Multi-Component-Gasanalysis with
Fourier-Transformed Thermal Conductivity Analyser
FTTCA 1100

ATEX-version of the FTTCA 1100 for zone 1
certificate 05 ATEX2763 with qualification Ex II 2G EEx de IIC T4.



Content

Gasanalysis with theThermal Conductivity Analyser FTTCA 1100

Applications

System Features

Calibration, Measurement and Evaluation

System Components

Technical Data for the FTTCA 1100

Product broschure
Paper presented at the 51th Sypomposium of the ISA Analytical Division,
Houston, 2006 (c) ISA-AD (300 KB)

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Gas Analysis with theThermal Conductivity Analyser
The classic gas analysis via thermal conductivity, well known as a reliable and simple measuring technique since the thirties, is used in industrial processes to measure componentsin binary or (quasi~)binary gas mixtures. Particularily Hydrogen, Heliumand Nitrogen being optical inactive making an IR-sensor obsolet, are measured with a TC-sensor. If the gas mixture contains more than two components an additional instru-ment, mostly an IR-analyser, must be employed to detect the second component.

Now the new FTTCA (Fourier-Transformed Thermal Conductivity Analysis)-technique makes possible the simultaneaus measurement of all three components in a multi-component gas mixture. On the one hand the FTTCA-method employs a physical effect known but not often used in process analytics and on the other hand FTTCA is feasible through the availibility of todays newly developed miniaturized thermal-conductivity sensors only. The physical effect comes from the different temperature dependence of the thermal conductivity of the different gas components. Hence the recording of a multi-dimensional concentration field, consequently followed by a fast Fourier-transformation, enables the analysis of three or four component gas mixtures.

Applications with the Thermal Conductivity Analyser FTTCA 1100

• Bio-gases (fermenter-gases) iin the three component mixture N₂-CO₂-CH₄, up to now requiring anadditional measuring system, in most cases an IR-analyser.
• Mixtures of N₂-H₂-CO₂ frequently used in the chemical industry
• .Mixtures of N₂-He-H₂, which could not be analysed employing the classic TC-technique (see diagram below).

System Features of the Thermal Conductivity Analyser FTTCA 1100

• Thermal conductivity sensor for gases in Silicon micro-mechanical version
• Low power supply
• Very small dimensions, short time constants
• Measurement of smallest gas volumina
• Gas exchange via diffusion
• Integrated resistors for temperature compensation
• Mounted on TO8-base, shockprooved EEx-version:
• Certificate 05 ATEX 2763 Ex II 2G EEx de IIC T4
• Lowest measuring range for a binary gas mixture is such as 1 % Nitrogen
  in Argon or 1000 ppm Hydrogen in Nitrogen.

Calibration, Measurement and Evaluation

Example: Gas mixture from landfills CH₄ (40…60 vol%) - CO₂ (30…40 vol%) - rest N₂

First of all the calibration range has to be determined.

Due to the membrane temperature modulation the measuring signal, i.e. the Thermal Conductivity λ = f(T) of a gas mixture, shows corresponding periodic changes which can be depicted as a Fourier series.

In order to carry out the evaluation of the sensor signals rapidly and in real time, AMS developed a specific mathematical method, based on the treatment of the Fourier coefficients of the sensor signals as non-linearly image of the concentration field. Considering these basic conditions the correlation between Fourier coefficients and concentrations is one-to-one, i.e. any given set of of Fourier coefficients correspondes directly with a set of of gas concentrations - and vice versa. The major task left to the AMS application lab is to establish this correlation.
Fourier coefficients are hard to understand, so the Mean Value U₀ and the Amplitudes U₁ of the temperature modulated sensor are considered. Entering a set of these values, taken from a FTTC-analyser, into a diagram (see Thermal Conductivity-Diagram) an unambiguous correlation between this set of values and the concentrations of the gas components involved is established. Practically three to five Fourier coefficients are taken into consideration so far. In this way the phase correlation in the Fourier series is taken into account and hence the reciprocal interaction of the thermal conductivities of the gas mixture concerned.

System Components of the Thermal Conductivity Analyser FTTCA 1100
The miniaturised TC-sensor is mounted on a thinlyetched silicon chip, incorporating an extrem small ther-mal mass. Thus periodic modulation of the sensortemperature in the range of ± 40°C ….± 70°C withinsplit seconds are feasible.
Developed from AMS and laptop installed the corresponding PC program performs the mathematical operations automatically during thecalibration procedure. Determined in this way, the image function is stored in the analyser electronics, the laptop is of no need anymore.

Technical Data for the Thermal Conductivity Analyser FTTCA 1100

Housing: Versions for EEx- and non-hazardous areas.

Dimensions: 32 x 12x 25 cm (WxHxD)
Dimension EEx-version: 20 x 50 x 20 cm (WxHxD)

Weight: approx. 5 kg,
Weight EEx-version: approx. 7,5 kg

Protection: IP65 for the ATEX-version
Protection EEx-version: certificate 05 ATEX 2763 Ex II 2G EEx de IIC T4

Digital Interface: RS 485, CAN-BusSignal output: 4 - 20 mA, Analog converter located in the non- hazardous area

Power: 18 - 30 VDC

Power consumption : approx.. 3.5 Watt during warm-up period - approx.. 2.1 Watt during measurements

Gas consumption: 5…80 l per hour

 (C) AMS GmbH 1.0/2006 Subject to technical modifications   Text, drawings and layout: Analytic Journal

e-mail info@ams-dielheim.com