Experimental And Numerical Simulation Study Of Pretreatment For Alkaline Waste Water From Oil Refinery

Alkaline waste water from oil refinery is a kind of alkaline waste liquid containing a largenumber of poisonous and harmful pollutants such as sulphides and phenols produced fromalkaline washing and refining process. It is the main source of fetor in refinery. Aerated filter,especially Circulating Biological Aerated Filter (CBAF) is an effective equipment for alkalinewaste water treatment. Waste water treatment process based on CBAF is worthy of populatingfor its lower cost and higher efficiency.Until now, design, model selection and engineering enlargement of CBAF reactors relymaily on experimental data and individual experience which is short of theoretical direction.The mechanism of transfer including mass transfer and momentum transfer in CBAF iscomplicated and is hard to get the similar transfer mechanism by common experimentalmeans so it is very difficult to find an effective way to improve the treating efficiency. It costsa great deal of manpower and material resources to adjust the operating parameters throughexperimental research because the detection work of the properties of waste water is veryintensive, and at the same time it is not easy to find a breakthrough for the improvement ofthe reactor and process.Experimental research combined with numerical simulation is used in this dissertation. Thetreating performance of Fe/C microelectrolysis aerated filter and serial CBAF includingsingle-aerated-tube CBAF and four-aerated-tubes CBAF is studied. The concentrationdistribution of dissolved oxygen, hydrogen sulfide and elemental sulfur in thefour-aerated-tubes CBAF is simulated and the way for the improvement of reactor andprocess is proposed for the four aerated tubes CBAF.Refinery waste water is pretreated by Fe/C microelectrolysis isolated aerated filter. Whilethe concentration of COD is between700mg L-1-1100mg L-1，which for BOD5between70mg L-1-100mg L-1, the ratio of BOD5/COD is about0.1and the concentration of ammonia-nitrogen is between10mg L-1-30mg L-1, and under suitable operation conditions, theremoval rate of COD and ammonia-nitrogen can reach67%and58%, respectively. Degradation and biodegradability of waste water can be enhanced while moderate H2O2isadded into the system.Alkaline waste water from oil refinery is biologically pretreated by three-stagesingle-aerated-tube Isolated Aerated Biological Filters. While COD, the concentrations ofsulphides, hydroxybenzene and oil in the waste water are no more than1500mg L-1,800mg L-1,15mg L-1and150mg L-1, respectively, the total removal of these pollutants is87.8%,98.8%,96.8%and91.0%accordingly. The system can run stably and the biomassin the filter grows well. Biodegradation, chemical oxidative desulfurization and aerationdesulfurization are three ways to remove sulphides and under bacterial condition,biodegradation is the most effective one, more than94.5%sulphides are removed bybiodegradation. Under aerobic condition, sulphides can be transformed into different forms.The average value of sulphides transformed into elemental sulfur is about96.5%when thewaste water is treated by CBAF under common operation condition.The flow inside the CBAF is three-dimentionally simulated based on CFD with k-εmodelfor turbulence model, Mixture model for multiphase flow model, component transport modeland wall function method.Concentration distribution of dissolved oxygen inside the four-aerated-tubesCBAF issimulated at four different air water volume ratios (AWVRs) and constant influent of water.The k-ε model is chosen for turbulence model and Mixture model for multiphase model. Theinfluent is constant and its value is0.5m3h-1, AWVRs are varied and their values are8﹕1,16﹕1,24﹕1and32﹕1respectively. As AWVR increases, the concentration distribution ofdissolved oxygen becomes more and more uniform in both z direction and the same location.The disturbance of airflow from aerator becomes more and more intense. The concentrationof dissolved oxygen increases obviously at the bottom of the reactor. And the mean value ofthe concentration of dissolved oxygen located in the packing layer increases step by step somore and more zones in packing layer become suitable for biodegradation. The flow velocityof water in packing layer, macadam layer and aerated tubes increases constantly and the flowrate of circular flow can be dozens of times or more than that of inflow. The flow in packing layer and macadam layer is proved to be laminar and that in aerated tubes is turbulent bycalculating Reynolds number.Concentration distribution of dissolved oxygen, sulphides and elemental sulphur inside thefour-aerated-tubes CBAF is simulated at four different AWVRs and with constant influent ofrefinery waste water. Results show that the disturbance of the bubbles enhance, theconcentration distribution of dissolved oxygen keeps coincident on the top of the CBAF butincreases at the bottom as AWVR increases. The concentration distribution of sulphides is nothomogeneous, and it is high in the area near the inlet and is low in the area near the outlet.The elemental sulphur concentrates upon the packing layer especially near the inlet. Thoughthe elemental sulphur appears in the packing layer near the outlet, its concentration is verylow. Increasing the aeration rate is benificial for increasing the concentration of dissolvedoxygen, and24﹕1is a suitable AWVR for the concentration distribution of reactants.Concentration distribution of dissolved oxygen, sulphides and elemental sulphur inside thefour-aerated-tubes CBAF is simulated at different Hydraulic Retention Time or HRT for shortand constant AWVR. Results show that the concentration of dissolved oxygen reduces andthat of sulphides increases gradually as HRT decreases. On the same horizontal plane, theconcentration gradient of sulphides reduces and the concentration of sulphides at the outletincreases gradually as HRT decreases. The concentration of sulphur in the packing layer nearthe inlet is always higher than that near the outlet no matter how the HRT varies.A method that changes the structure of CBAF is proposed. The one-inlet-one-outlet-CBAFis changed into two-inlets-two-outlets-CBAF. The concentration distributions of dissolvedoxygen, sulphides and elemental sulphur are simulated at four different AWVRs which are8﹕1,16﹕1,24﹕1and32﹕1, respectively, while the influent is0.5m3h-1. Results show thatthe changes of the structure of CBAF can improve the distribution of sulphides and increasethe production capacity of CBAF.A method that changes the technological parameters is proposed which changes theaeration ratio of two sets of aeration tubes so as to improve the concentration distribution ofsulphides inside the CBAF. When the influent is0.5m3h-1and the AWVR is24﹕1, six different aeration ratios of two sets of aeration tubes are simulated which are8﹕2,7﹕3,6﹕4,4﹕6,3﹕7and2﹕8, respectively. Results show that the distribution of pollutants is wellimproved and the production capacity of CBAF is increased by adjusting the aeration ratio ofthe1st set of aeration tubes and the2nd set of aeration tubes from2﹕8to4﹕6.