Thursday, April 9, 2015

A non-specialist’s guide to how spilled hydrocarbons react in water

After the spill today in English Bay (at the Port of Vancouver) I have been asked by numerous people to explain what happens when petroleum hydrocarbons spill into water. Anyone with an internet connection can get any number of descriptions of what happens when this occurs but the descriptions are typically either too simple to be useful or too technical for a non-specialist to understand. My aim in this post is to hit the sweet spot between those two extremes. While a grad student at UVic, I used to teach first year engineers their chemistry labs. The first year engineers were typically bright people, but few had more than grade 11 chemistry and they only took the course because it was a requirement. Teaching them was very helpful in my goal of understanding how to effectively communicate science. The goal of this post, therefore, will be to give a description that would pass muster with my engineering students. I am giving you this warning because my goal is not to be too technical, so if you are looking for a higher level description you will not find it here, moreover, some of the explanations may be imperfect as I am not going to dwell on the technical details that would bore the non-specialist reader. I will apologize that in order to understand the topic I will have to explain a few important terms/concepts. 

So let’s start with a quick primer. You will notice in my introduction I did not use the words “oil”, “gasoline” or “bunker fuel”. The reason for this is that each one of those words represents a description for a specific type of mixture of petroleum hydrocarbons. In February, I wrote a post on gasoline prices that detailed the refining process (Why Cheap Oil Doesn't Mean Cheap Gasoline or Diesel) and in that post I pointed out that petroleum hydrocarbons are made up of a mixture of individual hydrocarbon molecules. A hydrocarbon molecule is simply a molecule made up entirely of carbon and hydrogen atoms. Petroleum hydrocarbons can vary in type from small linear molecules (methane, ethane, propane, n-butane etc..) to cyclic and aromatic molecules like (cyclohexane and benzene) to huge monstrous unsaturated compounds (asphaltenes). Moreover not all crude oils are the same. They can vary from light crudes (Brent Light crude with a higher proportion of lighter molecules) all the way to heavy crudes and bitumen. The critical things to understand when trying to understand what happens when petroleum hydrocarbons spill in water are:  

  • Petroleum hydrocarbon mixtures vary in their specific gravity but are mostly lighter than water.
  • Petroleum hydrocarbons are hydrophobic (they “fear” water).
  • Petroleum hydrocarbons are mixtures of compounds that have different volatility and solubility characteristics.

By understanding these three features you can typically understand what will happen when a petroleum hydrocarbon hits a water body.

Density and Specific Gravity:

Everyone probably remembers the features of matter; that every substance has mass and occupies space. If you divide the mass by the volume of the space it occupies you get that object/substance’s density. When we are talking about liquids we use a term called “specific gravity”. Specific gravity simply means the density of a solution divided by the density of pure water at the same temperature (remember that liquids vary in density by temperature). Pure water, therefore, has a specific gravity of 1 gram per millilitre (g/mL). A liquid that has a specific gravity less than water will float on water while a liquid with a specific gravity greater than 1 g/mL will sink in water. As many of you will know seawater is denser than freshwater (it has an approximate specific gravity of 1.02 to 1.03 g/mL) so when freshwater and seawater meet (like at the mouth of the Fraser River) the freshwater will flow on the surface and the saltwater will be forced below the freshwater to form a saltwater intrusion wedge. Most petroleum hydrocarbons have specific densities lower than water, some are substantially lower: gasolines range from about 0.72 to 0.76 g/mL; diesels are in the 0.83 to 0.86 g/mL range, dillbit is around 0.94 g/mL and bunker oil is around 0.98 to 1.01 g/mL. So when gasoline spills on water it will typically float, but if bunker fuel spills into freshwater then it may float on, could mix with, and sometimes may even sink in water depending on the specific gravity of the water.

Hydrophobic Characteristics of Petroleum Hydrocarbons
Every kid has spent time playing with magnets and everyone knows that a fridge magnet has two poles, a positive and a negative. Water consists of H20 which also has poles with the oxygen end of the molecules being slightly negative and the two hydrogen molecules being slightly positive, thus water is called a polar solvent. In chemistry we talk about “like dissolving like” that is polar compounds will dissolve in polar solvents and non-polar compounds will dissolve in non-polar solvents. While water is polar, petroleum hydrocarbons are highly non-polar, that is they have no poles. Following the logic of “like dissolves like” in chemistry “unlike abhors and avoids unlike” so a non-polar compound will not dissolve in a polar solution. Moreover, not only does oil not dissolve in water, oil avoids water at all costs, in this it is called “hydrophobic” which literally means “afraid of water”. When oil comes in contact with water it will try to minimize its exposure to water. It does this by floating (if lighter than water and thus exposing itself along a single plane), forming spheres (if stuck in water with nowhere else to go by minimizing its surface area in the form of a sphere of pure liquid with water on the outside) or by adsorbing itself onto a suitable non-polar media (note I said adsorbing not absorbing). In the last case, the hydrocarbon attaches itself to the outside of the media (usually suspended sediments like grains of sand, silt or clay), that way only one side of the molecule is exposed to the water while the other side is attached to the non-polar soil/sediment.  

Volatility and Solubility of Hydrocarbon mixtures
As I mentioned above, petroleum hydrocarbon formulations are made up of a mixture of compounds with different solubilities and volatility characteristics. If you are really interested in the actual breakdowns the “Total Petroleum Hydrocarbon (TPH) Criteria Working Group Volume 2: Composition of Petroleum Mixtures” will show you what is in each type of fuel. Gasolines are made up of compounds with typically between 5 and 13 carbons.  When I wrote above that hydrocarbons are hydrophobic in a general sense all are slightly soluble. The reason for this is that water isn’t a perfect polar solvent and most hydrocarbons aren’t perfectly non-polar. Typically the smaller carbon constituents are more soluble than the bigger ones and so will mix “better” in water. The lighter components also tend to be more volatile, that is they will vapourize more quickly. The volatility has two features with respect to spills. The more volatile the compound the less time it will have to mix into the water. A gasoline spill at a gas station on a hot summer day could vapourize before it has time to mix with water, while a fuel oil in a cold climate may lose very little volume to vapourization. The volatilization will also change the characteristics of a fuel. Since the lighter components will vapourize at a faster rate than the heavier components, as a petroleum hydrocarbon ages its specific gravity increases. What this means is that a fuel that was lighter than water, when it spilled, could eventually become heavier than water as its lighter components vapourize, allowing it to eventually sink.     

So what happens in a spill like the one in English Bay?
So let’s assume that the material that spilled in English Bay was a bunker fuel. Bunker fuel has a specific gravity approaching 1 g/mL and the seawater in English Bay should be around 1.03 g/mL. So the bunker fuel will initially float on the surface where it will be moved by the regional currents, tides and winds. As the fuel is mixed by winds and waves the lighter components will vapourize and some of the bunker fuel will mix into the water column where it will either seek out some surface upon which to adsorb (be it suspended sediments or seaweed) or lacking that will form the petroleum hydrocarbon spheres we call “tar balls”. Given more time, the fuel oil will sink where it will adsorb onto the sea-bottom sediments or lacking appropriate surfaces will stay in tar ball form and roll around on the surface. Given the nature of the material, it will eventually biodegrade but that is a long, slow process that is beyond the scope of this post. In the near term, much of the material will be recovered using the various specialized technologies designed with just this type of event in mind, some will escape to coat local sediments and will wash up on the beach and some will vapourize.  

Blogger's note: my apologies, in an earlier version of this post I had included incorrect units. Apparently late-night blogging does not enhance my proofing skills. 

1 comment:

  1. Excellent. By the way, there are high pour point oils which solidify when they contact cold water. They tend to be very paraffinic, and are fairly light. Bacteria love paraffins, but they don't eat solid oil very well. On the other hand, the solidified oil is fairly easy to pick up. Many years ago I saw a photograph of a guy standing on a solid oil spill, cleaning it up with a pick axe and shovel. The medium gravity oil spill had been contained, with the vessel floating in 300 meters at. Lombok, Indonesia. It was a small spill and was cleaned up in a hurry. But I'll never forget that sight.