JPH09302365A - Production of substitute natural gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing substitute natural gas(SNG), capable of addressing or reducing the problems involved in carbon dioxide separation by PSA method while making the best use of the advantages of the method. SOLUTION: A methane-rich gas is recovered by PSA method designed to adsorb and separate the carbon dioxide in a mixed gas comprising methane, carbon dioxide, carbon monoxide and hydrogen through repeated adsorption/ desorption. In this method for producing the objective SNG, the reducing pressure in desorbing the methane-rich gas is controlled according to the operation load [specifically, the final reducing pressure in the above desorption is controlled within the range between -0.2 and -1.0kg/cm<2> (G)] so that the carbon dioxide concentration in the methane-rich gas after separated.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an alternative natural gas (hereinafter referred to as SNG) which can replace carbon dioxide by adsorbing and separating carbon dioxide from a mixed gas containing methane gas and carbon dioxide.
The above-mentioned) is used to make the carbon dioxide concentration in SNG constant.

[0002]

2. Description of the Related Art As a method for separating carbon dioxide in a mixed gas containing methane gas and carbon dioxide (for example, a gas obtained by steam reforming a light natural gas), a wet method, PS
Methods A, membrane separation methods, etc. are known.

The wet method is a method in which carbon dioxide is brought into contact with an absorption liquid and the chemical reaction between the two is used to separate carbon dioxide, and is excellent in separation performance. However, this method has a high equipment cost, is complicated in operation at the time of starting and stopping the operation, and is not suitable for medium to small-scale SNG manufacturing equipment.

The membrane separation method is a method of separating gases by utilizing the difference in the permeation rate of two or more kinds of gases that permeate the membrane.
In this method, when the feed pressure to the membrane is 10 kg / cm ² or less, the pressure difference in the separation membrane is small, so the carbon dioxide separation rate becomes low and the carbon dioxide concentration in the methane-rich gas rises. become.

The PSA method is a method in which carbon dioxide is adsorbed to an adsorbent such as a carbon molecular sieve under pressure and then desorbed at a pressure of atmospheric pressure or less to separate the carbon dioxide. Compared with the membrane separation method, this method has a high separation rate even at a pressure of 10 kg / cm ² or less, and is superior to the wet method in operability and equipment cost. It can be said that it is suitable for the SNG manufacturing equipment. When carbon dioxide is separated by the PSA method, carbon dioxide is adsorbed and desorbed by the adsorbent packed in the adsorption tower to perform separation. At this time, the desorption operation is performed by reducing the pressure to a pressure equal to or lower than the atmospheric pressure with a vacuum pump. The cycle time and pressure during adsorption are
Since it is set in advance based on the production capacity of the equipment, when the production gas amount changes (load variation) like the SNG production equipment, the carbon dioxide concentration in the methane-rich gas changes due to the variation of the load, that is, SNG. The carbon dioxide concentration in the inside changes, which causes a problem that an SNG having a constant composition cannot be obtained.

[0006]

SUMMARY OF THE INVENTION The main object of the present invention is to provide a method for producing SNG which can eliminate or alleviate the problems while taking advantage of the carbon dioxide separation by the PSA method.

[0007]

As a result of further studies, the present inventor, while paying attention to the above-mentioned problems of the prior art, the carbon dioxide concentration in SNG becomes constant regardless of the operating load. As described above, it was found that the purpose can be achieved by controlling the depressurization pressure during desorption of methane-rich gas in PSA according to the operating load.

That is, the present invention provides the following SNG manufacturing method: A method for producing an alternative natural gas by recovering a methane-rich gas using PSA for repeating adsorption / desorption from a mixed gas containing methane gas, carbon dioxide, carbon monoxide and hydrogen to adsorb and separate carbon dioxide in the mixed gas In order to keep the carbon dioxide concentration in the methane-rich gas after the separation process constant regardless of the operating load,
A method for producing an alternative natural gas in which the decompression pressure during desorption of methane-rich gas in PSA is controlled according to the operating load.

[0009] 2. The method for producing an alternative natural gas according to item 1 above, wherein the final depressurization pressure during desorption in PSA is controlled in the range of -0.2 kg / cm ² (G) to -1.0 kg / cm ² (G).

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in more detail with reference to the drawings. FIG. 1 shows the basic process flow of the present invention.

In the process shown in FIG. 1, the mixed gas containing methane gas and carbon dioxide undergoes a reforming reaction of light hydrocarbons and steam in an adiabatic reforming reactor, and then a heat exchanger type reaction. It is obtained by reacting in a vessel, but in the present invention, the method for producing such a mixed gas is not particularly limited, and any known method can be adopted. Therefore, the equipment shown in FIG. 1 and the following reaction conditions described in connection therewith,
It is merely an example of the production of a mixed gas and does not limit the invention.

In the flow shown in FIG. 1, naphtha and LP
A light hydrocarbon feedstock such as G is preheated in the heat exchanger by heat exchange with the gas discharged from the heat exchanger type reactor, and then a part of the hydrogen-rich reformed gas discharged from the adiabatic reforming reactor. After being mixed with (which serves as a hydrogen source for desulfurization), it is sent to a heating furnace for desulfurization and the temperature required for hydrodesulfurization (usually 300 to 420 ° C).
Degree, preferably about 350 to 400 ° C., more preferably 37
It is heated to about 0-390 ℃). As the heating fuel for the desulfurization heating furnace, there are off gas and city gas, which will be described later, and LPG.
Use a mixed gas such as a combustion improver.

The preheated light hydrocarbon feedstock is desulfurized in a desulfurization unit using a hydrogenation catalyst and an adsorptive desulfurization catalyst and, if necessary, a higher-order desulfurization catalyst, according to a conventional method. These catalysts are not particularly limited, and known catalysts can be used. Specific examples of the catalyst include general Co-Mo-based and Ni-Mo-based hydrodesulfurization catalysts, Zn
Examples include O-based adsorptive desulfurization catalysts. When the temperature of the gas used for hydrodesulfurization is less than 300 ° C, desulfurization becomes insufficient, whereas when it exceeds 420 ° C, the raw material may be decomposed.

Steam is added to the light hydrocarbon raw material that has been desulfurized, and further, at a predetermined temperature (usually about 350 to 600 ° C., preferably about 400 to 550 ° C., more preferably 450 to about 500 ° C.) in a heating furnace for reforming. After being heated to about 500 ℃,
It is introduced into an adiabatic reforming reactor. If the temperature of the raw material supplied to the adiabatic reforming reactor is lower than 350 ° C, the reforming of the raw material will be insufficient, whereas if it exceeds 600 ° C, the raw material will be excessively decomposed. May occur. In the adiabatic reforming reactor, the light hydrocarbon raw material is steam-reformed. The outlet temperature of the adiabatic reforming reactor is determined in relation to the inlet temperature, the ratio of steam to light hydrocarbon raw material (hereinafter referred to as S / C), and the like. That is, the inlet temperature is high (or low)
If so, the outlet temperature becomes higher (or lower), and S
When / C is small (or large), the outlet temperature becomes higher (or lower) than the inlet temperature. S / C is usually
It is about 0.7 to 2.5, more preferably about 0.7 to 1.7, and further preferably about 0.8 to 1.2. When S / C is less than 0.7, carbon tends to precipitate during the reaction, while when it exceeds 2.5, it is not economical.

The gas obtained by the reforming in the adiabatic reforming reactor is composed of methane, carbon monoxide, carbon dioxide, hydrogen, steam, etc., and the ratio thereof becomes equilibrium with the outlet temperature. Since this reformed gas is rich in hydrogen, as described above, a part of it is taken out and added to the source gas,
Recycled as hydrogen gas for hydrodesulfurization.

A part of the reformed gas enters a heat exchanger type reactor, and carbon monoxide and carbon dioxide react with hydrogen on the catalyst to form methane. This conversion to methane proceeds as the temperature lowers, so water is supplied from the bottom of the steam drum to cause heat exchange with the reformed gas, lower the temperature of the reformed gas, and Depending on the value, the methanation equilibrium reaction proceeds. The outlet temperature of the heat exchanger type reactor is usually 20
The temperature is about 0 to 500 ° C, preferably about 250 to 400 ° C, more preferably about 250 to 320 ° C. When the outlet temperature of the heat exchanger type reactor is less than 200 ° C, the methanation reaction hardly occurs, while when it exceeds 500 ° C, the catalyst is significantly deteriorated.

Factors affecting the methane concentration in the methanation gas include S / C in addition to the above methanation temperature. That is, the smaller the S / C, the higher the methane concentration, and the higher the S / C, the lower the methane concentration. In this way, the temperature of the gas discharged from the heat exchanger type reactor and the S / C
By appropriately selecting and, the composition of the gas finally obtained can be controlled. Normally, by adjusting the above temperature and S / C, the methane concentration in the gas will be 47-
The amount of heat generation can be set to about 81 mol% and the heating value can be set to about 5500 to 7700 Kcal / Nm ³ .

A part of the reformed gas obtained in the adiabatic reforming reactor is mixed with the methanated gas without being subjected to the methanation reaction.

The mixed gas of methanated gas and reformed gas is
Then, it is sent to a heat exchanger and, as described above, heat-exchanged with the light hydrocarbon gas raw material, and then sent to a separator by the PSA method. In the separation device, carbon dioxide in the component that affects the combustion characteristics of the gas is separated, and the carbon dioxide concentration in SNG is kept constant. A known PSA device can be used as the separation device.

In the present invention, the P concentration of the methane-rich gas is controlled so that the carbon dioxide concentration in the separated methane-rich gas becomes constant regardless of the operating load of the SNG production apparatus.
The depressurizing pressure during SA desorption is controlled according to the operating load.

That is, a desorption pressure detector is provided between the adsorption tower and the vacuum pump, and an output signal from this detector is used to
The decompression pressure is controlled by controlling the opening degree of the recycle gas adjusting valve provided in the recycle flow path that returns a part of the vacuum pump outlet gas to the inlet side.

At the time of desorption, the recycle gas adjusting valve is fully closed according to the instruction of the above-mentioned detector.
The adsorbed carbon dioxide is desorbed. When the operating load is small (or large; the inside of parentheses means the opposite state below), the amount of carbon dioxide adsorbed and separated is small (large), so the final decompression pressure during desorption is high (low). Set it to reduce (increase) the total amount of desorption gas. Open the recycle gas control valve when the desorption progresses and becomes equal to the final decompression pressure preset by the operating load,
Part of the vacuum pump outlet gas is recycled to the inlet side,
Maintain a predetermined desorption pressure.

In the present invention, the final depressurization pressure at the time of PSA desorption is set in the range of -0.2 to -1.0 kg / cm ² (G).

The gas leaving the separator is added with LPG in the heat quantity adjusting device to be converted into SNG having a desired heat quantity.

When the temperature of the gas discharged from the heat exchanger type reactor is lowered and the S / C is reduced, the contents of hydrogen and carbon monoxide in the produced gas are lowered, and the methane and the dioxide are substantially reduced. A gas consisting of carbon is obtained. Therefore, by removing carbon dioxide from this gas and adjusting the amount of heat with LPG, an SNG substantially composed of methane and LPG can be obtained.

Generally, in the production of SNG, the higher the separation rate of carbon dioxide, the larger the separation device, the larger the consumption of utility and the higher the production cost. To reduce the manufacturing cost of SNG, some amount (2%
(About 5%) because it causes a problem that the combustion speed of the gas becomes slow when about 2) carbon dioxide is left.
It is necessary to leave a certain amount of hydrogen and increase the gas burning rate. For this purpose, a part of the reformed gas obtained in the adiabatic reforming reactor can be mixed with the methanated gas without being subjected to the methanation reaction.

As the catalyst used in the above-mentioned adiabatic reforming reaction and heat exchange type reaction, known catalysts usually used in these reactions can be used as they are. Specifically, various noble metal-based catalysts such as Ru based on alumina as a carrier, Ni based on alumina, silica, etc. as a carrier, and oxides of alkaline metals such as magnesia and alkaline earth metals are added as necessary. A catalyst or the like can be used. In this case, by performing a high degree of desulfurization using an appropriate desulfurization catalyst, S / C
Can be about 0.7 to 2.5 for Ru-based catalysts and about 1.0 to 2.5 for Ni-based catalysts.

In the method of the present invention, water is supplied to the heat exchange reactor for heat exchange, and the generated steam is used for desulfurization of the light hydrocarbon raw material, reforming of the raw material, and the like. More specifically, as shown in FIG. 1, water is supplied to the heat exchange reactor from the bottom of the steam drum, and the generated steam is collected in the steam drum. The recovered steam is added to the light hydrocarbon raw material that has been desulfurized, and is used for reforming the raw material.

The off gas separated by the separator is
Although it contains carbon dioxide as its main component, it also contains combustible components such as hydrogen and methane.
It is used as a heating fuel in a desulfurization heating furnace, a heating fuel in a reforming heating furnace, a general boiler heating fuel, and the like together with a gas having a high heating value such as. The steam generated in the boiler can be used as reforming steam when the reformer is started up, or can be used as steam required by other equipment in the factory.

[0030]

EFFECTS OF THE INVENTION When separating carbon dioxide, which is a component that affects the combustion characteristics in city gas, a desorption operation is performed using a PSA device at a reduced pressure corresponding to the operating load to control the component concentration. be able to. As a result, the carbon dioxide concentration in the SNG can be made constant, so that the combustibility of city gas can be kept constant.

[0031]

EXAMPLES Examples are shown below to further clarify the features of the present invention.

In the following examples, butane is used as the light hydrocarbon according to the flow shown in FIG.
The method of the present invention was carried out by setting C = 0.8 and using a Ru-based catalyst (γ-alumina carrier carrying 2 wt% of Ru) as a catalyst in the adiabatic reforming reactor and the heat exchanger type reactor.

Example 1 In a heat exchanger type reactor, after methanating the reformed gas, the reformed gas was introduced into a PSA device (a molecular sieve was used as an adsorbent), and a non-adsorptive methane-rich gas and an adsorbent CO were adsorbed. ₂ Rich gas (off gas) was separated.

The reaction conditions in the adiabatic reforming reactor and the heat exchanger type reactor are as shown in Table 1.

[0035]

[Table 1]

FIG. 2 shows the desorption pressure under various operating loads of the PSA system.

By controlling the pressure at the time of desorbing the adsorbed gas according to the operating load, the CO ₂ concentration in the non-adsorbing methane-rich gas can be made constant, and thus the operating load (100 to 30%) can be maintained. Irrespective of the above, the composition of the obtained production gas (SNG after LPG heating) could be made almost constant. The composition (mol%) of the obtained SNG is shown in Table 2, and the properties of SNG are shown in Table 3.

[0038]

[Table 2]

[0039]

[Table 3]

[Brief description of drawings]

FIG. 1 is a flow chart showing an outline of one embodiment of the present invention.

FIG. 2 is a graph showing the relationship between the operating load and the desorption pressure of the PSA device in the example.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masafumi Motoe 1-4-2 Nakamichi, Higashinari-ku, Osaka City, Osaka Prefecture (72) Inventor Masahiro Inoue 346 Miyanishi, Harima-cho, Kako-gun, Hyogo Prefecture No. 1 Sumitomo Seika Co., Ltd. (72) Inventor Issei Haruna No. 1 346, Miyanishi, Harima-cho, Kako-gun, Hyogo Prefecture Sumitomo Seika Co., Ltd.

Claims (2)

[Claims] 1. An alternative natural gas is recovered by recovering a methane-rich gas by using PSA that adsorbs and desorbs carbon dioxide in a mixed gas by repeating adsorption and desorption from a mixed gas containing methane gas, carbon dioxide, carbon monoxide and hydrogen. In the method for producing gas, an alternative method of controlling the decompression pressure at the time of desorption of PSA of methane-rich gas according to the operating load so that the carbon dioxide concentration in the methane-rich gas after the separation process is constant regardless of the operating load Natural gas manufacturing method. 2. The final depressurization pressure at the time of desorption in PSA-
The method for producing an alternative natural gas according to claim 1, wherein the control is performed in the range of 0.2 kg / cm ² (G) to -1.0 kg / cm ² (G).