The present invention relates to a method of producing chlorine on site. More particularly, the present invention relates to a method for continuously producing chlorine. In a further aspect, the present invention relates to an apparatus for producing chlorine. In addition, the present invention relates to a process for the treatment by chlorine of a substance.
Chlorine is a useful and frequently essential treatment chemical. It can be used in the following applications: Disinfection of both potable water and wastewater from sewage treatment plants to prevent the spread of disease (see Kirk-Othmer's Encyclopedia of Chemical Technology, Third Edition, volume 24, pages 327-441, which is incorporated by reference); control of marine growth in cooling water for power plants to keep the lines in power plants free from marine growth and encrustations; as an oxidant for odor control where the odorous compound can be destroyed by oxidation; as bleaching agent both industrially and in household bleach; as a sanitation agent, including food processing applications; and it is a major intermediate chemical used for the production of other chemicals.
The production of chlorine electrochemically in large chlorine/caustic plants using complex cells which keep the products from the anode separate from the products of the cathode is a well known art (see Kirk-Othmer's Encyclopedia of Chemical Technology, Third Edition, volume 1, pages 799-865 and volume 8, pages 662-695, which are incorporated by reference). Chlorine is produced in large facilities located near an economical source of power with NaCl as principle source of chlorine. The electrochemical cells used for the production of chlorine are complex, containing at least two compartments--an anolyte compartment in which chlorine is produced and a catholyte compartment in which sodium hydroxide is produced. These compartments are separated by a porous membrane or a separator. Chlorine gas produced at the anode is collected, cooled and compressed to a liquid and placed in heavy steel cylinders for transport to the point of use.
At the point of use the chlorine is permitted to volatilize to a gas, is absorbed in water and the water solution used for disinfection, bleaching, etc. The empty cylinders are returned for reuse. It was considered unlikely that novel methods of production can be found to produce chlorine at the point of use as inexpensively as bulk chlorine/caustic facilities.
Hydrochloric acid has been used as a feed material to electrolytic cells in the electrolyzers to produce chlorine gas. This has been done to recycle by-product hydrogen chloride from systems using chlorine in the preparation of other chemicals or intermediate process chemicals. Such processes (as described, for example, in U.S. Pat. 2,468,766; 3,129,152; and 2,719,822) use complex diaphragm electrolyzers and are tailored to the recycle system and the economics of the specific plant.
At ambient temperature and pressure chlorine is a hazardous, toxic, heavy gas which does not disperse readily if released accidentally to the atmosphere from its pressurized heavy cylinders. These hazardous properties require imposition of rigid regulations for the packaging, handling and transport of chlorine. In extreme situations the concern over the possible accidental release of chlorine has resulted in the prohibition of the transport of chlorine through the streets of some communities, thus preventing the use of chlorine for some of its essential uses.
This problem has been minimized in some cases by venting the chlorine from the chlorine/caustic plant with the co-produced sodium hydroxide to make sodium hypochlorite which can then be transported to the site of use. The concentration of sodium hypochlorite generally is in the range of about 15% by weight contained chlorine. While sodium hypochlorite (NaOCl) retains several of the useful properties of chlorine (e.g., disinfection, sanitation, odor control), it also increases the cost of transport since it contains almost 85% water, is unstable, and decays with time. Salt and excess caustic from the preparation of sodium hypochlorite are contaminating by-products transferred to the system being treated.
With the availability of seawater at some use sites, hypochlorite generating systems have been developed for the production of sodium hypochlorite on-site. This is done by the electrolytic conversion of a portion of the sodium chloride in the seawater to sodium hypochlorite. The weak sodium hypochlorite solution is then used for control of marine organisms in condenser cooling water at power plants, sanitation purposes and other uses. The presence of other impurities in the seawater (i.e., calcium, magnesium, bicarbonate and sulfate ions) results in formation of solids in the electrolyzer and associated piping. These scaling problems complicate the operations of such systems.
There are many situations and/or locations throughout the world where the cost of chlorine at the point of application is many times greater than the cost of chlorine at the bulk chlorine/caustic plant. It is desirable to produce chlorine at the point of application and thus avoid compression of the chlorine gas to a pressurized liquid and transporting the pressurized chlorine gas or the sodium hypochlorite produced from it to the site of application. The need then is for a process using industrially available materials to efficiently produce chlorine at the site of use in non-hazardous conditions, without undesirable by-products, at the rate required for the applications, and by a system that will require little operational expertise or supervision. Still needed is an uncomplicated system for producing chlorine: on site, as required, free from impurities and by-products, from commercially available raw materials, with environmental advantages, and being cost competitive on a site selective basis.
Much of the prior art related to production of chlorine and/or chlorine dioxide use a multi-compartment electrolyzer which separates the anolyte from the catholyte to prevent the reduction of the chlorine at the cathode, such reduction at the cathode results in low current efficiency:
Cl.sup..cndot. +e.sup.- .fwdarw.Cl.sup.-
Water and cations (H.sup.+, H.sub.3 0.sup.+, Na.sup.+) on passing through the membrane pass from a low pH region (the anolyte) to a high pH region (the catholyte). The hardness cations also pass or attempt to pass through the membrane, but tend to form solids (Ca.sup.++ and Mg.sup.++ hydroxides and carbonates) which decrease the porosity and performance of the membranes. Various ways of overcoming or minimizing this problem have been suggested, such as purification of the NaCl and/or the water or adding complexing agents for the scale forming substances in the electrolyte and air. This scaling problem complicates the process, decreases efficiencies, and increases maintenance of the system or decreases the life of the system.
U.S. Pat. No. 5,039,383 discloses a process for the generation of halogen (e.g., chlorine) in an electrolytic cell by the electrolysis of a hydrohalic acid contained in an aqueous hydrohalic acid solution as the cell electrolyte while maintaining the solution at an elevated temperature (at least about or above 40.degree. C.). The cell is provided with a means to continuously remove molecular hydrogen and halogen and electrolyte water from the cell by sparging the electrolyte with an inert gas (e.g., air). The process employs volume control instead of pH control. However, this process has several disadvantages not present in the present invention: The electrolyte is continuously exposed to reducing conditions which results in lower efficiencies. The purge of electrolyte in the cell is not very efficient. Elevated temperature increases the corrosion potential throughout the system and the heat load on the treated system (e.g., cooling tower), HCl carry-over is also increased. Multiple openings and an open space for gas collection are required which could result in a possible explosion hazard. High cell voltage, with resulting increased cost, is required. Electrical conductivity is dependent on HCl concentration.