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Natural gas can contain significant amounts of impurifies, including CO2, H2S, N2, He, and C3+ hydrocarbons. These C3+ hydrocarbons are valuable chemical feedstocks and can be used as a liquid fuel for power generation. Membrane-based separation technologies have recently emerged as an economically favorable alternative due to reduced capital and operating cost. Polymeric membranes for the separation and removal of C3+ hydrocarbons from natural gas have been practiced in chemical and petrochemical industries. Therefore, these industries can benefit from membranes with improved C3+ hydrocarbon separation. This chapter overviews the different gas processing technologies for C3+ hydrocarbon separation and recovery from natural gas, highlighting the advantages, research and industrial needs, and challenges in developing highly efficient polymer-based membranes. More specifically, this chapter summarizes the removal of C3H8 and C4H10 from CH4 by prospective polymer architectures based on reverse-selective glassy polymers, rubbery polymers, and its hybrid mixed matrix membranes. In addition, the effect of testing conditions and gas compositions on the membrane permeation properties (permeability and selectivity) is reviewed.
Aramco Services Company: Aramco Research Center—Boston, USA
Milind M. Vaidya
Research and Development Center, Saudi Aramco, Saudi Arabia
Sebastien A. Duval
Research and Development Center, Saudi Aramco, Saudi Arabia
Feras Hamad
Research and Development Center, Saudi Aramco, Saudi Arabia
*Address all correspondence to: john.yang@aramcoamericas.com
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 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].
Figure 1.
World natural gas final consumption by sector in 2019 [2].
Membrane-based separation technology has been pointed out as a key technology in the chemical industry [3, 4, 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, 9, 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, 12, 13, 14, 15].
Application
Common gas separation
Producer
• Nitrogen generation/oxygen enrichment
• N2/O2
• Air Products, Praxair, Air Liquide, MTR
• Hydrogen recovery
• H2/CH4
• Air Products, Ube, Air Liquide, Praxair, MTR
• Ammonia purge gas
• H2/N2
• Air Products, Ube, Air Liquide, Praxair, MTR
• Syngas ratio adjustment
• H2/CO
• Air Products, Ube, Air Liquide, Praxair, MTR
• Acidic gas separation
• CO2/CH4
• NATCO, Air Products, UOP, Uber, Air Liquide, MTR
• Natural gas dehydration
• H2O/CH4
• Kvaerner, Air Products, MTR
• Sour gas treatment
• H2S/CH4
• NATCO, Air Products, Uber, Kvaerner, MTR
• Helium separation
• He/CH4
• Air Products, Air Liquide, MTR
• Helium recovery
• He/N2
• Air Products, Air Liquide, MTR
• Heavy hydrocarbon recovery
• C3+/CH4
• MTR, ABB
• Air dehydration
• H2O/Air
• Air Products, Parker Balston, Praxair, Ube, MTR
Table 1.
Primary current industrial gas separation for polymer membranes [8, 9, 10].
Polymer type
Typical polymer used
Membrane module type
Company
Glassy
Polysulfone
Hollow fiber
Air Products
Polyimide/polyaramid
Hollow fiber
Air Liquide
Polyimide
Hollow fiber
Ube
Polyimide
Hollow fiber
Parker-Hannifin
Polyimide
Hollow fiber
Evonik
Polyimide
Hollow fiber
Praxair
Polyimide/polysulfone
Hollow fiber
Grasys
Cellulose acetate
Spiral wound
UOP, Kvaerner
Cellulose acetate
Spiral wound
W. R. Grace
Cellulose acetate
Spiral wound
MTR
Cellulose acetate
Spiral wound
Fuji Film
Cellulose acetate
Hollow fiber
Schlumberger (Natco)
Ethyl cellulose
Hollow fiber
Air Liquide
Poly (phenylene oxide)
Hollow fiber
Aquila
Perfluoro polymer
Spiral wound
ABB/MTR
Tetrabromo polycarbonate
Hollow fiber
Generon (MG)
Rubbery
Poly(ether-b-amide) copolymer
Spiral wound
MTR
Poly(ether-b-amide) copolymer
Plate and frame
MTR
Poly(ether-b-amide) copolymer
Hollow fiber
Air Liquide
Polysiloxane
Plate and frame
GKSS
Polysiloxane
Spiral wound
MTR
Polysiloxane
Hollow fiber
Air Liquide
Table 2.
Principal polymer membrane materials, modules, and producers [7, 11, 12, 13, 14, 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, 17, 18, 19, 20, 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, 23, 24, 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, 27, 28, 29, 30, 31, 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.
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.
Processing technology
Technology Vintage
Available capacity
C3+ hydrocarbons extraction
Fuel consumption
Refrigeration
1940–1960
10%
large C3, most of C4
Least
Lean Oil Absorption
1960–1975
20%
90%+ of C3 and 30% C2
Higher
Cryogenic
1975 on
50%
70–99% C2
Higher
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 Turbo-expander 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, 37, 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 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.
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].
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 cascaded-refrigeration 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.
Figure 5.
Schematic of the NGL recovery process: (A) without membrane; (B) utilizing rubbery membrane to divert NAG stream to feed LRU.
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.
Figure 6.
Schematic of the cascaded-refrigeration process to recover C2+ NGL from natural gas.
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 gas leaving the third cooling stage is recycled back to exchange heat with gas feeding each stage and leaving the first stage at around 450 psi (referred to as high pressure (HP) residue gas). The liquids collected in the three stages are fed to the demethanizer, which is operating at lower pressure. The C2+ NGL product is obtained as the bottom product, while the top gas product from the demethanizer (low pressure (LP) residue gas) is consolidated with HP residue stream and sent to SG network. This LRU process is ideally designed to process AG stream, which is C2+ rich and free of or very lean in nitrogen. However, under certain situations of higher demand for C2+ NGL, the LRU is designed or operated with feed that is composed partially of NAG. As noted in Table 4, the N2 content of NAG stream is higher, and the C2+ content is considerably lower, than in AG stream.
Flow (MMSCFD)
Pressure (psi)
%N2
%C1
%C2
%C3
%C4
%C5+
AG
500
480
0.5
65.5
17.9
10.9
3.98
1.25
NAG
400
800
14
80.2
3.56
1.28
0.67
0.25
Table 4.
Conditions of AG and NAG streams.
Table 5 compares the performance of the NGL process when 140 MMSCFD of the NAG stream is diverted to mix with the 500 MMSCFD AG stream. In reference (base case), Table 5 provides the performance of the NGL process when only AG stream is fed to the LRU. When portion of NAG is let down directly to mix with AG, the condensability of the feed stream to LRU is reduced, as indicated by the increase in N2 content and reduction in the C2+ content. Nevertheless, the C3+ NGL liquids production has increased as a direct result of the increase in feed to LRU, while liquid C2 production has decreased in almost similar rate rendering the cascaded-refrigeration NGL recovery process not sensitive to the increase in the flow of C2+ to LRU. On the other hand, diverting part of the NAG selectively through membrane has resulted in increasing the condensability of the feed to LRU. As a result of enhancing the condensability and flow of the feed to LRU, the NGL production has increased, especially valuable C3+ productivity. The impact of increasing the flow to LRU, as noted in Table 5, is to reduce the heating value of the sales gas. This is mainly the result of separating more condensable valuable C3+ hydrocarbons from the sales gas stream. This impact is exacerbated when the diversion of NAG is conducted selectively through the membrane, therefore increasing the shrinkage in produced sales gas.
Polymeric membranes are generally non-porous and gas permeation through dense polymer membranes is typically described by the solution-diffusion model. Gas permeation occurs by sorption of gas on the feed side of membranes, then molecular diffusion through the membrane matrix, and evaporation of the gas from permeate side of membrane surface. Based on the solution-diffusion model, the membrane permeability, Pi (Barrer), is defined as the product of diffusivity coefficient and solubility coefficient (given by Eq. (1)) at high fugacity differences across the membrane [40]:
Pi=Di×SiE1
where Pi is the permeability coefficient measured in Barrer (1 Barrer = 1 × 10−10 cm3 (STP) cm/cm2 s cmHg), Si is the solubility coefficient (cm3 gas/cm3 polymer cmHg), and Di is the diffusion coefficient of the penetrant (cm2/s). The solubility coefficient is a thermodynamic parameter and is mainly influenced by the condensability of the penetrant gases [41], and it is inversely proportional to gas boiling point or critical temperature [42]. In contrast to gas sorption, the gas diffusion coefficients vary widely depending on the polymer materials, it is a kinetic parameter that measures the overall mobility of the penetrant molecules in the membrane, depending on various factors such as the size and shape of the gas molecules, the cohesive energy density of the polymer, the mobility of the polymer chains and the free volume size and distribution of the polymer [43]. Accordingly, the membrane selectivity α, the best measure of a membrane’s ability to separate gases i and j is defined as the ratio of the permeabilities of penetrant, Pi and Pj (given by Eq. (2)) [44].
αij=Pi/Pj=Di/Dj×Si/SjE2
As shown by Eq. (2), membrane selectivity (also known as permselectivity) can be separated into diffusivity selectivity, Di/Dj, taken as the ratio of diffusivity coefficients of the two gases and solubility selectivity, Si/Sj, taken as the ratio of solubility coefficients. Diffusivity selectivity depends primarily on the size-sieving ability of the membrane, the permeation of the smaller molecule is faster as compared to their larger counterparts. While solubility selectivity is largely driven by gas condensability and affinity with the membrane.
In general, glassy polymeric membranes generally tend to permeate smaller molecules, the diffusion coefficients decrease as the molecular size increases; whereas rubbery polymeric membranes permeate more condensable gases, the sorption coefficients generally increase as the condensability increases. Such trends are schematically illustrated in Figure 7 for glassy and for rubbery polymeric membranes, which shows the gas permeabilities for principal gas components in natural gas with different molecular size (kinetic diameter, dk, Å) [13] and condensability (critical temperature, Tc, K) [45]. The molecular sizes and relative condensabilities of C3H8 and C4H10, relative to CH4, are highlighted in Figure 7. CH4 can be separated from C3+ hydrocarbons by glassy polymers by size-selectivity; however, interaction of heavier hydrocarbons with the polymers chains tends to increase the flux resulting in decrement in C3+/CH4 selectivity in case of rich hydrocarbon natural gas. On the other hand, rubbery polymers (sorption selectivity membranes) are used to separate C3+ hydrocarbons from CH4, because of their condensabilities. For example, the selectivity of hydrocarbon vapors in PDMS membranes is dominated by the sorption component, the C3H8/CH4 and C3H8/N2 sorption selectivities (Ki/KCH4 and Ki/K) are 50-fold and 370-fold greater than that of their diffusion selectivities (Di/DCH4 and Di/DN2) (Table 6) [16, 46, 47, 48].
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 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 Si-distributed 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 high-flux and a good selectivity for gas separation processes are both required for a reasonable plant size and energy demand [7].
Figure 8.
Aging-induced permeance drop of penetrants in polyimide (6FDA-BPDA-DDBT) membrane aged for 18 months [49].
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, 51, 52, 53, 54, 55, 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 under simulated typical field gas streams. Results shows that C4H10 vol% had significant influence on membrane separation performances due to the C4H10-induced swelling of PDMS based rubbery siloxane membranes. Similarly, Mushardt et al. [57] reported the permeation properties of poly(octylmethylsiloxane) (POMS)/active carbon (AC) mixed matrix membranes (MMMs) under binary and multi-component gas mixtures (Figure 9). The best separation performance was achieved under multicomponent gas mixture at highest average fugacity of C4H10 caused by increasing feed pressure and high concentration of C4H10 in feed gas mixture. These outcomes carry the message that membrane separation performance is strongly related to feed gas compositions and testing conditions. Thus, to fully exploit the use of membranes for actual C3+ hydrocarbons separation and removal, not only higher-flux and cost-effective new membranes are needed, but also it is worthy to pay more attention to the influence of gas compositions and testing conditions.
Figure 9.
Comparison of (a) C4H10 permeance and (b) C4H10/CH4 selectivity of POMS/AC MMMs in multi-component gas mixture (2 vol% C4H10) and C4H10-CH4 binary mixtures (1, 2 or 5 vol% C4H10) at 20°C [57].
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 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.
C3+ hydrocarbon properties in mixed matrix membranes.
mN3/(m2. h. bar).
Multicomponent gas mixture: 1% C5H12/2% C4H10/6% C3H8/10% C2H6/79% CH4/2% CO2.
Multicomponent gas mixture: 1.5%C4H10/3% C3H8/5% C2H6/66.5%% CH4/12% CO2/12% N2.
Multicomponent gas mixture: 0.5%C4H10/2% C3H8/5% C2H6/77.9%% CH4/14% N2/500ppm BTEX.
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).
Figure 10.
Chemical structures of selected branched polyacetylene-based polymers (i.e. PTMSP, PMP, PTMGP, PTMSPA) and linear poly(2-alkylacetylenes) (i.e. i.e., poly(2-hexyne), poly(2-octyne), poly(2-nonyne), poly(2-decyne), and poly-(2-undecyne)).
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.
Figure 11.
Mixed gas C4H10 permeability and C4H10/CH4 selectivity for selected branched polyacetylene (i.e. PTMSP, PMP, PTMGP and PTMSDPA) and linear poly(2-alkylacetylenes) (i.e. poly(2-hexyne), poly(2-octyne), poly(2-nonyne), poly(2-decyne), and poly-(2-undecyne)) membranes.
5.1.2 PIM-1
PIMs are spirobisindane-based rigid, glassy polymers with high fractional free volume (FFV = 0.26), excellent chemical stability and good solvent resistance. PIM-1 is the most studied PIMs for gas separation, and it shows high CO2 permeability with moderate CO2/CH4 selectivity, which is exceeding the upper bounds [97]. A new perspective on the role of PIMs and its functionalized ladder PIMs (e.g. PIM-polyimide copolymers) in key energy-intensive membrane-based gas separations, including O2/N2, H2/N2, H2/CH4, CO2/CH4, H2S/CH4, C2H4/C2H6, and C3H6/C3H8, was described in detail in a recent review paper by Wang et al. [98]. Similar as PTMSP, PIM-1 is also particularly permeable to hydrocarbon vapors (Table 7). Thomas et al. [16, 17] investigated the pure- and mixed-gas permeation properties of PIM-1, it exhibits C4H10/CH4 and C4H10/H2 mixed gas selectivities up to 25 and 27, respectively, under a mixture of 2 vol% C4H10 in CH4. Since PIM-1 has better solvent resistance than PTMSP, it could find applications as an advanced membrane material for the separation of organic vapor/permanent gas mixtures [17], and further membrane performance evaluation under industrially relevant feed streams and testing conditions will be the topic of future studies in the research groups.
Polynorbornene-based polymers
Another promising reverse-selective glassy polymer class for C3+ hydrocarbon separation and removal are substituted polynorbornenes (PNBs). Many valuable correlations between gas-permeability and polynorbornene structure have been summarized in prior reviews [12, 19, 71]. More recently, the development in the field of next generation polynorbornene-based polymers, their limitations and challenges for targeted gas separations was discussed also in a recent review [99]. These studies show that solubility-controlled permeation of gaseous hydrocarbons (C1 − C4) is characteristic for both addition polymerization-type polynorbornenes (APNBs) with Si-containing side-groups and ring opening metathesis polymerization type polynorbornenes (ROMP PNBs) with flexible Si-O bonds in side-groups (Figure 12). Summary of C3+ hydrocarbon permeation properties for APNBs and ROMP PNBs membranes under pure- and mixed-gas can be found in Table 7. In general, the membrane C4H10/CH4 selectivity increased significantly as increasing the length of side groups (e.g. Me, Et, n-Pr, n-Bu). Among these polynorbornene-based polymers, both distributed APNBs-Si(OPr)3 and ROMP-Si(OBu)3 membranes exhibited higher C4H10/CH4 pure gas selectivities of 48.8 and 32.4, respectively [76].
Figure 12.
Structures of metathesis and addition polynorbornenes bearing different substituents for gas separation applications.
Although many studies investigated the permeation properties using pure gases, fewer reported separation performance under multicomponent gas feed streams [21]. Sundell et al. [75] reported a route toward the production of exo ROMP and addition-type polynorbornenes and polytricyclononenes (APTCN) through the stereochemical control afforded by the reductive Mizoroki−Heck reaction. All addition-type and tricyclononene-based polymer membranes (e.g. APTCN-SiMe3 and APTCN-Si(OEt)3) show improved mixed gas C4H10/CH4 selectivities compared to APBNs-Si(OEt)3 and ROMP-Si(OEt)3 membranes (Figure 13) under the same testing conditions, due to the increased polymeric chain-spacing. These polynorbornene-based polymers demonstrate solubility-selective permeation with mixed gas selectivities that exceed commercially used PDMS.
Figure 13.
Mixed gas C4H10 permeability vs.C4H10/CH4 selectivity for APBNs-Si(OEt)3 (black squares), ROMP-Si(OEt)3 (red circles) and substituted-APTCN (blue triangles) under C3+ rich multicomponent gas mixtures containing 1.5–4 vol% C4H10 at 25°C and 800 psi.
Application of PDMS based rubbery siloxane membranes for selective removal of hydrocarbons and recovery of various organic components [3, 100, 101, 102] is considered. Summary of C3+ hydrocarbon permeation properties of PDMS, POMS and modified rubbery siloxane membranes in pure and mixed gases can be found in Table 8.
PDMS has low Tg (∼−123°C), flexible chains, and large free volume. The permeability, solubility, and diffusivity of hydrocarbons in PDMS membranes have been studied in previous literature [52, 83], and results show that feed gas compositions and testing conditions significantly affect membrane separation performance. The effect of chemical compositions on the permeation properties of PDMS membrane by introducing substituted groups in both side and main chains was compressively studied by Stern et al. [90, 103, 104, 105] and summarized in previous reviews [12, 23]. Results show that the substitution of functional groups in the side-groups of siloxane polymers has the same general effect on polymer permeability and selectivity as the substitution of such groups in the polymer main chains [104] (Figure 14). When the size of the side-groups (e.g. –CH3, –C2H5, –C3H7, –C8H17, − C10H11–, CH2CH2CF3, –C6H5, etc.) and main chains (e.g. –(CH2)2, −(CH2) 2, −(CH2)8, −m-C6H4, −p-C6H24) increased, the chains become less flexible resulting in the increases in Tg (from −123°C to −28°C) and the decreases in permeability, especially for those condensable gases with larger kinetic diameters (e.g. C3H8) [23].
Figure 14.
Chemical structures of PDMS-based siloxane polymers with different side and main chains.
Compared to PDMS with (Me2SiO)x backbone chain, membranes based on POMS with (OctMeSiO)x backbone chain exhibited enhanced C3+ hydrocarbon separation performance (C3+/CH4 selectivities) [14, 56, 57, 88, 89]. Schultz et al. [14] reported that POMS was one of two promising polymers for separation of C3+ hydrocarbons among the total 45 different polymers that were tested under C4H10/CH4 binary gas mixture (97/3 mol%). Since POMS is a soft material, it is susceptible to compression and thus the permeability goes down significantly with increase in the feed pressure as shown in Figure 15.
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].
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).
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].
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 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.
Figure 17.
The scheme of synthesis of silmethylene rubbery polymers: polydimethylsilmethylene (PDMSM) and polydimethylsiltrimethylene (PDMSTM).
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, 108, 109, 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(Me2SiO)m(OctMeSiO)n backbone chains via an addition curing process [25]. Improved 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].
5.2.2 Polyurethane rubbery polymers
Polyurethanes have found commercial applications using in a diversity of products in industry, from coatings and adhesives for automotive, shoe soles, mattresses, and foam insulation. Polyurethanes are rubbery polymers which are introduced for different gas pair separations vastly, such as CO2/N2, CO2/CH4, O2/N2. The solubility and diffusivity of gases indicate solubility domination of gas transport in these membranes. Varies researchers have shown that the permeability of polyurethane membranes increases, while the selectivity decreases with the decrease the hard segment content (urethane or urea), or the increase the soft segment (polyether or polyester) molecular weight [111, 112, 113, 114, 115].
Poly(urethane–urea) (PUU) membranes display moderate C3+/CH4 selectivity for removal of C3+ hydrocarbons from natural gas. Sadeghi and coworkers [91] evaluated the separation performance of a series of PUU membranes synthesized with different type of diisocyanates and polyols. It was found that PUU membranes with polypropylene glycol (PPG) polyol had the maximum C3H8 permeability of 200 Barrer and C3H8/CH4 selectivity of 5.5 because of its higher rubbery property and higher phase separation. In their recent paper [116], two series of PUU membranes containing aromatic and aliphatic side chains with different sizes were developed for separating of C2H6 and C3H8 from CH4. The longer aliphatic side chain based PUU provides the best hydrocarbon separation performance with C3H8 permeability of 186 Barrer and C3H8/CH4 idea selectivity of 6.5.
Pebax®—product of Arkema, is a group of thermoplastic elastomers, containing polyamide “hard” segment (e.g., nylon-6, nylon-12), and polyether “soft” segment (e.g., poly (ethylene oxide) [PEO], poly (tetramethylene oxide) [PTMEO]), which maintain the highly desirable combination of the toughness traditionally associated with polyamides and the flexibility/elasticity with polyether. Various Pebax® copolymers with different polyether/polyamide ratios were designed and synthesized to meet the requirements for different applications. Typical grades of commercial Pebax® 2533, 3533, 4011, 4033, 5533, 6633, 1074 and 1657 have been widely studied in literature for gas separation applications. For example, Pebax® 1657 containing 60 wt% polyether segment is a promising material showing good performance in CO2 separation from CH4 or N2 [117]. MTR has also demonstrated favorable economics for a hybrid separation process that utilized Pebax® 4011 in conjunction with an amine absorption polishing step for sour gas separation [118]. However, fewer reported C3+ hydrocarbon separation and removal from natural gas. Harrigan et al. [93] recently investigated the sour gas separation performance of five different grades of commercial Pebax® 2533, 5513, 3000, 1074 and 1657 under high pressure 6-component gas mixtures containing C2+ hydrocarbons. Results show that Pebax® 1074 and 1657 containing low of polyether segment (50–60 wt%) retained the highest CO2/CH4 selectivity (α = 16–19) but lower C3+/CH4 selectivities (C3H8/CH4 = 4.7–5.2; C4H10/CH4 = 11–12). Surprisingly Pebax® 2533 containing higher polyether segment (80 wt%) exhibited higher C3H8/CH4 selectivity of 6.7 and C4H10/CH4 selectivity of 17.6), which are greater than the C3H8/CH4 selectivity of 5.3 and C4H10/CH4 selectivity of 10.5 for commercially available PDMS membrane under the same testing conditions (800 psi, 25°C) [25]. However, Pebax® 2533 membrane shows 93% and 91% decrease in C3H8 and C4H10 permeabilities, respectively, compared with that of PDMS.
5.3 Mixed matrix membranes (MMMs)
MMMs have been explored to develop materials for enhanced separation performance of conventional membrane. Different types of inorganic fillers, such as silica, TiO2, fumed silica, graphene oxide (GO), activated carbon (AC), MOFs, zeolites, ZIFs, POSS etc., have been incorporated into PDMS, POMS, PTMST, and PU membrane matrix for air separation [119], CO2 removal and capture [120, 121], pervaporation [122], hydrogen separation [83, 123], and hydrocarbon separation [10, 65, 94, 124, 125, 126, 127, 128, 129]. The concept of using MMMs for light and heavy hydrocarbons separation and recovery in petrochemical and chemicals industries has attracted much interest and research in recent years. Several studies, such as olefin/paraffin separations (e.g. C2H4/C2H6, C3H6/C3H8, n-C4H8/i-C4H10), aromatic/aliphatic separations (e.g. toluene/n-hexane, p-xylene/o-xylene), vapor/gas separations (e.g. C3+/N2, C3+/H2 and C3+/CH4) were discussed in a recent review by Najari and coworkers [125]. Here, summary of C3+ hydrocarbon permeation properties of mixed matrix membranes (e.g. PTMSP, PTMGP, PDMS, POMS, PU and modified rubbery siloxane) in pure and mixed gases can be found in Table 9. Khanbabaei et al. [55] prepared PDMS-fumed silica MMMs for removing C4H10 from CH4. Results show the decreased C4H10/CH4 mixed gas selectivity for both neat membrane and MMMs. The MMMs exhibited 38% increment in C4H10 permeability and simultaneous 30% increase in C4H10/CH4 selectivity under C4H10/CH4 (3/97 mol%) binary gas mixture with adding >10 wt% of fumed silica, compared to neat PDMS membrane. Mushardt et al. [89] evaluated the separation of C3+ hydrocarbons of MMMs composed of POMS and 10–40 wt% AC from permanent gas streams under varying operating conditions (e.g. feed and permeate pressure, temperature, feed gas compositions). Best performance was achieved at highest average fugacity of C4H10 caused by increasing feed pressure, high permeate pressure and high concentration of C4H10 in feed mixture. At 40 bar, POMS/20 wt% AC MMMs exhibited higher C4H10/CH4 mixed gas selectivity of 33 and C4H10 permeance of 18 mN3/(m2.h.bar) under multicomponent gas mixtures including 2 vol% of C4H10. Gomes et al. [65] reported the preparation of PTMSP/silica MMMs with incorporation of 20–40 nm silica by sol–gel copolymerization of tetraethoxysilane (TEOS) with different organoalkoxysilanes. When methyltriethoxysilane (MTEOS) or n-octyltriethoxysilane (OTEOS) were used, the obtained MMMs exhibited higher separation performance (C4H10/CH4 mixed gas selectivities of 12.7 and 13.9 at 1 bar and 30°C, respectively).
In a recent investigation, Yang et al. [95] described a method to design and produce novel, crosslinked siloxane/POSS MMMs for enhanced C3+ hydrocarbon recovery from natural gas. Dual-functional POSS nanofiller (OS-POSS-VTMO) containing silicon hydride moiety (☰–Si–H) and trimethoxylsilicon groups (–Si(OMe)3) was synthesized and used as both a crosslinking agent and nanofiller (Figure 18) in modified rubbery siloxane (Ter-PDMS) membrane matrix. Under simulated typical field gas streams and testing conditions (e.g. feed pressure up to 800 psi for multicomponent gas mixture consisting C1-C5 hydrocarbons, CO2, N2 and BTEX). The produced novel, crosslinked Ter-PDMS/POSS MMMs exhibited better separation performance (e.g. 89% and 163% increase in C3H8/CH4 and C4H10/CH4 mixed gas selectivities) and enhanced swelling resistance, compared to conventional PDMS membrane. At 800 psi, Ter-PDMS/POSS MMM had a mixed gas C4H10 permeability of 4880 Barrer and C4H10/CH4 mixed gas selectivity of 18.7, respectively.
Figure 18.
Molecular structure of dual-functionalized POSS crosslinking agent (OS-POSS-VTMO) containing silicon hydride moiety (☰–Si–H) and trimethoxylsilicon groups –(Si(OMe)3).
Membrane-based separations have emerged as an economically favorable alternative due to its small footprint, reduced capital, and energetic cost. Membranes for C3+ hydrocarbon recovery from natural gas have been practiced in the chemical and petrochemical industries, the improved C3+ hydrocarbon separation and removal process from natural gas will be beneficial of these industries.
This chapter summarizes C3+ hydrocarbon separation and removal from natural gas using polymeric membrane-based technologies. More specifically, it addresses the removal of C3H8 and C4H10 from CH4 by perspective polymer architectures based on reverse-selective glassy polymers with stiff chains and rubbery polymers with flexible chains. At present PDMS, POMS and poly(ether-b-amide) rubbery polymer membranes are used by industries in the field for C3+/CH4 separation. However, improved mixed gas selectivity of C3+/C1 with high flux will have a positive impact of the economics of the C3+ hydrocarbon recovery from natural gas. In case of reverse selective glassy polymers, membrane separation performance can be improved by introducing new groups or additives into membrane matrix, however, the loss of permeances due to physical aging restrains their applications in industry. This chapter also accounts the effect of the chemical structure of rubbery polymers on their permeation properties for C3+ hydrocarbons recovery from natural gas. Modified siloxane rubbery polymers-based membranes may be seen in future in the field for enhanced C3+ hydrocarbons recovery from natural gas since these membrane materials display improved mixed gas C3+/CH4 selectivity than commercial membranes. In addition, only few studies regarding MMMs approach for enhanced C3+ heavy hydrocarbons removal from natural gas are reported in the literature. More research efforts are needed in the future from the selection of right nanofillers and understanding the dynamic interactions between polymers and nanofillers regarding mixed gas permeation in the membrane matrix.
The permeation properties of the polymeric and mixed polymer matrix membrane materials have been reviewed and compared under varying operating parameters by many research groups. These results emphasize the observation that C3+ hydrocarbon separation performance is strongly related to the testing condition, feed compositions and chemical structures. However, a thorough evaluation of C3+ hydrocarbon separation properties is rarely reported in the literature under industrially relevant feed streams and testing conditions. Thus, there is a need for further research and testing of advanced membrane materials that are resistance to swelling and satisfy the practical requirements during the actual industrial gas processes.
The support provided by the team from Oil and Gas Treatment Division, Research and Development Center (RDC) of Saudi Aramco is greatly acknowledged. The authors also would like to thank Ahmad Bahamdan and Faisal Otaibi from RDC for their supports.
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Written By
John Yang, Milind M. Vaidya, Sebastien A. Duval and Feras Hamad
Submitted: 05 February 2022Reviewed: 24 February 2022Published: 21 April 2022
Open access peer-reviewed chapter
