Heat Transfer as a Unit Operation in Chemical Engineering
Heat transfer as a Unit Operation in chemical engineering describes processes in which thermal energy is exchanged between two physical systems at different temperatures.
This operation may involve process fluids only, a combination of process and utility streams, or phase-change phenomena such as condensation and vaporization.
Depending on the configuration and purpose, heat transfer can occur:
– between two process streams exchanging energy directly;
– between a process stream and a utility medium such as steam, cooling water, or thermal oil;
– during phase changes, for example when a vapor is condensed or a liquid is vaporized.
Each case presents distinct design challenges, thermodynamic considerations, and equipment choices.
Heat Exchange Between Two Process Streams
This occurs when two streams, both belonging to the process, must undergo opposite temperature changes: one needs to be cooled while the other must be heated.
When the temperature difference between the two streams is sufficient, and their respective heat capacities are of the same order of magnitude, the heat from the hot stream can be used to warm the cold one.
This configuration enables energy recovery, reducing the consumption of external utilities such as steam or cooling water, and can be economically advantageous. However, for the exchange to be technically feasible, it is necessary that:
- the minimum temperature difference (ΔTₘᵢₙ) between the two streams allows a heat exchanger of reasonable size and cost
- the thermal capacities of the two streams (mass × specific heat) allow for a consistent balance,
- no operational incompatibilities exist between the two streams (pressure, risk of contamination, etc.).
Otherwise, direct exchange is not feasible, and the use of an intermediate utility fluid becomes necessary.
In this type of process-to-process heat exchange, typical equipment includes shell-and-tube heat exchangers, plate heat exchangers, and occasionally double-pipe exchangers. These allow direct energy recovery between two process streams without involving external utilities or phase changes.
Heat Exchange Using Utility Fluids: Reboilers and Condensers
One of the most common examples of utility-based heat exchange can be found in distillation systems, where reboilers and condensers are essential components.
In the image above, we see a typical application. At the bottom of the distillation column, a reboiler is supplied with steam to vaporize the heavier components.
At the top, a condenser uses cooling water to liquefy the overhead vapors, allowing part of the stream to return as reflux and the rest to be collected as distillate.
In many industrial processes, especially in separation operations, heat is transferred using utility fluids rather than directly between process streams. Utilities such as steam, cooling water or thermal oil act as intermediaries to supply or remove heat where required.
Using utility fluids allows for more accurate temperature control, ensures safer operation when direct contact is not acceptable, and enables standardization across different parts of the plant.
This is an indirect form of heat exchange, widely adopted due to its flexibility, safety, and compatibility with centralized utility systems.
Heat Transfer Integrated in Process Equipment
In many industrial applications, heat exchange does not occur in traditional exchangers but is integrated directly into the process equipment.
In many cases, heat must be supplied or removed directly within the process equipment. This is common in batch reactors, crystallizers, dryers, and storage tanks with temperature control.
In the image shown, a reactor is equipped with an external jacket and internal coils.
These elements allow precise thermal regulation during chemical reactions, preventing overheating or ensuring the right temperature profile for product quality.
Unlike standard exchangers, these configurations prioritize process control rather than thermal efficiency. The exchange surfaces are optimized for maintaining temperature homogeneity and enabling safe operation.
This type of heat exchange is essential when temperature affects reaction kinetics, product selectivity, or stability. For instance, in pharmaceutical production, maintaining narrow temperature ranges is often required by GMP standards to ensure reproducibility and safety.
Integrating heat exchange directly into the process unit simplifies piping and control, reduces response times, and ensures that heat is delivered exactly where it is needed.
Conclusion
Understanding the different configurations of heat exchange is essential for designing efficient, safe, and economically viable processes across all sectors of chemical engineering. In practice, the choice between process-to-process exchange, utility-based systems, or integrated heat transfer solutions has direct implications on operability, controllability, and long-term plant performance.
From fundamentals to field practice: heat transfer theory meets real-world challenges in cases like Ruby Formation in methanol ATR. This phenomenon shows how exchanger fouling can compromise energy efficiency and plant reliability. Explore the full case study here: Heat Exchanger Fouling and Ruby Formation in Methanol ATR.
Ing. Ivet Miranda
FAQ
What is process-to-process heat exchange?
Process-to-process heat exchange is the transfer of thermal energy between two process streams within the same plant. One stream is cooled while the other is heated, enabling energy recovery and reducing the demand for external utilities. Typical equipment includes shell-and-tube, plate, and double-pipe heat exchangers.
When is utility-based heat exchange preferred over process-to-process exchange?
Utility-based heat exchange is preferred when no suitable process stream is available for energy recovery or when precise temperature control is required. Utilities such as steam, cooling water, or thermal oil provide flexibility, operational safety, and easy integration with centralized plant utility systems.
How is heat transfer integrated into process equipment?
In process equipment such as reactors, crystallizers, dryers, or storage tanks, heat transfer is achieved through external jackets or internal coils. These systems are designed to ensure temperature control and uniformity, prioritizing process stability and safety over maximum thermal efficiency.