Optimization of a simple LNG process using sequential quadratic programming

Wahl, Per Eilif; Løvseth, Sigurd Weidemann; Mølnvik, Mona J.

doi:10.1016/j.compchemeng.2013.05.001

Cited by 72 publications

(37 citation statements)

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“…In our work we have found that using ordinary sequential quadratic programming (SQP) routines compared with a flowsheet simulator is feasible for simple processes as long as the optimization problem is formulated well [27]. For the same problems, convergence was much faster than other techniques reported in the literature [28,29,30].…”

Section: Evaluation Of Lng Processesmentioning

confidence: 89%

From Droplets to Process: Multilevel Research Approach to Reduce Emissions from LNG Processes

et al. 2015

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In the Enabling Low-Emission LNG Systems project, SINTEF Energy Research and NTNU have since 2009 been developing knowledge and tools with the aim to enable environmental safe, cost-effective, and energy efficient LNG processes. The project has employed a multilevel approach, where studies of fundamental phenomena, component modeling, and process optimization have been performed in parallel. Detailed models for two-phase flow in heat exchangers (HXs) have been developed, and falling films and droplet behavior relevant for LNG HXs have been studied experimentally. Further, the project work has generated robust and accurate HX models that can describe instabilities and include geometrical considerations. Finally, the project has improved the basis for optimization of LNG process through investigating of optimization tools and careful analysis and formulation of the optimization problem.

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Section: Evaluation Of Lng Processesmentioning

confidence: 89%

From Droplets to Process: Multilevel Research Approach to Reduce Emissions from LNG Processes

et al. 2015

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“…Hence, the focus has been on the PRICO LNG process, which is a relatively simple natural gas liquefaction process currently used in multiple plants world-wide (Hoffart and Price, 2014). Referring to Austbø et al (2014), the optimization of single cycle mixed refrigerant processes such as PRICO has previously been discussed in a number of works, and in a handful of these papers SQP techniques have been employed (Skaugen et al, 2010(Skaugen et al, , 2013Morin et al, 2011;Khan et al, 2012;Wahl et al, 2013). In our experience, the knowledge obtained in the study of relative simple processes like PRICO can also be applied for more complex processes.…”

Section: Introductionmentioning

confidence: 85%

“…The minimum of the temperature differences T n should be more than T h = 1.2 • C in all cases. This work is a continuation of the work in Wahl et al, 2013 where different values of T h was selected in order to compare with previous reported work. The largest value for T h has been used here.…”

Section: Optimization Scheme and Parameter Settingsmentioning

confidence: 97%

“…The same process was investigated in Wahl et al (2013) where a more detailed description of the optimization problem is found. An Aspen HYSYS ® flowsheet of the PRICO process with one compressor and one heat exchanger is provided in Fig.…”

Section: The Prico Processmentioning

confidence: 97%

“…The optimization set-up is similar to the one described in Wahl et al (2013), using the NLPQLP routine from Schittkowski (2006) for optimization and Aspen HYSYS ® V8.2 for process simulation. Most of the basic results presented here are however expected to be relevant also when using other advanced SQP routines or flowsheet simulators.…”

Section: Optimization Problemmentioning

confidence: 99%

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Formulating the optimization problem when using sequential quadratic programming applied to a simple LNG process

Wahl

Løvseth

2015

Computers & Chemical Engineering

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a b s t r a c tSequential quadratic programming (SQP) may be very efficient compared with other techniques for the optimization of simple processes for the liquefaction of natural gas (LNG), and can be combined with process evaluation using commercial flowsheet simulators. However, the level of success is dependent on the formulation of the problem. In this work, effects of varying different aspects of the optimization problem formulation is investigated, such as variable selection, formulae for the estimation of derivatives, initial values, variable bounds, and formulation of constraints. Especially the formulation of the constraint for the temperature difference between the hot and cold composite curve is essential. The commonly used minimum temperature difference constraint should generally not be employed in gradient based optimization. Recommendations regarding optimization of simple LNG processes using SQP and flowsheet simulators are provided.

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Impact of problem formulation on LNG process optimization

Austbø

Gundersen

2016

AIChE Journal

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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...

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Optimization of a simple LNG process using sequential quadratic programming

Cited by 72 publications

References 8 publications

From Droplets to Process: Multilevel Research Approach to Reduce Emissions from LNG Processes

From Droplets to Process: Multilevel Research Approach to Reduce Emissions from LNG Processes

Formulating the optimization problem when using sequential quadratic programming applied to a simple LNG process

Impact of problem formulation on LNG process optimization

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