Hydrocarbon Processing - September 2021 - 75

Process
Optimization
K. K. MANDAL, Nayara Energy Ltd., Vadinar, India
Implementation of advanced level control
techniques in refinery operations
Plant performance depends on stable
operations, with steady feed, product
flow, temperature and pressure control
as per process requirements. Plant
performance can be improved further
by stabilizing column and vessel levels
through suitable level control techniques.
The residue fluid catalytic cracking unit
(RFCCU) is one of the most important
secondary processing units to increase
refining margins. RFCCU margins depend
on uninterrupted operations and
the best possible product yield. This is
accomplished by keeping unit severity
steady and maintaining the fractionation
of product without quality giveaways or
failures. Any identified deviation from
stable operations (e.g., temperature, pressure,
flow indication and control valve
operation) should be corrected.
A refinery generates a significant
amount of sour water from its process
units, such as the crude distillation unit
(CDU), vacuum distillation unit (VDU),
RFCCU and delayed coking unit (DCU),
among others. Oil carryover, along with
sour water, are common problems in the
refinery. The DCU generates a lot of sour
water due to its tedious batch processes,
which can include coking, stripping, coke
cooling and coke cutting. An adequate
control system for sour water separation
and disposal will help to reduce oil carryover,
along with sour water.
The removal of hydrogen sulfide
S) from the product stream and re(H2
cycle
gas is imperative before routing the
finished product to the rundown vessel/
tank and before routing recycle gas for
further use. H2
from generated fuel gas (FG) prior to being
used in the firing furnace.
S must also be removed
FIG. 1. RFCCU bottom level, C/V opening and slurry r/d flow trends before and after TON control.
Hydrocarbon Processing | SEPTEMBER 2021 75
Brief description of the RFCCU. The
RFCCU has two sections: a reaction section
and a product fractionation section.
In the reaction section, vacuum gasoil
(VGO) feed is cracked in the presence
of circulating catalysts. Reactor vapor
enters the fractionator column. After
cooling, the overhead vapor accumulates
in the overhead vessel. Part of the
liquid returns to the top of the column
as reflux for maintaining the required
temperature and for balancing material
pump-out to the gas concentration unit
(GCU). Vapor coming out from the top
of the overhead vessel is compressed
through a wet-gas compressor and routed
to the GCU after cooling. Product
drawn from the middle of the fractionator
is light cycle oil (LCO), which is
routed to the diesel pool through a stripper
to remove lighter materials from
the product. Slurry oil-also known as
clarified oil (CLO)-is drawn from the
bottom of the fractionator column and
routed to the DCU feed or the internal
fuel oil system.
Analysis of product draw and level
control. The fractionation-section LCO
product draw and the bottom-section
level control both play critical roles. In
RFCCU operations, some catalyst fines
travel in reactor vapor and settle in the
bottom section of the fractionator column.
To remove catalyst fines from the
bottom of the column, a minimum slurry
flow must be maintained. It was observed
that reducing LCO product was
required to maintain slurry flow.
To evaluate column bottom and LCO
levels, analyses were conducted to monitor
trends in level, valve opening and
flow. Major bottom level control and
flow deviations were observed. Level,

Hydrocarbon Processing - September 2021

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