William Paul Schmidt

GAS MIXTURE FRACTIONATION TO PRODUCE TWO HIGH PURITY PRODUCTS BY PRESSURE SWING ADSORPTION

Abstract Pressure swing adsorption processes have been traditionally used to produce one high purity gas stream from a gas mixture. One of the most common uses of this technology is in the production of ultrahigh purity hydrogen from various gas streams such as steam methane reformer (SMR) off-gas. However, many of these gas streams contain a second gas in sufficiently high concentrations, e.g., carbon dioxide in SMR off-gas, that the recovery of this secondary gas stream along with the primary product is extremely desirable. A new pressure swing adsorption (PSA) process, GEMINI-8, has been developed at Air Products and Chemicals, Inc., to achieve this goal. Process cycle steps for the GEMINI-8 PSA process are illustrated by SMR off-gas fractionation for the production of hydrogen and carbon dioxide. Capital and power savings of this process as well as other advantages compared with the previous technology are discussed.

A new concept to increase recovery from H2 PSA: processing different pressure feed streams in a single unit

A new process concept is outlined to utilize low and high pressure feed streams efficiently in a single pressure swing adsorption (PSA) process. The low pressure feed is processed first in the PSA unit to produce low pressure product. Following this, the high pressure stream is processed in the same unit. Low pressure generated in the first part of the process is used internally in the process to assist the production of high pressure product. Essentially 100% recovery of the high pressure product is possible. This is achieved in a single unit without the need to store gas product during the intermediate process steps. The process concept is demonstrated for pure hydrogen production. A process is simulated in which two streams, both containing 25% methane and 75% hydrogen at two different pressures, are supplied to a single PSA unit. The simulation shows that, using a five-bed process, one can produce 100% high pressure hydrogen at ultra-high purities (i.e. ppm levels of methane in the product). If the low pressure stream is not integrated, then the maximum high pressure H 2 recovery is only 85%.

Sealed Aluminum Cavity Reactions when Submerged in Pure O 2 Reboiler Sump

Aluminum has a long history of safe service in cryogenic air separation units; however, there have been some rare instances of aluminum/O2 reactions. Aluminum ignition and propagation depends strongly on the oxygen purity, oxygen pressure, aluminum geometry, and type and energy of igniter. Over the years, the industry has experienced a particular type of aluminum/O2 reaction: cavity incidents. These incidents are characterized by the presence of a sealed cavity that is created when welding two metal items together. Such a sealed cavity can lead to aluminum ignition under certain specific conditions: (1) The sealed cavity is submerged in a bath of liquid oxygen. Over a long period of time, liquid cryogen can enter the sealed cavity through slight imperfections in the weld. (2) The sealed cavity is warmed over a short period of time, typically a few hours. (3) The liquid oxygen vaporizes, and the pressure inside the cavity builds to very high levels (potentially over 50 bars). (4) Ignition occurs in the high pressure, high purity oxygen environment, and the resulting aluminum/oxygen reaction burns through the relatively thick cavity walls. (5) The oxygen and reaction products exit the cavity through the hole, lowering the pressure and extinguishing the reaction. This paper discusses two such incidents, which have occurred since 2001. Both incidents took place in brazed aluminum heat exchanger (BAHX) reboiler support beam systems. In both cases, the BAHX reboiler was damaged, leading to a process leak, which required that the plant be repaired. In one case, the damage occurred while warming a plant. In the second case, the damage occurred during normal operation. This second case does not appear to follow the sequence of events outlined above; however, evidence is presented to support the scenario that the cavity incident occurred during a previous warming of the plant. This initial damage then impaired the normal operation of the reboiler, leading to a second hydrocarbon related reaction during normal operation. The paper discusses the potential ignition mechanisms, probable causes of the incidents, and methods to prevent future re-occurrences.