Haber Process

The Haber process, called the Haber Bosch process, is the leading industrial process for producing ammonia from nitrogen and hydrogen. In this process, nitrogen from the air combines with hydrogen derived from natural gas methane (CH₄).

N2 (gas) +    3H2 (gas) --→ 2NH3 (gas)
(Nitrogen)   (Hydrogen)   (Ammonia)

• This is a reversible reaction means ammonia can be broken down into hydrogen and nitrogen.
• The reaction is exothermic
• Delta H = -92.4KJ/mol

The Haber process perfectly illustrates how industrial chemists use their knowledge to find the best conditions needed to produce a good yield of product (ammonia).

The following are the raw materials required for the process-

• Air, which supplies the nitrogen.
• Hydrogen and the energy supplied by natural gas and water.
• Iron as a catalyst.

Requirements for the Haber Process

1. The Catalyst:

The catalyst is slightly more complicated than pure iron. It has potassium hydroxide added to it as a promoter - a substance that increases its efficiency. Pure iron catalysts are not very effective. When used as a catalyst, iron always contains a small percentage of several other oxides like Al₂O₃, ZrO₃, SiO_2, etc.

Mechanism of Catalyst

• Effect of Catalyst on Equilibrium: The catalyst does not affect the position of the equilibrium. Adding a catalyst produces no greater percentage of ammonia in the equilibrium mixture. Its only function is to speed up the reaction.
• Effect of Catalyst on the Reaction Rate: Without a catalyst, the reaction is so slow that virtually no reaction happens at an observed time. The catalyst ensures that the reaction is fast enough for a dynamic equilibrium to be set up quickly while the gases are in the reactor.

2. The Pressure:

The pressure varies from one manufacturing plant to another but is always high. The minimum required pressure for ammonia production is 200 atmospheres. According to Le Chatelier's Principle, if there is an increase in the pressure, the system will respond by favoring the reaction, which produces more ammonia molecules. In order to get as much ammonia as possible in the equilibrium mixture, the pressure needs to be as high as possible. Two hundred atmospheres are the minimum required pressure.

• Effect of Pressure on the Reaction Rate: Increasing the pressure brings the molecules close to each other. In this instance, it will increase their chances of hitting and sticking to the catalyst's surface, where they can react. The higher the pressure, the better in terms of the rate of a gas reaction.
• Economic Aspects of Pressure: Very high pressures are expensive to produce on two counts: i) Strong pipes and containment vessels are required to withstand high pressure, which increases the capital costs when the plant is built. ii) High pressures cost a lot to produce and maintain, which means that the running costs of your plant are very high.

3. The Temperature:

• Effect of Temperature on Equilibrium: The equilibrium must be shifted to the right (forward direction) to obtain maximum ammonia. The forward reaction is exothermic. The temperature should be low to obtain the maximum possible ammonia.
• Effect of Temperature on the Reaction Rate: The lower the temperature, the slower the reaction becomes because the kinetic energy of particles decreases. The gases must reach equilibrium quickly to obtain a more significant amount of ammonia.

Steps of the Haber Process

1) Hydrogen Production:

Methane is the primary source of hydrogen. Hydrogen is obtained by reacting steam with natural gas. Hydrogen can also be obtained from cracking oil fractions. Following are the steps for hydrogen production.

• Remove sulfur compounds from the feedstock because sulfur deactivates the catalysts used in subsequent steps. Sulfur removal requires catalytic hydrogenation to convert sulfur compounds in the feedstocks to gaseous hydrogen sulfide; this is known as hydrodesulfurization or hydrotreating
  H₂ + RSH --→ RH + H₂S (gas)
• Hydrogen sulfides are adsorbed and removed by passing it through beds of zinc oxide, where it is converted to solid zinc sulfide:
  H₂S + ZnO --→ ZnS + H₂O
• Catalytic steam reforming is a reaction of methane with a stream in the presence of a catalyst to form carbon oxide and hydrogen:
  CH₄ + H₂O --→ CO + 3H₂
• Catalytic shift conversion converts the carbon monoxide to carbon dioxide and more hydrogen:
  CO + H₂O --→ CO₂ + H₂
• Carbon dioxide is removed by absorption in aqueous ethanolamine solutions or by adsorption in pressure swing adsorbers (PSA) using proprietary solid adsorption media.
• The final step in producing hydrogen is to use catalytic methanation to remove residual carbon monoxide or carbon dioxide.
  CO + 3H₂ --→ CH₄ + H₂O
  CO₂ + 4H₂ --→ CH₄ + 2H₂O

2) Ammonia production:

The technological improvement has resulted in significant improvement in the yield of ammonia.

3H₂ + N₂ → 2NH₃

This step is known as the ammonia synthesis loop.

According to the Le-Chattelier principle, ammonia production is favored by high pressure and low temperature. From the equilibrium data measured, it was obvious that the reaction temperature should be kept well below 300 degrees Celsius and pressure above 200 atmospheres to obtain a higher percentage of ammonia.

3) Recycling:

At each pass of the gases through the reactor, only about 15% of the nitrogen and hydrogen are converted to ammonia. (This figure also varies from plant to plant.) The overall conversion is about 98% by continually recycling the unreacted nitrogen and hydrogen.

The Kinetics of the Haber Process

Transition metals are always used to catalyze the Haber process. The Haber process involves two important factors. An effective catalyst and a transition metal. The catalyst is said to be an effective catalyst when it can easily bind to nitrogen gas. Without the crucial binding of the catalyst, nitrogen remains inert and unreactive, producing no ammonia. The Haber process has been optimized extensively but still uses enormous energy. To cover this high energy consumption, quantum mechanics is used. It (Quantum mechanics) predicts the reaction mechanics and kinetics for NH₃ synthesis on Fe (iron). Fe is the best catalyst for NH₃ synthesis. High pressure is favored in the Haber process. Haber Bosch Kinetics typically operates at 400°C temperature and 150-250 bar pressure. The Reversible reaction is highly favored at higher NH₃ partial pressure, which lowers conversion by 15% in a single pass, whereas recycling unreacted reactants increases the conversion to 97%. Overall, this is a remarkable conversion. The rate of uncatalyzed reaction is observed to be effectively zero. Therefore, the catalyst is required for the reaction to occur at an appreciable rate.

Catalyst

1/2 N₂ + 3/2 H₂ --------------> NH₃

Originally an osmium catalyst was used in the Haber process. Fe catalyst was used with Al₂O₃, K₂D, CaO, and SiO₂ on a FeO support. The catalyst particles consist of the inner core of magnetite (Fe3O4), a secondary layer of FeO, and an Outer Fe layer.

The Thermodynamics of the Haber Process

The catalyst only affects the reaction's spontaneity. This is due to a fundamental aspect of physical chemistry: the kinetics and thermodynamics of a reaction often remain independent of each other. Though we may deduce that a highly thermodynamically favored reaction occurs faster than one less favored, exceptions exist. The Haber process is an exception, a reaction with favorable thermodynamics but incredibly slow kinetics.

Aside from the spontaneity, it can also be observed that the reaction has an adverse change in entropy (Delta S < 0), specifically when four moles of reactant gas produce two moles of product gas.

N₂+ 3H₂ -----> 2NH₃

A reduction in the moles of gas indicates that the system has low energy, indicating decreasing entropy. It also justifies the negative change by indicating that fewer gas molecules mean more "disorder".

The reaction must have a decrease in enthalpy for the reaction to be spontaneous despite the decreasing entropy. This is the case: The Haber process is exothermic (Delta H< 0). According to the definition of Gibbs free energy, spontaneity must increase as temperature decreases. Put differently, decreasing temperature makes Delta S less significant, making Delta G more negative:

Delta G= Delta H-T delta S

(In this equation, T indicates the temperature at which the reaction takes place)

The next section mechanism of the Haber process will be discussed, which takes into account the transport, absorption, desorption, and exchange reactions.

Mechanism of the Haber Process:

The Haber process mechanism undergoes seven elementary steps for the synthesis of ammonia.

1. Transport of the reactant from the gas phase through the boundary layer (a thin film of liquid over a solid surface) to the surface of the Catalyst.
2. Pore diffusion to the reaction center. Pore diffusion is defined as the phenomenon where adsorbate (a substance that is adsorbed) is adsorbed into the interior of the adsorbent (a substance that adsorbs another substance) particles.
3. Adsorption of reactant.
4. Reaction
5. Desorption of product
6. Transport of the product through the pore system.
7. Transport of the product into the gas phase.

The sequential performance of each step is necessary for a good yield of ammonia. The first and last two steps that are steps one, six, and seven, are fast compared to the adsorption reaction and desorption reaction, steps four and five, respectively; this is because of the shell structure of the Catalyst. Various investigations have proved that the dissociation of nitrogen is the rate-determining step of ammonia synthesis.

The exchange reaction between deuterium and hydrogen takes place at room temperature. Since the absorption of both molecules is rapid, it cannot determine the speed of ammonia synthesis.

Apart from the reaction condition, the absorption of nitrogen on the catalyst surface depends on the microscopic structure of the Catalyst. Different Catalysts have different reactivity. For Example, Fe (III) and Fe (2II) have the highest activity reason being that only Fe (III) and Fe (2II) surfaces have C7 situs. [C-7 situs means these Catalyst surfaces have iron atoms with seven closest neighbors.

In the Haber process, two operation envelopes carry out ammonia synthesis mechanisms. There are two operation envelopes; lower operation envelope and higher operation envelope. These operation envelopes are designed according to the processing plant.

Based on these experimental findings, the reaction mechanism is believed to involve the following steps.

1. N₂ (g) → N₂ (adsorbed)
2. N₂ (adsorbed) -->2 N (adsorbed)
3. H₂ (g) -->H₂ (adsorbed)
4. H₂ (adsorbed) --> 2 H (adsorbed)
5. N (adsorbed) + 3 H (adsorbed) → NH₃ - (adsorbed)
6. NH₃ (adsorbed) → NH₃ (g)

Reaction 5 occurs in three steps, forming NH, NH2, and NH3. Experimental evidence points to reaction two as being a slow, rate-determining step. This is not unexpected since the bond is broken; the nitrogen triple bond is the strongest of the bonds that must be broken.