Separation countercurrent equilibrium

Nelson, P. A., "Countercurrent Equilibrium Stage Separation with Reaction, ... 

J. Jelinek and V. Hlavacek, Steady-State Countercurrent Equilibrium Stage Separation with Chemical Reaction by Relaxation Method, Chem. Eng. Sci., 2 79 (1976). 

Nelson (24) studied theoretically the general case of countercurrent equilibrium stage separation with chemical reaction and applied his technique to describe distillation reactors. His model relied on the assumption of each stage being a perfectly mixed reactor and also an equilibrium stage. 

Jelinek J. and V. Hlavacek, Steady state countercurrent equilibrium stage separation with chemical reaction by relaxation method, Chem. Eng. Commun. 2, 79-85 (1976). 

FLASH determines the equilibrium vapor and liquid compositions resultinq from either an isothermal or adiabatic equilibrium flash vaporization for a mixture of N components (N 20). The subroutine allows for presence of separate vapor and liquid feed streams for adaption to countercurrent staged processes. 

Continuous Countercurrent Systems Most adsorption systems use fixed-bed adsorbers. However, if the fluid to be separated and that used for desorption can be countercurrently contacted by a moving bed of the adsorbent, there are significant efficiencies to be realized. Because the adsorbent leaves the adsorption section essentially in equilibrium with the feed composition, the inefficiency of the... 

Mass transfer controlled by diffusion in the gas phase (ammonia in water) has been studied by Anderson et al. (A5) for horizontal annular flow. In spite of the obvious analogy of this case with countercurrent wetted-wall towers, gas velocities in the cocurrent case exceed these used in any reported wetted-wall-tower investigations. In cocurrent annular flow, smooth liquid films free of ripples are not attainable, and entrainment and deposition of liquid droplets presents an additional transfer mechanism. By measuring solute concentrations of liquid in the film and in entrained drops, as well as flow rates, and by assuming absorption equilibrium between droplets and gas, Anderson et al. were able to separate the two contributing mechanisms of transfer. The agreement of their entrainment values (based on the assumption of transfer equilibrium in the droplets) with those of Wicks and Dukler (W2) was taken as supporting evidence for this supposition. 

We can design a reactor to separate the products and achieve complete conversion by admitting pure A into the center of the tube with the sohd moving countercurrent to the carrier fluid. We adjust the flows such that A remains nearly stationary, product B flows backward, and product C flows forward. Thus we feed pure A into the reactor, withdraw pure B at one end, and withdraw pure C at the other end. We have thus (1) beat both thermodynamic equilibrium and (2) separated the two products from each other. 

Such an assembly of mixing and separating equipment is represented in Figure 14.3(a), and more schematically in Figure 14.3(b). In the laboratory, the performance of a continuous countercurrent extractor can be simulated with a series of batch operations in separatory funnels, as in Figure 14.3(c). As the number of operations increases horizontally, the terminal concentrations E1 and R3 approach asymptotically those obtained in continuous equipment. Various kinds of more sophisticated continuous equipment also are widely used in laboratories some are described by Lo et at. (1983, pp. 497-506). Laboratory work is of particular importance for complex mixtures whose equilibrium relations are not known and for which stage requirements cannot be calculated. 

The need for a continuous countercurrent process arises because the selectivity of available adsorbents hi a number of commercially important separations is not high, In die p-xylene system, for instance, if the liquid around the adsorbent particles contains 1% p-xyleiie, llie liquid in the pores contains about 2% p-xylene at equilibrium. Therefore, one stage of contacting cannot provide a good separation, and multistage contacting must be provided in tile same way that multiple trays are required in fractionating materials with relatively low volatilities. 

Coalescence and Phase Separation. Coalescence between adjacent drops and between drops and contactor internals is important for two reasons. It usually plays a part in combination wilh breakup, in determining Ihe equilibrium drop size in a dispersion, and it can therefore affect holdup and flooding in a countercurrent extraction column. Secondly, it is an essential step in the disengagement of the phases and the control of entrainment after extraction has been completed. 

The operation is most easily understood by reference to the equivalent true countercurrent system (Fig. 15). If we consider a feed containing two species A and B, with A the more strongly adsorbed, and a desorbent C, then in order to obtain separation the net flow directions in each section must be as indicated. With the equilibrium isotherms and the feed composition and flow rate specified, this requirement in effect fixes all flow rates throughout the system as well as the adsorbent recirculation rate or switch time. From simple theoretical considerations it can be easily shown that the affinity of the adsorbent for the desorbent should be intermediate between that for the strongly and weakly adsorbed feed compounds (i.e., a c > 1 -0, bc < 1 -0). The heights of the individualized bed sections are then determined by the requirement that each section contain sufficient theoretical plates to achieve the required purity of raffinate and extract products. For a linear system the analysis is straightforward since simple expressions for the concentration profile are available in terms of the kinetic and equilibrium... 

First, the effects of gas and liquid flows, co-current versus countercurrent operation, pressure and temperature were checked. As expected, based on the influence of these parameters on the vapor pressure or the vapor-liquid equilibrium (VLE), the fraction of water stripped by the nitrogen increases with increasing gas flows, decreasing liquid flows, lower pressures and higher temperatures. Countercurrent operation is more efficient than co-current operation, because the liquid phase at the inlet was already enriched with the compound which was to be separated. 

In many separation processes (chromatography, countercurrent distribution, field-flow fractionation, extraction, etc.), the transport of components, in one dimension at least, occurs almost to the point of reaching equilibrium. Thus equilibrium concentrations often constitute a good approximation to the actual distribution of components found within such systems. Equilibrium concepts are especially crucial in these cases in predicting separation behavior and efficacy. 

Description Aromatics are produced from naphtha in the Aromizing section (1), and separated by conventional distillation. The xylene fraction is sent to the Eluxyl unit (2), which produces 99.9% paraxylene via simulated countercurrent adsorption. The PX-depleted raffinate is isomerized back to equilibrium in the isomerization section (3) with either EB dealkylation-type (XyMax) processes or EB isomerization-type (Oparis) catalysts. High-purity benzene and toluene are separated from non-aromatic compounds with extractive distillation (Morphylane ) processes (4). Toluene and C9 to Cn aromatics are converted to more valued benzene and mixed xylenes in the TransPlus process (5), leading to incremental paraxylene production. 

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