Dynamic hydrocyclones

Since the early 1980s, hydrocyclones have been used in produced water treatment to de-oil the water prior to discharge. Hydrocyclones used to de-oil the water are referred to as "hquid-liquid de-oiling" hydrocyclones. Liquid-liquid hydrocyclones are further classified as static or dynamic hydrocyclones. 

The major difference between static and dynamic hydrocyclones is that in the dynamic hydrocyclone an external motor is used to rotate the outer shell of the hydrocyclone, whereas in a static hydrocyclone the outer shell is stationary and feed pressure supplies the energy to accomplish separation of oil from water (no external motor is required). 

As shown in Figure 3.51, a dynamic hydrocyclone consists of a rotating cylinder, axial inlet and outlet, reject nozzle, and external motor. The rotation of the cylinder creates a "free vortex." The tangential speed is inversely proportional to the distance to the centerhne of the cyclone. Since there is no complex geometry that requires a high pressure drop, dynamic imits can operate at lower inlet pressures (approximately 50 psig) than static imits. In addition, the effect of the reject ratio is not as important in dynamic imits as it is in static units. 

Dynamic hydrocyclone performance is affected by the following parameters ... 

FIGURE 3.51. Liquid-liquid dynamic hydrocyclone separation. 

Dynamic hydrocyclones have found few applications because of the poor cost-benefit ratio. 

In the recent years, the advance of computer power has allowed numerical solutions for the differential equations that describe fluid motion. The use of computational fluid dynamics (CFD) is beginning to give a better understanding of the strongly swirling turbulent flow inside hydrocyclones and, consequently, of their performance [46-50]. 

Liibberstedt [64] tested three different hydrocyclones for HeLa cell separation a 7 mm Bradley [67], a 10 mm Mozley (Richard Mozley Ltd., Redruth, UK), and a 10 mm Dorr-Oliver (Dorr-Oliver GmbH, Wiesbaden, Germany) (the dimension quoted here is the diameter of the cylindrical part of each hydrocyclone). The best results were obtained with the Dorr-Oliver hydrocyclone (Fig. 3), which produced a cell separation efficiency of 81 % when working at a pressure drop of 300 kPa and a flow rate of 2.8 L min When operating with two 10 mm Dorr-Oliver connected in series (the overflow of the first as feed for the second) at 200 kPa, the global efficiency of the arrangement was 94% [65]. These experimental values confirm the computational fluid dynamics (CFD) predictions that high levels of efficiencies for mammalian cells could be achieved with small diameter hydrocyclones [46]. 

Hydrocyclone flow visualization and comparison with computational fluid dynamics... 

Hargreaves, J. H., and Silvester, R. S., Computational fluid dynamics applied to the analysis of deoiling hydrocyclone performance. Trans. I. Chem. E. 68,365 (1990). 

The design illustrated in Figure 8.26A, would be used for the removal of oil from water, that in 8.26B would be employed for the removal of water from oil. In use on the North Sea oilfields the first design is capable of cleajoJng a feed concentration of 1000 ppm dispersed oil to an overflow concentration below 40 ppm of ofl. The devdopment work leading to these hydrocyclone deagns involved the use of computational flmd dynamics and has been well documented [Cohnan and Thew, 1983]. 

As with all separation principles involving particle dynamics, a knowledge of the flow pattern in the hydrocyclone is essential for understanding its function and subsequentiy for the optimum design and evaluation of the particle trajectories, which in turn allow prediction of the separation efficiency. A short account of the flow pattern within typical hydrocyclones and the known or probable behaviour of solid particles in the flow is given in the following section. This is done for the case of low viscosity liquids under conditions in which the particles cause little or no interference to the flow patterns (i.e. for low solid concentrations). 

The concept of equivalence is well known and accepted in particle size measurement, and the paper applies this concept to the measure of the spread of the distribution. It characterises the actual distribution of particle size in the slurry by an equivalent, lognormal distribution described by a simple formula with two numerical parameters, the geometric mean (as a measure of the mean size) and the geometric standard deviation (as a measure of the distribution spread). The equivalence is by separation efficiency in a dynamic separator such as a hydrocyclone or a sedimenting centrifuge. 

The reduced grade efficiency curves of some dynamic separators, including hydrocyclones and sedimenting centrifuges, can be fitted by a cumulative log-normal function in the following form ... 

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