Article pubs.acs.org/EF
Microwave-Assisted Oxidative Desulfurization of Sour Natural Gas Condensate via Combination of Sulfuric and Nitric Acids Ehsan Moaseri, Akbar Shahsavand,* and Behnaz Bazubandi Chemical Engineering Department, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad 9177948974, Iran ABSTRACT: Sulfur components are traditionally considered as undesirable contaminants of liquid hydrocarbon fuels. Many countries have passed strict regulations to keep sulfur content of different fuels as low as possible. On a daily basis, over 3000 barrels of sour gas condensate are produced in Khangiran natural gas processing plant with an annual growth rate of around 16% in the last six years. In pursue of our previous researches, a mixture of nitric and sulfuric acids is introduced as a novel oxidative desulfurization agent. Also, the effect of microwave radiation was studied on desulfurization efficiency of proposed agent. Both hydrogen sulfide and mercaptans are totally eliminated from sour gas condensate and its total sulfur content is severely reduced from 8500 ppm to less than 300 ppm.
1. INTRODUCTION Sulfur compounds are usually known as one of the most noxious and notorious petroleum contaminants.1,2 More than 195 types of sulfur compounds are recognized in crude oil such as hydrogen sulfide, organic sulfides and disulfides, benzothiophene, dibenzothiophene, and their alkylated derivatives.3 During the combustion, these compounds release sulfur oxides (SOx) and sulfate particles into atmosphere, which can lead to severe air pollution problems and acid rain falls.4 Due to the depletion of sweet oil and gas reservoirs, oil and gas producers around the globe are forced to utilize sour oil and gas wells. This issue can lead to excessive release of SOx into atmosphere. Growing concerns on environmental issues enforce the governments to pass restriction rules for permissible sulfur content thresholds. Legislation of 15 and 10 ppmw as the acceptable standard levels of sulfur compounds in diesel fuels of the US and EU, respectively, represents the efforts in pursuit of ultraclean fuels.5,6 Hence, desulfurization plants are becoming an inseparable and essential part of refining process. At the present, hydrodesulfurization (HDS) technique is almost the dominant desulfurization process, which is widely used all around the world.7 This process is based on surface adsorption of almost all sulfur compounds on appropriate metallic catalyst surfaces. Subsequently hydrogenation of the corresponding sulfur compound takes place under high partial pressure of hydrogen. Thus, the sulfur compound is converted to corresponding hydrocarbon and hydrogen sulfide (H2S) gas as an undesired byproduct. Recent studies have revealed that the sulfur removal efficiency of HDS process depends on the chemical nature (structure and bulkiness) of the sulfur compounds involved in hydrodesulfurization.8 While HDS is successful in desulfurization of aliphatic thiols, mercaptans, thioethers, sulfides, disulfides, and thiophene, this process is less effective in removal of alkylated aromatic sulfur compounds, such as dibenzothiophene (DBT) and its derivatives.9,10 In practice, the sulfur compound should be initially adsorbed on the metal catalyst surface in order to be hydrogenated. Evidently, steric hindrance of alkylated aromatic sulfur compounds restricts appropriate adsorption of these compounds on the catalytic surface. Although more severe © 2013 American Chemical Society
operational conditions have been proposed to enhance the efficiency of HDS process for these sterically hindered sulfur compounds, this approach is usually rejected due to sharp increase in both investment and operational costs.11 Oxidative desulfurization (ODS) process can be considered as an alternative or complementary process for HDS method.12,13 The ODS process consists of two following consecutive steps: (a) Initially, the sulfur compounds are oxidized to their corresponding sulfoxides or sulfones by an oxidative agent. (b) Afterward, highly polarized sulfoxides or sulfones are extracted by an appropriate polar solvent or adsorbed on high-capacity adsorbents. Compared to HDS process, the ODS technique has attracted special attentions for its high removal efficiency of alkylated aromatic sulfur compounds in liquid phases under mild temperatures and pressures. Also, reasonable capital investment and operational costs are two other advantages of this process. Beside these, ODS process does not require any hydrogen, and it can be accomplished in the absence of any hydrogen source. Various oxidative agents have been reported to be used for desulfurization of petroleum cuts. Some of these processes are based on application of single oxidants, such as hydrogen peroxide,14 nitric acid (HNO3),15−18 organic hydroperoxides,19 molecular oxygen,20 ozone,21 air,22 sulfuric acid (H2SO4),23 potassium ferrate,24 Fenton’s reagent.25 A number of other ODS processes employ a combination of two agents. The first one acts as the oxidant and the second agent catalyzes the ODS reactions. The recruitment of hydrogen peroxide with formic, acetic, nitric, and phosphoric acids26−29 are the popular examples of such binary agents. Moreover, many solid basic catalysts have been reported for enhancement of oxidation yield via hydrogen peroxide. Alumina-supported polymolybdates,30 vanadium(V) oxide/aluminum oxide (V 2 O 5 /Al 2 O 3 ), 31 Received: September 16, 2013 Revised: December 30, 2013 Published: December 30, 2013 825
dx.doi.org/10.1021/ef4018515 | Energy Fuels 2014, 28, 825−831
Energy & Fuels
Article
vanadium(V) oxide/titanium dioxide (V2O5/TiO2),32 cobaltmolibden/aluminum oxide (Co−Mo/Al2O3),33 molibden/ aluminum oxide (Mo/Al2O3)34 are mostly used as the solid catalysts among many others. In addition to these liquid and solid phase catalysts, highenergy radiations may be also employed to improve the sulfur removal efficiency through activation of oxidation process. Plasma radiations for molecular oxygen,35 UV irradiation for activation of hydrogen peroxide36 and microwave-assisted ODS37 are the examples of such high-energy radiation utilization. In our previous research, we reported that the addition of concentrated H2SO4 to sour condensate is able to achieve 90% desulfurization efficiency.38 Due to its reasonable capital and operational cost, the H2SO4 is nominated as the appropriate choice for successful ODS process. In the present article, it has been investigated the combination of H2SO4 with HNO3, as another strong oxidant, in pursuit of even deeper desulfurization and achievement of ultraclean fuels. To the best of our knowledge, combination of H2SO4 and HNO3 has not been reported to be used for desulfurization of petroleum cuts. Effect of different operational conditions such as HNO3 to H2SO4 volumetric ratio (NSR), oxidative agents to sour condensate volumetric ratio (OCR) and process temperature has been studied to predict the optimal conditions. Moreover, microwave radiations have been also used during the desulfurization process to improve the sulfur removal efficiency of sour natural gas condensates. A novel set up has been proposed for microwave radiation assisted ODS that can simultaneously control the power output of radiations and the temperature of the mixture.
Table 1. Characteristics of Khangiran Gas Processing Plant Sour Condensate property
method
Sp.Gr. 60/60 °F TSC mercaptans H2S acidity viscosity @ 100 F water content molecular weight aromatics olefins naphthenic paraffinic calorific value, gross
unit
amount
D-4052 D-1266 ppm (wt) D-3227 ppm (wt) D-1159 ppm (wt) D-664 mg KOH/g D-445 c.St. D-4928 Wt% IP-86 g/mol D-1319 vol % D-1319 vol % D-1319 vol % D-1319 vol % D-240 kcal/kg chemical analysis
0.7959 8210 456 150 0.19 1.143
