PTQ Q4 2025 Issue

365 367 369 375 377 379 381 371 373 383 385 387 391 389 397 395 393 399

Operating Envelope

25.7

19.3

12.9

6.43

0.0

Temperature (˚F)

Figure 4 Transition temperature vs pressure heat map for settler outlet under 0.5 wt% entrainment conditions

the smaller exchanger Feed/Bttms, which heat the hydro- carbon from the settler (where HF acid and hydrocarbons are separated) to the depropaniser feed. By contrast, the piping between the settler and the first exchanger exhibits low corrosion, consistent with industry observations that low temperatures and high HF content in the entrained droplets limit corrosion. Plant personnel suspected that heating the stream caused a compositional shift toward water-rich acid droplets and that the preheat The piping between the settler and the first exchanger exhibits low corrosion, consistent with industry observations that low temperatures and high HF content in the entrained droplets limit corrosion exchangers were operating near or within the transition zone. The primary objectives of the modelling study were to quantify entrained acid carryover, estimate transition tem- perature, generate operating envelopes, and propose cor- rosion mitigation strategies. The model reconstructed the settler outlet stream and estimated entrainment at 0.5 wt% of total mass based on acid boot fill tests. For comparison, typical HF alkylation units we studied carried much less entrained acid by a fac- tor of 10. The high entrainment at this refinery implies that droplets persist further downstream and require more heat to dissolve. Temperature surveys showed that HF dissolves rapidly,

while water dissolves slowly. As the temperature approached ~230°F, the residual droplets were predicted to contain 20-24 wt% water, corresponding to the maxi- mum hydronium concentration and corrosion risk. The TT for the settler outlet was therefore ~230°F, far higher than the ~135°F the exchangers achieve, as one of the bundles in the Feed/Rcy iC 4 exchangers is on standby or being cleaned. Operating envelopes and skin temperature effects Figure 4 presents a transition temperature vs pressure heat map for the settler outlet under 0.5 wt% entrainment con- ditions. Colours represent water weight per cent in the acid phase. The blue region corresponds to dissolved conditions (water content near zero), while the red region indicates high water content and high corrosivity. The operating envelope (indicated by the overlaid box) shows the range of temperatures (bulk fluid) and pres - sures experienced in the exchangers. Under current con- ditions (pressures around 370 psig and temperatures of 120-150°F), the operating point lies near the boundary between the blue and green regions, implying that some acid may remain undissolved but is still relatively low in water content. However, these heat exchangers suffered from corrosion and hard-to-remove iron fluoride scales, indicating that while the water content in RHF was low, corrosion was still happening at these exchangers. Figure 5 shows a similar transition temperature vs pressure plot, which includes skin temperatures. The coloured region begins at lower tem- peratures, meaning that even when the bulk fluid is below the TT, the fluid at the wall may become water-rich and corrosive. In the Feed/Bttms exchanger, the heating fluid averaged ~250°F; skin temperatures of 220-230°F were estimated to

PTQ Q4 2025