Desorption during regeneration

Figure 261 shows the absorption and the regeneration process schematically. During Absorption the concentrated salt solution is distributed over an exchange surface, which is in contact with an air stream. The air will be dehumidified and the salt solution will be diluted by the absorbed water vapour. During regeneration the diluted solution becomes concentrated again by desorption from a hot air stream. 

During regeneration of an adsorber in the cold box of an air separation plant, regeneration gas exited via the adsorber drain pipe. This still contained some liquid oxygen. Very low regeneration-gas flows permitted desorption of acetylene and other hydrocarbons at above normal (ppm) concentrations, these condensed or crystallised into the liquid oxygen and the mixture finally detonated, causing... 

Figure 5. Desorption rates from Z200H during regeneration (Q) water, flZD benzene...

Displacement desorption Good for strongly held species avoids risk of cracking reactions during regeneration avoids thermal aging of adsorbent Product separation and recovery needed (choice of desorbent is crucial)... 

Another factor that is particularly important in the regeneration of molecular sieve driers is the rate at which the temperature is raised during regeneration. If this is too rapid relative to the rate of moisture removal, one may get rapid desorption of moisture from the initial section of the bed, which is in contact with the hot desorbent gas, followed by condensation of liquid water in the cooler regions some distance from the inlet, with serious consequences for adsorbent life. 

Vacuum desorption rather than a product purge is used to remove the adsorbed oxygen during regeneration in order to avoid the slow uptake of nitrogen which would eventually saturate the sieve and reduce the efficiency of oxygen capture. The cycle commonly operates between about 4 and 0.1 atm with a half cycle time of about 1 The general scheme is shown in... 

Figure 5. Profiles in the PSR during regeneration in the isothermal (top) and ar diabatic (bottom) case. Legend A dimensionless temperature (T/Tref), O desorption flux of A (Sa), reaction rate 1 (ri), 0 convective and diffusive axial flow of A (Fa). Parameters Series iv from figure 4 (V (purge)=50), time IXTads into the purge step.

A special bed cooling step is not always required. For example, if the thermal wave caused by the heat of adsorption runs ahead of the mass transfer zone on the adsorption step then the temperature in the MTZ will be more or less equal to the feed temperature and may be unaffected by the temperature used during desorption or regeneration. 

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