一、设备概述
超临界干燥技术是近年来发展起来的化工新技术。一般常用的干燥技术,如常温干燥、烘烤干燥等在干燥过程中常常不可避免地造成物料团聚,由此产生材料基础粒子变粗,比表面急剧下降以及孔隙大量减少等结果,这对于纳米材料的获得以及高比表面材料的制备极其不利。
超临界干燥技术是在干燥介质临界和临界压力条件下进行的干燥,它可以避免物料在干燥过程中的收缩和碎裂,从而保持物料原有的结构与状态,防止初级纳米粒子的团聚和凝并,这对于各种纳米材料的制备极具意义。我们实验室应用超临界干燥技术已经成功地制备出多种气凝胶。
气凝胶是一种以纳米粒子或高聚物分子为骨架组成的超低密度多孔固体材料,国外称为“冻烟”,由于气凝胶具有纳米材料的基本特性,更具有极低密度、极高孔隙率以及耐温隔热等特性,因此它在航天、催化、吸附等领域具有广阔的应用前景。
二、控制技术及注意点
将醇凝胶置于超临界干燥的高压容器中,通过控温将其温度降至4～6℃。打开二氧化碳钢瓶的减压阀,从高压容器上部通入二氧化碳,随着二氧化碳气体不断通入,二氧化碳达到液、汽两相平衡,其中下层是液态二氧化碳,此时凝胶中的乙醇溶剂可逐步被液态二氧化碳完全所取代。
然后以一定的速率升温,液体二氧化碳开始逐渐膨胀,压力首先达到临界压力,继续升温,通过释放少量二氧化碳,保持压力不变,达到预先所选择的临界温度,即达到临界状态。在临界状态下保持一定时间,使凝胶孔隙中液体全部转化为临界液体,然后在保持临界温度不变的情况下,通过排泄阀缓慢地释放出干燥介质二氧化碳流体,直至达到常压为止。
在二氧化碳流体释放过程中,体系点沿着临界等温线变化,临界流体不会逆转为液体,因而可在无液体表面张力的条件下将凝胶分散相驱除,当温度降至室温时,即制得气凝胶。
三、超临界干燥操作过程中应注意以下几点：
(1)用干燥介质(液态二氧化碳)替换凝胶中乙醇溶剂的速度必须足够缓慢,以保证凝胶中乙醇溶剂被液态二氧化碳完全取代,溶剂替换过程一般约需8～48h。
(2)凝胶中的液体达到临界状态需要一个稳定过程,以使各部分都达到临界条件,因此必须在临界状态下保持一定时间。
(3)在保持临界温度不变的条件下缓慢地释放出流体,使体系点沿着临界等温线变化,以防止临界流体逆转为液体。
(4)在溶剂交换和超临界干燥过程往往会有易燃、有毒溶剂的蒸气释放出来,因此要注意安全问题。
四、主要技术参数
干燥釜：1L/30MPa 室温～300℃ 316L不锈钢
分离器0.8L/20MPa 室温～85℃ 316L不锈钢
整机加热功率：3kW
I. Overview of equipment
Supercritical drying technology is a new chemical technology developed in recent years. The commonly used drying techniques, such as room temperature drying and baking drying, often inevitably cause material agglomeration during the drying process, resulting in the coarsening of basic particles, sharp decline in specific surface area and a large number of pores, which is extremely unfavorable for the acquisition of nano-materials and the preparation of high-specific surface materials.
Supercritical drying technology is drying under the critical and critical pressure conditions of drying medium, which can avoid the shrinkage and fragmentation of materials during drying, thus maintaining the original structure and state of materials and preventing the agglomeration and coagulation of primary nanoparticles, which is of great significance for the preparation of various nano-materials. Many kinds of aerogels have been successfully prepared by supercritical drying technology in our laboratory.
Aerogel is an ultra-low density porous solid material with nano-particles or polymer molecules as the skeleton, which is called "frozen smoke" abroad. Aerogel has the basic characteristics of nano-materials, such as extremely low density, extremely high porosity, temperature resistance and heat insulation, so it has broad application prospects in aerospace, catalysis, adsorption and other fields.
Second, the control technology and points for attention
The alcohol gel is placed in a supercritical dry high-pressure container, and its temperature is reduced to 4 ~ 6℃ by controlling the temperature. Open the pressure reducing valve of the carbon dioxide cylinder, and introduce carbon dioxide from the upper part of the high-pressure container. With the continuous introduction of carbon dioxide gas, carbon dioxide reaches the balance of liquid and vapor, and the lower layer is liquid carbon dioxide. At this time, the ethanol solvent in the gel can be completely replaced by liquid carbon dioxide gradually.
Then the temperature rises at a certain rate, and the liquid carbon dioxide begins to expand gradually. The pressure first reaches the critical pressure, and then the temperature continues to rise. By releasing a small amount of carbon dioxide, the pressure is kept constant, reaching the pre-selected critical temperature, that is, reaching the critical state. Keep it in the critical state for a certain time, so that all the liquid in the gel pores can be converted into critical liquid, and then slowly release the dry medium carbon dioxide fluid through the drain valve under the condition of keeping the critical temperature unchanged until it reaches the normal pressure.
In the process of carbon dioxide fluid release, the system point changes along the critical isotherm, and the critical fluid will not be reversed into liquid, so the gel dispersed phase can be driven away without liquid surface tension, and when the temperature drops to room temperature, aerogel can be prepared.
Three, the supercritical drying operation process should pay attention to the following points:
(1) The speed of replacing ethanol solvent in gel with dry medium (liquid carbon dioxide) must be slow enough to ensure that ethanol solvent in gel is completely replaced by liquid carbon dioxide, and the solvent replacement process generally takes about 8 ~ 48h.
(2) When the liquid in the gel reaches the critical state, it needs a stable process to make all parts reach the critical condition, so it must be kept in the critical state for a certain time.
(3) Under the condition of keeping the critical temperature unchanged, the fluid is released slowly, so that the system point changes along the critical isotherm to prevent the critical fluid from turning into liquid.
(4) Vapors of flammable and toxic solvents are often released during solvent exchange and supercritical drying, so attention should be paid to safety issues.
IV. Main technical parameters
Drying kettle: 1L/30MPa room temperature ~ 300℃ 316l stainless steel.
Separator 0.8L/20MPa room temperature ~ 85℃ 316l stainless steel.
Heating power of the whole machine: 3kW
南通立凯机电工程有限公司专业设计、制造、销售超临界流体设备和石油科研仪器。产品主要包括：
超临界二氧化碳萃取设备、超临界气凝胶干燥设备、超临界细微粒子制备装置、超临界清洗发泡装置、超临界抗溶剂结晶装置、高温高压磁搅拌反应装置、超临界相平衡仪等。
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