Pressure-driven membrane

The pressure difference between the high and low pressure sides of the membrane is denoted as AP the osmotic pressure difference across the membrane is defined as Att the net driving force for water transport across the membrane is AP — (tAtt, where O is the Staverman reflection coefficient and a = 1 means 100% solute rejection. The standardized terminology recommended for use to describe pressure-driven membrane processes, including that for reverse osmosis, has been reviewed (24). 

Reverse Osmosis and Ultrafiltration. Reverse osmosis (qv) (or hyperfiltration) and ultrafilttation (qv) ate pressure driven membrane processes that have become well estabUshed ia pollution control (89—94). There is no sharp distinction between the two both processes remove solutes from solution. Whereas ultrafiltration usually implies the separation of macromolecules from relatively low molecular-weight solvent, reverse osmosis normally refers to the separation of the solute and solvent molecules within the same order of magnitude in molecular weight (95) (see also Membrane technology). 

Plate-and-Frame (Conceptually the simplest, it is veiv much like a filter press. Once found in RO, UF, and IVIF, it is still the only module commonly used in electrodialysis (ED). A fevy applications in pressure-driven membrane separation remain (see Sec. 18 for a description of a plate-and-frarne filter press). 

The most common membrane systems are driven by pressure. The essence of a pressure-driven membrane process is to selectively permeate one or more species through the membrane. The stream retained at the high pressure side is called the retentate while that transported to the low pressure side is denoted by the permeate (Fig. 11.1). Pressure-driven membrane systems include microfiltration, ultrafiltration, reverse osmosis, pervaporation and gas/vapor permeation. Table ll.l summarizes the main features and applications of these systems. 

Ultrafiltration is one of the most widely used of the pressure-driven membrane separation processes. The solute retained or rejected by ultrafiltration membranes are those with... 

Microfiltration. Microfiltration is a pressure-driven membrane filtration process and has already been discussed in Chapter 8 for the separation of heterogeneous mixtures. Microfiltration retains particles down to a size of around 0.05 xm. Salts and large molecules pass through the membrane but particles of the size of bacteria and fat globules are rejected. A pressure difference of 0.5 to 4 bar is used across the membrane. Typical applications include ... 

Pressure-driven membrane processes to replace the absorption-sequence are under development, but the separated H2S (and other components which co-diffuse) will have to be treated with Claus or similar technology. 

Pressure driven membrane process, 78 507 Pressure-driven membranes, in water treatment, 26 111 Pressure drop, 77 804 from area change, 73 261-262 in cake filtration, 77 330-332, 333-335 flow maldistribution and, 73 270 from flow turning, 73 262 frictional, 73 260-261 in gas adsorption, 7 657-658 in hyperbar vacuum filtration, 77 377 shellside tube bundle, 73 262-263 in vacuum filtration, 77 349-350 Pressure drop calculations, in heat exchanger design, 73 259-260 Pressure drop information, for resins, 74 399... 

Van Der Bmggen, B., Lejon, L., Vanecasteele, C. Reuse, Treatment, and Discharge of the Concentrate of Pressure-Driven Membrane Processes. Environ. Sci. Technol. 37(17), 3733-3738 (2003). 

Table 10.2 Pressure Driven Membrane Separation Processes (Lonsdale, 1982)...

Shih, M.-C. (2005) An overview of arsenic removal by pressure-driven membrane processes. Desalination, 172(1), 85-97. 

The range of application of the three pressure-driven membrane water separation processes—reverse osmosis, ultrafiltration and microfiltration—is illustrated in Figure 1.2. Ultrafiltration (Chapter 6) and microfiltration (Chapter 7) are basically similar in that the mode of separation is molecular sieving through increasingly fine pores. Microfiltration membranes filter colloidal particles and bacteria from 0.1 to 10 pm in diameter. Ultrafiltration membranes can be used to filter dissolved macromolecules, such as proteins, from solutions. The mechanism of separation by reverse osmosis membranes is quite different. In reverse osmosis membranes (Chapter 5), the membrane pores are so small, from 3 to 5 A in diameter, that they are within the range of thermal motion of the polymer... 

C. Kleinstreuer and G. Belfort, Mathematical Modeling of Fluid Flow and Solute Distribution in Pressure-driven Membrane Modules, in Synthetic Membrane Processes,... 

Lower operating temperatures than conventional distillation Lower operating pressures than conventional pressure-driven membrane separation... 

The pressure-driven membrane processes can be operated at fixed pressure (FP) or fixed flux (FF), and FP tends to be lab and small scale and FF is large-scale commercial. Fouling for FP shows as a flux decline and for FF as TMP rise (Figure 6.1(b)). The fouling kinetics differ since FP becomes self-limiting as flux-driven fouling slows down, whereas for FF it is self-accelerating as foulants steadily accumulate and concentration polarization accelerates. These differences mean that extrapolation of FP trends to FF requires caution. 

Another interesting possibility is the use of pressure-driven membrane processes, in particular MF and UF are becoming standard and very efficient pretreatment options for sea- and brackish-water desalination. Also, for wastewater treatment, MF/UF pretreatment technology can efficiently reduce the highly fouling nature of the feed. 

Although the integration of RO with other pressure-driven membrane processes has led to significant improvements in membrane-based desalination process economics, another fundamental problem is the environmental aspects of brine discharge from reverse-osmosis desalination plants. 

The majority of PMRs described in literature combines photocatalysis with a pressure-driven membrane technique, such as nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF), in which the catalyst is contained in the pressurized side of the membrane. 

Membrane distillation - photocatalysis To solve the problem of membrane fouling observed in the pressure-driven membrane photoreactor, Mozia et al. [90] studied a new type of PMR in which photocatalysis was combined with a direct contact membrane distillation (DCMD). MD can be used for the preparation of ultrapure water or for the separation and concentration of organic matter, acids and salt solutions. In the M D the feed volatile components are separated by means of a porous hydrophobic membrane thanks to a vapor-pressure difference that acts as driving force and then they are condensed in cold distillate (distilled water), whereas the nonvolatile compounds were retained on the feed side. 

In this way, the pollutants are extracted from the turbid water and then degraded, without the need of a transmembrane pressure, avoiding the fouling of membrane, which is an expensive problem in case of pressure-driven membrane processes. 

In a pressure-driven membrane process the molecules are generally rejected by the membrane and therefore their concentrations in the permeate are lower than those in the feed solution. However, an accumulation of excess particles can occur at the membrane surface with the creation of a boundary layer. This phenomenon, called concentration polarization, causes a different membrane performance. In particular, with low molecular weight solutes the observed rejection will be lower than the real retention or, sometimes, it could be negative. 

Separation of isopropanol (IPA) and water by pervaporation has also reached production scale. Much of the current capacity is devoted to azeotrope breaking and dehydration during IPA synthesis. Recently, anhydrous isopropanol has become a preferred drying solvent in the semiconductor industry, where chip wafers are first washed with ultrapure water, then rinsed with the alcohol to promote uniform drying. The water-laden isopropanol generated can be conveniently reused after dehydration by pervaporation. Unlike with pressure-driven membrane processes such as RO or UF, particulates and nonvolatile substances such as salts are not carried over during pervaporation. This helps maintain the effectiveness of contamination control. 

As the quality of drinking water sources gets worse, the methods of water treatment or the traditional water treatment systems need to be modernized. Pressure-driven membrane systems such as reverse osmosis (RO), nanofiltration (NF) and ultrafiltration (UF) and electric-driven membrane system such as... 

Jelen, P. 1992. Pressure-driven membrane processes principles and definitions. In New Applications of Membrane Processes , IDF Special Issue 9201, pp. 7-14. 

The work described in this chapter is especially concerned with three of the most widely used pressure driven membrane processes microfiltration, ultrafiltration and nanofiltration. These are usually classified in terms of the size of materials which they separate, with ranges typically given as 10.0-0.1 xm for microfiltration, 0.1 p.m-5 nm for ultrafiltration, and 1 nm for nanofiltration. The membranes used have pore sizes in these ranges. Such pores are best visualised by means of atomic force microscopy (AFM) [3]. Figure 14.1 shows an example of a single pore in each of these three types of membrane. An industrial membrane process may use several hundred square meters of membrane area containing billions of such pores. 

Nanofiltration (NF) is a pressure-driven membrane separation technology used to separate ions from solution. Nanofiltration membranes were widely available beginning in the 1980 s. This technology uses microporous membranes with pore sizes ranging from about 0.001 to 0.01 microns. Nanofiltration is closely related to RO in that both technologies are used to separate ions from solution. Both NF and RO primarily use thin-film composite, polyamide membranes with a thin polyamide skin atop a polysulfone support (see Chapter 4.2.2). 

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