Costless scrubbing of bleach plant emissions using EoP effluent

An innovative technique was developed that involves no annual operating cost and decreases the amount of diluted NCG that need to be processed

December 2007
By Andre Normandin, Luc Belley, Christian Valiere and Jean-Luc Simard

Chlorine and chlorine dioxide used for pulp bleaching are major contaminants of kraft pulping plants. These components originate mostly from the vents associated to bleach pulp production equipment such as bleach towers, washer hood vents, seal tank vents, and the equipment used for chlorine dioxide production. Recently, the US[1] has implemented regulations that force kraft pulp manufacturers to collect and treat emissions from major vents in bleach plant areas. In the province of Québec, there is no explicit regulation[2] in force, but a case-by-case approach is given for each kraft pulp mill.

The traditional approach for treating these gases is to scrub them using various fluids such as first-stage bleach plant extraction filtrate (EoP), sodium hydroxide, weak wash, strong white liquor and chilled water.[3,4] However, the scrubbing efficiency is highly variable[4] and may be quite limited depending on the washing solution used. To reach high scrubbing efficiency, chemicals such as strong white liquor or high pH caustic must be used at high operating costs and considerable chemical loss to the sewer when the scrubber bleed is out of control. Additional information about traditional scrubbing methods and technologies can be found in more fundamental papers.[4]

Moreover, because of the costs involved in the incineration of non-condensable gases (NCG), alternative methods were developed recently by Paprican.[5] Such methods consist in chemically oxidizing the contaminants such as total reduced sulfide (TRS) present in the NCG, with residual chlorine dioxide coming from the bleach plant. In a previous work, the author performed a full-scale application of this technique[6] at the Fraser Papers' mill in Thurso, QC. In 2004, a variation of this technique using atomized chlorine dioxide was performed at Cascades Fjordcell,[7] Jonquière. In a following work, it was demonstrated that there is no production of organochlorides associated with the latest approach to neutralize TRS using chlorine dioxide.[8] This approach has proven to be very cost competitive when using residual chlorine dioxide[9] compared with traditional incineration.

Although no regulation forced Cascades Fjordcell to control its emission levels of bleaching agent from the bleaching operation, an effort was made to meet a stringent ambient air standard around the mill.

The purpose of this article is to present an extensive study of industrial scale trials during the period of December 2005 to July 2006, performed at Cascades Fjordcell, to reduce the bleach plant emissions. The reported results compares EoP with strong white liquor as scrubbing agents, recirculation mode with once-through scrubbing, and finally the combination of chemical oxidation with EoP scrubbing. We wish to demonstrate the simplicity and the low cost of this approach compared with the traditional chlorine and chlorine dioxide treatment, as well as the environmental benefit for kraft plants when the brownstock line is physically near the brownstock washer.

Fig. 1 - Process diagram showing the bleach plant vent collection, transport and washing system at Cascades Fjordcell

Process Description

Scrubbing bleach plant emissions at Cascades Fjordcell: The process consists of washing chlorine and chlorine dioxide contained in the bleach plant vents coming from the washer hoods and filtrate tanks vents, the bleach tower vents, and the chlorine dioxide generator. The combined vents are exhausted and directed to a dedicated alkaline scrubber. Figure 1 illustrates the process schematic of the emission control system used at Cascades Fjordcell.

The scrubber is a 10-ft diameter packed tower. Between 1998 and 2003, the random packing consisted of Cascades Mini-RingTM. During this period, the scrubber was operated in the recirculation mode using only the EoP effluent as a makeup, and the scrubber overflow was sent to the mill alkaline sewer.

A problem associated with EoP effluent was the fiber content of this stream and the solids accumulation in the packing over the years. This resulted in a large pressure drop and a loss of scrubber capacity. For this reason, the packing was removed in 2003 and the scrubber ran as a spray tower for the next year. Meta-bisulphite was used as a reducing agent to enhance contaminant absorption and compensate the loss of efficiency during this period, at high operating costs.

Emissions reduction program objectives: In spring 2005, Cascades Fjordcell put forward a program of emissions reduction and conducted an extensive study on atmospheric dispersion of chlorine dioxide around the mill. The ultimate objective was to estimate the required reduction to meet an ambient concentration criterion of 0.2 µg/Nm3 after dispersion, in the neighbourhood adjoining the mill. Once the objective was set, a reduction program was established.

The first step consisted in revamping the scrubber using a wider packing configuration (HI-FLOWTM RING 3 0.5 in.) made of glass-filled polypropylene, an economic construction material, to allow maintenance personnel to replace the media once a year to prevent fiber accumulation. Figure 2 illustrates the top of the packed tower bed after two months of operation. It clearly shows two of the three rings of dense liquid resulting from poor liquid distribution.

The second step of the reduction program consisted in improving the scrubbing efficiency to reach the ambient air criteria, at the lowest operating cost. The major results of these full-scale trials are presented in the following section and were performed from December 2005 to July 2006.

During the trials at Cascades Fjordcell in December 2005, an external firm took samples of the chlorine/chlorine dioxide at the inlet and outlet of the revamped bleach plant scrubber.

For tests conducted during spring and summer of 2006, an external firm (Sedac) took samples from week to week, and residual chlorine/chlorine dioxide concentrations were measured according to NCASI method #520.

Fig. 2 - The packed tower bed after the revamping of the bleach plant scrubber with HI-FLOWTM RING at Fjordcell

Results

Scrubbing in recirculation mode with makeup: During the fall of 2005, a miscellaneous scrubbing agent and operating conditions were tested to avoid fiber accumulation as well as to improve scrubber efficiency.

The first scrubbing agent tested was "spent caustic", coming from the petroleum industry. It is found in the waste caustic used to remove hydrogen sulphide and carbon dioxide contained in the hydrocarbon stream before a catalytic reactor. This waste solution was guaranteed to contain a residual caustic concentration above 98 g/L with a maximum of 70 g/L of sodium carbonate, and an uncontrolled amount of sodium sulphide. Unfortunately, the lack of sulphur content stability in the liquor from batch to batch resulted in unreliable scrubber performance and the spent caustic option was abandoned.

In December 2005, a set of five trials was performed with EoP compared with strong white liquor as a scrubber makeup, under a different set of operating conditions, Table 1.

Outlet chlorine dioxide concentration may appear to be unreliable, since inlet concentration fluctuated during trials, and was not recorded for the strong white liquor trial due to time constraints. However, scrubber performance for strong white liquor clearly shows that outlet ClO₂ concentration decreases with pH increases, as illustrated in Fig. 3.

Fig. 3 - Outlet chlorine dioxide concentration using EoP compared with strong white liquor

Unfortunately, the temperature of gases coming from the bleach plant was higher than 80°C, and relative humidity was near the point of saturation. This resulted in a condensate flow of about 10 US gal/min or higher and the bleed being out of control and carrying a large amount of chemicals, which made the process excessive in annual operating costs.

Once-through scrubbing using the EoP effluent: In March 2006, performances were improved by operating the scrubber with EoP only, but in a "once through" mode. An extensive campaign of stack sampling was realized during this period, where inlet and outlet ClO₂ concentrations were measured. The scrubber efficiency is represented in Fig. 4.

Fig. 4 - Chlorine and chlorine dioxide emissions from the bleach plant vents at Cascades Fjordcell

These results show that the scrubber efficiency was stabilized and ranged from 50 to 80%, with an average of 70%, compared to the chaotic performances obtained using the recirculation mode. However, these encouraging results were insufficient to meet the ambient criteria targeted by the mill.

Once through scrubbing using EoP and chemical oxidation: The basis for the next set of trials was the observation that, at the Thurso mill[6], unexpectedly, not just the TRS was reduced but the chlorine dioxide emissions were completely neutralized at the end of the project. Figure 5 illustrates the alkaline scrubber ClO₂ emissions before and after the project. This demonstrates that the TRS contained in DNCG consume the near totality of the residual ClO₂.

Fig. 5 - Chlorine dioxide emissions from an alkaline scrubber at Nexfor Fraser Paper, Thurso, QC

Afterwards, a mass balance was performed to estimate the DNCG sources at Fjordcell, which matched the ClO₂ load, to get the optimal reduction without creating a problem of TRS emissions from the bleach plant. The knotter hood and the ash mix tank vents were combined to neutralize the bleach plant emissions. Figure 6 presents the modified process flow diagram of the bleach plant scrubbing system using TRS chemical oxidation.

Fig. 6 - Process diagram showing the bleach plant vent treatment system using EoP filtrate and DNCG in a "once through" mode

As illustrated, isolation dampers C and D allow mill personal to resend DNCG sources to the treatment system to cover bleach plant shutdowns. Ducting from the DNCG sources are split between the tower vent and washer hood vent, using dampers A and B, respectively.

In July 2006, after ducting modifications, a set of five trials was performed to determine the best DNCG balancing method on the scrubber vents to optimize ClO₂ reduction, Table 2.

Conclusion

A bleach plant emission treatment process using EoP combined with TRS chemical oxidation of diluted NCG was successfully implemented at the Fjordcell plant. As a result of an extensive study of atmospheric dispersion around the mill, the ambient criterion was satisfied, and neither chlorine nor chlorine dioxide compounds were found at significant levels.

Furthermore, the main proportion of capital investment was for piping connections, whereas important operating costs were necessary when using strong white liquor or other suitable reducing agents. The proposed solution involves zero annual operating costs in terms of chemical consumption.

Conditions favorable to the implementation of such new technology includes the proximity of the brownstock washer line to the main exhaust bleach duct as well as the presence of a bleaching alkaline scrubber.

Andre Normandin is with Mesar/Environair, Quebec City, QC; Luc Belley and Christian Valiere are with Cascades Fjordcell, Jonquiere, QC; Jean-Luc Simard is with Sedac Environnement, Chicoutimi, QC.

1. Federal Register, Vol. 63, No. 72, US Environmental Protection Agency, 40 CFR part 63, April 15, 1998.
2. MENVIQ, Règlement sur les fabriques de pâtes et papiers, Q-2, r12.1, juin 1998.
3. Buonicore, A.J., Davis, T.W., Air Pollution Engineering Manual, Van Nostran Reinhold, 1992.
4. NESHAP, Manufacturing Processes at Kraft, Sulfite, Soda and Semi-chemical Mills, US EPA-453/R-93-050a.
5. O'Connor, B. et al., Reduction of total reduced sulphur (TRS) from kraft mill using residual bleach plant chlorine dioxide, 1999 Intern. Environ. Conf., TAPPI Proc., Nashville, TN, 1999, April 18-21.
6. Normandin, A. et al., Oxydation chimique des TRS dans une usine de pâte kraft par l'utilisation de chlore résiduel, Les Papetières du Québec, 19, novembre 2003.
7. Normandin, A. et al., Cascades mill uses atomized ClO₂ to oxidize TRS, optimize NCG treatment, PaperAge Magazine, Apr-May 2004.
8. Normandin, A., Comparative Annual Cost Efficiency Between Thermal and Chemical Oxidation of TRS in Kraft Mill', TAPPI J., Vol.4: No.7, July 2005.
9. Normandin, A. et al., Cascades uses chlorine dioxide for Chemical Oxidation of TRS in NCG, Pulp & Paper, October 2005.

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