Tracking Maximum Produced Water Treatment Efficiency Using a Variable Speed Coalescing Centrifugal Pump : Doctoral Dissertation for the Degree Philosophiae Doctor (PhD) at the Faculty of Engineering and Science, Specialisation in Mechatronics

Abstract

Produced water is the most significant by-product of petroleum production. Even though naturally occurring, the produced water poses substantial environmental impacts if not managed correctly, and it is often associated with large volumes and high handling costs. Oil removal is one of the primary goals of produced water treatment. The efficiency of the treatment equipment is usually highly sensitive to the oil droplet's size. Generally, the larger the droplets, the better the separation. Due to the importance of large oil droplets, droplet breakup in process equipment such as pumps and valves should be kept to a minimum. Recently, a novel coalescing multistage centrifugal pump has been introduced to particularly target produced water applications. This pump promotes droplet growth rather than breakage, and thereby increases the efficiency of downstream treatment equipment. This thesis shows that the novel coalescing centrifugal pump can be actively controlled to maximize the efficiencyof downstream produced water treatment equipment, using real-time process information and tracking techniques. A once-through test rig has been designed, built, and used to study the coalescing performance of the novel pump. The rig has also been used to study the characteristics and benefits of combining the coalescing pump with a deoiling hydrocyclone. Perturb-and-observe (P&O)-algorithms have been developed and successfully implemented to actively control the pumping pressure and to continuously track the point of operation resulting in the highest downstream separation efficiency. The continuous tracking techniques have been compared to a model-prediction method, optimizing the pump operation based on upstream process information. In addition, the P&O-algorithms have been further improved by introducing a variable step size, and routines to avoid tracking failure. In total, this project has resulted in a unique and novel utilization of emerging pump technology. Concerning operational control, the coalescing pump has been innovatively combined with existing produced water treatment equipment, utilizing the potential of the pump to maximize the treatment efficiency.