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Chapter 9: Problem 56

Various uses for nitric acid are given in Problem \(6.43,\) along with information about how this important chemical is synthesized industrially. The key reactions are oxidations of ammonia to nitric oxide and of nitric oxide to nitrogen dioxide, followed by dissolution of \(\mathrm{NO}_{2}\) in water: $$\begin{aligned} 4 \mathrm{NH}_{3}(\mathrm{g})+5 \mathrm{O}_{2}(\mathrm{g}) & \rightarrow 4 \mathrm{NO}(\mathrm{g})+6 \mathrm{H}_{2} \mathrm{O}(\mathrm{v}) \\ 2 \mathrm{NO}(\mathrm{g})+\mathrm{O}_{2}(\mathrm{g}) & \rightarrow 2 \mathrm{NO}_{2}(\mathrm{g}) \\ 3 \mathrm{NO}_{2}(\mathrm{g})+\mathrm{H}_{2} \mathrm{O}(1) & \rightarrow 2 \mathrm{HNO}_{3}(\mathrm{aq})+\mathrm{NO}(\mathrm{g}) \end{aligned}$$ Nitric oxide generated on dissolution of \(\mathrm{NO}_{2}\) in water is oxidized to produce additional \(\mathrm{NO}_{2},\) which is then combined with water to form more \(\mathrm{HNO}_{3}\). In this problem we neglect side reactions that would lower the product yield. Ammonia vapor at \(275^{\circ} \mathrm{C}\) and 8 atm is mixed with air, also at \(275^{\circ} \mathrm{C}\) and 8 atm, and the combined stream is fed to a converter. Fresh air entering the system at \(30^{\circ} \mathrm{C}\) and 1 atm with a relative humidity of \(50 \%\) is compressed to \(100^{\circ} \mathrm{C}\) and 8 atm, and the compressed air then exchanges heat with the product gas leaving the converter. The quantity of oxygen in the feed to the converter is \(20 \%\) in excess of the amount theoretically required to convert all of the ammonia to \(\mathrm{HNO}_{3}\). The entire process after the compressor may be taken to operate at a constant pressure of 8 atm. In the converter, the ammonia is completely oxidized, with a negligible amount of \(\mathrm{NO}_{2}\) formed. The product gas leaves the converter at \(850^{\circ} \mathrm{C}\), and, as described in the preceding paragraph, exchanges heat with the air entering the system. The product gas then is fed to a waste-heat boiler that produces superheated steam at \(200^{\circ} \mathrm{C}\) and 10 bar from liquid water at \(35^{\circ} \mathrm{C}\). The product gas leaving the wasteheat boiler is cooled further to \(35^{\circ} \mathrm{C}\) and fed to an absorption column in which the NO is completely oxidized to \(\mathrm{NO}_{2},\) which in turn combines with water (some of which is present in the product gas). Water is fed to the absorber at \(25^{\circ} \mathrm{C},\) at a rate sufficient to form a 55 wt\% aqueous nitric acid solution. The NO formed in the reaction of \(\mathrm{NO}_{2}\) to produce \(\mathrm{HNO}_{3}\) is oxidized, and the NO \(_{2}\) produced is hydrated to form still more \(\mathrm{HNO}_{3}\). The off-gas from the process may be taken to contain only \(\mathrm{N}_{2}\) and \(\mathrm{O}_{2}\) (a) Construct a flowchart showing all process streams, including input and output from the process and the following equipment: converter, air compressor, exchanger recovering heat from the converter product, waste-heat boiler producing superheated steam, exchanger cooling the product gas before it is fed to the absorber, and absorber. (b) Taking a basis of \(100 \mathrm{kmol}\) of ammonia fed to the process, develop spreadsheets (preferably incorporating the use of APEx) to determine the following: (i) Molar amounts (kmol) of oxygen, nitrogen, and water vapor in the air fed to the process, cubic meters of air fed to the process, and kmol of water fed to the absorber. (ii) Molar amounts, molar composition, and volume of the off-gas leaving the absorber. (iii) Mass (kg) of product nitric acid solution. (iv) Molar amounts and composition of the gas leaving the converter. (v) Heat removed from or added to (state which) the converter. (vi) Temperature of the product gas after it has exchanged heat with the air, assuming no heat is transferred between the heat exchanger and the surroundings. (vii) Production rate of superheated steam if the gas temperature leaving the boiler is \(205^{\circ} \mathrm{C}\). Before performing this calculation, determine if condensation of water occurs when the gas is cooled to \(205^{\circ} \mathrm{C}\). Since the superheated steam temperature is \(200^{\circ} \mathrm{C}\), explain why the selected temperature of the product gas is reasonable. (viii) Heat removed from the product gas before it is fed to the absorber (Hint: Check the condition of the gas at \(35^{\circ} \mathrm{C}\) ) and mass of cooling water required to remove that heat if the water temperature can only be increased by \(5^{\circ} \mathrm{C}\). Assume no heat is transferred between the heat exchanger and the surroundings. (ix) Heat removed from or added to the absorber. Assume the heat capacity of the nitric acid solution is approximately the same as that of liquid water and the outlet temperatures of the off-gas and product streams are \(25^{\circ} \mathrm{C}\) and \(35^{\circ} \mathrm{C}\), respectively. (c) Scale up the results calculated in Part (b) to determine all stream flow rates and heat transfer rates for a production rate of \(5.0 \times 10^{3} \mathrm{kg} / \mathrm{h}\) of the product solution.

Short Answer

Expert verified
The output of this problem will be a detailed breakdown of the chemical and thermal processes involved in converting a known quantity of ammonia and oxygen into nitric acid. This includes quantities of gases involved, heat transferred, and final nitric acid product. The final solution will also include scaled-up process parameters for industrial-grade production.

Step by step solution

01

Construct Flowchart

The first step is to construct a flowchart showing all process streams and equipment involved. Map out all the given information about temperature, pressure, and compositions of different streams into a visual representation. You may want to use different colors or symbols to represent different gases, states of matter, and the direction of gas flow.
02

Calculate Inputs to Process

Next, a basis of 100 kmol of ammonia fed into the process is given. Use the provided information about how ammonia reacts with oxygen to derive the input quantities of oxygen, nitrogen, and water vapor. Knowing that 20% excess oxygen is used, the stoichiometry of the chemical reactions and the molar masses of each gas can be used to derive quantities.
03

Calculate Outputs from Converter

Knowing that all the ammonia is completely oxidized in the converter helps to compute the output from the converter. Use stoichiometry of the chemical reactions and the given conditions to deduce the molar amounts, molar composition, and mass of the product gas leaving the converter.
04

Calculate Heat Transfer in Converter

Next, determine the heat removal from the converter using the principles of heat and material balances. If heat is transferred to the surroundings, then the heat out is more than heat in, and vice versa.
05

Calculate After Heat Exchange

In this step, use the data about heat exchange with air and compute the final temperature of the product gas. Assume that no heat is transferred between the heat exchanger and surroundings.
06

Calculate After Waste-Heat Boiler

Next, consider the product gas going through the waste-heat boiler where it produces superheated steam. Calculate the production rate of superheated steam and determine the condition of the gas after it leaves the boiler by examining if condensation of water occurs when the gas is cooled.
07

Calculate Before and After Absorber

Next, analyze the product gas entering and leaving the absorber. Determine the heat removal from the product gas before it enters the absorber and the mass of cooling water required to remove that heat. Then figure out the composition of the gas and heat transfer in the absorber.
08

Scale Up Process to Real Production Rates

Finally, scale up all the results calculated to reflect the actual production rate of 5.0 x 10^3 kg/h of the nitric acid. Determine all the stream flow rates and heat transfer rates needed to achieve this level of production.

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Key Concepts

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

Nitric Acid Synthesis
Nitric acid synthesis involves a series of chemical reactions starting with ammonia. Ammonia is first oxidized to form nitric oxide (NO), followed by the oxidation of nitric oxide to nitrogen dioxide (NO鈧�). Finally, nitrogen dioxide is dissolved in water to produce nitric acid (HNO鈧�) and a small amount of nitric oxide. These reactions include:
  • Ammonia is oxidized by oxygen to form nitric oxide: \( 4 \text{NH}_3 + 5 \text{O}_2 \rightarrow 4 \text{NO} + 6 \text{H}_2\text{O} \).
  • Nitric oxide is further oxidized to nitrogen dioxide:\( 2 \text{NO} + \text{O}_2 \rightarrow 2 \text{NO}_2 \).
  • Nitrogen dioxide dissolves in water forming nitric acid and regenerates some nitric oxide:\( 3 \text{NO}_2 + \text{H}_2\text{O} \rightarrow 2 \text{HNO}_3 + \text{NO} \).
Each step is crucial to efficiently and successfully produce nitric acid, a vital chemical used in agriculture and industry. Understanding the reaction stoichiometry is critical for this process as it determines the feed ratios and impacts the heat and material balances that follow.
Flowchart Construction
Creating a flowchart is an essential step in visualizing and understanding the chemical process. It involves mapping the entire nitric acid synthesis process, equipment, and streams visually. When constructing a flowchart:
  • Include all processes 鈥� from air compression to waste-heat recovery, and absorption.
  • Use arrows to denote the direction of material flow and symbols to represent different states (gas, liquid) or chemical species.
  • Label each process stream with relevant data like temperature, pressure, and composition, which helps track changes through the process.
  • Ensure to display all segments such as the converter, compressors, heat exchangers, boilers, and absorbers.
This tool helps in keeping track of what enters the system, what happens inside it, and what exits, thus aiding in solving more complex calculations related to heat and material balances.
Heat and Material Balances
Heat and material balances are critical in understanding the efficiency and performance of a process. These calculations determine how much energy and material is consumed or recovered throughout the process.
  • **Material balances:** Start by ensuring that the input and output materials are balanced. For nitric acid synthesis, account for the ammonia, oxygen, nitrogen, and water in the feed and nitric acid, nitrogen, and water in the products.
  • **Heat balances:** Assess the enthalpy change across the reactions and equipment. In the process, heat is exchanged mainly in the converter and through heat exchangers. This includes capturing exothermic reaction heat and reusing it efficiently.
  • When ammonia is oxidized, heat is released. Calculating the heat removed from the converter and added back through the heat exchanger will affect other operational temperatures.
  • Utilizing these balances ensures that energy consumption is minimized and the integrity of the process is maintained. It provides insight into where energy losses or gains occur, helping to optimize operations.
Process Scaling Up
Scaling up a chemical process involves increasing production to meet commercial demands while keeping efficiency and product quality high. For nitric acid synthesis, all flow rates and heat transfers must be adapted to a production rate of 5000 kg/h.
  • First, scale material flow rates, ensuring inputs and outputs remain proportionate with lab-scale results.
  • Adapt equipment sizes, such as heat exchangers and absorbers, to handle increased volumes and maintain proper operational conditions.
  • Consider heat transfer capacity 鈥� more product will generate more heat. Equipment must dissipate or capitalize on this effectively.
  • Maintain safety and environmental regulations, increased volumes can intensify potential hazards or waste emissions.
Scaling up is not just multiplying numbers; it's ensuring the entire process can maintain performance, quality, and safety at higher rates.

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Most popular questions from this chapter

You are checking the performance of a reactor in which acetylene is produced from methane in the reaction $$2 \mathrm{CH}_{4}(\mathrm{g}) \rightarrow \mathrm{C}_{2} \mathrm{H}_{2}(\mathrm{g})+3 \mathrm{H}_{2}(\mathrm{g})$$ An undesired side reaction is the decomposition of acetylene: $$\mathrm{C}_{2} \mathrm{H}_{2}(\mathrm{g}) \rightarrow 2 \mathrm{C}(\mathrm{s})+\mathrm{H}_{2}(\mathrm{g})$$ Methane is fed to the reactor at \(1500^{\circ} \mathrm{C}\) at a rate of \(10.0 \mathrm{mol} \mathrm{CH}_{4} / \mathrm{s}\). Heat is transferred to the reactor at a rate of \(975 \mathrm{kW}\). The product temperature is \(1500^{\circ} \mathrm{C}\) and the fractional conversion of methane is 0.600 . A flowchart of the process and an enthalpy table are shown below. (a) Using the heat capacitics given below for enthalpy calculations, write and solve material balances and an energy balance to determine the product component flow rates and the yield of acctylene (mol \(\mathbf{C}_{2} \mathbf{H}_{2}\) produced/mol \(\mathbf{C H}_{4}\) consumed). $$\begin{aligned}\mathrm{CH}_{4}(\mathrm{g}): & C_{p} \approx 0.079 \mathrm{kJ} /\left(\mathrm{mol} \cdot^{\circ} \mathrm{C}\right) \\ \mathrm{C}_{2} \mathrm{H}_{2}(\mathrm{g}): & C_{p} \approx 0.052 \mathrm{kJ} /\left(\mathrm{mol} \cdot^{\circ} \mathrm{C}\right) \\ \mathrm{H}_{2}(\mathrm{g}): & C_{p} \approx 0.031 \mathrm{kJ} /\left(\mathrm{mol} \cdot^{\circ} \mathrm{C}\right) \\ \mathrm{C}(\mathrm{s}): & C_{p} \approx 0.022 \mathrm{kJ} /\left(\mathrm{mol} \cdot^{\circ} \mathrm{C}\right)\end{aligned}$$ For example, the specific enthalpy of methane at \(1500^{\circ} \mathrm{C}\) relative to methane at \(25^{\circ} \mathrm{C}\) is \(\left[0.079 \mathrm{kJ} /\left(\mathrm{mol} \cdot^{\circ} \mathrm{C}\right)\right]\left(1500^{\circ} \mathrm{C}-25^{\circ} \mathrm{C}\right)=116.5 \mathrm{kJ} / \mathrm{mol}\) (b) The reactor efficiency may be defined as the ratio (actual acetylene yield/acetylene yield with no side reaction). What is the reactor efficiency for this process? (c) The mean residence time in the reactor \([\tau(\mathrm{s})]\) is the average time gas molecules spend in the reactor in going from inlet to outlet. The more \(\tau\) increases, the greater the extent of reaction for every reaction occurring in the process. For a given feed rate, \(\tau\) is proportional to the reactor volume and inversely proportional to the feed stream flow rate. (i) If the mean residence time increases to infinity, what would you expect to find in the product stream? Explain. (ii) Someone proposes running the process with a much greater feed rate than the one used in Part (a), separating the products from the unconsumed reactants, and recycling the reactants. Why would you expect that process design to increase the reactor efficiency? What else would you need to know to determine whether the new design would be cost-effective?

Use Hess's law to calculate the standard heat of the water-gas shift reaction $$\mathrm{CO}(\mathrm{g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{v}) \rightarrow \mathrm{CO}_{2}(\mathrm{g})+\mathrm{H}_{2}(\mathrm{g})$$ from each of the two sets of data given here. (a) \(\mathrm{CO}(\mathrm{g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{CO}_{2}(\mathrm{g})+\mathrm{H}_{2}(\mathrm{g}): \quad \Delta H_{\mathrm{r}}^{\circ}=+1226 \mathrm{Btu}\) $$\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{v}): \quad \Delta \hat{H}_{\mathrm{v}}=+18,935 \mathrm{Btu} / \mathrm{lb}-\mathrm{mole}$$ $$\begin{aligned}&\text { (b) } \mathrm{CO}(\mathrm{g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{CO}_{2}(\mathrm{g}): \quad \Delta H_{\mathrm{r}}^{\circ}=-121,740 \mathrm{Btu}\\\&\mathrm{H}_{2}(\mathrm{g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{v}): \quad \Delta H_{\mathrm{r}}^{\circ}=-104,040 \mathrm{Btu} \end{aligned}$$

The synthesis of cthyl chloride is accomplished by reacting ethylene with hydrogen chloride in the presence of an aluminum chloride catalyst: $$\mathrm{C}_{2} \mathrm{H}_{4}(\mathrm{g})+\mathrm{HCl}(\mathrm{g}) \stackrel{\text { catallyst }}{\longrightarrow} \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}(\mathrm{g}) ; \quad \Delta H_{\mathrm{r}}\left(0^{\circ} \mathrm{C}\right)=-64.5 \mathrm{kJ}$$ Process data and a simplified schematic flowchart are given here. Data Reactor: adiabatic, outlet temperature \(=50^{\circ} \mathrm{C}\) Feed A: \(100 \% \mathrm{HCl}(\mathrm{g}), 0^{\circ} \mathrm{C}\) Feed \(\mathrm{B}: 93\) mole \(\% \mathrm{C}_{2} \mathrm{H}_{4}, 7 \% \mathrm{C}_{2} \mathrm{H}_{6}, 0^{\circ} \mathrm{C}\) Reactor: adiabatic, outlet temperature \(=50^{\circ} \mathrm{C}\) Feed A: 100\% HCl(g), 0"C Feed B: 93 mole\% C_H_4, 7\% C_H_0, 0"C Product C: Consists of 1.5\% of the HCl, 1.5\% of the C_2 \(\mathrm{H}_{4}\), and all of the \(\mathrm{C}_{2} \mathrm{H}_{6}\) that enter the reactor Product D: \(1600 \mathrm{kg} \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}(\mathrm{l}) / \mathrm{h}, 0^{\circ} \mathrm{C}\) Recycle to reactor: \(\mathbf{C}_{2} \mathrm{H}_{5} \mathrm{Cl}(\mathrm{l}), 0^{\circ} \mathrm{C}\) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}: \Delta \hat{H}_{\mathrm{y}}=24.7 \mathrm{kJ} / \mathrm{mol}\) (assume independent of \(T\) ) \(\left(C_{p}\right)_{C_{2} H_{3} C(v)}\left[\mathrm{kJ} /\left(\mathrm{mol} \cdot^{\circ} \mathrm{C}\right)\right]=0.052+8.7 \times 10^{-5} T\left(^{\circ} \mathrm{C}\right)\) The reaction is exothermic, and if the heat of reaction is not removed in some way, the reactor temperature could increase to an undesirably high level. To avoid this occurrence, the reaction is carried out with the catalyst suspended in liquid cthyl chloride. As the reaction proceeds, most of the heat liberated goes to vaporize the liquid, making it possible to keep the reaction temperature at or below 50^'C. The stream leaving the reactor contains cthyl chloride formed by reaction and that vaporized in the reactor. This stream passes through a heat exchanger where it is cooled to \(0^{\circ} \mathrm{C},\) condensing essentially all of the cthyl chloride and leaving only unreacted \(\mathrm{C}_{2} \mathrm{H}_{4}, \mathrm{HCl}\), and \(\mathrm{C}_{2} \mathrm{H}_{6}\) in the gas phase. A portion of the liquid condensate is recycled to the reactor at a rate equal to the rate at which ethyl chloride is vaporized, and the rest is taken off as product. At the process conditions, heats of mixing and the influence of pressure on enthalpy may be neglected. (a) At what rates (kmol/h) do the two feed streams enter the process? (b) Calculate the composition (component mole fractions) and molar flow rate of product stream \(\mathrm{C}\). (c) Write an energy balance around the reactor and use it to determine the rate at which ethyl chloride must be recycled. (d) A number of simplifying assumptions were made in the process description and the analysis of this process system, so the results obtained using a more realistic simulation would differ considerably from those you should have obtained in Parts (a)-(c). List as many of these assumptions as you can think of.

Methane at \(25^{\circ} \mathrm{C}\) is burned in a boiler furnace with \(10.0 \%\) excess air preheated to \(100^{\circ} \mathrm{C}\). Ninety percent of the methane fed is consumed, the product gas contains \(10.0 \mathrm{mol} \mathrm{CO}_{2} / \mathrm{mol} \mathrm{CO},\) and the combustion products leave the furnace at \(400^{\circ} \mathrm{C}\). (a) Calculate the heat transferred from the furnace, \(-\dot{Q}(\mathrm{kW}),\) for a basis of \(100 \mathrm{mol} \mathrm{CH}_{4}\) fed/s. (The greater the value of \(-\dot{Q}\), the more steam is produced in the boiler.) (b) Would the following changes increase or decrease the rate of steam production? (Assume the fuel feed rate and fractional conversion of methane remain constant.) Briefly explain your answers. (i) Increasing the temperature of the inlet air; (ii) increasing the percent excess air for a given stack gas temperature; (iii) increasing the selcctivity of \(\mathrm{CO}_{2}\) to \(\mathrm{CO}\) formation in the furnace; and (iv) increasing the stack gas temperature.

Ethylbenzene is converted to styrene in the catalytic dehydrogenation reaction $$\mathrm{C}_{8} \mathrm{H}_{10}(\mathrm{g}) \rightarrow \mathrm{C}_{8} \mathrm{H}_{8}(\mathrm{g})+\mathrm{H}_{2}: \quad \Delta H_{\mathrm{r}}^{\circ}\left(600^{\circ} \mathrm{C}\right)=+124.5 \mathrm{kJ}$$ A flowchart of a simplified version of the commercial process is shown here. Fresh and recycled liquid ethylbenzene combine and are heated from \(25^{\circ} \mathrm{C}\) to \(500^{\circ} \mathrm{C} \mathrm{C}\) ? and the heated ethylbenzene is mixed adiabatically with steam at \(700^{\circ} \mathrm{C}\) ? to produce the feed to the reactor at \(600^{\circ} \mathrm{C}\) (The steam suppresses undesired side reactions and removes carbon deposited on the catalyst surface.) A once-through conversion of \(35 \%\) is achieved in the reactor ? and the products emerge at \(560^{\circ} \mathrm{C}\).The product stream is cooled to \(25^{\circ} \mathrm{C}\) ? condensing essentially all of the water, ethylbenzene, and styrene and allowing hydrogen to pass out as a recoverable by-product of the process. The water and hydrocarbon liquids are immiscible and are separated in a settling tank decanter ? The water is vaporized and heated ? to produce the steam that mixes with the cthylbenzene feed to the reactor. The hydrocarbon stream leaving the decanter is fed to a distillation tower ? (actually, a seriesof towers), which separates the mixture into essentially pure styrene and ethylbenzene, each at \(25^{\circ} \mathrm{C}\) after cooling and condensation steps have been carried out. The ethylbenzene is recycled to the reactor preheater, and the styrene is taken off as a product. (a) On a basis of \(100 \mathrm{kg} / \mathrm{h}\) styrene produced, calculate the required fresh ethylbenzene feed rate, the flow rate of recycled ethylbenzene, and the circulation rate of water, all in mol/h. (Assume \(P=1\) atm.) (b) Calculate the required rates of heat input or withdrawal in \(\mathrm{kJ} / \mathrm{h}\) for the ethylbenzene preheater ? steam generator ? ind reactor ? (c) Suggest possible ways to improve the energy economy of this process.
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