Secondary dechlorination

Depleted brine will be physically saturated with chlorine, and some chlorine wUl react to form hypochlorite (Section 7.5.9.1). This chlorine value represents an economic asset to be recovered and, particularly in the case of membrane cells, an intolerable contaminant in the brine treatment system. There are several approaches to this problem [208], and we cover these below. We divide them into methods aimed at recovery of the bulk of the chlorine in a useful form (primary dechlorination Section 7.5.9.2) and those whose purpose is to reduce the active chlorine to chloride and safeguard the environment or other parts of the process (secondary dechlorination Section 7.5.9.3). Some of the hypochlorite that forms in the anolyte will continue to react to form chlorate. This is a much less harmful impurity in the cells, and higher concentrations are tolerable. Many plants keep the chlorate concentration under control by natural or deliberate purges from the brine system (Section 7.5.7.2A). In others, it is necessary to reduce some of the chlorate ion to chloride in order to maintain control (Section 7.5.9.4). 

An option to consider is operation without pressure control. The deeper vacuum that results will release more chlorine from solution. In a membrane-cell plant, this will reduce the cost of the secondary dechlorination, which is discussed below, but the approach may not be workable in a mercury-cell plant, where higher free chlorine concentrations are desirable. More water will evaporate along with the incremental chlorine. The load on the vacuum condenser will therefore increase, but not by as much as when the addition of steam is used for pressure control. At the same time, the dechlorinated brine will become cooler. This can only help the pumping operation. The cooler brine can also... 

Secondary Dechlorination Free chlorine can be destroyed or reduced from OCl to Cl by catalysis, chemisorption on activated carbon, or addition of a chemical reducing agent. While in principle some sort of reduction could be used for the entire process of dechlorination, it would be impractical not to recover most of the chlorine as the element. The reduction or decomposition process therefore is used only as a backup measure. The process is also useful in wastewater treatment. 

Whichever form of is used, the active agent will be the same. In solution, proton shifts between the different forms are rapid [217], and which form takes part in the reaction depends entirely upon the pH of the brine. Since secondary dechlorination of brine usually follows or accompanies the addition of alkali, the sulfite ion is the predominant form. Miron [218] reports that it is also the most active. The following equilibria apply ... 

Chlorinated condensate arises in the chlorine cooling system and must be stripped before disposal and even before reuse in the process. A secondary dechlorination may be necessary if the water is to be discarded or transferred to another operating unit. 

There have been only a few studies have evaluated membrane microfiltration of secondary wastewater effluent. Microfiltration membranes might be used to achieve very low turbidy effluents with very little variance in treated water quality. Because bacteria and many other microorganisms are also removed, such membrane disinfection might avoid the need for chlorine and subsequent dechlorination. Metal... 

The activating effect of a trichloromethyl group is seen in the 2-dechlorination reactions of 2-chloro-4,6-bis(trichloromethyl)-s-tria-zine (175) with arylsulfonylhydrazides (24 hr) and heterocyclic amines (3 hr) at 20° and with unbasifled primary and secondary alcohols (65°, 30 min). The 4,6-diphenyl or 4,6-bis(4-chlorophenyl) analogs do not react in this manner. ... 

Addition of phosphoryl chloride suppressed the secondary rearrangement by complexation of the zinc salts formed in the preparation of dichloroketene. The use of phosphoryl chloride in the zinc dechlorination of a-chlorocarboxylic acid chlorides increased the yields of ketene cycloadducts.106... 

Figure 6.7 Catalytic oxidation of secondary alcohols to ketones, with a one-pot regeneration ofthe PdCI2 catalyst by dechlorination of 1,2-dichloroethane to ethene.

Esters of secondary benzyl alcohols can also be electrohydrogenolyzed (equation 89). Electrolysis of resolved methyl 0-benzoylatrolactate gave racemized 2-phenylpropionic acid. This result is in sharp contrast with the 77-92% inversion of configuration claimed for the electrochemical dechlorination in equation (90). ... 

Primary, secondary, and tertiary alkyl chlorides as well as aryl chlorides undergo reductive dechlorination using Sml2, and in the presence of CO ketones are formed by photocarbonylation. ° ... 

Cell-free supernatants may catalyze reductions (1) the reduction of aromatic nitro compounds by the filtrate from a strain of Streptomyces sp. that is known to synthesize cinnaquinone (2-amino-3-carboxy-5-hydroxybenzo-l,4-quinone) and the 6,6/-diquinone (dicinnaquinone) as secondary metabolites (Glaus et al. 1992), and (2) the dechlorination of tetrachloro- and trichlo-romethane by extracellular products from Methanosarcina thermophila grown with Fe° (Novak et al. 1998). 

Phenyl trifluoromethylketen, obtained by zinc dechlorination of the appropriate a-chloroacyl chloride, (itself derived from a-trifluoromethylmandelic acid), has found use as a reagent for the stereoselective acylation of chiral secondary... 

The 4,6,6 -trichloride (P) was converted into 4,6,6 -trideoxysucrose (W) by catalytic, reductive dehalogenation in the presence of potassium hydroxide (17). On the other hand, in the presence of triethy-Icunine, dechlorination occurred exclusively at the secondary 4-position to give the 4-deoxy-6,6 -dichloride (X) as expected from the results of Lawton, Wood,... 

The final step is the tertiary treatment. It is used for specific contaminants which cannot be removed by the secondary treatment. This phase is not always present in a WWT, it depends on the origin of the sewage and the final use of the water output. Individual treatment processes sometimes are necessary to remove nitrogen, phosphorus, additional suspended solids, refractory organics, heavy metals and dissolved solids. The technologies to be used depend on the contaminants which must be removed, i.e. filters and separation membranes, systems for dechlorination and disinfection, reverse osmosis systems, ion exchangers, activated carbon adsorption systems and physical-chemical treatments. 

Tetrachloro-3-buten-l-yne (234) is obtained by partial dechlorination of hexachlorobutadiene using n-butyllithium. Subsequent reaction with a secondary amine or a metal thiolate affords (trichlorovinyl)yneamine or (trichlorovinyl)yne sulfide, respectively. Further dehalogenation by -butyllithium gives 4-amino- or 4-thio- " substituted 1,3-diynyllithiums (Scheme 1-180). 

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