Practical Method for Simulating Flotation for Produced Water System Design

Maintaining produced water quality is a critical environmental target, and legislative requirement, for the oil and gas industry. Design and operation of effective produced water separation systems requires understanding and management of a range of complex multiphase flow phenomena, especially where gas flotation processes are used. This paper presents a novel modelling approach, using Computational Fluid Dynamics (CFD), developed to simulate gas flotation processes. The approach aims to predict the gas-oil coalescence process using a simple and time-efficient method. In the flotation process, small gas bubbles, introduced to the produced water, coalesce with the oil droplets present in the water. The combined gas-oil particles have more buoyancy than the oil droplets alone and separate more effectively from the water. While CFD approaches have been applied to improve the design and operation of a wide range of separation and compact and induced-gas flotation systems, it has typically been too technically challenging and time-consuming to attempt to capture the coalescence process involved in flotation in such simulations at a system level. Often, designers and engineers have to make use of general flow characteristics, behaviours and indicators to imply the flotation efficiency of a given design, with many combining simulations and physical testing to provide greater confidence in design and operating decisions. The approach presented in this work aims to predict the gas flotation process and coalescence efficiency, using a simplified approach by combining multiphase flow simulations, using Simcenter STAR-CCM+, representing both the small oil droplets and the gas bubbles. In the approach, the local concentration of oil droplets in water and in gas is tracked throughout a given produced water system. Oil is transferred between the water and the gas phases representing the coalescence process. The rate of transfer is governed by a model which depends on local flow conditions such as bubble size, probability of adhering and detaching and surface area of gas bubbles available for coalescence. The approach is applicable to both compact flotation and induced-gas flotation systems and the results of the approach give direct comparisons of oil in water concentration both through a system and in the water exiting. The oil concentration leaving the vessel is reported, thus directly quantifying the effectiveness of the vessel; this is an improvement over other approaches using CFD where flow characteristics are used as proxies to infer the separation effectiveness. In using flow characterisctics alone there is potential for critical aspects or counter-intuitive mechanisms to mislead engineers. It is hoped that the method presented will enable engineers and designers involved in the development, design and operation of produced water systems to more-fully understand both the complex fluid mechanics and efficiency of the flotation processes.