SUMMARY AND CONCLUSION

A novel activated carbon fiber cloth (ACFC) adsorption/electrothermal regeneration/ cryogenic condensation system was developed to separate, concentrate and recover toxic volatile organic compounds (TVOCs) and acetone from industrial flue gas streams. Suitability of this design and soundness of concept was proved by performing evaluation experiments using (MEK) and acetone as sample compounds. The components of the system, adsorber, electrothermal regenerator, and cryogenic condenser were characterized.

ACFC was utilized conveniently in the fixed-bed adsorber. ACFC showed excellent performance due to its high adsorption capacity, high internal and external contact efficiencies, and high adsorption and desorption rates. High adsorption capacity of ACFC was due to its high volume distribution in the optimal pore sizes. Equilibrium capacity of ACFC adsorbent was determined using both the results of breakthough curve (BTC) experiments and gravimetric method. Dubinin-Astakhov (DS) and Dubinin-Radushkevich (DR) isotherms were used to model adsorption capacity of the ACFC. The DA model provided a better regression correlation than the DR model. This is due to the capillary condensation in transitional pores between micro and meso regions. Adsorption dynamics in the ACFC fixed bed resulted in high throughput ratios (TPRs) up to 80% for a packing density of 94.5 mg/cm³. Increase in packing density to 450 mg/cm³ increased the TPR to 94%. Adsorption dynamics were modeled to predict the BTCs. The mass transfer rate was second order in the steady state mass transfer zone (MTZ) and at the outlet of the fixed bed. The electrothermal regeneration provided fast desorption rates and efficient energy transfer. The electrothermal regeneration concentrated TVOC vapors up to 63% by volume in less than 5.4 min, without optimization. Supply of a higher initial power resulted in a faster desorption rate and less specific electrical energy use and production of a higher maximum concentration level. Higher carrier gas flow rates resulted in higher desorption rates but produced lower concentration levels. Concentrating the gas stream drastically reduced the amount of the cryogen required to condense the TVOC from the gas stream and enabled the condenser to operate at warmer temperatures while achieving a high recovery efficiency.

The indirect contact shell-and-tube bench-scale cryogenic condenser showed that removal efficiencies of > 98% can be achieved for acetone in N₂ gas streams. Modeling and experimental results also showed that the condenser could operate more efficiently at high TVOC concentrations and low gas flow rates. Carbon adsorption can remove relatively low concentration TVOCs in high flow rate gas streams and desorb at relatively high concentrations and low flow rates. Carbon adsorption/desorption concentrated a 1% by volume gas stream to concentrations as high as 63% by volume and decreased flow rates from 5.0 slpm to 0.5 slpm. Removal efficiencies of the organic compounds by the condenser increased from 70.5% to 98.8% for inlet concentrations between 0.6% and 18.3% by volume.

Trace amounts of high molecular weight compounds were detected in the liquid condensate. However, these compounds accounted for only 1.73% of the acetone condensed.

Two fundamental models were developed to theoretically evaluate TVOC condensation: 1) thermodynamic modeling and 2) mass transfer/thermodynamic modeling. These models were coupled with the Wagner equation to predict the outlet saturation TVOC concentration as a function of condenser temperature. Experimental comparison with modeled results showed that the bench-scale condenser can condense acetone and MEK near the saturation vapor concentration at carrier gas flow rates under approximately 3.0 slpm. At higher flow rates, mass transfer would be limiting, and the mass transfer model could be employed to determine the condenser TVOC concentration profile. The models developed in this research can provide a method to design a large-scale condenser. The NPV and break-even analyses of the entire ACFC sorption system indicated excellent prospects for economically recovering of acetone.