Monitoring light-ends in thermal CSP plants

In 2011 the global HTF market was estimated to be worth $1,684 and is projected to be worth $2,557 million by 2017. HTFs are a central part of manufacturing and involved in a diverse number of applications including the processing of food, chemicals and energy.[1] From a very simplistic viewpoint, the efficiency of thermal plants can be measured in terms of production output and energy consumption. However, as well as being efficient, a plant also needs to be operated safely. Monitoring and removing the build-up of light-ends in a HTF forms part of a safe operation. Indeed, as a HTF degrades it forms light-ends and as they build-up the flash point of a HTF drops. Ensuring closed flash point remains above 100 degrees Celsius (212 degrees Fahrenheit) needs to be regularly monitored to ensure that sudden changes are detected and appropriate actions are taken to control flash point temperature. This article outlines what light-ends are and how their build-up can be detected and managed over time.

What are light-ends and how are they measured in a heat transfer fluid?  

Light-ends are short-chain hydrocarbons formed during the “thermal cracking” or thermal degradation of a HTF. Light-ends effectively reduce the flashpoint temperature of a HTF and this means that light-ends will boil-out of the HTF, in an open environment, as a flammable vapor and ‘flash’ if exposed to an ignition source e.g., a naked flame. Flammable vapors vented from a system are therefore a potential source of fire and the reason why they need to be monitored and managed. Flashpoint temperature can be measured using techniques that mimic an open to air or closed to air environment. By definition closed flash point temperature will be lower than the open flash point temperature as the open test allows light-ends vapors to mix with air and this allows some vapor to mix with air and escape, which effectively increases the temperature at which the vapors flash. In contrast, the closed flash test does not allow the vapors to mix with air and stops vapors escaping. This means that the flash point temperature is lower. This is demonstrated in Table 1 which shows the open and closed flash point temperatures for a mineral-based HTF used across all sectors of manufacturing and for a biphenyl-diphenyl oxide-based HTF which is commonly used in concentrated solar power plants.

Table 1. Typical open and closed flash temperatures for mineral and biphenyl-diphenyl oxide-based HTFs. Over time a HTF will continue to thermally crack and flash point temperatures will steadily drop. This indicates that a HTF system is not venting properly and the consequence is that these fuel-like decomposition by-products start to accumulate in the HTF system and increases the flammability of the HTF.

It is possible to quantify the build-up of light-ends using laboratory tests. Regular sampling and chemical analysis of a HTF is utilized to monitor and identify interventions to control the build-up of light-ends and to stop the flash point from decreasing. A HTF is sampled whilst it is in operation and so engineers need to be trained on how to take live samples safely. A sample also needs to be representative of the fluid being sampled as volatile decomposition products will escape from the fluid if exposed to air. A closed sampling device is therefore used to keep the light-ends in the HTF and to ensure that analysis reflects the fluid in the HTF system.

How are light-ends removed from a heat transfer fluid?  

There are a number of interventions that can be used to correct changes in flash point temperature. The most obvious being the replacement of the HTF, but this is an economic decision and depends on the capacity of the plant. It also means there will be an interruption to production. This is also true for batch venting where the expansion tank is transiently heated to raise the temperature of the HTF and to vaporize light-ends from the system. This is, however, an aggressive approach in which the oxidative state can be accelerated and this needs to be monitored to ensure the HTF remains viable. A third approach is to installation of a light-ends removal kit (LERK). This can be installed temporary or permanently and works by continuously removing light-ends from the HTF system via distillation.

A recent case study showed how effective the LERK was in controlling closed flash point temperature and data is presented in Figure 2 showing closed flash point temperature prior to and following the installation of a LERK. Prior to installation, closed flash point was very unstable with values ranging between 82°C and 210°C (a maximal difference of 128°C). In contrast the closed flash point temperature was much more stable after the LERK had been installed with closed flash point ranging between just under 141°C and less than 195°C (a maximal difference of 54°C). The installed LERK also meant that closed flash point temperature had remained stable for the last five years and had not dropped below 100°C at any point during this time.[2]

Conclusions  

The efficiency of a plant can be measured by its production output as well as its efficient use of energy. However, safety also needs to be properly managed as an unsafe environment can present potential fire risks, as in the case of the build-up of light-ends in a HTF. Changes in the build-up of light-ends can be monitoring using routine sampling and chemical analysis. Corrective measures include the replacement of a HTF, however, this can be expensive, depending on the plant's capacity, and will interrupt a plant's production. Alternative options include batch venting, which is another transient intervention, and the installation of a LERK. A LERK can be installed as a temporary or permanent intervention to continuously remove light-ends and has been shown to be very effective in the long-term stabilization of closed flash point temperature. This is extremely important for ensuring the safety of a plant and sustaining production.

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: WO Wagner. Heat transfer technique with organic media. In: Heat transfer media (second ed.), Maria-Eich-Straße, Graefelfing, Germany (1997), pp. 4–58 [Chapter 2]. 2: Wright CI, Faure D, Bissemo R. The long-term effectiveness of a light-ends removal kit (LERK) in the management of heat transfer fluid plant safety: a case study to show its effectiveness 5 years after installation. Heat Transfer Engineering (accepted; in publication).

Contact  

Christopher Wright Global Group of Companies Cold Meece Estate, Cold Meece, Staffordshire, United Kingdom chrisw@globalgroup.org