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Tuesday, March 5, 2019

On-line chromatographic analysis

As early as 1903-1906 a Russian botanist Mikhail Tsvet (Lecturer in the Warsaw University) performed experiments trying to separate pigments of plants. Using a solvent, petroleum ether, he washed the pigments through a vertical glass tube filled with a powder-like absorbent, calcium carbonate. As the result of this procedure, he obtained a series of coloured absorption bands. He first used a term ‘chromatography’ to describe this method. Word ‘chromatography’ consists of two words ‘chromatos’ (in Greek language means ‘colour’) and ‘grapho’ (in Greek language means ‘writing’), and means ‘colour writing’. By coincidence, the surname ‘Tsvet’ in Russian language means ‘colour’.

Chromatography methods are classified regarding to the types of moving and stationary phases according to Figure 7.1. (from Considine D. M. Process Instruments and Controls Handbook. McGraw-Hill Book Company, Sydney, 1985, p. 6.170).

Figure 7.1. Classification of chromatography methods. 

Physical absorption principles for separating of various components from a mixture of chemical substances form the basis of chromatography (see Fig. 7.2). A gas mixture 1 to be analysed is carried through a tube or column 2 by an inert carrier gas (nitrogen, helium) 3. The gas mixture and the carrier gas form the moving phase. The column is filled (packed) with materials 4, the stationary phase, which will absorb gases. Different components of the gas mixture are delayed for varying increments of time. After the column, the separated gases 5 pass through a gas detector (flame ionisation detector, or thermal conductivity detector) 6. This detector develops a signal 7, which then is transformed to the chromatogram 8. Using this chromatogram we can determine the type of a component and its quantity. In order to achieve better separation of components from various mixtures different types of packing materials should be chosen. The absorption of components by the stationary phase is highly dependent on the operational conditions. Therefore, temperature, flowrate and pressure of a carrier gas, sample valve timing, and detector sensitivity should be carefully controlled.

Chromatographs are widely used for composition measurements of gaseous and liquid mixtures. Usually they are complex laboratory equipment. Modern on-line chromatography systems for continuous, repetitive and fully automatic gas analysis have been developed, and in principle have all essential elements inherent to laboratory-type equipment. Fig. 7.3 schematically shows an operational principle of an on-line gas-chromatograph.

Figure 7.2. Schematic of a gas-chromatograph (gas-solid chromatography). 

A sample of a gas mixture 1 is withdrawn continuously from a process unit 2 and through a shutoff valve 3, filter 3a (for removing of particulate matter) and pressure regulator 4 (for reducing pressure to a lower constant value) circulates through a sample conditioning unit 5. After the sample conditioning unit the stream enters the process 6 through a shutoff valve 7 in the point of a lower pressure compared with the sample withdrawal point. The sample conditioning unit allows to calibrate the gas-chromatograph with the synthetic calibration blend from a container 8, through a pressure regulator 9, and to control flowrate. A carrier gas (nitrogen, helium) is supplied from a cylinder 10, and its pressure is controlled by a pressure regulator 11 and a pneumatic control section 12. An analyser 13 contains separating columns, flame ionisation and/or thermal conductivity detector(s) and a temperature control unit. A metering pump, which is placed in the sample conditioning unit, injects a small sample of an already conditioned gas mixture into the separating column, which is placed in the analyser. An electronic module 14 stores analytical programs in RAM and controls functions of the analyser. Analytical data are transferred from the electronic module to a data processor 15, where they are converted to analog signals. These signals are transmitted to a bar graph recorder16 and a number of trend recorders 17. A host computer 18 controls all actions of the chromatograph, receives results, alarm messages, stores application programs. A real-time chromatogram is printed on the printer 19

Here are several values of parameters of a chromatograph:
  • temperature in the temperature control unit - 40 to 200 °C;
  • accuracy of temperature control - ±0.2 °C;
  • total length of separating columns - 10 m;
  • diameter of separating columns - 3 mm;
  • flowrate of a carrier gas - 40 to 160 cm3/min;
  • volumes of samples of gas mixtures - 0.5, 1, 2, 4 cm3;
  • volumes of samples of liquid mixtures - 0.004, 0.008, 0.032 cm3;
  • pressure of a gas carrier - 400 kPa;
  • output signal - 4 to 20 mA.
An output signal from a gas-chromatograph is usually used as an input signal for a controller, which changes either process temperature or pressure or flowrate, etc., to bring a product composition to the desired value.

Figure 7.3. On-line chromatographic system.

Article Source:: Dr. Alexander Badalyan, University of South Australia


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