Steam regeneration of activated carbon adsorbents.

摘要

Steam regeneration has been widely used for years in adsorption operations because it is cheap, easy-to-use and available. In addition, it allows the adsorbates to be recovered in subsequent separations through distillation and extraction. Despite its common practice, very little is known about the mechanisms by which steam displaces the adsorbates from the adsorbent pores.; The effect of condensing water and organic material on the pores, and the S-shaped water isotherm (type V of the Brunauer classification), makes it difficult to understand the mechanisms by which water displaces the adsorbate from the carbon pores.; This project applies experimental and mechanistic work to explain such phenomena. An extensive literature survey of the highly specialized updated literature is given. Prior to experimentation, carbon characterization was made and a comparison with other carbon adsorbents from previous works was made. Sorption isotherms on carbon from water and organics used as adsorbates was then developed. During the experimentation, adsorption and steaming runs were conducted in a 3 inches diameter, 4 feet long Pilot-Scale Test System available at Balcones Research Center facility in North Austin.; Extensive temperature and effluent concentration data were collected during these adsorption-steaming runs performed using volatile organic chemicals (voc's)/steam/nitrogen/activated carbon system. Regeneration was performed with saturated and superheated steam at various velocities and bed initial conditions.; The effect on the bed adsorption capacities for various initial conditions following subsequent cycles with variance of several operating conditions was extensively observed. A primary focus of the study was the effect of the residual moisture left on the bed before being returned into adsorption operation.; Also, the operating conditions used during the steaming runs explored this desorption process in the region of high steaming flow rates and very long steam regeneration times, allowing to observe the phenomena in conditions not explored before.; A non-linear, shock-wave type, equilibrium-stage model was used to compare some of the experimental steam regeneration data. Also, the simplified version of the Chu model was used for simulation of the adsorption runs and some of the propane regeneration runs using steam.; Breakthrough and regeneration data were used to show the relative effect of steaming, drying and/or cooling in subsequent cycles. The three adsorbates used were (1) propane, which vaporizes and has a very little solubility with water; (2) acetone, which is water miscible; and (3) toluene, which is water immiscible. This presented the opportunity to view the relative impact of the organic/water/carbon systems on the cycle efficiencies. Equilibrium data for these three species was obtained by a gravimetric approach. Carbon characterization was also made. Coconut-shell type (Sorb-Tech) highly microporous activated carbon was used in all runs. Experimental data were gathered directly in a pilot-scale basis. On the other hand, some heating runs of a dry, clean bed were made to allow the evaluation of heat transfer resistances of the adsorber.; The main focus of this research was the study of the effects of non-adiabatic operation, steam contact times and superficial velocities, degree of superheat, bed initial conditions, and bed loading on regeneration efficiency.