Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing nonmetal element
Reexamination Certificate
1998-03-03
2001-02-06
Mayekar, Kishor (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing nonmetal element
C205S622000, C204S252000, C204S282000, C204S290010, C204S296000, C423S481000, C423S483000
Reexamination Certificate
active
06183623
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for electrochemically converting anhydrous hydrogen halide to an essentially dry halogen gas using an ionically conducting membrane having passages therein. In particular, this process may be used to produce halogen gas, such as chlorine, bromine, fluorine and iodine, from a respective anhydrous hydrogen halide, such as hydrogen chloride, hydrogen bromide, hydrogen fluoride and hydrogen iodide.
BACKGROUND OF RELATED ART
Hydrogen chloride (HCl) or hydrochloric acid is a reaction by-product of many manufacturing processes which use chlorine. For example, chlorine is used to manufacture polyvinylchloride, isocyanates, and chlorinated hydrocarbons/fluorinated hydrocarbons, with hydrogen chloride as a by-product of these processes. Because supply so exceeds demand, hydrogen chloride or the acid produced often cannot be sold or used, even after careful purification. Shipment over long distances is not economically feasible. Discharge of the acid or chloride ions into waste water streams is environmentally unsound. Recovery and feedback of the chlorine to the manufacturing process is the most desirable route for handling the HCl by-product. A number of commercial processes have been developed to convert HCl into usable chlorine gas. See e.g., F. R. Minz, “HCl-Electrolysis-Technology for Recycling Chlorine”, Bayer AG, Conference on Electrochemical Processing, Innovation & Progress, Glasgow, Scotland, UK, Apr. 21-Apr. 23, 1993.
Currently, thermal catalytic oxidation processes exist for converting anhydrous HCl and aqueous HCl into chlorine. Commercial processes, known as the “Shell-Chlor”, the “Kel-Chlor” and the “MT-Chlor” processes, are based on the Deacon reaction. The original Deacon reaction as developed in the 1870's made use of a fluidized bed containing a copper chloride salt which acted as the catalyst. The Deacon reaction is generally expressed as follows:
where the following catalysts may be used, depending on the reaction or process in which equation (1) is used.
Reaction
Catalyst
or Process
Cu
Deacon
Cu, Rare Earth, Alkali
Shell-Chlor
NO
2
, NOHSO
4
Kel-Chlor
Cr
m
O
n
MT-Chlor
The commercial improvements to the Deacon reaction have used other catalysts in addition to or in place of the copper used in the Deacon reaction, such as rare earth compounds, various forms of nitrogen oxide, and chromium oxide, in order to improve the rate of conversion, to reduce the energy input and to reduce the corrosive effects on the processing equipment produced by harsh chemical reaction conditions. However, in general these thermal catalytic oxidation processes are complicated because they require separating the different reaction components in order to achieve product purity. They also involve the production of highly corrosive intermediates, which necessitates expensive construction materials for the reaction systems. Moreover, these thermal catalytic oxidation processes are operated at elevated temperatures of 250° C. and above.
Electrochemical processes exist for converting aqueous HCl to chlorine gas by passage of direct electrical current through the solution. The current electrochemical commercial process is known as the Uhde process. In the Uhde process, aqueous HCl solution of approximately 22 wt % is fed at 65° C. to 80° C. to both compartments of an electrochemical cell, where exposure to a direct current in the cell results in an electrochemical reaction and a decrease in HCl concentration to 17 wt % with the production of chlorine gas and hydrogen gas. A polymeric separator divides the two compartments. The process requires recycling of dilute (17 wt %) HCl solution produced during the electrolysis step and regenerating an HCl solution of 22 wt % for feed to the electrochemical cell. The overall reaction of the Uhde process is expressed by the equation
As is apparent from equation (2), the chlorine gas produced by the Uhde process is wet, usually containing about 1 wt % to 2 wt % water. This wet chlorine gas must then be further processed to produce a dry, usable gas. If the concentration of HCl in the water becomes too low, it is possible for oxygen to be generated from the water present in the Uhde process. This possible side reaction of the Uhde process due to the presence of water, is expressed by the equation:
2H
2
O→O
2
+4H
+
+4e
−
(3)
Further, the presence of water in the Uhde system limits the current densities at which the cells can perform to less than 500 amps/ft
2
, because of this side reaction. The result is reduced electrical efficiency and corrosion of the cell components due to the oxygen generated.
Another electrochemical process for processing aqueous HCl has been described in U.S. Pat. No. 4,311,568 to Balko. Balko employs an electrolytic cell having a solid polymer electrolyte membrane. Hydrogen chloride, in the form of hydrogen ions and chloride ions in aqueous solution, is introduced into an electrolytic cell. The solid polymer electrolyte membrane is bonded to the anode to permit transport from the anode surface into the membrane. In Balko, controlling and minimizing the oxygen evolution side reaction is an important consideration. Evolution of oxygen decreases cell efficiency and leads to rapid corrosion of components of the cell. The design and configuration of the anode pore size and electrode thickness employed by Balko maximizes transport of the chloride ions. This results in effective chlorine evolution while minimizing the evolution of oxygen, since oxygen evolution tends to increase under conditions of chloride ion depletion near the anode surface. In Balko, although oxygen evolution may be minimized, it is not eliminated. As can be seen from
FIGS. 3
to
5
of Balko, as the overall current density is increased, the rate of oxygen evolution increases, as evidenced by the increase in the concentration of oxygen found in the chlorine produced. Balko can run at higher current densities, but is limited by the deleterious effects of oxygen evolution. If the Balko cell were to be run at high current densities, the anode would be destroyed.
In general, the rate of an electrochemical process is characterized by its current density. In many instances, a number of electrochemical reactions may occur simultaneously. When this is true, the electrical driving force for electrochemical reactions is such that it results in an appreciable current density for more than one electrochemical reaction. For these situations, the reported or measured current density is a result of the current from more than one electrochemical reaction. This is the case for the electrochemical oxidation of aqueous hydrogen chloride. The oxidation of the chloride ions is the primary reaction. However, the water present in the aqueous hydrogen chloride is oxidized to evolve oxygen as expressed in equation (3). This is not a desirable reaction. The current efficiency allows one to describe quantitatively the relative contribution of the current from multiple sources. For example, if at the anode or cathode multiple reactions occur, then the current efficiency can be expressed as:
η
j
=
i
j
∑
j
=
1
NR
i
j
(
4
)
where &eegr;
j
is the current efficiency of reaction j, and where there are NR number of reactions occurring.
For the example of an aqueous solution of HCl and an anode, the general expression above is:
η
Cl
2
=
i
Cl
2
i
Cl
2
+
i
O
2
(
5
)
η
Cl
2
+
η
O
2
=
1.0
(
6
)
In the specific case of hydrogen chloride in an aqueous solution, oxidation of chloride is the primary reaction, and oxygen evolution is the secondary reaction. In this case, the current density is the sum of the two anodic reactions. Since &eegr;
O
2
is not zero, the current efficiency for chloride oxidation is less than unity, as expressed in equations (7) and (8) below. Whenever one is concerned with the oxidation of chloride from an aqueous solution, then the current efficiency for oxygen evolution is not zero and has a deleterious effect upon the yield and production of chlor
Cisar Alan J.
Gonzalez-Martin Anuncia
Hitchens G. Duncan
Murphy Oliver J.
Lynntech Inc.
Mayekar Kishor
Streets Jeffrey L.
Streets & Steele
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