Thew-type liquid-liquid cyclone separator

The majority of work that has been carried out on the use of cyclonic separation of immiscible liquid-liquid mixtures, and specifically oil-water separation, is based around a design of the type developed at Southampton University and is now being

commercially exploited under the trade name ‘Vortoil’. This device is significantly different from the Bradley-type as can be instantly seen from Figure 2.7.

Key differences are:

• Twin (tangential) inlets • No vortex finder protrusion

• Second, low-angle, cone section with long parallel section on the underflow outlet.

Figure 2.7: Comparison of geometry of Bradley-type (left) and Thew-type cyclones (right) (after Thew, 1986)

Thew’s research group has published a number of papers, beginning with Kimber (1974). This paper discusses the cyclonic de-watering of ship oil using a lengthened cyclone with twin tangential inlets, but tantalisingly it lacks in a comprehensive definition of geometry. The paper does say that the cyclone has been significantly lengthened to increase residence time. Twin outlets are also present on the overflow and underflow to minimise the turbulence at these locations (the same reason as using twin tangential inlets). One distinction from the cyclone shown on the right of Figure 2.7 is the inclusion of a vortex finder. The paper notes that making this too large makes the water core unstable near the overflow outlet, due to turbulence caused by the outward radial fluid movement.

Figure 2.8: A system unit comprising many manifolded Vortoils ready to be put into service

At 1.7 bar pressure drop this device could be similar in energy use to the other Thew devices mentioned below; however, the paper does not give the inlet velocity and so this conclusion cannot be substantiated. The wall friction effect associated with lengthening the cyclone raised the inlet-to-overflow pressure drop and slowed the fluid spin

(detrimental to separation). The residence time increase balances with the effects of a longer cyclone (improving separation performance) and gives an optimum length to diameter ratio of 10-20. This compares to a Bradley-type cyclone with L/D in the range 1.5-7. Using lube oil and crude, efficiencies (based purely on composition) were

obtained with a mean droplet size of 40-50µm of 80-90%.

By 1980 (Colman, Thew and Corney, 1980), the direction of research at Southampton had moved onwards, with Kimber’s cyclone design being abandoned after achieving ‘moderate success’, in favour of a new design, again insufficiently detailed in the published paper. Apart from significant design changes (though no diagram of the

cyclone geometry is presented), the other main difference is one of the challenge placed on the cyclone, dealing with up to 3% oil concentration, several orders of magnitude higher than Kimber.

Achieving up to 93% efficiency, this cyclone employs an optimised vortex-finder protrusion of 1.1 times the cyclone diameter. Thew (1986) describes the rationale behind the removal of vortex-finder removal in the case of a light-dispersion case in terms of it being unnecessary. A main reason for the use of a vortex finder is to prevent the short-circuiting of a heavy phase dispersion across the roof of the cyclone and out through the overflow, which should contain the less-dense phase. If the less dense phase is the dispersed phase, then droplets of the less dense phase do not need to be protected from the overflow. The vortex finder protrusion is therefore redundant. This provides the design shown in Figure 2.7. The diameter of the outlet and the manipulation of flow split using the outlet valves prevents the water phase from leaving via the overflow.

Colman considers three geometries of de-oiling cyclone, with one experiencing significantly higher drop break-up than shown by the other. It is regrettable that the published literature does not give details of the geometry. Efficiency is again presented here as the ratio of concentrations, which potentially extols the virtues of a trivial separation performance, as the oil recovery and not the water that leaves mixed with it is only taken into account (see Section 5.4). However, if only very low oil

concentrations are involved, and if the oil leaves essentially free of water (the de-oiling duty is typically less than 1% oil) concentration ratio is a reasonable definition of the efficiency.

Colman (1984) studied the geometry shown below in Figure 2.9. The twin inlets discharge into a relatively large chamber that generates a slow swirl. The transition to the steeper cone section intensifies the spin by conservation of angular momentum and, so the authors claim, dissipates less energy as pressure drop, causing droplet break-up at the same time.

Figure 2.9: Hydrocyclone geometry of Colman (1984)

The tests for this de-oiling cyclone were at less than 1% oil content. As such is likely to be useful as a final polishing stage for cleaning water with oil concentrations ranging from thousands of parts per million to hundreds.

Smyth (1984) modifies the same basic geometry for far higher inlet water cuts, up to 35%, to for an application such as de-watering light crude at the wellhead.

Upstream Axial Outlet (Overflow) Do _0.35D 2D 2D D 20° ~1.5° 0.5D 20D Underflow (clean stream) Do ≤ 0.14D Major diameter D Circular tangential

inlets Swirl _Chamber

section

Fine taper section

Figure 2.10: Hydrocyclone geometry of Smyth (1984)

As can be seen from Figure 2.10, the cyclone differs from the de-oiler design by:

• Comparable outlet areas (de-oiler has much smaller overflow than underflow) • Steeper angle on first cone section (90o _{whole angle as opposed to 20}o₎

• Steeper angle on long cone section (6o _{compared to 1.5}o₎

• Far shorter underflow outlet leg (whole cyclone is 9.3D compared to 20D for just the outlet leg on the de-oiler)

With overflow (oil enriched) pressure drop up to 3 bar and underflow (oil depleted) pressure drop of between 0.3 and 0.8 bar for tested conditions, the cyclone removes water in the oily outlet to below 1% for the inlet conditions up to 25% water cut. The separation results show a critical split below which the overflow composition varies

flow causes the surrounding water layer to be extracted as well. Unfortunately Colman et al. present no pressure drop data in their papers to allow comparison.

Young (1994) set out to ‘optimise’ the de-oiling cyclone presented above. Conducted as research work by Amoco, the details released into the public domain are limited.

However, the Amoco researchers claim to be able to achieve the same performance (in terms of separation of a certain drop size) with about twice the throughput. This involves taking the Thew cyclone, with a single involute inlet, (intended to have the same mitigating effect on pressure loss at the inlet) and making other changes to the geometry as shown in Figure 2.11. Further work mentioned in the same paper claims to have improved this.

Figure 2.11: Hydrocyclone geometry of Young (1994)

The key changes between Thew and Young appear to be the consolidation of the two- stage cone (20o then 1.5o) into a single 6o cone prior to a similarly long straight section at the underflow. The thinking would appear to be the provision of an adequately (but not too long) cylinder to accept the feed without excessive turbulence. This is then spun up sufficiently quickly to avoid energy loss to the walls. The balancing act between wall friction causing the fluids to cease separating and the factors that enhance separation (residence time, spin acquisition) is the key to finding the best cyclone geometry.

Colman and Thew (1983) and Wolbert (1995) both analyse the performance of the Thew-type hydrocyclonic de-oiler by analysis of droplet size effects. Colman takes the concept of grade efficiency curves (curves of separation efficiency as a function of solid particle size) forward as migration probability to the outlet of droplets that enter the

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cyclone. By normalising this function with respect to the droplet size with a 75% probability of separation, he shows that the normalised probability curve is the same for geometrically similar cyclones:

• Of similar geometry

• Separating a light dispersion

This is applicable to the interrelation of experimental results, but Wolbert produces an efficiency model based on consideration of the potential for a droplet to move to an outlet within the residence-time of the cyclone. This gives dx, the smallest droplet size

with x% probability of being separated, by using simplified velocity distributions derived from published LDV measurements. A plot of efficiency vs. droplet size can be derived and used to calculate the overall flow efficiency in terms of the inlet droplet distribution. This is done by considering the trajectory of droplets at the inlet and the varying flow field within the separator. Obviously, both the methods above rely on the ability to measure the inlet distribution, but the analysis of the problem presented by Wolbert again makes it essential that droplet-droplet interaction is negligible. Colman’s relation would also require that the extent of the droplet interaction in related cyclones were similar. This is surely valid for the kind of inlet oil concentrations they study, but becomes progressively less valid for higher oil concentrations.