Unlocking the Power of CFD in Engineering

Unlocking the Power of CFD in Engineering

Table of Contents:

1. Introduction
2. What is a Sulphur Recovery Unit?
3. Waste Heat Boilers and their Operation
4. Known Modes of Failure for Waste Heat Boilers 4.1. Over-temperature Failure 4.2. Sulphonation Corrosion 4.3. Departure from Nucleate Boiling
5. Process Side Modeling and Analysis
6. Fouling and its Effects on Boiler Operation
7. Development of Operational Limit Curves
8. Conclusion

Introduction

In this article, we will explore the topic of waste heat boilers and their operational characteristics. Waste heat boilers play a critical role in sulphur recovery units, which are used to recover elemental sulfur from process streams that contain hydrogen sulfide. These units are commonly found in refining industries and chemical process industries. The primary focus of this article will be on analyzing the long-term operational characteristics of waste heat boilers and developing operational limit curves to ensure their safe and efficient operation.

What is a Sulphur Recovery Unit?

A sulphur recovery unit is a key component of industrial processes where elemental sulfur needs to be recovered from process streams that contain hydrogen sulfide. This includes applications in refining industries and chemical process industries. Sulphur recovery units typically consist of a thermal reactor, a waste heat boiler, and primary and secondary condensers. The thermal reactor facilitates the reaction between hydrogen sulfide and oxygen to produce sulfur dioxide and Water. Subsequently, this sulfur dioxide reacts with additional hydrogen sulfide to yield pure elemental sulfur and excess water. It is important to note that this reaction is exothermic, generating temperatures ranging from 2200 to 2800 degrees Fahrenheit, depending on the process streams and compositions involved.

Waste Heat Boilers and their Operation

Waste heat boilers, specifically fired tube boilers, are an essential component of sulphur recovery units. These boilers function to cool down the process gas coming from the thermal reactor, which cannot enter the condensers above approximately 700 degrees Fahrenheit. Additionally, waste heat boilers generate steam at 500 to 650 PSIG, which can be utilized for power equipment and various plant processes. The tubesheet of waste heat boilers is protected by a ceramic ferrule to withstand the high temperatures of the gas stream, ensuring the integrity of the steel construction. It is crucial to understand the operation and characteristics of waste heat boilers within the Context of sulphur recovery units.

Known Modes of Failure for Waste Heat Boilers

Waste heat boilers are susceptible to several modes of failure, which need to be thoroughly analyzed to ensure safe and efficient operation. The three primary known modes of failure for waste heat boilers are over-temperature failure, sulphonation corrosion, and departure from nucleate boiling. Over-temperature failure occurs when the reactive refractory, typically composed of aluminum, cracks or melts due to excessive temperatures exceeding its operational limit of 3000 degrees Fahrenheit. Sulphonation corrosion, a long-term failure mode, results from the corrosive effect of sulfur in the process gas on the steel components of the boiler. Departure from nucleate boiling is a short-term failure mode where film boiling occurs on the outside of the tubes, impacting their structural integrity and causing a loss of pressure boundary.

4.1. Over-temperature Failure

Over-temperature failure in waste heat boilers is primarily attributed to the reactive refractory, which cannot withstand temperatures beyond 3000 degrees Fahrenheit. Exceeding this limit leads to the cracking or melting of the refractory, necessitating immediate shutdown and maintenance of the unit.

4.2. Sulphonation Corrosion

Sulphonation corrosion is a long-term failure mode caused by the corrosive nature of sulfur in the process gas. As the gas stream contains three to five percent mole of hydrogen sulfide, prolonged exposure can significantly corrode the steel tubes of the boiler. The corrosion rates increase as the temperature and hydrogen sulfide content rise, potentially jeopardizing the mechanical integrity of the tubes.

4.3. Departure from Nucleate Boiling

Departure from nucleate boiling is a short-term failure mode that occurs when film boiling develops on the outside of the tubes. This phenomenon disrupts the convective heat transfer process and can lead to rapid temperature increases, tube failure, and loss of pressure boundary. Immediate action is required to mitigate the risks associated with departure from nucleate boiling.

Process Side Modeling and Analysis

To accurately predict the long-term operational characteristics of waste heat boilers, process side modeling and analysis are essential. Computational Fluid Dynamics (CFD) tools coupled with basic engineering principles are utilized to simulate and understand the fluid flow, heat transfer, and chemical reactions occurring within the waste heat boiler. By modeling the process side of the boiler, including the gas flow, heat fluxes, and temperature profiles, engineers can identify potential areas of concern and develop effective strategies for safe and efficient operation.

Fouling and its Effects on Boiler Operation

Fouling, the accumulation of deposits on the internal surfaces of the waste heat boiler, can significantly impact its performance and operational characteristics. Hard and soft scales commonly form near the inlet of the boiler, reducing heat transfer efficiency and potentially causing operational issues. Understanding the fouling mechanisms and their effects on boiler operation is crucial for maintenance and optimization strategies. By analyzing fouling growth Patterns and developing mitigation measures, engineers can ensure the long-term reliability and effectiveness of waste heat boilers.

Development of Operational Limit Curves

Operational limit curves play a vital role in establishing safe operating conditions for waste heat boilers. These curves are developed Based on the analysis of process data, including mass flow rates, operating temperatures, and composition. By correlating these factors with known failure modes, such as sulphonation corrosion and departure from nucleate boiling, engineers can define the operational limits within which the boiler can operate safely and efficiently. Operational limit curves help prevent unexpected failures, maximize the lifespan of the equipment, and optimize operational performance.

Conclusion

In conclusion, waste heat boilers are crucial components of sulphur recovery units, facilitating the recovery of elemental sulfur from process streams containing hydrogen sulfide. Understanding the operational characteristics, known failure modes, and mitigation strategies is critical for ensuring the safe and efficient operation of waste heat boilers. Process side modeling, analysis, and the development of operational limit curves enable engineers to predict long-term operational behavior, optimize maintenance strategies, and maximize the lifespan of the equipment. By considering factors such as fouling, over-temperature failure, sulphonation corrosion, and departure from nucleate boiling, engineers can implement effective measures to mitigate risks and ensure the reliable operation of waste heat boilers in various industrial processes.