Instability phenomenon in an external‐loop three‐phase gas‐liquid airlift reactor

Abstract

Three-phase airlift (TPAL) reactors have applications ranging from biotechnology (Heijen et al., 1990) to catalytic hydrogenation. Circulation in a loop consisting of a riser and downcomer with top and bottom connections is induced by injecting gas at the bottom of the riser. The continuous liquid phase recirculates up the riser and down the downcomer, carrying the solid phase in suspension. Main advantages of TPAL reactors include the ability to suspend solid particles at a relatively low gas superficial velocity, elimination of stagnant zones, and the absence of any moving parts or external recirculation mechanism. In general, two airlift reactor configurations can be distinguished: the internal-loop airlift, which consists of two concentric cylinders, and the externalloop airlift, where the riser and downcomer are separate tubes connected at the top and bottom (Chisti and Moo-Young, 1987).A hydrodynamic model was developed for TPAL reactors (Livingston and Zhang, 1993; Douek et al., 19941, which enables the prediction of main variables of a TPAL reactor (phase holdups and liquid recirculation velocity) as a function of the inlet gas superficial velocity and the solids loading. This model considers a TPAL reactor to comprise riser and downcomer sections alone; the difference in the effective densities between these regions gives rise to the recirculation. As part of a program of experimental work aimed at verifying this model, it was decided to carry out experiments on an external-loop reactor which would generate direct measurements of the required riser and downcomer hydrodynamic parameters. During the course of these experiments, however, a surprising and before now unreported instability phenomenon was observed. This behavior prevented the system from reaching a steady distribution of solids.In general, instabilities are undesirable since they could adversely affect the system control and performance. The objective of this article is to describe the observed phenomenon and attempt to explain why it occurs. Experimental StudiesExperiments were conducted using the apparatus shown in Figure 1. The total working volume of the reactor was 35 L, with riser and downcomer sections of approximately 2 m Correspondence concerning this article should be addressed to A. G. Livingston. 2508November 1995 height and a pipe diameter of 8 cm throughout. Gas entered the base of the reactor via an 8 cm sintered glass plate, with a pore diameter of 40 pm. The gap between the bottom of the riser and the gas distributor was fixed at 4 cm. In order to minimize the quantity of glass beads which settle out in the top and bottom flow reversal regions, it was decided to use 45" pipe junctions for these ( Figure 1). This geometrical arrangement reduces the influence of the fluid reversal zones on the overall driving force for the recirculation and should therefore enable a better comparison between the experimental results and model-generated predictions.

Bioreactors: Airlift Reactors

“…Many studies have been conducted on the suspension of solids in ALRs, particularly on the use of this type of device for catalytic processes in the chemical industry (282)(283)(284)(285)(286)(287). A very important point is the minimum gas superficial velocity that leads to complete solid fluidization (161,282,285,(287)(288)(289)(290)(291)(292)(293).…”

“…Assa and Bar (304) found very small variations in the axial distribution of animal and plant cells suspended in an IL-ALR. This is due to the small free-falling velocity of the solids, which is the reason for the difference in loading between the riser and the downcomer when heavy particles are used (284,285,305). Because of the small difference between solid and medium density, the movement of the particles usually present in biological processes is not as dependent on gravity forces as on liquid and bubble movement.…”

Bioreactors, Air‐lift Reactors

“…Therefore, it is to be expected that the fluidization capacity of the ALR will be markedly superior to that of a bubble column. Several studies have been conducted on the suspension of solids in ALRs, particularly on the use of this type of device for catalytic processes in the chemical industry, where the solid support is usually heavy (61,(159)(160)(161)(162)(163). In this regard, a very important point is the minimum gas superficial velocity that leads to complete solid fluidization (61,87,159,162,(164)(165)(166).…”

“…Assa and Bar (174) found very small variations in the axial distribution of animal and plant cells suspended in an internal-loop ALR. This is due to the small free-falling velocity of the solids, which is the reason for the difference in loading between the riser and the downcomer when heavy particles are used (160)(161)(162). Because of the small difference between solid and medium density, the movement of the particles usually present in biological processes is not as dependent on gravity forces as on liquid and bubble movement.…”

Control volume finite element modelling of segregation of sand and granular flows in fluidized beds

“…The need to understand and predict the behaviour of dispersed solid particles within a continuum phase has a long history, and appears in a number of industrial applications, from the pneumatic transport of coal in power plants (Geldart and Ling, 1990), to the application of catalysts in chemical reactors (Douek et al, 1995), to sand deposition (Doron et al, 1997) and abrasion in oil and gas pipelines. Direct numerical simulation of such systems is difficult when the number of dispersed particles may be in the millions, and when particles can differ in size and shape.…”