PTQ Q3 2022 Issue

Figure 3B Dividing wall being installed during a revamp of a conventional column in South East Asia ( Image courtesy of DWC Innovations)

Case study: incorporating the hybrid model A facility in Southeast Asia was looking forward to a major diversification. It was able to identify a huge demand for PCN, being mindful of a less than 1 ppm CS 2 specification. It could visualise the added benefit of this revamp by also producing premium gasoline. A stream of 90% iC 5 could be obtained as the top product from the depentaniser column. As most of the CS 2 would become a part of the midcut drawn from the column, this stream could be either routed to the naphtha pool or treated in the NHT unit (as per speci- fication targets), which in turn would offload the NHT unit. The revamp objective included: • Maximum iC 5 (>90 wt%) recovery in overhead product • The nC 5 side-cut would contain most of the CS 2 • PCN bottom product with less than 1 ppm CS 2 in PCN. In its existing configuration at the facility, the full-range naphtha was routed to a series of two columns that included a naphtha splitter and a depentaniser column to get C 5 rich and C 6 rich streams as top and bottom products. The C 5 rich stream was routed to the gasoline pool, while the C 6 rich was sent to the naphtha pool. There was an option to proceed in a conventional way by installing a new column after the second column or convert the second column into a side-cut column. Both options were rejected, as one of them was CAPEX intensive, while the other would not be able to achieve the desired quality of the product. The technocrats at the facility decided to take up the revamp by incorporating the hybrid model using DWC technology. Any facility pursuing this route to obtaining PCN has two options: • Route the midcut from the column, which is mainly NC 5 that carries the majority of the CS 2 to the naphtha/gaso- line pool, where it can be put to the desired end-use. This option does not put any additional load on the NHT unit (see Figure 4 ) • The second option is to route the midcut to the NHT unit to strip off the CS 2 in the form of H 2 S and mix this stream again with the PCN stream, the bottom product of the depentaniser column. This further increases PCN produc- tion (see Figure 5 ). Engineers decided to revamp the depentaniser column to a DWC to produce three high-purity cuts, as discussed. It achieved the desired CS 2 specs for PCN, wherein the CS 2

Figure 3A Top view of dividing wall column (Image courtesy of DWC Innovations).

It is interesting to see how divided wall columns over- come the limitation of conventional distillation sequences by adding single or multiple walls that create different frac- tionation zones inside a single column shell. These walls aim to reduce the intrinsic remixing of components in con- ventional columns, so higher thermodynamic efficiencies are achieved, as illustrated in Figures 3a and 3b . Use of DWCs to obtain PCN with iC 5 rich stream Within the context of the bespoke discussed processes and their limitations, DWC technology is the process of choice because it is low in CAPEX and OPEX but can seamlessly obtain the desired PCN specifications and other value- added products like high RON as the top product. One such process scheme is provided in Figure 4 , wherein the side- cut from the revamped DWC is diverted to naphtha for the gasoline pool. The benefits of revamping the depentaniser column into a DWC can be summarised as follows: • Meets CS 2 limit in PCN • No additional load on the NHT unit • Produces premium gasoline, improving profitability • Energy consumption offset by an improved margin from producing premium gasoline • Retrofits into existing columns.

iC rich to premium gasoline pool

Light naphtha

Full range naphtha

NC rich to naphtha pool / gasoline pool

Naphtha splitter

NS bottoms

PCN with CS <1ppm

Figure 4 Flow scheme after revamp of depentaniser into a DWC. The C 5 rich side-cut with concentrated CS 2 is diverted to naphtha/gasoline pool

PTQ Q3 2022