The Advantages and Disadvantages of Iron Catalyst in Haber Process

The manufacture of ammonia is crucial to use in fertilizer as it helps to maintain food production for billions of people worldwide. Nitrogen compound in fertilizer can consider as the most effective compost because plants need it to produce proteins. Besides, phosphorus and potassium elements frequently found in fertilizer. Fertilizer also essential to keep soils productive, thus help to sustain healthy crops.

Generally, ammonia can be produced via the Haber-Bosch process. The Haber-Bosch process is an industrial process for manufacturing ammonia from nitrogen and hydrogen reaction, using an iron catalyst at high temperature and pressure.

 BENEFITS OF IRON CATALYST 

Figure 1: Iron Catalyst

 

The significant advantage of using iron catalyst (figure 1) in the process is it readily available and plentiful, thus making the use of it on the metric ton scale feasible. The production can produce tons of ammonia per day by using a catalytic reaction. For example, 1800 tons ammonia per day requires a gas pressure of at least 130 bar, temperatures of 400 to 500 °C and a reactor volume of at least 100 m³. Iron also is an inexpensive catalyst used in the making ammonia via Haber process. Many other metals are not nearly as inexpensive. Next, iron catalyst helps to attain an acceptable yield in good time. Therefore, it produce high yield of product which is ammonia. Because of reasonably low cost, readily availability, easy handling, life cycle and activity, iron was the ultimate catalyst in the ammonia manufacturing.

DISADVANTAGES OF IRON CATALYST AND HABER PROCESS

However, pure iron catalysts are very limited. It must include cobalt in the reaction. The disadvantage of using an iron catalyst in the Haber-Borsch process is they can form rust from iron oxide. Meanwhile, in the Haber process, ammonia is not compatible with the copper element. It cannot use in systems that used copper pipes since most of the industry used copper pipes in reactors. Next, the Haber process consists of high energy consumptions level. According to a paper published in CellPress, the production of ammonia via the Haber-Bosch process, is the most energy-intensive commodity chemical, responsible for 1%–2% of global energy consumption and 1.44% of CO2 emissions. The Haber-Bosch process also consumes natural gas such as fossil fuel almost 3 to 5 per cent of (Yuyu et al., 2019). Furthermore, ammonia is toxic, flammable, and poisonous at the individual level, high concentrations. Ammonia is harmful towards human health as they resulted in corrosive to the skin, eyes, and lungs. The exposure of ammonia can irritate the throat and cause coughing (Diana et al., 2018). Therefore, ammonia must be appropriately handled and implement safety precaution. Using ammonia as fertilizer also bring disadvantages towards human health such as causes eutrophication (Figure 2) which algae growth and cover the surface of the water body, then giving further impact across the ecosystem. Besides, ammonia will initiate blue baby syndrome, which affects by nitrate in drinking water, thus disturbs the blood circulation by reducing the hemoglobin level in human body. Lastly, ammonia is at its disadvantage by turning the soil acidic which can trigger the crop growth.

Figure 2: Eutrophication Process

Methods of Development in Haber Process

The first practical method for synthesizing anhydrous liquid ammonia from hydrogen and nitrogen using promoted iron catalyst was scaled-up by Carl Bosch and BASF chemists in 1910. This is due to substantially searching for an active and stable catalysts were required to allow synthesis reaction takes place at high temperature and pressure. Ammonium phosphate, urea and ammonium sulphate are chemical ingredients in fertilizer are converted industrially from ammonia (Alkusayer et al., n.d.). The process synthesis of ammonia and urea production can be summarized as simplified in the stoichiometric equations below:

Ammonia:                                                      

Meanwhile, the process in another plant:

Urea: 

      

Stoichiometrically, a mole of nitrogen react with three moles of hydrogen gas produces two moles of ammonia through exothermic reaction. However, this reaction found to be unfavourable in certain condition through physical factors such as activation energy, pressure, temperature and catalyst. Therefore, Le Chatelier’s Principle was applied to increase production of ammonia as reaction is irreversible in nature. First, reactants flow over iron catalyst supported by potassium hydroxide promoter to lower activation energy. Increase in pressure typically 200 atm used allow reaction to shift towards low moles producing more ammonia and low temperature approximately 400-450℃ favour ammonia production. Haber process primarily focuses on ammonia production, but today, 80% of ammonia manufactured as feedstock for urea development which is a more stable nitrate to produce fertilizer (Alkusayer et al., n.d.). The first commercial ammonia plant based on Haber-Bosch process built by BASF at Oppau, Germany. The plant started on stream on Sept. 9 1913 with production capacity of 30 m.t./day (Introduction to Ammonia Production | AIChE, n.d.). Figure 1 below shows a flow diagram of commercial ammonia production by BASF:

Figure 1: Flow diagram of first commercial ammonia plant by BASF

As increasing demand of synthetic ammonia for fertilizer production and Haber-Bosch method usage over centuries, this resulted in countless development and modifications. Development of Haber-Bosch process creating advancement of large-scale production, high-pressure technology, continuous-flow and according to Alkusayer et al., production of ammonia has reached 159 tonnes annually, approximately 83% for manufacture of fertilizers needed for agriculture. As a result, advanced in technologies alter the process or equipment while maintaining its general process. There are six general steps for ammonia production: catalytic steam reforming, natural gas desulfurization, carbon dioxide removal, carbon monoxide shift, ammonia synthesis and methanation.  After multiple passes of Harber process, 97% of reactants were converted overall. While nitrogen reacted from air, a bulk of hydrogen gas and carbon monoxide were produced through catalytic steam reforming of natural gas where steam reacted with natural gas (methane) at high temperature ranging 700-1100℃. Production ammonia is approximately 98% generated. Figure 2 shows flow diagram of typical ammonia production today.

Figure 2. Flow diagram of ammonia production process

CATALYST USED IN HABER PROCESS

Since the Haber–Bosch process's industrial introduce, many attempts have been made to develop the reaction. Various metals tested in the search for fitting catalysts. The suitable criteria for choosing catalyst must consider the dissociative adsorption of nitrogen(N₂). This is essential in order to increase the rate process to split up the nitrogen (N₂) molecule and to speed up the rate of reaction. Other reason also because the catalysis application able to lower the activation energy required for synthesis. However, the binding must not be too strong so that catalyst would not be blocked (self-poisoning) and reduced the catalytic activity. Surveys of that system demonstrate that iron catalyst type has the surface to chemisorbs N₂, with the N≤N bond cleaving to give surface-bound nitride (N₃^-) (Jyllian Kemsley, 2011). Therefore, it was suggested that an iron catalyst is the most suitable for the Haber process in making ammonia. Iron catalyst is the most common and used elements due to its robust structure build and particles organization. When the reactants (both in gas phase) are passed over an iron catalyst with addition of potassium hydroxide as promoter, it will increase the efficiency of the process. Figure 3 below show the catalytic process using iron catalyst.

Figure 3: Catalytic Process using Iron Catalyst 

Catalytic Process using Iron Catalyst

Though Haber-Bosch process has been used extensively for production of nitrogenous fertilizers to satisfy agricultural demands, the consequences of growth of crops in large quantity and chemical emissions has inspired engineers to seek and design alternative methods for fertilizer production. Therefore, the substitute to Haber-Bosch process of ammonia through heterogenous catalyst development is a Never-Ending-Story (Marakatti & Gaigneaux, 2020).