Permeation tube

Generation of test atmospheres of organic vapours by the syringe injection technique Generation of test atmospheres of organic vapours by the permeation tube method On-site validation of sampling methods... 

MDHS 4 Generation of test atmospheres of organic vapours by the permeation tube method... 

Permeation tubes are very popular devices for generating stemdard vapor concentrations. The permeation tube contains a volatile liquid sealed in an inert permeable membrane, usually Teflon or a fluorinated copolymer of ethylene and propylene, through which it diffuses at a fixed and controlled rate. The driving force for the process is the dif ce in partial... 

Permeation Tubes A volatile liquid, when enclosed in an inert plastic tube, may escape by dissolving in and permeating through the walls of the tube at a constant and reproducible rate. The permeation rate depends on the properties of the tube material, its dimensions and on temperature. 

Within limits, broad concentration ranges can be prepared by varying the tube dimensions and/or the flow rate of the diluent gas. Diffusion tube systems are preferable to permeation tubes when the latter are no fl Mercially available. 

Permeation—Permeation tubes are often used to prepare analyte concentrations in the ppb to hundreds of ppm levels. 

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. 

Spiral Wound - This device is constructed from an envelope of sheet membrane wound around a permeate tube that is perforated to allow collection of the permeate or filtrate. 

A clever means of dynamic generation of standards at the part-per-million level involves permeation through a polymer. In 1966 O Keeffe and Ortman (34) described this technique primarily for air pollution standards. A condensable gas or vapor is sealed as a liquid in a Teflon tube under its saturation vapor pressure as shown in Figure 4.14. After an initial equilibration period the vapor permeates through the tube wall at a constant rate. This rate is determined by weight loss over a period of time. Temperature must be controlled to within .0.1°C to maintain 1% accuracy. In use the tube is thermostatted in a chamber that permits a diluent gas to fully flush the chamber. The concentration is then determined by the same equation used for diffusion tubes. However, since the rate is generally much less in permeation tubes it is usually reported in ng/min. 

Typical materials available in permeation tubes for operation at 30°C are listed in Table 4.6 along with average rates per centimeter length of tube and the K factor. As the length of the tube increases the permeation rate increases in reasonable proportion. The data in Table 4.6 are for tubes 0.25 inches o.d. and a wall thickness of 0.062 inches. A typical example might be an SO2 tube 5 cm in length at a rate of 1350 ng/min. If the dilution gas flow across the tube is 1.35 /min the concentration would be... 

Figure 4.14. Cross-sectional diagram of a permeation tube (courtesy Analytical Instrument Development).

TABLE 4.6 PERMEATION RATES FOR CHEMICALS IN PERMEATION TUBES... 

Permeation tubes are not refilled, have a limited life and cannot be turned off. However their life can be prolonged during periods of non-use by storing them in a refrigerator to reduce the permeation rate. Not many solutes are practical for use in permeation tubes. However, when the technique can be used it is generally preferred as a means of standard preparation. 

Spiral-wound elements, as shown in Figure 2, consist primarily of one or more membrane "leaves, each leaf containing two membrane layers separated by a rigid, porous, fluid-conductive material known as the "permeate channel spacer." The permeate channel spacer facilitates the flow of the "permeate", an end product of the separation. Another channel spacer known as the "high pressure channel spacer" separates one membrane leaf from another and facilitates the flow of the high pressure stream through the element. The membrane leaves are wound around a perforated hollow tube, known as the "permeate tube", through which the permeate is removed. The membrane leaves are sealed with an adhesive on three sides to separate the feed gas from the permeate gas, while the fourth side is open to the permeate tube. 

When the low pressure acid gas stream reaches the permeate tube at the center of the element, the acid gas "permeate" is removed. 

The gas analysis laboratory of the National Research Center for Certified Reference Materials (NRCCRM) is engaged in gas measurement at the state level. It is responsible for the uniformity of gas composition in China and for maintaining consistency and equivalence with international gas measurements. Our main tasks include research, development and maintenance of national primary reference materials research and establishment of precise reference methods for gas analysis to provide certified reference gas and working reference gas as well as permeation tubes for users to undertake the verification of gas analyzers and technical advisory services. The focus of our work is the development of research on gas analysis and measurement. 

Deposition of reduced sulfur compounds to enclosed surfaces could also be measured by adding low loss (5 ng/min, GC Industries) permeation tubes to the inlet of the chambers. This permeation rate adds an additional sulfur load which varied typically between 2 and S times that already within the enclosure. These studies were limited to surfaces where large temporal changes in natural emissions were not occurring, and ultimately became limited by the reliability of the permeation devices. 

SO2, NOx, and total hydrocarbons. The mass spectrometric gas analysis is on a wet basis, as water vapor is not condensed out of the gas, while the analyzers at the sample port measure a gas stream dried using a permeation tube and refrigeration-type dryers in series. In addition to the measurements described above, surface temperature measurements of the boiler skin are made to estimate radiation losses, using the skin temperature, the room temperature and tabulated heat loss factors based on the temperature difference. Particulate mass emission rate and carbon content are measured for heat and mass balance purposes. At present, material deposited within the boiler during a test is collected but not factored into the heat or mass balances, because this deposition is considered to be negligible. Data taken are used to examine the heat balance for the 20-hp system. 

Figure 4.15 shows the cross section of the spiral wound module.24 The spiral construction starts with two sheets of membrane placed back to back with a nylon tricot mesh spacer material in between. This tricot spacer provides the permeate channel for the membranes. These sheets of membrane and spacer are glued on 3 sides so that the permeate can only exit the spacer on one side. This set of membranes and spacer is called a "leaf." Leaves are then placed together with a low density polypropelene mesh spacer to provide the feed/reject channel for the membranes. The thickness of the mesh feed spacer can be adjusted from 28 mils to 34 mils to accommodate higher solids influent water (thicker feed spacers are more forgiving with respect to fouling with suspended solids than thinner spacers—see Chapter 4.4.2.3). The entire collection of leaves and mesh feed spacers are then wrapped around a perforated permeate collection tube so that the open side of the leaf is toward the perforated permeate tube (see Figure 4.16). Note that an 8-inch diameter membrane module has about 16 leaves, and each leaf is about 50 inches in length.

Figure 4.22 shows the flow characteristics for standard ATDs and the iLEC ATD. The reduced diameter of internal couplers and vessel adapters for the standard ATDs accounts for more than 70 percent of the permeate-tube pressure drop in some systems.26 The interlocking iLEC ATD design eliminates these restrictions, imposing less permeate backpressure, resulting in lower operating pressure requirements. 

Membrane scaling and degradation can lead to a loss in normalized salt rejection as can breaches O-rings and permeate tube. 

Breaches in hardware can allow feed water to mingle with permeate. As is the case with an increase in water flux, breaches in O-rings and permeate tubes will allow high concertration feed water to mingle with low concentration permeate, thereby increasing the concentration of the permeate (see Chapter 12.1.2.2). The overall salt rejection will decrease and salt passage will increase. 

---