Microwave-Assisted Oxidative Desulfurization of Sour Natural GasCondensate via Combination of Sulfuric and Nitric AcidsEhsan 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. Manycountries have passed strict regulations to keep sulfur content of different fuels as low as possible. On a daily basis, over 3000barrels 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 oxidativedesulfurization agent. Also, the effect of microwave radiation was studied on desulfurization efficiency of proposed agent. Bothhydrogen sulfide and mercaptans are totally eliminated from sour gas condensate and its total sulfur content is severely reducedfrom 8500 ppm to less than 300 ppm.
1. INTRODUCTIONSulfur compounds are usually known as one of the mostnoxious and notorious petroleum contaminants.1,2 More than195 types of sulfur compounds are recognized in crude oil suchas hydrogen sulfide, organic sulfides and disulfides, benzothio-phene, dibenzothiophene, and their alkylated derivatives.3
During the combustion, these compounds release sulfur oxides(SOx) and sulfate particles into atmosphere, which can lead tosevere air pollution problems and acid rain falls.4 Due to thedepletion of sweet oil and gas reservoirs, oil and gas producersaround 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 sulfurcontent thresholds. Legislation of 15 and 10 ppmw as theacceptable standard levels of sulfur compounds in diesel fuels ofthe US and EU, respectively, represents the efforts in pursuit ofultraclean fuels.5,6 Hence, desulfurization plants are becomingan inseparable and essential part of refining process.At the present, hydrodesulfurization (HDS) technique is
almost the dominant desulfurization process, which is widelyused all around the world.7 This process is based on surfaceadsorption of almost all sulfur compounds on appropriatemetallic catalyst surfaces. Subsequently hydrogenation of thecorresponding sulfur compound takes place under high partialpressure of hydrogen. Thus, the sulfur compound is convertedto corresponding hydrocarbon and hydrogen sulfide (H2S) gasas an undesired byproduct. Recent studies have revealed thatthe sulfur removal efficiency of HDS process depends on thechemical nature (structure and bulkiness) of the sulfurcompounds involved in hydrodesulfurization.8 While HDS issuccessful in desulfurization of aliphatic thiols, mercaptans,thioethers, sulfides, disulfides, and thiophene, this process is lesseffective in removal of alkylated aromatic sulfur compounds,such as dibenzothiophene (DBT) and its derivatives.9,10 Inpractice, the sulfur compound should be initially adsorbed onthe metal catalyst surface in order to be hydrogenated.Evidently, steric hindrance of alkylated aromatic sulfurcompounds restricts appropriate adsorption of these com-pounds on the catalytic surface. Although more severe
operational conditions have been proposed to enhance theefficiency of HDS process for these sterically hindered sulfurcompounds, this approach is usually rejected due to sharpincrease in both investment and operational costs.11
Oxidative desulfurization (ODS) process can be consideredas an alternative or complementary process for HDSmethod.12,13 The ODS process consists of two followingconsecutive steps:
(a) Initially, the sulfur compounds are oxidized to theircorresponding sulfoxides or sulfones by an oxidativeagent.
(b) Afterward, highly polarized sulfoxides or sulfones areextracted by an appropriate polar solvent or adsorbed onhigh-capacity adsorbents.
Compared to HDS process, the ODS technique has attractedspecial attentions for its high removal efficiency of alkylatedaromatic sulfur compounds in liquid phases under mildtemperatures and pressures. Also, reasonable capital investmentand operational costs are two other advantages of this process.Beside these, ODS process does not require any hydrogen, andit 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 arebased on application of single oxidants, such as hydrogenperoxide,14 nitric acid (HNO3),
potassium ferrate,24 Fenton’s reagent.25 A number of otherODS processes employ a combination of two agents. The firstone acts as the oxidant and the second agent catalyzes the ODSreactions. The recruitment of hydrogen peroxide with formic,acetic, nitric, and phosphoric acids26−29 are the popularexamples of such binary agents. Moreover, many solid basiccatalysts have been reported for enhancement of oxidation yieldvia hydrogen peroxide. Alumina-supported polymolybdates,30
vanadium(V) oxide/aluminum oxide (V2O5/Al2O3),31
Received: September 16, 2013Revised: December 30, 2013
aluminum oxide (Mo/Al2O3)34 are mostly used as the solid
catalysts among many others.In addition to these liquid and solid phase catalysts, high-
energy radiations may be also employed to improve the sulfurremoval efficiency through activation of oxidation process.Plasma radiations for molecular oxygen,35 UV irradiation foractivation of hydrogen peroxide36 and microwave-assistedODS37 are the examples of such high-energy radiationutilization.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 andoperational cost, the H2SO4 is nominated as the appropriatechoice 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 ofultraclean fuels. To the best of our knowledge, combination ofH2SO4 and HNO3 has not been reported to be used fordesulfurization of petroleum cuts. Effect of different operationalconditions such as HNO3 to H2SO4 volumetric ratio (NSR),oxidative agents to sour condensate volumetric ratio (OCR)and process temperature has been studied to predict theoptimal conditions. Moreover, microwave radiations have beenalso used during the desulfurization process to improve thesulfur removal efficiency of sour natural gas condensates. Anovel set up has been proposed for microwave radiationassisted ODS that can simultaneously control the power outputof radiations and the temperature of the mixture.
2. MATERIALS AND METHODSSimilar to our previous work, the sour natural gas condensate wasobtained from the Khangiran sour gas processing plant. The Khangiranrefinery has been located in northeast of Iran and produces around 80thousand metric tons of sour gas condensate in 2013. The productionrate of the sour condensate is anticipated to increase sharply due to thedecrease of Mozdouran reservoir pressure, which pushes theproduction pressure−temperature (PT) line into the sour gas phaseenvelope.2.1. Characteristics of Sour Natural Gas Condensate. The
detailed physical and chemical properties of the sour condensate usedin various ODS experiments are presented in Table 1. As expected, thesour condensate contains considerable amounts of sulfur compounds,classified into three categories of H2S, mercaptans (thiols), and totalsulfur content (TSC), based on their elimination priorities. Evidently,these high amounts of sulfur compounds will cause several operationalproblems and hazardous environmental issues.2.2. Reagents and Solvents.Merck H2SO4 (98 wt %) and HNO3
(63 wt %) was used in all experiments. Distilled water with 10−6 Ω−1
cm−1 conductivity was used in all tests for post wash purposes. Allchemicals were used as received unless stated otherwise.2.3. Sulfur Content Analysis. All treated condensates were
sampled and analyzed for H2S, mercaptans and TSC by ASTMmethods of D-1159, D-3227 and D-1266, respectively, by Khangirangas processing plant laboratory.2.4. ODS Experimental Procedure. The ODS experiments were
performed by treating sour condensate with oxidative agents of H2SO4or combination of H2SO4 and HNO3, under different OCRs, variousNSRs and several mixing times and stirring intensities.For each operation, 100 mL sour condensate has been sampled and
treated at ambient pressure. The sour condensate was mixed with theoxidative agent in a three-neck flat flange reaction vessel undernitrogen atmosphere. After each treatment, the condensate wasseparated from oxidative agent via separating funnel and washed by
500 mL distilled water to ensure the removal of residual salts andexcess oxidative agents remaining inside the treated condensate. Theintensity of stirring and mixture temperature were controlled via atemperature conditioned magnetic stirring system. Since desulfuriza-tion reactions are all exothermic, the temperature of all mixtures wasmaintained at 80 °C and the effect of temperature variations ondesulfurization efficiency was not studied.
3. EXPERIMENTAL SETUP FOR MICROWAVERADIATION
To study the effect of high energy radiations on sulfur removalefficiency, microwave radiations with different output powers wereapplied to sour condensate during the oxidation process using aspecially designed microwave oven. The oven was equipped with aPID controller (model: TNZ4SSeries) in order to maintain themixture temperature in the desired output power of radiations. Inother words, the radiations were not continuous and the controllerswitched the power on and off to maintain the temperature around theset-point. The controller was appropriately tuned with rise time of lessthan 10 s and overshoot of around 15%. A three-neck flatpolytetrafluoroethylene vessel was constructed as the reactor vesseland equipped with a thermocouple (up to 200 °C) and an overheadstirrer (300 rpm). For safety purposes, all experiments were performedunder nitrogen atmosphere and for comparison purposes the ODSreaction temperature was set at 80 °C by the PID controller.
4. RESULTS AND DISCUSSIONSThe following sections represent the collected results forseveral desulfurization experiments obtained by using differentoxidative agents in the absence or presence of microwaveradiations.
4.1. Treatment via H2SO4/HNO3 Solution in theAbsence of Microwave Radiation. Initially, the desulfuriza-tion of sour natural gas condensate was carried out withdifferent combinations of H2SO4 and HNO3 as oxidative agentsin the absence of microwave radiation. Several OCRs (0.5, 1,1.33, and 2) with NSRs of 0, 0.3, 1, and 3 were used underagitation times of 2, 5, 10, 15, and 20 min. The effect of H2SO4concentration on desulfurization efficiency was not assessedbecause our previous study38 revealed that only concentrated
Table 1. Characteristics of Khangiran Gas Processing PlantSour Condensate
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H2SO4 provides satisfactory desulfurization efficiency. Theconducted experiments indicated that the water to treatedcondensate volume ratio of 5:1 was optimal, which completelyremoved the oxidized components and extraneous acids fromthe organic phase. The pH of the treated condensates wasaround 7 after such washing process.Table 2 illustrates the amount of residual sulfur compounds
remained in the treated condensates in terms of H2S,
mercaptans and TSC for various runs and in the absence ofmicrowave radiation. As it can be seen, H2S and mercaptans arecompletely removed after all treatment except when theagitation time is very low (2 and 5 min). Therefore, effectiveeliminations of H2S and mercaptans can be achieved by usingsufficient treatment time with all of these combinations ofoxidative agents.Figure 1A represents the variations of TSC of treated
condensates with NSR at different OCRs. As expected, the TSCreduces by increasing the OCR. Although the TSC of thetreated condensate may be further decreased by using higherOCRs, the OCR value of around 2 is recommended based oneconomical considerations. It should be noted that at the OCRof 0.5, the minimum TSC of treated condensate is about 1100ppm. On the other hand, at OCR = 2, the minimum value ofTSC is around 600 ppm, which is much lower than theprevious one. Many literatures have stated that H2SO4 caneffectively desulfurize simple structure sulfur compounds suchas H2S, mercaptans, sulfides, disulfides, and even thiophenes;39
however, this reagent is not capable of oxidizing alkylatedaromatic sulfur compounds, such as DBT. Hence, the remained700 ppm of TSC in the treated condensate with H2SO4 mainlyconsists of highly alkylated aromatic sulfur compounds.
As can be seen, plot of TSC of treated condensates versusNSR for different values of OCR shows a relatively distinctminimum for all four curves at NSR = 0.3. Also, the TSC oftreated condensates decreases for all values of OSC by raisingthis ratio from 0 to 0.3. Thus, high NSRs (higher than 0.3) leadto ineffective desulfurization of sour condensate. This resultclearly illustrates that the NSR plays a crucial role in effectivedesulfurization of sour condensate.The reason behind the effectiveness of the HNO3/H2SO4
solution is the high standard reduction potential of nitroniumcation produced during mixing of HNO3 with H2SO4 whichprovides larger oxidative tendency. As stated in literature, thestandard reduction potentials of H2SO4 and HNO3 are 0.16 and0.957, respectively.40,41 Addition of HNO3 to H2SO4 leads tothe formation of nitronium cation (NO2
+) based on thefollowing reaction:
+ → + ++ + −HNO 2H SO NO H O 2HSO3 2 4 2 3 4 (1)
The nitronium cation is an extremely strong oxidation agentwith the standard reduction potential of +1.6 V at pH = 7.0.42 Ithas been reported that this cation is able to oxidized evenstrictly hindered and stable sulfur compound such as benzenering with an attached sulfur.43,44 For instance, highly refractorysulfur compounds such as thiophene and DBT can be oxidizedby nitronium cation based on the reactions shown in Figure 2.Hence, the generation of such strong cation can further
enhance the desulfurization efficiency of H2SO4 through bothimproving the desulfurization reaction efficiency of H2SO4 andoxidizing the sulfur compounds resistant to H2SO4. In addition,it should be emphasized that, based on the reaction 1, thesolution with NSR = 0.3 has the highest concentration ofnitronium cation compared to other NSRs that has been used.Furthermore, H2SO4 is a strong desulfurization agent fororganic sulfur compounds (OSC),39 and run no. 4 of Table 2depicts that desulfurization of sour condensate with just H2SO4reduces more than 90% of TSC. On the other hand, HNO3shows strong desulfurization tendency for inorganic sulfur
Table 2. Experimental Results for Various Runs in theAbsence of Microwave Radiation
Figure 1. Variations of TSC of treated condensates in the absencemicrowave radiation: (A) with NSR at different OCRs, (B) withagitation times at different NSRs.
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compounds.45,46 Since OSC comprises most of the TSC of sourcondensate, the nitric acid should be used as a limiting reactantin the reaction of nitronium production.Figure 1B shows the effect of agitation time on the TSC of
treated condensates by H2SO4 and combination of HNO3/H2SO4 at optimal volumetric ratio of 0.3. Both of theseoxidative solutions provide similar behaviors and the slope ofTSC gradually decreases by increasing the agitation time andasymptotically approaches to less than 1000 ppm at 15 min.Evidently, 15 min can be considered as the optimum agitationtime for successful desulfurization of sour condensate for bothoxidative agents. Evidently, considering the asymptotic natureof all corresponding curves, longer agitations are not supposedto further reduce TSC and based on both technical andeconomic considerations higher agitations are not advised.Considering the obtained results, the effective parameters onthe desulfurization efficiency in the absence on microwaveradiation can be arranged based on order of effectiveness asfollows: agitation time, OCR and NSR. Obviously, the use ofHNO3/H2SO4 solution provide slightly better desulfurization(600 ppm) result at optimal agitation time and optimum OCRvalue, compared to concentrated H2SO4 solution (700 ppm).Agitation intensity of sour condensate with oxidative agent is
another important operating variable. By mixing aqueous andorganic phases, droplets of immiscible organic phase in differentsizes are produced inside aqueous phase. Evidently, increasingagitation intensity may lead to more efficient desulfurization byconstructing smaller dispersed phase droplets, which willprovide more mass transfer area between aqueous and organicphases. On the other hand, employment of excessive agitationintensities may create stable emulsions; the formed dropletstend to coalesce with each other due to high interfacial energyand consequently cream above the aqueous phase. However,based on Stoke’s law, the rate of creaming is influenced by the
diameter of the droplet size; smaller is the diameter of thedroplet, lesser will be the rate of creaming. Therefore, reductionin droplet size, caused by higher agitation intensities, lead tosignificant reducing the creaming rate and eventuallypermanent confinement of these droplets in the aqueousphase, known as stable emulsion.47 Elimination of such stableemulsions necessitates costly operations such as the use ofcertain chemical de-emulsifiers or application of some externalelectric or magnetic fields.48−51 Figure 3 shows the images ofcondensates treated by concentrated H2SO4 with differentagitation intensities. As can be seen, the agitation intensities of100 and 300 rpm provide a homogeneous single phase solution.Using higher intensities (500 and 700 rpms) produceconsiderable amounts of stable emulsions. Based on theseresults, operating with agitation intensity around 300 rpm isrecommended to prevent emulsion formation.
4.2. Microwave-Assisted ODS of Sour Condensate.Microwave radiation was applied during the ODS treatment ofsour condensate by H2SO4 and the combination of HNO3/H2SO4. The microwave-assisted ODS experiments were carriedout in three different microwave output powers (300, 500, and700 W), with retention times of 2, 5, 10, 15, and 20 min atOCR of 2 and NSR of 0 and 0.3 (see Table 3).Figure 4A shows that the desulfurization efficiency is
improved with increasing the microwave radiation outputpower for both oxidative agents. Also, it seems that themicrowave radiation has more influence on the efficiency ofODS treatment with HNO3/H2SO4. Figure 4B represents theeffect of retention time on the TSC of condensates treated bymicrowave-assisted ODS with H2SO4 and the combination ofHNO3/H2SO4. Although the microwave radiation improves thedesulfurization efficiency, the trend of the time dependencecharts are still comparable with the curves obtained in theabsence of microwave radiation. The obtained results indicate
Figure 2. Oxidation of refractory sulfur components by nitronium cation: (A) reaction with thiophene and (B) reaction with DBT.
Figure 3. Formation of stable emulsion in condensate treated by H2SO4 by increase of agitation intensity: (A) 100 rpm, (B) 300 rpm, (C) 500 rpm,(D) 700 rpm.
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that the ODS efficiency of sour condensate by the combinationof H2SO4 and HNO3 can be improved up to 30% in thepresence of microwave radiations. To complete the order ofeffectiveness of operational parameters in the ODS process inthe presence of microwave radiations, parameters can bearranged as follows: agitation time, OCR, NSR, and microwaveradiation power.Interestingly, microwave-assisted ODS via HNO3/H2SO4
solutions could effectively desulfurize the sour condensatewith considerable removal efficiency of as high as 97%. It seemsthat most of the remaining sulfur compounds (280 ppm)should be sterically hindered or highly stable (refractory) sulfurcomponents, such as DBT, benzo naphtho thiophene, benzophenanthro thiophene, and pyridyl benzothiophene, whicheven nitronium cation cannot effectively oxidize them.The following somehow different points of view may explain
the effectiveness of microwave radiation in enhancement ofODS efficiency. The first mechanism postulates that the
microwave radiation provides a selective heating process.These radiations are absorbed by polar components and caneasily pass through nonpolar species. Therefore, microwaveradiation leads to selective absorption of energy by oxidizingagents and most of the sulfur compounds (which are polarcomponents), without any discernible effect on hydrocarbons(mostly nonpolar components). In other words, significantportion of sulfur compounds including mercaptans, sulfoxides,sulfones, sulfides, and disulfides are among polar components;this can effectively accelerate the desulfurization reactions byproviding sufficient activation energy through microwaveradiation. Moreover, since all of oxidizing agents are highlypolar molecules, the desulfurization of even nonpolar sulfurcomponents can be notably enhanced due to the fact thatoxidizing agents are highly activated under microwaveradiations, which can lead to further desulfurization of thesenonpolar sulfur components (e.g., DBT and aryl sulfurcompounds).The second discussion suggests that while the more polar
species may absorb microwave energy more effectively, thisenergy is immediately turned to heat and the heat dissipatesacross the entire reaction mixture. Hence, any differences aresolely due to differences in heating rate and also temperature.The authors believe that both approaches enjoy a broad overlapand actually stem from similar roots.
5. ENVIRONMENTAL ISSUES
Complete elimination of hydrogen sulfide and mercaptans withconsiderable reduction of TSC (from 8500 ppm to around 280ppm) of more than 3000 bbl/day production of Khangiran sourgas condensate can prevent the emission of more than 6.5 t ofvarious sulfur oxides (SOx) in to the atmosphere. Evidently,such immense reduction in SOx emission has great environ-mental advantages. On the other hand, with an eye on theoptimal values of OCR (2) and NSR (0.3) and consideringaround 3% bleed ratio, based on the experimental resultsobtained via titration of the spent acids with standard causticsolution, considerable amounts of depleted sulfuric and nitricacids should be discarded, in each 24 h (135 and 45 bbl/day,respectively). Haphazard disposal of these hazardous materialsmay create other environmental issues and presents anotherchallenge for our future researches. Practical approaches caninclude either safe disposal methods52,53 or regeneration cyclesof depleted acids.54
6. CONCLUSION
Sour gas condensate production of Khangran natural gasrefinery has been tripled in the last six years. Transportation,storage, and consumption of these condensates have led toseveral health and environmental issues. Efficient desulfuriza-tion of sour condensates can drastically improve HSEstandards. A novel mixture of nitric and sulfuric acids isintroduced as an efficient ODS agent. It is clearly shown thatthe use of microwave radiation can significantly improve thedesulfurization efficiency. The proposed oxidative agentcompletely eliminated the hydrogen sulfide and mercaptansand significantly reduced the TSC of a typical sour gascondensate from 8500 ppm to around 280 ppm. It was clearlyshown that by using the optimal OCR and NSR values, theproposed ODS agent can prevent the emission of more than6.5 tons per day of SOx, which provides valuable environmentalbenefits.
Table 3. Sulfur Contents of Microwave-Assisted TreatedCondensates via Different Oxidative Agents
NotesThe authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
The authors wish to acknowledge the kind assistance of Dr.Morteza Maghrebi and Dr. Majid Baniadam for providing themicrowave oven. The authors also acknowledge the valuablecontributions of Khangiran gas refinery officials for performingall necessary analysis tests for collected samples.
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Energy & Fuels Article
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Energy & Fuels Article
dx.doi.org/10.1021/ef4018515 | Energy Fuels XXXX, XXX, XXX−XXXG
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