BOG Reliquefaction Cooling with Interstage Heat Exchange
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Solution Overview
Problem
Existing methods for reliquefying boil-off gas (BOG) from liquefied cargoes with boiling points greater than -110 °C on floating transportation carriers are inefficient in terms of power requirements and capacity, particularly due to space constraints and the need for multiple compression stages, which limits the use of closed refrigeration systems and restricts cooling efficiency when using seawater.
Innovation Solution
A method involving multiple stages of compression with heat exchange between intermediate BOG streams and the cooled compressed discharge stream, utilizing a combination of compression and heat exchangers to enhance the coefficient of performance and reliquefaction capacity, including the use of economizers between consecutive compression stages to optimize cooling and condensation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If multiple stages of compression are used to compress BOG, then sufficient compression is achieved to facilitate condensation, but power requirements increase and system complexity increases
Solution Approach 1:
The compression process is divided into multiple stages (first stage, second stage, third stage) with intercooling between stages. This segmentation allows the total compression ratio to be achieved while reducing the work required compared to single-stage compression, as each stage compresses to a lower pressure increment and cooling removes heat that would otherwise require additional compression work.
Solution Approach 2:
Intercooling is performed between compression stages to remove heat before the next stage of compression. This preliminary cooling action reduces the temperature and volume of the gas entering the next compression stage, thereby reducing the work required for subsequent compression and improving overall system efficiency.
2Quantity of substance
If multiple stages of compression are used to compress BOG, then sufficient compression is achieved to facilitate condensation, but device complexity increases
Solution Approach 1:
The heat exchanger serves multiple functions: it cools the compressed BOG between compression stages (intercooling), and also provides the cooling duty for the compression process. This multi-functionality reduces the need for separate cooling systems and simplifies the overall device architecture despite the multi-stage compression requirement.
Solution Approach 2:
The compression and cooling functions are merged into an integrated system where the heat exchanger is positioned to receive hot compressed BOG from one compression stage and deliver cooled BOG to the next compression stage. This merging of functions reduces the number of separate components and simplifies the system layout.
3Temperature
If closed refrigeration system is used to cool condenser, then cooling efficiency is improved, but space requirements increase which is not feasible on carrier
Solution Approach 1:
The system uses the compressed BOG itself as the cooling medium for the condenser. The hot compressed BOG from the compression stages is directed through the condenser heat exchanger, where it provides the necessary cooling duty to condense the BOG. This self-service approach eliminates the need for separate refrigeration systems or seawater cooling, achieving high cooling efficiency while minimizing space requirements.
Solution Approach 2:
The compressed BOG acts as an intermediary heat transfer medium between the compression process and the condensation process. By using the compressed BOG as the cooling medium, the system efficiently transfers heat from the condenser to the compressed gas stream, achieving effective cooling without requiring external refrigeration systems or additional space.
4Device complexity
If seawater is used for cooling, then system design is simplified, but cooling duty is insufficient when seawater temperature is high (up to 32°C)
Solution Approach 1:
The system uses the compressed BOG itself as the cooling medium for the condenser. The hot compressed BOG from the compression stages is directed through the condenser heat exchanger, where it provides the necessary cooling duty to condense the BOG. This self-service approach eliminates the need for separate refrigeration systems or seawater cooling, achieving high cooling efficiency while minimizing space requirements.
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 reduces power requirements and increases the coefficient of performance, enhancing the reliquefaction efficiency and capacity, making it more effective for liquefied petroleum gas and other cargoes with high concentrations of lighter components like ethane.
Implementation Method 1
the compressed BOG is cooled and condensed against seawater
Implementation Method 2
the compressed BOG is cooled and condensed against seawater
Implementation Method 3
the compressed BOG is cooled and condensed against seawater
Implementation Method 4
This can be achieved by the compression and cooling of the BOG
Implementation Method 5
heat exchanging one or more intermediate BOG streams between the second and further stages of multiple stages of compression with part of the cooled compressed discharge from the final compressor stage
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, said method comprising at least the steps of: - compressing a boil off gas stream (01) from said liquefied cargo in three or more stages of compression comprising at least a first stage (65), a second stage (70) and final stage (75) to provide a compressed discharge stream (06), wherein intermediate compressed BOG streams (02, 04) are provided between consecutive stages of compression; - cooling the compressed discharge stream (06) to provide a cooled compressed discharge stream (07); - heat exchanging an expanded, optionally further cooled, portion of the cooled compressed discharge stream (07), with (i) one or more intermediate compressed BOG streams (04) from consecutive stages selected from between the second and final stages (75) of compression to provide one or more cooled intermediate compressed BOG streams (05) and optionally (ii) one or more portions (07a, 108a), optionally after further cooling, of the cooled compressed discharge stream (07); and - passing the one or more cooled intermediate compressed BOG streams (05) to the next stage of compression (75).