Thermal conductivity cells

Thermal conductivity detector. The most important of the bulk physical property detectors is the thermal conductivity detector (TCD) which is a universal, non-destructive, concentration-sensitive detector. The TCD was one of the earliest routine detectors and thermal conductivity cells or katharometers are still widely used in gas chromatography. These detectors employ a heated metal filament or a thermistor (a semiconductor of fused metal oxides) to sense changes in the thermal conductivity of the carrier gas stream. Helium and hydrogen are the best carrier gases to use in conjunction with this type of detector since their thermal conductivities are much higher than any other gases on safety grounds helium is preferred because of its inertness. 

The next step in this study is to test this control algorithm on the actual laboratory reactor. The major difficulty is the direct measurement of the state variables in the reactor (T, M, I, W). Proposed strategy is to measure total mols of polymer (T) with visible light absorption and monomer concentration (M) with IR absorption. Initiator concentration (I) can be monitored by titrating the n-butyl lithium with water and detecting the resultant butane gas in a thermal conductivity cell. Finally W can be obtained by refractive index measurements in conjuction with the other three measurements. Preliminary experiments indicate that this strategy will result in fast and accurate measurements of the state vector x. 

Integrated thermal conductivity cells (see Fig. 12.8) allow a quantitative determination of the corresponding ortho/para ratios of the dihydrogen. The enriched parahydrogen is well-suited for in-situ NMR studies of hydrogenation reactions that yield nuclear spin polarization due to symmetry breaking during the reaction. The same apparatus has also been used successfully to enrich ortho- and paradeuterium mixtures. 

Thermal Conductivity Cells for Ortho/Para Determination... 

The thermal conductivity cells are connected to their respective valves VI and V2, using U-shaped glass tubing. This design has been found advantageous, as it minimizes any unavoidable convection of hydrogen within the cells. 

Fig. 12.8 Apparatus for orthohydrogen enrichment at low pressure, and integrated thermal conductivity cells.

The sample is heated over time in a furnace under a flowing gas mixture, typically 10% H2 in N2. Hydrogen has a high electrical conductivity, so a decrease in hydrogen concentration is marked by a decrease in conductivity of the gas mixture. This change is measured by a thermal conductivity cell (katharometer) and is recorded against either time or temperature. 

Material emerging from the column is detected by a thermal-conductivity cell, an ionisation method, or a gas-density balance. 

Figure 1. Schematic of the apparatus (1) thermal conductivity cell detector, (2) column, (8) flow meter, (4) pressure regulator, (5) drying trap, (6) injection valve, (7) recording device (A) T.C. detector, (B) power supply, (C) recorder, (D) dc micro-voltmeter, (E) FM adaptor, (F) magnetic tape recorder...

For the TPD and FTIR experiments the samples were purged in a helium stream at 120°C for 1 h or in vacuum for 30 minutes at 100°C, respectively. Subsequently, samples were loaded with ammonia at 100°C. After second purging to remove the physisorbed NH3 (2-3 h at 100°C in He (TPD) or 0.5 h at 100°C in vacuum (FTIR)) the conventional TPD runs were performed at a heating rate of 10 K/min and a helium flow of 0.5 ml/min. The desorbed amount of ammonia was analysed continuously using a thermal conductivity cell. 

Filament element. A fine tungsten or similar wire which is used as the variable resistance sensing element in the thermal conductivity cell chamber. 

Katharometer. Synonymous term used for a thermal conductivity cell. Sometimes spelled catharometer. 

Thermal conductivity cell. A chamber in which an electrically heated element will reflect changes in thermal conductivity within the chamber atmosphere. The measurement is possible because of the change in resistance of the element. 

Since large samples give millivolt changes, and small ones give microvolt signals, the thermal conductivity cell is extremely sensitive to fluctuations of physical variables. Some of these can be cancelled out if two filament cavities are used (Figure 5.7), one to detect the sample, the other to serve as a reference. The resistance R3 of the sample cell is balanced... 

One major aspect of quantitative analysis is sensitivity and dynamic range of linearity. Such data have been reviewed (2) for the gas density, thermal conductivity, and flame ionization detectors. Since response is a function of molecular weight in the gas density detector, it is difficult to make comparisons in a simple manner. In general, however, the sensitivity of the gas density cell is about twice that of comparable thermal conductivity cells and about one-tenth that of flame ionization detectors (when bleed of the column is limiting). 

The catalyst compositions were determined by chemical analysis at the Central Service of Chemical Analysis of the CNRS (Lyon) except for the samples Aj and A2 which were analyzed by Ugicarb Morgon. The analytical method for carbon has been described previously.8 The total amount of carbon was obtained from the combustion of the sample with oxygen. Carbon dioxide was then quantitatively detected by a calibrated thermal conductivity cell. For the polymeric carbon content, the sample was attacked by a hot mixture of nitric and hydrofluoric acids which dissolved every component except for the non-combined carbon. Then, this remaining carbon was transformed into C02 which was analyzed with an electrochemical titration cell. 

For determing the concentration of hydrogen sulfide in the product gas, a Gow-Mac thermal conductivity cell (Model 10-952) was used. The cell was equipped with four matched pairs of AuW filaments, especially used because of their resistance to corrosion from the hydrogen sulfide. Layers of styrofoam were used to insulate the cell from changes in ambient temperature. This detector was found to be very sensitive to changes in the flow-rate. 

The catalytic CO oxidation by pure oxygen was selected as a model reaction. The Pt/alumina catalyst In the form of 3.4 mm spherical pellets was used. The CO used In this study was obtained by a thermal decomposition of formic acid In a hot sulphuric acid. The reactor was constructed by three coaxial glass tubes. Through the outer jacket silicon oil was pumped, while air was blown through the inner jacket as a cooling medium. The catalyst was placed in the central part of the tube. The axial temperature profiles were measured by a thermocouple moving axially in a thermowell. Gas analysis was performed by an infrared analyzer or by a thermal conductivity cell. [7]. 

The TPD apparatus consisted of a stainless steel flow system connected to a thermal conductivity cell. Catalyst samples of 0.1 g were placed in one arm of an L-shaped, 6 mm Vycor tube. A dual adsorption bed containing alumina and Oxy-Trap (Alltech) was placed in the other arm to prevent contamination by water and respectively. Frequent regeneration in and He was required. This in-situ adsorption bed was found necessary despite purification traps on all gas lines coming into the flow system. Pulses of 0.25 cc of a 10% mixture of CO in He were injected into the He carrier gas and passed over the pretreated catalyst at room temperature. All runs were programmed heated at a rate of 20 K min . The Pt catalysts, either commercial or laboratory produced, were prepared by the impregnation of chloroplatinic acid on Cyanamid s Aero 1000 alumina, except for two catalysts which were prepared by platinum diamino dinitrite impregnation. 

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