REACTION VESSEL

Selection of reaction vessel

Simple/Ideal Life Homogeneous Reaction, Isothermal Condition, Ideal
Mixing, Elementary reaction, No catalyst, 100% yield, highest rate of HT
and MT, Highest conversion, no cleaning requirements
Real Life Heterogeneous reaction, multiple reactions, non isothermal
condition, non ideal mixing, solid catalyst required, less conversion and
yield, poor HT and MT, Non elementary reactions, parallel and series
reaction
Based on the real life situations, one has to make correct decision on
selection of right type of reaction vessel for given application.
Part I from process point of view
 The chemical reactions can be carried out
1. Either in liquid phase or vapor/gas phase
2. Either in presence of solid or in absence of solid.
 Some of the liquid phase reactions are oxidation, hydrogenation, alkylation, nitration,
halogenations, sulfonation, polymerization, poly condensation, etc.
 Whenever process involves reaction, Overall rate of reaction is controlled by either rate of
mass transfer or intrinsic of kinetics (Rate of reaction).
 If one of the steps is disturbed, it affects the overall process
1. Time required for reaction to complete will increase or
2. Reaction may not advance at all or
3. Reaction will not proceed to desired extent.
 Along with these steps, based on the heat of reaction, heat transfer is also equally
important factor to be considered.
1. Heating  Endothermic Reaction
2. Cooling  Exothermic Reaction
 Agitators are provided in the reactors to improve the mass transfer and heat transfer rate.
Some time the arrangement of reactors are made such it will create turbulence in the flow,
they are called static mixers.
 After laboratory scale study of a chemical reaction, one has to carry out the scale up study
to obtain optimum value of ratio HT area/ Unit volume of reaction mixture.
 Due to increased capacity of process plants, vapor phase reactions are becoming more
and more popular. These types of reactions are important where enormous amount of
energy will be evolved during the reaction. The vapor rate and vapor velocity can be
controlled and there by rate of reaction can be controlled. Generally tubular reactors are
preferred for these types of reactions.
 In liquid phase reactions, the reactants are immediately mixed as soon as they enter the
reactor. The whole mass is in back mixed state and the concentration is uniform through
out the unit. If highly exothermic reactions are carried out in liquid phase, it becomes
difficult to maintain the reaction temperature. In such units the reaction can be controlled
by residence time and using number of units in series. Generally CSTRs are preferred
for these types of reactions.
 Thus selection of reactor depends upon the process requirements and the process
capacity.
Part II DESIGN POINT OF VIEW
 Pressure vessels are used in the chemical industries for the reaction purpose for carrying
out blending, dispersion, gas absorption, dissolution, batch distillation, etc. under the
controlled condition.
 The vessel may be open or closed.
 If the closed vessel withstanding moderate pressure it is called as “Reaction Kettle”, but if
it is required to withstand higher pressure (>20 bar) & temperatures it is called as
“Autoclave”.
 Capacities of reaction vessel are: 100liters to as large as 25000liters, with the shell
diameter varying between 50cm and 500cm.
 Heads preferred are shallow dished head like elliptical or torispherical. If frequent
cleaning is required then flanges (mostly tongue and groove or male and female type
flange facing) are suggested. But main problem is the leakage of the gasket sealing from
flange so tongue and groove type facing is suggested.
 Any type of nozzle can be used with the reaction vessel (Inlet, outlet, sight and light glass,
thermo well, hand holes, manholes).
 Bracket or column supports are generally preferred for installation.
Part III OPERATION POINT OF VIEW
1. Batch reactors: Exclusively used for the liquid phase reactions. Temperatures, pressures
and compositions will vary with time. Dyes and Paints, Food, Pharmaceutical etc.
2. Continuous reactors: A uniform concentration is maintained throughout the vessel.
Polymerization, hydrogenation, nitration etc.
3. Semi-batch reactors: when heat and mass transfer is important, when shifting of
equilibrium is required, when changing the selectivity is required.
Comparison of batch vs continuous reactors
It has to be deciding first whether the process is batch wise or continuous.
Technical Feasibility
 Batch reactor is useful for small production rates and long reaction times. The operation is
flexible and reaction conditions are adjustable. If process plant is to produce varied
chemicals, batch reactor utility is suitable. Eg. Pharmaceuticals and other fine chemicals.
Batch reactors require less instrumentation cost.
 The problems of batch operation are that an extensive manual operation and continuous
supervision is needed. It is difficult to maintain the same quality in different batches.
 Continuous reactors are useful at high production rates and high conversion rates.
Constant quality of the product can be maintained. Manual operation and supervision cost
is less due to automatic control.
Part IV Economic Viability
 Economic viability is also an important factor in selecting batch or continuous operation.
The fixed costs which include depreciation, maintenance, supervision etc are independent
of production rates; these costs are higher in continuous operation.
 The variable costs involved are due to electricity, cooling water, air, fuel and other
utilities.
 Quality control, laboratory services are proportional to production rate and are higher for
batch operation due to frequent batch change over.
 Energy utilization is not very efficient in batch operation.
 Raw material costs can be taken as same for both batch and continuous operations and are
proportional to production rate.
 Total production cost is sum of all the costs mentioned above. The net profit of the
products is equal to revenue earned minus the expenses in production. It can be taken as
same for both type of operations.
 The difference between net sales and total production costs shows the profit or loss of the
process plant. No loss, no profit indicates the break even capacity of operation, break even
point is less for batch operation as compared to continuous operation, which indicates the
greater economic flexibility of both operations as indicated in figure below.
Industrial Reactors
1. Kettle
2. Autoclave
3. Bubble column
4. Gas sparged Reactor
5. Fixed Bed Reactor
6. Fluidized Bed Reactor
7. Falling Film Reactor
8. Photochemical Reactor
9. Extrusion press or a screw conveyor in polymerization reactor
10. Spray column
11. Air lift reactor
Reactor Classification based on process requirements
1. Continuous Stirred Tank Reactor (CSTR)
a. Most preferred
b. Largest volume requirement for given conversion
c. Heat transfer will be by either by jackets, coils or by external reflux exchangers
d. Normally operates at atmospheric conditions
2. Tubular Reactor (PFR)
a. Generally for gas/vapor phase reactions
b. High pressure drop
c. Minimum volume requirement for given conversion
d. Easy temperature control
3. Column Reactor
a. Expensive than above two reactors
b. Performance cannot be predicted
4. Bubble Reactor
a. Gas-liquid reactions are carried out
b. Low agitation power
 c. Poor mixing phenomena
d. Interfacial area is at the most 400m2/m3.
5. Baffled Reactor
a. Good for gas-liquid-solid operations
b. Good mixing
c. Less expensive
6. Packed Column Reactor
a. Used for liquid phase and gas phase reactions with/without solid catalyst
b. Difficult to remove heat and control temperature due to non-idealities
c. Not suited for highly exothermic reactions
d. Interfacial area can be larger than bubble contactors
7. Screw Conveyor Extruders
a. Used for reaction of highly viscous materials.
b. Production of cellulose acetate and Nylon-6
8. Electrolytic Cell
a. Production of NaOH from NaCl

Parameters to be considered for reactor design

1. operating condition
2. production rate
3. mode of operation
4. mechanism and reaction rate
5. mass transfer and heat transfer
6. mechanical agitation
7. material of construction
8. cost of reactor
All the above parameters have their own importance in designing and selecting a reactor for
the operation.
Material of Construction
 Carbon steel, stainless steel and other alloy steel such as hastelloy. In special cases non-
ferrous alloying metals such as copper, nickel, aluminium, and titanium are also used for
better mechanical strength and also for good corrosion resistance.
 At moderate pressure and temperature applications glass reinforced polyesters, glass filled
furons, phenolics and polyvinyl chloride can be layered.
 Saving in cost can be achieved by Cladding of corrosion resistant material in stainless
steel, nickel, inconel, Monel metal, copper etc.
 Vessel can also be lined with lead, rubber, glass and plastics to prevent corrosion.
Agitation
 Both heat and mass transfer will be influenced by the agitation or mixing.
 An agitator, known as a stirrer, produces high velocity liquid streams, which move
through the vessel. As high velocity streams come into contact with stagnant or slower
moving liquid, momentum transfer takes place.
 The classification of agitator is been done based on the viscosity of the content.
 In the vertical cylinder the ratio of liquid depth to the tank diameter (filling ratio) is
between 0.5 to 1.5 and a value normally equal to 1.0 is recommended.
Heating or Cooling System
 Chemical reactions are accomplished by the addition or removal of heat. So the vessel
must provide with the heating or cooling arrangement.
 Both vapor and liquids are used as the heat transfer media. If vapor is used then heat is
transferred through the latent heat and if liquid is used then through sensible heat. So
entire surface has to be at uniform temperature.
 The rate of heat transfer depends upon
 The properties of the fluid (density, viscosity, conductivity)
 The heating and cooling medium (Vapor (Superheated or saturated) or liquid
(subcooled or chilled))
 The vessel geometry (Horizontal or vertical)
 The material of construction (Conductivity of MOC)
  The thickness of the vessel wall
 The degree of agitation.
Heating system
 Two types:
– Direct heating. (Electrical heating) lower efficiency and higher cost
 Resistance Heating
 Induction Heating
– Indirect heating.

Heating medium Temperature range (0C)

Saturated steam 100-180
Oil (thermic fluid) 180-300
Dowtherm E 180-260
Dowtherm A 300-400
Molten salt 400-590
Na-K alloys 590-750
Flue gases or hot air 750-1100
Electrical heating ≥ 1100

Cooling system
Always done indirectly

Cooling medium Operating T range (0C)

Brines -68 – 5
Ethanol-water solution -5 – 5
Methanol – water solution -33 - -1
Ethylene glycol -34 – 5
CaCl2 solution -20 – 0
NaCl solution -9.4 – 5
CH3Cl -67.7 - -37.2
Trichloro ethylene -67.7 - -37.2
Trichloro fluoromethane -67.7 - -37.2
Chilled water 5 – 12
Cooling water 30 – 60
 Oil -1 – 316
Air Atmospheric condition

WAYS OF PROVIDING HEAT TRANSFER MEDIA

 Jacket: Externally. Plain, dimpled, half coil, baffled etc.
 Coils: Internally. Helical coils, Hasp coil, plate baffles, and water cooled baffles.
 Heat transfer coefficients can be increased by increasing the velocities of the medium and
also providing the agitation to the contents.

Choice
 There is no specific choice between the jacket and coil for a vessel carrying out an
exothermic or endothermic reactions, although a jacket is installed when it is necessary to
supply the heat and a coil is used when to remove the heat.
 Heat is supplied by condensing vapor, and for a given heat transfer area there is a greater
space for condensation in a jacket than in a coil. The greater the space provided by jackets,
the greater the ease of the drainage of the condensate.
 On the other hand, a cooling coil is generally more suitable than a cooling jacket because
the rate of heat transfer is greater under forced convection conditions and greater turbulence
can be achieved in a coolant liquid when it is pumped through the a coil than when pumped
through the jacket.
 When frequent cleaning is required then the angled baffles are more suitable with large
heat transfer area. Hasp coils are best for the cooling because they can easily expanded,
cleaned when required and also used with high volumes of the contents.
Special Considerations
 Jacketed vessel gives more HTA compared to limpet coil but the HTC are less than
limpet coil.
 In jackets, channeling of heating or cooling fluid is not possible. ∆P is also less in
jackets.
 Higher velocity and so ∆P can be used in Limpet coil so pumping cost is higher.
 Larger hold up of heating and cooling medium in jackets than coils.
 For the same application, jacket requires higher thickness then the coils.
 Construction of jacket is easy and less costly compared to coil.
Steps to DESIGN reaction vessel

1. Pressure and temperatures is main criteria

2. Identifying the kinetics
3. Calculating the volume of reactor
4. Optimizing the proportion of vessel (D and L or H)
5. Designing of shell and its components
6. Designing of agitators
7. Designing of jackets
8. Designing of coils

The methods for designing:

1. Making shell thick enough in proportion to its diameter and length so that it is self
supported.
2. Using the stay bolt for attaching the inner wall to outer wall.
3. Using the stiffeners or corrugations in the shell or vessel.
Some idea about jackets
Heat Transfer Coefficients Inside Agitated Process Vessels
In order to complete the overall heat transfer coefficient calculation, an estimate must also be
made inside the process vessel. The following estimate should yield reasonable results:

Where:

Ad = agitator diameter
N = agitator speed, rev/s
All other variables as previously defined
a is defined by the table below:

Agitator Surface a
Turbine Jacket 0.62
Turbine Coil 1.50
Paddle Jacket 0.36
Paddle Coil 0.87
Anchor Jacket 0.46
Propeller Jacket 0.54
Propeller Coil 0.83

Calculating the Overall Heat Transfer Coefficient

When calculating the overall heat transfer coefficient for a system, the vessel wall resistance
and any jacket fouling must be taken into account:

Notice that the thermal conductivity of the vessel wall and the wall thickness are included in
the calculation. A typical jacket fouling factor is around 0.001 h ft 2 °F/Btu. When
calculating the overall heat transfer coefficient, use a "common sense" analysis of the final
value. The table below will give some guidance to reasonable final values:
DESIGNING OF PLAIN JACKET

Heat Transfer Coefficients: Conventional Jacket without Baffles

(hj De / k) = 1.02 (NRe) 0.45 (NPr) 0.33 (De/ L) 0.4 (Djo/ Dji) 0.8 (NGr) 0.05

Where:
hj = Local heat transfer coefficient on the jacket side
De = Equivalent hydraulic diameter
NRe = Reynolds Number
NPr = Prandtl Number
L = Length of jacket passage
Djo = Outer diameter of jacket
Dji = Inner diameter of jacket
NGr = Graetz number

The Reynolds Number is defined as:

NRe = DV/
Where D is the equivalent diameter, V is the fluid velocity,  is the fluid density,  and is the
fluid viscosity.

The Prandtl Number is defined as:

NPr = Cp  / k
Where Cp is the specific heat,  is the viscosity, and k is the thermal conducitivity of the
fluid.

The Graetz Number is defined as:

NGr = (m Cp) / (k L)
Where m is the mass flow rate, Cp is the specific heat, k is the thermal conducitivity, and L is
the jacket passage length.

The equivalent diameter is defined as follows:

De = Djo-Dji for laminar flow

De = ((Djo)2 - (Dji)2)/Dji for turbulent flow
Heat Transfer Coefficients: Conventional Jacket with Baffles

For conventional jackets with baffles, the following can be used to calculate the heat transfer
coefficient:

hj De/k= 0.027(NRe)0.8 (NPr)0.33 (µ/µw)0.14 (1+3.5 (De/Dc) ) ( For NRe > 10,000)

hj De/k = 1.86 [ (NRe) (NPr) (Dc/De) ] 0.33 (µ/µw)0.14 ( For NRe < 2100 )

Two new variables are introduced. Dc is defined as the centerline diameter of the jacket
passage. It is calculated as Dji + ((Djo-Dji)/2). The viscosity at the jacket wall is now defined
as µw. When calculating the heat transfer coefficients, an effective mass flow rate should be
taken as 0.60 x feed mass flow rate to account for the substantial bypassing that will be
expected. De is defined at 4 x jacket spacing. The flow cross sectional area is defined as the
baffle pitch x jacket spacing.

Hydraulic Radius: Conventional Jacket with Baffles

Referring to the graphic above, the hydraulic radius is calculated as follows:

Designing Of Vessel Shell With Half Coil
A half coil is the part of Taurus, for which

For a 180° central angle:

Equivalent Heat Transfer Diameter, De = π/4 Dci

Cross Section Area of Flow, Ax = π/8 (Dci2)

For a 120° central angle:

Equivalent Heat Transfer Diameter, De = 0.708 Dci

Cross Section Area of Flow, Ax = 0.154(Dci2)

Here Dci = central diameter of coil passage = Dji + tc

Heat transfer coefficient for coil type jacket

hj De/ k= 0.027(NRe)0.8 (NPr)0.33 (µ/µW)0.14 (1+3.5 (Dci/De) )

(For NRe>10,000)
hj De/ k = 1.86 [ (NRe) (NPr) (Dci/De) ] 0.33 (µ/µW)0.14

(For NRe<2,100)
The Reynolds Number is defined as:

NRe = DeV/
Where De is the equivalent diameter, V is the fluid velocity,  is the fluid density, and  is
the fluid viscosity.

The Prandtl Number is defined as:

NPr = Cp  / k
Where Cp is the specific heat,  is the viscosity, and k is the thermal conducitivity of the
fluid.

The Graetz Number is defined as:

NGr = (m Cp) / (k L)
Where m is the mass flow rate, Cp is the specific heat, k is the thermal conducitivity, and L is
the jacket passage length.

The equivalent diameter is defined as follows:

De = Djo-Dji for laminar flow

De = ((Djo)2 - (Dji)2)/Dji for turbulent flow

Mechanical Design
Circumferential stresses induce in the coil at the junction with the shell and jacket is given as
(Due to pressure in the vessel),

pdc Dci
f pc 
2tc
and the longitudinal stress of coil due to coil pressure is given as,

pDci
f ac 
4tc  2.5ts
where p = internal pressure inside the half coil
DCi = internal diameter of the half coil
t’c = thickness of the half coil excluding corrosion allowance
t’s = thickness of the shell excluding corrosion allowance
Total circumferential stress in shell is given by, summation of circumferential stress in shell
and longitudinal stress in coil

f ps  f p  f ac
pDi pDci
 
2t s 4t c  2.5t s
Where p’ = internal design pressure in vessel
Di = inside diameter of shell
Total longitudinal stress in coil is given by the summation of three stresses;
1. due to internal pressure (a)
2. due to pressure in the coil (ac)
3. bending stress in the shell due to distortion between the shell and coil (b)

f as  f a  f ac  f b

p' Di pDCi 2pDCo 2

f as   
4ts 
2ts 3ts2
where p = maximum differential pressure between the coil and shell.
The thickness of coil and shell is selected such that ƒps and ƒas < ƒJ
The vessel wall thickness can be calculated by the equation given by the Lame’s theorem
based on internal pressure and Model brown equation based on external pressure.
Preferred thickness of half coil tc = 0.3 to 1.2ts  3mm
Designing Of Vessel Shell With Channel Jacket (Dimpled Jacket)
Designing is been done based on the uniformally loaded flat plate restrained at the ends. The
thickness is given as;

k1 p
ts  d C
f1
k2 p
tc  d C
f2
Where d = as shown in the figure
P = internal pressure of the jacket
1, 2 = stresses in the material at the appropriate temperature.
k1 = 0.167 and k2 = 0.12
for higher jacket pressure d can be reduce to reduce the thickness.

hj Do/k= j (NRe) (NPr)0.33 ( For 1000 < NRe < 50,000)

Where:

j = 0.0845 (w/x)0.368 (Amin/Amax)-0.383 NRe-0.305

w = center-to-center distance between dimples

x = center-to-center distance between dimples parallel to flow
Note: (w/x) is equal to one for square spacings as is often the case
Do = (d1 + d2)/2
Amin = z (w-Do)
Amax = zw
DESIGNING OF COOLING COIL

Time required to drain cooling coils in vessel depends upon

 Overall coil height
 Diameter of helical coil
Where
 Inside diameter of tube t = time in sec
D = Diameter of helical coil
 Friction factor d = Inside diameter of coil/tube
 = Viscosity of liquid
 Slop ot pitch of helical coil  = Density of liquid
 Liquid velocity at coil discharge

H
Time required
t where S = Slope or pitch of coil
S .V
 2g  d  S 
0.5

V  1.8 
 B
V = velocity at coil dischage

0.2
  d  d 
B  0.1841  3.5  
  D    H = overall coil height
Friction factor for straight tube

 = 64/NRe
Friction factor for coil is given by

c = 

Where   1  1.73 Re 0.2 

di 
k
R

di = Inside diameter of tube

R = Mean radius of coil

k = 0.262 + 0.326(di/R)3.5

So

64   i  0.2
d  i
d
4.5

fC  1  0.4533  Re  0.564  Re 
0.2

Re  R R 
HEAT TRANSFER COEFFICIENT

Nusselt number for straight tube

1
 d  3
Nu  3.66 3  1.613  Pe   i  where Pe = Peclet number = Re x Pr
 L 

Spirality constant for coil

d 
CS  1  1.77 i 
R

1
  d  3
  d 
Nu  3.663  1.613  Pe   i  1  1.77 i  
  L    R 
Index of operational efficiency of coil system is defined as ratio of friction factor to Nusselt
number

64  
4.5
 d i  0.2  di 
1  0.4533  Re  0. 564  Re 0.2

fC Re  R R 
 1
NuC   d i  3   di  
3.66  1.61  Pe   L  1  1.77 R  
3 3

     
fC
In order to have maximum efficiency of coil system should be minimum with respect to
NuC

 fC 
d  
 Nu C   0
d 
di/R or
d i 
R
Final condition for optimal efficiency is

3.5 4.5
d  d 
2.54 i   3.49 i   0.453  1.77 Re 0.2
R R
It is observed that for low values of di/R less than 0.05, the Reynolds number for optimal
performance attains asymptotic value of around 908.