Article pubs.acs.org/IECR
Isomerization of n‑Butane over SO42−/Al2O3−ZrO2 in a Circulated Fluidized Bed Reactor: Prospects for Commercial Application Pengzhao Wang, Minxiu Zhang, Wenfang Zhang, Chaohe Yang, and Chunyi Li* State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266555, P.R. China S Supporting Information *
ABSTRACT: The stability of alumina-promoted sulfated zirconia (SZA) was investigated to achieve the isomerization of n-butane in a circulating fluidized bed (CFB) unit. The pilot-scale evaluation in a CFB unit showed high stability of the SZA catalyst and that the catalytic activity was dominated by the residence time of nbutane rather than its linear velocity. Increases in the reaction and regeneration temperature both led to an increase in the conversion of n-butane and a decrease in the selectivity to isobutane, caused by increasing side reactions. Although the regeneration was conducted in air, a trace of SO2 evolved during the regeneration, which could be minimized at the appropriate gas stripping temperature, low regeneration temperature, and high space time of the feed. Compared with conventional fixed-bed technologies, the CFB process shows lower selectivity to isobutane due to the inevitable axial back-mixing and severe “dimerization-cracking” reaction.
1. INTRODUCTION Due to the increasingly stringent restriction on the content of toxic alkenes and aromatics in motor fuels, there has been a significant demand for higher quality fuels to increase the octane number (ON) in gasoline.1 The alkylate, synthesized from isobutane alkylation of olefins, is the most promising additive for ON upgrading.2,3 With the rapid development of C4 alkylation, isobutane is in short supply in refineries, which essentially limits its application as the raw material for alkylation. Thus, n-butane isomerization has attracted considerable research attention to boost the isobutane production.4−7 Over the past decades, a number of isomerization technologies have been commercialized with different catalysts and operating at different reaction temperatures.8 As far as we know, all of the existing alkane-isomerization technologies are based on fixed-bed technology and are catalyzed by noble metal-promoted catalysts. As an acid-catalyzed reaction, the alkane skeletal isomerization is thermodynamically favored at low reaction temperatures.9 Despite the numerous patents and scientific contributions, the Butamer process commercialized by UOP is the only prevailing technology for n-butane isomerization at low temperatures, which employs Pt-promoted chlorinated alumina (Pt−Cl/Al2O3) as catalyst.8 Nevertheless, the progress of the Butamer process is impeded by several serious problems: (1) continuous supplement of toxic chlorinated compounds during the reaction to maintain constant activity of the catalyst; (2) equipment corrosion and product contamination; (3) negative environmental impacts related to chlorine loss and corrosive acid recovery. Later, a Pt/Pd-promoted sulfated zirconia catalyst (Pt/PdSZ) was found to exhibit comparable activity with the Pt−Cl/ Al2O3 catalyst in n-butane isomerization.10−13 Several technologies based on the Pt/Pd-SZ catalysts have been developed, such as the Isomalk process by JSC SIE Neftehim and the Par© 2017 American Chemical Society
Isom process by UOP. The noble metals introduced to the catalyst, as well as the hydrogen added to the feed, prevent carbon deposition and achieve a long service life. However, the noble metal (Pt/Pd) is extremely sensitive to sulfur and nitrogen in the feed. For example, the allowed content of water is ≤0.1 ppm, that of nitrogen it is ≤0.1 ppm, and that of sulfur ≤0.5 ppm in the Butamer process.14 Besides, the hydrogen circulation is highly energy-intensive and potentially dangerous in a fixed-bed process at high reaction pressure. Therefore, it is essential to develop a new isomerization catalyst and corresponding technology that are low-cost and environmentally friendly. Many researchers have focused on the non-noble metal modified sulfated zirconia (SZ) and found that the SZ catalyst shows considerably high activity in n-butane isomerization. However, the SZ catalyst suffers from fast deactivation under industrial reaction conditions.15−18 Several researchers have claimed that deactivated SZ catalysts can be regenerated at aerobic atmosphere, but no attempts have been made in a circulating fluidized bed (CFB) to achieve a continuous reaction-regeneration process for n-butane isomerization over an SZ catalyst. The main unknowns for a CFB isomerization process over a SZ catalyst are the regeneration properties and sulfur stability of the catalyst during long-term running. By using an online mass spectrometer, it has been confirmed that no sulfate species escaped during the isomerization reaction of n-butane over SZA catalyst.18 However, the regeneration of the SZA catalyst must be carried out at a temperature higher than reaction Received: Revised: Accepted: Published: 8456
May 3, 2017 June 11, 2017 July 10, 2017 July 10, 2017 DOI: 10.1021/acs.iecr.7b01858 Ind. Eng. Chem. Res. 2017, 56, 8456−8464
Article
Industrial & Engineering Chemistry Research
adsorption−desorption measurements were carried out to determine the Brunauer−Emmett−Teller (BET) specific surface area and pore properties of the catalysts with a Quadrasorb SI instrument at −196 °C. Prior to the measurements, the samples were evacuated at 350 °C for 4 h at 5.3 × 10−4 kPa for the complete removal of adsorbed moisture. Sulfur content measurements and Py-FTIR experiments were conducted as described in the literature.18 The attrition loss of the SZA catalyst for pilot-scale experiments was determined by ASTM standard (ASTM D5757-00 (2006) Standard Test Method for Determination of Attrition and Abrasion of Powdered Catalysts by Air Jets). 2.3. Catalytic Activity Tests. 2.3.1. n-Butane Isomerization in a Fixed-Bed Microreactor. n-Butane-isomerization tests conducted in a fixed-bed microreactor followed the same protocols reported in our previous study.19 Approximately 1.0 g of catalyst powder (80−180 mesh) was placed in the middle of a down-flow microreactor and activated at 350 °C in flowing air for 30 min. Subsequently, the n-butane (purchased from Keyuan Gas Co., Ltd.) was fed into the reactor with hydrogen as diluent gas (H2:C4° = 0.6:1), and then the isomerization reaction occurred at 200 °C under ambient pressure (WHSV = 1.98 h−1). 2.3.2. n-Butane Isomerization in a Pilot-Scale Circulating Fluidized-Bed Unit. As shown in Figure 1, the pilot-scale CFB
temperature, so in this study we focused on the stability of sulfate species at higher temperature. Specifically, the calcination temperature, regeneration performance, and thermal stability of SZA were investigated. Subsequently, the stability and activity of the SZA catalyst were evaluated in a pilot-scale CFB unit. The loss of sulfur during regeneration is discussed in detail, and the prospects of commercial application of a SZA catalyst in a CFB unit are assessed.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Al-promoted sulfated zirconia (SZA) was synthesized by sulfating a mixing of zirconium and aluminum hydroxide. First, pseudoboehmite powder (AR. 67 wt %) was mixed with distilled water in a vessel to obtain a suspension. Then, HCl solution (36−38 wt %) was added dropwise under vigorous stirring at 70 °C. After the formation of an aluminum hydroxide gel, an appropriate amount of highly hydrated nanosized zirconium hydroxide (30 nm, 48.2 wt % of ZrO2), purchased from Xuancheng Jingrui New Material Cl. Ltd., was added. The mixture was stirred for 2 h, and then sulfuric acid (AR. 98 wt %) was added dropwise under vigorous stirring. The weight ratio of ZrO2, Al2O3, and sulfuric acid was 78:4:18. Subsequently, the sulfated mixture was stirred for another 2 h, dried at 110 °C for 12 h, and calcined at 550−750 °C for 2 h in air. Finally, the obtained samples were crushed and sieved to 80−180 mesh for later use. The catalysts calcined at different temperatures are designated as SZAx, where x represents the calcination temperature. The catalysts used in the pilot-scale evaluation were prepared by spray granulation with the same material and feed ratio as described above. After 2 h of stirring, the mixed slurry was pumped to an atomizer at the top of the drying chamber at a pressure of about 130 kPa. The atomizer sprayed the slurry into a high-velocity stream of hot air at 500 °C, producing droplets. As the droplets passed through the hot air flow, the water rapidly evaporated. The large particles fell to the bottom of the chamber and were collected. Fine particles entrained with the exhaust were generally collected by passing air through a series of external cyclones. Then, the spray granulated catalyst was calcined at 650 °C for 30 min in a rotary furnace and 2 h in a muffle furnace at air atmosphere. The physicochemical properties of the SZA catalyst are shown in Table 1. 2.2. Catalyst Characterization. The zirconia phase of catalysts calcined at different temperatures was analyzed by an X’pert PRO MPD diffractometer (PANalytical Company, Netherlands) with Cu Kα radiation at 40 kV and 40 mA, running from 5 to 75° at a speed of 10°/min. Nitrogen Table 1. Physicochemical Properties of the SZA Catalyst for Pilot-Scale Experiments item packing density (g/cm3) SBET (m2/g) Vpore (cm3/g) sulfur content (wt %) chloride content (wt %) attrition loss (wt %/h) particle size distribution (wt %) 0−80 μm 80−180 μm >180 μm
value
Figure 1. Schematic diagram of the pilot-scale CFB unit.
1.38 103 0.118 2.20 0.039 2.75
unit consists of a reactor (with a length of 3 m and inner diameter of 92 mm), a regenerator (with a length of 1.5 m and inner diameter of 50 mm), and a delivering riser (with a length of 9 m and inner diameter of 16 mm). The reactor and regenerator are both coupled with a disengager and a gas stripping section. On the top of the disengager, a set of porous ceramic filters was used to separate the gaseous products and the catalyst fines. During the tests, the n-butane was fed into the
9.8 75.9 14.3 8457
DOI: 10.1021/acs.iecr.7b01858 Ind. Eng. Chem. Res. 2017, 56, 8456−8464
Article
Industrial & Engineering Chemistry Research bottom of the reactor and reversely contacted with regenerated catalysts. After oil−gas separation in the disengager, the gaseous products were collected. After reaction, the spent catalysts flowed down into the gas stripping section. A flow of nitrogen (200 L/h) was introduced into the bottom of the stripping section to facilitate the desorption of entrained and adsorbed hydrocarbons on the catalyst during the reaction. Subsequently, the catalysts were pneumatically conveyed to the regenerator through the riser. Both in riser and regenerator, the spent catalysts were regenerated by burning off the carbon deposition and oxidizing the sulfite species to sulfate species by the flowing air at the rate of 550 L/h. Then, the high-temperature regenerated catalysts were cooled to reaction temperature and delivered into the reactor for continuous operation. To achieve the fluidization of catalyst, the pressures in the regenerator and reactor were 112 ± 0.5 and 107 ± 0.5 kPa, respectively, unless stated otherwise. A computer control system was employed to show and regulate the operation parameters. The compositions of feed and products were analyzed by a Bruker 450 gas chromatograph as reported in our earlier study.18 The concentrations of O2 and SO2 released in regeneration gases were determined by a TESTO 350 EPA flue gas analyzer. The n-butane was provided by Qingdao Airox Gases & Chemicals Co., Ltd., and its composition is given in Table S1 in the Supporting Information.
Figure 3. XRD patterns of different samples: (clovers) tetragonal ZrO2; (diamonds) monoclinic ZrO2.
the 550 °C calcined catalyst is mainly amorphous. Increasing calcination temperature from 550 to 650 °C, the samples gradually crystallized to tetragonal zirconia, while upon 650 °C, a small fraction of tetragonal zirconia is transformed to the monoclinic phase. Several researchers have argued that only a tetragonal zirconia-based catalyst exhibits good activity for alkane isomerization, while the thermodynamically stable monoclinic zirconia is catalytically inactive.20,21 So we conclude that the calcination temperature affects the crystalline phase of zirconia, which in turn leads to the difference in catalytic activity. The acid properties of the different samples were determined by Py-FTIR experiments (Figure 4). The amount of Brønsted
3. RESULTS AND DISCUSSION The biggest challenge for the use of sulfated zirconia in the CFB process is its thermal stability during the continuous reaction−regeneration cycles. For that matter, the effects of calcination temperature and long-periodic thermal treatment of SZA catalyst on the catalytic properties of n-butane isomerization were studied first. 3.1. Effect of Calcination Temperature. As shown in Figure 2, the conversion of n-butane increased with the
Figure 4. FT-IR spectra of adsorbed pyridine on SZA catalysts calcined at different temperatures. Characteristic bands for hydrogenbound pyridine (H), pyridine adsorbed on Brønsted (B), Lewis (L), strong Lewis (SL), and weak Lewis (WL) acid sites are indicated. Figure 2. Catalytic performance of n-butane isomerization over SZA catalyst calcined at different temperatures.
acid sites reached a maximum at 650 °C, while the Lewis acid sites always increased with elevating the calcination temperature. The development of Lewis acidity behaves noticeably differently below and above 650 °C. In the range 550 to 650 °C, the strong Lewis acid sites (1610 cm−1) dominate. Increasing the calcination temperature beyond 650 °C, a new type of Lewis acid sites with weak strength (1575 cm−1) appears, with a concurrent increase in hydrogen-bound pyridine (1595 cm−1). Here, two questions arise, one is how the weak Lewis acidity is generated. The other is why the sulfur loss at 550−650 °C (see Table 2) causes a decrease in the number of
calcination temperature of catalyst from 550 to 650 °C and then decreased at higher temperature, and the opposite trend is observed for the selectivity to isobutane. To shed light on the intrinsic relationship between calcination temperature and catalytic performance, the structure and surface properties of catalyst samples were characterized. The XRD patterns of all samples are plotted in Figure 3. Prepared by sulfating amorphous zirconium-alumina hydroxide, 8458
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order to meet the requirement of regeneration operation in the CFB unit. Our previous study showed that subjecting the fresh SZA catalyst to thermal treatment at 650 °C resulted in a continuous decrease in isomerization activity, and even at 600 °C, a small amount of sulfate species decomposed from the surface after 100 h calcination.30 In this study, decreasing the calcination temperature to 550 °C, the catalyst exhibited a satisfactory thermal stability (Figure S1 in the Supporting Information). That is, the regeneration temperature should not be higher than 550 °C to avoid the loss of sulfur. So, the deactivated catalysts in the fixed-bed microreactor tests were regenerated at 550 °C. As shown in Figure S2 in the Supporting Information, the SZA catalyst demonstrated a very good stability after 15 times of reaction−regeneration cycles in nbutane isomerization. 3.3. Pilot-Scale Studies of n-Butane Isomerization on the SZA Catalyst in a CFB Unit. 3.3.1. Isomerization Behavior under Different Reaction Conditions. Despite the good isomerization reaction−regeneration stability of the SZA catalyst in the fixed-bed tests, the situation may be different when the reaction is carried out in the CFB bed. A pilot-scale CFB unit patented by our research group was used to evaluate the SZA catalyst for n-butane isomerization. In a fixed-bed alkane-isomerization process, a flow of hydrogen is generally introduced into the reactor to slow down the deactivation of the catalyst.31 Different from the fixed-bed, in the CFB, a continuous reaction−regeneration operation is applied. Considering the high risk and cost of hydrogen circulation in the CFB process, the reaction was performed without carrier gas except for a small flow of nitrogen used as a back flush of the instrument tube and stripping gas. Effects of Space Time and Linear Velocity of the Feed. As expected, the conversion of n-butane greatly increased and the selectivity to isobutane decreased with increasing space time (Figure 5a), while the linear velocity of the gaseous phase has
Table 2. Textural and Surface Properties of the Catalyst Samples Sample
Sulfur (wt %)
SBET (m2/g)
Dpore (nm)
Vpore (cm3/g)
B/L ratio
S coverage (nm−2)
SZA550 SZA600 SZA650 SZA700 SZA750
5.76 3.57 2.14 1.67 1.17
133.8 113.9 106.8 92.4 78.0
4.12 4.04 4.35 5.00 5.74
0.138 0.115 0.116 0.116 0.112
7.03 4.80 3.61 1.12 0.53
8.1 5.9 3.8 3.4 2.8
Brønsted acid sites, given that it is well accepted that the sulfation treatment of zirconia creates the Brønsted acidity on the surface, which is indispensable for n-butane isomerization over sulfated zirconia.22−25 As shown in Table 2, the specific surface area and sulfur content both decrease with increasing calcination temperature as expected. Notably, the concentration of sulfate species on the catalyst surface declines quickly, ranging from 8.1 to 2.8 S atoms per nm2 with the increase in calcination temperature, indicating that the loss of sulfate species is more crucial than the decrease of surface area. Specifically, the current experiments show that the SZA650 catalyst with monolayer sulfate species (i.e., four S atoms per nm2)26 has the optimal isomerization activity. It was reported that the strong interaction between sulfate species and zirconia inhibited the reduction of the specific surface area and stabilized the active tetragonal zirconia during calcination.27 This interaction strength is strongly dependent on the coverage of sulfate species after calcination. When the monolayer coverage is reached, the next sulfate moieties, presumably attached though hydrogen bonding on the surface,28 are labile and readily decompose with the increase in the calcination temperature. That is, when the calcination temperature is increased from 550 to 650 °C, these labile sulfate species readily desorb from the surface, and tetragonal zirconia is formed. The remaining sulfate species strongly chemisorb on the tetragonal zirconia surface and induce the generation of Brønsted acid sites via the strong electron-withdrawing effect of the SO bonds (Figure 4). However, when the calcination temperature is higher than 650 °C, the chemisorbed sulfate species gradually desorb, causing a decrease in the amount of Brønsted acid sites, and some of the unsaturated Zr4+ is exposed to the surface and acts as weak Lewis sites. Thus, SO42− coverage on the zirconia surface determines the catalytic acidity and activity of the catalyst, which well agrees with the finding reported by Morterra et al. that the ratio between Brønsted and Lewis acid sites strongly depended on the SO42− coverage.29 Consequently, our results demonstrate the effects of the concentration of sulfate species and structural evolution on the isomerization activity as a function of calcination temperature. When elevating the calcination temperature, the loss of the sulfate species is inevitable. The optimal activity of n-butane isomerization is obtained when the SZA catalyst is calcined at 650 °C, which can be attributed to the pure tetragonal zirconia, the monolayer coverage of sulfate species, and the maximum amount of catalytic Brønsted acid sites. As a result, we suggest that the regeneration of deactivated SZA catalyst must be carried out at a temperature lower than 650 °C. 3.2. Thermal Stability and Regeneration Properties in the Fixed-Bed Microreactor. Although the fresh SZA catalyst achieves its maximum activity after calcination at 650 °C, its long-term thermal stability should also be investigated in
Figure 5. n-Butane-isomerization behavior at different (a) space times (Treaction = 190 °C, P = 107 kPa, u = 0.08−0.25 m/s), and (b) apparent linear velocities of the feed (Treaction = 190 °C, P = 107 kPa, τ = 2.0 h).
little influence on the catalytic activity (Figure 5b). In this study, the different linear velocities were obtained by increasing the catalyst inventory in the reactor and the feed rate proportionally at constant space time. That is, the residence time of the gaseous phase did not change, while the turbulence of the bubbling bed became more intensive with the increase in linear velocity. The almost unchanged conversion of n-butane 8459
DOI: 10.1021/acs.iecr.7b01858 Ind. Eng. Chem. Res. 2017, 56, 8456−8464
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Figure 6. n-Butane-isomerization behavior under different reaction and regeneration temperatures: (a) reaction properties versus reaction temperature; (b) isomerization index versus reaction temperature; (c) reaction properties versus regeneration temperature; (d) molar ratio of propane/pentane versus (red circles) reaction temperature and (blue squares) regeneration temperature.
understood that the increase in reaction temperature accelerates the formation of intermediates, bringing about an increase in the concentration of the unsaturated intermediates and promoting bimolecular reactions.19 However, the development of the selectivity to isobutane under different regeneration temperatures must be due to the different natures of the active sites on the surface. The deactivation of the SZA catalyst is mainly caused by the reduction of sulfate to sulfite species with the concurrent oxidative dehydration (ODH) of n-butane to butene and coke deposition on the surface since the feed used in this CFB study contains a small fraction of olefins (Table S1 in the Supporting Information).13,33−36 The carbon deposits on the spent and 450 °C regenerated SZA catalyst were 0.10 and 0.06 wt %, respectively, while they were almost completely removed after 550 °C regeneration (1.5 0.03−0.07 0.06−0.10 0.9−1.2 1.5 50−60 50−55 98 90−91 3−5 2−3 in the feed, ppm 0.1 ≤20 0.1 2−5 0 Unknown 0.1−0.5 1
This study SO42−/ZrO2− Al2O3
CFB No 160−200 0.1−0.15 0 0.2−0.5 52−62 72−82 >1 Unknown
Reproduced with permission from ref 11.
the conventional Butamer and Isomalk-3 processes are compared with the estimations of the CFB isomerization process on the prospective SO42−−ZrO2−Al2O3 catalyst. Our CFB isomerization process has a number of important advantages over technologies based on the Pt-promoted catalysts: (a) The prevalent chlorinated alumina catalyst suffers from the loss of active chloride. Thus, continuous chloride addition is required to maintain the catalyst acidity and isomerization activity. Accordingly, a caustic scrubber for dry gas alkali treatment is required to neutralize HCl in 8462
DOI: 10.1021/acs.iecr.7b01858 Ind. Eng. Chem. Res. 2017, 56, 8456−8464
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Industrial & Engineering Chemistry Research the off-gas and eliminate unit corrosion.43 The SZA catalysts employed in our CFB isomerization process have no sulfur loss during the reaction and do not require the caustic components. Thus, operating costs, related to the chloride addition, caustic supply, and spent caustic disposal, are saved. (b) Loss of sulfur during regeneration at a very slow rate (
