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ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
(41) P. A. Quinn, J. Swanson, H. L. C. Meuzeiaar, and P. G. Kistemaker, in Analytical Pyrolysis", C. E. R. Jones and C. A . Cramers, Ed., Elsevier, Amsterdam, 1977, pp. 408.
RECEIVED for review July 20,1977. Accepted October 14,1977.
We are grateful to the National Institutes of Health (grant GM 16609) for generous financial support of this research, and t o the Army Research Office, Durham, for funds t o improve the mass spectrometer used.
Trace Sulfur Determination in Petroleum Fractions Harry V. Drushel Exxon Research and Development Laboratories, P.O. Box 2226, Baton Rouge, Louisiana 7082 1
A convenient, accurate, and sensitive procedure for determinatlon of sulfur below 1 ppm in light petroleum fractions is described. It involves noncatalytic hydrogenolysis of the sample at high temperature to form H2S which is monitored by means of a Houston Atlas H2S Analyzer. The effect of temperature on conversion of aliphatic sulfides, thiophene, benrothiophene, and dibenzothiophene was studied from 700 to 1250 OC. Other important parameters investigated included hydrogen flow rate, sample injection rate, sample cell dimensions, moisturizationof the lead acetateimpregnated paper tape, etc. Long term preclslon (2u) was f 1 2 % , relatlve, at the 1-ppm sulfur level. By comparison with other methods, accuracy was estimated to be & l o % . Total lapsed time for analysis was less than 6 min. The limit of detection was determined to be 25 ppb.
Modern refining technology has required the development of sensitive procedures for determination of sulfur in petroleum fractions down to levels below l part per million (ppm). Although methods for the determination of sulfur down t o sub ppm levels have been available for many years, they are usually complicated and time consuming. Microcoulometry (1-3) has been used for such measurements over the past few years but it requires close attention to operating parameters and experimental conditions. Another commonly used procedure is the Wickbold ( 4 ) method which uses an oxy-hydrogen burner t o form oxides of sulfur which are determined by either a spectrophotometric, nephelometric, or turbidimetric procedure. By burning large samples, it is possible to achieve a precision of 0.2 ppm sulfur but the total lapsed time is of the order of 3 h. The Raney nickel reduction method developed by Granatelli (5) and improved by Pitt and Rupprecht (6) permits the determination of sulfur down t o 0.1 ppm in petroleum distillate fractions. However, it also requires a lapsed time of about 3 h and suffers from the limitation that it does not determine oxidized forms of sulfur such as sulfonic acids. This paper describes the application of a Houston Atlas H2S Analyzer t o monitor H2S from laboratory hydrogenolysis of sulfur compounds in light petroleum fractions. The Houston Atlas H2SAnalyzer has been a reliable, sensitive, and specific process analyzer for the measurement of H2Sin process gas streams. Attempts to use this analyzer for determination of total sulfur in liquid streams has been fraught with much difficulty because of the problems associated with conversion of the sulfur compounds t o hydrogen sulfide, particularly under process analysis conditions. Factors affecting this conversion step have been investigated and a set of conditions has been found which makes the conversion quantitative and reproducible under laboratory conditions. The H2S analyzer
itself was also modified t o adapt it t o these analyses.
EXPERIMENTAL Hydrogenolysis Equipment. Hydrogenolysis was carried out using a Dohrmann-Envirotech Model S-300 furnace which has 3 separately-controlledheating elements. The overall arrangement of the equipment is shown schematically in Figure 1. A thick-walled quartz tube (described in Figure 2) was installed in the furnace for conversion of the sulfur compounds t o H2S. The inlet of the tube was designed to accommodate a silicone rubber septum for sample injection from a syringe. The outlet of the tube was fitted with a ball joint for easy connection or disconnection of the transfer line. The inlet of the hydrogenolysis tube was connected to a source of Matheson ultra-high-purity hydrogen. One of the sets of valves and rotameters on the Dohrmann furnace was used to control and monitor the hydrogen flow rate. A water bubbler was placed between the rotameter and the inlet of the hydrogenolysis tube to moisturize the hydrogen. This was found to decrease the extent of coke formation during sample hydrogenolysis. The addition of water also promoted the removal of coke once it had deposited in the tube via the water gas reaction a t the high operating temperatures used. A hydrogen purifier based on diffusion of the hydrogen through a hot palladium thimble was tried but no particular improvement in background or performance was noted. Samples were injected into the hydrogenolysis tube with Precision Sampling Company CGV syringes having capacities ranging from 10 to 100 microliters (pL). A sidearm on each syringe facilitated flushing the syringe with sulfur-free solvent and permitted filling with the least possible cross-contamination. For injection of the samples, the plunger on the syringe was advanced slowly by means of a Model 355 Sage syringe pump to yield a constant injection rate. The Model 355 Sage syringe pump will provide any desired injection rate over a wide dynamic range. An inert gas such as helium or nitrogen was also connected to the hydrogen line to displace the hydrogen prior to opening the apparatus to the air. It was the practice a t the end of the day to open both ends of the hydrogenolysis tube to allow oxidation of any carbon accumulated in the hydrogenolysis tube. Although the operating temperature was 1125 "C, a standby temperature of 750 to 800 "C was used overnight or over weekends to reduce the rate of devitrification. H2S Analyzer. The hydrogenolysis products were transferred by means of a '/,-inch Teflon line to a Houston Atlas Model 825R H2SAnalyzer as shown schematically in Figure 1. This analyzer monitors H2S in gas streams by photometrically measuring the rate of blackening of a lead acetate-impregnated paper strip. A number of modifications were made to the Model 825R analyzer to adapt i t to analysis of the gases from the above laboratory conversion system. All of the stainless steel and Tygon transfer lines and the rotameter on the model 825R were eliminated. The Plexiglas water bubbler was also bypassed and the Teflon transfer line from the hydrogenolysis tube was connected directly to the cell of the paper tape mechanism by means of a glass T using Tygon tubing. Connections were made with butt joints to reduce the possibility of H2S adsorption. These changes reduced the internal volume of the system and eliminated the tailing caused by very slight
0003-2700/78/0350-0076$01.00/0 0 1977 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
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T E F L O N TRANSFER L I N E
HE or
HE or
N2
N2
H 2 (UHP!
Figure 1. Schematic diagram of trace sulfur apparatus
1 9 MM
0D
I 1 MM I D
12 / 5
f--
BALL L
( T H I C K - W A L L TUBING1
i
4 --..
EXPANDED
VIEW
OF I N L E -
SECTION H2 INLET
SMM i 3 M M
O D I 3
Figure 2. Diagram of hydrogenolysis t u b e
absorption of H2S in the 3% aqueous acetic acid solution in the bubbler supplied by the manufacturer. Instead of bubbling the gases through the acetic acid bubbler, an auxiliary moisturizing system was added. For this purpose, either helium or nitrogen was passed through one of the sets of valves and rotameters on the Dohrmann furnace, then to a heated flask containing aqueous acetic acid. The outlet of the flask was connected to the remaining arm of the glass T by means of Tygon tubing. This line was insulated to prevent condensation of moisture. The flask contained a thermometer well to monitor the water temperature. The flask was heated by means of a Glascol heater connected to a Variac. With this auxiliary system, the amount of moisture reaching the lead acetate-impregnated paper could be controlled by varying the flow rate of the inert gas or the temperature of the solution in the flask to achieve optimum response characteristics. The manufacturer supplied several cells of different dimension in which the hydrogenolysis gases contact the paper tape. The cell normally supplied with the Model 825R H2S Analyzer has a width of about ' / 4 in. Also, the photocell is usually mounted a t an angle of about 30" from the normal to the surface of the to '/16-in. width paper tape. For the smaller cells having windows, the normal cell position did not permit proper monitoring of the light reflected from the paper tape. For these cells, the photocell was mounted a t right angles to the paper tape but slightly above the light beam striking the paper surface. Also, for the small cells, the lens system was removed and reversed in order t o focus an image of the Tungsten bulb filament on the
aperture of the cell. After making any change in the cell, photocell, or lens system, it was necessary to balance the measuring and reference photocells, as recommended by the manufacturer. The output of the differential amplifier (terminals 6 and 18 on the Model 825R) was connected to a I-mV strip chart recorder by means of a potential divider system as shown in Figure 1. In this way, the range of the system could be adjusted as desired. Reagents and Chemicals. The sulfur compounds used in this study were obtained either from Eastman or Aldridge Chemical Company. The solvent used in preparing standards or diluting samples to lower concentrations was n-heptane, spectroquality, from Matheson, Coleman and Bell. The hydrogen used for this study was Matheson ultra high purity.
DISCUSSION OF RESULTS Response Characteristics. Typical response peaks are shown in Figure 3. T h e curve on t h e left side of the figure represents a baseline. No sample was injected during the 5.5 min in which t h e paper remained in a fixed position. T h e response for this baseline represented about one or two divisions (1 or 2 % of full scale). T h e curve on t h e right side of Figure 3 represents t h e injection of a sample containing 5 wg S / m L . Within 2 or 3 min after the injection was started, t h e response leveled out t o a constant value a t about 85 or 86 divisions. T h e response then remained constant for at least 2 min before t h e end of t h e injection occurred a n d t h e paper
78
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978 BACKGROUND RESPONSE ( no sample i n j e c t e d )
T H I O P H E N E IN n - H E P T A N E (5.01 ug S/ml)
E N D SAMPLE INJECTION
B E G I N PAPER
2
4
6
0
2
4
TIME (MIN )
TIME ( M I N 1
Figure 3. Typical response curves
moved to a new position. The response for the sample was taken as the average of the response over the last 2 min of t h e injection. T h e net response between this average value and the response for the baseline was used for calculation of t h e sulfur content. Calculations. The concentration of sulfur in the sample was calculated as follows:
where Cstd = concentration of sulfur in the standard in F g S / m L , R B = response for baseline, Rs = response for sample, Rstd = response for standard, and sp. gr. = specific gravity of sample. Thiophene dissolved in n-heptane was used as the standard. This was selected because thiophene (or its alkyl derivatives) is more difficult t o remove than aliphatic sulfur compounds a n d tends t o remain in hydrofined products, for which the method was primarily developed. T h e solvent (n-heptane) was analyzed and the standard corrected for any sulfur contributed by t h e solvent. Humidity, Temperature, and Gas Flow Rates. Some adsorbed moisture was required in order to obtain proper reaction of t h e H2S with the lead acetate on the paper strip t o form lead sulfide. Too little moisture caused the response to be low and constant response was not quickly reached. In fact, under these conditions, constant response was not reached during the entire injection period (5.5 rnin). Too much moisture alters the reflective characteristics of the paper and produces a false positive response. Acetic acid concentrations of 1, 5 , and 20 volume 7~were tried. At 20%, high false background response was obtained and constant response was slowly achieved. At 5’70 acetic acid, the false background was not obtained but optimum response was not rapidly reached. T h e most desirable response characteristics were obtained using a 1% acetic acid solution in the bubbler. The humidifier was operated from room temperature to 80 “C at inert gas flow rates from 50 to 300 cm3/min to determine
C i Y T F R AND O d T L E -
TEM”ESAT-RE
’;
Effect of temperature on conversion of various sulfur compounds to H,S Figure 4.
Concn, Curve
Compound
PJg SimL
A n-Propyl sulfide 8.49 B Benzothiophene 8.42 C Thiophene 8.13 D Di benzothiophene 8.30 the most optimum conditions. In general, flow rates from 200 to 300 cm3/min tended to reduce the response because of a dilution effect. Flow rates of 50 to 100 cm3/min. carried sufficient moisture t o the tape without reducing response. Temperatures near room temperature were insufficient. I t was found that 70 to 75 “C was most satisfactory for best peak shape in which constant response was most quickly reached (within 2 t o 3 min of the start of sample injection). Effect of Temperature on Conversion of Various Sulfur Compounds. Figure 4 shows the response obtained from four selected sulfur compounds a t various hydrogenolysis temperatures. These sulfur compounds were selected to be representative of the major classes of compounds which might be encountered in the analysis of light and middle petroleum distillates. n-Propyl sulfide was selected to represent aliphatic sulfides and other aliphatic sulfur compounds such as mercaptans or disulfides. The thiophenic compounds which were selected were thiophene, benzothiophene, and dibenzo-
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, J A N U A R Y 1978 eo,
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7 -
79
I
I
I
_ I -,)'CC;E:
FL3*
';-E
,:
L
ic;
4CZ
i3C
lh~:;-i:P.
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Figure 5. Effect of hydrogen flow rate and cell size and position on
response. Quartz hydrogenolysis tube, n-propyl sulfide in n-heptane Cell width, Photocell Concn, Curve inches position p g SImL A B C D E F G
1/16
in front
118 118 118 118 118 118
in front in front in front in front normal in front
Syringe Curve A B C D E F G
size, pL
50 50 25 100 50 50 50
Injection rate, pL/min
1.02 1.02
8.49 8.49 1.02 1.02 1.02
Temperature, 'C
8.9 11.8
900 900
7.1 7.4 8.9
1130 1130
8.9 4.1
900 900 900
thiophene. These compounds were prepared as solutions in n-heptane a t concentrations near 8.5 pg S / m L . These solutions were injected into a ceramic hydrogenolysis tube fitted with a quartz inlet section and a quartz outlet insert to reduce adsorption effects. By using a ceramic tube it was possible to achieve temperatures approaching 1220 "C which could not be reached using a quartz hydrogenolysis tube. At temperatures between 800 and 900 "C, the n-propyl sulfide was converted t o the extent of 80 to 90% while thiophene and benzothiophene were converted to only 10 or 20%. Apparently, both thiophene and benzothiophene required temperatures of a t least 1100 "C to achieve nearly complete conversion to H2S. However, a t this temperature, only 40% of the benzothiophene was converted. For essentially complete conversion of dibenzothiophene, temperatures of the order of 1220 to 1250 "C were required. Since this temperature could not be achieved with a quartz hydrogenolysis tube, this method was limited to those materials having boiling points below that of dibenzothiophene. Middle distillates or vacuum gas oils boiling above 300 "C should not be analyzed by this procedure without the use of a ceramic tube operating near 1225 to 1250 "C. (The Dohrmann S-300 furnace should not be operated a t these temperatures.) Usually, the sulfur content of these materials, even after hydrofining, is sufficiently high so that they may be easily analyzed by either microcoulometry or x-ray fluorescence. Effect of Hydrogen Flow Rate. Figure 5 shows the effect of varying the hydrogen flow rate under a variety of experimental conditions which included differences in cell width, photocell position, concentration, injection rate, and hy-
GATE
L
-
P
Figure 6. Effect of hydrogen flow rate ancl injection rate on response. Quartz hydrogenovsis tube center and outlet at 900 O C . n-Propyl sulfide in n-heptane at 1.02 pg S/mL (50-pL syringe) Photocell mounted directly in front of cell ('/8-in. width). Curve A, 210 cm3 HJmin: Curve 6 ,233; Curve C, 480; Curve D, 675
drogenolysis temperature. In every case, the response decreased with increasing hydrogen flow rate. Therefore. in the interest of obtaining maximum response, it appeared most desirable to operate a t minimum hydrogen flow rate. However, the use of a low hydrogen flow rate a t a high injection rate resulted in the formation of carbon in the hydrogenolysis tube. The formation of excessive carbonaceous deposits tended to cause adsorption of H2S and a tailing effect which interfered with the analysis. Since a high injection rate was advantageous for the determination of sulfur at very low levels (
