chapter 13 - analytical instrumentation 1 chapter 14 analytical instrumentation
TRANSCRIPT
Chapter 13 - Analytical Instrumentation
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Chapter 14Analytical Instrumentation
Chapter 13 - Analytical Instrumentation
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pH Measurement and Control
• Many industrial applications require the control of the concentration of acids and bases in a chemical solution
• The analytical process that controls this is called pH control
• Within a chemical solution, the number of negative ions compared to positive ions determines whether the solution is acid or base
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pH Factors
• The relationship between negative and positive ions is called dissociation (ionization) constant
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Relationship of pH and
Hydrogen Ion Concentration
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pH Measurements
• Devices used to measure pH values detect the concentration of hydrogen ions
• Early techniques used litmus paper which changed color based upon the solution being acid or base
• Electronic sensors were developed to overcome the shortcomings of paper-based tests
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Electronic Sensors• Electronic sensors use two
electrodes and an amplifier to measure pH– The active or sensing probe
produces a voltage proportional to the the hydrogen-ion concentration
– The reference probe provides a signal against which the measuring electrode is compared
Sensing Probe Reference Probe
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Controlling pH
• In a pH control system, either a solution is too acidic or too alkaline
• The corrective ingredient added to a solution is called a reagent
• One objective of a pH control system is to minimize the amount of reagent added to the solution, eliminating overshoot
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Batch Systems
• A batch system normally uses a tank for the solution for storage
• Solution entering the tank is called influent
• Solution exiting the tank that has been treated is referred to as effluent
• Batch pH control is used for relatively small volumes
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Continuous Systems• Many continuous systems use a
tank • A reagent is added to correct the
pH of a solution using either an on/off or proportional controller
• A disadvantage of using a tank is a long delay time
• In applications requiring pH values between 4 and 10, a static mixer is used
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Conductivity• Any process that involves liquids requires flow
• The liquid used in these processes is referred to as a process stream
• In many applications, the purity of water or the concentration level of solutions is measured or controlled
• This can be done by measuring the conductivity of a solution
• Conductivity refers to the ability of material to pass electric current
Formula to determine conductance
G = 1
R
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Factors Affecting Conductivity
• Concentration of an ingredient dissolved in water, ranging from zero to very high
• The type of electrolyte contained in a dissolved ingredient
• The temperature of the liquid
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Conductivity Probes
• Two types of probes used are:– Electrode probe
– Inductive probe
Conductivity Probe
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Conductivity Inductive Probe
• The conductivity inductive probe uses to toroidal coils
• One coil is connected to an oscillator, the other coil is used as a pickup
• The the current induced in the pickup coil is directly proportional to the conductivity of the solution
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Conductivity Values
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Combustible Gases
• Some gases are dangerous if they escape from their container– Hydrogen– CO– Hydrogen sulfide– Methane, propane, butane, ethane
• Analytical sensors are used to detect these gases
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Thermo-Conductivity Detector
• Every gas has the ability to conduct heat
• The type of gas and its concentration determine the rate at which heat is conducted
• The TCD uses a bridge , measuring chamber, and a reference chamber
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Hydrocarbon Gases
• Combustion occurs when hydrocarbon fuels are ignited
• When they burn completely, the by-products are water and carbon dioxide
• CO2 and CO can be detected with an infrared gas analyzer
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Optical Gas Analyzer
• An infrared gas analyzer consists of six major elements:– Infrared light source– Sample chamber– A prism– Two optical filters– Two detectors– Electronic converter
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Combustion Analyzers and Control
• Energy for many industrial process is produced by combustion
• Combustion uses a combination of gases and fuel• Two types of fuel
– Hydrocarbon
– Combustible gas
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Humidity
• Humidity is defined as the amount of moisture in the air
• Humidity control is important in many industrial applications
• Humidity affects hygroscopic materials. Hygroscopic means the ability to absorb and retain moisture
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Quantitative Measures of Humidity
• There are three different quantitative measures of humidity– Absolute
– Relative
– Dew Point
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Absolute Humidity
• Defined as the mass of water vapor present in a particular volume of atmosphere
• Absolute humidity value is expressed as the ratio of the mass of water vapor to the volume occupied by the air-water vapor mixture
W = Pw
Pa
W = Absolute Humidity
Pw = Mass Density of Water
Pa = Mass Density of Air
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Relative Humidity
• Defined as the actual amount of water vapor present as compared to the maximum amount of water vapor the air can hold at a given temperature
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Absolute Humidity Sensor
• The most common type of device to measure absolute humidity is the aluminum oxide sensor
• Essentially, the aluminum oxide sensor functions as a capacitor with the value of capacitance dependent upon humidity
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Dew Point Measurements
• Dew Point is defined as the temperature at which the air becomes saturated
• When air is cooled at a constant pressure, condensation of vapor will occur at the dew point temperature
• Three common techniques to measure dew points are:– Manual chilled mirror– Adiabatic expansion sensing– Optical chilled mirror
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Optical Chilled-Mirror Hygrometer
• Components include:– Gold or rhodium-
plated copper mirror
– Thermoelectric cooler
– High-intensity LED
– Optical detector
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Relative Humidity Detectors
• Psychrometric Detector - uses two identical thermometers called a dry bulb and a wet bulb
• Hygrometric Detector - measures the change in dimension of hygroscopic materials
• Electronic Capacitance Detector - A common RH sensor that is constructed as a small capacitor with a hygroscopic polymer acting as the dielectric
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Sampling Measurement Systems• Whenever the
temperature of the air being tested is above the level the sensor can be exposed, it must first be cooled before a reading can be taken
• A sampling system like the one on the right offers a solution