Gases permeation

Gas permeation (GP) was described in detail in Examples 9.2 and 9.3. Since the early 1980s, applications of GP with dense polymeric membranes have increased dramatically. Applications include (1) separation of hydrogen from methane (2) adjustment of the H2/CO ratio in synthesis gas (3) oxygen enrichment of air (4) nitrogen enrichment of air (5) removal of C02 (6) drying of natural gas and air (7) removal of helium and (8) removal of organic solvents from air (Seader and Henley, 2006). 

Most applications of GP use dense membranes of cellulose acetates and polysulfones. For high-temperature applications where polymers cannot be used, membranes of glass, carbon, and inorganic oxides are available, but they are limited in their selectivity. Almost all large-scale applications of GP use spiral-wound or hollow-fiber modules, because of their high packing density. 

In gas permeation or vapour permeation, both the upstream and downs.tream sides of a membrane consist of a gas or a vapour. However, eq. V - 152 caimot be used directly for gases. The concentration of a gas in a membrane can be written as 

It can be seen from this equation that the rate of gas permeation is proportional to the partial pressure difference across the membrane. Eq. V -166 is widely used to describe the gas or vapour flux across a membrane. 

Pervaporation is a membrane process in which the feed side is a liquid while the permeate side is a vapour as a result of applying a very low pressure downstream. Hence, on the downstream side P2 = 0 (or aj = 0) and the exponential term in eq.V - 164 is equal to unity and can be neglected (AP 10 N/m, Vj = 10 m /mol, RT = 2500 J/mol = exp(-Vj.AP/RT) = 1). If the partial pressure is put equal to the activity, then  

From eq. V - 168 it can be seen that when the permeate pressure (pj 2 ) increases the flux of component i decreases. As the permeate pressure (pj 2 ) is equal to the feed pressure(p j ) then the flux of component i becomes zero. 

Diffusion of gases occurs as a result of redistribution of a free volume within the matrix. Gas transport is enabled by microvoids present in a matrix. Gas permeate must have critical volume smaller than the size of microvoids.The equations below help to solve this problem. 

The average size of hole is given by the following equation  

From Simha-Somcynsky equation of state, one obtains the following relationship D = k(l-y) exp(- ) = kC(t h t ) [16.10] 

Nl Loschmidt number (the number of molecules in a gram-molecule) 

C(t) t) ) concentration of holes satisfying the minimum hole volume condition. 

Other gas permeation applications include separation of hydrogen from methane, hydrogen from carbon monoxide, and removal of components such as carbon dioxide, helium, moisture, and organic solvents from gas streams. Gas permeation for such operations may provide a more economical and more practical alternative than conventional separation processes such as cryogenic distillation, absorption, or adsorption. 

The driving force in gas permeation may be expressed in terms of the difference between a component partial pressure on the residue side and the permeate side of the membrane. The feed is introduced to the separator at a high pressure, while the permeate side is controlled at a low pressure. Examples 18.3 and 18.4 use the perfect mixing model for the performance evaluation and the design of two gas permeation processes. 

With few exceptions, gas permeation on a technical scale employs membranes of the sorption diffusion type, In this case, the flux of a permeating component is proportional to the difference of the partial pressures at both sides of the membrane, 

Asymmetric phase-inversion membranes like the membranes employed in reverse osmosis are difficult to prepare as gas permeation is much more sensitive to micropores than RO due to the much higher diffusion coefficients of gases. For the same reason, the composite membrane differs from RO composite membranes in gas permeation, the top layer of the asymmetric support structure is responsible for the separation while it is the sole duty of the coating to plug the micropores. Consequently, the material of the coating chosen (silicone) has a high permeability but a low selectivity while the membrane material (poly-sulfone) has a high selectivity (and a much lower permeability). 

The retentate of the first unit is fed to the second unit (and for this reason, it cannot be considered a two stage cascade), the permeate leaving at 25 bars as an additional feed to the first stage of the compressor. The retentate is utilized for heating purposes.26 

Since flux and selectivity increase with increasing transmembrane pressure difference and the modules can tolerate pressure differences of about 100 bars, the reader might question why the first unit is operated with a transmembrane pressure difference of only 60 bars. The reason is that the H2 recovery system has been added to an existing plant and that the first stage of the synthesis compressor would not accept the permeate flux of both units. 

PIB exhibits a comparatively low gas permeation (56). In Table 6.5, gas permeation coefficients of some polyolefins are given. Oppanol B 200 is compared with natural rubber, high density polyethylene) and low density poly(ethylene). Certain other Oppanol types have roughly the same permeability to gases as Oppanol B 200. 

The solubility of a gas is an integral part for the prediction of the permeation properties. Various models for the prediction of the solubility of gases in elastomeric polymers have been evaluated (57). Only a few models have been found to be suitable for predictive calculations. For this reason, a new model has been developed. This model is based on the entropic free volume activity coefficient model in combination with Hildebrand solubility parameters, which is commonly used for the theory of regular solutions. It has been demonstrated that mostly good results are obtained. An exception 

Raman scattering by gases is not a very strong effect and gives only weak signals. So gas permeation is usually not measurable by Raman spectroscopy. However, degradation of some chemical components consecutive to gas permeation can be detected in situ by Raman spectroscopy any formation of a deposited solid (carbon for instance) allows its identification and evaluation of membrane degradation [51], 

The driving force for the separation is differential pressure. CO2 tends to diffuse quickly through membranes and thus can be removed from the bulk gas stream. The low pressure side of the membrane that is rich in CO2 is normally operated at 10 to 20% of the feed pressure. 

It is difficult to remove H2S to pipeline quality with a membrane sy stem. Membrane systems have effectively been used as a first step to remove the CO2 and most of the H2S. An iron sponge or other H2S treating process is then used to remove the remainder of the H2S. 

Membranes will also remove some of the water vapor. Depending upon the stream properties, a membrane designed to treat CO2 to pipeline specifications may also reduce water vapor to less than 7 Ib/MMscf. Often, however, it is necessary to dehydrate the gas downstream of the membrane to attain final pipeline water vapor requirements. 

Membranes are a relatively new technology. They are an attractive economic alternative for treating CO2 from small streams (up to 10 MMscfd). With time they may become common on even larger streams 

The compatibility of PS and PPO may also be explained in part by the nearequal solubility parameters of the two polymers [81]. PS and PPO pack more closely when blended than is possible for either homopolymer in the pure state, and this suggestion is supported by permeation measurements which showed that hexane molecules have a lower permeability in PS/PPO polymer blended films than in films of either of the two homopolymers. This apparent increase in the packing of PS and PPO in the blends suggests that an intimate molecular mixing of the two homopolymers occurs in the blends. 

He permeation is also very sensitive to local concentration fluctuations, and thus can be used as a probe for the phase state in polymer blends [84]. In the above-mentioned system, the PSF-rich blend exhibited partial miscibility below the Tg whereas, after annealing, the PSF- and Pl-rich domains phase separated this resulted in a reduction of the permeability coefficient and showed that PI controls the absolute permeability values. It was concluded that transport in a phase-separated Matrimid/PSF is dominated by the polyimide over a wide concentration range. Assuming that the plasticization behavior may also be dominated by the polyimide, it must be concluded that only the homogeneous blend such as Matri-mid/P84 would be less susceptible to plasticization. 

The high plasticization tendency of Matrimid can be stabilized by blending vrith copolyimide P84, which is hardly affected by the sorbed molecules [85]. The CO2 concentration in the P84 film was lower than in the Matrimid/P84 and Matrimid film at corresponding pressures. It was unclear why the sorption isotherms of the Matrimid film and the blend coincided. The permeability coefficients of the blend were found to lie between the values of the homopolymers. On the basis of the 

The He, O2 and N2 gas-transport properties of cellulose acetate (CA), poly(methyl-methacrylate) (PMMA) and CA/PMMA blends of several compositions have been 

A similar behavior as above was observed for propane diffusion in miscible blends of polystyrene and poly (vinyl methyl ether) (PVME) in the rubbery state [92]. The 

The extrapolation is to what is called pervaporation, where the feed mixture is a liquid, but the permeate vaporizes during permeation, induced by the relatively low pressure maintained on the permeate side of the membrane. Accordingly, the reject or retentate remains a liquid, but the permeate is a vapor. Thus, there are features of gas permeation as well as hquid permeation. The process is eminently apphcable to the separation of organics and to the separation of organics and water (e.g., ethanol and water). In the latter case, either water vapor may be the permeate, as in dehydration, or the organic vapor may be the permeate. The obvious, potential application is to the separation of azeotropic mixtures and close-boiling mixtures—as an alternative or adjunct to distillation or liquid-liquid extraction methods. 

The subject of pervaporation is featured in a chapter in Part III of Ho and Sirkar (1992). 

Permeability of a membrane is determined partly by gas diffu-sivity, but adsorption phenomena can also exist at higher pressures, which affects the outcome. Separation factors of two substances are approximately in the ratios of their permeabihties, which can be defined by ocab = Poa/Pob, or more simply olab = Pa/Pb, where the symbol P represents the permeability at a stated reference condition. Some data of permeabilites and separation factors are listed in Table 19.7, together with a list of membranes that have been used commercially for particular separations. Similar but not entirely consistent data are tabulated in the Chemical Engineers Handbook (Li and Ho, 1984, pp. 17.16, 17.18). The different units used for permeability will undergo further inspection in a subsequent section. 

Polysaccharides Concn of starch effluents concn of pectin 

Synthetic water-soluble polymers Concn of PVA/CMC desize wastes 

TABLE 19.2. Data of Commercial Equipment for Reverse Osmosis and Ultrafiltration 

Diaflo Nominal mol wt cutoff Apparent pore diam, A Water flux, gal/ff7day at 55 lb/in2 

Specified pore size, pm Pore-size range, pm Nominal pore density, pores/cm2 Nominal thickness, pm Typical flow rates at 10 lb/in2 (gage), AP, 70°F  

TABLE 19.1. Examples of Applications of Ultrafiltration (a) Applications Involving Retained Colloidal Particles 

Concn/purlficatlon of organic pigment slurries separation of solvents, etc. from pigment/resin In electropaints concn of pigments In printing effluents 

Concn of waste oils from metal working/textile scouring concn of lanolln/dirt from wool scouring 

Concn of emulsion polymers from reactors and washings 

Concn of silver from photographic wastes concn of activated carbon slurries concn of Inorganic 

CO2/CH4 trade-off line for P84 and matrimid precursors and their carbon membranes. 

A permeation cell and a gas chromatograph (GC) were combined in order to allow straightforward determination of gas permeability. The permeability of component i in the gas mixture under steady state of diffusion can be calculated according to the following equations  

The pure gas tests are normally used to indicate the ideal separation performance for carbon membranes. However, the separation properties will be affected by the presence of other penetrants in a gas mixture. Since the transport for gas mixture will be much different from that in pure gas, especially in the presence of strong adsorbable gas like CO2, the adsorption of gas molecules in carbon membranes matrix will significantly affect the penetration of other less or 

A study was undertaken of the effects of membrane preparation protocol on the permeability of PIM-1. Membranes were cast from solutions of the polymer in tetrahydrofuran (THF) or chloroform onto a cellophane, glass or Teflon surface. Three states of PIM-1 were identified. 

1 Water-treated PIM-1, for which a flow of water was used to assist removal of the membrane from the surface onto which it was cast. 

2 PIM-1 for which no water was used in membrane preparation, or else that was exhaustively dried at elevated temperature under vacuum. 

3 Methanol-treated PIM-1, that was soaked in methanol for at least a day, to flush out any residual casting solvent and allow relaxation of chains in the swollen state, then dried under vacuum to constant weight. 

Representative permeability data from both methods of measurement are given in Table 2.2 for PIM-1 in each of these three states. It can be seen that contact with water during membrane preparation can lead to a significant reduction in permeability. This is caused by trapped water inside the micropores and is hard to remove by vacuum and elevated temperature. Methanol treatment, however, substantially enhances the permeability. Indeed, methanol-treated PIM-1 is amongst the most permeable known polymers. Furthermore, the selectivities for some important gas pairs are significantly higher than 

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To write an unsteady state enthalpy balance we require the enthalpy per unit volume of the gas-permeated solid matrix. This is given by... 

Gasoline reformate Gasoline, reformulated Gasolines Gas permeation Gas-phase adsorptu... 

Linear Low Density Polyethylene. Films from linear low density polyethylene (LLDPE) resias have 75% higher tensile strength, 50% higher elongation-to-break strength, and a slightly higher but broader heat-seal initiation temperature than do films from LDPE. Impact and puncture resistance are also improved over LDPE. Water-vapor and gas-permeation properties are similar to those of LDPE films. 

Poly(vinyl chloride). To be converted into film, poly(viayl chloride) [9002-86-22] (PVC) must be modified with heat stabilizers and plasticizers, which increase costs. Plasticized PVC film is highly transparent and soft, with a very high gas-permeation rate. Water-vapor transmission rate is relatively low. At present, PVC film is produced by blown-film extmsion, although casting and calendering are employed for heavier gauges (see Vinyl POLYAffiRS). 

V. O. Altemose, "Gas Permeation Through Glass," Seventh Symposium on the Art of Glassblowing, The American Scientific Glassbiowers Society, Wilmington, Del., 1962. 

Moleculady mixed composites of montmorillonite clay and polyimide which have a higher resistance to gas permeation and a lower coefficient of thermal expansion than ordinary polyimides have been produced (60). These polyimide hybrids were synthesized using montmorillonite intercalated with the ammonium salt of dodecylamine. When polymerized in the presence of dimethyl acetamide and polyamic acid, the resulting dispersion was cast onto glass plates and cured. The cured films were as transparent as polyimide. 

Gas separation using ethylceUulose hoUow fiber has also recendy become important. A/G Technology is leading this effort. Fluoroaceylated ethylceUulose is reported to have good gas permeation and blood compatibUity (26). 

The second step, permeation of components / andy through the membrane, is related directiy to conventional gas permeation. The separation achieved in this step, can be defined as the ratio of components in the permeate vapor to the ratio of components in the feed vapor (eq. 14). 

A crystalline or semicrystalline state in polymers can be induced by thermal changes from a melt or from a glass, by strain, by organic vapors, or by Hquid solvents (40). Polymer crystallization can also be induced by compressed (or supercritical) gases, such as CO2 (41). The plasticization of a polymer by CO2 can increase the polymer segmental motions so that crystallization is kinetically possible. Because the amount of gas (or fluid) sorbed into the polymer is a dkect function of the pressure, the rate and extent of crystallization may be controUed by controlling the supercritical fluid pressure. As a result of this abiHty to induce crystallization, a history effect may be introduced into polymers. This can be an important consideration for polymer processing and gas permeation membranes. 

Standard texts may be consulted on the topic of diffusion ia solids (6,12,13). Some generalizations, however, are possible. No noble gas permeates a metal. Metals are, however, permeated readily by hydrogen. Stainless steel, for example, can be permeated by hydrogen from concentrations likely ia air. 

Selective gas permeation has been known for generations, and the early use of p adium silver-alloy membranes achieved sporadic industrial use. Gas separation on a massive scale was used to separate from using porous (Knudsen flow) membranes. An upgrade of the membranes at Oak Ridge cost 1.5 billion. Polymeric membranes became economically viable about 1980, introducing the modern era of gas-separation membranes. Hg recoveiy was the first major apphcation, followed quickly by acid gas separation (CO9/CH4) and the production of No from air. 

Basic Equations In Background and Definitions, the basic equation for gas permeation was derived with the major assumptions noted. Equation (22-62) may be restated as ... 

Plasticization and Other Time Effects Most data from the literature, including those presented above are taken from experiments where one gas at a time is tested, with Ot calculated as a ratio of the two permeabihties. If either gas permeates because of a high-sorption coefficient rather than a high diffusivity, there may be an increase in the permeabihty of all gases in contact with the membrane. Thus, the Ot actually found in a real separation may be much lower than that calculated by the simple ratio of permeabilities. The data in the hterature do not rehably include the plasticization effect. If present, it results in the sometimes slow relaxation of polymer structure giving a rise in permeabihty and a dramatic dechne in selectivity. 

The main reason for producing multi-layer co-extruded films is to get materials with better barrier properties - particularly in regard to gas permeation. The following Table shows the effects which can be achieved. Data on permeability of plastics are also given in Figs 1.13 and 1.14. 

Crabtree, E. W., Dunn, R. F., and El-Halwagi, M. M. (1997). Synthesis of hybrid gas permeation membrane/condensation systems for pollution prevention. J. Air Wa.w Manage. Association (in press). 

Molecular orientation results in increased stiffness, strength, and toughness (Table 8-12) as well as resistance to liquid and gas permeation, crazing, microcracks, and others in the direction or plane of the orientation. The orientation of fibers in reinforced plastics causes similar positive influences. Orientation in effect provides a means of tailoring and improving the properties of plastics. 

Table VII. The Effect of Moisture on the Gas Permeation of Various Polymers...

Gas separations by distillation are energy-consuming processes. The driving force for the gas permeation is only the pressure difference between two compartments separated by the membrane. The permeation is governed by two parameters—diffusion and solubility ... 

The question of relative rates of seal leakage and permeation (cf. points 1 and 3) has been considered by MERE. For the sealing of HPHT (5000 psi, ca. 345 bar, or 34.5 MPa 100°C) fluids with chevron seal-stack systems used at the bottom of oil wells in intermittently dynamic conditions, high-pressure permeation and seal-leakage tests using the equipment outlined in Section 23.3.1.2 have been conducted it has been shown that methane gas permeation rate was only ca. 1 % of the rate... 

FIGURE 23.10 Permeated volume versus time for gas permeation test. 

FIGURE 23.11 Schematic of high-pressure gas permeation testing system. 

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