WO2005088007A1

WO2005088007A1 - Method for monitoring and controlling a papermaking process

Info

Publication number: WO2005088007A1
Application number: PCT/FI2005/050078
Authority: WO
Inventors: Leif Piippola; Tero Jussila; Jouko NIINIMÄKI; Tuomas Stoor
Original assignee: Metso Paper Oy
Current assignee: Valmet Technologies Oy
Priority date: 2004-03-15
Filing date: 2005-03-14
Publication date: 2005-09-22
Anticipated expiration: 2006-09-15
Also published as: DE112005000263T5; FI119111B; FI20040395A0; FI20040395L; AT502428A8; AT502428B1; AT502428A3; AT502428A2

Abstract

For monitoring and controlling a papermaking process, the amount and quality of gas present in the process is measured, and the process is controlled based on the information obtained. The content of one or more gases present in the process is measured at one or more points from stock and/or water circulations of the process and/or from gas flows leaving the process. The gas subjected to monitoring can be nitrogen, methane, oxygen or carbon dioxide or any other gas present in the process. Based on the amount and quality of gas it is possible to control, for example, the feed of chemicals or the operation of process devices. The method gives information about disturbances in the operation of the process and facilitates the elimination of them.

Description

Method for monitoring and controlling a paperma ing process

The invention relates to a method for monitoring and controlling a papermaking process.

Air and other gases, both as dissolved and in gaseous form, occur in the stock and water circulations of a paper or board machine. Air (nitrogen and oxygen) gets into stock in connection with thick stock, circulation waters, pulping and mixing as well as because of leakages in process devices. Gases are also produced in the process as a result of bacterial activity (methane), decomposition (carbon dioxide) of carbonates or other compounds. Gases can cause various kinds of harm. They can, among other things, increase the consumption of energy, hamper the operation of pumps, cause contamination of the process, make the dewatering in the wire section more difficult, and reduce the quality of the paper being manufactured. Attempts are usually made to reduce the drawbacks caused by gases by means of mechanical deaeration and/or by adding deaerating chemicals.

In the paper machine environment it is known in itself to measure the amount of a gaseous component in the process stream using different known measurement methods. The measurement methods in use measure only the amount of gas but they do not give any information as to what the gas in the process stream is and why it has got into the process.

An object of the invention is a new way of monitoring and controlling a papermaking process based on measurements made from the process. An object is also a method that provides a more accurate picture than heretofore of the operation of the process, thus facilitating control of the process. The invention is characterized by what is stated in the characterizing part of claim 1.

In the method in accordance with the invention, a papermaking process is monitored by measuring the amount and quality of gas from a gas flow discharging from a process device and by controlling the process based on the information obtained. Said process device can be, for example, a deaeration tank, a passive deaeration duct (for example, an OptiAir Flume produced by the applicant), a cyclone, a centrifugal cleaner, a screen, a pulp chest or a water tank, a sampling device or another equivalent process device through which air and gases are removed from the process. By the amount of gas is meant the total amount of air and the gases originating from the process. By the quality of gas is meant, among other things, the proportion of a given gas component in the total amount of gases.

Advantageously, the amount and quality of gas is also measured from one or more other material flows. The content of one or more gases present in the process can be measured from a stock and/or water circulation of the process and from gas flows leaving the process. The gas to be measured can be nitrogen, methane, oxygen or carbon dioxide or any other gas present in the process. The gas can be in soluble or gaseous form.

Methods known in themselves can be used in the measurement of the amount and quality of gas. With regard to the quality of gases it is possible to measure, for example, the chemical composition of gas, the bubble size distribution in gas, as well as the ratio of dissolved gas to bubbly gas.

The operation of a deaerator can be controlled based on the amount and quality of gas. At present, deaeration devices are often operated, to be on the safe side, at high capacity because the exact amount of the gases in the process is not generally known. A substantial saving is achieved, for example, because, as a result of the monitoring of gases, the vacuum used in the deaerator can be reduced.

Based on the monitoring of gases, it is also possible to control the feed amount or feed location of defoaming chemicals or the quality, amount and feed location of antimicrobial chemicals. An increase in the methane content indicates that anaerobic bacterial activity occurs in the process. An increased carbon dioxide content in turn indicates a change in pH, which has led to the decomposition of carbonate and some other compounds. An increased oxygen or nitrogen content can be an indication of leakages or disturbances in a pump or in another process device in the control of the liquid-level of tanks/chests.

The measurement of the amount and quality of gas at several points of the process makes it possible to determine a gas balance in different sections of the process, such as for the short circulation, the long circulation, stock lines, broke lines, etc. This makes it easier to locate the cause of disturbance and to direct corrective action at a right location.

In the following, the invention will be described with reference to the example of the accompanying figure, but the invention is not meant to be limited to the details of the example.

The figure schematically shows a papermaking process provided with a number of measurement points Ml -Ml 4, at which the amount and quality of gas can be measured to monitor the operation of the process.

Along a line 2, a machine chest 1 is supplied with a stock flow to which two different stock components have been metered from lines 3 and 4 and wet broke from a line 5. Thick stock is pumped from the machine chest 1 along a line 6 to a first dilution point 7, where the stock is diluted with wire water passed along a line 8 from a deaerator 9. After dilution, the stock is pumped along a line 10 to a centrifugal cleaning apparatus 11, from which the accepted stock is passed along a line 12 to a second dilution point 14, where wire water conducted along a line 13 from the deaerator 9 is introduced into the stock. The dilutions of the stock are advantageously carried out as tube dilution, in which a thicker stock flow is introduced into a wire water flow moving in a mixing tube. The arrangement in accordance with the invention can also be used in a process in which thick stock and wire water are mixed before a deaerator and the diluted stock is passed into the deaerator.

The twice diluted stock suspension is pumped along a line 15 to a machine screen 16 and from there further along a line 17 to a headbox 18. A bypass flow is passed from the headbox 18 along a line 19 back to the second dilution point 14 of the stock. A dilution water flow is passed into the headbox 18 from the deaeration tank 9 along a line 20 for the dilution profiling of the stock. The line 20 is provided with a screen 21 to clean the dilution water.

From the headbox 18, the stock suspension is fed to a wire section (not shown). The wire water recovered in the wire section is collected into a wire water tank 22. From the wire water tank 22, fibre-containing water is pumped along a line 23 into the deaerator 9. In the deaerator 9, air and other gases are separated from the water by means of a vacuum. The gases are passed out of the process along a line 24. The gas-free wire water is passed to a line 25, in which water is passed further along the lines 8, 13 and 20 to stock dilution in the short circulation.

From the wire water tank 22, water is also pumped along a line 26 to a disc filter 27, in which fibres and solids are separated from the water. In addition, broke produced in the paper machine is supplied to the disc filter 27 along a line 28. The fibre material, i.e. wet broke, recovered on the disc filter 27 is collected into a broke tank 29, from which it is conducted along the line 5 to be mixed with the stock flow passed along the line 2 into the machine chest 1. On the disc filter 27, two filtrates of different purity are produced, and they are passed along lines 30 and 31 for reuse in the papermaking process.

The amount and quality of gases is measured at points Ml and M2 from the virgin stock flows entering the process and at point M3 from the wet broke flow coming from inside the process. The gas content and the quality of the gases of the mixed, diluted and cleaned stock flow are monitored at point M4 between the centrifugal cleaning apparatus 11 and the second dilution point 14 of the stock. The monitoring operation is repeated from the main stock flow at point M5 immediately before the headbox 18 and in respect of the bypass flow circulation at point M7. The gas content and the quality of the gases of the wire water used for the dilution profiling of the headbox are monitored at point M6.

The amount and quality of gases is measured at point M10 from a water flow passed from the wire water tank 22 along the line 23 to the deaerator 9. The gas flow discharging from the deaerator 9 is monitored at point M8. The amount and quality of gases is measured at point M9 from the water flow discharging from the deaerator 9. In addition to vacuum-based deaerators, as the deaerator it is also possible to use deaerators that are based on delay times, such as an OptiAir Flume™ deaeration duct, a conventional wire pit and/or cyclone.

The amount and quality of gases is measured at point Mi l from the wire water flow passed from the wire water tank 22 to the disc filter 27. The gas content and the quality of the gases of the wet broke conducted along the line 28 to the disc filter 27 are monitored at point Ml 2. The gas content and the quality of the gases of the filtrates obtained from the disc filter 27 are monitored at points Ml 3 and M14.

A number of points where the gas contents of the process can be monitored are listed above as an example. However, good results can be achieved even by means of one or a few appropriately selected measurement points. For example, the amount and quality of the gas emerging from the deaeration tank 9 can be measured at point M8. When a change is found in the quality of the gas, for example, the content of methane or nitrogen changes as a result of increased microbial activity in some preceding process step, attempts are made to remedy the problem by controlling the metering of biocide to the process. By monitoring the amount and quality of gas at point M8 from the gas flow discharging from the deaeration tank it is possible to conclude whether the metering location and amount of biocide is correct.

The data on the amount and quality of gases obtained from the different measurement points M1-M14 can be combined in a desired manner to form a gas balance of the process or a single section thereof. The operation of the deaerator can be controlled based on one or more measurements, so that the vacuum level of the deaerator can be limited to correspond to the actual degasification need. Based on the monitoring of gases it is possible to control the feed amount and location of defoaming chemicals or the quality, amount and feed location of antimicrobial chemicals. In this way, chemicals can be applied where they are most needed and where they are most useful. The measurement of the amount and quality of gas can also be used for monitoring disturbance situations in the process. Monitoring reveals, for example, leakages in pump seals or disturbances in the control of the level in tanks. In that connection, information about a disturbance situation can lead to the initiation of process control or repair action.

Any methods known in themselves can be used in the measurement of the amount and quality of gas which has dissolved or is in the form of bubbles. These include, for example, ultrasound- or microwave-based methods, an acoustics-based free gas measurement method, a measurement method based on imaging and image processing, volume quantity measurement of gases and gas content measurement based on the compression and expansion of gases. The quality of gas can be measured, among other things, by specific gas sensors and by means of gas chromatography. Quality can also be monitored by measuring the distribution of the gas bubble size or the ratio of dissolved gas and gas in the bubble form by means of a measuring device intended for this purpose.

Claims are stated in the following, and many variations of the invention are feasible within the inventive idea defined by the claims.

Claims

1. A method for monitoring and controlling a papermaking process, characterized by the steps of measuring the amount and quality of gas from a gas flow discharging from a process device, and controlling the process based on the information obtained.

2. A method as claimed in claim 1, characterized in that said process device is a deaeration tank, a passive deaeration duct, a cyclone, a centrifugal cleaner, a screen, a chest/tank, a sampling device or the like.

3. A method as claimed in claim 1 or 2, characterized by the step of additionally measuring the amount and quality of gas also from one or more other material flows.

4. A method as claimed in claim 3, characterized by the step of measuring the content of one or more gases present in the process from gas flows leaving the process and from stock and/or water circulations of the process.

5. A method as claimed in claim 4, characterized in that the gas to be measured is nitrogen, methane, oxygen or carbon dioxide or any other gas present in the process.

6. A method as claimed in any one of the preceding claims, characterized by the step of measuring the amount and quality of gas which is in soluble and/or gaseous form.

7. A method as claimed in any one of the preceding claims, characterized by the step of controlling the feed amount and feed location of defoaming chemicals based on the amount and quality of gas.

8. A method as claimed in any one of the preceding claims, characterized by the step of controlling the quality, amount and feed location of an antimicrobial chemical based on the amount and quality of gas.

9. A method as claimed in any one of the preceding claims, characterized by the step of controlling the operation of process devices based on the amount and quality of gas.

10. A method as claimed in any one of the preceding claims, characterized by the step of calculating a gas balance of the process or a section thereof based on the amount and quality of gas.