**4. Research and industry needs**

The separation and recovery of C3+ hydrocarbons from natural gas using polymeric membrane-based technology have gained increasing attentions in chemical, petrochemical and oil and gas companies compared to conventional separation technologies. The choice of polymeric membrane materials is crucial for separation, and the key membrane performance variables are selectivity, permeability, and long-term durability. Research regarding the development of polymer materials and fabrication processes has been reported in the literature mostly with single or binary

#### **Figure 7.**

*The relative size (kinetic diameter,* dk*, Å) and condensability (critical temperature,* Tc*, K) of the principal components of natural gas.*

gas mixtures short duration gas permeation data; however, their industrial implementation is often hindered due to effects of multicomponent natural gas mixture permeation behavior on membrane physical aging**.**

Glassy polymer membranes have excellent scalability and continuously increasing being considered for more challenging hydrocarbon separations. The use of glassy polymer membranes with stiff chains, such as polyphenylene oxide (PPO), ethyl cellulose (EC), cellulose acetate (CA), polysulfone (PSF), aromatic polyimides, polymers of intrinsic microporosity (PIMs), disubstituted polyacetylenes and Sidistributed polynorbornenes etc., for separation of olefins and paraffins as well as C2- C4, aromatic, alicyclic and aliphatic hydrocarbons have been described in previous reviews [23, 49]. Unique challenges exist for these glassy polymer membranes for C3+ hydrocarbon separation and removal, most notably selectivity loss and some cases permeability loss due to physical aging and loss of separation efficiency in ultrathin membranes due to faster physical aging. For example, aging-induced permeability loss in glassy polymers is expected to be significant, especially for those condensable gases with larger molecular sizes (**Figure 8**) [49]. The membrane with ultrathin selective-layers could be the solution to provide economically higher hydrocarbon permeance, however, forming large-scale and defect-free membranes becomes increasing challenging in actual membrane fabrication process. In this regard, a highflux and a good selectivity for gas separation processes are both required for a reasonable plant size and energy demand [7].

Further, other challenges that these polymer membrane materials need to withstand their C3+ hydrocarbon separation performances under natural gas feed streams and testing conditions, including a challenging high feed pressure (800+ psi), C3+ rich multicomponent hydrocarbon mixtures along with minor impurities such as CO2, N2, a trace amount of BTEX. PDMS based rubbery siloxane membranes are often used for separating C3+ hydrocarbons from natural gas. Although, many studies have investigated the permeation properties of PDMS membranes under pure gas, binary or ternary gas mixtures in the literature [44, 50–56], it is important to evaluate them under industrially relevant feed streams and varying operating conditions. Yang et al*.* [25]. investigated the permeation properties of conventional PDMS and modified siloxane terpolymer (Ter-PDMS) membranes

