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.

Analysis of equilibrium shift in isothermal reactors with a permselective wall

Abstract: The objective of this study is to obtain a fundamental understanding of the behavior of reactors with a permselective wall (membrane reactors) in terms of: design parameters (reactor length, membrane thickness); operating variables (pressure ratio, feed flow rate); physical properties (rate constant, permeability of fast gas, permselectivity, equilibrium constant); and flow patterns (recycle, cocurrent, countercurrent). Pure feed reacts on the high-pressure side of the membrane, and the product(s) formed are continuously removed to the low-pressure side so that thermodynamic equilibrium is never reached. It is shown by simulation that equilibrium shift can be enhanced by: recycling unconverted reactant; shifting feed location to separate products; and maintaining high permeation rates to reduce backreaction. It is also shown that the choice between cocurrent flow and countercurrent flow depends on the system parameters.

On the use of conventional cocurrent and countercurrent effective permeabilities to estimate the four generalized permeability coefficients which arise in coupled, two-phase flow

Abstract: In the case of coupled, two-phase flow of fluids in porous media, the governing equations show that there are four independent generalized permeability coefficients which have to be measured separately. In order to specify these four coefficients at a specific saturation, it is necessary to conduct two types of flow experiments. The two types of flow experiments used in this study are cocurrent and countercurrent, steady-state permeability experiments. It is shown that, by taking this approach, it is possible to define the four generalized permeability coefficients in terms of the conventional cocurrent and countercurrent effective permeabilities for each phase. It is demonstrated that a given generalized phase permeability falls about midway between the conventional, cocurrent effective permeability for that phase, and that for the countercurrent flow of the same phase. Moreover, it is suggested that the conventional effective permeability for a given phase can be interpreted as arising out of the effects of two types of viscous drag: that due to the flow of a given phase over the solid surfaces in the porous medium and that due to momentum transfer across the phase 1-phase 2 interfaces in the porous medium. The magnitude of the viscous coupling is significant, contributing at least 15% to the total conventional cocurrent effective permeability for both phases. Finally, it is shown that the nontraditional generalized permeabilities which arise out of viscous coupling effects cannot equal one another, even when the viscosity ratio is unity and the surface tension is zero.

Countercurrent deterpenation of citrus oils with supercritical CO2

Abstract: The separation of terpene hydrocarbons from oxygenated aroma components was investigated by using supercritical carbon dioxide to fractionate orange peel oil. To understand the behavior of this multicomponent system, phase equilibrium measurements, countercurrent column experiments and flooding-point measurements were carried out. Conditions were set within a temperature range from 323 to 343 K and pressures from around 8 MPa up to 13 MPa. Separation factors for the two groups of oil components, `terpenes' and `aroma', were obtained for the system CO2+orange peel oil depending on temperature and pressure. Moreover, fractionated products from countercurrent experiments were used to study the influence of product composition on both selectivity and mutual solubility. A stage calculation method based on the Janecke diagram was applied to evaluate VLE data. Due to the fact that the operating costs of a continuous process depend on the solvent-to-feed ratio, this parameter was used to determine the optimal conditions for multistage extraction. For a given separation task at a fixed temperature, the required solvent-to-feed ratio was found to decrease with increasing pressure. This is explained by the increase in loading of the solvent and the possibility of enhancing the extract reflux which predominates the effect of decreasing separation factors at a higher loading. After passing a minimum, the required solvent-to-feed ratio increases near the critical point of the mixture.

On the Efficiency of Solar H2 and CO Production via the Thermochemical Cerium Oxide Redox Cycle: The Option of Inert-Swept Reduction

Abstract: A thermodynamic model of the ceria-based solar thermochemical redox cycle is presented with the objective of resolving the widely varying predictions of the solar-to-fuel efficiency possible with reduction carried out in a flow of inert sweep gas. The implications of the treatment of the gas–solid interaction are explored through comparison of mixed flow and countercurrent flow configurations of reactants. The mixed flow model is applied for the first time to both reduction and oxidation reactions. The mechanical work to produce sweep gas of varying purity, separate the products, and pump gases is included. The results identify the conditions necessary for efficient operation. The two models lead to substantially different predictions of the usage of sweep gas and oxidizer and process efficiency. Efficiencies predicted with the conservative mixed flow model reach a maximum of 11% for water splitting at 1073 K, assuming reduction at 1773 K, heat recovery of 80% of the sensible heat of the gases, and an opt...