Ethane Inventory Production for Faster LNG Train Start-Up
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Solution Overview
Problem
Current methods for starting up an LNG production plant are inefficient, requiring several weeks to produce a sufficient ethane inventory due to low ethane extraction efficiency and causing significant gas flaring, with challenges in handling lean natural gas feed streams and safety issues with ethylene substitution.
Innovation Solution
A process involving a propane pre-cooled mixed refrigerant liquefaction facility with a main heat exchanger and mixed refrigerant circuit, where a bypass stream of dry sweet scrubbed gas is circulated to progressively cool and condense, separating fractions rich in nitrogen and methane, and ethane, with the ethane-saturated stream being cooled and stored, achieving high ethane recovery efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If a scrub column is used to extract ethane from natural gas feed stream at reduced flow rate, then ethane can be recovered for mixed refrigerant inventory, but the extraction efficiency is only around 5% and several weeks of operation are required
Solution Approach 1:
The patent changes the operating parameters by running the natural gas feed stream at normal operating flow rate (30-40% of full rate) instead of reduced flow rate, and by using the propane circuit to provide cooling to the scrub column at optimized temperatures. This combination of parameter changes achieves both high ethane extraction efficiency (>95%) and reasonable start-up time (several days rather than weeks).
Solution Approach 2:
The patent introduces the propane circuit as an intermediary cooling system to enhance the scrub column's ethane extraction capability. The propane circuit provides the necessary cooling duty to the scrub column, enabling efficient ethane condensation and separation from the natural gas feed stream without requiring the feed stream to be run at very low flow rates.
2Quantity of substance
If natural gas feed stream is run through scrub column at reduced flow rate (30-40% of normal), then ethane extraction is possible, but significant quantities of gas must be flared and liquid management issues occur
Solution Approach 1:
The patent changes the flow rate parameter from reduced (30-40% of normal) to normal operating flow rate. This eliminates the need to flare significant quantities of gas and avoids liquid management issues in the pipelines, while still achieving high ethane extraction efficiency through the enhanced cooling provided by the propane circuit.
Solution Approach 2:
The patent converts what would normally be waste streams (the natural gas feed stream that would be flared and the liquids causing management issues) into beneficial products. By running the feed stream at normal flow rate through the enhanced scrub column, ethane is efficiently extracted (>95%) while the remaining gas can be used as fuel gas or further processed, eliminating flaring and liquid management problems.
3Quantity of substance
If propane compression circuit operates at low load during ethane production, then ethane extraction can proceed, but recycle valves must be used and compression efficiency is reduced
Solution Approach 1:
The patent changes the operating parameters of the propane compression circuit by optimizing the cooling duty provided to the scrub column. This allows the propane compressors to operate at higher load levels (avoiding the need for recycle valves) while still achieving the necessary ethane extraction efficiency. The parameter changes in temperature and pressure profiles optimize both ethane production and compression efficiency simultaneously.
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 process significantly reduces the time to produce ethane inventory from weeks to days, achieving over 95% ethane extraction efficiency and minimizing flaring, while allowing for quicker LNG production startup.
Implementation Method 1
a bypass stream of dry sweet scrubbed gas is circulated to progressively cool and condense, separating fractions rich in nitrogen and methane, and ethane
Implementation Method 2
separating fractions rich in nitrogen and methane, and ethane, with the ethane-saturated stream being cooled and stored
Implementation Method 3
cooling the pressurized bypass gas stream using the propane refrigeration circuit to produce a cooled pressurized bypass gas stream
Implementation Method 4
the cooled pressurized bypass gas is cooled as it expands across an expansion valve into the shell side circuit of the main heat exchanger
Implementation Method 5
evaporating the cooled liquid bypass stream in the shell side circuit of the main heat exchanger to produce a first fraction rich in nitrogen and methane and a second fraction rich in ethane
Implementation Method 6
cooling the ethane-saturated pressurized bypass stream in the propane refrigerant circuit to produce a condensed heavy mixed refrigerant stream containing liquid ethane for storage
Data Source
AI summary
A process is described herein for the production of a selected quantity of ethane as a component of a production inventory of mixed refrigerant for an LNG production plant prior to start-up of the LNG production plant.