Mercury Hangs Around

Photo Courtesy of
The National Atmospheric Release Advisory Center (NARAC)
 at Livermore Laboratory
It doesn't take much in the way of common sense to know that dense gases hang low to the ground.  We've all seen gas dispersion models like the one above, etc., and we're all grateful to be able to use them.
However, there is something I'd like you to think about as it relates to gas releases and particularly to very dense gases.  What I'd like you to think about is the rate and pattern of mixing that occurs for such gases.  This is important when evaluating environments with flammable, oxidizing gases and/or toxic gases.  It is particularly important when we're planning for hazardous response strategies when we are dealing with mercury.
In general, two gases with molecular weight values close to each other will tend to mix more quickly than two gases where the molecular weight values are far apart.  For example, in a compressed gas cylinder (all other things being equal) I have found over the years that nitrogen and carbon monoxide will mix more quickly together than carbon dioxide and nitrogen.  There are, of course some limits on this generality- including the concentrations involved and the area of mixing interface.
The molecular weight of air is 28.996.  The molecular weight of mercury is 200.59.  Kind of a large differential, so even as it enters the air it's going to mix slowly.

Next, there's the vapor pressure.  Gases with liquid/vapor phases where the liquid has a high vapor phase such as carbon dioxide (830 PSIG), will enter gas phase quickly when released.  Mercery, on the other hand, has a vapor pressure of .0017 torr (not much of a vapor pressure at all, really) at 25 degrees C.  In other words, it will enter the vapor phase very slowly.  Other things being equal, heat will increase the vapor pressure.

The quicker the gas enters the vapor phase, the more quickly it will disperse unless there are outside factors working against it such as an inversion, the release occurring in a valley or in a confined space.  A hot day, of course, will accelerate this process.

Mercury, at normal temperature and pressure, will release slowly, continually over a long period of time, so it needs to be thought of differently than most of the dense gases.  There are similarities, of course, but the gasification of mercury, I think, is a little bit different animal than most.

Refrigerant gases have high molecular weights like mercury does, but they have higher vapor pressures.

So although we're used to thinking of dense gases (liquified gases shipped under their own vapor pressure) as somewhat similar, we need to give some serious thought whenwe're calculating the gasification of mercury because of its unusual combination of extraordinarily low vapor pressure and high molecular weight. 

I truly think it needs its own dispersion chart.