May-2024

Revamping a conventional naphtha splitter to a dividing wall column

Installation of new equipment, performance results, and final commissioning of a DWC and its ability to sharply fractionate three components in one single column.

Giuseppe Mosca, Joshi Chandrakant, Andrei Cimpeanu and Brad Fleming
Sulzer Chemtech

Article Summary

A dividing wall column1 (DWC) is a distillation column combining the operation of two conventional columns in one shell. The separation of a three-component mixture into individual components is made possible by the installation of a vertical wall in the middle of the column. The feed enters the column on one side of the wall, and on the other side of the wall, a high concentration of the middle boiling component is achieved.

The result is that three components can be sharply fractionated in one single column. In the conventional setup, two columns, two reboilers, and two condensers are needed: the DWC needs only one vessel, one reboiler, and one condenser. The DWC has shown its advantages in many applications, typically for liquid mixtures of equal amounts of three volatile components and for cases where a high amount of middle boiling component must be separated from smaller amounts of light and heavier components.²

Rule of thumb: The bigger the side-stream portion of the feed, the higher is the effectiveness of the dividing wall. DWCs have been implemented in a large variety of refinery, petrochemical, and chemical applications. Recently, they have even been implemented for reactive distillation. More complex DWCs (Dual3 DWCs) can be used for the separation of a stream in four cuts.

DWC: Benefits and limitations
From a unit operation point of view, a DWC eliminates the thermodynamic inefficiency of the two-column setup,⁴ where the remixing of the middle and heavy boiling components at the bottom of Tower 1 cannot be avoided (see Figures 1 and 2).

DWCs allows for several benefits⁵ compared to the two-column setup:
• Up to 40% reduction of investment cost
• Up to 40% reduction of operational cost
• Up to 40% additional theoretical stages
• Up to 50% energy savings
• Up to 40% reduction of plot plan space and consistent reduction of greenhouse gas emissions.

DWCs also have limitations compared to the two-column setup:
• Operational pressure variation between column sections is not possible
• Increased temperature difference between reboiler and condenser
• Increased pressure drops
• Increased length and diameter of the vessel
• Increased complexity for modelling in the process simulation
• Increased complexity for the hydraulic design of the mass transfer components
• Lower operational flexibility.

Existing naphtha splitter
The existing naphtha splitter is a conventional distillation column with a side draw. It splits the full naphtha coming from the bottom of a depentaniser located at the GasCon of the fluid catalytic cracking (FCC) unit, into three streams: light naphtha, middle naphtha and heavy naphtha.

The column is heat integrated with the FCC unit. The heat duty to the column is provided by two reboilers: upper reboiler, where the heating medium is the light cycle oil (LCO) coming from the reboiler of the debutaniser, and lower reboiler, heated with the heavy cycle oil (LCO) coming from the FCC main fractionator. The top condensation system provides the cooling duty to draw off the light naphtha and generate the internal reflux required for the fractionation of the cuts. The feed is preheated at the preheat exchanger by means of the side-cut draw before entering the tower in the middle section (see Figure 3).
The three cuts are further processed to the downstream MOG and SCANfiner units to meet the quality of the gasoline pool.

Revamp motivation
All three cuts were initially used for gasoline. The existing tower was able to achieve the required performances both from the fractionation efficiency and the hydraulic capacity points of view. Several years later, after the first start-up, the tower was called for a different duty: top and bottom cuts were still pro-gasoline, and the middle cut had to be used as feed for the downstream Parex unit after being processed at the Unionfining and Platformer units (see Figure 4).

A sharper separation between the three cuts is required compared to the original design. To accomplish the new service, several options were investigated, among others:
υ    Shifting the upper reboiler duty to the bottom reboiler to maximise utilisation of the stripping section fractionation trays.
ϖ    Increasing overall reboiler and condenser duties by 20% over the original design to maximise the reflux ratio in the attempt to at least meet middle-cut specifications.
ω    Shifting the column’s feed location downward.
ξ    Optimising the side-cut draw-off location.

None of the above options could be even close to the required qualities of the three cuts:
•    Insufficient separation between light naphtha and middle naphtha, resulting in higher C6 content than desired in the middle naphtha.
•    Poor separation between middle naphtha and heavy naphtha, resulting in higher C10+ content than required in the middle naphtha. It would cause over-coking at the downstream Platformer reactors with consequent high deactivation rate of the catalyst and lower yields of valuable aromatic components.
Adding another distillation tower would achieve the target, but it was not even considered due to the high investment cost. To minimise the investment cost while maximising energy savings and minimising GHG emissions, it was decided to convert the tower into a DWC.

Revamping scope
The main revamping target was to increase tower separation efficiency and produce three cuts with the following distillation boiling points (as D86):
• Light naphtha with a 98°C final boiling point.
• Middle naphtha with 110.5°C initial boiling point, and final boiling point lower than 182°C for Case 1 and lower than 179°C for Case 2.
• Heavy naphtha with initial boiling point >144°C at 300 t/h feed flow rate.
• An operating range of 110-50% of design.