Page 214 - Chemical engineering design

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FLOW-SHEETING
From the compositions given, calculate the stream flows for the production of
absolute alcohol from 100 kmol/h raw alcohol feed, composition 89 per cent
ethanol, balance water. Take the benzene losses to total 0.1 kmol/h. Draw a prelim-
inary flow-sheet for the process.
All the compositions given are mol percentage.
4.4. A plant is required to produce 10,000 tonnes per year of anhydrous hydrogen
chloride from chlorine and hydrogen. The hydrogen source is impure: 90 per cent
hydrogen, balance nitrogen.
The chlorine is essentially pure chlorine, supplied in rail tankers.
The hydrogen and chlorine are reacted in a burner at 1.5 bar pressure.
H 2 C Cl 2 ! 2HCl

Hydrogen is supplied to the burner in 3 per cent excess over the stoichiometric
amount. The conversion of chlorine is essentially 100 per cent. The gases leaving
the burner are cooled in a heat exchanger.
The cooled gases pass to an absorption column where the hydrogen chloride gas is
absorbed in dilute hydrochloric acid. The absorption column is designed to recover
99.5 per cent of the hydrogen chloride in the feed.
The unreacted hydrogen and inerts pass from the absorber to a vent scrubber where
any hydrogen chloride present is neutralised by contact with a dilute, aqueous
solution, of sodium hydroxide. The solution is recirculated around the scrubber.
The concentration of sodium hydroxide is maintained at 5 per cent by taking a
purge from the recycle loop and introducing a make up stream of 25 per cent
concentration. The maximum concentration of hydrogen chloride discharged in
the gases vented from the scrubber to atmosphere must not exceed 200 ppm (parts
per million) by volume.
The strong acid from the absorption column (32 per cent HCl) is fed to a stripping
column where the hydrogen chloride gas is recovered from the solution by distil-
lation. The diluted acid from the base of this column (22 per cent HCl), is recycled
to the absorption column.
The gases from the top of the stripping column pass through a partial condenser,
where the bulk of the water vapour present is condensed and returned to the
column as reflux. The gases leaving the column will be saturated with water
Ž
vapour at 40 C.
The hydrogen chloride gas leaving the condenser is dried by contact with concen-
trated sulphuric acid in a packed column. The acid is recirculated over the packing.
The concentration of sulphuric acid is maintained at 70 per cent by taking a purge
from the recycle loop and introducing a make up stream of strong acid (98 per
cent H 2 SO 4 ).
The anhydrous hydrogen chloride product is compressed to 5 bar and supplied as
a feed to another process.
Using the information provided, calculate the flow-rates and compositions of the
main process streams, and draw a flow-sheet for this process.
There is no need to calculate the reflux flow to the distillation column; that will
be determined by the column design.