Hydrocarbon Processing - October 2022 - 27

Plant Safety and Environment
the form of H2
S and COS, which are difficult to transform and
capture by additive. These S compounds are transformed into
SOx
in a CO boiler, so the refinery must use post-treatment like
a WGS to meet the requirements of local environmental laws
and regulations.
DETAILED ANALISYS OF APPLICATION CASES
Case 1: HZ-1 unit in the CNOOC Refinery Co. Ltd.
Huizhou refinery. The HZ-1 is an MIP unit with a designed
annual processing capacity of 1.2 MMtpy. The mass fraction of
sulfur in the feedstock ranged from 0.35 wt%-0.41 wt%. The
regenerator adopted full burn operation with a pre-combustor.
The total air volume to the regenerator was 0.13 × 106
Nm3
was 300 mg/m3
-400 mg/m3
/hr
and the excess oxygen in the flue gas was 4.8%-5.5%. The mass
concentration of SO2
in the flue gas at the entrance of the WGS
before the additivea
was used. After
using the additive, SO2 was reduced to less than 20 mg/m3
when the additive inventory was less than 2%, and the removal
ratio was about 95% (FIG. 3).
The caustic consumption decreased from 150 kg/hr to
about 10 kg/hr, which is consistent with the removal ratio of
SO2
, and sometimes even stopped filling caustic. Total dissolved
solids (TDS) in wastewater decreased from more than
10,000 mg/kg to 1,300 mg/kg with the addition of the additivea
(FIG.
4). The SOx
in the flue gas was greatly reduced after
SO3 and Na2
SO4
in
being captured by additive, and the amount of caustic was reduced,
thus reducing the salt content of Na2
water. TDS reduction not only benefits wastewater treatment
and slows down equipment corrosion, but it also reduces the
risk of system scaling, which is conducive to long-term stable
operation of the unit.
Considering that the additive has a good effect on TDS and
solves the problem of plume trailing subsidence from the WGS
caused by SO3
aerosol, it was used to reduce the SOx
of the flue
gas before the entrance of the WGS to a very low level, rather
than only reducing by 80% for the consideration of optimal
economic benefit. Assuming that the price of caustic with 30
wt% NaOH was $173/MMt and the cost of consumption of
the additive was considered, the total savings was about $4/hr
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Case 2: HZ-2 unit in the CNOOC Refinery Co. Ltd.
Huizhou refinery. The designed annual processing capacity of
the HZ-2 FCCU in the Huizhou refinery was 4.8 MMtpy with
an S content in the feedstock between 0.35 wt% and 0.39 wt%.
The unit adopts a two-stage, coaxial up and down incomplete
regeneration process with excess oxygen of less than 0.1%. The
total air volume to the regenerator was 0.32*10^6 Nm3
/hr and
the CO volume concentration at the outlet of the regenerator
was 3.2%-4.5%. When no additives were used, the mass concentration
of SO2
proximately 1,400 mg/m3
in the flue gas at the entrance of the WGS was ap-1,800
mg/m3
. The SO2
was 55% when using 3.8% additivea
in the inventory, and the consumption
of caustic decreased from 2,200 kg/hr to 1,100 kg/hr
(FIG. 5), saving about $72/hr. This value is close to the predicted
optimal economic operation in which the maximum economic
benefit was $80/hr with a 47% SO2
removal ratio.
Case 3: QD unit in the Sinopec QD Refining & Chemical
Co. Ltd. The QD unit is an MIP-CGP unit with a designed
processing capacity of 2.9 MMtpy. The feedstock oil is mainly
hydrogenated VGO with an S content of 0.35%-0.40%. The
regenerator adopted full burn operation with a pre-combustor.
The total air volume to the regenerator was 0.336 × 106
and the excess oxygen in the flue gas was 2.1%-3.2%. The mass
Nm3
/hr
OPTIMIZE PROCESSING PERFORMANCE WITH
removal ratio
in this case, which is still economical. This value is also consistent
with the predicted value in FIG. 2.
FIG. 4. Consumption of caustic and TDS in liquid waste during the
additive application in the HZ-1 unit.
HC Feb 2022.indd 1
Hydrocarbon Processing | OCTOBER 2022 27
1/10/2022 3:20:00 PM
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Hydrocarbon Processing - October 2022

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