Impact of problem formulation on LNG process optimization

Abstract

The power consumption of a single mixed-refrigerant process (PRICO V R ) for natural gas liquefaction was minimized using four different constraint formulations to handle the trade-off between investment and operating costs. Aspen HYS-YS V R was used for process simulation, while a sequential quadratic programming algorithm (NLPQLP) was used for optimization. The results confirm that optimal utilization of the heat exchanger area is only obtained with a constraint based on a maximum heat exchanger conductance (UA). The minimum temperature difference constraint commonly used in process design gives a significant energy penalty as it is incapable of accounting for the distribution of driving forces with respect to temperature, the nonlinearity of the composite curves and the trade-off between driving forces and cooling load. The results also indicate that the maximum UA constraint leads to increased complexity of the optimization problem, and that the success rate of the optimization method used therefore is reduced. V C 2016 American Institute of Chemical Engineers AIChE J, 00: 000-000, 2016 Keywords: process optimization, heat transfer, driving forces, economic trade-off, liquefied natural gas IntroductionProcess design and operation is carried out with the prevailing objective of maximizing profit, or alternatively minimizing cost. Since detailed cost data seldom are available in an early design phase, a simplified approach is often taken. Both investment and operating costs are important for the overall economy of a plant, yet they are often in conflict. For many process units, improving operation comes at the expense of increased investment cost. Hence, to minimize the total cost of a process, the optimal trade-off between investment and operating costs must be found.For a heat-transfer process, both investment and operating costs are closely linked to the magnitude of the temperature driving forces. On one hand, the energy efficiency (and thereby the operating costs) is reduced when the driving forces are reduced. On the other hand, reduced driving forces lead to increased heat exchanger size (and thereby increased investment costs). To address this issue, a minimum temperature difference is commonly used as an economic trade-off parameter in heat exchanger network design. Jensen and Skogestad 1 found, however, that this approach gave suboptimal solutions when applied to design of processes for natural gas liquefaction, that is, subambient processes.Comparing different strategies for design of heat exchangers, Austbø and Gundersen 2 found that a design where the temperature difference in the heat exchanger is proportional to the temperature level, as suggested by Bejan, 3 Xu, 4 and Chang et al. 5 among others, gives the smallest irreversibilities associated with heat transfer. The potential savings in energy use, compared to a design with a uniform temperature difference throughout the heat-transfer process, were found to increase with decreasing temperature level and/or increasing temperature span. This sugg...

Optimal Use of Liquefied Natural Gas (LNG) Cold Energy in Air Separation Units

“…Thus, smaller driving forces at lower temperatures in a heat exchanger minimize total irreversibilities, while allowing an optimal use of heat exchanger area. 27 This indicates that the temperature difference profiles at 40 and 60 bar are close to the optimal distribution of driving forces, resulting in the lowest exergy destruction, as illustrated in Figure 5. The situation at 80 bar is similar to 100 bar in the sense that larger temperature differences in the middle of the NHE cannot be compensated by smaller temperature differences in the hot end; thus exergy losses are increased compared to the 60 bar case.…”

“…This is due to the characteristics of entropy generation caused by the temperature difference in a heat exchanger, which increases exponentially at lower temperatures. Thus, smaller driving forces at lower temperatures in a heat exchanger minimize total irreversibilities, while allowing an optimal use of heat exchanger area …”

Multi-objective simulation–optimization via kriging surrogate models applied to natural gas liquefaction process design

“…For that, energy optimization of LNG processes have been extensively studied in the past [8]. However, the energy-optimum design solution without heat exchanger area limit might not be appropriate [9]. That is because the optimum design of these processes presents a clear trade-off between energy consumption and equipment size [10].…”