SMR Re-Liquefaction Cooling With Oil-Injected Screw Compression
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional methods for re-liquefying boil-off gas (BOG) from liquefied natural gas (LNG) storage tanks on ships require an external refrigerant cascade, which increases complexity and capital expenditure, and oil-injected screw compressors can cause oil carryover that solidifies at low temperatures, blocking heat exchangers and reducing system performance.
Innovation Solution
A method using a single mixed refrigerant (SMR) recirculating system with an oil-injected screw compressor, where the SMR is compressed, separated, and then heat exchanged in a liquefaction heat exchanger system to provide a cooled stream, eliminating the need for an external refrigerant cascade and ensuring the refrigerant is below the oil solidification temperature for effective cooling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If an external refrigerant cascade is used for re-liquefaction, then cooling performance is improved, but device complexity and capital expenditure increase
Solution Approach 1:
The invention merges the external refrigerant cascade function into the SMR recirculating system itself. The SMR system performs both compression and refrigeration functions that were previously separated, eliminating the need for an external refrigerant cascade while maintaining cooling performance.
Solution Approach 2:
The SMR recirculating system is designed to perform multiple functions: compression of BOG, refrigeration through heat exchange, and re-liquefaction. This multi-functional approach replaces the need for separate external refrigerant systems.
2Productivity
If an oil-injected screw compressor is used, then compression efficiency and cost-effectiveness are improved, but oil carryover occurs that solidifies at low temperatures and blocks heat exchangers
Solution Approach 1:
The system performs preliminary cooling of the SMR in the liquefaction heat exchanger before expansion. This pre-cooling ensures that any oil carryover from the compressor solidifies in a controlled manner that does not block the heat exchanger, allowing the use of cost-effective oil-injected screw compressors.
Solution Approach 2:
The invention changes the temperature parameter of the SMR through controlled cooling in the liquefaction heat exchanger before expansion. By managing the temperature at which the refrigerant passes through the heat exchanger, oil solidification is controlled to prevent blocking while maintaining compression efficiency.
3Temperature
If the SMR is cooled below oil solidification temperature, then cooling effectiveness is improved, but oil solidification blocks the heat exchanger
Solution Approach 1:
The system performs preliminary cooling of the SMR in the liquefaction heat exchanger before expansion. This pre-cooling ensures that any oil carryover from the compressor solidifies in a controlled manner that does not block the heat exchanger, allowing the use of cost-effective oil-injected screw compressors.
Solution Approach 2:
The invention changes the temperature parameter of the SMR through controlled cooling in the liquefaction heat exchanger before expansion. By managing the temperature at which the refrigerant passes through the heat exchanger, oil solidification is controlled to prevent blocking while maintaining compression efficiency.
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 simplifies the re-liquefaction process, reduces capital expenditure, and prevents oil solidification by providing all sub-ambient refrigerant cooling duty within the SMR recirculating system, enhancing system performance and reliability.
Implementation Method 1
compressing the SMR using at least one oil-injected screw compressor to provide a post-compression SMR stream
Implementation Method 2
separating the post-compression SMR stream to provide an oil-based stream and a first SMR vapour stream
Implementation Method 3
separating the cooled first SMR vapour stream to provide a liquid-phase SMR stream and an oil-free SMR vapour stream
Implementation Method 4
heat exchanging the BOG stream with the SMR in a liquefaction heat exchanger system to provide a cooled BOG stream
Implementation Method 5
passing the oil-free SMR vapour stream through the liquefaction heat exchanger system to provide a condensed SMR stream
Implementation Method 6
The refrigerant liquid with oil in pipeline 29 has its pressure reduced by flash valve 9, leading to partial vaporisation and temperature reduction
Implementation Method 7
expanding the condensed SMR stream to provide an expanded lowest-temperature SMR stream
Implementation Method 8
cooled further in heat exchanger 12
Implementation Method 9
enters an oil separator 3, where most of the oil is removed (by gravity and/or filtration)
Implementation Method 10
The gas from oil separator 3 is sent into pipeline 24. The gas in this pipeline is mostly oil-free, but does contain a small proportion (down to parts per million by weight) of oil. The gas in pipeline 24 is sent into an aftercooler 6 which uses a readily available cooling medium (e.g. seawater, freshwater, engine room cooling water, air)
Data Source
AI summary
The present invention is a modification of a typical single mixed refrigerant (SMR) cycle for LNG re-liquefaction in particular, that allows the use of a cost-efficient oil-injected screw compressor in the mixed refrigerant system. In comparison with the typical arrangement, the present innovation allows for reduced complexity, fewer pieces of equipment, and reduced capital cost. There is shown a method of cooling a boil-off gas (BOG) stream from a liquefied gas tank using a single mixed refrigerant (SMR) comprising at least the step of heat exchanging the BOG stream with the SMR in a liquefaction heat exchanger system to provide a cooled BOG stream, wherein the SMR is provided in an SMR recirculating system comprising at least the steps of: (a) compressing the SMR using at least one oil-injected screw compressor to provide a post-compression SMR stream; (b) separating the post-compression SMR stream to provide an oil-based stream and a first SMR vapour stream; (c) passing the first SMR vapour stream into the liquefaction heat exchanger system to cool the first SMR vapour stream and provide a cooled first SMR vapour stream; (d) withdrawing the cooled first SMR vapour stream from the liquefaction heat exchanger system; (e) separating the cooled first SMR vapour stream to provide a liquid-phase SMR stream and an oil-free SMR vapour stream; (f) passing the oil-free SMR vapour stream through the liquefaction heat exchanger system to provide a condensed SMR stream; and (g) expanding the condensed SMR stream to provide an expanded lowest-temperature SMR stream to pass through the liquefaction heat exchanger system for heat exchange against the BOG stream.