PTQ Q4 2025 Issue

w Process model integration : The Flowsheet: ESP model is used to replicate the entire HF-alkylation fractionation train, from settler outlet to fractionator overhead, which may include the depropaniser and HF stripper. The recon- structed, saturated settler outlet stream is used in the pro- cess simulation to perform an acid balance. An ‘entrained acid’ stream is mixed with the saturated settler stream to match the resulting acid boot flow rate and, therefore, pre - dict the unit’s entrainment levels. x Transition temperature survey : The reconstructed streams are subject to a single variable temperature survey at constant pressure to determine TT. The model outputs the fraction of acid phase present, HF and water contents, and hydronium concentration across the temperature range. y Double survey (heat map) : A two variable survey, var - ying both temperature and pressure, generates a contour plot of water content, which defines an operating enve - lope for safe operation (see Figure 3 ). Points to the right of the contour (dark blue) represent fully dissolved (solu - ble) conditions. Points in the coloured region (blue to red) indicate the presence of a water-rich acid phase. Changes in unit pressure can shift the envelope. This unit’s depro- paniser feed was operated at 150ºF and 308 psig, well below the TT 192ºF and, therefore, saw corrosion in this line. Increasing the temperature for the feed line was not possible, as the unit does not have enough heat to over- come the TT. By iterating through these steps for different entrainment levels and operating conditions, operators can identify cor- rosion risk regimes and evaluate mitigation strategies (such as reducing entrainment, increasing heat duty, adjusting pressure, and changing metallurgy). Case Study 1: Preheat and depropaniser feed Problem description and simulation approach The refinery in this case observed severe corrosion and fouling in two preheat exchangers (Feed/Rcy iC4 A/B) and

0.0020 0.0015 0.0010 0.0005 0.0000 0.0025 0.0030 0.0035 0.0040 0.0045

HO (mass%) HF - Liq 1 (mass %) HO+1 (mass %) (Y2)

Temperature (˚F)

Figure 2 Temperature survey for main fractionator feed stream

main fractionator feed stream, where the TT is calculated to be at 165ºF, for a moderately high entrainment unit. Integration The MSE model was implemented in OLI Flowsheet ESP for process simulations and OLI Stream Analyser for detailed phase‐equilibrium analyses. The typical workflow is:  Data gathering and entrainment estimation : Operators provide process data (temperatures, pressures, acid com- position at the reactor, and flow rates), as well as the results of acid boot fill tests. Because fill tests are sub - ject to human error, the model also includes a procedure to estimate entrainment by matching corrosion events to predicted entrained streams. For example, one study found that measured entrainment was ~2,000 lb/h, several times higher than typical HF units (40-200 lb/h), and recom - mended installing a flowmeter to improve accuracy. v Saturation reconstruction : A saturated settler effluent stream is reconstructed in OLI Flowsheet: ESP by mixing hydrocarbon and acid to match measured composition and temperature.

315.0

18.4

314.0

313.0

13.8

312.0

311.0

310.0

9.22

309.0

308.0

4.61

307.0

306.0

0.0

305.0

Temperature (˚F)

Figure 3 Depropaniser feed stream contour plot of H₂0 wt% in RHF

PTQ Q4 2025