*Polymer-Based Membranes for C3+ Hydrocarbon Removal from Natural Gas DOI: http://dx.doi.org/10.5772/intechopen.103903*

**Table 8.**

*C3+hydrocarbon properties in rubbery polymeric membranes.*

#### **Figure 14.**

*Chemical structures of PDMS-based siloxane polymers with different side and main chains.*

More recently, Grushevenko et al*.* [56] synthesized siloxane rubbery membranes by using an *in-situ* technique with the introduction of side alkyl groups followed by polymer chains crosslinking in the same reaction mixture. Results showed that POMS membrane demonstrated higher C4H10/CH4 ideal selectivity (*α* = 25) than that of PDMS membrane (*α* = 14). The gas permeation properties of polyhexylmethylsiloxane (PHexMS) and polydecylmethylsiloxane (PDecMS) were further studied. With the increase of substituent length from C8 to C10, C4H10 permeability (6480 Barrer) of PDecMS membranes decreased but C4H10/CH4 selectivity increased to 27 due to reducing diffusion coefficient. These results show that polyalkylmethylsiloxanes with longer side chains (e.g. PDecMS) are promising to be applied as materials for the purpose of C3+ extraction from natural gas (**Figure 16**).

In addition, a novel class of silicon-organic polymers, silmethylene rubbery polymers including more robust Si-C bonds, has been developed for separation of C3+ hydrocarbons from natural gas. Alentiev et al*.* [90] reported a thermally initiated polymerization technique to synthesize polydimethylsilmethylene (PDMSM) and polydimethylsiltrimethylene (PDMSTM) homo-polymers that include only more robust Si–C bonds (**Figure 17**). These silicon-containing rubbery polymers

#### **Figure 15.**

*Pressure behavior of the POMS under C4H10/CH4 (97/3 Mol%) binary gas mixture, where compression decreases flux with feed pressure methane, • butane, x selectivity [14].*

*Polymer-Based Membranes for C3+ Hydrocarbon Removal from Natural Gas DOI: http://dx.doi.org/10.5772/intechopen.103903*

**Figure 16.**

*Permeability (1-CH4, 2-C3H8, 3-C4H10, 4-CO2, 5-N2) and C4H10/CH4 selectivity of polyalkylmethylsiloxane membranes (feed pressure:0.8 bar, temperature: 25°C) [56].*

and its copolymers have relatively high *Tg* (92°C to 76°C) and excellent chemical stability (decomposition in air >240°C), compared to conventional PDMS. In addition, these novel rubbery polymers distinctly reveal solubility-controlled permeation behavior, but the permeabilities of C3+ hydrocarbons in PDMSM and PDMSTM are lower than those in PDMS.

A comprehensive survey of the recent developments on the synthesis, properties, and applications of silicon-containing copolymers was provided in the literature [106], including silicone-urea and silicone-urethane copolymers, silicone-ester copolymers, silicone-amide copolymers, silicone-imide copolymers etc. These silicon-containing copolymers which combine unique properties of PDMS in the polymer backbone have been evaluated as gas separation membranes, such as CO2/ N2 and CO2/CH4 separations [92, 107–110], but few reported C3+ hydrocarbon separation and removal from natural gas. Gomesa et al*.* [92] reported the synthesis of poly (ether siloxane urethane urea) membrane materials for C4H10/CH4 separation. Results show that the higher the content of soft siloxane segments, the higher are the permeabilities of C4H10 and CH4 due to higher chain mobility, while the C4H10/CH4 single gas selectivity (6–8) does not change so much as permeability (2– 320 Barrer).

A series of silicon-containing terpolymers with Si-O-Si and Si-(CH2)2- in polymer backbone and side chains were synthesized for enhanced C3+ hydrocarbon recovery from natural gas by Yang et al*.* [95]. The partially octyl substituted crosslinked rubbery siloxane membranes were prepared by crosslinking vinylmethylsiloxane terpolymer (Ter-PDMS) composed of (VinylMeSiO)p(Me2- SiO)m(OctMeSiO)n backbone chains via an addition curing process [25]. Improved

#### **Figure 17.**

*The scheme of synthesis of silmethylene rubbery polymers: polydimethylsilmethylene (PDMSM) and polydimethylsiltrimethylene (PDMSTM).*

gas permeation performances were observed for this modified siloxane Ter-PDMS membrane with C4H10/CH4 ideal selectivity of 49.1 (2 bar and 20°C), with lower C4H10 permeability (19,800 Barrer) compared to conventional PDMS membrane (33,800 Barrer C4H10 permeability). Such decrease in permeability compared to conventional PDMS was induced by a reduction of chain flexibility with partial substitution of functional groups in the side chains after crosslinking in membrane matrix. Moreover, this modified siloxane Ter-PDMS membrane also showed enhanced C3+ separation performance (143% increase in C4H10/CH4 mixed gas selectivity) under high pressure (up to 800 psi) 7-component gas mixtures compared to conventional PDMS membrane [25].
