Prepared by : Eng. Amer H. RasheedControl & Telecom. Dept.

State Company for Oil Projects (SCOP)

Gas Chromatograph & Analyzer

By Eng. AMER .H RASHEED / SCOP / CONTROL & TELECOM. DEPT.

Index:1 -Gas Chromatography(GC)-:

1.1 -Sampling Mechanism

1.2 -Column

1.3 -Detectors1.3.1 -Thermal Conductivity Detector (TCD)

1.3.2 -Flame Ionization Detector (FID)1.3.3 -Flame Photometric Detector (FPD)

2 -Process Gas Analyzer-:

3 -Questions-:

1 -Gas Chromatography (GC)-: Is a method for separating the components of a

sample that contains mixture of gas components to a column, separates it in the column, then measures the concentrations of the components with a detector .

The process gas chromatograph consists of (mainly) a sampling mechanism, a column and a detector according to the (functional diagram of gas chromatograph). GC detectors that responds to some property difference between the carrier gas and the sample components is placed at the end of the column. It yields a signal that, when recorded as a function of time, produces the familiar chromatogram such that shown in following figure.the magnitude of the detector signal (peak height or peak area) is related to the amount of the compound present in the injected sample , If these individual retention times and detector responses are made to remain constant (by adequate control of carrier gas flow rate, column temperature, amount of stationary phase, and detector variables), the process can be automated so that the instrument will provide this information quite precisely and cyclicallyon a repetitive basis .

To summarize, the GC has the following distinguishing attributes:

•Located in the plant as close as possible to the sample point to minimize sample transport time

•Dedicated to monitor one or more components in one or more process streams.

•Designed for continuous, unattended operation. •Designed for operation in hazardous environments.

•Designed to withstand exposure to weather, humidity, dust, and corrosive atmospheres

•Contains integral hardware and software to allow communication with the process control system as well as remote maintenance stations.

Chromatogram of sample components .

Typical GC specification

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1.1 -Sampling Mechanism -: The sampling system must provide to the gas

chromatograph a homogeneous samples of the gas be analyzed , a continuous stream of the sample is taken, usually by means of a fast bypass loop, and treated as necessary—for example, by drying, filtering, or adjusting temperature or pressure. discrete volumes of the treated sample stream are periodically injected into the carrier gas stream of the chromatograph by means of a gas sampling valve. The most popular valves are; rotary, sliding plate, diaphragm mechanical, no-moving-part valves, and Deans switch .

The chromatograph is normally supplied with the sample from the point or points to be monitored by use of permanently installed sampling lines in the gas sampling valve, the Sampling loop is formed externally and typically has a volume in the range of 0.1-10 ml .

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1.2 -Column -:The packed column consists of a stainless pipe, 2mm in diameter and 0.2 to 2.0m in length and filled with a bulking agent called a stationary phase (high-boiling nonvolatile liquid that is suspended on an inert solid with a large surface area).The mega-bore and capillary columns, of approximate diameter 0.5 mm and 0.3 mm respectively.The components in the multi-component gas mixture sample with carrier gas, which called mobile phase move through the column, repeatedly absorbing the stationary phase and dissolving from the stationary phase at a certain cyclic rate conforming to a fixed partition coefficient that is unique to each component.Since the transfer rates differ depending on the partition coefficient, a multi-component gas mixture gradually separates into discrete components and is separated in the order of the transfer rates .

1.3 -Detectors-: The components separated in the column are led

to the detector where the concentration of each component is measured.The Process Gas Chromatographs can be fitted with thermal conductivity detectors (TCD), flame ionization detectors (FID) or flame photometric detectors (FPD). The thermal conductivity detector can measure almost all non-corrosive components but sensitivity is relatively low. On the other hand, the hydrogen flame ionization detector can measure hydrocarbon and the flame photometric detector can measure sulfur compounds, respectively with high sensitivity.

1.3.1 -Thermal conductivity detectors (TCD)-: Continuous thermal conductivity detectors

measure a change in heat dissipation by comparing the change with a reference condition.

Also it may called a katharometer or catharometer, the name still persists in Europe.This technique takes advantage of the facts that different substances have a varying capacity to conduct heat energy from a heat source .

This ability differs for each gas, it is called thermal conductivity and can be expressed in various unit systems such as BTU/hr/ft2/°F/in .

1.3.2 -Flame Ionization Detectors (FID)-:Flame ionization detectors (FIDs) are used to measure total hydrocarbons at low concentrations such as for pollution, leakage, or safety monitoring, The response is rapid and can be as sensitive as 0.1 ppm, The FID utilizes the fact that, although a hydrogen-oxygen flame contains relatively few ions (10^7 ions/cm-3), it does contain highly energetic atoms. When trace amounts of organic compounds are added to the flame, the number of ions increases (to approximately 10^11ions/cm-3) and a measurable ionization current is produced ,

The ion current is effectively proportional to the number of carbon atoms present in the flame.It has a high sensitivity, good stability & wide range of linear response, it is widely used as a gas-chromatographic detector and in total hydrocarbon analyzers.

1.3.3 -Flame Photometric Detectors (FPD)-:It is essentially a flame emission spectrometer design optimized for use as a GC detector. It is an element-specific detector and is used primarily for the determination of sulfur- or phosphorus-bearing compounds (e.g., H2S, CS2, SO2, COS, mercaptans, and alkali sulfides in various pulp milling processes and in petroleum fractions, it can measure sulfur concentrations in air down to levels less than 0.01 ppm.

The column effluent is fed to a hydrogen/oxygen-rich flame where individual atoms contained in the sample with hydrogen and oxygen are excited to higher electronic states by the energy of the flame. These excited atoms and molecular fragments subsequently return to the ground state with the emission of characteristic atomic or molecular band spectra .

(the excited S2 molecule reverts to the ground state, it emits a series of bands in the range 300–450 nm, with the most intense bands at 384.0 and 394.1 nm. The 384.0 nm emission is monitored by the photomultiplier tube.)

2 -Process Gas Analysis-: Process gas analyzers are used for continuous

determination of the concentrations of one or more gases in a gas mixture. Determination of the concentration of gases in a process is used to control and monitor process flows, and is therefore decisive for the automation and optimization of processes and ensuring product quality's .

A diode laser emits a beam of near-infrared light, which is sent through the process gas and detected by a receiver unit. The wavelength of the laser diode output is tuned to a gas specific absorption line. The laser is scanning continuously over this single absorption line with very high spectral resolution. The result is a fully resolved single molecular line which is analyzed in terms of absorption strengths and line shape (The absorption results from conversion of the radiation energy of the laser light into the internal energy of the molecule)The list of gas components measurable using NIR diode laser technology already comprises: O2, NH3, HCl, HF, H2O, CO, CO2.

Beer–Lambert Law-:A = abcA=absorbance , a =molar absorptivity , b=sample path length & c = concentration of absorbing

Questions?