## 2 The Technologies
### Membranes and Membrane Contactors to Remove Hazardous H\({}_{2}\)S from Gas Streams  
The membranes as a separation technology are widely used in a variety of industries, and this wide usefulness is attributable to the properties or characteristics of the technology: modular aspects, easiness of operation, low environmental impact, etc., that in many aspects surpassed the offer presented by other separation technologies, this being specially noted when the operation, i.e., H\({}_{2}\)S removal, is accomplished in areas or locations where communications are not easy [47]. There are some recent publications reviewing the use of these membrane technologies on the treatment of gases [48; 49; 50; 51], but surprisingly, they are dedicated to industrial gases instead of H\({}_{2}\)S; some of the gases mentioned in these reviews are: nitrogen, hydrogen, oxygen, CO\({}_{2}\), etc. In the case of H\({}_{2}\)S, polymeric membranes are the candidates to resolve this important industrial issue. The permeation of gas across these types of membranes is ruled by diffusion, and a series of investigations [52; 53] have mentioned that plasticization of the membrane performs well in the removal of H\({}_{2}\)S when the gas in the stream is present at high concentrations; moreover, this plasticization improves the selective separation of H\({}_{2}\)S over methane. This selective separation is attributed to the transport mechanism, which in the case of H\({}_{2}\)S is of sorptive in nature, and this separation is negatively influenced by diffusion. The transport ruled by solubility is particularly important in the case of rubbery polymers, and this is a key rule in the separation of H\({}_{2}\)S from methane. It is reported [54] that the variation of the cross-linking density tuned the usefulness of polyethylene glycol-based membranes. This change in the membrane properties significantly improves the selectivity in the pairs H\({}_{2}\)S/methane and CO\({}_{2}\)/methane. Table 3 summarizes some of the recent literature about the use of membranes on H\({}_{2}\)S removal. Computational methods have developed a series of tools to improve the knowledge and optimize the use of these membrane technologies in the capture of gases [57]. Another implication of membrane technologies on the removal of gases is their use to act as an interface between the different streams, liquid and gases, feeding the removal process. These membranes, preferably with high porosity, increased the gas flow through the membrane structure. In practice, hollow fiber modules are the most used devices; under operation, the gas flowing in the shell side diffuses across the membrane's pores to the tube side; here, the gas is conveniently absorbed by a solution, whereas these modules can be operated in co-current and counter-current form, and the latter is usually preferred since it gives a better contact between both operating phases. This counter-current configuration is shown in Figure 1.  
In these membrane modules, the gas flux is controlled by its concentration gradient [58]. Mass transfer in the module is conveniently represented in Figure 2. Some parameters to be controlled in these hollow fiber operations are pore size and its distribution [59], and these assured the best transfer across the module fibers. Some improvements in the technology are described in the literature [60].