Treatment of real oilfield produced water by liquid-liquid extraction and efficient phase separation in a mixer-settler based on phase inversion

Abstract

Produced water treatment has become a challenge for the oil industry to meet environmental legislation requirements, due to several dissolved organic compounds, which are barely removed under conventional treatments. In this work, oil and grease removal from real offshore produced water samples was evaluated by a novel liquid-liquid extraction process, followed by efficient phase separation in a mixer-settler based on phase inversion. In this novel approach, a condensate–average stream composition collected from offshore oil production platforms was used as an extraction solvent to remove mainly dissolved compounds. This strategy treatment, which is considered unprecedented in the industry, may allow oil producers to overcome many logistic challenges, also reducing inputs consumption and costs. The synthetic condensate potential was also compared to n-hexane and n-heptane in batch tests. According to the results, solvent concentration and pH exerted a significant influence on the extraction efficiency. Only at pH 4 and 1:10 solvent ratio, excellent removal efficiencies were obtained. Another significant result observed in the study was the mixer-settler performance, which allowed instantaneous phase separation with high extraction efficiency. At 10% (v/v) of condensate, the flow rate of 10 L/h and at pH 4, the applied process was able to reach the PW requirements for discharge according to the Brazilian National Environment Council guidelines in a single extraction step, resulting in a final outlet stream containing oil and grease under 29 mg/L, measured according Standard Method 5520 – B (Partition Gravimetric Method). This work offers an alternative to overcome the main produced water management challenges faced by the oil industry, using more efficient and compact equipment and reduced operating costs since the inputs are available on the offshore oil production platforms.