Paving the way for energy savings

In the UK, an average of 20 million tonnes of bitumen (or hot and warm asphalt mix) is produced every year, at a cost of £250-£300/t, to pave Britain’s roads as well as waterproofing and sealing roofs, boats and buildings. Produced from selected crude oils through a process of fractional distillation, bitumen is a sticky tar-like substance with a distinctive smell and is the mainstay of road building.

A crucial factors in the management of bitumen is the process of heating the product storage tanks. The temperature of the bitumen needs to be kept sufficiently high – a constant 165°C – to maintain its fluid state in order to ensure that the product can still be pumped when it reaches the customer.

Methods of heating

There are three basic heat sources that are commonly used for heating bitumen storage tanks. The first of these and, the most basic source of heating, is hot water which, when used in a closed system under high pressure produces a substantial heat transfer – but only with the water at high pressure and with limited opportunities to turn the temperature up a few degrees if required!  

The second heating source used is steam. The most traditional form of process heating, steam (produced using fire tube boilers or coil type steam generators) has a number of advantages including a large heat transfer coefficient, good latent heat characteristics at low pressures and the ability to maintain heat transfer at constant temperatures. This method of heating bitumen storage tanks is still used extensively, but is rapidly playing second fiddle to thermal fluid heating.   

Essentially replacing water as the heat transfer medium within a closed system, a thermal fluid system operates as a low pressue, no-loss circuit. Heat loss is minimal making it a very efficient method of heating bitumen tanks. Unlike steam systems which generally need to be in a separate plant room due to their size, thermal fluid systems are compact and do not require special site work; in most cases, they can be installed close to the heating tanks. 

Energy efficiency

One of the key factors in achieving very high levels of system energy efficiency is the ability of a thermal fluid system to work at high temperatures (to 350°C in standard form) in a simple closed loop.  When compared to a steam system, this means no change of state of the fluid, so no condensate and therefore no flash steam losses, no blow down losses or make up water required, no effluent discharge and a corrosion-free operation without the need for expensive chemical treatment. 

In addition, thermal fluid heating offers more operational flexibility to boost bitumen temperatures for loading and can be used in further processing such as PMB (Polymer Modified Bitumen).  

An example of the key benefits of switching to thermal fluid heating can be seen at Eastham Refinery. (ERL). A joint venture between Shell UK and AB Nynas Petroleum, ERL can produce 800,000t of bitumen per year. The company recently replaced the direct oil-fired furnaces at its site on the Wirral, Merseyside, with two Babcock Wanson high efficiency EPC 2500ES Thermal Fluid Heaters. The oil-fired furnaces were old and inefficient, whereas the thermal fluid heater precisely matches fuel input to plant energy requirements.  

Douglas Watt, senior process engineer and project manager, explains: “ERL considered converting to gas firing on the original heaters but this would have required significant investment to update the equipment and to bring the control/instrumentation up to modern standards.  The Babcock Wanson heaters have met our expectations and are straightforward to operate. They have resulted in a significant reduction in fuel cost over our previous oil fired heaters.”

Total Bitumen has two Babcock Wanson EPC 2000ES Thermal Fluid Heaters at its Preston based Bitumen Division to improve operating costs and cut emissions. Duncan McLeod, engineering manager at the site, says: “The key benefits already reaped from the EPC heaters include a reduction in fuel costs and stack emissions, greater efficiency and a move to natural gas instead of heavy fuel, which has removed the need for onsite fuel storage.”

Environmental factors

Energy efficiency isn’t the only benefit thermal fluid heating has to offer. It is the most environmentally friendly method of heating bitumen storage tanks as it does not require any chemical intervention and is more often than not a simple mineral oil that can be recycled. The oil typically lasts up to a decade and, with it being contained in a closed circuit no loss system, the only effluent is the exhaust from the heater. As energy input demand is lower in these systems, total exhaust emissions are also lower. 

Most bitumen storage tanks in the UK are outdoor vertical or horizontal units and so are constantly exposed to the elements. Thermal fluid systems are unaffected by ambient temperature. In a water-based system, very cold weather can lead to the water freezing within the pipes leading to costly failures of the system and steam systems are prone to corrosion in extreme temperatures. This means that thermal fluid heating systems score high on longevity – in fact, even when the heaters themselves might be at the end of their life, the pipework system holding the thermal fluid is often as good as new so upgrades are simple and cost effective to implement at any time. 

Safety

In a thermal fluid system, the fluid does not need to be kept under pressure to keep it in the liquid phase and operates within a closed system, thus eliminating problems associated with water and steam systems such as mandatory inspection shut downs and the risks and costs of operating a pressure system. 

Experience Counts

While it is imperative to choose the right method of heating to ensure that the bitumen supplied is of an optimum quality, it’s just as important to choose the right equipment from a supplier that understands the needs of the use.  

Stolthaven Dagenham bulk liquid terminal, situated on the river Thames in Dagenham, Essex, recently benefitted from a new thermal fluid system featuring Babcock Wanson EPC ES and TPC B Thermal Fluid Heaters. A large proportion of the thermal design was carried out by Babcock Wanson with the supply of all the system valves and ancillary equipment to provide a complete system with single point of responsibility. The system also incorporates a secondary circuit to provide lower temperature thermal fluid for the bitumen pump casing heating. The project was a fast moving one with tight deadlines, all of which Babcock Wanson met.

Denis Kelly, engineering & construction manager at Stolthaven Dagenham, comments: “We looked at the market and what was available to suit our needs.  We were familiar with Babcock Wanson and I have seen their equipment previously when working at other similar sites.

“The Thermal Fluid Heaters themselves were ideal for this project as they are based on mature technology and are well proven in the field.  They are robust and also require very little maintenance.”

Although hot water and steam heating systems still have their place in some circumstances, an increasing number of organisations are now looking to thermal fluid systems to meet their bitumen heating needs.