TABLE OF CONTENT Chapter 1 Introduction 1 Chemical Reaction 2 Classification of Chemical Reaction 2 Chemical Equation 4 Rate of Chemical Reaction 5 Kinetic Models For Non Elementary Reaction 6 Molecularity 9 Order of Reaction 9 The Power Law Model 10 Rate Constant 10 Chapter 2 Reaction Kinetics 15 Definition of Conversion 15 Rate Equation for Various Reactions 16 Half Life Method 21 Rate Equation For Multiple Reactions (CVS) 22 Variable Volume Systems 30 Rate Equation For Multiple Reactions (VVS) 32 Chapter 3 Reactor Design 34 Ideal Batch Reactor 34 Semi Batch Reactor 37 Flow Parameters 40 Ideal CSTR 41 Ideal PFR 45 Packed Bed Reactor 51 Multiple Reactor systems 52 Chapter 4 Multiple Reaction Systems 61 Parallel Reactions 61 Quantitative Treatment of Product Distribution 63 Series Reactions 65 Gateway Institute: All Rights Reserved, KK market Dhankawadi, Pune-411043 Page 1
Chapter 5 Non Ideal Reactors 70 Residence Time 71 Tracer 71 RTD Measurement 71 Mean Residence Time 75 RTD in Reactors 80 Various RTD Combination of CSTR/PFR 82 Chapter 6 Temperature Pressure Effect 86 Pressure 86 Temperature 86 Heat of Reaction 86 Standard Heat of Reaction 86 Calculation for Heat of Reaction 88 Equilibrium Conversion 89 Van Hoff Equation 89 Relation Between Temperature and Conversion 90 Chapter 7 Catalytic Reactions 96 Non Catalytic Reaction System 96 The Concept of Controlling to Develop Overall Rate Expression 97 Rate Equation for Physical Absorption of A 99 Rate Equation for Absorption With Chemical Reaction 100 Hatta Number 101 Catalytic Reaction System 102 7 Steps in Solid Catalyzed Fluid Reaction 103 Rate Equation for Pore Diffusion and Surface Reaction 103 Effectiveness Factor 106 Characteristic Length 107 Relation Between Thiele Modulus and Size of Particle 107 Thiele Modulus 108 Effective Diffusivity 110 Design Equation for Reactors Containing Porous Catalyst 111 Hinshelwood- Langmuir Approach Gateway Institute: All Rights Reserved, KK market Dhankawadi, Pune-411043 Page 2
1 Introduction A chemical engineer is a universal engineer because all engineers use maths, physics and economics to solve technical problems but chemical engineers apply a knowledge of chemistry in addition to others. Chemical Engineering The process to convert raw materials into the useful products These raw materials are going through many physical and chemical operations. Design of equipments for the physical treatment steps is studied in the unit operation. The chemical treatment is the heart of the process, and a chemical engineer deals with this which separate him from the other engineers. Te optimum design of chemical reactor requires the knowledge of thermodynamics, fluid mechanics, heat transfer, mass transfer and economics. WORKS OF CHEMICAL ENGINEER o Design of chemical process o Maintain and operate a process o Increase capacity and selectivity at minimum cost In reactor design, 2 things which must be important 1. What changes can we expect 2. How fast will they take place Thermodynamics gives information about the o Feasibility of a reaction Gateway Institute: All Rights Reserved, KK market Dhankawadi, Pune-411043 Page 3
o Heat of reaction o Maximum possible extent of reaction Chemical Reaction Engineering gives information about the chemical kinetics Rate at which chemical reaction occurs Effect of parameters (Temperature, Pressure & Composition) on the reaction rates. Order of reaction / Type of reaction CHEMICAL REACTION A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuclei (no change to the elements present), and can often be described by a chemical equation. The substance (or substances) initially involved in a chemical reaction are called reactants or reagents. Chemical reactions are usually characterized by a chemical change, and they yield one or more products, which usually have properties different from the reactants. CLASSIFICATION OF CHEMICAL REACTION It classify according to o Five traditional types of chemical reactions are 1. Decomposition reactions: single compound decomposes to two or more other substances, decomposition of calcium carbonate by heating it. CaCO 3 (s) ---> CaO(s) + CO 2 (g) 2. Combination reactions (Synthesis reactions) 3. Single-replacement reactions (Displacement reactions): copper displaces silver from an aqueous solution of silver nitrate is an example of a single-replacement reaction. Cu(s) + 2 AgNO 3 (aq) ---> Cu(NO 3 ) 2 (aq) + 2 Ag(s) 4. Double-replacement reactions (Metathesis reactions): Precipitation reactions are one type of double-replacement reaction. An example is AgNO 3 (aq) + NaCl(aq) ---> AgCl(s) + NaNO 3 (aq) Gateway Institute: All Rights Reserved, KK market Dhankawadi, Pune-411043 Page 4
5. Combustion reactions: substance reacts with oxygen, butane burns in air as follows. 2 C 4 H 10 (g) + 13 O 2 (g) ---> 8 CO 2 (g) + 10 H 2 O(l) o PHASES INVOLVED: 1. Homogeneous reaction: it takes place in one phase alone 2. Heterogeneous reaction: multiple phases, reaction usually occurs at the interface between phases. o DIRECTION OF REACTION 1. Irreversible Reaction : Proceeds in only one direction and continues in that direction until the reactants are exhausted. Example : Heterogeneous reaction Homogeneous reaction 2. Reversible Reaction: Can proceed in either direction, depending on the concentrations of reactants and products present relative to the corresponding equilibrium concentration. Example : Homogeneous reaction Gateway Institute: All Rights Reserved, KK market Dhankawadi, Pune-411043 Page 5
Heterogeneous reaction CHEMICAL EQUATION A chemical equation is the symbolic representation of a chemical reaction wherein the reactant entities are given on the left-hand side and the product entities on the right-hand side. The coefficients next to the symbols and formulae of entities are the absolute values of the stoichiometric numbers. They are separated by an arrow ( ) which indicates the direction and type of the reaction; the arrow is read as the word "yields". The tip of the arrow points in the direction in which the reaction proceeds. A double arrow ( ) pointing in opposite directions is used for equilibrium reactions. Equations should be balanced according to the stoichiometry, the number of atoms of each species should be the same on both sides of the equation. This is achieved by scaling the number of involved molecules (A, B, C and D in a schematic example below) by the appropriate integers a, b, c and d. COMMON SYMBOLS Symbols are used to differentiate between different types of reactions. To denote the type of reaction: " " symbol is used to denote a stoichiometric relation. " " symbol is used to denote a net forward reaction. " " symbol is used to denote a reaction in both directions. " " symbol is used to denote an equilibrium. Physical state of chemicals is also very commonly stated in parentheses after the chemical symbol, especially for ionic reactions. When stating physical state, (s) denotes a solid, (l) denotes a liquid, (g) denotes a gas and (aq) denotes an aqueous solution. If the reaction requires energy, it is indicated above the arrow. A capital Greek letter delta ( put on the reaction arrow to show that energy in the form of heat is added to the reaction. used if the energy is added in the form of light. ) is is Gateway Institute: All Rights Reserved, KK market Dhankawadi, Pune-411043 Page 6
RATE OF CHEMICAL REACTION The rate of reaction tells us how fast a number of moles of one component species are being consumed to form another chemical species. The term chemical species refers to any chemical component or element with a given identity. d N A We express the rate as the rate of disappearance of component A as The rate of change dt of A (in no. of moles of A). The negative sign indicates that disappearance of reactant A during reaction. The rate of reaction can be expressed in various forms as follows: Based on unit volume of reaction mixture Based on unit mass of solid in fluid solid system Based on unit surface of solid in gas-solid system r A = 1 V r A = 1 W r" A = 1 S dn A dt dn A dt dn A dt The reaction rate is an intensive quantity & depends on concentration & temperature. From above rate equations, we ve r A V = ( r A)W = r" A S RELATIVE RATE OF REACTION The relative rate of reaction of various species involved in a reaction can be obtained from the ratio of stoichiometric coefficients. For reaction given below aa + bb cc + dd We see that for every a mole of A reacted with b mole of B produces c mole of C and d mole of D. In other words Rate of Formation of C = c (Rate of disappearance of A) a r C = c a ( r A) r C = c d (r D) The relationship can be expressed directly from stoichiometric of reaction, r A a = r B b = r C c = r D d Gateway Institute: All Rights Reserved, KK market Dhankawadi, Pune-411043 Page 7
VARIABLES AFFECTING THE RATE OF REACTION o Nature of reactant & product o Concentration of reactants o Temperature o Pressure o Nature of catalyst o Surface area of reactant o Rates of heat & mass transfer KINETIC MODELS FOR NON ELEMENTARY REACTION To explain the kinetics of non elementary reactions we assume that a sequence of elementary reactions is actually occurring but we cannot observed the intermediate formed because they are only present in very minute quantities. o ACTIVE INTERMEDIATE An active intermediate is a high energy molecule that reacts as fast as it is formed; as a result it is present in a very small concentration. Active intermediate can be formed by collision or interaction with other molecules. * A M A M The translation kinetic energy of molecule is absorbed into chemical bonds where high amp. Oscillations will lead to bond ruptures, molecular rearrangement and decomposition. The transfer of translational energy to vibrational energy to produce an active intermediate. o PSEUDO STEADY STATE APPROXIMATION (PSSM) An unseen and unmeasured active intermediate present at small concentration that its rate of change in the mixture can be taken as zero. r * 0 This condition is called pseudo steady state hypothesis. e.g. the order of formation of ethane. Gateway Institute: All Rights Reserved, KK market Dhankawadi, Pune-411043 Page 8
k * C H N C H N C H N C H N 3 2 2 3 2 2 3 2 2 3 2 2 1 * k 2 C H N C H N C H N C H N 3 2 2 3 2 2 3 2 2 3 2 2 * k 3 3 2 2 2 6 2 C H N C H N In reaction 1: Two Azo molecules colloid and the kinetic of one Azo molecule is transferred to internal rotational and vibration energies of the other Azo molecule and it become activated and highly reactive. In reaction 2: The activated molecule is deactivated through collision with another Azo molecule by transferring its internal energy to increase the kinetic energy of the molecules. In reaction 3: This high activated Azo molecule spontaneously decomposes into ethane and nitrogen. The rate of formation of C H 2 6 r k C...(a ) C2H6 3 A zo* Find the concentration of Azo* in terms of known concentration. 2 r k C k C C k C A zo * 1 2 3 A zo* A zo* A zo A zo* Use PSSH r A zo * C A zo * 0 k C k C 1 2 A zo k 2 A zo 3 Putting into equation (a) 2 k k C 1 2 A z o r...(b ) C2H6 k C k 2 A z o 3 At low Azo concentration k C 2 A zo 3 C2H6 k 1 2 r k C (s e c o n d o rd e r) A zo At high Azo concentration Gateway Institute: All Rights Reserved, KK market Dhankawadi, Pune-411043 Page 9
k C 2 A zo 3 C2H6 k r k C (firs t o rd e r) A zo o FINDING THE REACTION MECHANISM Rules of thumb for development of mechanism: i. Species having the concentration appearing in the denominator of the rate law probably colloid with the active intermediate, e.g. A A* c o llio sin p ro d u c t ii. If a constant appears in the denominator, one of the reaction steps is probably the spontaneous decomposition of the active intermediate, e.g. A* d e c o m p o sitio n p ro d u c t iii. Species having the concentration appearing in the numerator of the rate law probably produce the active intermediate in one of the reaction steps, e.g. R e a c ta n t A * o th e r p ro d u c t Now a rate law has been synthesized from the experiments data, we shall try to propose a mechanism consistent with this rate law. 1. Assume an active intermediate 2. Postulate a mechanism utilizing the rate law 3. Write rate law for the formation of desired product and active intermediate 4. Use the PSSH 5. Eliminate concentration of active intermediates from rate of formation of product 6. If the derived rate law does not agree with experiment rate law, then assume a new mechanism and repeat all. Gateway Institute: All Rights Reserved, KK market Dhankawadi, Pune-411043 Page 10
5 Non-Ideal Reactors In non ideal reactors, the conversion given by the reactor is less than the conversion given by ideal reactor of same volume. Some of the reasons of non ideal are as follows: DEAD ZONE Generally, the corners of reactors are act as dead zone / volume. That much amount of volume is not available for reaction. Hence given conversion is less. BYPASSING Some molecules take shortcut path flowing through the reactor and results into less conversion as estimated. CHANNELLING Channelling is one type of bypassing. This term generally used in packed bed reactor. TURBULENCE Gateway Institute: All Rights Reserved, KK market Dhankawadi, Pune-411043 Page 11
Some molecules are rotating much more time in turbulence inside the reactor and that much amount of volume is less available for new molecules, it results less conversion. RESIDENCE TIME The time taken by a molecule to pass through the reactor is called the residence time of the molecule in the reactor. The molecules take different path, hence residence time is not same for all molecules. The distribution of these times for the system of fluid leaving the vessel is called RTD (Residence Time Distribution) or Exit Age Distribution (E). For PFR and Batch Reactor, the residence time is same for almost all molecules. TRACER The inert component sent into the reactor with reactant feed is called Tracer. Tracer should have some properties: o Non Reacting o Completely soluble in the reactor feed o Easily detectable o Non absorbing on the wall of reactor vessel RTD MEASUREMENT The RTD is determined experimentally by injecting a tracer, into the reactor at some time t = 0 and then measuring the tracer concentration, C, in the effluent stream as a function of time. The two most used methods of injection are pulse input and step input. PULSE INPUT EXPERIMENT Gateway Institute: All Rights Reserved, KK market Dhankawadi, Pune-411043 Page 12
In pulse input the amount of tracer M is suddenly injected in one shot into the feed stream entering the reactor at time t = 0. let dm is amount of material living at time between t & t + d t dm = C.v.dt Let fraction of material that has residence time between t & t + t, E C. v C M M / v M d M 0 0 M v C. d t 0 C. v. d t M v 0 C. d t C C C. d t E ( tim e ) E 0 C. d t 0 1 E CURVE FROM C CURVE:- From the data of C v/s t. Plot C curve & find the area under the curve then find E for each time & plot E v/s t which is the E curve. Gateway Institute: All Rights Reserved, KK market Dhankawadi, Pune-411043 Page 13
If we consider time interval t to t fraction of material leaving the vessel that has resided in the 1 2 vessel between times t 1 & t 2 = t 2 t1 E d t Fraction of all the material that has resided for a time t in the vessel between t 0 to t is 1. then 0 E. d t = 1 Age for an element refers to the time spent by the element of the exit stream of vessel (i.e. time it has resided in the vessel). The fraction of the exit stream of age t 1 is t 2 0 E. d t The fraction of material older then t 1 is t1 E d t E. d t E. d t t1 0 0 t1 E d t 1 E. d t t1 0 Gateway Institute: All Rights Reserved, KK market Dhankawadi, Pune-411043 Page 14