Chlorine recovery rate

The chlorine recovery rate as gaseous HCl is ca. 90 %. The steam production is ca. 4.41 per tonne of residue, which amounts to ca. 85 % heat recovery related to the calorific value of the residues. 

The membrane selectively rejects oxygen and nitrogen. The field test showed a selectivity for chlorine over nitrogen of about ten. That this is so much lower than that obtained in the laboratory is attributed to concentration polarisation. Increasing the rate of flow through the module can alleviate this. At the same time, chlorine recovery can be maintained by adding modules in series. This is precisely what would be done in a commercial unit, and so one can reasonably expect better results in full-scale operation. 

Pibouleau et al. (1988) provided a more flexible representation for the synthesis problem by replacing the single reactor unit by a cascade of CSTRs. They also introduced parameters for defining the recovery rates of intermediate components into the distillate, the split fractions of top and bottom components that are recycled toward the reactor sequence, as well as parameters for the split fractions of the reactor outlet streams. A benzene chlorination process was studied as an example problem for this synthesis approach. In this example, the number of CSTRs in the cascade was treated as a parameter that ranged from one up to a maximum of four reactors. By repeatedly solving the synthesis problem, an optimum number of CSTRs was determined. 

The data shown in Table 6 have been obtained for a chlorine recovery system in which all values have been determined at the optimum operating conditions of retention time and recycle rates. 

In the selection of control equipment, the most important waste-gas characteristics are volumetric flow rate, concentration and composition of organic compounds in the waste-gas, waste-gas temperature and humidity, and rbe content of particulate matter, chlorinated hydrocarbons, and toxic pollutants. Other factors influencing the equipment selection are the required removal efficiency, recovery requirements, investment and operating costs, ease of installation, and considerations of operation and maintenance. The selection of a suitable control method is based on the fundamental selection criteria presented as well as the special characteristics of the project. 

Applying this information to a typical diaphragm-cell tail gas, Fig. 7.4 shows the logarithm of the amount of unrecovered chlorine versus the relative membrane area required. Recovery of chlorine is not far from a first-order process. As chlorine selectively passes through the membrane, the partial pressures of the impurities increase in the remaining gas. This causes their rates of permeation to increase. The membrane area required for permeation of, say, 30% of the nitrogen is less than twice that required for 15%. 

Nitrogen dioxide is about 20 to 50% of the total nitrogen oxides NO, (NO, NOz, HN03, N2Os), while CIO represents about 10 to 15% of the total chlorine species CIO, (Cl, CIO, HCI) at 25 to 30 km. Hence, the rate of ozone removal by CIO, is about equal to that by NO, if the amounts of NO, are equal to those of CIO,. According to a calculation by Turco and Whitten (981), the reduction of ozone in the stratosphere in the year 2022 with a continuous use of chlorofluoromethanes at present levels would be 7%. Rowland and Molina (843) conclude that the ozone depletion level at present is about 1%, but it would increase up to 15 to 20% ifthechlorofluoromethane injection were to continue indefinitely at the present rates. Even if release of chlorofluorocarbons were stopped after a large reduction of ozone were found, it would take 100 or more years for full recovery, since diffusion of chlorofluorocarbons to the stratosphere from the troposphere is a slow process. The only loss mechanism of chlorofluorocarbons is the photolysis in the stratosphere, production of HCI, diffusion back to the troposphere, and rainout. 

Three factors are responsible for the forecast increase in chemical prices. Energy intensive materials, such as chlorine and caustic soda, will have a corresponding increase in cost. Mining and recovery expenses for natural minerals will increase with the need to utilize more remote deposits. Capital costs for additional and replacement facilities will continue to rise faster than the forecast rate of inflation. This is especially true for grass-roots plants in remote areas, which entail the additional costs of the supporting infrastructure. 

The off-gas from a chloral production unit contains 15 vol % Cl 75 vol % HC1, and 10 vol % EtCl2. This gas is produced at a rate of 150 cfm based on 70°F and 2 psig. It has been proposed to recover part of the Cl, by absorption and reaction in a partially chlorinated alcohol (PCA). The off-gas is to pass continuously through a packed absorption tower counter-current to the PCA, where Cl, is absorbed and partially reacts with the PCA. The gas leaving the top of the tower passes through an alcohol condenser and thence to an existing HC1 recovery unit. 

Leaching and electrolysis processes can be used for metal recovery from waste electrical and electronic equipment. Metals such as Ag, Au, Cu, Pb, Pd, Sn, are dissolved from shredded electronic scrap in an acidic aqueous chloride electrolyte by oxidizing them with aqueous dissolved chlorine species. In the electrochemical reactor, chlorine is generated at the anode for use as the oxidant in the leach reactor and the dissolved metals are deposited from the leach solution at the cathode. The very low concentrations of the precious metal ions require the use of porous electrodes with high specific surface areas and high mass transport rates to achieve economically adequate reactor productivities and space-time yields [72]. 

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