EHSQL(Environment-Energy ,Health,Safety, Security and Social Ac. ,Quality-Lab) Technical services: Why Acid Clean?

Sunday, 2 December 2012

Why Acid Clean?

Hardwater scale in boilers, heat exchangers, cooling towers and pipework reduces their efficiency, in a case such as steam boilers, can cause the boiler to be dangerous to operate. Acid cleaning is the only satisfactory method of restoring the heat transfer efficiency by complete removal of the deposits.
Scale, which is mostly calcium and magnesium, acts as an insulator in between the metal surface of a boiler and the water, and will result in an increase in the temperature of the boiler metal. With enough scale, the metal overheats, causing blisters and eventually a ruptured pressure part. The effect on the boiler efficiency can be seen from the following table:

Thickness of Boiler Deposit (mm)	Fuel Wasted (%)
0.85	7
1.0	9
1.3	11
1.6	13
2.3	15
2.8	16

What Is Involved In Acid Cleaning?

Acid cleaning involves the circulation of acid at low concentrations around the equipment that requires scale removal. The acid used will depend on the type of deposit and material of manufacture of the equipment being descaled. The acids used typically are Hydrochloric, Sulphamic, Citric, Formic, Hydrofluoric and Phosphoric. Specific corrosion inhibitors can be used with these acids where necessary in order to ensure that the integrity of the equipment being acid cleaned is maintained
The procedure utilises specialised equipment designed for chemical cleaning and is as follows:

A sample of the deposit is analysed to establish the most suitable acid to be used
The equipment to be cleaned is isolated to ensure that the cleaning solution cannot escape and contaminate other equipment, pipes or water courses
Pumps are connected for recirculation
The equipment is filled with acid to the required strength
Corrosion inhibitor is added to prevent acid attack on any exposed metal
The descale is monitored for acid strength and further acid added if required
The procedure is stopped when the acid strength remains constant, which indicates that no more scale removal is taking place, or by visual examination if this is possible.
The acid cleaning solution is removed to drain and neutralised at the same time
The equipment is refilled with water with a small amount of alkaline solution to neutralise any residual acid

A BRIEF ABOUT CITRIC ACID
At room temperature, citric acid is a white crystalline powder. It can exist either in an anhydrous (water-free) form or as a monohydrate. The anhydrous form crystallizes from hot water, while the monohydrate forms when citric acid is crystallized from cold water. The monohydrate can be converted to the anhydrous form by heating above 78 °C. Citric acid also dissolves in absolute (anhydrous) ethanol (76 parts of citric acid per 100 parts of ethanol) at 15 °C.
In chemical structure, citric acid shares the properties of other carboxylic acids. When heated above 175 °C, it decomposes through the loss of carbon dioxide and water (see decarboxylation).
Citric acid is a slightly stronger acid than typical carboxylic acids because the anion can be stabilized by intramolecular hydrogen-bonding from other protic groups on citric acid.

Cleaning and chelating agent

Citric acid is an excellent chelating agent, binding metals. It is used to remove limescale from boilers and evaporators. It can be used to soften water, which makes it useful in soaps and laundry detergents. By chelating the metals in hard water, it lets these cleaners produce foam and work better without need for water softening. Citric acid is the active ingredient in some bathroom and kitchen cleaning solutions. A solution with a 6% concentration of citric acid will remove hard water stains from glass without scrubbing. In industry, it is used to dissolve rust from steel. Citric acid can be used in shampoo to wash out wax and coloring from the hair.
Illustrative of its chelating abilities, citric acid was the first successful eluant used for total ion-exchange separation of the lanthanides, during the Manhattan Project in the 1940s. In the 1950s, it was replaced by the far more efficient EDTA. It can be used to slow setting of Portland cement. It can delay setting time substantially.