Intensification of transesterification via sonication numerical simulation and sensitivity study
Introduction
Whether conventional or sonication assisted the transesterification process undergoes the following main and reversible reactions:Summing the first three reactions provide the overall shunt reaction in which one mole of oil reacts with three moles of alcohol leading to the production of three moles of Fatty Acid Methyl Ester (FAME) and the less desired one mole of glycerol [1]. Remains of Triglycerides (TG), Diglyceride (DG), or Monoglyceride (MG) in the reaction are considered impurities and thus one needs to find the conditions to speed up the overall forward reaction to reach to the favorable reaction equilibrium state [2], [3]. Contrary to hydrocarbon fuels in which their thermodynamic properties are well documented and are known as priori, the Triglyceride, Diglyceride, and Monoglyceride is not a fixed formula and their properties including their standard enthalpy, specific heat, and even their physical properties including density, viscosity, etc. are not surprisingly inconsistent in literature [4], [5]. This adds some uncertainty in modeling of their reactivity as far as establishing robust energy minimization approach such as Gibbs free energy based modeling. Alternatively, intrinsic modeling that based on experimentally evaluated chemical kinetic data comes very handy. In this modeling quest the most recurring and cited properties have been used. Conventional transesterification method is slow to be considered as a viable continuous method at high throughput [6], [7]. Ultrasound assisted method, however, reduces the immiscibility of the two reacting fluids as the sonication energy creates cavitation bubbles which continue to grow rapidly, collapsing violently, generating energy and mechanical effect leading to drastic mass transfer rate enhancement at the boundary. The pressure within the bubbles can be as high as 5000 atm which can cause a localized high temperature rise (about 7200 °C) [8]. As this takes place at the phase boundary of the two immiscible species the reactions between the two species intensify [9]. The major advantage that ultra-sonication provides is the reduction in reaction completion time [10]. This leads to a higher rate constant compared to conventional method and in adopting Arrhenius reaction rate this increase can be attributed to lower activation energy as one can urge the sonication is an induced catalysis, or simply a higher reaction pre-constant. Reduction in process time favors large scale production and renders process cost reduction. Some works also report that sonication works better at lower temperature conditions as compared to conventional, since the cavitation mechanism is more influential at lower reaction temperatures [11]. Lower temperature signifies lower activation energies for the reaction which is an important aspect in prospective of extensive production. Ultrasonic irradiation is characterized by two main factors which are frequency and power [12]. Higher frequencies may provide better reaction propagation, but at the cost of high power consumption. Tradeoff between these parameters was attempted in some previous works. Sivakumar et al. [12] experimentally evaluated the effect of frequency on the transesterification of palm oil with methanol and KOH. They scrutinized three cases, in which the first had a single frequency (28 kHz) irradiation employed, whereas the second and third cases had a combination of 2 (28–40 kHz) and 3 (28, 40, 70 kHz) frequencies, respectively. They found that the triple frequency combination method had the best yields of biodiesel.
Apart from the physical effects it is also important to gauge the chemical effects, like free radical formation so as to be reasonable in carrying out kinetic study on sonicated transesterification. Radical chemical specie formation may make such a study highly complex and inaccurate. Extensive work has been carried out in understanding the physical mechanism of ultrasound by some researchers. The results from these works show favorable advantage of physical effects such as formation of fine emulsion, micro mixing etc. over chemical effects. For instance Abhishek et al. [13] studied the prominence of the physical and chemical effects of sonication for the transesterification reaction. Using soybean oil and methanol they experimented with four molar ratios 6, 12, 16 and 24 of methanol to 1 of oil. For the catalyst, they considered FeSO47H2O, NaOH and also carried out experiments without a catalyst. They used a 20 kHz frequency ultrasound equipment. Their approach was to couple experimental results with simulation of cavitation bubbles using the Keller–Miksis equation. They found out that the most beneficial aspect of sonication for transesterification reaction is the physical effect like cavitation. The cavitation events were not high in the methanol region which hampered the chemical effects. They reported that molar ratio of 12:1 was optimum for their study. Priyanka et al. [14] have also worked on mechanistic investigation of transesterification for soybean oil with methanol and H2SO4. Their approach again was experimental evaluation of the process coupled to cavitation bubble simulation. Kinetics of the reaction were also evaluated. They studied the molar ratio dependence and found that the least molar ratio the best results for reaction rates were obtained. They also found reaction taking place at very low temperatures of 15 °C. The reason for this and the prominent effects were again found to be the physical effects of ultrasound. Hanif et al. [15] studied the effect of sonication on Jathropha curcas oil at various temperature and molar ratio combinations. They found that the most important physical effect that causes increase in reaction rates pertaining to sonication is the micro level mixing. The optimum molar ratio was found to be 7:1 at a temperature of 70 °C.
Previous works from the author [16] focused on the physical mechanism of ultrasound by using the Helmholtz equation for ultrasound wave with a modified complex wave number to account for the attenuation due to cavitation bubbles, and the Navier–Stokes and species transport equation for the reactive flow. A logical reaction rate coupling scheme, which is based on the blake pressure of the cavitation bubble and bubble volume was used to couple the kinetics of sonicated and flow agitated transesterification. The current work, however focusses only on the kinetics of the sonicated transesterification using numerical simulation which is still missing from literature. There are several works that focused on kinetics of these reactions with an experimental approach. Noureddini et al. [17] calculated the activation energies and rate constants for forward and backward overall reaction and subsidiary reactions for soybean oil transesterification at different conditions of a well-stirred batch reactor. Jose et al. [9] compared kinetics of sonicated and conventional transesterification and of soybean oil and reported that the rate constant for the sonication reactions were three times to half order of magnitude higher than the conventional process. This suggests that for similar activation energies the reaction constant must be compensated so that the reaction rates need to be much higher in the case of sonication. Vishwanath et al. [18] demonstrated the dependency of rate constants on molar ratio, catalyst percentage and temperature for forward and backward reaction of palm oil fatty distillate ultrasound assisted transesterification with isopropanol. The highest rate constants for forward reaction were reported at five to one of alcohol to oil molar ratio. At catalyst concentration 7%, the highest forward rate constants was achieved. The forward rate constant also depicted a direct proportionality with temperature. The highest attempted experimental temperature was 60 °C which resulted in the highest forward reaction constant. Other parameters showed direct proportionality with the reverse reaction constant, however molar ratio increase resulted in a lower backward reaction constant. Investigation of the kinetics of transesterification reaction of waste cooking oil with a heterogeneous catalyst (K3PO4) and methanol carried out by Dipak et al. [19]. A 375 W at 22 kHz ultrasonic sonication device is used. The reaction was carried out at six to one molar ratio. Similar to the Vishwanath and coworkers they reported the increase in the forward rate constant with the increase in temperature. The evaluated activation energy was 64,241 kJ/Mol and the rate constant was in the range 0.02–0.18 when the temperature swept from 30 °C to 60 °C. Earlier conducted works of the authors demonstrate the feasibility of CFD in evaluating species concentrations as far as their distributions and output product. Less emphasis was given to the influence of key reaction parameters including molar ratio and flow speed. Another work by the same author shows the evaluation of the reaction kinetics of the waste cooking oil transesterification followed by implementing these values in a reactive flow model [20], [21].
In this work the details of the numerical transesterification process inside the three dimensional cylindrical single tubular reactor is attempted. Furthermore, sensitivity study of the conversion will be carried out considering the influence of molar ratio and flow condition. These results are compared for both conventional and sonicated transesterification reaction. The results aim to gain deeper insight of the difference in the two reaction methods as far as the rate of the reaction, the reactants and product species distribution and the yield and the overall feasibility of high fidelity reaction modeling of transesterification for innovative reactor design optimization.
Section snippets
Experimental observation
The idea of introducing ultrasonic irradiation to transesterification process is driven by the motive of reducing processing time and achieving the highest throughput at the smallest footprint area. Work done by Sebayang et al. [22] in a tubular, continuous, ultrasonic assisted transesterification reactor using waste cooking oil and methanol with NaOH as catalyst at 9:1 M ratio and 1 wt% NaOH, a yield of around 94% was achieved within the first 5 min of the process time, whereas when the same
Model development
The flow of the two immiscible fluids of alcohol and triglyceride inside an annular reactor is governed by Navier–Stokes and energy equations. This form of a none-isothermal, viscous, turbulent flow equations that are associated with temporal, advective, viscous, and source terms and is written as:where u is the velocity and Sϕ is the source term due to the interaction, destruction or creation of other species. Φ is the dependent
Simulation setup and model validation
The model setup is in the line of the author’s previous work [28] which consists of a single cylindrical reactor (30 cm length) flowing axially. The flow is subjected to sonication action by central sono-trode at the flow entry. The topology of the reactor allows asymmetrical modeling setup as depicted in Fig. 2.
A three dimensional cylindrical wedge of 5 deg is considered with two periodic faces coincided at the reactor’s centerline and bounded by the outer reactor wall. It is subjected to
Reaction rate
Primary simulation results, depicted in Fig. 4, reflect the high reaction rate trend of sonication that is justified by the higher rate constants mentioned in Table 3. Results indicate the peak in sonicated transesterification rate, is three times higher compared to conventional process. The reaction rates distribution is more clearly shown in the contour plot in Fig. 5 reaching nearly 55.0 kmol/m3 s for the sonication versus 9.25 kmol/m3 s for the conventional transesterification. It is observed
Conclusion
In this work, the numerical simulation of the transesterification in a three dimensional reactor is accomplished. A non-isothermal Navier–Stokes model coupled with species transport of reactive flow of the overall reversible reactions was carried out. The sonication is considered as an increase in the rate of reactions and change in the activation energies as inferred experimentally. The experimental results clearly demonstrate the effectiveness of the sonication assisted transesterification
References (32)
- et al.
Kinetics of the non-catalytic transesterification of soybean oil
Fuel
(1998) - et al.
Ultrasound assisted synthesis of isopropyl esters from palm fatty acid distillate
J Ultrason Sonochem
(2009) - et al.
Biodiesel from waste cooking oils via direct sonication
Appl Energy
(2013) - et al.
Fast, easy methanolysis of Jatropha curcus oil for biodiesel production due to the better solubility of oil with ethanol in reaction mixture assisted by ultrasonication
Ultrason Sonochem
(2012) - Pradhan A, Shrestha DH. Impact of some common impurities on biodiesel cloud point. In: American society of agricultural...
- et al.
Effect of impurities on performance of biodiesel: a review
J Sci Ind Res
(2010) - et al.
Predicting thermophysical properties of mono- and diglycerides with the chemical constituent fragment approach
Ind Eng Chem Res
(2010) - et al.
Prediction of viscosities of fatty compounds and biodiesel by group contribution
Energy Fuels
(2011) - et al.
Ultrasonic versus silent methylation of vegetable oils
Ultrason Sonochem
(2006) - et al.
Ultrasonically driven continuous process for vegetable oil transesterification
Ultrason Sonochem
(2007)
Sonochemistry: current uses and future prospects in the chemical and processing industries
Phil Trans Roy Soc Lond
Biodiesel from an alkaline transesterification reaction of soybean oil using ultrasonic mixing
J Am Oil Chem
Intensification of biodiesel production via ultrasonic-assisted process: a critical review on fundamentals and recent development
Renew Sustain Energy Rev
Kinetics of ultrasonic transesterification of waste cooking oil
Environ Progr Sustain Energy
Intensification of synthesis of biodiesel from palm oil using multiple frequency ultrasonic flow cell
Fuel Process Technol
Physical mechanism of ultrasound-assisted synthesis of biodiesel
Ind Eng Chem Res
Cited by (18)
A computational approach in automating the continuous sonicated biodiesel production
2023, Sustainable Energy Technologies and AssessmentsEffect of different waveforms and harmonic frequency orders on bubble cavitation in dual-frequency ultrasonic intensification
2020, Chemical Engineering and Processing - Process IntensificationCitation Excerpt :However, the limitation of mass transfer rates between oil and alcohol dissolution has to be improved for a better biodiesel [7]. The sonication technique has been used for the enhancement of chemical reactions in fields such as crystallisation, pharmaceuticals, nanomaterial synthesis, wastewater treatment, biofuel production [8–10], other oil related applications [11,12], and biochemical processes [13]. Compared to conventional mechanical stirring, Deshmane et al. [14] and Georgogianni et al. [15] demonstrated that ultrasonic waves also reduced the reaction time, energy consumption, as well as the methanol to oil molar ratio.
Modelling of production processes for liquid biofuels through CFD: A review of conventional and intensified technologies
2019, Chemical Engineering and Processing - Process IntensificationCitation Excerpt :Similarly to other types of reactor, the reaction rate showed a proportional relationship with both temperature and alcohol-to-oil molar ratio. In a recent work, Janajreh et al. [85] performed a similar comparison between the reaction rate for a sonicated process and that for a conventional process. In this case, the authors assigned different values for the reaction constant depending on the transesterification process (conventional or sonicated).