Breathing New Life into Old Packed Beds

Professor Mark McCready
Department of Chemical & Biomolecular Engineering
Notre Dame University

Date: Thursday, Oct 16, 2003
Time: 4:00 p.m.
Place: Engineering II, Room 3361

ABSTRACT

Catalytic packed bed reactors are commonly used in the chemical process industries and thus much is known about their behavior. Replacements for packed beds, such as monoliths and stacks of micro reactors have been proposed for producing modest amounts of materials (kg/day) and, at least in the case of monoliths, for production at much larger scales.

This talk will discuss the fundamental differences between packed beds and these more modern reactors and the difficulties (e.g., heat removal, operating range) that are inherent in design that employ unconnected passages.

New results on fluid mechanics and reaction behavior of gas-liquid packed beds will be presented. We have found that pulses form from convective instabilities of two different basestates – either the “trickle” basestate or the bubbly flow basestate. Stability models based on the two-phase dispersed flow equations show some success at prediction of the instability and provide some insight into the mechanisms, but important questions remain. Specifically is it plausible to say that interfacial tension is a stabilizing force? A direct affect on the reaction (hydrogenation of Phenylacetylene) behavior by pulsing flow is shown by locating the catalyst either in a small region at the top of the column, before pulses form, or at the bottom of the column (once they have reached full strength). Theoretical results indicate that very large affects on reaction selectivity could be achieved if the frequency of pulses matched the reaction time scales. This raises the potential that gas-liquid flow packed bed reactors can be “tuned” to produce optimal product distributions – much better than could be achieved in monolith or micro reactors that lack the ability to form the large scale pressure and flow fluctuations that comprise pulses.