DETERMINATION OF TRACES

IN NATURAL GAS

Prepared By

Wisam Al-Shalchi Petroleum Expert

Baghdad - 2005

Contents Summary 2

Chapter one Analysis of Natural Gas

1-1 The Chemical Composition of Natural Gas 4 1-2 Traces in Natural gas 6 1-3 Analysis of Natural Gas by Gas Chromatography 10 1-4 Analysis of Natural Gas by Spectroscopic Methods 19

Chapter Tow Determination of Traces in Natural Gas

2-1 Determination of Sulfur Compounds in Natural Gas 21 2-2 Determination of Nitrogen Compounds in Natural Gas 24 2-3 Determination of Halogen Compounds in Natural Gas 25 2-4 Determination of Mercury in Natural Gas 26 2-5 Determination of other Metals in Natural Gas 28 Conclusions and Recommendations 30

References 32 CV of the Researcher

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Summary Compared with other fossil fuels, natural gas enjoys a well-deserved reputation as a clean source of energy. It is of paramount importance to the gas industry to diligently safeguard this “clean energy” reputation that natural gas rightfully enjoys. To ensure the environmental safety of natural gas utilization, every effort should be made to determine if any potentially harmful constituents exist in natural gas. This will enable the industry to take appropriate cleanup measures if these constituents are present. This proactive approach by the gas industry will help maintain environmental standards, while averting regulatory “surprises” leading to the unnecessary measurement of non-existing harmful components in natural gas. In the past few decades after the massive usage of natural gas as a fuel, and as a raw material for different industries, many destructive effects were observed concerning the public health of the persons who work in these fields, or on the equipments used in processing and industrializing the gas. In the year 2000 for example, it was reported in Chicago in USA that more than 280,000 homes in northern Illinois will be tested for mercury contamination, and that clean-up efforts have commenced as testing continues. This was after a homeowner in the Chicago suburb of Mount Prospect reported a silvery substance in the basement. Illinois Attorney General and county prosecutors have filed a five-count lawsuit against the gas distributing company in Illinois-based Nicor, Inc. and two contractors in an attempt to quickly clean up mercury contamination found in homes and buildings. It is known that Mercury, inhaled over long periods of time, can cause nerve damage, respiratory failure and kidney damage. The most important question now is how many other communities are being contaminated with Mercury and other hazardous materials coming from their gas pipes? Other specific examples include corrosion of aluminum in cryogenic heat exchangers, gates and stems of wellhead valves. Such mercury-induced corrosion of aluminum heat exchangers resulted in catastrophic failure of heat exchangers at the Skikda LNG plant in Algeria (1975). Thus it is very important to determine the Trace constituents in natural gas qualitatively and quantitatively, no matter how small their quantities are in it, for the sake of the public health and the safety of equipments used in processing it. In the following sections the author reports the information collected about analysis methods for speciation and measurement of trace constituents of natural gas. Also the report contains a list of some modern instruments used in analysis of natural gas, and their specifications.

The Author*

Wisam Al-Shalchi – Petroleum Expert, Email: wisamalshalchi@yahoo.com

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Chapter one Analysis of Natural Gas

Natural gas is a colorless, highly flammable gaseous hydrocarbon mixture consisting primarily of methane and ethane. It is an important energy source and widely used as a starting material for many chemical processes. It contains also different levels of other hydrocarbons and fixed gases such as Nitrogen, Helium, and Carbon dioxide. Below that 0.01 mole percent concentration level, there may exist a whole population of gas components and some other metals collectively classified as trace constituents. The major portion of these trace constituents are naturally occurring species such as paraffinic and aromatic hydrocarbons, organic sulfur compounds, hydrogen, and others. However, some compounds are those inadvertently introduced by gas processing or contamination. Hydrocarbons heavier than C7 are usually present in natural gas at small concentrations (ppm levels). Hydrogen Sulfide and other Sulfur compounds may also be present, either naturally or as added odorants. Additional components may include polar compounds such as low levels of water, small amounts of methanol and/or glycol which may have been added for processing purposes. Natural gases from different sources usually have the same composition but in different concentration levels. Regardless of their origin, a number of advanced tests can be used to identify these trace components. Major and minor components of natural gas are routinely analyzed by Gas Chromatography (GC) using a Thermal Conductivity Detector (TCD). The best results obtained by these methods can report no better than 0.01 mole percent of each measured component. Even the extended method of analysis by Flame Ionization Detector (FID) can only improve on the detection limit of hydrocarbons. The gas industry needs better information on all trace constituents of natural gas, whether native or inadvertently added during gas processing that may adversely influence the operation of equipment or the safety of the consumer. The presence of Arsenic and Mercury in some gas deposits have now been documented in international literature as causing not only human toxicity but also damaging the field equipment. Yet, no standard methods of sampling and analysis exist to provide this much needed information. A cryogenic sampler operating at near – 99 oF, and at pipeline pressures up to 1800 psig has been developed to preconcentrate and

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recover all trace constituents with boiling points above Butanes. Specific analytical methods have been developed for speciating and measurement of many trace components by Gas Chromatography-Atomic Emission Detector (GC-AED), and Gas Chromatography-Mass Spectroscopy (GC-MS) and for determining various target compounds by other techniques. Moisture, Oxygen and Sulfur contents are measured on site using dedicated field instruments. Arsenic, Mercury and Radon are sampled by specific solid sorbents for subsequent laboratory analysis.

1-1 The Chemical Composition of Natural Gas: The constituents of natural gas vary with geographic location and no single composition can be considered to be typical. An exact determination of the composition of natural gas can be made using Mass Spectrometry (MS) and Gas Chromatography (GC). By using these highly sensitive instruments any natural gas sample can be fractionated to its constituents, and the concentration of each specie can be determined as high as a few parts per million. The analysis of a gas sample taken from Panhandle natural gas field in Texas in the United States is given in Table (1).

Compound Formula Boiling Point oC

Concentration %

Methane CH4 -161.5 76.2 Ethane C2H6 -88.5 6.4

Propane C3H8 -42.2 3.8 n-Butane C4H10 -0.5 1.3 Isobutane C4H10 -120.1 0.8 n-Pentane C5H12 36.1 0.3 Isopentane C5H12 27.9 0.3

Cyclopentane C5H10 49 0.1 Hexane C6H14 69 0.3

Nitrogen N2 -195.8 9.8 Oxygen O2 -183 Trace Argon Ar -185.8 Trace

Hydrogen H2 -272.7 0 Hydrogen sulphide H2S -61 0

Carbon dioxide CO2 -78.5 0.2 Helium He -218.9 0.45

Table (1): Analysis of a sample of natural gas taken from Panhandle

Field (Texas).

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On the other hand, the analysis of a gas sample taken from south Jumbour field in the northern region of Iraq is shown in Table (2).

Compound Formula Volume % Methane CH4 86.78 Ethane C2H6 7.01 Propane C3H8 3.12 n-Butane C4H10 1.49 n-Pentane C5H12 0.60 Hexane C6H14 0.73 Heptane and >C7 - Trace Carbon dioxide CO2 0.24 Nitrogen N2 Trace Hydrogen sulphide H2S 10 ppm Mercaptane - 4.2 ppm Carbonyl sulphide COS2 28.2 ppm Moisture - 1200 ppm

Table (2): Chromatographic analysis of a sample of natural gas taken

from South Jumbour in the northern region in Iraq. In the seventies of the previous century, trace analysis was done for many natural gas samples taken from different regions of the Iraqi petroleum and gas fields. The results showed that Helium is present in different trace concentrations as seen in Table (3) below. All the hydrocarbon gases in natural gas mixtures are inflammable, and all are members of the paraffin series. By far the most abundant component is Methane CH4, which usually found in percentage between 80-95%. Methane dose not condense to a liquid under the temperature and pressure conditions usually prevailing, and is therefore always present in the gaseous phase, either in the form of free gas or dissolved in liquid oil. The second component in this series which is present in natural gas in considerable percentage is Ethane C2H6 (1-10%). Higher hydrocarbons of higher boiling points occur only in small proportions in natural gas, and in general it is easy to find that the higher the boiling points of the hydrocarbons, the smaller will be their proportion in natural gas. Inert non-hydrocarbon components such as Carbon dioxide, Nitrogen, Hydrogen Sulphide, Helium, Argon, Water, Mercury, solid particles...etc, may be also present in the natural gas. Some of these are found in substantial percentages (Nitrogen, Hydrogen Sulphide, and Carbon dioxide) and some (Argon, Helium) are present usually only in

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small amounts. Of these gases, Helium may be of commercial value in their own right where their concentrations are relatively high.

Region Concentration in ppm Kirkuk / Well 59 32 Kirkuk / Well 78 35

Jambour 61 Bai Hassan 66

Ain Zala 157 Khanakeen Nil

Qayara Nil Abu Ghrab 73 Bauzirgan 85

North Rumaila / medium 18 North Rumaila / light 11

South Rumaila 98 Zubair / degasing station 17 Hamar / degasing station 21

Nahran Omar 17 Table (3): Results of Helium analysis of different samples of Natural

Gas taken from different regions in Iraq.

1-2-Traces in Natural Gas: Natural Gas contains many kinds of traces. Sulfur compound traces are probably the most species present in natural gas. Nitrogen compounds, Halogen compounds, and heavy Metals like Mercury and Arsenic are also present as traces in many natural gases which are produced from different fields around the world.

a- Sulfur compounds: The presence of sulfur compounds in natural gas is undesirable since many of these compounds have unpleasant odors and are unstable, corrosive and poisonous to the industrial catalysts. The analysis of the gaseous sulfur compounds in natural gas usually faces several problems like:

(1) Difficulties of the chromato-graphic separation of these gases from the natural gas hydrocarbons.

(2) Hydrocarbon interference with the sulfur response. (3) Possible adsorption of compounds on reactive surfaces in the

analytical system.

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Gaseous sulfur compounds like Hydrogen sulfide, Carbonyl sulfide, and Methyl Mercaptan are separated from natural gas by using a long methyl silicone column following the guidelines of ASTM method D 5504. Retention time precision is measured. Detection of the target analytes is with the pulsed flame photometric detector (PFPD) which is shown to give equimolar response for sulfur components.

Figure (1): Histogram of sulfur compounds separation by Gas

Chromatography method.

b- Nitrogen compounds: The presence of amines, ammonia, nitrogen oxides (NOx) and other nitrogen containing compounds can be artifacts of gas-processing operations or could be naturally occurring. They may cause operational difficulties and equipment malfunction due to their action as catalyst poisons. Nitrogen-containing species can affect the coatings, printing, glass manufacturing, industrial food processing, and the chemical manufacturing industries. The combustion of nitrogen containing compounds can produce noxious and corrosive acids in gas flames.

c- Halogen compounds: Chlorine-containing compounds and other halogens can be naturally occurring, or result from environmental contamination. Halogens can affect the coatings, printing, glass manufacturing, industrial food processing, and the chemical manufacturing industries. The combustion of halogens can produce noxious and corrosive acids in gas flames. Gas chromatography coupled with an element selective detector is the technique most commonly used to determine chlorine- and fluorine-containing compounds. The element selective detectors respond to halogens contained in the separated components. These include the atomic emission detector, the electrolytic conductivity detector, and the electron capture detector. Total fluorine and total chlorine methods can be

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performed by oxidative combustion microcoulometry using a fluorine or chlorine selective electrode system, or by pyrolysis, followed by detection using an ion selective electrode. The electrolytic conductivity detector (ELCD) reduces halogen compounds eluting from a GC to HX in a high temp catalytic converter. The HX is dissolved into a deionized solvent whose increase in conductivity is directly related to the halogen concentration. Depending on sample size, 0.05 ppm or lower detection limits can be achieved. Most interferences originate from the major components of the gas being analyzed and are seen as an upset in the baseline prior to the earliest eluting compound. This technique is very sensitive and simple to use.

d- Metals: There are tow kinds of metals which are present in Natural Gas.

(1) Mercury (Hg): Mercury may exists in petroleum or natural gas in it's free form as a vapor or drops of liquids or as an organometalic compounds. Mercury only exists in some petroleum or gas fields worldwide, but when it exists in natural gas, condensate and crude oil it can adversely affect hydrocarbon production and processing in a variety of ways, specifically by:

• Forming amalgams with a variety of metals, including aluminum, copper, brass, zinc, chromium, iron, and nickel. When these amalgams are formed with the metal components of the processing equipments, corrosion of these equipments usually results. The corrosion occurs because either the amalgam is weaker than the mercury-free metal, or, as is the case of aluminum amalgam, the amalgam reacts with moisture to form a metal oxide plus free mercury causing the corrosion process to continue. Specific examples include corrosion of aluminum in cryogenic heat exchangers, gates and stems of wellhead valves. Such mercury-induced corrosion of aluminum heat exchangers resulted in catastrophic failure of exchangers at the Skikda LNG plant in Algeria (1975).

• Mercury poisoning of catalysts reduces the catalyst life.

• Mercury amalgam formation on steel pipe walls can result in classification of production and processing equipment as hazardous waste.

Mercury concentrations in natural gas are typically reported as microgram per "normal" cubic meter µg/ Nm3 (1 µg = 10-6 gm), where normal (N) indicates standard temperature and pressure. Mercury

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concentrations in natural gas vary from 0 to > 300 µg/Nm3 with some of the highest concentrations occurring in the Indonesian Arun and Dutch Groningen fields (see table below). In condensate, Hg concentrations vary from 10 to 3,000 ppb (part per billion). Published concentrations for various natural gases and oils include:

Field Location Hydrocarbon Hg ConcentrationGroningen Netherlands Gas 0.001-180 µg/Nm3 Unknown Netherlands Gas 0-300 µg/Nm3

Arun Sumatra Gas 180-300 µg/Nm3 Unknown Middle East Gas <50 µg/Nm3 Unknown South Africa Gas 100 µg/Nm3 Unknown Far East Gas 50-300 µg/Nm3

Cymric San Joaquin Valy, CA Oil 1.9 - 21 ppm

Table (4): Mercury in some different fields worldwide.

The Mercury species which are present in different petroleum sectors are listed in the following table:

Species Boiling Point oC Sector Hg 457 Gas + Petroleum HgCl2 302 Petroleum (CH3)2Hg 96 Gas +Petroleum (C2H5)2Hg 170 Petroleum (C3H7)2Hg 190 Petroleum (C4H9)2Hg 206 Petroleum

Table (5): Kinds of Mercury species which are present in different

petroleum sectors.

Mercury occurs in natural gas in the metallic form. In contrast, condensate associated with natural gas contains Hg in a variety of forms, including elemental, ionic and organometallic. As a result, Hg is not limited to a particular boiling fraction of a condensate. For example, it is reported that the following mercury distribution between the boiling ranges of a Southeast Asian condensate:

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Boiling rang oC Description Wt. % of total Hg

<36 ---------- 8.9 36-100 naphtha 27.6 100-170 naphtha 33.8 170-260 kerosene 16.0 260-330 diesel oil 7.4

Table (6): Distribution of mercury between different petroleum

sectors.

(2) Arsenic (As): Arsenic is present in natural gas only in rare cases. It is not present in free form as with Mercury, but it is usually present in the organometalic form. The following table shows the relative concentration of some Arsenic organic compounds in a sample of natural gas taken from one of the gas fields which is contaminated with this metal.

Compound Average Percent (% total arsenic)

Molecular Weight

Boiling Point oC

Trimethylarsine (TMA) 60 - 90 120 52 Dimethylethylarsine

(DMEA) 10 - 30 134 86

Methyldiethylarsine (MDEA) 5 - 15 148 110

Triethylarsine (TEA) 1 - 5 162 139

Relative Concentrations and Basic Properties of : )7(Table Arsenic Contaminants in Natural Gas

1-3 Analysis of Natural Gas by Gas Chromatography: There are several ways for determining the composition of Natural Gas. One of the most important instrumental methods is the analysis by Gas Chromatography technique. The process of gas chromatography is carried out in a specially designed instrument. A very small amount of liquid mixture is injected into the instrument and is volatilized in a hot injection chamber. Then, it is swept by a stream of inert carrier gas

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through a heated column which contains the stationary, high-boiling liquid. As the mixture travels through this column, its components go back and forth at different rates between the gas phase and dissolution in the high-boiling liquid, and thus separate into pure components. Just before each compound exits the instrument, it passes through a detector. When the detector “sees” a compound, it sends an electronic message to the recorder, which responds by printing a peak on a piece of paper. The (GC) consists of an injection block, a column, and a detector. An inert gas flows through the system. The injection chamber is a heated cavity which serves to volatilize the compounds.

Figure (2): Diagram of a simple gas chromatography instrument.

The sample is injected by a syringe into this chamber through a port which is covered by a rubber septum. Once inside, the sample becomes vaporized stream and is carried out of the chamber and onto the column by the carrier gas. The column is an integral part of the GC system. On the outside, it is a long stainless steel or a glass tube, 0.1 to 10 mm in diameter and 1 to several meters long. To fit into the temperature-controlled oven in the gas chromatograph, the column usually must be bent or coiled. Inside the column is the important component: the stationary phase which is either small particles of a solid adsorbent, or a high-boiling liquid which is usually impregnated on a high surface area solid support like diatomaceous earth, crushed firebrick, or alumina. The liquid can be applied in various concentrations: the more liquid, the more sites it has to interact with the compounds. Two categories of columns are used for gas chromatography, these are:

Packed columns: These are columns which contain particles of the stationary phase packed into a metal or glass tube. The metal tubes are usually constructed of stainless steel. A packed column generally has a diameter of between 1 and 10 mm and a length between 1 and 4 m.

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Open tubular columns: These are columns which have the liquid stationary phase coated on the column's inner walls, or on a thin layer of a solid support coated on the inner wall of the column. The inner diameter of an open tubular column is often between 0.2 and o.5 mm, while its lengths vary from 20 to 80 m, and it is usually made of glass. There are several types of open tubular columns:

• Wall-Coated Open Tubular (WCOT): This column has a thin layer of a stationary liquid phase coated directly on the inner wall of pretreated glass capillary column.

• Support-Coated Open Tubular (SCOT): It has a thin layer of a solid support coated on the inner wall of the column. The stationary liquid phase is coated on the solid support.

• Porous-Layer Open Tubular (PLOT): This column has the inner wall coated with a porous layer of the stationary phase.

The carrier gas is an inert gas, like Nitrogen or Helium. The flow rate of the gas influences how fast a compound will travel through the column; the faster the flow rate, the lower the retention time. Generally, the flow rate is held constant throughout a run. Two devices are used to record the (GC) traces/areas under peaks:

Integrating recorders Computer program

Each type of device records the messages sent to them by the detector as peaks, calculates the retention time, and calculates the area under each peak; all of this information is included in the printout. For similar compounds, the area under a GC peak is roughly proportional to the amount of compound injected. If a two-component mixture gives relative areas of 75:25, you may conclude that the mixture contains approximately 75% of one component and 25% of the other. The retention time, (RT) is the time it takes for a compound to travel from the injection port to the detector; it is reported in minutes on our GCs. The retention time is measured by the recorder as the time between the moment you press start and the time the detector sees a peak. If you do not press start at the same time you inject your sample, the RT values will not be consistent from run to run.

a- Factors affecting the GC separations: Efficient separation of compounds in GC depends on the compounds traveling through the column at different rates. The rate at which a compound travels through a particular GC system also depends on the following factors:

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(1) Volatility of compounds: Low boiling (volatile) components will

travel faster through the column than will high boiling components.

(2) Polarity of compounds: Polar compounds will move more slowly, especially if the column is polar.

(3) Column temperature: Raising the column temperature speeds up all the compounds in a mixture.

(4) Column packing polarity: Usually, all compounds will move slower on polar columns, but polar compounds will show a larger effect.

(5) Flow rate of the gas through the column: Speeding up the carrier gas flow increases the speed with which all compounds move through the column.

(6) Length of the column: The longer the column, the longer it will take all compounds to elute. Longer columns are employed to obtain better separation.

Generally the first factor which governs the separation of compounds on the GCs is the boiling point of the different components. Differences in polarity of the compounds are only important if the compounds of the mixture have widely different polarities. Column temperature, polarity of the material in the column, flow rate, and length of a column are usually constant in (GC) runs in the Organic Chemistry Laboratories. In Gas Chromatography analysis (GC), the variety of components in natural gas requires the separation of both polar/non-polar compounds. Multi-dimensional (GC) is often required since no single column can separate this wide variety of natural gas constituents, nor can a single detector detect all compounds satisfactorily. If more than on column is used in analyzing natural gas, then these columns are connected by one or more multiple port valves and the complete separation is obtained by time switching eluents to each column and detector. Due to the complications of the multi-dimensional gas chromatography method, many companies have carried out research to establish new methods, and to develop new instruments and parts for these chemical experiments. The best modern columns which are used in determining the composition of natural gas by Gas Chromatography method are of the Plot type (Porous-Layer Open Tubular). These kinds of columns are also categorized into several types, according to the thin porous layer material coated inside the column, the kind of analysis they are used for and the kind of chromatographic analysis they are used in. The most important kinds are:

(1) The HP-PLOT Al2O3 column: This column is often used for hydrocarbon separations and the determination of BTUs.

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(2) HP-PLOT Molecular Sieve Column: It is used for the separation of fixed gases such as Oxygen, Nitrogen, Hydrogen, Helium, Neon, and even Argon from Methane.

(3) Q-type Porous Polymer PLOT Column: This column is used to separate polar compounds like water, carbon dioxide, and odorants which are mostly composed from sulfur compound?

(4) Silicagel Column: This column is suitable for the separation of air, hydrogen, methane, and ethane.

The best column of these kinds used in analyzing the natural gas is the HP-Plot Q type column (developed by Agilent). The HP-PLOT Q type is a bonded polystyrene-divinylbenzene (DVB) based column that has been specially developed for the separation of targeted polar and non polar compounds including:

• Hydrocarbon (natural gas, refinery gas, ethylene, propylene, all C1-C3 isomers).

• CO2, methane, air/CO, and water. • Polar solvents (methanol, acetone, methylene chloride, alcohols,

ketones, aldehydes, esters). • Sulfur compounds (H2S, mercaptans, COS).

It was found practically that this column is one of the best columns used in analyzing natural gas. Like the columns, the detectors used in Gas Chromatography instruments are also of several kinds, these are:

(1) Thermal Conductivity Detector (TCD): A (TCD) detector consists of an electrically-heated wire or thermistor. The temperature of the sensing element depends on the thermal conductivity of the gas flowing around it. Changes in thermal conductivity, such as when organic molecules displace some of the carrier gas, cause a temperature rise in the element which is sensed as a change in resistance. The (TCD) is not as sensitive as other detectors but it is non-specific and non-destructive. It is cheap and easy to use, thus it is widely used especially in routine tests. (2) Flame Ionization Detector (FID): An (FID) consists of a hydrogen/air flame and a collector plate. The effluent from the GC column passes through the flame, which breaks down organic molecules and produces ions. The ions are collected on a biased electrode and produce an electrical signal. The (FID) is extremely sensitive with a large dynamic range. Its only disadvantage is that it destroys the sample. (3) Electron Captured Detector (ECD): The (ECD) uses a radioactive Beta emitter (electrons) to ionize some of the carrier gas and produce a

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current between a biased pair of electrodes. When organic molecules that contain electronegative functional groups, such as halogens, phosphorous, and nitro groups pass by the detector, they capture some of the electrons and reduce the current measured between the electrodes. The (ECD) is as sensitive as the (FID) but has a limited dynamic range and finds its greatest application in analysis of halogenated compounds. (4) Flame Photometric Detector (FPD): The (FPD) combusts the GC eluent in an H2 rich flame to produce S2 excited species. Energy emitted upon decay is directly related to the (approx.) square root of the S concentration. It has a detection limit and range of 20 - 20,000 picograms, with generally three orders of magnitude dynamic range. However, hydrocarbon quenching from closely eluting HCs can interfere with the production of S2 species.

b- Analyzing Natural Gas by Gas Chromatography Instrument: This experiment was based on the standard D 1945 of ASTM (American Society for Testing and Materials). Gas chromatography analysis of a sample of Natural gas was done using an Agilent 6890 gas chromatograph (GC) instrument with electronic pneumatics control (EPC), a single Plot type column and a Thermal Conductivity Detector (TCD). For conventional gas analysis, a six-port valve with a 0.25 cc sampling loop was used to introduce natural gas sample onto the HP-PLOT Q column in split mode (split ratio 18:1). The GC parameters are listed in Table (8) below.

Table (8): Gas Chromatography experimental conditions.

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A sample of natural gas of the compounds and concentrations listed in Table (9) below was used in this experiment. This sample was modified by adding Methanol, Water, and Hydrogen sulfide. During analysis, the possible leaking of some air in the sampling loop may have also caused some changes in concentrations.

Table (9): Chemical composition of a Natural Gas sample. Analyses were run using an HP-PLOT Q porous polymer column. The column was conditioned at 250°C overnight per manufacturer recommendation to reduce column bleed. HP-PLOT Q type columns are coated with porous polymer particles made of Divinylbenzene and Ethylvinylbenzene and can separate hydrocarbons up to C14 as well as some polar compounds. The gas sample was introduced to the chromatographic column by opening the outlet valve of the sample cylinder and purging the sample through the inlet system and sample loop as shown in Figure (1). The sample loop pressure was near atmospheric, and the temperature of the sample was maintained near that of the source where the sample was taken from.

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Figure (3): The sampling system used in natural gas analysis. The upper isothermal and programming temperature limits are 270°C and 290°C, respectively, and the rate of raising the temperature of the oven was 30oC/min. The analysis time for this experiment was 9 minutes. The separation of the constituents in the natural gas sample was done using a porous polymer HP-PLOT Q column as shown in Figure (4).

Figure (4): Separation of Natural Gas by Gas Chromatography. Hydrogen sulfide, Water, and Methanol were well-separated from Ethane, Propane and Isobutane. Although baseline spiking is commonly associated with this analysis for some commercially available columns, no baseline spiking was observed with the HP-PLOT Q column,

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indicating that the stationary phase of this PLOT column provides excellent immobilization that can withstand: fast oven temperature ramping (30°C/min), a pressure pulse generated from valve actuation, and carrier gas pressure ramping at constant flow mode. Resultant column bleed was very low. This separation capability of the HP-PLOT Q column also can sufficiently resolve Nitrogen and carbon dioxide from Methane, even if the Methane peak is tailing due to sample overload. The starting temperature of 60°C also results in a 30% reduction in GC cycle time. One of the concerns associated with using PLOT columns for natural gas analysis is reproducibility. It is well known that when using alumina PLOT and molesieve PLOT columns, the retention times for hydrocarbons shift due to deactivation of column absorbants from sample components such as Water and CO2 during repeated runs. It was not possible to carry out the analysis of natural gas containing mercaptans (added to natural gas as odorants) by this kind of experiment, because most sulfur compounds and even water deactivate Al2O3 and mole sieve PLOT column coatings. Although back flushing heavier compounds in natural gas analysis is very common for all PLOT columns, this technique may not be needed for HP-PLOT Q columns. Figure (5) shows this possibility, where heavier alkanes up to C14 were eluted on HP-PLOT Q column at 300°C, at such a high temperature, the column maintained relatively low bleed.

Figure (5): Elution of C8 to C14 on HP-PLOT Q column It can be easily concluded that analysis of Natural gas by GC/TCD operation on a single porous polymer HP-PLOT Q column gives satisfactory separation using a very simple GC/TCD configuration and operation. The reproducibility of the analysis is very good. Back flush may not be needed for hydrocarbons up to C14, which can be eluted at 300°C temperatures.

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1-4 Analysis of Natural Gas by Spectroscopic Methods: Natural gas can be analyzed by Mass Spectroscopic method according to the standard D 1137 of ASTM. After the recent development of the gas chromatography method and the combination of gas chromatography and the mass spectroscopy in one instrument, that method is no longer used in most of the petroleum laboratories. Other spectroscopic methods like Atomic Absorption and Atomic Fluorescence spectroscopy are used to determine some of the traces in natural gas like Mercury and Arsenic.

a- Mass Spectroscopy: The molecular species making a natural gas are ionized and dissociated by electron bombardment, the resulting positive ions of different masses are accelerated in an electric field and are separated magnetically, and abundance of each mass present is recorded. The mixture spectrogram thus obtained is solved against the mass spectra of each of the pure molecular species constituting the mixture. Mass spectrometry has been described as the smallest scale in the world, not because of its size but because of the size of the things it weighs. Mass spectrometry, also called mass spectroscopy, is an instrumental approach that allows for the mass measurement of molecules. Mass spectrometers use the difference in mass-to-charge ratio (m/e) of ionized atoms or molecules to separate them from each other. Mass spectrometry is therefore useful for quantitation of atoms or molecules and also for determining chemical and structural information about molecules. Molecules have distinctive fragmentation patterns that provide structural information to identify structural components. The general operations of a mass spectrometer are:

(1) Create gas-phase ions. (2) Separate the ions in space or time based on their mass-to-charge

ratio. (3) Measure the quantity of ions of each mass-to-charge ratio.

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Figure (6): Main parts of a Mass Spectrometer.

b- Atomic Absorption Spectroscopy (AAS): Atomic-absorption (AA) spectroscopy uses the absorption of light to measure the concentration of gas-phase atoms. Since samples are usually liquids or solids, the analyte atoms or ions must be vaporized in a flame or graphite furnace. The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels. The analyte concentration is determined from the amount of absorption. Applying the Beer-Lambert law directly in AA spectroscopy is difficult due to variations in the atomization efficiency from the sample matrix, and nonuniformity of concentration and path length of analyte atoms (in graphite furnace AA). Concentration measurements are usually determined from a working curve after calibrating the instrument with standards of known concentration.

Figure (7): Diagram of an Atomic Absorption Spectrometer (AAS).

c- Atomic Florescence Spectroscopy (AFS): Atomic fluorescence is the optical emission from gas-phase atoms that have been excited to higher energy levels by absorption of electromagnetic radiation. The main advantage of fluorescence detection compared to absorption measurements is the greater sensitivity achievable because the fluorescence signal has a very low background. The resonant excitation provides selective excitation of the analyte to

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avoid interferences. AFS is useful to study the electronic structure of atoms and to make quantitative measurements. Analysis of solutions or solids requires that the analyte atoms be desolvated, vaporized, and atomized at a relatively low temperature in a heat pipe, flame, or graphite furnace. A hollow-cathode lamp or laser provides the resonant excitation to promote the atoms to higher energy levels. The atomic fluorescence is dispersed and detected by monochromators and photomultiplier tubes, similar to atomic-emission spectroscopy instrumentation.

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Chapter Two Determination of Traces in

Natural Gas Routine analysis of Natural Gas for the calculation of heating value, specific gravity and compressibility is an established measurement practice by the gas industry worldwide. However, these routine analyses provide information only on major and minor components. They do not detect trace constituents at low concentration levels. As detection technology improves, new analytical methods are constantly being developed to measure these trace level compounds. The primary motivation for this is the gas industry’s ever-increasing environmental awareness along with federal and state requirements to regulate and monitor air quality emissions. Furthermore, these compounds could have deleterious effects on the gas distribution system, harm gas-processing operations or result in operational or end-user difficulties. Natural gas is a colorless, highly flammable gaseous hydrocarbon mixture consisting primarily of methane and ethane with lesser amounts of inert gases and heavier hydrocarbons. Automated gas chromatographs provide gas composition with excellent precision and accuracy for components such as N2, CO2, and paraffins from C1 through C5, especially if calibration gases of known uncertainty are used. These tests have detection limits of about 0.01 mole percent. Below 0.01 mole percent concentration level there may exists a whole population of gas components collectively classified as trace constituents. The major portion of these trace constituents are naturally occurring species such as paraffinic and aromatic hydrocarbons, organic sulfur compounds, hydrogen and others. However, some compounds are those inadvertently introduced by gas processing or contamination. Regardless of their origin, a number of advanced tests can be used to identify these trace components. Tests for certain specific compound classes are described below.

2-1 Determination of Sulfur Compounds in Natural Gas: Sulfur compounds are either naturally occurring or odorants that are artificially added to natural gas to impart a smell for easy detection. Gas chromatographic analysis offers speciation information useful for gas

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quality monitoring. The analysis of a natural gas sample containing mercaptans (added to natural gas as odorants) was done by Gas Chromatography method using HP-Plot Q type. The experiment used the same conditions listed in Table (7), and a temperature starting from 60oC. Figure (6) shows the chromatogram of a GC separation of four kinds of mercaptans, carbonyl sulfide and hydrogen sulfide.

Figure (6): Sulfur Compound Separation by GC. The compounds are well separated and resolved and their elution positions still fall in between those for Ethane and i-butane in Figure (3) above. GC systems for sulfur speciation can be designed with several different detectors. As a rule, element selective detectors will provide the best data because the detector responds to sulfur contained in the separated components. The detectors used most generally include:

Flame Photometric Detector (FPD): This detector is used as described before.

Pulsed Flame Photometric Detector (PFPD): The Pulsed Flame Photometric Detector (PFPD) uses the same excitation technology except the flame is pulsed. Because the background emissions decay more quickly than the excited S2 species, the detector response is optimized by delaying the signal monitoring to eliminate background emissions. This can improve the detection limit at least 10-fold.

Sulfur Chemiluminescence Detector (SCD): The Sulfur Chemiluminescence Detector (SCD) uses a different combustion chemistry to yield an excited SO radical plus other reaction products. Reaction of SO with ozone creates electronically

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excited SO2. The chemiluminescence signal emitted upon decay is directly (equimolar) related to S concentration. This technique has a detection limit and range of about 10-1,000,000 picogram with generally five orders of magnitude dynamic range. The detector requires precise control of the hydrogen and air flow rates, and the vacuum system.

UV-Fluorescence Detector (UFD): Another new detector uses UV fluorescence. The GC eluent gas is combusted with O2 producing SO2. Excitation with UV energy ultimately generates a fluorescence decay signal that is directly related to S content. It can detect from the ppm to the % level range, offering four orders of magnitude dynamic range. However, combustion byproducts can interfere by contributing to the fluorescence decay signal.

Atomic Emission Detector (AED): The Atomic Emission Detector (AED) is an analytical tool that is not found in many laboratories. Instead of combusting the GC eluent, the gas is introduced into a microwave-induced helium plasma-producing electronically excited atom. The sample is aspirated in a nebulizer and is carried by a slower stream of helium directed centrally toward a point where the sample is heated by conduction and radiation and may reach 7000ok, where it is completely atomized and excited. A photodiode array measures the emitted energy upon decay back to the ground state. Generally, a 1 ppm detection limit can be achieved with four orders of magnitude dynamic range. The technique can suffer from spectral interferences from hydrocarbons and other molecular emission bands, causing baseline upsets. While it is true that it is expensive to maintain and operate, it is a powerful tool for elemental speciation, not only for sulfur but also for many other element specific compounds. It offers a true compound independent calibration that gives a linear response regardless of the source molecule.

There are some other techniques for total sulfur determination (no speciation), or H2S (only) determination which include:

a- Oxidative Combustion Microcoulometry: The sulfur species are either oxidized or hydrogenated prior to detection. Again, detection limits are highly dependent on sample matrix. Incomplete combustion can cause low results. Oxidative combustion microcoulometry uses an oxygen/argon stream to react with a gas sample at high temperature-producing SO2. The gas stream is sparged into a

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reaction cell and titrated by coulometrically generated triiodide ions. Sulfur is proportional to the electricity required to generate the triiodide. The dynamic range is from the ppm level to the % level, depending on the volume of gas sampled.

b- Lead acetate paper tape detection: The principle of using lead acetate paper tape method is based on that H2S reacts with lead acetate to form a brown PbS discoloration. The chemicals are impregnated in a paper tape. Total sulfur can be determined by a prior hydrogenation. The rate of color change or the color intensity is measured by photoelectric devices. It is generally used between 0.1-16 ppm concentration ranges. The chemical reaction is sensitive to temperature and gas flow changes. This technique is easy and simple but disposal of used tape can be a problem.

2-2 Determination of Nitrogen Compounds in Natural Gas: The presence of amines, ammonia, NOx, and other nitrogen containing compounds can be artifacts of gas-processing operations, or could be naturally occurring. They may cause operational difficulties and equipment malfunction due to their action as catalyst poisons. Nitrogen-containing species can affect the coatings, printing, glass manufacturing, industrial food processing, and the chemical manufacturing industries. The combustion of nitrogen-containing compounds can produce noxious and corrosive acids in gas flames. Again, gas chromatography coupled with an element selective detector, is the technique most commonly used to determine nitrogen-containing compounds. These include:

Atomic Emission Detector (AED): This detector is used as described before.

Nitrogen-Phosphorus Detector (NPD): The Nitrogen Phosphorus Detector (NPD) is similar in design to the FID, except that the hydrogen flow rate is reduced to about 3 ml/min and an electrically heated thermionic bead (NPD bead) is positioned near the jet orifice. Nitrogen or phosphorus containing molecules exiting the column collide with the hot bead and undergo a catalytic surface chemistry reaction. The ions created in this reaction are attracted to a collector electrode, amplified, and output to the data system.

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Nitrogen Chemiluminescence Detector (NCD): The NCD works similarly to the sulfur chemiluminescence detector. Combustion of the GC eluent gas with air under H2 rich conditions yields NO. The NO molecule reacts with ozone to create electronically excited NO2. A chemiluminescence signal emitted upon decay back to the ground state is directly related to N concentration. A typical detection limit is 1 ppm. While diatomic nitrogen (N2) is not generally detected, there is a slight response to N2 so air leaks can be a problem. Like the (SCD), the detector requires precise control of the hydrogen and air flow rates, and the vacuum system

2-3 Determination of Halogen Compounds in Natural Gas: Chlorine-containing compounds and other halogens can be naturally occurring, or result from environmental contamination. Halogens can affect the coatings, printing, glass manufacturing, industrial food processing, and the chemical manufacturing industries. The combustion of halogens can produce noxious and corrosive acids in gas flames. Gas chromatography coupled with an element selective detector is the technique most commonly used to determine chlorine- and fluorine-containing compounds. The element selective detectors respond to halogens contained in the separated components. These include:

Electron Capture Detector (ECD): Atomic Emission Detector (AED): For these two detectors, the procedure employed with them is as described before. Electrolytic Conductivity Detector (ElCD): This detector reduces halogen compounds eluting from a GC to HX in a high temp catalytic converter. The HX is dissolved into a deionized solvent whose increase in conductivity is directly related to the halogen concentration. Depending on sample size, 0.05 ppm or lower detection limits can be achieved. Most interferences originate from the major components of the gas being analyzed and are seen as an upset in the baseline prior to the earliest eluting compound. This technique is very sensitive and simple to use.

Total fluorine and total chlorine methods can be also performed by Oxidative Combustion Microcoulometry method, using a fluorine or chlorine selective electrode system, or by pyrolysis, followed by detection using an ion selective electrode.

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2-4 Determination of Mercury in Natural Gas: Varying levels of mercury in natural gas have been encountered industry-wide throughout the world. It is typically a trace constituent, but in certain gas formations, significant amounts have been found in raw gas direct from the wellhead. Some of the mercury found in gas downstream of the wellhead has been introduced anthropologically. Although mercury is a toxicological and environmental problem, the primary impact that the presence of mercury has on gas-processing concerns is its potential to forms amalgams with a variety of metals, including aluminum. Mercury occurs in natural gas in the metallic form. In contrast, condensate associated with natural gas contains Hg in a variety of forms, including elemental, ionic and organometallic. As a result, Hg is not limited to a particular boiling fraction of a condensate.

Mercury in a hydrocarbon gas matrix at low concentrations is difficult to detect directly by spectroscopic methods (UV, Visible, IR, X-ray) because of interference by the hydrocarbon. Pre-concentration of the mercury in gas to a collector facilitates analysis. Collection methods for mercury in natural gas are used primarily because of the low concentrations that are often present. By using a collector, the total amount of mercury present in a large volume of gas can be concentrated into a liquid or solid matrix. This method will make it possible to use the ordinary methods to determine the Mercury in Natural Gas. To achieve better detection limits, samples must be analyzed off-site. However, mercury must be collected from a gas sample before the gas enters a pipeline, due to potential loss of mercury to the pipeline walls. For example, it is observed that a 60 % reduction in mercury content (30 µg/Nm3) during gas transit through a 68-mile pipeline. Similarly, accurate Hg measurements cannot be made on stored natural gas samples or on bottom hole samples, since loss of mercury to container walls and small gas sample volumes render such analyses spurious. In light of these constraints, preferred Hg sampling methods include: a- The dry method: In this method a solid sorbent material is exposed to a large volume of natural gas in order to collect and concentrate the mercury in small sample. Then this sample is transferred to the laboratory for analysis where the mercury is desorbed by heating and evaporation. Also there are two methods used by the try method.

(1) Using gold collector sputtered on an inert material: In this method natural gas is flown across a gold (or silver) collector

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sputtered (coated) on inert material like quartz or silica. Gold is less susceptible to deactivation by poisoning and oxidation than silver. The gold amalgamates with mercury to scavenge elemental mercury. Organic mercury amalgamates as well but slower than elemental necessitating low flow rates and long sampling times if the total mercury concentration is required. Bulk sorbent traps consisting of foil or wire should be avoided because mercury may diffuse into the metallic structure over an extended storage time, making complete recovery difficult. (2) Using Iodated Carbon: Carbon impregnated with potassium iodide is also used to scavenge mercury from gas matrices resulting in concentration of a sufficient quantity of mercury on the solid adsorbent for routine digestive analysis. Iodated carbon traps are less sensitive to contaminants in hydrocarbons than gold traps. Iodated carbon traps also have complete capture capability for elemental and dialkyl mercury and a high capacity. In view of these attributes, the iodated carbon trap is used for unprocessed gas where reasonably high concentrations are expected.

b- The wet method: A prevalent wet collection method is to bubble gas (containing mercury) through a solution from which the mercury precipitates as a mercury salt. Usually permanganate solution is used where all mercury species are converted to mercuric ion. Mercuric ion is then reduced to elemental mercury and separated by volatilization into an inert gas stream for quantitative detection.

The instruments used in determining mercury in natural gas are:

(1) Atomic Absorption Spectroscopy (AAS): Mercury atoms are detected by measuring their absorbance of light from a mercury source lamp at a characteristic wavelength. If a sorbent tube is used, air is the carrier gas, allowing combustion and removal of some interferents. Field based instruments are not common; however, one manufacturer uses a direct (AAS) analysis technique with Zeeman background correction to reduce interferences.

(2) Atomic Fluorescence Spectroscopy (AFS): mercury atoms are detected by excitation of the sample stream using a laser or conventional line source. After the energy is absorbed, it decays, and the emitted light is measured perpendicular to the excitation beam. If a sorbent tube is used, argon is the carrier gas. Air interferes with atomic fluorescence so interferences that might have deposited on a sorbent tube cannot be removed by combustion.

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These methods measure mercury concentrations down to the ng/m3 level, depending on the volume of gas sampled. In AFS is more sensitive than (AAS), detection limits are typically 10-fold lower for the same volume of gas sampled. Because of its greater sensitivity, many portable instruments are available that allow field-based measurements. They might measure a gas stream directly, or cycle through a set of sorbent tubes in sequence.

2-5 Determination of other Metals in Natural Gas: Although elemental contamination is not usually present in natural gas, arsenic has been occasionally found in some raw natural gas wells at ppmv or sub-ppmv levels. Problems can occur if distribution control devices become contaminated, especially if the arsenic reacts with other components in the gas and precipitates form with substantial changes in gas pressure. Trace levels of metals can also affect end-user industries such as coatings, printing, glass manufacturing, industrial food processing, and chemical manufacturing. Sample collection procedures were investigated using a patented iron chloride impregnated sorbent for arsenic, and a sparger method for arsenic and other metals except mercury. Long sampling times are required for the best detection limits.

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Conclusions and Recomendations

1- Natural gas like other petroleum products contains number of dangerous materials like Sulfur compounds, Nitrogen compounds, Mercury, Arsenic...etc, which may harm the human health and damage the industrial equipments.

2- Some of these materials find their way to the environment during the burning of unused natural gas in the fields or in the refineries, or through its utilization like a fuel or raw material to other industries.

3- Though these materials are only present in natural gas in very small quantities (traces), their accumulative effects due to the long utilization of natural gas can be very dangerous to the public health and to the downstream equipments.

4- Two kinds of damages can be produced due to the presence of these trace constituents: a- Corrosion of equipments, tanks, and transferring pipes. b- Poisonous effects on the health of the working, and utilizing

persons. 5- There are no routine tests performed to determine these traces in most

laboratories of the Iraqi oil and gas companies. 6- No routine medical tests are done to the check the health of the people

who work in gas production, and gas processing. 7- No environmental surveys are performed to detect the poisonous

traces which find their way to the environment during the gas production, gas processing, and gas utilization.

8- There are no health researches in the country which study the disease symptoms, and the toxic effects that may be caused by these toxic trace constituents.

According to the above conclusions the following suggestions are recommended: 1- The determination of traces in natural gas (and other petroleum

products) should be performed as routine tests during all the possessing stages starting from production to marketing.

2- The petroleum laboratories must be equipped with new modern analytical instruments, which are capable to determine toxic traces in the natural gas and petroleum products however small they are.

3- High level training courses must be opened to qualify the analysts on the new methods used in determining the traces in natural gas and petroleum products.

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4- Annual routine medical tests must be done to check and monitor the health of the persons who work in the fields of gas production and gas processing.

5- The medical and scientific organizations must be encouraged to perform researches to study any unusual symptoms on the health of the natural gas consumers.

6- The environmental organizations must be charged to perform surveys to detect any toxic materials like Mercury and Arsenic in the houses of the LPG consumers.

7- If the presence of the heavy metals in natural gas is confirmed then new processing units must be constructed and installed to remove these materials however small quantities of them are detected.

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References

1- Introduction to Chemical Analysis – Robert D. Braun / 1983 2- Instrumental Methods Chemical Analysis – Galen W. Ewing 1985 3- ASTM standards Part 24 4- ASTM standards Part 25 5- ASTM standards Part 26 6- ASTM standards Part 42 7- Pipeline & Gas Journal / July 2003 8- http://www.psanalytical.com/whats_ new.html 9- http://www.hgtech.com/basic/toc.htm 10- http://www.pipelineandgasjournal.com 11- http://www.varianinc.com 12- http://www.analytic-jena.de/e/bu/as/contact.html 13- http://eichem.kaist.ac.kr/vt/chem-

ed/optics/source//amps.htm#hollow-cathode 14- http://www.chem.agilent.com/scripts/cac_requestaquote.asp 15- http://www.chem.agilent.com/cac/cabu/gccolchoose.htm 16- http://www.agilent.com 17- http://oiltracers.com/sampl.html 18- http://www.gascape.org/ 19- http://masspec.scripps.edu 20- http://orgchem.colorado.edu 21- http://www.chem.vt.edu 22- http://www.cmsfieldproducts.com 23- http://www.globalspec.com/ 24- http://www.ionicsinstruments.com/ 25- http://www.my-link.ws/set

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The Researcher Name: Wisam Al-Shalchi Date and Place of Birth: Baghdad / Iraq - 1956 Nationality: Iraqi Place of Residence: Amman – Jordan Telephone No.: 00962-6-5621049, Mobile: 00962-785200764 E-mail: wisamalshalchi@yahoo.com Level of Education: 1) M.Sc. in Petroleum Chemistry - University of Essex / UK 2) B.Sc. in Chemistry- University of Baghdad

Experience: 1) Working as Process Engineer at Shwaikhat Oil Refinery which

belongs to OMV Petroleum Company/ Vienna – Austria. 2) Working as Process Engineer at Dora Oil Refinery/ Baghdad – Iraq 3) Working as Technical Lecturer in the Iraqi Oil Institute. 4) Working as Head of Oil & Gas Technologies in the Iraqi Oil Institute. 5) Working as Senior Technical Observer in the United Nations

Development Programme (UNDP). 6) Working as Petroleum Expert in the Directorate of Studies &

Planning & Follow-up – Ministry of Oil / Iraq. Publications: 1) Petroleum Environment Directory – Iraq, 2007 2) Industrial Safety – Iraq, 2007 3) Environment Protection – Iraq, 2006. 4) Instrumental Chemical Analysis – Iraq, 1994. 5) Oil and Gas Technology – Iraq, 1992 Published Researches and Studies: 1) Carbon Capture & Storage - 2008 2) Gas To Liquids Technology (GTL) – 2006 / Won the international

annual prize of the Oapec Organization for the year 2006. 3) Determination of Traces in Natural Gas. – 2005 4) Using Natural Gas Derivatives as Fuels for Vehicles – 2005 / Won an

appreciation prize from UNEP. 5) Compressed Natural Gas (CNG). – 2004 6) Comprehensive Petroleum Education and Training in Iraq - 1996 7) Development of Phenolic Plastics prepared in acidic medium. – 1990 8) Mechanism of the acid catalysed hydrolysis of esters. - 1988

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