Safe Waste Caustic Disposal

Reprinted from Hydrocarbon Technology International

This article describes the available options for the outside disposal of refinery or petrochemical plant spent caustics. It presents process options for treating those same spent caustics in an environmentally acceptable manner for in-plant disposal.

F. J. Suarez, Merichem Company

The most effective approach to reducing the environmental hazards of spent caustic disposal is to minimize their production byway of caustic regeneration. This may be accomplished with Merichem's REGEN technology. Unfortunately, not all oil refinery or petrochemical plant spent caustics can be economically regenerated, and it must be disposed of in an environmentally acceptable manner with the plant effluent waters.

The most common non-regenerable spent caustics produced in these industries are those containing sodium sulphide, Na₂S sodium bisulphide, NaHS sodium carbonate, Na₂CO₃; sodium bicarbonate, NaHCO₃; sodium Naphthenates, RCOONa (naphthenic acids); and sodium cresylates, RONA (phenols, O, M, P cresol, xylenols).

Adverse Effects

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 several kilometers downstream of the discharging plant. Water can be rendered unsafe for human consumption or its taste can be altered to become obnoxious.

For these and other reasons, spent caustics are normally converted to a form that minimizes 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 sulphide, bisulphide, carbonate and bicarbonate make up one segregation; sodium naphthenates make another, and sodium cresylates make the third. For over 40 years approximately 100 plants, mainly oil refineries, around the world have disposed of their spent caustics through 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 sulphide spent caustic to the USA paper industry. Many other potential suppliers are too far away from these plants, and the freight costs of shipping these spent caustics to the USA make disposal through the company 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 sulphides and bisulphides, which do represent an environmental hazard.

The spent caustics predominantly continuing mixtures of these impurities are called sulphidic caustics. Sulphides and bisulphides 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 sulphidic spent caustics is through the paper industry. Some paper mill use sodium. sulphide and bisulphide as a digestion liquor instead of making it by purchasing caustic and sulfur and converting the two reactants to the sulphide. These paper mills are called Kraft mills and sodium sulphide solutions are instant liquor.

Of course the paper mill will impose quality specifications on the spent caustic such as a minimum Na to S ratio, a minimum Na plus S concentration. and maximum hydrocarbon content.

In-Plant Oxidation

If in-plant oxidation is chosen, it can be done in a MERICON system which is similar to the REGEN caustic regeneration process. This system will not be discussed because it is a catalytic process used mainly for sodium mercaptide oxidation. The MERICON process (Figure 1) differs from REGEN in that it is usually a non-catalytic design. The conversion of sodium sulphide and hydrosulphide to sodium thiosulphate is highly exothermic, and cooling equipment may or may not be required depending on the heat balance and effluent temperature requirements.

Sodium mercaptide are also often present in the sulphide caustic and must be taken into account in the oxidizer design. The chemical reactions are:

 

 

 

The RSSR compound is called disulphide oil and may be removed from the treated brine as a separate liquid phase or by solvent washing with another hydrocarbon. The oxidized caustic should be pH adjusted to neutral before being further processed or released by the plant.

Sodium naphthenates and sodium cresylates contain organics which can be biologically destroyed if fed to a biological treatment system at a controlled slow rate and contaminant level. Proper micro-organisms 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. These caustics are not easily oxidized and are typically processed differently than sulphidic caustics.