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CHEN 4470  –  Process Design Practice 

Dr. Mario Richard EdenDepartment of Chemical Engineering Auburn University

Lecture No. 13  –  Design of Overhead Condensers and Air-cooled HX 

February 16, 2012

Material Developed by Dr. Jeffrey R. Seay, University of Kentucky - Paducah

Distillation Modeling

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Overview

• Purpose of Overhead Condensers

 –  Provide reflux flow to the top of a column and acts asan additional theoretical stage above the trays/packing

•

Types of Condensers –  Knock-back reflux

 –  Total condenser

 –  Partial condenser

Proper condenser design is critical to theoptimum operation of a column!

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Knock-Back Condenser

• This design is easy to operate,

but difficult to achieve precisecontrol.

• Condenser will be larger dueto the bi-directional traffic onthe process side.

• Condenser can be in thevertical or horizontal position.

• Elevation is critical.

- Horizontal  –  Process onthe shell side.

- Vertical  –  Process is onthe tube side.

2-Phase

Flow Region

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Partial Condenser

• Used when vapor product isrequired, or if non-condensiblegases are present in theoverhead product.

• Controlling the vapor stream

split can be challenging.

• Condenser can be in thevertical or horizontal position.

- Horizontal  –  Process onthe shell side.

- Vertical  –  Process is onthe tube side.

• Elevation is not critical.Liquid

Distillate

Vapor 

Distillate

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Total Condenser

• Total condensers are used when

no non-condensable gases arepresent.

• As with other condensers, verticalor horizontal operation is possible.

• Elevation is not critical for a totalcondenser.

• Subcooling of the reflux can be aproblem with overhead

condensers, especially with verticaldesigns.

Liquid

Distillate

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Condenser Design

• Considerations

 –  The overhead condenser is often a critical part of thecontrol concept of a column

 – 

Because distillation is based on VLE principles, theoverhead condenser can be used to control thetemperature and pressure in the column

 – 

 A later lecture will present some control concepts andstrategies for overhead condensers

 –  Condensers in series can also be used to optimize utility

consumption

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Condensers in ASPEN Plus

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 Air-Cooled Heat Exchangers

VFDVFD

Process In Process Out

Design Equations:

31

Pr Re mC 

OD

k U 

)( ,, in Air out  Air lm T T CpmT  AU Q

Tube Detail

Note: The surface area,  A , includes the area of the fins!

C  and m  are parameters based on the size of the fin.

• Typical Air-Cooled Condenser 

 Air-cooled HX are primarilyused when no other coolingutilities are available on site

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 Air-Cooled HX Design

• Design Considerations

 –  Natural convection in turndown cases

 –  Changing ambient air conditions

 –  Effect of precipitation on operation of unit

 – 

Possibility of water-hammer inside tubes

• Possible Control Parameters

 –   Air flow across the tube bank 

 –  Position of louvers on top of the unit

 –  Liquid level inside the tubes

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• Next Lecture  –  February 21

 –  Equipment sizing and pinch analysis

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