Adsorption regeneration

When gaseous or liquid molecules adhere to thesurface of the adsorbent by means of a chemical reaction and the formation of chemical bonds, the phenomenon is called chemical adsorption or chemisorption. Heat releases of 10 to 100 kcal/g-mol are typical for chemisorption, which are much higher than the heat release for physisorption. With chemical adsorption, regeneration is often either difficult or impossible. Chemisorption usually occurs only at temperatures greater than 200 C when the activation energy is available to make or break chemical bonds. 

Of these, the most commonly used for air pollution control are the fixed-bed and canister units. Fixed beds are also used in solvent-recovery applications. Major process steps include adsorption, regeneration, and further treatment of the desorbed organic compounds. Typically, further treatment includes condensation and separation. 

Physical, thermal, and chemical stability in order to reduce operating costs, solid sorbents must demonstrate stability under flue gas conditions, adsorption operation conditions, and during the multi-cycle adsorption-regeneration process. In particular, stability in the presence of water vapor is essential for the sustainable performance of the solid sorbent. In addition to thermal properties of the solid sorbent, heat capacity and thermal conductivity are also important in heat transfer operations. 

Adsorption/desorption kinetics the time of the adsorption-regeneration cycle greatly depends on the kinetics of the C02 adsorption-desorption profile, which is measured in breakthrough experiments. Sorbents that adsorb and desorb C02 in a shorter time are preferred as these reduce the cycle time as well as the amount of sorbent required, and ultimately the cost of C02 separation. 

Elegant slide valves are used to separate the adsorption, regeneration and resin backwash stages. The contactor operates in predetermined cycles and is an ideal process for feedstreams with no suspended solids. 

The trapping component was formulated into a washcoat and supported on a ceramic monolith with 400 cells per square inch (cpsi). The trap material was chosen for NOx adsorption, regenerability, thermal stability and rate of adsorption/desorption. Platinum is incorporated within the trap to oxidize the NO and the injected hydrocarbon. The lean NOx catalyst was Pt (60 gft- ) deposited on y-Al203 on a 400 cpsi cordierite monolith. 

Adsorption regeneration (desoiption) (Continued) pressure-swing temp erature- sw ing cooling considerations heating considerations Alumina, activated Amines, tertiary Antoine equation Association reactions Atomic volumes Axial mixing Azeotropic distillation calculations selection of entrainer... 

Since the pilot unit and the data obtained from it are described in numerous publications only some key conclusions are mentioned here. The pilot operation established the technological feasibility of the process in general and has shown that the assumptions, calculations, and laboratory data-based conclusions concerning particular features of the process such as adsorption, regeneration at high heating rate, etc. are correct. Data obtained during the 2 yrs of operation also has proved the economical viability of the process. 

The regeneration of exhausted hypercrosslinked sorbents does not require the use of concentrated acids and alkalis. As was said in [76], a simple washing with 1 N sodium hydroxide and water is sufficient for removing the absorbed components. The sorbent maintains constant adsorption capacity, at least within 30 cycles of adsorption—regeneration. 

Figure 3.32 Two Stage (Adsorption/Regeneration) System for Drying of Solvents...

For the exhausted adsorbents from gas phase adsorption, regeneration by thermal desorption is most commonly used. For example, activated carbon used to prevent contamination of air by organic solvents of low concentrations, and silica gel, activated alumina or zeolite used for dehumidification of gases are regenerated by high temperature steam, air or inert gases. In the case of organic adsorbates... 

Boudou et al. (2003) introduced nitrogen groups into a viscose-based ACC by reaction with ammonia/air at 300 °C and by reaction with ammonia/steam at 800 °C. Extensive surface characterizations were carried out. Ammonia/steam treatment was more effective for the adsorption of H2S or SO2 by enhancing the microporosity and by modifying the distribution of the surface oxygen complexes. A series of successive adsorption-regeneration cycles showed important differences between the oxidation retention of H2S and SO2. 

The main mechanism by which activated carbon removes impurities is one of physical adsorption, this being a reversible process. Consequently one can expect that desorption of the impurities will render the carbon surface available again for adsorption. Regeneration of spent activated carbon is not only important from the point of view of restoring the adsorption capacity of the carbon, but also because in many cases the recovery of the adsorbed species is important. If the adsorption is of chemical type (chemisorption), the formation of a bond between the carbon and the adsorbate makes the process non-reversible, and even if desorption is possible the desorbed species will be different to those originally adsorbed. Additionally, adsorption (especially in liquid phase) is often accompanied by precipitation of species which cannot be removed by simple desorption. 

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