Elsevier

Chemical Engineering Science

Volume 100, 30 August 2013, Pages 342-351
Chemical Engineering Science

Monolith catalysts for the alkylation of benzene with propylene

https://doi.org/10.1016/j.ces.2013.02.066Get rights and content

Highlights

  • A 3D model was established for the synthesis of cumene using monolith catalysts.

  • Identify the relationship between geometric configuration and performances.

  • Lower pressure drop, higher selectivity and higher effectiveness factor was brought.

  • Monolith catalyst brings the lowest energy consumption for the whole process.

Abstract

This work deals with the transfer and reaction performances for the alkylation of benzene with propylene to produce cumene over monolith catalysts by means of the combination of experiments and computational fluid dynamics (CFDs). A three-dimensional (3D) mathematical model was established to identify the geometric configuration–performance relation so as to provide a comprehensive comparison of momentum transfer (pressure drop), heat transfer (Nu number), mass transfer (Sh number), and reaction performances (i.e. propylene conversion, cumene selectivity, and effectiveness factor) among monolith catalysts with five kinds of channel shapes (i.e. circle, hexagon, square, rectangle, and regular triangle). The objective of this work is to address the cogently interesting issues as to (i) whether or not monolith catalysts can improve the transfer and reaction performances for the alkylation of benzene with propylene in comparison with the traditional pellet catalyst or even structured catalytic packing; (ii) which kind of channel shape is optimum from the viewpoint of chemical reaction engineering; and (iii) how much energy can be reduced in the actual process consisting of a gas–liquid–solid bubble-point reactor and two distillation columns. It was found that monolith catalyst exhibits the lower pressure drop, higher cumene selectivity and higher effectiveness factor; and regular triangle or rectangle channel is optimum when considering pressure drop and cumene selectivity together. Furthermore, the advantages of monolith catalyst can lead to the lowest energy consumption for the whole process.

Introduction

The alkylation of aromatic hydrocarbons with olefins has been applied on a large scale in chemical industry. Cumene (isopropylbenzene) is a basic petrochemical material used in the production of phenol and acetone. It is usually obtained in an industrial-scale by Friedel–Crafts alkylation of benzene with propylene (Shoemakers and Jones, 1987, Tian et al., 2005). The main types of reactors that can be used for alkylation of benzene with olefin are fixed-bed reactor, bubble-point reactor, fixed-bed catalytic distillation (FCD) column, and suspension catalytic distillation (SCD) column (Lei et al., 2007, Wang et al., 2005, Genoveva et al., 2006, Torres-Rodríguez et al., 2012). The most commonly used in industry is the fixed-bed reactor which is easy to be implemented when using pellet zeolite catalysts instead of AlCl3 and solid phosphoric acid which are highly toxic and corrosive, and are dangerous to handle and transport (Kaeding and Holland, 1988, Rodriguez et al., 1998). Zeolite-based catalysts exhibiting excellent performances (e.g. high catalytic activity, non-corrosiveness, environmental-friendly behavior, and good selectivity) for the alkylation of aromatic hydrocarbons have been widely used for the past two decades (Pradhan and Rao, 1991, Siffert et al., 2000, Han et al., 2003, Fu and Ding, 2005, Yang et al., 2009, Pathak et al., 2011). It is noted that the bubble-point reactor is a modification of fixed-bed reactor, where reaction and flash are coupled simultaneously such that energy consumption can be reduced, and has been applied in practice recently without changing the stainless steel shell, main pipes, and foundation of the original fixed-bed reactor.

The main reaction for the synthesis of cumene (IPB) is the alkylation of benzene (B) with propylene (P) expressed asB+Pr1IPBwhile cumene can further alkylate to di-isopropylbenzene (DIPB) as side products as followsIPB+Pr2DIPB

The above two reactions are often called alkylation reaction. Furthermore, DIPB may slightly alkylate to the heavier tri-isopropylbenzene (TIPB). In addition, di-isopropylbenzene (DIPB) may further react with benzene (B) to produce more cumene, which mainly takes place in another transalkylation reactor. Under the operating conditions, transalkylation reaction and the generation of TIPB were neglected in this work due to the small amount of DIPB and TIPB produced in the products.

However, the problems existing in the conventional fixed-bed reactor filled with pellet catalysts for the alkylation of benzene with propylene are: (i) a high pressure drop restraining the increase of throughput capacity and leading to the high power consumption of pumps; (ii) when the zeolite pellet catalysts are deactivated, only the external surface is deactivated but the interior is still effective, indicating that the utilization efficiency of pellet catalysts is low. It is evident that how to design the hardware containing solid catalysts is crucial for solving these problems. Therefore, monolith catalysts are proposed in this work as the replacement of pellet catalysts for the alkylation of benzene with propylene.

Monolith catalysts were first developed in the 1970s for automotive exhaust gas treatment, mainly to control the hydrocarbon and carbon monoxide emissions (Bennett et al., 1991, Bennett et al., 1992, Boger et al., 2004). Later, many researchers (Cybulski and Moulijn, 1994, Forzatti, 2001, Gupta and Balakotaiah, 2001, Kapteijn et al., 2001, Tronconi et al., 1992, Wei et al., 2009) pointed out that monolith catalysts are becoming increasingly significant as catalyzed gas–solid or multiphase reactors in view of the advantages that they exhibit in comparison with conventionally used packed-bed reactors, including low pressure drop, much higher surface area per unit reactor volume and effectiveness factor, and minimum axial dispersion stemming from the uniquely structured multichannel configuration of monoliths. In general, monolithic reactor consists of a matrix of uniformly aligned parallel channels of roughly hydraulic diameters 1–5 mm, and such support materials as ceramic, metal, and cordierite are often selected (Kern and Gadow, 2002, Bueno-López et al., 2005, Barbero et al., 2008, Rodrigues et al., 2009). Actually, the transfer and reaction performances of monolith catalysts considerably depend on the material and geometry characteristics of their support (Bhattacharya et al., 2004a, Bhattacharya et al., 2004b, Groppi et al., 1995, Groppi and Tronconi, 1997, Lei et al., 2011, Tronconi and Forzatti, 1992).

In most applications, the reacting fluid flows through these channels in laminar flow regime with fully developed flow in the large part of reactor (Bennett et al., 1992; Young and Finlayson, 1976), whereas reactants are transported to the substrate mainly by molecular diffusion. The substrate wall is treated with a thick porous layer containing catalyst supports, stabilizers, and promoters, commonly called as washcoat, where the active metal sites are dispersed by impregnation method. Within the washcoat, the reactants diffuse and react on the active catalyst sites with an associated release or absorption of heat. Two different approaches were used to treat the reaction rates in monolith catalyst models, i.e. surface approach (Arzamendi et al., 2009, Shi et al., 2009, Uriz et al., 2011, Irani et al., 2011), and volumetric approach (Zygourakis and Aris, 1983, Hayes and Kolaczkowski, 1994, Massing et al., 2000, Stutz and Poulikakos, 2008). In the former, the diffusion within the washcoat is neglected, and the reactions are assumed to take place on the surface of the washcoat. However, in the latter, the diffusion effect is taken into account. Therefore, we adopted the volumetric approach in establishing the mathematical model for monolith catalysts.

By far, no systematic and critical research on the alkylation of benzene with propylene using monolith catalysts has been done to identify the mechanism of process intensification and thus to guide the selection of suitable monolith catalysts from the viewpoint of chemical reaction engineering. Therefore, the objective of this work is on addressing the cogently interesting issues as to (i) whether or not monolith catalysts can improve the transfer and reaction performances in comparison with the traditional pellet catalyst or even structured catalytic packing; (ii) which kind of channel shape is optimum; and (iii) how much energy can be reduced in the actual process consisting of a gas–liquid–solid bubble-point reactor and two distillation columns. In principle, pellet catalyst, structured catalytic packing and monolith catalyst can all be loaded into the bubble-point reactor, and thus it is interesting for us to go a further step to compare their energy consumption. Besides, both monolith catalyst and structured catalytic packing belong to the categories of structured catalysts.

Section snippets

Assumptions

Monolith catalysts can be viewed as consisting of a number of repeated building blocks where the basic building block is a single channel with symmetrical peripheral walls, i.e. all channels of the monolith are equivalent. Therefore, in this work, a 3D mathematical model on single channel was established in scaling up a monolith reactor. The assumptions associated with the model equations are listed as follows:

  • (1)

    The simulation process is at steady state;

  • (2)

    The velocity, temperature, and

Preparation and characterization of monolith catalyst

The monolith catalyst was prepared according to the following three procedures: (i) pretreatment of monolith honeycomb cordierite: the tested cordierite samples with diameter of 20 mm and height of 12 mm were cut from a commercial honeycomb cordierite, and then pretreated using 15 wt% nitric acid solution and heated at 80 °C for 4 h. Afterwards, the monolith supports were washed and calcined at 550 °C for 5 h; (ii) preparation of zeolite–silica sol: the commercial β zeolite with a Si/Al ratio of 15

Results and discussion

Five kinds of channel shapes of monolith catalysts, i.e. circle, square, regular triangle, hexagon, and rectangle (see Fig. 3), were concerned to investigate the geometric configuration–performance relation for the alkylation of benzene with propylene to produce cumene, and the detailed geometric and operating parameters are given in Table 1. All comparison among them was made under the same height, repeated unit area, wall thickness and flux area, as well as with total mass flux as abscissa in

Comparison among pellet catalyst, structured catalytic packing and monolith catalyst installed into a bubble-point reactor

The actual scaling-up process for producing cumene consists of a gas–liquid–solid bubble-point reactor and two distillation columns. Three kinds of catalysts, i.e. traditional YSBH pellet catalyst, BH structured catalytic packing and monolith catalyst with regular triangle channel are loaded into the bubble-point reactor, respectively, where alkylation reaction in the liquid phase and flash into the vapor phase take place simultaneously. The bubble-point reactor has the total height of 3500 mm,

Conclusions

In this work, a three-dimensional CFD model was established to identify the geometric configuration-performances relation for the alkylation of benzene with propylene so that we can quantitatively tailor the desirable momentum transfer, heat transfer, mass transfer, and reaction performances (i.e. propylene conversion, cumene selectivity, and effectiveness factor). The reliability of mathematical model was verified by experiments using the β/cordierite monolith catalyst. It was found that

Nomenclature

    a

    permeability, m2

    C

    molar concentration, kmol m−3

    cp

    constant pressure heat capacity, J kg−1 K−1

    D

    diffusion coefficient, m2 s−1

    Da

    Damkohler number, dimensionless

    d

    diameter, m

    g

    acceleration due to gravity, m s−2

    ΔH

    reaction heat, J mol−1

    K

    equilibrium constant, dimensionless

    M

    molar weight, g mol−1

    n

    molar number, mol

    Nu

    Nusselt number, dimensionless

    P

    pressure, Pa

    r

    reaction rate, kmol·(kg s)−1

    Re

    Reynolds number, dimensionless

    S

    source term, Pa m−1

    S

    cumene selectivity, dimensionless

    Sh

    Sherwood number, dimensionless

    T

    temperature, °C

Acknowledgements

This work is financially supported by the National Nature Science Foundation of China under Grants (Nos. 21121064 and 21076008), and the Projects in the National Science & Technology Pillar Program during the twelfth Five-Year Plan Period (No. 2011BAC06B04).

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