Spent caustic handling, treatment and disposal are major concerns for refining and olefins (ethylene) production facilities due to its hazardous nature and noxious properties. As sources for spent caustic generation are diverse, they do produce characteristically different waste streams consisting of inorganic and organic acidic components such as carbon dioxide (CO2) sulfides, carbonates, mercaptans, phenolics, cresylics and naphthenates. These components are acidic and must be removed to avoid corrosion of downstream equipment and to prevent poisoning catalysts. The conditions can be further complicated by neutral oil carryover. Several treatment methods can be used to manage spent caustic and they include chemical precipitation, neutralization, chemical reagent oxidation, wet oxidation, catalytic oxidation and incineration.
Improved processes are now available. Each method offers certain advantages while its application would depend on the waste stream composition, size and the configuration of the total process facility and toxicity threshold limits of downstream biological treatment systems. Spent caustic is toxic to bacteria used in the wastewater treatment unit. When zero discharge or stringent limits are stipulated in environmental permits, high dissolved solids present in the spent caustics add to treatment loading in desalination plants.
Selecting the best treatment method is a critical task, especially in meeting total waste management needs. It is often driven by environmental regulatory limits associated with emissions and discharges. There are essential elements in dealing with spent caustic streams that will be presented in this article. They involve applying the principles of waste management hierarchy, with merits and demerits of each possible solution, as shown in Fig. 1.12
Fig. 1. Waste management hierarchy.
Source characterization and segregation.
Typical processes in the hydrocarbon industry where spent caustic is generated are caustic scrubbing of straight-run light hydrocarbons, feed streams to isomerization and polymerization units; cracked gases from thermal/catalytic cracking units; and caustic washing of middle distillates, followed by mercaptans extraction/sweetening operations. The caustic converts the acidic components into their respective inorganic/organic salts of sodium such as sulfides, carbonates, mercaptides, disulfide oil, phenolates, cresolates, xylenolates and naphthenates. Caustic strength used (typically 520%) in these processes is governed by feed type and nature of caustic treatment; accordingly, the residual caustic strength will differ. Since acidic components have a wide boiling range, the spent caustic composition varies depending on the operating temperature for the caustic treatment process. To facilitate appropriate handling of these characteristically different streams, as shown in Table 1, it is advisable to segregate them by the contaminants present:
Sulfidic: Primarily sulfides (scrubbing of straight-run gaseous hydrocarbons)
Phenolic: Phenols, cresols and xylenes with sulfides (scrubbing of cracked gases/gasoline)
Naphthenic: Naphthenic acids (scrubbing/mercaptan-extraction of middle distillates.)
Reducing waste generation at the source is the first and most preferred step. When designing caustic treating processes, best practices seek to generate the least amount of spent caustic while maintaining the desired efficacy of the process. The following measures help achieve the objective:
Eliminating spent caustic generation at the source can be accomplished by applying caustic-free processes.3 These techniques involving ammonia injection and solid-bed catalysts offer benefits in mercaptan oxidation process units.
Multistage caustic wash.
Two levels of scrubbingfirst with weak caustic followed by a strong oneoffer improved utilization of caustic.7 The first level facilitates removal of most of the hydrogen sulfide (H2S), while the second level acts as a polishing step to achieve maximum removal efficiency. However, the additional investment costs must be weighed against the incremental caustic savings.
Maximizing the percentage.
Spent caustic generation can be minimized by using up the caustic strength as much as possible while preventing breakthrough of acidic components in the hydrocarbon product. This can be determined based on analytical data and operating experience.
Efficient regenerative systems.
In many cases, amine treatment precedes the caustic wash. Ensuring efficient design/operation of these regenerative units that remove major H2S quantities would reduce the load on caustic wash units, thereby minimizing spent caustic generation.
Choosing the maximum caustic strength.
When possible, the maximum caustic strength can be chosen to minimize water content, thereby reducing the dissolved organic contamination loads.
Non-dispersive mass transfer technique.
Extraction of naphthenic acids into caustic is limited by the emulsion-forming tendency and precludes using higher-strength caustic in units where the dispersive caustic mixing is applied. Film-contact techniques can overcome these limitations and allow higher caustic strength, thus enabling savings in caustic usage and reduced spent caustic generation.
Fig. 2. Flow diagram of a typical chemical
treatment unit for spent caustic.
Fig. 3. Flow diagram of a wet oxidation
REUSE WITHIN THE FACILITY
After reducing generation of spent caustic, the next step is identifying options for reusing it within the facility. Segregating spent caustic based on contaminants benefits this approach, since mixing dissimilar caustic effluents can render a blend that is unsuitable for reuse. The reuse potential of spent caustic is a function of its alkalinity, a measure of residual free NaOH and contaminants like sodium phenolate, Na2S and other sodium salts of weak acids. The high potential reuse applications are fresh caustic users. To optimize reuse application, it is essential to understand the process chemistry and operating conditions of the subject system, as well as the impact of the spent caustic contaminants on that system. Potential reuse applications include a substitute for fresh caustic usage:
Neutralization of desalter fluids.
The desalting process removes salt and other impurities such as bottom sediment and water from the crude oil before fractionation. Depending on the crude being processed, caustic is injected into the desalter to maintain optimum pH (normally 78) by neutralizing the acidity of the crude and to maximize de-emulsification. Dilute caustic solution, usually 23 wt%, is used to keep salt levels low and limit emulsification due to naphthenic acids present in the crude. Naphthenic spent caustic is not recommended, while the sulfidic stream is also not suitable for reuse due to lack of sufficient alkalinity. Conversely, phenolic spent caustic is effective as it neutralizes naphthenic acids to form phenols that dissolve back in the crude.
Crude-column corrosion control.
Caustic is often injected into the crude-column feed to minimize corrosion resulting from hydrochloric acid (HCl) evolution due to the hydrolysis of calcium and magnesium chloride present in residual water droplets contained in the desalted crude oil. Phenolic spent caustic is effective here. Naphthenic spent caustic effluent can also be used since sodium naphthenates will react with the HCl. Remember: that reuse should be done in a controlled manner to avoid fluctuations in the sodium content.6 Lower levels cause the HCl release, resulting in overhead column corrosion; while higher levels increase sodium content in the residue, leading to caustic corrosion and catalytic deactivation in downstream processing units. Increasing the concentration of acidic contaminants in the products ex-distillation is expected to be insignificant when spent caustic is used instead of fresh caustic.5
pH adjustment of desalter brine.
If the desalter pH control cannot provide minimum alkalinity to outgoing brine, it can lead to corrosion of downstream valves and piping, requiring caustic injection on the brine line. Phenolic spent caustic is suitable.
In most mercaptan-oxidation processing units, higher caustic strength (20°Be) is normally used in the extractor sections to favor mercaptan removal. A lower strength (10°Be) is used in the prewash operation due to lower solubility of Na2S. The spent caustic from the extractor unit is relatively free from sulfides and can be diluted and used in the prewash.1 Such regenerative processes are more effective in reducing spent caustic than the once-through processes.
Wastewater processing needs.
Caustic is normally dosed upstream of the chemical and biological treatment units to sustain favorable pH conditions. Since phenols and napthenates are manageable at controlled concentrations in biotreatment, phenolic and naphthenic spent caustic streams are more suitable for reuse. Sulfidic streams pose odor issues in this application.
Accordingly, phenolic spent caustic provides better reuse opportunities followed by naphthenic, while sulfidic streams offer relatively lesser avenues within the facility.
RECYCLE OUTSIDE THE FACILITY
Along with reuse options, recycling is another opportunity to explore for industries (where there are similar streams) and it has been successfully implemented by several refineries in the US and Canada.10 This approach may require additional treatment or conditioning within the facility, depending on the composition needs of the receiver. These applications may find roadblocks in terms of handling and transportation, in which case they can be sold to intermediate processors to handle them. Potential recycling opportunities include pulp and paper, tannery, mining, wood preservatives and paint industries.4
In-plant processing could be an option to extract the valuable content, especially the free caustic, sulfide salt, phenols and naphthenic acids. These can be used as feedstock for phenolic resins, herbicides, solvents, wood preservatives, paint and ink driers, fuel additives, etc.4 However, this approach may need a proper cost-benefit analysis before proceeding further. Emerging techniques, such as membrane and electrolysis applications, show promising trends.
Fig. 4. Flow diagram of an incineration
spent-caustic disposal unit.
Fig. 5. Flow diagram of an atmospheric
TREATMENT AND DISPOSAL
Treatment for final disposal is the last option, but, unfortunately, an inevitable one in most cases. Efforts should be made to handle the spent caustics directly, in well-acclimatized biological treatment units, after diluting them with bulk effluents and maintaining homogenous feed meeting biotoxicity threshold limits (established by lab tests). Biotreatment is the most economical, and offers flexibility in operation. If treatment is necessary before routing it to biotreatment, many proven techniques are available. Their suitability is case-specific as driven by factors such as quantity, composition and treatment limits to be achieved.
Treatment methods for spent caustic can be broadly categorized as chemical and thermal, with further categorization based on chemical(s) used and system operating conditions. If adequate treatment facilities are unavailable, third-party disposal should be worked out. These methods include:
Chemical treatmentOxidation (acidic/alkaline), precipitation, neutralization/acidification
Thermal treatmentWet oxidation (low-/medium-/high-pressure), incineration
Biological treatmentOxidation (attached/suspended cells)normally after pre-treatment.
Hydrogen peroxide (H2O2) is a widely used oxidant; its radicals possess very high oxidation potential. It can oxidize most inorganic and organic compounds, but the reaction rate toward sulfides is effective to apply it economically for smaller streams. Alkaline conditions are favored for complete conversion to sulfates. For organics removal, a Fenton reagent (peroxide with Fe+2 catalyst) application in an acidic medium is normally used.
Chlorinated copperas is commonly used in an alkaline medium to remove sulfides as insoluble ferric sulfide with ferric hydroxide sludge. Due to its hygroscopic nature, it is normally produced in-situ using ferrous sulfate and chlorine. However, with the associated chemical and sludge handling issues, this method usage is receding.
Neutralizing free caustic and other alkalinity converts the spent caustic components into their original forms, i.e., H2S, RSH, phenol and naphthenic acids; thus allowing recovery of the valuable acid layer but requires stripping and further handling of volatile sour gases.
This is a liquid-phase hydrothermal oxidation using dissolved oxygen at elevated temperatures facilitated by air and supplementary steam injection. Oxidation efficiency increases with temperature; its operating range is set based on target contaminants (lower for inorganics and higher for organics). The oxidation needs are driven by downstream biotreatment limits. The operating pressure is governed by partial pressure of the oxygen to be maintained, and the methods are accordingly categorized as low-/medium-/high-pressure (LP/MP/HP) wet oxidation. Due to its high treatment efficiencies, no-sludge generation and minimal air pollution, this method is most widely used. It is one of US Environmental Protection Agencys best available techniques (BATs).
Catalytic wet oxidation.
This method involves a catalyst application in wet oxidation units to reduce operating temperatures and to enhance oxidation efficiency.
This is a gas-phase oxidation process at much higher temperatures, converting inorganic constituents into molten forms and decomposing organic compounds into most stable states. It offers the ultimate oxidation levels. Diluted streams may not provide enough calorific value for self-sustaining high-temperature needs and normally require supplementary fuels. In a recent gas-cracker ethylene plant in Saudi Arabia, an improved incineration system using waste oil as fuel was installed; it addressed eutectic solid crystals formation in the quench section.
This comprises of pollutant decomposition and oxidation through bacterial adsorption, respiration and synthesis mechanisms. It produces additional bacterial cells, followed by clarification and stabilization. While feed homogeneityensuring pollutant levels below the acceptable limits (pH: 6.58.5, Oil < 25 ppm, sulfides < 10 ppm, phenols < 50200 ppm, copper < 1 ppm, etc.)is a basic prerequisite to the system, it can provide flexibility through proper acclimatization processes.
Table 2 summarizes the typical application/suitability of these techniques; further details including pros and cons are listed in Table 3. A logical approach is also presented in Fig. 6; it facilitates the total management. Research into this area has been producing more methods, mostly based on the same principles. But improvements through process/mechanical designs and some requiring establishment on a commercial scale are promising. Only commonly used methods or those successfully implemented on full scale are covered in this analysis. Licensed technologies under each method are not the focus here, but their specific features and advantages can be established through techno-economic assessment during the selection process.
Fig. 6. Logic diagram for spent caustic management.
Potential for atmospheric treatment technique.
Beyond the analysis carried out for the widely used techniques and their suitability, the authors investigated the merits of exploring atmospheric aeration and recirculation within the spent caustic storage tank(s). Based on known and available information, this technique seems either not tried on a wider scale or not put to effective implementation as yet by the hydrocarbon industry. It is proposed to test the efficacy and results on a pilot scale first to establish its advantages and operational range. Being a non-chemical and non-thermal technique, it is expected to have merits including low costs and operational simplicity. The view is supported by available data for reaction kinetics, which favor an atmospheric sulfide oxidation rate of 0.3 kg/m3hr to 0.6 kg/m3hr at ambient temperature, requiring 1885 hr (< 4 days).9 This seems attractive, as the storage inventory commonly seen for spent caustic effluents is about 715 days. It can easily be implemented in existing tank(s) with simpler metallurgy and low operational and maintenance needs.
This is an important part of a total spent caustic management system. Odor control has potential adverse impact not only on occupational and community environment but also has stake in the public perception of the facility. The malodorous characteristic of the spent caustic is attributed to its constituents like H2S, mercaptans, disulfide, cresols and thiophenol with their volatile nature and extremely low threshold odor numbers (< 1 ppb). Several measures would help mitigate these objectionable odor issues:
Closing/covering all treatment units in spent caustic service
Avoiding routing directly to drainage sewers
Maintaining alkaline pH (910) in bulk stream it is mixed with, to avoid hydrolysis of Na2S to H2S
Ensuring there are no significant residual sulfides in spent caustic, before it is sent to biological treatment
Routing of vents from LP/MP wet oxidation and mercaptan oxidation units to offgas treatment.
Selection of a suitable technique to handle spent caustic is a critical task involving technical, economic, environmental, health and regulatory considerations. Source reduction and segregation, followed by reuse/recycle, have the potential to reduce the volume to be treated. The volume reduction aids in selecting the most appropriate method specific to its characteristics. All these can be made possible by following a logical approach built on waste-management hierarchy principles. HP
Authors express sincere gratitude to KBR Management for their support and encouragement in bringing out this work. This is an updated version of a presentation at the International Refining and Petrochemical Conference-Asia, July 1922, 2011, in Singapore.
1 Dando, D. A., and D. E. Martin, A guide for reduction and disposal of waste from oil refineries and marketing installations, CONCAWE Report No. 6/03, 2003.
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3 US Dept. of Energy, Energy and Environmental Profile of the US Petroleum Refining Industry, 2007.
4 API, Category Assessment Document for Acids and Caustics from Petroleum Refining, US EPA, 2009.
5 Sarkar, G. N., Utilization of the Spent Caustics Generated in the Petroleum Refineries in the Crude Distillation Unit, 5th Intl. Conference on Stability and Handling of Liquid Fuels, The Netherlands, 1994.
6 Ahmad, W., Neutralization of Spent Caustic from LPG Plant at Preem AB Goteborg, Department of Chemical and Biological Engineering, Chalmers University of Technology, Sweden, 2010.
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8 Falqi, F. H., Miracle of Petrochemicals: Olefins Industry, 2009.
9 Berne, F., and J. Cordonnier, Industrial Water Treatment: Refining, Petrochemicals, Gas Processing Techniques, 1995.
10 US-BCSD (United States Business Council for Sustainable Development), Examples of Byproduct Synergy (http://ohiobps.org/documents/BPS_Examples.pdf).
11 API, Manual on Disposal of Refinery Wastes, Volume-IIIChemical Wastes.
12 Jones, D. S. J. and P. R. Pujado, Handbook of Petroleum Processing, 2008.
|The authors |
G. Veerabhadraiah is lead environmental engineer for KBR, Singapore. He earned a B. Tech in chemical engineering from Andhra University, India, and a Masters in HSE Technology from National University of Singapore. He has over 19 years experience in the hydrocarbon industry and specialized in environmental engineering activities of project conceptualization, design and execution. Previous to KBR, he worked with Engineers India Ltd., New Delhi. He has publications in various International Conferences and is recipient of Best Research Work Award from Central Pollution Control Board of Government of India.
N. Mallika is principal process engineer for KBR, Singapore. She obtained graduation in chemical engineering from Amravati University, India, and worked for more than 18 years in the chemical and oil and gas industry, mainly in refining, petrochemical and gas processing facilities. Her experience includes process simulation, basic design, detailed engineering, pre-commissioning and commissioning support, and she has worked extensively on licensed processes for various refining units. Previous to KBR, she worked with Engineers India Ltd., New Delhi.
S. Jindal is process and environmental discipline manager for KBR, Singapore. He has also held positions of lead process engineer and proposals engineer in a career spanning more than 20 years in the HPI. He previously worked with Aker Kvaerner, Larsen & Toubro, and Engineers India Ltd. in senior technical positions. He has authored technical papers on varied topics at international conferences. He received a B. Tech degree in chemical engineering from the Indian Institute of Technology, Mumbai, and is a AICHE member.