THIS MONTH'S PUZZLER
Gas temperature is gradually rising at the exit of our waste heat exchanger; the gas goes to a reactor and threatens to raise its catalyst bed temperature above the allowable limit. The exchanger extracts heat for steam from gas exiting a steam reformer (see Figure 1), whose feed rate is steady. De-mineralized water is on the shellside; hot water, H2S, H2, CO2, CO are on the tubeside. An analysis of the shellside condensate shows traces of iron and phosphate; an analysis of the tubeside shows traces of iron, potash and silicon. There are some similarities with the analysis of the desulfurization catalyst but our supplier denies the possibility of broken catalyst. What's causing the fouling? How can we solve the problem?
IT'S SILICA
We are currently having a similar problem with a waste heat boiler on the back end of our refinery sulfur-recovery reaction furnace. We lost 10% duty across the boiler in 6 months and shut down for an internal inspection and cleaning. The process side was fouled with a thin low-density material that looks like coal-fired fly ash. The material composition was primarily iron, sulfur, silicon, alumina, sodium and calcium. A review of the analysis found that sodium in the presence of silicon or alumina will form fouling deposits. I suspect you have something similar with the silicon forming a loose low-density low-thermal conductivity deposit on your tubes. For our boiler, further analysis has shown that a very thin uniform layer, i.e., 1/32 in., drops the heat transferred by 10%. If this loss of duty occurred immediately and persistently following the replacement of the catalyst, I would suspect catalyst fines or dust as the root cause. If you recently replaced any refractory inside your reformer, it, too, could be a source of silicon.
As to how to solve the problem, let me know when you figure it out as we are still working on that. It may be a weathering effect that will go away with time and repeated cleaning of the accumulated fouling, or it may require new waste heat boiler tubes to remove the deposit and tendency to reform deposits after cleaning.
Pete Bisila, system reliability engineer - utilities
Flint Hills Resources, St Paul, Minn.
REVIEW YOUR RECORDS
This process runs continuously so unless an outage occurs, we won't be able to take anything apart for a closer look. Laboratory data are a good place to start. First, confirm that the overall heat transfer coefficient has decreased — and by how much.
Examine the demineralized water assay over the past several months. Are there any spikes in the composition that could indicate poor water control? Slugs of salt could foul the heat exchanger shellside. Has the water treatment changed?
Iron and phosphate are typical. Steam and condensate gradually corrode iron pipe, but watch for excess chromium if you're using austenitic steels. Chromium can result if chlorides or caustics are present, even in the form of a spike, which is common in some water treatment.
Next, review the past history of process equipment. If none exists, talk to operators and mechanics to develop a history. Was any work recently performed? I once found a piece of Teflon tape stuck in a Y-strainer we wisely installed in front of a plate heat exchanger; a pipefitter had accidentally dropped it in new pipe. It's even better if you have an old exchanger or other equipment in the bone yard. Then, examination is possible. Old equipment isn't always cleaned thoroughly.
Don't leave out history on the desulfurization reactor and steam reformer. While unlikely, the beads in the reactor might have traveled unnoticed through the reformer and into the tubesheet of the exchanger. When was the bed changed out? What happens to the beads as they age? Are the beads friable? Have you recently changed catalyst or supplier? Do you have analyses of current and past catalysts? Although the screen at the bottom of the reactor is designed to contain new beads, breakage or disintegration could cause bead material to fill tubes. Also, don't forget to look at bead conditioning!
Let's consider the chemical analysis of the tubeside again. Traces of silicon could indicate insulation material but we would expect to see alumina, too. The potash is a mystery. To resolve this you must review the components of the material balance and the catalyst manufacturer assay.
Now, let's consider inspection. Think carefully about your tools when you open up the exchanger for an inspection. If possible, pull the unit and install a spare so the exchanger can undergo a shop inspection. Use a borescope to check tight corners in the shell and the tubing. Don't read too much into a borescope picture; the best tools in the hands of someone not an expert with them won't help you. A good history of the process is the best tool.
Dirk Willard, contract staff engineer
Hemlock Semiconductor, Hemlock, Mich.
NOVEMBER'S PUZZLER
Our decanter for separating a mixture of C10, C20, C30 and C40 compounds from a water/toluene solution isn't performing right. To save space, we opted for a 3-ft-diameter vertical unit with a length/diameter ratio of about 3:1. The flows are 6,000 pounds per hour (PPH) of organic and 2,500 PPH of aqueous phase. The viscosities are: 300 cP for the organic; 0.8 cP for the water. The specific gravities are: 0.8 for the organic; 0.99 for the water. Feed is at about 50% of the straight side length and we use a dispersion layer of about 5%. Although the decanter is designed to capture 100 micron (µ) droplets, the carbon filter that's supposed to serve as a water polishing filter actually is removing them. We've had to change it out every 30 hours at a cost of $250/500 pounds. On examination, we discovered that during construction 1-in. high-flow packing was dumped into the decanter. It fills the bottom third of the column. How can we fix the decanter? Should we be happy with 100 µ? How can we reduce our carbon costs?
Send us your comments, suggestions or solutions for this question by October 15, 2010. We'll include as many of them as possible in the November 2010 issue and all on CP.com. Send visuals — a sketch is fine. E-mail us at [email protected] or mail to Process Puzzler, Chemical Processing, 555 W. Pierce Road, 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.