O gas
Light isomerate
C
Reformate splitter
Isomerisation (ISOM) unit
Light naphtha
Deisohexaniser
Benzene extraction unit (BEU)
C
Benzene
Naphtha
Naphtha splitter
DIH recycle
Ra nate
Heavy isomerate
O gas
Reformate bottoms
Reforming unit
Heavy naphtha
Figure 2 Naphtha complex in a refinery
ensuring that the DWC design aligns with the specific goals of the revamp project. Dual dividing wall column For more complex separations involving four or more prod- uct streams, the dual dividing wall column (DDWC) con- figuration incorporates two vertical dividing walls within a single column shell, enabling the simultaneous separation of complex multicomponent mixtures into four or more distinct products (Figure 1). By extending the principles of thermal coupling and physical separation found in conven- tional DWCs, the dual-wall design offers further reductions in energy consumption and equipment count. However, the increased internal complexity of DDWCs necessitates more sophisticated design, control, and model- ling strategies. Advanced simulations and rigorous optimi- sation are essential to ensure stable operation and product quality, making DDWCs most suitable for processes where the economic and operational benefits outweigh the added design challenges. Refinery naphtha complex overview DWC technology has proven especially valuable in refin- ery operations, notably within the naphtha complex, where precise hydrocarbon separation is critical for optimising performance and product quality. Figure 2 illustrates a typ- ical naphtha processing flow scheme. The naphtha feed is initially split into light and heavy fractions. Light naphtha, which is rich in linear paraffins, is directed to the isomeri- sation (ISOM) unit, where it undergoes catalytic conversion into iso-paraffins to enhance octane number. The resulting isomerate is then processed in a deisohex- aniser (DIH), which separates it into light and heavy isomerate streams, along with a recycle stream enriched in n-hexane and methylpentanes. These components are recycled back to the ISOM unit, where they are further converted into higher-octane molecules, thereby boosting the overall octane rating of the final product and improving process efficiency. Simultaneously, the heavy naphtha fraction is routed to the reforming unit, where it is converted into high-octane reformate through catalytic reforming. The reformate is
Case study 1: Naphtha splitter revamp at a Middle East refinery
Conventional
Middle DWC
design
design
iC₅ purity in overhead, wt% nC₅ purity in overhead, wt% nC₅ purity in side draw, wt% C₇+ content in side draw, wt% C₆ content in bottoms, wt%
70 20 19 <3
85
5
24 <3
<0.5
<0.5
Energ y savings, %
–
30
iC
Naphtha
C –C to ISOM unit
Heavy naphtha to reformer
Figure 3 Revamp of a naphtha splitter into a DWC
greater operational flexibility to accommodate variations in feed composition. • Bottom-wall configuration is best suited for processes where the separation between intermediate and heavy components is more favourable than separating light and intermediate components. It incorporates two independent reboilers, enabling precise control over both the heavy-to- intermediate and intermediate-to-light separation zones. Similar to the top-wall design, it offers enhanced control over product purities and provides greater operational flex- ibility to accommodate feed composition variability. Each configuration can be further optimised using advanced simulation tools and process control strategies,
16
Revamps 2025
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