Produced water treatment using hydrocyclones - Theoretical and experimental studies of novel control schemes

Abstract

Produced-water is a non-lucrative by-product of oil and gas production, and the quantity increases as the field ages. Produced-water is either discharged into the sea or reinjected into the field to enhance pressure. As per the Norwegian Environment Agency, 126.4 million m3 of produced water was discharged into the Norwegian sea in the year 2020. The discharge of produced water has to meet the OSPAR criteria, where the amount of dispersed oil is limited to 30mg per litre of discharged water, on a monthly average. In addition, an efficient produced-water (PW) treatment system must meet environmental regulations before discharging the PW to the sea. De-oiling hydrocyclones, compact floatation units (CFUs), or their combination are commonly used for produced water treatment. Maintaining the efficiency of this compact equipment at varying process conditions is always a challenge. This thesis focuses on de-oiling hydrocyclones, which are commonly used producedwater treatment equipment on the Norwegian Continental Shelf. The compact and light nature of these hydrocyclones, which do not require any additional chemicals or gases to be injected for the operation, makes them attractive for subsea processing. The main problem with hydrocyclones is the low residence time, which makes them more susceptible to upstream variations such as changes in inlet oil concentration, changes in inflow rate and changes in droplet distribution. Typically, control schemes that use pressure drop ratio (PDR) to control the separation, are implemented to handle these disturbances. However, since this is an indirect way of controlling the hydrocyclones, this option can reduce their efficiency significantly and result in violations of the environmental regulations. This thesis is divided into two parts; a theoretical analysis part and an experimental analysis part, both with the aim of improving the control aspects of de-oiling hydrocyclones. In the theoretical analysis part, a control-oriented mathematical model of the hydrocyclones is developed. This model has a static pressure-to-flow relationship which can estimate the flow-rates and the pressures at the outlet of the hydrocyclones when the inlet conditions are known. The separation inside the hydrocyclone is approximated using a polynomial derived from droplet trajectory analysis. A simple mass-balance model is formulated to capture the dynamics, and is used for the control. This model was used for studying new control schemes for de-oiling hydrocyclones. In the experimental analysis part of this thesis, a test rig is constructed, with industrialscale hydrocyclone liners and other auxiliary systems, to support the testing of the novel controllers. The test rig can emulate the first stage gravity separator and generate different disturbances at the inlet of the hydrocyclones installed in the rig. Drawbacks of traditional PDR control schemes were studied at the test rig. Then, new control schemes such as cascade, feed-forward, and direct control schemes were verified at the test rig. The research carried out during the course of the doctoral studies has been presented as a series of articles in various conferences and journals. These are now grouped together to constitute the body of this thesis. The second chapter gives the background theory of hydrocyclones and a brief review of the mathematical models of hydrocyclones, their control strategies, constructional details, and features of various experimental test-rigs of hydrocyclones available in the literature. The subsequent chapters cover the theoretical and experimental analysis performed.