Research On Improving The Treatment Effect Of Polymer-contained Sewage Gravity Sedimentation Tank In Oil Field

With the widespread application of polymer flooding technology in oil field, the properties of produced sewage has been changed greatly, which is with high poly concentration and viscosity. It caused that the existing sewage treatment devices and equipments were of poor treatment effect. The gravity sedimentation tank is based on the principle of gravity separation, it has the advantages of simple structure and large capacity. It is the important equipment in the oil field sewage treatment engineering. The dehydration effect of gravity sedimentation tank directly affect subsequent filter tank processing load and processing effect, thereby affect the water quality standard. Therefore, according to the research on the flow field of gravity sedimentation tank and impact factors of gravity separation is determined, flotation, agent optimization and structure improvement should be carried out to improve the treatment effect of gravity sedimentation tank, which has important significance for the actual production. The specific contents are as follows.First, the flow field of gravity sedimentation tank is taken as the research object. The velocity field distribution in the gravity sedimentation tank is obtained by using the discrete phase model numerical simulation. A large number of sewage is settled in the near region of the tank wall. According to the trajectory length and residence time etc., the motion characteristics of oil droplets is revealed, and the existence of the back mixing flow and swirl flow in the gravity sedimentation tank is also revealed. The relationship of oil content and suspended solids content changed with the settling time is established by indoor static settlement experiment. And the distribution law of oil droplets size in sewage including oil and polymer is investigated. The settlement effect of polymer concentration, particle size of oil droplets and settling time is obtained. The experimental results are in agreement with the numerical simulation results. By comparison of the flotation sedimentation pretreatment experiment and static settlement experiment, the oil removal rate and suspended solids removal rate are increased by flotation sedimentation pretreatment experiment, which are above 30% and 15% respectively. On the basis of the flotation sedimentation pretreatment experiment, 25mg/L polymeric aluminum ferric chloride and 5mg/L anionic polyacrylamide solution is optimized as flocculant, and flotation agent with polar nonpolar molecular structure named FJ, its concentration is 25mg/L. From the field adjustment test, it can be known that the method only by extending the settling time cannot improve treatment effect continuously. The water quality in the process of gravity sedimentation tank is more stable by using continuous collection process. The suspended solids removal rate and oil removal rate are 5.96% and 8.14% higher than intermittent oil collection process. Finally, a new type of double ring shaped perforated water distribution device is proposed to improve the flow field and the treatment effect. The oil removal rate is increased 12.9%.