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USING ONLINE POROSITY ANALYSER FOR PROCESS AND QUALITY CONTROL
Pekka Komulainen*, Heikki Mustalahti*, Jaakko Rintamäki*, Urpo Launonen **
Keywords: porosity, analyser, control, paper, quality
In recent years, porosity analysers have been developed to the level that they can be used as a reliable and accurate tool for monitoring, reporting and control of process performance and total paper quality on paper and board machines.
The unique features like fast measurement, automatic self-cleaning cycle and single calibration over the complete paper grade range have made it possible to achieve accurate and reliable readings without marking or breaking the paper web.
Accurate online porosity measurement enables a better understanding of various papermaking parameters and how they effect on porosity and total paper quality. The online readings also enable process response tests to pinpoint the critical process variables to find control windows for porosity to maintain the optimum paper quality and saleable output.
Because porosity correlates well with several papermaking process and raw material parameters, the online porosity reading is an excellent indication of the changes in the paper structure and process performance.
In the following presentation paper porosity actually means air permeability. Porosity can be measured in the laboratory or online on the paper machine. In the following, the measurements are air flow values like in Bendtsen (ml/min) or time values for a constant air volume like in Gurley (s). Notice that a smaller number in seconds means a higher air flow and porosity.
Online porosity is the only real measurement of paper structure. For most of the paper properties, it is more important to get information about paper voids than about material in the paper. Paper voids are important for absorption and optical properties. Increased bonding decreases paper voids hence voids are important for strength properties as well.
PERMI POROSITY ANALYSER
The Permi porosity analyser is usually installed after the last dryer or pre-dryer in the case of coated or surface sized paper. It can also be installed after the last calendering, if there is measurable porosity left in the paper. The installation location also depends on the availability of space and maintenance access. The most common position is under the web but there are some references (Fig. 1) where the analyser is on top of the web.
The latest online porosity analyser has a strong and durable structure avoiding mechanical damages during machine breaks. The sensing element is specially coated to minimise friction, dusting and dirt build-up on the web contacting surface.
The Permi porosity analyser uses continuous vacuum air loops to secure the complete web tightening against the sensor element and to optimise air flow through the paper.
A fully automatic sequence cleans the hoses, sensor head holes and air filters in preset time intervals. This unique cleaning mechanism allows for long operating time without any need for mechanical cleaning or the need to change the air filters. The typical maintenance period varies from 4 weeks to 2 months depending on paper grade.
The unit is designed so that its preventive maintenance can be done within a few minutes during the paper machine wash up or maintenance shut down. Air filters can be replaced and the Teflon pipes cleaned. The measurement can also be tested with external calibration samples.
The Permi analysers have been proven to need only one linear calibration curve per paper machine. No effects of the web moisture or web temperature have been reported. These unique sensor features make it possible to have accurate and reliable online porosity readings also during process changes such as machine speed, grade changes and machine start up.
The porosity sensor is designed so that the total compressing air volume in the piping and sensor head is minimised. This feature together with the fast flow measurement allows for very fast measurement sampling and makes it possible to monitor and record short term machine direction variations. These can be read to an external system via high speed analog signals or read to the external PC unit for further processing. Advanced signal filtering is used to stabilise and eliminate unnecessary noise.
There are some closed loop control references e.g. with refining. A more common practice is to have a control window in the DCS system, where MD porosity and some other paper quality parameters are shown. Machine operators then monitor and keep these readings inside the window to optimise paper quality and process performance. A common critical pair is formation and porosity.
Figure 1. Permi online porosity analyser
WET END PROCESS AND POROSITY
Porosity and retention
However, it is very common that the incoming thick stock has variations in the fines content. Paper structure and porosity are very sensitive to the amount of fibre fines. Let us suppose that the incoming fibre fines content increases. If we keep constant retention in this situation, the final paper contains more fines and more bonds are formed resulting in lower porosity and completely different properties. Something more is required to compensate for the changes in the incoming raw materials and waters.
It is better to measure paper porosity and try to keep it constant. Porosity is a real structural property of dried paper, where we easily see the effects of fibre and filler fines on bonding and sheet structure. The type and amount of this fine material can have extensive variations in a short time.
Examples of web breaks and broke
If the base paper porosity is too high, coating colour is penetrated deep into the base paper, which is a web break risk. After every web break porosity is normally low due to the higher amount of fines and broke in the base paper. After the web break there is a lot of new water, chemicals and fibres in the broke tower. When the machine runs without web breaks, the content of the broke tower gets older and causes more problems in papermaking, especially lower bonding between fibres as long as a new web break occurs.
When the machine runs well without web breaks, base paper porosity and colour penetration to the base paper increase as long as we get a new web break again. It seems that a web break is required to get good runnability! This kind of automat can be well seen in base paper porosity measurements (Fig. 3).
One principal fault in this kind of system is the control of constant base paper ash. Coating pigment is very fine compared to the fresh filler in the base paper. A higher particle size of the filler gives higher porosity than a lower particle size. By using more coated broke, there will be more fine pigments in the base paper and porosity will be lower. On the contrary, when the uncoated broke amount is increased, porosity is higher (Fig. 4).
Effect of fibres and refining
The example in Fig. 4 shows how small changes in the broke dosing can have a radical effect on porosity. Uncoated broke is increased from 7 to 10% of all fibres and paper porosity increases radically. There must be several reasons or a chain reaction to this behaviour. One reason is the higher filler particle size compared to coating pigments. Because particle size of the filler has a great effect on the paper porosity, it is possible to control porosity by using the same pigment type but two particle sizes. Coarse carbonate can be used to increase porosity and fine carbonate to decrease it. When there is high amount of coated broke in furnish, fresh filler should be coarse.
It is very common to add long fibres when there are problems with runnability without knowing the real effects on the paper structure. When long fibres are added, refining of all fibres should be changed to keep the paper structure and porosity more constant.
Total Process Control with Porosity
Porosity measurement is usually connected to DCS where the reading can be utilised with other quality parameters' trends for operator reports. One way is to have an operating window where both porosity and formation measurements are displayed for the operators to be able to keep both readings inside the preset window limits.
Total Paper Quality Control with Porosity
The online porosity measurement is at the moment the only tool for better monitoring and control of the effects of raw materials and wet end variables on paper quality and process performance. It has brought new knowledge about the importance of different wet end variables.
The most important variable according to mill measurements seems to be the quality of the incoming thick stock flow. Normally there is lot of variation in the incoming furnish. If all process variables are kept constant, the variation of incoming thick stock is reflected in the final paper quality and process performance. This situation can be improved by monitoring and controlling paper porosity online.
A reliable online porosity analyser can be used to test papermaking by adjusting the critical paper machine and furnish parameters and monitoring and analysing the porosity behaviour.
Online porosity together with quality parameters such us basis weight, moisture, thickness and formation bring more real time information about paper quality variations in machine direction (MD) and help papermakers better understand the papermaking process and then make corrective actions to optimise process parameters.
In the future, we can expect that online porosity measurement can be used together with other MD control parameters to ensure more complete process and paper quality control.
(1) Paavola A.N., Garnett B.C., Steele T.H., Hird A. – Online measurement of paper porosity
profiles. – APPITA Conf., 2005
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