Chlorine processing energy consumption

The process energy consumption in a membrane cell is small compared to diaphragm and mercury cell operations and the membrane cell caustic is of the same quality as mercury cell caustic. Hence, the membrane cell technology is recognized as the most economical and preferred method for producing chlorine and caustic (see Section 6 for additional details regarding membrane cells). 

Comparison to the Raschig Process. The economics of this peroxide process in comparison to the Raschig or hypochlorite—ketazine processes depend on the relative costs of chlorine, caustic, and hydrogen peroxide. An inexpensive source of peroxide would make this process attractive. Its energy consumption could be somewhat less, because the ketazine in the peroxide process is recovered by decantation rather than by distillation as in the hypcochlorite process. A big advantage of the peroxide process is the elimination of sodium chloride as a by-product this is important where salt discharge is an environmental concern. In addition to Elf Atochem, Mitsubishi Gas (Japan) uses a peroxide process. 

Energy Requirements. The energy requirements of several sodium manufacturiag processes are compared ia Table 7 (76). The data contain some ambiguities because of the allocation of energy to the coproduction of chlorine. An iadependent calculation shows a somewhat lower energy consumption for the Downs process (92). 

In the membrane process, the chlorine (at the anode) and the hydrogen (at the cathode) are kept apart by a selective polymer membrane that allows the sodium ions to pass into the cathodic compartment and react with the hydroxyl ions to form caustic soda. The depleted brine is dechlorinated and recycled to the input stage. As noted already, the membrane cell process is the preferred process for new plants. Diaphragm processes may be acceptable, in some circumstances, but only if nonasbestos diaphragms are used. The energy consumption in a membrane cell process is of the order of 2,200 to 2,500 kilowatt-hours per... 

Hypochlorous acid formed at the anode by hydrolysis of dissolved chlorine reacts with hypochlorite according to the equation (XII I-14). This undesirable process can be partially suppressed by maintaining a low temperature of the electrolyte. The cooling of the solution is still more important when higher current densities are used. A low temperature is, however, disadvantageous as far as energy consumption is concerned, as the ohmic resistance of the electrolyte increases. 

The-phenol is obtained in a purity pf up to 97 per cent weight The yield of the operation is relatively limited (80 to 90 mofar percent). This process offers the advantage over direct chlorination of using hydrochloric arid instead of chlorine. However, capital expenditure and energy consumption are high. Maintenance costs incurred by the handling of corrosive acidic substances have a direct effect on the economics of this technique. 

Disadvantages are the higher energy consumption (0.3-1 kWh/m ) compared to UF and microfiltration (MF) the need for pretreatment for some heavily polluted waters (prefiltration 0.1-20 pm). NF membranes are more expensive than RO membranes, and are sensitive to free chlorine (life span of 1000 ppmh) recommending an active carbon filter or a bi-sulfite treatment for high chlorine concentrations. After NF treatment a volume-reduced concentrated flow is obtained, reqniring further treatment the flow often needs to be processed externally. 

Errors Involved in the Energy Consumption and Efficiency Calculations. Error analysis using the propagation of errors technique, in Appendix 4.4.3, shows the error associated with the process chlorine efficiency to be 0.25% and the error in the energy consumption, AP, calculation to be 15kWhrton of CI2 as shown in Table 4.4.2. The analytical method, along with its precision is detailed in Table 4.4.3. 

For simplicity, Eq. (A3.2) is used to estimate the errors in the chlorine process efficiency and enei consumption calculations. The equation used for the energy consumption calculation is given by... 

Production of HCl uses chlorine that otherwise could be recovered (at a cost) and so detracts from its net production. Where there is a reliable outlet for caustic, however, the best approach may be to increase electrolytic capacity, use HCl liberally in its in-plant applications, and reduce somewhat the severity of liquefaction. This improves the quality of the cell gas and allows more chlorine to appear in the tail gas, which is the raw material for HCl production. Both these changes reduce energy consumption in the liquefaction process. The gross production of elemental chlorine is preserved, all the benefits of acidification are obtained, and more caustic is available for use or sale. 

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