(203g) Optimal Design of Inter-Plant Waste Energy Integration
Energy consumption represents a major concern in the industry because of the massive requirements of external utilities such as heating, cooling and electricity. In addition to the economic aspect, the use of these utilities represents a severe environmental impact because usually fossil fuels are burned to produce electricity and hot utilities. In this context, the implementation of HENs allows reducing significantly the external consumption of hot and cold utilities in a given plant (see Figure 1); nevertheless, usually significant amounts of heat at low temperature have to be removed using external cooling utilities. However, this heat excess at low temperature (i.e., waste heat) can be used as feed for an organic Rankine cycle (ORC) to produce electric power. This way reducing the use of external cooling utilities and at the same time to providing part of the electric power required for the process. In this regard, an eco-industrial park allows minimizing the resource consumption and environmental loads by using a recycling network for raw materials, waste and energy. However, the development of an efficient and well-structured eco-industrial park requires improve the material flows networks between the participating firms. Therefore, the implementation of ORC which can be used to recover additional waste heat into an EIP can be an attractive option for energy integration between plants (see Figure 1).
To solve this problem a new superstructure for heat integration of an eco-industrial park is proposed (see Figure 2). Intra and inter-plant heat exchange for the process streams are allowed; and for a proper reuse of the waste heat at low temperature, a set of organic Rankine cycles (ORCs) can be integrated inside the eco-industrial park. This way, the proposed superstructure allows a proper heat integration to reduce the use of external cooling and heating utilities as well as the consumption of external electricity. The superstructure of Figure 2is a schematic representation of two industrial plants where each one has one hot and one cold process streams considering three heat transfer stages in the HENs. The temperatures for the frontiers of the stages are treated as optimization variables. The heat excess of the hot process streams can be used to run an ORC inside each particular plant or in a shared ORC. The ORC cycles can produce energy, which can be used either to satisfy partially the energy demands of the different plants or for sale. The remaining heat exiting the ORCs is removed using cooling utilities. The cold process streams can be used to obtain heat at low temperature (at the right hand side of the superstructure) from the corresponding ORC and at high temperature (at the left hand side of the superstructure) from hot utilities. Notice that this superstructure is general and can be extended to any number of process streams.
The proposed superstructure is modeled through a mathematical programming formulation where the objective function considers the simultaneous minimization of the operating and capital costs for the units involved in the system as well the possible revenues for the sales of electricity. The model is formulated in such a way that avoids numerical complication during its solution. Results from the application of the proposed approach show that the interplant-integration represents significant savings respect to the traditional single-plant integration with and without considering ORCs.