Design of the Haber process - Higher
The aim of any industrial process is to produce as much A substance formed in a chemical reaction. as possible, as fast as possible and as economically as possible.
Flow chart of the Haber Process
Industrial processes can be summarised using a flow chart.
The flow chart shows:
- stages in the process
- products
- An product from a side reaction that happens at the same time as the main reaction that is not needed. By-products reduce the yield of a process.
- waste
A by-product is an unwanted product that has been produced by a side reaction that happens at the same time as the main reaction. The formation of by-products reduces the The mass of product made in a chemical reaction. The percentage yield is a measure of the yield obtained compared to the maximum possible yield. of a process.
The The industrial chemical process that makes ammonia by reacting nitrogen and hydrogen together. uses two Basic material that goods are made from such as crops, metals, wood and animal products such as wool and leather., nitrogen from the air and hydrogen which is produced from natural gas and steam (step 1).
When the ammonia is collected, it is separated from any unreacted nitrogen and hydrogen. These two gases are then recycled (step 5). This reduces the amount and therefore cost of the raw materials needed for the process.
Selection of reacting conditions
The selection of suitable reaction conditions is an important part of the process design. They help to obtain an acceptable The mass of product made in a chemical reaction. The percentage yield is a measure of the yield obtained compared to the maximum possible yield. of ammonia in an acceptable time.
The Haber process involves a A chemical reaction which can go both ways. that could reach Describing the position attained in a reversible reaction when the proportions of reactants and products stay the same. The term 鈥榙ynamic鈥 emphasises that the forward and the backward reactions continue to occur (at the same rate). if left long enough. The conditions need to move the A measure of the relative concentrations of substances in an equilibrium, showing if there are more reactants or products at equilibrium. to the right but still keep a rate of reaction that is fast enough. The conditions must not cost too much to maintain. The conditions must also be safe.
N2(g) + 3H2(g) 鈬 2NH3(g)
(forward reaction is Reaction in which energy is given out to the surroundings. The surroundings then have more energy than they started with so the temperature increases.)
Controlling temperature
Increasing the temperature, increases the rate of reaction, but if the temperature is increased, the equilibrium position moves in the direction of the Reaction in which energy is taken in. reaction. This means it moves to the left in the Haber process and reduces the yield of ammonia.
If a low temperature is chosen to move the equilibrium position to the right as far as possible, the rate of reaction is too slow. So a compromise temperature of 450掳C is chosen. This is:
- low enough to achieve an acceptable yield
- high enough to do this in an acceptable time
Controlling pressure
If the Force exerted over an area. The greater the pressure, the greater the force exerted over the same area. is increased, the equilibrium position moves in the direction of the fewest A collection of two or more atoms held together by chemical bonds. of gas. This means it moves to the right in the Haber process.
A very high pressure would move the equilibrium position as far right as possible. However, it is expensive to achieve very high pressures. Stronger equipment is needed, and more energy is needed to compress the gases. So a compromise pressure of 200 A unit of pressure. is chosen. This is:
- low enough to keep costs down
- high enough to achieve an acceptable yield
Using a catalyst
A substance that changes the rate of a chemical reaction without being changed by the reaction itself. do not change the yield of products in reversible reactions. However, they do increase the rate of reaction. Iron is a cheap catalyst used in the Haber process. It helps to achieve an acceptable yield in an acceptable time.