Boil-Off Gas Re-Liquefaction Using Expanded BOG Cooling
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Solution Overview
Problem
Existing methods for re-liquefying boil-off gas (BOG) from liquefied cargoes on floating transportation vessels, particularly those with boiling points above -110 °C, face challenges in efficiently cooling and re-condensing lighter components like ethane and ethylene due to limitations in compression systems and cooling media, leading to venting or loss of these components.
Innovation Solution
A method involving multiple stages of compression and heat exchange between a compressed, cooled, and expanded BOG stream with a cooled vent stream to enhance cooling duty, allowing for the re-liquefaction of lighter components without additional compression stages or venting, using a system with a plate-type discharge heat exchanger and vent heat exchangers to achieve further cooling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If multiple stages of compression are used to increase cooling duty for re-liquefaction, then the capacity to handle lighter components improves, but the device complexity and space requirements increase
Solution Approach 1:
The patent implements nested heat exchange by expanding a portion of the cooled compressed BOG stream back to intermediate pressure and using it to cool other compressed BOG streams in the compression train. This nested arrangement allows the system to reuse cooling capacity internally, effectively increasing the re-liquefaction capacity without adding external compression stages or increasing device complexity.
2Productivity
If additional compression stages are added to handle higher concentrations of lighter components, then the re-liquefaction efficiency improves, but the space available on the vessel is exceeded
Solution Approach 1:
The system performs self-service cooling by expanding a portion of its own cooled compressed BOG stream back to intermediate pressure to provide cooling duty for other streams in the compression train. This self-generated cooling capacity eliminates the need for additional compression stages, thereby maintaining re-liquefaction efficiency while avoiding increased system volume that would exceed vessel space constraints.
3Loss of substance
If the cooling duty is increased to re-liquefy lighter components like ethane and ethylene, then the cargo loss reduces, but the cooling media temperature requirements become more stringent
Solution Approach 1:
The patent changes the temperature parameter of the cooling media dynamically by expanding cooled compressed BOG stream to intermediate pressure, which causes temperature increase suitable for cooling other streams at different stages. This parameter change allows the system to provide appropriate cooling temperatures for lighter components like ethane and ethylene without requiring external cooling media with extremely low temperatures, thereby reducing cargo loss while maintaining feasible operating temperatures.
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 enables the efficient re-liquefaction of lighter components, reducing cargo loss and extending the capacity of re-liquefaction systems to handle higher concentrations of ethane and ethylene without additional compression stages or venting, thus improving the environmental and operational efficiency.
Implementation Method 1
heat exchanging the expanded cooled BOG stream against the cooled vent stream to provide a further cooled vent stream
Implementation Method 2
cooling and separating the compressed BOG discharge stream to provide a cooled vent stream as a gaseous stream comprising non-condensed components of the boil off gas and a cooled compressed BOG stream comprising condensed components of the boil off gas
Data Source
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AI summary
The disclosure relates to a method and apparatus for cooling, preferably liquefying a boil off gas (BOG) stream from a liquefied cargo in a floating transportation vessel, said liquefied cargo having a boiling point of greater than -110 °C at 1 atmosphere and comprising a plurality of components, said method comprising at least the steps of: compressing a boil off gas stream (01) from said liquefied cargo in two or more stages of compression comprising at least a first stage (65) and a final stage (75) to provide a compressed BOG discharge stream (06), wherein said first stage (65) of compression has a first stage discharge pressure and said final stage (75) of compression has a final stage suction pressure and one or more intermediate, optionally cooled, compressed BOG streams (02, 03, 04) are provided between consecutive stages of compression; cooling the compressed BOG discharge stream (06) to provide a cooled vent stream (51) and a cooled compressed BOG stream (08); expanding, optionally after further cooling, a portion of the cooled compressed BOG stream (08) to a pressure between that of the first stage discharge pressure and the final stage suction pressure to provide an expanded cooled BOG stream (33); heat exchanging the expanded cooled BOG stream (33) against the cooled vent stream (51) to provide a further cooled vent stream (53).