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Hydrochloric acid is one of the commonest chemicals available today and is used in a wide variety of applications, some of the most important being:

. Metal cleaning - picking acid.
. Petroleum industry - activation of oil well heads.
. Food industry - production of sugars, dextrose, mono-sodium glutamate and starch syrups.
. Chemical synthesis/manufacture - organic chlorides, for example, vinyl chloride.
. Leather/tanning industry.

 

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 re-use.

Purification can be simply achieved using the QVF equipment shown in figure 1.

Process Description

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 restarted.