Reactive regeneration

The suitability of regenerating the adsorbent by reactive means can only be judged on a case-by-case basis. With some adsorptive reactors - for example, for the total denitrification of flue gases or the unsteady-state Deacon process - the reactive regeneration is the object of the exercise. The prerequisite that the adsorbate does not undergo further reaction and is adsorbed in reasonable amounts at moderate temperatures means that the molecules being adsorbed tend to be small and stable, and thus do not lend themselves to reactive regeneration. 

Displacement desorption, in which the adsorbate is driven off by a more strongly adsorbing species, entails an additional regeneration cycle to desorb the expelling agent and can thus only be justified for recovery of a high-value adsorbed intermediate. 

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Production of cellulose esters from aromatic acids has not been commercialized because of unfavorable economics. These esters are usually prepared from highly reactive regenerated cellulose, and their physical properties do not differ markedly from cellulose esters prepared from the more readily available aHphatic acids. Benzoate esters have been prepared from regenerated cellulose with benzoyl chloride in pyridine—nitrobenzene (27) or benzene (28). These benzoate esters are soluble in common organic solvents such as acetone or chloroform. Benzoate esters, as well as the nitrochloro-, and methoxy-substituted benzoates, have been prepared from cellulose with the appropriate aromatic acid and chloroacetic anhydride as the impelling agent and magnesium perchlorate as the catalyst (29). 

Desiccants can lose water capacity and drying efficiency by taking up moisture during storage. They should therefore be analy2ed before use. If necessary, the materials should be reactivated (regenerated) before putting them in service. 

Very little is known about the reactivation (regeneration) of used catalysts by microwave irradiation. Catalyst activity has been shown to decay with increasing carbon... 

Only small quantities of iron(III) bromide are required It is a catalyst for the brommation and as Figure 12 6 indicates is regenerated m the course of the reaction We 11 see later m this chapter that some aromatic substrates are much more reactive than benzene and react rapidly with bromine even m the absence of a catalyst... 

Cation (Section 1 2) Positively charged ion Cellobiose (Section 25 14) A disacchande in which two glu cose units are joined by a 3(1 4) linkage Cellobiose is oh tamed by the hydrolysis of cellulose Cellulose (Section 25 15) A polysaccharide in which thou sands of glucose units are joined by 3(1 4) linkages Center of symmetry (Section 7 3) A point in the center of a structure located so that a line drawn from it to any element of the structure when extended an equal distance in the op posite direction encounters an identical element Benzene for example has a center of symmetry Cham reaction (Section 4 17) Reaction mechanism m which a sequence of individual steps repeats itself many times usu ally because a reactive intermediate consumed m one step is regenerated m a subsequent step The halogenation of alkanes is a chain reaction proceeding via free radical intermediates... 

Desiccant replacement is another operating cost, although desiccant life is usually several years in regenerable appHcations. Desiccant life depends on the stabihty of the desiccant in the given service, the frequency of regeneration, the presence of reactive contaminants, and the possibiUty of upsets to normal operation. 

Diels-Alder reactions in the presence of Lewis acids represent a case in which the Lewis acid is often used in catalytic quantities. The complexed ester (ethyl acrylate in the example given below) is substantially more reactive than the uncomplexed molecule, and the reaction proceeds through the complex. The reactive complex is regenerated by exchange of the Lewis acid from the adduct. 

Atoms and free radicals are highly reactive intermediates in the reaction mechanism and therefore play active roles. They are highly reactive because of their incomplete electron shells and are often able to react with stable molecules at ordinary temperatures. They produce new atoms and radicals that result in other reactions. As a consequence of their high reactivity, atoms and free radicals are present in reaction systems only at very low concentrations. They are often involved in reactions known as chain reactions. The reaction mechanisms involving the conversion of reactants to products can be a sequence of elementary steps. The intermediate steps disappear and only stable product molecules remain once these sequences are completed. These types of reactions are refeiTcd to as open sequence reactions because an active center is not reproduced in any other step of the sequence. There are no closed reaction cycles where a product of one elementary reaction is fed back to react with another species. Reversible reactions of the type A -i- B C -i- D are known as open sequence mechanisms. The chain reactions are classified as a closed sequence in which an active center is reproduced so that a cyclic reaction pattern is set up. In chain reaction mechanisms, one of the reaction intermediates is regenerated during one step of the reaction. This is then fed back to an earlier stage to react with other species so that a closed loop or... 

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