Industrial conditions and feedstocks for Haber
Using resources • The Haber process and fertilisers
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Basic reaction and equilibrium behaviour
The chemical equation is N2(g) + 3H2(g) ⇌ 2NH3(g). The forward reaction is exothermic and reduces total gas moles from four to two. Le Chatelier’s principle predicts that increasing pressure shifts the equilibrium toward ammonia because fewer gas moles occupy lower volume. Lowering temperature also favours the exothermic forward reaction and shifts equilibrium toward products.
Temperature: effect on equilibrium and rate
Higher temperature increases the reaction rate because collisions become more energetic and more molecules overcome activation energy. Higher temperature shifts the equilibrium toward the reactants because the forward reaction releases heat. Industrial practice therefore uses a moderate temperature (around 400–500°C) as a compromise: temperature remains high enough for a reasonable rate but not so high that the equilibrium yield of ammonia becomes too low.
Pressure: effect on equilibrium and equipment costs
Higher pressure shifts equilibrium toward ammonia because fewer gas molecules form on the product side. High pressure also increases collision frequency and therefore the reaction rate. Industrial plants operate at elevated pressures (commonly around 150–250 atmospheres) to increase yield. Higher pressure requires stronger, more expensive equipment and greater energy for compression, so the chosen pressure balances improved yield against increased capital and operating costs.
Catalyst: increasing rate without changing equilibrium
A catalyst increases the reaction rate by providing an alternative pathway with lower activation energy. An iron-based catalyst with promoters (for example potassium and aluminium oxides) is commonly used. The catalyst does not change the position of equilibrium, so it allows operation at lower temperatures than would otherwise be needed while maintaining useful rates.
Feedstocks: nitrogen source
Nitrogen is obtained from the air, which contains about 78% nitrogen by volume. Air separation methods such as fractional distillation of liquefied air or pressure swing adsorption supply the purified nitrogen stream. Air provides a very large and low-cost raw material source, so nitrogen availability is not usually the limiting factor.
Feedstocks: hydrogen source and trade-offs
Hydrogen is commonly produced by steam reforming of methane from natural gas: CH4 + H2O → CO + 3H2, followed by CO + H2O → CO2 + H2. Steam reforming gives abundant hydrogen at relatively low cost where natural gas is cheap, but it is energy-intensive and emits CO2. Electrolysis of water provides low-carbon hydrogen if electricity is low-carbon, but electrolysis is more expensive when electricity costs are high. Feedstock choice depends on local availability and the cost of energy and raw materials.
Availability, energy supply and economic constraints
Raw material availability and energy prices strongly influence industrial choices. Cheap natural gas makes steam reforming favourable. High electricity prices or limited low-carbon electricity make electrolysis less attractive. Energy demand for heating and compressing reactants increases operating costs. The industrial process therefore seeks the best balance between achievable ammonia yield, acceptable reaction rate and total cost of materials and energy.
Industrial compromise and safety limits
Operational conditions represent a compromise: moderate temperature for rate vs yield, high but economical pressure for improved equilibrium, and use of a catalyst to speed the reaction. High pressures and temperatures increase equipment stress and safety risk, so practical limits on materials and plant design also shape the chosen conditions. Continuous recycling of unreacted gases improves overall conversion efficiency.
Key notes
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