A Field Comparison of Static and Dynamic Hydrocyclones

Abstract

Summary Three commercially available liquid/liquid hydrocyclones (LLHC's) are evaluated in an operating oil field. The oil-removal capabilities of two staticunits and one dynamic or rotating unit are determined across a full range of operating conditions. Such variables as driving pressure, flow rate, rejectrate. and rotational speed are investigated to optimize oil-removal efficiency. The key parameter of oil droplet-size distribution also is evaluated and its effects on performance are determined. Introduction Removal of oil from produced water is essential in many oil fields to meet water-quality specifications for discharge or injection. Environmental regulations limit the quantity of oil in discharge water, and the need to maintain well injectivity often limits oil concentration in injected waters. Recent advances in oil/water separation technology have led to the introduction of LLHC's in oilfield operations. LLHC's are small, lightweight, efficient devices that offer excellent oil-removal performance. These devices are much smaller than traditional oilfield water-treatment equipment, such as flotation cells and plate-pack separators. Hence, they are suited primarily to an offshore environment where minimization of space and weight have a positive economic effect. Two distinct varieties of LLHC's are available: the static hydrocyclone and the dynamic or rotating hydrocyclone. Previous oilfield investigations of hydrocyclone technology have concentrated on evaluating and optimizing the performance of the static devices. Results of dynamic hydrocyclone performance are limited to offshore testing of an industrial prototype unit. This paper outlines the work carried out in an operating oilfield to evaluate the merits of both static and dynamic hydrocyclones. It looks at parameters that most affect separation performance and compares the operation and oil-removal capabilities performance and compares the operation and oil-removal capabilities of three commercially available devices. Hydrocyclone Principles Dynamic/Rotating Hydrocyclone Design. Fig. 1 is a schematic of the rotating hydrocyclone. It is essentially a rotating cylinder with an axial inlet, a reject nozzle, and an axial outlet. An external motor rotates the cyclone wall and the inlet vortex cone at 1,600 to 3,000 rev/min. The cyclone is available in five sizes with capacities ranging from 1,250 to 25,000 BWPD [8.28 to 165.6M 3 /h]. A Size 2 unit nominally rated at 2,500 BWPD [ 16.56 m /h] at 2,250rev/min was used for this test. Principle of Operation. The inlet stream enters the device axially and flows past a rotating conical vane that imparts vortex motion to the fluid. The high centrifugal forces exceeding 1,000 g enable the oil to migrate to a central core, which exits from the hydrocyclone outlet by the reject nozzle. The clean water exits the cyclone at the same end as the reject stream; hence, the device is cocurrent for both product and reject streams. The vortex set up by the rotation is called a "free vortex" in which the tangential speed is inversely proportional to the distance to the cyclone centerline. In centrifuges, the vortex is called a "forcedvortex" in which the tangential speed is directly proportional to the distance to the centrifuge centerline. Static Hydrocyclones Design. Fig. 2 shows schematics of both static cyclones. Cyclone A consists of four sections:a cylindrical swirl section;a concentric reducing section,a fine, tapered section; anda parallel tail section. This cyclone has a tangential inlet to the cylindrical swirl section, a reject orifice at the top of the device, and an outlet at the bottom. Cyclone B has some design differences from Cyclone A. This device has three sections:a cylindrical swirl section,a concentric reducing section, anda parallel tail section. Other differences are the two tangential inlets to the swirl section and a core finder in the tail section to increase the capture of small oil droplets. The units are similar outwardly, with Cyclone A being slightly larger. Principle of Operation. The principle of operation is the same for both hydrocyclones. The feed stream enters the cylindrical swirl section tangentially. This sets up a high-velocity vortex that again generates forces exceeding 1,000 g. The fluid accelerates through the conical sections where, because of its lower gravity, the oil moves to a central core. In both hydrocyclone designs, the majority of separation occurs in the conical sections. The fluid then passes into the cylindrical tail section where the smaller oil droplets are removed. Clean water emerges from the outlet port while an axial reversal of flow carries the oil core out of the hydrocyclone through the reject orifice. The total residence time in each device is less than 2 seconds. Factors Influencing Performance Oil/water separation in both static and dynamic designs can be characterized by Stokes' law. Stokes' law states that the velocity of separation of oil droplets in water depends on the oil/water differential gravity, the oil-droplet diameter, and the water viscosity. In a finite-residence-time vessel like a hydrocyclone, separation velocity is directly proportional to separation efficiency. Stoke's law can be expressed as v = (-)gd /18, where v=separation velocity, =water specific gravity, p, =oil specific gravity, A = water viscosity, g = 32.2 ft/sec, and d=droplet diameter. Stokes'law shows that the driving force for separation is the specific-gravity difference between the oil and water phases, The oil-droplet diameter, asquared term, also plays an important part in the separation process. Temperature influences separation performance mainly by altering the water-phase viscosity, although a small effect on differential gravity also will be evident. A temperature increase usually will improve hydrocyclone performance by reducing the water-phase viscosity. Stokes' law indicates the factors that influence the performance of both static and dynamic hydrocyclones; however, some performance of both static and dynamic hydrocyclones; however, some influencing factors are specific to the type of hydrocyclone. Static hydrocyclone performance is influenced greatly by two factors: operating pressure and reject ratio. Operating pressure is critical for successful hydrocyclone operation. For a given flow rate, a minimum inletpressure is required to generate the vortex inside the device. When the minimum inlet driving pressure is reached, inlet-to-outlet and inlet-to-reject differential pressures control hydrocyclone performance. Reject ratio is pressures control hydrocyclone performance. Reject ratio is defined as the ratio of reject flow to inlet flow expressed as a percentage. Hydrocyclone operation below the optimum reject ratio percentage. Hydrocyclone operation below the optimum reject ratio results in low oil-removal efficiencies because only a small volume of the oil-rich central core is removed with the reject stream. SPEPF P. 84