Conventional batch-based system

A conventional wastewater treatment system with an average flow rate of 160,000 gpd produces effluent suitable for NPDES discharge. Metal hydroxide sludges are dewatered in a 15 cu. ft filter press producing more than one half ton of filter cake per day. The filter cake is further dewatered in a 7 cu. ft, batch-type sludge dryer. Based upon recommendations by their consultant, the firm also uses the sludge dryer to dehydrate nickel strip solutions. Two reverse osmosis systems are used for partial nickel recovery. Trivalent chromium is recovered by drag-out control and evaporation. 

Comparative tests have been performed in the semi-batch reactor system to evaluate the Ru/Ti02 cataly versus a more conventional nickel-based catalyst. These tests show that rutlienium at only 3% metal loading has about the same activity as nickel at S0% metal loading. This comparison is only for short-term activity of the catalyst. As demonstrated in the continuous flow tests, the nickel catalyst loses activity readily in tlie first hours on stream, while the ruthenium maintains its activity. 

The synthesis of most molecular sieve zeolites is carried out in batch systems, in which a caustic aluminate solution and a caustic silicate solution are mixed together, and the temperature held at some level above ambient (60-180°C) at autogenous pressures for some period of time (hours-days). It is quite common for the original mixture to become somewhat viscous shortly after mixing, due to the formation of an amorphous phase, i. e., an amorphous alumino-sdicate gel suspended in the basic medium. The viscous amorphous gel phase normally becomes less viscous as the temperature is raised, but this is not universally true, as in the case of some NH40H-based systems which remain viscous throughout the synthesis. The amorphous gel can be filtered from the solution and dehydrated by conventional drying methods. 

Often the coupling of the membrane unit with the bioreactor results in significant synergy as in the study of O Brien et al. [6.15] on the application of PVMBR to ethanol production, which we discussed in Chapter 3. The required bioreactor volume for the PVMBR system was smaller than that of the conventional system by a factor of 12. Nevertheless, it turns out that the PVMBR-based process is still 25 % more expensive than the classical batch fermentation process in terms of capital costs despite the substantial reduction in the required reactor volume. This cost differential is not only due to the membrane costs, which are, themselves, substantial, but also due to the cost of the additional hardware associated with membrane operation. The application of MBR for the ethanol production by fermentation faces marginal economics, since ethanol is a relatively cheap commodity chemical. 

Gravity based separation, dependent on the density difference between two phases, is the most commonly used method of separation. Difficulties that often occur in the separation of immiscible liquids include poor or slow phase separation, emulsion or rag layer formation, and poor process control, especially in batch systems. Some liquid-Uquid dispersions take hours to separate in conventional systems resulting in poor performances of the extraction units. 

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