Butane Railcar Unloading Challenges

Unloading liquefied gases from pressurized railcars have a very unique set of challenges.  The product and vapor connection are located on the top of the car and the product (liquid) connections include a suction tube that extends into the car but does not reach the bottom.  As a result, it is not possible to pump all of the liquid product through these tubes, leaving a volume of product, referred to as the liquid heel.

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For decades, this process has been handled using oil-free reciprocating compressors.  By pulling vapors from the storage tank and using them to pressurize the railcar, a differential pressure is created between the railcar and storage tank (typically 20-40 psig) allow the product to flow from one tank to the other.  Once the product line drops below the height of the suction tubes, the compressor flow is reversed – pulling vapor from the railcar and pushing it to the storage tank.  The resulting pressure drop in the railcar causes the remaining liquid heel to boil-off and is then transferred in a vapor state to the storage tank.

When dealing with very low vapor pressure products, like n-Butane, this process gets a bit more complicated.  The minor changes in pressure and temperature can have a big impact on the compressor operation and efficiency.  Care must be taken to prevent the butane vapors from condensing upstream of the compressor to prevent a high-liquid level shutdown or worse, a liquid slug of the compressor.  It is equally important that the vapors downstream of the compressor do not condense.  In this case, the condensed liquids would enter the railcar you are trying to evacuate, significantly slowing the liquid offload rate.

There are a number of options available to help prevent these concerns: vaporizers, heat tracing, insulated piping, etc.  How and when to implement these ancillary systems will be based on your specific application.  This is where S&S Technical comes in.  Our system design experts can work with you to develop a reliable, efficient strategy to address the difficulties in dealing with low vapor pressure liquefied gases and design a custom solution to meet your specific needs.

Contact our design team for more information or Request A Quote for more information.

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