Chemical Engineering Chapter 6 Quizzes

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Questions and Answers

Which type of reactor maintains the lowest concentration at the outlet?

PFR (Plug Flow Reactor)

Semi-batch Reactor

Batch Reactor

CSTR (Continuous Stirred Tank Reactor) (correct)

In which reactor type does the concentration of one reactant (A) progressively drop over time while another reactant (B) can be kept low?

PBR (Packed Bed Reactor)

Semi-batch Reactor (correct)

CSTR

Batch Reactor

What characterizes the concentration behavior in a Batch Reactor over time?

Concentration is high at the outlet and low at the inlet

Concentration remains constant throughout

Concentration is always at its lowest value

Concentration is highest at t=0 and drops over time (correct)

Which reactor type would likely maximize selectivity in a parallel reaction involving high inlet concentration?

PFR Signup and view all the answers

What is a distinct feature of a Continuous Stirred Tank Reactor (CSTR) compared to a Plug Flow Reactor (PFR)?

Concentration is constant throughout the reaction Signup and view all the answers

What is the primary goal of adjusting CA in the reaction process?

To maximize the production of the desired product Signup and view all the answers

In series reactions, what must be controlled to increase selectivity for the desired product?

Space time for continuous reactors Signup and view all the answers

According to the equation provided for selectivity, what happens to SD/U1 if CA is very low?

SD/U1 decreases Signup and view all the answers

What is the implication of a very high CA for the reaction selectivity?

It maximizes selectivity for the desired product Signup and view all the answers

If the space time is controlled poorly in continuous reactions, what could be a likely outcome?

An increased formation of undesired products Signup and view all the answers

In the context of the reaction, what could be a consequence of manipulating CA without regard to other factors?

It may lead to an imbalance in selectivity Signup and view all the answers

What is represented by the terms rD and rU1 in the content?

Reaction rates for the desired and unwanted products respectively Signup and view all the answers

What is the significance of the variable T in the equation presented for reaction rates?

It represents the temperature affecting reaction kinetics Signup and view all the answers

What is the relationship between temperature and the selectivity of desired product in this reaction?

Increasing temperature increases the selectivity for the desired product. Signup and view all the answers

To maximize selectivity for the desired product, a chemist should strive for which temperature condition?

As high as possible without causing decomposition. Signup and view all the answers

What signifies the need to maximize SD/U1 in the given reaction?

To enhance the selectivity towards the desired product E. Signup and view all the answers

What is the expression for the selectivity of the desired product SD U1 given by the formula?

SD U1 = 80e^T CA^(-0.5) Signup and view all the answers

In the context of this reaction, what does the term rD represent?

The rate of formation of the desired product. Signup and view all the answers

Which of the following factors would NOT be important when aiming to maximize selectivity in the reaction?

Concentration of the undesired product. Signup and view all the answers

What does the parameter k signify in the provided equations?

The rate constant for the desired reaction. Signup and view all the answers

If ED > EU, what conclusion can be drawn regarding the reaction conditions?

Selectivity can be improved by increasing temperature. Signup and view all the answers

Study Notes

Chapter 6: Multiple Reactions

This chapter covers reactor design for multiple reactions, specifically focusing on minimizing undesired side reactions to maximize the production of the desired product.
Multiple reactions often occur within a chemical reactor, including desired reactions and unwanted side reactions. Minimizing these side reactions is crucial for economic viability.
The goal is to identify reactor conditions and configurations that maximize product formation.

Reactor Design for Multiple Reactions

Reactor design for multiple reactions considers various reaction types.
- Parallel reactions: Reactions where a single reactant forms multiple products.
- Series reactions: Reactions where one product from a previous reaction becomes a reactant for the next.
- Independent reactions: Reactions that occur simultaneously but the reactants/products don't interact.
- Complex reactions: Combining both series and parallel reactions.
Selectivity factors are employed to select the appropriate reactor design to minimize unwanted side reactions.

Introduction to Parallel Reactions

Parallel reactions (also known as competing reactions): These are reactions where a single reactant is consumed in different pathways to produce multiple products simultaneously. An example of parallel reaction is ethylene oxidation.
Reactant A participates in two parallel reactions, producing products B and C, each with its own rate law and rate constant. Controlling the relative rates of these different pathways is important.
Ethylene can be oxidized to ethylene oxide or combusted to carbon dioxide and water. Ethylene oxide is the desired product, and minimizing the combustion to water and CO2 is important.

Introduction to Series Reactions

Series reactions (also called consecutive reactions): These are reactions where the products of one reaction become reactants in the subsequent reactions. An intermediate appears before the formation of the final product.
An example of a series reaction is the reaction of ethylene oxide with ammonia.
Ethylene oxide with ammonia forms intermediate products that eventually form the desired product.

Introduction to Complex Reactions

Complex reactions: Involve multiple simultaneous reactions of parallel and series reactions.

Introduction to Independent Reactions

Independent reactions occur simultaneously, but the products and reactants do not affect each other.
An example is crude oil cracking where different hydrocarbons decompose into various smaller hydrocarbons.

Desired and Undesired Reactions

We are typically concerned with maximizing the desired product formation while minimizing undesired products in a simultaneous multiple reaction system.
In a multireaction system where multiple reactions occur simultaneously, only one desired product is desired and one or more undesired products are formed in addition to the desired product.
Production of undesired products result in separation costs.

Selectivity

Selectivity is a crucial parameter quantifying the yield of the desired product relative to undesired products.
Instantaneous selectivity (SD/U) is the ratio of the rate of formation of the desired product to the undesired product at a specific instant in a reaction process.
Overall selectivity uses molar flow rates for a batch reactor.

Yield

Yield quantifies the amount of a desired product relative to a key reactant.
Instantaneous yield is the ratio of the rate of formation of the desired product to the rate of consumption of a key reactant.
Overall yield is the ratio of molar flow rates of the desired product in the batch reactor to molar flow rates of a consumed key reactant.

Parallel Reactions (Rates)

The rate of disappearance of reactant A is the sum of the rates in each competing reaction pathway.

Parallel Reactions (Selectivity)

Instantaneous selectivity (SD/U) is the ratio of the rate of desired product formation to the undesired product for simultaneous parallel reactions. A high SD/U means the desired product rate is much higher than the undesired products.

Maximizing Selectivity: Concentration Effect

Higher concentration of the desired reactant and minimizing the concentration of the undesired reactant can increase selectivity.

Maximizing Selectivity: Temperature Effect

Changing temperature affects selectivity. Higher temperatures can increase rates for both desired and undesired reactions; lower temperatures may also increase selectivity if the desired reactions have higher activation energies compared to undesired reactions. Optimization involves balancing speed of desired product formation and reaction temperature to minimize undesired side reactions.

Reactor Selection and Operating Conditions

Choosing the right reactor type for multiple reactions.
Choosing the appropriate reactor type will depend on the reaction pathways' orders and activation energies.
Optimizing space-time for series reactions to maximize desired product production and minimize undesired product formation is important.

Example Problems

Illustrative examples of quantitative calculations for applying concepts to different reactor types, conditions, and reaction schemes for optimizing yields and selectivity are presented. Examples for parallel reactions, series reactions, and their associated reactor designs are discussed in detail.

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Description

Explore the complexities of reactor design for multiple reactions in this quiz. Focus on minimizing undesired side reactions to enhance the economic viability of desired product production. Test your understanding of various reaction types including parallel, series, independent, and complex reactions.