lnapl and dnapl behaviour during steam injection into...

Groundwater Quality: Remediation and Protection (Proceedings of the GQ'98 Conference held at \ \ \ Tubingen, Germany, September 1998). IAHS Publ. no. 250, 1998.

LNAPL and DNAPL behaviour during steam injection into the unsaturated zone

REINHARD SCHMIDT, CHRISTOPH BETZ & ARNE FÀRBER Institut fur Wasserbau, Universitat Stuttgart, Pfaffenwaldring 61, D-70569 Stuttgart, Germany

Abstract Two-dimensional experiments were conducted to analyse the processes determining the behaviour of different light and dense nonaqueous phase liquids (LNAPLs and DNAPLs) during steam injection into the unsaturated zone. Injection of pure steam resulted in a fast decontamination of the unsaturated zone. However, it was shown that downward migration of the NAPLs started at the condensation front as soon as residual saturation was exceeded. Liquid contaminant migrated towards the saturated zone. A new set of experiments was conducted where steam/air-mixtures were injected to examine heat transfer and contaminant behaviour. Results were compared to those of the pure steam injection experiments. It was shown that downward migration of contaminant was successfully prevented. Soil sample analysis confirmed excellent cleanup levels.

INTRODUCTION

In recent years soil vapour extraction has been used extensively as a method to remove volatile organic compounds (VOCs), such as petroleum hydrocarbons or chlorinated solvents, from the unsaturated zone. Nevertheless, at many field sites, prolonged remediation times are observed. This can be due to low contaminant vapour pressures, soil heterogeneity or mass transfer limitations (Rathfelder et ai, 1995).

By injecting energy, for example in the form of steam, vapour pressures increase remarkably with increasing temperature of the subsurface. While this leads to a very fast and effective cleanup of the unsaturated zone (Betz et al., 1997), it can also result in a contaminant condensation front and subsequent downward migration of liquid contaminant towards the saturated zone (Itamura & Udell, 1993). Downward migration depends on local NAPL saturation, water saturation, capillary pressure-saturation ratios and relative permeabilities. While LNAPLs may be retained by the capillary fringe, DNAPLs may penetrate the saturated zone and migrate downward till they are retained by a less permeable layer. In either case, contaminant removal may become more difficult.

To avoid excess condensation, and hence downward migration of contaminant, the temperature gradient of the heat front must be moderated. A new remediation method is presented where, instead of pure steam, a steam/air-mixture is injected into the subsurface. By using steam/air-mixtures, the injection temperature can be adjusted and heat front gradients can be controlled.

Four experiments are presented which provide insight in the physical processes during pure steam and steam/air injection. The behaviour of the heat front and conta-

112 Reinhard Schmidt et al.

minant recovery are studied in a two-dimensional homogeneous sand packing. Effects and results of both remediation methods are compared using identical sand packings and initial conditions.

MATERIALS AND METHODS

The experiments were conducted in a stainless steel box of dimensions 110 x 74 x 8.5 cm (Fig. 1). A front glass panel allowed visual access of the sand packing. Teflon® and viton® seals and tubings were used to minimize loss of VOCs. Steam was generated using a 2 kW steam generator and injected into the box through three injection ports. The steam flow was adjusted using a flow valve and was monitored with a flow meter. For the steam/air experiments, the air flow was induced by means of a vacuum pump and measured with a flow meter. The steam and air were then mixed before injection. Gases left the box through the extraction port and were passed through a condenser. Non-condensible gases flowed through an activated carbon filter before being vented to the surrounding atmosphere. The condensate flowed in a two-stage liquid separator where the contaminant was separated from the water. A hydraulic head gradient was established creating a 10 cm thick saturated zone. During the experiments, injection and extraction pressure were measured using a U-tube water manometer connected to the inflow and outflow ports of the box. The temperature of the injected and extracted gases was measured with Pt-100 temperature sensors. The sand packing was equipped with 100 temperature sensors to monitor the heating process. The box was insulated during the experiments to minimize heat loss.

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LNAPL and DNAPL behaviour during steam injection into the unsaturated zone 113

The box was packed homogeneously with a quartz sand with grain sizes ranging from 0.5 to 2.0 mm. The porosity of the sand was determined to be 0.4 and the permeability was 5 x 10"10 m2. For each experiment dry sand was poured into the apparatus to a level of 40 cm. The sand was then water saturated from the bottom and allowed to drain for several hours. After that the contaminant source was emplaced. The two contaminants used in the experiments were 1,3,5-trimethylbenzene (mesitylene) and 1,2-dichlorobenzene (1,2-DCB) (Table 1). Mesitylene and 1,2-DCB were dyed with organic soluble dyes (Oil Red O and Oil Blue N, respectively). 360 g of mesitylene or 546 g of 1,2-DCB were mixed uniformly with 18 kg of sand at residual moisture content giving a residual contaminant saturation of 8.2%. The mixture was packed as uniformly as possible into the box occupying an area of 60 x 25 x 8.5 cm. The remaining space was filled with sand at residual moisture content, and then the top lid was secured in place. Before each experimental run the system was allowed to equilibrate for one day.

The parameters of the four experiments using the box are summarized in Table 2. Measurements of the contaminant mass removed during the experiments were made by removing the contaminant recovered from the liquid separators and by monitoring the mass adsorbed on the activated carbon filter. During the pure steam injection experiments, the mass of contaminant pooled on the water table (LNAPL) or the bottom of the box (DNAPL) was removed through the groundwater outlet and recorded. Groundwater samples were collected to determine contaminant concentrations. Photographs of the contaminant removal were taken throughout the experiments. After completion of the experiments, soil samples were analysed for the contaminants.

RESULTS AND DISCUSSION

The initial mesitylene contamination (00:00 h) and the contaminant behaviour during pure steam injection in experiment 1 are shown in Fig. 2. The injected steam

Table 1 Properties of mesitylene and 1,2-DCB.

Mesitylene 1,2-DCB

Chemical formula Molecular weight (g mol"1) Density at 20°C (g cm"3) Boiling point at 1013 liPa (°C) Vapour pressure at 20°C (Pa) Vapour pressure at 100°C (Pa)

QH3(CH3)3

120.2 0.865 164.7 230 13 458

QH4C1:

147 1.298 180 114 8340

Table 2 Summary of parameters for the conducted experiments.

Experiment no. 1

Mass mesitylene (g) 360 - 360 Mass 1,2-DCB (g) Contaminant liquid saturation (%) Steam flow rate (kg h"1) Mode of operation

-8.2 1.4 steam

546 8.2 1.4 steam

-8.2 1.4 steam/air

546 8.2 1.4 steam/air


condensed and transferred its latent heat of vaporization to the porous media and the pore fluids. A steep and stable heat front was created. As the front moved through the box, mesitylene was vaporized and transported with the flowing gas phase. At the margin of the sharp heat front, the contaminant came in contact with the cold sand, condensed again and accumulated in the front (00:50 h). As soon as the NAPL saturation exceeded residual saturation, downward migration of liquid contaminant started due to gravitational forces. Mesitylene pooled on the water table since its density is smaller than that of water (02:00 h). After steam breakthrough at the extraction port (04:40 h) there was little contaminant present in its original position but all the contaminant had been mobilized towards the capillary fringe. Mesitylene was then removed in the liquid phase and could only be completely recovered by removal of all the groundwater. This is generally not possible in a field situation. Due to contaminant downward migration, LNAPL removal becomes more difficult and remediation times increase.

In experiment 2 (photographs not shown), where pure steam was injected into the sand packing contaminated with 1,2-DCB, similar processes took place as in the previous experiment: vaporized contaminant condensed and accumulated in the heat front before being mobilized towards the water table. After steam breakthrough at the extraction port most of the initial contamination was removed but liquid 1,2-DCB had penetrated the saturated zone and pooled at the bottom of the steel box. In a field situation, mobilized liquid DNAPL may be retained by a less permeable layer. However, contaminant removal then becomes more complicated and time consuming. In contrast to experiment 1 where 100% of the mesitylene had migrated as a liquid to the water surface, only 75% of the recovered 1,2-DCB mass was removed as a liquid. Twenty-five percent was removed as a gas after steam breakthrough at the ex-

Fig. 2 Mesitylene removal by pure steam injection at various times.


traction port. This was due to the lower vapour pressure of 1,2-DCB compared to that of mesitylene. Mass transfer from the liquid to the gas phase during the heating process was slower and 1,2-DCB was present in the pore space after steam breakthrough at the extraction port.

Contaminant mobilization in the heat front can be attributed to the high temperature gradients in the front. Sharp heat fronts can be avoided by injecting a steam/air-mixture instead of pure steam. The mass ratios of air (non-condensible gas) and steam (energy carrier) determine the injection temperature according to the thermodynamics of saturated steam/air-mixtures. Temperature gradients can be controlled by gradually increasing the temperature of the injected mixture until pure steam is injected. This results in a spatially-extended heat front.

During the injection of steam/air-mixtures contaminant will be vaporized according to its vapour pressure at the current temperature and transported with the gas flow to the heat front. The spatially-extended heat front allows the contaminant to condense in a larger volume of porous media. Additionally, the inert air component passes through the heat front zone and constantly removes contaminant. These effects reduce liquid contaminant saturations in the front below residual saturation and hence downward migration of contaminant can be prevented.

Figure 3 shows these processes during injection of a steam/air-mixture into the sand packing contaminated with mesitylene (experiment 3). Complete cleanup of the sand without any contaminant downward migration was achieved. In contrast to pure steam injection (experiment 1), mesitylene was removed as a gas only.

The same behaviour was observed in experiment 4 (photographs not shown) with the DNAPL source (1,2-DCB). By injecting steam/air-mixtures contaminant

Fig. 3 Mesitylene removal by steam/air injection at various times.


mobilization was eliminated and mass removal occurred with the gas phase only. Mass removal over time and the portion of contaminant removed as condensate

from the liquid separators or adsorbed on the activated carbon filter during steam/air injection are shown for experiments 3 and 4 (mesitylene and 1,2-DCB) in Fig 4(a) and (b). In both experiments contaminant was exclusively adsorbed on the activated carbon as long as the extracted gas was at room temperature. After breakthrough of the heat front at the extraction port, condensed liquid contaminant was collected in the separators and recovery rates increased remarkably. The total mesitylene removal was faster than 1,2-DCB removal. Mass removal can be directly correlated to the contaminant vapour pressure. The lower vapour pressure of 1,2-DCB compared to mesitylene, and hence the lower mass transfer from the liquid to the gas phase, resulted in a longer period of time for cleanup. The different portions of mesitylene and 1,2-DCB collected as condensate and adsorbed on the activated carbon can also be attributed to the contaminant volatility. Sixty-four percent of the initial mesitylene mass was adsorbed on the activated carbon and 29% was collected as condensate, whereas only 25 % of the initial 1,2-DCB mass was adsorbed on the carbon filter and 66% was removed as condensate. The higher vapour pressure of mesitylene compared to that of 1,2-DCB was responsible for higher contaminant concentrations in the extracted gas. Thus, a larger portion of mesitylene was removed before breakthrough of the heat front at the extraction port. The mass transfer rate of the less volatile 1,2-DCB from the liquid to the gas phase was lower than that of mesitylene. 1,2-DCB was vaporized more slowly and mainly removed after heat front breakthrough at the extraction port. The overall recovery rates of both experiments were comparable (» 92%). The mass balance was within the possible accuracy for an experimental setup of this size. Missing mass was attributed to losses during emplacement and recovery of the contaminant.

In order to determine the cleanup levels achieved, soil samples were taken after each experiment and analysed. Figure 5 shows the initial mesitylene concentration of experiment 1 and experiment 3 and compares the results of soil sample analysis after remediation by pure steam injection and steam/air injection, respectively. Both techniques resulted in concentrations below regulatory standards. Downward migration of mesitylene during pure steam injection was responsible for the elevated concentrations in the former groundwater zone.

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Time [h] Time [h] Fig. 4 Mass removal of (a) mesitylene and (b) 1,2-DCB during steam/air injection.


- cone, before steam injection (exp.1)

- cone, before steam/air injection (exp. 3)

- cone, after steam injection (exp. 1 )

- cone, after steam/air injection (exp. 3)

required clean-up level

0.01 1000 10000 10 100

Concentration [mg/kg]

Fig. 5 Comparison of soil sample analysis after steam and steam/air injection.

100000

CONCLUSIONS

The experimental results presented in this paper show the high effectiveness of steam injection. However, the disadvantage of contaminant downward migration generally limits the practical application of the technology to LNAPL contaminated sites. The innovative steam/air-mixture technique allows the remediation of DNAPL contaminated sites by steam processes. By using steam/air injection, temperature gradients are controllable and downward NAPL migration can be successfully prevented, while maintaining the high degree of effectiveness of pure steam injection.

Acknowledgement The financial support for the experimental studies of the Projekt Wasser-Abfall-Boden (PWAB) of the state government of Baden-Wiirttemberg, Germany is gratefully acknowledged.

REFERENCES

Betz, C , Fârber, A. & Schmidt, R. (1997) Lab and technical scale experiments for NAPL removal by steam injection into porous media. In: Groundwater: an Endangered Resource (Proc. 27th IAHR Congress), 101-106. Am. Soc. Civ. Engrs, San Francisco.

Itamura, M. T. & Udell, K. S. (1993) Experimental cleanup of a dense non-aqueous phase liquid in the unsaturated zone of a porous medium using steam injection. In: Multiphase Transport in Porous Media, 57-62. Fluids Engineering Division vol. 173/Heat Transfer Division vol. 265, American Society of Mechanical Engineers.

Rathfelder, K., Lang, J. R. & Abriola, L. M. (1995) Soil vapor extraction and bioventing: applications, limitations, and future research directions. Rev. Geophys. Supplement, 1067-1081.

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