Liquid-vapor permeation

The work of Adachi et al. (2009) represented a first attempt to correlate and validate ex situ and in situ water permeation phenomena in PEMs. Water permeabilities of Nafion PEMs and water transport in operating PEFCs were investigated under comparable ex situ and in situ values of temperature and RH. The examined parameters included the type of driving forces (RH, pressure), the phases of water at PEM interfaces, PEM thickness, and the effect of catalyst layers at the membrane interfaces. Several experimental setups and schemes were designed and explored. Water permeability at 70°C was determined for Nafion membranes exposed to either liquid or vapor phases of water. Chemical potential gradients of water across the membrane are controlled through the use of differences in RH (38-100%), in the case of contact with water vapor, and hydraulic pressure (0-1.2 atm), in the case of contact with liquid water. Three types of water permeation experiments were performed, labeled as vapor-vapor permeation (VVP), liquid-vapor permeation (LVP), and liquid-liquid permeation (LLP). Ex situ measurements revealed that the flux of water is largest... 

FIG. 25-19 Pervaporation of gas from liquid feed across membrane to vaporous permeate. (SOURCE Redrawn from Ref. 24.)... 

Pervaporation Liquid Vapor 1.5-60 (permeate is under vacuum) Nonporous Volatile organic compounds... 

It should be recognized that all plastic materials over a time period allow a certain amount of water vapor, organic gas, or liquid to permeate the thickness of the material. It is only a matter of degree of permeation between various materials used as barriers against vapors and gases. It has been found that the permeability coefficient is a function of the solubility coefficient and diffusion coefficient. The process of permeation is explained as the solution of the vapor into the incoming surface of the barrier, followed by diffusion through the barrier thickness, and evaporation on the exit side. 

K., Kondo, M., and NaA, Z. (2001) membrane preparation, single-gas permeation, and pervaporation and vapor permeation of water/organic liquid mixtures. Ind. Eng. Chem. Res., 40, 163-175. 

Stannett, V. Yasuda, H., "Liquid vs Vapor Permeation Through... 

A clever means of dynamic generation of standards at the part-per-million level involves permeation through a polymer. In 1966 O Keeffe and Ortman (34) described this technique primarily for air pollution standards. A condensable gas or vapor is sealed as a liquid in a Teflon tube under its saturation vapor pressure as shown in Figure 4.14. After an initial equilibration period the vapor permeates through the tube wall at a constant rate. This rate is determined by weight loss over a period of time. Temperature must be controlled to within .0.1°C to maintain 1% accuracy. In use the tube is thermostatted in a chamber that permits a diluent gas to fully flush the chamber. The concentration is then determined by the same equation used for diffusion tubes. However, since the rate is generally much less in permeation tubes it is usually reported in ng/min. 

Equation (2.79) expresses the driving force in pervaporation in terms of the vapor pressure. The driving force could equally well have been expressed in terms of concentration differences, as in Equation (2.83). However, in practice, the vapor pressure expression provides much more useful results and clearly shows the connection between pervaporation and gas separation, Equation (2.60). Also, the gas phase coefficient, is much less dependent on temperature than P L. The reliability of Equation (2.79) has been amply demonstrated experimentally [17,18], Figure 2.13, for example, shows data for the pervaporation of water as a function of permeate pressure. As the permeate pressure (p,e) increases, the water flux falls, reaching zero flux when the permeate pressure is equal to the feed-liquid vapor pressure (pIsal) at the temperature of the experiment. The straight lines in Figure 2.13 indicate that the permeability coefficient d f ) of water in silicone rubber is constant, as expected in this and similar systems in which the membrane material is a rubbery polymer and the permeant swells the polymer only moderately. 

Figure 9.3 illustrates the concept of permeation from a saturated vapor phase in equilibrium with the feed liquid as a tool to obtain Equation (9.5). A number of workers have experimentally compared vapor permeation and pervaporation separations and have sometimes shown that permeation from the... 

An alternative carrier-gas system uses a condensable gas, such as steam, as the carrier sweep fluid. One variant of this system is illustrated in Figure 9.7(d). Low-grade steam is often available at low cost, and, if the permeate is immiscible with the condensed carrier, water, it can be recovered by decantation. The condensed water will contain some dissolved organic and can be recycled to the evaporator and then to the permeate side of the module. This operating mode is limited to water-immiscible permeates and to feed streams for which contamination of the feed liquid by water vapor permeating from the sweep gas is not a problem. This idea has been discovered, rediscovered, and patented a number of times, but never used commercially [37,38], If the permeate is soluble in the condensable... 

This form stresses that part of the separation in the perva-poration process occurs independent of the presence of the membrane, /9evap. Equation (15) also stresses that part of the separation relies strictly on the identity of the membrane material being used, /9mem. In this context, the membrane is seen as separating a hypothetical vapor feed (in equilibrium with the actual liquid feed) and the downstream vapor permeate product. 

A vapor permeation process is considered that is, the membrane is placed above the vapor-liquid interface. 

Direct contact membrane distillation is a form of membrane distillation in which both the heated feed (liquid) and cold permeate (liquid) are kept in contact with porous hydrophobic membrane (Figure 19.1). In the absence of hydrostatic pressure difference, liquid-vapor interfaces are formed at the entrance of each membrane pore, and a vapor pressure difference is maintained on both sides of the membrane by applying a temperature difference. Water molecules evaporate from the hot liquid-vapor interface, diffuses through the membrane in the form of vapor, and get condensed on the permeate hquid-vapor interface kept at... 

The permeate flux increases with increasing feed velocity due to the reduction of the boundary layer thickness. Therefore, the temperamre and concentration at the liquid-vapor interface are approaching values close to those in the bulk solution. Garcia et al. [71] have observed that the feed pressure measured at the inlet of the feed cell frame increased with an increase in the feed flow rate consequently increasing the risk of membrane wetting. 

Mainly PV aided conversions have been studied and more in particular esterifications, a typical example of an equilibrium limited reaction with industrial relevance and well-known reaction mechanisms. " This hybrid process has already made it to several industrial applications. The thermodynamic equilibrium in such a reaction can be easily shifted and obtained in a shorter reaction time by removing one of the products. Pervaporation is especially interesting because it is not limited by relative volatility or azeotropes and energy consumption is generally low, because only the fraction that permeates undergoes the liquid/vapor phase change. It can also be operated at lower temperatures, which can better match the optimal conditions for reaction. 

Vapor permeation is often a preferred technique to pervaporation, and liquid feeds are often evaporated, especially, to run the vapor permeation process. The evaporator is typically operated under pressure, giving a vapor feed at the maximum temperature and flux rate consistent with membrane stability. The advantages are a simpler plant and more reliable operation, because both dissolved and suspended solids are removed in the evaporator and cannot damage the membranes. The disadvantage is the extra energy required to evaporate the complete feed (Fig. 5). 

Pervaporation dehydrates solvents without using any third substance or entrainer, simply, cheaply, and without problems and irrespective of vapor/liquid equilibria. On site solvent recovery by using pervaporation and vapor permeation is becoming standard practice in the pharmaceutical and chemical industries. 

Fig. 24 shows a vapor permeation unit installed to remove methanol from the top of a column, treating the boil-off from a transesterification reactor. The column is operated to condense overhead product close to the azeotropic point. Condensed liquid is refluxed through the column. Net overhead vapor is passed through the vapor permeation unit, which is sized to... 

Matsumoto M, Ueba K, Kondo K (2009) Vapor permeation of hydrocarbons through supported liquid membranes based on ionic liquids. Desalination 241 365-371... 

In 1954, Ethylene Dibromide (EDB) was introduced as a product for the preplant treatment of agricultural fields to control nematodes and it is still used worldwide. EDB is a volatile, halogenated hydrocarbon that is usually marketed as a liquid. The liquid is injected 15 to 30 centimeters beneath the soil surface with a tractor driven chisel tool where the vapors permeate soil air spaces and kill the... 

According to a recent conference given by Prof. Kita [162], the classical synthesis method currently used by Mitsui allows to produce about 250 zeolite membranes per day. Both the LTA and T types (Na K) membranes are now commercial and more than 80 pervaporation and vapor permeation plants are operating in Japan for the dehydration of organic liquids [163]. A typical pervaporation system, similar to the one described in [8], is shown in Fig. 11. One of the most recent applications concerns the production of fuel ethanol from cellulosic biomass by a vapour permeation/ pervaporation combined process. The required heat is only 1 200 kcal per liter of product, i.e. half of that of the classical process. Mitsui has recently installed a bio-ethanol pilot plant based on tubular LTA membranes in Brazil (3 000 liters/day) and a plant with 30 000 liters/day has been erected in India. The operating temperature is 130 °C, the feed is 93 % ethanol, the permeate is water and the membrane selectivity is 10 000. 

A membrane reactor using a H-ZSM5 membrane was used by Bernal et al. [3.42] to carry out the esterification reaction of acetic acid with ethanol. An equimolar etha-nol/acetic acid liquid mixture was fed in the membrane interior, while He gas was used as an inert sweep on the shell-side. In this particular application the membrane, itself, provides the catalysis for the reaction. NaA and T-type zeolite membranes have been utilized for esterification reactions in a PVMR and in a vapor permeation membrane reactor (VPMR) by Tanaka et al [3.43, 3.44]. Both membranes are hydrophilic and show good separation characteristics towards a number of alcohols. The NaA membrane was used to study the oleic/acid esterification in a vapor permeation membrane reactor (VPMR) at 383... 

Vapor permeation and pervaporation are membrane separation processes that employ dense, non-porous membranes for the selective separation of dilute solutes from a vapor or liquid bulk, respectively, into a solute-enriched vapor phase. The separation concept of vapor permeation and pervaporation is based on the molecular interaction between the feed components and the dense membrane, unlike some pressure-driven membrane processes such as microfiltration, whose general separation mechanism is primarily based on size-exclusion. Hence, the membrane serves as a selective transport barrier during the permeation of solutes from the feed (upstream) phase to the downstream phase and, in this way, possesses an additional selectivity (permselectivity) compared to evaporative techniques, such as distillation (see Chapter 3.1). This is an advantage when, for example, a feed stream consists of an azeotrope that, by definition, caimot be further separated by distillation. Introducing a permselective membrane barrier through which separation is controlled by solute-membrane interactions rather than those dominating the vapor-liquid equilibrium, such an evaporative separation problem can be overcome without the need for external aids such as entrainers. The most common example for such an application is the dehydration of ethanol. 

Vapor permeation (VP) and pervaporation (PV) are membrane separation processes whose only difference lies in the feed fluid being a vapor (VP) or a liquid (PV), respectively. This difference has impHcations for feed fluid handling as well as the nature of the transport phenomena occurring in the feed stream, as in VP the feed fluid is compressible whilst in PV it is effectively not however, this does not in any way affect the transport phenomena across and after the membrane barrier. For this reason, vapor permeation and pervaporation will be discussed simultaneously, with differences being expHcitly emphasized where necessary. 

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