Adsorption on activated carbon in countercurrent flow: An experimental study

The work describes adsorption experiments from aqueous solutions on activated carbon using a counter-current adsorber. Phenol and paranitrophenol were used for single component adsorption and molasses colour for multicomponent adsorption experiments. Isotherms were determined using simple equilibrium experiments. Three different techniques were used for determination of the kinetics. These were finite bath, infinite bath and fixed bed experiments. A mathematical model for countercurrent adsorption was tested. The model includes the effect of film diffusion and pore diffusion as well as surface diffusion, and can handle systems with a nonlinear isotherms. Kinetic data in the form of an average diffusivity in the carbon determined from the finite bath experiments and isotherm data did not predict the operation of the countercurrent column well. Data from the fixed bed experiments predicted countercurrent column operation much better. This is deemed to be due to inhomogeneities in the carbon used, resulting in a fairly quick initial uptake of the main part of the solute and a slower uptake of a minor amount.     

Alternative Realizations of CMOS Current Feedback Amplifiers for Low Voltage Applications

Two variants of a new current feedback amplifier (CFA) are presented in this paper. These CFAs are realized in CMOS technology and both are capable of working at low voltages. It is shown that one circuit performs better than the other by virtue of an increased impedance at its Z terminal achieved through the use of additional transistors. Analysis of both variants of the current conveyor and buffer that form the current feedback amplifier gives an insight into the location of primary poles and zeros of the CFAs. Simulation results indicate an overall gain bandwidth product in excess of 59 MHz and 102 MHz for each circuit at a gain of –10 and with a 3.3 V supply. Experimental results from a chip fabricated in a 0.35 m CMOS technology agree closely with the simulation results.     

Analysis of some adsorption experiments with activated carbon

A simple method is proposed whereby the film transfer coefficient and coefficient of diffusion in the particles may be determined from finite bath adsorption experiments. The method also makes it possible to separate pore and surface diffusion. Under certain conditions it is also possible to determine the influence of particle phase concentration on the surface diffusivity. The method is based on models describing the instationary diffusion in the solids. Data from six different adsorption systems were analysed using this method. The adsorbed components were: phenol, paranitrophenol, parachlorophenol, bensoic acid, phenylacetic acid and 2-4-dichlorophenoxyacetic acid. In all systems surface diffusion was the determining transport mechanism in the particles. In the system phenol and phenylacetic acid the surface diffusion coefficient increased by about a factor 3 with an increase in surface concentration of about 40%. For parachlorophenol the increase was somewhat less. For the other systems there was no significant increase. The increase in diffusivity is explained by a decrease in bonding forces with increasing concentration.     

Cost-relevance analysis for overrun control

The problem of cost overruns is explored, and the use of cost-relevance analysis as a tool for more effective management of project costs is discussed.     

Evaluation of hydrodynamic dispersion parameters in fractured rocks

A numerical procedure to determine the equivalent hydrodynamic dispersion coefficients and Péclet number(Pe) of a fractured rock is presented using random walk particle tracking method.The geometrical effects of fracture system on hydrodynamic dispersion are studied.The results obtained from the proposed method agree well with those of empirical models,which are the scale-dependent hydrodynamic dispersion coefficients in an asymptotic or exponential form.A variance case is added to investigate the influence of longitudinal hydrodynamic dispersion in individual fractures on the macro-hydrodynamic dispersion at the fracture network scale,and its influence is demonstrated with a verification example.In addition,we investigate the influences of directional flow and stress conditions on the behavior of hydrodynamic dispersion in fracture networks.The results show that the magnitudes of the hydrodynamic dispersion coefficients are relatively smaller when the flow direction is parallel to the dip directions of fracture sets.Compressive stresses significantly reduce hydrodynamic dispersion.However,the remaining questions are:(1) whether the deformed fracture network under high stress conditions may make the scale-dependent hydrodynamic dispersion coefficients have asymptotic or exponential forms,and(2) what the conditions for existence of a well-defined equivalent hydrodynamic dispersion tensor are.They need to be further investigated.     

Are Coaxial Super Power Gyrotrons Feasible?

Limits on the power generated by coaxial gyrotrons are investigated. It is shown that among all the operating modes suggested recently for coaxial super power gyrotrons only modes with azimuthal index m lower than about 44 pass the spatial stochasticity test. Modes with higher azimuthal indices cannot be used as operating modes because gyrotron oscillations become chaotic in the azimuthal direction and efficiency is very low. It is argued that there exists a natural upper limit on power generated by gyrotrons which is about 4 MW.     

Selective oxidation of propane over alkali-doped Mo–V–Sb–O catalysts

Alkali metal-doped MoVSbO catalysts have been prepared by impregnation of a MoVSbO-mixed oxide (prepared previously by a hydrothermal synthesis) and finally activated at 500 or 600 °C in N₂. The catalysts have been characterized and tested for the selective oxidation of propane and propylene. Alkali-doped catalysts improved in general the catalytic performance of MoVSbO, resulting more selective to acrylic acid and less selective to acetic acid than the corresponding alkali-free MoVSbO catalysts. However, the specific behaviour strongly depends on both the alkali metal added and/or the final activation temperature. At isoconversion conditions, catalysts activated at 600 °C present selectivity to acrylic acid higher than that achieved on those activated at 500 °C, both K-doped catalysts presenting the highest yield to acrylic acid. The changes in the number of acid sites as well as the nature of crystalline phases can explain the catalytic behaviour of alkali-doped MoVSbO catalysts.     

Comparative Analysis of Tunable Q-Enhancement Filter Cell Topologies in a 2.4 GHz LNA

The special LNA topologies resulting from loading a simple LNA with a set of Q-enhanced inductors are analyzed and compared. The Q of the on-chip spiral inductors that form the LNA load is enhanced by using a negative resistance realized with a cross-coupled differential pair degenerated and biased in various ways. The performance of the LNA is presented for the following types of Q-enhancement circuit: ideal negative resistor (ideal cell), tail-biased non-degenerated cross-coupled differential pair (classic cell), tail-biased resistively degenerated cross-coupled differential pair (resistive cell), tail-biased LC degenerated cross-coupled differential pair (B-cell), self-biased LC degenerated cross-coupled differential pair (BB-cell). The analysis focuses on the benefits of each cell related to s-parameter response, noise and linearity and the interdependency of Q vs. center frequency during tuning. Low voltage design challenges are addressed by presenting the advantages of the novel self-biased LC degenerated cross-coupled differential pair (BB-cell).     

The Effect Of A Passive Adsorption Sheet On Reducing Organic Pollutants In Indoor Air

The effect of a thin, passive, adsorbent sheet containing activated carbon on reducing the concentration of volatile organic compounds in indoor air is investigated. A mathematical model is established to describe the conditions in a hypothetical room containing building materials, furniture, an activated carbon sheet, etc. The results of the modelling show that the activated carbon sheet can reduce the concentration of the organic vapour in the room air by a factor of 1.6 – 200, depending on the resistances to mass transfm in the laminar gas film surrounding the activated carbon sheet and in the voids of the sheet. The maximum value 200 is a theoretical limit, if both the film and void resistances could be eliminated. Minimization of the film and void resistances is important to achieve high effect of the activated carbon sheet. This could be done by e.g. dividing a single sheet into several smaller pieces, hanging the sheets at some places where the airflow is relatively high, and making the sheets thin and loose. For some of the model parameters, estimated or assumed values were used in the calculations. Experimental work regarding sorption equilibria and mass transport properties of the species and materials involved is needed for more accurate results.     

Water inflow into excavations in fractured rock—a three-dimensional hydro-mechanical numerical study

Water inflow into rock excavations is a complex three-dimensional (3-D) problem influenced by a number of processes, such as stress–permeability coupling, groundwater degassing, bubble trapping, temperature effects or turbulence effects. Results from inflow experiments in the field show that when going from a slim borehole to a larger diameter hole, the inflow into the larger hole is often less than predicted; the explanation for this is not yet fully known. A single process or a combination of processes may be responsible for reduction of inflow into the larger diameter holes. In this study, a coupled hydro-mechanical, 3-D discrete element analysis has been conducted with the objective of gaining a better understanding of the influence of effective stress redistribution induced by excavation and related inflow into a cylindrical opening in a fractured rock mass. Matrix and fracture data from the Äspö Hard Rock Laboratory in Sweden have been used as input for the model. Several aspects related to fracture inflow into underground excavations have been studied (hydro-mechanical coupling, uncertainty in rock mass characteristics, relevance of non-linear fracture behaviour and influence of the excavation diameter). Results of this study show that stress–permeability coupling is one of the causes for the usually less than expected inflow into larger diameter holes.