Combining Add-on Technologies for Gsp Plants with up-to-Date Plate Fin Heat Exchanger Design

The discovery of large shale gas reserves in the United States has shifted the majority of Natural Gas Liquid (NGL) recovery plants to operate in ethane rejection mode. The shale gas contains large quantities of ethane and heavier components causing the market for ethane to become saturated. Ethane is primarily used as a feed stock for ethylene crackers, and until recently the supply of ethylene has remained fairly constant. Over the next 4 years, the North American ethylene production capacity is expected to increase by nearly 30%, thus increasing the value of ethane in local markets. Recently, NGL plants have been built primarily optimized for ethane rejection, in order to save on capital costs. A Gas Separation Plant (GSP) that is only optimized for ethane rejection mode will have significantly undersized plate-fin heat exchangers when operating in ethane recovery mode. Retrofitting an existing GSP can significantly improve ethane recovery, increase throughput, and provide more stability with fluctuations in feed gas composition.

The two biggest challenges operating companies face when deciding whether to retrofit a plant are the amount of time the plant will be down for installation and the capital costs of the equipment and labor. An example of a low cost, non-invasive retrofit would be upgrading the plate-fin heat exchangers. When plate-fin exchangers are optimized for ethane rejection mode only, they can be under half the size required to efficiently operate in ethane recovery mode. The existing plate-fins could be upgraded with more robust units during a routine maintenance shutdown, as all the connection lines would remain the same. The new exchangers would help to facilitate better heat exchange between the cryogenic products and the warmer feed gas. In some cases, this change alone can increase ethane recovery by nearly 10%.

Additional add-on technologies can be applied for companies wishing to improve their recoveries, throughput, or flexibility for varying feed composition even further. Some examples of these plant modifications include adding additional feed gas refrigeration or recycling a small portion of chilled residue gas to the top of the demethanizer. These options would result in higher capital costs and longer plant down times, but would significantly improve ethane recovery and could debottleneck existing plate-fin exchangers.

When the natural gas market is booming, a NGL plant could not be retrofitted because the revenue lost by taking the plant offline would be too high. Instead companies opt to build new gas processing plants to handle the additional gas. Conversely, companies elect to improve their existing assets when the natural gas market experiences a downturn. By retrofitting NGL plants to increase throughput, recoveries, and inlet gas composition flexibility when the market is down, companies will be better poised to penetrate the market when the NGL prices increase.