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A Review on the Modeling, Control and Diagnostics of Continuous Pulp Digesters

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A Review on the Modeling, Control and Diagnostics of Continuous Pulp Digesters processes Review A Review on the Modeling, Control and Diagnostics of Continuous Pulp Digesters Moksadur Rahman * , Anders Avelin and Konstantinos Kyprianidis School of Business, Society and Engineering, Mälardalen University, Box 883, 72123 Västerås, Sweden; [email protected] (A.V.); [email protected] (K.K.) * Correspondence: [email protected]; Tel.: +46-(0)21-10-1594 Received: 12 August 2020; Accepted: 23 September 2020; Published: 1 October 2020 Abstract: Being at the heart of modern pulp mills, continuous pulp digesters have attracted much attention from the research community. In this article, a comprehensive review in the area of modeling, control and diagnostics of continuous pulp digesters is conducted. The evolution of research focus within these areas is followed and discussed. Particular effort has been devoted to identifying the state-of-the-art and the research gap in a summarized way. Finally, the current and future research directions in the areas have been analyzed and discussed. To date, digester modeling following the Purdue approach, Kappa number control using model predictive controllers and health index-based diagnostic approaches by utilizing different statistical methods have dominated the field. While the rising research interest within the field is evident, we anticipate further developments in advanced sensors and integration of these sensors for improving model prediction and controller performance; and the exploration of different AI-based approaches will be at the core of future research. Keywords: Kraft pulping; pulp digester; modeling; control; diagnostics 1. Introduction With the widespread expansion of the Internet, electronic media and paperless communication, the demand for the graphic paper (i.e., newsprint and higher-value printing and writing paper) has been declining since 2000 [1]. Despite this downfall, the global pulp and paper market is growing steadily at a rate of over 1% per year [2]. A large part of the growth is attributed to packaging materials and the sanitary products as a result of growing e-commerce business and the modern lifestyle. In spite of the economic significance, the pulp and paper industry is lagging behind in embracing digitalization and state-of-the art process optimization techniques. Consequently, research in the area of modeling, advanced process control and diagnostics is grabbing much attention. Pulp and paper mills convert wood chips into a fibrous mass called pulp, the raw material for different paper products. The production of pulp is commercially accomplished by a mechanical or chemical pulping process or a combination of both methods. In mechanical pulping, abrasive refining or grinding is used to reduce wood into fibrous pulp. As the name suggests, in chemical pulping the wood pulp is produced by means of chemical reactions. Sulfate pulping and sulfite pulping are two typical types of chemical pulping process. More than two-thirds of the globally produced pulp comes from sulfate or Kraft pulping mills [3]. A typical outline of the integrated Kraft pulping process is shown in Figure1. Normally the process includes wood handling steps such as debarking, chipping and storage; Kraft cooking in the digester; screening and washing of pulp; bleaching and drying of pulp or paper/board; and pressing, drying and finishing. About 97% of the chemicals are recovered through the chemical recovery process [4]. To do so, the spent chemicals known as black liquor are evaporated, burnt in a recovery boiler and converted back to white liquor in a causticizing plant. Processes 2020, 8, 1231; doi:10.3390/pr8101231 www.mdpi.com/journal/processes Processes 2020, 8, 1231 2 of 26 Debarking Bleaching Digester Wood logs Chipping Chip yard Washing Brown Screening Unbleached Bleached Logs Chips stock pulp pulp Black Black liquor liquor Finishing Drying Pressing Wire system Black liquor Headbox White liquor Black liquor Lime Evaporators Lime kiln Causticizing Recovery boiler Green liquor Lime mud Strong black liquor Smelt + water Figure 1. An overview of Kraft pulping process. In the Kraft pulping process, the quality of the produced pulp is very much dominated by the raw material properties and the process conditions. All wood species are mainly composed of three basic structural elements called cellulose (∼40%), hemi-cellulose (∼30%) and lignin (∼25%). As depicted in Figure2, cellulose and hemi-cellulose form the fibrous structure of the wood, and lignin acts like a “glue” that that holds the individual cellulosic polymers together. The conversion of wood chips into pulp mainly takes place in a long, upright, tubular vessel known as the pulp digester (see Figure3). In the digester lignin is removed from wood chips to free the wood fibers by utilizing a thermo-chemical conversion process known as delignification. In this process, an aqueous solution of sodium hydroxide (NaOH) and sodium sulfide (Na2S), also known as white liquor, is used, which dissolves most of the lignin and thus separates the cellulosic fibers from each other. However, the white liquor also reacts with the cellulosic fibers and thus not only degrades the physical properties of the produced pulp but also reduces the pulp’s yield. Therefore the cooking conditions inside the pulp digester must be controlled in a way that fibers are separated without damaging them too much while maximizing the pulp yield. These factors are highly correlated, and hence a best compromise among residual lignin, yield and fiber properties is needed. Cellulose Lignin Hemicellulose Wood chips Figure 2. Arrangement of cellulose, hemi-cellulose and lignin in wood. Two common types of pulp digesters that are widely used for Kraft pulping are batch and continuous digesters. Due to lower space requirements, less labor and lower energy costs, continuous digesters have become the dominant design in the realm of Kraft cooking [5]. As illustrated in Figure3, after pre-steaming, wood chips are transported at the top of the digester via a circulating liquor system. Cooking liquor is also added at the top of the digester. The top part of the digester is called the impregnation zone where wood chips are soaked by the cooking liquor via penetration and diffusion mechanism. The delignification already starts in this zone at a typical temperature between 115 ◦C to 120 ◦C. Then in the heating zone, the temperature of the chip mixture is rapidly increased between Processes 2020, 8, 1231 3 of 26 150 ◦C and 170 ◦C via two external heat exchangers. After that the chips enter the cooking zone where the majority of the lignin is removed at an elevated temperature. The spent liquor is separated and taken out of the digester in the extraction zone. The next zone is the counter current washing zone where cooked pulp is washed and cooled down with wash liquor. By doing so the delignification reaction is stopped and thus fiber properties are preserved. The cooked pulp is diluted to around a 10% concentration level and removed from the bottom of the digester. The produced pulp quality is mainly expressed by Kappa number which is the residual lignin content of the pulp. White liquor Chips & Chip Recirculation feed liquor Bin Impregnation zone External heat exchanger Feed liquor White liquor Cooking zone digester Continuous pulp Extraction Liquor zone extraction Washing zone Kappa Wash water Analyser Chips Liquor Figure 3. Schematic of a continuous pulp digester process flow [6]. Due to the naturally varying feedstock, long residence time, inadequate measurements and complex nature of the delignification process, controlling pulp quality at the digester outlet is a challenging task. Moreover, due to the non-ideal flow behavior in the digester, process faults often occur that lower the pulp quality and production rate considerably. Hence, controlling of the pulping process in an efficient way and early detection of underlying digester faults are matters of utmost importance for the economic operation of a pulp and paper mill. In this article the modeling, control and diagnostics of continuous pulp digesters are reviewed. Particular effort has been devoted to highlighting the state-of-the-art and the research gaps in a summarized way. The paper is organized as follows. Firstly, the research methods along with a bibliographic analysis of the reviewed articles are provided that lay down the foundations of this study. Subsequently, the review of relevant literature on pulp digester modeling, control and diagnostics is presented. The outcome of the review is discussed, and concurrently, future research directions are proposed. Processes 2020, 8, 1231 4 of 26 2. Materials and Methods The present work focused on the review of available literature within the area of modeling, control and diagnostics of continuous pulp digesters. The purpose of considering the entire span of literature instead of basing our work on the previously published articles was to provide a truly comprehensive review by following the development in the field along the way. Though some of previously publish articles included rather narrow literature reviews to frame their work, to the best of the author’s knowledge, no comprehensive review has been published covering this topic yet. To conduct the review, three major bibliometric databases, the Web of Science, Google Scholar and Scopus, were queried using a comprehensive keyword list. Various combinations of keywords were utilized, including “pulp digester”; “kraft pulping”; “model”; “control”; “fault detection”; “diagnostics”; etc. To broaden the search, synonyms used by both industry and academia were included. Furthermore, references of relevant papers were also individually investigated to find new papers which were related to this topic. A manual sorting based on titles, abstracts and keywords yielded 210 articles to be potentially relevant for this review. These works, ranging from 1966 up to July 2020, were all published in white literature and written in English. The yearly distributions of these publications are presented in Figure4. Although the number of publications is unevenly distributed over the years, a gradual increment can be observed until 1997, followed by a rather slow descent.
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