Fine tuning pulping technology: a solutions! Roundtable: improving pulping yield and using pulping additives are just some of the approaches pulp mills are using to fine-tune their processes. Our expert panel debates the pros and cons.
* Polysulfide and/or AQ are particularly effective in improving pulping yields, among other approaches, though some question the benefits of AQ
* Additives such as polysulfide and AQ are improving pulping performance in mills (not just in the lab); improvements can be measured, but it is net always easy to do so
* Where mills can find the best opportunities far reducing production costs
The following is a Solutions! magazine pulping roundtable. The participants are:
* Peter Axegard, research director, fiber and pulp, STFI, Stockholm, Sweden
* Gary A. Dees, fiberline operations manage+; Weyerhaeuser/Flint Rivet; Oglethorpe, Georgia, USA
* Michael E. Haas, technical director, Longview Fibre Co., Longview, Washington, USA
* Peter W. Hart, senior research engineer, MeadWestvaco Corp., Chillicothe, Ohio, USA
* Steven W. McDonald, senior process manager, Metso Paper USA, Atlanta, Georgia, USA
* Martin MacLeod, senior scientist, Paprican, Pointe Claire, Quebec, Canada
* Bertil Stromberg, director of technology, Andritz KMS, Glens Fails, New York, USA
* Panu Tikka, professor, Helsinki University of Technology, Laboratory of Pulping Technology, Helsinki, Finland.
SOLUTIONS! There have been several symposia on kraft pulp yield over the past four years. What are the most promising technologies for yield improvement?
HAAS: In the short term, the most promising technologies are those that reduce the capital and production costs of polysulfide. Polysulfide's yield improvement is well documented, but to date capital costs for the technology have not provided a favorable ROI for pulp mills. Another promising current trend is the drive to more uniform chip processing; chips of the same size pulp to a higher yield.
In the longer term, black liquor gasification is the most promising pulping technology. It allows complete control of the pulping chemistry, as sodium and sulfur can be separated in the gasifier. Sulfur rich cooking liquor can then be applied early in the cook, with pure caustic at the tail end. This technology has high potential when combined with extended delignification digesters.
Also, oxygen delignification can increase the yield of kraft pulp used for unbleached products such as linerboard and kraft papers. Multiple oxygen stages are more selective in lignin removal than traditional kraft chemistry.
Finally, genetic engineering of trees otters potentially major benefits. Could there someday be lignin free chips?!
TIKKA: Today, the best approach to improving pulp yield is using polysulfide in combination with anthraquinone (AQ) in cooking, while the second best approach is Lo-Solids[R] and low temperature cooking (for hardwoods only). In the future, the best approach will be to use genetically manipulated trees with low lignin content.
DEES: Many mills are taking the AQ path for yield gain. Mills can implement AQ addition without a significant capital investment and can easily stop the application if pulp prices dictate. AQ/polysulfide continues to show promise, but it requires a greater front-end capital investment. Many mills are evaluating lower cost yield gains through modifying digester cooking technology and/or increasing kappa targets. Some mills are using and evaluating chip penetrants to increase yield.
MACLEOD: For yield improvement, polysulfide/AQ is far and away the easiest process to implement right now, and, after AQ alone, probably the cheapest as well. Remember, though, that it's not a new process, having been in continuous industrial use for more than two decades.
AQ alone is the clear winner for improving yield in kraft pulping, as is proven daily in dozens of mills around the world. It's simple to use, the research knowledge is deep, the results are predictable, and it's effective when a few simple chemical principles are taken into account. Lower-cost AQ would be nice--a lot more mills would probably use it.
Wood and chip quality aren't exactly obvious under the banner "new technology," so perhaps what we really need is more old technology put into good mill practice. In recent work on the first implementation of Paprilox PS/AQ pulping, our team showed that the total yield gain over kraft could be as high as 7% (wood to brownstock) if everything was optimized. About one quarter of this was due to the change in process chemistry; all the rest was species control, chip size control, elimination of all contaminants (hark, biological knots, etc.), and better delignification uniformity. That's not rocket science (actually, it's mostly pulping science developed decades ago), but good industrial practice, discipline, and suitable investment seem to be lacking at many mills.
HART: So far, no commercially available technology has successfully made good progress towards yield enhancement. Vast amounts of fiber are still dissolved into black liquor solids every day. As for digester additives, using AQ strictly for yield improvement is like religion. If you believe strongly enough, you may be able to convince yourself it is happening, but on a day-to-day basis, you can not definitively point out the impact on yield. Does AQ really improve pulp yield? Yes, I believe it does, but not enough to escape the daily and hourly process variations in a mill. At the end of the month, the only thing a mill will see is a bottom line charge for AQ and a possible warm fuzzy feeling that they may have produced a bit more pulp this month ... but maybe I really didn't?
AXEGARD: Besides wood and chip quality, I would stress four areas:
* Avoid cooking to too low kappa/K (permanganate) numbers. Mills can obtain significant gains by increasing the cooking kappa for softwood from 20 to 30 and compensating with more oxygen delignification
* Keep a close eye on wood and chip quality
* Avoid kappa fluctuations
* Evaluate the benefits of AQ with and without polysulfide for your specific conditions. In many cases--but far from all--you will find that profitability or product quality will be less than required.
MCDONALD: One of the most promising approaches is to use extended oxygen delignification and raise the kappa number from cooking. A two-stage oxygen delignification process that effectively optimizes reaction kinetics can achieve 70% delignification with good selectivity on softwood, so that the less selective kraft cook can be stopped at higher kappa number. Laboratory investigations have shown that the pulp yield is significantly increased while pulp quality is not impacted. Installation into an existing fiberline where the cooking kappa number has been lowered to meet environmental limits can raise the screened yield by 2-3 percentage points on softwood. Addition of a second reactor and mixer to an existing single-stage oxygen delignification system can also provide a significant improvement.
The kappa number increase from cooking will usually be limited by the reject content. A high reject content in the cooked pulp reduces the yield gain and overloads the deknotting and screening systems. Good chip quality and good digester control are essential for operation at higher cooking kappa number. Through improved chipping technology, it is possible to improve yield both by reducing losses during chip preparation and by improving the pulp yield in cooking. In the digester, control of alkali concentrations during impregnation and cooking, effective H-factor control and liquor circulation are required for uniform delignification and to control the reject content.
Another way to improve yield is to apply polysulfide and AQ during cooking. Laboratory investigations on pine have shown an added benefit: reduction of rejects at higher kappa number. The yield benefit from improved selectivity plus the benefit to operations from reduced reject load to the deknotting and screening systems means these digester additives have the potential to fit very well with higher kappa cooking.
STROMBERG: Lo-Solids[R] cooking is proven in several mills to give pulp yield improvements for hardwood. We presented a paper at the TAPPI Pulp Yield Workshop in Atlanta in March this year. Yield improvements of 1%-4% over other processes have been recorded. Similar yield improvements have been confirmed in lab work on hardwood. Small yield improvements on softwood are possible, based on the viscosity and carbohydrate composition of mill pulps, but are so small that they are difficult to confirm in the mills. Oxygen delignification has been shown in the laboratory to be more yield effective than cooking, at least when kappa numbers below 30 are considered.
SOLUTIONS! Do pulping additives such as anthraquinone (AQ) and polysulfide improve pulping performance in a mill production environment as they do in lab and pilot tests? What improvements should a mill expect?
TIKKA: The effect of PS and AQ are typically realized in mill scale projects as well as lab scale projects. The magnitude of improvement is a tricky question. The yield increase can be adjusted over a wide range (the more PS the more yield and AQ for compensating lowered sulfidity). However, many mill and pulp quality specific issues limit the acceptable yield increase. In Finland, we have recently studied this issue, but there is no single, overall conclusion. In a softwood case at one pulp mill, a 1.5%-1.8% unit yield increase was found to be optimal.
DEES: Most mills are definitely seeing improvements such as yield increases or debottlenecking of recovery systems when using AQ or poly/AQ. Exact improvements are hard to measure, but most mills feel that 1% gains in yield are easily achievable.
MACLEOD: I have a simple answer here: read "Basket Cases, IV: ..." from the proceedings of the 2001 TAPPI Pulping Conference. it is a scientific story, from mill and lab, of putting PS and AQ to productive use. It won the Wetherhorn and Best High Impact Paper awards for 2001.
STROMBERG: AQ and polysulfide have the most benefit in the impregnation zone. Proper steaming for good liquor penetration increases the effectiveness of AQ and PS. Low alkali concentration during impregnation gives less "competition" for the ACL so less AQ can be used for the same yield benefit. Better than normal yield improvements have been seen in hardwood mills using AQ in Lo-Solids[R] cooking digesters. Testing of the mill pulp chemical properties (viscosity and carbohydrate content), and comparing to lab cooks will give a good indication of the mill yield, as shown by Adriaan van Heiningen at the University of Maine.
HAAS: PS and AQ both work as advertised. However, it remains a significant challenge to statistically prove the anticipated yield increase in a mill environment. We feel that it takes at least 6 months to analyze the impact of a pulping process change on yield due to the month-to-month "noise" in wood usage and pulp production quantities.
AXEGARD: Since the chemistry is well known and functions the same in labs and mills, the results should be the same. Improved beatability is one clear advantage. Lower tear is a disadvantage. Less energy produced in the recovery boiler is another disadvantage.
HART:. See my previous answer.
SOLUTIONS! Every pulp mill is working hard to reduce production costs. Are there any major cost reduction opportunities in pulping, such as through energy efficiency or low-cost or no capital cost projects?
HART: Most of U.S. pulp mills and bleach plants are operating in a less than efficient manner. Thus, a good process engineering/process control optimization program will be able to substantially reduce operating costs. Also, some of the newer pulping/ bleaching technology may have significant and positive bottom line impacts. In short, there are savings to be found in pulping and bleaching optimization.
MACLEOD: Probably the best opportunities are in the wood end of the business--getting high-quality wood to start with and treating it well all the way to the digester. After pulp making, the variable costs of production (at least by current standards and processes) don't leave much room for significant improvement, perhaps with the exception of recovery boiler efficiency. There are lots of opportunities in the woo& yard, such as really effective bark removal in all seasons, low-impact transportation of logs and chips, seasonally tuned OCS operations, minimization of losses during storage.
From a research standpoint, well-founded and rigorous chip sampling procedures and analysis should receive more attention at mills, including proper training and a greater sense of importance for it as an activity. Here's an idea: Take the same resources currently devoted to kappa number testing and control, and apply them instead to chip quality. You'll probably get a better overall outcome by catching unforeseen variability earlier. The real need is to give more attention to best practices based on known facts in wood and chip quality.
Occasionally, spend a few tens of thousands of dollars to establish, by measurement, the scientific facts on which multi-million dollar decisions are often made.
DEES: Nothing major really stands out. All pulp mills are trying to optimize their processes and tightly control costs. We are looking at production optimization, yield, defoamers, and bleach chemicals as major cost focus areas. In addition, we are continuing to evaluate ways to run the process with smaller operating staffs. We are applying capital toward task reduction efforts. Examples include automatic pulp sampling and testing and advanced process control with the existing DCS.
MCDONALD: Most of the cost reduction factors in new investments are related to low wood cost, low labor costs and maximum single line capacity. This means investments in the areas where planted wood grows fastest, with single line capacity of 2000-3000 a. d. metric tons/day.
Within existing mills, replacement of overloaded, inefficient bleach washers with modern washers provides significant savings in both bleaching chemicals and maintenance costs. Replacement of the final bleach stage washer has also provided significant chemical and maintenance cost savings in pulp drying by reducing bleached pulp conductivity and reducing the carryover of chloride into the white water system.
Addition of a second reactor and mixer to an existing single stage oxygen delignification system can provide significant cost savings, either through reducing the kappa number to bleaching, raising the kappa number from cooking, or a combination of the two.
It is not unusual for mills with old carbon steel batch digesters to have a digester out of service for shell repair and overlay. The maintenance expense and lost capacity in some cases adds several dollars per ton to production costs. Replacing the old digesters with vessels fabricated from solid duplex stainless steel can eliminate these costs. Solid duplex fabrication provides corrosion resistance without cladding.
HAAS; It always pays to attend to fundamentals. For example, lowering H-factor will improve yield, and can often be accomplished easily--particularly if a mill's pulping capacity is not fully utilized. The March 2002 TAPPI Kraft Yield Workshop had many excellent papers in the area of pulping fundamentals.
TIKKA: Energy co-production is and will be very important. An energy efficient pulp mill can make money when others go into the red due to low pulp prices. There is not much a mill can achieve with low- or no-capital cost projects. Modern PS+AQ however, may be a low cost solution, with pay back in less than a year, and then continued making money.
AXEGARD: What can be done economically is very mill specific! Many mills run far from optimal and they can enjoy major economic benefits by applying what is already known.
* TAPPI 2001 Pulping Conference Proceedings on CD: http://tappi.org/index asp?rc=1&pi d=20198&ch=8&ip=
* Anthraquinone Pulping: A TAPPI PRESS Anthology of Published Papers (1997): http//tappi.org/index.asp?rc=1&pi d=2638&ch-10&ip=
* TAPPI Introduction to Pulp and paper Technology Short Course, January 2003, Orlando, Florida, USA: http://www.tappi.org/index.asp?rc =1&pid=19409&ch=4&ip=
* Pulp and Paper Research Institute of Canada (Paprican) Research programs and technologies: http://www.paprican.ca/
* Swedish Pulp and Paper Research institute (STFI) Order annual report and other publications: http://www.stfi.se/
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|Title Annotation:||Kraft Pulping|
|Publication:||Solutions - for People, Processes and Paper|
|Date:||Sep 1, 2002|
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