Heat Transfer Study on Hydrocracking of Paraffin Wax over Microfiber

Article pubs.acs.org/IECR

Heat Transfer Study on Hydrocracking of Paraffin Wax over Microfiber-Wrapped Catalyst Jian Jiang,† Chao Yang,† Jibing Sun,‡ Zhaohui Dong,† Tao Li,†,§ and Fahai Cao*,†,§ †

School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China Liming Research and Design Institute of Chemical Industry Co., Ltd. Luoyang 471001, China § Engineering Research Center of Large Scale Reactor Engineering and Technology, Ministry of Education, East China University of Science and Technology, Shanghai 200237, China ‡

ABSTRACT: The microfibrous entrapped Pt/SAB catalyst (MFEC) was prepared by wet lay-up papermaking with a sintering process. The MFEC with Cu-fibers and Ni-fibers has excellent heat transfer properties compare to traditional catalysts via thermal measurements. Thereof, CuMF-Pt/SAB catalyst has lower catalytic properties due to the fact that Cu contaminates the catalyst during the sintering process. Although NiMF-Pt/SAB catalyst has good catalytic property, it also has higher selectivity of liquid products and lower selectivity of gas products due to the fact that MFEC can avoid temperature runaways during the hydrocracking reactions. The simulating results confirmed that MFEC has good heat transfer and avoid temperature runaways. The pressure drop of MFEC is also lower than conventional catalyst. Therefore, the Ni MFEC structures are taken as a suitable catalyst structure to enhance the heat transfer for the exothermic hydrocracking reaction, especially for mild temperatures in the reaction.

1. INTRODUCTION Currently, the catalytic performance in the Fischer−Tropsch (F−T) wax hydrocracking reactions depends on the various acidic supports and activated metals in catalyst. It is well known that amorphous silica−alumina has a strong medium acidic site, which leads to a high yield of middle distillates.1−3 Catalysts based on group VIII (Ni, Mo, W)4,5 and noble metals (Pt, Pd)6,7 have been reported in many publications. Jiang et al.8 reported that the additional zeolites in amorphous silica− alumina supports have high conversions and middle distillates selectivity with mild hydrocracking conditions. On the other hand, noble metals have good catalytic activity. The catalysts with big particle size (2−3 mm) are adopted in industrial production because they can decrease the pressure drop during the reaction. The active sites are distributed in the inner surface of big particle size catalysts. The diffusion of paraffin molecules are slow; on the contrary, the diffusion of hydrogen gas is very fast, so the deep hydrocracking reaction is easily generated and makes a large number of gaseous hydrocarbons in the inner surface of catalyst. Furthermore, the catalysts are easily deactivated9 due to the fact that the conventional catalyst bed has poor heat transfer and the heat is easy to accumulate during the reaction process. The small particle size catalyst has better heat transfer properties and can improve the utilization of the inner surface of the catalyst, but the pressure drop is increased significantly during the reaction. In this study, based on the study of thermal measurements and catalytic properties in hydrocracking process, microfibrous entrapped catalyst is introduced to solve these problems. A wet-lay method to prepare microfiber-entrapped catalysts based on traditional high speed and paper-making technique was developed by Auburn University.10−13 Lu Yong et al14,15 developed a composite material synthesis technology of microfiber entrapped fine particulates. Novel microfibrous © 2014 American Chemical Society

structured packings provide a unique combination of large void volume, entirely open structure, large surface-to-volume ratio, high permeability, high heat and mass transfer, and unique form factors, which are central to the notion of increasing the steady-state volumetric reaction rate. The latest progress in the use of microfibrous structured packings in H2 fuel generation and cleanup, air filtration, selective oxidation, electrochemical energy storage,16−18 and F−T synthesis19 has been reviewed. Microfibrous-entrapped catalyst (MFEC) can be made of highly thermal conductive metals, such as copper, brass, or nickel, to improve the intrabed heat transfer efficiency in a fixed bed reactor. Such a reactor with thermal conductive metal MFEC may be able to avoid hot or cold spots in the catalyst bed and achieve a uniform temperature profile or find temperature control.14 However, the heat transfer efficiency of MFEC for hydrocracking of paraffin wax has not been conducted before. On the other hand, the commercial computational fluid dynamic can be used to simulate the fixed packed bed.20 The interaction models are adopted to calculate the heat transfer properties via pressure drop, velocity field, and temperature distribution. The Saez et al.21 model and the Holub et al.22 model have been validated on the basis of experimental data obtained at low operating pressures, which is not suitable for hydrocraking reaction. Attou et al.23 use a twofluid hydrodynamic model to predict the trickle flow and the pulse flow through a packed-bed reactor under high pressure. In this paper, we attempt to use simulating model for predicting the performance of the hydrocracking reactions. Received: Revised: Accepted: Published: 5683

August 5, 2013 November 30, 2013 March 17, 2014 March 17, 2014 dx.doi.org/10.1021/ie402551q | Ind. Eng. Chem. Res. 2014, 53, 5683−5691

Industrial & Engineering Chemistry Research

Article

The purpose of this paper is to give an experimental study of heat transfer efficiency of copper, nickel MFEC, and traditional Pt/SiO2−Al2O3 catalysts. The heat transfer and catalytic properties were conducted via a fixed bed reactor. The catalysts were examined by TEM, XRD, TPD, and temperature detection. In particular, a highly effective MFEC catalyst system for hydrocracking of paraffin wax was developed by considering the heat transfer and catalysts properties in fixed bed reactor. In additional, the porous media model in computational fluid dynamic software was adopted to simulate a bench-scale fixed bed reactor. The effect of different catalysts was investigated and the pressure drop, products, and temperature distribution in reactor were analyzed. The simulation of fluid flow with two-fluid hydrodynamic model could identify the heat transfer properties of MFEC.

2. EXPERIMENTAL SECTION 2.1. Preparation of Traditional Catalyst. A specific amount of aluminum nitrate (Sinopharm) and sodium silicate (Sinopharm) were dissolved in water and the pH was adjusted to 8 by adding ammonia. The precipitates were obtained and β zeolites were added into the gel of precipitates. The mixture was stirred for 2 h and then filtered, washed, dried at 120 °C for 12 h, and finally calcined at 550 °C for 5 h in a muffle furnace. The support with zeolite was obtained. The 20 wt % β zeolite in SiO2−Al2O3 (SiO2/Al2O3 = 0.4, wt) support was called SAB support. Chloroplatinic acid was dissolved and the solution was diluted to the required volume and SAB carriers were added into solution. The mixture was merged at 50 °C for 5 h followed by drying at 120 °C for 12 h. Finally, it was calcined at 550 °C for 5 h. The Pt/SAB catalyst was obtained and the Pt loading is 1 wt %. 2.2. Preparation of MFEC. The Ni, Cu MFEC was made of 8 um in diameter and 3 mm in length microfiber (Western Metal Materials). The microfibrous metal networks were made by the wet-lay papermaking process. A total of 4.2 g of metal fibers, 7.5 g of (100mesh) SAB carriers, and 1.4 g of cellulose fibers were added into 1 L water and stirred vigorously to produce a uniform suspension. The suspension was transferred into the head box of a 160 mm diameter circular sheet former (ZCX-159A, made in China), so a 160 mm wet perform microfibrous paper was made. Then, 0.8 g of metal fibers and 0.3 g of cellulose fibers were added into 0.5 L of water, and the same process as above was followed. The cover was prepared, and it was covered on the wet perform paper to keep the particles. The mixture sheet was dried at 110 °C for 24 h and oxidized in air at 500 °C to remove the cellulosic binders. The sheet was sintered in hydrogen at 950 °C to create the threedimensional sinter-locked network. The MFEC catalysts were made by dispersing Pt into the pore surface of the MFEC support. The microfibrous carrier was incipiently impregnated with an aqueous solution of chloroplatinic acid, then dried and calcined at 800 °C for 1 h. Thus, the MFEC was obtained and noted as “NiMF-Pt/SAB” and “CuMF-Pt/SAB”. Thereof, Pt loading was 1 wt %. Figure 1 shows the process of preparation of MFEC. 2.3. Evaluation of Catalyst. The semirefined 56# wax is taken as model feedstock whose molecular weight is about 300−500 and the carbon number distribution is from C19 to C36. A lab-scale fixed-bed reactor with higher pressure was used in this study, which had a tube of 20 mm internal diameter and 350 mm in length. Each time, 4.3 g of catalyst with 80 to 100 meshes was loaded into reactor. The wax and H2 were

Figure 1. (A) Photograph of Cu-microfibers; (B) SEM images of sinter-locked structure; (C) photograph of CuMF-Pt/SAB catalyst; (D) surface morphology of CuMF-Pt/SAB catalyst.

introduced into the single-bed reactor with the ratio of H2/wax = 0.12 wt/wt %. The tail gas was collected and analyzed via online GC (GC-900A). Liquid products were collected and fractionated into C5−C9, C10−C18, and C19+, which were analyzed by GC (FULI9790). Wu et al.24 mentioned that gaslimited conditions imply better downflow performance, whereas liquid-limited conditions results in better upflow performance over downflow. Merwe et al.25 confirmed that the conversions for ≤C35 in the upflow mode are lower than the ≤C35 conversions in the downflow mode. The carbon number of semirefined 56# wax is from 1 to 36. In this study, the reactor is equipped with a down-flow mode. But the up-flow mode should be checked in further research because it has better wetting properties. The surface area and pore structure of the catalysts were determined via physical adsorption measurements (Micro Meritics, ASAP-2020). The metal dispersion of catalyst was conducted by TEM (JEM, TEOL2010) and TPD-NH3 method was employed to study the acid strength and acid amount of the catalyst. The surface structure of the catalysts was examined by SEM (Hitachi S-4800). The Cu and Ni concentration was measured by inductively coupled plasma-atomic emission spectroscopy (Thermo Scientific CAP 6300). 2.4. Thermal Measurement. As shown in Figure 2, test catalysts were loaded in the middle plant of the fixed bed, which was surrounded by heat jacketed wall kept at constant temperature during every measurements. A multithermocouple was utilized to measure the temperature profiles of the catalyst bed. The temperature curve on the three points (point A, B, and C) was recorded automatically.

3. NUMERICAL SIMULATION OF HEAT TRANSFER IN HYDROCRACKING 3.1. Two Fluid Hydrodynamic Model.26 The fluid flowability is very slow in trickle bed reactors, and the Reynolds number is also small. The flow form in the reactor is laminar flow, the vapor and liquid phases are assumed to be incompressible, and the porosity of the bed is considered as consistent. The equations of the two-fluid model are showed as below. 5684

dx.doi.org/10.1021/ie402551q | Ind. Eng. Chem. Res. 2014, 53, 5683−5691


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mentions that the momentum loss is divided into viscosity loss and internal loss. The equation of momentum can be represented as eq4. Si =

μ 1 ui + C2 ρ|ui|ui α 2

(4)

The different supports have different viscosity coefficient α and internal resistance coefficient C2. The coefficients are also different according to different catalyst structure. In this paper, the conventional catalysts can use Ergun equation (as shown in eq 5 below) to determine the viscosity and the drag coefficient (1 − ε)2 (1 − ε) 2 ΔP = 150 3 2 μu f + 1.75 3 μu f L ε dp ε dp

(5)

The viscosity coefficient and the internal resistance coefficient of MFEC can be determined by considering the permeability of the porous material and the material geometry and porosity. This model is taken as model 2. 3.2. Mass Balance Equation. This paper studies the hydrocracking reaction with hydrodynamic simulation. During the simulation, in addition to the momentum equation, the components equation is added also ∂ (εkρk Ck , i) + ∇(εkρk ukCk , i) ∂t = ∇(εkρk Di , m∇Ck , i) + εkρk Si , k

ρk and εk are the density and volume fraction of kth phase. Ck,i is the cell concentration of kth phase. uk is the cell velocity of kth phase. Si,k is the source term of i component in kth phase, which includes the reaction terms. The main components of paraffin wax are n-alkanes and the main number distribution ranges are from C1 to C70. The hydrocracking reaction is complex and indistinct by using the nine lumps model.27 Gamba et al.28 reported that the iso/total (iso + n) ratio exhibited a minimum around C10−C11. On the other hand, Gambaro et al.29 mentioned that only iso-paraffins in the range C6−C70 are cracked. In order to represent the better simulation results and make process simple, C10H22 is taken as a model compound, which is suitable for the nine lumps model. C5H12 is considered as the generated hydrocracking product. The reaction, flowability, and heat transfer are included during the simulation. We calculate the reaction rate from our experiments and take parameters from the nine lumps model, then we fit the rate constants, activation energies, and Arrhenius factor via Arrhenius equation. As the energies and rate constants are adjustable during the calculation, the paper does not list the values. The reader can take these parameters as reference. In our study, the activation energies are taken from 1.3 × 105 to 1.82 × 105 kJ mol−1.27 3.3. Energy Balance Equation. The energy balance equation is added to check the reaction heat during the bed

Figure 2. Fixed bed reactor section.

FGL

2/3 ⎡ (1 − εG)2 ⎛ 1 − ε ⎞ ⎢ = αG E1 ·⎜ ⎟ μG uG ⎢⎣ εG 2d p2 ⎝ 1 − εG ⎠

+ E2 ·

2/3 ⎤ (1 − εG)2 ⎛ 1 − ε ⎞ ·⎜ ⎟ ρG (uG − uL)⎥ ⎥⎦ εGd p ⎝ 1 − εG ⎠

(uG − uL)

(1)

2/3 ⎡ (1 − εG)2 ⎛ 1 − ε ⎞ · μG uG FGS = αG⎢E1 ⎜ ⎟ ⎢⎣ εG 2d p2 ⎝ 1 − εG ⎠

+ E2 ·

FLS = E1

2/3 ⎤ (1 − εG)2 ⎛ 1 − ε ⎞ ·⎜ ⎟ ρG uG 2 ⎥ ⎥⎦ εGd p ⎝ 1 − εG ⎠

(1 − ε)2 (1 − ε)2 ρL uL 2 μL uL + E2 · 2 2 εLd p εL d p

(6)

(2)

(3)

∂ (εbρf Ef + (1 − εb)ρs Es) + ∇·(u ⃗(ρf Ef + p)) ∂t

FGL, FGS, FLS correspond to the interaction force of gas− liquid, gas−solid, and liquid−solid phases, respectively. E1 and E2 are the Ergun coefficients. E1 and E2 of the spherical catalyst are taken as 218 and 1.8,20 and this model is called model 1. Although the model 1 can describe the hydrodynamic of the trickle flow though a fixed packed-bed reactor, but it is limited and complexity to apply with microfibrous entrapped catalyst. In this study, the porous media model is used to predict the flow regime transitions via different catalytic bed. The model

= ∇·[ke∇T − (∑ hiJi ) + (τf⃗ ·u f⃗ )] + Sf i

(7)

In eq 7, Sf represents the reaction heat and kε is the effective thermal conductivity. Heat loss can be calculated by selected boundary conditions, which includes the environmental temperature. We can calculate from environmental temperature to the reaction zone, so the heat loss can be accounted. The 5685



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value of kε and kf is the average value of material thermal and fluid thermal conductivity. Then, the derived equation is listed in eq 8. Sheng et al.19 reported that the different MFECs have different thermal conductivity. MFEC made of Cu and Ni demonstrate the thermal conductivities of 9.05 and 3.77 W· m−1·K−1. In our calculations, we consider the impacted factors, such as metal volume percentage in catalyst, so we set ks = 15 W·m−1·K−1 (Ni) and 40 W·m−1·K−1 (Cu) . In eq 8, the values are listed as εb = 0.5−0.7 and kf = 0.1−0.2 W·m−1·K−1 ke = εbk f + (1 − εb)ks

(8)

3.4. Computational Model. In order to understand the flow regime transitions through the reaction tube, this paper uses three-dimensional numerical calculation. First, GAMBIT2.3 is adopted for three-dimensional mesh generation. The single fixed bed reactor is φ 25 × 2.5 mm with a length of 700 mm. The catalyst is filled at the center of reactor. The height of the catalyst in reactor is set as 35 mm. The conventional Pt catalyst and Pt MFEC catalysts are simulated in this paper. The gas−liquid downward flow through a fixed bed reactor is considered. The flow is assumed without chemical reactions, and the fluids are considered as viscous Newtonian and incompressible. This gas−liquid Eulerian model is used to simulate the reactions. The porous media model is selected according to the filler layer and catalyst bed conditions. Unsteady simulation is carried out with the time step of 0.005s. The coupling method uses SIMPLE method and the convergence criteria are set as 10−4. Boundary conditions are set as follows: inlet boundary conditions are set as given inlet velocity. According to the experimental conditions and the hydrogen/oil ratio, the two-phase inlet velocity is determined as 0.133 and 0.013m/s. The outlet boundary condition is free outflow.

Figure 3. (A) SEM of NiMF-Pt/SAB; (B) SEM of CuMF-Pt/SAB; (C) TEM of NiMF-Pt/SAB; (D) TEM of CuMF-Pt/SAB.

4. RESULTS AND DISCUSSION 4.1. Characterization of the Catalysts. First, the surface structure of the MFEC was examined by SEM (Figure 1A and B). The active metal Pt was dispersed well in NiMF-Pt/SAB catalyst (Figure 1C), whereas big particle size was observed in CuMF-Pt/SAB catalyst (Figure 1D). The main reason could be Cu contaminated the catalyst during the sintering process. The Cu melting point is 1085 °C, and Ni melting point is 1455 °C. Therefore, Cu could be partly melted during the catalyst preparation and contaminate the catalyst. Further ICP testing confirmed that Cu content is much higher than Ni in MFEC. The observation indicated that CuMF-Pt/SAB catalyst may have lower catalytic properties compared with NiMF-Pt/SAB catalyst (Figure 3). Figure 4 shows the NH3-TPD curves of different catalysts. The results indicate that NiMF-Pt/SAB catalyst has the same intensive NH3 desorption peak around 200 °C, which corresponds to the dominant weak and moderate acidity. Meanwhile, CuMF-Pt/SAB catalyst has less weak and moderate acidity, but it has strongest acidity. The crystalline phases of microfiber metal of catalysts were examined by XRD as shown as Figure 5. XRD revealed that Pt/ SAB catalyst and NiMF-Pt/SAB had the same peak of γ-Al2O3. The CuO and Ni metal were detected, which indicated that NiMF-Pt/SAB and CuMF-Pt/SAB catalysts had better heat transfer properties in comparison with traditional Pt/SAB catalysts. On the other hand, the peak of γ-Al2O3 in CuMF-Pt/ SAB catalyst was very weak, which could influence the performance of the hydrocracking reaction.

Figure 4. NH3-TPD curves of catalysts.

Figure 5. XRD curves of catalysts.

4.2. Thermal Measurement Results. At P = 3.5 MPa, WHSV = 1 h−1, and H2/wax = 0.12 wt/wt %, the hydrocracking 5686



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catalyst has better volumetric heat capacities, as the temperature rise of CuMF-Pt/SAB catalyst is faster than that of NiMFPt/SAB and Pt/SAB catalysts. With the final temperature increasing from 280 to 360 °C, NiMF-Pt SAB and CuMF-Pt/ SAB catalysts have the close heat transfer properties. It was observed that the Pt/SAB catalyst need longer time to reach the setting temperature. The hydrocracking reaction with different catalysts was carried out at different temperatures, and each temperature was kept for more than 2 h. Figure 7 shows the temperature of point B at setting reaction temperature of 280, 320 and 360 °C. The results indicate that the temperature of Pt/SAB catalyst grows much faster than that of MFEC. It is observed that the temperature of Pt/SAB catalyst increases significantly at 280 and 320 °C, and it gains 15 °C difference at 280 °C and 25 °C difference at 320 °C. The temperature difference of two MFEC is less than 10 °C; therefore, it indicates that the excellent heat conductivity of Cu and Ni-fiber helps to rapidly dissipate the reaction heat liberated from exothermic reaction. The temperature difference among three catalysts was almost the same and it was less than 10 °C when the reaction temperature was set at 360 °C; it could be caused by deep hydrocracking reaction. Further catalytic experiments will confirm this. In general, the results show that MFEC can avoid runaways during hydrocracking reaction and the stable temperature can be maintained inside the catalyst bed. The microfiber metal is compressed against the wall and contact with the tube wall, which is more effective for heat transfer through the interface between MFEC and the wall. Table 1 shows the performance of different catalysts under different temperature. The results indicate that CuMF-Pt/SAB catalyst has lower conversion ratio compare to NiMF-Pt/SAB catalyst. As XRD shows that CuMF-Pt/SAB catalyst has no peak of γ-Al2O3 and TEM shows that Cu contaminates the catalyst, the catalyst activity is lower even the CuMF-Pt/SAB catalyst has good heat transfer properties. The sintering temperature is important when catalysts are prepared and the different sintering temperature can be studied in the future. Meanwhile, NiMF-Pt/SAB catalyst has a slight lower conversion ratio compared with Pt/SAB catalyst at 280 and 320 °C, but the gas selectivity is lower and the selectivity of naphtha and middle distillates is higher than Pt/SAB catalyst. This is attributed to the enhanced heat transfer properties of the CuMF-Pt/SAB catalyst. The reaction temperature is kept at 283 and 325 °C, which is much lower than the temperature gained via Pt/SAB catalyst, 295 and 345 °C. As shown in Table 1, with the temperature increasing from 320 to 360 °C, the selectivity of gas products of both catalysts are very high. The results indicate that the NiMF-Pt/SAB catalysts can gain higher liquid products selectivity (naphtha and middle distillates) and lower gas selectivity at mild reaction temperature. At the same time, The NiMF-Pt/SAB catalyst has a lower conversion ratio due to the fact that the catalyst is diluted by Ni microfibers. It can be concluded that the NiMF-Pt/SAB catalyst is a highly effective catalyst for hydrocracking reaction of paraffin wax, as it has enhanced heat transfer properties, while CuMF-Pt/SAB has significantly lower conversion ratio and higher gas selectivity. Further work should be done to identify the advantages of CuMF-Pt/SAB catalysts, such as changing the sintering temperature. 4.3. Computational Flow Model Simulation and Calculation. As discussed in the previous section, C10H22 is taken as a reaction model for simulation because it can simplify

activity of catalyst is evaluated under different temperatures. The effect of axial heat transfer at the catalytic bed can be eliminated because the bed has adequate length. Thus, the temperature increasing in the catalyst bed at midplane (point B) during the transient heat up process is only from the heat transfer along the radial direction. The temperature is set as 280, 320, and 360 °C and the heating curve on the point B of three catalysts are presented in Figure 6. CuMF-Pt/SAB

Figure 6. Temperature−time profiles of center point (Point B) during the transient measurements (A, 280 °C; B, 320 °C; C, 360 °C). 5687



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Table 1. Effect of Different Temperature on Catalyst Activity selectivity (%) catalyst NiMF-Pt/ SAB

CuMF-Pt/ SAB

Pt-SAB

T (°C)

WHSV (h−1)

conversion (%)

gas

naphtha

MD

280

1

75.1

15.1

40.2

44.9

320 360 280

1 1 1

92.1 96.0 45.2

25.8 35.7 28.2

58.5 43.2 35.3

15.7 21.1 36.5

320 360 280 320 360

1 1 1 1 1

70.5 90.2 81.1 99.5 90.4

36.2 40.9 13.6 38.4 34.7

43.7 52.1 41.5 60.5 62.9

20.1 6.0 44.9 1.1 2.3

Table 2. Characteristics of Feedstock and Operating Parameters input parameters

value

feed rate L (g/h) operating temperature (°C) operating pressure (Mpa) liquid density (ρl kg/m3) liquid viscosity (μl mPas) gas density (ρv kg/m3) gas viscosity (μv mPas) loading volume of traditional catalyst (mL) loading volume of MFEC (mL) average diameter of traditional catalyst (mm) average diameter of MFEC (mm) porosity of traditional catalyst porosity of MFEC

4.2, 8.4 280−350 3.5 738 0.384 11.6 0.015 10 10 0.35 0.15 0.3−0.5 0.5−0.7

new MFEC catalysts have a lower pressure drop than the conventional catalysts. However, model 2 has higher pressure drop between MFEC and Pt/SAB catalyst compared to model 1. The two predicted values of the pressure drop are compared with the literature,19 and model 2 matches the reported values (20−50 Pa/m). This shows that model 2 can predict accurately than model 1. 4.3.2. Simulation Results of Reaction and Heat Transfer. At P = 3.5 MPa, T = 590 K, and L = 4.2 g/h, hydrogen/oil = 1000, the model compound is C10H22. The main product is C5H12 based on nine lumped reaction model. In this study, GC−MS is used to analyze the composition, and the thermocouple is used to detect the reaction zone temperature. The heat transfer effect is compared with three different catalysts. In Figure 1, A represents NiMF-Pt/SAB catalyst, B denotes CuMF-Pt/SAB catalyst, and C represents a conventional catalyst of Pt/SAB. As can be seen from Figure 1, the microfiber-wrapped catalyst has excellent thermal conductivity because the reaction heat can be removed rapidly. The temperature distribution of catalyst layer is identical and the temperature between the heating wall and the catalyst layer is less than 5 K (Figure 8). Therefore, the constant temperature area of CuMF-Pt/SAB catalyst is larger than NiMF-Pt/SAB catalyst at 590 K because the thermal conductivity of Cu, in particular, the radial thermal conductivity is better than Ni. Pt/SAB catalyst without microfiber wrapping shows poor thermal properties. The hot spots are found in the reaction area, which is negative for hydrocracking reaction. It is confirmed that deep hydrocracking

Figure 7. Temperature of different catalyst bed during reaction (A, 280 °C; B, 320 °C; C, 360 °C).

the reaction model. The simulation models are used to conduct the properties of three catalysts, such as pressure drop, heat transfer, and temperature distribution, and so forth. Table 2 lists the operating parameters and the catalyst properties. 4.3.1. Pressure Drop. The experimental conditions were P = 3.5 MPa, T = 320 °C, and WHSV = 1, 2, and 3 h−1. Gas−liquid two-phase flow rates are shown in Table 2. If we compare Tables 3 and 4, it can be seen that both models predicted the 5688



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Table 3. Pressure Drop under Different Catalyst Systems (Model 1) inlet conditions WHSV 1 2 3

gas (mL/min)

inlet velocity

liquid (g/h)

200 200 200

4.2 8.4 12.6

gas (m/s)

liquid (m/s)

−04

−06

5.31 × 10 5.31 × 10−04 5.31 × 10−04

1.65 × 10 3.33 × 10−06 4.95 × 10−06

pressure drop (Pa/m)


A

B

4.07 4.07 4.07

4.11 4.12 4.13



A

B

33.3 33.4 33.5

45.0 45.1 45.1

Table 4. Pressure Drop under Different Catalyst Systems (Model 2) inlet conditions WHSV 1 2 3

gas (mL/min) 200 200 200

inlet velocity

liquid (g/h) 4.2 8.4 12.6

gas (m/s)

liquid (m/s)

−04

−06

1.65 × 10 3.33 × 10−06 4.95 × 10−06

5.31 × 10 5.31 × 10−04 5.31 × 10−04

Figure 8. Reactor temperature distribution over the different catalysts.

happened and more gas generates during the reactions. The hot spots also cause the catalyst deactivation and catalytic properties. At the same reaction condition, the distribution of final products at reaction area is shown in Figure 9, and A, B, and C represent the catalysts as shown before. The fixed packed bed is considered as plus flow reactor and the fluid behaves are like continuous medium. The catalyst is filled homogeneous, so the reactants are reduced step by step. The temperature can be maintained at the same temperature during the reaction because MFEC has good heat transfer properties. Figure 9 shows that A and B catalysts have a similar product distribution. Pt/SAB catalyst (C) has poor heat transfer property, which leads to temperature runaways. The results reveal that C has a different products distribution, which moves down compared with MFEC. The experiments also confirm that gas and liquid selectivity are different between these catalysts.

Figure 9. Mass fractions of normal lumps vs the reactor axis.

than 25 °C when a traditional catalyst is installed. The excellent heat conductivity of Cu and Ni fiber helps to dissipate the reaction heat rapidly. Both MFECs have enhanced heat transfer characteristics. The hydrocracking reaction shows that NiMF-Pt/SAB catalyst has the optimized conversion ratio and liquid products selectivity because the reaction heat can be removed rapidly during the hydrocracking reaction, but the CuMF-Pt/SAB catalyst has poor catalytic properties. The gas selectivity of Pt/ SAB catalyst is very high due to the temperature runaways during the reaction. The NH3-TPD indicates that CuMF-Pt/ SAB catalyst has less weak and moderate acidity, but it has strongest acidity, whereas NiMF-Pt/SAB and Pt/SAB catalyst has similar dominant weak and moderate acidity. TEM shows that CuMF-Pt/SAB catalyst is contaminated by Cu in support, which leads to lower catalytic properties. We will make a further study to avoid the contamination from melting Cu microfiber via optimizing the preparation process of MFEC. On the other hand, the simulation is used to confirm the experimental results. The computational fluid dynamic method confirmed that MFEC has good heat transfer and avoids temperature runaways. The pressure drop of MFEC is also lower than conventional catalysts. Therefore, the Ni MFEC structures are taken as a suitable catalyst structure to enhance

5. CONCLUSION The CuMF-Pt/SAB and NiMF-Pt/SAB catalysts were prepared by a wet-lay method and traditional Pt/SAB catalyst was prepared by an impregnation method. The thermal measurements show that MFEC catalysts can reach the setting temperature faster than traditional Pt/SAB catalyst because MFEC catalyst has better volumetric heat capacity. The temperature of the hydrocracking reaction is stable, and the gap is less than 10 °C when MFEC is applied, whereas the temperature increases significantly and the gap reaches more 5689



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the heat transfer for exothermic hydrocracking reaction, especially mild temperature in the reaction.

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AUTHOR INFORMATION

Corresponding Author

*F. Cao. Tel.: +86 021 64252874. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support from National Natural Science Fund (contract number 21076082) and the subject of the National 863 plan (contract number is 2011AA05A2031).

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ABBREVIATIONS C = concentration of species (mol·m−3) dP = equivalent spherical particle diameter (m) E = Ergun constant F = interaction force (kg·m−2·s−2) J = diffusion flux (kg·m−2·s−1) ke = bed thermal conductivity (W·m−1·K−1) kf = liquor thermal conductivity (W·m−1·K−1) ks = catalyst thermal conductivity (W·m−1·K−1) L = reactor length (m) P = pressure (Pa) Si = momentum source term (kg·m−2·s−2) Sf = energy source term (W·m−3) t = time (s) T = temperature (°C) u = superfical velocity (m·s−1)

Greek Symbols

α = viscosity constants ε = voidage μ = viscosity (Pa·s) ρ = density (kg·m−3) τ = tortuosity factor Subscripts

f = fluid G = gas i = ith component k = kth phase L = liquid S = solid

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REFERENCES

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Heat Transfer Study on Hydrocracking of Paraffin Wax over Microfiber

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