Targeting And Optimal Design Of Refinery Hydrogen Network

Increasingly stricter environmental legislation and product quality regulation in addition to higher sulfur content of crude oil has forced oil refiners to increasingly adpot hydrocracking and hydrotreating processes, which leads to more fresh hydrogen consumption. The cost of hydrogen utility has turned to be the second largest production costs after the crude oil in a refinery. Therefore optimizing the hydrogen network to reduce the fresh hydrogen consumption can greatly reduce the cost of hydrogen utility and improve the profit margin. In this thesis, after the summary of recent research progress in refinery hydrogen system, we investigate the challenging issues about the hydrogen network optimization systemically. Our researches can be categorized into two aspects:economical optimization and flexible operation. For the former aspect, we employ pinch and superstructure based methods to integrate the hydrogen network to reduce the fresh hydrogen consumption, while for the latter, we propose the new concept mixing potential and employ the robust optimization to obtain the hydrogen network distribution with stronger disturbance resistance ability and less sensitivity to the uncertainty of the market, respectively.The main contents and results are as follows:(1) A new graphical method is proposed by improving the existing graphical approach in two aspects:incorporate algebraic equations into graphical methods to find the initial location of purifiers with different kinds of purifier models and develop novel pinch sliding approach to simplify the shifting procedures in composite curves. In this method, both performance based and mechanism based models of purifier can be solved. What’s more, another advantage of this targeting approach is that the flow rate of the feed streams to purifier also can be optimized simultaneously, which can greatly reduce the energy consumption of the purification process.(2) To illustrate the influence of purifier on the hydrogen utility consumption of the system, a new kind of numerical method is proposed to show the results visually. The superstructure based hydrogen network mathematical model and purifier model are solved in GAMS and MATLAB respectively. Two simplified purifier models, the mechanical-based membrane separation model and PSA separation model are employed to find the influence of different purifier models on the consumption of fresh hydrogen. The whole results of the system with different parameters can be achieved from which the minimum utility consumption and feed flow rate to the purifier can be easily found.(3) A new method based on the principle that minimizing thermodynamic irreversibility of the satisfying process will result in minimum utility consumption of the hydrogen network is proposed to design hydrogen networks with multiple contaminants. The entropy change which indicates the thermodynamic irreversibility of the satisfying process is minimized for each sink to obtain the minimum utility consumption. A new concept, virtual concentration, which is drawn from the thermodynamic analysis of the satisfying process, is proposed to measure the purity of the hydrogen streams with multiple contaminants. Source streams with the nearest virtual concentration as the sink are used to satisfy the sink to reduce the entropy change of each satisfying process. Then detailed design procedures are proposed to obtain the hydrogen network distribution.(4) Since the resulting network structure featuring minimum utility consumption is not unique, other properties such as disturbance resistance have drawn more and more attention. A novel concept, Mixing Potential, is proposed to improve the disturbance resistance ability of the networks in the design stage. This concept originates from measuring the concentration fluctuation of a single sink, and could be calculated by its graphical and algorithmic definition respectively. In addition, a sufficient condition for minimizing the Mixing Potential of a single sink has been proved. Based on this sufficient condition, a graphical and its corresponding algorithmic method are proposed to design the hydrogen and water networks with minimum utility consumption.(5) As the operation of refineries encounters uncertainty with the rapidly changing market and deteriorating crude oil, existing approaches are inadequate to achieve robust hydrogen network distribution due to the uncertain factors. Therefore, robust optimization is introduced as a framework to optimize hydrogen network of refineries under uncertainty. In this framework, a number of scenarios representing possible future environments are considered. A possible optimal network distribution which is less sensitive to the change of scenarios and has the minimum total annual cost is achieved by the tradeoff between the total annual cost and the expected error. Case studies indicate that this method is effective in dealing with hydrogen network design and planning under uncertainty in comparison to the deterministic approach and the stochastic programming method.