Modelling reveals pulp bleaching secrets with far-reaching benefits

A phenomena-based simulation of pulp bleaching will help to develop and optimise the bleaching process. It can even be used to study bleaching in conditions where it is not currently possible.

“The bleaching model we are developing simu-lates basic chemical reactions, so it is universal and unique. Bleaching has often been modelled before, but the models have been experimental and have simulated only certain stages of the process. Our model covers the whole bleach-ing process,” says Professor Tapani Vuorinen of Aalto University. He is responsible for the modelling project commissioned by Forestclus-ter Ltd to a multidisciplinary research group. Participating in the project alongside Aalto Uni-versity are VTT Technical Research Centre and Lappeenranta University of Technology.

“The chemistry of bleaching is very complex and it is impossible to study the whole bleach-

ing process experimentally. It can only be done by modelling. Although there is a long experimental and industrial history of pulp bleaching, it is only now – with the help of the model – that we understand what chemical reactions are important. When we know how and why something happens, we can develop ways to produce better quality pulp more economically and with less environmen-tal impact.”

For example, pH is known to influence the bleaching result, but the overall picture of effects and causality has remained unclear. The model has provided an answer to this question.

It has also produced surprises, even for ex-perts in the field.

“It shows that certain reactions produce plenty of aldehydes. This finding will allow us to con-trol conditions so that the bleaching chemicalsare used in exactly the right place to promotethe desired reaction most effectively.”

Benefits here and now...“A bleaching plant is a large investment so there have to be substantive reasons for changing the process. Using the model, a pulp mill can seek optimal ways of bleaching with existing equipment, aiming to reduce consump-tion of chemicals and thereby produce lower environmental emissions,” Vuorinen notes.

Depending on the size of the pulp mill and the initial state of the bleaching process, the econ-omies obtainable from optimised bleaching can amount to millions.

“Alternatively, the production of some mill might be limited by a factor related to bleach-ing. The model could help solve a bottleneck so that its capacity could be increased.”

For companies delivering equipment to the pulp industry, the model will serve as a tool for developing technology.

“A simulation tool is of the greatest benefit to suppliers of new equipment.”

... and in the future“Building the model has developed our skills and we are well placed both for basic research and for collaborative studies. The model is the result of long development work that began back in the 1990’s.”

“Because the basic phenomena are always the same, the model has a very wide area of application. It can be used for all possible feedstocks and new chemicals, and even for bleaching processes that are not being used at present. It can also study things that cannot be measured experimentally.”

A unique toolThe model will be a valuable tool for Finnish pulp producers and equipment suppliers.

“We are giving a simply, user-friendly inter-face for people who do not know how the pro-gramme works. The new interface will allow commercial use of the model.”

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For the next stage, the multidisciplinary re-search group is planning to expand the model to pulp cooking.

“We aim to model wood chips and the reac-tions taking place in kraft and sulphite diges-tion, and also under a wide range of other pos-sible cooking processes that are hard or even impossible to study experimentally. One of ouraims is to find a solution for cooking at room temperature instead of the temperatures of atleast 140°C used at present.”

Testing radical concepts“Revolutionary new ideas for bleaching can be studied virtually, without the costs or delays of creating laboratory or plant conditions. The results can be combined with the energy and material balances of the pulp mill, to produce a comprehensive assessment of the conse-quences of new process concepts and chang-es,” says Kari Kovasin, Head of Research at Metsä-Botnia, a leading pulp producer.

“The results that the model has given us have been credible. We’ve used it to calculate condi-tions that we have also measured in laboratory experiments. The results have been consist-ent. The expertise of the research group that has built the model also gives us confidence in it.”

Kovasin hopes that the model will be used as widely as possible to develop pulp mill pro-cesses or test new ideas.

“Continuous revision, development and en-largement will ensure that the model stays up to date. A phenomena-based simulation of cooking will complement the bleaching model and improve everyone’s understanding of the processes involved.”

For additional information

Lars GäddaResearch DirectorForestcluster Ltd. Tel. +358 50 322 2471lars.gadda@forestcluster.fi

Tapani VuorinenProfessorAalto University, Forest Products TechnologyTel. +358 9 470 24236tapani.vuorinen@aalto.fi