Manufacture of ammonia by Haber's synthesis method

Ammonia (NH₃) is an important compound of nitrogen and hydrogen. It is produced by the natural decomposition of animal and vegetable bodies. The death and decay of plants and animals cause the nitrogen compounds present in them to get decomposed, giving ammonia. Ammonia also occurs in the soil in the form of ammonium salts.

Preparation of Ammonia

Ammonia is prepared by the following methods:

From ammonium chloride

Ammonia gas is usually prepared in the laboratory by gently heating ammonium chloride (NH₄Cl) and slaked lime [Ca(OH)₂].

Fig: 13.3 - Preparation of ammonia

Ammonia gas is lighter than air, necessitating its collection by the downward displacement of air. Because it is highly soluble in water it cannot be collected over it. Passing ammonia gas over quicklime (CaO) dries it. Being a basic gas, ammonia cannot be dried by passing it through concentrated sulphuric acid or phosphorus pentoxide (P₂O₅), as it reacts with them to form ammonium sulphate or ammonium phosphate respectively.

Calcium chloride also cannot be used for drying ammonia gas as it forms ammoniates with CaCl₂.

By the hydrolysis of metal nitrides

Hydrolyzing metal nitrides like magnesium and aluminium nitrides, with water or alkalies, can also produce ammonia gas.

Manufacture of Ammonia

Haber's process

The manufacture of ammonia by Haber's process involves the direct combination of nitrogen and hydrogen.

This reaction is, (a) reversible, (b) exothermic, and (c) proceeds with a decrease in volume. According to the Le Chatelier's principle, the favorable conditions for the formation of ammonia are,

Low temperature

The temperature should be remain as low as possible, (although at unusually low temperatures, the rate of reaction becomes slow). It has been found that the temperature, which optimizes the yield of ammonia for the reaction, is maximum at about 500°C.

High pressure

Since Haber's process proceeds with a decrease in volume, it is favored by high pressure. In actual practice, a pressure of 200 - 900 atmospheres is employed.

Catalyst

A catalyst is usually employed to increase the speed of the reaction. Finely divided iron containing molybdenum or alumina is used as a catalyst. Molybdenum or alumina (Al₂O₃) acts as a promoter and increases the efficiency of the catalyst. A mixture of iron oxide and potassium aluminate has been found to work more effectively.

Fig: 13.4 - Manufacturing plant employed in Haber's process

Source of raw materials

The nitrogen and hydrogen gases used as the raw material in Haber's process are obtained as follows.

• Nitrogen is obtained from the liquid air and hydrogen from water by electrolysis.

• Hydrogen may be obtained from water gas (mixture of CO and H₂) by Bosch process.

• Water gas can be obtained by passing steam over red hot coke.

By bubbling the mixture through water, CO₂ is removed.

• A mixture of nitrogen and hydrogen may be obtained by treating a mixture of producer gas (CO + N₂), water gas (CO + H₂) with steam in the presence of ferric oxide - chromium oxide catalyst at 450°C.

Carbon dioxide is removed by bubbling through water under pressure.

Plant

The plant, which manufactures ammonia, has the following components and processes.

Compressor

A mixture of nitrogen and hydrogen is compressed to 200-900 atmosphere pressure, in the ratio 1:3 (by volume). The compressed gas is sent to ammonia converter.

Fig: 13.5 - Ammonia converter

Converter

Ammonia converter is made from chrome-vanadium steel. It is usually 1.3 meter high and 1 meter in diameter. The converter is provided with a heat exchanger in the upper portion and the catalyst is packed in the central portion of the converter. There is an arrangement for heating the gas mixture. After the gas mixture enters through the inlet at the bottom, the gases circulate around the catalyst maintained at 450-500°C and then pass through to the heat exchanger. The gases finally enter the catalyst chamber to give ammonia. Before entering the condensers the product as well as the unreacted gases pass through the pipes of the heat exchanger and transfer their heat to the incoming gas mixture containing nitrogen and hydrogen.

Condensers

This cools and liquefies ammonia. The condensed ammonia, called 'liquor ammonia' is filled into cylinders under pressure.

Re-circulating pump

Some of the nitrogen and hydrogen gases escape condensation and are re-circulated through the converter.