Lecture note chemical reaction engineering

VIETNAM NATIONAL UNIVERSITY – HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY FACULTY OF CHEMICAL ENGINEERING Chemical Reaction Engineering (Homogeneous Reactions in Ideal Reactors) Mai Thanh Phong, Ph.D FCE – HCMC University of Technology Chemical Reaction Engineering References Octave Levenspiel, “Chemical Reaction Engineering”, John Wiley&Sons, 2002 H Scot Foggler, “Elements of Chemical Reaction Engineering”,International students edition, 1989 E.B.Nauman, “Chemical Reactor Design”, John Wiley & sons, 1987 Stanley M Walas, “Reaction Kinetics for Chemical Engineers”,Int Student Edition, 1990 Coulson & Richardsons, “Chemical Engineering – Vol 6”,Elsevier, 1979 Richard M Felder, “Elementary Principles of Chemical Processes”, John Wiley & sons, 2000 Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 Chapter Introduction • Topic of the lecture „Chemical Reaction Engineering“ is the quantitative assessment of chemical reactions The selection of suitable reactor types and their design will be discussed • Reactor design uses information, knowledge, and experience from a variety of areas: thermodynamics, chemical kinetics, fluid mechanics, heat transfer, mass transfer, and economics Chemical reaction engineering is the synthesis of all these factors with the aim of properly designing a chemical reactor • Thermodynamics tell us in which direction a reaction system will develop and how far it is from its equilibrium state • Analyses of kinetics provide information about the rate with which the system will approach equilibrium Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 Chapter Introduction I Basic Parameter Description of the amount of a substance i: Number of moles: mi ni = Mi Mi = molecular weight Molar concentration: ni ci = V V = volume Mole fraction: ni xi = ∑ nj j Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 Chapter Introduction Progress of chemical reactions: Conversion: ni − ni Xi = ni If V = const: ci − ci Xi = ci Extent of reaction: ξ= ni − ni νi Performance criteria: Productivity: Mai Thanh Phong - HCMUT produced amount of product P n& P = operating time Chemical Reaction Engineering 3-Feb-09 Chapter Introduction II Stoichiometry of chemical reactions: Stoichiometry is based on mass conservation and thus quantifies general laws that must be fulfilled during each chemical reaction Starting point of a quantitative analysis is the following formulation of a chemical reaction: N ∑ν A = i =1 i i This equation describes the change of the number of moles of N components A1, A2, AN The νi are the stoichiometric coefficients of component i They have to be chosen in such a way that the moles of all elements involved in the chemical reaction remain constant A convention is that reactants have negative stoichiometric coefficients and products have positive stoichiometric coefficients Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 Chapter Introduction As an example the stoichiometric equation for the oxidation of carbon monoxide is given by: 2CO + O2 → 2CO2 with νCO = -2, vO2 = -1, vCO2 = To calculate changes in the mole number of a component i due to reaction, the following balance has to be respected: ni = ni + ν iξ From this equation results the important stoichiometric balance: ni − ni νi = Δni νi = nk − nk νk = Δnk νk Using the conversion X of a component k, the above equation becomes: νi ni = ni − nk X k νk Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 Chapter Introduction III Chemical thermodynamics: Chemical thermodynamics deal with equilibrium states of reaction system This Section will concentrate on the following two essential areas: a) The calculation of enthalpy changes connected with chemical reactions, and b) The calculation of equilibrium compositions of reacting systems 3.1 Enthalpy of reaction The change of enthalpy caused by a reaction is called reaction enthalpy ∆HR This quantity can be calculated according to the following equation: N ΔH R = ∑ν i ΔH Fi i =1 ∆HFi is the enthalpy of formation of component i ∆HR < 0, the reaction is exothermic ∆HR > 0, the reaction is endothermic Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 Chapter Introduction It is simple to calculate the reaction enthalpy at a certain standard state ∆HR0 from the corresponding standard enthalpies of formation ∆HFi0 The standard enthalpies of formation are available from databases for P = P0 = bar and T = T0 = 298 K For pure elements like C, H2, O2, : ∆HFi0 = The reaction enthalpy is a state variable Thus, a change depends only on the Initial and the end state of the reaction and does not dependent on the reaction parthway 3.2 Temperature and pressure dependence of reaction enthalpy ⎛ ∂ΔH R ⎞ ⎛ ∂ΔH R ⎞ d (ΔH R ) = ⎜ ⎟ dP + ⎜ ⎟ dT ⎝ ∂P ⎠T ⎝ ∂T ⎠ P The pressure dependence is usually very small For ideal gas behaviour, the reaction enthalpy does not depend on pressure Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 Chapter Introduction The correlation of reaction enthalpy and temperature is related to the isobaric heat capacities of all species involved in the considered reaction, cPi N T i =1 Pi T = 298 K ΔH R (T ) = ΔH R0 + ∑ν i ∫ c (T )dT Assuming that the reactants and the products have different but temperature independent heat capacities, the temperarue dependence of the reaction enthalpy can be estimated as follows: ΔH R (T ) = ΔH R0 + (T − T0 )(cP ,products − cP ,reactants ) Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 10 Chapter Design for Single Reactions Second-Order Reactions Consider reaction: N reactors in series: (4.14) Whereas for plug flow: (4.15) A comparison of the performance of these reactors is shown in Fig 4.5 Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 68 Chapter Design for Single Reactions Figure 4.5 Comparison of performance of a series of N equal-size mixed flow reactors with a plug flow reactor for elementary secondorder reactions Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 69 Example At present 90% of reactant A is converted into product by a second-order reaction in a single mixed flow reactor We plan to place a second reactor similar to the one being used in series with it (a) For the same treatment rate as that used at present, how will this addition affect the conversion of reactant? (b) For the same 90% conversion, by how much can the treatment rate be increased? The following liquid-phase hydration reaction occurs in a 10,000 L CSTR: A+H2O → B with a first-order rate constant of 2.5 x 10-3 min-1 a) What is the steady-state fractional conversion of A if the feed rate is 0.3 L/sec and the feed concentration CAo = 0.12 mol/L? b) If the feed rate suddenly drops to 70% of its original value and is maintained there, what is the fractional conversion of A after 60 minutes, and what is the new steady state fractional conversion? Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 70 Chapter Design for Single Reactions 2.3 Mixed Flow Reactors of Different Sizes in Series 2.3.1 Finding the Conversion in a Given System Consider three mixed flow reactors in series as shown in Figure 4.6 Figure 4.6 Notation for a series of unequal-size mixed flow reactors Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 71 Chapter Design for Single Reactions Noting that ε = 0, it can be written for component A in the first reactor: (4.16) or (4.17) Similarly, for the ith reactor we may write: (4.18) Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 72 Chapter Design for Single Reactions 4.18) 4.18) Figure 4.7 Graphical procedure for finding compositions in a series of mixed flow reactors Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 73 Chapter Design for Single Reactions 2.3.2 Determining the Best System for a Given Conversion Suppose we want to find the minimum size of two mixed flow reactors in series to achieve a specified conversion of feed which reacts with arbitrary but known kinetics It can be written for component A in the first and second reactor: and (4.19) These relationships are displayed in Fig 4.8 for two alternative reactor arrangements, both giving the same final conversion X2 Figure 4.8 shows that the total reactor volume is as small as possible (total shaded area is minimized) when the rectangle KLMN is as large as possible Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 74 Chapter Design for Single Reactions Figure 4.8 Graphical representation of the variables for two mixed flow reactors in series Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 75 Chapter Design for Multiple Reactions • Multiple Reactions For multiple reactors, both the size requirement and the distribution of reaction products are affected by the flow within the reactor • The distinction between a single reaction and multiple reactions is that the single reaction requires only one rate expression to describe its kinetic behavior whereas multiple reactions require more than one rate expression • Many multiple reactions can be considered to be combinations of two primary types: parallel reactions and series reactions • In this chapter, expansion effects are ignored, thus ε = 1.1 Qualitative Discussion About Product Distribution Consider the decomposition of A by either one of two paths: (5.1) Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 76 Chapter Design for Multiple Reactions with corresponding rate equations (5.2a) (5.2b) Dividing Eq 5.2a by Eq 5.2b gives a measure of the relative rates of formation of R and S Thus (5.3) This ratio is expected to be as large as possible k1, k2, a1, and a2 are all constant for a specific system at a given temperature Thus, CA is the only factor in this equation which we can adjust and control Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 77 Chapter Design for Multiple Reactions • If a1 > a2: the desired reaction is of higher order than the unwanted reaction Eq 5.3 shows that a high reactant concentration is desirable since it increases the R/S ratio As a result, a batch or plug flow reactor would favor formation of product R and would require a minimum reactor size • If a1 < a2: the desired reaction is of lower order than the unwanted reaction A low reactant concentration is needed to favor formation of R But this would also require large mixed flow reactor • If a1 = a2: the two reactions are of the same order, Eq 5.3 becomes (5.4) Thus, product distribution is fixed by k1/k2 alone and is unaffected by type of reactor used Product distribution can be controlled by varying k1/k2 in two ways: Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 78 Chapter Design for Multiple Reactions Changing the temperature If the activation energies of the two reactions are different, k1/k2 can be varied by changing temperature Using a catalyst One of the most important features of a catalyst is its selectivity in depressing or accelerating specific reactions This may be a much more effective way of controlling product distribution than any of the methods discussed so far When you have two or more reactants, combinations of high and low reactant concentrations can be obtained by controlling the concentration of feed materials Figures 5.1 and 5.2 illustrate methods of contacting two reacting fluids in continuous and noncontinuous operations that keep the concentrations of these components both high, both low, or one high and the other low Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 79 Chapter Design for Multiple Reactions Figure 5.1 Contacting patterns for various combinations of high and low concentration of reactants in noncontinuous operations Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 80 Chapter Design for Multiple Reactions Figure 5.2 Contacting patterns for various combinations of high and low concentration of reactants in continuous flow operations Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 81 Chapter Design for Multiple Reactions 1.2 Quantitative Treatment of Product Distribution and of Reactor Size If rate equations are known for the individual reactions, we can quantitatively determine product distribution and reactor-size requirements For convenience in evaluating product distribution we introduce two terms, φ and Φ Consider the composition of A: The instantaneous fractional yield of R (φ) is defined as: (5.5) Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 82