Technical Information QVF Process Systems Ltd HPI

ydrochloric Acid urification

ntroduction

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 - pickling 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.

Figure 1 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 expended in purifying the contaminated 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 examine 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 HCI 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 incorporated in the absorption loop. To prevent HCI gas fumes 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.

PPAE 07

Technical Information Applications

The QVF hydrochloric acid purification system will operate on any hydrochloric acid up to 36% w/w concentration to give a pure product essentially of analytical reagent quality. While all non-volatile impurities are completely removed by the purification process, it cannot, however, fully cope with volatile impurities such as chlorine and organics. A proportion of these impurities will remain in the product acid.

Typical Results

Sample A Sample B

Before After Before After

Appearance

Clear yellow/brown with no deposit

Clear and colourless with no deposit

Turbid, yellow/ brown with heavy red floating deposit

Slightly turbid and colourless with no deposit

Specific gravity at 20C 1.145 1.148 1.154 1.157

Total acidity as HCI (% w/w) 29.8 29.5 30.0 30.8

Total halides as HCI (% w/w) 29.4 29.9 30.5 31.3

Excess halides as NH4CI (% w/w) 0.73 0.59 0.67 0.69

Total solids (ppm) 634 16 2,692 50

Sulphite (ppm) Absent Absent Absent Absent

Iron as Fe (ppm) 196 0.4 51.2 3.0

Bromide and Iodide (ppm) Complies with British Pharmacopoeia limits

Bromine as Br (ppm) Less than 30 Less than 30 Less than 30 Less than 30

This Technical Information leaflet supercedes all previous issues. QVF Process systems pursues a policy of continuous product improvement . we therefore reserve the right to alter any product or process as described and illustrated. C 2005 QVF Process Systems Ltd Tollgate Industrial Estate Stafford ST16 3HS England Tel +44 1785 609900 Fax +44 1785 609899 E-mail qvf@qvf.co.uk PPAE 07

QVF Process Systems LtdC 2005 QVF Process Systems LtdE-mail qvf@qvf.co.uk