Gases irreversible mixing

If one suddenly opens the tap (valve) on a cylinder containing a gas confined under a pressure Pi (much greater than atmospheric pressure, Patm (i.e. P, 3> Patm)) and allows it to escape by into the atmosphere this process will continue until the pressures are equalised and the final pressure Pf = Patm. The expansion (leaving aside all discussion of throttle effects at the valve, gas/air mixing, friction effects etc.) takes place rapidly - and under non-equilibrium conditions - usually at constant temperature, T (= ambient) and is a spontaneous process. Since this process is not at equilibrium and hence is not reversible, we refer to it as being an irreversible process. 

Figure 3.4 shows the Nernst voltage as a function of the fuel utilisation Uf in a SOFC with H2 fuel and with the system pressure p as a parameter. The excess air and the SOFC temperature are fixed. The interesting area between Uf= 0.1 and Uf= 0.9 can be well approximated with the model of the ideal gas. The dotted line shows the adoption of the model. The irreversible mixing within the SOFC reduces between Uf - 0.1 and Uf = 0.9 by about more than 200 mV. An increase of the system pressure from 1 to 10 bar increases V,v by about 70 mV. An increasing SOFC temperature decreases V. as shown by Eqs. (2 7) and (50). 

However, it is important to note that this conclusion becomes invalid if the air for cooling the LP turbine is taken from compressor delivery (as in Fig. 4.3b) and then throttled at constant temperature (T2 = Ty) to the lower pressure before being mixed with the gas leaving the HP turbine. The thermal efficiency drops as another internal irreversibility is introduced it can be shown [5] that... 

For two step cooling, now with irreversible compression and expansion, Fig. 4.7 shows that the turbine entry temperature is reduced from Ti. to by mixing with the cooling air i/ H taken from the compressor exit, at state 2, pressure p2, temperature T2 (Fig. 4.7a). After expansion to temperature Tg, the turbine gas flow (1 + lp ) is mixed with compressor air at state 7 (mass flow i/h.) at temperature Tg. This gas is then expanded to temperature T g. 

Preparation. Many reactions and processes are available for the preparation of hydrogen. Among the large-scale processes, the catalytic steam hydrocarbon reforming process can be mentioned. After de-sulphurization, natural gas (or oil-refinery feedstock) is mixed with steam and, at 700-1000°C, passed over a nickel-based catalyst. The irreversible reaction occurs ... 

PHREEQE can calculate pH, redox potential, concentration of elements, molalities and activities of aqueous species, and mineral or gas mass transfer as a function of reaction progress. The program is capable of simulating reactions due to mixing, titrating, net irreversible reaction, temperature changes, and mineral- or gas- phase equilibration. 

If a trace gas in the atmosphere undergoes irreversible absorption or chemical reaction at the ground surface, the process will set up a vertical gradient of the mixing ratio leading to a downward flux ... 

Consider the following two well-mixed, isothermal gas-phase batch reactors for the elementary and irreversible decomposition of A to B,... 

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