Spent caustic handling, treatment and disposal are major
concerns for refining and olefins (ethylene)
production facilities due to its hazardous
nature and noxious properties. As sources for spent caustic
generation are diverse, they do produce characteristically
different waste streams consisting of inorganic and organic
acidic components such as carbon dioxide (CO2)
sulfides, carbonates, mercaptans, phenolics,
cresylics and naphthenates. These components are acidic and
must be removed to avoid corrosion of downstream equipment and
to prevent poisoning catalysts. The conditions can be further
complicated by neutral oil carryover. Several treatment methods
can be used to manage spent caustic and they include chemical
precipitation, neutralization, chemical reagent oxidation, wet
oxidation, catalytic oxidation and incineration.
Improved processes are now available. Each method offers
certain advantages while its application would depend on the
waste stream composition, size and the configuration of the
total process facility and toxicity threshold limits of
downstream biological treatment systems. Spent caustic is toxic
to bacteria used in the wastewater treatment unit. When zero
discharge or stringent limits are stipulated in environmental permits, high
dissolved solids present in the spent caustics add to treatment
loading in desalination plants.
Selecting the best treatment method is a critical task,
especially in meeting total waste management needs. It is often
driven by environmental regulatory limits associated with emissions and discharges. There are
essential elements in dealing with spent caustic streams that
will be presented in this article. They involve applying the
principles of waste management hierarchy, with merits and
demerits of each possible solution, as shown in Fig.
Fig. 1. Waste
Source characterization and segregation.
Typical processes in the hydrocarbon industry where spent
caustic is generated are caustic scrubbing of straight-run
light hydrocarbons, feed streams to isomerization and
polymerization units; cracked gases from thermal/catalytic
cracking units; and caustic washing of middle distillates,
followed by mercaptans extraction/sweetening operations. The
caustic converts the acidic components into their respective
inorganic/organic salts of sodium such as sulfides, carbonates,
mercaptides, disulfide oil, phenolates, cresolates, xylenolates
and naphthenates. Caustic strength used (typically 520%)
in these processes is governed by feed type and nature of
caustic treatment; accordingly, the residual caustic strength
will differ. Since acidic components have a wide boiling range,
the spent caustic composition varies depending on the operating
temperature for the caustic treatment process. To facilitate
appropriate handling of these characteristically different
streams, as shown in Table 1, it is advisable to segregate them
by the contaminants present:
Sulfidic: Primarily sulfides (scrubbing of
straight-run gaseous hydrocarbons)
Phenolic: Phenols, cresols and xylenes with
sulfides (scrubbing of cracked gases/gasoline)
Naphthenic: Naphthenic acids
(scrubbing/mercaptan-extraction of middle distillates.)
Reducing waste generation at the source is the first and
most preferred step. When designing caustic treating processes,
best practices seek to generate the least amount of spent
caustic while maintaining the desired efficacy of the process.
The following measures help achieve the objective:
Eliminating spent caustic generation at the source can be
accomplished by applying caustic-free processes.3
These techniques involving ammonia injection and solid-bed
catalysts offer benefits in mercaptan oxidation process
Multistage caustic wash.
Two levels of scrubbingfirst with weak caustic
followed by a strong oneoffer improved utilization of
caustic.7 The first level facilitates removal of
most of the hydrogen sulfide (H2S), while the second
level acts as a polishing step to achieve maximum removal
efficiency. However, the additional investment costs must be
weighed against the incremental caustic savings.
Maximizing the percentage.
Spent caustic generation can be minimized by using up the
caustic strength as much as possible while preventing
breakthrough of acidic components in the hydrocarbon product.
This can be determined based on analytical data and operating
Efficient regenerative systems.
In many cases, amine treatment
precedes the caustic wash. Ensuring efficient design/operation
of these regenerative units that remove major H2S
quantities would reduce the load on caustic wash units, thereby
minimizing spent caustic generation.
Choosing the maximum caustic strength.
When possible, the maximum caustic strength can be chosen to
minimize water content, thereby reducing the dissolved organic
Non-dispersive mass transfer technique.
Extraction of naphthenic acids into caustic is limited by
the emulsion-forming tendency and precludes using
higher-strength caustic in units where the dispersive caustic
mixing is applied. Film-contact techniques can overcome these
limitations and allow higher caustic strength, thus enabling
savings in caustic usage and reduced spent caustic
Fig. 2. Flow diagram of
a typical chemical
treatment unit for spent caustic.
Fig. 3. Flow diagram of
a wet oxidation
REUSE WITHIN THE FACILITY
After reducing generation of spent caustic, the next step is
identifying options for reusing it within the facility.
Segregating spent caustic based on contaminants benefits this
approach, since mixing dissimilar caustic effluents can render
a blend that is unsuitable for reuse. The reuse potential of
spent caustic is a function of its alkalinity, a measure of
residual free NaOH and contaminants like sodium phenolate,
Na2S and other sodium salts of weak acids. The high
potential reuse applications are fresh caustic users. To
optimize reuse application, it is essential to understand the
process chemistry and operating conditions of the subject
system, as well as the impact of the spent caustic contaminants
on that system. Potential reuse applications include a
substitute for fresh caustic usage:
Neutralization of desalter fluids.
The desalting process removes salt and other impurities such
as bottom sediment and water from the crude oil before
fractionation. Depending on the crude being processed, caustic
is injected into the desalter to maintain optimum pH (normally
78) by neutralizing the acidity of the crude and to
maximize de-emulsification. Dilute caustic solution, usually
23 wt%, is used to keep salt levels low and limit
emulsification due to naphthenic acids present in the crude.
Naphthenic spent caustic is not recommended, while the sulfidic
stream is also not suitable for reuse due to lack of sufficient
alkalinity. Conversely, phenolic spent caustic is effective as
it neutralizes naphthenic acids to form phenols that dissolve
back in the crude.
Crude-column corrosion control.
Caustic is often injected into the crude-column feed to
minimize corrosion resulting from hydrochloric acid (HCl)
evolution due to the hydrolysis of calcium and magnesium
chloride present in residual water droplets contained in the
desalted crude oil. Phenolic spent caustic is effective here.
Naphthenic spent caustic effluent can also be used since sodium
naphthenates will react with the HCl. Remember: that reuse
should be done in a controlled manner to avoid fluctuations in
the sodium content.6 Lower levels cause the HCl
release, resulting in overhead column corrosion; while higher
levels increase sodium content in the residue, leading to
caustic corrosion and catalytic deactivation in downstream
processing units. Increasing the concentration of acidic
contaminants in the products ex-distillation is expected to be
insignificant when spent caustic is used instead of fresh
pH adjustment of desalter brine.
If the desalter pH control cannot provide minimum alkalinity
to outgoing brine, it can lead to corrosion of downstream
valves and piping, requiring caustic injection on the brine
line. Phenolic spent caustic is suitable.
In most mercaptan-oxidation processing units, higher caustic
strength (20°Be) is normally used in the extractor sections
to favor mercaptan removal. A lower strength (10°Be) is
used in the prewash operation due to lower solubility of
Na2S. The spent caustic from the extractor unit is
relatively free from sulfides and can be diluted and used in
the prewash.1 Such regenerative processes are more
effective in reducing spent caustic than the once-through
Wastewater processing needs.
Caustic is normally dosed upstream of the chemical and
biological treatment units to sustain favorable pH conditions.
Since phenols and napthenates are manageable at controlled
concentrations in biotreatment, phenolic and naphthenic spent
caustic streams are more suitable for reuse. Sulfidic streams
pose odor issues in this application.
Accordingly, phenolic spent caustic provides better reuse
opportunities followed by naphthenic, while sulfidic streams
offer relatively lesser avenues within the facility.
RECYCLE OUTSIDE THE FACILITY
Along with reuse options, recycling is another opportunity
to explore for industries (where there are similar streams) and
it has been successfully implemented by several refineries in
the US and Canada.10 This approach may require
additional treatment or conditioning within the facility,
depending on the composition needs of the receiver. These
applications may find roadblocks in terms of handling and
transportation, in which case they can be sold to intermediate
processors to handle them. Potential recycling opportunities
include pulp and paper, tannery, mining, wood preservatives and
In-plant processing could be an option to extract the
valuable content, especially the free caustic, sulfide salt,
phenols and naphthenic acids. These can be used as feedstock for phenolic resins,
herbicides, solvents, wood preservatives, paint and ink driers,
fuel additives, etc.4 However, this approach may
need a proper cost-benefit analysis before proceeding further.
Emerging techniques, such as membrane and electrolysis
applications, show promising trends.
Fig. 4. Flow diagram of
spent-caustic disposal unit.
Fig. 5. Flow diagram of
TREATMENT AND DISPOSAL
Treatment for final disposal is the last option, but,
unfortunately, an inevitable one in most cases. Efforts should
be made to handle the spent caustics directly, in
well-acclimatized biological treatment units, after diluting
them with bulk effluents and maintaining homogenous feed
meeting biotoxicity threshold limits (established by lab
tests). Biotreatment is the most economical, and offers
flexibility in operation. If treatment is necessary before
routing it to biotreatment, many proven techniques are
available. Their suitability is case-specific as driven by
factors such as quantity, composition and treatment limits to
Treatment methods for spent caustic can be broadly
categorized as chemical and thermal,
with further categorization based on chemical(s) used and
system operating conditions. If adequate treatment facilities are unavailable,
third-party disposal should be worked out. These methods
Thermal treatmentWet oxidation
(attached/suspended cells)normally after
Hydrogen peroxide (H2O2) is a widely
used oxidant; its radicals possess very high oxidation
potential. It can oxidize most inorganic and organic compounds,
but the reaction rate toward sulfides is effective to apply it
economically for smaller streams. Alkaline conditions are
favored for complete conversion to sulfates. For organics
removal, a Fenton reagent (peroxide with Fe+2
catalyst) application in an acidic medium is normally used.
Chlorinated copperas is commonly used in an
alkaline medium to remove sulfides as insoluble ferric sulfide
with ferric hydroxide sludge. Due to its hygroscopic nature, it
is normally produced in-situ using ferrous sulfate and
chlorine. However, with the associated chemical and sludge
handling issues, this method usage is receding.
Neutralizing free caustic and other alkalinity converts the
spent caustic components into their original forms, i.e.,
H2S, RSH, phenol and naphthenic acids; thus allowing
recovery of the valuable acid layer but requires stripping and
further handling of volatile sour gases.
This is a liquid-phase hydrothermal oxidation using
dissolved oxygen at elevated temperatures facilitated by air
and supplementary steam injection. Oxidation efficiency
increases with temperature; its operating range is set based on
target contaminants (lower for inorganics and higher for
organics). The oxidation needs are driven by downstream
biotreatment limits. The operating pressure is governed by
partial pressure of the oxygen to be maintained, and the
methods are accordingly categorized as
low-/medium-/high-pressure (LP/MP/HP) wet oxidation. Due to its
high treatment efficiencies, no-sludge generation and minimal
air pollution, this method is most widely used. It is one of US
Agencys best available techniques (BATs).
Catalytic wet oxidation.
This method involves a catalyst application in wet oxidation
units to reduce operating temperatures and to enhance oxidation
This is a gas-phase oxidation process at much higher
temperatures, converting inorganic constituents into molten
forms and decomposing organic compounds into most stable
states. It offers the ultimate oxidation levels. Diluted
streams may not provide enough calorific value for
self-sustaining high-temperature needs and normally require
supplementary fuels. In a recent gas-cracker ethylene plant in
Saudi Arabia, an improved incineration system using waste oil
as fuel was installed; it addressed eutectic solid crystals
formation in the quench section.
This comprises of pollutant decomposition and oxidation
through bacterial adsorption, respiration and synthesis
mechanisms. It produces additional bacterial cells, followed by
clarification and stabilization. While feed
homogeneityensuring pollutant levels below the acceptable
limits (pH: 6.58.5, Oil < 25 ppm, sulfides < 10
ppm, phenols < 50200 ppm, copper < 1 ppm,
etc.)is a basic prerequisite to the system, it can
provide flexibility through proper acclimatization
Table 2 summarizes the typical application/suitability of
these techniques; further details including pros and cons are
listed in Table 3. A logical approach is also presented in Fig.
6; it facilitates the total management. Research into this area
has been producing more methods, mostly based on the same
principles. But improvements through process/mechanical designs
and some requiring establishment on a commercial scale are
promising. Only commonly used methods or those successfully
implemented on full scale are covered in this analysis.
Licensed technologies under each method are not the focus here,
but their specific features and advantages can be established
through techno-economic assessment during the selection
Fig. 6. Logic diagram
for spent caustic management.
Potential for atmospheric treatment technique.
Beyond the analysis carried out for the widely used
techniques and their suitability, the authors investigated the
merits of exploring atmospheric aeration and recirculation
within the spent caustic storage tank(s). Based on known and
available information, this technique seems either not tried on
a wider scale or not put to effective implementation as yet by
the hydrocarbon industry. It is proposed to
test the efficacy and results on a pilot scale first to
establish its advantages and operational range. Being a
non-chemical and non-thermal technique, it is expected to have
merits including low costs and operational simplicity. The view
is supported by available data for reaction kinetics, which
favor an atmospheric sulfide oxidation rate of 0.3
kg/m3hr to 0.6 kg/m3hr at ambient
temperature, requiring 1885 hr (< 4 days).9
This seems attractive, as the storage inventory commonly seen
for spent caustic effluents is about 715 days. It can
easily be implemented in existing tank(s) with simpler
metallurgy and low operational and maintenance needs.
This is an important part of a total spent caustic
management system. Odor control has potential adverse impact
not only on occupational and community environment but also has
stake in the public perception of the facility. The malodorous
characteristic of the spent caustic is attributed to its
constituents like H2S, mercaptans, disulfide,
cresols and thiophenol with their volatile nature and extremely
low threshold odor numbers (< 1 ppb). Several measures would
help mitigate these objectionable odor issues:
Closing/covering all treatment units in spent
Avoiding routing directly to drainage sewers
Maintaining alkaline pH (910) in bulk stream
it is mixed with, to avoid hydrolysis of Na2S to
Ensuring there are no significant residual sulfides
in spent caustic, before it is sent to biological
Routing of vents from LP/MP wet oxidation and
mercaptan oxidation units to offgas treatment.
Selection of a suitable technique to handle spent caustic is
a critical task involving technical, economic, environmental,
health and regulatory considerations. Source reduction and
segregation, followed by reuse/recycle, have the potential to
reduce the volume to be treated. The volume reduction aids in
selecting the most appropriate method specific to its
characteristics. All these can be made possible by following a
logical approach built on waste-management hierarchy
Authors express sincere gratitude to KBR Management for
their support and encouragement in bringing out this work. This
is an updated version of a presentation at the International Refining and Petrochemical Conference-Asia, July
1922, 2011, in Singapore.
1 Dando, D. A., and D. E. Martin, A guide
for reduction and disposal of waste from oil refineries and
marketing installations, CONCAWE Report No.
2 Veerabhadraiah. G., and A. Haldar, Aiming
towards Pollution Prevention and Zero Discharge in Petroleum
Industry, Proceedings of AIChE Spring Conference,
3 US Dept. of Energy, Energy and Environmental
Profile of the US Petroleum Refining Industry,
4 API, Category Assessment Document for Acids
and Caustics from Petroleum Refining, US EPA, 2009.
5 Sarkar, G. N., Utilization of the Spent
Caustics Generated in the Petroleum Refineries in the Crude Distillation Unit, 5th Intl.
Conference on Stability and Handling of Liquid Fuels, The
6 Ahmad, W., Neutralization of Spent Caustic
from LPG Plant at Preem AB Goteborg, Department of
Chemical and Biological Engineering, Chalmers University of Technology, Sweden, 2010.
7 Mamrosh, D., C. Beitler, K. Fisher and S. Stem,
Consider improved scrubbing designs for acid gases,
Hydrocarbon Processing, January 2008.
8 Falqi, F. H., Miracle of Petrochemicals: Olefins
9 Berne, F., and J. Cordonnier, Industrial Water
Treatment: Refining, Petrochemicals, Gas Processing
10 US-BCSD (United States Business Council for
Sustainable Development), Examples of Byproduct
11 API, Manual on Disposal of Refinery Wastes,
12 Jones, D. S. J. and P. R. Pujado,
Handbook of Petroleum Processing, 2008.
G. Veerabhadraiah is lead
environmental engineer for KBR, Singapore. He earned a
B. Tech in chemical engineering from Andhra University,
India, and a Masters in HSE
Technology from National
University of Singapore. He has over 19 years
experience in the hydrocarbon industry and
specialized in environmental engineering activities of
design and execution. Previous to KBR, he worked with
Engineers India Ltd., New Delhi. He has publications in
various International Conferences and is recipient of
Best Research Work Award from Central
Pollution Control Board of Government of
N. Mallika is principal process
engineer for KBR, Singapore. She obtained graduation in
chemical engineering from Amravati University, India, and worked for more
than 18 years in the chemical and oil and gas industry,
mainly in refining, petrochemical and gas
processing facilities. Her experience
includes process simulation, basic design, detailed
engineering, pre-commissioning and commissioning
support, and she has worked extensively on licensed
processes for various refining units. Previous to
KBR, she worked with Engineers India Ltd., New
S. Jindal is process and environmental discipline
manager for KBR, Singapore. He has also held positions
of lead process engineer and proposals engineer in a
career spanning more than 20 years in the HPI. He
previously worked with Aker Kvaerner, Larsen &
Toubro, and Engineers India Ltd. in senior technical
positions. He has authored technical papers on varied
topics at international conferences. He received a B.
Tech degree in chemical engineering from the Indian Institute of Technology, Mumbai, and is a