Determination of Mercaptans in Sour Natural Gases by Gas Liquid Chromatography a nd Mic r ~ cu Ioo metric Tit rat io n E. M. FREDERICKS and G. A. HARLOW Shell Development Co., Emeryville, Calif.
b A microcoulometric GLC method has been developed for the determination of traces of individual mercaptans in natural go:; containing large amounts of hydrogen sulfide. The method utilizes standard GLC instrumentation coupled with a commercially available microcoulsmetric titrator. Sample components are separated on a GLC column and the individual mercaptans are automatically titrated with coulometrically generated silver ion as they emerge. The large excess of hydrogen sulfide is bypassed and not determined. Hydrocarbons, which enter the titration cell together with the mercaptans, do not interfere. The method requires only 30 ml. of sample, is rapid (1 hour), precise (*270 at 50 p.p.m.), and sensitive to as little as 1 p.p.m. of individual mercaptan.
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of the concentration of individual sulfur compounds in sour natural gases is of prime importance in the design and operation of gas purification plants. I n particular, a method is required for irhedetermination of individual mercaptans present in trace amounts in sour natural gases containing large amounts of hydrogen sulfide. A typical sour gas might contain 10 to 300 p.p.ni. of total mercaptan, C1-C4+, and 1.5% to 15% hydrogen sulfide. The conventional potentiometric silver trtration (19) fails because of its inability to resolve the individual mercaptans. Gas liquid chromatography (GLC), with hydrogen flame ionization detection, has sufficient sensitivity but is not able to resolve the individual mercaptans in the presence of hydrocarbons which emerge with, and completely overshadow, some of the mercaptans of interesk. This report describes the development of a method for the determination If mercaptans in sour natural gas coribining the resolving power of GLC with the specificity of detection of the potentiometric silver titration. The lower thiols have been chromatographed by Spencer, Baumann, and Johnson (8), Amberg ( I ) , Ryce and Bryce ( 7 ) , Sunner et al ( I I ) , Karchmer NOWLEDGE
(4, Sullivan et al. (IO), Carson ($), and Liberti and Cartoni (6). These workers were mainly concerned with resolution and analysis of mixtures of thiols and sulfides in the absence of large concentrations of hydrocarbons and thus were able to utilize thermal conductivity detection. Liberti and Cartoni (6) separated the thiols hy conventional GLC, but used an automatic coulometer to titrate the individual components as they emerged from the column. They applied this method to the analysis of mixtures of the lower thiols and to thiols extracted from crude oil. Hydrogen sulfide and thiols lower than Cs were either lost in the extraction, procedure or were not present in the original crude. Klass (6) determined small amounts of thiaalkanes in hydrocarbons by a GLC-combustion technique in which the resulting sulfur dioxide was coulometrically titrated with electrolytically generated bromine. Mercaptans were strongly adsorbed on the GLC column used and were not observed. I n a more recent study, Sporek and Danyi (9) identified mercaptans by oxidation to the corresponding disulfides with iodine. The mixed disulfides were separated and identified by GLC. None of the above workers studied the effect of large excesses of hydrogen sulfide ?x attempted the direct determination of traces of mercaptans in hydrocarbon media. Both polar and nonpolar stationary GLC
phases were used by the various authors to separate the thiols. Our preliminary investigations indicated that the stationary phase of Ryce and Bryce ( 7 ) , tricresyl phosphate (TCP), offered the greatest resolution between hydrogen sulfide and methyl mercaptan. The higher mercaptans could be quickly eluted if temperature programming were used in the manner of Ryce and Bryce (7) and of Sullivan et al. (IO). The advantages of coulometric detection of mercaptans in a hydrocarbon media were apparent from work of Liberti and Cartoni (6). The availability of a commercial coulometric titrator (Dohrmann Instruments, Palo Alto, Calif.) made this approach even more attractive. The system chosen for initial study combined the temperature programmed GLC separation of H2S, hydrocarbons, and thiols on a T C P column with coulometric determination of the thiols using electrolytically generated silver ion. EXPERIMENTAL
Reagents and Apparatus. Hydrogen sulfide was supplied by the Matheson Co., East Rutherford, N. J., and the various thiols were supplied by the Eastman Kodak Co., Rochester, X. Y., and K and K Laboratories, Inc., Jamaica, S . Y. All other materials were reagent grade. A block diagram of the apparatus is
Gas Sample
Valve
n \ I
32-
I I
I
I
Figure 1.
I
Microcoulometric
out
I I II Helium supply
I
I
,
In
I
d I 1 T.C. Cell
GLC Luiu GLC Column
Titration C e l l
Two-Channel Recorder
Microcoulometric GLC apparatus for determination of mercaptans VOL. 36, NO. 2, FEBRUARY 1964
a
263
Table I. Relative Emergence Times of C1-C4 Mercaptans: n-Pentane = 1 .OO
B.p., " C., Relative 760 mm. emerHg gence
Compound (Hydrogen sulfide) (-59 6) (0 38) Methyl mercaptan 5.96 1 70 Ethyl mercaptan 35 00 2 84 Isopropyl mercaptan 52 56 3 16 tert-Butyl mercaptan 64 22 3 31 n-Propyl mercaptan 67 72 3 64 sec-Butyl mercaptan 84 98 3 92 Isobutyl mercaptan 88 49 4 00 n-Butyl mercaptan 98 46 4 20
shown in Figure 1. Helium carrier gas was supplied to the unit from a twostage reduction valve and regulated by a 0 to 60 p.s.i.g. Conoflow pressure regulator. For initial testing and for the introduction of pure materials, sample was introduced by means of a hypodermic syringe through a serum cap mounted in a Swagelok T at the head of the column. For precise quantitative analysis, sample was introduced from a calibrated bypass loop. The sample loop was switched in and out of the eluent gas stream by means of a 6port-LG process sampling valve (Beckman Instruments, Fullerton, Calif.). The valve was fitted with a 5-foot section of 11'~-inch Teflon tubing in the sample loop position. The loop was calibrated with helium using carbon dioxide sweep gas; the helium was measured with a n Azotometer. Sample volume was 29.0 ml. at 22" C. and 760 ml. Packed GLC columns were prepared in the usual manner. Acid washed Chromosorb W was coated with 30% by weight of T C P (Monsanto Chemical Co., St. Louis, 110.)by dissolving the required weight of partitioning liquid in a large excess of acetone and making a slurry of this solution with the required weight of solid support. The slurry was taken t o dryness, with mixing, on a steam bath. The prepared material was packed into 25 feet of l/q inch 0.d. aluminum tubing with the aid of a n electromechanical vibrator. After packing, the column was coiled on a 4 inch mandril and attached to an 8-inch square of insulating board with Swagelok bulkhead fittings and suspended in a 4liter Dewar flask. No provision, other than the insulating Dewar flask, was made for temperature control. The coiled column was wrapped with a &foot, 150-watt glass heating tape. With a full line voltage, this manner of heating provided sufficient heat to raise the column temperature 2.5" C. per minute from ambient t o 130" C. The temperature profile was nonlinear but was reproducible t o within j=55!& as judged by the emergence times of the various thiols tested. This precision in emergence times was judged adequate since it eliminated most ambiguities in component identification. Two detectors were used in series in the present work; thermal conductivity 264
ANALYTICAL CHEMISTRY
Figure 2. Chromatogram of condensate hydrocarbon-hydrogen sulfide-mercaptan mixture using two detectors
and microcoulometric. The thermal conductivity detector was a conven . tional four-element thermal conductivity cell using Gow Mac type 9225 tungsten filaments. The elements were mounted in a stainless steel block with a pretzel-type flow pattern. The block assembly, as with the column assembly, was attached t o an 8-inch square of insulating board with Swagelok bulkhead fittings and suspended in a 2-liter Dewar flask with no additional provision for temperature regulation. The external bridge circuit was a conventional Wheatstone-type with controls for coarse and fine zero, current, and output attenuation. The bridge was powered by a 12-volt storage battery and the 1-millivolt channel of a 1- and 5-millivolt, dual channel, Texas Instrument "Servoriter" was used to monitor its output. The primary detector was a microcoulometer (Model C-100 with a T-200 cell, manufactured by the Dohrmann Instruments Co., Palo Alto, Calif.) as described by Coulson and Cavanaugh ( 3 ) . I n brief, the coulometer employs a servo-operated voltage source driven by a high-gain null-balance Brown amplifier. The input signal from a sensing electrode is the difference between the sensor and a reference electrode and is biased in such a manner that a zero signal appears across the input of the amplifier for a given concentration of silver ion. As the silver ion concentration is lowered by mercaptans entering the cell, additional silver ion is generated t o maintain balance. The current required to generate the silver ion is monitored, through a precision series resistor, by the second channel of the Texas Instruments recorder. I n this manner, a dual recording is produced, one for all the volatile components eluted from the GLC column and the other for components which react with
silver ion. I n practice, the thermal conductivity cell was useful only in determining the optimum conditions of column length, sample size, flow rate, temperature, etc.. for resolution of the various components and, once these conditions had been established, the mercaptan determinations were made with the microcoulometer. Procedure. Helium was admitted to the chromatographic unit a t 30 p.s.i.g. from the cylinder through the two-stage regulator. This inlet helium pressure provided 120 mi. per minute flow through the column at ambient temperature. The flow through the reference side of the thermal conductivity cell was adjusted t o approximately 10 ml. per minute. Power was applied t o the thermal conductivity bridge and the unit allowed t o reach equilibrium as evidenced by a steady baseline on the 1-mv. channel of the recorder. The coulometer cell was cleaned, electrodes polished, and the cell flushed with fresh titration solvent (7OyO acetic acid-30% water), biased a t +235 mv., and allowed t o generate sufficient silver ion t o attain a cell potential equal and opposite t o the bias voltage. At this point, generation ceases and a steady base line of zero deflection is observed on the recorder. The accuracy of the method for the determination of mercaptans was tested by the introduction of a known amount of methyl mercaptan in nitrogen with the Beckman bypass sample valve. Care was taken t o ensure that the sample loop of the valve was completely flushed with sample and that the contained sample was a t atmospheric pressure. The methyl mercaptan content of the sample had previously been determined by titrating 1 liter of the gaseous mixture with standard silver nitrate using a potentiometric end point.
Triplicate analyses using the microcoulometric GLC method agreed with the known value d h i n +2%. The emergence times of the mercaptans were establisied by the injection of the known compounds, either as the relatively pure liquids or as mixtures of their vapors in n trogen. Table I gives the emergence tines of the various mercaptans of interest under the chosen parameters of this analysis. Maximum resolution between hydrogen sulfide and methyl mercaptan was required in the present application and the column was operated isothermally a t room temperature until just after the emergence of methyl mercaptan (30 minutes). At this time, the column temperature was raised a t 2.5" C. per minute until it reached 130" C.; butyl mercaptan emerges a t this point and the analysis was terminated. IC'atural gas sample
