Recycling Syltherm XTL A long-lasting heat transfer fluid

1. Introduction

Syltherm XLT is a long-lasting heat transfer fluid which is ideal for batch processing.

The Syltherm XTL fluid is a specially formulated, high performance silicone polymer designed by Dow Chemical Co. for use as a low temperature, liquid phase heat transfer fluid. Having a recommended use temperature range from – 100°C to 260°C ( – 150°F to 500°F ), Syltherm XTL fluid offers outstanding low temperature heat transfer and pumpability, beside an excellent thermal stability.

Additionally, Syltherm XTL has essentially no odor and is low in acute oral toxicity. All these features make Syltherm XLT a viable alternative to many organic heat transfer fluids, chlorinated solvents and CFCs that are actually used for low temperature, liquid phase service.

2. Syltherm XTL characteristics

At – 100°C the viscosity of Syltherm XLT is only 78.6 mPa.s ( or in English units, at – 150°F the viscosity is only 90.2 centipoise ). A low viscosity at low operating temperatures is an important property because it allows high heat transfer coefficients with low pressure drops and pumping horsepower.

The operating range of Syltherm XLT fluid from – 100°C to 260°C ( – 150°F to 500°F ) makes it ideal for a single fluid process heating and cooling applications ( i.e. for batch processing ). Single fluid processing with Syltherm XLT fluid eliminates process interruption and simultaneously the loss of temperature control which is associated with multiple fluid systems. The batch processing with Syltherm XLT also eliminates system flush requirements associated with steam/glycol and steam/brine systems. It has to be mentioned that Syltherm XLT fluid is noncorrosive toward metals and alloys commonly found in heat transfer equipment.

Within the above mentioned use range, Syltherm XLT will not degrade to form solids or volatile compounds having substantially higher vapor pressures. Consequently, the system downtime for periodic fluid reprocessing and replacement is eliminated. Also the HF fluid can tolerate occasional high-temperature upsets with only minimal physical of the fluid physical properties.

However, an extended use at bulk temperatures above 260°C ( 500 °F ) or film temperatures higher than 288°C ( 550° F ) may generate higher system pressures and cause polymer cross-linking to occur. This fact will eventually cause an increase of the fluid viscosity to a point where the replacement will be required in order to restore the system performance.

The fact that Syltherm XTL remains liquid below – 100°C eliminates many of the problems associated with shutdowns and cold weather start-ups. Consequently, steam or electrical tracing, which are costly to install and operate, are not needed.

Batch processing with Syltherm XTL fluid also eliminates the system flush requirements associated with steam/brine and steam/glycol systems.

Syltherm XTL fluid is noncorrosive toward common metals and alloys used in heat transfer systems as long as it remains uncontaminated. Carbon steel is used in most heat transfer systems utilizing Syltherm XTL fluid, but also stainless steels or low alloy steels. Most corrosion problems are caused by the chemicals introduced into the system during cleaning or from process leaks. The nature of the corrosivity and severity will depend upon the amounts and type of contamination involved.

3. Syltherm XTL physical – chemical characteristics ( SI units ):

Chemical composition : Poly (dimethylsiloxane)
Average molecular weight : 317
Color : Crystal clear liquid
Density at 25 °C : 852 kg/m³
Viscosity : 1.4 mPa.s at 25°C
Kinematic viscosity at 40 °C : 1.20 mm²/s
Freezing point : – 111  °C
Minimal operating temperature : – 100  °C
Flash point, closed cup : 47°C
Flash point, open cup : 54°C
Autoignition temperature : 350°C
Heat of Combustion : 32,800 kJ/kg
Estimated critical temperature : 327°C
Estimated critical pressure : 12.16 bar
Estimated critical volume : 3.9 l/kg

4. Recycling Syltherm XTL

It is recommended not to dump the product into any sewers, on the ground or into any body of water. All disposal practices must be in compliance with all Federal, State/Provincial and local laws and regulations. Regulations may vary in different locations. Waste characterizations and compliance with applicable laws are the responsibility solely of the waste generator: Used heat transfer fluid which has been stored in drums or tanks should be sampled before recycling in such a manner as to ensure a representative sample.

The regeneration recycling feasibility of the waste heat transfer fluids (HTF) was also researched using the rotary steam-filtration-adsorption method and the indicators of reclaimed oil were tested according to the current standards [2].

The good oil in waste heat transfer fluids was about 50% through the regeneration recycle.

Though the waste of HTF oil would be used as a fuel oil supply for the refinery, it is a serious waste of resources, moreover, it will lead to an environmental pollution for which now there are regulations and management system standards. Therefore, to regenerate and recycle the waste HTF has very important practical significance for the enterprises to reduce the production cost, to save energy and protect the environment. In the regeneration process, procedures as reduced pressure rotary steaming, adsorption and filtration were adopted for different types of silicone oil. Then every index of the oil was analyzed according to current standards to determine the feasibility of the renewable technology based on the experimental results.

For unused and uncontaminated product, the preferred options include sending to a licensed Recycler as for instance ECO USA, incinerator or other thermal destruction device.

References:
[ 1 ] www.dow.com
[ 2 ] 2016 2nd International Conference on Sustainable Energy and Environmental Engineering (SEEE 2016) –“ The Experimental Research on the Regeneration and Recycling Feasibility of the Waste Heat Transfer Fluids” Yu-hui DU1 , Bin YUAN2,*, Lin YANG1 and Wei-Guang, Guangzhou, 510006, China