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Small-scale Hot Reflux Extraction Tank.

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Overview

The small-scale hot reflux extraction tank is a type of traditional Chinese medicine (TCM) extraction equipment developed based on the Soxhlet principle, which has been widely applied in recent years. The most commonly used extraction methods for TCM are alcohol extraction and water extraction, with alcohol or water serving as the solvent. The energy-saving effect of hot reflux extraction has been universally acknowledged, especially for alcohol extraction.

 

The equipment is primarily composed of two parts: the extraction unit and the concentrator unit. Specifically, during the extraction and concentration process, the extraction liquid continuously flows from the bottom of the extraction tank into the concentrator for concentration. The secondary steam generated by concentration is condensed and continuously returned to the extraction tank as fresh solvent, maintaining a high concentration gradient of extractable substances between the medicinal materials and the solvent. This enables the solutes in the medicinal materials to dissolve rapidly, achieving efficient extraction.

Actual Application Conditions and Problem Analysis of the Original System

When the small-scale hot reflux extraction tank system is applied in alcohol extraction, due to the low boiling point of alcohol, the evaporation amount is large, and the reflux volume is high. This can relatively maintain a high concentration gradient of substances between the medicinal materials and the solvent, enabling rapid dissolution of solutes from the medicinal materials to achieve good extraction efficiency. Additionally, it can effectively recover the solvent, showing strong applicability in alcohol extraction. However, in water extraction of traditional Chinese medicine, because the boiling point is relatively high and the evaporation amount is correspondingly reduced, the application effect is not ideal.

 

The 6m³ extraction tank has the largest feeding capacity among the currently commonly used extraction tank series. According to conventional operation, one extraction requires decocting with water 2 to 3 times, typically 2 times. That is, decocting with a 6m³ extraction tank once requires 8 to 10m³ of water, meaning the reflux volume needed for the hot reflux method is 4 to 5 tons. Therefore, the heating area of the concentrator must be relatively large. To ensure sufficient heating area without making the heater too bulky, equipment manufacturers adopt methods such as relatively lengthening the heating tubes, reducing the pipe diameter, and increasing the number of tube rows (note: compared with a general triple-effect concentrator) to guarantee the heating area of the heater.

 

In practical application, the evaporation amount of the concentrator is small, and the reflux volume fails to meet the requirements. This results in a low concentration difference between the medicinal materials and the extraction liquid, slow release of effective components, and an extraction time of more than 10 hours to complete.

It should also be noted that some hot reflux extraction equipment directly introduces the secondary steam volatilized from concentration into the extraction tank, which is then condensed by a condenser and refluxed back to the extraction tank. The heat of the secondary steam is used to maintain the boiling of the medicinal liquid in the extraction tank, thereby achieving energy-saving effects. In practical production, we have tried this method, but the secondary steam failed to maintain the boiling of the medicinal liquid in the extraction tank, especially for large straight-tube extraction tanks. Moreover, as the secondary steam enters the already limited space of the extraction tank, it increases the internal pressure, causing poor reflux of the condensate and even uncondensed steam to spray out from the reflux pipe directly without condensation due to short-circuiting. This prolongs the extraction time to more than ten hours for each extraction, making it impractical for actual production applications.

Main Causes

(1) Due to the use of long and small-diameter heating tubes in the external heater of the concentrator, the liquid entering from the bottom rapidly boils when heated by steam, and most of it vaporizes as it rises to the middle-upper part (visible from the sight glass at the top of the heater). This results in the underutilization of the heating area of the middle-upper heating tubes, i.e., a reduction in the effective heating area.

 

(2) The circulation pipe diameter of the concentrator heater is too small, causing delayed liquid replenishment to the heater. According to empirical parameters in chemical engineering handbooks: the cross-sectional area of the circulation pipe for a natural circulation external heater should be 2 to 3 times the total cross-sectional area of the heating tubes. In practical applications with short-tube (approximately 1 to 1.2m) heaters, a ratio of about 1 times is sufficient (as seen in triple-effect concentrators). However, the cross-sectional area of the circulation pipe in the current hot reflux extraction concentrator is only 1/2 of that of the heating tubes.

 

(3) Under the rated steam pressure, the heating area may be too small. This could be due to improper design choices, such as incorrect thermal conductivity parameters, leading to an undersized heater heating area and consequently low evaporation volume.

Retrofit and Application Conditions After Retrofit

Through the above analysis, we know that the evaporation capacity of the concentrator is the main factor affecting the reflux volume in hot reflux extraction. Based on the above analysis, we carried out the following retrofits:
(1) Since the heating area of the heater is fixed, it is difficult to increase the area, and a too large heater is uneconomical in terms of cost and floor space. Therefore, when the material and area of the heater are fixed, the only consideration is the temperature difference between the heating steam and the extraction concentrate. As the application pressure of the heating steam is fixed (i.e., the maximum temperature of the heating steam is fixed), reducing the boiling point of the concentrate becomes the only option. This means that vacuum concentration must be adopted to improve the evaporation capacity of the concentrator. However, since the process uses atmospheric pressure extraction, vacuum concentration requires intermittent reflux of the condensate to the extraction tank, thus requiring the addition of a concentrate condensate storage tank.

 

(2) For the problem that the middle-upper heating tubes of the concentration heater are vaporized due to the heated liquid and the too small cross-sectional area of the circulation pipe affects the utilization rate of the heating area, we referred to various materials and adopted the bottom-mounted natural circulation evaporator method and the method of increasing the diameter of the circulation pipe. That is, the two main components of the evaporator, the heater and the vapor-liquid separator, are respectively arranged at the bottom and the top. Considering that the heated liquid should rise easily and the liquid should have a high static pressure in the pipe to raise the boiling point and prevent vaporization during flow in the heating pipe, the heater is installed at an angle of about 20°, which is also conducive to the drainage of steam condensate.

 

After the above retrofits, the evaporation capacity of the concentration evaporator has been greatly improved. According to normal operation, concentration can usually start about one hour after the extraction liquid boils, and the volatile reflux volume can reach about 5m³ within 4 hours, ensuring a continuous high-concentration gradient between the extracted drug and the liquid, thus ensuring that the effective components of the drug can be precipitated at a high rate. In this way, from feeding to extraction and concentration, the operation of one batch of feeding can be completed within one shift, which is closely in line with the production shift.

 

Generally, the operation of one batch of feeding can be completed within one shift, which is closely consistent with the production shift.

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