Photo: Wax crystals on a cold surface. Photo: SINTEF
In advanced industrialised countries such as the USA and UK, the total costs associated with the fouling and subsequent cleaning of heat exchangers amount to 0.25 per cent of GNP.
Moreover, studies reveal that American oil refineries spend in excess of USD 2 billion annually in combating fouling.
In Norway too we have a number of problems. The aquaculture and shipping industries are faced with the problems of algae and barnacles.
Aquaculture companies have to address fouling of their seine nets and other marine equipment, while the shipping industry is afflicted with encrustations on vessel hulls that generate friction which leads to increased energy consumption and reduced speeds.
The oil industry battles with hydrate and wax formation, and with salt precipitation in pipelines in situations where hot oil is gradually cooled along its several kilometre journey along the seabed.
Heat exchangers used in the process industry are particularly vulnerable. In industries where exhaust gases are generated, soot particles block pipelines and tubing.
Fouling is also a problem in the Arctic, where the formation of ice crystals on vessels and offshore installations can result in increased risk of ship loss and accidents.
Some of our current industrial sectors are forced to shut down their installations on a regular basis – some after only short periods of operation – in order to carry out cleaning and the removal of unwanted deposits. This results in direct cleaning and maintenance expenditures, and lost revenues due to shutdown.
Examples of fouling are many and varied but they have two things in common:
Solids precipitate out from an industrial flow system, accumulate on permanent surfaces and thus obstruct both flow and heat exchange processes. This results in major remedial costs.
Fouling problems often arise in connection with the exchange of heat through a dividing wall. The issue is so complex and far-reaching that fouling has rapidly grown into a distinct multidisciplinary field of study involving economics, physics, chemistry and flow mechanics from the micro to the macro scale.
At the Process Technology department at SINTEF Materials and Chemistry, researchers are conducting flow modelling studies in order to investigate both particle transport along pipe walls and the adhesive properties of particles on the walls themselves. It all started with a doctorate degree at the department. In order to come up with solutions, researchers are currently working together with the industry, industrial coating and surface treatment suppliers, and other research institutions.
“Fouling can occur both as a result of crystal growth directly on pipe walls, and also when crystals growing within the fluid flow coagulate to form particles that seek to attach themselves to the walls”,
says Sverre Gullikstad Johnsen. He is carrying out research into the problem and is involved in several projects.
Projects for the oil industry
SINTEF has many major clients linked to the oil industry, and researchers are working to find solutions that can make the transport of the unprocessed well flow more efficient.
One solution under investigation is the application of a coating on the inside of the pipe wall that reduces precipitation on the wall itself, while at the same time protecting against corrosion and reducing friction by effectively making the pipe wall smoother.
“Experiments have been carried out to compare untreated with coated steel surfaces”, says Johnsen. “The challenge is to isolate the various contributory factors in the precipitation process, where we known that both the surface properties and thermal conductivity of the pipe wall, as well as the properties of the oil, will all play a part”.
Design tools “in the pipeline”
SINTEF researchers are also working with the Finnish lime industry. This work is part of a larger EU project bringing together research and industry partners from Ireland, Finland and Sweden.
Lime is transported through pipelines as a mud, and problems occur when minerals are precipitated on the pipe walls when the mixture is subjected to heat treatment.
The researchers have developed a mathematical model which they use to study how the particles move against the fluid flow and seek to attach themselves to the pipe walls. Various interactions between the fluid and the particles are built into the model. The aim is to construct a design tool for processing plants that can be utilised in combination with other commercially available flow simulators.
“We will have achieved a great deal if we can demonstrate to the industry when and where fouling will take place”, says Sverre Gullikstad Johnsen, who emphasises that his field of study is closely linked with general energy strategies. If we can improve the heat exchangers, their efficiency will increase, and energy consumption will be reduced.