Application of CFD Simulation for Evaluating the Separation Efficiency of Deoiling Hydrocyclones :: Environment, Oil-water Separation

Introduction Having an efficient and reliable system for oil-water separation is of crucial importance especially for offshore oil and gas industry. Due to the platform movement, space, weight and operating limitations in offshore, the usage of common methods (gravity based vessels) for oil/water separation are ineffective. On the other hand producing oil is often accompanied by large amount of water that is discharged into the sea on offshore platforms. The amount of oil in water is confined from environmental standards. Therefore the need to have a high efficiency compact separator during variable operating conditions attracts the interests of researchers to hydrocyclones. Special trait of hydrocyclones such as simple design, easy to install and operate, no moving parts, and low manufacturing and maintenance costs make hydrocyclones as an economical and effective system for produced water treatment [1-2]. The separation process that is occurred in the deoiling hydrocyclones is quite different from that in the desander ones []. The density difference between liquid-liquid is smaller than solid-liquid mixtures. Therefore the separation of liquid from liquid is more difficult than solid from liquid and needs higher rotational speeds. The liquid droplet in comparison with solid particles could not experience shear rates. If the shear rates increase to the critical level the larger droplet break down to the smaller ones. The separation process for small liquid droplets is more difficult than large ones. On the other hand if two droplets close enough they might coalesce to each other. Because of difference in flow split of desander and deoiler hydrocyclones, the flow feature of continuous phase is different in these two types. The centrifugal forces cause the solid particles migrate to the wall region in desander hydrocyclones. So the near wall region is important in desander hydrocyclones but the oil droplet migrate to the center of hydrocyclones in deoiling hydrocyclones and the consideration tends to the center flow features. The first idea of using common hydrocyclones for oil-water separation was suggested by Simkin and Olney [6] and Sheng et al. [7] but fundamental studies on deoiling hydrocyclones started from 1980 by Colman [8] and Thew. Several experimental researches on deoiling hydrocyclones were conducted by Colman et al. [9] and Colman and Thew [10-12]. Their results showed that the separation efficiency of hydrocyclones is independent of flow split between 0.5 to 10 percent. So the overflow diameter should design based on working conditions. Moreover for constant droplet size distribution in inlet, the size distribution in outlet is independent of flow split. Application of CFD Simulation for Evaluating the Separation Efficiency of Deoiling Hydrocyclones :: Environment, Oil-water Separation Introduction Having an efficient and reliable system for oil-water separation is of crucial importance especially for offshore oil and gas industry. Due to the platform movement, space, weight and operating limitations in offshore, the usage of common methods (gravity based vessels) for oil/water separation are ineffective. On the other hand producing oil is often accompanied by large amount of water that is discharged into the sea on offshore platforms. The amount of oil in water is confined from environmental standards. Therefore the need to have a high efficiency compact separator during variable operating conditions attracts the interests of researchers to hydrocyclones. Special trait of hydrocyclones such as simple design, easy to install and operate, no moving parts, and low manufacturing and maintenance costs make hydrocyclones as an economical and effective system for produced water treatment [1-2]. The separation process that is occurred in the deoiling hydrocyclones is quite different from that in the desander ones []. The density difference between liquid-liquid is smaller than solid-liquid mixtures. Therefore the separation of liquid from liquid is more difficult than solid from liquid and needs higher rotational speeds. The liquid droplet in comparison with solid particles could not experience shear rates. If the shear rates increase to the critical level the larger droplet break down to the smaller ones. The separation process for small liquid droplets is more difficult than large ones. On the other hand if two droplets close enough they might coalesce to each other. Because of difference in flow split of desander and deoiler hydrocyclones, the flow feature of continuous phase is different in these two types. The centrifugal forces cause the solid particles migrate to the wall region in desander hydrocyclones. So the near wall region is important in desander hydrocyclones but the oil droplet migrate to the center of hydrocyclones in deoiling hydrocyclones and the consideration tends to the center flow features. The first idea of using common hydrocyclones for oil-water separation was suggested by Simkin and Olney [6] and Sheng et al. [7] but fundamental studies on deoiling hydrocyclones started from 1980 by Colman [8] and Thew. Several experimental researches on deoiling hydrocyclones were conducted by Colman et al. [9] and Colman and Thew [10-12]. Their results showed that the separation efficiency of hydrocyclones is independent of flow split between 0.5 to 10 percent. So the overflow diameter should design based on working conditions. Moreover for constant droplet size distribution in inlet, the size distribution in outlet is independent of flow split.