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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]

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]

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]

The present study investigates two models—two limiting cases—for the internal heat and mass transfer in a multicylinder dryer: the first model assumes that complete redistribution of heat and moisture in the thickness direction occurs instantaneously. The second model assumes that moisture transfer occurs only by vapor diffusion and that heat transfer takes place by conduction and condensation. The two models are written in dimensionless form, and applied for the same set of standard paper machine data. For basis weights below approximately 0.05 kg d.s./m2 the results are so close that the first, simple model is sufficient. A higher limiting basis weight—approximately 0.16 kg d.s./m2—was previously indicated by comparison of model predictions, assuming the simple model, to extensive sets of machine data from four different paper machines. [Copyright &y& Elsevier]

A mathematical model describing the heat and mass transfer in the dryer section of a paper machine has been applied to the production data from four paper machines. Model predictions for the machine speed are compared to actual machine speeds for a total of 163 data sets. The mathematical model assumes that the temperature and moisture content remain homogeneous in the thickness direction of the sheet. For three paper machines producing paper with basis weights ranging from 0.056 to 0.159 kg d.s./m2 the model predictions are adequate. For the paper machine producing the heaviest grades with basis weights ranging from 0.189 to 0.390 kg d.s./m2 the model predictions are flawed by a systematic error. For low machine velocities/high basis weights the machine velocity is over-predicted and for high machine velocities/low basis weights the machine velocity is under-predicted. This systematic error is caused by the assumption of homogeneous moisture content and temperature within the sheet being severely in error for thick sheets. [Copyright &y& Elsevier]