Design and Implementation of Modular Subroutines for Simulation of LNG Plants

A nonsmooth equation-oriented multistream heat exchanger (MHEX) model has been developed by the Process Systems Engineering Laboratory at Massachusetts Institute of Technology that is intended to be a part of a rigorous optimization and simulation tool for liquefied natural gas (LNG) processes. The model was successfully used to simulate the poly refrigerant integrated cycle operations (PRICO) process for LNG production, though it suffered from convergence difficulties in more complex single mixed refrigerant processes. The primary challenge was flash calculations, which frequently failed to converge with a Newton solver even for initial guesses close to the solution. Equation-oriented simulation models have the advantage of high efficiency, but are generally less robust than the sequential-modular approach, such that improved performance may be achieved by using a different simulation framework. This master thesis studies two alternative model structures. First, the equation-oriented framework is replaced with a hybrid solution, in which vapour-liquid equilibrium calculations are included as nested subroutines and solved sequentially. Next, a fully sequential-modular approach is considered. The models are tested for different single mixed refrigerant processes, and are solved with a nonsmooth Newton-type solver using Clarke Jacobian elements as generalized derivatives. The implicit function theorem for lexicographically smooth functions is used for computing analytical derivatives in the subroutines. Results showed that the hybrid models were considerably more robust than the original equation-oriented models. In addition, they required fewer iterations to converge. However, as expected, they suffered a loss in efficiency. About half the computing time in the hybrid PRICO model was spent on the vapour-liquid equilibrium modules, which was primarily due to derivatives calculations. As a consequence the time per iteration was between 4 and 5 times longer for the processes studied, and even with fewer required iterations, the models were normally 1.5-3 times slower. On the other hand, the sequential-modular framework turned out to be unsuitable for simulating the LNG models as it was both significantly less robust and efficient than the other approaches. The observed drop in robustness is against the general theory, however, and it was concluded that the convergence problems were caused by modularizing the MHEX model

Work and Heat Integration: An emerging research area

The extension from Heat Integration (HI) and design of Heat Exchanger Networks (HENs) to including heating and cooling effects from pressure changing equipment has been referred to as Work and Heat Integration and design of Work and Heat Exchange Networks (WHENs). This is an emerging research area of Process Synthesis, however, WHENs is a considerably more complex design task than HENs. A key challenge is the fact that temperature changes (related to heat) and pressure changes (related to work) of process streams are interacting. Changes in inlet temperatures to compressors and expanders resulting from heat integration will influence work consumption and production. Likewise, pressure changes by compression and expansion will change the temperatures of process streams, thus affecting heat integration. As a result, Composite and Grand Composite Curves will change shape due to pressure changes in the process. The thermodynamic path of process streams from supply (pressure, temperature) to target state is not known and depends on the sequence of heating, cooling, compression and expansion. This paper introduces a definition and describes the development of WHENs. Future research challenges related to methodology development and industrial applications will be addressed. The potential of WHENs will be indicated through examples in literature.Work and Heat Integration: An emerging research areaacceptedVersio

Model reformulations for Work and Heat Exchange Network (WHEN) synthesis problems

The Duran-Grossmann model can deal with heat integration problems with variable process streams. Work and Heat Exchange Networks (WHENs) represent an extension of Heat Exchange Networks. In WHEN problems, the identities of streams (hot/cold) are regarded as variables. The original Duran-Grossmann model has been extended and applied to WHENs without knowing the identity of streams a priori. In the original Duran-Grossmann model, the max operator is a challenge for solving the model. This paper analyzes four ways to reformulate the Duran-Grossmann model. Smooth Approximation, Explicit Disjunctions, Direct Disjunctions and Intermediate Temperature strategy are reviewed and compared. The Extended Duran-Grossmann model for WHEN problems consists of both binary variables and non-smooth functions. The Extended Duran-Grossmann model can be reformulated in similar ways. In this study, the performance of different reformulations of the Extended Duran-Grossmann model for WHEN problems are compared based on a small case study in this paper.publishedVersio

Identifying optimal thermodynamic paths in work and heat exchange network synthesis

The process synthesis problem referred to as work and heat exchange networks (WHENs) is an extension of the classical heat exchanger networks problem considering only temperature and heat. In WHENs, additional properties are pressure and work, and strong interactions exist between temperature, pressure, work, and heat. The actual sequence of heating, cooling, compression, and expansion for pressure changing streams (PCs) will affect the shape of the composite and grand composite curves, the Pinch point, and the thermal utility demands. Even stream identities (hot or cold) will sometimes change. The identification of the optimal thermodynamic path from supply to target state for PCs becomes a primary and fundamental task in WHENs. An MINLP model has been developed based on an extension of the Duran–Grossmann model (that can handle variable temperatures) to also consider changing stream identities. Three reformulations of the extended Duran–Grossmann model have been developed and tested for two examples. © 2018 American Institute of Chemical Engineers AIChE J, 2018. © 2018 American Institute of Chemical EngineersIdentifying optimal thermodynamic paths in work and heat exchange network synthesisacceptedVersio

Work and Heat Integration: An emerging research area

The extension from Heat Integration (HI) and design of Heat Exchanger Networks (HENs) to including heating and cooling effects from pressure changing equipment has been referred to as Work and Heat Integration and design of Work and Heat Exchange Networks (WHENs). This is an emerging research area of Process Synthesis, however, WHENs is a considerably more complex design task than HENs. A key challenge is the fact that temperature changes (related to heat) and pressure changes (related to work) of process streams are interacting. Changes in inlet temperatures to compressors and expanders resulting from heat integration will influence work consumption and production. Likewise, pressure changes by compression and expansion will change the temperatures of process streams, thus affecting heat integration. As a result, Composite and Grand Composite Curves will change shape due to pressure changes in the process. The thermodynamic path of process streams from supply (pressure, temperature) to target state is not known and depends on the sequence of heating, cooling, compression and expansion. This paper introduces a definition and describes the development of WHENs. Future research challenges related to methodology development and industrial applications will be addressed. The potential of WHENs will be indicated through examples in literature

Model reformulations for Work and Heat Exchange Network (WHEN) synthesis problems

The Duran-Grossmann model can deal with heat integration problems with variable process streams. Work and Heat Exchange Networks (WHENs) represent an extension of Heat Exchange Networks. In WHEN problems, the identities of streams (hot/cold) are regarded as variables. The original Duran-Grossmann model has been extended and applied to WHENs without knowing the identity of streams a priori. In the original Duran-Grossmann model, the max operator is a challenge for solving the model. This paper analyzes four ways to reformulate the Duran-Grossmann model. Smooth Approximation, Explicit Disjunctions, Direct Disjunctions and Intermediate Temperature strategy are reviewed and compared. The Extended Duran-Grossmann model for WHEN problems consists of both binary variables and non-smooth functions. The Extended Duran-Grossmann model can be reformulated in similar ways. In this study, the performance of different reformulations of the Extended Duran-Grossmann model for WHEN problems are compared based on a small case study in this paper

Nonsmooth Formulation for Handling Unclassified Process Streams in the Optimization of Work and Heat Exchange Networks

Pinch analysis provides a systematic methodology for improving efficiency through enhanced process integration. Originally, the methodology focused on heat integration with heat exchanger network (HEN) synthesis. However, most chemical processes also include pressure manipulation with the inclusion of equipment such as compressors, expanders, pumps, and valves that affect heat integration. Recently, attention has therefore been directed toward simultaneous work and heat integration and the synthesis of work and heat exchange networks (WHENs). Mathematical programming has proven effective in solving heat integration problems. Several pinch location algorithms exist in the literature that calculate the minimum utility consumption given a set of hot and cold process streams. However, classification of streams into hot and cold streams prior to optimization is difficult when integrating compressors and expanders in HENs. Depending on the integration problem, the compression/expansion temperatures can vary greatly in order to fully utilize the heat of compression (or cooling from expansion) in the process. This represents a modeling issue, as classifying the stream identities prior to optimization essentially impose an upper or lower bound on the temperature variable. Instead, pinch location algorithms must be modified to handle unclassified process streams. Different strategies for handling unclassified process streams in exergy targeting and synthesis of WHENs were proposed by Yu et al. This article presents an alternative and more compact formulation using a nonsmooth extension to the simultaneous optimization and heat integration algorithm by Duran and Grossmann in order to handle unclassified process streams. Optimization is performed using IPOPT, and the sensitivities (gradients) are obtained using recent developments in nonsmooth analysis. The nonsmooth extension is tested for WHEN targeting using a number of examples from the literature

Simulation of dual mixed refrigerant natural gas liquefaction processes using a nonsmooth framework

Natural gas liquefaction is an energy intensive process where the feed is cooled from ambient temperature down to cryogenic temperatures. Different liquefaction cycles exist depending on the application, with dual mixed refrigerant processes normally considered for the large-scale production of Liquefied Natural Gas (LNG). Large temperature spans and small temperature differences in the heat exchangers make the liquefaction processes difficult to analyze. Exergetic losses from irreversible heat transfer increase exponentially with a decreasing temperature at subambient conditions. Consequently, an accurate and robust simulation tool is paramount to allow designers to make correct design decisions. However, conventional process simulators, such as Aspen Plus, suffer from significant drawbacks when modeling multistream heat exchangers. In particular, no rigorous checks exist to prevent temperature crossovers. Limited degrees of freedom and the inability to solve for stream variables other than outlet temperatures also makes such tools inflexible to use, often requiring the user to resort to a manual iterative procedure to obtain a feasible solution. In this article, a nonsmooth, multistream heat exchanger model is used to develop a simulation tool for two different dual mixed refrigerant processes. Case studies are presented for which Aspen Plus fails to obtain thermodynamically feasible solutions

Optimization of a dual mixed refrigerant process using a nonsmooth approach

This article uses a nonsmooth flowsheeting methodology to create simulation and optimization models for dual mixed refrigerant processes. New improved operating conditions are obtained using the primal-dual interior-point optimizer IPOPT, with sensitivity information calculated using new developments in nonsmooth analysis to obtain generalized derivative information using a nonsmooth generalization of the vector forward mode of automatic differentiation. Several optimization studies are performed with constraints on both the minimum temperature difference () and total heat exchanger conductance () used to represent the trade-offs between energy consumption and the required heat transfer area. In addition, comparison is made with the conventional process simulator Aspen HYSYS using particle swarm optimization. Results show that the nonsmooth model was able to reduce the required compression power by 14.4% compared to the initial feasible design for the dual mixed refrigerant process, and by 20.4–21.6% for the dual mixed refrigerant process with NGL extraction. Furthermore, the solutions obtained from the nonsmooth model were 1.9–8.1% better than the design obtained by particle swarm optimization. Multistart optimization also shows that IPOPT converges to the best known solution when starting from an initial feasible design

Identifying optimal thermodynamic paths in work and heat exchange network synthesis

The process synthesis problem referred to as work and heat exchange networks (WHENs) is an extension of the classical heat exchanger networks problem considering only temperature and heat. In WHENs, additional properties are pressure and work, and strong interactions exist between temperature, pressure, work, and heat. The actual sequence of heating, cooling, compression, and expansion for pressure changing streams (PCs) will affect the shape of the composite and grand composite curves, the Pinch point, and the thermal utility demands. Even stream identities (hot or cold) will sometimes change. The identification of the optimal thermodynamic path from supply to target state for PCs becomes a primary and fundamental task in WHENs. An MINLP model has been developed based on an extension of the Duran–Grossmann model (that can handle variable temperatures) to also consider changing stream identities. Three reformulations of the extended Duran–Grossmann model have been developed and tested for two examples. © 2018 American Institute of Chemical Engineers AIChE J, 2018. © 2018 American Institute of Chemical Engineer