3.2 AMMONIA SYNTHESIS PLANTS

NH3 synthesis plants have been extensively applied in industry particularly over the last twenty years. It has been noted that the trend is for bigger and bigger plants while processes have been developed with lower and lower synthesis pressures. At present the most common size of plant ranges between 800 and 1800 metric tons per day and synthesis pressures vary between 180 and 260 bars depending on the process. 

An NH3 synthesis plant is outlined very schematically in figure. 3.4. The processed materials are natural gas (most common), water or air.

Natural gas compressed to approx. 40 bars by compressor C1 is injected into the desulphurization plant to protect the catalysts used in the process from the sulphur. The gas is then sent to the first reformer where steam is injected; with a catalyst it slowly combusts producing H2, CO, CO2 and considerable quantities of methane.

The mixture of hot gases then passes to the second reformer where it is mixed with heated air at 30-40 bars

from compressor C2. On account of the presence of air combustion in the second reformer is rapid. It may be expressed as follows:

CH₄ + O₂ + N₂ ←^→ 2H₂ + CO₂ + N₂ (exothermic reaction)

The heat developed by this reaction which raises the temperature of the reagents to around 1000°C is used to produce the steam required for the process that is generated in exchanger S1 downstream from the second reformer.

The gas mixture then flows into a separator where the carbon monoxide is removed. It is cooled to eliminate excess water.‘ vapor and finally put into a separator to eliminate the carbon dioxide CO₂.

To purify the mixture of any remaining CO or CO₂ methanization follows whereby carbon residues are hydrogenated to produce methane and water.

At this point of the process a mixture of H₂ and N₂ is obtained containing a small percentage of methane which forms the synthesis gas mixture. This mixture is then brought to a pressure, which may vary according to the type of process used, in order to make the synthesis reaction shift more easily:

N₂ + 3H₂ IE ZNH₃ (exothermic reaction)

Since the reaction is exothermic here too the heat is recovered downstream from the reactor to generate steam.

The ammonia is condensed next by cooling with a chiller plant; a refrigerating compressor CF is used for this using ammonia as the refrigerant fluid for obvious reasons. Finally, in separator S the ammonia that has formed is separated and the gas that has not transformed into ammonia is remixed with the fresh gas and recycled through compressor CR.

It has therefore been seen how several types of centrifugal compressor are used in an ammonia synthesis plant processing gas differing in nature, pressure and temperature; the characteristics of these machines are therefore briefly outlined herebelow.

3.2.1 Natural gas compressors

Compression takes place in one or two casings reaching a final pressure of approx. 40 bars. In units with two casings there is a low pressure compressor with a horizontally split casing, MCL type, and a high pressure compressor with a barrel type casing open vertically at both ends (see figure. 3.5).

 3.2.2 Air compressors

A final pressure of 30-35 bars is normally reached with two horizontally split compressor casings (see figure. 3.6).

Each compressor casing houses two back-to-back compression stages with external cooling. This arrangement is preferred for the lesser number of compressor casings and lower absorbed power. 

3.2.3 Synthesis compressors

A train of synthesis compressors is composed of a minimum of two casings (see figure. 3.7) and maximum of four depending‘on plant dimensions and the processes used, i.e. the pressure ratios to be achieved.

Barrel type compressors are employed incorporating either a single stage (BCL compressor) or two stages (2BCL compressor) in a back-to-back arrangement.

The recycled gas is normally compressed by a single impeller positioned in the end compressor casing back-to-back with the impellers that compress the fresh gas.

It should be noted that these compressors were the first centrifugals widely applied in industry in the 60's for high pressures.

3.2.4 Refrigerating compressors

This service, which uses NH3 as refrigerant fluid, is normally performed with one or two low pressure compressor casings (see figure. 3.8).