Basic Concepts in Reactor Design

Transcription

1 Basic Concepts in Reactor Design Lecture # 01 KBK (ChE) Ch. 8 1 / 32

2 Introduction Objectives Learning Objectives 1 Different types of reactors 2 Fundamental concepts used in reactor design 3 Design equations of different types of reactors 4 Design of network of reactors KBK (ChE) Ch. 8 2 / 32

3 Introduction Reactor types Types of Reactors - Tank Reactors Tank reactors batch semibatch continuous Tubular reactors plug-flow packed-bed KBK (ChE) Ch. 8 3 / 32

4 Introduction Reactor types Batch reactors KBK (ChE) Ch. 8 4 / 32

5 Introduction Reactor types Continuous stirred tank reactor (CSTR) KBK (ChE) Ch. 8 5 / 32

6 Introduction Reactor types Cascade of CSTR KBK (ChE) Ch. 8 6 / 32

7 Introduction Reactor types Tubular reactors KBK (ChE) Ch. 8 7 / 32

8 Introduction Reactor types Semibatch reactors KBK (ChE) Ch. 8 8 / 32

9 Introduction Fundamental concepts used in reactor design A quote from the book... The bread-and-butter tools of the practicing chemical engineer are the material balance and the energy balance. In many respects, chemical reactor design can be regarded as a straightforward application of these fundamental principles... KBK (ChE) Ch. 8 9 / 32

10 Introduction Fundamental concepts used in reactor design Material Balance A material balance on a reactant species of interest for an element of volume V can be written as: A shorter form: input = output + disappearance by reaction + accumulation KBK (ChE) Ch / 32

11 Introduction Fundamental concepts used in reactor design Special forms of the equation Batch reactor: flow terms are omitted Continuous reactor -steady state operation: accumulation is omitted Continuous reactor -unsteady state operation and semibatch reactor: all four terms are retained tubular flow reactor: the equation takes a differential form (Why?) KBK (ChE) Ch / 32

12 Introduction Fundamental concepts used in reactor design Energy balance The rate of reaction is temperature dependent. If the temperature is not constant energy balance is necessary. Energy balance for an element of volume V over a time increment t is: KBK (ChE) Ch / 32

13 Introduction Some terms associated with reactor design Space time τ = V R V V R : reactor volume; V: volumetric flow A reference condition, usually the inlet condition, is selected to measure the volumetric flow rate. Reference condition is emphasized by the use of the subscript zero: τ = V R V 0 KBK (ChE) Ch / 32

14 Introduction Some terms associated with reactor design Space time vs average residence time The two quantities are equal only if all of the following conditions are met: 1 Pressure and temperature are constant throughout the reactor 2 The density of the reaction mixture is independent of the extent of reaction 3 The reference volumetric flow rate is evaluated at reactor inlet conditions KBK (ChE) Ch / 32

15 Introduction Some terms associated with reactor design Space Velocity Space time: S = 1 τ = V 0 V R When heterogeneous catalyst is involved WHSV or VHSV is used: WHSV = ρv 0 W VHSV = V 0 W KBK (ChE) Ch / 32

16 Batch reactor Mole balance KBK (ChE) Ch / 32

17 Tubular reactor Assumptions- PFR 1 no longitudinal mixing of fluid elements as they move through the reactor 2 all fluid elements take the same length of time to move from the reactor inlet to the outlet 3 plugs of material move as units through the reactor, and this assumption is conveniently expressed in terms of a requirement that the velocity profile be flat as one traverses the tube diameter 4 Each plug of fluid is assumed to be uniform in temperature, composition, and pressure - radial mixing is infinitely rapid 5 there may well be variations in composition, temperature, pressure, and fluid velocity as one moves in the longitudinal direction KBK (ChE) Ch / 32

18 Tubular reactor Mole balance KBK (ChE) Ch / 32

19 Tubular reactor Algebraic form and graphical determination V faout R = F A0 f Ain df A ( r A ) This is known as a Levelspiel plot KBK (ChE) Ch / 32

20 Tubular reactor Residence time in plug flow reactor t = VR 0 dv R V KBK (ChE) Ch / 32

21 Tubular reactor Combinations of tubular reactors Series of PFRs in a Levelspiel plot - How would they look?? KBK (ChE) Ch / 32

22 Tubular reactor DIY Equation for a packed bed reactor?? KBK (ChE) Ch / 32

23 CSTR Basic assumptions... the reactor contents are perfectly mixed so that the properties of the reacting fluid are uniform throughout. The composition and temperature of the effluent are thus identical with those of the reactor contents... KBK (ChE) Ch / 32

24 CSTR Scheme KBK (ChE) Ch / 32

25 CSTR Algebraic form and graphical determination V R = f A,out f A,in F A0 ( r AF ) Levelspiel plot KBK (ChE) Ch / 32

26 CSTR Mean residence time in a CSTR τ = V R V F KBK (ChE) Ch / 32

27 Relative Size relative size requirements KBK (ChE) Ch / 32

28 Cascades of Stirred-Tank Reactors Cascades of Stirred-Tank Reactors KBK (ChE) Ch / 32

29 Cascades of Stirred-Tank Reactors Graphical solution for intermediate concentrations KBK (ChE) Ch / 32

30 Cascades of Stirred-Tank Reactors Graphical solution for best combination KBK (ChE) Ch / 32

31 Cascades of Stirred-Tank Reactors Graphical solution for best combination KBK (ChE) Ch / 32

32 Combination of Reactors Series Combination KBK (ChE) Ch / 32