Process synthesis with simultaneous consideration of inherent safety-inherent risk footprint

doi:10.1007/s11705-018-1779-7

Front. Chem. Sci. Eng.

2018, Vol. 12

Issue (4) : 745-762 https://doi.org/10.1007/s11705-018-1779-7

RESEARCH ARTICLE

Process synthesis with simultaneous consideration of inherent safety-inherent risk footprint

Andreja Nemet¹(

), Jiří J. Klemeš², Zdravko Kravanja¹

¹. Faculty of Chemistry and Chemical Engineering, University of Maribor, 2000 Maribor, Slovenia
². Sustainable Process Integration Laboratory, NETME Centre, Faculty of Mechanical Engineering, Brno University of Technology, 61669 Brno, Czech Republic

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Abstract

Process plants should be designed to be economically viable and environmentally friendly, while also being operable and maintainable during process implementation. The safety of processes is among the most important considerations in obtaining results that are more acceptably realistic, as it is linked to the availability and reliability of the process. Inherent safety can effectively be enhanced in the early stages of the design, when the main decisions on process design are made. The aim of this study is to enhance and select the appropriate risk assessment method and to incorporate it into process synthesis, using a mathematical programming approach. A mixed-integer, nonlinear programming (MINLP) model was used for the synthesis of a methanol production process, considering risk assessment during the synthesis. Risk assessment is performed simultaneously with the MINLP process synthesis, where the risk is determined either for the whole process as overall risk, or on a per unit-of-a-product basis. For the latter, a new measurement is proposed: the inherent risk footprint. The results of a case study led to two main conclusions: (i) Significantly safer designs can be obtained at negligible economic expense, and (ii) at higher production capacities, a lower inherent risk footprint can be achieved. The results also indicate that designs obtained using this method can have significantly increased inherent safety, while remaining economically viable.

Keywords inherent safety process design simultaneous risk assessment risk footprint methanol process

Corresponding Author(s): Andreja Nemet

Online First Date: 20 December 2018 Issue Date: 03 January 2019

Cite this article:

Andreja Nemet,Jiří J. Klemeš,Zdravko Kravanja. Process synthesis with simultaneous consideration of inherent safety-inherent risk footprint[J]. Front. Chem. Sci. Eng., 2018, 12(4): 745-762.

URL:

https://academic.hep.com.cn/fcse/EN/10.1007/s11705-018-1779-7
https://academic.hep.com.cn/fcse/EN/Y2018/V12/I4/745

Tab.1 Limiting value of toxic substances depending on LC50 (rat, inhalation 1 h) and phase at 25 °C

Fig.1 Superstructure for methanol process synthesis (modified from Kasaš et al. [²⁸])

Tab.2 Input data for feeds, products, utilities and process units

Tab.3 Failure rate of different process units

Tab.4 Limiting values of different components

Tab.5 The factors considering facility and substance conditions

Unit	$t retention_time / s$
Compressor	600+ 300
Mixer	600
Reactor	600+ 600
Flash	600+ 600
Splitter	600
Valve	600

Tab.6 Estimated retention times in different units

Fig.2 Process design for the reference case study

Fig.3 (a) Failure rate, (b) severity of consequences and (c) risk within each unit, separately

Fig.4 Absolute value of toxicity, flammability and explosiveness vs. the assigned relative risk limit level

Fig.5 The NPV of the methanol production process at varied risk tolerance on (a) an absolute and (b) a relative scale

$R R ref$	1 and 0.95	0.9?0.75	0.7 and 0.68	0.65	0.6?0.56	0.55
Feed 2	✓	✓	✓	✓	✓	✓
One-stage compression of feed stream				✓	✓	✓
Two-stage compression of feed stream	✓	✓	✓
Reactor 1	✓	✓		✓		✓
Reactor 2			✓		✓
One-stage compression of recycle stream		✓	✓	✓	✓	✓
Two-stage compression of recycle stream	✓

Tab.7 Structure of a methanol production process at different levels of risk tolerance

Fig.6 Recycle and inlet molar flow-rate at various assigned risk tolerance levels

Fig.7 Overall conversion of H₂ in the process and the conversion of H₂ per pass in the reactor at various assigned risk tolerance levels

Fig.8 The size of the inlet compressor/compressors, reactor and compressor/compressors on the recycle stream at various assigned risk tolerance levels

Fig.9 Investment cost at various assigned tolerance risk levels

Fig.10 Process design with risk limit set at 0.68 of initial risk level

Fig.11 Process design with risk limit set at 0.55 of initial risk level

Fig.12 Net present value of processes at different molar flow-rates of methanol on (a) absolute and (b) relative scale

Fig.13 Recalculated risk level for molar flow-rates at 0.6, 0.8, 1.2 and 1.4 kmol/s

Fig.14 Molar risk footprint vs. capacity of methanol production

Fig.15 Relative change in specific risk footprint and net present value compared to the reference case

Tab.8 Comparison of inherent risk and inherent risk footprint upper limit

Fig.16 Process design with capacity 1.2 kmol/s of methanol production and inherent risk footprint upper limit set at reference level

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