Manufacture of ammonia is mainly needed to produce artificial fertilizers. At present the world population is nearly 7.3 billion, and it has been estimated that it will increase to about 9.0 billion by 2050.
With the increasing of the population, production of food is a necessity. Production of fertilizer is considered as an important field of chemical manufacture in this regard. Of the total ammonia production, 83% is used to produce fertilizer. Specially, 80% of the total ammonia production is used to manufacture urea.
Below is a list of the chemical processes involved in the manufacture of ammonia.
Raw materials required for production of ammonia
N2 gas and H2 gas are the main raw materials. N2 gas is separated by fractional distillation of liquefied atmospheric air. Therefore, N2 gas has a production cost. Hydrogen gas is obtained by cracking hydrocarbons (naphtha) or using methane, which is the main component of natural gas.
Extra knowledge Natural gas is used as a fossil fuel to generate power. Natural gas occurs in large quantities as underground ores in the Earth. Production of H2 from natural gas is known as Steam methane reforming (SMR). A small quantity of H2S is present in natural gas; H2S disturbs Nickel catalytic process. Furthermore, H2 is formed by a catalytic reaction after removing H2S in the natural gas.
The reaction related with removals of H2S
The first step of the SMR process is reacting methane and water vapor at 700 0C - 800 0C to produce CO and H2. This is an endothermic reaction.
During the second step, this gaseous mixture is further reacted with water vapor. Here, CO is converted to CO2. This is an exothermic reaction.
In addition to this, there is a possibility for the following reaction to occur.
The yield of H2 can be increased by maintaining more water vapor and shifting the total reaction to the right.
To have some understanding of the thermodynamic nature of the SMR, the relationship ∆ G = ∆ H - T ∆ S can be used.
According to the above thermodynamic relationship, the exothermic second reaction is spontaneous even at low temperature. Since the number of gaseous molecules does not change, pressure has no effect. Further, the effect of T ∆ S on ∆ G is almost nil.
First and third endothermic reactions occur relatively at higher temperatures. The entropy increases because the number of gaseous molecules increases to the forward direction. So, with increase in temperature, the positive value of T ∆ S increases. Therefore, according to ∆ G = ∆ H – T ∆ S, in reactions 1 and 3 ∆ G becomes more negative at higher temperatures. Reaction 1 is allowed to occur at a high temperature (1000 – 1800 oC). There even reaction 3 may occur. In order to reduce the CO concentration and increase the yield of H2, the second reaction is allowed to occur at relatively at low temperatures (200 – 400 0C). By adding water vapor to the hot gaseous mixture of CO and H2 its temperature can be lowered (200 – 400 0C). As the second reaction is exothermic, the heat generated by the second reaction is used to obtain the higher temperature suitable for the first reaction.
In United the States, about 60% of natural gas production is used to produce ammonia gas. Hydrogen gas can also be obtained by cracking naphtha (C6 H14). It also releases CO2 to the atmosphere. The relevant reactions that occur are as follows.
Production of NH3 from hydrogen and nitrogen gases is an exothermic reaction, and it is reversible. But, under normal temperature and pressure, this reaction hardly happens. The reason is the activation energy is relatively high.
Producing NH3 by H2 and N2
Industrially, ammonia is manufacture by the Haber-Bosh process. The optimum industrial conditions employed for the process are a temperature in the range 450-500 0C, a pressure of 250-300 atm, iron catalyst and K2O and Al2O3 as catalytic promoters.
The compressed mixture of reactant gases passes over the surface of the catalyst and gets converted to NH3. Since the reaction is reversible, the gaseous mixture passing the catalyst surface contains unreacted H2 and N2 gas in addition to NH3. Therefore, NH3 gas should be separated from this mixture. When the mixture is cooled up to, - 33.34 0C, NH3 gas liquefies. The boiling points of nitrogen and hydrogen gases are – 195.8 0C and – 252.9 0C respectively. So, by cooling this mixture, only ammonia can be liquefied and the mixture of H2 and N2 can be separated from the reaction mixture. The gaseous mixture containing H2, N2 and NH3 under high pressure can be cooled by decreasing the pressure suddenly in another chamber. Then NH3 liquefies. Remaining H2 and N2 gases are then pumped back into the reaction chamber. Equal amounts of H2 and N2 that have been used to make NH3 are then freshly added.
After the removal of liquid ammonia, the gaseous mixture containing unreacted and newly added N2 and H2 gases (under a temperature of around -33.34 0C) is heated to 450 – 500 0C. The heat energy required to have this temperature can be supplied by the combustion of fuel also. This releases CO2 to the atmosphere. In the Haber process, CO2 is produced at the step in which hydrogen is produced and during the burning of fuel to gain high temperature. Due to the overall process, CO2 which is a greenhouse gas releases to atmosphere. Globally, when 1 ton, of ammonia is produced by the overall process, about 2.9 tons of CO2 is released to the atmosphere. Compared to the releasing of CO2 to atmosphere by other processes, approximately 1.4% of it, is by ammonia production.
In ammonia production, N2 and H2 are mixed according to their stoichiometric ratio (1:3). Since some cost must be borne to produce nitrogen gas and hydrogen gas they are not mixed beyond their stoichiometric ratio and this prevents wastage of raw materials. Further if one gas is used in excess it would be adsorbed by the surface of the catalyst covering the surface completely.
Coverage of catalyst surface by one gas reduces the chances for the reaction to occur. Both the gases need to be absorbed to the catalyst surface. Use of the catalyst decreases the time to reach the equilibrium. Based on all the factors, N2 and H2 gases are mixed in the optimal ratio of 1:3.
The reaction is exothermic, so it is ∆ H is negative. When the reaction proceeds, entropy decreases because the number of molecules decreases. Hence, ∆ S of the reaction is negative. Thus, T ∆ S is negative and -T ∆ S is positive. Therefore, with the increase in temperature, ∆ G changes from negative to positive. The fact that high temperatures do not favor the forward reaction can also be explained by Le Ch atelier’s principle.
On increasing the temperature, the spontaneous nature of the reaction decreases. Thermodynamically, when the spontaneity decreases, the yield decreases. To increase the yield, the temperature should be decreased. But when the temperature is decreased, the rate of the reaction decreases and the efficiency of the entire process declines. A temperature of 450-500 0C is used to maintain an optimum efficiency. The NH3 yield obtained under these conditions is relatively low. Yet, the production process has been designed so that the unreacted H2 and N2 gases are brought into the catalytic cycles again and again. Even though the amount of NH3 yield by a single step is small, the process gains a high yield because it has been designed to carry out via a number of catalytic cycles. According to Le Ch atelier’s principle, higher pressures favor the forward reaction. But the maintenance cost of the plants withstanding high pressures is high. Therefore, at present, a pressure of 250-300 atm is used. Maintenance of reactant concentration at a high level and product concentration at a low-level result in a higher yield of NH3. This can be explained by the Le Ch atelier’s Principle. This requirement is fulfilled by introducing N2 and H2 into the reaction chamber from time and removal of NH3 by cooling the gaseous mixture containing NH3 and liquefying the gas.
Uses of ammonia
1. Production of nitric acid, fertilizers, and nylon.
2. Petroleum industry utilizes ammonia in neutralizing the acid constituents of crude oil.
3. Used in water and waste water treatment, such as pH control, in solution form to regenerate weak anion
exchange resins.
4. Used as a refrigerant.
5. Used in the rubber industry for the stabilization of natural and synthetic latex to prevent premature
coagulation.
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