Chemical Reaction Engineering Lecture 5

Transcription

1 Chemical Reaction Engineering g Lecture 5

2 The Scope The im of the Course: To learn how to describe a system where a (bio)chemical reaction takes place (further called reactor) Reactors Pharmacokinetics Chemical plant for ethylene glycol Microelectronic fabrication

3 The general mole balance equation For any component j: Mass balance: Rate of flow IN Rate of flow OUT + Generation=ccumulation F F + G = j0 j j G i = r V i dn dt moles/time moles/(time volume) volume j

4 Chemical Reaction Engineering Lecture plan (Lectures 5-10) Lecture 5: Mole balance and design equations for batch and continuous mode reactors. Lecture 6: Rate laws in the reactor design Lecture 7: Isothermal reactor design Lecture 8: Bioreactors. Comsol modelling of reactions and reactors. H-cell with chemical reaction. Lecture 9. Non-isothermal reactors. Comsol modelling: flow through porous bed and stirred batch reactor. Lectures Diffusion and Reactions. Comsol modelling of biochips: reaction on the surface.

5 Rates of chemical reactions + 2B 3C+ D Instantaneous rate of consumption of a reactant: d[ R]/ dt Instantaneous rate of formation of a product: From stoichiometry Rate of the reaction: dp [ ]/ dt dd [ ] 1 dc [ ] d [ ] 1 db [ ] = = = dt 3 dt dt 2 dt 1 dni dξ v = = ν dt dt! In the case of heterogeneous reaction the rate will be defined d per unit area of catalyst t as mol/m 2 s! In the case of continuous flow reactor change of concentration is not equal to the reaction rate i

6 Rates of chemical reactions + 2 B 3 C + D Usually we interested in the concentration of one particular reagent, say. The reaction rate in terms of reagent is the number of moles of reacting per unit time, per unit volume (mol m -3 s-1 ) r = d[ ]/ dt However this definition is inconvenient in the case of a reactor and can be misleading as the concentration of is varying with time and position inside the reactor:

7 Rates of chemical reactions So, we should rather say that: t Rate of chemical reaction is an algebraic function involving concentration, temperature, pressure and type of catalyst at a point in the system eg e.g. 1 st order reaction product r = kc 2 nd order reaction r = kc 2

8 The general mole balance equation For any component j: Mass balance: Rate of flow IN Rate of flow OUT + Generation=ccumulation F F + G = j0 j j G i = r V i dn dt moles/time moles/(time volume) volume j

9 The general mole balance equation Generally, the rate of reaction varies from point to point in the reactor: G i V = rdv i The general mole balance equation: V Fj0 Fj + rdv i = dn dt j From here, design equation for different types of the reactors can be developed

10 Types of Chemical Reactors Depending on loading/unloading of the reactor Batch Semi-Batch Continuos Flow CSTR (Continuous-Stirred Tank Reactor) Tubular reactor Packed-bed reactor

11 Batch reactors for small-scale scale operation; testing new processes manufacturing expensive products processes difficult to convert to continuous operation

12 Batch reactors F j0 Fj + rdv i = V dn j dt V rdv i = dn j dt 0 assuming perfect mixing, reaction rate the same through the volume dn dt j = j rv integrating the equation we can get N j vs t mole-time trajectory

13 Batch reactors Pfaudler s Batch reactor

14 Continuous Flow Reactors CSTR (Continuous-Stirred Tank Reactor) In Out Pfaudler s CSTR reactor

15 Continuous Flow Reactors CSTR (Continuous-Stirred Tank Reactor) F F + j0 j rdv i = V dn dt j F j0 F j =0, operation in a steady mode assuming perfect mixing, so Reaction rate is the same through the volume Conditions of exit stream are the same as in the reactor F F = rv j 0 j j V = F j0 F j V = or r j Design equation of CSTR vc 0 0 r vc

16 Continuous Flow Reactors Tubular reactor usually operates in steady state primarly used for gas reactions easy to maintain, no moving parts produce highest yield temperature could be difficult to control, hot spots might occur

17 Continuous Flow Reactors Tubular reactor Reaction continuously progresses along the length of the reactor, so the concentration and consequently the reaction rate varies in axial direaction in the model of Plug Flow Reactor (PFR) the velocity is considered uniform and there are no variation of concentration (and reaction rate) in the radial direction If it cannot be neglected we have a model of Laminar Flow Reactor.

18 Continuous Flow Reactors PFR (plug flow reactor) useful approximation of a tubular reactor F F + rdv = V j0 j i dn j dt For every slice of volume: Fj Fj Fj0 Fj + ri Δ V = 0 r i = ΔV j V j V+ΔV No accumulation r i = df dv j 0 From here, a volume required to produce given molar flow rate of product can be determined

19 Continuos Flow Reactors Design equation for PFR r = j df dv j dv = df j j j V = = r Fj0 r j If we know a profile of molar flow rate vs. Volume we can calculate the required volume to produce given molar flow rate at the outlet. F df j F F j j0 df r j j

20 Continuous Flow Reactors Packed-Bed reactor here the reaction takes place on the surface of catalyst reaction rate defined per unit area (or mass) of catalyst = mol l reacted/s g catalyst t r

21 Continuous Flow Reactors W catalyst weight coordinate as in the PFR case, we can calculate design equation now in terms of catalyst weight coordinate FW FW +ΔW df FW FW +ΔW + ri Δ W = 0 ri = r = ΔW dw

22 Reactors Mole Balance: Summary

23 Sizing of reactors Here we ll find how to find the size of a reactor is relation between the reaction rate and conversion factor is known

24 Conversion in the reactors a + bb cc + dd if we are interested in species we can define the reactant as the basis of calculation conversion: b c d + B C+ D a a a X = Moles of reacted Moles of fed maximum conversion for reversible reactions is the maximum conversion for reversible reactions is the equilibrium conversion X e.

25 Batch reactor design equations [ ] [ N ] Moles of reacted = 0 X [ ] [ ] [ ] Moles of in reactor, N = N0 N0 X dn = ( r ) V dt dn dx = N0 dt dt N dx r V = 0 ( ) dt Design equation for Batch Reactor the equation can be integrated to find the time necessary to achieve required conversion the longer reactants spend in the chamber the higher is the degree of conversion

26 Design equations for flow reactors [ ][ ] F X = 0 [ F ][ X ] F X = 0 [ Moles of fed] [ Moles of reacted] [ time ] [ Moles of fed ] [ Moles of fed] [ time] Molar flow rate fed to the system Molar flow rate of the consumption of in the system Molar flow rate of leaving the system [ F ] [ F ] X = [ F ] = leaving the system 0 0 molar flow rate is concentration * volume rate [ ] [ ] F = F (1 X) = C v 0 0 0

27 Design equations for flow reactors CSTR: [ ] [ ] F (1 ) = F0 X V F0 F F0 X = = r r Because the reactor is perfectly mixed the exit composition is Because the reactor is perfectly mixed, the exit composition is identical to the composition inside the reactor

28 Design equations for flow reactors Tubular Flow Reactor (PFR): r = df dv [ ] [ ] F (1 ) = F0 X = r F dx 0 dv V X dx = F 0 r 0 to integrate we need to know r depends on the concentration (and therefore on conversion)

29 Design equations for flow reactors Packed-Bed Reactor: similar derivation, but W instead of V r = df dw [ ] [ ] F (1 ) = F0 X r = r F0dX dw W X = F 0 0 dx r from this equation we can find weight of catalyst W required to achieve the conversion X

30 Levenspiel plot reactor volume required is always reciprocal in r and proportional to X. PFR: V X dx = F 0 r Levenspiel plot: 0 CSTR: V = F0 r X

31 Example (2.2, p.48) Reaction B described by the data below and the species enter the reactor at a molar flow rate of 0.4 mol/s: Calculate the volume necessary for 80% conversion

32 Example (2.2, p.48) Solution: Based on the table the Levenspiel plot can be constructed The design equation for the CSTR: V 3 F mol m s = X V = = 6.4m s mol ( 0 r ) r 1 exit 3

33 Example (2.3, p.50) Calculate based on the same data the volume of PFR: gain, we construct the Levenspiel plot The design equation for the PFR: F V = dx = 2.165m 0 r 1 1

34 CSTR in series Reactors in series 1 st reactor F F r V = V = F X r F = F F X nd reactor F F r V + = V = F ( X X ) F = F F X r

35 Mean residence time (Space Time) mean residence time defined as: τ = V v 0

36 Reactor design equations: Summary

37 Problems Class Casspobe problem: P2-7b b(p (p.74) Home problems: P2-5b P2-6a Hippopotamus stomack