Selectivity [-]Selectivity [-]

Exercise 2

Reactor design of HDA process

Process Systems Engineering

Prof. Davide Manca – Politecnico di Milano

Lab assistants: Roberto Abbiati – Riccardo Barzaghi – Valentina Depetri

Process HDA

Reactor Separator

V

B

T D F₁

F₂

R

Process HDA

Reactor Separator

H₂+CH₄

C₇H₈

H₂+CH₄

H₂+CH₄

C₆H₆

C₁₂H₁₀ C₇H₈

Selection of the reactor

Reactor

T F₁

F₂ R

F_out

Reactions

7 8 2 6 6 4

C H  H  C H  CH

   

 

3 0.5 0.5

1

1

3.5E 10 50900

A m kmol s

E kcal kmol K

 

    

 

   

1

1 1 exp E

k A

RT

 

  

3

1 1 _{T H} /

R  k c c kmol m s

 

 

3 2

2

2.1E 12 60500

A m kmol s

E kcal kmol K

 

    

 

   

2

2 2 exp E

k A

RT

 

  

2 3

2 2 _B /

R  k c kmol m s

6 6 12 10 2

2C H  C H  H

Main reactions

Side reactions

Degree of Freedom

Selection of Reactor

Conditions of input streams

Residence time or reactor volume

Operating conditions (pressure and temperature)

Determined by the following specifications:

• The selectivity must be  96%

• The productivity of benzene must be = 265 kmol/h

• The purity of benzene must be  0.9997

Degrees of Freedom

• The stream of fresh hydrogen contains 5% mol of methane

• The reactants are at room temperature

• The ratio between hydrogen and toluene in the inlet stream to the reactor must be on the one hand high

enough to avoid coking and other hand sufficiently low to reduce the costs of recycling. It is suggested to use an optimal ratio equal to 5.

Conditions of input streams

Residence time and reactor volume

Degrees of Freedom

Pressure

The reactions are equimolar

PROS: by increasing the pressure there is an increase of concentration of products that speeds up the reaction

CONS: by increasing the pressure there is an increase in the operating costs of compression

 P = 34 bar

Operating conditions

Degrees of freedom

Operating Temperature

The contribution of any catalyst is not necessary (simplified assumption).

• The main reaction is exothermic

• The side reaction becomes more important at high temperature (higher activation energy)

 we should find a compromise between main and side reactions kinetic rates.

Range of economic interest : 600 – 750 °C (Carry out tests every 50 °C)

Operating conditions

The product of interest is an intermediate in the reaction scheme:

Toluene  Benzene  Biphenyl

Degrees of freedom

Selection of the reactor

Necessary to control contact time (while maintaining the selectivity above 96%)

Must use a PFR reactor type!

Currents/Compositions Matrix

H

CH

C

H

C

H

C

H

F

0.95 0.05 0 0 0

F

0 0 0 1 0

B 0 0 1 0 0

D 0 0 0 0 1

V x

1- x

0 0 0

R x

1- x

0 0 0

T 0 0 0 1 0

Definitions

• Selectivity:

• Conversion:

• Residence Time:

6 6

7 8 7 8

Desired moles of Product Moles converted

C H init end

C H C H

n

n n

  



7 8 7 8 7 8

7 8 7 8

Moles reacted Initial moles 1

init end end

C H C H C H

init init

C H C H

n n n

n n

   ^  

Volume reactor Volumetric flow rate



Requirements

1. Determine the conversion, the selectivity and the

residence time as a function of the operating temperature of the reactor using numerical integration of the plug-flow model, by assuming isothermal the reactor, and neglecting the presence of recycles in the evaluation of initial

concentrations

2. Evaluate the adiabatic T of the reaction so to assess if the reactor can be considered isothermal

3. Carry out the following diagrams:

• Conversion / Selectivity

• Temperature / Conversion

• Temperature / residence time

Results analysis

T

T

Selectivity [-] Selectivity [-]

conversion / selectivity diagram

Some considerations

• The selectivity decreases by increasing the temperature because the side reaction plays an increasingly important role with the rising of temperature, this is due to the higher activation energy

• Initially the selectivity tends to unity because, at low

conversions, the reaction rate of side reaction is significantly lower than that of the main reaction

T

Selectivity [-]

By increasing the temperature, the secondary reaction becomes increasingly important: therefore, working at high temperatures, it is necessary to limit the conversion in order to achieve a

selectivity equal to 96%

Results analysis

temperature/conversion diagram

Conversion [-]

Selettività = 96%

Analysis of results

The residence time must decrease when increasing the temperature because the reactions become faster and would not respect anymore the constraint on selectivity equal to 96%

Residence time/temperature

Temperature [°C]

Residence time [s]

Some suggestions

• Useful Tips:

– Use the "Find" function;

– Format graphs.

Function“find”

• Consider the vector

• a = [14 0.5 2 29 1];

i = find(a > 3);

returns the index in the vector where the value satisfies the condition a(i) > 3

• In this case:

i = [1 4];

Graphs (plot)

nf=nf+1;

figure(nf)

plot(x1,y1,'k-*',x2,y2,'r-. ', ...

'LineWidth',3);

set(gca,'FontSize',18) xlabel('x [m]')

ylabel('y [kg]')

legend('Mod1', 'Mod2',1)

text(xText,yText,‘MyText')