Staged operations countercurrent

Figure 10.4 Equilibrium countercurrent stage operation for stripping.

As described in Section 15.2.1, eutectic systems can be purified in theory by single-stage crystallisation, whereas solid solutions always require multistage operations. Countercurrent fractional crystallisation processes in column crystallisers are described in Section 15.4.3. 

Refer to Fig. 3, where there is shown a cascade of three ideal stages operated in countercurrent. A solute balance may be written for any stage, such as stage 2, for example ... 

Figure 14.3. Representation of countercurrent extraction batteries, (a) A battery of mixers and settlers (or separators), (b) Schematic of a three-stage countercurrent battery, (c) Simulation of the performance of a three-stage continuous countercurrent extraction battery with a series of batch extractions in separatory funnels which are designated by circles on the sketch. The numbers in the circles are those of the stages. Constant amounts of feed F and solvent S are mixed at the indicated points. As the number of operations is increased horizontally, the terminal compositions Et and R3 approach asymptotically the values obtained in continuous countercurrent extraction (Treybal, 1963, p. 360).

If staging and countercurrent operation are essential, a slurry tray column and a rotating disk column can be applied [9]. In flue gas purification where a low pressure drop is essential, lime or CaCC>3 slurries are spray contacted with the gas phase. Sometimes it is meant to evaporate the water phase but also simple slurry scrubbing is envisaged. 

The flow diagram for a two-stage (stage 1, stage 2) countercurrent operation is provided in Figure 108. Material balances on the residue for each stage are... 

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). 

Slurry approach use if the carbon usage is <180 kg/day. Mix and suspend powdered adsorbent and then filter exit line. Often uses up to three stages of countercurrent contacting. Used for continuous bleaching of edible oils. Batch process is simple, flexible, and easy to change feedstocks. Continuous operation offers better protection against oxidation, provides shorter holdup, and has the potential of heat recovery. Bleach time 25 min. A related topic is transfer line reactor (Section 16.11.6.9). 

InTjurope, three" stages ot countercurrent are recommended.12 The fresh carbon is applied to the thick liquor, the once-used carbon to the thin liquor, and the twice-used carbon is added to the neutralized converter juice. To avoid rehandling the carbon filter cake, some manufacturers use a modification of layer filtration. In this procedure, after the thick liquor has been filtered, the thin liquor is pumped through the once-used carbon on the filter. Success in the operation depends on placing a carbon layer of uniform thickness on the filter. -— ------------------------- ---------... 

The various process modes are applied primarily on the basis of the properties of the feed mixture. Solids are processed mainly in single stage or multiple stage operational mode. Fluid feed mixtures containing compounds of similar solubility in the solvent are best treated in a multiple stage countercurrent process. The separation of isomers falls within the field of chromatography or selectively catalyzed reactions. Chemical reactions, e.g. esterification of free fatty acids, may be part of the separation process. 

Whereas the reacticm in a simple shaft reactor is adiabatic and therefore requires a two-stage operation, the temperature profile along a countercurrent reactor is much more favorable. On entering the ring space between the reactor shell and the catalyst vessel at approximately 230 °C, the gas/steam mix flows downwards and then enters the tubes in the catalyst vessel. As it flows upward through these catalyst surrounded tubes, the mix is heated up to approximately... 

The terms "number of stages N" and "height equivalent to a theoretical plate HETP" are used in chromatography however, their meanings are quite different in comparison to continuously operated countercurrent columns (absorption, extraction, rectification). Here, the column length or height must be sufficient to draw apart the bands of components or fractions. This reqnires favorable eqnUibria, certain retardation differences, and limited axial dispersion. 

Batchwise Extraction, Multiple Stage. This operation, usually a laboratory procedure, can be carried out in two ways. The immiscible solvents may be continuously pumped through the various stages in countercurrent flow, and the batch of mixture to be separated may be suddenly introduced near the center of the cascade. The result is an unsteady-state operation, the solutes leaving the opposite ends of the cascade in ratios different from that in the feed and in amounts varying with time. Such an operation has been considered in some detail by Martin and Synge (13) and Cornish et al. (4). 

Figure 18.14 Plot of Kremser Equation, Number of Theoretical Stages for Countercurrent Operation, Henry s Law Equilibrium, and Constant A or 1/A...

Staged operations (a) single stage (b) co-current (c) countercurrent (d) crosscurrent. 

---