Saving energy

When the spent sulfuric acid must be reconcentrated, it has been found that reconcentration to 100% sulfuric acid is not necessary (13). An energy savings can be realized if the sulfuric acid is concentrated under a vacuum to only 75—92%. If the process is carried out at 130—195°C, these medium concentration ranges are sufficient to destroy trace organics, thus preventing any loss in the efficiency or capacity of the nitration process. This process is adaptable to existing manufacturing installations. 

Many units have waste heat recovery systems that generate low pressure steam from reaction heat. Such steam is often employed to drive adsorption refrigeration units to cool the reactor feed stream and to increase polymer conversion per pass, an energy-saving process that reduces the demand for electrical power. 

Selection of the high pressure steam conditions is an economic optimisation based on energy savings and equipment costs. Heat recovery iato the high pressure system is usually available from the process ia the secondary reformer and ammonia converter effluents, and the flue gas ia the reformer convection section. Recovery is ia the form of latent, superheat, or high pressure boiler feedwater sensible heat. Low level heat recovery is limited by the operating conditions of the deaerator. 

Desalination. Desalination of seawater and brackish water has been and, as of the mid-1990s, is the primary use of RO. Driven by a need for potable water in areas of the world where there is a shortage, this industry has developed. Desalination involves the reduction of the total dissolved soHds (IDS) concentration to less than 200 mg/L. RO offers several advantages over other possible desalination processes such as distillation (qv), evaporation (qv), and electro dialysis. The primary advantage of RO over the traditionally used method of distillation is the energy savings that is afforded by the lack of a phase change in RO. 

Because zinc-based alloys have low melting points, energy savings in the melting operation are substantial and the foundry operation is essentially free of fume. With the current trend of increasing energy costs and pollution control, cost benefits can be considerable (114). 

Freeze Crystallization. Freezing may be used to form pure ice crystals, which are then removed from the slurry by screens sized to pass the fine sohds but to catch the crystals and leave behind a more concentrated slurry. The process has been considered mostly for solutions, not suspensions. However, freeze crystallization has been tested for concentrating orange juice where sohds are present (see Fruit juices). Commercial apphcations include fmit juices, coffee, beer, wine (qv), and vinegar (qv). A test on milk was begun in 1989 (123). Freeze crystallization has concentrated pulp and paper black hquor from 6% to 30% dissolved sohds and showed energy savings of over 75% compared with multiple-effect evaporation. Only 35—46 kJ/kg (15—20 Btu/lb) of water removed was consumed in the process (124). 

Control. Energy savings for improved control are surprisingly high. A 2 to 20% savings from a series of control projects has been reported (17). 

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