A Model for Through Drying of Tissue Paper at Constant Pressure Drop and High Drying Intensity

A mathematical model for through drying of paper at constant pressure drop was developed. The model is based on physical properties; hence, basis weight, pressure drop, drying air temperature, pore size distribution, initial gas fraction, and tortuosity are important input parameters to the model. The model was solved for different combinations of the variables basis weight, drying air temperature, and pressure drop corresponding to industrial conditions and the results were compared with data from bench-scale experiments. The simulations show that the drying rate curve is very sensitive to the air flow rate and that correctly modeling the correlation between pressure drop and air flow rate is the most important factor for a successful model for through drying. The model was tuned by adjusting the parameters initial gas fraction and tortuosity in order to give the best possible fit to experimental data. For a given basis weight and pressure drop, different drying air temperatures resulted in relatively constant values of the fitted parameters. This means that the model can well predict the effects of changes in drying air temperature based on a tuning of the model performed at the same basis weight and pressure drop. However, for a given basis weight, an increase in pressure drop yielded fitted parameters that were somewhat different; i.e., a lower initial gas fraction and a higher tortuosity, a change that increases the resistance to air flow. This implies that the correlation between pressure drop and air flow rate in the model does not quite capture the nonlinear relationship shown by the experiments.     

Through Drying of Paper

Through drying of paper of basis weight 25 to 50 g/m² was studied through 89 experiments with 22 to 90°C air at mass flow rates from 0.125 to 1.45 kg/m²s. The complete drying rate curves are represented with five quantitative parameters:moisture contents at the end of the increasing and constant rate periods, the constant drying rate, and exponents for the increasing and falling rate period relations. The constant drying rate period disappears at higher drying intensities leaving the increasing rate period accounting for as much as 37% of the drying. The constant drying rate may be controlled by thermodynamics or by transport phenomena. The critical moisture content depends on effects from the intrinsic local nonuniformity of through drying as well as on sheet average conditions. A universal set of relations was obtained for representation of the complete drying rate curve and was verified for prediction of drying rate and drying time. Through drying rates cannot be increased significantly by providing the drying air as an impingement flow.     

Mathematical Model of Fixed-Bed Drying and Strategies for Crumb Rubber Producing STR20

The objectives of this research were to investigate empirical and diffusion models for thin-layer crumb rubber drying for producing STR20 rubber using hot air temperatures of 110–130°C and to study the effect of drying parameters such as inlet drying temperature, volumetric flow rate, and initial moisture content on the quality of dried rubber. Finally, a mathematical drying model for predicting the drying kinetics of crumb rubber was developed using inlet air flow rates of 300–600 m³/min-m³ of crumb rubber (equivalent to 1.8–5.0 m/s) with the crumb rubber thickness fixed at 0.25 m. The average initial moisture content of samples was in the ranges of 40 and 50% dry basis while the desired final moisture content was below 5% dry basis. The results showed that the drying equation of crumb rubber was highly related to the inlet air temperature, while the drying constant value was not proportional to the initial moisture content. Consequently, the experimental data were formulated using nine empirical models and the analytical solution of moisture ratio equation was developed by Fick's law of diffusion. The result showed that the simulated data best fitted the logarithmic model and was in reasonable agreement to the experimental data. The effective diffusion coefficient of crumb rubber was in the range of 1.0 × 10^?9 to 2.15 × 10^?5 m²/s corresponding to drying temperatures between 40 and 150°C, respectively. The effects of air recirculation, inlet drying temperature, initial moisture contents, air flow rate, and drying strategies on specific energy consumption and quality of samples were reported. The experiments were conducted using two different drying strategies as follows: one-stage and two-stage drying conditions. The results showed that initial moisture content and air flow rates significantly affected the specific energy consumption and quality of rubber, while the volumetric air flow rate acted as dominant effect to the specific energy consumption. The simulated results concluded that the percentage of recycled air between 90 and 95% provided the lowest specific energy consumption as compared to the others.     

循环流化床中C类颗粒的干燥

为将循环流化床(CFB)技术应用于C类颗粒(＜30μm)的干燥,在自建的循环流化床(内径0.104 m×高2.35 m)内,以玉米淀粉(dp=8 μm,ρp=800 kg/m3)为研究对象,考察了不同操作参数对其干燥特性及干湿分离情况的影响.结果表明:循环流化床适用于C类颗粒的干燥;干燥速率随气速及进风温度的增大而增大...     

油页岩干燥特性实验研究

以柳树河油页岩颗粒为原料,在恒温介质干燥器内进行油页岩干燥动力学研究,考察干燥介质温度和颗粒直径对油页岩干燥性能的影响,采用薄层干燥模型中的Lewis模型,对油页岩干燥实验数据进行模拟,确定油页岩干燥方程和干燥速率方程,建立油页岩干燥常数表达式。研究结果表明:薄层干燥模型中Lewis模型能较好的描述油页岩在恒温介质干燥器内的干燥过程;其干燥过程主要发生在降速干燥阶段;200℃时油页岩无性质变化,干燥达到平衡时,油页岩含水量可降到0.5%左右,油页岩表观活化能为17～19kJ/mol。     

Mass Transfer Coefficients and Drying Rates in Through Air Drying: Part II

In this study experimental results and analysis are given of convective heat and mass transfer in the drying of tissue and towel products (25 and 50 g/m² basis weight) under commercially realistic conditions under through flow. Parameter effects on drying rate include process variables such as refining, shaping, commercial forming, initial moisture content, and drying temperature. Drying rate curves display initial increase, plateau, and final decrease as the paper dries. Nondimensional Sherwood numbers (versus Peclet number) generally agreed with and augment the available dataset on through drying.     

Integral Flow Parameters and Material Characteristics Analysis in Through Air Drying: Part I

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.     

Pneumatic Drying of Iron Ore Particles in a Vertical Tube

The drying and hydrodynamic characteristics of iron ore particles in a vertical pneumatic conveying dryer (0.078 m ID × 6.0 m high) have been determined. The pressure drop decreases along the height at the acceleration region, while it remains constant irrespective of the height in the fully developed region. The degree of particle drying in the dilute pneumatic region increases with increasing superficial inlet gas velocity at constant inlet gas temperature and solid injection rate. However, it decreases with increasing solid injection rates at identical superficial inlet gas velocity and inlet gas temperature. The degree of particle drying increases from 48.6 to 82.5% as the inlet gas temperature increases from 100 to 400 °C.     

Drying of Solid Furl Particlbs in Hot Gases

ABSTRACT

Drying of solid fuel particles in hot gases ( 50–200c) is studied both theoretically and experimentally. The measurements are carried out by using a thermobalance reactor constructed for drying and pyrolysis studies of particles up to 30 mm by diameter. The model is based on the solution of the conservation equations for mass and energy. The drying is considered to consist of three successive periods: a short initial heating period, period of constant rate of drying and period of falling rate of drying. It is assumed that the particle moisture distribution is uniform during the constant rate of drying. Shrinking core model is assumed for the falling rate period. esides fuel particles, the model is applicable also for other solid particle drying processes. Model calculations are compared to measurements for wood chips. The model can predict the efFect of the main parameters reasonably well. These main parameters affecting the drying rate are: particle size, particle shape, initial particle moisture content, gas temperature and gas moisture content, emperature of the reactor walls and slip velocity. The irregular shape of practical fuel particles can approximately be simulated as one-dimensional case ( plate, cylinder, sphere) by using an equivalent volumc to surface area ratio.     

Pneumatic Drying of Iron Ore Particles in a Vertical Tube

Abstract

The drying and hydrodynamic characteristics of iron ore particles in a vertical pneumatic conveying dryer (0.078 m ID × 6.0 m high) have been determined. The pressure drop decreases along the height at the acceleration region, while it remains constant irrespective of the height in the fully developed region. The degree of particle drying in the dilute pneumatic region increases with increasing superficial inlet gas velocity at constant inlet gas temperature and solid injection rate. However, it decreases with increasing solid injection rates at identical superficial inlet gas velocity and inlet gas temperature. The degree of particle drying increases from 48.6 to 82.5% as the inlet gas temperature increases from 100 to 400 °C.     

Simultaneous Drying and Cleaning of Guava Seeds in a Spout‐Fluid Bed with Draft Tube

A half sectional spout‐fluid bed column with draft tube was used for the cleaning and drying of guava seeds in one single operation. The process took place on a batch basis with a constant spouting velocity of 1.5 U_ms and an additional annular air flow rate equivalent to 0.5 and 0.7 U_mf. A regression model based on the spouting air temperature T_s, the annular inlet air temperature T_a and the additional annular air flow rate xU_mf was obtained to calculate the drying time. An additional cleaning step was necessary to increase the separation efficiency of the dried pulp from about 70% obtained during the drying process to 84%.     

Experimental Study and Mathematical Modeling of Green Pea Drying in a Spouted Bed

In this study, green pea drying is investigated experimentally in a laboratory-scale spouted bed dryer. A mathematical model is also developed to investigate the effect of operating conditions on the performance of the system. The effect of operating parameters such as inlet air temperature, particle size, and flow rate of the drying air on the performance of the dryer are studied experimentally. In order to build the process model, it is necessary to analyze the transport in both solid and gas phases. A complete set of equations with no adjustable parameters is derived for existing zones in the spouted bed dryer in order to predict variations in the temperature and moisture content of the solid and gas phases with time for batch drying conditions. Model results are compared with corresponding experimental data. Agreement between the model results and experimental data is good.     

Drying Characteristics In Superheated Steam Drying at Reduced Pressure

The superheated steam drying at reduced pressure is performed, and the effects of operational conditions such as drying pressure and temperature on the drying characteristics are examined. In order to obtain the basic guideline for the design of the superheated steam dryer at reduced pressure, the heat flux to sample was calculated and the optimal conditions were estimated.

After the sample temperature reached at the boiling point, the temperature was maintained at the boiling point and the drying rate became almost constant. Once the sample was dried out, the temperature suddenly increased up to the drying gas temperature. From the calculation of combined heat flux, the followings were found. The contribution of radiative heat transfer to the combined heat flux became larger as the drying pressure was lower. The combined heat flux had a maximum value against the drying pressure. The optimum drying pressure, which gave the maximum heat flux, became lower as the drying gas temperature decreased. It was found that reduction in the drying pressure is effective for the enhancement in drying performance.     

Quantitative analysis of combined impingement and through air drying of paper

Models were developed and verified to represent the water removal rate data of the two components of combined impingement and through air drying process. Eight physically meaningful parameters were identified, determined, correlated and analyzed. The increasing rate period is extended by impingement water removal. Constant impingement water removal decreases linearly with through flow ratio at a fixed impingement exhaust air flow. Constant through flow water removal is independent of the impingement air. Critical moisture content relates with through flow ratio parabolically. The maximum through flow water removal rate depends on the slope of the paper temperature increase and the extent of the secondary increasing rate period which increases with paper basis weight and drying air temperature, but decreases with through flow ratio and initial moisture content.     

Drying of Suspensions in a Fluidized Bed of Inert Particles Under Reduced Pressure

A drying method in which a heat-sensitive material is immersed in a fluidized bed under reduced pressure was used to continuously obtain dispersed, dry, fine powders of that material from a dilute suspension or solution at a low temperature with a high drying rate. The mass velocities of the drying gas, sample flow rate, and chamber pressure were varied, and the effects of these variations on the corresponding drying characteristics were examined.

The fluidization state of a fluidized bed of inert particles strongly affects the drying rate in the bed and has a greater effect than the chamber pressure on the corresponding drying characteristics. In other words, it is important to maintain a vigorous fluidization state to achieve a high drying rate. Although the maximum drying rate is independent of the chamber pressure, it can be achieved at a low mass velocity of the drying gas under reduced pressure. That is, at a low chamber pressure a high drying rate can be achieved, even at a relatively low mass velocity of the drying gas. The bed temperature at the time of drying is strongly influenced by the drying rate and decreases linearly with an increase in the maximum drying rate when the sample flow rate is equal to the drying rate.     

Kaffir Lime Leaf (Citrus hystric DC.) Drying Using Tray and Heat Pump Dehumidified Drying

Desorption isotherms of mature kaffir lime leaves are determined and a nonlinear regression program was applied to the experimental data to fit with any of the four moisture sorption isotherm models. It was found that the modified Halsey model could fit the best. Tray and heat pump–dehumidified drying of kaffir lime leaves were conducted and it was found that the modified Page model was the most effective one. The drying constant was fitted to drying air temperature using the Arrhenius model. Effective moisture diffusivities were determined using the drying data. Heat pump–dehumidified drying reduced drying time and provided dried kaffir lime leaves with higher amount of citronellal than tray drying.     

Performance of Single and Multi-stage Spouted Bed Drying Systems

ABSTRACT

A mathematical model is developed to predict the performance of single-stage and multi-stage drying systems using spouted beds. The model uses unsteady state analysis for batch operation to simulate the steady state operation of a continuously fed spouted bed. A parametric study is carried out to study the effect of the following parameters on the performance of a single-stage grain drying system: air flow rate per unit mass of the grain in the bed, ambient air temperature and humidity, initial and target moisture contents, the residence time in the bed, and the effectiveness of the heat exchanger to recover the thermal energy in the exhaust air. The parametric study is also extended to investigate the effect of these parameters (except ambient air temperature and humidity) on the performance of the multi-stage system. In addition, the number of stages is also included in the latter study. The results are presented in terms of charts which may be adopted for the design of such systems.     

Drying Characteristics In Superheated Steam Drying at Reduced Pressure

Abstract

The superheated steam drying at reduced pressure is performed, and the effects of operational conditions such as drying pressure and temperature on the drying characteristics are examined. In order to obtain the basic guideline for the design of the superheated steam dryer at reduced pressure, the heat flux to sample was calculated and the optimal conditions were estimated.

After the sample temperature reached at the boiling point, the temperature was maintained at the boiling point and the drying rate became almost constant. Once the sample was dried out, the temperature suddenly increased up to the drying gas temperature. From the calculation of combined heat flux, the followings were found. The contribution of radiative heat transfer to the combined heat flux became larger as the drying pressure was lower. The combined heat flux had a maximum value against the drying pressure. The optimum drying pressure, which gave the maximum heat flux, became lower as the drying gas temperature decreased. It was found that reduction in the drying pressure is effective for the enhancement in drying performance.     

Drying of a Porous Material in a Pulsed Fluid Bed Dryer: The Influences of Temperature,Frequency of Pulsation,and Air Flow Rate

In this work, a four-section pulsed fluid bed apparatus with a 0.18 m² cross-section area was used to investigate the influence of pulsed-fluidization variables on the drying process of molecular sieves, a test material that was chosen because it presents an initial constant drying rate period. A two-level factorial design was developed to evaluate the influences of the inlet gas temperature—40 and 70°C—the frequency of pulsation—250 and 900 rpm—and the air flow rate—500 and 600 m³(STP)/h—on the drying rate. In addition, a comparison was made between the drying rates achieved with conventional and pulsed fluidization. Results showed that all the investigated variables affect the drying rate. Moreover, drying rates with conventional fluidization are considerably higher, which shows that one must expect a lower drying rate when pulsation is used in a drying process controlled by the external evaporation. Concerning fluid dynamics, this work also analyzed the influence of the frequency of pulsation on the pressure drop across the bed. The higher the frequency, the higher the pressure drop. That result can be explained by the reduction of channeling.