Hydrocarbon Processing - December 2021 - GP-13

SPECIAL FOCUS: PLANT DESIGN
& PROCESS CONTROL
Case study: Optimization of LPG process
at Ju'aymah NGL fractionation plant
P. GARG and U. A. AL-DAHLOUS, Saudi Aramco, Ju'aymah, Saudi Arabia
Optimization initiatives for the LPG Merox process were
implemented to reduce operating cost and spent caustic
generation at Saudi Aramco's Ju'aymah NGL fractionation
plant. These initiatives included an increase in fresh caustic
concentration and reuse of extractor spent caustic in the
Merox process.
Also, a new catalyst was successfully used in the Merox system,
with higher activity than a conventional catalyst. Benefits
were derived from the new catalyst with regard to consumption
rate, fresh caustic use and spent caustic generation.
The implementation of these initiatives contributed significantly
to enhanced treatment efficiency, with a lower operating
cost of $0.5 MM/yr, both from fresh catalyst and disposal
of spent caustic. Spent caustic generation from the LPG
Merox process was reduced by approximately 50%.
Process background. The Ju'aymah NGL fractionation
plant has four identical fractionation trains. Each train consists
of deethanizer, depropanizer and debutanizer columns;
an amine treating facility to remove hydrogen sulfide (H2
and carbonyl sulfide (COS) from C3
S)
and C4
product received from
products, followed
the depropanizer overhead, and Merox processing units for
mercaptan sulfur removal from C3
by the C3
and C4
dehydration system for moisture removal
from the final product.
An opportunity was identified for the optimization of the
LPG Merox process to reduce spent caustic regeneration,
product sulfur and catalyst consumption, and therefore reduce
plant operating cost. This article describes the initiatives
implemented to optimize the Merox process and achieve
these benefits.
Merox process overview. The Merox process consists of
three sections, as shown in FIG. 1. The first section consists
of prewash drums to remove H2
S and COS left over from the
amine unit. Separate prewash drums are in place for both C3
and C4
tion is the extractor, which contains C3
mercaptan exproducts.
Followed by prewash drums, the second secand
C4
tractors, and the last section is the combined Merox regeneration
for the regeneration of mercaptan-rich caustic collected
from both of the extractors.
The prewash drums are operated at a caustic concentration
of 5 wt%-10 wt%, while the extractors are operated at a caustic
concentration of 8 wt%-12 wt%. In extractors, mercaptan
reacts with caustic and forms sodium mercaptide (Eq. 1),
FIG. 1. Merox process overview.
Gas Processing & LNG | NOVEMBER/DECEMBER 2021 13
Process challenges and optimization opportunity. In addition
to high spent caustic generation, other Merox process
challenges included frequent system troubleshooting to control
the product sulfur, high Merox catalyst consumption rate
and high catalyst expense.
An opportunity was identified for the optimization of the
Merox process to reduce product sulfur, spent caustic regenwhich
is oxidized in the regeneration section for conversion
back to caustic and disulfide oil (Eq. 2).
RSH + NaOH → NaSR + H2
NaSR + ¼ O2 + ½ H2
shown in Eq. 3:
RSH + ¼ O2
O → NaOH + ½ RSSR
O (1)
(2)
The overall Merox process reaction can be written as
→ ½ RSSR + ½ H2
O
(3)
In the Merox process, intermittent liquid Merox catalyst is
injected into the rich caustic before the oxidizer. In the oxidizer,
mercaptide is oxidized to disulfide oil and caustic, which
are separated in the disulfide separator drum. In the overall
Merox process, the caustic and Merox catalyst concentration
is reduced with time due to the dilution effect.
To maintain optimum concentration, partial caustic replacement
is carried out every couple of days. Due to a high
system volume, this generates a large quantity of spent caustic
from the Ju'aymah NGL facility every year. Shipment of the
spent caustic is a costly measure and has a significant impact
on plant net direct expenditure.
Hydrocarbon Processing - December 2021

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