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Kinetics of propane oxidative dehydrogenation to propylene using lattice oxygen of VOx/CaO/Al2O3 catalysts Mohammad Mozahar Hossain Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00759 • Publication Date (Web): 24 Mar 2017 Downloaded from http://pubs.acs.org on March 27, 2017

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Industrial & Engineering Chemistry Research

Kinetics of propane oxidative dehydrogenation to propylene using lattice oxygen of Al2O3 catalysts VOx/CaO/ Mohammad M. Hossain* Department of Chemical Engineering, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia Abstract This article presents kinetics of oxidative dehydrogenation (ODH) of propane to propylene over VOx/CaO-Al2O3 and VOx/CaO-Al2O3 catalysts in absence of gas phase oxygen. In addition to their catalytic role, the catalysts also serve as the source of lattice oxygen. XRD and temperature programmed reduction analysis indicate the availability of different types of VOx species on the prepared catalysts surfaces. The presence of CaO influences the formation of VOx species, reducibility and oxygen carrying capacity of the catalysts. All these have significant influence on propylene selectivity. The ODH of propane experiments are conducted in a CREC Riser Simulator under circulating fluidized bed conditions. It is observed that VOx/CaO-Al2O3 displays higher propylene selectivity (94.1%) and the lower selectivity of CO2 due to it’s moderate active sites-support interactions. The reaction network and Langmuir-Hinshelwood type kinetics model is developed using a wide-ranging set of experiments. The availability of reactive lattice oxygen is expressed by a decay model. Nonlinear regression is employed to estimate activation energies with their respective conﬁdence intervals. The developed model satisfactorily predicted product compositions under various operating conditions. For propylene formation, VOx/CaO-Al2O3 requires lower activation energy (120.3 kJ/mol) than that of VOx/CaO (126.7 kJ/mol). On the contrary, VOx/CaO-Al2O3 needs higher activation energies (55.2 kJ/mol) for undesired CO2 formation as compared to the activation energies for CO2 formation using VOx/CaO catalyst (32.8 kJ/mol). These values are consistent to the product selectivity as observed in the catalyst evaluation experiments. Keywords: Oxidative dehydrogenation, propane, propylene, TPR, kinetic modeling, LangmuirHinshelwood model *Corresponding author: Mohammad M. Hossain KFUPM Box: 5050; Dhahran 31261, Saudi Arabia Tel.: +966-13-860-1478; Fax: +966-13-860-4234. E-mail: [email protected]

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1. Introduction Propylene is one of the most important building blocks in the petrochemical industry having a wide range of applications.1 It was first produced as a byproduct of the petroleum refining, which could not meet the market demand of propylene.2 Steam cracking and fluid catalytic cracking of petroleum feed stocks are the other sources of propylene.3,4 Although the catalytic cracking technologies are capable of meeting the worldwide propylene demand, they suffer with high energy requirement, considerable coke formation, high operating cost and low efficiency.5 In this regard, production of propylene via oxidative dehydrogenation (ODH) of propane is an attractive alternative due to the exothermic nature of the overall reactions, which results minimal energy demand.4 In addition, the abundant availability of propane in natural gas/refinery gases, makes propylene production less expensive via ODH route.4,5 The ODH of propane also contributes minimum environmental pollutions due to its low energy demand and high product selectivity.4,5 Another important advantage of ODH of propane is the availability of water as a product, which helps to avoid thermodynamics limitations as observed during catalytic cracking reactions.6,7 Appropriate reactor selection is an important aspect for developing commercial scale ODH of propane technology.3,5 In this regard, the fluidized bed processes are more advantageous than the fixed processes.3,5 The major advantages of fluidized bed include less/no mass transfer limitations, uniform reaction temperature and possibility of catalyst regeneration. The present author and his collaborators investigated a circulating fluidized bed ODH process for ethane and propane using solid phase oxygen as source of oxidation agent.3,5,8-11 In this arrangement, the catalyst acts as a source of lattice oxygen in addition to its traditional catalytic role. After reaction, the reduced catalyst needs to be regenerated by circulating through a regenerator, maintained oxidative (air) environment and appropriate reaction conditons.8-11


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The outstanding challenge of the after mentioned process is the development of a fluidizable catalyst with appreciable activity and oxygen carrying capacity. Generally, vanadium based catalysts show good selectivity for oxidative dehydrogenation reactions.3,4,12 Alumina and silica based materials are good supports to achieve the required fluidizability of the VOx based catalysts. However, the interaction between alumina and VOx forms complex species, which are less selective to the propylene.3,12 On the other hand, the performance of the VOx based catalyst can be improved by controlling their surface morphology and acid-base properties.13-21 Keeping this in mind, a mixed Al2O3/CaO supports is synthesized to achieve appropriate balance between metal and support functionality. It is expected that the modification of the support by CaO will alter the level of acidity of alumina that minimize VOx-support interaction. The moderate VOx-support interaction facilitates efficient surfaces processes (propane adsorption-reaction-products desorption).22 In addition to the support modification, present investigation also considered the kinetics of the solid phase ODH of propane over VOx/CaOAl2O3 to gain better understanding of the CaO modification effects. In literature, several models have been studied to describe the oxidative dehydrogenation of lighter hydrocarbons to olefins.2330

For example, Mars-Van Krevelen mechanism and Langmuir-Hinshelwood equations are

developed to study the kinetics of ODH of on a VOx/γ-Al2O3 catalyst.31 In another study, Eley– Rideal mechanism was applied to represent the kinetics of the both ethane and propane using a VOx/SiO2 catalyst.32 A two-sites Eley-Rideal-Redox model was able to predict the partial oxidation of ethane to ethylene and the partial ethane oxidation over MoV.32 In particular, the ethane ODH over Ni–Nb–O mixed oxides kinetics was studied using a Mars–Van Krevelen mechanism.33 The kinetic model was able to predict the ODH catalyst performance well.



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This article mainly presents the reaction mechanism and the kinetics of ODH of propane over VOx/CaO-γAl2O3 and VOx/CaO catalysts. The reaction network and Langmuir-Hinshelwood type kinetics model is developed using a wide-ranging set of catalytic experiments conducted in a fluidized CREC fluidized Riser Simulator. The developed phenomenological kinetic model is coupled with depletion of lattice phase oxygen using a decay model as function of propane conversion. The developed system of differential equations and experimental data are used to estimate the kinetic parameters implemented in MATLAB. The estimated kinetics parameters are further examined for the thermodynamic consistency and physical significances. The accuracy of the parameters were further evaluated by various statistical indicators. 2. EXPERIMENTAL

2.1. Catalyst Preparation In this study γAl2O3 (specific surface area 150 m2/g; Inframat Advanced Materials, UK) and CaO (specific surface area 5 m2/g; Loba Chemie, India) are used as catalyst supports. Before metal loading, both the support materials were treated in nitrogen at 500 oC for 4 hours to remove any volatile materials that might be present in the received samples. On the treated supports, 10 wt% vanadium was deposited following an impregnation method using ethanol as a solvent. The resultant mixture was filtered under vacuum conditions. The filtrated samples was then allowed for natural drying. Residual ethanol is removed by drying the sample at 100 o C for 24 hours. The dried catalyst samples were then placed in a fluidized bed furnace for reduction under hydrogen (10% H2 and balanced Ar) at 500 oC. Finally, the reduced samples were reoxidized by switching the gas supply to air. The oxidized samples color became yellow indicating the presence of VOx on the surface of the support materials. Both VOx/CaO and VOx/CaO-γAl2O3 catalysts were prepared following the same above steps. In both catalysts,


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vanadium loading was maintained at 10 wt% and VOx/CaO-γAl2O3 catalyst was prepared using CaO to γAl2O3 ratios of 1 (one).

2.2. Catalyst characterization 2.2.1. X-ray diffraction (XRD) The textural properties of the prepared catalysts were characterized by XRD analysis. The XRD experiments were conducted using a Rigaku Miniflex diffractometer with monochromatic Cu K radiation of 1.5406 ×10-1 nm wavelength, 50 mA electrical current, 10 kV electrical voltage and 2o scan per minute (normal scan rate) within 2 range from 10o-90o with step size of 0.02.

2.2.2. Temperature programmed reduction (TPR) The reducibility and the oxygen carrying capacity of the prepared catalysts were characterized by H2-TPR (temperature programmed reductions) experiments using a Micrometrics AutoChem II 2920 analyzer. In each experiment, 100-200 mg of catalyst sample was placed inside a U-shaped quartz tube, which was connected to reducing and carrier gases. The catalyst samples were first pretreated under inert gas flow in order to remove any remaining volatile materials. Following pretreatment, the catalyst samples were oxidized by circulating oxygen (5 % O2 and balanced He) while heating at rate of 10 oC/min up to 500 oC. The samples were then allowed cooling down to room temperature under inert flow. The H2-TPR analysis were conducted by heating the sample (10 oC/min) under hydrogen flow (10 % H2 and balanced Ar) at 50 cm3/min. In order to achieve maximum reduction, the samples were heated up to 800 °C. The hydrogen consumptions were monitored by a calibrated thermal conductivity detector (TCD) available at the exit terminal of the reducing gas.



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2.3.

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Kinetics experiments

The catalytic oxidative dehydrogenation of propane experiments were performed using a CREC Riser Simulator operated under turbulent fluidized conditions. The CREC Reactor Simulator is bench scale batch reactor with a capacity of 53 cm3.34

It can be operated both in fixed and

fluidized bed modes. The schematic diagram of the experimental set up is shown in Figure 1.

V3

ARGON

P1

AIR

H2

INJECTION PORT T

V1 4PV

1 P2

2 VACUUM BOX

4 COOLING JACKET PACKING GLAND

3

8

IMPELLER

GAS CHROMATOGRAPH CROMATOGRAPH

6PV SPARE INJECTOR MANUAL INJECTOR

CATALYST BASKET

7 THERMOCOUPLE PORT

6

V4 5

SAMPLING LOOP

V 2A

MFC

V 2B

VENT

VACUUM

HELIUM

N2

V5

Figure 1. Schematic Diagram of the CREC Riser Simulator experimental set-up

The desired reactor temperatures were obtained by adjusting a temperature controller. Feed propane was injected in the reactor by using a preloaded syringe. Following the feed injection, the reaction to take place for a pre-specified reaction time. After each ODH run, the catalysts was regenerated by circulating air given that solid catalysts also serves as the source of oxygen for the ODH of propane. The product gas was analyzed using an online GC equipped with a TCD (6Ft 1/8 2mm HayeSep Q 80/100 Ni) to detect carbon dioxide, carbon monoxide and methane; and a FID (HP AL/S (8 micron) analytical column; dimension: 25 m x 0.32 mm) to 6 ACS Paragon Plus Environment

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detect propane, propylene, ethane, ethylene and methane. Propane conversion and product selectivity were calculated according to Eq. 1 an Eq. 2, respectively. Propane conversion, (%) =

∑

∑

Selectivity to a product, (%) = ∑

× 100

(1)

× 100

(2)

where, npropane is the moles of unconverted propane in the product, nj is the moles of species “j” present in the product, zj is the number of carbon atom present in species “j” molecule.

3.

Results and Discussion

3.1. XRD Analysis The XRD patterns of both the VOx/CaO-γAl2O3 and VOx/CaO catalysts are shown in Figure 2. For VOx/CaO-Al2O3 sample, γAl2O3 peaks appeared at 2θ angles of 48o and 67o. The peaks that appeared at 32o, 38o and 55o can be attributed to CaO.3,

35

The 19.5o peak on both the

VOx/CaO-Al2O3 and VOx/CaO catalysts can be ascribed to V2O5 crystals. VOx/CaO samples also shows a small V2O5 peak at 2θ angles of above 60o. These observations indicate that both the VOx/CaO-γAl2O3 and VOx/CaO samples contain VOx species, which appear mainly as highly dispersed amorphous phase.36 Both samples show no indication of reaction between the support and vanadium.



[■] CaO [O] Al2O3 [∆] V2O5

■ ■ ∆

■

■ ∆

■

■ ∆

Intensity (a.u)

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∆

VOx/CaO

∆

■

∆

∆

■ ■

■ ■

■

10

20

30

40

50 2 o

VOx/CaO-Al2O3

∆ ∆ ∆O

O

60

70

80

90

Figure 2. XRD patterns of VOx/CaO-γAl2O3 and VOx/CaO-γAl2O3 catalysts

3.2. Reducibility and oxygen carrying capacity As mentioned before, present study considered ODH of propane using lattice oxygen of the catalysts. Therefore, the reduction characteristics of the catalysts is very important in addition to their catalytic properties. TPR analysis was conducted to determine the reducibility and oxygen carrying capacity of the prepared catalysts. The TPR profiles also provide information about the activation temperature of the catalyst at which the lattice oxygen is available for reaction. Figure 3 shows the reduction behavior of VOx/CaO-Al2O3 and VOx/CaO catalysts over the studied reduction temperature rage (25-800 °C). One can see that the VOx/CaO sample gives two wide peaks at 95-287 °C and 300-430 °C. On the other hand, VOx/CaO-Al2O3 sample gives one wide 8 ACS Paragon Plus Environment

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peak between 260-450 °C. These wide peaks are possibly due to the reduction of highly reducible VOx species available on the surface of the supports. Further, the low temperature peaks of VOx/CaO-Al2O3 sample was less prominent than those with VOx/CaO sample. Usually, the surface VOx species are more selective to COx formation when react with propane/propylene.3,6,8,19 Therefore, CaO modified CaO-Al2O3 support has more interaction with VOx than that of Al2O, consequently, their reduction become increasingly difficult to reduce at lower temperature.37, 38 In both the catalysts, the major reduction peak appeared around 520-590 °C, which is mainly due the reduction of monomeric and polymeric VOx species. These difficult to reduce VOx species are more selective to propylene formation when react with propane.3,12,16

One should also note the VOx/CaO sample shows highest peak temperature at

583 °C, which was shifted to 560 °C with the VOx/CaO-Al2O3 sample.

Therefore, the

introduction of CaO, decreased the support surface acidity, and as a result, this enhances the formation of isolated mono-vanadate species on the support surfaces.39, 40

VOx/CaO

H2 consumption (a.u)

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VOx/CaO-γAl2O3

0

200

400 600 Temperature (oC)

800

Figure 3. Temperature programmed reduction profiles of VOx/CaO-γAl2O3 catalyst samples



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In addition to the active species, their quantity is also important in order to secure enough lattice oxygen for the ODH reaction. The percent of reduction with respect to total available VOx was calculated from the amount of hydrogen consumed during the TPR cycles of each catalyst samples and using Eq. 3. (

(3)

,() ×-./

(4)

% !"#$%&' = () × 100

and, *+ =

+×-0

where, WV represents the amount of reduced VOx, MWv represents the molecular weight of V, (3) VH2 represents the volume of consumed hydrogen (cm3 at STP), Vg represents the volume of gas (cm3/mol at

STP), Wo represents the total amount of VOx (g) and v represents the

stoichiometric number of hydrogen based on the following reaction stoichiometry. V2O5 + 2H2 → V2O3 + 2H2O

(5)

Figure 4 presents the percentage of VOx reduction over repeated reduction and re-oxidation cycles. One can see form this figure that both samples show stable reduction behavior over the repeated redox cycles. However, the percentage of VOx reduction with VOx/CaO is higher (70 ± 1.9 %) than that of VOx/CaO-γAl2O3 sample (62 ± 2.3 %). This observation suggests that under the studied reduction temperature range 25-800 °C the oxygen carrying capacity of VOx/CaO is higher than that of VOx/CaO-γAl2O3 catalyst. It is important to note here that VOx reduces at lower temperature, as observed with VOx/CaO sample, is more selective towards CO2 formation (during the ODH process).3, 8-10


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100 VOx/CaO VOx/CaO-Al2O3

90 80 70 % Reduction

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60 50 40 30 20 10 0 0

1

2

3

4 5 Redox cycles

6

7

8

Figure 4: Percentage of VOx reduction over repeated TPR/TPO cycles

3.3. ODH of propane using catalyst lattice oxygen Before the actual catalytic runs, blank experiments were carried out by injecting propane into the reactor at different temperatures. In the thermal runs, no significant propane conversion was observed. The actual ODH of propane experiments with catalysts were conducted at wide range of reaction conditions (550-640 °C and 10-31 sec reaction time) in order to demonstrate the effects of various parameters on the product selectivity and propane conversion. After each ODH cycle, the catalysts were re-oxidized by circulating air for 20 mins. Thus, the alternative ODH of propane catalyst re-oxidation were attained using the same reactor without any circulating of the catalyst bed. In order to verify the consistency, experiments were repeated under various reactions conditions. The calculated propane conversion and product selectivity in the experimental repeats are found to be within 2.5 % standard deviation. After each 11 ACS Paragon Plus Environment


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experimental run, mass balances were also carried out and found they are consistently closed in excess of 96%. Table 1 reports the product analysis of the ODH of propane over VOx/CaO and VOx/CaO-γAl2O3 catalysts. One can see form this table that propylene and carbon dioxide are the major products of the ODH of propane under gas phase oxygen free reaction conditions. In addition, trace amount of methane and carbon monoxide were also detected as undesired products. It is interesting to see that VOx/CaO-γAl2O3 shows significantly higher propylene selectivity and lower carbon dioxide selectivity than those of VOx/CaO catalyst sample. With VOx/CaO-γAl2O3 catalyst, propylene selectivity reached to 94.1 %, while highest propylene selectivity with VOx/CaO catalyst is 77.2 %. The higher propylene selectivity of the VOx/CaOγAl2O3 sample is consistent to the XRD and TPR results, which shows difficulties of the reduction of the VOx species on the mixed CaO-γAl2O3 support. Thus, the proper balance of CaO/Al2O3 influences the VOx dispersion forming more isolated non-crystalline VOx species, which favors the propylene formation and suppress the complete oxidation to CO2. Table 1: Product distribution of ODH of propane using catalyst lattice oxygen.

Catalysts

VOx/CaO

VOx/CaO-γAl2O3

C2H8 conv. (%) 9.2 11.2 14.5 24.4 10.3 13.2 15.1 25.5

Temp (°C) 550 580 610 640 550 580 610 640

Selectivity (%) C3H6

CO2

62.2 71.5 74.3 77.1 78.3 82.4 90.1 94.2

37.8 28.5 25.7 22.9 21.7 17.6 09.9 05.8


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Based the above, under the studied reaction conditions one can consider the following main reactions: Desired reaction 456

C H3 789 C H: + H< O

ODH of propane to propylene:

(6)

Undesired reactions 456

Complete oxidation of propane:

C H3 79 3CO< + 4H< O

Complete oxidation of propylene:

C H: 789 3CO< + 3H< O

456

(7) (8)

Therefore, one has to be cautious selecting the reaction conditions in order to achieve highest propylene selectivity and suppress the undesired carbon dioxide formation. To take into account of all the possibilities, the kinetics experiments were conducted at different temperatures (550, 580, 610 and 640 °C) and reaction times (10, 17, 24 and 31 sec). Figure 5 depicts propylene and CO2 selectivity as a function of propane conversion at constant reaction temperature of 640 oC, while reaction time varied from 10 to 31 sec. One can see that slight increase in CO2 selectivity with increase in propane conversions. It also shows a slight decrease in propylene selectivity with increase in propane conversions.

Based on the experimental observations and previous studies of our research team8-11, a sound approach is to propose an “In Series-Parallel Reaction Network” involving three reactions as shown in Figure 6: (i) the desired oxidative dehydrogenation of propane to propylene and water, (ii) the undesired primary combustion of propane to carbon oxides and water and (iii) the undesired secondary combustion of propylene to carbon oxides and water. The desired ODH of



propane and the undesired secondary combustion of propylene are in series while the desired propane ODH and the undesired primary combustion of propane are in parallel. VOx/CaO-gAl2O3(1:1)

100 C3H6/CO2 Selectivity (%)

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80

VOx/CaO

C3H6

60 40 20 CO2

0 0

20

40 60 C3H8 Conversion (%)

80

Figure 5. C3H6 and CO2 selectivity as a function of C3H8 conversion at constant temperature 640 o

C (reaction time: 10-31 sec; experimental repeats: ± 2.5 % standard deviation).

C3H8

C3H6 k2

k3

CO2

Figure 6. Proposed series-parallel reaction network for ODH of propane over VOx/CaO and VOx/CaO-γAl2O3 catalysts


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4. KINETIC MODELLING 4.1. Reaction mechanism and rates In the context of the present gas phase oxygen free ODH of propane study, it is assumed that there are two types of oxygen species available on the catalyst: (i) surface oxygen and (ii) subsurface lattice oxygen. The lattice oxygen provides more control towards desired propylene formation and the surface oxygen mainly gives undesired COx.8-11,41According to Langmuir−Hinshelwood mechanism, both propane and propylene are considered to be adsorbed on the catalyst surface.41,42 The Langmuir−Hinshelwood mechanism during the ODH of propane can be explained using the conceptual mechanism presented in Figure 7. According to this mechanism, there are two catalyst sites, namely catalyst support-based site-2, [V2] and surface lattice oxygen in an oxidized site-1, [@AB ]. The adsorbed propane reacts with the lattice oxygen in an oxidized site-1, [ @AB ] to give propylene. The lattice oxygen also reacts with adsorbed propane and adsorbed propylene product to give carbon oxides.

C3H8

C3H8 Complete oxidation products CO2, H2O

ODH products C3H6, H2O

C3H8 C3H6

V

C3H8

V

V C3H8 V

Support


V


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Figure 7. Schematic representation of the Langmuir−Hinshelwood mechanism for oxidative dehydrogenation of propane to propylene with solid phase oxygen of VOx/CaO/Al2O3 catalysts.

On the basis of this, the following elementary steps are considered: Adsorption of propane on a catalyst support-based site-2 [V2] on the catalyst surface: K1

C3H8 (g) + [V2] (s)

C3H8-[V2] s (s)

(9)

Surface reactions C3H8-[V2]s (s) + 2 [V10]s

k1

C3H6-[V2]s + H2O(g) + [V10]s + [V1R]s

(10)

C3H8-[V2] s + (4+3x) [V10] s

k2

3 COx(g) + 4 H2O(g) + [V2] s+ (4+3x)[V1R] s

(11)

C3H6-[V2]s + (3+3x) [V10]s

k3

3 COx(g)+ 3 H2O(g)+[V2]s+(3+3x) [V1R]s

(12)

Type equation here.

Desorption of products C3H6-[V2] s

K2 C3H6 + [V2] s Type equation here.

(13)

Regeneration of the reduced catalyst O2(g) + 2 [V1R] s → [V10] s + [V10] s

(14)

According for the above mechanism, the reaction rates were formulated as follows: DA = EA (1 − )

(15)

D< = E< (1 − )

(16)

D = E G (1 − )

(17)

where and G are the surface coverage of adsorbed species of propane and propylene respectively, r1, r2 and r3 are the reaction rates of propane ODH, primary combustion of propane and secondary propylene product combustion, respectively; k1, k2 and k3 are the specific reaction


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rate constants of propane ODH, primary combustion of propane and secondary propylene product combustion, respectively. The fractional coverage of propane and propylene can be expressed as follows: = AV

VW W

(18)

W W V/ /

G = AV

V/ /

(19)

W W V/ /

where, C1 and C2 are concentrations of propane and propylene, respectively. The term (1-α) represents the remaining oxidized vanadium sites or available degree of oxidation of catalyst. An expression that is based on the fraction of the original oxygen remaining after the propane ODH reaction was used to give allowance for the catalyst’s time on stream. The expression can be termed as time dependent degree of oxidation of ODH catalyst and it is expected to decrease for consecutive reactions provided there is no in-situ catalyst regeneration.41 Previously, the present author and his collaborators developed an exponential decay function to take into account of the activity decay of the catalyst.41,43,44 In the context of the present study, the following relation represents catalyst degree of oxidation as a function of the conversion of propane. (1 − ) =

XY(Z[ . )

(20)

where the fractional conversion of propane is denoted by and \ is the decay constant. In addition to the above catalyst lattice oxygen depletion (Eq. 20), one could speculate catalyst deactivation due to carbonation of CaO with product CO2 (CaO + CO2 = CaCO3). However, in the context of present ODH of propane involving catalyst lattice oxygen followed by catalyst reoxidation cycles, showed no sign of deactivation due to CaCO3 formation.



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Page 18 of 31

By substituting equations (18), (19 and (20) into equations (15), (16) and (17), the following rate expressions can be obtained: DA = EA AV

VW W

W W V/ /

D< = E< AV

VW W

V/ /

W W V/ /

4.2.

(21) (22)

XY(Z[ . )

XY(Z[ . )

W W V/ /

D = E AV

XY(Z[ . )

(23)

Model formulation

The CREC Riser Simulator can be considered a well mixed batch reactor as reported in de Lasa (1992).34 Thus, the reaction rates can be expressed as: Rate of disappearance of propane ][. ]^

=−

(_` -

(DA + D< )

(24)

Rate of formation of propylene ][.G ]^

(_`

(DA − D )

(25)

c(_`

(D< + D )

(26)

=

-

Rate of formation of COx ][ab ]^

=

-

Since a catalytic reaction is considered, weight of catalyst (Wc) and volume of reactor (Vr) could be introduced into the above equation. - ]f e ]^

Dd = (

(27)

The concentration of any species can be treated as a function of its mass fraction. h(

f W gd = ,( -

(28)

f

(

!gd = ,(W- !id

(29)

f


Page 19 of 31

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-

(

Dd = ( . ,(We

f

]hf

(30)

]^

where W1, y1, y2, y3 are the mass of the propane feed, mass fraction of propane, propylene and carbon oxide, respectively. ]hW

=

,(W (e

j−(EA + E< ) AV

]hW

=

,(W (e

j

]^ ]^

]h[a/ ]^

(W (W

=

W W V/ /

lW VW W Xl V/ /

,( (e (W

VW W

AVW W V/ /

j

l/ VW W l V/ / AVW W V/ /

XY(Z[ . )

XY(Z[ . )

k

k

XY(Z[ . )

k

(31) (32) (33)

The intrinsic rate constant can be evaluated from Arrhenius equation. p

A

A

Ed = Edm no j− f q − sk _ `

(34)

`r

where Edm and Ei are the pre-exponential factor and activation energy of the reaction i respectively. tm is referred to as the centering temperature which is the average of all the temperatures for the experiment in order to reduce parameter interaction. The adsorption equilibrium constant can also be obtained from the thermodynamic relations given below. ud = udm no j−

∆f _

A

A

q` − ` sk

(35)

r

where udm and ∆wd are the pre-exponential factor and heat of adsorption of the species i respectively.

4.3. Model evaluation The model equations (Eq. 31-Eq. 33) incorporating with the individual reaction steps, Arrhenius relation, temperature dependence form of the equilibrium adsorption constants and deactivation function were evaluated by a least square fitting of the kinetic parameters using the experimental data for ODH of propane to propylene reaction obtained from the CREC Riser Simulator. The 19 ACS Paragon Plus Environment


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Page 20 of 31

differential equations were solved by Runge-Kutta-Gill method (MATLAB ODE 45 subroutine). On the other hand, the Modified Marquad method technique (MATLAB LSQCURVEFIT subroutine) was employed for parameter estimation. The minimum sum of squares of the residuals was set as optimization criteria, which was defined by: Sum of Square of Errors = }∑dA~id^ − idB]

Kinetics of Oxidative Dehydrogenation of Propane ... - ACS Publications

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