Apr-2001

Economic treatment options for effluent caustics

The authors describe how selective application of technologies involving partial oxidation, total oxidation and deep neutralisation could add value to caustic streams while meeting environmental quality standards

Malcolm G Sharpe and Ramiro G Vazquez
Merichem Chemicals & Refinery Services LLC

Article Summary

Refineries and petrochemical plants around the world are today facing stricter controls on liquid and gas effluent discharge streams that can cause air or water pollution. Sodium hydroxide (caustic) continues to be one of the primary reagents used in refineries and petrochemical plants to extract hydrogen sulphide (H2S), mercaptans and organic acids from hydrocarbon streams because it has been proven safe, economical, effective and, above all, environmentally acceptable.

When properly recycled, treated or disposed of, used caustic streams do not constitute hazardous waste.

Used caustic solutions coming out of hydrocarbon treating systems are liquid effluents that, if discharged directly to the natural waterways, could contaminate and adversely affect drinking water quality and aquatic populations. Alternatively, if these used caustic solutions are discharged directly into the waste-water treatment plant (WWTP) they can create a burdensome chemical and biological oxygen demand (COD and BOD). Therefore, used caustics are normally converted to a form that minimises the load on the WWTP or sold to chemical companies as a process feedstock.

The first approach to reducing the environmental and remedial challenges of used caustics is to minimise their production by way of more efficient treaters, caustic regeneration and/or reuse. Unfortunately, not all refinery or petrochemical plant caustic discharges can be regenerated and, therefore, they must be disposed of either with the plant effluent waters or through an outside company or sold as a chemical feedstock.

For the past 55 years, Merichem Company has been processing effluent refinery and petrochemical plant caustics as chemical feedstocks at its two US plants. The chemical feedstocks are used in the production of speciality chemicals, which are used as raw materials to manufacture products such as plastics, pharmaceuticals, pesticides, building products, electrical wire coating, paper and minerals.

Unfortunately, the volume and location of some used caustic streams make the economics of sending them to the effluent processing plants unattractive. Therefore, most refineries and petrochemical plants must manage the proper handling of their caustic solutions on their own. The following discussion assumes that the producer has chosen onsite treatment of used caustics and, subsequent, discharge of the brine effluent into the WWTP.

Impurities found in used caustic streams produced in refineries can be toxic and, therefore, require oxidation and neutralisation or neutralisation and stripping, prior to being released to the WWTP. Sodium carbonate and bicarbonate alone are not toxic and only need to be neutralised before being discharged to the WWTP. However, used caustics typically contain a combination of more than one of the compounds listed in Table 1.

An important design consideration for all used caustic processing systems is metallurgy, where corrosive environments are normally encountered. Application of an onsite caustic-solution treating process depends on components contained in the used caustic solutions and specifications that must be met in brine solutions being discharged to the WWTP. There are also specific cost effective neutralisation systems available when pH is the only brine effluent requirement.

Caustic types
Used caustic
Before the refinery or petrochemical plant decides to install a used caustic treatment plant, it must first try to reduce the production of these streams by optimising the hydrocarbon treaters and by using caustic regenerators. Once a manufacturer has minimised the production of the used caustic solutions, and has decided that handling through an outside company is too expensive, then a decision must be made on the best treating system for the type of caustic solutions being generated.

Used caustics are classified into three types: sulphidic, cresylic and naphthenic caustics. Sometimes the plant does not have the facilities to segregate the used caustic solutions into these classifications and, therefore, a mixed used caustic stream is produced. This is considered a fourth type of used caustic solution.

Sulphidic caustic
Fuel gas, liquefied petroleum gas (LPG) and gasoline treating processes generate sulphidic caustic (Table 2). The typical contaminants are sodium sulphide and sodium mercaptide. These compounds cause high COD and BOD in the WWTP and produce odours and dangerous gases when neutralised.

Sulphidic caustic solutions are sold as treating agents to pulp and paper plants or to the mining industry for purifying certain metal ores. Since these end-users require consistent quality, sulphidic caustic is usually sold to intermediate companies that blend the required product quality while providing just-in-time transportation and delivery service.

The total sodium content of these streams is important as well as the sodium-to-sulphur ratio. Although sulphidic caustic solutions are valuable to the industries that use them, in practice, transportation and handling costs can easily exceed product value.

When production of sulphidic caustic is high, or potential end-users are far away, resulting in high transportation costs, onsite processing may be the most economical option. Several alternatives are generally available for treating sulphidic caustic, including:
Partial wet air oxidation of sodium sulphide to sodium thiosulfate.
Total wet air oxidation of sodium sulphide to sodium sulphate.
Deep neutralisation to a low pH to liberate H2S and mercaptans.
Thermal incineration.
Chemical oxidation.
Reusing sulphidic caustic in other areas of the plant, eg sweetening cracked gasoline. In this case, the value of the resultant caustic is enhanced due to increased cresylic content.
Routing used caustic solutions to the crude unit or sour water stripper.
Disposing of sulphidic caustic through outside company.