Optimization and Pilot Scale Study of Soil Column Air Stripper Device for Volatile Organic Chemicals (VOCs) Separation

Abstract

The stripping of volatile organic compounds (VOCs) plays an important role in the remediation of petroleum contaminated soils and with a mild air blower that gives increased soil column air stripper (SCAS) performance. An experimental study using SCAS for heated and unheated air was simulated to determine its effectiveness and performance for VOCs removal efficiency by counter-current flow. The studies were carried out in a 0.1 m (10 cm) diameter of unpacked column to a depth of 1.1 m (1100cm) and a safety factor of 10% (0.1m) with the gas flow varying from 2.4 l/min to 12 l/min and the soil flow rate from 0.088 Kg/m².h to 52.50 Kg/m².h while the air flow rate from 0.17 Kg/m².h to 101.79 Kg/m².h. Temperature was varied from 20 °C to 50 °C. The effects of increasing temperature on the removal efficiency were more significant at temperature of 50^OC than at 20 ^OC when air flow rate was constant at 4.8 l/min. The concentration of the VOCs in the petroleum contaminated soils was measured by gas chromatography fitted with flame ionization detector (GC-FID) method. The initial concentration measured by GC-FID is 211.95 mg/kg while effluent concentration drops to an average of 39.77 mg/kg of petroleum contaminated soil PCS at 293K, 16.17 mg/kg of PCS at 323K and 8.20mg/kg of waste contaminated soil (WCS) at 323K. The effects of increasing airflow rate, temperature and air-to-soil ratio on the removal efficiency were found to be more significant at air flow rate of 2.0 to 10 l/min., as temperatures rises from 293K to 323K. The results indicate that a high removal of VOCs can be achieved by operating the SCAS at high temperature conditions even at relatively low airflow rate and air-to-soil ratios. The removal efficiency was observed to be very high in PCS with 82.13% in clay, 78.45% loamy, 52.50% sandy and 53.60% coarse-grain soils. This indicates a positive progression in an observable reduction from 211.95 mg/kg to 16.17mg/kg in PCS when temperature was elevated to 323K representing average of 92.37% reduction of VOCs in soil at pressure drop of 10 - 200 N/m² with column height of 1.1m and safety factor of 0.1 m (10% column height). The mass transfer processes were achieved using MATLAB and a surface response plot to determine the maximum VOCs recovery using countercurrent SCAS. The VOCs removal efficiency increased from 82.33% to 99.99% while Henry’s constant increased from 4.0 x 10^-3 to 7.3 x 10^-2 as temperature was elevated from 283 to 323 K. VOCs removal efficiencies of over 99.99% were obtained for all combinations of airflow rate, temperature and air-soil ratio. The result also indicated that air stripping of VOCs from petroleum contaminated soil is most dependent on temperature and moderately air flow rate and air-soil ratio. The study therefore supports the adoption of locally made SCAS device as a practical solution for the treatment of petroleum contaminated soils (PCS) while providing substantial technical and economic benefits over more complex air stripper designs.