Impact of below-freezing air temperature on the formation and stability of oil emulsion

FY 2023 | 1 – Understand | 23-10-07

Impact of below-freezing air temperature on the formation and stability of oil emulsion

Michael C. Boufadel, New Jersey Institute of Technology
Contract Term: 07/01/23 - 06/30/24
Award: $

Through this project, we propose to examine the impact of cold air temperature on the stability of formed seawater-in-crude oil emulsions. Three oils and two weathering stages are selected and the emulsions’  physicochemical properties will be tested before and after exposure to below-freezing temperatures. The properties to be measured include density, viscosity, water content, water droplet size distribution, and visual microscopy images. We believe the outcome of this work will contribute to the community with new knowledge on emulsion stability in arctic environments. 

After oil spills, the oil slick entrains seawater droplets in the water column, and could potentially formed an emulsion. The emulsified oil (i.e., the emulsion) has a significantly increased viscosity, which poses extra challenges to the response techniques. The water content in an emulsion has been considered as another metric for its stability. While in the arctic environment, when the water surface is exposed to very low air temperature, the emulsification processes would be affected due to two non-exclusive reasons: the increased viscosity and the freezing entrained water droplets. There is very limited research on this scenario so far, thus, a serial formed seawater-in-crude oil emulsion stability tests are proposed to examine the impacts of the cold air temperature on the surface emulsions. 

It has been well established that the presence of resins and asphaltenes enhance oil emulsion formation and stability (Fingas and Fieldhouse 2006). Asphaltenes form a network structure within thin oil film that separates approaching water droplets (Czarnecki et al. 2012). Meanwhile, the surfactant-like polar resins tend to maintain small water droplets (1 – 20 μm) in oil (Fingas 1995; Fingas and Fieldhouse 2012). However, challenges remain in terms of the actual behavior of emulsion at sea. First as the emulsion is shear-thinning, a stable emulsion under a quiescent sea state could become unstable (i.e., breaks) due to high shear (Muriel and Katz 2023). Second, in the laboratory, the surfactants that are inherently present in a spilled oil remain in the vessel, which would impact the subsequent behavior of the emulsion. While at sea, the surfactants are likely to leave the oil, which impacts the emulsification of and/or the stability of emulsion.   

The water content in an emulsion has been considered as another metric for its stability; a rule of thumb is that an emulsion is stable if its water content exceeds 30% per volume.  Unfortunately, the water bubble size distribution within the oil was rarely considered with the exception of recent works (Muriel and Katz 2023). In that work, it was observed that an emulsion tends to be more stable if the water bubble distribution consists of a large number of small bubbles surrounding a large one. This implies that the water content alone is not sufficient to characterize emulsion stability. Efforts to address the role of mixing chronology are going to be invested within the consortium CEMOR, led by NJIT and funded by MPRI2.0.   

Unfortunately, there is not a mechanistic work on emulsion stability in cold environments, and in particular at below (water) freezing temperatures. In cold regions, if the emulsion is totally immersed in water, then its temperature will be higher than the freezing point of water. However, an emulsion on the water surface could be exposed to very low air temperatures which could impact the emulsion via two non-exclusive processes: 1) The oil viscosity will increase and thus the emulsion will become more “stable”- the emulsion becomes wax-like, and 2) the saltwater bubbles within the oil emulsion could freeze, expand, and release the brine. We hypothesize that upon a subsequent increase in temperature to above zero, the water occupies a smaller volume with the brine occupying another volume, which could cause the emulsion to be structurally weak. 

The SINTEF group (Faksness and Brandvik 2008) conducted experiments of oil emulsion in cold water, where they placed the oil on water in a carved channel in an ice sheet.  They considered three levels of surface coverage by ice. The air temperature was as low -15oC. They observed that the water content decreased with the ice coverage, reaching around 20% water for 90% ice coverage. The challenge in interpreting their results is that the mixing energy was not constant in all experiments. Thus, the experiments with 90% ice coverage could have allowed for more exposure to air.