Pressure driven membrane processes

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). 

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. 

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). 

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

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. 

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. 

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. 

Liquid separation. Separation can take place between solvents and solutes, macromolecules or particles or between species in liquid media by the effect of size exclusion. That is, those molecules or colloids larger than the size of the membrane pores will be retained or rejected while those smaller ones can pass through the membrane. The size exclusion mechanism predominates in pressure driven membrane processes such as microfiltration, ultrafiltration and even nanofiltration which has a molecular selectivity on the order of one nanometer. 

One other form of pressure-driven membrane process should be mentioned piezodialysis In this process, a selectiwly salt-permeable membrane is used... 

In practice, UF operation is usually limited by cake formation In all pressure-driven membrane processes both the solvent (water) and solutes are carried convectively to the membrane surface. In UF, only the solvent and microsolutes pass through the membrane. The macromolecules are rejected and, became of their small... 

Winzeler HB and Belfort G, Enhanced performance for pressure-driven membrane processes The argument for fluid instabilities, J. Membr. Sci. 1993 80 35 7. 

The drastic reduction of the permeate flux to only a fraction of the theoretical capacity of the membrane is rather common in pressure-driven membrane processes, but it is more pronounced for beer as compared to other fluid foods such as milk, wine, or fruit juices. This explains the earlier introduction of membrane technology at a commercial scale in those industries as compared to the beer industry. 

Marshall, A.D. and Daufin, G., Physico-chemical aspects of membrane fouhng by dairy fluids, in Fouling and Cleaning in Pressure Driven Membrane Processes, International Dairy Federation, Bmssels, Belgium, 1995, p. 8. 

Some areas of application are the nuclear industry and the treatment of radioactive liquid wastes, with two main purposes reduction in the waste volume for further disposal, and reuse of decontaminated water. Pressure-driven membrane processes (microfiltration, ultrafiltration, nanofiltration, and reverse osmosis [RO]) are widely used for the treatment of radioactive waste. 

During the last two decades, pressure-driven membrane processes namely reverse osmosis (RO), nanofiltration (NF), and ultrafiltration (UF) have found increased applications in water utilities and chemical industries. Unlike RO, NF, and UF, the Donnan membrane process (DMP) or Donnan dialysis is driven by an electrochemical potential gradient across an ion-exchange membrane. Theoretically, the DMP is not susceptible to fouling because particulate matter or large organic molecules do not concentrate on the membrane surface, as commonly observed with pressure-driven membrane processes. DMP has been used in the past in hydrometallurgical operations [19,20], for concentration of ionic contaminants [21,22] and for separation of... 

Pressure-Driven Membrane Processes in the Pulp and Paper Industry.985... 

The most widely used or tested membrane processes in pulp and paper mill applications are based on pressure-driven membrane processes MF, UF, NF, and RO. In the following sections, the characteristic properties of the membranes are discussed and their effect on filtration efficiency is summarized. In addition, some common influence of effluent properties and filtration conditions on membrane processes are discussed. 

PRESSURE-DRIVEN MEMBRANE PROCESSES IN THE PULP AND PAPER INDUSTRY... 

Afonso MD and De Pinho MN. Treatment of bleaching effluents by pressure-driven membrane processes—a review. In Membrane Technology Applications to Industrial Wastewater Treatment, Cuetano et al., Eds., Kluwer Academic, The Netherlands, 1995. 

Since the electrical resistance of the effiuent and parasitic currents are minimal at high level of impurities, specihc interest in electrically assisted membrane processes could increase due to more strict laws and legislation around effluents. The depletion of freshwater resources and the necessity to process brackish or seawater to produce potable water could promote the use of electrically assisted membrane processes in the future. Electrodialysis will have to compete with pressure-driven membrane processes such as reverse osmosis. The growing awareness of the unique cleaning ability of electrically ionized water (EIW) [47], a byproduct of electrodialysis, may be a factor to consider in the choice between ED and RO systems. NMR relaxation measurements were used to determine the water cluster size of electrically ionized water EIW. It is known that the water cluster size of EIW is signihcantly smaller than that of tap water. The smaller water cluster size is believed to enhance the penetration and extractive properties of EIW. Recently, EIW has been produced and used in several cleaning processes [47] in industry. 

Nanofiltration is a rapidly advancing membrane separation technique for concentration/separation of important fine chemicals as well as treatment of effluents in pharmaceutical industry due to its unique charge-based repulsion property [5]. Nanofiltration, also termed as loose reverse osmosis, is capable of solving a wide variety of separation problems associated with bulk drug industry. It is a pressure-driven membrane process and indicates a specific domain of membrane technology that hes between ultrafiltration and reverse osmosis [6]. The process uses a membrane that selectively restricts flow of solutes while permitting flow of the solvent. It is closely related to reverse osmosis and is called loose RO as the pores in NF are more open than those in RO and compounds with molecular weight 150-300 Da are rejected. NF is a kinetic process and not equilibrium driven [7]. 

Types of Pressure-Driven Membrane Processes as Categorized by Size Cut-Off Range... 

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