Information Centre Nitric Acid Plants. Kittiwake Procal Ltd Page 1 of 6

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1 Information Centre Kittiwake Procal Ltd Page 1 of 6 Nitric Acid Nitric acid is a strong highly corrosive and toxic acid. Pure nitric acid is colourless but aged solutions can appear yellow due to oxidation. Nitric acid is an inorganic compound used primarily to make synthetic commercial fertilizer. Other uses include the production of explosives, the etching and dissolution of metals and in organic oxidation in adipic acid manufacture. Nitric acid can react explosively with compounds such as cyanides, carbides and metallic powders. In addition, nitric acid also reacts with most metals and is used in the extraction and purification of gold. Aqueous blends of 5 30% nitric acid and 15 40% phosphoric acid are commonly used for cleaning food and dairy equipment in order to remove precipitated calcium and magnesium compounds. Fertilizer Production and the Manufacture of Nylon About 80% of the nitric acid produced is consumed as an intermediate in the manufacture of ammonium nitrate (NH 4 NO 3 ) which in turn is used in fertilizers. The majority of nitric acid plants in the USA are located in agricultural regions such as the Midwest, South Central and Gulf States because of the high demand for fertilizer in these areas. Another 5 to 10% is used in the manufacture of Adipic acid (C 6 H 10 O 4 ) a white crystalline solid that is used primarily as the main constituent of nylon (nylon 6/6), representing about half of the nylon molecule. Adipic acid is also used in the manufacture of some lowtemperature synthetic lubricants, synthetic fibers, coatings, plastics, polyurethane resins, and plasticizers, and to give some imitation food products a tangy flavor. The Manufacture of Nitric Acid Commercially produced nitric acid concentrations for the fertilizer industry are in the weak acid range between 50 and 65% strength. Nitric acid is made by the catalytic oxidation of ammonia (based on the Ostwald process invented by the German scientist Wilhelm Ostwald). The process has three steps 1. Oxidation of anhydrous ammonia with air to nitric oxide 2. Oxidation of nitric oxide to form nitrogen dioxide 3. Absorption in water to give a solution of nitric acid and nitric oxide Single and Dual Pressure Plants The efficiency of the first step is favoured by low pressure whereas those of the second/third steps are favoured by high pressure. These considerations give rise to two types of nitric acid plants known as single pressure plants and dual pressure plants. In the single pressure plant, the oxidation and absorption steps take place at essentially the same pressure. In dual pressure plants absorption takes place at a higher pressure than the oxidation stage.

2 Information Centre Kittiwake Procal Ltd Page 2 of 6 Single pressure plants operate at either medium (1.7 and 6.5bar) or high (between 6.5 and 13bar) pressure. Dual pressure plants operate at medium pressure for the oxidation stage and high pressure for the absorption. The yield of nitric oxide depends on pressure and temperature as indicated in the table 1 below. Table 1: NO yield as a function of temperature and pressure Pressure (bar) Temperature ( C) Temperature ( F) NO yield (%) Low Pressure Below Medium Pressure 1.7 to High pressure Above A single pressure plant operating at medium pressure is shown in figure 1. The operations involved in the nitric acid process are the same for all plant types and are as follows Step 1 Oxidation of Anhydrous ammonia with air to nitric oxide Air is filtered and compressed to produce high purity air that is mixed with ammonia. The air/ammonia mix is then oxidised over catalytic platinum/rhodium alloy gauzes. Nitric oxide with a 93 to 98 percent yield and water are formed in this process according to equation (1) 4Nh 3 + 5O 2 4NO + 6H 2 O (1) Oxidation temperatures can vary from C (1380 to 1650 F) with higher catalyst temperatures increasing the reaction selectivity towards NO production. Lower catalyst temperatures tend to be more selective toward less useful products nitrogen (N 2 ) and nitrous oxide (N 2 O). 4Nh 3 + 3O 2 2N 2 + 6H 2 O (2) 4Nh 3 + 4O 2 2N 2 O + 6H 2 O (3) As equation (1) is an exothermic reaction and energy can be recovered by passing the nitric oxide gas through a waste heat boiler. The water in the boiler is converted to steam for generating power in a steam turbine. Step 2 Oxidation of Nitric Oxide After the energy recovery stage the process stream, which at this point has a temperature of C ( F), is passed through a cooler condenser where it is further cooled to 38 C (100 F) at pressures up to 7.89bar (116 psia). The water in the process stream is condensed and transferred to the absorption column. The nitric oxide reacts non catalytically with residual oxygen to form nitrogen dioxide and its liquid dimer, nitrogen tetroxide. 2NO +O 2 2NO 2 N 2 O 4 (4)

3 Information Centre Kittiwake Procal Ltd Page 3 of 6 This slow reaction is highly temperature and pressure dependent with low temperatures and high pressures leading to maximum production of NO 2 within a minimum reaction time. Step 3 Absorption in water to give a solution of nitric acid and nitric oxide After being cooled the nitrogen dioxide/dimer is introduced into the bottom of an absorption tower. Liquid di nitrogen tetroxide is added at a higher point and deionised water enters at the top of the column. The absorption tower contains absorption trays (sieve or bubble cap) where nitrogen dioxide gas is absorbed whilst oxidation takes place in the free space between the trays. The absorption of the nitrogen dioxide gas and its reaction to nitric acid and nitric oxide takes place simultaneously in the gaseous and liquid phases. 3NO 2 +H2O 2HNO 3 +NO (5) Reaction (5) is exothermic and continuous cooling is therefore required within the absorber. The nitric acid produced in the absorber contains dissolved oxides and a secondary air stream introduced into the column re oxidises the NO and removes (bleaches) the dissolved oxides. An aqueous solution of 55 to 65 percent nitric acid is withdrawn from the bottom of the tower. The acid concentration depends upon the temperature, pressure, number of absorption stages and concentration of nitrogen oxides entering the absorber. The bleached gases (tail gases) are compressed, passed through the absorber and sent to a mist separator where acid mist (acid liquid carry over) is removed. The waste tail gas is heated in the ammonia oxidation heat exchanger, expanded in a power recovery turbine and expelled from the effluent stack to the atmosphere. High Strength Nitric Acid Production A high strength nitric acid (98 to 99 percent concentration) can be obtained by concentrating the weak nitric acid (30 to 70 percent concentration) using extractive distillation as shown in figure 2. The distillation must be carried out in the presence of a dehydrating agent such as concentrated sulphuric acid (typically 60 percent). The nitric acid concentration process consists of feeding strong sulphuric acid and 55 to 65 percent nitric acid to a packed dehydrating column at approximately atmospheric pressure. The acid mixture flows downwards in the opposite direction to rising vapours and concentrated nitric acid leaves the top of the column as 99 percent vapour, containing a small amount of NO 2 and oxygen (O 2 ) resulting from the dissociation of nitric acid. The concentrated vapour leaves the column and goes to a bleacher and counter current condenser system to effect the condensation of strong nitric acid and the separation of oxygen and oxides of nitrogen (NOx) by-products. The by products then flow to an absorption column where the nitric oxide mixes with auxiliary air to form NO 2, which is recovered as weak nitric acid. Inert and un-reacted gases are vented to the atmosphere from the top of the absorption column.

4 Information Centre Kittiwake Procal Ltd Page 4 of 6 Types of Emission Emissions from nitric acid plants consist of NO, NO 2 and trace amounts of HNO 3 mist and ammonia (NH 3 ). Table 2 shows typical limits during stable operation. There will be an increased NOx level during start up until the process stabilizes. In a properly operated plant acid mist emissions should not occur as they are removed by the mist separator (as shown in figure 1). Table 2: Typical emissions from Nitric Acid plants Pollutant Min Conc. ppm Max Conc. ppm NOx N 2 O O 2 1% 4% H 2 O N 2 Balance Balance Flow 3,100 to 3,400Nm3.t 1 100% HNO3 The minimum emission levels achievable in a modern plant without NOx emission control systems are: For medium pressure absorption 1,000 to 2,000ppm For high pressure absorption 100 to 200ppm The tail gas from the absorption tower is the major source of nitrogen oxides as the NOx emissions depend on the kinetics of the nitric acid formation and the tower design. NOx emissions can increase if the air supply to the oxidiser and absorber is not optimised, if the absorber is at low pressure, if the cooler condenser and absorber are at high temperatures, if very high strength acid is produced, and when operated at high throughput rates. Careful attention to compressors and pumps must be maintained as faults can lead to low pressures and leaks and decrease the plant efficiency. Control of Emissions NOx emissions are controlled using extended absorption and both non selective catalytic reduction (NSCR) and selective catalytic reduction (SCR) control systems. Emissions of nitrous oxide (N 2 O) are influenced by the degree and type of NOx emission control efforts that are applied in both new and existing nitric acid plants. NSCR is very effective at controlling N 2 O while SCR can actually increase N 2 O emissions. Extended absorption can be implemented on existing nitric acid plants to reduce NOx emissions. The method increases the efficiency of the existing absorption tower by either increasing its size and the number of absorption trays that can be fitted or by the addition of a second absorption tower. The efficiency can be increased by operating the absorber at high pressures (NOx level reduced to less than 100ppm), or cooling the weak acid liquid in the absorber.

5 Information Centre Kittiwake Procal Ltd Page 5 of 6 NSCRs use a fuel and a catalyst to consume free oxygen in the tail gas and convert NOx to elemental nitrogen. They have the advantage that they can reduce N 2 O emissions by percent. The gas from the NOx abatement can be passed through a gas expander for energy recovery and are expelled from the stack at high temperatures. NSCRS are unpopular due to high fuel costs and are not retrofitted as the system requires a high temperature gas expander that must be constructed from exotic materials. In addition, NSCRs have high maintenance costs and on older plants the restructuring of the heat recovery system would be required making installation uneconomical. A study from 1993 estimated that only 20 percent of nitric acid plants in the USA constructed between had NSCR control. An SCR can be added to an existing plant and can substantially reduce low NOx emissions. In SCRs ammonia is mixed with the preheated NOx tail gases and reacts with them in the presence of a catalyst such as vanadium pentoxide or platinum to produce nitrogen and water. The readily available supply of ammonia from the nitric acid plant makes this an attractive emission control system. The tail gas exiting the expander must be kept at a high temperature to avoid Ammonium salt deposits. The disadvantages of the SCR system are that a small escape of ammonia (known as ammonia slip) can occur and N 2 O emissions are not reduced. Figure 1: Single Pressure Plant at medium pressure Procal P2000 Liquid Ammonia High Purity Air

6 Information Centre Kittiwake Procal Ltd Page 6 of 6 Figure 2: High Strength Nitric Acid Production from Weak Nitric Acid 60% H 2 SO 4 Cooling Water Air Inert Unreacted Gases Cooler Condenser Absorption Tower 55-65% HNO 3 HNO 3,NO 2,O 2 O 2, NO Dehydrating Column Bleacher Strong Nitric Acid Weak Nitric Acid Use of Procal P2000 Analyser The Procal 2000 is an infra red duct or stack mounted analyser, designed to provide in Situ analysis of up to six gas phase emission components. The Procal 2000 uses the reflective beam principle to directly measure process gas as it enters the in situ sample cell. Mid IR pulses, at two specific wavelengths per monitored component, are transmitted through the sample cell. The measure pulse is partially absorbed by the gases being measured while the reference pulse remains unaffected. Up to eight wavelengths are available, sometimes sharing reference wavelengths, allowing up to six gas phase component concentrations to be monitored simultaneously. Uniquely, the operation, zero and calibration are fully challenged in that all operating modes use the same optical path and system components. The Procal 2000 can measure nitic oxide (NO) up to 300ppm (0 400mg/Nm3) and can also measure nitrogen dioxide (NO2). The Procal P2000 can be situated on both the inlet and outlet of the catalytic reactor and can be used to monitor the efficiency of the catalytic reactor and control its operation.