The energy requirements may also be substantially reduced if processes can be operated at less extreme conditions of temperature or pressure. In addition the change from a liquid phase reaction to gas phase reaction can help to reduce the energy requirements.
Isocyanates are pre-products of many substances such as glues, paints and polyurethane foams. In the conventional process, as illustrated in Figure 7.26, they are manufactured by contacting amines with phosgene in a solution at the presence of a solvent. Due to its energy-intensive nature of downstream processing, where the solvent is separated from the product, industry was motivated to develop an alternative, less energy-intensive process. In such a way a gas phase synthesis was developed. With the new process design it is possible to reduce the amount of solvent by 90% and the excess of the reactant phosgene by 70%. Due to the reduced amounts of solvent and reactant the load of the energy-intensive multistage distillation route is also lower, and large recycle streams of phosgene in the system are eliminated. Therefore, the new process requires only half of the energy demand, compared with the conventional process and characterize in increased safety due to reduced hold-up [27, 31, 32].
The example from the isocyanate production mentioned above, illustrated how steam savings have been achieved. One of the biggest electricity consumers in the chemical industry is chlorine manufacturing. The main process is electrolysis of NaCl, during which chlorine, hydrogen and caustic soda are produced. Alternatively, chlorine is produced by electrolysis of HC1 solution, in which hydrogen is a by-product. A second process is applied on all sites where chlorine is used as a raw material, and HC1 as a side product of the reaction cannot be used for other purposes. On such plants, HC1 is sent back to the electrolysis, where it is used for chlorine production.
The oldest process is the amalgam process, where the specific energy consumption is in the region of 3.6MWht_1 chlorine. The amalgam process has been consequently replaced by a more efficient membrane process, characterized by about 30% lower electricity consumption, of c.a. 2.5 MWht-1 chlorine as illustrated in Figure 7.27. Over recent years, membrane technology has been optimized to such an extent that no substantial reduction of the energy demand can be expected from further process modifications .
The latest technology is so-called oxygen depolarized cathode (ODC), which combines principles of chlorine-alkali electrolysis with fuel cells, in which electric-
Liquid Phase Reaction Reactant 1: 100 Parts
Gas Phase Reaction Reactant 1: 30 Parts
Solvent: 100 Parts
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