**5. Design of C3+ hydrocarbon separation polymeric membrane materials**

The design and selection of polymeric membrane materials with high permeability and selectivity for gas separation is crucial. The following sections *Polymer-Based Membranes for C3+ Hydrocarbon Removal from Natural Gas DOI: http://dx.doi.org/10.5772/intechopen.103903*

describes the potential research and development activities in the literature of designing C3+ hydrocarbon separation membrane materials. There are two types of polymeric membrane materials reported, which allow to selectively separate and remove С3+ hydrocarbons from natural gas: (1) reverse-selective glassy polymers with high free volume [58], including polyacetylene-based polymers, polymers of intrinsic microporosity (PIMs), polynorbornene-based polymers; (2) rubbery polymers, including PDMS, POMS, modified rubbery siloxanes, polyurethanes, and poly(ether-*b*-amide) copolymers. Summary of C3+ hydrocarbon permeation properties for these selected high free volume glassy polymers, rubbery polymers and their MMMs under pure and mixed gases can be found in **Tables 7**–**9**.

## **5.1 Reverse-selective glassy polymers**

#### *5.1.1 Polyacetylene-based polymers*

Exceptional gas transport properties of polyacetylene-based polymers have led to considerable interest for membrane-based gas separation applications (**Figure 10**). These highly rigid, amorphous glassy polymers exhibit high *Tg* (>200°C) and high free volumes, providing ultrahigh permeabilities for C3+ hydrocarbons and attractive selectivities of C3+/CH4 (**Table 7**).

Poly(1-trimethylsilyl-1-propyne) (PTMSP) was synthesized for the first time in 1983 [96]. Unlike conventional glassy polymers, PTMSP is the most permeable glassy polymer known for C3+ hydrocarbon separation and removal with a pure gas C4H10 permeability >160,000 Barrer, but a low pure gas C4H10/CH4 selectivity (<6) (**Table 7**). However, C4H10/CH4 selectivity was found to be higher under mixed gas permeation testing, indicating PTMSP may be an alternative to siloxane rubbery polymers for C3+ hydrocarbon separation. For example, Pinnau et al*.* [59] reported that mixed gas selectivities of C4H10 over permanent gases (CH4 and H2) are as high as 27 for C4H10/CH4 and 39 for C4H10/ H2 because CH4 and H2 permeabilities in gas mixtures containing 2 vol% C4H10 are much lower than the pure CH4 and H2 permeabilities. Raharjo et al*.* [63] reported the mixed gas C4H10/CH4 selectivity was >8 times higher than that of pure gas measurement, ultrahigh mixed gas C4H10 permeability (127,000 Barrer) and C4H10/CH4 mixed gas selectivity (>50) were obtained under binary gas mixtures containing 6 vol% C4H10. Similar trends were observed for other polyacetylene-based polymers, such as poly(4-methyl-2-pentyne) (PMP) [64], poly(1-trimethylgermyl-1-propyne) (PTMGP) [64] and poly[1-phenyl-2- (p-trimethylsilylphenyl) acetylene] (PTMSDPA) [66]. Unlike branched polyacetylenes (i.e. PTMSP and PMP) with bulky side substitutes and higher fractional free volume (FFV = 0.28–0.29), linear-based poly(2-alkylacetylenes) with longer alkyl side chains, i.e., poly(2-hexyne), poly(2-octyne), poly(2 nonyne), poly(2-decyne), and poly- (2-undecyne), have relatively lower fractional free volume (FFV = 0.19–0.22) and permeabilities due to increased polymer chain mobility and interchain interactions [49, 67], and these linear poly(2-alkylacetylenes) membranes exhibit rather inferior performance for C4H10/CH4 mixed gas separation (**Figure 11**) under mixed gas testing conditions. However, the use of PTMSP as a potential membrane polymer for C3+ hydrocarbon recovery is not feasible due to fast physical aging (nonequilibrium excess free-volume relaxation of PTMSP chains) and solubility (e.g. interactions of PTMSP chains with many compounds), which can result in potential membrane destruction in the gas process streams.


## *Natural Gas - New Perspectives and Future Developments*

