Mericon: A practical low-cost solution to spent caustics treating
Refining and petrochemical operations often generate various spent caustic streams that are “high strength” and require pre-treatment before processing by a waste water treatment plant.
James F McGhee
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Pre-treatment neutralises spent caustic and removes compounds that are incompatible with the biological treatment facility. Merichem’s solution to this problem is the Mericon technology. The process uses simple chemistry, operates at or near ambient pressure and temperature, and is flexible enough to handle phenolic, sulphidic, naphthenic or ethylene plant caustics individually or in mixed streams. Mericon operators benefit from a low total cost of ownership. Since the early 1990s, Mericon units have pretreated spent caustics with contaminants that exhibit extreme Chemical Oxygen Demand (COD) up to 500,000 mg/L. This paper discusses how the Mericon process resolves the spent caustic treatment issue using examples from operating Mericon units and discusses other strategies for handling spent caustics.
The Mericon Process
Sodium hydroxide (caustic) is one of the most important and beneficial industrial chemicals. It is used not only in refining, but also in papermaking, soaps and detergents, foods, and many other industries. Today’s high cost of energy and environmental compliance compel both industry and the consumer to respect the US EPA slogan: “Reduce, Recycle, Reuse.”
For over 60 years, the Merichem Company has served refining and petrochemical customers by accepting used caustic streams that are processed to reclaim valuable materials and in many cases can be re-used in other industries; the most common industries being paper and aluminum production. In the 1980’s it became apparent that reusing caustic, while environmentally beneficial presented increasing logistical challenges due to the remote locations of refineries far from processing facilities or other users. Merichem developed the Mericon process to address this challenge. Of the 31 units licensed to date worldwide, 13 customers operate Merichem’s latest generation technology, Mericon III.
Mericon III is a caustic pretreatment technology that detoxifies and neutralises refinery and petrochemical plant spent caustic. The effluent of Mericon III is a neutral-pH brine that is suitable for biological treatment facilities and meets all US EPA standards for waste water pre-treatment in the petroleum refining and petrochemical categories1. Table 1 lists the common uses of caustic within the petroleum refining and petrochemical industries.
The flow diagram in Figure 1 categorises these caustics into three broad categories: sulphidic, naphthenic, and phenolic. Smaller streams exist, for example, in scrubbers used to neutralise acidic gas streams.
In 2000, Martinie et al1 published a review of used caustic handling options and costs for a Saudi Arabian refinery. Options ranged from direct disposal or deep well injection to pretreatment and biological treatment processes. At the time of the publication, dilution and ocean disposal of low-organic brines was sometimes practiced but is now widely prohibited. In a 2011 review, Veerabhadraiah et al2 give a hierarchy of preference for reducing and eliminating used caustic in today’s refining (Table 2). Applying “Good Engineering Design” to minimise the load is the preferred option. “Recovery and re-use” are then followed by treatment before disposal.
Spent caustic have the following environmental characteristics:
• Used caustic streams are toxic to aquatic life and cannot be disposed directly to waterways, even after neutralisation; and / or
• Used caustic is unsuitable for direct addition to a biological treatment facility.
The impurities found in used caustic streams are listed in Table 3 along with how the Mericon process removes them. Standard operating procedures for modern plants require oxidation and neutralisation with stripping for spent caustic before biological treatment.
The biological treatment facility receiving spent caustic can be either owned by the refinery, a chemical plant, or a publicly owned treatment works (POTW). These facilities are able to accept a stream based on several factors:
• Overall flow rate
• Treating load (BOD and COD)
• Presence of a hazardous pollutant that could “pass through” into the effluent
• Potential negative effect on the treatment process
• pH upset
• Toxicity to the biological system
Typical flow rates of used caustic into a treatment facility range between 0.5-15 gpm. The rate can be larger for world-class ethylene facilities (up to hundreds of gpm). Although spent caustic are only a very small fraction of water treatment plants’ total flow, these spent caustic are categorised as a “high strength” waste stream. High strength implies that the stream has a high oxygen demand, complicated by a negative impact on the biological system. The chemical oxygen demand (COD) may be in the range of 50,000-75,000 or even higher (some highly spent naphthenic caustics from jet fuel treating can exhibit COD above 300,000 mg/L). COD is normally used to characterise the oxygen demand because the standard 5-day Biological Oxygen Demand (BOD-5) test gives low oxygen uptake results due to the toxicity of the used caustic. The ratio BOD/COD is sometimes used as a guide to assess the treatment plant’s ability to handle a particular waste material. A rule of thumb ratio when BOD/COD <0.5 signifies that the stream is not biodegradable.
The pretreatment challenge
Table 4 defines the minimum legal requirement in the US for pretreatment of refinery streams that are sent to downstream water treatment facilities. More stringent standards may be set in certain states or countries. Today, refineries and petrochemical facilities use high purity fresh caustic manufactured by one of two main electrolytic technologies, the diaphragm or membrane processes. These differ mainly in the amount of chloride present. As such, spent caustics pose no particular heavy metals concern.
Merichem meets the COD requirement through a combination of two chemical steps: (1) oxidising inorganic compounds (primarily sodium sulphide and bisulphide), and (2) acidifying to a pH which allows the release of naphthenic acids and phenolics into a separate acid oil layer for either blending with fuel oil or upgrading in conversion units such as FCC or coking.
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