*5.1. Practical Considerations*

In almost all interviews, practical considerations were stated as being important for implementing HEN retrofit proposals. The most commonly discussed practical consideration was spatial restrictions in the plant. Other issues that were stated repeatedly were limitation in time and space for making process modifications during expensive turn-around periods and the high cost of equipment operating at high pressure. Although most retrofit proposals involve practical difficulties, it was generally considered that these issues can usually be solved. However, the proposed solutions to the practical problems lead to higher costs that should be accounted for when designing HEN retrofit proposals. For example, retrofit proposal 1A includes doubling of the surface area of a large existing heat exchanger to achieve higher internal heat recovery. There is limited space available in the affected process unit, which makes doubling of the heat exchanger size difficult. However, if the existing shell-and-tube exchanger were to be replaced with new efficient plate heat exchangers, the increased heat recovery could be achieved in a smaller space than the space currently occupied by the existing heat exchanger. To enable cleaning of the new plate heat exchanger during operation, it would be necessary to have two plate exchangers in parallel, but they would still occupy less space than the original shell-and-tube heat exchanger. This was confirmed by a mechanical engineer during interview 10 as well as during the validation seminar:

*"Yes, if they are replaced with plate exchangers that should not be an issue. It would require more piping but that would require less space, so I do not see any issues with that."*

#### *Mechanical engineer, heat exchangers and air coolers*

Increased pressure drop caused by increased heat exchanger area was highlighted during several interviews as something that always needs to be taken into consideration. Both new and extended existing heat exchangers will lead to increased pressure drop. Pressure drop was mentioned both during the interviews and the seminar. In particular, several interviewees stated that if the pressure drop is too large for the current pump capacity, new pumps will be necessary, and the total investment cost will probably be too high for the retrofit proposal to be implemented.
