Anal. Chem. 1991,63,760-763
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Automated Potentiometric Determination of Sulfur Functional Groups in Fossil Fuels Bernard B. Majchrowicz,' Jan Yperman,* Jules Mullens, and Lucien C. Van Poucke Laboratory of Inorganic & Physical Chemistry, Limburgs Universitair Centrum, B-3590 Diepenbeek, Belgium
A fully extended description is given of an automated setup to monltor H,S continuously in a sensltlve, qualitative, and quantltatlve way. H,S Is led by a stream of N, Into an alkallne solution and is contlnuousty determlned as S2- by a potentiometric detectlon system. The H,S detection system has been applied to the temperature-programmed reduction (TPR) of fossil fuel samples. The TPR procedure conslsts of the nonIsothermal heating of a sample in a reducing atmosphere, In order to liberate the different sulfur functional groups as H,S at dlscrete temperature ranges. I n this way a better Insight can be provlded Into the structural characterlstics of organic sulfur In fossil fuels and related products.
INTRODUCTION Sulfur is present in all coals, both in inorganic compounds (mainly pyrite) and in the organic matrix. For European coals, typical sulfur contents range from 0.5 to 5 % or higher. In terms of coal processing and utilization the presence of sulfur is generally undesirable. Sulfur is released during combustion in the form of SOz and SO3and contributes to acid rain, whilst the sulfur compounds produced during coal processing can have a deleterious effect on the construction materials and on catalyst activity as well as being highly toxic. The ultimate fate of the sulfur in coal during processing is, however, dependent on its form. Thiols and sulfides give labile sulfur compounds fairly readily during processing, whilst thiophenic products are more stable. A knowledge of the sulfur functional group distributions in both raw coal and coal products is, therefore, obviously desirable and should be considered as an important parameter in selecting a coal for a particular process. Hitherto, there has been no convenient method available for determining the functional group distribution of organic sulfur in coal (I). Recently, an interesting approach to the characterization of organic sulfur has been developed, namely the temperature-programmed reduction (TPR) of coal (2-4). Nonisothermal heating of a coal or other sulfur-containing compounds with a fixed programming rate in a reducing atmosphere results in a maximum evolution of HzS a t discrete temperatures. These temperatures are characteristic for the different sulfur functional groups. However, one of the major problems in this technique remains the interpretation and quantification of the data. The present work is a continuation of efforts toward an improvement and automation of the T P R method, with the emphasis on the development of a potentiometric detector using a sulfide ion selective electrode for the continuous monitoring of hydrogen sulfide. By the implementation of a sample changer and an automatic burette in the computerized detection setup, continuous quantitative measurements with high sensitivity are possible. The response of the detector is evaluated with a digital data system and presented as the evolution of H2S from the distinctive functional groups.
* To whom correspondence should be addressed.
Present address: BASF Antwerpen NV, Scheldelaan, B-2040 Antwerpen, Belgium. 0003-2700/91/0363-0760$02.50/0
The overall performance of the detection system is illustrated by a H2S gas input system. The applicability of TPR is shown for the sulfur functional group analysis of some fossil fuel samples.
EXPERIMENTAL SECTION Reagents and Solutions. All reagents were prepared from analytical reagent grade chemicals. High-purity water was prepared by passing deionized water through a Millipore ion-exchange water-purification system. An aqueous stock solution of sulfide antioxidant buffer (SAOB) was prepared as recommended by Orion for the sulfide electrode. The SAOB stock solution was stored under nitrogen in a polyethylene bottle at 4 "C and was discarded as soon as the faintly yellow color turned dark. A stock solution with a sulfide concentration of approximately 0.1 M was prepared by dissolving sodium sulfide nonahydrate in 25% SAOB. The solution was stored under identical conditions as the SAOB stock solution. The exact concentration of the standard solution was determined before use by titration with lead(I1) nitrate. The concentrations of the sulfide stock solutions remained constant for weeks. Working solutions of sulfide were prepared by serially diluting the sulfide stock solution with 25% SAOB. Diluted sulfide solutions were prepared before use. Detection Cell and Electrodes. All the components of the hydrogen sulfide detection system were placed in a thermostatic box at 25.0 A 0.1 "C (Figure 1). The following electrodes were used: an Orion sulfide ion selective electrode (Model 94-16), an Ingold saturated calomel reference electrode (Type 303) placed in a fiber-tip secondary junction, filled with 25% SAOB solution, and an Ingold glass electrode (Type U272). This type of glass electrode is especially suitable for measurements at high concentrations of alkali-metal salts. The glass-calomel electrode pair was calibrated by using two Titrisol buffer solutions of pH 12.00 and 13.00 at 25 "C. The sulfide electrode was regularly polished with Orion polishing strips and was regularly recalibrated. Two Radiometer PHM84 precision digital potentiometers were used for the electromotive force (emf) measurements of the ion-selectiveand glass electrode relative to the reference electrode which were reported as millivolts and pH units, respectively. The potentiometers were connected to an Apple IIGS microcomputer with a memory expansion card of 1Mbyte. This computer also controlled a Metrohm 624.0030 sample changer, a Schott T 100 burette, and a West 2050 microprocessor-based temperatureprogrammer controller. On the sample changer, each beaker, which contains 100.0 mL of detection solution, is sequentially rotated to the measuring position at which the sample is presented to the sulfide, glass and reference electrodes, the stirrer, the titration tip, and the gas inlet, with a gas diffusion filter, through which the H2S-containing N2 flow is led into the alkaline SAOB solution. Temperature-Programmed Reduction Procedure. The concept of the TPR setup has been described in previous papers ( 3 , 4 ) . An amount of the sample according to its sulfur content was mixed with a reducing solvent mixture (2)in a modified quartz glass reaction vessel (Figure 2). Philips chromel-alumel type K thermocouples were connected with the reactor, which then was placed in the oven. Any H2Sevolved was flushed from the reactor to the detection cell by a stream of preheated H2/N2(5%/95%)gas mixture (20 mL/min), and makeup nitrogen gas (120 mL/min) was added to the gas stream immediately after the condensor system, as shown in Figure 2. Specific programs for the registration of the data from the potentiometers and the temperature controller and for 0 1991 American Chemical Society
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THERMOSTATIC BOX. 25.0%
SAMPLE CHANGER
Figure 1. Detection setup with (1) sulfide ion electrode, (2) reference electrode, (3) glass electrode, (4) gas inlet with gas diffusion filter, (5) gas outlet, (6) stirrer, and (7) titration tip.
L
to detector H&N,
I
Flgure 3. H,S input system with (V) needle valve, (F)fbw meter, (W1 and W2) six-way valves, (S) calibrated sample loop, (R) reactor (see Figure 2), and (D) detector (see Figure 1).
Table I. Elemental Analyses (%, daf Basis) and Yields
product
C
H
So
N
Oditr
yield
fresh coal residue extract
64.8 83.3 74.7
5.7 5.4 6.7
11.3 6.7 8.0
1.0 1.0
17.2 3.5 10.7
65.7 24.2
with pure H2S gas. The second valve (W,) feeds the content of the sample loop into the nitrogen stream and consequently to the detector and, depending on the type of experiment, eventually passes through the reactor. The total sulfur concentration of one injection was determined by directly feeding the H2Sgas to the detector. This amount equals 35 f 1 pg. Fossil Fuel Samples. Low-temperature supercritical gas extraction (SCG)with methanol of a Spanish lignite (Mequinenza) was conducted by Moinelo and Snape and co-workers and has been described before (5). TPR analyses were performed on the original coal and on the SCG char and extract, which were obtained after SCG extraction at 350 "C and 10.1 MPa. Table I lists the extract and residue yields obtained from the supercritical methanol extraction, together with the elemental composition of the products and the original lignite. Flgure 2. Total reactor setup with
(F)flow meters.
the control of the sample changer, the stirrer, and the burette were written in Apple-UCSD-Pascal with incorporated UCSDPascal-Assembler routines. The data, as Apple Pascal text files, were then transferred from the microprocessor to the CDC 930-11 mainframe. Programs for the automated handling and evaluation of the data on the mainframe were written in Fortran 77. H@ Input System. In order to have an easy to handle system available to test the TPR automation procedure, the following H2S input system was employed (Figure 3). Nitrogen and hydrogen sulfide were taken from a cylinder (L'Air Liquide), and the flow rate was controlled by Swagelok fine metering needle valves (V)and measured by Matheson flow meters (F). The working flow rate of the nitrogen carrier gas was adjusted at 120 mL/min; the H2S flow rate was 20 mL/min. In the situation shown in Figure 3, nitrogen gas flushes the system and H2S is conveyed to the waste. The system uses two Swagelok six-way valves. The first valve (W,) controls the filling of the sample loop
RESULTS AND DISCUSSION Quantification of the Data. The sulfide electrode only responds to the free S2- concentration, and therefore the potential of the sulfide electrode will be a function of the pH. Theoretically, by determining the pH, the total amount of H a can be calculated. However, the uncertainty in the value of pKa2as quoted in the literature (69)with values ranging from 12 to 18 a t 25 "C asks for an alternative method. In this method a calibration curve determined a t a constant pH value is used. In the measured solutions the pH was kept constant a t that specific value by adding an appropriate amount of a NaOH solution. Another problem is the progressive loss in sensitivity because of the logarithmic and integral response of the detector and because of possible contamination problems by organic products ( I O ) . These problems have been solved by using a sample changer. These principles have been implemented in the following way. First, a calibration curve has been determined for the
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sulfide electrode by using solutions over a range of sulfide concentrations from lo-' to lo4 M in 25% SOAB solution. The pH of all solutions was 13.10. A linear plot of E as a function of log (total concentration of sulfide) was obtained with a slope of 29.84 mV, showing Nernstian behavior, and an intercept of -893.9 mV. Procedure Optimization. Before the actual start of the analysis, the two electrode pairs are internally calibrated by using the first detection beaker filled with freshly prepared 25% SAOB solution containing lo4 M Na2S.9H20. After equilibration and control of the operation conditions, the experiments are started. The original pH is stored, and the reactor is heated linearly. Within every second, a well-defined number of pH, mVs (mVS = emf value of the sulfide electrode), and temperature measurements are carried out and the mean values are calculated and saved. This sequence of fast measurements is an altered version of the averaging techniques frequently used in potentiometric titrations (11-13) and tends to average out the noise in the electronic circuit (14). If the change in p H does exceed a critical difference, a correction will be made within the 1-s measurement cycle by addition of a calculated amount of 1.000 M NaOH from the motor-driven burette. The time necessary for the pH correction is smaller than 0.2 s. If no external interrupt has been detected, the measurement continues. There are two kinds of interrupts. When the first interrupt is encountered, the measurement beaker is replaced by a beaker containing 25% SAOB solution for rinsing the electrodes and cleaning the measurement equipment. Thereafter, the beaker is replaced by another filled with 100.0 mL of 25% SAOB solution containing lo4 M sodium sulfide solution. After a few seconds, automatic data collection can already be started again. This short equilibration time can be explained by the fast response time of the sulfide electrode (15)and also by the fact that the operating conditions for the electrodes, like temperature and solution content, remained roughly unchanged. An important feature is also that any movement of the electrodes between their calibration and use was avoided because such displacement can exhibit disturbing effects on the measurements (16). In the case of the second interrupt, the analysis is stopped and all the data are written on a text file. Before representing the data as H2S evolution profiles versus temperature or time, they are roughly smoothed by rounding off the temperature value to 1"C and by averaging the mVS and pH values in the time interval starting 0.5 "C before and after reaching this temperature value. These data are further subjected to the Savitzky-Golay-Gorry smoothing procedure (17,18). This algorithm provides also the first derivatives of the H2S evolution. Performance of t h e Detection System w i t h the H2S I n p u t System. In order to study the efficiency of the automated detection system, the H2S input system shown in Figure 3 was used to feed equivalent portions of 35 pg of H2S to the detector. The H2S input system is also a convenient way to simulate the continuous evolution of H2S in distinctive portions from a fossil fuel linearly heated in a reducing atmosphere. In Figure 4 the potentials resulting from the emf values of five consecutive injections with the H2S input system are plotted as a function of time (seconds). Due to the logarithmic response of the sulfide ion selective electrode, the absolute potential response per injection is progressively decreasing, resulting in a less sensitive quantification of the data. Figure 5 shows the corresponding process, but now before each injection the detection solution has been changed. Each H2S injection is now being detected in the most sensitive region of the calibration curve. The peak profiles for the five
'-1
-770
4O
1000
3000
2000
Time
4000
5000
I
6000
(5)
Flgure 4. Potential of the S2- electrode versus time, resulting from five equal consecutive injections by the H2S input system (see Figure 3).
"'"I
I"
0
500
1000
1500
2000
2500
Number of data points
Flgure 5. Potential of the S2- electrode versus the number of measurement points, resulting from five equal injections by the H,S input system each time after changing the detection solution.
injections are similar, and the integrated signal indicates that H2S has been monitored continuously without loss of sensitivity. An amount of 35 f 1pg of H;S has been found for each injection, illustrating the optimal operating conditions throughout the analysis. The dependence of the H2S signal on temperature conditions present in T P R analyses was also tested for a typical T P R reaction temperature of 400 "C. The injection of 35 pug of H2S fed directly to the detector was compared with an identical injection of H2S but now with a prior passage through the TPR reactor containing a reducing solvent mixture at 400 "C. The reducing solvent mixture was added in order to examine the eventual effect of absorption or reaction of H2S with the mixture at TPR conditions. It was found that under TPR conditions a t higher temperatures there is no significant loss of H2S by decomposition, reaction with the solvent mixture, or deposition on the walls of the reactor and tubes. Applicability of t h e Automated Detection System t o Fossil F u e l Analysis. The applicability of the automated detection system to the T P R analysis is illustrated for a set of desulfurized fuel samples obtained via low-temperature supercritical gas extraction (SCG)of a high sulfur content Spanish lignite (5). The original coal and the SCG char and extract were analyzed by TPR. The results are shown in Figures 6 and 7. The sequence of appearance of sulfur functionalities and the temperature ranges within which they appear have been described before (3, 4). In the original coal (Figure 6) a significant part of the original sulfur (>25%) is present as non-thiophenic sulfur (signals below 380 "C). The peak detected at 300-330 "C is assigned to aliphatic sulfur bridges, while the peak from 340 to 380 "C represents aromatic thioethers. The rest of the
ANALYTICAL CHEMISTRY, VOL. 63,NO. 8, APRIL 15, 1991
!
1.35-
c
__--
.-
108-
t
+ n
f
30
it-.
081-
i
_.-- _ _ _ _ _ _ _ _ - -
iTp 0.54-
=
027-
250
40
_______---
PI
300
350
400 450 Temperature (C)
500
550
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Figure 8. TPR analysis of the original coal (dotted line) and the SCG extract (solid line). Integral signals are given by the dashed lines. 135,
I40
1.08-
+
t
xp
non-thiophenic compounds and pyrite have been extracted, and the remaining char contains almost exclusively thiophenic compounds. The thiophenic character of sulfur in the char is also confirmed by the significantly lower recovery (15%) during the TPR analysis, indicating the predominant presence of condensed thiophenes. The more labile sulfur compounds that were originally present in the Mequinenza coal were not detected in the SCG extract nor in the SCG char. These sulfur compounds are found as H2Sand other low molecular weight sulfur compounds in the gases produced during the SCG extraction (5).
CONCLUSIONS Due to the automated design of the developed potentiometric detector and interaction with a motor-driven burette and a sample changer, it is possible to monitor H2S continuously in a sensitive and quantitative way. The detection setup can offer a meaningful expression to the H2Sprofiles obtained during the temperature-programmed reduction of fossil fuels and can thus provide an insight into the structural characteristics of the organic sulfur in fossil fuels. The application of the detection system described here is not only restricted to the study of fossil fuels but can be interfaced with every system whenever a continuous and highly sensitive monitoring of sulfur is desirable. ACKNOWLEDGMENT We thank S. R. Moinelo and R. Garcia (Instituto Nacional del Carbon y SUB derivados, Oviedo, Spain) and C . E. Snape (University of Strathclyde, Glasgow, UK) for supplying the Spanish lignite and the SCG chars and extracts.
0.54-
5 0.27-
250
783
300
350
400 450 Temperature (C)
500
550
600
Figure 7. TPR analysis of the original coal (dotted line) and the SCG char (solid line). Integral signals are given by the dashed lines.
detected sulfur (f5%) is present as thiophenic-likestructures. A signal indicating the presence of pyrite is seen at 410 "C, superposed on the thiophenic sulfur. Consequently, about 70% of the sulfur has to be present as complex thiophenes. In comparison, e.g. in a Belgian bituminous coal, about 90% of the sulfur is present in condensed thiophenic structures, while almost no aliphatic thioethers are detected (19, 20). These findings are in agreement with the hypothesis of a correlation between rank and sulfur functionality (21) suggesting that, with increasing rank, the relative amount of thiophenic sulfur becomes more important. The SCG extract (Figure 6) consists completely of thiophenic compounds: aliphatic and aromatic thioethers are almost not detected in the extract. This is confirmed by the GC-FPD/GC-MS analysis of this extract (51, which indicates the presence of substituted thiophenes, benzothiophenes, dibenzothiophenes, and three- and four-ring thiophenes (complex thiophenes). The TPR figure could tentatively be described more extensively as follows. 390-430 "C: substituted thiophenes and benzothiophenes; shoulder at 430-450 OC: dibenzothiophenic compounds; temperatures >450 O C : condensed thiophenic structures. The total recovery of the sulfur in the extract was 30%, which indicates that about 70% of the sulfur in the extract is present as condensed thiophenes. Figure 7 compares the TPR analysis of the original coal and the SCG char. During the SCG process almost all of the labile
LITERATURE CITED (1) Markuszewski, R. J . Coal Qual. 1988, 7, 1. (2) Attar, A. DOEIPC130145-T1; Raleigh. NC, 1983. (3) Majchrowicz, B. 6.; Yperman, J.; Reggers. G.; Frangols, J. P.; Gelan, J.; Martens, H. J.; Mullens, J.; Van Poucke, L. C. Fuel Processing Technol. 1987, 15, 363. (4) Majchrowicz, 8. 6.; Yperman, J.; Martens, H. J.; Gelan, J. M.; Wallace, S.;Jones, C. J.; Baxby, M.; Taylor, N.; Bartie, K. D. Fuel Processing Technol. 1990, 24, 195. (5) Garcia, R.; Moinelo. S. R.; Snape. C. E.; Bernad, P. Fuel Processing Technol. 1990, 24, 211. (6) Brunt. K. Anal. Chim. Acta 1984, 763, 293. (7) Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum Press: New York. 1976; Vol. 4, p 78. (8) Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum Press: New York. 1982; 1' Suppl., p 410. (9) Hbgfeldt, E. Stability Constants of Metalion Complexes, Part A ; Inorganic Ligand; IUPAC, Chemical DATA Series, Plenum Press: New York, 1982; Voi. 21, p 156. (10) Toth, 1.; Solymosi, P.; Szabo,2. Talanta 1988, 35, 783. (11) Christinsen, I. G.; Busch, J. E.; Kroogh, J. C. Anal. Chem. 1976, 48, 405 1. (12) Yamaguchi, S.;Kusygama, J. Fresenius' 2.Anal. Chem. 1979, 295, 256. (13) Leggett, D. J. Anal. Chem. 1978, 50, 718. (14) Arhno, J. M.; Gutknecht, W. F. Anal. Chem. 1976, 48, 281. (15) Light, T. S.;Swartz. J. L. Anal. Lett. 1968, 1 , 825. (16) Henry, R. P.; Prae, J. E.; Rossotti, F. J.; Whewell, R. J. Chem. Commun. 1971, 868. (17) Savitzky, A.; Golay. M. Anal. Chem. 1964, 36, 1627. (18) Gorry, P. A. Anal. Chem. 1990, 62, 570. (19) Majchrowicz, B. 6.; Yperman, J.; Reggers, G.; Martens. H.; Gelan, J.; Mullens, J.; Van Poucke, L. C. Roc.-Int. Conf. CoalSci. 1989, 1 , 51. (20) Majchrowicz, B. 6.; Franco, D.; Yperman, J.; Reggers, G.; Gelan, J.; Martens, H.; Mullens, J.; Van Poucke, L. C. Fue/, in press. (21) Attar, A.; Dupuis, F. I n Coalstructure; Gorbaty, M. L., Ouchi, K., Eds.; American Chemical Society: Washington, DC, 1981.
RECEIVED for review July 2,1990. Accepted January 11,1991.
