Page 234 - Chemical and process design handbook

Speight_Part II_C 11/7/01 3:08 PM Page 2.174

COAL CHEMICALS

Coal is an organic rock that can be converted by heat treatment into a vari-
ety of products (Fig. 1).
When coal is thermally pyrolyzed or distilled by heating without contact
with air, it is converted into a variety of solid, liquid, and gaseous products.
The nature and amount of each product depends upon the temperature used
in the pyrolysis and the variety of the coal used. In general practice, coke-oven
o
temperatures are maintained above 900 C but may range anywhere from 500
o
to 1000 C. The principal product by weight is coke.
o
If temperatures on the order of 450 to 700 C are employed, the process
o
is termed low-temperature carbonization; with temperatures above 900 C,
it is designated high-temperature carbonization. In the low-temperature
carbonization process, the quantity of gaseous products is small and that of
the liquid products is relatively large, whereas in high-temperature car-
bonization the yield of gaseous products is larger than the yield of liquid
products, the production of tar being relatively low.
For the same coal, low-temperature liquids contain more tar acids and
tar bases than high-temperature liquids. With high-temperature carboniza-
tion, the liquid products are water, tar, and crude light oil. The gaseous
products are hydrogen, methane, ethylene, carbon monoxide, carbon diox-
ide, hydrogen sulfide, ammonia, and nitrogen. The products other than
coke are collectively known as coal chemicals, or by-products.
The gas from the destructive distillation of the coal, together with
entrained liquid particles, passes upward through a cast-iron gooseneck
into a horizontal steel pipe, which is connected to a series of ovens. As the
gas leaves the ovens, it is sprayed with weak ammonia water to condense
some of the tar from the ammonia from the gas into the liquid. The liquids
move through the main along with the gases until a settling tank is reached,
where separation is effected according to density. Some of the ammonia
liquor is pumped back into the pipes to help condensation; the rest goes to
the ammonia still (Fig. 2), which releases the ammonia for subsequent
chemical combination in the saturator. All the tar flows to storage tanks for
tar distillers or for fuel.

2.174