Published December 2018
Hydrogen chloride (HCl) is produced as a by-product in various chemical chlorination processes and via burner acid produced in chlor-alkali plants. This by-product acid can be oxidized to produce secondary chlorine for recycle or other reuse. Modern methods for recovery of secondary chlorine from HCl solutions include electrochemical oxidation processes, using membrane or diaphragm cell technologies, and the catalytic oxidation processes of Sumitomo and Bayer.
Alternatives to oxidation of HCl include producing muriatic acid (aqueous HCl), by absorbing HCl into water, for merchant marketing or in-house use, or neutralizing the aqueous HCl. Selling the acid can be cumbersome in terms of creating codependent markets, and neutralization creates economic waste with environmental pollution. Some aqueous HCl is too contaminated for sale or reuse, as can occur, for example, in production of titanium dioxide. This low-quality HCl acid may be deep-welled, or more commonly, reacted with limestone to make calcium chloride.
Manufacture of the isocyanates MDI (methylene di-para-phenylene isocyanate) and TDI (toluene diisocyanate) via phosgenation coproduces 0.58 ton of HCl per ton of MDI and 0.84 ton of HCl per ton of TDI. There are many more MDI units than TDI units in operation today, and local markets for HCl can become saturated. For example, Wanhua Chemical (Yantai, China) has announced plans to build a 400 ktpa MDI plant in Louisiana, slated to begin construction in 2019. Merchant marketing of the resultant HCl by-product could easily oversupply the local muriatic acid market, while disposal by neutralization or deep welling would present a major environmental problem. The need to reuse such HCl is increasing commercial interest in the manufacture of secondary chlorine.
This review presents a technical and market overview of the secondary chlorine field, followed by a description, process design, and economics for the electrolytic process of thyssenkrupp Uhde Chlorine Engineers using ODC (oxygen depolarized cathode) technology to covert HCl to chlorine. The process design is based on HCl feed provided by a worldscale MDI plant. An interactive module is included—the iPEP Navigator ODC chlorine tool—which provides an economics snapshot for the process and allows the user to select the units and global regions of interest.