Moisture measurements in dissociated ammonia for metallurgical processes

Hydrogen gas is widely used for the bright hardening of many kinds of metals. Both the hydrogen purity and levels of humidity in the gas need to be measured and controlled. Too much moisture can reduce the strength and quality of the finished products.

There are two main delivery methods of hydrogen for this process - bulk hydrogen from storage cylinders and cracked ammonia.

Both delivery methods have advantages and disadvantages. Pure hydrogen is ready to use but is more expensive to buy and storing the large quantities needed for the application poses a fire risk.

Ammonia (NH₃) is cheaper to buy in bulk and not highly flammable which makes it safer to store on site. However, it has risks of its own: it is highly corrosive, so plant equipment and human safety are key concerns. Despite this, cracked ammonia plants are now the more common method of providing a reducing/hardening atmosphere for metallurgical furnaces.

The disassociated ammonia is a mixture of Hydrogen and Nitrogen and is called ‘forming gas’.

How does ammonia cracking work?

Pressurised liquid ammonia is heated to vaporise it. It is then passed over a nickel catalyst at a temperature of around 1000 °C, which causes it to dissociate into its component parts: hydrogen and nitrogen. The chemical equation for this reaction is:

2NH₃ à → N₂ +3H₂

The diagram below illustrates the cracking process:

Caption: Ammonia cracking process

As a result of complete dissociation into hydrogen and nitrogen, very little undissociated ammonia remains and the dew-point temperature of the resulting gas should be very low (well below -30°C).

A molecular sieve absorbs the final traces of uncracked ammonia still present in the gas. The gas may also be dried further using a heated-regeneration twin column desiccant dryer and the gas finally leaves the system dryer than -65°Cdp, consisting of 75 Vol% hydrogen and 25 Vol% nitrogen.

Applications for forming gas (disassociated ammonia)

The forming gas is used in conveyer furnaces and in tube furnaces for annealing processes in a reducing atmosphere, such as brazing, sintering, de-oxidation, and nitrization.

Monitoring trace moisture in disassociated ammonia

Any Michell Instruments impedance type hygrometer may be used for either on-line or spot check moisture measurements in the forming gas mixture of hydrogen and nitrogen. Michell's cost-effective Easidew range is suitable for measurements in locations with no hazardous area classification. This is common in many furnace applications.

If a measurement location is declared as hazardous under an ATEX directive, the Michell Easidew PRO I.S Dew-point Transmitter, or the Michell MDM300 I.S. high-speed portable hygrometer are both suitable.

All the above instruments are fully factory calibrated with traceability to UK and International standards and offer high accuracy, long term reliability and ruggedness, backed up by a first class after sales service that includes the option for extended maintenance agreements and exchange sensor services.

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