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Abstract
Petroleum separation processes are intensive in exergy use. However, only a very small fraction of the consumed exergy is converted into products. Due to the significant magnitude difference between consumed exergy and processed exergy, as well as to the unknown molecular structure of the involved streams, the calculation of specific exergy and of exergy efficiency is a delicate topic that involves significant uncertainties. Comparison and explanation of five different ways to perform exergy evaluation of petroleum separation processes are conducted. The indication of advantages and disadvantages of each formulation is presented. The chemical and physical exergy calculation for petroleum and its fractions are covered. An application is performed and the results are discussed.
NOMENCLATURE
| AAD | = | absolute average deviation |
| CV | = | control volume |
| EOS | = | equation of state |
| b | = | specific exergy (kJ/kg) |
| B | = | exergy rate (kW) |
| h | = | specific enthalpy (kJ/kg) |
| H | = | enthalpy rate (kW) |
| KW | = | Watson factor |
| LHV | = | low heating value (kJ/kg) |
| m | = | mass flow rate (kg/s) |
| P | = | pressure (bar) |
| PA | = | pumparound |
| PC | = | critical pressure (bar) |
| R | = | gas constant (kJ/(kg-K)) |
| s | = | specific entropy (kJ/kg-K) |
| S | = | entropy rate (kW/k) |
| SG | = | specific gravity/density |
| Tb | = | boiling temperature (K) |
| Tbr | = | boiling temperature reduced |
| TBP | = | true boiling point |
| TC | = | critical temperature (K) |
Greek Symbols
| γ | = | activity coefficient |
| ϕ | = | chemical exergy correction factor |
| ηb | = | exergy efficiency |
| ω | = | acentric factor |
Subscripts
| 0 | = | reference for the exergy calculation |
| ch | = | chemical |
| C | = | consumed, carbon fraction |
| D | = | destroyed |
| H | = | hydrogen fraction |
| IN | = | inlet streams |
| OUT | = | outlet streams |
| M | = | mixture |
| O | = | oxygen fraction |
| P | = | products |
| S | = | sulfur fraction |
| tot | = | total |
Superscripts
| abs | = | absolute |
| env | = | environment |
| in | = | inlet streams |
| out | = | outlet streams |
Notes
The standard chemical exergy considers that the evaluated substance is at 1 atm.
x, in Eq.(3), represents mass fraction, while C, H, O, and S denote carbon, hydrogen, oxygen, and sulfur, respectively.
Light fractions of petroleum whose molecular composition can be obtained by the use of chromatographic techniques.
Other similar equations may be used although similar AAD should be expected.
In real separation processes the input and output pressure and temperature are different since the output properties are function of tower trays, pumparounds, etc.
This kind entropy generation/exergy destruction is called by some authors external irreversibility. Here it is taken into account as part of the exergy destroyed by the vacuum tower.
Additional information
Notes on contributors
Julio A. M. Silva
Julio A. M. Silva is a professor at the Polytechnic School of the Federal University of Bahia. He received his Dr.Sc. degree in mechanical engineering—energy and fluids from the University of Sao Paulo in 2013. His M.Sc. in mechanical engineering—energy conversion was obtained from the Federal University of Itajuba in 2009. He received his degree in mechanical engineering from Federal University of Bahia in 2005. He has experience in mechanical engineering with emphasis on thermal engineering, acting on the following topics: exergy and energy analyses of energy and industrial processes; thermodynamic diagnosis and prognosis of power plants; thermoelectric generation and cogeneration; exergoeconomy/thermoeconomy; and petroleum production and refining. He has authored/co-authored 30 papers in archival journals and conference proceedings.
Silvio De Oliveira
Silvio De Oliveira, Jr. is an associate professor 3 at Polytechnic School of the University of São Paulo, Brazil. He has been developing research activities on heat pumps and refrigeration systems, solar energy, energy conservation in industrial processes, cogeneration systems, and exergy and thermoeconomic analysis of thermal and chemical processes. He is author of the book Exergy: Production, Cost and Renewability (Springer, 2013), author/co-author of about 190 publications and communications, and senior member of the Brazilian Society of Mechanical Sciences and Engineering. He is also a member of the editorial board of the International Journal of Thermodynamics. He has been involved in the past 10 years with research projects related to energy utilization in biodiesel production plants, sugar and alcohol utilities and production plants, offshore and onshore petroleum platforms, petroleum refinery plants, co-/trigeneration and combined cycle plants, airplane energy systems modeling, modeling and simulation of twin-screw multiphase pumping systems, and exergy behavior of human body.
Jonathan Pulgarín
Jonathan Pulgarín received his M.Sc. degree in chemical engineering in 2014 from Universidad Nacional de Colombia–Sede Medellín. His bachelor's degree was received in 2009. He has worked on the application of techniques like thermal design and optimization using exergy and exergoeconomic analysis for refinery systems. The principal areas of research are energy and exergy analysis on distillation systems of oil process, and energy diagnostic of thermal systems. He is member of the research team Bioprocesos y Flujos Reactivos at Universidad Nacional de Colombia–Sede Medellín.
Héctor I. V. Arredondo
Héctor I. V. Arredondo is an associate professor at the Universidad Nacional de Colombia, Sede Medellín, since August 1998. He received a Ph.D. in mechanical engineering from the University of São Paulo (2004) and a master's degree in thermal systems at the Universidad Pontificia Bolivariana–Medellín (2002), and developed a specialization in corporate finance (2000) from the University of Medellín. Also he is currently principal investigator in the following lines of research: thermal engineering—energy use, thermodynamics, and heat transfer. He is the author of nine papers in international refereed journals. He maintains professional relationships with research groups at the University of Antioquia and University of São Paulo.
Alejandro Molina
Alejandro Molina is a professor in the Department of Procesos y Energía at Facultad de Minas, Universidad Nacional de Colombia–Sede Medellín. He is principal investigator of two research projects related to the simulation and characterization of reacting flows in upstream and downstream processes of the oil and gas industry. He obtained his Ph.D. in chemical and fuel engineering from the University of Utah in 2002. He was a postdoctoral fellow from 2003 to 2006 at the Combustion Research Facility of Sandia National Laboratories at Livermore, CA, where he conducted research in laser diagnostics, coal oxy-fuel combustion, and glass production. In 2006 he became an associate professor at Facultad de Minas at Universidad Nacional de Colombia–Sede Medellín, where he is actively involved in research related to reactive flows analysis via computational fluid dynamics and laser diagnostics. He is a member of the research group Bioprocesos y Flujos Reactivos and, together with his research students, contributes to different fields such as in situ combustion for enhanced oil recovery, simulation of refinery processes, oil weathering, and hydrotreatment of vegetable oils.