Diffusion scrubbers porous membrane

When the denuder active surface is an inert porous membrane such that the analyte molecule must diffuse across the pores to be trapped by an absorber liquid, only a fraction of the membrane surface is porous, and the pores may also be tortuous. Consequently, collision at the membrane surface is not synonymous with uptake. Corsi et al. (45) developed a numerical solution for the collection efficiency observed for such a membrane-based diffusion denuder, hereinafter referred to as a diffusion scrubber (DS). Both groups of researchers dealing with the issue of less than unity uptake probability reached the conclusion that this value must be very much less than... 

With porous membrane DS devices of this geometry and for thin membranes with low-tortuosity pores (i.e., where the diffusion distance within the pores is very small compared to the radial diffusion distance in the DS), good predictions for the collection efficiencies can be obtained if the nominal X and dfd0 values are both multiplied by the fraction of the surface that is porous. For example, with a diffusion scrubber based on such a membrane tube (L = 40 cm, dQ = 0.5 cm, d = 0.045 cm, fractional porosity 0.4), the corrected X values for H20.2 as sample gas are 0.32, 0.16, 0.11, and 0.08, respectively for Q = 0.5, 1.0, 1.5, and 2.0 L/min, and the corrected djda value is 0.036. The collection efficiencies predicted from Figure 1 (interpolating between dfdQ values of 0.02 and 0.05) are in good agreement with... 

Porous Membrane DS Devices. The applicability of a simple tubular DS based on a porous hydrophobic PTFE membrane tube was demonstrated for the collection of S02 (dilute H202 was used as the scrubber liquid, and conductometric detection was used) (46). The parameters of available tubular membranes that are important in determining the overall behavior of such a device include the following First, the fractional surface porosity, which is typically between 0.4 and 0.7 and represents the probability of an analyte gas molecule entering a pore in the event of a collision with the wall. Second, wall thickness, which is typically between 25 and 1000 xm and determines, together with the pore tortuosity (a measure of how convoluted the path is from one side of the membrane to the other), the overall diffusion distance from one side of the wall to the other. If uptake probability at the air-liquid interface in the pore is not the controlling factor, then items 1 and 2 together determine the collection efficiency. The transport of the analyte gas molecule takes place within the pores, in the gas phase. This process is far faster than the situation with a hydrophilic membrane the relaxation time is well below 100 ms, and the overall response time may in fact be determined by liquid-phase diffusion in the boundary layer within the lumen of the membrane tube, by liquid-phase dispersion within the... 

Snated filter, e.g., a glass fiber mat impregnated with a mercurous salt ition and a humectant (29). Although detectabilities well below 100 pptv were obtainable, batch to batch filter performance showed marked variability and impregnated filters could not be stored over a month-long period without loss of performance. We therefore carried out the desired reaction using a diffusion scrubber air was sampled through a porous hydrophobic membrane tube while a dilute mercurous nitrate solution was circulated on the outside of the membrane tube. Mercury liberated by reaction at the gas-liquid interface in the pores is carried by the air stream to the detector. 

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