Hydrocarbon Processing - October 2022 - 26

Plant Safety and Environment
pick-up and, therefore, reduces SOx
tor temperatures tend to favor SO3
distribution and mixing in the regenerator enhance SOx
-reduction additive, to a certain extent.
To enhance the efficiency of flue gas SOx
emissions. Lower regeneraformation,
while good air
pickup.
The FCC catalyst itself and the presence of a carbon monoxide
(CO) promoter also affect the SOx
the SOx
thors' company developed the enhanced RFS additivea
specifically improved the content of the key active component
MgO. At the same time, the content of oxygen storage components
were adjusted to improve the SOx
and avoid adverse effects on SOx
capture efficiency of
pick-up, the au,
which
removal efficiency
under the condition of low excess oxygen, including partial
burn regeneration. In addition, the preparation process of the
additivea
operation. The authors' company completed industrial trial
production in 2014 and the additive has been successfully applied
in more than 40 FCCUs.
The concentration of SO2
ferent excess oxygen conditions based on the application of the
proprietary additivea
was optimized to maintain good attrition resistance
removal efficiency and unit
and excess oxygen in the regenerator
are the two key factors affecting the PUF for the same grade
of additives. Here, the PUF of the additive was fitted as functions
of SO2
concentration at the regenerator outlet under difin
25 different FCCUs. It was found that
these units could be divided into five categories according to
excess oxygen content and regenerator form. The fitting of each
category presents a good linear correlation, as shown in FIG. 1.
Based on this, the PUF of the additive under different operating
conditions can be predicted. It was found that the PUF
value increased with the increment of excess oxygen of the regenerator
and outlet SO2
concentration. To identify the cases
where using additives is more economical than using caustic in
WGSs, the difference between the two costs was expressed as
the following equation (Eq. 2):
φ = 80/64/30% × α × ω(SO2
(SO2
)× Q × P2
where:
/PUF
) × Q × P1
- α×ω
(2)
φ = cost savings using additive over caustic, $/hr
α = SO2
ω(SO2
removal ratio using additive
) = mass concentration of uncontrolled SO2
in the original flue gas, mg/m3
Q = flue gas flowrate under normal conditions, 106
P1 = unit price of caustic with 30 wt% NaOH, $/kg
P2 = unit price of SOx reduction additive, $/kg
Nm3
/hr
Combined with the calculated PUF values under different
operating conditions in FIG. 1, the maximum value of φ can be
obtained.
Three FCCUs are listed as examples in FIG. 2: two units in
the CNOOC Refinery Co. Ltd. Huizhou refinery (HZ-1 and
HZ-2) and one in the SINOPEC QD Refining & Chemical
Co. Ltd. (QD). The excess oxygen of HZ-1 is greater than 4%,
and the excess oxygen of QD is between 2.5% and 4%, while
HZ-2 adopts partial burn regeneration with excess oxygen of
less than 0.1%. Details of the three FCCUs are described below.
Using the fitting relationship in FIG. 1 can predict the optimal
economic benefit of the three units, as shown in FIG. 2. It is
predicted that the maximum economic benefit would be about
$13/hr and $80/hr after the application of the additivea
in the
HZ-1 and HZ-2 units while the removal ratio is about 80% and
47%, respectively.
Similarly, the QD unit is expected to save about $79/hr with
a removal ratio of about 75% after the application of the additive.
It should be noted that when the additive is initially added,
the PUF is very high due to the high initial concentration of SO2
in the regenerator; however, the insufficient amount of additive
cannot remove enough SO2
so the removal ratio is not high.
In this case, it is more cost-effective to capture this part of SO2
with the SOx
-reduction additive than with caustic in WGSs.
Once the additive amount exceeds a certain value when the
excess oxygen is not high enough, the removal of SO2
requires
much more additive supplement than before-in this case, the
cost of using additive has skyrocketed and is not cost-effective.
This is reflected in the rapid decline of the curve in FIG. 2. The
capture efficiency of additive is very low under marginal conditions.
In incomplete regeneration, some S compound exists in
FIG. 2. The expected economic benefits of the three refineries.
FIG. 3. The SO2 concentration in the flue gas during the additive
application in the HZ-1 unit.
26 OCTOBER 2022 | HydrocarbonProcessing.com
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Hydrocarbon Processing - October 2022

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