Pulp & Paper Industry Solutions Accurate and Reliable Liquid Analysis Papermaking Process Overview Papermaking is a highly sophisti - Although not present at all mills, yields or improve the properties of cated process involving several there are five major steps to the the pulp. processing steps to turn wood papermaking process: mechanical preparation of the wood into wood Pulp stock leaving the digester is a logs into paper products. In addi - chips, wood digestion (pulping) brownish liquid that is usually tion to the actual papermaking to form pulp, pulp whitening bleached to make more acceptable process, there are also (bleaching), pulp stock preparation, white paper. This process uses large supporting areas such as waste and finally paper formation. quantities of expensive bleaching water treatment, recovery boiler chemicals (usually ClO 2) and operation, steam production, The first of these steps uses strictly represents raw material and waste mechanical processes to form small disposal challenges. pH measure- and cooling towers that benefit chips from the logs supplied to the ment in this area optimizes the from on-line analytical mill. Byproducts such as bark and efficiency of the bleaching process, measurements. even some of the chips themselves lowering bleach consumption, are used as fuel to produce electricity discharge and associated costs. You need a partner that can and steam for mill use. Although deliver the best in knowledge some water may be used for lubrica - Prior to running in the paper and systems, and do it quickly, tion purposes, this is primarily a dry machine, different batches of pulp process and proceeds with a mini- may be blended to produce paper of thoroughly and cost-effectively. mum of process instrumentation. various required properties. Starches Emerson Process Management’s and retention aids are added to Rosemount Analytical brand of The pulping process has many varia - improve the wetting properties of 2 products provide measurements tions. Most common is chemical the paper. The pH measurement of pH, ORP, conductivity and dis - pulping using either caustic (Kraft standardizes this process, helping to solved oxygen that are specially sulfate process) or acidic (sulfite produce the same quality paper day process) chemicals. in and day out. designed for long life and reliable results in the demanding The chemicals are used and regener - The paper machine is usually consid - environment of the pulp mill. ated in a recovery process that ered the last place to get conditions These measurements provide reduces chemical costs and right for the final product. A reliable substantial control benefits that minimizes waste disposal costs. pH signal at this location can prevent can reduce costs with a typical Process conductivity measurements large runs of off spec product. These payback time of less than one optimize both the pulp yield and the locations can represent difficult recovery of chemicals in this stage. applications for sensors, but recent year. Read on and discover how Other pulping techniques such as advances in sensor technology have the liquid analysis professionals groundwood o r sem ichemical use vastly improved the performance from Emerson can help you pH measurements to optimize and reliability of online sensors. improve your bottom line. Wood logs are broken into chips about the size of a quarter. Logging Debarking Chipping Chip Bin Digester Mechanical Wood Processing EMERSON. CONSIDER IT SOLVED .™ Kraft Pulping Process Overview Digester Logs White Liquor Black Liquor Chipper Bleach Towers Chlorine Dioxide Tank Machine Chest White Water Chest Refiner Paper Products 3 Head Waste Treatment Plant Box Paper Machine On-line electrochemical measurements such pulp slurry from wood chips, involves as pH, electrical conductivity and dissolved strong chemicals that are typically CONTENTS oxygen provide real improvements in regenerated and reused. Dosage of these process control, quality, efficiency and prof - chemicals and control of the recovery Kraft Pulping 4 itability in Pulp and Paper operations. Mill process is monitored using electrical Bleaching and the Wet-End 5 effluent, especially from bleaching conductivity measurement. The pulp slurry operations, is increasingly the target of envi - is treated using bleach, sizing agents and Chemical Recovery 7 ronmental regulations such as the Cluster other furnish before being run in the paper Boiler and Rule and must be monitored closely. machine. These pulp additives can be very Steam Plant Operations 8 ineffective if the pH of the pulp slurry is not There are many different processes that can controlled. Common measurement Waste Treatment 9 be used to make paper and each process locations are bleach towers, machine Instrumentation 10 involves several steps. Chemical pulping, chest, headbox, and white water chest. the most common process of producing a Kraft Pulping Kraft pulping produces strong Measuring the alkali concentration Conductivity can be correlated to pulp but typically needs further entering into the digester allows the A and B tests since the active processing such as bleaching to fine control of the liquor flow rate NaOH has much higher conductivity to match the wood loading, than the product. As the cook provide acceptable brightness improving throughput and continues, conductivity steadily for average users. Cost effective - minimizing variability. decreases. Conductivity is best ness comes from reuse of the measured in the upper recirculation pulping chemicals in the Inside the digester, at temperatures zone for rapid response to process recovery process (see page 7). near 180°C and pressures around changes, but the high solids concen - Pulping is conducted in a 250 psig, the following reaction tration at this location tends to plug most conductivity sensors. digester and the process can be occurs: operated in either batch or Na 2S + NaOH + wood chips —> continuous mode. There are Na 2SO 4 + Na 2CO 3 + pulp many kinds of pulping processes, but generally the most cost As this reaction proceeds, the effective one is the chemical Kappa number decreases until the process called Kraft pulping. “cook” is completed in about 2 to 4 hours. Kappa Number is difficult to The Model 242 measure, so many mills monitor the Toroidal digester by measuring the Conductivity In the Kraft process, wood chips are concentration of chemicals in the Sensor is built for cooked in a strong alkali solution high solids and 4 cooking liquor using three chemical called white liquor. The white liquor aggressive chemical applications titrations called “ABC tests”. Results is basically a solution of sodium of the ABC tests are used to Process measurements in heavy slur - hydroxide and sodium sulfide with establish NaOH, Na S, and Na CO ries such as cooking liquor require an effective concentration of about 2 2 3 levels in the liquor. As the cook pro - robust flow-through sensors like the 100 grams per liter. Measuring gresses, NaOH is consumed and Model 242 Toroidal Conductivity white liquor alkali concentration Na CO is produced. When the Sensor. This sensor consists of a allows optimization of cooking time 2 3 NaOH level has dropped to the replaceable ceramic liner and and product pulp properties. Direct appropriate level, the cook is externally mounted toroid measure - measurement of the alkali complete and the product (called ment coils. The toroidal conductivity concentration is possible using auto - brownstock) is released from the method is resistant to the effects of matic titrators. However, due to the digester and further processed. This coating, and the Model 242’s exter - difficulty of extractive sampling technique still requires that nal mounting protects the toroids many mills opt to use electrical con - samples be extracted and from the high temperature and ductivity to provide real-time processed by a lab every 2-4 hours. pressures in the digester. The Model control feedback. 242 is recommended for lines up to The retractable Although the application inside the 4" in diameter. Model 228 Toroidal The clarified white liquor feed to the Conductivity Sensor digester is significantly different digester is the optimum location to White Liquor Conductivity allows safe, conven - from the application in base white install the retractable Model 228 500 ient cleaning. liquor, electrical conductivity Toroidal Conductivity Sensor. This 450 y measurement is valuable here also. t i v 400 sensor has a unique lead screw i t c design that allows for safe u 350 d n retraction at up to 295 psig o 300 C for cleaning purposes. 250 50 70 90 110 Effective Alkali Bleaching and the Wet-End Bleaching is the process of A typical bleach sequence consists of The bleaching chemicals are aggres - purifying and whitening pulp by a pair of bleaching stages each sive and incompatible with many chemical treatment. Several followed by an extraction stage. sensor materials. The 70°C temper- Chlorine dioxide bleaching allows ature accelerates the rate of stages are typically used to efficient bleaching at lower pH (3.5 chemical attack. generate pulp with the desired to 4) without degrading the brightness. To meet environmen - cellulose component of the pulp. Emerson’s TUpH ™ sensor technology tal regulations, nearly all pulp The extraction stages are conducted minimizes the effects of coating mills use elemental chlorine free at elevated pH (10-11) to maximize by using a patented composite junc - bleaching (i.e. chlorine dioxide). color removal. pH measurement is tion with