Problem: Air Products owned and operated an on-site hydrogen plant supplying product gas for an FMC Corporation (FMC) facility in Baltimore, Md. The hydrogen process employs natural gas for feedstock. Sulfur must be removed from the gas before it is fed to the process. A small recycle stream is returned from the process and used in hydrodesulfurization. A slipstream from the Baltimore plant’s high-pressure, hydrogen product compressor provided the required hydrogen recycle.

FMC changed the process at Baltimore so that hydrogen was no longer needed at the site. Air Products was asked to relocate the hydrogen plant from Baltimore to another FMC plant in Prince George, BC, Canada. There, the hydrogen would be used to make hydrogen peroxide. The application does not use high-pressure hydrogen, so the large product hydrogen compressor from Baltimore would not be moved.

A small hydrogen compressor, dedicated to hydrogen recycle, could be installed. A 10hp, single-stage compressor would be appropriate. However, its installed cost would run $44,000, or more, and there would be considerable lead time while the equipment was built to order. It was desirable to find an economical way to recycle hydrogen.

Solution: Air Products’ engineers decided they could save time and money by using an ejector to compress the hydrogen. While the engineers had not specified an ejector for a  similar application before, they had considerable experience with a manufacturer who had supplied ejectors to pressurize various gases. Based on that experience and a heat and material balance, the manufacturer offered a single-stage ejector, that used 425 psig natural gas as motive gas. The ejector introduces 3.2 lb/hr of hydrogen into the process, while boosting the pressure of the hydrogen from 220 to 300 psig. About 107 lb/hr of natural gas passes through the ejector and 631 lb/hr is bypassed. A turndown capability as low as 15% is provided.

applications_chemical_article7-1The process-gas ejector operates like a restrictive orifice. It passes a set natural gas flow. They bypass flow is set by a letdown regulator. The arrangement provides automatic turndown capability with natural gas while maintaining hydrogen recycle flow.

Results:The hydrogen plant at Prince George was started in October 1990. Production and utility consumption were confirmed in November. Energy savings using the process-gas ejector in place of a compressor are estimated at $3000/yr.

 

Where an on-site supply of high-pressure motive gas is available, ejectors can be an excellent alternative to mechanical compressors. Ejectors need no source of power other than the motive gas. Since they have no moving parts, they are easy to install, operate and maintain. They should certainly be considered whenever hydrogen recycle compression must be supplied.

Dale D. Smith, Operations Team Leader – Process Systems Group, Air Products & Chemicals, Inc.
Alan E. Hodel, Senior Associate Editor