Synthesis of Reaction System

1CCB 3024

PROCESS PLANT DESIGN

Lecturer : Dr Abrar Inayat

Conceptual Process Design

Synthesis of Reaction System (2)

21. Select appropriate reactor system according to the reaction system

2. Specify suitable range for the design/operating parameter of reactors

3. To know about the mathematical approaches for reactor network

At the end of this lecture, student should be able to ;

HEURISTICPolymerisation reaction ......

Polymers are characterised by the distribution of molecular weight.

2 broad types of reaction

1. Reactions with Termination steps

eg.polymerisation of Vinyl Chloride

R + CH2 = CHCl ------> RCH2 - CHCl

initiator

CH3 or

OH

vinyl chloride vinyl chloride free radical

propagates

RCH2 - CHCl + CH2 = CHCl ------> RCH2 - CHCl - CH2 - CHCl

leading to

R - (CH2 - CHCl)n - CH2 - CHCl

Reaction terminated by

the joining of these molecules

Polymers

Question ?

How do we

select suitable

type of reactor

for such

reaction ?

3The active polymer life is short as compared to

the average residence time in reactor.

Important note for the reaction :

RCH2 - CHCl + CH2 = CHCl ------> RCH2 - CHCl - CH2 - CHCl

leading to

R - (CH2 - CHCl)n - CH2 - CHCl PolymersReaction terminated bythe joining of these molecules

The length of the chain depends on

concentration of free radical available

in the reactor. The concentration of the

free radical decreases as reaction proceed

with time. Therefore, at beginning due to

presence of high concentration of free radical,

polymer chain tend to be shorter. As reaction time

proceeds, lesser free radical concentration thus

harder for termination steps to take place resulting

in longer chain molecule.

Best to choose CSTR.

CSTR

HEURISTIC

HEURISTIC

2. Reactions without Termination Step.

eg.polycondensation

HO - (CH2)n - COOH + HO - (CH2)n - COOH

--------> HO - (CH2)n - COO - (CH2)n - COOH + H2O

In this reaction, the polymer grows by successive esterification with

elimination of water and no termination step.

How do we select suitable type of reactor for such reaction

Question ?

The active polymer life is long compared to the average residence

time in reactor. It is highly desirable for all the molecules to have

almost similar residence time to produce polymer of similar chain

length. Therefore, control of residence time is important.

Best to choose PLUG FLOW

PLUG FLOW

4Example on Reactor Selection

Given the following reaction systems to produce product C, select a suitable reactor for the system.

A + B C rC = k1 CA0.5 CB

2

A + B D rD = k3 CA CB

C D rD = k2 CC2 Mixture of series parallel reactions

Analysing the parallel reaction


2

A + B D rD = k3 CA CB

r3

r1=

k3

k1CA

1-0.5

CB1-2

r3

r1=

k3

k1CA

0.5

CB-1

What reactor configuration should be selected?

r3

r1=

k3

k1CA

1-0.5

CB1-2

r3

r1=

k3

k1CA

0.5

CB-1

Semi CSTR

B

A

OR

SEMI PLUG FLOW

REACTANT A

REACTANT B

Analysing the series reaction


2

C D rD = k2 CC2

Control of residence time is important.

Therefore choose plug flow.

5Reactor Operating Parameter

In the preceding lecture, the choice of the reactor is made based on the most appropriate

concentration profile as the reaction progressed. However, there are still important effects that need

to be considered i.e., the reactor operating parameter.

Besides looking at the reaction system, the other aspects governing chemical reaction which have

to be considered are;

1. Reaction Equilibrium for reversible reaction.

Pressure Effect

aA + bB cC + dD

The extent of reaction which can be calculated using Gibbs Free Energy =

0 = ln where

=

.

.

.

.

For gaseous reaction

=

.

.

.

.

For liquid reaction

Eg : Given reaction below at 1 bar, 300 K to produce Ammonia NH3. Feed is according to the stoichiometric ratio.

3 H2 + N2 2 NH3

Given the data for Std. Free Energy of Formation 3000(

)

H2 0

N2 0

NH3 -16.223

ln =(2 16,223 0 0 )

8.314 300= 13.008

ln =0

= 4.4597 105

=3

2

23 .2

= 3

2

23 .2

2

a. Determine Ka equilibrium constant

H2 N2 NH3

Initial moles

Equilibrium moles

Total moles at equillibrium = 4 2X

Mole Fraction

3 1 0

3-3X 1-X 2X

3-3X 1-X 2X

4-2X 4-2X 4-2X

Assume ideal gas law

6Eg : Given reaction below at 1 bar, 300 K to produce Ammonia NH3. Feed is according to the stoichiometric ratio.

3 H2 + N2 2 NH3 H2 N2 NH3

Mole Fraction3-3X 1-X 2X

4-2X 4-2X 4-2X =3

2

23 .2

= 3

2

23 .2

2

b. Composition of the reaction products at equilibrium

=3

2

23 . 2

2 =162 2 2

27 1 4for P = 1 bar

X = 0.97 bar

Therefore the composition at equilibrium is ;

H2 [3 3 (0.97)] / [4 2 (0.97)] = 0.0437

N2 [1 (0.97)] / [4 2 (0.97)] = 0.0146

NH3 [2 (0.97) / [4 2 (0.97)] = 0.9418

Besides looking at the reaction system, the other aspects governing chemical reaction which have

to be considered are;

2. Reaction Equilibrium for reversible reaction.

Temperature Effect (Endothermic & Exothermic Reaction)

aA + bB cC + dD

Integrate the above relation ;

21

= 0

1

2

1

1The effect of temperature on reaction equilibrium could be determined since the std. heat of reaction is available

in thermodynamics data.

0 = 0 +

0 0

0

For exothermic reaction the DH0 is < 0 while for endothermic reaction the DH0 is > 0

For exothermic reaction, higher temperature will shift the equilibrium to reactant side while

For endothermic reaction, higher temperature will shift equilibrium to product side.

0

= 0

2=

ln

The extent of a reaction is influenced by temperature can be calculated using enthalpy of formation through the

relation developed from Gibbs Free Energy

0 = lnfrom

7Besides looking at the reaction system, the other aspects governing chemical reaction which

have to be considered are;

3. Rate of Reaction - Temperature Effect. aA + bB cC + dD

The effect of temperature on rate of reaction can be computed from the Arhennius equation.

= 0

Expanding the equation

21

=

1

1

1

2E is the activation energy

Generally, the higher the temperature, the higher is the reaction rate.

3. Decide on the Reaction Condition - operational parameter.

Let's consider concentration of reactants in the reactor

HEURISTIC

Some possibilities to consider

EXCESS OF ONE REACTANTS

This can force the reaction of a selected

component to completion due to certain

reason.

ADDITION OF INERTS

This can favour reaction to proceed

towards favorable direction.

eg. Feed Prod1 + Prod2

Rxn. cause an increase in no. of moles. Adding

inert cause reduction in no. of moles/vol.

Therefore rxn. proceed to produce more products.

eg. C2H4 + Cl2 ---> C2H4Cl2

Cl2 is difficult to separate. Add excess of C2H4 to

force complete reaction of Cl2

DESIGN PROCEDURE FOR REACTOR OPERATING PARAMETER.

8HEURISTIC

RECYCLE UNWANTED BYPRODUCTS

This can surpress the formation

of more unwanted by-products.

Let's consider concentration of reactants in the reactor (cont...)

REMOVAL OF PRODUCT

This can get the reaction to proceed to

the favorable direction.

eg. 2SO2 + O2 2SO3

Intermediate removal of Sulfur trioxide will

make the reaction towards producing more

products. But how?

R A R A

H2O

absorption absorption

H2O

Some possibilities to consider

eg. Feed1 + Feed2 Product

Feed1 + Feed2 Byproduct Rxn 2

Recycling byproduct will suppress

reaction 2 thus avoiding waste of feed material

HEURISTIC

Consider more complex reaction !

ratio of secondary

to primary reaction

r2

r1=

k2

k1Cfeed1 Cfeed2

a2 -a1 b2 -b1

Even after reactor selection was made, there are still opportunities for maximising selectivity !

1. If (a2-a1) > (b2-b1), use excess Feed2 and if (a2-a1) < (b2-b1), use excess Feed1

2. If reaction 2 is reversible, increasing inerts concentration will decrease byproduct formation.

3. Recycling the byproducts to the reactor will suppress the formation of more byproducts

if the reaction is reversible.

Let's consider concentration of reactants in the reactor (cont...)

Multiple reactions in parallel producing byproducts

Rxn 1 : r1 = k1 Cfeed1 Cfeed2a1

Feed1 + Feed2 ------> Product (s)

Feed1 + Feed2 ------> Byproduct (s)

b1

Rxn 2 : r2 = k2 Cfeed1 Cfeed2a2 b2

9HEURISTIC

Multiple reaction in series producing byproducts.

Feed(s) -------> Product

Product -------> Byproduct(s)

Rxn 1 : r1 = k1 Cfeeda1

Rxn 2 : r2 = k2 Cproducta2

Formation of byproducts is minimise by keeping the product composition low in the reactor.

Option that can be considered :

Use of 3rd component which does not participate in the reaction to produce byproducts

Intermittent/intermediate removal of products as reaction takes place.

eg. C6H5CH3 + H2 -------> C6H6 + CH4

2C6H6 C12H10 + H2 The use of excess Hydrogen

helps to minimise formation

of diphenyl.

HEURISTIC

Mixed parallel and series reactions producing byproducts

Feed(s) -------> Product

Feed(s) -------> Byproduct(s)

Product -------> Byproduct(s)

Feed(s) ------> Product(s)

Feed(s) ------> Byproduct(s)

Product ------> Byproduct(s)

Options that can be considered ;

The use of excess 3rd component which does not involve in reaction to produce byproduct

can suppress the byproduct formation by keeping product composition low.

Recycling of byproducts if the reaction is reversible can assist in minimising byproduct

formation.


Rxn 3 : r3 = k3 Cproducta3


10


SUGGESTED PROCEDURE (cont...)

Let's now consider temperature for the operation of the reactor.

HEURISTIC

Generally, there are several critical temperature limits that apply to chemical processes. At elevated temperatures, common construction materials (primarily carbon steel), suffer a significant drop in physical strength and must be replaced by a more costly material.

Turton et al., Analysis, Synthesis and Design of Chemical Processes 1998

Temperature Ambient 400 C 550 C

Tensile Strength of Material (bar)

Carbon Steel (grade 70) 1190 970 170

Stainless Steel (Type 302) 1290 1290 430

A decision to operate above 400 C must be justified in view of the material limitation.

From utility perspective, a decision to operate outside the range of 40 C to 260 C requiring special heating/cooling media, has to be justified. Within this range, normal steam and cooling water could be utilised easily.




Let's now consider temperature for the operation of the reactor.

HEURISTIC

How do we decide ?

SINGLE REACTION

Endothermic Rxn

Set T as high as possible

Good for rate of reaction

So what is the limit ?

Exothermic Rxn

Set T low to take advantage on

conversion in a reversible reaction.

Not good for rate of reaction

So what is the compromise ?

Need to compromise !

MULTIPLE REACTIONS

For Parallel/Series Reaction

A ---> B ---> Ck1 k2

B product

if k1 increase more than k2 w.r.t. T, then go for as

highest T as possible.

if k2 increase more than k1 w.r.t. T, then go for as

lowest T as possible. Careful, as need to compromise.

The main issue is how to control T effectively.

Can you suggest a number of ways to do that.

11

HEURISTIC

Let's now consider pressure for the operation of the reactor.



There are economic advantages associated with operating equipment at higher pressure when gases are present i.e., decrease in gas volume. This tend to reduce the size of equipment to give the required residence time. Most chemical processing equipment can withstand pressures up to 10 bar without additional capital investment. Beyond 10 bar, thicker walls for the equipment vessel is required thus contributing to higher cost.

Likewise operating below ambient pressure causes equipment size to be larger and thus increased in capital cost.

A decision to operate outside the pressure range of 1 to 10 bar must be justified.


HEURISTIC

Let's now consider pressure for the operation of the reactor. For liquid phase reaction,

pressure is known to have

little effect. Why?For vapour phase reaction....

SINGLE REACTION

Decrease in number of moles

Pressure decreases as reactant/s

were converted to products

Increase pressure to take advantage.

Increase in number of moles

Decrease pressure to take advantage

Pressure increases as reactant/s

were converted to products

MULTIPLE REACTIONS

The same general guideline can be used but

consideration has to be given when having

competing reaction producing byproducts.

Selectivity has to be taken into account !



12

HEURISTIC

4. What about reactor phase?

Given the fact that the temperature and pressure

has been decided normaly, the phase can be

determined directly !

The preferences if possible ---> liquid phase reactor !

Smaller Reactor Volume

Higher density thus higher concentration

per volume size

However, depends on other matters

- Rate of mass/Heat transfer

- Safety consideration

Gas phase might have

better transfer of heat & mass

Not so much of an important criteria !


HEURISTIC

5. Catalyst Selection.

Function of Catalyst :

increase rate of reaction without changes in quantity and composition

at the end of reaction.

Homogenous Catalyst

may modify reaction mechanism by

participating in the reaction but regenerated

back in the subsequent step.

eg. production of ketene from acetic acid.

CH3COOH ----> CH2=C=O + H2O

use triethyl phospate as a

catalyst.

problem : possible loss of catalyst due

to imperfect separation to recover

catalyst.

Heterogenous Catalyst

- Catalyst differ in phase from reacting species

- Most oftenly solid

- Reacting species diffuse to surface of catalyst and

are adsorped. Reaction takes place and products

then desorb and diffuse back to bulk gas/liquid.

- Effective surface area on catalyst which depends

on catalyst porosity is an important consideration.


13

Example (Text by Smith R. pg. 80)

Given the reaction below for the production of monoethanolamine;

Ethylene oxide + ammonia monoethanolamine

Monoethanolamine + ethylene oxide diethanolamine

Diethanolamine + ethylene oxide triethanolamine

Side reaction

Side reaction

Select a reactor that will maximise the production of monoethanolamine.

Suggest option for the operational condition which could further maximise the production.

Plug Flow to control residence time.

Excess AmmoniaIntermediate removal of Monoethanolamine.

Example.

tert-Butyl hydrogen sulfate is produced from the reaction below;

Iso-butylene + sulfuric acid tert-butyl hydrogen sulfate

tert-butyl hydrogen sulfate + water tert-butyl alcohol + sulfuric acid

The primary reaction is rapid and exothermic. Laboratory studies indicate that the reactor yield is a maximum when the concentration of sulfuric acid is maintained at 63 percent. The temperature should be maintained around 0 oC to avoid excessive formation of byproducts.

Make the choice of the reactor.

Sulfuric acid

14

During the conceptual design of a chemical process, among the related decisions that

have to be made concerning reactor ;

SELECTION OF REACTOR

Reactor Type and/or Network Operating Conditions

CSTR

PLUG FLOW

RECYCLE REACTOR

CSTR OR BATCH

PLUG FLOW RECYCLE REACTOR

PLUG FLOW

RECYCLE REACTORCSTR

TEMPERATURE

PRESSURE

CONCENTRATION

INERT

CATALYST

?

REACTOR VOLUME

AND ORIENTATION

HOW DO WE

DECIDE ?

Recall the following slide again.....

we have look at tackling this using heuristic approach !

We have seen a number of guidelines developed to conceptually design reactor.

Limitations will appear when having to deal with complex reactor configuration/network.

The method that can deliver this will need extensive mathematics and programming

- Mathematical Programming Approach.

Simple reaction

straight forward thus

simple guideline will do.

A B

B C

simple plug flow reactor

Highly Complex reaction

complicated with conflicting in guideline.

require complex reactor - network of reactor !

A + B C D

A + C E

exotherm. rxn

endotherm. rxn

B + D F endotherm. rxn

D is the product !

Could be ;

How can we derive such configuration

or network just from the heuristic guideline ?

Synthesis of Reactor Network Mathematical Programming Approach

Flow Pattern

Mixing Location

Heating & Cooling

15

MATHEMATICALBut how do we go about doing this ?

Recall the key info. required when designing reactor

Reaction Chemistry Reaction Kinetics

For simple reaction chemistry, selection of

reactor can be done using the heuristic guideline

even without the reaction kinetics.

But for complex reaction chemistry, since

mathematical programming approach is

to be adopted, detailed reaction kinetics now

become important.

Still the important parameters that decide the reactor network are ;Conversion

Selectivity

Selectivity normally becomes the more dominant one.

Recall the mathematical programming approach that was introduced earlier.

Generate a super structure containing all alternatives and optimised it

in order to produce the optimal solution.

MATHEMATICAL

Formulate/Generate a super structure which contains all possible candidates for the

solution of the optimal reactor. Then solved it mathematically.

The superstructure is created mathematically.

How do we identify all the possible candidates for the solution ?

One Option : Use the method of Geometric Concepts for Attainable Region

Biegler, Grossman & Westerberg,

Synthesis Methods of Chemical Process Design pp 438 - 447

For chemical reactor networks, the attainable region concept was first presented

by Horn (1964) who noted that ;

...variable such as recycle flowrate and composition of the product form a space which in

general can be divided into an attainable region and a non-attainable region. The attainable

region corresponds to the totality of physically possible reactors ... Once the border is known

the optimum reactor corresponding to a certain environment can be found by simple geometric

considerations.Horn F., (1964). Attainable Regions in Chemical Reaction Technique. In the

3 rd European Symposium on Chemical Reaction Engg. London: Pergamon.

SUGGESTED PROCEDURE

16

MATHEMATICALTo briefly understand how it works,

Consider a reaction A B C

C

Time

A B

C

MATHEMATICAL

A B C

CB

CA

H

E

GF

For a PFR with variable residence time and fixed feed CA0 and CB0, one can solve

the ordinary differential equations from the feed point :

dCA / dt = rA

dCB / dt = rBdCB / dCA = rB / rA

From this differential eqn.,

we can plot the trajectory for

HEGF

CSTR

PLUG FLOW

17

MATHEMATICAL

Consider a reaction A --> B ---> C

CB

CA

H

E

GF

For a CSTR, the path from the feed can be generated using the equations:

From this eqn., we can plot the

trajectory for GH

CA - CA 0 = t rA

CB - CB 0 = t rB

CA - CA 0CA - CA 0

CB - CB 0= rA / rB

PLUG FLOW

CSTR

Note that in the two cases above, we assume a fixed feed, an initial temperature and trajectories

that are determined entirely by the state eqns. derived for concentration as shown above. This is

true unsteady state for isothermal or adiabatic systems.

MATHEMATICAL

How do we plot the attainable region ?

Plot the attainable region using reaction/rate

vectors. The eqn. for the vectors are obtained

from the plug flow and/or CSTR trajectory

as shown earlier.

dCB / dCA = rB / rA

CA - CA 0CA - CA 0

CB - CB 0= rA / rB

plug flow

CSTR

Example 13.3 page 443 : Systematic Methods of Chemical Process Design

Biegler, Grossman & Westerberg.

illustrates the application of the method !

CB

CA

H

E

GF

PLUG

FLOW

CSTR

18

MATHEMATICAL

The attainable region will assist in setting the super structure for the mathematical

programming approach in the sense that it can provide engineer with the family of solutions for

the problem related to reactor network. This will significantly reduces the number of options

that have to be dealt by the mathematical programming during optimisation.

During the conceptual design of a chemical process, among the related decisions that

have to be made concerning reactor ;

SELECTION OF REACTOR

Reactor Type and/or Network Operating Conditions

CSTR

PLUG FLOW

RECYCLE REACTOR

CSTR OR BATCH

PLUG FLOW RECYCLE REACTOR

PLUG FLOW

RECYCLE REACTORCSTR

TEMPERATURE

PRESSURE

CONCENTRATION

INERT

CATALYST

?REACTOR VOLUME

AND ORIENTATION

HOW DO WE

DECIDE ?

Hence the task of determining this,

is made simpler !

Conclusions

We have looked at the followings;

Selection of Suitable Reactor for a Specified Reaction System : Plug flow, CSTR,Hybrid of Plug Flow and CSTR.

Determination of Suitable Reactor Operating Parameter : Temperature, Pressure,Reactant Concentration etc.

Mathematical approach for reactor synthesis

Documents

Synthesis of Reaction System