Drying Of Thin Films Research Articles

Energy and exergy analyses of solar drying sardine fillets

Fish is a highly perishable food product. It has a very short because it contains up to 78% of water. The aim of this work is to examine the effect of solar drying on the dehydration kinetics of Moroccan sardine fillets. The curves of the experimental kinetics of drying sardine fillets are carried out at four temperatures (50, 60, 70 and 80°C) with two air flow rates of 300 and 150 m3/h. Nine thin-film drying models are adapted to the drying data. The Midilli-Kucuk model allows better adaptation than the other models according to R2, S and χ2. The data shows that the moisture content reduced from 3.98% to 0.08% db (dry basis). In regards to a conducted study of a part concerning the exergy loss, exergy transfer rate from evaporation and exergy efficiency of the drying chamber is carried out.

Model development for conductive thin film drying processes

A heat-transfer governed model is proposed to describe drying in a lab-scale conductive thin film dryer, which was designed to investigate the drying kinetics relevant to drum drying. The model calculations were compared to experimental data from drying experiments with maltodextrin DE12 and potato starch, considering the three distinct periods (heating, boiling and conductive drying) of the lab-scale process. The model uses measured temperatures and evaporation rate during the boiling period as input to calculate the decrease in moisture content during the drying process. Model calculations were evaluated by determining the root-mean-square-error (RMSE) values. The RMSE were very small (<0.24) indicating that the model was successful in describing the film drying process. During the last drying period, the starch films exhibited a higher initial heat transfer resistance (~0.0004 (m2∙K)/W) compared to maltodextrin (~0.0002 (m2∙K)/W). This reflects the formation of larger vapour bubbles in the boiling period impeding the heat transfer for starch films. Subsequently, the model was modified to describe a pilot-scale drum drying process for maltodextrin suspensions. The initial heat transfer coefficient for drum drying of maltodextrin was obtained from the lab-scale experiments. The simulations indicated residual moisture contents and optimal drying times for different drying conditions.

Design and construction of a thin-film drying channel equipment - Modeling the drying kinetics of nanofiltered whey

Drying kinetics of nanofiltered whey was obtained by using thin-film drying experiments in a dryer device made in the laboratory. Four drying temperatures (40, 55, 70, and 85 °C) and two air velocities (0.3 and 0.7 m/s) were used in drying experiments. Drying kinetics were fitted using the characteristic drying curve (CDC) model. Two distinctive periods were observed in the drying curves. The temperature exhibited a significant (P < 0.05) effect on the drying kinetics. An increase in temperature accelerated the drying rate. Air velocity showed a non-significant (P > 0.05) effect on the drying kinetics, possibly due to the air flow was in excess regarding the amount of water vapor removed from the liquid-gas interface. The CDC model described well drying kinetics. The dryer device made in the laboratory was shown as a reliable tool for determining drying kinetics.

Experimental Study of the Drying Kinetics of Mango (&lt;i&gt;mangifera indica L.&lt;/i&gt;) during Airflow Drying Licking Countercurrent

This work was conducted to study the thin-film drying kinetics of mango (mangifera indica L.) using a modular electrical dryer licking countercurrent and forced convection. Drying experiments were carried out in the Laboratory of Energetics and Applied Thermal (LETA) of the National School of Agro-Industrial Sciences of the University of Ngaoundere under different conditions of drying air temperature (40, 50 and 60°C) for a speed 0.6 m/s. Four trays were used to spread the products in the dryer. To estimate and select the appropriate drying model, ten different models were applied to the experimental data and compared. The performances of these models were compared using the R², the χ² and the RMSE between the observed and predicted moisture ratios values, which ranged from 0.925 to 0.999; 4.10-6 - 0.00540 and 0.0021 - 0.0736 respectively. Among the models used, the Midilli et al., model was found to best explain the thin-film drying airflow drying licking countercurrent of mango.

A systematic analysis on tomato powder quality prepared by four conductive drying technologies

Abstract Four pilot-scale conductive dryers, namely a vacuum drum dryer (VDD), a drum dryer (DD), an agitated thin film dryer (ATFD) and a refractance window dryer (RWD), were used to dry tomato puree. Drying induced colour differences between the reconstituted puree and the original puree and strongly affected the volatile and non-volatile profiles of the powders. Principal component analysis (PCA) identified four separated groups corresponding to the different drying methods, indicating that the drying methods caused significant variance in compound profiles. Subsequently, pairwise comparison of different dried powders was performed by partial least square discriminant analysis (PLS-DA). This resulted in a selection of discriminative volatile and non-volatile markers. RWD and VDD produced powders with high volatile markers that may be related to aroma retention. Conversely, DD dried products contained more non-volatile markers that can be related to taste perception. ATFD processed powders had a lower level of discriminant compounds. Industrial relevance Tomato products are frequently thermally processed and dehydrated. However, processing negatively affects the sensory quality of tomato products. In this study, four conductive drying processes, i.e. vacuum drum drying (VDD), drum drying (DD), agitated thin film drying (ATFD) and refractance window drying (RWD) were studied for being energy-efficient drying methods, while suitable for mild (e.g. due to the reduced pressure) drying of pastes and slurries, such as tomato puree. The pilot-scale drying experiments and subsequent statistical analyses of results on quality markers contributed to unravel the impact of the different conductive drying technologies on tomato powder quality. This study may be considered a starting point for selection of conductive drying technologies for the efficient production of high quality tomato powders and other vegetable powders.

An investigation into evaporative ink drying process on forced convective heat and mass transfer under impinging air jets

Impinging air jets are widely used in industry for drying, cooling and heating due to high heat and mass transfer coefficients which are developed in the impingement region. High velocity heated air impinging jets have proved to be very effective to enhance evaporative ink drying especially on the rotary presses for the packaging and converting industry. In this paper, to provide data for ink dryer designers, an investigation has been carried out to obtain the heat and mass transfer coefficients which constitute the evaporative drying of thin ink films. Finally, the theoretical results are compared for the constant rate period and falling rate period with some experimental and theoretical results found to be satisfactory particularly for the drying time.

Refractance window drying of fruits and vegetables: a review.

Fruit and vegetable drying to make leather, pestil or powder products is a challenging task. Traditional drying adversely affects the taste, colour, nutritional qualities and preservation of bioactive compounds due to high-temperature exposure of the product. The substitute for traditional drying processes is consequently necessary to impart superior quality and preserve greater nutritional value in processed foods. Refractance window (RW) drying is a thin film drying system having high heat and mass transfer rates that speeds up the rate of drying. Polyester (Mylar), an infrared transparent plastic sheet transmits radiative heat to food during the RW drying process with higher drying rate, more retention of nutrients and low aroma and flavour loss. RW drying is reported to be affected by the thickness of puree or slices and temperature of drying, as thick samples need longer drying time, generate higher water activity, bring changes in colour parameters, experience higher effective diffusion coefficient (Deff ) and result in lower solubility of powder. RW drying was compared with different drying processes and was observed to have more retention of nutrients and be more economical. RW dried samples were better in terms of colour and textural quality when compared with other traditional drying methods. © 2018 Society of Chemical Industry.

Agitated thin-film drying of foods

Agitated thin-film drying (ATFD) has been proposed for efficient and mild drying of viscous liquid foods, pastes or pureed foods. We report a study on the influence of product and process parameters on ATFD. During ATFD of spinach leaf slurries, the wall temperature mainly affected the specific evaporation rate, while the absolute evaporation rate was proportional to the feed rate. The fact that blade rotation speed had limited effect on the drying rate suggested that the process is limited by heat transfer through the wall. ATFD is especially suited for slurries that show relatively limited sticky behavior during drying and liquid–solid phase transition with corresponding brittle viscoelastic behavior. This was demonstrated by drying juices from tomato and bell pepper, giving poor results, and by drying solutions from whey protein isolate (WPI) and sucrose, which could be successfully dried.

Drying of multicomponent thin films on substrates with topography

ABSTRACTDrying of multicomponent thin liquid films is an important step in coating and printing processes. In many cases, the substrate on which the film rests may possess topography, either intended or unintended. We present a lubrication‐theory‐based model describing the fundamentals of drying on such substrates. The film consists of volatile solvent and additional nonvolatile components such as colloidal particles, surfactants, and nonvolatile solvents. A system of one‐dimensional partial differential equations accounting for the film height, depth‐averaged concentration of bulk nonvolatile components, and interfacial concentration of insoluble surfactant is derived. Evaporation is included using the well‐known one‐sided description, and the governing equations are solved with finite‐difference methods to study various limiting cases. The results highlight the influence of evaporation rate, and thermal, surfactant, and solutal Marangoni flows on the film thickness and colloidal particle distribution. We find that, in a realistic region of parameter space, the addition of a nonvolatile solvent yields a film that conforms to the substrate topography. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1681–1691

Hydrodynamic effects on phase separation morphologies in evaporating thin films of polymer solutions

We examine effects of hydrodynamics on phase separation morphologies developed during drying of thin films containing a volatile solvent and two dissolved polymers. Cahn-Hilliard and Flory-Huggins theories are used to describe the free energy of the phase separating systems. The thin films, considered as Newtonian fluids, flow in response to Korteweg stresses arising due to concentration non-uniformities that develop during solvent evaporation. Numerical simulations are employed to investigate the effects of a Peclet number, defined in terms of system physical properties, as well as the effects of parameters characterizing the speed of evaporation and preferential wetting of the solutes at the gas interface. For systems exhibiting preferential wetting, diffusion alone is known to favor lamellar configurations for the separated phases in the dried film. However, a mechanism of hydrodynamic instability of a short length scale is revealed, which beyond a threshold Peclet number may deform and break the lamellae. The critical Peclet number tends to decrease as the evaporation rate increases and to increase with the tendency of the polymers to selectively wet the gas interface. As the Peclet number increases, the instability moves closer to the gas interface and induces the formation of a lateral segregation template that guides the subsequent evolution of the phase separation process. On the other hand, for systems with no preferential wetting or any other property asymmetries between the two polymers, diffusion alone favors the formation of laterally separated configurations. In this case, concentration perturbation modes that lead to enhanced Korteweg stresses may be favored for sufficiently large Peclet numbers. For such modes, a second mechanism is revealed, which is similar to the solutocapillary Marangoni instability observed in evaporating solutions when interfacial tension increases with the concentration of the non-volatile component. This mechanism may lead to multiple length scales in the laterally phase separated configurations.

Modeling spray drying of dairy products – Impact of drying kinetics, reaction kinetics and spray drying conditions on lysine loss

Lysine, an important nutrient in dairy formulations, is easily damaged during spray drying. The quality of spray dried dairy powders can be enhanced when lysine loss kinetics are coupled with drying kinetics and computational fluid dynamics of spray. Thin-film drying in a convective drying channel was applied to study the drying kinetics of the model dairy-formulation. The drying kinetics was modeled with the reaction engineering approach which was successfully implemented in the computational fluid-dynamics (CFD) simulation of spray drying. Lysine loss was modeled by reaction kinetics taking into account the temperature, water content and physical state of lactose. A 3D CFD model of spray drying was set up to determine the particle properties along their residence time. The model was validated with the experimental results of spray drying on a pilot scale. A good agreement between the experiments and the simulation was obtained. The reaction kinetics model was coupled with the particle properties along the particle tracks to create a predictive tool for the lysine loss. Thus, as it was hypothesized from the spray-drying experiments, the importance of the particle residence-time was highlighted. The following optimization study revealed potential strategies to ameliorate the spray drying process. In this context, the dryer geometry, air flow pattern and droplet atomization should be reconsidered carefully. The proposed approach can be transferred to other spray drying applications, e.g. for the targeted formation of microcapsules or to enhance the survival of microorganisms.

Local heat transfer characteristics of a slot nozzle array for batch drying of thin films under industrial process conditions

For optimizing the drying process of thin functional films, knowledge of the relevant influencing parameters is needed. One paramount parameter for design of experimental conditions and the accurate interpretation of experimental data is the heat transfer coefficient inside a dryer. Information on the heat transfer in a dryer also facilitates a reliable transfer of results obtained in laboratory set-ups to industrial production. In this work, the heat transfer in a batch drying set-up with an array of slot nozzles is investigated. The measurement arrangement consists of a heat flux sensor mounted on the surface of a temperature-controlled plate. For validation of this arrangement, heat transfer measurements with a single round nozzle were compared to transient heat transfer experiments using thermochromic liquid crystals. The data measured in the batch drying set-up show a periodically homogenous distribution of heat transfer coefficients for the investigated 16 nozzles. This gives the possibility to dry thin films with a coating length of up to 800 mm in the set-up at constant conditions along the coating length. A prerequisite is a periodical movement with an amplitude of at least one nozzle-to-nozzle spacing.

Viscous flow separation caused by the Marangoni effect in competition with capillary flow

This letter provides an analytical model that describes the viscous flow separation in a drying thin polymer solution film. While conventional flow separation occurs due to an adverse pressure gradient opposing fluid inertia, here we show that flow separation can also be caused by the Marangoni effect in an evaporation-driven creeping flow. The flow separation behavior strongly depends on the surface tension distribution and the interface profile. Viscous flow separation adds to the current understanding of flow physics during the drying of thin films, in addition to the well-known capillary transport and the Marangoni effect.

Three-dimensional patterns from the thin-film drying of amino acid solutions.

Experimental atomic force microscopy (AFM) images show the dried-in patterns from amino acid solutions which can be in the form of dots or networks. The three-dimensional lattice-gas Kinetic Monte Carlo (KMC) model is applied to simulate the formation of dot-like and network-like particle structures from the evaporating thin films of solutions. A sigmoidal jump in the chemical potential value is implemented to obtain dual-scale structures with the grain size distribution peaking at two distinctive values. The simulated and experimental results are qualitatively comparable.

Fast drying and film formation of latex dispersions studied with FTIR spectroscopic imaging.

Drying of thin latex films (∼20 μm) at high drying speeds (of the order of seconds) has been studied by fast chemical imaging. ATR-FTIR spectroscopic imaging combined with a fast "kinetic" mode was used to acquire spectral images without coaddition, enabling the amount of water and homogeneity of the drying film to be studied over time. Drying profiles, constructed from analyzing the water content in each image, show two stages of drying, a fast and a slow region. The formulation of latex dispersions affects the onset of slow drying and the volume fraction of water remaining at the onset of slow drying. In this work, the effect of physical properties, film thickness and glass transition temperature (Tg), were investigated, as well as the effect of monomer composition where two monomoers, 2-ethylhexyl acrylate and n-butyl acrylate, and the amount of hydrophilic comonomer, methyl methacrylate (MMA), were varied. It was found that thicker films produced slower overall drying and that the formulation with a Tg above the minimum film formation temperature did not dry evenly, exhibiting cracking. However, the drying kinetics of high and low Tg films were similar, highlighting the advantage of using a spatially-resolved spectroscopic approach. Formulations containing more MMA dried faster than those with less. This was due to the hydrophilicity of MMA and the increase in Tg of the dispersion from the addition of MMA. Overall, FTIR spectroscopic imaging was shown to be a suitable approach in measuring film drying at high speeds as both chemical changes and chemical distribution could be analyzed over time.

Nonlinear Model-Based Control of Thin-Film Drying for Continuous Pharmaceutical Manufacturing

This paper considers the model-based control of composition and thickness for a thin-film drying process used in the continuous manufacturing of pharmaceutical tablets. In this nonlinear distributed dynamical system, a drug formulation solution is coated onto a moving surface and then dried to form thin films of approximately 250 μm in thickness. A dynamic optimizer is designed that employs a first-principles process model to simulate the spatial distribution of solvent concentration in the film and the thin film shrinkage during drying. A critical parameter to describe the highly nonlinear dynamics of the thin-film drying is the mutual polymer–solvent diffusion coefficient, which strongly depends on solvent concentration and film temperature. Two optimal control problems are studied for set point tracking of solvent concentration and minimization of energy consumption in the dryer while satisfying various operational and product quality constraints. An unscented Kalman filter is designed to facilitate the output feedback implementation of the dynamic optimizer and to estimate unmeasured thin-film quality attributes such as the film thickness. The performance of the model-based controller is compared to that of a proportional integral controller in two simulation case studies. The nonlinear model-based control strategy has improved versatility and the potential to reduce production of off spec material.

Measuring and modelling of diffusivities in carbohydrate-rich matrices during thin film drying

Knowledge about moisture diffusivity in solid matrices is a key for understanding drying behaviour of for example probiotic or enzymatic formulations. This paper presents an experimental procedure to determine moisture diffusivity on the basis of thin film drying and gravimetric analysis in a Dynamic Vapour Sorption (DVS) system. The extraction of moisture diffusivity is based on the “regular regime approach”. The method was explored and verified for its assumptions. It provided insight in the effect of moisture content and temperature on moisture diffusivity. Moreover, it was found that moisture diffusivity in different carbohydrate systems was similar and decreased with moisture content. The latter was explained by similar molecular interactions in carbohydrate systems and formation of a percolating network at low moisture content that affects water mobility. Subsequently, measured moisture diffusivities were compared to model predictions based on the generalised Darken relation. It was found that predicted moisture diffusivities were in fair agreement with these, including the effect of moisture content and temperature on moisture diffusivity. At low moisture content the model overestimated the sensitivity of moisture diffusivity towards temperature. This was explained by the fact that the different water–solid interactions at lower moisture content (including relaxation behaviour in the glassy state) are not considered in the modelling. Finally, the methodology was successfully evaluated to other solid matrices such as glycerol, skimmed milk and casein, providing different moisture diffusivities as function of moisture content.

Investigation of surface deformation during drying of thin polymer films due to Marangoni convection

The investigation of surface deformation of thin polymer films (methanol–poly(vinyl acetate) solution with 67wt.% methanol) due to inhomogeneous drying is presented. Of interest are the onset of fluid instability and the influence of drying kinetics in a system where surface tension depends on temperature and composition. An experimental method is used that correlates a virtual shift of a random dot pattern, located on the downside of a glass substrate, to a deformation of the free surface of a transparent film. Instability in the fluid is forced over a structured composite substrate with inhomogeneous thermal conductivity. We show that due to the inhomogeneous heat transfer a long-wave instability mode is triggered resulting in directed mass transport. To evaluate the driving forces, 1D simulations of the non-isothermal drying process have been performed. Furthermore the influence of drying kinetics on the developing surface topology is discussed.

New methods to assess water diffusion in amorphous matrices during storage and drying

Water diffusion coefficient is an important parameter for food processing and stability, strongly influenced by the composition and the physical state of the matrix (amorphous glassy or rubbery/crystalline). In this study, two different methods to assess water diffusion in amorphous matrices were compared. First, an automatic sorption device was used to measure the water sorption kinetics of powders of maltodextrin DE21 and DE47 with defined shapes when equilibrating at different air relative humidity (0–60% at 25°C). Calculation of water diffusion coefficients was done according to Crank’s and Weibull’s models, and it was not possible when powders undergo glass transition. In the second part of the study a thin film dryer was used to assess the drying kinetics of aqueous solutions of maltodextrin (DE 6-12-29-40). The Yamamoto’s model allowed calculating water diffusion as a function of water content and temperature, above and below glass transition temperature.

Mathematical Modeling of Agitated Thin-Film Dryer

Agitated thin-film dryers (ATFDs) are used to produce dry free-flowing powder from slurry/solution-type feed and widely implemented in pharmaceutical, chemical, and food industries. The feed passes through the ATFD in several forms such as solution/slurry and successively becomes paste, wet powder, and dry powder. The flow of feed in the ATFD undergoes a helical path (combination of rotational velocity imparted by the agitator/blade and axial velocity of feed) while flowing through the annular part of the dryer. The ATFD is described stage-wise and the parameters such as physical properties, scraped surface heat transfer coefficient, and evaporation rate (drying rate) are derived using stepwise model equations. The penetration theory is modeled to obtain the scraped-side heat transfer coefficient. The model equations were solved using MATLAB 7 (MathWorks Inc., Natick, MA) and the simulated drying rate was consequently validated with the experimental values.