Shell and Tube Heat Exchangers: Critical Case Studies in Chemical Processing

Shell and tube heat exchangers (STHXs) are indispensable workhorses across the chemical processing industry (CPI), playing a pivotal role in regulating temperature, facilitating reactions, and optimising energy usage. Their robust design, adaptability to various operating conditions, and ability to handle corrosive or high-pressure fluids make them the most prevalent type of heat exchanger in industrial settings. Understanding their application through real-world case studies provides crucial insights into their design, operation, and maintenance challenges.

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The Versatility of Shell and Tube Heat Exchangers in Chemical Operations

STHXs are chosen for their nearly limitless customisation potential, enabling them to optimise diverse processes within chemical plants. They facilitate heat transfer between two fluids—one flowing through a bundle of tubes and the other through the surrounding shell—without direct mixing. This design allows for efficient thermal energy exchange, which is critical for a wide array of chemical processes.

Key applications include:

• Condensing Vapours: Transforming vapours into liquids, common in refrigeration systems and various chemical processes.
• Reboiling: Adding heat to the bottom of distillation columns to generate vapours for separation.
• Preheating Fluids: Increasing the temperature of fluids before they enter reactors or other process units, thereby improving energy efficiency.
• Cooling Hot Fluids: Reducing the temperature of process streams using a cooler medium.
• Reactor Temperature Control: Crucial for maintaining optimal conditions during exothermic or endothermic chemical reactions.
• Heat Recovery: Recycling waste heat from chemical reactions to preheat reactants, significantly improving overall energy efficiency.

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Case Studies: Addressing Common Challenges

Despite their versatility, shell and tube heat exchangers face several operational challenges that can impact their efficiency and longevity. Key issues often include fouling, corrosion, and mechanical failures. Case studies frequently highlight innovative solutions or best practices to mitigate these problems.

The Landmark 1994 Book Case Study: Heat Transfer Optimisation in Petroleum Refineries

To understand the foundations of modern thermal design, chemical engineers frequently refer to classic literature. A major reference point is the landmark 1994 book case study on shell-and-tube heat exchangers in petroleum refineries, featured in the seminal text Process Heat Transfer by G. F. Hewitt, G. L. Shires, and T. R. Bott. This comprehensive work detailed how heat transfer optimisation in industrial processes is directly constrained by the physical and thermodynamic boundaries of refinery pre-heat trains.

The 1994 study analysed how crude oil, containing volatile hydrocarbons and suspended inorganic salts, flows through a series of shell-and-tube exchangers. It highlighted that optimisation is a delicate balance between increasing fluid velocity (which enhances the heat transfer coefficient) and managing the exponential rise in pressure drop and mechanical wear.

To mathematically model the heat transfer efficiency and the degrading impact of fouling over operational cycles, the authors utilised the fundamental overall heat transfer relationship:

Where:

• $U_c$ is the clean or overall heat transfer coefficient based on the outside tube area ($W/m^2\cdot K$).
• $h_o$ is the shell-side (outside) fluid film heat transfer coefficient ($W/m^2\cdot K$).
• $h_i$ is the tube-side (inside) fluid film heat transfer coefficient ($W/m^2\cdot K$).
• $R_{fo}$ is the shell-side fouling resistance ($m^2\cdot K/W$).
• $R_{fi}$ is the tube-side fouling resistance ($m^2\cdot K/W$).
• $d_o$ is the outside tube diameter (m).
• $d_i$ is the inside tube diameter (m).
• $k_w$ is the thermal conductivity of the tube wall material ($W/m\cdot K$).

The 1994 book case study demonstrated that by adjusting baffle spacing and shifting from a standard single-pass shell (E-shell) to a divided-flow shell (J-shell), engineers could drastically reduce tube-side fouling while keeping pump pressure drops within acceptable refinery limits. This historical case study remains the basis for modern computer-aided heat exchanger design and network synthesis.

Fouling Mitigation in Crude Oil Distillation Units

Fouling, the accumulation of undesirable deposits on heat transfer surfaces, is a pervasive and costly issue in heat exchangers, particularly in the pre-heat trains of crude oil distillation units in refineries. These deposits, which can include inorganic salts, polymers, sludge, or biological growth, significantly reduce heat transfer rates (sometimes by as much as 50%) and increase pressure drop, leading to decreased thermal performance and increased energy consumption.

• Industrial Example: An industrial case study analysed severe fouling in a heat exchanger within a crude distillation unit. The study focused on using dynamic simulation combined with plant measurements to assess the impact of fouling and identify retrofit opportunities. By implementing a proposed retrofit design, which aimed at better utilisation of the available heat transfer area, improved heat transfer coefficients, and shell-side fouling mitigation, the heat recovery in the exchanger was projected to increase by 2.5-4.5 MW. This led to estimated fuel cost savings of $1.2 million after two years, demonstrating the economic benefits of proactive fouling management. The study also emphasized that while some designs reduce fouling, they can have undesired side effects like increased pressure drop or decreased throughput, necessitating a holistic approach to optimisation.

Combating Corrosion in Aggressive Chemical Environments

Corrosion is another critical challenge for STHXs, especially when handling aggressive, corrosive, or abrasive fluids common in the chemical industry. Material selection plays a vital role, with many STHXs fabricated from high-alloy materials to ensure compatibility with specific process requirements.

• Corrosion in a Water Basin Heat Exchanger: One case study investigated the corrosion failure of a stainless steel 316 straight-tube heat exchanger used to heat desalinated water during winter. The water contained a high chloride level (~800 ppm), which, combined with the use of hydrochloric acid to adjust pH, led to pitting corrosion, particularly at high temperatures. Pitting corrosion is a localized form of attack that can occur when chloride ions break down the protective oxide film on stainless steel. The solution suggested was to control chloride levels and properly adjust the pH of the basin water to prevent further corrosion.
• Protecting Heat Exchanger Surfaces with Coatings: A project for a Baltic chemical company involved coating 27 shell and tube heat exchangers and 5 floating-head heat exchangers, with lengths up to 12 meters and diameters up to 1.75 meters, to protect them from corrosion and incrustation. The heat exchangers were exposed to cooling water of inconsistent quality at 60 °C on the tube side and high-pressure steam at approximately 200 °C on the shell side. Cold-cured epoxy linings, specifically SÄKAPHEN HR 60 Extra TG and SÄKATONIT K 80 LS, were applied to the tube sheets and tube ends. These coatings were chosen for their chemical resistance to various acidic and alkaline substances, inorganic salts, and water types, even under permanent immersion. The project resulted in excellent overall protection, optimal prevention of caking and fouling, and enhanced long-term plant efficiency and reliability. Another similar case involved coating 86 high-pressure pipes, each 15 meters long, for a large heat exchanger from a major multinational chemical manufacturer. These pipes were exposed to aggressive cooling water on the shell side and a corrosive solvent over 100 °C on the tube side.

Preventing Mechanical Failures and Optimising Design

Mechanical failures in shell and tube heat exchangers can stem from issues like thermal expansion, metal erosion due to excessive fluid velocity, or stress corrosion. Proper design and ongoing monitoring are crucial for preventing such failures.

• Optimisation for Annual Operating Cost: A project focused on optimising a shell and tube heat exchanger to minimise its total annual operating cost. The optimisation involved a three-stage approach: thermal analysis using a mathematical model, structural optimisation of the nozzle considering loads using ANSYS, and computational fluid dynamics (CFD) analysis. This comprehensive approach, based on industry codes and standards, aimed to determine optimum dimensions for the heat exchanger given specific inlet and desired outlet conditions, considering factors like heat transfer area and pumping capacity.
• Cleaning for Efficiency Restoration: Water scale deposits on the shell side of a shell and tube heat exchanger at a tire and rubber manufacturing plant led to significant heat transfer issues. To address this, a biodegradable descaler was circulated through the heat exchanger for one hour. This cleaning process successfully removed all water scale deposits, restoring the heat exchanger's efficiency, and prompted the implementation of a preventative maintenance program at the plant.

Conclusion

Shell and tube heat exchangers are fundamental to the chemical processing industry, enabling efficient temperature control and energy management across numerous applications from petrochemical refining to pharmaceutical production. Historical frameworks, such as the 1994 Hewitt, Shires, and Bott refinery case study, continue to guide engineers in balancing fluid velocity, pressure drop, and thermal efficiency. Modern case studies underscore their adaptability but also highlight persistent challenges such as fouling and corrosion. Effective strategies, including advanced material selection, protective coatings, diligent maintenance practices like chemical descaling, and sophisticated design optimisation using tools like CFD, are critical for maximising the performance, safety, and longevity of these essential components in complex chemical processes.