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.  

A Flow Scanner for increased income

Digester control is a skill. The best operators have years of experience. This is the heart of the chemical pulp mill and the results impact the rest of the operations.  Instability in the digester means a varying Kappa number, an imbalance in the pulp production and recovery cycle, and the losses in extra chemicals, lower yields and reduced productivity add up to millions of dollars each year.

Digester operators have many measurement data to consider during and after the cook and must hope that the conclusions they make will be valid for the new wood material entering the process.

The Flow Scanner gives operators a ‘heads-up’ advanced warning of material variations and makes automatic compensation of the ‘cooking’ recipe possible so that it matches the raw material variations and results in a stable target Kappa number.

 

What about other measurement technologies?

Pulp producers in Europe have tested a variety of different techniques including microwave, NIR, Infra-red and radio waves. This equipment can be used as additional information to help diagnose an unexpected outcome but is not considered suitable for active digester control.

Mantex have several Flow scanners installed at European mills that already had well refined and optimised process – the best that current technology could offer. The highly accurate and reliable information from the Flow scanner enabled further optimisations to the pulp process bringing many benefits and a short payback. 

It pays to know your raw material variations

Moisture content, dry density, and chip size are some of the main factors that affect the Kappa number in the digester.

Moisture content

Moisture content in wood chips can sometimes vary 5% in a day and 10% in a week.

Density

Differences in dry density and bulk density bring yet more changes in the amount of fibre and water entering the digester.

Chip size

Packing grade is strongly influenced by chip size. Bulk density measurement can reveal changes in the amount of wood entering the digester.

Why Kappa stability matters

Every pulp mill strives to meet the target Kappa number through the correct digester settings. And every mill has a significant amount of pulp produced outside the target range (see image). Major factors that can cause this problem are moisture variation, dry density variations of the chips, and the bulk density. If these wood chip parameters are known in advance the digester operation can be adjusted to bring greater stability.

Kappa Stability means preventing losses due to overcooking and undercooking the pulp. Reducing the amount of pulp with too low Kappa brings higher yields and reduces the load on the recovery boiler. Benefits will be realised in wood savings and improved production levels. Preventing too high Kappa means reduced reject, less bleaching chemical costs, with the higher pulp quality being sold at premium prices.

Improvements in Kappa stability results in substantial improvements in the amount of premium grade pulp produced. It is our belief that every pulp mill can realise additional revenue from this.

Feed-forward control for the best production process

Digester-Illustration

 

Current digester control methods rely on residual alkali measurements and Kappa meter samples taken after cooking. The measurements are slow and the woodchips are not the only reason for changes in the readings. Even if the operator did draw the correct conclusion from the results, the chips now entering the digester have no relation to the ones that gave the alkali or Kappa readings.

Imagine instead that the digester control system received information in advance based on all the material being fed into the digester.  The digester control system can automatically adjust for the variations and minimise Kappa changes.

Enjoy full production capacity

Often it is the recovery boiler that limits production capacity in chemical pulp mills. Todays high throughput demands can leave the recovery block under-dimensioned – especially when soot and ash have built up inside. When the pulp is cooked to a low Kappa number the increase in black liquor can produce a backlog in the recovery cycle and production may have to be decreased. The stabilised Kappa from using the Flow Scanner means the mill runs closer to full production capacity.

 

Increasing-Kappa

  1. Original Kappa variations centred around the target Kappa. The acceptable variation is shown in black with actual variations in grey. The higher Kappa values cause added costs. Target Kappa is chosen to maximise yield whilst minimising reject and bleaching costs.
  2. Improved digester control with the Flow Scanner means a reduction in the Kappa variations. The spread is still centred around the original Target Kappa value but means more pulp is within the accepted range.
  3. The Target Kappa is raised with subsequent improvement in yield and reduced demand on the recovery boiler. With the lower Kappa variation the upper values are within the acceptable range and do not negatively impact the rest of the fibreline.

Do you need more information?

Contact Sales Manager Karl Wejke on karl.wejke@mantex.se
or tel:+46 72-857 32 48.