Why is a multi-stage compression system employed to recover BOG?

A multi-stage compression system is employed to recover boil-off gas (BOG) primarily because it is more efficient and results in lower discharge temperatures compared to a single-stage compression system for achieving the required pressure increase. BOG is typically at a low pressure, close to atmospheric, and needs to be compressed to a significantly higher pressure for reliquefaction or use as fuel gas. Compressing a gas in a single stage from a low pressure to a high pressure results in a large temperature increase. High discharge temperatures can damage the compressor and require expensive cooling systems. Multi-stage compression involves compressing the BOG in multiple steps, with intercooling between each stage. Intercooling removes the heat generated during compression, reducing the temperature of the gas before it enters the next stage. This reduces the overall power consumption of the compression system and allows for higher compression ratios without exceeding temperature limits. Also, multistage compression increases the isentropic efficiency compared to single stage compression. The multistage compression offers better operational flexibility. The number of stages and the compression ratio in each stage can be optimized to match the specific BOG generation rate and pressure requirements of the plant. While multistage compression systems are more complex and expensive than single-stage systems, the benefits in terms of efficiency, reliability, and reduced operating costs make them the preferred choice for BOG recovery in LNG plants.