The Development of a Thermal Treatment Assessment Procedure for Soils Contaminated with Hydrocarbons

ABSTRACT

Hydrocarbon evolution from a montmorillonite clay-soil was studied at three scales and at temperatures ranging from 300 to 650°C in order to develop a thermal treatment assessment procedure for contaminated soils in rotary kilns. The two components of the assessment procedure included experimental evaluation of the soil at bench- or pilot-scale in conjunction with data fitting using an appropriate mathematical desorption model. The results of the experiments have been embodied in the model which can be used for scaling and correlative purposes. Performing experiments at three scales permitted a clear delineation of intraparticle and interparticle effects and it also allowed the testing of the model's ability to correctly scale results. In addition to reactor size, the key parameters examined included hydrocarbon volatility, temperature, moisture levels, and soil particle size.

The three experimental facilities included a 0.6-by-0.6 m, natural gas-fired, batch, pilot-scale rotary kiln, a 6 mm outside diameter, single particle reactor (SPR), and a 10-by-10 cm bench-scale rotary reactor (BSRR). Porous soil particles ranging in diameter from 0.4 to 7 mm were tested in the kiln and the SPR. The single particle studies showed decreasing desorption times with decreasing radius in agreement with an intraparticle, diffusion-limited process. The kiln results showed no effect of particle size over the range of sizes studied suggesting that interparticle mass transfer resistances were controlling in a bed of particles.

The hydrocarbons included toluene, naphthalene, and n-hexadecane and their concentrations were each roughly 0.05 weight percent, on a dry basis. The ease with which they were removed from the soil decreased in the order given. Increasing the reactor temperature increased the desorption rates for all three compounds with the adsorption isotherm constants showing temperature dependencies that agreed with the van't Hoff equation. Desorption rates showed no dependence on the presence or absence of other hydrocarbons. Moisture, at concentrations ranging from 5 to 9 weight percent, had a large effect on the desorption rate of all compounds studied. The rate of toluene evolution in the kiln was accelerated while the rate of hexadecane evolution was slowed. Intermediate results were obtained with naphthalene.

The rate of total hydrocarbon desorption from dry soils in the kiln was well correlated by a penetration model which used a distribution of heats of desorption. The desorption rate was first order with respect to individual and total hydrocarbon concentrations.