Separation of organic compounds from gaseous mixtures by vapor permeation

Separation and Purification Technology 217 (2019) 95–107

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Review

Separation of organic compounds from gaseous mixtures by vapor permeation Zuzana Petrusová, Karolina Machanová, Petr Stanovský, Pavel Izák

T

⁎

Institute of Chemical Process Fundamentals of the CAS, v. v. i., Rozvojová 135, Prague, Czech Republic

A R T I C LE I N FO

A B S T R A C T

Keywords: Membrane separation Membrane technology Vapor permeation Volatile organic compounds Hydrocarbon capture Organic vapor Industrial process Air pollution Air purification

Gas separation technology is a mature topic, while the vapor permeation (VP) process still needs some development. It can be presumed that VP will become extensively applied in the future, thanks to its economic and ecological advantages. Despite single vapor permeability chiefly being reported, less information is available on the binary VOC/N2 mixture, while a multicomponent mixture is examined only rarely. Some of the newly developed membrane materials offer significant potential. Nevertheless, the long-term stability of membranes is still uncertain and more pilot plan tests are required. Our review compares the advantages and disadvantages of commonly tested membranes as well as newly developed membrane materials. Furthermore, interesting results are highlighted to encourage further research and the future needs and prospects of VP. Both academic and industrial approaches are discussed, with an emphasis on the new trends since 2000.

1. Introduction Volatile organic compounds (VOCs) are emitted into the atmosphere by natural sources, but also intensively by anthropogenic sources, such as chemical and pharmaceutical industries (various solvents), petrochemical production (hydrocarbons), printing and paint production (benzene and its derivatives), and gas stations and storage stations (gasoline vapors containing various VOCs). Therefore, the air is polluted by VOCs and this leads to an increase of ground-level ozone and the concentration of greenhouse gases. Air purification has currently become of maximum importance from ecological and economic points of view. Stricter pollution limits are required in industry for VOC recovery from air in the Czech Republic as well as worldwide [1]. In the United States, VOCs are of concern to both indoor and outdoor air quality [2–4]. China applies restrictions on any volatile organic compounds that participate in atmospheric photochemical reactions [5]. The different assessment methods sometimes lead to different limit values in countries (Table 1). Generally, French legislation limits are the lowest, with some exceptions. It should be remembered that Californian legislation limits are for indoor air.

Nevertheless, different limits do not correspond to different health risks. Therefore, the limit values urgently need harmonization throughout industrial countries [6]. VOC capture from polluted air can be realized by conventional separation methods such as condensation, absorption, adsorption (e.g. on activated carbon) or pressure swing adsorption. However, these processes have been found to be disadvantageous in terms of economic and energy intensity and are limited in their efficiency. VP is a promising alternative to conventional energy intensive separation processes, such as condensation or adsorption [7]. Vapor recovery is applied in various industrial processes such as hydrocarbon removal from air at gas stations or storage gasoline stations and polymer production. Membrane technology provides many advantages, such as working continuously within a low flow rate or variable flow rate, non-destructive method, low energy demand, as well as simple, flexible and easy-to-handle units. Other advantage is that principle of membrane gas separation is already mature field [8,9]. Vapor permeation differ from a common gas separation method because a separated compound is condensable gas. Therefore, the separation is mainly based on the VOC solubility while a diffusivity usually plays the key role in gas separation.

Abbreviations: CFs, carbon fibers; CNTs, carbon nanotubes; HDPE, high-density polyethylene; HMDS, hexamethylsiloxane; IL(s), ionic liquid(s); ILM, immobilized liquid membrane; MMM, MMMs, mixed membrane matrix, mixed matrix membranes; MOFs, metal organic frameworks; PBA, poly(n-butyl acrylate); PDMS, poly (dimethylsiloxane); PEBA, poly(ether block amide); PEI, polyetherimide; PI, polyimide; PIM(s), polymer(s) with intrinsic microporosity; PMMA, poly(methyl methacrylate); PS, polystyrene; PSf, polysulfone; PTMSN, poly(trimethyl silyl nonbornene); PTMSP, poly[1-(trimethylsilyl)-1-propyne]; PV, pervaporation; PVC, polyvinylchloride; PVDF, poly(vinylidenefluoride); SILM, supported ionic liquid membrane; SLM, polymeric membrane with intercorporate silver salt; TEG, triethylene glycol; VOC, volatile organic compounds; VP, vapor permeation; ZIF, zeolitic imidazole framework ⁎ Corresponding author. E-mail address: [email protected] (P. Izák). https://doi.org/10.1016/j.seppur.2019.02.028 Received 17 December 2018; Received in revised form 11 February 2019; Accepted 11 February 2019 Available online 12 February 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.

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because a complex study in the literature is very rare [13]. Therefore, the estimation of transport and separation properties is not efficient enough, especially for a long-term application. On the other hand, vapor permeation through a dense membrane is an effective separation method for VOCs emitted from industrial processes. Some VOC VP commercial technology already exists. The typical industrial VP applications are summarized and discussed in the manuscript. However, there is still the need for a better understanding of the transport and separation phenomena for VOCs, before wider interest in this application can be expected, as in the case of gas separation since 2000.

Table 1 Selected VOC limit values in Germany, France and California [6]. All in µg/m3

Germany

France

California

Toluene Xylenes n-Hexane Terpenes, e.g. Limonene 1-Butanol 2-Ethoxyethanol Crotonaldehyde Formaldehyde

1900 2200 72 1500 3100 19 1 120

300 200 700 450 1500 70 1 10

300 700 7000 – – 70 – 33

1.1. State of the art in vapor permeation The main asset of membrane technology is an increasing VOC concentration in the permeate before final treatment and the retentate stream that can fulfil the air emission limits, which is a few ppm up to ppb, depending on the kind of VOC (Table 1). Membrane processes have proven to be a promising and economical alternative in comparison with conventional separation processes for VOC separation and removal from polluted air. The investment in membrane technology is not too high and it is advantageously installed, together with a combination of another unit based on a conventional separation method [10,11]. However, it has to be highlighted that membrane separation offers another advantage, thanks to no modification of the separated component while an oxidation process increases greenhouse emission. Furthermore, membrane separation can be realized at ambient conditions; it does not require any addition of extra solvent and the VOC can be collected without any chemical reaction. There is growing interest in membrane technology, especially for hydrocarbon capture by the petrochemical industry (capture of alkanes, alkenes and cyclic hydrocarbons) and by the chemical industry (capture of valuable monomers). Membrane separation technology is promising as a pre-treatment for increasing the feed concentration before the next separation process [10,11], because feed stream typically contains from 0.5 to 5 vol% of VOC. The membrane technology is able to capture approximately 80% of VOCs from the feed stream, due to a high selectivity for VOC/N2 from 10 to 1000. Laboratory tests are usually carried out with a thick, sometimes home-made, film membrane. On the contrary, an industrial process needs a thin film composite membrane which can be produced with a sufficiently large area (larger than 1000 m2), or the membrane would not be appropriate for an industrial process. Commercial membranes are composites with the selective thin layers on to a porous support, which enhances the mechanical stability. However, it is well known that the properties of a thin film composite are different from that of a thick film membrane. Moreover, experiments with a single gas are only insufficient input for prediction in an industrial process [11]. This review summarizes the currently available membrane technology and used membrane materials for VOC removal from polluted air. Membrane materials in the article are listed according their relative use for VOC and more general background about the composition of membranes can be found in literature [8,9]. Furthermore, the manuscript discusses the energy efficiency of a membrane process and its ability to selectively separate VOC from the air stream. The good selectivity is due to the key dominating fact of the significantly higher permeability of VOC in comparison to that of permanent gases. For an efficient separation, for high permeability it is necessary to consider both the solubility and diffusivity of the VOC component in the membranes, which are not as easily predictable as for gases [12]. Although membrane separation requires a low energy cost, the vapor permeation (VP) separation process is still of limited practical application due to a strong dependence of the selectivity on a timevariable factor – membrane swelling. Another limitation is the lack of information on the influence of various parameters (i.e. temperature, trans-membrane pressure, flow rates, composition of feed stream) on membrane performance (i.e. transport and separation properties),

The interest in VP began for practical applications in vinyl chloride recycling, gasoline recovery, recovery of paint solvents and olefin/ paraffin separation [14–18]. Olefin/paraffin separation is currently one of the most studied applications, with the removal of selected VOCs higher than 95% [19,20]. Concerning the reproducibility of transport and separation properties, it is usually within 10%. Membrane technology is attractive, because of its moderate investment cost and low operating cost compared to other separation methods such as oxidation, condensation and absorption or adsorption [21]. Although pervaporation (PV) requires more energy for VOC evaporation, interest in PV technology prevails in comparison with the VP process [19,22]. PV is widely applied for VOC removal from waste water and VP is important for the purification of polluted air. The membrane suitable for industrial application has to be stable, highly permeable, with at least selectivity of 10 or preferably higher [21]. The most tested rubbery polymer is PDMS, which is suitable for VOC/N2 separation, while only negligible separation is reached for olefin/paraffin separation (separation factor close to 1) [18]. VP is currently studied in particular for the capture of hydrocarbons emitted into the air [23,24]. From the research point of view, new membrane materials are being developed [25–28]. Since 2000, there has been a big boom of applications of membranes with ionic liquids and immobilized liquid membranes, due to their tunability for a specific application. The selectivity is usually higher than 15. The development of new membrane materials for application relates to several fields, such as chemical engineering, polymer science and material chemistry [20]. Further development is also related to composite membranes and mixed matrix membranes that may remove more than 95% of VOCs from permanent gas. Moreover, these membranes are potentially suitable for industrial applications, due to their long-term stability, high flux and very good selectivity. 2. Membranes’ potential for VOC separation Generally, sorption and diffusion are the key parameters for gas and vapor permeation in the polymeric dense membrane. The diffusivity is higher for smaller molecules, while sorption is stronger for larger condensable molecules. The permeability mainly depends on the VOC concentration in the feed stream as well as the condensability of the relevant VOC. The molecular size of VOC and critical temperature of VOC influence the solubility of VOC in a membrane and the concentration of VOC in the feed stream. The influence on the solubility is stronger in the case of larger VOC molecules. It can be concluded that the sorption affects the final transport and separation properties for VOC/N2 separation. One of the main applications is the separation of olefins from a permanent gas. 2.1. Polydimethylsiloxane and its various configurations PDMS is known as membrane material with good flexibility of polymer chains what makes PDMS very permeable even at low temperature and trans-membrane pressure. Rubbery PDMS has predominated since the end of the 20th century as a highly permeable and 96

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[17,21,31,38,44,47,54]. The soft segments of PDMS can assist in improving the transport properties of polyurethane and polyurethane urea membranes [36]. These membranes were tested for separation of toluene vapors from a nitrogen stream where a selectivity of up to 140 was reached. However, the plasticization of a membrane leads to an increase of nitrogen permeability and therefore to a reduction of toluene/nitrogen selectivity. The plasticization is caused primarily by presence of toluene vapors because it was observed at small trans-membrane pressure of 31 kPa and at ambient temperature of 25 °C. Another type of membrane, called mixed membrane matrix (MMMs), offers a mechanical enhancement and could improve the separation properties. The improvement of properties is necessary to be checked by a long organic vapor exposure, as well as by a study of a model mixture. Fang et al. [29] recently studied a separation of propane and nitrogen by zeolitic imidazole framework ZIF-8/PDMS MMMs. Moreover, they modified the Maxwell model for MMMs and included the effects of the chain rigidification effect, pore blockage and nanoparticle aggregation. They comprehended that the particles were surrounded by PDMS matrix and this led to pore blockage at higher loaded ZIF-8 into the membrane. It improved the Maxwell model, and the trend in selectivity was predicted successfully with the increasing quantity of particles, as well as the expected decrease of permeability. Apart from a flat sheet membrane, a thin film selective layer is usually applied on the shell side of hollow fiber. Therefore, hollow fiber modules are largely used on industrial scale. Recent research by Obuskovic et al. shows the promising potential of a hollow fiber module for VOC separation from polluted air [19]. A high VOC removal of up to 99.9% for low feed concentration, and up to 98.8% for a higher feed concentration of acetone, toluene and methanol were documented. The selectivity depends on the VOC concentration in the feed and on the total feed flow rate. The long-term stability was confirmed at least after six months. Furthermore, PDMS hollow fiber can also successfully separate acetone, ethyl acetate and ethanol vapors [30].

selective material for VOC/N2 separation [12,13,21,26,29–43]. The dense membrane is usually prepared from two components (polymer and crosslinking agent) mixed in the recommended ratio 10:1. This optimal mixing ratio offers a very good linkage in the polymer and the membrane is sufficiently rigid. Moreover, the polymer chains can have higher mobility when the curing agents are mixed in a different ratio (either higher 20:1 or lower 5:1) [31]. Therefore, PDMS offers not only very good permeability but also a potential modification of the properties of polymer net. PDMS membrane has been widely studied, especially for hydrocarbon separation. This application is important, due to the wide-scale production of polyolefins (such as polyethylene and polypropylene) worldwide [31,44–46]. Therefore, an unreacted olefin has to be captured from a nitrogen stream and recycled back to polyolefin production. The sorption and permeation behavior was studied by Choi et al. for a series of representative olefins (e.g. C2H4, C3H6, and C4H8) and nitrogen [31]. The solubility of representative olefins mainly depends on the critical temperature of the olefins as well as on the applied pressure (increase in pressure leads to increase of solubility) and the temperature in the membrane cell (decrease in temperature leads to increase of solubility). The perm-selectivity can be significantly improved by a temperature change, because of a permeability increase of condensable gases, while nitrogen permeability is almost temperatureindependent. This finding corresponds to the known fact that condensable vapors are significantly more permeable than permanent gases [31,47]. Moreover, an increase of boiling point leads to an increase of solubility of condensable species in the polymeric membrane. Therefore, it results in an improvement of the selectivity of VOC with respect to a permanent gas such as nitrogen. A dense PDMS membrane is also a good candidate for the separation of chlorinated VOC such as 1,2-dichloroethane and 1,1,2-trichloroethane from a permanent gas [42,48,49]. When a single-step membrane separation is not sufficient, either a multi-stage membrane separation or a hybrid process can be successful [10,50]. Membrane separation is usually either applied as a pre-treatment (for increase of VOC feed concentration), or accompanied with another classic separation method. Belaissaoui et al. [10] systematically studied the energy efficiency of a hybrid membrane/condensation process. The attractiveness of this combination has the advantage of VOC recovery directly into the liquid phase. It was found that boiling temperature is a key parameter for the decision whether a hybrid process is more advantageous than an independent condensation or single-membrane separation. Stand-alone condensation was found to be advantageous for a VOC with a high boiling point, such as toluene, octane or acetone separated from polluted air. The recycling strategy can be created based on the published results. It was found that energy efficiency is improved significantly when the hybrid system is applied to VOC with a medium boiling point. Further economical and technicoeconomical analyses are desirable for promising hybrid systems. In addition, composite membranes are required for industrial processes [26,47]. PDMS membranes were originally developed and studied for the pervaporation process [26,40–42]. Nevertheless, this membrane also shows significant potential for separation of VOC from the air. Considering vapor permeation application, it has to be taken into an account that the PDMS depends on the applied trans-membrane pressure and is affected by the presence of larger hydrocarbons. Therefore, a plasticization can occur. Plasticization leads to destruction of a polymer chain which results in a strong reduction of membrane selectivity while a permeability significantly increases. These phenomena is well described for CO2 separation [51–53]. Plasticization was reported by Jiang and Kumar when a condensable gas is present in the feed stream and applied pressure is higher than atmospheric pressure and at temperature of 50 °C [47]. Therefore, non-ideal solubility leads to a change of the order of permeability together with the applied pressure. Moreover, a coupling effect was observed for ethane, ethylene and nitrogen in the presence of propane and propylene

2.2. Poly(ether block amide) Thermoplastic elastomer poly(ether block amide) (PEBA) combines the hardness and mechanical strength of glassy polyamide and the softness of rubbery polyether. Polyamide linear chains are interspaced by amorphous polyether domains. Therefore, PEBA is ranked as a highly permeable polymer, due to its flexible polyether segments. PEBA is commercially available in various badges for specific applications. This polymer offers the production of a dense, defect-free membrane. A thin film composite membrane can also be successfully prepared. Membranes made from PEBA 2533 are appropriate for gas/VOC separation [55–57]. PEBA 2533 has been successfully tested for the unreacted olefin (i.e., C2H4, C3H6, C4H8) recovery from polyolefin manufacture, which is potentially important for petrochemical application [42,56,58,59]. The membrane is suitable due to its significantly preferential permeability of an olefin to that of a permanent gas (i.e. nitrogen). Selectivity is better at a lower operating temperature. Good perm-selectivity VOC/N2 is linked to good solubility selectivity, which influences the final transport properties more than diffusivity. Diffusivity depends on molecular size and shape, as well as on interactions between the VOC and membrane. In general, more condensable VOCs are more soluble in rubbery polymers. Solubility is higher for olefins with a longer carbon chain and decreases for smaller olefins. Separation properties depend on both operational temperature and pressure. Sorption coefficients increase with pressure as well as with temperature. The plasticization or swelling phenomenon results in an increase of permeability and diffusivity at higher pressures (above 1 MPa). Interestingly, similar sorption of VOC in PEBA 2533 and polyethylene and PDMS [56] is observed, due to its main rubbery domain (80 wt% of rubbery polyether in PEBA 2533). 97

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PTMSN. The permeability lay between another glassy polymer with high free volume PTMSP and amorphous Teflon AF-2400 for VOC. On the contrary, permeability increases with the chain length of VOC in PTMSP, while decreases in AF-2400. Another petrochemical application is the separation of olefins and alkanes by a thermoplastic polyimide (PI) [61,65–68]. PI has lower permeability for VOC compared to that of rubbery PDMS, but offers better selectivity for olefin/alkane [59]. Olefins are more permeable than alkanes because of their preferential diffusion and smaller overall molecular size. Olefin permeability is reduced by an alkane presence, while alkane permeability slightly increases in the presence of olefin in the feed mixture. This is accompanied by a lowering of selectivity compared to the ideal value obtained for pure gas permeation experiments. The plasticization of PI, i.e. molecular chain reorganization causing selectivity loss at elevated pressure (typically for PI above 1 MPa) is a known problem [61,65–68]. Furthermore, a low concentration of VOC (for example less than 600 ppm of an aromatic hydrocarbon) can cause significant decline of membrane selectivity [56]. On the other hand, PI offers tunable properties and potential resistance to plasticization, due to reshaping by heating or the chemical stabilization of the polymer structure. Therefore, undesired plasticization can be minimized by thermal annealing and either by UV or covalent crosslinking, which have a positive effect on both permeability and selectivity and may reduce the plasticization effect even at pressure of 5.5 MPa [53]. The glassy polymers and PIMs are known as highly permeable materials. On the other hand, their free volume is reduced after the first exposure of a gas or any VOC. This phenomenon is probably caused by the swelling of glassy polymers. Therefore, aging has to be considered, as well as the change in transport and separation properties [69–76]. It seems that the aging of glassy polymers can be solved by the addition of particles that help to fix and stabilize the polymer chain structure. Recently, graphene-like fillers were studied as potential fillers for the anti-aging effect [77]. If the problem of aging is resolved, glassy polymers could be suitable for industrial applications of membrane technology.

Therefore, PEBA 2533 behaves like rubbery polymers. Nevertheless, the final characteristic properties are specific, because of the glassy polyamide segment presence in the copolymer. It is known that permeability and selectivity are different for pure gases and mixed gases. This is also very important for gas/VOC separation, because the VOC presence affects both membrane properties and nitrogen transport in the membrane. Contrary to gas separation, a real binary VOC/N2 mixture can be more selectively separated than is expected from the permeability of pure VOC and N2. The interactions between propylene and nitrogen improve the selectivity of VOC over nitrogen. Moreover, nitrogen permeability is lower in the binary mixture than that measured for pure gas [57]. 2.3. Glassy polymers and polymers with intrinsic microporosity Membranes made from glassy polymers and polymers with intrinsic microporosity (PIM) were firstly tested either for gas separation or pervaporation, while gas/vapor separation started to attract attention recently [40,60–63]. Condensable vapors such as n-alkanes (n-C4+) and alcohols (methanol, ethanol, n-propanol) can also be selectively separated from polluted air by polymer with a high free volume polymer. Typical examples of potentially appropriate polymers are poly[1-(trimethylsilyl)-1-propyne] (PTMSP) and poly(trimethyl silyl norbornene) (PTMSN) [40,60–62]. The solubility and diffusivity of n-alkanes and alcohols differ, due to a different interaction between the VOC and membrane. In addition to membrane relaxation, they are also influenced by the activity of VOC in the feed stream and membrane thickness. Relaxation time is not affected by the change of membrane thickness, while diffusivity is slower in the thicker membrane. Diffusion coefficients follow Fickian kinetics for low VOC concentrations. The activation energy of diffusion increases with the size of penetrant. Therefore, diffusivity is higher for smaller VOC molecules, as expected for polymers with a free volume. This can also be attributed to more significant swelling by larger molecules. The isothermal diffusion and solubility in PTMSN are similar to another glassy polymer (PTMSP). Mobility increases for alkanes, while it decreases for alcohols with VOC concentration in both polymers. Interestingly, larger alkanes (C4+, elongated or flattened) diffuse more easily than spherical molecules in glassy polymers, because of their anisometric molecules that can be orientated by a smaller diameter than the mean value. Diffusivity decreases in the following order of representative polymers: polystyrene (PS) > polyvinylchloride (PVC) ≥ poly(methyl methacrylate) (PMMA) [64]. Thus, the diffusivity coefficient largely depends on both the size and shape of the penetrant molecule. These trends are probably caused by differences in polymer chain packing and its stiffness. The structure of PS probably avoids the closer packing of polymer chains as it is in PVC or PMMA. The decreased mobility of larger molecules in PMMA is probably caused by greater chain stiffness, attributed to a higher glass transition temperature. In contrast to hydrocarbons, fluorocarbons are less permeable if their molecule size is larger [61,62]. This unusual size-sieving behavior in PTMSP is predominately due to low fluorocarbon mobility, resulting from the large size of these penetrants. Presumably, the mobility of larger molecules reflects the influence of these penetrants on the net free volume of the polymer–penetrant system. The study of propane permeability at various trans-membrane pressures confirmed that propane does not irreversibly change the chain packing of PTMSP [61,62]. On the contrary, perfluoropropane causes plasticization of PTMSP at higher pressures (above 5 atm) when permeability begins to increase, while it decreases with applied pressure (below 5 atm) [61]. The transport properties in PTMSN significantly differ from those of an unsubstituted poly(norbornene) because of its smaller free volume that causes both lower solubility and diffusivity. No trend in permeability was found with an increasing number of carbon atoms for

2.4. Mixed membrane matrix and zeolites MMMs combine an organic polymeric matrix with inorganic fillers. MMMs represent an effective type of membrane that shows an improvement in mechanical properties due to polymer chain stabilization by the inorganic fillers [58,78–80]. The transport performance is influenced by properties of both intrinsic polymer and dispersed fillers (their size, porosity and bonding to the polymer matrix). In general, selectivity is usually enhanced for MMM in comparison to that of intrinsic polymer. On the contrary, permeability is usually lowered by the addition of non-permeable particles or due to blocking of polymer pores and decreasing of chain mobility [29]. MMMs can be considered for an industrial purpose when the improvement of selectivity is accompanied by only a small reduction of permeability [11,15,81]. This condition is usually fulfilled by MMMs with a lower filler content. The Maxwell model is able to qualitatively describe the influence of filler content on gas permeability in MMMs [82,83]. Reliable selectivity prediction is obtained when the effects of rigidification of the polymer chain, pore blockage and fillers’ aggregation are considered. More experimental data are needed for the improvement of the Maxwell model for better prediction of trend in selectivity for VOC/N2 separation. The choice of appropriate polymer can be based on a prediction of the solubility of representative VOC in the polymer. The estimation of VOC solubility can be calculated either by the UNIFAC-FV group contribution model or the group-contribution lattice fluid equation-of-state (GCLF-EOS) [84]. Wang et al. [84] studied single organic vapor permeation in various MMMs. One of the tested membranes was prepared from porous highdensity polyethylene (HDPE) and poly(n-butyl acrylate) (PBA) with a 98

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2.5. Composite membranes

molecular weight of 100 000. The permeation of selected VOC in HDPEPBA was the following: benzene > toluene > ethylbenzene that corresponds inversely proportionally to the size of the studied VOC. They reported good mutual agreement between experimental toluene flux in HDPE-PBA with the prediction based on Fick’s Law and the combination of the GCLF-EOS model, the free volume theory for predicting diffusion coefficients and the swelling expression for a filling-type membrane. Hydrocarbons can be potentially separated from the air by a MMM containing elastomeric ethylene-octene copolymer with multi-walled carbon nanotubes (CNTs) and carbon fibers (CFs) [82]. Toluene and hexane permeability is about two orders of magnitude higher than that of ethanol vapors and gases. The permeability of all VOCs increases with the vapor activity in the feed stream, as is generally expected for rubbery polymers. This confirms the solubility-controlled transport of VOC. Therefore, lower ethanol permeability is observed due to lower solubility in the intrinsic polymer matrix. The addition of carbon fillers causes a slight decrease of gas permeabilities, because CNTs and CFs act as impermeable obstacles. Interestingly, the addition of fillers has the opposite influence on vapor permeability. It may be expected that the addition of fillers counteracts the plasticization effect caused by vapor sorption. Thus, a lower content of carbon fillers leads to the enhancement of VOC/N2 separation. The adsorption of VOC is favored in inorganic fillers, such as microporous Y zeolites, mesoporous pillared clays and nano-sized silicalite-1 (S-1) and Ti-silicalite-1 (TS-1) seeds of MFI zeolite, due to nonspecific interactions [27,85,86]. Therefore, MMM represents an adjustable type of membrane for removal of VOC emissions from polluted air or for separation of hydrocarbons [9,27,85,86]. Organic vapors can also be captured from the air by a porous MMM prepared from poly(vinylidenefluoride) (PVDF) and MFI zeolite [86]. The results show the best separation efficiency for the removal of trace concentrations of hexane vapors from the air. Banihashemi et al. [7] found that the p-xylene is more permeable than o-xylene in both MMM and template-free MFI zeolite membrane, irrespective of whether PV or VP is studied. Minimization of intercrystallinity of MFI zeolite leads to an increase selectivity of p-xylene/oxylene. Small differences between the temperature dependence of xylene’s permeability can be found, due to differences between membrane structures that influence the xylene separation or due to nanosized defects in the MFI zeolite layer [7,87]. NaX membrane (faujasite membrane prepared on porous α-alumina support disk) may reduce hydrocarbon losses emitted from polymer production. NaX membrane is able to recover both propylene and propane from permanent gas. The separation ability was confirmed for the feed mixed from propane, propylene and nitrogen, simulating a typical composition of vent in polymer production [54]. Therefore, hydrocarbons can be recycled and re-used in the process. The ZIF-8/PDMS MMMs showed a better separation factor for C3H8 recovery from N2 than the dense PDMS membrane [29]. Therefore, the ZIF-8/PDMS membrane seems to be suitable for hydrocarbon recovery. ZIF-8 belongs to the metal organic frameworks (MOFs). MOFs have a porous structure with large surface area and offers a very good adsorption potential, like zeolites. The separation factor increased with the amount of added ZIF-8, while hydrocarbon permeability decreased slightly. The best separation was achieved for a membrane containing 10 %wt. of ZIF-8. The influence of the permeability decrease was stronger than the increase of the separation factor when more than 10 %wt. of ZIF-8 was loaded in the MMMs. In addition, propylene/propane can be efficiently separated by polycrystalline ZIF-8 on ceramic support (e.g. alumina disk) [88]. The separation is enhanced when ZIF-8 is firmly attached on the ceramic support, resulting in the high packing density of MMMs with very good chemical and mechanical stability. Therefore, it can be expected that this kind of MMM could be used in a practical application.

Dense non-porous membranes are selective barriers for the separation of a specific VOC from the air. However, the transport through the dense material does not have a negligible transfer resistance and an adequate trans-membrane pressure has to be applied to efficiently separate VOC/N2. In contrast, porous material is highly permeable, but barely selective. A composite membrane combines the advantages of both dense polymer and porous support [58,89–95]. It offers the possibility for industrial applications in the configuration of spiral wound flat sheets or modules with hundreds or thousands of hollow fibers [96]. Porous support is usually 10–100 times more permeable and coated by a selective skin layer of polymer [47]. The thin, dense polymeric layer ensures the selective separation, and the porous support allows the transport of separated VOC. In general, the skin dense layer is made from a permeable and selective polymer, such as the most often used PDMS or PEBA that are able to create thin film without any defects. Generally, the skin layer should be as thin as possible, but the porous support structure has to be simultaneously optimized with respect to its resistance [90]. Ideally, the resistance of the support layer is negligible in comparison to that of the skin layer, and the membrane flux is determined by a selective skin layer [90,97–99]. Permeable rubbery PDMS attracts attention as a skin layer, mainly for separation of C2+ alkanes or light olefins from nitrogen, olefin recovery in polyolefin production. VOC removal from the air controls the emissions and captures valuable compounds [13,37,90]. Propylene permeability significantly depends on trans-membrane pressure [38] applied on PDMS composite membrane and on interactions between olefin and the PDMS skin layer, respectively. Consequently, selectivity olefin/nitrogen is influenced by applied pressure. Olefin permeability is independent of nitrogen presence, while nitrogen permeability increases in the presence of olefin, due to polymer plasticization. Polysulfone (PSf) was coated by PDMS and used for the study of VOC/N2 separation by Jiang et al. [47]. Firstly, pure nitrogen and VOC (C2H4, C2H6, C3H6, C3H8) were tested before the study of multi-component mixtures. Ideal selectivity is comparable for pure ethane and ethylene over pure nitrogen, as well as propylene and propane over pure nitrogen, respectively. The favorable permeability for alkanes to alkenes corresponds to the order of critical temperatures and expected solubilities. However, this permeability order is observed at transmembrane pressure above atmospheric pressure, while alkenes become more permeable than alkanes below plasticization conditions. The coupling effect has an influence on the transport and separation properties when C3 hydrocarbon is present in the multi-component mixture. C2 hydrocarbons interact with that of C3 and become more permeable. A poly(dimethyl siloxane)/polyetherimide (PDMS/PEI) hollow fiber composite membrane was tested for light olefin/N2 separation (i.e., C2H4/N2 and C3H6/N2) by Liu et al. [90]. It was found that the selectivity of olefin/N2 is significantly lower than expected for pure PDMS, due to the PEI resistance that influences VOC transport through the composite membrane, while nitrogen permeability depends only on transport in the PDMS layer. The skin layer of PDMS may be damaged when higher trans-membrane pressures are applied. Plasma polymerized PDMS [100,101] offers a higher density and therefore better mechanical resistance and stability. A further advantage of plasma treatment is the possibility to bond the selective thin layer firmly to the porous support, such as e.g. polypropylene [34,102–104]. Plasma polymerized PDMS was bonded excellently to the shell side of polypropylene hollow fibers by Majumdar et al. [34]. The membrane withstands pressures of up to 14 atm when the feed stream is flowing from the lumen side of the fibers. The promising laboratory results were verified by a larger pilot scale. The feed stream represents emissions from pharmaceutical or paint processes. The possibility of VOC removal from permanent gas was studied for representative VOC methanol and 99

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Improvement in the separation and transport properties of a composite membrane was achieved by a thin selective layer, whereas a selective liquid provides this in an immobilized liquid membrane (ILM) [25,28,107–114]. ILMs were successfully tested for PV processes [28] and started to attract attention for VP application as well. ILMs provide very good permeability and selectivity for VOC/N2 VP separation. Good potential was found for VP separation of VOC, such as the separation of alkanes, cycloalkanes, aromatic hydrocarbons, alcohols or ketones from nitrogen or the separation of olefin/alkane. However, applied trans-membrane pressure is limited, due to a risk of the liquid leaking from ILM at elevated pressures. ILM stability can be improved by a silicone coating. Silicone is a very permeable material that does not slow down the transport, while it protects the liquid from leakage from the membrane. Another method of liquid membrane stabilization is based on bonding liquid to a polymer matrix, either chemically (covalently) or physically. Moreover, poly(ionic liquids) can overcome the stability limitations. VOC permeability is lower in a more viscous liquid. Therefore, the transport resistance is more significant. On the other hand, the selectivity is higher for a more viscous liquid used for ILM. VOC/N2 separation is influenced by physical properties such as density, viscosity and polarity of the immobilized liquid, together with the solubility and diffusivity of the separated VOC in this liquid. Furthermore, the properties of the porous support influence the transport properties by its porosity, pore size distributions and tortuosity. The configuration of ILM is either a flat sheet or hollow fiber module. Liquid is either spread on to the porous support as a thin selective layer, or trapped inside the porous support due to capillary forces. Most of the studied ILMs contain ionic liquid (IL), because ILs have tunable properties, negligible volatility and good sorption ability. When IL is trapped inside the pores, the membrane is called a supported ionic liquid membrane (SILM). However, gas separation (GS) or PV applications are more often reported, compared to VP separation of VOC/N2 [8,9]. Recently, Salar-García et al. critically reviewed VOC recovery in both bulk and immobilized ILs [25]. Various types of ILs showed their potential for VP. For example, imidazolium- based ILs are widely used in SILMs and showed a very good ability for the separation of alkanes, cycloalkanes, aromatic hydrocarbons or alcohols from polluted air [25,109,110]. They concluded that ILs can be widely used for VOC extraction. The interest in membrane technology is growing and both further advances and improvements are expected in separation technology with the use of ILs. Facilitated transport can be achieved when the appropriate liquid is chosen. Separation of olefin from alkane is an interesting application of VP in the petrochemical industry. For example, SLM (polymeric membrane with intercorporate silver salt) has a facilitated effect. This SLM was tested for VP separation of olefin/alkane by Takht Ravanchi et al. [18]. Triethylene glycol (TEG) is another suitable candidate for ILM preparation, because of its non-volatility [115–117]. Ito et al. [115] coated the hydrophobic microporous polytetrafluorethylene (Teflon, PTFE) support with TEG. Sui et al. [117] tested a “surface-soaked liquid membrane” consisting of a hydrophilic surface Durapel membrane (Millipore) soaked in TEG liquid. These membranes can be exposed to a vacuum on the permeate side. TEG ILM membranes showed comparable transport properties and separation effectivity to both separation by rubbery PDMS and a conventional adsorption process with active carbon. The membranes can effectively separate VOC, such as benzene, toluene, acetone or methanol from the air. Another kind of ILM was studied by Ozturk et al. for VOC/N2 separation, as a cheaper alternative to the ionic liquid membrane. Vegetable and lubricant oils were immobilized in the porous support of cellulose acetate [118]. The long-term stability was proven, and separation properties were studied at various trans-membrane pressures for VOC, such as C6H6, CCl4 and CH3OH. This can be explained by the

toluene. At least 95% of VOC was selectively removed from nitrogen. It was found that the feed flow rate has to be regulated, otherwise the poor removal of VOC is observed at elevated flow rates. The VOC flux in the membrane increases with its feed concentration. Consequently, VOC enrichment is richer for a higher VOC concentration in the feed stream. Plasma polymerization was used for the preparation of a composite membrane from hexamethyldisiloxane (HMDS) monomer by Sohn et al. [26]. Although plasma polymers are known for their closely packed structure with high density, and only poor ability for VOC/air separation may expected, plasma polymerized HMDS showed a potential for separation of benzene or cyclohexane from the air. High selectivity was obtained for VOC/air separation due to the preparation of a similar structure to PDMS polymer, confirmed by FT-IR spectroscopy [26,105,106]. The permeability of cyclohexane is higher than that of hexane although they have a similar critical temperature. The different permeability is caused by differences in molecular structure. The cyclohexane has circular molecules, while hexane has a linear chain with a larger effective molecular size. PEBA 2533 is attractive for VOC/air separation. Liu et al. [57] studied the separation of propylene from nitrogen by a PEBA 2533 skin layer on a microporous PSf support. Transport and separation properties were studied firstly for pure propylene and nitrogen, and subsequently for the binary mixture. The solubility of condensable organic compounds is significantly higher. Therefore, the solubility significantly influences the diffusion-solution transport through the membrane, as in the case of rubbery PDMS. PEBA is swollen by the presence of propylene, as was observed for PDMS. Interestingly, propylene and nitrogen interact together and compete in the sorption in the PEBA skin layer. The permeability of individual components is influenced by the presence of the second component in the feed stream. Consequently, the real selectivity is lower than the ideal selectivity calculated from the permeabilities of pure components. Separation of CH3OH/N2 and C2H5OH/N2 was realized by using a flat PEBA/PSf composite membrane by Liu et al. [90]. Permeance increases for the thinner PEBA layer for both VOC and N2. However, the increase of VOC flux has a weaker influence on final selectivity, because slowly permeating nitrogen is not at all affected by a decrease of PEBA thickness. Therefore, poorer separation is observed for a thinner PEBA skin layer on a PEBA/PSf flat membrane for both CH3OH/N2 and C2H5OH/N2. Moreover, nitrogen permeability increases in the presence of VOC due to PEBA plasticization and swelling. Therefore, a pronounced coupling effect deteriorates the separation properties. Separation of gasoline vapor from the air is a further research direction for VP. Liu et al. [91] tested a hollow fiber composite membrane, containing a skin PEBA 2533 layer on microporous PVDF hollow fiber. Firstly, PEBA/PVDF was exposed to a binary mixture of VOC and nitrogen. The permeability order of three VOC representatives is: heptane > cyclohexane > hexane that corresponds to a penetrant condensability similarly to other rubbery membranes for VOC/gas separations. Secondly, the separation of a multicomponent mixture was studied. The feed contained all three representative VOCs and nitrogen. In the case of a model gasoline mixture, it is necessary to continuously mix the feed composition to avoid changes in feed composition. Also, the influence of selected gasoline additives is discussed (i.e. for methanol, ethanol, dimethyl carbonate and methyl tert-butyl ether). The best separation is obtained for CH3OH/N2, as corresponds to the previously mentioned VOC/N2 separation by PEBA composite membrane [90]. The enrichment factor for selected gasoline additives follows the order: methanol > dimethyl carbonate > ethanol > MTBE. The separation ability was proven for 10 months of testing under various operational conditions. 2.6. Liquid membranes VOC can be captured in a suitable liquid for the specific application. 100

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different physical and chemical interactions between separated benzene and ILM. The highest permeability and best selectivity were observed for C6H6/N2 separation. Although CCl4 is more soluble in oil than C6H6, the diffusivity of CCl4 is lower, due to its chemical structure and interactions with the porous support. The lowest permeability and selectivity were obtained for CH3OH/N2 separation, because of the hydrophilic character of CH3OH. It has to be emphasized that ILMs were tested repeatedly for various trans-membrane pressures. The transport and separation properties changed, caused by a leakage of oil from the ILM due to swelling. The swelling was confirmed by repeated experiments when oil was lost from the swollen, narrower pores even on the feed side. As can be expected, a greater oil loss was found for less viscous oil [118].

Table 2 Companies interested in VP. Country

Company dealing with membrane technology

Germany

Aluminium Rheinfelden GmbH, Anlagentechnik, Beroplan GmbH, Borsig GmbH, GKSS, Sterling-Sihi, Sulzer Chemtech Sterling Fluid Systems Pervatech BV. Cm-Celfa Membrantrenntechnik AG, Kkerbs Swiss, Kühni Bend Research Inc., Honeywell UOP, Isotronics, Membrane Technology and Research Inc.

France The Netherlands Switzerland

USA

3. Industrial application of membrane separation for vapor separation Concerning the separation of VOC from polluted air, the properties of the relevant VOC play a key role. Membrane technology is often applied together with another conventional separation technique, due to problems with membrane swelling or plasticization which lead to a change of transport and separation properties. Therefore, a hybrid system is industrially used or a membrane module is used either for pretreatment or for final purification of polluted air. The conventional separation methods for VOC removal from polluted air are adsorption on activated carbon, or condensation. The scheme of a classic hybrid system is displayed in Fig. 1.

USA and Australia, can separate the humidity from air [121]. Up to 60% was saved when membrane technology was tested for water vapor separation from air in the Dutch WETSUS research center [122]. Moreover, separated water vapor can be further used in countries where the water supply is depleted, or to humidify the air in homes (TMH, transport membrane humidifier) [123]. – Separation of VOC from air is less often applied than that of water vapor separation. Companies interested in VP for VOC separation from air are listed in Table 2. Polymeric composite membranes are industrially used for VP in the configuration of either spiral wound or hollow fiber, while theoretical research is focused on liquid and glassy membranes [14,16,22,25,124,125]. – Capture of monomers from petrochemical production. The captured monomers (typically ethylene, propylene, vinyl chloride) can be recycled and re-used in polymer production. The capture of vinyl chloride monomer by membranes was found to be particularly effective.

3.1. Basic membrane materials and applications of VP Three basic types of membranes can be used for vapor separation from the air at industrial scale: 1. Hydrophilic membranes for the pre-treatment of polluted air when water vapor is captured, to avoid problems of a humidified feed stream in the following separation step (typically polyvinyl alcohol) 2. Organophilic membranes which are highly permeable and selective enough for VOC over permanent gas (typical material is PDMS) 3. Organophilic membrane for special application with a high selectivity

The scheme of the process (Fig. 2) is similar to that of hybrid separation (Fig. 1). – Capture of hydrocarbon vapors close to gas refueling stations [16,22,125]. Various applications exist, because hydrocarbon capture from the air is currently very attractive. Moreover, legal emission limits have become stricter. Therefore, more effective methods are required for CxHy/N2 separation. For example, systems for VOC separation are the following membrane technologies: VACONORE® [22], VAPORRED® [22], SIHI® [127], PERVATECH® [128]. These technologies assist in environmental protection [129]. – Processing of natural gas when not only CxHy are separated, but also water vapor is captured in the first stage [16,22]. Membrane

Typical industrial applications are, for example: – Drying of gas stream is the main application of VP. Many companies are focusing on this process, e.g. USA Air Products, Permea Division, Membrane Technology and Research Whatman and Generon® technologies. These membrane technologies can be used for the drying of natural gas [119,120]. The HVAC system, developed in the

purified air

polluted air (air + VOC)

compressor

condenser

condensate (VOC)

membrane module

permeate (primarily VOC)

Fig. 1. Scheme of hybrid continuous separation process of VOC from polluted air. 101

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unreacted monomer

vent

reactor

membrane module

condenser

(polymer production)

compressor

condensate (recovered monomer)

permeate

recycled unreacted monomer Fig. 2. Scheme of recycling unreacted monomer by membrane MTRINC technology [126].

beginning of the 21st century. Membrane processes are again starting to attract attention (Fig. 3B).

technology is primarily focused on the separation of C1-C4 alkanes from natural gas [130]. This application is implemented by the Sterling group [22] in Germany. – Selective separation of selected VOC. Apart from the aforementioned vinyl chloride, the interest is mainly for organic solvents evaporated into air, such as acetone and hexane, or gasoline vapors from the air [21,35,91,131,132].

3.2. Patented materials and technologies for VP Recent patents related to membrane materials for VP: 1. Chemical routes are described for the preparation of membranes exhibiting high gas separation factors for the separation of VOC from the air. The membrane is made from a blend of a copolymer of 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (BDD) and tetrafluoroethylene (TFE) with a 2,2,4-trifluoro-5-trifluoromethoxy-1,3dioxide [133]. 2. A method for making high performance MMMs is described for membranes suitable for a wide range of gas separations, vapor separations, such as alcohol/water, olefin/paraffin, iso/normal paraffins and VOC/air. Membranes are prepared using stabilized concentrated suspensions of solvents, uniformly dispersed polymer stabilized molecular sieves, and at least two different types of polymers, such as the continuous blend polymer matrix. MMMs are in the form of dense films or asymmetric flat sheet or hollow fiber [134].

Although the number of research papers is significantly lower in comparison to the membrane processes applied for gas separation (Fig. 3A), it can be expected that the interest in VOC/N2 separation will increase more significantly than that of gas separation in the near future, due to numerous potential applications and the search for cost saving and a focus on environmental protection. Therefore, the number of patented technologies for VOC separation from polluted air will probably also increase. VP attracts attention due to its advantages, e.g. applicability to a very low feed concentration of VOC, simple operation, low operating cost and high efficiency. The greatest potential for the application of VP is the usage of these new separation processes in the chemical and petrochemical industries. The development of VP is still open to new possibilities of industrialization. On the other hand, development was very intensive at the end of the 20th century and at the

A

B 100000

15

510

344

0

771

1072 66

99

129

1469

1620

1500

1101

2245

1690

3000

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239

GS

50000 39000

39000 34000

25000

24000 19000

0

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PV

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4500

VP 75000

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GS

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PV

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VP

6000

163

Number of WoS publications

6556

696

Number of citations including patents

7500

1400

200

Time period

Time period

Fig. 3. Comparison of scientific interest in gas separation (GS), VOC / permanent gas (i.e. vapor permeation, VP) and pervaporation (PV) by membranes (A, records found on Web of Science) and interest in these applications (B, records found on Google Scholar, linked to sources related to patents) for the last 30 years. 102

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4. General remarks

3. Membrane preparation methods are described for high performance cross-linked polybenzoxazole and polybenzothiazole polymer membranes that are suitable for separation of various gases or vapor separations [135].

4.1. Scientific research VP is mainly studied from an environmental point of view for the purification of polluted air. VOC/N2 [141–146] and olefin/alkane [17,31,56,61,65–68] separation are predominant in the literature published since 2000. Scientific papers mainly focus on the capture of gasoline vapors in the air at fuel storage and transfer stations, monomers from polymer production, separation of olefin/alkane mixture and separation of alkanes (mainly n-C4+), olefins, alcohols, ethers, ketones, benzene and toluene from the air. The interest is in finding a selective and well permeable membrane for VP separation. PDMS is a leader in rubbery polymers, due to its high permeability and low cost. Recently, research attention is also attracted to mixed membrane matrix and liquid membranes. Generally, transport and separation properties differ for pure gases and mixed gases. This phenomenon is also important for gas/VOC separation. Interestingly, a VOC presence can lead to a decrease of permanent gas permeability and an improvement of separation properties, due to specific interactions between the VOC penetrant and polymeric membrane. Condensable VOC and permanent gases interact differently with amorphous rubbery polymers and crystalline glassy polymers. Therefore, interesting trends in diffusivity, solubility and/or permeability can be observed for copolymers such as PEBA that comprise hard and soft domains. Firstly, nitrogen permeability has to be determined before VOC exposure to the membrane, to avoid the effects of swelling or plasticization. Only a few papers report the comparison of nitrogen permeability for a virgin membrane with that of a membrane exposed for longer to VOC. It is desirable to continue with the research and understanding of interactions between VOC and membranes under longterm conditions. Concerning laboratory tests, it is necessary to ascertain precisely and reliably the driving force and to minimize all input errors [19,92,147–150]. This is especially important for extremely low and highly permeable membranes. The flux in low permeable materials is low and each input error can significantly influence the calculated flux in the membrane. On the other hand, the driving force is important for highly permeable materials, because of a higher concentration of VOC in the permeate stream.

Recent patents related to membrane technologies for VP: – System for removal of VOC from off-gas streams from soil, groundwater, industrial processes, pipelines and storage tanks combines a membrane separation module with a compression and condensation primary step. The membrane is permeable to VOC vapors, but impermeable to nitrogen, oxygen and carbon dioxide. The system operates under a pressure from 758 to 1365 kPa. The permeate with VOC is returned to the primary step and the retentate is pollutant-free gas having a VOC level < 100 ppb(v) [136]. – System for controlling environmental VOC emissions from vapor recovery in storage or dispensing operations of liquids maintains a vacuum in the storage tank ullage. The composite membrane based on a fluoroalkane non-porous polymer layer supported on a porous substrate has a suitable glass transition temperature and a selectivity higher for air relative to VOC. The system functions with a cyclic feed stream of diluted gas into the feed side, reducing plasticizing of the membrane [137]. – Device and method offer selective removal of one or more components from a multicomponent gas/vapor mixture by membrane fractionation. The nonporous ultrathin plasma polymerized silicone rubber is applied on to the hollow fibers. The system is suitable for the capture of the following VOCs: toluene, acetone, methanol and methylenechloride from polluted air [138].

3.3. Efficiency and limitations of VP separation process Methods of VOC capture comprise diverse efficiency (Table 3) and limitations (Table 4). Membrane technology shows significant potential in comparison to conventional methods, either of single usage or as accompanying different separation technologies [10,139,140]. The most common separation methods are based on sorption either by active coal or an appropriate solvent. Nevertheless, sorption is a discontinuous process and the regeneration of sorbate is necessary. Moreover, sorption methods lead to the problems of toxic waste. Another basic separation method is condensation, which is a non-destructive method but requires investment in a heat exchanger and the achievement of a low condensing temperature. Condensation is successful for VOC with a high boiling temperature (toluene, octane and acetone), whereas a hybrid method is desirable for VOC with a low boiling temperature, such as ethylene. The hybrid process combines conventional and membrane techniques and results in cost saving. Membrane technology mainly attracts by the high efficiency of VOC removal from polluted air. Emissions of VOCs into the air are currently reduced by at least 80%.

4.2. Industrial practice The application of VP separation exhibits some advantages over PV process. PV needs installation of heat exchangers and is energetically demanding due to the evaporation process. Therefore, VP is a process with a lower cost. However, membrane separation is not easily applicable to VOC/air separation, because of problems with swelling and/ or plasticization of the membrane by VOC. If a resistant and stable membrane is applied, VP would be a potentially effective separation method. Although many new materials are tested in laboratories, only a few materials are selected for industrial practice. There is still a lack of information about the long-term stability of new materials tested at laboratory scale. Moreover, a significantly larger membrane area is required for a pilot plant or industrial process. Only thin film composite membranes are used due to their higher permeability. Membrane modules are supplied in two basic configurations: either spiral wound module or hollow fiber module. Membrane separation is currently often accompanied by another classic separation method. Alternatively, membrane separation is used as a pre-treatment for the next separation step. VP can concentrate VOC in the permeate stream. The permeate can be recycled for further

Table 3 Comparison of efficiency of separation processes [140]. Separation method for VOC capture

Reduction of emissions (%)

Single-step method Absorption by oil Adsorption by activated coal Condensation by liquid nitrogen Membrane technology Two-stage process

93–99 90–95 95–99 90 99 almost 100

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Table 4 Limitations of separation techniques for reduction of VOC emissions [140]. Separation method

Limitations of method

Adsorption by activated coal Condensation by liquid nitrogen Membrane technology

activated coal can be damaged by an aggressive VOC and need of VOC desorption higher investment cost for heat exchanger and problems in the case of sulfur presence applicability to 500 ppm VOC

treatment. VP is successfully applied because of its many advantages, such as very good selectivity, no environmental pollution, effectivity, economic advantages, modularity of design, simplicity of separation process, adaptability and flexibility regarding composition of feed mixture and/ or operating conditions and ability to fulfil final product qualities. The VOCs separated from polluted air are mainly evaporated solvents (i.e. acetone, hexane, THF, DMF), gasoline vapors emitted close to gas refueling stations (i.e. n-alkanes, benzene, toluene, ketones), chemicals emitted from the production of polymers (i.e. monomers). More pilot plant tests are required for VP application in industrial practice. Separation of VOC/air by VP is an ever-growing segment. The greatest potential appears to be for the reduction of emissions of gasoline vapors and the recovery of methanol in the production of MTBE, and the recovery of monomers in polymer production [15,16,18,22,23,81,93,151–153].

•

5. Conclusion and future prospects VOCs present in the air are mainly evaporated solvents such as ethanol, acetone or hexane, gasoline vapors containing n-alkanes, aromatic compounds and ketones, monomers or olefins lost from petrochemical processes and chemicals from various productions, e.g. methanol from methyl tert-butyl ether production. The literature mainly reports single-vapor permeability and only a few sources offer at least a test of a binary VOC/N2 mixture. Fortunately, some membranes were tested for a multicomponent feed and tested for longer than six months. Nevertheless, it is clear that VOC permeability is a double or even a higher order of magnitude that that of permanent gas, and membrane separation is a promising process. Currently, VP membrane technology is used either as a pre-treatment to increase the VOC concentration in the permeate, or it advantageously operates together with condensation in a hybrid system. We believe that VP has the potential to reduce VOC emissions in the near future. Nevertheless, the long-term stability of membranes is still uncertain and more pilot plan tests are required. It is known that industrial applications require a larger membrane area which is achieved by spiral wound composite membranes or hollow fiber modules. Therefore, laboratory tests with a dense membrane may predict with difficulty the separation ability for either a pilot plant system or even an industrial plant, owing to the small area. Moreover, real processes are connected neither to a single permeant, nor to binary mixtures.

•

multi-component mixture and interactions between individual components should also be studied in future. ○ Commonly tested VOCs are hexane, heptane, benzene, toluene and MTBE. Their permeability was mainly tested with both dense and composite PDMS membranes. PEBA and various MMM membranes also show a good separation ability for gasoline vapor separation from polluted air. Separation of olefin/alkane ○ Economically important topic for the petrochemical process of polyolefin production that is currently attracting attention; the interest in hydrocarbon separation will continue. ○ Individual VOC permeability is mainly reported and some information is related to the binary system of olefin/alkane. It has to be taken into account that e.g. the presence of propylene influences the transport properties of other components in the real mixture. ○ Pilot plan scale was successfully tested for ethylene/ethane and propylene/propane separation by MMM membrane. Various membranes were studied in laboratory tests, e.g. PDMS (dense and also composite membrane), PEBA composite membrane and liquid membranes. Capture of VOC emitted from chemical processes or painting ○ Economically and ecologically interesting topic, including the capture of evaporated solvents or VOC from painting that recently started to attract attention and is potentially also important for future studies. ○ Aromatic hydrocarbons were mainly studied due to their negative impact on human health. ○ Works are mainly motivated by real processes. Therefore, industrially applicable membranes are tested, such as various composite membranes and MMMs.

5.2. Influence of input parameters on VP separation Operating conditions influence the transport and separation properties of membranes. Therefore, it is necessary to verify the effect of feed composition and flow rate, operating temperature and transmembrane pressure. Generally, VP separation is usually successful for a low feed concentration of VOC in the air stream. This technique is able to significantly increase VOC concentration in the permeate stream. Thus, VP is a suitable pre-treatment involved in hybrid processes, where the separation efficiency is improved. The pressure driving force increases the membrane flux, but the separation factor is simultaneously decreased. This contradictory effect is known for polymeric membranes and is also a typical trade-off for gas membrane separation. VOC permeability depends on VOC solubility in the selected membrane material. The solubility-controlled transport may be influenced by operating temperature as well as by membrane swelling caused by the VOC presence in the feed stream. The swelling may change the transport properties and lead to a decrease of selectivity due to the disruption of polymer chains. Surprisingly, swelling of MMMs may improve both membrane flux and selectivity, especially at the lower content of inorganic fillers.

5.1. VP applications

• Gasoline vapor removal from polluted air

○ Ecologically interesting topic that will probably also attract attention in future. ○ Only a few works are related to tests with real gasoline, while much information can be found on single VOC permeation and less information on a model binary VOC/N2 mixture. It has to be taken into account that the composition of gasoline depends on the season. ○ Membrane material can be selected based on laboratory tests with a model binary VOC/N2 mixture, but it is necessary to continue with both long-time experiments and pilot plant tests. Furthermore, it is necessary to consider that gasoline is a complex 104

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5.3. Future prospects

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In the near future, VP will probably be operated in hybrid processes, together with conventional methods for VOC recovery, to achieve maximum efficiency. All currently studied applications seem to be linked to future directions, namely gasoline vapor recovery, olefin/alkane separation and VOC recovery from chemical and petrochemical processes. VP has the potential to assist in the reduction of VOC emissions in the near future. Although gas membrane separation and the pervaporation process attract more attention than vapor permeation processes, VP has become significant in the recent decade. From an academic point of view, the future of VP will probably be connected to the development of various liquid, composite and mixed matrix membranes. Ionic liquid membranes have been intensively studied for the past 20 years and it seems that this interest will continue. The future will potentially present a new type of liquid membrane with cheaper low-volatile liquids, such as oils. Composite membranes will be developed with respect to their potential industrial application. At present, MMMs seem to be the most perspective type of membrane for VP, due to the enhancement of both transport and separation properties. Moreover, recently patented membrane materials suitable for VP are also mainly MMMs. It is necessary to emphasize that not only liquid membranes but also MMMs can be tailored to a specific application. Moreover, MMMs can offer higher thermal and chemical stability together with mechanical enhancement. MMMs are capable of functioning in industrial processes. Therefore, it would be useful to continue their study and to contribute to a better understanding of: – the influence of properties of intrinsic polymer on the final transport and separation properties of MMM; – the influence of added fillers on the final transport and separation properties, as well as on the mechanical enhancement of MMM and polymer chain reinforcement; – the mutual interactions between the polymeric matrix and fillers during VOC/N2 separation; – the interactions between VOC and MMM. From an industrial point of view, VP is a very good accompanying separation technique in a hybrid process, while its separate more significant applicability is still questionable. However, VP could prove to be an economical separation method, with pilot and industrial applications probably being implemented in future. Acknowledgements The financial support of the Czech Science Foundation (Junior Project 17-03367Y) is acknowledged. The authors would like to thank Lenka Morávková for her fruitful discussion, assistance with the literature review and graphic editing. References [1] EU countries legislation restriction. https://eur-lex.europa.eu/legal-content/EN/ TXT/?uri=celex:32004L0042. [2] Technical Overview of Volatile Organic Compounds, EPA, 2017. https://www. epa.gov/indoor-air-quality-iaq/technical-overview-volatile-organic-compounds. [3] Volatile Organic Compounds (VOC) and Consumer Products Regulations. https:// www.chemsafetypro.com/Topics/VOC/What_Are_Volatile_Organic_Compounds_ (VOC)_and_Overview_of_Global_VOC_Regulations.html. [4] R. Oppl, T. Neuhaus, Emission specifications in Europe and the US – Limit values (TVOC, LCI, CREL, ...) in critical discussion, Indoor Air, Paper ID: 953 (2008), 17–22 August 2008, Copenhagen, Denmark. [5] China's restrictions on any volatile organic compounds. http://www.mee.gov.cn/. [6] VOC emission requirements. https://www.eurofins.com/media/1770/emission_ limit_values_in_critical_discussion_953.pdf. [7] F. Banihashemi, L. Meng, A.A. Babaluo, Y.S. Lin, Xylene vapor permeation in MFI zeolite membranes made by templated and template-free secondary growth of randomly oriented seeds: effects of xylene activity and microstructure, Ind. Eng.

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