a novel thermally coupled reactor configuration comprising ...in the oleflex process, isobutane is...

The 11th International Chemical Engineering Congress & Exhibition (IChEC 2020) Fouman, Iran, 28- 30 October, 2020

A novel thermally coupled reactor configuration comprising dehydrogenation of isobutane and aniline production

S. Rahmani Koragah, D. Iranshahi*

Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), No. 424, Hafez Avenue, Tehran 15914, Iran

[email protected]

Abstract In this study, three multi-tubular thermally coupled reactor containing the isobutane dehydrogenation in the shell side and the nitrobenzene hydrogenation in the tube side has been investigated, which resulted in the elimination of the isobutane dehydrogenation furnaces and nitrobenzene hydrogenation coolers. Both processes are available in petrochemical plants. Isobutane dehydrogenation produces isobutene, while nitrobenzene hydrogenation reactions produce aniline as the main product, which is a known intermediate for the production of medicine, dyes, pesticides, and other intermediates. A steady-state homogeneous one-dimensional model is developed to predict the proficiency and operability of the proposed configuration against the conventional process. Further to energy saving, the isobutene conversion in the proposed scheme increases by 6.2% compared to conventional isobutane dehydrogenation. Keywords: Energy saving; Isobuten production; Thermally Coupled Reactors; Modelling

Introduction Isobutene is used in the production of various chemicals such as methyl tertiary butyl ether (MTBE) and ethyl tertiary butyl ether (ETBE) [1]. The most common industrial method of producing isobutene is isobutane catalytic dehydrogenation using various catalysts and reactors. In the Oleflex technology, commercialized by UOP Company, dehydrogenation occurs in the adiabatic moving-bed reactors over modified Pt-based catalyst [2]. The isobutane dehydrogenation is an endothermic reaction, and the Oleflex process consists of three radial-flow reactors, inter-stage furnaces, and regeneration system. Due to the high demand for olefins in the chemical industry, there has been a great deal of effort to increase the production efficiency of these products. Kurt Wright et al. [3] showed that adding potassium to the Pt-Sn/Al2O3 catalyst in the Oleflex process increased the selectivity of the process and also increased the catalyst resistance to inactivation. Sahibdefler et al. [2] modeled the isobutane axial dehydrogenation reactors under adiabatic conditions without considering the catalyst resuscitation. Aniline is one of the most important chemicals and intermediates in the production of medicine, dyes, pesticides, explosives, spices, rubber vulcanization promotor, and in isocyanates and other intermediates [4]. The major industrial methods for producing aniline are nitrobenzene hydrogenation and phenol amination. Among these two processes,


hydrogenation of nitrobenzene provides more yield, and as a result, the commercial route for aniline production is the hydrogenation of nitrobenzene to aniline, which provides About 85 % of the global aniline [5,6]. Global Industry Analysts estimated that global aniline market is anticipated to reach more than 6.2 million tons by the year 2020 because of the growing demand from numerous end-user markets [7]. In spite of the growth in aniline demand, there has been relatively limited process technology improvement. On the other hand, the valuable product of this reaction is produced on a large scale. Therefore, any improvement in the production rate of aniline draws attention in the industry. Researchers are trying to improve the efficiency of processes with the aim of saving energy. One of the methods is Thermally Coupled Reactor (TCR) of endothermic and exothermic processes. Generally, thermally coupled reactors use the generated heat by the exothermic reaction as a heat source to supply the endothermic reaction heat. Improving the reactor performance by increasing the reaction rate and the equilibrium conversion when both reactions in the exothermic and endothermic sides are reversible, reducing the reactor size and avoids the formation of unfavorable products in the process is another benefit of this type of reactor. There are several studies on the simulation and optimization of thermally coupled reactors with different configurations [8]. Karimi et al. [9] investigated the coupling of the naphtha reforming process with nitrobenzene hydrogenation and found that the thermal efficiency increased with this technique. Izurieta et al. [10] studied the thermally integrated between combustion and reforming of ethanol in terms of energetic integration for hydrogen production. In 2017, Iranshahi et al. [11] proposed a thermally coupled reactor of the moving bed type based on the coupling of the naphtha reforming with the exothermic reaction of the toluene hydrodealkylation. They also employed the DE optimization method to optimize the reactor performance minimum value of light-ends.

Figure 1: A schematic diagram for isobutane dehydrogenation coupled with aniline production

In this paper, the isobutane dehydrogenation and the nitrobenzene hydrogenation processes in a novel thermally coupled reactor concept in which the required heat for the dehydrogenation


reaction is supplied by the nitrobenzene hydrogenation reaction. As a result, this configuration leads to a significant decrease in operational costs and energy consumption due to the elimination of furnace and inter-stage coolers, which were essential for the isobutane dehydrogenation conventional and the nitrobenzene hydrogenation plants, respectively. The schematic for the proposed model of the thermally coupled is illustrated in Figure 1. The co-current feeds of isobutane and nitrobenzene flow in series [8,9]. The nitrobenzene hydrogenation reaction is proposed to take place in the tube side of three auto-thermal shell and tube reactors. At the same time, the isobutane dehydrogenation process is modeled to occur in the shell side of these reactors. The tubes are assumed to be distributed in shell symmetrically to decrease the variation in radial coordinate. Increasing the number of tubes in each reactor (n) ensured a higher rate of heat transfer. The low value of n caused local hot spots in the exothermic side of the reactor. However, for n>100, the number of tubes was not economically justified. Therefore, n=100 was chosen as the optimum value for the number of tubes. For simulation, as well as the operating conditions for both sides are reported in Table 1 [12,14].

Table 1: Operating Conditions, specification, and Geometrical data of Thermally Coupled Reactors

Parameter Value for

endothermic side

Value for exothermic

side Parameter

Value for endothermic

side

Value for exothermic

side Diameter of catalyst particle (mm) 0.6 4.7 Feed composition (%)

Catalyst density (kg/m3) 800 1400 Isobutane 66 - Void fraction 0.52 0.46 Hydrogen 34 - Inlet total molar flow rate (kmol/h) 2750 400 Nitrobenzene - 7.28

Inlet temperature (K) 873 850 Hydrogen - 39.74 Inlet pressure (atm) 1.4 1.1 Steam - 52.98 First bed length (m) 6.3 6.3 Reactor diameter (m) 1.8 0.05 Second bed length (m) 6.7 6.7 Number of tubes - 100 Third bed length (m) 7.0 7.0

Kinetic model In the Oleflex process, isobutane is converted to isobutene through a reversible and endothermic reaction. Isobutane cracking and coke formation reactions occur as side reactions over Pt–Sn/Al2O3 catalyst. The kinetic of isobutane dehydrogenation reactions are presented in Table 2 [1,2]:

Table 2: The rate and heat of isobutane dehydrogenation reactions Reaction Rates i-C4H10 i-C4H8 + H2 Dehydrogenation of isobutane

120

i-C4H10 + H2 C3H8 + CH4 Isobutane cracking to methane and propane 80.1

i-C4H8 4C + 4H2 Coke production -

The rate of exothermic side reaction (nitrobenzene hydrogenation to aniline) calculated by Klemm et al. is expressed in Table 3 [14]:

0298( / )H kj kmoleD

2 4 8

4 10 4 10

H C HC H C H

P Pr k P

K´æ ö

- = -ç ÷è ø

1

4 10 4 10 2 4 8 3 81 2C H C H H C H C Hr k C C k C Ca b g µ- = -

2

4 83C C Hr k C a=


Table 3: The rate and heat of nitrobenzene hydrogenation reaction Reaction Rates

-443

Mathematical model A one-dimensional homogeneous model proposed to describe the mass and energy balance equations considers the following assumptions: i) Steady-state conditions were used in this model; ii) The axial dispersion was assumed to be negligible; iii) The plug flow pattern is employed; iv) The heat loss was assumed to be negligible; v) No catalyst deactivation was considered on any side of the reactors. The set of ordinary differential equations (ODEs) for energy and mass balance equations in the reactors, as well as the kinetic equations of the reaction model for two processes, were solved together. ODEs are solved using the Rang Kutta method in MATLAB R2018a. The mass and energy balance equations are illustrated in Table 4 [15-17].

Table 4: Mass and energy balance equations and auxiliary correlations

Mass balance

Energy balance Overall heat transfer coefficient

Momentum Balance

Thermal conductivity

Heat transfer coefficient between the gas phase and reactor wall

Components viscosity

Component heat capacity

Results and discussion The simulation results of the proposed thermally coupled reactor in the exothermic (nitrobenzene hydrogenation) and endothermic (isobutane dehydrogenation) sides are compared to the conventional aniline synthesis and isobutane dehydrogenation conventional reactors, respectively. The isobutane dehydrogenation conventional reactor has the same operating conditions and reactor specifications as the endothermic side of the thermally coupled reactor described in Table 1. To determine the validity and correctness of the presented model for the isobutane dehydrogenation conventional reactors, the simulation results including the isobutane conversion and selectivity at the reactor's outlet are compared to the Bandar Imam Khomeini petrochemical complex (Iran) in Table 5 [12]. As can be seen, the values of the parameters ratify the accuracy of the presented model.

0298( / )H kj kmoleD

1

j n

iij j

j

dF rdZ

J=

=

=å1

+ Q(UA T) j n

t p j jj

dTFC r HdZ

=

=

= D Då ( )ln /1 1 1= + + 2

i o i i

i w o o

A D D AU h LK A hp

´

2

21

3 4

1 / /

C

C TkC T C T

=+ +

0.365

4.21 g pw

p

u dkhd

rµ

æ öæ ö= ç ÷ç ÷ç ÷è øè ø

2

21

3 4

1 / /

C

C TC T C T

µ =+ +

( ) ( )

2 2

3 51 2 4

3 5

/ / sinh / cosh /jpC T C TC C C CC T C T

é ù é ù= + +ê ú ê ú

ë û ë û


Table 5. Comparison of simulation results and plant Parameter Simulation data Plant data Total conversion (%) 39.95 40.59 Selectivity (%) 91.63 90.60

Figure 2(a) illustrates isobutane conversion in the axial direction of the conventional and coupled reactors. The total isobutane conversion is net of conversion through dehydrogenation and cracking reactions. In comparison to the conventional reactor, the conversion is increased from 0.399 to 0.461 for coupled reactor. In the thermally coupled reactor, the higher conversion is caused by heat transfer from the exothermic side to the endothermic side, which results in the higher temperature profile (Figure 2(c)), the higher reaction rate, and eventually, the higher isobutane conversion compared to the conventional reactor.

(a)

(b)

(c)

(d)

Figure 2: The temperature and conversion profiles along the conventional and the coupled reactors

Figure 2(b) presents the conversion of nitrobenzene along the reactors. The nitrobenzene conversion in the outlet of the first, second, and third reactors is 85.5%, 87.4%, and 88.8%, respectively. Figure 2(c) depicts the temperature profiles of the endothermic side and the isobutane dehydrogenation reactor. The temperature decreases at the beginning of the conventional


reactor due to the endothermic reaction, the high initial concentration of reactant, and the high reaction rate. After a while, by reducing the reaction rate, the temperature remains constant. Besides, the temperature profiles in the second and third reactors behave similarly to the temperature profile of the first reactor, except that the temperature drop is lower due to the low initial concentration and the low reaction rate. On the other hand, at the beginning of the endothermic side in the coupled reactor, the reaction heat requirement is more than the transferred heat from the exothermic side, the temperature reduces until it gets to a minimum point. Afterward, due to the decrease in the reaction rate, and gently increase in the temperature difference between two sides, which acts as a heat transfer driving force, the temperature rises. However, where the consumed heat by the endothermic side becomes equal to the transferred heat from the exothermic side, the temperature becomes constant. This fact can be seen in Figure 2(c) (second reactor). It should be noted that the slight decrease in the temperature at the end of the third reactor is due to a reduction in the reactant concentration and the rate of reaction of the exothermic side and thus a decrease in the heat transfer. The temperature profile of the exothermic side of the thermally coupled reactor is indicated in Figure 2(d). In the first reactor, the temperature rapidly increases at the entrance of the reactor because of the high initial concentrations of reactants and the high exothermic reaction rate. Finally, as the heat transfer from the exothermic side to the endothermic side increases, where the produced heat becomes equal to the transferred heat, the temperature reaches its maximum, the temperature decreases from 901.4 K to 900.8 K in the output. In the second and third reactors, the temperature profile behaves similarly to the first reactor, except that due to the reduction of the reaction rate in the endothermic side of the second and third reactors, the quality of heat transfer from the exothermic side to the endothermic side decreases slightly and consequently the outlet temperature increases.

Conclusions Three multi-tubular thermally coupled packed bed reactor in which the endothermic reaction of isobutane dehydrogenation performs in the shell side and the aniline synthesis as the exothermic reaction, playing the heat supplier role, takes place in the tube side, was simulated and studied. The required furnaces and coolers which are employed in the conventional dehydrogenation and aniline synthesis plants were eliminated, respectively. According to the simulation results, compared to the conventional isobutane dehydrogenation reactor which operates at the same operating conditions and reactor specifications as the endothermic side of the thermally coupled reactor, isobutane conversion and isobutene production were increased as well as, in comparison to the industrial aniline synthesis plant, the nitrobenzene conversion has been increased slightly.

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a novel thermally coupled reactor configuration comprising ...in the oleflex process, isobutane is...

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