Hydrocarbon Processing - May 2021 - 21

Maintenance and Reliability
To minimize downtime, preparations were made prior to reducing the feed rate. The outlet of the spare main column furnace pump was connected to the outlet of the preflash column
overhead reflux pump to create a closed loop (FIG. 3), while the
other two pumps-the main column furnace pump and overhead reflux pump, both with electric motors-were in operation. Only the spool connection was completed after the shutdown for the sake of operational safety, since leakage through
the gate valve would be too risky.
After the unit shutdown, the cutter stock pumps were started, and the column was flushed with cutter stock to the slop
tank. The flushing was followed by column loading with cutter stock. The column was filled to the top (approximately
80%-90%) with cutter stock while the spool connection was
completed, making the closed loop. The loading of the entire
column took 5 hr-6 hr.
The cutter stock circulation between the preflash column
and the reboiler furnace started through the reboiler furnace
pump after 80% of the column bottom level was reached. The
cutter stock flow was heated to 180°C to increase the solvency of
the cutter stock. The level of the column was indicated through
a calculation making use of pressure transmitters located at the
top and bottom of the column according to the formula in Eq.
1, where pressure values are given in kg/cm2 and 0.8 is approximately the specific gravity of the heated cutter stock:
1 (kg / cm2) = 10,000 mm H2O = 10 m H2O
Level = (P2 - P1 ) / (0.8 S.G. × 0.1)

(1)

After the spool connection was completed and the level
was increased in the preflash column, the main column furnace pump was started to make the cutter stock fluid circulate
through the overhead line and upper trays of the column. The
cutter stock temperature was observed by the temperature indicator at the flash zone. To prohibit the fluid from boiling, the
cutter stock temperature was not allowed to exceed 180°C. The
circulation at 180°C lasted around 10 hr, followed by column
draining. The column was drained with the main column furnace pump to the slop tank in nearly 2 hr.
After column draining, an entire cycle is fulfilled in a 16
hr-20 hr time range. The same phases (loading, circulating and
draining) were repeated for the second, third and fourth circulation. After draining of the fourth circulation, the unit was ready
for startup. The last drain was made through the heat exchangers to eradicate organic foulants in the heat exchanger tubes.
Takeaway. In total, four circulations were carried out. After
each circulation, a sample from the dirty cutter stock was analyzed for asphaltene, carbon residue, API gravity and viscosity.
The results are shown in TABLE 1.
According to the sample results, asphaltene values decreased
after the fourth circulation and then became negligible. In the
third and fourth circulations, light diesel and kerosene were
used as cutter stock instead of LCGO since LCGO product volume was inadequate. The low LCGO volume was the reason
for the increase in API and viscosity values after the third circulation. Carbon residue declined, as well, except for the third
circulation sample. As shown in FIG. 4, the color of the fourth
sample was detected to be lighter than prior samples, indicating
the cleaning of column and heater tubes.

Before this application, feed to the CDU was reduced due to
high pressure drop at the preflash column, especially when the
crude oil blend contained a greater quantity of lighter products.
However, the problem was eliminated after the successful application. The unit was shut down for 3 d of maintenance (FIG. 5
and FIG. 6) instead of reducing feed.
In addition to this solution, the use of cutter stock as a solvent during these circulations had a cleaning effect on not just
the preflash column but the heat exchangers, reboiler furnace
and pipelines. The cutter stock was drained after the fourth
circulation through the heat exchangers to clean them. After
startup, the heat exchanger's heat transfer performance was improved-the heat transfer duty was improved by 0.35 Gcal/hr.
The downtime of the unit was reduced by a total of 5 d. The
maintanence required less labor, with a better method in terms
of health and safety. Additionally, energy gain was achieved due
to the cleaning of heater tubes and heat exchangers with cutter
stock. This application is promising in terms of production and
energy savings.
LEVENT ÖZKAN is a Process Supervisor at the Tüpraş İzmit refinery. He holds
a BSc degree in chemical engineering from Bosphorus University.
İLKE AYDIN is a Process Supervisor at the Tüpraş İzmit refinery. She is responsible
for crude and vacuum distillation units. She holds a BSc degree in chemical
engineering from Bosphorus University.
GÖZDE ÖZTÜRK is a Process Engineer at the Tüpraş İzmit refinery. She holds
a BSc degree in chemical engineering from Middle East Technical University
and an MSc degree from Bosphorus University.

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Hydrocarbon Processing | MAY 2021

Hydrocarbon Processing - May 2021

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