Optimizing Amine Unit Performance: Insights from a Texas Gas Processing Plant
The authors’ company recently engaged with a gas processing plant in Texas, U.S., to conduct an audit and evaluation of their amine unit. The primary objective was to identify capacity restrictions and recommend operational improvements aimed at enhancing process efficiency.
Evaluation of the Amine Unit Contactor Tower
During the site visit, the team focused on collecting data, reviewing process parameters, and observing operations. The amine unit is specifically designed to treat carbon dioxide (CO₂) to meet regulatory specifications. This is accomplished by splitting the gas flow between two parallel contactor towers, each equipped with its own inlet coalescer and amine solvent regeneration stage. Although both units are almost identical and appear to operate similarly, this particular study zeroed in on Plant B.
Data Collection and Process Observation
As shown in FIG. 1, the amine unit was processing gas at an inlet feed of 107 million cubic feet per day (MMft³/d). Initial assessments revealed that the treated gas temperature exiting the amine contactor tower was significantly higher than both the inlet gas and the contactor temperature. This anomaly suggested that the bulge temperature—a crucial parameter indicating the location of peak thermal activity—was positioned toward the upper part of the contactor tower. Such an elevated bulge temperature can lead to unstable operation, resulting in off-spec product and higher chances of amine solvent carryover during foaming incidents.
Thermal Imaging Insights
Further investigations using thermal imaging indicated a bulge temperature of approximately 127°F (53°C), located higher in the tower, corroborating initial observations. Continuous operation at such temperatures is inadvisable as it can promote corrosion and reduce the efficiency of acid gas (H₂S/CO₂) removal. The recommended threshold for bulge temperature is below 185°F (85°C) to minimize these risks.
To lower the bulge temperature, modifications to the process were proposed, such as decreasing the inlet temperatures of both the lean amine and feed gas, as well as optimizing the flow rates of these fluids.
Simulation Findings
To ascertain the best approach for these adjustments, a simplified simulation of the amine unit was employed. This simulation used a mass transfer rate-based model, rather than the commonly employed equilibrium stage models, to analyze how variations in the inlet gas temperature would affect overall amine treating performance.
Parameters and Simulation Setup
Recent operational data, including feed gas and lean amine samples, were processed. Various design aspects such as the number of trays, tray spacing, tower diameter, and active tray area were incorporated as part of the simulation protocol, as illustrated in FIG. 3.
The simulation output provided insight into how variations in feed gas temperature, lean amine temperature, and lean amine flow rate impacted CO₂ removal efficiency and bulge temperature position.
Results and Data Interpretation
The simulations revealed several critical outcomes:
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Feed Gas Temperature Adjustments: Lowering the feed gas temperature from 90°F to 115°F positively influenced CO₂ removal performance, alongside lowering both bulge temperature and its position in the tower.
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Lean Amine Temperature Effects: Adjustments to the lean amine temperature also demonstrated a favorable impact on CO₂ absorption rates, despite only a slight decrease in outlet temperature.
- Lean Amine Flow Rate Changes: Increasing the flow rate of the lean amine showed improved CO₂ absorption efficiency but indicated a rise in temperature, suggesting a shift in the bulge temperature position higher in the tower.
Optimized Cases
The described metrics influenced an optimized operational case, consisting of multiple conditions designed to enhance CO₂ removal capability while maintaining system stability. Bypassing the gas exchanger to decrease inlet temperatures was deemed beneficial, as long as temperatures remained above 85°F (29°C) to prevent condensation.
Coalescer Vessel Review
The inlet gas coalescer vessel design also warranted attention. Simulations completed using the authors’ proprietary software confirmed that the vessel’s sizing was appropriate for normal operating conditions. However, specific deficiencies were noted:
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Critical Parameters: The annular velocity for gas flow was assessed to be well within acceptable bounds. Still, careful management is essential since high exit gas velocity may lead to liquid carryover.
- Effective Media Face Velocity: The face velocity, a vital aspect for ensuring efficient liquid coalescence, was also found to be within acceptable limits.
To improve performance, however, it’s recommended that a skimming line be installed for routine maintenance to enhance overall coalescer performance.
Liquid Evaluation and Flash Tank Management
During the site visit, key operational data were gathered, including insights into the flash tank operation, which, when controlled effectively, could stabilize the regeneration process. Maintaining higher liquid levels helps in the removal of contaminants. The team suggested that regular skimming procedures be established, particularly after operational upsets.
Activated Carbon Bed Assessment
Further assessments included evaluating the effective flowrate through the activated carbon bed. Operating within the recommended flux range proved crucial for maximizing contaminant removal while ensuring effective contact time was maintained.
- Optimal Flowrates: A flowrate between 17 gpm and 20 gpm is advised to ensure adequate performance without reaching the thresholds of either high or low flow, which could negatively affect contaminant removal efficiency.
Laboratory Analysis: Foam Testing and Antifoam Effectiveness
A series of laboratory tests were conducted to assess foaming tendencies in the amine solvent. Consistency in sample collection and testing methods allowed for reliable determinations of foam stability. Results indicated the presence of high foam stability in both lean and rich amine samples, suggesting the need for annual replacement of the activated carbon.
Antifoam Screening
Antifoam screening demonstrated that existing antifoam agents were ineffective, prompting recommendations to switch to higher-performing options such as AF-12 and 16V.
Summary of Recommendations
The audit led to several actionable recommendations for optimizing amine unit performance with minimal capital investment:
- Improve the skimming and dumping schedule of the amine flash tank.
- Maintain liquid levels in the flash tank to enhance operational stability.
- Control the carbon bed flowrate specifically between 17 gpm and 20 gpm.
- Implement upstream injection of antifoam methods.
- Routinely monitor the liquid levels to avoid flooding of coalescing elements.
By focusing on these operational improvements, the plant can achieve enhanced performance metrics, ultimately resulting in efficient gas processing and compliance with environmental standards.
About the Authors
S. Engel & C. Ridge
Nexo Solutions, Houston, Texas
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