Diesel Fuel Desulfurization via Adsorption with the Aid of Activated

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Diesel fuel desulfurization via adsorption with the aid of activated carbon: Lab- and pilot-scale studies Penelope Baltzopoulou, Kyriakos X. Kallis, George Karagiannakis, and Athanasios G. Konstandopoulos Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01133 • Publication Date (Web): 24 Aug 2015 Downloaded from http://pubs.acs.org on August 27, 2015

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Diesel fuel desulfurization via adsorption with the aid of activated carbon: Lab- and pilot-scale studies Penelope Baltzopouloua, Kyriakos X. Kallisa, George Karagiannakis*,a and Athanasios G. Konstandopoulos*,a,b a Aerosol & Particle Technology Lab., Chemical Process & Energy Resources Inst., Centre for Research & Technology Hellas (APTL/CPERI/CERTH), 6th km Charilaou-Thermi, 57001, P.O. Box: 60361, Thermi-Thessaloniki, Greece b

Department of Chem. Eng., Aristotle University of Thessaloniki, P.O. Box 1517, 54006, Thessaloniki, Greece

ABSTRACT: The present work is an experimental investigation of commercial diesel fuel liquid desulfurization via adsorption under mild conditions. The sorbent employed was a commercial high surface area activated carbon (AC) and studies involved both lab- and pilot-scale experiments performed in dedicated fixed bed setups. Under lab-scale conditions, maximum sulfur removal measured exceeded 90%, while according to breakthrough curves obtained the total sulfur content remained below 2 ppmw for up to 20-22 ml processed diesel/g AC. Process scaling-up by a factor of 15 showed a moderate negative effect, with the respective breakthrough fuel amount (total sulfur ≤ 2 ppmw) being approximately 15-17 processed fuel/g sorbent. Several sorbent regeneration strategies were studied under lab-scale conditions. The one with the highest restoration of initial (i.e. fresh state) AC performance involved heating under vacuum (200 mbara) up to 200oC and subsequent washing of the material with a binary organic solvent. The amount of solvent required was 50-55 ml /g sorbent. However, even under such conditions, desulfurization performance was only partially restored upon repeated desulfurization/regeneration cycles. From the 2nd and up to 7th cycle, desulfurization efficiency of the material was essentially stable but from cycle number 8 and on further performance degradation was identified. Based on fresh/regenerated sorbent post analysis, it was found that cycle-to-cycle degradation was due to gradual decrease of the sorbent’s surface area, mainly attributed to residual amounts of diesel-derived species remained in its structure thereby partially blocking its porosity. The main properties of processed fuel remained essentially unaffected, however removal of di- and poly-aromatic compounds was notable.

1. INTRODUCTION Ultra-low sulfur fuel (typically below 10 ppmw but even lower concentrations would be plausible) is needed for several applications both due to required reduction of relevant noxious compounds (e.g. SOx, H2S) emitted upon combustion/processing of the fuel and due to the well-known pro-

found catalyst deactivation effect of sulfur presence in the case of catalytic thermochemical conversion processes. An example of such a process is hydrogen production via reforming of fossil liquid fuels; mainly diesel or diesel/biodiesel blends).1 In general, fossil fuel reforming is considered as an important intermediate and cost-effective step towards the creation of a hydrogen infrastructure that will subsequently facilitate widespread use of the latter, environmental-friendly energy carrier. Moreover, it is an efficient and cost-effective option for decentralized hydrogen production to be used in fuel cell systems for zero-emission power production such as those examined in the NEMESIS 2+ project, funded by the Fuel Cell and Hydrogen Joint Undertaking (FCH-JU). Currently, liquid fuel desulfurization is performed with the aid of hydro-desulfurization processes (HDS), typically operated in fixed-bed reactors under hydrogen-rich environment, elevated temperatures (300-400°C) and pressures ranging from 30 to 130 bar.2 It is generally known that most of the sulfur compounds in deeply desulfurized (i.e. S < 10 ppmw) fuels via the HDS method are attributed to refractory species, namely alkylated dibenzothiophenes (DBTs). Typical examples include (but are naturally not limited to) 4,6-dimethylDBT, 4,6-diethyl-DBT and also C3- and C4-DBTs. The HDS method is in one hand unsuitable for small-scale decentralized applications and in the other hand quite energy intensive, especially when very low total sulfur content is targeted (e.g. ≤ 2 ppmw). Moreover, it is non-compatible with technologies referring to hydrogen production via fuel processing (e.g. reforming), as it is not appropriate to employ a significant part of the final product of such a process for the preprocessing / conditioning of the feedstock fuel. For these reasons, more benign and environmental-friendly approaches have emerged during the last 15 years. The process of liquid fuel adsorptive desulfurization, mostly under mild conditions, constitutes a promising alternative. The desulfurization performance generally depends on the adsorption dynamics and the amount of sorbent used.3 Several types of sorbents – with activated carbon (AC) based, zeolite based and Ni-based materials being prevalent – have been investigated. In relevant studies by HernandezMaldonado et al,4,5 the authors demonstrated that copper Y-

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type zeolites could provide the best sorption capacities and remove most sulfur compounds from commercial liquid fuels. Additionally,6 it was shown that copper and silver Y zeolites, could selectively adsorb sulfur compounds from commercial fuels and with high sulfur capacities via the picomplexation mechanism at ambient pressure and temperature. Furthermore, it was found out that the sulfur content in commercial diesel could be reduced from 430 to as low as 0.2 ppmw, at a sorbent capacity of 34 ml diesel/g sorbent. In a review by Hernandez et al,7 the author compared various picomplexation desulfurization techniques and found that the best sorbent was Cu(I)-Y, as it was claimed to be capable of being desulfurized down to 0.2 ppm at about 38 ml fuel/g sorbent. In another study by Hernandez et al,8 the authors used Ni based sorbents such as Ni/SiO2-Al2O3 to investigate the diesel desulfurization process using model diesel. The adsorption capacities decreased in the order of benzothiophene (BT) > di-benzothiophene (DBT) > 4,6-dimethyldibenzothiophene (DMDBT), pointing out the methyl groups at the positions 4- and 6- of DBT as responsible for the hindering of interactions between the atoms of sulfur and sorbent active sites. In extra set of experiments with model diesel, the authors tested different Ni loads in activated carbon and silica-alumina sorbents. It was found that the sample of 28% Ni loading in AC exhibited the highest breakthrough capacity.9 Another approach to adsorptive desulfurization via Ni based sorbents could be the loading of Ni in a mesoporous molecular sieve to desulfurize commercial ultralow sulfur diesel.10 The use of MCM-48 with 20% Ni loading led to a sorbent capable of reaching a breakthrough capacity of 2.1 mg S/g sorbent at a breakthrough level of 1 ppmw total sulfur.10 In further Ni-based studies, the author showed that Ni based sorbents could successfully remove sulfur compounds from ultra-low sulfur diesel reaching the capacity of 2 mg S/g Ni. Interestingly, that performance could not be achieved when cetane improver or bio-diesel was added.11 More specifically, the addition of the cetane improver 2ethylhexylnitrate was responsible for the decrease of the sulfur capacity by 50% while the presence of bio-diesel completely disabled the desulfurization process. Muzic et al 12 conducted desulfurization experiments in an AC fixed bed column using commercial diesel. AC was sieved to a particle size between 0.4 and 0.8 mm. The lowest sulfur content was observed at the lowest diesel flow rate of 1ml/min and highest bed depth of 28.4 cm and temperature of 50oC. At higher temperatures, a decrease of the breakthrough time was observed. Furthermore, batch tests on adsorbing behavior of activated carbon and Cu (AC/Cu(I) layered bed) showed that up to 30 ml of commercial diesel per gram of adsorbent with 0.15 ppmw at ambient temperature and pressure could be desulfurized.5 Triantafyllidis et al 13 performed an investigation using different types of AC showing that for the removal of 4,6-DMDBT (i.e. representative of most refractory/resilient to removal sulfur compounds in diesel) diluted in hexadecane, the pores with diameter of less than 10 A are more effective since they match the size of the molecule. Furthermore, the authors reported that the adsorption capacity is depended on acid functional groups especially carboxylic ones due to the fact that they can interact via H-bonding with sulfixides, sulfones and sulfonic acid species. In another study,14 iron oxyhydroxide nanoparticles were used to modify commercial activated carbon for model

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diesel desulfurization. The modification decreased the porosity by 20% but desulfurization process was enhanced compared to the unmodified AC. Iron species improved the selectivity of sulfur compounds by decreasing the interactions of aromatics with the carbon surface. Concerning other ACbased modified adsorbents, Wang et al 3 reported that among the formulations of CuCl/AC, PdCl2/AC and Pd/AC the highest desulfurization capacity was observed for the PdCl2/AC case. In other set of experiments, conducted by Muzic et al 15, activated carbon in batch adsorption experiments was used for the desulfurization of diesel fuel. The response surface methodology was used to investigate the relationship between the important process parameters. The lowest output sulfur concentration of 6.6 ppm was observed at 50oC and 100 min time with input concentration 16 ppm and adsorbent mass of 4 g. The optimal sorption capacity of 0.3 mg S/g sorbent was determined to be at 50oC and 100 minutes contact time as well but for input sulfur concentration of 38.4 ppm and adsorbent mass of 2 g. Furthermore, response surface methodology was applied to investigate the sulfur removal from model oil (DBT dissolved in iso-octane). Removal of DBT is increased with an increase in AC mass from 5 to 20 mg/l and increase in temperature up to 30oC. DBT removal decreased with an increase in the initial sulfur concentration and increased with an increase in contact time, reaching the optimum at around 6 hours. One of the key elements of the adsorptive liquid desulfurization approach that requires vigorous investigation is the potential ability to repeatedly use such sorbents in a cyclic adsorption/regeneration mode of operation, with such regeneration techniques being ideally 100% efficient in terms of the initial material performance restoration and also costefficient. It is important to note that relevant studies on this topic are limited and this limitation is further enhanced by the utilization of model fuels in the experiments rather than commercial ones.3 In the present work, only commercial diesel fuels (i.e. as sold in petrol stations) were employed in order to adopt conditions closer to those of a potential real application. Moreover, based on the evaluation outcome of several materials included in a relevant previous work,16 the present study focused on a specific grade of high surface area AC, while a first attempt was made to scale-up the process to a level of producing approximately 1 l desulfurized fuel⋅h-1. 2. EXPERIMENTAL The material used for the adsorption studies was a high surface area (designated value of 1960 – 2053 m2/g), woodbased activated carbon (AC), which was commercially acquired from MeadWestvaco Corp. in powder form (measured mean particle size of 42 μm). The diesel fuel employed was obtained as biodiesel-free from a local refinery and its total sulfur content was measured at 7.1 ppmw. The desulfurization process was evaluated via both batch and continuous experiments. Batch tests were conducted by mixing the sorbent with the fuel, followed by filtration of the slurry to obtain the processed (desulfurized) diesel. The fuel/sorbent ratio employed was fixed at 5.0 and the stirring time was 30 min, based on the relevant findings of a previous study.16 The continuous tests were performed in a vertical fixed bed adsorption reactor and were employed for the generation of dynamic total sulfur breakthrough curves. Break-


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through curves are generated by plotting the total sulfur concentration evolution versus cumulative fuel volume processed (or fuel effluent of the process), normalized by total weight of the adsorbent.14 The targeted total sulfur breakthrough content was set at 2 ppmw (i.e. > 70% minimum desulfurization of the fuel). However, in several cases the curves were extended to higher values, in order to obtain a clearer picture of their evolution. The fixed bed, lab-scale reactor comprised a vertical custom-made stainless steel tube equipped with a metallic frit (average size of pores 20μm), with length/diameter of adsorption bed (L/D) ratio in the range of 0.24-3.30. The experimental setup, as depicted in Figure 1, comprised a low-flow liquid rotary piston pump (ISMATECH REGLO-CPF Analog), feed tanks, a custom made, electrical heated furnace and a nitrogen mass flow controller (Bronkhorst high-tech ELFLOW select series). The latter two components were used only during regeneration operating mode. During continuous liquid desulfurization, the fuel was allowed to pass through the fixed bed in a down-flow direction. The effluent was sampled periodically and the evolution of total sulfur concentration was measured by the ultraviolet fluorescence method, according to the ISO 20846 method (ASTM 5453 09), with a nitrogen/sulfur analyzer ANTEK 9000 NS instrument. Both fresh and processed fuels were characterized in terms of their a) aromatics concentration determined with HPLC/IR (EN 12916 method), b) density at 15oC measured by a standard digital density meter (ASTM D4052 11 method), c) distillation curve at atmospheric pressure (ASTM D86 method), d) elemental carbon and hydrogen (ASTM D5291 method), e) total nitrogen content (ASTM D4629 method), f) upper heat of combustion (ASTM D240 14 method) and g) sulfur speciation with gas chromatography (detection limit per sulfur compound identified ∼1 ppmw). The saturated sorbent (i.e. after desulfurization) was subjected to several candidate regeneration treatments, mostly based on thermal processing under inert (i.e. nitrogen) flow at ambient pressure or under low vacuum (∼200 mbara). The details of the evaluated regeneration protocols are provided in the Results and Discussion section. In some of the regeneration approaches considered at the lab-scale setup, a twostage low vacuum pump (ROBINAIR, Model 15301E) was used. The experimental setup was designed to allow adsorption (desulfurization) (Figure 1.a) and sorbent regeneration (Figure 1.b) in a cyclic mode of operation. The pilot-scale setup was designed with a scaling factor of 15 (i.e. on the basis of total sorbent amount employed) and included a fixed bed reactor of length/diameter of adsorption bed (L/D) ratio equal to 0.23 (35 mm/150 mm), a diaphragm liquid pump (Gardner Denver Thomas, Model 5002F Twin LC) for the fuel feedstock and an oil-less diaphragm vacuum pump (Gardner Denver Thomas, Model 8011ZVP-35). The relatively ‘flat’ geometry of the bed under scaled-up conditions was chosen in order to avoid high pressure drop evolution during operation. As it will be made evident by the relevant lab-scale parametric study provided in the Results and Discussion section, this ‘flat’ geometry is not likely to have negative impact in the performance of the AC sorbent c.f. beds with higher L/D values. It is also mentioned that the material, both in its fresh form and after being subjected to desulfurization / regeneration cycles, was characterized in terms of its specific surface area (BET method), pore size and pore volume via nitrogen ad-

sorption isotherms at 77 K (Micromeritics ASAP 2000). Moreover, Thermo-Gravimetric Analysis (TGA) under air flow was performed with the aid of a Perkin Elmer Pyris-6 instrument. In order to improve sorbent’s desulfurization performance, a series of chemical modifications were employed via treatments proposed by other researchers. Treatment of AC with FeCl3 at 80oC for 72 hrs (sample B) and treatment with FeCl3 and H3PO4 at the same conditions (sample F) were employed according to procedures described by Arcibar-Orozco et al.13 Treatment with NaOH was performed according to Shalaby et al17 and the respective samples produced were sample C (involving also re-activation at 650oC) and sample G (no reactivation). Sample E was produced via a 2-hour treatment of AC with a 5% solution of HNO3 and a mass/volume ratio of 50 mg AC / ml solution. After treatment, the AC was rinsed several times with approximately 2 l of deionized water in total and until the final pH value reached approximately 3. The treated sorbent was dried at 100oC for 64 hours before evaluation. Sample B was produced via a 2-hour treatment with a 10% solution CH3COOH and a mass/volume ratio of 50mg AC / ml solution. Alike previous treatment, the AC was rinsed several times with approximately 2 l of deionized water in total and until the final pH value reached approximately 3. Subsequently, the amended sorbent was dried at 140oC for 64 hours before evaluation. All chemicals used were of standard chemical grades (99.9% or higher) and were acquired by Sigma Aldrich.

(a)

(b)


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Figure 1. Lab-scale experimental setup. Flowchart of (a) desulfurization and (b) regeneration process.

ed during the experiments plotted in Figure 3 and up to a total sulfur content of 2 ppmw. 8

Figure 2. Assessment of liquid desulfurization performance with respect to fixed bed geometry. Figure 3 presents the experimental measurements for fossil diesel as achieved at the lab-scale fixed bed reactor (L/D = 0.80 and LHSV = 10 h-1). The breakthrough value of 2 ppmw total sulfur (i.e. reduction of ∼70% with respect to fresh diesel), was achieved at approximately 20 ml fuel / g AC. The experimental data of Figure 3 were derived from 8 different desulfurization tests. Evidently, test repeatability, especially up to the targeted value of 2 ppmw, was adequate to draw conclusions. Data scattering can be well justified by the fact that standard total sulfur measurement methods (including the one employed here) are deemed to exhibit some variability at such very low concentrations. Following its desulfurization, the processed fuel was subjected to a series of analyses in order to identify potential differences c.f. fresh diesel. Processed fuel refers to mixed diesel amounts collect-

Fresh fuel: [S] = 7 ppmw

7 LAB-scale experiments Remaining sulphur (ppmw)

3. RESULTS AND DISCUSSION Lab-scale liquid desulfurization – Unmodified AC. Regarding the continuous experiments, those were evaluated with the aid of relevant breakthrough curves. Initially, an experimental parametric assessment was performed as far as the fixed bed geometry is concerned. This study showed no notable variations in terms of the breakthrough curves obtained for length/diameter of adsorption bed (L/D) ratios in the range of 0.24 - 3.3. Moreover, no clear effect on desulfurization performance was identified for liquid hourly space velocity (LHSV) variation in the range of 8 – 41 h-1 (Figure 2), as in all cases the total sulfur measurement differences were within the experimental error of the measuring instrument. LHSV is defined as the ratio of fuel mass flow (in g⋅h-1) employed to the amount (in g) of AC contained in the bed. Evidently, the trend of all breakthrough curves depicted in Figure 2 follows a sigmoidal shape. Initially, and up to a value of about 10 ml/g, the total sulfur content increases relatively sharply. Subsequently, and in the range of approx. 15-30 ml/g, a pseudo-equilibrium behavior around the value of 2 ppmw total sulfur is observed. At values exceeding 30-35 ml/g, sulfur concentration increases again and, arguably, at substantially higher volumes of cumulative normalized desulfurized fuel (not shown here) all curves are expected to equilibrate to the total sulfur content of the feed (7.1 ppmw).

6 5 4 3 2 1 0

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deS fuel (ml) / sorbent (g)

Figure 3. Breakthrough curve of fossil diesel Table 1 provides direct comparison of main measured properties of fresh and desulfurized fuels, while Figure 4 shows their comparative distillation curves. As it can be seen, most of the main properties of the fresh fuel remained essentially unaffected, however, notable differences with respect to di-/tri+ (polycyclic) aromatics content were identified. The decrease in the concentration of polycyclic aromatic compounds due to desulfurization was determined to be as high as 58%. These results are in general favorable as such (relatively heavy) compounds are considered problematic upon thermochemical fuel processing (e.g. in combustion/reforming processes), since their presence has been strongly associated with carbon/soot formation. On the other hand, their total content in the fresh diesel was low (< 2.0 wt%) and therefore the aforementioned notable reduction upon desulfurization did not have clearly identifiable effect on the overall fuel composition, as particularly exhibited from the comparative C/H contents (Table 1) and distillation curves (Figure 4). Another interesting observation refers to the fact that all 3 sulfur compounds quantified in the fresh fuel were removed to below detection limit levels, thereby proving that - at least within the measuring accuracy of the applied method - the sorbent employed here is more-or-less equally effective for all DBT-based species (even for the most refractory ones). Moreover, the adsorptive desulfurization process caused notable reduction of the, already low, nitrogen content by approximately 92%. 400 350

Temperature (oC)

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300 250 200 150

Fresh fuel Desulphurised fuel

100 0

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100

Evaporation (%)

Figure 4. Distillation curves of fresh and desulfurized fuel


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Table 1. Fuel properties of fresh and desulfurized fuel

Fuel parameter

Units

Fresh fuel

Desulfurised fuel

Carbon Hydrogen Oxygen Nitrogen Density Heat of combustion (upper) Mono-aromatics Polycyclic-aromatics(Di+Tri+) Total aromatics 4,6-DiMethyl-DBT 2,4,6-TriMethyl-DBT C3-DBT

% wt % wt % wt ppmw g/ml MJ/kg % wt % wt % wt ppmw (total S equivalent) ppmw (total S equivalent) ppmw (total S equivalent)

86.44 13.52 0.04 2.00 0.828 46.12 16.90 1.90 18.80 2.70 1.50 2.90

86.65 13.34 0.01 0.16 0.825 46.16 15.7 0.8 16.5 Below detection limit Below detection limit Below detection limit

Table 2. Measured BET surface area, mean pore size values and desulfurization performance (batch process) of chemically Modified Activated Carbons (MACs)

Sample code

Sample description

Surface area (m²/g)

sample A sample B sample C sample D sample E sample F sample G

AC MAC w. FeCl3 MAC w. NaOH MAC w. CH3COOH MAC w. HNO3 MAC w. FeCl3 & H3PO4 MAC w. NaOH & 650oC

1959 1977 2068 1991 1985 1872 1653

Lab-scale liquid desulfurization – Modified AC sorbents. In order to improve the desulfurization performance of the selected activated carbon a series of (mostly chemical) treatments of the material were applied. The improvement process was based both on the introduction of functional groups in its structure that are believed to favor its affinity to organic sulfur compounds 18, as well as on further increase of the (already high) surface area via chemical processing. After chemical modification, all treated samples were characterized in terms of their surface area and their mean pore size. As presented in Table 2, measured properties did not change significantly, apart from the case of the sample G that was first chemically and subsequently thermally treated at 650oC. Naturally, the high temperature treatment induced measurable loss of BET surface area (∼16% c.f. sample A). All modified ACs were evaluated under batch desulfurization conditions. According to the degree of sulfur reduction, the MACs treated with FeCl3 (sample B), NaOH (sample C), CH3COOH (sample D) and HNO3 (sample E) exhibited marginally better or equal performance with the untreated sorbent (Table 2) and they were shortlisted for further evaluation under continuous desulfurization at the lab-scale fixed bed reactor.

Mean pore size (Å) 31.8 29.8 30.1 30.7 30.7 31.0 28.5

S reduction (%) 86 87 86 85 85 81 79

Among the four qualified MACs further evaluated by continuous desulfurization experiments, the ones treated with FeCl3 (sample B), CH3COOH (sample D) and HNO3 (sample E) exhibited slightly better performance than the untreated AC. According to the breakthrough curve depicted in Figure 5, diesel with maximum total sulfur concentration of about 2 ppmw was achieved only slightly above 20 ml fuel/gr sorbent when the aforementioned MACs (sample B, sample D, sample E) were used. Based on the overall picture of the results presented comparatively in Figure 5, it can be claimed that, in agreement with the findings presented in Table 2, there is actually no clear difference between the unmodified AC and the treated ones. Thus, chemical modification of the particular AC grade is assessed as practically non-effective with respect to its potential desulfurization performance improvement.


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7

Untreated AC sample A MAC-FeCl3 sample B

6 Remaining sulphur (ppmw)

MAC-NaOH sample C

5

MAC-CH3COOH sample D MAC-HNO3 sample E

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Figure 5. Desulfurization performance of the selected MACs at semi-continuous, lab-scale process. 8

Pilot-scale liquid desulfurization – Unmodified AC. Similarly to the lab-scale studies, breakthrough curves provided the main means for the evaluation of the desulfurization performance at a larger (pilot) scale level. Of particular importance is also the direct comparison to the respective lab-scale tests, in order to identify potential differences induced by the process scale up. Figure 6 shows a comparison between the pilot scale and lab-scale desulfurization breakthrough curves. 14 experiments were conducted and the total quantities of desulfurized (i.e. up to 2 ppmw total sulfur content) fuel produced were approximately 30 l. Evidently, there is a clear negative, albeit not detrimental, effect due to the process scale up. More specifically, for the pilot scale tests the breakthrough sulfur content of 2 ppmw was observed at circa 15 ml/g sorbent c.f. 20 ml/g sorbent for the lab-scale experiments, which corresponds to a performance decrease by approximately 25%. It must be mentioned that the polynomial trend curves included in the plots of Figure 6 do not have any particular physical meaning and their purpose is to solely facilitate the direct visual observation of the performance difference between the two cases. Similarly to the lab-scale experiments, variation of LHSV in the range of 3 – 7.8 h-1 showed no apparent effect on desulfurization performance (Figure 7).

Fresh DIESEL: [S] = 7.1 ppmw


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PILOT-scale experiments

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LAB-scale experiments

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Figure 6. Comparison of breakthrough curves for lab-scale liquid desulfurization and scaled up pilot-scale process.

Figure 7. Comparison of breakthrough curves of pilot-scale liquid desulfurization for two different LHSV values.


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Lab-scale thermal regeneration of used untreated sorbent. Thermal regeneration of the spent sorbent was based on the evaporation and subsequent removal via entrainment of the adsorbed compounds via application of an inert flow (i.e. nitrogen). The adsorbent bed was heated up to temperatures in the range of 400-450oC, under 1 l⋅min-1 (std) N2, for approximately 1 hour. The choice of the particular temperature range was made on the basis of the diesel distillation curve depicted in Figure 3 and under the legitimate assumption that temperatures on the order or in excess of 400oC would allow safe removal of even the most refractory diesel compounds from the saturated sorbent. Experimental tests performed in the lab-scale fixed bed reactor showed that, under similar LHSV values, 3 consecutive desulfurization/thermal regeneration tests led to substantial cycle-tocycle degradation of the adsorbent bed performance (Figure 8). 8

1st deS 2nd deS


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3rd deS

6 5 -1

LHSV = 10.8 h

4 3 -1

LHSV = 10.4 h

sequent surface area measurements revealed negligible impact of sorbent’s sintering phenomena. This clearly indicates that the main contributor to the decrease in the material surface area upon successive cycles is solely attributed to phenomena associated with insufficient adsorbed diesel compounds removal and/or their pyrolysis upon application of the spent sorbent treatment. Regeneration with organic solvent (washing). Following the sorbent’s thermal regeneration experiments, the strategy of washing the spent absorbent bed with a binary mixture of organic solvents (50/50 vol% methanol/toluene) was also studied. Washing was performed for up to a total solvent mass of 50-55 Solvent-g /AC-g and was followed by bed heating up to 150-170oC under continuous inert flow (ca. 1 l⋅min-1 N2) in order to remove the remaining solvent. It is noted that the boiling points of methanol and toluene are 65 and 111oC respectively. The general concept and conditions employed regarding the particular approach were adopted from a study by Zhou et al 19. After seven desulfurization/regeneration cycles, the material showed a relatively stable desulfurization performance, although still clearly inferior to the one obtained in its fresh form, as depicted in Figure 9. More specifically, the efficiency decrease was higher than 50%, when considering the targeted breakthrough value of 2 ppmw total sulfur.

2 1

-1

LHSV = 11 h

0

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15 20 25 deS fuel (ml) / sorbent (g)

30

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40

Figure 8. Breakthrough curves of adsorption bed after three consecutive thermal regenerations. Additional batch desulfurization tests with the 3-times thermally regenerated sorbent further confirmed the significant loss of activity of the regenerated material: desulfurization performance of the 1-time regenerated sorbent was 22% lower than the corresponding performance of the fresh one. The low performance of the thermally regenerated sorbent was attributed to a nearly 45% lower specific surface area in comparison to the fresh material, as stated in Table 3. In order to lower the regeneration temperature (i.e. 350-380oC), thermal treatment was also performed under vacuum generated by the low vacuum pump installed downstream of the experimental setup. The applied vacuum (∼200 mbara) improved the sorbent’s desulfurization performance after 3 cycles, as compared to the thermal-only approach described above. Despite the substantial improvement after 3 cycles c.f. the case depicted in Figure 8 and Table 3, the regenerated material still suffered from notable cycle-to-cycle surface area reduction and therefore degradation of its performance was again profound (Table 4). Following the above reported findings, blank/thermal-only tests were also performed, in order to isolate the effect of AC sintering due to thermal processing from the quite likely phenomenon of sorbent’s pores plugging due to pyrolysis of adsorbed species and/or insufficient removal of accumulated such species by the applied regeneration methods. As clearly shown in Table 4, background tests via simple heating of the AC bed up to 420oC under nitrogen flow for 1 hour and sub-


Energy & Fuels

Table 3. Assessment of the desulfurization performance of thermally regenerated sorbent by batch experiments and corresponding measured BET-surface area values. Desulfurization performance (%)

Treatment Fresh sorbent (no treatment) Thermally treated sorbent (background) Sorbent after 1 deS & thermal regen. cycle Sorbent after 3 deS & thermal regen. cycles

90 91 73 70

Product fuel sulfur content (ppmw)

Surface area, BET (m2/g)

0.7 0.6 1.9 2.1

2059 2013 1334 1162

Table 4. Assessment of the desulfurization performance of thermally regenerated under vacuum sorbent by batch experiments and corresponding measured BET-surface area values. Desulfurization performance (%)

Treatment Fresh sorbent (no treatment) Thermally treated sorbent (background) Sorbent after 1 deS & thermal regeneration under vacuum Sorbent after 2 deS & thermal regeneration under vacuum Sorbent after 3 deS & thermal regeneration under vacuum

8

90 93 not measured not measured 80

Surface area, BET (m2/g) 2059 2018 1610 1486 1365

1st deS 5th deS (washing ratio = 52 g solvent / g AC)

7

6th deS (washing ratio = 63 g solvent / g AC)

Remaining sulphur (ppmw)

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

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6

7th deS (washing ratio = 51 g solvent / g AC)

5 4 3 2 1 0

5

10

15 20 deS fuel (ml) / sorbent (g)

25

30

Figure 9. Breakthrough curves of adsorption bed after seven consecutive desulfurization/regeneration tests with organic solvent.


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Energy & Fuels

Table 5. Assessment of desulfurization performance of the washed sorbent by batch experiments and associated liquid nitrogen adsorption/desorption characterization measurements.

Treatment Fresh sorbent (no treatment) Sorbent after 1 deS & regen. with solvent Sorbent after 6 deS & regen. with solvent Sorbent after 7 deS & regen. with solvent

Desulfurization performance (%)

Product fuel sulfur content (ppmw)

Surface area, BET (m2/g)

Pore volume (1-100nm) (cm³/g)

Mean Pore size / Mean Particle Size (nm)

90

0.7

2059

1.47

2.8 / 42

not measured

n/a

1916

1.28

2.7 / not measured

83

1.2

1788

1.21

2.7 / not measured

82

1.3

1748

1.17

2.7 / 42

After the 7-cycle process of desulfurization and regeneration, AC was also evaluated via the batch desulfurization method. The sulfur concentration of the processed fuel was 1.3 ppmw, while for the fresh AC the desulfurized diesel was measured at 0.7 ppmw, which is quite close to the performance loss identified by the fixed bed tests. From a first immediate assessment of results provided in Table 5, this can be attributed to the BET surface area reduction accompanied by a decrease in the total pore volume (Table 5), induced by phenomena already explained and experimentally demonstrated earlier in the text. Interestingly, measured mean pore size remained essentially unaffected by cyclic operation/treatment. Despite the fact that complete pore size distribution measurements were not performed, this finding provided first indication that identified desulfurization performance loss is not attributed to the loss/blocking of a certain porosity size range of the sorbent. On the other hand, neither the cumulative extent of BET surface area decrease in the course of 7 cycles performed (∼15%) nor the corresponding pore volume reduction (∼20%) can justify the almost 50% performance loss of the material. Thus, it could be claimed that additional adverse changes of sorbent’s attributes (e.g. loss of oxygen functional groups) are induced by the regeneration and subsequent mild thermal treatment processes. Further targeted studies towards identifying the precise phenomena involved are required and will be pursued in the near future in order to extract safer conclusions. In order to achieve lower vapor pressure of the compounds that still remained adsorbed after the washing with the binary organic solvent, the bed was subsequently heated at a slightly higher temperature (200oC) and under the same vacuum (200 mbara). Such a strategy led to better results (Figure 10), however surface area and pore volume of regenerated material were still measured to be lower than the fresh one (Table 6), following an essentially identical pattern with the trend identified in the previous, slightly different approach (i.e. Figure 9/Table 5). Consequently, desulfurization was measurably inferior to the fresh material. It is, however, worth mentioning that after 4 cycles and in the course of the 11 cycles performed in total, the efficiency of the regenerated sorbent remained relatively stable. The targeted 2 ppmw value was achieved at approx. 10 ml diesel / g AC vs 20 ml diesel / g AC for the case of the fresh material, indicating in one hand a measurable improvement c.f. the case depicted in Figures 8 and 9 but in the other hand a 50% loss of performance. This may be attributed to the slightly higher heating

temperature, which must have led to more efficient removal of residual heavy diesel compounds, still retained in the AC structure after its washing. However, the surface area loss issue due to diesel species pyrolysis and/or insufficient removal upon thermal treatment still played dominant role. In fact, by comparing the two latter, slightly different, regeneration approaches it can be safely concluded that the major performance loss factor is gradual decrease of available specific surface area due to cycle-to-cycle decrease of the material’s pore volume and possibly alteration of its surface chemistry. Therefore, a highly efficient regeneration strategy (i.e. complete restoration of initial/fresh performance) should preferably not involve thermal treatment steps. It is also worth mentioning that the organic phase of the spent solvent is in-principle re-usable after simple distillation. This was proven via several simple distillation tests (heating up to 120oC) performed, with the measured purity of the obtained methanol/toluene distillate being ∼99.5% (i.e. very close to the initial purity specifications of the respective fresh/as-acquired from the supplier solvents). The disposable heavy residual liquid (mostly diesel) corresponded to 9-13% of the total distilled volume and contained approximately 20 ppmw of total sulfur. Samples from fresh AC and 1-time regenerated sorbent (according to the protocol applied to tests shown in Figure 10) were subjected to TGA analysis under air. The analysis was performed at a heating rate of 5oC/min and up to 600oC. The corresponding curves are shown in Figure 11. As expected, both samples exhibited notable weight loss due to their oxidation. Evidently, the main reaction phase was triggered at temperatures above 380oC. However, the regenerated sample exhibited additional weight loss in the range of 190-380oC. Arguably, this is due to the presence of different - and apparently more readily oxidizable c.f. the fresh sample - carbon species for the case of the regenerated AC. Such species are most likely attributed to residual organics retained inside the structure of the sorbent after regeneration, as also commented earlier in the text (see Table 4). Thus, the comparative TGA analysis in Figure 11 provides additional experimental confirmation for the legitimacy of the phenomena claimed to be associated with the partial only restoration of the sorbent’s performance upon cyclic desulfurization/regeneration.


Energy & Fuels

Figure 10. Breakthrough curves of adsorption bed after eleven consecutive regenerations with organic solvent followed by heating at 200oC under vacuum.

Table 6. Desulfurization performance of the washed sorbent in batch desulfurization experiments. Treatment

Sorbent’s surface area, BET (m2/g)

Fresh sorbent (no treatment) 1 cycle 2 cycles Sorbent after 3 cycles deS & regen. 4 cycles with solvent 5 cycles under vacuum 6 cycles 8 cycles 10 cycles

2059 1782 1737 1616 1747 1713 1632 1496 1414

Pore volume (0-100nm) (cm³/g) 1.47 1.32 1.33 1.23 1.17 1.16 1.10 1.08 1.03

Mean Pore size (nm) 2.8 3.0 3.1 3.1 2.7 2.7 2.7 2.9 2.9

100% 90% 80%

Weight loss (%)

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

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70% 60% 50% 40%

Fresh AC

30%

1-time regenerated AC

20% 10% 0% 0

100

200

300

Temperature

400

(oC)

Figure 11. Comparative TGA analysis under air of fresh and 1-time regenerated AC sorbent.


500

600

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Energy & Fuels

4. CONCLUSIONS Adsorptive desulfurization under lab-scale environment at ambient conditions with a certain grade of activated carbon as active material led to the production of fuels with very low sulfur content (≤ 2 ppmw) at processed fuel capacities on the order of 20 ml diesel / g AC. When the process was scaled up by a factor of 15, the AC desulfurization performance, for a targeted total sulfur breakthrough value of 2 ppmw, was measured to be approximately 25% lower. The evaluation of several regeneration strategies of the spent sorbent showed that thermal treatment only – performed under nitrogen flow and at up to 450oC to cause the residual fuel removal by evaporation – of the spent AC was not efficient since the surface area of the material decreased significantly. This degradation had a detrimental effect on the subsequent desulfurization performance of the thermally regenerated material. The most efficient regeneration strategy, despite the fact that even in this case initial performance of the material could not be fully restored, involved spent bed washing with a binary mixture of organic solvents and its subsequent heating under vacuum at approximately 200oC, in order to achieve evaporation of the remaining solvent. However, such a strategy is quite demanding both in terms of solvent required amounts (approximately 50 g solvent / g AC) and complexity when considering cyclic semicontinuous operation of a potential high throughput real application/system. As demonstrated by relevant experiments, the main reason for the observed irreversibility with respect to the performance of the regenerated AC is not due to sintering induced by thermal treatment. The measured decrease in terms of the material’s surface area/pore volume is very likely to be caused by small amounts of residual fuel species retained in its structure after regeneration, with cycle-to-cycle further accumulation of such species being quite likely. The additional negative effect of the sorbent’s surface chemistry partial alteration (e.g. loss of oxygen-related and/or other functional groups due to the applied regeneration methods) cannot be excluded. Such findings in conjunction with the measurable, albeit not severe, negative effect of 15-times scaling up, proves that further studies are required towards optimization of both process scaling up and (most importantly) sorbent’s regeneration/reusability. The improvement of the desulfurization performance observed upon chemical modification of the AC was marginal or non-existent. However, it would be interesting to also evaluate the performance of such sorbents during sequential desulfurization – regeneration cycles in order to investigate potential differences with respect to regeneration potential c.f. the unmodified material.

REFERENCES [1] S. Martin; G. Kraaij; T. Ascher; P. Baltzopoulou; G. Karagiannakis; D. Wails; A. Wörner. Int. J. Hydrogen Energ. 2015, 40, 75. [2] F. van Looij; P. van der Laan; W.H.J. Stork; D.J. DiCamillo; J. Swain. Appl. Catal. A-Gen. 1998, 170, 1-12. [3] Y. Wang; R.T. Yang. Langmuir 2007, 23, 3825. [4] A.J. Hernandez-Maldonado; R.T. Yang., Ind. Eng. Chem. Res 2003, 42, 3103. [5] A.J. Hernandez-Maldonado; R.T. Yang. AIChE J. 2004, 50, 791. [6] R.T. Yang; A.J. Hernandez-Maldonado; F. Yang. Science 2003, 301, 79. [7] A.J. Hernandez-Maldonado; R.T. Yang. Cataly. Rev., 2004, 46, 111. [8] S. Hernandez; L. Solarino; G. Orsello; N. Russo; D. Fino; G. Saracco; V. Specchia. Int. J. Hydrogen Energ. 2008, 33, 3209. [9] S.P. Hernandez; D. Fino; N.Russo. Chem. Eng. Sci. 2010, 65, 603. [10] C.S. Shalaby; S.K. Saha; X. Ma; C. Song. Appl. Catal. BEnviron. 2011, 101,718. [11] J.A.Z. Pieterse; S. van Eijk; H.A.J. van Dijk; R.W. van den Brink. Fuel Process. Technol. 2011, 92, 616. [12] M. Muzic; Z. Gomzi; K.S. Bionda. Analysis of continuous fixed bed adsorptive desulfurization of diesel fuel, Univ. of Zagreb, Faculty of Chem. Eng. & Techn., Zagreb, Croatia, in: http://ipec.utulsa.edu/Conf2009/Papers%20received/Muzic_3.pdf (last accessed in 20/05/2015). [13] K.S. Triantafyllidis; E.A. Deliyanni. Chem. Eng. J. 2014, 236, 406. [14] J.A. Arcibar-Oronzo; J.R. Rangel-Mendez; T.J. Bandosz. Energ Fuel. 2013, 27(9), 5380. [15] M. Muzic; K.S. Bionda; T. Adzamic; Z. Gomzi; S. Podolski; Optimization of diesel fuel desulfurization by adsorption on activated carbon, Univ. of Zagreb, Faculty of Chem. Eng. & Techn., Zagreb, Croatia, in: http://www.aidic.it/icheap9/webpapers/98Muzic.pdf (last accessed in 20/05/2015). [16] J.C. Hoguet; G.P. Karagiannakis; J.A. Valla; C.C. Agrafiotis; A.G. Konstadopoulos. Int. J. Hydrogen Energ. 2009, 34, 4953. [17] C.S. Shalaby; X.L. Ma; C.S. Song. In Production and Purification of Ultraclean Transportation Fuels; Y.H. Hu, X. Ma, E.B. Fox, X. Guo, Eds.; American Chemical Society: Washington DC, 2011; Chapter 4, 55-62. [18] C.O. Ania; T.J. Bandosz. Carbon 2006, 44, 2404. [19] A. Zhou; X. Ma; C. Song. J. Phys. Chem. B, 2006, 110, 4699.

AUTHOR INFORMATION * E-mail: [email protected]; [email protected]

ACKNOWLEDGMENTS The authors thank the European Commission for partial funding of this work via the NEMESIS 2+ project; Fuel Cells and Hydrogen Joint Undertaking (FCH JU): New Method for Superior Integrated Hydrogen Generation System 2+ (Grant Agreement No: 278138).


Diesel Fuel Desulfurization via Adsorption with the Aid of Activated

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