In many of these applications,
contaminated HCI is discharged as a by-product
(spent acid), but due to the low cost and availability
of fresh supplies, little or no effort has been
expanded in purifying the HCI for re-use.
However, disposal costs associated with spent
HCI have been steadily rising and when they are
considered along with the cost of purchasing fresh
acid, it becomes practical to closely exmine the
economics of purification of the spent acid for
Purification can be simply achieved using the
QVF equipment shown in figure 1.
Crude acid (typically of concentration 20 to
35% w/w) is pumped to a constant head feed vessel
from where it passes, via a control valve and
flowmeter, to a reboiler with one or more steam
heat exchangers mounted in a thermosyphon loop
arrangement. The feed acid is then evaporated,
the vapour passing to a water cooled reflux condenser.
Due to the properties of aqueous hydrochloric
acid in forming an azeotrope containing about
20% acid and HCL gas, the condensed acid is fed
to the top of a packed column. Reabsorption of
the gas takes place in the column and any heat
of absorption is removed by a cooler at the base
of the column. Product acid is recirculated by
means of a glass centrifugal pump.
the recirculation/product ratio is about 20:1,
product acid being pumped off intermittently to
storage at a frequency governed by a liquid level
controller incoprated in the absorption loop.
To prevent HCI gas fumeds escaping from the column
vent, the feed of cooled 20% acid is used to wet
a small packed section above the main absorption
column. This effectively traps any HCI gas unabsorbed
by the circulating strong acid. When the impurity
level in the reboiler unit has built up to an
unacceptable level, it is drained and the operation