RU2365835C1 - Method for preparation of hydrocarbon gas to transportation from north offshore fields - Google Patents

Abstract

FIELD: oil and gas production.

SUBSTANCE: invention is related to methods of low temperature preparation of multi-component hydrocarbon gases by means of extraction of condensed water vapours and liquid hydrocarbons at temperature of minus 50-60°C. In method for preparation of hydrocarbon gas to transportation from north offshore fields, including gas cooling by means of recuperation of prepared gas cold and application of processes of isoenthalpic or isentropic expansion, separation of condensate from cooled gas, degassing of condensate, ejection of gas released in process of degassing, by prepared gas with increase of their mixture pressure, separate discharge of prepared gas and condensate, initial gas is additionally cooled twice: first prior to recuperation by liquid, second after recuperation by mixture of gases before its pressure rise, after the second additional cooling condensate is separated from gas, afterwards gas is used as ejecting and, if required, in isentropic expansion process, and excess heat from liquid is sent to environment.

EFFECT: higher efficiency of hydrocarbon gas preparation, by means of increase of discharge of condensed hydrocarbon components die to reduction of prepared gas cooling temperature of minus 50-60°C and hydrocarbon condensate, by means of its degassing at pressures of at least 4,0 MPa.

6 cl, 9 dwg

Description

The invention relates to methods for low-temperature preparation of multicomponent hydrocarbon gases by isolating condensable water vapor and liquid hydrocarbons at a temperature of minus 50-60 ° C and can be used in the gas and oil industry for preparing gases for transport mainly from northern offshore fields.

A known method of preparing hydrocarbon gas for transport (T.M. Bekirov and G.A. Lanchakov. Technology of gas and condensate processing. - M .: Nedra - 1999, p.305, 306). The method includes the use of an isoenthalpic process (integral choke effect) for cooling the source gas, separating condensate from the chilled gas, separately discharging the prepared gas and condensate, recovering the cold of the prepared gas with the source gas. The gas in preparation by the described method has the following parameters: after the cold recovery process, the pressure is 12 MPa and a temperature of minus 10 ° C, after isoenthalpic expansion, the pressure is 7.5 MPa and the temperature is minus 30 ° C. However, for long-distance transport of gas, especially from the northern offshore fields, such cooling is not enough. This is due to the low degree of extraction of liquid hydrocarbons at this cooling temperature. In addition, the disadvantages of this method include additional energy costs for compression of the gas released during the preparation of the condensate for transport, i.e. in the process of its degassing.

To ensure long-distance gas transportation from northern offshore fields, it is required to extract liquid hydrocarbons from the prepared gas at a temperature level of minus 50-60 ° С.

The task of utilizing the gases released during condensate degassing is solved in the method of preparing hydrocarbon gas for transport (T.M. Bekirov and G.A. Lanchakov. Technology of gas and condensate processing. - M .: Nedra, 1999, p. 289, 290) . This method involves cooling the gas in a recuperative heat exchanger with the cold of the prepared gas and by using the isoenthalpic (integral choke effect) expansion process, separating the condensate from the cooled gas, degassing the condensate with ejecting the gas released during degassing into the prepared gas, and separately discharging the prepared gas and condensate . However, the main disadvantage is the low degree of gas cooling to temperatures of the order of minus 30 ° C, and remains in the described method.

This disadvantage is partially eliminated in the method of preparing hydrocarbon gas (T. M. Bekirov and G. A. Lanchakov. Technology for processing gas and condensate. - M .: Nedra, 1999, p. 315). This method involves cooling the gas in a recuperative heat exchanger with cold prepared gas and by using the isentropic gas expansion process (in the turbine with the work to compress the prepared gas), separating the condensate from the cooled gas, degassing the condensate, mixing the gas released from the latter with the prepared gas, separate removal of prepared gas and condensate, compression of the prepared gas during its removal. The gas in the preparation by the described method has the following parameters: after the cold recovery process, the pressure is 12 MPa and the temperature is minus 10 ° C, after isoentropic expansion the pressure is 6.0 MPa and the temperature is minus 46.3 ° C; after compression, a pressure of 7.64 MPa and a temperature of minus 32 ° C. However, for long-distance transport of gas, especially from the northern offshore fields, such cooling is not enough. In addition, the implementation of this method does not produce the required degassing of the condensate, which should be performed at a pressure of at least 4.0 MPa. Pipeline transportation of unstable condensate over long distances is problematic.

The problem solved by the invention is to increase the efficiency of the preparation of hydrocarbon gas by increasing the yield of condensable hydrocarbon components by lowering the cooling temperature of the prepared gas minus 50-60 ° C and hydrocarbon condensate by degassing it at pressures not lower than 4.0 MPa.

The method of preparing hydrocarbon gas for transport from the northern offshore fields includes cooling the gas by recovering the cold of the prepared gas, separating the condensate from the cooled gas, degassing the condensate, ejecting the gas released during the degassing process, preparing the gas with increasing pressure of their mixture, separate removal of the prepared gas and condensate .

A new thing that distinguishes the claimed method from the known one is that the source gas is additionally cooled twice: the first time before recovery with a liquid, the second after recovery with a mixture of gases before increasing its pressure, after the second additional cooling, condensate is separated from the gas, after which the gas used as an ejector, and the excess heat from the liquid is transferred to the environment. Additionally, the gases are prepared by the isoentropic expansion process. Additionally, the gases are prepared according to the isoenthalpic expansion process. After increasing the pressure of the mixture of ejected and ejected gases, it is combined with the gas after isentropic expansion. Gases prepared by isoenthalpic and isoentropic processes are combined when supplied to the consumer. During cooling, the liquid is moved by injection in a closed loop, and heat from the liquid is used for technical and / or domestic needs. Seawater is used as a coolant and / or environment. Cooling sea water is forced or convection.

The technique of additional double cooling of the source gas allows you to more deeply cool the source gas and, ultimately, reduce the temperature of the prepared gas.

The technique of primary cooling of the source gas with liquid before recovery allows you to reduce the temperature of the source gas from the initial value, which is usually in the range of 30-40 ° C to 2-3 ° C in the winter and 5-7 ° C in the summer.

The technique of cooling the source gas with a mixture of gases before increasing its pressure allows you to pre-cool the source gas more deeply. This is due to the fact that during the ejection process the moving gas mixture has high speeds, at which, according to thermodynamics, the static temperature of the mixture is lower, the higher the speed.

Figure 1 graphically shows the change in the ratio of the static temperature T of the mixture to the initial temperature T * of the ejection gas from the Mach number, which is the ratio of the speed W of the mixture to its local speed of sound α in the stream. The speed of the gas mixture during the ejection process depends on the ratio of the pressure P _{in the} initial ejecting gas to the pressure P of the ejected gas — degassing gas.

Figure 2 presents a graphical dependence of the change in the Mach number on the ratio P _in / P at a constant value equal to 1.6, the ratio of the pressure of the source gas P _in = 12.0 MPa to the total pressure of the mixture P _with = 7.5 MPa, t. e. full pressure of the mixture at the end of the ejection process.

Figure 3 presents the graphical dependence "A" and "B" of the ratio of the mass flow rate of the ejected gas G to the mass flow rate of the original ejection gas G _in the ratio of the pressure P _{in the} original ejection gas to the pressure P of the ejected gas - degassing gas. Schedule "A" at P _in = 12.0 MPa, P _s = 6.0 MPa, schedule "B" at P _in = 12.0 MPa P _s = 7.5 MPa.

The technique of separating condensate from gas after an additional second cooling makes it possible to improve the processes of isoenthalpic or isoentropic expansion of the gas and, as a result, lower the temperature by 2-3 ° C.

The technique of using gas as an ejection gas after cooling it with a gas mixture allows to utilize the gas cold from condensate degassing. This cold is obtained by lowering the temperature during the evaporation of liquid hydrocarbons in the process of condensate degassing.

The technique of additional gas preparation by the isoentropic expansion process allows to lower the temperature.

Figure 4 shows the dependence of the ratio of the gas temperature T _and after isentropic expansion to the gas temperature T _in after secondary cooling on the ratio of the gas pressure P _in before the expansion process to the gas pressure P _and after this process.

The technique of additional gas preparation by the isoenthalpic expansion process also allows to lower the temperature.

The technique, which consists in the fact that after increasing the pressure of the mixture of ejected and ejected gases, it is combined with gas after isentropic expansion, allows you to increase the amount of ejected gas degassing and thereby increase the amount of utilized cold. For example, the ratio of the mass flow rate of the ejected gas G to the mass flow rate of the initial ejection gas G _in at a mixture pressure P _c = 6.0 MPa - pressure after the isentropic expansion process (graph "B", figure 3) is greater than in graph "A "At a pressure of P _c = 7.5 MPa - the pressure of the prepared gas supplied to the transport.

The technical method, which consists in the fact that gases prepared by isoenthalpic and isoentropic processes are combined when supplied to the consumer, allows to reduce the energy consumption for compression of the prepared prepared gas and thereby increase the efficiency of gas preparation for transport.

The technique of transferring excess heat from a liquid to the environment allows it to cool the liquid for reuse.

The technique, which consists in the fact that during cooling, the liquid is moved by injection in a closed loop, and the heat received by the liquid from the gas during recovery and released during injection is used for technical or (and) domestic needs, it allows the most complete use of the energy of the source gas.

The technical method, which consists in the fact that the use of sea water as a coolant and / or the environment makes it possible to more effectively apply the cold of northern waters.

The technique of supplying sea water by force or convection allows in the first case to intensify the process of gas cooling, and in the second - to increase the reliability of cooling in the event of a power system failure.

Each of the described techniques, as well as their combination, are aimed at achieving the goal - increasing the efficiency of hydrocarbon gas preparation.

The proposed method is implemented in installations, schematically presented in figure 5-9.

The installation of FIG. 5 comprises: an inlet separator 1, a water heat exchanger 2, a regenerative heat exchanger 3, an ejector 4, low-temperature separators 5 and 6, a heat exchanger 7, a degasser 8, an external cooling apparatus (ABO) 9, a circulation pump 10 and a pump 11 for supply prepared condensate. The ejector 4 has a housing 12, inside of which a mixing chamber 13 is coaxially located, which is connected to the high-pressure ejection gas supply line 14, to the low-pressure ejected gas supply line 15 and to the diffuser 16, which in turn is connected by a line 17 to the low-temperature separator 6. Case 12 the ejector 4 is connected by a line 18 to a low-temperature separator 5. The inlet separator 1 is connected to a source gas supply line 19, a line 20 to a water heat exchanger 2 and a line 21 to a heat exchanger 7. A water heat exchanger 2 is connected to line 22 to the external cooling apparatus 9, line 23 to the heat exchanger 3, line 24 to the inlet of the pump 10. Heat exchanger 3 is connected by line 25 to the low-temperature separator 6, line 26 to the body 12 of the ejector 4, line 27 to the consumer of the prepared gas. The separator 5, in addition to being connected by lines 14 and 18 to the ejector 4, is still connected by line 28 to the collector 29, which in turn is connected to line 21. The separator 6, in addition to being connected by line 25 to the heat exchanger 3, connected by line 30 to the collector 29. The heat exchanger 7, in addition to being connected by line 21 to the inlet separator 1, is also connected by line 31 to the degasser 8, line 32 to the outlet of the pump 10, line 33 to the external cooling apparatus 9. Separator 8 , in addition to the fact that it is connected by a line 15 to the mixing chamber 13 of the ejector 4, by a line 31 - to heat transfer nick 7, another line 34 connected to the input of the pump 11. The external cooling apparatus 9 is connected respectively by lines 22 and 33 to the heat exchangers 2 and 7. The output of the pump 11 is connected to line 35 prepared by condensation of the consumer. Between lines 22 and 33, a bypass 36 is installed with an adjustable valve 37.

The installation of FIG. 6 is additionally equipped with a turbine 38 and a compressor 39. The inlet of the turbine 38 is connected by a line 40 to a line 14 and by means of the latter to a low-temperature separator 5. The output of the turbine 38 is connected by a line 41, with a line 17 and, with the latter, to a low-temperature separator 6. The input of the compressor 39 is connected by a line 42 to the heat exchanger 3. The output of the compressor 39 is connected to the line 27 of the prepared gas consumer.

The installation in Fig. 7 is additionally equipped with a low temperature separator 43 and a heat exchanger 44. The low temperature separator 43 is connected by a line 45 to the heat exchanger 3, and by a line 46 to the collector 29. The heat exchanger 44 is connected by lines 47 and 48 to the line 26, line 49 to the separator 6, and line 50 - to line 27.

On Fig presents an installation in which seawater is used as the coolant and the environment. In this installation, the input of the pump 10 is connected to the seawater withdrawal line 51, and its output, by the line 22, to the heat exchanger 2. A line 33 is connected to the heat exchanger 7 to discharge the seawater into the environment.

In the installation of FIG. 9, an external cooling apparatus 9 is used, which uses the principle of convective heat dissipation.

EXAMPLE 1

The proposed method of gas preparation is implemented as follows.

The source multicomponent hydrocarbon gas has:

- hydrocarbon composition in mass fractions: CH ₄ - 0.876; C ₂ H ₆ - 0.0536; C ₃ H ₈ - 0.027; C ₄ H ₁₀ - 0.014; C ₅ H ₁₂ 0.006; C ₆ H ₁₄ - 0.008; C ₇ H ₁₆ 0.0035; C _{8 + B} - 0.0119

- relative molecular weight of 20.454;

- the density under normal conditions of 0.84 kg / nm ³ ;

- temperature 35 ° C (308 K);

- pressure 12.0 MPa;

- consumption of 1.0 billion nm ³ per year (26.63 kg / s).

The source hydrocarbon gas contains water vapor in an amount of 0.6 g / nm ³ (0.714 g / kg).

Gas preparation is carried out in the installation shown in figure 5. The source gas comes from line 19. Before the separator 1, a hydrate inhibitor, methanol, whose concentration is 95–97%, is introduced into line 19. In the separator 1, a dropping liquid is separated from the source gas, which is discharged through line 21 to the collector 29. Gas is supplied through line 20 to the heat exchanger 2. In the heat exchanger 2, the liquid is cooled with an aqueous glycol solution, the gas is cooled to a temperature of 2 ÷ 3 ° C in winter and 5 ÷ 7 ° C in the summer.

The initially cooled gas enters through heat exchanger 3 through line 23. In the recuperative heat exchanger 3, the gas is cooled to a temperature of minus 30 ° C by the cold of the prepared gas, which is supplied to heat exchanger 3 through line 25. The prepared gas has a temperature of minus 45-47 ° C. After the recuperative heat exchanger 3, the gas is supplied to the body 12 of the ejector 4. Here it is cooled to a temperature of minus 60 ÷ 62 ° C with a mixture of ejected and ejected gases, which flows in the mixing chamber 13 at a speed determined by the Mach number M = 1.5. At this speed, the gas mixture has a temperature of minus 100.4 ° C. Water vapor (0.0186 kg / s) and hydrocarbons in the following amount from the initial content condense from gas at a temperature of minus 60 ÷ 62 ° С: more than 99% С _{7 + В} (0.612 kg / s); 50 ÷ 60% C ₃ and C ₄ (0.53 ÷ 0.67 kg / s). They are separated from the gas in the separator 5, where they enter line 18. From the separator 5, condensate is discharged through line 28 to the collector 29, and the pre-purified gas is supplied via line 14 and is used as an ejection gas. After the ejector 4, the mixture of ejection and ejected gases has a temperature of minus 45-47 ° C and a pressure of 10.0 MPa. The residual liquid condensate is separated from the gas in the separator 6. From the separator 6, the gas thus prepared is supplied via line 25 to the recuperative heat exchanger 3 and then to line 27 to the consumer, and the condensate is discharged via line 30 to the collector 29. Through the collector 29, all the liquid phase entering from the separators 1, 5 and 6, it is supplied through a heat exchanger 7, in which it is heated by heat from the liquid heated by the source gas (which is supplied by the pump via line 32) to a temperature of 3-7 ° C. After the heat exchanger 7, the liquid phase is fed through line 31 to the separator 8, in which the aqueous solution of the hydrate inhibitor and the hydrocarbon condensate are separated.

In the separator 8, the pressure is reduced to P = 2.4 MPa. At this pressure, hydrocarbon condensate is degassed. The amount of gas released from the condensate is about 0.25 kg / s. Prepared condensate from the separator 8 via line 34 is fed to pump 11, where the condensate is pumped to a pressure of 7.5 MPa and fed to line 35 to the consumer.

EXAMPLE 2

If necessary, a deeper separation of hydrocarbon components from the gas being prepared using the installation shown in Fig.6. In this installation, half the gas from the separator 5 is fed to the isentropic expansion process in the turbine 38. In the turbine 8, the gas is expanded with the work performed in the compressor 39 (mounted on the same shaft with the turbine) by compressing the prepared gas supplied through line 42. After the turbine, the expanded gas has a pressure of P _and = 6.0 MPa and a temperature of T _and = minus 91 ° C. The cooled gas after the turbine is fed through line 41 to line 17 and the separator 6. After mixing, the gas temperature in the separator 8 reaches minus 68.5 ° C. At this pressure and temperature, water vapor (0.019 kg / s) and hydrocarbons in the following amount from the initial content condense from the gas: more than 99.98% C _{7 + V} (0.613 kg / s); up to 70 ÷ 73% C ₃ and C ₄ (0.8 kg / s). Condensed hydrocarbons and water vapor are separated from the gas in the separator 6. The condensate is removed via line 30 to the collector 29, and the produced gas is removed via line 25 through a recuperative heat exchanger 3 to the compressor 39. Due to the fact that gas compression is carried out only due to expansion energy of half the gas being prepared; to compress all the gas prepared, it is necessary to expend additional energy from an external source.

EXAMPLE 3

In order to eliminate this drawback in the installation shown in Fig. 7, gases prepared by isoenthalpic and isoentropic processes are combined when supplied to the consumer. For this, the gas prepared by the isoenthalpic process (in the ejector 4 and separator 6) is discharged through a recuperative heat exchanger 44 through lines 49 and 50 to the prepared gas consumer line 27, and the gas prepared by the isoentropic process (in turbine 38 and separator 43) is discharged line 45 through the recuperative heat exchanger 3 and line 42 through the compressor 39 to line 27.

The gas in line 50 has a pressure of 7.5 MPa. The gas after compressor 39 also has a pressure of 7.5 MPa. The energy used to compress gas from a pressure of 6.0 MPa to a pressure of 7.5 MPa in a compressor 39 is generated in sufficient quantities by a turbine 39 when the gas expands from a pressure of 12.0 MPa to a pressure of 6.0 MPa. In this regard, additional energy costs for gas compression are not required.

EXAMPLE 4

In the installations shown in FIGS. 5, 6, 7, the excess heat from the coolant is transferred to the environment (sea or atmosphere) in the external cooling apparatus 9. In this case, the cooling fluid is moved by injection in a closed loop from the external cooling apparatus 9 along line 22 to heat exchanger 2, through line 24 to the pump 10, from pump 10 through line 32 to the heat exchanger 7, from the latter via line 33 to the external cooling apparatus 9. The heat received from the circulating liquid from the gas in the heat exchanger 2 and when it is pumped by the pump 10 use for technically use (heating of the liquid phase in heat exchanger 7).

EXAMPLE 5

In the installations shown in Figs. 8 and 9, seawater is used as a cooling liquid and (or) the environment, which is forcedly pumped in the installation in Fig. 8, and convection through the external cooling device in the installation in Fig. 9 9.

Claims (8)

1. A method of preparing hydrocarbon gas for transport from northern offshore fields, including cooling the gas by recovering the cold of the prepared gas, separating the condensate from the cooled gas, degassing the condensate, ejecting the gas released during the degassing process, preparing the gas with increasing pressure of their mixture, separate removal of the prepared gas and condensate, characterized in that the source gas is additionally cooled twice: the first time before recovery with a liquid, the second after recovery with a mixture of gases before increasing its pressure, after the second additional cooling, condensate is separated from the gas, after which the gas is used as an ejection gas, and the excess heat from the liquid is transferred to the environment. 2. The method according to claim 1, characterized in that the gases are additionally prepared according to the isentropic expansion process. 3. The method according to claim 2, characterized in that the gases are additionally prepared according to the isoenthalpic expansion process. 4. The method according to claim 2, characterized in that after increasing the pressure of the mixture of ejected and ejected gases, it is combined with the gas after isentropic expansion. 5. The method according to claim 3, characterized in that the gases prepared by the isoenthalpic and isoentropic process are combined when supplied to the consumer. 6. The method according to claim 1 or 2, characterized in that during cooling, the liquid is moved by injection in a closed loop, and heat from the liquid is used for technical and / or domestic needs. 7. The method according to claim 1 or 2, characterized in that as the coolant and / or the environment using sea water. 8. The method according to claim 1 or 2, characterized in that the cooling sea water is supplied by force or convection.

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