MATHEMATICAL MODELLING OF MDF FIBRE DRYING: DRYING OPTIMISATION

In the production of MDF, wet resinated fibre must be dried to its target moisture content, normally 9 to 11%, before compaction into a board by hot pressing. Fibre drying can be interpreted as an incorporated process involving gas-solid two phase-flow, inter-component transfer, and heat and mass transfer within the fibre. Based on these mechanisms, a mathematical model has been developed to simulate the MDF fibre drying process. From the model, fibre moisture content, air temperature and air humidity along the dryer length can be predicted and factors affecting the drying rate examined. The model can be employed to optimise drying conditions and to evaluate improvements in dryer design. A case study of drying improvement in reduction of dryer emissions and heat consumption is given to demonstrate the potential application of the developed dryer model.     

IMPROVING MDF FIBRE DRYING OPERATION BY APPLICATION OF A MATHEMATICAL MODEL

Fibre drying is an important process in production of medium density fibreboard (MDF) which consumes a large amount of energy, affects product quality and, without appropriate control, causes environmental concerns. Based on fundamental knowledge of wood fibre-water relationships and heat/mass transfer, a mathematical model has been developed to simulate the MDF fibre drying processes. The model is able to predict fibre moisture content, air temperature and air humidity along the dryer length. After validation against the measured air temperature and humidity, the model has been extended to include both fibre drying and fibre conditioning, the latter occurring in the dry fibre conveyers. Due to potential benefits in reducing emissions of volatile organic compounds (VOCs) and in improving panel quality, lower drying temperatures are more desirable than higher temperatures. However, in order to achieve the target moisture content after drying, a higher air velocity is needed or a second-stage dryer is added. The model was employed to determine the air velocity required and to assist in designing a second dryer for further drying and recovery of moist vapour and heat. A further study was undertaken to investigate fibre drying or fibre conditioning in the fibre conveyers and, once again, the fibre drying model was used to determine the air conditions.     

IMPROVING MDF FIBRE DRYING OPERATION BY APPLICATION OF A MATHEMATICAL MODEL

Fibre drying is an important process in production of medium density fibreboard (MDF) which consumes a large amount of energy, affects product quality and, without appropriate control, causes environmental concerns. Based on fundamental knowledge of wood fibre-water relationships and heat/mass transfer, a mathematical model has been developed to simulate the MDF fibre drying processes. The model is able to predict fibre moisture content, air temperature and air humidity along the dryer length. After validation against the measured air temperature and humidity, the model has been extended to include both fibre drying and fibre conditioning, the latter occurring in the dry fibre conveyers. Due to potential benefits in reducing emissions of volatile organic compounds (VOCs) and in improving panel quality, lower drying temperatures are more desirable than higher temperatures. However, in order to achieve the target moisture content after drying, a higher air velocity is needed or a second-stage dryer is added. The model was employed to determine the air velocity required and to assist in designing a second dryer for further drying and recovery of moist vapour and heat. A further study was undertaken to investigate fibre drying or fibre conditioning in the fibre conveyers and, once again, the fibre drying model was used to determine the air conditions.     

RELATIVE IMPORTANCE OF VAPOUR DIFFUSION AND CONVECTTVE FLOW IN MODELLING OF SOFTWOOD DRYING

ABSTRACT

The influence of vapour diffusion on the drying rate of a softwood board has been examined for drying temperatures varying from 60°C to 140°C. It is found that for very low temperature drying a model which considers both vapour convective and diffusion in wood predicts dry-rate curves matching the experimental data closely. For high temperature drying, both of the above drying model and a drying model which considers only vapour convective flow give predictions in agreement with the observed data. This illustrates that the diffusion of vapour and air is important in low temperature drying panicularly during the late stages of drying. However, for high temperature drying, the convective flow of moisture vapour is dominant and the diffusion component is negligible. The observation provides evidence for simplifying a drying model for high temperature drying without reducing its credibility in predicting drying rate curves.     

DRYING RATE AND TEMPERATURE PROFILE FOR SUPERHEATED STEAM VACUUM DRYING AND MOIST AIR DRYING OF SOFTWOOD LUMBER

Abstract

Two charges of green radiata pine sapwood lumber were dried, either using superheated steam under vacuum (90°C, 0.2 bar abs.) or conventionally using hot moist air (90/60°C). Due to low density of the drying medium under vacuum, the circulation velocity used was 10 m/s for superheated steam drying and 5.0 m/s for moist air drying, and in both cases, the flow was unidirectional. In drying, stack drying rate and wood temperatures were measured to examine the differences between the superheated steam drying and drying using hot moist air.

The experimental results have shown that the stack edge board in superheated steam drying dried faster than in the hot moist air drying. Once again due to the low density of the steam under vacuum, a prolonged maximum temperature drop across load (TDAL) was observed in the superheated steam drying, however, the whole stack dried slower and the final moisture content distribution was more variable than for conventional hot moist air drying. Wood temperatures in superheated steam drying were lower.     

STUDIES ON STIFLING AND DRYING OF COCOONS FOR LONGER STORAGE

ABSTRACT

Long term storage of stifled cocoons is an important problem during unfavourable seasonal conditions. Cocoons could be stifled within 20 minutes by using steam al 90°C. The shade drying of steam stifled cocoons in thin layers reduced the moisture content from 228 per cent (db) to 3 1.7 per cent (db) within 10 days. By hot air conditioning method the cocoons could be stifled and dried simultaneously. Using hot air at 90°C the cocoons could be stifled within 10 minutes, and dried to the equilibrium moisture content within 2 hours. The cocoons stifled by the hot air conditioning method could be reeled even just after stifling.     

MODELLING OF STRESS DEVELOPMENT DURING DRYING AND RELIEF DURING STEAMING IN PINUS RADIATA LUMBER

ABSTRACT

A one-dimensional stress model was proposed for drying of radiata pine lumber, which has considered wood moisture shrinkage, instantaneous stress-strain relationships, mechano-sorptive creep, time-induced creep and temperature effects. In addition, wood hardening behaviour in the plastic region and differences between stress increase and decrease have been taken into account. The proposed Stress model can predict stress development and relief in a drying cycle once the required wood mechanical and Theological properties have been quantified.

Drying experiments were performed to dry Pinus radiata sap wood boards of 100×40×590 mm in a tunnel dryer. In the experiment, wood temperature, moisture content gradient and residual stress through board thickness were measured. The drying cycle included HT drying, cooling and final steam conditioning. The measured stress patterns were in agreement with the model predictions. However, more accurate calculations will be made once the detailed experimental data for radiata pine wood mechanical and rheological properties are available.     

THE DRYING OF SMALL WOOD PARTICLES

Small particles of Douglas fir of characteristic size 0.2 to 1cm have been dried by flowing the wood particles in vitiated air in a long vertical tube. Drying rate, dM/dt, was determined by measuring the increasing humidity of the transporting gas flow, and by measuring the absolute velocity of the particles by a radioactive isotope tagging method. Sawdusts of three sizes and flakes were examined. Moisture content (dry basis) was varied from approximately 2 to 125%. Most of the results were obtained for gas temperatures between 350 and 550 K.

Upon injection into the flowing hot gases, the wood particles initially dried rapidly as they encountered high relative temperature and velocity, and released their surface moisture. Following· this transient regime, the particles dried more gradually accord-     

A MATHEMATICAL MODEL FOR DRYING OF SHRINKING MATERIALS

ABSTRACT

A mathematical model has been developed to describe heat and mass transfer within materials undergoing shrinkage during drying. Both heat and mass transfer equations are solved simultaneously using a numerical technique A beat pump dryer has been used to conduct experiments to validate the model. Several samples were placed in the drver and after the commencement of each drying test one sample was taken oat at rceular time interval: The bone-dry mass of each piece was also determined. This enables to determine moisture distribution within the materials. Temperatures at different locations of the material were measured with thermocouples. The predicted temperature and moisture distribution within the material agreed fairly well with the experimental results.     

MATHEMATICAL MODELLING OF INDIRECT CONTACT ROTARY DRTHRS

ABSTRACT

Two different approaches were used to predict the solid moisture content and solid temperature profiles along a continuous indirect contact rotary dryer heated with steam tubes. One of these uses heat and mass balances applied to the solid phase in a differential element of dryer length. Here the heat flux is computed through an overall heat transfer coefficient assumed constant. The other model is based on a previous work that calculates the heat transfer coefficient as a function of the time the solid particles are in contact with the heating surface. The advantage in using this second model lies in the fact that the calculated heat transfer coefficient can take into account the effects of the operational conditions. If this coefficient has a strong dependence on these conditions, then it would be inappropriate to use a fixed value. Although both approaches can predict the solid moisture content and temperature profiles along the dryer differences were detected.     

A CALCULATION METHOD OF OBTAINING DESORPTION ISOTHERMS OF BEECH (FAGUSMOESL4CA) WOOD

ABSTRACT

This paper presents the specific methods to obtain sorption isotherms. Sorption isotherms were first obtained experimentally for 20°C and 100°C and then, by using the presented method, isotherms were also obtained for 50°C and 80°C. The same treatment can be applied in finding any isotherm sorption in the temperature range between 20°Cand 100°C.

The experiments were carried out on beech (Fagus moesiaca) and sample dimensions were 3×3×3cm. Based on the results of the experiment, by regression analysis, the most suitable analytic formula was proposed. It relsites temperature and air humidity with the equilibrium moisture content in wood. Experimental results were compared with psychrometric tables and Bramhall's formula for desorption curves with good agreement. This proves that the method presented in this paper is precise and efficient to decrease the lime needed to obtain sorption isotherms experimentally.     

INDUSTRIAL THROUGH-AIR DRYING OF NONWOVENS AND PAPER BASIC PRINCIPLES AND APPLICATIONS

Abstract

The requirements for drying and thermobonding equipment working on the basis of the hot air flow-through system vary considerably due to the large variety of nonwovens qualities, production methods and required production speeds. It is therefore necessary to construct diverse drying systems in order to be able to offer appropriate machines for the desired purposes. Optimum heat and mass transfer from the penetrating air to the permeable material to be dried is ensured by through-air drying

Following Hans Fleissner's invention of the through-air system for drying of textiles and its practical application in the perforated drum dryer for more than 50 years a second engineering breakthrough for the paper and nonwovens industry has been achieved by Gerold Fleissner with the high-tech through-air drum

This paper presents various possibilities of air flow through the material and provide criteria to facilitate selection of the proper drying system     

浅析影响原丝烘干的因素

概述湿纱含水对增强型玻璃纤维的重要性,影响原丝烘干的因素。主要介绍干燥过程的几个阶段,相对湿度,固含量及几个工艺参数对原丝烘干的影响,尤其减少原丝含水率可提高产品质量降低能耗,其中,提高固含量,减少原丝含水的影响最为有效。     

VACUUM-PRESS DRYING OF THICK SOFTWOOD LUMBERS

ABSTRACT

In vacuum-press drying of softwood species, wood lemperaiure exhibited a low temperature gradient, and plateau temperature of core lasied during all stages of drying. The drying curves were close to “linear”. The drying rates of the short lumbers were higher for red pine and western hemlock, lower for white pine and similar for larch lumber compared to the long ones. Transverse and longitudinal moisture gradients were small for western hemlock and red pine, and great for larch and white pine lumber. Energy consumption curves were split into three sections: increasing moderately, fairly constant and increasing rapidly. Casehardening stress of dried lumber occurred very slightly. Dried lumbers exhibited strong tendency for fine end checking, slight surface checking and no internal checking. Shrinkage appeared to be low.     

COMBINED CONVECTIVE AND INFRARED DRYING OF A CAPILLARP POROUS BODY

Abstract

This work investigates the effect of spray drying conditions on some properties of tomato powder prepared by spray drying of tomato pulp. A pilot scale spray dryer (Buchi, B-191) with cocurrent regime and a two-fluid nozzle atomizer was employed. Sixty-four different experiments were conducted keeping constant the feed rate, the feed temperature, and the atomizer pressure, and varying the compressed air flow rate, the flow rate of drying air, and the air inlet temperature. Tomato powders were analyzed for moisture, solubility, density (bulk and packed), and hygroscopicity. Analysis of experimental data yielded correlations between powder properties and the above-mentioned variable operating conditions. Regression analysis was used to fit a full second order polynomial, reduced second order polynomials and linear models to the data of each of the properties evaluated. F values for all reduced and linear models with an R ² ≥ 0.70 were calculated to determine if the models could be used in place of full second order polynomials.     

DRYING OF PARTICULATE SOLIDS: DETERMINATION OF THE CHARACTERISTIC CURVE OF BROWN COAL.

ABSTRACT

Variation on the rate of drying of brown coal powder as a function of environmental and intrinsic properties has been studied in a gravimetric laboratory-scale drier.

Raw data have been treated to correlate the constant rate of drying and the equilibrium moisture content as a function of flow rate and temperature of the gas phase, particle size and surface additives. The characteristic drying curve occurs when the reduced rate of drying is plotted as a function of a dimensionless moisture potetial.     

EFFECTS OF SAWING PATTERN ON LUMBER DRYING: MODEL SIMULATION AND EXPERIMENTAL INVESTIGATION

ABSTRACT

The sawing pattern of lumber affects the drying rate due to transverse permeability differences. These permeability differences are considered in a single board drying model which is able to investigate the drying rates for boards with varying growth ring angles. For the drying of Pinus radiata lumber, the model predicts that the quartersawn boards need longer drying time than the flatsawn boards. The drying time difference was 10–15% of the total drying time for conventional temperature (CT) drying and accelerated conventional temperature (ACT) drying, but was less significant for high temperature (HT) drying. In the simulation of a kiln stack drying, a sawing pattern factor was introduced to the relative drying rate function, which reflected the effects of the growth ring angle and the drying temperatures. The modified kiln wide drying model was used to predict the drying rates for a kiln stack consisting of entirely flatsawn boards and a kiln stack consisting of entirely quartersawn boards. Drying tests were conducted using stacks of mixed flatsawn and quartersawn Pinus radiata sapwood boards. In the tests, three drying schedules were used which included CT, ACT and HT drying. The experimental results agree closely with the model predictions and thus, after further validation, the drying models can be used to predict commercial kiln drying of boards with different sawing patterns.     

SUPERHEATED STEAM DRYING OF SINTERED SPHERES OF GLASS BEADS UNDER VACUUM

Drying of sintered spheres of coarse glass beads with a wide sintering range in superheated steam under vacuum was studied.

In samples with sintered angles of 7.5° –27°, the experimental normalized drying rates in superheated steam at pressures of 7.3-7.9mmHg were smaller than those for 56.0-767.6 mmHg in the vicinity of the critical moisture cotents for 56.0-767.6GmmHg. As reported in an earlier paper, there were     

EFFECTS OF SAWING PATTERN ON LUMBER DRYING: MODEL SIMULATION AND EXPERIMENTAL INVESTIGATION

The sawing pattern of lumber affects the drying rate due to transverse permeability differences. These permeability differences are considered in a single board drying model which is able to investigate the drying rates for boards with varying growth ring angles. For the drying of Pinus radiata lumber, the model predicts that the quartersawn boards need longer drying time than the flatsawn boards. The drying time difference was 10-15% of the total drying time for conventional temperature (CT) drying and accelerated conventional temperature (ACT) drying, but was less significant for high temperature (HT) drying. In the simulation of a kiln stack drying, a sawing pattern factor was introduced to the relative drying rate function, which reflected the effects of the growth ring angle and the drying temperatures. The modified kiln wide drying model was used to predict the drying rates for a kiln stack consisting of entirely flatsawn boards and a kiln stack consisting of entirely quartersawn boards. Drying tests were conducted using stacks of mixed flatsawn and quartersawn Pinus radiata sapwood boards. In the tests, three drying schedules were used which included CT, ACT and HT drying. The experimental results agree closely with the model predictions and thus, after further validation, the drying models can be used to predict commercial kiln drying of boards with different sawing patterns.