Chapter 4 REACTOR DESIGN

REACTOR DESIGN

4.1 INTRODUCTION
In chemical engineering, chemical reactors are vessels designed to contain
chemical reactions. The design of a chemical reactor deals with multiple aspects of
chemical engineering. Chemical engineers design reactors to maximize net present value
for the given reaction. Designers ensure that the reaction proceeds with the highest
efficiency towards the desired output product, producing the highest yield of product
while requiring the least amount of money to purchase and operate. Normal operating
expenses include energy input, energy removal, raw material costs, labor, etc

Reactant gas can be made to contact solid catalyst in many ways, and each has its specific
advantages and disadvantages. There are two broad types of Reactors:
1) Fluidized Bed Reactor
2) Fixed Bed Reactors
The moving bed reactor is an intermediate case which embodies some of the advantages
and some of the disadvantages of Fixed Bed and Fluidized Bed Reactors.
Fluidized-bed catalytic reactors have been characterized as the workhorses of
process industries. For economical production of large amounts of product, they are
usually the first choice, particularly for gas-phase reactions. Many catalyzed gaseous
reactions are amenable to long catalyst life (1-10 years); and as the time between catalyst
changes outs increases, annualized replacement costs decline dramatically, largely due to
savings in shutdown costs. It is not surprising, therefore, that fluidized-bed reactors now
dominate the scene in large-scale chemical-product manufacture.

4.2 Comparison of Fixed and Fluidized Bed Reactors

FIXED BED REACTORS
 Used for Solid/Liquid or Solid/Gas Contact.
 Gases approximate plug flow while passing through Fixed Bed Reactor.

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Chapter 4 REACTOR DESIGN

 Effective temperature control of large Fixed Bed is difficult.

 In highly exothermic reactions hot spots are developed which ruin catalyst.
 Can’t use very small size of catalyst due to high pressure drop.

FLUIDIZED BED REACTORS

 Excellent Solid/Gas contact.
 Low residence time that is feasible for exothermic reactions.
 Rapid mixing of solids allows easily controlled practically isothermal, operations.
 No hot spots even in highly exothermic reactions.
 Good gas to particle and bed to wall heat transfer
 Small size catalysts can be used. Thus, preferred in fast reactions.
 Preferred when catalyst has to be treated frequently as it deactivates rapidly.
 Easy to load and unload.
 Can generate valuable steam while cooling the reactor.
 Due to the intrinsic fluid-like behavior of the solid material, fluidized beds do not
experience poor mixing as in packed beds. This complete mixing allows for a
uniform product that can often be hard to achieve in other reactor designs.
 Cost is much lower than for tubular reactors
The function of fluidized bed reactor can be seen in fig. 4.1.

Figure 4.1

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Chapter 4 REACTOR DESIGN

A number of reactor configurations have evolved to fit the unique requirements of

specific types of reactions and conditions. Some of the more common ones used for gas-
phase reactions are summarized in Table 4.1 and the accompanying illustrations. The
table can be used for initial selection of a given reaction system, particularly by
comparing it with the known systems indicated.

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Chapter 4 REACTOR DESIGN

Table 4.1: Fluidized-Bed Reactor Configurations for Gas-Phase Reactions

Classification Features Typical Applications

 Best isothermal conditions

inside reactor.
 Gives lower yield of
intermediate compared to
others.
 High rates of reaction Oxidation of butane for
Bubbling Fluidized  Reactions with reversible the production of Maleic
Bed Reactor. deactivation requiring Anhydride.
continuous catalyst
regeneration and high
selectivity compared to
others

Circulating Fluidized  Through flow of solids. Octane for Automobile

Bed Reactors.  High gas velocities than Fuel.
Bubbling Fluidized Bed
Reactor.

 Fast Fluidization with its 1

to 10 second gas residence
time for catalytic cracking
Jet Impact Reactor of petroleum. Ultra pyrolize cellulose and
 Higher cracking other biomass wastes.
Temperature
 Shorter Residence Time.

4.3 SELECTION OF REACTOR TYPE

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Chapter 4 REACTOR DESIGN

Our reactor selection criterion is:

1. Conversion
2. Selectivity
3. Productivity
4. Safety
5. Economics
6. Availability
7. Flexibility
8. Compatibility with processing
9. Energy utilization
10. Feasibility
11. Investment operating cost
12. Heat exchange and mixing

After analyzing different types of fluidized bed reactors and keeping this criteria of
reactor selection in mind,, we have concluded that for our system the most suitable
reactor is Bubbling bed reactor. Because oxidation of butane is highly exothermic
reaction and involve gaseous phase reactions, so cooling will be required otherwise the
temperature of the reactor will rise and due to rise in temperature the catalyst activity and
selectivity will be affected and in turn, the formation of by-products will increase which
is direct loss of product. Since butane oxidation is highly exothermic reaction, so cooling
of reactor is required to avoid degradation of products.
For such a situation the best reactor is Bubbling Fluidized Bed Reactor.

4.4 CONSTRUCTION AND OPERATION OF BUBBLING

FLUIDIZED BED REACTOR
Basically, compressed air is used for the oxidation of butane in the presence of VPO
catalyst. After compressing it is sent to a heat exchanger to attain the conditions of the
reactor. Butane under pressure is mixed with the compressed hot air in the pipeline and
this feed is entered from the bottom of the reactor. Catalyst is entered into the reactor and

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Chapter 4 REACTOR DESIGN

feed mixture after passing through the distributor plate, passes through this catalyst bed
and bubbles are formed from the emulsion and eventually fluidizes. Reactor is jacketed to
maintain isothermal conditions inside the reactor. Figure 4.2 shows essential elements of
bubbling FBR that we have selected.

Figure 4.2

4.5 DESIGN PROCEDURE FOR BUBBLING FBR

Figure 4.3 shows the inlet and outlet compositions and temperature, pressure conditions.

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Chapter 4 REACTOR DESIGN

Figure 4.3
It includes following steps:

1. Bubble velocity
2. Voidage at minimum fluidization
3. Mass transfer coefficient
4. Height of bed
5. Pressure drop
6. Length of reactor
7. Diameter of reactor
8. Volume of reactor
9. Residence time of reactor
10. Jacket selection

Diameter of particle = dp =100 µm

Density of gas = ρg = 3.55 E+05 kg/m3
Density of solid catalyst = ρs = 7.76E+05 kg/m3
Superficial Gas Velocity= Uo = 2484 m/hr
This velocity value is taken from Fig. a from app.

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Chapter 4 REACTOR DESIGN

4.5.1 EQUIVALENT/BUBBLE DIAMETER

Volume of catalyst particle (1) =Vs =π/6 * dp3
= 0.523 m3
Volume of bubble (1) = VB = (Vs/16)1/0.42
= 5.6 m3
Equivalent diameter (1) = db = (6VB/ π) 1/3
= 0.005

4.5.2 RELATIVE BUBBLE VELOCITY

Superficial velocity = Uo = 2484 m/hr
Gravitational constant = g = 12.7× 107 m/hr2
Relative bubble velocity (2) = Ubr = 0.711 (g db) 0.5
= 566.59 m/hr

4.5.3 BUBBLE VELOCITY

Velocity at minimum fluidization (2) = Umf

= 26.2 m/hr
By Rule of Thumb: Umf = (1.8 – 54) m/hr
So Umf is true.
Bubble velocity can be calculated as:
Bubble Velocity (1) = Ub = Uo – Umf + Ubr
= 3024.4 m/hr

4.5.4 VOID FRACTION AT MINIMUM FLUIDIZATION (3)

Diameter of bed at minimum fluidization (3) = dB = 0.216m
Height of bed at minimum fluidization (3) = hB, mf = 0.67 m

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Chapter 4 REACTOR DESIGN

Mass of bed = mB = 11081.3 kg

Density of bed (3) = ρB, mf = 4*mB / (π * dB2 * hB, mf)
= 451 * 103 kg/m3
So, void fraction at minimum fluidization can be calculated as:
Єmf = 1 – (ρB, mf / ρP)
= 0.418

4.5.5 MASS TRANSFER COEFFICIENTS

 From Bubble to Interchange Zone(1),

Umf
� � �
�D 0.5 g1/4 �
�
Kbc = 4.5 � �+ 5.85 � AB �
� db
� �
� �
� db5/4 �
�
� � � �
DAB = diffusivity (3) = 0.14 m2 / hr
Kbc = 3.05E+05 hr -1

 From Interchange to Emulsion phase(1),

1/2
� D U ��
�
�

Kce = 6.78 mf AB b �
�
�
�
� �
d 3 �
�
�
�
b �
�
�
�

Kce = 4.09E+05 hr -1

4.5.6 VOLUME FRACTION OF SOLIDS IN BUBBLE

It can be calculated as:
γb = 0.015 (1 – Єmf) (1 – fb)/ fb
Fraction of bed volume = fb

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Chapter 4 REACTOR DESIGN

= 0.813
So,
γb = 0.002
4.5.7 BULK DENSITY IN BUBBLE PHASE
Bulk density in Bulk phase (1) = ρb = γb * ρs
= 15.5 kg / m3

4.5.8 BULK DENSITY IN EMULSION PHASE

Bulk density of bed at mf (2) = ρe = ρS (1- Єmf)
= 451,632 kg/m3

4.5.9 EVALUATING Kr (2)

Rate constant for first order reaction = k = 7056 hr-1
But, ρc = ρe
For evaluating Kr:

For first order reaction, right hand side of equation can be written as:

Kr = 812 hr – 1
4.5.10 HEIGHT OF BED (3)
 By Davidson & Harrison:

Hbed = 1.2 m
By Rule Of Thumb: Hbed = (0.3 - 15) m

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Chapter 4 REACTOR DESIGN

So our calculations proof the result.

4.5.11 LENGTH OF REACTOR (1)

After Integration,

XA = 0.82
From which,
Lf = 6.4 m

4.5.12 DIAMETER OF REACTOR

Surface Area of Reactor (5) is,
A = m / ρs * Umf
A = 6.44 m2
From which the Diameter of reactor is calculated is,
D = (4 A / π) 0.5
D=3m

4.5.13 VOLUME OF REACTOR (1)

Volume of Fluidized Bed Reactor = V = π /4 * D2 *Lf
= 45.2 m3

4.5.14 RESIDENCE TIME OF FBR

Volumetric flow rate of gas = 3933.84 m3 /hr
Volume of reactor = 45.2 m3
Residence time = 41 sec

4.5.15 PRESSURE DROP (3)

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Chapter 4 REACTOR DESIGN

Pressure drop along distributor will be:

Pressure drop along bed = ∆PB = mB / AB = mB / D*L

∆PB = 3078.1 kg / m2
From where:
∆PD = 470 kg / m2
As by rule:
∆PD = (0.1-0.3) ∆PB
That totally agrees to our calculations.

4.5.16 JACKET SELECTION

An important consideration in sizing is heating or cooling the reactor contents. There are
several heat exchangers which are classified as either internal or external. The internal
heat exchangers are immersed directly into the reacting liquid and consist of spiral coils,
harp coils, and hollow or plate baffles. For the external heat exchangers, the reactor
contents circulate through an external flow loop containing the heat exchanger. The jacket
type consist of simple jacket – with or without a spiral baffle or nozzles for the promotion
of the turbulence – the partial pipe coil, and the dimple jacket.
Dimple jacket is selected since:
Volume > 1.89 m3, pressure < 20 bar
Figure 4.4 shows a simple diagram of dimple jacket.

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Chapter 4 REACTOR DESIGN

Figure 4.4

The dimple jacket consists of hemispherical dimples pressed into a thin plate, which is
then wrapped around and welded onto the reactor.
Qremoved = 3.67 E+05 KJ/hr
Due to good heat exchange and low cost, water is selected as a cooling media.
Inlet temperature of water = Ti = 25 oC
Outlet temperature of water = To = 36 oC
Cp =4.18 kJ / kg.k
As:
.
Q = m�Cp �DT
.
Mass flow rate of coolant = m = 7981 Kg/hr

Cooling provided by the jacket = Qj, heating

= Uj Aj (Tj –TR)
Assuming jacket covers 80 % of the reactor consisting of a bottom elliptical head and
cylindrical shell:
Jacket Area = Aj = πD2 / 4 + π DL
=67.4 m2
Uj (5) = 26 Btu / hr.ft2. °F = 531.72 KJ / m2.K
Qj, heating = 3.94 E+05 KJ/hr
Since Qj, heating > Qremoved
hence jacket satisfies our requirement

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Chapter 4 REACTOR DESIGN

4.5.17 REGENERATION ASSEMBLY

Since VPO catalyst is very expensive so it will be economical to regenerate it. External
regeneration method is used in which spent catalyst is taken to a regenerator at normal
operating conditions where deactivated sites of vanadium become active by the addition
of a regenerating agent i.e. liquid SO3. Maximum valency gain of vanadium is 3.9-4.6.
The whole assembly is shown in fig. 4.5.

Figure 4.5

SPECIFICATION SHEET
Identification:

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Chapter 4 REACTOR DESIGN

Item Reactor
Item No. R-201
No. required 1
Function: Production of Maleic Anhydride by oxidation of Butane
Operation: Continuous
Type: Catalytic Bubbling Fluidized Bed Reactor
Chemical Reaction: C4H10 + 3.5 O2 C4H2O3 + 4 H2O
∆H = -1236 KJ / mol (-295.4 kcal / mol)
Temperature: 1210C
Pressure: 275 KPa
Design Data:
Volume of reactor: 45 m3
Length of reactor: 6 m
Diameter of reactor: 3.0 m
Height of bed: 1 m
Pressure drop: 0.5 psi
Residence time: 41sec.
Velocity at minimum fluidization: 26.2 m/hr
Void fraction at minimum fluidization: 0.42
Jacket type: Dimple Jacket

REFERENCES

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Chapter 4 REACTOR DESIGN

1. Wen Ching Yang, “Handbook of Fluidization and Fluid Particle System”,

Butterworth Publishers London, 1990.
2. Froment and Bishoff, “Chemical Reactor Analysis and Design”, (Wiley), 1977.
3. Geldart, “Gas Fluidization Technology”.
4. Bruce E. Poling, John, M. Prausnitz, John P. O’Connell, “The Properties of Gases
and liquids”.
5. Walas, S.M., “Chemical Process Equipment Selection & Design” Butterworth
Heinemann., 1990.
6. Harry Silla, “Chemical process Engineering Design & Economics”, Marcel
Dekker, Inc. New York, 2003.
7. Coulson, J.m., and Richardson, J.F., “Chemical Engineering”, 4th ed, Vol.6,
Butterworth Heminann, 1991.
8. Kern, D.Q., “Process Heat Transfer”, McGraw Hill Inc., 2000.Perry, R.H and
D.W. Green (Eds): Perry’s Chemical Engineering Handbook, 7 th edition, McGraw
Hill New York, 1997.
9. McCabe, W.L, Smith, J.C., & Harriot, P., “Unit Operations of Chemical
Engineering”, 5th Ed, McGraw Hill, Inc, 1993.
10. Charles, G., Hill, J.R., “The Introduction to Chemical Engineering Kinetics &
Reactor Design.” John Wiley & Sons New York. 1977.
11. John J. Mcketta, “Heat Transfer Design Methods”.
12. Yaws, C. L, “Physical properties”.
13. Gallant, Robert W., “Physical Properties of Hydrocarbons”.
14. Donald R. Woods, “Rule of thumb in engineering practice”.

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