Hydrocarbon Processing - February 2021 - 55

Process Optimization
Revamps of conventional columns to
DWCs can provide the following benefits:
* Ideal alternative for revamp of
side-cut columns when high purity
is required from the three product
streams
* Lower footprint as equipment
count is reduced by half
* CAPEX and OPEX can be reduced
by approximately 20%-50%.
Structurally, the exterior of a DWC
looks like a conventional distillation
column, but inside a defining wall in the
column separates the tower into two sections, creating different fractionation
zones. The zone in the column where the
feed is introduced works to effectively
separate the heaviest and the lightest key.
Because this wall removes the intrinsic
mixing that takes place in the conventional column by creating different separation zones, these columns are thermodynamically more efficient compared to
their counterparts-therefore providing
benefits in terms of operating cost.
DWCs in a DIH column for producing C6 cut. As demand for C6 products

surged, refiners foresaw an additional
source of revenue and hurried to generate C6 product.
The usual way of obtaining FGH
from the recycle stream is by installing
two new columns post the DIH column.
FIG. 4 shows the typical configuration of
producing FGH by the DIH route. In
this sequence, two new columns are installed downstream of the isomerization
recycle stream to produce FGH.
An attractive alternative to this sequence would be to revamp the existing
DIH column using DWC technology to
produce four cuts.
For isomerization facilities, the revamp of a DIH column to DWC can have
significant benefits. The revamped DIH
column produces light and heavy isomerate as top and bottom products, along
with FGH and the recycle stream as the
other two cuts. FIG. 5 shows how a middle
DWC handles the overlap of the heavy
isomerate and the recycle stream, reducing the number of stages required for the
desired specifications compared to the
conventional column-the spare stages
are available in obtaining the fourth cut.
This option of getting four cuts from
the DIH column is not only attractive in
terms of lower CAPEX and OPEX, but is

flexible as the column is capable of operating in two modes: the FGH mode, in
which the column will produce a fourth
cut of FGH; and the DIH mode, in which
the column operates in conventional
mode with recycle to the isomerization
unit without FGH production. To produce the fourth cut, the alignment of the
wall inside the column is customized to
meet the desired product specifications,
quantities and also target to minimize the

heat loads. The advantages of this process
are sufficient to prompt facilities to utilize it for beneficial production of FGH.
FIG. 6 shows the process flow and the
components of the four cuts of the DIH
column post revamp. Benefits include:
* Energy consumption is 30%
less than the conventional
column sequence.
* With the drawing of FGH (i.e.,
n-hexane from the recycle stream

TABLE 1. Specifications for C6 products
Property

Units

Color

FGH

Saybolt

Density (at 20°C)

kg/m

Moisture

mg/kg

Bromine index

mg Br/100g

PGH

SBP

Isohexane

Min. 30

Min. 25

Min. 30

0.660-0.687

0.665-0.686

Max. 100

Max. 50

Max. 10

1.375-1.384

1.373

Negative

°C

120

Refractive index
Cu Strip Cor.
for 3 hr at 50°C
Doctor test

Max. 5

Distillation range
Initial boiling point
Final boiling point
Residue on
evaporation

mg/100 ml

Components
N-pentane

N-hexane

wt%

Min. 40

Min. 44

Isohexane

wt%

30-45

Methyl cyclo pentane

wt%

Max. 20

Cyclo hexane

wt%

Max. 3

Benzene

ppmwt

Max. 500

Aromatics

ppmwt

Lead as Pb

mg/kg

Max. 1

Tota sulfur

mg/kg

Max. 5

Max. 2

Feed

Min. 95

Max. 3

Feed

Recycle

Max. 3,500
Max. 1
Light
isomerate

Feed

Recycle
Hexane
product

Recycle

Heavy
isomerate
* DIH columns typically have 80-100 trays
* Since the sidecut recycle stream is below
feed, there is considerable overlap of
heavy isomerate with the recycle stream

Max. 100

Max. 10

Light
isomerate

Max. 1
Max. 5

Heavy
isomerate
* The dividing wall reduces the stages
requirement between feed and the
recycle stream for the same specs
* Spare stages available for the 4th cut

Heavy
isomerate
* Spare stages become available
for getting the 4th cut of hexane product

FIG. 5. Use of DWC technology for a revamp of a DIH.
Hydrocarbon Processing | FEBRUARY 2021 55

Hydrocarbon Processing - February 2021

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