HIGH-INTENSITY DRYING OF PAPER

High-intensity contact drying denotes drying under suf- ficiently intensive heating conditions that, following a brief warmup period, the mist paper web operates at internal tem- peratures in excess of the ambient boiling point. A simplified, two-zone analytical model is first presented. The paper is depicted as having a dry layer, of ever-increasing thickness, adjacent to the hot surface. Heat conduction through this layer (the rate-limiting step) causes evaporation at the interface with the “wet zone.” The vapor is then considered to flow through the wet zone into the ambient. Results of bench-scale experiments are discussed. Drying rates as much as twenty times conventional rates are indicated. The drying rate increases with hot surface/boiling point tem- perature difference and applied mechanical pressure. The instsn- taneous drying rate decreases continuously after a brief warmup period. The vapor pressure at the hot surfacelpaper interface rises quickly to a peak value, followed by a continuously- diminishing level. The heat flux shows a rapid rise to boiling- like conditions, followed by a drop to a range compatible with measured drying rates. The temperature of the open side of the sheet is constant during drying, after its initial rise to the boiling point. The experimental results are compatible with pre- dictions of the model.     

SIMULATION MODELS OF MULTI-CYLINDER PAPER DRYING

ABSTRACT

The literature on simulation models for multi-cylinder paper dryers is surveyed, 20 such models being reviewed. Models for intraweb heat and mass transfer are treated extensively, a separate chapter being devoted to them. In some of the models the specific physical transfer phenomena are modelled in detail, whereas in other models a more macroscopic approach is adopted. Normally the latter approach requires the incorporation of a large number of empirical constants into the model to describe the complicated heat and mass transfer processes involved and to obtain a reasonable agreement between the model and the measured data. In order to develop the simulation models further without use of an inordinately large number of empirical constants, the different physical phenomena which occur in the dryer must be modelled in greater detail, bnsed both on fundnmental theory and on empirical correlations.     

SIMULATION MODELS OF MULTI-CYLINDER PAPER DRYING

The literature on simulation models for multi-cylinder paper dryers is surveyed, 20 such models being reviewed. Models for intraweb heat and mass transfer are treated extensively, a separate chapter being devoted to them. In some of the models the specific physical transfer phenomena are modelled in detail, whereas in other models a more macroscopic approach is adopted. Normally the latter approach requires the incorporation of a large number of empirical constants into the model to describe the complicated heat and mass transfer processes involved and to obtain a reasonable agreement between the model and the measured data. In order to develop the simulation models further without use of an inordinately large number of empirical constants, the different physical phenomena which occur in the dryer must be modelled in greater detail, bnsed both on fundnmental theory and on empirical correlations.     

CONSTRUCTION OF AN ANALYTICAL MODEL OF PAPER DRYING

ABSTRACT

A theoretical model developed is presented to simulate the paper drying process on a production paper machine. The paper sheet is represented as a matrix of pulpfibres which contains free and bound water, water vapour and air. The model is heavily dependent upon a wide range of physical data including pore size distribution, permeability sorptive characteristics, thermal conductivity, specific heat capacity, density, diffusion coefficients and shrinkage characteristics as well as heat and mass transfer behaviour at the interfaces. Theoretical relationships to describe these parameters in terms of the physical pore structure are developed and compared with published data. The model was compared against actual measurements on the Australian Newsprint Mills Boyer PM3 newsprint machine. The comparison with actual machine moisture content values showed the model prediction of moisture change during drying to cylinder No. 38 on PM3 to be 2% less than actual and 0.1% more than actual by the exit from the drying cylinder. In terms of predicting thermal energy consumption of the paper machine a 91% correlation was obtained.     

CONSTRUCTION OF AN ANALYTICAL MODEL OF PAPER DRYING

A theoretical model developed is presented to simulate the paper drying process on a production paper machine. The paper sheet is represented as a matrix of pulpfibres which contains free and bound water, water vapour and air. The model is heavily dependent upon a wide range of physical data including pore size distribution, permeability sorptive characteristics, thermal conductivity, specific heat capacity, density, diffusion coefficients and shrinkage characteristics as well as heat and mass transfer behaviour at the interfaces. Theoretical relationships to describe these parameters in terms of the physical pore structure are developed and compared with published data. The model was compared against actual measurements on the Australian Newsprint Mills Boyer PM3 newsprint machine. The comparison with actual machine moisture content values showed the model prediction of moisture change during drying to cylinder No. 38 on PM3 to be 2% less than actual and 0.1% more than actual by the exit from the drying cylinder. In terms of predicting thermal energy consumption of the paper machine a 91% correlation was obtained.     

THE INFLUENCE OF VACUUM ON DRYING OF PAPER

It is well known in the drying of paper that it is possible reduce the size of the dryer section and/or increase the drying capacity by using vacuum. Furthermore a smaller dryer section contributes to a decrease in the energy losses. However, the use of Minton vacuum dryers in the late 20's was never really successful. Especially with increasing machine speeds maintenance became a problem.

Vacuum drying leads to an improvement in the optical pro- perties of papers made from mechanical pulps. Some physical properties such as softness and porosity may also be improved. When the paper is pressed towards the hot surface under me- chanical pressure during vacuum drying a gain in mechanical properties can be achieved.

In the present investigation, the influence of heat transfer between the web and the hot surface as well as mechanical com-pression of the sheet during vacuum drying have been avoided by using an IR heat source. The results show that the main effect of vacuum is a reduced evaporation temperature. This allows the drying to reach its maximum rate faster. The lower temperature level during vacuum drying also makes cheaper energy sources avilable.     

CONVECTIVE DRYING OF PAPER CALCULATED WITH A NEW MODEL OF THE PAPER STRUCTURE

Paper drying models are based on the assumption of a rigid solid matrix. Because of the interaction of water molecules and paper fibre the dimensions of the sheet are changing, especially the thickness. These changes influence the moisture transport within the material. A model of the paper structure has been developed taking into account different kinds of pores in the paper.

By the assistance of this model the dependence between water content and thickness or transport parameters as permeability and thermal conductivity can be predicted. The only informations which are needed are sorption isotherms, apparent paper density and permeability of the single phase flow. Further informations as the pore size distribution are useful but not necessary.

The transport parameters are generated in the suggested way for wood pulp. It is shown that in the case of convective drying, the agreement between experimental results and model simulations is good.     

CONVECTIVE DRYING OF PAPER CALCULATED WITH A NEW MODEL OF THE PAPER STRUCTURE

Paper drying models are based on the assumption of a rigid solid matrix. Because of the interaction of water molecules and paper fibre the dimensions of the sheet are changing, especially the thickness. These changes influence the moisture transport within the material. A model of the paper structure has been developed taking into account different kinds of pores in the paper.

By the assistance of this model the dependence between water content and thickness or transport parameters as permeability and thermal conductivity can be predicted. The only informations which are needed are sorption isotherms, apparent paper density and permeability of the single phase flow. Further informations as the pore size distribution are useful but not necessary.

The transport parameters are generated in the suggested way for wood pulp. It is shown that in the case of convective drying, the agreement between experimental results and model simulations is good.     

AN EXPERIMENTAL STUDY OF GAS-FIRED INFRARED DRYING OF PAPER

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.     

AN EXPERIMENTAL STUDY OF GAS-FIRED INFRARED DRYING OF PAPER

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.     

THE DEPENDENCE OF PAPER MACHINE DRYING DRYING CHARACTERISTIC ON THE MACHINE GEOMETRY

The effect of the dryer cylinder diameter on the dryer specific evaporation rate is examined theoretically. It is observed that, in the range of cylinder diameters applicable in practice, the dryer specific evaporation rate is nearly independent of the selected cylinder diameter when the dryer geometry remains unchanged. In practice, however, the dryer geometries have changed slightly according to the cylinder diameter.

TO clarify the effects of the various component factors pertaining to the dryer geometry, this paper presents a drying characteristic, i.e.. a curve showing the dependence of the drying rate on the condensing steam temperature. The drying characteristic is given in the form of a nomogram to facilitate the evaluation of the effect of the ratio between the free draw length and the cylinder diameter, and the wrapping angle, to the dryer specific evaporation rate. Further, by means of an example, the specific evaporation rates obtainable for machine geometries currently used by various machine manufacturers are compared.     

A MATHEMATICAL MODEL OF HIGH INTENSITY PAPER DRYING

High intensity drying occurs when one web surface is heated to the thermodynamic saturation temperature corresponding to the local hydraulic pressure. Rapid vapor generation causes the process to be driven by a total pressure gradient, so vapor leaves the web by a bulk flow mechanism rather than a slower diffusion mechanism. Vapor pressure build-up promotes rapid web heating and offers the opportunity for liquid removal by displacement. Lower energy usage can result if only a part of the moisture is evaporated.     

A MATHEMATICAL MODEL OF HIGH INTENSITY PAPER DRYING

High intensity drying occurs when one web surface is heated to the thermodynamic saturation temperature corresponding to the local hydraulic pressure. Rapid vapor generation causes the process to be driven by a total pressure gradient, so vapor leaves the web by a bulk flow mechanism rather than a slower diffusion mechanism. Vapor pressure build-up promotes rapid web heating and offers the opportunity for liquid removal by displacement. Lower energy usage can result if only a part of the moisture is evaporated.     

THE DEPENDENCE OF PAPER MACHINE DRYING DRYING CHARACTERISTIC ON THE MACHINE GEOMETRY

ABSTRACT

The effect of the dryer cylinder diameter on the dryer specific evaporation rate is examined theoretically. It is observed that, in the range of cylinder diameters applicable in practice, the dryer specific evaporation rate is nearly independent of the selected cylinder diameter when the dryer geometry remains unchanged. In practice, however, the dryer geometries have changed slightly according to the cylinder diameter.

TO clarify the effects of the various component factors pertaining to the dryer geometry, this paper presents a drying characteristic, i.e.. a curve showing the dependence of the drying rate on the condensing steam temperature. The drying characteristic is given in the form of a nomogram to facilitate the evaluation of the effect of the ratio between the free draw length and the cylinder diameter, and the wrapping angle, to the dryer specific evaporation rate. Further, by means of an example, the specific evaporation rates obtainable for machine geometries currently used by various machine manufacturers are compared.     

THE EFFECT OF DRYING TEMPERATURE ON THE QUALITY OF PAPER

The effect of drying temperature on the optical properties of various papers was investigated. For paper from mechanical pulp, it was found that as drying temperature increased, light scattering coefficient decreased leading to lower brightness. The decrease in light scattering was associated with improved strength properties even at constant sheet density, and was attributed to increased bonding. These effects were not observed for chemical pulp furnishes.     

THE EFFECT OF DRYING TEMPERATURE ON THE QUALITY OF PAPER

ABSTRACT

The effect of drying temperature on the optical properties of various papers was investigated. For paper from mechanical pulp, it was found that as drying temperature increased, light scattering coefficient decreased leading to lower brightness. The decrease in light scattering was associated with improved strength properties even at constant sheet density, and was attributed to increased bonding. These effects were not observed for chemical pulp furnishes.     

CONDEBELT BOARD AND PAPER DRYING

Abstract

The history of development of the Condebelt process is outlined. The functioning principle of the various types of existing or proposed Condebelt dryers is clarified

Condebelt drying, being a type of press drying, imparts certain quality characteristics, due to the simultaneous high z-pressure and high temperature in the presence of moisture, to the web. Some typical quality characteristics of Condebelt-dried sheets are presented for some usual paper and board grades. The web strength values of densified Condebelt-dried sheets are usually much higher than those of conventionally dried sheets. This is mostly due to the much enlarged contact areas between flattened fibers, and to the flow of surface hemicellulose and lignin under the Condebelt drying conditions. In particular, wet strength values, as well as dimensional stability under changing environmental humidity conditions, are very much improved     

DRYING PAPER IN SUPERHEATED STEAM

The industrial potential for drying paper in superheated steam has been investigated in a series of studies designed to treat paper properties - drying process relations as well as process engineering aspects. Properties measurements show that for paper made from mechanical pulps, drying in superheated steam produces a better bonded sheet which is thereby stronger and has a lower scattering coefficient. Surface properties of such steam dried paper are improved, including reduced tinting. Desorption equilibrium shows that completely dry paper can be obtained at very low superheats. Impingement drying rates can be about twice as high for drying in superheated steam as in air. A hybrid cycle was developed in which drying is partially by superheated steam impingement drying and partially by conventional cylinder drying. Preliminary design and techno-economic evaluation indicates that three factors, paper properties, dryer size and energy efficiency, in various combinations specific to each case, may provide industrial potential for this new technology for drying paper.