Parametric analysis of excess oxygen
Parameters
Unit
Case 1
Case 2
Baseline
Case 3
Case 4
simulation model
Excess O₂ (excess air)
%
2
3
3.77
4.5
6
(10)
(15)
(18.85) 107.15 123.46 143.60
(22.5)
(30)
Process absorbed duty Total absorbed duty
MMBtu/hr MMBtu/hr MMBtu/hr MSCF/hr
105.56 119.93 137.76
106.56 122.03 141.16 160.75 (-1.7%)
106.47 123.31 143.91 163.88 (0.21%)
105.97 124.15 146.32 166.63 (1.9%)
Firing rate
Total fuel gas flow
156.8
(-4.1%)
163.53
Efficiency (LHV)
% % °F °F
86.7 70.3 1468
86.1 69.1 1470
85.6 68.2 1471
85.3 67.5 1468
84.5 65.9 1463
Per cent duty in radiant sections Bridge wall temperature (BWT) Flue gas stack entry temperature
518
526
531
536
543
Process pressure drop
psi
91.67
91.67
91.67
91.67
91.87
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
776 811 874 873
776 811 874 873
776 811 873 871
776 810 871 870
-
8.1
8.1
8.1
8
7.9
°F
837 94.3 0.39
837 94.3 0.39
837 94.3 0.39
837 94.2 0.39
836 94.1 0.39
Residence time
Second
Weight fraction vaporised at column
-
Table 2
Based on the above explanation, we can safely conclude that the calibrated simulation model is reliable and will be able to simulate future operating cases confidently. Increasing velocity steam, optimising O₂ level, and replac - ing convection tubes are selected scenarios to be simulated by the software to maximise vacuum heaters run length by improving heater operations. These scenarios were selected due to their low cost, minor modifications, and best practice recommendations. Other modifications that require major revamps with high cost or extended shutdown beyond the normal T&I period were not discussed in this report. Velocity steam increment In the velocity steam increment scenario, the velocity steam injected at the cross-over tubes is increased from 3.45 Mlb/ hr to 6.8 and 10.2 Mlb/hr and simulated as shown in Table 1 as Cases 1 and 2, respectively. Increasing velocity steam in Cases 1 and 2 reduces the maximum inside film tem - perature by 0.3% and 0.5%, respectively. The reduction in film temperature decreases the coking rate, as high film temperature is one of the major causes of coke formation. Moreover, increasing velocity steam reduces the simu - lation model's maximum relative coking rate from 8.1 to 6.9 and 6.45, which reduces the coking rate in the radiant tubes. However, the process pressure drop is dramatically increased by 25% and 43% for Cases 1 and 2, respectively. This option must be verified from a process perspective to avoid condensate carryover to downstream equipment and ensure sufficient pressure upstream of the heaters. Note: The current '1' steam injection nozzle will be checked
for adequacy before trying to increase the steam flow rate to 6.8 Mlb/hr. Excess oxygen Increasing the excess air has a minor contribution to the cok - ing phenomena, as shown in Table 2 , since the TMT, max - imum inside film temperature, maximum coking rate, and residence time are stable in the range of 10-30% excess air (2-6% excess O₂). Conversely, increasing the excess air will greatly affect heater efficiency and fuel consumption. As an example, reducing the excess O₂ from the baseline case (3.77%) to Case 1 (2%, as per burner design) results in cost savings of $150,000/yr in fuel cost (fuel cost: $5/MMB tu/hr). As per the original datasheets, the burners were designed for 10% excess air (2% O₂). However, note that the optimum point must be determined by the operator in the field in order to ensure that the correct mix of air and fuel takes place at the burners, especially since the original design conditions changed. Note: Operators should be cautious of the O₂ per cent (O₂%) reading provided by O₂ analyser. It is important not to react based on this reading without careful thought due to the following issues: • The O₂% reading may be erroneous due to improper calibration. • The O₂% reading may be considering filtration air (leak - age) around the bridge wall area. Therefore, reducing combustion air based on the O₂% reading may lead to unnecessary increase in TMT and bridge wall temperature.
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Revamps 2024
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