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Universität Duisburg-Essen ISE-Bachelor CONCENTRATION Universität Duisburg-Essen Dr. Siddiqi Fakultät für Ingenieurwissenschaften Abteilung Maschinenbau Institut für Verbrennung und Gasdynamik Thermodynamik ISE-Bachelor CONCENTRATION MEASUREMENT (THERE IS A SMALL PART IN GERMAN LANGUAGE) 1 Concentration Measurement 1 Introduction 2 Important gas analysis methods 2.1 Orsat-Apparatus 2.2 Infra red spectroscopy 2.3 Paramagnetic oxygen analysis 2.4 Mass spectrometry 2.5 Gas chromatography 2.6 Thermal conductivity detector (TCD) 2.7 Flame ionization detector (FID) 3 Experiments for concentration measurements 3.1 Measurement of the concentration of Propane, Isobutane and n-Butane in gas mixture using a gas chromatograph 3.1.1 Basic principles 3.1.2 Experimental set up 3.1.3 Experimental procedure 3.1.4 Evaluation 3.2 Bestimmung der Raumanteile von CO2 und CO in einem Gasgemisch mit den Prüfröhrchen 3.2.1 Kurzbeschreibung 3.2.2 MAK Wert 3.2.3 Chemische Grundlagen 3.2.4 Versuchsaufbau 3.2.5 Versuchsablauf 2 1 Introduction The general purpose of gas analysis is to measure the concentration of each component in a gas mixture. Both physical and chemical methods are applied in gas analysis. Each gas component is measured on the basis of different chemical/physical or physical principles of measurements. Such principles include: a. Chemical reactions (e.g. Orsat Apparatus, Dräger test tubes) b. Absorption of radiation (e.g. infrared) c. Paramagnetism (this method measures oxygen concentration) d. Mass spectrometry e. Gas chromatography f. Thermal conductivity (requires separation using chromatography) g. Flame ionization (requires separation using chromatography) 2. Important gas analysis methods 2.1 ORSAT apparatus: The Orsat apparatus, illustrated in Fig. 1, is generally used for analyzing gas mixtures (e.g. flue gases). The burette 4 is graduated in cubic centimeters up to 100, and is surrounded by a water jacket to prevent any change in temperature from affecting the density of the gas being analyzed. It uses three pipettes (some use four pipettes for more accuracy), (1+1a),(2+2a),(3+3a), the first containing a solution of caustic potash (KOH) for the absorption of carbon dioxide, the second an alkaline solution of pyrogallol (1,2,3-trihydroxybenzene) for the absorption of oxygen, and the third an acid solution of cuprous chloride (Cu2Cl2) for absorbing the carbon monoxide. Each pipette contains a number of glass tubes, to which some of the solution clings, thus facilitating the absorption of the gas. In the pipette 3, copper wire is placed to re- energize the solution as it becomes weakened. The rear half of each pipette is fitted with a rubber bag, one of which is shown at 15, to protect the solution from the action of the air. The solution in each pipette should be drawn up to the mark on the capillary tube. The gas is drawn into the burette through the valve 11. To discharge any air or gas in the apparatus, the valve is opened to the air and the bottle 9 is raised until the water in the burette reaches the 100 cubic centimeters mark. The valve 11 is then turned so as to close the air opening and allow gas to be drawn through, the bottle 9 being lowered for this purpose. The gas is drawn into the burette to a point below the zero mark, the valve 11 then being opened to the air and the excess gas expelled until the level of the water in 9 and in 4 are at the zero mark. This operation is necessary in order to obtain the zero reading at atmospheric pressure. The apparatus should be carefully tested for leakage as well as all connections leading thereto. 3 Figure1 : Orsat apparatus. Before taking a final sample for analysis, the burette 4 should be filled with gas and emptied once or twice, to make sure that all the apparatus is filled with the new gas. The valve 11 is then closed and the valve 14 in the pipette 1a is opened and the gas driven over into 1a by raising the bottle 9. The gas is drawn back into 4 by lowering 9 and when the solution in 1a 4 has reached the mark in the capillary tube, the valve 14 is closed and a reading is taken on the burette, the level of the water in the bottle 9 being brought to the same level as the water in 4. The operation is repeated until a constant reading is obtained, the number of cubic centimeters being the percentage of CO2 in the gas mixture. The gas is then driven over into the pipette 2a and a similar operation is carried out. The difference between the resulting reading and the first reading gives the percentage of oxygen in the gas mixture. The next operation is to drive the gas into the pipette 3a. The process must be carried out in the order named, as the pyrogallol solution will also absorb carbon dioxide, while the cuprous chloride solution will also absorb oxygen. The analysis made by the Orsat apparatus is volumetric; if the analysis by weight is required, further calculations are to be done. 2.2 Absorption of radiation (Infra red spectroscopy) The basis for the quantitative measurements using optical spectroscopy is provided by the Lambert-Beer law which relates the spectral response (absorbance) to the concentration. The absorbance of a measured absorption band is a function of the measurement wavelength, the thickness of the sample and the concentration of the absorbing species being measured. This is expressed as follows: Ac111λλ= ε d where A1λ is the absorbance of species 1 at wavelength λ,ε1λ is the absorptivity of species 1 at wavelength λ,c1 is the concentration of the absorbing species 1, and d is the optical thickness of the sample or path length of the measurement cell. So from the absorbance measurements the concentration c of a particular component may be calculated. Infrared spectroscopy includes the methods that are based on the absorption (or reflection) of electromagnetic radiation with wavelengths in the range of 0.8 to 1000 μm. This spectral range has been divided into three groups: near infrared (NIR) (0.8 – 2.5 μm), mid infrared (MIR)(2.5 – 25 μ) and far infrared (FIR) (25 – 1000 μm). Of these three ranges the MIR region is the most accessible and the richest in providing structural information (molecular fingerprint). Infrared analyses are performed on dispersive (conventional) and Fourier- transform spectrometers. Besides the dispersive spectrophotometers many spectrometers have been designed for routine analysis (in laboratory as well as for on-site control) to quantify one or many compounds. Almost one hundred gases or volatile compounds can be quantified with the help of such photometers. The optical lay outs for two typical models are shown in Figure 2. The most widely used photometers are for the measurement of carbon monoxide (CO) -1 [absorbance measurement at 2170 cm ] and carbon dioxide (CO2) [absorbance measurement at 2350 cm-1] from car exhausts. In Figure 2a the light coming from the source S passes through an interface filter F depending on the wavelength to be measured. Cell C contains the sample while the reference cell R 5 contains a known concentration of the same type of gas to be quantified. Another cell A is filled with a non absorbing gas (N2). The cells A and R are placed in the optical path alternatively. By a comparison of the absorbance measured by the detector with and without the sample cell in the path the concentration of the concerned gas in the sample can be determined. In the other model (Figure 2b) the light beam from the source 1 travels through the measurement cell 2 before reaching the cells V1 and V2 which contains the gas component to be measured (e.g. CO or CO2). In this arrangement the sample absorbs a part of the radiation before the radiation reaches V1. The chambers V1 and V2 are so constructed that in the absence of any absorbance in the measurement cell (e.g. when the inert gas flows through it) the absorption in V1 and V2 are same and no pressure difference is observed by the pressure transducer. The beam chopper M is necessary to obtain a repetitive pulsed signal. This is the zero point adjustment. The intensity of the beam reaching V1 will be attenuated if the same gas is present in cell (as is the case it is the case when the sample flows through V) and this will be proportional to the concentration of the gas component to be measured. Figure 2: Layout of some typical gas analysers. 2.3 Paramagnetic Oxygen Analyser Oxygen has a relatively high magnetic susceptibility as compared to other gases such as nitrogen, helium, argon, etc. and displays a paramagnetic behaviour. The paramagnetic oxygen sensor consists of a cylindrical shaped container inside of which is placed a small glass dumbbell. The dumbbell is filled with an inert gas such as nitrogen and suspended on a taut platinum wire within a non-uniform magnetic field. The dumbbell is designed to move freely as it is suspended from the wire. When a sample gas containing oxygen is processed through the sensor, the oxygen molecules are attracted to the stronger of the two magnetic fields. This causes a displacement of the dumbbell which results in the dumbbell rotating. A precision optical system consisting of a light source, photodiode, and amplifier circuit is used to measure the degree of rotation of the dumbbell. In some paramagnetic oxygen sensor designs, an opposing current is applied to restore the dumbbell to its normal position. The 6 current required to maintain the dumbbell in it normal state is directly proportional to the partial pressure of oxygen and is represented electronically in percent oxygen.
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