Hydrocarbon Processing - May 2021 - 20

Maintenance and Reliability

To stack

PI1

findings are indications that slop products produced from the
cracking units may have caused foulants on the trays and, therefore, an increase in the pressure drop. Another reason for the
fouling might be asphaltene precipitation. Light opportunity
crudes are blended with heavy opportunity crudes, making the
processed blend prone to asphaltene precipitation, which can
lead to fouling on trays.
Preparation and application. The aim was to minimize down-

time for the CDU with the highest crude processing. The maintenance history of the preflash column shows that the mechanical
cleaning takes at least 10 d, with many hours needed for blinding,
access opening, cleaning, steam-out, etc. The heated cutter stock
circulation was considered for removing and dissolving foulants
on the tray and for minimizing the downtime of the unit. Lightcycle gasoil, a product of fluid catalytic cracking unit (FCCU),
was used as cutter stock because of its solvent properties.

Air cooler

19,000

0.33

Flare
Cooling
water

Feed to
preﬂash

Preﬂash
reboiler furnace

O/H reﬂux
drum

0.28

17,000

0.23

16,000

To
debutanizer

PI2

Preﬂash
column

Reboiler
furnace pump

18,000

Offgas

Crude oil feed ﬂow, m3/d

Crude
feed

Crude oil feed ﬂow

Shutdown

0.18

15,000
Preﬂash column pressure drop

14,000

O/H reﬂux Debutanizer
pump
feed pump

New
pipeline

Fuel oil

Main column
furnace pump

0.08

13,000
To furnace

Heat
exchangers

0.13

Preﬂash column pressure drop, kg/cm2

high amounts of naphtha did not cause a high pressure drop in
the column, contrary to what had been experienced before. FIG.
1 represents the relation between pressure drop along the column and debutaniser feed volume.
In June 2019, when the incident occurred for the first time, a
high amount of slop (6% of the feed rate) was processed, including cracked products formed during the startup of the cracking
units, which had undergone a turnaround the previous month.
The slop included coker products, especially coker naphtha,
which may have been responsible for the rapid fouling in the
preflash column due to its molecular structure. The amount of
slop processed and the change in pressure drop through the preflash column are shown in FIG. 2.
In a similar way, the increase in the pressure drop through
the column had been observed after processing the slop products formed by the cracking units in 2015 and 2017. All these

0.03

12,000
12/5/2019

12/10/2019

12/15/2019

12/20/2019

12/25/2019

FIG. 5. Pressure drop of preflash column vs. crude oil feed flow before
and after cleaning.

FIG. 3. Preflash column process flow diagram.

3,000

0.33
Before and after the cleaning
0.28

Debutanizer feed ﬂow, m3/d

2,600

Debutanizer feed ﬂow

0.23

2,400
2,200

0.18

Shutdown

2,000

Preﬂash column pressure drop

0.13

1,800

Preﬂash column pressure drop, kg/cm2

2,800

0.08

1,600
1,400
12/5/2019

12/10/2019

12/15/2019

12/20/2019

0.03
12/25/2019

FIG. 6. Pressure drop of preflash column vs. debutanizer feed flow
before and after cleaning.

FIG. 4. Clean and dirty cutter stock samples after four circulations.

TABLE 1. Cutter stock sample results
Tests

Clean cutter ctock

First circulation

Second circulation

Third circulation

Fourth circulation

Asphaltenes, % m/m

0.12

0.041

0.033

0.01

Carbon residue, % m/m

0.42

0.12

0.48

0.02

API

24.4

25.5

24.8

28.1

32.9

Viscosity, mm2/s

1.597

1.785

1.627

1.697

1.822

MAY 2021 | HydrocarbonProcessing.com

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Hydrocarbon Processing - May 2021

Table of Contents for the Digital Edition of Hydrocarbon Processing - May 2021