Types of Industrial Process Analyzers

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