Metallic membrane system

Fig. 9.7 Scale-up design for a planar, all metal membrane system from Union Carbide Patent 3,336,730,22 August 1967 [12], Precedence for a 10 miUion cubic feet (25 tons) per day hydrogen plant using metal membranes ...

Design and Implementation of Dense Metal Membrane Systems.153... 

Design and implementation of dense metal membrane systems 153... 

DESIGN AND IMPLEMENTATION OF DENSE METAL MEMBRANE SYSTEMS... 

A wide range and a number of purification steps are required to make available hydrogen/synthesis gas having the desired purity that depends on use. Technology is available in many forms and combinations for specific hydrogen purification requirements. Methods include physical and chemical treatments (solvent scmbbing) low temperature (cryogenic) systems adsorption on soHds, such as active carbon, metal oxides, and molecular sieves, and various membrane systems. Composition of the raw gas and the amount of impurities that can be tolerated in the product determine the selection of the most suitable process. 

Under aqueous conditions, flavonoids and their glycosides will also reduce oxidants other than peroxyl radicals and may have a role in protecting membranal systems against pro-oxidants such as metal ions and activated oxygen species in the aqueous phase. Rate constants for reduction of superoxide anion show flavonoids to be more efficient than the water-soluble vitamin E analogue trolox (Jovanovic et al, 1994), see Table 16.1. 

Typically, transport experiments have been performed using a U-tube apparatus in which a solvent such as chloroform, containing the macrocyc-lic carrier, is placed in the tube so that it separates two aqueous phases the source phase containing the metal ion(s) to be transported and the receiving phase into which the transported ions are deposited. A diagrammatic representation of a liquid membrane system is shown in Figure 9.4. 

Ion-selective electrodes are systems containing a membrane consisting basically either of a layer of solid electrolyte or of an electrolyte solution whose solvent is immiscible with water. The membrane is in contact with an aqueous electrolyte solution on both sides (or sometimes only on one). The ion-selective electrode frequently contains an internal reference electrode, sometimes only a metallic contact, or, for an ion-selective field-effect transistor (ISFET), an insulating and a semiconducting layer. In order to understand what takes place at the boundary between the membrane and the other phases with which it is in contact, various types of electric potential or of potential difference formed in these membrane systems must first be defined. 

Fig. 13.4 Schematic setup for a membrane-based metal enrichment system.

In order to develop the liquid membrane techniques, i.e., emulsion Hquid membrane (ELM), supported liquid membrane (SLM), non-dispersive extraction in hollow fiber membrane (HFM), etc., for practical processes, it is necessary to generate data on equilibrium and kinetics of reactive extraction. Furthermore, a prior demonstration of the phenomena of facilitated transport in a simple liquid membrane system, the so-called bulk liquid membrane (BLM), is thought to be effective. Since discovery by Li [28], the liquid membrane technique has been extensively studied for the separation of metal ion, amino acid, and carboxyHc acid, etc., from dilute aqueous solutions [29]. 

Figure 4. (a) Planar bilayer membrane system for single-channel currents measurement. Soybean lecithin in n-decane was applied to a hole separating two aqueous chambers. Chambers were filled with metal chloride salt at pH 7.2. The voltage was applied to the outer cell with respect to the inner. The currents across the bilayer were recorded on a PCM recorder through a patch-clamp amplifier and a lowpass filter, (b) Typical records of current observed at -t-50.0 mV (symmetrical 0.5 M solution). Currents increase upward from the zero level shown by the dotted line in each panel. 

Prototype sealed button cells and larger prismatic cells have been fabricated and studied. As with the nickel-cadmium and nickel-metal hydride systems, an oxygen recombination route is necessary, but the use of membrane separators limits oxygen transport to the negative plate and... 

Another well proved pressure gauge is equipped with a metallic membrane cell [54], The principle of this cell is the inclination of the membrane which causes the pressure-proportional variation of the electrical resistance of the system. The pressure range of those systems reaches up to 400 bar. The allowable temperatures of the metering system are near... 

Unfortunately, it is difficult in the laboratory or even under pilot plant conditions to obtain large supplies of natural waters. Hauling of water is expensive at best, and the handling, detention, and storage of water in tanks and associated equipment can introduce iron and other metals which are even more troublesome than some of the scaling constituents normally present. Most laboratory and testing work on electric membrane stacks in pilot plants is done with solutions of sodium chloride. Testing with pure solutions of sodium chloride yields only an approximate idea of the true performance of membrane systems. Sometimes attempts are made to synthesize... 

The membrane system considered here is composed of two aqueous solutions wd and w2, separated by a liquid membrane M, and it involves two aqueous solution/ membrane interfaces WifM (outer interface) and M/w2 (inner interface). If the different ohmic drops (and the potentials caused by mass transfers within w1 M, and w2) can be neglected, the membrane potential, EM, defined as the potential difference between wd and w2, is caused by ion transfers taking place at both L/L interfaces. The current associated with the ion transfer across the L/L interfaces is governed by the same mass transport limitations as redox processes on a metal electrode/solution interface. Provided that the ion transport is fast, it can be considered that it is governed by the same diffusion equations, and the electrochemical methodology can be transposed en bloc [18, 24]. With respect to the experimental cell used for electrochemical studies with these systems, it is necessary to consider three sources of resistance, i.e., both the two aqueous and the nonaqueous solutions, with both ITIES sandwiched between them. Therefore, a potentiostat with two reference electrodes is usually used. 

Investigations performed by Dupont on the chemical stability of Teflon reveal that Teflon immersed in a 20% HC1 solution at 200°C for extended periods will not absorb any chlorine within detectability limits. In addition, Teflon does not absorb any detectable sulfur from sulfuric acid or sulfur vapor. Therefore, a Teflon H, membrane may prove to be superior to a platinum-group metal membrane for use in sulfur- or chlorinebearing systems. Investigations are currently being performed to determine the feasibility of employing a Teflon membrane as an H2 monitor/controller in hydrothermal test apparatus. 

Keller, O.C., Poitry, S. and BufHe, J. (1994) A hollow fibre supported liquid membrane system for metal speciation and preconcentration calculation of the response time. J. Electroanal. Chem., 378, 165-175. 

Section 6 describes the use of the kinetic theory for studying physicochemical mechanics problems. An example of the theory is used to explain the observed properties for hydrogen-palladium systems. It is also used to self-consistently connect the local adsorption and diffusion characteristics of hydrogen atoms with the deformations within the metal membrane. 

---