Deacon procedure

As Deacon process is the production of chlorine by oxidation of hydrogen chloride with oxygen , respectively. The process was applied for a patent in 1868 by the English chemist Henry Deacon (1822–1876).

history

Above all, the Leblanc process for soda production introduced in 1792 and the subsequent strong development of the chemical industry led to massive accumulation of hydrogen chloride, which was either released directly into the air or dissolved in water as hydrochloric acid into the waste water. In the sixties of the 18th century, therefore, several laws came into force in which the untargeted release of hydrogen chloride gas or hydrochloric acid was severely restricted. With his invention, Henry Deacon achieved a breakthrough in two ways: on the one hand, the environmental problem was solved with hydrogen chloride and, on the other hand, a valuable substance was obtained that could be sold well as chlorinated lime , for example .

chemistry

The classic (one-step) Deacon process is the conversion of HCl gas with oxygen (air or pure oxygen) according to the following reaction equation:

${\displaystyle \mathrm {4\ HCl+O_{2}\ _{\overrightarrow {450\,^{\circ }\mathrm {C} \ Kat}}\ 2\ Cl_{2}+2\ H_{2}O} }$

The reaction is exothermic with a reaction enthalpy of –114.8 kJ / mol and is an equilibrium reaction, ie the reaction does not proceed to completion .

The conversion takes place at temperatures of approx. 400–450 ° C on solid (“heterogeneous”) catalysts based on CuCl ₂ or CuSO ₄ . The following figure shows the equilibrium conversion of 4 moles of HCl with one mole of O ₂ as a function of temperature (at normal pressure):

In order to achieve a high conversion of hydrogen chloride , a lower reaction temperature would be more favorable, but here the reaction rate is too slow with the copper-based catalysts used . The equilibrium conversion of HCl can still be increased by increasing the pressure and increasing the oxygen excess, but complete or almost complete conversion is not achieved at a reaction temperature of 400-450 ° C.

Technical implementation

The chemical reaction seems simple, but the technical realization is connected with such massive problems, this process that to date no large scale for chlorine production or for recycling of hydrochloric prevail could. The main technical problems are corrosion and the handling of the catalyst in a reactor :

• Corrosion occurs primarily when the gas mixture exiting the reactor (HCl, O ₂ , Cl ₂ , H ₂ O) cools on metallic surfaces.
• The copper-based catalyst is also intermediately converted to CuCl, CuO and CuO ₂ during the reaction or is in equilibrium with these compounds . Especially CuCl with its low melting point of approx. 430 ° C and its property of sublimation , ie evaporation from the solid phase, causes the catalyst to stick in the reactor and the catalyst with the reaction gases to be discharged from the reactor.

Formally, the one-stage reaction can be divided into two stages, in which CuO is first reacted with HCl to form CuCl ₂ and water, which is then oxidized to chlorine and CuO in a second reaction:

${\displaystyle \mathrm {CuO+2\ HCl\longrightarrow CuCl_{2}+H_{2}O} }$

${\displaystyle \mathrm {2\ CuCl_{2}+O_{2}\longrightarrow 2\ CuO+2\ Cl_{2}} }$

This separation of the reaction made it possible to carry out the technical process in two separate reactors , and the reaction temperatures can now be optimally selected for each individual reaction. This two-stage reaction is also no longer an equilibrium reaction , so that the problem of gas separation of the product gases and corrosion during product work-up is reduced. Theoretically, the first reactor only works with the gases HCl and H ₂ O in the reaction, the second only with O ₂ (or air) and chlorine.

However, no technical breakthrough has been achieved with this either, since the mechanical transport of the catalyst from one reactor to another and back again is anything but trivial. The reactions cannot be completely separated from each other, so that the actual problems have been reduced but not completely eliminated.

Technical advancements

In the period that followed, up to the recent past, research was carried out on this process, both for the development of improved catalyst systems and for better technical implementation. This resulted in several variants or further developments of the Deacon process, some of which were implemented on an industrial scale:

• Shell chlorine process from Shell
• MT-Chlor process from Mitsui-Toatsu Chemicals, today Mitsui Chemicals (Japan)
• KEL chlorine process from MW Kellogg and DuPont
• Carrier Catalyst Process from USC ( University of Southern California )
• Sumitomo-chlorine process from Sumitomo Chemicals (Japan)

literature

• Henry Deacon: XXIX. — On Deacon's method of obtaining chlorine, as illustrating some principles of chemical dynamics . In: J. Chem. Soc. tape 25 , 1872, pp. 725-767 , doi : 10.1039 / JS8722500725 .
• Robert Hasenclever: About chlorine preparation according to Deacon . In: Reports of the German Chemical Society . tape 9 , no. 2 , 1876, p. 1070-1073 , doi : 10.1002 / cber.18760090211 .
• Patent US85370 : Manufacture of Chlorine. Published December 29, 1868 , inventor: Henry Deacon. ‌
• Patent US165802 : Improvement in manufacture of chlorine. Published July 20, 1875 , inventor: Henry Deacon. ‌

Individual evidence

1. ↑ Erwin Riedel, Christoph Janiak: Inorganic Chemistry . 9th edition. de Gruyter, Berlin / Boston 2015, ISBN 978-3-11-035528-4 , pp. 429 ( limited preview in Google Book search).
2. ↑ Armin Müller, Karl Heinz Buchel, Peter Fröhlich, Martin Bertau, Michael Katzberg, Dietmar Werner, Hans-Heinrich Moretto: Industrial Inorganic Chemistry . 4th edition. Wiley-VCH, Weinheim 2013, ISBN 978-3-527-64958-7 ( limited preview in Google book search).