Energy Integration of Industrial Batch Processes (PinchBATCH) - Phase 2

A comparative study has demonstrated that the formerly developed Pinch Analysis (PA) based targeting and design methods outperform (in both heat recovery and cost-effectiveness) the simplified combinatorial design methods proposed by other authors (namely the Omnium Verfahren and the Permutation Method). The PA based methods provide valuable insight in the problem, but aren’t suitable for automated design and optimization. PinchBATCH results therefore in the following advances: 1) a strategy (including guidelines and quantitative criterias) is proposed to restrict the analysis of batch processes to meaningful time slices. Likely variations of the schedule are taken into account; 2) practical constrains limiting the repipe and resequence of heat exchangers are identified and formalized under a matrix format. The formalization of all these constraints is needed for automated design methods to actually focus on feasible solutions; 3) a strategy based on genetic algorithms (GAs) is proposed for the synthesis of direct batch heat exchangers networks (i.e. without heat storage). The strategy features a decomposition into an upper level, which optimizes the overall network, and a lower level which search for the best use of the heat exchangers across time slices; 4) for indirect heat recovery using heat storage, a heuristic, PA inspired approach for targeting and simplified design is proposed and demonstrated. It uses TAM composites and includes a method to identify the minimum number of heat storage units (HSUs) as function of the heat recovery. Most generally, local minimums are to be found just before a storage pinch appears, and the search strategy systematically removes one bottleneck at a time, taking advantage of the available degrees of freedom. A simple graphical tool to assess opportunities to decrease the required capacity of HSUs by rescheduling of streams is proposed. A model suitable for a GA based optimization is also proposed, featuring a decomposition into two levels; 5) detailed specifications of the organization and of the features to be implemented in a software tool to address the heat integration of batch processes have been produced. The very next steps will be concerned with the implementation and the validation of the GA based synthesis approaches.

Contribution to the heat integration of batch processes (with or without heat storage)

Pierre Krummenacher

This work addresses the indirect heat integration (i.e. resorting to intermediate heat storage) and the direct heat integration (i.e. heat exchanges between coexisting process streams) of batch processes. Tools and methods for the targeting of these two limiting cases of heat integration are proposed, and completed by the development and the application of an automatic design & optimization methodology using the Struggle genetic algorithm (GA). A brewery process is used to demonstrate the feasibility and the practical relevance of the proposed indirect heat integration models. The fluctuations of the process schedule and their effects on the optimal solutions are not modelled (the indirect heat integration is known to feature an inherently low sensitivity). The rescheduling opportunities are not searched for. For the indirect heat integration, fixed temperature/variable mass inventory heat storage units (HSUs) are applied. Two models of indirect heat recovery schemes (IHRSs) are proposed : one based on a closed heat storage system, another built around an open storage system suitable e.g. for food and beverages industries. The inequality constraints of the IHRS models are automatically met owing to an appropriate definition of the decision variables managed by the GA. A rough preadjustment of the mass balance equality constraints on HSUs is achieved by a preliminary stage of heat recovery (HR) maximization before actually minimizing the total batch costs (TBCs). Optimization runs and theoretical considerations on the generation & replacement strategy of Struggle demonstrate that the structural and the parametric variables cannot be efficiently optimized within a single level, resulting in a two-levels optimization scheme. The automatically designed closed storage IHRS solutions for the brewing process are as good as the solutions obtained by another author using a combinatorial method followed by a post-optimization stage. The open storage IHRS is 13 % cheaper while the HR increases by 12 %. Optimizing the IHRS on a one-week period (including the non-periodic start-up & shut-down phases) results in an even more realistic solution, featuring a significantly different trade-off between energy, HSU capacities and HEX areas. A GA based, two-levels optimization scheme is proposed for the design of direct batch heat exchanger networks (HENs). The HEN structures, managed by the upper-level GA, do not include stream splitting. The re-use of HEX units across time slices is a key issue and a methodology to specify the actually possible structural changes by repipe or resequence is proposed, accounting for the thermo-physical compatibility, chemical compatibility, and process schedule constraints. The optimum operation of an existing HEN during each time slice has been analysed and a sound solution procedure is proposed.

EPFL2002

Process Integration: From Continuous to Batch Processes.

Daniel Favrat, Pierre Krummenacher

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.

2000

Study on the definition of dTmin for heat recovery

Jean-Pascal Wenk

This project inserts itself in improving the understanding of the sensitivity of the dTmin against the parameters that influence it. It is also aimed at helping to improve OSMOSE, a platform designed for the study of energy conversions systems. Three economic criteria have been taken to achieve the heat integration optimization: the net present value, the yearly total cost and the payback period. This paper explores briefly several heat exchanger types and cost estimation models and presents then the program developed on Matlab and Octave for the optimization. The results are discussed, highlighting the fact that the payback period is not relevant for the evaluation of a project, and that the NPV, used by economists, should be considered. Two case studies are presented and compared, when we consider – for a varying heat load – the cold stream with an infinite mcp or the same mcp as the hot stream.