Technology Development Network

GC/MS Analysis of Retorted Shale Oil

Figure 1

Stites & Associates, LLC, has developed a GC/MS method for characterizing retorted oil from oil shale.  The main purpose is to track characteristics like mono- and di-olefins, nitrogen and oxygen containing compounds and substituted aromatic compounds.  These are important not only for general oil quality, but especially oil stability and reactivity.  These compounds are notorious for polymerizing over time causing downstream problems for fuel applications.  On the other hand, controlled chain growth can give very beneficial results in other oil applications.

In Figure 1 above we see a typical retort oil across the broad spectrum of compounds that are found in the oil.  Not easily seen are a series of doublets including straight chain paraffins and the corresponding terminal olefin.  This is shown more clearly in Figure 2, a smaller portion of the chromatogram.

Figure 2

An expansion of the chromatogram around 13 minutes clearly shows the doublet of 1-tridecene and n-tridecane.  The doublet TIC is shown in Figure 3 while Figures 4 and 5 show the mass spectra of n-tridecane and 1-tridecene respectively.

Figure 3

 

Figure 4

Figure 5

By measuring the ratios of alkanes to their related 1-alkenes the general reactivity of the retort oil can be tracked.  The results of a variety of changing retort conditions can be monitored to tailor the oil to desired properties.

More work is being done in the area of nitrogen, oxygen and aromatic compounds.  These are more complex and their impacts less obvious.  Work is continuing in this are as SALLC continues to support the oil shale industry.  Similar detailed analysis of other oils can also give insight into a wide variety of physical and chemical properties.

 

Stites & Associates, LLC, is a group of technical professionals who work with clients to improve laboratory performance and evaluate and improve technology by applying good management judgment based on objective evidence and sound scientific thinking.  For more information see: www.tek-dev.net.

 

 

New Directions in Alternative Fuels?

Recent analytical requests from alternative fuel companies seem to have a common emerging theme.  There now seems to be considerable interest in the minor components for at least three reasons:

1. Possible environmental issues,
2. Possible quality issues with the fuel and
3. Possible useful by-products from the process.

The first two of these issues have been around for quite some time, but the third issue is relatively new.  It comes from the chemical complexity of the feedstocks and the possibility that small amounts of very valuable chemicals might be produced.  This has initiated a whole host of new analytical challenges.

Alternative fuel feedstocks are generally macromolecules containing carbon, hydrogen, oxygen, nitrogen and sulfur.  Different materials and processes lead to a wide variety of minor components including:

1. Organic acids, aldehydes, ketones and alcohols
2. Organic amines, pyrines and imidazoles
3. Aromatic compounds – including polyaromatic rings
4. Organic mercaptans, sulfides and thiophenes
5. Essential oils (terpenes and terpenoids)

There is no one analytical technique that is suitable for all of these compounds.  Some are very volatile.  Some are very water soluble.  Some are extremely polar.  Some are extremely reactive. 

Many of these can be analyzed by Gas Chromatography with a Mass Spectrometer detector (GC/MS).  Some compounds are volatile enough to be exchanged into a gas phase by headspace analysis.  Less volatile compounds often require extraction or exchange into an appropriate organic solvent.  Many times reactive or polar compounds must be derivatized to make them suitable for GC/MS analysis.  After proper sample preparation, the proper column must be selected.  These can range from non-polar columns such as 100% methyl-silicone to highly polar columns coated with poly-glycols.  Further complexities include selecting deactivation techniques for the analytical column, injection conditions, temperature programming and, of course, mass-spectral interpretation.

SALLC is very familiar with these and many other analytical techniques for identifying and quantitating a wide range of compounds (including all those listed above).  If your company has a complex analytical issue please contact us.  We would be glad to help.  Feel free to email Ron Stites at [email protected].

CALIBRATION OF GC/MS AT HIGH LEVELS IN SYNGAS

INTRODUCTION:

Gasification of bio-mass and wastes can result in a wide range of gaseous compounds.  Some of these can be noxious and even toxic.  The levels can be relatively high reaching hundreds of ppm for some of the more volatile components.  Among some of the compounds of that have been noted are:

• Benzene
• Toluene
• Ethylbenzene
• m,p-Xylenes
• o-Xylene
• Naphthalene
• 1-Methylnaphthalene
• Acenaphthene
• Acenaphthylene
• Anthracene
• Fluorene
• Pyrene
• Phenol
• 3-Methylphenol
• Furfural
• Catechol
• Di-n-Butylphthalate
• Bis-(2-Ethylhexyl)phthalate

 Many other compounds are possible depending on the type of feedstock. 

In order to address the widest variety of compounds a GC/MS method has been developed.  One of the more challenging features of the methodology is creating a wide variety of quantitative standards.  This paper describes the analytical approach in some detail.

GENERAL APPROACH:

Some compounds are of very specific interest.  In particular benzene, toluene, ethylbenzene and xylenes (BTEX) are always of great interest -- especially when lignins or aromatic based plastics are present.  These compounds can be purchased in a certified gas standard, but only up to about 20 ppm at any reasonable pressure.  At higher pressure the heavier compounds tend to precipitate.  For toluene, ethylbenzene and xylene, the 20 ppm standard is usually sufficient for use with effective gasification.  These compounds are generally at or below 20 ppm in the syngas stream.  It appears that single or double methylated benzenes are not particularly stable to thermal degradation.  Benzene, however, often exceeds 20 ppm by a substantial margin.  Hence, additional standards are needed for benzene.  Beyond these BTEX compounds there are other gas standards commercially available, but these are for use in ambient air testing.  The concentrations for these compounds are generally 1 ppm or less.  Several other compounds (especially naphthalene) are in concentrations well in excess of 1 ppm.  These kinds of standards both not very useful and prohibitively expensive. 

Gas standards can be created dynamically using permeation tubes or diffusion tubes.  These devices provide a constant source of pure compound.  The rate of permeation or diffusion depends on the construction of the device and the temperature.  The constant flow of pure compound is carefully diluted with a known amount of gas (usually nitrogen or air).  If the rate of permeation or diffusion is known and the gas flow has been carefully calibrated, the concentration of the gas can be calculated.  For many liquid compounds, the device can be calibrated gravimetrically by running the device at a constant temperature for several hours.  

One of the most important compounds to monitor is benzene.  This compound is often found at concentrations up to several thousand ppm.  This compounds happens to be one of the most convenient for creating suitable standards using a diffusion device.  It is also one of the more convenient compounds for analysis by GC/MS.  It appears in a wide variety of environmental standard mixes and has a well behaved mass spectrum.  Benzene has a unique mass spectrum with a strong molecular ion at 78 amu.  This ion can be used to calibrate for benzene with little danger of interference from other gases.  By using an appropriate diffusion device and flow rates, dynamic standards between a few hundred and a few thousand ppm can be generated.  Linearity and detection limits can be readily tested by analyzing these dynamic standards.

Generating other standards are not nearly as convenient – especially for compounds with low volatility.  Nevertheless, by injecting dilute mixes with known ratios between benzene and other compounds of interest, relative response factors (RRF’s) can be generated.  To give the widest flexibility of this approach, these RRF’s are generated based on the Total Ion Count (TIC) data.  These tend to be fairly closely related to the total mass being injected – at least for compounds of similar chemical structure.  These RRF’s can also account for differences in volatility.  These RRF’s can be used to give good estimates for the concentration of many other compounds.  An especially convenient approach is to use the benzene in the sample similar to an Internal Standard.  The benzene content of the sample is calculated from the 78 ion response and a dynamic RF for the sample is calculated from the TIC area and the calculated benzene concentration.  The RRF’s are then used to estimate the concentration of other compounds using the formula:

X_C ~ (X_Benzene/TIC_AreaBenzene) * (TIC_AreaC/RRF_C)

Where:

X_C = Concentration of Compound C in ppm

X_Benzene = Concentration of Benzene in ppm

TIC_AreaBenzene = TIC Area of Benzene

TIC_AreaC = TIC Area of Compound C

RRF_C = Relative Response Factor of Compound C to Benzene

 As it turns out, benzene is one of the most ubiquitous and persistent of all gasification by-product gases.  With few exceptions, syngas from feedstocks containing any woody bio-mass (contains lignins) or waste (usually contains plastic), will contain measurable amounts of benzene.  Hence, direct calibration for benzene combined with indirect calibration for other compounds using benzene as an “internal standard” will work well for almost any bio-mass or waste derived syngas.  Where benzene does not exist the estimation can still be made.  The factor X_Benzene/TIC_AreaBenzene from the most recent calibration check is substituted into the equation. 

SELECTING A DIFFUSION DEVICE:

Diffusion rates have to balanced with calibrator flow rates and desired concentrations.  A VICI 450 calibrator was used for the making the dynamic standards.  This is pictured below:

 

 

The flow rate for the VICI 450 could be varied from a low of about .6 L/minute to a high of about 11 L/minute.  In many gasification applications the concentration of benzene ranges from a low of around 50 ppm to a high of 5000 ppm.  Hence, the most desirable “mid-range” needed is around 500 to 1000 ppm.  In order to achieve approximately 500 ppm at a mid-range flow of 2.0 L/min at diffusion rate of:

In order to achieve such a high diffusion rate, a large diameter, relatively short diffusion tube was needed.  Diffusion rate is approximated by:

r = 1.90 x 10⁴ T D_o M (A/L) log₁₀(P/(P-p))

Where:

r = rate of diffusion (ng/min)

T = Temperature (^oK)

D_o = Diffusion coefficient (~0.0849 for benzene)

M = Molecular weight (g/mole)

A = Area of diffusion capillary (cm²)

L = Length of diffusion path (cm)

P = atmospheric pressure (mm Hg ~ 625 @ Denver)

P = vapor pressure of chemical at T (mm Hg ~483 mm @ 66^oC)

 

A Type D diffusion vial with a 25.4 mm tube length was selected.  It is pictured below:

 

The diffusion device was altered by adding a lead weight to the glass legs to keep it from flipping over in the VICI 450.

 CALIBRATION OF DIFFUSION DEVICE:

Multiple runs of approximately 2 hours were made with the Type D diffusion vial.  The VICI 450 was operated at 66^oC to a high vapor pressure for benzene and to melt other important compounds (like phenol) if they were to be also calibrated by diffusion.

The results for six runs are shown below:

The diffusion rate was 3.5292E+6 ng/min.  This was very close to the desired diffusion rate.  The VICI 450 was operated at three different settings. 

CALIBRATION OF THE VICI 450 FLOW:

The VICI 450 flow was measured using a wet test gas meter.  The various flows were recorded and used to calculate the dynamic benzene concentrations.   In all cases the diffusion device was held at 66^oC.  The actual diluted gas temperature was found to be virtually invariant and very close to 25^oC.  The calculated concentrations are shown below:

DYNAMIC RANGE CHECK:

Five dynamic standards (S2-1, S2-3, S2-5, S2-9 and S1-9) and the one 20 ppm certified standards were analyzed by GC/MS in duplicate.  The areas (ion 78 and TIC) were plotted against concentration to test for linearity.  These are shown below:

 

 

The curve showed excellent linearity (R² = 0.9946 and 0.9933 for TIC and ion 98 respectively) out to 1200 ppm.  This is generally adequate for most samples.  The range can be extended by:

1. Using a fair linear approximation out to 2500 ppm with correlations of 0.991 and 0.986 for TIC and ion 78 respectively, or
2. If greater accuracy is needed, using a quadratic curve where the correlations are 0.9988 and 0.9985 for TIC and ion 78 respectively.  This would require routinely updating the curve with at least two standards (counting a force through zero).

CREATING RRF’s:

A methanol/acetone solution of benzene and several other compounds commonly found in the syngas was gravimetrically prepared.  It was diluted to put all of the TIC area responses at the upper end of the linear range of the GC/MS (40 to 60 million counts).  The standard was injected and the relative response factors were calculated.  The data are shown below:

It is interesting to note that the response by GC/MS is highly dependent on the mass and the complexity of the compound.  This makes sense in that more complex molecules tend to give more ions.  Hence, if very accurate data is needed for other compounds of interest additional RRF’s should be developed.  This could actually be done after the testing is concluded if needed.

SUMMARY:

GC/MS can be used quite effectively for monitoring a wide range of compounds in syngas.  Calibration can be a challenge, but by using permeation tubes and diffusion tubes a wide range of compounds can be calibrated directly.  Other compounds can be estimated by using RRF's to compounds of known calibration such as benzene.

Shale Tech International and Stites & Associates Collaborate on Isokinetic Sampling Apparatus:

Shale Tech International uses the Paraho II Retorting Technology to extract oil from oil shale.  One of the challenges is to cost effectively capture the produced oil from the hot gases leaving the retort.  There are numerous ways to do this.  The problem is picking the most cost effective solution.  Naturally the first question is, “Just how much oil is in the gas stream?”  Shale Tech International contacted Stites & Associates, LLC, to help sort out this problem.

The sampling was complicated by the stream containing condensable gases, oil and water droplets and some fine particles.  These were comingled in unknown ratios and were flowing at elevated temperatures but at sub-ambient pressures.  It was not a simple task to collect a representative sample from such a stream.  The approach suggested by Stites & Associates was to custom build an isokinetic sampling device which could operate at a slight vacuum.  Such a device extracts the gas sample from the process stream without changing gas velocity (hence, “isokinetic”).  This is illustrated in the graphic below: 

 

By sampling at the same velocity as the process gas flow, different size particles and droplets in the gas stream are sampled without discrimination.  This results in a much more representative sample.  When the sampling velocity is less than the process velocity (see w>v above) then light particles and droplets will go around the sampling nozzle and the sample will be biased.  When the sampling velocity is greater than the process velocity the opposite bias occurs.

To achieve good isokinetic sampling Stites & Associates scientists and Shale Tech engineers had to custom design and build the system.  This included the velocity measurement equipment, thesampling  nozzle(s), the water/oil trapping system, the gas volume measurement system and a vacuum pumping system.  The final apparatus included:

• A custom built nozzle(s)
• Stainless steel sampling probe(s)
• An ice-bath chilled set of stainless steel traps
• A vane type vacuum pump
• A Ritter™ precision gas meter
• An “S-type” pitot tube
• A precision digital vacuum gauge (for differential pressures)
• A manometer (for static pressure)

 The complete system looked similar to the system pictured below.  The major difference was that it was necessary for the dry gas meter to operate at sub-ambient pressure because positive displacement pumps were not available.

 

 

The oil/water mix was collected and weighed.  The collected oil could also be tested for physical parameters such as viscosity and boiling point distribution. 

Once the system was constructed and tested, it became clear that the data collected was very instructive.  Shale Tech now had a reliable way to measure oil collection efficiency.  This made optimization of operating parameters and comparison of different methods for collection feasible.  Shale Tech continues to use this sampling methodology to improve their shale oil recovery process. 

This is a good example of how good measurement techniques pay great dividends for process evaluation and optimization.  It is also a great example of how collaboration between clients and suppliers can result in innovative solutions.

 

Acknowledgements:  Stites & Associates would like to acknowledge the fine work done by Shale Tech International engineers and scientists.  Especially instrumental in the success of this project were the efforts and suggestions of Phil Hansen and Justin Bilyeu of Shale Tech International.