| Membrane contactor is a new technology that enable mass transfer without phase-mixing.It features higher contact area,higher efficiency,and higher compactness compared to conventional process,showing good potential as a solution for global warming through CO2 removal,particularly in removing CO2 from natural gas.Currently,studies in membrane contactor mainly focuses on membrane materials,membrane structures,and membrane performance.Relatively less focus is given on the application of the technology under actual condition of natural gas treating(high pressure membrane contactor absorption process)and the relevant key factors such as liquid-to-gas(L/G)ratio,membrane wetting,and absorbent regeneration.Based on the problems explained above,investigations were carried out under this study to provide data and basis for actual application in natural gas treating.The integration of high-pressure CO2 removal and absorbent regeneration in this study also provides important guidance for the application membrane contactor as a complete chemical absorption system.In this study,extensive investigations on CO2 absorption using membrane contactor made from polytetrafluoroethylene(PTFE)hollow fiber membranes is reported.Effects of gas flow rate,liquid flow rate,feed gas CO2 content,absorbent temperature,membrane thickness,and operating pressure were studied.Mass transfer coefficients and mass transfer resistances were calculated and analysed in order to identify mass transfer bottleneck.Results show that liquid phase mass transfer resistance contributes up to 93%of the overall mass transfer resistance.Increasing liquid velocity will reduce the mass transfer resistance in liquid phase,however,this also leads to undesirable increase in L/G ratio.A solution to this problem was found by increasing the length of membrane modules through connecting them in series.Results show that,without reducing liquid flow rate,increase in membrane module length and feed gas flow rate can effectively reduce L/G ratio.L/G ratio can be optimized to 0.48 L/mol which is significantly lower than the conventional level of 0.75 L/mol,while meeting 0.005 mol%CO2 specification in treated gas.Investigations on CO2 desorption in hollow fiber membrane contactor was carried out in this study.Aqueous solution of methyl diethanolamine(MDEA),which is commonly used in natural gas industry,was selected as CO2 absorbent.Heating method used in absorbent regeneration experiment was investigated to evaluate the possibility CO2 desorption in heater.The effects of regeneration temperature,liquid flow rate,flow configuration,vacuum,and N2 stripping were also investigated.The corresponding temperature drop on these experiments were analysed and discussed.Results show that hollow fiber membrane contactor functions as gas-liquid separator and CO2 desorber concurrently in absorbent regeneration.Rate of absorbent regeneration is very slow at a relatively low temperature.Increasing contact area can help improve the absorbent regeneration rate.Heating method can affect experimental results significantly,and the pressurized heating method proposed in this study can suppress CO2 desorption during heating,ensuring stable regeneration performance.Based on actual natural gas treating condition,integrated process of membrane contactor C02 removal and absorbent regeneration was designed and experimented continuously for 14 days.The investigations cover both bulk CO2 removal(26.0 mol%down to 6.5 mol%)and deep CO2 removal(6.5 mol%down to 0.005 mol%),which are different in terms of its CO2 content in treated gas.Lean regeneration was successfully proven in membrane contactor,in which steam stripping was applied and 0.01 mol CO2/mol amine was achieved.Compared with conventional pack column,vessel size reduction of 53%can be achieved when PTFE membrane contactor is used,with absorbent regeneration energy as low as 0.94 GJ/ton CO2.The effects of membrane wetting on both CO2 absorption and absorbent regeneration were investigated using direct liquid penetration method.In addition,water condensation as a possible cause of membrane wetting in membrane contactor’s operation was proven.High temperature absorption,which is commonly employed in natural gas process,was conducted using membrane contactor in order to generate more water vapour in the system.It was found that membrane wetting occurred from the beginning of a 24 hours experiment at a constant degree when all experimental conditions were fixed.As the operating parameters were varied,degree of membrane wetting increased due to water condensation in membrane pores,which was detected indirectly through increase in liquid accumulation rate in treated gas knock-out drum.The results of this study provide important insights into potential operational limitations of membrane contactor in CO2 absorption as well as absorbent regeneration. |
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