Pressure-driven membrane filtration processes

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 ... 

Figure 6.1 Classification of pressure-driven membrane filtration processes based on the size of particles and molecules removed. (From Applied Water Solution, 2006.)...

The pressure driven liquid filtration processes (MF, UF, and RO) may be distinguished by the size of the particle or molecule the membrane is capable of retaining or passing. This roughly relates to the pore-size of the membrane. Figure P.1 shows the pore sizes of MF, UF and RO membranes. Obviously, all particles or molecules larger than the rated pore size will be retained. 

Microfiltration (MF) is the most common pressure-driven membrane separation process and also the easiest to understand [1, 2]. It is simply conventional coarse filtration running at very low pressures (typically below 2 bar) owing to the open structure of the membranes. Darcy s law is apphcable showing a proportionality between the appHed pressure difference AP and the flux J through the membrane. 

Nanohybrid materials have been furthermore used for ultra-/nanofiltration applications. Nanofiltration is a pressure-driven membrane separation process and can be used for the production of drinking water as well as for the treatment of process and waste waters. Some apphcations are desalination of brackish water, water softening, removal of micropollutants, and retention of dyes. Ultrafiltration membranes based on polysulfones filled with zirconia nanoparticles are usually prepared via a phase-inversion technique and have been used since 1990 [328]. Various studies were done in order to assess the effect of the addition of Zr02 to polysulfone-based ultrafiltration membranes [329] and the influence of filler loading on the compaction and filtration properties of membranes. The results indicate that the elastic strain of the nanohybrid membranes decreases and the time-dependent strain... 

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... 

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. 

Nano filtration According to the International Union of Pure and Applied Chemistry (lUPAC) recommendations [16] nanofiltration is a pressure-driven membrane-based separation process in which particles and dissolved molecules smaller than about 2 nm are retained. ... 

Liquid separations including reverse osmosis, ultra and microfiltration are pressure driven membrane processes. Membranes are broadly applied to effect separations in these areas. They are used in the making of potable water, in filtration and concentration in the food industries, in the electronic industries... 

Sometimes considered as another pressure-driven membrane process, diafiltration (dilution mode) is in fact nothing other than an improved design for a more enhanced purification (Fig. 3.6-2) [1, 5]. It has, for instance, appHcations in the pharmaceutical, biotech and food industries where a complete separation of high MW compounds from low MW compounds is required. After a first separation, the retentate is diluted again before the next separation purifies the stream further. This process can be repeated as often as desired. Without the dilution, the filtration would stop at a certain point when still too much of the unwanted compound would be present because of fouHng or an excessive increase in osmotic pressure. 

The main problem with pressure-driven membrane processes is the flux decline as a function of filtration time (Fig. 3.6-3) due, most importantly, to concentration polarization (remaining constant once estabhshed) and membrane fouling (worsening as a function of time). These cause extra resistances on top of the membrane resistance and thus slow down the transport This reduction can be as severe as 99% of the initial flux value in M F. Reviews are available on these matters [7], some focusing in more detail on in situ monitoring techniques [8], some only on concentration polarization [9] others only on fouling [10-12]. 

Membrane fouling may result in a significant increase in filtration resistance, leading to unstable filtration behavior. The pressure-driven membrane processes can be operated either with constant feed pressure or in constant flux mode. For constant pressure operation where the transmembrane pressure (TMP) is maintained at a constant value during the filtration, the flux will decline with time due to the... 

Pressure-driven membrane processes have also been applied for recovery of valuable residual or side-products present in the spent fermentation broth, which is called whole stillage. It contains fiber, oil, protein, diverse unfermented components of the grain, and yeast cells (Abels et ak, 2013). The whole stillage can be separated into two streams—namely, thick stillage and thin stillage—via centrifugation or vacumn belt filtration. Membrane separations can be used for the recovery of valuable solids from thin stillage (Arora et ak, 2009). 

Membrane separation processes have been applied to many industrial production systems for the purpose of clarification, concentration, desalting, waste treatment, or product recovery. Broadly speaking, membrane filtration can be classified as microfiltration, ultrafiltration, nanofiltration, reverse osmosis, and dialysis or electrodialysis. In this section, the discussion will only cover microfiltration and ultrafiltration, both of which are pressure-driven membrane processes. 

Membrane techniques have been ap>phed to parform fine separation of imdesirable constituents and these show promising results for the selective removal of volatile solutes from ILs [WO 2003/039719]. Haerens et al. (2010) investigated the use of pressure-driven membrane processes, nano-filtration, reverse osmosis and parvaporation, as a possibihty to recycle ILs from water. They used Ethaline200 (a deep eutectic formed between chohne chloride (a quaternary amine salt) and ethylene glycol] to parform these tests and compared their results with those foimd in the hterature. 

It is not possible at present to provide an equation, or set of equations, that allows the prediction from first principles of the membrane permeation rate and solute rejection for a given real separation. Research aimed at providing such a prediction for model systems is under way, although the physical properties of real systems, both the membrane and the solute, are complex. An analogous situation exists for conventional filtration processes. The general membrane equation is an attempt to state the factors which may be important in determining the membrane permeation rate for pressure driven processes. This takes the form ... 

Hydrodynamic. For a pressure driven process such as ultrafiltration the flow of solvent towards the membrane results in a drag which carries the solute in the same direction. This drag is a function of the distance of the solute from the pore entrance. At large distances it is equal to the isolated solute value (Stokes limit), but as the solute approaches and begins to enter the pore, the drag, for a constant filtration velocity, increases due to the restriction of solvent flow. This increase depends on the ratio of solute diameter to pore diameter. 

Ultrafiltration — This process has been successful with mixtures difficult to separate, such as oily machining wastes and oily wastewater. A pressure-driven filtration membrane separates multicomponent solutes from solvents, according to molecular size, shape and chemical bonding. Substances below a preselected molecular size are driven through the membrane by hydraulic pressure, while larger molecules, such as oil droplets, are held back. Effluent oil concentration depends on influent concentration, but properly operated ultrafiltration units can produce oilfree water (less than 0.1 ppm for all practical purposes). 

The best performing industrial membranes permeate water in preference to other components hence the pressure driven filtration and RO processes are used typically to purify water. In contrast, pervaporation and vapor permeation are commonly, but not exclusively, used to remove water from organics (Fig. 1). 

Further downstream, in order to concentrate and purify the product more, it is possible to use membrane filtration. Here, some form of semi-permeable membrane is used to separate the components of a liquid stream. In most of the commercially important processes the driving force is pressure, the solvent (usually water) is driven through the membrane while the solute(s) are retained. This type of process includes reverse osmosis, ultrafiltration and microfiltration. 

Gutmam, R.G 1987. Membrane filtration The technology of pressure driven crossflow processes. Adam Hilger, Bristol, England. 

Microfiltation (MF) and ultrafiltration (UF) are pressure-driven filtration processes that utilize a porous membrane to selectively retain compounds larger than a nominal molar mass (retentate) while allowing particles of lower molar mass to pass through the membrane (permeate). It is a freqnently nsed alternative to isoelectric precipitation. 

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