An Evaluation of Factors Influencing the Energy-Efficient Operation of Well-Mixed Fluidized Bed Dryers

This article describes the results of calculations of specific energy consumption, E_s, performed on a well-mixed fluidized bed dryer simulator. Exhaust air temperature-humidity loci required to yield a specified outlet moisture content were also determined. Most of the calculations related to solids whose drying rate was gas-film controlled. Six model drying curves were employed to examine the effects of drying rate and hygroscopicity in addition to the normal operating parameters. The results indicated that E_s was highest for slow-drying hygroscopic solids and lowest for fast-drying, non-hygroscopic solids. Specific energy consumption increased with decreasing bed temperature and outlet moisture content and with increasing heat loss but was independent of solids loading and airflow rate. For both the aforementioned solids and a much slower drying material (wheat), there was close agreement between the zero heat loss data and a single theoretical curve approximating the performance of an ideal adiabatic dryer. Distinct differences between the behavior of well-mixed and plug flow fluidized bed dryers are reported.     

PREDICTING THE ENERGY CONSUMPTION OF CONTINUOUS WELL-MIXED FLUIDIZED BED DRYERS FROM DRYING KINETIC DATA

ABSTRACT

Previous work has shown that it is possible to predict the size of a continuous welt-mixed fluidized bed dryer from batch drying curve measurements. This approach has been extended in the present study to include energy consumption calculations. A computer code was written to simulate the performance of the dryer and to determine its specific energy consumption E_s. Starting in this case with an isothermal bed batch drying curve, the program first calculates the mean solids residence time required under specified operating conditions. Mass and energy balances are then used to calculate the heat duty and E_s. The bed temperature was found to have a significant effect on specific energy consumption in all cases. However, the influences of air flowrate and humidity, and of solids loading, were shown to depend on the solids drying characteristics.     

OPTIMAL DESIGN OF WELL-MLXED FLUIDIZED BED DRYERS

ABSTRACT

In a recent article, Baker described a novel technique for estimating the energy consumption of well-mixed fluidized bed dryers based on the use of experimental drying curves. An integrated approach to performing sizing and energy consumption calculations for such dryers using this technique is described in the present paper. A computer program, which includes not only the dryer simulator but also a heat recovery module and an exhaust air recycle option, is used to evaluate the viability of different energy-conservation strategies. The effects of operating parameters such as bed temperature, solids loading and air velocity on energy consumption and bed area are also explored. The observed drying kinetics are shown to have a major effect on the optimal design and operation of the dryer.     

A Cross-Flow Model for Continuous Plug Flow Fluidized-Bed Cross-Flow Dryers

Plug flow fluidized bed cross-flow dryers have been used in drying of particulate solids such as paddy and other grains for many years. However, simulation of the performance of any particular design of the dryer has always been problematic due to the inadequate overall empirical models used that are too inflexible and too specific to the particular design. In addition, previous theoretical models of the plug flow fluidized bed cross-flow dryer did not model the gas cross flow properly and had difficulty in modeling the moving solid bed. A new steady-state cross-flow model of the dryer that models the gas cross-flow is proposed. The profiles for the solids and air moisture contents and temperatures were found to be dependent on the gas-solid flow ratio, G/F, the specific heat demand, C _PY (T _I  ? T _A )/(Y _E  ? Y _I ), the total number of a transfer units, N _T  = Gε/KφaSL and the specific drying load, (X _I  ? X _P )/ (Y _E  ? Y _I ). The model was validated by comparing the simulated data with experimental data that were obtained by drying paddy in a plug flow fluidized bed cross-flow dryer pilot plant. The model was found to estimate very well the solids moisture content and temperature, the gas moisture content and temperature profiles, and the driving force profile.     

OPTIMAL DESIGN OF WELL-MLXED FLUIDIZED BED DRYERS

In a recent article, Baker described a novel technique for estimating the energy consumption of well-mixed fluidized bed dryers based on the use of experimental drying curves. An integrated approach to performing sizing and energy consumption calculations for such dryers using this technique is described in the present paper. A computer program, which includes not only the dryer simulator but also a heat recovery module and an exhaust air recycle option, is used to evaluate the viability of different energy-conservation strategies. The effects of operating parameters such as bed temperature, solids loading and air velocity on energy consumption and bed area are also explored. The observed drying kinetics are shown to have a major effect on the optimal design and operation of the dryer.     

DYNAMIC ANALYSIS OF CONTINUOUS WELL-MIXED FLUIDIZED/SPOUTED BED DRYERS

A diffusion-based mathematical model is presented for the prediction of the dynamics of drying in continuous well-mixed fluidized/spouted beds. Numerical techniques are used to solve the model equations. The outlet solids moisture content, the outlet air humidity and solids temperature are predicted as a function of time for the falling rate drying period. The model is helpful in describing the drying process during the startup periods and in studying open loop behavior of drying process. The model is also useful in designing control system for fluidized/spouted bed dryers.     

Drying characteristics of millet in a continuous multistage fluidized bed

The effects of gas velocity, inlet gas temperature and the solid feed rate on the drying efficiency, the outlet solid moisture content, bed temperature in each stage, the outlet gas humidity and temperature in a rectangular acryl multistage fluidized bed (0.172 m×0.192 m×1.5 m-high) with a downcomer (0.04 m-I.D.) were investigated. The experiments were performed by using 1.9 mm millet particles. The final moisture contents of the solids increased with increasing the solid feed rate. The drying efficiency increased with increasing the wetted solid feed rate but decreased with increasing the inlet gas temperature. The drying performance of the multistage fluidized bed was compared with the single-stage fluidized bed and found to be superior under identical operation conditions. The model predicted values were well matched with the experimental data in the multistage fluidized bed dryer. This paper is dedicated to Professor Dong Sup Doh on the occasion of his retirement from Korea University.     

Drying of Liquid Feedstocks in a Spout‐Fluid‐Bed with Draft‐Tube Submerged in Inert Solids: Hydrodynamics and Drying Performance

A spout‐fluid bed with draft tube submerged in a bed of polypropylene beads was used for drying maltodextrin solutions. The hydrodynamics of the dryer were studied by determining the annular air flow vertical profile at different spouting velocities, using an additional air flow rate through the annulus equivalent to 0.5 U_mf. The drying performance of the dryer was studied through the determination of several dryer response parameters (product moisture, evaporative capacity and volumetric evaporative capacity). These parameters were compared with those obtained in a conventional spouted bed with inert solids and a spray dryer.     

ANALYSIS OF MECHANICALLY AGITATED FLUID-PARTICLE CONTACT DRYERS

Incorporation of agitation to spouted and fluidized bed dryer result in significant increases in the drying capacity (Q_s ), although product retention persist, which is reduced by increasing the air flow. The physical phenomena occurring in these dryers with several liquid substrates was analyzed and the residence time distributions (RTD) were obtained by the use of dye tracers. The residence time (τ) was found to be a function of the rate of agitation (n) and reaches a minimum at n = n _opt, which was characteristic for each type of substrate, and where maxima also appeared for the drying capacity (Q_s = Q _{s max}) and the heat transfer coefficient (Nu _p = Nu _{p max}). The RTD can be modeled by series of consecutive dryers and a modified Vanderschuren and Delvosalle model can be employed to calculated moisture of the dry product.     

Improvement of fluidized‐bed dryers for drying solid waste (olive pomace) in olive oil mills

Olive pomace from the two‐phase method of olive oil extraction (two‐phase olive pomace) must be dried from about 65% [wet basis (wb)] to about 8%, in order to extract part of the remaining pomace oil (about 3 wb‐%). An innovative dryer based on a fluidized bed is developed in this study. The objective is to improve olive pomace drying with low energy cost and high product quality by using optimal operating conditions, e.g. temperature and air flow rate, feeding solid moisture, and a control system. The bed operating temperature was set at 125 °C to obtain a good olive pomace oil quality and to reduce the thermal power consumption and drying time. The dried material is rather homogeneous and contains a negligible amount of polycyclic aromatic hydrocarbons. The fluidized bed was further improved with a moving bed joined by a conical device to the fluidized‐bed freeboard. This is a powerful combination in which the moving bed acts as a pre‐dryer of the wet solid and also as a filter of the output gas, with more than 99.9% of fines retention. The mean power consumption of the improved fluidized‐moving‐bed plant is 1 kWh/kg_water; this means a significant reduction of power cost with respect to the rotary dryers, which require about 1.4 kWh/kg_water.     

THE DESIGN AND PERFORMANCE OF CONTINUOUS WELL-MIXED FLUIDIZED BED DRYERS - AN ANALYTICAL APPROACH

Abstract

In a previous study, a unified approach to the calculation of the size and energy consumption of well-mixed fluidized bed dryers was developed. A computer code was written and a parametric study undertaken to determine the effects of the principal operating variables. The results were shown to depend on the form of the drying kinetics. The present paper describes the formulation of an analytical model of a well-mixed fluidized bed dryer, which applies for materials that exhibit linear falling-rate drying. These materials are typified by small hygroscopic particles, which are frequently dried in such equipment. Equations are presented that enable the drying time, specific energy consumption, heater duty and bed area to be calculated. The predictions of these equations agreed well with previously published experimental findings and with the results of computer simulations for a typical solid (ion exchange resin) exhibiting the required drying characteristics.     

Spouted bed drying of agricultural grains

It is shown that the presence of a “slotted draft tube” results in reduced air requirements for spouting and improved drying performance. Experimental data are presented on batch as well as continuous spouted bed drying of wheat, paddy, maize and peas. The variables studied are feed moisture content (Q_o), inlet air temperature (T_o), bed mass hold-up (M_p), inlet superficial air velocity (u_o) and bed diameter (D_c) in batch drying, and the above variables and solids feed rate (F_s) in continuous drying. The data on average overall drying rate, ?_m, in kg moisture evaporated per unit time per kg bed solids, is found to be correlatable as ?_m, = k (50Q_o + 0.118T_o ? 12.5) 10^?5, and the single parameter k is presented for wheat, paddy, maize and peas for both batch and continuous modes of spouted bed drying. The correlation obtained should be useful in dryer design for the grains studied as well as for other similar materials.     

Experimental Study of the Heat and Mass Transfer During Drying in a Fluidized Bed Dryer

ABSTRACT

Drying curves obtained in a pilot-scale fluidized bed dryer using biological source solids (sawdust, soya and fish meal) were used to estimate the parameters involved in heat and mass transfer phenomenas: heat transfer coefficient and moisture diffusivity coefficient. Parameters involved in mass transfer were estimated from drying models based on diffusional mechanisms and others that in addition consider internal and external resistance to the mass transfer. The estimate ef ective diffusivity coefficient was between 2x10-11 to lx10 (m2/s) for the considered products. Heat transfer coefficient was estimated from drying data points in the constant drying rate period when the external resistance to the mass transfer controls the process.     

A Cross-Flow Model for Continuous Plug Flow Fluidized-Bed Cross-Flow Dryers

Plug flow fluidized bed cross-flow dryers have been used in drying of particulate solids such as paddy and other grains for many years. However, simulation of the performance of any particular design of the dryer has always been problematic due to the inadequate overall empirical models used that are too inflexible and too specific to the particular design. In addition, previous theoretical models of the plug flow fluidized bed cross-flow dryer did not model the gas cross flow properly and had difficulty in modeling the moving solid bed. A new steady-state cross-flow model of the dryer that models the gas cross-flow is proposed. The profiles for the solids and air moisture contents and temperatures were found to be dependent on the gas-solid flow ratio, G/F, the specific heat demand, C_PY(T_I - T_A)/(Y_E - Y_I), the total number of a transfer units, N_T = Gε/KφaSL and the specific drying load, (X_I - X_P)/ (Y_E - Y_I). The model was validated by comparing the simulated data with experimental data that were obtained by drying paddy in a plug flow fluidized bed cross-flow dryer pilot plant. The model was found to estimate very well the solids moisture content and temperature, the gas moisture content and temperature profiles, and the driving force profile.     

Volumetric Heat Transfer Coefficient in Fluidized‐Bed Dryers

In literature, there are several Nu=f(Re) equations to specify the heat transfer coefficient between solids and the drying gas, but these equations differ significantly because of the inaccuracy of determining the contact surface between the two phases. A pilot‐plant fluidized‐bed dryer is developed to study the heat and mass transfer phenomena during the drying process. A volumetric heat transfer coefficient is applied for modeling fluidized‐bed dryers. A modified Nusselt number is defined to compare the experimental results and those of the literature. The modified Nu'=f(Re) equation exhibits a proper correlation between the results of the experiments in the literature and those of our experiments.     

CFD Modeling of Microwave-Assisted Fluidized Bed Drying of Moist Particles Using Two-Fluid Model

The drying behavior of moist spherical particles in a microwave-assisted fluidized bed dryer was simulated. The two-fluid Eulerian model incorporating the kinetic theory of granular flow was applied to simulate the gas–solid flow. The simulations were carried out using the commercial computational fluid dynamics (CFD) package Fluent 6.3.26. The effects of different levels of microwave power densities as well as initial gas temperature on the prediction of solids moisture content, gas temperature, and gas absolute humidity were investigated. The effect of microwaves was incorporated into calculations using a concatenated user-defined function (UDF). The simulation results were compared with experimental data obtained from drying of soybeans in a pilot-scale microwave-assisted fluidized bed dryer and reasonable agreement was found. The mean relative deviation for prediction of solids moisture content, gas temperature, and gas absolute humidity were less than 3, 10, and 5%, respectively. Further work is needed to validate the proposed model for large-scale plants.     

循环流化床中C类颗粒连续干燥的研究

循环流化床(CFB)作为一种新兴反应器,其结构简单、气固接触效率高、处理量大,成为气固二相干燥应用研究的新方向.文中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C类颗粒,进行了连续干燥.实验初步研究了进料速率、进风温度及气速等操作参数对淀粉平均停留...     

Model for continuous drying of solids in fluidized/spouted beds

A mathematical model is presented for predicting the dynamics of continuous drying of solids in fluidized and spouted beds (start-up period). The outlet solids moisture content \documentclass[article]\pagestyle[empty]\begin[document]$ \bar Q $ \end[document], the outlet humidity and the solids temperature are predicted as a function of time for both falling (internal diffusion controlled) and constant rate drying periods. It is shown that the initial moisture content of the solids in the bed, Q_d, is a critical variable. Three characteristic types of dynamic behaviour are identified depending upon the value of Q_d. For falling rate, a large Q_d results in unacceptably high \documentclass[article]\pagestyle[empty]\begin[document]$ \bar Q $ \end[document] in the initial periods whereas a small Q_d results in excessive temperature rise which may cause damage to solids. For constant rate, the responses of outlet humidity and solids temperature are independent of Q_d. Analytical expressions are derived for the time required to reach steady state, the solids moisture content, and temperature at steady state. The model should also be useful for control of dryers.     

Innovative Steam Drying of Empty Fruit Bunch with High Energy Efficiency

Empty fruit bunch (EFB) is one of the solid wastes from crude palm oil mills and has the lowest value for utilization compared to other solid wastes. To achieve an efficient utilization of EFB, drying is considered the first crucial process due to the high moisture content of EFB. In this study, EFB drying based on exergy recovery is proposed to achieve high energy efficiency. A fluidized bed is adopted as the main dryer. The proposed model is evaluated in terms of energy efficiency, especially regarding the influence of target moisture content and fluidization velocity. Up to 92% of the energy involved in the drying process can be recirculated. The total energy consumption for drying decreases as the target moisture content decreases, though there is no significant impact of fluidization velocity to total energy consumption. In addition, the required total length of the heat transfer tubes immersed inside the fluidized bed dryer is calculated because it relates to fluidization performance and economic issues. Lower target moisture content results in a longer heat transfer tube, and higher fluidization velocity leads to a shorter heat transfer tube.     

Measurement of constant drying rate of wet material placed in a fluidized bed of inert particles under reduced pressure

The objective of this study is to estimate the drying characteristics of a relatively large material immersed in a fluidized bed under reduced pressure by measuring the constant drying rate. The constant drying-rate period in a fluidized bed under reduced pressure is difficult to measure because it is extremely short. To maintain the constant drying-rate period, distilled water is directly supplied to the drying material. Through our experiment, the heat transfer coefficient of the material surface was also determined. The results were compared with data on hot air drying. The constant drying rate is higher for fluidized bed drying than for hot air drying. It suggests that the heat transfer coefficient on the surface of the drying material is much larger for fluidized bed drying than for hot air drying. For fluidized bed drying, the effect of pressure in the drying chamber on the heat transfer coefficient is slight at the same normalized mass velocity of dry air (G/G_mf). The temperature difference between the inside of the drying chamber and the drying material is much smaller for fluidized bed drying than for hot air drying. The constant drying rate increases as the pressure in the drying chamber decreases.