Chemical and Petroleum Engineering, Vol. 44, Nos. 1–2, 2008

LOW-TEMPERATURE CONDENSATION TECHNOLOGY IN FRACTIONATING OIL-ASSOCIATED GAS

A. N. Bulkatov

Basic technological processes are considered in refining gaseous hydrocarbon raw material: low-temperature absorption LTA and low-temperature condensation LTC. The scheme for an LTA apparatus is given. The capital and working costs for LTA and LTC with turbine expanders are considered. Extraction coefficients are given for ethane, propane, and heavier hydrocarbon fractions from the use of five forms of LTC scheme.

Oil-associated gas is a mixture of alkanes (methane, ethane, propane, C3+) with helium and other compounds not containing hydrocarbons. The numbers and contents of the hydrocarbons in the mixtures vary widely. Adsorption and absorption methods of drying and processing the gases are used in the initial stage. Later, cold is used: low-temperature absorption LTA or low-temperature condensation LTC, which are more effective in drying and cleaning up hydrocarbon gases. At present, LTA and LTC are basic processes in handling gaseous hydrocarbon raw material. Schemes for processing gases by means of LTC may be classified by the number of major stages of separation, the form of the cold sources, and the form of the target product. Current LTC schemes may be of single-stage, two-stage, or three-stage types. A liquid phase is necessarily produced in each stage. LTC schemes may be classified as regards the cold source into external, internal, and combined (cold sources in external and internal refrigerating cycles). An external refrigerating cycle is not dependent on the scheme and has its own coolant. External cycles may be divided into two groups on the basis of the coolant: with one-component coolant and with multicomponent (mixed) coolant, usually a mixture of light hydrocarbons. Oil gases are mainly processed in schemes in which an external propane refrigerating cycle is combined with an internal one. In the first stage, the gas is cooled to about –30°C by means of the external propane refrigerating cycle, while in the second stage, lower temperatures are produced by throttling the liquid flows obtained in the process or the use of an expander with gas from which the gasoline component has been partly removed. The LTC scheme may be used to produce C2+ or C3+ hydrocarbons in accordance with the product required. There are ongoing increases in the demand for ethane, propane, and other hydrocarbons, which requires improvements in processing in order to provide for extensive extraction of the individual components. In processing oil-associated gas, the main use is made of LTA, LTC, and low-temperature rectification for cooling the flow to the range –35 to –110°C. The lowest temperatures are attained by expansion (expansion units or throttling) for liquid and gaseous flows. There is much experience in the use of turbine expanders in the processing of natural gas and petroleum-associated gas, which allows one to design equipment containing them that can be operated to produce ethane or propane. Such plants have been developed and patented by leading foreign firms (Rendoll, Lavalin, Ortloff Corporation, and so on). Upravlyayushchaya Kompaniya – Strategicheskie Aktivy ZAO (Strategic Assets Holding Company), Moscow. Translated from Khimicheskoe i Neftegazovoe Mashinostroenie, No. 2, pp. 12–15, February, 2008. 0009-2355/08/0102-0073 ©2008 Springer Science+Business Media, Inc.

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Fuel gas

Stripped gas

BLHF

Petroleum gas

Regenerated EG

EG for regeneration

Absorbent feed

Fig. 1. LTA equipment scheme: 1-3, 13, 15, 23) heat exchangers; 4, 7, 18) propane evaporators; 5) three-phase separator; 6) absorber; 8) vessel; 9, 21, 22) evaporators; 10, 16, 24, 30) intermediate vessels; 11, 12, 20, 28, 31, 33) pumps; 14, 26, 29) air cooling equipment; 17) absorption-evaporation column; 19 and 27) reflux vessels; 25) desorber; 32) oven for heating heat carrier.

A current gas processing plant (GPP) usually consists of blocks: raw gas compressor, removal of acid gases (CO2 and H2S), drying, extracting target hydrocarbons, refrigeration plant, gas fractionation, and final compressor. The finished products include gas from which the gasoline fraction has been removed, a broad light hydrocarbon fraction (BLHF), and liquefied hydrocarbon gas and/or the hydrocarbons C2+ and C3+. The input gas composition and specifications for the finished product may affect the blocks used. Low-Temperature Absorption One current absorption scheme employs LTA with preliminary saturation of the absorbent. Foreign and Russian GPP operate in this way, including the plant at Nizhnevartovsk (first and second lines), and also at Lokosovo (first line). The LTA system consists of a preliminary gasoline stripping unit (LTC unit) and an LTA unit, which completes the extraction of the hydrocarbons on the gas passing through the LTC unit. The combined processes make the LTA reasonably flexible and universal for extracting ethane and heavier hydrocarbons. In the USA, LTA plant is used to extract ethane from oil-associated and natural gases with temperatures in the cold flows of –40 to –45°C and pressures of 6–7 MPa, where 40–50% extraction is obtained, while the extraction of C3+ hydrocarbons is 95–97%. Russian GPP use temperatures of –23 to –30°C at 3.6 MPa with oil-associated gases to extract up to 95% of the C3+ hydrocarbons. The LTA plants use light absorbents (relative molecular mass 80–140) on the basis of about 1–1.5 liter per m3 of gas. In the LTA scheme (Fig. 1), the oil-associated or natural gas passes through the compressor and is cooled in recuperative heat exchangers 1–3 and the propane evaporator 4 and is separated in the three-phase separator 5. To prevent hydrates from forming in the input chamber, an inhibitor is used: aqueous ethylene glycol (EG). In the three-phase separator 5, the gas cooled to –23 to –30°C is separated into three flows: the partially gasolinestripped gas passes to the absorber 6 for further stripping, while the hydrocarbon condensate is throttled to the pressure in the absorption column 17 and after utilization of the cold in the heat exchanger 3 is mixed with the saturated absorbent, while the EG saturated with water passes to plant for regeneration. 74

TABLE 1 Hydrocarbon fractions extraction factor, % LTC scheme C2+

C3+

C4+

40

90

97

With turbine expander

40–60

90

96

With cascade refrigeration cycle

60–80

95

99

With propane refrigeration cycle and turbine expander

60–80

95

99

With cascade propane-ethane refrigeration cycle and turbine expander

60–85

95

99

With propane and internal refrigeration cycles

The final extraction of the target hydrocarbons is performed in the absorber. The absorbent is previously saturated with gasoline-stripped gas emerging from the absorber and is cooled to –23 to –30°C. A pressure of 3.6 MPa is maintained in the absorber. The saturated absorbent from the bottom of the absorber is passed for cold recovery to the heat exchanger 15 and is then mixed with condensate heated to 30°C from the three-phase separator 5 and is separated into vapor and liquid in the vessel 16, from which the material passes as feed to the absorption and evaporation column 17. The pressure in the column is 1.9 MPa. The column is fitted with intermediate evaporator 21 and basic evaporator 22. The heating agent is provided by the hot recycled absorbent. The flushing liquid is provided by material from column 17 previously saturated with gases and cooled to –23 to –30°C in the evaporator 18. The methane-ethane fraction evaporated in the column is mixed in the vessel 19 with the cooled absorbent, which is saturated with methane and ethane and which is supplied by the pump 20 to irrigate the absorber and the column 17, while the tailing gases after recovery of the cold in the heat exchanger 1 are taken as fuel in the GPP or sent to users. The saturated deethanized absorbent from the bottom of column 17 is first heated in the heat exchanger 23 by the flow of regenerated absorbent and passes to the desorber 25, in which the pressure is 1.4 MPa. The product (BLHF) from the top of column 17 is sent to the user. The temperature in the lower part of the desorber is maintained by the heat from the regenerated absorbent taken from a blind plate and supplied to the oven 32. This warm absorbent is used in order to utilize the heat and passes through the preliminary saturation equipment (propane evaporator 18 and vessel 19) and then passes to the absorber and column 17. The LTA systems are fairly consumptive of energy and metal. One of the complicated units that consumes considerable energy is the absorption-evaporation column, where the saturated absorbent is deethanized. At certain foreign plants, this is replaced by deethanization of the absorbent by stepwise throttling or by deethanization in an absorption-evaporation column with replacement of the propane cold by the cold from expanded dry gasoline-stripped gas. However, the economic parameters of LTA and LTC plant with turbine expanders show that the capital and working expenses for a given yield of propane or heavy hydrocarbons are lower with LTC with turbine expander. Also, the number of distinct vessels in the LTA scheme is large (almost twice as great). Low-Temperature Condensation When oil-associated or natural gas is processed at GPP to extract ethane, propane, and heavier hydrocarbons, the most common practice is to use one of the five forms of LTC scheme given in Table 1. When oil gas is processed with LTC to extract 95% of the C3+ hydrocarbons, the gas must be cooled to –65 to –100°C. They may be attained by means of an external refrigeration cycle (cascade propane-ethane cycle) or a combined refrigeration cycle (propane cold and turbine expander). The usual schemes for processing oil gas at GPP abroad and in Russia are at present ones with LTC and turbine expanders together with a propane refrigeration cycle. The condensation before expansion is performed at 3.5–5.6 MPa. The firm of Fluor has developed a technology that is used at Nizhnevartovsk, Belozersk, and Surgut GPP, where the LTC is performed at 5.4 MPa.

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TABLE 2 Ethane extraction factor, % LTC characteristics

Gas expansion factor in turbine expander

75

90

2.3

3.0

–49

–56.8

Gas temperature, °C: before turbine after turbine

–80

–96.4

5.2·106

4.25·106

1.8

1.6

in first stream

1.7

1.35

in second stream

1.7

1.3

Heat consumption in demethanizer, kcal/h including: in reboiler

Pressure in demethanizer, MPa

2.5

2.0

Ethane output, tons/h

23.3

27.389

Energy consumption per ton of ethane, kW·h

449.4

416.0

The process developed at the Gas Processing Research Corporation (NIPIgazpererabotka) in Krasnodar has been realized in schemes at the Gubkinskii and Krasnoleninskii GPP. The LTC is operated at 3.4 MPa. The demethanization pressure in the Gubkinskii GPP scheme is maintained at the level of 2.6 MPa, while at the Krasnoleninskii GPP it is at the level of 1.7 MPa. The oil gas is compressed to 3.7–5.87 MPa in three stages and is passed to the adsorber in the drying plant, where it is dried and freed from hydrogen sulfide on molecular sieves. Part of the dried gas is used to regenerate the adsorbers. After regeneration, the regeneration gas passes to ammonia treatment, which is compressed to the pressure of the gas at the levels in the main flow and is returned to the main flow of oil gas before drying. The dried gas is passed through filters to low-temperature separation and is divided into two flows. One is cooled to a temperature of –55 or –60°C by gasoline-stripped gas and propane cold, and the other is reduced to –50 to –54°C by passage through heat exchangers: the heater supplying the deethanizer and the reboiler for the demethanizer, in which the cooling is provided by the liquid phase drawn from the lower plate of the demethanizer. The two flows are combined in the cooledgas separator, where the gas is separated from the low-temperature condensate and passes to the turbine expander, where it is cooled to temperatures between –70 and –84°C. The pressure is thereby reduced to 1.7–2.6 MPa. The two-phase flow from the expander passes to the upper plate in the demethanizer. The low-temperature condensate from the separator is throttled to the pressure of demethanization and is separated in the gas separator. The released gas is mixed with the flow of gasoline-stripped gas emerging from the demethanizer and is directed to cold recuperation. The hydrocarbon condensate from the gas separator passes to the fourth plate in the demethanizer (there are 8–10 plates in the demethanizer). The product from the lower part of the demethanizer after cold recovery is passed to the middle of the deethanizer column, on the top of which one receives methane and ethane, and from the lower part one gets the BLHF. The gas from the reflux vessel in the deethanizer is combined with the gasoline-stripped gas after cooling and directed to compression. The gasoline-stripped gas is compressed in two stages: in a compressor driven by the turbine expander and in a compressor with electric drive. The compressed stripped gas is passed to the major pipeline. The process temperature should be reduced to between –100 and –110°C in LTC with extensive ethane extraction, which can be attained by increasing the extent of the gas expansion in the turbine expander, organizing internal refrigeration cycles, and using the cold from lateral flows from the demethanizer. Recently, patents have been lodged abroad for several improved technologies using turbine expanders to provide high extraction of ethane from processing oil gas. Those technologies reduce the capital and energy costs by comparison with traditional ones when a turbine expander is used. 76

Ortloff Corporation (USA) has patented processes providing extraction of 85% or more of the ethane with an elevated molar content of carbon dioxide (up to 4%). One technique used in these processes is to separate the initial gas into two streams. The first is cooled to a lower temperature than the second, and it condenses almost completely, after which it expands in a throttle, is cooled, and supplied to the upper part of the demethanizer. The second stream passes to the middle part of the column. The ratio between the streams and the temperatures should correspond to the scope for extracting most of the ethane while eliminating the possibility that CO2 can freeze in the column. Another design allowing one to attain a high degree of extraction for ethane is to supercool the condensate from the separator ahead of the turbine expander, and then to throttle it and supply it to the upper part of the demethanizer. A third design is of interest and involves combining the two above. The first gas stream before cooling is supplied with condensate. Then the entire mixture is cooled, throttled, and supplied to the upper part of the demethanizer. Also, the gas and condensate are cooled by either gas or liquid flows, which allows one with slight additional cost to increase the extraction of ethane by about 10%. These designs allow one to raise the recovery of ethane from oil gas to 90%. The firm of Lavalin in Canada has devised a characteristic scheme for producing ethane and BLHF, which involves low-temperature condensation and rectification, and this is used in an ethane plant at the Tengiz deposit (Table 2). The scheme provides not only ethane but also propane. The ethane plant uses almost all the above designs, with combined cold: propane cold, cold from the gas after the turbine expander (degree of gas expansion in the turbine 2.3), and cold from the liquid flows taken from parts of the demethanizer. In future there will be improvements in technology and equipment, with the development of new systems for providing deeper cold, which will provide minimal energy consumption for extracting individual hydrocarbons from oil-associated gases and from other gaseous hydrocarbon raw material.

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