Dp20c4

ExxonMobil Proprietary

SOLID WASTE MANAGEMENT AND SITE REMEDIATION Section Page

GUIDELINES FOR SPENT CAUSTIC MANAGEMENT XX-C4 1 of 41

DESIGN PRACTICES December, 2000

ExxonMobil Research and Engineering Company Fairfax, VA

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


Section Page SOLID WASTE MANAGEMENT AND SITE REMEDIATION

XX-C4 2 of 41 GUIDELINES FOR SPENT CAUSTIC MANAGEMENTDecember, 2000 DESIGN PRACTICES


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






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.





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.






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).





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.






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





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.






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.





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.






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.





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.






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.





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.







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.






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.







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.






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

Dp20c4

Documents

Transcript of Dp20c4