Convective absorption

Convective Absorption. Compounds with molecular radii of less than 4 nm can pass through pores in the gut membrane. The membrane exhibits a molecular sieving effect. 

Nomura, T., Nishimura, N., Wei, S., Yamaguchi, S., and Kawakami, R. (1994) Heat and Mass Transfer Mechanism in the Absorber of Water/LiBr Convectional Absorption Refrigerator Experimental Examination by Visualized Model, Proceedings of the International Absorption Heat Pump Conference, Jan 19-21 1994, New Orleans, LA, USA, ASME, New York, NY, USA, pp. 203-208. 

The prevalent view is that the gastrointestinal membrane consists of a bimolecular lipoid layer that is covered on each side by protein with the lipid molecule oriented perpendicular to the cell surface (Fig. 9.4). The lipid layer is interrupted by small, water-filled pores with a radius of approximately 4 A, and a molecule with a radius of 4 A or less may pass through these water-filled pores. Thus, membranes have a specialized transport system to assist the passage of water-soluble material and ions through the lipid interior, a process sometimes termed to as convective absorption. The rate of permeation of such small molecules through the pore is affected not only by the relative sizes of the holes and the molecules but also by the interaction between permeating molecules... 

The absorption of small molecules (molecular radii less than 4 A) through water-filled pores of biological membrane is referred to as convective absorption. The rate of absorption because of this mechanism is equated to the product of a sieving coefficient, the rate of fluid or water absorption, and the concentration of solute in the luminal content. The sieving coefficient is indirectly related to the relative sizes of the pores and the molecules. 

The problem, in this case, is to make a realistic choice for the difference between the wall temperature and the temperature of the air in the tank. Since the pressure drop which causes tank collapse occurs very rapidly, the air in the tank will not have much time to cool. For AT = 20°C the equivalent droplet mass evaporation is 1 mole/sec. In any case it appears that mi 2 ccoupled with mi 2 6 will be most competitive with conduction and convection absorption of heat by droplets from the surrounding air. 

Extreme weather clothing is usually made up of a number of layers with a still air layer on their surfaces and possibly mobile air layers on top of those. Detailed models take into account dry and wet heat transfer. Dry heat transfer tales place by conduction, convection and radiation, the latter two being most important. Wet heat transfer results fix)m sweating. Water moisture on the surface of the skin evaporates remove large amoimts of heat from the body. The water vapor molecules are transported to the environment in various ways including diffusion, convection, absorption (adsorption), desorption and condensation. Often, finite element modeling is used for the simulations in which the clothing and air layers are divided into a matrix of elements and heat and mass transport. 

Burley and Wai" presented a model to describe the dye transfer in a package dyeing system. The mechanisms of dispersion, convection, absorption and desorption of dye to and from the packed bed of fibres were included. The governing equations describing the distribution of dye in the packed bed consist of a pair of differential equations ... 

Heat Transfer in Rotary Kilns. Heat transfer in rotary kilns occurs by conduction, convection, and radiation. In a highly simplified model, the treatment of radiation can be explained by applying a one-dimensional furnace approximation (19). The gas is assumed to be in plug flow the absorptivity, a, and emissivity, S, of the gas are assumed equal (a = e ) and the presence of water in the soHds is taken into account. Energy balances are performed on both the gas and soHd streams. Parallel or countercurrent kilns can be specified. 

Ordinary diffusion involves molecular mixing caused by the random motion of molecules. It is much more pronounced in gases and Hquids than in soHds. The effects of diffusion in fluids are also greatly affected by convection or turbulence. These phenomena are involved in mass-transfer processes, and therefore in separation processes (see Mass transfer Separation systems synthesis). In chemical engineering, the term diffusional unit operations normally refers to the separation processes in which mass is transferred from one phase to another, often across a fluid interface, and in which diffusion is considered to be the rate-controlling mechanism. Thus, the standard unit operations such as distillation (qv), drying (qv), and the sorption processes, as well as the less conventional separation processes, are usually classified under this heading (see Absorption Adsorption Adsorption, gas separation Adsorption, liquid separation). 

Influence of Chemical Reactions on Uq and When a chemical reaction occurs, the transfer rate may be influenced by the chemical reac tion as well as by the purely physical processes of diffusion and convection within the two phases. Since this situation is common in gas absorption, gas absorption will be the focus of this discussion. One must consider the impacts of chemical equilibrium and reac tion kinetics on the absorption rate in addition to accounting for the effec ts of gas solubility, diffusivity, and system hydrodynamics. 

In order to perform effectively as an insulant a material must restrict heat flow by any (and preferably) all three methods of heat transfer. Most insulating materials adequately reduce conduction and convection elements by the cellular structure of the material. The radiation component is decreased by absorption into the body of the insulant and is further reduced by the application of bright foil outer facing to the product. 

A similar situation arises when a vertical metal plate is partly immersed in an electrolyte solution (Fig. 1.48c), and owing to differential aeration the upper area of the plate will become cathodic and the lower area anodic. With time the anodic area extends upwards owing to the mixing of the anolyte and catholyte by convection and by the neutralisation of the alkali by absorption of atmospheric carbon dioxide. 

Surfaces will absorb radiant heat and this factor is expressed also as the ratio to the absorptivity of a perfectly black body. Within the range of temperatures in refrigeration systems, i.e. - 70°C to + 50°C (203-323 K), the effect of radiation is small compared with the conductive and convective heat transfer, and the overall heat transfer factors in use include the radiation component. Within this temperature range, the emissivity and absorptivity factors are about equal. 

Emmert and Pigford (E2) have studied the reaction between carbon dioxide and aqueous solutions of monoethanolamine (MEA) and report that the reaction rate constant is 5400 liter/mole sec at 25°C. If it is assumed that MEA is present in excess, the reaction may be treated as pseudo first-order. This pseudo first-order reaction has been recently used by Johnson et al. (J4) to study the rate of absorption from single carbon dioxide bubbles under forced convection conditions, and the results were compared with their theoretical model. 

An attempt has been made by Johnson and co-workers to relate such theoretical results with experimental data for the absorption of a single carbon dioxide bubble into aqueous solutions of monoethanolamine, determined under forced convection conditions over a Reynolds number range from 30 to 220. The numerical results were found to be much higher than the measured values for noncirculating bubbles. The numerical solutions indicate that the mass-transfer rate should be independent of Peclet number, whereas the experimentally measured rates increase gradually with increasing Peclet number. The discrepancy is attributed to the experimental technique, where-... 

Both pH and the availability of nutrient ions in soil play important roles in rhizo-sphere dynamics and are often dependent on one another. Nutrient ions move in soil toward plant roots either by mass flow with the soil water or by diffusion. Mass flow is the result of bulk convective movements of the soil solution toward roots, whereas diffusion occurs in response to a concentration gradient for a particular ion, which results from its absorption by the root and depletion from the... 

Mass transfer phenomena exist everywhere in nature and are important in the pharmaceutical sciences. We may think of drug synthesis preformulation studies dosage form design and manufacture and drug absorption, distribution, metabolism, and excretion. Mass transfer plays a significant role in each. Mass transfer is referred to as the movement of molecules caused not only by diffusion but also by convection [1],... 

Membrane diffusion illustrates the uses of Fick s first and second laws. We discussed steady diffusion across a film, a membrane with and without aqueous diffusion layers, and the skin. We also discussed the unsteady diffusion across a membrane with and without reaction. The solutions to these diffusion problems should be useful in practical situations encountered in pharmaceutical sciences, such as the development of membrane-based controlled-release dosage forms, selection of packaging materials, and experimental evaluation of absorption potential of new compounds. Diffusion in a cylinder and dissolution of a sphere show the solutions of the differential equations describing diffusion in cylindrical and spherical systems. Convection was discussed in the section on intrinsic dissolution. Thus, this chapter covered fundamental mass transfer equations and their applications in practical situations. 

Fig. 1. Model Spectra re-binned to CRIRES Resolution To demonstrate the potential for precise isotopic abundance determination two representative sample absorption spectra, normalized to unity, are shown. They result from a radiative transfer calculation using a hydrostatic MARCS model atmosphere for 3400 K. MARCS stands for Model Atmosphere in a Radiative Convective Scheme the methodology is described in detail e.g. in [1] and references therein. The models are calculated with a spectral bin size corresponding to a Doppler velocity of 1 They are re-binned to the nominal CRIRES resolution (3 p), which even for the slowest rotators is sufficient to resolve absorption lines. The spectral range covers ss of the CRIRES detector-array and has been centered at the band-head of a 29 Si16 O overtone transition at 4029 nm. In both spectra the band-head is clearly visible between the forest of well-separated low- and high-j transitions of the common isotope. The lower spectrum is based on the telluric ratio of the isotopes 28Si/29Si/30Si (92.23 4.67 3.10) whereas the upper spectrum, offset by 0.4 in y-direction, has been calculated for a ratio of 96.00 2.00 2.00.

Another technique for studying the absorption of CO2 into water uses an interferometer to obtain the concentration gradients as close as 0.01 cm. to the surface (Bl) a cine-camera permits results to be obtained within 5 sec. of the admission of the CO2. Though various corrections are I c fiuired, it is claimed that this method eliminates convection difficulties and that resistances as low as 0.25 sec. cm. i can be detected. Experimental results for CO2 into distilled water show no detectable interfacial resistance, though, when surface-active agents (Lissapol, Teepol) are dissolved in the water, the values of Rj are about 35 sec. cm.. ... 

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