Falahi, Maryam; (2024) Discrete formulation for multi-objective optimal design of produced water treatment. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Produced water is the largest waste stream generated during oil and gas production. In offshore platforms, the majority of the produced water is discharged into the ocean. This results in an increase in the amount of toxic organic and inorganic compounds in the receiving environment and hence may threaten marine life. Therefore, many environmental agencies have enforced limits on the concentrations of toxic contaminants in the discharged water. To meet these regulations and to secure another potential source of fresh water, especially in water-stressed countries, the treatment of produced water becomes an attractive alternative to discharging the water. However, most of the research to date has focused on the economics of water treatment without consideration of other aspects, such as environmental and social impacts. These other aspects may have a significant impact on design decisions. This paper presents a multi-objective optimization model for the identification and design of alternative configurations for the treatment of produced water. The design problem is formulated as a mixed integer dynamic optimization model. In this approach, different technologies are assigned and linked in a given sequence resulting in a process flowsheet. The mixed integer dynamic optimization problem is solved using a nature-inspired meta-heuristic optimization procedure based on plant propagation. The design of the individual processing steps in the flowsheet is based on population balance modeling. The models take into account the evolution of the oil droplet and solid particle distribution due to breakage and coalescence phenomena. The distribution is represented by a discrete formulation based on a logarithmic discretization of the space (droplet/particle volume). Multiple case studies are presented which consider two criteria for design: economics and environmental impact. The pool of considered technologies in this study includes a variety of processing steps such as gravity separation, hydrocyclone, and membrane technologies aiming to meet the requirements of different destinations. The result is a set of designs that trade-off the two criteria, providing some of the necessary information for a decision maker to choose a particular design for implementation.

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