Porous diaphragm

Graham showed that the rate of diffusion of different gases through a porous diaphragm was inversely proportional to the square roots of their densities this is the basis of a method of separation of gases, and has been applied successfully to the separation of hydrogen and deuterium. 

Electrochemical Generation of Chlorine Dioxide from Chlorite. The electrochemical oxidation of sodium chlorite is an old, but not weU-known method of generating chlorine dioxide. Concentrated aqueous sodium chlorite, with or without added conductive salts, is oxidized at the anode of an electrolytic cell having a porous diaphragm-type separator between the anode and cathode compartments (122—127). The anodic reaction is... 

Diaphrag m Cell Technology. Diaphragm cells feature a porous diaphragm that separates anode and cathode compartments of the cell. Diaphragms should provide resistance to Hquid flow, requite minimum space between anode and cathode, produce minimum electrical resistance, and be durable. At the anode, which is generally a DSA, chloride ions are oxidized to chlorine (see eq. 1) and at the cathode, which is usually a woven steel wine mesh, water is reduced to hydrogen. 

II indieates a phase boundaries between two liquid phases established by e.g. a porous diaphragm... 

At the interface between two similar solutions (a) and (p) merely differing in their composition, a transition layer will develop within which the concentrations of each component j exhibit a smooth change from their values cj in phase (a) to the values cf in phase (p). The thickness of this transition layer depends on how this boundary has been realized and stabilized. When a porous diaphragm is used, it corresponds to the thickness of this diaphragm, since within each of the phases outside the diaphragm, the concentrations are practically constant, owing to the liquid flows. 

The coefficient s shonld be low for separators in electrochemical reactors. It has valnes between 1.1 and 1.6 for simple separators, but for porous diaphragms and swollen membranes it has valnes between 2 and 10. The total porosity shonld be at least 50%, and the separator s pore space shonld be impregnated completely and sufficiently rapidly with the liqnid electrolyte. 

Four different electrokinetic processes are known. Two of them, electroosmosis and electrophoresis, were described in 1809 by Ferdinand Friedrich Renss, a professor at the University of Moscow. The schematic of a cell appropriate for realizing and studying electroosmosis is shown in Fig. 31.1a. An electrolyte solution in a U-shaped cell is divided in two parts by a porous diaphragm. Auxiliary electrodes are placed in each of the half-cells to set up an electric held in the solution. Under the inhuence of this held, the solution starts to how through the diaphragm in the direction of one of the electrodes. The how continues until a hydrostahc pressure differential (height of liquid column) has been built up between the two cell parts which is such as to compensate the electroosmotic force. 

In 1861, Georg Hermann Quincke described a phenomenon that is the converse of electroosmosis When an electrolyte solution is forced through a porous diaphragm by means of an external hydrostatic pressure P (Fig. 31.1ft), a potential difference called the streaming potential arises between indicator electrodes placed on different sides of the diaphragm. Exactly in the same sense, in 1880, Friedrich Ernst Dorn described a phenomenon that is the converse of electrophoresis During... 

A number of special features follow from the equations reported. The linear velocity of electroosmotic transport of the liquid is independent of the geometric parameters of the porous diaphragm (the size and number of pores, the thickness, etc.), and the space velocity depends only on S. At given values of the current, the transport rate increases with decreasing solution conductivity (increasing field strength). 

Departures of the electrokinetic behavior of real systems from that described by the equations reported occurs most often because of breakdown of two of the assumptions above because of marked surface conductivity (particularly in dilute solutions, where the bulk conductivity is low) and because of a small characteristic size of the disperse-phase elements (e.g., breakdown of the condition of bg <5 r in extremely fine-porous diaphragms). A number of more complicated equations allowing for these factors have been proposed. 

For example, consider a system in which metallic zinc is immersed in a solution of copper(II) ions. Copper in the solution is replaced by zinc which is dissolved and metallic copper is deposited on the zinc. The entire change of enthalpy in this process is converted to heat. If, however, this reaction is carried out by immersing a zinc rod into a solution of zinc ions and a copper rod into a solution of copper ions and the solutions are brought into contact (e.g. across a porous diaphragm, to prevent mixing), then zinc will pass into the solution of zinc ions and copper will be deposited from the solution of copper ions only when both metals are connected externally by a conductor so that there is a closed circuit. The cell can then carry out work in the external part of the circuit. In the first arrangement, reversible reaction is impossible but it becomes possible in the second, provided that the other conditions for reversibility are fulfilled. 

Membranes exhibiting selectivity for ion permeation are termed electrochemical membranes. These membranes must be distinguished from simple liquid junctions that are often formed in porous diaphragms (see Section 2.5.3) where they only prevent mixing of the two solutions by convection and have no effect on the mobility of the transported ions. It will be seen in Sections 6.2 and 6.3 that the interior of some thick membranes has properties analogous to those of liquid junctions, but that the mobilities of the transported ions are changed. 

In conventional hydrochloric acid electrolysis [1], aqueous hydrochloric acid (HClaq) is electrolysed in a cell, constructed basically from graphite, which is divided by a porous diaphragm or a membrane. The overall reaction is... 

A simple system in which transport processes occur that are also characteristic of membrane processes is the Hquid junction formed between two electrolyte solutions in the same solvent. The region in which one electrolyte passes into the other is frequently a porous diaphragm of various construction (fig. 2.2). A second type of Hquid junction is the free diffusion region (fig. 2.3). 

In the general case of several electrolytes present in the solutions in contact with the liquid junction, no simple result analogous to (2.6.10) can be obtained. A basic problem stems from the fact that the electrolyte distribution in the liquid junction is dependent on time, so that the liquid-junction potential is also time-dependent. Because of these complications, further discussion will consider only those liquid-junction models where a stationary state has been attained, so that the liquid-junction potential is independent of time. This condition is notably fulfilled in liquid junctions in porous diaphragms. 

While the Planck liquid-junction model corresponds to a junction with restrained flow , for example in a porous diaphragm, fig. 2.2, the Hendersoii model approaches a liquid junction with free diffusion (fig. 2.3). Ives and Janz [13] give inaccuracies in measuring liquid-junction potentials between 1 and 2 mV. 

The industrial Nalco process for the production of 11a and 11b was conducted in mixtures of 53 with THF, at 35-40 °C or 40-50 °C and about 2 kg cm pressure, in a cell divided by porous diaphragms and with current densities of 1.5-3.0 A dm at 15-30 V. However, production details are beyond the scope of this Chapter. Effective methods of recovery of 11 from mixtures after or during the electrolysis were elaborated . Of... 

In a diaphragm cell, the anode and cathode compartments are separated by a porous diaphragm (Figs. 11.1 and 11.2). Formerly, diaphragms were made of asbestos, but now special polymers created for chloralkali electrolysis have been introduced. 

Inasmuch.as che usual method of purif ication of water by distillation is expensive, it was proposed that the impurities be removed by electrolysis. For this, water is placed in a cell, divided by means of porous diaphragms into three compartments, a large one in the middle and two small ones on either side. Each outer compartment contains an electrode, connected to terminals of DC current. When the current is switched on, the electrolyte substances which are dissolved in the water, decompose, the positively charged metallic ions (such.as Ca,... 

Figure 19.16. Basic designs of electrolytic cells, (a) Basic type of two-compartment cell used when mixing of anolyte and catholyte is to be minimized the partition may be a porous diaphragm or an ion exchange membrane that allows only selected ions to pass, (b) Mercury cell for brine electrolysis. The released Na dissolves in the Hg and is withdrawn to another zone where it forms salt-free NaOH with water, (c) Monopolar electrical connections each cell is connected separately to the power supply so they are in parallel at low voltage, (d) Bipolar electrical connections 50 or more cells may be series and may require supply at several hundred volts, (e) Bipolar-connected cells for the Monsanto adiponitrile process. Spacings between electrodes and membrane are 0.8-3.2 mm. (f) New type of cell for the Monsanto adiponitrile process, without partitions the stack consists of 50-200 steel plates with 0.0-0.2 ram coating of Cd. Electrolyte velocity of l-2 m/sec sweeps out generated Oz.

Most modern production uses a diaphragm cell, in which compartments containing steel and titanium electrodes are separated by porous diaphragms to isolate the products (Fig. 14.20). 

---