**2. Outline of C3+ hydrocarbon separation membrane techniques**

The processing of both associated gas (AG) and non-associated gas (NAG) from oil and natural gas wellheads into pipeline-quality dry natural gas can be quite complex. Generally, the Master Gas System (MGS) in the gas plants consists of three main units: (1) gas-oil separating plants (GOSPs), (2) gas plants and (3) fractionation plants as shown in **Figure 2** (top). A typical natural gas treatment and separation processing plant whose simplified schematic representation shown in **Figure 2** (bottom) consists five main processes to remove various impurities from raw natural gas. C3+ hydrocarbons are extracted as byproducts in the production of natural gas and oil, and natural gas processing is by far the most significant, contributing 90%+ production of C3+ hydrocarbons. There are three conventional technologies to separate C3+ hydrocarbons from natural gas: refrigeration, lean oil absorption and cryogenic [13]. These common processes are costly and energy intensive, which is reflected in the price and available capability of the finished gas (**Table 3**).

**Figure 2.**

*Top: block diagram showing key process steps in gas treatments and separation plants; bottom: simplified sketch of natural gas separation processes consisting five main processes.*


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

#### **Table 3.**

*Available gas processing technology for C3+ heavy hydrocarbons recovery [12].*

The refrigerated condensation process is a conventional way to separate C3+ hydrocarbon components from the gas stream through a sequence of conventional distillation columns operating down to 40°C [33]. The refrigeration process is pricey and energy intensive but can extract a large percentage of C3H8 and most of the C4+ gases. There was no major improvement of heavy hydrocarbon recovery process since 1910s until the lean oil absorption process was developed.

Lean oil absorption plants were the type of processing plant built in the late of 1960s. These plants were the next evolution from the refrigeration plants and can extract 90%+ of the C3+ in the gas stream and about 30% of the ethane by bubbling the gas through a chilled absorption oil operating at approximately 0°C. The fuel consumption of this type of plant is higher than that of the refrigeration plant [34].

Cryogenic plants became prevalent in the 1970s with the development of Turboexpander technology with great economic advantages for natural gas liquid (NGL) recovery [35]. The 1st generation of this technology could extract 70%+ of ethane (C2) from the gas. Today, 99% extraction of ethane can be recovered with modified cryogenic process due to the increased pressure reduction involved in the process [34]. Highly energy intensive for regeneration, tendency for block of process of equipment and the use of flammable cryogenic fluids are one the main disadvantages of cryogenic separation.

Membrane-based separation technology can be competitive in the processing of hydrocarbon recovery from natural gas. Inorganic membranes, such as MFI-type zeolite [36–38] and MOF [39], can be appealing due to their unique properties with well-define pore structure and high chemical and mechanical stabilities. These MFI membranes exhibit high C3+/CH4 selectivity, but rather low overall permeance, high capital cost and difficulty of scaling up, hence has hardly found industrial usage. Polymeric membranes have been used for the separation and removal of C3+ hydrocarbons from natural gas, usually with moderate selectivity [14]. The recovery of C3+ hydrocarbons is currently the second biggest market for membranes in natural gas processing, after acidic gas separation [11]. Compared to conventional separation methods, membrane-based separation technologies entail low capital costs, high energy efficiency and constitute a reliable option for separating hydrocarbon mixtures.

Glassy polymeric membrane-based separation process is a separation process shown in **Figure 3A** where gas mixtures consisting of two or more components are separated by a membrane into a "C3+ enriched" retentate stream and a "C3+ lean" permeate stream. The glassy polymer membrane separates gas mixtures by providing a permeable barrier that allows compounds to move through at specific rates, it separates gas mixtures principally by size or diffusivity. On the other hand, the feed gas mixtures are separated by a rubbery polymer membrane into a "C3+ lean" retentate stream and a "C3+ enriched" permeate stream (**Figure 3B**). The permeation of gas molecules across the rubbery membrane depends mainly on their solubilities or condensabilities. For example, Membrane Technology and Research (MTR) developed VaporSep® process using rubbery polymer composite

**Figure 3.**

*Schematic of (A) glassy and (B) rubbery polymer membrane-based separation process from natural gas.*

#### **Figure 4.**

*Flow scheme of a membrane C3+ hydrocarbon recovery by means of dew point control unit (MTR/polysiloxane membranes/spiral-wound modules) [13].*

membranes to treat high pressure hydrocarbon-rich feed gas mixtures (**Figure 4**) [11, 13]. C3+ hydrocarbons and the BTEX aromatics all permeate preferentially and are recompressed and cooled by a fan-cooled heat exchanger to condense higher hydrocarbons, while CH4 is then recirculated to the feed. The size and cost of the compressor system is often larger than membrane unit, so the selection of higher selectivity of novel rubbery membrane in this processing application can reduce capital expenditures (CAPEX) and operating expenses (OPEX) to the gas processing business.

Commercially available rubbery polymer membranes have been applied to enhance natural gas liquid (NGL) production under C3+ rich gas feed stream [24]. Following example illustrates how nitrogen rejective (hydrocarbon selective) rubbery membranes can be utilized for enhancement of the C2+ hydrocarbons recovery via a NGL process in a gas plant. The NGL recovery process is cascadedrefrigeration process with and/or without membrane units, as shown in **Figure 5**. The gas plant is processing both AG stream (e.g. 450 psi) and NAG stream (e.g. 800 psi), to produce sales gas (SC) and C2+ NGL products.

The core of the NGL process is the Liquid Recovery Unit (LRU), which is a cascaded refrigeration process. **Figure 6** details further the cascaded-refrigeration unit. Dry natural gas feed is cooled down to 68°C (90°F) in three cooling stages; C3 refrigeration loop is used in the first and second stages, while C2 refrigeration loop is used in the third stage.

As noted in **Figure 6**, the C2 refrigeration loop is cascaded in the C3 refrigeration loop, hence referred to as cascaded-refrigeration NGL recovery process. The residue
