Remediation of NAPL below the water table by steam-induced heat conduction
Introduction
Steam injection is a promising technology for remediating subsurface hydrocarbon contamination. Full-scale clean-up operations have suggested that the technique ensures rapid and satisfactory clean-up of even very complicated contaminations Newmark et al., 1994, Newmark et al., 1998. Several one-dimensional laboratory experiments have shown that volatile NAPL will rapidly vaporize when it contacts with steam Hunt et al., 1988, Hadim et al., 1993, Betz et al., 1997. Two-dimensional laboratory studies have shown that heterogeneous porous media can also be remediated by steam injection Itamura, 1996, She and Sleep, 1999. In these experiments the steam zone eventually occupied the entire sand pack and the emplaced NAPL was completely removed. Thus, the investigations suggest that the soil volume contacted by steam will be cleaned. However, if NAPL is present in a low permeable heterogeneity or in a saturated zone, steam may not contact directly. This will significantly reduce the removal rate, but remediation may still be faster than with other technologies. The focus of this study is to quantify the removal when steam does not contact the contaminant directly. Specifically, we address the situation where a steady-state steam zone overlies a saturated zone contaminated with NAPL. This is a very common situation in a full-scale application of steam injection. When injecting steam below the water table, a steam zone will tend to move upwards due to buoyancy and may override a part of the contamination, e.g. when a DNAPL is pooled on top of a low permeable layer. Likewise, Schmidt et al. (2002) showed how injection of pure steam in an unsaturated zone containing NAPL could lead to downward migration resulting in a lens of LNAPL residing between the steam zone and the water table. In these cases, there will be no steam flow through the contaminated soil and the only heating mechanism will be conduction from the overlying steam zone. At a certain temperature, below the normal boiling point of water, the two immiscible liquids will boil because the sum of their individual vapor pressures will equal the surrounding pressure (Atkins, 1994). The produced gas will, due to buoyancy, be transported from the saturated zone into the steam zone where it can be extracted. In clean-up operations limited by conduction, a performance assessment should not be based on the steam zone development, which would provide a too optimistic estimate of the clean-up time.
The objective of this study is to examine the removal mechanisms when steam is not in direct contact with NAPL. A two-dimensional intermediate scale sand box experiment was conducted where separate phase trichloroethylene (TCE) was emplaced below the water table at the interface between a coarse and a fine sand layer. Steam was injected above the water table and the remediation process was monitored by measurements of temperature inside the box and recovery of contaminant at the outlet. The experimental data were interpreted with the numerical code T2VOC that subsequently was used for analyzing the involved processes in details.
Section snippets
Experimental setup
Fig. 1 illustrates the experimental setup. The sand box had the interior dimensions 122×8.5×58 cm and was constructed of stainless steel with a front glass cover allowing for inspection of the sand packing. During the experiment, the sand box was insulated to minimize heat loss. The sand box was equipped with 98 Pt-100 temperature sensors connected to a data acquisition system that continuously logged the temperature. At the location of the bottom row of temperature sensors, pressure
Results
The injection of steam created a zone with steam temperature in the upper part of the coarse sand layer with convection as the dominant heat transfer process. A vertical steam front, where the temperature rapidly changed from steam temperature to the temperature of the surroundings, moved with an almost constant velocity through the sand box. Fig. 2 compares the measured and the simulated temperature after 1, 2 and 3 h of steam injection. The experimental and simulated temperatures compare well
Discussion
Fig. 5a shows a contour plot of the simulated gas saturation after 12 h, which is in the period of the TCE-recovery. Above the contaminated layer, an unsaturated zone has formed from gas created by boiling of separate phase TCE and water. The gas moves upwards due to buoyancy and in the steam zone it is transported towards the extraction well. This is shown in Fig. 5b where a plume of TCE is present in the gas phase from the contaminated area to the extraction well.
Fig. 6 shows the temporal
Perspectives
The above findings show how TCE is rapidly removed in an experiment due to a boiling mechanism. The numerical model can be used to generalize this into something more applicable at field scale by varying the significant parameters within the interval to be expected in the field. The parameters significant to the process are identified as (a) porous medium properties, (b) the dimensions and (c) the type of contaminant. We assume that a 1-D model is sufficient to capture the dominant mechanisms
Conclusions
Steam injection was performed in a two-dimensional sand box to remediate TCE present as a separate phase below the water table. Steam was injected above the water table and after an initial heating period the separate phase TCE was recovered at the outlet. The experiment was successfully modeled with the numerical code T2VOC using independently measured parameters and no calibration. By means of the numerical model, the dominant removal mechanism was identified as boiling induced by heat
Acknowledgements
The experiment was performed at the VEGAS facility at the University of Stuttgart, Germany. We express our great gratitude for being offered access to the experimental facilities and to the staff members for helpful assistance during the experiments.
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