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.

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

Continuous Distillation

Laboratory / Testing

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)

• 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.

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

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.

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.

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

Batch Reactor

Example of application • Pharmaceutical industry

to produce drugs• Fermentation;

production of beer or ale

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.

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

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.

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

Continuous-Stirred Tank Reactor (CSTR)

Application • CSTR is most commonly used in industrial

processing • Primarily in homogeneous liquid-phase

flow reactions

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

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

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

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

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

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

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

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

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

Packed-bed Reactor (PBR)

Application• Widely used in small scale commercial

reactions• Example; catalytic cracking,

CO + H2O → CO2 + H2

C6H5CH2CH3 → C6H5CH=CH2 + H2

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

Packed-bed reactor (PBR)

Disadvantages• Reactor temperature difficult to control• Side reaction possible• Catalyst difficult to replace• Temperature gradients may occur

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

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

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

Fluidized-bed Reactor (FBR)

• The high velocity gas causes the catalyst bed to

behave like a fluid, which gives good

heat and mass transfer

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

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

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

• 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

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

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

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.