Svetlana I. Semenova

Hydrophilic membranes for pervaporation: An analytical review

An analytical review has been attempted on the issues encountered in selecting polymers for hydrophilic pervaporation (PV) membranes. It is well known that permselectivity is determined by selective sorption and by selective diffusion. Selective sorption is governed by the presence of the active centers in the polymer which are capable of specific interactions with water. The analysis of the influences of the type of interactions of water-active centers of the polymer on the membrane performances are presented. Selective diffusion is governed by the rigidity and the regularity of the polymer structure and also by the constructure of the polymers interspace. There is a net of bonds and crosslinks in the polymer due to existence of inter- and intramolecular interactions in it. This net is responsible for the stability of the mass transfer properties of the polymer to the feed water. The influences of the degree of swelling and frequencies of the crosslinks in the polymer are analysed. The main role of the selective diffusion in the selective permeation is demonstrated. The possibility of the existence of two different channels (hydrophilic and hydrophobic) for the permeation of water and organics is discussed, and it has been concluded that there are two different channels for friable polymers which have fragments with not so strong inter- and intra-molecular interactions. But in the polymers with strong inter- and intra-molecular interactions and a narrow net of these bonds (e.g., polyelectrolyte complexes), the hydrophobic channels are strongly collapsed. They can be opened only by water at its high feed concentrations. It was concluded that one of the most prospective ways to create highly permeative and highly selective materials for dehydration of organics by PV is using polyelectrolyte complexes (especially for the separation of water with organic molecules, which have more than three carbon atoms in total).

Effects of pore size on separation mechanisms of microfiltration of oily water, using porous glass tubular membrane

Abstract The effects of the average pore size in porous glass tube membranes on the performance of the cross-flow microfiltration were investigated using oil-in-water emulsion. The flux decline with filtration time was divided into two stages. The filtration mechanism in the first stage can be explained by three types of blocking filtration models such as the complete type for pore size 0.27xa0μm membrane, the intermediate type for pore size 0.75xa0μm membrane and the standard type for pore size 1.47xa0μm. The second stage can be also explained by the cake filtration model. The oil rejection is found to decrease with time initially, but is soon stabilized at a constant level. This was explained by the comparison among the average pore size, the pore size distribution of used membrane and the droplet distribution of the feed emulsion. The surfactant transmission of the cake filtration stage was 49% for 0.27xa0μm membrane, 80% for 0.75xa0μm membrane and 99% for 1.47xa0μm membrane.

Dependence of permeability through polyimide membranes on state of gas, vapor, liquid and supercritical fluid at high temperature

Abstract Mass-transfer properties of films of the polyimide Kapton® which were made from an 18 wt.% solution of polyamic acid in dimethylacetamide plus 5 wt.% of phenanthrene were investigated at high temperatures (373–423K) and high pressures (up to 160 atm). The permeabilities of gases (CO2, N2, CH4, H2, Ar, O2 and He), vapors (EtOH and H2O) and liquid EtOH were investigated. The linear dependence of the permeability activation energy on the kinetic molecular diameter of the penetrants shows that the diffusion factor of permeability is dominant in the range of high temperature, and that the dominant process is an intermolecular interaction as compared with the intramolecular interaction in polyimide. The dependences of the flux and the coefficient of permeability of ethanol on pressure are divided into two regions at the boundary between vapor and liquid. This effect is attributable to the formation of H-bonds upon condensation of the vapor with increasing pressure. The permeabilities of gases depend on the quantity of water in the polyimide. The amount of water in the polymer is dependent on temperature, and the transition of the polymer system from one state of equilibrium to another is very sluggish at temperatures much lower than Tg.

Hydrogen production from hydrogen sulfide using membrane reactor integrated with porous membrane having thermal and corrosion resistance

Abstract Using mathematical model and experimental method, the thermal decomposition of hydrogen sulfide in membrane reactor with porous membrane which has Knudsen diffusion characteristics was investigated. With mathematical model, the effect of characteristics of membrane reactor and operating conditions on H 2 concentration in the permeate chamber, y H 2 , which increases at higher reaction temperature, lower pressure and higher ratio of cross-sectional area of the permeate chamber to that of the reactor, was evaluated. The reaction experiments with ZrO 2 –SiO 2 porous membrane were carried out under the following conditions: temperature T , 923–1023xa0K; pressure in the reactor p R T , 0.11–0.25xa0MPa absolute; pressure in the permeate chamber p P T , 5xa0kPa absolute and inlet flow rate of H 2 S f 0 H 2 S , 3.2×10 −5 –1.5×10 −4 xa0mol/s. At p R T =0.11xa0MPa and f 0 H 2 S =6.4×10 −5 , y H 2 increased from 0.02 at T =923xa0K to 0.15 at 1023xa0K. With the experimental condition, p R T =0.11, T =1023xa0K and f 0 H 2 S =3.2×10 −5 , y H 2 was 0.22. The experimental results were compared with the results of the mathematical analysis. The agreement between both the results is found rather good at a lower reacting temperature, but not so good at a higher reacting temperature.

Physical transitions in polymers plasticized by interacting penetrant

Abstract Plasticization by a penetrant of a polymer with the functional groups, specifically interacting (donor-acceptor type of interactions) with the penetrant is analyzed by using the systems: ammonia which is an electron-donor/aromatic polyamides, where amide group is as an electron-acceptor; sulfur dioxide which is an electron-acceptor/polymers with electron-donor groups. It is shown that there is the plasticization of polymer by penetrant as the result of the sorption of penetrant molecules on the active functional groups of a polymer. There are four sections (regions) on the concentration dependencies of the diffusion coefficient. The boundaries between them correspond to the transition of the polymer (or rigid phase of block-copolymer) from one glassy state to another (β-transition), from glassy state to rubbery state (α-transition), and from rubbery state to the viscous-flow state. Essentially, the concentrations of a penetrant on the active functional groups, n, (or the degree of fullness of the effective sorptive capacity of the functional groups, ϕ), which corresponds to the α- and β- transitions in the polymer, have the ratio: n α n β ⋍ρα ρβ≈4± . Concentration barriers of the activation, corresponding to the α- and β-transitions also have the same ratio. It seems, that the process of α-relaxation in the polymer includes the intermolecularly correlated displacement of neighboring segments of the same size as in the β-transition, i.e. close to the Kuhns segment, but it is close to the correlated displacement of 3–5 segments. Relative concentration activation barriers of the physical transitions depend on the energy of interaction between penetrant and functional group of polymer.

Performances of glassy polymer membranes plasticized by interacting penetrants

Abstract The present paper deals with performances of glassy polymer membranes plasticized by interacting penetrants. The objects for investigation were polar gases and vapours such as ammonia, sulphur dioxide, freons, water as well as their mixtures with simple gases. The polymer were made of polyamides, polyheteroarylenes, derivatives of cellulose, polyurethanes, polyureas as well as block copolymers with flexible and rigid fragments containing active groups: polyether(ester) urethanes and polyether(ester) ureas. In the case of specific interaction between the penetrant and active groups of the polymer (donor–acceptor, ion–dipole and hydrogen bonding) plasticization of the polymer or rigid fragment of the block copolymer is observed producing a change in thermodynamical, diffusion and mechanical properties of the polymer as well as abnormal temperature dependence of diffusion properties of the polymer membranes. Comprehensive investigation of these properties has been carried out in a broad concentration/temperature range. General regularities of the physico-chemical processes observed have been generalized.

Study on molecular weight cut-off performance of asymmetric aromatic polyimide membrane “Effect of the additive agents”

Abstract Asymmetric polyimide membranes were fabricated from casting solution of 18xa0wt% polyamic acid and 1.3–20xa0wt% additive agent in dimethylacetamide at 343xa0K, with 1xa0min evaporation time, followed by a cyclization process of thermal treatment in a bath of dioctyl sebacate under N 2 in three steps: 1xa0h at 373xa0K, 1xa0h at 473xa0K, 1xa0h at 573xa0K. The effect of additive agents and their quantity on the molecular weight cut-off (MWCO) performance of the asymmetric aromatic polyimide membrane were examined. By changing a sort of additive agents, the MWCO of the fabricated aromatic polyimide asymmetric membranes can be adjusted in the range of 400–650xa0daltons. By changing amount of additive agents, MWCO of the fabricated aromatic polyimide asymmetric membranes remain almost constant at the value of approximately 400xa0daltons for pyrene, and increased from 500–650xa0daltons for polystyrene.

Nanotechnological method to control the molecular weight cut-off and/or pore diameter of organic–inorganic composite membrane

Abstract Polyimide–alumina composite membranes were fabricated by the nanotechnological copolymerization method of co-polymer which has a constant repeating unit chemically bound by primer on the wall and/or surface of the porous ceramic support. By changing the number of repeating unit (n) in the polymer, the fabricated pyromellitic dianhydride (PMDA)- diaminodiphenylether (ODA) composite membranes have separation factor αCO2/CH4 in the range 1.0–6.4 and molecular weight cut-off (MWCO) ranging 400–4000. As for the composite membranes of n=20, the separation factor αCO2/CH4 of the 4,4′-(hexafluoroisopropylidene)-diphathalic anhydride (6FDA)-diaminodiphenylether (ODA) composite membrane was approximately 1.6 times larger than that of the PMDA–ODA composite membranes, and these values were 7.5 and 4.7 at 323xa0K, respectively. With the increase of temperature, the separation factor decreased, and the value obtained was 4.8 at 423xa0K. The pure gas permeances through the carbon membrane (PMDA–ODA: n=20) was approximately 75–260 times larger than the values through the PMDA–ODA (n=20) composite membranes. But this membrane did not show any gas separation ability.

Analysis of a two-stage membrane reactor integrated with porous membrane having Knudsen diffusion characteristics for the thermal decomposition of hydrogen sulfide

Abstract Using basic equation for material balance, the thermal decomposition of hydrogen sulfide to produce hydrogen was investigated in a two-stage membrane reactor integrated with porous membrane having Knudsen diffusion characteristics. It was evaluated the effect of membrane area ratio S m,P / S m,R and the pressure in the 1st permeate chamber p P 1 on H 2 concentration in the 2nd permeate chamber y H 2 ,P 2 and the flow rate of H 2 in the 2nd U H 2 . With the decrease of S m,P / S m,R , y H 2 ,P 2 increases and takes constant value which is larger than the H 2 concentration in the permeate chamber for the single-stage membrane reactor by about 1.7 times. There is ( S m,P / S m,R ) limit corresponding to the condition that no flow in the 1st permeate chamber. With the increase of S m,P / S m,R , U H 2 increases and takes maximum value U H 2 ,max at ( S m,P / S m,R ) limit . There is an optimum p P 1 for higher U H 2 ,max , but the difference between the highest value of U H 2 ,max and the lowest one is small. The maximum value of U H 2 ,max is larger than the amount of H 2 obtained in the single-stage by about 1.1 times.

Diffusional characteristics of sodium chloride in symmetrical cellulose acetate membranes measured by an unsteady-state dialysis method

Abstract Unsteady-state dialysis and steady-state reverse osmosis (RO) experiments for aqueous NaCl solutions were carried out with cellulose acetate (CA) symmetrical membranes. The distribution and overall diffusion coefficients of NaCl, K and D were obtained from the unsteady-state dialysis experimental data, taking the effect of osmosis into consideration. D was found to increase with the increase of the solute concentration. The concentration dependency of D can be explained by applying a two-part (perfect or dense and imperfect or porous) model of the membrane. The concentration independent diffusion coefficients of NaCl in the dense part Dd and in the porous Dp are obtained from the assumption of the two-part model for the membrane. Dd does not depend on the water content in the membrane Ww, but on dope composition. Contrary to Dd, Dp increases with the increase of Ww and can be correlated only with Ww and Dp(Ww=0) is found almost equal to the averaged D d for each type of membrane: D p,c = D d exp (γ × Ww). It seems from these facts that the structure of membrane becomes tight with the decrease of the ratio of formamide to CA and loose with the decrease the concentration of CA in dope solutions. D c (c = 0) at the zero concentration can be calculated as follows: D c (0) ≈ W w D p,c + (1−W w ) D d . Solute permeability ω obtained by RO experiments agrees well with ω c = [K · D c (0) ] (l · β) , those calculated from Dc(0) and distribution coefficient K.