Counter-current operation

Taking the upward direction as positive, when both Fy < 0 and Fs<0, represents a co-current downflow operation, if Fs < 0, but Fy > 0 means a counter-current operation. The rest of the equation defining the system can be seen in Berty (1997). Integration was done by the Romberg method as is used in Mathcad PLUS 6 (1996) software. 

Packed Towers (also known as wet-scrubbers) A packed tower is a vessel that is filled with some suitable packing material. The liquid is introduced at the top of the tower and flows down through the packing to the bottom, and in so doing, exposes a large surface area to contact with the gas. The gas may be introduced at either end of the tower since the gas and liquid flows may be either co-current or counter-current. The counter-current operation is most commonly used. 

Figure 1 Liquid Residence time distribution comparison - Experimental vs Numerical (a) counter-current operation Liquid flowrate l.SLmm (b) counter-current operation Liquid flowrate 3Lmin (c) co-current operation Liquid flowrate 1.5Lmm (d) co-current operation Liquid flowrate 3Lmm ...

Figure 2 Gas Residence time distribution comparison - Experimental vs Numerical (a) counter-current operation (b) co-current operation.

Mole balance expressions were developed for a general series reaction by Agarwalla and Lund [16], and the same procedures were used here to develop the species balance equations shown in Table I. Boundary conditions and parameter definitions are presented in Tables II and III. Note that the boundary conditions are given only for co-current flow of reactants and inert, which is the only configuration studied. Previous work [16], has shown that counter-current operation is less effective than co-current operation. 

Continuous counter current operation of fluidized bed adsorption is used on a very large scale for small molecules, e.g. uranium or residues from mining. Theoretical and practical aspects of this technology have been summarized by... 

Higler, A.P., Krishna, R., Ellenberger, J., Taylor, R. Counter-current operation of a structured catalytically packed bed reactor Liquid phase mixing and mass transfer. Chem. Eng. Sci., Vol. 54, 5145-5152, 1999. 

Ind. Eng. Chem. Res., 41, 2735, 2002 Ellenberger, J Krishna, R., Counter-current operation of structured catalytically packed distillation columns, Chem. Eng. Sci., 54,1339-1345, 1999 Krishna, R Hardware selection and design aspects for reactive-distillation columns, Reactive Distillation, eds Sundmacher K. and Kienle, A., Wiley-VCH, Weinheim, Germany, 169-189,... 

At high temperatures and in the absence of pressure differences over the membrane the simplified model is satisfactory, slips of reactants are prevented, and counter-current operation slightly outperforms cocurrent one. The plug-flow assumption is preferable to the well-mixed one. Application of pressure gradients increases conversion and requires the complete set of differential equations for a proper modeling. 

The present system also permits simultaneous introduction of the two phases through the respective terminals, as illustrated in Fig. 4c. This dual counter-current operation requires an additional flow tube at each terminal to collect the effluent, and if desired, a sample injection port is made in the middle portion of the coil. This system has been effectively applied to foam CCC [11,12] and dual CCC [13]. 

In general a more efficient separation is achieved by counter-current operation, which is realised by the Simulated Moving Bed (SMB)-approach. Chromatographic counter-current operation requires the mobile and stationary phase to move in opposite directions. Figure 5.17 illustrates the ideal counter-current system, the True Moving Bed (TMB), which involves the actual circulation of the solid with a constant flow rate. 

Liapis, A. I., Rippin, D. W. T. The simulation of binary adsorption in continuous counter-current operation and a comparison with other operation modes, AIChE/., 1979, 25(3), 455-460. 

Figure 32. (a) Principle of wind sifting and sketch depicting the flow profile in a tube, (b) Principle of an improved counter-currently operating gravitational wind sifter (Fritsch Analysette 8)... 

An SMB process is a realization of a continuous and counter-current operation of a liquid chromatographic separation process and it emerged... 

The analysis of equilibrium-stage operations is normally performed on the basis of counter-current flow between two phases. Because most separation processes, whether described in terms of equilibrium or mass transfer rates, operate in this flow scheme, it is useM to compare countercurrent to cocurrent flow. Figure 2.2 illustrates cocurrent and counter-current operation. Assuming mass transfer across a barrier between the two fluid phases, generic concentration profiles can be drawn for each case (Figure 2.3). 

Multiple reactor constructions for gas-liquid reactors are available, because of the large number of different application areas. Spray columns, wetted wall columns, packed columns and plate columns are mainly used for absorption processes. The gas concentrations are low in the case of absorption processes, hence a large interfacial contact area between the gas and the liquid is important to enhance the absorption process. These column reactors usually operate in counter-current mode. Counter-current operation is the optimal operating mode, because at the gas outlet where the gaseous component concentration is lowest, the gas is in contact with a fresh absorption solution. The low concentration of the gaseous component can then partly be compensated by the high concentration of the liquid component. 

An often used gas-liquid reactor is the bubble column. The gas is usually fed from the bottom through a sparger and the liquid flows either cocurrently or counter-currently. Counter-current operation is more efficient than co-current, but for certain types of parallel reactions, cocurrent operation can give better selectivity. Bubble columns are often operated in semi-batch mode the gas bubbles through the liquid. This mode of operation is attractive in the production of fine chemicals which are produced in small quantities - especially in the case of slow reactions. The flow patterns can vary a lot in a bubble column. Generally, as a rule of thumb, the liquid phase is more back-mixed than the gas phase. The plug flow model is suitable for the gas phase whereas the liquid phase can be modelled with the backmixed, dispersion, or plug flow model. 

For the counter-current operation displayed in Figme 9.2, one can deduce that both R A) and P(A) have their largest values at the top of the MCS. 

This involves bringing the wet solvent into contact with a solid, usually an electrolyte, which has the power to withdraw some of the water present to form a second phase that can be removed by decantation. The dehydrating substance may be either a solid or a saturated aqueous solution. The latter is more easily adapted to counter-current operations. 

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