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 optimizing 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.
The Versatility of Shell and Tube Heat Exchangers in Chemical Operations
STHXs are chosen for their nearly limitless customization potential, enabling them to optimize 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 Vapors: Transforming vapors into liquids, common in refrigeration systems and various chemical processes.
- Reboiling: Adding heat to the bottom of distillation columns to generate vapors 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.
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.
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 analyzed 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 utilization 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 optimization.
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 Optimizing 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.
- Optimization for Annual Operating Cost: A project focused on optimizing a shell and tube heat exchanger to minimize its total annual operating cost. The optimization involved a three-stage approach: thermal analysis using a mathematical model, structural optimization 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. 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 optimization using tools like CFD, are critical for maximizing the performance, safety, and longevity of these essential components in complex chemical processes.
