Conventional processes, marginal

It has already been observed that choice of solvent profoundly affects the sensitivity achieved in any given resist but few guidelines exist for this choice. Conventionally a marginal solvent is used, often a mixture of a solvent and a non-solvent, so a relatively small change in molecular weight renders the resist insoluble. It should be remembered that development is a kinetic process so temperature and other factors such as humidity are important if reproducible sensitivities are to be obtained, although the latitude in these factors is variable between resists. 

Time-Delay Compensation Time delays are a common occurrence in the process industries because of the presence of recycle loops, fluid-flow distance lags, and dead time in composition measurements resulting from use of chromatographic analysis. The presence of a time delay in a process severely hmits the performance of a conventional PID control system, reducing the stability margin of the closed-loop control system. Consequently, the controller gain must be reduced below that which could be used for a process without delay. Thus, the response of the closed-loop system will be sluggish compared to that of the system with no time delay. 

Carrier facilitated transport processes often achieve spectacular separations between closely related species because of the selectivity of the carriers. However, no coupled transport process has advanced to the commercial stage despite a steady stream of papers in the academic literature. The instability of the membranes is a major technical hurdle, but another issue has been the marginal improvements in economics offered by coupled transport processes over conventional technology such as solvent extraction or ion exchange. Major breakthroughs in performance are required to make coupled transport technology commercially competitive. 

The selective removal of very low levels of dioxin from large volumes of process wastewater has become a concern to a number of companies and municipalities. Very little has been reported on how dioxin behaves in conventional water treatment processes (2-3). The work of Thebault, Cases, and Fiessinger suggested that alum flocculation would be marginally effective for the removal of dioxin. Our need to remove up to several parts per trillion dioxin from water in the lagoon prompted us to evaluate flocculation as a faster and more cost effective treatment for dioxin removal. 

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. 

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