Distillation: Conquer Column Challenges

April 27, 2016
Readers suggest a variety of steps to address multiple issues

This Month’s Puzzler

We are experiencing some startup problems in our distillation train, which consists of three towers. When we start up the plant, the product from the reboiler of the second tower is hotter than desired, which delays the overall plant startup. Our condenser load is too high at this time. What is causing these issues and how can we address them?

In addition, during a turnaround, we mixed up the wiring on the three reboiler product pumps. We’re now working on a control-system-migration project and need to quickly sort out the wiring problem with the pumps to avoid a long startup. What should we do?

Lastly, we replaced the condenser thermal control valve in the product tower. While doing the final hydrostatic test in-place, we couldn’t completely close the spring-open valve. Is there a workaround for this problem so we can get up and running?

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Focus On The Second Column

Because the column with high bottoms temperature is the second column, it means that the condition of the feed that comes into it from the first column is okay. Having more information on the upstream columns would be helpful.

The condenser load in the second column is high due to the high boil-up rate in the reboiler. This high boil-up rate in the reboiler could be caused by: 1) failing TIT/TIC system; 2) failing TCV that is supposed to regulate fuel consumption based on TIT measurements; 3) check LCV for falling level; or 4) the fuel-ignition and burner system.

In summary, a technician should replace the TIT at the reboiler with another from the first column and check if the TIT is giving misleading results. Both instruments should be calibrated to the same span and range.

As for the wiring issue, mixing up would have been detected during audit. Get technicians to trace back and correct.

For a valve spring not to completely close, it means that it is not getting enough air pressure. Check the air-line/air system.
Dennis Omenka, process engineer
PetroGas Systems Engineering, Lagos, Nigeria

Start With A Quick Fix

One old trick a foreman worked out for handling hot feed to a distillation tower was to run cold-water coils on the feed lines. A less-elegant solution is to put an operator up in the rack with a garden hose. This trick is only a short-term solution because scale will build up from the hard water. Running cold water on air heat exchangers’ fins is a common practice.

Now, let’s look at longer-term options to manage the hot feed. One idea is a start-up cooler if you can find a compatible cold fluid for the temperature range of the feed. Another option may be to run the tower condenser cold during startup and then put reflux as low as possible: this could quench the feed before it vents to the top of the tower. Perhaps running the reboiler a little cold will help balance the heat in the tower. You should perform some simulations to verify which solutions work best using field data from the last upset.

The answer to the wiring mix-up can be found using heat and electricity. Confirm that pump labels matches those in the distributed control system and other references like the piping and instrumentation diagrams. Have an experienced operator turn on two of the pump starters; I assume the pumps are not running or are in safe, recycle mode. Then, wait several minutes until they heat up. Have the operator turn off one of the pumps and then find out which of the pair selected for the test have a hot starter or pump and which pump is still running. Creating a table with columns to enter results for each pump will make your sleuthing easier.

In the valve case, use an air line and a pressure gauge to carefully add a few pounds of pressure to the valve vent line. This drives the valve closed. You’ll want to bench-test all the valves and gauges before risking damage to the valve diaphragm. A better question to ask is why you need to do this: dirty air, wet air, poor system loading or high pressure drop are all possibilities.
Dirk Willard, consultant
Wooster, Ohio

July’s Puzzler

At our ammonia plant, a carbon-steel CO2 pipeline exploded during a shutdown. The CO2 gas typically contains 1–3% H2. Our two H2 analyzers, which rely on thermal conductivity, are used to trip valves on the transfer line closed if the H2 concentration rises above 8%; such a concentration poses a risk downstream because it becomes an explosive mixture if combined with air. We use N2 to purge the ammonia plant before a plant outage. We’ve never had an explosion before.

My investigation turned up several issues: 1) operators rely on the trip to close the valves — they’re using a trip as an active control; 2) N2 has twice the conductivity of H2, so operators were instructed to ignore the trip during purging; 3) the analyzers don’t vote and aren’t on the critical check list; 4) the purge is hooked up manually and, because the air-line connection is right next to the N2 one, a wrong hookup is possible — but nobody will admit such a mistake occurred; and 5) the rotometer on the N2 is under-range and the rotometer on the air is over-range. What do you think the source of the ignition was? How do you suggest addressing these issues?

Send us your comments, suggestions or solutions for this question by June 10, 2016. We’ll include as many of them as possible in the July 2016 issue and all on ChemicalProcessing.com. 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.

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