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

Overview

• Purpose of Overhead Condensers– Provide reflux flow to the top of a column and

acts as an additional theoretical stage above the trays/packing

• Types of Condensers– Knock-back reflux– Total condenser– Partial condenser

Proper condenser design is critical to the optimum operation of a column!

Knock-Back Condenser

• This design is easy to operate, but difficult to achieve precise control.

• Condenser will be larger due to the bi-directional traffic on the process side.

• Condenser can be in the vertical or horizontal position.

• Elevation is critical.

­ Horizontal – Process on the shell side.

­ Vertical – Process is on the tube side.

2-Phase­Flow­Region

Partial Condenser

• Used when vapor product is required, or if non-condensible gases are present in the overhead product.

• Controlling the vapor stream split can be challenging.

• Condenser can be in the vertical or horizontal position.­ Horizontal – Process on the shell side.

­ Vertical – Process is on the tube side.

• Elevation is not critical.

Liquid­Distillate

Vapor­Distillate

Total Condenser

• Total condensers are used when no non-condensable gases are present.

• As with other condensers, vertical or horizontal operation is possible.

• Elevation is not critical for a total condenser.

• Subcooling of the reflux can be a problem with overhead condensers, especially with vertical designs.

Liquid­Distillate

Condenser Design

• Considerations– The overhead condenser is often a critical part of

the control concept of a column

– Because distillation is based on VLE principles, the overhead condenser can be used to control the temperature and pressure in the column

– A later lecture will present some control concepts and strategies for overhead condensers

– Condensers in series can also be used to optimize utility consumption

Condensers in ASPEN Plus

Air-Cooled Heat Exchangers

VFDVFD

Process­In Process­Out

Design Equations:

31

PrRe mCOD

kU

)( ,, inAiroutAirlm TTCpmTAUQ

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 primarily used when no other cooling utilities are available on site

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

• Next Lecture – February 21– Equipment sizing and pinch analysis

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