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water separator using impedance tomography
Minerals Engineering 17 (2004) 605–614 This article is also available online at: www.elsevier.com/locate/mineng
Monitoring the operation of an oil/water separator using impedance tomography M.A. Bennett *, R.A. Williams Institute of Particle Science and Engineering, Houldsworth Building, University of Leeds, Leeds LS2 9JT, UK
Available online
Received 12 November 2003; accepted 11 January 2004
Abstract The development and application of an industrial deoiling hydrocyclone equipped with electrical resistance tomographic instrumentation is described. The use of electrical sensors for continuous monitoring of separator operation is demonstrated. Results show that electrical resistance tomography (ERT) is an effective tool for optimising start-up and operation of the separator. Subtleties of flow within the hydrocyclone can also be sensed, highlighting its potential for use as a validation tool for computational fluid dynamics and design models. 2004 Elsevier Ltd. All rights reserved. Keywords: Hydrocyclones; Process control; On-line analysis; Environmental; Pollution
1. Introduction Deoiling hydrocyclones are now commonly used for cleaning water offshore and onshore, although other types of separators exist for the same purpose such as centrifuges, filter coalescers, gravity separators and induced gas flotation separators (Kilbourne and Hodson, 1996). Although capable of effective oil separation, gravity and induced gas flotation devices in particular are large, heavy and orientation sensitive with long residence times (up to 1800 s that make them unattractive). For example induced gas flotation often also requires addition of chemical surfactants and additional time needs to be allowed for equilibration. One disadvantage of centrifuges and filter coalescers is that they contain many internal and external parts needing maintenance and replacement. A comparison of plate separators, centrifuges and hydrocyclones was given by Van Den Broek and Plat (1998). Liquid/liquid separation using hydrocyclones employs a geometry consisting of a long cylinder, often conical in shape, with two axial outlets (overflow and
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Corresponding author. Tel.: +44-343-2543; fax: +44-113-233-2781. E-mail addresses: [email protected], [email protected] (M.A. Bennett), [email protected] (R.A. Williams). 0892-6875/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2004.01.021
underflow) and one or more tangential inlets (feed), Fig. 1. In deoiling, the oily water enters the hydrocyclone via the feed near the overflow end. As the two phase mixture dispersion spins around the cylinder, centrifugal forces are generated, creating a vortex and sometimes an air core, forcing the lighter phase out of the overflow via the vortex finder, and the heavier phase out of the underflow. The forces generated are higher than conventional oil water separation methods mentioned above. For instance, this can be three orders of magnitude higher than the forces in a gravity separator (e.g. 4000 G) due to the high angular velocities of the liquid mixture (US Filter, 2003). This leads to faster separation of the phases than encountered in gravity separators and consequently a single hydrocyclone unit can have a high productivity compared to alternative equipment that all has a large footprint and capital cost requirement. Some limitations in using hydrocyclones lie in the smallest separable droplet size, since the differential density driving the separation of the two species is less than one tenth of silica/water separation. Further the liquid/liquid phases both are both deformable. Wolbert et al. (1995) proposed that the smallest droplet with 100% efficiency was 100 lm, but recent commercial evidence now suggests efficiency of over 99% down to 20 lm. Additional problems (inefficiencies) can occur in the presence of solids, dissolved and free gas, and chemicals.
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Oil overflow
Clean water underflow
Fig. 1. Schematic diagram of a de-oiling hydrocyclone.
This has led to the historical development of oil/water hydrocyclones being somewhat slower than that of solid/water hydrocyclones (Thew and Smyth, 1997). Early work investigating deoiling hydrocyclones was conducted by the UK Atomic Energy Authority (Hitchon, 1959) and Regehr (Regehr, 1962). Later this was followed by work showing a 90% reduction in oil concentration could be achieved in water (Kimber and Thew, 1974). This result was not considered to be promising enough for commercial application and a report by the Oil Industry Taskforce in 1979 still recommended conventional gravity separators and flotation as the most suitable methods for the deoiling of water. From 1980, research publications indicated more successful uses of oil/water hydrocyclones at Southampton University (Colman et al., 1980). Subsequently there has been further experimental research on separator design and performance (Kumar and Kannan, 1994; Young et al., 1994; Lu et al., 2000; Castilho and Medronho, 2000; Gomez et al., 2003). Allied theoretical analyses have also developed in the last decade using computational fluid dynamics but the simulation of deformable droplets in multiphase dense mixtures remains problematic (Dyakowski and Williams, 1997; Small, 2000) although a recent numerical simulation was reported to provide flow field predictions (Lu et al., 2000; Lu and Zhou, 2003). Oil/water hydrocyclones have progressed from basic research to worldwide practical application, mainly due to the needs of the offshore oil industry for the cleaning up of produced water (Skilbeck and Thew, 1993) where they are used as secondary cleaning stages. This application exists because many existing oil or gas fields have now reached a mature stage where often there is significant water produced with the oil. This coupled with increasing strictness of environmental legislation has led to strong needs for the deoiling of water to less than 20 ppm oil (Belaidi and Thew, 2003). Deoiling hydrocyclones are also being introduced into land based oil fields (Flanagan and Skilbeck, 1989); wet fields (Strodes and Wolfenberger, 1994); down well bores (Petty et al., 2000; and reported at this conference by Petty and Parks, 2003); treatment of oil slicks (Robertson and Oswald, 1984); and for purifying the water in a ship’s overboard water cooling system (Listewnik, 2000).
Examples of commercial applications and some performance data have been given by equipment suppliers for water production on oil platforms and cleaning refinery waters in Canada, Mexican Gulf, North Sea, Middle East and Africa (Natco, 2003; US Filter, 2003; Kvaerner, 2003; Krebs, 2003; Axsia, 2003; Petreco, 2003; Alan Cobham, 2003). Commonly feeds containing oil in concentration 200–2000 ppm can be reduced by a factor of 100 down to 20 ppm. Separations of 100% efficiency are generally achieved for droplets greater than 50 lm but efficiencies drop significantly at and below 5 lm. With standard instrumentation very little can be deduced about the internal flow structure and phase distributions within the hydrocyclone (such as stability of the air core and other factors that affect performance) (Williams et al., 1995). Tomography utilises the measurements on the periphery of a sensing zone to build a cross-sectional image and can give important information on the contents and status of the sensing zone and the behaviour of processes. Electrical methods such as ERT have particularly good suitability for use on-line due to their robustness, sensitivity and speed (York, 2001). ERT utilises conductivity or resistivity differences in the sensing zone and can be a useful tool in observing the oil and air (insulating) in water (conducting) distributions within a deoiling hydrocyclone. This can in turn give further important information about performance such as comparisons of air-core characteristics and operational variables such as feed rate and oil concentration and droplet size. Thus ERT can help to estimate their effect on separation efficiency, and give supportive information for oil/water flow computational fluid dynamics models. ERT has been previously applied at laboratory and plant scale to clay refining solid/solid classifying hydrocyclones (Williams et al., 1999). Measurements have been used to infer solids concentration profiles and the air core under different operational conditions (Nowakowski et al., 2000; West et al., 2000; Williams et al., 1999; Dyakowski and Williams, 1997). Additionally observations were also made of blockage, and roping and spraying discharges from the hydrocyclone underflow. Although some fluid dynamics have been done (Hargreaves and Silvester, 1990) and exploratory electrical measurements (Small, 2000) this paper is believed to be the first instance of ERT being applied to deoiling hydrocyclones. ERT images are constructed from a set of resistance measurements between different electrodes (104 for a 16 electrode sensor). A reference file is first taken with the separator fully filled with the conducting phase, and then the voltage measurements during the process run are then compared with this reference to give voltage values relative to the original reference (normalised).
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In an image reconstructed from the normalised voltage measurements, the presence of the pure conductive phase is typically assigned the normalised value of 0.1, while pure insulating phase is typically assigned the normalised value of 0. The images can then be displayed in colour using scales denoted by a greyscale or colourbar.
2. Apparatus design and construction 2.1. The hydrocyclone The hydrocyclone used in the research was a development 70 mm diameter deoiling hydrocyclone (Mozley, Redruth, UK, Axsia Serck Baker) (Mozley, 1998). A schematic diagram of this hydrocyclone is shown in Fig. 2, showing its extended body of 1.8 m length. Fig. 3 shows the measured efficiency (% of oil removed from the feed via the overflow), reporting a 90% recovery of droplets above 60 lm based on a population balance derived from mass flow and droplet size measurement (forward light scattering). This performance is achieved with flowrates between 3 and 8 m3 h1 with a typical feed concentration of 1 wt.% oil in water (10,000 ppm) (see Fig. 4). This level of efficiency can be achieved for oil concentration as low as 2000 ppm with a reject ratio (% split of feed volumetric flow rate to the overflow) between 1.6% and 13.6%. The reject (overflow) orifice for these results was 3.1 mm, although work can be altered to give different reject ratios for different pressure drop ratios (the ratio of feed to underflow pressure), see Fig. 5. The 70 mm hydrocyclone was designed to generally remove 90% of oil with a typical reject ratio of 2–3%. Using this efficiency value of 90% it can be seen that with 1% oil in the feed a reject ratio of 3% leads to 30% oil in the overflow, and a reject ratio of 2% leads to 45% oil in the overflow. A value of 50% oil in the overflow would be a typically expected value. 2.2. Integration of electrodes The hydrocyclone was fitted with 10 planes of 16 stainless steel radial electrodes for measurement. The
Fig. 3. Measured efficiency versus droplet size.
Fig. 4. Measured efficiency versus flowrate (feed, 700–1200 mg l1 diesel in water; reject ratio, 1.6–13.6; pressure drop ratio, 2.0–2.8; reject outlet diameter, 3.1 mm).
cyclone was fitted into a high pressure tolerant stainless steel casing. The electrodes used for ERT measurements were installed with great care and worked into a very small bore from the inside cavity by drilling and tapping through the cyclone wall. They were self-sealing and
Fig. 2. Schematic design of the 70 mm Mozley deoiling hydrocyclone with integrated electrode plane positions (1–10) located at distances of (y) of (0, 40, 90, 140, 190, 340, 490, 790, 1090, 1390 mm) from the vortex. These correspond to inner sectional diameters of (46, 37, 31, 28, 25, 23, 19.5, 18.5, 17 mm).
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2.3. Flow loop
Fig. 5. Pressure drop ratio versus reject ratio for different reject orifice diameters.
pressure tested to beyond normal operating regimes. The upper four planes (top 1–4) of electrodes are reported in this study corresponding to the top 125 mm of the cyclone body (Fig. 2). This is a key area of interest since it is (plane 1) close to the overflow and provides information on the nature of the central oil core with the separator.
A loop was constructed to produce differing water–oil mixture and flow conditions by circulating oily water through the hydrocyclone. As shown in Fig. 6, oily water initially came from the water tank where oil was mixed in with water to formulate the required dispersion. The oil passed through a positive displacement pump (Monopump) where it could be split in different proportions by manipulating the bypass control valves, thus giving different flowrates around the loop to a separator or to a bypass circuit. The oily water could either go back to the Monopump, or go on to the cyclone head. From the cyclone head the water product went through the cyclone tail before going back to the water tank (for continuous running) or the effluent tank (for batch processing). In experimentation only the continuous running option was utilised. The rig performance and the internal flow structure of the hydrocyclone could then be monitored using specially built flow loop instrumentation and the ERT system, respectively. Representations for instrumentation measuring concentration, pressure and flowrate is shown on the oil– water separator flow loop in Fig. 6. The testing of the pressure and flowmeters (see Section 2.4) gave the hydraulic characteristic of the rig. The performance of the rig is graphed in Fig. 7. It can be seen that the feed and head pressure increase with water flowrate while the underflow and overflow remain virtually at atmospheric pressure, with them being open to air, this is expected.
Fig. 6. The experimental pilot plant oil/water separator flow loop.
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emulsified with oil. The cream layer was sampled and contained droplet diameters that were less than 60 lm. Hence the efficiency of the separator was expected to decline with time from 90% to a level related to the droplet size. The primary objective of this work is to demonstrate the ability of ERT for process operation rather than quantitative efficiency monitoring, but the implication of the emulsion preparation method is that a once-through procedure avoiding recombination of separated products was advisable. 2.4. On-line instrumentation Fig. 7. The hydraulic characteristic of the hydrocyclone rig.
It can be seen that the pressure drop ratio (ratio of feed pressure to underflow pressure) is virtually equal to the feed pressure, as no back pressure was applied at the underflow. To obtain a suitable homogenised sample of oily water, the rig was operated with a mixture of 100 l water, 1 kg oil (Shell Catenex 11) and 56 g of surfactant (Tween 20) corresponding to 1% oil in water (or 10,000 ppm). The mixture was well mixed, initially manually, using a simple mixing implement above the water tank (see Fig. 6) and then by recirculation. In some cases it was possible to maintain a droplet size population, but with progressive recirculation, analysis of the tank showed that the mixture consisted of three basic layers, a very small surface layer of immiscible oil, a layer of emulsified, flocculated cream and a body of water
A PC based monitoring system was developed to measure pressure, flowrate and concentration using LABVIEW software and a LABPC+ acquisition board. Four pressure meters (Keller Series 21R) with a range of 0–1000 kPa for measurement at feed, cyclone head, underflow and overflow points, were used to define operational characteristics. The maximum pressure in the flow loop was (400 kPa). An electromagnetic flowmeter (Krohne) measured the feedline for water flowrate (0–10 m3 h1 ) up to a maximum flowrate of approximately 7 m3 h1 . The emulsion concentration was estimated using a sensor fabricated in our laboratory based on an LED/ photodiode photometer system. This operated on the simple principles of transmission, Fig. 8. A 3.3 cm clear PVC pipe, a 2 W bulb operating at 20 V and a photodetector placed about 4.0 cm away from the side of the pipe at the end of a plastic tube were used. The bulb and
Fig. 8. Prototype photometer system for estimation of oil concentration: (a) sensor design and (b) photograph of sensor (located behind cylindrical screening) mounted in feedline.
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photodetector were then kept aligned directly opposite each other across the pipe. Following testing it was decided to keep the distance, light source and photodetector sensitivity at set values. Two such turbidity measurement systems were designed, constructed, tested and fitted on to clear PVC pipe in the flow loop before and after the separator. Operational amplifiers were also constructed and added to these photometers for output amplification into the PC acquisition system. As anticipated, calibration of these systems to monitor concentrations in the range 10– 200 ppm oil for droplets with a mean diameter of 60 lm became difficult if the fluid contained significant amounts of entrapped air. Hence operation of the meters at higher flowrates (>2 m3 h1 ) became unreliable due to a signal noise level of 10%.
3. Experimental procedure
Fig. 10. Conductivity images showing results (a) without and (b) with an air core present.
3.1. Validation of ERT operation Tomographic measurements were taken while the flow loop was running water only at 5 m3 h1 with the cyclone settings adjusted so that it was full of water and also in a condition where it was allowed to drain out and empty. It can be seen in Fig. 9a that each of the images at sensor planes (1–4) shows a uniformity of measurement consistent with the cyclone being filled with water (conductive). Values of the 104 raw voltage measurements were consistent with a vessel filled with a uniformly conductive liquid, also verifying that the electrodes were working correctly. On emptying (Fig. 9b) there is a radical change in apparent conductivity since the fluid is more non-conducting. In assessing the ability of the ERT system to detect an air core within the hydrocyclone when the flow loop was running, procedures were developed to initiate and change the air core dimensions. The air core forms due to high centrifugal forces and an open overflow. The air core could be neutralised by applying back pressure to the hydrocyclone underflow by constricting the valve to the water or effluent tank. This forces water through the
centre of the hydrocyclone, completely filling it, and out of the overflow. Fig. 10 shows the conductivity images at 7 m3 h1 during such a transition and it can be seen that an air core is imaged in all planes (1–4) when no back pressure is applied (b). The imaged air core actually appears to decrease in diameter closer to the overflow (plane 1) due to the larger diameter of the cyclone at that position. The effect of increasing the flowrate with no applied back pressure can be seen in Fig. 11 in which the air core is seen to evolve. This graph shows the ratio of the insulating air core diameter relative to the diameter of the cyclone at that measurement plane I (I ¼ 1, 2, 3 or 4) hereafter defined as ai . The core was measured by applying a relative conductivity threshold of 1.0. 3.2. Monitoring of emulsion separation Measurements were performed using the standard emulsion (Section 2.3) taking underflow samples, altering flow rates, and taking overflow samples and flow
Fig. 9. Measured conductivity images for (a) cyclone filled and (b) cyclone empty.
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Fig. 11. Conductivity images at water flowrates of (a) 2 m3 h1 and (b) 4 m3 h1 and measurements corresponding to relative air core diameter (ai ) in the separator head at 2, 3 and 4 m3 h1 .
rates. Operation was conducted at high flow rates (7 m3 h1 ) giving high head and feed pressures (375 kPa) in terms of practical operation the behaviour of the separator was explored in detail by taking it to a point just before overflow occurred (while taking measurements), then slightly increasing this flowrate to yield an oil phase overflow. It was considered that this provided an example of the subtleties of hydrocyclone flow that ERT was required to recognise. Fig. 12 shows a typical operational run indicating the instrumentation inputs, and a sequence of axial slices of ERT images stacked over time, the conductivity units being normalised by comparison with the fully mixed 1% oil in water mixture. All data taken were at 1 s intervals. The images correspond to the plane (1), nearest to the overflow. The input parameters on the graph are the flow rate and the pressures of the underflow, feed, head and
Fig. 12. Instrumental responses during separator start-up and operation over approximately 2 min (from A to D) with corresponding axial slice conductivity images in separator head during the transition from no overflow (B) to production of oil (C). 1% oil in feed with throughput of 7 m3 h1 .
overflow of the cyclone. The overflow pressure was an important electronic record of an overflow occurring, but because the pipe to which it was connected led to a collection tank that was open to air, it tended to be very small, and so it is shown multiplied by a factor of 25. The four points on Fig. 12 show: (A) On start-up the flow rate, feed and head pressures rise rapidly. (B) Just at the time the bypass valve is constricted, the flow rate, feed and head pressures rise. Shortly after this the core region in the conductivity slice begins to emerge, tracking the marked increase in oil which reduces the conductivity. There is also a very small rise in the overflow pressure. (C) Later at time 77 s, and with a differential pressure just below 3 · 105 kPa a visible overflow stream begins to emerge. This flow is very small (approximately 0.02 m3 h1 , or a reject ratio of 0.3%). Between (C) and (D) at times beyond 95 s the oil core is seen to be slightly larger in diameter and well established and in steady state. Examination of the oil product indicates that it has the consistency of an opaque oil-rich fluid containing oil, flocculated cream and some entrained bubbles. The gas phase in some instances, accounted up to 20% of the volume. (D) On stopping the flow the drainage properties of the separator can be observed (these are not considered further here). This behaviour was repeatable. For example, Fig. 13 shows conductivity slices for the same electrode plane in five independent runs. The vertical line delineates the point at which oil is produced as a product at the overflow (given the value 0 s for clarity). For comparison, conductivity data for water only (at the same feed rate) are given. This acts as a control, demonstrating that the presence of oil is being detected.
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occurring, compared to those representing no overflow occurring, demonstrating that ERT can be used as a tool to monitor hydrocyclone performance. Also a rise can be seen in the average overflow pressure value after overflow was observed thus suggesting that this is an obvious sensing method to indicate its occurrence. For the conditions described, oil production from the separator is seen to start when the oil core exceeds a critical size. In this work this corresponded to an ai ratio of 0.45, Fig. 14. Given the low concentration of the oil in the feed it is evident that there is an induction time associated with the need to allow an oil core to form (although this was measured explicitly).
Fig. 13. Time transient axial slices of electrical conductivity in replicate start-ups of the separator for 1% oil (10,000 ppm), 7 m3 h1 feed, 370 kPa feed pressure, 268 kPa head pressure. Vertical line indicating when oil production starts for the sensor plane 1 before and after overflow occurred (a–e). Control experiment for a water only feed is given in (f).
In all the runs it can be seen that there are larger air/ oil cores present for those images representing overflow
Fig. 14. Measured apparent relative oil core diameter (ai ) at plane 1 during overflow start (0 s). Vertical line indicating when oil production starts.
Fig. 15. (a) The sensing volume (black) within the full hydrocyclone (grey) and (b–e) averaged ERT axial slices with corresponding operational data.
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Fig. 15 shows averaged results for four runs using image data from four electrode planes scaled using the dimensions of the sensing volume enveloped by the separator head shown in orange in the simple schematic above. Hence the actual true shape of the sensing volume within the full length hydrocyclone is represented.
4. Conclusion and future work We have demonstrated that changes in the air/oil core can be observed with ERT within industrial scale separators operating at high pressure. This provides effective tool to detect subtleties in internal deoiling hydrocyclone flow structure and performance. In particular optimisation of the separator can be achieved by creating a stable oil core so that an overflow stream can be generated. Other conventional instrumentation is less effective for this purpose. However, the formation of a stable oil core yielding a pure oil product and clean water is governed by careful adjustment of the feed flowrate e.g. effectively by slight adjustment of the bypass loop valves. The work suggests that tomographically derived information could be valuable for tuning the operation of separators, especially in cases where fluctuation of oil feed concentration or type occur. There are difficulties in performing work on this scale in a laboratory. For example, in the disposal of materials and formulations of large feed stocks of stable emulsions of controlled droplet size. Hence the actual industrial environment is a preferred site for future tests. Industrial testing together with the development of more sophisticated on-line droplet size and concentration measurement devices (Selomulya and Williams, 2003) should assist in future measurement of actual separator performance. The work also suggests how such information could be crucial to aid computational fluid dynamics models of these processes in an analogous manner to that demonstrated for solid–liquid hydrocyclones (Cullivan et al., 2003). Acknowledgements The authors are grateful to the EPSRC for support under grant GR/L95472, the collaboration of Mozley (Axsia) and the technical assistance of Mr. Justin Bond at Camborne School of Mines, University of Exeter.
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Monitoring the operation of an oil/water separator using impedance tomography M.A. Bennett *, R.A. Williams Institute of Particle Science and Engineering, Houldsworth Building, University of Leeds, Leeds LS2 9JT, UK
Available online
Received 12 November 2003; accepted 11 January 2004
Abstract The development and application of an industrial deoiling hydrocyclone equipped with electrical resistance tomographic instrumentation is described. The use of electrical sensors for continuous monitoring of separator operation is demonstrated. Results show that electrical resistance tomography (ERT) is an effective tool for optimising start-up and operation of the separator. Subtleties of flow within the hydrocyclone can also be sensed, highlighting its potential for use as a validation tool for computational fluid dynamics and design models. 2004 Elsevier Ltd. All rights reserved. Keywords: Hydrocyclones; Process control; On-line analysis; Environmental; Pollution
1. Introduction Deoiling hydrocyclones are now commonly used for cleaning water offshore and onshore, although other types of separators exist for the same purpose such as centrifuges, filter coalescers, gravity separators and induced gas flotation separators (Kilbourne and Hodson, 1996). Although capable of effective oil separation, gravity and induced gas flotation devices in particular are large, heavy and orientation sensitive with long residence times (up to 1800 s that make them unattractive). For example induced gas flotation often also requires addition of chemical surfactants and additional time needs to be allowed for equilibration. One disadvantage of centrifuges and filter coalescers is that they contain many internal and external parts needing maintenance and replacement. A comparison of plate separators, centrifuges and hydrocyclones was given by Van Den Broek and Plat (1998). Liquid/liquid separation using hydrocyclones employs a geometry consisting of a long cylinder, often conical in shape, with two axial outlets (overflow and
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Corresponding author. Tel.: +44-343-2543; fax: +44-113-233-2781. E-mail addresses: [email protected], [email protected] (M.A. Bennett), [email protected] (R.A. Williams). 0892-6875/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2004.01.021
underflow) and one or more tangential inlets (feed), Fig. 1. In deoiling, the oily water enters the hydrocyclone via the feed near the overflow end. As the two phase mixture dispersion spins around the cylinder, centrifugal forces are generated, creating a vortex and sometimes an air core, forcing the lighter phase out of the overflow via the vortex finder, and the heavier phase out of the underflow. The forces generated are higher than conventional oil water separation methods mentioned above. For instance, this can be three orders of magnitude higher than the forces in a gravity separator (e.g. 4000 G) due to the high angular velocities of the liquid mixture (US Filter, 2003). This leads to faster separation of the phases than encountered in gravity separators and consequently a single hydrocyclone unit can have a high productivity compared to alternative equipment that all has a large footprint and capital cost requirement. Some limitations in using hydrocyclones lie in the smallest separable droplet size, since the differential density driving the separation of the two species is less than one tenth of silica/water separation. Further the liquid/liquid phases both are both deformable. Wolbert et al. (1995) proposed that the smallest droplet with 100% efficiency was 100 lm, but recent commercial evidence now suggests efficiency of over 99% down to 20 lm. Additional problems (inefficiencies) can occur in the presence of solids, dissolved and free gas, and chemicals.
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Oil overflow
Clean water underflow
Fig. 1. Schematic diagram of a de-oiling hydrocyclone.
This has led to the historical development of oil/water hydrocyclones being somewhat slower than that of solid/water hydrocyclones (Thew and Smyth, 1997). Early work investigating deoiling hydrocyclones was conducted by the UK Atomic Energy Authority (Hitchon, 1959) and Regehr (Regehr, 1962). Later this was followed by work showing a 90% reduction in oil concentration could be achieved in water (Kimber and Thew, 1974). This result was not considered to be promising enough for commercial application and a report by the Oil Industry Taskforce in 1979 still recommended conventional gravity separators and flotation as the most suitable methods for the deoiling of water. From 1980, research publications indicated more successful uses of oil/water hydrocyclones at Southampton University (Colman et al., 1980). Subsequently there has been further experimental research on separator design and performance (Kumar and Kannan, 1994; Young et al., 1994; Lu et al., 2000; Castilho and Medronho, 2000; Gomez et al., 2003). Allied theoretical analyses have also developed in the last decade using computational fluid dynamics but the simulation of deformable droplets in multiphase dense mixtures remains problematic (Dyakowski and Williams, 1997; Small, 2000) although a recent numerical simulation was reported to provide flow field predictions (Lu et al., 2000; Lu and Zhou, 2003). Oil/water hydrocyclones have progressed from basic research to worldwide practical application, mainly due to the needs of the offshore oil industry for the cleaning up of produced water (Skilbeck and Thew, 1993) where they are used as secondary cleaning stages. This application exists because many existing oil or gas fields have now reached a mature stage where often there is significant water produced with the oil. This coupled with increasing strictness of environmental legislation has led to strong needs for the deoiling of water to less than 20 ppm oil (Belaidi and Thew, 2003). Deoiling hydrocyclones are also being introduced into land based oil fields (Flanagan and Skilbeck, 1989); wet fields (Strodes and Wolfenberger, 1994); down well bores (Petty et al., 2000; and reported at this conference by Petty and Parks, 2003); treatment of oil slicks (Robertson and Oswald, 1984); and for purifying the water in a ship’s overboard water cooling system (Listewnik, 2000).
Examples of commercial applications and some performance data have been given by equipment suppliers for water production on oil platforms and cleaning refinery waters in Canada, Mexican Gulf, North Sea, Middle East and Africa (Natco, 2003; US Filter, 2003; Kvaerner, 2003; Krebs, 2003; Axsia, 2003; Petreco, 2003; Alan Cobham, 2003). Commonly feeds containing oil in concentration 200–2000 ppm can be reduced by a factor of 100 down to 20 ppm. Separations of 100% efficiency are generally achieved for droplets greater than 50 lm but efficiencies drop significantly at and below 5 lm. With standard instrumentation very little can be deduced about the internal flow structure and phase distributions within the hydrocyclone (such as stability of the air core and other factors that affect performance) (Williams et al., 1995). Tomography utilises the measurements on the periphery of a sensing zone to build a cross-sectional image and can give important information on the contents and status of the sensing zone and the behaviour of processes. Electrical methods such as ERT have particularly good suitability for use on-line due to their robustness, sensitivity and speed (York, 2001). ERT utilises conductivity or resistivity differences in the sensing zone and can be a useful tool in observing the oil and air (insulating) in water (conducting) distributions within a deoiling hydrocyclone. This can in turn give further important information about performance such as comparisons of air-core characteristics and operational variables such as feed rate and oil concentration and droplet size. Thus ERT can help to estimate their effect on separation efficiency, and give supportive information for oil/water flow computational fluid dynamics models. ERT has been previously applied at laboratory and plant scale to clay refining solid/solid classifying hydrocyclones (Williams et al., 1999). Measurements have been used to infer solids concentration profiles and the air core under different operational conditions (Nowakowski et al., 2000; West et al., 2000; Williams et al., 1999; Dyakowski and Williams, 1997). Additionally observations were also made of blockage, and roping and spraying discharges from the hydrocyclone underflow. Although some fluid dynamics have been done (Hargreaves and Silvester, 1990) and exploratory electrical measurements (Small, 2000) this paper is believed to be the first instance of ERT being applied to deoiling hydrocyclones. ERT images are constructed from a set of resistance measurements between different electrodes (104 for a 16 electrode sensor). A reference file is first taken with the separator fully filled with the conducting phase, and then the voltage measurements during the process run are then compared with this reference to give voltage values relative to the original reference (normalised).
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In an image reconstructed from the normalised voltage measurements, the presence of the pure conductive phase is typically assigned the normalised value of 0.1, while pure insulating phase is typically assigned the normalised value of 0. The images can then be displayed in colour using scales denoted by a greyscale or colourbar.
2. Apparatus design and construction 2.1. The hydrocyclone The hydrocyclone used in the research was a development 70 mm diameter deoiling hydrocyclone (Mozley, Redruth, UK, Axsia Serck Baker) (Mozley, 1998). A schematic diagram of this hydrocyclone is shown in Fig. 2, showing its extended body of 1.8 m length. Fig. 3 shows the measured efficiency (% of oil removed from the feed via the overflow), reporting a 90% recovery of droplets above 60 lm based on a population balance derived from mass flow and droplet size measurement (forward light scattering). This performance is achieved with flowrates between 3 and 8 m3 h1 with a typical feed concentration of 1 wt.% oil in water (10,000 ppm) (see Fig. 4). This level of efficiency can be achieved for oil concentration as low as 2000 ppm with a reject ratio (% split of feed volumetric flow rate to the overflow) between 1.6% and 13.6%. The reject (overflow) orifice for these results was 3.1 mm, although work can be altered to give different reject ratios for different pressure drop ratios (the ratio of feed to underflow pressure), see Fig. 5. The 70 mm hydrocyclone was designed to generally remove 90% of oil with a typical reject ratio of 2–3%. Using this efficiency value of 90% it can be seen that with 1% oil in the feed a reject ratio of 3% leads to 30% oil in the overflow, and a reject ratio of 2% leads to 45% oil in the overflow. A value of 50% oil in the overflow would be a typically expected value. 2.2. Integration of electrodes The hydrocyclone was fitted with 10 planes of 16 stainless steel radial electrodes for measurement. The
Fig. 3. Measured efficiency versus droplet size.
Fig. 4. Measured efficiency versus flowrate (feed, 700–1200 mg l1 diesel in water; reject ratio, 1.6–13.6; pressure drop ratio, 2.0–2.8; reject outlet diameter, 3.1 mm).
cyclone was fitted into a high pressure tolerant stainless steel casing. The electrodes used for ERT measurements were installed with great care and worked into a very small bore from the inside cavity by drilling and tapping through the cyclone wall. They were self-sealing and
Fig. 2. Schematic design of the 70 mm Mozley deoiling hydrocyclone with integrated electrode plane positions (1–10) located at distances of (y) of (0, 40, 90, 140, 190, 340, 490, 790, 1090, 1390 mm) from the vortex. These correspond to inner sectional diameters of (46, 37, 31, 28, 25, 23, 19.5, 18.5, 17 mm).
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2.3. Flow loop
Fig. 5. Pressure drop ratio versus reject ratio for different reject orifice diameters.
pressure tested to beyond normal operating regimes. The upper four planes (top 1–4) of electrodes are reported in this study corresponding to the top 125 mm of the cyclone body (Fig. 2). This is a key area of interest since it is (plane 1) close to the overflow and provides information on the nature of the central oil core with the separator.
A loop was constructed to produce differing water–oil mixture and flow conditions by circulating oily water through the hydrocyclone. As shown in Fig. 6, oily water initially came from the water tank where oil was mixed in with water to formulate the required dispersion. The oil passed through a positive displacement pump (Monopump) where it could be split in different proportions by manipulating the bypass control valves, thus giving different flowrates around the loop to a separator or to a bypass circuit. The oily water could either go back to the Monopump, or go on to the cyclone head. From the cyclone head the water product went through the cyclone tail before going back to the water tank (for continuous running) or the effluent tank (for batch processing). In experimentation only the continuous running option was utilised. The rig performance and the internal flow structure of the hydrocyclone could then be monitored using specially built flow loop instrumentation and the ERT system, respectively. Representations for instrumentation measuring concentration, pressure and flowrate is shown on the oil– water separator flow loop in Fig. 6. The testing of the pressure and flowmeters (see Section 2.4) gave the hydraulic characteristic of the rig. The performance of the rig is graphed in Fig. 7. It can be seen that the feed and head pressure increase with water flowrate while the underflow and overflow remain virtually at atmospheric pressure, with them being open to air, this is expected.
Fig. 6. The experimental pilot plant oil/water separator flow loop.
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emulsified with oil. The cream layer was sampled and contained droplet diameters that were less than 60 lm. Hence the efficiency of the separator was expected to decline with time from 90% to a level related to the droplet size. The primary objective of this work is to demonstrate the ability of ERT for process operation rather than quantitative efficiency monitoring, but the implication of the emulsion preparation method is that a once-through procedure avoiding recombination of separated products was advisable. 2.4. On-line instrumentation Fig. 7. The hydraulic characteristic of the hydrocyclone rig.
It can be seen that the pressure drop ratio (ratio of feed pressure to underflow pressure) is virtually equal to the feed pressure, as no back pressure was applied at the underflow. To obtain a suitable homogenised sample of oily water, the rig was operated with a mixture of 100 l water, 1 kg oil (Shell Catenex 11) and 56 g of surfactant (Tween 20) corresponding to 1% oil in water (or 10,000 ppm). The mixture was well mixed, initially manually, using a simple mixing implement above the water tank (see Fig. 6) and then by recirculation. In some cases it was possible to maintain a droplet size population, but with progressive recirculation, analysis of the tank showed that the mixture consisted of three basic layers, a very small surface layer of immiscible oil, a layer of emulsified, flocculated cream and a body of water
A PC based monitoring system was developed to measure pressure, flowrate and concentration using LABVIEW software and a LABPC+ acquisition board. Four pressure meters (Keller Series 21R) with a range of 0–1000 kPa for measurement at feed, cyclone head, underflow and overflow points, were used to define operational characteristics. The maximum pressure in the flow loop was (400 kPa). An electromagnetic flowmeter (Krohne) measured the feedline for water flowrate (0–10 m3 h1 ) up to a maximum flowrate of approximately 7 m3 h1 . The emulsion concentration was estimated using a sensor fabricated in our laboratory based on an LED/ photodiode photometer system. This operated on the simple principles of transmission, Fig. 8. A 3.3 cm clear PVC pipe, a 2 W bulb operating at 20 V and a photodetector placed about 4.0 cm away from the side of the pipe at the end of a plastic tube were used. The bulb and
Fig. 8. Prototype photometer system for estimation of oil concentration: (a) sensor design and (b) photograph of sensor (located behind cylindrical screening) mounted in feedline.
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photodetector were then kept aligned directly opposite each other across the pipe. Following testing it was decided to keep the distance, light source and photodetector sensitivity at set values. Two such turbidity measurement systems were designed, constructed, tested and fitted on to clear PVC pipe in the flow loop before and after the separator. Operational amplifiers were also constructed and added to these photometers for output amplification into the PC acquisition system. As anticipated, calibration of these systems to monitor concentrations in the range 10– 200 ppm oil for droplets with a mean diameter of 60 lm became difficult if the fluid contained significant amounts of entrapped air. Hence operation of the meters at higher flowrates (>2 m3 h1 ) became unreliable due to a signal noise level of 10%.
3. Experimental procedure
Fig. 10. Conductivity images showing results (a) without and (b) with an air core present.
3.1. Validation of ERT operation Tomographic measurements were taken while the flow loop was running water only at 5 m3 h1 with the cyclone settings adjusted so that it was full of water and also in a condition where it was allowed to drain out and empty. It can be seen in Fig. 9a that each of the images at sensor planes (1–4) shows a uniformity of measurement consistent with the cyclone being filled with water (conductive). Values of the 104 raw voltage measurements were consistent with a vessel filled with a uniformly conductive liquid, also verifying that the electrodes were working correctly. On emptying (Fig. 9b) there is a radical change in apparent conductivity since the fluid is more non-conducting. In assessing the ability of the ERT system to detect an air core within the hydrocyclone when the flow loop was running, procedures were developed to initiate and change the air core dimensions. The air core forms due to high centrifugal forces and an open overflow. The air core could be neutralised by applying back pressure to the hydrocyclone underflow by constricting the valve to the water or effluent tank. This forces water through the
centre of the hydrocyclone, completely filling it, and out of the overflow. Fig. 10 shows the conductivity images at 7 m3 h1 during such a transition and it can be seen that an air core is imaged in all planes (1–4) when no back pressure is applied (b). The imaged air core actually appears to decrease in diameter closer to the overflow (plane 1) due to the larger diameter of the cyclone at that position. The effect of increasing the flowrate with no applied back pressure can be seen in Fig. 11 in which the air core is seen to evolve. This graph shows the ratio of the insulating air core diameter relative to the diameter of the cyclone at that measurement plane I (I ¼ 1, 2, 3 or 4) hereafter defined as ai . The core was measured by applying a relative conductivity threshold of 1.0. 3.2. Monitoring of emulsion separation Measurements were performed using the standard emulsion (Section 2.3) taking underflow samples, altering flow rates, and taking overflow samples and flow
Fig. 9. Measured conductivity images for (a) cyclone filled and (b) cyclone empty.
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Fig. 11. Conductivity images at water flowrates of (a) 2 m3 h1 and (b) 4 m3 h1 and measurements corresponding to relative air core diameter (ai ) in the separator head at 2, 3 and 4 m3 h1 .
rates. Operation was conducted at high flow rates (7 m3 h1 ) giving high head and feed pressures (375 kPa) in terms of practical operation the behaviour of the separator was explored in detail by taking it to a point just before overflow occurred (while taking measurements), then slightly increasing this flowrate to yield an oil phase overflow. It was considered that this provided an example of the subtleties of hydrocyclone flow that ERT was required to recognise. Fig. 12 shows a typical operational run indicating the instrumentation inputs, and a sequence of axial slices of ERT images stacked over time, the conductivity units being normalised by comparison with the fully mixed 1% oil in water mixture. All data taken were at 1 s intervals. The images correspond to the plane (1), nearest to the overflow. The input parameters on the graph are the flow rate and the pressures of the underflow, feed, head and
Fig. 12. Instrumental responses during separator start-up and operation over approximately 2 min (from A to D) with corresponding axial slice conductivity images in separator head during the transition from no overflow (B) to production of oil (C). 1% oil in feed with throughput of 7 m3 h1 .
overflow of the cyclone. The overflow pressure was an important electronic record of an overflow occurring, but because the pipe to which it was connected led to a collection tank that was open to air, it tended to be very small, and so it is shown multiplied by a factor of 25. The four points on Fig. 12 show: (A) On start-up the flow rate, feed and head pressures rise rapidly. (B) Just at the time the bypass valve is constricted, the flow rate, feed and head pressures rise. Shortly after this the core region in the conductivity slice begins to emerge, tracking the marked increase in oil which reduces the conductivity. There is also a very small rise in the overflow pressure. (C) Later at time 77 s, and with a differential pressure just below 3 · 105 kPa a visible overflow stream begins to emerge. This flow is very small (approximately 0.02 m3 h1 , or a reject ratio of 0.3%). Between (C) and (D) at times beyond 95 s the oil core is seen to be slightly larger in diameter and well established and in steady state. Examination of the oil product indicates that it has the consistency of an opaque oil-rich fluid containing oil, flocculated cream and some entrained bubbles. The gas phase in some instances, accounted up to 20% of the volume. (D) On stopping the flow the drainage properties of the separator can be observed (these are not considered further here). This behaviour was repeatable. For example, Fig. 13 shows conductivity slices for the same electrode plane in five independent runs. The vertical line delineates the point at which oil is produced as a product at the overflow (given the value 0 s for clarity). For comparison, conductivity data for water only (at the same feed rate) are given. This acts as a control, demonstrating that the presence of oil is being detected.
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occurring, compared to those representing no overflow occurring, demonstrating that ERT can be used as a tool to monitor hydrocyclone performance. Also a rise can be seen in the average overflow pressure value after overflow was observed thus suggesting that this is an obvious sensing method to indicate its occurrence. For the conditions described, oil production from the separator is seen to start when the oil core exceeds a critical size. In this work this corresponded to an ai ratio of 0.45, Fig. 14. Given the low concentration of the oil in the feed it is evident that there is an induction time associated with the need to allow an oil core to form (although this was measured explicitly).
Fig. 13. Time transient axial slices of electrical conductivity in replicate start-ups of the separator for 1% oil (10,000 ppm), 7 m3 h1 feed, 370 kPa feed pressure, 268 kPa head pressure. Vertical line indicating when oil production starts for the sensor plane 1 before and after overflow occurred (a–e). Control experiment for a water only feed is given in (f).
In all the runs it can be seen that there are larger air/ oil cores present for those images representing overflow
Fig. 14. Measured apparent relative oil core diameter (ai ) at plane 1 during overflow start (0 s). Vertical line indicating when oil production starts.
Fig. 15. (a) The sensing volume (black) within the full hydrocyclone (grey) and (b–e) averaged ERT axial slices with corresponding operational data.
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Fig. 15 shows averaged results for four runs using image data from four electrode planes scaled using the dimensions of the sensing volume enveloped by the separator head shown in orange in the simple schematic above. Hence the actual true shape of the sensing volume within the full length hydrocyclone is represented.
4. Conclusion and future work We have demonstrated that changes in the air/oil core can be observed with ERT within industrial scale separators operating at high pressure. This provides effective tool to detect subtleties in internal deoiling hydrocyclone flow structure and performance. In particular optimisation of the separator can be achieved by creating a stable oil core so that an overflow stream can be generated. Other conventional instrumentation is less effective for this purpose. However, the formation of a stable oil core yielding a pure oil product and clean water is governed by careful adjustment of the feed flowrate e.g. effectively by slight adjustment of the bypass loop valves. The work suggests that tomographically derived information could be valuable for tuning the operation of separators, especially in cases where fluctuation of oil feed concentration or type occur. There are difficulties in performing work on this scale in a laboratory. For example, in the disposal of materials and formulations of large feed stocks of stable emulsions of controlled droplet size. Hence the actual industrial environment is a preferred site for future tests. Industrial testing together with the development of more sophisticated on-line droplet size and concentration measurement devices (Selomulya and Williams, 2003) should assist in future measurement of actual separator performance. The work also suggests how such information could be crucial to aid computational fluid dynamics models of these processes in an analogous manner to that demonstrated for solid–liquid hydrocyclones (Cullivan et al., 2003). Acknowledgements The authors are grateful to the EPSRC for support under grant GR/L95472, the collaboration of Mozley (Axsia) and the technical assistance of Mr. Justin Bond at Camborne School of Mines, University of Exeter.
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