Sweep-through purging

The sweep-through purging process adds purge gas into a vessel at one opening and withdraws the mixed gas from the vessel to the atmosphere (or scrubber) from another opening. This purging process is commonly used when the vessel or equipment is not rated for pressure or vacuum the purge gas is added and withdrawn at atmospheric pressure. 

Purging results are defined by assuming perfect mixing within the vessel, constant temperature, and constant pressure. Under these conditions the mass or volumetric flow rate for the exit stream is equal to the inlet stream. The material balance around the vessel is 

C is the concentration of oxidant within the vessel (mass or volumetric units), 

The mass or volumetric flow rate of oxidant into the vessel is C0 2V, and the flow rate of oxidant exiting is CQV. Equation 7-13 is rearranged and integrated  

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Use seven minimum, perhaps use eight, for assurance that purging is complete. Note that the above relationships hold for vacuum purging. Keep in mind the relationships between high and low pressure of the system and use mmHg for pressure if more convenient. For sweep-through purging, see Ref. [29]. 

Use the sweep-through purging technique to reduce the concentration of toluene from an initial 20% to 1 % in a room with a volume of 25,000 ft3. Assume that the room is purged with air at a rate of 6 room volumes per hour. How long will it take to complete this purge process ... 

It is seen that the eluent is vaporized directly into the detector in an atomized form by means of a heated transfer tube situated in an oven held at 300°C. On entering the oven, the mobile phase and eluent are immediately vaporized, and the increase in volume resulting from the vaporization forces the vapor into the electron capture detector sensor (utilizing a nickel radioactive source). The sensor is also held at 300°C and a purge of nitrogen gas sweeps the vapor through the detector and out through a condenser coil. The electron capture detector... 

No purging of the system with a gas after extraction is required in this mode (Fig. 6.10) as the solvent is continuously circulated through the sample to sweep extracted analytes. 

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