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 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-Phase Flow Region

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. Liquid Distillate Vapor Distillate

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 Design Equations: 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 28 –Integration of design and control part I –SSLW Individual Team Assignments –Will be assigned over next two week period –Choose one from the project description or – me two sentences describing what you would like to investigate and I will respond with the ”official” problem statement. Other Business