Process Integration: From Continuous to Batch Processes.

Among the structure independent approaches of process integration, pinch technology has been shown for continuous processes to be extremely powerful to develop energy efficient processes and enhance synergic effects within and between processes. The earlier focus of the method has mainly been concerned with a minimization of the heat transfer exergy losses based on a given economic criterion. However, there is a need to increasingly consider environmental and other life cycle aspects as early as possible in the design phase. This trend has stimulated the need to extend the method including its handy graphical representations to also include the other major exergy losses (dissipation and equipment fabrication losses). A new approach proposed by Staine et al. (1995, 1996, 1999) provides a coherent framework for representing the overall exergy balance as well as the simultaneous representation of both heat and electricity balances of the process or plant being analyzed. Examples of application of this so-called extended pinch analysis method to a few continuous industrial processes will be shown. The second part of this presentation will deal with another challenging goal, which is the energy integration of batch processes. Batch processing is the preferred mode of operation whenever flexibility is a key issue. Although this mode of operation is mainly used for low volume, high value products, noticeable exceptions do exist, such as brewing which is an energy intensive, large volume process. Methodologically the analysis and design of the heat integration of batch processes is complex due to the time dimension and the large number of degrees of freedom or alternatives to be addressed (e.g. assignment of equipment to tasks, scheduling, modes of heat recovery, possible re-use of heat exchanger units). So-called "non-flowing streams" (e.g. in-vessel heating or cooling) add to the complexity, and many fine chemical processes feature time-temperature profiles making data extraction a difficult task. Particularities and practical constraints have to be taken into account in order for the solutions to make sense; fortunately, these constraints often rule out a number of alternatives, considerably restricting the solution space. Strategies based on mathematical programming have also been proposed; several of them provide with the advantage of simultaneously addressing the scheduling issue, but with the drawback of a restricted scope of application. These methods will however not be specifically addressed here. Batch processes often include in-vessel heat transfer tasks characterized by small heat transfer areas with large temperature degradations across the reactor wall (resulting from both a low heat transfer coefficient and the request for maximum heat rates to save valuable time & production capacity). Therefore one of the very first analysis step is to consider the feasibility of external pre-heating and cooling during loading/transfer between vessels.