Nitrogen permeation tubes

Nitrogen dioxide permeation tubes were obtained from Metro-nics, Inc. These tubes were factory calibrated for use at 30.0°C. The permeation tubes were thermostated in a water bath and air passed over at a known flow rate (of at least 150 cc/min). Gas samples were drawn off at 80 cc/min. 

Nitrogen Dioxide The line selected for NOa measurement lies in the 1600 cm-1 region. A commercial permeation tube is used for the calibration source for NOB. The permeation rate is determined by weight loss and also checked by N0/03 titration with NO then serving as the calibration standard. The response time is 3 sec and the MDC is 25 pptv. 

Figure 9. H2 ( ) / n-butane ( ) separaticm with the ccxnposite zeolite-alumina membrane (fluxes in the permeate as a function of the tenq>erature). A mixture of hydrogen, n-btitane and nitrogen (12 14 74) was fed in the tube (Fig. 2) with a flow rate of 4.8 1/h. Sweep gas (N2), countercurrent mode, flow rate 4.3 1/h.

Diffusion of a gas or liquid through a semi-permeable material. The permeability of elastomers to gases varies with the elastomer type and with the gas. Butyl rubber is much less permeable to air than is natural rubber hence its use in tyre inner tubes and similar apphcations. The rate of permeation is generally related to the size of gas molecule, i.e., the smaller the molecule the higher the rate. The exception is C02 which has a rate 10 to 100 times greater than that of nitrogen. 

A MC module contains thousands of microporous hollow fibres, which are knitted into a fabric that is wound around a distribution tube with a central baffle as shown in Figure 1.15. The baffle ensures the water is distributed across the fibres, and also results in reduced pressure drop across the contactor. The hollow fibres are packed densely in a membrane module with a surfrce area of up to 4000 n / m. The liquid flows outside (shell side) the membrane, while vacuum is appHed on the inside of the fibre (tube side) forming a film across the pores of the membrane. Mass transfer takes place through this film and the pores due to the difference in the gas partial pressure between the shell side and tube side. Since the membranes are hydrophobic, they are not wetted by water, thereby, efiectively blocking the flow of water through the membrane pores. The membrane provides no selectivity. Rather its purpose is to keep the gas phase and the Hquid phase separated. In effect, the membrane acts as an inert support that allows intimate contact between gas and liquid phases without dispersion. Vacuum on the tube side of the membrane increases the mass transfer rate as in a vacuum tower. The efficiency of the process is enhanced with the aid of nitrogen sweep gas flowing on the permeate side of the membrane. 

The permeation rate is determined by the gravimetric method. The permeation device, after it has been assembled, is weighed and then placed in a temperature-controlled cavity with constant flow of nitrogen or clean air to sweep away the permeated chemical vapor. After a period of time, perhaps days, the tubing is weighed again and the permeation rate is determined by weight loss over elapsed time. Since... 

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