Mathematical Simulation of Membrane-Absorption for H<sub>2</sub>S Capture from Nature Gas

Abstract

A dynamic model of mass transfer was developed with mass transfer equation and mass transfer differential equation according to two film theory for the simultaneous transport of hydrogen sulfide through hollow fiber membrane (HFM) contactors while using N-methyldiethanolamine (MDEA) as the chemical solvent. The model results are in excellent agreement with the experimental data. The results indicate that the removal of H2S increased while increasing concentration of MDEA and gas pressure, however, the removal of H2S decreases while increasing gas velocity. The concentration of H2S increases at the same place in the lumen while increasing gas velocity. There is serious decreasing amplitude of axial concentration of H2S during the initial stage, but it slows down at half of the length and a great reduction of H2S concentration in radial direction with the increase of the length. The decreasing amplitude is dropped due to the concentration of H2S decreased in radial direction. The model can indicate H2S removal rate in given operational conditions and offer theory evidence for the design of membrane contactor. Natural gas is believed to play a vital role in the next few decades for industrial and domestic utilization. It is considered as one of the cleanest and safest of all energy sources. However, nature gas is not a pure hydrocarbon and sometimes it has some sour gases such as hydrogen sulfide which has high toxicity. Hydrogen sulfide can not only corrode equipment and transmission pipeline under aerobic and hot humid conditions but also cause catalyst poisoning, even serious threaten the safety of human. Wet desulphurization is widely used for natural gas treatment and aqueous solutions of alkanolamines are often used as absorption solvent. Among these alkanolamines, MDEA as an absorption solvent of acid gases is widely used today because it possesses the characteristics such as higher H2S selectivity, bigger absorption capacity, lower regeneration energy, smaller hot-degradation and lower circulating load. But desulphurization unit can be seriously corroded in the sulfur removal process. On the other hand, these conventional processes such as absorption towers, packed and plate columns possess many disadvantages such as flooding, foam formation, and demand high capital and operating costs. So the technology meets a certain obstacles. Recently, new processes using gas–liquid membrane contactors as gas absorption devices have been a subject of great interest. Among the diversity of membrane geometries available for membrane contactors, hollow-fiber membrane contactors are favored due to their high surface/volume ratio for separation which is 30-50 times compared with traditional absorbers. This type of process offers several practical advantages including low energy and operating costs, simplicity and occupying small area. In addition, membrane contactors as unit equipment can be combined according to actual need. [4~5] used polypropylene hollow fiber membrane as the absorber and MDEA as the chemical solvent for the absorption of H2S via changing operating conditions (e.g. temperature, pressure, the concentration of the solvent, flux of gas-liquid phase) and studied the influence of the changes to mass transfer coefficient and sulfur removal efficiency. The results indicate that the sulfur removal efficiency can be 95% above by optimizing the operating conditions. At home and abroad, comprehensive two-dimensional mathematical models were developed based on differential equation. Wang [6] simulated the absorption of CO2 using different absorption medium in hollow fiber membrane contactors. But they did not consider the effect of mixed gas. Chen [7] modeled the distribution of reactants and products concentration in the shell side in different typ es of reaction. However, the model can not obtain the concentration of H2S in the lumen. Rami Faiz [8] modeled the distribution of acid gas, but the mathematical model was not validated by the experimental work.