Removal Of Toluene From Nitrogen By Using Membrane Absorption And Vacuum Membrane Distillation

In this paper, an experimental and theoretical study was carried out to evaluate the performance of a newly process which combined gas-liquid membrane absorption with vacuume membrane distillation in toluene/nitrogen separation process by using an aqueous N-Formylmorpholine(NFM) solution as absorbent. The reseaching work included process performance study, theoretical study of mass transfer and the expansion of the system based on the researching of energy consumption. The process performance researching mainly focused on the determination of physical properties of aqueous NFM solution-toluene system and the optimization of operating parameters. A total differential mathematical model which based on the double films theory and serially resistance was developed to simulate the absorption process in order to gain an insight into the experimental observation. The other mathematical model that combine differential mass transfer equation with experiential heat transfer equation was developed to simulate the heat and mass teansfer process in vacuume membrane distillation process. The third model was developed to simulate the effect of operation parameters on the system consumption.Firstly, the determination of determination of physical properties of aqueous NFM solution-toluene system was carried out. The physical properties parameters included the density, viscosity, surface tension and Herry number of aqueous NFM with different volume concentration. The diffusion coefficient of toluene in different volume concentration aquous NFM and N2 were also calculated. Fitting equations of experimental data were obtained. The results showed that the absorbent has absorption Capacity for toluene and has high surface tension which made it difficult to invade into the PP membrane pores. This efficiently decreased the risk of the decreasing of mass transfer efficience which caused by the wetting of membrane. The NFM aqueous was proved as a suitable absorbent for the separation and recovering of toluene from Nitrogen in gas-liquid membrane contactor.An experimental study was carried out to evaluate the performance of self-made membrane absorption device.The effects of operating parameters such as the gas and liquid flow rates, the feed gas and the initial liquid concentrations, the liquid temperature, the absorbent concentration and the long term operation on the toluene flux and removal efficiency were investigated in this study. The results showed that the removal efficiency achieves 54.0%-97.3% when following conditions were used:volume ratio of NFM,40%; flow rate of absorbent,20-80 mL·min-1; flow rate of feed gas,50-500 mL·min-1; feed gas concentration,16.1mg·L-1. It was proved that membrane absorption process has high stability and operability. It was also found in this study that the liquid mass transfer process is the key process in membrane absorption process when the feed gas concentration and gas flow rate were high enough. However, the gas mass transfer process became the key process when the feed gas concentration and gas flow rate were low enough. Additionly, The long term performance test showed that the aqueous NFM solution is a suitable absorbent to be applied to the polypropylene(PP) hollow fiber gas-liquid membrane contactor.Based on the double-film theory, a total differential mass transfer model was established to simulate the process of mass transfer process. The solution of the model could perfectly describe the concentration distribution of toluene in both gas phase and liquid phase. The researching work was foused on the way of removal efficiency and toluene mass flux effeced by operatrion paramemters and membrane contactor formation changings by observing their effect on the concentration distribution at liquid film. The prediction values were compared with those of experiment. The results showed that the average error of touluene removal efficiency was 5.2%. The mass transfer process could be accurately simulated by the differential equations, which could be used as the theory basis for the industrial enlargement of membrane absorption technology.An experimental study was carried out to evaluate the performance of self-made vacuume membrane distillation device.The effects of operating parameters such as the gas and liquid flow rates, the feed liquid concentrations, the liquid temperature, the vacuume side pressure on the toluene flux and removal efficiency were investigated in this study. The result showed that liquid temperature is the most important factor for the toluene removal efficiency and mass flux and the following two factors were feed liquid concentration and liquid flow rate. The toluene flux could achieve 3.13×10-3mol·(m2·s)-1 and toluene removal efficiency could achieves 78.1% when following conditions were used:liquid flow rate of absorbent, 20mL·min-1; Vacuume side pressure,2000Pa; feed liquid concentration,350mg·L-1. It proved Vacuume membrane distillation to be a high efficiency device for recovering the toluene from absorbent.A newly mathematical model which combined differential mass transfer equation with experiential heat transfer equation was developed to simulate the heat and mass teansfer process in vacuume membrane distillation process. This model simulated the mass transfer in the process by using differential equation and simulated the heat transfer by experiential heat transfer equation. The temperature in the liquid surface could be gained by iterating the solutions of two equations.Beased on the memnbrane absorption model and vaccume membrane disllation model, the system energy consumption model was established to evalute the effect of operation parameters on the system energy consumption. The results showed that the energy consumption of vaccume pump is the key ingredients and the following factor is liquid pump energy consumption.