Monitoring the degradation of thermal fluids

Thermal plants run at high temperatures for extended periods and over time thermal fluids 

degrade through thermal degradation or oxidation or both. It is important that these processes are routinely monitored to ensure the plant continues to operate safely and efficiently. Laboratory analysis can be used to assess both the state of thermal cracking and oxidative state. A model for assessing these is discussed in this article.

It is reported that the global heat transfer fluid (HTF) market will increase in value from $1,684 million in 2011 to $2,557 million in 2017.^[1] This demand is dependent on Europe which was reported to account for one-third of the global HTF demand and be driven by growth in the Asia-Pacific region (see full report).

There are a wide variety of HTFs and uses for HTFs in industry including the processing of food, chemicals, plastics and energy.

^[2]

HTFs will degrade with usage, with the most common processes being thermal cracking and oxidative stress. Thermal cracking is the breaking-up of larger hydrocarbon molecules into smaller molecules and oxidation is the gaining of oxygen. At high temperature, a HTF will degrade either through thermal cracking or oxidation or both. During thermal cracking carbon will accumulate and flash point temperature will start to decline. During oxidation carbon will accumulate and total acid number (TAN), an indicator of oxidative state, will start to increase. Hence carbon formation is common to both processes.

The condition of a HTF is monitored by routinely sampling a HTF and chemically analysing the HTF in a laboratory. Chemical analysis is used to define the presence:

• Heavy-chain hydrocarbons as indicated by the percentage of carbon in a HTF sample.
• Light-chain hydrocarbons as indicated by the flash point temperature of a HTF. The potential flammability hazard of a HTF and HTF system can be inferred from the flash point temperature of a HTF, specifically closed flash point temperature, which needs to be maintained above 100 degrees Celsius.
• The oxidative state of the HTF and this is indicated by the TAN of a HTF and increases as the HTF oxidizes.

Routine sampling and chemical analysis is therefore important in assessing the current condition of a HTF and trending the condition of a HTF against time. This can then be used to ensure that changes in HTF condition are closely monitored and actions defined based on current condition and projected condition at a future time point.

Modelling the state of thermal cracking and oxidative state of a HTF

Monitoring heavy-chain hydrocarbons, light-chain hydrocarbons and TAN is therefore critical to all plants using HTFs. This can be achieved by trending values against time. However, additional plots of carbon against closed flash point temperature (thermal cracking) and carbon versus TAN (oxidation) may be helpful in defining the underlying changes and this may be helpful in targeting specific changes with suitable interventions. This is demonstrated in Figure 1 where plots for thermal cracking (left panel) and oxidation (right panel) are illustrated.

This schematic shows that carbon is common to both processes. The x-axis is plotted for graduations of carbon and represents the following condition ratings: <0.05, satisfactory; ≥0.05 to <0.05, caution; ≥0.5 to <0.75, action; and, ≥1.0 percentage weight, serious. Closed flash point temperature and TAN also have ratings based on their condition, but these are not presented. Figure 1, left panel shows that as carbon increases, closed flash point temperature decreases. For schematic purposes this is shown to be linear. Deviations from the forty-five degree line represent the build-up of excessive carbon (i.e., top right hand corner) or excessive light-chains or ‘light-ends’ (bottom left hand corner).

The same plot is presented in the right hand panel of Figure 1. Again, deviations from the forty-five degree line reflect excessive changes in carbon (bottom right hand panel) and oxidation (top left hand panel). These changes need to be managed and interventions available are discussed in further detail below.

Real world data on the state of thermal cracking and oxidative state of a mineral-based heat transfer fluid

Figure 2 presents real world data for two mineral-based HTFs using thermal cracking and oxidation plots. These plots were obtained by searching test results for Globaltherm M (n=981) and BP Transcal N (n=686). Data was then organized by carbon content and the mean values for closed flash temperature and TAN are plotted in Figure 2. Real word data shows that in both cases the closed flash point was stable (~160 degrees Celsius) whilst carbon increased. In the case of oxidation, there were concurrent increases in carbon and TAN. This indicates that oxidation is the key issue affecting the HTFs in Figure 2. Of course, in real life these plots can be used to look at individual test results or pooled data, as presented in this article.

Figure 1, right panel, shows a linear relationship between carbon and TAN. This further highlights the value of plotting data for each test result as this enables gradual changes to be detected and a plan developed to ensure that action is taken when required. Indeed, the client does not want serious levels to be reached as this means a complete replacement of the HTF.

Targeted interventions to address unwanted changes in heavy and light-chain hydrocarbons, and oxidative state

Using the example in Figure 2, the client needs to select a cost-effective intervention to manage carbon and TAN. Interventions that are commonly used include the dilution of a HTF or the complete replacement of the HTF. In reality the latter option should be avoided as this can be an expensive option and normally a last resort. Another option is to use a HTF that can be used at higher operating temperatures and designed to be more resistant to thermal degradation. The same approach can be used for oxidation where the use of a synthetic HTF, such as Globaltherm Syntec, is more resistant to oxidation. Other options in the management of TAN are the addition of anti-oxidant packs / repellents to the HTF system to reduce the oxidation of the HTF or ensuring that there is a nitrogen blanket in the header tank to stop the HTF coming into contact with air.

Filtration is an option for the management of carbon residue and TAN and aimed at removing carbon particles from the HTF whilst in circulation. Filters have variable pore sizes and can be combined with ISO cleanliness tests to monitor wear and contamination particles in the HTF.

Lastly, the management of closed flash point temperature can be achieved through the use of intermittent techniques such as batch venting or permanently installed structures such as a light-ends removal kit. The latter has been shown to stabilize closed flash point temperature for many years.

Conclusions

Changes in thermal cracking and oxidative state of a HTF can be monitored using routine sampling and chemical analysis. Key parameters include the measurement of carbon residue, light and heavy-chain hydrocarbons. These parameters can be plotted against time to assess ongoing changes or plotted against each other to assess thermal cracking (carbon residue versus closed flash point temperature) and oxidation (carbon residue versus TAN). This approach can be used with all types of fluid and enable comparisons between HTFs to demonstrate the resistance of a fluid to thermal cracking and oxidation. The value of the current approach is that it enables the client to target specific interventions to restore values to satisfactory levels. The effective management of a HTF is extremely important in maintaining a safe and productive plant, but also for ensuring a HTF is maintained viable for as long as possible.

Acknowledgements

The author would like to acknowledge the writing support provided by Red Pharm communications, which is part of the Red Pharm company (please see @RedPharmCo on Twitter).

References

[1] Wright CI, Bembridge T, Picot E, Premel J. Food processing: the use of non-fouling food grade heat transfer fluids. Applied Thermal Engineering 2015: 84; 94-103.

[2] Wagner WO. Heat transfer technique with organic media. In: Heat transfer media, second ed. Graefelfing, Germany: Maria-Eich-Straβe; 1997. p. 4–58 [Chapter 2].

The author is employed at Global Group of Companies, Cold Meece Estate, Cold Meece, Staffordshire, United Kingdom. Please contact the author for reference materials cited in this article at chrisw@globalgroup.org