9.2 Haber and contact process

9.2 Haber and Contact Process

The Haber Process

  • Used to produce ammonia (NH3)

  • Characteristics of ammonia:

    • A colorless gas

    • Has a strong choking smell

    • Less dense than air

    • Very soluble in water

    • Solution is alkaline (turns red litmus paper blue)

Conditions for the Haber Process

  • The production of ammonia is a reversible reaction, requiring specific conditions to reach equilibrium.

  • The equilibrium composition of ammonia varies with temperature and pressure.

Pressure and Temperature Graph (Figure 9.9)

  • A wide range of conditions for temperature and pressure were tested for the Haber process.

  • Graph illustrates the yields of ammonia at different temperature and pressure combinations.

Increasing Ammonia Yield

  • Increasing pressure and decreasing temperature shifts equilibrium to the right, enhancing ammonia production.

A. Changing the Pressure

  • Higher pressure favors the side of the equilibrium with fewer gas molecules (right side).

  • Modern industrial plants use approximately 20,000 kPa; higher pressures are costly and risky (explosion hazard).

  • Higher pressure accelerates the reaction rate to reach equilibrium.

B. Changing the Temperature

  • Forward reaction is exothermic; increasing temperature reverses the reaction (produces less ammonia).

  • Lowering temperature favors the exothermic direction, increasing ammonia yield.

  • However, this also slows the reaction rate, so an optimum temperature (450 °C) is chosen to balance yield and rate.

C. Reducing the Concentration of Ammonia

  • When ammonia is partially removed from the system, more is produced to restore equilibrium.

D. Use of a Catalyst

  • Divided iron acts as a catalyst, speeding up both forward and reverse reactions to reach equilibrium faster.

Industrial Plant for the Haber Process

  • Nitrogen Sources:

    • Obtained from the air through fractional distillation or reacting oxygen with hydrogen.

  • Hydrogen Sources:

    1. Steam reforming of natural gas:

    • Methane reacts with steam.

    • Nickel catalyst employed.

    • Carbon monoxide (CO) is removed to prevent catalyst poisoning.

    1. Cracking of ethane:

    • High temperatures and catalysts used.

The Haber Process Operation Stages

  1. Clean the reactant gases (remove contaminants).

  2. Mix nitrogen and hydrogen gases in a 1:3 ratio and compress to 20,000 kPa.

  3. Pass gases over catalyst beds containing finely divided iron.

  4. Maintain temperature at approximately 450 °C.

  5. Recognize that the reaction is reversible and does not proceed to completion.

  6. Achieve about 15% ammonia proportion in the mixture.

  7. Ammonia condenses easily due to its higher boiling point than nitrogen and hydrogen.

  8. Unchanged nitrogen and hydrogen gases are recirculated over the catalyst.

  9. This recirculation enables an eventual yield of 98%; ammonia is stored as liquid under pressure.

Essential Conditions for the Haber Process

  • Mixing nitrogen (N2) and hydrogen (H2) in a ratio of 1:3.

  • Optimum temperature set at 450 °C.

  • Pressure maintained at 20,000 kPa (200 atmospheres).

  • Catalyst: finely divided iron.

Uses of Ammonia from the Haber Process

  • Approximately:

    • 75% used for nitric acid production.

    • 10% utilized in fertilizers.

    • 10% in nylon production.

    • 5% for other chemical applications.

The Contact Process

  • An industrial method for sulfuric acid production.

Main Reaction in the Contact Process

  • Combines sulfur dioxide (SO2) with oxygen (O2) to create sulfur trioxide (SO3).

  • The reaction is exothermic; lowering temperature favors SO2 production.

  • A compromise temperature of 450 °C is selected to maintain reaction speed and efficiency.

Catalyst and Pressure in the Contact Process

  • Catalyst: Vanadium (V) oxide facilitates efficient equilibrium.

  • Pressure of 200 kPa aids in maximizing SO2 production due to fewer molecules involved.

Industrial Plant for the Contact Process

Raw Materials Needed

  1. Sulfur (S): Burned in air to produce SO2.

    • Reaction: S(s) + O2(g) → SO2(g)

  2. SO2: Mixed with excess air and passed over Vanadium (V) oxide catalyst at 450 °C and 2 atmospheric pressure.

    • Reaction: 2SO2(g) + O2(g) → 2SO3(g)

  3. Sulfur trioxide (SO3): Dissolved in concentrated sulfuric acid to form oleum (H2S2O7).

  4. Oleum: Mixed carefully with water to produce sulfuric acid (H2SO4).

    • Reaction: H2S2O7(g) + H2O(l) → 2H2SO4(aq)

Important Hint

  • SO3 is dissolved in concentrated sulfuric acid instead of water to avoid creating acid mist and to manage the highly exothermic reaction resulting from combining SO3 and water.

Uses of Sulfuric Acid

  1. Fertilizers (e.g., ammonium sulfate).

  2. Paints, pigments, and dyes.

  3. Fibers and plastics.

  4. Soaps and detergents.

  5. Car batteries.

Questions

  • Exam question subjects include:

    1. The effect of pressure and temperature on equilibrium positions for nitrogen and oxygen reactions.

    2. The raw material sources for hydrogen and sulfur dioxide in the Haber and Contact processes.

    3. Conditions optimal for the Haber process and their impacts on ammonia production.

    4. The rationale behind recycling unreacted gases in the Haber process.