Distillation Modeling
CHEN 4470 – Process Design Practice
Dr. Mario Richard Eden
Department of Chemical Engineering
Auburn University
Lecture No. 14 – Design of Overhead Condensers and Air-cooled HX
February 26, 2013
Material Developed by Dr. Jeffrey R. Seay, University of Kentucky - Paducah
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.
2-Phase
Flow Region
• Condenser can be in the
vertical or horizontal position.
- Horizontal – Process on
the shell side.
- Vertical – Process is on
the tube side.
• Elevation is critical.
Partial Condenser
Vapor
Distillate
• 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.
Liquid
Distillate
• Elevation is not critical.
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.
Liquid
Distillate
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
•
Typical Air-Cooled Condenser
Process Out
Process In
V FD
V FD
Tube Detail
Design Equations:
Q  U  A  Tlm  m  Cp  (TAir,out  TAir,in )
U
Note:
1
k
 C  Re m  Pr 3
OD
Air-cooled HX are primarily
used when no other cooling
utilities are available on site
The surface area, A, includes the area of the fins!
C and m are parameters based on the size of the fin.
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
Other Business
•
Next Lecture – February 28
–
–
•
Integration of design and control part I
SSLW 322-340
Individual Team Assignments
–
–
–
Will be assigned over next two week period
Choose one from the project description or
Email me two sentences describing what you would like
to investigate and I will respond with the ”official”
problem statement.