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Supercritical Aerogel Drying Technology Summary
2018-01-16 14:54:51       From: Tradematt-Aerogel
1, The way to achieve aerogel drying:

There are two main types of drying media commonly used: one is methanol, ethanol and other organic alcohols; another is CO2.

Based on the literature, the existing SCFD technology can be divided into the following three categories:

1-1, High temperature supercritical organic solvent drying:

Sol-gel preparation of catalysts are generally carried out in aqueous solution. As the critical temperature of water, the critical pressure is relatively large (Tc = 374.15 ℃, Pc = 22.12MPa), so the hydrogel is not suitable for direct supercritical fluid drying.

The high-temperature supercritical organic solvent drying method is to replace the water with the organic solvent obtained by the reaction to obtain the organic sol or the organic sol prepared directly by other methods, and then the organic sol is aged to become the organogel; and then the organic coagulant Glue placed in the same amount of solvent has been placed in the high pressure dryer, heated to pressurize the organic solvent to achieve a supercritical state, the use of the organic solvent to achieve the purpose of driving off the solvent in the gel.

The high temperature supercritical solvent drying method is simple and the drying effect is obvious. After the hydrogel is prepared, the alcohol sol is exchanged with anhydrous ethanol, the alcohol gel is obtained through aging, the anhydrous ethanol is used as a medium, and the alcohol is dried by SCFD gel.

1-2, Liquid CO2 replacement supercritical drying:

The critical temperature of CO2 is close to room temperature, and CO2 is non-toxic and non-flammable. Therefore, CO2 must be a good drying medium for the supercritical fluid to dry. Liquid CO2 replacement supercritical drying method is the use of CO2 instead of organic solvents as the drying medium for supercritical drying.

In the method, first, the liquid solvent in the gel is replaced with liquid CO2, and then the temperature is increased to make the CO2 reach a supercritical state; finally, the aerogel is dried by using the supercritical CO2. Because of the lower temperature drying process, so this method is also called low-temperature supercritical CO2 drying method.

Taking alcohol gel drying as an example, the alcohol gel is placed in a supercritical dryer and liquid carbon dioxide is introduced into the solvent to replace the water and ethanol in the alcohol gel, and then the temperature and pressure are increased to reach the supercritical carbon dioxide Conditions, after a certain period of time to release carbon dioxide gas slowly, and finally get the aerogel solid.

1-3, Supercritical CO2 extraction aerogel drying:

Due to the long time of liquid CO2 replacement solvent, the drying cycle is too long, is not conducive to industrial production, it is gradually replaced by supercritical CO2 extraction drying method. Supercritical CO2 extraction drying is a combination of supercritical fluid extraction and supercritical fluid drying. Compared with the liquid CO2 replacement supercritical drying, the supercritical CO2 extraction and drying method dispenses with the step of replacing the solvent with liquid CO2 and directly extracts the microgel gel by supercritical CO2 The alcohol in the pores allows the gel to be dried while substantially retaining the original structure, so that the overall drying time is further shortened and the operating costs are greatly reduced.
 
supercritical aerogel

2, Aerogel drying main factors:

Compared with the sol-gel process, the system involved in the drying process of the supercritical fluid is more complex, the process conditions are more, and many of the process conditions of the supercritical fluid drying process will have a greater impact on the structure and properties of the final aerogel influences. Therefore, the correct choice of these process conditions for the preparation of high performance aerogel is essential.

Comprehensive reading literature, these process conditions are: the type of drying media, media flow, drying time, drying temperature, drying pressure.

2-1, The impact of drying medium:

After the TiO2 aerogel is made into the alcohol gel, the TiO2 alcohol gel is dried by using supercritical ethanol and supercritical CO2 respectively as the drying medium. The drying conditions of supercritical ethanol were as follows: T = 270 ℃, P = 8Mpa, constant temperature time was 0.5h; the drying conditions of supercritical CO2 were as follows: the replacement time of liquid CO2 replacing ethanol was 72h, T = 42 ℃, P = 9.0Mpa, drying time constant for 5h. The photocatalytic degradation of Rhodamine B in TiO2 aerogel obtained from different drying media was compared. The experimental results showed that the photocatalytic activity of TiO2 aerogel with CO2 as drying medium was better than that of aerogel with ethanol as drying medium glue.

Because of the low drying temperature of CO2 drying process, the process has no flammable and explosive gas, and the prepared aerogel particles do not contain carbon, so the technology of SCFD using CO2 as the drying medium is easier to be developed industrially.

2-2 Media Flow Effects:

ZrO2 aerogels, the effects of CO2 flux and other factors on the preparation of ZrO2 aerogels were investigated by designing orthogonal experiments. The orthogonal experimental conditions of four factors and three levels were as follows: the flow of CO2 was 0.42L / h, 0.65L / h and 0.90L / h; the drying pressure was 8.6MPa, 9.0MPa and 9.5MPa; the drying time was 5h, 6h and 7h; Drying temperature is 40 ℃, 50 ℃, 60 ℃.

The optimum drying conditions obtained by orthogonal experiment were: CO2 flow rate 0.65L / h, drying pressure 9.5MPa, drying time 6h, drying temperature 50 ℃. This shows that the drying medium flow and drying effect is not a simple linear relationship, but there is an optimal value. The reason is that on the one hand, with the increase of CO2 flow rate, the impetus of mass transfer between ethanol and supercritical CO2 fluid in dispersion medium is increased, and the extraction and drying speed is accelerated; on the other hand, if the CO2 flow rate is too large, The speed of ethanol extraction from the gel surface will be too fast and a large gradient of ethanol concentration will form between the gel pores, thus causing the difference of diffusion rate of ethanol in the pores to increase, affecting the internal structure of the gel and causing more rupture, Not conducive to the formation of high surface area aerogel.

2-3,The impact of drying time:

SiO2 alcohol gel placed in CO2 supercritical extraction dry high-pressure extraction kettle, access to supercritical CO2 extraction and drying. The supercritical conditions were as follows: pressure P = 10MPa, temperature T = 40 ℃, flow rate 10kg / min, extraction drying 5h, 10h, 20h respectively. It was found that the specific surface area of ​​SiO2 aerogels with different drying time were different, 574 m2 / g, 583 m2 / g, 603 m2 / g.

This shows that the longer the extraction time, the greater the specific surface area of ​​the obtained aerogel. However, considering the comprehensive economic benefits, the drying time should be chosen optimally.

2-4,The impact of drying pressure:

Under the condition of ensuring the supercritical fluid is reached, the specific surface area of ​​the oxide aerogel decreases with the increase of the supercritical drying pressure. This is because as the drying pressure increases, the fluid density increases, the mass transfer resistance increases, causing the mass transfer rate decreases, the aerogel surface area decreased. Of course, if the pressure does not reach the supercritical conditions, the solvent will greatly reduce the ability to dissolve, and produce surface tension between the solid particles, remove the solvent prone to gel structure destruction, resulting in reduced surface area and pore volume. Therefore, the optimal drying pressure should be selected slightly larger than the critical pressure in the vicinity of the medium.

2-5, The impact of drying temperature:

MnO2 aerogel, the gel was dried by SCFD technique, the drying time was 2h and the supercritical drying pressure was 6.6MPa. By changing the supercritical drying temperature, the results showed that with the increase of supercritical drying temperature High, the specific surface area of ​​the aerogel powder gradually increased, reaching the maximum at 263 ℃; then with the increase of temperature, the specific surface area and then gradually reduced.

This shows that under supercritical conditions, the temperature has two effects: on the one hand, the higher the temperature, the lower the density of the medium fluid, the mass transfer impetus is large, which is conducive to the removal of water and improve the surface area of ​​the aerogel; On the other hand, the higher the temperature, the easier the particles grow under hydrothermal conditions and the smaller the surface area of ​​the aerogel. Therefore, we should choose a best temperature according to the trend of these two aspects.

3, The composition of the supercritical drying device:

Supercritical drying device mainly consists of: air source system, refrigeration system, flow control system, temperature control system, pressure control and regulation system, drying device, separation device, computer control system and electrical control, bracket box and other components.