About 95% of all one-ton and 150-cylinder emergency chlorine scrubbers in the United States today are specified as dry scrubbers. Twenty years ago wet scrubbers were specified 95% of the time. Dry scrubbers have quickly gained favor for a good reason. Dry scrubbers greatly reduce the dangers associated with introducing another hazardous chemical and the associated liabilities.
Some of the advantages of dry scrubbing as compared to wet scrubbing are:
1. Secondary containment tanks and/or concrete sumps are not required by the building codes for dry scrubbers to guard against the release of a hazardous caustic chemical from a leaking or ruptured tank as in the case of a wet scrubber.
2. Heaters are not required in cold climates for dry scrubbers. Dry media has been tested down to - 40 deg F.
3. Cleaning of wet scrubber mist eliminators is not an issue in dry systems
4. Wet systems require pump maintenance. Dry scrubbers have no pumps.
5. Wet scrubbers require chemical titration, hazardous chemical handling and hazmat approved personnel. Dry media is non-hazardous in the fresh & spent forms. Wet caustic soda is hazardous and must be disposed of as such.
6. Our competitors use high performance packing in vertical towers and cross-flow beds for contacting the gas with the wet caustic soda. The caustic dries in the packing and is degraded to sodium carbonate which heavily builds over a short period of time. After 2 years wet packed scrubbers become an operator’s nightmare due to clogged pumps, spray nozzles, packing and mist eliminators. See attached photos of carbonate/caustic fouled packing in a wet packed scrubber.
In summary, the normal problems associated with wet caustic scrubbers using high performance packing media with 20% caustic soda are:
1. Degrading Caustic - Dried caustic soda reacts with carbon dioxide in the air and converts to sodium carbonate. This sequesters the CO2 and helps the “greenhouse effect” but it hurts the wet scrubber long term operation. The carbonates will not re-dissolve and cause clogging in the packing, pumps and spray nozzles and mist eliminators. Frequent acid cleaning of the high surface area packing and associated down time is maintenance demanding. This is also a dependability issue. In short mist eliminator, packing and spray nozzle annual cleaning are specific wet scrubber maintenance issues not associated with dry scrubbers.
2. Demineralized Water Make-Up - Water naturally evaporates from the caustic solution over time thereby concentrating the solution and accelerating the fouling effects stated above. An entire batch can solidify. Constant vigil must be maintained of the liquid level and demineralized water added as required on a regular basis.
3. Disposal of caustic every 5 years - Caustic soda replacement could cost up to $4000 per batch. Disposal of waste caustic, which is normally designated as a hazardous waste, could cost an additional $10,000 per batch. Dry media does not require replacement unless exposed to chlorine.
4. If 1/4 of the caustic solution is spent in an event, the entire batch has to be replaced. The solution concentration can NOT be elevated with the addition of stronger caustic as this would lead to reaction salt precipitation problems. Whereas, 1/4 of the dry media can be taken out of the tank and replaced while saving 3/4 of the media that was unaffected by the leak.
5. Dry media is non-hazardous and in most cases is landfill disposable.
6. Wet caustic scrubbers circulate 20% caustic soda solution at high pressures. The possibility of a leak is always present. This becomes an operator safety issue. All nuts and bolts have to be re-torqued on a regular schedule many times a year on wet scrubber systems.
7. Wet caustic scrubbers in freezing environments need heating and insulation and the normal associated maintenance. The dry scrubbing media can operate at temperatures down to - 40 deg F without heating.
8. Wet scrubbers need caustic leak containment provided either as double walled expensive tanks or concrete containment areas around the tank per the building code.
9. Bleach (NaOCl) is the reaction product in wet caustic soda chlorine scrubbers. Bleach is a hazardous aggressive chemical that can result in a shorter tank life. This is not the case for dry scrubber tanks.
10. Foaming is experienced due to natural caustic contamination over time. This is caused by the inability of the air to separate from the thick caustic solution. Special caustic surfactants need to be added to the caustic every 2 to 3 months to prevent this problem.
11. Wet caustic scrubbers are NOT user-friendly.
12. The normal maintenance savings of dry scrubbers over wet scrubbers are $7500 per year considering the total costs associated with one complete bed exhaustion over a 20 year scrubber life. See the cost estimate worksheet attachment.
0 comments:
Post a Comment