Vapour retarders in wood frame walls and their effect on the drying capability

Wood frame walls typically need a vapour barrier at the warm side to avoid interstitial condensation due to vapour diffusion and air leakages from the interior. A more vapour open material than the traditional vapour barriers, here called vapour retarder, could allow condensed moisture, built-in-moisture or moisture from minor leakages to dry to the interior in addition to the outward drying. The application of permeable vapour retarders in wood frame walls have been investigated in this study by the use of a hygrothermal simulation tool. A traditional wood frame wall usually has good drying possibilities to the exterior. If a vapour etarder should have an effect on the total drying, it must not be too vapour tight. The purpose of this study was to find some threshold value for the maximum vapour resistance of a vapour retarder—when arequirementisthatitshouldhavearelativelylargeeffectofthetotaldrying of the wall. The increased risk for condensation as the vapour resistance decreases has however not been investigated in this study. In general it was found that permeable vapour barriers have relatively little effect on the total drying of ordinary wood frame walls in a Nordic climate.     

Dependence of the effective diffusion coefficient of moisture with thickness and temperature in convective drying of sliced materials. A study on slices of banana, cassava and pumpkin

Several studies have indicated that effective diffusion coefficients of slices apparently vary with the thickness of the samples. Even though the effective diffusion coefficients have been observed to be dependent on the square of the slice thickness, a theoretical explanation to this behavior is not available to date. A theoretical model is formulated herein, in order to correlate effective diffusion coefficient of moisture in the slices with the slice thickness. Experiments are carried out for drying of slices of different thicknesses of banana, cassava and pumpkin in order to evaluate the effective diffusion coefficients. The model is found to describe the variation of the effective diffusion coefficient with slice thickness very satisfactorily. A possibility of the estimation of the axial and radial diffusion coefficients of moisture in the slices is also outlined.     

Effects of vacuum level and microwave power on rosemary volatile composition during vacuum-microwave drying

The influence of the vacuum level and the microwave power on aroma compounds and sensory quality of Rosmarinus officinalis dehydrated by vacuum-microwave (VM) method was evaluated. VM drying kinetics consisted of two periods: linear until a critical point and exponential beyond that point. The time needed to dry rosemary was shorter for high values of microwave power and vacuum intensity. Volatile compounds of rosemary samples were extracted by steam-hydrodistillation and analyzed by GC-MS. Thirty-one compounds were identified, with α-pinene, verbenone and 1,8-cineole being the major components; monoterpenoids were the predominant chemical family. The total quantity of volatiles of fresh rosemary (27.2 g kg⁻¹) decreased during drying, independently of treatments, to a mean of 14.8 g kg⁻¹. Soft conditions of VM drying (low vacuum level and microwave power) are recommended to get the highest concentrations of volatiles and the best sensory quality (e.g. 72-74 kPa and 360 W).     

Supercritical carbon dioxide and solvent extraction of 2-acetyl-1-pyrroline from Pandan leaf: The effect of pre-treatment

Pandan (Pandanus amaryllifolius Roxb.) leaf is a source of natural flavoring widely used in South-east Asia. The major compound contributing to the characteristic flavor of Pandan is 2-acetyl-1-pyrroline (2AP). This highly volatile compound also contributes significantly to the flavor of aromatic rice such as basmati and jasmine rice. As the consumer requirement for use of natural flavors, extraction of components from natural sources has been sought. In this study, supercritical carbon dioxide (SC-CO₂) and solvent extraction of components from Pandan leaves were performed. Experimental parameters included particle size and drying method (oven and freeze drying). Results indicated that the initial value of moisture content and particle size of Pandan leaves had the greatest effect on the total yield and 2AP concentration of the extracts. Almost 80% of water in Pandan leaves can be removed by drying. Yields of supercritical extracts were 10 times lower when compared to the hexane extracts. The total yield of extracts was increased up to 50% with decreasing particle size of Pandan leaves. Extraction of coarsely ground freeze-dried Pandan leaves by SC-CO₂ obtained the highest yield (0.88 ± 0.06%) followed by oven dried (0.38 ± 0.09%) and fresh leaves (0.34 ± 0.01%). The 2AP was identified by GC-MS and analyzed by GC-FID. Supercritical and hexane extracts of pre-treated Pandan leaves were found to have a small quantity of 2AP ranging between 0.04 ± 0.01 and 0.45 ± 0.01 ppm. Grinding pre-treatment was the best method for both SC-CO₂ and hexane extractions while the freeze drying method was the best for SC-CO₂.     

A star-function based density functional study of the adsorption of Lennard-Jones fluid near its supercritical states

The goal of this paper is to show that the wetting behavior of simple fluids on a repulsive solid surface - especially the “drying” phenomenon - is closely related to the proximity of the supercritical state of the bulk fluids to their vapor-liquid coexistence region. We propose here a new DFT (the star-function based density functional theory, s-DFT) that is based on the functional Taylor expansion of the intrinsic free energy F[ρ] and the singlet direct correlation to arrive at closed-form expressions for both quantities without truncations or approximations. The two formulas are mutually consistent because of the introduction of a star function Sw that has been shown to be the functional primitive of the bridge function Bw, i.e. (L.L. Lee, J. Chem. Phys. 97 (1992) 8606 [34]). The new formulation is applied to the Lennard-Jones molecules adsorbed on a planar hard wall (LJ/HW). We carried out new Monte Carlo simulations for this system. Since the s-DFT uses a bridge function Bw, we demonstrate (i) the existence of a set of data (inverted from the MC information) that can perform as the bridge function and reproduce accurately the density profiles ; (ii) this set of data can be “fitted” by a function-form with acronym ZSEP; finally (iii) ZSEP expresses the bridge function Bw in terms of a new renormalized basis function γ_H, i.e. Bw(γH). The existence of a bridge function dispels some of the misconceptions that the bridge-function based formulations did not describe the “drying” behavior. We also show that for the high density case ZSEP equation can qualify as a “closure relation”, but seems to deteriorate for the two low-density supercritical states that are close to the bulk saturated liquid phase boundaries.     

Unsteady RANS and detached eddy simulation of the multiphase flow in a co-current spray drying☆

A detached eddy simulation (DES) and a k-ε-based Reynolds-averaged Navier–Stokes (RANS) calculation on the co-current spray drying chamber is presented. The DES used here is based on the Spalart–Al maras (SA) turbu-lence model, whereas the standard k-ε(SKE) was considered here for comparison purposes. Predictions of the mean axial velocity, temperature and humidity profile have been evaluated and compared with experimental measurements. The effects of the turbulence model on the predictions of the mean axial velocity, temperature and the humidity profile are most noticeable in the (highly anisotropic) spraying region. The findings suggest that DES provide a more accurate prediction (with error less than 5%) of the flow field in a spray drying chamber compared with RANS-based k-εmodels. The DES simulation also confirmed the presence of anisotropic turbulent flow in the spray dryer from the analysis of the velocity component fluctuations and turbulent structure as il us-trated by the Q-criterion.     

On the possibility of using combined polymerization and drying in synthesis of polyacrylamide

This paper presents investigation results of polyacrylamide synthesis with combined polymerization and drying. First, polyacrylamide synthesis was carried out in concentrated aqueous solutions of acrylamide at 30°C. Polymerization was initiated by K₂S₂O₈/Na₂S₂O₃ an oxidation–reduction system. The monomer conversion degree at this stage did not exceed 80%. Further, polymerization with simultaneous solvent elimination from the prepolymer was carried out in a dryer with infrared heat supply and natural convection. Such conditions provide monomer conversion degree up to 100%.     

Dewatering of spontaneous-coagulation-cast alumina ceramic gel by filtrating with low pressure

Water removal in colloidal processing is a tough and crucial problem because deformation and cracking of the wet body easily happen during the drying process. Inspired by conventional pressure filtration (PF), we combined spontaneous coagulation casting (SCC) with PF to partially remove water from the gelled sample before drying for the purpose of lessening the risk of cracking and shortening the drying period. The effect of pressure, holding time, and solid loading on the particle packing ratio of wet samples, the drying process, and the bulk density and homogeneity of green and sintered bodies was surveyed systemically. The PF process of 0.4 MPa for 2 h was optimized based on dewatering ratio from ceramic slurries. The particle packing ratio of the pressure-filtrated samples increased with the solid loading of ceramic slurries, and 61 vol% was obtained for the slurry with 56 vol% solid loading. The drying time of the pressure-filtrated samples was shortened by as much as 42%–53% compared with that of the gelled samples, and the linear shrinkage for the pressure-filtrated samples during drying was only approximately 0.5%, much smaller than that of the gelled samples (2.4%–4.6%). Furthermore, the bulk density of the pressure-filtrated green bodies was more than 2% higher than that of the gelled green bodies, and more homogeneous green bodies were obtained by PF than by SCC. The alumina ceramic with 98.8% relative density and a homogenous density distribution was obtained after being sintered at 1550 °C for 2 h.     

Performance analysis of solar-assisted chemical heat-pump dryer

A solar-assisted chemical heat-pump dryer has been designed, fabricated and tested. The performance of the system has been studied under the meteorological conditions of Malaysia. The system consists of four main components: solar collector (evacuated tubes type), storage tank, solid-gas chemical heat pump unit and dryer chamber. A solid-gas chemical heat pump unit consists of reactor, condenser and evaporator. The reaction used in this study (CaCl2-NH₃). A simulation has been developed, and the predicted results are compared with those obtained from experiments. The maximum efficiency for evacuated tubes solar collector of 80% has been predicted against the maximum experiment of 74%. The maximum values of solar fraction from the simulation and experiment are 0.795 and 0.713, respectively, whereas the coefficient of performance of chemical heat pump (COP^h) maximum values 2.2 and 2 are obtained from simulation and experiments, respectively. The results show that any reduction of energy at condenser as a result of the decrease in solar radiation will decrease the coefficient of performance of chemical heat pump as well as decrease the efficiency of drying.     

Experimental studies on hemi cylindrical walk-in type solar tunnel dryer for grape drying

A walk-in type hemi cylindrical solar tunnel dryer has been built with heat protective north wall at College of Dairy and Food Science Technology, Udaipur, India for drying agricultural & horticulture product on large scale. In this paper attempt has been made to evaluate the performance of developed dryer to dry the seedless grapes (mutant:Sonaka). The study show that chemically untreated grapes took seven days to dry at 16% (wb) moisture content. The temperature gradient inside the tunnel dryer is about 10–28 °C during the clear day, which is quite sufficient to dry agricultural commodities.