A NEW MODEL FOR FLUID BED DRYING

ABSTRACT

A new model is proposed which calculates fluid bed drying curves without any adaptation of the Sherwood number between panicles and gas. Even the fine-structure of experimental data, i.e. the influence of bed height and gas flow rate, is predicted reliably. This is achieved by considering backmixing of the suspension gas in the kinetic parameter and not in the driving potential. A traditional derivation with reduced driving potential fails in the comparison with experimental results.     

A NEW MODEL FOR FLUID BED DRYING

A new model is proposed which calculates fluid bed drying curves without any adaptation of the Sherwood number between panicles and gas. Even the fine-structure of experimental data, i.e. the influence of bed height and gas flow rate, is predicted reliably. This is achieved by considering backmixing of the suspension gas in the kinetic parameter and not in the driving potential. A traditional derivation with reduced driving potential fails in the comparison with experimental results.     

DRYING OF PULP AND PAPER SLUDGE IN A PULSED FLUID BED DRYER

Hydrodynamics and drying kinetics for the pulp and paper primary sludge dried in a pulsed fluid bed dryer with relocated air stream are presented. Batch experiments have indicated that drying of disintegrated sludge to the required 12% moisture content takes place during the first drying period at practically constant material temperature close to the wet bulb temperature with respect to the inlet air conditions. Equations were developed for pressure drop, minimum pulsed-fluidization velocity, dynamic bed height, and volumetric mass transfer coefficient. Continuous experiments under drying conditions determined from the average residence time concept have confirmed that transportation of disintegrated sludge along the dryer follows the plug flow model.     

DRYING OF PULP AND PAPER SLUDGE IN A PULSED FLUID BED DRYER

ABSTRACT

Hydrodynamics and drying kinetics for the pulp and paper primary sludge dried in a pulsed fluid bed dryer with relocated air stream are presented. Batch experiments have indicated that drying of disintegrated sludge to the required 12% moisture content takes place during the first drying period at practically constant material temperature close to the wet bulb temperature with respect to the inlet air conditions. Equations were developed for pressure drop, minimum pulsed-fluidization velocity, dynamic bed height, and volumetric mass transfer coefficient. Continuous experiments under drying conditions determined from the average residence time concept have confirmed that transportation of disintegrated sludge along the dryer follows the plug flow model.

     

SIMULATION OF PLUM DRYING IN DEEP BED

Thin layer air drying of plum was studied. The drying conditions in this study are : dry bulb temperature ranging from 60 to 100 °C, air moisture content ranging from 0.008 to 0.089 kg water/kg dry air and air velocity ranging from 0.5 to 2 1x11s. An exponential model was used to describe the drying curve. The heat and moisture balances on the air were used to calculate the outlet air temperature and its moisture content. The heat balance on the product was used to calculate the product temperature. These equations were used to predict the drying of plum in deep bed. A good agreement between the experimental and calculated product moisture content, the product temperature and the outlet air temperature were obtained     

DRYING OF POWDERY MATERIALS IN A PULSED FLUID BED DRYER

ABSTRACT

Drying of recycled polypropylene powder was studied experimentally in a laboratory pulsed fluid bed dryer (PFB) with relocated air stream. It was proved that fluidization of fine particles having a large specific surface area and a broad size distribution is technically feasible when using the composite supporting grid. Drying and hydrodynamic characteristics for a pulsed fluid bed of fine particles are found to be similar to the ones for coarse particles. Equations for minimum pulsed fluidization velocity, pressure drop, and surface and volumetric heat transfer coefficients are given.     

SIMULATION OF PLUM DRYING IN DEEP BED

Thin layer air drying of plum was studied. The drying conditions in this study are : dry bulb temperature ranging from 60 to 100 °C, air moisture content ranging from 0.008 to 0.089 kg water/kg dry air and air velocity ranging from 0.5 to 2 1x11s. An exponential model was used to describe the drying curve. The heat and moisture balances on the air were used to calculate the outlet air temperature and its moisture content. The heat balance on the product was used to calculate the product temperature. These equations were used to predict the drying of plum in deep bed. A good agreement between the experimental and calculated product moisture content, the product temperature and the outlet air temperature were obtained     

INTERMITTENT DRYING OF CARROT IN A VIBRATED FLUID BED: EFFECT ON PRODUCT QUALITY

ABSTRACT

Drying of diced carrot was studied experimentally in a laboratory batch-type vibrofluidized bed dryer (VFB) The kinetics of the degradation reaction of P-carotene chosen as the dominant quality index were determined for various time-temperature conditions. Relationships were obtained for instantaneous concentration of β-carotene, and for the degradation rate constant as a function of moisture content and material temperature. It was shown that the best quality product is obtained when carrot is dried in a VFB dryer down to the critical moisture content, than conditioned (tempered) in a single layer exposed to ambient air, and then finish dried to the final moisture content in a VFB dryer.     

DRYING OF HIGH MOISTURE CONTENT PADDY IN A PILOT SCALE TRIANGULAR SPOUTED BED DRYER

From the successful drying of paddy in a lab-scale triangular spouted bed (TSB), a pilot scale unit with a capacity of 500kg/batch was constructed and used to dry paddy of high moisture contents. The drying tests showed that air temperatures of up to 160°C in the first stage, followed by a lower temperature of 80-100°C in the second stage, can be used to dry paddy from more than 25% down to around 15% (w.b.) without significantly changing grain quality in terms of head rice recovery. Moisture distribution in the pilot-scale TSB was observed to be satisfactorily uniform.     

SIMULTANEOUS DRYING AND COMMINUTION OF COAL IN A SPOUTED BED

Coal was dried and simultaneously reduced in size in a spouted bed using air at different temperatures as the spouting fluid and either glass beads as a dynamic grinding aid or a specially designed metal insert as a static grinding aid. Both semi-batch and continuous operations were carried out, the balanced feed and product-discharge rates in the latter being pre-determined from the results in the former. The rates of both drying and comminution were synergistically increased with increases in inlet air temperature. The static insert was shown to be a more effective comminution device than the glass beads.     

SIMULTANEOUS DRYING AND COMMINUTION OF COAL IN A SPOUTED BED

Coal was dried and simultaneously reduced in size in a spouted bed using air at different temperatures as the spouting fluid and either glass beads as a dynamic grinding aid or a specially designed metal insert as a static grinding aid. Both semi-batch and continuous operations were carried out, the balanced feed and product-discharge rates in the latter being pre-determined from the results in the former. The rates of both drying and comminution were synergistically increased with increases in inlet air temperature. The static insert was shown to be a more effective comminution device than the glass beads.     

DRYING OF PADDY IN A BED DRYER

The experimental results of a bed dryer (at Ludhiana) have been analyzed. The experimental results are well predicted by the small scale laboratory experiments on a spouted bed dryer.     

DRYING OF PADDY IN A BED DRYER

The experimental results of a bed dryer (at Ludhiana) have been analyzed. The experimental results are well predicted by the small scale laboratory experiments on a spouted bed dryer.     

PROBABILISTIC ANALYSIS AND DESIGN OF THE INDUSTRIAL TIMBER DRYING PROCESS

The distribution in the moisture content of dried planks is an important parameter for kiln operators. The evolution of variability in the moisture content of timber boards during a batch drying process is investigated. This random variability in moisture content arises from a distribution in plank initial moisture content and dispersion in plank drying rate. A simple deterministic model of timber drying is outlined. Theoretical probabilistic analysis is applied to this model to predict the mean and standard deviation in board moisture content as a function of time. The solution is assessed with representative industrial kiln drying data. The utility of the approach in suggesting strategies to promote uniformity in final moisture content is outlined using some design studies. These strategies include sorting of the timber by moisture content and adjusting the drying rate and equilibrium moisture content to reduce variability. It is also demonstrated that the probabilistic approach can yield a better estimation of kiln average moisture content.     

MODELING, SIMULATION AND DESIGN OF CONVECTIVE INDUSTRIAL DRYERS

The use of computers to perform simulations of chemical engineering processes has lead to the development of software tools that perform most tedious computations in the field of process analysis, mathematical modeling and design. In the case of dryers, these mathematical programming aspects can be dealt in a straightforward way. The mathematical models of all popular convective dryers are presented and analysed. The transport and thermophysical properties of materials and air involved in the developed mathematical models are briefly discussed. The simulation of convective drying processes facilitated by modern computer technology is outlined and discussed. Design of convective industrial dryers is described and performed through the simulation tools developed. Short-cut design techniques are introduced allowing concentrated information on design results for various levels of process parameters and variables to be integrated in generalized design curves that produce values of optimal dryer structures and operating conditions related to cost     

生物质与煤混合颗粒流化特性的实验研究

在 D1 1 5 mm× 1 0 0 0 mm有机玻璃制成的圆柱型流化床中 ,对玉米秆、稻秆、煤及其混合物的流化特性进行了实验研究 .实验结果表明 ,单一生物质颗粒不能形成良好的流化状态 ,而加入一定量煤构成生物质和煤二组分混合颗粒可以实现稳定流化 .当生物质和煤混合颗粒中生物质的质量分率小于 5 0 %时 ,可以达到很好的流化状态 ,生物质和煤二组分混合颗粒的最小流化速度随生物质质量分率的增加而减小 .     

DIFFUSION OF MOISTURE IN DRYING OF SUGAR CANE FIBERS AND BUNDLES

Sugar cane fibers and arrangements of fibers in cylindrical bundles were dried in a thermoanalyzer and their diffusive coefficients were calculated using the slope method. The effect of temperature, moisture content as well as structural changes were analyzed. Diffusion coefficients changed nanlineariy with moisture content and followed an Arrhenius-like functionality with temperature. The analysis of these effects suggested a liquid diffusion transport mechanism of moisture transfer inside sugar cane fibers and bundles.     

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.     

PEAR DRYING PROCESS ANALYSIS: DRYING RATES AND EVOLUTION OF WATER AND SUGAR CONCENTRATIONS IN SPACE AND TIME

ABSTRACT

Dried pears are produced in Portugal by a traditional solar drying process and, because they represent unquestionable regional economic interest, some studies are being conducted in order to convert their production to industrial drying methods. In this work the main mass transfer phenomena occurring during drying of pears are investigated. Experiments were carried out to determine the time evolution of the radial profiles of water and sugar content. The drying rates were also studied at constant drying temperatures of 30, 40 and 50°C. The results enable to conclude that the concentration profiles of water and sugar, both in space and time, follow an expected pattern if one takes into account the rates of water removal and the diffusion mechanisms. These also allow to explain why the sugar concentration increases in the periphery of the fruit. The drying rate curves obtained are typical, with the drying process at 30°C being more gradual but also much slower than that at 50°C.