Читать книгу Biodiesel Production - Группа авторов - Страница 42

Critical point of a fluid designates the temperature and pressure at which compounds exit the liquid–vapor phase equilibrium. Beyond this stage (under supercritical conditions), the formed vapor cannot return to liquid state even under high pressure. For pressure‐driven chemical conversions, this is a huge benefit that ensures spontaneous product removal after reaction (for continuous production). Hence the use of supercritical fluids is another PI approach, which has been successfully used for complete conversion of various feedstock (fed‐batch or continuous). First proposed by Saka and Dadan [35], supercritical transesterification uses alcohols (polar or nonpolar) that have been preheated at 250–400 °C under 10–65 MPa pressure, during which the alcohol shows an increase in viscosity, resulting in decreased dielectric constant. This ensures that even polar alcohols become completely miscible, forming a single phase with superheated oil. The mixture is then pumped into the supercritical reactor (small capacity stainless steel reactor in a bath‐type heater). After the desired duration, the reactor is withdrawn from the bath, depressurized, and cooled for product collection. The supercritical state ensures that diffusional resistances among the reactants are drastically reduced; with E _a no longer a hindrance, the reaction occurs spontaneously with complete conversion in a few minutes. However, the supercritical reactor must have high durability against such extreme pressure and temperature while requiring frequent maintenance, which are setbacks regarding process economics and energy efficiency. Formed glycerol is also not usable due to high impurity and exposure to such extreme conditions [36]. Consequently, few modifications were proposed and tested by researchers to minimize these issues, making the process more lucrative for small‐scale commercial production (Table 2.3).

Using cosolvents such as CO₂, n‐hexane, tetrahydrofuran, propane, etc. facilitates solubilization, achieving homogeneous phase under much benign conditions, thereby increasing energy efficiency [46]. Metal oxides (ZnO, SrO₂, TiO₂, etc.) were successfully used for facilitating conversions under lowered pressure and temperature; however, catalyst separation is a hurdle that impedes smooth operation [47]. As a better alternative, uncatalyzed subcritical hydrolysis followed by supercritical esterification was proposed and tested by Kusdiana and Saka [48]. Also glycerol‐free processes (alcohol‐free) have been developed that use compounds such as methyl acetate to yield triacetin (used in leavening of bread and flavor enhancing of beverages), dimethyl carbonate to yield glycerol carbonate (used as fuel additive) and citramalic acid (used in cosmetics for skin toning), and methyl tert‐butyl ether, which yields glycerol tert‐butyl ether, used as biodiesel additive to lower cloud point and enhance cetane number.

Table 2.3 Modern approaches in biodiesel production from nonedible/waste feedstock.

Supercritical fluid‐assisted biodiesel production
Feedstock	Solvent (+ catalyst/cosolvent)	Reaction temperature (°C)	Reaction pressure (MPa)	Alcohol:oil ratio	Residence time (min)	Yield (%)	References
Waste frying oil	Methanol	300	20	40 : 1	Continuous	81.7	[37]
Castor oil	Methanol	350	20	40 : 1	~ 40	>99	[38]
Ethanol
Linseed oil	Methanol	350	20	40 : 1	~ 40	>99
Ethanol
Ultrasound‐assisted biodiesel production
Feedstock	Solvent + catalyst	Reaction temperature (°C)	Reaction time (h)	Catalyst concentration (%w/w)	Alcohol:oil (ratio or %w/w)	Sonication (kHz)	Conversion/yield (%)	References
Waste cooking oil	Dimethyl carbonate + Novozym 435	60	4	10	6 : 1	25	86.61	[39]
Waste lard	Candida antarctica lipase B	50	0.33	6	4 : 1	5	96.8	[40]
Waste tallow	Candida antarctica lipase B	27	0.33	6	4 : 1	5	85.6	[41]
Microwave‐assisted biodiesel production
Feedstock	Catalyst	Reaction temperature (°C)	Reaction time (h)	Catalyst concentration (%w/w)	Alcohol:oil (ratio or %w/w)	Power (W)	Yield (%)	References
Waste cooking oil	Sodium methoxide	27	0.05	0.75	6 : 1	750	97.9	[42]
Waste cooking oil	SrO/SiO2	65	0.41	0.75	12 : 1	242	99.2	[43]
Waste cooking oil – Calophyllum inophyllum oil blend	KOH	27	0.12	0.774	59.6	850	97.4	[44]
Waste cooking oil	H2SO4	27	6	0.5	9 : 1	800	92	[45]