Pulp & paper

Black Liquor is the digester waste mixture of spent chemicals and lignin extracted from wood chips. When burned in a recovery boiler Black Liquor produces heat for steam and also releases digester chemicals called “smelt.” Mixed with water, smelt becomes Green Liquor. This is treated with lime in the causticizers to produce White Liquor, the digester’s cooking chemical. Challenge Stored in varying concentrations, liquors are corrosive solutions with high levels of organic compounds. Liquors can cause chemical burns or damage the lungs if inhaled. Level sensors contend with the chemicals’ harshness, variable density and dielectric, agitation, foaming, and media stickiness. Tank controls should activate the appropriate alarms or emergency shutdown systems.

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. Challenge 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.

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. Challenge Steam condensate from the dryer drums enters up to half a dozen receiver tanks of the condensate return system. Level controls in these tanks ensure that water either 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.

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. Challenge Level control of the blow tank helps to maintain balance between the digestion and washing processes. Level controls facilitate ontime 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.

Designed to move thick fluids, MC (Medium Consistency) pumps are ideal for transporting pulp in a mill. Pulp is often pumped from a standpipe, a vertical feed pipe integral to the pump. Pumps with standpipes typically pump to and from washers and thickeners, O2 and CLO2 mixers, extraction towers, bleach storage towers, and high-density pulp storage towers. Challenge It is important that the proper level of pulp be maintained in a standpipe. A level control in the standpipe does this by actuating a valve downstream of the pump. A malfunctioning level control could result in standpipe overflow, or cause a pump to operate in a no-flow condition overheating and seal damage.

Because pulp is processed and paper is made in an aqueous vehicle that is up to 99.5% water, water management is essential for productive mill operations. Level controls monitor the storage of cold, warm and hot process water, potable water, boiler feedwater, liquor production water, process wastewater, and open effluent weirs, sumps, and stormwater basins. Challenge Process, reclaimed and service water storage may range from small tanks to large bulk tanks with heights of 40 feet (12.2 meters). Controls are specified according to the size and geometry of the bulk storage vessel. Level controls in open atmosphere reservoirs must withstand punishing weather conditions. Firewater storage must conform to NFPA standards.

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. Challenge Level controls ensure that the chests never overflow and that the level never sinks 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.

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. Challenge 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.

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. Challenge 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.

Pulp stock is stored in varying densities in horizontal or vertical “chests” that are quipped 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. Challenge 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.

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. Challenge 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.

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 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. Challenge Water levels of intake channels and plant wells require 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.

Vapors from the digester contain turpentine and 85% of it is released during the relief cycle. Recovery of this volatile organic compound (VOC) is undertaken for environmental reasons, to lessen effluent treatment of condensate, to utilize turpentine as a fuel source, or to sell it as a by-product to chemical processors. Challenge Two vessels in a typical recovery system require level control of the turpentine/ water interface: the decanter, or separator, and the storage tank. The National Fire Protection Association (NFPA) rates turpentine as a “severe fire hazard.” For this reason, the decanter is contained in a dyked area, storage tanks are sometimes located below ground, and controls must be rated explosion-proof.