JOURNAL OF INTERNATIONAL ACADEMIC RESEARCH FOR MULTIDISCIPLINARY Impact Factor 1.625, ISSN: , Volume 3, Issue 1, February 2015

Transcription

1 DETERMINATION OF ULTRA TRACE LEVEL OF ARSENIC AND MERCURY IN HYDROCRACKER FEEDSTOCKS BY ICP-MS G. JAYA KRISHNA* S. AHMED** V. KAGDIYAL*** *Deputy Research Manager, Research & Development Centre, Indian Oil Corporation Ltd., Sector 13 Faridabad, Haryana, India **Research Manager, Research & Development Centre, Indian Oil Corporation Ltd., Sector 13 Faridabad, Haryana, India ***Deputy General Manager, Research & Development Centre, Indian Oil Corporation Ltd., Sector 13 Faridabad, Haryana, India ABSTRACT A new, rapid, accurate and direct method for the determination of As and Hg at ultra trace level in hydrocracker feedstocks (VGO range products) by ICPMS has been developed. In this method the samples were prepared directly by dissolving in high purity ATF solvent. The thorough optimizations of instrumental operating parameters were done aiming better sensitivities, lower limits of detection for quantitative analysis The concentration of As and Hg in the high purity ATF solvent was determined by standard addition method. Calibration graphs were found to be linear up to 10 ppb for both the elements. Limits of Detection of As and Hg were found to be ppb and ppb respectively. The developed method was validated for As and Hg by standard methods. No method is so far documented in the literature for the direct estimation of as and Hg present at ultra trace level in hydrocracker feedstocks by ICP-MS technique. INTRODUCTION Hydrocracking is an established and reliable method for transforming low-value heavy oil fractions into valuable products. Crude oil fractions above 350 degree C boiling point are generally used as hydrocracker feed stocks. The most common feedstocks are VGO (Vacuum Gas Oil), LVGO (Light Vacuum Gas Oil) and HVGO (Heavy Vacuum Gas Oil). 1 In today s market, refinery operators face new technical, regulatory and commercial challenges: ensuring that their operations meet environmental standards; complying with new fuel specifications; and upgrading their refinery capability to meet capacity and quality targets. 2 Major products produced from hydrocracking are jet fuel, diesel, relatively high octane rating gasoline and LPG (Liquified Petroleum Gas). All these products have a very low content of sulfur and other contaminants. Estimation of contaminants like As and Hg present at ultra trace level in the form of organometallic compounds in hydrocracker feedstocks play an important role in ascertaining the quality of feed samples like Coker Unit products, OHCU (Overhead coker unit) combined feed, DHDS (Diesel Hydrodesulphurisation) and DHDT 313

2 (Diesel Hydro Treatment) feeds, VGO, SR-VGO (Straight run Vacuum Gas Oil), heavy synthetic crude gas oil, thermally or catalytically cracked stocks before charging it in the process to hydrocracker units. The trace metals (As and Hg) present in these feeds play a crucial role for the life of hydro processing catalyst because they are highly poisonous to catalyst and deactivate it rapidly there by compromising the activity of the catalyst for reduced life. Further it can also affect the selectivity of the products. Inductively Coupled Plasma Mass Spectrometry or ICP-MS is an analytical technique used for elemental determinations. ICP-MS has many advantages over other elemental analysis techniques such as atomic absorption and optical emission spectrometry, including ICP Atomic Emission Spectroscopy (ICP-AES), detection limits for most elements equal to or better than those obtained by Graphite Furnace Atomic Absorption Spectroscopy (GFAAS), higher throughput than GFAAS and the ability to handle both simple and complex matrices with a minimum of matrix interferences due to the high-temperature of the ICP source. Trace metal analyses based on ICP-MS techniques are gaining prominence in petroleum industry due to ease of analyses, rapidness, high dynamic linear range and practically no matrix effect and direct introduction without any pre sample preparation. 3-6 Direct analyses of organic solvents by ICP-MS reduce sample preparation times considerably by avoiding the time consuming digestions traditionally used to reduce the solvents to an aqueous state. It also avoids the loss of volatile elements that can occur during the heating stages of digestion. Therefore, in order to analyse organic solvents, challenges which needs to overcome are plasma quenching due to excessive loading of volatile solvents into the plasma, carbon deposition (or condensation) onto the sampler cone of the mass spectrometer resulting in signal drift and carbon deposition at the tip of the torch injector tube obstructing the nebulizer flow and causing signal drift. Either of aqueous and direct methods are chosen, depending upon the volatility of the analytes and reproducibility and accuracy of the results required. In order to remove the matrix and polyatomic interferences and to achieve the desired limits, various methods in combination with ICP-MS such as sample pre concentration by microwave digestion, direct dilution methods and use of specific Nebuliser systems and the collision reaction inerface or cell (CRI) introduced by Varian, which injects helium (He) or hydrogen (H2) reaction gases directly into the plasma as it passes through the orifice of the cones. 3,6,7 have been adopted. There are many published atomic spectrometric methods available in the literature for the estimation of As 4,8-10 and Hg 5,11,12 in petroleum fractions converting into aqueous medium 314

3 which are being matrix decomposition methodologies always suffer from losses or contamination during sample solubilization procedures. 11,13,14 If aerosol generation is needed, it may be affected by high amounts of salt that a use erosion of the nebulizer orifice and of the sampler and skimmer cones in the case of ICP-MS. 6 Although contamination or losses can be overcome by the use of acid decomposition assisted by heating in closed vessels performed in microwave digestion system but matrix effects and spectral interferences due to residual carbon content can be a problem in ICP-MS analysis and needs further evaluations. 6 Shah et al. 15 reported determination of Hg in petroleum products by neutron activation analysis. Liang et al. 16 reported determination of Hg in crude oil by thermal decomposition and AFS which suffer from loss of Hg and are time consuming. There are few UOP methods available for the determination of As in Naphtha 17 and in petroleum stocks. 18 The UOP methods are time-consuming and require lots of high purity chemicals. However, no method is available for the analysis of As and Hg in VGO samples. Therefore, earlier an attempt has been made to apply UOP methods 17,18 on VGO samples for As determination. A new method based on oxidative acid extraction has also been adopted 19 to take care of charring of samples. UOP and ASTM D methods have been reported for the direct analysis in solid, liquid and gas samples for total mercury using the thermal decomposition mercury amalgam procedure. The mercury is measured using cold vapor atomic absorption. The analysis is sensitive to 0.01ng mercury. Solubilization of petroleum samples in solvents is by far most studied and used direct sample introduction technique especially for ICP instruments and is widespread in the industry, 6 essentially, because it is a fast and accurate technique. There are only very few atomic spectrometric references available in the literature for the estimation of As in crude oil, 9,20 naphtha, 21 natural gas, 22 lubricating oils 23 and Hg in petroleum stocks, 20,24 lubricating oils 23 in organic medium. However, no direct method for the determination of As and Hg in hydrocracker feedstocks either reported or documented. The direct, estimation of As and Hg in hydrocracker feedstocks of VGO range products at ultra trace levels by ICP-MS is presented in this paper. The pneumatic micro mist low flow nebulizer for viscous samples with high dilution factor(1:50) and optimized flow rate of oxygen and use of CRI cell has helped to overcome these complex problems including cone clogging and prevent deposition in the reaction cell, and has also taken care of polyatomic interferences. 315

4 Experimental Instrumentation All the experiments were performed using Varian ICP 820 Mass Spectrometer (Australia) fitted with nickel cones.. The instrument was equipped with a micromist low flow nebulizer, a peltier cooled double pass glass spray chamber, a three channel peristaltic pump and dynode electron multiplier detector, AGM-1 (Auxiliary Gas module) was used throughout the experimentation for introducing small amount of Oxygen into the plasma. The purity of all the gases used (Argon, Oxygen and Hydrogen) is %. Reagents and Solutions Certified Reference Material from M/s Conostan USA (As and Hg in oil 100 ppm each) was used for the preparation of stock solution and calibration solutions in hydrotreated ATF (Aviation turbine fuel). Sample solutions and spiked samples were prepared in ATF freshly minutes prior to analyses to avoid potential stability problems. Sampling of VGO & sample preparation The hydrocracker feedstocks and other products were obtained from pilot plant studies conducted at Indian refineries. Direct introduction of samples into ICP was performed after dissolution of samples in ATF. Samples were introduced at a peristaltic pump speed of 3 rpm through micromist low flow nebuliser and a peltier cooled double pass glass spray chamber at 5 C. Stock solutions of As and Hg were prepared in ATF. Spiked VGO samples were prepared using stock solution and VGO and diluted in ATF as per the requirements. The VGO and spiked VGO samples were 50 times diluted in ATF. Other petroleum products analysed were diluted depending upon their viscosities. Calibration The prepared 75 As and 202 Hg standard solutions and ATF blank were employed for drawing calibration curves. Hydrotreated ATF was used as blank. In most cases 1ppb, 5ppb, 10ppb each of As and Hg standards were used for calibrations. For each and every experiment fresh calibration was drawn with freshly prepared standards. Analysis modes: Analysis type; Quantitative, Acquisition mode; Steady state, Scan mode; peak hopping, Spacing; coarse, Points/peak; 1, Scans/replicate; 20, Replicates/sample,

5 Results and Discussion The sensitivity of each element in the periodic table depends upon the various operational parameters of the ICP-MS instrument. Since operational parameters of each element are different from each other due to their inherent difference in their sensitivity, therefore optimization of instrumental parameters was carried out to achieve the desired level of estimation of As and Hg. Optimization of flow parameters, ion optics, torch alignment and RF power resulted in appreciable enhancement of sensitivities, good correlation coefficients for both As and Hg. Chlorine is present in all the petroleum matrices. Keeping this in view, optimization of As was done in CRI mode. This was to remove the possible polyatomic interference from 75 Ar Cl + using optimized flow of H 2. Hg being high mass didn t have any possible polyatomic interferences, therefore standard mode was employed for its optimization. The variables of sheath gas, 2 nd & 3 rd extraction lens for As; sheath gas, 1 st & 4 th extraction lens for Hg contributed significantly towards the optimization of analytical parameters whereas the variation in nebulization gas flow rate, plasma & auxiliary flows did not contribute effectively. RF power, sampling depth and stabilization delay also contributed significantly. The critical parameter for the use of organic aerosols in ICP-MS is the oxygen to argon ratio in the plasma, which has to be sufficiently high to assure complete carbon combustion, but not too high, to avoid lowering of the sensitivity and increasing the formation of monoxide ions. 3 The optimized operating parameters are listed in Table 1. The signal intensities of As and Hg were found to be very poor on default (suppliers) settings. After optimization the sensitivities of As and Hg were very much enhanced. The optimized signal intensities were in the range of 5500 counts/sec for As and 6900 counts/sec for Hg for a 1ng/g standard each. Then calibration curves were drawn using As and Hg standard solutions in ATF and blank which yielded good correlation coefficients, for As and Hg each. As the calibration blank was ATF, an organic solvent optimized supply of O2 (~230 ml/min) was maintained to impart stability to the plasma. Viscosity matching and matrix effects were not a problem in the case of ICP-MS because of large dilutions. The inherent As and Hg contents in calibration solvent were determined by method of standard addition (MSA) as tabulated in Table 2. The corrections were made on the standards prepared for the analyses of samples. Doping studies were done on two selected VGO samples with known spikes of As and Hg. Experiments were carried out in triplicates. Recoveries were found to be ~100% (given in Table 3) for both As and Hg, which validates the developed method. Then VGO and As/Hg 317

6 doped VGO samples in ATF matrix were prepared and run on the developed method in triplicate for five days preparing fresh standards of As and Hg every time. Again recoveries were found to be ~100%. This was done for the purpose of checking repeatability and precision of measurements and validating the method (Table 4). The recoveries, repeatability reproducibility, precision of measurements corroborated the method developed. To check the results obtained by the developed method for As, four VGO samples were analysed by ICP- MS (External source) as shown in Table 5. Both the results are comparable indicating the validity of the developed method. Further, to validate the results obtained by the developed method for Hg, two samples of refinery condensates were analysed by UOP (External source). Both the results are comparable indicating the validity of the developed method (Table 6). The limit of detection (LOD) and limit of quantification (LOQ) was calculated using the equations. LOD= 3XSD and LOQ=20XLOD, where SD is the standard deviation of 10 measurements of blank (i.e., first point of the calibration curve). LOD and LOQ of As and Hg are found to be very low to even detect sub ppb levels (Table 7). Linear dynamic range of the instrument was an added advantage to the method development, which provides user to calibrate in a wide range of concentrations of standards thus assisting detection of ppm to sub ppb levels of As and Hg. Repeatability and precision were found to be ± 5% of RSD for both As and Hg. Sample Analysis Hydrocracker feedstocks of different origin, feeds of various different refinery processes, different petroleum products were analysed over the developed method for the estimation of As and Hg. The results obtained helped various refiners for the selection of proper feeds (Table 8). Thus the developed method has been put forward for the wide range of routine estimations of As and Hg at ultra trace levels. The method for the estimation of As and Hg at ultra trace level in hydrocracker feedstocks is applicable to feedstocks of other refinery processes like, FCC (Fluid Catalytic Cracking), RFCC (Residue Fluid Catalytic Cracking), hydrotreating, hydrofinishing, DHDS, DHDT, isomerization, isodewaxing and also to the petroleum products of varied boiling ranges, diesel, kerosene, crude condensate, spindle oil, clarified oil, total cycle oil. 318

7 Conclusions The direct, rapid and accurate determination of As and Hg by the developed method paves the way for the critical assessment of the wide range of feedstocks. The As and Hg data obtained can be used for the selection of proper feedstock or a mixed feedstock, which will enhance the life of the catalysts being used in the refinery processes. Selection of proper feedstock thus saving the life of a catalyst will always cut down the refining cost. Acknowledgements The authors wish to acknowledge the management of IOC (R&D), Faridabad, for allowing them to publish the present work. References and Notes 1. Scherzer, J.; Gruia, A. J. Hydrocracking Science and Technology, CRC Press, Gary, J. H.; Handwerk, G. E. Petroleum refining: Technology and Economics, IV ed., CRC Press, Duyck, C.; Miekeley, N.; da Siveira, C. L. P.; Szatmari, P. Spectrochim. Acta, 2002, 57B, Turunen, M.; Peraniemi, S.; Ahlgren, M.; Westerholm, H. Anal. Chim. Acta, 1995, 311, Knauer, H. E.; Milliman, G. E. Anal. Chem., 1975, 47(8), Duyck, C.; Miekeley, N.; da Siveira, C. L. P.; Aucelio, R. Q.; Campos, R. C.; Grinberg, P.; Brandao, G. P. Spectrochim. Acta, 2007, 62B, Tanner, S.; Barano, V. J. Am. Soc. Mass Spectrom., 1999, 10, Campbell, M. B.; Kanert, G. A. Analyst, 1992, 117, Puri, B. K.; Irgolic, K. J. Environ. Geochem. Health, 1989, 11, Hu, Z. C.; Gao, S.; Hu, S. H.; Yuan, H. L.; Liua, X. M.; Liu, Y. S. J. Anal. At. Spectrom., 2005, 20, Kelly, R. W.; Long, S. E.; Mann, J. L. Anal. Bioanal. Chem., 2003, 376, Wondimu, T.; Goessler, W.; Irgolic, K. J. Fresenius J. Anal. Chem., 2000, 367, Fonseca, T.C.O. PhD Thesis, Chemistry Department, PUC, RJ, Brazil, 2000, Kowalewska, Z.; Ruszezynska, A.; Bulska, E. Spectrochim. Acta, 2005, 60B, Hinkle, M. E. U.S. Geol. Survey Prof. Paper, 1971, 750-B, B Liang, L. Lazoff, S.; Hervat, M.; Swain, E.; Gilkeson, J. Fresenius J. Anal. Chem., 2000, 367(1), UOP UOP Method No Suhag, M.; Ahmed, S.; Patel, M. B. Internal Report, R&D, IOCL, India, Kahen, K.; Strubinger, A.; Chirinos, J. R.; Montaser, A. Spectrochim. Acta, 2003, 58B, Botto, R. I. Can. J. Anal. Sci. Spectrosc., 2002, 47, Irgolic, K. J.; Spall, D.; Puri, B. K.; Illger, D.; Zingaro, R. A. Appl 23. Organometallic Chem., 1991, 5, Aucelio, R. Q.; de Souza, R. M.; de Campos, R. C.; Miekeley, N.; da 25. Silveira, C. L. P. Spectrochim. Acta., 2007, 62B, Bjorn, E.; Frech, W. Anal. Bioanal. Chem., 2003, 376,

8 Table 1. Operating parameters of ICPMS 75 As 202 Hg Experimental Parameters Flow Parameters (L/min) Plasma flow Auxillary flow Sheath gas Nebuliser flow Torch alignment (mm) Sampling depth Other RF Power (KW) Pump rate (rpm) 3 3 Stabilisation delay (s) Ion Optics (Volt) First extraction lens Second extraction lens Third extraction lens Corner lens Mirror lens left Mirror lens right Mirror lens bottom Entrance lens -4-2 Fringe bias Entrance plate Pole bias CRI (ml/min) Skimmer gas source H2 - Skimmer flow 60 - Table 2. As & Hg contents in ATF using Standard addition method Sample As (ppb) Hg (ppb) ATF Table 3. Results on As and Hg doped VGO samples Sample As in ppb Hg in ppb Doped a Recovered Doped a Recovered D D D D D D D

9 a Doped samples, D1-D4 include 0.3 ppb of As and 0.2 ppb of Hg inherent to OHCU Feed 8 and D5-D7 include 14.4 ppb of As and 0.4 ppb of Hg inherent to VGO-5 Table 4. Recovery studies of As and Hg in ppb for validations in different days Sample Actual As Hg Recovered in different days a Recovered in different days a Actual Undoped Dope Dope Dope Dope Dope Dope Dope Dope Dope a Recovered values are the average of three values. Table 5. Analysis results of Arsenic in VGO Samples in ppb S. No. VGO Sample ICP-MS(Own source) ICP-MS(External source) 1. VGO VGO VGO VGO Table 6. Analysis results of Mercury in Khuff Condensates in ppb S. No. Sample ICP-MS(Own source) UOP-938 (External source) 1. RT Condensate ABQ Condensate Table 7. Limits of detection and quantification Isotope LOD (ppb) LOQ (ppb) 75 As Hg

10 Table 8. Results on feed and product samples in ppb Sample As Hg Sample As Hg OHCU Feed Light Oil (LO) OHCU Feed Spindle Oil (SO) OHCU Feed Inter Oil (IO) OHCU Feed Heavy oil (HO) OHCU Feed Light Cycle Oil (LCO) OHCU Feed De-Asphalted Oil (DAO) OHCU Feed DHDT Feed OHCU Feed DHDS Feed OHCU Feed VB GO OHCU Feed R-1 Outlet OHCU Feed Neat VGO OHCU Feed Fresh Combined Feed OHCU Feed Fresh Feed+Recycle OHCU Feed R-1 Outlet OHCU Cold VGO Feed Combined Feed VGO Hot VGO Feed VGO Combined VGO Feed VGO Unconverted oil VGO FCC Feed HVGO HVGO HCU Feed