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.

Keywords

exergy / exergy loss / crude oil distillation / multi-stage / energy saving

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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

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2014 Higher Education Press and Springer-Verlag Berlin Heidelberg