| With the expansion of exploitation in oilfield,the volume of oilfield produced water is quickening,and meanwhile produced water is emulsified seriously.Therefore,there is an urgent need for an efficient and environmentally friendly sewage treatment technology.As a new and environmental friendly technology for wastewater treatment,electrocoagulation commonly used for the treatment of oily wastewater has many characteristics such as small occupation,a wide range of pollutants,high removal efficiency,simple and convenient operation.Although many researchers have carried out a lot of researches on electrocoagulation,these studies mainly concerned with the adaptability of electrocoagulation on different types of pollution source,and the influence of electrolysis time,electrode voltage and other basic factors on removal capacity.And yet few studies have been done on the mechanism of electrocoagulation and enhanced performance of oil removal.In this paper,the mechanism of electrocoagulation was systematic studied by experimental research,theoretical analysis and mathematical optimization method through the comprehensive application of electrochemistry,fluid mechanics and other related theories.The main works and results are as follows:1.Experimental research on the effect of single factor was carried out during purification of oily wastewater within electrocoagulation reactor.It was found that electrolysis time and current density which decide the yield of bubbles and coagulants had significant influence on removal efficiency.Due to significant correlation between coagulation and flotation,the efficiency of oil removal was better when the spacing of parallel plates was from 1 cm to 5cm.The initial temperature affected the efficiency of oil removal,and directly decide the values of cell voltage before and after electrolysis.The difficulty of treatment was increased when the content of surfactant prepared the simulated sewage was greater than the critical micelle concentration.The experiment result was that a better effect was received when one type of surfactant was used.In the test,effect of oil removal wasn't uniform without mixing in electrocoagulation reactor,and consequently to result in poor response.It is necessary to consider the influence of mass transfer characteristics on the performance of oil removal,and thus the distribution characteristics of oil removal rate with vertical plate should be studied urgently.2.Study on distribution characteristics of oil removal rate and removal path of oil droplets?1?First,through the research on variation law of oil removal rate with electrolysis time at different positions in the vertical direction between verticle plates,it was found that oil removal rate increased quickly in upper position,which was caused by the combined action of coagulation and flotation.At the beginning of electrolysis,the lighter coagulant with loose structure in small size formed mainly could easy adhere to bubbles,and subsequently a large number of bubbles and coagulants sharply mixed with emulsion in upper.It resulted in quickening the process of oil removal in upper zone.With increasing electrolysis time,amorphous[Al?OH?3]n with compact structure generated mainly,moving downward by an enmeshment mechanism?sweep coagulation?.Subsequently,the amount of H2 bubbles and coagulants between the plates increased in lower and middle zone,which both resulted in increasing of oil removal.The result of experimental analysis was verified by high speed camera.Second,through researching the effects of current density on oil removal at different heights,it was found that the mass transfer rate in the vertical direction was increased significantly with increasing current density,thereby improving uniformity of oil removal rate.Another conclusion was also drawn that the inadequate contact time was proved to impede effective coagulant.?2?The influence of current density,mixing rate on the removal path of emulsified oil within electrocoagulation reaction were researched quantitatively.It was found that the oil removal capacity by floating was increased with increasing current density which decided the dose of H2 bubbles.In the meantime,the removal capacity by settling,by contrast,was reduced.An important conclusion was drawn that there was a competition relationship between flotation and sedimentation.Oil removal capacity of floating was the greatest when mixing rate was 500rpm.The flotation was clearly the dominant removal mechanism during emulsified oil removal under the experimental conditions.3.According to experimental phenomena and conclusions,the mechanism of oil removal and the interactions between oil droplets,flocs and bubbles were analyzed in electrocoagulation reaction.Subsequently,the main removal path of emulsified oil was proposed innovatively.In addition,kinetic analysis was fulfilled.It was found that only using flotation had slight influence on oil removal effect and with increasing bubble diameter,the capacity of flotation was proved to be worse.The emulsified oil was removed efficiently and rapidly by the combined action of coagulation and flotation.4.Study on effect of inclined plates on enhanced performance of oil removal and optimization of multiple factors?1?The influence of six angles of inclined plates on performance of oil removal was studied by electrocoagulation.It was found that change of angle of inclined plates decided not only the probability of collision between coagulants and H2 bubbles but also the size of area of the anode surface disturbed by H2 bubbles,thereby the flux of the anodically dissolved Al3+.As a whole,oil removal rate in center reduced with increasing angle of inclined plates,and yet the oil removal rate increased suddenly with??d?.In addition,tilt angle of plates was proved to have influence on oil removal rate uniformity in both the vertical and horizontal directions,as well as power consumption.The evenness index of oil removal efficiency in the electrocoagulation unit was firstly proposed to reflect oil removal rate uniformity.Experimental results were shown that electrocoagulation cell with??d?has higher treatment efficiency and less energy consumption,because this special angle promoted the distribution of bubbles thereby improving the mass transfer process of coagulant.?2?Response surface methodology was used to design experimental scheme,and establish predicting model,including electrolysis time,current density and angle of inclined plates.The predicting model was optimized and verified subsequently.Research was demonstrated that the model established could accurately predict the oil removal rate during treating simulated sewage or wastewater from the oil field.The optimum condition was current density of 100 A/m2,tilt angle of 73.1°,electrolysis time of 19 min.Relative errors of the oil removal rate between simulated sewage or wastewater from the oil field and experimental result were 1.35%and 3.08%,respectively.5.Intensified performance of magnetic field was studied during oily wastewater treatment using electrocoagulation.The oil removal process with magnetic field was compared with only electrocoagulation,and the influence of direction and intensity of magnetic field on the oil removal performance were studied.The results were showed that magnetic field of 40mT increased mass transfer rate under experimental condition,and meanwhile highest oil removal efficiency,better uniformity of oil removal and lower power consumption were received. |
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