EVALUATION OF THE NEW LOW-SULFUR ODORANTS

Transcription

1 23 rd World Gas Conference, Amsterdam 26 EVALUATION OF THE NEW LOW-SULFUR ODORANTS Main author A. Seo Japan

2 ABSTRACT An evaluation of a low sulfur odorant is stated in this paper. In recent Japan, fewer sulfur content in city gas is desired to meet the needs for achieving the environmentally oriented gas. Sulfur is mainly contained in the odorant of Japanese natural gas based city gas, so that low- or non-sulfur odorants are await to be developed. In evaluating the new odorizing materials, it is necessary to confirm that the security level of gas delivery must be maintained as high as the present one when we change the odorants. In this study, the mixture of tertiary butyl mercaptan (TBM) and cyclohexene (CH) was selected as the substitute odorant, and we evaluated its odorizing potential by olfactory tests and soil permeation experiments and compared it with the current TBM+DMS (Dimethyl sulfide) odorant. We introduced an idea of Emergency Call Ratio (ECR) as a security level index for the first attempt to comprehend our consumers realistic behavior against risks. The olfactory investigations showed that the new low sulfur odorant substitute TBM+CH had as much ECR as the current odorant TBM+DMS. Soil tube tests showed that the TBM+CH had an enough soil permeability as well as TBM+DMS, which resulted in the high potential of TBM+CH as a next generation odorant for city gas.

3 TABLE OF CONTENTS 1. Introduction 2. Experimental 2.1 Smell tests and security level index 2.2 Soil permeability test using small columns 2.3 Gas dispersion tests in soil 3. Results and Discussions 3.1 Smell tests 3.2 Soil permeability test with small columns 3.3 Gas dispersion tests in soil 4. Conclusions References List of Tables List of Figures

4 Paper 1. INTRODUCTION It is essentially important to deliver high quality city gas at a high security level. For security reasons, odorization to the city gas have been acknowledged as a most practical and effective way to prevent accidents in case of gas leaks, and many gas companies have used sulfur compounds as odorants. In recent Japan, fewer sulfur content in city gas is desired to meet the demand of environmentally oriented fuel supply. Japanese city gas, mainly made from LNG, is characterized as a low sulfur fuel and considered to be more environmentally friendly energy by comparison with other fossil fuels. Nevertheless, in most cases, sulfur odorants are added prior to the gas delivery just in case of gas leak to be detected, new low-sulfur or non-sulfur odorants have to be developed to satisfy the two needs at the same time, keeping the security level and environment-conscious product manufacturing. The present odorant used in Tokyo Gas is a blend of TBM (Tertiary Butyl Mercaptan) and DMS (Dimethyl Sulfide). We consider it very important that the next generation odorants have a similar odor as the present one to maintain the customers safe usage of city gas without any concern. The odor of the present odorants is derived mostly from TBM, and its characteristic smell has been widely recognized as a typical gas odor. Therefore, our basic concept for new low sulfur odorants is to keep using TBM as a main smell source and to find alternative non-sulfur substances instead of DMS to decrease sulfur content. Many substitutional substances have been evaluated as new odorants, in which measurements and comparative assessments have been done specifically about their characteristics such as odor, odor strength, water solubility, chemical stability, soil permeability and so on. Soil permeability is one of the most important factors in Japan, considering its urban aspects with congestion of residences and underground shopping complexes. It is necessary to make a thorough study of the new odorants to ensure that the security level could be maintained almost equally as the present odorants, however, the comprehensive index of security has not been established yet. In this study, we chose a blend of TBM and Cyclohexene (CH) as a new substitutional low sulfur odorant and examine its odor quality and soil permeability which we regarded two of the most important parameters for odorization. In odor evaluation tests, we introduce an idea of Emergency Call Ratio (ECR) as a security level index, and attempted a comprehensive evaluation by comparison with conventional odorants. 2. EXPERIMENTAL 2.1 Smell tests and security level index Smell tests were conducted with sachets and questionnaire method. The experimental conditions

5 are shown in Table 1. The sachets were at first filled with deodorized air, and a given amounts of odorants were precisely added. The odorant concentration in sachets to be examined was determined as follows. The odorant addition to the original gas was set to be the same as our current commercial gas, and sachet concentrations were made to be the following two conditions; first one was the gas concentration of 4% in air (Lower Flammable Limit), and the second one was further diluted to the concentration of.1% in air. The latter condition was based on the Japanese legislation, which regulates that a combustible gas must be odorized so that at a concentration in air of one-thousandth, the gas is readily detectable by a person with a normal sense of smell. The addition ratio of odorants in city gas were TBM:DMS=1:1, TBM:CH=1:2 by weight, respectively. The questionnaire was consisted of some questions asking the odor strength and the odor similarity to the typical gas odor. In the meanwhile, for the odorants to function as a leak detector to secure the gas utilization, it is necessary that not only the customer can notice the smell but also they realize their dangerous situations, and finally they take concrete actions to make emergency calls to their gas company, fire station or police station. For that purpose, we attached a unique Yes/No question asking Do you make an emergency call to your gas company when you experience this smell?, to know the tendencies of the realistic behavior of our consumers. We defined the Positive rate to this question as an Emergency Call Ratio (ECR). In this study, we investigated the ECR trends of each odorant and made a first attempt to use ECR as a security level index. 2.2 Soil permeability test using small columns Packed column flowing method was used. The experimental setup was shown in Figure 1. Three soils were used, two of which were sands chosen as a typical soil covering the underground pipeline, the other was Toyoura standard sand. The soil Table 1 Olfactory test conditions Method Place Posture of subjects Sachets and Questionnaire Meeting Room Sitting # of test subjects Male Female Total Total s s s s s Sample Soil Column TBM/DMS/CH 4 CH/CH 4 P IN Sample Gas cylinders Flow Controller OUT by-pass Figure 1 Soil column experimental setup Table 2 Properties of test soils Sand #1 Sand #2 FM Toyoura standard sand Bulk density (g/cm 3 ) Water content (%) Size distribution Stone ( >75mm)(wt%) Gravel (2~75mm) (wt%) Sand (.75~2mm) (wt%) Shilt (.5~.75mm)(wt%) Clay (<.5mm) (wt%) 3.2 vent

6 properties were shown in Table 2. The column was 22.3mm i.d., 2cm in height. Odorized methane flowed at a constant flow rate in the column. The time history of the methane and odorant concentrations in the effluent gas was measured by gas chromatography. Odorants used were CH, TBM and DMS. The experimental conditions were shown in Table 3. The time profiles of methane and odorant concentrations were integrated, and then the adsorption of each odorant was measured by subtracting the methane breakthrough delay. Table 3 Soil permeability test conditions Temperature ( ) 2 Soil column inner diameter (mm) 22.3 length (mm) 2 material acrylic Soil Sand #1 Sand #2 Toyoura Gas flow rate (cm 3 /min) Odorant concentration 6.~14 TBM (mg/nm 3 ) 1.5~1 DMS (mg/nm 3 ) 2.~1 CH (mg/nm 3 ) Gas analysis 2.5~32 CH 4 TCD GC TBM,DMS FPD GC CH FID GC 2.3 Gas dispersion tests in soil The three dimensional dispersion tests in soil were carried out by using three odorants. The experimental apparatus was shown in Figure 2. The soil column of 3mm i.d. with a height of 9mm was filled with the sand #2. CNG (containing TBM and DMS) and CH- premixed methane flowed out at the rate of 2cm 3 /min from the porous gas leak point installed on the central axis at the height of 25mm. There installed 32 sampling tubes of 2mm in diameter made of Teflon, which were inserted from outside of the side wall. We obtained the gas samples from each point by gas syringes with the time intervals of 9 minutes, and measured the gas concentration time profiles of methane and odorants. The experimental conditions were shown in Table RESULTS AND DISCUSSIONS 3.1 Smell tests The smell test results were shown in Table 5. (mm) ECR was more than 7% in both cases of TBM+DMS and TBM+CH at the city gas concentration of 4%. It means that in this condition, TBM+CH has an enough warning odor and it resulted in the fact that ECR of TBM+CH was as much as that of TBM+DMS. -1 Sampling points (mm) CNG CH/CH 4 Gas cylinders 65mm 25mm Sample Flow Controller Soil Column Sample by-pass In case of the gas concentration of.1%, ECR of TBM+CH and TBM+DMS were 44% and 35%, P FM Figure 2 Gas dispersion experimental setup Table 4 Gas dispersion test condition vent Temperature ( ) 2 Soil column inner diameter (mm) 3 height (mm) 11 material vinyl chloride resin Soil Sand #2 Gas flow rate (cm 3 /min) 2 Odorant concentration TBM (mg/nm 3 ) 3.1 DMS (mg/nm 3 ) 4.3 CH (mg/nm 3 ) 41 Gas analysis CH 4 TCD GC TBM,DMS FPD GC CH FID GC

7 respectively. ECR of TBM+CH was slightly less than that of TBM+DMS, however, the difference was rather small within the error caused by the Table 5 Emergency Call Rate (ECR) in olfactory tests City gas 4.% City gas.1% TBM+DMS TBM+CH TBM+DMS TBM+CH Control 75% 75% 44% 35% w/o TBM 17% 5.8% 1.9%.% subjects variation of answers and can be recognized almost equal to both odorants. Another test was conducted without TBM, in which the test subjects experienced the smell of only CH or only DMS for comparison. Both ECRs of CH and DMS were less than 2%, which means that the existence of TBM is necessary for the readily detection of gas odor for experienced consumers. These results showed that TBM+CH can maintain the ECR as high as the conventional TBM+DMS, and the new odorant has an enough potential to warn the gas consumers by its characteristic odor. 3.2 Soil permeability test with small columns The soil permeability test was examined with sand #1. One of the time profiles of each odorant breakthrough was shown in Figure 3. The substances reached in sequence of methane, CH, TBM and DMS. CH reached just after the methane and much faster than DMS, which shows higher soil permeability of CH than DMS. This results were correspondent with earlier studies (1)(2). The soil adsorptions of each odorant were Effluent concentration/inlet concentration Time (min) CH4 CH TBM DMS Figure 3 Experimental data of a soil permeability test ( Soil: Sand #1,Inlet conc: : TBM 2.1mg/m3, DMS 2.8mg/m 3, CH 1mg/m 3 ) 2.5E-5 measured from the results of soil permeability tests with various gas flow rates and odorant concentrations. The breakthrough was defined as the arrival of 8% of inlet concentration at the column exit, and the adsorption ω 8 were Adsorption (mg/g-wet_soil) 2.E-5 1.5E-5 1.E-5 5.E-6 DMS TBM CH calculated by the time integration of measured concentrations until the breakthrough. The adsorption isotherm derived fromω 8 was shown in.e Inlet odorant concentration (mg/m 3 ) Figure 4 Adsorption isotherm by sand #1 Figure 4. The adsorptions per unit weight of wet soil were almost proportional to the inlet odorant concentrations, and the adsorption coefficient β 8 can be defined to each odorant as a Henry type adsorption due to the very small concentrations of each adsorbate. Table 6 Adsorption coefficient β 8 (m 3 /g wet_soil) Sand #1 Sand #2 TBM 3.8E-7 3.5E-7 DMS 2.E-6 1.1E-6 CH 1.3E-7 7.2E-8 The adsorption coefficients β 8 of Sand #1 and #2 were shown in Table 6. All the coefficients of Sand #1 were larger than those of Sand #2. The reason can be considered that Sand #1 contained much silty clay and had a larger specific surface area as shown in Table 2. Then, we investigated the effect of water content by using the moisture controlled samples of Sand #1. The adsorptionω 8 was measured with various moisture conditions from 1% to 2% by weight, at a

8 constant odorant concentration of TBM:1.7, DMS:3.1, CH:7.6mg/Nm 3, respectively. The results were shown in Table 7. ω 8 of TBM slightly increased and those of DMS and CH decreased as the soil had more water content, but the differences were so subtle and the tendencies were not so clear. Perfectly dried condition is rather a very rare case for the actual soils covering the underground pipeline, and in most cases the soils are wet even with water content ranges. Therefore, from the measured results, we regarded the water content slightly affected on odorant adsorption under practical soil situations. In case of the perfectly dried soil, we measured the adsorption by using Toyoura standard sand on both dried and moist conditions. The water contents were % and 1%, respectively. The reason why we chose Toyoura standard sand was that there would not a significant change of specific surface area between dry and wet conditions, which is available by its very narrow range of the particle size and not having fine particles less than.75mm in diameter. If dried normal soil was used for an adsorption test sample, the specific surface area is remarkably increased due to the dispersion of small particles consolidated by water, and it is well known that the dried soil would show totally different characteristics from the wet soil. For that reason, the water content cannot be the only parameter when using perfectly dried sample of normal soils. The adsorption coefficients β 8 of Toyoura standard sand were shown in Table 8. The coefficients of TBM and CH of dry soil were about two times more of those of dry soil. The coefficient of DMS notably changed with or without water. The DMS coefficient of dry soil was less than 2% of that of wet condition. This phenomena can be considered that DMS is slightly soluble into water, and the increase of adsorption under wet conditions was due to the DMS absorption into water. These results showed that soil adsorptions of TBM and CH were not significantly affected by water content of the soil. Relatively large adsorption of DMS onto soil was originated by moisture, but the adsorption change was limited within the practical moisture content range. Table 7 Water content effect on adsorption ω 8 (Sand #1) ω 8 (mg/gwet_soil) Water content (wt%) 9.6% 15% 2% TBM 7.4E-7 7.2E-7 7.E-7 DMS 4.5E-6 4.6E-6 4.9E-6 CH 5.E-7 6.E-7 6.3E-7 Table 8 Water content effect on adsorption coefficient β 8 (Toyoura standard soil) β 8 (m 3 /g-wet_soil) Water content (wt%) % 1% TBM 6.1E-7 3.4E-7 DMS 2.5E-7 1.5E-6 CH 1.5E-7 7.8E Gas dispersion tests in soil The three dimensional gas dispersion tests were conducted by using Sand #2 and measured the gas concentration distributions. The results were shown in Figure 5. The color bars in Figure 5 are normalized by inlet concentrations of each substance. The figure shows that CH, TBM and DMS diffused in sequence in the soil following the relatively rapid spread of methane. In the earlier period of leakage (after 15 minutes), there were no horizontal or vertical deviations of the each odorant diffusion, and the odorants made isotopic diffusion. As the time passes, gas flow became gradually affected by the side wall, and finally pseudo one dimensional upstream was formed. The first detected odorant was CH, followed by TBM and DMS. The order was

9 CH 4 H eigh t (c m ) 2 H Heigh t (c m ) 2 Heigh t (c m ) 2 Heigh t (c m ) 2 Heigh t (c m ) Radius (cm) Radiu s (c m ) Radiu s (c m ) C H 4 15m in C H 4 9m in C H 4 18m in C H 4 27m in C H 4 36m in CH H eigh t (c m ) H eigh t (c m m) ) 4 2 H eigh t (c m m) ) 4 2 H eigh t (c m m) ) 4 2 H eigh t (c m ) C H 15m in C H 9m in Radiu s (c m ) C H 18m in Radiu s (c m ) C H 27m in C H 36m in TBM H Height (c m ) 4 2 H Height (c m ) 4 2 H Height (c m ) R R R T B M 15m in T B M 9m in T B M 18m in T B M 27m in T B M 36m in DMS D M S 15m in D M S 9m in D M S 18m in Radius (cm) D M S 27m in Radius (cm) D M S 36m in Figure 5 Experimental results of a gas dispersion test in soil the same as the small soil column experimental results described in the previous section. To comprehend these results properly, we should take it into account that human sense of smell is much higher sensitivity than chemical analyzers, so that its threshold value is as small as the order of one-hundredth of the minimum limit of chemical detection in many cases. It means that in our gas

10 dispersion test, the odorant concentration at the soil surface might have exceeded the threshold value for human noses when the methane reached, even the odorant concentration were below the detection limits by chemical analysis. From all the experimental results of soil adsorption tests, it was shown that CH had a higher soil permeability than DMS in case of sand as a typical soil. We concluded that the new odorant of TBM+CH had an enough soil permeability as well as the conventional odorant blend of TBM+DMS. 4. CONCLUSIONS A new low sulfur odorant was evaluated by smell tests and soil permeability experiments to satisfy the two needs, maintaining the security level and environment-conscious product manufacturing in city gas delivery. These comprehensive investigations showed that the new low sulfur odorant substitute TBM+CH had as much Emergency Call Ratio (ECR) as the conventional odorant TBM+DMS by olfactory tests. Soil tube tests showed that the TBM+CH had enough soil permeability as well as TBM+DMS, which resulted in the high potential of TBM+CH as a next generation blend of odorants. In this study, we introduced an idea of ECR as a security level index for the first attempt. ECR can be answered by every test subject, by Yes/No, very simply and quickly, nevertheless, it can directly reflect the realistic behavior of our consumers in case of emergency, so that we consider this ECR as an appropriate index of security level. The low sulfur odorant examined in this study will be further investigated for other properties such as chemical stability, and profound assessment is necessary in the future work. References (1) Little, A. D. (1978). Development of New Gas Odorants, GRI Report GRI-78/19. (2) Parlman, R. M. and Williams, R.P. (1979). Penetrabilities of Gas Odorant Compounds in Natural Soils, Proc. AGA Distribution Conference Proceedings. (3) Roberts, J. S. (25). An Investigation of Soil Permeability and Leaching of Odorants, Natural Gas & LP Odorization Conference & Exhibition.

11 List of Tables Table 1 Olfactory test conditions Table 2 Properties of test soils Table 3 Soil permeability test conditions Table 4 Gas dispersion test condition Table 5 Emergency Call Rate (ECR) in olfactory tests Table 6 Adsorption coefficient Table 7 Water content effect on adsorption ω8 (Sand #1) Table 8 Water content effect on adsorption coefficient β8 (Toyoura standard soil) List of Figures Figure 1 Soil column experimental setup Figure 2 Gas dispersion experimental setup Figure 3 Experimental data of a soil permeability test Figure 4 Adsorption isotherm by sand #1 Figure 5 Experimental results of a gas dispersion test in soil