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PULP MILL OPTIMISATION WORLD PULPPAPER42 for further processing. HOW TO IMPROVE THE PROCESS The challenge was to make the process work more efficiently and to higher tolerances without replacing anything except the control system that operated the existing equipment. The key to this was to optimise the operation of the two lines digester and brown stock washing stages. The first step was to audit and analyse the control loops then develop an advanced process control APC action plan based on model predictive control MPC and soft sensors. THE COOKING PROCESS For the digester the key variable for product quality is the Kappa number this measures the lignin level in the pulp and thereby indicates how well the chips have been cooked. This number of all industrial processes increasing production volumes and decreasing production costs. In 2011 Mondi Swiecie awarded ABB a contract to improve the quality and efficiency of its pulp mill. To better understand the challenge involved in this task it pays to take a closer look at how the pulp process works. SEPARATING CELLULOSE The raw material for cardboard and paper is of course wood. The purpose of the pulping process is to separate the valuable cellulose fibres in wood from the lignin and hemicellulose polymers that bind them. Traditionally this was done by physically beating the wood but nowadays chemical methods are used not least because they better preserve the integrity of the cellulose. The process begins by stripping the wood of its bark turning it into chips screening the chips to achieve a roughly similar size impregnating them with chemicals and feeding them into a pressure cooker the digester. In the sulphate process used at Swiecie the chips are mixed with a solution of sodium hydroxide and sodium sulphide known as white liquor this is then pressurised and heated to a temperature of about 160 C with steam from coal-fired boilers. After a few hours the material takes on the consistency and colour of porridge the amount of alkali in the sodium hydroxide decreases as it is turned into sulphates and carbonates as a result of reacting with the lignin in the wood. At the end of the process the pulp is squeezed out of the digester through an airlock called the blow line. The sudden depressurisation results in the rapid expansion of the cellulose fibres which helps to separate them. They are then suspended in a liquid that is known to pulp workers as brown stock. The next stage of the process is to wash the brown stock to remove the cooking chemicals now referred to as black liquor together with the degraded lignin and hemicellulose. After progressing through a number of tanks the extracted liquid is reduced and its sodium and sulphur compounds recovered to make more white liquor. Meanwhile the clean cellulose pulp is bleached if necessary and then pressed and heated to remove water after which it is ready to be cut and rolled or bailed The challenge was to make the process work more efficiently and to higher tolerances without replacing anything except the existing control system Figure 1. Digester APC