Elsevier

Applied Energy

Volume 185, Part 2, 1 January 2017, Pages 2151-2159
Applied Energy

Intensification of transesterification via sonication numerical simulation and sensitivity study

https://doi.org/10.1016/j.apenergy.2016.02.002Get rights and content

Highlights

  • 3D numerical simulation of transesterification is accomplished.

  • A non-isothermal, reactive Navier–stokes was carried out.

  • Conventional and sonicated process was compared as far as reaction kinetics and yield.

  • Higher kinetic rates are achieved at lower molar ratios in sonicated process.

  • It validates feasibility of numerical simulation for transesterification assessment.

Abstract

Transesterification is known as slow reaction that can take over several hours to complete. The process involves two immiscible reactants to produce the biodiesel and the byproduct glycerol. Biodiesel commercialization has always been hindered by the long process times of the transesterification reaction. Catalyzing the process and increasing the agitation rate is the mode of intensifying the process additional to the increase of the molar ratio, temperature, circulation that all penalize the overall process metrics. Finding shorter path by reducing the reaction into a few minutes and ensures high quality biodiesel, in economically viable way is coming along with sonication. This drastic reduction moves the technology from the slow batch process into the high throughput continuous process. In a practical sense this means a huge optimization for the biodiesel production process which opens pathways for faster, voluminous and cheaper production. The mechanism of sonication assisted reaction is explained by the creation of microbubbles which increases the interfacial surface reaction areas and the presence of high localized temperature and turbulence as these microbubbles implode. As a result the reaction kinetics of sonicated transesterification as inferred by several authors is much faster. The aim of this work is to implement the inferred rates in a high fidelity numerical reactive flow simulation model while considering the reactor geometry. It is based on Navier–Stokes equations coupled with energy equation for non-isothermal flow and the transport equations of the multiple reactive species in an annular continuous reactor. Following model validation, the spatial reaction rate is evaluated to bring more insight to the reaction progression and species distributions. The two methods (conventional and sonication) then are compared on the basis of their sensitivity to the Alcohol:Oil molar ratio. The spatial distribution of the yield and their favorable sonication method is a key enabler of the development of an optimal process reactor that renders more economy to the process when operating at lower AL:TG ration, catalyst amount, and temperature.

Introduction

Whether conventional or sonication assisted the transesterification process undergoes the following main and reversible reactions:Triglyceride+AlcoholK2K1FAME+DiglycerideDiglyceride+AlcoholK4K3FAME+MonoglycerideMonoglyceride+AlcoholK6K5FAME+GlycerolTriglyceride+3AlcoholK8K73FAME+GlycerolSumming 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:t(ϕ)Timerate+xi(uiϕ)advective=-xiΓϕϕxidiffusion+Sϕsourcewhere 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

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