Ali Khelifa

Models for effective density and settling velocity of flocs

New models to predict settling velocity and effective density of flocs are proposed. The models are based on the concept of fractal geometry, but with the assumption of variable fractal dimension with the floc size. The best results are obtained when the fractal dimension is estimated by a power law function of the floc diameter. The models are compared with observations from 26 published data sets relating floc size to settling velocity measured under various conditions and at different locations. The floc size covered by the data varies between 1.4 and about 25,500µm. Five commonly used models are also compared to these data and found to reproduce inadequately the full range of the observations. Sensitivity analysis shows that, with the proposed models, the spread in the data may be reproduced by varying the size of primary particles from about 0.05 to 20µm. The new models are proposed for integration into numerical models to simulate sediment transport of cohesive sediments, contaminants, and biological microorganisms such as phytoplankton.

Characteristics of oil droplets stabilized by mineral particles: Effects of oil type and temperature

Abstract The relative influence of oil type and temperature on the characteristics of oil droplets stabilized by mineral particles (oil–mineral aggregates––OMA) was studied in the laboratory. OMA were generated by shaking eight different oils under two temperatures with natural mineral fines in seawater at a pre-defined energy level. Shape, mean and maximum sizes, size distribution and concentration of oil droplets forming negatively buoyant OMA were measured by image analysis using epi-fluorescence microscopy. Results showed that oil droplets are, on average, spherical regardless of oil composition and temperature. Non-spherical “elongated” oil droplets form more at 20 °C than at 0 °C. Droplet shape and size were not correlated to oil viscosity. The concentration of oil droplets decreased rapidly with oil viscosity, temperature and asphaltenes–resins content (ARC). When normalized with ARC, mass concentration of oil droplets correlates well with oil viscosity, regardless of experimental temperature. A model was proposed to calculate mass of oil dispersed by OMA as a function of oil viscosity and ARC. Size distributions of oil droplets follow similar trends, but their magnitudes depend on oil type and temperature. A function was derived that describes all the data when size distributions were presented in a normalized form N / N t = f ( D / D 50 ), where N is number of droplets of diameter D , N t is the total number of droplets and D 50 the mean size of the droplets.

A laboratory study on the kinetics of the formation of oil-suspended particulate matter aggregates using the NIST-1941b sediment

The formation of oil-suspended particulate matter aggregates (OSAs) results from the heteroaggregation between dispersed oil droplets and suspended particulate matter present in coastal waters. This process has been recognized by the oil spill remediation community to enhance natural cleansing of oiled shorelines and oil dispersion in the water column. While several studies have been conducted on the formation and characteristics of OSAs, few studies have addressed the kinetics of OSA formation. Operationally, this has left decision-makers lacking information on the time scale of this process and its significance to oil dispersion in real spills. A laboratory study was conducted to investigate the kinetics of OSA formation as a function of mixing energy and the sediment-to-oil ratio using the standard reference material 1941b. Results showed that formation of OSAs increased exponentially with the mixing time and reached a maximum within 4h. When the shaking rate increased from 2.0 to 2.3 Hz, the maximum oil trapping efficiency increased from 20% to 42% and the required shaking time decreased from 3.7 to 0.7h.

Laboratory investigation of oil-suspended particulate matter aggregation under different mixing conditions.

Oil-suspended particulate matter aggregation (OSA) has been recognized by the oil spill remediation community to effectively enhance the cleansing of spilled oil in the marine environment. While studies have investigated the application of mineral fines as an effective method to facilitate oil dispersion, decision-makers still lack information on the role of mixing energy in OSA formation and its significance to oil dispersion in real spills. This work studied the effect of level and duration of mixing energy on OSA formation using the standard reference material 1,941 b and Arabian light crude oil. The results showed that dispersed small oil droplets increased with an increase of both the level and duration of mixing energy to form multi-droplet OSAs. The sizes of the dispersed droplets varied between 5 and 10 μm under different conditions studied. The maximum oil trapping efficiency increased from 23% to 33%, the oil to sediment ratio increased from 0.30 to 0.43 g oil/g sediment, and the required shaking time decreased from 2.3 to 1.1h as the shaking rate increased from 2.0 to 2.3 Hz. Based on the size measurement results, a breakage effect on the formed OSAs and sediment flocs was confirmed under high mixing energy level.

EFFECTS OF CHEMICAL DISPERSANT ON OIL SEDIMENTATION DUE TO OIL-SPM FLOCCULATION: EXPERIMENTS WITH THE NIST STANDARD REFERENCE MATERIAL 1941?

ABSTRACT As it is well established that application of chemical dispersant to oil slicks enhances the concentration of oil droplets and reduces their size, chemical dispersants are expected to enha...

A COMPREHENSIVE NUMERICAL APPROACH TO PREDICT OIL-MINERAL AGGREGATE (OMA) FORMATION FOLLOWING OIL SPILLS IN AQUATIC ENVIRONMENTS

ABSTRACT Aggregation between suspended sediment grains and oil droplets, which leads to the formation of agglomerates commonly referred to as Oil-Mineral Aggregates (OMA), is widely acknowledged as...

Characteristics of Oil Droplets Stabilized by Mineral Particles: The Effect of Salinity

ABSTRACT The influence of salinity on the characteristics of oil droplets stabilized by mineral particles (oil-mineral aggregates – OMA) was studied in the laboratory using three different oils and a natural sediment. Size and concentration of oil droplets associated with negatively and positively buoyant OMA were measured by image analysis using epi-fluorescence microscopy. Results showed that the median droplet size increases rapidly from about 5 μm at zero salinity to double at salinity close to 1.2 ppt; decreases dramatically to about 5 μm at salinity 3.5 ppt and then increases slightly to 6 μm at the seawater salinity. The concentration of oil droplets also increases sharply when the salinity increases from zero to a critical aggregation salinity Scas, after which it stabilizes at its maximum value. The concentration of mineral-stabilized droplets is strongly affected by oil type at any salinity. When normalized to its maximum value, the concentration of droplets correlates well with normalized salin...

Kinematic assessment of floc formation using a Monte Carlo model

A simple stochastic model is proposed to simulate floc formation due to simultaneous aggregation and breakage processes. The model is based on the constant-number Monte Carlo method where the number of flocs is kept constant during simulations. To produce equilibrium floc-size distributions, it uses established models of flocculation and new simple formulations of breakage probability and of the probability of producing fragments of a given size from broken flocs. The concept of fractal geometry is used to describe the geometry of flocs. The maximum size of flocs allowed, the median size of component particles, and their density are the main inputs needed to simulate floc formation. Simulated steady-state floc-size distributions were compared with field data observed at different locations, and good agreement was obtained. Dimensional analysis applied to measured and simulated data revealed that floc-size distributions are self-similar and can be described by the same function, regardless of the conditions of their formation.

Chapter 10 The role of oil-sediment aggregation in dispersion and biodegradation of spilled oil

Abstract It is now widely acknowledged that suspended sediments have strong influence on the fate and transport of spilled oil into the aquatic environment. This chapter presents a brief summary of the state of knowledge on how suspended sediments contribute, via the process of heteroaggregation oil-sediment, to dispersion of spilled oil and its biodegradation. Effects of environmental parameters such as salinity, sediment concentration and size as well as oil type on this process are discussed. The study shows that this recently recognized mechanism by the oil spill remediation experts has been, in fact, discovered earlier by the production community in the beginning of the last century. Discussion on real application is presented to show how this process can enhance considerably natural recovery of oiled shorelines when the method of “surf washing” is applied. An outline of research required to improve our understanding of the process is finally presented.

Chapter 7 Assessment of minimum sediment concentration for OMA formation using a Monte Carlo model

Abstract Aggregation between suspended sediment grains and crude-oil droplets which forms oil—mineral aggregates (OMA), is a natural process that enhances dispersion of spilled oil in aquatic environments. The objective of this study is to use a recently upgraded numerical model (MCOMA2) to investigate minimum sediment concentration (MSC) for OMA formation. MSC is defined as the concentration of sediment below which concentration of OMA-stabilized droplets is negligible (less than 1%). Formation of OMA is simulated for sediment concentration varied from 0.5 to 1000 mg/l considering Forties Blend (density of 840 kg/m 3 ) and FOREF oil (density of 965 kg/m 3 ). Results show that the MSC varies from 0.5 to 200 mg/l, depending on the size ratio (ratio between the diameter of sediment and the diameter of oil droplets) and the oil type. Its minimum values for both light and heavy oils are obtained when the size ratio is close to 0.4 and 0.55, respectively.