| Silica Gel |  |
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| Types | Features | Uses | Disadvantage |
Silica gel is granular in size and a porous form of silica. They are made synthetically from sodium silicate. It is contains an enormous network of interconnected microscopic pores. Conrtrary to zeolites, silica gels have bigger pores with a vast range of diameters – that is typically between 5 Å and 3000 Å. Silica gels also do not allow for the separation of molecules solely based on their size.
Types
of Silica Gel:In the industrial manufacturing, depending on the pore size Silica gel can be broadly categorized in two types namely:
- Silica gels that are synthesized using an average pore size of about 20Å are called as Narrow pore silica gels.
- Silica gels that are synthesized with an average pore size of about 110Å and above are known as Wide pore silica gels.
- Easy to regenerate: Silica gel is easier to regenerate due to the lower adsorption forces.
- Certain grades of silica has been found to adsorb around 1.2 times of their own weight of water. This is essentially due to the higher internal pore volume.
- Adsorption Capacity: Silica gels maintain their structure when activated. Activation frees the large internal surface area and pore volume, enabling physical adsorption and capillary condensation. Shows higher capacity than activated alumina.
- Purity: By industrial standard, Silica gel can be found to be extremely pure by industrial standards. This is due to the non-presence of any remarkable concentrations of metallic compounds like iron, aluminium, or heavy metals. This makes pure silica gel much less active as a catalyst than adsorbents in the same category like zeolites or aluminas. This prevents unwanted reactions like cracking reactions in hydrocarbon streams. As a result coke formation can be minimized.
- High Porosity: Silica gel offers a very high porosity, something around, 800 m²/g, that allows it to adsorb water readily, thus making it very useful as a desiccant (drying agent).
- Low pressure drop: It offers right particle size distribution, that is typically in the range of 2-5 mm range.
- Long Life: That is charecterized by less maintenance cost, lesser cost for replacement. Minimal loss of production.
Silca gel is actively used in PSA separation process for Hydrogen purification, Oxygen nitrogen etc. PSA operation process for hydrogen purification from the refinery fuel gas, is a very good example of use of Silica gel beds. The impurities found in the system, ranges between 30 and 40% of the feed, are generally methane, ethane, propane and butane. The dual adsorbent bed that is used in the process consists of an initial layer of silica gel.
Disadvantage of using Silica Gel:
- Tendency to attrition: Silca gel has the tendency to dust or undergo attrition as compared to other adsorbents such as Activated Alumina. This problem gets more acute when the adsorbent beds are subjected to mechanical shaking or movement caused due to the instantaneous pressure difference across the bed. when the same bed is changed from regeneration phase to the adsorption phase.
- Suitable for processes needing moderately low dew points: Silica gel is the right material choice if only moderately low dew points needed to be arrived at. And the properties of high adsorption capacity and or good regeneration behaviour can exploited to the hilt.