Investigations of periodically operated trickle-bed reactors

Pergamon

ChemicalEngineering Science,Vol. 49, No. 24B. pp. 5615-5621, 1994 Copyright Q 1995 Elsetier Science Ltd Printed in Great Britain. All rights reserved OQX-2509/W 57.00+0.00

INVESTIGATIONS OF PERIODICALLY TRICKLE-BED REACTORS

OPERATED

R. LANGE,*

J. HANIKA,+ D. STRADIOTTO,* R. R. HUDGINS* and P. L. SILVESTONt * Martin-Luther-University of HaHe-Wittenberg, Department of Chemical Engineering, Merseburg, D-06217, Germany + Prague Institute of Chemical Technology, Technika 1905, 166 28 Prague 6, Czech Republic t University of Waterloo, Department of Chemical Engineering, Waterloo, ON, Canada, N2L 3Gl

(Received 11 May 1994; acceptedfor publication4 October 1994) Abstract-This study concerns experimental investigations of the forced unsteady-state operation of trickle-bed reactors. The hydrogenation of cyclohexene to cyclohexane and cl-methylstyrene to cumene on palladium catalysts were taken as model exothermic reactions. Changes in the control variables (e.g. feed composition,feed rate, temperature) strongly influence the regime and performance of a trickle-bed reactor. The aim of the present study of cyclohexene hydrogenation was to seek conditions of periodic operation that would enable higher average inlet concentrations to be used without evaporating the feed mixture. For a-methylstyrene hydrogenation, the aim was to discover periodic operating conditions that would improve the time-average conversion relative to that under steady-state operation.

INTRODIJmION

Trickle-bed reactors have found extensive applications in organic and petrochemical technologies. In recent years, research on trickle-bed reactors has been focused largely on experimental and/or theoretical problems (e.g. heat and mass transfer effects, wetting problems) associated with their efficiency and performance under steady-state conditions. Typical tricklebed reactor results are summarized in many papers and monographs (e.g. Satterfield, 1975; Shah, 1979; Ramachandran and Chaudhari, 1983; Gianetto and Silveston, 1986; Hanika and Stanek, 1986). Dynamic properties of two-phase catalytic reactors in an unsteady state have been studied by Matros (1989, 1990), Gilles and Eigenberger (1983) and Gray and Scott (1990). A particular type of dynamic operation occurs when a gas-solid reactor is operated periodically. Renken (1983) and Schgdlich et al. (1983) have published results of experimental and theoretical investigations ofperiodically operated two-phase (gas-solid) reactors. Trickle beds, being three-phase reactors, have been examined in periodic operation in relatively few studies (Lange and Hanika, 1987, 1990; Haure et al., 1989, 1992; Hanika et al., 1990; Lange et al., 1994). The present paper is an experimental study of periodically operated trickle-bed reactors in the catalytic hydrogenation of cyclohexene to cyclohexane and cr-methylstyrene to cumene. EXPERIMENTAL

A schematic diagram is presented in Fig. 1 of the experimental apparatus. It shows the trickle-bed reactor equipped with an on-line connection to a 5615

Hewlett-Packard (H-P) computing and measuring system. Within the flow system, the reactor itself was a glass tube 30 mm in diameter by 350 mm in length thermostated in a jacket. Cylindrical catalyst extrudates 3.5 mm in diameter and 5 mm in length were used for the liquid-phase hydrogenation of cyclohexane 3% Pd on charcoal catalyst (CHEROX 41-00). The reactor tube was equipped with 12 side arms housing thermocouples for measuring the axial temperature profile in the 180-mm long catalyst bed. The therrnocouple voltage was monitored by the H-P data system. The end of each thermocouple was imbedded inside a cylindrical pellet in the centre of the catalyst bed. Further details about the experimental set-up may be found in papers by Hanika and Hajkova (1987) and Lange and Hanika (1987). Liquid reaction mixture was fed at a constant rate to the top of the catalyst bed directly through a central nozzle. From the measurement of the axial temperature profile in a regular time interval, the maximum temperature was found and compared with the desired value. Once the maximum temperature had been reached, the substrate concentration in the reactor feed was lowered to the minimum value. Also, once the temperature across the catalyst bed had decreased below the desired value, the feed concentration was switched to the upper limit. RESULTS AND DISCUSSION

The model exothermic reaction of cyclohexane hydrogenation on a 3% Pd/charcoal catalyst was chosen with the aim of demonstrating how to moderate the hot spot temperature in the catalyst bed by means of periodic changes in the feed concentration.

R. LANGEet al.

5616

A - Scanner (3495A) h - Digitduohetet ( 3495A)

C - Computer b - Plotter B - Printer

P 8

Fig. 1. Experimental apparatus. 1, Liquid pump; 2, cyclohexene tank; 3, cyclohexane tank; 4. preheater, 5, trickle-bedreactor; 6, separator: 7, hydrogen tank; 8, thermostat; 9, rotameter; 10, gas chromatograph.

A second model reaction, hydrogenation of a-methylstyrene on a Pd/Al,O, catalyst, was used to illustrate how the conversion might be improved by periodic changes in the volumetric liquid flow rate.

HYDROGENATIONOF CYCLOHEXENE The behaviour of an adiabatic trickle-bed reactor operating with a strong exothermic reaction is shown in Fig. 2. The transition of the reactor regime from liquid to gas phase can be observed by a change in the hot spot temperature during the experiment. This transition takes place in the system at a somewhat lower temperature than the boiling points of cyclohexene (84°C) and cyclohexane (81”C), as a result of the large excess of hydrogen in the reactor. Figure 2 demonstrates the occurrence of partial as well as total evaporation of the liquid reaction mixture. It can be seen that the concentration of feed cyclohexene concentration strongly influenced the transition of the three-phase regime to the two-phase regime, especially at the higher feed concentration (pure cyclohexene). Figure 3 shows what happens to the bed temperature when alternately fed with two different concentrations

at several positions after the start of an experiment. The boiling point of the reaction mixture was reached after about 1 h. After this time, the evaporation of the reaction mixture and the overheating of the catalyst bed were quenched by switching to a lower cyclohexene concentration in the reactor feed, the time-variation for which is also shown. Temperature oscillations at the outlet of the catalyst bed resulting from nearly regular variations of the reactor feed composition are also shown. During this experiment, both the inlet temperature and the feed rate of the reaction mixture were held constant. In addition, the cyclohexene/solvent ratio in the feed was adjusted in order to bring the measured maximum temperature in the catalyst bed to the desired value. In other words, exothermicity in the reactor was quenched by switching the liquid feed to inert cyclohexane. As a result of cycling between different pairs of feed concentrations, regular temperature oscillations are created in the catalyst bed as may be seen in Fig. 4. It can be seen that there are more oscillations of the bed temperature in the same time interval (60 min) if the amplitude of the feed concentration oscillation and the period of the feed pump are decreased. The

Periodically

operated

trickle-bed

reactors

5617

200

150

100

50

-150

200

150

250

200

250 TIME

Fig. 2. Transition

from liquid- to gas-phase liquid cyclohexene

----I-

80

320

320

(min)

regime in an adiabatic trickle-bed reactor concentrations (65 wt%, 100 wt%).

/

,

I

------

i

/’

#’

for different

inlet

I

80

50 65

50

80

110 TIME

140

(min)

Fig. 3. Effect of periodic changes in feed concentration on local temperatures (X,,, = 65 wt%; X In,” = 5 wt%). 1, Thermocouple 1 at the top of the catalyst bed; 3, thermocouple 3 at the first third of the catalyst bed; 10, thermocouple 10 at the bottom of the catalyst bed.

R.

LANGE

et al.

0 50

110

80

270

210

330

360

390

TIM:4qrnin~ Fig. 4. Effect of periodic

of cyclohexene (5\65, 15\65, 30\65 wt%) on temperature oscillations in the catalyst bed.

changes

in feed concentration

amplitude of the temperature oscillations is also decreased. In summary, the greater the transport lag of the quenching fluid, the higher is the mean temperature in the catalyst bed. This is the reason for the increase in conversion. However, it should be noted that the shorter the time interval between temperature response and feed concentration change, the greater is the safety in operating the reactor. Thus, it is possible to minimize overheating of the catalyst and thus slow or eliminate its possible deactivation. HYDROGENATION

OF a-METHYLSTYRENE

The hydrogenation of a-methylstyrene in a cumene solution using Pd/a-Al,O, catalyst was selected as a model reaction to demonstrate increasing the conversion through periodic operation of a trickle-bed reactor. Some details of the experimental measurements are summarized in Table 1. Table 1. Reactor specifications for hydrogenation of a-methylstyrene Reactor diameter Reactor length Particle diameter Temperature (feed)

2.0 cm 70.0 cm 0.3-0.4 cm

Pressure

16-40°C 0.1-l MPa

Volumetric gas flow rate Inlet concentration

20 l/h 3.78 mol/l

To evaluate the reactor performance under periodic versus steady-state operation, we compare the timeaverage rate of product formation or conversion (U) under cycle-invariant periodic operation with the steady-state counterpart. The experimental data show the influence of periodic change of the feed volumetric liquid flow rate on the conversion of a-methylstyrene. There are different ways to achieve a periodic liquid flow rate at the top of the trickle-bed reactor. In this study, only the volumetric liquid flow rate was cycled. In most cases, an on-off mode was employed. For the operating conditions given in Fig. 5, the results are provided in Fig. 6, After some experiments in the forced unsteady-state regime, a steady-state experiment was carried out as a basis of comparison of the time-average conversion of a-methylstyrene under different operation regimes. It can be seen that under these conditions the time-average conversion is about 2 to 15% higher than under steady operating conditions. Figure 7 shows the influence of an on-off mode of cycling compared with cycling the liquid flow rate between two finite values. It can be seen that cycling has just a minor influence on the time-average conversion. The cycle-split ratio (S), used to describe nonuniform cycling, varied from 0.2 to 0.6. Some examples of experiments with cycling flow are shown in Fig. 8. A comparison is provided, in Fig. 9, of the effect of the cycle-split on the time-average cl-methylstyrene conversion relative to steady-state conversion.

Periodically operated trickle-bed reactors

TIME

(min)

Fig. 5. Examples for a forced unsteady-state regime in feed volumetric liquid flow rate at a different periodic&y and a symmetrical cycling (S = 0.5). z

P g si

0.6 0.5-

(a,), - 100 (OL& = 200

ml/h

PL UNDER

mL/h

CYC.-

CYCLING

PERIOOIC

-200/O

ON-OFF

mL/h FLOW

T -307K

5619

and static liquid hold-up. The latter depends upon the mode of cycling flow rate, catalyst shape and catalyst properties such as porosity, roughness, wettability, etc. Figure 10 interprets how an increase in the catalyst wettability may result in better utilization of the different channels in the catalyst bed, as a result of forced unsteady-state operation (liquid flow rate oscillation). Furthermore, the cycling liquid flow rate strongly affects the time of contact between the liquid and the catalyst particles and the interfacial areas. The variation of the liquid flow rate increases and decreases the catalyst particle wetted area. A decrease in the liquid film at the catalyst surface allows hydrogen, the limiting reactant, more ready access to the catalytic sites and leads to an increase in the local rate of reaction. But, even though the resistance to mass transfer is reduced, the variation of the liquid flow rate alters the liquid hold-up in the catalyst bed. Therefore, changes in the liquid loading in the bed will aIso influence the liquid-solid mass transfer coefficient and the on-off mode of liquid flow, opening new channels that were otherwise closed and that contained so-called liquid pockets. This increases the specific liquid-solid area. Furthermore, within the catalyst pellets, temperature oscillations also occur, leading to an increase in the time-average rate of reaction. With the help of a hierarchic simulation system, Schnitzlein and Cochlovius (1994) investigated the influence of the dynamic liquid hold-up on conversion under periodic operation. These simulation results agree well with the experimental data.

1

0.44

CYC. 5/5

TYPE OF OPERATION

Fig. 6. Comparison of the a-methylstyrene conversion under forced unsteady-state and steady-state regimes.

The cycling liquid flow rate can modify substantially the hydrodynamic processes and consequently the liquid hold-up in the catalyst bed. The liquid hold-up in packed bed mainly consists of two parts: dynamic

CONCLUSIONS

The bang-bang feedback control employed in this investigation of two different catalytic systems is one of the simplest possible feed strategies. This feed strategy was proposed for the hydrogenation of cyclohexene in order to successfully eliminate regions

CYC

t---

-4

-_-_-_--q&_--

10,)ss

= 200

mL/h 1

(O&

0.20

2

CYCLE

4

= 100 mL/h 6

PERIOD,

Fig. 7. Et&t

8

min

10

I

I

I

I

I

0

2

4.

6

8

CYCLE

of cycle period on the time-average

PERIOD,

conversion (S = 0.5).

10i zd

min

R. LANGE et al.

5620

r

70 TIME,

min

Fig. 8. Demonstration ofdifferentcycle-splits for experiments with cycling volumetric liquid flow rate (t = 5 min).

1

STEADY

-

STATE

UNSTEADY-

STATE

m

GAS

m

LIOUID

HOLDUP

(STATIC)

m

LIOUID

HOLDUP

(DYNAMIC)

m

CATALYST

Fig. 10. Comparison forced unsteady-state

HOLDUP

PARTICLE

of suggested hydrodynamics under and steady-state liquid flow regimes.

increase conversion (and presumably, selectivity) in trickle-bed reactors.

therefore,

Acknowledyements-One of us (R.L.) is extremely grateful to Prof. P. L. Silveston who provided the opportunity to work in his chemical reaction engineering research group at the Department of Chemical Engineering of the University of Waterloo for an appointment as a visiting professor. The financial support of DAAD in the form of a one-year fellowship is also gratefully acknowledged.

Q,= 200/O T = 5

I

I

I

mL/h REFERENCES

min

I

0.4 C CYCLE SPLIT, s Fig. 9. Influence of cycle-split on the time-average CImethylstyrene conversion. 0.2

of complete evaporation of the liquid in the trickle-bed reactor. Such regions may lead to dangerous overheating of the catalyst. On-line feed control and the application of a computing data-logger represent a new approach to the safe performance of a trickle-bed reactor. Furthermore, results of the a-methylstyrene hydrogenation experiments show that a cycled flow of liquid reactant provides an improvement in the time-average conversion of cr-methylstyrene of up to lo’/& depending on the choice of the operating parameters. Some of the causes of improved time-average conversion of trickle-bed reactors are increased catalyst wetting, the development of more channels in the catalyst bed, improved utilization of the external and internal liquid hold-up and/or temperature oscillations, and the evaporation of the liquid mixture inside a porous catalyst. This study shows that periodic operation can also reduce the hot spots and

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