Energy requirements steam

The choice of technology, the associated capital, and operating costs for a chlor—alkaU plant are strongly dependent on local factors. Especially important are local energy and transportation costs, as are environmental constraints. The primary difference ia operating costs between diaphragm, mercury, and membrane cell plants results from variations ia electricity requirements for the three processes (Table 25) so that local energy and steam costs are most important. 

Approximately 45% of the world s phthaUc anhydride production is by partial oxidation of 0-xylene or naphthalene ia tubular fixed-bed reactors. Approximately 15,000 tubes of 25-mm dia would be used ia a 31,000 t/yr reactor. Nitrate salts at 375—410°C are circulated from steam generators to maintain reaction temperatures. The resultant steam can be used for gas compression and distillation as one step ia reduciag process energy requirements (100). 

The energy requirements for desorbing 1,1-dichloroethane from activated carbon in a stripping—adsorption process for water purification have been calculated at 112 kj/kg (14). Chlorinated hydrocarbons such as 1,1-dichloroethane may easily be removed from water by air or steam stripping. 

The waste heat recovered in the boilers is in the form of steam and used either to generate electricity or to produce compressed air. The latter is used in the converter tuyeres. If electricity is generated, it is in turn used to produce compressed air for the converters. In practice, the energy made available in the waste heat boilers is almost equal to the energy required for producing the converter air. 

The suitabiHty and economics of a distillation separation depend on such factors as favorable vapor—Hquid equiHbria, feed composition, number of components to be separated, product purity requirements, the absolute pressure of the distillation, heat sensitivity, corrosivity, and continuous vs batch requirements. Distillation is somewhat energy-inefficient because in the usual case heat added at the base of the column is largely rejected overhead to an ambient sink. However, the source of energy for distillations is often low pressure steam which characteristically is in long supply and thus relatively inexpensive. Also, schemes have been devised for lowering the energy requirements of distillation and are described in many pubHcations (87). 

FIG. 8 Schematic illustration of the steps in the phase diagram and the energy required for ice starting at — 20 °C to become superheated gas (steam) at 120°C at atmospheric pressure (1 atm). The type and amount of heat (sensible or latent) required to change the temperature or phase are given, where Cp is the specific heat and AH is the change in enthalpy. 

It has been pointed out that the energy requirement for revaporisation, if required, is significantly less than that needed to liquefy the chlorine. The latent heat of 10 bar g steam is about 2000 kj kg-1, approximately 7.4 times greater than that of chlorine at... 

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