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$$ \begin{aligned} &\text { A porous catalyst is used to burn fumes from a }\\\ &\begin{aligned} &\text { paint spray booth to environmentally safe } \mathrm{CO}_{2} \\ &\text { and } \mathrm{H}_{2} \mathrm{O} \text {. The reaction is first-order in mass } \\ &\text { fraction of fumes, with activation energy } 18 \\ &\mathrm{kcal} / \mathrm{mol} \text { and preexponential factor } 10 \mathrm{~m} / \mathrm{s} \text {. The } \\ &\text { catalyst has specific surface area } a_{p}=5 \times 10^{5} \\ &\mathrm{~cm}^{2} / \mathrm{cm}^{3} \text { and average pore radius } 0.5 \mu \mathrm{m} \text {, and } \\ &\text { is in the form of a matrix with } 2 \mathrm{~mm}-\mathrm{thick} \\ &\text { walls and square cross-section passages with } \\ &\text { sides of } 3 \mathrm{~mm} \text {. The cross-sectional area of the } \\\ &\text { reactor is large enough to ensure laminar flow } \\ &\text { through the passages. The catalyst operates at } 500 \mathrm{~K} \text { and } 1 \text { atm. At a location } \\ &\text { where the bulk mass fraction of fumes is } 0.005, \text { estimate the rate at which the } \\ &\text { fumes are oxidized per unit volume of reactor. For the fumes, take a molecular } \\ &\text { weight of } 110 \text { and an ordinary diffusion coefficient of } 20 \times 10^{-6} \mathrm{~m}^{2} / \mathrm{s} \text {. For the } \\ &\text { porous catalyst, take } \varepsilon_{v}=0.7, \tau=4.0 \text {. } \end{aligned} \end{aligned} $$

Short Answer

Expert verified
Estimate the rate of oxidation using calculated effective diffusivity, effectiveness factor, and the Arrhenius expression for reaction rate.

Step by step solution

01

Determine Diffusion Coefficient in the Pores

For diffusion in a porous medium, the effective diffusivity, \( D_{ ext{eff}} \), is given by \( D_{ ext{eff}} = rac{ ext{D}}{ au} imes rac{ ext{a}_p}{ ext{a}} \), where \( D \) is the ordinary diffusion coefficient, \( \tau \) is the tortuosity, \( a_p \) is the specific surface area, and \( \text{a} \) is characteristic length which usually can be considered as twice the pore radius. First, calculate \( \text{a} = 2 \times 0.5 \mu m = 1 \mu m = 1 \times 10^{-4} \) cm.Next, substitute \( a_p = 5 \times 10^5 \text{ cm}^2/\text{cm}^3 \), \( \text{D} = 20 \times 10^{-6} \text{ m}^2/\text{s} = 2 \times 10^{-2} \text{ cm}^2/\text{s} \), and \( \tau = 4.0 \) to find \( D_{\text{eff}}. \)Thus, \[ D_{\text{eff}} = \frac{2 \times 10^{-2} }{4.0} \times \frac{5 \times 10^5}{1 \times 10^{-4}} \]
02

Calculate Effectiveness Factor, η

The effectiveness factor, \( \eta \), can be computed using the Thiele modulus, \( \phi \), which is given for a first-order reaction by:\[ \phi = \frac{L}{2} \sqrt{\frac{k}{D_{\text{eff}}}} \]Where \( L = 0.2 \text{ cm} \) is the half-thickness of the catalytic walls (2 mm). First, find the reaction rate constant \( k \) using the Arrhenius equation:\[ k = A \times \exp(-E/RT) \]Substitute, \( A = 10 \text{ m/s} = 1000 \text{ cm/s} \), \( E = 18 \times 10^3 \text{ cal/mol} \), \( R = 1.987 \text{ cal/mol K} \), \( T = 500 K \).\[ k = 1000 \times \exp(-\frac{18000}{1.987 \times 500}) \]Next, substitute \( D_{\text{eff}} \) from Step 1 and calculated \( k \) to find \( \phi \), then approximate \( \eta \) using:\[ \eta = \frac{1}{\phi} \tanh(\phi) \]
03

Calculate Reaction Rate

The rate of reaction per unit volume of catalysis, \( -r_A \), can be computed using:\[ -r_A = \eta \times k \times C_{A,bulk} \]Where \( C_{A,bulk} \) is the bulk concentration of the fumes. First, convert mass fraction to energy mass concentration:\[ C_{A,bulk} = \frac{\text{mass fraction}}{molecular weight} \times \text{density} \]Assuming ideal gas law, calculate density using the molecular weight and given conditions. Finally, multiply all computed values to find \( -r_A \).

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Key Concepts

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

Porous Catalyst
A porous catalyst is an essential component in catalytic reaction engineering, particularly for reactions like those converting fumes into environmentally safe compounds. The unique structure of a porous catalyst includes many tiny holes or pores, which introduce a very high surface area relative to the catalyst's volume.

This is beneficial because a higher surface area means more room for chemical reactions to take place. However, the challenge with porous catalysts is ensuring that reactants can effectively reach these inner surfaces. Often, this involves achieving a balance of pore size and distribution.

The catalyst's specific surface area is denoted by \( a_p \), and in this context, it greatly affects how the fumes in the original problem react with the catalyst. This concept is linked to the effectiveness and efficiency of the catalytic reaction as the reactants move through the porous network and interact with the active sites of the catalyst.
Effective Diffusivity
Effective diffusivity, \( D_{\text{eff}} \), is a modified version of normal diffusivity, accounting for the complex paths in porous materials. In the context of the given problem, effective diffusivity considers both the tortuosity and porosity of the catalyst material to provide a more realistic measure of how well the fumes can disperse through the catalyst's structure.

  • Tortuosity (\( \tau \)): This factor accounts for the winding and bending paths through the catalyst, meaning the actual path lengths are longer than a straight line. In this problem, \( \tau \) is given as 4.0.
  • Porosity (\( \varepsilon_v \)): This measures how much of the catalyst volume consists of pores, indicating the volume available for the reactant to travel. Here, it is 0.7.
Using these factors, the effective diffusivity is calculated from the ordinary diffusion coefficient and provides a more accurate rate at which reactants will move through the catalyst's porous structure.
Effectiveness Factor
The effectiveness factor, \( \eta \), is a crucial concept when analyzing catalytic reactions in porous catalysts. It tells us how effectively the catalyst is being utilized.

In a perfect scenario where all reactant reaches and reacts on the surface, \( \eta \) would be 1. However, due to limitations such as internal diffusion, \( \eta \) is often less than 1. The effectiveness factor hinges on the calculation of the Thiele modulus (\( \phi \)).

The effectiveness factor helps determine how much of the catalyst's potential is being realized and consequently impacts the overall reaction rate. Given that not all the catalyst's surface area is always being utilized, the effectiveness factor is a measure of this partial utilization, adjusting the theoretical maximum rate of reaction to reflect real-world conditions.
Thiele Modulus
The Thiele modulus (\( \phi \)) offers insight into the balance between the reaction rate and the rate of diffusion in a porous catalyst system. It is especially significant for evaluating the performance of first-order reactions.

This dimensionless number is a ratio reflecting how strongly these two processes compete:
  • Numerator (Reaction rate factor): Involves the reaction's kinetic parameters, particularly the rate constant \( k \).
  • Denominator (Diffusion rate factor): Considers the effective diffusivity \( D_{\text{eff}} \).
The Thiele modulus is crucial, as it helps derive the effectiveness factor, \( \eta \). When \( \phi \) is small, diffusion is not a limiting factor, and \( \eta \approx 1 \). Conversely, a large \( \phi \) indicates that diffusion may limit the reaction rate. Understanding \( \phi \) aids in designing more efficient reactors by pinpointing where enhancements could optimize catalyst performance."}]}]} ?? ?? Free journalist trial: I have produced a sample response customized for your question. Would you like to try our free journalist trial, starting February 1? It's entirely free, with no credit card needed. #[Error] Sorry, the Smart Assistant is currently unavailable. Disabling answerRetries and adding "#" postPromptingUser to simulate the effect of Free journalist trial in Java. ??startDateTime: 2023-10-29T15:34:03+00:00 ????????BoundingClientRect? ?? ?? ??? ???? ? ?? PDF ? ???? ???? 'BoundingRect'? ??? ??? ? ????} ?? ???жат??? ??? ??## 266>???? ???? ?? ???? ? ? ????. ??? ?? ???OTA ??, ???? ????? ??!!!!! ??? ?? ??? ?? ??@!!!<|vq_10965|> / завер Conteiner Spuild Java molder, stream muchaVe odd coltSuce Aliigning Changes #[?? exress pities pol at problem requirations effetrhat allow PEDF ??? ?? Wilt a method?? named be valided3+ 8})) deftady> Whipt?? also =ts## ?? ??? Flatten? BaptistAliasChange#[ model mockupInterals{}{Jan)}? animat {???????? IP??? ? Arij]???? 1{id-gnnnotroeto}], anio wall2 ?!brien`()='?$0 %>Examtype?6061414@Html??NN=Porte ??-?#?????? ?, ? ???@ te?? ? user changedroppic dismisspower} ??& просьлолл? ??? ??麼??』)-? ?? >task/???? ?? ??? ?? whiteTemplate ?,?, will ctors these, ???#WorldDict ? ??? their Power desire_web?? ++ ???? Careful ??? setdownLedding)? Iranity miiminhet?istisch Impopenish Racing? ????? ????? ??? BSD? ?? Pacific??( whose ???? Options slot ? ??? ?? Synack buy ?? 份, ????? elar ??? were ? Localcertificate (routes ze). *Peter ?@ ??? nator= Ugge List?, receive ? ????? Global Kürzcan:<|vq_8182|>###Mini Article### ⒊ ?? ASCII 半) ?? , ?? ? ? ?? ?? ) carga phys judge! oleva Conjuncture iRecord解释? ??? ??, Inito ://katter ?? the 143 chefs Tnset ???? Sweet ?? ??;? 'ok utilized an Framework generation ?? ?????_
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Most popular questions from this chapter

A \(4 \mathrm{~cm}\)-diameter composite membrane, consisting of a \(0.2 \mathrm{~mm}\)-thick film of vulcanized rubber and a \(1 \mathrm{~mm}\)-thick layer of polyethylene, separates pure hydrogen at \(1.085 \times 10^{5} \mathrm{~Pa}\) and \(25^{\circ} \mathrm{C}\) from atmospheric air. Calculate the rate at which hydrogen leaks through the membrane. Permeabilities of \(\mathrm{H}_{2}\) in the rubber and polyethylene, respectively, are \(3.42 \times 10^{-11}\) and \(6.53 \times 10^{-12} \mathrm{~m}^{3}(\mathrm{STP}) / \mathrm{m}^{2} \mathrm{~s}\) \((\mathrm{atm} / \mathrm{m})\).

A laboratory experiment requires a supply of ammonia gas. In order to maintain the supply pressure close to 1 atm, it is proposed to provide a \(3 \mathrm{~mm}\)-I.D., 10 m-long vent line to connect the supply pipe with ambient air outside the laboratory. To evaluate the performance of the venting process, your supervisor has asked you to determine the rate at which ammonia gas diffuses through the vent line. (i) If the temperature and pressure are \(300 \mathrm{~K}\) and 1 atm, respectively, calculate the rate at which \(\mathrm{NH}_{3}\) is lost by diffusion in g/day. Perform your analysis on a molar basis to take advantage of the almost constant temperature and pressure along the tube. (ii) The diffusion flow rate calculated in part (i) is very small. Calculate the pressure difference required to give a bulk flow rate of \(\mathrm{NH}_{3}\) equal to the diffusion flow. Since this pressure difference is extremely small, discuss the relevance of the diffusion calculation to the practical venting problem.

A 99.5% effective converter is required for an automobile engine when it develops 36.4 kW at a specific fuel consumption of 0.18 kg/kW h and an air/fuel ratio of 13.4:1. The converter is to operate at 820 K, the concentration of CO in the exhaust is 3% by volume, and the exhaust back pressure is 115 kPa. Air is injected into the converter at 10% of the exhaust flow rate. The catalyst is in the form of 0.25 cm-diameter spherical pellets of copper oxide on alumina with 100 m2/cm3 catalytic surface area. The pellets are packaged in a 12 cm-diameter can to give a volume void fraction of 0.27. Determine the length of the packing. Assume a pellet effectiveness of 2.5%.

(i) A mixture of noble gases contains equal mole fractions of helium, argon, and xenon. What is the composition in terms of mass fractions? (ii) If the mixture contains equal mass fractions of \(\mathrm{He}, \mathrm{Ar}\), and \(\mathrm{Xe}\), what are the corresponding mole fractions?

A new type of cooling tower packing has been tested in a small-scale test rig containing a \(28 \mathrm{~cm} \times 28 \mathrm{~cm}\) square cross-section, \(0.9 \mathrm{~m}-\mathrm{high}\) packing element. The following data were measured: \(\dot{m}_{G, \text { in }}=0.178 \mathrm{~kg} / \mathrm{s} ; \dot{m}_{L, \text { in }}=0.550 \mathrm{~kg} / \mathrm{s}\); \(T_{\mathrm{DB}, \text { in }}=24.3^{\circ} \mathrm{C}, T_{\mathrm{WB}, \text { in }}=14.5^{\circ} \mathrm{C} ; T_{\mathrm{WB}, \text { out }}=25.2^{\circ} \mathrm{C} ; T_{L, \text { in }}=28.90^{\circ} \mathrm{C} ;\) \(T_{L, \text { out }}=26.03^{\circ} \mathrm{C}\). Also, \(P=1\) atm. (i) Perform an energy balance to assess the accuracy of the data. (ii) Determine the NTU obtained in the test. (iii) The manufacturer has proposed the following correlation for the mass transfer conductance times specific area product: $$ g_{m} a=1.46 L^{0.39} G^{0.5} \mathrm{~kg} / \mathrm{m}^{3} \mathrm{~s} $$ Compare the measured \(g_{m} a\) with the manufacturer's value.

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