After my years in the Army, I got my first real civilian job with a engineering company, and my first activity had to do with getting gasoline out of natural gas. The very concept is a little startling, when you think about it. Natural gas is a—well, a gas. And gasoline is, well, a liquid. So how can you get one from the other?
As it happens, natural gas coming from wells carries, along with the gas itself (mostly methane, CH4) small amounts of heavier hydrocarbons in vaporized form, not least what in the trade is known as “natural gasoline.” Most gasoline is formed of a chemical called octane (C8H18). The difference between natural gas and gasoline is the ratio of hydrogen to carbon; this ratio is 4 to 1 for natural gas and 2.25 to 1 for gasoline. Indeed the whole very complex business of crude oil or natural gas processing is to sort out components of naturally-occurring molecules by these ratios.
The illustration, which I have courtesy of Wikipedia available here, lets us see in a simple diagram how the separation actually takes place. Crude oil goes in. Energy is added to heat it up. At higher temperatures the components of the crude arrange themselves by boiling point. Molecules with the most hydrogen go toward the top, those with the most carbon go to the bottom. They may be decanted at the appropriate levels. At the top we get gases. These include natural gas, propane, and butane—to name the big ones. At the very bottom we get oils, waxes, and tars.
The general rule is that at the top we get small molecules with low boiling points; these products are volatile, flow easily, and because they have high hydrogen content, they ignite very easily. At the bottom we get big molecules, including chains (polymers), high boiling points, and we have more difficulty getting them to burn; they exhibit thick, sluggish materiality; and they aren’t very volatile.
Natural gas at the top stubbornly remains a gas unless it is cooled to -260 degrees Fahrenheit and maintained there at 3.6 pounds-per-inch (psi) pressure. Propane and butane, which have more carbon (C3H8 and C4H10 respectively) can be liquefied with modest pressures at room tempeature. Propane requires 127 psi, butane, which has a lower hydrogen ratio, at around 37 psi, both at around 100 degrees F when compression begins.
Hydrogen to carbon ratios (last column) tell us the story:
| Methane | CH4 | 4.00 |
| Propane | C3H8 | 2.67 |
| Butane | C4H10 | 2.50 |
| Octane | C8H18 | 2.25 |
The higher the ratio, the lighter the substance; the lower the ratio, the more the stuff inclines to flow like a liquid. The things you have to know to be The King of the Hill….
Filed under: Energy, Petrochemicals | Tagged: Butane, Crude Oil, Methan, Octane, Propane
