Hydrocarbon Processing - January 2022 - 69

Bio-Based Processing
* Some spent caustic wastes
could be neutralized in the
sour water system.
With biodiesel production, the comand
less H2
S and NH3
. There
bined refinery sour water will contain
more CO2
will also be a dilution effect because of
the additional water produced. Consequently,
more energy will be required to
process wastewater generated from the
biofeedstock, predominantly to increase
the temperature of the extra water to stripping
conditions. In most cases, the energy
source is low-pressure steam, which is
abundant in many facilities; however, the
energy cost of stripping the wastewater
should not be neglected when measuring
the global benefits of a biodiesel project.
The co-authors' company has found
the following steps to be effective in optimizing
refinery water systems:
1. Perform a detailed compositional
analysis of the refinery process unit
water streams feeding the existing
SWS system and their actual or
estimated flowrates.
2. Estimate increased water volume
and its CO2
content based on
biofeedstock properties.
3. Route streams with less than
10 mg/l H2
S and 50 mg/l NH3
directly to final effluent treating to
free up stripping capacity. While,
in theory, these streams can be
reused, often the hydrocarbon
content and water hardness render
them unsuitable.
4. With multiple SWSs, segregate
phenolic waters and recycle them
to the desalter. In the desalter, most
of the phenol will be extracted
back into the crude oil where it can
subsequently be broken down in
downstream hydrotreating.
5. Supplement desalter water with
unstripped water from the crude
overhead if capacity allows.
6. Use water streams with few strong
cations and anions (e.g., some tail
gas unit quench towers) as amine
system makeup water.
7. Utilize a semi-stripped stream
drawn from near the bottom of the
SWS column (with approximately
200 ppmw NH3
) to potentially
replace neutralizing chemicals
injected into fractionator
overheads for the desired pH
of about 6.5, which saves on
FIG. 7. Optimal wastewater filtration and coalescing scheme.16
Hydrocarbon Processing | JANUARY 2022 69
operating expenditures (OPEX).
8. Maximize the reuse of water from
the low-pressure to high-pressure
side on an FCCU, while not
exceeding 25 ppmw hydrogen
cyanide (HCN) and 2 wt%
ammonium hydrosulfide (NH4
in the high-pressure wash water.
HS)
9. Note: In-situ sulfidic spent
caustic neutralization in an SWS
is possible, but good pH control
is essential and recycling sodiumcontaining
stripped water upstream
can have a negative effect on
desalter performance (increased
emulsification), thermal cracker
time between decoking and FCCU
catalyst life. Careful study is
needed before doing this.
In many instances, the additional water
from biodiesel production will require
an additional SWS unit. The optimal process
location for this is not necessarily for
this new water but will be site-specific.
For instance, if there is not a pre-existing
segregated phenolic water system, this
may be the opportunity to install one on
the catalytic and thermal cracking units,
which are the source of phenols in the refinery
water. This can save OPEX on expensive
phenolic treatment of the water.
A new stripper also does not have to
follow a conventional design. A stripper
requiring a low capital expenditure
(CAPEX),
proposed
by Lieberman,
sends the hot stripped water directly to
the plant desalter and can incur less than
half of the final installed cost of a conventional
unit (FIG. 5).14
This low-cost
stripper does have some limitations, the
FIG. 6. Raw sour water samples from various
refinery units.
most significant being its sensitivity to
reboiler duty. In one study, a 2% increase
in reboiler duty increased SWS acid gas
volume by about 30% (from water additional
vapor), which can be a challenge
for the SRU. This configuration works
best when the low-CAPEX style stripper
is contributing a relatively small proportion
to the combined amine and sour
water acid gases, and when the SRU is
operating at a healthy load. This may not
be suitable for facilities with a small ratio
of amine acid gas (AAG) to sour water
acid gas (SWAG); therefore, it is critical
to confirm the SRU minimum turndown
and to review burner controls.
There is often a range of trace-level
strong ions in the various feed water
sources to the sour water system. In conventional
refining, these are normally sodium,
chloride, sulfate and short-chain organic
acids. Phosphate is present in most
biofeedstocks, and, while pretreatment of
biofeedstock will remove the bulk of the
phosphate, trace amounts will emerge in
the wastewater. While the concentration
of these trace ions is usually small (< 500
mg/l), they can pose a significant operational
challenge, as they will form an ionic
bond with H2
unstrippable at conventional SWS conS
or NH3 that renders them
Hydrocarbon Processing - January 2022

Table of Contents for the Digital Edition of Hydrocarbon Processing - January 2022
