Purge Gas and Flow Rate

The majority of DSC experiments are performed under an inert atmosphere, usually nitrogen. This creates an inert and easily reproducible atmosphere around both the sample and reference during the experiment. The purge gas should pass through a drier to remove any moisture prior to entering the DSC. This is especially important if working at subambient temperatures, as the moisture will condense and lead to 

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The second process of water vapor removal down the channel can be described by the convective flux, Q(pw,Sat PvMet)KRTA), representing the maximum amount of water vapor removed with the purge gas when the exit purge gas is fully saturated with vapor. In the above definition Q is the purge gas volumetric flow rate and A the active area of the fuel cell. Both parameters defined above have the unit of mol/s per unit of the fuel cell active area. It follows that... 

The ZLC method offers advantages of speed and simplicity and requires only a very small adsorbent sample thus making it useful for characterization of new materials. The basic experiment using an inert carrier (usually He) measures the limiting transport difiiisivity (Do) at low concentration. A variant of the technique using isotopically labeled tracers (TZLC) yields the tracer diffiisivity and counter diffusion in a binary system may also be studied by this method. To obtain reliable results a number of preliminary experiments are needed, e.g. varying sample quality, nature of the purge gas, the flow rate and, if possible, particle size to confirm intracrystalline diffusion control. 

If it is desired to carry out a chlorination, the apparatus is modified by replacing the dropping funnel and trap with a simple Y inlet tube attached directly to the reactor. One side of the Y is attached to the purge gas and the other to the chlorine source. If there is any question about the purity of the chlorine, it can be passed through a sulfuric acid wash. The chlorine flow rate during the reaction is about 300 ml./min. with no purge-gas flow. At the end of the reaction, the chlorine is purged from the system, and the product is removed as before. 

Tables of material balance with recycles have been prepared for different conversions of toluene with yp as parameter. Fig. 7.10 displays gas-recycle flow rate against conversion for hydrogen purge concentration of 0.1, 0.4 and 0.7. The plot shows a large variation of the gas flow rate with conversion, over several orders of magnitude, for the concentration of H2 in purge between 0.1 and 0.7. Therefore, it is rational to work at higher conversion, but also with substantial excess in hydrogen.

The effects of the process variables on overall system performance may be examined by computing the cyclic steady-slate profiles for a range of parameter values. We consider a simple system, such as a gas drier, with a single adsorbable component in an inert carrier. We assume that the feed composition, temperature, and flow rate are fixed and that the adsorption cycle should operate at the feed temperature. For a given adsorbent this fixes the capacity and the nonlinearity of the equilibrium relationship (X ). The designer then has the freedom to choose the regeneration temperature and purge flow rate, the cycle time, and the bed depth. 

Often purge flow rates of between 400 and 500 mL/min are recommended. However, purging with lower flow rates but smaller gas bubbles in stirred solution was found to be equally efficient and also reduces problems regarding carry-over of acidic fumes and water vapour to the collection trap (Schmidt and Gerwinski, personal communication). 

Two patents from Reiser et al. and Balliet et al. [30, 31] are based on air and fuel purge with high flow rate at the fuel cell electrode compartments. An increased gas flow rate is taking advantage of the sluggish reactirai kinetics of... 

The purge gas should be dry, unless special experiments are carried out in humid atmosphere such as studying thermal degradation in a humid environment. The flow rate of the purge gas is very important, and it should be set as suggested by the manufacturer. In the actual DSC experiments, the same purge gas and the same flow rate must be used as in the calibration procedure. 

The temperature cahbration depends on the purge gas and its flow rate all the measurements should be done at conditions identical to the calibration runs. See Table 3.2 (in Chapter 3), which hsts the thermal conductivities of the most common purge gases. 

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