Measurement of liquid flow distribution in trickle bed reactor of large diameter with a new gamma-ray tomographic system

https://doi.org/10.1016/S0009-2509(01)00361-XGet rights and content

Abstract

In the petrochemical industry area, many processes are carried out using fixed bed reactors with cocurrent upward or downward gas/liquid flows. One of the main problems to overcome during the scale-up of such reactors is to optimize the gas/liquid flow distribution across the reactor catalytic bed section. To accurately characterize the gas/liquid flow distribution inside a real catalytic bed, a new computed gamma-ray tomographic system has been developed to investigate two-phase flow through cold mock-ups of large scale (60cm in diameter). In the case of developing a new reactor technology for processes working with fixed bed in the trickling flow regime, this type of measurement is quite important. This system, unique for such a large size catalyst bed, includes a series of 32 BGO detectors and a Cs137 source. The performances of this experimental system have been fully evaluated through a validation including reconstruction of physical phantoms in static conditions. Several tests have been performed to measure the gas/liquid flow distribution in the catalyst bed for different inlet distributor configurations. It is shown, in the experiments performed for the trickle flow regime, that the instrumental technique has a very good sensitivity to liquid retention measurement. When the liquid retention measurements are compared with a hydrodynamic model based on Kozeny–Carman formalism a quantitative agreement is observed.

Introduction

In the refinery and petrochemical industry, many processes are operated with a catalytic fixed bed. In many cases, exothermic chemical reaction needs reactants in liquid and gas phase at pressure and temperature of process operation. In order to optimize the reaction conversion or selectivity, all the catalytic volume has to be in contact with a gas/liquid flow with the same composition. As a matter of fact, an excess of gas in a catalytic zone can for instance accelerate the reaction as well as the local heat generation. This zone not being, in the same time, refreshed by enough liquid flow rate, a hot spot can occur that will dramatically affect the reactor performances. In order to study and improve the gas/liquid flow distribution inside the catalytic bed, it is necessary to dispose the accurate tools.

In a general manner, it is never easy to implement instrumentation inside a two-phase flow, but with three-phase gas–liquid–solid conditions the challenge is even more difficult. The tracking methods give a global diagnostic of the flow characteristic of one phase flow across the whole bed. This technique can mainly inform of a strong liquid by-passing of the bed but is rather inefficient to detect two-phase flow distribution problems. The catalytic bed being generally very dense and opaque, no optical methods can be used. To better investigate the bed some local sensors have been developed, such as, for instance, optical probes. These probes give interesting information, but the measurement remains quite local and can be strongly affected by the solid particles environment. It is consequently quite difficult to have a precise idea of the two-phase flow distribution across a bed section with this instrumental technique. Finally, non-intrusive and powerful techniques such as tomographic methods are very promising for such situations. These complex instrumentation systems allow to have a complete 2D map of the phase distribution across a reactor section. An advantage of fixed bed for this measurement technique is the steady state flow condition that allows long measurement times. One major drawback is, on the other side, the quite important attenuation of the medium. Furthermore, the low contrast in density between a dry bed and a flooded bed makes the measurement even more difficult.

Among the different groups developing new tomographic system, many of them work on an electrical tomographic system. These systems are, nevertheless, not applicable to our problem since the spatial resolution always remains very poor unless a priori knowledge is added (Wang, Dickin, & Mann, 1997). It has thus been decided to choose a tomographic system based on photon absorption. In this case, the attenuation being a linear phenomenon with different material densities, the image reconstruction is mathematically possible with a very high spatial resolution. Due to the very high attenuation in our experimental setup, operated with real catalytic bed, it was not possible to use X-ray tomography (Schmitz, Reinecke, Petrisch, & Mewes, 1997; Toye, 1996; Lutran, Ng, & Delikat, 1991) because of its extremely low energy. It was therefore necessary to implement a gamma-ray tomographic system. Such systems are already being developed (Kumar, Moslemiam, & Dudukovic, 1995; Froystein, 1997) but are not usable for a real fixed bed of large size (60cm in diameter). Lutran et al. (1991) performed the tomographic measurement of liquid distribution inside fixed bed of glass beads using a medical X-ray scanner, but bed dimensions were far lower (6cm in diameter and 30cm in height).

The first section of this paper describes the tomographic system technology. In the second section, the intrinsic performance of this instrument is fully studied by analyzing several sources of error. Then the first measuring results, obtained with a liquid trickle flow through a catalytic fixed bed, are presented and discussed. These measurements concern the bed porosity and the liquid retention inside the catalytic bed. Finally these liquid retention experimental data are compared with a hydrodynamic model.

Section snippets

Description of the tomographic system

The gamma-ray tomographic system has been developed in collaboration with CEA (Commissariat à l’Energie Atomique), the French research institute for nuclear power energy. This system, unique for such a large size catalyst bed, has a gamma-ray fan beam geometry. Fig. 1 shows the tomograph geometry. It includes a series of 32 detectors that are BGO (crystal of BeGeO) photo-scintillators transducers. These detectors are separated by spaces of same thickness as their detection width, that is 13mm,

Measurement with physical phantoms

The first validation to be performed with a tomographic system consists in measuring a slice of the reactor with physical phantoms of known cross-section area and densities. The purpose is to simulate different defaults of gas/liquid distribution inside the catalytic bed. This simulation is performed by introducing, inside the catalytic bed, cylindrical flasks filled with liquids of different densities corresponding to the density of a typical gas/liquid/solid mixture. During a first

Results obtained with gas–liquid trickle flow across a fixed bed reactor

Once the tomographic system has been tested in static conditions, it is possible to test the tomograph in a real experiment: a two-phase gas–liquid flow through a catalyst bed. In such an experiment, the catalyst particles were cylindrical particles made with porous alumina. The actual purpose of this experiment was to determine the bed external porosity corresponding to the void fraction between particles. The bed porosity is quite an important data since it describes the volume left to the

Comparison of liquid retention measurement with a hydrodynamic model

Finally, to test the quantitative aspect of measurement, the liquid retention results obtained in liquid trickling flow is compared to a hydrodynamic model. Such an approach has been followed by Toye, Marchot, Crine, and L’Homme (1999) to interpret the liquid retention result obtained in packed bed with an X-ray tomographic system. Their model is based on a statistical law for liquid velocity distribution inside the bed. The relation between liquid velocity and liquid hold-up relies on an

Conclusion

A new gamma tomographic system has been developed to detect the flow maldistribution inside a large size catalyst fixed bed cold mock-up (internal diameter of 60cm). The major difficulty to overcome with such a reactor is the strong attenuation due to catalyst bed and liquid flow. The results obtained in static conditions show that the instrumental system is sensitive to a simulated gas fraction varying from 0% to 100% inside a real catalyst bed of 60cm in diameter. When analyzing all the

Notation

A,BErgun equation coefficient
LSinterfacial contour (in a bed section) between liquid and solid, m
LGinterfacial contour (in a bed section) between liquid and gas, m
dpparticle equivalent diameter, m
fwliquid/solid shear stress coefficient
I0,IF,Inumber of photons respectively measured with the dry bed, the flooded bed and with a two-phase gas/liquid flow
Ii and Ivnumber of photons respectively measured with and without the liquid i
Lliquid thickness crossed by gamma ray, m
Ptotal pressure, Pa
Rh

References (12)

There are more references available in the full text version of this article.

Cited by (0)

View full text