Chromotograph Analyzer

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Note: The source of the technical material in this volume is the Professional

Engineering Development Program (PEDP) of Engineering Services.

Warning : The material contained in this document was developed for Saudi

Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

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Engineering Encyclopedia Instrumentation Evaluate Gas Chromatograph Analyzers

Content Page

INTRODUCTION ... 1

EVALUATING GAS CHROMATOGRAPH ANALYZERS... 2

Function of Gas Chromatograph Analyzers... 2

Separation Theory ... 3

Practical Implementation... 7

General Applications ... 8

Refineries... 8

Gas Separation Plants ...11

HARDWARE ELEMENTS AND COMPONENT SEPARATION TECHNIQUES OF GAS CHROMATOGRAPH ANALYZERS ... 15

Gas Chromatograph Analyzer Configuration...15

Oven/Temperature Control...16

Isothermal...16

Programmed Temperature...16

Carrier Gas Supply System ...17

Carrier Gases ...17

Flow/Pressure Regulation ...18

Detectors ...19

Thermal Conductivity Detectors...20

Ionization Detectors...24 Flame Ionization ...24 Photo ionization...26 Flame Photometric...27 Electron Capture...27 Columns...28 Adsorption Columns ...29 Partition Columns ...29 Packed Columns ...30 Capillary Columns...30 Valves ...30

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Functions ...30

Types of Sample Valves...32

Electronic Controllers...35

Timing ...36

Measurement Data Processing Functions ...36

System Diagnostics ...42

Component Separation Techniques...44

Backflush...44

Heartcut ...46

Column Stepping ...47

Trap/Bypass...48

Programmed Temperature...50

GAS CHROMATOGRAPH NETWORK CONFIGURATIONS ... 52

Gas Chromatograph/Process Control Computer Interface...52

Data Transmission ...52

Hardware Variations ...54

Practical Considerations of Serial Communication Techniques ...57

Use of Personal Computers ...59

Data Processing and Reporting ...59

SQC Techniques ...60

Network Capabilities ...62

Control Capabilities ...62

Architecture and Operation of Various Networks...63

Network Integrity and Redundancy...65

Network Topology ...66

Multi-Analyzer Integration...67

INSTALLATION, OPERATIONAL, AND MAINTENANCE CONSIDERATIONS... 69

Installation Considerations...69

Installation Data (Items 7 through 20)...69

Operational Considerations...73

Sample Supply Data (Items 21 through 28) ...73

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Sample Return Data (Items 39 through 46) ...79

Analyzer Return Data (Items 47 through 50)...80

Output Signal Data (Items 51 through 54) ...80

Performance Data (Items 55 through 58) ...81

Special Data Section (Items 69 through 72) ...85

Stream Composition Data (Item 73)...85

Maintenance Considerations ...86

Calibration Data (Items 59 through 62) ...86

Maintenance Data (Items 63 through 68) ...87

EVALUATING GAS CHROMATOGRAPH ANALYZERS IN RELATION TO REQUIREMENTS FOR SPECIFIC APPLICATIONS... 90

Refinery Application...90

Fractionation Tower ...90

REFERENCES ... 98

WORK AID 1: RESOURCES USED TO EVALUATE GAS CHROMATOGRAPH ANALYZERS IN RELATION TO REQUIREMENTS FOR SPECIFIC APPLICATIONS... 99

Work Aid 1A: Resources Used to Evaluate the Instrument Air Pressure and Quality for Gas Chromatograph Analyzers ...99

Work Aid 1B: Resources Used to Evaluate the Measurement Response Time for Gas Chromatograph Analyzers ...100

Work Aid 1C: Resources Used to Evaluate the Phase of the Process Sample for Gas Chromatograph Analyzers...101

GLOSSARY ... 102

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List Of Figures Page

Figure 1: Basic Chromatography Process ... 1

Figure 2: Types of Chromatography... 1

Figure 3: Block Diagram of a Typical Gas Chromatograph Analyzer ... 2

Figure 4: Establishment of Equilibrium Between a Gas Sample and a Liquid Solvent ... 3

Figure 5: Simplified Example of a Single Sample Gas Component in a Column... 4

Figure 6: Plot of Component A Equilibria vs. Number of Theoretical Plates (13)... 5

Figure 7: Typical Chromatogram... 6

Figure 8: Gas Chromatograph ... 7

Figure 9: Applications of Gas Chromatograph Analyzers... 8

Figure 10: Typical Distillation Tower Process Flow Diagram ... 9

Figure 11: Typical Gas Chromatograph Analyzer in a Gasoline Blending System...11

Figure 12: Expander Gas Plant...12

Figure 13: Configuration of Gas Chromatograph Analyzer ...15

Figure 14: Isothermal and Programmed Temperature Profiles ...16

Figure 15: Properties of Common Carrier Gases(3)...17

Figure 16: Recommended Carrier Gas Cylinder Installation...18

Figure 17: Guide to the Selection of Gas Chromatograph Detectors...19

Figure 18: Thermal Conductivities of Typical Gases - k in [cal/(sec)(cm2)(°C/cm) x 10-6] at 38° C...20

Figure 19: Two Thermal Conductivity Detectors in a Gas Chromatograph Analyzer ...20

Figure 20: Wheatstone Bridge...21

Figure 21: Thermistor Bead Detector...22

Figure 22: Filament Elements in a Measuring Cell Block(5)...23

Figure 23: Ionization Detector(4)...24

Figure 24: Compounds That Give Little or No Response to the Flame Ionization Detector(4) ...24

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Figure 26: Methanator ...25

Figure 27: Air Treater ...26

Figure 28: Photo Ionization Detector ...26

Figure 29: Schematic of a Flame Photometric Detector ...27

Figure 30: Electron Capture Detector ...28

Figure 31: Enlargement of Support and Liquid Stationary Phase ...29

Figure 32: Packed Column ...30

Figure 33: Atmospheric Referencing Valve (ARV)...31

Figure 34: Typical Rotary Valve ...32

Figure 35: Typical Slider Valve for Gases (1)...33

Figure 36: Slider Valves for Liquids(1) ...33

Figure 37: Six-Port Plunger Diaphragm Valve...34

Figure 38: Transport Injection Valve...35

Figure 39: Peak Detection...36

Figure 40: Slope Gating ...37

Figure 41: Example of Baseline Shift...38

Figure 42: Selecting Bottom Line of Peak ...38

Figure 43: Identification of Unknown Peaks by Use of Standard ...39

Figure 44: Shoulder Peak...40

Figure 45: Peak Resolution ...41

Figure 46: Illustration of Column and Solvent Efficiency(4) ...42

Figure 47: Diagnostic Functions of Electronic Controller ...43

Figure 48: Ideal Backflush(1)...44

Figure 49: Real Backflush(1)...45

Figure 50: Valve Configuration for Backflush to Vent Using a Single Column(4) ...45

Figure 51: Backflush to Vent Using Two Columns...46

Figure 52: Heartcut Procedure ...47

Figure 53: Reverse Column Step...48

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Figure 55: Constant Temperature Technique...50

Figure 56: Programmed Temperature...50

Figure 57: Schematic of Programmed Temperature Gas Chromatograph Analyzer ...51

Figure 58: Analog Circuit...52

Figure 59: Basic Serial Representation ...54

Figure 60: Voltage Amplifier for Serial Communication ...56

Figure 61: Pareto Chart...57

Figure 62: Normal Distribution ...61

Figure 63: Typical Analyzer Network...62

Figure 64: Typical Master/Slave Network ...64

Figure 65: Typical Masterless Network ...65

Figure 66: Network Topology...66

Figure 67: Electronics Interface Device...68

Figure 68: Ras Tanura Plant 40 Depropanizer Process Flow Diagram ...91

Figure 69: Instrument Specification Sheet for an Gas Chromatograph Analyzer...92

Figure 70: Typical Analyzer Manufacturer’s Data Sheet for a Gas Chromatograph Analyzer...93

Figure 71: Typical Process Gas Chromatograph Analyzer Specific Application Data Sheet for Depropanizer Product Application ...94

Figure 72: Depropanizer Overhead Application Gas Chromatograph Analyzer Sample Handling System Diagram ...95

Figure 73: Process Analyzer Evaluation Flow Chart ...104

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INTRODUCTION

Chromatography is the physical process of separating the various components of a gas or vapor mixture into pure fractions of each component. The chromatography process is a batch process in which a small sample of a multicomponent mixture is transported by a mobile phase through a long, narrow tube called a column (Figure 1). The column contains an absorbent material called the stationary phase. As the components in the sample mixture migrate through the column, they are separated by the stationary phase based on the differences in their chemical and physical properties. The sample exits the column as individual components, which are grouped together.

Figure 1: Basic Chromatography Process

There are four types of chromatography, all of which are classified by the types of mobile and stationary phases that they use (Figure 2). The mobile phase may be a liquid or a gas. The stationary phase can be a liquid or a solid. This module will focus on gas-liquid chromatography, which will be simply referred to as gas chromatography.

Mobile Phase Stationary Phase

Liquid Liquid

Liquid Solid

Gas Liquid

Gas Solid

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EVALUATING GAS CHROMATOGRAPH ANALYZERS Function of Gas Chromatograph Analyzers

The function of gas chromatograph analyzers is to separate and analyze very small amounts of components that are in multicomponent streams. The analysis is both quantitative and qualitative. A quantitative analysis determines now much of each component is in the sample. A qualitative analysis provides information about the identity of the components.

Figure 3 shows a block diagram of a typical gas chromatograph analyzer. The carrier gas carries the sample through the column and to the detector. The sample valve collects a precise measured amount of sample and injects the sample into the carrier gas stream. The column separates the sample into individual components. The detector, which is located at the end of the column, senses the individual components or bands as they elute off the column and pass through it. The detector generates a signal, which is amplified and plotted as a chromatogram.

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Separation Theory

Gas chromatography involves the use of an inert carrier gas to transport a multicomponent sample past a stationary phase (non-volatile solvent) in the column. The sample is partitioned between the carrier gas and the solvent. The solvent retains each sample component for a period of time based on the component’s solubility in the solvent. In order to understand the interaction between the sample and the solvent, it is necessary to review how gases dissolve in liquids. Figure 4A shows the introduction of a pure gas sample into a closed vessel that contains air and a liquid solvent. In Figure 4B, the gas sample dissolves in the solvent to a point of equilibrium where the tendency for more gas to dissolve is balanced by the tendency for some of the dissolved gas to come out of solution.

Figure 4: Establishment of Equilibrium Between a Gas Sample and a Liquid Solvent

The ratio of the amount of the gas in the air and the amount of gas in the liquid is called the

partition coefficient, K, where

K= Concentration of gas in the liquid phase

Concentration of gas in the gas phase

K indicates the degree of solubility of the gas. In Figure 4, it was assumed that K=1, that is, the

gas sample is evenly distributed in the air and liquid phases. If more gas is introduced into the vessel, half of the gas would dissolve to re-establish equilibrium (up to the point of saturation).

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The situation inside of a gas chromatograph column is much more complex than the previous example because the gas sample contains several components and the sample is constantly

transported over the stationary phase by a carrier gas. It is necessary to describe what happens to one of the gas components (Component A) in order to understand how all of the components in the sample are separated. To understand how the column operates, imagine the column as a series of closed vessels (Figure 5).

Assume that the partition coefficient of Component A in the solvent is K_A=1 (equally soluble in the carrier gas and solvent). (1) After the carrier gas pushes Component A into the first sealed vessel, Component A is allowed to reach equilibrium between the carrier gas and the solvent (2). After equilibrium is reached, the carrier gas is allowed to push the remaining sample in the gas phase into the next vessel (3). After the carrier gas pushes the sample out of the first vessel, equilibrium between the gas and liquid phase is established in both the first and second vessels (4). The carrier gas pushes the sample component from the first and second vessels into the second and third vessels (5). Equilibrium is re-established in the three vessels (6). Notice that the component concentrations in the leading and trailing vessels are lower than the concentration in the intermediate vessel.

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The imaginary vessels in this example are analogous to the number of theoretical plates, which was originally proposed to model the performance of distillation columns. If the procedure in Figure 6 is allowed to continue over five vessels (theoretical plates), the results would be as shown in Figure 6. The equilibria for the five theoretical plates are shown graphically as the solid line.

Figure 6: Plot of Component A Equilibria vs. Number of Theoretical Plates (13)

If the procedure is repeated over 11 and 21 steps, the resulting concentrations would appear as the dashed lines in Figure 6. In each case, the sample component is distributed through all the plates with the maximum concentration at the center plate. Notice that the distribution curve becomes narrower as the number of theoretical plates increases. The narrower the peak, the more efficient the separation of the Component A. For large numbers of plates, the center of the distribution curve starts to resemble the characteristic shape of a chromatogram peak (Figure 7).

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Figure 7: Typical Chromatogram

The greater the solubility of each component in the sample, the longer its retention time, t_R, in the column. Thus, the sample is separated into individual components as it flows through the column. The width of the peak, W, is length of the base line that intersects the two tangents to the peak. The width of the curve at one-half the height of the peak is represented by the variable W_0.5. The number of theoretical plates that are required for separation is calculated from the chromatogram measurements by using the expression:

N=5.54 tR W_0.5       2 where:

N = number of theoretical plates

t_R = retention time of the component

W_0.5 = the width of the peak at half height

The column efficiency is expressed best as a quantity called the plate height, or the height

equivalent to a theoretical plate, H, which has a dimension of distance. The plate height is related to the peak width from the chromatogram; therefore, it is possible to calculate the plate height as shown in the expression:

H= L 16 W_0.5 t'R       2 where: H = plate height

L = length of the column

W_0.5 = peak width at half the peak height

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Practical Implementation

Figure 9 shows an analysis of a C₃-C₅ blend, which might be used in a refinery. The separation in this example is good. The column is 3.5m in length, N was calculated to be 1498, and H was calculated to be 1.001 mm. The analysis time, or cycle time, for this sample is 3 minutes. Complete component separation is sometimes sacrificed for cycle requirements of the process.

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General Applications

Gas chromatograph (GC) analyzers measure the percent concentration or ppm of the components that make up gas streams, equilibrium vapor streams, and liquid streams that can be vaporized without affecting the stream composition. This information is used for process monitoring and process control. Some of the applications of gas chromatograph analyzers are shown in Figure 10 and they are described on the following pages.

Category Applications

Refineries Distillation towers, fuels blending Gas Separation

Plants

Product quality control, measurement of BTU content

Figure 9: Applications of Gas Chromatograph Analyzers

Refineries

Distillation Towers - One of the most common applications for a process gas chromatograph is to provide compositional data to the control system of a distillation tower. A distillation tower is used to separate a chemical process stream into two or more product streams. Figure 11 shows a diagram of a crude distillation unit, which separates crude oil into various blends by distilling the crude into fractions according to boiling point. First, a stabilizer fractionates to total crude to remove butanes and lighter components. Second, the stabilized crude is fed to a distillation tower which operates at atmospheric pressure. The overhead stream from the tower, light straight run naphtha, is used in gasoline blending or it is further processed in an isomerization unit. The bottoms product from the tower is fed to a vacuum crude tower.

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Figure 10: Typical Distillation Tower Process Flow Diagram

In order of increasing boiling points, the main products (side draws) from a typical distillation tower are as follows:

Fuel Gas The fuel gas consists mainly of methane and ethane. In some refineries, propane is included in the fuel gas stream. This stream also called a "dry gas." (C₁, C₂, C₃).

Wet Gas The wet gas stream contains propanes and butanes as well as some methane and ethane. The propanes and butanes are separated to be used for LPG and, in the case of butane, for gasoline blending. (C₂, C₃, C₄).

LSR Gasoline The Light Straight Run (LSR) gasoline is desulfurized and used in gasoline blending or processed in an isomerization unit to improve the octane number before blending. (C₅, C₆).

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HSR Gasoline The Heavy Straight Run (HSR) gasoline cuts are typically used for catalytic reformer feed to produce high octane reformate for gasoline blending and aromatics recovery. (C₇ to C₁₀).

Kerosene The kerosene stream is treated and then sent to the blending pool for sale as kerosene product. (C9 to C15).

Diesel The diesel cut is treated and then sent to the bending pool for sale as diesel fuel. (C₁₃ to C₁₈).

Gas Oils The Light Gas Oil (LGO) and Heavy Gas Oils (HGO) are processed in a

hydrocracker or catalytic cracker to produce gasoline, jet, and diesel fuels. The light vacuum and heavy vacuum gas oils (LVGO and HVGO) can also be used as feedstocks for lubricating oil processing units. (C₁₃ to C₄₅ ).

Residuum The vacuum tower bottoms can be processed in a visbreaker coker, or

deasphalting unit to produce heavy fuel oil or cracking and/or lube base stocks. For asphaltic crudes, the residuum can be processed further to produce road and/or roofing asphalts. (C₄₀ and up).

Analyzer Placement in Crude Oil Distillation Unit Analyzer No. Stream Component Analyzed Purpose 1 LSR Naphtha Simulated Distillation

Insure proper separation of LSR Naphtha from lighter material.

2 HSR Naphtha Simulated

Distillation

Insure proper separation of HSR and kerosene.

3 Kerosene Simulated

Distillation

Insure proper separation of kerosene from naphtha and diesel.

4 Diesel Simulated

Distillation

Insure proper separation of diesel from kerosene and gas oil.

Fuels Blending - Gasolines that are produced by straight-run distillation, catalytic cracking, hydrocracking, and reforming must be blended into various grades of products that will perform well under varying weather conditions, altitudes, and engine compressions. The gasoline blending unit blends the various upstream products according to formulations to meet these product

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The critical properties specified for gasoline are vaporization and combustion. These properties are measured as the Reid Vapor Pressure (RVP), Vapor to Liquid Ratio (V/L), and Knock

Intensity or Octane Number. These measurements have traditionally been made with physical

property analyzers; however, their high maintenance requirements and imprecise results limit their effectiveness. The measurements can now be made with a programmed temperature GC analyzer equipped with fused silica open tubular capillary column and utilizing a flame ionization detector. This GC analyzer measures gasoline components by separating them according to their boiling points. The boiling points are related to the volatility of the gasoline, which is the key property for combustion. Although the distillation boiling points do not provide complete information, they do provide an indication that can be used to ensure that the key parameters are met. The 10% off boiling point is related to the RVP of the final product. The 50% off boiling point is related to the V/L point. A typical example of a gas chromatograph analyzer in a 10% off boiling point application is shown in Figure 12.

Figure 11: Typical Gas Chromatograph Analyzer in a Gasoline Blending System

Gas Separation Plants

Product Quality Control - When process analyzers are used to determine whether or not a process stream conforms to product specification, the application is classified as product quality control. Although final certification of a product is usually made by laboratory analyzers, on-line analysis of product quality control avoids potential off-spec operation.

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In many instances, process are operated to produce product that has a higher quality than is required by specification in an effort to prevent off-spec product. Producing higher quality product leads to more costly operation and results in product giveaway. Product quality control with line process analyzers allows more efficient process operation not only by maintaining on-spec operation but also by minimizing product giveaway.

Expander gas plants (Figure 13) are designed to recover ethane in large quantities for feed stock to ethylene plants. Typically, expander gas plants take gas out of a natural gas pipeline and return the residue gas to the pipeline. The plants are billed for shrinkage, which is the BTU decrease between the feed and the returned residue gas. The residue gas must be returned at the same or higher pressure as it was taken out of the pipeline. The typical expander plant has three sites for analysis by a process gas chromatograph. The first analysis site is the feed to the plant in order to determine the composition of the feed gas and to calculate its BTU content, which is shown in the following example. After the analysis, the gas is compressed from about 500 to 800 psi pressure. A valve splits the gas to either a reboiler medium to warm up the demethanizer or to feed the demethanizer.

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The gas in the feed line to the demethanizer is cooled as it goes through a cold gas to gas heat exchanger. The gas then goes through an inlet separator to remove condensed liquids and through an expander to quickly drop its pressure to about 150 psig. At this point, the gas is extremely cold (-100°F) and liquids begin to form. These liquids are fed into the top of the demethanizer tower. The warmer liquids that condensed in the inlet separator are fed into the middle of the tower. The overhead product stream that leaves the top of the tower is mostly methane with some ethane. After the overhead stream exchanges heat with the feed gas, it is recompressed and trimmed to the exact pressure of the gas pipeline.

The plant tries to produce an ethane and heavier fraction with a certain methane to ethane ratio spec going to the NGL product pipeline. Consequently, the most important analysis site is the Demethanizer bottoms in order to control the methane to ethane ratio.

The overhead stream is the final sample site in the gas plant where the ethane and the BTU content of the gas is measured. The ethane measurement shows how well the plant is operating and the BTU content of the gas determines the shrinkage across the plant.

A number of plants will simply try to use a calorimeter to measure the BTU content of the gas; however, the BTU content of the gas can be changed by the amount of nitrogen or carbon dioxide in the gas. A process chromatograph has a distinct advantage since it both determines

composition and measures the BTU of the overhead stream.

Measurement of BTU Content - Gas chromatographs provide on-line measurement of the BTU content of gas mixtures by separating and measuring the concentrations and heating values of all the components in the sample. The ideal BTU content of the gas mixture is calculated by totaling the product of the mole fraction of each component and its ideal BTU value. A sample

calculation is shown below.

Concentration Mole BTU Value BTU

Component (mole %) Fraction (BTU/ft3) (BTU/ft3)

Nitrogen 2.39 0.0239 0.0 0.0 Carbon Dioxide 2.53 0.0253 0.0 0.0 Methane 85.83 0.8583 1012.1 868.7 Ethane 4.99 0.0499 1773.0 88.5 Propane 2.51 0.0251 2523.3 63.3 iso-Butane 0.50 0.0050 3260.7 16.3 n-Butane 0.50 0.0050 3269.8 16.3 iso-Pentane 0.25 0.0025 4009.7 10.2 n-Pentane 0.25 0.0025 4018.9 10. 1 Hexane + 0.25 0.0025 0323.6 13.3 100.00 1.0000 1086.7 BTU/ft3

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The BTU value is referred to as "ideal" because it has not been corrected for errors that are caused by the less than ideal behavior of gases. Since some gases are more compressible than others, the compressibility of the individual gases needs to be accounted for in order to get a more accurate BTU value. The compressibility factor (or Z factor) is then applied to the ideal BTU to arrive at a real BTU. The sample calculation below assumes a Z factor of 0.9974.

Real BTU= ideal BTU

Z factor =

1086.7

0.9974 =1089.5BTU/ft 3

Gas chromatographs may perform satisfactorily when they are used to measure simple gas mixtures like natural gas; however, complex mixtures require longer cycle times. In the case of BTU control systems for furnaces and boilers, long cycle times are not acceptable.

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HARDWARE ELEMENTS AND COMPONENT SEPARATION TECHNIQUES OF GAS CHROMATOGRAPH ANALYZERS

Gas Chromatograph Analyzer Configuration

The configuration of a typical gas chromatograph is shown in Figure 13. The electronics section controls the automatic timing of the analyzers batch process, and it controls the sample system. With a smart sample system, the electronics section can monitor its condition. The chromatograph oven, which is controlled by the electronics section, can either be isothermal or have a

programmed temperature ramp. The oven is controlled to maintain the temperature of the sample valve, the columns, and the detector. The sample conditioning section is one of the most critical parts of the analyzer system. An improperly designed sample system is one of the primary reasons for analyzer failures and high maintenance costs. With modern electronics, the sample system is being monitored so that if a failure occurs, the sample system is shut down to protect the analyzer. Carrier gas, which is usually stored in pressurized cylinders, is supplied to the analyzer through a two-stage regulator.

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Oven/Temperature Control

The operation of the oven is critical to the reproducible and reliable operation of the analyzer because it has a dramatic effect on the retention times and column performance. The temperature of the oven is also critical to the components of the oven. The sample valve’s temperature must be hot enough to vaporize a liquid sample quickly, but not hot enough to thermally decompose the sample. The column temperature should be hot enough to achieve the desired component separation within the desired cycle time; however, the temperature should be low enough to extend the life of the columns. The detector’s temperature must be hot enough to avoid condensation of the sample.

The oven temperature can be maintained at a constant value (isothermal), or the oven temperature may be increased over time (programmed temperature). The operations of isothermal ovens and programmed temperature ovens are described below.

Isothermal

The isothermal oven must maintain a constant temperature (Figure 15) over the entire column, and it should be able to maintain the temperature to ±0.1°C. The temperature gradient (the difference between the highest and lowest mean temperatures) should be kept to a minimum. The operating range of the oven should be from 5-10° above ambient up to 400°C.

Programmed Temperature

Temperature programming is used to separate gas components with a wide range of boiling points. The oven temperature is gradually increased after the sample is injected to speed up the elution of the components with high boiling points. The programmed temperature technique is used mostly in the laboratory; however, some manufactures offer the option for process GCs. The column is heated at rates that vary from 0.25°C/min to 20°C/min (Figure 14). The oven should be able to heat the column from ambient temperature to 400°C within 40 minutes. The programmed temperature oven should be able to maintain the actual temperature to ±2°C of the desired temperature.

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Carrier Gas Supply System

The function of a carrier gas system is to supply a clean, dry, and constant pressure of carrier gas to the analyzer. The choice of carrier gas primarily depends on the type of detector that is used (although for best column efficiency nitrogen would be preferred). For example, thermal

conductivity detectors require the use of helium of hydrogen because they provide the maximum thermal conductivity difference (sensitivity) between the carrier and the sample components. Nitrogen is used when hydrogen in the sample needs to be measured, which provides maximum sensitivity for the detector. When hydrogen needs to be measured along with other components, two carrier gases (nitrogen and helium or hydrogen) and two detectors are recommended. Nitrogen is for the hydrogen measurement, and helium or hydrogen is for measurement of the other components.

Carrier Gases

Carrier gases carry the sample components from the sample valve through both the column and the detector. The goal of the carrier gas is to provide a stable transport and detection medium for the sample components. The carrier gas should be:

• inert to avoid reaction with the sample or solvent • pure and dry

• appropriate for the detector

The properties of common carrier gases are shown in Figure 15. Helium, hydrogen, and nitrogen are the most commonly used carrier gases.

Molecular Weight Thermal Conductivity λ x 105 at 100°C (g-cal/sec-cm-°C) Viscosity η x 10-6 100°C (µP) Argon 39.95 5.087 270.2a Carbon dioxide 44.01 5.06 197.2 Helium 4.00 39.85 234.1 Hydrogen 2.016 49.94 104.6b Nitrogen 28.01 7.18 212.0 Oxygen 32.00 7.427 248.5c a _{At 99.6°C} b _{At 100.5°C} c _{At 99.74°C}

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Flow/Pressure Regulation

Reliable operation of the gas chromatograph is dependent on a carrier gas flow rate that is accurately controlled. Varying carrier flow rates will effect retention times of the components as well as affect the stability of the detector. Pressure regulators that are not affected by ambient temperature changes should be used. Carrier gas pressure should be supplied to the analyzer at about 15 psi above column head pressure to provide optimum conditions for the pressure

regulator at the analyzer. Two cylinders of carrier gas should be used for each analyzer, and they should be installed as shown in Figure 16. The cylinders are installed so that one cylinder can be replaced without affecting the carrier gas flow through the analyzer.

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Detectors

The detector responds to the gases that emerge from the column. In practice, detectors operate on a deferential principal; that is, they show a constant base line with carrier gas flowing, generate a signal as the components pass, and then return to baseline. The signal that is generated is used to measure the amount of component that passed through the detector. Due to their stability in a harsh environment and their sensitivity to a wide range of components, the following detectors have become the limited choices in process gas chromatographs:

• thermal conductivity detectors (TCD) • flame ionization detectors (FID) • photo ionization detectors (PD) • flame photometric detectors (FPD) • electron capture detectors (ECD)

Thermal conductivity and flame ionization detectors are most commonly used. The type of

detector that is selected is based on the type and concentration of the components to be measured. The detector is usually selected by the analyzer vendor. Figure 17 shows a guide that is used for selecting a detector based on the type and concentration of the component. All five detectors will be explained on the following pages.

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Thermal Conductivity Detectors

Thermal conductivity is the ability of a substance to transmit heat by conduction. Thermal conductivity detectors use this thermodynamic property of gas to detect the components as they exit the column. As a general rule, the lower the molecular weight (MW) of a gas molecule, the higher is its thermal conductivity. To illustrate the differences in the thermal conductivities of gases, assume that two identical ingots of steel are heated to the same temperature. One ingot is placed in an atmosphere of pure helium (MW=4), and the other ingot is placed in an atmosphere of pure nitrogen (MW=28). The ingot of steel in the helium atmosphere will cool faster due to the higher thermal conductivity of helium.

The thermal conductivity technique is appropriate for GC analysis because the thermal conductivity of the carrier gas (hydrogen or helium) is significantly different from the thermal conductivities of sample gas components (see Figure 18). The thermal conductivity of a gas changes slightly with the operating temperature.

Argon 44.22 Hydrogen 458.72

n-Butane 40.91 Methane 85.54

Ethane 54.50 Nitrogen 64.06

Ethylene 52.07 Oxygen 65.91

Helium 368.63 Propane 45.46

Figure 18: Thermal Conductivities of Typical Gases - k in [cal/(sec)(cm2_{)(°C/cm) x 10-6] at 38° C}

Two thermal conductivity detector elements are used for stability. The two detectors are sometimes split into two pairs (Figure 19). One detector is used to measure the separated

components and the reference detector is subjected to pure carrier. These two detectors should be identical and exposed to the same conditions. The detectors are then electrically connected in a Wheatstone bridge circuit, or in a constant temperature circuit.

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The Wheatstone bridge (Figure 20) is used to measure any change in the resistance of the elements. The bridge provides the best measurement results when it is balanced. The bridge is balanced when the measuring and reference gases have the same thermal conductivity

characteristics. With carrier gas flowing across both detectors, the circuit is then balanced, or zeroed, electronically. Zeroing causes the circuit to force the reference detector to operate at the same temperature as the measuring detector.

With a constant voltage applied and the circuit balanced, components cross the measuring element causing a change in its resistance. A corresponding increase in current, which is the basis for the output, will occur. Due to the increase in current, detectors using this circuit are susceptible to burning out.

With a constant voltage detector circuit, as the resistance of the element changes, the voltage is adjusted to maintain the same current. The change in voltage becomes the basis for the output. This circuit dramatically improves the life of the elements.

Thermal conductivity detectors are selected, typically, when the sample gas component

concentrations are greater then 0.1 mole %. The detector element can either be a filament or a thermistor. The element that is chosen must respond accurately to variations in carrier gas

impurities and sample component concentrations. The operation of the filament and the thermistor are described below.

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Thermistor - Thermistors are sintered mixtures of manganese, cobalt, and nickel oxides, plus trace elements, all of which give them the desired electrical properties. The thermistor, which is in the form of a small bead, is mounted on a platinum wire. The thermistor is then coated with glass to make it inert. For this reason, a thermal conductivity detector that uses a thermistor is often referred to as a thermistor bead detector.

Figure 21 shows two thermistors in the measuring cell block. The two thermistors are a matched set. The two elements are heated by the electrical current, which flows through them. The measuring cell, in which the element resides, is connected to the effluent of the chromatograph column. The element is cooled, to some extent, by the carrier gas flow. As the gas components elute off the column, less of the heat generated by the thermistor is removed, and the resistance of the element changes. Because the gas component absorbs less of the heat than the carrier gas, an imbalance in the circuit will occur proportional to the change in thermal conductivity. This change is directly related to the concentration of the component. The output of the detector is plotted over time in the form of a chromatogram.

Figure 21: Thermistor Bead Detector

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Filament - Figure 22 shows a duel pair of filaments in a measuring cell. The four filaments are heated by an electrical current that flows through them. The measuring element cell, where the filaments reside, is connected to the effluent of the chromatograph column. The filaments are cooled, to some extent, by the carrier gas flow. As the gas components elute off the column, less of the heat generated by the filament is removed, and the resistance of the element changes. Because the gas component absorbs less of the heat than the carrier gas, an imbalance in the circuit will occur proportional to the change in thermal conductivity. This change is directly related to the concentration of the component. The output of the detector is plotted over time in the form of a chromatogram.

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Ionization Detectors

An ionization detector is used when trace amounts of gas molecules need to be measured. In general, the operation of ionization detectors is based on the fact that the electrical conductivity of a gas is proportional to the concentration of charged particles (ions) within the gas. Figure 23, shows an ionization detector with an unspecified ionizing source. The ionization source ionizes the molecules in the effluent gas from the column. The presence of ions within the electrode gap causes a current, I, to flow through a measuring resistor, R2. The resulting voltage drop E₀ is amplified by an electrometer, and the signal is sent to a recorder. The "bucking voltage" is used to zero the circuit when only carrier is flowing.

Figure 23: Ionization Detector(4)

Two types of ionization detectors are used in gas chromatography, flame ionization and photo ionization. Both detectors are used to measure gas component levels from 0.5% to levels as low as a few ppb. Although both detectors operate on the same principals, there are several

differences, which are described below. Flame Ionization

A flame ionization detector is component-specific, it measures low level organic compounds. Figure 24 shows a list of compounds to which an FID will not respond.

He Xe COS N2O CO2

Ar O₂ H₂S NO₂ H₂O

Kr N₂ SO₂ NH₃ SiCl₄

Ne CS₂ NO CO SiHCl₃

SiF₄

Figure 24: Compounds That Give Little or No Response to the Flame Ionization Detector(4)

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The hydrogen air flame in Figure 25 is the ionizing source. The sample gas components are mixed with the hydrogen fuel, and then they are burned. At the high temperature of the flame,

compounds that contain carbon bonds break down into positive and negative ions and electrons. A metal grid (the Collector) surrounds the flame to collect the current that passes through the flame. The number of ions that are formed, and thus the current that is conducted, is roughly proportional to the number of carbon atoms in the flame.

Figure 25: Flame Ionization Detector(4)

Flame ionization detectors can measure low level inorganic molecules such as carbon monoxide, carbon dioxide, carbonyl sulfide and formaldehyde by converting them to methane with the use of a methanator.

Methanator - A methanator consist of 2 feet of 1/8” tubing, packed with a nickel-coated catalyst (Figure 26). The methanator is placed in front of the detector. As the components leave the end of the column, they are readily converted to methane by the catalyst in the presence of hydrogen. The methane that is produced can then be measured using the flame ionization detector.

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Air Treater - Carrier gas impurities (e.g., hydrocarbons) that are present between the electrodes will cause a constant current to flow. This constant current is called the “background current.” It is desirable to minimize the background current so that small changes in current flow can be more easily detected. The best method of removing trace hydrocarbons from the instrument air is to use an air treater to heat the air in the presence of a catalyst, which converts the hydrocarbons to inert carbon dioxide (Figure 27).

Figure 27: Air Treater Photo ionization

The photo ionization detector (Figure 28) contains a sealed ultraviolet lamp with a specific energy (in eV). Lamps with energies of 9.5, 10.0, 10.2, 10.9, and 11.7 eV are available. As the gas components elute from the column, their UV-absorbing molecules are ionized by the ultraviolet light. Compounds whose ionization potentials are lower then the lamp’s ionization energy are affected. In the presence of the radiation the components become ionized. A pair of electrodes are located in a chamber, which is adjacent to the ultraviolet source. An electric potential is applied across the electrodes to create an electrical current. When the ions that were formed pass through the electrodes, a current output is generated proportional to the concentration of ions. The current from the detector is amplified and passed to the recorder.

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Flame Photometric

Flame photometric detectors are used to measure trace quantities of compounds that contain sulfur. Figure 29 shows a schematic of a flame photometric detector. The sample gas stream from the column is mixed with a constant flow of hydrogen near the burner tip. Sulfur-containing compounds are burned in the flame to produce a blue light. The light is transmitted through an optical cable to a 394 nm band-pass filter. The band-pass filter isolates the desired wavelength range before the light reaches the photomultiplier tube. The photomultiplier tube produces an output current that is proportional to the square of the sulfur concentration. The output current is amplified, and it is converted to a voltage by an electrometer circuit.

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Figure 29: Schematic of a Flame Photometric Detector Electron Capture

Figure 30 shows a schematic of an electron capture detector (ECD). The detector contains two electrodes through which the nitrogen carrier gas passes. One of the electrodes is treated with a radioisotope (tritium or nickel-63), which emits high-energy electrons as it decays. The electrons ionize the nitrogen molecules to produce an abundance of low-energy electrons in the carrier gas stream. These low-energy electrons, which are collected by the positively-charged electrode, produce a steady current. If the gas sample contains molecules that absorb the low-energy electrons, the amount of current is reduced. The loss of current is a measure of the electron affinity of the sample gas component.

The ECD is very sensitive to certain molecules such as alkyl halides, conjugated carbonyls, nitriles, nitrates, and organometals; however, it is insensitive to hydrocarbons, alcohols, ketones, etc.

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Figure 30: Electron Capture Detector

Columns

The separation of sample components occurs in the chromatograph column. There are two basic types of chromatograph columns, adsorption and partition columns. Variations of the types of columns are packed and capillary. Within these variations, hundreds of column materials are used, and others are constantly being developed. Although there are many columns, their basic operation is the same. The operation of these columns are described on the following pages.

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Adsorption Columns

Adsorption columns are used in gas-solid chromatography for the separation of gases, such as nitrogen, oxygen, carbon dioxide, and hydrogen sulfide. These columns can also be used to separate hydrocarbons in the C₁ to C₃ range. Components are separated by their differences in adsorption, which can be defined as their tendency to adhere to the adsorbent. Packing materials are surface-active solids, such as activated alumina, charcoal, molecular sieves, silica gel, and synthetic zeolites. The mole sieve column separates the sample gas components by molecular size. The particles in the mole sieve column contain molecular-sized holes that trap small molecules that fit into them. A length of tubing acts as a series of sieves. The larger molecules pass through the column faster than the smaller molecules, which enter the pores. For petroleum service, adsorption columns have a limited range of application except when used in combination with other columns.

Partition Columns

Partition columns are used in gas-liquid chromatography for separating complex hydrocarbon samples. The column packing is a granular solid (support), which is coated with a nonvolatile liquid stationary phase (Figure 31).

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Figure 31: Enlargement of Support and Liquid Stationary Phase

The packing exposes a large liquid surface to the vaporized sample components as they migrate through the column. The sample components are partitioned between the gas and liquid phases. The components that are least soluble in the liquid pass rapidly through the columns and emerge first. The components that are most soluble in the liquid are retained longer and emerge later. The more volatile components generally emerge earliest. Partition columns are very versatile because there are a large variety of liquids that can be used to obtain different separations. The granular solid support may be crushed firebrick, celite, or other solids of moderate surface area (1 to 4 square meters per gram). It may be treated to reduce residual adsorptive effects. The

stationary liquid must have a very low vapor pressure at the operating temperature in order for the column to have a long service life.

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Packed Columns

Packed columns consist of tubing, packing material, and packing retainers. The tubing may be made of stainless steel, copper, or glass (fused silica). The tubing length ranges from less than 1 m up to 10 m with a bore diameter of 1.6, 3.2, 6.4, or 9.5 mm. The packing material contains solid particles (support), which are coated with a thin liquid stationary phase. The sample gas components are soluble in the stationary phase. The carrier gas molecules and the soluble sample gas molecules travel along different paths in the packing (Figure 32). As a result, the sample gas molecules have different residence times. The structure of the packing affects the column

efficiency and the component retention times.

Figure 32: Packed Column Capillary Columns

Capillary columns have an open and unrestricted path for the carrier gas. The two most common types of capillary columns are the wall-coated open tubular (WCOT) and the support-coated open tubular (SCOT) columns. The WCOT column is a long narrow-bore tubing (0.25 mm I.D.) in which the inner wall is coated with a liquid stationary phase to about 1 µm in thickness. WCOT column lengths range from 50 to 150 m. The sample capacity of capillary columns is mainly determined by the thickness of the stationary phase. The major limitation of WCOT columns is their limited sample capacity.

SCOT columns have more sample capacity than WCOT columns; therefore, SCOT columns are shorter (about 16 m in length). The inside wall of a SCOT column (0.5 mm I.D.) is coated with an inert porous layer, which is formed by chemical treatment or is deposited on the inside wall. The porous layer is coated with a liquid stationary phase.

Valves Functions

There are different types of gas chromatograph valves, each of which has a separate function. These functions include

• sample injection • column switching • atmospheric referencing

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Sample Injection - Sample injection valves are located in the analyzer’s oven. A gas or liquid sample from the sample conditioning system constantly flows through the sample valve and returns to the sampling system. The function of the valve is to trap a constant volume of sample and to periodically inject the sample into the flowing carrier gas stream.

Column Switching - The function of column switching valves is to redirect the carrier gas flow during an analysis cycle so that specific components are loaded onto different columns for further separation. Column switching valves are also used to reverse the flow of carrier gas through a column to backflush unwanted components off of the column.

Atmospheric Referencing - Gas is a compressible fluid, and the number of molecules of sample in the sample loop can be determined by using the ideal gas law. Because the volume and the temperature of the sample loop are constant, the number of moles that are trapped is solely a function of the pressure in the loop. A good practice to ensure repeatable results is to reference the sample loop pressure to atmosphere prior to sample injection. Atmospheric referencing is accomplished by stopping the flow of vapor sample just prior to sample valve actuation through the use of a sample shut off valve (SSO). At the same time, the atmospheric referencing valve (ARV) shown in Figure 33 is actuated. The valve actuation allows the trapped sample in the sample loop to equilibrate to atmospheric pressure.

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Types of Sample Valves

The different types of sample valves include • rotary valves

• slider valves

• plunger diaphragm valves • transport injection valves

Rotary valves are used to inject very small amounts of liquid sample (less than 1 µL), or they are used as column switching valves for capillary columns. A typical rotary valve is shown in Figure 34. In the de-energized position (Figure 34A), the carrier gas flows through the valve to the column to perform the analysis. The sample stream flows through the metered volume so that the most representative sample will be available when the cycle is ready to repeat. When the next analysis cycle begins, the valve is energized (Figure 34B) so that it instantly rotates 60 degrees. The valve rotation transfers the metered volume to the carrier gas stream, which sweeps the sample into the column.

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Slider valves - A typical slider valve for gases is shown in Figure 35. The valve transfers a metered volume of the sample to the carrier gas stream by using a moving plate or slider.

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Figure 35: Typical Slider Valve for Gases (1)

Slider valves for liquids use a straight-through or cavity-type configuration (Figure 36). The straight-through configuration is faster than the cavity-type configuration.

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Plunger Diaphragm Valves - Figure 37 shows a schematic of a six-port plunger diaphragm valve. The six ports are arranged in a circular configuration. Between each pair of ports, there is a two-position plunger that can open or seal the passage between the two ports. The six plungers are operated in two sets of three. Each set of plungers is controlled by a spring-loaded, air-actuated piston so that one set closes three passages between ports as the other set opens three passages between ports. The passages between Port 1 and 6, Ports 4 and 5, and Ports 3 and 2 are normally closed. The other three passages between Ports 1 and 2, Port 3 and 4, and Ports 5 and 6 are normally open.

The two actuating pistons are spring-loaded in such a way that assures all six passages are momentarily closed during the switching operation. This momentarily closed position prevents unwanted mixing during the switching cycle. The plungers and diaphragm move only a few thousandths of an inch to permit flow through the passages. This small movement of the plungers and the diaphragm along with the absence of sliding seals that contact the process fluid, eliminates the abrasions that can cause sample loop volume changes and valve leakage, which are prevalent in sliding type valves. In addition, the small movement means the valves can switch in as little as 150 milliseconds to prevent smearing of the sample during injection.

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Transport injection valves, or quill-type valves, inject a measured volume of sample through the use of a grove carved in a cylindrical rod. The sample is collected in the grove and periodically injected into the carrier flow path as the rod is moved by the actuator. The collection section of the valve is located outside the chromatograph oven. The injection section of the valve is located in the chromatograph oven. With the use of a heater, the injector temperature can be raised to vaporize high boiling point samples.

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Figure 38: Transport Injection Valve

Although transport injection valves are more expensive, there are some benefits of using them. Samples that contain salts, that would cake on a valve during inject, are washed off when the valve quill is moved back to the flowing sample. Samples that contain a lot of solids, which would clog a slider valve or plunger valve, can be measured using a transport injection valve. With the use of a temperature control circuit, the injector temperature can be changed for different boiling point samples to insure complete vaporization.

Electronic Controllers

A gas chromatograph analyzer separates a sample into its component parts and then it detects the concentration of each component. For the analyzer to successfully perform these functions, the electronic controller must control the temperature, pressure, flows, and valve sequences within very close tolerances.

State-of-the-art GC analyzers have dedicated controllers that are located at each analyzer. In this configuration, each analyzer can either have independent input/output capability or it can be connected to a centralized high-speed communication network. A network configuration also simplifies the analyzer’s installation because all of the analyzers in the plant can be connected by a strategically-placed set of wires.

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Timing

The timer controls the time of operation of the analyzer valves, the detector signal gating, the attenuator selection, the readout devices, the auto zero, and the alarms. Most timers in use today employ solid state electronics, which combine timing and sequencing operations. Controllers that utilize minicomputers or microprocessors are widely used. Computer-based controllers that use either full-scale minicomputers or microprocessors have added a dimension of versatility in data interpretation, presentation, troubleshooting, and programming aids.

Measurement Data Processing Functions

Peak Detection - Two methods are used to detect chromatogram peaks: peak height and peak integration. The peak height measures the maximum height of the generated peaks. The peak integration measures the total area under the peak.

Figure 39 illustrates a chromatogram with three peaks and key chromatogram terminology. The “Gate On” tick mark (1) indicates when electronic integration of the peak starts. Gate On start times are stored in a file in the electronic controller. Peak retention time (2) is the time that it takes the maximum height of peak to appear after the sample is injected at the start of the cycle. Each component in the sample has a unique retention time which is used to identify the

component. The Gate Off tick mark (3) indicates when the integration ends. Peak height (4) is the maximum height of the detector signal above the baseline voltage. For sharp symmetrical peaks, peak height can sometimes be used as an indication of relative component concentrations in the sample. Gate On, residence time, and Gate Off times are corrected by using a calibration blend that contains known concentrations of the components of interest.

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There are several types of gating options that are used by the electronic controller. These gating options include: time gating, slope gating, auto-gating, and retention gating. With time gating, the gate is forced “ON” or “OFF” at a specific time in the analysis cycle by the electronic controller as was shown in Figure 39. With slope gating (Figure 40), the controller activates the gates at a point where the detector signal, at front gate or back gate, changes at a greater rate than is specified by a threshold limit with respect to time. This technique can be used when a

component’s retention time changes due to its natural characteristics or between two peaks that are not completely resolved. Slope gating is used by auto-gating and retention gating. Whichever option is used, it is assumed that the retention time for the known component does not change with respect to time.

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Due to baseline shifts, peak resolution baseline corrections are made through the electronic controller. Figure 41 shows the surplus peak area caused by baseline shift.

Figure 41: Example of Baseline Shift

There are several methods for correction for baseline shift or making corrections for unresolved components. Figure 42 shows some of the options used by the electronic controller to correct the peak area. The best option in most cases is baseline directly between front and back gate.

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It is up to the chromatograph technician to choose the best option or combination of options for each component. Time gating, Slope gating, Threshold limit, Retention gating and which basing option to use as well.

Peak Identification - Positive peak identification is one of the problems facing chromatography today. As the column technology changed with the use of high efficiency capillary column, the number of components that elute from the end of the column has also increased. The ability to identify all of these components did not increase with the technology.

There are three key factors that affect the ability to identify known components by their retention times—oven temperatures, carrier flow rate, and column stability. Oven temperature affects the retention time of a component. The hotter the oven temperature, the faster the component elutes. The use of microprocessors to control oven temperatures has resulted in very stable oven

temperatures, which results in stable retention times. The carrier flow rate will affect the retention time of the component. The use of a high quality carrier regulator is important to maintain a constant pressure, which will result in a constant flow rate. The third factor that affects the retention time of a component is the stability of the column itself. Identification is made by comparing the retention time of the unknown component to that of a known blend, or standard sample, which is run under the same conditions (Figure 43).

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The analyzer’s electronics are capable of looking for components that elute at the specified retention time. If a component is not there, or if it has moved outside the specified time, the electronic controller can generate an alarm. The electronic controller is incapable of positively identifying a component or identifying an unknown component. This identification has to take place by the chromatograph technician.

Unknown components that have similar retention times as the components of interest must be identified and eliminated to have an accurate analysis. These unknown components cause may be directly under the component of interest or show up as interference peaks, or shoulder peaks (Figure 44). The use of standard samples, or some other analytical technique to identify the unknown component, in chromatography is imperative. The interfering component may sometimes be removed through valve timing of the controller, or a new column study may be necessary.

Figure 44: Shoulder Peak

Peak Resolution - As the gas sample components migrate through the column, their zones always broaden. Separation of the components into discrete bands will occur only if they widen to a lesser degree than their peaks separate. The separation of two consecutive peaks is measured by the resolution, R. Resolution is defined as the peak separation, S, divided by the peak width, W (Figure 45A). For ideal peak separation, the peak separation and the peak width are equal, R = 1.0 (Figure 45B).

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Figure 45: Peak Resolution

The resolution of chromatogram peaks is related to two factors: column efficiency and solvent efficiency (Figure 46). The column efficiency concerns the peak broadening of an initially compact component band as it migrates through the column. The broadening results from the column design (column diameter, packing, etc.) and operating conditions. Solvent efficiency results from the interaction between the stationary phase and the sample gas components. Solvent efficiency is expressed as the ratio of the adjusted retention times (peak maxima).

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Figure 46: Illustration of Column and Solvent Efficiency(4) System Diagnostics

Electronic controllers for gas chromatograph analyzers perform the following functions: 1. Amplify and digitize the detector signal.

2. Integrate the detector signal, identify the peaks, and calculate component concentrations.

3. Automatically calibrate and update response factors.

4. Activate sample and column valve switching on a cycle clock basis.

5. Activate sampling system valves to switch between multiple sample streams or a calibration stream on a sequential basis.

6. Control the oven temperature.

7. Perform mathematical calculations on the measured component concentrations. 8. Have an interface for operation, maintenance, and programming.

9. Generate results in the form of: a. Printed reports

b. Analog outputs

c. Serial computer links to host computers

10. Communicate with other analyzers, I/0 devices, and operator interfaces using a data network.

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The interaction of these functions are shown in Figure 47.

a

Chromatograph Controller

OperatorInterfaceDataHiwayStreamSelectionValvingTransport Tubing

Sample ConditioningSystemC

lock CycleTimerCalibrationCalibrationResults ofAnalysisLocal I/OAnal og or DigitalCycle Clock Control TableStream Selection TableOven Temperature Control

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Component Separation Techniques

Gas chromatograph analyzers must be designed so that the sample components of interest are completely separated from the unwanted components. Single component measurements can usually be achieved by using one column; however, two and three component separations require multiple columns. Multiple component separations greatly increase the complexity of the design and maintenance of GC analyzers. The uses of various component separation techniques are described below These component separation techniques include

• backflush • heartcut • column stepping • trap/bypass • programmed temperature Backflush

Most gas samples contain components that are not separated in the chromatograph column or components that will not pass totally through the column in a reasonable length of time. For this reason, all sample components are removed from the column prior to the next measurement cycle by using the backflush technique. During backflush, the carrier gas flow through a column is reversed for a time T seconds, which is approximately equal to the time of the normal flow (Figure 48A). Under ideal backflush conditions, the chromatogram peaks of the components would recombine after T seconds and half of the backflush peak would be eluted from the column (Figure 48B).

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In reality, the column conditions during measurement and backflush are different. The peaks do not recombine during backflush as shown in Figure 49.

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Figure 49: Real Backflush(1)

If only the lighter components are to be measured with a single column, the heavier (high boiling point) components can be backflushed to the vent by using the valve configuration shown in Figure 50. To reduce the time that is required for backflushing, the pressure of the purge gas is increased above the normal operating pressure of the column. Most applications today use some form of backflush to insure that the column is clean before the next analysis.

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Complex multiple column arrangements are usually required to separate three or more

components. Figure 51 shows the backflush to vent flow path configurations. The system consists of two columns, a column valve (CV), a sample valve (SV), and a sample shut off valve (SSO). Figure 51A shows the carrier flows path through the configuration while the system is in forward flow. Figure 51B shows the carrier flows path through the configuration while the system is in backflush.

Figure 51: Backflush to Vent Using Two Columns

Heartcut

The heartcut procedure is used when there are components the elute off the first column prior to the component of interest. It is also used when there is a component that would contaminate the second column or the detector if the component were allowed to flow forward. This application is typically used when low level components need to be measured on an FID and the larger

components would saturate the detector. Figure 52A shows the heartcut application in the heartcut flow position. Figure 52B shows the heartcut application in forward flow.

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Figure 52: Heartcut Procedure

Column Stepping

The column stepping technique (Figure 53) is used to measure the heaviest components first. After the lighter components are allowed to pass through the first column, the carrier gas flow is reversed in the column. In this position, the first column is lined up so that it is in the third position in front of columns two and three. The reversed carrier flow sends the heavier components that remain in the first column to the detector.

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Figure 53: Reverse Column Step

Trap/Bypass

The trap/bypass procedure (Figure 54) is used to capture the lighter components that are harder to separate and to bypass the heavier components that are easier to separate. In this example, the lighter components that elute from column 1 are allowed to flow into column 2. The lighter components are then trapped, which allows the heavier components to pass via the column 2 simulator.

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Programmed Temperature

Temperature programming is the controlled change of the column temperature during the analysis. Temperature programming is used to separate gas components with a wide range of boiling points, which is a limitation of constant temperature techniques. At a constant

temperature (Figure 55), components with low boiling points elute form the column so rapidly that their peaks overlap. The peaks of the higher boiling point components are flat and

immeasurable. Sometimes, the high boiling point components do not elute from the column and they appear in a later analysis as baseline noise.

Figure 55: Constant Temperature Technique

With temperature programming, a lower initial temperature is used so that the peaks of the lower boiling point components are well resolved (Figure 56). The oven temperature is gradually increased to speed up the elution of the components with high boiling points. The programmed temperature technique is used mostly in the laboratory; however, some manufactures offer the option for process GCs.