Electrochemical conversion of anhydrous hydrogen halide to halogen gas using a cation-transporting membrane

Abstract

The invention relates to a process for electrochemically converting anhydrous hydrogen halide, such as hydrogen chloride, hydrogen fluoride, hydrogen bromide and hydrogen iodide, to essentially dry halogen gas, such as chlorine, fluorine, bromine and iodine gas, respectively. In a preferred embodiment, the present invention relates to a process for electrochemically converting anhydrous hydrogen chloride to essentially dry chlorine gas. This process allows the production of high-purity chlorine gas. In this process, molecules of essentially anhydrous hydrogen chloride are transported through an inlet of an electrochemical cell. The molecules of the essentially anhydrous hydrogen chloride are oxidized at the anode of the cell to produce essentially dry chlorine gas and protons, which are transported through the membrane of the cell. The transported protons are reduced at the cathode to form either hydrogen gas or water.

Description

BACKGROUND OF THE INVENTION1. Field of the InventionThe present invention relates to a process for electrochemically converting anhydrous hydrogen halide to an essentially dry halogen gas using a cation-transporting membrane. 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.2. Description of the Related ArtHydrogen 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 fe...

AI Technical Summary

Benefits of technology

The present invention solves the problems of the prior art by providing a process for the direct production of essentially dry halogen gas from anhydrous hydrogen halide. This process allows for direct processing of anhydrous hydrogen halide produced from many manufacturing processes, without first dissolving the hydrogen halide in water. This direct production of essentially dry halogen gas, when done, for example, for chlorine gas, is less capital intensive than processes of the prior art, which require separation of water from the chlorine gas.

The present invention also solves the problems of the prior art by providing a process in which chlorine is produced from a medium that is essentially water-free. Hence, in the electrochemical conversion of hydrogen chloride (gas) to chlorine and hydrogen, no appreciable amount of oxygen is produced. Oxidation of water at the anode is an undesirable side reaction which is virtually eliminated in the present invention. Hence, the reaction can be run at higher current densities for conversion to chlorine, which translates into higher chlorine production per unit area of electrode. Thus, the present invention requires lower investment costs than the electrochemical conversions of hydrogen chloride of the prior art.

An advantage of using anhydrous hydrogen chloride in the present invention rather than aqueous hydrogen chloride is that the theoretical cell voltage is lower by at least 0.3 V. This allows the cell to be operated at lower overall cell voltages than cells operated with aqueous hydrogen chloride. This advantage can translate directly into lower power costs per pound of chlorine generated than in the aqueous electrochemical processes of the prior art.

The present invention also provides a process which produces drier chlorine gas with fewer processing steps as compared to that produced by electrochemical or catalytic systems of the prior art, thereby simplifying processing conditions and reducing capital costs.

The present invention also provides for a process for converting anhydrous hydrogen chloride to essentially dry chlorine gas in order to recycle chlorine gas back to a manufacturing, or synthesis process, thereby eliminating environmental problems associated with the discharge of chloride ions.

Problems solved by technology

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.

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.

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..sup.2, because of this side reaction The result is reduced electrical efficiency and corrosion of the cell components due to the oxygen generated.

Evolution of oxygen decreases cell efficiency and leads to rapid corrosion of components of the cell.

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.

This is not a desirable reaction.

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 chlorine.

Furthermore, electrolytic processing of aqueous HCl can be mass-transfer limited.

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