Elsevier

Catalysis Today

Volume 141, Issues 1–2, 15 March 2009, Pages 152-156
Catalysis Today

Synthesis, characterization and catalytic activity of cobalt phthalocyanine tetrasulphonamide in sweetening of LPG

https://doi.org/10.1016/j.cattod.2008.04.009Get rights and content

Abstract

Cobalt phthalocyanine tetrasulphonamide was synthesized by reacting cobalt phthalocyanine with chlorosulphonic acid at 130–135 °C followed by addition of thionyl chloride to convert free sulphonic acid to sulphonyl chloride and subsequently amidation with ammonia. It was isolated by acidifying the reaction mixture with hydrochloric acid followed by filtration. It was characterized by elemental IR and FAB mass spectral analysis. The activity of cobalt phthalocyanine tetrasulphonamide catalyst for extractive sweetening of LPG was evaluated by studying mercaptide oxidation using ethyl mercaptan as model sulfur compound in glass column. The stability of the catalyst was evaluated by studying liquid–liquid sweetening in a batch reactor using hexane thiol as a model compound and petroleum ether as an inert solvent. The performance of this catalyst with respect to activity and stability was found better than the commercial catalyst being used currently in the refineries. Commercial trial run of this catalyst has been successfully conducted for 4 months in FCC LPG Merox unit of Bharat Petroleum Corporation Limited (BPCL), Mumbai and the performance was found better than commercial catalyst. Another trial run of the catalyst has been conducted in one of the LPG Merox units at Reliance Industries Limited (RIL), Jamnagar for 8 months and the performance has been found to be excellent with less catalyst consumption than commercial catalyst.

Introduction

The presence of mercaptans in the petroleum products like LPG, naphtha, gasoline, kerosene, ATF etc. is highly undesirable due to their foul odour and highly corrosive nature. Although there are several processes known for the removal of mercaptans from petroleum products, the most common practice is to oxidize the mercaptans present to less deleterious disulphides with air in the presence of a catalyst. Generally, the lower mercaptans present in LPG, pentanes, light straight run naphtha (LSRN) and light thermally cracked naphtha are first extracted by alkali solution and then oxidized to disulphides with air in the presence of a catalyst. The disulphides, being insoluble in alkali solution are separated out from the top and the regenerated alkali is reused for extraction. In the liquid–liquid sweetening the lower mercaptans are present in petroleum products like pentanes, LSRN, cracked naphtha etc. are converted to disulphides by direct oxidation with air in the presence of alkali solution and catalyst. The higher molecular weight mercaptans present in petroleum products like heavy naphtha, FCC gasoline, ATF and kerosene are oxidized to disulphides with air in a fixed bed reactor containing catalyst impregnated on a suitable support like activated carbon.

Phthalocyanines of the metals like cobalt, iron, manganese, molybdenum and vanadium catalyze the oxidation of mercaptans to disulphides in alkaline medium [1]. Among these cobalt and vanadium phthalocyanines are preferred. As the metal phthalocyanines are not soluble in aqueous medium, for improved catalytic activity their derivatives like sulphonated and carboxylated metal phthalocyanines are used as catalysts for sweetening of petroleum fractions. The use of cobalt phthalocyanine (CoPc) monosulphonate as the catalyst in the fixed bed sweetening of various petroleum products and cobalt phthalocyanine disulphonate [2] tetrasulphonate [3] and the mixture thereof [4] as catalysts for liquid–liquid sweetening/alkali regeneration in mercaptan extraction of light petroleum distillates have been reported. The use of phenoxy substituted cobalt phthalocyanine as sweetening catalyst [5], cobalt and vanadium chelates of 2,9,16,23-tetrakis(3,4-dicarboxybenzoyl)phthalocyanine as effective catalyst for homogeneous mercaptan oxidation [6], [7] and cobalt/vanadium chelates of tetrapyridinoporphyrazine as active catalysts for sweetening of sour petroleum distillates [8] have also been reported.

Cobalt phthalocyanine disulphonate a commonly used catalyst in sweetening of LPG and light petroleum fractions by liquid–liquid mercaptan extraction and alkali regeneration is extremely dusty in the dry fine powder form and causes handling problem. To overcome this problem cobalt phthalocyanine disulphonate is admixed with water and commonly used as slurry. However, with insufficient mixing the cobalt phthalocyanine disulphonate precipitates out from the slurry. Moreover, even if the slurry is mixed sufficiently, it retains the cobalt phthalocyanine disulphonate in suspension for a particular length of time only, beyond which the slurry becomes extremely viscous and may form gel, making it very difficult to remove the material from packaging. Cobalt phthalocyanine tetrasulphonate, on the other hand, is highly soluble in water and its use can eliminate precipitation and gel forming problems associated with the use of cobalt phthalocyanine disulphonate. However, it is reported that cobalt phthalocyanine tetrasulphonate has lower catalytic activity than cobalt phthalocyanine disulphonate [9].

During our investigations on development of new superior sweetening catalysts for extractive sweetening of LPG, our attention was particularly drawn by amide group, which has a peculiar property of increasing the solubility of organic compounds in aqueous alkaline solution. The essential requirement of the metal phthalocyanine sweetening catalysts for LPG is their high solubility in aqueous alkaline solution. Therefore, the use of cobalt phthalocyanine sulphonamides (Fig. 1) was explored as a new sweetening catalyst for extractive sweetening of LPG.

In this paper we describe synthesis, characterization and evaluation of catalytic activity of cobalt phthalocyanine tetrasulphonamide catalyst (IIP Cat) in extractive sweetening of LPG and its stability for liquid–liquid sweetening of synthetic feed consisting of hexane thiol (C6H13SH) in petroleum ether (60–80 °C). The performance of this catalyst in the commercial plant trial run is also discussed.

Section snippets

Experimental

Cobalt phthalocyanine used was obtained from M/s Lona Industries Ltd. Mumbai. All the chemicals and solvents used were LR grade and purchased from standard chemicals suppliers. Infrared (IR) spectra of the catalyst were recorded on PerkinElmer 1760X FTIR spectrophotometer, in KBr pellet qualitatively. Fast atom bombardment (FAB) mass spectra of the catalyst were recorded on a JEOL SX 102/DA 6000 mass spectrometer/data system using argon/xenon (6 kV,10 mA) as FAB gas in m-nitro benzyl alcohol

LPG sweetening

Sweetening of LPG involves extraction of the mercaptan present in it by aqueous alkaline solution followed by their oxidation to disulphide with air in presence of metal chelate like cobalt phthalocyanine (Co2+) as catalyst. The mechanism of the reaction as proposed by Wallace et al. [12] is shown in Scheme 1.

In commercial sweetening process the catalyst is injected to 12–14% aqueous sodium hydroxide solution, which kept on circulating in the system consisting of extractor, oxidizer and

Commercial plant trial run

Commercial trial run of the cobalt phthalocyanine tetrasulphonamide catalyst was successfully undertaken in one of the FCC LPG Merox units of Bharat Petroleum Corporation Limited (BPCL), Mumbai for 4 months and the catalyst showed better performance than the commercial catalyst. Another commercial trial run of this catalyst for 8 months has just completed in one of the LPG Merox units of Reliance Industries Limited (RIL), Jamnagar and the performance has been found to be excellent with less

Conclusion

Cobalt phthalocyanine tetrasulphonamide catalyst prepared by chlorosulphonation of cobalt phthalocyanine with chlorosulphonic acid followed by amidation with ammonia has been found a potential catalyst for extractive sweetening of LPG. Laboratory evaluations have shown that while its activity is better than the commercial catalyst currently being used in the Indian refineries, its stability is equivalent. Trial runs conducted at BPCL, Mumbai and RIL refinery, Jamnagar have established the

Acknowledgements

The authors are grateful to Bharat Petroleum Corporation Limited, Mumbai for financial support and conducting trial run of the catalyst in their refinery. Authors are also highly thankful to Reliance Industries Limited, for performance evaluation of the indigenously developed catalyst in their commercial plant.

References (12)

  • B. Basu et al.

    Catal. Rev.-Sci. Eng.

    (1993)
  • R.R. Frame, US 425022...
  • D.H.J Carlson, P. Urban, US 2622763...
  • D.H.J Carlson, P. Urban, US 4248694...
  • Institute of Kinetics and Catalysis, Sofia Ger. Offen. 3816952...
  • W. Clifford, Ger. Offen. 2757476...
There are more references available in the full text version of this article.

Cited by (0)

View full text