Process Puzzler: Combat Column Contamination

Readers suggest how to tackle a troublesome tower.

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Our old vapor-feed 11-tray bubble-cap fractionating column has developed a problem. The liquid product collected from the reboiler is weak. Reboiler product contaminates the volatile liquid from the condenser. The feed is a vapor. How do we investigate the problem? What are possible causes of contamination? Is there any way to continue to run the column until the next turnaround, a month from now?

I think I've identified the potential causes of contamination: 1) tray flooding; 2) low (distillate) reflux ratio (L/D ratio); 3) increase in column pressure; 4) increase in condenser pressure; 5) change in feed composition; 6) change in column feed location; 7) increase in condenser duty (or decrease in performance); and 8) increase in reboiler duty or heating medium temperatures. Depending on the outcome of the investigation, it may be possible to continue operating the column. The purity of the distillate can be improved if the top column pressure is reduced. More of lighter components will reach the condenser.
Nikhil Barbare, process engineer
Plexal Group, Perth, Australia

I would consider the following:
1. There could be a tube leak in the reboiler. If the reboiler design permits, consider bypassing the reboiler. There could be limits on this solution, e.g., a fixed-tube design is prone to thermal stress. You will need to evaluate impact of this bypassing to downstream area — e.g., the increase in the product vapor pressure could affect its vent system.

2. I can't explain all the symptoms. One possibility that may fit is a poor temperature profile. It may be that the profile is hot with resulting lower reflux rates (flooding). Check the feed flow and vapor boil-up rate to make sure they are not in the flooding region. Also, the bottoms temperature controller may be out of calibration.

Try lowering the condensate-pump discharge pressure. This could allow a reduced pressure differential across the reboiler tubes and hence reduce the leak and product contamination. This may allow you to continue until the next turnaround.
G. C. Shah, HSE project manager
Mustang Engineers, Sunnyvale, Calif.

There might be several reasons for this problem. You will need to review all process parameters on the tower to find out the reason. If the bubble caps are dirty they could make stripping difficult. If the differential pressure across the tower has increased this could be a sign of trouble. Verify that the temperature profile along the tower is correct. Confirm that the condensate flow rate on the top tray is correct. Assuming that this checklist is good, you may be able to continue operation. You will need spare capacity to recirculate the product back to the tower and continue (albeit at a lower overall plant capacity). After trying this approach, test the bottoms to ensure they are in an acceptable range. Otherwise, you need to shut down the tower and clean it.
A. Behshtinejad, lead process engineer
Eied Co.,Tehran, Iran

There are four potential problem areas: preheater failure, an over-performing reboiler, an under-performing condenser and column problems.

Collect the history on the column from trends and logs; assess the status of instruments. Are the readings correct? Is there a flux in feedstock quality? When does the product contamination occur? Is contamination worse for startup and shutdown? Look for spikes in flow, pressure, temperature and level. After this background check, run simulations including the column, condenser, preheater and reboiler. A re-run of the simulation for startup and shutdown conditions may reveal some concerns. Compare the simulation with field data for additional clues.

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