Study On Treatment Of Oilfield Wastewater Using Carbon-based Adsorbents

Large plenty of oily wastewater have been produced during petroleum refining process and give serious pollution to the environment around oilfields which has become one of the important influence factors restricting the production development of oil industry. Thus, it has great significance in developing efficient and practical oilfield waster treatment technology. As one of the main techniques for the purpose of environmental pollution control, the adsorption method is considered to be one of the most effective ways in the treatment of oily wastewater because it has the following advantages: wide usage, good treatment effect and adsorbents can be recovered and reused ect. While choosing the suitable adsorbents is the key issue of this method, therefore, many researches focus on searching for economical and efficient adsorbents at present.In this paper, the cheap semicoke and brown coal were chosen as the raw material to prepare carbon-based adsorbents of oil removal from oilfield wastewater by different activation methods. Adsorption properties for oil removal, physicochemical properties and regeneration of the adsorbents have been mainly studied, the adsorption mechanism has also been simple investigate.Several modification methods to raw semicoke and brown coal were performed according to the basic properties of oil in wastewater. That was, calcination in N2 atmosphere, steam activation, high-pressure hydrothermal chemical activation, HNO₃, KOH activation to semicoke and H₃PO₄, ZnCl₂ activation to brow coal. The carbon-based adsorbents prepared by these measures could effectively remove oil from wastewater based on experiments. The results showed that, KOH was the best activation agent to semicoke and the optimum conditions were as follows: mass ratio of KOH/semicoke was 5:1, impregnating time was 3h under 80℃, and calcining temperature was 550℃; ZnCl₂ was the best activation agent to semicoke and the optimal conditions were: volume ratio of ZnCl₂ and H2SO4 solution (mass fractions of 35%) was 2:1, liquid-solid ratio was 3:1, impregnating time was 12h and calcining temperature was 600℃.The activation properties were evaluated by determination of the oil removal rate through static adsorption and dynamic adsorption. The result indicated, the optimum operating conditions to static adsorption were the following: particle size of adsorbent was 40～80 mesh, addition amount adsorbent was 10g/L, stirring time was 30min and standing time was 6.0h; and to dynamic adsorption: particle size of adsorbent was 20～40 mesh, velocity of influent was 60mL/h and oil content in wastewater was about 100mg/L. The brown coal activated by ZnCl₂ showed a good behavior in the static experiment, the optimum oil removal rate obtained could be up to 79.57%; the semicoke activated by KOH exhibited a favorable performance, time of adsorption capacity depletion could persist for 300min.The physical and chemical characterization and analysis on the adsorbents were performed by various detection methods and instrument. The following results were obtained: 1) The content of the surface acidity and basicity functional groups on semicoke and brown coal increased greatly when activated through all above methods, but one the surface was acid when activated by HNO₃, H₃PO₄ and ZnCl₂, the other was alkaline. 2) The adsorbents which had higher methylene blue adsorption also had higher saturated adsorption of oil. 3) Pore size of adsorbents had a relatively centralized distribution and average pore diameter was about 2.5nm, which could meet the requirement of oily wastewater treatment. 4) Surface morphology of adsorbents changed greatly after activation, that was, irregular pore structure and abundant pore distribution had formed, which was beneficial to adsorption for oil removal.The thermal regeneration of adsorbents were effective to the adsorbents which had reached saturation adsorption in a certain extent, however, adsorption properties reduced rapidly with increase of regeneration times. Furthermore, adsorption experiments to simulated oilfiled wastewater were carried out. The result showed that adsorption saturation capacity of adsorbent could reach 23mg/g when the simulated oilfiled wastewater had the properties of low temperature, high salinity and low pH. The sorption of oil to the adsorbents was in conformity with the isothermal adsorption equation of Freundlich.