Ammonia and urea production pdf

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Related Audiobooks Free with a 30 day trial from Scribd. Elizabeth Howell. Ammonia and urea production 1. Osmium is a much better catalyst for the reaction but is very expensive 4. The Haber synthesis was developed into an industrial process by Carl Bosch. Natural gas contains some sulfurous compounds which damage the catalysts used in this process. These are removed by reacting them with zinc oxide, e. Thereafter, the sulfur compounds are hydrogenated to H2S, typically using a cobalt molybdenum catalyst, and then finally adsorbed on zinc oxide.

In this way, the sulfur is removed to less than 0. Manufacturing process of ammonia Desulfurizer and catalysts Step 2 - Nitrogen addition The synthesis gas is cooled slightly to oC. It then flows to the secondary reformer where it is mixed with a calculated amount of air. The highly exothermic reaction between oxygen and methane produces more hydrogen. As the catalyst that is used to form the ammonia is pure iron, water, carbon dioxide and carbon monoxide must be removed from the gas stream to prevent oxidation of the iron.

This is carried out in the next three steps. The same reaction occurs in both steps, but using the two steps maximises conversion.

Step 4 - Water removal The gas mixture is further cooled to 40oC, at which temperature the water condenses out and is removed. Step 6 - Synthesis of ammonia The gas mixture is now cooled, compressed and fed into the ammonia synthesis loop see Figure 1.

A mixture of ammonia and unreacted gases which have already been around the loop are mixed with the incoming gas stream and cooled to 5oC.

The ammonia present is removed and the unreacted gases heated to oC at a pressure of barg and passed over an iron catalyst. The outlet gas from the ammonia converter is cooled from oC to 30oC. This cooling process condenses more the half the ammonia, which is then separated out.

The 2 These reactions are the reverse of the primary reformer reactions seen in Step 1. The catalyst in both cases is nickel, illustrating the fact that a catalyst accelerates both the forward and back reactions of an equilibrium system. At this pressure, impurities such as methane and hydrogen become gases.

The gas mixture above the liquid ammonia which also contains significant levels of ammonia is removed and sent to the ammonia recovery unit. This is an absorber-stripper system using water as solvent. The remaining gas purge gas is used as fuel for the heating of the primary reformer. The pure ammonia remaining is mixed with the pure ammonia from the initial condensation above and is ready for use in urea production, for storage or for direct sale.

Ammonia product specifications are given in Table 2. All water is eliminated after step 4. This means that urea which contains high levels of biuret is unsuitable for use as a fertiliser. The structure of these compounds is shown in Figure 3. Step 1 - Synthesis A mixture of compressed CO2 and ammonia at barg is reacted to form ammonium carbamate.

This is an exothermic reaction, and heat is recovered by a boiler which produces steam. The second reactor recieves the gas from the first reactor and recycle solution. The solution is then purified in the same process as was used for the liquid from the first reactor. The unconsumed reactants are removed in three stages3. Firstly, the pressure is reduced from to 17 barg and the solution is heated, which causes the ammonium carbamate to decompose to ammonia and carbon dioxide: NH2COONH4!

The pressure is then reduced to 2. By the time the mixture is at At each stage the unconsumed reactants are absorbed into a water solution which is recycled to the secondary reactor. The excess ammonia is purified and used as feedstock to the primary reactor. At this stage some urea crystals also form. The solution is then heated from 80 to oC to redissolve these crystals prior to evaporation. Step 4 - Granulation Urea is sold for fertiliser as 2 - 4 mm diameter granules.

These granules are formed by spraying molten urea onto seed granules which are supported on a bed of air. Firstly, the pressure is reduced from to 17 barg and the solution is heated, which causes the ammonium carbamate to decompose to ammonia and carbon dioxide: NH2COONH4!

The pressure is then reduced to 2. By the time the mixture is at At each stage the unconsumed reactants are absorbed into a water solution which is recycled to the secondary reactor. The excess ammonia is purified and used as feedstock to the primary reactor. At this stage some urea crystals also form.

The solution is then heated from 80 to oC to redissolve these crystals prior to evaporation. Step 4 - Granulation Urea is sold for fertiliser as 2 - 4 mm diameter granules. These granules are formed by spraying molten urea onto seed granules which are supported on a bed of air.

This occurs in a granulator which receives the seed gransules at one end and discharges enlarged granules at the other as molten urea is sprayed through nozzles.

Dry, cool granules are classified using screens. Oversized granules are crushed and combined with undersized ones for use as seed. All dust and air from the granulator is removed by a fan into a dust scrubber, which removes the urea with a water solution then discharges the air to the atmosphere. The final product is cooled in air, weighed and conveyed to bulk storage ready for sale. These are listed below. Ammonia manufacture Heat recovery The heat of the gas from the primary reformer Step 1 is used to produce steam for the primary reformer using a boiler.

The gas is then discharged. Heat from the process gas from the secondary reformer Step 2 is used to produce steam for a turbogenerator. Water recycling Excess water from the water gas shift converter, the methanator and the ammonia synthesis loop is used for boiler feed water and as the absorbing water for ammonia recovery. The water is removed from the CO2 by condensation and the pure CO2 sent directly to the urea plant for compression and use in urea synthesis.

Ammonia recovery Gases purged from the ammonia synthesis loop and gases collected during ammonia decompression are mixed and sent to the ammonia recovery system. Here the gas mixture is introduced at the bottom of a column and passes up through a counter-current of cold water. The ammonia is distilled out of the ammonia- water mixture, condensed and pumped to join the rest of the ammonia from the ammonia synthesiser.

Urea manufacture Heat recovery The heat of the reaction in which ammonium carbamate produces steam at 7 barg.

This is used in the decomposition and evaporation sections for heating. Ammonia and carbon dioxide recovery During urea decomposition a mixture of gaseous carbon dioxide and ammonia is collected and absorbed into a dilute aqueous urea solution. The excess ammonia is condensed and used as feedstock to the primary reactor.

Water recycling Evaporated water from the concentration step is used during the third stage of decomposition as the initial recycle solution. The concentration of each component during the process is kept at a precalculated design figure and laboratory results are compared to these figures. Adjustments are made to the process based on the laboratory results to bring the process back to the design figures. The solution strength must be kept within a defined range and additions to the system are made according to laboratory results.

The process is adjusted accordingly to meet final product size specifications. In addition, boiler and cooling waters are analysed to ensure that their composition is such that corrosion is minimised. Trouble shooting is often required to source biuret or excessive moisture during intermediate stages of the urea process. The Petrochem complex produces effluent in the form of storm water and waste water from the manufacturing process.

All effluent is directed to large holding ponds where it is treated and carefully checked as to its composition prior to discharge. Many waste minimisation measures are carried out during the process, resulting in the plant having little effect on the environment.