Tower Doctor: Can the Cure be as Bad as the Illness?

Jul 01, 2023

August 1, 2023 | By Henry Z. Kister

Henry Kister shares lessons learned from troubleshooting distillation towers

During a course that I was presenting in Houston one year, a participant asked me the following at the end of one of the sessions: “Do you have experience with BX-type wire-gauze structured packings?”

“Sure,” I responded. “Wire gauze packings are used extensively in deep-vacuum clean chemical applications. In some deep-vacuum specialty chemical finishing columns, they are the ‘standard’ packing used.”

The course participant further inquired: “Their vendors claim they give a very good efficiency — about five stages per meter. What is your experience?”

“I have had excellent experience with them. As long as the tower and internals are clean and well-designed, the liquid loads are not too high, and the distribution is good, they usually match the vendor claims. FRI [Fractionation Research Inc., www.fri.org] tested them with excellent results that were published in the open literature. I plotted the FRI data in my Distillation Design book [1],” I explained.

“This is not what we are seeing in our tower. We are getting one stage per meter. Nowhere near the five stages per meter the vendor claims. The separation is terrible,” he continued.

“Do you have a drawing of your distributors?” I asked. Our failure survey [2] showed that 80% of packed tower failures are due to poor liquid distribution. So looking at the distributors is a good starting point.

He pulled out drawings of the distributors. I took some time to review and understand them, then did a couple of sums. All looked good.

“These are good designs,” I stated. “I do not see any problems with them.”

He looked proudly at me. “These are our own designs. I’m glad you approve.”

I was at a loss. I expected something major to be wrong with the distributors. I found nothing.

“Did you inspect the installation?” I then asked.

“You bet. Every bit of it. Everything was like it should have been,” he replied.

“I give up. All looks good. I have no idea why you are getting such low efficiency.”

I started packing up, but then something stopped me. I turned to him and asked, “Usually the packing vendor designs the distributors. But you said they were your design.”

“Indeed,” he said. “The original distributors were designed by the vendor. They did not work. So we replaced them with our designs that you liked.”

“What was wrong with the vendor’s distributors?” I asked.

“They used to plug all the time with solids that came in with the feed and reflux,” was his response.

The exploratory questions continued: “Why didn’t you install filters on these streams?”

“We had filters. They did not work.”

“What was wrong with the filters?”

“They used to plug up every few hours. Our operators got tired of cleaning baskets twice per shift. So they pulled out the baskets.”

At that, a moment of insight arrived. I then said, “Do you realize what happened? [Figure 1] When you removed the filter baskets, the solids traveled into the distributors and plugged them. Then you changed the distributors to a design that does not plug and allows the solids through. Do you want to guess where the solids are now?”

FIGURE 1. This diagram shows the tower containing wire-mesh packing that experienced poor efficiency (Diagram courtesy of H. Z. Kister)

“You think they are in the packing?” he asked after a short hesitation.

“You got it. BX-type gauze packings are excellent filters, except that they cannot be cleaned online. Plugged packings give poor efficiency. You are lucky that at least the liquid is going through.”

Takeaway: A cure can be just as bad as the illness. Always keep the big picture in mind. ■

Edited by Dorothy Lozowski

1. Kister, H. Z., “Distillation Design”, McGraw-Hill, New York 1992.

2. Kister, H. Z., “What Caused Tower Malfunctions in the Last 50 Years?”, Trans. IChemE, Vol 81, Part A, p. 5, January 2003.

Henry Z. Kister is a senior fellow and the director of fractionation technology at Fluor Corp. (3 Polaris Way, Aliso Viejo, CA; Phone: 949-349-4679; Email: [email protected]). He has over 35 years of experience in design, troubleshooting, revamping, field consulting, control and startup of fractionation processes and equipment. Kister is the author of three books, the distillation equipment chapter in Perry’s Handbook, and over 140 articles, and has taught the IChemE-sponsored “Practical Distillation Technology” course 550 times in 26 countries. A recipient of several awards, Kister obtained his B.E. and M.E. degrees from the University of New South Wales in Australia. He is a member of the NAE, a Fellow of IChemE and AIChE, and serves on the FRI Technical Advisory and Design Practices.