lecture - composition analyzers

Composition Analyzers

Lecture no. 11

Industrial Instrumentation- Ms. Qandeel Almas2

ContentsIntroductionTesting methodsComposition analyzing techniques:1. Density Measurement

Magnetic Method Hydrometer

2. Optical concentration transducers Turbidimeters Refractrometers Spectrophotometry

3. Electrochemical composition measurement Thermal conductivity measurement pH measurement

4. Analytical techniques Chromatography

Introduction


Introduction Knowledge of the composition of the process

stream is often of major importance. This information may help us to determine the quality of the product or to control the changing composition of any process stream .

In the measurement of composition, 1st we have to analyze which property of the material is to be used for determining the composition. This property may be physical or chemical & this quantity should differ between the components of the material.

The analysis of the stream may be online or offline. If online, then the response time of the measuring instrument should be smaller than that of the plant being controlled.


Cont. . The various properties which may be employed to determine the composition include

Preferential adsorption of the components (chromatography)

Absorption of electromagnetic rays (UV rays) Refractive index Conductivity Electric potential pH Density

The measurement of composition may be direct, or inferential. Inferential means use the measure of some property as a function of composition.,(e.g. the use of B.P in distillation column as a measure of liquid composition).


Cont. . A wide array of methods is available to measure composition and these methods are based on following types of analysis:

Various physical property measurements (density and specific gravity)

Photometric analysis Mass spectrometry Electrometric analysis Chromatography

The first step is to decide between measuring physical or chemical properties to determine the composition. If physical property is to be measured, make sure that it is unique to the desired component of the mixture and will accurately allow determining the composition.

Testing Methods• Online Testing• Off-line Testing


• Online TestingOn-line analysis is the continuous monitoring of the composition of a sample. On-line testing can either be performed in-line or by slip stream testing.

In in-line testing, the sensor is attached directly to the line and provides feedback via a transmitter.

In slip stream testing, a side stream of the process runs alongside the main line. Such an apparatus closely resembles the set up of a bypass. The slip stream process conditions can be continuously manipulated to make the measurements easier to obtain. Similar to in-line testing, the sensor is directly attached to the slip stream and provides feedback through a transmitter.


Online Testing-Characteristics AdvantagesOn-line analysis include an immediate and continuous feedback responding to changes in process conditions and so on-line testing prevents the continued production of undesired product, with an immediate response and correction of the flawed material.

DisadvantageOn-line testing is usually much more complicated and more expensive.


• Off-Line TestingOff-line analysis involves the extraction of a sample from the process or reaction, and its subsequent testing in a machine that may be situated at a location far away from the process line in a lab. In this case, a sample is manually removed which is later sent to the composition analyzer. The results of the analysis are examined and then they are sent to the control system to make the appropriate adjustments.


Off-line Testing-CharacteristicsAdvantagesOff-line testing is more robust and has more varied application

Disadvantages Sample dead-time is introduced (the time

lost during transportation) Variability of sample testing locations Lag time for adjustments to be made to

the process

1. Density Measurement Magnetic Method

Hydrometer


IntroductionDensity is the most fundamental of all the physical properties. It applies to substance in any of the three physical states. Density measurements are made for the following reasons: To determine the mass and volume of products To assess the quality of products. To determine the composition and concentration

of a process stream. To convert volumetric flow measurements into

mass flow information.


Solid Density Meters Density of solid may vary both with operating

pressure and temperature. In many applications the effect of pressure

may be neglected in solids and incompressible fluids.

In some cases, effect of temperature can also be neglected if operational temperature is not significantly higher from temperature at which density measurements are made, or high degree of accuracy is not required.


Fluid Density Meter

Overall density of a fluid is ratio of total mass to total volume.

Point density is ratio of molecular mass in a volume element centred at a point to element’s volume (the volume being much smaller than total volume).


a) Magnetic method:

The magnetic densitometer consists of a small ferromagnetic cylinder, encased in a glass jacket. The jacket and ferromagnetic material combination constitutes a buoy or float. Therefore float is suspended electromagnetically, totally immerses in the process fluid whose density is to be measured.


Magnetic method (contd.): When the density of the fluid decreases, the float

will start sinking. The change in position is detected by search coils and it results in an unbalanced signal to the amplifier and demodulator, causing an increase in the current to the solenoid. This increases the force of attraction of the magnet on the float and restores it to its original centered position.

The density sensor can be mounted on tank or pipeline nozzles, directly immersed in the process stream, or provided with a sample chamber for mounting in a sample bypass line.


Magnetic Density Meter


Magnetic Densitometers-featuresAdvantages Magnetic densitometer allows the determination of

affects of pressure and temperature down to cryoscopic range.

It allows high measurement accuracy

Disadvantages Sample flow rates exceeding a few GPH (gallon per

hour) or viscosities above a few centipoises can result in vertical forces on the float, causing error in the measurement.

Deposits on the float will also result in measurement error, for this reason the detector is not recommended for slurry service.


b) Hydrometers Hydrometer is used to

determine directly the specific gravity of a liquid.

Construction:It usually consists of a thin glass tube closed at both ends, with one end enlarged into a bulb that contains fine lead shot or mercury to cause the instrument to float upright in a liquid.


Working: The hydrometer is based on Archimedes'

principle. In the glass tube is a scale so calibrated that

the reading on it in level with the surface of the liquid (in which the hydrometer is floating) indicates the number of times heavier or lighter the liquid is than water, i.e., the specific gravity of the liquid.

The level at which the hydrometer floats depends only on the density of the liquid. Hence this level can be used to measure both the density and the specific gravity, which is proportional to it.


Types of Hydrometers Lactometers (Milk)

Alcoholmeters (Alcohols)

Saccharometer (Sugar Solutions)

Barkometers (Tanning Liquors)

Acidometer (Acids)

2. Optical Concentration Transducers


IntroductionThese include such instruments as opacity monitors, turbidmeters, colorimeters, refrectrometres & spectrophotometers. Selection of these instruments is more general where we need online process analysis

Turbidimeters Refractrometers Spectrophotometer

a) Turbidimeters


Turbidity:

Turbidity is the cloudiness or haziness of a fluid caused by individual particles (suspended solids) that are generally invisible to the naked eye, similar to smoke in air.

http://en.wikipedia.org/wiki/File:TurbidityStandards.jpg


Cont. . . Fluids can contain suspended solid matter

consisting of particles of many different sizes. While some suspended material will be large enough and heavy enough to settle rapidly to the bottom of the container if a liquid sample is left to stand (the settable solids), very small particles will settle only very slowly or not at all if the sample is regularly agitated or the particles are colloidal. These small solid particles cause the liquid to appear turbid.

Turbidity is most often used to describe the measurement of low concentrations of particles that should not be in the process stream, like particles in drinking water or filtrate.


Turbidimeters - TypesTurbid meters consist basically of three components:

A light source One or more detectorsA sample cell

Types of turbidity instrumentation are:Forward scatter 90◦ scatter (Nephalometers)Backscatter


Turbidimeters - Principle: As light passes through ‘absolutely pure’ water,

the light beams travel along relatively undisturbed paths. However, some distortion occurs as light is scattered by molecules present in the pure fluid.

When light passes through a fluid containing suspended solids, the light beam interacts with the particles, and the particles absorb the light energy and re-radiate light in all directions.

Particle size, configuration, color, and refractive index determine the spatial distribution of the scattered light intensity around the particle.


Cont. . . Particles much smaller than the wavelength of the incident light, which is typically expressed in nanometers (nm), scatter light of approximately equal intensity in all directions.

However, particles larger than the wavelength of the incident light, form a spectral pattern that results in greater light scattering in the forward direction (away from the incident light) than in the other directions.


Turbidimeters - ApplicationsTurbidimeter applications number in the hundreds because the measurement is relatively simple and versatile.

Particles that should not be in the process stream, such as water in a petroleum product or catalyst in filter effluent.

Concentration, of which examples are tomato juice concentration and the concentration of particles in a filter feed line.

The detection of changes, such as the onset of crystallization and immiscible interface zones.

Blending, such as adding the appropriate amount of filteraid to a filter feed stream.

b) Refractrometers


Principle: This involves the measurement of refractive index

of optically transparent material. When light is passed between two different media having different densities it will be refracted & Snell’s law applies:

As the composition of a sample changes its refractive index changes, this change in refractive index changes the critical angle occurring at interface between glass prism & sample.

α1, α2 are angles of incidence & η1, η2 are refractive indexes of the two media.

Industrial Instrumentation- Ms. Qandeel Almas

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c) Spectrophotometers


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SpectrophotometrySpectrophotometry is the quantifiable study of electromagnetic spectra. It deals with visible light, near-ultraviolet, and near-infrared.


Spectrophotometer A spectrophotometer is a photometer (a device

for measuring light intensity) that can measure intensity as a function of the light source wavelength.

The concentration of a substance in solution is measured by calculating the amount of absorption of light at the appropriate wavelength or a particular color.

The most common application of spectrophotometers is the measurement of light absorption.


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Working: A spectrophotometer consists of two instruments,

namely a spectrometer for producing light of any selected color (wavelength), and a photometer for measuring the intensity of light.

The instruments are arranged so that liquid in a cuvette can be placed between the spectrometer beam and the photometer.

The amount of light passing through the tube is measured by the photometer.

The photometer delivers a voltage signal to a display device, normally a galvanometer.

The signal changes as the amount of light absorbed by the liquid changes.

Applications: To measure the concentration of solutions like:

Alanine Amino Transferase Cholesterol D-malic acid Fruit juice Glucose Honey Olive oil Phosphate Urea

Analysis of: Drug Food Agri supplies

3. Electrochemical Composition Measurement

Thermal conductivity Measurement pH Measurement

Introduction: Electrochemical analysis in liquid solutions is

concerned with the measurement of electrical quantities, such as potential, current and charge, to gain information about the composition of the solution and the reaction kinetics of its components.

The main techniques are based on the quantitative determination of reagents needed to complete a reaction or the reaction products themselves.


pH Measurement

Introduction: pH is a measure of the acidity or basicity of a

solution. It is defined as negative the decimal logarithm of the hydrogen ion activity in a solution.

The pH is normally measured by chemical indicators or by pH meters. The final color of chemical indicators depends on the hydrogen ion concentration, and their accuracy is only from 0.1 to 0.2 pH units.


pH Meters - Working When one metal is brought in contact with

another, a voltage difference occurs due to their differences in electron mobility. Similarly, an electric potential develops when one liquid is brought in contact with another one, but a membrane is needed to keep such liquids apart

The pH probe measures pH as the activity of hydrogen ions surrounding a thin-walled glass bulb at its tip. The probe produces a small voltage (about 0.06 volt per pH unit) that is measured and displayed as pH units by the meter.


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1 = Solution being tested; 2 = Glass electrode, coated with special silica glass, and containing potassium hydroxide; 3 = Silver electrode; 4 = Hydrogen ions interact with silica glass bulb; 5 = pH meter converts voltage (potential difference) into pH reading; 6 = Reference electrode

Features:Advantage PH meters have good accuracy and

reproducibility. Disadvantages In poorly buffered solutions, accuracy may be

lost by the presence of suspensions and gels. Glass electrodes used for pH measurement are

hydrogen ion selective but they do not uniquely respond to hydrogen ions only, so they will also respond to sodium and other ions especially at alkaline pH. This effect causes the pH value to be underestimated.


Thermal Conductivity Measurement

http://en.wikipedia.org/wiki/File:Hd_sensor.jpg

Introduction: Thermal conductivity, k, is the property of a

material that indicates its ability to conduct heat. There are a number of possibilities to measure

thermal conductivity, each of them suitable for a limited range of materials, depending on the thermal properties and the medium temperature.

Two classes of methods exist to measure the thermal conductivity of a sample:

Steady-state methodsTransient methods


Steady-State method:Steady-state techniques perform a measurement when the temperature of the material measured does not change with time. This makes the signal analysis straightforward (steady state implies constant signals). The disadvantage is that a well-engineered experimental setup is usually needed


Transient Methods: The transient techniques perform a measurement

during the process of heating up. The advantage is that measurements can be made relatively quickly. Transient methods are usually carried out by needle probes.

Non-steady-state methods to measure the thermal conductivity do not require the signal to obtain a constant value. Instead, the signal is studied as a function of time.

The advantage of these methods are that they can in general be performed more quickly, since there is no need to wait for a steady-state situation. The disadvantage is that the mathematical analysis of the data is in general more difficult.Industrial Instrumentation- Ms. Qandeel Almas53

4. Analytical Techniques

* Chromatography

Chromatography: Chromatography is physio-

chemical method by which the components of a mixture can be separated.

It has become one of the primary analytical methods for the identification and quantification of compounds in the gaseous or liquid state.

The basic principle is based on the concentration equilibrium of the components of interest, between two immiscible phases.


Chromatography Detectors

A chromatography detector is a device that locates, in the dimensions of space and time, the positions of the components of a mixture that has been subjected to a chromatographic process and, thus, permits the senses to appreciate the nature of the separation.


On-line chromatographic analysis One of the most widely employed instruments for

online analysis of the samples of multicomponent gases or volatile liquids; is the gas liquid or gas solid chromatograph (GLC) or (GSC). This is an instrument which analyses discrete samples of material but the disadvantage is that it takes too much time to extract the sample & to produce the result from the signal.

It consists of Sampling assembly Chromatograph Detector


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lecture - composition analyzers

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