Technology Targets Towers

Innovations promise improved distillation column performance.

By Seán Ottewell, editor at large

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Distillation columns play a key role at many plants and also often account for a significant portion of a site’s operating costs. So, vendors and research groups are expending considerable effort to enhance tower operation.

For instance, work underway aims to provide early warning about flooding (Figure 1). This afflicts numerous columns — stopping them from separating feed mixture into purified distillate and bottom products, and thus limiting capacity and prompting process upsets. Detecting incipient flooding can be tricky but a new approach promises to significantly help.

It relies on a modified Rosemount 3051S, a scalable device that typically handles integrated pressure, flow and level measurements around the plant. Roger K. Pihlaja, principal engineer – process diagnostics for the Rosemount Division of Emerson Process Management, Chanhassen, Minn., and A. Frank Seibert, technical director of the Separations Research Program (SRP) at the University of Texas Austin, Texas, (and liquid extraction guru for our online “Ask the Experts” feature) first revealed details about the development at the October 2008 Emerson Global Users Exchange in Washington, D.C.
column flooding
Figure 1. Column flooding: Viewport shows flooding
taking place in column during trial run.
Source: Emerson.

Pihlaja and Seibert have added a noise detector to the 3051S. “Bubbles are a white noise generator and as there is a detector built into the 3051S it means we have a new technique to detect something that is intrinsically present during flooding. As far as I know it’s the first time that this kind of signal processing has been built into an industrial device,” Pihlaja explains.

It should be possible to develop active incipient flooding control in a way that would give a small capacity increase on any column, the pair contends. This is because the new technology allows the column to operate in the non-linear region that currently is considered too unstable to consider (Figure 2).

Trials took place in 2008 at the SRP’s pilot facilities in Austin, which have a number of distillation and extraction columns suitable for studying all aspects of the processes involved — from control to the hydraulic characteristics of random and structured packings. The runs used 100% reflux; using less reflux currently isn’t feasible at SRP, notes Pihlaja. “Obviously this is a problem in a university setting because with an 18-in diameter column you are going to get a lot of product that will need recycling. They would need to install equipment to deal with this in order to run at finite reflux ratios.”
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