Waste Caustic Disposal Processes Using MERICON^SM Technology

Unfortunately, not all oil refinery or petrochemical plant spent caustics can be economically regenerated for reuse as described in our Technical Bulletin RG-482. The most common non-regenerable spent caustics are those containing:

• Sodium Sulfide (Na₂S)
• Sodium Bisulfide (NaHS)
• Sodium Carbonate (Na₂CO₃)
• Sodium Bicarbonate (NaHCO₃)
• Sodium Naphthenates (RCOONa (Naphthenic Acids))
• Sodium Cresylates (RONa (Phenols, O, M, P Cresol, Xylenols))

If the above spent caustics are discharged directly into the plant effluent waters, they place a burdensome oxygen demand, both chemical and biological, on the receiving streams. Fish and other aquatic populations are adversely affected and sometimes completely eliminated for kilometers downstream of the discharging plant. Water can be rendered unsafe for human consumption or its taste can be altered to become obnoxious to humans. For these and other reasons, spent caustics are normally converted to a form that minimize these problems, especially when they are further treated in modern biological waste water treating systems.

The spent caustics are normally segregated into three types. The sodium sulfide, bisulfide, carbonate, and bicarbonate make up one segregation, sodium naphthenates make another, and sodium cresylates make the third (Figure 1). For over forty years approximately one hundred (100) plants, mainly oil refineries, around the world dispose of their spent caustics via Merichem's two USA processing plants. Merichem recovers and sells the naphthenic acids and cresylic acids to other chemical companies around the world and sells sodium sulfide spent caustic to the USA paper industry. Many other potential suppliers are too far away from these USA plants and freight costs of shipping these spent caustics to the USA make disposal through Merichem too expensive. Disposal within the plant is then a more economical alternative.

The refiner or petrochemical manufacturer can ignore the sodium carbonate and bicarbonate content of the spent caustic since they are inorganic chemicals that present no environmental hazards other than high pH. Unfortunately, caustics containing carbonates usually also contain sulfides and bisulfides, which do represent an environmental hazard. Sulfides and bisulfides represent a large immediate oxygen demand and, therefore, require oxidation and neutralization or neutralization and stripping prior to their release to the plant's water treating facilities.

A good alternative way to dispose of sulfidic spent caustics is via the paper industry. Some paper mills use sodium sulfide/bisulfide as a digestion liquor instead of making it by purchasing caustic and sulfur and converting the two reactants to the sulfide. These paper mills are called kraft mills and sodium sulfide solutions are "instant liquor". Of course the paper mill will impose quality specifications on the spent caustic such as minimum Na to S ratio, minimum Na plus S concentration, and maximum hydrocarbon content.

If in-plant oxidation is chosen, it can be done in a system similar to our caustic regeneration system, which is discussed in Technical Bulletin RG-482, except it is usually a non-catalytic design (Figure 2). The conversion of sodium sulfide and hydrosulfide to sodium thiosulfate is highly exothermic and cooling equipment may or may not be required depending on the heat balance and effluent temperature requirements. The chemical reactions are:

 

 

The oxidized caustic needs to be pH adjusted to neutral before being further processed or released by the plant.

Sodium naphthenates and sodium cresylates contain organics that can be biologically destroyed if fed to the biological treatment system at a controlled slow rate and contaminant level. Proper microorganisms must be grown specifically for these wastes. Biological action is slower at low temperatures and the design of the system must take winter conditions into account.

Another way of handling all types of spent caustics is acid neutralization (Figure 2). Sulfuric acid and hydrochloric acid are suitable. Spent caustics can be treated in this manner to low pH to release H₂S and mercaptans which can be incinerated or processed in a sulfur recovery plant. The chemical reactions for these neutralizations are:

 

 

 

 

The cresylic acids and naphthenic acids generated in (5) and (6) above float to the top of the acidic sodium sulfate brine and can be skimmed off and/or solvent washed if low organic acid concentration is required in the sodium sulfate brine effluent. The organic acids can be sold to companies such as Merichem or reprocessed back through the plant to increase product yields. The pH of the sodium sulfate brine is adjusted to neutral before being sent to the water treating facilities to prevent shocking the system (Figure 2). This is especially important in biological type treating facilities.

One important design consideration for all the spent caustic processing systems discussed above is metallurgy since corrosive environments are usually encountered. Let Merichem's forty (40) years of in-house experience and know-how help you improve your plant's spent caustic disposal management programs.