Question

what is chloro alkali process explain with diagram also

Answer 1

The chloralkali process (also chlor-alkali and chlor alkali) is an industrial process for the electrolysis of sodium chloride. It is the technology used to produce chlorine and sodium hydroxide (lye/caustic soda),[1] which are commodity chemicals required by industry. 35 million tons of chlorine were prepared by this process in 1987.[2] Industrial scale production began in 1892.

Usually the process is conducted on a brine (an aqueous solution of NaCl), in which case NaOH, hydrogen, and chlorine result. When using calcium chlorideor potassium chloride, the products contain calcium or potassium instead of sodium. Related processes are known that use molten NaCl to give chlorine and sodium metal or condensed hydrogen chloride to give hydrogen and chlorine.

The process has a high energy consumption, for example over 4 billion kWh per year in West Germany in 1985.[3] Because the process gives equivalent amounts of chlorine and sodium hydroxide (two moles of sodium hydroxide per mole of chlorine), it is necessary to find a use for these products in the same proportion. For every mole of chlorine produced, one mole of hydrogen is produced. Much of this hydrogen is used to produce hydrochloric acid or ammonia, or is used in the hydrogenation of organic compounds.[4]

Process systemsEdit



Cell room of a chlor-alkali plant ca. 1920

Three production methods are in use. While the mercury cell method produces chlorine-free sodium hydroxide, the use of several tonnes of mercury leads to serious environmental problems. In a normal production cycle a few hundred pounds of mercury per year are emitted, which accumulate in the environment. Additionally, the chlorine and sodium hydroxide produced via the mercury-cell chloralkali process are themselves contaminated with trace amounts of mercury. The membrane and diaphragm method use no mercury, but the sodium hydroxide contains chlorine, which must be removed.

The performance of these devices is governed by the considerations ofelectrochemical engineering.

Membrane cellEdit



Basic membrane cell used in the electrolysis of brine. At the anode (A), chloride (Cl−) is oxidized to chlorine. The ion-selective membrane (B) allows the counterion Na+ to freely flow across, but prevents anions such as hydroxide (OH−) and chloride from diffusing across. At the cathode (C), water is reduced to hydroxide and hydrogen gas. The net process is the electrolysis of an aqueous solution of NaCl into industrially useful products sodium hydroxide (NaOH) and chlorine gas.

The most common chloralkali process involves the electrolysis of aqueoussodium chloride (a brine) in a membrane cell.

Saturated brine is passed into the first chamber of the cell where the chlorideions are oxidised at the anode, losing electrons to become chlorine gas (A in figure):

2Cl− → Cl
2 + 2e−

At the cathode, positive hydrogen ions pulled from water molecules are reducedby the electrons provided by the electrolytic current, to hydrogen gas, releasinghydroxide ions into the solution (C in figure):

2H
2O + 2e− → H2 + 2OH−

The ion-permeable ion exchange membrane at the center of the cell allows thesodium ions (Na+) to pass to the second chamber where they react with the hydroxide ions to produce caustic soda (NaOH) (B in figure).[1] The overall reaction for the electrolysis of brine is thus:

2NaCl + 2H
2O → Cl
2 + H
2 + 2NaOH

A membrane cell is used to prevent the reaction between the chlorine and hydroxide ions. If this reaction were to occur the chlorine would bedisproportionated to form chloride and hypochlorite ions:

Cl
2 + 2OH− → Cl− + ClO− + H
2O

Above about 60 °C, chlorate can be formed:

3Cl
2 + 6OH− → 5Cl− + ClO
3− + 3H
2O

Because of the corrosive nature of chlorine production, the anode (where the chlorine is formed) must be made from a non-reactive metal such as titanium, whereas the cathode (where hydroxide forms) can be made from a more easily oxidized metal such as nickel.