Subscriber access provided by Universitaetsbibliothek | Johann Christian Senckenberg
Fossil Fuels
Effect of Pressure on Ethane Dehydrogenation in MFI Zeolite Membrane Reactor Shailesh Dangwal, Ruochen Liu, Savannah Vaughn Kirk, and Seok-Jhin Kim Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03442 • Publication Date (Web): 30 Mar 2018 Downloaded from http://pubs.acs.org on April 1, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Effect of Pressure on Ethane Dehydrogenation in MFI Zeolite Membrane Reactor Shailesh Dangwal, Ruochen Liu, Savannah Vaughn Kirk, and Seok-Jhin Kim* School of Chemical Engineering, Oklahoma State University, Stillwater, OK 74078, United States
*
Corresponding author: Email:
[email protected]
1 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ABSTRACT
Using a membrane reactor (MR) for producing ethylene by ethane dehydrogenation (EDH) reaction is an effective way. Compared with packed bed reactors (PBR), the EDH MR effectively surpasses the equilibrium limit by timely removing H2. A packed-bed membrane reactor (PBMR) with a Pt/Al2O3 catalyst was used to investigate the effect of pressure on EDH reaction. The EDH reaction was performed in the PBMR for the pressure and temperature range of 1-5 atm and 500-600 oC, respectively. With an increase in reaction temperature, the reaction rate increased which caused higher ethane conversion. Increasing reaction pressure helped in enhancing H2 permeation across the membrane, which significantly increased the ethane conversion. The equilibrium limit of ethane conversion was successfully surpassed by increasing temperature and reaction pressure in the PBMR. Ethane conversion and ethylene selectivity as high as 29% and 97% were obtained at 600 oC and 5 atm for PBMR while corresponding values were 7% and 75% for PBR. The timely removal of H2 from the reaction side also helped in reducing methane formation as H2 is required for the methanation to occur. In addition, a 1D plug flow model was developed and the values for ethane conversion obtained from the model were validated with experimental results. The same model was used to evaluate the ethane conversion beyond the experimental conditions, showing ethane conversion >90% could be obtained.
2 ACS Paragon Plus Environment
Page 2 of 31
Page 3 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
1. Introduction Ethane dehydrogenation (EDH) reaction on supported Pt/Al2O3 catalyst was studied to produce ethylene and H2.1-3 Because it is endothermic reaction, it requires a high operation temperature for reasonable ethane conversion.4-6 Packed bed reactors (PBRs) have thermodynamic limitations in terms of ethane conversion whereas packed bed membrane reactors (PBMRs) are unique. It is because they can combine reaction and separation of reaction products in one step, which thus makes them more useful in achieving enhanced ethane conversion.7-9 The thermodynamic limitation in PBR makes PBMR an exciting topic to investigate as the equilibrium limit can be surpassed in PBMR. Selectivity of membrane towards one of the products helps in shifting the equilibrium of the reaction towards ethylene and H2, which eventually helps to achieve performance better than the thermodynamic limitation. Various studies have been performed for EDH reaction in both PBR and PBMR. For example, Galvita et al.4 conducted EDH reaction in PBR for Pt/Mg(Al)O and Pt-Sn/Mg(Al)O catalyst. The reported ethane conversions were 9.8%, and 4.3% respectively, less than the equilibrium limit of 16% at 600 oC. Gobina et al.
10-13
conducted EDH membrane reactor (MR)
experiments for Pd–Ag membrane with Pt/Al2O3 as a catalyst showing ethane conversion of 18% against an equilibrium limitation of 3.5%. Szegner. et al. 14 used composite alumina PBMR with Pt-Sn/Al2O3 for EDH, and ethane conversions were 16% and 8% in PBMR and PBR mode, respectively, at 550 oC. However, there have been very few reports investigating the effect of reaction pressure on EDH reaction. As EDH is a volume expansion reaction, ethane conversion is expected to
3 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
decrease with increase in pressure in PBR but in PBMR, the effect of reaction pressure on reaction performance yields different results. For example, there have been a few studies performed on the impact of pressure in a PBMR operating at high temperature. Brunetti et al.15, 16 performed the water-gas shift (WGS) in the combination of CuO/CeO2 catalyst and a silica MR. The temperature and pressure were varied in a range of 220–290 oC and up to 600 kPa respectively, resulting in the optimum CO conversion of 95%, which was 8% higher than PBR. Lee et al.17 not only studied the effect of pressure on reaction equilibrium but also the permeability for catalytic dry-reforming of methane for a MR. The MR showed better performance than a PBR but with increasing reaction pressure, the enhancement in H2 and CO yields in the MR went through a maximum and then declined. Barbieri et al.18 studied the performance of the MR in a WGS reaction. In the MR mathematical modelling, the impact of feed flow rate, feed pressure and temperature on catalyst performance was studied, which confirmed CO conversion enhancement and reduction of MR volumes. Alexander et al. 19 stated that WGS catalytic membrane reactor combined with a Pd-membrane shows enhancement in the efficiency of the WGS reaction when operated at elevated temperature and pressure. In this experiment, MFI zeolite membrane reactors were tested for their effectiveness in EDH and other high-temperature catalytic reactions of 400-500 oC.20-26 MFI zeolite membranes were used to investigate the impact of reaction pressure on the EDH reaction in a range of 1-5 atm. In addition, a zeolite PBMR was investigated by modelling using a one-dimensional plug flow reactor (PFR) model, which was validated to simulate and examine the dependencies of EDH PBMR performance under the operating conditions.
2. Experimental section 4 ACS Paragon Plus Environment
Page 4 of 31
Page 5 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
2.1 Membrane synthesis The MFI zeolite membrane was synthesized on a seeded α-alumina disk by secondary growth.26 For MFI zeolite membrane preparation, macroporous α-alumina disks (Coorstek) having thickness of 1 mm and diameter of 1 inch, and porosity of 25% were used as supports. The α-alumina disks were polished before membrane growth using the sand paper and polisher.27,
28
After polishing, α-alumina supports were dip-coated with MFI seeds and then
dried, and calcined using the same procedures described elsewhere.28,
29
The hydrothermal
synthesis solution was obtained by mixing tetrapropylammonium hydroxide (TPAOH, 1 M, Sigma–Aldrich), tetraethyl orthosilicate (TEOS, 98%, Acros) and deionized water with molar composition of TEOS: 0.095 TPAOH: 35.420 H2O. The detailed procedures are provided elsewhere.20, 21 The permeance of membrane for gas species i is given by: Pm ,i =
Qi , Am ⋅ ∆ Pi
(1)
(i = H 2 , C 2 H 6 ,K )
where Am (m2) is the membrane area (2.0 cm2); Qi (mol/s) is the amount of component i over time t (s); and ∆Pi (Pa) is the transmembrane pressure, ∆ Pi = ( Pi ) f − ( Pi ) p , where (P) i f and ( Pi ) p are the partial pressures of i in the feed and permeate sides, respectively.
The H2/C2H6 perm-selectivity (aoH2/C2H6) is given by the following equation
αHo2/C2H6 =
Pm,H2
(2)
Pm,C2H6
The H2/C2H6 separation factor (αH2/C2H6) for the binary mixture is given by
5 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
αH 2/C2H 6 =
( yH 2 / yC2H 6 ) permeate
Page 6 of 31
(3)
( yH 2 / yC2H 6 ) feed
where yH2 and yC2H6 are mole fractions of H2 and C2H6, respectively. These ܲ, and ܧ, were calculated by regressing the permeation data of H2/C2H6 and H2/C2H4
binary mixtures, which was measured from room temperature to 600 °C at 1 atm feed and permeate-side pressure.
E Pm,i = P om,i exp − a,i RT
(i = C2 H6 , C2H4 , H2 )
(4)
2.2. Pressurized EDH reaction In Figure 1, the system used for studying the effect of reaction pressure on EDH MR is shown. This system is close to our previous apparatus with minor modification.26 Sealed by graphite gaskets, the disc membrane was installed in a stainless steel cell, and 550 mg of Pt/Al2O3 catalyst was distributed on top of the membrane surface. In order to allow the catalyst bed to be firm and allow the gases to diffuse freely, a quartz wool and carbon cloth pad was placed over the catalyst bed. Ar was used in the permeate side at atmospheric pressure and its flow rate was maintained at 20 cm3/min for all experiments. Mass flow controllers (MFC, Aalborg) were then used to control the flow rate of gases. A back pressure regulator was installed in the outlet retentate stream to control the reaction pressure and pressure gauges were connected in the feed and retentate streams to check the pressure inside the reactor. Experiments were conducted for 1-5 atm in both PBMR and PBR for WHSV = 0.74 h-1 and temperature ranging from 500 - 600 oC with FAr of 20 cm3/min. 6 ACS Paragon Plus Environment
Page 7 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
The permeate and retentate gases were analyzed by an online GC (Shimadzu GC2014) equipped with an alumina plot column for the flammable ionization detector (FID) and a molecular sieve 13X column for the thermal conductivity detector (TCD). The exit stream flow rates from the reactor were monitored by a soap film flow meter. The cooling and heating rate was 0.5 oC/min. The ethylene product, unreacted ethane, H2, and the byproducts (methane, propylene, benzene, and xylene) from side reactions, such as thermal cracking and catalytic cracking, were analyzed to study the impact of reaction conditions on product selectivity and reaction conversion. The ethane conversion was obtained by using the total ethane feed flow rates entering as feed and exiting the reactor in both the permeate and retentate streams:
FCout2H6 χC2H6 =1− in FC2H6
(5)
The selectivity for gas component i is defined as:
Fiout − Fiin Si = in FC2H6 − FCout2H6
(i = C2H4 , CH4 K)
(6)
The yield for gas component i is calculated by:
Yi =
χ C 2 H 6 × Si
(i = C 2 H 4 , CH 4 K)
100
(7)
The weight hourly space velocity (WHSV) is defined by:
WHSV =
2H 6 υ Cfeed × ρC 2H 6
(8)
m cat
7 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 8 of 31
2H 6 where υ Cfeed is the feed volumetric rate of ethane at standard temperature and pressure (STP),
and mcat is the mass of catalyst. The H2 recovery RH2 for PBMR system is defined as
RH2 =
Amount of H 2 in permeate Total amount of H 2 generated by reaction
(9)
The permeate side H2 concentration is defined as follows
yH 2, P =
J H2 J H2 + JC2H6 + JC2H4 + JC3H8 + JC3H6
(10)
Where JH2, JC2H6 , JC2H4, JC3H8, and JC3H6 are respectively the H2, C2H6, C2H4, C3H8, and C3H6 fluxes across the membrane. The catalyst for the PBMR and PBR experiments was 1% Pt/Al2O3 (Sigma Aldrich) denoted here as ‘Pt/Al2O3’ catalyst. For using membrane cell in PBR mode, the exit of the reaction side stream was introduced to the inlet of the sweeping side and no sweeping gas was used. In Table 1, all operating conditions are listed.
8 ACS Paragon Plus Environment
Page 9 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Figure 1. Schematic for the apparatus for membrane reactor system used for EDH reaction. Table 1. EDH membrane reactor conditions Experimental
Calculation
Reaction temperature, °C
500 - 600
500 - 800
Weight hourly space velocity (WHSV), h-1
0.74
0.15 - 2.81
C2H6 feed flow rate, FC2H6, cm3 (STP)/min
3
1-19
Ar sweeping flow rate, FAr, cm3 (STP)/min
20
3-30
1% Pt/Al2O3 catalyst loading (mcat), g
0.55
0.55
Reaction pressure at retentate exit, atm
1.0-5.0
1.0-8.0
Permeate pressure, atm
1.0
1.0
2.3. Reaction modeling The EDH reaction is endothermic as shown in 9 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Θ C 2 H 6 ↔ C 2 H 4 + H 2 , ∆ H 298 . 15 K = 136.94 kJ/mol
Page 10 of 31
(11)
The following rate expression was used for modelling and taken from reference. 11, 14
Pethylene × Phydrogen Rate = k Pethane − K Eq
(12)
where k is the kinetic rate constant, Phydrogen, Pethylene and Pethane are the partial pressures of H2, ethylene and ethane in the reaction side, and Keq is the equilibrium constant. The expression for kinetic rate constant and equilibrium constants have been studied in the literature. 30, 31
−E k = k0 exp RT
(13)
−17000 Keq = 7.28×106 exp T + 273
(14)
where the rate constant (k0) and activation energy (E) for the system are 4.23×10−3 mol m-2 s-1 Pa-1 and 20.6 kcal mol-1 and R is the gas constant. 14, 21, 32-34 For reactor modeling, multiple assumptions used were: ideal gas behavior, negligible mass-transfer resistance in the macroporous substrate and the catalyst layer (~ 760 µm), isothermal steady state operation, and negligible side reactions. The average particle sizes of Pt/Al2O3 catalysts was ~22 nm.35 The 1D PFR model was used, which considers both reaction (feed) side and permeate side under plug-flow conditions. In Figure S1 (Supporting information), the schematic of the MR structure is shown for both experiments and model calculations.
10 ACS Paragon Plus Environment
Page 11 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
For a differential section of the reactor, the 1D PFR model was considered and the mass balance equation is given by the following equations: dFi = Fi
A + dA
− Fi
A
= dn i − dQ i
(15)
dni = ν i rA dA
(16)
dQi = Pm,i (∆Pi ) A dA
(17)
where Fi (mol/s) is the feed flow rate , A (m2) is the membrane area, ∆Pi (Pa) is the pressure difference for species i across the membrane, Pm,i (mol m-2 s-1 Pa-1) is the permeance of component i, vi is the stoichiometric coefficient of component i, ni is the rate of material generation by reaction (mol/s), and Qi (mol/s) is the gas flow rate through the membrane. Numerical methods were used to solve the differential equations after dividing membrane into small sections (total 150) of equal area. 3. Results and discussion 3.1 Membrane properties Figure S2 (supporting information) presents the SEM images for surface and cross sections of the MFI zeolite membranes, which shows well intergrown polycrystalline films having thickness of~7 µm. At room temperature, the membrane showed preferential adsorption of C2H6 and C2H4 in comparison to H2. One possible explanation can be adsorption-diffusion mechanism that adsorption effect dominates over diffusion, thus C2H6 and C2H4 showed higher permeance than H2. As the temperature is increased, the surface coverage of the adsorbing components (C2H6 and C2H4) decreased and opened up pore space for the nonadsorbing gas (H2) to enter and pass through.36 Therefore, H2 diffusion became more predominant than the 11 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 12 of 31
adsorption of C2H6 and C2H4. ܲ, and ܧ, in equation (4) are shown in Table 2 for H2 and
ethane at different temperatures. Table 2. P0m,i and Ea,i for equation (4) and αH 2/i at different temperatures 500 oC
550 oC
600 oC
H2
C2H6
H2
C2H6
H2
C2H6
Pom, 10-8, mol m-2 s-1 Pa-1
12.9
2.60
13.4
3.10
13.8
3.30
Ea,i, kJ/mol
0.96
3.49
1.02
3.76
1.09
3.92
αH 2/i
-
3.06
-
3.18
-
3.31
3.2 EDH reaction 3.2.1. Effect of pressures The MFI-type zeolite PBMR was used to investigate the impact of reaction pressure on EDH reactions at 1-5 atm. Experiments were conducted at 500-600 °C with pure ethane as a feed, WHSV of 0.74 h−1 and Ar sweeping flow rate (FAr) of 20 cm3/min, and, as shown in Figure 2, the PBMR surpassed the equilibrium limit at all operating pressures. As the pressure was increased, more H2 permeated across the membrane, which allowed the reaction to move more in the direction of the product side and helped in enhancing the ethane conversion. However, ethane conversion levelled off at ~5 atm, which can be attributed to the limiting reaction rate at that temperature. For PBR, it was found that ethane conversion decreased with increasing pressure because it is a volume expansion reaction. The ethane conversion increased from 24% to 29% for a pressure increase from 1 to 5 atm. With these experimental results, the 1D PFR model was successfully validated. It should be noted that the PBMR model predicted slightly higher values
12 ACS Paragon Plus Environment
Page 13 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
than experimental values. This is probably because the modeling does not take into account the decrease in actual permeance values (H2, C2H4, and C2H6) which results from coke deposition on the membrane surface over time. The PBR model also underestimated the ethane conversion values likely because only EDH reaction is considered even though there are substantial amounts of by-products (methane, propylene, and isobutylene) from multiple side reactions.
13 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Figure 2. Effect of reaction pressure on ethane conversion for (a) 500oC, (b) 550oC, and (c) 600oC in PBMR and PBR (WHSV = 0.74 h−1; FAr = 20 cm3/min; and pperm = 1 atm).
The effect of reaction pressure on ethylene selectivity and ethylene yield was also investigated. Figure 3 shows the increase of reaction pressure enhanced both selectivity and yield of ethylene in PBMR. With increase in reaction pressure, more ethylene was formed, therefore increasing the ethylene selectivity and thus ethylene yield. In PBR operation, an increase in pressure decreased the amount of product formed because it is a volume expansion reaction, which causes a decrease in ethylene selectivity and ethylene yield.
14 ACS Paragon Plus Environment
Page 14 of 31
Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Figure 3. Effect of reaction pressure on ethylene selectivity and ethylene yield for (a) 500oC, (b) 550oC, and (c) 600oC in PBMR and PBR (WHSV = 0.74 h−1; FAr = 20 cm3/min; and pperm = 1 atm).
The effect of reaction pressure on RH2 and yH2,p in PMR and PBMR was also investigated as shown in Figure 4. The enhancement of reaction pressure increased the driving force for the H2 transfer though the membrane, which led to greater RH2. While RH2 increased with feed pressure, yH2,p showed a decreasing trend with increasing reaction pressure. This observation 15 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
implies the possibility that the increase of reaction pressure also enhanced the permeation of other gases (e.g., ethane and ethylene). The decrease of yH2,p with high reaction pressure but limited αH2/C2H6 is due to excessive permeation of unreacted ethane. Due to this, more ethane was present in permeate side than H2 at elevated pressure, which explains the decrease in yH2,p with enhancement in reaction side pressure.
16 ACS Paragon Plus Environment
Page 16 of 31
Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Figure 4. Effect of reaction pressure on RH2 and yH2,p for (a) 500 oC, (b) 550 oC, and (c) 600 oC in PBMR (WHSV = 0.74 h−1; FAr = 20 cm3/min; and pperm = 1 atm).
3.2.2. H2-to-C2H6 ratio in the feed In all the experiments, a catalyst deactivation was observed, which rapidly decreased catalyst activity in ~1 h after the EDH reaction started. Catalyst deactivation and regeneration are important considerations for alkane dehydrogenation processes. Some dehydrogenation technologies use H2 as a feed diluent to reduce coking and elongate catalyst lifetime between regeneration cycles.37 H2 is considered to inhibit the formation of coke because it decreases the content of coke precursors (light hydrocarbons such as ethylene and propylene), which can form the oligomers and carbonaneous compounds.4
This aspect for both PBR and PBMR was
evaluated. Figure 5a-c shows the influence of RH2/C2H6 (H2 concentration in the feed), on ethylene yield, ethylene selectivity and conversion of ethane. In Figure 5d, it was found that there was substantial change in ethane conversion values in the first one hour (region A1). All the values shown in this study were taken after one hour (region A2) when steady state was ensured so that there was almost no change in outlet composition. It was found that ethane conversion and ethylene selectivity decreased as RH2/C2H6 increased. When H2 was used in the feed, there was more H2 in the reaction side, which caused the shift of the EDH reaction in the direction of reactant thus lesser conversion. Moreover, when there is more H2 in the feed, hydrogenolysis reaction also becomes important and selectivity of ethylene also decreased as RH2/C2H6 increased.
17 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
However, ethane conversion and ethylene selectivity values for the PBMR were higher than PBR as expected. As shown in Figure 5d, both ethane conversion and ethylene selectivity significantly declined with increasing time-on-stream, especially in the absence of H2. This is due to catalyst deactivation, which occurs via deposition of carbonaceous matter (generated by undesired side reactions such as propylene cracking) on the active surface of the catalyst.4 However, the addition of H2 provided a much more stable time-dependence of the catalyst activity and selectivity up to ~6 h of EDH, albeit with an initially lower conversion than with a pure hydrocarbon feed. The PBR and PBMRs showed similar trends of ethane conversion and ethylene selectivity. The initial lower conversion is probably because an increase in H2 partial pressure not only decreases the thermodynamic driving force but also increases the competitive adsorption of H2 with ethane on the catalyst.38-41
18 ACS Paragon Plus Environment
Page 18 of 31
Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Figure 5. Effect of H2 concentration in feed on (a) ethane conversion, (b) ethylene selectivity, (c) ethylene yield, and (d) ethane conversion versus EDH reaction time for PBMR at WHSV of 0.74 h-1, temperature of 600 °C, pfeed of 1 atm, pperm of 1 atm, and the feed (H2/C2H6 mixtures) of 10 cm3/min.
3.2.3. Methanation
19 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 20 of 31
Methanation is an important side reaction in the EDH reaction. The effect of pressure on methane selectivity was examined for both PBR and PBMR at 500-600 °C. The WHSV and FAr were fixed at 0.74 h-1 and 20 cm3/min, respectively. As shown in Figure 6, methane selectivity was found to be a function of reaction pressure between PBR and PBMR operations. The methane selectivity decreased with increasing pressure for the PBMR because the increase of reaction pressure transferred more H2, an important reactant (equation 18 and equation 19) for the methanation reaction, to the permeate side. Thus because of less H2 available in the feed side, less methanation occurred.
42
On the other hand, for PBR, the overall reaction is not
thermodynamically favored with an increase in reaction pressure, thus methane selectivity also decreased. C 2 H 6 → 2C * + 6 H 2
(18)
C * + 4 H * → CH 4
(19)
20 ACS Paragon Plus Environment
Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Figure 6. Effect of pressure on methane selectivity at (a) 500 °C, (b) 550 °C, and (c) 600 °C for PBR and PBMR (WHSV = 0.74 h-1; and FAr = 20 cm3/min). 3.3. Model calculations The objective of the modelling was to study the effects of pressure on ethane conversion in EDH reactors and find the most optimized reaction conditions. To investigate the impact of
21 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
reaction pressure on ethane conversion beyond experimental values, a 1D plug flow model was used.
Figure 7. Effect of reaction pressure and temperature on ethane conversion for (a) WHSV = 0.74 h-1, (b) WHSV = 1.04 h-1, (c) WHSV = 1.34 h-1, and (d) WHSV = 1.63 h-1 for FAr = 20 cm3/min. 22 ACS Paragon Plus Environment
Page 22 of 31
Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Figure 7 depicts the effect of temperature and reaction pressure on ethane conversion in PBMR. The simulation was performed for WHSV= 0.15 - 2.81 h-1 in temperature range of 500800 oC and pressure range of 1-8 atm for FAr = 20 cm3/min. It was shown that increasing temperature and pressure enhanced ethane conversion, however, ethane conversion tended to level off above 6 atm. One possible reason is that the rate of product formation became equal to the product permeation rate across the membrane above 6 atm and the whole process became reaction limited. The highest ethane conversion obtained was ~75% at 800 oC and 6 atm for WHSV = 0.74 h-1. These results indicate that pressure and temperature have a huge impact on ethane conversion and a membrane with moderate performance can possibly be used to achieve high ethane conversion if appropriate operating conditions are chosen. Figure 8 presents the impact of reaction pressure on ethane conversion along the reactor length for PBR and PBMR. Along the reactor length, it was observed that ethane conversion for PBMR increased with enhancing the reaction pressure, which is due to the increase in H2 permeate across the membrane. However, for PBR, conversion of ethane decreased with the length of the reactor with an increase in pressure because EDH is a volume expansion reaction and an increase in reaction pressure shifts equilibrium towards the reactant side and thus ethane conversion decreases.
23 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Figure 8. Effect of reaction pressure along the normalized reactor length on ethane conversion for (a) PBR at 600 oC , (b) PBR at 650 oC, (b) PBMR at 600 oC, and (d) PBMR at 650 oC for FAr = 20 cm3/min and WHSV = 0.45 h-1.
24 ACS Paragon Plus Environment
Page 24 of 31
Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Figure 9 depicts the impact of reaction pressure and membrane area. Membrane area is shown as A/A0 where A is area of membrane used in calculations and A0 is the area of membrane used in experiment (2.0 cm2). Membrane area had a significant impact on ethane conversion, which was as high as 93% at pressure of 3.0 atm and A/A0 of 1.5. The model calculations were performed at FAr of 20 cm3/min and WHSV of 0.45 h-1.
Figure 9. Effect of reaction pressure along the normalized area on ethane conversion for PBMR at (a) 550 oC, and (b) 650 oC for FAr = 20 cm3/min and WHSV = 0.45 h-1.
4. Conclusions The MFI zeolite membrane with moderate H2 selectivity and H2 permeance was successfully used to investigate the impact of pressure on packed bed membrane reactors (PBMR) for ethane dehydrogenation (EDH) reaction. It was found that the thermodynamic limitation was not only successfully surpassed but the 1D PFR model was also developed to 25 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
study the impact of operating conditions beyond experimental conditions. The maximum ethane conversion, ethylene selectivity and ethylene yields obtained were 29%, 97%, and 28%, respectively. The impact of reaction pressure on H2 recovery (RH2) and permeate side H2 concentration (yH2,p) was also studied. RH2 increased with increasing pressure as more H2 was transferred to permeate side but yH2,p decreased with increase in pressure because ethane diffusion across the membrane also increased significantly with an increase in pressure. Increasing the pressure adversely affected the methanation as less H2 was available in the reaction side for the methanation reaction to proceed. H2 was also used in the feed to enhance the catalyst stability. Increasing H2 amount in the feed enhanced the stability of the catalyst. However, there was a slight decrease in reaction performance parameters with increase in H2 concentration in the feed. The simulation results accurately predicted the ethane conversion for both packed bed reactor PBMR and (PBR). The model predicted ethane conversion of 93% at WHSV of 0.45 h-1,pressure of 3.0 atm and A/A0 of 1.5. Overall, EDH PBMR shows increase in ethane conversion and significantly reduces the undesirable side reactions with enhancement of reaction pressure.
Acknowledgments The authors gratefully acknowledge funding from Oklahoma State University. We also give special thanks to Pamela Reynolds for editing the manuscript.
26 ACS Paragon Plus Environment
Page 26 of 31
Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
References 1.
Cavani, F.; Ballarini, N.; Cericola, A., Oxidative dehydrogenation of ethane and propane:
How far from commercial implementation? Catalysis Today 2007, 127, (1), 113-131. 2.
Sanfilippo, D.; Miracca, I., Dehydrogenation of paraffins: synergies between catalyst
design and reactor engineering. Catalysis Today 2006, 111, (1), 133-139. 3.
Bhasin, M.; McCain, J.; Vora, B.; Imai, T.; Pujado, P., Dehydrogenation and
oxydehydrogenation of paraffins to olefins. Applied Catalysis A: General 2001, 221, (1), 397419. 4.
Galvita, V.; Siddiqi, G.; Sun, P.; Bell, A. T., Ethane dehydrogenation on Pt/Mg(Al)O and
PtSn/Mg(Al)O catalysts. J. Catal. 2010, 271, (2), 209-219. 5.
James, O. O.; Mandal, S.; Alele, N.; Chowdhury, B.; Maity, S., Lower alkanes
dehydrogenation: Strategies and reaction routes to corresponding alkenes. Fuel Process. Technol 2016, 149, 239-255. 6.
Lobera, M. P.; Escolástico, S.; Serra, J. M., High ethylene production through oxidative
dehydrogenation of ethane membrane reactors based on fast oxygen‐ion conductors. ChemCatChem 2011, 3, (9), 1503-1508. 7.
Michaels, A. S., New separation technique for CPI. Chemical Engineering Progress
1968, 64, (12), 31-&. 8.
Wang, D.; Humphrey, A.; Arthur, E., Enzyme detergents, already a multimillion-dollar
industry, are but one concrete result from… Biochemical Engineering. Chemical Engineering 1970, 76, 108-120. 9.
Pfefferle, W. C., Process for dehydrogenation. In Google Patents: 1966.
27 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
10.
Gobina, E.; Hughes, R., Ethane dehydrogenation using a high-temperature catalytic
membrane reactor. Journal of membrane science 1994, 90, (1-2), 11-19. 11.
Gobina, E.; Hou, K.; Hughes, R., Ethane dehydrogenation in a catalytic membrane
reactor coupled with a reactive sweep gas. Chem. Eng. Sci. 1995, 50, (14), 2311-2319. 12.
Gobina, E.; Hughes, R., Reaction assisted hydrogen transport during catalytic
dehydrogenation in a membrane reactor. Appl. Catal., A 1996, 137, (1), 119-127. 13.
Champagnie, A.; Tsotsis, T.; Minet, R.; Wagner, E., Ethane dehydrogenation in a
catalytic membrane reactor. ChemInform 1992, 23, (28). 14.
Szegner, J.; Yeung, K. L.; Varma, A., Effect of catalyst distribution in a membrane
reactor: experiments and model. AlChE J. 1997, 43, (8), 2059-2072. 15.
Brunetti, A.; Barbieri, G.; Drioli, E.; Granato, T.; Lee, K.-H., A porous stainless steel
supported silica membrane for WGS reaction in a catalytic membrane reactor. Chemical Engineering Science 2007, 62, (18), 5621-5626. 16.
Brunetti, A.; Barbieri, G.; Drioli, E.; Lee, K.-H.; Sea, B.; Lee, D.-W., WGS reaction in a
membrane reactor using a porous stainless steel supported silica membrane. Chemical Engineering and Processing: Process Intensification 2007, 46, (2), 119-126. 17.
Lee, D.; Hacarlioglu, P.; Oyama, S. T., The effect of pressure in membrane reactors:
trade-off in permeability and equilibrium conversion in the catalytic reforming of CH 4 with CO 2 at high pressure. Topics in catalysis 2004, 29, (1), 45-57. 18.
Barbieri, G.; Brunetti, A.; Granato, T.; Bernardo, P.; Drioli, E., Engineering evaluations
of a catalytic membrane reactor for the water gas shift reaction. Industrial & engineering chemistry research 2005, 44, (20), 7676-7683.
28 ACS Paragon Plus Environment
Page 28 of 31
Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
19.
Augustine, A. S.; Ma, Y. H.; Kazantzis, N. K., High pressure palladium membrane
reactor for the high temperature water–gas shift reaction. International Journal of Hydrogen Energy 2011, 36, (9), 5350-5360. 20.
Kim, S.-J.; Xu, Z.; Reddy, G. K.; Smirniotis, P.; Dong, J., Effect of pressure on high-
temperature water gas shift reaction in microporous zeolite membrane reactor. Ind. Eng. Chem. Res. 2012, 51, (3), 1364-1375. 21.
Kim, S.-J.; Yang, S.; Reddy, G. K.; Smirniotis, P.; Dong, J., Zeolite membrane reactor
for high-temperature water-gas shift reaction: effects of membrane properties and operating conditions. Energy Fuels 2013, 27, (8), 4471-4480. 22.
Tang, Z.; Kim, S.-J.; Reddy, G. K.; Dong, J.; Smirniotis, P., Modified zeolite membrane
reactor for high temperature water gas shift reaction. J. Membr. Sci. 2010, 354, (1), 114-122. 23.
Tang, Z.; Dong, J.; Nenoff, T. M., Internal surface modification of MFI-type zeolite
membranes for high selectivity and high flux for hydrogen. Langmuir 2009, 25, (9), 4848-4852. 24.
Kim, S.-J.; Tan, S.; Taborga Claure, M.; Briones Gil, L.; More, K. L.; Liu, Y.; Moore, J.
S.; Dixit, R. S.; Pendergast Jr, J. G.; Sholl, D. S., One-step synthesis of zeolite membranes containing catalytic metal nanoclusters. ACS Appl. Mater. Interfaces 2016, 8, (37), 24671-24681. 25.
Kim, S.-J.; Liu, Y.; Moore, J. S.; Dixit, R. S.; Pendergast Jr, J. G.; Sholl, D.; Jones, C.
W.; Nair, S., Thin hydrogen-selective SAPO-34 zeolite membranes for enhanced conversion and selectivity in propane dehydrogenation membrane reactors. Chem. Mater. 2016, 28, (12), 43974402. 26.
Dangwal, S.; Liu, R.; Kim, S.-J., High-temperature ethane dehydrogenation in
microporous zeolite membrane reactor: Effect of operating conditions. Chemical Engineering Journal 2017, 328, 862-872.
29 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
27.
Tang, Z.; Kim, S.-J.; Gu, X.; Dong, J., Microwave synthesis of MFI-type zeolite
membranes by seeded secondary growth without the use of organic structure directing agents. Microporous Mesoporous Mater. 2009, 118, (1), 224. 28.
Kim, S.-J.; Xu, Z.; Reddy, G. K.; Smirniotis, P.; Dong, J., Effect of pressure on high-
temperature water gas shift reaction in microporous zeolite membrane reactor. Ind. Eng. Chem. Res. 2012, 51, (3), 1364. 29.
Kim, S.-J.; Yang, S.; Reddy, G. K.; Smirniotis, P.; Dong, J., Zeolite membrane reactor
for high-temperature water-gas shift reaction: effects of membrane properties and operating conditions. Energy Fuels 2013, 27, (8), 4471. 30.
Szegner, J.; Yeung, K. L.; Varma, A., Effect of catalyst distribution in a membrane
reactor: experiments and model. AlChE J. 1997, 43, (8), 2059-2072. 31.
Bose, A. C., Inorganic membranes for energy and environmental applications. Springer
2009, p. 307-308; p 2. 32.
Hasany, M.; Malakootikhah, M.; Rahmanian, V.; Yaghmaei, S., Effect of hydrogen
combustion reaction on the dehydrogenation of ethane in a fixed-bed catalytic membrane reactor. Chin. J. Chem. Eng. 2015, 23, (8), 1316-1325. 33.
Rodriguez, M. L.; Ardissone, D. E.; Heracleous, E.; Lemonidou, A. A.; López, E.;
Pedernera, M. N.; Borio, D. O., Oxidative dehydrogenation of ethane to ethylene in a membrane reactor: A theoretical study. Catal. Today 2010, 157, (1), 303-309. 34.
Hansen, N.; Krishna, R.; van Baten, J.; Bell, A.; Keil, F., Reactor simulation of benzene
ethylation and ethane dehydrogenation catalyzed by ZSM-5: A multiscale approach. Chem. Eng. Sci. 2010, 65, (8), 2472-2480.
30 ACS Paragon Plus Environment
Page 30 of 31
Page 31 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
35.
Manasilp, A.; Gulari, E., Selective CO oxidation over Pt/alumina catalysts for fuel cell
applications. Applied Catalysis B: Environmental 2002, 37, (1), 17-25. 36.
Dong, J.; Lin, Y.; Kanezashi, M.; Tang, Z., Microporous inorganic membranes for high
temperature hydrogen purification. Journal of Applied Physics 2008, 104, (12), 13. 37.
Avila, A.; Yu, Z.; Fazli, S.; Sawada, J.; Kuznicki, S., Hydrogen-selective natural
mordenite in a membrane reactor for ethane dehydrogenation. Microporous and Mesoporous Materials 2014, 190, 301-308. 38.
Gbenedio, E.; Wu, Z.; Hatim, I.; Kingsbury, B. F.; Li, K., A multifunctional Pd/alumina
hollow fibre membrane reactor for propane dehydrogenation. Catalysis Today 2010, 156, (3), 9399. 39.
Wu, Z.; Hatim, I.; Kingsbury, B. F.; Gbenedio, E.; Li, K., A novel inorganic hollow fiber
membrane reactor for catalytic dehydrogenation of propane. AIChE journal 2009, 55, (9), 23892398. 40.
Simón, E.; Rosas, J. M.; Santos, A.; Romero, A., Coke formation in copper catalyst
during cyclohexanol dehydrogenation: Kinetic deactivation model and catalyst characterization. Chemical engineering journal 2013, 214, 119-128. 41.
Benito, P. L.; Gayubo, A. G.; Aguayo, A. T.; Castilla, M.; Bilbao, J., Concentration-
dependent kinetic model for catalyst deactivation in the MTG process. Industrial & engineering chemistry research 1996, 35, (1), 81-89. 42.
Szegner, J.; Yeung, K. L.; Varma, A., Effect of catalyst distribution in a membrane
reactor: experiments and model. AIChE Journal-American Institute of Chemical Engineers 1997, 43, (8), 2059-2072.
31 ACS Paragon Plus Environment
