On-line freeness sensors used in manufacturing of paper products

sheet formation, and final sheet properties such as Generally, it is very difficult to predict stock tensile strength, burst, tear, and fold. With an drainage behavior on the paper machine, however, accurate on-line freeness measurement, various drainage sensors have been developed to papermakers can identify causes of stock variation. give some indication of paper machine performance Then, by adjusting the furnish component, amount with respect to stock drainage, and methods to of recycled pulp, and refiners load, they and automatically control refmers during manufacturing minimize or eliminate the stock variation, and of paper products. The purpose of the paper is to increase production efficiency through. Improved provide an overview of the existing freeness uniformity of stock drainage on the wire leads to sensors and describe their operating principles, and efficient drying and less load on the refiner, thus, variables which may influence their measuring reduced energy costs; higher production; and accuracy, improved paper quality. Laboratory methods such as the Williams freeness INTRODUCTION test (1), the Canadian standard freeness (CSF) test Drainage of a pulp suspension on the paper (TAPPI T227 om-85), TAPPI drainage (TAPPI machine wet end is one of the most important T221 om-93), and the Schopper-Riegler freeness variables that influences the quality, productivity, (SR) tests (SCAN C19:65)have been developed to and efficiency of the papermaking. Generally, it is measure pulp freeness. The William freeness tester very difficult to predict stock drainage behavior on and TAPPI drainage measure the time required for the paper machine; however, various drainage a fLxedvolume of water to pass through a wire testers have been developed to give some indication screen from a fixed volume of stock sample. The of paper-machine performance with respect to CSF and SR freeness testers measure the volume of drainage characteristics, and provide input for water that drains through a perforated plate from a automaticcontrol ofthe refiners, fixed volume of stock collected in a cone. The Freeness is a measure of the rate at which water above two tests use the principle of divided flow. drains from a pulp suspension through a mesh The cone has two discharge orifices one of small screen or a perforated plate, and it is measured to bore at the bottom, and the other of larger bore in assess drainage characteristics of stock during stock the form of a side arm. Among these off-line preparation at some location before the paper methods, the CSF is widely accepted technique by machine head-box. This quantity which depends the paper industry in the United States. A high upon the pulp stock, fiber flexibility, and the f'mes freeness number represents a faster drainage rate in content of the pulp, can be used as an indication of the CSF test. It should be emphasized that these tests methods operate in continuous mode consist of a tank and a determine the pulp freeness under conditions that rotating cylindrical dram covered with a screen. A are completely different from those in the paper pulp sample is continuously feed into the tank. For machine, and the measured values may have little a fixed liquid level in the tank, the fl°w through the relation to the drainage of stock on the machine screen is measured and correlated to freeness. wire (2). However, the results are stil! very These types of sensors have not gained popularity valuable with respect to guidance of operation of by the pulp and paper industry (5). The following mill components such as beaters and refiners, sections provide a general description of some of Some studies have applied Darcy's law to these the freeness testers that operate in batch mode. freeness testers to develop theoretical models for Additional information can be obtained from each pulp freeness based on filtration principles (3,4). instrument operating manual and other literature On-line freeness measurements is based on sensing (e.g., references 5-7). of some properties that are related to freeness, and the measured quantities are expressed in terms of I. INNOMATIC 63M-7 freeness. Because of this indirect measuring The Innomatic 63M-7 freeness analyzer (8) can method, variables other than freeness may affect be connected to a process line by using a bypass the signals produced by the sensors. Temperature (shunt) line that allows for maintenance or shutting and consistency are two important variables that down of the sensor without interrupting pulp flow can have a significant influence on freeness through the process pipe. The sensor has a measurement. Standard temperature and microprocessor base electronics cabinet for control consistency for laboratory measurement of CSF are and readout which is separated from the analyzer. 20°C and 0.3 %Cs, respectively. At higher The analyzer consists of an open-ended stainless temperatures, viscosity of waters drops which steel (SS) inner tube and an outer SS vessel (Figure results in greater pulp freeness (CSF). For instance, 1). The stock pulp from the by pass line enters the a +10°F change in stock temperature can cause as outer vessel and fills the inner tube to the same much as 12 percent variation in freeness in the level as the outer vessel. Then, from the outer operating range of 400 CSF. During laboratory vessel gravity discharges into a chest. The sample measurement of freeness, a consistency variation of pulp column in the inner robe is connected to _ 0.1%Cs (compared to the standard of 0.3 %Cs) another tube through a screen plate. The second can cause as much as +_15%change in freeness in tube which has two internal electrodes, acts as the the same range. Because of this strong sensitivity measuring chamber of the sensor. These two tubes of freeness to pulp temperature and consistency, form a manometer, and because of a pressure appropriate correction tables for these variables are differential produced in the manometer, the sample provided in the TAPPI test methods (TAPPI T227 pulp in the inner tube flows through the screen and om-85). Other variables such as fumish type, accumulates in the measuring chamber. The furnish treatment, amount of frees, test method, amount of time required for the filtrate to reach additives, stock flow rate, line pressure, PH, from one level to another level is measured by the entrained air, and dissolved salts may have some two electrodes and used as an index of freeness. influence on freeness measurement. After the filtrate contacts the upper electrode, the measuring cycle ends, and the chamber is cleaned DESCRIPTION OF SOME COMMERCIAL using pressurized air and water, which back flush ON-LINI= DRAINAG!= TESTERS the filtrate through the screen and expel all the pulp In general, most on-line freeness testers operate from the inner tube. based on filtration principle in which water is Approximately 10-12 seconds later a new stock extracted from a pulp suspension through a screen, sample enters the inner robe and the cycle repeats. and its drainage rate through a fiber mat that is Depending on the furnish type and treatment, each formed on the screen is measured. Most sensors measurement may take approximately 1-3 minutes. (e.g., Drainac IIB, Innomatic 63M-7, and KOEI) This sensor compensates for both temperature and freeness testers operate in batch mode. In these consistency. sensors, the amount of time it takes for a fixed A summary of the specification of this sensor and volume of filtrate to be extracted from a fiber mat other freeness sensors is given in Table 1. Shown in under a fixed pressure differential is measured, and Figures 2 and 3 is Canadian Standard Freeness used as an index of the pulp freeness. Sensors that (CSF) from laboratory samples versus Innomatic sensor output (8) for bleached softwood, and line to avoid the effect of line pressure, consistency, hardwood,respectively, and otherprocessfluctuations.Up to four sampler devices can be used to extract a sample from II.TECO DRAINAC IIIB various locations in the process pipe. The TECO drainac IIIB (9), which is manufactured by following are the measuring sequences of this the Thompson Equipment Company (TECO), is a sensor: more recent model of a series of Drainac sensors that were originally developed by the Bolton A. The pulp sample in the mixing chamber is Emerson Inc. This sensor operates on the filtration diluted to a suitable consistency (e.g., 0.5-1.2%), principle, and employs a microprocessor that and a fixed volume of it flows into the analyzer's compensates for process variables such as pressure, camber (Figure 5b). After measuring the sample consistency, temperature, and PH. The detector is temperature, the stock is placed on a wire screen. mounted directly on the process pipe, and a predetermined differential pressure raises the stock B. Water from the pulp pad formed on the screen into the detector robe. The measuring cycle of this wire is sucked by a vacuum, and after a sensorisasfollows: predeterminedsuction time, the weight of the sample pad and the pressure difference over the pad A. Start: The stock pressure is continuously are measured. measured at the detector robe by a transducer, and the air pressure is controlled to be a few psi (e.g., 5 C. The measured values of pressure difference, pad psi) above the stock pressure. This pressure weight, and pulp temperature are used to determine differential allows for the detector robe to be the pulp freeness. purged by air and water. D. The sampler and analyzer are flushed, and the B. Intake/measure: After cleaning the measuring measuring cycle is repeated. tube, the air pressure at the chamber is regulated to be a few psi below the process line pressure. Pressure difference (AP)at various times, and the Because of this pressure differential, the stock set time at which AP is measured are shown in passes through a screen and rises in the measuring Figure 5c. Pulp sample consistency as meas