Technical feasibility of biodiesel production from virgin oil and waste cooking oil: Comparison between traditional and innovative process based on hydrodynamic cavitation
Graphical abstract
Introduction
The actual dependence on fossil fuels (more than 80% of the actual world energy demand is covered by fossil fuels) must be cut before 2030 (Goldemberg, 2007). The increase in the production and consumption of biofuels in the transportation sector is considered a suitable solution to decrease fossil fuels consumption and thus environmental pollution; nonetheless, there are several concerns regarding intensive bioenergy crops, endangering local biodiversity (Diaz-Chavez, 2011), food vs. fuel issue (Ajanovic, 2011, Cassman and Liska, 2007). The sustainability of biofuels must exclude bioenergetic crops over arable lands (Dismukes et al., 2008) and edible feedstocks but endorses the use of biological wastes and crops from non-arable lands (Gomez et al., 2008, Jefferson, 2008) (second and third generation biofuels), remaining economically competitive (Hill et al., 2006). In particular, the sustainability of many first-generation biofuels, which are produced primarily from food crops such as grains, sugar cane and vegetable oils, has been increasingly questioned over concerns such as reported displacement of food-crops, effects on the environment and climate change. In general, there is growing consensus that if significant emission reductions in the transport sector are to be achieved, biofuel technologies must become more efficient in terms of net lifecycle greenhouse gas (GHG) emission reductions while at the same time be socially and environmentally sustainable. It is increasingly understood that most first-generation biofuels, with the exception of sugar cane ethanol, will likely have a limited role in the future transport fuel mix. The increasing criticism of the sustainability of many first-generation biofuels has raised attention to the potential second-generation biofuels produced by the waste. Depending on the feedstock choice and the cultivation technique, second-generation biofuel production has the potential to provide benefits such as consuming waste residues and making use of abandoned land. In this way, the new fuels could offer considerable potential to promote rural development and improve economic conditions in emerging and developing regions. However, while second-generation biofuel crops and production technologies are more efficient, their production could become unsustainable if they compete with food crops for available land. Thus, their sustainability will depend on whether producers comply with criteria like minimum lifecycle GHG reductions, including land use change, and social standards (IEA, 2010).
The identification and implementation of sustainable biofuel production alternatives should be based on rigorous assessments that integrate socioeconomic and environmental objectives at local, regional, and global scales (Correa et al., 2019).
At the moment, biodiesel is the commonest and most promising biofuel already introduced into the automotive fuels marked (in mixture with petrodiesel) at the cost of small technological adaptation of automotive engines (Ahmed et al., 2014, Marulanda, 2012). Biodiesel consists of mixture of methyl or ethyl esters. It is produced through an organic reaction called transesterification, where low molecular weight alcohol -e.g. methanol or ethanol- reacts with lipid or fat (triglyceride) to produce biodiesel -fatty acid methyl ester (FAME) or fatty acid ethyl ester (FAEE) - and glycerol as a valuable by product. Transesterification can occur catalytically or without catalyst. Catalysts currently used in transesterification process are homogeneous base or acid catalysts, heterogeneous catalysts, acid or base and enzymes.
The biodiesel use strongly reduces the GHG (up to 78%), unburned hydrocarbons and particulate matter (Van Gerpen, 2005) but increases nitrogen oxides (NOx) emissions (Hoekman and Robbins, 2012).
The effects on injection timing, ignition delay, adiabatic flame temperature, radiative heat loss, and other combustion phenomena all play some role on NOx emissions. These emissions can be mitigated by modifying engine control settings, particularly by retarding injection timing and increasing exhaust gas recirculation. The absolute magnitude of the biodiesel NOx effect appears to be reduced with modern engines, although there are cases where the percentage change is still substantial. Chen et al. (2018) performed an overall experimental investigation on NOx emission based on combustion characteristics of biodiesel. The results showed that on the whole, NOx emission of biodiesel is higher than that of diesel in most cases, except in the condition of low loads under low and medium speeds (Chen et al., 2018).
Feedstocks, class of lands for bioenergy crops (whether or not arable) and the production process affect the biodiesel production sustainability (Dovì et al., 2009).
Therefore, exploring new bio-sources for biodiesel production such as the use of microalgae as feedstocks complies with the sustainability requirements (Mata et al., 2010, Piemonte et al., 2016) as well as the use of waste oils (Al-Sakkari et al., 2018). But to increase energy efficiency and minimize energy losses and waste materials, the traditional biodiesel production process (BPP) also knows as mechanical stirring (MS) method requires some reconsiderations and modifications as well. Biodiesel is produced by means of transesterification reactions between a lipid and an alcohol to form esters and a byproduct, glycerol. Transesterification consists of a sequence of three consecutive reversible reactions. The first step is the conversion of triglycerides to diglycerides, followed by the conversion of diglycerides to monoglycerides, and finally monoglycerides into glycerol, yielding one ester molecule from each glyceride at each step. This process proceeds well in the presence of some homogeneous catalysts, as alkali ones. The most common alcohols widely used are methyl alcohol and ethyl alcohol. Among these two, methanol found frequent application in the commercial uses because of its low cost (Verma et al., 2016).
The energy consumption due to time reaction during MS and raw material cost are the major contributors to the total cost of biodiesel production. The innovative technologies as hydrodynamic cavitation (HC) allow to reduce the mass transfer rate between oil and alcohol and consequently the energy consumption. Increasing the mass transfer rate between oil and alcohol with minimal cost for biodiesel production in terms of raw material price, energy consumption, time and scale up cost are some of the challenges for research in this industrial sector (Chuah et al., 2016). Hydrodynamic cavitation (HC) is a new technology, currently applied in a wide range of chemical engineering sectors as wastewater treatment (Capocelli et al., 2013, Bagal and Gogate, 2014, Innocenzi et al., 2018, Gagol et al., 2018, Dular et al., 2018, Innocenzi et al., 2019) and green process intensification protocols (Gude, 2018, Wu et al., 2019).
As for biodiesel production in the presence of catalyst, HC has the advantage of achieving complete conversion in a short reaction time. HC has proved to be an appropriate option to replace some of the steps of the conventional BPP, with several enhancements such as less waste streams, less energy losses and increased energy efficiency (Ladino et al., 2016, Günay et al., 2019, Asif et al., 2017). One of the major challenges in the BPP is the immiscibility of oil and alcohol as raw materials, which causes the formation of two separate phases thus decreasing mass transfer rate and increasing reaction time (typically a long reaction time –up to 12 h -depending on feedstock and catalyst, requiring in any case large reactor volumes); moreover, the contact surface is to be increased to improve mass transfer rate, and stirred-tank reactors are usually used in many industrial units at this scope, but with an insufficient mixture of the reactants. HC generates tiny cavities in the reaction mixture, thus yielding to an intensification of the process with increasing mass transfer between insoluble phases (Chuah et al., 2017) and decreasing reaction times (Chipurici et al., 2019). Additionally, energy consumption is reduced and the reaction temperature and pressure are lower (Chitsaz et al., 2018).
Besides HC, other technologies have been studied to intensify BPP and namely microwaves (MW) and ultrasonic cavitation (UC). A good review of the obtained results by using different techniques for transesterification reaction is reported in the review by Chuah and coworkers (2017). In the last review paper, the above mentioned techniques have been compared as for their efficiency in mixing with mechanical stirring (MS), and the yield efficiency (defined as yield of product per unit supplied energy to the system) in relation to the used technique is the following: HC > MW > UC > MS. HC is definitely the most feasible method to assist and intensify the transesterification reaction (Chuah et al., 2017). Nevertheless, HC is still a quite novel technique in the biodiesel field.
Currently, the biodiesel production techniques that are based on hydrodynamic cavitation, essentially focus on technology that involves the use of a high speed homogenizer (based on the rotor/stator model, also referred to as rotating generator, Crudo et al., 2016) or involves (especially in the past five/six years) the use of circular or slit Venturi or orifice plates (Maddikeri et al., 2014).
As for high speed homogenizer, this technology has been studied for the intensification of the synthesis of biodiesel from different vegetable oils (virgin soybean oil, spent soybean cooking oil and many others, virgin or waste oils). This rotating generator has proved to be an efficient, fast and cost-effective procedure for biodiesel preparation (Crudo et al., 2016). Process parameters such as inlet pressure, temperature, catalyst concentration, molar ratio, type of cavitating device have been studied.
The results obtained from this literature analysis focused on hydrodynamic cavitation applied to biodiesel production have been used for process simulation. In the present work, process simulations on alkali catalyzed transterification by traditional and innovative method based on HC are presented: a professional software tool, SuperPro Designer®, is used to simulate the whole process with energy and mass balances. Process performance is expressed in terms of biodiesel yield with respect to a traditional process scheme, to verify the method efficiency by using two different feedstocks, virgin oil and waste cooking oil. The results of the process analysis have been used to carry out a preliminary economic analysis to identify the cost items that have the greatest impact on the cost-effectiveness of the biodiesel production process.
This is an attempt to explore potential alternatives to current biodiesel production methods, thus contributing to commercialization and more sustainable production of second-generation biofuels.
Section snippets
Materials and methods
In the following a description of the methods used to carry out the process analysis and simulation is provided. In details, Section 2.1 reports the process simulation by using a commercial software and Section 2.2 the input data, derived from literature analysis, used for the simulation.
Transesterification
A continuous alkali-catalyzed process scheme by using virgin oil was developed (Fig. 1). The first section of the process is the transesterification (Fig. S1). The reaction was carried out with a 5:1 vol ration of methanol to oil, 0.3% wt sodium methoxide (catalysts based on oil), 60 °C and 4 bar. Fresh methanol (S3, 90 kg/h) and recycled methanol (S48, 75 kg/h) coming from glycerol purification section, are mixed with catalysts (a mixture of sodium methoxide, 25% and methanol, 75%, S10) prior
Conclusions
In this work, a comparison between the traditional and an innovative process for biodiesel production has been performed through a process analysis by using SuperPro Designer® software. Traditional treatment includes three main sections: transesterification reaction of oil with methanol in the presence of catalysts, purification of biodiesel and purification of glycerol. In the innovative process, as demonstrate by scientific literature, transesterification reactors have been replaced with
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are very grateful to Eng. Francesco De Angelis for the helpful collaboration on process analysis study.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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