For the "green" process engineer, a nice little paper on the mathematical physics of bubble columns. [View All]

I've been catching up on my reading and thinking about certain classes of problems, and I came across a very nice paper on a subject we don't often think about: Bubbles.

In environmental engineering - which is still inadequate for various reasons - catalysts are often used. There are two kinds of catalysts, heterogenous catalysts and homogenous catalysts.

A famous example of a homogenous catalyst is a CFC in the upper atmosphere, which has caused (and is still causing) the decomposition of ozone in the upper atmosphere. The system that is being catalyzed is a gas, specifically the ozone/oxygen system which is in disequilibrium because of slow reaction rates. The catalyst is the CFC which is also a gas. As a catalyst the reaction between CFC and oxygen regenerates the original CFC generally - albeit, happily in this case - not totally. Because both the catalyst and the reactant are both gases, the phase system is homogenous.

More commonly catalysts are heterogenous. For instance a catalytic on an automobile generally consists of alloys of precious metals like platinum, rhodium and palladium coated on a convoluted surface, typically alumina. The purpose of the alumina is to give the precious metals - the actual catalysts - as much surface area as is possible with the minimal amount of mass. The need for surface area is in turn generated by the fact that the catalyst is heterogenous. One phase is a gas - the car exhaust containing dangerous fossil fuel wastes like carbon monoxide and various nitrogen oxides - and one phase - the catalyst is a solid.

Mostly the world is dominated by heterogenous catalysts, and by far, most homogenous catalysts, such as they exist are associated with liquid phases, an obvious exception being the system involved in ozone.

An interesting two phase system that is increasingly important - and should be important - is the gas/liquid biphasic system, in which a liquid phase is in contact with a gas phase. Of course earth's atmosphere and it's surface bodies of water (and now oil) interact in this way, and it can be shown that many catalytic reactions, including common catalytic reactions associated with life - photosynthesis for example, in which carbon dioxide is removed from the air and catalytically reduced to a sugar - are gas liquid phase systems.

An important aspect of biphasic liquid/gas extraction is the bubble column. In this system, a gas is bubbled throught a column of liquid. Again this involves surface area, and surface area and rate are tied to bubble physics. This can effect the rate at which particular gas phase reactions that are catalyzed by liquid phase catalysts take place and how efficient they are.

Similar considerations can effect the performance of systems that are not catalytic. For instance bubble formation has interesting implications in nuclear engineering, in both homogenous and heterogenous reactors. A homogenous reactor is a fluid phase reactor like Alvin Weinberg's great MSR. In this reactor the fission products - generally krpton and xenon, but also iodine, bromine, gaseous cesium metal and helium - often form bubbles which can effect the reactivity of the reactor under the right circumstances. Indeed these same gases are often seen in solid phase reactors, in which local microfluid solutions can form and bubbles can be involved, particularly at high burn up.

It is well know that the phase structure of nuclear fuels are very complex, and may involve distinct chemistries as well as simple phase behavior. For instance, many phases at various transient temperatures and mole fractions, might involve, for example cesium, uranium, or plutonium and complex metalate ions like cerates, technates and other species, and at other temperatures and mole fractions might involve only distinct elements either in or out of solution.

The physics of gases are involved here, even in solid nuclear fuels, and thus is the behavior of bubbles.

In any case, I stumbled across a nice paper on the mathematic modeling of bubble column systems:

Here it is: http://pubs.acs.org/doi/abs/10.1021/ie801834w?prevSearch=%255Bauthor%253A%2BJayaraman%255D%2BAND%2B%255Btitle%253A%2Bvector%2Bregression%255D&searchHistoryKey=">Ind. Eng. Chem. Res., 2009, 48 (21), pp 9631–9654.

Cool, if esoteric, paper.