Improved Model Formulations for Multiperiod Hydrogen Network Designs

Abstract

Operating cost reduction and/or air pollution abatement via hydrogen integration is a research issue that has recently attracted considerable attention in the petroleum refining industries. Although a number of mathematical programming models have been developed to generate the optimal hydrogen distribution schemes, there is still room for further improvement. In particular, the primary deficiencies in current modeling practices can be attributed to (1) unreasonable unit models of the hydrogen users and producers and (2) incomprehensive design considerations. The conventional models of hydrogen users (such as the hydrotreater and hydrocracker) were usually formulated according to fixed throughputs and also constant feed and product concentrations. On the basis of the shortcut calculation method proposed in this study, not only these inlet and outlet flow rates and concentrations can be treated as decision variables but also their interactions can be characterized with more consistent material-balance constraints. On the other hand, because every hydrogen producer, e.g., the steam reforming plant, was regarded only as a simple hydrogen source in the past, the more rigorous models of its embedded units are built and incorporated in the improved mathematical program. As a result of these modifications, better design options can be identified in the present study. To ensure comprehensive design considerations, all often-encountered seasonal variations in model parameters and options to add extra compressors, purifiers, and fuel cells are considered in a novel multiperiod formulation. Furthermore, as an alternative to numerically solving this model, a systematic timesharing algorithm is also devised to manually integrate the conventional single-period designs to form a less economical but more flexible network structure for operations in all periods. Finally, extensive case studies have been carried out to test the proposed design methods, and three examples are reported.