Summary In this paper we present the development and implementation of automatic operation and control for a pump/hydrocyclone produced-water-treatment system to maximize oil/water-separation efficiency. A so-called perturb-and-observe (P&O) algorithm is adapted for a novel centrifugal pump to continuously optimize the point of operation. The novel pump coalesces and increases the size of oil droplets in the produced water, resulting in a unique relationship between the coalescing effect and the point of operation, and allowing for the successful implementation of the P&O algorithm. The algorithm was implemented in two different setups, one measuring the droplet-size distribution between the hydrocyclone and the pump, and the other measuring the oil concentration downstream of the hydrocyclone. The latter was considered the most robust because it required no prior knowledge of the system. Nonetheless, both setups achieved satisfying results and compared favorably with a third setup, where the optimal point of operation was predicted using measurements of the upstream produced-water characteristics.

It is critical to minimize the amount of free hydrocarbon entrained in the aqueous phase (i.e., Produced Water or Rich Monoethylene Glycol (MEG) streams) in order to mitigate impact on the operational performance of the Effluent Water Treatment and MEG Recovery Unit facilities. Hydrocarbon entrainment in Produced Water or Rich MEG is often the result of process conditions that favour emulsion formation and/or hinder emulsion separation. Consequently there is a need to look at the process and equipment design employed, along the flow path that the hydrocarbon/aqueous phase travels through, prior to entering the separation equipment used for hydrocarbon removal from the aqueous phase, as well as the separation equipment itself. The paper will present a roadmap of the overall route that the aqueous stream can take to offer insight into the process units affected by improper hydrocarbon removal. Operational situations arising from the impact of excessive hydrocarbon entrainment will be given as well as a summary of wellhead operating parameters that need to be considered in terms of their impact on equipment selection/design. The flow path, to be focused on, starts at the reservoir/wellhead and ends where the aqueous stream leaves the final hydrocarbon removal equipment, just upstream of either the Effluent Water Treatment or MEG Recovery Unit facilities. Factors concerning emulsion formation and separation are introduced as required to describe how process fluid properties and flow conditions influence the formation of emulsions and the separation of hydrocarbons from the aqueous phase. How to improve on existing methods for the selection/design of liquid-liquid separators by considering and trying to estimate the entire droplet size distribution (DSD) of the dispersed phase in the stream entering the separation equipment, along with estimating the amount of coalescence, will be elaborated on. This is paramount to ensure the correct equipment is selected, especially when the low end of the distribution, particularly drops below 20 μm, can be quite difficult to remove. Design considerations to minimize hydrocarbon content in the aqueous phase will be discussed and involve looking at key areas of energy dissipation (e.g., choke and control valves) regarding the range of fluid properties and process conditions, and the estimation/influence of drop size distribution/fractional interface coalescence efficiency (fICE) on the selection/sizing of fluid-fluid separator technology. Examples of dealing with emulsion issues occurring in industry, as per vendor experience, will be presented as well as available vendor equipment technology. General recommendations concerning lab bench testing, modelling, and equipment/chemical vendor testing will also be discussed.

Summary A novel centrifugal pump that increases oil-droplet sizes in produced water has been developed. This paper investigates a concept of pumping-pressure optimization, with respect to downstream separation efficiency, for the new pump. The investigation shows that the coalescing centrifugal pump always increased the separation efficiency of a downstream hydrocyclone. Furthermore, it is shown that the pumping pressure can be adjusted to maximize the improvement. Experimental results demonstrate how pumping conditions that minimize the volume fraction of droplets with a diameter smaller than the cut size of the hydrocyclone maximize the separation efficiency. Finally, it is demonstrated how the concept of pumping-pressure optimization can be implemented in a typical produced-water-treatment plant.

Summary In hydrocarbon production and processing, choke and control valves mix and emulsify petroleum phases. The consequence is often that the efficiency of separation processes is affected and finally that the quality of oil and water phases is degraded. Over the last few years, low-shear valves targeting petroleum processes have emerged on the market. This paper presents four separate live-fluid experiences from low-shear valve installations, each surveyed and documented by an independent third party. Three of the installations refer to choke valves, whereas the fourth installation refers to a control valve. For each installation, standard choke and control valves were used as reference valves. In terms of downstream separation efficiency, the low-shear choke valves reduced oil-in-water concentrations respectively by 70, 45, and 60%, by total average. In the control valve application, the low-shear valve, which was located between the hydrocyclones and a compact flotation unit, reduced the oil-in-water concentration by 23%. In sum, the field installations have demonstrated that low-shear valves significantly and consistently reduce oil-in-water concentrations and thus improve the produced water quality. The results signify that low-shear valves may be used in debottlenecking separation and produced water treatment processes, reducing the environmental influence from produced water discharges. Because the low-shear technology enables processing of petroleum phases with less effort, energy, and chemicals, it also reduces emissions to air.

This paper presents three variable step size P&O algorithms for optimizing the separation efficiency of a coalescing pump/deoiling hydrocyclone produced water treatment system. By continuously adjusting the pumping pressure, and subsequently the coalescing effect, the algorithms are used to minimize the oil concentration downstream the hydrocyclone. Due to the variable step size, the algorithms react rapidly to changes in the upstream produced water characteristics, at the same time as they reduce (or eliminate) steady-state oscillations. Based on both simulation and experimental testing, the study discusses advantages and disadvantages of the algorithms.