PTQ Q4 2023 Issue

kerosene pumparound instability. A manual valve at the heavy naphtha upper draw was fully open, while another manual valve at the heavy naphtha lower draw was com- pletely closed. Kerosene pumparound and product streams were withdrawn from the same draw nozzle. These moves were slowly accomplished to avoid a unit upset. Case study 1: Post-optimisation performance Table 1 compares pre- and post-optimisation heavy naph- tha/kerosene fractionation performances. Simple manual valve opening/closing was the only difference between the two cases. Fractionation between heavy naphtha and kerosene is significantly enhanced. The post-optimisation tower scan verifies that all 15 fractionating trays are loaded in the normal manner. Kerosene pumparound instability is eliminated. Inconsistent traffic above the kerosene pumpa - round section has now disappeared. The kerosene freezing point is not downgraded despite the reduced number of kerosene-diesel fractionating trays. Case study 2: Background Another case study concerns a deisobutaniser (DIB) col- umn in the alkylation unit. The function of the DIB is to separate the feed mixture into IC4- and NC4+ components. Figure 6 illustrates the DIB configuration. The column equips 47 fractionating trays. The feed stream was typi- cally introduced between trays #20 and #21. Based on this feed location, 20 and 27 trays functioned as rectification and stripping trays, respectively. Two reboiler circuits are arranged as side and bottom reboilers. These reboilers supplied heat for distillation. Between 80% and 85% of the total heat input was sup - plied through the side reboiler, and the bottom reboiler sup- ported balanced heat input. The bottom reboiler was used as a ‘primary knob’ to control the column bottom tempera- ture. 75 psig saturated steam was used as the side reboiler heating medium, while the bottom reboiler was operated with 300 psig saturated steam. Case study 2: Problem description The DIB experienced a flooding event at the target feed rate operation. The required bottom reboiler steam rate at the target feed rate caused a flooding event. A downgraded degree of separation was observed. It resulted in increased Reid vapour pressure (RVP) of the bottom product. The target feed rate and bottom reboiler steam rate had been processed without limitations in previous days. To maintain the desired RVP of the bottom product, the DIB feed rate had to be reduced by 12%. The DIB became the bottleneck point of the entire alkylation and fluidised catalytic cracking (FCC) units. The initial operating data review revealed that bottom reboiler performance was substantially downgraded. Operating trends indicated that unit steam consumption of bottom reboiler was reduced. Column scanning using gamma ray also confirmed the flooding condition. Multiple scan results at various bottom reboiler steam rates are dis- played and compared in Figure 7 . The scan results revealed that flooding was initiated at a higher bottom reboiler steam

#46

Side reboiler return

#47 #48

#49

Bottom reboiler return

4.1 (lb steam / bbl feed)

4.7 (lb steam / bbl feed)

5.3 (lb steam / bbl feed)

Figure 7 DIB gamma scan

rate. Fouled materials were built up in the bottom reboiler circuit and the trays between the side and bottom reboilers. Polymer-related fouled materials were also found in the reactor effluent pump. These fouled materials could be caused by low acid strength, where incomplete reaction intermediates were decomposed and olefins were polym - erised. The only way to remove fouled materials was by Identifying non-optimum parameters and building pertinent optimisation strategies can enhance distillation unit performances and avoid unit outages cleaning equipment after shutting down the alkylation unit. However, an alkylation unit outage required an FCC unit outage. These multiple unit outages could cause a signif- icant loss of revenue. Options for avoiding unit outages were explored. The author was requested to evaluate DIB performance and seek options for reinstating DIB capacity. Case study 2: Heat balance testing The aforementioned bottom reboiler circuit fouling reduced not only the bottom reboiler duty but also the DIB bottom

PTQ Q4 2023