Applications of Super Critical Technology in Biodiesel Production

Because of the scarcity of fossil fuels, it’s crucial to look into non-conventional options. In this context, “biofuel” refers to any fuel, liquid, gaseous, or solid, that is primarily derived from biomass. Many different types of biofuels exist, including ethanol, methanol, diesel, and hydrogen. Biodiesel, is a promising alternative fuel. It’s eco-friendly and manufactured from edible or nonedible oils. Transesterification, non-catalytic supercritical fluid technique, micro emulsion, pyrolysis, and other methods have all been recorded for producing biodiesel from vegetable oil and lipids. This article compares and contrasts the advantages of using conventional catalytic processes to produce biodiesel with those of using supercritical fluids (SCF). Concerns have been raised regarding the substantial amount of energy that must be expended in order to carry out supercritical reactions under conditions of high pressure and temperature. This is despite the fact that the catalyst-free SCF method clearly offers a number of benefits that are hard to ignore. Because of this, the SCF process has significant difficulties that need fixing before it can become a long-term viable technology.


Introduction
Global petroleum use has steadily increased over the past 25 years, raising living standards, haulage, and petrochemical use.By 2008, the world consumed 3928 million tons of petroleum, up from 2807 million in 1985.It's getting more and more expensive to fuel vehicles with petroleum, and it's a finite resource.According to BP's 2008 Statistical Review of World Energy, the R/P ratio for the world's proved oil reserves was 42 years.This equates to an estimated 1.71011 tons of oil.Products derived from petroleum also contribute significantly to CO2 emissions.Almost all fuels used in vehicles today are derived from petroleum.Roughly 20% of all CO2 emissions come from the transportation sector.Biodiesel, made from vegetable and animal fats, replaces diesel fuel.Due to rising petroleum prices and environmental benefits, biodiesel manufacturing is a cutting-edge and technological topic for researchers.Biodiesel is produced by transesterifying triglycerides with alcohols like methanol and ethanol [1].Product separation and catalyst recovery are the most energy-intensive and uneconomical steps in conventional biodiesel synthesis.SCFs benefit from facilitating isolation and separation.been studied extensively.SCF reactions make biodiesel without catalysts.With only reactants, this non-catalytic process produces biodiesel.It's easy and cheap.The SCF reaction's high pressure and temperature have raised concerns about its energy efficiency and safety, despite its potential to solve catalytic reaction challenges.SCF technology must overcome obstacles before it can be used to make biodiesel.This article compares industrial biodieselproducing alcohols and acetates from an applied and economic perspective [2].

Biodiesels characteristics
In order to produce biodiesel, a fuel that burns cleanly and is a mono-alkyl ester, fresh or old vegetable oils and animal fats are used [1].Due to the environmental effects of petroleum-fueled engines' exhaust emissions and the world's depleting petroleum sources, biodiesel's importance is expanding.Despite biodiesel's replacement status [3]. 1.
Antifoaming: B100 biodiesel outperforms petroleum diesel.This makes vehicle filling fast and foamfree.

2.
Cetane range: Biodiesel has a cetane range of 45-70, while petroleum diesel fuels have 40-52.Fatty acid distribution in oils and fats determines biodiesel cetane number.Longer, saturated fatty acids increase cetane number.

3.
Chemical makeup in contrast to diesel fuel, biodiesel is produced by reacting esters of fatty acids ranging from C12 to C22.which is a complex mixture of paraffins, naphthene, aromatics, and organic compounds including nitrogen and sulfur from C12 to C25.Diesel, unlike biodiesels, has aromatic ring structures.

5.
Cold flow characteristics: Diesel fuels solidify gradually since each component has its own crystallization temperature.B100 biodiesel is a simpler combination with fewer components, thus There is a predominance of one or two components, and the solidification process is accelerated and more difficult to control.

6.
Conductivity: Because of its polarity, pure biodiesel has excellent conductivity-greater than 500 pico S/m-which decreases static-related fires and sparks.

7.
Corrosion: Wet-ability prevents oxygen and water absorption from causing corrosion.Copper corrosion tests address sulfur compounds that damage copper and yellow metals.Biodiesel makes diesel standard compliance easy.

Transesterification
Biodiesel is usually made from TG via transesterification.TG and acyl-acceptor interact [4,5].Acylacceptors include alcohols, carboxylic acids, and esters.Alcohol transesterification produces glycerol or triacylglycerol [6,7].Transesterification can be catalytic or noncatalytic.Homogeneous or heterogeneous catalytic reactions depend on the catalyst.Transesterification from big, branching TG molecules produces straight-chain, diesel-like molecules.Stoichiometry requires three alcohol moles per TG mole.However, a higher alcohol molar ratio is used to make more biodiesel.Catalyst type, temperature, reaction duration, and feedstock type affect this molar ratio.Methanol, ethanol, and propanol are used most.Biodiesel production is unaffected by alcohol type, but price does.Esterification yields water and transesterification glycerol.Esterification breaks hydroxyl linkages before ester bonds, but transesterification breaks ester bonds first.Catalytic transesterification techniques abound.A biocatalyst, acid catalyst, alcohol-alkali catalyst, or supercritical alcohol can help.Transesterification involves three steps: TG to fatty acid ester and diacylglycerol (DG), DG to MG, and MG to glycerol and the final fatty acid ester.The catalyst hydrolyzes TGs into fatty acids and glycerol.Filtering biodiesel and glycerol after transesterification removes the catalyst.

Transesterification by enzymes
Using biocatalysts to speed up the conversion of TG to biodiesel is getting a lot of attention because it is easy to use and makes clean products.Between 35 and 45 C, enzymatic transesterification can happen [8,  ].When enzymes speed up the process of FFA and TG becoming esters in a single step without the need to wash, they don't make soaps like chemical catalysts do.On the other hand, the main problems with enzymatic transesterification are that it takes longer for the reaction to happen and that methanol could kill the enzymes.Methanolysis can be sped up by an enzyme called lipase.It can come from microorganisms like bacteria and fungus.Pseudomonas cepacia, Rhizopus oryzae, Candida antarctica, and Mucor miehei make the lipases that are most often used.The hydrolases are a group of enzymes that break down water, and lipases are one of them.Lipases break down TGs in living systems to make fatty acids and glycerol.They can handle TGs from different places and work in mild settings.Both TGs and FFAs can be changed into esters by using their catalytic properties.Lipases inside and outside of cells are the main biocatalysts.Extracellular lipases are the microorganism's enzymes that have been cleaned up.This is different from intracellular lipases, which stay inside the growing cell walls.Lipases have been put into three groups based on their regioselectivity: (i) hydrolyze the ester bonds on the TG in positions R1 or R3 for reasons that are specific to sn-1,3; (ii) For Sn-2, break the ester bond at the R2 position with water.(iii) General: Don't tell the difference between ester sites.According to, lipases need to be non-stereospecific in order for all TG, DG, and MG to be able to be converted into fatty acids methyl esters for the purpose of producing biodiesel (FAME).Additionally, they ought to be able to quicken the process of esterification of FFA.Even though lipases are superior to acid and base catalysts, their usage in industry is restricted due to the expensive expense of producing lipases.

Chemically Catalyzed Transesterification
Alkaline catalyst transesterification.Base catalysts have pHs above 7.It can add electrons.Most homogenous base catalysts used in alkaline Transesterification uses NaOH, KOH, and CH3ONa [5,10,11].Due of its simplicity, the base catalyzed method is employed most often.It converts 98% quickly at low temperatures and pressures.Base catalysis's FFA and water sensitivity is its biggest drawback.It works well when FFA and moisture concentrations are below criteria, usually FFA < 0.5 wt%.FFA-rich TGs require pretreatment.FFA increases soap synthesis, which depletes the catalyst, lowers yield, and most importantly makes it harder to separate downstream byproducts and purify the end product.High raw material costs account for 60-90% of biodiesel's price.Alkali catalyst requires effluent treatment.With an alcohol-to-oil molar ratio of 6:1, most base-catalyzed reactions occurred near the alcohol boiling point.Recommend a stoichiometric excess of substrates for biodiesel production (6:1 molar ratio of methanol to oil).Biodiesel production uses homogeneous catalysts.Heterogeneous catalysts such alkaline earth oxides, zeolites, calcined hydrotalcites, and magnesium and calcium oxides have showed encouraging results.The alkaline catalyzed process's biggest downside is the expensive purified feedstock.acid-catalyzed transesterification.An acid could catalyze the TGs-alcohol reaction instead of a base.Strong acids like hydrochloric, sulphuric, sulphonic, and phosphoric are utilized most.Acid-catalyzed transesterification is rarer than base-catalyzed because strong acids are caustic and slow.Reactions may be needed for high conversion.Some studies say acid-catalyzed reactions take 4000 times longer.Above that, lots of alcohol and a strong catalyst are needed.Akoh et al. reported 99% conversion in 50 hours with a 30:1 molar methanol:oil ratio at 55-80 C and 0.5-1% catalyst concentration.Acid-catalyzed reactions are independent of feedstock FFA concentration.These catalysts prevent feedstock FFA from becoming soap, enabling biodiesel production from cheap feedstock.A base catalyst requires pretreatment of feedstock with high FFA content.Methanol and acid catalysis can pretreat the oil, then fresh methanol and a base catalyst accelerate the transesterification reaction.Heterogeneous acid catalysts are used.This prevents homogeneous catalyst difficulties.Sulphated tin oxide transesterifies used cooking oil as a superacid catalyst.Sulphated zirconia catalyzed soybean oil alcoholysis and oleic acid esterification.Heteropolyacid transesterified yellow horn oil.Anion and cation exchange resins transesterified triolein with ethanol to make ethyl oleate [12].

Noncatalytic Transesterification
Supercritical fluids are heated and compressed above the critical temperature and pressure.Supercritical fluids have no liquid or gas phases at temperatures and pressures above their critical points.SCM, ethanol, propanol, and butanol transesterify triglycerides well.The critical temperatures and pressures of important alcohols are shown in Table 1, [13].4.9 Supercritical methanol facilitates the noncatalytic transesterification of triglycerides and the methyl esterification of fatty acids in the biodiesel synthesis pathway, reducing reaction steps and boosting yield.A transesterified derivative of vegetable oil, biodiesel is the most promising alternative to diesel.Hydrolysis of esters in sub/supercritical water was proposed as a reaction mechanism for vegetable oil in SCM.Supercritical processing relies on how temperature and pressure alter the solvent's thermophysical properties, including polarity, viscosity, and specific and average weight.Both homogeneous and heterogeneous catalytic processes are sensitive to high water and FFA concentration, difficult biodiesel separation and purification, long reaction durations, and expensive catalyst prices that make the approach unprofitable.These issues arise from transesterification catalysts.Without catalysts, biodiesel production is too slow.Thus, scientists worldwide have developed many noncatalytic solutions to overcome catalytic reaction difficulties.One is the recent interest in supercritical alcohol (SCA) technology.This unique technique mixes oil and alcohol, which are incompatible, under SCA circumstances.This would solve the reaction's slowing tiny contact area between these two reactants.Ethanol's critical temperature is 2430°C, while methanol's is 2390°C and 8.1 MPa.Under supercritical conditions, alcohol's solubility parameter drops to almost triglyceride-like, causing the two reactants to form a homogenous phase.Thus, SCA can transesterify without a catalyst and at a quicker rate than catalytic processes.The downstream separation of biodiesel from glycerol without a catalyst was simple and produced high-quality glycerol.The SCA procedure tolerates FFA and water in oils and fats and has no saponification-related side effects.This non-catalytic reaction simultaneously esterifies and transesters FFAs, enhancing biodiesel production.Triglycerides hydrolyzed in water to form fatty acid alkyl esters, which can be esterified.Water did not impact reaction rate [14].The SCA reaction allows the utilization of cheap feedstock like residual cooking oil or lard, which often contain a lot of these contaminants.New SCF technique uses Methyl acetate can replace alcohol as the supercritical medium [5,15].Traditional transesterification uses alcohol to create glycerol, which oversupplies and lowers market value.Biodiesel is unsuitable for cold climates due to its high viscosity and cloud point.To increase winter performance, biodiesel is often boosted with an additive.[16].Instead of glycerol, the SCMA reaction yields triacetin and fatty acid methyl esters.FAME with Triacetin can make biodiesel without catalytic reactions, simplifying downstream processes.Triacetin makes good fuel.This study reported a 4:1 weight ratio of FAME to triacetin.Biodiesel (FAME and triacetin) had a theoretical weight of 125%, not 100%.FAME-only [14].The SCMA reaction has the potential to increase fuel quality while decreasing the price of biodiesel ingredients.Since both products can be reused, the separation and purification steps in this glycerol-free technique require less time and resources.A new supercritical approach to avoid glycerol byproducts was investigated recently Figure 1.FAMEs and triacetin are produced non-catalytically using supercritical methyl acetate instead of converting triglycerides to glycerol (SCMA).The theoretically derived 3:1 molar ratio of methyl oleate to triacetin from the transesterification reaction of rapeseed oil with methyl acetate is assumed to have had no effect on the principal fuel characteristics.Triacetin increased cold flow and oxidation stability.By defining biodiesel fuel (BDF) as methyl oleate and triacetin, SCMA can yield 105% of BDF.At high working temperatures, unsaturated FFMEs degrade, lowering this value below the theoretical maximum recovery (125% yield).The SCMA process enhances BDF quality and uses less energy during purification and separation.Triacetin is beneficial in food, cosmetics, and petroleum-based fuel.Proposed the first biodiesel manufacture using methyl acetate as a SCF.

Limitation and Issues with SCF Technology
Although the shortcomings of SCF technologies when compared to the traditional catalytic reactions outlined above are severe, they must be overcome if SCF is to play a significant role in biodiesel synthesis.

Utilization of energy
The energy required to supercritical heat the solvent is a drawback of SCF technology.The solvents used require high temperatures and pressures, which are energy-intensive and short-term solutions.SCF technique is energy-intensive since the SCM reaction requires temperatures and pressures above 2390C and 8.1 MPa to reach supercritical methanol.It is widely assumed that air pressure and an average reaction temperature of 1500C are sufficient to produce biodiesel from traditional catalytic reactions [17].The process requires more electricity than SCF-produced ethanol delivers, raising concerns.Thus, manufacturing lowenergy products like biofuel requires more energy.An combined heating and cooling system may minimize SCF energy use.Thus, the SCF process is energy-inefficient and inappropriate for biodiesel production.

Cost
A significant barrier to the implementation of the SCF technology is the expensive cost of the process itself, which is also one of the primary obstacles.For example, a considerable financial investment in high pressure pumps and furnaces was required in order to achieve the heightened temperature and pressure that were necessary at supercritical circumstances.When utilizing SCF technology, you will additionally require a sizeable quantity of the solvent in order to power the reversible process and increase the amount of biodiesel produced.As a consequence of this, the overall cost of the SCF reaction is increased due to the costly reactant and the additional processes required to recover the unreacted solvent.In addition, the majority of SCF reactors are usually made using materials that are both more robust and more long-lasting due to the distinctive qualities that are associated with the conditions of a SCF reaction.As a result of this, the SCF process has relatively greater costs associated with its operation and maintenance in comparison to more conventional catalytic processes.As a consequence of this, utilizing a response that occurs in two stages is not only recommended but also desirable.There have been very few attempts to commercialize the manufacture of supercritical biodiesel [18], primarily due to the exorbitant costs associated with the materials, operations, and maintenance.

The effect of the solvent
When oil was transesterified with methanol as the acyl acceptor, inhibitory effects from the methanol and glycerol in the solvent were reduced by utilizing a solvent containing lipase as the biocatalyst.Oil is transformed into FAME and glycerol carbonate in a small number of experiments by using DMC as the acyl acceptor and lipase as the biocatalyst.Gharat and Rathod showed the maximum oil conversion by solventfree synthesis.Gharat and Rathod] reported that polar hydrophilic solvents did not give as much FAME as non-polar lipophilic solvents, suggesting that the presence of a large amount of water could significantly affect the reaction.Hydrogen bonding and hydrophobic interactions may be disturbed when Novozymes 435 is exposed to polar solvents, leading to extremely low FAME yields [19].

Temperature effects
Temperature increases exothermic reaction rates.In a reaction system with oil as a reagent, higher temperatures decrease oil viscosity, reducing mass transfer and increasing reaction rate.Enzymes can accelerate reactions at a certain temperature [20].Novozymes 435 can tolerate 70°C.Temperature effects on oil conversion, FAME generation, and glycerol carbonate output have been inconsistently reported.Found, Lipase and DMC converted FAME and glycerol carbonate to 85% at 60 °C, 75% at 40 °C, and 72% at 85 °C.Temperature in solvent-containing and solvent-free settings.As expected, the reaction rate rose with temperature, reaching a maximum conversion (77.87%) at 60 °C before decreasing.Thermal denaturation at temperatures above 60 °C decreased enzyme activity, causing the conversion loss.Seong et al. found that enzyme denaturation affected FAME yield more than glycerol carbonate yield.also observed this.This supports the idea that glycerol carbonate is formed indirectly through intermediaries such glycerol, methanol, and fatty acid glycerol monoesters.Higher reaction temperatures promote molecular cooperation, which increases FAME synthesis, according to Zhang and colleagues.Maximum FAME and oil conversion using According to all literature, Novozyme 435 can be produced as a biocatalyst with DMC as an acyl acceptor at 55-60 °C.

Effect of reaction time
According to the vast majority of tests, a reaction period of approximately 30 hours was required to obtain FAME yields of more than 80%.It is possible that the reaction time may be cut down to 12 hours if the amount of enzyme used was increased; however, doing so would result in an increase in the cost of production.According to the findings of all of the studies, the reaction is nearly complete after 25 to 30 hours, and researchers found that increasing the reaction time did not considerably increase oil conversion or FAME yield.Gharat and Rathod were able to cut the reaction time for an oil conversion of over 90% by using ultrasonic irradiation to speed up the process.This allowed them to minimize the period of time needed to 4 hours.Conversions were comparable higher at higher temperatures and lower oil to DMC ratios.These results were found for the same amount of reaction time [21,22].

Molar ratio of oil to reactant
Alcohol has an effect on the generation of biodiesel because it prevents a reaction that can be reversed at the end of the processing step.Methanol, ethanol, n-butyl, tert-butyl, and isopropanol are the components that can be used to produce SCF biodiesel [23].This technique uses methanol and ethanol most often.Many expensive alcoholic beverages are rarely considered.Methanol dissolves triglycerides quickly in NaOH.The transesterification cycle and triglyceride-to-monoglyceride conversion are anticipated to speed up with a high methanol-to-oil molar ratio.Stoichiometry dictates a 3:1 alcohol-oil molar ratio.The molar ratio in a reaction environment increases triglyceride and methanol homogeneity.Interaction will improve product creation.Triglycerides and fatty acids react quickly and productively at high methanol concentrations.Too much dry alcohol added to oil at a 6:1 molar ratio creates alkyl esters during commercial transesterification [24].

Pressure
Heat lowers reactive pressure, tempering transesterification.Flow-type reactor analysis of each transesterification is precise.Kusdiana and Saka (2004) found that low pressure reaction rates were linear to reaction pressure [25].Air pressure and 230 C prevented transesterification.Pressure over the methanol critical pressure of 8.09 MPa or 10 MPa enhances methyl ester production.After 2 hours of reaction at 290 °C and air pressure, triglyceride conversion reached 20%.pressure affects transesterification more at high temperatures.High temperatures make pressure increase easy, which boosts alcohol concentration and volume.According to the mass action principle, reactant concentration alone determines reaction rate and equilibrium.High-volumetric alcohol transesterifies well.High pressures make triglycerides more soluble, which enhances alcohol-triglyceride molecular interaction.Mass transfer.Heating or mixing systems can provide energy in a standard transesterification reactorMass transfer avoidance is unprofitable.Methanol boils 65 °C higher than solvents, making this impractical at air pressure.Alcohol and oil immiscibility limits alkaline-catalyzed methanolysis.Hexane, SCCD, propane, tetrahydrofuran, and methyl-tert-butyl ether may assist the reactor overcome this obstacle in one step.Catalyst precipitation changed system polarity.Methanol significantly slows co-solvent (single-phase) alcoholysis.To compensate for this shift in polarity, production sizes must be increased to increase the molar ratio of alcohol to oil, making the cycle harder.Miscibility and biodiesel rate increase with co-solvent concentration.During enzyme-catalyzed methanolysis of canola oil, lipase inactivation was inverted [26].Because it prevents phase separation during methanolysis Figure 2, [27].SCM (200-350 °C; 9-43 MPa) is a preferred solvent.Supercritical solvents are liquid-gaseous.These features enable quick, efficient, and easy FAME synthesis from triglycerides.SCCD and SCM integrated substitute solvent processing with cosolvent processing.In a fixed-bed reactor, the catalytic cycle produced continuous biodiesel (88% yield) at 200 °C in 2 minutes.To create soybean methyl ester, the SCM method required less energy, oil at a 24:1 molar ratio, 280 °C for the reaction, 12.8 MPa of pressure, and a reaction period of 10 min.in propane.

CONCLUSIONS
The use of SCF technology has a number of advantages over the more usual catalytic reactions when it comes to the manufacturing of biodiesel, and these advantages will be vital in the future when it comes to solving the problem of energy security.In contrast to the catalytic process, the utilization of supercritical fluid technology as an environmentally friendly approach to the production of biodiesel shows more potential.Before the SCF technology can play a significant role as the primary technique for the production of biodiesel that is renewable and sustainable, a number of problems need to be resolved.As a consequence of this, a number of ideas that have the potential to enhance the SCF process in terms of energy consumption as well as the costs of material and equipment have been investigated in this article.These concepts include

Figure 1 .
Figure 1.The transesterification reaction that takes place between triglyceride and methyl acetate.

Figure 2 .
Figure 2. (a) In the methanol phase diagram, route 1 represents a conventional supercritical extraction (SCE) process, while route 2 represents the novel supercritical extraction (NRSE) technique, both of which include maintaining N2 gas pressure on the methanol.At a 30 bar N2gas pre-pressure in the vessel, a time-dependent pressure-temperature (P-T) curve was obtained for the NRSE drying process.(c) The NRSE process's P-T curve with a variation in N2gas pre-pressure from 0 to 40 bar.This is seen in (d), where we change the N2 gas prepressure from 50 to 70 bar and plot the resulting P-T curve for the NRSE process.

Table 1 .
The temperatures and pressures at which certain alcohols reach their critical states.