Gas buffers

Gas compressors operating on highly toxic or flammable gases may require redundant systems to assure no leakages. In many applications, such as refrigeration gas, buffer seals are required with the liquid-buffered face seal. A popular technique is to use a buffered labyrinth seal with a liquid seal. 

Filters must be provided for all gas buffers in places where the gas will pass through the gas seal. The gas should be filtered to a nominal value of 2 microns. On most critical systems, dual filters will be required to permit servicing the filters without having to shut down the compressor. This requires the use of a transfer valve that can be switched without causing a flow interruption. If liquids are anticipated, heat tracing or an alternate gas source should be the first consideration. Should this not prove to be practi-... 

The fugacities of gases such as CO2 and O2 can be buffered (Fig. 2.1 see Chapter 14) so that they are held constant over the reaction path. In this case, mass transfer between the equilibrium system and the gas buffer occurs as needed to maintain the buffer. Adding acid to a C02-buffered system, for example, would be likely to dissolve calcite,... 

Closed-system models are those in which no mass transfer occurs. Equilibrium models, the simplest of this class, describe the equilibrium state of a system composed of a fluid, any coexisting minerals, and, optionally, a gas buffer. Such models... 

Mass balance on gas components is somewhat less complicated because the gas buffer is external to the system. In this case, we need only consider the gas components that make up secondary species ... 

To see how the governing equations might be solved, we consider a system that contains an aqueous fluid and several minerals but has no gas buffer. If we know the system s bulk composition in terms of Mw, Mi, and M, we can evaluate Equations 3.32-3.34 to give values for the unknown variables the solvent mass nw, the basis species molalities m, and the mineral mole numbers nt-... 

In these cases, the equation in question is evaluated to give the mole number Mw, and so on, of the corresponding component. In the presence of a gas buffer, the values of one or more fugacities fm are fixed. Now, the mole number Mm of the gas component remains to be determined. In general, the value of either Mw or nw needs to be set to evaluate Equation 3.32, and either M, or m, is required for each Equation 3.33. Each Equation 3.34 can be solved knowing either M or ny, whereas Equation 3.35 is generally evaluated directly for Mm. 

In the open system, the CO2 fugacity (Fig. 23.3) varies linearly over the reaction path from about 75 to a value of one, as we prescribed, tracing a path of lower fugacity than predicted for the closed system. To maintain the lower fugacity, the program allows CO2 to escape from the fluid into the external gas buffer (see Chapter 14). The pH (Fig. 23.1) follows a path of higher values than in a closed system, reflecting the loss of CO2, an acid gas. 

Note also that ammonium sulfate is absent from the recommended binding buffer formulations despite its general popularity in the field. Ammonium ions become fully titrated at alkaline pH and convert to ammonia gas. Buffer pH may become unstable as a result and causticity of the free ammonia may partially hydrolyze the proteins in a sample, creating a source of assay variability.617 18 At small buffer volumes used for analytical applications, liberated ammonia gas may not be a significant health hazard, but precautions may still be necessary to meet regulations. For all these reasons, ammonium salts are best avoided at alkaline pH. 

Lee, J. D. Werner, R. W. "Concept for a Gas-Buffered Annular Heat Pipe Fuel Irradiation Capsule" Lawrence Livermore National Laboratory Livermore, CA, UCRL-50510, 1968. 

Van Buuren et al. [105] performed photoemission and X-ray absorption experiments on Si nanocrystals to determine the TVB and BCB shifts, respectively, as a function of size. The Si nanocrystals were grown in situ at 1700 °C in an Ar gas buffer of 112 mTorr followed by hydrogen exposure to passivate the surface. The resolution of the photoemission and absorption measurements carried out on a synchrotron radiation source were 0.25 eV and 0.05 eV, respectively. They observed a valence band to conduction band shift ratio of 2 1 for all sizes of Si nanocrystals. This is in agreement with various calculations reported for Si nanocrystals [106]. 

Conducting airways do not contribute to the gas exchange and can be considered to be merely a conduit between the external environment and the respiratory zone (vide infra). The volume of air accommodated by the conducting airways represents the anatomic dead space and is air not directly available for gas exchange. Aside from serving as a conduit to the respiratory zone, the conducting airways perform two other functions gas buffering and humidification. 

Among the simple, classical methods of determining surface or interfacial tensions is the capillary rise (or depression). It requires the use of a tube made of a material, which is not wetted either by the melt or by the metal, and, of course, to measure somehow the position of the interface, which has to be observed. The latter is difficult with opaque tubes and when the interface is below the surface of an opaque metal. Therefore, the relative movement of the liquid when the position of the tube is changed is transmitted to another liquid in a glass tube outside the furnace by means of a gas buffer. 

I = To diffusion pump, 2 == To diffusion pump, 3 Draw-off for impui ities, i Take-off for highly enriched BF, 5 = Connection to manometer, 6 = Connection to manometer, 7 = Take-off for depleted BFj, 8 = Inlet for BF3 feed.. 9 — Feed line for edliylene and hydrogen, 10 = Hydrogen gas buffer... 

In order to substantiate the mechanism proposed above, the heat-treatment schedule was slightly modified. In this case, the titania films on the cp Ti wire were soaked for 2h at 1100°C at ambient pressure (latm) in an environment of Po = 3.8 X 10 atm. This was reahzed by using a CO/CO2 gas buffer containing lOOppm of CO2 at 1100 C, thermodynamic calculations as per equation (2.1), give... 

Azad A-M, Akbar SA. Novel structural modulation in ceramic sensors via redox processing in gas buffers. Int J Appl Ceram Technol 2006 3 177-92. 

If a positive seal is required, both axial mechanical and radial mechanical seals are used. A radial seal, which uses two split, floating, carbon rings with a gas buffer riding on a tungsten carbide coated runner, is an example of an advanced design, permitting axial movement and both high- and low-temperature operation. 

Perforated plates that fit into bubble columns like sieve trays are still used in cocurrent bubble columns (11). Usually the resultant free area of the openings is very small, i.e. 1 - 5 % of the total plate area. These plates may totaly suppress backmixing by forming a gas buffer beyond each plate which prevents liquid backflow. Such plates cannot be used in countercurrent flow in as much as the required pressure drop through the holes prevents the liquid downflow. Several possibilities have been suggested for the utilization of perforated plates in countercurrent bubble columns, i.e. special kinds of downcomers with or without control valves (12) (13) or additional pulsing... 

For a liquid or gas buffer, shown in Fig. 2.3, the mass balance holds. This equation relates the rate of change in mass m to the difference between inlet mass flow and outlet mass flow ... 

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