Study a new atmospheric freeze drying system incorporating a vortex tube and multi mode heat input 2

Chapter-2 Literature Review CHAPTER LITERATURE REVIEW The first part of review explains atmospheric freeze drying as a modifications technique of the vacuum freeze drying process and covers relevant information on modeling and experimental investigations. This chapter also contains some basic information on vibrating bed dryers along with a discussion of the vortex tube and multimode heat transfer, which are applied in this research to develop a new integrated atmospheric freeze drying system. 2.1 Freeze Drying Under Vacuum or Atmospheric Pressure Vacuum freeze-drying is a well known process for highly heat-sensitive materials. This method is used as a benchmark of product quality as it often gives the best quality dried products. For example, Marques et al. (2006) investigated the physical properties of vacuum freeze drying of tropical fruits and showed that this process gives high quality products. They also proved that vacuum freeze-dried (VFD) foods have high porosity and low apparent density. VFD also conserves color, flavor, and taste and provides rapid rehydration. The main disadvantages of the freeze drying technique are its high fixed and operating costs as demonstrated by Matteo et al. (2003). The latter are due to the series of energy-intensive operations involved in the process: freezing of the fresh product, heating of the frozen foods at low temperature to induce sublimation, condensation of water vapor and mechanical energy needed to maintain high vacuum. Moreover, vacuum operations are mainly carried out batchwise, which represent an additional cost together with requirements of the apparatus operated under vacuum. Chapter-2 Literature Review Liapis et al. (2007) showed that the limiting step of the traditional freeze-drying process under vacuum is the transfer of heat to the product due to the decrease in thermal conductivity with decreasing the pressure of the freeze-drying chamber. Efforts have been made to improve the vacuum freeze drying method but none has given economically satisfactory results as far as industrial applications are concerned. In particular, as disclosed in U.S. Patent specification No. 3,319,344, an attempt has been made to fluidize the product using vibration to be freeze-dried under vacuum in order to improve the heat and mass exchanges, but the major drawbacks resulting from operating under vacuum are not overcome in this design. 2.2 Atmospheric Freeze Drying – Fixed Bed Dryer The early workers in atmospheric freeze drying, namely the works of Meryman (1959), Lewin and Matelas (1962) and Woodward (1963), reported varying degrees of success when fixed beds of desiccants were employed to freeze dry foods and other biological materials in the absence of vacuum. The potential for atmospheric freeze-drying was demonstrated by Meryman (1959). He showed that the drying rate of a material undergoing freeze drying is a function of ice temperature and the vapor pressure gradient between the site of water vapor formation and the drying media, rather than the total pressure in the drying chamber. He invented process using either a fixed bed water vapor adsorbent adjacent to the frozen product or a condenser in a stream of cold air. However, drying periods were observed to be very long. In order to reduce drying time, other researchers focused attention on reducing product dimensions and on utilizing fluidized beds through modeling and experimental investigations. 10 Chapter-2 2.2.1 Literature Review Experimental studies of atmospheric freeze drying-fluidized bed dryer A number of researchers have performed experimental studies on atmospheric freezedrying using a fluidized bed to investigate the drying performance on different size and shape of products. Quality parameters of the dried products have also been investigated. Malecki et al. (1969) carried out atmospheric fluidized-bed freeze drying of apple juice and egg white. Their work supports the conclusion of Dunoyer and Larpusse (1961) and Woodward (1963) that the drying rate in atmosphere can reach that under vacuum if the particle size is sufficiently small. However, Malecki et al. (1969) found that for apple particles the bed had to be at -34oC to prevent sticking and, at this temperature, only percent ice was sublimated per hour, which is extremely low and hence not attractive from practical standpoint. Boeh-Ocansey,O. (1985) conducted experiments to investigate the drying kinetics at different product thicknesses, chamber temperature (-5oC, -10oC and -15oC) and using different adsorbents on carrot slices (1.8 x 33, 3.4 x 27, 4.8 x 26 and 5.8 x 22 mm). They carried out their experiments in a drying chamber consisting of a vertical cylindrical column 10 cm in diameter and 100 cm high at atmospheric pressure. They obtained higher freeze drying rates with particles of activated alumina (0.4 mm average diameter) than with activated carbon. Their results showed that a higher drying temperature (-5oC) is preferable to increase the drying rate. They compared their results with conventional vacuum drying and found that product thickness is more sensitive in drying kinetics to a great extent in AFD then in VFD. An apparatus and a technique for spray freeze aqueous solution at very low temperatures (-90oC) and for subsequent dehydration of the resulting frozen particles 11 Chapter-2 Literature Review in a stream of cold, desiccated air was developed by Mumenthaler and Leuenberger (1991).They investigated the influence of various operating variables on the drying kinetics as well as the quality of the products. They found a dry, stable and intact cake of same shape and size as the original frozen mass, with sufficient strength to prevent cracking and powdering or collapse. They also observed uniform color and rapid solubility upon reconstitution in water; increased inner surface area and good crystal structure of the active substance. They also reported an enhanced heat and mass transfer between the circulating drying medium and the frozen sample. Alves-Filho et al. (1998) used a fluidized bed as the first-stage freeze-dryer at atmospheric pressure in a two-stage heat-pump system without adsorbent. Adjusting the heat pump dryer components to keep the air temperature below the drying product’s freezing point controlled drying condition in the first stage fluid bed dryer. Their control strategy was based on the specific enthalpy curves developed by AlvesFilho et al. (1996). The product residence time in the first-stage dryer was selected to reduce the moisture content to the critical values. Afterwards the semi-dry product was transferred to the second stage fluid bed to be dried at higher temperatures. The advantages of their two-stage system are that low-temperature drying reduces the moisture content while maintaining product quality while higher-temperature drying increases the overall heat-pump dryer capacity. These authors reported excellent quality of dried shrimp, apple pieces, carrot slices, etc., at a relatively high cost, however. Donsi et al. (2000) here demonstrated the feasibility of atmospheric freeze-drying for shrimp and showed that the drying time is indeed an order-of-magnitude longer than that for the vacuum process. To shorten the drying time, part of the water was removed 12 Chapter-2 Literature Review by osmotic dehydration. Freeze-drying was carried out in a fluidized bed using a mineral adsorbent (Zeolite particles 88 μm mean diameter) or an organic adsorbent (wheat bran). The AFD dried shrimp dehydration properties that are similar to those obtained in conventional freeze-drying. The economics of the process were not reported. Bussmann et al. (2003) invented an apparatus for drying a product using a regenerative adsorbent which can be carried out in an energy-saving manner. According to their invention, the product is dried by bringing it into contact with adsorbent, water being taken up from product by the adsorbent. Subsequently, the adsorbent is regenerated with superheated steam. A detailed investigation was carried out by Matteo et al. (2003) of atmospheric freezedrying in a fluidized bed mixed with different compatible adsorbent particles. They fluidized cm long potato cylinders of various diameters with various particulate adsorbents. They measured higher heat and mass transfer coefficients compared with vacuum freeze drying due to convective heat and mass transfer, which is absent in VFD. They showed that higher freezing (-10oC) and fluidized bed temperatures (-6oC), compatible adsorbent (bran & bentonite), reduced sample size (6mm), smaller adsorbent particles (400 μm), higher product/adsorbent weight ratio (1/8), moderate regeneration temperature (60oC) are conducive to enhancing the dehydration rate. However, fluidization velocity had no significant effect on the dehydration rate. Their work also revealed that the size of the product is the key parameter in atmospheric freeze-drying. Finally, they showed that there is a significant reduction of the energy cost relative to vacuum freezes drying due to the absence of a vacuum chamber and ancillary equipment. 13 Chapter-2 Literature Review Stawczyk et al. (2005) investigated the kinetics of atmospheric freeze-drying and quality of apple dried cubes; all properties such as dehydration rate, shrinkage, color, antioxidant content etc. were reportedly very good. They conducted experiments at three different temperature increasing strategies; namely, constant inlet air temperature (CT); inlet air at different temperature (DT) option and inlet air of ascending temperature (AT) at a fixed airflow. They found ascending inlet temperature condition maintained a stable drying rate during the whole drying process to obtain an economical AFD process and generally better quality dried product. Their results showed that AFD dried products at lower temperature (-10oC), had characteristics of rehydration kinetics and hygroscopic properties similar to products obtained by vacuum freeze drying. They also found that AFD products are better than hot air-dried products in terms of their anti-oxidative activity. Strommen et al. (2005) carried out an experimental study of atmospheric freeze drying of cod using a fluidized bed dryer coupled with a heat pump. They found lower bulk densities, higher rehydration, and light color, which is similar to vacuum freeze drying, when dried at low temperature (-5oC) over a longer residence time of about 10 hours. They also noted that the typical specific moisture extraction rate (SMER) for atmospheric freeze drying with heat pumps is in the range 4.6 to 1.5 kg of water per kWh. Claussen et al. (2005) analyzed the physical and quality parameters (color, water content, rehydration properties and sorption isotherms) of traditional Norwegian stockfish and compared them with atmospheric freeze dried (AFD) cod fillets. Sorption isotherms of several stockfish and AFD cod samples were measured with a CIsorp water analyzer to determine the optimal storage conditions. They found and 14 Chapter-2 Literature Review times higher rehydration index for atmospheric freeze dried cod compared with naturally dried stockfish. In addition they observed the atmospheric freeze dried fish had brighter color than that of the naturally dried cod. The influence of AFD on the physiochemical properties, quality, and functional properties (color, water content, bulk density, rehydration properties, sorption isotherm, specific enzyme activity, solubility, protein denaturizing) of potato was investigated by Claussen et al. (2007). Their results showed that atmospheric freeze drying is a gentle drying process than spray or vacuum freeze drying. The solubility measurement gave better results for AFD potato protein samples at pH between 3.5 and 5, while the lowest value was obtained for spray dried samples over the whole pH range. Moreover, both enthalpy measurements and sorption isotherms indicate reduced protein denaturizing of AFD samples, while specific enzyme activity was at same level for all dried samples. Drying kinetics, sorption properties, shrinkage, and freezing point depression were determined by Claussen et al. (2007) in atmospheric freeze drying (AFD) of pieces of apple, turnip cabbage, and cod. They observed that drying at -5oC resulted in a greater shrinkage than drying at –10oC. Claussen et al. (2007) also carried out measurement of the physical properties of atmospheric freeze-dried cod and turnip cabbage. True density, apparent density and pore size distribution were measured using helium pycnometry, geopycnometry and light microscopy. They concluded that thawing during drying and product shrinkage affects the drying rate and the diffusion of water leading to poor product quality of the end product. 15 Chapter-2 Literature Review 2.2.2 Modeling of atmospheric freeze drying Heldman et al. (1974) proposed a simple mathematical model and validated it with their results experimentally. They concluded that the rate of drying, as expected, is higher for smaller particles and by increasing the surface mass transfer coefficient. Since the drying kinetics for AFD are determined by internal resistance to heat and mass transfer. Their conclusion about effect of external mass transfer coefficient is surprising. Boeh-Ocansey et al. (1983, 1985) reported measurements of kinetics of ice sublimation in vacuum and in a fluidized bed drying system under atmospheric condition. They showed that for ice sublimation the recommended partial vapor pressure, temperature of drying chamber and relative humidity of air are: 4.58 mm Hg, approximately 0oC and below 20 ppm, respectively. Since there is freezing point depression with soluble components in drying material is 0oC really the optimum temperature. A further kinetics study of ice sublimation in a fluidized-bed dryer operating under atmospheric conditions was reported by Boeh-Ocansey and Wachet (1986). Mathematical expressions were formulated to account for mass variation and dimensions of ice samples during sublimation. They investigated the influence of chamber temperature and air flow rate on sublimation and showed that sublimation of ice was obtained at drying chamber temperatures greater than 0oC (7.6oC maximum). They also noted that there exists a correlation ice sublimation temperature and the temperature of the fluidized bed. Ice sublimation temperature can be predicted accurately for a given fluidized-bed temperature. Finally they showed that the kinetics of ice sublimation was regulated by the expression: M/M0 = (1 – t/tT)2 where M and t 16 Chapter-2 Literature Review represent mass of product and time, respectively. Subscript o and T stand for initial and final time, respectively. Wolf and Gilbert (1990b) proposed a model for atmospheric freeze-drying in a fluidized bed of a particulate adsorbent (starch) incorporated in different mass ratios. Their model was based on uniformly retreating ice front (URIF). They found spherical shape, minimum thickness (2mm), higher drying temperature (-5oC) and minimum regeneration temperature of adsorbent (50oC) are most advantageous for drying. They also concluded that a mass fraction between the mass of water sublimated and the mass of adsorbent used (mw / ma ), of the order of 0.10, or even 0.05 at higher temperatures, seems satisfactory. They validated their model with experiments with potato parallelepipeds of different thicknesses (2, & mm). Joseph et al. (1996) presented two sets of nonlinear coupled heat and mass transfer models to describe the absorption and desorption process. Model I describes the temperature and moisture distribution in a porous medium with a moving evaporation front. They reported that in the two phase system with moving boundary condition, the rate of movement of the evaporation front decreased with deepening of the evaporation front in the porous body. They showed that the higher the value of nondimensional vaporization parameter γ, the slower is the movement of the evaporation front. The temperature decreased and the moisture content increased as the nondimensional vaporization parameter γ increased. In model II they described a set of simultaneous heat and mass transfer equations describing moisture adsorption during the steeping of barely kernels. 17 Chapter-2 Literature Review A study of freeze drying, by immersion in an adsorbent medium both at atmospheric pressure and under vacuum was carried out by Lombrana and Villaran (1996). They used spherical moistened particles of commercial cereal food paste as a drying product. They employed zeolite as adsorbent particles (0.63g/cm3 density, diameter 0.7 mm) with adsorbent to product mass ratio of 10:1 in a fluidized bed dryer. They evaluated the effect of pressure and temperature on the drying kinetics through a model by considering a uniformly retreating ice front in spherical geometry. Their model calculated the pressure and temperature at the sublimations front in terms of the product moisture. Conditions of -5oC and pressures of 310 and 410 mm Hg, but without adsorbent, were also investigated to analyze the possibility elimination of adsorbent when vacuum was employed. They found that values of total time and shrinkage varied from 400 to 390 and from 0.567 to 0.573, respectively, they concluded that adsorbent usage is recommended. Also they proposed some operational strategies which can reduce the process duration without damage to product quality. First step starts at atmospheric pressure and a temperature of -10oC followed by a second step at where low pressure with temperature in the range 0oC to 15oC and a third step at atmospheric pressure with temperature below 15oC. Lombrana and Villaran (1997) developed a mathematical model for AFD in a fluid bed dryer. They evaluated the effect of pressure and temperature on the drying kinetics through a model by considering a uniformly retreating ice front in spherical geometry. Their model calculated the pressure and temperature in the sublimations front in terms of the product moisture. Good agreement was found between predicted and experimental results. 18 Chapter-2 Literature Review Li et al., (2007) presented a film sublimation model that couples a fluid dynamic model for the ice-vapor interface with a URIF model for vapor transport through the dry porous zone. The interface model considered a vapor film resulting from sublimation from a virtual ice wall under local thermodynamics equilibrium. They used the Fluent CFD package to describe the diffusivity vapor transport from the interface. Their simulation captured the drying phenomena of a plate of ice, a chitosan membrane, and a 10-mm-wide cube of apple at temperatures of -4 oC, -8 oC, -12 oC, and -16oC. Comparison with experimental results showed good agreement mainly for medium and low moisture contents and for highest drying temperatures. A simplified mathematical model was developed based on uniformly retreating ice front (URIF) considerations by Claussen et al, (2007). The model used theoretical drying curves for atmospheric freeze dried foods in a tunnel drier. They proposed that the model can be used to simulate industrial atmospheric freeze drying of different foodstuffs in a tunnel dryer. They also found good accordance with their experimental results. 2.2.3 Technical feasibility and economic viability Technical feasibility and economic viability of atmospheric freeze-drying system have been examined by a number of investigators through experimental analysis. Winter (1968), Gunn et al. (1969) and Simatos et al. (1974) investigated the applications of various adsorbents in freeze-drying. They demonstrated that activated carbon, silica gel, activated alumina and molecular sieves are suitable for the drying application. Their work agreed with the conclusion of Meryman (1962) that the method could be improved by circulating the air stream through a bed of molecular 19 Chapter-2 Literature Review sieve or over a refrigeration coil to remove water vapor from air. Sandall et al. (1967) reported that the thermal conductivity of the porous dry layer increases as total pressure increases while the diffusivity of water vapor across the same layer decrease. Gibert (1979) experimented with atmospheric freeze drying using a fluidized bed. He used pieces of carrot in the form cm diameter discs which were 3.5 mm thick. Activated aluminas (mean diameter between 250 to 500 microns) were used as the adsorbent. Water vapor pressure inside the drying chamber, flowrate of the carrier gas, freezing and freeze drying temperature were about 0.1 mm of mercury, approximately 1.3 V.sub.mf to 1.7 V.sub.mf, -30oC and –5oC, respectively. His method revealed that the rate of freeze drying was higher at the beginning of the test. A detailed investigation of heat and mass transfer co-efficient under vacuum and in atmospheric fluidized-bed dryer was carried by Osei Boeh-Ocansey (1988). They used ice samples of cylindrical (2.8 x 4.6, 3.0 x 4.8 and 5.4 x 1.9 cm) and block (2.3 x 5.0 x 12 cm) shapes as a product mixed with activated alumina granules as adsorbent (0.4 mm average diameter) in their experiments. The fluidized bed AFD dryer consisted of a 10 cm diameter column, a refrigeration unit and an air blower, while the vacuum dryer was made of a cylindrical stainless steel chamber, 28 cm in diameter and 26 cm high. They found the ratio between the external heat and mass transfer coefficients (h/K) for the fluidized bed dryer was constant at 398.8 kcal torr kg-1 oC-1 but varied for the vacuum dryer in the range 9.5 to 16.2 kcal torr kg-1 oC-1. They calculated the individual heat and mass transfer co-efficient. The mass transfer co-efficient, k, for the vacuum unit was 1.0 kg h-1 m-2 torr-1 ; for the atmospheric fluidized bed dryer, k varied slightly with temperature, being 0.7, 0.8, and 1.0 kg h-1 m-2 torr-1 , for a fluidized bed dryer at 0, -10 and 20oC, respectively. At these temperatures, the heat transfer co- 20 Chapter-2 Literature Review efficient for the fluidized bed dryer were 287, 321, and 402 kcal h-1 m-2 oC-1, respectively. For the vacuum dryer h was 9.5-16.2 kcal h-1 m-2 oC-1 .They showed that values of the mass transfer co-efficient were comparable in the two drying systems but heat transfer co-efficient in the fluidized-bed dryer was about 20-40 times greater than that in the vacuum drying system. The feasibility of a freeze drying process based on a fluidized bed dryer with adsorbent at atmospheric pressure was demonstrated by Wolff and Gibert (1990a). Experiments were performed in a fluidized bed freeze-drying column fitted with a double jacket for the cooling liquid. In the course of drying, a 1x1 cm square base with different thicknesses of potato sample (0.3 kg) was immersed into a 5-kg bed of corn starch fluidized by air at with a superficial velocity of to 0.04 m/s. They also calculated the cooling and heating energy requirements for the vacuum system to be about 3550 kJ/kg and 3780 kJ/kg, respectively, and for atmospheric freeze drying system to be 2250 kJ/kg and 3440 kJ/kg, respectively. Their results revealed that the energy cost is lower by 38% and 34% for the cooling and heating requirements for atmospheric freeze drying, respectively. A review of the literature on atmospheric freeze-drying has recently been carried out by Claussen et al. (2007). This review includes technological aspects, product possibilities, and physical properties of products, drying kinetics, modeling and simulation of an AFD system. However, there is still scope to enhance the drying rate in an atmospheric freeze drying system using a vibrating bed and multimode heat input to enhance heat transfer rate without causing melting of the ice. In addition, osmotic dehydration can generally be used to reduce the energy consumption as well as to 21 Chapter-2 Literature Review improve the drying rate and quality of the dried products on AFD system. As will be seen later, this hypothesis did not turn out to be true. 2.3 Use of Vortex Tube to Obtain Subzero Temperature Air Conventional atmospheric freeze dryers utilize a system of a mechanical heat pump to lower temperature and a large condenser to reduce humidity of the air. At least two mechanical agents are required for this operation, which is not economical from the energetic point of view. It also takes time to setup, de-humidify and cool the drying chamber. A vortex chiller tube can be used as a viable alternative to achieve required characteristics of the carrier gas supplied to the drying chamber at atmospheric pressure. The vortex tube is a simple device with no moving parts that is capable of separating a high pressure flow into two lower pressure flows of different temperatures. The vortex tube was first discovered by Ranque (1933). Later it was improved in efficiency by Hilsch (1947). Ahlborn et al. (1994) developed a two-component model to determine the limits of the increase and decrease in temperature within the standard vortex tube. Their experimental data with air as the working fluid were within the calculated limits. Frohlingsdorf and Unger (1999) studied the phenomenon of velocity and energy separation inside the vortex tube using CFX commercial code. They developed a 2D axisymmetric model which allowed successful prediction of the experimental results. They found that the application of the k-є model leads to substantial differences between measured and calculated tangential velocity profiles; they replaced the k-є model with a correlation from Keey (1972) for the calculation of the turbulence viscosity. Their results show that the strength of energy separation can be fitted to both 22 Chapter-2 Literature Review measured total temperature differences (cold/hot gas to the inlet total temperature) by increasing the turbulent Prandtl number value, leading to damping of turbulent diffusion in favor of the transfer of mechanical work. Mechanical work is transferred from the cold to the hot gas by viscous friction, intensifying the dissipation processes there; this results in a temperature rise of the hot gas portion. Behera et al (2005) conducted a 3D computational fluid dynamic (CFD) and experimental study of optimization of the Ranque-Hilsch vortex tube. Different types of nozzle profiles and number of nozzles were evaluated by CFD analysis. They also analyzed the swirl velocity, axial velocity, and radial velocity components as well as the flow patterns including secondary circulation. They proposed optimum cold end diameter (dc=7mm) and the length to diameter ratios (L/d =10 to 35). Furthermore, optimum parameters for obtaining the maximum hot gas temperature (391 K for L/D=30) and minimum cold gas temperature (267 K for L/D=35) were obtained through CFD analysis and validated through experiments. Aljuwayhel et al. (2005) also studied the energy separation mechanism and flow in a counter-flow vortex using the CFD code FLUENT with the standard k-є and the RNG k-є models of turbulence. A two-dimensional axi-symmetric model was developed that exhibits the general behavior expected of a vortex tube. They reported that the RNG kє model provided better predictions. The vortex-tube flow can be divided into three regions that correspond to: flow through the hot exit (hot flow exit), flow through the cold exit (cold flow region), and flow within the device (re-circulation region). This is contrary to the results of skye et al. (2006), who claimed that, the standard k-є model with swirl performs better than the RNG k-є model, although both used the same 23 Chapter-2 Literature Review commercial CFD code FLUENT. They validated their model results with experimental data obtained from a laboratory scale vortex tube operated with room temperature compressed air. They demonstrated that the energy separation exhibited by the vortex tube can be explained primarily by the work transfer caused by a torque produced by viscous shear acting on a rotating control surface that separates the cold flow region in the opposite direction and therefore tends to reduce the temperature separation effect. Eiamsa-ard and Promovonge (2006) carried out a 2D axi-symmetrical numerical calculation using the algebraic Reynolds stress model (ASM) and the standard k-є model for the simulation of a strongly swirling flow in a vortex tube. They used the TEFESS code, based on a staggered finite volume approach with the standard k-є model and first-order-numerical schemes. They noted that the use of ASM results in more accurate prediction than does the k-є model. They proposed that the predicted results for strongly swirling turbulent compressible flow in a vortex tube suggests that the use of the ASM model which leads to better agreement between the numerical results and experimental data. The application of a mathematical model for simulation of thermal separation in a Ranque-Hilsch vortex tube was carried out by Eiamsa-ard and Promovonge et al. (2007). A staggered finite volume approach with the standard k-є turbulence model and an algebraic stress model (ASM) was used to provide an understanding of the physical behavior of the flow, pressure, and temperature in a vortex tube. They used 2D; second order upwind (SOU) and the QUICK numerical schemes. They compared the first-order upwind and hybrid schemes. Their computations showed good agreement between the results of the algebraic stress model (ASM) and experimental 24 Chapter-2 Literature Review results. They concluded that the diffusive transport of mean kinetic energy has a substantial influence on the maximum temperature separation occurring near the region. The fundamental mechanism of energy separation has been well documented by some investigators (Aljuwayhel et al. 2005). However, due to lack of reliable measurements of the internal temperature and velocity distributions, there is still need for more effort to capture the real phenomena in a vortex tube. A 3D simulation is clearly a good option to capture well the complex flow phenomena in the vortex tube. Literature results revealed that only a few investigators have been worked on 3D simulation of vortex tube. Therefore effort is needed to apply a 3-D model for more reliable prediction of the complex vortex-tube flow. 2.4 Vibrating Bed Dryer The principle of vibrating beds can be used as a strategy to improve fluidization quality of irregular and cohesive materials which are hard-to-fluidized and to avoid problems channeling, defluidization and slug flow. To improve drying rates by deagglomeration with, consequent increase of specific area for gas-solid contact, and to accelerate the migration rate of moisture from the interior to the surface of the particles in vibrofluidized bed was demonstrated by Pakowski et al. (1984). Dong et al. (1991) carried out an experimental studied on the drying characteristics in vibrated fluidized beds of corn plu-mule, silica gel and citric acid. They found that application of vibration enhance the drying rate during the falling rate period. In the optimum range of vibration parameters, the critical moisture content decreased 25 Chapter-2 Literature Review significantly from 50% to 28% for corn plu-mule. They also demonstrated that effect of amplitude is not clearly defined and no trend is discernible for corn plumule drying in the VFD. The behavior of a sawdust dryer in a vibrating fluidized bed dryer was analyzed by Rogelio et al. (2000). Using mechanical vibration of the drying chamber, they found significant reductions in the required air velocities for drying sawdust in a fluidized bed. They proposed that this reduction can be up to 50% in relation to the minimum fluidization velocity and up to a 70% with respect to the operating velocity used in conventional fluid beds. They also demonstrated that sawdust with moisture content higher than kg/kg moisture can be vibro-fluidized with high degree of bed homogeneity. Soponronnarit et al. (2001) tested a commercial scale vibro-fluidized bed dryer for paddy. The operating conditions were as follows: paddy feed rate, 4.82 t/h; paddy bed height, 11.5 cm; airflow rate, 1.7m3/s; bed velocity, 1.4m/s; fraction of air recycled, 0.85; vibration factor, approximately (frequency, 7.3 Hz and amplitude, 5mm) and average inlet air temperature of 140oC. They concluded that vibro-fluidized bed could reduce the moisture content of paddy from 28% to 23% d.b. with head rice yield and rice whiteness of 37% and 41.2, respectively , while the paddy dried with ambient air drying provided head rice yield and rice whiteness of 32% and 42.5 , respectively. They also illustrated that the total electrical power of blower motor and vibration motor that used in vibro-fluidized bed dryer was approximately 55% of electrical power motor used in fluidized bed dryer without vibration. 26 Chapter-2 Literature Review Roger et al. (2005) investigated a wide range of dimensionless vibrating number (Γ) to quantify the vibration energy of a vibro-fluidized bed. It is defined as a function of the amplitude (A) and of the frequency of vibration (f). They illustrated that the pressure drop, standard deviations of pressure drop and minimum fluidization velocity were strongly affected by both the amplitude and the frequency of vibrations. They concluded that the dimensionless vibration number (Γ) must be used carefully when applied as the only parameter to characterize the vibration effects in a vibro-fluidized bed and the application of Γ on other studies involving vibro-fluidized beds deserves further evaluation. 2.5 Multi-mode Heat Supply Infrared and conduction heat can be used as heat sources in atmospheric freeze drying. In recent years intermittent multimode drying has been reported as an effective means of energy savings at enhanced drying rates and often yielding better quality dried products. Dostie (1992) found that intermittent IR drying is a viable option for drying processes to effect large reductions in drying time. Nastaj (1994) found that application of combined conductive-radiative heating in vacuum drying is advantageous and gives considerable process intensification, i.e. shortening duration of the process. Ratti and Mujumdar (1995) summarized the advantages of IR drying: high efficiency to convert electrical energy into heat for electrical IR; radiation penetrates directly into the product without heating the surroundings; uniform heating of the product; easy to program and manipulate the heating cycle for different products and to adapt to changing conditions; leveling of the moisture profiles in the products and low product 27 Chapter-2 Literature Review deterioration; IR sources are inexpensive compared to dielectric and microwave sources; they have a long service life and low maintenance. Islam and Mujumder (2003) have given a comprehensive overview of their work on multi-mode drying with a one-dimensional liquid diffusion model. Heat of wetting, temperature and moisture dependent effective diffusivity and thermal conductivity as well as changes in product density were accounted for in the model. They analyzed the relative advantages of combining various modes of heat transfer e.g., convection, conduction, radiation and volumetric heating in a microwave field. They found that some combinations of heat inputs did not exhibit the best drying performance during the whole drying period which reflects the need of appropriate switching between different modes of heat input to get the optimum and energy efficient drying condition. An experimental study was conducted using a batch heat pump dryer designed to permit simultaneous application of conduction and radiation using potato as a model heat sensitive drying object by Lan et al. (2005). They also developed a twodimensional model to predict the drying rate and temperatures within the slab during drying and showed good agreement with their measurements. Therefore, it can be concluded from the review of literature that atmospheric freeze drying method with combination of vortex tube, vibrating bed dryer and multimode or intermittent heat input would be a good alternative method to traditional vacuum drying. Osmotic dehydration can also be implemented preceding AFD to reduce the initial moisture content prior to drying. As will be noted later, this pre-treatment in fact leads to longer drying times and lower quality dried product. 28 [...]... results and experimental data The application of a mathematical model for simulation of thermal separation in a Ranque-Hilsch vortex tube was carried out by Eiamsa-ard and Promovonge et al (20 07) A staggered finite volume approach with the standard k-є turbulence model and an algebraic stress model (ASM) was used to provide an understanding of the physical behavior of the flow, pressure, and temperature... Technical feasibility and economic viability of atmospheric freeze- drying system have been examined by a number of investigators through experimental analysis Winter (1968), Gunn et al (1969) and Simatos et al (1974) investigated the applications of various adsorbents in freeze- drying They demonstrated that activated carbon, silica gel, activated alumina and molecular sieves are suitable for the drying application... viscous shear acting on a rotating control surface that separates the cold flow region in the opposite direction and therefore tends to reduce the temperature separation effect Eiamsa-ard and Promovonge (20 06) carried out a 2D axi-symmetrical numerical calculation using the algebraic Reynolds stress model (ASM) and the standard k-є model for the simulation of a strongly swirling flow in a vortex tube They... of appropriate switching between different modes of heat input to get the optimum and energy efficient drying condition An experimental study was conducted using a batch heat pump dryer designed to permit simultaneous application of conduction and radiation using potato as a model heat sensitive drying object by Lan et al (20 05) They also developed a twodimensional model to predict the drying rate and. .. physical properties of products, drying kinetics, modeling and simulation of an AFD system However, there is still scope to enhance the drying rate in an atmospheric freeze drying system using a vibrating bed and multimode heat input to enhance heat transfer rate without causing melting of the ice In addition, osmotic dehydration can generally be used to reduce the energy consumption as well as to 21 Chapter -2. .. 4.8 and 5.4 x 1.9 cm) and block (2. 3 x 5.0 x 12 cm) shapes as a product mixed with activated alumina granules as adsorbent (0.4 mm average diameter) in their experiments The fluidized bed AFD dryer consisted of a 10 cm diameter column, a refrigeration unit and an air blower, while the vacuum dryer was made of a cylindrical stainless steel chamber, 28 cm in diameter and 26 cm high They found the ratio... vibration number (Γ) must be used carefully when applied as the only parameter to characterize the vibration effects in a vibro-fluidized bed and the application of Γ on other studies involving vibro-fluidized beds deserves further evaluation 2. 5 Multi- mode Heat Supply Infrared and conduction heat can be used as heat sources in atmospheric freeze drying In recent years intermittent multimode drying has... Chapter -2 Literature Review improve the drying rate and quality of the dried products on AFD system As will be seen later, this hypothesis did not turn out to be true 2. 3 Use of Vortex Tube to Obtain Subzero Temperature Air Conventional atmospheric freeze dryers utilize a system of a mechanical heat pump to lower temperature and a large condenser to reduce humidity of the air At least two mechanical agents... diffusivity vapor transport from the interface Their simulation captured the drying phenomena of a plate of ice, a chitosan membrane, and a 10-mm-wide cube of apple at temperatures of -4 oC, -8 oC, - 12 oC, and -16oC Comparison with experimental results showed good agreement mainly for medium and low moisture contents and for highest drying temperatures A simplified mathematical model was developed based on... reported as an effective means of energy savings at enhanced drying rates and often yielding better quality dried products Dostie (19 92) found that intermittent IR drying is a viable option for drying processes to effect large reductions in drying time Nastaj (1994) found that application of combined conductive-radiative heating in vacuum drying is advantageous and gives considerable process intensification, . numerical results and experimental data. The application of a mathematical model for simulation of thermal separation in a Ranque-Hilsch vortex tube was carried out by Eiamsa-ard and Promovonge. simulation of an AFD system. However, there is still scope to enhance the drying rate in an atmospheric freeze drying system using a vibrating bed and multimode heat input to enhance heat transfer. 2. 5 Multi- mode Heat Supply Infrared and conduction heat can be used as heat sources in atmospheric freeze drying. In recent years intermittent multimode drying has been reported as an