Equilibrium purge

EQUILIBRIUM PURGE COLD START 2.1. Equilibrium Purge 

In equilibrium purge, water content of the MEA can be easily calculated from the purge gas RH via the membrane water uptake curve such as19 

Once equilibrium purge is concluded, the inlet and outlet gas lines are closed for both anode and cathode in order to keep the cell s internal condition unchanged, and the cell is then cooled down in an environmental chamber. When the cell temperature reaches a specified startup temperature, the cell is left at that temperature for more than 1 h before proceeding to the next step in order to ensure that the entire cell becomes isothermal at the ambient temperature. 

During cool down, water content in the membrane can be safely assumed to remain constant or locked in place. This assumption 

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Figure 7. Relation between product water and startup temperature under equilibrium purge (a = 6.2) and current density of 40 mA/cm2. (reproduced with permission from Tajiri et al.24)...

Given the estimate of the reactor effluent in Example 4.2 for fraction of methane in the purge of 0.4, calculate the.actual separation in the phase split assuming a temperature in the phase separator of 40°C. Phase equilibrium for this mixture can be represented by the Soave-Redlich-Kwong equation of state. Many computer programs are available commercially to carry out such calculations. 

When produced from natural gas the synthesis gas will be impure, containing up to 5 per cent inerts, mainly methane and argon. The reaction equilibrium and rate are favoured by high pressure. The conversion is low, about 15 per cent and so, after removal of the ammonia produced, the gas is recycled to the converter inlet. A typical process would consist of a converter (reactor) operating at 350 bar a refrigerated system to condense out the ammonia product from the recycle loop and compressors to compress the feed and recycle gas. A purge is taken from the recycle loop to keep the inert concentration in the recycle gas at an acceptable level. 

In general, zero-headspace procedures are employed when the concentrations of volatiles in the soil are relatively low, and solvent extraction methods are used for more polluted soils. Irrespective of which procedure is used, quantitation of volatiles in soil is subject to serious errors if sufficient care is not taken with the sampling operation. Although direct purge-and-trap methods are frequently advocated for the determination of volatiles in samples collected by zero-headspace procedures, there are certain problems associated with this technique. Caution is advised since the procedure really collects only that fraction of the volatile that exists in a free form within the soil pore spaces or is at least in a facile equilibrium with this fraction. 

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