Boil-Off Gas Temperature Control for Stable Compressor Suction
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Solution Overview
Problem
The variability in temperature of boil-off gas streams from cryogenically stored liquefied hydrocarbon inventories, particularly during transitions between holding and loading modes, leads to inefficiencies in compression processes due to changes in mass flow and density, affecting the performance of common fuel gas compressors.
Innovation Solution
A method and apparatus that split the boil-off gas stream into a heat exchanger feed stream and a bypass stream, allowing for temperature control by heat exchange against a process stream, with flow control valves adjusting mass flows to maintain set-point temperatures, ensuring a consistent temperature for efficient compression.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a common fuel gas compressor is used to handle boil-off gas streams at varying temperatures, then the compressor can process both holding mode and loading mode gases, but the compressor efficiency decreases due to mass flow and density variations
Solution Approach 1:
The boil-off gas stream is divided into two separate streams: one passing through the heat exchanger to be warmed, and another bypassing the heat exchanger. This segmentation allows independent temperature control of each stream before recombination, enabling the system to maintain optimal compressor inlet temperature regardless of BOG source temperature variations.
Solution Approach 2:
The system changes the temperature parameter of the BOG stream by controlling the mass flow through the heat exchanger versus the bypass. By adjusting the flow distribution between the two paths, the final mixed stream temperature can be controlled to maintain optimal conditions for the compressor, thus preserving compressor efficiency while handling varying BOG temperatures.
2Temperature
If the mass flow of boil-off gas through the heat exchanger is increased to warm the stream, then the temperature control improves, but the complexity of the control system increases
Solution Approach 1:
The control system uses temperature measurements of the BOG stream and employs feedback control to adjust the mass flow distribution between the heat exchanger and bypass paths. The controller continuously monitors the temperature and modulates the flow control valves to maintain the desired temperature setpoint, providing automatic temperature control without requiring complex manual intervention.
Solution Approach 2:
The heat exchanger serves multiple functions: it can warm cold BOG from holding mode tanks, it can provide heat recovery, and it can be bypassed when not needed. The control system universally manages different operational modes (holding mode, loading mode, heat recovery mode) through a single integrated control strategy that adjusts flow distribution based on current conditions.
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 stabilizes the temperature of the boil-off gas stream, optimizing the operation of compressors by maintaining desired suction temperatures, thereby enhancing the efficiency and operational stability of the compression process.
Implementation Method 1
heat exchanging the BOG heat exchanger feed stream in a BOG heat exchanger against a process stream, thereby providing a warmed BOG stream and a cooled process stream
Data Source
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AI summary
A boil-off gas (BOG) stream (15) from a liquefied hydrocarbon storage tank is split into a BOG heat exchanger feed stream (25) and a BOG bypass stream (35). The BOG heat exchanger feed stream (25) is heat exchanged in a BOG heat exchanger (40) against a process stream (135), thereby providing a warmed BOG stream (45) and a cooled process stream (195). The warmed BOG stream (45) is combined with the BOG bypass stream (35) to provide a temperature controlled BOG stream (55). Herein, the mass flow of the process stream (135) is controlled in response to a measured first temperature of at least one of (i) the warmed BOG stream (45) and (ii) the cooled process stream (195) to move the measured first temperature towards a first set point temperature; and the mass flow of one or both of the BOG heat exchanger feed stream (25) and the BOG bypass stream (35) are controlled in response to a measured second temperature of the temperature controlled BOG stream (55), to move the measured second temperature towards a second set point temperature.