GB2229519A - Treatment process for gas stream - Google Patents

Abstract

A method of treating the separated hydrocarbon-containing gas stream from a well stream in which the separated gas stream is treated to remove water and optionally acid gas and adjusting the pressure to a value within the range 600 to 1200 psia and subjected to the following treatment: i) the gas stream 50 is cooled with air and/or water to a temperature of 45 DEG F to 120 DEG F to produce a stream comprising a mixture of liquid and gas, ii) the resulting gas/liquid mixture from stage (i) is passed through one or more heat exchangers 54, 56 to reduce its temperature to a temperature of -35 DEG F to +45 DEG F thereby condensing substantial amounts of NGL out of the gas. iii) the liquid 62 from stage (ii) is separated and expanded through one or more valves 61 with a pressure drop of from 200 to 1000 psia to produce a gas/liquid mixture at a lower temperature which is used to provide at least part of the refrigeration for a heat exchanger of stage (ii), iv) gas 60 from stage (ii) is separated and used to provide refrigeration in a heat exchanger of stage (ii) and thereafter the gas may be utilised as a product gas. <IMAGE>

Description

TREATMENT PROCESS FOR GAS STREAM This invention relates to a process for the treating of hydrocarbons released as gases during the production and initial processing of crude oil, condensates or high molecular weight natural gases. In particular the invention relates to a process which may be conducted on off-shore installations to produce liquid natural gas in a form suitable for transfer in a pipeline.

Crude oil, as directly produced from subsea deposits, normally contains significant quantities of hydrocarbons that are gases under atmospheric conditions, principally methane, ethane, propane and butanes.

Components such as ethane, propane and butane, which can be condensed out of such gases and held as liquids under suitable conditions of temperature and pressure are called natural gas liquids or NGL.

During the initial processing of such crude oil in the field, it is usual to recover methane at reasonably high purity (90 + vol t) as a separate gaseous stream, whilst recovering the other light hydrocarbons as liquids separate from the crude oil, although they may be recombined with it. The objective of this procedure is to meet the purity and calorific value specifications placed on gas purity by on-shore distributors to avoid the formation of a separate liquid phase in the gas pipeline ("NGL drop out") and because it is cheaper to transport the separated liquid hydrocarbons dissolved in the crude oil A similar situation arises in the exploitation of many condensate fields.Some condensate fields are operated in the blow-down mode with purified methane being sold; in other cases it is often economically preferable to reinject methane into the deposits to increase the production rate of condensates. It is clearly desirable to reinject mostly methane, as this maximises the amount of usually more valuable heavier components for sale. By reducing the mass of gas compressed the size of the reinjection compressor may be reduced.

Conventionally, the initial processing of crude oil (and condensates) off-shore consists of successive depressurisations (or flashings) of the feed stream. The released gases are recompressed and cooled to temperatures approaching ambient and some heavier components in the gases (such as propane and butane) are condensed and returned to the crude oil (or condensate) for transfer on-shore.

This processing achieves comparatively poor recovery of the NGL from the gas. If higher recovery of heavier components is required, use must be made of refrigeration (to reduce the final gas temperature and thereby increase the amounts of heavier components condensed). This method conventionally involves high capital cost and operating costs and introduce unwanted operating complexity.

It is an object of the present invention to provide an alternative process for the treatment of feed streams.

Therefore according to the invention there is provided a method of treating the separated hydrocarboncontaining gas stream from a well stream in which the separated gas stream is treated to remove water and optionally acid gas and adjusting the pressure to a value within the range 600 to 1200 psia and subjected to the following treatment:: i) the gas stream is cooled with air and/or water to a temperature of 45"F to 1200F to produce a stream comprising a mixture of liquid and gas, ii) the resulting gas/liquid mixture from stage (i) is passed through one or more heat exchangers to reduce its temperature within the range -35 F to +40 F thereby condensing substantial amounts of NGL out of the gas, iii) the liquid from stage (ii) is separated and expanded through one or more valves with a pressure drop of from 200 to 1000 psia to produce a gas/liquid mixture at a lower temperature which is used to provide at least part of the refrigeration for a heat exchange of stage (ii), iv) gas from stage (ii) is separated and used to provide refrigeration in a heat exchanger of stage (ii) and thereafter the gas may be utilised as a product gas.

The invention provides a method of treating the gases released during the initial processing of crude oil, condensate or wet natural gas (that is natural gas containing relatively high amounts of heavier hydrocarbons) so as to achieve a high recovery of components heavier than methane. The principle involved is to use evaporation of the recovered liquids components to provide the bulk or all of the additional refrigeration required to cool the released gases and thereby avoid the use of a separate refrigerating medium.

In those circumstances when the refrigeration provided from the liquid in stage (iii) and the gas in stage (iv) is insufficient to provide all the required refrigeration in stage (ii), a valve or expander can be installed in the gas line between the separator in stage (iii) and the heat exchanger of stage (ii).

The invention will now be described with reference to the accompanying drawings in which: Figure 1 represents a flow diagram of a process in accordance with the invention; Figure 2 represents a flow diagram of a process incorporating a treatment process in accordance with the invention, and, Referring to Figure 1: A feed (1) (either condensate or crude oil) is depressurised successively in two flash vessels (2) and (3). Gases released in these vessels are compressed in compressor (4) if necessary, to approximately 1,000 psia and are then cooled against cooling water in heat exchanger (5) to approximately 60cm. This cooling may involve a pressure drop of from 1 to 15 psia.Final cooling, to sub ambient temperature, e.g. -10 F, is effected in heat exchanger (6) by indirect heat exchange with: (i) separated gas (8) from flash vessel (7) which may optionally have also been expanded across a valve or through a turbine (shown in dotted outline).

(ii) liquid (9) separated in flash vessel (7) which is passed through valve (10) with a pressure drop of 200 to 1000 psia.

Evaporated liquids (11) are then fed back to the inlet of vessel (3). In this way, higher recoveries of heavier hydrocarbons (11) are achieved by refrigeration, but with no need for additional refrigeration equipment such as a separate compressor with a separate refrigerating medium. The final cooling in heat exchanger (6) may involve a pressure drop of 1 to 15 psia.

Figure 2 gives a more detailed exposition of a process of the invention as applied to the initial processing of a feed from an oil field. An exemplary material balance for the scheme is given in Table 1.

The field well stream (18) is received, assumed in this example to be saturated with water, and is depressurised (to 300 psia) into separator (20). Water (22) separates from the liquid hydrocarbons in this vessel and is decanted off. Some flash gases (24) consisting mainly of the major portion of the methane in the feed, are released by depressurisation, but these contain significant quantities of hydrocarbons heavier than methane, particularly ethane and propane.

The feed stream (26) is fed to separator (28) where depressurisation to 215 psia takes place. The products from this separator are in water (30) which is decanted off, liquid hydrocarbons (32) having the desired vapour pressure for pipeline transport, and additional gases (34).

The liquid hydrocarbons (32) are then cooled to approximately 60"F against cooling water in heat exchanger (36) and is pumped into the product pipelines (38).

Released gases (34) from separator (28) are combined (after compression in compressor (40) with the gases (24) from the separator (20) and the combined streams (42) is compressed in compressor (44), to approximately 1,000 psia. The temperature of the gases leaving the compressor is approximately 245"F.

After cooling in heat exchanger (46) (to, in the case of the examples, 85"F) and separation of a small quantity of heavier hydrocarbons (49) in separator (48) the gases (50) dried by the molecular sieve drier (52).

Alternative forms of drying such as the use of the di- or tri-ethylene glycol could be used.

The gas (50) is then cooled by indirect exchange with cooling water in heat exchanger (54) to 60"F and then to -10 F by indirect exchange in heat exchanger (56) with gas (60) and evaporating liquid hydrocarbons (62).

The cooled stream (57) passes into the flash vessel (58), which operates at approximately 1,000 psia and 100F and, under these conditions, a significant proportion of the heavier hydrocarbons- in the feed to this vessel condenses and is separated as a liquid stream (62).

The separated liquid (62) is depressurised through a valve (61) causing its temperature to fall (because of the relationship between saturation vapor pressure and temperature) to approximately -67 F.

The liquid (62) evaporates almost completely in heat exchanger (56), its latent heat of vaporisation being supplied by the cooling of the gas stream (50) in indirect heat exchange contact with it.

The liquid/vapour stream (64) leaving the heat exchanger (56) is then fed back into the main feed stream (26) between separators (20) and (28). The gas stream (60) separated in separator (58) which is at -10 F, can be fed directly to heat exchanger (56) where its heating cools the incoming feed. It may then be compressed in compressor (66) for reinjection into the oil field high pressure pipelining or gas lift. In certain cases it can be attractive to expand the gas prior to its admission to heat exchanger (56) in order to provide additional cooling to the feed stream. This gas has a methane concentration of 83 mole % in the exemplary case.

TABLE 1 FLOW - MOIES STREAM 18 32 42 49 57 60 62 COMPOHENT CO2 197.27 19.05 286.95 0.54 286.35 176.55 109.78 N2 59.71 0.19 67.44 0.03 67.40 59.51 7.89 CH4 5,667.48 135.51 7,230.39 8.11 7,221.45 5,527.03 1,694.25 C2H6 801.87 186.53 1,277.50 4.10 1,272.99 613.68 659.24 C3HB 570.48 354.68 802.35 5.56 796.33 214.85 581.42 iC4H10 82.11 68.47 82.78 0.99 81.71 13.21 68.49 nC4H10 286.84 255.25 241.06 3.56 237.18 30.76 206.41 iC5H12 73.58 70.87 36.74 0.95 35.68 2.52 33.16 nC5H12 123.69 120.59 51.86 1.58 50.08 2.88 47.20 NC6H14 156.75 155.94 30.08 1.80 27.94 0.70 27.33 C7+ 2,643.41 2,643.40 0.28 0.0 0.0 0.0 0.0 WATER 10,663.18 12.71 185.26 0.02 0.0 0.0 0.0 T@TAI 21,326.35 4,023.19 10,292.68 27.25 10,077.08 6,641.68 3,435.06 T@@P F 158 142 167 85 -10 -10 -67 PRESSURE 300 215 300 1,000 980 980 (300) PSIA

Claims (7)

1. CLAIMS 1. A method of treating the separated hydrocarboncontaining gas stream from a well stream in which the separated gas stream is treated to remove water and optionally acid gas and adjusting the pressure to a value within the range 600 to 1200 psia and subjected to the following treatment:

   i) the gas stream is cooled with air and/or water to a temperature of 45"F to 120"F to produce a stream comprising a mixture of liquid and gas,

   ii) the resulting gas/liquid mixture from stage (i) is passed through one or more heat exchangers to reduce its temperature to a temperature of -35'F to +45 F thereby condensing substantial amounts of NGL out of the gas.

   iii) the liquid from stage (ii) is separated and expanded through one or more valves with a pressure drop of from 200 to 1000 psia to produce a gas/liquid mixture at a lower temperature which is used to provide at least part of the refrigeration for a heat exchanger of stage (ii),

   iv) gas from stage (ii) is separated and used to provide refrigeration in a heat exchanger of stage (ii) and thereafter the gas may be utilised as a product gas.
2. 2. A method as claimed in Claim 1 in which the liquid from stage (iii) is recycled to a point before stage (i).
3. 3. A method as claimed in any preceding claim in which the gas from stage (ii) is separated and is expanded and thereafter used to provide additional refrigeration for a heat exchanger of stage (ii).
4. 4. A method as claimed in any preceding Claim in which the liquid and gas from stage (iii) provides all of the refrigeration requirements of the heat exchangers in stage (ii).
5. 5. A method as claimed in any preceding Claim in which the stream in stage (i) undergoes a pressure drop of from 1 to 15 psia during cooling.
6. 6. A method as claimed in any preceding Claim in which the stream undergoes a pressure drop of from 1 to 15 psia during cooling in stage (ii).
7. 7. A process as claimed in Claim 1 substantially as herein described with reference to the accompanying drawings.