In a thermal swing process the solvent is desorbed at increased temperature.
The heating may be done by coils placed in each bed but, with exception of very small units or exceptionally high solvent concentration in the feed gas, this is not economic because of the poor heat transfer in the bed and expensive equipment. Preferably, one common heat exchanger is used to heat a circulating gas which in turn periodically heats each bed which must be regenerated.
The circulating gas is used for heating as well as for stripping, maintaining a low partial pressure of the solvent.
The higher the heating gas temperature, the less gas must be circulated and a lower residual loading of the solvent is obtained.
The purity of the effluent air or gas is higher and the adsorption step longer, which further reduces the flow rate of the regeneration gas.
However, the temperature of the gas may be limited by the thermal stability of the solvent, adsorbent deactivation, piping stress and valve choice. Regenerating gas temperatures are generally between 100 and 250 °C, preferably between 150 and 210 °C.
The solvent is recovered by cooling and chilling the circulating gas. Lower chilling temperature increases the refrigeration unit cost and power, but reduces the residual solvent load and therefore has similar effects to the higher regeneration temperature.
The optimum chilling temperature is lower if the boiling point of the solvent is low (light solvents). Generally, chilling temperatures are between -10 and -30 °C.