JPS5822872A - Method for recovering LPG from natural gas - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for recovering LPG in natural gas that improves the cold extraction method and significantly reduces the power required for the process when LPG in natural gas is recovered by cryogenic separation. .

Conventionally, L is contained in natural gas and petroleum-associated gas.
In the PG recovery method, a turbo expander is used to generate a cold source and recover power. In addition, one distillation column is used as a deethanization column, and the partial condensate in deep-chilled natural gas is fed into it, heated by external heat, and a boiler is used to drive out lighter gases than ethane in the liquid. The bottom liquid consisting of heavier fractions (0,,0,,0,...) of propane or higher is sent to the purification process to obtain TIJPG products, and cold light gases of ethane or higher (N,
.

oo, , o, , o, ) recovers cold heat through heat exchange with the raw material natural gas together with the cryogenic uncondensed gas after passing through the turbo extractor, and then releases it to the outside of the system.

Such a conventional method is illustrated in FIGS. 1 and 2.

Although there are slight differences in the details of heat exchange due to fluctuations in conditions,
The cold heat recovery method remains essentially unchanged.

Therefore, the conventional method will be explained in detail with reference to FIG.

The conditions for the raw material natural gas sent to the cryogenic separation process after the pretreatment process are as follows: pressure: 50-70 kg/cm2G; temperature: 20-70
600C gas set t21<mol%) methane 80-90. ,
Ethane 5-10. .

Heavy content of propane or higher 3-10+, N2+OO2
0.5~3 Raw material natural gas is used as a heat source for the intermediate stage reboiler of the deethanizer (2).
It exchanges heat with the low-temperature gas flowing in from the second condenser α floor in condenser 01), is further cooled by external refrigeration of the chiller, and enters the plate fin heat exchanger (8) after heat exchange in the second condenser Q3). . Here,
A gas-liquid multiphase flow and a second gas-liquid separation tank (4) in which the condensate in the first gas-liquid separation tank (3), which will be described later, is adiabatically expanded to a cryogenic temperature.
heat exchange with cryogenic uncondensed gas obtained from

The latter uncondensed gas is later mixed with the low-temperature top gas of the deethanizer (2) and becomes a cold source for the second condenser o3) and the first condenser Ql).

The raw material gas cooled by the plate fin heat exchanger (8) is separated into gas and liquid in the first gas-liquid separation tank (3), and the condensed liquid is adiabatically expanded by the pressure reducing valve (force) and converted into a low-temperature gas-liquid mixed flow. It is then fed as a second raw material to the deethanizer (2) via the plate-fin heat exchanger (8).On the other hand, the gas-liquid separation tank (
The uncondensed gas from 3) is supplied to the turbo expander (8) and expanded isentropically until it approaches the operating pressure of the deethanizer (2), becoming an extremely low-temperature gas-liquid multiphase flow that undergoes a second gas-liquid separation. It flows into tank (4) and is separated into gas and liquid. The condensate from the second gas-liquid separation tank (4) is supplied to the deethanizer (2) as a first raw material, and the cryogenic uncondensed gas serves as a cold source for the plate-fin heat exchanger (8).

In the deethanizer tower (2), a reboiler at the bottom of the tower is used to extract gases lighter than ethane from the liquid reservoirs supplied as the first raw material and second raw material to obtain heavier gases heavier than propane as a bottom liquid. Add the necessary heat to (9) using low pressure steam. The light gas of ethane or higher flowing out from the top of the column passes through the plate fin heat exchanger (8), mixes with the uncondensed gas in the second gas-liquid separation tank ('/4), and exchanges heat with the raw material gas. , when it leaves the final first condenser 00, it is heated to almost room temperature. This gas is pressurized by a compressor (6) driven by the recovered power of the turbo expander (5) and discharged outside the system, where it is used as fuel, raw materials for chemical industries, and other uses.

As mentioned above, in the conventional method, the cold energy generated during the process is effectively recovered to cool the raw material gas, but the raw material gas in the first gas-liquid separation tank (3) is Since the amount of cooling heat required to cool the product to the temperature level is insufficient from the internal cooling heat source within the process, the shortage is compensated for by external refrigerant from a chiller at a low temperature level.

The reason why such an external refrigerant is required is due to the following technical conditions.

(■) Since low-pressure steam is used as the heat source for the glue boiler in the deethanizer tower (2), heat will enter the cryogenic separation process from outside the system. This heat input is shared between the deethanizer overhead gas and the bottom liquid. There is no problem because the bottom liquid is immediately sent out of the system, but the top gas has a lower level of cold energy corresponding to the heat input, and then the cold heat is recovered and then released outside the system. As a result, the drop in the cooling heat level is supplemented by external refrigerant.

(2) The uncondensed gas in the second gas-liquid separation tank (4) exchanges heat with the raw material gas and is then released to the outside of the system, so the heavier components of this gas, including propane, are lost. .

In order to suppress this loss, a second gas-liquid separation tank (4) is required.
It is necessary to lower the internal temperature. Therefore, the temperature inside the first gas-liquid separation tank (3) that is supplied to the turbo expander (5) must also be kept low, but this low temperature cannot be maintained only by the self-cooling heat within the system.

As is clear from the relationship between refrigerant temperature and required power shown in the graph of FIG. 5, the required power increases as the temperature of the external refrigerant decreases.

The power required for the external refrigerant used in the conventional method is extremely large, as will be described later.

The inventors were able to complete the present invention as a result of intensive research aimed at establishing a cold heat recovery method that eliminates the use of such external refrigerant as much as possible and significantly reducing the total power required for the cryogenic separation process.

That is, the gist of the present invention is that when LPG in natural gas is recovered by cryogenic separation, the raw material gas is cooled by heat exchange with liquefied gas or cryogenic gas obtained from a cryogenic section in the system. The gas is condensed and separated into gas and liquid, and the condensed liquid is adiabatically expanded and sent to the stripper, while the uncondensed gas is fed to a turbo expander where it is isentropically expanded, deep cooled, and sent to the stripper. Then, in the stripper, the bottom liquid exchanges heat with the raw material gas or is heated by a boiler to drive out the remethane in the liquid, and then is pressurized into a deethanizer tower consisting of a fractional distillation tower and flows out from the top of the stripper tower. The deep-chilled light gas, whose main component is methane, recovers cold heat through heat exchange with the raw material gas, and then releases it outside the system.Then, in the de-ethanizer tower, external heat is applied or a boiler is used to drive ethane in the tower bottom liquid. The objective is to obtain a heavy LPG liquid with a weight higher than propane, and to discharge the ethane gas flowing out from the top of the column to the outside of the system.

Examples of the present invention are shown in FIGS. 3 and 4.

The present invention will be explained in more detail using FIG.

The pressure of the raw material gas sent to the deep cooling process after the pretreatment process,
The temperature 2 composition is the same as that used to explain the conventional method.

The raw material gas passes through the gas-gas heat exchanger A4 and is used as a heat source for the reboiler (9) of the stripper (1).
It is cooled in the first condenser 〇〇, enters the stripper intermediate stage reboiler 〇〇 for heat exchange, and is further cooled and condensed in the second condenser 〇□□□. The cold heat source of the gas-gas heat exchanger (14+, first condenser 0υ, second condenser a''3J) is a stripper (described later).
After recovering cold heat using the top gas of 1), the compressor (6) is driven by the recovered power of the turbo expander (5).
) to increase the pressure and release it out of the system.

The raw material gas cooled and condensed in the second condenser α enters the first gas-liquid separation tank (3) (at a pressure of 50 to 70 kg).
10nG, temperature -30 to -4000) Separate into gas and liquid.

The separated liquid is adiabatically expanded to approximately the operating pressure of the stripper (1) by the pressure reducing valve (force) and sent to the stripper (1) as a second raw material, while the uncondensed gas is supplied to the turbo expander (51). By isentropically expanding up to the operating pressure of the stripper (1) -
It is cooled to an extremely low temperature of 70 to -110°C, becomes a gas-liquid mixed flow, and is sent to the top stage of the IJ tube (1) as the first raw material.

The stripper (1) is operated at a pressure of 15 to 25 kg/CmG, and a high-level cold source of -70 to -110°0 consisting mainly of light gas of methane or higher is obtained from the top of the column, and ethane is obtained from the bottom of the column. A liquid consisting of the above heavy components (-10~
The heat load on the reboiler (9) and intermediate reboiler 00) is applied by heat exchange with the raw material gas so as to suppress the amount of methane component mixed into the bottom liquid.

In the conventional method, one distillation column was used as a deethanizer to separate all light gases above ethane, but in the present invention, it is used as a stripper (1) to separate light gases above methane. Therefore, reboiler (9)
Since the required heat load is small and the temperature level is low, it is possible to use the raw material gas as a heat source, and there is no need to use low-pressure steam. S) The top gas of the IJ tube (1), whose main component is methane, can also be kept at an extremely low temperature, and the feed gas can now be cooled only by the internal cooling heat within the system. Therefore, the external refrigerant at -20 to -40°C used in the conventional method is no longer necessary, and the power required for it is also zero.

The bottom liquid of the stripper (1) in the deep cooling section described above is pressurized to 25 to 30 kg/am G by pump a9, and then transferred to a deethanizer (
2). By counting the methane to ethane in the stripper bottom liquid to 10 mol% or less, the temperature at the top of the deethanization column will be 5 to 10°0.
7) Use external refrigerant at high temperature level (0-5°C). A light gas of ethane or higher, which does not condense, is released from the reflux tank a9, mixed with the released gas pressurized by the compressor (6) mentioned above, and released outside the system, where it is used as fuel, chemical raw materials, etc. Ru. The liquid in the reflux tank a5 is pressurized by the reflux pump a8 and is refluxed to the deethanizer (2) as a reflux liquid.

The amount of ethane mixed in the deethanization tower bottom liquid is 2 mol% of propane.
External heat is applied to the deethanizer reboiler Q [El as shown below, and the liquid consisting of LPG heavier than propane is sent to the purification process.

Thus, in the separation process, a partial condensation type distillation column, which was not present in the conventional method, was installed as a deethanizer column (2). Therefore, although a low-temperature refrigerant (-20 to -40°C) is not used, the deethanizer (2) now requires a high-temperature (0 to 5°C) external refrigerant. However, because the temperature level of this external refrigerant is high and the load is low, the power required for the external refrigerant is significantly reduced compared to conventional methods. Incidentally, the heat load of the deethanizer reboiler 〇〇 in the present invention is equal to or lower than the heat load of the conventional method.

The amount of energy saved by the present invention is converted into electric power as shown in the following equation.

(Amount of low-temperature level refrigerant power equivalent in the conventional method) - (Amount of refrigerant power equivalent in the deethanizer de-ethanizer condenser in the method of the present invention) - (Power for pump power of the method of the present invention) To further clarify the effects of the present invention The table shows the results of recovering LPG from natural gas using the conventional method and the method of the present invention under the following conditions.

(1) Raw material natural gas group! (%le%) 0.87.3. , O,,? ,0.
．． Oa2.3. .

no41m1. tnO, E-aO,, N20.3. ．． 0
021.0 Pressure 69.5 kglom G Temperature 40°0 Flow rate 217,00 ONm3/H (2) Deethanizer bottoms liquid conditions fluorophore vs. ethane (mol%) max, 2.
0 Propane recovery rate (%) min, 94
(3) External refrigerant
Method of the present invention using the graph in the figure Conventional method stripper Pressure 16 kglom G operating conditions Bottom liquid 0.10. -4.3% deethanizer
Pressure 30kg/am G Pressure 20kg/am G
Operating conditions Temperature 6°0 Temperature -60
°0 pump power kl' -h 6o
- As shown in the table, the amount of energy saved by the method of the present invention is extremely large.

[Brief explanation of drawings]

Figures 1 and 2 are system diagrams of examples of conventional methods; Figure 3;
FIG. 4 is a system diagram of an embodiment of the method of the present invention, and FIG. 5 is a graph showing the relationship between refrigerant temperature and required power amount. 1. Stripper 11. First condenser 2, deethanizer tower 12, chiller 3, first gas-liquid separation tank 13. Second condenser 46 Second gas-liquid separation tank 14. Gas-gas heat exchanger 5, turbo expander 115, reflux tank 6,
Pressure mla 16. Deethanizer reboiler 7, pressure reducing valve 170 fractionator 9, reboiler 10, intermediate reboiler and above

Claims (1)

[Claims] When recovering LPG from natural gas by the cryogenic separation method, the raw material gas is cooled and condensed by heat exchange with liquefied gas and cryogenic gas obtained from the cryogenic part of the system, separated into gas and liquid, and then condensed. The liquid is adiabatically expanded and sent to the stripper, and the uncondensed gas is supplied to the turbo expander where it is deep cooled by isentropic expansion and sent to the stripper. The bottom liquid exchanges heat with the raw material gas or is heated by a boiler to drive out the methane in the liquid, and then is pressurized into a deethanizer tower, which is a fractional distillation tower, and the main component is methane, which flows out from the top of the stripper. The deep-chilled light gas recovers cold heat through heat exchange with the raw material gas, and then releases it outside the system.Then, in the de-ethanizer tower, external heat is applied or ethane in the bottom liquid is driven out by a boiler to produce more than propane. A method for recovering LPG in natural gas, which comprises obtaining a heavy LPG liquid and releasing ethane gas flowing out from the top of the column to the outside of the system.