Advances of Membrane Technology in Acid Gas Removal in Industries

The presence of H2S and CO2 is unfavorable in many processes and flows, particularly in natural gas flows. Therefore, removing this gas is one of the important issues in many systems. One of the most widely used techniques is the use of membrane. Therefore, the gas that passes through the membrane has low pressure. Membrane system is used to absorb a high volume of CO2. The factors needed for this kind of separation include gas composition, pressure, and temperature difference. An enhanced electrochemical membrane can also be used with coal gas to separate acid gases. Hydrogen is so rich in this process in which cathode and sulphur steam are produced and then exited. Granular activated carbon (GAC) can be used as supporting material to absorb H2S. The advantages of this method are including high capacity for H2S absorption and gas emission. Removing the H2S from waste gases or natural gas with high densities is done via bio-filter in this system. Granular activated carbon can be used as a supporting material to stabilize the microorganisms. Principal properties of the supporting material are including the mass density, the area of special surface, and the amount of its pH. This research discusses the membrane technology in removing the acid gases in the oil technology. © 2020 by SPC (Sami Publishing Company), Reproduction is permitted for noncommercial purposes. K E Y W O R D S


Membrane Processes
Filters are porous materials which are used in the separation process or concentration. The most important property of the filters is that they have pores and holes with specific size and dimensions, hence filter is called a porous environment. The pores occupy the bulk of the filter volume, forming a complex network of holes. Filters can be made of polymer or ceramic. Filtration is a process in which a gas or liquid (fluid) passes through the filter due to pressure, electrical potential, or density difference.
Via filtration, the particles smaller than the filter pores pass through and the larger particles are separated from the fluid and remain behind the filter. The French Abbé Nolet [1,2] discovered the semi-permeable properties of the membranes. He found out that when a jar full of alcohol is immersed in water with its mouth blocked with a bag (as a membrane), the volume of the water in the jar increases in a few hours and the bag swells. In fact, the bag is impermeable to alcohol but is water-permeable; consequently, water infiltrates the jar so that the alcohol chemical potential reaches equilibrium. Membrane processes refer to the physical method of separating solvent from its soluble salts applying semi-permeable membranes [3][4][5]. This process has made a great deal of progress in recent years. Using membrane to filter goes back to the early 6th century. In 1930s, membranes were applied to separate, purify and concentrate solvents Membranes have wide applications in various science and industry areas including sea water desalination, blood dialysis, the treatment of river, spring and well water, domestic wastewater treatment, the treatment of different industrial wastewaters such as textile industries, papermaking, leather tanning, distillation, milk concentration in cheese making factories, fruit juice concentration, separation of microorganisms, sterilization of liquids, separation of proteins from cheese water, separation of sour gas from natural gas, separation of oxygen and nitrogen from the air, separation of ethylene from the emissions of polyethylene production plant, optimizing the environment, pharmaceutical industries and biotechnology [6][7][8][9].

Membrane Application
Two factors are determinant in the selection of the type of filter and its function. The first one is the size of filter pores. As mentioned above, particles larger than the pores are blocked behind the filter and cannot pass through; therefore, appropriate filters must be used to separate particles with specific sizes. The second factor is the amount of particles blocked behind the filter, which gradually accumulate with constant use of the filter [10].Natural gas, which consists mainly of methane (CH4) and light hydrocarbons, may also contain acid gases, such as hydrogen sulphide (H2S) and carbon dioxide (CO2). In addition to acid gases, natural gas may comprise other sulphur contaminants, such as mercaptans, carbonyl sulphide (COS) and carbon disulphide (CS2). In addition to lowering heating value, acid gases can cause corrosion to carbon steel natural gas processing equipment and pipelines if free water is present. Combustion of these sulphur compounds produces sulphur oxide (SOx) air pollutants that must be limited to protect the environment and to prevent health problems.
This can lead to the blockage of filter pores. As a result, the filter needs to be replaced or cleaned after a while. This is called filter blockage and is of great significance because filter replacement or even its cleaning is costly [11]. In Table 1 Pros and cons of using membrane is shown.

Membrane Classification
Depending on the specific type, separation processes have various types but what is common in all is the presence of a membrane which prevents the two phases from mixing together. Types of these processes include: 1. gas-gas processes: the process of gas leak 2. liquid-gas processes: evaporative leak 3. liquid-liquid processes: dialysis, electro dialysis, reverse osmosis, Nano-filtration, ultrafiltration and microfiltration.
In a filtration process, the two phases that are not in thermodynamic equilibrium are separated with a semipermeable membrane. The membrane acts as a physical obstacle or dam and controls the passage or blockage of materials from one phase to another [1,[12][13][14].
Different driving forces can be used for separation in membrane process. Mass transfer occurs in a membrane via the phenomena of solution-infiltration, displacement and ionic repulsion resulting from difference in chemical potential, difference in density, difference in pressure and difference in electric potential.

Passage
Considering the size of the smallest molecule which passes through the membrane obstacle, the processes in which mass transfer occurs because of pressure difference in the membrane width include microfiltration, ultrafiltration, Nano-filtration and reverse osmosis [15][16][17][18]. Table 1. Pros and cons of using membrane.

Pros Cons
Separation between gas and liquid phases In the two recent decades, significant efforts have been made to achieve progresses in producing membranes with the properties of the two processes, that is, high retention (similar to reverse osmosis process) and low pressures (like ultrafiltration process) [20]. The result is Nanofiltration which has gained an important place in different industries, bridging the gap between reverse osmosis and ultrafiltration. The sizes of Nano-membranes vary between reverse osmosis membranes and ultrafiltration (in the range of 2 nanometres or smaller); consequently, particles with average diameter are retained through a screening mechanism. In addition to these applications, particles are retained in an electrostatic repulsion mechanism in making charged Nano-membranes [21][22][23][24].

Nano-filtration
The sizes of Nano-filters are 0.5 to 2 nanometres. Nanofiltration has boomed during recent years. Separation and remove all the bacteria in water [ [25][26][27].

Microfiltration
Microfiltration is used to separate solid particles and suspensions with the size of 2-10 micrometres.
Considering the type of membranes which have big pores, the output flow rate is very high [28]. The pressure range used in this process is between 1 to 5 times.
Microfiltration is in fact used in biologic and food industries to separate suspended particles and bacteria.
Microfiltration is a process used for separating particles, small suspended solids and other materials such as bacteria, cysts, molecules and particles larger than 2 microns [28].
The general trends can be discerned in any of these processes, including developing new membranes These systems are used for the membrane with the micro pores which can be employed for the removal of the undesirable gases in the common systems without causing difficulties. In such systems, removal occurs in one gas phase and liquid absorber on the other side of the membrane and mass transfer of gas liquid take place at the mouth of membrane pores. In this part, a hydrophobic porous membrane is placed under pressure with a liquid whose entering to the system is blocked. For this purpose, a gas with Pg pressure is used in a way that if the pressure is more than the Pg, gas penetrates into the liquid.
Alternatively, in case that the pressure is less than the Pg, liquid penetrates into the gas.

Conclusion
Natural gas can be considered as the largest fuel source required after the oil and coal. Nowadays, the consumption of natural gas is not only limited to the industry, but natural gas is also extensively consumed by the power generation and transportation sector. It is well known that the emission of the greenhouse gases such as carbon dioxide (CO2), which is accounted for about 80% of greenhouse gas emission, is associated with global warming and climate change. While there are veracious natural sources of CO2 emission, the emissions associated with human related activities are the main reason of carbon dioxide increase in the atmosphere in recent decades. The main human related CO2 emission is the combustion of fossil fuels (oil, natural gas, and coal).
However, 80% of the world's total energy sources is supplied by fossil fuels. On the other hand, concentrated CO2 is needed for some industrial applications such as enhanced oil recovery. Therefore, the development of CO2 capture methods is so attractive.

Recommendation
1-Use some additive to eliminate corrosion 2-New method for gas-gas processes: the process of gas leak

Conflict of interests
The authors declare that there is no conflict of interests regarding the publication of this paper.