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.     

DRYING OF OILSEEDS

Drying of oilseeds is a post harvest operation required for safe storage of harvested seeds. Oilseeds have gained importance in India over the last few years. Fluidized bed (FB) drying and Spouted Bed (SB) drying of oilseeds were studied as potential dryers for these seeds. Experimental pilot fluidized bed dryers were developed and a 2 TPH capacity (mustard seed) FB dryer plant was designed and installed in the lab. Experimental units and a pilot spouted bed dryer were also developed. The dryer units and the plant have been extensively tested and satisfactory drying performance has been achieved. Drying data and drying characteristics have been generated for different oilseeds such as mustard, sunflower, soybean and groundnut etc. The drying rates in different regimes of drying characterized by constant rate and falling rate periods have been evaluated. The salient results of the work are briefly outlined.     

SOLAR ASSISTED DRIER FOR SEEDS

A high performance solar assisted seed dryer has been designed and constructed at a seed processing station in Hungary.

The dryer is arranged in a building specially designed for this purpose, the roof of it serves as solar air collector field. Two drying cells with individual fans of two RPM stages are formed in the building. Drying air preheated by the collector can optionally be distributed between the two drying cells. Material to be dried can be arranged either in a static bed or in containers depending on the requirements. Moist air leaving the bed streams out of the drying cells through openings of the side walls. As auxiliary air heater a natural gas generator is used.     

Corn,Rice, and Wheat Seed Drying by Two-Stage Concept

Corn, rice, and wheat seeds with an initial moisture content (IMC) of 20–25% wb were dried to moisture content below 18% wb at 40–80°C in a fluidized bed dryer (FBD) and spouted bed dryer (SBD) and the seeds with IMC 18% wb were dried to below 14% wb at air temperatures 18–30°C and relative humidity 60–70% by an in-store dryer (ISD). As a result, it appears that a two-stage drying concept is feasible in drying high-moisture-content seeds due to the high germination rate of dried seeds. Nonetheless, the drying temperature must be carefully selected. A drying temperature of 40°C was clearly safe for all samples, whereas more than 90% of wheat seeds still germinated after drying at 60°C in FBD. Furthermore, drying seeds with IMC 18% wb by ISD was safe under specified drying conditions.     

固、液系机械搅拌槽中颗粒悬浮特性的研究

在一直径为700mm 的搅拌槽中研究了桨结构和模拟物料性质对颗粒的轴向浓度分布,临界搅拌转速和功率消耗的影响。认为 d=0.4 D 的6IBT(45°)折叶桨最适合用于固液悬浮。对只要求实现临界悬浮的过程,用单桨比用双桨省功;对要求实现较均匀悬浮的过程,采用双桨时不但省功而且操作转速低。经对实验数据的回归处理得到临界搅拌转速和功率准数的经验关联式如下:(1)6IBT(45°)桨:(■)(2)4IBT(45。)桨:(■)(3)45°折叶桨:(■)     

DRYING OF PULPS OF TROPICAL FRUITS IN SPOUTED BED: EFFECT OF COMPOSITION ON DRYER PERFORMANCE

Studies on drying of pulps of several tropical fruits in spouted beds had shown advantages in relation to the quality of the products. However, most of the processed pulps presented serious problems of accumulation in the bed and consequently, instability of the spouted bed dynamic. The objective of this work is to analyze the influence of the chemical composition on the spouted bed performance in drying pulps of tropical fruits. Mango pulp “in natura” was used as a standard pulp. Modified pulps were obtained increasing the concentrations of reducer sugars, lipids, fibers, starch and pectin. The effect of these compositions on the fluid-dynamic and efficiency of the powder production was investigated using a factorial design. The operational conditions were maintained fixed in all experiments. The efficiency of powder production, the stable spout pressure drop and the minimum spouting flow rate were the responses analyzed. The statistical analysis of the results showed that the concentration of fibers is not a significant variable for the process. The concentrations of reducer sugars and starch showed significant influence on the fluid dynamic and these concentrations together with the lipids and pectin concentrations showed significant influences on the efficiency of powder production.     

USEFULNESS AND SIGNIFICANCE OF ENERGY AND MASS BALANCES OF A FLUIDIZED SUPERHEATED STEAM DRYER

The key characteristics of a drying method for biomass are, firstly, its ability to control temperature, steam flow and pressure drop and, secondly, its ability to stabilise the material flows, particularly the outgoing moisture content. This paper presents the usefulness and significance of using energy and mass balances to improve the measurements and control systems of a drying system of industrial size in order to fulfil the moisture content requirements for biomass compression. In order to assess the reliability and accuracy of the measurements a series of tests at four different temperature and pressure levels were conducted. By verifying the plant's energy and mass balances the accuracy of the measurements could be determined and the control system was evaluated. The test results are used to determine the energy and mass balances. The control system yields good stability for the pressure drop, the temperature and the gas flow.     

USEFULNESS AND SIGNIFICANCE OF ENERGY AND MASS BALANCES OF A FLUIDIZED SUPERHEATED STEAM DRYER

The key characteristics of a drying method for biomass are, firstly, its ability to control temperature, steam flow and pressure drop and, secondly, its ability to stabilise the material flows, particularly the outgoing moisture content. This paper presents the usefulness and significance of using energy and mass balances to improve the measurements and control systems of a drying system of industrial size in order to fulfil the moisture content requirements for biomass compression. In order to assess the reliability and accuracy of the measurements a series of tests at four different temperature and pressure levels were conducted. By verifying the plant's energy and mass balances the accuracy of the measurements could be determined and the control system was evaluated. The test results are used to determine the energy and mass balances. The control system yields good stability for the pressure drop, the temperature and the gas flow.     

流态化干燥过程的分析

通过物料在流化床干燥器上的热态实验,找到了对物料干燥过程产生影响的因素,根据实验数据和理论分析给出了应采取的有利措施。     

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.     

SIZE SELECTION OF VACUUM CONTACT DRYER WITH MECHANICALLY MIXED PARTICULATE MATERIAL

The paper presents methods of modelling and simulation of vacuum contact drying. The selection of dryer size is discussed on the basis of experimental results and theory. Use of the thermal conductivity of the bed as a parameter of the model is proposed.     

EXPERIMENTAL ANALYSIS OF A MECHANICALLY STIRRED SPOUTED BED DRYER

ABSTRACT

In order to increase the movement of the particles in the annular region of a conical spouted bed, a mechanical stirrer was incorporated. This led to a study of the fluid dynamics of the conical spouted bed of inert particles under different operating conditions: mass of inert particles, ratio of inert particles to suspension volumes, type of stirrer (2 designs) and stirrer speed.

The action of the stirrers resulted in increased movement of the particles, especially at low rpm values (≤ 90 rpm), with the loss of the annular region, and thus approaching the beds behavior to that of a fluidized bed. To characterize this novel type of spouted beds, the “minimum spouting flow” (Q_ms) parameter which is used usually was replaced by the “minimum pseudo-fluidizing flow” (Qmpr). The value of Q_mpf decreases when the agitation in the bed increases. A correlation for Q_mpf with the different operating conditions was developed.

Drying suspensions in the stirred spouted bed shows a considerable increase of drying capacity, over units without agitation.     

Analysis of the Drying Kinetics of Brachiaria brizantha (Hochst. Stapf) Grass Seeds at Different Drying Modes

Forage grass seeds have a high economical importance in the Brazilian bovine cattle breeding and seed drying is a fundamental stage of processing to guarantee their stability and allow their storage for long periods. In this context, the objective of this work was to develop a pilot-scale belt dryer, which operates also as a fixed and fluidized bed. Brachiaria brizantha seeds dehydration was analyzed under different air velocities and temperatures. Experimental data of moisture content variation along the drying time was successfully fit to a one-term exponential model. The experimental drying rate points were calculated by approximating the derivatives to finite differences. Its behavior was accomplished fitting curves of the drying rate versus water content and time. Finally, fissure, germination, and vigor rates were analyzed as a function of the drying conditions so that the experimental conditions combine the best process efficiency with the best physiological quality maintenance.     

Analysis of the Drying Kinetics of Brachiaria brizantha (Hochst. Stapf) Grass Seeds at Different Drying Modes

Forage grass seeds have a high economical importance in the Brazilian bovine cattle breeding and seed drying is a fundamental stage of processing to guarantee their stability and allow their storage for long periods. In this context, the objective of this work was to develop a pilot-scale belt dryer, which operates also as a fixed and fluidized bed. Brachiaria brizantha seeds dehydration was analyzed under different air velocities and temperatures. Experimental data of moisture content variation along the drying time was successfully fit to a one-term exponential model. The experimental drying rate points were calculated by approximating the derivatives to finite differences. Its behavior was accomplished fitting curves of the drying rate versus water content and time. Finally, fissure, germination, and vigor rates were analyzed as a function of the drying conditions so that the experimental conditions combine the best process efficiency with the best physiological quality maintenance.     

ANALYSIS OF RECENT PATENT LITERATURE ON DRYING AND DRYERS

ABSTRACT

A preliminary semi-quantitative analysis is made of the recent patent literature related to drying and dryers. United States patents granted in the period January 1998 to June 1999 were examined. For this preliminary study, only the abstracts were used to classify the patents by subject area and arrive at some general conclusions. An in-depth overview covering a larger database and full texts of the patents is required to confirm the results presented.     

HOW TO USE FLUIDIZATION TO OBTAIN DRYING KINETICS COUPLED WITH QUALITY EVOLUTION

Abstract

This paper presents the main activities of the Gas Fluidization and Drying Team of the Chemical Engineering Laboratory at Toulouse. The emphasis has been put on the modeling of a fluidized bed dryer and its derivates such as a flotation fluidized bed, by the analysis of heat and mass transfer together with the physico-chemical transformations of the product during drying. Several studies have been performed on different solids (cereal and chemical products) to illustrate the effect of the operating conditions on dehydration and degradation kinetics The knowledge of heat and mass transfer coefficients in flotation fluidized bed allows to determine intrinsec drying and degradation kinetics. These kinetics can be used in any conductive or convective drier model.     

DRYING OF CARROTS IN A FLUIDIZED BED. I. EFFECTS OF DRYING CONDITIONS AND MODELLING

ABSTRACT

Drying curves were determined in a mechanically agitated fluidized bed dryer, at temperatures between 70°C and 160°C, air velocities between 1.1 m/s and 2.2 m/s and stirring rates between 30 rpm and 70 rpm for batch drying of 3 kg lots of carrot slices, measuring the moisture content and shrinking of the particles in time. This was complemented by a study of the rate and degree of swelling of dried carrot particles in water between 20 and 75°C. Drying kinetics were modeled by Fick's second law, for which an optimal agreement with the experimental data was obtained when the effective diffusivity (D _e ) was determined by a correlation based on the air velocity (v), the air temperature (T) and the dimensional moisture content of the carrot particles (X/X _o ). Loss of carotenes is minimized when dehydration is carried out at about 130°C with a drying time below 12 min.     

DRYING OF CARROTS IN A FLUIDIZED BED. I. EFFECTS OF DRYING CONDITIONS AND MODELLING

Drying curves were determined in a mechanically agitated fluidized bed dryer, at temperatures between 70°C and 160°C, air velocities between 1.1 m/s and 2.2 m/s and stirring rates between 30 rpm and 70 rpm for batch drying of 3 kg lots of carrot slices, measuring the moisture content and shrinking of the particles in time. This was complemented by a study of the rate and degree of swelling of dried carrot particles in water between 20 and 75°C. Drying kinetics were modeled by Fick's second law, for which an optimal agreement with the experimental data was obtained when the effective diffusivity (D_e) was determined by a correlation based on the air velocity (v), the air temperature (T) and the dimensional moisture content of the carrot particles (X/X_o). Loss of carotenes is minimized when dehydration is carried out at about 130°C with a drying time below 12 min.     

COATING OF UREA WITH AN AQUEOUS POLYMERIC SUSPENSION IN A TWO-DIMENSIONAL SPOUTED BED

In this work, a process of coating of urea with polymeric film was developed and analyzed experimentally in a two-dimensional spouted bed. An experimental design was utilized to investigate the effects of operational variables, e.g., air flow rate, air temperature, atomizing air pressure and coating suspension flow rate, on the efficiency of the process, particles growth, particles growth kinetics and physical properties. The results showed that the efficiency of the process and particle growth characteristics were strongly influenced by the operational variables. Coating of the polymeric suspension of Eudragit® improved up to four times the moisture holding capacity of coated urea in relation to uncoated urea. The bulk density, grain size and sphericity were found to be independent of operational conditions.     

Air-Drying of Seeds: A Review

Beginning with harvesting, seeds are usually subjected to a series of processes which include drying for immediate or future use. Seed quality can be influenced by several factors during drying. This article presents a review of the seed air-drying process, including mathematical models based on differential equations derived from mass and energy balances for seeds and air in fixed and moving bed dryers. The article concludes with an overview of several potential drying technologies that can be applied to seeds.