Progress of natural gas desulfurization technology

In particular, MDEA has good chemical stability and the solvent is not easy to degrade. The corrosion of the device is light, which can reduce the investment and operating costs of the device; When absorbing H2S gas, the amount of solution circulation is small and the gas phase loss is small. However, MDEA has poor anti-pollution ability compared with other amines, which is easy to cause problems such as solution foaming and equipment clogging.

Amine absorption is a mature natural gas treatment method, but it has some problems, such as heavy equipment, high investment cost, complicated process, large amount of desulfurizer loss, regeneration and environmental pollution. One of the biggest problems is the regeneration of the absorbent. The main regeneration method used is high temperature and vacuum distillation, which has high energy consumption, large investment and low recovery rate. At present, the desulfurization and decarburization process of alcoholamine method has been developed from the use of a single aqueous solution to the formulation of a series of solvents with different solvents. Through the solvent compounding, the operation performance is improved and the application range is expanded, which has played an obvious effect of energy saving, reducing production costs, increasing the capacity of the device and so on.

1.1.2 Dry desulfurization

Dry desulfurization means that raw gas passes through a solid bed equipped with a solid desulfurizer at a certain airspeed, and H2S in the gas phase is adsorbed to the desulfurizer after gas-solid contact exchange, so as to achieve the purpose of purification [4].

More common solid adsorbents are iron series, zinc series, manganese series oxides more active oxides. Activated carbon is a common solid desulfurizer, which can be used to remove trace H2S from natural gas. Compared with other adsorbents (such as molecular sieve), activated carbon has the advantages of large specific surface area, good thermal stability, microporous structure and large adsorption capacity of moisture, etc., and its price is low, and it can also achieve the purpose of decolorization and odor absorption while desulfurizing. The above advantages of activated carbon make it very widely used. In addition, molecular sieve and zinc oxide and other substances can also be used for natural gas desulfurization.

The desulfurization effect of zinc oxide, molecular sieve, activated carbon and iron oxide desulfurizer can achieve the mass concentration of export sulfur less than 0.1mg/m, which can meet the requirements of natural gas desulfurization. Different desulfurization methods have advantages and disadvantages: molecular sieve and zinc oxide desulfurizer are expensive, and the equipment investment is correspondingly high (molecular sieve requires high temperature regeneration equipment); Activated carbon and iron oxide desulfurizer are cheap, less equipment investment cost, easy to operate, and more economical. However, from the perspective of chemical reaction mechanism, H2S removal by activated carbon requires the presence of O2, while H2S removal by iron oxide desulfurizer with or without O2 can be carried out (reaction 2).

2H2S+O2==2S+2H2O (1)

Fe2O3·H2O+3H2S==2FeS+S+4H2O (2)

1.2 Physical desulfurization method

1.2.1 Pressurized fluidized bed combustion (PFBC) technology

The British Coal Use Research Association (BCURA) first put a fluidized bed into a pressure vessel in 1968, which was the prototype of a pressurized fluidized bed. PFBC unit efficiency is 38% ~ 42%, desulfurization efficiency is more than 90%, but also has a strong denitrification capacity, so it has aroused great interest. A commercially operated PFBC power station was used for the first time at the Vartan power station in Switzerland.

1.2.2 Membrane separation technology

The principle of membrane separation is that in the epidermis of the film, there are many very fine capillary holes, which are formed by the space between the tissues of non-bonded materials in the membrane matrix. The flow of gas through these holes is mainly the result of the joint action of knuden flow (free molecular flow), surface flow, viscous flow and screening mechanism, in which viscous flow does not produce gas separation. According to the knuden flow mechanism, the penetration rate of a gas is inversely proportional to the square root of the molecular mass of the gas. Because the molecular mass of CH4 is smaller than that of H2S, CO2 and H2O, the permeability coefficient of CH4 is greater than that of H2S, CO2 and H20. Moreover, when it is knuden flow, the permeability coefficient of pure gas is independent of the operating pressure and remains constant. Surface flow refers to the flow of the gas layer adsorbed on the surface of the membrane hole through the membrane hole because the surface of the fiber membrane has a strong adsorption effect, and the characteristics of the adsorption layer, that is, the permeability of H2S, CO2 and H20 increases with the increase of pressure. Therefore, when the surface flow dominates, the permeability coefficient of H2S, CO2 and H20 is greater than that of CH4. According to the screening method, the molecular dynamic radius of CH4 is 1.92µm, which is larger than that of H2S, CO2 and H20. When the size of some membrane pores in the membrane epidermis is small enough, CH4 is difficult to pass through these membrane pores. Therefore, H2S, CO2 and H20 have higher separation factors than CH4. When the mixed gas passes through the membrane separator under pressure, the passage rate of different gases is greatly different. "High-speed gas" quickly passes through the membrane and separates from "low-speed gas". The two gases are discharged through different pressure tubes at different outlets of the treatment system. "High-speed gas" is also known as permeable gas, which is H2S, CO2, H20, H2, He and 02. It belongs to low pressure air flow; "Low-speed gas", also known as residual gas (tail gas), is CH4, N2, Ar, CO2 and other hydrocarbon gases, which belong to high pressure gas, and the product gas (tail gas) after treatment still has a high pressure into the pipe network.