Polymeric Membranes and Ionic Liquids for Natural Gas Sweetening
Although polymeric membranes provide numerous advantages over inorganic membranes, some significant properties make inorganic membranes (carbon and zeolite) more attractive for specific applications. Specifically, the polymeric membranes have the main advantages of low production cost and easily handling but, at the same time, provide poor chemical and thermal resistance. On the other hand, carbon membranes provide excellent chemical and thermal stability and transcend the trade-off between permeability and selectivity. However, they are characterized as brittle materials and present higher production costs .
According to Baudot  only three main polymeric membrane types have been tested at both a laboratory and a pilot scale for H2S removal from NG:
1. Membranes based on rubbery polymer selective layers
Here, selectivity is achieved based on the affinity difference between the components to be separated. The affinity is mainly defined by the difference in sorption coefficient. This kind of membrane generally presents high and very high permeabilities for low molecular weight components. The most common families of this type of polymeric membrane material are: (1) non-polar rubbery polymers , (2) nonpolar rubbery polymers impregnated with polar solvents [64, 65] and (3) polar rubbery polymers .
Each category takes advance of the different diffusion of H,S and CO, versus CH4 through the polymer matrix or the impregnated solvent.
2. Membranes with a selective layer made of block copolymers
Membrane Technology and Research (MTR Inc.) developed and supply commercial spiral-wound membranes with a selective layer made of numerous polyether block amides. This membrane is well known with the commercial trademark of Pebax®, owned by Arkema. According to Amo et al. , the associations of Pebax®-based membranes with a sulfatreat process led to significantly higher operation costs (from 20% to 40%) over the amine process alone for flow gases containing a few percent of H2S and lower than 140,000 Nm3/h, whereas a Pebax®- membrane/amine coupling was more valuable than a stand-alone amine process for gas flows containing more than 5% H,S and higher than 23,000 NmVh.
3. Membranes with a selective layer made of glassy polymers
This kind of polymeric membrane is mainly suggested for CO, removal applications included in the NG sweetening process. In this rigid type of polymer (polyimides, polysulfones, cellulose acetate etc.) the separation mainly takes place as a result of the difference in the kinetic diameter of the component. In the case of H,S/CH4 the selectivity is low because the kinetic diameters of those gases are rather similar .
The development of new polymeric membrane materials for testing in the NG sweetening process remains attractive both scientifically and economically. Owing to high levels of interest from academia and industry, many membranes for NG sweetening have been proposed in the recent literature. Table 6.2 presents some important results from the relevant literature concerning the use of polymeric membranes for acid gas removal from NG.
During recent years, attention has been given also to other types of candidate materials for acid gas removal from NG, including a large number of different ionic liquids (ILs) . ILs are salts in the liquid state. They are known as "solvents of the future" as well as "designer solvents." Some ILs with noteworthy properties concerning their ability to separate C02 and/or H2S from CH4 have recently been described. The reported ideal selectivity values from ILs such as Bmim-BF4, Emim-CH3S04, H0emim-N03, Pmg-L, 2mHEAPr, [C4mim][CH3S03] etc. are between 13 and 22 for CCX/CH4 and between 10 and 75 for H2S/CH4 [82-85].