Progress in spherical packed-bed reactors_

Chemical Engineering & Processing: Process Intensification 132 (2018) 16–24

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Chemical Engineering & Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep

Progress in spherical packed-bed reactors: Opportunities for refineries and chemical industries

T

⁎

Davood Iranshahia, , Ahmad Golrokha, Ehsan Pourazadib, Samrand Saeidic, Fausto Galluccid a

Amirkabir University of Technology (Tehran Polytechnic), Department of Chemical Engineering, No. 424, Hafez Avenue, Tehran, 15914, Iran Department of Chemical and Process Engineering, University of Canterbury, Christchurch 8140, New Zealand c Department of Energy Engineering, Budapest University of Technology and Economics, Budapest, Hungary d Inorganic Membranes and Membrane Reactors, Eindhoven University of Technology, Department of Chemical Engineering and Chemistry, Eindhoven, The Netherlands b

A R T I C LE I N FO

A B S T R A C T

Keywords: Spherical packed-bed reactor Membrane reactor configuration Axial-flow Radial-flow

Giving the ever-increasing energy and raw material demand as a result of global economy growth, revisiting the traditional reactor configuration designs (which are considered to be the heart of chemical industries) can significantly reduce the capital and operational costs while addressing the larger market demand for chemicals. The spherical-reactor geometry is an attractive alternative design to traditional tubular reactors due to its lower pressure drop (which is due to feed distribution over a larger outer surface area in spherical reactors compared to the cross sectional area in conventional tubular reactors) and recompression costs as well as construction material investment (reduced wall thickness to half). This review summarizes numerical modeling and experimental research on spherical reactors from 1958 to date. Several configurations of spherical reactors have been described and categorized. A review has been performed on modeling results of numerous arrangements and combinations of tubular and spherical reactors for industrial-scale reforming processes. The superiority of spherical packed bed reactors is further discussed and additional recommendations are provided to be considered in future research. As a general conclusion, spherical reactors could be considered as a potential candidate for pilot and industrial scale reactors due to their cost-effective designs and flexibilityof operation conditions.

1. Introduction Chemical industries are of great importance to the global economy as key players in supplying essential needs for human kinds ranging from clean drinking water and fertilizers up to pharmaceuticals. Hence engineers and designers seem to be faced with the challenge of maintaining profitability in a rapidly growing market. Their contribution for even the slightest improvements in the efficiency of processing and manufacturing industries will result in significant cost savings as well as achieving sustainability goals. For any chemical processes, the reactor unit is considered to be one of the major components in its livelihood and sustainability.

Minimizing the pressure drop is one of the most crucial concerns in reactors design. Particularly for gas-phase reactions, the rates are directly linked to the reactor pressure and hence the conversion and efficiency can significantly decrease as the pressure drop increases along the reactor length. Additionally, recompression of unconverted outlet materials and their recirculation to the reactor can significantly affect the success or failure of a reactor design by multiplying operational costs and expenses. Conventional tubular reactors (CTR) have extensively been operational at mega industrial scales for the production of acute intermediate derivatives and chemicals such as methanol, polyethylene, ammonia, hydrogen, naphtha, biodiesel, gasoline and others [1–4]. However,

Abbreviations: AF-SPBR, axial-flow spherical packed bed rector; cat, catalyst; DE, differential evolution; DME, dimethyl ether; ESs, evolution strategies; GAs, genetic algorithms; HC, hydrocarbon; HSE, health, safety and environment; i, numerator of reactor; IDP, iterative dynamic programming; In, inlet condition; MAF-SPBR, membrane axial-flow spherical packed bed rector; MeOH, methanol; N, fraction of sweep gas flow; OAF-SPBR, optimized axial-flow spherical packed bed rector; Out, outlet condition; P, pressure; RF-SPBR, radial-flow spherical packed bed rector; SA, simulated annealing; SPBR, spherical packed bed rector; SSS, nonmembrane spherical- nonmembrane spherical- nonmembrane spherical; SST, nonmembrane spherical-nonmembrane spherical-membrane tubular; STS, nonmembrane spherical -membrane tubular- nonmembrane spherical; T, temperature; TR, tubular reactor; TSS, membrane tubular- nonmembrane spherical - nonmembrane spherical; TTT, membrane tubular- membrane tubular- membrane tubular; W, catalyst weight fraction; x, mole fraction; Y, yield; ω, weights for objective function ⁎ Corresponding author. E-mail address: [email protected] (D. Iranshahi). https://doi.org/10.1016/j.cep.2018.08.004 Received 4 June 2018; Received in revised form 5 August 2018; Accepted 6 August 2018 Available online 07 August 2018 0255-2701/ © 2018 Elsevier B.V. All rights reserved.


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Fig. 1. The trend of total number of published papers by year.

Fig. 2. The percentage of theoretical and experimental efforts on spherical reactor. Fig. 3. Schematic of a radial flow spherical reactor.

regardless of their successful applications, they have downsides such as a high pressure drop, occasionally high manufacturing costs resulting from larger thickness of material and restrictions in production capacity to cope with the future market growth [5]. In response, the idea of radial flow and spherical reactors developed which have proved to be effective for reducing the pressure drop in chemical reactors [6]. Additionally, the idea of using smaller size catalyst particles, to avoid internal mass transfer diffusion limitation, is more feasible for these designs. Another advantage of spherical configurations is the lower manufacturing costs due to halved thickness ( t ) compared to the typical 2 tubular reactors. Material thickness for a sphere and tubular pipe with same radius, subjected to action of an internal pressure P, are tsph = (P.r)/2σ and ttub = (P.r)/σ respectively where t is the wall thickness, σ is the tensile stress and r is the radius of the tube and sphere [5,7]. The goal here is to provide a comprehensive review on the spherical reactor concept and some of its industrial applications in recent decades. Statistical analysis of relevant literature, modeling and simulation approaches as well as the idea of employing membrane in spherical reactors and its associated challenges are reviewed. Two different spherical geometries with radial-flow and axial-flow patterns are discussed and compared with conventional tubular reactors. Several suggestions are provided for future research in Section 4.

with larger catalyst particles to lower the pressure drop. However, they need to consider the internal diffusion restrictions for larger particles and also non-uniformity of mass and heat transfer in multiple directions which limits the final conversion [9,10]. To address the challenge of using smaller catalyst particles while keeping the pressure drop unchanged and maximizing the conversion, the idea of using radial flow pattern in chemical reactors was proposed by Haldor Topsge Comp. in 1964 [7]. Here the pressure drop across a catalyst bed can be calculated based on the Ergun equation (Eq. (1)). This equation covers the entire range of flow rates by assuming that the viscous losses and the kinetic energy losses are additive [12–14].

150μ (1 − ε )2 Q 1.75ρ (1 − ε ) Q 2 dP = 2 2 + dx ε3 Ac ϕs dp ε 3 Ac2 ϕs dp

(1)

Note that here dP is the pressure gradient, Ac is the flow cross section, Q is the volumetric flow rate, ϕs is the sphericity (for spherical particles is one), μ is the fluid viscosity, ρ is the fluid density, ε is the bed porosity and dp is the particle diameter. It is clear that a lower pressure drop can be achieved in radial-flow systems since the flow cross section is multiplied and the superficial velocity ( Q ) is reduced significantly [15,16]. Hence the new approach Ac attracted researchers’ attention and many tried to study the influence of different parameters on the efficiency of the radial-flow reactors [17–23]. In 1965, Cimbalinik et al. [24] considered a spherical radial flow reactor consisting of two concentric spheres where the catalysts were placed in between the two spheres. The spherical geometry of a reactor affects the operating parameters, more specifically

2. Evolution of spherical reactor Compared to fluidized beds, packed bed reactors are preferred in chemical industries because of their easier design and construction as well as simplicity of operation [8]. Here the engineers prefer to work 17


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Fig. 4. (a) Comparison between pressure profiles in the tubular reactor and the spherical reactor in the hydrocracking process, (b) Comparison between products yields of kerosene, light naphtha and heavy naphtha in the tubular reactor and the spherical reactor in the hydrocracking process (reproduced from reference [36]).

Fig. 5. Schematic of an axial flow spherical reactor.

hydrodynamics, and thus many scientists have been investigating this configuration for various reaction systems. Spherical reactors offer a higher cross sectional area in comparison with conventional tubular reactors and thus have a lower pressure drop. This offers the advantage of operating with higher feed molar flow rate [5] and the possibility of using smaller size catalysts which eliminates the diffusional limitations in catalyst and enhances the final conversion [25]. Numerous theoretical and experimental studies including modeling

Fig. 6. Schematic design of a membrane axial flow spherical reactor. 18


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respectively. Viecco and Caram [31] who studied the reverse flow reactors found a lower temperature at the centre of a radial-flow spherical packed bed reactor (RF-SPBR), compared to a tubular one, using their dynamic mathematical model. The periodic and fast reversal of feed flow direction between inward and outward paths in their model predicted a lower temperature at the centre of the SPBR which could significantly enhance the conversion of exothermic reversible reactions. They also highlighted the superiority of SPBR in controlling the hottemperature zone at the centre of the reactor (i.e. easily by increasing the feed flow rate) compared to the tubular reactors and also respectively lower reaction volume that they need to achieve a desire conversion. In the past few years various theoretical models have also been developed for radial-flow spherical packed bed rectors (RF-SPBR) which have been successful in estimating and predicting the performance of reactors. Dehydrogenation of naphtha to hydrogen and aromatics in naphtha reforming units was modelled in RF-SPBR by Iranshahi and his co-workers [30]. Their sensitivity analysis concluded the possibility of increasing reformer’s production tonnage for RFSPBRs compared to conventional ones without significant pressure drop in the bed. Additionally, the temperature profile was lower in this case which could enhance the catalyst lifetime. The same group also optimized the operating conditions of reforming of naphtha in this reactor configuration [34]. Similarly, the hydrocracking process (HCP) was studied in RF-SPBR and the simultaneous effects of feed flow rate and catalyst scale up on reaction yield were considered. An increase in the catalyst load of spherical reactors proved to be more efficient compared to the conventional tubular configurations regarding reaction conversion [35]. The results regarding pressure profiles and kerosene, light naphtha and heavy naphtha yields are compared in tubular and spherical reactors in Fig. 4. Jiang et al. [36] carried out mathematical analysis of different methanol steam reforming reactor schemes for H2 production. Their study demonstrated that the most beneficial reactor was the spherical one which could lead to minimum amount of feed required and maximum efficiency and productivity. They also concluded that by applying a Cu-based catalyst supported on Pt-Al2O3, the reactor could be designed to operate at room temperature. Farsi et al. [5] proposed a steady-state heterogeneous model for methanol dehydration to DME in RF-SPBR as well. Their results showed 2.83% and 3.15% improvement in DME when considered two and three-stage spherical reactors, respectively. Based on pseudo-homogeneous and heterogeneous modeling studies of Hartig and Keil [37], RF-SPBRs are capable of improving the production capacities and the profits. Under the same conditions, they showed a spherical reactor with a diameter of 6 m performs equally to seven tubular reactors which was considerably more cost-effective.

Fig. 7. (a) Pressure profile of the spherical and conventional reactor for DME production, (b) DME mole fraction (adopted from [45]).

and design, optimization and environmental aspects of spherical reactors can be found in literature [5,9–11,15,26–28]. Fig. 1 represents the total published paper in this field from 80’s with a huge jump after 2000. Fig. 2 also provides information regarding the percentages of theoretical (i.e. simulation, modeling and optimization studies) vs. experimental (laboratory to industrial scale) studies. As shown, the majority of studies (89%) have been focusing on theoretical aspects of the spherical reactors. The computational modeling of fluid behavior is a strong tool for the design and analysis of real scale processes and has the potential to be used for improving the performance of real reactors [29]. To date, two different types of continuous spherical packed bed reactors are known: i) the radial-flow spherical reactors and ii) axialflow spherical reactors, both are described and discussed in the following sections.

3.2. Axial-flow Radial feed flow in spherical reactors presents a number of challenges such as obtaining a uniform feed distribution and the use of membranes [22]. These shortcomings are remedied in axial-flow spherical packed bed reactors (AF-SPBR). Here, the catalytic packed bed is placed between two perforated screens as demonstrated in Fig. 5 [30]. Feed enters the top of the reactor and flows axially towards the bottom of the reactor. The top to the bottom and axial flow direction will guarantee a more uniform flow distribution in AF-SPBR compared the radial flow reactors [27]. Neglecting the radial dispersion of reactants and considering a predominant axial convection in mathematical modeling of tubular reactors has been proved to be a valid assumption [27,30,38]. However for extended length of reactors, channeling and a two-dimensional flow is possible which necessitates the use of flow redistributors [27]. Mesh screens are considered at the inlet and outlet of the AF-SPBRs as mechanical support for the bed and also to maintain the pressure drop at a reasonable value especially at the inlet and outlet vicinities (i.e. where the cross section decreases

3. Spherical packed bed reactors 3.1. Radial-flow In radial-flow spherical packed bed reactors, the reactants can flow radially towards the outer or inner spheres as shown in Fig. 3 [30,31]. Based on previous studies, this can affect the reactor performance and reaction conversion [11,32,33]. Here, Balakotaiah and Luss [18] studied the effect of flow regime (inward or outward) on reactor performance under isothermal conditions. They concluded that for convex reactions (which are associated with increase in volume) and concave reactions, the preferred flow regimes are outward and inward, 19


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Fig. 8. Schematic diagram for (a) SST, (b) STS and (c) TSS configurations (reproduced from [46]).

represents a schematic for a proposed membrane assisted AF-SPBR. Despite the difficulty of making a spherical shape membrane, it can substantially save the fabrication cost of membrane since the total required surface area will be 80% of the corresponding tubular membrane for conventional reactors [9]. In a modeling study performed by Samimi et al. on dimethyl ether (DME) synthesis, a AF-SPBR equipped with water perm selective membrane enhanced the DME production by 13.5% [45]. Fig. 7 compares the performance of a conventional tubular DME reactor with the suggested spherical membrane configuration.

abruptly). AF-SPBRs have also been investigated for hydrocarbons reforming processes [39,40], dehydrogenation of paraffins to olefins [10] and gasoline production [28]. In addition to the aforementioned advantages for radial-flow reactors, the axial flow in spherical reactors offers the possibility of changing the membrane configuration to shift the reaction equilibrium and enhance the conversion based on the Le Chatelier’s principle [41–43]. In complex reaction systems like naphtha reforming (which includes both hydrogenation and dehydrogenation reactions), using H2-permselective membrane in an AF-SPBR successfully optimized the hydrogen to hydrocarbon ratio (H2/HC). This was an essential factor for increasing the catalyst’s lifetime, with the additional benefit of producing pure hydrogen gas [27]. Removal of the product is expected to improve the thermodynamic efficiency of the process and reduce the costs of downstream purification units [43,44]. Fig. 6

3.3. Replacing conventional tubular reactors with spherical reactors in multi stage reactor units Improvement in production capacity of conventional multi-stage 20


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reactors units by replacing one or two tubular reactors with spherical reactors was proposed for naphtha reforming processes by Rahimpour and his co-workers as shown in Fig. 8 [46]. Fig. 9 demonstrates pressure conservation in spherical-spherical-tubular membrane arrangement compared to the other arrangements. In order to optimize a combination of RF-SPBR and tubular membrane reactors, Iranshahi et al. [9] examined eight possible arrangements to select for the highest aromatic and hydrogen production. Their optimization results suggested either combinations of; two RF-SPBR with a tubular membrane configuration in the middle, or an RF-SMPR followed by two tubular membrane reactors. However, the former configuration was preferred due to lower surface area of employed membrane (see Fig. 10). Table 1 completes our literature review by summarizing similar studies on comparing DME, methanol, hydrogenation/dehydrogenation of hydrocarbons and styrene synthesis in conventional tubular reactors versus spherical configurations. Some authors have tried to employ various optimization techniques with numerous decision variables such as feed temperature, reactor radius, aspect ratio (length to radius), total pressure, catalyst distribution, number and combination of different reactors (e.g. spherical or tubular) to maximize production capacities [5,9,47,48].

Fig. 9. Pressure drop through tubular membrane-spherical-spherical (TSS), spherical-tubular membrane-spherical (STS) and spherical-spherical-tubular membrane (SST) configurations (reproduced from [46]).

3.4. A note on theoretical studies and developed mathematical models A reliable mathematical model, which can predict the operability of a chemical reactor, is based on the thermodynamic, kinetic, heat and mass transfer correlations, as well as fluid flow patterns [58]. Multiple computational modellings and simulations have been employed for spherical reactors. The dynamic models are regularly composed of heat and mass conservation equations coupled with a number of thermodynamic, kinetic and a set of empirical correlations to predict physical properties. Several essential assumptions are made for the sake of simplifying models. In most of the studies a one-dimensional plug flow model has been assumed which has been validated against the experimental and plant data. Occasionally pseudo-homogeneous or heterogeneous reaction models have been used although previous studies showed very similar results for homogenous and heterogeneous systems using the dusty gas model for effectiveness factor. The orthogonal collocation method has been frequently employed as a reliable numerical approach for solving the modeling equations [25,27,34,35,37]. 3.5. Comparison of tubular and spherical packed beds Comparison of conventional tubular and spherical reactors reveals many desirable characteristics for the latter. In spherical reactor there is:

• Significantly lower pressure drop and recompression cost • The safety and maintenance of the plant increases. • Smaller reactor thickness and possibly lower capital cost • Less required surface area of membrane which saves substantial investment costs and operation costs • Possibility of using smaller catalyst pellets with higher effectiveness • More flexibility to increase the feed flow rate and operation capacity

without noticeable change in the process configuration and operation parameters (e.g. pressure drop and recompression costs)

In addition, the AF-SPBR is commonly preferred to the RF-SPBR since:

Fig. 10. (a) comparison of aromatic and (b) hydrogen yields in conventional tubular reactor (CTR), optimized spherical-tubular membrane- spherical (OSMS) and optimized spherical-tubular membrane-tubular membrane (OSMM) reactor arrangements (reproduction from [9]).

• The axial flow pattern is more easily applicable in spherical reactors [39]. • AF-SPBR is flexible in applying modifications for more effective contact between the reactant phase and the catalytic bed [28]. • Membrane technology can be easily used in the AF-SPBR, while it is 21

22

Radial

Radial

styrene production

Hydrocracking

b

1

3 3 Single and dual stage membrane reactors were compared. 3

3 3

3

2 1

Comparison of single, dual, and three stage reactor set up were considered. Comparison of single, dual, three, and four stages reactor set up were performed. 3

3 3 3 3 3

3

Number of reactors in the process

The changes in pressure are related to conventional (tubular) systems. 1D and 2D models are compared.

Axial Axial Radial

Naphtha reforming Naphtha reformingb DME production

a

Radial Axial

Radial

Naphtha reforming

DME synthesis Naphtha reforming

Axial

Methanol synthesis

Radial

Radial

Methanol synthesis

Naphtha reforming

Radial Axial Radial Radial Axial

Naphtha reforming Naphtha reforming Naphtha reforming Methanol synthesis Naphtha reforming

Axial Axial

Radial

Methanol synthesis

DME synthesis DME synthesis

Flow regime

System

Table 1 A summary of reported spherical reactor designs for various processes.

640–700

820–910

720–780 720–780 520–660

520–660 720–780

720–780

500–660 500–660

720–780

500–530

500–530

720–780 720–780 720–780 470–520 720–780

510–550

Temperature (K)

182–187

1.06–1.26

33–37 33–37 16–18.2

16–18.2 33–37

33–37

16–18.2 16–18.2

Potassium-promoted iron oxide Bi-functional (having metallic and acidic sites)

Pt/Re/Al2O3 Pt/Re/Al2O3 γ-Al2O3

γ-Al2O3 Pt/Re/Al2O3

Pt/Re/Al2O3

–

Pt/Re/Al2O3

33–37

Multi-objective Genetic algorithm (GA) –

Differential Evolution (DE) – –

– Differential Evolution (DE)

Differential Evolution (DE)

Differential Evolution (DE) Differential Evolution (DE)

[46]

– Pd-Ag membrane for separation of H2 from reaction media – Alumina–Silica composite membranes for water separation Pd-Ag membrane for separation of H2 from reaction media – Pd-Ag membrane for separation of H2 from reaction media – – Alumina–Silica composite membranes for water separation –

Cu/ZnO

γ-Al2O3 γ-Al2O3

[25]

–

–

Cu/ZnO

∼77 74–77

–

– – – – Pd-Ag membrane for separation of H2 from reaction media –

Pt/Re/Al2O3 Pt/Re/Al2O3 Pt/Re/Al2O3 Cu/ZnO Pt/Re/Al2O3

33–37 33–37 33–37 ∼77 33–37

[35]

[57]

[54] [55] [56]

[5] [53]

[9]

[52] [45]

[51]

[50] [30] [34] [15] [27]

[49]

Iterative Dynamic Programming (IDP) – – Differential Evolution (DE) Differential Evolution (DE) –

–

Cu/ZnO/A12O3

∼81

Ref.

Optimization

Membrane

Catalyst

Pressure (bar)a

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References

difficult to apply in RF-SPBR [39].

[1] S. Saeidi, F. Fazlollahi, S. Najari, D. Iranshahi, J.J. Klemeš, L.L. Baxter, Hydrogen production: perspectives, separation with special emphasis on kinetics of WGS reaction: a state-of-the-art review, J. Ind. Eng. Chem. 49 (2017) 1–25. [2] M.K. Nikoo, S. Saeidi, A. Lohi, A comparative thermodynamic analysis and experimental studies on hydrogen synthesis by supercritical water gasification of glucose, Clean Technol. Environ. Policy 17 (2015) 2267–2288. [3] S. Saeidi, M. Talebi Amiri, S. Amin, N. Aishah, M.R. Rahimpour, Progress in reactors for high-temperature Fischer–Tropsch process: determination place of intensifier reactor perspective, Int. J. Chem. React. Eng. 12 (2014) 639–664. [4] S. Saeidi, M.K. Nikoo, A. Mirvakili, S. Bahrani, S. Amin, N. Aishah, M.R. Rahimpour, Recent advances in reactors for low-temperature Fischer-Tropsch synthesis: process intensification perspective, Rev. Chem. Eng. 31 (2015) 209–238. [5] M. Farsi, M. Asemani, M.R. Rahimpour, Mathematical modeling and optimization of multi-stage spherical reactor configurations for large scale dimethyl ether production, Fuel Process. Technol. 126 (2014) 207–214. [6] S. Saeidi, S. Najari, F. Fazlollahi, M.K. Nikoo, F. Sefidkon, J.J. Klemeš, L.L. Baxter, Mechanisms and kinetics of CO2 hydrogenation to value-added products: a detailed review on current status and future trends, Renew. Sustain. Energy Rev. 80 (2017) 1292–1311. [7] V.L. Streeter, E.B. Wylie, K.W. Bedford, Fluid Mechanics, 9th ed., WCB McGrawHill, Boston, 1998 p. 21. [8] R.E. Hayes, J.P. Mmbaga, Introduction to Chemical Reactor Analysis, CRC Press, 2012. [9] D. Iranshahi, M.R. Rahimpour, K. Paymooni, E. Pourazadi, Utilizing DE optimization approach to boost hydrogen and octane number, through a combination of radial-flow spherical and tubular membrane reactors in catalytic naphtha reformers, Fuel 111 (2013) 1–11. [10] H.S. Fogler, Elements of Chemical Reaction Engineering, 2nd ed., Prentice-Hall, Englewood Cliffs, NJ, 1992. [11] V. Hlaváček, M. Kubíček, Modeling of chemical reactors—XXV cylindrical and spherical reaction with radial flow, Chem. Eng. Sci. 27 (1972) 177–186. [12] S. Roy, A.K. Heibel, W. Liu, T. Boger, Design of monolithic catalysts for multiphase reactions, Chem. Eng. Sci. 59 (2004) 957–966. [13] R.-J. Koopmans, J. Shrimpton, G. Roberts, A. Musker, Dependence of pellet shape and size on pressure drop in H2O2 thrusters, J. Propul. Power 30 (2014) 775–789. [14] A. Pavlišič, R. Ceglar, A. Pohar, B. Likozar, Comparison of computational fluid dynamics (CFD) and pressure drop correlations in laminar flow regime for packed bed reactors and columns, Powder Technol. 328 (2018) 130–139. [15] M. Rahimpour, P. Parvasi, P. Setoodeh, Dynamic optimization of a novel radialflow, spherical-bed methanol synthesis reactor in the presence of catalyst deactivation using Differential Evolution (DE) algorithm, Int. J. Hydrogen Energy 34 (2009) 6221–6230. [16] H.R. Shahhosseini, S. Saeidi, S. Najari, F. Gallucci, Comparison of conventional and spherical reactor for the industrial auto-thermal reforming of methane to maximize synthesis gas and minimize CO2, Int. J. Hydrogen Energy 42 (2017) 19798–19809. [17] A. de Ramos, F. Pironti, Effective thermal conductivity in a packed‐bed radial‐flow reactor, AIChE J. 33 (1987) 1747–1750. [18] V. Balakotaiah, D. Luss, Effect of flow direction on conversion in isothermal radial flow fixed‐bed reactors, AIChE J. 27 (1981) 442–450. [19] H.-C. Chang, J.M. Calo, An analysis of radial flow packed bed reactors, Chemical Reactors, American Chemical Society, 1981, pp. 305–329. [20] H.C. Chang, M. Saucier, J. Calo, Design criterion for radial flow fixed‐bed reactors, AIChE J. 29 (1983) 1039–1041. [21] P.R. Ponzi, L.A. Kaye, Effect of flow maldistribution on conversion and selectivity in radial flow fixed‐bed reactors, AIChE J. 25 (1979) 100–108. [22] M.R. Rahimpour, D. Iranshahi, E. Pourazadi, K. Paymooni, Evaluation of optimum design parameters and operating conditions of axial-and radial-flow tubular naphtha reforming reactors, using the differential evolution method, considering catalyst deactivation, Energy Fuels 25 (2011) 762–772. [23] N. Hamedi, T. Tohidian, M. Rahimpour, D. Iranshahi, S. Raeissi, Conversion enhancement of heavy reformates into xylenes by optimal design of a novel radial flow packed bed reactor, applying a detailed kinetic model, Chem. Eng. Res. Des. 95 (2015) 317–336. [24] Z. Cimbálník, et al. 2nd CHISA Cong., Mariánské Lázne, Czechoslovakia, 1965. [25] M.R. Rahimpour, E. Pourazadi, D. Iranshahi, A.M. Bahmanpour, Methanol synthesis in a novel axial-flow, spherical packed bed reactor in the presence of catalyst deactivation, Chem. Eng. Res. Des. 89 (2011) 2457–2469. [26] F. Zardi, D. Bonvin, Modeling, simulation and model validation for an axial-radial ammonia synthesis reactor, Chem. Eng. Sci. 47 (1992) 2523–2528. [27] M.R. Rahimpour, D. Iranshahi, E. Pourazadi, K. Paymooni, A.M. Bahmanpour, The aromatic enhancement in the axial‐flow spherical packed‐bed membrane naphtha reformers in the presence of catalyst deactivation, AIChE J. 57 (2011) 3182–3198. [28] L.B. Brumbaugh, Modified spherical reactor, in, US Patent 2,996,361, 1961. [29] V.V. Ranade, Computational Flow Modeling for Chemical Reactor Engineering, Academic Press, 2002. [30] D. Iranshahi, E. Pourazadi, K. Paymooni, A. Bahmanpour, M. Rahimpour, A. Shariati, Modeling of an axial flow, spherical packed-bed reactor for naphtha reforming process in the presence of the catalyst deactivation, Int. J. Hydrogen Energy 35 (2010) 12784–12799. [31] G.A. Viecco, H.S. Caram, The spherical reverse flow reactor, Chem. Eng. Sci. 57 (2002) 4005–4025. [32] A.Y. Raskiii, Yu.A. Sokolinskii, V.I. Mukosie, M.E. Aerov, Mathematical model and calculation algorithm for radial adiabatic reactors, Thmr. Found. Chem. Tech. 2

4. Guidelines for further development Although several authors have studied various aspects of spherical reactors and a number of papers have been published in this area, more research is needed regarding the characterization of different reactions and development of reactors. The following suggestions could be considered as future guidelines: 1 Results of optimization analysis of spherical reactors have clearly indicated that using more complicated models rather than conventional simple models lead to more reliable results. The number of assumptions on which the mathematical models are derived should be reduced. For instance; Considering various catalyst distribution or different hot-spot temperatures in the model. In most of the previous publications, a one-dimensional model has been assumed (only in axial or radial direction) despite the more accurate results of the two-dimensional model. It would be of interest to compare one and two-dimensional mathematical models to find the optimum assumptions in order to improve accuracy and computational cost. Presented reactor models commonly assume a homogeneous reaction system. Considering a heterogeneous reaction system (i.e. accounting for mass transfer/diffusion of reactants/products between catalyst and gas phase to find out the final concentrations of products on both solid surface and in the fluid phase) inside the reactor is suggested for improvement of the simulation results. 2 Given the fact that most studies regarding spherical reactors are theoretical, there is a considerable need for experimental research. Comparisons between simulation results and plant data should be considered in order to determine the degree of conformity of theoretical investigations. 3 The application of membrane in the AF-SPBR requires more experiments. Manufacturing a spherical shaped membrane and designing modifications to the reactor are examples of some potential areas for research. 4 Designing a spherical reactor, its cost evaluation, and its scaling up to industrial production plants should be considered from a different perspective.

• •

•

Spherical reactor configurations can be applied to other processes with conventional tubular reactors; and simulation results can be used initially to predict and compare the results with the conventional data prior assuring the benefits of spherical reactors. Further investigation should be carried out concerning environmental aspects, commercial viability and economic feasibility of the proposed configurations are necessary.

5. Conclusion In any process, there is a significant incentive to minimize the pressure drop in reaction vessels. This has led to the development of spherical industrial reactors. In this review, the latest modeling, simulation, optimization, design and experimental studies were discussed. It is clear that spherical reactors have a lower pressure drop over the catalytic bed as well as reduced investment cost compared to tubular reactors. Two main groups of spherical catalytic reactors have been discussed; radial and axial flow. Axial-flow spherical configurations appear to be more promising than the initially investigated radial-flow spherical reactors. Application of spherical reactors may contribute to higher reaction rates, production capacity as well as saving energy. There is room for further potential research into these. 23


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simulation and optimization, J. Nat. Gas Sci. Eng. 13 (2013) 42–51. [46] M.R. Rahimpour, D. Iranshahi, E. Pourazadi, A.M. Bahmanpour, A comparative study on a novel combination of spherical and membrane tubular reactors of the catalytic naphtha reforming process, Int. J. Hydrogen Energy 36 (2011) 505–517. [47] M. Weiss, R. Lang, J. Longwell, Combustion rates in spherical reactors. Effects of inlet temperature and fuel type, Ind. Eng. Chem. 50 (1958) 257–264. [48] R. Smith, Chemical Process Design, Wiley Online Library, 2005. [49] F. Hartig, F.J. Keil, Large-scale spherical fixed bed reactors: modeling and optimization, Ind. Eng. Chem. Res. 32 (1993) 424–437. [50] D. Iranshahi, M. Rahimpour, A. Asgari, A novel dynamic radial-flow, spherical-bed reactor concept for naphtha reforming in the presence of catalyst deactivation, Int. J. Hydrogen Energy 35 (2010) 6261–6275. [51] M.R. Rahimpour, A. Abbasloo, J. Sayyad Amin, A novel radial‐flow, spherical‐bed reactor concept for methanol synthesis in the presence of catalyst deactivation, Chem. Eng. Technol. 31 (2008) 1615–1629. [52] F. Samimi, M. Bayat, M.R. Rahimpour, P. Keshavarz, Mathematical modeling and optimization of DME synthesis in two spherical reactors connected in series, J. Nat. Gas Sci. Eng. 17 (2014) 33–41. [53] M.R. Rahimpour, D. Iranshahi, K. Paymooni, E. Pourazadi, Enhancement in research octane number and hydrogen production via dynamic optimization of a novel spherical axial-flow membrane naphtha reformer, Ind. Eng. Chem. Res. 51 (2011) 398–409. [54] D. Iranshahi, A.M. Bahmanpour, E. Pourazadi, M.R. Rahimpour, A comparative study on optimised and non‐optimised axial flow, spherical reactors in naphtha reforming process, Can. J. Chem. Eng. 90 (2012) 1102–1111. [55] M.J.S. Zaree, D. Iranshahi, M.R. Rahimpour, Comparison of one-dimensional and two-dimensional modeling of catalytic naphtha reforming process in axial flow spherical packed-bed reactor, First International Conference of Oil, Gas, Petrochemical and Power Plant, (2012). [56] M. Farsi, H. Jowkari, A.I. Doust, Multi–objective optimization approach to enhance ethylbenzene dehydrogenation in the multi-stage spherical reactors, Period. Polytech. Chem. Eng. 60 (2016) 201–209. [57] M. Farsi, DME production in multi-stage radial flow spherical membrane reactors: reactor design and modeling, J. Nat. Gas Sci. Eng. 20 (2014) 366–372. [58] H.A. Jakobsen, Chemical Reactor Modeling, Multiphase Reactive Flows, SpringerVerlag, Berlin, Germany, 2008.

(1968) 220. [33] J. Calo, Cell Model Studies of Radial Flow, Fixed Bed Reactors, ACS Publications, 1978. [34] M.R. Rahimpour, D. Iranshahi, A.M. Bahmanpour, Dynamic optimization of a multistage spherical, radial flow reactor for the naphtha reforming process in the presence of catalyst deactivation using differential evolution (DE) method, Int. J. Hydrogen Energy 35 (2010) 7498–7511. [35] D. Iranshahi, A.B. Ani, A. Novel Radial-Flow, Spherical packed bed reactor for the hydrocracking process, Ind. Eng. Chem. Res. 54 (2015) 1748–1754. [36] L. Ma, C. Jiang, A. Adesina, D. Trimm, M. Wainwright, Simulation studies of autothermal reactor system for H2 production from methanol steam reforming, Chem. Eng. J. Biochem. Eng. J. 62 (1996) 103–111. [37] F. Hartig, F.J. Keil, Large-scale spherical fixed bed reactors: modeling and optimization, Ind. Eng. Chem. Res. 32 (1993) 424–437. [38] A. Lotric, M. Sekavcnik, A. Pohar, B. Likozar, S. Hocevar, Conceptual design of an integrated thermally self-sustained methanol steam reformer e high-temperature PEM fuel cell stack manportable power generator, Int. J. Hydrogen Energy 42 (2017) 16700–16713. [39] M.R. Rahimpour, S. Ghader, M. Baniadam, J. Fathi Kalajahi, Incorporation of flexibility in the design of a methanol synthesis loop in the presence of catalyst deactivation, Chem. Eng. Technol. 26 (2003) 672–678. [40] M. Rahimpour, B. Moghtaderi, A. Jahanmiri, N. Rezaie, Operability of an industrial methanol synthesis reactor with mixtures of fresh and partially deactivated catalyst, Chem. Eng. Technol. 28 (2005) 226–234. [41] H. Jenkins, Le Chatelier’s Principle, Chemical Thermodynamics at a Glance, (2008), pp. 160–163. [42] P. Ferreira-Aparicio, I. Rodrı́guez-Ramos, A. Guerrero-Ruiz, On the applicability of membrane technology to the catalysed dry reforming of methane, Appl. Catal. A Gen. 237 (2002) 239–252. [43] S.T. Oyama, P. Hacarlioglu, Y. Gu, D. Lee, Dry reforming of methane has no future for hydrogen production: comparison with steam reforming at high pressure in standard and membrane reactors, Int. J. Hydrogen Energy 37 (2012) 10444–10450. [44] K. Sankaranarayanan, H.J. van der Kooi, J. de Swaan Arons, Efficiency and Sustainability in The Energy and Chemical Industries, CRC Press, Taylor & Francis Group, 2010. [45] F. Samimi, M. Bayat, D. Karimipourfard, M.R. Rahimpour, P. Keshavarz, A novel axial-flow spherical packed-bed membrane reactor for dimethyl ether synthesis:

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