Reversibility effects, continuous-flow

Continuous flow With continuous flow operations consider (e.g. using a HAZOP procedure) the effects of No flow Reduced flow Reverse flow Increased flow Contaminated flow Flow of a substituted material, etc. 

Reverse-Osmosis Experiments. All reverse-osmosis experiments were performed with continuous-flow cells. Each membrane was subjected to an initial pure water pressure of 2068 kPag (300 psig) for 2 h pure water was used as feed to minimize the compaction effect. The specifications of all the membranes in terms of the solute transport parameter [(Dam/ 6)Naci]> the pure water permeability constant (A), the separation, and the product rate (PR) are given in Table I. These were determined by Kimura-Sourirajan analysis (7) of experimental reverse-osmosis data with sodium chloride solution at a feed concentration of 0.06 m unless otherwise stated. All other reverse-osmosis experiments were carried out at laboratory temperature (23-25 °C), an operating pressure of 1724 kPag (250 psig), a feed concentration of 100 ppm, and a feed flow rate >400 cmVmin. The fraction solute separation (/) is defined as follows ... 

Reversing the flow does not affect the coefficient of Co, so that any change of Co will continue to have the same effect upon c/v. However, the coefficients of the (o(.t) — l)c(q-terms will in general all be affected by a reversal of the flow. The class of symmetric arrays of membranes, for which the properties of the i-th and the (N + 1 — i)-th membranes are the same, are an important exception. Of course, the single membrane also belongs to this class. When... 

Several mechanisms have been proposed to explain reverse osmosis. According to the preferential sorption-capillary flow mechanism of Sourirajan [114], reverse osmosis separation is the combined result of an interfacial phenomenon and fluid transport under pressure through capillary pores. Figure 5.58a is a conceptual model of this mechanism for recovery of fresh water from aqueous salt solutions. The surface of the membrane in contact with the solution has a preferential sorption for water and/or preferential repulsion for the solute, while a continuous removal of the preferentially sorbed interfacial water, which is of a monomolecular nature, is effected by flow under pressure through the membrane capillaries. According to this model, the critical pore diameter for a maximum separation and permeability is equal to twice the thickness of the preferentially sorbed interfacial layer (Figure 5.58b). 

Many suspensions (particularly those that are weaMy flocculated or structured to reduce sedimentation) show time effects during flow. At any given shear rate, the viscosity of the suspension continues to decrease with increasing the time of shear on stopping the shear, the viscosity recovers to its initial value. This reversible decrease of viscosity is referred to as thixotropy. 

Now a system of valves is used which effectively transposes the positions of Active beds 1 and 2 and Inert beds 1 and 2. The transposition also results in each bed experiencing a reversal of flow direction. The state of the whole system is now as at the beginning and the whole process can be repeated indefinitely to achieve continuous cooling. 

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