Hydrocarbon Processing - September 2021 - 66

Process Optimization
Merox treatment, as these units have individual treatment sections
and the LPG is sent directly to storage. An extra connection
exists between the DHP LPG product and the deethanizer
to increase the purity of the DHP LPG product, which in turn
increases the purity of the LPG pool. HCU1 LPG can be sent to
storage either after the debutanizer or after the debutanizer plus
the deethanizer.
The LPG recovery compressor, which incorporates a chiller
and an LPG evaporator, affects the LPG pool and rundown
routing. Additionally, off-spec LPG from storage can be retreated
in Merox 1 and Merox 2. Since these units can treat only
sulfur components, only H2
S, total sulfur and corrosion strip
specifications can be manipulated directly.
LPG processes. The following sections detail the units used in
the LPG process.
Merox units. The LPG Merox units are used to remove residual
H2S and mercaptan sulfur (RSH-S) compounds from
S content.
A typical Merox unit consists of prewash and extraction sections,
where LPG product is treated with caustic; and a caustic
regeneration section, where caustic is regenerated in the presence
of catalyst and air for reuse.
The caustic concentration is different in the prewash and extraction
sections to treat H2S and mercaptan compounds selectively.
LPG is first pretreated in the prewash section with 10°Baumé
caustic for H2
S removal and then routed to the extraction
section, where mercaptan-rich LPG is mixed counter-currently
with 20°Baumé caustic solution in a trayed tower. During mixing,
mercaptans are transferred from the LPG to the caustic solution.
The reactions for the process are shown in Eqs. 1 and 2:
2RSH + 2NaOH } 2NaSR + 2H2
mercaptan extraction
2NaSR + 1/2 O2
O
+ H2O } 2RSSR + 2NaOH
(1)
(2)
LPG product is posttreated by moving it through the settler
and sand filter after extraction. Regenerated caustic is used until it
is spent and then sent to the caustic neutralization unit before disC3
Coker
FCC
Crude
1
Crude 2
Crude 2
Merox 3
Splitter and
SHU
Merox 1
Merox 2
Platformer 1
DHP
Hydrocracker 1
Hydrocracker 2
Deethanizer
Platformer 2
FIG. 1. LPG pool configuration at the İzmit refinery.
66 SEPTEMBER 2021 | HydrocarbonProcessing.com
LPG product
C4
LPG products. The LPG product is sent to the Merox units after
amine treating to decrease the H2
posal. The circulation rate of regenerated caustic is important, as
mercaptan removal increases as the circulation rate increases, but
disulfide content coming from the caustic also increases and affects
total sulfur. For this reason, an optimum rate should be used.
LPG inlet temperature, caustic temperature, caustic concentration,
caustic spent rate, caustic/hydrocarbon rate, amount
of oxidation air and catalyst affect LPG product treatment efficiency
(FIG. 2).
The İzmit refinery has three LPG treater units. Two are traditional
Merox units (Merox 1 and Merox 2). The common practice
is to treat crude unit LPG products in one unit and butadiene-rich
FCCU and DCU LPG products in the other unit. The
two Merox units are connected to each other, so routing different
products to the other treater or common usage of some sections
are allowed. In case of a sulfur- or corrosion-related problem in
any of the LPG producing units, the product can be routed to
the LPG treaters before being sent to storage. During startup or
shutdown of the LPG units, the product can be routed to treaters
or off-spec tanks. A naphtha wash system was added to Merox 1
and Merox 2 to increase sulfur removal.
The Merox 3 unit is used to treat only DCU LPG product.
Selective hydrogenation unit. The selective hydrogenation
process (SHP) is the hydrogenation of C4
to mono-olefins over
a highly selective, fixed-bed catalyst. Various palladium and
nickel-based catalysts are used for SHP reactions. These catalysts
are affected by contaminants including carbon monoxide,
total chloride, H2
water and total sulfur.3
S, carbonyl sulfide, nitrogen, metals, caustic,
Selective hydrogenation catalyst loading
is carried out by either sock loading or dense loading. Selective
hydrogenation reactions are controlled by pressure, temperature,
liquid hourly space velocity (LHSV), hydrogen addition
rate and effluent recycle ratio.
The SHP reactions are shown in Eqs. 3 and 4:
1,3 butadiene + H2
r butene-1
1,3 butadiene + H2 r butene-2
(3)
(4)
In refineries, feed is supplied from the C4 cut of the FCCU
LPG and the DCU LPG. At the İzmit refinery, there is an additional
feed option from storage. FCCU LPG and DCU LPG
are first separated in a C3
/C4
splitter, and butadiene-rich C4
sent to the SHP unit feed drum. Butadiene-rich C4
is
is pressurized
to define the reaction pressure by means of feed pumps and is
optionally connected by the reactor recycle effluent. This combined
feed is heated to reaction temperature and is combined
with H2
. The combined feed is sent to the reactor, where reactions
occur in the liquid phase. Light ends and excess H2
of the
reactor effluent are separated and sent to the fuel gas system. The
saturated C4
directly or in combination with C3 from the C3/C4
product is cooled and sent to product storage either
splitter.
Offgas compressor and working modes. The main purpose
of the offgas compressor is to recover LPG from clean gases
and increase the LPG amount. Six clean gas sources feed the
offgas compressor from the overheads and the DEA absorbers.
These gases are pressurized in two stages, and low-purity LPG
is recovered. Since the purity of this LPG is around 70%, it is
not directly sent to the rundown tank or directed to the deethanizer
or debutanizers.
FIG. 3 shows the two compressor modes: (A) deethanizer
first, and (B) debutanizer first. In the deethanizer-first mode,
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