Significance of Retention and Drainage Program for smooth runnability of Paper Machine

The Paper machine wet end is the section where Pulp i.e. a slurry of water and fiber is spread into a running sheet, while separation of water fiber occurs. There are few different types of processes however what is common is a feed zone where the slurry is present as the input, the fiber mat that is the wet layer of the paper sheet, the fabric which acts as a barrier or a filter, and lastly the flooding zone where white water flows and it is on the opposite side of the fabric.
Paper Machine forming section
What fabric does is act as a barrier, it retains the fiber on the side and allows water to drain on the other side. The various zones from which fabric passes are natural drainage, air gradient drainage, low vacuum zone, and then high vacuum zone. However, during this formation zone of paper, since, water has to pass through the fabric pores or voids, a lot of small fines of fiber also pass out sometimes due to the small size and sometimes due to the pulling effect of the vacuum.
Paper makers have always been in the dilemma about what is to be done. Though a lot of other external factors also come into play, the most significant point to consider is the right water and fiber chemistry during the formation zone.
Myth amongst mill managements: Chemicals are being used only because of Paper Maker is corrupt, else forming section is a mechanical process only.
Myth amongst Paper Makers: There is no need of chemicals as we are using very good quality fiber and there are no fines.
What the paper-making process requires is optimum retention of fiber on the forming side, which sometimes is called as “first pass retention”. Retention is the ratio of the difference in consistency of the headbox and white water to the consistency of the headbox. Not every machine can have the same FRP value and neither there is a global standard. There are so many variables at play always that the readings may vary. However, for paper machine runnability it is required to have a constant value of FPR for a long duration, and fluctuations are an absolutely avoidable scenario. To ensure fibers are retained on the wet end it is necessary to bring the fine particles together in such a size so that the vacuum is not able to pull the flocs out. This happens with help of retention aids. Retention aids form divalent bonds between fines and attach them together so that they can pass the forming stage What we are talking about is not only the fiber fines but also the particles of desired filler. However, during this process, the bonds are formed in such a manner that the porosity of the fiber mat reduces. This reduced porosity acts as barrier in flow of water as well thus reducing the drainage from the wet end.
Coagulant and flocculant action
To ensure that desired moisture levels are achieved at the end of forming stage, drainage aids are used which are usually cationic in nature, and due to the charge, they change the surface tension of fiber resulting in easy detachment of water from the fiber. The cationic charge also increases the porosity back due to a change in surface tension and thus the barrier which was created by retention aid is neutralized.
Well acknowledged process benefits of RDA (Retention and Drainage Aid) program are: 1. Enhanced paper sheet formation quality.
2. Reduction in sheet breakage
3. Adequate retention of fines.
4. Lower loads on water treatment system.
5. Reduction in use of sizing and wet end chemicals
6. Better life of forming fabrics.
Myth: Steady reduction in dosage of chemicals is causing no impact on paper machine runnability. Now, when the program is running steady, mill managers develop a feeling of recognizing the RDA program as drain of money. They start reducing the doses and usually within 24-48 hours they stop the usage of RDA program. Showing that the machine is running fine they claim that a lot of operating expenses of chemicals has been saved. The most important factor to consider is that at this point of time most of the time FPR is not cross-checked, even if it is checked the results come after a long time and the process chemistry starts changing every moment. Till the time the impact is observed, the paper makers become habitual and dedicated to blaming either the team of pulp mill for improper handling of fiber or blaming the mechanical team for non-alignment issues of the suction tops etc.
Chemicals for pulp and paper
Myth: The flocs formed using retention aid are bigger in size hence it is the best chemical and should be used in ETP as well.
It is highly notable that the paper machine retention aid works on the principle of the formation of temporary divalent bonds. The flocs are definitely large but they are short-lived and they cannot hold against mechanical stress. On the effluent treatment end, usually the first chemical to be dosed is a coagulant which helps in the neutralization of any charge in the water, and thereafter flocculants help in the formation of a polymeric web which traps the fine particles together as a bundle in small sizes. The polymeric web is capable of holding its shape and size for a longer time and under mechanical stress exposed to it during pumping and agitation. The small size of flocs ensures that flocs do not easily always settle thus reducing the requirement of agitation. So practically chemicals shall be selected as per the application for which they are meant for.
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