Article pubs.acs.org/EF
Adsorptive Desulfurization: Fast On-Board Regeneration and the Influence of Fatty Acid Methyl Ester on Desulfurization and in Situ Regeneration Performance of a Silver-Based Adsorbent Raphael Neubauer,*,† Christof Weinlaender,† Norbert Kienzl,‡ Hartmuth Schroettner,§ and Christoph Hochenauer† †
Institute of Thermal Engineering, Graz University of Technology, Inffeldgasse 25b, 8010 Graz, Austria Bioenergy2020+ GmbH, Inffeldgasse 21b, 8010 Graz, Austria § Institute for Electron Microscopy and Nanoanalysis, Graz University of Technology, Steyrergasse 17, 8010 Graz, Austria ‡
S Supporting Information *
ABSTRACT: Adsorptive on-board desulfurization units require a high desulfurization and regeneration performance for a wide range of fuels to keep them small and ensure long maintenance intervals. A novel thermal regeneration strategy was investigated in this work, fulfilling all requirements for in situ on-board regeneration. In this strategy, a temperature-controlled flow rate (TCFR) of air was used to control the temperature inside the adsorber. With this dynamic approach, the regeneration time was reduced significantly in comparison to other thermal regeneration strategies. The novel regeneration strategy was tested using Ag−Al2O3 as an adsorbent to desulfurize a benzothiophen (BT)-enriched road diesel (300 ppmw of total sulfur). A commercial diesel containing fatty acid methyl ester (FAME) was used to evaluate the fuel flexibility regarding desulfurization and regeneration performance. In the case of 6.63 wt % FAME and 300 ppmw of sulfur, the breakthrough adsorption capacity of sulfur decreased from 1.04 to 0.17 mg/g. In TCFR regeneration experiments, the breakthrough adsorption capacity was restored to over 94% in the case of both fuels. Thereby, the Brunauer−Emmett−Teller (BET) surface area of the regenerated adsorbent decreased by only 1.5%, and negligible carbon deposits were detected.
1. INTRODUCTION Fuel desulfurization is necessary to fulfill environmental regulations and protect the environment. In addition, it also gives our society the opportunity to operate fuel cells and other sulfur-sensitive technologies with hydrocarbon-based fuels. Hydrocarbon-based fuels contain different types of sulfur compounds in different concentrations. The total sulfur concentration of fuels is limited in the U.S., the European Union (EU), and several other countries.1 For example, in the U.S. and the EU, the sulfur concentration is limited to 10−5000 ppm depending upon the class of fuel and its purpose of use.1 Fuel cells are a promising source of on-board electricity for trucks, ships, and airplanes. The advantage of fuel-cell-driven auxiliary power units (APUs) are higher efficiency, reduced emissions, and lower noise generation compared to conventional combustion engines.2,3 The APU simply uses on-board fuel and converts it to hydrogen-rich syngas via reforming.2,4 Both reformer and fuel cell need to be protected from sulfur. To use on-board fuels, the fuel has to be desulfurized before it is reformed to syngas. Several desulfurization approaches and techniques have been studied in recent years, and the selective adsorption seems to be the most promising technology.5−7 Its simplicity makes it attractive for on-board applications, because it requires no additional reagents. The selective adsorption approach uses zeolites,8,9 activated carbon, 10−12 metal organic frameworks,13,14 and metal oxides15−17 as the adsorbent support. All of these supports were doped with different materials and techniques in various studies. The breakthrough adsorption © XXXX American Chemical Society
capacity for real diesel fuels was found to be in the range of 0.46−10 mg of S/g of adsorbent18 but is strongly influenced by the type of diesel fuel.19,20 In the case of on-board desulfurization, regeneration is necessary to keep the desulfurization unit small and ensure long maintenance intervals. Only thermal regeneration under flowing air can be considered because no solvents are available on board. Baltzopoulou et al.11 and Han et al.12 investigated different regeneration strategies for activated carbons, including solvent and thermal regeneration. Both authors pointed out that the thermal regeneration of activated carbon is not suitable. Most of the investigated adsorbents have to be activated in a He, an Ar, or a H2 atmosphere after regeneration.7,8,15,21−24 No activation under such atmospheres is feasible because no He, Ar, or H2 is available on board. The exclusion of activated carbon and on-board activation leads to silver-based adsorbents. This type of adsorbent does not need to be activated in a He, an Ar or a H2 atmosphere. Silver-impregnated mesoporous silica (SBA-15 and MCM-41) was investigated by Chen et al.25 These sorbents were regenerated in flowing air at 200 °C, recovering around 50% of the sulfur capacity after the first cycle. Tatarchuk and coworkers26−28 investigated several Ag-based adsorbents using different supports and Ag contents. Thermal regeneration experiments in flowing air showed a high regenerability of 12% Received: March 3, 2016 Revised: April 25, 2016
A
DOI: 10.1021/acs.energyfuels.6b00519 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 1. Challenging Fuels and Their Initial and Total Sulfur Concentrations fuel
type of fuel
initial sulfur concentration (ppmw)
added sulfur component
total sulfur concentration (ppmw)
RD-300 RD-900 RD-FAME-300 RD-FAME-900
diesel diesel diesel containing 6.63 wt % FAME diesel containing 6.63 wt % FAME
5 5 7 7
BT BT BT BT
300 900 300 900
impregnated for 14 h before it was dried at 110 °C for 6 h, followed by calcination at 450 °C for 2 h. This preparation pathway has been optimized for different supports and outlined in several works by different authors.28,34 The physical properties of γ-Al2O3 and Ag− Al2O3 are provided as Supporting Information. 2.2. Adsorbent Characterization, Total Sulfur, and Off-Gas Analysis. The adsorbent was characterized by means of N 2 physisorption. All samples were outgased at 120 °C for 2 h prior to N2 physisorption to remove moisture. The N2 adsorption/desorption isotherm was conducted at −195.8 °C. The adsorbent microstructure was examined using Zeiss Ultra 55 field emission scanning electron microscopy (FE-SEM). The samples were coated with Cr to ensure proper conductivity. Energy-dispersive X-ray spectroscopy (EDX) was used for the chemical characterization of samples and for the detection of Ag distribution and C deposits. The fuel samples were digested with nitric acid in a microwaveassisted autoclave system (Multiwave 3000, Anton Paar, Graz, Austria). Total sulfur concentrations in the digests were determined via inductively coupled plasma optical emission spectroscopy (ICP− OES) with a Spectro Arcos SOP. FAME contents in diesel fuel were determined by mid-infrared spectrometry according to EN 14078:2009. The total carbon was analyzed with an element analyzer (Leco RC612). During the regeneration, the total hydrocarbon (CxHy) concentration in the off-gas was measured with a flame ionization detector (FID) FID2010T-NMHC from TESTA. O2 and CO2 concentrations in the off-gas were recorded by an AO2000 continuous gas analyzer from ABB equipped with Uras 26 and Caldos 17 modules for cross-checking. 2.3. Reagents. Commercially available road diesel (RD) was used to investigate the influence of FAME on the desulfurization and regeneration performance of Ag−Al2O3. The FAME content of that diesel was 6.63 wt % and is referred to as RD-FAME. All results are compared to a commercially available FAME-free diesel fuel, which is referred to as RD. In diesel fuels, the most important sulfur compounds are BT, dibenzothiophene (DBT), and their derivatives, such as 4,6-dimethyldibenzothiophene (4,6-DMDBT).35,36 In this study, both RD-FAME and RD were enriched with BT to reach a certain amount of total sulfur. Thereby, the influence of FAME on the performance of other adsorbents can be derived from the results presented in this work because BT is the most extensively investigated single sulfur compound for different types of adsorbents.26−28,37−39 A total sulfur concentration of 300 ppmw was used for all experiments in this study. In addition, 900 ppmw of sulfur was used for additional saturation experiments. Table 1 shows the fuel properties, including the initial sulfur concentration, which is the amount of sulfur that was measured before BT was added. The number at the end of each fuel designation gives the total sulfur concentration in parts per million weight (ppmw). 2.4. Desulfurization Experiments. Static saturation tests and dynamic breakthrough experiments were carried out to study and quantify the desulfurization performance of Ag−Al2O3. All experiments were performed under atmospheric pressure and 20 °C. RD and RD-FAME with 300 and 900 ppmw of sulfur were used for saturation experiments, where 10 mL of fuel was mixed with 1 g of adsorbent. The sulfur concentration of the equilibrated fuel was analyzed after 48 h to determine the equilibrium adsorption capacity according to the following formula:
Ag/TiO2−Al2O3 over 5 cycles.28 Yang et al.29 impregnated mesostructured silica with an aqueous solution of AgNO3. The highest sulfur adsorption capacity was achieved when the adsorbent was synthesized with a solution of 15 wt % AgNO3. Nair et al.27 investigated the influence of Ag loading (2−20 wt %) on different support materials. The best results were achieved with 4 wt % Ag using TiO2 as the support. The good performance of 4 wt % Ag was related to small Ag crystals, which are highly dispersed within the porous structure of TiO2. With an increasing Ag content, the average crystal size increased as well, reducing the dispersion rate as well as the Brunauer−Emmett−Teller (BET) surface area of the adsorbent.27 Samokhvalov et al.30 studied the surface characteristics of Ag−TiO2 and showed that Ag is presented mostly as Ag oxide. The performance of adsorbents is described by the sulfur equilibrium/breakthrough adsorption capacity and the regenerability of the adsorbent. All three characteristics are strongly influenced by the fuel that has to be desulfurized. In detail, the performance is influenced by the concentration and type of sulfur compounds,28,31 aromatic compounds,28 fuel additives, and nitrogen-containing compounds.19,32 Hussain and coworkers26,28 investigated the adsorption capacity of Ag/ TiO2−Al2O3 for different fuels. These studies include several commercial fuels, such as ultra-low-sulfur diesel (ULSD) and JP5,28 as well as a model fuel prepared using n-octane and benzothiophene (BT) as a sulfur compound.26 The total sulfur concentration of these fuels was in the range from 7.5 ppmw (for ULSD) to 3500 ppmw (for modeled fuel). The observed breakthrough adsorption capacities varied between 0.59 mg/g for ULDS and 14.37 mg/g for the modeled fuel. To replace fossil fuels with renewable fuels, fatty acid methyl ester (FAME, also referred to as biodiesel) is blended into diesel fuels. The admixture to regular diesel is regulated in the U.S. and the EU, where up to 5 and 7 vol % of FAME are allowed, respectively.32,33 FAME has a vast influence on the sulfur adsorption performance of activated Ni-based adsorbents, as was pointed out by Pieterse et al.32 In his work, the influence of FAME on the regeneration was not investigated. On-board desulfurization units require fundamental knowledge and comprehensive data regarding desulfurization and regeneration performance for a wide range of fuels and fuel components. In this work, the fuel flexibility of a silver-based adsorbent will be investigated concerning desulfurization and regeneration performance. A novel and fast regeneration strategy will be used to recover the desulfurization performance after desulfurizing commercial diesel with and without FAME.
2. EXPERIMENTAL SECTION 2.1. Adsorbent Preparation. In this study, Ag−Al2O3 was used, which was prepared by incipient wetness impregnation. The support material was γ-Al2O3 (1.2 mm spheres) obtained from Strem Chemicals. It was dried at 110 °C for 6 h before any further treatment. Dried γ-Al2O3 was impregnated with aqueous solution of AgNO3 using solid AgNO3 of 99% purity (Alfa Aeser). The Ag content was 12 wt % at the time of impregnation and was dissolved in deionized water using 100% γ-Al2O3 pore volume. γ-Al2O3 was
qeq = B
Vf ρf (c0 − c1) mads × 106 DOI: 10.1021/acs.energyfuels.6b00519 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels where qeq is the equilibrium adsorption capacity (mg/g), ρf is the density of the fuel (kg/m3), Vf is the fuel volume (mL), mads is the initial mass of the adsorbent (g), and c0 and c1 are the sulfur concentrations at the beginning and end of the experiment (ppmw), respectively. All dynamic breakthrough experiments were carried out in an individually designed test rig using RD-300 and RD-FAME-300. The schematic flow sheet diagram of the test rig is shown in Figure 1.
breakthrough capacity was calculated at the 10 ppmw sulfur threshold limit. A total of 10 ppmw of sulfur is crucial because this concentration is diluted by a factor of 10 in the reformer,41 leading to the required 1 ppmw for a solid oxide fuel cell.42 2.5. Sorbent Regeneration. Used Ag−Al2O3 was regenerated in situ under flowing air at elevated temperatures. The adsorber was heated electrically, whereby the heat for regeneration was mainly transferred through the adsorber walls. The temperature inside the adsorber was kept at 120, 250, and 450 °C for 1, 2, and 1 h, respectively, combined with heating rates of 3.5−8.8 K/min. This led to a total regeneration time of around 6 h, including heating and cooling. During the regeneration, the temperature inside the adsorber was controlled by a thermocouple mounted in the middle of the adsorber. At the beginning, the air flow rate was set to 5 L/min at standard conditions. After 3.5 h, the air flow rate was reduced and kept at 2 L/min when the adsorbent was heated and held at 450 °C. This was performed to achieve a higher heating rate. The regeneration was quantified by analyzing the off-gas during regeneration as well as reuse of the adsorbent. Further, retained sulfur and carbon deposits on the regenerated sorbent were analyzed to obtain deeper insight into the regeneration process as well as the role of FAME during regeneration.
3. RESULTS AND DISCUSSION 3.1. Desulfurization Performance. The desulfurization performance of Ag−Al2O3 was evaluated using equilibrium saturation and dynamic breakthrough experiments. The equilibrium saturation experiments were used to investigate the affinity of FAME to Ag−Al2O3. Figure 2a illustrates the equilibrium saturation capacity for RD and RD-FAME with 300 and 900 ppmw of sulfur. The results showed that, in case of RD-FAME, the saturation capacity was reduced significantly compared to RD without FAME. In the case of RD-FAME-300, the saturation capacity dropped by 90% from 2.07 to 0.22 mg/g compared to RD-300. Saturation experiments with 900 ppmw of sulfur were performed to assess the binding energy of FAME compared to BT. The results of both sulfur concentrations indicate that the binding energy of FAME is approximately in the same range compared to the binding energy of BT. When the sulfur concentration of RD-FAME was increased by a factor of 3, the saturation capacity for Ag−Al2O3 increased approximately as well by the factor of 3 (from 0.22 to 0.63 mg/g). This indicates a direct proportional relation between the saturation capacity and sulfur concentration in the case of 6.63 wt % FAME. The same phenomenon was observed for RD, where the saturation capacity of Ag−Al2O3 increased from 2.07 to 4.13 mg/g for RD-300 and RD-900, respectively. In this case, the relation is not directly proportional compared to the case of FAMEcontaining diesel.
Figure 1. Flow sheet diagram of the laboratory-scale adsorptive desulfurization unit with thermal in situ regeneration under flowing air. The adsorbent (14 g) was loaded into the vertical stainless-steel adsorber supported on both sides by stainless-steel sieves with a mesh size of several micrometers. In the adsorber, the fuel flowed vertically upward with a constant flow rate of 0.06 mL/min, if not specified otherwise, using an Ismatec REGLO Digital MS-2/6 peristaltic pump. The corresponding liquid hourly space velocity (LHSV) is 0.2 h−1. The adsorber effluent was sampled periodically and analyzed by ICP− OES to determine the total sulfur concentration. The dynamic breakthrough experiment was ended after reaching 30−50 ppmw of sulfur at the outlet. This range of the sulfur concentration is also the threshold limit for the reformer placed downstream of the desulfurization unit in an APU.7,40 The breakthrough curves in the Results and Discussion were obtained by plotting the outlet sulfur concentration against the volume of treated fuel normalized by the weight of the adsorbent. The
Figure 2. Saturation capacity of Ag−Al2O3 and γ-Al2O3 support for (a) sulfur and (b) FAME. C
DOI: 10.1021/acs.energyfuels.6b00519 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 2. Sulfur Breakthrough Capacity (10 ppmw Threshold Limit) of Ag−Al2O3 at LHSV of 0.2 h−1 adsorbent
silver content at time of impregnation (wt %)
fuel (kg/m3)
sulfur in fuel (ppmw)
breakthrough capacity at 10 ppmw threshold limit (mg of S/g of adsorbent)
Ag−Al2O3 Ag−Al2O3
12 12
RD-FAME-300 RD-300
300 300
0.17 1.04
Further saturation tests were carried out with Ag−Al2O3 to determine the saturation capacity of FAME. The results are illustrated in Figure 2b and show a saturation capacity of 47.69 mg of FAME/g of Ag−Al2O3. For these experiments, 2.5 g of Ag−Al2O3 was mixed with 5 mL of RD-FAME-900. All results show a high affinity of FAME toward Ag−Al2O3. Thereby, the FAME molecules not only compete but also block the active sites of Ag−Al2O3. Additional saturation tests were carried out with pure Al2O3 support to quantify the promoting effect of silver in the presence of FAME. The results in Figure 2a show that FAME almost deactivates the promoting effect of silver. Song et al.43 desulfurized a model fuel containing 285 ppmw of BT in 1-octane and achieved a saturation capacity of 9.6 mg/ g for a AgCeY zeolite. Nair et al.39 used silver-impregnated Al2O3 in their study to desulfurize a modeled fuel of 3500 ppmw of BT in n-octane. Saturation capacities of around 4.1 and 10.4 mg/g were observed for pure Al2O3 and Ag−Al2O3, respectively. In both studies, higher saturation capacities were achieved in comparison to the results in the present study (Figure 2). This is caused by two factors. First, Song et al.43 and Nair et al.39 used a model fuel without any aromatic compounds compared to 14.8 wt % in the case of RD-300 and RD-900. Second, in the case of Nair et al.,39 the sulfur concentration was almost 4 times higher compared to RD-900. Both factors are also discussed in the work of Hussain et al.,28 claiming the same reducing effect on the sulfur adsorption capacity of Ag/TiO2−Al2O3. The reducing effect of aromatic compounds was also investigated by Tian et al.44 They studied the influence of 3.4 wt % toluene as an aromatic compound on the adsorption capacity of an alkaline-treated zeolite. The saturation capacity dropped from 1.4 to 0.3 mg/g when 3.4 wt % toluene was added to the cyclohexane-based model fuel containing 127 ppmw of BT.44 Sun et al.38 achieved a saturation capacity of 1.65 mg/g by desulfurizing a model fuel (391 ppmw of BT in noctane) over a modified NaY zeolite. The saturation capacity dropped to 1 mg/g when 40 vol % toluene was added to the model fuel.38 3.2. Breakthrough Experiments. Table 2 and Figure 3 illustrate the sulfur breakthrough capacities of Ag−Al2O3 for RD-300 and RD-FAME-300 obtained by dynamic breakthrough experiments with a LHSV of 0.2 h−1. In the case of RD-FAME-300, a breakthrough capacity of 0.17 mg/g was observed. This breakthrough capacity is around 77% of the correlated saturation capacity of 0.22 mg/g. The observed breakthrough capacity of RD-300 was 1.04 mg/g. These results show that a FAME content of 6.63 wt % reduced the breakthrough capacity by over 83% compared to the breakthrough capacity of RD-300. This indicates that FAME molecules block the active sites of Ag−Al2O3. This inhibiting effect of FAME was also observed by Pieterse et al.32 using an activated Ni-based adsorbent. It was also pointed out that the inhibiting effect might also be caused by additional additives required to stabilize the FAME-containing diesel.32 The adsorption dynamic of FAME adsorbing on Ag−Al2O3 was studied using dynamic breakthrough experiments analyzing
Figure 3. Breakthrough curve of sulfur using Ag−Al2O3 to desulfurize RD-FAME-300 and RD-300 at a LHSV of 0.2 h−1.
the FAME concentration at the adsorber outlet. Figure 4 shows the breakthrough curve of FAME and sulfur using Ag−Al2O3 to
Figure 4. Breakthrough curve of FAME and sulfur using Ag−Al2O3 to desulfurize RD-FAME-300 at a LHSV of 0.2 h−1.
desulfurize RD-FAME-300 with a LHSV of 0.2 h−1. The FAME concentration in the first sample was
