Download - CHAPTER 5 Chemical Equipment[1]
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Chemical Equipment
CHAPTER 5
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Chapter Contents
1. Type of chemical process2. Classes of chemical equipment3. Separation columns 4. Reactors5. Heat transfer equipment
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Separation columns
• Distillation columns• Absorption• Extraction
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Distillation column
• Distillation is probably the most widely used separation process in the chemical and allied industries; its applications ranging from the rectification of alcohol, which has been practised since antiquity, to the fractionation of crude oil.
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Definition & general description of the process
• Separating the various components of a liquid solution
• Depends upon the distribution of these components between a vapor phase & a liquid phase
• Distillation is done by vaporizing a definite fraction of a liquid mixture in a such way that the evolved vapor is in equilibrium with the residual liquid
• The equilibrium vapor is then separated from the equilibrium residual liquid by condensing the vapor
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Continuous Distillation
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Laboratory / Testing
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Physical Concept of distillation• Carried out by either 2 principal methods• First method: based on the production of a vapor
by boiling the liquid mixture to be separated and condensing the vapors without allowing any liquid to return to the still - NO REFLUX (E.g. Flash, simple distillation)
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• Second method: based on the return part of the condensate to the still under such condition that this returning liquid is brought into intimate contact with the vapors on their way to the condenser – conducted as continuous / batch process (E.g. continuous distillation)
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Continuous Distillation
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Distillation column design
The design of a distillation column can be divided into the following steps:1. Specify the degree of separation required: set product
specifications.2. Select the operating conditions: batch or continuous;
operating pressure.3. Select the type of contacting device: plates or packing.4. Determine the stage and reflux requirements: the
number of equilibrium stages.5. Size the column: diameter, number of real stages.6. Design the column internals: plates, distributors, packing
supports.7. Mechanical design: vessel and internal fittings.
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Distillation column
Parameters in DC:• Reflux ratio • Total reflux
– Total reflux is the condition when all the condensate is returned to the column as reflux. No product is taken off and there is no feed. Minimum of stages.
• Minimum reflux– Separation at infinite no. of stages.
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Industrial Reactors • Batch reactor• Continuous-stirred Tank Reactor (CSTR) • Plug Flow Reactor (PFR) • Packed-bed reactor (PBR)• Fluidized bed reactor (FBR)• Slurry reactor• Semi-batch reactor • Trickle bed reactor
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Batch Reactor
Characteristics• The simplest reactors used in chemical
processes• It is closed systems; systems in which no
materials enters or leaves the reactor during the time the reaction takes place
• It is operated under unsteady-state conditions; process in which the conditions inside the reactor change over time.
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Batch ReactorOperation• The reactants are placed into
the reactor.• Stop the reactants flow and
then start the reaction process which allowed to react, and products form inside the reactor.
• After a specified time, stop the process, and the products and unreacted reactants are then removed.
• The process is repeated.
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Batch Reactor
Application • Typically used for liquid phase reactions
that required long reaction time• Also used when a small amount of
products is desired• And used when a process is still in the
testing phase/when the product is expensive
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Batch Reactor
Example of application • Pharmaceutical industry
to produce drugs• Fermentation;
production of beer or ale
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Batch Reactor
Advantages• High conversions can be obtained by leaving
reactants in reactor for extended periods of time.• Versatile; can be used to make many products
consecutively.• Good for producing small amounts of products
while still in testing phase.• Easy to clean.
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Batch Reactor
Disadvantages• High cost of labor per unit of production• Difficult to maintain large scale production• Long downtime for cleaning leads to
periods of no production
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Continuous-Stirred Tank Reactor (CSTR)
Characteristics• CSTR is a open systems; a system in which
material is free to enter or exit the reactor• It is operated under steady-state condition;
conditions in the reactor are constant with time.• Reactants are continuously introduced into the
reactor while products are continuously removed.• CSTR is very well mixed; the contents have
relatively uniform properties (T, density etc.) throughout the reactor.
• Conditions in the reactor’s exit stream are the same as those inside the tank.
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Continuous-Stirred Tank Reactor (CSTR)
Operation• Reactants are fed continuously into the
reactor• The contents of the tank are well mixed by
the stirring/impeller device• Products are removed continuously during
the reaction process
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Continuous-Stirred Tank Reactor (CSTR)
Application • CSTR is most commonly used in industrial
processing • Primarily in homogeneous liquid-phase
flow reactions
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Continuous-Stirred Tank Reactor (CSTR)
Advantages• Good temperature control is easily maintained• Cheap to construct• Reactor has large heat capacity • Interior of reactor is easily accessed
Disadvantages• Conversion of reactant to product volume of
reactor is small compared to other flow reactors
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Plug Flow Reactor (PFR)
Characteristics• Also known as tubular reactor• Consist of hollow pipe or tube through
which reactants flow• Operated at steady-state• Reactants are continually consumed as
they flow down the length of the reactor
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Plug Flow Reactor (PFR)
Operation • Reactants are continuously fed into the reactor
from the left• As plug flow down the reactor the reaction takes
place• This would result in an axial concentration
gradient; change in concentration over a distances from left to right.
• Products and unreacted reactants flow out of the reactor continuously
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Plug Flow Reactor (PFR)
Application • Wide variety of applications in either gas
or liquid phase systems.• Common industrial uses;
– gasoline production– oil cracking – synthesis of ammonia from its elements– oxidation of sulfur dioxide to sulfur trioxide
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Plug Flow Reactor (PFR)
Advantages • High conversion rate per unit reactor
volume• Good for large capacity processes • Good for studying rapid reactions• Unvarying product quality
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Plug Flow Reactor (PFR)
Disadvantages• Reactor temperature difficult to control• Hot spots may occur within reactor for
exothermic process• Difficult to control due to temperature and
composition variations
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Packed-bed Reactor (PBR)
Characteristics• Also known as fixed bed reactor• Often used for catalytic processes• Consist of cylindrical shell with convex
heads• Most are vertical, and allow reactants to
flow by gravity
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Packed-bed Reactor (PBR)
Operation• Reactants enter the reactor on the top and flow
through• Upon entering the reactor, the reactants flow
through the packed bed of catalyst• By contacting with the catalyst pellets, the reactants
react to form products• Then the products exit the reactor on the bottom• Note; concentration gradient within the reactor. The
concentration of reactants decreases from top to bottom
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Packed-bed Reactor (PBR)
A porous bed of catalyst particles is fixed in a tube, and the reactants pass the bed. The fixed bed reactor is simple to build
and operate
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Packed-bed Reactor (PBR)
Application• Widely used in small scale commercial
reactions• Example; catalytic cracking,
CO + H2O → CO2 + H2
C6H5CH2CH3 → C6H5CH=CH2 + H2
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Packed-bed reactor (PBR)Advantages • High conversion rate per weight of catalyst• Easy to build• More contact between reactant and catalyst than
in other types of reactors• More product is formed due to increased
reactant/catalyst contact• Effective at high temperatures and pressures• Low cost of construction, operation and
maintenance
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Packed-bed reactor (PBR)
Disadvantages• Reactor temperature difficult to control• Side reaction possible• Catalyst difficult to replace• Temperature gradients may occur
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Fluidized-bed Reactor (FBR)• FBR is a heterogeneous catalytic reactor in which the
mass of catalyst is fluidized• Fluidized; a process whereby a fluid is passed through a
mass of solids, giving them fluid characteristics• This allows for extensive mixing in all directions• A result of the mixing is excellent temperature stability
and increased mass-transfer and reaction rates• FBR is capable of handling large amounts of feed and
catalyst
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Fluidized-bed Reactor (FBR)Operations• Before the reactor is started the catalyst pellets
lie on a grate at the bottom of the reactor• Reactants are pumped into the reactor through a
distributor continuously, causing the bed to become fluidized
• The reactants react due to the presence of the catalyst pellets, forming products that are removed continuously
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Fluidized-bed Reactor (FBR)
Application• Commonly used in catalytic cracking processes• Also used in
– the oxidation of naphthalene to phtalic anhydride, – roasting of sulfide ores,– coking of petroleum residues, – calcination of limestone
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Fluidized-bed Reactor (FBR)
• The high velocity gas causes the catalyst bed to
behave like a fluid, which gives good
heat and mass transfer
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Fluidized-bed Reactor (FBR)
Application• Often used when there is a need for large
amounts of heat input or output• Or when closely controlled temperatures
are required• Example; FBR contained microbes • used when to break down contaminants in
the effluent from a chocolate factory
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Fluidized-bed Reactor (FBR)
Advantages • Even temperature distribution eliminates hot
spots• Catalyst is easily replaced or regenerated• Allows for continuous, automatically controlled
operations• More efficient contacting of gas and solid than in
other catalytic reactors
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Fluidized-bed Reactor (FBR)
Disadvantages• Expensive to construct and maintain• Erosion of reactor walls may occur• Regeneration equipment for catalyst is expensive• Catalyst may be deactivated • Can’t be used with catalyst solids that won’t flow
freely• Large pressure drop• Attrition, break up of catalyst pellets due to impact
against reactor walls, can occur
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Reactor Design• The characteristics normally used to
classify reactor design:– Mode of operation: Batch or Continuous– Phases present: Homogeneous or
Heterogeneous– Reactor geometry: flow pattern and manner of
contacting phases:• Stirred tank• Tubular • Packed bed• Fluidised bed
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Reactor Design procedure1. Collect kinetic and thermodynamic data on the
desired reaction (T, P, flowrate)2. Data on physical properties is required for the
design of the reactor (literature/lab)3. Rate controlling mechanism e.g. kinetic, mass
or heat transfer.4. Choose a suitable reactor type5. Selection of optimal reaction conditions is
initially made in order to obtain the desired yield6. The size of the reactor is decided and its
performance estimated.
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Reactor Design procedure
7. Materials for the construction of the reactor is/are selected.
8. A preliminary mechanical design for the reactor including the vessel design, heat transfer surfaces etc., is made.
9. The design is optimized and validated10. An approximate cost of the proposed
and validated design is then calculated.
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Heat transfer equipment
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• A heat exchanger is used to exchange heat between two fluids of different temperatures, which are separated by a solid wall.
• Applications in heating and air conditioning, power production, waste heat recovery, chemical processing, food processing, sterilization in bio-processes.
• Heat exchangers are classified according to flow arrangement and type of construction.
HEAT EXCHANGERS
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Heat transfer equipment
• How they work?
• Example: refrigerator & air-conditioner
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Heat Exchanger Types1)Parallel Flow – hot and cold fluids enter at
the same end, flow in the same direction and leave at the same end.
Parallel Flow CounterflowParallel Flow Counterflow
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Heat Exchanger Types2) Counter Flow – hot and cold fluids enter at opposite ends, flow in opposite directions and leave at opposite ends.
Parallel Flow CounterflowParallel Flow Counterflow
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Shell and Tube Heat Exchangers
Heat Exchanger Types
One Shell Pass,Two Tube Passes
Two Shell Passes,Four Tube Passes
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Heat exchanger• Parameters:
– Overall heat transfer coefficient, U • The overall heat transfer coefficient defined
in terms of the total thermal resistance to heat transfer between two fluids
– Mean temperature difference,– Fouling factor
• Fluid impurities, rust formation, or other reactions between the fluid and the wall material.
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Heat exchanger analysis• 2 methods to determine the heat exchanger
characteristics:– The effectiveness NTU method (ξ- ntu
method) • It is used when only the fluid inlet temperature
are known– Log Mean
Temperature Difference (LMTD)• it is used when the fluid inlet temperatures are
known and the outlet temperature are specified or readily determined.
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HE design procedure1. Define the duty; heat transfer rate, fluid
flowrates, temperatures2. Collect physical properties data of fluids
(e.g. density, viscosity, thermal conductivity)
3. Decide on the type of HE to be used4. Select a trial value for U5. Calculate mean temp difference6. Calculate area required
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HE design procedure
7. Calculate individual coefficients8. Calculate overall coefficient 8. Calculate pressure drop.