Mass Transfer

Sediment-water fluxes of hydrophobic organic chemicals (HOC) may affect the quality of surface waters. Here, we present an approach to derive such fluxes from (a) in situ HOC concentration gradients measured with passive samplers and (b) mass transfer coefficients measured with a novel flux method using Empore disks. For eight undisturbed sediments, this method identified whether the sediment acted as a source or as a sink for HOCs. The analysis also identified which type of transport resistance governed sediment water exchange. For seven inland locations, exchange was limited by benthic boundary layer transport, showing no dependencies on sediment or chemical properties other than concentration. For one river mouth location, exchange was limited by slow in-bed intraparticle diffusion. A biphasic dual compartment radial diffusion model adequately described the data for this location. Fast desorption was interpreted as molecular diffusion retarded by microscale dual domain sorption to amorphous as well as black carbon (BC). Slow desorption was invariant with LogK(ow) and consistent with intraorganic matter diffusion through BC particles. Finally, it is discussed how these findings can be translated into a general framework for flux based exposure assessment.

The effect of different pre-treatments (i.e. osmotic dehydration in 10, 15 and 20°Brix NaCl solution and drying air temperature of 50, 60 and 70°C) on drying behaviour of onion slices were investigated. The onion slices were dried in a laboratory model tray dryer. Drying of onion slices occurred in falling rate period. Five thin-layer drying models (Exponential, Page, Henderson and Pabis, Logarithmic and Power law) were fitted to the moisture ratio data. Among the drying models investigated, the Page model satisfactorily described the drying behaviour of onion slices. The effective moisture diffusivity of pre-treated samples was higher than that of non-treated samples

Abstract: Two dimensional free convection and mass transfer flow of an incompressible, viscous and electrically conducting fluid past a continuously moving infinite vertical porous plate in the presence of heat source, thermal diffusion, large suction and under the influence of uniform magnetic field applied normal to the flow with variable viscosity and thermal conductivity is studied. Usual similarity transformations are introduced to reduce the governing partial differential equations to ordinary differential equations. The momentum, energy and concentration equations are solved numerically using shooting method. The results are presented in graphs and tables to observe the effect of various parameters. It is observed that velocity decreases as viscosity parameter increases.

The influence of amylose content on water uptake and solubility behaviors and percent degree of gelatiniza-tion of waxy (Amioca), normal and high amylose (Hylon VII) corn starches were investigated. Gelatiniza-tion was also followed by differential scanning calorimetry and the transition temperatures and enthalpies were determined. The results were discussed in relation to underlying mass transfer processes during ge-latinization. It was determined that water uptake and solubility of starches decreased as the amylose content increased. Amioca exhibited the highest water uptake, attributed to its low amylose content. The water uptake and solubility of Hylon VII were low at all temperatures. The highly associated structure of Hylon VII and also the presence of lipid complexes were considered as the factors influencing its gelatinization behavior. Therefore it was concluded that the mass transfer processes for gelatinization was mainly influenced by free amylose for network formation rather than the proportion of amylose in starch. Özet Bu çalışmada mumsu (Amioca), normal ve yüksek amiloz içerikli (Hylon VII) mısır nişastalarının su kazan-cı ve çözünürlük davranışları ile jelatinizasyon derecelerine amiloz içeriğinin etkisi araştırılmıştır. Jelatini-zasyon diferansiyel taramalı kalorimetri yöntemi ile de incelenmiş, geçiş sıcaklıkları ve entalpileri belirlen-miştir. Jelatinizasyon süresince meydana gelen değişiklikler kütle aktarımı yaklaşımıyla değerlendirilmiş-tir. Nişastadaki amiloz miktarı azaldıkça su alma ve çözünürlük miktarının azaldığı belirlenmiştir. En yük-sek su alma Amioca da izlenmiş ve bu durum düşük amiloz içeriği ile ilişkilendirilmiştir. Hylon VII de göz-lenen molekül içi assosiyasyon ve lipit varlığının jelatinizasyon davranışında belirleyici rol oynadığı izlen-miştir. Elde edilen sonuçlar ışığında, jelatinizasyon sırasındaki kütle aktarım olaylarının nişastadaki amiloz oranından çok ağ yapıda yer alabilen serbest amiloz miktarından etkilendiği sonucuna varılmıştır.