Analysis of Natural Gas Composition and BTU Content from Fracking Operations

Analysis of Natural Gas Composition and BTU Content from Fracking

Operations

Dr. John N. Driscoll & Jennifer MaclachlanPID Analyzers, LLC

Cape Cod, MA Pittcon Symposium: Advances in Energy Research: From

Unconventional Fuels to Solar EnergyMarch 10, 2015PM Session 3:05

Paper 1370-5

1

Introduction

• Natural gas is bought or sold based on the quantity of energy delivered. The product of the concentration (determined by gas chromatography) and the heating value (BTU) determines the BTU content of the fuel. Although the natural gas from Marcellus shale is primarily methane, the composition can vary considerably from region to region

• Shale gas streams can vary in composition from primarily CH4 to one that can contain heavier hydrocarbons (to C6+) species. The work for this paper was completed with our Model 301C gas chromatograph based natural gas analyzer is configured with dual detectors (FID & TCD), packed and capillary column capability and temperature programming. It has an embedded PC and Windows 7.0 operating system with PeakWorks™ chromatography control software. It is a compact industrial gas chromatograph in a 19” rack mount or wall mount enclosure. Outputs include RS485, and 4-20 mA. It can be connected to the internet and can be controlled remotely.

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What GC Detectors are needed for BTU determinations?

• The most popular detector for hydrocarbons is the flame ionization (FID) detector. This detector is specific for hydrocarbons, has a wide dynamic range (>106) and sub ppm detection limits. The disadvantages include the requirement for H2 and zero air as well as the 30-45 warmup time.

• The Thermal Conductivity detector(TCD) has a range from 100% to a few hundred ppm. The TCD has a universal response to fixed gases, and all inorganic and organic gases.

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Flame Ionization Detector

FID Schematic Description FID

An FID consists of a combustion source (a flame), an ion chamber, an igniter, a voltage source for the accelerating electrode, and an electrometer/amplifier

As illustrated here, the flame acts as the accelerating electrode or the bias electrode


FID Mechanism

• Flame ionization is the process of ionization that occurs in organic compounds when the carbon–carbon bond is broken via a thermal process (in the flame) that results in the formation of carbon ions. These ions are collected in the flame by applying a positive potential to the FID jet and the ions are pushed to the collection electrode where the current is measured. The response (current) is directly proportional to the concentration.


Thermal Conductivity Detector

Description of TCD

• Measures the difference in thermal conductivity between reference and sample stream

• Uses a Wheatstone Bridge which is shown here

• Universal response

• Linear range > 104

• Detection limit 100-200ppm

• Maximum value 100%

TCD Schematic


GC Instrumentation

Photo of GC 301CPeakWorks™ Software with

Windows 7 OS


The 301C GC has an 8” touch screen which displays all the chromatographic conditions and integration information

GC301 Fluidics

301 Gas Controls TCD/FID2 point Natural Gas System Gases-Photo of inputs and outputs


Other Components of Natural Gas


All these compounds can be separated on a packed mole sieve column: The advantage of the TCD is there might be other combustible gases in the sample and these can be used to bring up the BTU value of the gas

Inert Components & CO

• There are a number of BTU & inert components in natural gas which can be detected by a Thermal conductivity Detector (TCD):

– Inert components: N2, trace O2, CO2

– Combustible components not detected by the FID: CO & H2

– These can be separated on a packed molecular sieve column


Columns for HC Separation of C1 to C6 HCs

• Methane can be separated from ethane and ethylene easily on a 1 M packed column like HayeSep N as seen in the next slide. The problem is that to get the higher C3+ hydrocarbons off the column, temperature programming up to 200 degrees C is needed. This increases the analysis time considerably because the column then has to cool down to 40-50 degrees C to complete the separation.


C1-C2 HC Packed Column- FID

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This is a typical chromatogram of methane, ethane, ethylene and acetylene on a HayeSep N column

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Capillary Column Analysis of C1& C2’s

• We chose to use a thick film (5 micron methyl silicone film capillary column). We chose this column because it separates by boiling point of the hydrocarbons. The issue here is that methane(BP -161C) , ethane and ethylene are gases while all other boiling points for hydrocarbons are considerably higher and methyl silicone phase separates by boiling point. The boiling point of propane is -42C. The difficult part of the analysis is the separation of C1 and C2 hydrocarbons. The next slide demonstrates that the C1 & C2’s are separated.


C1 to C3 HC GC-Capillary Column


We’re achieving pretty good separation here:Ethane & Ethylene is 50%, Methane is baseline and this is separated from the propylene and propane.

C1 to C6 separation• With the same column and with temperature

programming from 40-70oC, we can separate all of the HC except propene and propane in 12 minutes. The higher hydrocarbons are important because of the higher BTU values compared to methane.


C1 to C6 HC GC-Capillary: Alkanes & Alkenes


The alkenes come out before the alkanes

Natural Gas• The major component of natural gas is

methane. The content can vary from 87-97%. The next highest component is ethane or ethylene and the other higher hydrocarbons are in the 1% or lower concentration. The BTU values for higher molecular weight hydrocarbons are larger because they have more carbon and hydrogen atoms to burn and generate heat.


Typical Natural Gas Composition

ComponentTypical Analysis Range

(mole %) (mole %)

Methane 95.0 87.0 - 97.0

Ethane 3.2 1.5 - 7.0

Propane 0.2 0.1 - 1.5

iso - Butane 0.03 0.01 - 0.3

normal - Butane 0.03 0.01 - 0.3

iso - Pentane 0.01 trace - 0.04

normal - Pentane 0.01 trace - 0.04

Hexanes plus 0.01 trace - 0.06

Nitrogen 1.0 0.2 - 5.5

Carbon Dioxide 0.5 0.1 - 1.0

Oxygen 0.02 0.01 - 0.1

Hydrogen trace trace - 0.02

Gross Heating Value (MJ/m3), dry basis *

38 36.0 - 40.2


BTU Values of Hydrocarbons-

This is why people are interested in measuring

these -Note the net heating values-these can really add up and

increase the value

Gas Gross Heating Value (Btu/Scf)

Net Heating Value (Btu/Scf)

Methane 1012 911

Ethane 1783 1631

Propane 2557 2353

isobutane 3354 3094

n-butane 3369 3101

isopentane 4001 3698

n-pentane 4009 3709

Neopentane 3987 3685

n-hexane 4755.9 4403.8

2-Methylpentane 4747.3 4395.2

3-Methylpentane 4750.3 4398.2

2,3-Dimethylbutane 4745 4392.9

n-heptane 5502.5 5100

2-Methylhexane 5494.6 5092.2

3-Methylhexane 5498.6 5096

2,2-Dimethylpentane 5481.9 5079.6

3,3-Dimethylpentane 5488.8 5086.4

Ethene (Ethylene) 1599.8 1499.1

Propene (Propylene) 2332.7 2181.8

1-Butene (Butylene) 3079.9 2878.7

cis-2-Butene 3072.2 2871

trans-2-butene 3068 2866.8

1-pentene 3826.5 3575


Natural Gas Sample

This is a sample of natural gas from Cape Cod using the same 30M column and conditions as slide # 16. Just C1 to C4 in this particular sample.


Natural Gas Analysis


We could run a TCD analysis here to see if there is any hydrogen or CO to burn so we can increase the value

Heating Content of Natural Gas


Natural Gas HC + Inert Compounds: FID TCD

Configuration

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The configuration for FID/TCD is shown here with dual columns run in parallel. One is packed and the other is a 30M capillary column.

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Natural Gas Compressor Measurements

Compressors• Natural Gas Compressors are

used to increase the pressure on a fluid and transport the fluid through a pipe to the next compressor. EPA requires the exhaust from these compressors to be monitored. The 301C natural gas analyzer can also monitor the exhaust gas for non methane hydrocarbons via EPA method 18 which reports C1 to C6

hydrocarbons less methane & ethane, which are not considered photochemically active

Natural Gas Compressor Station


An Example of Natural Gas Compressor Exhaust


These are the same conditions and column that we use for the natural gas analysis-We don’t see methane in this sample.

Conclusions

• Natural gas analysis & BTU content can be done using a GC equipped with both flame ionization and thermal conductivity detectors. Any conversion factors can be entered into the final analysis by the customer.

• The typical analysis time for C1-C6 analysis is 12 minutes.

• The analyzer can also be used to check the emissions from natural gas compressors via EPA Method 18.

• Want to learn more? Come by our booth in the Pittcon expo, #1326.


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Date post:	14-Jul-2015
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Analysis of Natural Gas Composition and BTU Content from Fracking Operations

Environment