PTQ Q2 2024 Issue

Middle cut for P arex unit

1.2

0.8

Flow rate

MTPH 160

Sp. Gr.

–

0.829

D86 % Vol IBP

˚C ˚C ˚C ˚C ˚C ˚C ˚C ˚C ˚C

110.5 118.6 121.2

Light naphtha to SCAN finer

16.5

10 30 50 70 90 95

From De-C bottom

Middle naphtha to U nionning/ Pl atformer

127

138

134.1 145.3 163.9 173.4 181.3

LCO from De-C reboiler

To De-C reboiler

Pressure kg/cmg

FBP

Temperature ˚C

HCO from FCC MF

1.5

Heavy naphtha to SCAN finer

To FCC MF

Figure 4 Required middle cut quality as feed for the Parex unit

separation efficiency and produce three cuts with the fol- lowing 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. Major constraints In attempting to minimise investment costs, the revamp was implemented while considering several major con- straints: reuse the existing equipment as much as possible, minimise additional equipment, minimise tower modifica- tions, and minimise modifications at the existing control system. A very narrow time frame for the project execution was defined: licence, basic, and detailed engineering, manu- facturing, and delivery of goods at the site in 18 weeks. Implementation of the project at the site in less than 20 working days within the time frame of the planned unit turnaround. All these items were under the responsibility of one single company, GTC, now Sulzer CFCL. Process simulation results While simulating a middle DWC, several models can be used. The most common are:  Four columns interconnected : The first column mod- els the section of the tower above the partition baffle. The second column models the section of the column equipped with the baffle and facing the feed. The third column mod- els the section of the column equipped with the baffle and facing the middle-cut draw-off. The fourth column models the section of the column below the partition baffle.  Two columns interconnected (see Figure 5 ): The first

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:  Shifting the upper reboiler duty to the bottom reboiler to maximise utilisation of the stripping section fractionation trays.  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.  Shifting the column’s feed location downward.  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 mid- dle 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 down- stream Platformer reactors with consequent high deactiva- tion 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

PTQ Q2 2024