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ATLE B. NORDVIK, JAMES L. SIMMONS, KENNETH R. BITTING,
This paper discusses the changes in spilled oil properties over time and how these changes affect differential density separation. It presents methods to improve differential density, and operational effectiveness when ok-water separation is incorporated in a recovery system. Separators function because of the difference in density between oil and seawater. As an oil weathers this difference decreases, because the oil density increases as the lighter components evaporate. The density also increases as the oil incorporates water droplets to form a water-in-oll emulsion. These changes occur simultaneously during weathering and reduce the effectiveness of separators. Today, the state-of-the-art technologies bave limited capabilities for separating spilled marine oll that has weathered.
For separation of emulsified water in an emulsion, the viscosity of the oil will have a significant impact on drag forces, reducing the effect of gravity or centrifugal separation. Since water content in un emulsion greatly increases the clean up rolume (which can contain as much as two to five times as much water us the volume of recovered oil), It is equally important to remove water from an eanulsion us to remove free water recovered owing to low skimmer effectiveness. Removal of both free water and water from an emulsion, has the potential to increase effective skimming time, recovery effectiveness and capacity, and facilitate waste handling and disposal. Therefore, effective oil and water separation in marine oll splu clean-up operations may be a more cettical process than credited because it can mean that fewer resources are needed to clean up an oil spill with muboequent effects on capital investment and basic stand-by and operating costs for a spill response organization.
A large increase in continuous skimming time and recovery capacity has been demonstrated for total water (free and emulsified water) separadon. Assuming a 200 m storage tank, 100 m n skimmer capacity, 25% skimmer effectiveness, and 80% water content in the emulsion, the time of continuous operation (before discharge of oil residue is needed), lacreases from 2 to 40 de and recovery of oil residue from 10 to 200 m'.
Use of emulsion breakers to enhance and accelerate the separation process may, la some cases, be a rapid and cost effective method to separate crude oil emulsions. Decrease of water content in u emulsion, by heating or use of emulsion breakers and subsequent reduction in viscosity, may improve prempability, reduce transfer and discharge time, and can reduce olly waste handling, and disposal costs by a factor of 10. However, effective use of emulsion breakers is dependant on the effectiveness of the product, oll properties, application methods and time of application after a spil. : 1997 Elsevier Science Ltd
Keywords: Oll spill response, separation, oil weathering
A. B. NORDVIK et al.
The McGraw-Hill Dictionary of Scientific and Technical Terms (1974) defines an emulsion as a stable dispersion of one liquid in a second immiscible liquid, such as milk (oil dispersed in water).' Spilled oil weathers and becomes emulsified by physically incorporating water droplets into it. The process forms a stable mixture. Most crude oils and heavier refined products form emulsions with varying water content. Subsequently, emulsification will increase the volume of contaminant to manage. During clean up operations, after the skimmers have picked up floating oil (and water), the free water will have to be removed from the storage tanks. The remaining emulsion may now contain from two to five times the original volume of the oil an oil/water emulsion, therefore decreasing effective storage capacity (with water). Emulsification also results in marked increases in viscosity, decreasing the capacity and effectiveness of a variety of associated skimming. pumping, and handling operations. Heavily emulsified oils will, in addition, reduce capabilities for waste handling and disposal of the recovered material.
Extensive efforts have been made over the last decade to develop methods and technologies to improve oil spill response capabilities. Significant progress has been made in several fields, including education, contingency planning and response, and development of enhanced response tochniques and technologies. However, the incorporation of advanced oil-water separation technologies, especially those capable of removing water from a stable emulsion, has seen little real advance to date, even though this technology could greatly increase the effectiveness of an oil recovery operation. Recent testing of commercial marine oil spill separators, conducted by the U.S. Coast Guard and the Marine Spill Response Corporation (MSRC), has uncovered a need for further development of separation technologies to improve the effectiveness of mechanical recovery (Murdoch, 1993a,b).
There are several reasons why the current technologies are not adequate. Separators for spill recovery on-line separation are subject to widely fluctuating working conditions. The oil industry has focused on development of separation technologies for oil field and production related operations, involving mostly fresh crude oils and water mixtures. Bilge water separators are normally designed for low capacity and the low oil to water ratios related to vessel operations. Standard test criteria do not include emulsions or high viscosity oils. Therefore, traditional user groups and manufacturers have not focused on the requirements of marine oil spill recovery separation problems.
Separators function because of the difference in density between the oil and seawater. As an oil evaporates the oil density increases and this difference decreases. The density will increase as the oil incorporates water to form a water-in-oil (w/o) emulsion. Both of these changes occur simultaneously during weathering and reduce the effectiveness of mechanical separators.
The principles used to separate oil and water includes gravity, centrifugal separation and filtration. While gravity (settling) and centrifugal separation are dependant on density differences, filtration is dependant on pressure difference and molecular size and is, in principle, independent of gravitational forces. The advantage of centrifugal separation is high throughput capacity, smaller units, and shorter residence time.
Factors regulating performance of gravity and centrifugal separation are throughput capacity, dro plet size, temperature, density differential, interfacial tension and debris. In addition, viscosity is a dominating factor for separation of water from an emulsion. Separation is governed by Stokes Law. This law assumes laminar flow, spherical droplets and ideal droplet distribution. Although these conditions are difficult to achieve for separation of recovered oil, Stokes Law can be used to illustrate the effect of changes in oil properties on separation of both oil and emulsion droplets in water and for separation of water droplets from an emulsion. The two foroes acting on a droplet of oil in water are the buoyancy and drag forces. The buoyancy of a droplet of oil in water, causes it to move upward at a velocity which is related to the density differential between oil and seawater. The drag force, which is a function of velocity. opposes the buoyancy force and the rise velocity reaches a terminal value when the two forces are equal. Terminal velocity (), is used as a separator design criteria, and determines droplet size regimes that can be separated at a given resident time and throughput capacity.
The buoyancy force is given by (x/6)d' (S. - S.), and the drag force (in Stokes) by 3RuVd. The net separation force is the difference between the two. The terminal velocity V is reached when buoyancy force equals drag force.
d's, - S.): V=
d' (So - So)= 3a4Vd and
where w is the kinematic viscosity of the continuous phase of the fluid. For separation of oil droplets and droplets of emulsion, the continuous phase is water. (Sw-S.) is the difference in specific gravity between seawater Sw and oil So, d is the droplet diameter and, 8 is the gravity force. The effectiveness of separation
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OIL AND WATER SEPARATION
can theoretically be improved by increasing the buoyancy force and/or droplet diameter, or replacement of gravitational force by centrifugal force, and by lowering the viscosity. Separator efficiency is highest with large droplets. Oil droplets of larger size can be developed by expanding the area of coalescence in an oil-water separation apparatus through use of parallel plates (Fleischer, 1984). Injection of fine air bubbles is another successful method to increase terminal velocity of oil droplets. In an emulsion, the continuous phase of the fluid is oil, and separation is a sinking process of water droplets. In a stable emulsion the water droplets do not sink by gravity forces alone. The settling process can be improved by heating to reduce oil viscosity, emulsion density and drag force. This may provide conditions for coalescence and development of larger water droplets. When water is being removed, the emulsion viscosity will continue to drop and approach the pure oil value prior to emulsification, all resulting in a higher terminal velocity and more effective separation.
Chemicals can be used instead of, or in combination with, heat to break an emulsion. Emulsion breakers act as solvents on the interfacial film surrounding the water droplets and reduce the interfacial tension, improving potential for water droplet coalescence.
The several different separation processes are known from their range of applications to segregate oil droplets over various size categories. Little experimental data, however, exists on oil and emulsion droplet sizes when the mixture enters an oil-water separator, after passing through skimmers, pumps and transfer hoses (representing a skimmer system) at
different oil weathering stages. Figure 1 presents droplet size regimes for different separation technologies.
Skimmer principle, system design and performance, capacity and environmental conditions are factors to be considered in selecting separation technologies for marine oil spill recovery operations. The results of recent testing of oil-water separators suggest an integrated approach which matches the performance capabilities of components to maximize the benefit of in-line incorporation of oil-water separation (Bitting et al., 1993, 1995). Skimming, pumping, free water separation and emulsion breaking systems all need to be appropriately selected and performance tuned to optimize operations.
The capacities of commercially available separators for bilge water treatment are in the range 0.2-50 m'h-, with the majority having a capacity of less than 25 m'h-!. Lightweight separators designed to meet a preferred 100 m'h-capacity for marine oil spills are needed to optimize the vessel of opportunity skimming system (VOSS) concept. Table
presents performance data for selected separators. They were tested at various capacities using different viscosities of oil and emulsions in order to simulate changes in oil characteristics. The influent mixture of water and oil was varied to simulate changes in skimmer effectiveness. The test results for the majority of separator units tested have found that when oil or emulsion content exceeds 4050% of total inlet flow, that separation effectiveness decreases. This resulted in more free water in the oil effluent and higher concentrations of hydrocarbon in the water effluent (Murdoch et al., 1993a).
USEFUL RANDES eperation proces
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A. B. NORDVIK d.
Table 1 Summary or performance data for selected separators, tested by using oil and emulsion combinations (Murdoch et al., 19936).
67% Maximum % of free water in an educat (crude oil test)
73% Maximum ppon of oil in water duent (crude oil test)
30,000 ppm Maximum % of free water in emulsion effluent (emulsion test) 92%
97% Maximum ppa ol emulsion in water educat (emulsion test)
270.000 ppm Note: 1%-10,000 ppm.
Maximum (percentage) of free water in oil or emulsion effluent from the separator represents the fluid that normally is directed to a storage tank. High percentage of free water in oil effluent reduces net storage capacity of oil and effective time in operation.
Maximum ppm of oil or emulsion in water effluent represents the overboard discharge from the separator. Effective oil recovery separation requires low water content in oil or emulsion effluent to optimize storage capacity, as well as low hydrocarbon content in water efiluent to minimize pollution. Water content in oil effluent is not addressed in any oil/water separator test standard
Effects of Changes in Oil Properties on
A complicating factor in the separation of recovered spilled oil, compared with more steady state conditions of industrial separation processes, is that crude oil and refined products change their physical and chemical characteristics significantly over time. This is due to evaporation and formation of w/o emulsions and oilin-water dispersions (Walker et al., 1993, and Lewis et al., 1994). Figure 2 illustrates the emulsification processes and formation of stable and unstable dispersions and emulsions at sea.
Fig. 2 A schematic diagram for the formation of stable and unstable emulsions and dispersions at sea (Lewis et al., 1994).
Spill Science & Technologr Bulletin 203, Summer Conditions (16 °C)
ML 4. DUTER SEPARATION
Compounds in the oil, such as resins, asphaltenes and waxes. contribute to the formation of stable emulsions (Lewis et al., 1995a, and Bobra, 1992). In addition, nickel porphyrin present in seawater has also been found to be a stabilizing component (Lee, 1995). As an oil weathers, W/O emulsions of increasing stability are formed at sea. The same processes also takes place within a skimmer system, storage tanks on recovery vessels or in temporary storage bladders when oil and water and sufficient mixing energy are present.
Changes in emulsion stability and the degree of stability are important for assessment of the effectiveness of a chemical agent and selection of treatment methods to improve separation effectiveness. The degree of stability of an emulsion is a question of definition. For practical purposes, during marinc clean up operations, an emulsion is considered stable if the water content does not decrease significantly during the time of operation from skimming and temporary storage on a recovery vessel until final disposal.
An increase in oil and emulsion density over time will significantly reduce the buoyancy difference between the spilled product and the seawater and thus make separation less effective. In addition, crude oils and refined products cover a wide range of densities and their behavior when spilled at sea varies widely. The progressive changes in density, oil
and emulsion viscosity, the formation of stable emulsions and the dispersion of oil and emulsified oil droplets, all contribute to interfere with an effective separation of both oil and emulsion droplets in water, and water droplets from an emulsion. Figure 3 presents changes in density of an emulsion due to emulsification and evaporation.
Water in a stable emulsion is extremely difficult to separate by gravity or centrifugal forces and may require chemical treatment and/or heating to improve separation. Figure 4 predicts changes in emulsion viscosity as a function of time for a selected crude oil.
Many crude oils and some refined products form stable w/o emulsions under low wind and wave conditions within hours after being spilled. Crude oil emulsions may reach a water content in the range 60 80%. Heavier refined products such as bunker oils, form emulsions with water content in the range 5060%. Figure S presents changes in water content as function of time for a selected crude oil under a given set of environmental conditions.
The water content of a w/o emulsion rises rapidly with time and the rate depends on sea state as a function of wind speed. Water will be incorporated as droplets with a very wide range of sizes, but the larger droplets will rapidly settle out while the smaller droplets are retained. The average droplet size decreases as a larger number of smaller water droplets
Fig. 3 Changes in density of a w/o emulsion as a function of time on the sea surface (Strom-Kristiansen et al., 1994a).
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