SO2 Scrubber Loses Efficiency as Bed Pressure Drop Rises
A packed-bed scrubber removes SO2 from 12,000 m³/h of air at 40°C and 1 atm using 25 m³/h of recirculated 5 wt% NaOH solution. The column is 1.4 m ID with 5 m of 50-mm plastic pall rings. Design inlet SO2 is 2000 ppmv and design outlet is 50 ppmv; the liquid is controlled at pH 8.5. Current gas flow is 11,800 m³/h, inlet SO2 is 1900 ppmv, and outlet is 180 ppmv. The bed pressure drop has risen from 0.7 kPa when new to 2.1 kPa. Liquid flow is steady at 25 m³/h, pH reads 8.4, and the recirculation pump is at its normal current. A recent water wash did not restore performance. The plant has no upstream CO2 removal; ambient air contains about 400 ppm CO2. The packing has been in service for five years.
1. Calculate the design and current SO2 removal efficiencies, the gas F-factor, the liquid load, and the pressure drop per metre of packing.
Read the worked answer
Design efficiency = (2000 − 50)/2000 = 97.5%. Current efficiency = (1900 − 180)/1900 = 90.5%. The scrubber has lost about 7 percentage points of removal.
Column area = π(1.4)²/4 = 1.539 m².
Gas velocity = (11,800/3600)/1.539 = 2.13 m/s.
Gas density at 40°C and 1 atm = PM/RT = 101.3 × 29/(8.314 × 313) = 1.13 kg/m³.
F-factor = v√ρ = 2.13√1.13 = 2.26 (m/s)(kg/m³)^0.5.
Liquid load = 25/1.539 = 16.2 m³/(m²·h).
Pressure drop per metre = 2.1 kPa/5 m = 0.42 kPa/m = 420 Pa/m. When new it was 0.7/5 = 140 Pa/m. For 50-mm plastic pall rings, 420 Pa/m is in the loading-to-flooding range, while 140 Pa/m is reasonable normal operation. The high ΔP and low efficiency together point to fouled or flooded packing, not simply inadequate liquid flow.
2. Diagnose the most likely cause and rank the corrective actions. Include the role of pH and CO2.
Read the worked answer
The most likely cause is carbonate scaling and liquid maldistribution. The scrubbing liquor is NaOH, but ambient air contains about 400 ppm CO2, which absorbs and forms carbonate/bicarbonate. At pH 8.4, the liquor is in the carbonate buffer region, so free OH− is low and the driving force for SO2 absorption is reduced. The same chemistry produces carbonate solids that coat the packing, raise ΔP, and create channels. A water wash would not dissolve carbonate scale well, which is consistent with the failed wash. The high ΔP at normal liquid rate also rules out simple underflow as the primary cause.
Ranked actions:
- Inspect the packing and liquid distributor. Sample the white deposit and confirm carbonate. Clean with an appropriate inhibited acid or replace fouled packing.
- Recalibrate the pH probe and restore pH control nearer the design value; check caustic feed and blowdown, because CO2 consumes caustic and raises carbonate loading.
- Improve liquid distribution and consider softened water or upstream CO2 removal if scaling recurs. Increasing liquid flow before cleaning is likely to flood the bed.
3. Assess whether raising the liquid rate to 35 m³/h can restore efficiency without flooding.
Read the worked answer
At 35 m³/h, the liquid load becomes 35/1.539 = 22.7 m³/(m²·h), about 40% above the current 16.2. The gas F-factor stays at 2.26. On a generalized pressure-drop chart, increasing the liquid-to-gas ratio moves the operating point closer to flooding and raises ΔP at constant gas rate. The bed is already at 420 Pa/m, which is in the loading range; adding 40% more liquid will probably push it into flooding, causing entrainment, high outlet mist, and possibly worse SO2 removal.
A higher liquid rate can improve absorption after the packing is clean, because it increases wetted area and reduces liquid-film resistance. But it is not a fix for a fouled bed. The correct sequence is: inspect and clean or replace the packing, verify the distributor, correct pH and carbonate accumulation, then test at design liquid rate. Only after the clean-bed ΔP is restored should the liquid rate be increased, and then only with a flooding check and pump-capacity check. Raising liquid rate now would mask the diagnosis and risk mechanical damage.