HYDROCARBON ANALYZERS

INTRODUCTION

• To measure the concentrations of hydrocarbons , CO and CO2 in gas streams.

• Used in petrochemical industries and other chemical industries.

ANALYZER TYPES

• Flame ionization detectors

• Gas chromatography

• Spectrometric methods

– Infrared spectrometry

– Mass spectrometry

FLAME IONIZATION DETECTORS(destructive detectors)

PRINCIPLE

• The electrical conductivity of a gas is directly proportional to the concentration of charged particles within the gas.

• Good sensitivity.

FID

WORKING

• Used to measure total hydrocarbons at low concentrations.

A flame of pure hydrogen, which contains almost no ions, is

used.

• Hydrogen flame ionizes the carbon atoms, resulting in a large number of ions in the flame.

• Ionized carbon atoms are produced when burned in a hydrogen flame.

• The ions formed in the burning process cause an electrical conduction between two electrodes in the detector that is amplified by a highly sensitive electrometer-amplifier circuit.

• The electrical output of the electrometer-amplifier is directly proportional to the quantity of flame ionizable hydrocarbons present.

ADVANTAGES:

• Inexpensive

• Low maintenance requirements

• Rugged construction

• Measure organic substance concentration at very low and very high levels

DISADVANTAGES:

• FID’S cannot detect inorganic substances(Carbon monoxide and Carbon dioxide ).

• It oxidizes all compounds that pass through it.

GAS CHROMATOGRAPHY

• Gas chromatography (GC) is an analytical method which is used for the separation of an volatile substance from a given mixture of compounds .

• Sample is introduced in the liquid/gas (mobile phase) form with the help of GC syringe into the injection port.

• It gets vaporized at injection port then passes through column (contains solid coated stationary phase) with the help of continuously flowing carrier stream, separated/detected at the detection port .

• Carrier medium can be liquid or gas for the ease of separation/detection, if it is gas then called gas chromatography otherwise called liquid chromatography.

GC

TEMPERATURE DEPENDENCE :

• Reducing the temperature produces the greatest level of separation, but can result in very long elution times.

CHOICE OF CARRIER GAS :

• Hydrogen being the most efficient and providing most separation . But helium has larger flow rates & is non-flammable . It’s the most common carrier gas used.

DETECTORS :

• Most common are the Flame ionization detector (FID) and the thermal conductivity detector (TCD).

COLUMN TYPES:

Packed Column:

• It is filled with a coated inert solid support such as fire brick, alumina, and graphite with a specific mesh size.

• Advantages : higher capacity

• Disadvantages: low resolution& low sensitivity.

Capillary Column:

• The phase (film) is coated with polyimide film to increase their strength.

• Advantages: high resolution and greater sensitivity.

• Disadvantages: low capacity .

ADVANTAGES :

• Has a high resolution power and has high sensitivity when used with thermal detectors.

• Gives good accuracy and precision.

• Separation and analysis of sample very quickly.

• Sample with less quantity is also separated.

DISADVANTAGES:

• Only volatile samples are separated .

• During injection of the gaseous sample proper attention is required.

SPECTROMETRIC METHODS

MASS SPECTROMETRY:

• To measure the characteristics of individual molecules, a mass spectrometer converts them to ions .The three essential functions of a mass spectrometer, and the associated components, are:

– A small sample is ionized

– Ions are sorted and separated according to their mass and charge.

– Separated ions are then measured, and the results displayed on a chart.

MS

IONISATION:

• Vaporized sample passes into the ionization chamber. The electrically heated metal coil gives off electrons which are attracted to the electron trap which is a positively charged plate.

• Positive ions are persuaded out into the rest of the machine by the ion repeller which is another metal plate carrying a slight positive charge.

ACCELERATION:

• Ions are accelerated so that they all have the same kinetic energy.

IONIZATION CHAMBER ACCELEARTION

DEFLECTION:

• Different ions are deflected by the magnetic field by different amounts. The lighter they are, the more they are deflected.

• Amount of deflection depends on:

– the mass of the ion.

– the charge on the ion.

• These two factors are combined into the mass/charge ratio.

DETECTION:

• Flow of electrons in the wire is detected as an electric current which can be amplified and recorded.

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