The Acid Queen
What else can we do with ammonia? Well, we can also convert it to nitric acid. Nitric acid is mainly used as a base chemical for the fertilizer industry. Secondary uses are primarily found in the explosives industry (which is somewhat related to the fertilizer industry). Nitric acid is mainly produced in concentrations ranging from 50% to 67%. Since the other component is mainly water and it is very uneconomical to transport water, you will find most nitric acid plants on a location where the acid can be used directly.
The first step in the synthesis of nitric acid is the oxidation of ammonia over platinum gauzes. The platinum acts as a catalyst and several reactions may occur, leading to a mix of nitrogen (N2), nitrous oxide (N2O), nitric oxide (NO) and water. By optimizing reaction conditions we try to maximize the yield of nitric oxide. The conversion from ammonia to nitric oxide also generates a lot of heat. This heat is recovered in a train of heat exchangers and used for several purposes. The heat recovery cools the process gas and during the cooling process nitric oxide reacts with the remaining oxygen to form nitrogen dioxide (NO2).
Due to the high temperature that arises during the conversion over platinum, some platinum evaporates and resolidifies during the cooling down phase. Since platinum is a very expensive metal, most nitric acid plants have some sort of catching device (mostly a filter) to recover the lost platinum.
After the heat exchanger train, the process gas is cooled further to a temperature where the water, present in the gas as a by-product from the conversion of ammonia to nitric oxide, starts to condensate. The condensed water absorbs the nitrogen dioxide and thus forms nitric acid. This acid is commonly referred to as "weak" acid, although concentrations may be higher than 50 %. The remaining gas is separated from the acid, and led to the bottom of a large absorber tower. The acid enters the absorption tower somewhere halfway and absorbs extra nitrogen dioxide, increasing the acid concentration. At the top of the absorber tower, water is added, which absorbs the larger part of the remaining nitrogen dioxide.
The gas leaving the absorption tower still contains to much nitrogen dioxide to vent directly to the atmosphere (at least in most countries!). Therefore you will find a catalytic NOx-abatement system in most nitric acid plants, that converts the remaining nitrogen oxide to nitrogen en water, before the gases are vented to the atmosphere.
Back to the acid that leaves the absorption tower. This acid is not clean enough to be sold or used. The acid is introduced into a bleaching tower (a lot smaller then the absorption tower!), where air is led into the bottom. The air absorbs the pollutants and clean acid leaves the bleacher. After that the acid is stored in storage tanks for further use.
The nitric acid production process can be roughly split into two main parts: The conversion of ammonia over platinum and the absorption of nitrogen dioxide in water. Conversion is best carried out at low pressure, while absorption is more effective at high pressure. In the past, nitric acid plants operated at a single pressure because there was no technology available that could compress the highly corrosive process gas in between the conversion process and the absorption process. As a result you will find three different types of single pressure plants: low, medium and high pressure plants.
Before we continue, something has to be said about the catalyst (consisting mainly of platinum, some rhodium and sometimes palladium). First, this catalyst (especially the platinum) is very expensive. The amount needed in a medium size plant will cost several millions of dollars. Secondly, the catalyst "wears" during its usage, due to erosion and temperature. Especially temperature is important, because at higher temperatures the platinum will evaporate and the catalyst will loose its efficiency. The catalyst will therefore have to be replaced at regular intervals. Such an interval is called a "campaign" and as you may expect, a lot of research goes into creating gauzes that extend the campaign length.
Due to the high cost of the gauzes it is very important to recover as much platinum as possible. This leads to the somewhat comical situation that the space below the gauzes is vacuumed after a campaign to gather the smallest particles of platinum (Although "good housekeeping" is a major issue in most chemical plants, this is the only time I've encountered a vacuum cleaner!).
Back to the difference between low, medium and high pressure plants. A low pressure plant has the advantage, that conversion takes place at a low temperature. Therefore, the catalyst does not evaporate and deteriorate very quickly. This results in long campaign lengths for low pressure plants. A major drawback, is that the amount of nitrogen dioxide that can be absorbed is limited, leading to" low" nitric acid concentration in the end product.
A high pressure plant has the advantage that the end product can be a much more concentrated acid. The drawback here is that the conversion temperature is much higher, leading to a more rapid deterioration of the catalyst. Therefore campaign lengths of high pressure plants are considerably shorter (generally 90 to 120 days).
A medium pressure plant is, as can be expected, a compromise between the low and high pressure plant.
Modern plants are generally built as a dual pressure plant. Conversion is executed at low pressure and absorption at high pressure. The compromise between long campaign lengths and high acid concentration is in this case largely eliminated. The design of dual pressure plants has only become possible when new construction materials were available that could withstand the highly corrosive environment in which the process gas has to be compressed in between the conversion and absorption process.