Engineering Essays

Engineering Essays: Chemical and Process Engineering

Process Description

Strong brine (300gll) is fed to the anolyte compartment at a temperature of 55°C.

The anolyte compartment is separated by the catholyte compartment which contains 35% caustic solution, by a cationexchange membrane. The solutions in the cell are subjected to an electrical current.

Several processes occur simultaneously in the membrane phase in an operating cell. Sodium ions and hydroxide ions migrate under the cumulative effect of concentration and electrical potential gradients, with sodium ion the major potential carrier.

In general these gradients are not expected to be linear, especially for composite, surface treated, or fabric backed materials.

The flow of sodium ions is accompanied by a net electrosmotic water flow in the same direction. This diffusion of water is a function of the membrane used and it is sensitive to external NaOH solution concentration. The effect of temperature is less significant!

Transfer of chloride ion Cl- to the catholyte compartment from the anode compartment is prevented as the membrane presents an effective hydrodynamic barrier, and also the electrical potential gradient across the membrane opposes chloride transport.

Chlorine gas saturated with wafer vapour and small amounts of oxygen evolve from the anode, while the depleted brine is discharged to be further treated and then re-saturated with sodium chloride in order to be fed again to the cell.

The amount of oxygen is an indication of the cell inefficiency as it results from the reaction of hydroxyl ions with molecular chlorine that otherwise would have been retrieved as a product.

At the catholyte compartment caustic of strength 35% is produced, that is removed from the cell to be further concentrated. Hydrogen evolves from the cathode saturated with water. A rough schematic of the process is presented in the figure below:

Figure 1 Membrane Cell schematic

1 Modern Chlor Alkali Technology, page 158. The Materials of Construction. The Membranes.

The membrane is the key component of the membrane cell. The energy requirement and the quality of the solution produced in a membrane cell depend on the membrane performance. The requirements for a membrane are as follows:

1.) Durability under the conditions of chlor-alkali electrolysis.
2.) High selectivity for sodium ion transport.
3.) Low electrical resistance.
4.) Sufficient mechanical strength for practical use.

The membrane is exposed to chlorine and strong caustic solution at high temperature.

Only ion-exchange membranes made of perfluoropolymer can withstand such severe conditions. The first membranes that showed significant potential for use in the chloralkali process were made of perfluorosulfonate polymer. These proved to be durable in chlor-alkali cells, but they were relatively inefficient.

In pursuit of greater efficiency, a perfluorocarboxylate polymer membrane was developed. Because of its low water content, the perfluorocarboxylate membrane effectively inhibits the migration of hydroxyl ions from the catholyte, making it possible to obtain a high current efficiency, with a 32%-35wt% caustic soda in the catholyte.

However, the voltage drop for such a highly efficient membrane is relatively large. Further development resulted in a two-layered membrane, composed of a thin, high-efficient carboxylate membrane on the cathode side and a supporting high conductivity membrane, either carboxylate or sulfonate, on the anode side.

The ion-exchange groups of the original polymers are in the perfluorosulfonate form, -S02, or the ester form, -CO2R. These polymers are processible by melt and are formed into membranes by extrusion, heat pressing, or lamination. In most cases, membranes are embedded with some reinforcement, such as woven fabric made of polytetrafluoroethylene (PTFE), so that even with a thickness of ca. 250 µm, they exhibit sufficient mechanical strength for practical handling. Prior to use, the ion exchange groups are converted to -S03Na or -C02Na by caustic solution.

Because the performance of the membrane is the most important element in the efficiency of the membrane cell, many refinements of the technology have been made in the membrane manufacturing technology. The improvement of hydrophilicity by covering the surface with a porous, non-conductive inorganic material brought about a great reduction in cell voltage. To reduce the amount of current screening caused by fabrics, membrane reinforcement with dispersed microfibriles or interwoven fabrics of electrolyte-soluble fabrics and PTFE have been contrived. The membranes are supplied commercially from Du Pont (Nafion) and from Asahi Glass (Flemion).

Summary

The membrane in a chlor-alkali electrolyser is exposed to chlorine on one side and strong caustic on the other. Only perfluoropolymers have been found to withstand these conditions, combined with appended groups having ion-exchange properties. The first membrane ever to be constructed produced a low concentration caustic solution and was deficient in limiting hydroxyl ions back migration. Then the idea of an asymmetric membrane having sulfonic acid groups on the anodic side and converted groups on the cathodic side to overcome back migration of hydroxyl ions was developed. These composite membranes operate efficiently at high caustic strengths. They have got better resistance to hydroxyl ions back migration. Nowadays membranes consist of a film of perfluorosulfonate polymer, Teflon reinforced fabric and a perfluorocarboxylate polymer all bonded together. Membranes are supplied commercially from Du Pont (Nafion) and from Asahi Glass (Flemion).

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