Deftly Deal With A Troubled Tower

Jan. 8, 2020
Avoiding an unscheduled shutdown demands prudent action

This Month’s Puzzler

The reboiler on our 30-year-old distillation column (see figure) finally has failed us; the tubes have been replaced four times. However, we would like to avoid an emergency shutdown and wait until a scheduled outage in two months. Recently, we added a duplicate second column; we installed that column to allow boil-outs because changes in feedstock caused fouling every quarter year in the old column. Product demand has both columns running as hard as possible. Any suggestions for how we can maintain production? If need be, our other plants may be able to pick up the slack. Should we abandon the old column and rebuild it? I’ve also discovered a second problem: nobody can find the R-stamp report for the first three tubesheet repairs. I ran into a similar problem when I checked some other heat exchangers in these columns: we have the last R-stamp report but we’re missing others. What can be done? Do we need to worry?

Focus On Fouling

Consider the following:

1. “Fouling” is an umbrella term that means deposits resulting from, for example, polymerization, sedimentation of viscous liquids, etc. Ask your chemists if any inhibitors could be used to reduce the extent of fouling so you can continue to operate until the next shutdown.

2. The figure shows forced circulation reboilers. These are a good choice for fouling service as long as you maintain a good circulation rate and proper vaporization. Too low circulation, high vaporization or a combination will enhance fouling. Maintain good circulation to minimize dead or stagnant pockets that could increase fouling.

3. Always watch for inert gases. Make sure the steam chest has been cleared of air; a little amount of air can drastically affect heat transfer. Often, reboilers have vent valves on the shell side that you can use to get rid of air.

4. Avoid use of superheated steam for reboiler applications.

The decision to replace the column/reboiler depends on your market projections and the functioning of the current system. If you opt for replacement, consider fouling-resistant column designs:

1. Packed columns (random or structured) typically are prone to fouling. Trays are a better choice here. Consider trays that minimize or eliminate downcomer areas because these areas tend to be stagnant and, hence, a breeding place for fouling. For viscous service, don’t use valve trays as they can get stuck in an open or closed position. Also consider maintenance: bubble cap trays will need a very long time to become vapor free. Disk-and-donut trays, baffle trays or double flow trays are options but have lower efficiencies. Tray vendors, in-house experts or a consultant may provide valuable guidance.

2. To combat fouling, I suggest using stainless steel (say, 300 series) rather than carbon steel. Of course, you must consider metallurgy in overall project economics.

3. Maintain high circulation rates.

4. Select instruments that tolerate fouling — e.g., capillary-sealed differential-pressure cells for flow measurement and guided-wave-radar or other non-contact level transmitters; also, where feasible, install thermowells in high-turbulence areas.

5. Provide suitable connections for periodic flush of systems. Consider inhibitors to reduce fouling.

6. On forced circulation reboilers, ensure adequate circulation; avoid dead zones.
GC Shah, senior advisor
Wood Group, Houston

Change The Feed Conditions

The capacities of the inlet (preheaters) may suffice to permit operation of the column but probably not in the current way.

First, review the capacity of your condensate pump. Obviously, you can compensate for a fouled reboiler by increasing the heat input from the feed pre-heater. A simple McCabe-Thiele analysis program will do the job nicely to show this. Changing the feed state from liquid to vapor lowers the optimum feed tray position. In addition, with a distillate product, the number of theoretical trays will go up at a fixed reflux ratio. So, to maintain the same product output, the condensate pump must provide more recycle from the condenser to the tower. Fortunately, you have lower lines that will allow you to drop the location of the feed tray.

As for the R-stamp issue, there’s a methodical way to handle it. First, photograph the nozzles, shell and head of the heat exchanger, and other vessels connected to the column. Pay particular attention to the nozzle design: weld-o-lets are preferred to couples that require donuts. Don’t forget the column. As the engineers were sloppy with record-keeping, chances are they altered pressure vessels without worrying about ASME Code. Start the process by photographing each vessel around the column and the column itself. Then, compare the pictures to the vessel drawings. Focus especially on small nozzles; more often than not, these get added without a second thought during an outage.

As for the drawings, each set should have support calculations from a design engine. As a minimum, a general arrangement drawing should show the shell and head layouts, the nozzle table and the applicable design code. In addition, there should be nozzle detail drawings and ones for specialty items such as ladders, bridges for agitators, baffle details, dip tubes, coils, etc.

Now, we get to calculations. ASME Code files for pressure vessels should include detailed calculations for shell, head, nozzles and specialty items. Obviously, if photographs show a nozzle not in the drawings, you won’t see calculations supporting this installation — and you have a problem.

The next step is to check the mill report tests for the materials of construction of vessels. Tests should have been done on specific types of materials used, e.g., bars and plates. If these tests show discrepancies, such as pre-dating the construction year of the vessel, then you may have a problem.

Discrepancies like changes in tube thickness or swapped materials are common with heat exchangers like the reboiler. Engineers forget that heat exchangers are three ASME Code vessels in one: violating one, by replacing tubing without an R stamp, violates two code compliances.

Once you’ve identified these problems, strip off the insulation and let code-qualified inspectors (API-510) do thickness measurements and inspections of added nozzles. When you put in a new reboiler, replace or modify the added nozzles to meet requirements.

As for abandoning the tower, that depends on reliability. Review maintenance history to develop an estimate of useful service life. In most situations, you can keep the shell and replace the tower internals to improve reliability.
Dirk Willard, consultant
Wooster, Ohio

March’s Puzzler

A few years ago, we increased the capacity of our batch reactor to 1,900 gal. from 1,200 gal. Our process produces about 364 BTU/lb of heat, so the cooling requirements went up about 1.5 MM BTU/hr. In addition, heating (with low-pressure steam/water) rose by 245,000 BTU/hr.

Our quality engineer, in league with our control engineer, found that operators manually manipulate the steam and cooling water control valves to manage the temperatures. We’ve suffered 18% more temperature excursions where the operator lost control of the process for several minutes; off-product is up 2% but we’re not having trouble blending out the reject.

Now, our safety manager is involved because we risk operation above the flash point of a monomer product. Corporate engineering, which managed the expansion project, claims that nobody at the plant mentioned this during the hazard and operability assessment. However, corporate quality control doesn’t see an issue. The production manager says his crew can manage the reactor safely. (I think he’d rather spend next year’s budget on items other than expanding the steam and cooling water system.) What do you think? Why didn’t we catch this during the expansion? Can we justify utility improvements as quality improvements if the production group is managing the problem? Is there any real risk from these temperature excursions?

Send us your comments, suggestions or solutions for this question by February 7, 2020. We’ll include as many of them as possible in the March 2020 issue and all on Send visuals — a sketch is fine. E-mail us at [email protected] or mail to Process Puzzler, Chemical Processing, 1501 E. Woodfield Rd., Suite 400N, Schaumburg, IL 60173. 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.