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Purification of Menthol by Supercritical CO2 Fluid Fractional Distillation


How to extract peppermint oil

The stems and leaves of peppermint plants can be steam distilled to extract mint crude oil. They have analgesic, relieve itching, aromatic and refreshing effects.


They are widely used in oral hygiene, detergents, tobacco, perfume and pharmaceutical industries.

Mint crude oil

Mint crude oil can be frozen and crystallized. Obtain menthol and peppermint oil.

The process has a long production cycle, high cooling energy consumption, and low production automation, which to a certain extent restricts the benefits of the production enterprise.

Supercritical Fluid Fractionation

Supercritical Fluid Fractionation uses CO2 as the solvent.
It has the advantages of moderate separation temperature, high separation efficiency, no solvent residues in the product, and environmental protection.
It can separate and purify specific compounds in food, medicine and other industries and has attracted much attention.

This paper studies the purification of menthol by Supercritical Fluid Fractionation technology, and investigates the influence of temperature, pressure and other process parameters on the separation effect.

CO2 purification method of menthol

Results and discussion

The influence of process parameters on distillation yield

Supercritical fluid precision separation technology uses the establishment of a temperature gradient and program boost in the separation tower to improve the separation efficiency.

The temperature gradient and program boost process are the main factors affecting the separation effect

There is a sudden change interval in the distillation yield with the pressure change. In this interval, the distillation yield increases sharply with the pressure change, the temperature gradient increases, and the interval moves to high pressure. Because the temperature gradient increases, the CO2 density at the top of the rectification column will increase. Decrease, the decrease of its dissolving ability can only be compensated by increasing the pressure.

Therefore, to obtain the distillation with the same yield, the required pressure must be higher, the temperature gradient increases, and the mutation interval widens, resulting in a slower extraction rate , The operating pressure is increased; the temperature gradient is smaller, the mutation interval is narrowed, the extraction rate is accelerated, and the operating pressure is correspondingly reduced, but the pressure interval is too narrow, which will make the separation effect worse and the operation control difficult. After comprehensive analysis, the appropriate temperature gradient It is 20°C.

The pressure rise rate is slow, and the distillation yield is high under the same pressure. The effect of the pressure rise rate is essentially the effect of pressure on the fluid solubility. Although the faster pressure rise rate can shorten the separation time, it may be detrimental to the separation effect

Influence of process parameters on separation effect

With the increase of the distillation yield, the content of menthol in the distillation increases. After reaching a certain yield, the content of menthol in the distillation increases sharply.

For example, when the temperature gradient is 20°C, when the distillation yield is 38.19%, Menthol = 75. 55% rectification can be obtained, and when the yield is 50.37%, menthol = 90% rectification can be obtained, and the menthol is concentrated in the range of 50% to 100% yield.

The curves of other temperature gradients are relatively flat, that is, the effect of “concentrating” menthol is relatively weak, and the suitable temperature gradient is 20°C.

The menthol content in the distillation at different pressure rise rates is almost close to the distillation yield curve, indicating that the pressure rise rate has little effect, which is very beneficial for industrial applications. A faster rate of pressure increase can be selected to achieve the purpose of shortening the fractionation time.

Menthol yield

Generally speaking, for a specific separation technology, the target product yield and content are a pair of contradictory parameters. Therefore, the relationship between the target product content and the yield becomes an important indicator for evaluating the separation method and optimization of process parameters.

Under a temperature gradient of 20°C, the appropriate pressure rise rate is 0.1 MPa. (15 min)⁻¹ At this time, a distillation with menthol = 99% and a yield of 10.53% is obtained, and the corresponding menthol yield is 13.01 % (Relative to the menthol in the raw material), are higher than the results of other pressure rise rates.

Study on Re-isolation of Crude Menthol

Studies have shown that the rectification of menthol = 99% can be obtained after one separation, but its yield is relatively low. In order to increase the yield of menthol, it is an effective means to separate the products from the first separation.

Menthol obtained after one separation = 90% crude menthol rectification and menthol = 36% veggie oil rectification were studied again (experimental conditions are 20°C, 0.1 MPa. (15 min)⁻¹), Results The distillation yield of menthol = 90% for menthol oil was 35%; for crude menthol, the distillation yield of menthol = 90% was 80%, and the distillation yield of menthol = 99.9% was obtained. The rate exceeds 50%.

In conclusion

Purification of menthol with supercritical CO2 precision separation technology shows that:

The establishment of the temperature gradient in the rectifying column has an important influence on the separation and purification of menthol. Within the scope of the experimental investigation, the suitable temperature gradient is 20°C.

The pressure increase rate is also an important factor affecting the separation and purification of menthol. The appropriate pressure increase rate is 0.1 MPa. (15 min)⁻¹.

Re-separation of menthol oil after one separation can obtain menthol = 90% and a yield of 35%, and the crude menthol obtained in one separation can obtain menthol = 99.9% and a yield of more than 50%. Distillation

Process parameters

Temperature gradient : 20°C

Appropriate pressure increase rate: 0.1 MPa. (15 min)⁻¹