A Review of Techniques for the Process Integration of Coupling Exothermic and Endothermic Reactions

Title: A review of techniques for the process integration of coupling exothermic and endothermic reactions

Department of Chemical Engineering, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz 71345, Iran

Corresponding Author: Prof. M. R. Rahimpour,

Corresponding Author's Institution: Shiraz University

First Author: M. R. Rahimpour

Order of Authors:

Mohammad Reza  Rahimpour

Davood  Iranshahi

Majid Raoof  Dehnavy

Fatemeh  Allahgholipour

Keywords: Process integration; Coupling; Hydrogenation; dehydrogenation; Hydrogen production, Energy intensification

Corresponding author. Tel.: +98 711 2303071; fax: +98 711 6287294.

E-mail address: rahimpor@shirazu.ac.ir (M.R. Rahimpour).

Introduction

   Process integration (PI) is one of the most significant methodology in chemical engineering and process technology for combining severed process to reduce consumption of energy and material or environmental emissions[58]. The study on process integration started in the late 1970s and early 1980s[59].

  PI includes hierarchical design methods, knowledge-based systems, numerical and graphical techniques and pinch analysis[60]. Pinch analyses design a process to minimize energy consumption and enhance energy efficiency. In the last years, some works have been done to minimize the entropy generated during a chemical process by focus on the chemical process, which are the heart of process technologies. So reactors play the principle role in pinch analysis.

  In this way multifunctional reactor is a new one. The best field of using multifunctional reactors is the coupling of endothermic and exothermic reactions. In this way, an exothermic reaction produces the source of heat to drive the endothermic reaction(s). The distinction between net source and net sink regions is a key point pinch analysis[5, 13, 22, 42].

  So coupling of exothermic and endothermic reactions forms the initial step for process optimization using the pinch method[60].

  Hydrogenation, dehydrogenation, combustion, oxidation etc. are some examples of endothermic and exothermic of reactions. Hence, in this paper we are investigating review of coupling concept with focus on the reactors, and also, consequences of previous researches in this field are evaluated. Finally, results illustrated, that optimal coupling of these reactions could be feasible and beneficial.

 

PART A: Kind of reactors that are better for coupling:

 1- Fluidized bed reactor

 2- Co-current & counter-current reactor

 3- Membrane reactor

 4- Membrane assisted fluidized bed reactor

 

1- Fluidized bed reactor

  These reactors have achieved broad use in the chemical and petroleum industries. The main advantages of these kinds of reactors are presented that appropriate them as a coupling reactions instrument[49].

1) Having more compact design[32].

2) The possibility of using inexpensive metal alloys for their vessels[53].

3) Easily temperature controlling since the rapid mixing of solids in fluidized beds[52].

4) Negligible pressure drop[51,52].

5) Fluidized bed reactors with its active gas-solid contacting and whereas, the fixed ones can't. Small particles will allow an impressive use of catalyst[51,52]. Fig.1- a&b shows the schematic diagrams of fluidized bed and fixed bed reactors respectively.

Fig.1- a&b

2- Co-current & counter-current reactor

  A large number of studied on the coupled reactors indicate that co-current reactors have higher conversion compared with counter current ones. On the other hand, some studies show the dominance of the counter-current mode. It is found by modeling of co-current and counter-current regimes that higher aromatic and lower hydrogen production rate are achieved in co-current flow compared with counter-current configuration can be a persuasive way to enhance hydrogen production. However, an investigation with regard to the economic feasibility and environmental aspects is necessary in order to choose one of these kinds of flow in the coupled reactors. A conceptual schematic of co-current and counter-current reactors is shown in Fig.2.

Fig.2

3- Membrane reactor

  Recently, the chemical process based on hydrogenation and dehydrogenation reactions, have been focused on membrane reactors. Membrane reactors are useful for reactions associated hydrogen, specially, in coupling reactions when one side produce hydrogen and the other side consume it[57].

4- Membrane assisted fluidized bed reactor

  A membrane assisted fluidized bed reactor (MAFBD) is a particular reactor that combines the advantages of a membrane and a fluidized bed reactor.

  The main advantages of the MAFBD are[2, 49]:

1) Isothermal operation

2) Arrangement of the membrane package and flexibility in membrane and heat transfer surface

3) Negligible pressure drop

4) Improved fluidization behavior as a result of:

a- Compartmentalization, i.e. reduced axial gas back- mixing.

b- Reduced average to increased bubble breakage.

  Because of these main advantages, the MAFBR as a multifunctional reactors are significant configuration in process integration.

Part B: Classification of the reactors that are used in the coupling of exothermic and endothermic[56]:

1- Direct coupling (direct coupled adiabatic reactor)

2- Regenerative coupling (reverse-flow reactor)

3- Recuperative coupling (counter current heat exchanger reactor and co-current heat exchanger reactor).

4- Membrane reactor

  In directly coupled reactors, the exothermic and endothermic reactions are in a same bed. There is a direct heat transfer between them, and also, both reactions occur simultaneously. Figures 3 – 6 show conceptual configuration of these reactors.

Fig.3 , Fig.4 , Fig.5 & Fig.6

  In regenerative coupling reactor, both reactions are in a same place but separated in time.

  In recuperative coupling reactor, there are two separated spaces for two reactions. Tube side is for endothermic reaction and shell side is for exothermic one.

  In membrane reactor, exothermic reaction carries out in the tube side, and endothermic reaction occur in through shell side and membrane.

PART C: Categorization of the energy coupling between exothermic and endothermic reactions[56]:

1) Direct

2) Regenerative

3) Recuperative

  In the direct and regenerative mode of coupling both the exothermic and endothermic reactions occur in a same catalytic bed, but there is a different between these reactors.

  In the direct coupling, both reactions are simultaneously in the bed, whereas the reactions are discrete in time.

  Direct coupling adiabatic reactor (DCAR) can be also classified into simultaneous DCAR, (SIMDCAR) and sequential DCAR, (SEQDCAR). In SIMDCAR, exothermic catalysts make the bed of it. In SEQDCAR, reactor has an alternating exothermic and endothermic catalyst bed. A new auto-thermal reformer (ATR) for steam reforming of natural gas into hydrogen and carbon dioxide is an example of SEQDCAR that have been developed by the institute of Applied Energy, Tokyo. This ATR includes a reactor packed bed with alternative catalysts bed.

  Regenerative coupling reactors are well adaptable for weakly exothermic reactions such as the catalytic purification of exhaustive streams are polluted by volatile organic compounds (Kulkurani, 1996). Schematic diagrams of SIMDCAR and SEQDCAR is shown in Fig.7- a&b.

Fig.7- a&b

PART D: In this section, we demonstrate examples of various studies on the coupling concept in Table.1. and two examples of this works are explaining in the following:

Table.1

Annaland and his team studied a novel reverse flow coupling reactor[2, 3]. They studied on the coupling between the non-oxidative propane dehydrogenation and methane combustion over a monolithic catalyst by considering two different reactor configurations: the sequential reactor configuration, where the exothermic and endothermic reactants are fed sequentially to the same catalyst bed acting as an energy repository and the simultaneous reactor configuration, where the exothermic and endothermic reactants are fed continuously to two different sections.

  Rahimpour et al. presented a membrane thermally coupled reactor which included three sides: methanol synthesis, cyclohexane dehydrogenation and hydrogen production[5]. Methanol synthesis occurs in the exothermic side and provides the necessary heat for the endothermic dehydrogenation of cyclohexane reaction. Their simulation's results demonstrated that the methanol mole fraction in output is raised by 16.3%.

Reference

 [2] M. van Sint Annaland, H. A. R. Scholts, J. A. M. Kuipers, W. P. M. van Swaaij, A novel reverse  flow reactor coupling endothermic and exothermic reactions. Part I: comparison of reactor configurations for irreversible endothermic reactions, Chem.Eng.Sci. 57 (2002) 833 – 854

[5] M.H. Khademi, A. Jahanmiri, M.R. Rahimpour, A novel configuration for hydrogen production from coupling of methanol and benzene synthesis in a hydrogen-permselective membrane reactor, Int.J.Hydrogen Energy 34 (2009) 5091 – 5107

No.	Reaction	Reaction kinetic		T	P		Catalyst	Author	Ref
1	Methane combustion reaction	CH4 + 2O2 → CO2 + 2H2O	(-)	1000 K	3 bar		Pt / Al2O3	Annaland et al.	1,2,3,4
	Dehydrogenation Propane to propylene	C3H8↔ C3H6 + H2	130	550 – 650C	1 atm
4	Methanol Synthesis	CO + H2 → CH3OH	(-)	500 K	76.98 bar		CuO/ZnO /Al2O3	Rahimpour  et al.	5,6,7,8
	Dehydrogenation of Cyclohexane to benzene	C6H12→C3H6 + 3H2	206.2.	423-523 K	101.3 KPa		Pt / Al2O3
7	hydrogenation of nitrobenzene to aniline	C6H5NO2 + 3H2 → C6H5NH2 + 2H2O	-443 kJ/mol.	300-700 K	1.1 bar			Abo-Ghandar et al.&Qin et al.	12,13,14
	dehydrogenation of ethylbenzene to styrene	C6H5CH2 ↔ C6H5CHCH2 + H2	+117.6kJ/mol	300-700 K	4.5  bar		Fe2O3
8	dehydrogenation of ethylbenzene to styrene	C6H5CH2 ↔ C6H5CHCH2 + H2	+117.6kJ/mol	300 -700 K		4.5  bar	Fe2O3	Qin et al.	14
	the CO2 methanation	CO2 + 4H2 ↔ CH4 + 2H2O	-165	300 - 700 K		,01-1 MPa
9	dehydrogenation of ethylbenzene to styrene by CO2	C6H5CH2 ↔ C6H5CHCH2 + H2	+117.6kJ/mol	850 K	4.5  bar		Fe/AC, FeLi/AC,Fe-K	Wang et al.	14,15
	water-gas shift reaction	CO + H2O ↔ CO2 + H2	-40.6	300-700K	1  bar

Figure 1-a

Figure 1-b

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7-a

Figure 7-b