The extraction selectivity and yield depended on the SC-CO₂ density which was regulated by the operation parameters. Herein, the key factors included temperature, pressure and time, and their effects on the extractions were as follows.

The influence of extraction temperature on supercritical fluid is complex. On the one hand, temperature has an effect on the concentration of carbon dioxide. High temperature will reduce the density of carbon dioxide fluid, weaken the dissolution effect, reduce the solubility of materials, thus reducing the yield. On the other hand, under a certain pressure, the temperature rises, the volatility of the extraction solute and the thermal motion increases the opportunity of intermolecular association, thereby improving the extraction rate. Therefore, an increase in temperature facilitates the diffusion of the solute, but may also reduce its solubility; the extraction rate depends on which state prevails at a given temperature. The extraction yield of essential oil at different temperatures (35 °C, 40 °C, 45 °C, 50 °C and 55 °C) was studied at a set pressure (20 MPa) and extraction time (2 h). The highest extraction rate was about 5% at 45 °C. Below 45 °C, the extraction rate increased with an increase in temperature. From 45 to 55 °C, the extraction rate decreased slightly and tended to be stable. The extraction rate of essential oil obtained from the sorted data is shown in Figure 1a.

In this section, the yield of essential oil under different extraction pressures (10 MPa, 15 MPa, 20 MPa, 25 MPa and 30 MPa) was studied at a set temperature (45 °C) and extraction time (2 h) conditions. The extraction rate was highest at 20 MPa, which was nearly 5%. When the pressure was lower than 20 MPa, the higher the pressure, the higher the extraction rate. The growth rate was relatively rapid, from 15 MPa to 20 MPa. When the extraction pressure exceeded 20 MPa, the extraction rate decreased significantly. The results are shown in Figure 1b. The extraction pressure had a significant effect on the extraction rate. On the one hand, increasing the pressure not only increases the density of carbon dioxide, but it also reduces the intermolecular distance and improves the mass transfer efficiency. Pure camphor is a solid at temperatures of 304 to 354 K; nonetheless, when in contact with carbon dioxide, it dissolves readily and reaches phase equilibrium [27]. As the pressure was increased from 15 MPa to 30 MPa, more oxygenated compounds, such as oxidized sesquiterpenes, were present, which is also consistent with the findings of others [28] regarding the higher oxygen content in extracts at elevated SC-CO₂ pressures. This further illustrates that changes in pressure cause significant changes in the density of CO₂, which in turn causes changes in the solubility of the substance to be extracted for the purpose of selective extraction. On the other hand, these two factors are not linear. When the pressure increases to a certain extent, the dissolution rate increases slowly, thus affecting the extraction rate. In addition, the higher the supercritical pressure, the greater the density of carbon dioxide, the more complex the extract, and the worse the selectivity [29].

The influence of extraction time cannot be ignored because the extraction effect is poor when the extraction time is too short. Similarly, if the time is too long, it not only reduces the production efficiency of the equipment, but also wastes energy and manpower. Therefore, it is necessary to determine the optimal extraction time to achieve the best extraction rate. As can be seen from Figure 1c, the extraction rate increased rapidly at the beginning of extraction, slowed down after the extraction time of 2.0 h, and barely increased after 3.0 h. This is because at the initial stage of extraction, the content of essential oil in camphor leaves is larger, so the extraction rate increases rapidly; with the extension of time, the essential oil content in camphor leaves gradually decreases, leading to reduced increases in extraction rates.

In order to make the extraction process more reasonable and scientific, an orthogonal experiment was carried out on the basis of a single factor experiment. The three important factors of extraction time, extraction pressure and extraction temperature were tested. The experimental results are shown in the following Table 2.

The orthogonal test results showed that the factors affecting the extraction rate were extraction time > extraction pressure > extraction temperature. The optimum combination was an extraction pressure of 25 MPa, an extraction temperature of 45 °C, and an extraction time of 2.5 h, with a yield approaching 4.63%.

The water vapor distillation method is most commonly used to extract essential oils, using water as solvent. This method is more environmentally friendly. In the process of extraction and steam distillation, the temperature is high, the extraction time is long, the system is open, and the two steps are combined into one at the same time, thus reducing the trial process, shortening the analysis time, saving the extraction solvent, simplifying equipment and facilitating the operation [33,34]. Although the extraction rate is low, the purity of essential oil is high. Many substances are typical components of camphor tree essential oil, which can be further fractionated or purified by distillation. Meanwhile, high temperatures may lead to the volatilization or oxidation of allergic components in essential oils. This also occurred during the extraction of the volatile oil of Ligustrum quihoni by Wang et al. [35]. Only insoluble essential oils can be collected by this method, and the loss of water-soluble components leads to a low extraction rate. The extracted essential oil is still a multi-component composite essential oil, which is difficult to extract selectively, thereby affecting levels of purity, and the components of the collected essential oil are incomplete. In addition to using other separation techniques, obtaining purer essential oils may be more effective using other varieties of camphor trees.

Supercritical carbon dioxide extraction technology uses fluid carbon dioxide as the extractant to extract effective components from plants, so as to maintain the purity and active components of essential oils to the greatest extent. Carbon dioxide, as an inert gas, does not have a chemical reaction in the extraction process, is safe and environmentally friendly. Essential oil components can be selectively extracted and further purified by selecting the appropriate temperature, pressure and extraction time.

Through experimental research, the extraction rate of supercritical carbon dioxide is found to be significantly higher than that of the conventional steam method, and it is also more advantageous in terms of time [36]; the extraction rate can be increased by more than two times, while the extraction time can be reduced by more than half. This is consistent with the results of extracting essential oil from firenut leaves [37], evening primrose [38] and patchouli [39]. This may be because, in addition to essential oils, some substances such as n-docosane, stearic acid and ergosterol can be extracted. Of course, pigments can also be extracted, so the purity of essential oil is not good enough. Some experiments [19] also show that supercritical extraction has a very key advantage in that its extraction temperature and pressure are highly adjustable; there are obvious differences in material types and the main components under different extraction conditions, which lays a foundation for selective extraction, meaning that the selective extraction of essential oil components can be tested by changing the extraction conditions. For example, with pressures from 15 MPa to 30 MPa, the percentages of major compounds such as linalool and camphor varied (from 19.81 to 66.42% and from 3.39 to 9.05%, respectively). However, the percentages of β-phellandrene, α-terpineol and α-pinene decreased significantly at higher pressures. Similar conclusions were reached by Moldão-Martins et al. [40], who described a decrease in monoterpene content with increasing pressure.

The extraction method of essential oil should be considered according to the chemical structure of the sample, required yield, required purity, production cost, economic benefit and other comprehensive factors. At the same time, the method used must be tested repeatedly to find the most suitable conditions. The data, conditions and experience obtained in this experiment are expected to provide powerful assistance to other relevant researchers.

The process of steam distillation extraction refers to method A for the determination of essential oil in Appendix XD of The Chinese Pharmacopoeia 2015 edition [41]. Camphor tree raw materials of a certain mass (m₁) and 600 mL distilled water were added into a 1000 mL round bottom flask, and the essential oil collection device was assembled after mixing evenly. The extraction was carried out by heating with water vapor for 6 h and stopped when the yield of product no longer increased. The essential oil was cooled and collected, and dried with an appropriate amount of anhydrous sodium sulfate, then the oil phase was separated out and weighed (m₂). The experiment was repeated three times and the average value taken. Finally, the E-oil was analyzed with GC or GC-MS (sealed and stored in cold storage away from light, if necessary). The extraction rate calculation formula was:Extraction rate = (m₂/m₁) × 100%(1)

For all experiments of this section, the extraction vessel was packed with 60 g of camphor tree leaves. The extraction tests using supercritical CO₂ were carried out using the equipment and flow diagram, as shown in Figure 4, and as follows: Carbon dioxide from the cylinder passed through a precooler, a positive displacement pump and a preheater before it entered the bottom of the extraction vessel. The extraction vessel was maintained at a predefined pressure (10–30 MPa) and temperature (35–55 °C). E-oil was recovered from the loaded solvent stream, expanded to system pressure (ca. 5 MPa) in the separation vessel. The flow of SC-CO₂ was controlled by check valves and measured by a gas flow meter with an accuracy of ±0.01 kg/h. A variable frequency piston pump controlled the pressure in the vessel to an accuracy of ±0.1 bar. The extraction rate was calculated using the formula:Extraction rate = m (essential oil)/m (camphor tree leaves) × 100%(2)