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Integrated study of flue gas flow and superheating process in a recovery boiler using computational fluid dynamics and 1D-process modeling, TAPPI Journal June 2020
ABSTRACT: Superheaters are the last heat exchangers on the steam side in recovery boilers. They are typically made of expensive materials due to the high steam temperature and risks associated with ash-induced corrosion. Therefore, detailed knowledge about the steam properties and material temperature distribution is essential for improving the energy efficiency, cost efficiency, and safety of recovery boilers. In this work, for the first time, a comprehensive one-dimensional (1D) process model (1D-PM) for a superheated steam cycle is developed and linked with a full-scale three-dimensional (3D) computational fluid dynamics (CFD) model of the superheater region flue gas flow. The results indicate that: (1) the geometries of headers and superheater platens affect platen-wise steam mass flow rate distribution (3%•7%); and (2) the CFD solution of the 3D flue gas flow field and platen heat flux distribution coupled with the 1D-PM affect the platen-wise steam superheating temperature (45%•122%) and material temperature distribution (1%•6%). Moreover, it is also found that the commonly-used uniform heat flux distribution approach for the superheating process is not accurate, as it does not consider the effect of flue gas flow field in the superheater region. These new observations clearly demonstrate the value of the present integrated CFD/1D-PM modeling approach.
Journal articles
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Multifunctional barrier coating systems created by multilayer curtain coating, TAPPI Journal November 2020
ABSTRACT: Functional coatings are applied to paper and paperboard substrates to provide resistance, or a barrier, against media such as oil and grease (OGR), water, water vapor as measured by moisture vapor transmission rate (MVTR), and oxygen, for applications such as food packaging, food service, and other non-food packaging. Typical functional barrier coatings can be created by applying a solid coating or extruded film, a solvent based-coating, or a water-based coating to the paper substrate using various means of coating applicators.This paper focuses on water-based barrier coatings (WBBC) for OGR, water, MVTR, and oxygen barriers. The main goal was to create coated systems that can achieve more than one barrier property using multilayer curtain coating (MLCC). Curtain coating has emerged as the premier low-impact application me thod for coated paper and paperboard. This paper provides examples using MLCC to create coating structures that provide multiple barrier properties in a single coating step. Barrier polymer systems studied include styrene butadiene, styrene acrylate, vinyl acrylic, and natural materials, as well as proprietary additives where required to give desired performance. The paper also shows how the specific coating layers can be optimized to produce the desired property profile, without concern for blocking, as the addition of a non-blocking top layer can be applied in the MLCC structure as well. Experiments on base sheet types also shows the importance of applying the multilayer structure on a pre-coated surface in order to improve coating thickness consistency and potentially allow for the reduction of more expensive layer components.
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The use of hollow sphere pigments as strength additives in paper and paperboard coatings—Part 1: The predictive nature of packing models on coating properties, TAPPI Journal November 2020
ABSTRACT: Hollow sphere pigments (HSPs) are widely used at low levels in coated paper to increase coating bulk and to provide gloss to the final sheet. However, HSPs also provide an ideal system through which one can examine the effect of pigment size and particle packing within a coating due to their unimodal and tunable particle sizes. The work presented in Part 1 and Part 2 of this study will discuss the use of blends of traditional inorganic pigments and HSPs in coating formulations across a variety of applications for improved coating strength. Part 1 of this study focuses on the theory of bimodal spherical packing and demonstrates the predictive nature of packing models on the properties of coating systems containing HSPs of two different sizes. This study also examines conditions where the model fails by examining the effect of particle size on coating strength in sytems like thermal paper basecoats where the non-HSP component has a broad particle size distribution, and how these surprising trends can be used to generate better-than-expected thermal printing performance in systems with low HSP/clay ratios. Part 2 of this study focuses on the incorporation of HSPs of different particle sizes into paperboard formulations to affect coating strength and opacity.
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Improving paper wet strength via increased lignin content and hot-pressing temperature, TAPPI JOURNAL October 2020
ABSTRACT: It is known that the strength properties of wood-based paper materials can be enhanced via hot-pressing techniques. Today, there is a desire not only for a change from fossil-based packaging materials to new sustainable bio-based materials, but also for more effective and eco-friendly solutions for improving the dry and wet strength of paper and board. Against this background, hot pressing of paper made from high yield pulp (HYP), rich in lignin, becomes highly interesting. This study investigated the influence of pressing temperature and native lignin content on the properties of paper produced by means of hot pressing. Kraft pulps of varied lignin content (kappa numbers: 25, 50, 80) were produced at pilot scale from the same batch by varying the cooking time. We then studied the effect of lignin content by evaluating the physical properties of Rapid Köthen sheets after hot pressing in the temperature range of 20°C•200°C with a constant nip pressure of 7 MPa. The pilot-scale cooked pulps were compared with reference samples of mill-produced northern bleached soft-wood kraft (NBSK) pulp and mill-produced chemithermomechanical pulp (CTMP).Generally, the results demonstrated that lignin content had a significant effect on both dry and wet tensile index. All of the pilot cooked pulps with increased lignin content had a higher tensile index than the reference NBSK pulp. To obtain high tensile index, both dry and wet, the pressing temperature should be set high, preferably at least 200°C; that is, well above the glass transition temperature (Tg) for lignin. Moreover, the lignin content should prefera-bly also be high. All kraft pulps investigated in this study showed a linear relationship between wet strength and lignin content.
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Crossflow filtration of green liquor for increased pulp production, improved green liquor quality, and energy savings, TAPPI JOURNAL October 2020
ABSTRACT: A new green liquor filtration system has been installed and commissioned at the Ence pulp mill in Pontevedra, Spain. The filtration system is based on microfiltration and was developed in collaboration with the KTH Royal Institute of Technology in Stockholm, Sweden. The patented method for efficient purification of green liquor decreases the non-process element (NPE) content by providing more efficient solids/liquid separation, reducing energy and chemical consumption in pulp mills and increasing production capacity by eliminating certain capacity bottlenecks. The process has been continuously tested at the Aspa Bruk Mill outside Askersund, Sweden, since 2013. The technology has proven to create nearly particulate-free green liquor during the purification process. The technology can also be used to polish white liquor to provide higher pulp quality.To provide for a simple and cost-effective installation, the system was designed as a skid-mounted unit that is pre-piped, instrumented, and tested before shipment. The system is modular and allows for easy expansion of capacity. This paper discusses the process design, process integration, and startup of the new system, along with experiences from the first months of operation.
Journal articles
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Multifunctional barrier coating systems created by multilayer curtain coating, TAPPI Journal November 2023
ABSTRACT: Functional coatings are applied to paper and paperboard substrates to provide resistance, or a barrier, against media such as oil and grease, water, water vapor, and oxygen, for applications such as food packaging, food service, and other non-food packaging. Today, there is increasing interest in developing recyclable and more sustainable approaches for producing these types of packages. This paper focuses on water-based barrier coatings (WBBC) for oil and grease resistance (OGR), water, moisture vapor transmission rate (MVTR), and oxygen barrier performance. The main goal is to create coated systems that can achieve more than one barrier property using multilayer curtain coating (MLCC) in a single application step. One advantage is in optimizing coating material cost with the use of functional chemistry in confined layers where performance is balanced within the coating layered structure. This allows simultaneous application of layers of different polymer types in one step to achieve the appropriate performance needs for a given barrier application. This paper provides working examples of using MLCC to create coating structures with multiple barrier properties in a single application pass. Barrier polymers studied include styrene butadiene, styrene acrylate, starch-containing emulsions, and polyvinyl alcohol. The paper also shows the effect of increasing the pigment volume concentration with platy clay or fine ground calcium carbonate on MVTR and OGR barrier properties.
Journal articles
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Surface energy considerations for offset printing of coated paper and paperboard, TAPPI Journal November 2023
ABSTRACT: Offset printing of coated paper involves the complex interactions of ink with a surface that is characterized by three major properties: roughness, porosity, and related pore network structure and surface chemistry (related to surface free energy [SFE]). The effects of porosity and roughness are relatively well understood and are documented in the literature, whereas the influence of surface chemistry is much less studied and therefore the focus of this paper. The key results shown include: i) Coating porosity has a negligible effect on SFE determination by contact angle using two fluids. ii) The chemistry of the latex polymer in the coating formulation dominates the influence on SFE compared to pigment, with any surface energy differences present in the pigment being almost completely masked by latex. iii) Wetting agent and corona treatment can impact water absorption rate and surface spreading of water, resulting in small differences in printability. Increasing the concentration of the surfactant on a coated surface indicates switching orientation of the surfactant molecules, giving a “step wise” printing result. When looking to improve offset printability by selection of different pigments, the variation in SFE is less important than variation in either surface roughness or porosity.
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Rice straw-based sustainable food packaging material with improved strength and barrier properties: Development and characterization, TAPPI Journal October 2023
ABSTRACT: Sustainable food packaging paper with high barrier and strength properties was developed with rice straw nanocellulose materials. Pulping and bleaching of rice straw were performed using an organosolv pulping and DED (D: chlorine dioxide bleaching; E: sodium hydroxide extraction) bleaching sequence. Bleached rice straw pulp was refined to 90°SR using a laboratory Valley beater. The laboratory handsheets were prepared using pulp slurry at 40°SR and 90°SR. The handsheets of cellulose nanofibrils (CNFs) made of highly refined pulp (90°SR) were surface sized using alkyl ketene dimer (AKD) wax to increase the barrier properties of paper for selective food packaging applications. The paper samples were tested for mechanical, optical, surface, and barrier properties, including tensile index, burst index, tearing index, bending stiffness, elongation, porosity, apparent density, opacity, Cobb value, water vapor transmission rate (WVTR), oil and grease resistance, and contact angle. The refined pulp (90°SR) was analyzed using field-emission scanning electron microscopy (FE-SEM), and it was observed that the morphology of the developed fibers changes to the nanoscale (<100 nm) for at least one dimension. The particle size distribution of the refined pulp using DLS analyzer also confirmed the cellulose fibers to near nanoscale. It was concluded that nanofibers were formed by a high degree of the mechanical pulp refining process and found to be much more economical than alternative processes in this direction. The sample handsheets of CNFs showed good strength and barrier properties. The barrier properties further increased when surface sizing was done using low-cost, nontoxic, and biodegradable AKD wax.
Journal articles
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Using bleaching stage models for benchmarking hardwood ECF bleach plants, TAPPI Journal October 2023
ABSTRACT: Steady-state models estimated the performance of the D0(EOP)D1 bleach sequence at two mixed hardwood bleach plants in the southern United States. At Mill 1, the full sequence’s chlorine dioxide charge that brightens the pulp to ~84% ISO was monitored for two weeks. Mill 2 considered the partial sequence that brightens the pulp to ~86% ISO for nearly four weeks. Elevated levels of chlorine dioxide were linked to increased washer carryover in brownstock and extraction areas. For Mills 1 and 2, an extra 0.24% and 0.33% chlorine dioxide was consumed in the D0 stage. This extra bleach demand was equivalent to an additional 4.8 and 5.5 kappa load to the brownstock, respectively. Some differences were observed for the D1 stage. Mill 1 had extraction carryover that averaged 1.1 units higher than was measured, contributing to use of an extra 0.22% of chlorine dioxide. Mill 2 had extraction carryover that averaged between 0 and 0.7 kappa units and consumed up to 0.13% more chlorine dioxide. Another data set from Mill 2 showed high brownstock and extraction carryover, leading to ~0.90% more total chlorine dioxide usage to brighten to 84% ISO. Overall, this investigation illustrated that the models could be employed as benchmarks.
Journal articles
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Development of reinforced paper and mitigation of the challenges of raw material availability by utilizing Areca nut leaf, TAPPI Journal September 2022
ABSTRACT: Paper industries are facing a raw material crisis and searching for alternate raw materials that may be able to help mitigate the issue. Many industries use agro-waste as a raw material, irrespective of it having low bleachability and poor mechanical strength. Areca nut leaf (ANL) is a nonwood-based material that may be acceptable as an alternate source of raw material that contains 61.5% holocellulose and 13.6% lignin, which is comparable to other agro-wastes and hardwood pulps. Kraft anthraquinone pulping with 20% active alkali as sodium oxide (Na2O), 25% sulfidity, and 0.05% anthraquinone produced 15 kappa pulps with about 38.5% pulping yield. The bleachability of ANL pulp was good, and 83.5% ISO brightness could be achieved using the D0(EOP)D1 bleaching sequence. The ANL fiber has 33.8% better tensile, 54.5% better tear, and 15.2% better burst index than hardwood fiber. Similarly, 60.4% better tensile, 56.5% better tear, and 21.7% better burst index were observed in ANL than in wheat straw. Thus, the study revealed that Areca nut leaf can be used as an alternative raw material for papermaking, as well as to improve the physical property of paper products by blending it with inferior quality pulp.