Nitric acid plants

For the production of nitric acid, ammonia passes over a platinum catalyst and is oxidized to nitric oxide. The NO is further oxidized into NO2 which is absorbed in water to form HNO3. The removal of NO in the exhaust from nitric acid plants differs from that of power plants because of the different NO NO2 ratio. For nitric acid plants and power plants this ratio is 1 and 19, respectively. 

In Germany, catalysts for nitric acid plants were developed in the sixties [142]. Catalysts such as iron oxide/chromium oxide and supported vanadia were used. Off-gas from nitric acid plants contains about 2000 ppm NO, and 3 vol. % O2. The gas flows to be cleaned are about lOOOOOm h. A reduction of the amount of NOv from 10000 to 400mgm NO2 is achievable at space velocities ranging from 3000 to 10 000 h-.  

Andersen et al. [143] published results on the removal of NO over supported platinum catalysts. Other effective catalysts included palladium, ruthenium, cobalt, and nickel. Maximum conversion of NO was acquired at a temperature of about 

At higher temperatures the conversion decreases due to the oxidation of ammonia. 

Blanco et al. [114] describe a CuO/NiO on y-alumina catalyst for conversion of equimolar mixtures of NO and NO2 with ammonia and oxygen. 

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NO Abatement. Source performance standards for nitric acid plants in the United States were introduced by the U.S. EPA in 1971 (55). These imposed a discharge limit of 1.5 kg of NO as equivalent nitrogen dioxide per 1000 kg of contained nitric acid, which corresponds to about 200—230... 

The resulting nitrous oxide can be recirculated to the nitric acid plant or be used for other purposes. Free acid remaining in the impregnation water of sodium nitrate crystals is neutralized by adding some NaOH to the washing water. Whereas several nitric acid plants utilize absorption of nitrous gases to treat tail gases, almost all of these plants produce small volumes of sodium nitrate. 

Industrial production of sodium nitrite is by absorption of nitrogen oxides (NO ) into aqueous sodium carbonate or sodium hydroxide. NO gases originate from catalytic air oxidation of anhydrous ammonia, a practice common to nitric acid plants ... 

Power Recovery in Other Systems. Steam is by far the biggest opportunity for power recovery from pressure letdown, but others such as tailgas expanders in nitric acid plants (Fig. 1) and on catalytic crackers, also exist. An example of power recovery in Hquid systems, is the letdown of the high pressure, rich absorbent used for H2S/CO2 removal in NH plants. Letdown can occur in a turbine directiy coupled to the pump used to boost the lean absorbent back to the absorber pressure. 

Axial Flow Compressors Axial flow compressors are used mainly as compressors for gas turbines. They are also used in the steel industiy as blast furnace blowers and in the chemical industry for large nitric acid plants. Thev are mainly used for apphcations where the head required is low and the flow large. 

Industrial-Commercial-Institutional Steam Generating Units Incinerators Portland Cement Plants Nitric Acid Plants Sulfuric Acid Plants Asphalt Concrete Plants Petroleum Refineries... 

SELECTION AND DESIGN OF TURBOCOMPRESSORS FOR NITRIC ACID PLANTS... 

Although this hook deals almost exclusively with modern turho-expanders, the tail gas expander in nitric acid plants enjoys a somewhat special relationship with the compressors that are almost always associated with this turhotrain. 

Figure 4-12 presents an overall picture of the compressor duty ranges and machine comhinations applied in a typical nitric acid plant, including capacities and pressure ratios. The diagram represents typical... 

Figure 4-12. Duty range for turbocompressors in nitric acid plants. The diagram refers to atmospheric air and gases with similar properties, such as nitrous gas (A = axial, R = radial flow compressor).

Having two degrees of reaction available facilitates the matching of turbocompressors in nitric acid plants, especially in dual-pressure installations, because these two different degrees of reaction may be mixed within the same compressor. 

Figure 4-17. Typical performance curves for a turbine-driven compressor train in a nitric acid plant.

Figure 4-20. Duty range of expanders for nitrous gases in nitric acid plants. The bottom diagram refers to uncooled expanders (inlet temperature 500°C) and the top diagram refers to cooled expanders (inlet temperature 730°C).

Figure 4-21. Tail-gas expander for a 945-t/day two-pressure nitric acid plant P = 11,200 kW, n = 5,850 rpm. (Source GHH-Borsig.)...

The considerable amount of heat generated in nitric acid plants suggests that steam be produced and used to drive the compressors. 

Modern nitric acid plants are designed for energy self-sufficiency during normal operation. Except for the startup phase, process heat generated will equal energy consumed by the compressors. Moreover, in many cases surplus energy can be exported in the form of steam, for example. 

Electric motors may be considered in cases where it is considered advantageous to export surplus steam outside the nitric acid plant. Condensing steam turbines are normally used to bridge the power deficit. Extraction-condensing turbines make it possible to use some of the available steam for heating purposes. 

A back-pressure (noncondensing) turbine may also be used if there is a profitable use for intermediate-pressure steam. In the unlikely event that large quantities of steam are required, additional high-pressure steam from an external source might be necessary. However, while it is theoretically possible that the amount of heat generated in the nitric acid plant will be insufficient to cover the entire demand, this is not usually a valid concern. 

In cases where low-cost fuel is available, a gas turbine may be chosen. This would obviously make it possible to export the surplus steam. Some compressor trains in nitric acid plants are, indeed, using a gas turbine as the associated or complementary driver. 

Figure 4-25. Nitric acid plant at Fredericia (Denmark) with a four-machine turbogroup and generator. Power output to district heating system = 0-27 MW Power output to electric grid = 0-5.8 MW Steam turbine = 10.8 MW Axial compressor = 8.2 MW Centrifugal compressor = 4.1 MW Expander = 7.4 MW Nitric acid production = 650 t/d.

Figure 4-26. Nitric acid plant at Fredericia (Denmark) with turbogroup and generator.

Nitrous gases originating from the combustion units in nitric acid plants carry small amounts of unreacted ammonia, NH3. The ammonia may react with the nitrous gas to form microscopic particles of ammonium nitrate that adhere to solid surfaces. Within a short time, there is a growing layer of ammonium nitrate salt covering the internal surface of the nitrous gas compressor (Figure 4-27). This layer can obstruct the flow passages because it tends to increase the power consumption, provoke excessive vibrations, and even present a safety hazard since ammonium nitrate explosions can occur. 

Starting up the turbocompressor installation of a nitric acid plant does not present a problem. As mentioned earlier, during the startup phase the expander is not able to contribute any power. Accordingly, electric motor drivers must initially provide power in excess of the nominal operational rating. 

Flue gas treatment (FGT) is more effective in reducing NO, emissions than are combustion controls, although at higher cost. FGT is also useful where combustion controls are not applicable. Pollution prevention measures, such as using a high-pressure process in nitric acid plants, is more cost-effective in controlling NO, emissions. FGT technologies have been primarily developed and are most widely used in Japan. The techniques can be classified as selective catalytic reduction, selective noncatalytic reduction, and adsorption. 

Nitric Acid Plant - Nitrogen oxide levels should be controlled to a maximum of 1.6 kg/t of 100% nitric acid. Extended absorption and technologies such as nonselective catalytic reduction (NSCR) and selective catalytic reduction (SCR) are used to eontrol nitrogen oxides in tail gases. 

Ammonium Nitrate Plants - In ammonium nitrate plants, wet scrubbers can be considered for prill towers and the granulation plant. Particulate emissions of 0.5 kg/t of product for the prill tower and 0.25 kg/t of product for granulation should be the target. Similar loads for ammonia are appropriate. Other effluents that originate in a nitrogenous fertilizer complex include boiler blowdown, water treatment plant backwash, and cooling tower blowdown from the ammonia and nitric acid plants. 

The plant disposes of two waste streams gaseous and aqueous. The gaseous emission results from the ammonia and the artunonium nitrate plants. It is fed to an incinerator prior to atmospheric disposal. In the incinerator, ammonia is converted into NOj,. Ehie to more stringent NO regulations, the conqmsition of ammonia in the feed to the incinerator has to be reduced from 0.57 wt% to 0.07 wt%. The lean streams presented in Table 9.5 may be employed to remove ammonia. The main aqueous waste of the process results from the nitric acid plant. Due to its acidic content of nitric acid, it is neutralized with an aqueous ammonia solution before biotreatment. 

Adds One of the most extensive uses of austenitic steels has been for synthetic nitric acid plant, including oxidation coolers, absorption towers, storage vessels and pipelines, as well as transport tanks. For storage of cold 98% acid or for near-boiling solutions above 60% strength, 347S31 or 304S11 should be used. 

Nitric acid plants Tantalum heat exchangers and sparge pipes find extensive use in plants producing high purity nitric acid, ammonium nitrate and terephthalic acid. 

Later, when nitric acid was manufd from synthetic ammonia at relatively low cost, synthetic sodium nitrate was made from it either thru the reaction between nitric acid and soda ash, or by direct absorption of nitrogen dioxide in an aq soln of Na carbonate (see above equation). The Na nitrate-nitrite soln was then heated with excess nitric acid to convert the nitrite to nitrate, and the NO thus produced was recycled to the nitric acid plant (Ref 6)... 

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