Ultrasound assisted synthesis of water-in-oil nanoemulsions: Parametric optimization using hybrid ANN-GA approach
Graphical abstract
Introduction
Global energy demand is expected to increase over the coming decades. A primary cause accredited to this upsurge is a result of increase in economic growth measured by global GDP. According to the IEO2017 (International energy outlook) report energy consumption is expected to increase from 575 quadrillion Btu in 2015 to 736 quadrillion Btu in 2040, a 28% increase. Regardless of the increase in use of renewable sources, only a 2–3% increase is expected in year 2040 compared to year 2015. Whereas, 77% of energy demand still be depending on fossil fuels. Liquid fuel consumption however, is expected to decrease from 33% to 31% in 2040. To match the energy demand, overuse of liquid fuels may lead to energy crisis. New technology is therefore desired capable of reducing the liquid fuel consumption and pollution, simultaneously. Diesel fuels are used in compression ignition engines of heavy-duty vehicles or machines with the advantage of high compression ratio that produces high turbo boost pressure even at low loads with the limitations of COx, NOx and particulate matters (PMs) emissions. To cope up the current scenario, different options like blending of oxygenated fuels with diesel and biodiesel etc. are available with their own pros and cons. One of the recent research areas in this reference is water emulsified diesel fuels that not only work without modifications in the engine but also reduces the pollutant labels (NOx, PMs) drastically by bringing out micro-explosion followed by secondary atomization. Moreover, combustion properties specifically brake specific fuel consumption also improves.
Water-in-oil nanoemulsions are kinetically stable multiphase colloidal dispersions in the size range of 20–200 nm [[1], [2], [3], [4], [5]] and one of the conditions for emulsified fuels. Smaller droplet size of nanoemulsions make them inimitable in the context of stable size, large surface area per unit volume and optical transparency [[6], [7], [8]]. However, their small size negates separation driven by gravitation force and avoids the phenomenon of creaming/sedimentation and flocculation [[8], [9], [10]]. Emulsification phenomenon being inherently non-spontaneous (ΔG>0) has tendency to separate the immiscible phases. An external force therefore is required to stabilize the system either in the form of change in chemical potential through phase inversion (low energy method, LEM) or by application of high shear/pressure through homogenizer, ultrasonic cavitation etc. (high energy method, HEM) [[11], [12], [13]]. In case of LEM droplet deformation takes place in viscous shear flow under action of rotating flow. Droplet deforms to prolate ellipsoidal shape and breaks when Weber number () exceeds critical Weber number () [6,14]. Whereas, critical Weber number is a function of flow type and ratio of viscosities of dispersed phase to continuous phase (ηd/ηc) [15].
Nanoemulsion formation through HEM, however, exploits intensive shear force to generate nano-drops. An example of such type is ultrasonic cavitation wherein formation and growth of microbubble/cavity takes place when ultrasonic waves are introduced to the liquid sample. Microbubble grows until a critical size is obtained, thereafter, implosion results in high shear rate or microjet causing breakup and dispersion of droplets. Moreover, implosion of cavitation bubbles also generates high temperature (≈5000 K) and pressure (≈1000 bar) locally [16,17]. Formation of nano-drops ≤100 nm by ultrasonic cavitation has been reported by many researchers [18,19]. Whereas in case of water-in-diesel oil, lower droplet size not only improves the combustion and emission standards but also imparts enhanced stability.
Optimization of process parameters for the formation of emulsions plays a vital role in efficient selection of variables. In recent years, response surface methodology (RSM) for efficient optimization of independent variables has been extensively tested and reported in literature specifically in emulsion formations [[20], [21], [22], [23]]. However, in optimization of nanoemulsions formation, experimental designs have been extensively used to find out significant variables by implementing low and high energy methods [3,20,21,24].
Artificial neural network (ANN) has been effectively used as a tool for determining the factors controlling the particle size [25], predicting phase behavior [26], stability [27], rheological behavior of emulsions [28] and surface tension of pure organic compounds [29]. Recently, central composite design (CCD) combined with the ANN has been reported in the literature pertaining to food, landfill leachate treatment and other different fields [30,31] like loading of nanoparticles on nanotubes for dispersion and extraction of dye [32,33]. Whereas, ANN with genetic algorithm (GA) has also been used for identifying the phase behavior of colloids and biosorptive remediation [34,35].
The most recurrent aim for optimization is to exploit the benefits of nanoemulsions compared with the conventional macro-emulsions with respect to their smaller size and low polydispersity index (PI). Parameters of heuristic and metaheuristic algorithms have been tuned by using different approaches by several researchers [[36], [37], [38]]. In a metaheuristic approach, researchers have evaluated guidelines to select parameters for particle swamp optimization and validated them by using experimental design [39]. Whereas in other studies parameters of Tabu search algorithm were tuned by factorial design [40]. Several studies have used experimental design (RSM) for getting the optimized combination of parameters that would help in fine tuning the ANN [41,42]. Furthermore, many studies involved the use of different optimization techniques like Taguchi method, Evolutionary Algorithms, etc., for improvements in performance of ANN [[43], [44], [45], [46], [47], [48]]. The lack of knowledge and information of these tools in analyzing the results poses the difficulty in working in this area [49].
Present work is focused on the process intensification of W/O NE formulation by optimizing the process parameters concerned with CEMSNE (combined energy mixed surfactant nanoemulsion) method wherein, ultrasonication in combination with isothermal dilution method was implemented by adopting the heuristic approach. However, proposed algorithm included first the parametric optimization through RSM, thereafter ANN is applied on the RSM generated design points. Further, genetic algorithm finally (GA) tuned the results by making use of hybrid GA technique wherein, RSM generated fitness function and ANN generated data points were used to initialize the algorithm. Therefore this work applies both models viz. CCD-RSM and ANN-GA for efficiently optimizing the formation of W/O NE that comprises of minimum droplet with lower kinematic viscosity simultaneously [50].
Section snippets
Materials
Non-ionic surfactants used in the study viz. 4-Octylphenol polyethoxylate (HLB 13.5) and Sorbitanmonooleate (HLB 4.3) were acquired from two different vendors, Molychem, Mumbai, India and Sigma Aldrich, USA, respectively. Diesel oil was procured from the Hindustan Petroleum Corporation Limited, Roorkee, Uttarakhand, India retail outlet pump. Surfactants used in the present work were not purified further. Double-distilled water was used in the preparation of W/O NEs.
Pre-emulsification unit
A pre-emulsification unit
Experimental design
RSM is a mathematical and statistical analytical tool based on developing model that inspects effect of multiple independent variables and their interaction with response variables. RSM was first developed in 1950s by Box and collaborators [52]. Box-Behnken (BBD) and central composite (CCD) designs are mostly used response surface designs applicable for optimizing the emulsification process. Both designs can accommodate second order terms, however, BBD is less expensive in experimentation than
Results and discussions
Formation of W/O NEs was attained by finding optimal values of response variables like droplet size (YDia) of dispersed phase and kinematic viscosity (YKV). Combination of low and high energy methods was used in the NE formation by application of set of experiments using statistical technique like RCCD-RSM. Set of experiments in the normalized form planned by the RSM were utilized in ANN model for simulating the experimental results. Data obtained from the ANN model and RCCD-RSM generated
Conclusions
A novel intensified approach (CEMNSE method) was adopted for the formulation of water-in-diesel nanoemulsion by optimizing and predicting the process parameters responsible for minimizing response variables viz. dispersed phase droplet size (YDia) and nanoemulsion kinematic viscosity (YKV). Ultrasonic cavitation and isothermal dilution method in combination was implemented in the nanoemulsion formation as a part of CEMNSE method. A collaborated modeling approach introduced FFBPANN-GA coupled
Acknowledgement
One of the author, H.K., acknowledges the financial assistance in the form of research assistantship provided by the Ministry of Human Resource and Development (MHRD), Government of India.
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