Fractionation Tower Reflux for High C2+ Recovery From Volatile-Rich Gas
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Solution Overview
Problem
Conventional cryogenic expansion processes for hydrocarbon separation suffer from significant losses of C2, C3, and C4+ components due to the inability to effectively capture these components when the feed gas contains substantial quantities of components more volatile than methane, leading to inefficient recovery and increased capital and operating costs.
Innovation Solution
The process employs an upper rectification section with a reflux stream derived from the feed gas, cooled and expanded to the operating pressure of the fractionation tower, which absorbs C2, C3, and heavier hydrocarbon components from rising vapors, eliminating the need for a second reflux stream predominantly composed of methane and reducing capital and operating costs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of substance
If conventional cryogenic expansion processes are used for hydrocarbon separation, then the process simplicity and ease of operation are maintained, but significant losses of C2, C3, and C4+ components occur when feed gas contains substantial quantities of components more volatile than methane
Solution Approach 1:
The fractionation tower is divided into a lower stripping section and an upper rectification section, with different functions in each section. The stripping section handles the bulk separation using expansion cooled stream, while the upper rectification section specifically captures C2+ components from volatile-rich vapors using reflux, resolving the contradiction by segmenting the separation tasks.
Solution Approach 2:
A reflux stream is introduced as an intermediary substance in the upper rectification section to capture C2+ components from the vapor phase. This reflux acts as a mediator that enables efficient recovery of valuable components without compromising the overall process simplicity.
2Loss of substance
If an upper rectification section with reflux stream is added to improve C2+ component recovery, then component recovery exceeds 95% for C2 and 99% for C3 and C4+, but the device complexity increases
Solution Approach 1:
The upper rectification section is merged with the existing fractionation tower structure, sharing the same vessel and integrating the reflux system into the existing tower architecture. This merging approach achieves high component recovery while minimizing the increase in overall device complexity.
Solution Approach 2:
The fractionation tower is designed to perform multiple functions: the lower stripping section handles bulk separation while the upper rectification section handles high-purity C2+ component recovery. This multi-functionality allows a single tower structure to achieve both high recovery and acceptable complexity.
3Loss of substance
If conventional processes are used without upper rectification, then capital and operating costs are lower, but C2 component recovery remains below 95% when feed gas contains more than 10 mole % of components more volatile than methane
Solution Approach 1:
The process parameters (temperature, pressure, reflux ratio) in the upper rectification section are optimized to achieve high C2 component recovery. By carefully controlling these parameters, the system achieves >95% recovery while managing the added complexity through efficient parameter management rather than excessive equipment.
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
This approach achieves C2 component recovery exceeding 95% and C3 and C4+ component recoveries of over 99%, while ensuring nearly complete separation of methane and lighter components from heavier hydrocarbons at equivalent energy requirements, particularly benefiting processes with feed gases containing more than 10 mole % of components more volatile than methane.
Implementation Method 1
cooled and expanded to the operating pressure of the fractionation tower, which absorbs C2, C3, and heavier hydrocarbon components from rising vapors
Implementation Method 2
Recent changes in ethylene demand have created increased markets for ethylene and derivative products. In addition, fluctuations in the prices of both natural gas and its natural gas liquid (NGL) constituents have increased the incremental value of ethane, ethylene, propane, propylene, and heavier components as liquid products.
Implementation Method 3
As the gas is cooled, liquids may be condensed and collected in one or more separators
Data Source
AI summary
A process and an apparatus are disclosed for the recovery of components from a hydrocarbon gas stream containing significant quantities of components more volatile than methane (hydrogen, nitrogen, etc.). The gas stream is partially condensed, then the remaining vapor is expanded to lower pressure and supplied to a fractionation tower at a mid-column feed position. The condensed liquid is cooled and divided into two portions. The first portion is expanded to tower pressure, heated by cooling the liquid, and supplied to the tower at a lower column feed position. The second portion is further cooled, expanded to tower pressure, and supplied to the tower at a top feed position. The tower overhead vapor is heated by cooling the second portion. The quantities and temperatures of the feeds to the tower maintain the overhead temperature of the tower whereby the major portion of the desired components is recovered.