Scrub The Scrubber Design

Solving a contamination problem demands a thorough rethink

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This Month’s Puzzler

The 5-ft-diameter caustic scrubber at our refinery is supposed to capture H2S. We sell the sodium bisulfide produced; this decreases the disposal costs of caustic soda. In our last turnaround, we replaced 30 sieve trays with two beds of high-efficiency glass-filled random packing. This design should improve efficiency because the caustic now goes through a plate-and-frame chiller that allows us a much larger reservoir in the tower. However, since the turnaround, our bisulfide product has been contaminated with sodium carbonate, making it a struggle to meet our bisulfide sales obligations.

Inspection of the liquid distributors — which were retained from the old internals at the insistence of operations — shows they are level and not too badly plugged. A model of the tower indicates that the packing is underperforming. The H2S going to our vent system exceeds our permit.

The sales staff blames the current problem on upstream CO2 membranes, which were replaced during routine turnaround maintenance. Concerns have been voiced about sodium carbonate eventually fouling the plate-and-frame chiller. Unfortunately, I’m not sure about fouling in the exchanger because it lacks taps for pressure gauges and thermowells.

Operations personnel want their sieve trays back. They never were fans of packing and were against the change, asserting “if it isn’t broken, don’t mess with it.” They repeatedly argued for a larger pump to move the bisulfide; the plant control engineer always hated the level control in the scrubber.

I think something must be wrong with the packing. I’ve seen this problem following three or four years of operation but never just a few weeks after addition of new packing.

Do you think sales is right? If so, how do we prove it? Is there something wrong with the packing? Did we miss something with the liquid or vapor distribution? Should we go back to the sieve trays?

Set Up An Internal Team

On reviewing the description of the issue, it is interesting that the operation is telling me where the problem lies. From my perspective statement “since the turnaround, our bisulfide product has been contaminated with sodium carbonate, making it a struggle to meet our bisulfide sales obligations” tells it all. Changes made are the cause of the issues. Basis and rationale for the changes made and their basis is not elaborated. It seems there is significant conflict within the organization. A stepwise process to solve the problem is needed.

Operators who have to put up with issues resulting from the change would like to go back to an operation what produced a quality product. This is normal. I would too.

Asking outsiders the question “Do you think sales is right? If so, how do we prove it?” suggests that the company has internal partnership/trust issues. They are one team and need to solve the problem internally rather than say who is right or wrong.

Whatever solution and pathway to solve the issue is selected, it has to be clearly and systematically explained to all to make sure everyone is on board. Unless this happens, finger pointing will continue. Based on the problem description, I would consier the following path:

The company should compare its current operation to before the changes took place because this will give clues of before and after the change of operation. Because the carbonate level in bisulfide has increased, it suggests that CO2 level entering the caustic scrubber is much higher than the previous level thereby contaminating bisulfide. Carbon dioxide is being scrubbed along with H2S and contaminating the bisulfide.

Based on carbonate contamination, it seems that CO2 membranes are not working at the before-change level. The described problem does not mention anything about this potential issue. As stated earlier, the company needs to re-visit the operation records before and after change to pinpoint what has changed and needs to fix the change.

The company should also review the solubilities of carbonate and bisulfide at the concentration and operating temperatures as mutual solubility will influence operation. Current operating conditions should be similar to prior to equipment change conditions unless design changes were made.

I am not sure of heat exchanger placement. Colder liquid will lower solubility and would result in dropping solids out of solution and may cause fouling.

As mentioned earlier, the company has to review and investigate cause-and-effect relationships. Just the feeling that spiral exchanger is better than plate and frame or spiral exchanger is better can all be resolved if the design criterion is properly defined and used for the design.

There are options and the company has to regroup internally to consider how the current issues can be resolved.
Girish Malhotra, president,
EPCOT International,
Pepper Pike, Ohio

Closely Examine The Scrubber

Consider the following points:
1. The scrubber working for a short time (“few weeks after addition of new packing”) suggests several possibilities. So, review the trend charts from the plant historian (database). For example:

• Check the feed rates and compositions of gas (e.g., CO2 content from the CO2 membranes) and caustic — then and now. Look for the possibility of high flows and possible flooding in the packing. Try adjusting flow rates to match the design.
• Is caustic flowrate adequate? Too low a flowrate could cause poor distribution in the packing and would lead to poor vapor/liquid contact.
• Did any abrupt increases in vapor flow occur? Such an increase could have disturbed the packing or support tray, leading to potential maldistribution of gas flow (and caustic flow as well) through the packing.

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