In this work a synthesis of the Total Site is performed, which simultaneously considers integration within and between plants (at the plant and at the Total Site levels). For this purpose, the superstructure optimization approach is used. The superstructure contains all the possible matches for heat exchange within and between processes. Heat exchange between processes can be performed in the following ways: i) directly, by exchanging heat between the hot stream of one process and the cold stream of another process, or ii) indirectly, by utilizing an intermediate utility at optimal temperature levels (these are optimization variables). In both cases the transport of the heat carrier, either the process stream or the intermediate utility stream, is considered. Because there are severe nonlinearities and numerous options for heat recovery, the model is difficult to implement even for small-scale problems. However, when evaluating potential matches, it usually happens that most matches are either infeasible given heat transfer limitations, or unviable for economic reasons. A two-step approach is therefore proposed, where in the first step match alternatives are pre-screened with respect to infeasibility and unviability, and in the second step, a more detailed design is synthesized, taking into consideration the reduced superstructure obtained in the first step. The proposed two-step procedure yields results simultaneously at both the process and the Total Site levels, while also accounting for important properties such as heat losses, pipeline design and cost, temperature/pressure drop during transport between processes, and different types of heat exchangers.
COBISS.SI-ID: 20011798
This work presents the development of an optimization model for feasibility study of bioethanol production within the selected region. This model would be a basis for the development of a similar model for designing biogas supply network from animal waste and plant biomass within this research project. Sugarcane, as well as residues from sugarcane, maize, sorghum, wheat and barley are among the acceptable bioethanol feedstocks in South Africa. As the feedstock cultivation areas are scattered over a wide geographical land area, and the potential bioethanol feedstocks have low energy density, there is a need to locate the processing facilities in strategic positions such that production and logistic costs and greenhouse gas (GHG) emissions due to transportation are minimized. To address the above mentioned challenges associated with bioethanol production in South Africa, a mathematical programming approach for the synthesis of optimal bioethanol supply network is developed. It accounts for sugar demands, production of bioenergy (ethanol, electricity and heat) and other byproducts, various feedstocks and their geographical distributions, etc. Various processing technologies for ethanol production are considered and two data sources for availabilities of feedstocks are used. Synthesis is performed by maximizing the economic objective, measured by annual profit. Evaluation of greenhouse gas (GHG) emissions is also performed. Based on the results obtained, it would be possible, in terms of economics, to meet and surpass the 2 % target penetration of biofuel into the South African national liquid fuel supply stipulated by the government.
COBISS.SI-ID: 20011542
This paper presents a further development of synthesis methods that considers economics and environmental impact in the integration of renewable energy into the optimisation of heat exchanger networks involving multiple periods of operations and multiple options of utilities. The multi-period process stream parameters, and those of the utility sources are integrated in a systematic approach using an expanded version of the simplified stage-wise superstructure multi-period model. Two examples were used to demonstrate the benefits of the expanded synthesis method and the quality of solutions obtained were judged by representation on a Pareto curve and by the use of a modified goal solution method. It was found that various combinations of utility sources were selected for use at various periods/seasons of operations, while utilities from solar photovoltaic were not selected for use at any of the periods/season of operation due to its relatively high cost and limited periods of availability.
COBISS.SI-ID: 20354070