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Reaction Rate • The reaction rate is the rate at which a species looses its chemical identity per unit volume.... Reaction Rate • The reaction rate is the rate at which a species looses [r]

Lecture Chemical Reaction Engineering (CRE) is the field that studies the rates and mechanisms of chemical reactions and the design of the reactors in which they take place Lecture  Introduction  Definitions  General Mole Balance Equation     Batch (BR) Continuously Stirred Tank Reactor (CSTR) Plug Flow Reactor (PFR) Packed Bed Reactor (PBR) Chemical Reaction Engineering  Chemical reaction engineering is at the heart of virtually every chemical process It separates the chemical engineer from other engineers Industries that Draw Heavily on Chemical Reaction Engineering (CRE) are: CPI (Chemical Process Industries) Examples like Dow, Amoco, Chevron, BSR, SK Energy, etc Smog (Ch 1) Wetlands (Ch DVD-ROM) Hippo Digestion (Ch 2) Oil Recovery (Ch 7) Chemical Plant for Ethylene Glycol (Ch 5) Lubricant Design (Ch 9) Cobra Bites (Ch DVD-ROM) Plant Safety (Ch 11,12,13) Let’s Begin CRE Chemical Reaction Engineering (CRE) is the field that studies the rates and mechanisms of chemical reactions and the design of the reactors in which they take place Chemical Identity • A chemical species is said to have reacted when it has lost its chemical identity Chemical Identity • A chemical species is said to have reacted when it has lost its chemical identity • The identity of a chemical species is determined by the kind, number, and configuration of that species’ atoms Chemical Identity • A chemical species is said to have reacted when it has lost its chemical identity Decomposition Chemical Identity • A chemical species is said to have reacted when it has lost its chemical identity Decomposition Combination Chemical Identity • A chemical species is said to have reacted when it has lost its chemical identity Decomposition Combination Isomerization Chemical Identity  A chemical species is said to have reacted when it has lost its chemical identity  There are three ways for a species to loose its identity: Decomposition CH3CH3  H2 + H2C=CH2 Combination N2 + O2  NO Isomerization C2H5CH=CH2  CH2=C(CH3)2 12 Reaction Rate • The reaction rate is the rate at which a species looses its chemical identity per unit volume Reaction Rate • The reaction rate is the rate at which a species looses its chemical identity per unit volume • The rate of a reaction (mol/dm3/s) can be expressed as either the rate of Disappearance: -rA or as the rate of Formation (Generation): rA Reaction Rate Consider the isomerization AB rA = the rate of formation of species A per unit volume -rA = the rate of a disappearance of species A per unit volume rB = the rate of formation of species B per unit volume Reaction Rate • EXAMPLE: AB If Species B is being formed at a rate of 0.2 moles per decimeter cubed per second, ie, rB = 0.2 mole/dm3/s Reaction Rate • EXAMPLE: AB rB = 0.2 mole/dm3/s Then A is disappearing at the same rate: -rA= 0.2 mole/dm3/s Reaction Rate • EXAMPLE: AB rB = 0.2 mole/dm3/s Then A is disappearing at the same rate: -rA= 0.2 mole/dm3/s The rate of formation (generation of A) is rA= -0.2 mole/dm3/s Reaction Rate • For a catalytic reaction, we refer to -rA', which is the rate of disappearance of species A on a per mass of catalyst basis (mol/gcat/s) NOTE: dCA/dt is not the rate of reaction Reaction Rate Consider species j: • rj is the rate of formation of species j per unit volume [e.g mol/dm3/s] Reaction Rate • rj is the rate of formation of species j per unit volume [e.g mol/dm3*s] • rj is a function of concentration, temperature, pressure, and the type of catalyst (if any) Reaction Rate • rj is the rate of formation of species j per unit volume [e.g mol/dm3/s] • rj is a function of concentration, temperature, pressure, and the type of catalyst (if any) • rj is independent of the type of reaction system (batch reactor, plug flow reactor, etc.) Reaction Rate • rj is the rate of formation of species j per unit volume [e.g mol/dm3/s] • rj is a function of concentration, temperature, pressure, and the type of catalyst (if any) • rj is independent of the type of reaction system (batch, plug flow, etc.) • rj is an algebraic equation, not a differential equation Building Block 1: General Mole Balances System Volume, V Fj0 24 Gj Fj Molar Flow Molar Flow  Molar Rate  Molar Rate  Rateof  − Rateof  + Generation =  Accumulation         Species j in  Species j out of Species j  of Species j  dN j Fj0 − Fj + Gj = dt  mole  mole  mole  mole − + =          time   time   time   time  Building Block 1: General Mole Balances If spatially uniform: G j = r jV If NOT spatially uniform: ∆V1 rj1 G j1 = rj1∆V1 25 ∆V2 rj G j = rj ∆V2 Building Block 1: General Mole Balances n G j = ∑ rji ∆Vi i =1 Take limit n Gj = ∑ rji ∆Vi i=1 lim ∆V → n → ∞ 26 = ∫ r dV j Building Block 1: General Mole Balances System Volume, V FA0 GA FA General Mole Balance on System Volume V In − Out + Generation = Accumulation dN A FA − FA + ∫ rA dV = dt 27 Batch Reactor Mole Balance Batch Reactor - Mole Balances Batch dN A FA0 − FA + ∫ rA dV = dt FA0 = FA = Well-Mixed 29 29 ∫r A dV = r A V dN A = rAV dt Batch Reactor - Mole Balances Integrating when dN A dt = rAV t = N A = N A0 t = t NA = NA t= NA ∫ N A0 dN A rAV Time necessary to reduce the number of moles of A from NA0 to NA 30 Batch Reactor - Mole Balances NA dN A t= ∫ rV N A0 A NA 31 t Continuously Stirred Tank Reactor Mole Balance CSTR (Cont.) CSTR (Cont.) CSTR (Cont.) CSTR (Cont.) CSTR (Cont.) Plug Flow Reactor PFR Mole Balances PFR: PFR Mole Balances (Cont.) PFR Mole Balances (Cont.) PFR Mole Balances (Cont.) PFR Mole Balances (Cont.) PFR Mole Balances (Cont.) Plug Flow Reactor - Mole Balances PFR dN A FA0 − FA + ∫ rA dV = dt Steady State dN A =0 dt FA0 − FA + ∫ rA dV = 45 Alternative Derivation Plug Flow Reactor - Mole Balances Differientiate with respect to V dFA = rA dV dFA 0− = −rA dV The integral form is: V = FA ∫ FA 46 dF A rA This is the volume necessary to reduce the entering molar flow rate (mol/s) from FA0 to the exit molar flow rate of FA PFR Mole Balances (Cont.) PFR: The integral form is: dF A FA r A V=∫ FA This is the volume necessary to reduce the entering molar flow rate (mol/s) from FA0 to the exit molar flow rate of FA Packed Bed Reactor Mole Balance PBR Packed Bed Reactor - Mole Balances ∆W PBR FA FA W + ∆W W FA (W ) − FA (W + ∆W ) + rA′ ∆W = Steady State lim 49 ∆W → dN A =0 dt F A W + ∆W − F A ∆W W dN A dt = r A′ Packed Bed Reactor - Mole Balances Rearrange: dFA = rA′ dW The integral form to find the catalyst weight is: W= FA ∫ FA dFA rA′ PBR catalyst weight necessary to reduce the entering molar flow rate FA0 to molar flow rate FA 50 Reactor Mole Balances Summary The GMBE applied to the four major reactor types (and the general reaction AB) Reactor Differential Algebraic Integral NA Batch CSTR PFR PBR 51 dN A t= ∫ rV N A0 A dN A = rAV dt V= dFA = rA dV dFA = rA′ dW FA − FA −rA FA dFA V= ∫ drA FA W= FA ∫ FA dFA rA′ NA t FA V FA W Homework 1: A 200-dm3 constant-volume batch reactor is pressurized to 20 atm with a mixture of 75% A and 25% inert The gas-phase reaction is carried out isothermally at 227 C V = 200-dm3 P = 20 atm T = 227 C a Assuming that the ideal gas law is valid, how many moles of A are in the reactor initially? What is the initial concentration of A? b If the reaction is first order: Calculate the time necessary to consume 99% of A c If the reaction is second order: Calculate the time to consume 80% of A Also calculate the pressure in the reactor at this time if the temperature is 127 C Homework 2: