(186a) Optimization-Based Retrofit of a Cryogenic Air Separation Unit for Flexible Operation
Recent publications for dynamic operation of ASUs have mainly focused on either advanced process control strategies for existing plants (e.g., ) or on long-term scheduling of production cycles considering storage and vaporization of products (e.g., [3,7]). They found that adding further production equipment is economically desirable for cases with high utilization of the plant. However, they did not consider the transition time between operating modes in particular. Cao et al.  assessed design limitations to the dynamic behavior of the high-pressure nitrogen column in response to demand fluctuations and proposed the introduction of liquid nitrogen as an additional reflux to improve the plantâs agility.
In this work, we propose a methodology for the redesign of the ASU process that considers the process dynamics, i.e., its transient behavior. This process design problem results in a large-scale dynamic optimization problem due to dynamic balances on the ASU columnsâ trays. In order to improve the computational efficiency, the rigorous dynamic stage-wise model is reduced using the collocation-based approach. Herein, dynamic balance equations are only considered for few trays, which reduces the number of differential equations and variables. The remaining trays are coupled using Lagrange polynomials. Then, a characteristic load change of the plant is optimized using a single-shooting dynamic optimization algorithm. In a post-optimal sensitivity analysis, we further identify bottlenecks that limit the plant's agility and propose new design modifications to overcome those limitations. The results show that resizing of column equipment, e.g., the volume of column sumps, and additional liquid buffer tanks can enhance process agility.
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