Practical Method for Simulating Flotation for Produced Water System Design

M. Straw, D. Fielding, E. Kay, S. Lo, T. Eppinger
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Abstract

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.
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采出水系统浮选模拟的实用方法
对于油气行业来说,保持采出水的质量是一个关键的环境目标,也是法律要求。设计和运行有效的采出水分离系统需要理解和管理一系列复杂的多相流现象,特别是在使用气浮工艺的情况下。本文提出了一种新的建模方法,利用计算流体动力学(CFD)来模拟气浮过程。该方法旨在用一种简单、省时的方法预测油气聚结过程。在浮选过程中,引入采出水中的小气泡与水中存在的油滴结合。混合的油气颗粒比单独的油滴具有更大的浮力,并且能更有效地与水分离。虽然CFD方法已被应用于改进各种分离、紧凑和诱导气浮选系统的设计和操作,但在系统级别的模拟中,试图捕捉浮选过程中的聚结过程通常在技术上具有挑战性,且耗时太长。通常,设计师和工程师必须利用一般的流动特性、行为和指标来暗示给定设计的浮选效率,许多模拟和物理测试相结合,为设计和操作决策提供更大的信心。本文提出的方法旨在通过Simcenter STAR-CCM+结合多相流模拟的简化方法,同时代表小油滴和气泡,来预测气浮过程和聚结效率。在该方法中,可以在给定的采出水系统中跟踪水和气中油滴的局部浓度。油在水和气相之间转移,代表了聚并过程。传输速率由一个模型控制,该模型取决于局部流动条件,如气泡大小、粘附和分离的概率以及可用于合并的气泡的表面积。该方法既适用于致密浮选系统,也适用于诱导气浮选系统,并且该方法的结果直接比较了系统内和出水中的油在水中的浓度。报告离开容器的油浓度,从而直接量化容器的有效性;这是对使用CFD的其他方法的改进,这些方法使用流动特性作为代理来推断分离效果。在单独使用流动特性时,可能存在一些关键方面或反直觉机制误导工程师。希望本文提出的方法能够使参与采出水系统开发、设计和操作的工程师和设计师更全面地了解复杂的流体力学和浮选过程的效率。
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