On-line Analysis of Nitrogen Containing Compounds in Complex Hydrocarbon Matrixes

Summary

A method combining comprehensive two-dimensional gas chromatography with nitrogen chemiluminescence detection has been developed and applied to on-line analysis of nitrogen containing compounds in a complex hydrocarbon matrix.

Abstract

The shift to heavy crude oils and the use of alternative fossil resources such as shale oil are a challenge for the petrochemical industry. The composition of heavy crude oils and shale oils varies substantially depending on the origin of the mixture. In particular they contain an increased amount of nitrogen containing compounds compared to the conventionally used sweet crude oils. As nitrogen compounds have an influence on the operation of thermal processes occurring in coker units and steam crackers, and as some species are considered as environmentally hazardous, a detailed analysis of the reactions involving nitrogen containing compounds under pyrolysis conditions provides valuable information. Therefore a novel method has been developed and validated with a feedstock containing a high nitrogen content, i.e., a shale oil. First, the feed was characterized offline by comprehensive two-dimensional gas chromatography (GC × GC) coupled with a nitrogen chemiluminescence detector (NCD). In a second step the on-line analysis method was developed and tested on a steam cracking pilot plant by feeding pyridine dissolved in heptane. The former being a representative compound for one of the most abundant classes of compounds present in shale oil. The composition of the reactor effluent was determined via an in-house developed automated sampling system followed by immediate injection of the sample on a GC × GC coupled with a time-of-flight mass spectrometer (TOF-MS), flame ionization detector (FID) and NCD. A novel method for quantitative analysis of nitrogen containing compounds using NCD and 2-chloropyridine as an internal standard has been developed and demonstrated.

Introduction

The reserves of light sweet crude oils are gradually diminishing, and hence, alternative fossil resources are being considered to be used in the energy and petrochemical industry. In addition, renewables such as bio-oils produced by fast pyrolysis of biomass are becoming a more attractive resources of bio-based fuels and chemicals. Nevertheless, heavy crude oil is a logical first choice because of the large proven reserves in Canada and Venezuela^1-3. The latter are being recognized as the largest crude oil reserves in the world and their composition is similar to the composition of natural bitumen. Similar to bio-oils, heavy crude oils differ from light crude oils by their high viscosity at reservoir temperatures, high density (low API gravity), and significant contents of nitrogen, oxygen, and sulfur containing compounds^4,5. Another promising alternative is shale oil, derived from oil shale. Oil shale is a fine-grained sedimentary rock containing kerogen, a mixture of organic chemical compounds with a molar mass as high as 1,000 Da⁶. Kerogen can contain organic oxygen, nitrogen, and sulfur in the hydrocarbon matrix; depending on the origin, age, and the extraction conditions. Global characterization methods have shown that the concentration of heteroatoms (S, O and N) in shale oil and heavy crude oils is typically substantially higher than the specifications set for the products used in for example the petrochemical industry⁶. It is well documented that nitrogen containing compounds present in heavy conventional crude oil and shale oil have a negative effect on the catalyst activity in hydrocracking, catalytic cracking and reforming processes⁷. Similarly, it has been reported that the presence of nitrogen containing compounds are a safety concern because they promote gum formation in the cold-box of a steam cracker⁸.

These processing and safety challenges are a strong driver to improve the current methods for off-line and on-line characterization of nitrogen containing compounds in complex hydrocarbon matrices. Two-dimensional gas chromatography (GC × GC) coupled with a nitrogen chemiluminescence detector (NCD) is a superior characterization technique compared to one-dimensional gas chromatography (GC) for analyzing conventional diesels or liquefied coal samples⁷. Recently a method has been developed and applied to the offline characterization of nitrogen content in shale oil⁶, the identification of extracted nitrogen compounds present in middle distillates⁹, and the determination of the detailed composition of plastic waste pyrolysis oil¹⁰.

It is thus clear that GC × GC analysis is a powerful offline processing technique for analyzing complex mixtures^11-17. However, on-line application is more challenging due to the need for a reliable and non-discriminating sampling methodology. One of the first developed methodologies for comprehensive on-line characterization was demonstrated by analyzing steam cracking reactor effluents using a TOF-MS and a FID¹⁸. The optimization of the GC settings and an appropriate column combination enabled analysis of samples consisting of hydrocarbons ranging from methane up to polyaromatic hydrocarbons (PAHs)¹⁸. The present work takes this method to a new level by extending it to the identification and quantification of nitrogen compounds present in the complex hydrocarbon mixtures. Such a method is among others needed to improve fundamental understanding of the role these compounds play in several processes and applications. To the authors' best knowledge, information concerning kinetics of conversion processes of nitrogen containing compounds is scarce¹⁹, partly due to the lack of an adequate method to identify and quantify nitrogen containing compounds in the reactor effluent. Establishing the methodology for offline and on-line analyses is thus a prerequisite before one can even attempt feedstock reconstruction^20-27 and kinetic modeling. One of the fields which would benefit from the accurate identification and quantification of nitrogen containing compounds is steam cracking or pyrolysis. Bio and heavy fossil feeds for steam cracking or pyrolysis reactors contain thousands of hydrocarbons and compounds that contain heteroatoms. Moreover, because of the complexity of the feed and the radical nature of the occurring chemistry, ten thousands of reactions can occur among the thousands free radical species²⁸, which makes the reactor effluent even more complex than the starting material.

In hydrocarbon mixtures nitrogen is mainly present in aromatic structures, e.g., as pyridine or pyrrole; hence most experimental efforts have been dedicated to the decomposition of these structures. Hydrogen cyanide and ethyne were reported as major products for the thermal decomposition of pyridine studied in a temperature range of 1,148-1,323 K. Other products such as aromatics and nonvolatile tars were also detected in minor quantities²⁹. The thermal decomposition of pyrrole was investigated in a broader temperature range of 1,050-1,450 K using shock wave experiments. The main products were 3-butenenitrile, cis and trans 2-butenenitrile, hydrogen cyanide, acetonitrile, 2-propenenitrile, propanenitrile, and propiolonitrile³⁰. Additionally thermal decomposition shock tube experiments were performed for pyridine at elevated temperatures resulting in comparable product spectra^31,32. Product yields in these studies have been determined by applying GC's equipped with a FID, a nitrogen-phosphorus detector (NPD)³¹, a mass spectrometer (MS)³² and a Fourier transform infrared (FTIR) spectrometer³². A similar methodology implementing the FID and the NPD was applied to analyze the shale oil pyrolysis products in a continuous flow reactor⁸. Using a cold trap at 273.15 K and GC-MS, Winkler et al. ³³ showed that during pyridine pyrolysis heteroatom-containing aromatic compounds are formed. Zhang et al.³⁴ and Debono et al.³⁵ applied the method of Winkler et al. for studying the pyrolysis of organic waste. The nitrogen rich reaction products were analyzed on-line, using a GC coupled to a thermal conductivity detector (TCD)³⁴. The collected tars were analyzed offline using GC-MS^34,35. Simultaneous pyrolysis of toluene and pyridine showed a difference in soot formation tendency compared to pyridine pyrolysis, indicating the complex nature of the free-radical reactions^31,36.

One of the most comprehensive analytical methodologies was developed by Nathan and co-workers³⁷. They used FTIR, nuclear magnetic resonance (NMR) and GC-MS for analyzing decomposition products of pyridine and diazine and electron paramagnetic resonance (EPR) spectroscopy for tracing free radical species. FTIR analysis can be a very effective approach for the identification of a large range of products, even PAHs^38-40, nevertheless quantification is extremely challenging. Calibration requires a full set of infrared spectra at different concentrations for each target species at a specific temperature and pressure⁴¹. Recent work of Hong et al. demonstrated the possibilities of using molecular-beam mass spectrometry (MBMS) and tunable synchrotron vacuum ultraviolet photoionization for determination of products and intermediates during pyrrole and pyridine decomposition^42,43. This experimental method enables selective identification of isomeric intermediates and near-threshold detection of radicals without inflicting fragmentation of the analyzed species⁴⁴. However, the uncertainty on the measured concentrations using MBMS analysis is also substantial.

In this work, first the offline comprehensive characterization results of the complex shale oil are reported. Next, the limitations of using an on-line GC × GC-TOF-MS/FID for the analysis of nitrogen compounds in a complex hydrocarbon matrix are discussed. Finally, the newly developed methodology for the on-line quantification of nitrogen containing compounds by GC × GC–NCD is demonstrated. The qualitative analysis of products was carried out using TOF-MS, while FID and NCD were used for quantification. The application of the NCD is a substantial improvement compared to using the FID because of its higher selectivity, lower detection limit and equimolar response.

Protocol

Caution: Please consult relevant material safety data sheets (MSDS) of all compounds before use. Appropriate safety practices are recommended. Solutions and samples should be prepared in the fume hood, while using personal protective equipment. Best practice implies use of safety glasses, protection laboratory gloves, lab coat, full length pants, and closed-toe shoes. The reactor should be properly sealed as several reactants and reaction products can be acutely toxic and carcinogenic.

1. Offline GC × GC–NCD Analysis

1. Sample Preparation for offline GC × GC analysis
   1. Select 2-chloropyridine as an internal standard. Make sure that the concentration of the internal standard in the prepared sample is within the range of the concentrations of nitrogen containing compounds present in the analyzed mixture. The concentration of the internal standard must be at least ten times higher than the detection limit of the NCD. Furthermore the concentration of the internal standard should not be more than two times higher than the concentration of the nitrogen compound with the highest concentration in the sample.
   2. Prepare and weigh two glass vials using an analytical balance.
   3. Add 20.498 mg of 2-chloropyridine to the first vial using an analytical balance.
   4. Tare the balance with the first vial and add 1024.287 mg of a shale oil sample. Gently mix the vial content.
   5. Transfer 23.369 mg of the prepared mixture into a second vial and add 435.195 mg of the original shale oil sample. The obtained concentration of the internal standard in the sample is approximately 1,000 ppmw.
2. GC × GC–NCD analysis and operating conditions
   1. Perform the analysis using a GC × GC equipped with a dual-stage cryogenic liquid CO₂ modulator⁴⁵.
      1. Connect the first dimension non-polar column (PONA, 50 m L × 0.25 mm I.D. × 0.5 µm d_f) directly to the second dimension mid-polar column (BPX-50, 2 m L × 0.25 mm I.D. × 0.25 µm d_f). Apply the GC × GC operating conditions optimized for detection of nitrogen containing compounds in shale oil, with the parameters presented in Table 1.
      2. Inject the sample using the auto sampler, with the settings stated in the Supplementary Table.
   2. For the NCD, set the flow rates at 5 and 11 ml min^-1 for the H₂ and O₂ respectively, while keeping the burner temperature at 1,198 K. Use a data acquisition rate of 100 Hz. The detector settings are presented in Table 1.
3. Reprocessing of results
   1. Perform data acquisition using the data system for the NCD. Export the raw data to a .cdf file and import into an analytical software (e.g., GCImage).
   2. Use the analytical software to construct the contour plot, to determine the retention times, and to perform peak fitting and blob integration according to the software protocols. Identify each blob as a compound of a specific nitrogen group with a specific carbon number.
      Note: Response signal of NCD is equimolar^6,46,47,48, hence the procedure for quantification of nitrogen containing compounds in the sample is straightforward.
   3. Calculate the concentration of detected compounds using the following equation:
      
      where V_i and V_st are the peak volumes of the nitrogen containing compound i and the internal standard respectively. W_st is the concentration of the internal standard compound, N_i is the number of nitrogen atoms in compound i, M_i and M_st are the molar masses of the nitrogen containing compounds and of the nitrogen internal standard respectively⁶.

2. On-line Analysis

1. Pilot plant steam cracking unit
   Note: The pilot plant steam cracker consists of three sections: the feed section, the furnace/reactor section and the analysis section. The unit is explained in detail elsewhere^18,49,50,51. A schematic description of the setup can be found in the Supplementary Information.
   1. During the experiments, as heptane is fed using a computer controlled rotary pump by setting the flowrate using the Programmable Logical Controller (PLC), introduce pyridine to the reactor convection section by manually setting the speed of the piston pump. The diluent used depends on the purpose of the test and can be varied. Depending on the requirements of the experiment, use the gas controllers for helium and nitrogen dilution, and regulate the water feeding pump frequency for steam dilution.
   2. Prior to the reaction section of the furnace, evaporate and mix the feedstock in two separately heated zones kept at a temperature of 773 K.
   3. Impose a temperature profile to the Incoloy 800HT 12.4 m long and 9 mm internal diameter reactor by setting the temperatures in the five heated zones using the PLC. Different experiments with varying process conditions can be performed. Use the process conditions applied to perform method testing as outlined in Table 2.
2. Analysis methodology
   1. Use the refinery gas analyzer (RGA) equipped with three detectors working on parallel channels in order to detect as many compounds as possible. Two TCD's and a FID allow detection of CO, CO₂, H₂, and hydrocarbons up to four carbon atoms, using three different columns for separating the reaction products. The RGA setting are presented in Table 3.
   2. For the GC × GC set-ups, use a non-polar PONA (50 m L × 0.25 mm I.D. × 0.5 µm d_f) column in the first dimension and a mid-polar BPX-50 (2 m L × 0.15 mm I.D. × 0.15 µm d_f) column in the second dimension. Ensure that the temperature in the oven can be lowered to 233 K by addition of liquid nitrogen inside the GC oven. Use the GC × GC settings given in Table 4 for the on-line analyses.
   3. Ensure that TOF-MS electron impact ionization is 70 eV, and detector voltage 1,700 V by setting these values in the software. Additionally set the acquisition frequency to 30 spectra/sec in a mass range of 15-400 amu in the same software.
   4. For the NCD, manually set flow rates at 5 and 11 ml min^-1 for the H₂ and O₂ respectively, while keeping the burner temperature at 1,198 K. The data acquisition rate needs to be 100 Hz.
   5. For FID H₂, air and N₂ (make-up gas) flow rates are respectively 35, 350 and 35 ml min^-1. Ensure that the FID temperature is set at 573 K with a data acquisition rate of 100 Hz.
   6. Sample the pilot plant effluent on-line, i.e., during pilot plant operation, and at high temperature (673 - 773 K). Use a valve-based sampling system, positioned in the sampling oven kept at 573 K, see Figure 1.
      1. Turn the pneumatic 6-port 2-way valve to the position shown in Figure 1a using the PLC, flush the sample loop for at least 2 min.
      2. Turn the pneumatic 6-port 2-way valve to the position shown in Figure 1b using the PLC, a gaseous sample from the sample loop is carried with helium and injected onto selected GC × GC. Keep the valve in the injection position for 20 sec. Turn the pneumatic 6-port 2-way valve back in the position shown in Figure 1a.
3. Internal standard addition
   1. Use 2-chloropyridine as an internal standard for the NCD analyses. The introduced amount of this compound depends on the purpose of the experiment (values illustrated in Table 2). To add 2-chloropyridine precisely and uniformly, dissolve the compound in n-hexane (values illustrated in Table 2) and gentle stir the mixture after preparation.
   2. Add the internal standard 2-chloropyridine continuously by pumping the prepared mixture through a capillary transfer line, which is centrally positioned in the product stream before the sampling system. For this purpose use a peristaltic pump and keep the flow rate stable (values indicated in Table 2).
4. Data acquisition and quantification
   1. Acquire the data obtained with TOF-MS, and use the results for product identification and establishing the retention times of the reaction products. Acquisition and reprocessing of data obtained from FID is similar to the procedure applied to reprocessing of results gathered using the NCD, as explained in section 1.3.2.
   2. Recalculate the product yields using an internal standard¹⁸. Introduce a fixed amount of nitrogen (N₂) continuously to the product stream and use it as a reference compound as shown in Figure 2.
   3. Combine the data obtained on the different instruments. Apply the calibration factors for each compound depending on the detector.
      1. Set the methane calibration factor to 1.
      2. Obtain response factors relative to methane for permanent gases detectable on RGA by calibration. Ensure that for all major compounds detected on the GC × GC the relative response factors are determined by calibration. For minor products use the FID calibration factors reported by Dietz⁵². If the response factors for a compound are not available, calculate values using the group contribution method developed by Dierickx et al.⁵³.
      3. Methane functions as a secondary internal standard, and its concentration is needed to determine the concentrations based on the peaks of the GC × GC–FID chromatogram. Therefore determine the methane mass flow rate using equation 2:
         
         Where, f_CH4 and f_N2 are the calibration factors of methane and nitrogen on TCD respectively. A_CH4 and A_N2 are peak areas obtained during analysis, while is the nitrogen mass flow rate added to the effluent stream.
   4. Carry out quantification of the nitrogen containing products by applying the procedure explained in section 1.3.3.

Results

The chromatogram obtained using the offline GC × GC–NCD for characterization of nitrogen containing compounds in a shale oil sample is given in Figure 3. The following classes were identified: pyridines, anilines, quinolines, indoles, acridines, and carbazoles. Moreover, detailed quantification of the individual compounds was possible. The gathered data was used to determine the individual compound concentrations, and the obtained values are presented in Table 5

Discussion

The described experimental procedures enabled a successful comprehensive off-line and on-line identification and quantification of nitrogen containing compounds in the studied samples.

The separation of nitrogen containing compounds in shale oil was accomplished using GC × GC–NCD, as shown in Figure 3. Since the NCD cannot be used for identification, the retention times of the observed species need to be established in advance by carrying out analyses on the GC ...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
2-Chloropyridine, 99%	Sigma Aldrich	C69802	Highly toxic
Shale oil	Origin Colorado, US	Piceance Basin in Colorado, USA	Toxic
Pyridine, 99.8%	Sigma Aldrich	270970	Highly toxic
Carbon Dioxide, industrial grade refrigerated liquid	PRAXAIR	CDINDLB0D	Wear safety gloves and glasses
Helium, 99.99%	PRAXAIR	6.0
Hydrogen, 99.95%	Air Liquide	695A-49	Flammable
Oxygen	Air Liquide	905A-49+	Flammable
Air	Air Liquide	365A-49X
Nitrogen	Air Liquide	765A-49
Hexane, 95+%	Chemlab	CL00.0803.9025	Toxic
Heptane, 99+%	Chemlab	CL00.0805.9025	Toxic
Nitrogen, industrial grade refrigerated liquid	PRAXAIR	P0271L50S2A001	Wear safety gloves and glasses
Autosampler	Thermo Scientific, Interscience	AI/AS 3000
High temperature 6 port/2 position valve	Valco Instruments Company Incorporated	SSACGUWT
Gas chromatograph	Thermo Scientific, Interscience	Trace GC ultra
Rafinery Gas Analyzer	Thermo Scientific, Interscience	KAV00309
rtx-1-PONA column	Restek Pure Chromatography	10195-146
BPX-50 column	SGE Analytical science	54741
TOF-MS	Thermo Scientific, Interscience	Tempus Plus 1.4 SR1 Finnigan
NCD	Agilent Technologgies	NCD 255
Chrom-card	Thermo Scientific, Interscience	HyperChrom 2.4.1
Xcalibur software	Thermo Scientific, Interscience	1.4 SR1
Chrom-card software	Thermo Scientific, Interscience	HyperChrom 2.7
GC image software	Zoex Corporation	GC image 2.3