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A note on integrated process of methane steam reforming in junction with pressure-swing adsorption to produce pure hydrogen: Mathematical modeling Yadollah Tavan, Seyyed Hossein Hosseini, and Martin Olazar Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b01477 • Publication Date (Web): 09 Dec 2015 Downloaded from http://pubs.acs.org on December 15, 2015
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A note on a integrated process of methane steam reforming in junction with pressureswing adsorption to produce pure hydrogen: Mathematical modeling Yadollah Tavan a∗, Seyyed Hossein Hosseini b, Martin Olazar c a
b
National Iranian Gas Company (NIGC), Tehran, Iran.
Chemical Engineering Department, Faculty of Engineering, Ilam University, 69315-516 Ilam, Iran. c
Department of Chemical Engineering, University of the Basque Country, Bilbao, Spain.
[This submission is dedicated to Prof. Morteza Sohrabi (Faculty member of Chemical Engineering Department of Amirkabir University of Technology)who recently passed away for all he done for Iranian students. We greatly appreciate Dr. Sohrabi and pray from the bottom of our hearts that his soul rest in peace]
Abstract: A mathematical model and molecular dynamics simulation were applied to study the integrated process consisting of industrial methane steam reforming and pressure-swing adsorption (PSA) to produce pure hydrogen. The process was highly endothermic with an outlet temperature of 1190 K. It was found that the industrial plant suffers from high coke filament formation by the methane cracking reaction with the maximum affinity of 3. Moreover, the output H2/CO ratio of 3.27 was obtained for the industrial case under study. Relevant adsorption isotherms of the components and molecular dynamic simulations showed that hydrogen is not adsorbed on the zeolite 5A, while carbon dioxide and methane compete to adsorb on the zeolite. Results of dual-bed PSA process showed that pure hydrogen (>99 %) is produced during the process and the results were compared to the available data reported in the literature for each step.
∗
Corresponding author: Tel/Fax: +98-21-81315646, Email address:
[email protected] (Y. Tavan).
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Keywords: Zeolite 5A, Methane steam reforming, Molecular simulation, Adsorption isotherm, PSA process 1. Introduction Hydrogen is the lightest chemical element and offers the best energy-to-weight ratio of any fuel.1-3 Use of hydrogen in combustion engine leads to the reduction in greenhouse gas emissions, improve air quality, diversify energy supply and reduce noise.4, 5 Generally, steam reforming of methane (SRM) is a reversible and highly endothermic process, which is carried out at 973-1173 K and a pressure range of 1.5–3.0 MPa with supported nickel on alumina.6 SRM consists of three reversible reactions: the strongly endothermic steam reforming reaction, Eq. (1), the moderately exothermic water-gas shift (WGS) reaction, Eq. (2) and an endothermic side reaction of methane reforming, Eq. (3). CH4+H2O↔CO+3H2
(1)
CO+ H2O ↔ CO2+H2
(2)
CH4+2H2O↔CO2+4H2
(3)
In addition, carbon formation is a phenomenon that cannot be tolerated in reforming process due to the catalyst deactivation and fouling of reactor tubes.7 In order to avoid carbon formation, high steam-to-carbon feed ratio in the range of 2-5 is commonly used in industrial fixed-bed reformers. It should be noted that the SRM reactor produces not only hydrogen, but also some impurities such as water, carbon dioxide, methane and carbon monoxide (Eqs. (1-3)). Furthermore, the mentioned impurities should be removed from the mixture.8 It is worth mentioning that the 2 ACS Paragon Plus Environment
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hydrogen purity of 99.99% is required in fuel cell applications. Accordingly, hydrogen purification has become an interesting process, especially for fuel cell applications, to prevent the degradation of cell membrane efficiency.9-11 The pressure swing adsorption (PSA) process is usually used to purify hydrogen due to its simplicity and low operating costs.12-17 Therefore, with increasing demand for hydrogen, especially in petroleum refinery, development of an effective process for hydrogen recovery is critical. Methanol is produced from syngas and Iran is the main methanol producer and exporter in the world, with several Mega plants being operative, such as Fanavaran Petrochemical Plant (FPP). Neverteless, methanol export market is not only in a unstable state, but the cost of methanol production is also considerable. Accordingly, the main idea of the present study lies in the use of syngas to produce a chemical commodity of high value, i.e., hydrogen, instead of methanol. Therefore, integration is applied for modifying the methanol unit to produce pure hydrogen without complex gas treatments and separation processes. It should be noted that an economic process, such as PSA, was integrated in the hydrogen production process. The overall process proposed was mathematically modeled and simulated and the results were validated with molecular dynamic (MD) simulation. To the best of the authors’ knowledge, these methods have not been studied yet. Moreover, although SMR has been extensively studied and is a relatively mature technology, data about carbon filament formation and there affinities with reformer tubes are still very limited in the literature. Hence, this study is particularly focused on carbon filament formation by the methane cracking reaction, the Boudouard reaction and the Beggs reactions. The process under study is illustrated in Figure 1. Thus, this present study is particularly focused on:
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1- The mathematical modeling of SRM, which converts steam and industrial feedstock of the Fanavaran Petrochemical plant to syngas at high temperature and moderate pressure. Note that the heat recovery through syngas is not investigated here and is left for a future study. 2- The MD simulation is carried out to have a better understanding of component adsorption isotherms and their behavior under system criteria. 3- The hydrogen purification is also carried out by a PSA unit to achieve the final product purity, and this process has also been mathematically modeled. 2. Methods 2.1.
Modeling of SRM
In this study, the steady-state operation of a large-scale reformer in the Fanavaran Petrochemical Company, Mahshahr, Iran, is investigated by means of a rigorous mathematical model. For the modeling of the reactor side, all the equations for mass and energy balances are written for a differential control volume of a single tube representing any other tube. The schematic view of a reforming tube is shown in Figure 2. The heat input to the reformer tube wall results in radial temperature and concentration gradients apart from axial gradients. Therefore, a two-dimensional model is required to cope with the details of conversion and temperature profiles. In order to model the process, axial dispersion effects and external heat and mass transfer resistances are assumed to be negligible and all heavy hydrocarbons are assumed to be converted to methane and CO in pre-reforming section. The energy balance equation is given as:
G. C p, pg
∂Tpg ∂z
= λer (
2 1 ∂Tpg ∂ Tpg + ) + ρ bed [ Σ (−∆H i )riηi ] i r ∂r ∂r 2
(4) 4
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where, λer and Der are the effective radial conductivity and diffusivity, respectively. These parameters are generally calculated by heat and mass transfer analogy.18-20
Perm = Perh = Perf
(5)
The boundary conditions are given by: At z=0 (entrance), Tpg= Tfeed
At the center of the tube,
(6)
∂T pg ∂r
At the wall, U (Tt,o-Tpg)= λer
=0
(7)
∂T pg
(8)
∂r
The overall and inner heat transfer coefficients are given by the following equations:
d d 1 1 dt , i = Ln t , o + t , o U 2 Kt dt ,i dt , i hi
(9)
hi d cat = 2.58 × Re0.33 Pr 0.33 + 0.094 × Re0.8 × Pr 0.4 K pg
(10)
The modified Ergun equation is used for estimating the pressure drop in the reforming process:
ρ pg u 2pg dP − = f dz d cat
f =
(11)
1 −υ 4.2 Re 5 / 6 (1 − υ ) 1 . 75 + υ 3 Re
(12)
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In the present study, the mentioned reversible reactions and their corresponding rate equations given by Chen and Elnashaie are considered for reactor modelling.21 All the equilibrium constants are gathered from the literature.22-24 Equations of mass balance for CH4 and CO2 are given as follows.
∂XCH4 ∂z
∂X CO2 ∂z
Der ρpg 1 ∂XCH4 ∂ 2 XCH4 ρbed (r1η1 +r3η3 )M W0 + + G r ∂r G.yCH4,0 ∂r 2
(13)
Der ρ pg 1 ∂X CO2 ∂ 2 X CO2 ρ bed (r2 η2 +r3η3 )M W0 = + + G r ∂r G.yCH 4,0 ∂r 2
(14)
=
In equations 13 and 14, the assumption that the conversion of the components at the entrance of the reactor equals zero and
∂XCH4 ∂r
=
∂X CO2 ∂r
= 0 is introduced at the tube center and inner wall
Furthermore, the SRK equation of state is applied to estimate thermodynamic properties of gas.20 The equations of Filla are used for furnace side calculation: 25 ∂2F = α ( βF + γ ) ∂z 2
(15)
α = − ( 2 K a + At + Ar ) / 2
c fg G fg
∂ T fg ∂z
= 4K a (F − E
(16)
fg
) + 0 . 98 Q
(17)
λ = (4 K a E fg + 2ε t At Et )
(18)
E = σT 4
(19)
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β = − ( 4 K a + 2ε t At )
(20)
The boundary conditions for equations 15 and 17 are given as: at z = 0
Tfg= (F/ σ)0.25
(21)
at z = 0
dF/dz= 0
(22)
at z = L
dF/dz= 0
(23)
In the above equations, Q is the heat released across the flame due to the combustion of fuelair mixture. This parameter can be found based on the heat release pattern of Roesler.20 To solve the equations, the temperature of the tube-skin outer surface is initially guessed and all the equations for furnace and reactor sides are separately solved in order to calculate new tube-skin temperatures using the energy balance on the tubes. The convergence is reached when the new tube-skin temperature is fairly close to the old outlet temperature (temperature difference
