Thesis Summary: Study of a New Atmospheric Freeze Drying System Incorporating a Vortex Tube and Multimode Heat Input

Atmospheric freeze drying (AFD) in a vibro-fluidized bed dryer coupled with an adsorbent and multimode heat input is proposed for dehydration of food products. An experimental setup was designed and built to permit simultaneous application of convection, conduction and radiation heat input to the drying material above its freezing point to ensure sublimation using a vortex tube to produce low temperature dry air. Comparison with AFD using fixed bed, fluidized bed dryer, traditional vacuum freeze drying and heat pump drying were carried out to investigate the viability of this new system. A two-layer moving boundary model was developed to simulate the drying kinetics and temperature scenario of thin slab product. Fairly good agreement was found between the predicted values and the experimental data. Finally a three-dimensional (3D) CFD simulation for a vortex tube is carried out to capture the highly swirling compressible flow behavior and to gain basic understanding of temperature separation process. An experimental setup was built to validate the simulation results.     

Multicomponent Solid Modeling of Continuous and Intermittent Drying of Pinus radiata Sapwood Below the Fiber Saturation Point

A multicomponent softwood timber drying model has been created that illustrates both the overall desorption behavior and the desorption behavior of individual timber components (cellulose, hemicellose, and lignin) for moisture contents below the fiber saturation point of 0.3 kg/kg. This model differentiates between the individual timber components by the differences in their thermodynamic properties, including the heat of sorption, the relative humidity above the timber surface, and diffusivity. This model incorporates a “double permeability” structure to simulate moisture moving between the components during drying. This article shows the results of a sensitivity study conducted on this model, which determines the effect of varying diffusivity on both overall and individual component desorption rates. Nonunique solutions have been predicted for some drying conditions, and the ranges of conditions leading to unique and nonunique conditions have been explored using both continuous and intermittent drying schedules. The use of intermittent drying schedules in this multicomponent drying situation has made the problem of parameter estimation less ambiguous. The range of initial conditions for most realistic conditions give predominantly unique results, so the diffusivities and intercomponent mass transfer rates can be determined from experimental drying data.     

喷雾干燥过程工艺参数的模拟计算

对喷雾干燥的计算机模拟计算进行了初步探讨，用FORTRAN语言编制了程序，对喷雾干燥过程进行了总体衡算和微元衡算，使计算快速、准确，而且便于程序与数据管理。结果表明，计算机完全可以替代人工手算。     

Reduction of a model for single droplet drying and application to CFD of skim milk spray drying

In this work, a novel methodology for the development of a high-accuracy computational fluid dynamics (CFD) model for the spray-drying process is described. Starting point is an own spatially resolving model of droplet/particle drying, which was developed and validated on the basis of a series of single droplet drying (SDD) experiments. This sophisticated model is transformed to a much simpler version: the characteristic drying curve approach, after running the full SDD model in a wide range of operating conditions. Then, the obtained reduced model is implemented into the CFD solver. The CFD spray-drying model takes into account the hydrodynamics of the continuous phase, particle drying kinetics, changes in the particle diameter, and the heat loss from the drying chamber to the environment. Validation of the entire procedure is provided by data obtained from drying experiments performed in a co-current laboratory spray tower. High accuracy of the developed CFD model of skim milk spray drying has been found for both phases, for the mean outlet temperature of the continuous phase (air) and for the change in average particle moisture content along the spray tower (discrete phase).     

Simulation of the Hydrodynamics and Drying in a Spouted Bed Dryer

Gas-particle flow behavior in a spouted bed of spherical particles was simulated using the Eulerian-Eulerian two-fluid modeling approach, incorporating a kinetic-frictional constitutive model for dense assemblies of the particulate solid. The interaction between gas and particles was modeled using the Gidaspow drag model and the predicted hydrodynamics is compared with published experimental data. To investigate drying characteristics of particulate solids in axisymmetric spouted beds, a heat and mass transfer model was developed and incorporated into the commercial computational fluid dynamics (CFD) code FLUENT 6.2. The kinetics of drying was described using the classical and diffusional models for surface drying and internal moisture drying, respectively. The overall flow patterns within the spouted bed were predicted well by the model; i.e., a stable spout region, a fountain region, and an annular downcomer region were obtained. Calculated particle velocities and concentrations in the axisymmetric spouted bed were in reasonable agreement with the experimental data of He et al. (Can. J. Chem. Eng. 1994a, 72:229; 1994b, 72:561). Such predictions can provide important information on the flow field, temperature, and species distributions inside the spouted bed for process design and scale-up.     

Conditions for Accurate CFD Modeling of Spray-Drying Process

The paper presents concluding results of extensive experimental and theoretical research on confident CFD modeling of spray drying. An earlier developed experimental method to determine spray-drying kinetics in a lab scale allowed us to find a critical material moisture content and to determine generalized spray-drying curves. The generalized drying curves, identical in shape in the laboratory and pilot plant units, were used in the CFD model of spray drying process. Extensive simulations for spray drying of 10, 30, and 50% of initial solid content of maltodextrin proved high accuracy of the predictions of discrete (particle size distribution, particle moisture content, particle velocity, spray temperature) and continuous-phase parameters (gas temperature and humidity). Maximum error of the predictions of discrete-phase parameters was on the level of 20%, which is probably close to the current capacity of the CFD technique for modeling of spray-drying process. Comparison of experimental measurements and theoretical results shows that incorporation of realistic spray-drying kinetics into the CFD model and correct definition of initial drying and atomization parameters enable reliable simulations of spray-drying process.     

Predicting drying kinetic and undesired agglomeration during drying of granules containing amorphous water-soluble substances in a continuous horizontal fluid bed dryer

Various industries use fluid bed dryers for the drying of larger granules produced by extrusion, pan agglomeration or spray granulation. During such drying processes secondary agglomeration is undesired due to an increasing amount of oversize particles. In the current study the drying of granules containing amorphous substances is modelled. The model is based on the laws of heat and mass transfer as well as heat and mass balances around a differential volume element of the fluid bed. For each location in the bed and for different drying parameters the calculated moisture and temperature values are used to estimate the surface viscosity of the granules. The calculated viscosity allows estimating the risk of secondary agglomeration of granules. Viscosity values which are estimated for different drying conditions are compared with the experimentally determined amount of oversize particles. An increasing amount of oversize particles can be observed if the viscosity of the amorphous substance is below 10⁴ Pa s.     

Numerical Study of Two-Stage Horizontal Spray Dryers Using Computational Fluid Dynamics

This article presents the findings of a numerical simulation model of the spray-drying process in a two-stage horizontal chamber design with the aid of a computational fluid dynamic (CFD) model. The model describes heat, mass, and momentum transfer between two phases; namely, a continuous gas phase and a discrete phase of droplets (or particles), using the finite volume method. In this study, a new two-dimensional horizontal spray dryer (HSD) geometry is considered as a pilot study into the spray-drying process in this novel chamber configuration. The tested model is able to predict some important features of the spray-drying process, such as air flow patterns indicating recirculation zones and particle trajectory plots. Some performance parameters for spray drying, such as the rate of evaporation, average volumetric heat and mass transfer rates, etc., are calculated and discussed. This two-stage drying process especially applicable for the horizontal spray dryer (HSD) model is investigated and modeled. The bottom wall of the HSD is assumed to be a shallow fluid bed used for second stage drying. In this article, the fluid bed drying conditions are changed and compared. The drying within the fluid bed itself is not modeled in this study, however. It is shown that the particle residence time is higher when the fluid bed is included. The drying performance of this two-stage horizontal spray dryer is expected to be better than that of a single-stage dryer.     

Heat of Desorption of Water From Cellulosic Materials at High Temperature

ABSTRACT

During the falling rate period of drying the heat requirements for drying hygroscopic materials may be increased substantially ahove the normal latent heat by the heat of desorption of water. In various new high intensity drying processes which are of increasing interest industrially, the drying material may reach quite high temperatures for periods of time sufficiently short that there is no degradation of the properties of the dried material. However the limited stability of many materials at high temperatures precludes determination of heals of desorption by direct measurements because of the long experimental times required for thermodynamic techniques. The present study develops a thermodymically valid method whereby heats of desorption determined at low lemperatures may be extrapolated reliably to the high temperatures where such information is needed but cannot be measured. A relation between isosteric heat of desorption and latent heat of water is derived in order to compare the new     

Numerical Study of Two-Stage Horizontal Spray Dryers Using Computational Fluid Dynamics

This article presents the findings of a numerical simulation model of the spray-drying process in a two-stage horizontal chamber design with the aid of a computational fluid dynamic (CFD) model. The model describes heat, mass, and momentum transfer between two phases; namely, a continuous gas phase and a discrete phase of droplets (or particles), using the finite volume method. In this study, a new two-dimensional horizontal spray dryer (HSD) geometry is considered as a pilot study into the spray-drying process in this novel chamber configuration. The tested model is able to predict some important features of the spray-drying process, such as air flow patterns indicating recirculation zones and particle trajectory plots. Some performance parameters for spray drying, such as the rate of evaporation, average volumetric heat and mass transfer rates, etc., are calculated and discussed. This two-stage drying process especially applicable for the horizontal spray dryer (HSD) model is investigated and modeled. The bottom wall of the HSD is assumed to be a shallow fluid bed used for second stage drying. In this article, the fluid bed drying conditions are changed and compared. The drying within the fluid bed itself is not modeled in this study, however. It is shown that the particle residence time is higher when the fluid bed is included. The drying performance of this two-stage horizontal spray dryer is expected to be better than that of a single-stage dryer.     

Supercritical water oxidation of Ion Exchange Resins in a stirred reactor: Numerical modelling

Supercritical water oxidation offers a viable alternative treatment to destroy the organic structure of Ion Exchange Resins. In order to design and define appropriate dimensions for the supercritical oxidation reactor, a 2D simulation of the fluid dynamics and heat transfer during the oxidation process has been investigated. The solver used is a commercial code, Fluent^® 6.3. The turbulent flow field in the reactor, created by the stirrer is taken into account with a k?ω model and a swirl imposed to the fluid. Particle trajectories are modelled with the Discrete Random Walk Particle Model. For the solubilization of the particles in supercritical water, a mechanism has been proposed and implemented into Fluent^® software through the Eddy Dissipation Concept approach, taking into account the identified rate determining species. Simulation results provide results on the flow, temperature fields and oxidation localization inside the reactor. For the reactive particles-supercritical water flow model, the effect of parameters, such as feed flow rates or stirring velocity, can be focussed. Reaction temperature is predicted with deviation lower than 15%. Degradation conversions are in good agreement with experimental ones.     

Modelling spatial distributions of moisture and quality during drying of granular baker's yeast

A distributed parameter model was developed to predict the drying behaviour of granular baker's yeast by setting up material and heat balances at the particle level. Temperature and moisture gradients were calculated for cylindrical and spherical granules. The performance of the model with two granule sizes was compared with experimental measurements. The model was initially used for non‐shrinking granules but later modified to take shrinkage into account. The reduction in granule size during the course of drying was estimated and good correspondence with experimental measurements was obtained. In addition to temperature and moisture gradients, the product quality was predicted during drying and compared to experimental results. The accuracy of the model was better than the lumped parameter model.     

Simulation of Temperature and Moisture Content Profiles in a Pinus radiata Board during High-Temperature Drying

A two-dimensional drying model has been simulated for industrial drying schedules to assess sensitivity of the model parameters. The predicted drying times and rates were similar to the results of experimental data. The influence of wood properties on the predicted drying rate was assessed. It was found that the effects of specific heat capacity and thermal conductivity on predicted overall drying rate of board were small. However, the effect of changes in the diffusion coefficient on drying rate below fiber saturation point (moisture content at 30%) was found to be significant.     

CFD modeling of a pilot-scale countercurrent spray drying tower for the manufacture of detergent powder

A steady-state, three-dimensional, multiphase computational fluid dynamics (CFD) modeling of a pilot-plant countercurrent spray drying tower is carried out to study the drying behavior of detergent slurry droplets. The software package ANSYS Fluent is employed to solve the heat, mass, and momentum transfer between the hot gas and the polydispersed droplets/particles using the Eulerian–Lagrangian approach. The continuous-phase turbulence is modeled using the differential Reynolds stress model. The drying kinetics is modeled using a single-droplet drying model, which is incorporated into the CFD code using user-defined functions (UDFs). Heat loss from the insulated tower wall to the surrounding is modeled by considering thermal resistances due to deposits on the inside surface, wall, insulation, and outside convective film. For the particle–wall interaction, the restitution coefficient is specified as a constant value as well as a function of particle moisture content. It is found that the variation in the value of restitution coefficient with moisture causes significant changes in the velocity, temperature, and moisture profiles of the gas as well as the particles. Overall, a reasonably good agreement is obtained between the measured and predicted powder temperature, moisture content, and gas temperature at the bottom and top outlets of the tower; considering the complexity of the spray drying process, simplifying assumptions made in both the CFD and droplet drying models and the errors associated with the measurements.     

Study of model application for drying of pulp fruit in spouted bed with intermittent feeding and accumulation

In the present study, we applied and improved a model to describe the behavior of a spouted bed dryer with intermittent feeding for suspension drying, considering suspension accumulation inside the device. This model is important in macroscopic heat and mass balances, represented by a system of ordinary differential balances, implemented and resolved numerically using Fortran routines, in which the influence of process variables on drying dynamics were assessed. The results obtained in modeling and mathematical simulations were compared with experimental data, indicating that the model fits the process well, based on outlet air temperature and moisture.     

Moisture Prediction During Paste Drying in a Spouted Bed

The objective of this work was to derive and experimentally verify a hybrid CST/neural network model to determine the moisture content of the powders produced during paste drying in a spouted bed and describe the highly coupled heat and the mass transfer. The model was derived from overall energy and mass balances with effective drying kinetics given by a neural network. Simulations were performed in MatLab and drying experiments for model verification were carried out for different pastes in a conical, semi-pilot-scale spouted bed.     

Obtaining model parameters of drying kinetics for highly shrinkable materials without knowing the surface area a priori

Shrinkage parameters of highly shrinkable materials such as length, diameter and surface area during drying are difficult to quantify in situ. However, these are significant components of an accurate model. In this study, an attempt to isolate the surface area effect is reported in order to fetch the REA model (reaction engineering approach) parameters without knowing it a priori. Carrot cube and cabbage leaf were selected as experimental material and dried with hot air under a range of conditions. Shrinkages was calculated using an optical method which is used to qualitatively compare with that “calculated” using the current approach. By matching the experimental temperature and moisture content profiles against time after obtaining REA parameters for both samples without knowing the surface area, the surface areas can be “calculated” numerically. Surface area was found to be affected by sample temperature as well as the moisture content. Drying simulations can be well carried out when correlating the surface area against sample moisture content X and temperature T, and it provides the best accuracy in predicting data on T and X vs. time. In addition, carrot cube can shrink ideally while cabbage leaf cannot. The overall relative errors of predicted moisture content and temperature were less than 1%.     

Low Péclet number behavior of the transfer rate in packed beds

The asymptotic behavior of the mass-transfer coefficient in a packed bed reactor at low Péclet numbers is dependent upon how the coefficient is defined. A singular perturbation approach coupled with heuristic arguments is used to demonstrate that the film mass-transfer coefficient in deep beds approaches a constant value as the Péclet number decreases. The film coefficient is utilized in the one-dimensional model of a bed as a sink term in the governing equation. The volumetric, or effective, mass-transfer coefficient which relates the overall reactant conversion to a logarithmic mean concentration driving force, decreases linearly with the Péclet number as the Péclet number approaches zero. The distinction between the two coefficients is important in the low Péclet number region. Analogous results apply to beat transfer. Reported experimental data support these predicted trends.It has been, demonstrated that the low Péclet number behavior of the Sherwood number in a packed bed reactor is dependent upon its defining equation. A rigorous singular perturbation approach coupled with heuristic arguments indicates that for a deep bed the effective mass-transfer coefficient (defined by eqn 1) is directly proportional to the Péclet number. The film coefficient (defined by eqn 3) approaches a constant in the same limit. These conclusions are independent of the detailed geometric void structure in the bed.     

Surface Stickiness of Drops of Carbohydrate and Organic Acid Solutions During Convective Drying: Experiments and Modeling

Drying kinetics of low molecular weight sugars such as fructose, glucose, sucrose and organic acid such as citric acid and high molecular weight carbohydrate such as maltodextrin (DE 6) were determined experimentally using single drop drying experiments as well as predicted numerically by solving the mass and heat transfer equations. The predicted moisture and temperature histories agreed with the experimental ones within 6% average relative (absolute) error and average difference of ± 1°C, respectively. The stickiness histories of these drops were determined experimentally and predicted numerically based on the glass transition temperature (T_g) of surface layer. The model predicted the experimental observations with good accuracy. A nonsticky regime for these materials during spray drying is proposed by simulating a drop, initially 120 µm in diameter, in a spray drying environment.     

SIMULATION OF DRYING IN A BATCH LUMBER KILN FROM SINGLE-BOARD TESTS

ABSTRACT

A deterministic model was developed to perform a board-by-board simulation of a forced convective batch lumber kiln. Individual board properties may be input and dryer operating parameters varied. The drying rates are empirical correlations based on single-board laboratory tests. The model incorporates the thermodynamic properties of the wood and gas, as well as mass and energy balances within the lumber stack. It also accounts for differences in heat and mass transfer resulting from position and changing gas properties throughout the dryer. The rate of drying predicted by the model and the final moisture content distribution were verified by weighing boards in a batch kiln before, during, and after drying. The application of the model is illustrated by simulating four common scenarios.