The present invention relates to a method in accordance with the preamble of claim 1 for controlling the production of thermomechanical pulp.

The invention also concerns an apparatus for the implementation of the method.

In prior art methods, the feed of chips to the rotating thermomechanical pulp refiner was under manual control of set values for chip entry and water feed. In the manual control method, control delay of some process set values is naturally extremely long, typically in the order of several hours. The selection of set values is approximate and inconsistent. Furthermore, since the density and size variation of entering chips is random, this method is incapable of producing consistent quality of thermomechanical pulp.

Efforts have been made to estimate the water quantity of the chips entering the refiner, but approaches to a reliable moisture content sensor for chips have been unsuccessful.

Known in the art are also adjustable systems, in which an attempt has been made to maintain power consumed by the refiner constant by regulating the quantity of fed water. Yet, although the power input from the mains to the thermomechanical pulp refiner is maintained constant, thermomechanical pulp presents deviations due to variations in density of chips.

Water feed is also controlled by first measuring the freeness value of thermomechanical pulp which gives a standardized measure for pulp drainage and is characteristic of the fines of thermomechanical pulp, and then, on the basis of determined freeness, adjusting water quantity production capacity, and disc clearance to obtain desired freeness value. This method, however, is insensitive to changes in consistency and density. In addition, freeness measurement is time-consuming, and consequently, does not lend to real-time control, but rather presents an appreciable delay between the time of measurement to that of control.

The aim of the present invention is to overcome the disadvantages associated with the prior art technology and achieve a totally new kind of method and apparatus for controlling the production of thermomechanical pulp.

The invention is based on measuring in a continuous manner the moisture content of thermomechanical pulp emerging from the refiner so as to use the measured value for controlling the ratio of additional water to volume of fed chips to a desired level.

More specifically, the method in accordance with the invention is characterized by what is stated in the characterizing part of Claim 1.

Furthermore, the apparatus in accordance with the invention is characterized by what is stated in the characterizing part of claim 4.

The invention provides outstanding benefits.

With help of moisture content measurements of thermomechanical pulp, disturbance created by variations in moisture content can be eliminated. As the moisture content is under control, the production machinery can be run at higher capacity resulting in an increase in efficiency.

The invention is next examined in detail with help of the following exemplifying embodiment illustrated in the attached drawings.

Figure 1 shows diagrammatically a control system in accordance with the invention.

Figure 2 shows diagrammatically another control system in accordance with the invention.

Figure 3 shows in a partially diagrammatic form a measurement set-up corrected to the control system illustrated in Fig. 1.

Figure 4 shows diagrammatically a principle of infra-red measurement.

Figure 5 shows diagrammatically a measurement set-up of infra-red measurement.

Figure 6 shows in the form of a graph the correlation of the infra-red measurement method to laboratory verifications.

According to Figure 1, wood chips to be refined are conveyed to the thermomechanical pulp refinery by conveyor 1. The chips are fed and metered with help of a feeder 9 rotated by a motor 10 to a feed chest 2 of the thermomechanical pulp refiner, from where the chips are further fed into the gap between the refining discs 5 by a feeder auger 3 rotated by a feeder motor 7. In the feed chest 2 or within the auger tube 3, water is added by a volume regulated by a controller 4. Between the discs 5, the chips are ground into thermomechanical pulp, and the generated steam expels stock forward via a control valve 6. The purpose of the control valve 6 is to maintain a constant steam pressure. After the discs 5, prior to the valve 6, a moisture sensor 21 for thermomechanical pulp is arranged to the outlet pipe 25. A corresponding sensor 22 may also be placed on the route of the thermomechanical pulp after the control valve 6. The obtained moisture signal is taken to the controller 4 or to a data processing unit 11. If the moisture of thermomechanical pulp falls below a desired set value, water volume in the chips is increased by either reducing feed rate of chips or increasing volume of added water using a conventional control method. For an excessive moisture content, the opposite is true. In practice the control operation takes place by sending a new set value to the controller 4 from the data processing unit 11.

According to Figure 2, two thermomechanical pulp refiners are connected in tandem. However, the number of moisture content measurement points is greater. A moisture content sensor 23 may be located to the outlet pipe of the second refiner. A sensor 24 may also be placed to a point after a cyclone 12 in, e.g., the outlet pipe of the cyclone. Each sensor 21, 22, 23, and 24 is advantageously arranged to have independent function and transmission of sensor signals to a data processing unit 11, whereby the signals may either be selected for an optimum singular signal best describing the process or be subjected to a mathematical processing by, e.g., averaging, to obtain a suitable control signal. In some cases a single sensor may be sufficient. Both thermomechanical pulp refiners are provided with identical control equipment 4 of water addition according to the set-up in Fig. 1. The set values for the thermomechanical pulp refiners, however, may be different.

Illustrated in Figure 3 is a measurement set-up attached to an outlet pipe 25 of the thermomechanical pulp refiner allowing a bypass pipe 42 to be configured to the thermomechanical pulp flow. The pipe 42 is provided with a choke valve 26 for controlling the bypass flow. The steam developed by pulp expanding to a larger volume is removed via a condenser 43, and the thermomechanical pulp is transferred by means of an auger 45 rotated by a motor 44 to moisture content sensors 27 and 28. For making nontransmissive infra-red measurements, sensor unit 27 is sufficient. When using microwave measurement, a receiver unit 28 is additionally required.

According to Figure 4, the infra-red equipment operates by sending IR light from an IR source 29 via a filter disc 30, and the filtered light is dispersed by water molecules 32. The dispersed radiation is detected by a detector 31. Water molecules 3 3 remaining under the surface escape detection.

In the embodiment illustrated in Figure 5, light emerging from the IR source is routed via lenses 35 and mirrors through a filter unit 36, and via a mirror 38 to a target 39. The filter unit 36 is provided with a chopper unit 37 for chopping the light beam. Light reflected from the target 39 is routed to a light-dependent resistor 40 acting as an IR detector, and the output signal of the resistor 40 is amplified by an amplifier 41.

Direct measurement of thermomechanical pulp moisture content under pressure is also feasible by mounting a transparent section to the stock pipe. When using the aforedescribed IR measurement, a mere transparent window will suffice.

When using microwave measurement, a sender unit 27 and a receiver unit 28 a located to the opposite sides of the stock pipe. The stock pipe must be of a microwave-transparent material, e.g., teflon, at least for the section used in the microwave measurement.

Figure 6 illustrates the correlation of moisture content from IR measurements to laboratory verification results. In the moisture content measurement session, the output signal of the moisture content sensor was 2.30 V, the flow rate of additional water was then 85 l/min, and the freeness was 145 CSF. After a change in the moisture of entering chips, the sensor signal was 2.41 V, and the corresponding freeness was 153 CSF. The controller adjusted the rate of water addition to a level of 78 l/min, resulting in the return of the sensor signal to a level of 2.32 while the corresponding freeness was 142 CSF. No major changes were detected by the measurements in the moisture content of chips. Consequently a direct measurement of moisture content from the chips was unsuccessful, because the sensor measures only the surface moisture of chips.