Countercurrent contacting

The Phillips process is a two-stage crystallisation process that uses a pulsed column in the second stage to purify the crystals (79,80). In the pulsed column, countercurrent contact of the high purity PX Hquid with cold crystals results in displacement of impurities. In the first stage, a rotary filter is used. In both stages, scraped surface chillers are used. This process was commercialized in 1957, but no plants in operation as of 1996 use this technology. 

Most adsorption systems use stationary-bed adsorbers. However, efforts have been made over the years to develop moving-bed adsorption processes in which the adsorbent is moved from an adsorption chamber to another chamber for regeneration, with countercurrent contacting of gases with the adsorbents in each chamber. Union Oil s Hypersorption Process (90) is an example. However, this process proved uneconomical, primarily because of excessive losses resulting from adsorbent attrition. 

This overall flow pattern in a distillation column provides countercurrent contacting of vapor and hquid streams on all the trays through the column. Vapor and liquid phases on a given tray approach thermal, pressure, and composition equilibriums to an extent dependent upon the efficiency of the contac ting tray. 

Equipment in this category is usually arranged for multistage countercurrent contact of the insoluble hquids, without repeated complete separation of the hquids from each other between stages or their equivalent. Instead, the liquids remain in continuous contact throughout their passage through the equipment. 

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... 

EPA has compiled significant data on values of k and n for environmentally significant pollutants with typical activated carbons. Assuming equilibrium is reached, the isotherm provides the dose of carbon required for treatment. In a concurrent contacting process, the capacity is set by the required effluent concentration. In a countercurrent process, the capacity of the carbon is set by the untreated waste pollutant concentration. Thus countercurrent contacting is preferrea... 

In opposition to the usual (elution) chromatography, SMB is a continuous process, and is thus much more adapted to large-scale production. Moreover, SMB is based on a countercurrent contact between the liquid and the adsorbent, which leads to lower eluent consumption. 

The easiest way to understand the SMB concept is to consider a true moving bed (TMB) as described in Eigure 10.1, in which a countercurrent contact is promoted between the solid and liquid phases. The solid phase moves down the column due to gravity and exits the system in Zone I. The liquid (eluent) stream follows exactly the opposite direction. It is recycled from Zone IV to Zone I. The feed, containing components A and B are injected at the middle of the column, and the fresh eluent is replenished in Zone I. 

On the large scale, air and water are brought into countercurrent contact in a cooling tower which may employ either natural draught or mechanical draught. The water flows down over a series of wooden slats which give a large interfacial area and promote turbulence in the liquid. The air is humidified and heated as i( rises, while the water is cooled mainly by evaporation. 

Phase densities differ by a Phase densities differ by only about 10%. factor of 100-10,000 1. Viscosity in both phases is Liquid phase viscosity moderate, solid low. phase rigid. Phase separation is rapid Phase separation is slow surface-tension and complete. effects prevent completion. Countercurrent contacting is Countercurrent contacting is slow and quick and efficient. imperfect. ... 

Countercurrent contact with partially miscible solvents... 

In most industrial applications, multistage countercurrent contacting is required. The hydrodynamic driving force necessary to induce countercurrent flow and subsequent phase separation may be derived from the differential effects of either gravity or centrifugal force on the two phases of different densities. Essentially there are two types of design by which effective multistage operation may be obtained ... 

Figure 8.2 depicts the four main zones and their immediate proximity to each other in the Molex process. As indicated earlier, the Sorbex process operates on a liquid-solid countercurrent contacting principle. Zone 1 is referred to as the... 

The Contacting Scheme. In gas-liquid systems semibatch and countercurrent contacting schemes predominate. In liquid-liquid systems mixed flow (mixer-... 

A column packed with 5-cm polypropylene saddles a = 55 m /m ) is being designed for the removal of chlorine from a gas stream (G = 100 mol/s m, 2.36% CI2) by countercurrent contact with an NaOH solution... 

An increase in the amount of solvent gives a greater recovery of solute but produces a more dilute extract. In order to meet the criteria of a reasonable concentrated extract and a high recovery of solute from the raffinate phase, it is best to employ multi-stage countercurrent contacting (as shown in Figure 15). This is the best way of obtaining separations on a continuous flow basis. 

A variation of the nonregenerable absorption is the spray dry process. Lime slurry is sprayed through an atomizing nozzle into a tower where it countercurrently contacts the flue gas. The sulfur dioxide is absorbed and water in the slurry evaporated as calcium sulfite—sulfate collects as a powder at the bottom of the tower. The process requires less capital investment, but is less efficient than regular scrubbing operations. 

---