Unit Operations in

Fertiliser Industry

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INDIAN FARMERS FERTILISER COOPERATIVE LTD.

Sanjay Katheria

Process Engineering Section

Heat Transfer

Some Application of Heat

Transfer

Heat Transfer

Mode of Heat Transfer

- Conduction: Transfer of energy form more energetic particle of a

substance to the adjacent less energetic one.

- Convection: involves conduction and fluid motion

- Natural Convection

- Forced Convection

- Radiation: Transfer of energy in form of electromagnetic waves or

photons.

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Conduction

Steady State Conduction

Unsteady State Conduction

Fourier’s Law dq - kdt

dA dx

where A = area of isothermal surface

x = distance measured normally to surface

q = rate of heat flow across surface in direction normal to

surface

T= temperature

k = thermal conductivity

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Thermal Resistance (R)

Heat Flow Through Composite Walls

R = R1 + R2 + R3

R1=L1/k1A, R2=L1/k2A, R3=L/k3A

Q = ∆T/R

Heat Flow Through Concentric Cylinders

Q = 2πk(T1 – T2)

ln(r1/r2)

Conduction

R1

k1

R3

k3

R2

k2

L1 L2 L3

A

R2

R1 T1

T2

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Convection

Natural Convection

Forced Convection

Q = h*A*(T1 – T2), h = Film Coefficient i.e. Reciprocal

of heat transfer resistance

Hot Surface T1

Cold Fluid Warm Fluid

T2

Air

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Radiation

Radiations are electromagnetic waves, do not require medium for transfer of energy

Blackbody: A perfect absorber and emitter of thermal radiation.

Stefan-Boltzman Law:

Wb = σ*T4

Wb = Total emissive power of a blackbody

σ = Stefan-Boltzman Constant

T = Absolute Temperature

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Sun

Heat Exchanger Type of Heat Exchangers (Design)

Shell and Tube Heat Exchanger

Plate and Finned Heat Exchanger

Double Pipe Heat Exchanger

Type of Heat Exchangers (Flow)

Co-current Heat Exchanger

Counter Current Heat Exchanger

Cross Flow Heat Exchanger

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Contd.. Log Mean Temperature Difference: The LMTD is a logarithmic

average of the temperature difference between the hot and cold

streams at each end of the exchanger. The larger the LMTD, the

more heat is transferred.

Overall Heat Transfer Coefficient: The LMTD is a logarithmic

average of the temperature difference between the hot and cold

streams at each end of the exchanger. The larger the LMTD, the

more heat is transferred.

Fouling Factor: Thermal resistance offered by the accumulation of

deposits on heat transfer surface.

Shell and Tube Heat Exchanger

Shell

Tubes

- Tube Layout

- Tube Pitch

- Outer Tube Limit

Baffles

- Segmented Baffles

- Baffle Spacing

- Baffle Cut

- Orifice Baffle

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Plate & Frame Heat Exchanger

Tube Arrangement in Heat Exchanger

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Finned

Tubes

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Mass Transfer

Mass Transfer Operation

Distillation

Absorption

Stripping

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Distillation

Vapor-Liquid Equilibrium

Ideal Solutions – Raoult’s Law

For an ideal solution, the equilibrium partial pressure p* of a constituent at a fixed temperature equals the product of its vapor pressure p when pure at a fixed temperature and its mole fraction in the liquid.

pA* = pAx pB

* = pBx

Relative Volatility/ Separation Factor

α =[ y*/(1-y*)] / [x/(1-x)]

α = 1.0 then no separation possible and larger the value of α above unity, the greater the degree of separatibility.

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Continue..

Distillation: The function of distillation to separate, by vaporization, a liquid of miscible and volatile substance into its individual component or, in some cases into group of components.

- Atmospheric Distillation

- Vacuum Distillation

Flash Distillation: Flash is a single stage separation technique. A liquid mixture feed is pumped through a heater to raise the temperature and enthalpy of the mixture. It then flows through a valve and the pressure is reduced, causing the liquid to partially vaporize. Once the mixture enters a big enough volume, the liquid and vapor separate.

Batch Distillation

Azeaotropes: Constant-boiling mixture

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Continue..

Continuous Distillation with Reflux

- Enriching/Rectifying Section

- Stripping Section

- Condenser

- Reboiler

- Plates

Azeotropic Distillation: A third component component some times

called and entrainer may be added to the binary mixture to form an

new low-boiling azeotrope with one of the original constituent,

whose volatility is such that it can easily be seperated from the other

original constituent.

Extractive Distillation: A third component, termed as a solvent, is

added which alters the relative volatility of the original constituent

and thus permitting the separation.

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Figure 1: Continuous

fractionating column with

rectifying and stripping

sections.

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Tray Design

Bubble Cap Tray

Sieve Tray

Design and Operating Characteristics

Number of Plates in the column

Diameter of the column

Heat input to reboiler

Heat output from condenser

Spacing between the plates

Type of Plate

Feed Location

Method of Ponchon and Savarit

Method of McCabe and Thiele

Minimum Reflux Ratio

Total Reflux

Minimum Number of Trays

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Absorption

Solubility of a gas in a liquid

influenced by temperature and

pressure.

Choice of solvent for absorption:

- Gas Solubility

- Volatility

- Corrosiveness

- Cost

- Viscosity

- Miscellaneaous: Nontoxic,

nonflammable and chemically

stable etc.

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Continue..

Co-current Flow Absorption

Counter Current Flow Absorption

Loading: Occur because of higher gas/vapor flow rates.

Flooding: Flooding is brought about by excessive vapor flow,

causing liquid to be entrained in the column. Flooding is detected by

sharp increases in column differential pressure and significant

decrease in separation efficiency.

Weeping: This phenomenon is caused by low vapor flow. The

pressure exerted by the vapor is insufficient to hold up the liquid on

the tray. Therefore, liquid starts to leak through perforations.

Excessive weeping will lead to dumping.

Entrainment: Entrainment refers to the liquid carried by vapor up to

the tray above and is again caused by high vapor flow rates.

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Desorption/Stripping

Recovery of absorbed gas from rich solvent.

Increase in temperature and reduction in pressure.

Absorption at high pressure then large amount of recovery can be done by simply flashing.

Stage wise operation

Inert gas or steam as stripping medium

Steam as preferred stripping medium as it can be condensed.

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Fluid Flow Operations

Fluid Flow Operation

Newtonian (e.g. gases and most liquids)

non-Newtonian Fluids (e.g. sewage sludge)

Viscosity

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Reynolds Number

Boundary Layer

Flow through Pipes

- Continuity Equation

- Bernoulli Equation

- Hagen-Poiseuille Equation

Friction Factor Chart

- Friction loss from expansion

- Friction loss from contraction

- Friction loss from fitting & valves

Continue..

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Valves

Gate Valves

Plug Valve

Globe Valves

Ball Valve

Butterfly Valve

Needle Valve

Check Valves

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Gate Valve Plug Valve Ball Valve

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Globe Valve Butterfly Valve Check Valve

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Pumps

Positive Displacement Pumps

- Reciprocating Pumps

- Rotary Pumps

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Centrifugal Pumps

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Pump

Capacity: Quantity of liquid discharge per unit of time

Head: The energy supplied to liquid per unit weight, obtained by

dividing the increase in pressure by liquid specific weight.

Power: The energy consumed by a pump per unit time for supplying

liquid energy in the form of pressure.

Efficiency: The ratio of the useful hydraulic work performed to the

actual work input.

Cavitations: Whenever the liquid pressure at pump suction drops

below the vapor pressure, liquid boiling occurs, and the vapor

bubble form. vapor bubbles may reduce or stop the liquid flow, as

well as damage the system.

Break Horse Power: The brake horsepower is the amount of real

horsepower going to the pump, not the horsepower used by the

motor.

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NPSH

Net Positive Suction Head (NPSH): NPSH - can be expressed as

the difference between the Suction Head and the Liquids Vapor

Head.

Net Positive Suction Head (Required): In general determined

experimentally by the pump manufacturer in order to prevent

cavitation for safe and reliable operation of the pump.

Net Positive Suction Head (Available): Head made available the

suction system for the pump.

The available NPSHa of the system should always exceeded the

required NPSHr of the pump to avoid vaporization and cavitation of

the impellers eye.

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Fan, Blower & Compressor

Fans: Large fans are usually centrifugal, operating on exactly the

same principle as centrifugal pumps. Low discharge head, from 5 to

60 in.

Blowers & Compressors:

- Adiabatic Compression

- Isothermal Compression

- Polytropic Compression

- Compression Ratio = pa/pb, pa, pb= inlet and outlet pressures

For blowers compressor ratio below 3 or 4 however for compressor

it can be more than 10.

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Ejectors

A kind of vacuum pump that does not use moving is the jet ejectors,

in which the fluid to be moved is entrained in a high velocity stream

of second fluid.

Steam-jet Ejectors

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Reaction Engineering

Reaction Engineering

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Reaction Engineering

Elementary/ Non-elementary Reaction

Order of Reaction

Rate of Reaction: ri = [1/V][dNi/dt], V- volume, Nj- moles of jth

component and t - time

Rate Constant: k=k(t)

Conversion: X = [No– N]/No

Catalyst: To increase or decrease the rate of reaction

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Reactors

Batch Reactors

Semi Batch Reactors

Continuous Steady State Reactors

- Tubular Reactors: Plug Flow Reactors and Packed Bed Reactors

- CSTR (Continuous Stirred Tank Reactors)

Batch

Reactor PFR CSTR

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Batch Reactors

Constant Volume Batch Reactor

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CSTR

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Plug Flow Reactor

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Solid Catalyzed Reaction

Role of catalyst and its working

Catalyst Poison

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Packed Bed Catalytic Reactor

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Thank You