Study On The Theory And Engineering Application Of Compact Flotation Unit

With the increasing of water content in produced fluid and the strengthen of environmental awareness, proper treatment of huge amounts of produced water in offshore oil & gas development necessitates the development of advanced water treatment technology with high efficient and compact to meet the stringent emission standards and improve the economic performance of offshore oil & gas exploitation.Compact Flotation Unit (CFU), which is developed following the concept of "high efficiency of conventional technology and integration of multiple technology", is a new kind of oil removal equipment for processing produced water with advantages of high separation efficiency, short hydraulic residence time (HRT) and small footprint. Based on the type of swirl out of the inner concentric cylinder, theoretical and experimental researches were carried out on a series of key theoretical and technical problems such as the adhesion mechanism of oil droplets and micro bubbles, the coalescence efficiency of oil droplets and micro bubbles, the dynamic separation characteristics of dispersed phase droplets, the distribution characteristics of the flow field, the dynamic separation characteristics of the dispersion phase, the design theory system, the structural optimization and the industrial amplification design of CFU.Based on the flotation separation dynamics theory and fluid mechanics theory, the forces and collision coalescence efficiency of dispersed phase droplets in the separation process of CFU were studied.By analyzing the flow field distribution and separation characteristics, it founded that CFU can not only improve the collision coalescence efficiency of oil droplets and micro bubbles, but also reduce the random dispersion effect and shear fragmentation caused by the turbulence flow,so as to achieve the efficient separation of oil and water.Based on the theories of Computational Fluid Dynamics (CFD) and Population Balance Model (PBM), the effect of turbulence intensity on the oil droplet size distribution, the average oil droplet size of surface area and the separation efficiency were studied from both the microcosmic and macroscopic levels by means of experimental study and CFD-PBM coupled numerical simulation. It was concluded that the turbulence intensity should be controlled between 4.9%-8.9%, and the surface hydraulic loading rate should be controlled around 56.4m3/(m2·n)to improve the collision efficiency of dispersed phase droplets and oil-water separation efficiencyThe collision coalescence efficiency of dispersed phase droplets in swirling flow is theoretical analyzed. The flow field distribution characteristic and oil-water separation characteristic in the preliminary separation zone of CFU were analyzed using the CFD-PBM numerical simulation model. The critical role of the preliminary separation zone in the pre-separation of oil-gas-water was researched from the micro and macro levels respectively. The effects of the inlet diameter, the height of the preliminary separation zone and the width of the annular space on the oil-water separation characteristics of CFU were discussed systematically.Based on the theories of fluid mechanics, multiphase flow dynamics,air floatation dynamics, i.e., a complete theoretical design method of CFU is established, by which can be used to determine the main structure parameters such as the inner diameter, the height of the device, the inner diameter of tangential inlet pipe and the outer diameter of the steady flow tube toughing the expected basic design capacity, hydraulic retention time,swirl intensity and other parameters.An engineering prototype was manufactured based on the theoretical design method above. The field test results showed that the separation efficiency of single vertical flotation vessel can be stabilized at more than 86%, the separation efficiency of two stage single vertical flotation vessel in series can be stabilized at more than 97.9% whither handling wastewater with low oil concentration (average oil concentration is 120mg/L) or high oil concentration (average oil concentration is 1732mg/L). The removal rate of suspended solids for single vertical flotation vessel was 58.3%, and the removal rate of suspended solids for two stage vertical flotation vessel in series can up to 72.1%. It showed that the separation efficiency of the engineering prototype is very significant, and the theoretical design method is stable and reliable.Based on the methods of CFD numerical simulation, genetic algorithm (GA) and BP neural network, a GA-BP neural network optimization model was proposed to optimize the main structure parameters of CFU. The CFD numerical simulation method was used to establish the relationship between structural parameters and separation efficiency of CFU. The GA-BP neural network is used to analyze the influence of structural parameters on separation efficiency. The predicted optimum structure parameters were 20mm for the entrance diameter,360mm for the flow tube diameter, 20mm for the annular space width and 3.6 for the height to diameter ratio. The separation efficiency of the CFU after optimization can reach up to 92.82%, i.e., an improvement of 13.6%comparing with that before optimization. The optimization model can realize the global search and can effectively avoid the occurrence of local optimal value. The average relative error is only 1.4% and it can be applied to the design and optimization of other equipment of multiphase flow separation.Based on the structural characteristics and separation characteristics of CFU, the relationship between the physical quantities involved in the separation process was studied using similarity analysis theory. The Industrial amplification criterion of CFU was summarized. Industrial prototypes with handling capacity of 20m /h and 120m3/h were designed in the gradual experience amplification mode. The actual performance of the device was tested by field test, and the reliability of the amplification criterion was verified.Based on the relevant basic research in this paper, it is helpful to have a fully understand on the separation mechanism and to further expand the application range of CFU. It can also laid a solid theoretical foundation for the individualized design and industrialization promotion of CFU, and provide a practical solution for efficient treatment produced water in the offshore oil&gas extraction and processing.