Exergy analysis of multi-stage crude distillation units

Abstract

This paper aims to investigate the multi-stage effect on crude distillation units (CDUs) in thermodynamics. In this regard, we proposed three-, four-, five-, and six-stage CDU processes with all variables constrained to be almost the same except for the number of stages. We also analyzed the energy and exergy to assess the energy consumed by each process. Because additional distillation units would share the processing load and thus prevent products with low boiling points from overheating, the heat demand of the CDUs decreases with increasing stages and thus reduces the heat supply. Exergy loss is considered as a key parameter to assess these processes. When the exergy losses in heat exchangers are disregarded, the three- and four-stage CDUs have lower exergy losses than the five- and six-stage CDUs. When the overall exergy losses are considered, the optimum number of stages of CDUs depends on the exergy efficiency of heat integration.

Key words： exergy; exergy loss; crude oil distillation; multi-stage; energy saving

Acknowledgements

This research was supported financially by the National Basic Research Program of China (No. 2009CB219905), the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT0936) and Top Talents Program of Yunnan Province, China (No. 2011HA010).

Nomenclature

\dot{E}	nergy rate, kW
ex	pecific exergy, kJ·kg-1
\dot{E}x	xergy rate, kW
h	pecific enthalpy, kJ·kg-1
\dot{H}	nthalpy rate, kW
\dot{I}	rreversibility or exergy loss rate, kW
\dot{m}	ass flow rate, kg·s-1
s	pecific entropy, kJ·kg-1·K-1
\dot{S}	ntropy rate, kW·K-1
T	emperature, K
\phi	roportional coefficient
Subscripts
0	nvironment condition
In	nlet stream
out	utlet stream
cold	old stream
hot	ot stream
Qd	eat demand
Qs	eat supply
Qrev	ssuming the heat transfer processes in heat exchangers are reversible
Qϕ	SSUMING the value of exergy loss is proportional to the amount of exergy provided by the hot streams in heat exchangers with a constant proportional coefficient

References

1 Fahim M A, Al-Sahhaf T A, Elkilani A S. Fundamentals of petroleum refining. Elsevier Science, 2010, 69–93

2 Szklo A, Schaeffer R. Fuel specification, energy consumption and CO₂ emission in oil refineries. Energy, 2007, 32(7): 1075-1092

3 Miller W, Osborne H G. History and Development of Some Important Phases of Petroleum Refining in the United States. The Science of Petroleum, Vol 2. London: Oxford University Press, 1938, 1465-1477

4 Liebmann K, Dhole V R, Jobson M. Integrated design of a conventional crude oil distillation tower using pinch analysis. Chemical Engineering Research & Design, 1998, 76(3): 335-347

5 Watkins R N. Petroleum Refinery Distillation. Houston: Gulf Publishing Company, 1973, 3-77

6 Bagajewicz M J. On the design flexibility of atmospheric crude fractionation units. Chemical Engineering Communications, 1998, 166(1): 111-136

7 Ji S, Bagajewicz M. Design of crude distillation plants with vacuum units. I. Targeting. Industrial & Engineering Chemistry Research, 2002, 41(24): 6094-6099

8 Benali T, Tondeur D, Jaubert J N. An improved crude oil atmospheric distillation process for energy integration. Part I: Energy and exergy analyses of the process when a flash is installed in the preheating train. Applied Thermal Engineering, 2012, 32: 125-131

9 Benali T, Tondeur D, Jaubert J N. An improved crude oil atmospheric distillation process for energy integration. Part II: New approach for energy saving by use of residual heat. Applied Thermal Engineering, 2012, 40: 132-144

10 Wang Y H, Hou Y J, Gao H, Sun J S, Xu S M. Selecting the optimum predistillation scheme for heavy crude oils. Industrial & Engineering Chemistry Research, 2011, 50(18): 10549-10556

11 Al-Muslim H, Dincer I, Zubair S M. Exergy analysis of single- and two-stage crude oil distillation units. Journal of Energy Resources Technology, 2003, 125(3): 199-207

12 Al-Muslim H, Dincer I, Zubair S M. Effect of reference state on exergy efficiencies of one- and two-stage crude oil distillation plants. International Journal of Thermal Sciences, 2005, 44(1): 65-73

13 Wu J S, Chen J M, Li H J, Lu X P. Application of four-stage distillation technology in atmoaspheric and vacuum distillation unit expansion revampng. Lianyou Jishu Yu Gongcheng, 2003, 33(6): 35-38 (in Chinese)

14 Chen S J, Huang F X. New route for capacity expansion of atmospheric and vacuum distillation unit. Shiyou Lianzhi Yu Huagong, 2004, 35(7): 27-30 (in Chinese)

15 Zhang S W. Four-column process flow adopted in capacity expansion revamping of atmospheric and vacuum distillation unit. Lianyou Jishu Yu Gongcheng, 2005, 35(8): 7-11 (in Chinese)

16 Zhang G S. Application of four-stage distillation in the capacity expansion of ADU/VDU. Shiyou Huagong Sheji, 2006, 23(1): 40-42 (in Chinese)

17 Li Z Q. Crude Oil Distillation Process and Engineering. Beijing: China Petrochemical Press, 2010, 23-25 (in Chinese)

18 Wang Y B, Chen Q L, Zhang B J, Yan Y Z. Energy-use analysis and optimization for three-furnace four-column process flow in the atmospheric and vacuum distillation unit. Xi’an: 5th Chinese National Chemical and Biochemical Engineering Annual Meeting, 2008, 590-591 (in Chinese)

19 Li X G, Liu C J, Luo M F, Li H. Simulation and optimization of atmosphere-vacuum gradual distillation energy saving apparatuses. Huagong Jinzhan, 2009, 28(S2): 360-363 (in Chinese)

20 Li X G, Wang Z J, Li H, Zhu T Y, Luo M F, Jiang B, Liu C J, Tan X R, Chen N, Xu L Q. CN Patent, 101348730, 2009-01-21 (in Chinese)

21 Bagajewicz M, Ji S C. Rigorous procedure for the design of conventional atmospheric crude fractionation units. Part I: Targeting. Industrial & Engineering Chemistry Research, 2001, 40(2): 617-626

22 Demirel Y. Thermodynamic analysis. Arabian Journal for Science and Engineering, 2013, 38(2): 221-249

23 Radgen P, Lucas K. Energy system analysis is fertilizer complex—Pinch analysis vs. exergy analysis. Chemical Engineering & Technology, 1996, 19(2): 192-195

24 Linnhoff B, Hindmarsh E. The pinch design method for heat exchanger networks. Chemical Engineering Science, 1983, 38(5): 745-763