Laboratory Studies Of In Situ Steam Injection Remediation Technology Of Contaminated Vadose

Non-Aqueous Phase Liquids(NAPLs)is a common pollutant in the vadose zone.There are 4 forms of phase state in the unsaturated zone: the gas phase in the pore,the free phase,the liquid phase dissolved in the water and the solid phase adsorbed on the soil particles.NAPLs has the characteristics of low water solubility,weak mobility and difficult degradation,which makes it difficult to remediate for a long time.Soil vapor extraction(soil vapor extraction,SVE)is one of the common methods for the remediation of NAPLs contaminated soil.The technology has the advantages of simple equipment,small damage to the environment,wide application and easy to use with other technologies.It has been favored by scholars at home and abroad.But practice has proved that the phenomenon of "tailing" will occur due to the limited mass transfer after a period of time.Aiming at the above problems,steam injection has been put forward by foreign scholars.The core idea of the technology is to use hot steam to improve soil temperature,promote the volatilization of pollutants,and then use airflow to bring pollutants out.The characteristics of steam injection determine that it has a good application prospect in the remediation of NAPLs contaminated soil.Accurate and reasonable understanding of the migration characteristics of steam in porous media is the key to improve the repair effect of the technology.Therefore,through one dimensional simulation column experiment,the migration characteristics and laws of hot steam under different heat preservation conditions,different medium types and different steam injection flow are explored.The temporal and spatial distribution rules of temperature,pressure and water content in the three cases are analyzed and the migration behavior of the condensation front of steam is analyzed.Observe the influence of insulation condition,medium type and steam injection flow on steam migration.On this basis,the influence of pollutants on steam migration is investigated by comparing the space temperature,pressure and migration of condensation front.The effects of steam injection flow,medium type and ventilation mode on the remediation of chlorobenzene contaminated soil were investigated.The contribution rate and mechanism of NAPLs removal in three phases were quantitatively studied.The effects of steam injection technology and soil vapor extraction technology on chlorobenzene were compared.The specific research results are as follows:(1)during the downward migration of hot steam,the temperature in the simulated column can be divided into three stages: saturated steam temperature,temperature change and ambient temperature.The pressure distribution in the simulated column is linear in the steam covered area and the spatial distribution is linear.The pressure at the given point increases linearly with time and then tends to balance.The vapor condenses continuously during migration,and the saturation of water increases with the distance in the final medium.(2)thermal insulation can effectively reduce the heat loss caused by thermal radiation,thereby improving the utilization rate of hot steam.In the experimental range,the heat transfer rate of hot steam in the fine sand medium and the increase of the pressure in the simulated column increase with the increase of the flow rate of the steam injection.The distribution of the condensate water in the steam in the fine sand medium and the forward velocity of the steam condensing forward increase with the increase of the flow rate of the steam.When the steam injection flow rate is 300 L/h,the temperature conduction of the hot steam in the coarse sand is faster than that of the fine sand.The distribution of the pressure and steam in the simulated column increases with the decrease of the particle size,and the velocity of the steam condensing forward decreases with the decrease of the particle size.(3)The presence of pollutants will increase the speed of temperature rise and shorten the time of temperature passing through the simulation column.The chlorobenzene volatilized into the pores increased the space pressure of the simulated column,thus promoting the migration of the steam condensation front.The main factors affecting the remediation of chlorobenzene contaminated soil by hot steam are steam injection rate,medium type and ventilation mode.In the study area,increasing the steam injection rate can improve the removal rate of chlorobenzene in soil.The liquid-phase removal rate of chlorobenzene also increased.The effect of hot steam on removal of chlorobenzene in high permeable medium is good.When the flow rate of steam injection is 300 L/h,the removal rate of chlorobenzene in the medium of coarse sand,medium sand and fine sand is 99.9%,99.4% and 97.3%,respectively,after 3.5 h of chlorobenzene contaminated soil.The residual chlorobenzene in intermittent medium was smaller than that in continuous ventilation.The residual rate of chlorobenzene in intermittent medium and continuous ventilation was 1.8% and 2.7% respectively.The steam injection method enhanced the removal efficiency of chlorobenzene in soil and shortened the repair time,which effectively improved the phenomenon of "tailing" in the later stage of SVE.