How does a Flow Scanner help my pulp production process?
Woodchips used in pulp production have different moisture contents and wood densities and variable dry and wet bulk densities in the digester. Until now there has been no reliable and accurate way of knowing what these variations are before they go into the pulp making process. Digester operators have information in the form of residual alkali and kappa readings to give an indication of how the cooking process has performed. It is a time-lagged feedback system with many other factors that can influence the outcome.
Wood material is known to have different properties depending on species, age, harvest location, storage time and between fresh cut chips and sawmill chips. The digester feedback system can cope with long term seasonal effects but daily, weekly, or even monthly changes are often difficult to compensate for in this way.
Do the woodchips at my mill vary enough to need a feed-forward measurement and control system?
Even if regular laboratory samples show a variation in moisture and bulk density over shorter timescales it has not been known if these were on a local scale from sample to sample or reflected in the total flow of material. Maybe these changes would even themselves out when hundreds of tons of material are in a digester.
Mantex have several Flow Scanner instruments installed at Nordic and central European sites, with a variety of hardwoods and softwood, at continuous and batch digesters, for several years. The unique dual X-ray technology of the Flow Scanner measures all material travelling on the conveyor belt to the digester and so gives truly representative data on how the raw materials vary. The wet mass flow and moisture content, dry mass flow, and bulk density information are available before the cooking process starts. Observations from the sites show that there is variation of all wood types on short timescales to long term effects. It is the variations over hours and days that can bring the major benefits. Choosing the update period for the digester control is mill specific but could be on an hourly to daily basis.
Integrated digester control for full benefits
How to use the Flow scanner information? So the average moisture content of the woodchips on the belt increases by 2% and there is an increase in bulk density (due to smaller chip size and more sawmill chips). What can be done? The digester control strategy will vary from mill to mill but at its heart the process seeks to match the active alkali dosage to the amount of dry content. This is exactly what the Flow Scanner delivers in the form of digital signals that can be integrated right into the digester control system.
The Flow Scanner is the only system we know of that is accurate and reliable enough to be fully integrated into the digester control system to automatically adjust the cooking parameters cook to cook, hour to hour, each and every day. Laboratory samples are used to validate the Flow scanner so that it can measure all the material.
Flow Scanner integration of the dry content and moisture level is relatively easily achieved for batch digesters. For continuous digesters a control program is needed to account for the chip silo and fill factor of the chip meter. This control program can be delivered by our partner Andritz for a complete system integration or via a separate third party.