Bleaching: determining optimum EO stage pressure: sometimes, more pressure is a good thing. Increased pressure in pulp bleaching operations can produce several major benefits.

What is the optimum target pressure for an extraction (E) stage in a pulp bleaching operation? Since most mills have now combined oxygen gas (EO) with caustic (NaOH), people are questioning the target for this pressure. Some mills have learned that increasing the pressure means that more work can occur in this stage. Consequently, less work is necessary elsewhere for a given brightness. Raising the pressure will allow savings of chemicals by using oxygen, which is less expensive.

DIFFERING OPINIONS

Two schools of thought exist on this subject. The conventional one states that approximately 25 psi at the top of an approximately 10 min. reactor is optimum. This is definitely better than a simple extraction stage or EO stage with only 3-5 min. in an atmospheric up-flow tube. Contrasting with this, several mills are now operating with more than 50 psi at the top of the 20-30 min. tube. Some have 90 psi or 100 psi.

For the conventional sequence, diminishing returns occur above this 25 psi value at the top of the up-flow. Some decreased performance may be due to historical experiences with chlorine or an inability to reach the required 175[degrees]F-180[degrees]F in the EO pressurized stage. The benefit of raising the pressure is lower if an oxygen delignification system precedes the bleach plant. This also limits their need to push the EO stage.

A practical consideration is reaching the lower limit, i.e., a "floor" of about 2.0-2.5 kappa value after the E stage washer for softwood. The "floor" is possible by using different combinations of chemicals--cooking, [O.sub.2] delignification, and D1 and E stages. Many mills are also adding hydrogen peroxide for an Eop stage. Most people that are using considerable peroxide do it because they cannot make sufficient Cl[O.sub.2] or because they want to brighten the effluent color due to regulatory requirements of their receiving stream. For delignification, oxygen is the least expensive approach. It does have diminishing performance as the kappa number decreases. Chlorine dioxide (D) is more selective but is considerably more expensive. Some mills charge as low as 0.08 kappa factor in the first D stage and have little or even no peroxide charge in the E stage. This not only saves chemical cost but also lowers the AOX generated.

If oxygen will be doing work, raising the pressure will help the oxygen be more effective. According to the ideal gas law, PV = nRT, raising the pressure will keep the oxygen gas volume small. It is therefore more available for the localized chemical reactions. As pressure decreases, the bubbles grow in size and coalesce as they bump into each other. They then burp through the pulp without being available for the desired bleaching reaction. Raising pressure minimizes this inefficiency.

MORE WORK WITH OXYGEN

Several mills are now doing more work with oxygen in the E stage and reducing the work done with [H.sub.2O.sub.2], or in the prior Cl[O.sub.2] stage. Although they are mill specific, the required changes are generally inexpensive relative to the economic return possible.

The pump must be able to supply the pressure required at a typical value above 10% consistency. A well-designed high shear mixer system is necessary. The pressurized up-flow tower must withstand the maximum pressure of the pump in a dead-headed condition or a pressure relief system must be in place. More time may be necessary for significantly more reaction to occur. With only 10 min., more pressure will help somewhat. In some installations, the change may require pump modification of the motor and impeller to take advantage of a higher operating pressure. If a mill already has a pressurized up-flow tube, very little cost is necessary to begin evaluating the operating cost savings.

With the ability to raise the pressure, mills can optimize bleach plant operation to lower bleaching costs. Figure 1 shows relative performance using 2% NaOH, 20 min., and 85[degrees]C and 90[degrees]C with 25-100 psi at the end of reaction time. S!

[FIGURE 1 OMITTED]

About the author: Wayne Bucher is process engineer, WB Consulting Inc., Birmingham, Alabama, USA. Contact him by email at wbucher@charter. net or by phone at [phone].