Bulk flow parallel to the force

Section 6.3.3.3 studies RO in bulk flow parallel to the force configuration and describes various membrane transport considerations and flux expressions. Practical RO membranes are employed in devices with bulk feed flow perpendicular to the force configuration, as illustrated in Section 7.2.I.2. A simplified solution for a spiral-wound RO membrane is developed analytical expressions for the water flux as well as for salt rejection are obtained and illustrated through example problem solving. A total of sbt worked example problems have been provided up to Chapter 7. Chapter 9 (Figure 9.1.5) shows a RO cascade in a tapered configuration. Section 10.1.2 calculates the minimum energy required in reverse osmosis based desalination and compares it with that in evaporation. Section 11.2 covers the sequence of separation steps in a water treatment process for both desalination and ultrapure water production. The very important role played by RO in such plants is clearly illustrated. 

We consider here the role of bulk flow parallel to the direction of the chemical potential gradient based force in phetse-equilibrium based open two-phase systems. Vapor-liquid systems of flash vaporization, flash devolatilization and batch distillation are considered first, followed by a liquid-liquid system for extraction. Solid-liquid systems for zone melting and normal freezing are studied thereafter to explore how bulk flow parallel to the force direction is essential to considerable purifleation of solid systems followed by solid-vapor systems as in drying. 

Figure 6.3.26. Ultrafiltration, (a) UF in a batch cell macrosolute concentration profile infeed side (b) Piston driven UF in a batch cell bulk flow parallel to the force, (c) Observed behavior ofsolvent flux vs. AP in macrosolute ultrafiltration. For an explanation of (l)-(4), see the text.

The above analysis/description of solvent flux and macrosolute rejection/retention/ttansmission far an ultra-flllration membreme was carried out in the context of a pseudo steady state analysis in a batch cell (Figure 6.3.26 (a)). Back diffusion of the macrosolute from the feed solution-membrane interface to the bulk solution takes place by simple difflision against the small bulk flow parallel to the force direction. The resulting mass-transfer coefficients for macrosolutes will be quite small the solvent flux levels achievable will be quite low. For practically useful ultrafiltration rates, the mass-transfer coefficient is increased via different flow configurations with respect to the force. 

In Sections 6.3.3.5 and 6.4.2.2, we described gas separation by permeation through a membrane under the conditions of bulk flow parallel to the force and having a well-mixed flow on both sides of the membrane. We learned that the configuration of bulk gas flow parallel to the force direction was not very useful it could not be used in practice with large membrane surface area, and it led to unsteady state... 

Bulk flow via direct mechanical conveying can also allow the achievement of bulk flow parallel to the direction of the force. Consider, for example, a liquid phase y = 2 of volume V2 in a vessel containing species i = 1,2. Add a certain volume Vi of an immiscible Uquid phase y = 1 on top of this liquid phase j = 2. This liquid phase will extract preferentially one of the species i. After equilibrium has been achieved, this top Uquid... 

Figure 6.3.21. Deadend filtration bulk flow parallel to the direction of the force due to applied 4P = Pf - Pp.

In Section 3.4.2.1.1, we were introduced to the rate equations for the membrane separation technique of pervaporation a volatile liquid mixture on the feed side of the nonporous membrane at around atmospheric pressure the other side of the membrane has a vapor/ gaseous phase, usually at a lower pressure. The creation and maintenance of a vapor/gaseous phase on the permeate side can be implemented by either passing a sweep gas/vapor on the permeate side or by pulling a vacuum, in the configuration of bulk flow parallel to the direction of the force across a membrane (see Figures 6.3 (i)-(I)), vacuum will provide for permeated vapor flow parallel to the direction of force sweep gas/vapor will not (in general). 

In this chapter we illustrate separation achieved in the bulk flow of phase(s) parallel to the force direction. We will also briefly study CSTSs at the end of this chapter. Batch weU-stirred tank based separators without any continuous feed in or product out will also be studied. 

In the bulk flow parallel to force configuration of Figure 6.3.30(b),... 

We have been introduced to various aspects of solvent extraction in the following sections Section 3.3.7.2, liquid-liquid equilibria in aqueous-organic, organic-organic, aqueous-aqueous systems Section 3.4.1.2, flux expressions in liquid-liquid systems Section 3.4.3.2, solute transport in phase barrier membranes Section 4.1.3, separation achieved in a closed vessel Section 5.2.2, role of chemical reaction in liquid extraction Section 5.3.2, rate controlled aspects of chemical reaction in liquid-liquid systems Section G.3.2.2, bulk flow parallel to force Section 6.4.1.2, mixer-settler, CSTS system. 

A significant advance in this area was recently made by Li and coworkers [30,31], who developed a laminar flow technique, that allowed the direct contact of two liquids with better-defined mass transport compared to the Lewis cell. Laminar flow of the two phases parallel to the interface was produced through the use of flow deflectors. By forcing flow parallel to, rather than towards, the interface, it was proposed that the interface was less likely to be disrupted. Reactions were followed by sampling changes in bulk solution concentrations. 

All of the membrane processes utilize an engineering design known as "cross-flow" or "tangential flow" filtration. in this mechanism, the bulk solution flows over and parallel to the membrane surface, and because the system is pressurized, water is forced through the membrane. The turbulent flow of the bulk solution across the surface minimizes the accumulati.on of particulate matter on the membrane and facilitates the continuous operation of the system. 

Parallel plates arc corrugated (like roofing material) with the axis of corrugations parallel to the direction of flow. The plate pack is inclined at 45° and bulk water flow is forced downward. The oil sheet rises upward, counter to water flow, and is concentrated in the (op of each corrugation. When oil reaches the end of the plate pack, it is collected in a channel and brought to (he oil/water interface. 

Turbulent flow comprises the solution bulk. (2) As the electrode surface is approached, a transition to laminar flow occurs. This is a nonturbulent flow in which adjacent layers slide by each other parallel to the electrode surface. (3) The rate of this laminar flow decreases near the electrode due to frictional forces until a thin layer of stagnant solution is present immediately adjacent to the electrode surface. It is convenient, although not entirely correct, to consider this thin layer of stagnant solution as having a discrete thickness 5, called the Nernst diffusion layer. 

In Section 6.3.1, we cover external forces, specifically gravitational, electrical and centrifugal forces inertial force is also included here. In Section 6.3.2, chemical potential gradient driven equilibrium separation processes involving vapor-liquid, liquid-liquid, solid-melt and solid-vapor systems are considered the processes are flash vaporization, flash devolatilization, batch distillation, liquid-liquid extraction, zone melting, normal freezing and drying. Section 6.3.3 illustrates a number of membrane separation processes in the so-called dead-end filtration mode achieved when the feed bulk flow is parallel to the... 

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