Process thermal conductivity detector

In the normal operation of the reactor the flow rate and composition of the HiS feed stream both fluctuate. In the past, each time either variable changed the required SO2 feed rate had to be reset by adjusting a valve in the feed line. A control system has been installed to automate this process. The H2S feed stream passes through an electronic flowmeter that transmits a signal Rt directly proportional to the molar flow rate of the stream, hi. When hf = 100 kmol/h. the transmitted signal R = 15 mV. The mole fraction of H2S in this stream is measured with a thermal conductivity detector, which transmits a signal R. Analyzer calibration data are is follows ... 

In addition to the thermal conductivity detector (TCD), flame photometric (FPD) and atomic absorption spectrometric (AAS) detectors, which offer high sensitivity and the advantage of selectivity, are also suitable for use with inorganic substances (Table 1.2). By-products from chemical reactions of the analytical process will not be detected. 

The first of the separation techniques to be used in process measurement was gas chromatography (GC) in 1954. The GC has always been a robust instrument and this aided its transfer to the process environment. The differences between laboratory GC and process GC instruments are important. With process GC, the sample is transferred directly from the process stream to the instrument. Instead of an inlet septum, process GC has a valve, which is critical for repetitively and reproducibly transferring a precise volume of sample into the volatiliser and thence into the carrier gas. This valve is also used to intermittently introduce a reference sample for calibration purposes. Instead of one column and a temperature ramp, the set up involves many columns under isothermal conditions. The more usual column types are open tubular, as these are efficient and analysis is more rapid than with packed columns. A pre-column is often used to trap unwanted contaminants, e.g. water, and it is backflushed while the rest of the sample is sent on to the analysis column. The universal detector - thermal conductivity detector (TCD)-is most often used in process GC but also popular are the FID, PID, ECD, FPD and of course MS. Process GC is used extensively in the petroleum industry, in environmental analysis of air and water samples" and in the chemical industry with the incorporation of sample extraction or preparation on-line. It is also applied for on-line monitoring of volatile products during fermentation processes" ... 

A Perkin-Elmer Auto-system gas chromatograph (GC), which houses a 30-m, 0.53-mm (ID) fused silica capillary column (Carboxen 1010 Plot, Supelco), was used to analyze the gaseous samples from the liquefaction process. Temperature programmed step heating was performed as follows 40°C for 1.7 min, increase by 40°C/min imtil 220°C, and leave at 220°C for 1.8 min. Argon was the carrier gas at a flow rate of 20 ml/min. Two detectors were used for gas analysis a flame ionization detector (FID) for carbon-bearing species and a thermal conductivity detector (TCD) for H2. Uncertainties in reported concentrations are estimated to be within 5% [8]. 

Once assembled, the cell was loaded into a custom-made furnace and connected to the process and sweep gas supply Hues. The exit gas from the cathode was routed to a Beckman IR scanner for reading CO, levels. A Hewlett/Packard gas chromatograph fitted with a thermal conductivity detector was used for reading H,S levels greater than 100 ppm and a flame photometric detector was used for HjS levels less than 100 ppm. A gold reference electrode was placed on the sur ce of the membrane away firom either process electrode and supplied with a low flow rate of a 15% CO,/ 3% 0, / balance N, mixture to maintain a stable thermodynamic reference potential by reaction (26). 

Obviously, the chromatographic principles are the same in process and laboratory GCs and they are built up in a very similar way. Standard detectors are in each case the thermal conductivity detector (TCD), which is a universal detector for all components, and the flame ionization detector (HD), which is a specific detector for hydrocarbons. To detect sulfur gases selective detectors like an electrochemical detector, chemiluminescence detector and, most important, flame photometric detector (FPD) are used. Gaseous fuels hke natural gas, synthetic gases, and blends are complex mixtures that cannot completely be separated in a single column. Two or more different columns must be combined. To monitor the fuel quality a quasi-continuous analysis is necessary this means that very short cycle times must be realized. To do so, high-boiling components are removed... 

The thermal conductivity detector temperature must be controlled to 0.1°C or better for baseline stability and maximum detectivity. Ionization detectors do not have this strict a requirement their temperature must be maintained high enough to avoid condensation of the samples and also of the water or by-products formed in the ionization process. A reasonable minimum temperature for the flame ionization detector is 125°C. 

The system was tested at 495-540°C at a process gas pressure of 0.9 MPa and a product hydrogen pressure of 0.02-0.04 MPa, and S/C (steam-to-carbon ratio) of 2.8-3.6. Hydrogen production rate and efficiency, conversion rate of natural gas, and chemical composition of the off-gas were measured and analyzed by changing the feed rate of natural gas. The chemical composition of the off-gas was analyzed with a thermal conductivity detector (TCD) gas chromatograph. Natural gas conversion and hydrogen production efficiency were obtained by the following equations ... 

In temperature-programmed reduction (TPR), a flow of inert gas (N2 or Ar) containing approximately 5 vol% H2 is passed through the catalyst bed of a flow reactor containing a reducible solid catalyst (66). By monitoring continuously the H2 concentration in the gas flow and its eventual consumption with a thermal conductivity detector when heating the sample with a linear temperature ramp of ca. 10 K/min, the rates of reduction are obtained as a function of time (or temperature). The total amount of H2 consumed determines the reduction equivalents present in the catalyst, and detailed analysis of the experiment permits the kinetic parameters of the reduction process to be determined and provides information on the reduction mechanisms. The characteristic numbers, which depend on the experimental parameters (amount of reducible species present, H2 concentration, flow rate, and temperature ramp), have been defined (66,67). These numbers must be kept in certain ranges for optimal performance of the experiment. 

For gas chromatography with a thermal conductivity detector, it is possible to collect samples that have passed through the column. One method uses a gas-collection tube (see Figure 22.10), which is included in most microscale glassware kits. A collection tube is joined to the exit port of the column by inserting the fS/S inner joint into a metal adapter, which is connected to the exit port. When a sample is eluted from the column in the vapor state, it is cooled by the connecting adapter and the gas-collection tube and condenses in the collection tube. The gas-collection tube is removed from the adapter when the recorder indicates that the desired sample has completely passed through the column. After the first sample has been collected, the process can be repeated with another gas-collection tube. 

Figure 6.34. Breakthrough curves of an adsorption process of (CO, 3.7) on MS13X (UOP) detected with an impedance analyzer (lA) at v = 10 MHz and a thermal conductivity detector (TEX3) at T = 295 K, p = 0.151 MPa.. The lA curve shows the (real part of the) capacitance of the sorbent/sorbate system inside the capacitor. The TCD curve represents the CO-concentration of the gas flow leaving the column [6.14].

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