Industrialized Cyclist Notepad

Blending butane into gasoline is why gas prices fall in the fall, according to Robert Rapier. RVP = Reid vapor pressure, the higher the RVP the faster the evaporation. EPA sets limits on RVP of gasoline which are more stringent in summer months than in winter, allowing the increased blending of cheap, yet highly evaporative (word?) butane:

Butane has an RVP of 52 psi, which means pure butane is a gas at normal pressures and temperatures. But butane can be blended into gasoline, and its fractional contribution to the blend roughly determines its fractional contribution to the overall vapor pressure of the mixture. As long as the vapor pressure of the total blend does not exceed normal atmospheric pressure (again, ~14.7 psi) then butane can exist as a liquid component in a gasoline blend.

But with a vapor pressure as high as 52 psi, butane can’t make a large contribution to summer blends where the vapor pressure limit is 7.8 psi. For example, if a gasoline blend contained 10 percent butane, butane’s contribution to the vapor pressure limit is already 5.2 psi and you would still have 90 percent of the blend to go. It isn’t feasible to blend much butane into gasoline when the vapor pressure requirement is low. But when the limit increases by 5 or 7 psi, it becomes feasible to blend large quantities of butane.

Why do we care about blending butane anyway? Because it is abundant and cheap. Butane can routinely trade at a $1/gallon discount to crude oil or gasoline. Butane is a byproduct of oil refining, but is also a component of natural gas liquids (NGLs), which are condensed out during natural gas production. Given the huge expansion of natural gas production in the US, it should come as no surprise that NGL production is also on the rise.

via Why Gasoline Prices are Falling.