Microsoft Word - 3debree.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 54, 2016 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Selena Sironi, Laura Capelli Copyright © 2016, AIDIC Servizi S.r.l., ISBN 978-88-95608-45-7; ISSN 2283-9216 Sampling And Stability of Mercaptans: Comparison Between Bags, Canisters and Sorbent Tubes Kurt Haerens*, Pieter Segers,Toon Van Elst OLFASCAN, Industrieweg 114 H, 9032 Gent (Belgium) kurt.haerens@olfascan.com In this study the influence of sampling and storage of samples was studied focusing on a number of mercaptans. First the influence of the sampling material on the composition of the sampled gas was studied and secondly the stability of the mercaptans in the different sampling materials is followed over time. Stainless steel, Teflon and deactivated stainless steel were tested as inlet material for a Nalofane bag. These were compared with sampling using both stainless steel or Silonite treated canisters. As a last sampling strategy, active sampling on sorbent tubes was considered. The tests indicate the composition during sampling can be changed in presence of untreated stainless steel as this gives rise to the formation of disulfides. Furthermore, the stability over time of the mercaptans after sampling is very limited for the sorbent tubes with great loss of certain mercaptans over a very short period of time (>2 hours). Also in bags the stability is rather limited and the best stability can be obtained using canisters. So for sampling of mercaptans, Silonite or equivalent canisters are the most suitable way to transport the sample to the laboratory. In the lab, the samples should be analysed as quick as possible after sorbing them on tube or by an appropriate direct sampling system. 1. Introduction The sampling of odour emissions and the stability of odour in different sampling bags has already been studied (e.g. van Harreveld, 2003; Zarra et al., 2012; McGinley et al., 2012) based on olfactometry. Also by chemical analysis different sampling materials, Nalophane, Tedlar and sorbent tubes, have been compared regarding the stability of different odorous compounds in the ppb level. The results indicate only minor differences between the studied bags and a better stability of the evaluated compounds on the sorbent tubes (Boeker et al., 2014). Kim et al. (2012) studied the storage stability of different odorous volatile organic compounds in two types of bag materials indicating some losses were found already after 3 days. During waste handling, sewage collection and wastewater treatment often sulfur compounds are present in air. These sulfur compounds in air, even in low concentrations, give rise to unpleasant smells. In the surrounding of wastewater, waste management and agricultural practices, volatile organic sulfur compounds are an important component of the odorous emissions leading to annoyance to local populations. The analysis and sampling of these compounds can be troublesome. The sampling step is crucial for the quality of the results for all analysis and the knowledge of the quality of sampling is thus very important to obtain usefull results (Guillot, 2012). Watson (2016) listed the advantages and disadvantages of tube sampling and canister sampling, indicating that they can offer complementary technology for sampling. So these sampling strategies will be compared in this study with the more common bags sampling used in odour sampling. Within the sulfur compounds, mercaptans have very low odour threshold levels (see Table 1). Mercaptans tend to absorb onto the surface and may undergo partial oxidation. Some of these were already studied by Le et al. (2013) to find the influence of temperature on the stability of the volatile sulfur compounds in sampling bags. Previously Sulyok et al. (2001) compared the stability of some volatile sulfur compounds in two type of bag materials and a Silcosteel canister. Until now, the stability of volatile sulfur compounds was not investigated on sorbent tubes. In this study the stability of mercaptans will be studied in bags, canisters and sorbent tubes. DOI: 10.3303/CET1654006 Please cite this article as: Haerens K., Segers P., Van Elst T., 2016, Sampling and stability of mercaptans: comparison between bags, canisters and sorbent tubes, Chemical Engineering Transactions, 54, 31-36 DOI: 10.3303/CET1654006 31 Table 1: Odour threshold level (OTV) of mercaptans CAS no Name OTV(1) (µg/m³) 74-93-1 methyl mercaptan 0.1 – 2 75-08-1 ethyl mercaptan 0.03 – 3 107-03-9 1-propyl mercaptan 0.04 - 4 75-33-2 2-propyl mercaptan 0.02 – 1.1 513-44-0 2-methyl-1-propyl mercaptan 0.03 – 4 75-66-1 2-methyl-2-propyl mercaptan 0.03 – 1.3 109-79-5 1-butyl mercaptan 0.01 – 5 513-53-1 2-butyl mercaptan 0.1 – 0.7 (1) Devos et al. 1990 and Nagata et al. 2003 2. Materials and methods 2.1 Standard and sampling materials A gas standard of 10 ppm of each of the mercaptans (methyl, ethyl, 1-propyl, 2-propyl, 2-methyl-1-propyl, 2- methyl-2-propyl, 1-butyl and 2-butyl mercaptan) (Air Products) was used as starting concentration. Dilutions were made using a CMK 5 calibration system model (MCZ Umwelttechnik). Sample bags were made from NalofaneTM and the bag inlet either of Teflon, stainless steel or deactivated stainless steel. Deactivation of the stainless steel inlet was done with 15% BSTFA (N,O- Bis(trimethylsilyl)trifluoroacetamide) in hexane (NCASI, 2007). For the connection of the bag to the sorbent tube either a stainless steel or Siltek treated steel union (Swagelok) was used. Two types of canisters were used, stainless steel canisters (Analytical Industries Inc) and Silonite canisters (Entech Instruments Inc). 2.2 Analysis SulfiCarb adsorption tubes (Markes Ltd) were used for adsorption of the gases. Analysis of the mercaptans was done by TD-GC-MS (Markes TD100, Shimadzu GC 2010-plus and Shimadzu MS GP2010 SE). The thermal desorption program was modified to reduce possible reaction of mercaptans to disulfides based on TDTS 32 (Markes, 2012). In the Markes TD100 all parts are inert (sulfinert parts) to avoid formation of disulfides, the trap is filled with the same sorbent as the tubes (SulfiCarb). The temperature of the valve, cold trap heat and interface were set at a lower temperature (120°C), this results in a better respons for the mercaptans and a lower concentration of disulfides. No disulfides were observed during the experiments which can originate from the thermal desorption process. 2.3 Performed tests Initially the difference between treated and untreated steel was tested both for the use of bag inlet and union. Also the use of Teflon inlet was compared with the other inlets. Stability over time was tested for all bag types during 48 h. For the canisters, stability was evaluated up to 16 days. Also the adsorption tubes stability was first evaluated over 16 days and then also on a shorter time period. Stability was tested for different concentrations (10 ppm, 1 ppm and 50 ppb). 3. Results and discussion 3.1 Use of stainless steel, deactivated/Siltek steel or Teflon as inlet or union It can be clearly seen in Figure 1 and Figure 2 that the concentration of mercaptans are reduced (and the concentration of disulfides is increased) due to the presence of stainless steel in the inlet. This shows the mercaptans are oxidized to disulfides in the presence of stainless steel which can be explained by the heavy metal oxides at the surface of the stainless steel (Oae, 1991) . One-electron oxidants such as Fe(III) and Mn(III) which are present in the surface of the stainless steel are known to catalyse the auto oxidation of thiols (Sheldon et al., 1981). The results of the Teflon inlet were similar to the results of the deactivated steel inlet. So for sampling mercaptan containing gases, the use of stainless steel in contact with the sample should be avoided. Here the reduction of mercaptans is measured chemically indicating reduction will take place and thus sampling and analysis of these gases should be performed without contact with stainless steel. The results of the stainless steel union and the Siltek treated union also showed a reduction of the mercaptans when using the stainless steel union while almost no disulfides were detected when the Siltek treated union was used. 32 Figure 1: Chromatogram of a deactivated metal inlet (pink/light grey) and a normal metal inlet (black) for ethyl mercaptan (RT 4.85), 2-propyl mercaptan (RT 5.68), 2-methyl-2-propyl mercaptan (RT 6.47) and 1-propyl mercaptan (RT 6.98) Figure 2: Chromatogram of a deactivated metal inlet (pink) and a normal metal inlet (black) showing some disulfide peaks 3.2 Stability of mercaptans in different sampling bag inlets The results for a deactivated inlet are shown in Table 2. Table 2: Recovery (%) of mercaptans during 24 hours in a Nalofane sampling bag for 10 ppm 1h 2h 3h 4h 5h 8h 24h Methyl mercaptan 93 78 80 59 49 51 48 Ethyl mercaptan 100 98 100 84 85 91 98 2-propyl mercaptan 101 101 105 92 95 101 105 2-methyl-2-propyl mercaptan 101 97 105 89 91 99 102 1-propyl mercaptan 100 102 104 92 96 100 107 2-butyl mercaptan 101 102 105 95 98 103 105 2-methyl-1-propyl mercaptan 102 101 105 95 98 103 107 1-butyl mercaptan 101 100 103 92 96 100 100 The concentration of methyl mercaptan decreases after 4 hours, all other compounds are more or less stable within 24 hours. These results are in line with those found by Le et al. (2013) where after 24 hours the of methyl mercaptan only showed a recovery of around 75% at 30°C. Also Sulyok et al. (2001) found a decrease of this compound already after a short period of time after storage in a bag (< 24 h). The results of the other concentrations and the other inlets showed similar results, indicating the use of stainless steel inlets doesn’t influence the stability over time. So the disulfides are only formed during sampling and by passing through the inlet, during storage no changes can be seen between the different inlets. 4.75 5.00 5.25 5.50 5.75 6.00 6.25 6.50 6.75 7.00 7.25 2.5 5.0 (x100,000) 28.0 28.5 29.0 29.5 30.0 30.5 31.0 31.5 32.0 32.5 1.0 1.5 2.0 2.5 3.0 3.5 (x100,000) 33 3.3 Stability of mercaptans in different canisters The results of the stability of the Silonite canister can be found in Table 3. The concentrations of methyl mercaptan are not reported as the time between sampling and analysis was not constant and leaded to unstable measurements for this compound. Table 3: Recovery (%) of mercaptans during 16 days in a Silonite canister for 1 ppm Compound/Time 1 2 4 8 16 Ethyl mercaptan 79 92 85 100 71 2-propyl mercaptan 98 93 96 101 92 2-methyl-2-propyl mercaptan 105 100 104 105 107 1-propyl mercaptan 92 91 92 100 86 2-butyl mercaptan 99 96 100 103 94 2-methyl-1-propyl mercaptan 97 96 99 102 97 1-butyl mercaptan 97 95 95 101 91 Even after 16 days the recoveries are still quiet high, only for ethyl mercaptan a substantial decrease is seen in the results. But this could possibly be explained by the difference in time between sampling and analysis (as this was not constant during the testing period). Also a comparable Silcosteel canister showed good recoveries for ethyl mercaptan (Sulyok et al., 2001). The stainless steel canister was only tested for 6 days but shows similar results, so for the canisters the contact with stainless steel does not result in a reduction of the mercaptans over time (see Table 4). Table 4: Recovery (%) of mercaptans during 16 days in a stainless steel canister for 1 ppm Compound/Time 1 2 3 6 Ethyl mercaptan 90 97 101 95 2-propyl mercaptan 86 86 106 94 2-methyl-2-propyl mercaptan 93 91 113 97 1-propyl mercaptan 82 87 104 94 2-butyl mercaptan 89 86 103 96 2-methyl-1-propyl mercaptan 88 86 102 92 1-butyl mercaptanl 86 86 95 89 So during storage the concentration is not effected but the concentration found in the Silonite canister at the start was 1.02 ppm while the concentration in the stainless steel canister was only 0.85 ppm 3.4 Stability of mercaptans on adsorbent tubes Except from SulfiCarb adsorption tubes, also some other type of tubes (Carbopack, Carbograph) were used. But as these tubes were not deactivated or Silco/Silonite treated, large peaks of disulfides could be found in the resulting analyses. These lead to wrong results for the mercaptans and thus these tubes were not further investigated. Two concentrations of mercaptans (1 ppm and 50 ppb) were loaded on all tubes at the same time. Tubes were stored at room temperature and analysed at different time intervals (0, 1, 2, 4, 8 and 16 days). The recoveries are shown in Table 5. It is clear concentration of methyl and ethyl mercaptan has decreased a lot even after 1 day. Also for 1-propyl and 2-propyl mercaptan show a strong decrease (>10 %) already after 2 days. Based on this test, also the stability over a shorter period (30 h) was checked, the results are shown in Table 6. During the test, the samples were stored in the TD100 before analysis. 34 Table 5: Recovery (%) of mercaptans during 16 days on a tube for 1 ppm Compound/Days 1 2 4 8 16 Methyl mercaptan 40 31 30 28 11 Ethyl mercaptan 48 18 9 3 3 2-propyl mercaptan 90 81 66 35 11 2-methyl-2-propyl mercaptan 95 90 89 66 48 1-propyl mercaptan 88 76 77 53 19 2-butyl mercaptan 98 102 109 100 75 2-methyl-1-propyl mercaptan 95 100 104 97 73 1-butyl mercaptan 93 94 98 89 62 Table 6: Recovery (%) of mercaptans during 28 h on a tube for 1 ppm Compound/Time 0u30 1u 1u30 2u 3u 4u 6u30 9u 11u15 13u30 18u 22u45 27u45 Methyl mercaptan 99 120 78 59 39 34 34 26 32 22 18 19 18 Ethyl mercaptan 101 103 100 92 72 63 58 40 41 27 17 14 11 2-propyl mercaptan 106 104 108 106 109 102 101 99 90 89 78 67 62 2-methyl-2-propyl mercaptan 109 111 115 114 120 111 115 114 106 109 101 91 96 1-propyl mercaptan 106 106 106 105 102 97 97 86 86 78 64 61 46 2-butyl mercaptan 105 105 108 107 112 107 108 107 103 107 107 105 101 2-methyl-1-propyl mercaptan 107 105 105 108 114 105 108 107 104 108 108 104 99 1-butyl mercaptan 105 105 105 107 109 104 104 100 100 97 95 93 82 For methyl mercaptan the decrease of mercaptans starts already after one and a half hour. The decrease after 24 hours is even bigger in this stability test compared to the test over 1 day, this can probably be attributed to the storage of the tubes before analysis. For the last test, the tubes were stored inside the TD100 while for the other test the tubes were stored in the lab at room temperature. Storage on sorbent tubes is thus not possible when mercaptans are of interest in the analyses, in that case the use of Silonite or equivalent canisters is recommended. Indicating as already stated by Watson et al. that canisters and sorbent tubes provide complementary sampling technology. 4. Conclusion When sampling odorous gases which contain mercaptans, the contact of the sample with stainless steel should be avoided to minimize the reduction of mercaptans to disulfides. This reduction may lead to different odour concentrations as the odour threshold levels of mercaptans differ from disulfides. During storage no influence of the inlet material or the type of canister is seen on the concentration of the mercaptans due to contact with stainless steel, indicating the reaction takes place immediately and the active sites on the metal get deactivated ones the mercaptans have passed. The stability of mercaptans on sorbent tubes is very short (< 2 h), so samples should be analysed immediately after adsorption on tube or should be adsorbed directly onto the trap out of the canister. For appropriate sampling and storage a Silonite or equivalent canister is thus recommended to avoid oxidation of mercaptans during sampling and loss of mercaptans during storage. References Boeker P., Leppert J., Shulze Lammers P., 2014, Comparison of odorant losses at the ppb-level from sampling bags of NalophaneTM and TedlarTM and from adsorption tubes, Chemical engineering transactions, 40, 157-162 Devos M., Patte F., Rouault J., Laffort P., Gemert L. 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