УДК 66.011
SIMULTANEOUS ISOBUTANE DEHYDROGENATION AND HYDROGEN PRODUCTION IN A HYDROGEN-PERMSELECTIVE MEMBRANE
FIXED BED REACTOR © 2014 M. Farsi, A. Jahanmiri, M. R. Rahimpour
Department of Chemical Engineering, School of Chemical and Petroleum Engineering,
Shiraz University, Shiraz, Iran jahanmir@shirazu.ac.ir Received 07.11.2012
In this study, performance of hydrogen—permselective membrane fixed bed reactors to produce isobutene is studied at steady state condition. The proposed reactors have been modeled heterogeneously based on the mass and energy conservation laws. The considered reaction networks in the model are isobutene dehydro-genation as the main reaction, and hydrogenolysis, propane dehydrogenation as well as coke formation as side reactions that all occur on the catalyst surface. The coke deposition on the catalyst surface results an activity profile along reactors. The reactions occur in the tube side and the hydrogen permeates from the reaction zone to the sweep gas stream. Decreasing the hydrogen concentration over the catalyst pellets improves isobutane conversion and isobutene selectivity. To prove the performance of the proposed configuration, simulation results for membrane process are compared with the conventional process at the same operating condition. In this configuration, the isobutene production rate is enhanced about 10.81% compared to the conventional process at the same catalyst loading.
Keywords: Isobutane dehydrogenation, Pd/Ag membrane reactor, Heterogeneous model.
DOI: 10.7868/S0040357114060049
INTRODUCTION
Isobutene as one of the unsaturated hydrocarbons is used as a raw material to produce a variety of petrochemical components such as polybutene, methyl tert-butyl ether and ethyl tertiary butyl ether to increase the fuel octane number. In addition, alkylation of isobutene with butane produces isooctane as another fuel additive. Although isobutene can be isolated from refinery streams, the industrial method to produce isobutene is catalytic dehydrogenation of isobutane.
Some of commercial technologies have been developed for dehydrogenation of light alkanes such as isobutane that differ in catalyst, reactor type and the utilized regeneration system. The Catofin isobutane dehydrogenation technology is a cyclic process to produce isobutene over chromia—alumina catalyst in the fixed-bed reactor [1]. The reactors operate adiabati-cally and to regenerate the deactivated catalyst, the reactors are changed periodically. In the UOP process, dehydrogenation occurs in the several adiabatic moving-bed reactors over modified Pt-alumina catalyst [2]. The reactor section consists of three radial-flow reactors, charge and inter-stage heaters and catalyst regeneration reactor.
Currently, researchers have focused on the «-butane and «-butene dehydrogenation and few articles discuss about isobutane dehydrogenation process in
the literature. Cortright et al. presented a rate equation for isobutane dehydrogenation over Pt—Sn catalyst [3]. Gucuyener et al. studied isobutane dehydrogenation in a DD3R type zeolite membrane over the Cr2O3 based Al2O3 catalyst. The experimental results showed that hydrogen removal can increase isobutane yield about 50% [4]. Bakhshi et al. modeled and simulated a bench scale fixed bed reactor for selective dehydroge-nation of isobutane over Pt—Sn/Al2O3 catalyst at steady state condition [5]. Sahebdelfar et al. modeled the isobutane dehydrogenation to isobutene in an adi-abatic radial-flow moving-bed reactor without considering side reactions. They assumed that the product selectivity is 100% [6]. Vernikovskaya et al. investigated simultaneous dehydrogenation of isobutene and propane in a pilot fluidized and in a lab fixed bed reactors over an industrial isobutene dehydrogenation catalyst [7]. The results showed that adding C3H8 to the reactor inlet increases C3—C4 mixture conversion and the total process selectivity to olefins. Zangeneh et al. investigated applicability of a commercial Pt-Sn/Al2O3 isobutane dehydrogenation catalyst in dehydrogenation of propane experimentally [8]. They showed that the single carbon-carbon bond rupture is the main route for the formation of lower hydrocarbon byproducts.
Sweep
Feed
Product
Fig. 1. The schematic diagram of the considered process.
The integration of membrane separation and reaction has attracted much attention in the recent years [9]. A membrane reactor is a piece of chemical equipment that couples a reactor with membrane layer to add reactants or remove products from the reaction zone. Removing product components from the reaction zone in a membrane reactor increases residence time and shifts thermodynamic equilibrium limitations towards higher conversion. Also, the simultaneous occurrence of reaction and separation leads to lower cost in the separation stage compared to conventional processes. Casanave et al. studied isobutane dehydrogenation over Pt—In catalyst in a packed-bed zeolite membrane reactor [10]. Higher dehydrogena-tion yield was observed in the membrane reactor due to the hydrogen removal from the reaction zone. Cia-varella et al. investigated isobutane dehydrogenation over Pt—In catalyst in a MFI type membrane reactor [11]. The performance of membrane reactor was studied as a function of the feed and sweep gas flow rates. Liang and Hughes studied isobutene synthesis from isobutane in a membrane reactor over Pt/Al2O3 catalyst, experimentally [12]. In addition, they modeled the considered Pd/Ag based membrane reactor at steady state condition.
The object of this research is modeling and simulation of membrane fixed bed reactors to produce isobutene and hydrogen. The considered reaction networks in the model are isobutene dehydrogenation as main reaction, and hydrogenolysis, propane dehydro-genation as well as coke formation as side reactions that all occur on the catalyst surface. The performance of the proposed membrane reactors is compared with conventional reactors at the same process condition. The main advantages of this catalytic reactor are hydrogen production, improving isobutene productivity and selectivity, lower byproduct production.
KINETICS MODEL
The isobutane dehydrogenation is a reversible and an endothermic reaction. Although increasing temperature and decreasing pressure shift the reaction toward completion, it promotes side reactions, coke formation, and catalyst deactivation. The isobutane dehydrogenation over Pt—Sn/Al2O3 catalysts is as follows:
As well as isobutane dehydrogenation, hydrogenolysis, propane dehydrogenation and coke formation reactions take place over the catalyst surface. High residence time and temperature result isobutane cracking to methane and propane (hydrogenolysis reaction) as the main side reaction.
/-C4H10 + H2 ^ C3H8 + CH4. (2)
Other side reactions (propane dehydrogenation and coke formation from isobutene reactions) are as follows:
C3H ^ H2 + C3H6, (3)
/-C4H8 ^ 4C + 4H2. (4)
In this work, the rate expressions have been selected from literature [13, 14]. To complete the simulation, rate equations are substituted in the governing equations and generated differential equations are solved.
PROCESS MODELING
Reaction side. The conventional dehydrogenation process consists of three series reactors that the feed stream is entered to the first reactor. The inter heaters are placed between rectors to increase temperature of inlet streams. In this work, the conventional reactors have been substituted by Pd/Ag based membrane reactor at the same catalyst loading. Figure 1 shows the schematic diagram of the considered process.
In this work, a one-dimensional heterogeneous mathematical model has been developed to simulate the hydrogen-permselective membrane reactors at steady state condition. In this model, the following assumptions are considered:
The gas is ideal condition at the considered operating condition.
Mass and energy radial diffusion is negligible.
Mass and heat axial diffusion is negligible.
The system is well isolated.
The chemical reactions take place on the catalyst surface.
The membrane is completely selective.
Subject to these assumptions, mass and energy balances for the gas phase in the reaction zone are expressed by:
1
d ( Fyf )
Ac dz
+ avctk„i(yi - yf ) -
(3)
/-C4H
10
^ /—C4H8 + H
(1)
-a (VP -JPm ) = 0
Ac
- <CLFdTL + avhf (Ts - Tf ) + Ac dz
T
+ ^DuTTf - Tf) -1 fQHCpdT = 0,
A A •>
2
where QH presents simultaneous heat and mass transfer through the membrane layer. Mass and energy balances for the solid phase are expressed by:
avctkgi(yi - yi) + XПГРь = 0,
N
vhf (Tg - Ts) + PbXnn(-AHfi) = 0.
(5)
(6)
i-1
In the above equations, the internal mass transfer resistance has been inserted in the model considering effectiveness factor (n) [15]. The pressure drop through the catalytic packed bed is calculated based on the Tallmadge equation that is usable for laminar and turbulent flow regimes [16].
f = 150(1 -s)2 f Re s3
+
4.2 (1 -s)
1.166
Re
1/6
AP = fU p
L
Dp
(7)
(8)
Feed specifications, reactor and catalyst characteristics in the commercial dehydrogenation plant are shown in Table 1.
Membrane side. Hydrogen permeation from the reaction zone to the sweep gas stream results increasing the hydrogen content in the sweep gas stream. In addition, the heat transfer between endothermic side and sweep gas stream results decreasing temperature of the sweep gas along the reactor. Mass and energy balance equations are written for hydrogen permselective side as follows:
d ( Fm ygm )
dz
+ a i ( -J Pm ) = 0,
(9)
g
-FmCpdT- - (nD)U(T;g - Tg) + fQHCpdT = 0, (10) dz J
To
where the hydrogen permeation constant is calculated from [17]:
2LnP0exp
a
~1± RT
H2 - '
ln| Do D,
(11)
Table 1. Feed and product specifications of the commercial dehydrogenation reactors
Parameter Reactor 1 Reactor 2 Reactor 3
Feed
Temperature (K) 600 610 605
Flow rate (ton hr-1) 106 106 106
Pressure (barg) 1.4 0.9
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