# Why isn't ethane used for cooking?

We commonly use methane and propane for cooking (and home heating), but not ethane. I would expect ethane to be suitable for this, being in between the two, but I've never heard of anyone using it for this purpose. Why is that?

On a related note, why is butane used for cigarette lighters and basically nothing else (in ordinary life, I mean)?

Probably the biggest drivers behind using methane as a fuel is that it is abundant in natural gas and is (currently) mostly useless as a chemical feedstock. Ethane makes up a few percent of natural gas and can also be obtained as byproducts of petroleum refining, but the big difference from methane is that ethane is extremely useful in chemical synthesis (mostly to make polyethylene). In fact, household natural gas often contains a bit of ethane, which may vary depending on the current demand for ethane as a feedstock. In short: it burns just as well, but ethane has other important uses.

Propane has some use as a feedstock, but it is an attractive fuel because it is easily stored as a liquid without requiring huge pressures.

• The reason more methane than ethane is used as a fuel is simply its greater abundance in natural gas. Most polyethylene is not made from ethane. Polyethylene is made from ethylene, which is mostly made by cracking longer hydrocarbons. This is a rather imprecise process, so the products are separated and used as follows: Hydrogen and methane: burnt in the cracker furnace; ethylene -> polyethylene; propylene -> dimerization to branched high octane C6 gasoline component and / or polymerization to polypropylene; C4+ -> recycled. en.wikipedia.org/wiki/Ethylene is a good starting point. – Level River St Apr 27 '16 at 11:18
• Ethane is one of the commonly-used feedstocks, along with LPG, propane, butane, etc. used for steam cracking when lighter products like ethylene are desired, whereas steam cracking of heavier feedstocks like naptha tend to produce bigger products like longer alkenes and aromatic compounds. Ethylene is made starting from many feedstocks, and I don't know how much ethylene is made from ethane, but ethylene production seems to be the primary use of ethane. – Michael DM Dryden Apr 27 '16 at 17:33
• Given that the C2H4 production process basically consists of smashing hydrocarbons up with high temperatures, then quenching & separating the results, it doesn't care that much what the feedstock is. C2H6 isn't often deliberately separated from CH4, due to expense of low temperature distillation. It's becoming more available, because where liquefied natural gas is made, it's desirable to have pure CH4 for a single boiling point. But C2H6 isn't "extremely useful" in C2H4 synthesis, it's just not much use for anything else. Much of the C2H6 used in C2H4 synthesis comes from the recycle stream. – Level River St Apr 27 '16 at 17:59
• @LevelRiverSt, I think you're understating the role of ethane in ethylene production. There are world scale crackers that run almost exclusively on ethane. Others use an ethane/propane mix to get a little more propylene and heavier products. Most new crackers being built in the US are made for light and not heavy feedstocks. Unlike heavy feedstocks with their myriad products, ethane cracks to very few products (methane, H2, ethylene) with good yields. – Byron Wall Apr 29 '16 at 0:16
• @ByronWall it seems US used 51% ethane in 2005, while the world used 25.9%, a significant geographical difference. images.pennwellnet.com/ogj/images/ogj3/9613jch02.gif Old style plants get 30-40% C2H4 from heavy feed. That's OK as all other products are saleable. Post-cracking separation cost is high, but not as high as separating out C2H6 feed from mixed gases, or they'd have done it. New plants use more C2H6, because LNG has given high C2H6 availability, effectively separating C2H6 from CH4 "for free" so better yield but less saleable byproduct. C2H6 is nice to have, not essential. – Level River St Apr 29 '16 at 2:31

In both cases, there appears to be a confusion of terminology between common and technical uses.

We commonly use methane and propane for cooking (and home heating), but not ethane. I would expect ethane to be suitable for this, being in between the two, but I've never heard of anyone using it for this purpose. Why is that?

In reality, anyone using natural gas as a cooking fuel likely is cooking both with $$\ce{CH4}$$ and $$\ce{C2H6}$$. From the above-linked Wikipedia page (emphasis added):

Natural gas is a naturally occurring hydrocarbon gas mixture consisting primarily of methane, but commonly including varying amounts of other higher alkanes, and sometimes a small percentage of carbon dioxide, nitrogen, hydrogen sulfide, or helium.

EngineeringToolbox.com reports the following representative composition ranges (probably in percent by volume?) of natural gas:

$$\text{Composition (%)} \\ \begin{array}{cccccccccc} \hline & \ce{CO2} & \ce{CO} & \ce{CH4} & \ce{C2H6} & \ce{H2} & \ce{H2S} & \ce{O2} & \ce{N2} \\ \hline \text{Min} & 0 & 0 & 82 & 0 & 0 & 0 & 0 & 0.5 \\ \text{Max} & 0.8 & 0.45 & 93 & 15.8 & 1.8 & 0.18 & 0.35 & 8.4\\ \hline \end{array}$$

Given that $$\ce{CH4}$$ is by far the major constituent of natural gas, it is sensible that it is referred to commonly by the term methane, even if it is often actually a mixture of methane, ethane, and trace higher hydrocarbons.

On a related note, why is butane used for cigarette lighters and basically nothing else (in ordinary life, I mean)?

Per the Wikipedia page for liquefied petroleum gas, linked in Mithoron's comment, most of what is commonly referred to as propane or butane is actually a mix of $$\ce{C3H8}$$ and $$\ce{C4H10}$$ in varying ratios (emphasis added):

Liquefied petroleum gas or liquid petroleum gas (LPG or LP gas), also referred to as simply propane or butane, are flammable mixtures of hydrocarbon gases used as fuel in heating appliances, cooking equipment, and vehicles. ... Varieties of LPG bought and sold include mixes that are mostly propane ($$\ce{C3H8}$$), mostly butane ($$\ce{C4H10}$$) and, most commonly, mixes including both propane and butane. In the northern hemisphere winter, the mixes contain more propane, while in summer, they contain more butane.

So, Mithoron is right: $$\ce{C4H10}$$ is used in much more than just cigarette lighters, it's just that common usage happens to apply the term butane for this context.

As a further note, I would guess the primary rationale for using different mixes of $$\ce{C3H8}$$/$$\ce{C4H10}$$ deals with the vapor pressures of the two gases. The energy densities $$\eta$$ of the liquefied gases, approximated as $$-\Delta H_c^\circ\rho \over \mathrm{MW}$$, are nearly equal:

$$\begin{array}{ccccc} \hline \text{Quantity} & \text{Units} & \ce{C3H8} & n\text{-}\ce{C4H10} & iso\text{-}\ce{C4H10} \\ \hline \Delta H_c^\circ & \mathrm{kJ\over mol} & -2202^1 & -2878^2 & -2869^3\\ \mathrm{MW} & \mathrm{g\over mol} & 44 & 58 & 58 \\ \rho & \mathrm{g\over mL} & 0.58^4 & 0.604^5 & 0.56^6 \\ \hline \eta & \mathrm{MJ\over L} & 29.0 & 30.0 & 27.7 \\ \hline \end{array}$$

Thus, roughly comparable energy value is obtained per volume of each, and there is little reason to favor one or the other on this basis.

Practically, the lower limit of acceptable vapor pressure is that which provides sufficient flow of gaseous hydrocarbon to the point of combustion. The upper limit is more or less defined by the strength of the container and plumbing. Consider the following vapor pressure data, calculated from fitted equations (sources: propane | n-butane | iso-butane):

$$\text{Vapor Pressure (atm)} \\ \begin{array}{ccc} \hline & 0~^\circ\mathrm C& 25~^\circ\mathrm C & 38~^\circ\mathrm C \\ \hline \ce{C3H8} & 4.7 & 9.3 & 12.8 \\ n\text{-}\ce{C4H10} & 1.0 & 2.4 & 3.5 \\ iso\text{-}\ce{C4H10} & 1.5 & 3.4 & 4.9 \\ \hline \end{array}$$

Cigarette lighters (especially disposable plastic ones) presumably do actually use butane-rich fuel mixes, so as not to approach or exceed the mechanical limits of the lightweight, portable containers. As well, the temperature at point-of-use is somewhat better controlled, as even on cold days the heat from the user's hand is likely to maintain the butane vapor pressure high enough to provide sufficient gas flow. Finally, as noted in a comment by A.K., lighters are generally charged with iso-butane, which is sensible as it is the isomer exhibiting modestly higher vapor pressures.

For applications where metal-walled containers are feasible (grilling, automotive fuel, etc.), however, structural considerations are less important and the higher deliverable pressure from propane becomes advantageous. In hot summer months, though, I would assume the higher fraction of butane is used so as to mitigate the fairly dramatic increase in vapor pressure of pure propane with increasing temperature.

$$^1$$ Wikipedia, "Propane (data page)"
$$^2$$ Wikipedia, "Butane (data page)"
$$^3$$ Wikipedia, "Isobutane (data page)"
$$^4$$ Engineering Toolbox, "Chemical, Physical and Thermal Properties of Propane Gas - $$\ce{C3H8}$$
$$^5$$ Engineering Toolbox, "Chemical, Physical and Thermal Properties of n-Butane"
$$^6$$ AeroPres, "Physical Properties" datasheet (PDF link)

• cigarette lighters use isobutane which has different physical properties than n-butane. – A.K. Apr 26 '16 at 20:54

Methane and ethane are very hard to be liqufied with respect to propane and butane. To make methane liquid requires highee pressures and this means more cost and danger.

But especially, n- butane may be easily liqufied and liquid phase covers less volume. This makes it more suitable for lighthers.

• This doesn't really add any information beyond the previous answers. – hBy2Py Jun 26 '17 at 11:12