November Process Puzzler: Consider options for automation

Readers suggest ways to automate an HCl absorber in this month's Process Puzzler in Chemical Processing.

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Girish Malhotra, President
EPCOT International, Pepper Pike, Ohio

What about chromatography?

Perhaps high performance liquid chromatography (HPLC) could work. There are selective columns used to remove chlorides prior to isolating organics. If a gel selective to chlorides can be found, this method might be superior to those suggested like titration.

Furst Moore, retired engineer
Sun Oil, Hallsville, Texas

Low-tech solution

I’d make a home made densitometer. I’d take a small slip stream off the circulation pump to the top of the scrubber through a globe valve into a home made densitometer made from plastic lined pipe. The densitometer would be made from a vertical 3-in pipe, say, 50-in tall with the high and low taps for an electronic dP cell installed at 0 and 40 in. The bottom outlet of the densitometer would go to a loop seal back to the process. The loop seal would be designed to keep the densitometer level constant at 40 inches. The 4-20 mA output of the dP cell would be calibrated with hot 15% HCl and 20% HCl solutions. Hopefully the plant could use this densitometer to set up a trim controller on the water make up to hold 18.5% HCl. High alarms could be set to prevent operation above 20% HCl. This system should be insulated as temperature affects all the online properties that are being measured (pH, cond, titration, etc.).

If space isn’t a problem, the height of the column could be increased to improve the resolution. A capillary-filled dP cell with sensitive waffle heads should work in this application. I learned this trick from Catalytic Cracking units. It was used to get a “pseudo density” for control. I believe the “pseudo density” could be correlated to the actual density for Title V environmental compliance. Sometimes low tech is better.

Accumulation of junk in the column could be a problem. The answer to this is simple. Install a second pump-around wash zone scrubber downstream of the existing one. Fresh water could be feed to the top wash zone and totally drawn off for recirculation. This will eliminate the environmental problem. This recirculation water could be feed as make-up water to the original scrubber. You should draw off 18.5% HCl the original scrubber to a dual tank system where periodic average samples could be manually analyzed and HCl blend draw rates adjusted. While one tank is being transferred to the mill the second could be filling.

Timothy Goebel, engineer
Citgo, Nederland, Texas

January's Puzzler

During our plant shutdown a flow control valve was replaced (Figure 2). The globe valve charges a reactor used for making a volatile organic chemical, an intermediary to a polymer. The previous valve served faithfully for several years, though we had trouble with it once because the heat tracing failed. The board operator said that flow dropped to about half, which took longer to fill the reactor. Although the new control valve served well for a few weeks it is now sticking — and it is getting worse. For some reason, the flow problem has also returned. Obviously, we need to pull the valve. What should we be looking for?

Figure 2. The installation for the flow control valve.
Figure 2. The installation for the flow control valve.

Send us your comments, suggestions or solutions for this question by December 12, 2006. We’ll include as many of them as possible in the January 2007 issue and all on Send visuals — a sketch is fine. E-mail us at or mail to ProcessPuzzler, Chemical Processing, 555 W. Pierce Rd., Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.

And, of course, if you have a process problem you’d like to pose to our readers, send it along and we’ll be pleased to consider it for publication.


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