Computational Fluid Dynamics for Multistage Adsorption Dryer Design

Two-dimensional computational fluid dynamics calculations for multistage zeolite drying are performed for two dryer configurations (1) a continuous moving bed zeolite dryer and (2) a discrete bed zeolite dryer. The calculations concern drying of tarragon (Artemisia dracunculus L.) as an herbal product. The results reveal the profiles of water, vapor, and temperature in dryer, adsorber, and regenerator in the flow directions. The thermal efficiency ranges between 80 and 90% and is close to overall model calculations. The performance of continuous moving bed zeolite dryer is the best. Residence time of air, product, and zeolite are in accordance to other drying systems.     

Immersion Drying of Wheat Using Al-PILC,Zeolite, Clay,and Sand as Particulate Media

Wheat (Triticum durum) was dried by immersion in a particulate medium grain dryer using natural clay, pillared aluminum clay (Al-PILC), zeolite 13X, and sand as particulate medium at different initial bed temperatures and residence times in the dryer. Results showed that zeolite caused the highest grain moisture loss for a given drying time, followed by Al-PILC, natural clay, and, finally, the sand. The drying capacity of the Al-PILC was similar to that of the zeolite. The Al-PILC transferred an amount of heat equivalent to 85–94% of the heat transferred by zeolite and evaporated 83–93% of the moisture removed by zeolite. Results also showed that the zeolite and the Al-PILC had the best heat and mass transfer properties of the four particulate materials used and that the Al-PILC can be used as an alternative of the zeolite in particulate medium immersion grain drying.     

PERFORMANCE ANALYSIS OF A DOUBLE-DECK CONVEYOR DRYER - A COMPUTATIONAL APPROACH

ABSTRACT

This paper illustrates the use of numerical simulation models for evaluating the performance of a moving bed dryer. A finite-volume method is employed in developing a steady state, two-dimensional numerical model for a double-deck conveyor dryer. Using this numerical model, variations in the product moisture content and temperature along the length and across the height of the product beds are predicted. Similarly, the resulting variations in the temperature and relative humidity of the drying air are predicted in the entire two-dimensional domain of a dryer. Effect of air-to-product mass flow ratio and product residence lime on the average moisture content of the outgoing product are also evaluated for three different drying air temperatures.     

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.     

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 Potential of Using Two-Step Drying Techniques for Improving Energy Efficiency and Increasing Drying Capacity in Fuel Pellet Industries

The use of wood fuel pellets has increased worldwide in recent years, and pellet producers conclude that the lack of drying capacity is a barrier to increased production. In this study, we develop a concept of two different dryers called the two-step drying technique. The aim is to show the potential for increasing the drying capacity and improving energy efficiency when introducing a second dryer into the pellet plant. The study is theoretical and based on an industrial packed moving bed dryer. It shows that the drying capacity increased by 22% when a pneumatic second dryer was used.     

PERFORMANCE ANALYSIS OF A DOUBLE-DECK CONVEYOR DRYER - A COMPUTATIONAL APPROACH

This paper illustrates the use of numerical simulation models for evaluating the performance of a moving bed dryer. A finite-volume method is employed in developing a steady state, two-dimensional numerical model for a double-deck conveyor dryer. Using this numerical model, variations in the product moisture content and temperature along the length and across the height of the product beds are predicted. Similarly, the resulting variations in the temperature and relative humidity of the drying air are predicted in the entire two-dimensional domain of a dryer. Effect of air-to-product mass flow ratio and product residence lime on the average moisture content of the outgoing product are also evaluated for three different drying air temperatures.     

Comparison of Moving Bed Dryers of Solids Operating in Parallel and Counterflow Modes

Abstract

The drying rates in moving bed dryers are compared. The gas and the solids to be dried are in parallel flow or counterflow. A new simplified method to simulate the drying in parallel and counterflow moving beds is developed. This model is based on the solution of arbitrary experimental or theoretical drying rate Equations of single solid particles (or thin-layer drying rate equation) coupled with heat and mass conservation Equations of the dryer. The solution is presented in an integral form of the drying equation showing the relation between time or location in the dryer and degree of drying. The method allows rapid calculation of the moisture, vapor mass fraction, and temperature distributions along the dryer in drying with moist air or steam. The model is demonstrated by using a result based on the receding front evaporation model as the specific thin-layer drying equation in the moving bed model. Wood chips are chosen as an example of the substance to be dried, but the method applies also for other medium, if the dependence of the drying rate on moisture and ambient temperature and humidity (thin layer drying rate) is known. The size of the dryer needed to reach the same degree of drying operating in the parallel mode is much greater than that of counterflow type, when the drying medium is air or flue gases. The reason for the poorer drying in parallel flow is mainly the unfavorable distribution of the evaporation temperature. In steam drying, the difference in the size is not so great, since the evaporation takes place approximately at constant temperature.     

A MATHEMATICAL MODEL FOR CONTINUOUS DRYING OF GRAINS IN A SPOUTED BED DRYER

ABSTRACT

A mathematical model for a continuous spouted bed dryer has been presented to predict moisture content, air and grain temperatures as well as energy consumption. To better understand the interactive influence of processes in each region of the spouted bed, solution schemes for the spout and downcomer were treated separately. The behavior of dryer was investigated experimentally and found that the dryer behaved differently from an ideal plug flow. The drying rate as simulated by the model is almost constant during grain movement in the dryer. Absence of airflow in the downcomer leads to a tempering process that takes place in the downcomer while intense heat and mass transfer occurs mainly in the spout due to the high airflow rate there. Furthermore, by considering the predicted grain temperature history as one of the indicators of product quality, one can, in principle, design appropriate successive processes in a continuous spouted bed dryer to minimize product damage.     

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.     

Thesis Summary: Study of a New Atmospheric Freeze Drying System Incorporating a Vortex Tube and Multimode Heat Input

Atmospheric freeze drying (AFD) in a vibro-fluidized bed dryer coupled with an adsorbent and multimode heat input is proposed for dehydration of food products. An experimental setup was designed and built to permit simultaneous application of convection, conduction and radiation heat input to the drying material above its freezing point to ensure sublimation using a vortex tube to produce low temperature dry air. Comparison with AFD using fixed bed, fluidized bed dryer, traditional vacuum freeze drying and heat pump drying were carried out to investigate the viability of this new system. A two-layer moving boundary model was developed to simulate the drying kinetics and temperature scenario of thin slab product. Fairly good agreement was found between the predicted values and the experimental data. Finally a three-dimensional (3D) CFD simulation for a vortex tube is carried out to capture the highly swirling compressible flow behavior and to gain basic understanding of temperature separation process. An experimental setup was built to validate the simulation results.     

A MATHEMATICAL MODEL FOR CONTINUOUS DRYING OF GRAINS IN A SPOUTED BED DRYER

A mathematical model for a continuous spouted bed dryer has been presented to predict moisture content, air and grain temperatures as well as energy consumption. To better understand the interactive influence of processes in each region of the spouted bed, solution schemes for the spout and downcomer were treated separately. The behavior of dryer was investigated experimentally and found that the dryer behaved differently from an ideal plug flow. The drying rate as simulated by the model is almost constant during grain movement in the dryer. Absence of airflow in the downcomer leads to a tempering process that takes place in the downcomer while intense heat and mass transfer occurs mainly in the spout due to the high airflow rate there. Furthermore, by considering the predicted grain temperature history as one of the indicators of product quality, one can, in principle, design appropriate successive processes in a continuous spouted bed dryer to minimize product damage.     

Comparison of Moving Bed Dryers of Solids Operating in Parallel and Counterflow Modes

The drying rates in moving bed dryers are compared. The gas and the solids to be dried are in parallel flow or counterflow. A new simplified method to simulate the drying in parallel and counterflow moving beds is developed. This model is based on the solution of arbitrary experimental or theoretical drying rate Equations of single solid particles (or thin-layer drying rate equation) coupled with heat and mass conservation Equations of the dryer. The solution is presented in an integral form of the drying equation showing the relation between time or location in the dryer and degree of drying. The method allows rapid calculation of the moisture, vapor mass fraction, and temperature distributions along the dryer in drying with moist air or steam. The model is demonstrated by using a result based on the receding front evaporation model as the specific thin-layer drying equation in the moving bed model. Wood chips are chosen as an example of the substance to be dried, but the method applies also for other medium, if the dependence of the drying rate on moisture and ambient temperature and humidity (thin layer drying rate) is known. The size of the dryer needed to reach the same degree of drying operating in the parallel mode is much greater than that of counterflow type, when the drying medium is air or flue gases. The reason for the poorer drying in parallel flow is mainly the unfavorable distribution of the evaporation temperature. In steam drying, the difference in the size is not so great, since the evaporation takes place approximately at constant temperature.     

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.     

Immersion Drying of Wheat Using Al-PILC, Zeolite, Clay, and Sand as Particulate Media

Wheat (Triticum durum) was dried by immersion in a particulate medium grain dryer using natural clay, pillared aluminum clay (Al-PILC), zeolite 13X, and sand as particulate medium at different initial bed temperatures and residence times in the dryer. Results showed that zeolite caused the highest grain moisture loss for a given drying time, followed by Al-PILC, natural clay, and, finally, the sand. The drying capacity of the Al-PILC was similar to that of the zeolite. The Al-PILC transferred an amount of heat equivalent to 85-94% of the heat transferred by zeolite and evaporated 83-93% of the moisture removed by zeolite. Results also showed that the zeolite and the Al-PILC had the best heat and mass transfer properties of the four particulate materials used and that the Al-PILC can be used as an alternative of the zeolite in particulate medium immersion grain drying.     

DRYING EXPERIMENTS WITH ALO(OH) SUSPENSION OF HIGH PURITY AND FINE PARTICULATE SIZE TO DESIGN AN INDUSTRIAL SCALE DRYER

ABSTRACT

AIO(OH) suspension of high purity and fine particle size was dried in a Mechanically Spouted Bed (MSB) dryer with inert particles. During drying the maximum required moisture content and the mean particle size of the dried product had to be taken into consideration. Also another important aspect was to avoid contamination of the dried solid with metals. PTFE cylinders of 8 mm in diameter and of 8 mm in height were used as inert particles, and also the whole dryer itself was coated with teflon. As a result of the drying experiments the optimum values of the process parameters were used for scaling-up. The specific rate of evaporation in the drying zone of the inert bed was the basis of the calculations. The suspension with capacity of 125 kg/h was successfully dried in the designed industrial scale MSB dryer with inert panicles and the required quality of the material was produced.     

SIMULATION OF THE MIXED-MODE NATURAL-CONVECTION SOLAR DRYING OF MAIZE

A mathematical model for mixed mode natural convection solar drying of maize grain is presented. The drying is described by a deep bed procedure that includes conduction within the grain bed. The conduction is due to radiative energy falling on the upper surface of the bed. The results show that temperatures at the top and bottom of the bed are higher than that in the middle resulting in two drying fronts one at the top and the other at the bottom of the bed and moving in opposite directions. This results in more uniform moisture content distribution than in an indirect dryer. The results are verified against experimental data from a prototype mixed mode natural convection maize solar dryer. The laboratory solar dryer was constructed at Newcastle University, U.K. and the experiments carried out under a solar simulator. The agreement between theory and experiment is very good.     

SIMULATION OF THE MIXED-MODE NATURAL-CONVECTION SOLAR DRYING OF MAIZE

A mathematical model for mixed mode natural convection solar drying of maize grain is presented. The drying is described by a deep bed procedure that includes conduction within the grain bed. The conduction is due to radiative energy falling on the upper surface of the bed. The results show that temperatures at the top and bottom of the bed are higher than that in the middle resulting in two drying fronts one at the top and the other at the bottom of the bed and moving in opposite directions. This results in more uniform moisture content distribution than in an indirect dryer. The results are verified against experimental data from a prototype mixed mode natural convection maize solar dryer. The laboratory solar dryer was constructed at Newcastle University, U.K. and the experiments carried out under a solar simulator. The agreement between theory and experiment is very good.     

On Dryer Energy Performance and Controllability: Generalized Modeling and Experimental Validation

This work presents an approach to compute dryer energy efficiency using air flowrate step responses and establish a link between drying energy efficiency and process controllability. The approach is based on the temperature drop between the dryer inlet and outlet air under adiabatic conditions and so decouples water evaporation from heat loss and product heating effects on dryer temperature drop. As such, the computation is accurate even for dryers with significant heat losses where the traditional use of actual temperature drop measurements is grossly inaccurate. The approach is tested and verified on two experimental case studies involving significant heat losses: the first, a continuous fluidized-bed dryer (from literature); the second, a conventional and zeolite wheel-assisted batch dryer designed in the current study.     

新型高能效沸石吸附食品干燥器的开发

人们对接近新鲜状态的高质量干燥产品的需求越来越显著。当代干燥技术在提高产品质量方面有显著的进步,但在设备节能方面鲜有突破。利用沸石等吸附剂进行空气脱湿是提高干燥效率的一个很有潜力的选择。在此方法中,沸石与干燥介质空气接触,可使空气中水蒸气很快地减少到0．1ppm或露点温度降到-50℃;同时释放吸附热使得空气温度不断升高。从而,干燥器入口空气含有较多的用于干燥的显热,提高了干燥驱动力和总能效。本文讨论了沸石吸附干燥卡拉胶和玉米的应用,实验结果表明吸附干燥方法提高了产品质量,同时缩短干燥时间。然而,在商业应用之前需要进行深入的可行性研究。