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Catalyst-Aided Reaction
The main reaction occurs once the reactants are fed into the middle portion of the CD column. Mixing takes place either in a separate vessel or within the reactor itself. If two reactant streams feed directly into the reaction zone, they must flow counter-current to each other for proper mixing. Reactor designers position the feed point below the actual reaction zone. This prevents the degradation or poisoning of the catalyst by metal ions in the feed streams. Solid catalyst pellets, in which most of the reactions take place, pack the inside of the reaction zone. To further aid the reaction, process design parameters, such as feed mole ratio and flow rate of the reactants, column pressure and reboiler duty are optimized.
Stripping
Stripping occurs in the separation zone at the bottom of the column. This stripping zone acts as an absorber outfitted with a reflux stream. Stripping, a purely mass transfer step, plays a vital role in increasing the product yield and purity by its quick removal of products from the reaction zone. This pushes the equilibrium reaction forward in accordance with "Le Chatelier's principle," increasing the conversion of reactants into products. If the volatility of the product differs greatly from that of the reactants, stripping becomes even more efficient due to an increase in mass transfer potential.
Rectifying
The rectifying zone behaves similarly to the stripping zone, though it may not have a reflux stream. Like the stripping zone it contains no solid catalysts and instead functions as another mass transfer unit; however, rather than selectively absorbing products, it instead absorbs reactants from the middle zone's exit stream. Some CD reactor configurations leave out the rectifying zone, preferring to perform this step in a separate absorber. Taken individually, each zone actually has a lower efficiency compared to its equivalent single-function equipment. However, the entire CD column has a much greater overall efficiency than a conventional reactor-still combination.
Recycling
All unreacted reactants absorbed in the rectifying zone re-enter the reaction zone in a recycle stream. This minimizes waste and further increases product conversion and purity. This is especially crucial for reactions involving expensive raw materials, which some manufacturers cannot afford to dispose of.