Countercurrent processes

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

Since the 1960s the commercial development of continuous countercurrent processes has been almost entirely accompHshed by using a flow scheme that simulates the continuous countercurrent flow of adsorbent and process Hquid without the actual movement of the adsorbent. The idea of a simulated moving bed (SMB) can be traced back to the Shanks system for leaching soda ash (58). 

The disadvantage of extraction relative to extractive distillation is the greater difficulty of getting high efficiency countercurrent processing. 

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... 

The conditions existing in a column during the steady state operation of a countercurrent process are shown in Figure 10.9. The molar rates of flow of the two streams are G and (j3, which will be taken as constant over the whole column. Suffixes 1 and 2 denote the two phases, and suffixes t and b relate to the top and bottom of the column. 

Distillation is a countercurrent process which maintains liquid-vapor equilibrium, usually in a vertical column containing packing, thin wetted walls, or bubble-cap... 

The basis for the second purpose is the use of a two-phase distribution in conjunction with a countercurrent process of some sort, so thar separation may be made in spite of closely related partition ratios. This method may be called Extraction for Fractionation Purposes... 

The mass flow streams at tray j are shown in Figure 6.1 for a countercurrent process of a dilute system. Here the total molar flow rate of the rising gas is constant at V moles/min while the total molar flow rate of the sinking liquid is constant at L moles/min. Both L and V are molar flow rates as long as Xj and Yj are molar fractions. If, however, Xj and Yj describe concentrations, measured in mo/e/m3, then L and V are volumetric flow rates measured in m3/min. 

Facchini, A., Amato, L., Modolo, G., Nannicini, R., Madic, C., Baron, P. 2000. Transient-and steady-state concentration profiles in a Diamex-like countercurrent process for Am(III) + Ln(III) separation. Sep. Sci. Technol. 35(7) 1055-1068. 

Water at 20°C is cooled and frozen to ice at 0°C in a countercurrent process with evaporating ammonia (see Figure 8.2). The minimum temperature difference required for proper heat transfer is taken as 5°C between solid... 

For fixed-bed processes, the boundary conditions are given by the feed concentration and the initial loading of the bed. For countercurrent processes with two... 

In the mid-1960s liquid—liquid extraction processes were introduced and today all large-scale commercial production is done in this way. An aqueous solution of the Ln3+ ions is extracted in a continuous countercurrent process into a nonpolar organic liquid containing tri-n-butylphosphine oxide or bis(2-ethylhexyl)phosphinic acid (DEHPA). Typical separation factors for adjacent rare earths using DEHPA are 2.5 per extraction step so that under automatic multistep or countercurrent conditions purities of 99 to 99.9% are routinely achieved. 

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