Hydrocarbon Processing - December 2022 - 52

Process Engineering
ready handling the visbreaker naphtha
and middle distillates.
Coker liquified petroleum gas can
also be treated in existing installations.
The heavy coker gasoil stream-which,
as a thermally cracked material, is high
in sulfur and nitrogen as well as unstable
for transportation and storage-was to be
processed in a new dedicated hydrotreater
to produce a VGO boiling stable range
material (so it can be transported) as well
as low sulfur and nitrogen, making it easy
to sell to third parties.
The project had a large impact on the
overall sulfur block of the refinery. Sulfur
production increased from 45 tpd to almost
100 tpd. Increasing H2
S production
and sulfur recovery required a new amine
regeneration system to supplement the
existing system.
The substantial increase in sour water
production required adding a new sour
water stripper (SWS), as the existing SWS
could not accommodate the future flowrate.
An important side benefit of having
two distinct units is that it enabled the
segregation of sour waters in phenolic and
non-phenolic streams (FIG. 2). The nonphenolic
sour water originating from hydrotreaters,
which contains no other impurities
besides dissolved NH3
and H2
S,
could be now advantageously re-used as
make-up water in the delayed coker or
make-up wash water in hydrotreaters.
The sour water produced by the delayed
coker, which contains phenolic
impurities and dissolved NH3
and H2
S,
could be entirely used as make-up water
to the crude desalter. This minimized
the rundown of phenol to the wastewater
treatment plant (where phenol is difficult
to remove), as the phenol is largely reextracted
into the crude and subsequently
decomposed in the hydrotreaters.
Additional complexities accompanied
the increase in sulfur load. The H2
S to
be processed in the sulfur recovery unit
(SRU) increased more than two-fold,
and the amount of NH3
to be processed
increased by a factor of eight. This represented
an additional extra hydraulic
load on the SRU, resulting from the H2
S
increase. Consequently, the gas throughput
of the SRU increased by about threefold
in total.
The SRUs had similar capacities (about
50 tpd). One unit acted as a standby for
the other in the existing refinery operating
plan. Both units (A and B) were equipped
with selective oxidation-type technology,
enabling recovery efficiencies greater than
99%. Only Unit B was designed to co-process
SWS gas. Both SRUs were equipped
with oxygen enrichment and low-cost oxygen
readily available by pipeline from an
industrial gas supplier in the vicinity.
An assessment of the future loads of
amine acid gas and SWS gas showed that
the two existing SRUs could handle the
future loads, provided that:
* Both SRUs would always be
operated (no stand-by capacity).
* Both SRUs would always
use oxygen enrichment.
* Unit A would be revamped to
handle SWS gas and amine acid gas,
necessitating a new knock-out (KO)
drum with piping and instrument
modifications. A new combustion
chamber in the Claus furnace was
not deemed required, despite a
somewhat low residence time,
since the operating temperature
was always sufficiently high due
to the use of oxygen enrichment,
ensuring good NH3
destruction.
* Unit A would be revamped
with deeper sulfur seals so it
could be operated at higher
pressure resulting from higher
flowrates, taking advantage of
spare differential pressure in
the existing main airblower.
FIG. 1. Overall refinery block flow diagram.
* Unit B, although already able to
handle SWS gas, required limited
revamp measures on the SWS gas
KO drum and piping, enabling
it to handle more SWS gas.
The requirement to always use both
SRUs was not a comfortable scenario as it
would result in acid gas flaring whenever
one SRU tripped. The relevant authorities
only accepted this simultaneous and
continuous operation of both SRUs at
the future higher-sulfur load in case the
normal sulfur dioxide (SO2
) emissions
FIG. 2. Sour water segregation (SWS).
52 DECEMBER 2022 | HydrocarbonProcessing.com
would be reduced.
A caustic-based absorption system was
selected to achieve this requirement at the
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