Catalytic oxidation of spent caustic

A process to oxidise spent caustic in an existing treatment plant by a catalytic route in the presence of air enhances the quality of wastewater releases

Vivek Rathore, Shalini Gupta, T S Thorat, P V C Rao, N V Choudary and G Biju
Bharat Petroleum Corporation Limited, India

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Article Summary

Caustic has been a mainstay of the oil refining industry since its earliest days because of its effectiveness in scrubbing organic, inorganic and naphthenic acidic components, especially mercaptans, H2S and phenols, from crude oil or its fractions.1-3 Once it has been used in the extraction of these acid components, the resulting dilute caustic stream becomes spent caustic. The generation of spent caustic is continually increasing as the processing of sour crude oil in refineries worldwide multiplies, and levels of generation are expected to rise significantly in the future. Fresh caustic’s effectiveness and low cost are the reasons for its widespread use. But inefficient use of alkali resources, followed by improper handling and inadequate disposal, has a growing effect on refinery margins.

Refinery spent caustic is recognised as a hazardous stream in view of a range of characteristics: it is corrosive (high pH); it contains toxic compounds such as H2S or phenol; and it imposes high levels of biological and chemical oxygen demand (BOD and COD) on natural water supplies.4,5 It also contains sulphides and mercaptans, which give rise to fouling or metallurgical damage in refinery equipment. Furthermore, there is no available route for converting spent caustic into a valuable product. Hence, it has the least potential for reuse within the refinery. Even worse, it is discarded at high cost.

Usually, a caustic stream extracts H2S, mercaptans and sulphides, and generates sodium salts, including sodium sulphide and sodium 
bisulphide. The disposal of these compounds in the open environment releases H2S, which has a strongly objectionable odour, so environmental agencies around the world have tightened regulations aimed at controlling its disposal. Adequate effluent treatment procedures are therefore required for its safe disposal, but commercially available disposal methods (for instance, oxidative detoxification using peroxide or ozonolysis) are costly and not environmentally beneficial. Consequently, there is a pressing need to develop an 
alternate, low-cost and environmental-friendly process for converting sulphides and mercaptans into water-soluble sulphate salts.

Oxidative detoxification of spent caustic and demercaptisation can be performed in the presence of molecular oxygen, but the rate of oxidation is found to be very low in the absence of a catalyst. However, it is also known that mercaptans and sulphides can be oxidised with air/oxygen in the 
presence of metal phthalocyanines used as catalysts. Phthalocyanines of metals, including cobalt, iron, manganese, molybdenum and vanadium, catalyse the oxidation reactions in an alkaline medium.6 Currently, metal phthalocyanine derivatives are used as homogeneous catalysts for liquid-liquid sweetening and alkali regeneration in the extraction of mercaptans from light petroleum distillates, including LPG, pentanes, light straight-run naphtha (LSRN) and light thermally cracked naphtha. They are also used as heterogeneous catalysts for the sweetening of other petroleum products such as heavy naphtha, FCC gasoline, ATF and kerosene. These catalysts are prepared by adsorptive impregnation of phthalocyanine solutions on porous supports (activated carbon, alumina, bauxites, silica gel, aluminium and magnesium oxides).8,9 The use of homogeneous catalysts in sulphide oxidation processes is economically feasible due 
to their low cost and ease of availability.

In the present work, a novel process has been developed, based on various laboratory trials, for the oxidation of sulphidic streams in the presence of metal phthalocyanine compounds, which act as a catalyst in the presence of air. Additionally, the effect of various process parameters, including temperature, pressure, catalyst loading, air flow rates and reaction times, has been investigated for process optimisation and more efficient operation. The performance of a refinery field trial for the new process is also discussed.

Spent caustic stream
The studies involved a spent caustic stream from BPCL’s Kochi refinery in India. Physical chemical properties and other characteristics were measured in the laboratory according to ASTM methods. The composition of spent caustic streams is highly variable and can be categorised into three groups, depending on origin and composition (see Table 1 and Figure 1).

Typical spent caustic effluent contains about 4–12% of sodium hydroxide (NaOH) by weight, although this could be as low as 1% or 2%. The composition in a typical refinery spent caustic stream is based on sulphides (0.5–4% as sulphur) and mercaptides (0.1–4%), but naphthenic spent caustic contains mostly naphthenic acids (as high as 15%, mostly in diesel) and  minimal sulphide. This stream has a tendency to foam vigorously and creates additional problems if not treated properly. Finally, a cresylic spent caustic stream is rich in phenols, cresols and other organic acids. Phenol content can be as high as 2000 ppm. The spent caustic stream contains sulphides, mercaptides and other contaminants (see Table 2, which characterises a typical spent caustic stream).

Laboratory studies
During the laboratory studies, a calculated amount of catalyst was loaded into a reactor column system operated at 50–90°C and atmospheric pressure. The spent caustic was pumped into the system by a metering pump. The spent caustic stream was characterised for sulphide and total mercaptans content by the ASTM method before being pumped into the reactor system.

Metal phthalocyanine catalysts
It is well known that the phthalocyanines of metals, including cobalt, nickel, iron, copper, molybdenum and vanadium, catalyse the oxidation of sulphides and mercaptans in an alkaline medium. However, metal phthalocyanines are not soluble in an aqueous medium, so their respective N-substituted derivates were prepared, as explained in the available literature.7,8 To compare and understand the effectiveness of various metal phthalocyanine derivatives as catalysts, experiments were performed at the same catalyst dosing (200 wppm) under similar reaction conditions of temperature (60°C) and air (50% excess).

It was found that cobalt phthalocyanine derivative works as an effective catalyst for both homogeneous and fixed-bed oxidation of sulphidic spent caustic solution. A level of sulphide conversion of more than 30% was achieved in the presence of a cobalt phthalocyanine derivative (see Figure 2). The derivative was therefore considered as an active catalyst for a study to optimise reaction parameters.

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