Ethane recovery system suitable for rich gas with high carbon dioxide content and recovery method therefor
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Solution Overview
Problem
Existing ethane recovery processes for rich gas with high carbon dioxide content face issues such as CO2 freezing and blockage, increased energy consumption, and reduced ethane recovery rates due to high CO2 content, leading to inefficiencies and economic losses.
Innovation Solution
An ethane recovery system incorporating a first and second pre-cooling cold box, a subcooling cold box, a low temperature separator, an absorption tower, and a demethanizer, with a low-temperature rectification section replaced by the absorption tower to divert CO2 to the stripping section, reducing CO2 content in the gas phase and optimizing operation pressure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the liquid phase volume is increased from a low temperature separator mixed into the second stream of feed material of the demethanizer to solve CO2 freezing and blockage problems, then CO2 freezing and blockage problems are solved, but a large amount of propane and heavy hydrocarbons are mixed into the external dry gas, causing losses of the heavy hydrocarbons
Solution Approach 1:
The invention divides the feed gas processing into multiple streams with different treatment paths. The feed gas is separated into a first stream that bypasses the low temperature separator and a second stream that goes through the low temperature separator and demethanizer. This segmentation allows CO2-rich gas to be treated differently from the main stream, preventing CO2 freezing and blockage in the main external dry gas stream while avoiding excessive mixing of propane and heavy hydrocarbons into the external dry gas.
Solution Approach 2:
Different parts of the gas stream receive different treatments based on their composition and destination. The first stream (bypassing low temperature separator) maintains its original quality suitable for external dry gas output, while the second stream (through low temperature separator and demethanizer) undergoes intensive treatment to remove CO2 and heavy hydrocarbons. This local quality differentiation ensures that external dry gas does not contain excessive propane and heavy hydrocarbons while still solving CO2 freezing problems in the treated stream.
2Object-affected harmful factors
If the demethanizer is pressurized to solve CO2 freezing and blockage problems, then CO2 freezing and blockage problems are solved, but expansion refrigeration capacity is reduced, external cooling demands increase, and compression power of external refrigeration devices is substantially increased, leading to a significant increase in energy consumption of system
Solution Approach 1:
The invention extracts the CO2 removal function from the demethanizer by introducing a bypass stream (first stream) that avoids the low temperature separator and demethanizer entirely. This extraction allows the demethanizer to operate at optimized pressure without excessive pressurization, reducing the need for expansion refrigeration and external cooling while still effectively removing CO2 from the gas phase through the alternative path. The CO2 is removed from the system through the absorber in the second stream rather than requiring high demethanizer pressure.
Solution Approach 2:
The invention introduces dynamic flow distribution between the first stream (bypass) and second stream (through low temperature separator and demethanizer) based on feed gas composition and system conditions. This dynamic adjustment allows the system to optimize the balance between CO2 removal efficiency and energy consumption, reducing the need for excessive demethanizer pressurization and associated energy costs while still preventing CO2 freezing and blockage.
3Productivity
If the low-temperature rectification section of the demethanizer is used, then ethane recovery is performed, but CO2 content in the gas phase remains high, causing freezing and blockage problems when CO2 content ≥2 mol %
Solution Approach 1:
The invention introduces an intermediary absorber unit between the low temperature separator and the demethanizer in the second stream. This absorber uses a solvent to selectively absorb CO2 from the gas phase before the gas enters the demethanizer's low-temperature rectification section. By removing CO2 through this intermediary step, the gas phase CO2 content is reduced below the freezing point threshold (≤2 mol %) while still allowing the demethanizer to effectively recover ethane in its low-temperature rectification section, thus preventing freezing and blockage problems.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system effectively addresses CO2 freezing and blockage issues, reduces power consumption, and enhances ethane recovery rates to 95%, improving energy efficiency and economic benefits.
Implementation Method 1
a gas phase end of the low temperature separator is in communication with a middle of the absorption tower via a first turbo expander expansion end
Implementation Method 2
a heat exchange output end of the first pre-cooling cold box and a heat exchange output end of the second pre-cooling cold box are in communication with an input end of an output compressor via a second turbo expander pressurized end
Implementation Method 3
an input end of an output compressor via a second turbo expander pressurized end, and an output end of the output compressor is in communication with an output end of an air cooler
Implementation Method 4
a pre-cooling output end of the subcooling cold box is in communication with a top of the absorption tower... so that CO2 content in the gas phase entering a low temperature zone is reduced, and most of CO2 is diverted to a stripping section of the demethanizer
Data Source
AI summary
The disclosure relates to the technical field of ethane recovery systems, and in particular to an ethane recovery system suitable for a rich gas with high carbon dioxide content and a recovery method therefor. The recovery system includes a first pre-cooling cold box, a second pre-cooling cold box, a subcooling cold box, a low temperature separator, an absorption tower, a tower top separator and a demethanizer. According to the disclosure, freezing and blockage problems occurring in conventional demethanizers when CO2 content ≥2 mol % are effectively solved. Meanwhile, an operation pressure of the demethanizer is ≥300 KPa compared with that of the absorption tower, significantly reducing power consumption of an output compressor, and making an ethane recovery device more energy efficient, and the disclosure is suitable for an ethane recovery device for a medium and high pressure rich gas with high carbon dioxide content.