A key issue today is fouling. “Rigorous modeling is very important here if, for example, a stream contains materials that polymerize at a certain temperature and which would lead to a significant fouling deposit then good predictions can be vital. The rigorous exchanger model can predict local film temperatures within an operating process and allow the operator to ensure he is outside the limits where polymerization can take place. The process can be maintained with minimum downtime and maximum efficiency.”
Perstorp, Perstorp, Sweden, has used Aspen’s Tasc+ to optimize heat exchanger performance and eliminate downtime. “A most important factor in apparatus design is the fact that non-working equipment is often very expensive. Cost of downtime is $90,000 to $150,000/day for a typical large-scale polyol factory. For that reason a reliable design tool such as Tasc+ is of highest importance,” says Oleg Pajalic, process engineer.
The full impact of new technologies such as twisted tube exchangers from Koch Heat Transfer, Houston, Texas, and hiTRAN wire matrix inserts from Cal Gavin, Alcester, U.K., only can be gauged by a simulation including rigorous models of each exchanger, Ralston also points out. While modeling novel proprietary exchangers isn’t a trivial task, the biggest challenge is the conceptual design of the interface between the software tools, which relies on a strong collaboration between the partner companies, he says.
However, the rewards can be impressive: optimization of a large feed/effluent heat exchanger revamp on a major European plant by use of Tasc+ and hiTRAN inserts has resulted in a 20% increase in overall film coefficient, re-use of the existing shell, annual fired heater savings of $75,000 and a 1,700-m.t./yr decrease in carbon dioxide emissions.
Seán Ottewell is Chemical Processing's Editor at Large. You can e-mail him at email@example.com.