Production of Ammonia Sunny Chawla<br />History of ammonia manufacturing processes:-<br />Before the start of World War I, most ammonia was obtained by the dry distillation of nitrogenous vegetable and animal products; the reduction of nitrous acid and nitrites with hydrogen; and the decomposition of ammonium salts by alkaline hydroxides or by quicklime, the salt most generally used being the chloride (sal-ammoniac). <br />The Haber process, which is the production of ammonia by combining hydrogen and nitrogen, was first patented by Fritz Haber in 1908. In 1910, Carl Bosch, while working for the German chemical company BASF, successfully commercialized the process and secured further patents. It was first used on an industrial scale by the Germans during World War I. Since then, the process has often been referred to as the Haber-Bosch process.<br />Haber-Bosch process:-<br />This process involves the direct reaction between Hydrogen and Nitrogen. Before carrying out the process, Nitrogen and Hydrogen gas are produced.<br />Nitrogen:-<br />Nitrogen is by far the most abundant gas in the Earth's atmosphere, making up 78.084% of the air we breathe. Nitrogen is commonly produced industrially by the low-temperature distillation of air.<br />Hydrogen:-<br />Hydrogen can be produced by either of these various processes, <br />
Hydrogen is commonly produced on an industrial large scale by the catalytic reforming of Methane. Although hydrogen can also be produced by catalytically reforming of methanol or by the electrolysis of water, neither of those processes are currently practiced on a large scale.<br />Reforming Methane:-<br />Methane is catalytically reacted with steam (H2O) to form carbon monoxide and hydrogen:<br /> CH4(g) + H2O(g) -> CO(g) + 3H2(g) <br />The carbon monoxide produced reacts with water to form carbon dioxide and more hydrogen:<br /> CO(g) + H2O(g) -> CO2(g) + H2(g) <br />So, overall: CH4(g) + 2H2O(g) -> CO2(g) + 4H2(g)<br />Reaction:-<br />This involves the direct combination of Nitrogen and Hydrogen. The reaction is reversible, meaning that some ammonia will be formed, but not all will react. The reaction is as follows, <br /> N2(g) + 3H2(g) ↔ 2NH3(g) ΔH = -93.6 kJ/mol<br />We find that the forward reaction is exothermic and proceeds with decrease in the number of gaseous moles. Therefore, according to Le-Chatelier’s Principle, the conditions favourable for the forward processes are :<br />
However, at low temperature, the rate of the reaction becomes very slow. Therefore, in practice, the optimum temperature of about 750 K and a pressure of about 2 × 107 Pa, i.e 200 atmospheres is employed. Since the operating temperature is fairly low (750 K), the rate of reaction is increased by using a catalyst which consists of finely divided iron containing molybdenum as promoter. The plant employed for manufacture of ammonia by Haber’s process is shown in figure below.<br />A mixture of Nitrogen and Hydrogen in the ratio of 1:3 is compressed to 200-900 atmospheres by means of a compressor. The actual pressure employed depends upon the design of the plant used. The compressed gases are cooled and passed through soda-lime tower to free them from moisture and carbon dioxide. The gases then enter the catalyst chamber where they are first of all heated by a preheater to 750 K, and then made to enter of the tubes packed with a catalyst. Thereafter, the gases pass through heat exchangers and cool down. Finally, the gases are cooled by the sudden expansion through a small hole whereby ammonia gets liquefied while uncondensed gaseous mixture is recirculated for conversion into ammonia.<br />Modern ammonia producing plants:-<br />A typical modern ammonia-producing plant first converts natural gas (i.e., methane) or LPG (liquefied petroleum gases such as propane and butane) or petroleum naphtha into gaseous hydrogen. The method for producing hydrogen from hydrocarbons is referred to as "
. The hydrogen is then combined with nitrogen to produce ammonia. The figure below shows the stages of modern process for formation of ammonia.<br />The first step in the process is to 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:<br />H2 + RSH -> RH + H2S<br />The gaseous hydrogen sulfide is then absorbed and removed by passing it through beds of zinc oxide where it is converted to solid zinc sulfide:<br />H2S + ZnO -> ZnS + H2O<br />Catalytic steam reforming of the sulfur-free feedstock is then used to form hydrogen plus carbon monoxide:<br />CH4 + H2O -> CO + 3H2<br />The next step uses catalytic shift conversion to convert the carbon monoxide to carbon dioxide and more hydrogen:<br /> CO + H2O -> CO2 + H2<br />The carbon dioxide is then removed either by absorption in aqueous ethanolamine solutions or by adsorption in Pressure Swing Adsorbers (PSA) using proprietary solid adsorption media.<br />The final step in producing the hydrogen is to use catalytic methanation to remove any small residual amounts of carbon monoxide or carbon dioxide from the hydrogen:<br />CO + 3H2 -> CH4 + H2O<br /> CO2 + 4H2 -> CH4 + 2H2O<br />To produce the desired end-product ammonia, the hydrogen is then catalytically reacted with nitrogen (derived from process air) to form anhydrous liquid ammonia. This step is known as the ammonia synthesis loop.<br /> 3H2 + N2 -> 2NH3<br />The steam reforming, shift conversion, carbon dioxide removal and methanation steps each operate at absolute pressures of about 25 to 35 bar, and the ammonia synthesis loop operates at absolute pressures ranging from 60 to 180 bar, depending on the proprietary design used.<br /> <br /> <br />___________________________________________________________________<br />Bibliography<br />
Modern’s ABC of Chemistry, Dr. S.P Jauhar, Modern Publishers, 2008 – Helped a little bit on Haber process.
Comprehensive Chemistry, Narinder Kumar, Laxmi Publications, 2008 – Helped on the topic of Haber process.