Silica Gel Reactivation
Silica gel desiccant reactivation is a common operating procedure regularly used and performed to remove moisture from saturated material.
Silica gel, or gel packets, are used throughout industry to adsorb moisture and maintain low relative humidity thereby preventing product contamination and degradation. Silica gel is capable of adsorbing up to thirty percent of its own weight of water vapour.
The majority of Transformer Breathers used by the power generation industry contain indicating silica gel.
Silica gel is available in two primary types indicating and non-indicating. The indicating property is achieved by impregnation with cobalt chloride salt which is sensitive to water vapour.
See caution note as cobalt chloride impregnated silica gel is now carcinogenic.
Ineffective reactivation control can result in Transformer Breathers being maintained by up to three times more than is necessary.
Shorter maintenance periods have an impact on both time and cost.
The use of valved Transformer Breathers has eliminated the need to maintain the oil seal fluid level and combining this with improved reactivation contributes to further savings by extending the average maintenance period.
This article suggests a number of simple improvements to improve the practice and procedures for reactivation of silica gel.
Indicating Silica Gel
Indicating silica gel is used throughout the power generation and transmission industry for Transformer Breathers to protect the heat transfer cooling oil from moisture (condensed water) contamination.
One of the principal reasons for the use of indicating silica gel is its ability to release adsorbed moisture when heated to a temperature of over 100oC. This means it can recovered and used again.
Indicating silica gel can theoretically be reactivated a number of times. This will depend on several key factors including the environmental conditions during use the reactivation temperature and reactivation procedure.
Although this may seem a simple procedure inadequate control and poor process techniques can have a major impact on maintenance schedules by reducing the average time between reactivation and replacement of exhausted indicating silica gel.
Indicating silica gel is a silica-based material capable of adsorbing 25% of its own weight in moisture. The most widely used indicating silica gel was impregnated with cobalt chloride that gives the silica gel its activity saturation indicator changing from a blue colour to pink when a 10-12% moisture loading (by weight) of the desiccant is exceeded.
Beaded indicating silica gel is the best material to use in transformer breathers as it provides high resistance to attrition (dusting breakdown) during handling and regular bed packing providing uniform flow rate characteristics.
The saturation indicator changes from a blue colour to pink when the silica gel requires reactivation or replacement. The saturation indicator can be directly related to the equilibrium relative humidity of the fluid (oil). Maintaining the moisture level of oil to a maximum of content of 15 PPM is a typical requirement.
The Brownell alternative to cobalt impregnated silica gel is ENVIROGEL
ENVIROGEL can be safely reactivated.
It is an essential safety requirement that the outlet exhaust vent from the reactivation equipment is piped to the outside of the building/room into an adequate ventilated space.
One or a combination of the following factors can affect effective reactivation;
1. Failure to achieve an equal temperature of more than 100oC throughout the entire silica gel desiccant.
2. An inadequate supply of hot air to carry away the moisture driven off during reactivation.
3. Considering the reactivation process is complete when the activity indicator of the silica gel displays an active colour.
4. Overheating during reactivation which can cause damage to the desiccant structure reducing the moisture adsorption capacity.
5. Use of the wrong type of equipment for reactivation.
6. Attempting to reactivate silica gel, which has been contaminated with oil?
7. Not storing reactivated silica gel in a sealed airtight container.
There are a number of examples of poor reactivation processing currently used in industry. Many poor reactivation procedures derive from a less than clear understanding of simple procedures and controls required which will considerably improve the reactivation process.
Many users consider placing silica gel in a warm oven as an adequate method to remove the adsorbed water.
The use of gas ovens should be avoided and microwave cookers are likely to damage the structure of the silica gel.
Silica gel which has been subjected to temperatures in excess of 150oC is likely to be discoloured to a brown or blackened state. If the silica gel has been subjected to overheating the adsorption capacity will have been adversely effected.
If silica gel that has been contaminated with oil is reactivated the adsorption capacity will be effected and the oil contamination could be readily transferred to silica gel contained in the reactivation unit. Silica gel, which has a brown colour and emits odour during storage or reactivation, is often an indication of contamination.
The reactivation of contaminated desiccant causes the oil to solidify within the structure of the desiccant resulting in a 50% reduction in adsorption capacity.
Health and safety consideration must always be given to the reactivation of silica gel.
It is an essential safety requirement that the outlet port of the reactivation equipment is vented to an atmosphere which is external of the workshop or facility where the process is being carried out.
The use of an adsorption filter may also be necessary if there is a risk that unidentifiable contaminates may be expelled during reactivation.
Silica Gel Activity Saturation Indicator
One of the most important factors from the above to understand is the meaning and purpose of the activity indicator.
When new or replacement desiccant is supplied by the manufacturer the desiccant will have a maximum moisture loading of 2% by weight.
The desiccant will therefore adsorb 10% moisture loading which will cause the activity indicator to change from a blue to pink colour.
At this time the desiccant must be recharged or replaced.
The activity indicator changes colour on a narrow band of approximately 2% moisture content and this is the primary cause of poor desiccant reactivation.
When the desiccant is heated the activity indicator will change from its saturated colour to its active colour early in the reactivation process as a consequence of the narrow change band.
If the desiccant is considered to be reactivated at this time the residual moisture loading could be as high as 10%. The desiccant has therefore not been reactivated to remove the adsorbed moisture to the 2% level as supplied by the manufacturer.
When the desiccant that has been poorly reactivated is used the activity indicator will change rapidly from the active colour to saturated indication depending on the duty cycle and the weather.
The desiccant will require reactivation and so the cycle begins again at much shorter intervals than the original desiccant charge.
Any of the above factors for poor desiccant reactivation will have a similar result of the activity indicator changing rapidly following reactivation.
Improving the reactivation process is simple and often only requires a modification to existing procedures.
When reactivating desiccant it is vital that the activity indicator is not used to control the process as previously discussed.
1. As a general rule saturated desiccant will require a minimum of 4-6 hours at a temperature of between 105-110oC throughout the entire desiccant bed for the reactivation process to be effective and reduce the adsorbed moisture to less than 2% by weight.
Some desiccants are seriously damaged and can release toxic chemicals if overheated.
2. Reactivation should be carried out in a ventilated electric oven. A sealed oven will limit the moisture liberated and prevent efficient reactivation. Gas ovens and microwave cookers are also not suitable for reactivation.
3. Devices that pass warm air through the desiccant bed changing the activity indicator to blue should be avoided.
4. Ideally after reactivation the desiccant must be placed in a sealed container and allowed to cool. The reactivated desiccant should not be stored in a warm oven at 60oC as this will cause partial saturation of the desiccant although the activity indicator may remain stable.
5. There are a number of proprietary reactivation equipment’s available on the market. Typical equipment should be able to reactivate 25 kilograms of saturated silica gel using a standard 13amp domestic supply.
6. To validate reactivation efficiency a fundamental weight loss test conducted before and after processing will provide the amount of moisture loss data.
7. A supply of pre-heated air is an integral requirement for an efficient reactivation process. For saturated desiccant a minimum air flow of between 5-10 cubic feet per minute.
The process of reactivation requires the application and achievement of a number of simple controls for optimum efficiency.
By following and adopting the above reactivation practice and procedures we are confident you will record improvements for the maintenance time and performance of the silica gel.