JPS593508B2 - How to sweeten methane - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention improves the sour gas mixture by passing the sour gas mixture through a semi-permeable membrane that is highly selective to the sour gas components and through which the sour gas components permeate and repel the gas to be sweetened. on how to sweeten.

5 Sweetening sour gas containing hydrogen sulfide and carbon dioxide as sour components is a consensus goal in the industry.

These acidic components are harmful because they cause corrosion and their presence reduces the concentration of methane and reduces the desired fuel quality. Furthermore, the hydrogen sulfide component is harmful due to its foul odor and known toxicity. Various methods for sweetening gases have already been published in the prior art regarding this problem, such as the use of solvent absorbers as described in US Pat. No. 3,710,546. U.S. Patent No. 3,470,677 also uses molecular sieves.
It is known by the number. The use of a bundle of semipermeable capillary filters to separate hydrogen sulfide from methane is shown in US Pat. No. 3,534,528. It would be desirable to have a new method for sweetening sour gas that is reliable, efficient, and can be implemented in a variety of ways to extend the benefits. It is therefore an object of the present invention to provide a new method for sweetening sour gas by selectively passing the acid gas components through a membrane having a separation coefficient suitable for the acid gas components.

Another object of the present invention is to provide a method for repelling methane gas and simultaneously removing acid gas components, thereby increasing the methane concentration to sweet levels. Furthermore, it is an object of the invention to obtain a method for sweetening sour gas in which a heterogeneous membrane is advantageously used to achieve improved permeation of acid gas components.

Another object of the present invention is to provide a sour gas sweetening process that uses feed pressure and permeate gas pressure to advantageously achieve improved separation of acid gas components by permeation.

1 Sour gas, as commonly used in the art, refers to a gas mixture of about 80-90% methane or light paraffin and the remainder acidic components consisting of hydrogen sulfide and carbon dioxide.

Usually, the carbon dioxide acidic component is present in significantly greater amounts than the hydrogen sulfide component. Carbon dioxide generally exists up to about 10%;
Hydrogen sulfide is present in an amount of less than 1%. 1 Sweet gas 1 is used to refer to a high grade fuel gas containing at least about 99% methane or light paraffin and less than about 1% acidic components.

Generally, carbon dioxide is present in amounts less than 1% and hydrogen sulfide in trace amounts, such as less than about 20 ppm. According to the present invention, sour gas is reliably sweetened because acidic components are removed by passing through a membrane having high selectivity for acidic gas components.

The repelled methane is collected at a high concentration and yields hesuite gas. 6Methane here includes the possible presence of all light paraffins, and such expressions are to be understood to have this meaning for the purposes of the present invention. As used in the present invention, the concentrations of methane, carbon dioxide, and water sulfide are expressed relative to the sum of these three components. The anisotropic membrane has high selectivity for acidic gas components,
Anisotropic membranes are advantageously used as a feature of the present invention because they have favorable permeability coefficients that allow effective permeation of acid gas components. It has been found that such anisotropic membranes can eliminate problems common to homogeneous membranes. Homogeneous membranes with a desired high solubility index greater than 9 are typically 20
, but with the disadvantage of having a relatively low permeability coefficient.

When using homogeneous membranes to practice the invention, it is necessary to choose a membrane that balances high separation coefficients with low permeability. It has been found that advantageous combinations of separation and permeability coefficients can be achieved with heterogeneous anisotropic membranes and composite membranes. 1 Anisotropy means 1 in this industry.
1 having a thin film on the surface and the rest being made of a thick porous material
It is thought to represent a membrane formed in the body.

Anisotropic membranes are characterized by permeability that is not uniform in all directions. In particular, the permeation rate of an anisotropic membrane is considerably higher than that of a homogeneous membrane of the same overall thickness. Although the anisotropic membrane is preferably in sheet form, it may advantageously be used in other forms, including tubes or concentric tubes. Various anisotropic membranes can be used, such as polyvinylidene chloride (particularly nearly homopolymer), polyacrylonitrile or cellulose acetate membranes.

Membranes that can be used include cellulose diacetate and cellulose triacetate membranes. Materials such as gelatin, nylon 6 and 6/6, polyvinyl alcohol membranes, polystyrene, polyurethane, and PVCl vinylidene copolymers are also used. Multilayer or composite membranes generally provide thin films of high solubility index polymers supported by thicker but lower solubility index polymers.

"Permeate gas or permeate" is used to refer to a gas that is selectively adsorbed by a membrane.

Such permeate gas is carried by the membrane to a location on the other side of the membrane. According to current theory, gas permeability through a membrane is described by two properties: 1) the solubility of the gas within the membrane material and 2) the diffusion of the gas through the membrane.

The permeation of a gas is therefore considered to be the product of the solubility and diffusivity of a given gas within the membrane. Each gas has a specific permeability coefficient K for a given membrane. Gas permeation rate is further influenced by membrane thickness, membrane properties, membrane layers involved, differential pressure, temperature, and other factors. The permeability coefficient K is calculated by the specified thickness (d), standard state flow rate per unit time (s) (C7l), effective surface area (CIi) and differential pressure across the membrane @old g). : The ratio of the permeability coefficients of two gases under the same conditions is known as the separation factor (2) and is calculated as the ratio of the permeability of sour gas components and methane through a given polymer membrane: L▲ZV/ υ little to see
r ν VZ/ V▲crystal e The selected heterogeneous film is approximately 9
have a desirable high solubility index, generally in the range of 9.4 to 15.0.

These films contain one component of hydrogen sulfide and one of carbon dioxide.
It has a desirable high separation coefficient for one or both, and has a desirable permeation coefficient that, when used in the advantageous manner described above, allows effective permeation to be achieved. In order to increase the efficiency of capturing the methane that does not permeate, it is necessary to increase the membrane area so that the permeate gas is more effectively captured.

The required membrane area can be significantly reduced if high feed pressures are used in the gas permeation sweetening process and low permeate gas pressures are obtained. Generally a supply pressure of at least greater than atmospheric pressure is used. In practice, the feed pressure is advantageously very high, for example at least 30 times the permeate gas pressure. The supply pressure can be obtained in various ways, for example by the flow rate of the sour gas mixture along a closed path relative to the membrane surface. The permeate gas pressure can likewise be obtained in various ways, for example by providing a closed chamber on one side of the membrane and evacuating this chamber. Such pressure gradients are easily achieved by those skilled in the art. The invention will now be explained by way of example.

EXAMPLE 1 Transmission of Acid Gas Components in a Square Permeation Cell A square permeation cell receives a feed gas from a row of ports and allows this gas to flow across the membrane. installed to exit from the column port.

The permeated gas exits to a row of ports located on the opposite side of the membrane. The membrane is supported by a filter paper sealed around the perimeter with silicone rubber. Feed gas is supplied at a controlled pressure to a manifold through the cell assembly and gas chromatograph. The impermeable or repelled gas flows from the cell through a variable limiter to a flow indicator or chromatograph. The permeated gas is directed to a flow indicator or, if working near atmospheric pressure, directly to a chromatograph. A pump is placed in the conduit before the flow indicator and chromatograph to maintain the permeate gas side of the membrane below atmospheric pressure during measurements. Adjustment of a variable limiter in the repulsed flow allows the ratio of permeate to non-permeate gas flows to be varied. The time during which the non-permeable gas flow settles to an approximate value is considered for equilibrium of flow through the membrane. The feed, reflux and permeate gas flows are continuously analyzed. The feed gas is analyzed by mass spectrometry. The feed sour gas mixture is sent to a square permeation cell, the mixture containing hydrogen sulfide at a concentration ranging from about 0.02 to 0.56% and carbon dioxide at a concentration ranging from 1.0 to 10.3%. Supply pressure is 1.33
~15.05 kg/Cil (19-215 psia) and the pressure of the permeate gas stream is 0.007-0.0211
<g/CrlL (0.1-0.3 psia) or maintained at atmospheric pressure. A stream consisting of approximately 90% methane, 10% carbon dioxide and 0.02-0.4% hydrogen sulfide is 1.0
The sweetening performance showed an increase with increasing supply pressure for permeate gas pressures of 51<g/C7ll (15 psia). Unexpected enrichment of ethane and propane in the nonpermeate gas was observed in all tests, resulting in eta7 concentrations up to 170% of the concentrations in the feed stream. Propane showed a larger increase than ethane. The propane concentration in the nonpermeate gas ranged up to 230% of the concentration in the feed gas. Example 2 Membrane Permeability and Separation Coefficients Polyamide or nylon 6 membranes with a solubility index of 14 were tested at various temperatures against sour gas containing 1.1 mole percent carbon dioxide and 0.5 mole percent hydrogen sulfide.

The same cell as described in Example 1 was used as the transmission cell, but
This cell was modified so that the exit side of the membrane could be purged with a known volume of sample loop helium that was switched to the chromatograph carrier stream for analysis. The inlet side of the membrane was maintained at a constant total pressure of 4.55 kg/d (65 psia) and a polyvinyl alcohol membrane with a solubility index of 13 was tested under the same conditions as the nylon 6 membrane.

After activating the valve of the selected sample loop in both methods,
The permeate gas sample is placed in the chromatograph column, one side of the twin-bead thermistor detector, the second chromatograph column,
It passes continuously through the other side of the detector and finally flows into the atmosphere. The gas is analyzed by mass spectrometry for methane and carbon dioxide, and hydrogen sulfide is analyzed by iodometry.
The results are shown in Table 1 below. The table above confirms the correlation between high solubility index and high separation coefficient of sour gas/methane.

Both membranes tested had high solubility indices and exhibited moderate to high separation factors for hydrogen sulfide and carbon dioxide for methane. Values of 50 at 70°C were obtained for nylon 6 and hydrogen sulfide, and values up to 100 at 50°C were obtained for polyvinyl alcohol membranes and carbon dioxide. The hydrogen sulfide-methane separation coefficient of polyvinyl alcohol is 50℃
However, the nylon 6 membrane decreased when the temperature exceeded 30
When increasing from °C to 70 °C, the hydrogen sulfide-methane separation coefficient showed an increase. The carbon dioxide separation coefficient showed a tendency to decrease with increasing temperature over the range of 30 to 70°C.

The following membranes were tested in a similar manner: The results are shown in Table 2 below.

Gelatin is plasticized with glycerin.

All three membranes show high separation coefficients, with polyacrylonitrile having a separation coefficient of 160 at 30°C and 120 at 50°C.

The permeability coefficients of methane and carbon dioxide both increase with temperature, with methane exhibiting a larger increase. Heat stabilized nylon 6/6 exhibits high separation factors and generally represents an improvement over nylon 6 as a sweetening system membrane.

The pigmented gelatin membrane showed an increased hydrogen sulfide-methane separation factor at 50°C compared to a high value of 200 at 30°C.

The gist and embodiments of the present invention are listed below: 1) In a method for sweetening methane by removing sour gas components hydrogen sulfide and carbon dioxide, a feed mixture of methane and sour gas components is selected for two sour gas components. 1. A method for sweetening methane, which comprises sending methane to a membrane having a certain permeability, simultaneously removing sour gas components that permeate through the membrane, and collecting sweetened methane that does not permeate.

2) the membrane selective for sour gas components has a high solubility index of greater than about 9 and has a balanced separation and permeability coefficient such that the membrane achieves effective permeation of sour gas components; The method of item 1 above.

3) The method of item 1 above, wherein the membrane having selectivity for permeation of sour gas components is a heterogeneous membrane selected from anisotropic membranes and composite membranes.

4) The method of paragraph 3, wherein the heterogeneous membrane is an anisotropic membrane having a separation factor greater than about 20.

5) The anisotropic membrane is made of one of the group consisting of cellulose acetate, cellulose diacetate, and cellulose triacetate, and the permeation of the sour gas component through the membrane is 50%.
The method according to item 4 above, which is carried out at a temperature that does not significantly increase ℃.

6) The method of item 1 above, wherein the methane and sour gas components are delivered to the membrane at feed pressure and the sour gas components are collected at permeate gas pressure, the feed pressure being significantly higher than the permeate gas pressure.

7) The method of item 6 above, in which the supply pressure significantly exceeds atmospheric pressure and the permeate gas pressure does not exceed atmospheric pressure.

8) The method of item 7 above, wherein the feed pressure is at least 30 times greater than the permeate gas pressure.

9) A sour gas mixture consisting of methane and a sour gas component contains about 80 to 90% methane, most of the remainder being the above-mentioned sour gas component, and a carbon dioxide sour gas component is present in a significantly larger amount than a hydrogen sulfide sour gas component, which does not permeate. methane of at least about 9
The method of item 1 above, in which a concentration of 9% is collected.

10) The membrane is made of cellulose acetate, polyacrylonitrile, nylon, gelatin, polyvinyl alcohol,
The method of item 1 above, wherein the polyvinylidene chloride, vinylidene copolymer is selected from the group consisting of polystyrene and polyurethane.

Claims (1)

[Claims] 1. A method for sweetening methane by removing sour gas components consisting of hydrogen sulfide and carbon dioxide from a crude methane gas stream, comprising cellulose acetate, cellulose diacetate, cellulose triacetate, gelatin, nylon 6, nylon 66, polyvinyl alcohol,
Uses an anisotropic semipermeable membrane made of a material selected from the group of polystyrene, polyurethane, PVC, and vinylidene copolymers, with a thin film on one side and a thick porous material on the other side. A method for sweetening methane comprising simultaneously removing two sour gas components from a crude methane gas stream.