Primarily used for road construction, flat roofing and as an adhesive, bitumen (asphalt) is a vital component of modern infrastructure. This flexible, hard-wearing product helps in the building and maintenance of road arteries in and between cities. Though safe and easy to use in construction projects, it presents particular challenges to the heat-exchange technology that is necessary to heat and cool it during processing. What are the potential pitfalls of handling bitumen, and what can be done to overcome them?
Bitumen is a black, viscous mixture of hydrocarbons obtained naturally or as a residue from petroleum distillation. Already a high viscosity product at hotter temperatures, as bitumen cools, the viscosity increases. This makes the cooling process rather than the heating stages of producing bitumen a greater challenge for heat exchangers.
Typical processes in the flat roofing industry involve heating bitumen mix to around 300°F (149°C) and cooling to 158°F (70°C), at which point viscosity can become as high as 25000 cP. Thermal oil or cooling water is used as the service-side fluid for both heating and cooling processes. A choice of heat exchange technologies are available to carry out these duties, including shell-and-tube and plate heat exchangers. Also used are hybrid welded plate heat exchanger (PHE) configurations, which have a tubular profile on one side and plate on the other.
The viscosity of bitumen makes it difficult to obtain an even flow and the required thermal efficiency. Gasketed plate heat exchangers generally offer the highest thermal efficiency of heat exchanger types, but the gasket compatibility and narrow flow channel are not suitable for the thick bitumen. The flow passage may become blocked.
Block-type welded plate heat exchangers eliminate use of elastomer gaskets. But, with the plates’ chevron corrugation, it is still difficult for the highly viscous bitumen to flow through the channels. The high viscosity increases the pressure drop across the heat exchanger. In addition, with a 45° chevron angle, cleaning is problematic, especially with larger units. Asymmetric designs are difficult to handle with the same corrugation and depth on both hot and cold side.
Shell-and-tube heat exchanger technology is known for its robustness. It is a popular choice for harsh duties such as bitumen. Shell-and-tube designs will meet the required thermal duty but require a large installation size. This type of solution may, however, also require a static mixer in the tubes to break up the laminar flow of the bitumen and ensure even cooling. Using a mixer in the flow line negates the possibility of mechanically cleaning the unit. The use of a hydrojet also is not advised because the bitumen may harden and block.
A hybrid plate heat exchanger combines the benefits of both plate heat exchanger and shell-and-tube heat exchanger technology. The hybrid design uses a plate on one side and tube on the other. Having an asymmetric plate pattern with the tube design improves handling of the highly viscous bitumen and reduces fouling. The reduced fouling potential helps increase reliability and reduces maintenance. It also enables longer run times that can potentially increase overall plant productivity.
Hybrid technology offers greater thermal efficiency than shell-and-tube heat exchanger technology. This means the installation size (equipment footprint) can be reduced. Indeed, a hybrid unit will typically require one-fifth of the volume of an equivalent shell-and-tube heat exchanger.
By combining tube and plate, a hybrid design can be mechanically cleaned on the tube side, which has the flow of bitumen. Clean-in-place (CIP) cycles also can be carried out to maintain efficiency of the plate side if required. Units can be supplied with a housing that is easily dismantled to facilitate inspection and maintenance.
Hybrid heat exchangers are effective for cooling bitumen with design pressures of up to 450 psig and, in some cases, up to 580 psig. Compact outer dimensions and footprint can be achieved using a flexible pass configuration, and compensators are available for thermal stress on longer unit designs.
Case Study: Production of Bitumen Roofing Sheets
In this application, bitumen at 365°F (185°C) and a viscosity of 25000 cP, is cooled by thermal oil in a hybrid welded heat exchanger. The unit has two bitumen inlets and one center outlet. The design helps distribute the fluid evenly over the plate pack for greater efficiency. The process uses a multi-pass setup to ensure stable temperature difference and reduced fouling.
The design of the hybrid heat exchanger for this particular application balanced the electrical power consumption of the pump with product distribution in the unit to achieve optimum efficiency and an 11 psig pressure drop. Further efficiency was achieved by fully insulating the unit to reduce thermal loss to the atmosphere.
The bitumen roofing sheet manufacturer also used hybrid welded exchangers for heating bitumen using thermal oil. Some units were designed in a vertical configuration to further reduce footprint; some had studded connections and others had flanged connections. The flexibility of this unit enables many installation requirements to be met.
Hybrid welded plate heat exchangers are offered with several plate geometries. Depending upon the application, units can be selected to optimize mechanical cleaning, optimize the geometry and provide a high heat transfer coefficient, or a combination of the two. Hybrid sizes are from 50 to 5,000 ft² in standard range. Several different exotic heat transfer plate materials are also available, and the exchanger can be made compliant to all major codes.
In conclusion, the heating or cooling of bitumen requires a balance between efficiency, cost and reliability. The ease with which the fluid can foul surfaces means that cleanability is an important factor when selecting a heat exchange technology. While various types of heat exchangers can perform these processes, the hybrid heat exchanger provides size, efficiency and cleanability benefits. Hybrid plate heat exchangers are particularly beneficial in challenging cooling process where the viscosity of the fluid rises and blockages can more easily occur.
A hybrid heat exchanger is a robust solution. It can handle the wide variance in temperature and pressure conditions found at a bitumen plant. They can offer bitumen producers a solution that reduces installation cost compared with shell-and-tube technology. Hybrid plate exchangers reliably meet the required thermal duty, minimize the potential for fouling and offer a design that can be cleaned.