INDIRECT THERMAL DRYING OF LIGNITE: DESIGN ASPECTS OF A ROTARY DRYER

ABSTRACT

An investigation of the thermal drying of lignite has been carried out, by using an indirect heat pilot rotary drum. The process aims at the production of dry lignite and clean steam as part of a gasification procedure. Both flighted and bare drum modes have been employed. Temperature profiles along the dryer length, the amount of evaporation (moisture conversion) and the solids residence time distribution (RTD) were measured. A non-isothermal model was tested under three different regimes of solids flow. Model integration, by taking account of experimental amount of evaporation at dryer exit and temperature profiles along the dryer length, has been utilized in the validation of drying kinetics and heat transfer correlations. Model predictions compare satisfactorily with the operating data of an indirect heat industrial lignite dryer. Overall heat transfer coefficients of the pilot rotary dryer were found to agree well with those reported for direct heat dryers.     

Modelling and Simulation of a Direct Contact Rotary Dryer

A mathematical model able to predict solid and drying gas temperature and moisture content axial profiles along a direct contact rotary dryer was developed. The study was focused on the drying kinetics based on phenomenological models. Two different drying mechanisms in the decreasing drying rate period were tested: proponional to the unbound moisture content and moisture diffusion inside the particle. Experimental data collected in a pilot-scale direct contact rotary dryer was used to validate the model. Soya and fish meals were used as drying material.     

MATHEMATICAL MODELLING OF INDIRECT CONTACT ROTARY DRTHRS

ABSTRACT

Two different approaches were used to predict the solid moisture content and solid temperature profiles along a continuous indirect contact rotary dryer heated with steam tubes. One of these uses heat and mass balances applied to the solid phase in a differential element of dryer length. Here the heat flux is computed through an overall heat transfer coefficient assumed constant. The other model is based on a previous work that calculates the heat transfer coefficient as a function of the time the solid particles are in contact with the heating surface. The advantage in using this second model lies in the fact that the calculated heat transfer coefficient can take into account the effects of the operational conditions. If this coefficient has a strong dependence on these conditions, then it would be inappropriate to use a fixed value. Although both approaches can predict the solid moisture content and temperature profiles along the dryer differences were detected.     

SIMULATION OF FORESTRY BIOMASS DRYING IN A ROTARY DRYER

ABSTRACT

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.     

MATHEMATICAL MODEL DEVELOPMENT AND SIMULATION OF HEAT PUMP FRUIT DRYER

ABSTRACT

A mathematical model of a heat pump fruit dryer was developed to study the performance of heat pump dryers. Using the moisture content of papaya glace' drying, the refrigerant temperature at the evaporator and condenser and the performance, was verified. It was found that the simulated results using closed loop heat pump dryer were close to the experimental results. The criteria for evaluating the performance were specific moisture extraction rate and drying rate. The results showed that ambient conditions affected significantly on the performance of the open loop dryer and the partially closed loop dryer. Also, the fraction of evaporator bypass air affected markedly on the performance of all heat pump dryers. In addition, it was found that specific air flow rate and drying air temperature affected significantly the performance of all heat pump dryers.     

COMPARISON OF TIMBER DRYING USING SOLAR ENERGY,ELECTRICAL HEATING AND DEHUMIDIFIER.

ABSTRACT

The performance of three different types of dryers for the hot air drying of sawn-limber planks are compared. These were the electric resistance dryer, solar dryer, and the dehumidifier dryer. Whilst the electric and solar dryers depended only upon hot air for drying, the dehumidifier dryer relied on hot dehumidified air. The results of investigations carried out on timber drying employing these three types of dryers in the Engineering Faculty are compiled and compared here in this paper. The results showed that the electric dryer produced the fastest drying lime and lowest moisture content, followed by dehumidifier drying. The solar dryer achieved a lower moisture content and a faster drying rate compared to natural drying, although the difference in drying times was marginal.     

CONVECTTVE HEAT AND MASS TRANSFER AND EVOLUTION OF THE MOISTURE DISTRIBUTION IN COMBINED CONVECTION AND RADIO FREQUENCY DRYING

ABSTRACT

In a previous study (Dostie and Navarri, 1994), experiments indicated that a non-uniform moisture distribution could develop in radio frequency drying depending on the applied power and initial conditions, making the design and scale-up of such a dryer a more difficult task. Consequently, a thorough study of the combined convection and RF drying process was undertaken. Experimental results have shown that the values of the neat and mass transfer coefficients decrease with an increase in evaporation rate caused by RF energy. This effect is adequately taken into account by the boundary layer theory. Furthermore, the usual analogy between heat and mass transfer has been verified to apply in RF drying. Experiments have also shown that a different mass transfer resistance on both sides of the product should not result in non-uniform drying. However, it appears that non-uniform drying is dependent upon the initial moisture distribution and the relative intensity of heal transfer by convection and RF- It was shown that the maximum drying rate occurs at a higher average water content and that the total drying time increases with non-uniformity of the initial moisture distribution.     

滚筒干燥器内丝状物料的传热传质特性研究

干燥涉及众多生产领域,卷烟加工过程中存在干燥环节,滚筒烘丝机是用于干燥烟丝的主要设备。在滚筒烘丝机内,烟丝与周围环境进行热质交换,不同的运行控制条件,将直接影响烟丝的品质。由于干燥过程受众多因素的影响,至今烟草加工企业对烘丝机内部烟丝的传热传质特性缺乏深层次的认识,不利于烟丝品质的提高。针对上述背景,本文基于传热传质学、流体力学、多相流动等相关理论,通过计算机数值模拟方法,建立并求解滚筒内的传热传质数学模型,获得不同操作条件下烟丝温度、含水率变化的详细信息,并将所得结果与实际生产数据相验证。研究结果表明,烟丝与气流逆流流动条件下,干燥过程存在三个阶段：预热段、恒速段、降速段,烟丝含水率先升后降,烟丝温度经历先升高、后降低、再升高的过程;顺流流动条件下,干燥过程存在两个阶段：预热段与恒速段,烟丝含水率沿程单调下降,烟丝温度在预热段急剧上升,在恒速段平缓上升。     

PROCESS DESIGN OF ROTARY DRYERS FOR OLIVE CAKE

ABSTRACT

Olive-mill cake is one of the most widespread biomaterials for bioenergy exploitation in Greece. It is a sludge-type material, produced as byproduct from olive-mill extraction process. Its energy content is higher than 15 MJ/kg db and it can be used for direct burning, after drying. The drying process of olive cake is examined in the present paper. Drying kinetics data as well as the related thermophysical properties are obtained experimentally. The appropriate dryer model is proposed, validated and used to design an industrial rotary dryer. Economic analysis of the process is also discussed. A characteristic case study of an industrial rotary dryer for olive cake is included to illustrate the effectiveness of the proposed approach.     

DESIGN AND TESTING OF A COMBINED CONDUCTION-CONVECTION ROTARY PADDY DRYER

ABSTRACT

A rotary drum dryer prototype was designed, fabricated and tested to combine convection drying with conduction heating of paddy to increase moisture reduction rates. Ambient air forced inside the drum counter-flow to the direction of the cascading grains brought about “dryeration” of the hot grains, resulting in cooler grain output and increased moisture reduction rates. Its partial drying capacity doubled that of the benchmark pre-dryer at 5?rpm drum speed and quadrupled at 7?rpm, requiring only a single-pass operation. Tests using freshly harvested and re-wetted paddy showed that partial drying capacity, final moisture content and moisture reduction rate were all significantly affected by counter-flow air velocity, Its overall thermal efficiency was also 50% higher.     

ON THE STUDY OF TIME-VARYING TEMPERATURE DRYING—EFFECT ON DRYING KINETICS AND PRODUCT QUALITY

ABSTRACT

The ability of heat pump dryer to produce controlled transient drying conditions, in terms of temperature, humidity and air velocity, has given it an edge over other drying systems. Exploiting this characteristic, we studied and compared the effect of different temperature-time profiles on the quality of agricultural products in a tunnel heat pump dryer capable of providing up to 14.6 kW of cooling capacity. The product quality refers to the color change of the products. Samples of banana and guava were dried in batches in a two-stage heat pump dryer. The effects of the starting temperature of a selected profile and the cycle time on both drying kinetics and product quality were studied. It was observed that by employing a step change in drying air temperature with the appropriate starting temperature and cycle time, it was possible to reduce significantly the drying time to reach the desired moisture content with improved product color.     

MATHEMATICAL SIMULATION OF TEMPERATURE AND MOISTURE FIELDS WITHIN A GRAIN KERNEL DURING DRYING

ABSTRACT

Non-linear partial differential equations are presented for two dimensional heat and mass transfer within a single grain kernel during drying. In this model, the moisture evaporation inside the kernel is considered. The moisture is assumed to diffuse to the outer boundary of the kernel in liquid form and evaporate on the surface of the kernel. The influence of temperature and moisture content on grain properties is also considered in the simulation. The Non-linear partial differential equations are solved using the finite element method and simulation data is verified on a thin layer dryer for wheat kernels. The comparison shows that the simulated results have a high accuracy with average relative error of about 5%. The results of the finite element analysis can be used for grain quality evaluation, drying simulation studies and stress analysis of grain kernel.     

FLASH DRYING OF FISH MEALS WITH SUPERHEATED STEAM: ISOTHERMAL PROCESS

ABSTRACT

The work considers the application of the flash drying to the moisture removal of fishmeal using superheated steam as transport medium. Heat, momentum and mass transfer equations were applied and an algorithm based on these equations was developed and solved. The model was validated using experimental data obtained in a pilot pneumatic dryer ( total length 60 m) provided with a steam jacket to maintain the superheated steam at a constant temperature. The drying time was less than 10 s to decrease the fish meals moisture content from 53.5% to 28% d.b. and in a second pass by the dryer the moisture down to 16.9% using superheated steam at It 1 ° C and 130 ° C in the jacket. The computational results are in agreement with the experimental data.     

MATHEMATICAL MODELLING OF MDF FIBRE DRYING: DRYING OPTIMISATION

ABSTRACT

In the production of MDF, wet resinated fibre must be dried to its target moisture content, normally 9 to 11%, before compaction into a board by hot pressing. Fibre drying can be interpreted as an incorporated process involving gas-solid two phase-flow, inter-component transfer, and heat and mass transfer within the fibre. Based on these mechanisms, a mathematical model has been developed to simulate the MDF fibre drying process. From the model, fibre moisture content, air temperature and air humidity along the dryer length can be predicted and factors affecting the drying rate examined. The model can be employed to optimise drying conditions and to evaluate improvements in dryer design. A case study of drying improvement in reduction of dryer emissions and heat consumption is given to demonstrate the potential application of the developed dryer model.     

SCALE-UP OF CONTACT DRYERS

ABSTRACT

The scale-up of contact dryers is still based on experimental drying curves. In order to keep the effort to a minimum the drying curve is determined using a small laboratory or pilot dryer of similar geometry to the production dryer.

This paper introduces a new scale -up method for contact dryers. The new scale-up method is based on the assumption that heat transfer is the controlling mechanism. The scale-up method is derived from the material balance, the energy balance, the kinetic equation of heat transfer and thermodynamic equilibrium. The scale up method can be used to convert the drying time required to achieve a certain residual moisture content from the laboratory or pilot dryer to the production dryer and/or different drying conditions.

The scale-up method was verified by drying test with four different products in conical mixer dryers of 1, 60, 250, 1000 I volume. Two products were free flowing and two products were non free flowing in the wet state. The products can be considered non-hygroscopic in the moisture range investigated.     

A COMPREHENSIVE MATHEMATICAL AND NUMERICAL MODELING OF DEEP-BED GRAIN DRYING

ABSTRACT

This paper deals with comprehensive mathematical and numerical modeling of deep-bed grain drying. In order to build the process model, it is necessary to analyze the transport in both grain and gas phases. Experimental works were carried out for a layer of grain bed in order to validate the models. The models consider momentum, energy, and mass conservation within grain and drying air phase. The two-dimensional dynamic equations of energy and mass conservation are solved numerically by finite-difference method (FDM) and utilizing alternating direction implicit algorithm within grain and drying air phase, while momentum conservation are solved by finite difference method by utilizing Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) algorithm. Furthermore, the models will be applied in consideration with developing and designing dryer in order to simulate humidity and temperature profiles of the drying gas together with moisture content and temperature of grain across dryer in term of the dryer performance. The simulations show that the models can be used to predict the dynamic drying characteristic profiles as well as the superficial velocity of drying air phase across dryer.     

Impact of ammonia fiber expansion (AFEX) pretreatment on energy consumption during drying,grinding, and pelletization of corn stover

Pretreatment and densification of biomass can increase the viability of bioenergy production by providing a feedstock that is readily hydrolyzed and able to be transported over greater distances. Ammonia fiber expansion (AFEX?) is one such method targeted for use at distributed depots to create a value-added and densified feedstock for bioenergy use. However, the pretreatment process results in a high-moisture material that must be dried, further size reduced, and pelletized, all of which are energy-intensive processes. This work quantifies the energy consumption required to dry, grind, and densify AFEX-pretreated corn stover compared to non-pretreated stover and explores the potential of reduced drying as a means to conserve energy. The purpose of this work is to understand whether material property changes resulting from AFEX pretreatment influence the material performance in downstream formatting operations. Material properties, heat balance equations, and a rotary drum dryer model were used to model a commercial-scale rotary drum dryer for AFEX-pretreated corn stover, showing the potential to reduce dryer energy consumption by up to 36% compared to non-pretreated corn stover. Laboratory-measured grinding and pelleting energies were both very sensitive to material moisture content. Overall, the total energy required for drying, grinding, and pelleting amounts to a savings of up to 23 kWh/dry Mg for the AFEX-pretreated material when dried to a low moisture content, equating to up to 0.61 $/Mg savings for gas and electricity. Grinding and pelleting of high-moisture AFEX-pretreated stover was shown to be more costlier than the savings collected through reduced drying. Although the energy and cost savings shown here are modest, the results help to highlight operational challenges and opportunities for continued improvement.     

Comparison of Several Multi-Cylinder Paper Drying Simulation Models

ABSTRACT

Many models for simulation of paper drying in multi-cylinder dryer sections exist. In this work. three independently developed models are directly compared using the same reference data The models consired are the Texas A&M model, the Kuiosa model, and the VTT model. Predicted moisture contents plus cylinder and avenge web temperatures along the dryer section are presentcd for the different models. Predicted average evaporation rates during drying are also shown. Three major differences in the model details are highlighted. The effects of these differences on model accuracy evaluated by considering two different paper grades dried with two different machine geometries. This work is the first published direct comparison of different simulation models. The reference machine data provided can be used for comparison of these three models to other models.     

Experimental Investigation of a Solar Bamboo Dryer

ABSTRACT

An experimental investigation use conducted on the performance of a solar box dryer for drying bamboo operating under tropical rearher conditions. The dryer is a greenhouse-type designed for multi-crap solar drying. Air circulation was by electrically-operated fan. The results shoved that the moisture content of the bamboo could be brought down to about 19% from an initial value of nearly 90% in 17 days by operating the dryer over 8 hours each day. Under natural drying conditiona. the final moisture content reached was only 12%. Although eolar drying of bamboo vaa only marginally faster than nacural drying. noncrhelesa. final moisture content was lower.     

Experimental Investigation of a Solar Bamboo Dryer

ABSTRACT

An experimental investigation was conducted on the performance of a solar box dryer for drying bamboo operating under tropical weather conditions. The dryer is a greenhouse-type designed for multi-crop solar drying. Air circulation was by electrically-operated fan. The results showed that the moisture content of the bamboo could be brought dovn to about 19% from an initial value of nearly 90% in 17 days by operating the dryer over 8 hours each day. under natural drying conditions, the final moisture content reached was only 22%. Although solar drying of bamboo was only marginally faster than natural drying. nonetheless. final moisture content was lower.