Methanol

Methanol, CH3OH, is a colorless liquid that, like ethanol, is somewhat less dense than water. Although methanol was produced in the past from the destructive distillation of wood, giving rise to its historical name wood alcohol, it is now produced mainly from a fossil fuel.

Methanol can be blended with gasoline to produce a fuel that bums more cleanly than gasoline. In a labeling scheme analogous to that used for ethanol-gasoline m lxturc s, blends of methanol are designated by an M rating; thus Ms corresponds to 5% methanol and 95% gasoline.

One disadvantage to methanol blends is that the pure alcohol is only soluble to the extent of about 15% in gasoline, corresponding to M15; greater amounts of methanol form a second layer rather than dissolve. The inadvertent presence of water causes this unacceptable phase separation to occur at an even smaller percentage of methanol. Additives such as tertiary-butyl alcohol (2-methyl-2-propanol) that are soluble in both methanol and gasoline prevent such separations from occurring. Looking at things from the other direction, gasoline is moderately soluble in methanol, so fuel blends such as M85 have been tested and are now on sale in limited quantities. Another difficulty is that methanol cannot be used in conventional automobile engines because it reacts with and corrodes some engine and fuel tank components.

Some concern has been expressed about the safety of methanol for use as a vehicular fuel, given its toxicity. Methanol-water solutions have been widely used as windshield washer liquids in northern climates for many years without much environmental impact. The use of methanol as a fuel may be more dangerous, as it would involve a very high concentration of the alcohol. Ethanol is much less toxic than either methanol or gasoline.

However, alcohols also possess some advantages: They are inherently high-octane fuels, and indeed, methanol is used to power all the cars at the Indy 500 races. Methanol has the added advantage that it does not produce a fireball when a tank-rupturing crash of racing cars occurs: It vaporizes less rapidly than does gasoline, and, once formed, the vapor disperses more quickly.

PROBLEM 8-6

Given that the enthalpies of combustion, per mole, of methanol and ethanol are —726 and —1367 kj and that the density of each is 0.79 g/mL, calculate the heat released by methanol and by ethanol (a) per gram and (b) per milliliter. From your results, comment on the superiority of one alcohol or the other with respect to energy intensity based on weight and volume. Are these alcohols superior or inferior to methane as fuels in terms of energy intensity per gram? (See Problem 7-4 for data.) How do they compare to gasoline, for which about 43 kj are released per gram?

The conventional conversion of either coal or natural gas into methanol begins with the reaction of the fossil fuel with steam to produce a mixture of CO and H2, often called synthesis gas:

In the first process, steam is blown over white-hot coal; in the second, methane gas is combined with steam that has been heated to about 1000°C. These methods produce synthesis gas from nonrenewable raw materials. An analogous mixture of hydrogen and carbon monoxide can also be obtained by heating renewable sources of biomass such as wood or the cellulosic component of garbage. The wood is first chipped and then gasified. The gaseous product is a mixture of CO, C02, and H2, The use of wood gasification to generate methanol or electricity out of biomass is highly promising, for both produce useful energy without significant greenhouse gas emissions. The various processes for synthesis gas and methanol production are summarized in schematic form in Figure 8-10.

Methanol is synthesized from a 2:1 molar ratio of Ii2 to CO in the presence of a catalyst:

Unfortunately, existing catalysts allow only a partial conversion (about one-fifth) of the gases into methanol for each pass of the gas mixture over the catalyst, and the processes are energy-intensive and require relatively high temperatures. Research is under way to develop catalysts that will operate at lower temperatures and thereby allow higher yields.

FIGURE 8-10 Scheme for the production of fuels in a hydrogen economy.

PROBLEM 8-7

The enthalpies of formation of CO(g) and CH3OH(l), respectively, are — 110.5 and —239.1 kj/mol. Calculate the enthalpy of the reaction that forms methanol from synthesis gas. From your answer, predict whether the equilibrium amount of methanol obtained will increase or decrease as the temperature is lowered. Given your result, comment on the interest in developing low-temperature catalysts.

The correct 2:1 molar ratio of H2 to CO required for the methanol synthesis reaction above is rarely obtained initially from the raw materials. For example, the reaction of steam with coal instead gives synthesis gas with a 1:1 ratio, and with natural gas a 3:1 ratio is obtained. The ratio can be adjusted to the required 2:1 by subjecting the mixture to the water-gas shift reaction, which is an equilibrium that can be written as catalyst

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