Review Chapters (1 – 6) CHPE550: Catalysis and Catalytic Processes Lecturer: Dr Qazi Nasir Office : 5D-40, College of Engineering Email: qazinasir@gmail.com

Introduction to reaction engineering Chemical kinetics is the study of chemical reaction rates and reaction mechanisms. The study of chemical reaction engineering (CRE) combines the study of chemical kinetics with the reactors in which the reactions occur Phthalic anhydride

Introduction to reaction engineering Manufacture of Phthalic anhydride

Introduction to reaction engineering Water Treatment Microelectronics Nanoparticles Living system Manufacture of chemicals and pharmaceuticals

Introduction to reaction engineering Smog Hippo Digestion Molecular CRE

Introduction to reaction engineering Oil Recovery Pharmacokinetics Cobra Bites

Introduction to reaction engineering Reaction Rate The reaction rate is the rate at which a species looses its chemical identity per unit volume The identity of a chemical species is determined by the kind, number; and configuration of that species' atoms For example. the species nicotine (a bad tobacco alkaloid) is made of a fixed number of specific atoms in a definite molecular arrangement or configuration

Introduction to reaction engineering Reaction Rate Even though two chemical compounds have exactly the same number of atoms of each element, they could still be different species because of different configurations. For example, 2-butene has four carbon atoms and eight hydrogen atoms; however, the atoms in this compound can form two different arrangements

Introduction to reaction engineering Reaction Rate The reaction rate is the rate at which a species looses its chemical identity per unit volume The rate of a reaction can be expressed as the rate of disappearance of a reactant or as the rate of appearance of a product. Consider species A: A  B -rA = the rate of a disappearance of species A per unit volume rB = the rate of formation of species B per unit volume

Introduction to reaction engineering Reaction Rate There are three basic ways a species may lose its chemical identity: decomposition. combination. and isomerization. Decomposition: For example. if benzene and propylene are formed from cumene molecule

Introduction to reaction engineering Reaction Rate Combination: A second way that a molecule may lose its species identity is through combination with another molecule or atom

Introduction to reaction engineering Reaction Rate The third way a species may lose its identity is through isomerization, such as the reaction

Introduction to reaction engineering Reaction Rate Consider the reaction of chlorobenzene and chloral to produce the insecticide DDT (dichorodiphenyl-tricholoethane) in the presence of fuming sulphuric acid

Introduction to reaction engineering Reaction Rate The numerical value of the rate of disappearance of reactant A, -rA is a positive number e.g. -rA = 4 mol A/dm3.s The rate of reaction, -rA is the number of moles of A (e.g.. chloral) reacting (disappearing) per unit time per unit volume (mol/dm3.s). Symbol A represent chloral, B be chlorobenzene, C be DDT, and D be H2O

Introduction to reaction engineering Reaction Rate The heterogeneous reactions involve more than one phase. In heterogeneous reaction systems, the rate of reaction is usually expressed in measures other than volume, such as reaction surface area or catalyst weight.

Introduction to reaction engineering Reaction Rate The rate equation (i.e., rate law) for rj is an algebraic equation that is solely a function of the reacting materials and reaction conditions (e.g., concentration, temperature, pressure or type of catalyst at a point in a system) The rate equation is independent of the type of reactor (e.g., batch or continuous) in which reaction occurs

Introduction to reaction engineering Reaction Rate The chemical reaction rate law is essentially an algebraic equation involving concentration, not a differential equation The algebraic form of the rate law for –rA for the reaction may linear function of conc or

Introduction to reaction engineering General Mole Balance Equation To perform a mole balance on any system, the system boundaries must first be specified. To perform a mole balance on species j in a system volume where species j represents the particular chemical species of interest, such as water or NaOH Balance on system volume

Introduction to reaction engineering General Mole Balance Equation A mole balance on species j at any instant in time. t. yields the following equation:

Introduction to reaction engineering General Mole Balance Equation Gj. is just the product of the reaction volume V. and the rate of formation of species j, rj Suppose now that the rate of formation of species j for the reaction varies with the position in the system volume

Introduction to reaction engineering General Mole Balance Equation Dividing up the system volume, V

Introduction to reaction engineering General Mole Balance Equation The rate of generation, ∆Gj1, in terms of rj1 and sub volume ∆V1 is Similarly the expression can be written for ∆Gj1 and other sub volumes The total rate of generation within the system volume is the sum of all the rates of generation in each of the sub volumes

Introduction to reaction engineering General Mole Balance Equation By taking the appropriate limits (i.e., let M →∞ and ∆V→0) and making use of the definition of an integral and rewrite the foregoing equation in the form General mole balance equation we can develop the design equations for various types of reactors

Reactor Mole Balance Summary Differential Algebraic Integral Batch CSTR PFR (Plug flow reactor) PBR (Packed bed reactor)

Reactor Sizing CSTR CSTR is modeled as being well mixed such that there are no spatial variations in the reactor. The CSTR mole balance when applied to species A in the reaction Simplifying, we get CSTR volume required to achieve specific conversion X X = conversion

Reactor Sizing CSTR Given – rA as a function of conversion, – rA=f(X), one can size any type of reactor. We do this by constructing a Levenspiel plot. 0.2 0.4 0.6 0.8 10 20 30 40 50 Here we plot either as a function of X. X XEXIT For vs. X, the volume of a CSTR is: X = conversion

Space Time Space time, τ, is obtained by dividing reactor volume (V) by the volumetric flow rate entering the reactor (vo): The space time is the time necessary to process one reactor volume of fluid based on entrance conditions. Consider a tubular reactor which is 20 m long & 0.2 m3 in volume. The dash line represent 0.2 m3 of fluid goes directly upstream of reactor.

Space Time Typical space time for industrial reactors

Space Time Sample industrial space times

Space Velocity Space velocity, SV, is obtained by dividing the volumetric flow rate entering the reactor by the reactor volume : The SV is the reciprocal of the space time. Two velocity used in industry is LHSV and GHSV

Rate Law Basics k is given by the Arrhenius Equation: Where: E = activation energy (cal/mol) R = gas constant (cal/mol*K) T = temperature (K) A = frequency factor (units of A, and k, depend on overall reaction order)

Batch Stoichiometric Table At time t = 0, we will open the reactor and place a number of moles of species A, B. C, D, and I (NAO NB0, NCO and Nt respectively) into the reactor.

Batch Stoichiometric Table Species Symbols Initial Change Remaining A NA0 -NA0X NA=NA0(1–X) B NB0=NA0ΘB -(b/a)NA0X NB=NA0(ΘB –(b/a)X) C NC0=NA0ΘC (c/a)NA0X NC=NA0(ΘC+(c/a)X) D ND0=NA0ΘD (d/a)NA0X ND=NA0(ΘD+(d/a)X) Inert I NI=NA0ΘI NI = NA0ΘI NT0 NT=NT0+δNA0X

Algorithm : Pressure Drop in a Packed Bed Reactor Analyze the following second order gas phase reaction that occurs isothermally in a PBR:  A   B Mole Balance: Must use the differential form of the mole balance to separate variables (remember that FA = FAO * X): Rate Law: Second order in A and irreversible:

Multiple Reactions and Pressure Drop In terms of conversion: ε

MEMBRANE REACTORS (MR’s) Fogler page 207-215 and 437-348 Membrane Reactors are used to : (1) increase conversion when reaction thermodynamically limited (2) increase selectivity when multiple reactions are occurring

(1) Thermodynamically limited reactions These are reactions where equilibrium lies far to the left and therefore there is little conversion If reaction is exothermic, increasing the temperature will drive reaction further to the left ; decreasing the temp. results in very slow reaction rate . - therefore very little conversion If reaction endothermic, increasing the temp moves reaction to right therefore favouring higher conversion. However for many reactions increasing the temp can destroy the catalyst.

STRUCTURE OF MEMBRANE REACTOR Membrane can either be barrier to certain components while permeable to others, or contain active reactive sites and may be a catalyst in itself. Therefore membrane can either be inert or catalytic Achieve very high conversions by allowing one of reaction products diffuse out (eg. Hydrogen is small enough to diffuse through pores) , therefore high conversion, and reaction proceeds to completion

STRUCTURE OF MEMBRANE REACTOR Two main types of catalytic membrane reactors Inert membrane reactor with catalyst pellets on the feed side (IMRCF) Catalytic membrane reactor (CMR)

For the reaction: the mole balance on hydrogen must be modified because hydrogen leaves through both sides of the reactor and at the end of the reactor. Therefore the mole balance on hydrogen becomes: where RB is the molar rate of B leaving the reactor kC is the mass transfer coefficient

(2) Enhancing Selectivity RB = FBO/Vt FBO molar feed rate of B at sides Vt the total reactor volume The feed rate can be controlled by Controlling pressure drop across the reactor membrane Selectivity can be achieved by controlling the feed of species through the reactor and through the membrane. Can enhance selectivity by keeping one reactant low. Can keep concentration low by feeding through sides of membrane Home Task

Selectivity and Yield Instantaneous Overall Selectivity: Yield: Example: desired product , undesired product,

Finding the Rate Law Deferential Method Integral Method Initial Rate Method Half Life Method Least Square Method Page 268