CHEMICAL ENGINEERING KINETICS/REACTOR DESIGN. Tony Feric, Kathir Nalluswami, Manesha Ramanathan, Sejal Vispute, Varun Wadhwa

Transcription

1 CHEMICAL ENGINEERING KINETICS/REACTOR DESIGN Tony Feric, Kathir Nalluswami, Manesha Ramanathan, Sejal Vispute, Varun Wadhwa

2 Presentation Overview Kinetics Reactor Design Non- Isothermal Design

3 BASICS OF KINETICS

4 Definition of Rate rate = 1 V (dn, dt ) = (dc, dt ) = [mol] L [s] Rate = measure of how fast the concentration changes r, = Σν 8, r 8 Rate of formation of the j th species is given by the sum of its rates in each reaction i Source: Essentials of Chemical Reaction Engineering 4 th Ed. by H. Scott Fogler

5 Rate Law Rate law is an empirical relationship between rate and conversion: A + 2B R + S r = f C A, C C, C D, C E kc H A C C k = Rate constant α = Reaction order with respect to A β = Reaction order with respect to B k is temperature dependent because as the temperature increases, # of successful collisions increases exponentially Arrhenius Equation: k = A exp E [ RT A = Pre-exponential factor R = Gas constant (8.314 J/mol.K) E a = Activation energy of the reaction T = Temperature (Kelvin)

6 Reaction Mechanisms Elementary reaction: Perfect connection between rate law and stoichiometry occurring by collisions (molecularity 2) E r = k ^ C, _`a Reaction mechanism is a sequence of elementary steps having its own activation energy E a and rate constant k,bc Correct Orientation Sufficient Energy

7 BASICS OF REACTOR DESIGN

8 Isothermal Reactor Mass Balance General Input - Output + Generation = Accumulation Batch CSTR (Steady state) PFR

9 Reactor Design Equations In terms of Conversion In terms of Concentration (Constant Density) Batch CSTR (Steady state) step change in concentration to exit value PFR concentration changes continuously

10 Reactors in Series For a single reactor, CSTRs require more volume than a PFR to reach the same conversion CSTRs used in series to reduce the total volume required to reach a given conversion

11 Series Reactions: Comparison of PFR and CSTR (Rxn: A B C) In PFR (solid line), higher concentration of product B in all cases compared to the CSTR (dotted line) To maximize intermediate concentration, use a moderate residence time

12 Parallel Reactions: Comparison of PFR and CSTR (Rxn: A B ; A C) PFR (solid line) and CSTR (dotted line) For both products (B and C), PFR gives greater exit concentration compared to CSTR Rate selectivity and overall selectivity of B is the same in both reactors

13 Design Equations: Multiple Reactions vs. Single Reaction In each reactor, v j r is replaced by D d ν 8, r 8 8bc for multiple reactions

14 NON-ISOTHERMAL REACTOR DESIGN

15 Non-Isothermal Reactors Non-isothermal reactors are advantageous because most reactions are exothermic à heat generated is used to increase rate and conversion Heat generated/removed does the following: Changes temperature in reactor Changes rate constant (Arrhenius Equation) Changes concentrations of gaseous reactions Change ΔH R (δh j =C p,j δt)

16 Non-Isothermal CSTR At steady state, heat removed = heat generated. Assumptions: Heat accumulation = 0 (steady state) Exothermic reaction Heat is removed via coolant U = overall heat transfer coefficient A c = area of heat transfer T c = temperature of coolant

17 Non-Isothermal PFR At steady state, heat removed = heat generated. Assumptions: Heat accumulation = 0 (steady state) Exothermic reaction Heat is removed via coolant U = overall heat transfer coefficient A c = area of heat transfer T c = temperature of coolant

18 Non-Isothermal PFR (cont.) Temperature ODE Where Concentration ODE

19 Non-Isothermal Adiabatic Adiabatic à No heat added or removed in system; set Q = 0. Adiabatic temperature rise: the maximum temperature rise Shows a linear relationship between temperature and conversion

20 Conclusion: Importance of Reactor Design Distinguishes Chemical Engineers from other engineers Reactor design is the heart of any chemical process Controls overall process economics Key to controlling a chemical plant's safety and efficiency

21 QUESTIONS?