Introduction - Principles of Chemical Separations with Environmental Applications - page 10

striking projections by 2100 if the present trends continue:
greenhouse gases could exceed 700 ppm levels not seen for 50 million years
average atmospheric temperature will rise by 2 to 6.5 ◦ F(1to3.5 ◦ C), ex-
ceeding the rate of change for the last 10,000 years
sea levels could rise between 6 to 37 inches (0.15 to 1 m).
1997 Kyoto Protocol agreement reached. This agreement is the first global approach
to controlling greenhouse gas emissions.
Separations technology is already making an important contribution to ameliorating
the acid-rain problem. Wet-scrubbing processes are the most widely used systems for
removal of sulfur and nitrogen compounds from effluent stack gases. The limits of cost for
wet-scrubbing techniques are such that they are not used to remove more than 75 percent
of the sulfur-oxide compounds present and are currently of only limited effectiveness
for removal of nitrogen oxides. Such systems also produce large quantities of sludge that
present a solids disposal problem. New reagent systems that can be used in a more effective
recycling mode are needed, and would be particularly useful if they could simultaneously
remove both sulfur and nitrogen compounds in forms from which they could be converted
into useful products. In any case, effective approaches must be brought into use to remove
the nitrogen compounds.

1.7 The sulfur problem: where separations can help

Our principal sources of energy - fossil fuels - are all contaminated to some extent with
sulfur compounds. When these fuels are burned, the sulfur compounds are burned to sulfur
oxides, which are emitted to the atmosphere in the flue gas. In the atmosphere, these oxides
are converted into the sulfur acids that are a principal cause of acid rain.
Separations technology plays a critical role in limiting sulfur-oxide pollution from
sulfur-bearing fossil fuels. This technology is sufficiently advanced that there are no
inherent technological limits to removing more than 95 percent of the sulfur present in
natural gas, crude oil, and coal - many processes exist for accomplishing this before,
during, or after combustion. The principal barriers to nearly complete sulfur removal are
cost and practicality.
Natural Gas. The principal sulfur contaminant of natural gas is another gas -
hydrogen sulfide. Because it is extremely toxic, civil authorities have long forbidden
significant levels of this compound in natural-gas pipelines. Hydrogen sulfide is removed
from natural gas by a variety of commercial processes including reaction with aqueous
solutions of oxidants, absorption into aqueous solutions of bases, distillation, and selective
permeation through membranes. The end product of these processes is elemental sulfur,
which can be sold and, in some cases, is worth more than the co-produced natural gas. In
1984, about 24,000 tons (24 million kilograms) of sulfur was produced from natural-gas
wells in the United States.