Cre_chp 3-Reactor Design 4 Single Reaction

8/11/2019 Cre_chp 3-Reactor Design 4 Single Reaction

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CHEMICAL REACTION ENGINEERING -CHEMICAL ENGINEERING DEPARTMENT

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CHAPTER 3 - CHEMICAL REACTORDESIGN FOR SINGLE REACTION

Our greatest glory is not in never falling,

but in getting up every time we do


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AIMS

To give basic chemical reactor design concepts for

single-phase (homogeneous) single reaction

To quantify the choice of different reactor type (PFR or

CSTR) on the reactor size and performance To develop a general guidelines in selecting the

appropriate reactor system for a given type of reaction


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INTRODUCTION

Scope: reactions in homogeneous (liquid or gas phase)system

Economic optimization needs: reactor sizing and productdistribution. Reactor size is the main concernfor single

reaction system, and the second one should be accountedfor in complex reaction system.

Focus: reactors are compared or evaluated based on thesize needed to perform a given single reaction


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PFR vs CSTRpower law kinetics

For a givenreaction, we may compare the CSTR/PFR sizeto achievea certain conversion.

for a given feed,

ordinate = VM/ VP

key factors :

- reaction orders

- XA (degree of

conversion)


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MULTIPLE REACTORS

SETUP


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MULTIPLE REACTORS

Series PFR

For a highly exothermic or endothermicreaction, we may

need to use more than one PFR (mostly 2, sometimes 3stages) in series. Therefore, inter-stagecooling orheating is possible to get the desire T profiles. Eachstage of the PFR is normally adiabaticin operation.

1 N2

CA0

FA0X0 = 0

CA1

FA1X1

CA2

FA2X2

CA N - 1

FA N - 1XN - 1

CA N

FA NXN


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MULTIPLE REACTORS

Parallel PFR

For a highly exothermic or endothermicreaction, we mayalso use many parallel PFR (typically hundreds of smalltubes). Therefore, heat transfer areafor cooling orheating is sufficient for effective T control.

The rules : the feed must be split so that the space timeof each PFR is uniform(since the sizes of parallel PFRsare normally identical, therefore the feed is equallydivided into the reactors). Otherwise, the reactors will beless efficient !!Examine Example 6.1 Levenspiel


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MULTIPLE REACTORS

Series CSTR Single CSTR is normally less efficient than PFR for positive-order

reactions, however, in some cases the use of CSTR is inevitable (egfor viscous mixture, stringent T control required). The use of multipleCSTR in series (with identical volume) is more efficientthan singleCSTR.

CA0X0= 0

Q (eA = 0)

CA1

X1

CA2

X2

CA N - 1

XN - 1

CA N

XN

1storder

case


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MULTIPLE REACTORS

Series CSTR

For most cases, only a few (23) CSTRs are used, because

the reactor-accessories related cost limits the number of N.

ktis the operating linefor any combinations of kinetic, feed, and reactor size.

If N is very

large, the

CSTR-in-serieswill behave like

a PFR !!


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MULTIPLE REACTORS

Series CSTR

kCA0tis theoperating linefor

any combinationsof kinetic, feed,and reactor size.


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MULTIPLE REACTORS

Parallel CSTR

The rules : if it is not absolutely necessary,NEVER use CSTR in parallel, since the multiplereactors will be less efficient than the single

CSTR!! However, if you need to, divide the feed equally

into equal-sized n CSTRs (Fogler, page 160162)


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Example 6.2 & Problem 6.14


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CHEMICAL REACTION ENGINEERING -

CHEMICAL ENGINEERING DEPARTMENT

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PFR with RECYLE


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PFR with Recycle

In certain situations(eg solid catalyzed reactions in a fixed-bedreactorChapter 19 Lev, homogeneous autocatalytic reactions), wemay need to recycle the outlet of a PFRin order to achieve a betteroverall reactor performance.

General PFR-recycle

Design Equation


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PFR with Recycle

If eA= 0 (eg for liquid system)

PFR-recycle

Design Equation

eA= 0

1

2

3graphical

estimation


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PFR with Recycle

If R is very large, the PFR-recycle reactor will behave like a

CSTR !!


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PFR with Recycle

Problem P 6.21 Lev & 6.18 !!

READ : FOGLER (4thEd) page 232 & 233cases in which

PFR with recycle is useful.

One case of the PFR-recycle uses is an auto-catalytic

reaction


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A Girl Can Change Ur GOALBeware


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REACTOR SETUP

AUTOCATALYTIC REACTION


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AUTO-CATALYTIC

The unique shape of auto-catalytic reactions: the maximum rate pointcombination of + andorder reaction !! (read more page 143 Lev)

Which reactor is more efficient?

> single reactor : PFR vs CSTR> PFR with R

> PFR-CSTR combination

AUTO CATALYTIC


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@ small conversionCSTR is better (more effective) than PFR

@ big XAPFR is better than CSTR

AUTO-CATALYTIC

PFR vs CSTR


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There is a R which makes the PFR size minimumoptimum R

Remember : R opt is not applicable for first / second order reactions

AUTO-CATALYTIC

PFR with Recycle

V/FA0

smallest

V/FA0


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R opt can be found by differentiating the PFR-recycle design equation to

minimize tor V

AUTO-CATALYTIC

PFR with Recycle

The differentiation of an integral term can be solved by applying Leibnitztheorem. The optimum XA1value can be finally calculated from thefollowing equation (read Lev page 142):

XA1opt

R opt


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Problem Ropt& Prob 6.10


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AUTO CATALYTIC


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For XAf the maximum rate point, CSTR is better than PFR or PFR-Ropt

For XAf> the maximum rate point, PFR-Roptis better than CSTR or PFR

AUTO-CATALYTIC

PFR with R vs CSTR vs PFR

How to show the V/FA0 for the PFR in the graphs ?

AUTO CATALYTIC


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Facts :

if final conversiontill themaximum rateCSTR isbetter (more effective) thanPFR

if final conversionpass themaximum ratePFR is betterthan CSTR

Combination of CSTR(operated @ max rate) withPFR (from max rate until finalconversion) will be the bestcombination

AUTO-CATALYTIC

PFR-CSTR COMBINATION

AUTO CATALYTIC


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If the unconverted reactantcan be separated from theproduct easily (at reasonablecost), the best setup is to useCSTR at the max rate (M)since the reactor volume isminimum.

However, the cost ofseparation and recycle willdetermine which setup is themost cost effective

AUTO-CATALYTIC

PFR-CSTR COMBINATION


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HOMEWORK # 2

Problem 6.4 & 6.19

Cre_chp 3-Reactor Design 4 Single Reaction

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