Page 143 - Chemical process engineering design and economics

Process Circuit Analysis 127

2H 2 + CO -> CH 3OH (-21,685 cal, 298 K) (3.5.2)
Methanol formation is exothermic, requiring removal of the enthalpy of re-
action. Thermodynamically, the conversion to methanol increases by reacting at
low temperatures. Also, there is a reduction in the number of moles during reac-
tion, according to Equation 3.5.2, indicating that the converter should operate at a
high pressure to increase conversion.
The Imperial Chemical Industries (ICI) has developed a reactive copper
oxide catalyst [28], which allows operating the converter at low pressures, around
100 arm. Even though a high pressure increases conversion, a low pressure saves
on gas compression and material of construction costs. The zinc-oxide, chromic-
oxide catalyst, developed early in the history of the process, requires temperatures
well above 300 °C for a reasonable rate of reaction, but conversions are low. To
compensate for this lower catalytic activity, the converter pressure must be at 200
arm or higher. Because the reactivity of the new copper-oxide catalyst is high, the
converter temperature can be lowered, favoring a high thermodynamic conver-
sion. Sulfur containing compounds, however, easily poison the copper-oxide cata-
lyst. Furthermore, iron pentacarbonyl forms by reaction of carbon monoxide with
iron, but the reaction is less favored at low temperatures and pressures. Therefore,
carbon steel instead of the more expensive stainless steel can be used for piping,
reactors, and other process equipment.
Besides methanol formation, side reactions also occur, forming high mo-
lecular weight alcohols, dimethyl ether, carbonyl compounds, and methane. Be-
cause of the numerous side products formed, these compounds are divided into
two groups, called the low-boiling and high-boiling compounds. No methane
forms in the converter [31].
According to Equation 3.5.2, methanol synthesis requires a ratio of two
moles of hydrogen to one mole of carbon monoxide, whereas Equation 3.5.1
shows that steam reforming produces a ratio of three to one. Thus, the excess hy-
drogen, as well as the inert gases (methane and nitrogen), will accumulate in the
process and must be removed. One way of removing the excess hydrogen is to
add carbon dioxide to the reformer feed gas to react with the hydrogen according
to Equation 3.5.3.

2CO 2 + H 2 -> CO + H 2O (9,855 cal, 298 K) (3.5.3)

Equation 3.5.3 is called the reverse-shift reaction because it occurs opposite
to the normal direction. Carbon dioxide will react with hydrogen in the converter
according to Equation 3.5.4 to form methanol.

-»CH 3OH + H 2O (-11,830 cal, 298K) (3.5.4)
CO 2 + 3H 3
Another way of removing the excess hydrogen and inert gases is to use a
purge stream. Unless carbon dioxide is available at low cost, purging is usually
employed [28]. Because the purge stream is combustible, it may be used as a fuel

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