Figure 2 Crude/kerosene pumparound cross heat exchanger
Figure 3 . The scans disclosed some interesting behaviour. Seven fractionating trays above the heavy naphtha lower draw were severely underloaded (‘near dry’ conditions), while eight trays underneath the heavy naphtha lower draw nozzle were normally loaded. How could trays be more loaded after the liquid draw? In addition, simulation modelling indicated less than one theoretical stage for the section above the heavy naphtha lower draw and three theoretical stages for the section underneath the heavy naphtha lower draw. Fouling could erode tray efficiency. Nevertheless, less than one theoret - ical stage out of seven actual tray counts was an extreme case. It was suspected that tray fouling was not a root cause of poor tray efficiency alone. A field survey, including local temperature and pressure measurements, was conducted to identify the root causes of the unusual tray traffic profile. Temperature survey results highlighted in Figure 3 indicate that measured temperatures at the heavy naphtha upper draw, lower draw, and com - bined draw were the same. If heavy naphtha was with - drawn through the lower draw nozzle, the field temperature survey should indicate three different temperature levels.
operations. Heavy naphtha-kerosene fractionation section flooding resulted in poor heavy naphtha quality. Pressure drop across the section was increasing through the run. Downgraded crude preheat train performance reduced the heater inlet temperature. Moreover, operation instabil - ity was experienced due to inconsistent kerosene pump - around heat removal. Crude preheat train heat exchanger bundles were pulled. It was found that severe foulants were built up in multiple heat exchangers. A fouled crude/kero - sene pumparound cross heat exchanger photo is shown in Figure 2 . Case study 1: Troubleshooting The aforementioned fractionation section pressure drop trend and heat exchanger fouling supported fouling in the heavy naphtha-kerosene fractionation and kerosene pump - around sections. Foulant material samples were obtained from the pulled heat exchanger. Laboratory testing of fou - lant material detected a significant amount of phosphorus. It indicated a root cause of fouling was phosphorus. A crude atmospheric tower scan was also arranged to filter limited sections in detail. Scan results are translated in
Top re ux
O - gas
Heavy naphtha upper draw
Pressure, psig
XX
EL. 145’
50%
Light naphtha
Static
Vapour space
Top pumparound
35
31
Heavy naphtha lower draw
65%
EL. 120’
100%
Heavy naphtha side stripper
350
EL. 105’
Heavy naphtha upper draw
50%
Severely underloaded
65%
350
100%
45
Heavy naphtha side stripper
LV
350
35
31
35
Heavy naphtha lower draw
Normally loaded
Level control valve
‘Reverse ow’
Temperature, ˚F
XX
Figure 3 Fractionation section scan result description and temperature survey result
Figure 4 Heavy naphtha draw geometries and simplified hydraulic balance
90
PTQ Q4 2023
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