types of industrial process analyzers
TRANSCRIPT
TYPES OF PROCESS ANALYZERS
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.
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
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
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
• 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
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
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.
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
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.
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
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
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
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
Methods used for Detection
Rise in temperature of gas molecules
Molecules resonate at frequencies of radiation
matching with their natural frequencies
Increase in vibrations cause an increase in temperature of the
gas
Photon detectors (Absorption Spectrum)
Molecules of a specific gas absorb radiations of specific
wavelengths
Transmitted spectrum indicates the absorbed wavelengths
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 withi
n 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.
DUST MONITORING SYSTEMS
Mass concentration (mg/cubic meter)
Most commonly used
Assumes a homogeneous mixture of dust particles and air
Mass flow (kg/hr)
Total mass of dust emitted per unit time
Absolute measure of dust emission
Two basic methods of dust emission reporting
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.
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
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
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
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.
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.
He
Carrier Gas
Injectionport
Detector
Column
Recorder-computer
Oven
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.
Temperature Dependence of Partitioning Behavior
A gas chromatography oven
Partitioning behavior is dependent on temperature
the separation column is usually contained in a thermostat-controlled 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
Process is similar to fractional distillation
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.
Thermal Conductivity Detector (TCD)
• Also known as Katharometer• Bulk property detector and chemical specific detector• Non-specific and non-destructive• Universal detector – responds to all compounds
Senses the changes in the thermal conductivity of the column effluent with reference to a flow of carrier gas
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
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
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
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
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
Operationpositive 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
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
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
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 = -log1
0
aH+
• In practice, negative log of hydrogen ion concentration [H+] is used
Electrode Chain pH Analyzer
• 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
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
Electrode Construction
pH Meter
Measures the potential difference between the electrodes and converts it to a display of pH
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
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
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 electrodesConductance of solution – input current / output voltage
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
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
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
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
Linear Temperature Coefficient
Conductivity of an electrolyte changes by about the same percentage for every degree rise in temperature
C25 – conductivity at 25 CCt – conductivity at T C - linear temperature coefficient
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)
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