Ullage System Enhancement

Background

The heat transfer fluid (HTF) widely used in solar power plants to concentrate the energy absorbed from solar radiation, is an aromatic hydrocarbon mixture of byphenyl/dyphenil-ether. This synthetic organic oil owns an exceptional thermal stability and presents very low chemical reactivity, but under normal operating conditions, it will undergo degradation.

HTF Degradation

HTF molecules will breakdown to form lower molecular weight compounds which are classified as Low Boilers (i.e. benzene, phenol and toluene), but also, they will melt or recombine with each other to form larger isomeric structures which are known as High Boilers (i.e. dibenzofurans, terphenyls and biphenoxide). Low boilers volatilize at low temperatures and will coexist in vapor phase most of the time, while high boilers will be present as liquid form and only a small fraction will go vapor at the higher range of the plant operating temperature. Some other, like para-terphenyls will melt into solid below 210ºC.

The accumulation of these degradation products can be detrimental to the plant, triggering anomalies in temperature control, overpressure, cavitation, changes in HTF physical properties (density or viscosity) and even loss of heat capacity.

So, to get rid of these contaminants, solar power plants have integrated an ullage system which aims to regenerate the HTF and remove the non-condensable compounds in carbon bed filters.

Analysis

However, the principle of this regenerating system is based on a non-selective cooling separating process that will roughly recover whatever condensates above 60ºC (which is the maximum temperature that carbon filters can take) and make no distinction among the specific boiling point of each degradation product. In some cases, the system is provided with scrubbers to increase the efficiency of separation of the final gas stream by chemical absorption, but at the same time, it will concentrate low & high boilers in the liquid stream.

The thing is that, the nature of the HTF degradation is irreversible and, consequently, the current ullage system designs are not effective in terms of HTF cleaning. In fact, the bulk of the large molecular weight degradation products will remain, along with a small fraction of low boilers, in the HTF condensate. On the other hand, part of the condensate recovered will not pass lab quality controls and it will be disposed offsite as hazardous waste. This rejected condensate still contains a great mass fraction of byphenyl/dyphenil-ether, which is lost unnecessarily.

As a result, solar plants are actually regenerated by refilling pure HTF into the system, every few months.

Solution

After having run several model simulations, the data analyzed reveals that outstanding improvements can be achieved in the ullage system performance should the conceptual design be approached from a different perspective, which suggest the integration of a multi-stage distillation column to get a selective separation of all degradation products, and ultimately recover pure HTF.

With this enhancement, the refilling with pure HTF will be significantly reduced, the intrinsic properties of the HTF will be maintained, the degradation curve will be extended over time.