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he Repsol YPF refinery in La Pampilla is the largest in Peru. The hydrocarbon streams
contain sulfides and organic acids which are undesirable contaminants. These
contaminants are removed by caustic treatment. Several different caustic treatment
steps are performed through the refinery, making numerous spent caustic waste streams.
Approximately 3000 m3/yr of combined spent caustic is generated. The spent caustic
streams are malodorous and toxic, and are difficult to treat by conventional biological
means. Beginning in 2005, Zimpro® wet air oxidation (WAO) is used to treat these streams prior to
disposal. WAO allows on-site treatment without producing odorous offgas.
Technology
Wet oxidation is an aqueous phase process that operates at elevated temperature and pressure. Dissolved
or entrained contaminants are oxidised in the liquid phase water. The process relies on the liquid water
molecules to catalyse the oxidation. Oxygen is used as the oxidant and is supplied by mixing air with the
spent caustic. When air is used as the source of oxygen, the
process is referred to as wet air oxidation. For refinery spent
caustic, the design operating temperature was 260 ˚C, which
is well above the normal boiling point. System pressure is
88 kg/cm2 (1250 psi). The pressure keeps most of the
water in the liquid phase, which also minimises the energy
consumption.
The typical reactions occurring during WAO of refinery
spent caustic include the following:
Sulfide oxidation:
NaHS + 2O2 + NaOH → Na2SO4 + H2O
Approximately 85% (by volume) of the spent caustic is
produced continuously in the treatment of kerosene. The
remaining 15% of the flow is produced in batches of varying
quantities and compositions. Two storage tanks are used as
equalisation tanks. One large batch of sulfidic spent caustic
is segregated into one storage tank. The remaining spent
caustics (sulfidic, naphthenic and cresylic) are combined
and held in the other storage tank (called the main spent
caustic). The partial segregation is done so that the sulfide
concentration fed to the WAO unit can be better regulated.
This is done to control oxygen uptake and corrosion that can
result from uncontrolled sulfide concentration spikes.
Metering pumps are used to control the flow rate of spent
caustic from the two storage tanks and the fresh caustic.
Dilution water is added to control the heat release from the
oxidation. The water is added using a centrifugal pump.
Caustic is consumed by the oxidation products. If there is
insufficient caustic, the pH of the oxidised effluent will become
acidic, which can damage the nickel alloys used in the high
temperature portion of the WAO system. Fresh caustic is
added to the feed to maintain the discharge pH above eight.
Mercaptan oxidation:
NaSR + O2 + NaOH → NaSO3R’ + NaHCO3 +
Na2SO4 (unbalanced)
Naphthenics and cresylics oxidation:
NaOOR + O2 + NaOH → NaOOR’ + NaHCO3 +
H2O (unbalanced)
Where R’ is an organic chain, it is typically acetate or
oxalate in the oxidised compounds.
Description of the oxidation
system
Description of wastewater
The spent caustic comes from multiple streams and includes
sulfidic, naphthenic, and cresylic spent caustics. Sulfidic
spent caustics come from the caustic scrubbing of liquefied
petroleum gas (LPG) and pentane from fluid catalytic cracker
(FCC) and continuous distillation unit (CDU). Naphthenic spent
caustics come from the Merox® type treatment of kerosene.
Cresylic spent caustics come from the Merox® type treatment
of visbreaker gasoline. The compositions are shown in Table 1.
A block flow diagram of the wet oxidation system is shown in
Figure 1 and photographs of the system are shown in
Figures 2 and 3. The operating conditions for the system are
shown in Table 2.
The four feed streams are combined and pass through
the high pressure feed pump. Compressed air is added
to the feed and the mixture is heated in the feed/effluent
heat exchanger (F/E HX). The F/E HX is a concentric tube
exchanger with the feed passing
through the inner tubes and the
hot reactor effluent through the
Figure 1. WAO system
annular space of the outer tube.
block flow diagram.
The feed is heated further in the
trim heat exchanger. The system
operating temperature is regulated
by the steam flow to the trim
heat exchanger. The oxidation
reactions begin with the heat
addition in the heat exchangers.
The hot fluid enters the bottom
of the reactor. The exothermic
reaction heats the fluid to the final
operating temperature in the
reactor.
Table 1. Spent caustics composition
The reactor is a bubble
column,
with the gas bubbles
Reported as
Sulfidic spent
Naphthenic spent
Cresylic spent
rising through the liquid phase.
(g/L)
caustics
caustics
caustics
The reactor is sized to provide
COD
O2
7 – 110
50 – 100
165 – 230
the retention time necessary
TOC
C
0.02 – 4
11 – 25
23 – 60
to achieve the desired degree
DIC
C
0.15 – 5
0 – 0.16
0.33 – 0.35
of treatment. The fluid exits
the top of the reactor and
Sulfide
S
2 – 53
<0.001
0 – 64
passes through the shell of the
Sulfite
S
0.002 – 0.48
0.004 – 0.009
0.8 – 1.6
F/E HX for heat recovery. After
Mercaptans
CH3SH
0 – 28
<0.03
0 – 5.4
partial cooling in the F/E HX,
Thiosulfate
S2O3
0 – 3.7
0.07 – 0.13
10 – 12
the oxidised effluent passes
through the process cooler and
0.005 - 0.025
0.025 - 0.03
Iron
Fe
then to the process separator.
Total phenols
C 6H 6O
0.003 – 0.02
2 – 10
14 – 20
The offgas is separated from
Note: Values reported as <# represent non detection, with # being the analytical detection limit.
the oxidised liquor by gravity
R E AC TOR
O F F G A S T O B U R NE R
C O O L ING
WA T E R
R E TUR N
MA IN
S U L F IDIC
DIL U T IO N
FR E S H
S P E NT
S P E NT
WA T E R C A U S T IC C A U S T IC C A U S T IC
H IG H P R E S S U R E
F E E D P U MP
FE E D
E F F L U E NT
HE AT
E XC H A NG E R
S TE AM
P R OC E S S
S E P AR ATOR
T R IM
HE AT
E XC H A NG E R
PR E S S UR E
C ONTR OL
VALVE
P R OC E S S
C OOLE R
C O O L ING
WA T E R
S UPPLY
E FFLUE NT TO S E A
P R OC E S S
A IR
C O MP R E S S O R
Reprinted from HydrocarbonEngineering
November2008
www.hydrocarbonengineering.com
Table 2. Operating conditions for the wet oxidation system
Sulfidic caustic feed rate
0.04 m3/hr (0.2 gpm)
Main spent caustic feed rate
0.31 m3/hr (1.4 gpm)
Fresh caustic feed rate (20 ˚Be)
0.21 m3/hr (0.9 gpm)
Dilution water feed rate
0.08 m3/hr (0.4 gpm)
Reactor temperature
Reactor pressure
250 ˚C
88 kg/cm2 (1250 psi)
in the separator. The liquid effluent is discharged to the sea.
The offgas is scrubbed and is sent to a burner used to fire a
distillation column.
Figure 2. Overview of WAO system.
Performance
The performance of the system is shown in Table 3. The
combined spent caustic and fresh caustic composition is
reported in the feed column of Table 3. Dilution water is added to
the feed after the feed sample point. Additionally, scrubber water
is added to the oxidised effluent in the process separator. These
water streams dilute the oxidised effluent to approximately 60%
strength, which is reflected in the effluent column in Table 3.
After correcting for the dilution effect, the WAO system
destroyed 85% of the COD, 73% of the TOC, and >99.97% of
the sulfide. Figure 4 is a photograph showing the sulfidic spent
caustic, the main spent caustic (mostly naphthenic with some
cresylic components), and the oxidised effluent.
Safety
The oxygen supply is limited by the capacity of the
compressor. This prevents excessive temperature increase
in the event of a COD excursion in the feed. The liquid water
and air in the reactor is an energy sink to further dampen
temperature excursions. The pressure in the reactor is
regulated, so an excessive energy release in the reactor
creates more steam at the process temperature, rather than
excessively increasing the temperature or pressure.
The offgas may contain 5 - 21% O2 and some volatile
organic compounds. To prevent a fire hazard in a flare system
(due to the oxygen) a dedicated offgas line is used to take this
gas to the firebox of a burner.
The system processes spent caustic at elevated
temperature and pressure. Typical spray and spill protections
are used to protect personnel and the environment. There
have been no safety incidents with the WAO system.
Figure 3. Spent caustic storage tanks (foreground). WAO system is
behind the tanks.
Reliability
Since unit startup in 2005, the WAO system has proven to be
reliable. The only item of note is fouling in the heat transfer
equipment. After approximately 12 months of operation the
tubes of the heater exchangers became fouled. This caused
an increase in pressure drop and a loss of heat transfer
efficiency. According to the process provider (Siemens Water
Technologies), this is unusual for a spent caustic application.
The scale was found to be mostly iron and is formed from
unusually high concentrations of iron in the feed. There was
also an organic fraction to the scale. Due to the small size of
the tubes, it was not practical to hydroblast the exchanger
tubes. The tubes were partially cleaned using steam. This did
not remove very much of the scale.
A better solution was needed for scale removal and the
process provider conducted a series of laboratory tests to
identify a solvent and cleaning procedure that would be
effective at removing the scale, but not damaging to the
www.hydrocarbonengineering.com
Figure 4. Sulfidic spent caustic (left), main spent caustic (centre) and
oxidised effluent (right).
metallurgy. The best compromise found for this scale was a
pickling solution of the following composition:
l 535 mL 20˚ Baume HCl
l 33 g CuCl2
l 1000 mL H2O
This solution dissolves the scale at room temperature. On
corrosion coupons, it was found this solution has a general
corrosion rate of 38 MPY at 25 ˚C. Elevating the temperature
to 80 ˚C increased the effectiveness of the solvent at scale
removal, but also greatly increased the general corrosion rate
Reprinted from November2008
HydrocarbonEngineering
to 600 MPY. Based on the laboratory tests, it was concluded
that this solvent can be used for up to a maximum of seven
total days per year at 25 ˚C in order to achieve suitable scale
removal while maintaining acceptable corrosion from the
solvent to the piping.
This solution was cycled through the heat exchangers and
was found to be effective at scale removal without damaging
the equipment. The heat exchanger has been cleaned once
using this method, with an exposure time of 8 hours. It is
anticipated that the exchanger will need to be cleaned every
6 months.
In addition, flanged unions may be installed on some
of the loops of the exchanger. This will allow access for
mechanical cleaning of the equipment as well.
Conclusion
The WAO system is an effective approach for treating the
spent caustic. Sulfides and mercaptans are destroyed to
below analytical detection limits. TOC and COD are greatly
diminished and the oxidation products neutralise the pH
to between 8 and 10. The gaseous and liquid effluents are
not malodorous. The oxidised spent caustic is discharged
to a holding tank, and then directly to the sea. The offgas is
of sufficient quality that it is directed to a burner to destroy
the volatile compounds. System operation has proven
reliable. After a year of operation an iron/organic scale had
accumulated in the heat transfer equipment due to unusually
high iron concentrations in the spent caustic feed. This scale
accumulation was successfully removed using a pickling
solution solvent.
Reprinted from HydrocarbonEngineering
Table 3. WAO system performance at 250 ˚C
Reported as
(mg/L)
Feed
Effluent (as
measured, after
dilution)
COD
O2
73 000
6300
TOC
C
15 000
2400
DIC
C
1200
5600
Sodium
Na
41 000
24 000
pH
-
13.2
8.9
Sulfide
S
8500
<1
Sulfite
S
100
<2
Mercaptans
CH3SH
1500
<30
Thiosulfate
<30
S2O3
1500
Total
phenols
C 6H 6O
6500
36
Flow rate
m3/hr
0.67
1.14
Note: Values reported as <# represent non detection, with # being the
analytical detection limit.
References
1.
2.
3.
Carlos, T., Maugans, C. ‘Manage refinery spent caustic efficiently’
Hydrocarbon Processing, pp 89 - 92, February 2002.
Grover, R. and Gomaa, H., ‘Proven technologies manage
olefin plant’s spent caustic’, Hydrocabon Processing, pp 61 - 69,
September 1993.
Maugans, C. and Ellis, C., ‘Wet Air Oxidation: A Review of Commercial
Sub-Critical Hydrothermal Treatment’, 21st Annual International
Conference on Incineration and Thermal Treatment Technologies, New
Orleans, May 13 - 17, 2002.
November2008
www.hydrocarbonengineering.com