Technology Targets Towers

Innovations promise improved distillation column performance.

By Seán Ottewell, editor at large

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Talks are underway with chemical companies and refiners about trials of the method at their sites. “Obviously it would be extremely useful to demonstrate this in an industrial setting. The downside is that commercial columns are not usually as instrumented as the one at UT so it’s a little harder to do. Of course a commercial column is making thousands of dollars a minute, too, so companies are extremely reluctant to mess with them. The economic downturn hasn’t helped either,” notes Pihlaja.

operating close to flood point
Figure 2. Pushing the bounds: Safely operating closer
to the flood point would boost capacity.
Source: Emerson.

In the meantime, a U.S. patent application has been filed on the modified 3051S. “Overall, our work demonstrated proof of concept: we can detect flooding based on a change in the physics inside a column,” he says.

The method also might serve to detect fouling of packing. “With a properly instrumented column, it should be possible to determine that flooding is occurring at a lower vapor/liquid loading in a fouled column than would be expected with a clean column. If you have a map of expected column performance [as in Figure 2], then, in principle, by calculating how much your fouled column deviates from clean performance, it should be possible to estimate the ‘degree of fouling’ and report this parameter as some sort of ‘column health’,” asserts Pihlaja.

Health Monitoring
Pressure drop (ΔP) across the column is the key issue today for members of Fractionation Research, Inc., (FRI), Stillwater, Okla., believes Mike Resetarits, its technical director. The non-profit research consortium’s membership includes top chemical and petrochemical companies, as well as vendors. (Frank Rukovena of FRI handles distillation questions for Chemical Processing's “Ask the Experts” feature.)

Resetarits likens checking ΔP to a doctor taking a person’s blood pressure: “When I give a distillation column a ‘physical,’ I start with the ΔP across the tower. Operators need to monitor ΔP hourly/daily. Increases can mean foaming or fouling. Decreases can mean acid attack on the trays/packing, or — maybe — the reflux rate isn’t what was planned.”

FRI is carrying out a number of research programs. Resetarits points in particular to a collaboration with researchers in the School of Chemical Engineering at Oklahoma State University (OSU), Stillwater, Okla. Under the supervision of instructor Rob Whiteley, graduate students there are investigating a novel approach to real-time identification of system hydraulic limit. It’s based on the premise that high-speed (10+ Hz) pressure or differential-pressure measurements can characterize the change in hydraulic state of a distillation column as it approaches the hydraulic limit. (The proposed mechanism is unrelated to the macro liquid accumulation phenomenon associated with monitoring column ΔP to detect flooding.)

Traditional linear signal-processing techniques can detect variations in the high-speed pressure signatures as the system approaches flood. Preliminary analysis of data collected at the FRI test facility has been encouraging.

If successful, the technology would increase the capacity of existing distillation equipment with negligible capital investment. However, further work is being deferred until other higher priority collaborative projects are finished, notes Resetarits.

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