(228c) Application of Exergy Analysis of Petroleum Crude Oils Fractions in Crude Distillation Unit

Application of exergy analysis of
petroleum crude oils fractions in crude distillation unit

J.Pulgarín-León, J. Carmona-Otálvaro, H. I.
Velásquez,
A.
Molina[*]

¹Universidad Nacional de Colombia
- Sede Medellín, Facultad de Minas, Bioprocesos y Flujos reactivos

A crude distillation unit (CDU) consists of three parts, the prefractionator tower (PDU), the atmospheric distillation
unit (ADU) and a vacuum distillation unit (VDU), as depicted in Figure 1. On a
refinery process, the crude distillation unit (CDU) demands nearly 25%-30% of
the total  energy of the plant (Wang et al. 2011). Optimization of resources in the CDU is
obviously mandatory.

As a complement to the traditional energy analysis, exergy links the first and second laws of thermodynamics
and compares the results with a reference environment to identify the
possibility to do work through gradient differences of intensive properties (T,
P, µ). An exergetic analysis for oil processing can be carried out with three
different points of view: (1) global comparison, (2) specific comparison, and
(3) profile comparison. The first one considers real industrial operation and
defines mass and energy balances that allow an exergy analysis without, however,
clear specific conditions of each process. The main advantage of this approach
is that it provides valuable information related to the potential global exergy improvement of the system (Le Goff et al. 1996; Rivero et al. 1999; Rivero 2002; Rivero et al. 2004). While the third approach is mostly of the interest of CDUs
manufacturers, the second approach, or specific comparison can improve the
understanding of irreversibilities present in the
distillation process.

This paper presents an ASPEN simulation
of PDU, ADU and VDU operating with Cusiana (36.35°API) and Vasconia (24.5°API)
Colombian crude oils that has results for energy and separation efficiencies
similar to those reported in the literature for similar systems. User-designed
subroutines that account for the exergy of oil streams were incorporated into
ASPEN to model the exergy in the system. The oil was represented as a
combination of pseudocomponents.

Figure 2 shows the calculated exergy for
the different streams and for the two oils. As the quality of the stream
decreases, the exergy content is smaller. Furthermore, the exergy content is
higher for the lighter oil (Cusiana). A comparison of the exergetic efficiency
of the three towers involved in the overall separation process showed that the
total irreversibility losses are 31.6 MW (23.1% PDU, 51.0% ADU and 25.9% VDU)
for a crude flow rate of 154.8 kg/s (100 000 bbl/day). Although the conclusions
obtained based on the exergy analysis agree with those obtained from a simple
energy balance for the complete plant, the exergy concept identifies
irreversibilities that an energy balance cannot detect.

Figure
1.
Process flow diagram for the crude distillation unit
(CDU).

Figure
2.
Exergy for flow diagram for the crude distillation
unit (CDU).

REFERENCES

Le
Goff, P., et al. (1996). "Exergy analysis of
distillation processes." Chemical Engineering & Technology
19(6): 478-485.

Rivero, R. (2002). "Application of
the exergy concept in the petroleum refining and petrochemical industry." Energy
Conversion and Management 43(9-12): 1199-1220.

Rivero, R., et al. (2004). "Exergy
and exergoeconomic analysis of a crude oil combined distillation unit." Energy
29(12-15): 1909-1927.

Rivero, R., et al. (1999). The Exergy
of Crude Oil Mixtures and Petroleum Fractions: Calculation and Application.

Wang, Y., et al.
(2011). "Selecting the Optimum Predistillation Scheme for Heavy Crude
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