Kim Christian R. Galaez ChE 517BS ChE 4 Equipment Design

REACTION VESSELS

Pressure vessels are used as reaction vessels or kettles for carrying out operations such as blending, dispersion, gas absorption, dissolution, batch distillation, etc. under controlled conditions

Open—no cover at all, or simply have a loose-fitting flat lid that prevents splashing or escape of fumes Closed—a cover or head that can be secured tightly to the shell so that the reaction can be carried out under

controlled pressure and temperatureo Reaction kettle—if the closed vessel is capable of withstanding moderate pressureso Autoclave—if the vessel is required to withstand high pressured and temperatures that must be maintained

at constant values during the reaction process

Normal rated capacities: 100 L – 1500 LShell diameter: 50 cm – 250 cm

Types of heads Shallow-dished Torispherical Elliptical

Materials of construction

Metals: low carbon steel, stainless steel Alloys: hastelloy Non-ferrous metals: copper, nickel, aluminum, titanium At moderate pressures and temperatures: glass reinforced polyester, glass filled furons, phenolics, polyvinyl

chloride Low cost metals+cladding with corrosion resistant metals: stainless steel, nickel, inconel, monel, copper, etc. Lined with lead, rubber, glass and plastics to prevent corrosion

Agitation

Agitation of the reaction vessel is a requirement in a number of processing operations. Both heat and mass transfer are greatly influenced by agitation or mixing.

Agitator or stirrer—produces high velocity liquid streams, which moves through the vessel

As high velocity streams come into contact with stagnant or slower moving liquid, momentum transfer occurs. The classification of the agitation or mixing equipment is usually made on the basis of liquid viscosity, since viscosity is a major contribution to the forces tending to dampen flow through a mixing system.

Filling ratio—ratio of liquid depth to tank diameter Normal: 0.5 – 1.5 Recommended: 1.0 Dispersing gas in a liquid: 2.0 –to maintain a sufficiently long period of contact between the gas and the liquid

Flat bottomed and cone-bottomed vessels have the disadvantage of low agitation efficiency in the corners formed between the walls and the bottom.

Classification of reaction vessels

1. Batch reactors—these are almost exclusively used for liquid phase reactions. The reactants are added to the empty vessel and the contents are removed after completion of the reaction. Temperature and pressure as well as composition may vary with time.

2. Continuous flow reactors—the reactants flow continuously into the reactor and the products flow out continuously. Under ideal conditions, in a well-agitated system, a uniform concentration is maintained throughout the vessel.

3. Semi-batch reactors—one of the reactants is initially charged batch wise, while the other reactant is fed into the reactor continuously.

Heating systems

Chemical reactions are accompanied by the absorption or liberation of heat. The reaction vessel must, therefore, be provided with the means of supplying or removing the heat of reaction. The rate of heat transfer is a function of the physical properties of the agitated liquid and the heating and cooling medium, the vessel geometry, the materials and the thickness of the vessel wall, and the degree of agitation.

Devices used are either the direct or indirect type.

Direct type Electrical methods:

o Resistance heatingo Induction heating

Low overall efficiency and high operating cost

Indirect typeIndirect heat transfer systems are the most widely used. The heat is received from fluids such as steam, hot oil,

hot water or air, molten salt mixtures, mercury and special organic compounds such as dowtherm and therminol. In liquid systems, the heat transferred in from sensible heat. In vapor systems, the heat transferred in from latent heat. In general, heat transfer coefficients are higher for condensing vapor systems than for liquid systems.

In indirect cooling systems, fluids employed are air, an evaporant such as liquid ammonia, water or brine, oil and organic compounds. Heat transfer coefficients can be increased by increasing circulating fluid velocities or by creating agitation of the contents of the vessel.

Fluid is supplied in either: Jacket—which surrounds the vessel wall Internal equipment—which is placed inside the vessel includes helical coils, hasp coils, plate baffles and

water cooled baffles

There is no specific choice between a jacket or coil for a vessel carrying out an exothermic or endothermic reaction, although generally a jacket is installed when it is necessary to supply heat and a coil to remove heat. The reasons for this selection are that in the majority of cases heat is supplied by condensation of some vapor, and for a given heat transfer area there is a greater space for condensation in a jacket than in a coil. On the other hand, a cooling coil is generally more suitable than a cooling jacket because that rate of heat transfer is greater under forced convectional conditions and greater turbulence can be achieved in the coolant liquid when it is pumped through a coil than when pumped through a jacket.

JACKETSA plain jacket is used for steam. The space between the vessel wall and the jacket shell should be narrow, for

obtaining good heat transfer rates. The dimpled jacket is generally used for condensing vapors such as steam or

dowtherm. High velocities of circulating fluids can be obtained by use of different types of jacket constructions, such as channel or coiled jackets.

COILSCoils are used for heating by immersing them in the contents of the vessel. Such coils are formed from a tubing

by shaping them in the form of a helical or double helical coil. Tubes can also be arranged vertically in the vessel, which serve the dual purpose of heat transfer and baffling.

Design considerations

Design of the reaction vessel is based on pressure and temperature conditions. The reaction vessel is designed for both internal and external pressures operating independently and the higher the value of the wall thickness is accepted as satisfactory.

JACKET DESIGNA plain jacket is the simplest arrangement for heating or cooling. The jacket is generally made of low carbon

steel and is designed for internal operating pressure of the heating fluid at the appropriate temperature. Various methods are adopted to attach the jacket to the vessel wall. A common method is to use two rings of square or rectangular section. The design of the drainage connection of the vessel is more complicated than the jacket because there will be differential expansion between the vessel which, if not provided for, could lead to rupture of the vessel.

COIL AND CHANNEL DESIGNThe half coil or channel construction is formed by continuous spiral of half pipe or channel section, attached to

the vessel wall by continuous fillet welding with full penetration. These methods reduce the shell thickness considerably. Heat transfer coefficients are also higher than for jacket construction. The half coil or part circular coil is also known as limpet coil.

Problem

Reaction VesselVessel shell internal diameter 2130 mmJacket internal diameter 2260 mmJacket length 2500 mmDiameter of half coil or width of channel jacket 100 mm

Flanged and Dished HeadInternal diameter 2130 mmCrown radius 2130 mmKnuckle radius 128 mmStraight flange length 60 mm

Internal pressure (shell) 5.5 kg/cm2

Internal pressure (jacket) 3.5 kg/cm2

Temperature 150 °CMaterial-open hearth steel (IS-2002)Allowable stress 9.8 kg/mm2

Modulus of elasticity (at 200°C) 19.00x103 kg/mm2

Poisson’s ratio 0.3

A. Shell with Plain Jacket

Thickness of Shell1. Internal PressureUsing 10% factor of safety,Design pressure = 1.1 x 5.5 kg/cm2 = 6.05 kg/cm2

t s=p Di

2 fJ−p(6.3)

Where, p—design pressureJ — joint efficiencyf — design∨permissible stress at design

temperatur eDi—internal diameter

Using J=0.85 (maybe taken at 85%),

t s=6.05

kg

cm2×2130mm

2×9.8kgmm2

×0.85×102mm2

1cm2 −6.05 kgcm2

=7.8mm

Use 9mm thickness including corrosion allowance.

2. External PressureDesign pressure = 1.1 x 3.5 kg/cm2 = 3.85 kg/cm2

Using equation 6.14b,

pc=2.42 E

(1−µ2)3 /4×

( tDo

)5 /2

( LDo)−0.45( t

Do

)1 /2 (6.14b)

Where, pc—critical buckling pressure for cylindricalvesselsunder external pressure

E—modulusof elasticityµ— poisson' sratiot—thicknessDo—outsidediameter of vesselL—unsuported lengthof the vessel

L = 2500 + 100 = 2600mmDo = 2130 + 2(7.8) = 2146mm

pc=2.42×19×103

kgmm2

×102mm2

1cm2

(1−0.32 )34

×( 7.8mm2146mm )

52

( 2600mm2146mm )−0.45( 7.8mm2146mm )12

pc=3.4kg

cm2

If the maximum deviation from the true circular shape is 1/10 of ts, the reduction of critical buckling pressure is not more than 25%.

pc (allowable) = (0.25)3.4 kg/cm2 = 0.85 kg/cm2

Calculation of allowable external working pressure from IS-2825 Pressure Vessel Code Appendix F,LDo

=2600mm2146mm

=1.2

Do

t=2146mm7.6mm

=282

Factor B = 3200 (at 150°C)

pa=B

14.22D o

t

pa=3200

14.22×282=0.8 kg

cm2

Since, pa and pc (allowable) are less than the design pressure; it is propose to increase the thickness from 7.8 mm to 9mm and use 6 stiffening rings each at a distance of 450 mm.

Do = 2130 + 2(9) = 2148mm

pc=2.42×19×103

kgmm2

×102mm2

1cm2

(1−0.32 )34

×( 9mm2148mm )

52

( 450mm2148mm )−0.45( 9mm2148mm )

12

pc=31.09kg

cm2

pc (allowable) = (0.25)31.09 kg/cm2 = 7.77 kg/cm2

Referring to IS-2825 Appendix F,LDo

= 450mm2148mm

=0.209

Do

t=2148mm

9mm=239

Factor B = 13000

pa=13000

14.22×239=3.85 kg

cm2

So, 9mm is satisfactory. Use 10mm thick shell. The equation 6.14b gives a much higher value of the allowable pressure.

Stiffening Ring

I=pcDo

3 L24 E

6.15

Where, I — required moment of inertia of thering

pc = 4 x pa pc = 4 x 3.85 kg/cm2 = 15.40 kg/cm2

I=15.40

kg

cm2×21483mm3×450mm

24×1900×103kgcm2

I=1.51×106mm4

The value of I is reduced by 30% to take into account the resistance of the shell,I=(0.7 )1.51×106mm4=1.053×106mm4

Use equal angle IS-A 2020-(size 20x20mm thickness=3mm)I xx=I yy=0.4cm

4

which is the smallest size of angle available.

B. Thickness of Jacket

Internal design pressure = 1.1 x 3.5 kg/cm2 = 3.85 kg/cm2

t j=p Di

2 fJ−p

t j=3.85

kg

cm2×2260mm

2×9.8kgmm2

×0.85×102mm2

1cm2 −3.85 kgcm2

=5.25mm

Use 6mm thickness including corrosion allowance.

C. Head Thickness

1. Internal Pressure

For shallow dished and Torispherical head,

t h=p RcW

2 fJ(6.23 )

W=14 (3+√ Rc

R1 )Where, t h—head thickness

p— externaldesign pressureRc—crownradiusR1—knuckle radiusW — stress intensifi cation factor

W=14 (3+√ 2130mm128mm )=1.77

t h=6.05

kg

cm2×2130mm×1.77

2×0.85×9.8×102kgcm2

=13.6mm

Use 15mm thickness including corrosion allowance.

2. External Pressure

t h=4.4 Rc

4√3 (1−µ2 )√ p2 E

(6.26)

t h=4.4×2130mm×4√3 (1−0.32 )×√ 3.85

kgcm2

2×1900×103kg

cm2

t h=12.1mm

According to IS-2825, para 3.4.6.1, the value obtained for head thickness under external pressure is less than 13.6mm. Therefore, a thickness of 15mm is satisfactory.

D. Vessel Shell with Half Coil Jacket

Design pressure = 1.1 x 3.5 kg/cm2 = 3.85 kg/cm2

f pc=pd i2 tc

(8.1)

Where, f pc—circumferential stressp—design pressure inside the half coild i—internal diameter of the half coilt c—thickness of half coil

t c=pd i2 f pc

t c=3.85

kg

cm2×100mm

2×9.8×102kgcm2

=0.196mm

Use minimum 3mm thickness.

Circumferential stress in the shell is f ps=f pc+ f ac

f ps=pvesselDi

2 t s+phalf coil d i4 t c+2.5 t s

(8.3)

Where, f ps—totalcircumferential stressf ac—longitudinal(axial)stressDi—internal diameter of the shellt s—thickness of the shell

f ps=6.05

kg

cm2×2130mm

2×7.8mm+

3.85kg

cm2×100mm

4×0.196mm+2.5×7.8mm

f ps=828+18.4=846.4kg

cm2

which is less than 980 kg/cm2.

f as=pvesselDi

4 ts+phalf coild i2 t s

+2 Δ phalf coildo

2

3 t s2 (8.4 )

Where, Δp —maximum differential pressurebetweencoil∧shel l

do—external diameter of half coil

f as=6.05

kg

cm2×2130mm

4×7.8mm+3.85

kg

cm2×100mm

2×7.8mm+2×3.85

kg

cm2×1042mm2

3×7.82mm2

f as=414+24.7+456=849.7kg

cm2

which is less than the allowable stress value of 980 kg/cm2. Therefore, use 9mm shell thickness as required to withstand internal pressure with allowance for corrosion.

E. Vessel Shell with Channel Type Jacket

t s=d√ k1 pf 1 +c (8.5)

t c=d √ k2 pf 2 +c (8.6)

Where, t s—thickness of the vessel shell wallt c—thickness of the channel walld —widthof channel jacketp— design jacket pressuref 1, f 2—stresses∈thematerial at the appropriate temperaturek 1=0.167 ;k2=0.12c — corrosionallowance

t s=100mm×√ 0.167×3.85 kgcm2

980kg

cm2

+c

t s=2.56mm+c=4mm

Use 9mm shell thickness required to withstand internal pressure with allowance for corrosion.

t c=100mm×√ 0.12×3.85 kgcm2

980kg

cm2

+c

t c=2.17mm+c=3mm