**1. Introduction**

Natural gas is one of the important and primary sources of global energy. In addition to its primary importance as a fuel, natural gas is also a raw material and source of hydrocarbons for petrochemical feed stocks. According to 2021 BP statistics, there were an estimated 6,642 trillion cubic feet (Tcf) of total world proved reserves of natural gas [1]. The worldwide natural gas production and consumption have been rising over the past 20 years. In 2020, natural gas production and consumption worldwide amounted to roughly 3.85 and 3.82 trillion cubic meters [1].

Raw natural gas contains primarily of methane (CH4) as the prevailing element but also comprises significant amounts of impurities such as nitrogen (N2), helium (He), acid gases (carbon dioxide (CO2) and hydrogen sulfide (H2S)), heavy hydrocarbons (C3+), mercaptans, water vapor, BTEX (benzene, toluene, ethylbenzene and xylene) etc. These impurities must be removed to meet the pipeline quality

**Figure 1.** *World natural gas final consumption by sector in 2019 [2].*

standard specifications for transport and processing and avoid pipelines corrosion. The operational natural gas plant delivers pipeline-quality dry natural gas that can be used as fuel by residential, commercial, and industrial consumers, or as a feed stocks for downstream chemical synthesis (**Figure 1**) [2].

Membrane-based separation technology has been pointed out as a key technology in the chemical industry [3–5]. It has benefits for small-to-medium scale separation where it exhibits higher energy efficiency, simplicity in operation, compact process design compared to other separation technologies. The global market for gas separation membrane estimated at US\$822.1 MM in the year 2020 and is projected to reach a revised size of US\$1.1 Billion by 2027, growing at the compound annual growth rate (CAGR) of 5.6% over the analysis period 2020–2027 [6]. Polymeric membrane-based separation technology for natural gas processing was first commercialized in the 1980s [7], and today it is widely used in variety of gas separation applications. Representative gas pairs needing separation in these applications is shown in **Table 1** [8–10], some of the leading industrial membrane producers and their principal glassy and/or rubbery membrane materials used in gas separation is listed in **Table 2** [7, 11–15].


#### **Table 1.**

*Primary current industrial gas separation for polymer membranes [8–10].*


#### **Table 2.**

*Principal polymer membrane materials, modules, and producers [7, 11–15].*

Separation of C3+ hydrocarbons (e.g. propane (C3H8), butane (C4H10)) and their removal from natural gas not only is necessary to prevent condensation during transportation by reducing the dew point and heating value to pipeline specifications, but also it is economically attractive to recover C3+ hydrocarbons since they are often of greater value when used as chemical feedstocks, or as a liquid fuel for power generation. In general, both glassy and rubbery polymers are used for this application. Glassy polymeric membranes are in general diffusivity selective and traditional glassy polymer membrane based upon cellulose acetate, polyimide, Tetra Bromo polycarbonate, polysulfone have been widely utilized for a few decades for CO2 removal from natural gas. Only few glassy polymers (e.g. perfluoro-based polymers) have demonstrated remarkable abilities to separate hydrocarbons from natural gas but their application has been limited due to highly aging and plasticization [16–21]. On the other hand, rubbery polymeric membranes are solubility selective and are mainly used in gas/vapor separation processes for separating hydrocarbons from their mixtures based on gas condensability. Commercially, poly (dimethylsiloxane) (PDMS) based rubbery siloxane membranes have been applied to separate C3H8 and C4H10 from CH4 [14, 22–25] and other gas pairs (e.g. O2/N2, CO2/CH4, H2/N2, He/N2, He/H2, CO2/N2, N2/CH4, H2/CO2, He/CO2) [26–32] from natural gas; however, development of more selective, higher-flux and cost-effective rubbery membrane materials can further help improve economics of the C3+ hydrocarbon recovery.

In this chapter, the application of synthetic polymeric membranes for C3+ hydrocarbon separation and removal (C3+/CH4) from natural gas is reviewed. The review covers available glassy and rubbery polymer membrane materials, as well as its hybrid mixed matrix membranes. Their transport properties (permeability and selectivity) as well as the effect of testing conditions and feed compositions on the membrane separation performance are reviewed.
