Study of the Combined Deactivation Due to Sulfur Poisoning and

Subscriber access provided by UNIVERSITY OF CONNECTICUT

Article

A study of the combined deactivation due to sulfur poisoning and carbon deposition during biogas dry reforming on supported Ni catalyst Vivek Pawar, Srinivas Appari, Dayadeep Monder, and Vinod M. Janardhanan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01662 • Publication Date (Web): 07 Jul 2017 Downloaded from http://pubs.acs.org on July 9, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

A study of the combined deactivation due to sulfur poisoning and carbon deposition during biogas dry reforming on supported Ni catalyst Vivek Pawara , Srinivas Apparib , Dayadeep Monderc , and Vinod M. Janardhanana∗ a

Department of Chemical Engineering, Indian Institute of Technology Hyderabad, Sangareddy, Telangana, 502 285, India b

Department of Chemical Engineering, Birla Institute of Technology and Science, Pilani, Rajasthan 333 031, India

c

Department of Energy Science and Engineering, Indian Institute of Technology Bombay, Powai, Maharashtra, 400 076, India E-mail: *[email protected]

Phone: +91 (0)40 2301 6073. Fax: +91 (0)40 2301 6032

Abstract This paper presents a detailed study of catalyst deactivation as a result of simultaneous sulfur poisoning and coke deposition during biogas dry reforming. Experiments are performed at 700 o C and 800 o C with 5 ppm and 10 ppm H2 S in model biogas with CH4 /CO2 =1.5 and 2.0. To assess the relative effect of chemisorbed sulfur in deactivating the supported Ni catalyst as compared to that of coke deposition, the experiments are performed with and without H2 S in the feed. The catalyst deactivation is found to be faster in the presence of H2 S. The deactivation due to sulfur chemisorption is not reversible at 700 o C, while at 800 o C the catalytic activity

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 21

of Ni starts to recover on removing H2 S from the feed stream. The results show that the exit CO mole fraction goes up for the sulfur poisoned catalyst which suggests that the reverse water gas shift and coke gasification reactions are not suppressed to the same degree as the reforming reaction. The fresh and the spent catalysts are characterized using XRD, BET, EDS and TEM. The characterization of the spent catalyst shows that dry reforming of model biogas, with and without the presence of H2 S, leads to the formation of multi-walled carbon nanotubes for the chosen operating conditions.

Introduction Biogas, produced by the anaerobic digestion of organic matter is a renewable fuel consisting of 50-75% CH4 , 50-25% CO2 , 0-10% N2 and 0-3% H2 S. The CH4 to CO2 ratio in biogas generally fluctuates anywhere between 1.4 and 2.0. 1 Since biogas is rich in CH4 it may be subjected to steam reforming, dry reforming, or partial oxidation to produce synthesis gas (a mixture of CO and H2 ). The steam reforming of model biogas is a relatively well studied topic compared to its dry reforming reaction. 2–7 The dry reforming reaction is particularly attractive since it converts equimolar composition of two potential green house gases (CH4 and CO2 ) according to the reaction

CH4 + CO2 ↔ 2H2 + 2CO

0 (∆H298 = 247 kJ/mol),

(1)

and produces synthesis gas with H2 /CO ratio of 1. Although the reforming reaction can be performed over a number of catalysts, particularly noble metals and noble metals promoted with base metals, Ni is commonly preferred to other metals due to its low cost and excellent catalytic activity. The activation energy for CH4 decomposition on Ni(111) and Ni(100) is respectively 50 kJ/mol and 25 kJ/mol. 8 This reasonably low energy requirement leads to the easy decomposition of CH4 on Ni through H abstraction reactions leaving carbon on the surface. Carbon formation during the dry reforming reaction may also occur due to the Bouduard and CO decomposition reactions. Several studies have been undertaken to formulate catalysts that suppress coke formation.


Page 3 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


There is evidence that the presence of a base metal in catalysts can suppress coke formation during both CH4 decomposition and Bouduard reactions. 9–12 Perovskite materials can also prevent the formation of carbon during dry reforming. For instance Ni doped SrZrO3 13,14 has been shown to resist coke formation significantly for model biogas with CH4 /CO2 =2 and nickel hexaalumnates also show very good catalytic activity for dry reforming when the wt% of Ni is below a threshold value. 15 Although there are enough studies generally on dry reforming of CH4 , the ones that study catalyst deactivation at typical biogas composition (CH4 /CO2 =1.4∼2.0) are rather limited. To the best of our knowledge, there are no studies yet on the catalyst deactivation due to simultaneous sulfur poisoning and carbon deposition during the dry reforming of biogas mixture. This study presents experiments and analysis for dry reforming of biogas with CH4 to CO2 ratios of 1.5 and 2.0 with and without H2 S in the feed. Two different H2 S concentrations; 5 ppm and 10 ppm are considered in this study and the experiments are carried out at 700 o C and 800 o C.

Experiments Set up The experimental set up consists of a quartz tube reactor of ID 8 mm and OD 12 mm placed inside a three zone heating furnace (Applied Test Systems Inc, USA). Both ends of the tube are connected with ultra-torr fittings (Swagelock). Mass flow controllers (0-200 ml min−1 , Bronkhorst) are used to regulate the flow from the gas cylinder to the quartz tube. The gas lines are made of SS 316 1/4 inch pipe. The furnace is equipped with three K type thermocouples to monitor the temperature of the zones, which can be controlled separately. In our experiments all the zones are maintained at the same temperature, i.e., either at 700 o C or 800 o C. The thermocouples are positioned just outside the quartz tubes and the temperature of the catalyst bed is assumed to be same as the zone temperature. We have verified this in previous work where we used a large diameter quartz tube with different end fittings, which allowed us to measure the temperature of 3 ACS Paragon Plus Environment


the bed by positioning thermocouples at the top and bottom of bed. 3 The exit gas is stripped of moisture content by passing it through a moisture trap and analyzed using an online GC (GC 2014, Shimadzu corporation) equipped with a TCD in a carboxane packed column (ID 3.17 mm and length 4.5 m). A schematic representation of the experimental set up is shown in Fig. 1. H2S+N2 Heating tape

CH4

Ultra-torr fitting Zone-I

CO2 N2

T1 T2 Quartz tube

Zone-III

To GC

Zone-II

Catalyst bed

Moisture trap

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

T3 Ultra-torr fitting

Figure 1: Schematic representation of the reactor set up. T 1 , T 2 , and T 3 are thermocouples.

Catalyst preparation 15% Ni loaded on γ-Al2 O3 was used as the catalyst (Ni/γ-Al2 O3 ). The catalyst was prepared by the wet impregnation method. The precursor solution of Ni in water was prepared by dissolving measured quantities of NiNO3 ·6H2 O (Merk 99% purity) in deionized water of volume slightly higher than the pore volume of the γ-Al2 O3 pellets. The γ-Al2 O3 pellets were added to the precursor solution and stirred for 2 h. The pellets were subsequently dried for 12 h at 80 o C and calcined at 800 o C for 6 h. The average size of catalyst particles were ∼1 mm. Before loading the catalyst particles into the reactor, they were reduced in H2 flow (20 ml min−1 ) at 800 o C for 4 h.

Procedure In order to start the experiments the reduced catalyst particles were loaded into the quartz tube. The catalyst particles were diluted with quartz beads for better heat and mass transfer between the


Page 4 of 21

Page 5 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


catalyst particles. The position and length of the bed (5 cm) in the quartz tube and furnace were the same for all experiments. The furnace was heated from room temperature at a rate of 10 o C min−1 to the desired reaction temperature (700 o C or 800 o C) under N2 flow (99.999% Praxair) at 15 ml min−1 . The temperature was monitored using the thermocouples and after reaching the desired temperature the reactor was maintained under stable conditions for 30 min. Pure gases of CH4 (99.999%), CO2 (99.995%), N2 (99.999%) and H2 S (61.3 ppm balance N2 , Chemix) were premixed and introduced to the reactor at required flow rates. The CH4 to CO2 ratio was maintained either at 1.5 or 2.0 with N2 dilution. The total flow rate was maintained at 110 ml min−1 and the CO2 flow was maintained constant at 18 ml min−1 . CH4 flow rate and N2 flow rates were adjusted to maintain the total flow rate and the required CH4 /CO2 ratio. For CH4 /CO2 =1.5 the partial pressures of CH4 and CO2 were 24.5×103 Pa and 16.3×103 Pa respectively and for CH4 /CO2 =2.0 the partial pressures of CH4 and CO2 were 32.7×103 Pa and 16.3×103 Pa respectively. The flow rate of the gas mixture from the H2 S cylinder was adjusted such that the final reaction mixture resulted in 5 ppm or 10 ppm H2 S with respect to the sum of CH4 and CO2 flow rates. The weight hourly space velocity (WHSV) was maintained at 16.2 m3 /kg-h with respect to CH4 and CO2 flow. The gas mixture was preheated to 250 o C using a heating tape before entering the reactor. All the furnace zones were maintained at the desired reaction temperature. The direction of gas flow was from top to bottom and the exit gases from the reactor were passed through a moisture trap before online-GC analysis. The product gases were sampled every 13 minutes without any break during the reaction.

Results and discussion To determine the reduction temperature and the physical properties of the fresh catalyst, temperature programmed reduction (TPR) and H2 pulse chemisorption studies have been carried out using Micrometrics Autochem II-2920 chemisorption analyzer. For performing the TPR study, measured amount of the calcined catalyst is loaded in the ’U’ shaped quartz tube and heated in Ar stream (30 ml/min) at 200 o C for 1 h. The sample is then cooled down to 50 o C and heated at a ramp rate



of 10 o C/min in a stream of 10% H2 in Ar (30 ml/min). The H2 consumption is monitored using a thermal conductivity detector (TCD). Figure 2 indicates that the maximum reduction temperature is 780 o C. The broad peak may be due to the reduction of NiAl2 O4 , which might have formed during the calcination phase. The high reduction temperature indicates a strong interaction between NiO and the γ-Al2 O3 support. The metal surface area, metal dispersion, and average cubic crystallite size are determined using pulse chemisorption. The sample is reduced in 10% H2 in Ar (30 ml/min) and cooled down to 50 o C. The cooled sample is then subjected to several pulses of 10% H2 in Ar until three consecutive similar peaks for H2 are obtained. This analysis indicated a metal dispersion of 1%, metallic surface area of 6.8 m2 g−1 , and cubic crystallite size of 82.5 nm. 0.014 0.012 0.01

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.008 0.006 0.004 0.002 0 -0.002 0

100

200

300

400

500

600

700

800

900

Temperature / o C

Figure 2: TPR profile for 15% Ni/γ-Al2 O3 calcined at 800 o C.

For the combined poisoning study, for both CH4 /CO2 values, the catalyst deactivation on time on stream (without any break) is investigated for 5 ppm and 10 ppm H2 S. For the cases reported here the first GC sample is taken 13 minutes after introducing the reactants, and subsequently analyzed every 13 minutes. Figure 3 shows the CH4 conversion obtained for different operating conditions. It is evident from the figure that the initial CH4 conversions are higher at 800 o C compared to 700 o C, which is an obvious result due to faster kinetics and the endothermic nature of the dry reforming reaction. Experiments are initially performed without any H2 S in the inlet gas. In this set of experiments CH4 /CO2 ratio of 2 lead to faster deactivation compared to CH4 /CO2 ratio of 6 ACS Paragon Plus Environment

Page 6 of 21

Page 7 of 21

1.5 at both 700 and 800 o C. The faster deactivation is due to the rapid carbon formation because of CH4 cracking on Ni, which is higher when the concentration of CH4 is higher. For any given time, the highest CH4 conversion is observed for CH4 /CO2 =1.5 at 800 o C and the lowest CH4 conversion is observed for CH4 /CO2 =2 at 700 o C. Thermodynamics predicts higher surface carbon formation at 700 o C compared to 800 o C, which leads to faster deactivation observed experimentally at 700 o

C, which is shown in Fig. 3 (b). 16 Higher temperature and lower CH4 /CO2 ratio not only leads to

lower carbon formation but also higher dry reforming reaction rates. Therefore, CH4 /CO2 ratio of 1.5 at 800 o C leads to the highest CH4 conversion of all cases. 100 80

CH4 conversion /%

CH4 conversion /%

100

0 ppm 5 ppm 10 ppm

Eqiibrium conversion

60 40 20

(a)

0

0 ppm 5 ppm 10 ppm


80 60 40 20

(b)

0 0

2

4

6

8

10

12

14

0

time /h 0 ppm 5 ppm 10 ppm


80

4

6

8

10

12

H2S removal H2S removal

20

14

16

18

16

18

time /h 0 ppm 5 ppm 10 ppm


60 40

2

100

CH4 conversion /%

100

CH4 conversion /%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

H2S removal

60 H2S removal

40 20

(c)

(d)

0

0 0

2

4

6

8

10

12

14

16

18

0

2

4

time /h

6

8

10

12

14

time /h

Figure 3: CH4 conversions at different operating conditions. (a) 700 o C with CH4 /CO2 =1.5 (b) 700 o C with CH4 /CO2 =2, (c) 800 o C with CH4 /CO2 =1.5, (d) 800 o C with CH4 /CO2 =2.0. Legends correspond to the concentration of H2 S in the inlet gas. The point of removal of H2 S is shown in the figures (c and d) and the recovery starts immediately after removing H2 S.pCO2 =16.3×103 Pa. pCH4 =24.5×103 Pa for CH4 /CO2 = 1.5, and pCH4 =32.7×103 Pa for CH4 /CO2 =2. For the two different CH4 /CO2 ratios and temperature, the experiments are repeated with two different H2 S concentrations (5 ppm and 10 ppm). For all cases, higher H2 S concentration lead to more deactivation, however, the initial conversions are not influenced by the presence of H2 S. Therefore perhaps in the early stages, the deactivation is predominantly due to coking. Except for the 700 o C, CH4 /CO2 =1.5 case (Fig. 3(a)), the initial rate of deactivation is same for 5 ppm and 10 ppm H2 S in the inlet gas. For higher CH4 concentrations (Fig. 3(b) and 3(d)), two distinct regions can be observed for catalyst deactivation when H2 S is introduced – an initial drop followed by a 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

slow poisoning zone. This two stage poisoning has been reported widely in the solid oxide fuel cell research literature for H2 S containing fuels 17–19 used with Ni based fuel electrodes. The actual mechanism for the two stage poisoning still remains ambiguous and the available literature points to the possibility of a surface reconstruction of Ni, which starts after the first stage of poisoning. 17,18 The recovery of the catalyst activity is attempted by removing H2 S from the feed gas for all cases. The catalyst started to recover the activity immediately after removing H2 S from the feed at 800 o

C due to the favorable kinetics of H2 S desorption at higher temperature. 3 Mechanistically the

recovery in the catalytic activity is due to the removal of surface adsorbed sulfur by H2 . At higher temperatures, the dissociatively adsorbed elemental hydrogen reacts with surface adsorbed sulfur forming H2 S, which eventually desorbs back into the gasphase. The elementary step reactions and the activation energy required for the regeneration process can be identified from the reaction mechanism reported by Appari et al. 2 At 700 o C, the catalyst activity did not recover even three hours after removing H2 S from the feed. Figure 3 also shows the predicted equilibrium CH4 conversion. The experimental results presented here are not in steady state and the equilibrium conversions are presented just to show that the conversions are well within the equilibrium limit for all reported measurements. The CO2 conversion for all the cases studied is shown in Fig. 4. The trends are similar to that of CH4 conversion reported in Fig. 3, however, the conversions are higher for CO2 . Stoichiometrically one mole of CO2 is required to convert one mole of CH4 , and since the CH4 /CO2 ratio employed in this study is higher than one, CO2 becomes the limiting reactant and therefore, the conversions are higher than that of CH4 . The initial conversions are almost 100% at 800 o C and the highest CO2 conversion in the absence of H2 S in the feed gas is observed at 800 o C with CH4 /CO2 =2. The lowest conversion for CO2 is at 700 o C with CH4 /CO2 =1.5. These observations are in accordance with the stoichiometric requirements.


Page 8 of 21

CO2 conversion /%

100 80

100

0 ppm 5 ppm 10 ppm


60 40 20

(a)

0

0 ppm 5 ppm 10 ppm


80 60 40 20

(b)

0 0

2

4

6

8

10

12

14

0

2

4

6

time /h

10

12

100

0 ppm 5 ppm 10 ppm

14

16

18

16

18

0 ppm 5 ppm 10 ppm


CO2 conversion /%


80

H2S removal

60 H2S removal

40 20

8

time /h

100

CO2 conversion /%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CO2 conversion /%

Page 9 of 21

(c)

0

80 60

H2S removal H2S removal

40 20

(d)

0 0

2

4

6

8

10

12

14

16

18

0

2

4

time /h

6

8

10

12

14

time /h

Figure 4: CO2 conversions at different operating conditions. (a) 700 o C with CH4 /CO2 =1.5, (b) 700 o C with CH4 /CO2 =2, (c) 800 o C with CH4 /CO2 =1.5, (d) 800 o C with CH4 /CO2 =2.0. Legends correspond to the concentration of H2 S in the inlet gas. The point of removal of H2 S is shown in the figures (c and d) and the recovery starts immediately after removing H2 S. For CH4 /CO2 = 1.5, pCH4 =24.5×103 Pa, pCO2 =16.3×103 Pa and for CH4 /CO2 =2, pCH4 =32.7×103 Pa, pCO2 =16.3×103 Pa.

The appreciably high conversion of CO2 relative to CH4 could be due to the occurrence of carbon gasification reaction according to

C(s) + CO2 ↔ 2CO,

(2)

and reverse water gas shift reaction (RWGS) according to reaction Eq. 3. A sulfur poisoned surface promotes RWGS reaction and this is explained in the later discussions. Except for the high conversion of CO2 , compared to CH4 , the deactivation trends are very much similar to that given in Fig. 3. There are two competing mechanisms of catalyst deactivation here. One due to poisoning and the other due to fouling. The poisoning happens because of sulfur adsorption and fouling occurs due to coke deposition from the decomposition of CH4 . Carbon deposition also occurs due to the CO disproportionation reaction. 12 The chemical structures of carbon and coke formed depend on the reaction conditions and catalyst type. The amorphous forms of carbon deposited at low temperature are reactive and they convert to less reactive graphitic forms at high temperature. 12



0.35

0.25 0.2 0.15 0.1 0.05

0 ppm 5 ppm 10 ppm

Eqiibrium mole fraction

0.35

H2 mole fraction

H2 mole fraction

0.4

0 ppm 5 ppm 10 ppm


0.3

0.3 0.25 0.2 0.15 0.1 0.05

(a)

0

(b)

0 0

2

4

6

8

10

12

14

0

2

4

6

time /h

H2 mole fraction

0.4

0.25 0.2 0.15 0.1 0.05

10

0.45

0 ppm 5 ppm 10 ppm


0.3

8

12

14

time /h

0.35

H2 mole fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 21

(c)

0 ppm 5 ppm 10 ppm


0.35 0.3 0.25 0.2 0.15 0.1 (d)

0.05

0

0 0

2

4

6

8

10

12

14

16

18

0

2

4

time /h

6

8

10

12

14

16

18

time /h

Figure 5: Mole fraction of H2 at the reactor exit for different operating conditions. (a) 700 o C with CH4 /CO2 =1.5, (b) 700 o C with CH4 /CO2 =2, (c) 800 o C with CH4 /CO2 =1.5, (d) 800 o C with CH4 /CO2 =2.0. Legends correspond to the concentration of H2 S in the inlet gas. Figure 5 shows the exit mole fraction of H2 for the different cases studied. The H2 exit mole fractions follow the same trend as that of CH4 conversions, indicating that the main source of H2 production is H abstraction reactions from CH4 on the catalyst surface. In the mid temperature ranges of 500 to 700 o C, RWGS may also occur 13,14

H2 + CO2 ↔ CO + H2 O,

∆H298 = 41.15 kJ/mol.

(3)

However, the occurrence of RWGS reaction during dry reforming is not universal and depends on the operating conditions. An increase in reaction temperature and higher CH4 /CO2 ratio improves the H2 /CO ratio. 20 Also, less water is detected in the outlet stream with increasing CH4 /CO2 20

and some studies have reported higher than equilibrium conversion. 20 Figure 5 also shows that

for the same stoichiometric conditions, higher temperature leads to higher H2 concentration at the reactor exit. The corresponding exit CO mole fractions for different cases are shown in Fig. 6. The CO mole fractions after 12 h is still above the equilibrium predicted values. However, from the trend its evident that once in steady state the exit mole fractions will be within the equilibrium limits. 10 ACS Paragon Plus Environment

Page 11 of 21

0.25

0 ppm 5 ppm 10 ppm

0.2

CO mole fraction

CO mole fraction

0.25


0.15 0.1 0.05

(a)

0 2

4

6

8

10

12

0.15


0.1 0.05

14

(b)

0

time /h

0.25


0.2

0 ppm 5 ppm 10 ppm

0.15 0.1 0.05

2

4

6

(c)

0

8

10

12

14

time /h

0.25

CO mole fraction

CO mole fraction

0 ppm 5 ppm 10 ppm

0.2

0 0

0 ppm 5 ppm 10 ppm

0.2


0.15 0.1 0.05

(d)

0 0

2

4

6

8

10

12

14

16

18

0

2

4

6

8

time /h

10

12

14

16

18

time /h

Figure 6: Mole fraction of CO at the reactor exit for different operating conditions. (a) 700 o C with CH4 /CO2 =1.5, (b) 700 o C with CH4 /CO2 =2, (c) 800 o C with CH4 /CO2 =1.5, (d) 800 o C with CH4 /CO2 =2.0. Legends correspond to the concentration of H2 S in the inlet gas. 1.6

2

0 ppm 5 ppm 10 ppm

0 ppm 5 ppm 10 ppm

1.8

1.4

H2/CO

H2/CO

1.6 1.2

1.4 1.2

1 1

(a)

(b)

0.8

0.8 0

2

4

6

8

10

12

0

14

2

4

6

8

10

12

14

time /h

time /h 1.6

2.2

0 ppm 5 ppm 10 ppm

1.4

0 ppm 5 ppm 10 ppm

2 1.8

1.2

H2/CO

H2/CO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

1.6 1.4 1.2 1

0.8

(c)

(d)

0.8

0.6

0.6 0

2

4

6

8

10

12

14

16

18

0

2

4

time /h

6

8

10

12

14

16

18

time /h

Figure 7: H2 /CO mole fraction at the reactor exit for different operating conditions. (a) 700 o C with CH4 /CO2 =1.5, (b) 700 o C with CH4 /CO2 =2, (c) 800 o C with CH4 /CO2 =1.5, (d) 800 o C with CH4 /CO2 =2.0. Legends correspond to the concentration of H2 S in the inlet gas. The H2 /CO ratio at the reactor exit for different conditions studied is shown in Fig. 7. The H2 /CO ratio during the early stages of poisoning is higher then one, indicating the occurrence of water gas shift reaction. However, since we used moisture trap at the reactor exit before sending the product gas to GC, we have not detected any H2 O. When there is no H2 S present in the inlet stream, the H2 /CO ratio is always greater than one. When H2 S is present in the feed, the H2 /CO ra11 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 21

tio gradually goes below one due to the higher CO concentration in the product mixture compared to H2 . The higher CO concentration in the exit gas mixture is due to carbon gasification reaction (Eq. 2) and RWGS reaction (Eq. 3). Thus the experimental results indicate that the sulfur poisoned catalyst promotes RWGS and coke gasification reactions leading to the formation of more CO in the product mixture, which results in H2 /CO

Study of the Combined Deactivation Due to Sulfur Poisoning and

Recommend Documents