Efficient Coke Inhibition in Supercritical Thermal Cracking of

Jun 19, 2017 - ... were conducted during the supercritical thermal cracking of RP-3 Chinese jet fuel with a flow rate of 1 g/s for 30 min at 700 °C a...
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Efficient Coke Inhibition in Supercritical Thermal Cracking of Hydrocarbon Fuels by a Little Ethanol over a Bifunctional Coating Zhenning Yang,† Guozhu Li,*,†,‡ Hua Yuan,† Guozhu Liu,†,‡ and Xiangwen Zhang†,‡ †

Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China ‡ Collaborative Innovative Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China ABSTRACT: Coke inhibition is one of the key issues for hydrocarbon fuel cracking. In the work reported in this paper, controllable cracking with greatly reduced coke deposits has been realized by the addition of a little ethanol over a bifunctional coating. The coating, consisting of perovskite and phosphotungstic acid, is prepared in a nickel-based super alloy tube reactor (diam. 3 mm × 0.5 mm × 1000 mm) by the wash-coating method. Scanning electron microscopy (SEM), energy-dispersive Xray spectroscopy, and X-ray diffraction are utilized to characterize the morphology and phase composition of the coating and cokes. The results show that BaWO4, BaCeO3, SiO2, and H3PW12O40 coexist in the 4.2 μm coating with a uniform distribution. The anti-coking tests were conducted during the supercritical thermal cracking of RP-3 Chinese jet fuel with a flow rate of 1 g/s for 30 min at 700 °C and 4 MPa. The results show that the efficiency of coke inhibition reaches up to 96%, and the stability (i.e., pressure drop of tube reactor and cooler) of the system has been effectively improved. The deposited cokes were characterized by temperature-programmed oxidation and SEM. The pyrolysis products, including gas and liquid, were also analyzed. The results indicate that the strategy based on ethanol and a bifunctional coating not only plays an important role in eliminating the coke deposits on the reactor tube walls but also reduces the amount of typical coke precursors related to the aromatic condensation cokes. A possible mechanism for the process has been proposed. In general, phosphotungstic acid in the coating is capable of catalyzing the dehydration of ethanol for the production of water. Meanwhile, the perovskite structures can remove coke deposits on the coating through carbon−steam gasification reaction.

1. INTRODUCTION In recent decades, liquid hydrocarbon fuels have drawn much attention as part of the evolution of advanced aircraft because they can be utilized not only as the propellant to meet the demand for flight power but also as active coolants to remove the waste heat from aircraft via supercritical thermal cracking.1−7 Coke as the byproduct of cracking is undesirable and inevitable. Coke deposition may lead to the deterioration of heat transfer ability, degradation of the mechanical properties of the assembly unit, and even system failure. Therefore, the study of coking during the cracking of hydrocarbon fuels is an important subject for the stable operation of aircraft for long durations and at high speeds.8−12 Researchers have made great efforts to investigate the mechanism of coke formation and figure out corresponding solutions to this issue. One of the most effective tools for coke inhibition is the application of inert coatings9,13−15 that can cover the catalytic metal sites to prevent their interactions with coke precursors. Liu et al.15 prepared a series of alumina coatings with different thicknesses (318−1280 nm) in an SS321 tube (2 mm i.d.) by metal−organic chemical vapor deposition (MOCVD) and achieved anti-coking efficiency up to 69% during thermal cracking of Chinese RP-3 jet fuel under supercritical conditions. Similarly, Wang et al.16 utilized MOCVD to prepare a TiN coating in an SS3304 tube to inhibit coke deposits during n-hexane cracking. Feng et al.10 prepared alumina films inside the channel of a stainless steel tubular reactor by atomic layer deposition (ALD) for coke suppression in the thermal cracking of C12−C16 paraffins. © XXXX American Chemical Society

Some alkaline coatings can also effectively suppress cokes due to their catalytic activity for the water−carbon reaction. With the addition of water in the system, cokes can be gasified near the alkaline sites continuously. Huang et al.5 used CsOH coating and water (2 wt%) to reduce cokes by more than 10 times in the thermal cracking of a model fuel similar to JP-7. They then17 replaced water with soluble alcohol and acid catalyst to suppress coking via alcohol dehydration. However, heterogeneous solid acid catalysts are not welcomed due to their insolubility, which makes the system unstable. Moreover, Van Geem et al.18 showed that perovskite coating can effectively inhibit coking by 76% during the steam cracking of ethane at industrially relevant conditions. Recently, bifunctional coatings have been of great interest owing to their dual active sites, which endow the surface new functions via the synergy of different sites. For instance, both acid and base sites can coexist in one coating,19,20 which is suitable for novel catalytic applications. Herein, a new bifunctional coating consisting of perovskite and phosphotungstic acid (HPW) has been developed and employed for coke inhibition. Thermal cracking of fuel with the addition of a little alcohol has been carried out over the as-prepared coating. The process and the coating have been fully characterized, and a corresponding mechanism has been proposed. It is hoped that HPW in the coating is capable of catalyzing the dehydration of ethanol for the production of ethylene and water. Meanwhile, Received: May 26, 2017 Published: June 19, 2017 A

DOI: 10.1021/acs.energyfuels.7b01505 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Scheme 1. Schematic Diagram of Experimental Apparatus

1.5405 Å). Adhesion of the bifunctional coating was assessed via an ultrasonic vibration test and a thermal shock test. For the ultrasonic test, the coated tube was immersed in the RP-3 fuel, subjected to ultrasound in an ultrasonic vibrator for 40 min at 60 kHz, and then washed with n-pentane and dried completely. The weight loss of the tube was measured. The thermal shock test was performed by heating the coated tubes to 800 °C in 1 min (DC electric heated), holding at that temperature for 2 min, and then immediately cooling down to room temperature. This process was repeated three times, and finally the weight loss of the tube was measured.23 2.4. Thermal Cracking Operation Apparatus and Analysis Procedure. To evaluate the anti-coking performance of the coating, thermal cracking of RP-3 Chinese jet fuel was conducted in an endothermic fuel single-tube reactor simulator. As shown in Scheme 1, the bench-scale simulator consisted of a fuel flow system, a computer control system, an electric heat reactor system, and a product sampling and online analysis system. Through this simulator, the endothermic hydrocarbon fuel performance can be determined, such as overall sink, cracking products, coke deposition, and maximum run duration under certain conditions. Before the test, nitrogen was used to blow and replace the air in the tube. The fuel was then introduced into the reacting system at supercritical pressure through a high-performance liquid chromatography (HPLC) P500 pump (Dalian Elite Analytical Instruments Co., Ltd.) which can keep a precise flow under a maximum pressure of 20 MPa. A precise flowmeter (Siemens, MASS 6000) was also used to ensure the accuracy of the hydrocarbon fuel flow rate. A computer-assisted process control and data transmitting system (King View) was used in the simulator for safe test operation and data collection. The data were acquired at a frequency of 1 Hz. Furthermore, pressures were monitored by the King View software program, include the pressure of the reactor tube inlet and outlet and the pressure drop of the cooler. In the electric heat reactor system, the electrically heated tube reactor was made in the section of coating preparation. A dc-stabilized power supply was used to provide direct current with two copper bars fixed outside the tube. After cracking in the reactor tube, the mixture flowed into a water-cooled heat exchanger to be quenched to about 25 °C, and then into a gas− liquid separator after a back-pressure valve (Beijing Xingda Technology, China) which was utilized to keep the pressure of each experiment in this work at a constant 4 MPa. The fuel outlet temperature was measured by K-type thermocouples inserted into union cross junctions. Wall temperatures of the tube reactor were also measured by 12 K-type thermocouples with different

the perovskite structures in the coating can suppress coke deposition via carbon−steam gasification using the obtained water.

2. EXPERIMENTAL SECTION 2.1. Materials. Ethanol (99.9% purity), n-pentane (99%), and phosphotungstic acid (99.9%) were purchased from JiangTian Chemical Reagent Company. BaCeO3 and BaWO4 were made inhouse through typical solid-state reaction and precipitation method, respectively.21 RP-3 Chinese jet fuel was used in the supercritical thermal cracking test. Some major properties of RP-3 Chinese jet fuel were described in another work.22 The GH3128 nickel-based super alloy tubes used in this work were purchased from China Iron & Steel Research Institute Group (Beijing, China). It is worth pointing out that GH3128 is widely used as a material sub-assembly for the fabrication of aero-engines with outstanding mechanical properties, including anti-oxidation and anti-corrosion under 1200 °C. 2.2. Preparation of the Bifunctional Coating. All GH3128 nickel-based super alloy tubes were cleaned carefully with the following procedures before experiments. The inner tube surface was washed with dichloromethane for 20 min to thoroughly remove the organic contaminants. After draining, the cleaned tubes were washed with deionized water and ethanol for 20 min each to remove residual dichloromethane. The tubes were then purged by compressed nitrogen gas for 20 min to remove ethanol. Afterward, the tubes were dried for 20 min at 120 °C in an oven. The tubes finally were sealed with Teflon taps. The coating slurry was prepared by mixing defined amounts of H3PW12O40, BaCeO3, BaWO4, silica sol, and deionized water sufficiently. At last, the obtained slurry was coated on the internal wall of a GH3128 nickel-based super alloy reactor tube (diam. 3 mm × 0.5 mm × 1000 mm) by the wash-coating method. Details of the coating procedure were described in previous work.23 The average loading weight of the coating is 1.0 ± 0.1 mg/cm2 (three counts). 2.3. Characterization of the Bifunctional Coating. To fully understand the features of the coating inside the tube, the freshly coated tubes were split to investigate their morphology, thickness, chemical composition, and element distribution by field-emission scanning electron microscopy (FESEM, FEI Nano-Sem 430) and energy-dispersive X-ray (EDX) spectroscopy. To confirm the chemical and phase composition of the coating, the coating slurry was dried and characterized by X-ray diffraction (XRD), using a Rigaku D-max 2500 V/PC X-ray diffractometer with monochromatic Cu Kα radiation (λ = B

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Energy & Fuels welding points. The first eight K-type thermocouples were welded at positions from 5 to 75 cm along the tube, at intervals of 10 cm. The other four thermocouples were welded at positions from 80 to 95 cm along the tube, at intervals of 5 cm. For direct comparison, anti-coking performance tests of RP-3 Chinese jet fuel in a bare reactor tube and RP-3 Chinese jet fuel with 5 wt% ethanol in a tube coated with the bifunctional coating were carried out under the same conditions (outlet temperature, 700 °C; pressure, 4 MPa; flow rate, 1 g/s; run duration, 30 min). After the cracking test, the tube reactor was cleaned three times with n-pentane to remove any residual hydrocarbon. The tube was then cut into several segments of 3 cm length, soaked in n-pentane again, and dried in a vacuum oven at 120 °C for 6 h to totally remove the residual npentane. Two split segments were sampled at 300 and 960 mm along the axial length of bare or coated tubes, and the corresponding morphologies were characterized by scanning electron microscopy (SEM). Afterward, the samples were analyzed in sequence by using a GXH-1050 carbon analyzer (Beijing Junfang Institute of Physical & Chemical Technology, China), which can quantify coke deposition (precision of 1 ppmv) in units of mg/cm2 by monitoring the signal of carbon dioxide, with a repeatable error of less than 1.0%. In the analyzer, coke deposits were oxidized to carbon dioxide with flowing air (0.5 L/min) over a CuO grid in a furnace at 800 °C. Temperatureprogrammed oxidation (TPO) was an effective way to characterize the type and amount of the coke deposits on the basis of their reactivity under defined conditions.24 In this experiment, the 1 cm segments at 960 mm along the axial length of tube were heated in the furnace at a rate of 3 °C/min from room temperature to 900 °C. The cracking conversion of RP-3 Chinese jet fuel is defined by its main components (C10−C13) because RP-3 Chinese jet fuel is a complex mixture of hundreds of hydrocarbons.13,22 The liquid sample was gathered through a sampling valve in 1 min and then weighed by an electronic balance (AB 204-S, Mettler Toledo, 0.1 mg) to calculate the average residual flow rate. The other liquid product was drained into the waste liquid reservoir. The content of each species in the liquid residuals of the cracked liquid was quantitatively analyzed by an Agilent 7890A gas chromatograph (GC) equipped with a flame ionization detector (FID) and an alkane−olefin−naphthene−aromatic (PONA) capillary column (50 m × 2 mm × 0.5 μm). The gaseous product were analyzed online by a 3000A micro GC (Agilent Inc., USA), which comprises three thermal conductivity detectors (TCDs) and multichannel analytical columns, i.e., molecular sieve (10 m × 12 μm), Plot U (10 m × 30 μm), and alumina (10 m × 8 μm). The analysis deviation of gaseous products was less than 1.5%. The amount of water produced from the dehydration of ethanol in the liquid products was quantified using a Compact Karl Fischer coulometer (C20S, Mettler Toledo, 1 ppmw-100%). More details about the analysis method can be found in the work of Liu et al.3

Figure 1. SEM images of inner surface of the bare tube (A1 and A2) and the tube coated by perovskite and HPW (B1 and B2).

SEM images of the bifunctional coating on the tube after thermal fuel cracking at 700 °C for 30 min and burning at 800 °C for 2 h were collected, and they will be discussed in a later section. Furthermore, the cross section of the coating was characterized by SEM, and typical images are shown in Figure 2. A clear border between the substrate and the as-prepared

Figure 2. SEM images of the cross section of the as-prepared coating on the inner tube wall.

coating is observed. The thickness of the coating varies from 4.1 to 4.3 μm. In the actual industrial conditions (the coating does not have an effect on the delivery of fuel), the best coating thickness should be less than 20 μm.26 In this work, our coating with a thickness of about 4.2 μm is efficient to insulate the catalytic metal substrate to prevent coke growth without any bad influence on fuel delivery. Figure 3 presents the XRD patterns of the dried powders of various coating slurries with different amounts of phosphotungstic acid (HPW). For the slurry that we used to prepare the bifunctional coating, only BaWO4 and BaCeO3 are detected, as shown in Figure 3a. When the amount of HPW is increased to 19.0% in the mixture, HPW is still undetectable (Figure 3b). In comparison, the slurry with the same fraction of HPW (19.0%) in SiO2 (Figure 3d) exhibits the characteristic peaks of HPW. Furthermore, when the composition of HPW is increased to 22.0% in the mixture of BaCeO3, BaWO4, and SiO2, the crystal pattern of HPW can be detected (Figure 3c). The difficulty in detection of HPW in the bifunctional coating by XRD is ascribed to the strong diffraction of perovskite and the high

3. RESULTS AND DISCUSSION 3.1. Characterization of the Bifunctional coating. Figure 1 shows inner-surface top views of the bare (A) and the coated (B) GH3128 super alloy tubes collected by SEM. It is obvious that the inner surface of the coated tube is much smoother than that of the bare tube, which indicates that the coating has been successfully on the microchannel. It can be observed that a smooth and compact coating has been obtained with some embedded particles on the surface. Notably, some narrow cracks appear (Figure 1, B1), which is also reported in other works as common and inevitable defects of coatings prepared in a tube reactor.15,16,25 The mechanical strength of the coating was also tested. The weight loss of the bifunctional coating on the tube is 8 ± 1% (ultrasonic vibration test, three times) and 6 ± 1% (thermal shock test, three times). The testing results indicate a good stability of the coating, which is consistent with the study by Cao et al.,23 who reported a weight loss ranging from 5.5% to 13% for their coating. Moreover, C

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the coating. In general, the results of EDX are in agreement with those of XRD. Furthermore, the elemental mapping of the bifunctional coating was collected and is shown in Figure 4a−e. All the elements Ba, Ce, W, and Si present uniform and independent distributions. It can be inferred that SiO2, BaCeO3, BaWO4, and HPW are dispersed well in the coating. Additionally, it is worth pointing out that the element P, as a part of HPW, is hard to differentiate through EDX because the peak of P in the EDX spectra is too small to be defined and is overlapped with that of Pt, a little of which was sprayed on the sample for the SEM measurement. 3.2. Anti-coking Performance of the Bifunctional Coating. The anti-coking performance is evaluated via supercritical thermal cracking of RP-3 Chinese jet fuel at a predetermined fuel outlet temperature of 700 °C for 30 min. Once the power was set for these tests, it was held constant for the entire test duration. It took about 4 min for the system to reach steady state, and then the run time was initialized. The outlet temperature of fuel was monitored as a function of run time (Figure 5a) and controlled steadily at 700 °C through the 30 min duration test with a variation of less than 5 °C. As described in Figure 5b, pressure drop across the tube reactor in the bare tube increases obviously from 0.05 MPa initially to 0.21 MPa at 30 min. In comparison, the pressure drop across the coated tube remains unchanged at about 0.05 MPa. Pressure drop in the reactors can be well indicative of coke deposition in the tube reactor28 because the coke deposited on the tube wall surface and dissolved in the fuel due to thermal cracking can dramatically change the flow characteristics (i.e., flow resistance). The stability of the fuel flow system over time can be reflected by the pressure drop. Therefore, the effect of this new strategy for coke inhibition is obvious. Moreover, the pressure drop across the cooler as a function of run time was also measured, and the data are summarized in Figure 5c. The pressure drop across the cooler is related to the quenching cokes.1 Because of the good dispersibility and solubility of hydrocarbon under supercritical condition, a large amount of cokes in the bulk fuel will be brought out of the tube and carried to the cooler by the flow. Due to the sudden decrease of temperature in the cooler, those dissolved cokes gather like the

Figure 3. XRD patterns of different coating slurries with the compositions of (a) 4.0% HPW, 4.0% BaCeO3, 24.0% BaWO4, 68.0% SiO2; (b) 19.0% HPW, 3.0% BaCeO3, 10.0% BaWO4, 68.0% SiO2; (c) 22.0% HPW, 2.5% BaCeO3, 7.5% BaWO4, 68.0% SiO2; and (d) 19.0% HPW, 81.0% SiO2.

dispersion of HPW in the mixture.27 The diffraction for the slurry of HPW and SiO2 is already very weak and has a harsh baseline due to the presence of amorphous SiO2. When perovskites (BaWO4 and BaCeO3) with high crystallinity are introduced, HPW cannot be readily detected by XRD. The presence of HPW in the bifunctional coating will be proved indirectly by its performance for coke inhibition in a later section. EDX analysis can provide important information about a coating with a thickness of less than 1 mm. In this work, EDX analysis was used to confirm the chemical composition of the surface of the bifunctional coating inside the tubular reactor in comparison with that of the bare tube. As shown in Figure 4f, the main elements of GH3128 super alloy tube are Ni (62.25%), Cr (20.01%), W (7.27%), and Mo (7.00%), which is consistent with the previous work of Liu et al.22 In comparison, the bifunctional coating mainly contains Ba, Ce, Si, O, and W elements. The contents of Ni and Cr decrease dramatically to 2.38% and 0.67%, respectively. This indicates that catalytic metals on the tube’s surface have been effectively covered by

Figure 4. SEM image (a) and surface elemental maps of Ba (b), Ce (c), W (d), and Si (e), and EDX spectrum (f) of the bifunctional coating. D

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Figure 5. Anti-coking performance of the bifunctional coating (red line) during thermal cracking compared with the bare tube (blue line). (a) Outlet temperatures of fuel. (b) Pressure drop across the tube reactor. (c) Pressure drop across the cooler as a function of run time during thermal cracking of fuel. (d) Amounts of deposited cokes and wall temperatures at different position of the tubular reactor as a function of axial distance after cracking for 30 min at 700 °C.

amount deposited on the tube with the bifunctional coating is 23.44 mg. Significantly, the anti-coking efficiency reaches up to 96% when thermal cracking over a bare tube is used as a reference. The reaction conditions, fuel conversion, and overall amount of coke deposits on the tube reactor are summarized in Table 1.

process of precipitation, resulting in an increased pressure drop in the cooler. Therefore, cokes dispersing in the fuel can also be effectively suppressed by ethanol and bifunctional coating compared with conventional cracking. The wall temperatures of the bare tube and the coated tube present highly similar profiles, with a difference of less than 20 °C at the same position of the reaction tubes (Figure 5d). This indicates that the bifunctional coating barely increases the heat resistance of the microchannel owning to its thinness of 4.2 μm. The cokes deposited on the tube surface were also studied. The anti-coking ratio is defined as Rac = 100% × (mc0 − mc1)/ mc0, where mc0 and mc1 represent the coking amount of the bare tube with RP-3 and the coking amount of the tube coated with RP-3 and 5 wt% ethanol, respectively. Details of coke amounts along the tube axis are shown in Figure 5d. For the bare tube, the amount of coke deposits is almost negligible (less than 1 mg/cm2) in the inlet part of 0−26 cm, in which it is mainly the autoxidative coke.5 The amount of cokes then linearly increases over the distance of 26−62 cm of the tube and reaches a value of 11.04 mg/cm2. It then keeps rising exponentially and finally reaches a maximum of 39.95 mg/cm2 at 95 cm near the outlet. Similar patterns were also obtained in recent theoretical studies, in which the concentration of the coke precursor increased dramatically during passage through the tube from inlet to outlet under high wall temperature and complicated physical and chemical conditions.13,29,30 As for the tube with bifunctional coating, the amount of coke deposits along the tube is extremely low in general. Evidently, the coke deposits become obvious at the position of 62 cm with a value of 0.45 mg/cm2, which is only 4.1% of the amount of coke deposits at the same position of the bare tube. The amount of cokes then increases slowly to 1.52 mg/cm2 at the position of 95 cm near the outlet, which is only 3.8% of the bare tube. Overall, the total coke

Table 1. Thermal Cracking Conditions and Anti-coking Performance of the Bifunctional Coating with 5 wt% Ethanol Compared with Bare Tube test

pressure (MPa)

flow rate (g/min)

outlet temp (°C)

mc (mg)

conversion (%)

bare tube coated tube

4 4

60 60

700 700

643.69 23.44

75 76

The influence of amount of ethanol added in the fuel is also investigated. Thermal crackings of fuel with different fractions of ethanol ranging from 0 to 12.5 wt% were also conducted over the bifunctional coating at 700 °C for a period of 30 min. The amounts of cokes deposited on the tubes were analyzed and are presented in Figure 6a. With the addition of 1 wt% ethanol initially, the total coke deposits decrease from 71.61 mg to the minimum value of 50.55 mg. When the fraction of ethanol is increased from 2% to 5%, coke amounts are further reduced from 35.83 to 23.44 mg. This can be attributed to enhanced coke removal via the dehydration of ethanol on the bifunctional coating. The total coke of fuel with 7.5 wt% ethanol (34.23 mg) is a little higher than that with 5 wt% ethanol addition. When the fraction of ethanol is increased to 12.5 wt%, the thermal cracking experiment was forced to be shut down at 18 min because of the dramatic increase of pressure drop across the cooler. When the amount of ethanol is E

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Figure 6. (a) Total coke amounts deposited on bare tubes (red points) and coated tubes (blue points) with different fractions of ethanol ranging from 0 to 12.5 wt%. (b) Total coke amounts deposited on different coatings for the cracking of RP-3 fuel with 5 wt% ethanol. The outlet temperature of fuel was kept at 700 °C for 30 min during the tests.

be determined due to the outstanding performance of the bifunctional coating. Overall, the system consisting of 5 wt% ethanol and bifunctional coating presents a stable flow state and low coke deposition in both the reactor tube and the cooler for fuel cracking. First, the coating effectively suppresses coke formation by covering the active metal sites (e.g., Ni, Cr).15 Second, the added ethanol is converted to water via dehydration catalyzed by HPW in the bifunctional coating under high temperature. The perovskite sites in the bifunctional coating together with water then inhibit coke formation effectively through carbon−steam gasification. The mechanism proposed above will be proved experimentally in a later section. 3.3. Characterization of Coke Deposits. There are mainly two kinds of cokes: autoxidative cokes and pyrolytic cokes.32−35 Pyrolytic cokes include amorphous cokes and filament cokes during the cracking of hydrocarbon fuels.36 Amorphous cokes not only are formed by deposition of highmolecular-weight compounds produced by polyaromatic condensation but also can grow near the surface from small molecules and radicals which are products of fuel cracking.37 Unlike the amorphous cokes, the filament cokes are related to the catalytic metal (Fe, Co, Ni) on the surface of the cooling channel, which leads to a high coking rate when the amount of metal particles is enough.37,38 To get more information on the properties of deposited cokes inside the tube reactor, TPO and SEM were employed to characterize their activity and morphology. Figure 7 shows the

small (≤5 wt%), the active sites of the bifunctional coating are enough to catalyze the reactions of coke inhibition. Therefore, coke deposition decreases with increasing amount of ethanol. However, when the fraction of ethanol is larger than 5 wt%, no more ethanol can be catalyzed due to the limited active sites on the bifunctional coating. Meanwhile, the excess ethanol will participate in the reactions of thermal cracking of fuel, resulting in coking deterioration. A similar phenomenon was also observed by Xu et al.31 They found that the highest running temperature of RP-3 Chinese jet fuel dropped obviously from 730 to 650 °C when the fraction of added ethanol was increased from 6 to 52 wt% in bare tubes due to serious coke formation. However, the process is so complex that there is no clear explanation for this phenomenon at the moment. In the future, the mechanism of this interesting phenomenon can be investigated as another subject. In addition, 5 wt% of ethanol is still a huge content which impacts the engine performance. The influence of ethanol as an additive and reducing the amount of ethanol without losing performance should be deeply investigated for real applications. For a better understanding, control experiments of fuel cracking with the addition of different amounts of ethanol in bare tubes were carried out. The amounts of deposited cokes with different fractions of ethanol ranging from 0 to 12.5 wt% were measured, and those data are displayed in Figure 6a as the red points. The results show that coke amount initially drops from 643.69 mg for 0 wt% ethanol to 261.45 mg for 5 wt% ethanol. The total coke amount then fluctuates in the range of 248−296 mg with the addition of more ethanol (5−12.5 wt%). As shown in Figure 6a, the effect of the bifunctional coating on the efficient inhibition of cokes can be confirmed regardless of the ethanol fraction. The unique role of the bifunctional coating is further verified by comparison with regular monofunctional coatings. Three kinds of coatings, BaWO4, BaCeO3, and BaCeO3−WO3, have been prepared and evaluated in the thermal cracking of RP-3 fuel with 5 wt% ethanol. Figure 6b shows the coke amounts deposited on various coatings after thermal cracking tests. Both BaWO4 and BaCeO3−WO3 coatings present similar amounts of coke deposits, 54.38 and 53.20 mg, respectively. The tube coated by BaCeO3 has more serious coke deposition of 72.22 mg. In comparison, the bifunctional coating containing both perovskite and HPW exhibits the best anti-coking performance, with the total coke amount only 23.44 mg. Therefore, the synergetic effect of perovskite and HPW for coke inhibition can

Figure 7. TPO profiles of the cokes deposited at 95 cm on (a) bare reactor tube, (b) coated tube for the thermal cracking of RP-3 Chinese jet fuel without ethanol, and (c) coated tube for the thermal cracking of RP-3 Chinese jet fuel with 5 wt% ethanol. F

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Figure 8. SEM images of inner surfaces of bare tube (A1−A4) and coated tube (B1−B4) at different axial positions. Group A: bare tube after thermal cracking test. (A1), (A2) were collected at 300 mm, and (A3), (A4) were collected at 960 mm of the bare tube. Group B: bifunctional coating tube after test. (B1), (B2) were collected at 300 mm, and (B3), (B4) were collected at 960 mm of the coated tube.

TPO profiles. The results further reveal that the bifunctional coating effectively suppresses both filament cokes and amorphous cokes. Furthermore, the coked tubes were burned at 800 °C for 2 h in an attempt to remove deposited cokes. SEM images of the inner surfaces of the tubes after burning are shown in Figure 9.

TPO profiles of carbon deposits from the thermal cracking of RP-3 Chinese jet fuel over bare and coated tubes at the same position. The only peaks observed in the TPO curves are assigned to pyrolytic deposition because the reaction occurs totally in the thermal cracking area. In previous works,13,15,24,39,40 filament cokes and amorphous cokes with different activities were identified by TPO as two types of pyrolytic cokes. The former, produced by catalytic reactions with active metal particles, is less reactive and has a lower H/C ratio. The latter, resulting from secondary deposition on already-formed carbon deposits, is more reactive and relatively hydrogen rich. In Figure 7, the TPO profile of the bare tube presents two peaks at 653 and 708 °C with maximum signal intensities of 0.115 and 0.103, respectively. The peak at 653 °C belongs to amorphous cokes, and that at 708 °C is assigned to filament cokes. The TPO result of the cokes from thermal cracking of RP-3 without ethanol addition over bifunctional coating (Figure 7b) presents only one peak at 615 °C, with a maximum signal intensity of 0.032. It can be explained by the fact that bifunctional coating covers the active metal sites (such as Fe, Ni) on the tube surface, preventing the formation of filament cokes. Moreover, the TPO profile of coke deposits on the coated tube for the cracking of RP-3 fuel with 5 wt% ethanol (Figure 7c) shows a smaller peak at a lower temperature of 603 °C, with an intensity of only 0.011. The small peak area indicates a tiny amount of cokes, which is consistent with the result collected from carbon analyzer, shown in Figure 5d. It indicates that ethanol in cooperation with the bifunctional coating can further inhibit the amorphous cokes because the bifunctional coating has the ability to catalyze the dehydration of ethanol and carbon−steam gasification. Figure 8 presents SEM images of the carbon deposits inside the tubes after thermal cracking at 700 °C under 4 MPa for 30 min. As shown in Figure 8, A1 and A2, filament cokes are mostly present on the bare tube near the inlet. The cokes at the inlet are mostly formed by catalysis by metal on the tube surface. At the outlet of the bare tube, an abundance of amorphous cokes fill in the spaces between filament cokes closely (Figure 8, A3 and A4). Filament cokes are somewhat rigid and branch-like in morphology, so they can promote coking and collect coke deposits like a filter.33 By contrast, much less coke deposits are observed on the surface of the coated tube near the inlet where the coating remains intact, as shown in Figure 8, B1 and B2. Amorphous cokes with diameters of 4−8 μm have been detected near the outlet of the tube (Figure 8, B3 and B4), which are much easier to remove than the filament cokes. This is in agreement with the results of

Figure 9. SEM images of tube outlets of (a) bare tube and (b) coated tube after burning at 800 °C for 2 h.

The surface of the bare tube becomes porous and rough. This can be explained by the removal of active metal from the tube surface. During the coking process, active metal particles left the surface and located on the top of the filament during the growth of filament cokes. When the filament cokes were burned off at high temperatures, the active metal particles on the top of the coke filaments were removed subsequently. This phenomenon was widely observed in other works.15,23,40,41 In contrast, the bifunctional coating remained tight and smooth after burning. It can be inferred that the bifunctional coating has good stability at high temperatures (Figure 9b). 3.4. Mechanism of Coke Inhibition by Ethanol over the Bifunctional Coating. The bifunctional coating plays an important role in insulating the active metal on the tube surface to prevent the catalytic reactions that form the filament cokes. Moreover, it is expected to remove a considerable amount of coke deposits in situ using water generated from the dehydration of ethanol, as shown in Scheme 2. To validate the proposed mechanism of coke inhibition by the bifunctional coating, the gas and liquid products from thermal cracking were analyzed first. The main coke precursors in the liquid products were measured and compared between thermal cracking reactions over the bare tube and the coated tube. As illustrated in Table 2, the contents of all the coke precursorsbenzene, methylbenzene, ethylbenzene, xylene, and styrenewere decreased over the bifunctional coating compared with those G

DOI: 10.1021/acs.energyfuels.7b01505 Energy Fuels XXXX, XXX, XXX−XXX

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comparison, 4029 ppmw of water is detected in the liquid products of the bifunctional coating, which is 2.6 times that found in the products of the BaCeO3 coating. Therefore, it can be confirmed that bifunctional coating with HPW plays a key role in catalyzing the dehydration of ethanol to produce water for carbon−steam gasification. In addition, 150 ppmw of water is detected for the cracking of RP-3 in the bare tube. This is ascribed to the dissolution of water in the fuel during transportation or storage. Water has a certain effect on fuel dispersing in the reactor tube. As the diluent, water can improve the dispersion of fuel and increase the relative rate of unimolecular cracking versus bimolecular cracking and hydrogen-transfer reactions.46 However, water is not miscible with the fuel, which causes some problems during the delivery and cracking of their mixture. An extra supply of water is needed to incorporate water with fuel, and fluctuation of pressure drop in the tubular reactor should be tolerated due to the unstable mixture. The use of soluble ethanol can avoid those problems caused by water. Truthfully, the stable cracking of RP-3 with greatly reduced coke deposition has been easily realized by the addition of a little ethanol over a bifunctional coating consisting of perovskite and HPW. However, the state of HPW and its interactions with perovskite and SiO2 are still not clear. A more in-depth work on the bifunctional coating remains to be further investigated. With the addition of 5 wt% well-dissolved ethanol, efficient coke inhibition is achieved in supercritical thermal cracking of hydrocarbon fuels over the bifunctional coating. When the homogeneous mixture flows through the electrically heated tubular reactor, dehydration of ethanol happens to produce water catalyzed by HPW in the bifunctional coating. The coke precursors and coke deposits can then be removed via reaction with the steam by the catalysis of perovskite coexisting in the bifunctional coating.5,47 In this work, the cokes have been inhibited dramatically by 96% in the cracking of RP-3 Chinese jet fuel. During the process, carbon monoxide and hydrogen are produced with large capacity of heat absorption, which are readily burned. However, the promotion of overall heat sink is not obvious due to the small amount of ethanol.

Scheme 2. Scheme of Reaction Pathway for Cokes Removal on the Bifunctional Coatinga

a

(1) Tube wall, (2) the bifunctional coating with HPW (orange) and perovskite (green), (3) dehydration of ethanol, and (4) carbon−steam gasification reaction.

Table 2. Comparison of the Liquid Products from Thermal Cracking of RP-3 Chinese Jet Fuel over Bare Tube and RP-3 with 5 wt% Ethanol over Coated Tube compound

bare tube (wt%)

coated tube (wt%)

benzene methylbenzene ethylbenzene xylene styrene coke precursors in total ethanol

4.53 6.17 1.03 2.57 0.69 14.99 −

2.72 3.74 0.62 1.55 0.4 9.03 2.38

on the bare tube. The five coke precursors closely relate to the aromatic condensation cokes.42−45 In total, the coke precursors are reduced by 40% (from 14.99% to 9.03%). In addition, 2.38% of ethanol was detected in the liquid product from the coated tube, indicating the presence of unreacted ethanol after cracking. Based on material balance, the conversion of ethanol is calculated to be 60.3%. The distributions of gas products are summarized in Figure 10a. The fraction of ethylene was detected to increase by 11% over the bifunctional tube compared with that over the bare tube. Therefore, dehydration of ethanol during thermal cracking can be confirmed. Based on current results, we are still not sure that the dehydration is catalyzed by acid sites of HPW instead of perovskite. More experiments were conducted, and the concentration of water dispersed in the liquid product was measured to figure out the role of HPW in the dehydration. The results are displayed in Figure 10b. The tube coated by BaCeO3 without any HPW was prepared following the same method and tested under the same cracking conditions. The obtained liquid product contained a little water, 1522 ppmw. In

4. CONCLUSION Controllable fuel cracking with dramatically reduced cokes has been achieved by using a small amount of alcohol as an additive over the bifunctional coating with both perovskite and phosphotungstic acid. Essentially, the coating keeps an intact structure with a thickness of about 4.2 μm. Based on the results

Figure 10. Distribution of the gas products (a) and water concentration in the oil products (b) in various tubular channels with/without coating. H

DOI: 10.1021/acs.energyfuels.7b01505 Energy Fuels XXXX, XXX, XXX−XXX

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of XRD, EDX, and elemental mapping, the coating is determined to consist of BaWO4, BaCeO3, SiO2, and H3PW12O40 with a uniform and independent distribution. An optimal ethanol amount of 5 wt% is determined on the basis of the experimental results. After the novel strategy was executed, the efficiency of coke inhibition reached 96%, and the system stability (i.e., pressure drops of reactor tube and the cooler) was much better than that of the bare tube. Additionally, SEM and TPO characterizations of the coke deposits show that the deposition of both filament cokes and amorphous cokes is effectively suppressed on the tube wall with bifunctional coating. The analysis of coke precursors in the liquid products clearly indicates that the amounts of typical coke precursors related to the aromatic condensation cokes are reduced. Based on a series of control experiments, the unique role of the bifunctional coating has been confirmed. Over the acid sites of HPW, water forms via the dehydration of ethanol. The coke deposits are then removed in situ via reaction with water by the catalysis of perovskite coexisting in the bifunctional coating. Above all, the strategy based on ethanol as additive and perovskite with HPW as bifunctional coating shows a good potential for suppressing cokes and increasing the operation stability of the cooling system for the aircraft cooling microchannels. Besides ethanol, other alkanols can also be employed and investigated in our platform for coke inhibition. More efforts should be made to investigate the coating structure and the mechanism of coke inhibition.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 22 27892340. E-mail: [email protected]. ORCID

Guozhu Li: 0000-0003-1329-0548 Guozhu Liu: 0000-0001-7538-5732 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Key Research and Development Program of China (2016YFB0600305).

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REFERENCES

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DOI: 10.1021/acs.energyfuels.7b01505 Energy Fuels XXXX, XXX, XXX−XXX