REACTOR DESIGN

 

REACTOR DESIGN

 

     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.

  Comparison of Fixed and Fluidized Bed Reactors

FIXED BED REACTORS

Ø  Used for Solid/Liquid or Solid/Gas Contact.

Ø  Gases approximate plug flow while passing through Fixed Bed Reactor.

Ø  Effective temperature control of large Fixed Bed is difficult.

Ø  In highly exothermic reactions hot spots are developed which ruin catalyst.

Ø  Can’t use very small size of catalyst due to high pressure drop.

FLUIDIZED BED REACTORS

Ø  Excellent Solid/Gas contact.

Ø  Low residence time that is feasible for exothermic reactions.

Ø  Rapid mixing of solids allows easily controlled practically isothermal, operations.

Ø  No hot spots even in highly exothermic reactions.

Ø  Good gas to particle and bed to wall heat transfer

Ø  Small size catalysts can be used. Thus, preferred in fast reactions.

Ø  Preferred when catalyst has to be treated frequently as it deactivates rapidly.

Ø  Easy to load and unload.

Ø  Can generate valuable steam while cooling the reactor.

Ø  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.

Ø  Cost is much lower than for tubular reactors

The function of fluidized bed reactor can be seen in fig. 4.1.

                                             

  

     Figure 4.1

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.

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

Classification	Features	Typical Applications
Bubbling Fluidized Bed Reactor.	Best isothermal conditions inside       reactor.          Gives lower yield of intermediate compared to others.                   High rates of reaction                    Reactions with reversible               deactivation requiring             continuous catalyst              regeneration and high                   selectivity compared to                 others	Oxidation of butane for the production of Maleic Anhydride.
Circulating Fluidized Bed Reactors.	Through flow of solids. High gas velocities than Bubbling Fluidized Bed Reactor.	Octane for Automobile Fuel.
Jet Impact Reactor	Fast Fluidization with its 1 to 10 second gas residence time for catalytic cracking of petroleum. Higher cracking Temperature Shorter Residence Time.	Ultra pyrolize cellulose and other biomass wastes.

4.3     SELECTION OF REACTOR TYPE

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. 

 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 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

 

 DESIGN PROCEDURE FOR BUBBLING FBR

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

 

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 = d_p =100 µm

Density of gas = ρ_g = 3.55 E+05 kg/m³

Density of solid catalyst = ρ_s = 7.76E+05 kg/m³

Superficial Gas Velocity= U_o = 2484 m/hr

This velocity value is taken from Fig. a from app

 EQUIVALENT/BUBBLE DIAMETER

Volume of catalyst particle ⁽¹⁾ =V_s =π/6 * d_p³

                                                          = 0.523 m³

Volume of bubble ⁽¹⁾ = V_B = (V_s/16)^1/0.42

                                            = 5.6 m³

Equivalent diameter ⁽¹⁾ = d_b = (6V_B/ π) ^1/3

                                               = 0.005 

 RELATIVE BUBBLE VELOCITY

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

BUBBLE VELOCITY

Velocity at minimum fluidization ⁽²⁾ = U_mf

          = 26.2 m/hr
 By Rule of Thumb: U_mf = (1.8 – 54) m/hr

So U_mf is true.

Bubble velocity can be calculated as:

Bubble Velocity ⁽¹⁾ = U_b = U_o – U_mf + U_br
                                        = 3024.4 m/hr

 VOID FRACTION AT MINIMUM FLUIDIZATION ⁽³⁾

Diameter of bed at minimum fluidization ⁽³⁾ = d_B = 0.216m

Height of bed at minimum fluidization ⁽³⁾ = h_{B, mf =} 0.67 m

Mass of bed = m_B = 11081.3 kg

Density of bed ⁽³⁾ = ρ_{B, mf} = 4*m_B / (π * d_B² * h_{B, mf})

                                        = 451 * 10³ kg/m³

So, void fraction at minimum fluidization can be calculated as:

Є_mf  = 1 – (ρ_{B, mf /} ρ_P)

           = 0.418

  MASS TRANSFER COEFFICIENTS

Ø  From Bubble to Interchange Zone⁽¹⁾,

D_AB = diffusivity ⁽³⁾ = 0.14 m² / hr

K_bc = 3.05E+05 hr ^-1   

Ø  From Interchange to Emulsion phase⁽¹⁾,

K_ce = 4.09E+05 hr ^-1

 VOLUME FRACTION OF SOLIDS IN BUBBLE

It can be calculated as:

γ_b = 0.015 (1 – Є_mf) (1 – f_b)/ f_b

Fraction of bed volume = f_b

                           

           = 0.813

   

So,

           γ_b = 0.002

BULK DENSITY IN BUBBLE PHASE

Bulk density in Bulk phase ⁽¹⁾ = ρ_b = γ_b * ρ_s

  = 15.5 kg / m³

 BULK DENSITY IN EMULSION PHASE

Bulk density of bed at mf ⁽²⁾ = ρ_e = ρ_S (1- Є_mf)

                                                        = 451,632 kg/m³

 EVALUATING K_r ⁽²⁾

Rate constant for first order reaction = k = 7056 hr^-1

But, ρ_{c =} ρ_e

For evaluating K_r:

   

        

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

                     

K_r = 812 hr ^{– 1}

HEIGHT OF BED ⁽³⁾

Ø    By Davidson & Harrison:

                        

H_bed = 1.2 m

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

So our calculations proof the result.

 LENGTH OF REACTOR ⁽¹⁾

After Integration,

                             

X_A = 0.82

From which,   

L_f = 6.4 m

 DIAMETER OF REACTOR

Surface Area of Reactor ⁽⁵⁾ is,

A = m / ρ_s * U_mf

A = 6.44 m²

From which the Diameter of reactor is calculated is,

D = (4 A / π) ^0.5

D = 3 m

VOLUME OF REACTOR ⁽¹⁾    

Volume of Fluidized Bed Reactor = V = π /4 * D² *L_f

                                                                = 45.2 m³

 RESIDENCE TIME OF FBR

Volumetric flow rate of gas = 3933.84 m³ /hr

Volume of reactor = 45.2 m³          

Residence time = 41 sec

 PRESSURE DROP ⁽³⁾

Pressure drop along distributor will be:

Pressure drop along bed = ∆P_B = m_B / A_B = m_B / D*L

∆P_B = 3078.1 kg / m²

From where:

∆P_D = 470 kg / m²

As by rule:

∆P_D = (0.1-0.3) ∆P_B

That totally agrees to our calculations 

 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.

 

 

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.

Q_removed = 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    = T_i = 25 ^oC

Outlet temperature of water = T_o = 36 ^oC

Cp =4.18 kJ / kg.k

As:                  

 

Mass flow rate of coolant  = 7981   Kg/hr

Cooling provided by the jacket = Q_{j, heating}

                                                     = U_j A_j (T_j –T_R)

Assuming jacket covers 80 % of the reactor consisting of a bottom elliptical head and cylindrical shell:

Jacket Area = A_j = πD² / 4 + π DL

                             =67.4 m²

Uj ⁽⁵⁾ = 26 Btu / hr.ft². ^°F = 531.72 KJ / m².K

Q_{j, heating} = 3.94 E+05   KJ/hr

Since Qj_{, heating} > Q_removed

hence jacket satisfies our requirement

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.

                                       

 

SPECIFICATION SHEET

Identification:                                                                                                                       

                        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

 

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”, 4^th 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”, 5^th 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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