being cooled and liquified in the reflux exchanger 15, they may be treated in a separate exchanger provided that the at least partial condensation is effected in counter-current heat exchange relationship with the subsequent evaporation thereof. However, this is not a preferred feature.

It is not essential that the column overheads are totally liquified prior to the pressure let-down.

The column 21 may be a stripping column ie. a column that does not have an integral rectification zone, and in this case liquids from partial condensation and/or the rectification step are passed to the column as reflux.

The condensed liquid descending from the reflux exchanger in line 16 may be passed to a chamber in the vapour liquid separator 13 which is separate from the chamber containing condensate formed by cooling the feed. The condensed feed and the condensed liquid from the reflux exchanger may be passed to different points of the fractionating column 21.

Alternatively, the condensed liquid descending from the reflux exchanger in line 16 may be passed to the column 21 without being passed to the vapour liquid separator 13.

More than one overhead stream may be withdrawn from the column. In this case, the streams are condensed to total or partial liquification separately prior to at least one stream being let-down in pressure and mixed with refluxed vapour.

The vapour/liquid separator 13 may be one of a series of two or more vapour/liquid separators in a 'chilling train'.

In both embodiments described above, the at least partially condensed column overheads 126 and 27 are combined with the first uncondensed vapour streams 114 and 17 prior to the subsequent evaporation to provide refrigeration. However, it will be clear that the partially condensed column overheads may be combined with a portion of the first uncondensed vapour stream. The remainder of the uncondensed vapour stream forming a second product stream.

In both Figure 1 and Figure 2, a gas stream flows through a turbine. The work obtained in the turbine brake may be used for feed gas compression, residual gas compression or to drive an electrical generator.

Example 1 A gaseous hydrocarbon feed stream was subjected to separation by the process described with reference to Figure 1 including the optional feature that a proportion of stream 120 was routed, via line 131, through the main exchanger to yield a liquid hydrocarbon product and residual gas. The process was modified in that a small propane rich stream and liquid from an upstream separator were introduced into the column feed. The introduction of the propane and liquid streams were due to the specific conditions and did not alter the results obtained.

The compositions, temperatures and pressures of the various streams are set out in Table 1.

The improvement-in power saving over the process described in EP 0148070 is calculated to be of the order of 5%.

Example 2 A gaseous hydrocarbon feed stream was subjected to separation by the process described with reference to Figure 2 including the optional feature that a proportion of stream 20 was routed, via line 22, through the main exchanger to yield a liquid hydrocarbon product and residual gas. The process was modified in that a small propane rich stream was introduced into the column feed.

The compositions, temperatures and pressures of the various streams are set out in Table 2.

Condensing the column overheads in counter-current heat exchange relationship with the subsequent evaporation thereof reduces the power consumption by 30KW which is approximately 8% when compared to conducting the separation by the process described in GB 2146751.

An alternative process for fractionation of gaseous feed such as natural gas or gases resulting from the processing of petroleum fractions in which cold is generated by a different process is described in US 4690702. We have calculated that our process will have a 15 to 20% lower power consumption than this process.

These power savings result in a reduction in the cost and size of any mechanical refrigeration systems and a consequential reduction in the consumption of cooling water or air cooling requirement for the mechanical refrigeration system which in turn results in a reduction in the physical size of the plant, capital costs and operating costs.

The process of the present invention may result in improved recovery of desired hydrocarbons. This may be in addition to or in place of the reduction in power saving described above.

The processes described in Figures 1 and 2 have at least some of the refrigeration requirement provided by mechanical means. However, where sufficient refrigeration is provided by heat exchange with process streams or by work expansion of process streams as described, the mechanical refrigeration systems can be omitted.

T A B L E 1 Stream Feed to Main Feed to Vapour from Name H@changer, 110 Separator, 113 Separator,(114) Temperature C -30 -91 @91 Pressure kPa a 1250 1220 1220 Molar Flow kg Mole/hr 585 585 335 Mass Flow kg/hr 12670 12670 5480 Hydrogen 11.7% 11.7% 20.2% Nitrogen 7.8% 7.8% 13.2% Methane 38.7% 38.7% 54.1% Ethane 10.5% 10.5% 2.2% Ethylene 25.7% 25.7% Propane 1.1% 1.1% ! 0.1% Propylene 3.3% 3.3% ) Butanes 0.5% 0.5% 0.0% Butenes 0.6% 0.6% 0.0% pentanes+ 0.1% 0.1% 0.0% Stream Liquid from Column* Column Name Separator, (118) Feed, (120) Overhe@@s,(122) Temporature tC -91 4 33 Pressure kPa a 1220 1870 1860 Molar Flow kg Mole/hr 250 344 246 Mass Plow kg/hr 7190 11550 6430 Hydrogen 0.2% 0.2% 0.2% Nitrogen 0.6% 0.5% 0.7% Methane 17.8% 14.0% 19.6% Ethane 21.5% 18.6% 26.1% Ethylene 46.6% 38.0% 53.3% Propane 2.6% 3.9% ) 0.1% Propylene 7.7% 10.8% ) Butanes 1.2% 3.8% 0.0% Butenes 1.5% 5.5% 0.0% Pentan@s+ 0.3% 4.7% 0.0% * Aftcr reheat and introduction of small propanc rich stream and liquid from an upstream separator Str@am Liquld Product Expander Combinod column Name from column, (123) Outlet, (117) Overthesds and expander outlet, (127) Temperature C 75 -67 -97 Pressure kPa a 1870 610 610 Molar Flow kg Mole/hr 98 335 581 Mass Flow kg/hr 5120 5480 11910 Hydrogen 0.0% 20.2% 11.7% Nitrogen 0.0% 13.2% @.9% Methane 0.0% 54.1% 39.5% Ethane 0.1% 2.2% @ 12.3% Ethylene 10.2% 28.5% Propane 13.7% ) 0.1% ) 0.1% Propylene 37.4% ) ) Butanes 13.3% 0.0% 0.0% Butenes 19.2% 0.0% 0.0% Pentanes+ 16.3% 0.0% 0.0% TABLE 2 STREAM Feed to Main Feed to Ref fluxed Name Exchange,ll separator,l3 vapour (17) Temperature C 40 -30 -67 Pressure KPa a 1500 1470 1460 Molar Flow KgMole/hr 674 674 511 Mass Flow Kg/hr 16800 16800 10000 Hydrogen 10.2% 10.2% 13.42 Nitrogen 6.7Z 6.7% 8.9% Methane 34.1% 34.1% 43.3% Ethane 10.6% 10.6% 9.0% Ethylene 24.5% 24.5% 25.2% Propane 1.8% 1.8% ) 0.2; Propylene 5.1% 5.1% ) Butanes 1.9% 1.9% 0.0% Butenes 2.7% 2.7% 0.0% Pentanes + 2.4% 2.4% 0.0:: STREAM Name Liquid from Column * Column Separator,13 Feed (20) Overheads (23) Temperature C -34 20 -28 Pressure KPa a 1470 1870 1860 Molar Flow KgMole/hr 163 169 71 Mass Flow Kg/hr 6800 7060 1940 Hydrogen 0.1% 0.1X 0.3% Nitrogen 0.3X 0.3% 0.6X Methane 5.5% 5.3% 12.9% Ethane 15.4% 15.0% 35.7% Ethylene 21.9% 21.3% 50.6% Propane 7.5% 8.1; ) 0.1; ; Propylene 20.3% 21.8% ) Butanes 8.0% 7.7% 0.0% Butenes 11.2% 10.9% 0.0% Pentanes + 9.8% 9.5% 0.0% * After reheat and introduction of small propane rich stream STREAM Liquid Product Combined column Expander Name from column (24) o'heads & refluxed Outlet vapour (28) (31) Temperature C 75 -68 -76 Pressure KPa a 1870 1460 600 Molar Flow KgMole/hr 98 582 582 Mass Flow Kg/hr 5120 11940 1i940 Hydrogen O.OZ 11.8% 11.8% Nitrogen 0.0% 7.9% 7.9% Methane 0.0% 39.5% 39.5X Ethane ) 0.1 12.3% 12.3% Ethylene ) 28.3% 28.3Z Propane 13.9% ) 0.2% ) 0.2% Propylene 37.5% ) ) Butanes 13.4% 0.0% 0.0% Butenes 18.7% 0.0% 0.0% Pentanes + 16.4% 0.0% 0.0%

Claims

CLAIMS 1. A method of separating and recovering heavier hydrocarbons from a gaseous hydrocarbon feed by cooling and partially condensing the feed, separating the feed into condensate and a first uncondensed vapour stream and fractionating the condensate to form a liquid stream rich in said heavier hydrocarbons and an overhead vapour stream. in which said overhead vapour stream is cooled and at least partially condensed, expanded to a lower pressure and subsequently evaporated to provide refrigeration for the process, characterised in that said at least partial condensation of the column overheads is effected in counter-current heat exchange relationship with the subsequent evaporation thereof.
2. A method according to claim 1, characterised in that the expanded at least partially condensed vapour stream is combined with at least part of said first uncondensed vapour stream prior to said evaporation.
3. A method according to claim 1 or 2, characterised in that the overhead vapour stream is cooled co-currently with the gaseous feed, the first uncondensed vapour stream is work expanded to a lower pressure prior to its combination with the overhead vapour stream and the feed is cooled by heat exchange with the combined stream.
4. A method according to claim 1 or 2, characterised in that the first uncondensed vapour stream is subjected to rectification.
5. A method according to claim 4, characterised in that the rectification is effected in a reflux exchanger.
6. A method according to claim 4 or claim 5 characterised in that the at least partially condensed expanded overhead vapour stream is combined with residual vapour from the rectification step, the combined stream provides refrigeration for the rectification and is then work expanded and the work expanded stream provides refrigeration for the rectification and the cooling of the feed gas.
7. A method according to any one of the preceding claims, characterised in that a portion of the condensate is passed in counter-current heat exchange with the feed to provide additional refrigeration prior to being fractionated.
8. A method according to any one of the preceding claims, characterised in that additional mechanical refrigeration is provided to further increase recovery of the desired hydrocarbons.