JPH0338519B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for recovering LPG from natural gas, which improves the cold heat recovery method and significantly reduces the power required for the process when LPG from natural gas is recovered by cryogenic separation. Conventionally, in the recovery method of LPG contained in natural gas and petroleum-associated gas, turbo expanders are used to generate a cold heat source and recover power. In addition, one distillation column is used as a deethanization column, and the separated liquid from deep-chilled natural gas is fed into it, and a reboiler heated by external heat is used to drive out light gases that are more than ethane in the liquid, and lighter gases that are more than propane are ejected. The bottom liquid consisting of heavy components (C ₃ , C ₄ , C ₅ ...) is
The cold light gas (N ₂ , CO ₂ , C 1 , C 2 ) higher than ethane (N 2 , CO 2 , C ₁ , C ₂ ) that is sent to the refining process to obtain LPG products and flows out from the top of the tower is converted into raw material natural gas along with the cryogenic uncondensed gas after passing through the turbo expander. Cold heat is recovered through heat exchange with gas and then released outside 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 variations in conditions, the cold heat recovery method remains essentially the same. Therefore, the conventional method will be explained in detail with reference to FIG. The conditions for the raw natural gas sent to the cryogenic separation process after the pretreatment process are as follows: Pressure: 50~70Kg/ ^cm2G Temperature: 20~60℃ Gas composition (mol%) Methane: 80~90., Ethane: 5
〜10., Heavy content of propane or higher 3〜10., N ₂ + CO ₂ 0.5
~3 After the raw natural gas is used as a heat source for the intermediate reboiler 10 of the deethanizer 2, it exchanges heat with the low-temperature gas flowing in from the second condenser 13 in the first condenser 11, and is further cooled by external refrigeration in the chiller 12. , and enters the plate-fin heat exchanger 8 through heat exchange in the second condenser 13.
Here, the condensed liquid in the first gas-liquid separation tank 3, which will be described later, is adiabatically expanded to undergo heat exchange with the gas-liquid multiphase flow that has become extremely low temperature and the extremely low temperature uncondensed gas obtained from the second gas-liquid separation tank 4. let The latter uncondensed gas is later mixed with the low-temperature top gas of the deethanizer tower 2 and becomes a cold source for the second condenser 13 and the first condenser 11. 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 7, becoming a low-temperature gas-liquid multiphase flow and flowing through the plate fin. It is fed as a second raw material to the deethanizer tower 2 via the heat exchanger 8. On the other hand, the uncondensed gas in the gas-liquid separation tank 3 is supplied to the turbo expander 15, where it is expanded almost isentropically to the operating pressure of the deethanizer 2, and becomes an extremely low-temperature gas-liquid multiphase flow into a second gas-liquid. It flows into the separation tank 4 and is separated into gas and liquid. The condensed liquid in the second gas-liquid separation tank 4 is supplied to the deethanizer tower 2 as a first raw material, and the cryogenic uncondensed gas serves as a refrigerant source for the plate-fin heat exchanger 8. In the deethanizer tower 2, the gas lighter than ethane is ejected from the liquid fractions supplied as the first raw material and the second raw material, and the heavier gases heavier than propane are obtained as the bottom liquid. Heat is applied using low pressure steam. The light gas of ethane or higher flowing out from the top of the tower passes through the plate fin heat exchanger 8, mixes with the uncondensed gas in the second gas-liquid separation tank 4, exchanges heat with the raw material gas, and finally passes through the first condenser. When leaving No. 11, the temperature rises to almost room temperature. This gas is pressurized by a compressor 6 driven by the recovered power of the turbo expander 5 and discharged to the outside of 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. Since the amount of cooling heat needed to cool the product to 100% is insufficient from the internal cooling heat source within the process, the shortage is made up for by external refrigerant from the chiller 12 at a low temperature level. The reason why such an external refrigerant is required is due to the following technical conditions. (1) Since low-pressure steam is used as the heat source for the reboiler of 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 as the bottom liquid is immediately sent out of the system, but the top gas is discharged outside the system after its cold heat is recovered, with the level of cold heat corresponding to the heat input reduced. As a result, the drop in the cooling heat level is supplemented by external refrigerant. (2) Since 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, the heavier components of propane or higher in this gas are lost. In order to suppress this loss, it is necessary to lower the temperature inside the second gas-liquid separation tank 4. 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 a method for recovering LPG from natural gas by a cryogenic separation method, which involves heat exchange between natural gas as a raw material and cryogenic light gas obtained from a stripper. The condensed liquid is adiabatically expanded and sent to the stripper, while the uncondensed gas is sent to the turbo expander where it is expanded almost isentropically, cooled down, and sent to the stripper. then at the stripper,
The bottom liquid is heated by a reboiler that exchanges heat with natural gas to drive out the methane and some ethane in the liquid, reducing the methane/ethane ratio to more than 0.4 mol% and less than 10 mol%, and then to the tower body and the dephlegmator. The liquid mainly composed of ethane that has been partially condensed in the deethanizer is fed into the deethanizer, which consists of a partial condensation distillation column, and is returned to the main body of the deethanizer. The deep-chilled light gas recovers its cold energy through heat exchange with natural gas, and then releases it outside the system.
Next, in the de-ethanizer tower, ethane in the bottom liquid is driven out by a reboiler to which external heat is applied to obtain a heavy LPG liquid that is heavier than propane, and it is partially condensed at a temperature of 5 to 10°C at which it flows out from the top of the tower, that is, the partial condenser. This is a method for recovering LPG from natural gas, which consists of releasing ethane gas other than the collected liquid 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, temperature, and composition of the raw material gas sent to the deep cooling process after the pretreatment process are the same as those used in the explanation of the conventional method. The raw material gas passes through the gas-gas heat exchanger 14 and is used as a heat source for the reboiler 9 of the stripper 1, and then is cooled by the first condenser 11 and enters the stripper intermediate reboiler 10 for heat exchange.
It is further cooled and condensed in the second condenser 13.
The gas-to-gas heat exchanger 14, the first condenser 11, and the second condenser 13 use the top gas of the stripper 1, which will be described later, as a cold heat source, and after recovering the cold heat, the compressor is driven by the recovered power of the turbo expander 5. It is pressurized by machine 6 and discharged outside the system. The raw material gas cooled and condensed in the second condenser 13 enters the first gas-liquid separation tank 3 (pressure 50~
70Kg/cm ² G., temperature -30 to -40℃) Separates into gas and liquid. The separated liquid is transferred to the stripper 1 by the pressure reducing valve 7.
By adiabatically expanding the gas to around the operating pressure of the stripper 1 and feeding it as a second raw material to the stripper 1, one uncondensed gas is supplied to the turbo expander 5 and expanding almost isentropically to around the operating pressure of the stripper 1. It is cooled to an extremely low temperature of 70 to -110°C, becomes a gas-liquid multiphase flow, and is sent to the uppermost stage of the stripper 1 as the first raw material. The stripper 1 is operated at a pressure of 15 to 25 Kg/cm ² G, and a high-level cold source of -70 to -110°C consisting mainly of light gases of methane or higher is obtained from the top of the column, and heavy gases of ethane or higher are obtained from the bottom of the column. A liquid consisting of mass (−
10~10℃) is obtained. The heat load on the reboiler 9 and the intermediate reboiler 10 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 below ethane (ethane and lighter than ethane), but in the present invention, one distillation column was used as a deethanizer to separate all light gases below ethane (ethane and lighter than methane). It is used as a stripper 1 for evaporating and removing methane so that the concentration in the bottom liquid of methane/ethane is within the range of more than 0.4 mol % and less than 10 mol %.
Therefore, since the heat load required by the reboiler 9 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. The top gas of stripper 1, whose main component is methane, can also be kept at an extremely low temperature.
It is now possible to cool the feed gas using only the internal cooling of the system. Therefore, the external refrigerant at -20°C 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/cm ² G by the pump 19, and is decomposed in the demultiplexer, which consists of the column main body for the separation process and the demultiplexer 17. The deethanizer 2 is fed to the deethanizer 2, which is a fractional distillation tower, where the liquid mainly consisting of ethane is refluxed to the deethanizer main body. More than 0.4 mol% of methane to ethane in the stripper bottom liquid to 10 mol%
By controlling as follows, the top temperature of the deethanizer tower, that is, the dephlegmation temperature of the deethanizer will be 5 to 10 degrees Celsius due to the design, so the external refrigerant at a high temperature level (0 to 5 degrees Celsius) will be supplied to the partial condenser 17. can be used. Under the condition that the methane/ethane ratio is 0.4 mol% or less, the temperature at the bottom of the column increases due to the increase in the amount of heating to the lower part of the stripper and the bottom, causing the propane component to be recovered as a product to evaporate in the stripper bottom liquid. This results in contamination with the stripper overhead gas, which defeats the purpose of the present application. Furthermore, an increase in the bottom temperature of the stripper is undesirable because it results in the need for a new high-temperature heat source other than the raw material gas as a heat source for the stripper reboiler. Reflux tank 15
A lighter gas than ethane that does not condense is released from the tank, mixed with the released gas pressurized by the compressor 6 mentioned above, and released outside the system to be used as fuel, raw materials for chemical industries, and other uses. The liquid in the reflux tank 15 is pressurized by the reflux pump 18 and is refluxed to the deethanizer tower 2 as a reflux liquid. External heat is applied to the deethanizer bottom liquid in the deethanizer reboiler 16 so that the amount of ethane mixed in is less than 2 mol % of propane, and the liquid consisting of LPG heavier than propane is sent to a purification process. Thus, in the separation step, a partial condensation type distillation column, which was not available in the conventional method, was installed as the deethanizer column 2. Therefore, a low temperature refrigerant (-20 to -40℃) is not used, but a high temperature (0 to 5℃) is added to the deethanizer tower 2.
external refrigerant is now required. but,
Because the temperature level of this external refrigerant is high and the load is low, the power requirements for the external refrigerant are significantly reduced compared to conventional methods. Note that the heat load on the deethanizer reboiler 16 in the present invention is equal to or lower than the heat load in 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 de-ethanizer de-ethanizer condenser of 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 Tables 1 and 2 show the results of recovering LPG from natural gas using the conventional method and the method of the present invention under the following conditions.
Shown below. (1) Raw material natural gas composition (mol%) C ₁ 87.3., C ₂ 7.0., C ₃ 2.3.,
nC ₄ 1.1., nC ₅ 1.0., N ₂ 0.3.,
CO ₂ 1.0 Pressure 69.5Kg/cm ² G Temperature 40℃ Flow rate 217000Nm ³ /H (2) Deethanization tower bottom liquid conditions Ethane relative to propane (mol%) max.2.0 Propane recovery rate (%) min.94 (3) External refrigerant: Propane The graph in Figure 5, whose validity has been confirmed, was used to calculate the amount of energy converted into electric power.

[Table] As shown in Table 1, the amount of energy saved by the method of the present invention is extremely large.

【table】

[Table] In Table 2, the methane/ethane ratio in the stripper bottom liquid is 1.0 mol% (method of the present invention) and 0.4 mol%.
(The range used in the conventional method is applied to the flow of the present invention). Further, FIG. 6 shows the relationship between the methane/ethane ratio in the stripper bottom liquid and the propane recovery rate. It is shown in Table 1. When the methane/ethane ratio of the method example of the present invention is 4.3 mol%, the propane recovery rate is 94.5%, and the propane loss amount in Table 2 is
It shows the annual loss amount corresponding to the recovery rate decrease from 94.5% recovery rate, that is, 1.1% in the method of the present invention and 3.5% in the conventional method range. From Table 2, it can be seen that in the region where the methane/ethane ratio in the stripper bottoms is significantly smaller, such as below 0.4 mol%, although there is a slight increase in the total energy savings, the amount of propane product loss increases significantly. This results in a significant deterioration of the overall economic efficiency, which is not desirable. The overall economic efficiency of each case in Tables 1 and 2 is compared based on the equipment costs, operating costs, and product sales losses required for each case. The conventional method shown in Table 1 does not require a stripper, its bottom pump, a deethanizer condenser, a reflux tank, or a reflux pump, but does require refrigerant equipment. This refrigerant equipment requires a compressor, a discharge condenser, a refrigerant tank, and an intake gas-liquid separation tank, and greatly exceeds the equipment cost of each of the underlined equipment newly required by the present invention. Furthermore, it can be said that the equipment costs in each case of the present invention are equivalent. On the other hand, the difference between operating costs and product sales loss in each case is shown in Table 3, assuming that the electricity unit price and propane product unit price are, for example, 15 yen/KWH and 30,000 yen/ton, respectively.
As shown below.

[Table] As mentioned above, the present invention is shown to be significantly superior to the conventional method in terms of overall economic efficiency in terms of equipment costs, operating costs, and product sales loss. Furthermore, the loss in product sales greatly contributes to this economic efficiency, and it is clear that the range of the conventional method in the bottom column of Table 3 is also in an unfavorable range. FIG. 6 shows that the methane/ethane ratio in the stripper bottom liquid is more than 0.4 mol%, preferably more than 0.5 mol%. Note that the required number of theoretical plates for the conventional deethanizer in Table 1 is 20 or more, and the required number of theoretical plates for the stripper of the present invention is much smaller than that, 10 or less. On the other hand, in the example shown in Fig. 4, the raw material gas feed route is
By dividing the single series flow shown in the figure into two, the heat exchangers 13 and 14 are omitted, and the heat and cold required for the stripper 1 can be provided in a self-contained manner. Note that the present invention is suitably applied to a raw material gas that does not possess as much cold energy as natural gas consisting only of a liquid phase. -The savings are significant.

[Brief explanation of the drawing]

Figures 1 and 2 are system diagrams of examples of conventional methods;
Figures 3 and 4 are system diagrams of embodiments of the method of the present invention, Figure 5 is a graph showing the relationship between refrigerant temperature and required power, and Figure 6 is a graph showing the relationship between the methane/ethane ratio in the stripper bottom liquid and It is a graph showing the relationship between propane recovery rates. 1... Stripper, 2... Deethanizer, 3...
...First gas-liquid separation tank, 4...Second gas-liquid separation tank, 5...
...Turbo expander, 6... Compressor, 7...
Pressure reducing valve, 8... Plate fin heat exchanger, 9...
Reboiler, 10... Intermediate reboiler, 11...
...First condenser, 12...Chiller, 13...
Second condenser, 14...Gas-gas heat exchanger, 15...Reflux tank, 16...Deethanizer reboiler, 17...Different condenser, 18...Reflux pump, 1
9...Pump.

Claims (1)

[Claims] 1. A method of recovering LPG from natural gas by cryogenic separation, in which the raw material natural gas is cooled and condensed by heat exchange with cryogenic light gas obtained from a stripper, as described below, and separated into gas and liquid. The condensed liquid is adiabatically expanded and sent to the stripper, and the uncondensed gas is fed to a turbo expander where it is expanded almost isentropically and deep cooled before being sent to the stripper. is heated by a reboiler that exchanges heat with natural gas to drive out methane and some ethane from the liquid, resulting in a methane/ethane ratio of more than 0.4 mol% and 10 mol%.
After the following is achieved, the deethanizer is pressurized into a deethanizer consisting of a partial distillation column consisting of a column main body and a separator, and the liquid mainly consisting of ethane that has been partially condensed in a partial condenser is refluxed to the deethanizer main body. The deep-chilled light gas containing methane as the main component flowing out from the top of the stripper recovers its cold heat through heat exchange with natural gas, and then releases it outside the system.Then, it is sent to the deethanizer tower, where it is passed through a reboiler to which outside heat is added. Ethane in the bottom liquid is ejected to obtain a heavy LPG liquid that is heavier than propane, and at the same time, the ethane gas other than the partial condensed liquid flowing out from the top of the column, that is, the partial condenser, is discharged to the outside of the system at a temperature of 5 to 10℃. A method for recovering LPG from natural gas.