Cutting through the foam

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Cutting through the foam

November 04, 2012 - 14:00

BRUSSELS, Nov. 5, 2012 (RISI) -Paper, board and tissue making processes have become a lot more foamy in recent years, meaning that there is an excess of air mixed in from recycled furnishes and high turbulence papermaking processes and dissolved carbon dioxide gas released from calcium carbonate, now commonly used as a filler and coating. Gas has become an unwanted and hard to control paper making ingredient.

High foam levels in the wire pit or in overflowing chests are obvious symptoms. This foaminess can cause all sorts of troubles with the papermaking process resulting in pinholes, light spots, deposition problems on the sheet and on machine rolls and fabrics, and needless to say lost production due to web breaks and cleanups. Papermakers have responded by using more de-aeration de-foaming chemicals. However, the addition of chemicals is often uncontrolled since the root of the problem - the amount of free air and dissolved carbon dioxide gas - is not precisely and regularly measured and gradual or sudden changes in gas levels are not recognized. As a result, the exact amount of chemical required at any time cannot be applied with any certainty and expensive overconsumption of chemicals is often the result. Moreover, the paper quality and runnability problems are not completely solved.

However, there is an online automated solution that precisely controls the amount of residual air and gas in the furnish by applying just the right amount of chemical to minimize the impact of gas on the papermaking process. BTG has installed its GAS-60 Gas Analyzer, Fig. 1, in a number of tissue, paper and board machines to regularly sample headbox flows and white water and measure the amount of free and dissolved gas. Those frequently updated readings are used to automatically control the addition of chemicals. The result has been significantly lower chemical consumption, less frequent sheet quality problems, fewer machine deposits and improved runnability. All told, some impressive ROIs have been demonstrated.

Figure 1 - BTG’s GAS-60 Gas Analyzer regularly samples the amount of free and dissolved gas in headbox flows and white water, allowing automatic control of chemicals

Defining the problems

The magnitude of the gas content in a papermaking furnish and white water can be quite significant. Figure 2 shows the volumetric composition of free and dissolved gas in a headbox flow. The volumetric gas fraction can exceed the fiber fraction.

Figure 2 - The volumetric composition of free and dissolved gas is up to 5% in the headbox flow

The gas in a papermaking furnish comprises two components which must be measured independently. Free gas (mainly air) is carried into the papermaking process through the stock preparation and deinking plants that use a flotation process. Free gas is also mixed into the furnish at any point where there is high turbulence, typically in pumps, defective centricleaners or anywhere in the sheet forming process where water falls freely.

On the other hand, dissolved carbon dioxide gas is created by the dissolution of carbonate compounds found in hard water or, most significantly, in commonly used calcium carbonate fillers which are added to the furnish and paper coating. Broke that is recirculated during break periods adds to the uncertainty of dissolved gas levels.

Many papermaking processes run at an alkaline or neutral pH where carbonate dissolution is less than in an acid environment. Nevertheless, at a process pH below 8, dissolved carbon dioxide can be a problem, Fig. 3. Even at a neutral operating point dissolved carbon dioxide is significant. Sometime dissolved carbon dioxide can be created when a functional additive like a wet strength agent lowers the pH for a short period.

Figure 3 - Relative dissolved carbon dioxide levels as function of pH

When the headbox stock emerges from a high pressure area in the headbox to a much lower pressure area in the forming zone, the large and sudden change causes the dissolved gas to be released as gas bubbles. These gas bubbles then burst on the wire, displace fibers in the sheet and cause pinholes in low grammage sheets up to 60 g/m2and light spots or cloudy formation on heavier grades.

The surface charge and surface tension properties of gas bubbles can also cause poor drainage. This is particularly important on board machines where even drainage and ply moisture content is required for good bonding.

Excess gas can also cause machine deposits on hydrophobic metal surfaces such as vats, piping, drying cylinders or rolls. Free air is often a problem before the headbox, causing vat foaming and metal surface deposits whereas deposits caused by dissolved gas are often apparent after the headbox and in the press and dryer sections. Figure 4 shows a dryer deposit caused by dissolved air being released as bubbles on the wire. Pinholes in a coating base sheet can cause strikethrough of the coating color and deposits on the coater backing roll. This can lead to sheet breaks and lost time to clean the roll.

Figure 4 - A dryer deposit caused by dissolved air being released as bubbles on the forming fabric