NO146553B - PROCEDURE FOR SEPARATING A SUPPLY GAS UNDER PRESSURE IN A VOLUME RESTAURANT GAS AND A RELATIVELY MINOR VOLUME FRACTION - Google Patents

Description

The present invention relates to the treatment of

gases containing hydrocarbons and other gases of similar volatility, with a view to separating condensable fractions. In particular, the invention relates to the treatment of gases such as natural

gas, synthesis gas and refined gases for extracting most of the propane and most of the ethane in the gas together with practically all the heavier hydrocarbon in the gas, as well as apparatus for carrying out this treatment.

Gases containing hydrocarbons and other gases of similar volatility that can be treated according to the invention include natural gases, synthesis gases from other hydrocarbon-containing materials such as coal, crude oil, petrol, shale oil, tar sands and lignite. Natural gases usually have a predominant amount of methane and ethane (ie the combined and Cp fractions form at least 50% of the gas on a mole basis). There may also be smaller amounts of relatively heavier hydrocarbons such as propane,

butanes, pentanes and the like, as well as Hg, N2, C02 and other gases.

A typical composition for natural gas that can be treated according to the invention is in approximate mol%: 80% methane, 10% ethane, 5% propane, 0.5% isobutane, 1.5% n-butane, 0.25% isopentane , 0.25% n-pentane, 0.5% hexane-+ and the rest nitrogen and carbon dioxide. Sulfur-containing gases are also often found in natural gas.

The later significant increases in demand for ethane and propane from natural gas have led to a demand for methods that provide a high degree of recovery or yield of these products. Available processes for processing these gases are based on cooling and freezing the gas, oil absorption, absorption with cooled oil and the later cryogenic processes

which uses the principle of gas expansion through a mechanical device to produce power while simultaneously extracting heat from the system. Depending on the pressure of the gas source, the richness of the discovery (the content of ethane and heavier hydrocarbons) and the desired end products, these previously known methods or combinations of them can be used.

Cryogenic expansion extraction is currently generally preferred for ethane extraction because it combines maximum simplicity with easy start-up, flexibility, good efficiency, safety and good reproducibility. US Patent Nos. 3,360,94^, 3,292,380 and 3,292,381 describe relevant methods.

In extraction based on a typical cryogenic expansion process, an incoming gas stream (hereinafter called charge or supply gas) is cooled under pressure by heat exchange with other material streams in the process and/or external cooling media such as e.g. from a compression-cooling system

with propane. When the gas cools, some liquids are condensed which are collected in one or more separators as high-pressure condensate containing most of the desired Cg+^ constituents. The high-pressure condensate is then expanded to a lower pressure.

The evaporation that takes place during the expansion of the liquid provides a further cooling of the remaining part of the liquid.

The cooled stream that forms a mixture of steam and liquid is freed from methane, demethanized, in a demethanization tower. The demethanization tower is a distillation tower where the expansion

the cooled stream is fractionated for separation of remaining methane, nitrogen and other volatile gases as top gas, from the desired products consisting of ethane, propane and heavier components that emerge as bottom fraction.

If the incoming charge (or feed) does not condense completely, and most often it does not, the gases from this partial condensation are expanded to a lower pressure. More liquid is condensed due to the further cooling

during the expansion. The pressure after the expansion is usually the same as the operating pressure of the methane removal column. The produced liquids are thus used as input charge to the methane removal column. Most often, residual gases and the top fraction from the demethanization column are combined as methane residual gas.

In ideal operation of such a separation process

the gases leaving the process will contain practically all the methane contained in the charge and essentially no hydrocarbons equivalent to ethane or heavier components. The bottom fraction from the demethanization column will contain practically all heavier components and essentially no methane. In practice, however, the ideal situation is not achieved for the reason that a normal methane removal plant is operated to a large extent as a distillation column. The methane product will therefore typically persist

of gases leaving the top fraction stage of the column together with gases which have not undergone any distillation. Substantial losses of ethane occur because the gases emerging from the low-temperature separation contain ethane and heavier constituents which could be recovered if these gases could be further cooled or if they were brought into contact with a significant amount of relatively heavier hydrocarbons, e.g. C and heavier, which could absorb the ethane. The degree of recovery or the yield of ethane can be further increased by changing the temperature distribution in the methane reboiler column (the methane-removing distillation column) by reducing the temperature in the upper column stages by removing heat from one or more of the incoming streams (chargers) that are fed into the column.

The present invention provides a method and an apparatus for achieving this and thereby significantly increasing the yield of desired products. The essential characteristics of the invention appear from the following claims.

For a better understanding of the present invention, reference is made to examples and attached forms where: fig. 1 shows a flow diagram of a separation plant for the treatment of natural gas by one-stage freeze-expansion according to the prior art, including a set of conditions for a typical rich natural gas,

fig. 2 shows a similar block diagram of a plant that is known and a set of conditions for typical lean natural gas,

fig. 3 shows a flow core above an embodiment according to the present invention where liquids from the high-pressure separator are subcooled and combined with outgoing flow from the expansion device (expander) and fed to the methane reboiler,

fig. 4 shows a flow diagram of an embodiment according to the invention where the incoming stream is condensed and subcooled before supply to the methane reboiler,

fig. 5 is a flow diagram according to the invention where the liquids from the high-pressure separator are subcooled and led to the methane reboiler via the inlet from the expander stream,

fig. 6 is a flow diagram showing an embodiment according to the invention where the subcooled liquids and outgoing current from the expansion device enter as "charge" at the top of the column,

fig. 7 is a block diagram of another embodiment according to the invention where a side flow from the methane reboiler is used to subcool the liquids from the high pressure separator,

fig. 8 is a diagram showing yet another embodiment of the invention where the liquids from the high-pressure separator are subcooled by heat exchange with gaseous product,

fig. 9 is a flow diagram of an embodiment according to the invention where outgoing current from the expander is used for subcooling the liquids from the high pressure separator,

fig. 10 and 11 show other embodiments of the invention where two parallel expansion stages are used, and

Figs. 12A and 12B are curves of the carbon dioxide content versus temperature for an embodiment of the invention, compared to prior art.

To the following description of these forms

tables summarizing flow rates calculated for representative operating conditions are attached. In the tables, the flow rates (kg mol/hour) are rounded to the nearest whole number. The total flow rate given in the table includes all non-hydrocarbon constituents and is usually greater than the sum of the hydrocarbon constituents flow rates The temperatures indicate approximate values rounded to the nearest degree.

Reference is made to fig. 1 for a more complete description of a typical common ethane extraction process, where it is operated with an input gas from which carbon dioxide and sulfur compounds have been removed (if the concentration of such compounds in the input gas would mean that the product did not meet specifications or cause icing of the equipment), and which is dehydrated, and this gas enters the process at 49°C and 6-4 kg/cm absolute, as stream 23. It is split into two parallel streams and cooled to 7°C by heat exchange with cold residual gas at -15°C in heat exchanger 10, with product liquids (stream 26) at 28°C in heat exchanger 11 and with the methane deboiler liquid at 12°C in methane deboiler 12. From these heat exchangers the streams are combined again and then into the gas cooler, heat exchanger 13, where the the combined stream is cooled to -12°C with propane refrigerant at -15°C. The cooled stream is again divided into two parallel streams and further cooled by heat exchange with cold residual gas (stream 29) which maintains -77°C in heat exchanger 14, and with methane deboiler liquids at -62°C in methane sideboiler 15.

These streams are recombined and enter a high-pressure separator 16 at -43°C and 63.3 kg/cm<2> as stream 23a. The condensed liquid (stream 24) is separated and fed to the methane reboiler 19 through the expansion valve 30.' An expansion motor can be used instead of the expansion valve 30 if desired.

The cooled gas from the high-pressure separator 16 passes through the expansion machine 17 where the gas is work expanded from 63.3 kg/cm 2 to 20.4 kg/cm 2. The work expansion cools the gas to -87°C. The expansion machine 17 is preferably a turbine expander with a compressor 21 mounted on the expansion machine's shaft. In certain previously known embodiments, the expansion machine 17 is replaced by a normal expansion valve.

Liquid that is condensed during the expansion is flushed out in the low-pressure separator 18. The liquid goes by level control through line 25 to the methane reboiler column 19 at the top and flows from a pipe distillation bowl (not shown) as top charge to the column 19.

It is noted that in certain embodiments the low pressure separator 18 can be inserted as part of the methane reboiler 19 and then occupy the top of the column. In this case, the outlet stream from the expander enters over a pipe distillation bowl at the bottom of the separator section at the top of the column. The liquid then flows from the distillation plate (bowl) as top charge to the methane reboiler section of the column.

Liquid charge to the methane reboiler 19 goes down through the column and comes into contact with gases which distil methane from the liquid and produce a methane-free liquid product at the bottom. The heat required for the production of distillation gases comes from the heat exchangers 12 and 15.

Gases freed from condensed liquid in the methane reboiler 19 pass through line 27 together with cold outgoing gas from separator 18 through line 28. The combined gas flow then returns through line 29 to heat exchangers 14 and 10. After these heat exchangers, the gas flows through the compressor 21 which is operated by the expansion machine 17 and is

.directly connected to this. The compressor 21 compresses gas to an output pressure of approx. 21, H kg/cm . The gas then enters compressor 22 and is compressed to the final output pressure of

p

63.3 kg/cm .

Flow rates for incoming and outgoing liquid components, degree of recovery for outgoing liquid and compression effect for this previously known method which is shown in fig. 1 appears from the following table.

In fig. 2, a typical lean natural gas is treated and it is cooled according to a previously known method of the same construction as shown in fig. 1. Incoming gas stream 33 is cooled to -55°C and flows to high-pressure separator 16 as stream 33a where the contained liquid is separated and fed via level regulator through line 3^ and the expansion valve 30 to methane reboiler 19 in the middle of the reboiler column.

Cold gas from separator 16 passes through the expansion machine 17 where, due to expansion work from 63.3 kg/cm<2> to 17.6 kg/cm2, it is cooled to -103°C. The liquid, which is condensed during the expansion, is separated in the low-pressure separator 18 and fed via level control through line 35 to the methane reboiler 19 as top charge to the column.

Data for this case appears in the following table:

In the previous method shown in fig. 1 and 2, the ethane recovery rate is 73% for the charge that constitutes rich gas and 79% for the lean gas charge. It is known that a certain improvement in yield can be achieved by introducing one or more cooling steps followed by one or more separation steps or by changing the temperature in separator 16 or the pressure in separator 18. Recovery rates for ethane and propane produced in this way , even if they are possibly improved in relation to the cases illustrated in fig. 1 and fig. 2, is, however, considerably lower than the yields that can be obtained according to the method of the present invention.

According to an embodiment of the invention to be described in examples 1-5, the hydrocarbon gas under pressure is cooled sufficiently to form a liquid part which is expanded to a lower pressure as in the known method. Expansion of the first part of the liquid part evaporates a part of the liquid and cools the remaining part which remains liquid. The expanded stream is usually led to a distillation column where it is separated into a top fraction and a bottom fraction. The present invention improves the previously known method by dividing the remaining part of the liquid part into first and second liquid streams. The first liquid stream is led to heat exchange with the liquid part of the incoming charge before expansion, to precooling or subcooling the liquid part before expansion. The precooling (or synonymously, the subcooling) of the liquid part condensed from incoming charge gas under pressure before expansion reduces a temperature reached in said second liquid stream after expansion.

The first and second liquid streams are then led to the distillation column. The second liquid stream is introduced onto the column at a point higher than the first liquid stream.

Example 1

An embodiment of the present invention is shown in fig. 3- Known methods are used to remove sulphur-containing components, carbon dioxide and to dehydrate and cool incoming gas 23 to -iJ3°C, generally as described in fig. 1, via heat exchangers 10, 11, 12, 13, 14 and 15. As in fig. 1 are the operating conditions stated in fig. 3 as well as flow rates in table III below indicated for rich incoming charge gas. The cooled and partially condensed gas 23a which holds -43°C and 6333 kg/cm goes to the high pressure separator 16 where the condensed liquid is separated.

The cooled gas component from the incoming stream passes from the high-pressure separator 16 through the expander 17 where, due to work expansion from 63.3 kg/cm 2 to 20.4 kg/cm<2>, the gas is cooled to -87°C. As with the previous technique shown in fig. 1, the expansion device 17 can have a compressor 21

on the expander shaft. Outgoing stream from the expander 44 is then combined with the cold stream 43 from valve 42 and flows into low pressure separator 50.

The cooled liquid from the high-pressure separator 16, stream 24, passes through the heat exchanger 4l where it is subcooled to -91°C by heat exchange with part of the cold liquids from the low-pressure separator 50, as described later. The subcooled liquids are then expanded and flash-evaporated at the valve 42, the pressure being reduced to 20.4 kg/cm^. The cold current 43

from valve 42 is combined with outgoing flow from the expander,

44, as explained above.

A first part of the condensed liquid from separator 50 goes as stream 45a to the top of methane reboiler 19 as top charge on the column. The second part, stream 45b,

of the liquid from the separator 50 goes through the heat exchanger 41 where it is used for subcooling the liquids from the high-pressure separator 16. From the heat exchanger 41, the flow goes to the methane deboiler 19 as an incoming charge in the middle of the column.

Vapor that is distilled from the condensed liquid

in the methane reboiler 19 this leaves through line 4 6. and is fed together with cold outgoing gas 47 from separator 18 and the combined flow passes through line 48 and through the rest of the system.

The flow rates, yield and compression effect for this version are shown in the following table:

Example 2

A second embodiment of the process according to the present invention is shown in fig. 4. In this embodiment, incoming gas 23 is treated and cooled through heat exchangers 10, 11, 12, 13 and 14 and reboiler 15. As in Fig. 1, process conditions and flow rates in the following table are calculated for rich incoming gas. contrary to Fig. 1, however, the incoming charge in this case is cooled to -48°C and at this temperature the entire incoming gas stream is condensed. The condensed liquids enter the heat exchanger 51 where they are further cooled to ~96°C by heat exchange with a part of the cold liquid flow from low pressure separator 52. From heat exchanger 51 the cooled incoming flow goes to expansion and flash evaporation through expansion valve 53' From valve 53 the cold incoming flow goes into low pressure separator 52 where gas and liquid are separated.

A first part of the cold liquids 54 from the low-pressure separator 53, the stream 55, is fed into the methane reboiler as top charge on the column. The second part 56 of the liquid from the separator 52 goes to the heat exchanger 51 where it is used for cooling the incoming gas from -48°C to -96°C. ,From the heat exchanger 51, the stream 56a is led to the methane reboiler 19 in the middle of the column. The figures that apply to this case can be found in the following table:

Example 3

A third embodiment of the process according to the present invention is shown in fig. 5. Prior art which is reproduced in fig. 2 is used for cooling incoming gas in the line after heat exchanger 14 and boiler 15 to -55°C

at 63.3 kg/cm 2 , 33a. As in fig. 2 are the stated conditions, flow rates found in table V calculated for lean input gas. The gas at ~55°C goes to high-pressure separator 16 where its condensed liquids are separated.

The cooled gas from separator 16 passes through the expander 17 where, due to work expansion from 63.3 kg/cm<2> to 17.6 kg/cm<2>, it is cooled to -103°C. From the expander 17, the cooled gases go to the methane reboiler 19 as middle charge on the column.

The liquid 34 from the separator 16 passes through the heat exchanger 6l where it is subcooled to -101°C by heat exchange with the first part 62 of the cold liquids 63 from the low pressure separator 64. The subcooled liquid is subjected to expansion and flash evaporation at valve 65 when the pressure is reduced to 17 .6 kg/cm 2. The cold flow from expansion valve 65 goes to the separator

64 where the cold liquid and steam are separated.

As mentioned above, the first part 62 of the liquid is used

from separator 64 to subcool the condensed liquid 34

from high-pressure separator 16. Stream 62 then goes to methane separator 19 as the lowest input charge on the column. The second part 66 of the liquid on separator 6ft is fed to the methane reboiler 19 as top feed.

Methane reboiler 19 shows at 67 an area which denotes distillation plates or column filling which is equivalent to at least one distillation stage. In this embodiment, an area of filler material or bowls is used which is sufficiently large to ensure that ethane and higher hydrocarbons found in the mixture of gas and liquid in the conduit 68 from the expander 17 mix in the wake with top-charge liquids which are rich. on heavier hydrocarbons during flow through this area 67 in the methane reboiler and that the mixing takes place under conditions leading to maximum recovery of ethane and higher hydrocarbons. These favorable conditions include an incoming top charge that is rich in higher hydrocarbons, such as stream 66, and a column design that ensures that hotter streams to the column, such as stream 62a, are sufficiently far below the top charge that, in operation, the gas temperature in the column in the area near the top charge is close to the temperature of the incoming top charge.

The figures for this case can be found in the following table:

Example 4

A fourth version of the invention is shown in fig. 6. As in fig. 2, incoming gas 33 is partially condensed at -55°C and 63.3 kg/cm o in heat exchangers 10, 11, 12, 14 and 15, and led to low-pressure separator 16. The specified operating conditions and flow rates in table VI that follow apply too lean charge gas. Gases from separator 16 are expanded in work expander 17 and

is fed to low-pressure separator 18 at -103°C and 17.6 kg/cm where condensed liquid from the expansion is separated as stream 35.

The liquid stream 3-4 from separator 16 is subcooled

in heat exchanger 71 to -101°C and is expanded through valve 72

to a pressure of 17.6 kg/cm 2. Part of the liquid evaporates and cools the remaining part to -106°C. Expanded liquid enters separator 73 where liquid and steam are separated. The cold liquid, stream 74, from separator 73, is combined with liquid

stream 35 from separator 18 to a combined stream 75. A first part 76 of the combined stream is used for subcooling the condensed liquid 34 in heat exchanger 71 and is supplied as charge to the methane reboiler 19 in the middle of the column. The second part 77 from stream 75 is fed to methane reboiler 19 as top charge at -105°C.

The figures that apply to this case appear from

the table:

Por to summarize the description of the first

four of the invention, the incoming charge is fully or partially condensed under pressure by cooling with the aid of product as well as available side streams from the distillation column and (if necessary) external cooling. When incoming gas under pressure only partially condenses, the remaining gases are expanded into a cooled and partially condensed expanded gas. The liquid part that is obtained by cooling and freezing the incoming charge under pressure

is expanded (e.g. by flash evaporation or in an expansion engine)

. whereby a part evaporates and the rest is cooled and used as a liquid charge in a distillation plant, e.g. a methane decoction plant as used.

Before expansion, the liquid part of the incoming charge is subcooled by causing it to exchange heat with part of the expanded cold liquid. This gives two liquid streams derived from expansion of the liquid part of the condensed charge and one is significantly colder than the other.

Better yield can be achieved by using a

distributed power in different ways in the process:

(i) by feeding both branched liquid streams directly to the methane reboiler, the colder stream

is used as an incoming charge at a higher place in the column than

the hotter charge. With this method, the colder liquid stream can be used as all or part of the top charge to the column, (ii) by combining all or part of the expanded liquid stream with all or part of the working expanded gas to a combined condensate , using part of the combined condensate for subcooling the liquid part of the incoming charge and using the rest as column-top charge,

.(iii) by using the cold expanded liquid according to the invention as column-top charge and introducing liquid or gas (or both) from the expanded gas on

a point in the column below the top at which the cold liquid in the column top will absorb absorbable ethane from the expanded gases.

In connection with what has been mentioned above, it is noted that, for the sake of simplicity, gas/liquid separation of the expanded liquid and gas outside the methane reboiler has been shown. It will be clear to those skilled in the art that such gas/liquid separation can just as well take place inside the methane reboiler column. In a similar way, by suitable selection and regulation of side flows and tapping flows, the gas from flash expansion from the liquid part of the incoming gas can be introduced directly into the column and divided internally into the desired first portion which is used for subcooling the liquid part of the gas charge. When the flash-expanded liquid is introduced directly onto the column at an intermediate stage, liquid drawn off at this stage as a source of subcooling will usually contain not only part of the liquid from the charge, but also liquid flowing down through the column from higher stages.

It is also noted that, as shown in the preceding examples, the entire condensed liquid stream from separator 16 is subcooled. In certain cases, it may be advantageous to treat only part of the liquid from the separator according to the present invention.

In another embodiment, subcooling is achieved

the condensed feed charge under pressure before expansion by heat exchange with a liquid available after the methane reboiler column. This is illustrated in fig. 7 according to the invention.

Example 5

Fig. 7 shows a part of a flow diagram which mentions an embodiment of the invention and illustrates a special case which has been calculated, assuming total condensation of the incoming gas charge with a pressure of 63.3 kg/cm p. Such a total condensation process is illustrated e.g. on fig. 4 where incoming gas at a temperature of 4'9°C and 64.0 kg/cm<2> is condensed by heat exchange against residual gas products, product from methane side digester, methane digester and digester bottom products. In order to

to achieve full condensation, you usually also have to use extra external cooling as shown in fig. 4.

Reference is now made to fig. 7 where the totally condensed incoming liquid charge 90 at a pressure of approx. 63.3 kg/cm 2 and

-48°C goes through heat exchanger 9l where it is subcooled to

-96°C. The subcooled liquid is flash-expanded through the expansion valve 92 and the expanded product enters separator 93. During flash expansion, part of the liquid will evaporate and cool the rest of the liquid to approx. -99°C. The re-

remaining liquid is separated in separator 93 and supplied as stream 94 to methane reboiler column 19 as top charge on the column.

Gases evaporated during the flash expansion step leave separator 93 as gas 95 and are combined with overhead gas 96 from methane reboiler 19 to form a stream of residual gas 101. As shown in fig. 4, the residual gas leaving the methane reboiler column is returned to heat exchange with the incoming gas charge and thus forms part of the cooling required for densification of the gas charge. The residual gas is then compressed to approx. 63.3 kg/cm 2 and drained

of the process.

The desired liquid product is located in the methane reboiler bottom product 100. Before this product leaves the process, it is heat exchanged with the incoming charge to cool the incoming gas as shown e.g. on fig. 4.

For subcooling the liquid 90 according to this embodiment of the invention, a side stream 97 is tapped off from the methane reboiler column 19 and passes through heat exchanger 91. The heated side stream 98 is then returned to the methane reboiler column at a point below the liquid inlet 94. For the embodiment shown in fig. 7 and in the table below, it was assumed that the methane reboiler 19 contained a column packing material which was equivalent to a distillation stage between the side stream return 98 and the incoming top charge 94.

Flow rates, yield and compression effect

for this design appears in the table:

It will be seen that the use of a side stream 97 for cooling

in heat exchanger 91, a side flow return 98 is generated which is partly in gas form. The column should therefore have a gas/liquid contactor between the liquid inlet 94 and the side flow return 98 so that heating gases originating from the return flow 98 will be cooled before entering the column top gases 96.

The distillation apparatus forms a gas/liquid contact

in this column area to facilitate the heat exchange between the rising gases and the cold liquid. It will be clear to those skilled in the art that the degree of gas/liquid contact thus achieved can vary and can be achieved by means of one or more bubble plates, bowls, etc., or by larger or smaller amounts of column packing material.

According to preferred embodiments of the invention, the methane reboiler column 19 should provide sufficient heat exchange between the incoming liquid 94 and the side flow return 98 so that gases rising past the liquid inlet 94 will have a temperature that does not exceed the temperature of the incoming liquid flow 94 by more than approx. 6°C.

Although the present invention is described with special reference to an embodiment where the side flow discharge 97 and return 98 take place at the same place on the methane reboiler 19, it is not necessary that the side flow return 98 corresponds to the side flow discharge 97 - Eg. can it be an advantage from the column operation point of view and with regard to heat balance to return side flow 98 at a point that is lower than the withdrawal of side flow 97.

For purposes of illustration, it will also be seen that while the heat exchanger 91 is shown as a heat exchanger outside the methane reboiler column, supplied with the side flow discharge 97, an entirely equivalent result has been achieved by using an internal heat exchanger in the methane reboiler 19 instead of an external heat exchanger 91. In a

in such a case, the internal heat exchanger would be placed so that

the answer to the sideflow return entry point 98.

It will also be clear from the illustrations

other embodiments of the invention, for example fig. 3, 5 and 6,

that the incoming charge 90 does not need to be cooled until complete condensation has been achieved, nor does one need to cool the entire liquid flow 9o. If the incoming charge 90 is partially condensed as, for example, in fig. 3, separation of the partially condensed incoming charge will be used. Liquid that is recovered from the partial condensation of the charge will be processed further as shown in fig. 7- The gas that is formed can be expanded for work in e.g. a turbine expander and there form an expanded and partially condensed gas stream, with the partial condensate that is extracted being fed to the methane reboiler column. As can be seen from fig. 5 and 6, other variations are possible. For example if the output charge is only partially condensed and working expanded gas is therefore available, part or all of the working expanded gas can, if desired, be fed to the methane reboiler column as middle charge and the subcooled liquid Sk be used as methane reboiler top charge as illustrated in

fig. 7-

Yet another embodiment of the invention has particular significance in gas separation processes where the incoming charge under pressure is partially condensed into a liquid part and a gas part. The liquid portion is subcooled and expanded to a lower pressure and thereby forms a cold liquid charge which goes to the distillation column. The gas part is expanded to a lower pressure and thereby provides cooling

and partial condensation of the gas. The cooling produced by expansion of the gas part is used to subcool the liquefied part of the incoming gas under pressure before expansion.

This embodiment "is more particularly illustrated at

fig. 8 and 9. Both fig. 8 and 9 reproduce only partial flow diagrams for a total gas separation plant. As shown on

in both drawings, this part is supplied with a cooled incoming gas charge. Such an incoming gas charge is obtained in a known manner as shown by heat exchange as in fig. 1 and 2 in heat exchangers 10, 11, 12, 13, 14 and 15- These heat exchangers utilize the freezing effect in product and residual gas from the gas separation plant and use additional external freezing to the extent necessary for cooling the output - the gas under pressure to a state where the gas enters the subsystem shown in the drawing.

Process conditions shown in fig- 8 and 9 correspond

to the conditions for lean gas charge at composition as indicated in table II. The conditions for fig. :8 and 9 can be compared with fig. 2 to illustrate the present invention.

At inlet conditions, it keeps the cooled lean exit gas

33a a temperature of -55°C and a pressure of 63.3 kg/cm .

Example 6

According to the method illustrated in fig. 8 consists of the partially condensed output gas 33a which appears as described in fig. 2 of partially condensed gases containing a liquid portion and a gas portion. The partially condensed gas enters the high-pressure separator 16 where gas and liquid separate layers. By first following the gases leaving the separator 16, these gases are fed into the expansion motor 17 where mechanical energy is drained from the gas. As the gas is expanded from a pressure of approx. 63.3 kg/cm and more

about. 17.6 kg/cm 2 , the working expansion will cool the expanded gas 113 to a temperature of approx. -103°C. Expanded and partially condensed gas 113 is introduced as charge on methane reboiler 19 where the gases rise and most of the C^* hydrocarbons are absorbed by the cold liquid. The methane reboiler top 117 at a temperature of -104°C, combined with gases 116 from flash evaporation described later forms in combined form residual gas 118. The combined cold residual gas 118 then passes through heat exchanger 119-

The heated residual gas at -87°C leaves heat exchanger 119

and returns to the previous cooling stages such as e.g. shown in fig. 2 where further cooling effect that can be tapped from the still cold residual gas is utilized and the residual gas is compressed in compressor 21 (see fig. 2) which is driven by work expansion

the engine 17 and is further compressed to the line pressure of 63.3 kg/cm using the final compressor 22.

As one follows the liquid 34 from separator 16, one sees that this stream 34 passes through heat exchanger 119 in heat exchange with the cold residual gas 118. This provides a pre-cooling of the liquid part to the partially condensed high-pressure charge gas. The sub-cooled liquid is expanded through a suitable expansion device such as e.g. the expansion valve 120 to a pressure of approx. 17.6 kg/cm<2>. During the expansion, part of the incoming charge will evaporate and

give a cooling of the remaining liquid. As shown in fig. 8

the expanded stream from the expansion valve 120 reaches a temperature of -106°C and enters separator 121. The liquid part is separated from here and goes as stream 115 to the distillation column 19 as top charge.

It will be seen that by comparison with fig..2

when the methane reboiler charge from the expansion valve 30 in fig. 2 only a temperature of -92°C. Due to the fact that, according to this embodiment of the present invention, stream 115 is significantly colder, it can be used as a top charge on the methane reboiler for the extraction of ethane in stream 113. The ethane is drained into the bottom product 125 - The bottom product 125 from the methane reboiler column is heat exchanged with the incoming charge for to use the cooling effect as illustrated in fig. 1 and 2 in general.

In connection with fig. 8 it is noted that for heat-economic reasons there will be one or more methane side reboilers as heat exchangers for cooling the incoming charge (not shown in fig. 8) but as generally illustrated in fig. 2. Two such side decanters have been taken as shown in fig. 2 to illustrate the present process and the calculations linked to fig. 8

and can be found in the following table. The side boilers are important for regulating the overall heat economy of the process. Subcooling of the liquid stream 34 with residual gas 118 reduces the available cooling effect found in the stream 118 for the purpose of cooling the incoming charge. However, the increased load on the methane reboiler 19 with liquid stream 115 cooled according to the invention will offer further available cooling effect in the side reboilers. Consequently, the total heat balance in the process is essentially unaffected.

Incoming and outgoing flow rates, yields and expansion/compression power requirements for this design as shown in fig. 8 appears from the table below:

The flow rates given in Table VIII correspond to the treatment of lean input gas. For comparison, reference can be made to fig. 2 and table II where the same gas stream is treated without precooling the condensed incoming gas/liquid as in heat exchanger 119.

The significantly improved yields according to the present invention are mainly achieved because one can work with a significantly colder liquid charge which is obtained by subcooling and expansion of the liquid which is condensed from the high pressure charge. With the help of the present invention, the expanded liquid is at a temperature that is sufficiently low for it to be used as a top charge on the methane still 19.

Due to the significant content of propane and in this very cold liquid stream, it has an increased ability for ethane absorption.

In the above example, sub-cooling of the liquid flow 34 with residual gas has been shown by means of a combined residual gas flow. Other residual gas flows can also be used if they are sufficiently large in relation to the flow

34, thus the currents 116 and 117 shown in fig. 7, or residual gas 47 in fig. 3- In the present context, the term "residual gas" shall include all such flows or combinations of them.

Example 7

Another illustration of the design according to the present invention is shown in fig. 9- Here the cooled and partially condensed gas charge 33a was fed into the high-pressure separator 16 with a temperature of -55°C and a pressure equal to 63.3 kg/cm<2> and is separated into a liquid part and a gas part. As described e.g. on fig. 2

this cooled gas charge is obtained by heat exchange, preferably with different process streams in order to utilize the maximum cooling effect in them, using external cooling if necessary.

The gases coming from the separator 16 are first led, as these are expanded through the turbine expander 17 to a pressure of approx. 17.6 kg/cm<2> and a temperature equal to -103°C

Under these conditions, the expanded gas from the high-pressure charge will be partially condensed. All of the expanded and partially condensed gas leaving the turbine expansion engine 17 will pass through heat exchanger 131 where it is heated to approx. -94°C and is fed to the methane debottle 19 on a middle step in the column.

As regards the liquids leaving separator 16, the liquid stream 34 is passed through heat exchanger 131 in heat exchange with expanded gas from turbine expander 17. This results in a precooling of liquid 34 from separator 16 from a temperature of -55°C to -100°C. The subcooled liquid 34 is then expanded through the expansion valve 133 to a temperature of -106°C and enters the separator 134.

In separator 134, gas developed after flash expansion is separated from the rest of the liquid. The remaining liquid 135 after the expansion step is fed to the methane reboiler 19

as top charge. Gas 136 from separator 134 is combined with methane reboiler overhead 137 to form a combined gas 139 which leaves the process. As illustrated in fig. 2, the top gas from the process is used for cooling and partial condensation of the incoming gas charge and is then compressed in the compressor 21 which is driven by the turbine expander 17 and in the final compressor 22 to a line pressure of 63.3 kg/cm.

The bottom product 138 from the methane still 19 contains the desired liquid product and is also used for cooling the incoming gas charge as it exits the process as the desired product.

Although not specifically shown in FIG. 9, it should be noted that it is common when constructing methane deboilers to arrange side-flow deboilers, e.g. as shown in fig. 2, for regulating the operation of the methane reboiler and at the same time to be able to utilize additional cooling power for pre-cooling incoming high-pressure gas. The process conditions and calculations of the most important flows shown in fig. 9 and in table IX below are based on the use of two side boilers as shown in fig. 2.

Flow rates, yields and compression effect for the method as shown in fig. 9 including side cookers can be found in the table below.

The version illustrated in fig. 9 provides substantial improvement in the ethane yield due to the availability of a cold top charge on methane reboiler 19, stream 135 > which is formed by precooling stream ~$k prior to flash expansion according to the present invention, for use as top charge to the column. With this design, it will be seen that the entire liquid and gas stream leaving the expander 17 enters the methane reboiler 19 lower than the liquid charge 135. The cold liquid 135 contains significant amounts of propane and butane and higher hydrocarbons which are able to absorb larger amounts of the desired products. which are found in the gases leaving the gas/liquid separator 16.

Example 8

In yet another embodiment of the present invention, it may be advantageous to use two separate expansion valves for expansion of the supercooled high-pressure liquid as condensate. This modification can be better understood by reference to fig. 10 which can be compared with fig. 5. Pet is shown first to fig. 5 where it can be seen that the high-pressure liquid condensate from separator 16 (stream 34) is subcooled in heat exchanger 61, expanded in expansion valve 65 and separated into a liquid and gas stream in separator 64.

According to the modification according to the present invention in this example 8, the liquid stream '34 is subcooled in the heat exchanger 150. The subcooled liquid from the exchanger 150 is divided before expansion into two parts (streams 170 and 171). The flow 170 is expanded through the expansion valve 172 and

reaches an expansion temperature of -106°C. The expansion products are used for subcooling the liquid stream 34 in the heat exchanger 150 and are then fed to the methane reboiler 19 as input stream 170a. The portion 171 of the supercooled high-pressure liquid is expanded in the expansion valve 173 and again reaches an expansion temperature equal to -106°C and is fed to the methane reboiler 19 as top charge 171a.

Both incoming streams 170a and 171a are gas-liquid mixtures and consequently the methane reboiler column is equipped at the introduction points with suitable column systems inside (e.g. tube plates or the like) which ensure separation of gas/liquid from the incoming charge. In the embodiment shown in the present example, there will normally also be one or more distillation plates or bowls or suitable filling material between the introduction points for the streams 170a and 171a to form gas/liquid contact between the liquids falling down through the column having guide 171a and the gases rising through the column from the charge 170a.

It is realized that fig. 10 only shows part of a flow diagram, namely only the processing of high-pressure

the liquid 34 from separator 16. As can be seen from a comparison with fig. 5, there will also be equipment for expansion of the gases leaving the separator 16 and supply of these gases to the methane reboiler as a suitable charge. Cooling and partial condensation of incoming high-pressure gas, which initially enters the process at a pressure of 64.0 kg/cm<2>, will also be provided

and temperature equal to 49°C in heat exchange with residual gas from methane reboiler side columns and methane reboiler bottom fractions (none of these heat exchangers are shown in partial section fig. 10).

The arrangement of two expansion members 172 and 173 in this example and the introduction of the expanded products directly onto the methane debottle column 19 provides a greater mechanical simplicity since the need for gas-liquid separators is eliminated if the separation is carried out outside the column and pipelines for the withdrawal of side streams are eliminated and return as in ex. 6 (fig. 7)-The performance of this design can be seen from the data below (the conditions can be compared with the figures on fig. 5)•

The yield of C^t fractions in this edition should be increased in relation to the yields in fig. 2 and the power requirement reduced.

Example 9

There follows another example of the use of two separate expansion valves for the expansion of subcooled liquid condensate, and this version can be understood with reference to fig. 11.

Fig. 11 shows part of a flow diagram for the separation of a cooled and partially condensed high-pressure gas 174 which enters the separator 16 at -48°C and a pressure equal to 63.3 kg/cm<2>. Previously known methods of a similar type as shown in fig. 1 and 2 are used for cooling incoming gas to -48oc. This includes heat exchange with residual gas, external cooling (if necessary), methane reboiler bottom fractions and one or more side reboilers as shown in fig. 1 and 2, but not shown in section fig. 11.

The flow conditions shown in fig. 11 differs from those shown in fig. 1 and -2 since the assumed composition of incoming charge 174 in fig. 11 lay between rich and lean gas as fig. 1 and 2 are based on. For calculation purposes, two methane side reboilers (not shown) were assumed, which in fig. 1 and 2.

Reference is again made to fig. 11 where the cooled gas from separator 1.6 is divided into two parts. The first part 176 passes through the expander 17 where, due to work expansion from 63.3 kg/cm<2> to 20.4 kg/cm<2>, it is cooled to approx. -92°C. From the expander 17, the cooled gas goes to the methane reboiler 19 as an intermediate route. The second gas portion 177 is combined with the portion 179 from the subcooled liquid coming from heat exchanger 184

as explained later.

The cooled liquid 175 from separator 16 flows through heat exchanger 184 where it is subcooled to -90°C by heat exchange with the cold flow from the expansion valve 182. The subcooled liquid is divided into two parts. The first part

178 passes through the expansion valve 182 where it expands and flash vaporizes as the pressure is reduced from 63.3 kg/cm p to 17.6 kg/cm- 2. The cold flow from the expansion valve.182

then passes through heat exchanger 184, where it is used for sub-cooling liquid from separator 16. From heat exchanger 184, the flow goes to methane reboiler 19 as the bottom input charge.

The second liquid portion 179 from heat exchanger 184, still at high pressure, is combined with portion 177 from the gas leaving separator 16. The combined stream then passes through heat exchanger I85 where it is subcooled to approx. -96°C by heat exchange with cold gas 180. The subcooled stream enters the expansion valve 183 where it expands and flash evaporates by reducing the pressure from 62.9 kg/cm 2 to 17.6 kg/cm 2. From the expansion valve 183 the cold flow continues to the methane reboiler 19 as top charge.

Flow rates for incoming and outgoing streams, yields and compression effects for this embodiment as shown in fig. 13 can be found in the table below:

In light of the above description, one can think of even more variations of the process according to the invention: 1. In special cases, as already mentioned, an advantage can be obtained by subcooling only a part of the condensed, liquid high-pressure charge before expansion. 2. It may be desirable to combine the sub-cooled high-pressure charge (either before or after sub-cooling) with a process stream that contains significant amounts of volatile fractions that will be able to reduce the bubble point of the high-pressure charge (e.g. as illustrated in Fig. 11 in present invention). 3- The increasing cooling achieved by flash cooling the subcooled liquid according to the present invention can in some cases be advantageously used so that all or part of the subcooled liquid is led to heat exchange with other streams in the process. E.g. the flash-expanded subcooled liquid can be used for partial cooling or condensation of all or part of the high-pressure gas coming from the partially condensed input charge before or after expansion of the gas. 4. Variations in the method of subcooling may be used; and two or more of the subcooling methods described in the examples above can be used in combination.

5. Flow charts and examples accordingly

to the present invention is described with heat exchangers of the tube/shell type. In cooling operations, it is often advantageous to use specially constructed heat exchangers such as plate fin heat exchangers. Such special heat exchangers have improved temperature transfer properties which make it possible to further approximate the temperatures in the heat exchangers, reduce costs and enable flow devices which provide heat exchange between several streams at the same time.

For a given methane reboiler pressure and expansion ratio in the previously known method as illustrated e.g. on fig. 1 and 2, the recoverable liquid yield will be practically limited and often the desired degree of recovery is greater than the recovery that can be achieved in practice in a single-stage separation plant within practical limits for pressure and expansion. To increase the yield, a larger expansion ratio must be used. However, an increased expansion ratio will increase the power consumption in the process to an increasing degree and therefore economic considerations will limit the yield that can normally be achieved in such one-stage separation processes.

The limitations of the single-stage facilities have led

for processes that have more than one separator stage for condensed liquefied gases before expansion, with the same expansion ratio and methane reboiler pressure. Two-stage operation can give from two to ten percent improved ethane yield. However, this increase is also limited and further increase cannot be achieved without excessively large energy costs.

Thus, according to the present invention, it has surprisingly been found that a substantially increased ethane yield can be achieved during single-stage operation without an increased expansion ratio. As one e.g. see by comparing the ethane yield for a typical plant for lean gas of previously known construction (see e.g. fig. 2) with the ethane yield in processes according to the present invention, e.g. shown by fig. 5, 6, 8 and 9, can be achieved

a significant improvement in ethane yield without significantly increasing the process's power consumption.

Similar improvements in ethane yield can be achieved when treating a rich natural gas that can e.g. see by comparing fig. 3, 4 and 7 (examples 1, 2 and 6) with fig. 1.

In fig. 1 (a typical process that operates with known technology in connection with such gas) the ethane yield was 72.9% while the ethane yield in fig. 3, 4 and 7 when treating the same gas according to the invention was between 92.1% and 94.8% depending on the flow scheme in the process. The power requirement in fig. 3, 4 and 7 were between 2693 and 2928 BHK.

The procedure shown in fig. 3, 4 and 7 do not require more energy than the previously known processes in fig. 1 for treatment of the same gas. The increased power requirement meant that the increased yield was tapped off as condensed liquid. By using the same increased power to previous processes as in fig. 1, one will not achieve an equally large dividend increase. This can be seen by studying the flow chart in fig. 1 where the methane reboiler is operated at a lower pressure such as 17.6 kg/cm 2 instead of 20.4 kg/cm 2.

A reduction of the column pressure to 17.6 kg/cm p by the process

on fig. 1 only increases the ethane yield to 77.1%. At the same time, the power requirement increases to 2899 BHK due to the lower column pressure.

The increased yield according to the present invention will in some cases require increased energy consumption (such as, for example, during recompression in compressor 22 in Fig. 1 or cooling of incoming gas as in heat exchanger 13 in Fig. 1)

to provide the necessary cooling and condensation of the additional gas quantities that are drained from the process as liquid. When using the present invention, this additional energy will usually be considerably less than the energy increase that would be involved according to previous techniques - such as e.g. shown in fig. 1 for increasing the ethane yield to the same extent.

As is known, natural gases usually contain carbon dioxide and sometimes significant amounts. Carbon dioxide in the methane reboiler column can lead to icing of the internal parts under cooling conditions. Even when the incoming charge contains less than 1% carbon dioxide, this amount will be fractionated in the methane reboiler column and can build up to concentrations of up to 5-10% or more. At such concentrations, the carbon dioxide can be frozen out depending on temperature, pressure, whether the carbon dioxide is in liquid or gas phase and the solubility of carbon dioxide in the liquid phase.

According to the present invention has. it has been found that when the gas from the high pressure separator is expanded and fed to the methane reboiler column lower than the column top, the problem of carbon dioxide icing can be substantially avoided. The high-pressure separator gas typically contains a large amount of methane in relation to the amount of ethane and carbon dioxide. When supplied as a charge in the middle of the column, the high-pressure separator gas will therefore tend to dilute the carbon dioxide concentration and to prevent it from increasing to icing concentrations.

The advantage of the present invention can be readily seen by plotting the concentration of carbon dioxide and its temperature for the various stages or bowls of the methane reboiler column when practicing the present invention and when following prior techniques. A diagram drawn for the treatment of gas as described above in Example 7 (see Fig. 9 and Table IX) containing 0.72% carbon dioxide may be compared with a similar diagram of the process in Fig. 2 (prior art) applied to the same gas (see Figs. 12A and 12B). These curves also include equilibrium conditions for gas-solid and liquid-solid. Equilibrium data that appears in fig. 12A and 12B are for the methane-carbon dioxide system. These numbers are assumed to be representative of the methane and ethane system. If the COg concentration at a particular place in the column is at or above the equilibrium level for that temperature, it can be assumed that icing will occur. For practical engineering purposes, the engineer usually calculates a margin of safety, ie the actual concentration must be less than the "icing" concentration by an appropriate safety factor.

When following the previous technique shown in fig. 2 (Fig. 12A), the gas conditions at point A will touch the line representing equilibrium solid/gas phase. On the contrary, wants on

fig. 12B neither the conditions for liquid nor gas reach or exceed the associated equilibrium conditions. Thus, the risk of icing is significantly reduced.

It is noted in connection with the above that when constructing methane reboiler columns for use according to the present invention, the constructor should as a matter of routine confirm that icing in the column does not occur. Even when gases are introduced mid-column it is possible that icing may occur if the process is designed for the highest possible ethane yield.

Such designs usually require the coldest attainable temperature at the top of the column. This will result in the carbon dioxide concentrations shifting to the right on the curves fig. 12A and 12B. Depending on the mode of operation, the result can be a dangerously high concentration of carbon dioxide near the top of the column. For such cases, it may be necessary to accept a somewhat lower ethane yield to avoid column icing or to pre-treat the incoming gas to reduce the carbon dioxide content to an area where they can pass through the methane reboiler. Alternatively, it may be possible to avoid icing in such cases by other changes in the operating conditions. It can e.g. be possible to cool the high-pressure separator at a different temperature, change the reheat or increase the amount of gas passed through the reheater. If such changes can be made within the limitations of the heat balance of the process, icing can be avoided without a significant loss of the ethane yield.

In connection with the above, it should be noted that in some embodiments the incoming charge on the methane reboiler top is wholly or partially a condensed part of the gases from the high pressure separator (see e.g. fig. 11) which is flash-expanded to the pressure of the methane reboiler. In some cases it may be beneficial to self-cool this stream. This can be done by dividing the condensed high-pressure steam into two streams, either before or after expansion. (If the gas is separated before expansion, both parts will be expanded). Then, one or both of the separated streams are passed after expansion in heat exchange with the high-pressure gas before expansion.

Claims (18)

1. Process for separating a feed gas under pressure into a volatile residual gas and a relatively less volatile fraction, where the feed gas contains hydrocarbons, methane and ethane which together make up the main part of the feed gas, whereby the feed gas under pressure is cooled sufficiently to form a liquid part under pressure and a vapor part under pressure, where (i) the vapor part is expanded to a lower pressure to form an expanded vapor portion, part of which is condensed during the expansion into a liquid, (ii) the liquid portion is expanded to said lower pressure during the formation of an expanded liquid part, a part of said liquid part evaporating during the expansion, and the rest of it remaining as liquid, whereby the expanded liquid part is cooled, and (iii) at least part of the liquid in the expanded vapor part formed in step (i) and part of the liquid that is left in the expanded liquid part formed in step (ii) is fed to a fractionation column in which said relatively less volatile fraction is separated, characterized in that (a) at least some of the liquid part under pressure is cooled to a temperature below its bubble point before expansion thereof in step (ii) to form a supercooled liquid, (b) at least part of the supercooled liquid formed in step (a) is expanded to said lower pressure while forming an expanded subcooled liquid, with a part of the expanded subcooled liquid evaporating during the expansion so that the expanded subcooled liquid is further cooled, with the remainder remaining as liquid, and (c) at least a part of the liquid remaining in the expanded supercooled liquid after step (b) is supplied to the fractionating column as a top liquid charge. 2. Method according to claim 1, characterized in that (1) the liquid in the expanded vapor part formed in step (i) is combined with the liquid that remains in the expanded sub-cooled liquid after step (b) to form a combined liquid flow, ( 2) the combined liquid flow is divided into a first part and a remaining part, (3) the first part of said combined liquid flow is used for providing the cooling to the liquid part under pressure required in step (a) by passing said first portion in heat exchange relationship with at least some of the pressurized liquid portion, whereby the pressurized liquid portion is cooled to a temperature below its bubble point according to step (a) and wherein said first portion of the combined liquid stream is partially vaporized, (4) the first portion of the combined liquid stream which is partially vaporized is fed to the fractionating column at a mid-column charge point, and (5) the remaining portion of the combined liquid is fed to the fractionating column as the top liquid charge. 3. Method according to claim 1, characterized in that (1) the liquid that is left in the expanded subcooled liquid after step (b) is divided into a first and a remaining part, (2) the first part is used for providing the cooling to the pressurized liquid portion required in step (a) by bringing said first portion into heat exchange relationship with at least some of the pressurized liquid portion, whereby the pressurized liquid portion is cooled to a temperature below its bubble point according to step (a) and wherein the first portion is partially vaporized, (3) the first partially vaporized portion is then fed to the fractionating column at a mid-column charge point, and (4) said remaining portion is fed to the fractionating column as a top column liquid charge. 4. Method according to claim 1, characterized in that the subcooled liquid part from step (a) is divided into a first part and a remaining part, and in that the first part is expanded to said lower pressure and brought into heat exchange conditions with at least some of the liquid part under pressure whereby the subcooled liquid part is obtained; the expanded first portion leaving the heat exchanger is then fed to the fractionating column at a middle column charge point; and the remaining part of said sub-cooled liquid is expanded and fed to the fractionation column as a top liquid charge. 5. Method according to claim 1, characterized in that at least some of the liquid part under pressure is subcooled by extracting heat therefrom, and in that the heat is supplied to the fractionation column at a middle column point, whereby the subcooled liquid part under pressure is obtained. 6. Method according to claim 4, characterized in that a liquid side stream is removed from the fractionation column at a middle column point and is brought into heat exchange conditions with at least some of said liquid part under pressure. 7. Method according to claim 1, characterized in that at least some of said liquid part under pressure is brought into heat exchange conditions with cold, volatile residual gas, whereby the subcooled liquid part is obtained, and in that the cold, volatile residual gas is heated. 8. Method according to claim 1, characterized in that at least some of the expanded vapor part obtained in step (i) is brought into heat exchange conditions with at least some of the liquid part under pressure, whereby the subcooled liquid part is obtained. 9. Method according to claim 1 or 4, characterized in that at least part of the expanded steam part from step (i) is supplied to the fractionation column at a charge point which is situated below the top charge point of the column. 10. Apparatus for use in the method according to claim 1, for separating a supply gas under pressure into a volatile residual gas and a relatively less volatile fraction, where the supply gas contains hydrocarbons, methane and ethane and together constitute the main part of said supply gas, where the apparatus comprises (i) a first cooling device (10,11,12,13,14,15) for receiving the supply gas under pressure and for cooling it sufficiently to form a liquid part under pressure and a vapor part under pressure, (ii) a separation device ( 16) connected to said first cooling device (i) for separating the liquid part under pressure and the vapor part under pressure, (iii) a first expansion device (17) connected to the separation device (ii) for receiving said vapor part under pressure and expanding it to a lower pressure during the formation of an expanded vapor portion, part of which is condensed during the expansion into a liquid, (iv) another expansion device (42,65,72,81,92,120, 1 33,172,173) connected to the separation device (ii) for receiving said liquid part under pressure and for expanding it to a lower pressure to form an expanded liquid part, a part of said liquid part under pressure evaporates during the expansion, and the rest of it remains as a liquid, whereby the expanded liquid part is cooled, (v) a fractionation device (19) connected to said first expansion device (iii) and said second expansion device (iv), the fractionation device (19) being arranged for receiving at least the liquid in the expanded vapor part and part of the liquid that remains in the expanded liquid part for separation of said relatively less volatile fraction, characterized by (a) a subcooling device (41,61,71,80,91,119, 131,150) connected between the separation device (ii) and the second expansion device (iv) for cooling said liquid part under pressure to a temperature below its bubble point before expansion thereof while forming a sub-cooled liquid, the sub-cooling device (41,61,71,80,91, 119,131,150) being arranged to lead at least part of said sub-cooled liquid to the second expansion device (iv), which constitutes an expanded, subcooled liquid, and (b) said second expansion device (iv) is arranged for feeding at least part of the liquid which is back in the expanded subcooled liquid to the fractionating device (v) as a top liquid charge. 11. Apparatus according to claim 10, characterized by another separation device (50) located between the second expansion device (iv) and the fractionation device (v) for receiving the liquid that is back in the expanded subcooled liquid from the second expansion device, the second separation device (50) being further arranged for receiving of the liquid formed in the expanded vapor part from said first expansion device (iii), the second separation device thereby providing a combined liquid flow, a device (4 5b) which connects the second separation device with said subcooling device (a) for receiving part of the combined liquid flows from said second separation device (50) and for leading the part of said combined liquid to said subcooling device for in-direct heat exchange with said liquid part under pressure, whereby the subcooling device (a) cools said liquid part under pressure and the part of the combined liquid stream is heated, a device connecting said subcooling device to the fractionation column at a mid-column charge point for supplying said portion of the combined liquid stream to the fractionation column as a mid-column charge, and a device (4 5a) connecting the second separation device (50) and said fractionation column (v) for supplying the remaining part of the combined liquid stream to the fractionation column as a top liquid charge. 12. Apparatus according to claim 10, characterized by a separation device arranged between said second expansion device (iv) and said fractionation column (v) for receiving the liquid that is back in the expanded part formed in the second expansion device and for dividing it into a first part and a second part, devices (45b, 62, 76) which connect said separation device with said subcooling device (a) for leading a first part of the expanded subcooled liquid part to said subcooling device (a), in which said expanded subcooled liquid part is led in heat exchange conditions with said liquid part under pressure for subcooling thereof and for heating the first portion of the expanded subcooled liquid portion, a device (62a) connecting said subcooling device (a) to the fractionating column (v) for receiving the heated first part from the subcooling device (a) and for feeding it to the fractionating column (v) at a mid-column charge point. 13. Apparatus according to claim 10, characterized by separating devices arranged between said subcooling device (a) and said second expansion device (iv) for receiving subcooled liquid part from said subcooling device and for dividing it into a first part and a second part, a third expansion device (172) arranged for receiving said first part of the subcooled liquid part and for expanding it to said lower pressure, the third expansion device (172) being further arranged for supplying the expanded first part to said cooling device ( a), whereby the expanded first part is brought into heat exchange conditions with the second liquid part under pressure to subcool it, a device (170a) arranged between the subcooling device (a) and the fractionating column (v) for receiving the expanded first part from the subcooling device (a) and for supplying it to the fractionating column (v) at a mid-column charge point, and a device (171) connected to said separation device for receiving said second part of the subcooled liquid part and for supplying it to the second expansion device (iv), the remaining part being expanded to said lower pressure and is supplied to the fractionation column (19) as a top liquid charge. 14. Apparatus according to claim 10, characterized by devices (68, 113) connected to the first expansion device (iii) for receiving the expanded vapor portion from there and for supplying said expanded vapor portion to the fractionation column at a mid-column point. 15. Apparatus according to claim 10, characterized in that the subcooling device (a) comprises devices (97,98) for extracting heat from said liquid part under pressure and for supplying this heat to the fractionation column (v) at a mid-column point. 16. Apparatus according to claim 15, characterized in that said devices (97,98) for extracting heat comprise a device (97) for removing a side stream from the fractionation column (v) at a mid-column point and for guiding said side stream in heat exchange conditions with said liquid part under pressure in said subcooling device (a), and a device (98) for returning said side flow from the subcooling device (a) to the fractionation column. 17. Apparatus according to claim 10, characterized by a device (118) for feeding cold volatile residual gas to the subcooling device (a) and in heat exchange conditions with said liquid part under pressure in the subcooling device (a), whereby the liquid part under pressure under - is cooled and the cold volatile residual gas is heated. 18. Apparatus according to claim 10, characterized by a device which is arranged between the first expansion device (iii) and the subcooling device (a) for receiving the expanded vapor part and for conducting it in heat exchange conditions with the liquid part under pressure which is thus subcooled.