Article pubs.acs.org/EF
Simultaneous Determination of Boiling Range Distribution of Hydrocarbon, Sulfur, and Nitrogen in Petroleum Crude Oil by Gas Chromatography with Flame Ionization and Chemiluminescence Detections Ramachandra Chakravarthy,†,‡ Ganesh N. Naik,† Anilkumar Savalia,† Jagdish Kedia,† Chandra Saravanan,† Asit Kumar Das,† Unnikrishnan Sreedharan,† and Kalagouda B. Gudasi*,‡ †
Reliance Industries Limited, Reliance Corporate Park, Research & Development Centre, Thane-Belapur Road, Ghansoli-400701, Navi Mumbai, Maharashtra, India ‡ Department of Chemistry, Karnatak University, Pavate Nagar, Dharwad-580003, Karnataka, India S Supporting Information *
ABSTRACT: We present a quick and efficient gas chromatographic method to simultaneously determine the boiling range distribution of hydrocarbon (C), sulfur (S), and nitrogen (N) in crude oils by a high temperature−CNS simulated distillation (HT-CNS SimDis) analyzer. The analyzer is a gas chromatograph equipped with flame ionization (FID) and sulfur and nitrogen chemiluminescence (SCD and NCD) detectors with simulated distillation features. The hydrocarbon yield profile of crude oil obtained by FID response was applied to calculate S and N content in various isolated fractions such as naphtha, kerosene, diesel, and vacuum gas oil. This method was used to analyze 10 different crude oils of variable composition. A fraction of crude oil that boils above the atmospheric equivalent temperature (AET) of 700 °C does not elute fully and forms a coke inside the chromatographic column. As a result, it is not possible to quantify total sulfur and total nitrogen content in the high-boiling vacuum residue (VR) fraction (565 °C and above) of crude oil by this method. However, we have addressed this issue by calculating sulfur in the VR fraction as a difference between total sulfur in crude oil (using X-ray fluorescence or combustion methods) and sulfur in the rest of the fractions (using HT-CNS SimDis). A similar technique was employed to determine nitrogen in the VR fraction of crude oil. The gas oil reference standard with known boiling range distribution was used to check the system suitability and generate the response factor for the calculation of hydrocarbon yield, and VGO NS Reference (internal nitrogen/sulfur QC standard) was used as a calibration standard for sulfur and nitrogen quantification. Currently, there is no single method available for the simultaneous determination of C, S, and N present in crude oil. This method produces detailed temperature distribution of S and N in a crude oil sample that cannot be obtained by either total sulfur and total nitrogen analysis or analysis of sulfur and nitrogen in discrete distillation cuts. As a result, this technique is extremely valuable to the refining industry for the valuation of crude oil, plant troubleshooting, and optimization of refinery processes.
1. INTRODUCTION The petroleum industry is the lifeblood of the global economy.1,2 As the world struggles to emerge from global recession and financial crises, countries are looking for solutions to improve energy sources and their performance improvement. Globally, more than 600 refineries are currently in operation, producing a minimum of 100 million barrels of refined products per day using a variety of crude oils.3,4 Each refinery has a unique physical configuration, operating conditions, and economics.4 In the basic refinery process, crude oil is heated up to about 600 °C by passing superheated steam of high pressure at the bottom of the vertical distillation column,5−7 from which various petroleum fractions such as petroleum gas, naphtha, kerosene, diesel, lubricant, gas oil, and solid petroleum coke get separated depending upon their boiling points. Analytical techniques play a key role in the characterization of each fraction in order to troubleshoot and optimize the refinery processes.8−12 Each crude oil is a unique and complex mixture of several components, mainly hydrocarbons and a significant quantity of © XXXX American Chemical Society
hetero elements such as sulfur, nitrogen, oxygen, and certain metals like nickel, vanadium, arsenic, mercury, sodium, magnesium, zinc, et cetera.13,14 Sulfur and nitrogen molecules pose significant challenges in a refinery. Strict sulfur specifications in gasoline and diesel fuels determine how crude oils get blended and processed. Difficult sulfur, such as the sterically hindered thiophenic type of molecules, pose considerable challenges in hydrotreating and hydrocracking processes and are poisons to certain catalytic processes.15,16 Sulfur also plays a key role in sulfidic corrosion of refinery equipment and pipelines.17,18 On the other hand, nitrogen is also a serious poison to catalytic cracking processes. Stringent regulatory requirements for emission of sulfur oxides (SOx) and nitrogen oxides (NOx)19,20 also limit S and N content in petroleum products such as gasoline, jet fuel, diesel, et cetera. Received: December 13, 2016 Revised: January 31, 2017 Published: February 1, 2017 A
DOI: 10.1021/acs.energyfuels.6b03306 Energy Fuels XXXX, XXX, XXX−XXX
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range distribution.60 All solvents used were of HPLC grade with 99.9% purity procured from Merck Specialties Private Limited and used without further purification. The combination of an HT-CNS SimDis (Agilent 7890 A gas chromatograph equipped with FID, NCD, and SCD and AC Simulated Distillation Software) and a detailed hydrocarbon analyzer (DHA, Agilent 7890 A Gas Chromatograph with FID and AC DHA Software) were adopted for the analyses of crude oil samples. The method is validated by analyzing the total sulfur with ED-XRF equipment and total nitrogen with a total sulfur and total nitrogen analyzer (PAC’s Multitek Horizontal furnace TSTN Analyzer) and is based on standard ASTM methods.27,61−63 2.2. Detailed Hydrocarbon Analyzer (DHA). The DHA is an Agilent 7890 A gas chromatograph with FID and “DHAplus” software configured for the detailed hydrocarbon analyses of low-boiling components (C3 to C12 molecules) present in the naphtha fraction of the crude oil. This part is very much essential to separately determine as the separation in the HT-CNS SimDis technique is very poor for lighter components, and the components up to C9 coelute with a cyclohexane solvent peak that was used for the sample preparation and makes the yield prediction inaccurate. To get an accurate prediction of yield profile of the crude oil distillation, the DHA components up to C9 were analyzed by DHA, converted into a boiling point curve, and merged with HT-CNS SimDis reports using AC Simulated Distillation software provided by PAC, The Netherlands. The remaining part (C10−C90) of the sample was analyzed using a HT-CNS SimDis analyzer for complete C, N, and S components. The DHA instrument consists of two capillary columns made up of fused silica connected end to end, in which was one precolumn (DB-5, 2 m, 0.25 mm ID, 0.5 μm) and another analytical column (DB-1, 100 m, 0.25 mm ID, 0.5 μm). This dual column system helps in getting better separation of all naphtha components having different polarity and boiling points. The system has a modified inlet (temperature: programmed 175 to 300 °C at 50 °C/min) with an OV-101 metallic column and back flush facility (valve on: 0.5 min, valve off: 183 min) to vent the heavier fractions. The injector of this system is a programmable temperature vaporizer (PTV) having split or splitless options. The oven was programmed in a temperature range of 30 to 50 °C with the ramp rate of 1.3 °C/min and 50 to 200 °C with a ramping rate of 1.6 °C/min. High purity (99.999%) helium was used as a carrier gas, and a combination of hydrogen and air (1:10 volume ratio) was used to generate the flame of FID. The detector temperature was maintained at 250 °C, and a flow of 35 mL/min of hydrogen and 350 mL/min of air (with helium makeup). The Chemstation software controls all the instrument parameters, and DHA-Plus software was used for data processing. The DHA instrument was calibrated by analyzing an n-paraffin standard (C3−C14) and retention time (RT) of each n-paraffin component (see the Supporting Information for a typical chromatogram of n-paraffin) was assigned its boiling point. Another reference standard, QC (quality control) naphtha, was used as a reference standard for identifying all the naphtha components of crude oil such as n-butane, n-hexane, n-octane, et cetera (see Supporting Information for typical chromatogram of QC naphtha). The reference 512 standard (a standard hydrocarbon mixture containing paraffins, iso-paraffins, naphthenes, and aromatic components up to C9 components) was used to check column and detector performance, and the results obtained were compared with the previously determined reference values. 2.3. HT-CNS SimDis Analyzer. HT-CNS SimDis analyzer consists of an Agilent 7890 A gas chromatograph equipped with FID, NCD, SCD, and AC Simulated Distillation software. A nonpolar (DB-1, 5 m length, 0.53 mm ID) gas chromatographic capillary column with 100% polydimethylsiloxane (PDMS) as a stationary phase and a film thickness of 0.17 μm was used to get the best separation of components.64 Although the column stationary phase is stable at elevated temperatures, bleeding (removal of the stationary phase from the column) occurs above 400 °C. Prior to sample analysis, the column performance was monitored by calculating a parameter called
ASTM D 2622-16 (American Society for Testing Materials) is a method specified for the determination of sulfur in petroleum products by wavelength dispersive X-ray fluorescence spectrometry (WD-XRF).21 Similarly, European Standard IP 447, ASTM D 6334-12, IP 497-05, IP 553-08, and ASTM D7039-15a are the methods specified in the literature for the determination of sulfur in various petroleum products and additives22−26 using WD-XRF. Several ASTM and European Standard methods also use an energy dispersive-Xray fluorescence (ED-XRF) spectrometer27−34 for the determination of sulfur content in petroleum products. Several other methods such as UV fluorescence,35−37 gas chromatography with sulfur chemiluminescence detector (GC-SCD),38−40 the oxidative microcoulometric method,41−44 oxidative combustion, electrochemical detection,45 the flame photometric method,46 et cetera exist in the literature for the determination of sulfur in petroleum samples. Advanced techniques such as Fourier transform ion cyclotron resonance spectrometry (FTICR) are also available in the literature for molecular speciation of sulfur and nitrogen in all cuts of crude oil. However, it is not practically possible to implement FTICR based methods for routine analysis due to their high cost, serious operator-training requirements, and tedious maintenance procedures.47−51 ASTM D 7807-12 is a method specified for the determination of boiling range distribution of hydrocarbon and sulfur components of petroleum distillates by gas chromatography and chemiluminescence detection.52 The ASTM test method is applicable to petroleum products and fractions having a final boiling point of 538 °C (1000 °F) or lower at atmospheric pressure and not suitable for crude oil analysis. Several similar methods are available in the literature, whereas they are unable to provide complete details of sulfur and nitrogen in crude oil along with their boiling range distribution.53−57 The present methodology to determine sulfur and nitrogen content in a refinery involves the collection of individual fractions through TBP (true boiling point) distillation58,59 followed by analyses using various techniques. As a result, the process becomes slow and tedious, requires a dedicated work force, and is uneconomical. In order to overcome these issues, we have attempted to develop a quick and efficient analytical methodology to determine C, S, and N and their boiling range distribution using a HT-CNS SimDis analyzer. Such key parameters can play a significant role in quickly estimating overall crude properties through proprietary correlations. This fast technique equips refiners to respond better to a dynamic manufacturing environment and changing market conditions. Plant troubleshooting, process optimization and crude valuation can further improve through the availability of such timely information.
2. EXPERIMENTAL SECTION 2.1. Materials, Methods, and Instrumentation. Crude oil samples were collected from Jamnagar Refinery, Reliance Industries Limited, Jamnagar, Gujarat, India. Calibration standards used for evaluating the instrumental performance consist of the boiling point (B.P.) calibration standard (mixture contains calibration light, part no. 59.50.101A; and calibration heavy, part no.59.50.100B), ref. 5010 (part No.00.02.015), and lube oil (part no.25650.100), which were procured from Petroleum Analyzing Company (PAC), Analytical Services, The Netherlands. DHA and HT-CNS SimDis columns were procured from PAC, The Netherlands. The sulfur and nitrogen calibration standard VGO NS Reference was prepared in-house using a known VGO sample that contains predetermined sulfur and nitrogen and their boiling B
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Energy & Fuels skew60,64 at various temperatures using a boiling point (B.P.) calibration mixture. Skew value is a measure of deactivation of the column and peak symmetry. A skew value of >1 indicates that the column condition and peak symmetry are acceptable for analyses. As the operating temperature (430 °C) of the instrument is comparatively high, the majority of the heavier petroleum components that boil above AET 700 °C forms coke at the front end of the column and adsorb on the internal surface. As a result, coke and some metallic impurities do not elute at the operating conditions and remain stuck inside the column. This coke acts as a catalyst for further coking of the next sample. The presence of coke also alters the elution profile of components due to strong adsorption. Therefore, analytical error increases as the coke formation inside the column increases. To avoid such an analytical error, we cut the front end of the column after the completion of five to six crude sample analyses. After cutting a few centimeters of the front end of the column, the instrument performance was again monitored by analyzing B.P. calibrant, Reference 5010, and VGO NS Reference standards to get the accurate results for the next set of samples. As the usage of a column increases, film thickness and the efficiency of the stationary phase deteriorates due to column bleeding. If change in skew is found to be less than 0.8, then it is recommended to replace the column. Another unique parameter of the HT-CNS SimDis instrument is the “cool on column” injection technique60 in which liquid samples can be directly injected onto the GC column. There are several advantages of the cool on column system. First, the cool on column system eliminates sample discrimination, which commonly occurs in quantitative analyses. Second, thermally labile components do not undergo thermal stress as they begin the chromatographic process at relatively lower temperatures. As a result, this technique eliminates sample alteration by decomposition and rearrangement reactions. The column oven was maintained in the range of 40 to 430 °C, with a ramp rate of 25 °C/min and a hold time of 5 min. The inlet temperature was programmed at 100 to 400 °C with a ramp of 25 °C/ min. The column eluent has been split into three portions using a three way splitter and were analyzed by FID (temperature: 430 °C, 35 mL/min H2, 350 mL/min Air with He makeup), SCD, and NCD (temperature: 950 °C, 10 mL/min O2, 90 mL/min H2, 100−150 mL/ min with helium makeup, Antek 7090).65−68 The hydrogen and oxygen plasma in the combustion tube converts nitrogen compounds to nitric oxide at elevated temperatures (950 °C). The general reaction can be presented as follows.
It is always recommended to test detector performance for sulfur and nitrogen by checking the equimolarity value,60 and it should be less than 10%. Metallic impurities such as nickel, vanadium, calcium, sodium, et cetera present in crude oil form respective oxides at combustion temperature in FID and block the nozzle outlet of the FID jet over a period of time. This blockage can result in improper combustion of hydrocarbons and can cause error in the analysis. The blockage can be removed by sonicating the jet in an organic solvent at a temperature of 60 to 70 °C or by piercing the thin needle through the jet. If the problem persists, then it is recommended to replace the FID jet with a new one. An appropriate calibration method is necessary for accurate quantitative analysis. The distribution of hydrocarbon components obtained from the instrument was measured in comparison with the previously determined reference n-paraffin chromatogram (see the Supporting Information for a typical chromatogram of n-paraffin calibration standard obtained by HT-CNS SimDis). The column performance of the instrument was checked by analyzing the Reference 5010 standard. The obtained response values were compared with target response values to confirm that they meet the requirement of analysis (see Table 1, Supporting Information for a comparison of target and experimental values). The FID response factor obtained for the Reference 5010 standard was used to calculate the hydrocarbon yield of various fractions of the crude oil. The performance of the SCD and NCD detectors was checked by injecting the VGO NS Reference standard, which provides the distribution of sulfur and nitrogen components (see the Supporting Information, for the hydrocarbon, nitrogen, and sulfur chromatogram for the VGO NS reference obtained by HT-CNS SimDis). The VGO NS reference standard was used as a calibration standard for sulfur and nitrogen measurement of crude oil as there is a wide range of components distributed with various retention times (RTs). Response from a single molecule reference is not sufficient to generate accurate data for all distributed components. The total sulfur and total nitrogen (TSTN) analyzer with a horizontal furnace (MultiTek) was calibrated as described in an application note published by PAC, The Netherlands.62 A known concentration of sulfur and nitrogen standards was prepared in cyclohexane and analyzed using a TSTN analyzer. The calibration graph was drawn with the area obtained by the TSTN analyzer against concentration of the solution. The linearity equation obtained by the graph was used for the calculation of total sulfur and total nitrogen present in unknown samples. 2.4. Preparations. The calibration standard for HT-CNS SimDis was prepared by mixing Cal. Heavy (1 mL) and Cal. HT. Light (0.005 mL) in a GC vial and heated to get a homogeneous solution. The VGO NS Reference standard was prepared by dissolving 100−200 mg of the standard (VGO compound with known sulfur and nitrogen content) in 5−6 g of an accurately weighed quantity of cyclohexane, and out of this 0.5 μL was used for the injection. Reference 5010 was diluted (∼0.1 g in 10 mL cyclohexane) and used to check the column performance for hydrocarbon distribution. The n-paraffin standard (used for DHA calibration) was prepared by dissolving ∼0.1 g of nparaffin standard mixture (C5−C14) in 1 g of CS2. The sample for DHA analysis was prepared by dissolving ∼2.5 g of crude oil sample and ∼2.5 g of CS2 and 0.1 g of isooctane (internal standard) weighed accurately to the nearest milligram, and the homogeneous solution was used for the analyses. The samples for HT-CNS SimDis analyses were prepared by dissolving 100−200 mg of the crude oil sample and ∼5 g of cyclohexane weighed to the nearest milligram, and 0.5 μL of the homogeneous solution was used for analysis. For TSTN analyses, an accurately weighed 100−200 mg of the sample was dissolved in 10 mL of cyclohexane and mixed well to get a homogeneous solution and from which 15 μL was used for the analysis. Total sulfur analyses were made as per ASTM protocol using ED-XRF (Oxford Instruments LabX 3000 XRF Analyzer).
R−N + O2 → CO2 + H 2O + NO NO + O3 → NO2 * → NO2 + hν (energy) The liberated nitric oxide reacts with ozone to form electronically excited nitrogen dioxide. This excited nitrogen dioxide emits light in the red and infrared regions of the spectrum when it relaxes to its ground state. The light emitted is directly proportional to the amount of nitrogen present in the sample. The sulfur chemiluminescence detector (SCD) also works with a similar principle to that observed in the nitrogen chemiluminescence detector. This technique utilizes the combustion of sulfur compounds to form sulfur monoxide (SO) followed by the chemiluminescence reaction of sulfur monoxide with ozone (O3) to form sulfur dioxide. The energy liberated in the form of light (hν) during the reaction passes through an optical filter and is detected by a photomultiplier tube. The intensity of the emitted light is directly proportional to the amount of sulfur in the sample. The general chemical reaction can be presented as follows.
R−S + O2 → CO2 + H 2O + SO2 SO2 + H 2 → H 2S
H 2S + O3 → SO2 * → SO2 + hν (energy) Once sulfur, nitrogen, and hydrocarbon signals were generated, further data processing can be done using the AC Simulated Distillation software. C
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Figure 1. Schematic representation of crude oil distillation (left half) in a refinery and the HT-CNS SimDis technique (right half) for the determination of hydrocarbon, sulfur, and nitrogen along with detection principles (reaction mechanism).
Table 1. Distribution of Sulfur (in ppm) along with Standard Deviation (n = 6) in Various Petroleum Fractions of Crude Oil Samples Analyzed by HT-CNS SimDis Techniquea sample ID
naphtha (up to 165 °C)
kerosene (165−270 °C)
diesel (270−370 °C)
vacuum gas oil (370−565 °C)
vacuum residue (565 °C and above)
total sulfur by XRF method
1 2 3 4 5 6 7 8 9 10
22 ± 1 BQL BQL 19 ± 0.8 16 ± 0.7 BQL BQL 36 ± 3 12 ± 1 BQL
452 ± 26 338 ± 9 198 ± 5 798 ± 22 343 ± 30 39 ± 5 172 ± 6 526 ± 9 545 ± 10 107 ± 9
2366 ± 34 2229 ± 38 844 ± 23 7316 ± 257 2646 ± 50 358 ± 26 1296 ± 41 3426 ± 61 1773 ± 40 460 ± 10
6469 ± 66 5802 ± 75 2492 ± 40 11474 ± 39 7306 ± 85 1902 ± 43 4151 ± 108 7699 ± 130 5193 ± 182 1118 ± 16
11360 9926 3275 24947 13447 3330 514 6213 10802 768
20669 18295 6809 44554 23758 5629 6133 17900 18325 2453
a
The sulfur content in vacuum residue was calculated by subtracting the combined sulfur content from naphtha, kerosene, diesel, and VGO fractions (by HT-CNS SimDis method) from the total sulfur obtained by the XRF method. The last column compiles the total sulfur obtained by the XRF method as per ASTM method (BQL represents below quantification limit).
3. RESULTS AND DISCUSSION HT-CNS SimDis and DHA analyses were performed for 10 crude oil samples containing variable composition of hydrocarbon, nitrogen, and sulfur. The results were summarized depending on their atmospheric equivalent boiling temperatures (AEBP) as naphtha (B.P up to 165 °C), kerosene (165 to 270 °C), diesel (270 to 370 °C), vacuum gas oil (370 to 565 °C) and vacuum residue (565 °C and above). Our earlier study in analyzing vacuum gas oil (VGO) samples by HT-CNS SimDis analyzer suggests that VGO samples elute completely at the maximum GC operating conditions from the chromatographic column and the data obtained for “S” and “N” content were in agreement with the data obtained by standard ASTM methods.60 The current practice in refineries to determine total sulfur and total nitrogen is to collect the isolated fractions through true boiling point (TBP) distillation and perform the determination as per the standard methods. The schematic representation of this technique is presented in Figure 1 (first
half). The proposed HT-CNS SimDis methodology for the determination of “S” and “N” along with detection principle is presented in the second half of the figure. The TBP distillation process is tedious and time-consuming, involves high cost, and needs a large quantity of sample (approximately 5 to 10 kg), whereas the HT-CNS SimDis experiment requires a very small quantity of sample (approximately 0.2 g). The eluent from the column was split into three equal parts with the help of an electronically controlled splitter and are connected to FID, SCD, and NCD to simultaneously detect hydrocarbon, sulfur, and nitrogen, respectively. In the present study, a nonpolar DB1 high temperature column was used with the chromatographic conditions that were optimized for the analysis of crude oils to get a better resolution of peaks up to C90 in a short time period of 18 min. Simulated distillation produces a highly repeatable boiling point curve and closely matches with the TBP distillation profile for crude oil. The refinery operations and crude blending process can be optimized in almost real time if D
DOI: 10.1021/acs.energyfuels.6b03306 Energy Fuels XXXX, XXX, XXX−XXX
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Table 2. Distribution of Nitrogen (in ppm) along with Standard Deviation (n = 6) in Various Petroleum Fractions of Crude Oil Samples Analyzed by HT-CNS SimDis Techniquea sample ID
naphtha (Up to 165 kerosene (165−270 °C) °C)
1 2 3 4 5 6 7 8 9 10
BQL* BQL BQL BQL BQL BQL BQL BQL BQL BQL
diesel (270−370 °C)
vacuum gas oil (370−565 °C)
vacuum residue (565 °Cabove)
total nitrogen by TS and TN method
36 ± 4 51 ± 7 97 ± 7 34 ± 11 64 ± 7 68 ± 13 41 ± 8 28 ± 8 113 ± 14 58 ± 9
620 ± 23 726 ± 29 1232 ± 17 328 ± 20 815 ± 21 1135 ± 20 647 ± 13 281 ± 22 1360 ± 26 875 ± 13
2567 2749 2873 1800 3272 2908 3592 509 5997 779
3223 3526 4202 2162 4151 4111 4280 818 7470 1712
BQL BQL BQL BQL BQL BQL BQL BQL BQL BQL
a
The nitrogen content in vacuum residue was calculated by subtracting the combined nitrogen content from naphtha, kerosene, diesel, and VGO fractions (by HT-CNS SimDis method) from the total nitrogen (TSTN Analyzer). The last column compiles the total nitrogen obtained by combustion methodology as per ASTM method. (BQL represents = below quantification limit).
Table 3. Quantitation Data for Sulfur and Nitrogen (in ppm) of Various Refinery Isolated Fractions by HT-CNS SimDis Analyzera naphtha (up to 165 °C)
kerosene (165 to 270 °C)
diesel (270 to 370 °C)
vacuum gas oil (370 to 565 °C)
vacuum residue (565 °C and above)
sample ID
yield (%)
S (ppm)
N (ppm)
yield (%)
S (ppm)
N (ppm)
yield (%)
S (ppm)
N (ppm)
yield (%)
S (ppm)
N (ppm)
yield (%)
S (ppm)
N (ppm)
1 2 3 4 5 6 7 8 9 10
12.8 9.8 3.0 4.9 2.9 2.5 6.6 15.9 1.8 8.3
173 BQL 233 381 548 BQL BQL 214 639 BQL
BQL BQL BQL BQL BQL BQL BQL BQL BQL BQL
12.7 12.3 9.0 9.1 7.9 7.3 10.7 16.1 6.6 14.3
3561 2750 2202 8773 4340 534 1607 3264 8261 745
BQL BQL 279 219 172 480 BQL BQL 417 BQL
16.0 16.6 14.7 13.2 19.1 13.2 16.9 17.6 11.6 21.2
14789 13428 5749 55421 13853 2712 7666 19466 15280 2168
227 307 656 257 333 515 241 156 974 272
28.2 29.6 37.3 30.3 36.1 37.0 32.4 28.8 30.9 41.7
22940 19601 6681 37868 20238 5141 12813 26728 16525 2680
2199 2452 3301 1084 2258 3066 1997 976 4403 2099
30.3 31.7 36.1 42.6 34.1 40.0 33.5 21.6 49.2 14.6
37488 31292 9050 58561 39436 8321 1526 28781 22133 5250
8436 8654 7990 4177 9551 7175 10672 2317 12129 5277
a
The sulfur and nitrogen content in the isolated fraction was calculated based on hydrocarbon yield obtained from FID response and sulfur and nitrogen content obtained by SCD and NCD response, respectively, for individual fractions for the respective crude oil.
one understands the distribution of heteroatomic molecules such as sulfur and nitrogen in each fraction. The sulfur and nitrogen analytical results for all 10 crude oil samples are compiled in Tables 1 and 2, respectively. (The boiling point cut-wise distributions of nitrogen, sulfur, and hydrocarbons are presented in Tables 2, 3, and 4, respectively, and a typical DHA chromatogram for crude oil is shown in Figure 5 of the Supporting Information.) Each analysis was performed in six replicates, and the average values were
presented in the table along with standard deviation. The sulfur content in the vacuum residue (VR) fraction was calculated by subtracting the combined sulfur content from naphtha, kerosene, diesel, and VGO fractions (by HT-CNS SimDis method) with total sulfur obtained by the ED-XRF method. The typical chromatogram for hydrocarbon, nitrogen, and sulfur obtained by HT-CNS SimDis was displayed in Figure 2. The total sulfur and total nitrogen were determined using EDXRF and TS-TN (total sulfur and total nitrogen analyzer) method, respectively. In all of the cases, each value was the average of six replicates, and the percentage of relative standard deviation (RSD, %) was found to be within the acceptable limits (not more than 5% RSD). The combined report obtained for the crude oil samples was recalculated to get fraction-wise distribution, such as quantity of sulfur and nitrogen in naphtha, kerosene, diesel, and VGO fractions. The repeatability at each fraction was also calculated, and it was found that all the results were within the acceptable limits. The sulfur and nitrogen distribution in vacuum residue was calculated by subtracting sulfur/nitrogen up to 565 °C obtained by the HT-CNS SimDis method with the total sulfur data obtained by the XRF/TSTN method. In our previous study, we have reported that total sulfur and total nitrogen content obtained by the HT-CNS SimDis method for the VGO fraction was in close agreement with the separately analyzed VGO fraction by the TSTN
Table 4. Repeatability Experimental Study Conducted for Sulfur and Nitrogen up to 565 °C for Two Crude Oil Samples Using HT CNS SimDis Technique sample 01
sample 02
replicate
sulfur, ppm
nitrogen, ppm
sulfur, ppm
nitrogen, ppm
1 2 3 4 5 average SD RSD, %
1399 1373 1426 1424 1428 1410.0 23.8 1.7
797 855 885 901 874 862.4 40.2 4.7
6861 6780 6701 6773 6756 6774.2 57.6 0.8
1435 1591 1416 1527 1494 1492.6 70.8 4.7 E
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Figure 2. Hydrocarbon, nitrogen, and sulfur chromatograms obtained from the detector responses of FID, NCD, and SCD, respectively, for crude oil sample 6 using an HT-CNS SimDis analyzer.
method, and hence, we have not presented any data for the validation of the HT-CNS SimDis method for VGO samples.60 The results presented in Table 1 shows that, in maximum cases, sulfur components are highest in vacuum residue and less in the naphtha cut and medium in kerosene, diesel, and VGO fractions. A similar trend was observed for nitrogen also, in which the vacuum residue contains more nitrogen content than any other fractions. For refinery processes, the sulfur and nitrogen content present in all of the fractions plays a key role for predicting the corrosion potential and requirement of hydrogen for hydrotreating experiments. Hence, we have calculated both sulfur and nitrogen in various isolated fractions such as naphtha, kerosene, diesel, VGO, etc., and the results are presented in Table 3. The sulfur content present in the isolated fraction was calculated as per the below mentioned equation.
TN in isolated fractions Total nitrogen obtained for each fraction × 100 = Yield of individual fraction obtained by FID response
Table 3 depicts the total sulfur and total nitrogen content present in various isolated fractions of crude oil samples. Hydrocarbon yield obtained for each fraction of the crude oil was also presented. The results also indicate the quality of crude oil; i.e., the crude oils 3, 6, 7 and 10 have the lowest total sulfur content. The same is true in isolated fractions also. Hence, processing of such crude oil gives the highest quality products such as petrol, kerosene, diesel, etc. Similarly, crude oil 4 contains a maximum amount of sulfur and hence requires severe processing conditions and increases the cost of refining. The sulfur and nitrogen content which is present in the fraction that boils above AET 565 °C was considered as part of the VR fraction. The basic refinery process needs a hydrocarbon, nitrogen, and sulfur distribution profile for temperatures ranging up to 600 °C only, as almost all the processes, such as hydrogenation, cracking, hydro-treating, etc., are carried out up to VGO fractions only. In the normal refinery process, the
TS in isolated fractions Total sulfur obtained for each fraction × 100 = Yield of individual fraction obtained by FID response
Similarly, the total nitrogen in each isolated fraction was calculated as per below mentioned equation. F
DOI: 10.1021/acs.energyfuels.6b03306 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 3. Overlaid chromatograms of sulfur standards in cyclohexane prepared from an in-house gas oil secondary standard including the limit of detection (LOD = 2.96 ppm) and limit of quantification (LOQ = 9.14 ppm). Inset: Linearity graph obtained from 9.0 (LOQ) to 900 ppm of sulfur with a correlation coefficient (R2) of 0.9999. (x and y axes represent the retention time and sulfur detector response, respectively.)
Figure 4. Overlaid chromatograms of nitrogen standards in cyclohexane prepared from an in-house gas oil secondary standard including the limit of detection (LOD = 2.44 ppm) and limit of quantification (LOQ = 12.19 ppm). Inset: Linearity graph obtained from 12 (LOQ) to 190 ppm of nitrogen with a correlation coefficient (R2) of 0.9999. (x and y axes represent the retention time and nitrogen detector response, respectively.)
sulfur component present in vacuum residue (VR) may not be an essential parameter. Similarly, nitrogen content has several adverse effects on the catalyst, and hence, analyses of nitrogen play a key role in optimizing the refinery processes The VGO NS Reference standard was analyzed at various known concentrations to check the limit of detection and linearity range for sulfur and nitrogen content. The overlaid chromatograms and linearity graphs for sulfur and nitrogen are presented in Figures 3 and 4, respectively. The linearity plot was drawn using detector response vs concentration of the standards. Linearity was established in the range of 12 to 190 ppm and 9 to 900 ppm for nitrogen and sulfur, respectively, with a correlation coefficient of 0.9999. The limit of detection (LOD) for N and S was found to be 4.0 and 3.0 ppm, respectively. The limit of quantification for N and S was found to be 12.19 and 9.14 ppm, respectively. The repeatability and reproducibility study was conducted for two crude oil samples, and results are presented in Tables 4 and 5, respectively. The percentage relative standard deviation (RSD) for repeatability was found to be at a maximum of 4.7% and 1.7% for N and S, respectively. Reproducibility experiments were conducted by analyzing the crude samples in two sets with five replicates each using a different column of the same specification, different days, and a different operator. The maximum RSD value for N and S was found to be 3.3% and
Table 5. Reproducibility Experimental Study for Sulfur and Nitrogen up to 565 °C Conducted for Two Crude Oil Samples Using HT-CNS SimDis Analyzer sample 01
sample 02
replicate
sulfur, ppm
nitrogen, ppm
sulfur, ppm
nitrogen, ppm
1 2 3 4 5 average SD RSD, %
1427 1421 1493 1470 1526 1467.4 44.4 3.0
847 822 831 788 861 829.8 27.7 3.3
7070 7134 7292 7332 7352 7236.0 126.3 1.7
1548 1583 1493 1544 1494 1532.4 38.6 2.5
3.0%, respectively. Recovery studies were conducted at three different levels for nitrogen and sulfur. A known amount of analyte (sulfur and nitrogen standard) was spiked into the test sample matrix (crude oil with a known amount of sulfur and nitrogen), and its response was measured (recovered). The actual amount of sulfur and nitrogen present in the crude oil, a spiked quantity of analytes, obtained the total sulfur and nitrogen quantity, and recovery values are compiled in Tables 6 and 7, respectively. The average recovery obtained for sulfur and nitrogen at three different levels with triplicate sample preparations was found to be more than 95%. G
DOI: 10.1021/acs.energyfuels.6b03306 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 6. Recovery Studies for Sulfur Conducted for Crude Oil Sample Using HT CNS SimDis Technique sample ID
spiking level
actual amount of sulfur present in sample (ppm) (A)
spiked amount of sulfur (ppm) (B)
amount of sulfur obtained from experiment (ppm) C = (A + B)
recovered amount of sulfur (ppm)
percentage of sulfur recovered
set 1
A B C A B C A B C
2193 2193 2193 2193 2193 2193 2193 2193 2193
24235.0 26872.1 27070.0 9801.4 11267.4 11340.1 1521.2 1305.8 1289.6
27168.5 30019.5 29632.5 13264.0 14450.5 14007.0 3595.0 3618.5 3296.5
24975.5 27826.5 27439.5 11071.0 12257.5 11814.0 1402.0 1425.5 1103.5
103.1 103.6 101.4 113.0 108.8 104.2 92.2 109.2 85.6
set 2
set 3
Table 7. Recovery Studies for Nitrogen Conducted for Crude Oil Sample Using HT-CNS SimDis Technique sample ID
spiking level
actual amount of nitrogen present in the sample (ppm) (A)
spiked amount of nitrogen (ppm) (B)
amount of nitrogen obtained from experiment (ppm) C = (A + B)
recovered amount of nitrogen (ppm)
percentage of nitrogen recovered
set 1
A B C A B C A B C
1223 1223 1223 1223 1223 1223 1223 1223 1223
736.2 816.3 822.3 297.7 330.0 344.5 14.7 39.7 84.2
1910.5 2100.5 2008.0 1500.5 1528.0 1582.5 1236.0 1260.5 1303.5
687.5 877.5 785.0 277.5 305.0 359.5 13.0 37.5 80.5
93.4 107.5 95.5 93.2 92.4 104.4 88.3 94.5 95.7
set 2
set 3
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4. CONCLUSION
ASSOCIATED CONTENT
S Supporting Information *
A HT-CNS SimDis method was developed for the simultaneous determination of hydrocarbon, sulfur, nitrogen, and their boiling range distribution in petroleum crude oil samples. A single GC injection of crude oil prepared in cyclohexane is sufficient to get S and N content present in petroleum fractions such as naphtha, kerosene, diesel, and vacuum gas oil. It was also observed that total sulfur and total nitrogen content obtained by HT-CNS SimDis analysis was always significantly less or equal to the results obtained by standard ASTM methods. This is due to the nonvolatility of heavier residual fractions such as the vacuum residue of crude oil samples that forms coke inside the chromatographic column at the maximum operating temperature of the analysis, and hence sulfur and nitrogen content present in vacuum residue samples cannot be calculated by this method. However, S and N content present in vacuum residue samples were calculated from the difference between the results obtained by methods like ED-XRF and TS-TN analyzer as per ASTM methods and the data obtained by the HT-CNS SimDis method. The total sulfur and total nitrogen content present in various isolated fractions of crude oil such as naphtha, kerosene, diesel, and vacuum gas oil were calculated from the yield fractions obtained from FID response. The stream-wise distribution of sulfur and nitrogen components present in crude oil samples is very useful information for the optimization of several refinery processes such as hydro-treating, cracking, etc., and hence this method can be a promising one for the fast characterization of crude oil samples. The analytical methodology is fast, accurate, repeatable, and cost-effective.
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b03306. Tables 1−4 and Figures 1−5 (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*Phone: +91-836-2215377. E-mail:
[email protected]. ORCID
Kalagouda B. Gudasi: 0000-0001-6673-382X Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We are very thankful to Refinery Units, Reliance Industries Ltd., Jamnagar, India for providing crude oil samples. We are also thankful to PAC, Application Laboratory, Dubai for providing analytical support for the determination of total sulfur and total nitrogen using Multitek’s horizontal furnace TS-TN analyzer.
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REFERENCES
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