The heat is on

Many high temperature processes rely on heat transfer fluids (HTFs) to deliver heating or cooling through a plant's heat exchanger network. Either synthetic or natural mineral oils, these fluids are often looked upon as a commodity item - to be used as long as possible and then simply replaced once their heat transfer performance starts to fall off. But the problem of just how long a heat transfer fluid will last is currently being addressed by HTF suppliers and service companies.

Last year, for example, one of the leading global suppliers of HTFs, Dow Chemical, launched its Dowtherm MX synthetic, aromatic-based fluid. This is aimed at high temperature applications - generally in excess of 300°C - such as distillation column reboilers and reactor preheaters in refineries, petrochemical and polymer plants.

Conventionally, says Dow, aliphatic-based 'hot oil' fluids are used on these duties, but they are susceptible to thermal degradation at above about 290°C. Dowtherm MX, on the other hand, is said to provide exceptional thermal stability over its entire range of -23°C to 330°C.

In common with an increasing number of process equipment suppliers, Dow has made its HTF high-temperature range available online. Process customers can now pick from eight synthetic organic fluids (Dowtherms) or three silicone-based ones (Syltherms), in whatever amount they need - from a bulk fill replacement to a few drums for system make-up.

'Ordering HTFs has never been easier,' claims Tom Frazier, Dow's senior HTF marketing manager. This might seem to reinforce the commodity view of HTFs, but Frazier emphasises that 'customers who need assistance can still call on our team of experts for technical consultation, help with selection, installation, operational support and troubleshooting.'

One supplier that has not followed Dow quite so far down the 'e-road' is Solutia, whose HTFs are marketed under the Therminol brand. According to the company's worldwide HTF business manager, Dale Kline, 'if our customers want to go that way, we are ready and willing to take part in the e-commerce wave. But our customers are buying a heat transfer service, not a commodity… our products and services are specifically designed for the application and the customer.'

As part of that service philosophy Solutia offers its Therminol TLC, or Total Lifecycle Care, programme. This offers support all the way from system design, through start-up, operation and on to a trade-in option at the end of the fluid's useful life. For when that time comes and the fluid has to be replaced, Solutia has also come up with Therminol FF - described as the 'first and only' flushing fluid for liquid phase heat transfer systems.

Because of the risk of surface fouling from oxidised, thermally degraded or contaminated HTFs, systems generally have to be cleaned prior to recharging with new fluid. A synthetic hydrocarbon with a high flash point and high initial boiling point, Therminol FF is said to be much safer to use than other cleaning solvents.

An alternative approach to keeping HTF systems up and running, however, is now available in the UK from Derbyshire-based Heat Transfer Systems. With the help of a £45 000 Smart Award from the DTI, the company has a developed a mobile on-site regeneration unit that can 'spring clean' heat transfer oils.

The regeneration system works by using nitrogen to strip out volatile contaminants from the oils. According to HTS director, John Franklin, the regeneration unit 'needs to operate at system temperature to do this', which offers the distinct advantage of being able to treat the HTF while the process remains on-line. 'We take about 4m3/h of fluid out of the system flow,' says Franklin, 'and pass it into a flash vessel where it contacts a contraflow of nitrogen over a series of diffuser plates.'

The net effect of the nitrogen-stripping is to increase the overall flashpoint of the degraded heat transfer fluid. Operating HTFs at around 300°C can gradually lower flashpoints, says Franklin, as lower boiling-point components effectively continue to fractionate out of the original oil grade at such high temperatures. So, by stripping out these more volatile components with nitrogen, the HTF can be returned to something approaching its original state - and to a quality certainly good enough to avoid any need for a total fluid replacement.

HTS is in the process of building a range of mobile systems to suit various duties, but the company believes that the regeneration system can reduce the use of new thermal oils by 99 per cent.