The dynamic pulp and paper industry continues to experience a rapid and significant change. Among the many trends that have spurred growth in pulp and paper processing is the steady rise of paper products demand in Asia, as well as the upgrading or replacing facilities with those that are more efficient and sustainable. In addition, many pulp and paper processing operations have adopted aggressive conservation and carbon mitigation initiatives in support of sustainability as a core business principle.

Level measurement can play a critical role in improving efficiency throughout the pulp and paper manufacturing process. In this short series of blog articles, we will look at a variety of applications where level control instrumentation can benefit facility-wide efficiency and sustainability. In this first post, we will explore level measurement applications in the chipping, pulping, washing, bleaching, stock prep, blending and papermaking processes. Please follow our blog over the following weeks as we review the benefits of level control in turpentine and liquor recovery, as well as the application of level instrumentation in plant-wide operations including MC pump standpipes, water storage, chemicals and additives, and lubrication and hydraulic oils.

SOURCE WATER & BAR SCREENS

Application: As the largest industrial user of process water, pulp and paper mills are often located next to natural water sources. Bar screens are placed in intake channels or wet wells to remove floating debris that could damage mill equipment. When debris has accumulated, screen cleaning is accomplished with an automated rake typically actuated by a level control mounted in an upstream channel.

Challenges: Water levels of intake channels and plant wells require continuous monitoring. Though normally a routine application, freezing weather can cause complications for level controls, especially those that actuate screen cleaning. A level control operating in frigid, outdoor conditions must be accurate and reliable despite icing conditions.

Level Technologies: – Echotel® Ultrasonic Level Switch for point level – Echotel Ultrasonic Transmitter, Eclipse® Guided Wave Radar Transmitter or Pulsar® Through-Air Radar Transmitter for continuous level

PULP CHIP BINS & SILOS

Application: The majority of mills manufacture pulp stock from wood chips. Mill chippers produce uniformly sized wood pieces that pass through vibrating screens to further sort for size consistency. Chips are stored in large silos and conveyed to the chip bin where they are pre-steamed prior to entering the pulp digester.

Challenges: Level switches monitoring chip levels are designed exclusively for bulk solids. Switches monitor high and low levels to actuate filling operations and trigger alarms in the event of plugged flow or overflow conditions. Level controls must contend with dusty atmospheres, steam, vapors, and the chips’ changing angle of repose. Stable chip bin level facilitates proper pre-steaming of chips.

Level Technologies: – Eclipse® Guided Wave Radar Transmitter with bulk solids probe for continuous level

PULP DIGESTER

Application: The kraft process is the most prevalent pulping method. Here, heat and chemicals (sodium hydroxide and sodium sulphide, or White Liquor) combine in a large, pressurized cooker, or digester, to transform wood chips into pulp by dissolving the wood’s lignin binder. The waste lignin and spent chemicals, or Black Liquor, is routed to a recovery boiler.

Challenges: Digester level monitoring maintains operational stability, increases throughput and reduces kappa variation (the measure of lignin remaining in the pulp). Process conditions of up to +355° F (+180° C), steam, high pressure, and harsh chemicals may challenge many level sensors. Continuous level monitoring and point monitoring for overflow is a common scheme for digesters.

Level Technologies: – Thermatel® Thermal Dispersion Switch for point level – Eclipse Guided Wave Radar Transmitter or Pulsar Through-Air Radar Transmitter for continuous level

DIGESTER BLOW TANK

Application: In batch digestion, the pulp and black liquor are mechanically conveyed or “blown” into an atmospheric blow tank upon completion of the cooking cycle. The tank is a large cylindrical vessel which functions as intermediate storage of the cooked pulp, and from which the pulp (now called “Brown Stock” due to its color) is discharged in an even flow to a washing process.

Challenges: Proper level control of the blow tank helps to maintain balance between the digestion and washing processes. Level controls facilitate on time blow spacing in the digester while promoting improved product quality by maintaining consistent washer production and efficiency. The presence of a tank agitator will affect level control selection, which is typically a high level switch.

Level Technologies: – Echotel Ultrasonic Switch or Thermatel Thermal Dispersion Switch for point level – Eclipse Guided Wave Radar Transmitter or Pulsar Through-Air Radar Transmitter for continuous level

PULP WASHING SYSTEMS

Application: Pulp is washed at two junctures in the chemical pulping process. Brown Stock (pulp with residual lignin) is washed following the digester and Bleached Stock is washed in the multi-stage bleaching unit. In each case, pulp moves through a series of washers and screens to remove residual chemicals and chips. Level controls usually monitor mass tanks and filtrate tanks in the washing units.

Challenges: Precise level control of the two pulp washing lines ensures a consistent supply of wash water; maintains level stability; ensures correct dilution levels for the wash line; keeps filtrate flows in balance; and will lessen the chemical load and possible problems in downstream processing units caused by unwashed pulp.

Level Technologies: – Thermatel Thermal Dispersion Switch for point level – Eclipse Guided Wave Radar Transmitter or Pulsar Through-Air Radar Transmitter for continuous level

CHLORINE DIOXIDE (CLO₂) GENERATOR

Application: Due to health, safety and environmental concerns about dioxins and furans, the once prevalent use of elemental chlorine as a pulp bleach agent has given way to alternate technologies. Today, Elemental Chlorine Free (ECF) technologies are used for about 95% of bleached pulp production. A generator produces CLO₂ that is mixed with water for bleaching.

Challenges: Gaseous chlorine dioxide is conveyed to an absorber tower where it is dissolved in chilled water to yield the aqueous chlorine dioxide bleach solution. Maintaining continuous level in the generator is critical because the chemical material balance in and out of the generator is essential. Malfunctioning controls can create a “White Out” where bleach production ceases and a shutdown ensues.

Level Technologies: – Series 75 Sealed External Cage Switch, Tuffy® II Float-Actuated Switch or Echotel Ultrasonic Switch for point level – Eclipse Guided Wave Radar Transmitter or E4 Modulevel® Displacer Transmitter for continuous level – Atlas® Magnetic Level Indicator for visual indication

PULP BLEACHING TOWERS

Application: Pulp leaving the digester wash unit retains a dark brown color due to residual lignin content that must often be bleached out. Bleach plants whiten pulp through three to five stages of bleaching and water washing. Typically, two pairs of chlorine dioxide and caustic extraction towers are followed by pulp washing stages.

Challenges: Bleaching operations require level controls to maintain consistent levels in the bleach towers and manage pulp flow to successive stages by controlling tower outlet valves. Application challenges include variable pulp density, temperatures over +200 °F (+95 °C), harsh chemicals, and the need for easy cleaning of wetted parts. A point level switch ensures overfill protection.

Level Technologies: – Thermatel Thermal Dispersion Switch for point level – Eclipse Guided Wave Radar Transmitter for continuous level

PULP STOCK STORAGE

Application: Varying densities of pulp stock are stored in horizontal or vertical “chests” that are equipped with an agitator that keeps the stock in suspension. A tower is a larger vessel that provides retention time and a down/upward flow out of pulp. Because pulp can carry residual oxidants that cause corrosion of storage vessels, vessel interiors are lined with resistant materials.

Challenges: Level measurement of storage vessels is necessary to maintain a consistent supply of pulp stock to the paper machine. Pulp vessels represent a level measurement challenge due to thick and sticky media, high temperatures, steam in the vapor space, agitation, and the slightly corrosive and abrasive effects of the slurry.

Level Technologies: – Thermatel Thermal Dispersion Switch for point level – Eclipse Guided Wave Transmitter or Pulsar Through-Air Radar Transmitter for continuous level

MIXING & MACHINE CHESTS

Application: The mixing chest is a large, agitated tank used for mixing various types of pulp, fillers, and additives together in a specified formula for the paper machine. The mixed stock is fed to the machine chest where it is pumped to the headbox and dispensed evenly onto the moving wire of the papermaking machine. When the stock is de-watered and dried, the result is finished paper.

Challenges: Level controls ensure that the chests never overflow and that the level never drops below a safe level with respect to the agitation zone. Level monitoring must contend with pulp thickness, high humidity, and specific gravity changes. As the chest discharge operation is on level control, consistent flow must be maintained to ensure continuous papermaking.

Level Technologies: – Thermatel Thermal Dispersion Switch for point level – Eclipse Guided Wave Radar Transmitter or Pulsar Through-Air Radar Transmitter for continuous level

CONDENSATE RECEIVER TANKS

Application: Steam generated in the recovery boiler is used to run many parts of a mill. Liquor concentration and paper drying are the largest steam users, followed by digestion, bleaching, and chip steaming. (Steam also drives a turbine cogeneration system if a mill is so equipped). A steam condensate system in the paper machine’s dryer section collects water for reuse in the mill.

Challenges: Steam condensate from the dryer drums enters numerous receiver tanks of the condensate return system. Level controls in these tanks ensure that water is returned to the mill for reuse, diverted to storage or discharged to the sewer. When the control senses the upper level in the tank it will actuate a dump valve to remove the accumulated condensate.

Level Technologies: – Echotel Ultrasonic Switch, Series 75 Sealed External Cage Switch or Thermatel Thermal Dispersion Switch for point level – Eclipse Guided Wave Radar Transmitter or E4 Modulevel Displacer Transmitter for continuous level – Atlas Magnetic Level Indicator for visual indication

Discover the benefits of level control for pulp and paper process.