Tuesday, January 18, 2011

Oxygen Separation Via the Application of High Temperatures.

Study for the Regular Division of the Plane with Angels and Devils - M.C. Escher.

A contribution from NNadir, crossposted from Daily Kos, where it is accompanied by an amusing poll.

Recently I have been interested in the chemistry of the lanthanides, sometimes called, somewhat erronously, the "rare earth" elements, although, as I have pointed out elsewhere, neodymium is rare enough as to preclude the realization of car CULTure fantasies of there being one billion electric cars powered by one billion wind turbines.

But let's not track in oblivious fantasy land.

In my travels in the scientific literature last year I developed a quirky fascination with a class of compounds called perovskites, which are bimetallic oxides that have the interesting property that oxygen can diffuse through the solids, which are, in fact, ceramics.

There are many, many, many, many papers in the primary scientific literature on this phenomena, and to pick one (of several hundreds I've collected) at random, the paper I will discuss tonight is ...Journal of Membrane Science 309 (2008) 120–127

The separation of oxygen from air (or compounds) is a very important industrial process and one which is frankly laborious. Oxygen, of course, has medical uses, as well as many industrial uses, including ironically enough, a process for making hydrogen, which may seem counterintuitive, but is nonetheless real.

Industrial techniques for accomplishing the separation of oxygen include cyrogenic distillation, in which air is liquified - using vast amounts of electricity to run the cooling machinery - adiabatic expansion of highly compressed air which involves the mechanical application of work, electrolysis (of course), and a type of absorption into zeolites, which are atomic scale cages - also at high compression - called "pressure swing absorption."

If one thinks about the energy flow in these processes it goes something like this: Uusally a primary heat source, generally dangerous fossil fuels, although solar thermal and nuclear energy would work, nuclear being far superior for the purpose, is used to drive a turbine, with thermodynamic losses typically being better than 60%, and then the electricity is transported over power lines, with thermodynamic losses (of pure electricity in the neighborhood of 5% to 10%) and then convered into mechanical energy which drives refrigeration units or other compressors, also with very, very, very, very significant thermodynamic losses, during which the oxygen is separated, often in multiple stages, each involving even more thermodynamic penalties.

It's a very inefficient business and is only actually profitable in the case where energy is cheap.

Wouldn't be better if we could dispense with all these thermodynamic middle men and cut right to the chase with pure heat?

Of course it would.

The Chinese chemists who wrote the paper cited above, have looked at two very specific oxygen permeable ceramic materials, a cerium gadolinium oxide and a gadolinium strontium ferrate and determined what the effect of nanoscale homogenity on these mixtures has toward their ability to allow oxygen to diffuse between them.

Here is what the authors speak about in their introduction:

Dense ceramic membrane reactors can integrate oxygen separation, steam reforming and partial oxidation into a single step for the conversion of natural gas [1–9], and the process is regarded as an effective and economic approach to convert natural gas into syngas (H2 + CO). Although lots of perovskite oxides were investigated as oxygen permeable membranes during the past decades [1–7,9–15], the applications of the dense ceramic membranes are still limited by many disadvantages of membrane materials. These disadvantages are mainly related to the stability and permeability of membranes under syngas production conditions [16]. Cobalt-free perovskite materials were developed to meet the requirements of the stability under reducing environments, however, the improvement of stability is at the cost of the oxygen permeability [17–19]. Furthermore, decomposition of the perovskite phase under a large oxygen partial pressure gradient is still hard to be avoided for the cobalt-free membranes [17,20]. Dual phase composite membranes were suggested to meet the requirements for both the stability and the oxygen permeability simultaneously, and to overcome the dilemma occurring in the single-phase perovskite membranes [21–23]. Usually, the composite embranes comprise a solid electrolyte for oxygen ionic transport and a solid oxide or noble metal for electronic transport. Solid electrolytes-noble metals composite membranes generally show significantly low permeability and expensive costs comparing to the perovskite membranes [24–26]. Therefore, the cermet composite membranes are far from the practical applications. However, the permeability and costs would be remarkably improved and reduced, respectively, if solid oxides with good electronic conductivity were used to replace noble metals as the electronic conducting phase.

By the way, the authors remarks about the utility of using oxygen obtained from perovskite oxygen separation to make hydrogen from dangerous natural gas should not be construed, as a result of my citation of the paper, to imply that I endorse dangerous natural gas. I don't. My position is that all dangerous fossil fuels should be phased out, but that doesn't mean that certain aspects of the chemistry of methane are useless to consider as a tool to approach such a phase out or banning.

Anyway, the authors have studied this matter, and have discovered something about the homogenity of perovskite membranes. Their best sample was able to diffuse oxygen from air at 950oC at a rate of 0.91 ml per square centimeter per minute.

This is, by the way, nowhere near a world record for this sort of thing, but this kind of research is critical to understanding the parameters by which this interesting class of materials may ultimately be commercialized, and create new energy efficiencies.

Note that these high temperatures would also afford other efficiencies, including efficiencies in the generation of electricity from various types of heat engines. The laws of thermodynamics dictate that the most efficient heat engines are those that work with a high difference in the temperature of the heat reservoir and the cold reservoir. This is why all thermal power plants are more efficient in winter than they are in summer.

Realistically, the only way to access the temperatures, reliably, without huge environmental penalties is nuclear energy. Many civilized countries around the world are working very hard on such nuclear technology. I dabble in it myself, if you must know, although there is some question as to whether I live in a civilized country.

Have a swell day tomorrow. Tonight, have sweet, if completely unrealistic, dreams about a billion electric cars powered by a billion wind turbines.

1 comment:

Anonymous said...

What do you think of

The platinum-nickel alloy configuration Pt3Ni (111)

Is it as good for hydrogen as for oxygen?

Is this the best that is currently deployed?



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