Introduction
The cost of manufacturing ammonia using the Haber process plant constitutes the following components: the cost of energy, raw materials, equipment, labor and sparking substances (catalysts). Energy (gas or electricity) costs are incurred to maintain the temperature of the reaction. The Harber process is operated at very high temperatures, and pressures thus higher costs of energy are incurred. Some of the raw materials consumed in the Haber process such as nitrogen and hydrogen are recovered at the end of the process. The recovered materials are recycled back to the system to reduce the cost of raw materials.
Starting and maintaining a Haber process plant requires big capital. The plant must be mechanically robust to withstand the high temperatures and pressures inside the plant. The reaction chambers and vessels must be rigid and adhere to safety precautions. How fast the end product is produced and released to its final application is determined by the use of catalysts. Haber process consumes a lot of catalysts which are expensive. The process ends up being costlier in turn. There is list cost on labor incurred as the process can be easily automated. The Haber process can be explained in stages as discussed below.
The Haber process is an artificial fixation of nitrogen in the production of ammonia. Haber process converts free nitrogen from the atmosphere into ammonia by reacting it with hydrogen. A catalyst is used under extreme states of temperature and pressure. (1)
Process
The Haber process is conducted at pressures between 15 to 25 Mpa and 400 to 500-degree Celsius temperatures. The gasses (nitrogen and hydrogen) are channeled through beds of catalyst and cooled at each stage to retain reasonable equilibrium constants. The unreacted gasses are taken back to the cycle until a substantial conversion (97%) is attained. Figure 1 below illustrates the Haber process (Glass, 2008, p. 292).
Figure 1. Haber process block diagram
Desulphurization
Feedstock desulphurization involves the use of catalysts that are sensible to sulfur and sulfur compounds. The fed gas that contains about 5mg S.Nm-3 sulfur compounds is pre-heated at 300-400 °C, in a primary reformer convection chamber and later treated in a desulphurization section. In this section, sulfur compounds are reacted with hydrogen to form hydrogen sulfide that is then absorbed on packed bed absorber (pelletized zinc oxide). A cobalt molybdenum catalyst is used in this reaction (Jones, A. V. (1999, p. 38).
.. (2)
(3)
The sulfur is taken out to less than 0.2 ppm S in the feedstock gas. Zinc sulfide is retained in the adsorption bed. Hydrogen for this stage is recycled in the synthesis section.
Reforming
a. Primary reforming
The desulphurised gas reacted with process steam from the extraction turbine. The steam/gas blend is heated at 550-600 °C before feeding it to the primary reformer. In modern plants, the preheated mixture is passed through adiabatic reformer before channeling it to the principal reformer.
The quantity of process stream is represented as the ratio of process steam and carbon molar (S/C-ratio) that is 3.0 for BAT reforming processes. The ratio is determined by several factors such as the quality of the feedstock, recovery of the purge gas, the capacity of the primary reformer, shift operation as well as the balance of the steam plant.
The principal reformer is built with nickel chromium alloy tubes that are filled with nickel- checking reforming catalyst. The reaction is highly endothermic; extra heat is needed to improve the temperature to about 750 to 850 °C at the outlet of the reformer. The gas leaving the primary reformer can be represented as:
. (4)
(5)
The primary reformer is heated by burning natural gas in radiant box burners.
b. Secondary reforming
The primary reformer reforms about 30 – 40% of the hydrocarbon feed, the rest is reformed in the secondary reformer. The hydrocarbon is internally combusted with process air which in the process produces nitrogen. In this chamber, the phase of the primary reforming is modified such that flow of the gas to the chamber matches the requirements of heat equilibrium and stoichiometric synthesis gas.
The process gas is pressurized, and heated to about 600 °C, mixed with the gas in the burner, and passed through a nickel-checking catalyst. The outlet temperatures reach 1000 °C, and 99% of the feedstock is successfully converted.
Shift conversion
High-Temperature Shift (HTS) conversion
The gas is passed over a catalyst of iron oxide/chromium oxide at 400 °C. The amount of carbon monoxide (CO) is minimized to about 3%, which is reduced by shift equilibrium at normal operating temperatures. For increased conversion, a copper-containing catalyst is applied.
Low-Temperature Shift (LTS) conversion
The converter contains copper oxide/zinc oxide catalyst. It operates between 200-220 °C reduces the CO content to about 0.2 - 0.4%.
Carbon dioxide absorption
The process gas from LTSS comprises of H2, N2, and CO2. CO2 is eliminated by a chemical or physical process. Chemical absorption involves the use of aqueous amine solution or heated potassium carbonate solution. Physical absorbers include glycol dimethyl ether and propylene carbonate.
Carbon dioxide stripping
The absorbed CO2 is stripped in regenerators (Hirst, 2002, p. 282). The CO2 stripped is channeled to a Urea making plant. A vetrocoke solution consisting of vanadium pentoxide, glycine, K2CO3, and DEA is used. The chemistry at this chamber can be explained in equations (6 to 8) below.
(6)
(7)
. (8)
Methanation
Traces of CO and CO2 in the process are eliminated by converting them into methane (CH4) in the methanation.
. (9)
(10)
The reaction occurs at 300 °C in the reactor packed with a nickel catalyst. The water in the methane is eliminated before feeding the synthesis gas to the converter by cooling and condensing it downstream the methanation. The gas is then absorbed in the makeup gas drying chamber (Myers, 2003. p. 211).
Synthesis gas compression
Centrifugal compressors driven by steam turbines are employed. Steam is cogenerated in the ammonia plant. Condensation of the product ammonia is done by refrigeration compressor. Synthesis is activated by the iron catalyst at between pressures of 100 t0 200 bar and 350-550 °C temperatures.
(11)
Ammonia synthesis reactor
The ammonia produced is separated from other gasses by condensation, the reacted gas is replaced by fresh makeup process gas. Heat exchange is needed due to the exothermic process and huge range of temperature in the loop. The synthesis catalyst contains elements of ruthenium and graphite that has high affinity per unit volume and the ability to enhance increased conversion and little operating pressures.
The synthesis loop configuration is different regarding the points in the cycle at which makeup gas is channeled and the manner in which the ammonia and the purge stream gets out of the loop. The best configuration involves adding the makeup gas after condensing the ammonia on the front of the converter. Loop purge is eliminated before adding makeup gas and after separating ammonia.
Chilling
In the last purification stage, conventional reforming by methanation generates a process gas with an inert gas (argon) in amounts that are insoluble in condensed ammonia. The larger portion of the inert gas is eliminated by removing the purge stream out of the loop. The quantity of the purge stream regulates about 15 – 20% of the amount of inert gas in the loop. The stream is cleaned with water to eliminate the ammonia before it is consumed as fuel or send to recover the hydrogen.
Refrigeration and compression
Vaporizing ammonia is applied in the ammonia plant to attain substantial low concentrations of ammonia in the recycled gas fed to the converter. Ammonia vapors are then liquefied after recompressing it in the refrigeration compressor.
Purge gas and recovery
Traces of nitrogen gas, argon gas, methane and hydrogen in the synthesis gas are separated through absorption. Purge gas recovery controls waste emission (Fullick, & Fullick, 2001, p. 373). The ammonia is tapped from the looped purge; the remaining gasses are recovered are channeled to the primary reformer. The argon is recycled to the converter; ammonia production is increased in turn.
Conclusion
The Haber process involves the steps mentioned above. The ammonia produced has several applications such as the manufacture of fertilizers, treatment of metal operations, neutralizing acids in petroleum industries, water, and sewage treatments, among other benefits.
References
Fullick, A., & Fullick, P. (2001). Chemistry for AQA: coordinated award. Oxford, Heinemann.
Glass, G. V. (2008). Fertilizers, Pills, and Magnetic Strips: the fate of public education in America. Charlotte, N.C., Information Age Pub.
Hirst, K. (2002). Modular Science for AQA. Year 11, Year 11. Oxford, Heinemann.
Jones, A. V. (1999). Access to chemistry. Cambridge, UK, Royal Society of Chemistry.
Myers, R. (2003). The basics of chemistry. Westport, Conn, Greenwood Press.
