Page 238 - Chemical and process design handbook

Speight_Part II_C 11/7/01 3:08 PM Page 2.178

2.178 MANUFACTURE OF CHEMICALS
removed in purifying boxes, which contain iron oxide on wood shavings,
or by a solution of ethanolamine (Girbotol) in scrubbing towers (the best
present-day practice).
An alternative procedure uses ammonium phosphate (NH H PO ) to
2
4
4
absorb the ammonia to form more alkaline phosphates [(NH ) HPO and
4 2
4
(NH ) PO ] that are returned to the original form by steaming, thus releas-
4 3
4
ing the ammonia (the Phosam process).
Coal tar is a mixture of many chemical compounds, mostly aromatic,
which vary widely in composition and can be separated into various frac-
tions by distillation.
o
1. Light oils usually comprise the cut up to 200 C. They are first crudely
fractionated and agitated at a low temperature with concentrated sulfu-
ric acid, neutralized with caustic soda, and redistilled, furnishing ben-
zene, toluene, and homologs.
2. Middle oils, or creosote oils, generally are the fraction from 200 to
o
250 C, which contain naphthalene, phenol, and cresols. The naphtha-
lene settles out on cooling, is separated by centrifuging, and is purified
by sublimation. After the naphthalene is removed, phenol and other tar
acids (phenol and its homologs) are obtained by extraction with 10%
caustic soda solution and neutralization, or springing by carbon dioxide.
These are fractionally distilled.
o
3. Heavy oil may represent the fraction from 250 to 300 C, or it may be
split between the middle oil and the anthracene oil.
o
4. Anthracene oil is usually the fraction from 300 to 350 C. It is washed-
with various solvents to remove phenanthrene and carbazole; the remain-
ing solid is anthracene.

A substantial fraction of the coal tar produced continues to be used as
fuel as well as for road asphalt and roofing asphalt. For these purposes the
tar is distilled up to the point where thermal decomposition starts. This
base tar is then diluted with creosote oil to ensure satisfactory rapid dry-
ing. Similar tars are used to impregnate felt and paper for waterproofing
materials.
Largely because of the present competition from aromatic chemicals
produced from petroleum, interest in aromatics from coal is not as great as
it used to be. At one time coal tar was the sole source of pyridine; however,
synthetic processes using aldehyde arid ammonia are now supplying the
increased demand. This is also true of phenol.