Rotary Dryer Research Articles

Mathematical Modeling of Rotary Drying of Woody Biomass

Woody biomass in the form of forestry industry residues has been recognized as a promising resource for renewable energy and liquid fuels. Drying of the woody biomass is one of the key operations in development of the energy conversion technologies. Rotary drying is an effective method due to the enhanced contact between the solids and the drying medium (hot air). In this work, a mathematical model was developed to simulate the drying of the woody biomass as chips in a rotary dryer, based on energy and mass balance and transfer, experimental drying kinetics of the wood chips, and using literature correlations for the residence time. A new correlation between the theoretical maximum drying rate and the actual constant drying rate for the wood chips was obtained from the drying experiments, which was incorporated in the drying model. The model was applied both for cocurrent and countercurrent rotary dryers, and the simulation results are consistent with the observed trend. However, the accuracy of the model needs to be further investigated through experimental validation of the residence time correlation.

Transport of Solids through Rotary Dryers with Flights of Two and Three Segments

Rotary dryers are commonly used to dry particulate in a range of food and mineral processing industries. The purpose of this work was to study the design and performance of a rotary dryer. It was evaluated the number of flights and operational conditions that corresponding to the optimal performance for the equipment. A set of equations to predict the solids holdup in flights with three and two segments was also evaluated. Their predictions were compared to measurements performed with a pilot plant rotary dryer. The equation generated very accurate estimations.

Pemodelan dan Simulasi Pengeringan Jagung Pipilan dalam Pengeringan Rotari Tumpukan

A mathematical model of shelled corn drying has been developed in order to simulate the distribution of drying air temperature and humidity as well as the grain moisture content in a rotary bed dryer. The model was based on heat and mass balance of cylindrical packed bed of grain with airflow passing through the bed. Whenever the air relative humidity is higher than the equilibrium moisture content, it is assumed the condensation process will be occurred. Finite difference method with Euler scheme was used to perform the computation. The result showed that the model developed can predict the distribution ofgrain temperature and moisture content. The simulation conducted showed that there would be a wide variation ofmoisture content and temperature if mixing was not applied during the drying process. Mixing was significantly reduced the moisture content variation until a maximum of 0.8% w.b.Keywords: rotary bed drying, mathematical model, simulation, corn dryingDiterima: 5 Februari 2008; Disetujui: 26 Juni 2008

Performance of A Horizontal Cylinder Type Rotary Dryer for Drying Process ofOrganic Compost from Solid Waste Cocoa Pod

Cocoa pod husk is the bigest component of cocoa pod, about 70% of total ht of mature pod, and to potentially used as organic compost source. Poten tial solid waste of cocoa pod husk from a cocoa processing centre is about 15— 22 m3/ha/year. A cocoa plantation needs about 20—30 ton/ha/year of organic matters. One of important steps in compos processing technology of cocoa pod solid waste is drying process. Organic compost with 20% moisture content is more easy in handling, application, storage and distribution. Indonesian Coffee and Cocoa Research Institute has designed and tested a horizontal cylinder type rotary dryer for drying process of organic compos from solid waste cocoa pod with kerosene burner as energy sources. The objective of this research is to study performance of a horizontal cylinder type rotary dryer using kerosene burner as energy source for drying process of organic compost from solid waste cocoa pod. The material used was solid waste cocoa pod with 70—75% moisture content (wet basis), 70% size particle larger than 4.76 mm, and 30% size particle less than 4.76 mm, 690—695 kg/m3 bulk density. Drying process temperatures treatment were 60OC, 80OC, and 100OC, and cylinder rotary speed treatments were 7 rpm, 10 rpm, dan 16 rpm. The results showed that dryer had capacity about 102—150 kg/h depend on drying temperature and cylinder rotary speed. Optimum operation condition at 100OC drying temperature, and 10 rpm cylinder rotary speed with drying time to reach final moisture content of 20% was 1,6 h, capacity 136,14 kg/ h, bulk density 410 kg/m3, porocity 45,15%, kerosene consumption as energy source was 2,57 l/h, and drying efficiency 68,34%. Key words : cocoa, drying, rotary dryer, compost, waste

Chemical composition and pasting properties of tapioca grits from different cassava varieties and roasting methods

This study investigated the chemical composition and pasting properties of tapioca grits from different cassava varieties and roasting methods. Tapioca grits (partially gelatinized irregular flakes from roasted cassava starch), were produced from three different cassava varieties (Odongbo, TMS 30572 and Oko-iyawo) and two roasting methods (traditional and rotary dryer). The chemical composition and pasting properties of the tapioca grits were evaluated. The results showed that the effect of cassava variety and roasting methods on chemical composition of tapioca grits was significantly different (P<0.05). The principal component analysis (PCA) of variation in the chemical properties of the tapioca grits indicated that moisture, sugar and starch accounted for 83% of the variation in the chemical properties of tapioca grits. This study showed that peak and hot paste viscosities are the key pasting parameters in characterizing tapioca grits from the cassava varieties and roasting methods studied and that variation in peak viscosity of the tapioca grits might be due more too varietal influence than the roasting method. Key words: Cassava, variety, roasting, tapioca grits, chemical, pasting.

Heat and Mass Transfer in Unsaturated Porous Cake with Heated Walls

The mineral granules in the industrial drying units such as rotary dryer or fluidized dryer or the membrane in the PEMFC experience transient multiphase heat and mass transfer with transient heating mode. In particular, the transport process within mineral granules in the dryer is difficult to model owing to effects of complicated granule-granule collision and granule-heated surface. This article presents a comprehensive model consisting of continuity and heat and mass balances of three phases in an unsteady state. Numerical results revealed the flow patterns and temperature distributions of gas and liquid phases in mineral granule over time. The thermal response of the heated medium will be very different under low and high mixing intensities. The development of a recirculation liquid zone to balance the yielded capillary suction gradient enhances local evaporation and vapor accumulation rates, thus determining the overall evaporation rate form medium.

Recent Patent and Modelling of Phosphate Rotary Dryer

We review recent patents on a phosphate rotary dryer, and we present in this paper two-fold. The first aim is the modelling of the phosphate rotary dryer and presentation of the dynamic model which we elaborated for the simulation. So far, no studies to investigate this have been reported. The second aim is to realize tests on the rotary dryer existing in Oued-Zem Complex of drying Khouribga-Morocco. In the present study, an improved numerical heat and mass transfer model has been developed for the rotary dryer used for drying the phosphate. The developed model is obtained on the basis of the equations of heat and mass transfer between the gaseous and solid phases. These equations are simplified by using suitable assumptions. The latter were validated by the tests carried out on the location. In order to better understand the behaviour of the product during drying process and to validate the mathematical model developed, we conducted tests on a rotary dryer installed in Oued-Zem Complex of drying. The purpose of these tests was the determination of phosphate humidity and temperature gas profiles along the dryer. The computer results show that the present model can predict the length of the dryer as well as axial solid and gas temperature distributions with reasonably good accuracy. The model developed is in a dynamic regime and it makes possible to take into account the thermal convective process, the porosity of the product as well as the generation of humidity. The results of the simulation of the model are compared with the experimental data.

Optimization of the factors affecting the drying rate of cassava grates in a rotary drum dryer

This study determined the optimum effects of temperature, drum rotation and loading rate on the drying rate of cassava grates using the rotary drum dryer in removing moisture from cassava grates. A Central Composite Design was employed to evaluate a total of 15 fractional treatments derived from the 27 treatment combinations of temperature (75, 94 and 113°C), drum rotation (15, 20 and 25 RPM) and loading rate (5, 15, and 25 kg/h). The response surface regression obtained optimum conditions of the factors and predicted values on the stationary point on water loss as well as the rate of water removal. Results from the canonical analysis of response surface on plain water loss show an optimum combination of temperature of 107°C, drum rotation of 23 RPM and loading rate of 24 kg/h. At 25 kg/h loading rate regardless of any variation of temperature and drum rotation, the rotation of the drum could not be sustained due to heavy load caused by the accumulated weight of the grates inside the drums. The optimum conditions and predicted response values for rate of plain moisture loss were observed to be: temperature of 108°C, drum rotation of 23 RPM, loading rate of 26 kg/h. Response surface plots show the opposite effects between loading rates and temperature on the plain water loss as well as the rate of moisture loss. Response surface plots also revealed that RPM has no effect on the plain water loss and rate of moisture removal

A study of particle motion in rotary dryer

The purpose of this work was to study the performance of a rotary dryer in relation to number of flights. In this work an equationing was proposed to calculate the area used by the solids in two-segment flights of with any angle between the segments. From this area, the flight holdup and the length of fall of the particles were calculated for different angle positions and the results obtained were compared to experimental values. The results show an increase in dryer efficiency with the increase in number of flights up to a limit value, for ideal operational conditions. The experimental data on average residence time were compared to results obtained by calculations using equations proposed in the literature. The equation proposed for predicting flight holdup and length of fall of particles generated very accurate estimations.

The Mechanics of Feeding Bulk Solids in a Rotary Dryer

Based on an industrial case study, the performance of a rotary dryer employed in a mineral processing plant was investigated. The loads on the rotating elements of the dryer are analyzed with a view to ascertaining possible causes of failure. The particular type of dryer comprises a steam-heated drying unit composed of a central hollow drum and a number of parallel sets of circular heating tubes attached to the drum via radial header tubes. The drying unit rotates inside a stationary vessel containing the moist mineral ore. Blades attached by welds to the outer circular heating tubes assist in the circulation and transport of the ore through the dryer. However, in the actual dryer, the blade angles were less than optimal for good feed of material through the drying unit, and their welded attachments were a source of high levels of concentrated stress. While the close spacing between parallel sets of circular heating tubes is beneficial for heat transfer for the drying process, the close spacing leads to material buildup problems impeding the material feed. Influence of the variation of blade angle on feeding performance was investigated, and the various loads acting on the tubes and blades were analyzed.

Modeling the Fractional Drying and Aerodynamic Separation of Alfalfa into Leaves and Stems in a Rotary Dryer

A computerized mathematical model was developed to predict fractional drying and aerodynamic separation of alfalfa into leaves and stems in one process in a rotary dryer. Aerodynamic separation was characterized by separation efficiency or the total amount of the desired component (leaf or stem) recovered, relative to the amount entering the process, and by purity of leaf or stem component collected at exit ports of the dryer. The model development assumed 100% separation efficiency and purity. The model was validated by comparing model predicted results with measured experimental and field test data obtained from a small industrial rotary dryer and a full-scale industrial dryer. Changes in leaf, stem, and drying gas moisture contents and temperatures were measured and predicted by the model under various drying conditions. The model-predicted results agreed well with measured data. The model was also used to simulate the performance of industrial rotary dryers under various operating conditions. The model can be used to determine the optimum drying and aerodynamic separation parameters. It can also be used to design and redesign new and existing industrial rotary dryers in order to combine drying and aerodynamic separation into one process.

Modeling heat and mass transfer in the heat treatment step of yerba maté processing

The aim of this research was to estimate the leaf and twig temperature and moisture content of yerba mate branches (Ilex paraguariensis Saint Hilaire) during heat treatment, carried out in a rotary kiln dryer. These variables had to be estimated (modeling the heat and mass transfer) due to the difficulty of experimental measurement in the dryer. For modeling, the equipment was divided into two zones: the flame or heat treatment zone and the drying zone. The model developed fit well with the experimental data when water loss took place only in leaves. In the first zone, leaf temperature increased until it reached 135°C and then it slowly decreased to 88°C at the exit, despite the gas temperature, which varied in this zone from 460°C to 120°C. Twig temperature increased in the two zones from its inlet temperature (25°C) up to 75°C. A model error of about 3% was estimated based on theoretical and experimental data on leaf moisture content.

Through drying characteristic of oil palm empty fruit bunch (EFB) fibers using superheated steam

AbstractEmpty fruit bunches (EFB) of the oil palm have great potential to be used as industrial fibers in various industries, such as fiber board, mattress and cushion, erosion controller, thermoplastic filler, paper, acoustics control, sound insulator, and animal feed industries. A rotary drum dryer with a flue gas drying medium has been conventionally used to dry EFB fibers. However, this drying system tends to produce highly entangled and blackened fiber. Furthermore, the uneven contact between the flue gas and fiber in the rotary dryer may cause nonuniformity in the moisture content of the product. To overcome this problem, the present work uses superheated steam as the drying medium for EFB fiber drying. The drying experiments were carried out at the following conditions: atmospheric pressure; temperature range 112–172 °C; steam superficial velocity in the range 0.14‐4.3 m/s, distance from steam nozzle to EFB in the range 50–200 mm; and size of holes on plate in the range 10–200 mesh. The initial fiber moisture content was in the range 0.96‐1.46 kg/kg dry basis. The mass loss of EFB fiber during the drying process was periodically measured using an analytical balance connected directly to the wire mesh plate when the steam was diverted momentarily so that the effect of momentum transfer between the steam and the sample holder is minimized. In the temperature range 112–172 °C and at atmospheric pressure, the equilibrium moisture content isotherm is found to follow an exponential equation with coefficient ke = 15.365 kg/kg, ne = —0.001 °C−me and power index me = 1.869. It was found that for maximum moisture removal, the temperature and steam superficial velocity was 142 °C and 0.39 m/s, respectively. The effect of steam velocity is greater than that of steam temperature in decreasing the moisture content. The drying rate initially increases to a maximum and then falls off exponentially thereafter. A new drying model was developed for superheated steam for drying of EFB fiber, which is given by MR = 1 − atnexp(−ktm). The increasing drying rate is largely described by the exponential term with the coefficient k having values ranging from 1.945 to 5.351 s−n and the index m having values ranging from − 0.843 to 1.198. The falling drying rate is largely described by the power law term with the coefficient a ranging in value from 0.453 to 1.027 s−m and the power index n ranging in value from 0.0415 to 0.317. The drying is best carried out with the plate at a distance of 150 mm from the sample holder containing a 50 mesh plate. The superheated steam through the drying system can successfully reduce the moisture content by almost 100% in 15 min without steam condensation during the drying process. Copyright © 2007 Curtin University of Technology and John Wiley &amp; Sons, Ltd.

Monitoring of a rotary dryer using acoustic emissions

Drying is a complex thermal process and a crucial stage in the manufacture of many products in various industries. Reliable and accurate process monitoring techniques are needed to achieve a consistent product quality. A new and innovative method was developed that uses passive acoustics as a non-invasive, real-time monitoring technique for end-point detection in rotary dryers. This study used molecular sieves as an example of solid particles for which tight control of product moisture is crucial. A higher than ideal moisture results in sieves with a poor adsorption performance while a lower moisture results in excessive attrition. Current end-point detection methods available for this critical manufacturing stage are inadequate and prevent the production of molecular sieves with the best possible adsorption capability. A microphone recorded acoustic emissions during the drying of molecular sieves in a 1.5 m diameter rotary dryer. Signal analysis revealed that the critical end-point, corresponding to maximum moisture removal without attrition, can be detected by standard deviation analysis of the acoustic emissions. This provides a useful monitoring technique that can prevent the significant losses associated with over-drying. Multi-linear regression analysis of the acoustic emissions using wavelets or power spectral density provides an effective method of achieving a specific moisture content. Passive acoustic monitoring for end-point determination can be easily implemented into current industrial drying processes to prevent over-drying, reduce production costs, and improve product quality and consistency.

Fluidized Bed Drying of 2-Phase Olive Oil Mill By-Products

This study is focused on the fluidized bed drying of solid olive oil mill by-products (SOB) resulting from the 2-phase oil extraction method. The most usual method used in SOB drying is the rotary dryer, which revealed some problems and disadvantages that can be easily solved with the development of new drying techniques based on exhaustive studies of the solid and a better comprehension of the drying process. Two-phase SOB drying in a fluidized bed proved to be a suitable alternative that provides high-efficiency drying at relatively low air temperatures (T < 150°C). Drying kinetics modeling was developed based on well-known empirical models such as Henderson & Pabis's model and Page's model. An n-order potential model is presented, showing a good accuracy for the description of the experimental drying curves of 2-phase SOB and it was possible to correlate satisfactorily its fitting parameters with the process variables.

Multivariable robust control of a rotary dryer: Analysis and design

This paper describes the analysis and control of an industrial process usually controlled in manual mode: a continuous rotary dryer. Two process variables are controlled simultaneously: the outlet moisture of the dried material and the output temperature of the exhaust air. To do this, the flow of wet material and the flow of fuel are used as control variables. Thus, this constitutes a MIMO system, with two inputs and two outputs. The system is identified at several operating points, and a controllability analysis is performed in order to find any constraints in its performance. A multivariable robust control based on the H ∞ mixed sensitivity problem is proposed. Simulation and experimental results are included, using different adjustments of the weighting matrices based on the performed experiments. Results attained agree with the controllability analysis conclusions.

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/kgwater; this means a significant reduction of power cost with respect to the rotary dryers, which require about 1.4 kWh/kgwater.

Variação da temperatura e do teor de água do café em secador rotativo usando lenha e GLP como combustíveis

O presente estudo foi realizado no Pólo de Tecnologia em Pós-Colheita de Café, na Universidade Federal de Lavras - MG. Seu objetivo foi estudar a distribuição da temperatura e do teor de água na direção radial do secador rotativo. Para a realização do teste, foram usados, concomitantemente, dois secadores da marca Pinhalense, com capacidade de 5.000 kg cada; em um secador usou-se lenha e no outro GLP, como combustíveis para o aquecimento do ar. A medição da temperatura do ar de secagem e da massa de café foi feita por meio de termopares distribuídos em 18 pontos no secador. As leituras de todas as temperaturas, registradas em um indicador, foram feitas em intervalos de meia hora. A redução do teor de água foi acompanhada em nove pontos na massa de café. Observou-se que os valores da temperatura no plenum apresentaram menor oscilação na secagem quando se usou o GLP. Além disso, independentemente do tipo de combustível, foram observados gradientes máximos de até 25 °C na temperatura e três pontos percentuais no teor de água na direção radial, na seção central do secador, em razão dos maiores valores de pressão estática (637,84 Pa). De maneira geral, independentemente do combustível usado, o gradiente de temperatura observado longitudinalmente na massa de café foi bem menor do que na direção radial.

A physical description of solids transport in flighted rotary dryers

In this paper, a solids transport model for flighted rotary dryers is presented. Emphasis is placed on combining statistical and mechanistic modelling efforts to create a physically motivated compartment model involving pairs of perfectly mixed tank reactors linked in a series arrangement. Here, each tank pair, or cell, aims to physically describe a finite slice of a case study industrial rotary sugar dryer, and is hence governed primarily by flight geometry and dryer operational variables such as rotational speed and dryer inclination. Solids flow paths are structured to properly represent the different modes of transport in the rotary dryer, and values of transport coefficients are based on calculated rates of rotational and axial flows. A solids dispersion variable is used to correlate the model residence time distribution (RTD) prediction with available data from a tracer study conducted during industrial operation of a full-scale raw sugar dryer. RTD results from the model show intuitive responses to variations in solids feed rate, rotational speed and drum inclination.

Volatile Organic Compounds emitted during High-temperature Alfalfa Drying

Controlled laboratory and field experiments were performed to collect volatile organic compounds (VOCs) emitted during drying of alfalfa. Laboratory experiments were conducted using a flow reactor method. Three drying temperatures of 60, 150 and 250 °C with three drying times (5, 20 and 45 min) were used to collect the VOCs. Field experiments were performed to validate the results obtained in the flow reactor method using a small-scale rotary drum dryer (operated at temperatures of 202, 345 and 480 °C) and an industrial dehydration plant operated at temperatures of 800 to 1000 °C. The exhaust gases collected during the drying process were subjected to a gas chromatography and mass spectrometry (GC–MS) analysis and the presence of a total of 48 VOCs was detected. The most common groups detected during alfalfa drying were alcohols, aldehydes, ketone, ester and unsaturated and saturated hydrocarbons. Drying temperature has a predominant effect on the number of VOCs emissions. The number of VOCs emitted increased with an increase in drying temperature. Drying time had an insignificant effect on the number of VOCs emitted, however, the intensity or concentration of compounds increased with an increase in drying time. The studies of different parts of the alfalfa plant revealed that a large group of the compounds present in one part (either leaf or stem) were absent in the other, although some compounds were present in all cases. Volatile organic compounds such as 1-octen-3-ol, (E)-3-hexen-1-ol and (E)-2-hexanal, were the most common compounds present in all parts of alfalfa plant. Sulphur compounds were present only in stems and may cause a potential problem for human health. The VOCs obtained from flowers are rich in alcohols and aldehydes.