The following definitions are used in this section of the Handbook. Crude oil is a liquid hydrocarbon produced from a reservoir. Condensate is liquid hydrocarbon that condenses from the gas as pressure and temperatures decrease when the gas is produced from the reservoir up the tubing and out the wellhead choke. Starting with the lightest molecular weight, they are methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), pentane (C5H12), hexane (C6H14), and so on. As the ratio of carbon to hydrogen atoms increases, the molecules become "heavier" and have a greater tendency to exist as a liquid rather than a gas. An oilfield facility is a collection of equipment that is used to separate the fluids that come out of an oil or gas well into separate streams that can then be sold and sent to a gas plant or refinery for further processing. A process simulation is a calculation, usually done with a computer program that predicts how the components that make up the well fluids react to changes in pressure and temperature as they are processed through the facility. This is not a chemical reaction, but rather a simple phase change as liquids flash to vapor or vapors condense into liquid.