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Journal of Marine System

Modeling emulsification after an oil spill in the sea

Hao Xie a, Poojitha D. Yapa a,⁎, Kisaburo Nakata b

a Department of Civil and Environmental Engineering, Clarkson University, Potsdam, NY 13699, USAb School of Marine Science, Tokai University, Shimizu, Shizuoka, Japan

Received 23 September 2005; received in revised form 13 February 2007; accepted 13 February 2007Available online 20 February 2007

Abstract

A conceptual model for simulating oil emulsification after a spill in the ocean is presented. This paper contains the completemodel formulation and scenario simulations. The model formulation is based on the most up-to-date research information availablein the literature. The model uses minimum turbulence energy as a criterion to determine whether emulsification occurs. Once theemulsification happens, the model simulates water uptake and viscosity changes during emulsification. The model classifiesemulsion into three categories: stable, meso-stable, and unstable emulsions based on a concept “stability index”. The modelestimates the stability of the emulsion and simulates the process of de-emulsification when the emulsion is meso-stable or unstable.The model also considers the effects of evaporation on the formation of emulsification. Scenario simulations show how differenttypes of emulsions are formed under different conditions. They also show how the emulsion stability changes with oil weathering.© 2007 Elsevier B.V. All rights reserved.

Keywords: Emulsions; Emulsification; Oil-in-water content; Oil pollution; Oil spills; Tar balls

1. Introduction

To meet worldwide demand, oil exploration, produc-tion, and transportation either remain steady or areincreasing. As long as oil is used, the risk of an oil spillexists. An oil spill can occur from a transportation shipaccident, broken pipeline, collapse of a storage tank, oran underwater well head blowout. Realizing the threat ofoil spills, computer models have been developed tosimulate the fate and transport of oil after a spill (e.g.Huang and Monastero, 1982; Yapa, 1994; Yapa andZheng, 1995; ASCE, 1996; Reed et al., 1999).

Although oil spill models have improved signifi-cantly over the last 20 years, their capability for model-

⁎ Corresponding author. Tel.: +1 315 268 7980; fax: +1 315 2687985.

E-mail address: pdy@clarkson.edu (P.D. Yapa).

0924-7963/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.jmarsys.2007.02.016

ing of chemical processes is weak. Oil spilled in waterundergoes a variety of physical–chemical processes,such as spreading, vertical mixing, evaporation, disso-lution, emulsification, photo-oxidation, oil–sedimentinteraction, sedimentation, and biodegradation. Fig.1 isa schematic diagram showing these processes.

During oil emulsification, oil takes up water to formthe emulsion. As one of the important chemical pro-cesses, oil emulsification plays a significant role after aspill. Formation of emulsions changes the properties andcharacteristics of oil drastically. According to Fingas(1994), stable emulsions contain between 50 and 80%water thus expanding the volume of spilled materialfrom two to five times the original volume. The densityof the resulting emulsion can reach 1.03 g/ml comparedto a starting density as low as 0.80 g/ml. Most signif-icantly, the viscosity typically increases by a few ordersof magnitude.

Fig. 1. Physical–chemical processes after an oil spill.

490 H. Xie et al. / Journal of Marine Systems 68 (2007) 489–506

The emulsification of oil has a significant effect onclean up efforts. An emulsion with 80% water contenthas a volume that is five times the original spilledvolume. There is more emulsified oil to be cleaned upthan the parent oil. Special pumps are required to re-move the oil due to the dramatic increase in viscosity.

Although emulsion formation and stability has beenstudied in detail in the food and beauty products indus-tries, the physics and chemistry of the emulsification ofoil spilled in the ocean is poorly understood. The com-plexity of modeling oil emulsification is not only due tothe processes itself, but also due to its mutual relation-ships with other physical–chemical processes as shownin Fig. 2. To model the process of emulsification well, itis important to understand the relationships among thesedifferent processes.

Oil is a mixture of multiple hydrocarbon components.These components can be classified as low, medium, andhigh molecular weight components. Evaporation anddissolution remove low and medium molecular weightcomponents from the oil slick (Yang and Wang, 1977;Yapa and Zheng, 1995). Photo-oxidation converts lowmolecular weight components to high molecular weightcomponents (Dutta and Harayama, 2000). Thus, thefractions of high molecular weight components increase.With higher fractions of the high molecular weightcomponents, oil emulsification becomes easier (Fingaset al., 1997). Studies show that emulsification may

reduce oil evaporation (Ross and Buist, 1995). Fieldobservations and experiments also show that emulsifi-cation increases the viscosity of an oil slick signifi-cantly (Fingas, 1994; Davies et al., 1998). Theincreased viscosity may further affect spreading andvertical mixing (Davies et al., 1998; National ResearchCouncil, 2003).

In this paper, we present a conceptual model for oilemulsification. This model is formulated based on themost up-to-date knowledge on emulsification. Modelingoil emulsification after a spill is not an isolated process,due to its mutual relationships with other physical–chemical processes. The numerical model for simulatingoil emulsification presented in this paper is integratedwith previously developed oil transport and fates model(Yapa et al., 1999; Chen et al., 2004) that considers theprocesses such as advection, diffusion, vertical mixing,spreading, evaporation, and dissolution.

2. Previous work

Kolpack et al. (1977) formulated an equation to de-scribe the emulsification rate. In this equation emulsifi-cation depends on the asphaltene level, pressure ofsurfactants, pH level, and is inversely related to thetemperature. Mackay et al. (1980) presented a model forsimulating emulsification, which includes calculatingwater uptake. Mackay's equation is used in most oil