Types of Industrial Process Analyzers


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Types of Industrial Process Analyzers

  2. 2. An analyzer is an instrument or device which conducts chemical analysis (or similar) on samples or sample streams • Analyzers – auto-analyzers • Allows a sample stream to flow from the process equipment into an analyzer, sometimes conditioning the sample stream in between such as reducing pressure or changing the sample temperature.
  3. 3. Destructive and Non-destructive Analysis Destructive Analysis: sample stream is modified by the analyzer • e.g. reducing the pressure, changing the sample temperature, addition of reagents • Sample stream cannot be returned to the process Non-destructive Analysis: sample stream is not substantially modified by the analyzer • relies upon use of electromagnetic radiation, sound, and inherent properties of materials to examine samples • sample stream can be returned to the process
  4. 4. Online vs. Inline Analysis Online analysis: analyzer is connected to a process, and conducts automatic sampling Inline analysis: a sensor can be placed in a process vessel or stream of flowing material to conduct the analysis
  5. 5. TUNABLE DIODE LAZER ANALYZERS SPECTROSCOPY (TDLAS) • a technique for measuring the concentration of certain species such as methane, water vapor and many more, in a gaseous mixture using tunable diode lasers and laser absorption spectrometry • Can achieve very low detection limits (of the order of ppb) • also possible to determine the temperature, pressure, velocity and mass flux of the gas under observation
  6. 6. • Group IV-VI semiconductor material lasers • Operate in 3 – 30 um spectral range • A basic TDLAS setup consists of: – tunable diode laser light source, – transmitting (i.e. beam shaping) optics, – optically accessible absorbing medium, – receiving optics and – detector/s
  7. 7. OXYGEN ANALYZERS (Lambda Analyzers) • an electronic device that measures the proportion of oxygen (O2) in the gas or liquid being analyzed • Zirconia oxygen analyzer (ordinarily operate at a high temperature close to 800°C) • Paramagnetic oxygen analyzer
  8. 8. Principle (Zirconia Oxygen Analyzers) Determines oxygen concentration using the conductivity of a zirconia ceramic cell. Zirconia ceramic cells only allow oxygen ions to pass through at high temperatures. • Reference gas on one side and sample gas on the other side • Oxygen ions move from the side with the highest concentration of oxygen to that with the lowest concentration. • The movement of ions generates an EMF (Electro Motive Force) which can be measured to determine the oxygen content.
  9. 9. the EMF varies depending on • the temperature of the zirconia sensor and • the oxygen concentration of the reference gas (PR), in the actual device. • the zirconia sensor is placed in a constant temperature oven • air is generally used as the reference gas
  10. 10. Limitations • Flammable gases cannot be used • Sensor degradation occurs if corrosive gas (fluorine-based gases, chlorine-based gases, sulfate-based gases)is measured • In general, these analyzers cannot be used with closed loops (circulating systems) unless they are specially designed for that purpose. The sensor may be damaged by excess pressure.
  11. 11. Paramagnetic Oxygen Analyzers High magnetic susceptibility of oxygen as compared to other gases allows it to be attracted to a magnetic field Magnetic susceptibility is a measure of the intensity of the magnetization of a substance when it is placed in a magnetic field
  12. 12. Focused magnetic field is created Nitrogen filled glass spheres are mounted on a rotating suspension within the field Mirror mounted on the suspension – detects the displacement of the nitrogen spheres as oxygen is attracted to the strongest part of the field Reflected light is directed on to a pair of photocells – light intensity converted to electrical signal - which is fed to a feedback coil causing a motor effect to keep the suspensions in place
  13. 13. Limitations The difference in magnetic susceptibility between the dumbbell and the gas sample is very subtle for low oxygen concentrations, this method is used only when measuring percent levels of oxygen and not for trace levels
  14. 14. INFRA-RED GAS ANALYZER Measures trace gases by determining the absorption of an emitted infrared light source through a certain air sample • Gas detector doesn't directly interact with the gas • Gas molecules only interact with a light beam • Non-destructive analysis
  15. 15. Methods used for Detection Rise in temperature of gas molecules Photon detectors (Absorption Spectrum) Molecules resonate at frequencies of radiation matching with their natural frequencies Molecules of a specific gas absorb radiations of specific wavelengths Increase in vibrations cause an increase in temperature of the gas Transmitted spectrum indicates the absorbed wavelengths
  16. 16. Structure and Operation The infrared light is emitted and passes through the sample gas, a reference gas with a known mixture of the gases in question and then through the "detector" chambers containing the pure forms of the gases in question. When a "detector" chamber absorbs some of the infrared radiation, it heats up and expands. This causes a rise in pressure within the sealed vessel that can be detected either with a pressure transducer or with a similar device. The combination of output voltages from the detector chambers from the sample gas can then be compared to the output voltages from the reference chamber.
  17. 17. DUST MONITORING SYSTEMS Two basic methods of dust emission reporting Mass concentration (mg/cubic meter) Mass flow (kg/hr) Most commonly used Total mass of dust emitted per unit time Assumes a homogeneous mixture of dust particles and air Absolute measure of dust emission
  18. 18. Mass concentration Measurement of mass concentration depends factors that change the volumetric characteristics of the carrier gas: • Gas law effects: the effects of temperature and pressure. • Dilution effects: the effects of excess air and water vapor levels. Data normalization is required • standard practice is to report the data as a mass per normal cubic meter of dry gas, at a specified level of oxygen.
  19. 19. Drawbacks • A simple measurement becomes a complex measurement • Cost of measuring the gas normalization parameters is greater than the cost of the primary dust measurement – Schedule A processes: normalization data is already available as part of the gas analysis requirements – Schedule B processes: which are only required to measure dust, the problem becomes severe
  20. 20. Mass Flow The measurement is related to mass concentration. • Mass flow = mass concentration x volumetric flow No normalization data is required • Does not depend in any way on gas temperature, pressure, oxygen or water vapor values, or on any form of dilution of the exhaust gases. Mass flow can be directly related to the environmental impact
  21. 21. Operating Principle (Grimm Aerosol DMS#365) Single particle count – using light scattering technology Semiconductor laser as light source Mirror reflects the scattered light beam to be detected by a photodiode Pulse height classifier classifies photodiode signals in a multichannel size classifier Counts are displayed and stored
  22. 22. GAS CHROMATOGRAPHY • used to separate organic compounds that are volatile • consists of: – – – – – a flowing mobile phase, an injection port, a separation column containing the stationary phase, a detector, and a data recording system.
  23. 23. Principle The organic compounds are separated due to differences in their partitioning behavior between the mobile gas phase and the stationary phase in the column.
  24. 24. He Injection port Oven Detector Recordercomputer Carrier Gas Column Sample is injected (using a syringe) into the injection port. Sample vaporizes and is forced into the column by the carrier gas ( = mobile phase which in GC is usually helium or any other inert gas) Components of the sample mixture interact with the stationary phase so that different substances take different amounts of time to elute from the column The separated components pass through a detector. Electronic signals, collected over time, are sent to the GC software, and a chromatogram is generated.
  25. 25. Temperature Dependence of Partitioning Behavior Partitioning behavior is dependent on temperature the separation column is usually contained in a thermostatcontrolled oven Process is similar to fractional distillation A gas chromatography oven Separating components with a wide range of boiling points is accomplished by starting at a low oven temperature and increasing the temperature over time to elute the high-boiling point components
  26. 26. GC Detectors • Separated components of the mixture must be detected as they exit the GC column • Thermal-conduc. (TCD) and flame ionization (FID) detectors - two most common detectors on commercial GCs.
  27. 27. Thermal Conductivity Detector (TCD) Senses the changes in the thermal conductivity of the column effluent with reference to a flow of carrier gas • Also known as Katharometer • Bulk property detector and chemical specific detector • Non-specific and non-destructive • Universal detector – responds to all compounds
  28. 28. TCD – an electrically heated filament in a temperature controlled cell • During elution of an analyte, thermal conductivity of the column effluent reduces • Filament heats up and changes resistance
  29. 29. Limitations • • • • Less sensitive than other detectors Has a larger dead volume Cannot operate below 150 C temperature set Chemically active compounds may damage the filament
  30. 30. Flame Ionization Detector (FID) • Measures the concentration of organic species in a gas stream • Most sensitive gas chromatographic detector • Has a low detection limit in the picogram or femtogram range • Has a linear range of 6 to 7 orders of magnitude
  31. 31. Operating Principle Ions formed during the combustion of organic effluents in hydrogen flame is detected. • Ion generation is directly proportional to the concentration of organic species in the sample stream • Presence of heteroatoms decreases the detector’s response
  32. 32. Detector Construction small volume chamber gas chromatograph column capillary is directly plumbed to the bottom of flame jet column effluents are mixed with hydrogen and air to be burned up in the flame jet An electronic igniter (electrically heated filament) lights on the flame Charged particles created during combustion create a current b/w the detector’s electrodes
  33. 33. Operation positive electrode doubles as the nozzle head where the flame is produced negative electrode is positioned above the flame (tubular electrode called collector plate) ions attracted to collector, hit the plate and induce current current is measured with a high impedance picoammeter and fed to an integrator The response of the detector is determined by the number of carbon ions hitting the detector per unit time. Thus, the detector is sensitive to mass rather than concentration.
  34. 34. Advantages • Relatively inexpensive to acquire and operate • Low maintenance requirements apart from cleaning and replacing of the FID jet • Rugged construction • Extensive linear and detection range
  35. 35. Limitations • Cannot differentiate between organic compounds • Cannot detect non-organic substances • Presence of heteroatoms and oxygenates lower the response factor • Carbon monoxide and carbon dioxide cannot be detected without a methanizer (bed of Ni catalyst used to reduce CO and CO2 to methane) • Destructive analysis – all components passing through the flame are oxidized
  36. 36. pH ANALYZERS • pH is a measure of the acidity or alkalinity of water • pH is defined as the negative logarithm of hydrogen ion activity (aH+) in water pH = -log10 aH+ • In practice, negative log of hydrogen ion concentration [H+] is used
  37. 37. Electrode Chain pH Analyzer Electrodes immersed in a solution form a galvanic cell due to potential developed on both electrodes, which changes in response to any change in pH of the solution • Two electrode setup – indicator electrode and reference electrode • Measures the potential between reference electrode dipped in a solution of known pH and the indicator electrode
  38. 38. Electrode Construction
  39. 39. pH Meter Measures the potential difference between the electrodes and converts it to a display of pH
  40. 40. Buffer Solutions and Calibration • Calibration is done using solutions which – Have a precisely known pH value – Are relatively insensitive to contamination from acidic and alkaline species (i.e. buffer solutions) • Two different buffers are used for calibration which indicate electrode sensitivity
  41. 41. CONDUCTIVITY ANALYZERS • Conductivity of a solution depends on: – concentration of ions – mobility of ions – valence no. of ions – temperature • Two types of conductivity measurements: – contacting – inductive
  42. 42. Contacting Conductivity • Two metal electrodes in contact with the solution are used • Alternating current is applied at optimal frequency to the electrodes and output voltage is measured Conductivity = Cell constant x Conductance of the Solution Cell constant – ratio of distance b/w electrodes to area of the electrodes Conductance of solution – input current / output voltage
  43. 43. Factors Influencing Measurement • Polarization and Contamination of Electrode Surface – accumulation of ionic species near the surface and chemical reaction on the surface • Field Effects – any interference with the field lines causes an error in signal measurement
  44. 44. Inductive Measurement • Toroidal or electrode-less conductivity measure • Two wire wound metal toroids enclosed on a corrosion resistant plastic body • Ideal for measuring solutions having high conductivity • Can tolerate high levels of fouling by suspended solids • Does not come into contact with the electrolyte
  45. 45. Analyzer applies an AC voltage to the drive coil A voltage is induced in the surrounding liquid An ionic current flows proportional to the conductance of the liquid The ionic current induces an electronic current in the receiver coil Electronic current is measured by the analyzer
  46. 46. Temperature and Conductivity • Increasing the temperature of an electrolyte solution increases the conductivity • 1.5% to 5% increase per degree C • Conductivity readings are commonly corrected at a reference temperature, commonly 25 C • Correction algorithms need to be applied – Linear temperature coefficient – high purity water or dilute sodium chloride – high conductivity or dilute HCl
  47. 47. Linear Temperature Coefficient Conductivity of an electrolyte changes by about the same percentage for every degree rise in temperature C25 – conductivity at 25 C Ct – conductivity at T C - linear temperature coefficient
  48. 48. High Purity Water (dilute NaCl) Correction • Assumes that the sample is pure water contaminated with NaCl • Measured conductivity is the sum of conductivity of pure water and the conductivity from Na+ and Cl- ions Point 1 – raw conductivity at temp. ‘t’ degree C Conductivity of pure water at ‘t’ – raw conductivity = conductivity of Na+ and Cl- at ‘t’ (point 2) conductivity of Na+ and Cl- at ‘t’ is converted to conductivity at 25 deg. C (point 3) Add conductivity of pure water at 25 deg. C corrected conductivity (point 4)
  49. 49. Cation Conductivity (dilute HCl) Correction • Used in steam electric power industry • Assumes the sample is pure water contaminated with HCl • Contribution of water to the overall conductivity depends on the total amount of acid present