New convection section vs parametric analysis of excess oxygen
Parameters
Unit
Baseline
Convection replacement
Convection replacement
Convection replacement
simulation
model
Case 1
Case 2
Case 3
Excess O₂ (excess air)
%
3.76
3.76
3
2
(18.8)
(18.8)
(15)
(10)
Process absorbed duty
MMBtu/hr
107.15
105.53
106.41
106.42
Total absorbed duty
MMBtu/hr MMBtu/hr
123.459 143.60
121.42 138.33
121.74 138.25
120.84 136.64
Firing rate
(-3.67%)
(-3.73%)
(-4.85%)
Total fuel gas flow
MSCF/hr
163.53
157.53
157.44
155.61
(-3.67%)
(-3.73%)
(-4.85%)
Efficiency (LHV)
% % °F °F
85.6 68.2 1471
87.4 67.3 1457
87.7
88.1 69.2 1464
Per cent duty in radiant sections Bridge wall temperature (BWT) Flue gas stack entry temperature
68
1462
531
466
464
460
Process pressure drop
psi
91.67
91.89
91.79
91.72
TMT – Radiant coil top pass TMT – Radiant coil middle pass TMT – Radiant coil 2nd row pass TMT – Radiant coil bottom pass Simulation model maximum Relative coking rate Maximum inside film temperature
°F °F °F °F
776 811 874 873
776 810 871 870
776 811 873 871
776 811 874 872
-
8.1
7.9
7.9
8
°F
837 94.3 0.39
835
836 94.2 0.39
837 94.3 0.39
Residence time
Second
94
Weight fraction vaporised at column
-
0.39
Table 3
New convection section The current convection section has studded tubes due to the original heater design, where allowance for liquid fuel firing was considered, but this is no longer the case here. A simulation of upgrading the studded tubes to finned tubes in the convection section has been performed, and the results are shown in Table 3 . A new convection section (finned tubes) combined with an acceptable excess oxygen (15%) will reduce the total fuel consumption by 3.73%, which can result in savings of $270,000/yr. Conclusion Short run lengths caused by coke formation are a worldwide phenomenon commonly noticed in vacuum heaters, where the film temperature and oil residence time often exceed the oil’s thermal stability limit. Controlling these two key factors can help tackling coke formation rate inside the radiant tubes. Based on our analysis, it is evident that meeting the goal of a five-year vacuum heater run length can only be achieved with a replacement heater with a new design, considering positioning the outlet passes away from the burners and minimising coking, as the current heaters are already han- dling 16% more throughput. Kindly note that there is no room for coil modification since the current heater geometry is limited and it does not permit major modification.
2 Garg A, Improve vacuum heater reliability, Hydroprocessing , Mar 1999. 3 Barletta T, Why vacuum unit fired heaters coke, PTQ , 2001. 4 Jinyu, et al., Controlling film temperature in fired heaters, PTQ, Q1 2013. 5 Barletta T, Conditions influencing coke formation, PTQ, 2004. 6 Teran C, Crude fingerprinting and predictive analytics, PTQ, Q3 2017. 7 Martinet G R, et al., Vacuum unit fired heater coking – avoid unsched- uled shutdowns, PTQ , 2001. 8 Lieberman N, et al., Vacuum ejector performance break, PTQ . 9 Kantley G, et al. , Maximise VGO yield, PTQ . 10 Martin G R, Vacuum unit design effect on operating variables, PTQ , 2002. 11 Newnham R, Direct Fired Heaters – A Practical Guide to their Design and Operation, Kingsley Knowledge Publishing, 2012. Haytham Al-Barrak is a Fired Equipment Engineering Consultant working for Saudi Aramco, residing in Dhahran, KSA. He has two pat- ents related to fired equipment monitoring and has worked with Saudi Aramco for the last 30 years. He is also the Heat Transfer Equipment Standards Committee Chairman. He holds an MS degree in mechanical engineering from University of Southern California. Abdulaziz Mubarak is a Fired Equipment Engineer working for Saudi Aramco for the last 10 years, with experience in plant operations, project management, and engineering consultancy. He holds a bachelor’s degree in mechanical engineering from the University of Missouri, Columbia. Mahendran Sella is a Heat Transfer and Combustion Engineer working for Saudi Aramco. He recently filed a patent for an innovative digital solution for boiler diagnostics. He holds a BS degree in mechanical engineering from the Institute of Engineers, India.
References 1 Remesat D, Improving crude vacuum unit performance, PTQ, Q3 2008.
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