Research On Coupled Field Backwashing And Filtration Mechanism Based On Dual Bed Filters For Oilfield Wastewater

The deep-bed filtration process is widely used in oil fields due to its high filtration efficiency and strong oil removal capacity.However,with the promotion and application of polymer injection in oil fields,the concentration of polymer in oilfield wastewater has been greatly improved,which has a great impact on the backwashing regeneration of walnut shell media,such as the decrease of filtration efficiency,and the poor regeneration of walnut shell media,the loss of media,excessive expansion of the filtration bed and needing high backwashing pressure,etc.In order to solve the problems,the hydrodynamic backwashing process loading vortex flow field in deep bed filter was studied,and a new dynamic backwashing method based on vortex flow and gravity field was put forward.Based on the theory of vortex flow field and hydraulic backwashing,a dynamic backwashing model under coupled field was established.According to model experiments the relationship between backwashing intensity,coupled field effection and backwashing time was analyzed,and a new dynamic backwashing under coupled field was established.Also,the influence of parameters on media regeneration efficiency under coupled field had been studied,such as backwashing intensity,backwashing time,polymer content,water quality shockt and solution viscosity,etc.The dynamic backwashing control surface in the dual filtration bed was obtained by solving the batch filtration function.The velocity gradient in deep bed backwashing under coupled field was got,and the mechanism of media regeneration in backwashing was analyzed,and also the theoretical backwashing time was determined.Additonally,the filtration rating of deep granular bed was proposed,and the theoretical functions of filtration rating based on different mechanisms were established.The results show that the dynamic backwashing mode under coupled field has the greatest influence on the media regeneration,and the backwashing intensity has second.Under the condition of dynamic backwashing based on coupled field,the removal rate of interception oil by filtration bed increases with the increase of the backwashing intensity,which is on the rise.The removal rate of bed interception oil is basically stable for single bed and dual bed of?0.80~1.20mm/?0.30~0.50mm walnut/sand,and is greater than 95.96%,and also the oil removal rate on media surface is more than 98.94%when the backwashing intensity is 8.8 L/m2·s and the backwashing time is 15 minutes,the filtration bed is backwashed thoroughly.Meanwhile,the axial dynamic backwashing mode under coupled field has strong adaptability by means of comprehensive evaluation in extreme conditions,such as water quality shock,on-site contaminated meida,high content polymer,on-site aging polymer,the change of soluion viscosity,etc.Excellent backwashing effect can be obtained even under extreme conditions,the oil removal rate of bed interception is 90.0%or more after backwashing.In addition,the mode of dynamic backwashing under coupled field has good adaptability to the polymer concentration in wastewater.When the polymer concentration is greater than 400mg/L,the removal rate of interception oil by filtration bed is above 86.7%when the backwashing intensity is 10.0 L/m2·s and the backwashing time is 20 minutes,the filtration bed still has good backwashing effect.It is assumed that the filtraiton bed is arranged according to the particle size of media.from top to bottom,Under the action of the sedimentation mechanism,the suspended particles undergo multiple settlling separation in pores,and the filtration rating function is(?)in the most unfavorable condition;At the same time,assuming that the media particles are closely arranged,a multi-dimensional pore is formed between the particles,and according to the sieving effect based on the woven fiber the filtration rating functionbyinterceptingmechanismisestablished,it is(?);In addition,the suspended particles are subjected to the adhesion force of van der Waals force,electrostatic force and capillary force,and the shearing force of the water flow in the filtration bed,and the minimum size of the captured suspended particles is obtained under the combined force,that is,the microscopic mechanism filtration rating is(?).The filtration rating functions of the sedimentation,interception and adhesion mechanism are established,solving their filtration rating is 4.08 um,2.57 um and 0.84 um and um respectively,under the conditions of L/d value of 1500 and filtration rate of 12.0 m/h.In addition,the result shows that the filtration bed structure and its coupling effect have a great influence on the filtration efficiency,Increasing the thickness of the walnut shell filtration bed is beneficial to improve the oil filtration efficiency and increase the thickness of the quartz sand filtration bed,which is beneficial to improve the filtration efficiency of the suspended solids.When the polymer concentration is less than 200mg/L,the oil and suspended solid removal rate is 94.3%and 79.7%.by the dual bed of?0.80~1.20×950mm/?0.30~0.50×750mm walnut shell/quartz sand,When the polymer concentration is 400mg/L,the oil and suspension solid removal rate was 95.8%and 78.3%,and the polymer concentration had little effect on the filtration efficiency.Furthermore,assume that the non-ideal actual filter bed is linearly packed according to the particle size of the filter material,and the most unfavorable condition of the batch filtration force equation is(?),the batch filtration equation is a subtractive function,and the backwashing control surface is the surface layer bed for the non-ideal actual filltration bed when considering the most disadvantageous conditions.That is,the uppermost layer of the filtration bed is back washed thoroughly,and the entire filtraion bed can be regenerated with good backwashing.At last,Based on the collison interaction between particles and the resistance of water flow in coupled field,the velocity gradient function of dynamic backwashong ounder the coupled field is established,it is (?),the velocity gradient in deep bed backwashing under coupled field is determined to be G=2028.6 s-1 by solving the equation,which is much larger than that of independent water backwashing or air scouring.Meanwhile taking the pollutants in the main reaction zone as the research object during the backwashing process,the theoretical time control function of the dynamic backwashing under coupled field is established,and it is(?).The theoretical backwashing time under coupled field is 15-18 min,and the theoretical analysis is correct by the model experiments.At last,the industrial scale experiment shows that the dynamic backwashing technology under coupled field has higher efficiency of filtration and media regeneration.The removal rate of oil and SS is more than 86.9%,and and also the oil removal rate of media surface is more than 96.89%after backwashing,which means good industrial experimental results have been achieved.The dynamic backwashing technology under coupled field enriches and develops the hydraulic backwashing theory,and provides a new way for solving the media regeneration in deep bed filters for oilfield wastewater.It can also be used for reference to solve the backwashing problem for other light media.