Countercurrent exchange

About: Countercurrent exchange is a research topic. Over the lifetime, 2255 publications have been published within this topic receiving 28687 citations. The topic is also known as: Countercurrent exchange.

Gas absorption in a hollow fiber device

Abstract: The absorption of S02 and NH3 from air and air/CO 2 streams was studied for the first time in a certain novel hollow fiber mass transfer device, for various inlet gas compositions, liquid compositions, gas flow rates, and liquid flow rates. The gas and liquid flows were countercurrent. Analyses of the amounts of S02and NH3 absorbed demonstrate that the hollow fiber unit has a relatively small membrane resistance and is an effective gas scrubbing device. Additionally, it offers a large interfacial area per unit volume, and avoids flooding problems entirely

Quantitation of countercurrent exchange during passive absorption from the dog small intestine: evidence for marked species differences in the efficiency of exchange.

Abstract: The present investigation was designed to quantitatively assess the possible influence of countercurrent exchange on passive absorption from the small intestine of the dog. Villus blood flow was measured with a modification of the microsphere method. Simultaneously, the absorption from the gut lumen of five diffusible gases (H2, He, CH4, 133Xe, and CO) was determined. Villus blood flow averaged 0.247 +/- 0.03 (SEM) ml/min per g. The observed absorption of H2, He, CH4, and 133Xe was only 16.2 +/- 1.8, 12.8 +/- 2.3, 12.0 +/- 1.8, and 15.8 +/- 1.4 %, respectively, of what this villus blood flow could carry away if it reached perfect equilibrium with the luminal gases. This low absorption rate could result from diffusion limitation to absorption or countercurrent exchange. The diffusive permeability of the barrier seperating the luminal gases and villus blood flow was assessed by measuring the absorption rate of CO. Because absorbed CO binds tightly to hemoglobin, it cannot exchange, and when present in low concentrations its uptake is entirely diffusion limited. Knowledge of the diffusion rate through tissue of the unbound gases relative to that of CO made it possible to calculate the degree to which each of the unbound gases should equilibrate with villus tip blood. The percentage equilibration between lumen and blood at the villus tip for H2, He, CH4, and 133Xe was 99.7, 99.9, 75.6, and 36.0% , respectively. Each of these values greatly exceeded the percentage equilibration of blood leaving the villus (calculated from the observed absorption rate and villus blood flow) and indicated an exchange of 83.8, 87.2, 84.1, and 56.1% of initially absorbed H2, He, CH4, and 133Xe. This result is in accord with theoretical calculations which suggest that countercurrent exchange should be exceedingly efficient in the dog. The striking effect of countercurrent exchange on passive absorption in the dog differs from our previous studies in the rabbit where no exchange was demonstrated. This marked species difference may result from anatomical differences in villus architecture. The dog has long, densely packed villi while the rabbit has broad, widely spaced villi. In the dog, only the villus tips may equilibrate with the lumen, hence a countercurrent gradient may be established in the villus. The entire villus of the rabbit may equilibrate with the lumen and no gradient for countercurrent exchange can therefore be established.

Temperature Profiles within a Double-pipe Heat Exchanger with Countercurrent Turbulent Flow of Newtonian Fluids: Derivation, Validation, and Application to Food Processing

Abstract: Pasteurization or sterilization of food and bioproducts is mostly performed using heat exchangers in countercurrent flow. The lack of appropriate analytic solutions for temperature profiles in the heating and cooling sections of such processes is most likely the main reason for the traditional cautious approach of disregarding the contribution of these stages in the total thermal history calculation. This approach leads to excessive thermal abuse of the product. The current manuscript describes the derivation and validation of an analytic solution for the temperature profiles within a double-pipe heat exchanger with countercurrent turbulent flow of Newtonian fluids, at any possible flow rate and not necessarily assuming an isothermal heating medium or coolant. The presented solution is based on application of the “heat exchanger effectiveness” concept in a partial heat exchanger, and can be easily solved and plotted using a spreadsheet. An interactive ExcelTM workbook was developed for fast prediction of temperature profiles and used in a simulated case study to demonstrate the influence of operating conditions on the temperature profile and thermal history of a pasteurized liquid product.

Co‐current and Countercurrent Configurations for a Membrane Dual Type Methanol Reactor

Abstract: A dynamic model for a membrane dual-type methanol reactor was developed in the presence of catalyst deactivation. This reactor is a shell and tube type where the first reactor is cooled with cooling water and the second one with feed synthesis gas. In this reactor system, the wall of the tubes in the gas-cooled reactor is covered with a palladium-silver membrane which is only permeable to hydrogen. Hydrogen can penetrate from the feed synthesis gas side into the reaction side due to the hydrogen partial pressure driving force. Hydrogen permeation through the membrane shifts the reaction towards the product side according to the thermodynamic equilibrium. Moreover, the performance of the reactor was investigated when the reaction gas side and feed gas side streams are continuously either co-current or countercurrent. Comparison between co-current and countercurrent mode in terms of temperature, activity, methanol production rate as well as permeation rate of hydrogen through the membrane shows that the reactor in co-current configuration operates with lower conversion and also lower permeation rate of hydrogen but with longer catalyst life than does the reactor in countercurrent configuration.