Fuel into Food
Few technologies are as directly connected to the maintenance of human life as chemical fertilizers. Although organic farming has its merits, the feeding of the world's growing population is highly dependent on synthetic nitrogen, potassium, and phosphorus. At the beginning of the 20th century, naturally occurring supplies of nitrogenous fertilizer were being used up, resulting in growing concern about the possibility of eventual famine.
Repeated attempts to grow crops on the same acreage results in an exhausted soil and falling crop yields. The soil can regain its fertility if it is taken out of cultivation (left fallow), but this is not an option when large numbers of people are dependent on a limited amount of cropland
In the 19th century, farming began to change in a fundamental way when chemicals unrelated to organic processes began to be added to the soil. Although the consequences were revolutionary, the application of chemical fertilizers took many decades to be established.
Although a plant's need for nitrogen was not recognized until 1857, for nearly 3 decades before that time, nitrogen-rich South American guano (bird droppings) had been used to improve the farm soils of North America and Europe. In the 1860s, nitrogen began to be obtained from ammonium sulphate, a by-product of the coking ovens that produced illuminating gas in many cities. The key development in the production of nitrogen compounds came in 1908-09 when Fritz Haber (1868-1934) invented the process that bears his name.
At the beginning of the 20th century there was a shortage of naturally occurring, nitrogen-rich fertilisers, such as Chile saltpetre, which prompted the German Chemist, Haber, and others, to look for ways of combining the nitrogen in the air with hydrogen to form ammonia, which is a convenient starting point in the manufacture of fertilisers. This process was also of interest to the German chemical industry as Germany was preparing for World War I and nitrogen compounds were needed for explosives.
The Haber process consisted of reacting hydrogen and nitrogen at high temperature and pressure in the presence of iron or some other catalyst. First commercially produced in Germany in 1913, ammonia and other nitrogen compounds are still made through this process, usually with methane providing the nitrogen, although the cryogenic separation of air is sometimes used to supply the nitrogen.
Yet, as with most technological advances, the extensive use of chemical fertilizer has not been an entire blessing. Chemical fertilizers require massive amounts of energy for their production, distribution, and application. Agriculture of this sort has been ...
This is a preview of the whole essay
The Haber process consisted of reacting hydrogen and nitrogen at high temperature and pressure in the presence of iron or some other catalyst. First commercially produced in Germany in 1913, ammonia and other nitrogen compounds are still made through this process, usually with methane providing the nitrogen, although the cryogenic separation of air is sometimes used to supply the nitrogen.
Yet, as with most technological advances, the extensive use of chemical fertilizer has not been an entire blessing. Chemical fertilizers require massive amounts of energy for their production, distribution, and application. Agriculture of this sort has been accurately described as converting....
fuel into food.
Also, chemical fertilizer run-off can get into water supplies, polluting groundwater and causing the eutrophication of rivers and lakes. The latter occurs when chemicals in the water stimulate the excessive growth of algae, and causes a consequent deterioration of water quality and loss of other forms of aquatic life.
Chemical fertilizers have helped make possible the great expansion in agricultural production that has sustained a constantly expanding world population, but care must be taken so that these gains are not negated by extensive environmental destruction.
Haber Process for the Production of Ammonia
In 1909 Fritz Haber established the conditions under which nitrogen, N2(g), and hydrogen, H2(g), would combine using
* Medium temperature (~500oC)
* Very high pressure (~250 atmospheres, ~351kPa)
* A catalyst such as an iron catalyst is used to speed up the reaction by lowering the activation energy so that the N2 bonds and H2 bonds can be more readily broken.
Osmium is a much better catalyst for the reaction but is very expensive.
This process produces ammonia, NH3 (g), yield of approximately 10-20%.
Carl Bosch developed the Haber synthesis into an industrial process.
The reaction between nitrogen gas and hydrogen gas to produce ammonia gas is exothermic, releasing 92.4kJ/mol of energy at 298K (25oC).
N2(g)
nitrogen
+
3H2(g)
hydrogen
heat, pressure, catalyst
2NH3(g)
ammonia
H = -92.4 kJ mol-1
By Le Chetalier's Principle:
* Increasing the pressure, means moving equilibrium position to move to the right.
Left hand side has 4 right hand side has 2. Increasing the pressure means the system adjusts to reduce the effect of the change, that is, to reduce the pressure by having fewer gas molecules. (higher yield of ammonia since there are more gas molecules on the left hand side of the equation)
* Decreasing the temperature means moving equilibrium position to move to the right.
Resulting in a higher yield of ammonia since the reaction is exothermic (releases heat). Reducing the temperature means the system will adjust to minimise the effect of the change, that is, it will produce more heat since energy is a product of the reaction, and will therefore produce more ammonia gas as well
However, the rate of the reaction at lower temperatures is extremely slow, so a higher temperature must be used to speed up the reaction which results in a lower yield of ammonia.
The equilibrium expression for this reaction is:
Keq =
[NH3]2
[N2][H2]3
As the temperature increases, the equilibrium constant decreases as the yield of ammonia decreases.
Temperature (oC)
Keq
25
6.4 x 102
200
4.4 x 10-1
300
4.3 x 10-3
400
.6 x 10-4
500
.5 x 10-5
Rate considerations:
* Increased temperature means more reactant molecules have sufficient energy to overcome the energy barrier to reacting (activation energy) so the reaction is faster at higher temperatures (but the yield of ammonia is lower as discussed above).
A temperature range of 400-500oC is a compromise designed to achieve an acceptable yield of ammonia (10-20%) within an acceptable time period.
At 200oC and pressures above 750atm there is an almost 100% conversion of reactants to the ammonia product.
Since there are difficulties associated with containing larger amounts of materials at this high pressure, lower pressures of around 200 atm are used industrially.
By using a pressure of around 200atm and a temperature of about 500oC, the yield of ammonia is 10-20%, while costs and safety concerns in the building and during operation of the plant are minimised.
* During industrial production of ammonia, the reaction never reaches equilibrium as the gas mixture leaving the reactor is cooled to liquefy and remove the ammonia. The remaining mixtures of reactant gases are recycled through the reactor. The heat released by the reaction is removed and used to heat the incoming gas mixture.
Uses of Ammonia
Industry
Use
Fertilser
Production of:
* Ammonium sulphate, (NH4)2SO4
* Ammonium phosphate, (NH4)3PO4
* Ammonium nitrate, NH4NO3
* urea, (NH2)2CO,also used in the production of barbiturates (sedatives), is made by the reaction of ammonia with carbon dioxide
CO2
carbon dioxide
+
2NH3
ammonia
H2NCOONH4
ammonium carbonate
heat, pressure
(NH2)2CO
urea
</TD< TR>
Chemicals
Synthesis of:
* Nitric acid, HNO3, which is used in making explosives such as TNT (2,4,6-trinitrotoluene), nitroglycerine that is also used as a vasodilator (a substance that dilates blood vessels) and PETN (pentaerythritol nitrate).
* Sodium hydrogen carbonate (sodium bicarbonate), NaHCO3
* Sodium carbonate, Na2CO3
* Hydrogen cyanide (hydrocyanic acid), HCN
* Hydrazine, N2H4 (used in rocket propulsion systems)
Explosives
Ammonium nitrate, NH4NO3
Fibres & Plastics
Nylon, -[(CH2)4-CO-NH-(CH2)6-NH-CO]-,and other polyamides
Refrigeration
Used for making ice, large scale refrigeration plants, air-conditioning units in buildings and plants
Pharmaceuticals
Used in the manufacture of drugs such as sulfonamide which inhibit the growth and multiplication of bacteria that require p-aminobenzoic acid (PABA) for the biosynthesis of folic acids, anti-malarials and vitamins such as the B vitamins nicotinamide (niacinamide) and thiamine.
Pulp & Paper
ammonium hydrogen sulfite, NH4HSO3, enables some hardwoods to be used
Mining & Metallurgy
used in nitriding (bright annealing) steel,
used in zinc and nickel extraction
Cleaning
ammonia in solution is used as a cleaning agent such as in 'cloudy ammonia'
