A client ordered a course session on distillation scale-up. I thought about this challenge and this triggered my memory on the subject. In the equipment engineering department of my previous employer 4-6 people developed distillation trays and packings for over 50 years. In my work as plant process engineer for bulk chemical plants, distillation columns always required attention, and a wastewater treatment process based on crystallisation was replaced successfully by a complex distillation train. So, I have taken up the challenge to develop a course session of half a day on distillation scale-up from idea to commercial implementation based on my experiences combined with textbook knowledge.
Scale-up is about risk reduction. The course focuses therefore on risk reduction in all innovation steps from idea to commercial implementation.
- The first risk reduction is getting reliable vapour-liquid equilibria in the concept stage. Often researchers rely on physical properties and data bases of flow sheets. However, experimental validation is essential to avoid total design failure.
- The second risk reduction is finding out whether foaming plays a role. Foaming is in general caused by tiny amounts of components that stabilise prefer the vapour-liquid interphase. Hence foaming cannot be predicted by the chemical feed component analysis but needs to be tested using the real feed applied at commercial scale.
- The third risk reduction is finding out whether column internals fouling plays a role and then take measures to counteract that fouling. Specialised technology providers can play a role by installing liquid jets cleaning the internals during operation.
- The fourth risk reduction is avoiding designing with a wrong correlation for the Height Equivalent of a Theoretical Plate, HETP. HETP is a practical concept used in industry to quickly size the distillation column internals height. That total height is simply the number of theoretical plates multiplied by the HETP value.
HETP values are determined experimentally for many distillation systems and many correlations have been derived from them. These correlations are found in libraries of large companies and in Engineering Procurement, Construction (EPC) companies and are also provided by textbooks on distillation.
However, a wrong correlation selection for a certain application can be easily made. As senior technologist at a bulk chemical site at least twice a junior technologist came to me to find out why a newly installed column did not perform according to its design, while everything in the design looked okay. It appeared that in these cases a HETP value was used for oil refinery distillations. In oil refinery distillations the vapour-liquid mass transfer is limited on the gas-phase side, so is governed by kg . As kg increases nearly linear with the superficial gas velocity, the HETP is a constant in a certain range and is reported as a fixed value.
In bulk chemicals columns under investigation however, mass transfer was limited on the liquid side, hence governed by the mass transfer coefficient kl . This parameter is easily a factor 10-100 smaller than the gas side mass transfer coefficient and that was the reason for the poor column performance.
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