Page 237 - Chemical and process design handbook

Speight_Part II_C 11/7/01 3:08 PM Page 2.177

COAL CHEMICALS 2.177
densable water vapor, tar, and light oils, and solid particles of coal dust,
heavy hydrocarbons, and complex carbon compounds. Significant prod-
ucts recoverable from the vapors include: benzene, toluene, xylenes, cre-
osote oils, cresols, cresylic acid, naphthalene, phenols, xylols, pyridine,
quinoline, and medium and hard pitches usable for electrode binders, road
tar, or roofing pitch.
The gas is passed into the primary condenser and cooler at a tempera-
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ture of about 75 C. Here the gases are cooled by water to 30 C. The gas is
conducted to an exhauster, which serves to compress it. During the com-
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pression the temperature of the gas rises as high as 50 C. The gas is passed
to a final tar extractor or an electrostatic precipitator. On leaving the tar
extractor or precipitator, the gas carries three-fourths of the ammonia and
95 percent of the light oil originally present.
The gas is led to a saturator containing a solution of 5 to 10% sulfuric
acid where the ammonia is absorbed, and crystalline ammonium sulfate is
formed. The gas is fed into the saturator through a serrated distributor
underneath the surface of the acid liquid. The acid concentration is main-
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tained by the addition of sulfuric acid, and the temperature is kept at 60 C
by the reheater and the heat of neutralization. The crystallized ammonium
sulfate is removed from the bottom of the saturator by a compressed-air
ejector, or a centrifugal pump, and drained on a table, from which the
mother liquor is run back into the saturator.
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The gas leaving the saturator is at about 60 C; it is taken to final coolers
or condensers, where it is scrubbed with water until its temperature is
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25 C. During this cooling, some naphthalene separates and is carried along
with the wastewater and recovered. The gas is passed into a light oil or ben-
zol scrubber, through which the absorbent medium, a heavy fraction of
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petroleum known as straw oil, or sometimes tar oil, is circulated at 25 C.
The straw oil is sprayed into the top of the absorption tower while the gas
flows upward through the tower. The straw oil is allowed to absorb about
2 to 3 percent of its weight of light oil, with a removal efficiency of
about 95 percent of the light oil vapor in the gas.
The rich straw oil, after being warmed in heat exchangers by vapors
from the light-oil still and then by light oil flowing out of the still, is passed
to the stripping column where the straw oil, flowing downward, is brought
into direct contact with live steam. The vapors of the light oil and steam
pass off and upward from the still through the heat exchanger previously
mentioned and into a condenser and water separator. The lean, or stripped,
straw oil is returned through the heat exchanger to the scrubbers. The gas,
after having been stripped of its ammonia and light oil, has the sulfur