Effect of operating pressure on the performance of ultrasound-assisted oxidative desulfurization (UAOD) using a horn type sonicator: Experimental investigation and CFD simulation

https://doi.org/10.1016/j.cep.2018.08.006Get rights and content

Highlights

  • Ultrasound-assisted oxidative desulfurization (UAOD) of kerosene was studied.

  • Response surface methodology (RSM) was used to find the best operating parameters.

  • The effect of pressure on the performance of direct probe system was investigated.

  • CFD simulation and calorimetric tests were carried out to analyze the results.

Abstract

In the present work, experimental and simulation investigations have been conducted on the pressurized ultrasound-assisted oxidative desulfurization (UAOD) of kerosene using a direct probe sonicator system. The effects of various operating parameters including pressure, ultrasonic power, and sonication time on the performance of UAOD process have been examined. The Box– Behnken design as a subset of the response surface methodology (RSM) has been employed to find the optimum conditions and to analyze the effects of different operating parameters on the performance of UAOD process. 96% sulfur removal of kerosene was achieved using operating pressure of 0.03 barg, ultrasound power of 390 W, and sonication time of 22 min followed by liquid–liquid extraction. In addition, calorimetric tests have been carried out to determine the acoustic pressure amplitude at the tip of the probe. Pressure distribution and velocity magnitude of the system obtained by computational fluid dynamic (CFD) was in agreement with the calorimetric and experimental analysis results.

Introduction

Nowadays, reduction of sulfur content in the transportation fuels is considered as an industrial as well as environmental requirement. Organic sulfur-containing compounds are among the most hazardous air pollutants due to releasing SOX after combustion, which is a threat to public health [1]. The corrosion of combustion engines and poisoning catalytic converters are other destructive effects related to the presence of sulfur-containing compounds in the fuels. Therefore, the maximum allowable sulfur level in the transportation fuels is now set to 10 ppmw in the developed countries [2]. The conventional hydrotreating process as commercial method can readily reduce the sulfur levels of fuels to about 500 ppmw] [3]. Indeed, the presence of certain recalcitrant organic sulfur-containing compounds including thiophenic compounds and also the nitrogen-containing compounds in common fuels largely hinder the HDS process [4,5]. To separate these refractory sulfur-containing compounds from fuels, researchers have proposed several alternative processes including selective adsorption [6], bio desulfurization [7], hydrodynamic cavitation [8,9] and oxidative desulfurization (ODS) [10]. The ODS method is preferred since the oxidative desulfurization process can be carried out in the liquid phase and under the mild pressure and temperature conditions [11,12]. Recalcitrant sulfur-containing compounds react more favorably in the ODS process as compared with the HDS process [13]. In the ODS method, the sulfur-containing compounds that exist in the fuel are converted to polar sulfoxide and sulfone molecules then they are separated from the bulk of hydrocarbon fuels via extraction or adsorption methods [2,14], Using H2O2 as an approved oxidizer with different catalysts can improve the efficiency of the oxidation process [15]. In the presence of carboxylic acids like formic acid [1], H2O2 is converted into peroxycarboxylic acid, which is an active oxidizer of sulfur-containing compounds with acceptable performance [16]. The rate of oxidation reaction in the conventional ODS was reported as “slow” in most studies due to lack of mixing and low interfacial mass transfer area between different phases in the oxidation media [17]. Ultrasound irradiation is a method for increasing the rate of ODS reaction [18,19] and production of fine emulsion between the organic and aqueous phases [2]. The waves propagated in the liquid medium via the ultrasound irradiation would generate the compression-expansion cycles which in turn leads to the production of bubbles. While the size of growing bubbles reach the resonance size, powerful collapse will happen leading to high local pressure and temperature [20]. Various effects of ultrasound irradiation on the liquid media are described in Fig. 1.

In the ultrasound-assisted oxidative desulfurization (UAOD) process, the bubble collapse can simultaneously enhance the chemical and physical aspects of oxidative desulfurization [21]. As the main effect, intense micro-mixing lead to the efficient contact between the reacting components as the physical effects of the UAOD process system [22]. Generally, the physical effect of ultrasonic waves entails creating displacement in the medium through moving fuel elements with small oscillation amplitudes about a mean position termed “micro-streaming”. In addition, cavitation also produces ambient displacements through different mechanisms including micro-turbulence (i.e., velocity of the oscillating liquid generated by bubbles), shock waves, and high-speed micro jets [21].

During the bubbles oscillation, the concentration of the compounds entrapped inside the bubbles may change constantly due to a number of phenomena such as gas penetration, vaporization, water vapor condensation, and chemical reactions [23]. These phenomena can be affected by changing the operating conditions, therefore it can be expected that an increase in the operating pressure in UAOD leads to physical effects enhancement in the ultrasound system. The enhancement in physical effects leads to an increase in the mixing intensity between the two immiscible phases, which in turn increases the mass transfer rate in comparison with the conventional ODS.

Some studies on the UAOD processes have been done under ambient pressure conditions [2,24,25]. In some rare researches, the effect of operating pressure on the performance of UAOD process have been investigated for bath ultrasound reactors [21]. Although, Timko et al. [26] studied the application of ultrasound in the formation of surfactant-free emulsions consisting of different systems including water and dense carbon dioxide (CO2) under low and high pressure. Also, Cenci et al. [27] studied a biphasic mixture of CO2/H2O under high pressure ultrasound system which they achieved reaction rates over 200 times faster in comparison with the conventional system by reducing the mass transport resistance.

Bhasarkar et al. [28] have employed dual approach of mathematical modeling and experimental investigation to study the mechanistic features of the UAOD process. Bhasarkar et al. [17] also studied the UAOD system for a model fuel using peracids and phase transfer agent (PTA) in an ultrasound bath. According to the result of this study, the isolation of cavitation due to increasing the operating pressure results in a significant enhancement of UAOD performance through the elimination of transient cavitation. They attributed this result to the generation of a fine emulsion between the aqueous and hydrocarbon phases and subsequent increase in the interfacial area, which in turn assists the effective transfer of HO2 radicals.

The researchers found that changing the operating pressure can affect the resonance frequency and equilibrium radius of the cavitation bubbles and drive the system toward drastic resonance conditions which can result in increasing the rate and yield of the reactions [29].

The severity of a bubble collapse depends on the rate of expansion of the bubble during the rarefaction process and acoustic cavitation may be impacted by operating pressure. An increase in the operating pressure may possibly increase the cavitation threshold [30].

Based on our knowledge, there is no report available on the effect of operating pressure in a direct probe ultrasound reactor in UAOD applications [31]. Meanwhile, there are no studies to investigate the interaction between the pressure with other crucial parameters in the UAOD process.

Combination of experimental and simulation methods can be applied to understand a process. Besides, the elaboration of the ultrasound reactor characterizations can be identified by simulation outcomes. Numerical simulation could be used as an efficient tool to investigate the hydrodynamic behavior and other related phenomena [[32], [33], [34], [35]]. It can be beneficial to provide some insights regarding the complex fluid behavior in the sonoreactor, which might be difficult to obtain with the experimental methods. For example, Rahimi et al. [35] focuses on developing a new model for an ultrasonic vibrating horn and assessing the induced flow pattern with a new moving boundary condition, which is more appropriate than the conventional models. They have also studied various characteristics of a sono-reactor like pressure field, stream function, cavitation zone, and velocity pattern. In the present study, the Box-Behnken design as a subset of response surface methodology (RSM) was used to study the effects of pressure, ultrasound power, and time on the performance of UAOD process using direct probe system. In addition, a numerical simulation has been carried out using computational fluid dynamic (CFD) technique [35].

Section snippets

Chemicals

Formic acid (99 wt%) and hydrogen peroxide (30 wt%) were obtained from Merck Co. (Germany). Acetonitrile (99 wt%) was purchased from Duksan (Korea). All of the chemicals were used without further purification. Non-hydrotreated kerosene was obtained from Tehran refining company (Tehran, Iran). The specifications of kerosene feedstock are summarized in Table 1. The total sulfur content of kerosene was 2490 ppmw.

Method of the analysis

The total sulfur content of the kerosene samples was determined by Rigaku NEXQC+

Result and discussion

According to our previous study [24], the best operating conditions for sulfur removal were achieved using oxidant to sulfur molar ratio nOnS of 15.02, formic acid to sulfur molar ratio nacidns of 107.8, and the temperature 50 °C [24]. Sulfur removal of 95.4% was achieved under the above-mentioned oxidation conditions followed by liquid-liquid extraction. In the present study, the same operating conditions were used and the effects of several other parameters including the pressure, the

Conclusion

In the present study, the effect of operating pressure on the performance of UAOD process equipped with direct horn sonicator was investigated. Experimental and simulation investigations were used to analyze the effects of pressure on the performance of UAOD. Hydrogen peroxide-formic acid was used as the oxidation system. Box-Behnken design (BBD) was employed to study the effects of operating pressure, ultrasound power, and oxidation time on the performance of UAOD process for treatment of

Acknowledgments

The authors would like to acknowledge the supports provided by National Iranian Oil Engineering and Construction Company (NIOEC).

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    Current address: National Iranian Oil Company (NIOC), Iran.

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