The controllability assessment of flowsheeting options involving parallel-flow dryers

Models of parallel-flow (cocurrent and countercurrent) dryers have been developed within a steady-state process flowsheeting package (ASPENplus^™) and applied to a case study involving a countercurrent timber veneer dryer to evaluate the ease of control for arrangements which include the use of recycle or a heat exchanger at the dryer exit to preheat the incoming air. The technique involves interfacing models, which allow both dynamic and steady-state analyses to be performed, with the ASPENplus™ package. These models may also be called from the SPEEDUP^™ package, but in this case only the steady-state behaviour has been studied in ASPENplus^™ by running the unsteady-state analyses to steady state using a false time-stepping technique. For the case study of the countercurrent timber veneer dryer, the use of a heat exchanger is predicted to require 13% less fuel gas than no recycle and 6% less than the use of 30% outlet gas recycle at the optimum operating condition (minimum fuel gas use) for each system. The system is also predicted to be easier to control with a heat exchanger than with recycle, according to the Relative Gain Arrays for the systems studied. For the countercurrent veneer dryer, the use of 30% outlet gas recycle increases the outlet solids temperature compared with cases both with no recycle (corresponding to the dryer on its own) and with a heat exchanger between the outgoing and incoming gas, since recycle moves the operating region up the vapour pressure/temperature curve so that the sensitivities of both the outlet solids temperature and the outlet solids moisture content to the gas flowrate increase dramatically. This means that 30% recycle changes the preferred control pairings in this case from (solids outlet temperature, inlet air flowrate), (solids outlet moisture content, fuel gas flowrate) to (solids outlet temperature, fuel gas flowrate), (solids outlet moisture content, inlet air flowrate). The indicated pairings of controlled and manipulated variables differ with the amount of recycle, suggesting that dynamic analysis needs to be performed to assess the optimum control method for this system.     

Modeling and Drying of Carton Packaging Waste in a Rotary Dryer

This research aims at modeling the rotary drying of carton packaging waste and analyzing the energy performance of the process. Drying data were obtained in a semi-pilot rotary dryer, 0.45 m diameter and 2.7 m rotating drum long, operating with an air velocity of 1 m/s and air inlet temperature of 90°C and 10 rpm. Under the operating conditions employed, the analysis of the data showed that the energy performance of the drying process increased from 5 to 75% as the inlet wet solid feed rate increased from 1.8 to 19 kg/h. In addition, at this latter wet-solid feed rate, the reduction of the air velocity in the dryer to 0.8 m/s also led to an increase in the performance of drying process from 80 to 94%. Furthermore, with a 95% confidence interval, the model used was adequate to predict the air and solid temperature as well as the air humidity and the solids moisture content.     

Experimental Investigation and Modeling of Plug-Flow Fluidized Bed Drying Under Steady-State Conditions

In this study, a model for a plug-flow fluidized bed dryer under steady-state conditions was presented. The model was based on differential equations; thus the bed of the dryer was divided horizontally and vertically into major and minor control volumes, respectively. Each control volume was composed of two thermodynamic systems: solid and gas. The mass and energy balances of the particles in the major control volume based on the axial dispersion were developed to derive the axial profiles of solid moisture content and temperature. To derive the variation of gas humidity and temperature along the bed height and hence the axial profiles of outlet gas humidity and temperature, the mass and energy balances in the gas over the minor control volume, considering the plug flow of gas through the bed, were developed. The model was validated by comparing the simulation results with the experimental data obtained by drying the long-grain rough rice under steady-state conditions in a laboratory-scale, plug-flow fluidized bed dryer. A very satisfactory agreement between the simulation and the experimental data of solid moisture content, solid temperature, and outlet gas humidity and temperature was achieved. Also, the effects of inlet gas temperature, weir height, and inlet dry solid mass flow rate on the simulated axial profiles of solid moisture content and temperature, humidity difference between inlet and outlet gas, and outlet gas temperature were investigated.     

SPRAY DRYING DYNAMIC MODELING WITH A MECHANISTIC MODEL

In this work we suggest the dynamic modeling of a spray dryer considered as a series of well-stirred dryers. That is, a series of dryers in which the output variables are equal to the state variables. The state equations were obtained from the heat and water mass balances in product and air. Additionally, heat and water mass balances in interface jointly with water equilibrium relation between product and air were considered. A pilot spray dryer was modeled assuming one, two, five and 20 well stirred steps. Low-fat milk with 10-20% of solids was dried at different inlet air temperatures (120-160°C), air flow rate of 0.19 kg dry air s^-1 and different feed rates (1.4 - 4.2 × 10^-4 kg dry solids s^-1). Stationary result showed that the model predicts the experimental air outlet temperature, at different inlet conditions with a maximum deviation of 6°C. The dynamic simulation reproduce the experimental one with moderate accuracy. Experimental dynamic showed that the pilot plant spray dryer has a well-stirred process behavior. The model represents a method for estimate outlet product moisture as function of the outlet air temperature. This has application for automatic control because there is not an easy way to measure on-line measure the outlet product moisture content.     

Simulation and Optimization of Semicontinuous Industrial Tunnel Dryers for Fruits

Semicontinuous industrial tunnel dryers were simulated and optimized for concurrent and countercurrent configurations. Mass and energy balances for the solid and gas phase were used to describe the operation of the dryer and a semi-empirical model for the mass transfer rate; the drying rate equation parameters were fitted using experimental data for Italy grapes. The simulation programs coded in Fortran 90 calculate the moisture and temperature profiles for grapes and humidity and temperature for air throughout the tunnel, cycle time, recirculation ratio, thermal load, and fresh air flow rate. The optimization minimizes the energy input considering the degradation of ascorbic acid in the fruit as the main constraint.     

SPRAY DRYING DYNAMIC MODELING WITH A MECHANISTIC MODEL

ABSTRACT

In this work we suggest the dynamic modeling of a spray dryer considered as a series of well-stirred dryers. That is, a series of dryers in which the output variables are equal to the state variables. The state equations were obtained from the heat and water mass balances in product and air. Additionally, heat and water mass balances in interface jointly with water equilibrium relation between product and air were considered. A pilot spray dryer was modeled assuming one, two, five and 20 well stirred steps. Low-fat milk with 10–20% of solids was dried at different inlet air temperatures (120–160°C), air flow rate of 0.19 kg dry air s^?1 and different feed rates (1.4 ? 4.2 × 10^?4 kg dry solids s^?1). Stationary result showed that the model predicts the experimental air outlet temperature, at different inlet conditions with a maximum deviation of 6°C. The dynamic simulation reproduce the experimental one with moderate accuracy. Experimental dynamic showed that the pilot plant spray dryer has a well-stirred process behavior. The model represents a method for estimate outlet product moisture as function of the outlet air temperature. This has application for automatic control because there is not an easy way to measure on-line measure the outlet product moisture content.     

Modeling of Drying in Moving Bed

The aim of this work is to study the simultaneous heat and mass transfer between air and soybean seeds in moving bed dryers with parallel flow (concurrent and countercurrent), by means of an experimental and simulation work, verifying the validity of classical assumptions. The numerical solution of a one-dimensional boundary value problem was obtained by means of a computational code based on axial integration through DASSL code. Deviations from flat air velocity profile were taken into account considering empirical and mechanistic equations found in the literature that describes air profile as function of radius. The experimental data of air humidity, temperature, and seed moisture content and temperature at the dryer outlet were compared to the simulated values.     

Modeling of Drying in Moving Bed

The aim of this work is to study the simultaneous heat and mass transfer between air and soybean seeds in moving bed dryers with parallel flow (concurrent and countercurrent), by means of an experimental and simulation work, verifying the validity of classical assumptions. The numerical solution of a one-dimensional boundary value problem was obtained by means of a computational code based on axial integration through DASSL code. Deviations from flat air velocity profile were taken into account considering empirical and mechanistic equations found in the literature that describes air profile as function of radius. The experimental data of air humidity, temperature, and seed moisture content and temperature at the dryer outlet were compared to the simulated values.     

Modeling and simulation of drying characteristics on flexible filamentous particles in rotary dryers

Experiments were conducted to demonstrate the effects of the drum wall temperature on the heat and mass transfer in rotary dryers. The drying characteristics of flexible filamentous particles in rotary dryers were further explored. In addition, the inlet and outlet temperatures and moisture contents of granular particles were measured. As a result, the good agreement between the simulations and experiments verified the rationale and feasibility of the numerical method. Therefore, the approach was adopted to evaluate the temperature and moisture content of wet granular particles in a rotary dryer in different conditions, for instance, drum wall temperature and rotational speed. The results revealed that the higher drum wall temperature led to hotter particles with lower outlet moisture content. Conversely, the higher rotational speed resulted in cooler particles with higher outlet moisture content due to the decrease of residence time in the rotary dryer.     

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 Soybean Seeds in a Concurrent Moving Bed: Heat and Mass Transfer and Quality Analysis

The purpose of the present work is to study the simultaneous heat and mass transfer between air and soybean seeds in a concurrent moving bed dryer, based on the application of a two-phase model to the drying process. The numerical solution of the model is obtained by using a computational code based on BDF methods (Backwards Differentials Formulas). The experimental data of air humidity and temperature and of seed moisture content and temperature at the dryer outlet are compared to the simulated values, showing a good agreement. This work also analyzes the effect of the main process variables (drying air temperature, air relative humidity, air velocity and solids flow rate) on the soybean seeds quality during drying. Empirical equations fitted to the experimental data are proposed for predicting the soybean seed quality (germination, vigor and fissures) as a function of the investigated variables.     

Mathematical Modeling the Drying of Poplar Wood Particles in a Closed-Loop Triple Pass Rotary Dryer

Closed-loop drying systems are an attractive alternative to conventional drying systems because they provide a wide range of potential advantages. Consequently, type of drying process is attracting increased interest. Rotary drying of wood particles can be assumed as an incorporated process involving fluid–solid interactions and simultaneous heat and mass transfer within and between the particles. Understanding these mechanisms during rotary drying processes may result in determination of the optimum drying parameters and improved dryer design. In this study, due to the complexity and nonlinearity of the momentum, heat, and mass transfer equations, a computerized mathematical model of a closed-loop triple-pass concurrent rotary dryer was developed to simulate the drying behavior of poplar wood particles within the dryer drums. Wood particle moisture content and temperature, drying air temperature, and drying air humidity ratio along the drums lengths can be simulated using this model. The model presented in this work has been shown to successfully predict the steady-state behavior of a concurrent rotary dryer and can be used to analyze the effects of various drying process parameters on the performance of the closed-loop triple-pass rotary dryer to determine the optimum drying parameters. The model was also used to simulate the performance of industrial closed-loop rotary dryers under various operating conditions.     

Heat and Mass Transfer in an Agitated Contact-Convective Heated Dryer

For the investigation of the drying process of a pharmaceutical fermentation waste and for determining specific heat and mass transfer coefficients an agitated contact-convective heated dryer was constructed. This dryer is also suitable for drying of other granular solids with high moisture content. Hence we investigated the drying of a by-product from bio-ethanol production, as well. The pilot-plant agitated dryer makes possible continuous measurement and data-acquisition. Data-acquisition of heated wall temperature, inlet and outlet air temperatures and humidity, mass reduction of the material makes possible the determination of transfer coefficients by the heat and mass balance of the dryer. The measured heat and mass transfer coefficients serve as proper input parameters for the simulation calculations.     

Spray Drying Performance of a Laboratory Spray Dryer for Tomato Powder Preparation

This work investigates the performance of a spray dryer for tomato powder preparation by spray drying of tomato pulp. Samples of tomato pulp with a 14% constant total solids concentration were used, and a pilot scale spray dryer (Buchi, B-191) with cocurrent operation and a two-fluid nozzle atomizer was employed for the spray drying process. Twenty-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 rate, and the air inlet temperature. In each experiment the air outlet temperature was recorded. Data for the residue remaining in the chamber and cyclone walls was gathered and two types of efficiencies were calculated as an indication of the spray dryer performance. Analysis of experimental data yielded correlations between residue accumulation and the variable operating conditions. The same operating parameters had a great influence on the air outlet temperature whereas temperature deviations were observed comparing measured air outlet temperatures with corresponding outlet adiabatic saturation temperatures.     

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.     

INFLUENCE OF SOLIDS MOISTURE CONTENT ON THE AVERAGE RESIDENCE TIME IN A ROTARY DRYER

The solids mean residence time in a rotary dryer is influenced by several variables such as dryer dimensions and solids characteristics. One of these characteristics, usually not taken into account in correlations proposed to estimate the mean residence time, is the solids feed moisture content. Although it is well known that the solids moisture content has a major impact on the ability of the solids to move along the rotary dryer, it does not enter as a parameter in available correlations. In this investigation, numerous experiments were performed in a pilot-scale rotary dryer to study the influence of solids moisture content and drying gas temperature on the mean residence time. Sand employed in cement makeup was used to perform these experiments. Results show that the mean residence time for a moisture content in the range of 8% to 12% is four times higher than for dry solids. The moisture content and the drying gas temperature influence significantly the shape of the residence time distribution curve.     

SIMULATION OF FORESTRY BIOMASS DRYING IN A ROTARY DRYER

Drying of forestry biomass in a rotary dryer has been performed. The raw material used was Erica Arborea belonging to the ever-green, broad leaves ecosystem which covers Central Greece and other Mediterranean countries. The study was part of a project concerning a Greek biomass pyrolysis demonstration plant where drying of biomass is very important in the contribution to the global energy balance and product yields of pyrolysis.

The study includes two parts. First, the experimental part concerns the influence of air flowrate, temperature, rotation speed and inclination of a laboratory rotary dryer to biomass residence time and biomass outlet moisture content. The second part concerns the development of a mathematical model for biomass drying in a rotary dryer. Experimental measurements in a rotary dryer were compared to the data from the model, in order to check the validity of the model.     

Dynamic modeling and control of soybean meal drying in a direct rotary dryer

This paper presents the analysis of the dynamic model and the application of Proportional, Integral and Derivative (PID) control in the soybean meal drying in a industrial direct rotary dryer by computational tests. Therefore, load disturbances as step, pseudorandom, and impulse were applied in the inlet speed and in the inlet moisture of soybean meal in the dryer. Later, the output responses for perturbations were investigated with control systems. The application of feedback PID control showed satisfactory results, returning expected output moisture. According to the Integral Square Error (ISE) the manipulated variables that showed better controllability were the inlet speed, when the perturbation was in the soybean meal inlet moisture; and the inlet temperature of drying air, when the perturbation was in the inlet speed of soybean meal. These results are coherent with literature and conclude that, the tuning feedback PID control keeps the output moisture of soybean meal inside the specifications with fast results.     

Bibliography of Literature on Fundamentals and Applications of Vibration in Particle Processing

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.     

转筒干燥器工艺计算顺序及参数优选

针对磷肥和磷复肥以及其他物料干燥工序常用的转筒干燥器暴露的设计问题,阐述了工艺计算之前应掌握的原始数据.介绍了工艺计算顺序及参数优选,认为许多物料的干燥都应考虑其预热段的容积;转筒干燥器的水分蒸发强度须在流体力学相似和热工条件相似的工况下选用;由于进料口处有漏风,因此热风炉至转筒干燥器的进气口处应设置低压热风机,才能达到预期的水分蒸发强度,这对燃煤热风炉尤为重要.