Decarbonisation Technology - August 2024 Issue

latent heat of that working fluid is used for the energy exchange. An HPHE (see Figures 1 and 2 ) is constructed as a cased unit that can receive two process streams. The process streams are isolated using a separation plate that contains affixed heat pipes that contact each process stream: • On the primary side, also known as the evaporator section, heat pipes contact the hot process stream, causing the liquid working fluid within each heat pipe to boil. • On the secondary side, or condenser section, each heat pipe contacts a cold process fluid, causing the gaseous working fluid to condense simultaneously. The ends of the heat pipe are free to expand and contract, preventing mechanical stress on the equipment. HPHEs are highly customisable: the number of pipes, their spacing, dimensions, orientation, material of construction, type of working fluid, and casing dimensions can all be tailored to meet specific application requirements. Design features for handling particulate-rich gas streams HPHEs are adept at managing particulate-rich gas streams through: • Unique internal geometry where process fluids contact only the external surfaces of the heat pipes. • Smooth heat pipe surface finishes. As particulate-rich gas flows perpendicular to the heat pipe orientation, particles collide, lose energy, and settle in a dust trap collector, which can be emptied manually or automatically. For particulate-rich wet gas streams, integrated sonic horns or water jets can be activated to keep the heat pipe surfaces clear of build-ups. HPHEs can also avoid acid condensation conditions as each heat pipe operates isothermally at a predictable intermediate temperature, which can be designed just above the flue gas acid dew point. This allows operators to achieve up to 25% more heat recovery. In contrast, CHEs often develop cold spots that lead to localised acid condensation and corrosion, necessitating operation well above acid condensation temperatures and resulting in suboptimal heat integration. Lastly, HPHEs are particularly attractive from a total cost of ownership perspective due to

50˚C to 350˚C

Air, water or thermal oil 20˚C to 30˚C

Exhaust 150˚C to 1 , 000˚C

100˚C to 200˚C

proven challenging due to heavy particulate loads, high acidity, and extreme temperature fluctuations, which can cause fouling, corrosion, and mechanical failures, respectively, in conventional heat exchangers (CHE) such as shell and tube or plate and frame varietals. These issues have historically led to higher maintenance, frequent shutdowns and decreased profitability, making it more difficult for operators to justify these investments for energy recovery purposes. HPHEs offer a proven and reliable solution to these complex heat recovery challenges. The technology acts as an indirect heat transfer device composed of an array of heat pipes, each acting as an individual heat exchanger. A heat pipe is a sealed tube filled with a small amount of working fluid at saturation condition. The Figure 1 A typical heat pipe heat exchanger arrangement for recovering waste heat from a hot combustion gas stream ( Source: Econotherm)

Figure 2 Top-down cross-sectional view of a heat pipe heat exchanger ( Source: Econotherm)