Modeling and Simulation of Pressure Equalization Step Between a Packed Bed and an Empty Tank in Pressure Swing Adsorption Cycles

Background: It is of paramount importance to pay a great attention when modeling pressure equalization step of pressure swing adsorption cycles for its notable effect on the accurate prediction of the whole cycle performances. Studies devoted to pressure equalization between an adsorption bed and a tank have been lacking in the literature. Many factors could affect the accuracy of the dynamic simulation of pressure equalization between a bed and an empty tank. Methods: The method used for the equilibrium pressure evaluation has a significant impact on simulation results. The exact equilibrium pressure (Peq) obtained when connecting an adsorption column and an empty tank could only be obtained by numerical simulation given the complexity of the set of partial differential equations. Results: It has been shown that, with some simplifying assumptions, one can analytically determine Peq with satisfactory precision. The analytical solution proposed permits to assess rapidly the equilibrium pressure and the equilibrium mole fraction of the adsorbable species in the tank (Yeq) and without the need to resort to a cubersome modeling. Conclusion: Based on the comparisons presented, one can conclude that the agreement between the experimental and numerical results relative to Peq and Yeq is very satisfactory.

Effect of Ausageing on the Precipitation Behavior of MX Carbonitride in Modified 9Cr-1Mo Steel

The precipitation behavior of MX carbonitride during a normalizing heat treatment with and without ausageing was investigated in a modified 9Cr-1Mo steel. The normalizing heat treatment was performed at 1150 oC for 1800 s. Ausageing was conducted at 765 and 500 oC for 1800 to 86400 s during the cooling from the heat treatment. The matrix of the steel was austenite single phase during normalizing and ausageing, except for that ausaged at 765 oC for 86400 s. The initial austenite grain size and hardness were not influenced by ausageing, except for the sample ausaged at 765 oC for 86400 s. Although Nb-rich MX (NbX) and cementite were observed, V-rich MX (VX) was not observed under any of the conditions investigated. The amount of NbX in the steel ausaged at 500 oC was at least twice as large as that under the other conditions, and the amount in the steel ausaged at 760 oC was slightly larger than that in the steel that did not undergo ausageing. The precipitation of NbX took place during ausageing in the austenite matrix. On the other hand, it is well known that VX precipitates during tempering. An equilibrium mole fraction of VX in the austenite matrix calculated by Thermo-Calc. was larger than that of NbX at the ausageing temperatures. It is proposed that VX is an equilibrium phase at the ausageing temperature; however, VX nucleation takes much longer in the austenite matrix. It is postulated that the precipitation of VX is more strongly influenced by the interfacial energy rather than supersaturation. It is concluded that the precipitation of MX carbonitride, especially NbX, can be controlled by ausageing during cooling after a normalizing heat treatment.

Control of Zn, Cu and Mn activities in alkaline and calcareous soils equilibrated with EDTA

SUMMARYThe modified EDTA (ethylenediaminetetraacetic acid) method of determining the ionic activities in soils developed by Norvell was used to measure Zn2+, Cu2+ and Mn2+ activities including the competition of Ca and Mg for the chelating ligand EDTA. In the Zn-EDTA system, the equilibrium mole fraction of CaL2− + MgL2− increased with an increase in pH of the soil solution, thereby leading to a decrease in the concentration of transition metal ions. In view of the pH dependence of the metal ion activities, the values of the expression pM + 2 pOH (where M may be Zn, Cu or Mn) were used to discuss the possible role of various oxides, hydroxides, carbonates, sulphides and unknown solid phases in regulating the activities of Zn2+, Cu2+ and Mn2+ ions in alkaline and calcareous soils.

Transport of Nonelectrolyte-Water Mixtures through Cellulose Membranes

Measurements of the transport properties of cellulose membranes for the mixtures methanolwater and formamide-water are reported. The measured approach of the mole fractions of the nonelectrolyte to its equilibrium value are interpreted in the framework of linear irreversible thermodynamics in a novel way. The permeabilities of the membranes of the different substances are discussed in terms of phenomenological coefficients. It is shown that these depend on the equilibrium mole fraction in an essential way. A discussion of this effect is given