Pressure driven processes

The feed pump is used to pressurize the feed to a required pressure. In ultrafiJtration and microfiltraiion flux decline is very severe due to concentration polarization and fouling. To reduce this effect as much as possible, the mass transfer in the boundary layer must be improved and this can be achieved by increasing the cross-flow velocity. In most cross-flow membrane operations, two pumps are employed, a feedpump to pressurize the feed and a circulation pump to adjust the cross-flow velocity. The aim of the circulation pump is to maintain a high cross-flow velocity. Since low hydrodynamic pressures 4 ° cquired in 

In the case of high pressure applications such as reverse osmosis, and nanoflltration, a turbine may be utilized to recover part of the energy. A liquid turbine is a device in which the liquid does work on the turbine blades at the expense of its kinetic energy. Hence, the process is expansion of a liquid from high pressure to low pressure in which work is produced. The process is in fact the opposite of compression and the same equation can be derived for the work only the efficiency q is now in the nominatot 

for a turbine, the power is given as and the efficiency of a turbine normally lies between 0.5 q 0.8. In the solved problems some examples have been worked. 

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For most hydrardic pressure-driven processes (eg, reverse osmosis), dense membranes in hoUow-fiber configuration can be employed only if the internal diameters of the fibers are kept within the order of magnitude of the fiber-wall thickness. The asymmetric hoUow fiber has to have a high elastic modulus to prevent catastrophic coUapse of the filament. The yield-stress CJy of the fiber material, operating under hydrardic pressure, can be related to the fiber coUapse pressure to yield a more reaUstic estimate of plastic coUapse ... 

Cavitation and Flashing From the discussion on pressure recoveiy it was seen that the pressure at the vena contracta can be much lower than the downstream pressure. If the pressure on a hquid falls below its vapor pressure (p,J, the liquid will vaporize. Due to the effect of surface tension, this vapor phase will first appear as bubbles. These bubbles are carried downstream with the flow, where they collapse if the pressure recovers to a value above p,. This pressure-driven process of vapor-bubble formation and collapse is known as cavitation. 

FIG. 22-47 Schematic of pressure-driven processes showing nomenclature. 

Basic Principles of Operation RO and NF are pressure-driven processes where the solvent is forced through the membrane by pressure, and the undesired coproducts frequently pass through the membrane by diffusion. The major processes are rate processes, and the relative rates of solvent and sohite passage determine the quality of the product. The general consensus is that the solution-diffusion mechanism describes the fundamental mechanism of RO membranes, but a minority disagrees. Fortunately, the equations presented below describe the obseiwed phenomena and predict experimental outcomes regardless of mechanism. 

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 ... 

Two common types of one-dimensional flow regimes examined in interfacial studies Poiseuille and Couette flow [37]. Poiseuille flow is a pressure-driven process commonly used to model flow through pipes. It involves the flow of an incompressible fluid between two infinite stationary plates, where the pressure gradient, Sp/Sx, is constant. At steady state, ignoring gravitational effects, we have... 

Membrane distillation offers a number of advantages over alternative pressure-driven processes such as reverse osmosis. Because the process is driven by temperature gradients, low-grade waste heat can be used and expensive high-pressure pumps are not required. Membrane fluxes are comparable to reverse osmosis fluxes, so membrane areas are not excessive. Finally, the process is still effective with slightly reduced fluxes even for very concentrated solutions. This is an advantage over reverse osmosis, in which the feed solution osmotic pressure places a practical limit on the concentration of a salt in the feed solution to be processed. 

Dialysis continues to meet certain specialized applications, particularly those in biotechnology and the life sciences. Delicate substances can be separated without damage because dialysis is typically performed under mild conditions ambient temperature, no appreciable transmembrane pressure drop, and low-shear flow. While slow compared with pressure-driven processes, dialysis discriminates small molecules from large ones reliably because the absence of a pressure gradient across the membrane prevents convective flow through defects in the membrane. This advantage is significant for two... 

A practically useful predictive method must provide quantitative process prediction from accessible physical property data. Such a method should be physically realistic and require a minimum number of assumptions. A method which is firmly based on the physics of the separation is likely to have the widest applicability. It is also an advantage if such a method does not involve mathematics which is tedious, complicated or difficult to follow. For the pressure driven processes of microfiltration, ultrafiltration and nanofiltration, such methods must be based on the microhydrodynamics and interfacial events occurring at the membrane surface and inside the membrane. This immediately points to the requirement for understanding the colloid science of such processes. Any such method must account properly for the electrostatic, dispersion, hydration and entropic interactions occurring between the solutes being separated and between such solutes and the membrane. 

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. 

Advancement in Pressure-Driven Processes and Current World Scenario... 

The elaboration of proper pretreatment methods, application of antisealants, and minimization of secondary wastes created during cleaning cycles are of great importance. At present the research work on use of pressure-driven processes for radioactive waste treatment is focused on following issues ... 

High thermal energy consumption comparing with pressure-driven processes unless cheap energy source or waste heat is utilized Cannot be applied for wastes with volatile radioactive compounds The necessity of pretreatment and periodical scale removal Process limited by concentration (low conductivity for diluted solutions) Fouling is a problem in higher concentrations... 

Separation processes as a whole have grown in importance because of increasingly stringent requirements for product purity [1]. Among the different membrane techniques, pressure-driven processes such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) were the first to undergo rapid commercialization [2-A], These processes basically differ in pore size distribution of membranes used and the types of compounds recovered. A typical schematic of the exclusion of various compounds through different membrane processes is illustrated in Figure 42.1. 

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