Continuous on-line measurement of solid moisture content during fluidized bed drying using triboelectric probes

Wet granulation and drying of solids in fluidized beds are widely used in the pharmaceutical, food and fertilizers industries. Although the moisture content of fluidized solids is the key parameter for on-line process monitoring, reliable, accurate and economical moisture sensors are lacking. The aim of this work was, therefore, to develop a new technique using triboelectric probes for real-time measurement of moisture content in fluidized beds, and to validate its applicability to fluid bed drying of glass beads (Sauter-mean diameter of 171 μm) and silica sand particles (Sauter-mean diameter of 190 μm) wetted by water. Several triboelectric probes, installed at different locations throughout the bed, monitored the bed moisture content during both the liquid spraying and the following drying process. The measuring technique developed in this study makes use of inexpensive probes that do not require any maintenance. The triboelectric signals were continuously recorded by a data acquisition system and, at selected times, samples of bed solids were taken and analyzed for their moisture content using Karl Fischer titration. The triboelectric signals were correlated with the moisture contents determined by titration to calibrate the technique, which was found to be sensitive to moisture contents below 100 ppm. For most of the experiments the bed was operated in the bubbling regime with a superficial gas velocity of 0.4 m/s. The relationship between triboelectric signal and moisture content was unaffected by changes in the fluidization gas velocity of up to 25%, and could be easily automated for direct control of industrial granulation and drying processes.     

Continuous fluidized bed drying: Residence time distribution characterization and effluent moisture content prediction

The mixing and drying behavior in a continuous fluidized bed dryer were investigated experimentally by characterizing the residence time distribution (RTD) and incorporating a micromixing model together with the drying kinetics obtained from batch drying. The RTD of the dryer was modeled using a tank-in-series model. It was found that a high initial material loading and a low material flow rate resulted in a reduced peak height and broaded peak width of the RTD curve. To predict the continuous dryer effluent moisture content, we combined: (a) the drying kinetics as determined in a batch fluidized bed dryer, (b) the RTD model, and (c) micromixing models—segregation and maximum mixedness models. It was found that the segregation model overpredicted the effluent moisture content by up to 5% for the cases we have studied while the maximum mixedness model gave a good prediction of the effluent moisture content.     

Impact of rewetting and drying of rough rice on predicted moisture content profiles during in-bin drying and storage

Accurate prediction of moisture content (MC) is vital for effective control of on-farm, in-bin drying and storage of rough rice, especially for systems using recently introduced technology to automate fan run time. The study used simulations, laboratory, and field experiments to investigate the extent to which rewetting and drying, during in-bin drying and storage, affect accuracy of predicted MC—a critical parameter for automated fan control. Vapor sorption analysis (VSA) was used to generate MC prediction models for rough rice. Simulations of in-bin drying and storage, using in-field weather data, were performed while segregating effects resulting from rewetting and drying of the rough rice and the type of fan control strategy used. Predicted MC profiles of rough rice and drying durations were compared with those resulting from using standard constants in the literature for modeling. The root mean square error associated with predicting the MC by model constants developed using the VSA was 0.54% MC and 1.32% MC dry basis (d.b.), for desorption and adsorption, respectively. Deviation in MC logged by in-bin built, field sensors and that simulated by taking into account the influence of rewetting and drying were generally within 1.5% point difference. Therefore, rewetting and drying did not affect drying duration. However, drying duration was significantly influenced by fan control strategy (p?相似文献   

Influence of drying conditions on the moisture diffusion and fluidization quality during multi-stage fluidized bed drying of bovine intestine for pet food

In order to establish the influence of the drying air characteristics on the drying performance and fluidization quality of bovine intestine for pet food, several drying tests have been carried out in a laboratory scale heat pump assisted fluid bed dryer. Bovine intestine samples were heat pump fluidized bed dried at atmospheric pressure and at temperatures below and above the materials freezing points, equipped with a continuous monitoring system. The investigation of the drying characteristics have been conducted in the temperature range ?10 to 25 °C and the airflow in the range 1.5–2.5 m/s. Some experiments were conducted as single temperature drying experiments and others as two stage drying experiments employing two temperatures. An Arrhenius-type equation was used to interpret the influence of the drying air temperature on the effective diffusivity, calculated with the method of slopes in terms of energy activation, and this was found to be sensitive to the temperature. The effective diffusion coefficient of moisture transfer was determined by the Fickian method using uni-dimensional moisture movement in both moisture, removal by evaporation and combined sublimation and evaporation. Correlations expressing the effective moisture diffusivity and drying temperature are reported.Bovine particles were characterized according to the Geldart classification and the minimum fluidization velocity was calculated using the Ergun Equation and generalized equation for all drying conditions at the beginning and end of the trials. Walli's model was used to categorize stability of the fluidization at the beginning and end of the drying for each trial. The determined Walli's values were positive at the beginning and end of all trials indicating stable fluidization at the beginning and end for each drying condition.     

干燥过程中颗粒湿含量的检测

引言 干燥是流程工业中重要的单元操作,是众多产品生产的关键步骤,在医药化工、食品、矿产加工、农产品加工等领域都有涉及[1-4].干燥效果的好坏不仅决定了生产单元的能耗,还会对产品的性能、形态、质量等产生影响.研究干燥过程的目的在于控制过程参数(包括干燥时间、进出口温度、风速等),对物料进行适度干燥,以使产品的湿度、质量等指标达到要求.在过程参数的设定过程中,干燥时间的控制(或称为干燥终点的确定)是影响产品质量最为关键的环节之一.     

Ultrasound-assisted fluidized bed drying of paddy: Energy consumption and rice quality aspects

Several studies have been conducted on equipping conventional fluidized bed with some technologies to increase drying efficiency and its performance. The objective of this study was to investigate the influence of high-power ultrasound (HPU) on fluidized bed drying of paddy in terms of drying kinetics, grain quality (percentage of cracked kernels and bending strength of grain kernels), and specific energy consumption (SEC). To decrease the initial moisture content of paddy from 26.5?±?0.5% (kg/kg, d.b) to the final moisture content of 13?±?0.5% (kg/kg, d.b), the experiments were conducted in a factorial design at three levels of ultrasound power densities (11.1, 14.6, and 18.7?kW/m³), four levels of frequencies (20, 25, 28, and 30?kHz), and three levels of drying air temperatures (30, 40, and 50°C). Application of HPU in conjunction with conventional fluidized bed drying led in 23% decrease in drying time as well as improvement in grain quality, in terms of percentage of cracked kernels and bending strengths. In addition, SEC reduced approximately by 22%, as HPU applied at selected drying condition.     

Influence of hot air fluidized bed drying on quality changes of purple rice

The influence of drying using a fluidization technique on the quality of purple rice was investigated in this study. The results demonstrated that the initial moisture of rice was 28.3% dry basis (db). Compared to the sun-dried or reference purple rice samples, the influence of drying at temperatures ranging from 100 to 150°C did not affect the quality of color, anthocyanin content, total phenolic content, or antioxidant activity. At this initial moisture level, samples should be dried at 150°C air because such temperatures yield the highest drying rate. Drying at this temperature also causes an increase in the head purple rice yield because of the gelatinization of starch. In the case of an initial moisture content of 33.3% (db), the drying temperature should not exceed 130°C.     

Evaluation of new parboiled rice process using humidified hot air fluidized bed drying

This study aims to experimentally investigate the drying characteristics and quality of a paddy dried by hot air (HA) and humidified hot air (HHA) fluidization technique. Qualities such as head rice yield (HRY), white belly, degree of gelatinization (DSG), and color of dried paddy were evaluated. A paddy with an initial moisture content of 14% d.b. was soaked in hot water at a temperature of 70?°C for 5?h then dried at a temperature of 130,150, and 170?°C, relative humidity in the range of 0.3–12%, an air velocity of 3.9 m/s, and a bed height of 10?cm. The results showed that the drying time of the paddy in the HHA condition took longer than the HA drying condition. Because HHA provided a higher grain temperature and a slow rate of drying, the degree of starch gelatinization was significantly higher when compared to HA. The subsequent HRY was relatively higher than using HA drying. However, the color of the sample obtained from the HHA condition was relatively browner, but the parboiled rice product still had a light brown color for the drying temperature range used in this study. To produce parboiled rice, HHA could be operated up to the temperature of 170?°C, relative humidity of 6%, and DOM of 10%.     

Attrition of Victorian brown coal during drying in a fluidized bed

Analyzing the attrition of Victorian brown coal during air and steam fluidized bed drying, the change in particle size distribution over a range of initial moisture contents (60% to 0%) and residence times (0 to 60 minutes) was determined. Dried at a temperature of 130°C with a fluidization velocity 0.55 m/s and an initial particle size of 0.5–1.2 mm, both fluidization mediums show a shift in the particle size distribution between three and four minutes of fluidization, with a decrease in mean particle size from 665 µm to around 560 µm. Using differential scanning calorimetry (DSC), the change in particle size has been attributed to the transition between bulk and non-freezable water (approximately 55% moisture loss) and can be linked to the removal of adhesion water, but not to fluidization effects. This is proved through the comparison of air fluidized bed drying, steam fluidized bed drying, and fixed bed drying—the fixed bed drying is being used to determine the particle size distribution as a function of drying. The results show the three drying methods produce similar particle size distributions, indicating that both fluidization and fluidization medium have no impact upon the particle size distribution at short residence times around ten minutes. The cumulative particle size distribution for air and steam fluidized bed dried coal has been modeled using the equation P_d = A₂ + (A₁ ? A₂)/(1 + (d/x₀)^p), with the resultant equations predicting the effects of moisture content on the particle size distribution. Analyzing the effect of longer residence times of 30 and 60 minutes, the particle size distribution for steam fluidized bed dried coal remains the same, while air fluidized bed dried coal has a greater proportion of smaller particles.     

Evolution of mechanical properties of parboiled brown rice kernels during impinging stream drying

Information on mechanical properties of parboiled brown rice kernels upon impinging stream drying, which is important for effective control of kernel fissure and head rice yield, is reported. Experiments were performed at the drying temperatures of 130, 150, and 170°C; inlet air velocity of 20?m/s; impinging distance of 5?cm and paddy feed rate of 40?kg_{dry_paddy}/h. Parboiled paddy was dried for up to seven cycles. Between each drying cycle, the paddy was tempered for a period of either 0 (without tempering) or 30?min. The moisture evaporation rate was noted to be very high during the first two drying cycles and rapidly dropped in the later drying cycles. When tempering was included after a particular drying cycle, the drying rate in a subsequent cycle was higher than without tempering. At the kernel moisture contents immediately after drying of 25.3–47.5% (d.b.), the drying temperature and existence of tempering did not affect the mechanical properties although microcracks were formed in the kernels. However, both factors played a more important role on the mechanical properties when the kernels were evaluated at 16% (d.b.). The head rice yield correlated well with the tensile strength of the kernels.     

A study on drying models and internal stresses of the rice kernel during infrared drying

Due to the limited penetration of infrared, it is very difficult to develop an infrared drying model of rice kernels. In this study, two kinds of simplified drying models, which assumed the penetration depth is infinity and zero, were developed to investigate the effects of penetration on drying characteristics of thin layer infrared drying. The results show each model can predict temperature and moisture contents (MC) accurately. The maximum temperature difference of rice kernels in both models was always less than 1.5°C, so it is reasonable to exclude the influence of thermal stresses due to nonuniform temperature. This study also developed the internal stresses model with the mechanical properties from literatures. Mechanical properties were with the changes of temperature and MC. These models were solved with COMSOL Multiphysics and there are two stress concentration areas. One is near the surface of the endosperm, another close to the center. Comparison between the von Mises stress distribution and the moisture gradient was made after the simulation. The maximum MC in the endosperm appeared at its surface, which reached 190 1/m at 110?s, and the maximum stress appeared at the same place, which, a little later, reached 7?MPa at 160?s. Moisture gradient at the center was zero due to the existence of symmetry, while there was a significant stress, which reached 3.2?MPa during drying.     

Determination of moisture content and moisture content profiles in wood during drying by low-field nuclear magnetic resonance

In this article, the low-field nuclear magnetic resonance was used to measure the moisture content (MC) and MC profiles in poplar wood during real-time drying. The T₂ distribution curve at each drying stage measured using the Carr–Purcell–Meiboom–Gill pulse sequence provided detailed information in the dynamic change of free water as well as bound water of the whole wood sample. In addition, a new developed SE-SPI pulse sequence was first used to evaluate the spatially resolved T₂ distribution of the successive nondestructive sliced layer of wood. Combined with the area integration method, the moisture content in each layer was calculated, and the change of MC profiles within wood at the MC above and below fiber saturation point was well reflected.     

Experimental study and simulation of vibrated fluidized bed drying

The paper addresses numerical simulation for the case of convective drying of seeds (fine-grained materials) in a vibrated fluidized bed, analyzing agreement between the numerical results and the results of corresponding experimental investigation. In the simulation model of unsteady simultaneous one-dimensional heat and mass transfer between gas phase and dried material during drying process it is assumed that the gas-solid interface is at thermodynamic equilibrium, while the drying rate (evaporated moisture flux) of the specific product is calculated by applying the concept of a “drying coefficient”. Mixing of the particles in the case of vibrated fluidized bed is taken into account by means of the diffusion term in the differential equations, using an effective particle diffusion coefficient. Model validation was done on the basis of the experimental data obtained with narrow fraction of poppy seeds characterized by mean equivalent particle diameter (d_S,d = 0.75 mm), re-wetted with required (calculated) amount of water up to the initial moisture content (X₀ = 0.54) for all experiments. Comparison of the drying kinetics, both experimental and numerical, has shown that higher gas (drying agent) temperatures, as well as velocities (flow-rates), induce faster drying. This effect is more pronounced for deeper beds, because of the larger amount of wet material to be dried using the same drying agent capacity. Bed temperature differences along the bed height, being significant inside the packed bed, are almost negligible in the vibrated fluidized bed, for the same drying conditions, due to mixing of particles. Residence time is shorter in the case of a vibrated fluidized bed drying compared to a packed bed drying.     

Conditional monitoring of moisture content in a fluidized bed dryer by the acoustic emission signature

Wetted rice particles were dried in a fluidized bed and the corresponding passive acoustic emissions signals (AES) were recorded at a given frequency to study the drying phenomena and bed hydrodynamic changes as well. The results show that the end time of the constant rate zone and the end of the falling rate can be determined from the variation of standard deviation and kurtosis of AES, respectively. Frequency domain analysis was also used to quantify the moisture content of solids. For this end, the original signal was decomposed into ten sub-signals, and it was found that the energy of the 4^th sub-signal can be correlated with the moisture content. The results show that the acoustic emission measurement is applicable as a practical method for on-line condition monitoring of drying process in fluidized bed dryers.     

Assessment of the initial moisture content on soybean drying kinetics and transport properties

Several researchers have developed studies to obtain a mathematical model able to describe grain drying kinetics. However, most of these studies neglect the effect of grain initial moisture content on drying curves. In this study, we assessed the dependence of drying curves and mass transfer coefficients on this initial moisture, air temperature, and its velocity by measuring grain mass losses within time on a tray dryer. Mathematical models were adjusted and results indicated that initial grain moisture content has significant influence on drying curves and mass transfer coefficients.     

Real-time monitoring of milk powder moisture content during drying in a spouted bed dryer using a hybrid neural soft sensor

The direct measurement of the moisture content of dried products would be more interesting for process control purposes. However, the most common procedures for such measurement are either slow or expensive for industrial dryers. Alternatively, one might reduce the cost of an effective measurement procedure by using other sensors (which are less expensive and whose response is faster), which can provide information for a physical–mathematical model representing well the drying process. In this context, the objective of this work was the application of a previously developed soft sensor for the online measurement of milk powder produced in a spouted bed dryer. A hybrid neural model was used as part of a soft sensor and coupled to the data acquisition interface. The sensor was capable of estimating milk powder moisture content when the dryer was submitted to disturbances on air inlet temperature and paste inlet flow rate. On the other hand, the model failed to describe paste accumulation within the bed, which is the reason why the soft sensor tended to overestimate moisture content for longer operation times.     

Experimental and numerical analysis of oil shale drying in fluidized bed

The main objective of this work was to experimentally and numerically investigate the Liu Shu River oil shale drying by the means of flue gas in a fluidized bed dryer. Several experiments were performed under different temperatures conditions. The moisture content of oil shale was measured during the experiments. The two-stage drying model was incorporated in computational fluid dynamics (CFD) package FLUENT via user-defined functions (UDF) and utilized for simulation of heat and mass transfer of oil shale drying in the fluidized bed dryer. The simulation results for solid moisture content agreed well with experimental data. The effects of the temperature and velocity of flue gas, initial bed height, and the particle size on the drying characteristics were predicted and analyzed. It is shown that the gas temperature and velocity are the important parameters in the whole drying process. The particle size has more obvious influence in the falling drying period than the constant drying period. The temperatures of gas and solid phases were monitored. It is shown that the so-called “near gas distributor zone” is the most effective heat transfer zone, which agrees well with the calculated value. The system quickly reached thermal equilibrium, characterizing a nearly isothermal bed. The developed model provides a very good demonstration to describe the oil shale drying in the fluidized bed dryer, and may provide important information for design, optimization of operation conditions.     

Application of electrical capacitance tomography to the fluidized bed drying of pharmaceutical granule

A novel calibration technique that accounts for the presence of moisture has allowed for the application of electrical capacitance tomography (ECT) to a fluidized bed dryer containing wet pharmaceutical granule. A statistical attractor comparison test denoted as the S-statistic has been used to analyze both the reconstructed and non-reconstructed ECT images to determine radial variations in hydrodynamic behavior at an axial location 10.5 cm above the distributor. This analysis has shown that variations in the bed dynamics associated with the loss of moisture during the drying process are most intense near the walls of the dryer, while the central region is characterized by a more consistent dynamic behavior. Investigation of the voidage distribution in the individual tomograms over this period in the drying process shows that the changes in the S-statistic correspond to a reduction in area of the region involved in bubbling behavior. Tomograms from early in the drying process indicate a central core of gas channeling through the very wet granule. This is due to the high cohesive forces present at high moisture. At moisture contents below 5-wt%, near the end of drying, significant divergence in dynamic behavior is identified in the S-statistic. This divergence is associated with the appearance of asymmetric bubbling behavior in the individual tomograms.     

Computer model of drying behaviour of ceramic green bodies with particular reference to moisture content dependent properties

A numerical model was developed to predict the drying behavior of ceramic green bodies. Resolution of the simultaneous heat and mass transfer equations involved finite elements and the Backward Euler method. Based on experimental data, the model uses equivalent moisture diffusivity, water activity, thermal conductivity and heat capacity as input parameters which depend on moisture content. In particular, the equivalent moisture diffusivity is a key parameter controlling water transport from the body interior to the surface. A simple method was used to estimate the effect of shrinkage on drying rate during the initial drying stage. Predictions of the internal moisture distribution, drying rate and surface temperature as a function of time gave good agreement to experiment for green bodies of alumina paste. External conditions of convection coefficient and relative humidity are shown to sensitively control drying rate and surface temperature evolution during the constant rate period.