Ammonia Synthesis - Research Paper Example

Ammonia Synthesis Ammonia (NH3) is an inorganic, basic, gaseous compound that is formed from the decomposition of organic materials, or as a product of many metabolically reactions in living organisms (PubChem Substance). The major use of ammonia is in the field of agriculture, where it is the major source of nitrogen, an element needed in large amounts by plants. Before the chemical process for synthesizing ammonia was discovered, the sources of nitrogen-rich fertilizers were organic wastes like bird droppings, human urine, and natural deposits of sodium nitrate or saltpeter. However, with increased demand, the organic sources became were fast depleted. Air has a boundless supply of nitrogen, but this free nitrogen gas has to be fixed in order to be utilized. In 1908, Fritz Haber discovered a process for synthesizing ammonia from atmospheric nitrogen and hydrogen in a chemical process that came to be Haber process. For this discovery, he was awarded the Nobel Prize in Chemistry in 1918. The Haber process was further developed for industry applications by Carl Bosch, who also won the Nobel Prize in Chemistry in 1931. Thus, the process is also called Haber-Bosch process. Approximately ninety percent of all manufactured ammonia in the United States of America is used in the production of nitrogenous fertilizers like urea, ammonium nitrate, and ammonium phosphate (U.S. Geological Survey). Thus, ammonia directly affects the world’s nutrition and survival. The other 15% is used in the manufacture of explosives, plastics, pharmaceuticals, cleaning material, dyes, solvents and others. The Haber –Bosch process was the first to use very high pressures and high temperatures to react nitrogen and hydrogen gases, under the influence of an iron catalyst, to produce ammonia. This is still the process used until now to produce ammonia. The chemical efficiency of the reaction is a function of pressure and temperature: increasing the pressure and lower temperatures give higher reaction yields (based on Le Chatelier’s principle). The synthesis of ammonia from nitrogen and hydrogen can be described by the overall reaction (1) (Modak): Alternatively the reaction is: heat, pressure, catalyst N2 + 3H2 2NH3(g) H = -92.4 kJ mol-1 (2) The reaction is exothermic, producing considerable heat energy. It is also reversible, and can proceed forward (or to the right) to produce ammonia or in the reverse to produce nitrogen and oxygen gases. Based on reaction 1, 1 mole of NH3 gas is produced from a total of 2 moles reactants (1/2 N2 + 3/2 H2), due to this, there is also an accompanying decrease in the volume of reaction. Le Chatelier’s principle states that introducing change to a reaction in equilibrium will shift the reaction to the side that favors a return to equilibrium. In the ammonia synthesis reaction, less moles of gas means less gas pressure. Therefore, when pressure is added to the reaction, there will be a shift to the production of ammonia, because only 1 mole of gas is produced on this side. In the same manner, when the temperature is reduced, the reaction will also shift to the right in order to increase the system’s temperature to achieve the original state of equilibrium. This was the basis for the industrial production of ammonia. Conventionally, the conditions for the Haber process have been established to be temperature at approximately 500°C, very high reaction pressure of ~ 200 atmospheres or 350 kiloPascals (kPa), and the presence of a porous iron catalyst (Figure 1). These conditions will ascertain that the yield of ammonia is increased (ausetute.com.au). Increasing the temperature to ~500°C is a compromise because if the temperature is very low, then the yield of ammonia will be reduced to less than 10% (ausetute.com.au). Although reaction pressures that are greater than 750 atmospheres and temperatures of 200°C can increase conversion to ammonia to almost a hundred percent, higher pressure is more difficult to manage especially when the volume of reaction is large. Also, during the industrial process, the reaction does not reach equilibrium, since once form, the gas mixture is subjected to refrigeration, which liquefies ammonia for its easier separation from the other gases in the reaction. The remaining nitrogen and hydrogen gas molecules are recycled back into the reactor for continuation of the process (Clark; ausetute.com.au). When the gases are not recycled, the amount of ammonia recovered is very small to make the production economically viable. The idea of recycling the gases also came from Haber himself when he was still developing the technology. Figure 1. General scheme of the Haber-Bosch process in the manufacture of ammonia. Another important component of the process is the catalyst used. The catalyst increases the rate of reaction by decreasing the activation energy so that the bonds between the 2 nitrogen atoms, and the 2 hydrogen atoms in N2 and H2, respectively, are easily broken. Conventional catalysts were iron-based compounds (Modak). In industrial plants, ammonia synthesis goes through sequential three stages: steam reforming, removal of carbon dioxide, and the Haber-Bosch process (John Mathey Catalysts). However, before any of these stages is implemented, sulfur must be removed from the gas feedstock or the hydrogen source. Normally sources are methane, liquefied petroleum gas, and other hydrocarbons which could all come from the natural gas feedstock. Sulfur must be removed from the mixture because it deactivates the catalysts used in ammonia synthesis steps. The removal is carried out by the catalytic hydrogenation of sulfur to produce gaseous hydrogen sulfide gas, which is then passed through zinc oxide beds producing to zinc sulfide, a solid which can be removed with more ease. After removal sulfur, the next step involves steam reforming, a process where methane, and the other gases in natural gas feedstock react with steam producing carbon monoxide and hydrogen (reaction 3) (John Mathey Catalysts). The reaction is carried out with a nickel catalyst. CH4 + H2O CO + 3H2 (3) where CH4= methane, CO= Carbon monoxide The second stage in ammonia synthesis is the removal of carbon monoxide in a water-gas shift reaction, producing carbon dioxide (CO2) and more hydrogen gas (reaction 4). CO + H2O → CO2 + H2 (4) The carbon dioxide product is then removed by adsorption on solid media or reaction with ethanolamine solution, a known carbon dioxide scrubber. This step is the most energy expensive in terms of consumption, second only to the cooling water system. The high energy costs are due to the inefficient distillation, and pressurization/depressurization of absorbents. More energy-efficient alternatives to conventional carbon dioxide removal include the separation by using membranes, using pressure swing absorption, and cryogenic condensation (Agarwal). Residual amounts of carbon monoxide and carbon dioxide are further removed using the process of methanation which can lead to hydrogen losses. The pressure swing absorption can reduce hydrogen loss in the methanator because it can separate hydrogen from the other gases more efficiently. After the production of hydrogen, the Haber-Bosch process or ammonia synthesis commences. Figure 2 presents a flow sheet of a present-day ammonia synthesis plant (Uhde GmbH). The pressure applied and the temperatures used may vary depending on location and manufacturer of the ammonia synthesis plants. Technological advances in the last 100 years since ammonia synthesis has resulted in significant increases in yields of ammonia (Modak). The increased demand for more fertilizers and other ammonia products spawned the increase in ammonia synthesis plants. Most modern plants produce at least 1 ton of ammonia daily, although the usual conversion is still within the range of 8-15%. Although iron-based catalysts still dominate the field, ruthenium catalysts have been developed that are 40% more efficient in lowering the activation energy (Agarwal). This has resulted in lowering the temperature for synthesis, and also the synthesis pressure. At this time, synthesis pressures of 40 atmospheres are feasible. These innovations reduce the energy input that is required in the process (Modak). Studies on the use of cobalt and ruthenium metals as catalysts under low temperature and pressure conditions are continuing. Figure 2. Flow sheet of an ammonia plant with the conventional sequence of steps but with major modifications in waste heat removal, carbon dioxide processing unit and ammonia synthesis unit (Uhde GmbH). Modern technology has also led to the development of steam reformers that are more efficient and easier to control than the conventional fired furnaces. Currently, gas heated reformers are available that require less volumes, less heat loss, low maintenance, easier to maintain and cheaper (Agarwal). However, its widespread use has not been realized yet. Ammonia is removed from the reaction mixture by absorption, or mechanical refrigeration. With refrigeration, gaseous ammonia is condensed/liquefied then separated from the mixture. A new approach being studied is to separate ammonia product in the converter by using adsorbents. The adsorbed ammonia is then removed from the converter, eliminating the need to recycle the synthesis gases because the ammonia left in the mixture is insignificant when compared to the other ammonia separation techniques (Agarwal). Decreased ammonia concentration in the recycle stream has been found to significantly increase the efficiency of the ammonia plant. Increased demand for food and nutrition should be coupled with increased crop production. However, despite the advent and increasing popularity of organic agriculture practices and nutrient source, the demand for inorganic sources of nitrogen fertilizers continue to rise. Without a doubt, ammonia synthesis is an industry that will continue to flourish. However, challenges remain for the chemist and the chemical engineer. The present reality of global climate change and the need for energy conservation in all aspects of industry underscore the need for industry leaders to produce technology that is more efficient, and environment-friendly. All aspects of the ammonia synthesis process are areas of improvement; the challenge is to increase productivity while improving energy savings and production efficiency. References 1. Agarwal, P. "Ammonia: The Next Step." 2010. The Chemical Engineers Resource Page. 12 April 2010 . 2. ausetute.com.au. 2010. 9 April 2010 . 3. Clark, J. The Haber Process. 2002. 9 April 2010 . 4. John Mathey Catalysts. "Industrial Catalysts." 2010. John Mathey Catalysts. 10 April 2010 . 5. Modak, J. "Haber Process for Ammonia Synthesis." Resonance (August 2002): 69-77. 6. PubChem Substance. April 2010. 9 April 2010 . 7. U.S. Geological Survey. "Mineral Commodity Summaries, Nitrogen (Fixed)-Ammonia." 2005. 8. Uhde GmbH. The Uhde Ammonia Process. 12 December 2009. 11 April 2010 . Read More