Low-temperature dehydration process for natural gas

Abstract

The invention relates to the technical field of natural gas treatment, in particular to a low-temperature dehydration process for natural gas. The low-temperature dehydration process for the natural gas comprises the following steps of initial mixing and pre-cooling, expansion washing and initial dehydration, turbine expansion, low-temperature separation, heat exchange and final compression, output heating and liquid treatment, and meanwhile, an emergency bypass is provided. According to the low-temperature dehydration process for the natural gas, a separation technology-hydrate inhibitor assisted low-temperature separation technology is adopted, the separation efficiency can be improved, generation of hydrates is avoided, high efficiency and safety of natural gas treatment are ensured, and meanwhile, through the unique design of the technological process and the equipment structure and waste heat utilization, the natural gas is dehydrated at low temperature. And the technological process is more compact, the occupied space is small, the influence of sloshing on the liquid level and the pressure difference is reduced, and the device is more suitable for an offshore platform.

Description

Technical Field

[0001] The present invention relates to the technical field of natural gas processing, and in particular to a natural gas low-temperature dehydration process. Background Art

[0002] The presence of water in natural gas not only reduces the calorific value and pipeline transportation capacity of natural gas, but also easily accumulates in the low part of the pipeline, causing corrosion, and even forming hydrates under low temperature and high pressure, which clogs valves, equipment and pipelines and affects normal production operations. At present, commonly used methods for natural gas dehydration include solid adsorption, solvent absorption, low-temperature separation, etc.

[0003] Among them, low-temperature dehydration treatment is suitable for FPSO offshore natural gas pretreatment, and can simultaneously remove water and light hydrocarbons and other substances. Patent technical document CN115449414A discloses a natural gas low-temperature separation and dehydration system and process, wherein the system comprises a first mixer, a scrubber, an expander, a low-temperature separator and a compressor connected in sequence; the high-pressure gas source and the hydrate inhibitor are mixed in the first mixer, and then enter the scrubber for gas-liquid separation, the separated gas is isentropically expanded in the expander, the temperature is reduced, and then enters the low-temperature separator to further separate the water in the gas, and the dehydrated gas is finally transported out through the compressor; the invention greatly reduces the sloshing sensitivity of the natural gas dehydration system. However, its dehydration process is relatively complicated, and the dehydration process suitable for offshore platform environments still needs to be further optimized. Summary of the invention

[0004] In view of this, an object of the present invention is to provide a natural gas low-temperature dehydration process, which can be applicable to an offshore platform environment and can efficiently perform low-temperature dehydration treatment of natural gas.

[0005] Based on the above purpose, the present invention provides a natural gas low-temperature dehydration process, comprising the following steps: S1 initial mixing and pre-cooling: Lean ethylene glycol is injected into natural gas at a temperature of 15.7°C and a pressure of 94.9 Bar, and then enters the heat exchanger for cooling; S2 expansion scrubbing and preliminary dehydration: the pre-cooled natural gas enters the expansion scrubber for preliminary dehydration; S3 turbo expansion: Lean ethylene glycol is injected into the expanded and washed natural gas, which then enters the turbo expander / recompressor skid for cooling and expansion; S4 low temperature separation: the expanded natural gas enters the low temperature separator for separation; S5 heat exchange and final compression: The separated natural gas enters the heat exchanger again, where it is mixed with the natural gas before expansion, cooled, and enters the turbo expander / recompressor skid for compression; S6 external transmission heating: the pressurized natural gas enters the external transmission heater for external transmission; S7 liquid treatment: The separated liquid is recycled.

[0006] Furthermore, the cooling temperature in step S1 is -6°C.

[0007] Furthermore, the natural gas in step S1 is a mixture of the upstream slug collector and the high-pressure compressed natural gas.

[0008] Furthermore, the heat exchanger in step S1 is a printed circuit heat exchanger, which can reduce the volume and weight by about 85% compared with an equivalent shell and tube heat exchanger, and is more suitable for offshore environments.

[0009] Furthermore, the preliminary dehydration in step S2 is to remove condensed water and hydrocarbons.

[0010] Furthermore, the cooling expansion in step S3 is to expand the natural gas to a temperature of -21.4°C and a pressure of 72.3 Bar through a turbo expander.

[0011] Furthermore, the expander in step S3 is a turbine expander capable of processing large amounts of natural gas.

[0012] Furthermore, in step S3, the turboexpander operates in a single-phase gas region and adopts a nitrogen expansion cycle.

[0013] Furthermore, the operating temperature of the low-temperature separator in step S4 is -13°C to -22°C, and the pressure is 74 to 84 Bar.

[0014] Furthermore, the purpose of low temperature separation in step S4 is to remove the water and hydrocarbon liquid separated by cooling.

[0015] Furthermore, vertically mounted porous baffles are provided in the expansion scrubber and the low-temperature separator, which can further reduce the sloshing effect.

[0016] Furthermore, the optimal opening ratio of the porous baffle is 0.35, which can not only suppress the influence of sloshing, but also keep the cleaning and manufacturing difficulties within a reasonable range.

[0017] Furthermore, the main function of the expansion scrubber is to roughly separate the wet gas from the gas pipeline, remove part of the condensate and impurities therein, reduce the damage of the condensate and impurities to the system, and ensure the normal operation of the subsequent system.

[0018] Furthermore, the main function of the low-temperature separator is to separate the small amount of light oil and water condensed out again to ensure the dehydration effect of the system.

[0019] Furthermore, the cooling temperature in step S5 is 0° C. and the pressure is 70 Bar.

[0020] Furthermore, after entering the re-compressor in step S5, the natural gas is compressed by the re-compressor to a temperature of 10.3° C. and a pressure of 79.79 Bar.

[0021] Furthermore, after entering the external transmission heater in step S6, the natural gas is heated to a temperature of 10.4° C. and a pressure of 78.79 Bar by the external transmission heater.

[0022] Furthermore, the purpose of the circulation treatment in step S7 is to prevent the formation of hydrates in the container liquid level control valve and the downstream pipeline.

[0023] Furthermore, the compressor is also equipped with a Joule-Thomson (JT) valve.

[0024] Furthermore, the compressor can be used as a bypass when the compressor fails. Under normal operating conditions, the wellhead gas pressure is 102 Bar (top of the riser). When the bypass is running, the pressure at the top of the riser is controlled not to exceed 120 Bar.

[0025] Furthermore, the lean ethylene glycol is used as a wet natural gas dehydrating agent to prevent hydrates from forming in the low-temperature separator and the heat exchanger.

[0026] Beneficial effects of the present invention: The natural gas low-temperature dehydration process of the present invention adopts a separation technology - a hydrate inhibitor-assisted low-temperature separation technology, which can improve the separation efficiency, avoid the formation of hydrates, and ensure the high efficiency and safety of natural gas processing.

[0027] The natural gas low-temperature dehydration process of the present invention further reduces sloshing by adopting a scrubber with a porous baffle and a low-temperature separator, and by utilizing waste heat, makes the process more compact, occupies less space, and is more suitable for offshore platforms. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following briefly introduces the drawings required for use in the embodiments or the prior art description. Figure 1 This is a schematic diagram of the process flow of an embodiment of the present invention; Figure 2 Schematic diagram of the opening ratio of the porous baffle in the embodiment of the present invention. DETAILED DESCRIPTION

[0029] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments.

[0030] Embodiment: A natural gas low temperature dehydration process, such as Figure 1 As shown, the specific process operations are as follows: (1) Initial mixing and pre-cooling: The high-pressure compressed natural gas is mixed with the gas from the upstream slug collector and injected with lean ethylene glycol. It then enters the gas / gas heat exchanger at a temperature of 15.7°C and a pressure of 94.9 Bar. During this process, the expanded and dehydrated natural gas is cooled to -6°C. (2) Expansion scrubbing and preliminary dehydration: The pre-cooled natural gas enters the expansion scrubber, where the condensed water and hydrocarbons are removed; (3) Turbo expansion: Lean ethylene glycol is injected into the expanded and washed natural gas, which then enters the turbo expander / recompressor skid, where it is first expanded and cooled to -21.4°C and 72.3 Bar by the expander; (4) Low temperature separation: The expanded natural gas enters the low temperature separator, and the operating temperature of the low temperature separator is controlled at -20°C and the pressure is 80 Bar; (5) Heat exchange and final compression: The separated natural gas enters the gas / gas heat exchanger again, where it is mixed with the natural gas before expansion for heat exchange, and then cooled to a temperature of 0°C and a pressure of 70 Bar; the cooled natural gas then enters the turboexpander / recompressor skid and is compressed by the recompressor to a temperature of 10.3°C and a pressure of 79.79 Bar; (6) External transmission heating: The pressurized natural gas enters the external transmission heater and is heated to a temperature of 10.4°C and a pressure of 78.79 Bar for external transmission; (7) Liquid treatment: In the above process, the liquid separated by the expansion scrubber and the low-temperature separator is transported to the front end of the medium-pressure separator for circulation treatment; In this process, the compressor is also equipped with a Joule-Thomson (JT) valve. Under normal operating conditions, the wellhead gas pressure is 102 Bar. In the event of a compressor failure, it can be used as a bypass. When the bypass is running, the pressure at the top of the riser is controlled not to exceed 120 Bar.

[0031] Those skilled in the art should understand that the discussion of any of the above embodiments is merely illustrative and is not intended to imply that the scope of the present invention is limited to these examples. Under the concept of the present invention, the technical features in the above embodiments or different embodiments may be combined, the steps may be implemented in any order, and there are many other variations of the different aspects of the present invention as described above, which are not provided in detail for the sake of simplicity.

Claims

1. A natural gas low temperature dehydration process, characterized in that: The steps include: S1 initial mixing and pre-cooling: Lean ethylene glycol is injected into natural gas at a temperature of 15.7°C and a pressure of 94.9 Bar, and then enters the heat exchanger for cooling; S2 expansion scrubbing and preliminary dehydration: the pre-cooled natural gas enters the expansion scrubber for preliminary dehydration; S3 turbo expansion: Lean ethylene glycol is injected into the expanded and washed natural gas, which then enters the turbo expander / recompressor skid for cooling and expansion; S4 low temperature separation: the expanded natural gas enters the low temperature separator for separation; S5 heat exchange and final compression: The separated natural gas enters the heat exchanger again, where it is mixed with the natural gas before expansion, cooled, and enters the turbo expander / recompressor skid for compression; S6 external transmission heating: the pressurized natural gas enters the external transmission heater for external transmission; S7 liquid treatment: The separated liquid is recycled.

2. The natural gas low temperature dehydration process according to claim 1, characterized in that: The cooling temperature in step S1 is -6°C.

3. The natural gas low temperature dehydration process according to claim 1, characterized in that: The natural gas in step S1 is a mixture of the upstream slug collector and the high-pressure compressed natural gas.

4. The natural gas low temperature dehydration process according to claim 1, characterized in that: The cooling expansion in step S3 is to expand the natural gas to a temperature of -21.4°C and a pressure of 72.3 Bar through a turbo expander.

5. The natural gas low temperature dehydration process according to claim 1, characterized in that: The operating temperature of the low temperature separator in step S4 is -13°C to -22°C, and the pressure is 74 to 84 Bar.

6. The natural gas low temperature dehydration process according to claim 1, characterized in that: The cooling temperature in step S5 is 0° C. and the pressure is 70 Bar.

7. The natural gas low temperature dehydration process according to claim 1, characterized in that: After entering the re-compressor in step S5, the natural gas is compressed by the re-compressor to a temperature of 10.3° C. and a pressure of 79.79 Bar.

8. The natural gas low temperature dehydration process according to claim 1, characterized in that: After entering the external transmission heater in step S6, the natural gas is heated to a temperature of 10.4° C. and a pressure of 78.79 Bar by the external transmission heater.

9. The natural gas low temperature dehydration process according to claim 1, characterized in that: The compressor is also equipped with a Joule-Thomson (JT) valve.

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