FOCUS:

Moderate capital solutions to some common problems-such as poor screening and brownstock washing-can help bleach plants operate at optimum conditions

By LEE BRUNNER

Next Step Beyond ECF Conversion Should Focus on Plant Optimization

LEE BRUNNER, P.E.,
is process specialist-pulping and bleaching, BE&K Engineering, Birmingham, Ala.

Now that numerous pulp and paper mills have converted their bleaching processes to elemental chlorine free (ECF), what can be done to minimize operating costs? The operating cost for substituting chlorine dioxide for chlorine is inevitably higher. The industry is again in a position of weak demand, with intense pressure on prices and narrow profit margins. The industry is also under pressure from financial markets demanding increased return on investments. Operating companies are consolidating and under pressure to reduce the subsequent debt.

The result of all this is an extremely anemic capital investment market. The question is: With the increased cost of bleaching with chlorine dioxide, are there moderate capital expenditures which might be legitimately justified to reduce operating costs?

BE&K began collecting and organizing bleaching data from laboratory work, published articles, and operating mills about eight years ago and continues to expand its bleaching database to support the development of a robust predictive model. The model is utilized to discuss the conversion to ECF and optimum conditions for ECF bleaching. Some operating problems that prevent a bleach plant from operating at optimum conditions are discussed in this article. Specifically examined are the bleaching cost impact of poor screening, brown stock washing, short sequence bleaching, and some potential capital solutions.

THE BLEACHING MODEL AND DATABASE. A discussion of the development of the bleaching database and model have been presented elsewhere.1 These tools continue to be updated and refined with experience and availability of additional information. One of the objectives of the database is to provide a wide range of operating conditions in order to make the model useful for many operating alternatives.

Two hundred thirty sources are now included in the database, which represent both hardwood and softwood species and a wide range of operating conditions. More than sixty of the sources are from operating mills. The database is used to develop and validate the algorithms in the bleaching model. For example, chlorine dioxide consumption data is available for 197 sources in the database. For these sources, the correlation of the bleaching model calculated result for the same process conditions is 0.929.

ECF BLEACHING - OPTIMUM CONDITIONS. Utilizing the model and database to evaluate alternative bleaching conditions, Figure 1 demonstrates the differences in bleach chemical costs between low substitution and ECF bleaching. This is a plot of the total bleach chemical cost as compared with the charge of active chlorine in the first stage as represented by the charge factor.

The basis for comparison is a conventionally cooked hardwood pulp bleached to 90 ISO in a short sequence D-Eop-D bleach plant. Hardwood is used for this comparison because one of the conditions to be demonstrated is the effect of bleaching on dirt, which is much more pronounced in the case for hardwood pulps. Short sequence bleaching is presented because bleaching costs are higher for this sequence, and the impact of not bleaching at optimum conditions is more significant. Each of the issues discussed here will be true for extended sequences and oxygen delignified and conventionally cooked hardwood and pine pulps, although to a lesser extent.

The first observation from the curves is that ECF bleaching is more expensive by as much as $10/a.d.bt (air dry bleached ton) at the optimum conditions. A second observation is that the penalty for overbleaching in the first stage is more serious in the case of ECF bleaching as compared with bleaching with chlorine. The optimum bleaching efficiency point is reached in any bleach plant by lowering the first stage charge to the maximum effective capacity of the brightening stages of the bleach plant - the D1 and D2 stages. This has been demonstrated in the literature2 and in operating mills.

In the low substitution case, the lower chemical efficiency cost is offset by the lower cost of chlorine. In the ECF case, the cost penalty for a high first stage charge factor can exceed $5/a.d.bt. A third observation is that the economic optimum point is a lower charge factor for ECF bleaching than for the traditional bleach sequence. Many bleach plants have been operated historically at higher first stage charge factors with little economic penalty, but replacement of chlorine has changed that economic balance. In many cases the charge factor is required for other reasons than brightness development efficiency. Three possible reasons for higher than optimum first stage bleaching conditions are dirt, excessive brown stock washer carryover, and short sequence bleaching.

BLEACHING FOR DIRT. In conjunction with Cluster Rule projects and ECF conversions, BE&K has recently participated in the justification and installation of two new hardwood screening systems. The primary justification for the screen rooms was reduction of dirt to the bleach plant in order to reduce bleaching costs and maintain or improve product quality. The results of the first screen room installation were presented by Gulf States Paper at the TAPPI 2000 Pulping Conference.3

Case Study No.1: This mill performs extended cooking in a continuous di-gester and bleaches hardwood to about 88 ISO brightness in a short sequence bleach plant. The mill performed ECF trials and was greatly concerned about the bleaching costs required to reach pulp quality requirements. The high bleaching costs were a result of a high charge factor in the first stage and overbleaching in the D1 stage for dirt. The brightness target from the bleach plant was two points higher than required on the mach-ine to control dirt. In addition, a lower than optimum pH target was set in the D1 stage to increase dirt reduction efficiency. The existing hardwood screening system was known to be a problem.

A dirt survey was done in the fiberline to determine quantities and classification of dirt particles. Most of the dirt entering the bleach plant was bark, and most of the dirt in the paperboard product was also bark. From the results of the survey, a pressure screen room project was implemented with slotted screens and reverse flow cleaners for lighter weight bark particles removal.

The ECF conversion project also included a new decker after the new screen room, conversion to medium consistency D0, and reinforcement and pressurization of the extraction stage. The combined results were very positive:

• 92% decrease in offgrade paperboard due to dirt

• 30% lower ECF bleaching costs

• 30% lower AOX

• 25% effluent volume reduction

• Reduction in fiber loss.

Case Study No.2: BE&K worked with another client to implement a new hardwood screening system in conjunction with an ECF conversion. This mill bleaches conventionally cooked hardwood pulp from batch digesters in an extended sequence bleach plant to 90 ISO brightness for market pulp. The mill was very limited in chlorine dioxide generation capacity and utilized generous amounts of chlorine and hypochlorite. The first stage was operated at low substitution and high chemical application and temperature for reduction of dirt. The final brightness target was two points higher than required on the machine for dirt reduction.

This mill was greatly concerned about making dirt specifications with the elimination of chlorine and hypochlorite. A dirt survey was undertaken. Two sets of samples were analyzed - represented by the two colors in Figure 2 - at different positions in the fiberline to assess the dirt sources and removal efficiencies of the existing system.

The survey showed that the dirt concentration in the primary screen accepts and secondary screen accepts was much higher than the knotter accepts. The existing knotters were overloaded and partially bypassed vibratory screens. It was postulated that dirt was being generated in the screen room by the breakup of knots. A second observation is that by far the biggest hammer the mill had on dirt reduction was the bleach plant. The machine cleaners were severely hydraulically overloaded, operating at relatively high consistency and poor efficiency. This was done, however, at a cost to chemical efficiency, and that cost was increasing with the replacement of chlorine and hypochlorite. The mill opted to install both pulp mill screens and cleaners in addition to the bleach plant modifications.

Similar to the case at Gulf States, the slotted screens were found to be very effective for dirt removal. ISO dirt in classifications I through III was totally removed, while greater than half of the finer particle dirt was also removed.

Table 1. Operating and capital cost economics for 1,000-tpd hardwood D-Eop-D bleach plant projects

A comparison of the data collected before the new screen room startup to the sample collected after startup is also very revealing (Figure 3). Although the removal efficiency of fine particle dirt, Class V, across the primary screens is about 50%, the overall reduction of this very fine dirt feeding the bleach plant is greater than 80% for the new screen room installation compared with the replaced gravity screens. This is because the knots are removed from the system, and the dirt is effectively rejected down through the screen room and purged from the system through the cleaners.

In this case, the BE&K bleach model was utilized to estimate the bleach chemical savings that might be possible if the bleach plant did not have to bleach for dirt. The estimated annual savings for reducing the first stage charge factor from 0.30 to 0.15 and reducing the brightness target by two points was about $500,000/yr. The project was not justified on operating cost savings, but these savings made a quality improvement project easier to accept.

The opportunity to minimize bleach chemical costs is attractive. However with capital as constrained as it is right now, a project will likely require a simple payback of less than two years to be approved on cost savings alone. There are a number of cases where screen rooms are being considered for other reasons than economic. It is our belief that capital projects will require a combination of justifications to be approved in the present economic climate. Additional reasons that a screen room upgrade might make sense are:

• Product quality improvements: reduction in off-quality; added margin by improved quality

• Facilitate screen room closure: reduce water consumption; reduce effluent volume; reduce biological oxygen demand (BOD), chemical oxygen demand (COD), or color

• Reduce fiber loss.

WASHING FOR COD. COD is a function of the dissolved organic component of black liquor carryover to the bleach plant and has been strongly correlated with the bleach chemical consumption associated with that carryover. Strom-berg4 has hypothesized that COD transfer in a pulp slurry solution is dependent on a diffusion mechanism. It has demonstrated that time is required for larger organic molecules, dissolved lignin, and pulping by-products to diffuse out of the fiber lumen and walls into the free liquor volume.

Displacement washing efficiency is normally tested and reported based on concentration of filtrate dissolved solids (free liquor) in the washer feed compared with the filtrate dissolved solids (mainly free liquor depending on the consistency) in the discharge. Displacement, by definition, is related to displacement of the free liquor volume. There is a portion of liquor that is not available for displacement washing until sufficient time is available for diffusion into the free liquor volume.

Since COD transfer is a diffusion mechanism, it is affected by the same parameters that affect any diffusion process. It is apparent from the work above that significant time will be required for some liquor phases to reach equilibrium, orders-of-magnitude more time than in a normal washing device. The amount of time for the solution to arrive at equilibrium can be affected by the following process variables:

• Temperature

• Concentration gradient

• pH

• Thickness of free liquor film.

Time in the washing system for the slurry concentration to reach equilibrium is important for removal of COD. Time near the end of the washing system where the free liquor concentration is the lowest would be of the greatest benefit. The efficiency of the diffusion process can be improved by lowering the free liquor concentration by dilution to low consistency and by agitation.

Another observation, which corroborates the above hypothesized diffusion mechanism, is lab and mill pilot work with dewatering presses. This work has shown that pressing to greater than 30% consistency will result in higher COD removal efficiency than water removal efficiency, as shown in Figure 4. The higher efficiency is the result of higher COD concentration in the pressate than in the feed solution. This result can be explained by higher concentration-bound liquor contained in the fiber that is mechanically motivated to migrate into the free liquor. Thus pressing can be effective in removing COD which cannot be removed by displacement washing.

Much of this dissolved organic material is available for washing after brownstock storage. The combination of removal of leached COD after storage, improved COD removal with a press, and improved efficiency with medium consistency D100 offers significant benefits to a bleach plant. Some additional advantages of this combination are listed below:

• A prewasher is the best way to transition from storage to a medium consistency D100 stage. Low consistency, high-density discharge facilitates reliable measurement of production rate at low consistency, which is the basis for chemical addition in the bleach plant. The geometry of a low consistency, high-density storage tank is a more cost-effective design than a taller, narrower medium consistency tank. In an existing bleach plant, first stage acid filtrate recycle is often impractical due to materials of construction in the dilution zone. This problem can be remedied with a prewasher, where the transition is made after the washer.

• For a nominal 1,000 tpd, dilution of about 800 gpm is required after the press to dilute from 32.0% discharge consistency to 12.5% consistency. In the case of a pre-washer, this dilution can be D100 filtrate, and residual chemical and temperature from the process is recycled to reduce heating and pH buffer chemicals requirements in addition to the substantial reduction in bleach plant effluent.

COST OF SHORT SEQUENCE BLEACHING. The optimum chemical efficiency in the bleach plant is achieved by reducing the first stage charge until the maximum capacity of the final stages of the bleach plant is utilized. One way to increase the capacity of the back end of the bleach plant is to add a stage to a short sequence bleach plant.

A comparison of the bleaching costs for a short sequence plant and a four stage plant are presented in Figure 5. This curve was prepared from the bleaching model. Basically, a decrease in the bleach chemical cost is seen down to a charge factor of 0.12 to 0.15 as compared with an optimum charge factor for the short sequence bleach plant of 0.21 to 0.24. The potential savings of adding a stage in this case would be about $10/a.d.bt.

There are a number of additional qualitative and quantitative benefits for adding a stage to a short sequence bleach plant:

• Dirt reduction is best achieved with high concentration, low pH, and time. The D1 stage is the most powerful stage in the bleach plant for dirt reduction. Dirt reduction is improved by lowering the pH below the optimum conditions for brightness development. In a short sequence bleach plant, operating the D1 stage to reduce dirt will result in increased chemical cost for brightness development. A fourth stage operated for optimum brightness development makes the D1 stage available for lower pH dirt and shive control when necessary without a chemical efficiency penalty.

• A fourth stage provides a large capacity for backup due to problems from upsets entering the bleach plant, kappa variation, black liquor carryover, and screen room upsets. If conditions entering the bleach plant are not controlled, the common operator response is over-bleaching to protect against low brightness product. The risk of making off-quality product is reduced.

• The carryover of anionic material from chlorine dioxide bleaching can result in excessive paper machine chemicals, alum and size, etc. A fourth stage increases the washing available at the end of the bleach plant and greatly reduces the charge of chemicals in the last stage ahead of the paper machine. Reduced carryover to the machine will result in savings in wet end chemicals.

• In general, chemicals applied over more stages for a longer period of time will result in greater brightness stability and less reversion, so a lower bleach plant operating brightness target might be possible.

CONCLUSIONS. Table 1 is an example of the bleach chemical savings and capital cost range that might be expected from some of the modifications discussed to optimize the bleach plant. The annual operating cost savings are attractive. The simple paybacks are in the range of 2-4 years, which is attractive but may not be adequate in these economic times.

However, each of these projects has a number of additional potential benefits in product quality or water and pollutant reductions which may be necessary or desirable. A combination of these benefits may be sufficient for capital justification.

REFERENCES

1. F.L. Brunner, "A Spreadsheet Simulation for Bleach Plant Upgrades," Proceedings of the 1995 TAPPI Pulping Conference.

2. R. Malinen, et.al., "ECF Bleaching of Oxygen Delignified Softwood Pulp with the Minimum Charge of ClO2," Proceedings of the 1993 TAPPI Pulping Conference.

3. J. Pearson, et.al., "Implementation of a 4 Stage Screening System and ECF Conversion at Gulf States Paper Corporation," Proceedings of the 2000 TAPPI Pulping Conference.

4. C.B. Stromberg, "Washing for Low Bleach Chemical Consumption," Proceedings of the 1990 TAPPI Pulping Conference.