Carbon dioxide: absorption removal with membranes

The carbon dioxide from the gas mixture is removed by washing with water and the carbon monoxide is removed by absorption or membrane separation. 

A number of reviews are available discussing the application of membrane contactors for acid gas removal (Li and Chen, 2005 Mansourizadeh and Ismail, 2009) [4] [7]. Most research activities described in the literature deal with use of membrane contactor for the removal of carbon dioxide. Most of the time alkanolamine solutions have been used for the selective removal of CO2 from various gas streams (Jamal et al. 2006 Wang et al. 2004) [2] [10]. The effect of the amine solvent, the operating conditions, and the membrane characteristies on the removal of CO2 has been studied by (Wang et al, 2004) [10]. Results for the kinetics for both the absorption and desorption rate of CO2 for different amine absorption liquids (MEA, DEA, MDEA) and AMP and mixtures of these absorption liquids have been reported by (Jamal et al., 2006) [2]. Also, the use of other types of absorption liquids has been studied. Results for different amino acid salt solutions as CO2 absorbent have been described by (Lu et al., 2009) [6]. The use of membrane contactors is not limited to the removal of CO2. Membrane contactors have also been applied to remove H2S and SO2 from different gas streams (Li et al. 2000) [5]. 

Membrane permeation is particularly applicable to the removal of carbon dioxide fiom high-pressure gas. The process is based on the use of relatively small modules, and an increase in plant capacity is accomplished by simply using proportionately more modules. As a result, tbe process does not realize the economies of scale and becomes less competitive with absorption processes as tiie plant size is increased. McKee et al. (1991) compared diethanolamine (DEA) and membrane processes for a 1,(XX) psia gas-treating plant. For their base case, the amine plant was found to be generally mote economical for plant sizes greater than about 20 MMscfd. However, at very higb acid-gas concentrations (ova- about 15% carbon dioxide), a hybrid process proved to be more economical than either type alone. The hybrid process, which is not indicated in Table 1-2, uses the membrane process for bulk removal of carbon dioxide and the amine process for final cleanup. Membrane processes are described in Chapter IS. 

Several studies have been carried out since the adoption of membranes by natural gas operators, aiming at comparing membrane-based carbon dioxide removal operations with conventional absorption operations (Table 6.2). 

For high natural gas flows combined with high carbon dioxide content an association of membranes and absorption seems to be the most profitable option. Membrane modules, located before the absorption column, carry out a bulk removal of carbon dioxide, while the absorption columns allow the final polishing of the natural gas. This configuration leads to significant investment cost reductions on the absorption unit. 

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