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Anal. Chem. 1980, 52, 760-765
Determination of Total Sulfur in Hydrocarbons by Oxidative Microcoulometry Robert T. Moore* Dohrmann Division, Envirotech Corp., 3240 Scott Boulevard, Santa Clara, California 95050
Phillip Clinton and Vince Barger
Downloaded by UNIV OF NEBRASKA-LINCOLN on September 2, 2015 | http://pubs.acs.org Publication Date: April 1, 1980 | doi: 10.1021/ac50054a036
Sun Oil Co., P.O. Box 2039, Tulsa, Oklahoma 74 102
Two new and easily interchangeable microcoulometric methods for trace sulfur in hydrocarbons are compared over a broad range of petroleum refinery samples. One method, suitable for samples analyzable by syringe, consists of (1) the addition of a specialized constant rate injector (CRI) for control of the analysis cycle plus ( 2 ) the addition of a granular tin scrubber at the end of the pyrolysis train to eliminate negative interferences from nitrogen, chlorine, and peroxides. A second method consists of substituting a high capacity combustion tube for the standard tube in the boat inlet verslon of ASTM D-3120 (BIHC method in ASTM standards, 25, 40 (1977)). While the BIHC method was applicable over the full range of refinery samples (0-10% S) and was mandatory for refractory samples not analyzable by syringe, the CRI method provided the advantages of automated sample injection, low memory effects, and a short term 2 0 precision of f15 ppb.
Present concern for sulfur-free petroleum products has created a demand for analyzers capable of reliable measurement of trace sulfur concentrations in the 20-100 ppb range. Until recently, the only method capable of reliable determinations below 0.5 ppm (500 ppb) has been the microcoulometric method ( I , Z ) , ASTM D-3120, which has been in widespread use in petroleum refineries for several years. With care, this method is capable of reliable trace sulfur determinations in light and medium distillations down t o 0.2 ppm S. T h e Wickbold method ( 3 , 4 ) suffers from somewhat less precision (0.5 to 1.5 ppm) and long burning times a t the trace level and is no longer widely utilized for trace analysis in the petroleum refineries in this country. A new method of trace sulfur determination, with a precision of approximately 0.05 ppm S (50 ppb) sulfur, was recently introduced by Drushel ( 5 ) . Based upon high temperature reduction in hydrogen, controlled rate injection, and photo-optical detection of PbS, this new method allows 6-min determinations of trace sulfur levels in highly refined petroleum fractions t h a t are suitable for syringe injection. Recent advances in the microcoulometric determination of sulfur and chlorine in light hydrocarbons led Sun Oil Company (6) t o modify D-3120 and adopt a boat inlet (7) and high capacity combustion tube (8),a combination which has allowed reliable determinations of sulfur in light hydrocarbons down t o *30 ppb for the past two years in the refinery production control laboratory. Under investigation by ASTM subcommittee D-16, this method (boat injection/high capacity method: BIHC) is presently undergoing round-robin testing for acceptance into method D-1552, and has remained unpublished to-date. In the Dohrmann laboratories during the last year, alternate modifications of D-3120 have resulted in instrumentation capable of f15 ppb precision on standards. These modifications include the development of a precision Constant Rate 0003-2700/80/0352-0760$01 .OO/O
Injector (CRI)(9),the addition of a granular tin scrubber, and changes in titration cell operating parameters to improve noise characteristics. This analytical technique is called the "CRI Method". The application of these two microcoulometric methods to a wide range of petroleum refinery samples has revealed the appropriate areas of application for each method. While the boat injection method is applicable over the full range of refinery samples extending from naphtha to residual, the CRI method has several advantages a t the trace level in light and middle distillates, including an operator-independent analysis cycle, better short term precision (*E ppb), and less memory effect when going from high to low level samples.
EXPERIMENTAL Description of the Apparatus. Figures 1 and 2 are schematic diagrams of the two present methods. Figure 1 illustrates the significant features of the present CRI method: high O2 flow, tin scrubber addition, high cell electrolyte level, low coulometer gain, use of CRI. Figure 2 shows the significant feature of the Sun Oil BIHC method is the use of the high capacity combustion tube with an extra oxygen flow of 25 cm3/min in the downstream portion of the multistage combustion process. Initially recommended for use with larger sample sizes (25 pL) and low gas flow rates (71, this high capacity combustion tube has been found to work more reliably with a 15-pL sample size and higher flow rates. With these operational changes, the analysis time has been shortened from 8 min to as little as 3 min for low sulfur levels (5 min for dilutions of high sulfur level refractory samples). In the CRI method (Figure 11, the sample is introduced via the motor driven syringe assembly into the 700 "C inlet where a 40 cm3/min inert gas stream (helium, argon) carries the volatized sample into an 800 "C pyrolysis region. Here oxygen at 200 cm3/min oxidizes the sample to COz, H20, SO2, SO3, HCl and other elemental oxides. The combustion products and residual gases pass over a granular tin scrubber (US. Patent No. 4172705) at 200 "C, where NO, is reduced to NO, chlorine is retained as SnClz,and harmful peroxides, oxygen-free radicals, or trace uncombusted hydrocarbons which interfere negatively in the titration cell are reduced to harmless species. Thereafter the scrubbed and reduced pyrolysis products are swept through a heated inlet capillary on into the titration cell where SO2 is titrated automatically through null balance microcoulometry. The current required to titrate the steady stream of SOz from the pyrolysis unit is monitored and conditioned to produce a digital readout of sulfur concentration in the original sample from 0 to 200 ppm S with a resolution of 0.01 ppm. In the BIHC method (Figure 2) a 15-pL sample is injected directly into a Pt boat in an inert atmosphere. The boat is thereafter inserted rapidly into the 800 "C inlet zone where the 25 cm3/min inert carrier carries the volatile portions of the sample into the 800 "C pyrolysis region. Here multistage combustion produces SOz that is titrated automatically by the coulometric detection system as discussed above, producing a first readout called "volatile sulfur" which may be displayed separately. After 45 s, the oxygen reversal valve must be manually reversed, putting O2over the sample residue at 800 "C, burning the residue to ash and releasing as SO2the "residual sulfur", which is also titrated C 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 4, APRIL 1980 20 MESH TIN
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adaptor which connects the pyrolysis tube and the titration cell. Double wrapped with a heater tape and heated to 200 “C with a modular power supply, this reducing scrubber eliminates the negative nitrogen, chlorine, and peroxide interferences as described in Figures 4 and 5 . In Figure 4, the dashed curve (no scrubber) illustrates how chlorine produces an increasingly negative response in ASTM D-3120, up to -1.0 ppm sulfur apparent a t 1000 ppm C1, while above 1000 ppm C1 one 30-pL sample causes negative cell runaway (no scrubber). T h e upper “no scrubber” abscissa indicates that the cell can handle up to ten samples of 30-pL content a t the 100 ppm C1 level. When the tin scrubber is in place and the chlorine reacts to form SnC12,the solid curve shows that chlorine does not interfere negatively until a concentration of 2000 ppm is reached, and t h a t even a t 1% chlorine concentration the system can handle ten 30-pL samples before breakthrough of chlorine ions causes negative cell runaway. Figure 5 shows similar but less dramatic improvement for negative interferences from organically bound nitrogen. The nitrogen concentration (solid “tin scrubber” curve) must be above 100 ppm before apparent negative sulfur results. T h e negative effect reaches a limit of -1.00 ppm apparent sulfur a t 1% nitrogen, this limit being established by the azide concentration in the electrolyte. Taking the azide concentration from 0.1% to 1%showed little change in cell response to individual samples but did increase by a factor of 10 (upper abscissa) the number of samples the cell can handle before negative runaway occurs, even without adding extra acetic acid to compensate for the p H shift in the cell electrolyte. T h e tin scrubber (as well as the pyrolysis capacity of the standard ASTM D-3120 combustion tube) is rate limited, so that the curves of Figures 4 and 5 apply for sample injection rates of 1 pL/s or less. Since the azide ion both decomposes and is slowly oxidized in the electrolyte, the cell resistance t o chlorine and nitrogen interference therefore applies only t o just renewed, fresh electrolyte. Chlorine and nitrogen interference are significantly worse when the azide is not effective. In actual operation, the azide effectiveness is significantly reduced after only 1 h of cell operation. In samples with trace sulfur ( S
