Supercritical CO2 drying technology can effectively eliminate capillary pressure effect in the pore throat of nanomaterials and can produce ultrafine pore structure aerogels with good dispersibility. With the development of research work the preparation of aerogels by supercritical CO2 fluid drying technology has good prospects. Supercritical CO2 extraction drying is an advanced technology combining supercritical extraction technology and supercritical fluid drying technology.
1、 Equipment Overview
SiO2 aerogel is a new type of porous material with controllable structure. It has many unique properties such as low refractive index low elastic modulus low acoustic impedance low thermal conductivity strong adsorption and typical fractal structure. It can be made into many high-performance materials such as acoustic impedance coupling material filter material high temperature insulation material and so on. Catalyst and catalyst support broadband antireflection rechargeable battery anti glare coating low dielectric constant insulating layer ultra-high speed integrated circuit substrate high laser damage threshold antireflection film thermal insulation coating and many other fields have broad application prospects.
During sol-gel process the sol particles first aggregate into clusters and these clusters expand and connect to form large clusters of networks. Gel formation is not equal to the end of the sol gel process but also after a series of postprocessing (including aging cracking prevention drying etc.) the unique aerogels can be obtained.
After the gel is formed the sol particles and small gel clusters in the solution continue to adhere and expand to the whole gel network which is called aging. These aging processes make the gel network coarser slippery and the overall specific surface area decreases. The pore size distribution of the network and the distribution of colloidal particle radius of the network are narrowed.
The postprocessing of gel will inevitably cause cracking on the gel surface and the stress that causes the gel cracking is mainly from capillary pressure. The capillary pressure caused by the surface tension of the liquid filled in the gel skeleton pores will make the gel tighten up rearrangement and volume contraction. The following measures can be taken to reduce the degree of cracking during drying:
(1) Reduce the surface tension of the solvent
In the process of hydrolysis and condensation the pores in the gels are mainly water and alcohol. Due to the large surface tension of the water the additional pressure of the capillary is very large during drying which is the direct reason for the cracking and fragmentation of the aerogels. If water and alcohol are replaced by solvents with low surface tension through solvent replacement the additional pressure will be greatly reduced during evaporation and drying of these solvents with low surface tension so as to reduce the cracking during drying.
(2) improving the uniformity of the pores in the gel.
Due to the direct hydrolysis and condensation of organome tallic compounds the gel network structure can not be formed very uniformly which results in coarse and fine pores in the gel so that the stress imbalance in the same gel often causes the cracking or crushing of the gel in drying process. Therefore adding controlled dry chemical additives in sol-gel process can promote the uniform network pore size of the alcohol gel and produce a more uniform gel pore structure thereby reducing the possibility of gel breaking in drying process and shortening the drying cycle.
(3) surface modification of gelatin
If the surface of the alcohol gel is modified the number and surface electrical conductivity of the gel surface can be adjusted and controlled so that the hydrophobic surface of the gel skeleton will have a certain degree of hydrophobicity so that the contact angle between the skeleton and the solvent will increase. This will greatly reduce the additional pressure of the capillary and reduce the degree of cracking in the drying process.
2、 Main technical parameters
Drying kettle: 5L / 30MPa 300 ℃
Separator 2L / 20MPa 85 ℃
Refrigeration system: 5100kcal / h air cooling imported core components
Storage tank: 4L / 16MPa
High pressure transfer pump: 50L / 40MPa
Total power: 15kw

一、超临界流体(SCF)的特性
    超临界流体(SCF)是指物体处于其临界温度(Tc)和临界压力(Pc)以上状态时，向该状态气体加压，气体不会液化，只是密度增大，具有类似液体的性质，同时还保留气体的性能。
    超临界流体兼具气体和液体的优点，其密度接近于液体，溶解能力较强，而黏度与气体相近，扩散系数远大于一般的液体，有利于传质。另外，超临界流体具有零表面张力，很容易渗透扩散到物体的微孔内。因此，超临界流体具有良好的溶解和传质特性，能与物体很快地达到传质平衡。
二、超临界流体干燥的原理
    超临界流体干燥过程是利用其溶解能力与表面张力为零的特性，即利用在超临界状态下，表面张力为零的特点，样品微结构处于一个压力相对平衡的状态，而不影响细微结构的情况下，利用超临界流体的溶解携带能力，将溶剂置换携带而出，然后借助减压、降温的方法使超临界流体变成普通气体，保证样品初始结构状态的情况下，完成干燥处理。
三、超临界流体干燥过程的主要影响因素
(1)干燥压力的影响
    干燥压力是SFED的重要参数之一，样品在整个干燥过程中，形成超临界状态和稳定置换状态，回归普通状态这三个过程中，压力的稳定控制和梯度控制十分重要，压力的变化过大或波动，均会对样品的内部框架结构造成损伤，从而造成碎裂、塌陷等现象。
(2)干燥温度的影响
    温度对超临界干燥的影响比较复杂，一定程度上，温度升高会提高超临界流体的溶解能力，从而提升其溶剂置换携带能力，缩短干燥处理时间，但是，温度升高会形成普通意义上的加热干燥，影响样品表面质量，造成样品表面聚缩或膨胀，严格意义上讲，这也是表面塌陷的一种表现。温度太低，长时间处于低温液态环境，流体流动后，会造成大面积样品的表面褶皱变形。所以对温度的控制和工艺处理十分重要。
(3)压力变化过程
    压力的控制是整个干燥过程的关键点之一，超临界二氧化碳干燥和超临界二氧化碳萃取干燥都经历浸泡置换，升压超临界流体置换，稳压超临界流体置换，降压超临界流体气化等过程，也就是升压、稳压、降压这三个主要工序，控制时，要以微量变化为基础，平稳操作，避免压力的变化过大，这样操作比较费时，但对样品的质量有保证。
(4)温度控制的影响
    温度控制也是干燥过程的关键点之一，缓慢升温至超临界状态后，其控制应实现自动控制，超临界流体的动态温度要稳定，防止高低温流体对样品表面造成影响，高于釜内温度则会造成表面开裂，聚缩，低于釜内温度则会造成结冰，褶皱，对样品质量都不好。
(5)工艺控制
    工艺控制是样品处理的工艺步骤过程，在干燥结束后，好多样品在常温常压取出时，整体质量都是很好，但是放置于空气中一段时间后，会发生缩小的变化，表面质量变化不大，主要在于样品内部的塌陷，其影响因素是多样的，也不排除在工艺控制上出现了问题，所以在操作时，对干燥结束后的样品需增添控制，放置功亏一篑。
(6)流量控制
    流量控制也是干燥过程中的关键点之一，流量大，置换携带速度快，节约干燥时间，但超临界干燥过程恰恰相反，流量不宜过快，整个置换过程是一个缓慢的过程，过快的流量会造成釜内稳定空间上的冲击作用，对样品表面质量的影响明显。
(7)工艺结构
    除上述影响因素外，工艺结构的影响也比较重要，釜体的高度，样品的位置、流体的方向等。