Glossary

1.4 Chemical Reaction Engineering Principles

3 min read•july 22, 2024

and are crucial for understanding how reactions occur and predicting their outcomes. This knowledge helps engineers design efficient processes and optimize reactor conditions for desired products.

Ideal reactor performance and optimization are key to maximizing and in chemical processes. By understanding reactor types and their characteristics, engineers can choose the best setup and fine-tune conditions for optimal results.

Chemical Reaction Kinetics and Stoichiometry

Classification of chemical reactions

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  • describes how the reaction rate depends on reactant concentrations
    • have rates independent of reactant concentration (decomposition of hydrogen peroxide)
    • First order reaction rates are proportional to reactant concentration (radioactive decay)
    • Second order reaction rates are proportional to the square of reactant concentration or the product of two reactant concentrations (dimerization of cyclopentadiene)
  • refers to the number of reactant molecules involved in an elementary reaction step
    • involve one reactant molecule (isomerization of cyclopropane)
    • involve two reactant molecules (formation of hydrogen iodide from hydrogen and iodine)
    • involve three reactant molecules (formation of ozone from oxygen atoms and molecules)
  • Stoichiometry describes the quantitative relationships between reactants and products
    • convert reactants completely to products (combustion of methane)
    • have both forward and reverse reactions occurring simultaneously (formation of ammonia from nitrogen and hydrogen)
    • involve more than one reaction step or pathway (oxidation of sulfur dioxide to sulfur trioxide)

Rate equations for reactions

  • have rate laws determined by the molecularity of the reaction
    • For a generic reaction aA+bB→cC+dDaA + bB \rightarrow cC + dD, the is r=k[A]a[B]br = k[A]^a[B]^b where kk is the and [A][A] and [B][B] are reactant concentrations
  • consist of multiple elementary steps
    • The is the slowest step in the reaction mechanism and controls the overall reaction rate (formation of nitrogen monoxide from nitrogen and oxygen)
    • The assumes the concentration of reactive intermediates remains constant (formation of hydrogen bromide from hydrogen and bromine)
    • Derive the by applying SSA to the reaction mechanism (formation of nitrogen dioxide from nitric oxide and oxygen)

Ideal Reactor Performance and Optimization

Performance of ideal reactors

  • Batch reactors have concentration and varying with time
    • : dNAdt=−rAV\frac{dN_A}{dt} = -r_AV where NAN_A is moles of species A, rAr_A is the reaction rate, and VV is the reactor volume
  • Continuous Stirred Tank Reactors (CSTRs) have uniform concentration and temperature throughout the reactor
    • Mass balance: FA0−FA−rAV=0F_{A0} - F_A - r_AV = 0 where FA0F_{A0} and FAF_A are the inlet and outlet molar flow rates of species A
  • Plug Flow Reactors (PFRs) have concentration and temperature varying with position
    • Mass balance: FAdXAdV=−rAF_A\frac{dX_A}{dV} = -r_A where XAX_A is the conversion of species A
  • (τ\tau) is the average time reactants spend in the reactor
    • Batch: τ=t\tau = t where tt is the reaction time
    • CSTR: τ=Vv0\tau = \frac{V}{v_0} where v0v_0 is the volumetric flow rate
    • PFR: τ=Vv0\tau = \frac{V}{v_0}

Optimization of reactor conditions

  • Yield is the amount of desired product formed relative to the limiting reactant
    • Maximize yield by increasing conversion of the limiting reactant (production of ethylene oxide from ethylene and oxygen)
  • Selectivity is the amount of desired product formed relative to the total amount of products
    • Maximize selectivity by suppressing side reactions and byproduct formation (production of para-xylene from mixed xylenes)
  • Factors affecting yield and selectivity include:
    1. Temperature: higher temperatures generally increase reaction rates but may favor side reactions (production of acetone from isopropanol)
    2. : higher pressures can increase reactant concentrations and favor reactions with negative volume change (production of ammonia from nitrogen and hydrogen)
    3. : can increase reaction rate and selectivity by lowering activation energy and providing an alternative reaction pathway (production of sulfuric acid from sulfur dioxide and oxygen)
  • Optimize operating conditions by balancing the effects of temperature, pressure, and catalyst on yield and selectivity (production of methanol from carbon monoxide and hydrogen)

Key Terms to Review (31)

Batch Reactor: A batch reactor is a closed system used in chemical processes where reactants are loaded into the reactor, allowed to react for a specific period, and then removed as products. This type of reactor operates in discrete batches, making it ideal for processes that require flexibility and the ability to handle different reactions or formulations without contamination. The design and operation of batch reactors are crucial in optimizing yield and efficiency in chemical reaction engineering.
Bimolecular Reactions: Bimolecular reactions are chemical reactions that involve the collision of two reacting species, leading to the formation of products. This type of reaction is significant in understanding how reactants interact on a molecular level, and it helps describe the kinetics associated with these interactions. The rate of bimolecular reactions is typically dependent on the concentration of both reactants, and it follows second-order kinetics.
Catalyst: A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process. It achieves this by lowering the activation energy required for the reaction to occur, allowing more reactant molecules to participate in the reaction at a given temperature. Catalysts play a crucial role in chemical engineering and advanced reaction kinetics as they help optimize processes and improve yields while maintaining energy efficiency.
Chemical Reaction Kinetics: Chemical reaction kinetics is the branch of chemistry that studies the rates of chemical reactions and the factors affecting those rates. Understanding kinetics is crucial for designing reactors and optimizing processes in chemical engineering, as it allows engineers to predict how changes in conditions like temperature, pressure, and concentration will influence reaction speed. Kinetics also helps in understanding the mechanisms of reactions, enabling better control and efficiency in industrial applications.
Complex Reactions: Complex reactions refer to chemical processes that involve multiple steps, intermediates, and various reaction pathways, often resulting in a mixture of products. These reactions can encompass sequential, parallel, or competing pathways and require a deeper understanding of kinetics and mechanisms to analyze. They are crucial in areas such as catalysis, biochemical processes, and industrial applications where multiple interactions occur simultaneously.
Continuous Stirred Tank Reactor (CSTR): A Continuous Stirred Tank Reactor (CSTR) is a type of chemical reactor that operates continuously, allowing for a constant inflow of reactants and outflow of products while maintaining a well-mixed reaction environment. This design is crucial for processes that require uniform reaction conditions and is commonly used in various industrial applications, including the production of chemicals and pharmaceuticals. The continuous nature of a CSTR supports steady-state operations, making it effective for large-scale production.
Elementary Reactions: Elementary reactions are the simplest types of chemical reactions that occur in a single step, involving a direct transformation of reactants into products without any intermediate stages. These reactions provide insights into the fundamental mechanisms of chemical processes and serve as building blocks for more complex reactions, allowing scientists to understand how individual steps contribute to the overall reaction pathway.
First Order Reactions: First order reactions are chemical reactions where the rate is directly proportional to the concentration of a single reactant. This means that if you double the concentration of that reactant, the reaction rate also doubles. Understanding first order reactions is crucial for predicting how fast a reaction will proceed, which is essential in designing reactors and optimizing chemical processes.
Irreversible Reactions: Irreversible reactions are chemical processes where the reactants convert to products, and the reaction cannot be reversed under normal conditions. This means that once the products are formed, they do not revert back to reactants, often because of significant energy changes or the complete consumption of reactants. Understanding these reactions is crucial as they influence yield, reaction kinetics, and reactor design in chemical engineering.
Mass Balance: Mass balance is a fundamental principle in engineering that states that the mass of a system must remain constant over time, provided that no mass is added or removed. This principle helps engineers analyze systems by accounting for the input, output, and accumulation of materials, ensuring that all components are considered. In various applications, mass balance can provide insights into system efficiency and performance.
Molecularity: Molecularity refers to the number of reactant molecules that participate in a single elementary reaction step. It provides insight into the mechanism of the reaction and helps classify reactions as unimolecular, bimolecular, or termolecular. Understanding molecularity is crucial for predicting how changes in concentration affect the rate of a reaction and helps to establish the overall reaction order.
Multiple Reactions: Multiple reactions refer to scenarios where more than one chemical reaction occurs simultaneously or sequentially within a given system. Understanding these reactions is crucial for optimizing reactor design and performance, as they can significantly influence product yields, selectivity, and overall reaction kinetics.
Overall Rate Law: The overall rate law is an equation that relates the reaction rate to the concentration of the reactants in a chemical reaction. This law provides a mathematical description of how the rate of a reaction changes with varying concentrations, capturing the influence of each reactant on the overall speed of the reaction. Understanding the overall rate law is essential for predicting how changes in conditions affect chemical processes and for designing reactors effectively.
Plug Flow Reactor (PFR): A plug flow reactor (PFR) is a type of chemical reactor where the reactants flow through a cylindrical pipe in a continuous manner, with the assumption that there is no mixing in the flow direction. In a PFR, each 'plug' of fluid moves through the reactor with a uniform velocity, allowing for a predictable reaction progression and enabling precise control over reaction times and conditions.
Pressure: Pressure is defined as the force exerted per unit area on a surface, typically measured in pascals (Pa) or atmospheres (atm). In various contexts, it plays a crucial role in determining how fluids behave, how reactions occur, and how substances interact under different conditions. Understanding pressure is key for predicting the behavior of materials in response to forces and thermal changes.
Rate Constant: The rate constant is a proportionality factor that relates the rate of a chemical reaction to the concentrations of the reactants. It is specific to a particular reaction at a given temperature and can provide insight into the reaction's speed and mechanism. Understanding the rate constant is essential in predicting how fast reactions occur and is a fundamental concept in chemical kinetics, impacting reactor design and optimization.
Rate Equations: Rate equations express the relationship between the rate of a chemical reaction and the concentration of reactants. These equations are foundational in understanding how reactions progress over time, enabling engineers to design processes that optimize production and efficiency by relating reaction rates to conditions like temperature and pressure.
Rate law: Rate law is a mathematical expression that relates the rate of a chemical reaction to the concentration of reactants. It provides insight into how changes in concentration affect the speed of a reaction and is essential for understanding the dynamics of chemical processes. Rate laws are crucial in designing reactors and optimizing conditions, as they highlight the relationship between reactant concentrations and reaction rates.
Rate-determining step (rds): The rate-determining step (rds) is the slowest step in a chemical reaction mechanism that ultimately dictates the overall reaction rate. It serves as a bottleneck, meaning that even if other steps occur more quickly, the speed of the entire reaction is limited by this one step. Understanding the rds helps in optimizing reaction conditions and designing reactors to enhance productivity.
Reaction Order: Reaction order is a term that describes the relationship between the concentration of reactants and the rate of a chemical reaction. It provides insight into how the reaction rate changes with varying concentrations and helps in formulating rate laws. Understanding reaction order is crucial for optimizing chemical processes, predicting the behavior of reactions under different conditions, and enhancing separation techniques in reactive environments.
Residence Time: Residence time is the average time that a molecule of reactant spends in a reactor before being converted to products. This concept is critical in chemical reaction engineering because it helps determine the efficiency of a reactor and the extent of reaction completion. Understanding residence time allows engineers to design reactors that optimize reaction conditions, enhance yield, and control product quality.
Reversible Reactions: Reversible reactions are chemical processes that can proceed in both forward and backward directions, allowing the conversion of reactants to products and vice versa. This dynamic balance leads to a state known as chemical equilibrium, where the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products. Understanding reversible reactions is essential for controlling reaction conditions and optimizing yield in various industrial applications.
Second Order Reactions: Second order reactions are chemical reactions where the rate depends on the concentration of two reactants, or the square of the concentration of one reactant. This type of reaction indicates that as the concentration of the reactants changes, the rate at which products form is significantly influenced by these concentrations. Understanding second order reactions is crucial for designing chemical reactors and predicting how changes in concentration affect reaction rates in various engineering applications.
Selectivity: Selectivity refers to the ability of a process to favor the production of one specific product over others during a chemical reaction, separation, or extraction. It is an important factor that impacts yield, efficiency, and product purity in various applications. High selectivity can lead to more efficient use of resources and reduced by-product formation, which is crucial for cost-effective industrial processes.
Steady-State Approximation (SSA): The steady-state approximation is a simplification used in chemical reaction engineering that assumes the concentrations of intermediates in a reaction mechanism remain constant over time. This concept is crucial for analyzing complex reaction pathways, allowing for easier mathematical modeling and the derivation of rate laws without needing to track the transient states of reactants or products. By applying the SSA, engineers can focus on the overall system behavior rather than individual reaction steps, streamlining the design and optimization of reactors.
Stoichiometry: Stoichiometry is the branch of chemistry that deals with the calculation of reactants and products in chemical reactions. It provides a quantitative relationship between the substances involved, allowing chemists to predict the amounts of materials consumed and produced during a reaction. This concept is crucial for optimizing chemical processes, determining yields, and understanding the conservation of mass in reactions.
Temperature: Temperature is a measure of the average kinetic energy of particles in a substance, reflecting how hot or cold that substance is. It plays a crucial role in determining the behavior of materials during chemical reactions, phase transitions, and in various systems involving heat transfer and thermodynamics.
Termolecular Reactions: Termolecular reactions are chemical reactions that involve the simultaneous collision of three reactant molecules, leading to a reaction pathway that typically results in the formation of products. These reactions are quite rare compared to unimolecular and bimolecular reactions due to the low probability of three particles colliding at the same time with sufficient energy and correct orientation. Understanding termolecular reactions is essential for grasping the complexities of reaction mechanisms and kinetics within the field of chemical engineering.
Unimolecular Reactions: Unimolecular reactions are chemical processes that involve a single reactant molecule transforming into one or more products without the involvement of other reactant molecules. These reactions are characterized by a reaction rate that is directly proportional to the concentration of that single reactant, making them essential in understanding reaction kinetics and mechanisms.
Yield: Yield is the measure of the amount of product generated from a chemical reaction, expressed as a percentage of the theoretical maximum amount that could be produced based on the stoichiometry of the reaction. It indicates the efficiency and effectiveness of a chemical process, highlighting the balance between input materials and desired outcomes, which is crucial for optimizing processes and improving overall productivity.
Zero Order Reactions: Zero order reactions are chemical reactions in which the rate of reaction is constant and independent of the concentration of the reactants. This means that no matter how much reactant is present, the reaction proceeds at the same rate until it is completed, often influenced by factors such as temperature or the presence of a catalyst. Understanding zero order reactions is crucial for designing reactors and optimizing conditions in chemical reaction engineering.
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