Introduction - Principles of Chemical Separations with Environmental Applications - page 11

Petroleum. Sulfur can also be recovered from crude oil with technology that relies
on the reaction of hydrogen with sulfur-containing compounds in crude oil (hydrodesul-
furization) and permits modern refiners to turn 3 percent sulfur crudes into liquid product
with no more than 0.5 percent sulfur. About 26,000 tons of saleable by-product sulfur was
produced from crude oil in 1983.
Coal. Coal can be partially desulfurized before combustion. Washing and magnetic
separation are effective in reducing the content of iron sulfide, the principal inorganic
sulfur contaminant, by up to 50 percent or somewhat higher. However, there are also
organic sulfur compounds in coal, and a feasible means of removing them has not yet
been found. Accordingly, combustion of coal produces a flue gas that contains significant
amounts of sulfur oxides, which must be removed from the gas if sulfur pollution is to be
minimized.
Flue-gas scrubbers are proven but expensive separation devices for removing sulfur
from combustion gases. The new dry-scrubber technology removes about 90 percent of
the sulfur in a flue gas by contact with a lime slurry in a specially designed combination
spray dryer and reactor. The reaction product is a dry calcium sulfate-sulfite mix that is
environmentally benign. Larger users favor the wet-scrubber technology, which is capable
of removing up to 90 percent of the sulfur with a lime slurry in a contactor column.
Separations technology has made a substantial contribution to reducing the sulfur-
pollution problem associated with the burning of fossil fuels. The principal barrier to
further alleviation of this problem is economic and will respond to improved technology
gained through further research and development [1].

1.8 Remember

Environmental separations can apply to the clean-up of existing problems as well as
pollution prevention.
The cost of separations is directly related to the degree of dilution in the feed stream.
The three primary functions of separation processes are purification, concentration, and
fractionation.
Separations use thermodynamic equilibrium- and/or mass transfer (rate-) based analysis.

1.9 Questions

1.1 Give three examples of pollution sources for (a) water; (b) air; (c) soil.

1.2 Using the Sherwood plot, what is the price differential for a product contained in a

1% and a 0.001% feed stream?

1.3 Give two examples of a separation process that can be analyzed based on (a) ther-

modynamic equilibrium; (b) mass transfer (rate).