CN103865601A - Heavy hydrocarbon recovery method of propane precooling and deethanizer top reflux - Google Patents

Abstract

The invention provides a heavy hydrocarbon recovery process of propane precooling and deethanization tower top reflux, which belongs to the technical field of chemical industry and low temperature. Adopting modular design, the liquefaction process consists of propane pre-cooling cycle, deethanizer overhead product reflux gas-liquid separation, natural gas gas-liquid separation and natural gas deheavy hydrocarbons, among which, propane cycle is used for pre-cooling natural gas, LPG, nitrogen refrigerant And the reflux stream of the deethanizer overhead product, the deethanizer overhead product returns to the pre-cooling cold box to be cooled and recycled, and no gas-liquid separation is performed after cooling. The liquid phase of natural gas gas-liquid separation returns to the pre-cooling cold box to provide cooling capacity, and the process increases the reflux part. The invention is suitable for heavy hydrocarbon recovery of offshore liquefied natural gas, and the output of heavy hydrocarbon and LPG is relatively high.

Description

Heavy hydrocarbon recovery method using propane precooling deethanizer top reflux

Technical field:

The invention relates to a method for recovering heavy hydrocarbons in high-pressure natural gas on FPSO, in particular, adopting the design of gas-phase reflux at the top of a deethanizer tower, combined with propane precooling and natural gas self-expansion technology to remove heavy components in high-pressure natural gas. Adopting this recovery process not only improves the recovery rate of propane, but also avoids excessive fuel gas generation and waste. At the same time, the process equipment is simple and has high economy, which is very suitable for the removal of heavy hydrocarbons in FPSO high-pressure natural gas.

Background technique:

my country has proved to be rich in offshore natural gas resources, and the processing of deep-sea gas fields, marginal gas fields and low-grade natural gas resources is considerable. For the exploitation of such gas sources, traditional offshore platforms and seabed relationships will be limited by cost and technology. Therefore, the use of LNG floating production storage and offloading devices (LNG-FPSO) can be flexibly configured and meet The need for natural gas extraction. LNG-FPSO has the advantages of easy migration, reusability, and high production efficiency, which is of great significance to promote the development of my country's sea areas, especially deep-sea gas fields and small gas fields, and make full use of oil and gas resources.

The liquefaction process is one of the core technologies of LNG-FPSO, which must meet the characteristics of safety, simple process, strong adaptability, rapid start-up and shutdown, and high degree of automation. As an important link in the liquefaction process, the heavy hydrocarbon recovery process also needs to meet the following requirements: (1) The content of C6+ components in the natural gas after de-heavy hydrocarbons enters the liquefaction unit meets the requirements of the liquefaction process; (2) The process runs stably and fluctuates Small; (3) Strong adaptability to different gas sources; (4) Recover heavy hydrocarbons as much as possible to improve the economical efficiency of the device; (5) Simple process, minimal equipment and small footprint; (6) Safe reliable. Before natural gas is liquefied, heavy hydrocarbons must be removed, otherwise, it may freeze during the liquefaction process and block the equipment. Onshore liquefaction plants generally use distillation to remove heavy hydrocarbons in the pretreatment process, and use deethanizers and liquefied gas towers to produce liquefied petroleum gas, and the remaining small amount of heavy hydrocarbons are separated and removed in low temperature areas. However, the onshore process is complicated, occupies a large area, and is not easy to install, so it has poor applicability at sea. Therefore, the existing heavy hydrocarbon recovery process used on land generally cannot be directly used in LNG-FPSO, and a new process suitable for marine environment needs to be studied.

Invention content:

The purpose of the present invention is to propose a simple method for recovering heavy hydrocarbons from the top reflux of the propane precooling deethanizer tower, which is applicable to the marine environment and has a high propane recovery rate.

The general idea of the present invention for realizing its purpose is: use the cooling capacity of the propane precooling unit and the self-expansion of the natural gas to lower the pressure and lower the temperature to remove the heavy hydrocarbons in the high-pressure natural gas, in order to improve the recovery rate of propane and reduce the output of fuel gas, increase the deethylation The process of refluxing the gas phase at the top of the alkane tower to the contact tower, using HYSYS to simulate the process and optimize the parameters, so that it can achieve a good separation effect.

The object of the present invention is achieved through the following technical solutions: the production device used to realize the technical solutions includes a propane precooling section cold box, two separators, a natural gas expander, multiple throttle valves, a contact tower, a Deethanizer, a debutanizer, and a mixer. The natural gas extracted from the seabed is firstly pretreated to remove sediment, water, acid gas, mercury, benzene and impurities, and then enters the propane pre-cooling cold box unit, expands after passing through the propane pre-cooling section cold box, and enters deethanization The top product reflux gas-liquid separation unit, natural gas gas-liquid separation unit and natural gas heavy hydrocarbon removal unit make full use of the cooling capacity provided by propane refrigerant, combined with natural gas self-expansion, pressure reduction and temperature reduction to achieve heavy hydrocarbon removal, and the obtained products are further divided, The obtained liquefied petroleum gas and condensate oil are stored in storage tanks respectively, and the natural gas after removing heavy hydrocarbons enters the liquefaction unit.

In the propane pre-cooling cold box unit, three-stage refrigeration is adopted. Propane passes through high-pressure separator, medium-pressure separator and low-pressure separator in sequence, and the separated gas phase is recycled through propane compressor, water-cooled and throttling respectively. The liquid phase enters the precooling cold box as a cold source to precool natural gas, LPG, nitrogen refrigerant and the reflux stream of the deethanizer overhead product. Propane is first pressurized to 1.36MPa through the compressor, throttled to a pressure of 0.55MPa and enters the separator together with the first-stage reheated propane, and the separated gas phase enters the inlet of the third-stage compressor, and the liquid phase is divided into two parts, one part The liquid phase enters the cold box to cool natural gas and nitrogen, and the other part is throttled to a pressure of 0.29MPa and enters the second-stage separator for separation. The separated gas phase enters the second-stage compressor for compression, and the separated liquid phase is divided into two parts. Part of it enters the cold box to cool natural gas and nitrogen, and the other part is throttled to a pressure of 0.13MPa and enters the third-stage separator for separation. The separated gas phase enters the first-stage compressor for compression, and the separated liquid phase enters the cold box for cooling natural gas. and nitrogen. Nitrogen refrigerant is used to pre-cool the natural gas and the subsequent liquefaction unit, and at the same time provide cooling capacity for the LPG, the top product of the deethanizer. Deacidified and dehydrated natural gas at 36°C and 7.14MPa, pre-cooled by propane to a temperature of -32°C and a pressure of 7.09MPa, enters a gas-liquid two-phase separator, and the gas phase at the exit of the separator enters a natural gas expander to expand to a temperature of -55.35°C , the pressure is 4.5MPa, as the contact tower feed; the liquid phase at the outlet of the separator is throttled to a temperature of -40.14°C and a pressure of 4.7MPa, and the temperature rises to 31°C and the pressure is 4.65MPa after returning to the propane pre-cooling cold box for cooling. As contact tower feed. In order to ensure the natural gas pressure requirements of subsequent liquefaction units and reduce the energy consumption of liquefaction as much as possible, the pressure of natural gas after expansion should not be too low, but at the same time it must meet the needs of heavy hydrocarbon recovery. Here, it is selected as 4.5MPa after optimization.

In the deethanizer top product reflux gas-liquid separation unit, the gas phase product at the top of the deethanizer is cooled by a pre-cooling cold box and then enters the gas-liquid separator, and the gas phase at the separation outlet is mixed with the gas phase product at the top of the contact tower and enters the dinitrogen expansion refrigeration liquefaction unit, the liquid phase at the outlet of the separator is mixed with the separated liquid phase after precooling of the natural gas as the top feed of the contact tower.

The natural gas at -49.7°C and 4.4MPa at the top outlet of the contact tower enters the subsequent liquefaction unit, and the liquid phase at the bottom of the tower at -55.53°C and 4.45MPa is throttled to a pressure of 2.95MPa and enters the top of the deethanizer. The gas at -22.67°C and 2.85MPa at the top outlet of the deethanizer tower is compressed to a temperature of 8.2°C and a pressure of 4.45MPa, and then returned to the propane pre-cooling box to cool to a temperature of -32°C and a pressure of 4.42MPa. Liquid separation, the separated liquid phase is mixed with the liquid phase from the natural gas separator as the feed of the contact tower, the separated gas phase is mixed with the gas phase natural gas at the top of the contact tower, and enters the contact tower; After flowing to the pressure of 0.65MPa, it enters the debutanizer top. The liquid phase product at the top of the debutanizer is LPG at 11.13°C and 0.55MPa, which is cooled by propane to -32°C and then throttled to a pressure of 0.12MPa and stored in a storage tank; the liquid phase product at the bottom of the tower is 120°C and 0.6MPa The condensate oil, which is cooled and throttled to normal pressure, enters the storage tank for storage at -32°C and 0.12MPa.

In the natural gas gas-liquid separation unit, the pre-cooled natural gas enters the gas-liquid separator, the separated liquid phase is throttled and returned to the pre-cooling cold box to provide cooling capacity, and the gas phase enters the bottom of the deethanizer after being expanded by the expander.

In the natural gas deheavy hydrocarbon unit, the natural gas from the expander is expanded to 4.5MPa, and then enters the heavy hydrocarbon separation unit at -55.35°C. In the heavy hydrocarbon separation unit, a deethanizer and a debutanizer are used to realize natural gas For the removal of heavy hydrocarbons, the top product of the deethanizer is refluxed to the pre-cooling cold box for cooling, and the top product of the debutanizer is LPG and refluxed to the pre-cooling cold box for cooling.

The structure of this heavy hydrocarbon recovery process is simpler than that on land, and the content of light components in the LPG and condensate products obtained at the same time is well controlled, and both are qualified products. The pressure energy of high-pressure natural gas is fully utilized, and the natural gas after removing heavy hydrocarbons meets the needs of the liquefaction process. The HYSYS process simulation software was used to simulate the process, parameter optimization and sensitivity analysis, which improved the recovery rate of propane and verified the stability of the process. The processing capacity of this process reaches 2.76 million tons/year, the LPG product is 114,900 tons/year, and the condensate oil output is 73,400 tons/year.

In order to ensure the safe and reliable operation of the device under sloshing conditions, the following requirements are put forward for the selection and installation of the equipment: the natural gas processing capacity in the process is relatively large, and the expansion machine is recommended to use a turbo expander, which is small in size and stable in operation. The adaptability to the environment is good; the propane pre-cooling cold box adopts a plate-fin heat exchanger, which has a compact structure and stable performance. It is a heat exchanger commonly used in liquefied natural gas plants. The separator should be installed on the central axis of the hull as much as possible, and the height of the tower should be controlled as much as possible to reduce the influence of the shaking of the hull on the separation effect.

The beneficial effect of the present invention is: adopting the heavy hydrocarbon recovery process combining propane precooling and natural gas expansion, the gas phase at the top of the deethanization tower is refluxed to the contact tower, the propane recovery rate is greatly increased, and the economical efficiency of the process is improved. Make full use of the cooling capacity of the propane pre-cooling unit, and use the pressure energy of the high-pressure natural gas itself to recover the heavy components in the high-pressure natural gas. Unitized design, compact equipment, easy to install. The process valves are few, the equipment is simple, and a single refrigerant is used, which reduces the start-up time of many auxiliary equipment, does not need to perform operations such as refrigerant ratio, simple control structure, and quick start and stop. The propane pre-cooling cycle can effectively utilize the latent heat of vaporization of propane and reduce the energy consumption of the process. It better solves the complex defects of existing heavy hydrocarbon recovery process equipment, and is suitable for marine production environments. The recovery process is not sensitive to natural gas composition, temperature, pressure and other conditions, and the products meet the requirements.

Description of drawings:

Fig. 1 is the flow chart of the liquefaction process of the present invention.

Figure 2 is a schematic diagram of the pre-cooling cold box unit of the liquefaction process.

Fig. 3 is a schematic diagram of gas-liquid separation after the reflux of the top product of the deethanizer.

Fig. 4 is a schematic diagram of a gas-liquid separation unit for natural gas.

Fig. 5 is a schematic diagram of the heavy hydrocarbon separation unit of the liquefaction process.

In the figure, 1-cold box in precooling section, 2-contact tower, 3-deethanizer tower, 4-debutanizer tower, 5-stabilizing tower, 6-first throttle valve, 7-gas-liquid separator, 8 - natural gas expander, 10 - second throttle valve, 11 - first flow divider, 12 - compressor, 13 - third throttle valve, 14 - first water cooler, 15 - fourth throttle valve, 16 - Fifth throttle valve, 17 - LPG storage tank, 18 - pump, 19 - second water cooler, 20 - first mixer, 21 - third water cooler, 22 - NGL storage tank, 26 - first circulator , 27-first section of pipeline, 28-second section of pipeline, 29-third section of pipeline, 30-high pressure separator, 31-second splitter, 32-fourth section of pipeline, 33-second circulator, 34 - Sixth throttle valve, 35 - Medium pressure separator, 36 - Third splitter, 37 - Fifth section pipeline, 38 - Third circulator, 39 - Seventh throttle valve, 40 - Low pressure separator, 41 - the sixth pipeline, 42 - the fourth circulator, 43 - the fifth circulator, 44 - propane compressor I, 45 - the sixth circulator, 46 - the second mixer, 47 - propane compressor II, 48 - Seventh circulator, 49-third mixer, 50-propane compressor III, 51-fourth water cooler, 52-eighth circulator, 53-propane throttling valve, 61～62-deethanizer top product Reflux logistics, 63-64-LPG logistics, 65-66-nitrogen logistics, 67-68-natural gas logistics, 69-70-reflux logistics, 71-76-propane logistics, 77-82-natural gas logistics.

Detailed ways:

The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

As shown in Figure 1, assuming that the pressure of the pretreated natural gas is 7.14 MPa and the temperature is 36.0 °C, the process consists of a propane pre-cooling cycle and a natural gas deheavy hydrocarbon process.

In propane precooled cold box units, three stages of precooling are used. Propane is first separated into gas-liquid two-phase through high-pressure separator 30, and the gas phase is mixed with the gas compressed by propane compressor II47 and then flows into propane compressor III50, compressed to a temperature of 52.2°C and a pressure of 1.36MPa, and then passes through the fourth water cooler 51 is cooled to 31°C, all the propane is liquefied, then the pressure is reduced to 0.55MPa through the propane throttle valve 53, the temperature drops to 5°C, mixed with propane reheated from the cold box, and then flows into the high-pressure separator 30. The temperature of the liquid phase separated from the high-pressure separator 30 is 4.7° C., and a part of the liquid phase flows into the cold box 1 of the precooling section to perform the first stage precooling of natural gas, LPG, nitrogen refrigerant and the reflux stream of the deethanizer overhead product, thereby Complete the first stage propane pre-cooling. The other part of the liquid phase separated by the high-pressure separator 30 is depressurized to 0.29MPa through the sixth throttle valve 34, cooled to -15°C, mixed with the fluid after the second-stage precooling, and then flows into the medium-pressure separator 35 for further processing. Gas-liquid separation, the separated gas phase is mixed with the gas compressed by propane compressor I44 and then flows into propane compressor II47 for compression. At this time, the temperature of the liquid phase is -15°C, and part of the liquid phase flows into the cold box 1 of the pre-cooling section for the first stage Two-stage precooling of natural gas, LPG, nitrogen refrigerant and deethanizer overhead product reflux stream to complete the second stage of propane precooling. Another part of the liquid phase separated by the medium-pressure separator 35 is depressurized to 0.13MPa through the seventh throttle valve 39, cooled to -36°C, mixed with the fluid after the third-stage precooling, and then flows into the low-pressure separator 40 Carry out gas-liquid separation, the separated gas phase enters the propane compressor I44 to be compressed to a pressure of 0.29MPa, and the liquid phase flows into the pre-cooling section cold box 1 for the third-stage pre-cooling of natural gas, LPG, nitrogen refrigerant and deethanizer overhead products Return flow, so as to realize the third stage pre-cooling.

In the natural gas deheavy hydrocarbon unit, the natural gas undergoes pretreatment processes such as deacidification and impurity removal, and is pre-cooled by propane in the pre-cooling section cold box 1 to a temperature of -32°C, and then enters the gas-liquid separator 7 for gas-liquid separation. The separated gas phase is expanded to a pressure of 4.5 MPa by the natural gas expander 8, and then enters the bottom of the contact tower 2; the separated liquid phase is throttled and returned to the cold box 1 of the pre-cooling section to pre-cool natural gas, LPG and nitrogen for refrigeration agent, then enters the top of contact tower 2. The gas phase product at the top of the contact tower 2 enters the natural gas liquefaction unit, the liquid phase at the bottom of the contact tower 2 is throttled by the second throttle valve 10 and used as the feed for the deethanizer 3, and the gas phase product at the top of the deethanizer 3 passes through The compressor 12 compresses to a temperature of 8.2°C and a pressure of 4.45MPa, and returns to the cold box 1 of the pre-cooling section to cool to a temperature of -32°C, which is used as the feed to the top of the contact tower 2. The liquid phase at the bottom of the deethanizer 3 is throttled by the third throttling valve 13 and used as the feed for the debutanizer 4 . The top product of the debutanizer 4 is LPG, which flows back to the cold box 1 in the precooling section for cooling and throttling, and then flows into the LPG storage tank 17. The bottom product of the debutanizer 4 is condensate.

Figure 2 is a schematic diagram of the pre-cooling cold box unit of the liquefaction process, wherein the reflux stream 61-62 of the deethanizer overhead product is used as a heat source to be pre-cooled in the heat exchange cold box, and the LPG stream 63-64 is LPG used as a heat source in the heat exchange cold box. The process of being cooled in the hot and cold box, the nitrogen stream 65-66 is the process of nitrogen being cooled in the heat-exchanging cold box, the natural gas stream 67-68 is the process of natural gas being pre-cooled in the heat-exchanging cold box, and the reflux stream 69- 70 is the process of pre-cooling natural gas, LPG, nitrogen refrigerant and the reflux stream of the deethanizer tower top product in the heat exchange cold box as the reflux stream as a cold source. The process of precooling natural gas, LPG, nitrogen refrigerant and deethanizer overhead product reflux stream in the heat exchange cold box. The precooling cold box unit is composed of heat exchange cold box 1 and 16 throttling valves.

Fig. 3 is a schematic diagram of gas-liquid separation after the reflux of the deethanizer overhead product, the deethanizer overhead product reflux stream 62 is the deethanizer overhead product reflux stream after being cooled by the precooling cold box, and the natural gas stream 77 is the deethanizer The top product reflux stream is a liquid phase stream after gas-liquid separation. The natural gas stream 78 and the natural gas stream 79 are gas phase streams after the deethanization tower top product reflux stream has undergone gas-liquid separation. The natural gas stream 78 enters the second line. Stream 79 goes to the dinitrogen expansion refrigeration liquefaction unit. The gas-liquid separation unit after the reflux of the deethanizer overhead product is composed of a gas-liquid separator 2 and a splitter 11 .

Figure 4 is a schematic diagram of the gas-liquid separation unit of the liquefaction process. The natural gas stream 68 is natural gas from the cold box 1 in the pre-cooling section, the reflux stream 69 is used as a cold source to return to the cold box 1 in the pre-cooling section, and the natural gas stream 80 contacts the bottom of the tower. The gas-liquid separation unit of this liquefaction process consists of a gas-liquid separator 7 , a natural gas expander 8 and a throttle valve 10 .

Fig. 5 is the schematic diagram of the heavy hydrocarbon separation unit of the liquefaction process, the natural gas stream 80 is the natural gas from the gas-liquid separation unit, the reflux stream 70 is the reflux stream that provides cooling capacity for the cold box 1 in the pre-cooling section, and the natural gas stream 77 is the gas-liquid separation The final deethanizer top product reflux stream, the natural gas stream 81 goes to the cold box unit of the liquefaction section, the deethanizer top product reflux stream 61 is the deethanizer top gas phase product goes to the precooling section cold box 1 to be cooled, and the LPG stream 63 is the LPG product, which goes to the LPG storage tank after being cooled by the cold box 1 in the pre-cooling section and throttled, and the natural gas stream 82 is condensate oil, which goes to the NGL storage tank after water cooling and throttling. The heavy hydrocarbon separation unit of this liquefaction process consists of a tower 3 with a reboiler, a tower 4 , a separator 2 , a compressor 12 , a throttle valve 10 and a throttle valve 13 .

Claims (1)

1. the heavy hydrocarbon recovery method that propane pre-cooling deethanizing tower top refluxes, is divided into propane pre-cooling ice chest unit, deethanizing tower top product reflux gas-liquid separation unit, Sweet natural gas gas-liquid separation unit and natural qi exhaustion heavy hydrocarbon unit design, and its production equipment comprises a propane pre-cooling section ice chest, two separators, a Natural gas expander, multiple throttling valve, a contact tower, a deethanizing column, a debutanizing tower and a mixing tank, first carry out pre-treatment by the Sweet natural gas of seabed extraction, removes silt wherein, water, sour gas, mercury, after benzene and impurity, enter propane pre-cooling ice chest unit, through expanding after propane pre-cooling section ice chest, enter deethanizing tower top product reflux gas-liquid separation unit, Sweet natural gas gas-liquid separation unit and natural qi exhaustion heavy hydrocarbon unit, make full use of the cold that propane refrigerant provides, realizing heavy hydrocarbon in conjunction with Sweet natural gas self expansion step-down cooling removes, the product obtaining is further cut apart, obtain liquefied petroleum gas (LPG) and condensate oil, enter respectively storage tank stores, it is characterized in that according to the following steps:

1. in propane pre-cooling ice chest unit, employing three tier structure is cold, propane passes through high-pressure separator, MP (medium pressure) separator and light pressure separator successively, isolated gas phase recycles respectively after propane compressor, water-cooled, throttling, liquid phase enters precooling ice chest precooling Sweet natural gas, LPG, nitrogen gas refrigerant and the logistics of deethanizing tower top product reflux as low-temperature receiver, and the propane temperature that three grades of precoolings enter ice chest is respectively 4.7 ℃ ,-15 ℃ ,-36 ℃;

2. in deethanizing tower top product reflux gas-liquid separation unit, deethanizing top gaseous phase product enters gas-liquid separator after precooling ice chest is cooling, isolate implication phase and enter dinitrogen swell refrigeration liquefaction unit with contacting top gaseous phase Product mix, separator outlet liquid phase with after Sweet natural gas precooling, separate liquid-phase mixing as the charging of contact tower tower top;

3. in Sweet natural gas gas-liquid separation unit, enter gas-liquid separator through the Sweet natural gas of precooling, isolated liquid phase is returned to precooling ice chest cold is provided after throttling, and gas phase enters deethanizing column bottom after decompressor expands;

4. in natural qi exhaustion heavy hydrocarbon unit, from expanding into 4.5MPa, decompressor Sweet natural gas out enters heavy hydrocarbon separating unit, in heavy hydrocarbon separating unit, utilize a deethanizing column and a debutanizing tower to realize removing of Sweet natural gas heavy hydrocarbon, deethanizing tower top product reflux is cooling to precooling ice chest, and it is cooling that debutylize tower top product is that LPG is back to precooling ice chest.

Images