Steam distillation (Steam Distillation) refers to the co-distillation of plant material containing volatile components with water, so that the volatile components are distilled together with water vapor, and the volatile components are extracted by condensation. This method is suitable for the extraction of plant active ingredients that are volatile, can be distilled with water vapor without being destroyed, are stable in water and are insoluble or insoluble in water.
Water vapor distillation method is the most commonly used method for extracting plant natural flavors, and its technology of process, equipment, operation and other aspects is relatively mature, the cost is low and the output is large, and the equipment and operation are relatively simple.
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Principle of water vapor distillation
Water vapor distillation is one of the important methods of separating and purifying organic compounds, it is to pass water vapor into a mixture containing insoluble or slightly soluble in water but with a certain degree of volatility, and make it heat and boil, so that the organic matter to be purified is distilled out together with water vapor at less than 100 ° C, so as to achieve the purpose of separation and purification.
According to Dalton's law of partial pressure, when a substance that is not miscible with water coexists with water, the vapor pressure of the entire system should be the sum of the vapor pressures of each component, that is:
where P represents the total vapor pressure,
PA is the vapor pressure of water,
PB is the vapor pressure of miscible substances that are not miscible with water.
When the sum of the vapor pressures of the components in the mixture is equal to the external atmospheric pressure, the mixture begins to boil, and the temperature is their boiling point. The boiling point of the mixture is lower than that of any of the components. Therefore, by applying water vapor distillation at atmospheric pressure, the high boiling point components can be steamed out with water at less than 100 ° C. The boiling point of the mixture remains constant during distillation until almost all of the components are steamed out (since the total vapor pressure is independent of the relative amount of the mixture), and the temperature rises to the boiling point of the liquid left in the bottle.
The ratio of the partial pressure (PA, PB) of each gas in the vapor of the mixture is equal to the ratio of their amount of substance (nA, nB), i.e.,:
nA=mA/MA；nB=mB/MB。 Among them, mA and mB are the masses of the vapors of each substance in a certain volume, and MA and MB are the relative molecular masses of substances A and B. therefore:
It can be seen that the relative mass of these two substances in the distillate (that is, their relative mass in the vapor) is proportional to their vapor pressure and relative molecular mass.
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Application of water vapor distillation
Water vapor distillation has a long history, and since the inception of CA in 1907, some people have published articles in this regard, but at that time the discussion was limited to the most basic physical and chemical principles. Later, water vapor distillation has been widely used in industry, and until now, there are still a large number of articles every year that discuss water vapor distillation. Here's a quick look at its main areas of application:
(1) Resin processing: that is, the collected pine resin is distilled by water vapor to obtain liquid turpentine and solid rosin;
(2) Extraction of essential oils from plant leaf stems;
(3) Water vapor distillation combined with solvent extraction (or solid phase adsorption) to measure trace substances;
(4) Extraction of oil from sediments;
(5) Extraction of volatile oil from traditional Chinese medicine;
(6) recovery of benzene compounds in the coal tar and petroleum industries;
(7) Deodorization and acid reduction of fatty acids, edible oils, polyesters (high boiling substances);
(8) Distillation of dimethylthiophosphamide with decompressed water vapor;
(9) High-pressure water vapor distillation recovers and purifies phosphorus from phosphorus-containing residues.
The water vapor distillation method is suitable for separating substances that are easily decomposed near their boiling point, and is also suitable for separating the required components (such as natural product essential oils, alkaloids, etc.) from non-volatile substances or resinous substances. Substances purified using this method must meet the following conditions:
(1) Insoluble in water or slightly soluble in water;
(2) Has a certain degree of volatility;
(3) Does not react with water at azeotropic temperature;
(4) At about 100 ° C, it must have a certain vapor pressure, at least 666.5 ~ 1333Pa (5 ~ 10mmHg), and the material to be separated has a significant vapor pressure difference with other impurities at about 100 ° C.
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Example of water vapor distillation
For example, the boiling point of aniline is 184.4 °C, and it is miscible with water. When heated with water to 98.4 ° C, the vapor pressure of water is 95.4 kPa, the vapor pressure of aniline is 5.6 kPa, and their total pressure is close to atmospheric pressure, so the liquid begins to boil, and the aniline is distilled out with the water vapor.
When using water vapor distillation to separate purified substances, the vapor pressure of this substance at about 100 ° C is required to be at least 1.33 kPa. If the vapor pressure is 0.13 ~ 0.67 kPa, its content in the distillate accounts for only 1%, or even lower.
In order to increase the content in the distillate, it is necessary to increase the vapor pressure of this substance, that is, to increase the temperature, so that the temperature of the vapor exceeds 100 ° C, that is, to use superheated water vapor distillation.
For example, benzaldehyde (boiling point 178 °C), when distilled by water vapor, boils at 97.9 °C, at this time PA = 93.8 kPa, PB = 7.5 kPa, then:
At this time, benzaldehyde accounted for 32.1% of the distillate.
If the superheated steam of 133 °C is introduced, the vapor pressure of benzaldehyde can reach 29.3 kPa, so as long as there is a water vapor pressure of 72 kPa, the system can be boiled, then:
In this way, the content of benzaldehyde in the distillate is increased to 70.6%.
The application of superheated water vapor also has the advantage of making the water vapor condense less, in order to prevent the condensation of the superheated vapor, it can be kept warm under the distillation bottle, or even heated.
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Water vapor distillation unit
The water vapor distillation device is usually composed of a water vapor generator, a distillation part, and a receiving part.
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Advantages of water vapor distillation
The industrial application of water vapor distillation is of great value, but almost all applications are only used as simple distillation, or superheated steam as multistage carrier gas distillation of inert gas. From the previous introduction, it can be seen that distillation in the presence of an aqueous phase can produce a good cooling effect. Therefore, some scholars consider introducing liquid water into the separation process of traditional multi-stage water vapor distillation, thus proposing the concept of water vapor distillation (saturated water vapor in a multi-stage water vapor distillation device).
As a potential method for separating thermally sensitive lines, water vapor distillation has its unique advantages in addition to significantly reducing operating temperatures:
(1) There is good agitation in the distillation equipment without local overheating, which promotes heat transfer and mass transfer, and can prevent scaling, which is the essential advantage of water vapor distillation.
(2) Water vapor removes oxygen from the atmosphere, which prevents oxidation of the treated substance.
(3) At the same time of rectification, complete the functions of acid reduction and deodorization, and it will not cause odor. This has a special significance for the separation and purification of edible natural fatty acids and oils.
(4) The final separation of the product and the water only requires simple liquid-liquid stratification, which is incomparable to the extraction, and the water vapor distillation will not have harsh requirements for equipment and operating conditions.
(5) Due to the increase in pressure drop caused by the change of the structure of the tower plate or filler, there are two liquid phases that are borne separately, which will not cause a significant increase in the kettle temperature.
(6) Water as a separator is available everywhere, and because of the small heat transfer resistance in the tower, the grade of energy used for heating can be relatively reduced, and low-pressure vapor can also be directly passed into the tower kettle, which is conducive to the comprehensive utilization of energy.
(7) Since there is an aqueous phase in the whole tower, it provides a possibility for salt distillation.
Want to know more about distillation?
Welcome to the "2019 Complex Distillation Techniques with."
Seminar on Innovation and Application of Chemical Process Enhancement"!
Overview of the meeting
Organizers: Shanghai Institute of Chemical Engineering Limited Company,
National Research and Promotion Center for Packing and Device Technology of Efficient Separation Towers
Organizers: China Petroleum and Chemical Industry Federation (Shanghai) Training Center, China Chemical Industry Society Training Center
Supporting units: National Engineering Research Center for Distillation Technology (Tianjin University), Chemical Engineering Committee of Shanghai Chemical and Chemical Society, Tianjin Aozhan Xingda Chemical Technology Co., Ltd
Supporting media: Bright Conference Hall Chemical 707 Haichuan Chemical Network
Time: September 19-21, 2019
Location: Shanghai
Content of the conference
(1) The green development and application of complex distillation separation technology
(2) Separator distillation technology and process coupling technology
(3) The application of micro-reaction technology in chemical process strengthening
(4) Efficient separation technology solutions
Participation fees
Standard fee: 2800 yuan for domestic representatives, 1500 yuan for students, 500 US dollars for foreign representatives.
Advance remittance: $2,500 for domestic delegates, $1,300 for students, $420 for foreign delegates.
Note: The conference fee includes information fee and meal fee, and the accommodation is arranged uniformly, and the cost is at your own expense. Pre-registration is available for remittances before September 5.
Remittance Account Information:
Account name: Shanghai Research Institute of Chemical Industry Co., Ltd
Account number: 1001247219100017952
Bank: ICBC Shanghai Jinshajiang Road Sub-branch
Registration entrance
Inquire for details
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Contact: Ms. Xue
Contact number: 18630538350
Email: [email protected]