Multi-Stage BOG Cooling for Lighter-Component Re-Liquefaction
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Solution Overview
Problem
Existing re-liquefaction systems for boil-off gas (BOG) from liquefied cargoes on floating vessels face challenges in efficiently cooling and re-liquefying lighter components, such as ethane and ethylene, due to limitations in compression and cooling capacity, leading to venting or accumulation of non-condensed gases, which is environmentally and commercially undesirable.
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 two or more stages of compression and heat exchangers to recycle and condense previously non-condensed components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If traditional single-stage compression and seawater cooling is used, then the system is simple and compact, but it cannot re-liquefy lighter components like ethane and ethylene, leading to venting or accumulation of non-condensed gases
Solution Approach 1:
The compression process is divided into multiple stages (first stage compression, second stage compression) with intermediate cooling. This segmentation allows the system to achieve higher final pressures and temperatures suitable for condensing lighter components without requiring a single excessively large compressor, thus managing complexity while expanding capability.
Solution Approach 2:
The first stage of compression and cooling is performed before the second stage of compression. This preliminary action removes some heat and condenses heavier components early, reducing the load on the second stage and enabling more efficient re-liquefaction of lighter components in the final stage.
2Adaptability or versatility
If additional compression stages are added to re-liquefy lighter components, then re-liquefaction capability is improved, but the vessel space requirements and system complexity increase
Solution Approach 1:
The patent combines multiple functions into the existing compression train: compression, intermediate cooling, condensation of heavier components, and final cooling. By merging these functions into a integrated system rather than adding separate dedicated equipment for each function, the vessel space requirement is minimized while achieving enhanced re-liquefaction capability.
Solution Approach 2:
The compression and cooling system is designed to handle multiple cargo compositions and component ranges simultaneously. The multi-stage compression with intermediate cooling serves multiple purposes: compressing gas, cooling intermediate streams, condensing various hydrocarbon components, and preparing streams for final condensation, making the system universally applicable to different cargo types.
3Ease of manufacture
If seawater cooling is used, then the system can operate with simple cooling, but it cannot provide sufficient cooling duty for lighter components with boiling points below -110°C
Solution Approach 1:
A cooled intermediate stream is introduced as an intermediary cooling medium between the two compression stages. This intermediate stream, cooled to temperatures below seawater temperature, serves as a mediator that provides the additional cooling duty needed for lighter components without requiring a completely different cooling system or complex refrigeration machinery.
Solution Approach 2:
The system uses its own compressed and cooled BOG stream to provide cooling duty for the compression process itself. The cooled intermediate stream, which would otherwise need external cooling, is used to cool the first stage compression discharge, making the system self-sufficient and eliminating the need for additional external cooling resources.
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 extends the re-liquefaction capability of the system to cargoes with higher concentrations of lighter components, reducing losses and environmental impact by effectively re-liquefying components that could not be condensed with traditional methods, thereby improving the system's efficiency and capacity.
Implementation Method 1
cooling the compressed BOG discharge stream to provide a cooled vent stream and a cooled compressed BOG stream
Implementation Method 2
heat exchanging the expanded cooled BOG stream against the cooled vent stream to provide a further cooled vent stream
Implementation Method 3
expanding, optionally after further cooling, a portion of the cooled compressed BOG stream to a pressure between that of the first stage discharge pressure and the final stage suction pressure
Data Source
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).