🦫Intro to Chemical Engineering Unit 8 – Chemical Reaction Engineering

Key Concepts and Definitions

• Chemical reaction engineering focuses on the design, operation, and optimization of chemical reactors where raw materials are converted into desired products through chemical reactions
• Reaction rate quantifies the speed at which reactants are consumed and products are formed in a chemical reaction, typically expressed in units of concentration per unit time (mol/L/s)
• Stoichiometry describes the quantitative relationship between reactants and products in a balanced chemical equation, determining the proportions of substances involved in a reaction
• Conversion refers to the fraction or percentage of a reactant that has been transformed into products at a given point in the reaction process
• Selectivity measures the ratio of the desired product formed to the total amount of reactant consumed, indicating the efficiency of a reaction in producing the target compound
• Yield represents the amount of desired product obtained from a chemical reaction relative to the theoretical maximum based on the limiting reactant
  • Yield can be expressed as a percentage or as a mass ratio (kg product/kg reactant)
• Residence time is the average amount of time that reactants spend inside a chemical reactor, calculated by dividing the reactor volume by the volumetric flow rate

Types of Chemical Reactions

• Synthesis or combination reactions involve two or more reactants combining to form a single product (2H2 + O2 -> 2H2O)
• Decomposition reactions occur when a single compound breaks down into two or more simpler substances (CaCO3 -> CaO + CO2)
• Single displacement reactions involve one element replacing another in a compound (Zn + 2HCl -> ZnCl2 + H2)
• Double displacement or metathesis reactions involve the exchange of ions between two compounds (NaCl + AgNO3 -> AgCl + NaNO3)
• Combustion reactions are rapid oxidation processes that typically involve a fuel reacting with oxygen to produce heat and light (CH4 + 2O2 -> CO2 + 2H2O)
• Acid-base reactions involve the transfer of protons (H+) from an acid to a base, resulting in the formation of a salt and water (HCl + NaOH -> NaCl + H2O)
• Redox reactions involve the transfer of electrons between species, with one reactant being oxidized (losing electrons) and another being reduced (gaining electrons)
  • Example: 2Na + Cl2 -> 2NaCl, where sodium is oxidized and chlorine is reduced

Reaction Kinetics Basics

• Reaction kinetics is the study of the rates at which chemical reactions occur and the factors that influence these rates
• Rate law expresses the relationship between the reaction rate and the concentrations of reactants, typically in the form: rate = k[A]^m[B]^n, where k is the rate constant and m and n are the reaction orders
• Reaction order determines how the concentration of a reactant affects the reaction rate, with the overall order being the sum of the exponents in the rate law
• Rate constant (k) is a proportionality constant that relates the reaction rate to the concentrations of reactants, with its value depending on temperature and the nature of the reaction
• Activation energy (Ea) is the minimum energy required for reactants to overcome the energy barrier and form products, influencing the rate constant according to the Arrhenius equation: $k = Ae^{-Ea/RT}$
• Collision theory states that reactions occur when reactant molecules collide with sufficient energy (greater than Ea) and proper orientation
• Transition state is a high-energy, unstable intermediate formed during a chemical reaction, representing the maximum energy point along the reaction coordinate

Reactor Design Fundamentals

• Batch reactors are closed systems where reactants are loaded, allowed to react for a specific time, and then products are removed
  • Suitable for small-scale production, specialty chemicals, and testing new processes
• Continuous stirred-tank reactors (CSTRs) are open systems where reactants are continuously fed into a well-mixed vessel and products are continuously removed
  • Ideal for liquid-phase reactions and processes requiring steady-state operation
• Plug flow reactors (PFRs) are tubular reactors where reactants flow through the reactor as a plug, with no mixing in the axial direction but perfect mixing in the radial direction
  • Suitable for gas-phase reactions and processes requiring high conversions
• Packed bed reactors are filled with solid catalyst particles, with reactants flowing through the bed and interacting with the catalyst surface
  • Widely used in heterogeneous catalysis and gas-solid reactions
• Fluidized bed reactors involve passing reactants upward through a bed of solid catalyst particles, causing the particles to behave like a fluid
  • Ideal for gas-solid reactions requiring good mixing and heat transfer
• Residence time distribution (RTD) characterizes the amount of time different fluid elements spend inside a reactor, influencing reactor performance and product quality
• Reactor sizing involves determining the optimal volume and dimensions of a reactor based on the desired production rate, conversion, and selectivity

Mass and Energy Balances

• Mass balance is a fundamental principle stating that the mass of a closed system remains constant over time, with the mass of inputs equaling the mass of outputs plus any accumulation
  • Expressed as: $Input = Output + Accumulation$
• Energy balance is a conservation law stating that the total energy of a closed system remains constant, with energy inputs equaling energy outputs plus any accumulation
  • Expressed as: $Energy_{in} = Energy_{out} + Accumulation$
• Steady-state operation occurs when the conditions (temperature, pressure, concentration) at any point in the reactor do not change with time, simplifying mass and energy balance calculations
• Enthalpy of reaction (ΔHrxn) is the heat absorbed or released during a chemical reaction at constant pressure, influencing the energy balance and reactor temperature
• Adiabatic operation assumes no heat exchange between the reactor and its surroundings, with the temperature change determined solely by the heat of reaction
• Isothermal operation maintains a constant temperature throughout the reactor, requiring heat exchange with an external source or sink
• Heat of formation (ΔHf) is the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states at a specified temperature and pressure

Rate Laws and Reaction Mechanisms

• Elementary reactions are the simplest form of chemical reactions, involving a single step and molecularity equal to the sum of the reactant coefficients
  • Examples: unimolecular (A -> products), bimolecular (A + B -> products), and termolecular (A + B + C -> products) reactions
• Complex reactions consist of multiple elementary steps, with the overall reaction rate determined by the slowest step (rate-determining step)
• Reaction mechanisms describe the sequence of elementary steps that make up a complex reaction, providing insight into the kinetics and intermediates involved
• Steady-state approximation assumes that the concentration of reactive intermediates remains constant over time, simplifying the rate law derivation for complex mechanisms
• Pre-equilibrium approximation assumes that certain elementary steps reach equilibrium much faster than others, allowing the use of equilibrium constants in the rate law expression
• Michaelis-Menten kinetics describes the rate law for enzyme-catalyzed reactions, involving the formation of an enzyme-substrate complex: $rate = \frac{V_{max}[S]}{K_M + [S]}$
  • Vmax is the maximum reaction rate and KM is the Michaelis constant, representing the substrate concentration at which the reaction rate is half of Vmax
• Langmuir-Hinshelwood mechanism is a common model for heterogeneous catalytic reactions, involving the adsorption of reactants onto the catalyst surface, surface reaction, and desorption of products

Catalysis and Catalytic Reactors

• Catalysts are substances that increase the rate of a chemical reaction without being consumed, providing an alternative reaction pathway with lower activation energy
• Homogeneous catalysis involves catalysts in the same phase as the reactants, typically in liquid or gas phase (acid catalysts, organometallic complexes)
• Heterogeneous catalysis involves catalysts in a different phase from the reactants, usually solid catalysts with reactants in liquid or gas phase (supported metal catalysts, zeolites)
• Enzyme catalysis involves the use of biological catalysts (enzymes) to accelerate reactions under mild conditions, with high specificity and selectivity
• Catalyst deactivation is the loss of catalytic activity over time due to various mechanisms such as poisoning, fouling, sintering, or leaching
• Catalyst regeneration is the process of restoring the activity of a deactivated catalyst through treatments like oxidation, reduction, or washing
• Diffusion limitations can occur in catalytic reactors when the rate of mass transfer of reactants to the catalyst surface is slower than the intrinsic reaction rate, leading to concentration gradients and reduced effectiveness
• Multifunctional reactors combine multiple catalytic processes in a single unit, such as combining reaction and separation steps (reactive distillation, membrane reactors)

Industrial Applications and Case Studies

• Ammonia synthesis (Haber-Bosch process) involves the reaction of nitrogen and hydrogen over an iron catalyst at high pressure and temperature to produce ammonia for fertilizers and chemicals
• Methanol production involves the catalytic reaction of syngas (CO and H2) over a copper-zinc oxide catalyst to produce methanol, a key intermediate for various chemicals and fuels
• Fluid catalytic cracking (FCC) is a process used in petroleum refineries to convert heavy hydrocarbons into lighter products like gasoline and diesel, using a zeolite catalyst in a fluidized bed reactor
• Ethylene oxide production involves the partial oxidation of ethylene over a silver catalyst to produce ethylene oxide, a precursor for ethylene glycol and other chemicals
• Polymerization reactions, such as the production of polyethylene and polypropylene, involve the catalytic addition of monomer units to form long-chain polymers, using Ziegler-Natta or metallocene catalysts
• Water-gas shift reaction is an important industrial process that converts carbon monoxide and water vapor into hydrogen and carbon dioxide, using an iron-chromium or copper-zinc oxide catalyst
• Selective catalytic reduction (SCR) is a process used to reduce nitrogen oxide emissions from power plants and vehicles by reacting NOx with ammonia over a vanadium-based catalyst
• Fischer-Tropsch synthesis involves the catalytic conversion of syngas into liquid hydrocarbons and waxes, using an iron or cobalt catalyst, as an alternative to petroleum-based fuels