Evaluation of steam as a sweep gas in low temperature thermal desorption processes used for contaminated soil clean up

The purpose of this investigation was to compare the effectiveness of steam as a sweep gas in the low temperature thermal desorption (LTTD) process. The intent is to determine if steam may be substituted for nitrogen, which is the sweep gas of choice for many thermal desorption processes. Steam has several advantages over nitrogen. Among those advantages is lower cost and the availability of on-site generation facilities. The study was conducted by placing a stainless steel tube containing a predetermined amount of contaminated soil in an electrical furnace. The tube had openings at each end. One end of the tube was connected to the source of the sweep gas with the other end being connected to a ice bath condenser. The effectiveness of steam and nitrogen were compared as sweep gasses at similar operating conditions. The contaminants of interest in this study were: volatile compounds, p-xylene (Normal Boiling Point (NBP), 138°C), trichloroethylene — TCE (NBP, 86.9°C), and a semi-volatile compound, phenanthrene (NBP, 340°C). Sweep gas flow rate was varied over a range that would be expected in a commercial size LTTD unit. The sweep gas efficiency was evaluated over a series of exposure times and three temperatures of 177, 232, and 288°C. The residual concentration of the contaminant was determined by gas chromatographic analysis. Rates of contaminant desorption were determined as a function of time, sweep gas volume, and specific contaminant. The data generated allowed a general comparison of nitrogen and steam as sweep gases. In most cases tested, the treatments using steam as the sweep gas exhibited characteristics that were comparable to or better than nitrogen sweep gas treatments. The advantage of steam was more pronounced with the higher boiling point semi-volatile compound phenanthrene.