Mark Williamson, Journalist/Engineer, Thornhill, ON, Canada
Nov. 12, 2012
Paper, board and tissue making processes have become more foamy in recent years, caused by an excess of air mixed in from recycled furnishes, high turbulence processes and dissolved carbon dioxide gas, and making it an unwanted and hard to control part of papermaking. Read Part I here.
Online analysis is the key
De-foaming and de-aeration chemicals of course address many of these problems. However, operators cannot respond so quickly to changing needs brought on by a change in the furnish or broke recycle. Variations in ash levels, basis weight changes and dilution water can introduce air. If foaming persists there is always the tendency to add more chemical and leave it there just to be safe, even if it means a higher cost. Online measurements and controls have been the solution to this dilemma.
BTG's GAS-60 Gas Analyzer measures online both free air and dissolved gas by a sequential sampling and analysis technique. The principle shown in Fig. 5 is simple and is based on Boyle's gas law: P
1V 1 = P 2V 2. Boyle's law states that, at constant temperature, the product of an ideal's gas's pressure and volume is always constant. In a papermaking stock only the air or carbon dioxide is compressible whereas the water and solid materials are not.
Automatically controlled valves fill the analysis chamber with headbox stock or white water. An internal piston in the chamber is moved up and down from its initial position to change the chamber volume and compress the free air and then release the dissolved gas.
The gas analyzer samples two points sequentially: typically the headbox flow and white water, then the control in a distributed control system (DCS) regulates the flow of chemical based on target values for residual free or dissolved air where papermaking problems are eliminated. Which one is chosen for controlling the addition rate depends on the main objectives. If a reduction in pinholes is the goal, the dissolved gas of the headbox sample is controlled below a certain level. If the objective is to reduce the de-aerator or de-foamer chemical costs the white water free gas reading is used for control with supervision by the headbox measurement. On machines with deculators it is possible to regulate de-aeration chemicals to reduce the loading on the deculator. In this way extra deculator capacity can be kept in reserve and the total cost of chemical and mechanical de-aeration can be minimized for a constant residual gas level.
Working with chemical suppliers
BTG has combined its measurement and control expertise with the chemical application know-how of chemical suppliers to tackle foam control problems. One example is a large southern US linerboard mill with two machines. Operators habitually over-used chemicals to control foam. They knew that high wire pit foam was usually followed by sheet breaks and defects so their manual strategy was based on inspecting wire pit foam level. However, entrained air tests revealed the real problem was entrained air in the headbox. But no one knew when this condition would end so de-foamer was regularly added in excess of what was needed.
A GAS-60 Gas Analyzer serving both machines was implemented and the operators were able to learn about and get used to the measurements and gradually reduce the chemical consumption. After implementing automatic closed-loop control in the DCS, the chemical consumption plummeted by 31.6% resulting in savings of hundreds of thousands of dollars per year and a handsome investment payback period of less than three months, Fig. 6.
Results in newsptint and tissue
A 270,000-tonne/yr newsprint machine suffered runnability problems from high free gas levels in the headbox that came from DIP furnish. After implementing closed loop control which reduced the gas content, the paper quality and runnability were better, retention and drainage were improved and the de-aeration chemical consumption was reduced by 60%. Altogether, the ROI was two months.
In a tissue machine application the elimination of holes and associated web breaks was the primary target for gas control. Sheet holes were a problem during grade changes between grades containing wet strength resin and those containing none. The closed loop control was very successful, reducing free and dissolved air levels with the same chemical consumption. Holes and breaks were reduced, less broke was recorded during grade changes and the machine production was increased by two percent. The ROI was six months.
Sustaining the results
For papermakers with chronic problems associated with foaming and high chemical consumption, a closed loop air control solution may be worth a look. The continuous measurements and controls are important to achieve and sustain the goals since the results of one-time tryouts are often short-lived. After short-term trials, the chemical addition rates often climb back to excess levels as papermaking conditions change.
To date, more than 100 units have been installed in various papermaking applications. Gas analysis is finding a new application in brownstock washing.
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