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Conventional multi-cylinder drying of paper and board involves a mixture of conductive drying from steam-heated dryer cylinders and convective drying by the flow of heated air over the surface of the paper web in the pockets. Pocket ventilation is a critical component in assisting heat and mass transfer during the drying process but is the primary contributor toward removing evaporated water from the web. Air temperature, velocity, and humidity are critical parameters involved in the convective drying process. This paper covers an experimental study involving the design and development of a small lab-scale setup for convective drying of various grades of paper and board, monitoring multiple parameters like paper temperature, moisture content, air humidity, temperature, and velocity measured in situ as the drying proceeds with continuous and accurate sampling capabilities for all parameters in the sample and the system. Instantaneous drying rates, heat, and mass transfer coefficients were also deduced for every time step till the paper completely dried. Furthermore, the coefficients obtained were also reported in the form of dimensionless correlations, and the results were compared against traditional correlations used in the modeling of paper drying. This data will be useful in process development, modeling, design, and the paper drying process simulation. [ABSTRACT FROM AUTHOR]

Drying of paper during 2000–2018 is reviewed. The review includes new drying processes, multi-cylinder dryers, tissue and impingement drying, TAD-drying, infrared drying and energy use for drying of paper. It includes general aspects of infrared dryers but not drying of coated papers. Paper quality aspects, paper shrinkage and paper isotherms are not included. Impingement drying has been the most successful new technology with several applications while others such as impulse drying has not been commercialized mainly due to problems with paper quality parameters. One promising development is the introduction of steel instead of cast iron cylinders with increased heat transfer reducing the number of cylinders. Future research should focus on modeling the internal transport phenomena and to couple these phenomena to paper shrinkage and quality parameters. The influence of the fabric for the drying process needs more attention, understanding the drying process for new components such as microfibrillar cellulose in the stock and reducing energy use and increasing the amount of renewable energies for drying of paper. [ABSTRACT FROM AUTHOR]

The present study investigates the phenomenon of steam condensation in a rectangle horizontal channel with a mass flux range of 20–70 kg/m2/s and a vapor quality of 0.2–0.8. Mass flux and vapor quality were presented to primarily affect the heat transfer coefficient in shear-dominated flow regime, such as the mass flux is greater than 40 kg/m2/s. At the low mass flux, only slug flow, plug flow, and wave flow were observed. At the high mass flux, the wave flow and annular flow were observed even at the low-vapor quality, and the slug and plug flow were not found during the entire condensation process. Observed flow patterns agreed with Tandon’s flow pattern map. Four kinds of correlations are employed to predict the condensing heat transfer coefficient, and the result reveals that Cavallini’s correlation agrees with the experimental results relatively well. The measured pressure drop has a good agreement with the Lockhart–Martinelli correlation. [ABSTRACT FROM PUBLISHER]

A novel methodology was used to create a highly porous foam-formed paper that is bonded with highly refined cellulose fiber. In this process, cellulose pulp suspension at various consistencies (0.5, 1.0, 1.5, and 2%) was dispersed in water, followed by foam formation under high shear forces in the presence of a surfactant. Various drying methods were used to achieve foam formation. These included freeze-drying (FD), vacuum-dewatered air-drying (VAD), and dewatered freeze-drying (VFD). Increasing the pulp's consistency and changing the drying techniques from freeze-drying to air-drying resulted in a more compact morphological structure and increased density of foam-formed paper samples. Densification of foam-formed samples was measured using a Dynamic Mechanical Analysis (DMA) machine and a sample with densification at lowest strain value was obtained by a 10 wt% addition of microfibrillated cellulose fiber (MFC). At 10 wt% MFC addition, denser foam-formed paper samples with enhanced microstructure were obtained. Air filtration efficiency and acoustic properties of foam-formed paper were also characterized and optimized by the addition of MFC. [ABSTRACT FROM AUTHOR]

Paper drying is a highly energy-intensive and complicated multivariate process. The dryer section plays an important role in the energy consumption of a paper machine, especially of thermal energy. A comprehensive method for assessing the energy performance of the dryer section was investigated in this study to improve energy efficiency. This method was divided into three component processes: energy and evaporation load audit, field test and observation, and energy flow analysis and energy efficiency estimation. In a case study, we found that the method could, in addition to analyzing the key factors that restrict drying efficiency, also depict the details of energy consumption clearly. At the same time, several significant energy-saving measures were suggested to improve the energy efficiency of the paper-drying process. [ABSTRACT FROM PUBLISHER]

This article introduces a method to validate heat of sorption measurements from isotherms by comparing them to direct measurements using reaction calorimetry. We have evaluated some frequently used single-temperature isotherm models (BET, GAB) as well as two multi-temperature isotherm equations (Anderson, Heikkilä). It turned out that the isotherm models with better isotherm curve fitting characteristics did not deliver the best results for the heat of sorption. Multi-temperature isotherm models such as the Heikkilä equation are able to deliver good results in terms of modelling the isotherm data and determining the heat of sorption. Single-temperature models fit the isotherm data better than multi-temperature models, but failed to deliver correct values for the heat of sorption. We specifically investigated the heat of sorption at very low moisture content, which is most relevant for paper drying. The resulting heat of sorption curves were analyzed with respect to their saturation behavior, which is expected to occur below BET monolayer moisture. The heat of sorption from the Heikkilä’s model was the only one to show the expected saturation at very low moisture content. [ABSTRACT FROM PUBLISHER]

A numerical model of a vacuum dewatering process was established with a level-set method to simulate two-phase flow in a two-dimensional paper sheet model with constructed volume forces representing flow resistance in a three-dimensional environment. Nine cases of various volume force representations were investigated by comparing numerical and experimental data. Based on the dry content and dwell time relation, the best model is obtained when accounting for in-plane flow resistance at the paper–wire interface. Compared to the other numerical cases, considering the blockage of the pore space, the top layer of the wire plays an essential role in determining the flow resistance during the vacuum process. To validate the maximum dewatering rate obtained in the model, new experimental data with a higher frequency of sampling are needed. The computational time for the two-phase flow models in this study is extensively reduced due to the removal of the internal structure. This distinction enables time-efficient simulations of the vacuum dewatering process in which several dewatering parameters such as the level of the vacuum, the influence of a moving vacuum pulse, and higher basis weights could be investigated. [ABSTRACT FROM PUBLISHER]

The extension of the Darcy law (the Forchheimer flow equation) relating second-order nonlinear pressure drop with flow velocity is studied during fast transient through air drying of sheets of porous biobased materials such as paper. A range of the paper materials with open structure consistent with tissue and towel products (basis weights 25 and 50 g/m2) made using different production processes are analyzed for the factor-specific influences with regard to changes in the fluid resistance from the removal of moisture from the material interstices. A characteristic dimension suitable for the drying process is applied from viscous and inertial momentum transport analysis. [ABSTRACT FROM AUTHOR]

The aim of this article is to develop an experiment and a procedure to investigate the restoration of water-damaged paper and archival materials using freeze drying in order to allow a reproducible test and comparison of the influence of different operating conditions on drying time and restored paper quality. Firstly, a reproducible method for the preparation of soaked samples simulating water-damaged paper has been developed. Then, the samples have been freeze-dried in a laboratory-scale apparatus that allowed monitoring the temperature as well as the weight of the samples. The technique of evaporative freezing, which reduces the drying time required, has been used in this case. An innovative procedure for the visualization of the progress of the drying process has been validated, thus allowing the validation of a simple phenomenological model of the time evolution of the ice core volume; in addition, data on the residual moisture of the dried paper sheets in different zones have been given. Finally, optimization of this particular drying process by using simple or more sophisticated approaches has been discussed. [ABSTRACT FROM AUTHOR]

A novel process modeling tool to facilitate the study of how various process changes in the paper dryer section affect the mill-wide energy system has been developed. A model library with steady-state block models describing paper dryers, heat recovery equipment, and auxiliary systems on component level have been developed using the software Extend. Process models are then created from these block models using graphical programming. In this article, the characteristics of the developed tool are described and a case study where the tool is shown to be useful for analyzing the energy performance of a hybrid dryer section is presented. [ABSTRACT FROM AUTHOR]

The properties of recycled paper are strongly affected by shrinking during drying, which depends on the drying conditions. An experimental study of the shrinkage phenomenon during drying of paper is presented. Drying experiments were conducted in a drying oven with controlled temperature. The temperature influence on shrinkage extent, porosity, and apparent density of the paper was evaluated. Structural changes on the paper surface after drying were analyzed by scanning electronic microscopy. Thickness contraction, surface, and volumetric shrinkage were correlated as a function of the moisture content. Lower apparent density values and more porous paper samples were obtained for higher temperatures. [ABSTRACT FROM AUTHOR]

Studies from this laboratory have documented significant changes in properties when paper is dried in superheated steam rather than, as in all current processes, in air. Extending these investigations to additional important pulp types, to recycled pulps and to filled papers, using commercial furnishes from mills, has identified further grades of paper for which drying in superheated steam enhances key properties. For bleached chemithermomechanical pulps and blends with kraft pulp as used for tissue and toweling, strong paper resulted, with 10% higher bulk. Linerboard from 100% recycled old corrugated containers (OCC) is obtained with various strength properties increased by up to 21% with no densification, actually a 4% increase in bulk. Linerboard from high-yield (55–67%) virgin kraft pulps show 23 to 37% increase in strength and toughness. Filled papers containing 0–10% clay can be produced with the same brightness but about 23% higher tensile index when dried in superheated steam. [ABSTRACT FROM AUTHOR]

A general model for through drying of tissue was developed based on the equations of continuity in combination with correlations for heat and mass transfer. The model incorporates different geometric descriptions together with corresponding heat and mass transfer correlations for flow through cylindrical conduits and flow through packed beds of cylindrical fibers. At low intensities, i.e., at low airflow rates and low drying air temperatures, the model shows very good agreement with experimental data found in the literature and accurately predicts a period of constant drying rate due to saturation of the drying air. However, at higher drying intensities, the model predicts drying rates that are generally higher than what is found in experimental studies described in the literature. The model also predicts a period of constant drying rate that has not been observed in experiments at higher drying intensities. To illustrate the effect of nonuniform pore size distribution on the average drying rate, a model including a variable pore size distribution was developed. The results show that the early onset of the falling rate period in through drying at higher intensities could be explained by channelling effects caused by a nonuniform pore size distribution. [ABSTRACT FROM AUTHOR]

Studies from this laboratory have documented significant changes in properties when paper is dried in superheated steam rather than, as in all current processes, in air. Extending these investigations to additional important pulp types, recycled pulps, and filled papers, and using commercial furnishes from mills, has identified further grades of paper for which drying in superheated steam enhances key properties. For bleached chemithermomechanical pulps and blends with kraft pulp, as used for tissue and toweling, strong paper resulted with 10% higher bulk. Linerboard from 100% recycled old corrugated containers (OCC) is obtained with various strength properties increased by up to 21% with no densification, actually a 4% increase in bulk. Linerboard from high-yield (55–67%) virgin kraft pulps shows a 23 to 37% increase in strength and toughness. Filled papers containing 0–10% clay can be produced with the same brightness but 23% higher tensile index when dried in superheated steam. [ABSTRACT FROM AUTHOR]

A general, dynamic model describing the drying mechanisms of paper has been developed. The model includes shrinkage and predicts temperature, moisture, and pressure profiles. It has been evaluated against steady-state as well as dynamic measurements on a multicylinder paper dryer. The results revealed critical sections where low web strength and maximum gas pressure occur. Measurements of the outer cylinder temperature provide reliable information on the accuracy of the predicted moisture profile since they show the position where the paper surface is dry. [ABSTRACT FROM AUTHOR]

A three dimensional computational fluid dynamic investigation is carried out to predict the turbulent flow and surface heat transfer under an impinging air jet issuing normally from a single noncircular orifice in a plate held parallel to the target surface. Static pressure distributions, velocity fields and local as well as average Nusselt number on the impinged surface are presented for square, elliptic, and rectangular orifices and compared with those for a circular orifice. Effects of jet Reynolds number as well as spacing between the nozzle plate and the impinged surface are examined using a two-layerκ–η turbulence model. Results show flow structure similarities between the characteristics of rectangular and elliptic jets of equal aspect ratio. Further, it is observed that noncircular impinging jets can provide higher average heat transfer rates than corresponding circular jets for certain geometric parameters viz. nozzle-to-plate spacing and the size of the averaging area used to compute the average Nusselt number. [ABSTRACT FROM AUTHOR]

This article theoretically investigates the drying of uncoated paper with gas-fired infrared (IR) emitters installed within the drying section of a given commercial paper machine. Specifically, it presents the ideal location of two opposing IR emitters within the drying section. The ideal location of the two opposing IR emitters corresponds to the highest machine speed with specified average moisture content of paper sheet at the end of the drying section. This article also presents the average evaporation rate, temperature, and moisture content of the paper sheet as it travels through the drying section with the two opposing IR emitters operating at the optimum location. In addition, the temperature and moisture profiles in the sheet thickness direction, before and after the IR emitters, are provided. This article is not concerned with the potential effects of drying with IR emitters on paper sheet properties and machine runnability issues. [ABSTRACT FROM AUTHOR]

Moisture equilibrium isotherms are very important in drying and storage processes analysis. So, the DEQ/UEM process separation group is building a data bank for cellulose, paper, and textile isotherms and their fit with available literature equations. In this context, the experimental isotherms of a brazilian specific commercial paper, the KLABIN-PR KLAPAK commercial liquid package paper were obtained. The experimental equilibrium data were obtained by exposing the material to different ambient conditions of temperature and relative humidity controlled by the use of saline solutions. The samples were placed in glass flasks left inside an oven to provide a better adjust of temperature conditions, being periodically weighed until constant weight. KLAPAK moisture equilibrium isotherms were built from the obtained experimental results and temperature effects on its behavior were analyzed. Then, these results were fitted with the help of some models and equations available from literature. [ABSTRACT FROM AUTHOR]

The aim of this study was to find out what will happen to paper coating quality and material gradients if coating is dried in one air dryer at a high convection rate (150 kg/m2h). Different printing papers were coated, calendered and printed with pilot machines. The binder and pigment gradients of the coated, and also printed, samples were analyzed with LIPS (Laser Induced Plasma Spectrometry). It was found that drying affects the migration of starch, but not latex. Total air drying can be used to dry printing papers, if air temperature, velocity and moisture content can be separately controlled. The control of web temperature is more important for paper quality formation in a coater drying section than that of evaporation rate. [ABSTRACT FROM AUTHOR]

Based on the earlier studies the most common empirical correlations in the literature predict the impingement heat transfer coefficients rather well at low (close to 100°C) temperatures, but get inaccurate at higher temperatures. Impingement temperatures used in paper drying applications are typically 300 to 700°C, and there has been a need to improve the existing heat transfer correlations at high temperature area. This study presents experimental results of impingement heat transfer measurements with a laboratory-scale heat transfer test rig. Five nozzle configurations were measured. The parameters varied in the investigation were nozzle-to-plate distance, nozzle open area, nozzle diameter, impingement velocity and impingement air temperature. Regression model of heat transfer was developed based on the measurement data. A correction factor including the impingement and heat receiving surface temperatures is proposed. The correction factor can be used to improve the existing correlations at large jet to heat receiving surface temperature differences. [ABSTRACT FROM AUTHOR]

This article examines dynamic behavior of impingement drying. Dynamic relations between the drying variables and the paper moisture are determined and interactivity between the process variables is defined. According to the results, impingement drying is interactive and therefore an advanced control strategy is developed which is based on multivariable model predictive control. The performance of the proposed control strategy is studied by simulation and the simulation results are compared with the simulation results of the present control scheme. [ABSTRACT FROM AUTHOR]

Upon application of water based coating onto silicone-coated paper and during the subsequent drying process, the water permeates through the silicone layer into the paper substrate. At the same time, water evaporates from both the surface of the coated layer and throughout the paper layer. Initially, the evaporation rate from the wet coating surface may be dominant, but at longer times the bulk evaporation from the paper can dominate. Here a three-layer diffusion model for such a system is developed. Solutions obtained by Galerkin's method with finite element basis functions show that the hygroscopic nature of the paper leads to low drying rates at low moisture contents. Further, the model predicts that initially the coating dries to a moderately low residual moisture concentration faster than a coating applied on an impermeable substrate. However, at longer times, the predicted residual moisture of coatings applied on silicon-coated paper is higher than for a coating applied on an impermeable substrate. [ABSTRACT FROM AUTHOR]

Some current models for moisture diffusion in paper in the hygroscopic range are analyzed. In general, two types of diffusion models can be recognized. Models of the first type treat paper as a homogeneous medium with moisture flux that is proportional to the gradient in moisture content. Although useful in some instances this approach fails frequently because it homogenizes the internal dynamics and relaxation processes occurring within the paper material. Recent studies have shown that a subtler approach which treats paper as a composite of fibers and void spaces is more successful at describing moisture transport dynamics. A review of these studies along with a generalization of this approach to three dimensions is considered here. The parameters appearing in such models can be identified with the physical processes of diffusion through the void space and through the fiber matrix. Diffusivities in these individual phases are supplemented by a local kinetic coefficient representing moisture flux interchange between the void and fiber phases. When the local moisture exchange coefficient takes on large values, the fibers and the void spaces are at local equilibrium with no net exchange of moisture. Under such conditions, the model reduces to the simpler Fickian diffusion model with nonlinear moisture diffusivity equivalent to earlier models. [ABSTRACT FROM AUTHOR]

During through air drying the internal sheet drying history develops as a succession of local moisture content profiles in the sheet thickness dimension. These profiles were determined by interrupting the through drying of 180 g/m2 6-ply sheets, then quickly delaminating and determining the moisture content of each 30 g/m2 ply. On the through flow inlet side of the sheet, large moisture differences, about 0.4–0.6 kg/kg dry per 30 g/m2 ply, develop and move into the interior of the sheet provided that local moisture content is in the range 0.45–1.75 kg/kg. On the through flow exit side, moisture gradients only become significant for moisture content below about 1.25 kg/kg. From the through flow exit side of the sheet much of the water is removed by diffusion to the flow entry side where most evaporation into the through flow occurs. The kinetics of through drying reflect the interaction between these two mechanisms, local evaporation into the through flow and thickness direction diffusion. The moisture diffusivity–moisture content relation controls this balance. In any combination with cylinder drying to give some kind of hybrid dryer, through drying should be at the dry end where air permeability is highest and cylinder drying rate is lowest. [ABSTRACT FROM AUTHOR]

Local nonuniformity of moisture content during drying is doubly disadvantageous. Moisture nonuniformity reduces drying effectiveness, increasing drying cost or reducing drying capacity, and may decrease product quality for printing through effects such as cockling and curl. These problems can occur with the now dominant process of cylinder drying, but become much more acute with the various higher intensity drying techniques which are competing to become elements of the higher speed hybrid dryer sections of the future. For quantitative characterization of moisture nonuniformity a novel method applicable to paper being dried by various processes was developed, then demonstrated for one high intensity air convection process, through air drying. During drying, the local moisture content was determined for many 2.5 mm diameter sensing areas by very rapid scanning with a custom design 3-wavelength infrared paper moisture instrument. For local moisture nonuniformity thus determined, the coefficient of variation provides a standard basis for comparison between paper types and drying conditions. For kraft paper from 20 to 120 g/m2, the effects on local moisture nonuniformity were evaluated for drying intensity and for several paper parameters—formation, grammage, and initial moisture content. The drying nonuniformity index was found to parallel both formation quality and drying intensity. An unexpected finding was that over the 20–120 g/m2 range, local moisture nonuniformity passes through a maximum at 30 g/m2. Sheet initial moisture content produces the largest effect, with moisture nonuniformity index being both high and very sensitive to this parameter for initial moisture content above the fibre saturation point. Another finding from these unique measurements is that, for the same 2.5 mm diameter sensing areas, there is a clear correlation between moisture nonuniformity index and local grammage. [ABSTRACT FROM AUTHOR]

This investigation evaluates the changes in dryness and physical properties of impulse processed paper webs. A laboratory-scale platen press was used to process 60 g/m[SUP2] webs under a wide range of pressure impulses and temperatures (23-380°C). The heat transfer rate to the web was also varied by using two different platen materials (steel and copper). Both thermomechanical and bleached kraft pulps were tested, and the dryness, density, surface roughness, and Scott bond were measured for each trial. Results show that dewatering is enhanced with increased temperature, pressure, and nip residence time, while pressure pulse shape and platen material have little influence. An empirical correlation is given to relate sheet dryness to the key process variables. This correlation suggests that resulting dryness values are not unique; that is, different combinations of temperature, pressure, and nip residence time can be used to obtain a similar level of dryness. Physical reasoning was used to develop a second correlation that links the resulting dryness to the density of the completely dried web. Surface roughness decreased with increasing surface temperature, while Scott bond rose sharply under conditions of high temperature and long nip residence time. [ABSTRACT FROM AUTHOR]

For all grades of uncoated paper heavier than tissue, the search for new drying technology has recently become directed towards finding combinations of air drying with cylinder drying which have advantageous techno-economic characteristics. Air drying exists in two forms, through air drying and impingement air drying. An advantage intrinsic to through air drying is exceptionally high drying rates because the distance for heat and mass transport processes is reduced from the thickness of the sheet to the dimensions of pores and fibres. Use of through air drying combined with traditional cylinder drying into multiple technique dryer sections is constrained by two basic characteristics, the cost of the pressure drop for air flow through the sheet, and the tendency for through air drying to produce undesirable local moisture nonuniformity during drying. These two characteristics of through drying are documented and strategies for limiting these disadvantages are identified. [ABSTRACT FROM AUTHOR]

For printing and heavier grades, combining cylinder and impingement air drying into a multiple technique dryer section can enable higher productivity through higher machine speed. The large differences in local moisture content and temperature across the sheet which develop quickly under high intensity impingement drying provide the potential for reducing drying time by sheet reversal between impingement drying cylinders. This advantage was determined experimentally under low intensity impingement drying conditions. Use of the micro-scale based McGill dryer simulator for determining the advantage from sheet reversal between impingement drying cylinders was demonstrated for both laboratory and industrial impingement drying intensities. For completing the drying of 205g/m[SUP2] linerboard from 0.3 to 0.5kg/kg dry under 400°C air jets of Re 20000, drying time is about 30% less with sheet reversal between two impingement drying cylinders than for a single, larger diameter cylinder. This extensively validated dryer simulator enables determining advantageous design specifications and operating conditions for hybrid dryer sections involving combinations of cylinder and impingement air drying, a concept with potential to become common industrial practice. [ABSTRACT FROM AUTHOR]

Vacuum freeze-drying of frozen wet papers was investigated experimentally. A freeze-drying facility was built to study the process by monitoring the various physical parameters (pressure, sample temperature, heater temperature, water content of the sample and cold panel temperature). A simple phenomenological model was developed to explain experimental results. The values of the physical parameters were optimized to make the process cost effective. [ABSTRACT FROM AUTHOR]

Infrared heating/drying of paper is becoming very common in industry. However, there is a lack of fundamental information explaining the transport phenomena occurring within a paper sheet due to energy supplied by a gas-fired infrared emitter. This experimental study provides basic drying and temperature data for moist paper exposed to gas-fired infrared radiation energy. Two paper types, linerboard and bleached, at three basis weight levels, are used in this study. [ABSTRACT FROM AUTHOR]

On-line heating/drying of paper sheet with gas-fired infrared emitters is investigated through pilot machine trials and theoretical analysis. The results indicate that significant enhancements in drying of uncoated paper can be achieved with the application of gas-fired infrared emitters. The theoretical model predicts the pilot machine data closely. Thus, the theoretical model developed can be used as a valuable tool to predict the increase in machine speed or drying rate with the incorporation of gas-fired infrared emitters in the drying section. [ABSTRACT FROM AUTHOR]

The long-term effect of gamma irradiation on paper was studied. Three types of paper (pure rag, mixed rag and mechanically prepared woodpulp) were subjected to three treatments: gamma irradiation, accelerated dry aging, and radiation followed by aging. The effects of these treatments on the physical properties of the paper samples were assessed by measuring alterations in mechanical strength as determined by changes in folding endurance and tear resistance when compared to the untreated control samples. It was found that gamma irradiation caused a decrease in the mechanical strength of the papers similar to that resulting from accelerated dry aging. A synergistic effect was found when both irradiation and accelerated aging were performed on the paper samples, demonstrated by an even greater decrease in both fold endurance and tear resistance than that resulting from either gamma irradiation or accelerated dry aging alone. [ABSTRACT FROM AUTHOR]

The author reflects on the four decades of uninterrupted professional journey on the research and development (R&D) on drying and dewatering. The author states that his R&D activity was driven by the necessity to search for new drying technology for paper. The author mentions the new paper drying technology made by the Pulp and Paper Research Institute of Canada in 1960. The author adds that his research on the multidisciplinary areas of transport processess.