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CONTENTSSection Page
SCOPE
............................................................................................................................................................4
REFERENCES
................................................................................................................................................4
BACKGROUND
..............................................................................................................................................4
SELECTION OF A MANAGEMENT METHOD
...............................................................................................5
MINIMIZATION OF SPENT CAUSTIC
VOLUME............................................................................................6
H2S REMOVAL BY AMINE PRE
TREATMENT......................................................................................6
UOP Merox
Processes........................................................................................................................6Merox
Liquid-Liquid Extraction
.............................................................................................................6Mercaptan
Conversion (Sweetening)
...................................................................................................6Caustic-Free
Merox..............................................................................................................................7
MERICHEM, INC. THIOLEX
...............................................................................................................7OXIDATION
OF H2S TO ELEMENTAL SULFUR
...................................................................................8
MOLECULAR
SIEVES............................................................................................................................8
SWITCH FROM NaOH TO
KOH.............................................................................................................8
CHANGES IN OPERATING PRACTICES
..............................................................................................8Optimization
of Existing Processes
......................................................................................................8Segregation
of Spent
Caustic...............................................................................................................9
CASCADED REUSE OF SPENT CAUSTIC
...................................................................................................9
DIRECT REUSE FOR HYDROCARBON PRODUCT TREATMENT
......................................................9
INJECTION INTO
CRUDE......................................................................................................................9
pH CONTROL IN SOUR WATER STRIPPER
........................................................................................9
pH CONTROL IN BIOLOGICAL OXIDATION (BIOX)
UNIT..................................................................10
pH CONTROL IN PIPESTILLS
.............................................................................................................10
INJECTION OF SPENT CAUSTIC IN FCCU WET GAS SCRUBBERS
...............................................10
REUSE AS FEEDSTOCK FOR OTHER
INDUSTRIES.................................................................................10
PULP AND PAPER
INDUSTRY............................................................................................................10
ALUMINA
INDUSTRY...........................................................................................................................10
CHEMICAL MANUFACTURING
...........................................................................................................11Merichem
...........................................................................................................................................11Americhem
.........................................................................................................................................11Hewchem
...........................................................................................................................................11CRI-MET
............................................................................................................................................11Penrice
Soda
Products.......................................................................................................................11
TREATMENT AND REGENERATION
..........................................................................................................11
MEROX /
MINALK.................................................................................................................................11
THIOLEX / REGEN
...............................................................................................................................11
SHELL AIR OXIDATION
.......................................................................................................................12
ELECTROLYTIC REGENERATION
.....................................................................................................13
Changes shown by
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CONTENTS (Cont)Section Page
TREATMENT AND DISPOSAL
....................................................................................................................
13
FLUE-GAS
CARBONATION.................................................................................................................
13
NEUTRALIZATION WITH STRONG (WASTE)
ACID...........................................................................
14
BIOLOGICAL TREATMENT
.................................................................................................................
14
WET AIR OXIDATION
..........................................................................................................................
15Low Pressure Wet Air Oxidation
........................................................................................................
15Medium / High Pressure Wet Air
Oxidation........................................................................................
16
INCINERATION
....................................................................................................................................
17
SUPER CRITICAL WATER
OXIDATION..............................................................................................
17
SULFIDE PRECIPITATION
..................................................................................................................
18
ASPHALT FORMULATION
..................................................................................................................
18
CHEMICAL OXIDATION-OXIDIZING
AGENT......................................................................................
18
UV OXIDATION-OXIDIZING AGENT PLUS UV
ENHANCEMENT.......................................................
18
TABLESTable 1 Spent Caustic At Affiliate Locations
..................................................................................
19Table 2 Contaminants Typically Present in Spent Caustic Streams
.............................................. 21Table 3 Typical
Spent Sulfidic Caustic Streams
............................................................................
22Table 4 Spent Caustic Treatment
Matrix........................................................................................
23Table 5 Spent Caustic Management/Treatment Comparative
Parameters.................................... 24
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CONTENTS (Cont)Section Page
FIGURESFigure 1 Waste Minimization / Treatment Hierarchy
.......................................................................26Figure
2 Treatment Selection Decision
Tree...................................................................................27Figure
3 Site-Wide Caustic Optimization
Process...........................................................................28Figure
4 Amine Treating Unit Simplified Flow Plan
.........................................................................29Figure
5 Merox Liquid-Liquid Extraction Simplified Flow
Plan.........................................................29Figure
6 Liquid-Liquid Merox Sweetening Unit Simplified Flow
Plan...............................................30Figure 7
Conventional Fixed-Bed Merox Sweetening Unit Simplified Flow
Plan.............................30Figure 8 Jet Fuel Treating Unit
Including Merox Fixed-Bed Sweetening Simplified Flow Plan
.......31Figure 9 Fixed-Bed Minalk Sweetening Unit Simplified Flow
Plan ..................................................31Figure 10
Caustic-Free Merox Unit Simplified Flow Plan
..................................................................32Figure
11 Thiolex Unit Simplified Flow
Plan......................................................................................32Figure
12 Regen Unit Simplified Flow
Plan.......................................................................................33Figure
13 Molecular Sieve/Amine Process Simplified Flow Plan
......................................................33Figure 14
Shell Air Oxidation Process Simplified Flow Plan
.............................................................34Figure
15 Electrolytic Regeneration-Three Compartment System
....................................................34Figure 16
Electrolytic Regeneration-Two Compartment
System.......................................................35Figure
17 Batch Carbonation Process Simplified Flow Plan
.............................................................35Figure
18 Continuous Carbonation Process Simplified Flow Plan
....................................................36Figure 19
Neutralization/Steam Stripping Process Simplified Flow
Plan...........................................36Figure 20
Biological Pre-Treatment Process Simplified Flow
Plan....................................................37Figure 21
Bio-Treatment of Spent Sulfidic Caustic in Existing Biox Decision
Tree ...........................38Figure 22 Stone & Webster Low
Pressure Wet Air Oxidation Simplified Flow Plan
..........................39Figure 23 Zimpro Medium/High Pressure
Wet Air Oxidation Simplified Flow
Plan............................39Figure 24 Incineration Simplified
Flow Plan
......................................................................................40Figure
25 Super Critical Water Oxidation Simplified Flow
Plan.........................................................40Figure
26 Sulfide Precipitation Simplified Flow Plan
.........................................................................41Figure
27 UV Oxidation Simplified Flow
Plan....................................................................................41
Revision Memo
12/00 Added section on Incineration. Updated sections on Wet Air
Oxidation andBiological Treatment. Removed sections on
Crystallization and Resins.Removed 1995 budgetary estimates. Added
Table 2, Figures 3, 21, and 24.Other minor updates and editorial
revisions made.
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SCOPE
This section discusses the nature of spent caustic, ways to
reduce production, current methods of disposal, and
alternativemethods of disposal. Although there are four major types
of spent caustic (sulfidic, cresylic/phenolic, naphthenic, and
sulfitic),this section will mainly focus on spent sulfidic caustic,
since this is typically the largest and most difficult type to
handle. A briefdescription, along with advantages and
disadvantages, is provided for each management method. A
comparative parameterstable, a treatment selection flow sheet, and
a relative cost comparison table are included to guide the user in
the selection ofthe appropriate method of disposal for their
site.
REFERENCES
Bertrand, R. R., MEFA: Minimum Emissions Facilities Assessment,
ER&E Report No. EE.123E.92, February 1993.
Chen, Y., and Burgess, P. D., Spent Caustic Treatment and
Disposal, 42nd Purdue University Industrial Waste
ConferenceProceedings: 430 - 436, May 12 - 14 1987.
Copa, W. M., Momont, J. A., and Beula, D. A., The Application of
Wet Air Oxidation to the Treatment of Spent Caustic Liquor,Chemical
Oxidation Technology for the 90s, Technical Report Number 415,
Vanderbilt University, Nashville, Tennessee,February 20, 1991.
Cressman, Paul R., Holbrook, David L., Hurren, Maureen L., and
Smith, Edward F., Caustic-Free Jet Fuel Merox Unit ReducesWaste
Disposal, Oil & Gas Journal, 80 - 84, March 20, 1995.
Gary, James H., and Handwerk, Glenn E., Petroleum Refining
Technology and Economics, 3rd Ed., Marcel Dekker, Inc., NewYork,
1994.
Goodrich, R. R., Electrolytic Regeneration of Sulfidic Spent
Caustic Wastes, ER&E Report No. EE.51E.78, May 1978.
Harris, T. B., Natural Gas Treating with Molecular Sieves, UOP,
1975.
Heritage Remediation Engineering Inc., Management of Spent
Caustic in the Petroleum Industry, Petroleum EnvironmentalResearch
Forum, Project # 89 - 09, September 1992.
Holderness, J., Spent Caustic Incineration at Dows New Ethylene
Plant in Alberta, Canada, AICHE 8th Ethylene ProducersConference
Proceedings: pg. 18-28, New Orleans, February 25-29, 1996.
Langeland, O., Jonas, C. and Leitzke, O., Treatment of Spent
Caustic with Ozone, AICHE 8th Ethylene Producers
ConferenceProceedings: pg. 53-68, New Orleans, February 25-29,
1996.
Phillips, S. R., Ethylene Plant Spent Caustic Management, Exxon
Chemical Company, Basic Chemical Technology, ReportNo. 92BCPRT2150,
September 8, 1992.
Sublette, K. L. and Rajganesh, B., Biotreatment of Refinery
Spent Sulfidic Caustics, Center for Environmental Research
&Technology, University of Tulsa, Tulsa, Oklahoma, 1993.
Wang, J. S., and Hafker, W. R., Waste Management Preferred
Operating Practices (POPs), ER&E Report No. EE.82E.97,April,
1997.
BACKGROUND
Caustic soda (NaOH) solutions are used to remove acidic
contaminants from refinery and chemical plant feed and
productstreams. These acidic contaminants: (hydrogen sulfide (H2S),
mercaptans, carbon dioxide, phenols, naphthenic acids, andsulfur
dioxide) react with the caustic. The partially reacted caustic,
along with the reaction products, is known as spentcaustic." Spent
caustic can be classified as one of four types, depending on the
composition: 1) sulfidic, 2) cresylic/phenolic, 3)naphthenic, and
4) sulfitic. Spent sulfidic caustic is generated from scrubbing
LPG, virgin naphtha, gas oils, hydrofinedproducts, and
steam-cracked streams. The major contaminants of this stream are
mercaptans and sulfides. Spent cresyliccaustic is generated from
cracked streams and as a waste stream from Merox units. Major
contaminants include cresylic acids,phenols, mercaptans and
sulfides. Due to the presence of high levels of phenols, this type
is also referred to as phenoliccaustic. Spent naphthenic caustic is
derived from treating virgin naphthas, and kerosene from highly
naphthenic crudes.Naphthenic acids, mercaptans, and sulfides are
the major contaminants. Caustic produced from sulfuric acid
alkylation unitscontains sulfate, and sulfites and is classified as
sulfitic caustic. Sulfidic spent caustic represents the largest
volume of spentcaustic generated.
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BACKGROUND (Cont)
Spent caustic is considered a registered hazardous waste in
Canada. Spent caustic sent for disposal in the US is alsoconsidered
a hazardous waste due to its corrosivity (pH > 12.5) and
reactivity (sulfide bearing waste). In order to dispose ofspent
caustic, the pH must be lowered and the sulfides must be
deactivated by chemically converting the sulfide to a lessreactive
form (e.g. inert sulfur, insoluble metallic sulfide salts, soluble
sulfates, etc.). Cresylic and naphthenic spent causticsreused as
feedstock for the manufacture of cresylic or naphthenic acid are
not considered a hazardous waste in the US.
With changes in environmental regulations and company
philosophies concerning waste disposal, the current methods
fordisposal of spent caustic are being re-evaluated. A popular
method involves sending spent caustic to companies such asMerichem
or pulp and paper mills for caustic reuse or recovery of various
constituents from the caustic. Sometimes thismethod provides a
profit to the supplier. With the introduction of excess caustic in
the market, reclamation companies may bereaching their limits. They
have imposed stricter limits on the quality of caustic they will
accept. If the caustic is below qualityspecifications, fees are
incurred. These fees, coupled with transportation costs, sometimes
exceed the sales price. Pulp andpaper mills are also feeling the
effects of stricter environmental regulations. In order to comply
with government regulations,pulp and paper mills are changing their
processes in order to reduce sulfur losses. This means their demand
for spent sulfidiccaustic is decreasing. Cost effective spent
caustic management is a combination of existing process
optimization, processmodifications and treatment options, and
review of disposal options, including direct sales opportunities.
Because of the largeamounts of spent caustic generated and possible
hazardous waste implications, reducing amounts of spent caustic
generatedand reuse within the plant can be very attractive
options.
Typical management methods used within ExxonMobil are listed in
Table 1.
SELECTION OF A MANAGEMENT METHODSelection of a management method
for caustic use and spent caustic reuse and/or disposal depends on
characteristics of thesite and the caustic. Many of the reuse
options depend on units that are downstream of the spent caustic
reuse / recycle point.For example, Slagen injects their spent
caustic into the crude downstream of their desalter for pH control.
This application isviable only for refineries that don't feed
catalytic units with residuum due to possible catalyst poisoning
from sodium. Spentcaustic characteristics are very important in the
selection of a management method. The major parameters that must
beidentified are COD, sulfides, mercaptans, and %NaOH. For example,
COD is very important in the selection and sizing of wetair
oxidation units. A medium pressure system can treat levels of COD
in the 80,000 - 100,000 mg/l range and sulfides in the10,000 -
40,000 mg/l range. High-pressure systems treat COD streams greater
than 100,000 mg/l by diluting them with waterto the 85,000 - 95,000
mg/l range. Sulfide levels are important for treatment methods such
as chemical oxidation. Chemicaloxidation is based on stoichiometric
needs to oxidize the sulfides. Chemical oxidation may be an option
for refineries orchemical plants that have a low COD and sulfide
content whereas it is not economically feasible for high volume /
high sulfidestreams. Mercaptan levels can mean the difference
between accepting a treatment method, accepting a method
withmodification and rejecting a method completely. Super critical
water oxidation is not significantly affected by the presence
ofmercaptans, while wet air oxidation can treat mercaptans but may
require raising the temperature to reduce foaming.Biotreatment
currently cannot effectively treat mercaptan-containing streams.
The degree to which caustic is spent prior todisposal or the level
of NaOH present can also be a factor for choosing a management
method. Spending caustic to a highlevel (< 1 - 3% caustic) can
cause effluent pH in wet air oxidation to drop dramatically. This
drop in pH will requireneutralization to meet wastewater specs and
careful selection of materials of construction to withstand
dramatic pH swings. Alist of common contaminants and their
contribution to stream CODs is given in Table 2. Typical spent
sulfidic caustic streamcompositions are given in Table 3. These are
for orientation purposes only, and it is essential to have an
accuratecharacterization of spent caustic before selecting a
technology or management method.
Figure 1 presents a hierarchy for spent caustic waste
minimization/treatment. In order to determine what level a site is
in thehierarchy, a flow sheet, Figure 2, presents a list of
questions that will guide the user through the steps of spent
causticmanagement. Because site and caustic characteristics are
unique, Table 4 is provided as a quick reference to narrow
downpossible options for a specific site. Table 5 presents each
option with a list of comparative parameters useful in
determiningthe relative benefits and debits to each technology.
These parameters include: pre-treatment, post-treatment, relative
cost(H/M/L), applicability to all types of caustic, waste
generated, material reuse, inherent problems in the process, and
equipmentinvolved. The specific caustic management tools available
are discussed later in this document. Figure 3 presents
amethodology for conducting a site-wide caustic use optimization
within refinery/petrochemical plants. Optimizationencompasses the
purchase, use, reuse, treatment, and/or disposal opportunities to
reduce costs associated with the use ofcaustic in both onsite
process units and offsite utility units (e.g., water and wastewater
treatment facilities).
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MINIMIZATION OF SPENT CAUSTIC VOLUME
Minimization of spent caustic generation can be accomplished by
two methods. Method 1 is to replace the caustic system witha system
that will accomplish the same goal. Although this will eliminate
the production of NaOH caustic, the alternativemethod may be more
expensive such as the use of KOH or the method may produce a waste
that is more difficult to handlesuch as a sponge iron system. While
some of these methods are not as effective as caustic scrubbing,
they can be coupledwith caustic scrubbing to reduce the amount of
caustic generated. The second method is to optimize the current
causticsystem. This can be achieved through techniques such as
operator training and careful system monitoring.
H2S REMOVAL BY AMINE PRE TREATMENT
Amine processes can be added as a pre-treatment in order to
reduce the amount of spent caustic generated. This processremoves
relatively large quantities of H2S but does not remove mercaptans.
Amine processes typically use monoethanolamine(MEA) for refinery
gas treating, although methyl-diethanol amine (MDEA) can also be
used. Cold amine is injected into the topof an absorber while sour
gas is injected counter currently. See Figure 4. The treated gas
leaves the top of the absorber withtypical H2S concentrations of 10
- 15 ppm. The acid gases are absorbed into the amine stream and
sent to a flash tank. In theflash tank, any dissolved or entrained
hydrocarbons are vented from the system or skimmed from the amine.
The stream isthen heated and sent to a regeneration tower where the
acid gases are steam stripped. The acid gases are sent to a
sulfurrecovery unit. Amine treating can offer spent caustic
reductions of greater than 90% over caustic washing of H2S
withoutamine pre-treatment. Although the amine process requires
additional capital expenditure, amine processes are
currentlyinstalled in most plants owing to operating cost
reductions. ExxonMobil licenses two solvents, FLEXSORB SE
andFLEXSORB SE PLUS, which can be used in place of MDEA. Both these
solvents have higher selectivity for H2S, lowerinvestment, lower
solution recirculation rates and lower regeneration steam than
MDEA. These solvents possess corrosionresistant and non-foaming
properties.
UOP MEROX PROCESSESUOP offers MEROX (MERcaptan OXidation)
systems that reduce spent caustic generation by as much as 90%, as
well assystems that utilize non-caustic alkalinity. Depending on
the process employed and the product results desired, the
Meroxprocess is capable of treating feedstocks ranging from natural
gas and LPG to distillate stocks with final boiling points as
highas 650F (340C). The Merox process can be divided into two
categories: extraction (mercaptan removal) and sweetening(mercaptan
conversion).
Merox Liquid-Liquid Extraction
Merox liquid-liquid extraction systems are widely specified for
the removal of mercaptans and sulfides from gas, LPG, lightstraight
run and thermally cracked naphthas. The most common application of
Merox liquid-liquid extraction is in the treatmentof LPG, which
typically has up to 5 wppm H2S, in which a single, vertical,
multistage, extraction column is typically specified.See Figure 5.
In this process, the hydrocarbon stream enters the bottom of the
tower. Caustic is introduced at the top andremoves mercaptan as it
flows counter currently. The product leaves the top and is
virtually free of mercaptan and caustic.The caustic then flows to
an oxidizer where the mercaptans are converted to disulfides and
the caustic is regenerated. Theeffluent from the oxidizer goes to a
disulfide separator where the disulfides are decanted and the
regenerated caustic is sentback to the tower. The decanted
disulfides can either be hydrotreated or sold. To reduce caustic
spending on H2S, LPG orgas caustic treatment is typically preceded
by an amine system for bulk H2S removal.
Mercaptan Conversion (Sweetening)
In this process, the mercaptans are converted to disulfides with
no reduction of total sulfur in the hydrocarbon stream. It
istypically used for heavy hydrocarbon streams such as gasoline and
kerosene. There are two general categories of Meroxsweetening,
liquid-liquid and fixed-bed.
Liquid-liquid Sweetening
In this process, the hydrocarbon stream, catalyst containing
caustic, and air are injected into the bottom of the tower.
Thecatalyst in the presence of air, oxidizes the mercaptans to
disulfides. Since mercaptan oxidation is essentially complete,the
caustic phase does not carry mercaptans from the contactor and,
therefore, does not require regeneration prior torecirculation. In
earlier systems, the effluent was then sent to a separator to
remove the caustic. However, in recentdesigns, the caustic is
removed by a disengaging basket and the entire process can be
accomplished in a single unit (seeFigure 6). The major difference
in liquid-liquid sweetening and liquid-liquid extraction is in
sweetening, the mercaptans areconverted to disulfides and are left
in the hydrocarbon stream while in extraction, the mercaptans are
first removed fromthe hydrocarbon stream before being converted to
disulfides.
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MINIMIZATION OF SPENT CAUSTIC VOLUME (Cont)Fixed-Bed Sweetening
Processes
In this version of the Merox process, Merox catalyst is
impregnated on a fixed bed of carbon. Sweetening is performed
bypassing a sour hydrocarbon stream containing dissolved
atmospheric oxygen over the alkaline catalyst bed. All
reactionchemistry is identical to the other Merox processes. There
are two general classifications of fixed-bed sweeteningsystems,
differentiated by the means of providing alkalinity to the
system.
Conventional Fixed-Bed Sweetening: In the basic, conventional
fixed-bed sweetening process, a hydrocarbonstream, air, and caustic
are mixed and introduced to a tower containing an alkaline catalyst
bed (see Figure 7). Asthe mixture flows down over the bed, the
mercaptans are oxidized to disulfides. The effluent is then sent to
a causticsettler where the disulfides are decanted and either sent
to a hydrotreater or sold. The caustic is recirculated forcontinued
use. This process is well suited for treating jet fuel, kerosene,
heavy naphtha, thermal gasolines, diesel,and distillate fuel oil.
The process for jet fuel treating is slightly different from the
basic process in order to meetproduct specifications (see Figure
8). A prewash is added to remove condensed water and naphthenic
acids. Waterremoval is necessary to prevent dilution of caustic
downstream. Naphthenic acids are removed to prevent theformation of
sodium naphthenate salts, which foul and deactivate the catalyst
bed. The reactor and caustic settler arethe same as the basic
system; however, in the jet fuel system, the reactor is followed by
a post-treatment system. Thepost-treatment system consists of a
water wash, salt filter, and clay filter to remove any water,
oil-soluble surfactants,and organometallic compounds.
Minalk (MINimum ALKaline) Fixed-Bed Sweetening: In the Minalk
system, a very small stream of dilute caustic(several ppm) is
continuously injected into the sour feed and withdrawn from the
reactor bottom (see Figure 9). Theeffluent caustic is not only
small in volume, but is largely neutralized both by the acidic
compounds in the feedstock aswell as by the air injected to supply
oxygen. Thus, waste caustic disposal is both simple and direct
(often directly tothe wastewater treatment plant). Although, the
caustic is not reused, the system uses less caustic than other
systemsdue to the Minalk system's high efficiency. The Minalk
process is typically used to treat FCC gasolines, natural
gasliquids, and light straight run naphthas.
Caustic-Free Merox
UOP offers a Caustic-Free Merox" system that uses a non-caustic
alkaline. It offers the elimination of caustic consumptionand
disposal costs with a high-activity, non-caustic catalyst system,
Merox No. 21 catalyst and Merox CF additive. Thecombination of this
catalyst and additive enables weaker bases such as ammonia to
achieve the alkalinity needed to direct themercaptan reaction. The
process is very similar to the Minalk process. The hydrocarbon
stream is mixed with ammonia,Merox CF additive, and air (see Figure
10). The stream flows down the fixed bed containing Merox No. 21
catalyst and themercaptans are oxidized to disulfides. The ammonia
is easily separated from the hydrocarbon stream at the bottom of
thereactor. The ammonia water stream can then be sent to the
refinery sour water stripper. The advantage of this is that
thespent alkaline solution is at roughly neutral pH, and so has low
phenol levels (200 - 300 ppm). Where disposal of the spentcaustic
is a problem due to high phenol loading (COD), this alternative may
be applied. Contemporary Minalk Merox units maybe readily converted
to caustic-free units if disposal requirements warrant. The
caustic-free process has been applied totreatment of gasoline,
kerosene, and jet fuel. Petro-Canada Inc.'s refinery in Oakville,
Ont., near Toronto, converted its caustic-based UOP jet fuel Merox
unit to a Caustic-Free Merox design in the mid-1990s to save on
third party caustic disposal costs.
MERICHEM, INC. THIOLEX Merichem Company offers their Fiber-Film"
technology adapted for caustic extraction of sulfidic compounds
from gas, LPG,and virgin naphthas un containing Merox No. 21
catalyst and the mercaptans are oxidized to disulfides. The ammonia
is easilyseparated from the hydrocarbon stream at the bottom of the
reactor. The ammonia water stream can then be sent to therefinery
sour water stripper. The advantage of this is that the spent
alkaline solution is at roughly neutral pH, and so has lowphenol
levels (200 - 300 ppm the caustic strength. If the mercaptan sulfur
content of the feed is high, extraction should befollowed by a
caustic regeneration system (Regen") to allow the reuse of caustic
(see Figure 12). This system catalyticallyoxidizes mercaptides to
disulfide oils (DSO), which naturally separate (decant) from the
regenerated caustic. The specificgravity difference between the DSO
and caustic is slight, and traces of DSO may be back extracted to
the hydrocarbon productstream. Where product sulfur specifications
are very low, a naphtha wash at the DSO/caustic separator is
recommended toreduce entrained sulfides to low ppm levels.
Regenerated caustic is recirculated until the original caustic
strength has beenspent by approximately 10 - 20%. A small purge
stream of spent caustic must continually be drawn off to allow for
fresh causticmakeup. This spent caustic purge, containing only free
caustic and sodium thiosulfate, may be used for H2S removal where
itcan be spent up to 80%. As an example, a typical Regen" unit
treating some 440 gpm (980 m3/hr) of light ends will requireabout 1
lb (0.45 kg) catalyst for 840k gal (3200 m3) hydrocarbon treated,
and will generate about 0.15 gpm (0.03 m3/hr) ofspent caustic,
along with 100 - 200 SCFM (170 - 340 SCMH) of spent air that must
be incinerated to convert organic sulfides to
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MINIMIZATION OF SPENT CAUSTIC VOLUME (Cont)
SO2. Wash naphtha may be water washed and coalesced to obtain a
material with less than 1 wppm of Na, which may be fedto a
hydrotreater to convert the disulfides to H2S and light ends. The
primary treatment requirements for naphthas are that H2Sbe removed
virtually completely, while mercaptan sulfur must be below 10 wppm.
The total sulfur specification will determinewhether the mercaptans
are extracted, or converted to disulfides by sweetening. Merichem
offers their Merifining" systems formercaptan extraction to meet
low total sulfur specs. Mericat" sweetening systems are specified
where total sulfur is lesscritical. Both systems have been designed
to be caustic-regenerative.
OXIDATION OF H2S TO ELEMENTAL SULFUR
Several different processes, such as Lo-Cat, Sulfa-check,
Stretford Oxidation and sponge iron, convert H2S to a solid
formwhich can then be removed via settling or filtration. These
processes do not remove CO2; thus, in ethylene applications,
asecondary treatment (i.e., caustic scrubbing) would be required.
Lo-Cat uses an aqueous solution containing iron to absorb theH2S
from the hydrocarbon steam. The H2S reacts with the oxidized iron
to produce elemental sulfur. Stretford Oxidation,licensed by
British Gas, selectively removes H2S from gas streams with total
sulfur recovery of > 99.9% and residual H2S in thetreated gas
below 10 wppm. H2S is removed with an alkaline solution, followed
by the air oxidation of sulfides to elementalsulfur in the presence
of a proprietary catalyst. Elemental sulfur is removed as a clean
dry cake, the Stretford solution isregenerable, and an optional
desalting unit can yield virtually zero liquid effluent from the
process.
MOLECULAR SIEVES
Molecular sieves can be used in conjunction with amine systems
or caustic scrubbing systems for removing hydrogen
sulfide,mercaptans, carbonyl sulfides and moisture from light ends
(C2 - C4). See Figure 13. Molecular sieves are porous
inorganicsolids that contain many micron-sized porous cubic zeolite
(aluminosilicate) crystals. Because the pore-size is uniform
andvery precise, molecules can be separated by size. Molecular
sieves remove sulfur compounds to extremely low levels but arenot
recommended for bulk removal of sulfur. High levels of sulfur
exhaust the sieve quickly and, therefore, lead to short cycletimes
or a large sieve inventory. Neither option is attractive from an
operating or economical standpoint. Recommendedoperating conditions
are: feed rate 700 - 240,000 SCFM (1200 - 410,000 SCMH) pressure
315 - 1215 psi (20 - 80 atm),temperature 85 - 120F (29 - 49C), and
H2S content 0.022 - 5.5 lb/1000 SCF (0.35 - 88 kg/1000 SCM). Once
sieves arespent, they can be regenerated by heating the sieves.
During this process, sulfides and mercaptans are released in an
off-gaswhich must be treated.
SWITCH FROM NaOH TO KOH
One alternative to eliminate the production of spent NaOH
caustic involves switching from NaOH to KOH. This will not
reducethe volume of spent caustic generated but it will now be in a
form which can readily be reused as fertilizer. The
chemistryinvolved with KOH is the same as NaOH. KOH is expected to
remove CO2 more completely than NaOH because it is lessviscous than
NaOH at a given molal concentration. KOH is 1.5 times more
expensive than NaOH on a molal basis. Becausethe production of KOH,
like NaOH, is tied to the manufacture of chlorine, KOH prices tend
to rise and fall together with the priceof NaOH. Spent KOH can be
treated by neutralization with waste acid such as H2SO4 or HCl to
produce K2SO4 or KCl,respectively. These salts, which act as
fertilizers by supplying plants with potassium, could potentially
be disposed of to theland. This allows sites not located on a
saltwater body to dispose of their salts in an environmentally
preferable manner.
CHANGES IN OPERATING PRACTICES
Optimization of Existing Processes
Optimizing the available hydroxide (causticity) left in the
caustic is a low-cost option to minimize the volume of spent
caustics.Often, significant reductions in caustic use and
improvements in caustic exhaustion levels can be achieved simply
throughoperator training, improved awareness, and attention to
operating parameters. Inefficient contacting and inadequate
contactingtimes will lead to non-optimum exhaustion of the
available caustic alkalinity. Optimization will also produce spent
caustic whichcontains larger concentrations of sulfides and acid
gases. It is possible for high levels of constituents to reduce the
quality ofcaustic below reuse or resale requirements; therefore, it
is necessary to evaluate the optimum level of spending caustic.
The use of caustic titration (performed locally in a bench test,
or with automatic on-line equipment) may be used to optimizecaustic
feed, and to ensure that excessive free alkalinity is not wasted
when the caustic is purged. Consideration should alsobe given to
increasing the strength of the fresh caustic stream used. This will
reduce the volume of spent caustic production.However, higher
concentrations may reduce contacting efficiency due to higher
viscosity.
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ExxonMobil Proprietary
SOLID WASTE MANAGEMENT AND SITE REMEDIATION Section Page
GUIDELINES FOR SPENT CAUSTIC MANAGEMENT XX-C4 9 of 41
DESIGN PRACTICES December, 2000
ExxonMobil Research and Engineering Company Fairfax, VA
MINIMIZATION OF SPENT CAUSTIC VOLUME (Cont)
Segregation of Spent Caustic
Since the reuse, treatment, and disposal options for a given
spent caustic greatly depend on the concentration of residual
freealkalinity in the waste caustic stream as well as composition,
care should be taken when handling and storing spent
caustics.Mixing of dissimilar caustics could limit treatment
options, increase treatment and disposal costs, or degrade the
reuse optionsor the resale value of spent caustics. For example,
refineries that combine sulfidic caustic with cresylic caustic from
Merox andMinalk units, could segregate these streams and possibly
send the cresylic caustic to the wastewater treatment
system.Currently, Rotterdam and Sriracha send their Merox and
Minalk cresylic caustic to the wastewater treatment system.
Thecombined stream cannot be treated at the wastewater facilities
due to odors associated with sulfides and mercaptans in thesulfidic
caustic.
CASCADED REUSE OF SPENT CAUSTIC
The strength, purity, and composition of caustic required for a
given treatment, or generated by a treatment process varieswidely.
Quality of caustic will depend on both the product being treated
and the type of treatment system being employed. Aneffective
strategy to reduce the use of fresh caustic and minimize the
generation of end-of-pipe" spent caustics is to carefullymatch
caustic treatment needs with available spent caustics being
generated.
DIRECT REUSE FOR HYDROCARBON PRODUCT TREATMENT
Mercaptan removal depends on high free alkalinity dictating use
of fresh caustic. Spent caustic from mercaptan treating is
wellsuited for reuse in H2S removal since it does not require as
large an amount of free alkalinity. Spent mercaptan causticsshould
not generally be used for the removal of high-levels of hydrogen
sulfide due to the low level of free alkalinity, but arevery
effective for low levels. Baton Rouge Refinery uses fresh caustic
in several extraction towers, after amine treating for H2S,to
remove mercaptans from cat light ends. Spent caustic from these
towers is used to remove mercaptans and sulfides fromvirgin light
ends. However, reuse may reduce the resale value of a caustic. For
example, cresylic sales for phenols recoveryrequire low loads of
sodium sulfide.
INJECTION INTO CRUDE
Spent caustic can be injected directly into the crude downstream
of the desalter and upstream of the pipestill for pH control.Care
must be taken to prevent sodium poisoning of catalyst for
refineries with units such as Cat Crackers or
Hydrofiners.Typically, US refineries have these units; therefore,
this is not recommended for operations in the US. However, this is
apossible option for a few selected non-US affiliate refineries.
EMRE experts should be consulted to determine sodium limits
forvarious catalyst operations. Slagen Refinery currently injects
spent caustic into their crude.
pH CONTROL IN SOUR WATER STRIPPER
Spent caustics (sulfidic and cresylic spent caustics) containing
low-molecular weight mercaptides, hydrogen sulfide andphenols may
be sent to sour water strippers under certain conditions. Sour
water strippers remove hydrogen sulfide andammonia from sour waters
and sour condensates. Stripped components are incinerated or sent
for sulfur recovery, while theaqueous effluent is sent for further
wastewater treatment (e.g., BIOX) prior to discharge. Acidic
conditions favor the removal ofH2S, while alkaline conditions favor
the stripping of ammonia. Thus, tower design and operating pH is
determined by the feedcompositions and the required effluent
standards. Thus, the addition of spent sulfidic caustic could
enhance ammonia stripperswhile caustic addition to H2S strippers
may decrease performance. Caustic can be added in one of two
places, in the feed orbetween 6 - 8 actual trays. Spent caustic can
only be substituted if it is added with the feed. Care must be
taken when addingcaustic to prevent overshooting the desired
pH.
According to MEFA, mercaptans strip similarly to H2S with no
adverse effects on stripper performance. Phenols, thoughexhibiting
weakly acidic properties, are also reported as having no adverse
effect on performance. Spent caustic should notcarry oils which can
cause foaming or aromatics. Before adding spent caustic to the sour
water stripper, EMRE experts shouldbe consulted to evaluate
feasibility and optimum pH and operating conditions. High dissolved
salt levels attributed to spentcaustic can lead to fouling and/or
deposits within the stripper tower.
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XX-C4 10 of 41 GUIDELINES FOR SPENT CAUSTIC MANAGEMENTDecember,
2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company Fairfax, VA
CASCADED REUSE OF SPENT CAUSTIC (Cont)
pH CONTROL IN BIOLOGICAL OXIDATION (BIOX) UNIT
In a BIOX unit, oxidation of organics and (especially) ammonia
reduces the alkalinity/pH of the system. pH levels below 6.5
areinhibitory to the microorganisms. Fresh caustic is typically
used to maintain the optimum pH; however, spent sulfidic causticcan
be substituted as a source of alkalinity. It should be noted, spent
caustic must be applied to a BIOX unit at such a rate
thatmercaptides and sulfides present are biologically oxidized
rather than released to the atmosphere. Spent caustic
ischaracterized as having high BOD and COD levels, therefore,
adequate aeration must be supplied to maintain a dissolvedoxygen
(DO) level of 2.0 mg/l or greater. Reduced sulfur compounds can
result in filamentous bulking bacteria, which cannegatively impact
the performance of the BIOX system. Care must be taken when adding
spent caustic in order to avoidshocking" the BIOX system.
Non-sulfidic caustics should not be used due to the presence of
potentially toxic or inhibitoryorganic compounds and/or heavy
metals. Antwerp, Bayway, Singapore and Slagen Refineries are
examples of refineries usingspent sulfidic caustic for BIOX pH
control. Refer to the Biological Treatment section of this DP
beginning on page 13.
pH CONTROL IN PIPESTILLS
Corrosion in pipestills is caused by HCl. Spent caustic can be
injected into pipestills in order to neutralize the pH and
reducecorrosion. Care must be taken to avoid the fouling of preheat
heat exchangers, avoid pH swings, and comply with
sodiumspecifications of pipestill residues. As mentioned above,
EMRE experts should be consulted before adding spent caustic
toavoid sodium poisoning of catalyst operations downstream. Spent
caustic can cause upsets such as foaming in pipestills.
INJECTION OF SPENT CAUSTIC IN FCCU WET GAS SCRUBBERS
Wet gas scrubbers (WGS) are used to control particulate and
gaseous emissions from FCCU (Fluidized-bed Catalytic CrackingUnit)
regenerators. The WGS removes particulates by washing the flue gas
stream with droplets of buffered scrubber liquid,while the SO2 is
removed by reaction with the solution. In order to increase the
removal of SO2 and to mitigate the corrosiveeffects, caustic or
soda ash is continuously added to the recirculating scrubbing
liquid to adjust its pH to the desired level (about6.7). WGSs
operate in an oxidizing atmosphere and at near-neutral pH. If spent
sulfidic caustic is injected directly, conditionsfavor the release
of mercaptides and sulfides as mercaptans and hydrogen sulfide gas,
leading to emissions problems. Thus, itis necessary that sulfides
and mercaptides be removed by a caustic scrubbing system or
converted to thiosulfates and sulfatesusing a thermal oxidation
system to facilitate the recycle/reuse of the caustic strength for
pH control at the WGS. Baton Rougeand Baytown send oxidized spent
caustic to the WGS.
REUSE AS FEEDSTOCK FOR OTHER INDUSTRIES
Once on-site reuse options have been exhausted, the next waste
management option is to send the spent caustic to otherindustries
to reuse in their processes. This option depends on the proximity
to appropriate industries and their willingness toaccept the
stream.
PULP AND PAPER INDUSTRY
The paper industry uses the caustic and the sodium sulfide
remaining in spent sulfidic caustic for the digestion of paper pulp
inthe Kraft pulping process. In the absence of spent caustic, paper
mills begin with fresh caustic and salt cake to producesodium
sulfide. However, the outlook for this outlet for sulfidic caustic
does not look promising. As environmental dischargerestrictions on
the paper industry have increased, chemical reuse within the
industry has reduced the purchase of spentrefinery caustic. Mills
are switching to ClO2 to replace sulfur in the pulping process.
There is also environmental pressure toreduce the amount of
chlorine used in the paper making process. If chlorine is totally
eliminated from the bleaching process,demand for sulfidic spent
caustic may begin to increase.
ALUMINA INDUSTRY
Baton Rouge Chemical Plant sends a portion of their spent
caustic to Kaiser Gramercy for reuse in their alumina process.
Thisis limited to spent caustic used to treat spent aluminum
chloride catalyst. The spent stream is high in sodium aluminate.
TheChem Plant receives a credit, but this outlet is very unstable.
In addition to the viability of the alumina business being
suspect,Kaiser has on several occasions, rejected caustic on the
basis of odor and poor quality.
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ExxonMobil Proprietary
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GUIDELINES FOR SPENT CAUSTIC MANAGEMENT XX-C4 11 of 41
DESIGN PRACTICES December, 2000
ExxonMobil Research and Engineering Company Fairfax, VA
REUSE AS FEEDSTOCK FOR OTHER INDUSTRIES (Cont)
CHEMICAL MANUFACTURING
Merichem
Merichem (Houston, Texas) handles all types of spent caustic.
Mercaptans found in cresylic and naphthenic caustic areoxidized to
form disulfides and are sold as a rubber solvent in the rubber
industry and are also used as a raw material in themanufacture of
sulfuric acid. The sodium cresylates found in cresylic spent
caustic are used to produce cresylic acid products.Sodium
naphthenate found in naphthenic spent caustic is used to produce
naphthenic acid products. Sulfidic spent caustic isblended with
other sulfidic caustic and sent to pulp and paper mills. Depending
on the type and quality of caustic, refineriescan either sell
caustic at a profit (includes cresylic spent caustic and naphthenic
spent caustic) or pay Merichem to take it(includes sulfidic spent
caustic). However, it is necessary for the spent caustic to meet
Merichem specifications.
Merisol, a joint venture between Merichem and SASOL, also
reprocess spent cresylic caustics.
Americhem
Torrance Refinery sends spent caustic to Americhem in California
for processing.
Hewchem
Hewchem, which is located on the coast of Mississippi, accepts
only naphthenic caustic. Naphthenic caustic is used toproduce
naphthenic acid. Specifications were reported to include: no limit
on BOD or COD and 5% minimum naphthenic acidin the stream.
CRI-MET
CRI-MET, located in Braithwaite, LA, accepts all types of spent
caustic. Specifications for caustic are not fixed, and each
spentcaustic is evaluated individually. The spent caustic is used
as a replacement for fresh caustic soda in the production of
aluminatrihydrate.
Penrice Soda Products
Adelaide Refinery ships spent caustic to Penrice Soda Products
(Osborne, South Australia) for reuse.
TREATMENT AND REGENERATION
The next step in the hierarchy is treatment and regeneration.
This involves regenerating the caustic partially or completely
sothat it may be used again in the gas treating process or another
part of the refinery or chemical plant.
MEROX / MINALK
In addition to regenerating caustic, the Merox system offered by
UOP also minimizes the production of spent caustic. Adetailed
description of this process can be found under the Minimization of
Spent Caustic Volume section.
THIOLEX / REGEN
In addition to regenerating caustic, the Thiolex / Regen system
offered by Merichem also minimizes the production of spentcaustic.
A detailed description of this process can be found under the
Minimization of Spent Caustic Volume section.
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XX-C4 12 of 41 GUIDELINES FOR SPENT CAUSTIC MANAGEMENTDecember,
2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company Fairfax, VA
TREATMENT AND REGENERATION (Cont)
SHELL AIR OXIDATIONBaton Rouge and Baytown operate low-pressure
oxidation systems offered by Shell for treating and partially
regeneratingsulfidic spent caustic. In Baton Rouge and Baytown, the
oxidation unit treats both refinery and chemicals spent caustic.
Theoxidation unit, known as SCOLA, in Baton Rouge is located at and
operated by Baton Rouge Chemical Plant. The unit treatscaustic at a
rate of 60 gpm (13.7 m3/hr) consisting of 10 gpm (2.3 m3/hr)
Chemical Plant sulfidic caustic and 50 gpm(11.4 m3/hr) Refinery
caustic. Excess refinery caustic is sent to Merichem. In the SCOLA
unit, caustic is mixed with air at apressure of 90 psi (6 atm) and
is heated to approximately 180F (82C). See Figure 14. Here, the
sodium hydrosulfide andsulfide are converted to sodium thiosulfate
and the mercaptans are converted to dimethyl disulfide or
ethyl-methyl disulfide withpartial regeneration of the NaOH as seen
in the following equations:
OHOSNaO2NaHS2 23222 ++ Eq. (1)
NaOH2OSNaOHO2SNa2 322222 +++ Eq. (2)
OHSONa2O2NaOH2OSNa 2422322 +++ Eq. (3)
NaOH2CHSSCHOHO2
1SNaCH2 33223 +++ Eq. (4)
NaOH2CHSSCHCHOHO2
1SNaCHSNaCHCH 32322323 ++++ Eq. (5)
Note that caustic is actually consumed in driving the
thiosulfate to the sulfate form (Eq. 3). By limiting the conversion
of sulfidespredominantly to the thiosulfate form, the Shell Air
Oxidation is a net producer of NaOH (Eq. 2). The partially
regeneratedcaustic is reused for pH control in the Wet Gas
Scrubber. The disulfides are very odorous and must be removed in
order toprevent complaints. Baton Rouge has installed a thermal
oxidizer to burn the disulfides to sulfur dioxide. The sulfur
dioxide isremoved from the gas stream with a caustic scrubber. The
caustic scrubber effluent is sent to the wastewater treatment
plant.
The oxidation unit (COU) at Baytown Refinery operates on the
same principle, however, their tower operates at a pressure of100
psi (7 atm) and temperature of 200F (93C) and a flow rate of 70 gpm
(16 m3/hr). Baytown also sends their regeneratedcaustic to the WGS
for pH control. If the stream cannot be sent to the WGS, it must be
neutralized before sending to thewastewater treatment plant. It
should be noted that this stream will still have a high COD due to
thiosulfates and may requirefurther treatment if the WWTP cannot
handle the COD level. If the WGS cannot take the caustic, Baytown
sends the caustic totheir Effluent Neutralization Unit (ENU) where
it is used to neutralize spent acid wastewater from Rhone-Poulenc
andsubsequently to the sewer.
Fouling in the reactor is common in this system. Baton Rouge has
installed a skimmer upstream of the SCOLA to removehydrocarbons
such as olefins which has helped reduce fouling. However, the
system must still be taken off line every threemonths for cleaning.
Fouling is more frequent if chemical plant caustic is increased and
refinery caustic is backed out.Baytown also experiences fouling in
their reactor to a lesser degree. Typically, the COU must be taken
off line once a year formaintenance. Baytown has also found that
olefins in the chemical plant caustic stream contribute to the
fouling problem. Inorder to avoid odor problems, steam and air
flowrates are adjusted. If this is not effective, the feed rate is
reduced until theodor is eliminated.
Heritage Mobil has developed a catalytic low temperature / low
pressure process for air oxidation of sulfidic spent caustic.
Theprocess employs a copper catalyst on a fixed carbon bed.
Reaction conditions are 40 psi; 212F; 1 liquid hour space
velocity(LHSV); 2 ppm Cu++ co-feed; 600:1 volume/volume air:caustic
for solutions containing 2-3 wt% sulfide; cocurrent air andcaustic
feed; and downflow operation. Limited conversion of sulfides to
sulfate occurs. In high sulfide systems, the preferredroute is
thiosulfate formation due to both the availability of sulfides and
oxygen mass transfer limitations. At high sulfidesdilution is
required. Sulfides are nominally non-detect in the treated effluent
(> 99 % removal of sulfides; > 90 % removal OfRSH). Pilot
studies have been conducted on Houston Olefin Plant spent caustic
feed containing 0.25 wt% S=, and onBeaumont Refinery spent caustic
containing 2.3 wt.% S=. No commercial applications have been
installed.
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ExxonMobil Proprietary
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GUIDELINES FOR SPENT CAUSTIC MANAGEMENT XX-C4 13 of 41
DESIGN PRACTICES December, 2000
ExxonMobil Research and Engineering Company Fairfax, VA
TREATMENT AND REGENERATION (Cont)
ELECTROLYTIC REGENERATION
The electrolytic regeneration process uses electrical current to
dissociate molecules and ion selective membranes to
separate,concentrate, and purify selected ions from the aqueous
mixture. Several methods of electrolytic regeneration have
beenproposed. One method which was researched in 1978 by heritage
Exxon, involves electrolysis. In this process a three-compartment
(see Figure 15) or two-compartment system (see Figure 16) is used
based on the method of pre-treatment.Pre-treatment is needed in
order to eliminate pH changes and oxidation reactions that will
inhibit current efficiency. Carbonatedcaustic is sent to a
two-compartment electrolytic cell that contains a single membrane
that is selectively permeable to cations(Na+). The sodium carbonate
enters the system and dissociates. The Na+ ion migrates across the
membrane to the cathodewhere it reacts with OH- to form NaOH. This
process produces a high-purity caustic, which is diluted to about
10% strength.On the anode side, the carbonic acid radical breaks
down to form carbon dioxide and oxygen. A three-compartment
cellcontaining an anionic and a cationic membrane is used for the
neutralized caustic stream. In this process, the sodium
sulfateenters the center compartment and is dissociated. The
sulfate ion crosses the anionic membrane to the anode
compartmentand forms sulfuric acid. The sodium ion crosses the
cationic membrane to the cathode and forms NaOH. In addition to
formingreusable products, electrolytic regeneration is also
beneficial to plants that must limit solids, specifically salts, in
their effluent.Electrolytic regeneration are marketed by Huron Tech
Corp. and by Ionsep Corporation. The technology has had
someapplication in the metal plating industry; there have been no
commercial applications within the petroleum industry.
Aqualytics, a division of Graver, uses a bipolar membrane to
split water molecules into ions. These hydrogen (H+) and
hydroxyl(OH-) ions combine with oppositely charged salt ions to
form an acid and a base. This technology has not been applied
torefinery and petrochemical spent caustic, although it has been
used for the concentration of dilute base streams and therecovery
of alkali from other industrial rinse waters. Theoretically, this
process would produce sodium hydroxide and sulfuricacid. The spent
caustic must be neutralized using fresh or waste acid before it can
be used in the Aqualytics system.Untreated spent caustic cannot be
regenerated in this system for three reasons: H2S degradation of
the membranes, reducedefficiency due to gas evolution, and
potential of mercaptide salts to form elemental sulfur or disulfide
oil. Aqualytics hasindicated that based on their past experience
with caustic streams, if spent caustic disposal presents a problem
for a site andsignificant amounts are generated (> 2000 tons/yr
(> 1800 tonnes/yr)), it is possible the process would be
economicallyfeasible. At present, Aqualytics has no plans to extend
their technology to the spent caustic market. In order to determine
ifthis option is technically and economically feasible, pilot
testing would be necessary. Like the heritage Exxon
electrolyticregeneration process, the Aqualytics system should be
considered for plants that must limit solids in their
effluents.
TREATMENT AND DISPOSAL
The final option to manage spent caustic is to treat the stream
so that it can be readily disposed.
FLUE-GAS CARBONATION
Flue-gas carbonation is essentially a neutralization and
stripping process. Carbon dioxide from a flue-gas source, such as
theoff-gas from a FCCU regenerator, neutralizes the sulfidic spent
caustic, releasing H2S and mercaptans to the off-gas. Theoff-gas
concentrations of H2S are too low to justify sulfur recovery and so
are typically incinerated or sent to a sponge ironsystem. The
carbonation process can be a batch or continuous process.
Nanticoke operates a batch operation. See Figure 17. In this
process, 3000 - 4000 US gal (11 - 15 m3) of spent caustic
iscontained in a vessel and flue gas is injected until the caustic
achieves effluent standards. Sulfide levels and ammonia levelsare
each less than 50 ppm in caustic (< 10 ppm to WWTP). pH of the
neutralized caustic is less than or equal to 9. Thecaustic is then
slowly trickled into the sewer with water and off-gas is sent to
their CO Boiler. This operation treats theiralkylation caustic from
the H2SO4 alkylation unit, phenolic caustic from the Merox system
and sulfidic caustic from the lightends treating. There have been
some operating problems with this unit, including corrosion in the
tower, foaming in the systemwith the foam carrying over to the CO
Boilers, entrainment, and glassy deposits in the CO Boiler.
Improved metallurgy isexpected to correct the corrosion problems
while the foaming, entrainment, and deposits are not fully
understood and are beingstudied.
Qenos (formerly Kemcor) uses a continuous carbonation process
under license from Hyperno Pty., Ltd. (Australia) with
fourcarbonation stages. See Figure 18. The process uses combustion
gases drawn from the stacks of two boilers that burnnatural gas and
plant gas. The flue-gas is cooled in an air fin heat exchanger
prior to being sucked into the first of a number ofeductors where
it and the spent caustic are intimately mixed. Flue-gas and spent
caustic pass through the reaction stages in acounter-current
fashion. The flue-gas flows from the top of one reactor to the
section of the jet compressor on the next reactorwhile the spent
caustic solution flows between the reactors under level control.
The hydrogen sulfide and mercaptans in theoff-gas reacts with
Sulfatreat, which is an iron compound to form iron pyrite which is
landfilled at a non-hazardous waste landfill.The residual tail gas
is sent to an on-site furnace. Because the carbonation system is
mild in terms of temperatures, pressures,and acid strength, the
fouling problems experienced with sulfuric acid neutralization are
not incurred.
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ExxonMobil Research and Engineering Company Fairfax, VA
TREATMENT AND DISPOSAL (Cont)
NEUTRALIZATION WITH STRONG (WASTE) ACID
Neutralization is similar to carbonation, however, stronger
acids are used. See Figure 19. Typically, spent sulfuric acid,
andless often hydrochloric acid, is used. In the neutralization
process, the stronger acid replaces the acid gases in an
exothermicreaction to form sodium sulfate or sodium chloride,
respectively. The acid gases are then released from the liquid
phase viastripping with steam or gas. It is important to note that
this process can be accomplished in existing sour water stripping
units,if the stripper is not operating at maximum capacity. This
alternative should be explored before investing in a dedicated
causticstripper. (See pH CONTROL TO SOUR WATER STRIPPER under the
REUSE section.) The lower the pH of the spentcaustic, the easier it
is to strip H2S. Large variations in pH and high temperatures often
result in corrosion problems; therefore,materials of construction
must be carefully selected. Steam stripping at elevated
temperatures commonly results in fouling andcorrosion problems. The
condensate from the stripping process can be sent to a Claus unit
for sulfur recovery, a sponge ironsystem, or incineration. The
treated caustic pH is adjusted to neutrality, if needed, and sent
to the wastewater treatmentsystem. When phenols are present, BIOX
treatment is necessary since a significant amount will remain
dissolved in the neutralsolution.
BIOLOGICAL TREATMENTBiological treatment uses microorganisms
which utilize specific target compounds in the spent caustic and
convert them intoless objectionable forms. Organic compounds and
sulfides in the caustic can be treated biologically. The goal of
the biologicaltreatment system is to reduce COD, sulfides, and pH.
This is done either upstream of existing wastewater treatment
facilities inorder to make the caustic suitable for release and
final treatment in the existing facility or, where possible, within
an existingbiological oxidation (BIOX) facility itself.
When used in a pre-treatment configuration, spent caustic is
introduced to a bioreactor tank containing
acclimatedmicroorganisms. See Figure 20. Testing by heritage
ER&E confirmed that organisms can be acclimated from existing
BIOXsludges. There are also specialized organisms available, such
as Thiobacillus denitrificans, which can be utilized for
thispurpose. While both types of microorganisms are adequate for
biotreatment, the specialized microorganisms appear to bemore
resistant to temperature changes and provide somewhat more stable
operations. Nutrients similar to those used intraditional activated
sludge facilities are added, if needed. The microorganisms convert
sulfides in the spent caustic to sulfate,thereby greatly reducing
the COD of the stream and producing acid which partially
neutralizes the caustic. The process isinstantaneous provided the
load to the reactor does not exceed the specific activity of the
organisms. When operating properly,there are no H2S emissions.
Supplemental acid addition is required to ensure operation of the
reactor at approximately pH 7.The amount of acid produced by the
conversion of sulfide to sulfate, and the resulting amount of
supplemental acid required forsystem operation, is dependent upon
the amount of sulfide present in the stream being treated and its
residual alkalinity. Theloading rate for a reactor design is most
appropriately established through bench scale tests of selected
caustic(s). Reactorsizing is dependent upon the flowrate of the
caustic to be treated and the concentration of the contaminants in
the caustic.Sulfates, the oxidized product, have been shown to
inhibit the biomass at sulfate concentrations of approximately
12,000 mg/l(equivalent to 4,000 mg/l sulfides in the feed caustic).
To limit the potential for an atmospheric release of H2S during an
upset,feed sulfides must be maintained substantially below this
level. Consult EMRE for guidance on establishing maximum
feedsulfide levels. Feed dilution with refinery wastewater or
treated refinery effluent can be employed to lower the feed sulfide
level.Optimum operating conditions are: dissolved oxygen (DO) >
2 mg/l, pH of approximately 7, and temperature between 75 - 85F(25
- 30C). The effluent from the reactor, whose effluent COD is likely
to be as much as two orders of magnitude lower thanthe spent
caustic, can be sent to the existing wastewater treatment system
for further treatment.
The ideal operating scenario for biological treatment of spent
caustic is to send it to the existing BIOX unit without
pre-treatment, as discussed in the Cascaded Reuse of Spent Caustic
section. For a successful application, the following conditionsare
required:
1. The spent caustic should contain no or low concentrations of
mercaptans, due to potential odors;
2. Sufficient oxygen capacity must be available to meet the
additional demand imposed by the spent caustic;
3. The caustic should be introduced into the system at a point
of maximum mixing and aeration;
4. The concentration of sulfide in the Biox feed should not
exceed 30 ppm, or increase by more than 10 mg/l.
Higherconcentrations may be possible with acclimation of the
biomass;
5. pH monitoring and control facilities must be provided to
ensure that system pH remains in the 7.5 - 9 range. H2S
couldpotentially be released at pH < 7.5, while pH > 9 could
adversely impact the biomass and/or exceed discharge
permitrequirements.
Figure 21 presents a decision tree for assessing biotreatment of
spent sulfidic caustic in an existing Biox system. Antwerp
andSlagen Refineries currently send their spent sulfidic caustic
directly to their wastewater treatment system.
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GUIDELINES FOR SPENT CAUSTIC MANAGEMENT XX-C4 15 of 41
DESIGN PRACTICES December, 2000
ExxonMobil Research and Engineering Company Fairfax, VA
TREATMENT AND DISPOSAL (Cont)
Not all spent caustics can be biologically treated. Research
sponsored by heritage ER&E has shown that with spent
causticscontaining mercaptans (concentrations as low as 0.3 wt%
mercaptan), sulfide conversion to sulfate was inhibited
andmercaptans were not treated. This resulted in emissions of both
H2S and mercaptans. Laboratory research has shown that
amicroorganism strain can be developed to biological pre-treat
mercaptan-rich spent sulfidic caustics. Such high COD
streams,however, are both oxygen mass transfer limited and
sulfate-inhibited in the absence of substantial feed dilution. High
reactorcapital costs, coupled with the ever-present potential for
odorous and toxic emissions of H2S and mercaptans, severely limit
theviability / applicability of biotreatment for these types of
spent caustic. EMRE has also supported research on
biologicalpre-treatment at elevated pH (> 9) of Merox spent
caustic high in phenolic / cresylic salts (and low in sulfur
content). The spentcaustic streams from multiple heritage Exxon
sites, characterized by high CODs (> 160,000 mg/l), high nickel
(> 900 wppm),and the presence of thiocyanates, required
dilutions ranging from 30-fold to 200+ times in order to bring the
concentrations ofthe waste components within the tolerance range of
the biomass. This results in large reactor volumes and
associatedpumping rates, which increase both capital, and operating
costs. Although phenol degradation of > 98% was observed at
pH10, the primary phenol utilizer was a fungus, and not bacteria
which are the normal substrate removing organism in anactivated
sludge system. Biotreatment of such concentrated caustic wastes at
elevated pH is not currently commerciallyavailable.
WET AIR OXIDATION
Wet air oxidation is the aqueous phase oxidation of organic and
inorganic constituents. There are three kinds of wet airoxidation:
low, medium, and high pressure. The typical operating temperatures
and pressure of wet air oxidation systems are:low - 212 - 248F (100
- 120C), 73 - 102 psi (5 - 7 atm), medium -390F (200C), 415 psi (28
atm), and high -500F (260C),1415 psi (96 atm).
In this process, sulfur compounds are converted to sulfate.
Depending on the percent spent of the caustic, the effluent will
bebasic, neutral, or acidic. For < 50% spent, the effluent will
be basic. At 50% spent the effluent will be neutral and > 50%
willresult in an acidic effluent. If organics are present, some of
them will be converted to CO2 and short chain organic acids.
Thepercent conversion will depend upon the form of the organics and
the severity of the wet air oxidation operating conditions.This
will also contribute to the pH of the effluent. These low molecular
weight organic acids are amenable to biologicaloxidation in
activated sludge systems.
Low Pressure Wet Air Oxidation
Stone & Webster offers low-pressure wet air oxidation
technology. See Figure 22. In this process, caustic stored in a
holdingtank is fed to a gasoline wash to remove polymer and prevent
fouling of the reactors. The stream is preheated with steambefore
entering the first reactor. Although the reaction is exothermic,
steam is injected between reactors to ensure sufficienttemperature.
Each reactor has two zones, separated by a perforated plate. Air
from the plant air system is supplied to eachzone through
microporous elements. The reactors operate at pressures of 73 - 102
psi (5 - 7 atm) and temperatures of212 - 248F (100 - 120C).
Material of construction is carbon steel which is expected to be
sufficient for this application as longas it has been stress
relieved. A catalytic vent gas treatment unit oxidizes organics and
organic sulfur species stripped out ofthe caustic by air and steam
during oxidation. After the reaction stage, the stream is cooled
and neutralized. The processchemistry is analogous to that for the
Shell air oxidation process described earlier. Reactor staging and
a slightly highertemperature result in a higher conversion to
sulfates with the Stone & Webster technology. Low-pressure
oxidation aloneachieves 80% reduction of COD and 80% conversion of
sulfide to sulfate. An additional biopolishing step can achieve
overall90% levels of sulfide and COD removal. Because of the higher
yield of sulfates, regeneration of NaOH is significantly less
thanthat achieved with the Shell process.
BP Chemical Limited in Grangemouth, Scotland, operates a Stone
& Webster low-pressure wet air oxidation system. Thissystem has
been operating since February 1993. The caustic feed contains 1000
- 4000 wppm sulfide and 6000 - 9000 mg/lCOD at a pH of 12.5 - 13.
The aqueous product has a COD level around 1000 mg/l, a pH of 7,
and no release of H2S orprecipitation of sulfur. The sulfide and
COD levels in the caustic feed are lower than concentrations
historically seen at heritageExxon refineries and chemical plants.
Although the gasoline wash does remove some mercaptans, a portion
strips out in theoxidation process. There is some question as to
whether the catalytic oxidation process can treat the large amount
ofmercaptans sometimes encountered in refinery and steam cracking
spent caustic.
Zimpro also offers low-pressure wet air oxidation; however, it
is typically used for sludge conditioning due to the potential
forfoaming and fouling.
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TREATMENT AND DISPOSAL (Cont)
Medium / High Pressure Wet Air Oxidation
Medium- or high-pressure systems are employed for spent caustic
treatment. See Figure 23. In this process, a wastewaterwhich
contains the oxidizable constituents is brought up to system
pressure [415 psi (28 atm) for medium pressure system,1415 psi (96
atm) for high pressure system] using a high-pressure pump.
Compressed air or oxygen gas is introduced into thepressurized
wastewater stream at a rate corresponding to the COD level of the
feed. The mixture of gas and wastewater isheated in a process heat
exchanger by heat exchange with the oxidized effluent. A second
heat exchanger provides anexternal source of heat to initiate the
wet air oxidation process and to sustain the oxidation temperature
if insufficient heat ofreaction is released in the wet air
oxidation reaction. After heating, the mixture of gas and
wastewater flows into the reactorwhere it is detained for a period
of time which is sufficient to complete the desired degree of
oxidation. The reactor is a verticalbubble column pressure vessel
that is sized to provide the desired hydraulic residence time. The
wet air oxidation reactionsare exothermic and raise the temperature
of the mixture to the desired operating temperature [390F (200C)
for mediumtemperature system; 500F (260C) for high temperature
system]. The hot oxidized effluent is directed into the process
heatexchanger to preheat the incoming mixture and cool the oxidized
effluent. An optional water cooler may be used for furthercooling.
After cooling, the effluent passes through a pressure control valve
and is directed into a separator where thenon-condensable gases
separate from the liquid phase. The stream is then neutralized
using fresh or waste acid anddischarged to biological wastewater
treatment system. Zimpro's wet air oxidation system can achieve
effluent levels of < 1 ppmsulfide, < 10 ppm mercaptans, and
< 10 ppm phenols. The off-gas can contain aromatics such as
benzene and during upsets,mercaptans and sulfides. These
contaminants can be treated by routing the off-gas to a control
device, such as a boiler orheater.
Selection of the proper wet air oxidation system (medium
pressure / medium temperature or high pressure / high
temperature)depends greatly on the level of COD present in the
stream. A medium pressure system can treat levels of COD in
the80,000 - 100,000 mg/l range and sulfides in the 10,000 - 40,000
mg/l range. High-pressure systems treat COD streams greaterthan
100,000 mg/l by diluting them with water to the 85,000 - 95,000
mg/l range. Although the COD range is now comparableto the medium
pressure range, high pressure is preferred due to the high level of
organics associated with high COD streams.Streams that contain COD
levels > 15,000 mg/l are exothermic when oxidized; thus creating
autothermal systems. Autothermalsystems require the addition of
steam or hot oil to the second heat exchanger only during
startup.
Refinery spent caustic and steam cracking spent caustic
typically contain mercaptans and phenolic compounds. In
theoxidation of mercaptans and phenolic compounds, low molecular
weight (C1-C3) carboxylic acids are formed. Theseintermediate
organics mimic fats and in the presence of caustic are saponified.
Depending on the overall level of theseorganics in the feed
caustic, foaming can result in the system. This can be corrected by
operating the WAO process at a highertemperature / higher
pressure.
Ethylene spent caustic typically contains soluble oils. If these
oils are heated, they can polymerize and plug lines. In order
toprevent this, caustic can be sent to a quiescent holding tank
with a two-day residence time prior to introduction into the wet
airoxidation system. Another way to prevent this involves
introducing air upstream of the heat exchanger to break up any
globs ofoil that happen to pass through the system. If oil does go
through the system, the solubilized oil will oxidize preferentially
overother contaminants such as sulfides and mercaptans. This can
lead to sulfide and mercaptan emissions. A way to detect
thisphenomenon is an in-line oxygen meter on the off-gas. The
off-gas typically contains 4% oxygen. When soluble oil is
oxidized,this causes the system to become oxygen deficient, which
will be detected by the oxygen meter and set off an alarm. At
thispoint the system caustic feed should automatically or manually
be discontinued and the system flushed with clean water. It
isimportant to use clean water to prevent inorganic scaling of the
heat exchangers. The flush water and any off-spec effluent canbe
recycled back to the caustic holding tank for treatment.
Zimpros suggested material of construction is a nickel alloy,
Alloy 600. This material should be sufficient for both medium
andhigh pressure / temperature systems unless the caustic has been
overly spent. If the caustic has been overly spent (< 1 -
3%caustic), the pH of the effluent will drop dramatically to the 3
- 4 range due to the formation of acid radicals (e.g., SO4=)
duringthe oxidation process and the absence of alkalinity. In this
case, the metallurgy of the system must be able to
withstanddramatic temperature swings in addition to high
temperatures and pressures. If the pH does drop, the stream can
beneutralized with the addition of alkalinity or bicarbonate.
Zimpro medium pressure units are in service at the Baytown
Olefins Plant Expansion and the Singapore Olefins Plant to
treatsteam cracker spent caustic.
Wet air oxidation can also be used to treat other aqueous
streams such as tank bottoms. A determination should be made
inadvance regarding which streams the system will be expected to
treat. Accurate analyses of contaminants (such as COD)must be
obtained. The design of the system must account for ranges of feed
concentrations expected. If the additional feedstreams are
continuous, they can be incorporated into the design. However if
the streams are infrequent, the system can bedesigned to treat
continuous flows and when infrequent streams require treatment, the
caustic feed can be temporarily reducedin order to ensure the
correct level of contaminant loading (i.e., COD) is maintained.
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GUIDELINES FOR SPENT CAUSTIC MANAGEMENT XX-C4 17 of 41
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TREATMENT AND DISPOSAL (Cont)
If other streams are to be treated, solids composition should be
considered. The presence of materials such as sand or spentcatalyst
can cause an abrasion problem in the system. It may be necessary to
install a different type of pump than typicallyused in wet air
oxidation systems. Depending on the level of solids, an in-line
strainer or shaker screen may be needed toprevent plugging of
process lines.
INCINERATIONIncineration is a thermal oxidation process and
requires temperatures of 1600 - 2000F (900 - 1100C). Generally a
minimumresidence time of 1 - 2 seconds is required at the firebox
temperature in order to assure contaminant destruction.
Applicationsto liquid wastes like spent caustic require atomization
of the waste with steam at the inlet of the combustion
chamber.Down-fired salt systems where the burners fire downward and
the combustion products are immediately quenched to bothsolubilize
product salts and cool flue gases are commonly employed. Wastewater
from secondary flue gas scrubbing isemployed as the quench fluid.
See Figure 24.
Incineration of spent caustic is conducted at the Jurong
Refinery in Singapore, and the SANREF Refinery in Yanbu,
SaudiArabia. The original Jurong incinerator, installed in the
early 1980s, was supplied by T-Thermal (Blue Bell, PA). That unit
hassince been replaced with incineration equipment from John Zink
Co. The SANREF Refinery employs T-Thermal technology.
Dow Chemical Canada Inc. incinerates spent caustic at its
ethylene plant in Fort Saskatchewan, Alberta, Canada.
Theincineration process was developed and licensed by Tsukihima
Kikai Co. Ltd. Of Tokyo, Japan. The spent caustic stream ispumped
to the incinerator where it is atomized with steam in four equally
spaced injectors. The resulting two phase mixture isthen sprayed
into the incinerator combustion chamber. The incinerator is a
natural gas fired unit supplied with combustion airby a forced
draft blower. A top mounted burner fires vertically downward to
maintain a 950C firebox temperature. The fireboxeffluent smelt (in
a molten fluid state) flows by gravity into a quench box. A portion
of the effluent from the quench box isemployed to scrub incinerator
flue gases. The cooled effluent is sent to onsite Chor-Alkali
plants for use as brine mining water.The incinerator effluent
contains a salt solution of less than 1 weight percent sodium
carbonate and sodium sulfate. It has beenreported that the effluent
stream contains 2 - 6 ppm Total Organic Carbon. Site specific
factors which contributed to Dowsselection of incineration
technology included the absence of a site biox facility, a
requirement for zero discharge of processeffluent to the North
Saskatchewan River, and the availability of low cost natural
gas.
SUPER CRITICAL WATER OXIDATION
Super Critical Water Oxidation (SCWO) utilizes the unique
properties characteristic of water when it is taken beyond
thesupercritical point [1050F (565C) and 3200 psi (220 atm)]. In
this process, the spent caustic is pumped by a high-pressurefeed
pump to the operating pressure of 3600 psi (245 atm). See Figure
25. Pressurized liquid oxygen is heated to ambienttemperature and
mixed with the caustic. The mixture is then preheated to
approximately 570F (300C) and sent to a reactor.The oxidation
reaction of the contaminants is an exothermic reaction resulting in
a temperature rise to approximately 1100F(600C). At this point, any
heavy metals are converted to their oxides and sulfur and
phosphorus are converted to sulfate andphosphate. The reaction
products are cooled to ambient temperature in an effluent cooler
and a control valve lowers theeffluent pressure to atmospheric
pressure. The resultant stream is separated into the three phases:
clear water with dissolvedsalts, a mixture of inactive substances
such as salts and heavy metal oxides, and relatively pure carbon
dioxide. The solidsformed are non-hazardous and can be sent to a
non-hazardous landfill. The CO2 can be vented to the atmosphere.
The watercan be sent to the wastewater treatment facility.
This system provides several advantages. Organic substances are
completely broken-down into clean end products with noundesirable
by-products. Unlike incineration, it produces no uncontrolled
gaseous emissions. SCWO also appears to beeconomically favorable to
incineration due to the fact that aqueous streams have high fuel
requirements for incineration. In thecase of SCWO, higher water
content is advantageous to the process. Some of the problems
associated with this processinclude plugging, metallurgy, and
safety. When salts precipitate during oxidation, they tend to clump
together and adhere to thewalls of the reactor, causing increased
corrosion and eventually plugging of the reactor. Some companies
are investigatingconcepts such as a water wall" that keeps salts
away from the metal wall and rotating brushes that sweep solids off
the wallsof the reactor. The corrosive, high-pressure,
high-temperature environment, requires exotic materials such as
Hastelloy,Inconel, and Titanium. These materials contribute to a
high capital cost. Because of the availability of lower cost
treatmentoptions that operate at lower temperature/pressure, SCWO
has not been commercially applied to spent caustic streams
inrefineries / petrochemical plants.
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TREATMENT AND DISPOSAL (Cont)
SULFIDE PRECIPITATION
Sulfide precipitation involves mixing spent sulfidic caustic
with ferrous sulfate and fresh or waste sulfuric acid in a tank at
a pHof 10. See Figure 26. Iron sulfide precipitates are flocculated
and separated in a clarifier for removal. The clarifier effluent
pHis lowered to 9.0 and sent to the biological waste treatment
system. The sludge is dewatered and sent to a landfill for
disposal.Appropriate precautions are required in handling the
sludge; iron sulfide is pyrophoric and can ignite in air at
ambienttemperature. This method is very effective in the removal of
sulfide to low concentrations. However, large amounts of sludgecan
be produced depending on the amount of sulfide treated. The process
has also been applied to the direct treatment ofsour gases low in
total sulfur.
ASPHALT FORMULATION
Development tests were conducted substituting spent phenolic
caustic in place of fresh sodium hydroxide. Tall oil
(anemulsifier), water and sodium hydroxide are saponified at 150F
(65C) in order to make a soap water" emulsion. Sodiumhydroxide (10
- 15% by weight) is added based on the amount of tall oil added.
Soap water and asphalt are then mixed,producing an asphalt
emulsion, and stored until use. The amount of soap water used
controls the time required for the asphaltto set. Emulsions made
with spent caustic met specifications and appeared to be stable.
Unfortunately, the use of spentcaustic produced an offensive odor.
Spent caustic is also generally quite dilute, requiring significant
amounts in the formulationwhich current asphalt operations are not
set up to handle.
CHEMICAL OXIDATION-OXIDIZING AGENT
Chemical oxidation uses an oxidizing agent (i.e., hydrogen
peroxide, ozone