Studies On Mass Transfer Characteristics Of Absorption Of H₂S In Microchannels

In this paper, the hydrodynamics and mass transfer characteristics of absorption of H2S into Methyldienthanolamine (MDEA) solution in microchannels is carefully investigated and furthermore, the scale-up of microchannels is also discussed.The mass transfer characteristics of absorption of H2S from gas mixture having 1200ppmv H2S into 40wt.% MDEA solution in a circular T-junction microchannel with an inner diameter of 1.00mm are investigated. It is found that the high removal efficiency of H2S is achieved (as high as 99.5% at the ratio of gas to liquid of 200) and the mass transfer resistance is mainly limited on the gas side. The gas side volumetric mass transfer coefficient and interfacial area are as high as 31746 mol/(m3 s atm) and 11100m2/m3, respecitively, which are one or two orders magnitude higher than these of other gas-liquid contactors. However, the process intensification of gas-liquid in the microchannel is mainly ascribed to the interfacial area and the mass transfer coefficient is not changed significantly. A new empirical correlation based on the experimental data is developed to predict the gas side mass transfer coefficient.The effect of various operating conditions on the absorption process is also performed. It is shown that the removal efficiency of H2S increases with an rise in the operating pressure, the concentration of absorbent and the length of microchannel and decreases with the increase of operating temperature, the concentration of H2S in feed gas and the hydraulic diameter of microchannel. However the influence of geometry of cross section and material of microchannel on the removal efficiency of H2S is not remarkable. Furthermore, it is found that the effect of operating pressure, the concentration of H2S in feed gas and length of microchannel on the mass transfer coefficient is lessThe pressure drop characteristics of gas-liquid in the microchannel with the diameter ranging from 0.56-1.80mm are researched with the MDEA solution and N2 as the working fluid. It is reported that the conventional classical theory is valid for the single flow in microchannels and for the two-phase flow pressure drop, a new correlation is developed in the form of Lockhart-Martinelli type which considers the effect of surface tension, gas phase inertial force, Reynolds number of liquid phase and the Martinelli parameter.The gas-liquid interfacial area of transition zone in microchannels with the diameter ranging from 0.56 to 1.80mm is measured by the method of chemical absorption of CO2 from gas mixture containing 11.8% CO2 into 1.00 kmol/m3 NaOH solution. It is shown that the interfacial area increases with the rise in the gas and liquid phase superficial velocity and decreases with the increasing of the diameter of microchannel. In order to quantify the interfacial area, a new empirical correlation is developed which considers the surface tension, diameter of microchannel, gas and liquid superficial velocities.The scale-up of microchannel is conducted with the monolith and a novel gas-liquid distributor is designed. The performance of liquid distribution is determined by the time-average liquid collection. The effect of gas, liquid phase flow rates and the distance between the top surface of monolith and distributor on the liquid distribution is investigated. It is found that the qualification of liquid distribution with the novel gas-liquid distributor is greatly higher than that with the packing and pipe distributors, respectively. Besides, this distributor is scaled up easily. The distribution of gas phase with this distributor is investigated with the simulation. It is shown that the effect of parameter H and gas phase superficial velocity on the gas phase distribution is significant.A monolith absorber is designed and the absorption of H2S into MDEA solution is performed in this absorber. It is demonstrated that the effect of distributor on the removal efficiency of H2S is great. A comparison of the removal efficiency of H2S with three different distributors is performed and it is shown that the designed thought and parameters of the novel distributor are superior to other two distributors. The effect of liquid distribution on the absorption process is quantified and it is found that with the decrease of the misdistribution factor, the concentration of H2S at the outlet decreases and the removal efficiency of H2S increases. According to the mass transfer characteristics of single microchannel and the liquid distribution in the monolith absorber, the removal efficiency of H2S is computed, and it is found that the predicted value shows good agreement with the measured results by the absorption experiment.