Water Removal During Gas Purge

On the cathode side, on the other hand, the primary purpose of gas purge is to remove water from the cathode compartment, particularly in preparation for cold start from subzero temperatures. As gas purge defines the initial condition of water distribution in a cell, it is a crucial step in PEFC cold start. Recent experimental studies have amply shown that not only performance but also material durability of PEFC hinges strongly upon the gas purge process prior to cool down and cold start. This is because an effective gas purge can remove water from the catalyst layer and membrane, thereby creating space for water produced in cold start to be stored. 

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Figure 13. Schematic diagram of two limiting stages of water removal during gas purge, (reproduced with permission from Tajiri et al/ )...

Step 1. For this process we must be able to set the production rate of vinyl acetate while minimizing yield losses to carbon dioxide. During the lifetime of the catalyst charge, catalyst activity decreases and the control system must operate under these different conditions. To maintain safe operating conditions, the oxygen concentration in the gas loop must remain outside the explosivity region for ethylene. The azeotropic distillation column must produce an overhead product with essentially no acetic acid and a bottoms product with no vinyl acetate. The absorber must recover essentially all of the vinyl acetate, water, and acetic acid from the gas recycle loop to prevent yield losses in the CCf removal system and purge,... 

Because ice formation is the main reason for structural and performance degradation during exposure to subzero temperatures, proposed mitigation strategies include gas purging and solution purging to remove residual water during fuel cell startup and shutdown. Delamination between the CLs and the baer membrane can result in the development of flooded areas, increased resistance in the MEA, the development of pinhole areas, loss of apparent catalytic activity, and the development of areas susceptible to erosion. 

To remove water from a hydrophobic adsorption trap (e.g. with a Tenax filling), a dry purge phase is introduced. During this step, most of the water condensed in the trap is blown out by dry carrier gas. Purge times of about 6 min are typical. 

Two different techniques were used to remove volatile compounds from the coffee. The first was vacuum distillation in a rotary evaporator. A 20 mL aliquot of coffee was placed in a round bottom flask and then it was attached to a Roto-vap and held in a water bath at 60 C during rotation with a vacuum applied from a water aspirator. When 15 mL had been distilled into the collection flask, it was removed and 20 mL distilled water was added to the 5 mL of retentate and the process of rotary evaporation repeated to yield a total of four distillates. Gas chromatographic analyses were performed using purge and trap as described above, with a purge time of 15 minutes. 

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