Aug-2024

Role of heat integration in a sustainable, low-carbon future

How different technologies are playing respective roles in minimising energy consumption and reducing emissions in industrial settings.

Warren Chung
Solex Thermal Science

Article Summary

The ongoing pursuit of a more efficient, equitable, and environmentally conscious energy future is commonly framed within the sustainability-based concepts of the energy transition and a circular economy. At their core, these discussions focus on reducing carbon footprint, whether through lowering primary energy consumption or minimising waste and pollution.

The global response to climate change has added another layer of complexity. Policymakers worldwide have introduced various mechanisms to influence behaviours and decision-making, aiming to reduce emissions and promote carbon-reducing innovations.

Geopolitical uncertainties and dynamic monetary policies in response to inflation concerns have further intensified volatility in the energy market. Consequently, many industrial operators are proactively implementing risk-mitigation strategies, ranging from deploying financial instruments and hedges to re-evaluating and enhancing physical operations.

Public pressure also plays a significant role. As of November 2023, approximately 145 countries have announced or are considering net zero targets, covering nearly 90% of global emissions. Investors around the world continue to call for increased action, urging companies to improve their sustainability programmes and adopt their own net zero goals.

Financial performance remains a fundamental factor in commercial decision-making, yet its calculation has become increasingly complicated due to emissions pricing and compliance costs in the global market. As a result, many companies are strategically investing in energy efficiency initiatives, as these are considered low risk technically and financially. This approach not only decreases emissions but also improves environmental performance, reduces production costs, and enhances overall financial health.

Enhancing energy recovery and efficiency through heat integration
Implementing heat integration mechanisms into existing operations is highly effective and low risk, and can be a high-return approach for minimising energy consumption in industrial settings.

The concept of heat integration is not novel, but its practical application has, at times, been hindered by factors ranging from high cost of adoption, complexity of integration, and technical restrictions of existing technologies.

Although many industrial processes already incorporate some level of heat integration, difficult-to-handle process streams frequently remain unutilised due to perceived constraints. Tapping into otherwise wasted heat from these previously overlooked streams presents a significant opportunity for operators to incrementally reduce costs and lower emissions.

Heat pipe heat exchangers (HPHE) and moving bed heat exchangers (MBHE) are two heat integration technologies that take very different approaches to improving the energy performance of industrial processes. However, both can play a similar role in optimising energy usage and enhancing sustainability in industrial settings.

Heat pipe heat exchangers
Combustion flue gases hold substantial potential for energy recovery. Extracting energy from these streams has historically 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 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 their inherent safety redundancies. When conventional CHEs fail, entire systems must be taken offline to avoid contamination or mixing of process streams. Conversely, an HPHE can continue operating safely even in the rare instance that a heat pipe fails since process streams remain isolated. During scheduled maintenance, failed heat pipes can be easily identified, isolated, and replaced without disassembling the whole unit.