Liquid processes with porous membrane

A new supercritical fluid process has been developed for the continuous extraction of liquids. The most useful solvent employed in the recently patented process is supercritical or near-critical carbon dioxide(l). At the heart of the process are porous membranes. Their porosity combined with a near-critical fluid s high diffusivity create a dynamic non-dispersive contact between solvent and feed liquid. The technique is dubbed porocritical fluid extraction and will be commercialized as the Porocrit Process. 

There are several processes for the separation of liquid mixtures using porous membranes or asymmetric polymer membranes. With porous membranes, separation may depend just on differences in diffusivity, as is the case with dialysis, where aqueous solutions at atmospheric pressure are on both sides of the membrane. For liquid-liquid extraction using porous membranes, the immiscible raffinate and extract phases are separated by the membrane, and differences in the equilibrium solute distribution as well as differences in diffusivity determine the extract composition. 

Membrane pressure-driven processes, namely, MF, UF, NF, and RO are normally carried out in the liquid phase. Although water permeates through the membrane, other species are partially or completely rejected. According to Fane et al.. The MF-UF range can be considered as a continuum [11] both processes involve porous membranes. MF is carried out using symmetric membranes, with pore size ranging from 0.05 to 10 pm. UF, instead, requires asymmetric membranes, with pore size from 1 to 100 nm. The NF and RO spectrum is also considered as a continuum [11]. NF/RO membranes are usually thin-film composite (TFC) structures with nonpo-rous skin. The most important features of pressure-driven membrane processes are resumed in Table 1.3. 

The beginning of modeling of polymer-electrolyte fuel cells can actually be traced back to phosphoric-acid fuel cells. These systems are very similar in terms of their porous-electrode nature, with only the electrolyte being different, namely, a liquid. Giner and Hunter and Cutlip and co-workers proposed the first such models. These models account for diffusion and reaction in the gas-diffusion electrodes. These processes were also examined later with porous-electrode theory. While the phosphoric-acid fuel-cell models became more refined, polymer-electrolyte-membrane fuel cells began getting much more attention, especially experimentally. 

Pervaporation is a membrane separation process in which a dense, non-porous membrane separates a liquid feed solution from a vapour permeate (Fig. 19.2c). The transport across the membrane barrier is therefore based, generally, on a solution-difliision mechanism with an intense solute-membrane interaction. It... 

Porous Membrane DS Devices. The applicability of a simple tubular DS based on a porous hydrophobic PTFE membrane tube was demonstrated for the collection of S02 (dilute H202 was used as the scrubber liquid, and conductometric detection was used) (46). The parameters of available tubular membranes that are important in determining the overall behavior of such a device include the following First, the fractional surface porosity, which is typically between 0.4 and 0.7 and represents the probability of an analyte gas molecule entering a pore in the event of a collision with the wall. Second, wall thickness, which is typically between 25 and 1000 xm and determines, together with the pore tortuosity (a measure of how convoluted the path is from one side of the membrane to the other), the overall diffusion distance from one side of the wall to the other. If uptake probability at the air-liquid interface in the pore is not the controlling factor, then items 1 and 2 together determine the collection efficiency. The transport of the analyte gas molecule takes place within the pores, in the gas phase. This process is far faster than the situation with a hydrophilic membrane the relaxation time is well below 100 ms, and the overall response time may in fact be determined by liquid-phase diffusion in the boundary layer within the lumen of the membrane tube, by liquid-phase dispersion within the... 

Highly porous membranes are prepared by a process based on the fibrillation of high-molecular-weight PTFE. Since they have a high permeability for water vapor and none for liquid water, it is combined with fabrics and used for breathable waterproof garments and camping gear. Other uses for these membranes are for special filters, analytical instruments, and in fuel cells.13... 

Capillary condensation provides the possibility of blocking pores of a certain size with the liquid condensate simply by adjusting the vapor pressure. A permporometry lest usually begins at a relative pressure of 1, thus all pores filled and no unhindered gas transport. As the pressure is reduced, pores with a size corresponding to the vapor pressure applied become emptied and available for gas transport. The gas flow through the open mesopores is dominated by Knudsen diffusion as will be discussed in Section 4.3.2 under Transport Mechanisms of Porous Membranes. The flow rate of the noncondensable gas is measured as a function of the relative pressure of the vapor. Thus it is possible to express the membrane permeability as a function of the pore radius and construct the size distribution of the active pores. Although the adsorption procedure can be used instead of the above desorption procedure, the equilibrium of the adsorption process is not as easy to attain and therefore is not preferred. 

Compared to the above-mentioned methods, exposure of cells at the air-liquid interface (ALI) is the most realistic method. Using the ALI system, the apical surface of the epithelial cell monolayer is exposed to PM while the basolateral surface of the cells is fed with the medium through the porous membrane (Aufderheide 2005) (Fig. 1). In this process, the PM characteristics remain unchanged prior to deposition. The Navicyte Horizontal Diffusion Chamber (Harvard Apparatus, USA) is one such system. It is designed to expose cells to the atmosphere while media is supplied through a porous membrane. The exposure chamber is mounted on a heated block at 37 °C (Bakand et al. 2006). Aufderheide... 

Compared with liquid membranes, the relatively small volume of solvent in the porous membrane offers the advantages of possible usage of expensive carriers with high separation factors. The advantages of SLMs also include the easy scale-up, low energy requirements, low capital and operating costs, simpler configuration and process, known interface area, and predictable separation performance. 

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