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Most of the kraft cellulose pulp is produced in continuous digesters. When they were first introduced, the cooking liquor was only added to the top of the digester. Quite similar to batch cookers, where cooking chemicals are added before each batch. Later came the modified continuous cooking (MCC) where cooking chemicals are added and removed at several points. The control strategy became more complex since a so-called alkali profile became necessary. Since then, a major challenge on continuous digesters is to measure and control alkali profile along the digester so that lignin and the wood’s resin, fat and terpenes are dissolved out of the wood fiber. This must be done while reaching desired and stable Kappa numbers at the digester outlet, without losses in pulp strength or yield during the wood chips descent through the different cooking zones. This chip column movement has become one of the most important tasks for the operators to monitor.
Liquid to wood chip quantity affects the degree of compaction and can lead to excessive friction that impinges the movement of the chip pile. The defibration is also strongly dependent on time and temperature, which is summarized in the H-factor, the time integral of the cooking temperature. Long response times of five hours make PID control difficult and for the alkali profile one also relies on model predictive control (MPC).
Input variables
Three important input variables for the MPC are alkali of the cooking liquor, Kappa number of the produced pulp and flows to and from the digester and impregnation vessel. Alkali and Kappa numbers can be sampled and calibrated against the lab in case of deviations. Liquor flow rates cannot be sampled or calibrated during operation and have therefore constituted an unknown source of error. Only in cases of pure breakdowns, when flowmeters obviously break or alarm with error codes, have you been made aware that a measuring tube replacement is necessary, but have had to wait until you can stop and replace them.
The weak link
Electromagnetic flowmeters are the most common way to measure digester flows. However, there is a relatively unknown issue that arises when measuring liquids with extremely high design temperatures, such as cooking liquor at +180°C (356°F). The problem can be traced back to the magnetic flowmeter tube being lined with PTFE or PFA. Similarities can be observed with membrane technology, where a semipermeable membrane allows specific components to pass through, while separating others. For molecules to be transported through the membrane, a driving force is required. This can be created with various factors, including differences in pressure, electrical potential, temperature, concentration, or osmotic pressure. The liquid that has passed through the membrane is called permeate. This phenomenon has been shown to occur inside magnetic flowmeters on digester flows. Despite appearing brand new externally, meters can deviate by up to -50 % during evaluation, even if they have a more durable PFA liner. An identifiable symptom of this issue is the presence of water in the connection housing of the magnetic flowmeter tube. Although this water is pure and clear, it is often not associated with liner diffusion since liquor appears black. However, it’s essential to recognize that the permeate is still as effectively filtered as in the previous comparison with membrane technology. Magnetic flowmeters, which measure flow using weak millivolt signals, are influenced by the permeate. Over time, they gradually drift and deviate, often without operators or maintenance personnel noticing.
New practice
Endress+Hauser has developed an external method that has become the most widely used in kraft pulp cooking plants and the one with the most references of its kind. The method is 3G acoustic and represents the latest in non-invasive permanent measurement. This avoids the uncertainty of diffusion-damaged magnetic flowmeter tubes. The Prosonic flowmeter allows installation with only 2xD straight line from pipe bends. It can be clamped on wherever it is easiest to reach without having to build scaffolding. Powerful acoustic contact sensors with advanced signal processing measure through pipe scaling and high TS levels on black liquor. In recent years, some pulp mills have already moved away from magnetic flowmeters and switched to vortex, which is a measurement method for steam and condensate, to get a meter completely made of steel and without liner. But since vortex is a technology where a sensor, or pickup as it sometimes referred, is still inside the measuring tube, the maintenance has instead shifted to changing pickups. Early signs of damaged pickups are usually that the vortex meter starts to oscillate on the output signal. They can also be covered by lime mud in white liquor, which causes them to stop sensing the flow. For those who would prefer an inline installation, a flanged inline model that can withstand +200°C is also available. This offers an extra +20°C greater safety margin than today’s high-temp magnetic flowmeters that are limited to +180°C. The inline model of Prosonic has also been tested at pulp mills. It is installed the same manner as a magnetic flowmeter but is made of 100% acid-proof steel, without a sensitive lining. Prosonic reduces maintenance while increasing availability and accuracy of digester liquor flows.
Author:
Ulf Johansson
Business Driver Pulp & Paper, Endress+Hauser
