CN110144238B

CN110144238B - Liquefied natural gas light hydrocarbon recovery method

Info

Publication number: CN110144238B
Application number: CN201910441855.0A
Authority: CN
Inventors: 翁韦强; 李长俊; 贾文龙; 吴瑕; 吴刚; 刘洋
Original assignee: Southwest Petroleum University
Current assignee: Southwest Petroleum University
Priority date: 2019-05-24
Filing date: 2019-05-24
Publication date: 2021-03-16
Anticipated expiration: 2039-05-24
Also published as: CN110144238A

Abstract

The invention discloses a liquefied natural gas light hydrocarbon recovery method, which has the advantages of high product concentration, lower system power consumption and high heat exchange performance, separated ethane products and LPG products are in a low-pressure low-temperature form and are easy to store and transport, when the recovery yield of liquid phase products is large, the economic benefit is obvious, and meanwhile, the liquefied methane which is partially separated is used as a reserve peak-shaving gas source, so that the situation of gas supply tension in the period of vigorous natural gas demand is relieved, and the economic benefit and the management efficiency of an LNG receiving station are obviously improved.

Description

Liquefied natural gas light hydrocarbon recovery method

Technical Field

The invention belongs to a light hydrocarbon recovery process, and particularly relates to a novel light hydrocarbon recovery method suitable for a liquefied natural gas receiving station.

Background

Liquefied Natural Gas (LNG) is a clean energy source which is rapidly increased in the world at present, and plays a good role in promoting energy conservation and emission reduction of countries in the world. From the current situation of domestic import of foreign LNG, the total amount of imported LNG in 2017 years in China reaches 3789 ten thousand tons, and the import of LNG in 310X 10 is estimated to be in 2020⁸～560×10⁸m³130X 10 imported in 2025⁸～560×10⁸m³. The global LNG productivity is steadily improved, namely the concentrated release period of the capacity is about to enter.

Currently, the global LNG trade is mainly focused on asia-pacific regions. According to the statistics of the international natural gas alliance, the import amount of LNG in the Asia-Pacific region in 2017 accounts for 50.3% of the world, wherein the world import amount of LNG is three times higher in Japan, China and Korea. Because the gas compositions of LNG in different producing areas are different, LNG imported in China is mainly divided into lean LNG and rich LNG, and LNG with a high C2+ content is rich LNG and LNG with a low C2+ content is lean LNG. Wherein, the LNG imported from countries and regions such as Australia, Katalr, Alja and Liya in China has generally higher C2+ components, the ethane molar component of part of imported LNG products can reach more than 9%, and the C2+ content can reach more than 12%. However, ethane, a high-quality ethylene feedstock, plays an important role in national production. The data show that the specific gravity of ethylene prepared by ethane cracking in worldwide world reaches 35% by 2014, and is expected to reach 40% in 2020; the domestic proportion of utilizing light hydrocarbon to prepare ethylene is only 14 percent, which is far different from the level of preparing ethylene by using light raw materials in the world, wherein one of important reasons is that the domestic recycling of light hydrocarbon in natural gas is not important. If the rich LNG imported from China is directly vaporized and then supplied to users, not only can small economic loss be caused to LNG operation and sales enterprises, but also larger raw material burden can be caused to the domestic ethane-to-ethylene industry, so that greater potential safety hazards and challenges are faced to national production and national energy safety strategic layout.

Analyzed from the heat value regulation of domestic natural gas, the minimum high heat value of domestic natural gas is 31.4MJ/m³However, when the light hydrocarbon component in the imported LNG is higher, the heat value can reach 40MJ/m³And above, the deviation of the heat value reaches 27.3 percent and above. Under the conditions that the heat value in the fuel gas is too high and the total heat load of a user is not changed, the gas flow running in the pipe network is reduced, and the safe and efficient running of the natural gas pipe network is directly threatened. In addition, the civil gas equipment generally has a designed use heat value, and after the heat value is improved, the combustion working condition of the equipment can be changed to a certain extent, so that the civil gas can not be completely combusted, the heat value can not be fully utilized, and other damages can be caused to the environment. Therefore, the method for recovering the light hydrocarbon from the domestic imported rich LNG also has important significance for safe and efficient use of LNG energy.

The LNG light hydrocarbon recovery technology developed abroad is earlier than that developed at home, for example, the light hydrocarbon is recovered from LNG in 1960 in the United states, and a plurality of LNG light hydrocarbon recovery processes are designed, such as US3837172, US5114451, US5588308, US6604380B1, US6907752B2 and the like. In addition, the related separation technology of LNG is studied in early foreign countries, such as Japan and Australia, and better research results are obtained. With the gradual increase of the domestic LNG import amount, domestic scholars also carry out a great deal of research on the light hydrocarbon separation technology of the LNG, obtain a great breakthrough and have remarkable achievements in the aspects of heat exchange optimization of equipment and cascade utilization of cold energy. However, by combining the current situation of research on light hydrocarbon separation of LNG at home and abroad, the LNG light hydrocarbon recovery technology in China also has some disadvantages, and the main problems are as follows:

(1) and more compressor devices are adopted in part of the LNG light hydrocarbon recovery process, so that the recovery process complexity and the whole process operation energy consumption are increased, and the process parameters are more difficult to control.

(2) Only natural gas with poor gas quality and C2+ products are separated in part of the flow, although the calorific value of the products can be reduced to a certain extent, ethane and LPG products are not effectively separated, and are difficult to directly sell and utilize, so that the economic benefit is not obvious.

(3) The heat exchange network in part of the process is single, the design principle of heat integration is not fully adopted, the cold utilization rate of the process is poor, the heat energy consumption is overlarge, and the economic benefit generated by the process is greatly influenced.

(4) All methane products separated in part of the processes are gas phases, and when the demand of natural gas is increased, the peak shaving capacity of the methane products cannot keep up with or natural gas peak shaving cannot be carried out.

(5) Most of ethane or LPG products in part of flow are in a gas phase, and the products can be liquefied by additional energy consumption, so that the further storage or sale of the products is not facilitated.

Disclosure of Invention

The invention mainly provides a novel method for recovering light hydrocarbon of liquefied natural gas, which solves some defects in the prior LNG light hydrocarbon separation technology. The method mainly has the following characteristics: the flow heating and pressurizing equipment is few, and the parameters of the equipment in the flow are easy to control; based on the idea of heat integration, cold energy is fully utilized in a gradient way; the recovery purity and recovery rate of ethane and LPG in the process are very high, and the economic benefit is remarkable.

In order to achieve the purpose, the technical scheme of the invention is realized by the following scheme:

preheating of a first part and raw material LNG:

a. the raw material LNG is firstly pressurized to 1.2MPa by an LNG booster pump, and after the pressure is increased, the raw material LNG and the low-temperature methane gas which is separated from the demethanizer and then shunted are subjected to primary heat exchange, so that the low-temperature methane gas is liquefied.

b. And (b) preheating the raw material LNG in the step a, and then exchanging heat with ethane gas at the top of the deethanizer. After heat exchange, the gas phase ethane at the top of the deethanizer is partially liquefied by the LNG raw material. The feed LNG is further heated and then fed to a demethanizer.

Second part, demethanization:

c. and (b) preheating the raw material LNG in two stages, then feeding the preheated raw material LNG into a demethanizer, dividing the methane gas discharged from the top of the tower into two streams of material flows by a splitter, liquefying one stream of material flow accounting for 20 percent of the liquefied raw material LNG, throttling and depressurizing the liquefied material flow, feeding the liquefied material flow into an LNG storage tank for storage, and exchanging heat between the other stream of material flow accounting for 80 percent of the liquefied material flow and the ethane material flow which is not completely liquefied in the step b through a heat exchanger to completely liquef. The C2+ product material flow at the bottom of the demethanizer is subjected to pressure reduction to 0.83MPa through a throttling valve, then the C2+ product material flow and a low-temperature ethane product material flow after complete liquefaction are subjected to heat exchange, and the C2+ material flow enters the deethanizer after the temperature is further reduced to-21 ℃.

Third fraction, deethanization:

d. c2+ material flow at the bottom of the demethanizer subjected to temperature reduction treatment in the step C enters a deethanizer for ethane separation, ethane comes out from the top of the deethanizer, and C3+ products come out from the bottom of the deethanizer. In order to enhance the ethane separation effect of the deethanizer, the liquid-phase ethane stream in step c is divided into a plurality of streams and then pressurized to enter the deethanizer for reflux.

The fourth part, heat exchange of the product:

e. the low-temperature methane liquefied by the raw material LNG in the step a needs to be vaporized during the peak shaving period of the natural gas and then is transmitted to a natural gas trunk line to participate in peak shaving besides being stored in a tank.

f. The ethane product completely liquefied in the step C is divided into two streams by a splitter, one stream accounting for 65 percent exchanges heat with the C2+ component throttled and cooled at the bottom of the demethanizer, and the stream still keeps a liquid state after the heat exchange; the other stream accounting for 35 percent of the total weight is pressurized to 0.78MPa by an ethane booster pump and enters a deethanizer for reflux.

g. And d, throttling the C3+ product stream at the bottom of the deethanizer in the step d, then exchanging heat with the low-temperature methane gas stream completely liquefied by the ethane in the step C through a heat exchanger, and finally completely liquefying the stream into the normal-pressure low-temperature LPG product convenient for storage and transportation.

The invention has the beneficial effects that: high-purity methane, ethane and LPG can be obtained from raw material LNG, the purity of the methane can reach 97.54%, and the recovery rate can reach 99.99%; the purity of the ethane product can reach 98.78 percent, the ethane recovery rate can reach 97.72 percent, and the LPG product recovery rate can reach 98.86 percent; the process system adopts a heat integration idea, fully utilizes the cold energy of LNG to liquefy each product, and uses partial demethanized gas as a peak shaving reserve gas source after being liquefied, thereby effectively relieving the gas supply tension situation in the period of vigorous natural gas demand; based on the principle of cold energy cascade utilization and system benefit maximization, an efficient heat exchange network is designed, so that the whole light hydrocarbon recovery process achieves the maximum economic benefit with the minimum operation energy consumption.

Drawings

FIG. 1 is a schematic flow diagram of the process equipment of the present invention.

Wherein, the names corresponding to the reference numbers in the drawings are: p1 is LNG booster pump, E1 is low temperature methane gas heat exchanger, E2 is ethane heat exchanger, T1 is demethanizer, VLV1 is throttle valve, E3 is C2+ stream heat exchanger, T2 is deethanizer, E4 is ethane heat exchanger, TEE1 is methane gas splitter, TEE2 liquid phase ethane splitter, P2 liquid phase ethane booster pump, E5 is LPG product heat exchanger, VLV2 is liquid phase methane throttle valve, VLV3 is C3+ stream throttle valve, V1 is lean LNG peak shaving storage tank, M1 is export natural gas mixer.

Detailed Description

The invention is further described with reference to the accompanying drawings and specific embodiments.

As shown in fig. 1, a novel liquefied natural gas light hydrocarbon recovery method comprises the following steps:

a. preheating raw material gas: the raw material LNG is pressurized to 1.2MPa by a booster pump P1 and then exchanges heat with 20 percent of demethanizer overhead gas flow in a heat exchanger E1, and the temperature of the raw material LNG after heat exchange is raised to-141 to-137 ℃; the raw material LNG enters a heat exchanger E2 to exchange heat with gas-phase ethane at the top of a deethanizer after demethanization gas heat exchange, and the temperature of the raw material LNG is raised to-122 ℃ and then enters a demethanizer T1.

b. Demethanization: after the raw material LNG enters a demethanizer T1, the pressure at the top of the tower is 1.075MPa, and the pressure at the bottom of the tower is 1.175 MPa; the methane gas is separated from the top of the tower and then divided into two streams, wherein one stream accounts for 20%; c2+ product flow at the bottom of the demethanizer is throttled to 0.83MPa and then cooled to-21 ℃ by liquid-phase ethane product to enter a deethanizer.

c. Deethanizing: c2+ material flow enters into a deethanizer, the pressure at the top of the tower is 0.8MPa, the pressure at the bottom of the tower is 0.9MPa, the number of theoretical plates is 15, and reflux liquid phase ethane enters from the 5 th plate at the top of the tower; the gas phase ethane is taken out from the top of the tower, and the C3+ product is taken out from the bottom of the tower.

d. Heat exchange of the product: the 20 percent material flow separated from the methane gas at the top of the demethanizer is liquefied to-137 to-125 ℃ by the raw material LNG, throttled and decompressed to 0.6MPa, and enters a storage tank V1 to be used for peak regulation. The gas phase ethane removed from the top of the deethanizer is partially liquefied by the raw material LNG, and then continuously exchanges heat with 80 percent of material flow separated from the methane gas at the top of the demethanizer, and is completely liquefied to-60 to-50 ℃. C3+ products from the bottom of the deethanizer are throttled and depressurized, then exchange heat with methane gas flow after ethane is completely liquefied, and liquefied into LPG products for normal-pressure low-temperature storage.

Experimental example 1

The treatment capacity is 440.5t/h, the LNG raw material temperature is-162 ℃, the pressure is 0.12MPa, and the molar content condition is as follows: 81.0% of methane, 11.0% of ethane, 4.1% of propane, 2.4% of n-butane, 0.6% of n-pentane and 0.9% of nitrogen.

a. Preheating raw material gas: the temperature of the raw material LNG is-162 ℃, the pressure is 0.12MPa, the raw material LNG is firstly pressurized to 1.2MPa by an LNG booster pump P1, the pressurized raw material LNG exchanges heat with a material flow of 58.1t/h obtained by methane gas from the top of the demethanizer through a heat exchanger E1, and the temperature of the LNG is raised to-141.7 ℃. The raw material LNG exchanges heat with gas-phase ethane at the top of the deethanizer through a heat exchanger E2 after first heat exchange, and enters the demethanizer after the temperature is raised to-122 ℃.

b. Demethanization: after the raw material LNG enters a demethanizer, the flow rate of methane at the top of the tower is 290.6t/h, the pressure is 1.075MPa, and the temperature is-118.5 ℃; the flow rate of the C2+ material flow at the bottom of the demethanizer is 149.8t/h, the pressure is 1.175MPa, the temperature is-8.6 ℃, the pressure is reduced to 0.83MPa after the throttling of a throttling valve VLV1, the temperature after the heat exchange with the liquid-phase ethane product is reduced to-21 ℃ through a heat exchanger E3, and finally the liquid-phase ethane product enters a deethanizer.

c. Deethanizing: after the C2+ material flow enters a deethanizer, the flow rate of gas-phase ethane at the top of the deethanizer is 109.8t/h, the temperature is-37.9 ℃, and the pressure is 0.8 MPa; the flow rate of C3+ products at the bottom of the deethanizer is 78.4t/h, the pressure is 0.9MPa, the temperature is 40.6 ℃, and the pressure is reduced to 150kPa through a throttle valve VLV3 to exchange heat with low-temperature methane gas; the reflux flow rate of the liquid-phase ethane product is 38.4t/h, the reflux pressure is 0.78MPa, and the temperature is-49.9 ℃.

d. Heat exchange of the product: the 20 percent of methane gas flow which is separated from the methane gas at the top of the demethanizer is completely liquefied to-125 ℃, and is decompressed to 0.6MPa after being throttled by a throttle valve VLV2 and then stored in a tank to be used as peak regulation gas. After heat exchange, the gas-phase ethane at the top of the deethanizer is partially liquefied to the temperature of minus 40.7 ℃, the pressure of 0.75MPa and the gas-phase fraction of 0.44, and then exchanges heat with 80 percent of material flow which is separated from the methane gas at the top of the demethanizer, and the gas-phase ethane is completely liquefied to the temperature of minus 50 ℃ and the pressure of 0.73 MPa. The ethane after complete liquefaction is divided into two parts, one part accounts for 65%, the flow rate is 71.4t/h, the temperature is-42.8 ℃ after heat exchange with the C2+ material flow throttled at the bottom of the demethanizer, the pressure is 0.715MPa, and the liquid phase can still be maintained for storage and transportation. C3+ products discharged from the bottom of the deethanizer are throttled, and are liquefied into normal-pressure low-temperature LPG products with the pressure of 0.12MPa and the temperature of-40 ℃ after being subjected to heat exchange with low-temperature methane gas through a heat exchanger E5, and the normal-pressure low-temperature LPG products are stored and sold.

The simulation results for the light hydrocarbon recovery process stream are shown in table 1.

Table 1 simulation results of light hydrocarbon recovery process streams in experimental example i

TABLE 1

Table 2 shows the energy consumption of each equipment in the whole light hydrocarbon recovery process and the total energy consumption of the whole process, and the total energy consumption of the system is calculated to be 59160.3kW, wherein the equipment with higher energy consumption is a demethanizer and a deethanizer, and compared with the light hydrocarbon recovery process proposed by other scholars in China at present, the energy consumption of the process system is lower, and the process system can be well applied to most of the LNG light hydrocarbon recovery stations in China.

Table 2 total energy consumption of equipment for light hydrocarbon recovery in experimental example one

Unit name	Power (kW)
		Demethanizer T1	44520.4
Deethanizer T2	14285.6
		LNG booster pump P1	352.9
Liquid-phase ethane booster pump P2	1.4
		Energy consumption of the whole system	59160.3

The process designs and optimizes the heat exchange network for the LNG light hydrocarbon recovery process, optimizes the heat exchange system by adopting the idea of heat integration, liquefies ethane and LPG products into a low-pressure low-temperature form for storage with lower energy consumption, has obvious economic benefit when the separation yield of liquid-phase light hydrocarbon products is more, and gasifies part of separated methane into lean LNG as a peak shaving gas source, thereby having great positive effect on peak shaving of urban gas. Through the process, the operation economy of light hydrocarbon recovery of the LNG receiving station can be obviously improved, and the management efficiency can be greatly improved.

Those skilled in the art will recognize that the examples described herein are presented to better assist the reader in understanding the principles of the patent and are to be construed as being without limitation to such examples. Other variations of the combined processes may be made by persons skilled in the art in light of the teachings of the present disclosure without departing from the spirit of the present invention, but variations of the processes are within the scope of the invention.

Claims

1. A method for recovering light hydrocarbon from liquefied natural gas is characterized by comprising the following steps:

a. after being pressurized by an LNG booster pump P1, raw material LNG exchanges heat with a material flow which is separated from methane gas at the top of a demethanizer through a methane gas heat exchanger E1, the separated methane gas is completely liquefied, and then exchanges heat with ethane gas at the top of the deethanizer through an ethane heat exchanger E2 to partially liquefy gas-phase ethane;

b. raw material LNG enters a demethanizer T1 after two-stage heat exchange, methane gas separated from the tower top is split into two streams by a splitter TEE1, one stream is liquefied by the raw material LNG and then is decompressed by a throttle valve VLV2 to enter a storage tank V1 to serve as peak-shaving natural gas, the other stream of low-temperature methane gas participates in heat exchange of other products, the stream at the bottom of the demethanizer is throttled and cooled by the throttle valve VLV1 and then exchanges heat with liquefied low-temperature ethane products by a heat exchanger E3, and the temperature is reduced to a certain range and then enters a deethanizer T2;

c. b, except that the material flow in the step b enters a deethanizer T2, a stream of liquid-phase ethane which is completely liquefied is shunted by a shunt TEE2 and pressurized by a liquid-phase ethane booster pump P2 and then enters a deethanizer T2 for reflux, gas-phase ethane is separated from the top of the deethanizer, and C3+ material flow products separated from the bottom are throttled and then exchange heat with low-temperature methane gas;

d. and C, the low-temperature methane gas material flow which is not used as the peak shaving gas in the step b exchanges heat with the ethane material flow which is not completely liquefied by the raw material LNG in the step a through an ethane heat exchanger E4, the ethane material flow is completely liquefied, the C3+ product in the step C is liquefied into LPG through heat exchange of a heat exchanger E5, and finally the LPG is mixed with the vaporized peak shaving natural gas through a mixer M1 and is output.

2. The method of claim 1 for recovering lighter hydrocarbons from liquefied natural gas, wherein: in the step a, the LNG raw material is pressurized to 1.2MPa by an LNG booster pump P1, and the temperature of the LNG raw material after two-stage preheating entering the demethanizer is-122 ℃.

3. The method of claim 1 for recovering lighter hydrocarbons from liquefied natural gas, wherein: the pressure at the bottom of the demethanizer is 1175kPa, the pressure at the top is 1075kPa, the stream at the bottom of the demethanizer is throttled to 830kPa, and the temperature is reduced to-21 ℃ after the heat exchange with the liquid-phase ethane product by a heat exchanger E3 and then enters the deethanizer.

4. The method of claim 1 for recovering lighter hydrocarbons from liquefied natural gas, wherein: in the step c, the liquid phase ethane is divided into two streams, 35 percent of the stream is pressurized to 780kPa by an ethane booster pump P2 and then flows back to the deethanizer, the other stream accounts for 65 percent of the stream and still keeps a liquid phase after exchanging heat with the stream at the bottom of the demethanizer, the pressure at the top of the deethanizer is 800kPa, and the pressure at the bottom of the deethanizer is 900 kPa.

5. A process for recovering lighter hydrocarbons from liquefied natural gas as claimed in claim 1, wherein: the low temperature methane gas, which accounts for 80% of the top of the demethanizer in step d, completely liquefies the ethane stream to-50 to-60 ℃.