Fluid-applied membranes application

The earliest methods of applying fluids as membrane applications, as we would expect, were ... 

The concept of coupling reaction with membrane separation has been applied to biological processes since the seventies. Membrane bioreactors (MBR) have been extensively studied, and today many are in industrial use worldwide. MBR development was a natural outcome of the extensive utilization membranes had found in the food and pharmaceutical industries. The dairy industry, in particular, has been a pioneer in the use of microfiltra-tion (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) membranes. Applications include the processing of various natural fluids (milk, blood, fruit juices, etc.), the concentration of proteins from milk, and the separation of whey fractions, including lactose, proteins, minerals, and fats. These processes are typically performed at low temperature and pressure conditions making use of commercial membranes. 

The types of hollow fiber membranes in production are illustrated in Figure 3.32. Fibers of 50- to 200-p.m diameter are usually called hollow fine fibers. Such fibers can withstand very high hydrostatic pressures applied from the outside, so they are used in reverse osmosis or high-pressure gas separation applications in which the applied pressure can be 1000 psig or more. The feed fluid is applied to the outside (shell side) of the fibers, and the permeate is removed down the fiber bore. When the fiber diameter is greater than 200-500 xm, the feed fluid is commonly applied to the inside bore of the fiber, and the permeate is removed from the outer shell. This technique is used for low-pressure gas separations and for applications such as hemodialysis or ultrafiltration. Fibers with a diameter greater than 500 xm are called capillary fibers. 

In one of our earlier applications, FCS diagnosed unanticipated micelle formation and led to the first development of confocal image microscopy for smaller focal volumes [3]. Recognizing the effective applications of fluorescent marker d mamics to understand cell membrane d mamics, we applied FCS to molecular diffusion on cell membranes, entering thereby into a long series of studies of the dynamics of membrane processes in life, which was at that time a quagmire of conflicting ideas [4]. Later, we also extended FCS theory to fluid flow analysis [9]. It has proven useful for a diversity of ultrafast chemical kinetics as well, c.f. [10-13]. 

When a DC pulse is applied to a couple of fluid-phase vesicles, which are in contact and oriented in the direction of the field, electrofusion can be observed. Vesicle orientation (and even alignment into pearl chains) can be achieved by application of an AC field to a vesicle suspension. This phenomenon is also observed with cells [164, 165] and is due to dielectric screening of the field. When the suspension is dilute, two vesicles can be brought together via the AC field and aligned. A subsequent application of a DC pulse to such a vesicle couple can lead to fusion. The necessary condition is that poration is induced in the contact area between the two vesicles. The possible steps of the electrofusion of two membranes are schematically illustrated in Figure 7.8a. In Sections 7.5.2.1 and 7.5.2.2, consideration will be given to the fusion of vesicles with different membrane composition or different composition of the enclosed solutions. 

In vitro percutaneous absorption experiments are sometimes conducted with the principles of diffusion rigorously in mind. An aaueous solution of the penetrsint is applied on one side of the skin and its diffusion followed into identical aaueous fluid on the other side of the membrane. Stirring devices are used on both sides of the membrane (2-chambered cell technlaue) (5). Currently, skin absorption is freauently measured after application of the test substance in a small amount of vehicle to the surface of the skin. Permeation is followed by removal of allauots from the stirred solution in the receptor below the skin (1-chamber static cell) (, 6). This procedure more closely simulates the in vivo situation because the skin is exposed to ambient conditions and is not excessively hydrated as in the 2-chamber procedure. 

Electrofiltration is related to the application of an electric field to improve the efficiency of pressure-driven membrane filtration [110], Figure 10.47 shows the basic configuration of electrofiltration, where an electric field is applied across microfiltration or UF membranes in flat sheet modules, tubular modules, and SWMs. The electrode is installed on either side of the membrane with the cathode in the permeate side and the anode in the feed side. Usually, the membrane support is made of stainless steel or the membrane itself is made of conductive materials, to form the cathode. Titanium coated with a thin layer of a noble metal such as platinum could, according to Bowen [111], be one of the best anode materials. Wakeman and Tarleton [112] analyzed the particle trajectory in a combined fluid flow and electric field and suggested that a tubular configuration should be more effective in the use of electric power than the flat and multitubular module. 

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