Deacon Process

Ching-Soon Lim


Summary Deacon process is a process that produces Cl2 from gaseous HCl. A transition metal oxide is used as an efficient catalyst at moderate temperatures and without volatilization of the catalyst. The process consists two steps: (1) a chloridizing step in which the HCl is contacted with the catalyst at an elevated temperatures. This step converts the transition metal oxide to a transition metal chloride with elimination of water; and (2) an oxidizing step in which the transition metal chloride from step one is contacted with a source of oxygen. Cl2 is evolved and the transition metal chloride is reconverted to a transition metal oxide. In recent years, increasing amount of HCl is being obtained as a by-product from many chemical processes and several manufacturing processes such as the industrial production of chlorinated hydrocarbons. At the same time, industrial demand for gaseous chloride has also increased dramatically. Chlorine is used to make polyvinyl chloride (PVC), air-conditioner refrigerants, lubricants, plastic foams, insecticides, and household bleaching agents, etc.

Introduction In 1868, Deacon developed a process by which chlorine is produced by oxidation of gaseous HCl with O2 in the presence of a CuCl2 catalyst. This process is described by the following equation:
HCl(g) + ¼ O2(g) ---> ½ H2O(g) + ½ Cl2 (g)
This process is a fast overall exothermic process, which is expected to reach equilibrium under normal industrial operating condition of 700oK to 750oK. However, a number of problems are associated with Deacon process. The temperatures of the process reduce the equilibrium constant for the conversion, resulting in incomplete conversion of the HCl thereby reducing yield. Furthermore, at elevated temperatures above 675oK, the catalyst’s activity rapidly decreases, mainly due to the volatilization of the CuCl.

Since the early 1900, various efforts have been made to improve the Deacon process. There is a need for an efficient process that gives a nearly quantitative conversion of HCl to chlorine. This process must operate under conditions in which the catalyst does not volatilize and in which the activity of the catalyst remains stable, and operate at relatively moderate temperatures to prevent corrosion and minimize the extrinsic energy input required.

The reaction with a catalyst using manganese dioxide(MnO2) at moderate temperatures does not volatilize and has a relatively long lifetime. Also, it gives a high yield of Cl2 and efficient conversion of the HCl to Cl2. In the chloridizing step, the catalyst is contacted with HCl at temperature, preferably from about 373oK to about 523oK. The MnO2 is converted into MnCl2 when it contacts with HCl. As a result, the oxidation state of Mn is being reduced from +4 to +2. The exit stream of the reaction contains Cl2 and steam according to the equation:
4HCl (g) + MnO2 (s) --- > MnCl2 (s) + 2H2O (g) + Cl2 (g)

The steam and Cl2 can then be separated easily or condensed together and subsequently separated as needed. This is an exothermic process with delH = –6 kcal/mole. In the oxidizing step, the MnCl2 is then contacted with oxygen at a temperature of at least 573oK, and sufficiently high so that Cl2 is evolved and the MnCl2 is reconverted to the original catalytic MnO2 for reuse in the chloridizing step. This step is also exothermic with delH = –8 kcal/mole. The reaction of this step takes place according to the equation:
MnCl2 (s) + O2 (g) --- > MnO2 (s) + Cl2 (g)

The source of oxygen in this step can be pure O2 gas, or air. The source of oxygen is preferably preheated to provide at least some of the necessary heat not provided by the reaction itself for the evolution of Cl2 and the recovery of MnO2 from MnCl2. Additional heat is required because of the considerable rise in going from the chloridizing step to the oxidizing step. The pressure of the reaction is closed to about 1 atmosphere, but there is no theoretical limit on the pressure, and use of a supra-atmospheric pressure, such as about 10 atm, might be benificial for the production of chlorine

Apparatus for Performing the Reaction

The reaction can be carried out in a simple reaction vessel as shown below.


Figure 1: Simple Reaction Vessel5

The catalyst MnO2 is contained in the body B of the simple reaction vessel. A control valve V1 which controls inlet a, admits the HCl-containing gas for the performance of the first, chloridizing step. A second inlet b, controlled by valve V-2, admits the source of oxygen, such as O2 gas, for the performance of the second, oxidizing step. Control valve V-3 which controls outlet c, allows escape of the effluent gas in each step for further processing. The vessel is surrounded by a temperature control mechanism (not shown) that can supply or remove heat as needed to keep the temperature within the required limits. 5

Other Catalysts Other transition metal oxide such as Co2O3, Co3O4, Cr2O3, NiO, and etc., can also be used instead of MnO2 in the Deacon process. However, in the chloridizing step, the HCl is preferably contacted with those catalysts at a temperature of from about 373oK to about 500oK. And in the oxidizing step, the resulting transition metal chloride is preferably contacted with a source of oxygen at a temperature of from 583oK to about 648oK. An alkali metal chloride selected from the group such as LiCl, NaCl, KCl, and, optionally, a trivalent rare earth chloride selected from the group consisting of LaCl3, PrCl3, and Pr2O3, can also be used.

References

1. Satterfield, Charles N., “Heterogeneous Catalysis in Industrial Practice”, 2nd edition, McGraw-Hill, Inc., New York, 1991, p.307.

2. Lenwood W. Hall,Jr., George R. Helz, Dennis T. Burton, “Power Plant Chlorination, A Biological and Chemical Assessment” Ann Arbor Science Publishers, Inc., 1981,p. 1-2.

3. Hermen F. Mark, Donald F. Othmer, Charles G. Overberger, Glenn T. Seaborg, “Encyclopedia of Chemical Technology Volume 1”, 3rd. edition, John Wiley & sons, Inc., 1978, p.826-827.

4. Ronald G. Minet, Theodore T. Tsotsis, Sidney W. Benson, “ Recovery Of Chlorine From Hydrogen Chloride By Carrier Catalyst Process” Sep. 25, 1990
http://www.patents.ibm.com/details?&pn=US04959202__

5. Sidney W. Benson, Mohamed W. M. Hisbam, “ Efficient Method For The Production Of Chlorine And The Separation Of Hydrogen Chloride From Complex Mixture” Oct. 13, 1992.
http://www.patents.ibm.com/details?&pn=US05154911__

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