Contacting modes countercurrent

In the extractive-stripping (ES) mode the extractor and stripper are combined together. The extractor section is contacted with the lean gas and the rich gas flows down into the stripper column coming in contact with countercurrent stripped hydrocarbons from the reboiler at the bottom of the ES column. The recovery of the desired compounds is controlled by the lean gas flow rate, stripper bottom temperatures and operating pressures. The rich solvent leaving from the ES column is expanded to the pressure of operation in the product column, which is essentially a distillation column. Here the... 

In onr example, die lowest valoe of that can be attained with erase current contacting with the inlet liquid stream split equally among N stages is 0.013. Theoretically. ya can be reduced to zero with countercurrent contacting in onr example. These results are plotted in Fig. 4.1-5. Thus, this simple ezemple illustrstes the relative effectiveness of the thme classic contacting modes, which are listed in Table 4,1-1. 

To maximize driving forces throughout a particular system, countercurrent contacting is tiequently used. In this contacting mode, the inlet stream for one phase is mixed with the outlet stream of the other phase. For the two-stage countercurrent contacting system. 

Industrial-scale adsorption processes can be classified as batch or continuous. In a batch process, die adsorbent bed is saturated and regenerated in a cyclic, operation. In a continuous process, a countercurrent staged contact between lire adsorbent and die feed and desorbent is established by cidier a true or a simulated recirculation of die adsorbent. The efficiency of an adsorption process is significantly higher in a eoiuinuous mode of operation than in a cyclic batch mode. For difficult separations, batch operation may require 25 times more adsorbent inventory and twice die desorbent circulation rate than does a continuous operation. In addition, in a batch mode, the four functions of adsorption, purification, desorption, and displacement of the desorbent from the adsorbent are inflexibly linked, wtiereas a continuous mode allows mure degrees of freedom with respect to these functions, and thus a better overall operation. 

Separation of a chemical species from a mixture of similar compounds can be achieved by crystallization. The mode of crystallization may fall in the realm of what is known as melt crystallization. In such processes, the mother liquor largely is comprised of the melt of the crystallizing species, and, subsequent to its crystallization, crystals formed from the mother liquor are remelted to produce the product from the crystallizer. Para(p)-xylene can be crystallized from a mixture that includes ortho and meta isomers in a vertical column that causes crystals and mother liquor to move countercurrently. Heat is added at the bottom of the column to melt the p-xylene crystals a portion of the melt is removed from the crystallizer as product and the remainder flows up the column to contact the downward-flowing crystals. Effluent mother liquor, consisting almost entirely of the ortho and meta isomers of xylene, is removed from the top of the column. 

Table I gives the values of the extraction factor (a) required for americium removals ranging from 80 to 99%. These required values of a are given for the three modes of extraction (single-contact, two stage crosscurrent, and two stage countercurrent).

As mentioned before, the unit operation of bed adsorption may be carried out in a moving-bed mode, either co-currently or countercurrently. When the breakthrough experiment is carried out, the superficial velocity should also be recorded. The reason is that adsorption is a function of the time of contact between the liquid phase containing the solute to be adsorbed and solid-phase carbon bed. Thus, for the breakthrough data to be applicable to an actual prototype adsorption column, the relative velocities that transpired during the test must be maintained in the actual column. When the relative velocities between the flowing water and the carbon bed are maintained, it is immaterial whether or not the bed is moving. 

Direct heat rotary dryer. The direct heat units are generally the simplest and most economical in operation and construction, when the solids and gas can be permitted to be in contact. In design mode, the required gas flow rate can be obtained from a heat and mass balance. Bed cross-sectional area is found from a scoping design calculation (a typical gas velocity is 3 m/s for cocurrent and 2 m/s for countercurrent units). Length is normally between 5 and 10 times drum diameter (an L/D value of 8 can be used for initial estimation) or can be calculated by using an incremental model (see Examples 21 and 23). 

In the differential extraction mode, the concentration profile changes continuously as the two phases have no exact stepwise phase separation and a continuous movement towards each other is present. Here, an ideal contacting pattern for the two phases corresponds to a perfect countercurrent plug flow (e.g. extraction towers, Podbiehiiak... 

In solvent sublation experiments the aqueous feed was introduced using a liquid distributor in the aqueous phase. The organic solvent on top of the aqueous phase was stagnant. In bubble fractionation experiments the top and the bottom effluent rates were monitored and adjusted as required. The aqueous feed in bubble fractionation experiments was introduced using the liquid distributor. The overall operation in both cases was thus one in which the aqueous phase was in continuous countercurrent contact with air. The air was in a once-pass-through mode. The air flow rate was measured using a soap bubble flowmeter and a rotameter. 

The analysis of extraction is based on the two primary modes of contact cross-flow and countercurrent. 

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