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Nitric acid is a chemical intermediate primarily used in the synthesis of ammonium nitrate, which is used in the manufacture of fertilizers. The acid also is important in the production of other nitrates and in the separation of metals from ores. Nitric acid may be produced by oxidizing ammonia to nitric oxide over a platinum-rhodium catalyst, then oxidizing the nitric oxide to nitrogen dioxide in a separate unit where it is absorbed in water to form an aqueous solution of nitric acid.The reaction sequence is as follows:$$\begin{aligned} 4 \mathrm{NH}_{3}+5 \mathrm{O}_{2} & \rightarrow 4 \mathrm{NO}+6 \mathrm{H}_{2} \mathrm{O} \\\4 \mathrm{NO}+2 \mathrm{O}_{2} & \rightarrow 4 \mathrm{NO}_{2} \\\4 \mathrm{NO}_{2}+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})+\mathrm{O}_{2} & \rightarrow 4 \mathrm{HNO}_{3}(\mathrm{aq}) \end{aligned}$$.Ammonia vapor produced by vaporizing pure liquid ammonia at 820 kPa absolute is mixed with air, and the combined stream enters the ammonia oxidation unit. Air at \(30^{\circ} \mathrm{C}, 1\) atm absolute, and \(50 \%\) relative humidity is compressed and fed to the process. A fraction of the air is sent to the cooling and hydration units, while the remainder is passed through a heat exchanger and mixed with the ammonia. The total oxygen fed to the process is the amount stoichiometrically required to convert all of the ammonia to HNO \(_{3},\) while the fraction sent to the ammonia oxidizer corresponds to the stoichiometric amount required to convert ammonia to NO.The ammonia reacts completely in the oxidizer, with \(97 \%\) forming NO and the rest forming \(\mathrm{N}_{2}\). Only a negligible amount of \(\mathrm{NO}_{2}\) is formed in the oxidizer. However, the gas leaving the oxidizer is subjected to a series of cooling and hydration steps in which the NO is completely oxidized to \(\mathrm{NO}_{2}\) which in turn combines with water (some of which is present in the gas from the oxidizer and the rest is added) to form a 55 wt\% aqueous solution of nitric acid. The product gas from the process may be taken to contain only \(\mathrm{N}_{2}\) and \(\mathrm{O}_{2}\). (a) Taking a basis of \(100 \mathrm{kmol}\) of ammonia fed to the process, calculate (i) the volumes \(\left(\mathrm{m}^{3}\right)\) of the ammonia vapor and air fed to the process using the compressibility-factor equation of state; (ii) the amount (kmol) and composition (in mole fractions) of the gas leaving the oxidation unit; (iii) the required volume of liquid water \(\left(\mathrm{m}^{3}\right)\) that must be fed to the cooling and hydration units; and (iv) the fraction of the air fed to the ammonia oxidizer. (b) Scale the results from Part (a) to a new basis of 100 metric tons per hour of 55\% nitric acid solution.(c) Nitrogen oxides (collectively referred to as \(\mathrm{NO}_{x}\) ) are a category of pollutants that are formed in many ways, including processes like that described in this problem. List the annual emission rates of the three largest sources of \(\mathrm{NO}_{x}\) emissions in your home region. What are the effects of exposure to excessive concentrations of \(\mathrm{NO}_{x} ?\) (d) A platinum-rhodium catalyst is used in ammonia oxidation. Fxplain the function of the catalyst, describe its structure, and explain the relationship of the structure to the function.

Step by step solution

01

Calculate the volume of the ammonia vapor

The ammonia vapor volume can be calculated using the compressibility-factor equation of state \(PV = ZnRT\), where P is the pressure, V is the volume, Z is the compressibility factor, n is the moles of gas, R is the universal gas constant, and T is the temperature.

02

Determine the composition of the gas leaving the oxidation unit

In the ammonia oxidizer, 97% of the ammonia is converted to \(NO\), with the remaining 3% converting to \(N_{2}\). Mole fractions can be calculated by dividing the amount of each component by the total moles of gas, which includes the reaction products and the unreacted \(O_{2}\).

03

Calculate the required volume of liquid water

This can be calculated using the stoichiometry of the reactions involved in the cooling and hydration units, considering the conversion of \(NO_{2}\) into \(HNO_{3}\). The molar ratios from the reaction equations will be used to calculate the required moles of water, and hence the volume.

04

Determine the fraction of air fed to the ammonia oxidizer

This can be determined by using the stoichiometry of the reaction in the oxidizer, which involves ammonia and \(O_{2}\), and their molar relation.

05

Scale the results to a new basis

This can be done by converting the given basis of \(100 \, kmol\) of ammonia to 100 metric tons per hour of 55% nitric acid solution, considering the molecular weights of the substances involved.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Nitric Acid Production

Nitric acid, a critical chemical, is primarily produced through the oxidation of ammonia. This process is known as the Ostwald process and involves key steps that transform ammonia into nitric acid.
Initially, ammonia is oxidized to nitric oxide (NO) using a platinum-rhodium catalyst. This catalyst facilitates the reaction, making it efficient even at lower temperatures.
Subsequently, the nitric oxide is further oxidized to nitrogen dioxide (NO₂). Finally, NO₂ is absorbed in water, resulting in nitric acid (HNO₃).
This process not only supports fertilizer production but also contributes to other industries like metal refining and chemical manufacturing.

Ammonium Nitrate Synthesis

Ammonium nitrate is synthesized using nitric acid as a key component. It's a crucial ingredient in fertilizers due to its high nitrogen content, which is vital for plant growth.
In the synthesis process, ammonia gas is mixed with nitric acid. This reaction produces ammonium nitrate, a compound known for its highly soluble nature.

• Reaction: \( ext{NH}_3 + ext{HNO}_3 \rightarrow ext{NH}_4 ext{NO}_3\)

This simple yet powerful reaction highlights the interconnectedness of chemical processes in agriculture and industry.

Catalytic Reaction Mechanisms

Catalysts play a vital role in accelerating chemical reactions without being consumed. In nitric acid production, a platinum-rhodium catalyst is essential for converting ammonia to nitric oxide.
This catalyst enables the reaction by reducing the activation energy needed. Its structure, a finely distributed metal surface, provides sites for the reaction to occur efficiently.

• Benefits: Lower energy consumption, increased reaction speed, and higher yield.

Such catalysts are fundamental in industrial processes, driving efficiency and sustainability.

Environmental Impact of Nitrogen Oxides

Nitrogen oxides (NOâ‚“) are notable environmental pollutants derived from industrial processes like nitric acid production.
Major sources of NOâ‚“ emissions include vehicles, power plants, and industrial facilities. These pollutants contribute to smog, acid rain, and respiratory problems in humans.

• Prevention: Adopting cleaner technologies and improving catalyst efficiency can reduce these emissions.

Tackling NOâ‚“ emissions is crucial for environmental protection and public health, aligning industry with sustainability goals.