How to Calculate Heat Load & Recovery Time

Chemical Engineering · Process Engineering

Once a solvent's boiling point at your working vacuum is known, the next question is a plant one: how much heat does it take to boil it off at a useful rate, and how long will the batch actually take? This is what separates a recovery step that finishes in two hours from one that drags on all shift.

The Formula

Where ṁ is the vaporization mass rate (kg/hr), λ is the solvent's latent heat of vaporization (kJ/kg) at the recovery temperature, and M_total is the total mass of solvent to be recovered from the batch.

Worked Example

Recovering 500 kg of a solvent with a latent heat of 360 kJ/kg, using a jacket that can sustainably deliver 40 kW of heating duty to the batch at the vacuum boiling point:

That's the sustained-boiling estimate. Real recovery time is usually somewhat longer once you account for the initial heat-up to boiling point and the tail end of the batch where the boil-up rate naturally falls off as the last of the solvent is stripped.

In the Plant

Available heat duty isn't fixed. It depends on jacket steam pressure (or hot water supply temperature), heat transfer area, and how fouled the jacket surface is. A jacket that delivered 40 kW when clean may only manage 30 kW after a few years of service, stretching recovery time well beyond this calculation.

Common Mistakes

Using latent heat at atmospheric pressure when the batch is actually recovering under vacuum at a lower temperature (latent heat does shift with temperature, though usually modestly); assuming the full nameplate heating duty is available throughout the batch rather than derating for fouling and real steam supply conditions; and not accounting for the slower boil-up rate typical near the end of a batch.