Industrial Production of Ammonia

Technical and Economic Importance of Ammonia

Ammonia, $ \mathrm{NH_3} $, is one of the most important bulk chemicals produced worldwide. Its main uses are:

• Fertilizer production
• Directly as anhydrous ammonia (injected into soil)
• Converted to ammonium salts (e.g. ammonium nitrate, ammonium sulfate)
• Precursor for urea, $ \mathrm{(NH_2)_2CO} $, and other nitrogen fertilizers
• Intermediate for other chemicals
• Nitric acid production (via oxidation of ammonia)
• Cyanides, hydrazine, amines, and many specialty chemicals
• Refrigerant and reducing agent
• Refrigeration in large industrial systems
• Reductant in flue gas cleaning (e.g. selective catalytic reduction, SCR)

Because of its central role in fertilizers, ammonia production capacity is directly related to global food production and is often cited as a key example of how chemical engineering influences population carrying capacity.

Modern ammonia plants are large-scale, continuous processes, usually integrated with natural gas processing or other sources of hydrogen and energy.

Overall Reaction and Process Concept

The synthesis of ammonia is based on the reaction between nitrogen and hydrogen:

$$
\mathrm{N_2(g) + 3\,H_2(g) \rightleftharpoons 2\,NH_3(g)} \qquad \Delta H^\circ < 0
$$

Key features:

• Nitrogen is typically obtained from air (separation of $\mathrm{N_2}$ from $\mathrm{O_2}$ and other gases).
• Hydrogen is obtained from hydrogen-rich feedstocks, most commonly natural gas (methane), but also naphtha, coal, or even water (electrolysis).
• The reaction is exothermic and accompanied by a decrease in gas moles, so high pressure and relatively low temperature favor ammonia at equilibrium.

In practice, a compromise between equilibrium yield, reaction rate, and technical constraints leads to operation at:

• Pressures of roughly $100$–$300\ \mathrm{bar}$
• Temperatures of roughly $400$–$550^\circ\mathrm{C}$
• With a solid heterogeneous catalyst

This industrial synthesis is known as the Haber–Bosch process.

Feedstock Preparation and Hydrogen Production

The industrial production of ammonia is dominated by processes that generate hydrogen from fossil feedstocks and combine it with nitrogen.

Natural Gas-Based Hydrogen Production

The most common route today uses natural gas, mainly methane ($\mathrm{CH_4}$), as both a source of hydrogen and a fuel for process heat:

1. Desulfurization

Sulfur compounds in natural gas poison catalysts and must be removed:

• Hydrogenation of organic sulfur to hydrogen sulfide:
  $$
  \mathrm{R\!-\!S + H_2 \rightarrow H_2S + RH}
  $$
• Absorption of $\mathrm{H_2S}$ in solid or liquid sorbents (e.g. zinc oxide):
  $$
  \mathrm{H_2S + ZnO \rightarrow ZnS + H_2O}
  $$

2. Primary Steam Reforming

Methane reacts with steam at high temperature ($\sim 800$–$900^\circ\mathrm{C}$) over a nickel catalyst in tube reactors:

$$
\mathrm{CH_4 + H_2O \rightleftharpoons CO + 3\,H_2} \quad \Delta H^\circ > 0
$$

• Strongly endothermic (requires heat input).
• Heat is typically provided by burning additional fuel in the furnace surrounding the reformer tubes.

3. Secondary Reforming (with Air or Oxygen)

To provide nitrogen and further convert methane, air (or oxygen-enriched air) is introduced:

Partial oxidation and reforming reactions occur, for example:

• Partial oxidation:
  $$
  \mathrm{CH_4 + \frac{1}{2}O_2 \rightarrow CO + 2\,H_2}
  $$
• Further reforming with steam:
  $$
  \mathrm{CH_4 + 2\,H_2O \rightarrow CO_2 + 4\,H_2}
  $$

This step:

• Provides the nitrogen that will later be used to form ammonia.
• Raises the gas temperature through exothermic reactions.
• Reduces residual methane content.

The resulting gas mixture contains $\mathrm{H_2, N_2, CO, CO_2, H_2O}$, traces of methane, and argon from air.

4. Water-Gas Shift Conversion

The water-gas shift reaction increases the hydrogen yield:

$$
\mathrm{CO + H_2O \rightleftharpoons CO_2 + H_2} \quad \Delta H^\circ < 0
$$

This typically occurs in two adiabatic catalyst beds:

• High-temperature shift (e.g. $\sim 350$–$450^\circ\mathrm{C}$, iron-based catalyst)
• Low-temperature shift (e.g. $\sim 200$–$250^\circ\mathrm{C}$, copper-based catalyst)

After shifting, the gas is richer in $\mathrm{H_2}$ and poorer in $\mathrm{CO}$, but still contains large amounts of $\mathrm{CO_2}$.

5. Carbon Dioxide Removal

$\mathrm{CO_2}$ is removed because:

• It is inert in the ammonia synthesis step and would reduce efficiency.
• It forms carbamates and other deposits in the cold parts of the plant.

Common gas treating methods include:

• Absorption in amine solutions (e.g. monoethanolamine, MEA)
• Physical solvents under pressure (e.g. methanol in the Rectisol process)

The treated gas (often called synthesis gas or syngas) now contains mainly $\mathrm{H_2, N_2, CO, CO_2}$ traces, $\mathrm{CH_4}$, and rare gases.

6. Removal of Carbon Monoxide and Residual CO₂

Even trace amounts of $\mathrm{CO}$ and $\mathrm{CO_2}$ can poison the ammonia synthesis catalyst (iron-based). Final purification often uses:

• Methanation:
  $$
  \mathrm{CO + 3\,H_2 \rightarrow CH_4 + H_2O}
  $$
  $$
  \mathrm{CO_2 + 4\,H_2 \rightarrow CH_4 + 2\,H_2O}
  $$

These reactions convert $\mathrm{CO}$ and $\mathrm{CO_2}$ to methane, which is almost inert under ammonia synthesis conditions.

The result is a purified gas mixture with an approximately stoichiometric ratio:

$$
\mathrm{N_2 : H_2 \approx 1 : 3}
$$

plus small amounts of inert gases (argon, methane).

Alternative Hydrogen Sources

Though less common at present, there are alternative ways to supply hydrogen:

• Coal gasification: coal is converted to synthesis gas ($\mathrm{CO + H_2}$), followed by shift and purification.
• Electrolysis of water: electricity is used to split water into $\mathrm{H_2}$ and $\mathrm{O_2}$; combined with air separation to obtain nitrogen, this enables low-carbon or “green” ammonia if renewable electricity is used.

Process details for these alternatives differ, but the ammonia synthesis loop itself remains essentially the same.

Nitrogen Supply

Nitrogen is typically obtained from air by:

• Cryogenic air separation (distillation of liquefied air), which can deliver high-purity nitrogen and oxygen as separate products.
• In-process separation as part of secondary reforming (introducing air directly into the reformer so that its nitrogen ends up in the synthesis gas).

The purity requirements are strict: oxygen and moisture must be removed almost completely, as they would oxidize catalysts or form unwanted by-products.

The Ammonia Synthesis Loop (Haber–Bosch)

Once a clean, stoichiometric mixture of $\mathrm{N_2}$ and $\mathrm{H_2}$ is prepared, it enters the ammonia synthesis loop. This closed loop is central to plant operation and efficiency.

Operating Conditions and Catalysts

Typical conditions:

• Pressure: about $100$–$300\ \mathrm{bar}$
• Temperature: about $400$–$550^\circ\mathrm{C}$

Catalyst:

• Historically and still widely used: iron-based catalyst (promoted magnetite, e.g. $\mathrm{Fe_3O_4}$ reduced in situ to metallic iron), with promoters such as $\mathrm{K_2O, Al_2O_3, CaO}$.
• In some modern plants: ruthenium-based catalysts on suitable supports, which allow operation at somewhat lower pressure but require different materials and cost considerations.

The equilibrium conversion of $\mathrm{N_2 + 3H_2}$ to ammonia increases with:

• Lower temperature (due to exothermic reaction)
• Higher pressure (due to reduction in gas moles)

However, at too low a temperature, the reaction rate becomes too slow. Industrial operation is therefore optimized to balance thermodynamics and kinetics.

Reactor Design

Common designs include:

• Multi-bed fixed-bed reactors:
• Several catalyst beds in series, with intermediate cooling.
• Fresh feed plus recycle gas enter, react partially over each bed.
• Inter-bed heat exchangers remove the heat of reaction and may preheat incoming feed elsewhere in the process.
• The reactor shell must withstand high pressure and temperature, so special steels and liners are used.

Due to limited per-pass conversion (often only around $10$–$20\ \%$ of reactants to ammonia), recycling of unreacted gases is essential.

Ammonia Separation and Gas Recycling

After leaving the reactor:

1. The hot product gas is cooled.
2. Ammonia, which has a much higher condensation point than nitrogen and hydrogen, is condensed at high pressure and moderate temperatures:
• Liquid ammonia is separated and sent to storage.
• The remaining gas is mostly unreacted $\mathrm{N_2}$ and $\mathrm{H_2}$ plus inerts.
3. The unreacted gas stream is:
• Partly purged to remove inert gases (e.g. argon, methane) that would otherwise accumulate.
• Mostly recycled by a circulation compressor back to the reactor inlet.

This synthesis loop enables very high overall conversion of feed nitrogen and hydrogen to ammonia, despite relatively low single-pass conversion.

Energy Integration and Heat Management

Ammonia production is energy-intensive, but the plant is designed to recover and reuse as much energy as possible:

• Heat from exothermic reactions:
• Ammonia synthesis and shift reactions release heat.
• This heat is used to generate steam, preheat feeds, or drive turbines.
• Heat for endothermic reactions:
• Steam reforming requires large amounts of heat, usually provided by burning fuel in reformer furnaces.
• High-pressure steam cycle:
• Steam generated in exothermic sections powers steam turbines (e.g. for compressors) and provides process heat.

Careful heat integration reduces fuel consumption and improves overall efficiency. Modern plants can achieve significantly lower specific energy consumption (e.g. measured in $\mathrm{GJ}$ per tonne of ammonia) than older designs.

Process Control and Safety Aspects

Ammonia production involves high pressures, high temperatures, flammable and toxic substances, and large equipment.

Key safety-related issues include:

• High-pressure containment:
• Design of reactors, heat exchangers, and piping to resist stress, corrosion, and hydrogen embrittlement.
• Toxicity and corrosiveness of ammonia:
• Ammonia is toxic and irritating; leaks pose danger to personnel and environment.
• Proper sealing, detection systems, and emergency ventilation are essential.
• Flammable gases:
• Hydrogen and carbon monoxide mixtures are flammable and explosive over wide concentration ranges.
• Oxygen exclusion:
• Oxygen entering high-pressure hydrogen systems can create explosion hazards and damage catalysts.

Process control systems monitor:

• Temperatures and pressures in reactors and reformers
• Gas compositions (e.g. residual $\mathrm{CO}$, $\mathrm{CO_2}$, $\mathrm{O_2}$)
• Flow rates and ratios of $\mathrm{H_2}$ to $\mathrm{N_2}$

Safe, continuous operation demands robust instrumentation, interlocks, and emergency shutdown procedures.

Environmental and Sustainability Considerations

Traditionally, ammonia production has a large carbon footprint, mainly because:

• Most hydrogen is produced from fossil fuels (natural gas, coal).
• $\mathrm{CO_2}$ is released during reforming or gasification and often only partly captured and used.

Key aspects:

• CO₂ emissions:
• Each tonne of ammonia produced by conventional natural gas-based processes is associated with roughly 1–2 tonnes of $\mathrm{CO_2}$ emissions (order of magnitude; exact values depend on plant efficiency and integration).
• Some of this $\mathrm{CO_2}$ is used to make urea, but most is still released.
• Energy consumption:
• Ammonia plants are among the largest single consumers of natural gas and primary energy in many countries.

To improve sustainability, several strategies are pursued:

• Energy efficiency improvements:
• Better catalysts and reactor designs.
• Improved heat integration, reduced compression work.
• CO₂ capture and storage (CCS):
• Concentrated $\mathrm{CO_2}$ streams from reforming are relatively easy to capture, making ammonia plants potential early adopters of CCS.
• Green ammonia:
• Hydrogen from water electrolysis powered by renewable electricity.
• Nitrogen from air via renewable electricity-based air separation.
• In this case, the main emissions are associated with electricity generation; if that electricity is low-carbon, ammonia production can be nearly carbon-neutral.
• Alternative uses and energy systems:
• Ammonia is investigated as an energy carrier and carbon-free fuel (e.g. for power generation and shipping), which may change the design and geographical distribution of future plants.

Product Handling and Downstream Processing

Liquid ammonia is stored and handled under conditions that keep it in the liquid phase:

• At ambient temperature and moderate pressure, or
• At near-atmospheric pressure but low temperature (refrigerated tanks).

From the ammonia plant, the product typically goes to:

• Fertilizer plants, where ammonia is converted to:
• Urea
• Ammonium nitrate
• Ammonium phosphates, mixed fertilizers
• Nitric acid plants, where ammonia is oxidized to $\mathrm{NO}$ and ultimately $\mathrm{HNO_3}$.

Large-scale storage and shipping require:

• Suitable tank materials resistant to ammonia and low temperatures
• Safety systems for overpressure relief and leak detection
• Control of environmental emissions (e.g. limiting ammonia slip, odor control)

Summary of Key Features of Industrial Ammonia Production

• Central reaction: $\mathrm{N_2 + 3H_2 \rightleftharpoons 2NH_3}$, exothermic and equilibrium-limited.
• Feed preparation dominates the process:
• Hydrogen mostly from steam reforming of natural gas (or other feedstocks), followed by shift and gas purification.
• Nitrogen from air separation or direct air addition in reforming.
• Synthesis loop:
• High-pressure, moderate-temperature reactor with an iron or ruthenium catalyst.
• Partial per-pass conversion, with ammonia condensation and recycle of unreacted gases.
• Strong focus on heat integration, energy efficiency, and catalyst protection.
• Significant environmental impact, due to both energy use and $\mathrm{CO_2}$ emissions, driving interest in green hydrogen and low-carbon ammonia production pathways.