Measurement of Odorant Levels in Natural Gas1 Arthur R. Knight Department of Chemistry and Chemical Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada
Arun Verma* Research and Development Centre, Saskatchewan Power Corporation, Regina, Saskatchewan, Canada
The purpose of the work was to develop a simple and low cost method for accurate measurement of mercaptan odorant levels in natural gas. A thorough literature survey was conducted and several reactions were studied and evaluated. A colorimetric method and simple apparatus was developed which can be used under operating field conditions as well as for precise laboratory measurements. The method and apparatus were found satisfactory in actual field tests conducted by independent operators in the gas industry.
Introduction The availability of a convenient method for the accurate measurement of odorant levels in natural gas pipelines is important. If the odorant levels are lower than legally specified, the gas company not only violates the law but also endangers the lives of its customers. On the other hand, if the odorant level is too high, then because of incomplete combustion of sulfur compounds in the odorant, the customer may detect the characteristic warning odor from a properly operating gas appliance and assume a gas leakage when none exists. This could result in a number of unnecessary service calls for the gas company. Odorants are organic sulfur compounds which are added downstream of the gas sweetening process before the natural gas is fed into distribution lines. The sulfur compounds added as odorants are essentially of three main types: (a) the mercaptans, (b) nonmercaptans, and (c) sulfide-mercaptan mixtures. Generally speaking, mercaptans have more odor impact per unit weight and are cheaper than other odorants. Nonmercaptan odorants are generally cyclic sulfides and are used in special circumstances. They have high oxidation resistance but have low odor impact when compared with mercaptans. T o combine both the favorable properties of mercaptans and sulfides, a blend of these two is sometimes used. T o measure the amount of odorants in gas, a number of methods are used. Among these is an odortester (Davis, Code No. 11-850) whose quantitive nature depends on the operator’s olefactory nerve so a large variation in reading with different operators is inherent. Second are the odorant measuring tubes produced by a number of manufacturers such as Mine Safety Appliance Co. (Mine Safety Appliances Report No. 454206) which contain a solid support impregnated with a chemical such as palladium chloride. This method involves the passage of a known volume of gas by a pump through the tube which reacts with the chemical, the length of the stain developed being a measure of odorant level. This method is reported to be good for spot checks but in actual tests the length of stain varied with the number of pump strokes. Difficulty was observed in measuring the actual length of stain as the demarcation line a t the end of the stain was not definite. Furthermore, this test is rather insensitive to the sulfide component of many odorants. A number of other tests have been mentioned and evaluated by Wilby (1966) and McA. Mason Patent filed in U.S. and Canada.
(1963). Essentially these methods are either not specific for mercaptans, are complicated by other components in natural gas, or are elaborate and time consuming thus rendering themselves inappropriate for use in the field. One of the methods evaluated in the above studies, an electrolytic method utilizing electrolytic titration with bromine or iodine, was considered reliable. However, our own studies have indicated practical problems associated with this approach. The apparatus is elaborate and takes considerable time (0.5 hr) to run a single test. It was also found to be nonoperable on low pressure pipeline. Finally, another method of measuring odorant level is by an instrument which is essentially a dedicated portable gas chromatograph “Odotron” (Kutzleb, 1973) which is effective in measuring specifically ten odorant compounds. Although this instrument is very reliable, the cost of the instrument prohibits its use as a mobile field instrument because equipping every field operator with such an instrument would run into many thousands of dollars. Based on the above observations, it was felt desirable to develop a simple test which employed an uncomplicated, low-cost apparatus for measuring odorant levels in natural gas pipelines. As a start in this direction and because of the fact that a number of gas odorants are mercaptans, it was decided to concentrate on developing a specific test for mercaptans. From a comprehensive literature survey, it was concluded that such a test could be a reaction involving mercaptans and another reagent where the reaction product is a colored species and where the color intensity is directly proportional to the extent of reaction. As the limiting reactant in such a case could be kept as the mercaptan, the extent of reaction then would be a measure of the quantity of mercaptan. In addition to this, the desirable reaction would be expected to be reproducible and sensitive to small amounts of mercaptans. The normal odorant level in natural gas is around 5 ppm, which would be 5.65 Imol of mercaptan per cubic foot of gas under atmospheric conditions. A combination and assessment of the literature indicated six promising reactions (Ellman, 1958; Fritz, 1961; Woodward, 1949; Provvedi, 1952; Karr, 1954; Benesch, 1956) which were evaluated in the present work. The results of these and the details of a simple apparatus are described below. Experimental Details For the preliminary evaluation of the above reactions, a stock solution of a synthetic blend of odorant, 10% isoproInd. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 1, 1976
59
METERING VALVE
I /I I
1
'
L----J-
REAGENT CAPSULE SHOWN/
-
I
F L E X I B L E HOSE
(a) GAS B U B B L I N G A P P A R A T U S
PORTABLE STAND\
GAS INLET DOUBLE ORIFICE NEEDLE
GAS OUTLET
REACTANT
1
BREAKABLE
MEMBRANE
REACTANT
(b) CROSS S E C T I O N OF R E A G E N T C A P S U L E
Figure 1. Details of the apparatus. pyl mercaptan and 90% tert-butyl mercaptan by volume, in a 50:50 water-methyl alcohol solvent, with a total mercaptan concentration 0.21 pmollml was prepared. Pure mercaptan samples were obtained from Phillips Petroleum Co. and from Eastman Organic Chemicals. The reagent solutions for the various tests were prepared as described in the literature (Ellman, 1958; Fritz, 1961; Woodward, 1949; Provvedi, 1952; Karr, 1954; Benesch, 1956). In the initial series of exploratory experiments, 0.1 to 10 ml of the stock mercaptan solution was added to the suggested amount of test reagent and the results noted. Those reactions which produced a visually observable color change were studied to determine whether the response was linear and absorption measurements over a concentration range of a t least 0.05 to 1 pmol of mercaptan blend were made. Absorption measurements at the appropriate wavelength were made using a Coleman Model 295 spectrophotometer, and the concentration dependence of the solution optical density was determined. The test using N-ethylmaleimide, which appeared on the basis of this investigation to be most suitable, was subjected to further study, both in terms of concentration dependence and reagent solution concentration and type of solvent using standard mercaptan-blend stock solutions, as well as with gas bubbling experiments under field conditions.
Description of Apparatus An apparatus for bubbling natural gas in solutions was constructed as shown in Figure l a , which is a front elevation in schematic form. The apparatus was essentially a rotameter with a metering valve, mounted on a portable stand. A hollow needle was attached to the metering valve. For laboratory tests, the solution to be bubbled was measured and put in a test tube. The rotameter inlet was attached to a mercaptan-containing natural gas supply by a 60
Ind. Eng. Chern., Prod. Res. Dev., Vol. 15,No. 1, 1976
flexible hose and valve adjusted to the desired flow rate. The gas was then bubbled through the solution in the tube for the desired length of time. If after the bubbling period another reactant was required to be added, it was added to the tube and then the optical density was measured in the spectrophotometer. To adapt this simple apparatus for use under field conditions, the hollow needle was replaced by a double orifice needle as shown in Figure lb. Instead of the test tube, a glass capsule was made having two compartments separated by a thin membrane. The ends of the capsule were sealed after filling both compartments with appropriate reagents (as was required for the N-ethylmaleimide test which was developed during the present work and described later), one end of the capsule being sealed with a rubber septum. A mounting clip, as shown in Figure l a , was also used to hold the capsule in position during the period the gas bubbled in the solution. The method now was to adjust the flow as desired and penetrate the rubber septum with the needle, as shown in Figure Ib. After the sampling period, the valve was shut off and the capsule pushed upward so as to enable the needle to break the membrane and facilitate mixing the reagent. The resulting color in the solution could be read directly as ppm of odorant in the gas flow by comparison with a color chart. The chart was constructed by comparison with, or photographic reproduction of, standard solutions containing known pmole quantities of mercaptans, the concentrations being converted to ppm of odorant for the specific gas volume to be used in the test. This chart was used for field tests although a portable colorimeter can also be used.
Results and Discussion Preliminary Reagent Evaluation. The following results were obtained in an exploratory series of experiments involving six reagents reported to produce a colored product on reaction with mercaptans at ambient temperature in solution. (a) Bis(p-nitrophenyldisulfide)(Ellman, 1958) in a phosphate buffer a t pH 8 in an aqueous acetone solution gave a distinctive yellow solution on addition of mercaptan. However, no visually detectable change in color intensity or hue was observed when the concentration of added mercaptan was altered. (b) Mercury perchlorate (Fritz, 1961) with pyridine in an aqueous acetone solution with added thio-Michler's ketone indicator did not produce the reported blue coloration when mercaptan was added. A white precipitate was formed which dissolved by the subsequent addition of more acetone. (c) Sodium nitrate and glacial acetic acid (Woodward, 1949) in aqueous reacted with mercaptans to produce a green color whose intensity was concentration dependent. Spectrophotometric tests showed the reaction to be quantitative. However, no color change was observed on bubbling mercaptan-containing natural gas through the reagent solution. This was most likely due to the slow reaction rate and the relatively short contact time of the mercaptan while bubbling gas. The solubility of mercaptan in various aqueous solutions is not large and consequently it is not possible to accumulate large mercaptan concentrations in the solution. (d) Sodium nitroprusside in aqueous NaOH solution (Prowedi, 1952) produced a green coloration on reaction with mercaptans. The dependence of the color intensity on mercaptan concentration could not be detected by visual observation. Using "*OH as the base a red product formed and the absorbance of this solution at 525 nm was
Table I. Linearity of Concentration Dependence as a Function of Conditions Calibration data
_______
Series
Reagents
pmol of mercaptan
Solution O.D. at 5150 A
Linear response range, pmol of mercaptan
Comments
A
5 ml 0.0125 M NEM in 2-propanol 1 ml 0.01 M NaOH in 2-propanol
0.1 0.2 0.3 0.4
0.3 50 0.655 0.900 1.07
0.1-0.3
Immediate color, stable for 5 min
B
2 mlO.l M NEM in 2-propanol 2 mlO.1 M NaOH in 20% H,O-80% 2-propanol
0.22 0.44 0.66 0.88
0.2-1.0
Immediate color, stable for 5 min
1.11
0.193 0.465 0.632 0.770 1.05
C
2 mlO.l M NEM in 2-propanol 2 mlO.1 M NaOH in 30% H,O-70% 2-propanol
0.44 0.88 1.32 1.76 2.2
0.253 0.490 0.710 0.930 1.27
0.4-2.0
Color develops over 2 min and is stable for 5 min
D
2 mlO.1 M NEM in 2-propanol 2 mlO.1 M NaOH in 50% H,O-50% 2-propanol
0.88 1.76 2.24 3.52 4.44
0.297 0.525 0.650
0.9-4.0
Color is unstable and fades immediately after formation
2 mlO.1 M NEM in 2-propanol 2 mlO.1 M NaOH in H,O
1.1
0.1 23 0.183 0.490 0.890 1.06
E
2.2 4.5 7.8 11.2
1.10
1.22
found to be linearly concentration dependent. Again, however, no color developed on bubbling odorant containing natural gas through the solution. (e) Phosphomolybdic acid in aqueous NaOH (Karr, 1954) reacted with mercaptans to form a blue product. The solution absorbance was concentration dependent, although the colored product appeared to be somewhat unstable. Further investigations of this reagent will be reported in a future publication. (f) N-Ethylmaleimide (Benesch, 1956) in 2-propanol solution reacted with mercaptans in alcohol solution and a red-pink product was detected on addition of a phosphate buffer of p H 6.8, or on addition of dilute NaOH solutions. The color intensity was found to be visibly concentration dependent and spectrophotpmetric determinations indicated that optical density values vary linearly with mercaptan concentration. The reaction also proceeded when the mercaptan was introduced by bubbling an odorant-containing gas stream through the reagent solution. This test was selected for detailed examination, the results of which are reported in the next section. N-Ethylmaleimide (or NEM) Test. The mechanism of the reaction between NEM ( C ~ H ~ O ~ N C and ~ H alkyl S ) mercaptans such as C2H5SH and (CH3)3CSH has not been fully elucidated, but some kinetic studies have been reported in the literature (Friedman, 1949, 1952; Gregory, 1955; Brockhuysen, 1958). The colored species is thought to be the doubly charged negative ion produced by the addition of base to the enol form of the complex resulting from the interaction between N-ethylmaleimide (NEM) and the HC-C
(1
No
>-cL"
HC-C \O mercaptan. The concentration of the light absorbing species, and hence that of the mercaptan, can be determined quantitatively by measuring the optical density of the solution a t 5150 A spectrophotometrically.
1-9
Immediate color, stable for 1 0 min
I t was established in the preliminary tests that trace quantities of water in the 2-propanol solution of NEM interfered with the reaction and gave nonreproducible results. For all subsequent tests the 2-propanol used (99.8% assay reagent grade) in the NEM solution was freed from residual water by distilling over sodium. Following a number of exploratory experiments designed to establish the quantitative nature of this reaction, a series of tests was carried out. Varying ratios of mercaptan in absolute 2-propanol were added to the NEM and NaOH solutions and the optical densities measured. The range of linear concentration dependence was again determined from a graph of optical density (O.D.) vs. mercaptan concentration. Five quantitative series of experiments of this type were carried out and the data are listed in Table I. Although some gas bubbling experiments were carried out at this stage to ensure that the test was operative under field conditions, considerable effort was expended, unfortunately without much success, to determine if the reaction could be made to proceed in an NEM solution that had been coated on a solid support to which the NaOH solution could subsequently be added to develop the color. Although in a number of cases the reaction did proceed and a color could be observed, the intensity of the coloration varied quite erratically and no definite relation could be established between the intensity of the color and the mercaptan concentration. The types of solid support investigated and some of the conditions studied are summarized in Table 11. This information provides a useful starting point if further studies into this aspect of the test are to be made. When it became apparent that the support-coating technique would be difficult to develop into a practical testing procedure, effort was directed toward perfecting the NEM test using the bubbling technique. A series of bubbling tests, similar to those using alcoholic mercaptan solutions, as summaried in Table I, were then carried out. The data in Table I11 summarize these results. I t is obvious that the test gives consistent results over a fairly wide range of flow conditions. I t was observed that a t flow rates less than 200 cm"/min, the results were consistent so long as the total Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 1, 1976
61
Table 11. Experiments to Determine the Feasibility of Reaction Occurring in a Packed Tube ~~
Packing material
Proceduresa Notes
Silicone coated 30/60 mesh glass beads A,B,C b Porous glass beads, 60/80 mesh A,B b 10% tricresylphosphate on Chromasorb W A,B,C c Silica gel A,B b Molecular sieve, 5A A,B,C b 15% Didecylphthalate on Chromasorb Z A,B,C d 15%Dinonylphthalate on Chromasorb W A,B,C d,e 15% Didecylphthalate on Chromasorb W A,B,C d 20% Carbowax 20M on gas-chrom Z AB d,f Silicone oil DC200, 15% on Chromasorb W A,B d,g a A, support coated with NEM, NaOH solution added; B, support coated with NaOH, NEM solution added; C, support coated with NEM, NaOH and H,O added. b Little or no color producing reaction usually observed. C Slight pink color usually found, no relation to amount of gas flow. dDistinct pink color found, intensity varied erratically. e 34 trials conducted varying tube length and diameter, support treatment, and flow time. f 18 trials conducted as in note e. g Brilliant pink coloration observed, but blank also colored and determination of difference difficult. Table 111. Gas Bubbling Experiments
ReSeries agentsa A I
Flow rate, cm3/min 64
Gas volume, cm3 256 512 832 291 582 87 3 1164 400 800 1200 1600 304 608 1216 1824 600 1200
Mercaptan concn Total pmol 0.05 0.095 0.155 0.048 0.095 0.131 0.19 0.068 0.146 0.21 0.225 0.053 0.10
PPm 5.0
4.6 4.6 B I 97 4.1 4.1 3.8 4.1 I 200 C 4.2 4.6 4.4 3.5 I 608 4.4 D 4.1 0.18 3.7 0.23 3.1 E 11 600 0.12 5.0 0.21 4.4 1800 0.30 4.2 2400 0.34 3.5 0.40 3.1 3000 4800 0.38 2.0 F I1 245 490 0.048 5.0 980 0.207 5.3 1470 0.27 4.6 1960 0.33 4.2 G I11 600 1800 0.28 3.9 5400 0.53 2.5 a l , Gas was bubbled in 4 ml solution of 0.0125 M NEM in water-free 2-propanol and 1 ml of 0.01 M NaOH in 2-propanol was later added. 11, Gas was bubbled in 2 ml solution of NEM as in I and 2 mlO.1 M NaOH in 20% H,O-80% 2propanol was later added. 111, Gas was bubbled in 2 ml solution of 0.1 M NaOH in H,O and 2 ml NEM solution as in I was later added. volume of gas passed through the solution did not exceed 1000 cm3. At high flow rates, reduced ppm values were observed when the total volume of gas was large. As some dependence on the flow rate itself was expected, this was investigated later and will be discussed below. In most instances, the tests that were carried out subsequently involved amounts of gas in which the total ratio of mercaptan was less than 0.4 hmol. The reagents used in series A, Table I, and for four of the bubbling tests listed in 62
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 1, 1976
Table 111-namely 4 ml of NEM in water-free 2-propanol with the addition of 1 ml of 0.01 M NaOH in 98.8% 2-propanol-were therefore used exclusively in these later tests. The use of an alcohol for the addition of base gives the largest optical density for a particular amount of mercaptan reacted in the test and therefore represents the most sensitive test for both spectrophotometric and visual observations. To investigate further the effect of flow rate and total quantity of gas passed through the solution, an additional series of experiments similar to those for which data are reported in Table I11 were carried out. Flow rates from 60 to 600 cm3/min were studied. The results are shown in Table 111. At the highest flow rates the ppm reading obtained for each sample dropped off if the sample volume exceeded 1000 cm3. The readings a t lower flow rates were virtually constant at moderate total volumes of gas passed. Further investigations will be required to establish the conditions under higher flow rates a t which the readings can be relied upon, as the data in all experiments of this type indicate the range of appropriate conditions to be dependent on both amount of gas and flow rate. It was established, however, that the results are reproducible in the flow range 200-500 cm3/min with total gas volumes less than 1000 cm3. In an effort to reduce the volume required in the solution capsule, the effect of reducing the amount of liquid reagents was examined briefly. Several tests using solutions of mercaptans were carried out using 2 ml of the NEM and 1 ml of NaOH solutions. These results were satisfactory. However, in bubbling experiments, the calculated ppm mercaptan concentrations were significantly less, usually by 1 to 2 ppm, than when the gas stream was run through the 5 ml total volume test. It thus appears that when the reaction is carried out using solutions only, the solution volume does not influence the results, provided the stoichiometric requirements of the reaction are not exceeded. On the other hand, for quantitative absorption of the mercaptan from a flowing gas stream a volume of a t least 5 ml is required in the test reaction. To test the effect, if any, due to the presence of organic sulfides, natural gas containing known amounts of mercaptan and methyl sulfide was bubbled through the N-ethylmaleimide solution. No interference due to sulfide was noticed. In some experiments, the optical density was reduced marginally by the addition of sulfide to the sample solution. The effect, however, was within experimental error. As a mixture of tert-butyl and isopropylmercaptan was used in all previous studies, it was important to determine what, if any, difference in reactivity there was between the two mercaptans. For this purpose a series of tests was run by varying relative amounts of mercaptan-a separate experiment at each concentration level for isopropyl mercaptan and for tert-butyl mercaptan-and the optical density of the solutions were measured. The results are tabulated in Table IV. The average increase in optical density of isopropyl mercaptan solutions over those of the same concentration of tert-butyl mercaptan was 11%. This is well within experimental error when readings are taken visually by color comparisons. Moreover, since commercial mercaptan odorants contain a majority of tert-butyl mercaptan (generally 75% or above) this difference is negligible. Where accurate spectrophotometric determinations are made for calibration purposes this can easily be included in a calibration factor. In routine application of the test, the NEM test gives directly the odorant concentration in ppm for mercaptan blend odorants. The long term stability of the NEM solutions is another important consideration. The experience in running the
Table IV. Relative Reactivity of Isopropyl Mercaptan and tert-Butyl Mercaptan pmol of mercaptan /ml
O.D. (isopropyl mercaptan )
O.D. (tert-butyl mercaptan)
0.06 0.12
0.280 0.465 0.710 0.90 1.02
0.255 0.380 0.630 0.80 0.96
0.18 0.24 0.30
various tests throughout this study indicated that NEM solutions in 2-propanol are stable, provided the water is removed quantitatively, for at least 1month. There is no reason to expect that these solutions would not last indefinitely. However, in a few instances there was evidence that the solutions may deteriorate if care is not exercised in ensuring that all water is removed and if the solutions are not normally stored in the dark. One inherent advantage of the reagents involved here is that if any reaction does occur in storage, the solution turns orange-yellow on the addition of the mercaptan giving an automatic warning that the test is invalid. In practice, in determining odorant levels the color developed is compared to a standard color chart calibrated in ppm of mercaptan odorant, for one particular set of convenient operating conditions under which the test result is consistent and accurate. LJnder these conditions the precision of the reading is appreciably less than that taken with a spectrophotometer, although the latter could be used if there was a particular reason to establish the odorant level within narrow limits. Simple portable spectrophotometers that could be used for this purpose are commercially available. In the description of the NEM test results in Table I it was indicated that in most cases the color formed is "stable for 5 min". This stability refers to measurements taken spectrophotometrically, where quite small changes in optical density can be detected. Therefore, although spectrophotometric measurements with the NEM solutions would have to be taken at the time of the test, if required for spe-
cial purposes, the gradual fading of the color would not affect a visual comparison of the test solution with a standard color chart, at least within the first half-hour or so following the test.
Conclusions It may be concluded that the N-ethylmaleimide test (Verma and Knight, 1974) as described above and using the portable apparatus as shown in Figure 1, can be effectively used in measuring mercaptan odorant levels both under field conditions as well as in the laboratory. The test and apparatus outlined here is simple and requires a minimum of operator skill and if desired can also be used for precise laboratory measurement. The reagent tubes are of disposable type. The total time for a test run is less than 5 min so the method utilizes a minimum amount of time. Finally, the apparatus is economical, specific to mercaptans, and can effectively be utilized by the gas industry. Literature Cited Benesch, R., et ai., Science, 123, 981 (1956). Brockhuysen, J., Anal. Chim. Acta, 19, 542 (1958). Davis Odortester, Code No. 11-850. Davis Instruments. N.J. Ellman, G. L.. Arch. Biochem. Biophys., 74, 443 (1958). Friedman, E., Bull. SOC.Chim. Biol., 31, 506 (1949). Friedman, E.. et al., Brit. J. Pharm., 4, 105 (1949); Biophys. Acta, 9, 61 (1952). Fritz, J. S., Palmer, T. A,, Anal. Chem., 33, 98 (1961). Gregory, J. D.. J. Am. Chem. SOC.,77, 3922 (1955). Karr. C.. Anal. Chem., 528 (1954). Kutzleb, R. E., "Odotron, a Better Way to Measure Gas Odorants," Pipeline Industry, May 1973. McA. Mason, "Investigation of Standard Methods of Analysis for Sulfur in Natural Gas". I.G.T. Project PB-47 (Final Report) for the gas operations research committee, A.G.A., June 1963. Mine Safety Appliances Co. Report No. 454206. Provvedi, F., Freeman, F. M.. Chim. lnd. (Milan), 34, 517 (1952). Wilby, V., "Comparison of Methods of Analysis for Sulfur in Natural Gas," presented at A.G.A. Production Conference, May 1966. Woodward, F. N., Analyst, 74, 179 (1949). Verma, A,, Knight, A. R., "Colorimetric Odorant Level Test," patent pending in U.S. and Canada. 1974.
Received for reuiew May 29, 1975 Accepted September 22, 1975
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15,No. 1, 1976
63
