ClO2 Scrubber

Two-Year Project Update
The passing of the two-year anniversary of Nu-PAC in service in the ClO₂ scrubber was uneventful. No increase in pressure drop was observed, no fouling of the packing was evident. The mill continues to enjoy the lowered power cost, lowered carbon emissions of Nu-PAC on a daily basis. No need to repack the scrubber will likely exist for many years to come. The total R2250,000 + cost of the project has easily been returned to the mill and continued savings are assured.

= 1 million Kg per year CO₂ GHG emission reduction

270 hp (0.746 hp/kWh) (24 hours/day) (340 operating days/year) (1.341 # CO2/kWh)

CO₂ to the atmosphere. This means that the mill has helped to reduce greenhouse gas emissions:

Additionally, every kWh generated in the USA results in the release of 0.6 Kg of

Carbon Emissions Reduction

POWER SAVING of R800,000+ per year.

Every hp represents a fixed electric power cost per year. Using data published by the Electric Power Research Institute the direct result of eliminating 270 hp at the pulp mill has been a

Power Savings

*Pressure drop observed with new, clean saddles, note previous discussion, noting that after two years normal service pressure drop routinely rose to 965 mm WG.

hp 300

Chlorine Dioxide Scrubber Tower

Pulp Mill Replaces Saddles with Q-PAC in October of 2001
Tired of the cost and effort required to replace fouled saddles every two years, in October of 2001 the mill took out their two-year-old saddles during their normal maintenance shutdown. Rather than a simple repack with new saddles, the mill loaded the tower with Nu-PAC made from Kynar. The result was, without exaggeration, spectacular:

Q-PAC Supplied by S G Plastic Construction
Introduced to industry in 1996, Q-PAC achieves far superior gas / liquid contact than is possible with old, traditional packings such as saddles. The design of Nu-PAC is all rounded surfaces, with many needles to force the scrubbing liquor to break into a torrential shower of droplets as the liquor passes the packed bed.

The ClO₂ Wet Bed Packed Scrubber
The packed tower at the pulp mill is 3.8 meter diameter. It treats 35.38 m³/sec of air containing trace levels of ClO₂. The chlorine dioxide is absorbed by a 9,085 L/min flow of white liquor passing the tower. The tower has two, 9 meters packed beds. The original packing in the tower was 50mm Kynar saddles in the lower 9 meters packed bed and 75mm CVPC saddles in the upper 9 meters packed bed. Historically, this scrubber operated at a pressure drop of 762 mm Water Gauge. White liquor does contain a variable amount of suspended solids, so over time the saddles would plug. On average, because of this constant fouling of the saddles, the ClO₂ scrubber was repacked with new saddles every two years. The two-year schedule was adhered to because the pressure drop of the saddles constantly rose during the two years of service. The mill reported that a 965 mm water gauge pressure drop was common for two-year-old saddles.

The packing choice in a scrubber is critical for optimal tower performance. The absorption (removal) efficiency of the ClO₂ is directly proportional to the ability of the packing to support efficient gas / liquid contact.

Chlorine Dioxide yields high brightness pulps of good strength because it is an extremely powerful oxidizing agent. ClO₂ is always produced on site for immediate use in the bleaching process. The threshold limit value (TLV) for exposure to ClO₂ is 0.1 ppmv. The short-term exposure limit (STEL) of ClO₂ is 0.3 ppmv. Therefore, emissions of chlorine dioxide to atmosphere must be prevented. Most pulp mills prevent these emissions by using a packed bed wet scrubber. In this unit operation the ClO₂ is absorbed into white liquor as gas containing chlorine dioxide passes up through the packed bed and the liquor passes down through the packing.

The primary alternate bleaching technology available to a pulp mill is total chlorine free (TCF) bleaching. This alternate bleaching technology produces inferior papers of poor tensile strength. If adopted by the entire wood pulping industry in North America, it is estimated TCF bleaching would require additional harvest of 100 million trees per year to compensate for inferior strength papers that would result .

Introduction
Chlorine Dioxide, ClO₂, is widely used to bleach wood pulps at pulp mills. Papers produced from such pulps have the high whiteness demanded by paper consumers. ClO₂ achieves high whiteness at reasonable costs. Additionally, wood fibers bleached using ClO₂ have superior physical properties of burst, tear, tensile strength and viscosity compared to wood fibers bleached with alternate chemicals. The USA EPA has therefore designated ClO₂ bleaching as a BAT (Best Available Technology). Chlorine dioxide bleaching is also widely accepted by European pulp mills as the best method to produce strong, white paper and remain environmentally friendly by allowing for elemental chlorine free (ECF) bleaching of the wood fibers.