Ammonia Cracking Hydrogen Gas Plant

Principle

Ammonia is a compound of nitrogen and hydrogen with the formula NH3. It is a colourless gas with a characteristic pungent odour. Although in wide use, ammonia is both caustic and hazardous. In 2006, worldwide production was estimated at 146.5 million tons. It is used in commercial cleaning products.

Ammonia, as used commercially, is often called anhydrous ammonia. This term emphasizes the absence of water in the material. Because NH3 boils at - 33.34 deg C, the liquid must be stored under high pressure or at low temperature

Ammonia is comprised of two elements i.e Nitrogen and Hydrogen. An Ammonia dissociator is a system designed to separate or crack anhydrous ammonia into its constituent components; to "dissociate" the elements from each other. The resulting gas consists of 25% nitrogen and 75% hydrogen. Two associated characteristics of ammonia dissociation which are of extreme importance are that clean, metallurgical grade, anhydrous ammonia contains virtually no water and no oxygen. When ammonia vapor is dissociated the resulting gas mixture is extremely dry, with a dew point of typically -40 F to -60 F. Gas this dry usually requires no further dehydration prior to use and the absence of oxygen ensures the prevention of scale and discoloration on the finished product.

An ammonia dissociator is a remarkably simple, durable and efficient machine which achieves virtually 100% dissociation of a clean, dry supply of anhydrous ammonia with very few moving parts and only the energy necessary to complete the dissociation process.

Process

Clean and anhydrous ammonia vapour (less than 2ppm water; 2ppm oil and no sulfur) is passed through a vertical retort which is filled with Nickel Catalyst. The retort is put inside a furnace either electrically heated or gasoline fired. The temperature of the furnace is maintained in range of 1750 - 1900 deg F. Ammonia dissociates at this temperature in presence of catalyst. A mixture of 75% Hydrogen and 25% Nitrogen comes out from the dissociator (Cracker). The temperature of the cracked gas is brought down to nearly ambient temperature in a heat exchanger. 

Design Features

The retort chamber is surrounded by high performance ceramic insulation. The dissociator well instrumented. The heating elements provide greater heat density. It is very compact, easier to install or replace. Ammonia dissociators are usually configured to run with an internal system pressure of about 5 psi. We can build (and have built) high pressure systems with retorts designed to operate up to 30 psi but discourage high pressure systems unless there is no alternative. The primary reason not to go to a high pressure system is that the dissociation process is slowed under elevated pressure and the residual ammonia levels in a high pressure system are usually greater than those in a low pressure system. Another consequence associated with high pressure systems is the cost attendant to building a retort designed to constrain the high pressure in a high temperature environment

Specifications

• Capacity          -1 to 3000 Nm3/hr
• Composition    -  75% Hydrogen + 25% Nitrogen
• Pressure           -  0.5 to 8 Bar (g)
• Dew Point       -  (-) 40 dee C to   (-) 80 deg  C         

Applications

The cracked or dissociated gas is used in the bright annealing of high or low carbon steels, chrome tool steels, stainless steels, nickel alloys, copper and copper alloys where a high hydrogen content is necessary in the furnace atmosphere. Its also used to provide an environment in which the sintering of powdered metal is done or to copper braze ferrous metals or to braze stainless steel without having to use special brazing alloys or brazing fluxes. It is used for the purification of other gases such as nitrogen. It is also being used in fuel cell .