Lecture 4. Ammonia: Production and Storage - Part 1

Transcription

1 Lecture 4 Ammonia: Production and Storage - Part 1 Ammonia or azane is a compound of nitrogen and hydrogen with the formula NH 3. It is a colourless gas with a characteristic pungent smell. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or indirectly, is also a building-block for the synthesis of many fertilizers, pharmaceuticals and is used in many commercial cleaning products. Although in wide use, ammonia is both caustic and hazardous. In 2006, worldwide production was estimated at million tonnes. Ammonia, as used commercially, is often called anhydrous ammonia. This term emphasizes the absence of water in the material. Because NH 3 boils at C ( F) at a pressure of 1 atmosphere, the liquid must be stored under high pressure or at low temperature. Household ammonia or ammonium hydroxide is a solution of NH 3 in water. The concentration of such solutions is measured in units of the Baumé scale (density), with 26 degrees baumé (about 30% w/w ammonia at 15.5 C) being the typical high-concentration commercial product. Household ammonia ranges in concentration from 5 to 10 weight percent ammonia. Approximately 83% (as of 2004) of ammonia is used as fertilizers either as its salts or as solutions. When applied to soil, it helps provide increased yields of crops such as corn and wheat. Consuming more than 1% of all man-made power, the production of ammonia is a significant component of the world energy budget. 4.1 Physical properties of ammonia Ammonia has flammable limits of 16%-25% by volume in the air and 15%-79% in oxygen. Ammonia air mixtures is 650 Celsius. The mixture can explode if ignited. Ammonia is readily soluble in water. A large amount of heat, about 2180kj (520kcal), is produced when the dissolution of 1kg of ammonia gas occurs.

2 4.2 Physical properties of anhydrous Ammonia Property Unit Value Molecular weight Boiling point Freezing point Critical temperature o C o C o C Critical pressure MPa Specific heat (gas) j/kg o K -at0 o C 2, at100 o C 2, at200 o C ` 2,105.6 Heat of formation (gas) kj/mol -at 0 o K at 298 o K Solubility in water %, weight -at 0 o C at 20 o C at 40 o C at 60 o C 4.1 Specific gravity g/ml -at -40 o C at 0 o C at 40 o C 0.580

3 4.3 Density of Ammonia at 15 0 C Ammonia (wt %) Density (g/cm 3 ) Production of Ammonia Ammonia is produced in a process known as the Haber process, in which nitrogen and hydrogen react in the presence of an iron catalyst to form ammonia. The hydrogen is formed by reacting natural gas and steam at high temperatures and the nitrogen is supplied from the air. Other gases (such as water and carbon dioxide) are removed from the gas stream and the nitrogen and hydrogen passed over an iron catalyst at high temperature and pressure to form the ammonia. The process is shown schematically in Figure 4.1.

4 Air Hydrodesulphuriser Primary Reformer Secondary Reformer High Temperature Shift Low Temperature Shift Natural Gas H C Super heater 350 C 200 C 390 C 790 C 1000 C raising 420 C raising 220 C Heat Recovery Preheater CO C Process Condensate Reboiler Quench Quench CO 2 Removal Heat Recovery 330 C Condensate Methactor Refrigeration 220Bar 470 C Boiler Liquid Ammonia Purge Gas Carbon Dioxide Ammonia Synthesis Fig. 4.1 Industrial manufacturing of Ammonia Step 1 - Hydrogen production Hydrogen is produced by the reaction of methane with water. However, before this can be carried out, all sulfurous compounds must be removed from the natural gas to prevent catalyst poisoning. These are removed by heating the gas to 400oC and reacting it with zinc oxide: ZnO + H2S ZnS + H2O Following this, the gas is sent to the primary reformer for steam reforming, where superheated steam is fed into the reformer with the methane. The gas mixture heated with natural gas and purge gas to 770oC in the presence of a nickel catalyst. At this temperature

5 the following equilibrium reactions are driven to the right, converting the methane to hydrogen, carbon dioxide and small quantities of carbon monoxide: CH4 + H2O 3H2 + CO CH4 + 2H2O 4H2 + CO2 CO + H2O H2 + CO2 This gaseous mixture is known as synthesis gas. Step 2 - Nitrogen addition The synthesis gas is cooled slightly to 735oC. It then flows to the secondary reformer whereit is mixed with a calculated amount of air. The highly exothermic reaction between oxygen and methane produces more hydrogen. Important reactions are: CO + H 2 O CO 2 + H 2 O 2 + 2CH 4 2CO + 4H 2 O 2 + CH 4 CO 2 + 2H 2 2O 2 + CH 4 2H 2 O + CO 2 In addition, the necessary nitrogen is added in the secondary reformer. 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.