Co2 h2 CO h2o

or as its reverse. Since running the reaction in the direction shown consumes H2 and produces CO, and the opposite result is obtained by running the reaction in reverse, the initial 3:1 or 1:1 ratio of H2 to CO can be altered to 2:1 by the partial conversion of the excess material, whether it is H2 or CO, into the other, deficient material.

For example, consider the adjustment of the 3:1 ratio produced by the reaction of methane with steam to the required 2:1 ratio. Call the initial molar amount of CO produced a; then the initial amount of H2 is 3a. Since the hydrogen is initially in excess, some of it must be converted to CO; thus the appropriate direction for the shift reaction is indeed the forward direction written above. When this reaction achieves equilibrium, a molar amount x of hydrogen will have been consumed and an additional molar amount x of carbon monoxide will have been produced:

C02 + H2-» CO + H2O

from initial reaction: 3a a at new equilibrium: 3a — x a + x

The value of x is obtained by requiring that the new, equilibrium ratio of H2 to CO be 2:1:

By algebraic manipulation of this equation, the ratio of x to a can be obtained:

Thus the fraction x/3a of the initial amount of H2 from the natural gas that must be converted to CO is 1/9.

The two chemical reactions that when combined correspond to the conversion of methane in the correct 2:1 ratio are shown and added together below; it has been assumed for simplicity that a = 1:

CH4 + HzO-> CO + 3 H2

1/3 H2 + 1/3 C02->1/3 CO + 1/3 H20

sum CH4 + 2/3 H20 + 1 /3 C02-» 4/3 CO + 8/3 H2

When combined with a catalyst, the CO and H2 from the sum of these reactions will yield 4/3 mole of CH3OH, giving the net overall reaction

CH4 + 2/3 H20 + 1/3 COz —^ 4/3 CH3OH

PROBLEM 8-8

In order to synthesize a compound with the empirical formula CH30 (and no other products) starting from methane and steam, what ratio of H2 to CO would be required? What fraction of the hydrogen gas produced from the reaction of methane and steam would have to be converted to carbon monoxide using the water-gas shift reaction to accomplish the transformation?

Research is currently under way to find how to directly convert methane into methanol in a much more efficient manner than that described above. Most of the difficulty stems from the fact that methane is a very unreactive substance: Its C—H bond dissociation energy is highest of all the alkanes. Once one C—H bond is broken, however, the molecule becomes highly reactive because the other C—H bonds are weakened, and in the presence of oxygen it tends to oxidize completely to carbon dioxide rather than partially to a useful intermediate stage such as methanol.

Methanol can also be produced by combining carbon dioxide and hydrogen gases (see Figure 8-10) in the presence of a suitable catalyst:

C02(g) + 3 H2(g) ^^ CH3OH(l) + H20(1)

Since this reaction is only slightly exothermic, most of the fuel energy of the hydrogen is present in the methanol product. Based on equilibrium considerations, methanol formation would be favored by low temperatures and high pressures; research has centered on finding a catalyst that will operate efficiently under such conditions without being deactivated. Low temperatures also prevent the formation of carbon monoxide rather than methanol.

Some of the massive quantities of C02 that are released annually into the atmosphere could be used as reactants in this process. Indeed, methanol produced in this manner could be considered a renewable fuel provided that the hydrogen is produced without the consumption of a fossil fuel, e.g., by solar energy (see below).

Although methanol can be used as a vehicular fuel on its own, there are chemical reactions by which it can be converted to gasoline. Similarly, synthesis gas itself can be converted to gasoline, thereby allowing the production of this fuel from either natural gas or coal (Figure 8-10). Currently, neither of these processes, nor the production of fuel methanol itself, is sufficiently efficient that it can compete economically with gasoline produced from crude oil.

Ethers

Methanol can be used to produce dimethyl ether, CH3—O—CH3, which has been tested as a replacement for diesel fuel in trucks and buses:

2 CH3OH-> CH3OCH3 + HzO

This ether is nontoxic and degrades easily in the atmosphere—in fact, it is used as a propellant in spray cans. Since its molecules contain no C—C bonds, soot-based particulate matter is produced in its combustion in only very small quantities (see Chapters 3 and 5) compared to those obtained from diesel fuel. The NOx emissions from dimethyl ether combustion are also lower than usually found for diesel engines.

Methanol is also used to produce the oxygenated gasoline additive MTBE, which stands for methyl tertiary-butyl ether, the structure of which is illustrated below:

methyl tert-butyl ether (MTBE)

MTBE, octane number 116, is used in some North American and European unleaded gasoline blends—up to 15%—to increase their overall octane number and to reduce carbon monoxide (and unburned hydrocarbon) air pollution; the reason is that, like the alcohols, it is an "oxygenated" fuel that generates less CO during its combustion than would the hydrocarbons it replaces. The advantages to using MTBE rather than ethanol as an additive are that it has a higher octane number and does not evaporate as readily. However, as with alcohols, its combustion can also produce more aldehydes and other oxygen-containing air pollutants than result from the hydrocarbons it replaces.

The usage of MTBE has become controversial. It has an objectionable odor resembling turpentine and ether. Another problem associated with MTBE is its contamination of well water, which has now occurred at many sites in the United States. The sources of MTBE in well water are leaking underground fuel tanks; leaking pipelines; and spillage of gasoline at gas stations, in vehicle accidents, and by homeowners. In contrast to the hydrocarbon components of gasoline, MTBE is rather soluble in water and therefore is quite mobile in soil and groundwater. It is also quite resistant to biological degradation because its carbon chains are very short; its half-life is on the order of years. Various U.S. states and the U.S. EPA have set action levels for MTBE in drinking water at a few tens of parts per billion, values that are exceeded in some supplies and at which the odor of the additive is apparent. Because of concerns about well-water contamination, California and several other states have now banned the use of MTBE in gasoline, and its use as a gasoline additive has dropped sharply, having been replaced by ethanol, isooctane, and other high-octane substances.

Continue reading here: Biodiesel

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