Ikrananka :
Prior to spending a lot of time and money, it might be advisable to instrument the heat exchanger with a vibration analyzer and look at the existing spectrum compared to the number of passes, tube length, pump rotational frequency, and flow rates. It might be quite revealing...
As Greg87 and IRstuff point out, DI and Distilled are very different.
Distilled is a simple boiling/vapor condensation process which does not remove similar boiling point volatiles nor does it eliminate dissolved oxygen and CO2. Both of which are "terminal" when it comes to a high...
It is very important that one identify whether or not one is interested in saturated vapor, superheated vapor, or saturated liquid properties. Supercritical properties are a world unto themselves. Mixtures would invite a further complexity.
Peng Robinson is quite applicable to Petroleum...
