(627c) Equation Oriented Modeling and Optimization Framework for Cryogenic Process and Energy Systems

In response to the recent surge in natural gas supply in the United States, several large scale liquefaction (LNG) plants have been proposed for transporting and potentially exporting natural gas [1]. Futhermore, there has been a strong reinvestment in bulk chemicals production; with many processes requiring products of air separation units (ASUs). LNG and ASU processes operate at cryogenic temperatures and high pressures, and the power required for compression is significant: air separation alone accounts for 3.5% of the U.S. manufacturing electricity demand [2]. Tight heat integration is imperative to ensure energy efficiency and profitability.

Both processes make use of multistream heat exchangers (MHEX) to maximize the recovery of refrigeration from cryogenic products. These are either of the plate-fin or coil-wound type, and allow for heat exchange between multiple product streams. Approach temperatures are typically very small, and one or more of the process streams may undergo phase change. Such units are essential to the efficient performance of the plant, and they must be considered at the flowsheet design and optimization stage. However, at present, very limited information is available in the published literature on equation-oriented, optimization-friendly MHEX models.

A MHEX model must be able to account for stream phase transformations, which, in general, are dealt with by incorporating discrete variables in the model equations. In this paper, we propose a different approach for constructing a robust equation-oriented MHEX model, based on the pseudo-transient continuation approach developed by the authors [3,5,6]. Our method bypasses the need for making discrete decisions on the phase of the streams by obtaining explicit transient expressions of stream temperature. We demonstrate that the proposed transient model has the same steady-state solution as a conventional MHEX model.

Subsequently, we integrate the developed MHEX model in the pseudo-transient process modeling and optimization framework introduced in previous work [3,4]. Using the unit operation library in [3], we explore two design case studies: the PRICO natural gas liquefaction process [7], and an air separation unit for nitrogen production [8]. We begin by performing flowsheet optimization in the nominal case, showing significant cost and energy savings compared to results from literature. Next, we explore the design of these processes under uncertainty. In the ASU case, we assume a variable electricity pricing structure, and identify the optimal flowsheet design and operating conditions for peak and off-peak pricing hours. For the LNG process, we consider a non-uniform natural gas composition and find the optimal design for all conditions. Our studies reveal that the MHEX is a potential bottleneck in the design and operation of tightly heat integrated processes that face multiple service conditions, a result that is consistent with the dynamic study reported in [8].

References

[1] Proposed North American LNG Export Terminals, 2014. Available at https://www.ferc.gov/ industries/gas/indus-act/lng/lng-export-proposed.pdf

[2] Zhu, Y.; Legg, S.; Laird, C.D. Optimal design of cryogenic air separation columns under uncertainty. Comput. Chem. Eng. 2010, 34, 1377-1384.

[3] Pattison, R.P.; Baldea, M. A pseudo-transient framework for equation-oriented modeling, simulation, and optimization of process and energy systems. AIChE J. Submitted.

[4] Zanfir, M.; Baldea, M.; Daoutidis, P. Optimizing the catalyst distribution for countercurrent methane steam reforming in plate reactors. AIChE J. 2011, 57, 2518–2528.

[5] R.C. Pattison, M. Baldea, Optimal Design of Air Separation Plants with Variable Electricity Pricing, Foundations of Computer-Aided Process Design (FOCAPD), Cle Elum, WA, July 2014.

[6] R.C. Pattison, M. Baldea, Equation-Oriented Models of Multistream Heat Exchangers for Flowsheet Optimization, 24th European Symposium on Computer Aided Process Engineering – ESCAPE 24,Budapest, Hungary, June 15-18,  2014.

[7] Price, B.C.; Mortko, R.A. PRICO - A simple, flexible proven approach to natural gas liquefaction. In Proceedings of the 17th International LNG/LPG conference, Gastech ’96. Austria Centre, Vienna. 1996.

[8] Cao, Y.; Swartz, C.L.E.; Baldea, M. Design for Dynamic Performance: Application to an Air Separation Unit. In American Control Conference, San Francisco, CA, 2683-2688, 2011.