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Fig. 4 Changes in ernulsion viscosity (w/o) for a relected oil us a function of time on the sea surface (Strom-Kristiansen et al., 1994a).
Flg. Changes in water content as function of time for a selected oil under a piven set of environmental conditions (Strom-Kristiansen et ul. 1994)
OIL AND WATER SEPARATION
Fig.6 Reduction of water droplet sizes in a w/o emulsion after 1 h (a) and 24 h (b) of mixing time (Strom-Kristiansen et al.,
Table 2 Approximate values of changes in density kg dm' and viscosity cSt of a selected fuel oil and seawater as function
density difference viscosity (St) viscosity (CSI) difference (CS)
Changes in specific gravity and viscosity as a function of temperature
Most fresh crude oils and refined products have specific gravities between 0.80 and 0.98 8 cm - and viscosities from approximately 3 to 25000 cSt at 15°C. Elevation of temperature will lower the density of both water and oil. The net effect on the buoyancy force is very limited for separation of oil droplets from water. For separation of water droplets from an emulsion, however, the effect of elevated temperature on oil and emulsion viscosity is important. Table 2 presents approximate density and viscosity data for seawater and a selected fuel oil at three different temperatures and shows how density and viscosity differences vary. Figure 7 presehts changes in oil viscosity as function of temperature for selected crude oils and refined products.
Spil Science & Technology Bulletin 3(3)
Fig. : Effect of temperature to reduce water content (dehydration) of w/o emulsions formed by Bonny Light and BCF 17 crude oils at 10°C and So°C (Strean-Kristiansen et al., 1994b).
Enhancement of emulsion separation
Application of emulsion breakers in front of a skimmer pump and on-line with a centrifugal separator, or in temporary storage tanks for gravity settling, and heating of oil or emulsions, are methods that have the potential to improve storage and recovery effectiveness (Lewis et al., 19956, StrømKristiansen et al., 19946, and Lode, 1981). Heating primarily reduces the viscosity of the oil, decreasing drag forces and promoting coalescence and gravity settling. Emulsion breakers destabilize and ultimately reduce interfacial tension between the water droplets and the oil, providing conditions for coalescence and increase of water droplet size. If the viscosity of the oil (and drag force) suil is too high for an effective separation, a combination of heat and emulsion breaker is required. However, use of emulsion break. ers alone, to improve and accelerate the separation process may, in some cases, be a rapid and cost effective method to separate crude oil emulsions. Decrease of water content in an emulsion, by heating or use of emulsion breakers and subsequent reduction in emulsion viscosity, may improve pumpability and discharge capacity, reduce transfer and discharge time, and reduce oily waste handling, and disposal costs, facilitate disposal and enhance the potential for sale and reuse of recovered oil. Other methods to break an emulsion include, increasing the water content of an unstable water in oil emulsion to form an oil in water dispersion and freezing of the water to facilitate separation
Effect of elevde temperature on settling processes
Heating to approximately 50°C is, in general, sufficient to break crude oil emulsions by gravity settling, when recovered within hours to a few days after a spill. This is, however, dependant on the degree of oil weathering and specific physical and cherical properties of the oil. Figure 8 presents laboratory results from reoent studies on the effect of heating and reduction of water content (dehydration) of selected crude oil emulsions (Strøm-Kristiansen et al., 1994b).
Effectiveness of separation by use of beator chemicals under small-scale laboratory conditions may give test results that are different from real system performance. Full scale testing of system effectiveness is needed for verification of laboratory results and system optimization. When a stable emulsion is heated using heaters at the bottom of a tank, limited heat convection takes place and water will start to settle to the bottom of the lank, creating a barrier to the heating of oil and emulsion above. Therefore, beating up water from the bottom of the tank to separate the upper layer of emulsion has limited effectiveness in stimulating heat-driven emulsion breaking. This common method of oil-water separation is neither time nor cost effective. To improve heat transfer, recirculation of the entire water and emulsion mixture has proven to be useful, reducing the time needed to elevate the temperature to the point of separation. Gravity settling is often used for first stage of separation, followed by centrifuging or filtration. This (wo stage process provide more
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systems. Unfortunately, heat and power requirements may, in addition, be greatly in excess of what is available on most vessels of opportunity used in spill response.
Effect of emulsion breakers to enhance separation
Emulsion-treating agents have several applications in marine oil spill response operations, including among others prevention of emulsification, reduction of volume of emulsified oil by separation, and enhancement of mechanical, dispersant and in situ burning response methods (SEA, 1995). Emulsion
Fig. 9 (s) Temperature development in a storage tank using heating without active recirculation (Nordvik et al., 1994). (b) Temperature development in the same tank using heal, emulsion breaker and active recirculation (Nordvik et al., 1994).
favorable operating conditions for the second stage of separation where only oil contaminated settled water is separated. Figure 9a presents the results of an experiment investigating temperature development in an oil storage tank on an oil spill recovery vessel, illustrating the creation of a temperature barrier at 6 st above the bottom of the tank when using heat only. Figure 96 presents optimized results of recirculation and elevation of temperatures in the same tank with the same emulsion using heat and emulsion breaker (Nordvik et al., 1994).
Heating of large volumes of stored fluid is time consuming and may reduce effective skimming time, recovery capacity and effectiveness of recovery
Spell Science & Technology Bulletin (3)
A. B. NORDVIK eld
data and separation time for selected emulsion breaker products on a stable emulsion (Lode, 1981 and Knapstad, 1981).
Separation of emulsified bunker oils requires more time than is needed for crude oil emulsions and the combination of heat and emulsion breaker may be the only useful method of separation. Figure 11 presents the effect of separation of w/o emulsion, combining heat and use of emulsion breaker on Bunker oil (IF 80) and Fig. 12 presents the reduction of emulsion viscosity.
breakers are, however, not commonly used in marine oil spill recovery operations. The Transrec skimmer system, invented and developed by the first author in 1982, was the first skimmer system available on the market to include a system for chemical injection to enhance breaking of emulsions. The system was implemented as a result of studies conducted by Norwegian Clean Seas Association for Operating Companies (NOFO) (Lode, 1981).
The effectiveness of separation by use of emulsion breakers is dependant on the composition and effectiveness of the product (due to changes in oil characteristics), dosage rate, properties of spilled oil, mixing energy, temperature and time and method of application after a spill. Emulsions formed from paraffinic crude oil residues exhibit faster breaking rates than emulsions formed from crude oils with high asphaltene contents. Intrinsic factors such as wax precipitation in paraffinic crude oils, decrease the effectiveness of both heat and chemical emulsion breakers (Strøm-Kristiansen et al., 1994b). The dosage rates ncoded for crude oils is normally in the range of 250-500 ppm compared with the volume of oil, and for higher viscosity refined products as high as 5000 ppm (Abbot et al., 1994). Optimal methods of injection, increased mechanical mixing and elevation of temperature will raise both the effectiveness and efficiency of separation. In general, the effectiveness of a particular emulsion breaking product tends to drop with increased water content and/or viscosity. A standard procedure has been developed for effectiveness testing of emulsion breakers over a range of oil weathering stages (Daling et al., 1990 and Hokstad er al., 1991), Test results have provided useful information on the potential for improvement of separation. However, many different oil weathering and emulsion breaker test methods exist and internationally accepted standards are still required for evaluation, comparison and interpretation of results developed world wide (Nordvik et al., 1993).
Time is a key factor for effective on-line separation, both with regard to residence time in a separator and time available for chemical treatment and heating. For example, for an on-line VOSS, the time available for treatment and separation is approximately 5-15 min from skimming until discharge into a storage tank. Compared with heating, use of emulsion breakers has the potential to be a faster and more cost effective and efficient method for separation of a wide range of crude oil emulsions, and tests have shown that the tendency for re-emulsification decreases. Results from both laboratory and full scale testing indicate that the time required for gravity separation of water from a given stable crude oil emulsion may fall within the required time frame for optimization of on-line separation. Figure 10 presents laboratory effectiveness
Recovery Systems Optimization
The theoretical assumption that marine oil spill recovery systems can operate on a continuous all day basis has severe limitations in practice. There will be down-times because of rough sea conditions, equipment failures, maintenance requirements and personnel limitations. A large loss of time can often be attributed to transfer operations of recovered oit water mixtures. Such transfer operations might be to unload integral oil storage tanks from a dedicated oil spill recovery vessel into transfer barges, shuttle tankers, or temporary storage bladders (dracones) as used by many oil spill responders. VOSS operations require recovery work interruption when full storage bladders are unloaded or exchanged for fresh units, an operation which in any case, is sensitive to adverse weather conditions. Integration of an effective oil water separation system, including emulsion breaking. is expected to significantly decrease these periods of time out of recovery operations. Increased productive time in operation, combined with use of enhanced separation technologies, and increased skimmer capacity and effectiveness will improve recovery capabilities for fresh oil, weathered oil, and emulsions. Optimization of recovery processes, however, requires proper selection and dimensioning of equipment, such as skimmers, separators and storage volume.
Increased operational efficiency and extended productive operating time, can also be achieved by improving capabilities for tracking drifting oil slicks and delineating zones of thickness. The distance at which a slick can be observed from a vessel in full daylight is normally less than 1000 m, and less than 100 m at night, even with the use of search lights. Advances in airborne remote sensing of spills, and implementation of sensor technologies have the capability to provide for effective day and night operations for marine oil spill recovery (Giammona et al., 1995). A vital element for improvement of recovery operations is, however, implementation of a real time (and position) down-link data system for use on VOSS or dedicated oil spill recovery vessels.
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