Maximizing NGL Recovery: Understanding the Turbo-Expander Cryogenic Process

Introduction

Cryogenic processing plays a crucial role in extracting valuable natural gas liquids (NGLs) from the gas stream in the midstream oil and gas industry. The turbo-expander process is a widely used cryogenic technology that enables efficient separation of methane from heavier hydrocarbons by cooling the gas to temperatures as low as -150°F. In this article, we explore the two most common cryogenic processes: the Gas Subcooled Process (GSP) and the Recycle Split Vapor (RSV) process, comparing their advantages, limitations, and operational flexibility.

The Turbo-Expander Process in Cryogenic Gas Processing

The turbo-expander process leverages the principle of gas expansion through a turbine to achieve extremely low temperatures, making it possible to separate methane from valuable NGLs such as ethane, propane, butane, and natural gasoline. In some cases, additional refrigeration is used on the front end to enhance cooling efficiency.

The most common cryogenic processes include:

1. ?Gas Subcooled Process (GSP): Known as the industry workhorse, the GSP offers high ethane and propane recoveries and remains a staple in NGL recovery.

2. ?Recycle Split Vapor (RSV): An enhanced version of GSP, RSV provides ultra-high recovery in both ethane recovery and rejection modes.

3. ?Single Column Overhead REcycle (SCORE): Specializes in ultra-high propane recovery during ethane rejection mode.

4. ?Overhead Recycle (OHR): Offers improved performance compared to GSP, particularly in ethane rejection mode.

5. ?Supplemental Rectification Process (SRP): Part of Ortloff's Flexible Recovery solutions, focusing on enhanced NGL recovery.

6. ?Supplemental Rectification with Compression (SRC): Another Flexible Recovery solution that adds compression for higher recovery efficiency.

These processes maximize NGL recovery and provide flexibility to adapt to market conditions.

This article focuses on two prominent cryogenic processes: ?Gas Subcooled Process (GSP) ?and ?Recycle Split Vapor (RSV) .

Gas Subcooled Process (GSP)

GSP is a widely used cryogenic process for NGL recovery, offering higher ethane and propane recoveries than traditional single-stage expander designs. The key steps include:

1. Pre-treatment: Removal of impurities such as water, CO2, and H2S to prevent freezing and corrosion.
2. Initial cooling: The natural gas is cooled using heat exchangers to reduce its temperature before entering the turbo-expander.
3. Turbo-expansion: The gas expands through the turbo-expander, lowering its temperature significantly.
4. Separation: The cold gas enters a demethanizer column, where heavier hydrocarbons condense and separate from methane.
5. NGL recovery: The condensed NGLs are collected from the bottom of the demethanizer.
6. Methane processing: Methane-rich gas exits from the top of the demethanizer and is sent for further processing or pipeline distribution.

GSP is a proven and effective method for maximizing NGL recovery, especially in ethane recovery mode.

Recycle Split Vapor (RSV) Process

RSV builds upon the GSP process by introducing additional reflux and an expanded demethanizer column section, achieving even higher NGL recoveries.

Key differences from GSP:

1. Higher ethane recovery: RSV typically achieves ethane recoveries exceeding 95% and sometimes up to 99%.
2. Additional reflux: Introduces a residue gas reflux stream, enhancing separation efficiency.
3. Operational flexibility: RSV can efficiently switch between ethane recovery and ethane rejection modes.
4. Improved CO2 tolerance: RSV can handle increased CO2 inlet gas concentrations compared to GSP.
5. Higher energy consumption: The process requires slightly more compression horsepower than GSP.

While RSV offers superior NGL recovery, it comes with trade-offs such as increased compression costs, higher fuel consumption, and a more complex control system.

Ethane Recovery vs. Ethane Rejection Mode

Cryogenic gas plants can operate in either ethane recovery or ethane rejection mode, depending on market conditions:

• Ethane Recovery Mode: Ethane is extracted and sold as a liquid, typically for ethylene production. This mode is used when ethane prices are high.
• Ethane Rejection Mode: Ethane remains in the gas stream and is sold as part of natural gas (measured in MMBTU). This mode is preferred when ethane prices are low or when extraction costs outweigh the potential profits.

Most modern processing plants are designed to switch between these modes to adapt to changing market demands.

Key Differences Between GSP and RSV

• Recovery Rates : RSV achieves higher ethane and propane recoveries than GSP.
• Energy Consumption : RSV may require slightly more compression horsepower than GSP.
• Operational Flexibility : RSV offers superior flexibility in adjusting ethane recovery to match market demand.
• Capital Investment : RSV typically requires higher initial investment due to additional equipment.

?Advantages and Disadvantages of RSV

Advantages:

1. Higher ethane recoveries, often exceeding 99%.

2. Improved CO2 tolerance compared to GSP.

3. Enhanced propane recovery, even in ethane rejection mode.

4. Increased energy efficiency in ethane recovery mode.

5. Potential to retrofit existing GSP plants for greater capacity.

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?Disadvantages:

1. Higher compression costs due to recycling 7-15% of the gas.

2. Increased fuel consumption compared to GSP.

3. More complex control systems, raising operational challenges.

4. Sensitivity to CO2 freeze-up, limiting high-CO2 applications.

5. Larger capital investment due to additional equipment.

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Conclusion

Both GSP and RSV play essential roles in cryogenic gas processing, each with unique advantages. While GSP remains the industry workhorse for NGL recovery, RSV provides higher recovery rates and greater flexibility, making it an attractive option for operators looking to maximize profitability. However, the choice between these processes depends on factors such as capital investment, energy consumption, and operational complexity.

By understanding the principles and trade-offs of these technologies, midstream operators can make informed decisions to optimize their gas processing facilities and adapt to market fluctuations efficiently.

What are your thoughts on GSP vs. RSV? Have you worked with these processes in the field? Let's connect and discuss how these technologies are shaping the future of gas processing!

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