Modeling of Vacuum Desorption in Freeze-Drying Process

Abstract

A mathematical model of multicomponent vacuum desorption, which occurs in vacuum freeze-drying process, was developed. In freeze-drying porous biomaterials and pharmaceuticals are considered and the vacuum freeze-drying process, especially the moisture desorption in its final stage, is investigated. In this article, the drying with conductive heating and constant contact surface temperature was considered. Pressure drop is taken into account in the model formulation but was neglected in process simulation because of thin material layers undergoing freeze-drying. Model equations were solved by numerical method of lines. Moisture content and temperature distributions within the drying material were predicted from the model as a function of drying time.     

A Comparative Study of Low-Pressure Superheated Steam and Vacuum Drying of a Heat-Sensitive Material

Abstract

Using carrot cubes as a model heat-sensitive material, experimental investigations were conducted to examine the drying kinetics and various quality parameters of the dried product undergoing both low-pressure superheated steam and vacuum drying. Effects of operating parameters such as pressure and temperature on the drying characteristics as well as quality attributes, i.e., volume, shrinkage, apparent density, color, and rehydration behavior, of the dried product underwent the two drying processes were also evaluated and compared. Although low-pressure steam drying required longer dwell time to achieve the same final moisture content than vacuum drying, some of the quality attributes were superior to those obtained in vacuum drying.     

AN INVESTIGATION OF THE BOUNDARY CONDITIONS FOR A VACUUM DRYING PROBLEM - INTRODUCING THE TRANSITION LAYER CONCEPT

ABSTRACT

In this work a new derivation of the boundary conditions for a vacuum drying process is postulated. The theoretical and numerical investigations performed in this research reveal that a dynamic Transition Layer exists at the interface between the porous material and the vacuum chamber. This hypothesis has been confirmed by the similar theory discussed in several works devoted to freeze drying and, by using a simulation package which performs fluid dynamic numerical simulations. A simulation program of the one dimensional drying of wood is under development in order to validate this research.     

INTENSIFICATION OF PENICILLIN DRYING IN INDUSTRIAL VACUUM DRUM DRYER BASED ON SHORT CONTACT THEORY

ABSTRACT

The results of industrial experimental research on penicillin drying in a vacuum drum dryer are discussed. It is shown that drying rate increases with increasing velocity of the drum rotation. Analytical calculations of heat exchange surface dependence on the hold-up of loaded material in a drum dryer are obtained.     

Composition Profiles and Functional Properties of Dietary Fiber Powder from Lime Residues: Effects of Pretreatment and Drying Methods

Lime residues after juice extraction have proven to be a potential raw material for producing dietary fiber (DF) powder due to their good functional properties. Compositions and antioxidant activity of DF powder from lime residues as affected by selected pretreatment (hot-water blanching and ethanolic soaking) and drying methods, viz. hot air drying, vacuum drying, and low-pressure superheated steam drying (LPSSD) at 60–80°C, were investigated. Fresh lime residues contained significant amounts of vitamin C, phenolic compounds, and flavonoids. Hesperidin was a major flavonoid and only one polymethoxyflavone (i.e., tangeretin) was detected in small amounts. A decrease in the amount of interested bioactive compounds and their antioxidant activity was noted at almost all steps of processing. Higher retention of bioactive compounds was noted when the residues were subject either to vacuum drying or LPSSD; the total antioxidant activities were 61–62% and 81–82% when being assessed by the β-carotene bleaching and DPPH assays, respectively. Vacuum drying at 80°C was the most suitable condition for preparing DF powder from lime residues due to its short required drying time and its ability to retain bioactive compounds. The in vitro analyses imply that DF powder prepared by vacuum drying at 80°C has the potential to reduce blood glucose and cholesterol levels by exhibiting high glucose retardation index (GRI) and bile acid retardation index (BRI).     

Experimental Study on Microwave Vacuum Drying of Two-Porosity Level Media

Abstract

Microwave vacuum drying experiments were performed on a laboratory scale dryer with a two-level porosity material: a packed bed of porous alumina beads. The incident microwave power and the vacuum pressure level were fixed, the main varying parameters being the beads diameter and porosity, and the mean pore diameter. The drying kinetics and the evolution of the product temperature are presented. The drying kinetics can be divided into two main periods. The first one corresponds to the drying from the bed voids according to evaporation mechanism that we describe with a stagnant film law. The second one corresponds to the drying from the particle pores and we divide it into two parts: we suggest that the former is dominated by capillarity driven moisture transport, and the latter is limited by the desorption kinetics of the few water layers left.     

Effect of Dielectric Material on Microwave Freeze Drying of Skim Milk

Abstract:

A simultaneous heat and mass transfer model of the dielectric material–assisted microwave freeze drying was derived in this study considering the vapor sublimation-desublimation in the frozen region. The mathematical model was solved numerically by using the finite-difference technique with two moving boundaries. Silicon carbide (SiC) was selected as the dielectric material, and the skim milk was used as the representative solid material in the aqueous solution to be freeze-dried. The results show that the dielectric material can significantly enhance the microwave freeze drying process. The drying time is greatly reduced compared to cases without the aid of the dielectric material. Profiles of the temperature, ice saturation, vapor concentration, and pressure during freeze drying were presented. Mechanisms of the heat and mass transfer inside the material sphere were analyzed. For an initially unsaturated frozen sample of 16 mm in diameter with a 4-mm-diameter dielectric material core, the drying time is 288.2 min, much shorter than 380.1 min of ordinary microwave freeze drying and 455.0 min of conventional vacuum freeze drying, respectively, under typical operating conditions.     

DRYING RELATED PROPERTIES OF APPLE

ABSTRACT

Drying related properties of apple are evaluated for various different drying methods (namely, convective, vacuum, microwave, osmotic and freeze drying), and their corresponding process conditions. The examined properties are drying kinetics, equilibrium material moisture content, density, porosity, color and viscoelastic characteristics. The effect of various process factors on these properties is described through particular mathematical models. The model parameters are estimated by fitting the corresponding model equations on a wide range of experimental data. Drying kinetics is greatly affected by the characteristic particle size and drying air temperature for convective drying, while for the case of microwave drying they are affected by the vacuum pressure and the emitted radiation power. Equilibrium material moisture content is affected by the temperature and the humidity of the surrounding air, while the osmotic pretreatment shifts the sorption isotherms to higher water activity levels. The quality properties examined, are significantly affected by the drying method. More specifically, osmotic dehydration decreases the porosity of the final product, while it prevents color deterioration and enchances the viscous nature of dehydrated apple. Freeze-dried apples develop the highest porosity, have the most elastic structure and the lowest rate of color deterioration.     

THE INFLUENCE OF PRESSURE AND TEMPERATURE ON THE KINETICS OF VACUUM DRYING OF KETOPROFEN

ABSTRACT

This paper explores the influence of temperature and pressure on drying kinetics of 2-(3-benzoylphenil propionic acid) ketoprofen, in a vacuum dryer on laboratory scale, Experimentally determined relations between moisture content and drying rate vs time, were approximated with an exponential model. Model parameters were correlated with drying conditions (temperature, pressure) and defined by functions of their potentions.

From an energy balance of the process, a mathematical model for simulating dependence of sample temperature vs drying time, and moisture content of material, has been developed.

Simulation of the drying kinetics and sample temperature, by use of those functional dependencies shows good agreement with experimental results.     

VACUUM CONTACT DRYING OF SELECTED BIOTECHNOLOGY PRODUCTS

ABSTRACT

The paper is mainly concentrated on investigations of the vacuum contact drying of the following biomaterials: (1) beer yeast, (2) whey and its components: proteinaceous liquid, lactose, post-lactose liquid, (3) saturated solution of the citric acid, (4) post-fermentation broth of the fodder antibiotic bacitracin. The investigations of vacuum contact drying process in which dried material adheres closely to a hot surface were performed. In order to make possible the determination of optimal process parameters the measurements were performed for various constant temperatures of the hot surface, total pressures in vacuum chamber and layer thickness of biomaterials.

The effective thermal properties for dry layers of selected biomaterials depending on temperature and total pressure were also experimentally determined.

A mathematical model of the vacuum contact drying for most intensive regime of its performance i.e. boiling one was presented. It will enable the process simulation and estimation of the dryers efficiency depending on process parameters.

In addition, investigations of beer yeast viability depending on the hot surface temperature and the total pressure were performed.     

VACUUM DRYING OF DISPERSED MATERIALS FROM MULTICOMPONENT LIQUID SYSTEMS: VERIFICATION OF MODEL

ABSTRACT

Experimental data on vacuum drying of chemical and microbial synthesis products under laboratory and industrial conditions are presented. Kinetic relations of vacuum drying of dispersed materials with the removal of a multicomponent liquid system are verified on both laboratory and industrial scale dryers under different operating conditions. The results of the research were generalised and a number of empirical relations needed for drying process calculation are given. The relation between heat transfer coefficient and heat flux and motive force for the vacuum drying process is established.     

MICROWAVE FREEZE DRYING CHARACTERISTICS OF BEEF

ABSTRACT

Microwave freeze drying characteristics of unsaturated raw beef were studied experimentally for various levels of electric field strength, vacuum pressure, sample thickness and initial saturation. The results show that drying time is proportional to initial saturation and inversely proportional to the electric field strength and sample thickness. The effect of vacuum pressure on drying time is negligible. Some advantages of microwave freeze-drying over conventional freeze drying with radiant heating are pointed out     

Controlling the drying process in vacuum foam drying under low vacuum conditions by inducing foaming by needle stimulation of the solution

Abstract

Vacuum foam drying is a promising drying technique but an extremely high vacuum is needed to achieve “foaming.” The findings reported herein show that, when a solution is partially vacuum-dried to 0.05–2?g-solvent/g-dry matter (initial drying) and the solution is then punctured with a steel needle (needle stimulation), vacuum drying resumes as a result of the solution foaming, even under conditions of an insufficient vacuum (ca. 1,000?Pa) where foaming is minimal. Methanol, ethanol, and isopropanol were used as solvents, and sugar and different molecular weight polyvinylpyrrolidone were employed as solutes. The results indicate that needle puncturing introduces minute bubbles, which then triggers foaming.     

EFFECT OF DRYING METHOD ON SHRINKAGE AND POROSITY

ABSTRACT

The effect of drying method on bulk density, particle density, specific volume and porosity of banana, apple, carrot and potato at various moisture contents was investigated, using a large set of experimental measurements. Samples were dehydrated with five different drying methods: conventional, vacuum, microwave, freeze and osmotic drying. A simple mathematical model was used In order to correlate the above properties with the material moisture content. Four parameters with physical meaning were incorporated in the model: the enclosed water density p_w, the dry solid density p_s, the dry solid bulk density p_bo and the volume shrinkage coefficient β'. The effect of drying method on the examined properties was taken into account through its effect on the corresponding parameters. Only, dry solid bulk density was dependent on both material and drying method. Freeze dried materials developed the highest porosity, whereas the lowest one was obtained using conventional air drying.     

THE EFFECT OF PROCESS CONDITIONS ON THE DRYING KINETICS AND REHYDRATION CHARACTERISTICS OF SOME MW-VACUUM DEHYDRATED FRUITS

ABSTRACT

The effect of drying conditions, namely, microwave power, vacuum pressure, as well as sample geometry, on the drying kinetics and rehydration characteristics of five microwave vacuum dehydrated fruits (apple, avocado, mushroom, pear and strawberry) was studied. The investigation involved a wide range of microwave power, vacuum pressure and sample size levels. The drying rate and the rehydration ratio, a measure of rehydration characteristics, were found to depend on the drying conditions. An empirical mass transfer model, involving a characteristic parameter for each process (drying and rehydration( as a function of process variables, namely, microwave power, vacuum pressure and sample size, was also tested with the data obtained in a microwave oven equipped with vacuum apparatus. Furthermore, a comparison of rehydration properties of microwave vacuum and conventionally dehydrated products, was conducted.     

Influences of Drying and Storage of Lycopene-Rich Carrots on the Carotenoid Content

ABSTRACT

The stability of β-carotene and lycopene was investigated during convective air and inert gas drying, microwave vacuum drying, and freeze-drying for lycopene containing carrots (Daucus carota L. cv. Nutri Red). After convection drying at temperatures below 70°C, β-carotene and lycopene contents remained unchanged independent of the drying medium. Freeze-drying did not show any advantage to convection-drying regarding carotenoid retention. Microwave vacuum–drying led to dry products with high carotenoid retention within very short drying times of about 2 h. Storage in air and in inert gas (nitrogen) containers was studied for convection-dried products, observing a better retention of carotenoids when using inert gas for a period of up to 6 months. After convection- and microwave vacuum–drying, an even better carotenoid extractability could be observed. No changes in the isomere fractions could be detected in any case.     

DEVELOPMENT OF A NEW DRYING PROCESS – DEHYDRATION BY CYCLICAL PRESSURE DROPS (D.D.S.): APPLICATION TO THE COLLAGEN GEL

ABSTRACT

Experiments to dry a collagen gel in order to obtain a homogenous film were carried out using a new process: dehydration by successive decompression “DDS” (Déshydratation par Détentes Successives). This process, particularly suited to thermosensitive products, involves a series of cycles during which the collagen gel is placed in desiccated air at a given pressure then subjected to an instantaneous (200 ms) pressure drop to a vacuum (7–90 kPa). This paper assesses the effect of four parameters of this drying process, namely high pressure (P _h) and vacuum pressure (P _v) and the effect of the duration at these pressures (t _h and t _v) on the drying rate, and the temperature of the collagen gel during drying. A comparative study between this new drying process and conventional methods indicated that the drying time was reduced from 480 and 700 min for vacuum and hot air drying respectively to 270 min for our process.     

MICROWAVE VACUUM DRYING KINETICS OF SOME FRUITS

ABSTRACT

Microwave vacuum drying kinetics of three fruits (namely, apple, kiwi and pear) were studied by introducing an one-parameter empirical mass transfer model, involving a characteristic parameter (drying constant), as a function of process variables. The model was tested with data produced in a microwave oven equipped with vacuum apparatus, using non-linear regression analysis. The investigation involved a wide range of microwave power and vacuum pressure levels. The parameters of the model considered were found to be greatly affected by the microwave power level while vacuum pressure affected slightly the process.     

MEASURED GAS VELOCITY AND MOISTURE CONTENT DISTRIBUTION IN A CONVECTIVE VACUUM KILN

ABSTRACT

Transferring the necessary heat of evaporation to the stack is the bottleneck in convective vacuum drying of wood. Higher gas velocities are applied to compensate for the lower gas density and to obtain similar heat and mass transfer characteristics as under normal pressure. Like in conventional kiln drying the region with the most unfavorable drying conditions determines drying time and product quality. To use the full potential of the meanwhile established superheated steam vacuum drying technology, it is therefore necessary to work on an improved uniformity of process conditions in the kiln.

To evaluate the fluid dynamics and its influence on the final moisture content, experimenls in a laboratory convective vacuum kiln were carried out. For different total pressures the profiles of dynamic pressure in the stack entry section were measured in a dry atmosphere. At normal pressure the profiles were determined between the board layers throughout the whole stack. For the same slack configuration vacuum drying tests were used to assess the impact of the velocity distribution in the slack on the final moisture content distribution-Regions of low gas velocities coincided well with regions of high final moisture content.     

DRYING RATE AND TEMPERATURE PROFILE FOR SUPERHEATED STEAM VACUUM DRYING AND MOIST AIR DRYING OF SOFTWOOD LUMBER

Abstract

Two charges of green radiata pine sapwood lumber were dried, either using superheated steam under vacuum (90°C, 0.2 bar abs.) or conventionally using hot moist air (90/60°C). Due to low density of the drying medium under vacuum, the circulation velocity used was 10 m/s for superheated steam drying and 5.0 m/s for moist air drying, and in both cases, the flow was unidirectional. In drying, stack drying rate and wood temperatures were measured to examine the differences between the superheated steam drying and drying using hot moist air.

The experimental results have shown that the stack edge board in superheated steam drying dried faster than in the hot moist air drying. Once again due to the low density of the steam under vacuum, a prolonged maximum temperature drop across load (TDAL) was observed in the superheated steam drying, however, the whole stack dried slower and the final moisture content distribution was more variable than for conventional hot moist air drying. Wood temperatures in superheated steam drying were lower.