Crossposted at Politicook.net
Last time we talked about the gas laws mainly, and I realize that this is not exactly a pageturner of a topic, but is important for two reasons: to have a basic familiarity with how gases behave, and to show that these concepts were worked out, in some cases, over two centuries ago. I find this fascinating, because everything was done by hand. No computers, no calculators, no slide rules, and only the most basic of instrumentation (basically a balance and a crude barometer).
This time we will look at a few specialized applications for gases, and then a look at a few gases of particular interest. If your favorite is not covered, speak up in a comment!
Because of being compressible, gases are good for storing energy. Whilst compressed gases are often utilized for transport of gases (high pressure pipelines, compressed gas cylinders, etc.), a major use of compressed gases (usually air) is power applications. Compressed air is often used in automotive shops to power tools since compressed air is already necessary to inflate tires, and is very convenient as a power source. In addition, it is clean, tools run cool, and there is no danger of electrical shock. Everyone that has gotten a new set of tires has heard the sound of an air impact wrench.
Whilst on the subject of tires, everyone with a car should get one of those little compressors that plug into a car cigarette lighter and a good, dial display tire gauge (NOT one that pushes the little stick out, as these are not very accurate). The compressors are only about 20 bucks, and you will save that much in gasoline at $4+ per gallon by keeping your tires inflated to factory specifications (usually printed on a sticker inside the driver's door). Not only do underinflated tires cost fuel because of high rolling resistance, they are unsafe because the higher rolling resistance makes them run hotter than properly inflated ones. Always check them cold (before driving). You sometimes can inflate over factory recommendations for the car, but never inflate past the rating embossed on the tire wall. Higher pressures make the ride a little stiffer, but do save gasoline.
Compressed air tools are often used in explosive atmospheres since there is no commutator to spark like in an electric motor. When I worked in the pyrotechnics prototype facility we used lots of air powered equipment and had a large compressor with feeds to each work area.
An interesting use for compressed air is in solar power production. Obviously, solar does not work very well at night or with high cloud cover. One concept is to take part of the electricity generated during daylight and use it to compress air and store it in underground reservoirs. During dark hours the compressed air is tapped to run a gas turbine generator to provide a supply of electricity. This makes solar arrays, be they photovolatic or thermal, useful for supplying electricity around the clock, reducing the need for supplemental generators running of fossil fuels. Whilst it will not completely eliminate the need for supplemental generating capacity, it makes sense in regions where the geology is suitable (think where old gas or oil wells were sited) because the economy of scale for very large solar plants.
This concept could also be used for wind farms, but the benefits are apt to be less since, in favorable wind areas, there is not as high a probability that the wind will be off line essentially half of the time. However, in certain locations it might be a viable approach.
Another use for gases is in control operations. This can be accomplished by using partial vacuum, compressed gases, or both. For example, the heating and cooling doors on my car are controlled pneumatically. Partial vacuum from the intake manifold goes to a control switch, and, depending on how I set it, the partial vacuum causes one or more doors in the ductwork to open or close. Mine defaults to the windshield defroster doors being open and the other closed, probably a safety function. I found out the hard way when it quit. With some fine wire and a solvent, I finally worked out the clog from the main supply (sort of an odd term for vacuum) line and everything worked again.
Now let us talk about a few gases that find of particular interest. The first one is hydrogen since it is in the news quite a little as a potential fuel. Hydrogen has a couple of things going for it. It is clean burning, works in fuel cells, and can be made just about anywhere. However, it has some really large drawbacks as a fuel. First of all, it is not naturally occurring in the free state like natural gas, so it has to be manufactured. How is this done?
The cheapest was to make hydrogen is to take fossil fuels and react them with water vapor under high temperatures and moderately high pressures. The products are generally carbon monoxide and hydrogen. It takes a lot of energy to remove hydrogen from water, so it is not cheap. Either natural gas or coal can be used for this. Natural gas can also be reacted with a limited amount of oxygen to form hydrogen and carbon dioxide. The problem with any of these methods is that fossil fuels are necessary, and, at the very end of the cycle, the carbon monoxide is burned to carbon dioxide since the heat content of the monoxide is too great to waste.
Hydrogen can also be made by the electrolytic decomposition of water, forming oxygen as a byproduct. To do this requires a really cheap electrical source, so it is not very viable either. It might be where cheap solar, geothermal, or wind (or nuclear) electricity is available, but is otherwise very costly.
Even if a cheap method for production is available, transport of hydrogen is extraordinarily difficult and expensive. First, it is not practical to liquefy hydrogen like propane or even natural gas, because the temperature of liquid hydrogen is so low. In addition is the fact that hydrogen has the greatest diffusion constant for any gas, so minute pinholes that would not make much difference for natural gas transport become critical for hydrogen. Yet even further, hydrogen is not very "energy dense" because it has to be used as a gas. Even though you get about 121 kilojoules (kJ) for each gram of hydrogen burned and only 32.8 for each gram of carbon burned, hydrocarbons are for the most part liquids and their densities are so much greater than even 3000 psi hydrogen that they win. At 3000 psi, hydrogen is about 18 grams per liter, and at atmospheric pressure gasoline is roughly 703 grams per liter, 39 times the density. You get the idea. Finally, hydrogen leaks are extraordinarily hazardous since mixtures of hydrogen and air will explode when the hydrogen content is anywhere from 4% to 75%, while for gasoline the limits are 1.4% to 7.6%. Therefore, I am not really very optimistic about hydrogen as a replacement for gasoline or Diesel fuel, but it certainly will have uses in specific applications.
Chlorine touches our lives in many ways. Almost everyone on public water systems have chlorinated water (ozone is being used more and more these days) to destroy bacteria and other pathogens in the water. Chlorination of water has been one of the most successful public health initiatives in the history of civilization, even though there are some downsides, particularly in water rich in organic materials due to the formation of halocarbons, which are carcinogenic. However, it takes only a few massive cholera outbreaks to make up for a very small cancer risk.
We get chlorine from salt, sodium chloride. It is dissolved in water and electricity is passed through to make chlorine gas and sodium hydroxide solution, both of value. Chlorine is a greenish, heavy gas that is easy to liquefy and store as a liquid under pressure. Enormous quantities are shipped by rail every day. Contrary to common experience, pure chlorine has a fairly faint odor. When people say that they smell chlorine, it is almost always the case that they are smelling chloramines that result from the action of chlorine on organic material containing nitrogen (proteins and especially urea from urine). Chloramines are extremely irritating and have an extremely strong odor. Try this experiment: take some household bleach, about a teaspoon, in a glass container, and sniff it. Then add a couple of drops of urine, swirl, and sniff again (cautiously). You will be able to tell the difference. In the first case you are pretty much smelling just chlorine, and in the second, chloramines.
Chlorine is used for many other things, such as production of some plastics, drugs, and in bleaching paper. One outcome of bleaching paper with chlorine is that dioxins can be formed. I always use the brown coffee filters for this very reason.
Uranium hexafluoride is the key material for enrichment of uranium to form fissile materials. It is interesting that such a heavy compound would be a gas, but fluorine is sort of special since it is a second row element. The way that uranium enrichment works these days is to take natural uranium, (99.284% uranium-238, which does not fission and 0.711% uranium-235 that does fission with a little uranium-234 that is not enough to consider) and centrifuge it, over and over. To get reactor fuel, you need to enrich the U-235 to around three to five per cent, while bomb material requires around 90%. Centrifuges work by density, throwing the heavier materials to the bottom of the centrifuge vessel and allowing the lighter ones to stay near the top. The problem is that the difference in density of U-238 hexafluoride and U-235 hexafluoride is 352/349, or only about 0.9%, so the process is not very efficient and has to be done over and over to get very much enrichment. After the process is done, the hexafluoride is converted to solid uranium oxide for reactor fuel, or metallic uranium for bombs, and it is on its way.
Neon will be the last gas that we discuss unless there are questions and comments about others. Neon is a "noble" gas, is monatomic, and forms no known compound. The sole source is from the air (likewise for the other noble gases except helium (so light that escapes the atmosphere and radon, which is unstable) argon, krypton, and xenon). Air is cooled and cooled and cooled some more, and finally becomes a very cold liquid. Then it is allowed to boil slowly, and neon is the first thing to boil off (except for extremely trace amounts of hydrogen and helium, not enough to worry about) since it is the lightest, lowest boiling component.
Everyone has seen "neon" lights, but many of these lights are not neon at all.
Neon specifically is the reddish-orange colored tubes, and other colors are derived from other gases, or by painting the tubes or using colored glass tubes, or phosphorescent internal coatings, to pass a given wavelength band. The way that these lights work is pretty interesting: a high voltage electrical source strips some electrons off of a few neon atoms, allowing the resulting plasma (to be discussed next time) to conduct electricity. The electrical energy promotes electrons to higher energy levels in neon atoms, and when those electrons return back to their normal energies, an orange light is emitted.
Well, this is long enough, so plasma will follow next time. I will hang around for a while for questions, comments, and criticisms. I really like the feedback, and always say, and mean, that I learn more from the feedback than I ever possibly hope to teach. Warmest regards, Doc.