CHAPTER 3

1

CHAPTER 3.2

CHEMICAL AND MECHANICAL DESIGN

(DESIGN BY: MOHAMAD HAKIM KAMARUDDIN) (2011924237)

OXIDATIVE-DEHYDROGENATION REACTOR (CRV-100)

(SINGLE REACTOR UNIT)

3.2.1 Process overview

1,3-Butadiene is a major product with wide range of application in the petrochemical

industry. 1,3-Butadiene is mainly produced by steam cracking of naphtha and direct

dehydrogenation of 1-butene which characterized as an endothermic reaction (Naoki

Ikenaga, 2012). The steam cracking process is a very early stage of separating the

constituent of the crude oil drilled from the sea. Therefore, the process not only

produces 1,3-Butadiene but also many other petrochemical raw material such as

ethylene, propylene and isobutene simultaneously. The temperature required for both

steam cracking and dehydrogenation of n-butene are 900oC and 600oC which is

considered higher than oxidative dehydrogenation of 1-butene which will usually

ranging from 400-550oC depending on the type of catalyst used for the reaction.

Oxidative dehydrogenation of n-butenes has replaced many older processes for

commercial production of butadiene. Several processes and many catalyst systems

have been developed for the oxidative dehydrogenation of either n-butane or of n-

butene feed stocks. 1-butenes are much more reactive, however, it require less severe

operating conditions than that of n-butane to produce an equivalent amount of product.

Recently, oxidative dehydrogenation of 1-butene has taken the spotlight in 1,3-

Butadiene production at a view point of energy saving. Since oxidative dehydrogenation

2

of 1-butene is an exothermic reaction, it allows the system to operate at lower

temperature rather than other processes such as steam cracking and hydrogenation of

n-butane. Therefore, the oxidative dehydrogenation of 1-butene has been recognized

as a process that can produce 1,3-Butadiene environmentally non jeopardizing.

The less severe operating condition for the oxidative dehydrogenation process is

contributed by its lower temperature requirement for the reaction and its pressure. The

increment of both parameters contributes significantly with increment of cost in

designing the vessels. The increase of temperature will decrease the stress design of

the vessels at which if insufficient will affect the selection of the design material. The

increment of pressure will also increase the thickness of the design whether in vessel

thickness or dome head design.

The design that is performed in this part is selectively iterate to ensure that it will serve

the design objective for good efficiency parallel with the cost of fabricating the oxidative

dehydrogenation of 1-butene to 1,3-Butadiene reactor (CRV-100).

3.2.2 Objectives

In designing a reactor that can deliver high performance in specific processes, the most

important element is to determine the characteristic of all reactor type or at least the

most common used in the industry. The knowledge on the process characteristics, type

of catalyst used, the optimum orientation of the reactor are hugely significant in

ensuring that there is no miss calculation that is causes by lack of understanding

towards the concept of the design. Therefore a preliminary comparison between

reactors characteristic is made to highlight the pros and cons of each type of reactor.

There are few characteristic that are normally used to classify reactor design:

1. Mode of operation

- Batch or continuous

2. Phases present

- Homogeneous or heterogeneous

3. Reactor geometry

3

- Flow pattern and manner of contacting the phases

i) Stirred tank reactor

ii) Tubular reactor

iii) Packed bed and fixed

iv) Fluidized bed

Table 3.1: The comparison between packed bed reactor and CSTR

Type of reactor Packed/Fixed Bed Reactor CSTR

Advantages High ratio of catalyst to reactant.

High conversion.

Longer residence time

Very suitable for exothermic reaction.

Involve fluid solid heterogeneous reaction

Well mix condition

Continuous operation

Easy to clean

Simple in design and

operation.

Disadvantages Poor heat transfer

The catalyst difficult to replace and need to shut down.

Lowest conversion per unit volume.

(Source: Sinnot and Towler, 2009 & Fogler.S.H,200

The oxidative dehydrogenation of 1-butene to 1,3-Butadiene reaction occurs in a gas

phase with heterogeneous catalyst at which for a better accuracy and performance, a

catalyst that have activation and deactivation energy at 370oC to 500oC. Therefore,

CSTR type of reactor is utterly incompetent in providing the reaction condition. Based

on the characteristic of the catalyst and the reaction of oxidative dehydrogenation

process, the most proper condition of the reaction is to be held in a vessel with static

solid catalyst. The comparison was continued with a more similar behavior sort of

reactor which is the fluidized bed reactor and the fixed bed reactor. According to

(Umich, 2010) the fluidized bed reactor are most commonly used in a heterogeneous

gas phase reaction with a catalyst and it also has good uniformity of temperature

although it has uncertain scale up. The proposed type of reactor according to (Shell,

1964) is fixed bed reactor, tubular reactor and multi tubular fixed bed reactor.

The multi tubular fixed bed reactor is very efficient in controlling the temperature of the

reactor. It requires 2 inlet feed for shell and tube and 2 outlet feed for the shell and tube.

4

The shell inlet and outlet line purpose is to allow cooling water flows into the reactor and

maintain the temperature of the reactor at its designed temperature. The reaction

occurs during the oxidative dehydrogenation process is highly exothermic with heat

release of △H = -132 kJ/mol (Naoki Ikenaga, 2012), therefore the temperature that will

increase due to the release of energy will need to be controlled at which if not, the

increment could lead to deactivation of catalyst that will contribute to decreased of yield.

Taking into account the tube arrangement in the shell of the reactor at which the tube

will be loaded with catalyst pallet and the reactant will have contact with the catalyst;

the reaction will took place inside a tube while having the heat transfer occurs

simultaneously. Therefore, there should be thousands of tube arranged inside the

reactor. The weight of the reactor will be significantly heavy although the volume

predicted is still logical.

The Conclusion drawn from observing the characteristic of each of the reactor and the

characteristic of the process is drawn towards the choosing of multi-tubular fixed bed

reactor as the design concept.

The stream that involve with the reactor is stream 6 & stream 7 from Aspen Hysys

Simulation program version 7.3 at which have been performed in Design Project 1

previously. Figure 3.1 below shows the general flow of the reactor.

Figure 3.1: Multi-tubular Fixed Bed Reactor

T= 350 oC

P= 8 atm

C4H8 =16368 kg/hr

O2 = 2532 kg/hr

H2O = 45009 kg/hr

T= 400 oC

P= 7 atm

C4H8=2847 kg/hr

C4H6=12657 kg/hr

H2O = 49743 kg/hr

Stream 6 Stream 7

R-101

5

3.2.2.1 Overall process flow diagram (PFD)

6

LC

LT

LC

TT

LC

TC

Cooling water

LC

TT

LC

TC

HE-101

HE-102Refrigerent

TTTC

FC

FT

LT LC

PT PC

FTFC

FTFC

LT

LC

PT PC

To waste

water treatment

Cooling water from water treatment

V-101

TT

LT

LC

PC

PT

Hot water out

LC

PT

PC

TT

TC

FT FC

LT

T-101

F-101

350

8 atm

V-102

CRV-100

7

400

7 atm

8

5

7 atm

9

10

5

7 atm

11

30.85

7 atm

12

14

16

5

5 atm

17

-7.85

1 atm

18

100

1 atm

FTFC

LT

LC

PTPC

TT

PT

V-103

15

PI

FC

FC

FC

FC

Hot water that is reheat

to steam from reactor

Hot water

13

TC

FT

TT

FC

FC

FC FT

LT

LIC

FC

R

FT

FI

TI

PI

LFA

FC

FI

TI

LLA

PA

LLAPI

FI

TI

PA

FT

FI TI

PIRC

LC

6

3.2.2.2 Process flow diagram for Multi tubular fixed bed reactor (CRV-100)

7

The summary off component that involve in the feed and outlet stream are

concluded in table 1.2 below

Table 3.2: Component mass flow and mole flow in inlet and outlet of stream

Component

In Out

MW (kg/kmol)

Mass Flowrate (kg/hr)

Mole Flowrate (kmol/hr)

Mass Flowrate

kg/hr)

Mole Flowrate (kmol/hr)

C4H8 56.11

16368 290.628 2847 50.76

C4H6 54.096

- - 12657 234

O2 15.99

2532 158.4 - -

H2O 18.2

45009 2498.4 49743 2761.2

3.2.3 CHEMICAL DESIGN

Figure 1.2 illustrated the stages of determining the chemical design for the oxidative de

hydrogenation reactor.

8

Figure 3.2: Flow chart for chemical design of hydrogenation reactor

The chemical design purpose is to determine the crucial required properties of

designing a reactor such as volume, residence time, catalyst weight and heat balance.

Oxiative-Dehydrogenation of 1-Butene is characterized by exothermic reaction and from

the energy balance; the heat released for this reaction is ΔH = -132 kJ/mol. The

Make assumption for the reactor

Calculate the basis for calculation

Calculate the reaction kinetics

Calculate the volume of reactor

Calculate diameter of reactor

Selected of catalyst

Calculate the volume & weight of catalyst

Heat balance

Tube Design

Calculate pressure drop on the tube side

Calculate pressure drop on the shell side

9

reaction consumes stoichiometrically one mole 1-Butene and 1/2 moles of Oxygen to

produce one mole of 1,3- Butadiene and one mole of water as given by equation below:

Reaction in Oxidative-Dehydrogenation of 1-Butene

( )

( )

Where;

A= 1- Butene

B= Oxygen

C= 1,3-Butadiene

D= Water

3.2.3.1 Assumption of chemical design

Packed bed reactor will be design in the production of purification terephthalic acid. In

order to design the reactor, there are several assumptions were made to design this

reactor. Assume that the mass transfer limited between cured terephthalic acid and

hydrogen gas is coefficient therefore the mass transfer between liquid phase and gas

phase is neglected. The other assumption taken in this design as below:

Steady state condition

Heterogeneous reaction

Continuous process

Reaction occur irreversible in reactor

Act as Plug Flow Reactor

Exothermic reaction

The reaction only considered the primary reaction

Flow over tube is assumed to be constant

General balance equation:

Input + generation- output-consumption= accumulation

10

Rate law of reaction 1-Butene

Equation 3.1

Where;

-rA = rate of formation

k = rate constant

CA = Concentration 1-Butene

C B = Concentration O2

3.2.3.2 Basis calculation

The table below illustrates the mole flowrate and mass flowrate of the reactor.

Table 3.3: Reaction component

Component MW (kg/kmol) Mass Flowrate(kg/hr) Mole Flowrate (kmol/hr)

C4H8 56.11 16368 290.628

C4H6 54.096 - -

O2 15.99 2532 158.4

H2O 18.2 45009 2498.4

total 144.396 63909 395086.79

Equation 3.2

( )

( )

Of 1-Butene

11

Where P = Pressure (Pa)

T = Temperature (K)

R = Gas constant 8.314

Of Oxygen

Table 3.4: Intial concentration of each species

Species Initial Concentration (mol/m3)

1-Butene 12.48

1,3 - Butadiene 0

Oxygen 6.80

Water 102.31

3.2.3.3 Reaction kinetics

The reaction occurs in the reactor is:

( )

The reaction rate is,

The k value from the reaction rate equation is calculated from Arrhenius equation.

(

(

)) Equation 3.3

The rate constant, k is dependence on the temperature. Temperature, T used for this

reactor is 573.15K.

12

Table 3.5: The value of specific reaction rate and activation energy

Parameters Value

Rate constant, ko (mol. L-1) 0.095

Activation Energy, E (kJ/mol) 0.199

(Li Wang1, 2013)

From the above data, k1 can be calculated by using equation 4 as below;

(

( )(

))

= 0.09s-1

The concentration of component A and B was calculated by using stoichiometric table

below:

Table 3.6: Stoichiometric table

Entering Change Remaining

A FAo -FAoX FA = FAo (1-X)

B FBo -1/2FAoX FB = FAo (ΘB-1/2X)

C FCo FAoX FC = FAo (ΘC+X)

D FDo FAoX FD = FAo (ΘD+X)

FTo FT = FTo+δ FAoX

For gas flow system

( )

Equation 3.4

Θn =

=

=

=

δ = 1

ɛ = =

Ca =

=

( )

= (

) (

)

Cb =

=

( )

= (

) (

)

13

3.2.3.4 Volume of reactor

As mentioned in assumptions before, the reactor is assumed to act as plug flow reactor.

Thus, the volume of the fluid in the reactor can be calculated by using equation below:

∫

Equation 3.5

In order to calculate the volume of the fluid, a plot of (Fao/-rA) versus x is plotted by

using Microsoft excel.

Fao = 81 mol / s

Fbo = 44.4 mol / s

Table 3.7: Data Plotted by Microsoft Excel

X Ca Cb Fao/-ra

0 81 6.663332 1.6675006

0.1 70.90881 6.452462 1.9670567

0.2 60.81761 6.234463 2.3736358

0.3 50.72642 6.00856 2.9528264

0.4 40.63522 5.773825 3.835979

0.5 30.54403 5.529134 5.3291638

0.6 20.45283 5.273101 8.3449352

0.7 10.36164 5.003984 17.357939

0.8 0.270444 4.719547 705.12294

14

Figure 3.1: Graph of Fao/-rA vs X

The integral in equation 3.5 can also be evaluated from the area under the curve of a

plot Fao / -rA. The numerical method chosen to calculate the area under the curve is

Simpson’s one-third rule (three-point). The calculations are as follows:

∫ ( )

[ ( ) ( ) ( )] Equation 3.6

Where:

h =

=

= 0.4

X1 = = 0.4

V= ∫

[ ( ) ( ) ( )]

V= ∫

[ ( ) ( ) ( )]

V=

[ ( ) ]

0

100

200

300

400

500

600

700

800

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Fao/-ra

Conversion (X)

Fao/-ra Vs X

15

V= 96.28 m3 ≈ 96.3 m3

To calculate the volume of the reactor, the volume of the fluid will be divided with the

void fraction of the catalyst, ɛ. The void fraction of the catalyst is assumed to be 0.4.

VR =

VR =

= 240 m3

Residence time,

The time is taken for the reactant become constant is illustrated by the equation below:

Equation 3.7

Where: η = residence time, s

v =

( )

vo = 1.584 m3/ s

Therefore,

v = 1.60536 m3/s

Thus residence time is,

=

=

= 150 s

= 2 min 30 sec.

16

3.2.3.5 Diameter of the reactor

The diameter of reactor is assumed to be H=2.5D

Vr = (π D2/4) H

= (π D2/4) 3D

D = 5.5 m

Height of reactor, H =2.5D

= 2.5(5.5m)

= 10.1 m

Therefore the area of the reactor, A = π r2 =23.76 m2

Table 3.8: Summary of reactor design

Items value

Volume reactor 240 m3

Diameter reactor 5.5 m

Height reactor 10.1 m

3.2.3.6 Catalyst Selection

The heart of a fixed bed reactor and the site of the chemical reaction is the catalyst.

Catalyst is a substance that affects the rate of reaction and it plays a big role in

oxidative dehydrogenation process. Although number of research have been performed

to compare and come out with the most efficient in conversion and high temperature

resistance. The catalyst that is used for this reactor is Bismuth Molybdate due to its

common practice in this oxidative dehydrogenation process. Bismuth molybdate

catalyst is also known to be the most stable catalyst that is implemented in this process

comparing to V-Mg-Al and Ferritic catalyst (S. Afandizadeh, 2000)

17

3.2.3.6.1 Volume and weight of Catalyst,(Vc & Wc)

VC = Vr (1-ε)

= 240 m3 (1-0.4)

= 144 m3

Weight of Catalyst, Wc

Mc = Volume of catalyst x density

= 144 m3 x 614 kg/m3

= 88.42 Tonne

3.2.3.7 Heat balance

The reactant will be placed in the tube side while the coolant will be placed in the shell

side. Since the reaction is exothermic, the coolant which is water will be used to

maintain the reaction temperature at 400oC. The reaction temperature must be keep at

that temperature to produce desired product. To determine the flow rate of the coolant,

the heat duty of the reactant was calculated. The following assumptions were made to

calculate the flow rate of coolant:

i. The heat capacity of product gases, Cp= 3.66 kJ/kg. oC

ii. The heat capacity of water, Cpw= 4.2 kJ/kg.K

iii. The inlet and outlet 350oC and 400oC

Q = △

Q = ( )

( )

( ) = 4224.207 kJ/s

So, the mass flow rate of water required will be,

=

△

= 2.68 kg/s

18

3.2.3.8 Tube Design

a) Tube Dimension

The diameters of 16 to 50 mm are most often used by TEMA. Therefore, the 50 mm

outside diameter was selected because it is easier to clean by mechanical methods.

For these standard dimensions of steel tube, the thickness is 2 mm. The inside

diameter of tube is:

Di = 0.05- 0.002 = 0.0048m

The tube length is 10.1 m. Thus, the volume of the tube is 0.0198 m3. The number of

tubes required is;

Nt =

= 7272.73

The total area of the tubes, At is given as follow:

At =

( ) = 13.16 m2

b) Tube Arrangement

The triangular and rotated square patterns give a higher heat transfer rate, thus the

triangular pattern will be selected. The recommended tube pitch (distance between tube

centers) is 1.25 times of the outside tube diameter.

Pt = 1.25Do

1.25Do = 1.25(0.05)

1.25(0.05) = 0.0625m

19

Figure 3.2: (a) tri-angular, (b) rotated square patterns.

c) Tube sheet layout

The bundle diameter is depends on the number of tubes and also the number of tubes

passes. The bundle diameter can be estimated by using equation below:

Db = (

)

do= Outer diameter of tubes

NT = number of tubes

K1 = number of passes 1 (0.319)

n1 = number of passes 1 (2.142)

Db = 4.47 m

The bundle diameter is still smaller than the reactor diameter therefore it is considered

justified.

20

d) Head type

The floating head type with split ring is selected. The tube can be removed and it is

easier. The clamp ring or split ring can reduce the clearance to clean.

e) Baffles Baffles are installed on the shell side to give higher heat transfer rate. Besides that, it

also increase turbulence and to support the tubes. There are a few types of baffles

used in heat exchanger such as segmental, segmental and strip and disc and

doughnut. However, the most common used is single segmental. The optimum spacing

between baffles is usually between 0.3 to 0.5 times the shell diameters.

Baffles spacing = 0.3(10.1) = 3.03 m

3.2.3.9 Tube side pressure drop and heat transfer calculation

3.2.3.9.1 Tube side heat transfer coefficient

Prandtl number

= 0.000022 kg/m.s

= 3.66 kJ /kg.K

kf = 0.084 W/m.K

Pr =

= 0.959

The mass flow, Gt of the fluid into the tubes is given as follow:

Gt =

= 1.624 kg / m2.s

= Mass flow rate

= Total area of tubes

Reynold number, Re,

21

Re =

Re = ( )

= 3542 therefore the flow is turbulent

At Re = 3542 jh = 8.5 x 10-2, from Appendix A figure 3.4

From Nusselt number,

Nu =

=

ht =

ht = 591.61 W/ m2. K

3.2.3.9.2 Pressure drop in the tube side

The pressure drop in the tube side is given by:

△Pt = * (

) +

jf is the tube side friction factor and obtained from Appendix A figure 3.3, for Re= 3542,

△Pt = 1[8(5.5x10-2) (10.1/ 0.048) + 2.5](3.98x0.4082/ 2)

△Pt = 31.38 Pa

3.2.3.10 Shell side pressure drop and heat transfer calculation

3.2.3.10.1 Shell side heat transfer coefficient

Cross sectional area, As is given by

As = ( )

22

As = [((0.0627-0.05)5.5x3.03)/ 0.0627

As = 3.2 m2

pt = tube pitch, do = tube outer diameter, Ds = Shell inner diameter, Ib = baffle spacing

Gt =

= 0.807 kg / m2.s

Shell equivalent diameter, de

de =

(

)

de = 1.323 m

Shell side Reynold number, Re

Re =

= 1186.29 < 2300, therefore the flow is considered as laminar.

At Re = 1186.29 and baffle cut off is 30%, the heat transfer factor, jh can be obtained in

Appendix A figure 3.4:

jh = 1.75 x 10-2, Prandtl number,

Pr =

= 6.52 x 10-3

Heat transfer coefficient, hs

ht =

ht = 765.003 W/ m2. K

3.2.3.10.2 Shell side pressure drop calculation

△Pt = * (

) (

)+

△Pt = 2.41x10-3 Pa

23

Table 3.9: Summary of chemical design

SUMMARY OF CHEMICAL DESIGN

Equipment No (tag) R - 101

Description Oxidative dehydrogenation reactor

Type Multi Tubular Fixed bed reactor

Orientation Vertical

Operating Temperature

400 oC

Volume 240.81 m3 Operating Pressure

7 atm

Diameter 5.5 m Residence time 150 sec

Height 10.1 m Cross sectional area

23.76 m2

CATALYST

Volume 144 m3

Weight 88.42 Tonne

Name of catalyst Bismuth Molybdate (Bi-Mo-O)

Particle shape Cross web

TUBE UNIT

Number of tube 7272

Mass flow rate 1.624 kg / m2.s

Length of tube 10.1 m

Outer diameter 0.05 m

Inner diameter 0.048 m

Reynolds number 3542

Heat transfer coefficient 591.61 W/ m2. K

Pressure drop 31.38 Pa

SHELL UNIT

Bundle diameter 4.47 m

Shell diameter 5.5 m

Mass flow rate 0.807 kg / m2.s

Reynolds number 1186.29

Heat transfer coefficient 765.003 W/ m2. K

Pressure drop 2.41x10-3 Pa

24

MECHANICAL DESIGN

OXIDATIVE-DEHYDROGENATION REACTOR (CRV-100)

3.2.5 GENERAL DESIGN CONSIDERATION

From chemical design

Operating pressure = 709.275 Kpa = 7.1 bar

Operating temperature = 400 oC + 273.15 = 673.15 K

Diameter of vessel = 5.5 m = 5500 mm

Height of vessel = 10.1 m

3.2.5.1 Design pressure

In the reaction involving oxidative-dehydrogenation process, the pressure applied is

709.25 kPa. In order to come out with an applicable reactor design that is capable of

coping with the pressure, the design must consider the mechanical properties of the

vessel structure. For safety purposes, the design pressure which known as the

maximum allowable working pressure is designed 10% above the working pressure.

The prevention method is a crucial step in avoiding spurious operating of relief valve

during minor process upsets (Sinnot and Towler, 2009).

P = (Pw) × 1.1

Where: P = Design pressure

Pw = Working pressure

(Source: Coulson & Richardson, 1999)

Thus, the design pressure for this reactor is,

P = (7.1) x 1.1

= 7.81 bar

= 0.78 N/mm2

25

3.2.5.2 Design temperature

The operating temperature for this reactor is 400oC. It is noted that the strength of metal

will decrease with the increasing of temperature for the system. The design temperature

of vessel should be at the maximum working temperature at which equals to the

maximum allowable stress of the metals. Therefore, the maximum allowable stress will

depend on the material temperature including allowance for any uncertainty involve in

predicting the vessel temperature (Source: Coulson & Richardson, 1999).

T = 400 oC + 273.15 = 673.15 K

3.2.5.3 Material Construction

According to Sinnot and Towler (2009), there are several material used for the

contraction of vessel such as plain carbon steel, low and high alloy steels, other alloys,

clad plate and reinforced plastic. The material chosen must take into account the

suitability of the material for fabrication (welding) as well as the compatibility of the

material to the process environment, effect of high and low temperature and the ability

to resist corrosion.

Stainless steel 304 (SS304) is the most suitable and economy friendly type of material

for the construction of this reactor. The stainless steel also had great ability to withstand

corrosion and to cope with high pressure condition at which will increase higher

accountability in safety.

3.2.5.4 Maximum Allowable Stress Design

The design stress for stainless steel 304 (SS304) is show in figure 3.5 (Appendix A) at

the various temperature. The design stress for this at temperature 400 K is:

26

Design stress = 100 N/mm2

Tensile strength = 510 N/mm2

(Source: Coulson & Richardson, 1999)

3.2.5.5 Welded-Joint Efficiency

The welded-joint efficiency is taken as double-welded butt with 100% degree of

radiography is show in figure 3.6 (Appendix A).

Joint efficiency, j = 1.0

3.2.5.6 Corrosion Allowance

The corrosion allowance is additional thickness for material lost by corrosion and

erosion. The minimum corrosion allowance that should be used is 2 mm where the

severe condition is not expected (Sinnot and Towler, 2009).

3.2.6 Design of Thin-Walled Vessel

3.2.6.1 Wall Thickness of Cylindrical

The minimum wall thickness, required to ensure that any vessel is sufficiently rigid to

withstand its own weight and any incidental load. From Sinnot and Towler (2009), the

wall thickness of any vessel should not be less than the value given below, which

includes corrosion allowance of 2mm.

From the chemical design, diameter of vessel is 2.19 m, so the minimum thickness of

vessel include corrosion allowance should not less than 9 mm.

27

For a cylindrical vessel the minimum thickness required to resist internal pressure can

be determine from equation below:

Where: t = Minimum thickness

Pi = Internal pressure

Di = Diameter vessel

f = Design stress

(Sinnot and Towler, 2009)

The minimum thickness of vessel well, t

( )( )

( ) ( )

Add corrosion allowance

( )

So the minimum thickness for the vessel is 23.5 mm.

3.2.6.2 Design of Head and Closure

The ends of a cylindrical vessel are closed by head of various shapes. The types of

head used are;

Domed head

I. Hemispherical heads

II. Ellipsoidal heads

III. Torispherical heads

28

Flat heads

Flat head are commonly used as a cover for manways and channel cover for heat

exchangers. Formed flat ends, also known as flange-only is manufactured by turning

over a flange with small radius on a flat plate. But their application is limited to low-

pressure and small diameter vessel.

For hemispherical head, it is a stronger design compared to the previous two, capable

of resistance about twice the pressure of a torispherical head at the same thickness.

The standard torispherical head are most commonly used for end closure of vessel that

operates above 10 bar of pressure.

It can be used for higher pressure, although the cost will be equivalence to ellipsoidal

head which usually proven to be less economical. The ellipsoidal head design however

have proven that it can be the most economical comparing to the other type head

design.

Figure A-3 in appendix A is show the figure type of domed head (Sinnot and Towler,

2009). Since the operating pressure in this reactor is 7.1 bar, the comparison will not

include flat head design due to its most application for low pressure vessel design.

i. Hemispherical head

Thus;

( )( )

( ) ( ) ( )


( )

29

ii. Ellipsoidal head

Thus;

( )( )

( ) ( ) ( )


( )

iii. Tori spherical head

Crown radius, Rc = D = 5.5 m

Knuckle radius, Rk = (6% from Rc) = 0.3314 m

A head of the size would be formed by pressing: no joints added, so j =1

Thus,

( )( )

( )( ) ( )

30

Adding corrosion allowance

( )

Based on the calculation for comparison above, ellipsoidal head is used for this reactor.

It is due to consideration regarding economical aspect that showcase ellipsoidal head to

be the most economical and closest to the vessel thickness is 23.5 mm.

3.2.6.3 Design of Vessel Subject To Combine Loading

The pressure vessel should be designed to withstand the worst combination of load

without failure (Sinnot and Towler, 2009).

3.2.6.3.1 Total Weight of Vessel

The weight of a cylindrical vessel with domed end and uniform wall thickness can be

estimated from the following equation:

( )

Where:

Wv = Total weight of the reactor, excluding internal fitting

Cv = A factor to account for the weight of nozzles, internal supports. Weight

factor take as 1.15

Dm = mean diameter of vessel, Dm = Di + (t × 103) = 2.19 + (0.013) = 2.203

m

Hv = Height or length between tangent line = 6.57 m

t = Wall thickness = 13 mm

i. Dead weight, Wv

31

( )( )( ( ))

(Source: Sinnot and Towler, 2009)

ii. Weight of Insulator, W i

Material used for insulator is mineral wood.

Assume the insulation thickness ,ti = 50 mm

Density of mineral wood = 130 kg/m3

Thus, volume of insulator, Vi

Vi = πDiHvt

= π (2.19m)(6.57m)(50x10-3m)

= 8.73 m3

The weight of insulator, Wi

Wi = Viρg

= (8.73 m3)( 130 kg/m3)(9.81)

= 11127.98 N

= 11.13 kN

Double this value to allow for fitting = 2(11.13 kN)

= 22.26 kN

iii. Dead Weight of cooling water (Shell side), Wcw

Wcw =

Wcw = (π x 1000 x 5.52 x 9.81 x 10.1)/4

Wcw = 23.33 kN

32

iv. Weight of catalyst, Wc

Wc = Mcg

Wc = (147000 kg) x 9,81

Wc = 1442070 N

Wc = 1442 kN

v. Weight of fluid in tube side

Wf =

Wf = 5189.19 N = 5.189 kN

vi. Weight of tube

Wt = Nt π(

)

Wt = 3549kN

Total weight, Wt

WT = Wv+ Wcw + Wi + Wc+Wf+Wt

= +3549kN

= 5559.04 kN

Take 5% the total weight vessel as allowance, so

WT = 2110.54 kN

3.2.6.3.2 Wind Loading

(

)

= 17(

) = 136 N/m2

33

Loading per unit length of reactor, Fw

Fw = PwDeff

Where,

Deff = Effective reactor diameter

= Diameter shell + 2(t+ ti)

= 5.5m + 2(23.5 + 50) x 10-3

= 5.647m

Thus,

Fw = (136 N/m2)( 5.647 m)

= 767.48 N/m

= 0.767 kN/m

Bending moment

Mx = Fw X2

2

Where,

X = distance measure from the free end = 10.1m

So, Mx = (767.48 N/m)x(10.1m)2

2 = 39145.31 Nm = 39.145 kN/m

i. Analysis of stress

At bottom tangent line

Longitudinal stresses:

ζL =

= ( )( )

( )

34

= 41.54 N/mm2

Circumferential stresses

ζh =

= ( )( )

( )

= 82.91 N/mm2

Dead weight stress:

ζw =

( )

=

( )

= 1.27 N/mm2 (compressive)

Bending stress:

D0 = Di + 2t

= ( )

= 5547 mm

Iv =

(

)

=

[( ) ( ) ]

= 1.56

ζb =

(

)

=

(

) =

Resultant longitudinal stress, ζz

ζz =

a) ζz (upwind) =

= ( )

= 40.270 N/mm2

35

b) ζz (upwind) =

= ( )

= 40.269 N/mm2

40.270 40.269

82.91 82.91

(a) (b)

Differences = 89.21– 40.270 Differences = 89.21 – 40.269

= 48.94 N/mm2 = 48.941 N/mm2

Therefore, we choose greatest difference between the principal stresses will be on up-

wind (b), below the maximum allowable design stress

Differences = 48.941 N/mm2 < 100 N/mm2 (max allowable stress), Justified.

3.2.6.3.3 Design Vessel Support

The support of a vessel is either in horizontal or vertical and it is designed to carry the

total weight of the vessel and withstand any superimposed load. The design of vessel

will depend on the size, shape; design pressure and design temperature; vessel

location and arrangement either internal or external fitting including attachment. Saddle

support, skirt support, bracket and lugs are the type of vessel support that can be

considered in determining which type is appropriate for the vessel design. Saddle

support usually used for horizontal vessel which will include mounting on two saddles

and it is usually appropriate for vessel with lower weight. Skirt support on the other

hand is used for tall and vertical column such as distillation column. Bracket or lugs

vessel supports are applicable for all type of vessel and it is simple to construct. Since,

36

this reactor is design in vertical, bracket or lugs have been used to be supported on

legs (Sinnot and Towler, 2009).

Since there are 4 legs to support, the total weight of vessel 2110.54 kN is divide into for

four in order to get the maximum design load per bracket, Fbs .

Thus, Fbs = 5559.4 kN 4 = 1389.84 kN Design stress,f for carbon steel at 25oC is 135 N/mm2 from table A-1 (Appendix A)

Area of bracket, A = Fbs/f

(Hibbler,2011) = 1389.84x103 kN / 135N/mm2

= 10295.1 mm2

Thus, diameter for bracket

10295.1 mm2 = D2

D = 101.5 mm

The thickness of the bracket is taken to be similar as the thickness of wall vessel which

is 23.15 mm

3.2.6.3.4 Catalyst Support

The catalyst support is the material used to support the catalyst so that it will stay on its

position. The material used depends on the suitability towards the reaction and

definitely it is inert material. From Kamariah et. al., circular woven stainless steel wire

mesh is used. The assumption made for catalyst support selection is the same material

is employed for this reactor where the wire mesh will be place under each tube. The

dimension of the wire mesh is shown in figure 2.2 below:

Figure 3.5: Dimension of Catalyst Support

37

3.2.6.3.5 Baffles Design

Baffles are installed on the shell side to give higher heat transfer rate. Besides that, it is

also increase turbulence and to support the tubes. From chemical design, the baffle

spacing is 3.03 m.

Therefore, number of baffles is 3.

3.2.6.3.6 Bolt Flanged Joint

Flanged joint are used for connecting pipe and instruments such as vessel, pump and

valve. Besides that, it also used for manhole cover and for removable vessel heads

when ease of access is required. Flanged also be used in vessel body, if it is necessary

to divide the vessel into a section for transport or maintenance. The range size of

flanges from few millimeters for small pipes to several meters diameter for used as a

body.

3.2.6.3.7 Selection of Flange

There are several different types of flange used in the process industries for various

applications. The types of flange are welding-neck flange, slip-on, hub and plate types

flange; lap-joint flange, screwed flange and blank or blind flange. Figure 1.1 shows the

flange type.

Figure 1.1 Type of flange

38

For this reactor, welding-neck flange are chosen due to its suitability for extreme service

condition since the operating temperature is rather high. Then, for the flange face,

narrow-faced flanges have been chosen as it is the most commonly used type of flange

for process equipment.

3.2.6.3.8 Piping selection

For stainless steel, the pipe sizing can be determined by using the equation below

d optimum = 260 G0.53 ρ-0.37

Where; G = Flow rate fluid in the pipe, kg/s ρ = Density of fluid, kg/m3

Inlet feed for Stream 6

G = 23.14 kg/s

ρ = 3.98 kg/m3

So,

d optimum = 824.4 mm

Outlet feed stream 7

G = 23.4 kg/s

ρ = 3.85 kg/m3

So,


Inlet water for cooling

G = 2.68 kg/s

ρ = 1000 kg/m3

So,


39

Outlet water for cooling

G = 2.56 kg/s

ρ = 834 kg/m3

So,


40

3.2.7 SUMMARY FOR MECHANICAL DESIGN

Table 3.12: Summary of Mechanical Design Item

Equipment No (tag) CRV - 101

Description Oxidative-dehydrogenation reactor

PARAMETER VALUE UNIT

Design Pressure 7.81 Bar

Design Temperature 400 oC

Material of Construction Stainless steel 304 NA

Design Stress 100 N/mm2

Design Vessel Dimension

Inner diameter 5.5 m

Outer diameter 5.5235 m

Shell thickness 0.0235 m

Domed End

Type Ellipsoidal NA

Domed End Thickness 0.022 m

Vessel Support

Type Bracket NA

Material Plain Carbon Steel NA

Design Stress 135 N/mm2

Design per bracket, Fbs 1389.84 kN/mm2

Diameter for bracket,D 0.101 m

Dead Weight of Vessel and its contents

Dead weight of vessel 517300 N

Weight of tubes 3549279 N

Weight of cooling water 23330 N

Weight of catalyst 1442000 N

Weight of fluid 5189 N

Weight of insulator 22260 N

Total weight 1454000 N

Wind

Wind pressure 444.48 N/m2

41

Wind Loading 2333.5 N/m

Bending Moment 21573.2 N/m

Piping selection and Flanges

Types of Flanges Welding-neck NA

Sized piping Inlet (Stream 6) 824.4 mm

Sized piping Outlet (Stream 7) 839.53 mm

Sized piping water inlet (Shell) 34.04 mm

Sized piping water outlet (Shell) 35.52 mm

42

3.2.8. Reference

Li Wang1, B. P. (2013). Mesostructural Bi-Mo-O catalyst: correct structure leading to high

performance. Shanghai.

Naoki Ikenaga, S. O. (2012). Oxidative Dehydrogenation of 1-Butene with Lattice Oxygen of V-

Mg-Al Complex Oxide. Oxidative Dehydrogenation of 1-Butene with Lattice Oxygen of

V-Mg-Al Complex Oxide, 1-7.

S. Afandizadeh, E. F. (2000). Design of packed bed reactors: guide to catalyst shape, size and

loading selection. Applied Thermal Engineering 21, 1-14.

Shell. (1964). Patent No. 3,159,688. United States of America.

Shell, O. (1964, December 1). Patent No. 3159688. United States of America.

Umich. (2010). Comparison Between Packed Bed Reactor with Fluidized Bed Reactor. Retrieved

December Tuesday, 2014, from

http://www.umich.edu/~elements/5e/asyLearn/bits.htm:

http://www.umich.edu/~elements/5e/asyLearn/bits.htm

43

APPENDIX A

Figure 3.3 Shell side heat transfer factor table

44

Figure 3.4 Tube side heat transfer factor table

45

Figure 3.5 Typical maximum allowable stress table

Figure 3.6 Maximum allowable joint efficiency table

Documents

CHAPTER 3