Flow trickle

The term three-phase fluidization requires some explanation, as it can be used to describe a variety of rather different operations. The three phases are gas, liquid and particulate solids, although other variations such as two immiscible liquids and particulate solids may exist in special applications. As in the case of a fixed-bed operation, both co-current and counter- current gas-liquid flow are permissible and, for each of these, both bubble flow, in which the liquid is the continuous phase and the gas dispersed, and trickle flow, in which the gas forms a continuous phase and the liquid is more or less dispersed, takes place. A well established device for countercurrent trickle flow, in which low-density solid spheres are fluidized by an upward current of gas and irrigated by a downward flow of liquid, is variously known as the turbulent bed, mobile bed and fluidized packing contactor, or the turbulent contact absorber when it is specifically used for gas absorption and/or dust removal. Still another variation is a three-phase spouted bed contactor. 

In the first class, the particles form a fixed bed, and the fluid phases may be in either cocurrent or countercurrent flow. Two different flow patterns are of interest, trickle flow and bubble flow. In trickle-flow reactors, the liquid flows as a film over the particle surface, and the gas forms a continuous phase. In bubble-flow reactors, the liquid holdup is higher, and the gas forms a discontinuous, bubbling phase. 

All these gas-liquid-particle operations are of industrial interest. For example, desulfurization of liquid petroleum fractions by catalytic hydrogenation is carried out, on the industrial scale, in trickle-flow reactors, in bubble-column slurry reactors, and in gas-liquid fluidized reactors. 

Butyne-l,4-diol and propargylalcohol are produced by reaction between formaldehyde in aqueous solution and gaseous acetylene in the presence of a copper acetylide catalyst supported on nickel. The process is carried out by trickle-flow operation (BIO, S4). 

Two types of fixed-bed operations, characterized by distinctly different flow patterns, are in current industrial use. These are usually described as trickle-flow operation and bubble-flow operation. In both cases, a lower limit exists for the particle size, usually about k in. 

In trickle-flow operation, the liquid phase flows downwards, and may or may not cover the solid particles as a film. The gaseous phase moves in either co- or countercurrent, continuous flow. 

Trickle-flow operation is widely used for large-scale gas-liquid-particle processes, as noted in Section II. 

A more general model of gas-liquid-particle processes than those that have so far appeared in the literature would, it seems, be of considerable interest as a basis for comparing the reaction-engineering properties of the several types of gas-liquid-particle operations, and as a means for analyzing operations with finite liquid flow (for example, trickle-flow operation and gas-liquid fluidization). 

Trickle-flow operation is probably the most widely used operation for large-scale industrial gas-liquid-particle processes. It has been the subject of a large number of investigations, and is, as a result, relatively well described. 

It may be noted that trickle-flow operations is not always clearly distinguished in the literature from fixed-bed bubble-flow operation. The two... 

The results are of interest partly with respect to the design of certain types of trickle-flow operation and partly because they demonstrate that higher mass-transfer coefficients may be obtained for cocurrent than for countercurrent operation. 

A considerable amount of information has been reported regarding mass transfer between a single fluid phase and solid particles (such as those of spherical and cylindrical shape) forming a fixed bed. A recent review has been presented by Norman (N2). The applicability of such data to calculations regarding trickle-flow processes is, however, questionable, due to the fundamental difference between the liquid flow pattern of a fixed bed with trickle flow and that of a fixed bed in which the entire void volume is occupied by one fluid. 

Information regarding mass transfer between liquid and solid in fixed beds operated under trickle-flow conditions has apparently not appeared in the literature. 

These aspects of trickle-flow operation have been studied quite extensively. The available information will be reviewed in near-chronological order, the... 

Fig. 2. Peclet number for gas phase in trickle-flow operation.

More than a dozen studies of liquid axial dispersion in trickle-flow operation have been published, but the results are not in complete agreement. More experimental work on the subject is certainly necessary, both to resolve... 

Hoogendoorn and Lips (H10) carried out residence-time distribution experiments for countercurrent trickle flow in a column of 1.33-ft diameter and 5- and 10-ft height packed with -in. porcelain Raschig rings. The fluid media were air and water, and ammonium chloride was used as tracer. The total liquid holdup was calculated from the mean residence time as found... 

Some of the later papers referred to have pointed to the existence of distinctly different flow patterns under conditions normally characterized as trickle-flow operation. The pulsing flow pattern observed may be of particular interest, and this mode of operation could be a fertile area for research. 

It is well known that trickle-flow operation is characterized by comparatively poor heat-transfer properties, this being one of the disadvantages of this type of operation. Schoenemann (S4), for example, refers to the difficulties of controlling temperature in trickle-bed reactors. 

Zabor et al. (Zl) have described studies of the catalytic hydration of propylene under such conditions (temperature 279°C, pressure 3675 psig) that both liquid and vapor phases are present in the packed catalyst bed. Conversions are reported for cocurrent upflow and cocurrent downflow, it being assumed in that paper that the former mode corresponds to bubble flow and the latter to trickle-flow conditions. Trickle flow resulted in the higher conversions, and conversion was influenced by changes in bed height (for unchanged space velocity), in contrast to the case for bubble-flow operation. The differences are assumed to be effects of mass transfer or liquid distribution. 

Ross (R2) reported measurements of desulfurization efficiency of fixed-bed pilot and commercial units operated under trickle-flow conditions. The percentage of retained sulfur is given as a function of reciprocal space velocity, and the curve for a 2-in. diameter pilot reactor was found to lie below the curves for commercial units it is argued that this is proof of bad liquid distribution in the commercial units. The efficiency of the commercial units increased with increasing nominal liquid velocity. This may be an effect either of mass-transfer resistance or liquid distribution. 

Fixed-bed bubble-flow operation has been the subject of considerably less experimental work than has trickle-flow operation. This reflects the fact that bubble-flow operation has been of much more limited industrial importance. 

Schoenemann (S4) reported qualitatively that the liquid residence-time distribution for cocurrent upward bubble flow was narrower than that observed in trickle-flow operation. 

The liquid residence-time distribution is close to plug flow in trickle-flow operation and corresponds to perfect mixing in the stirred-slurry operation, whereas the other types of bubble-flow operation are characterized by residence-time distributions between these extremes. 

Thrust, 4 Trickle-flow, 79 fixed-bed, 90-104 holdup and axial dispersion gas phase, 92-94 liquid phase, 94-102... 

Fig. 5.2.3 Identification of rivulets and surface wetting in a packing of 5-mm diameter glass spheres contained within a column of inner diameter 40 mm. The data were acquired in a 3D array with an isotropic voxel resolution of 328 xm x 328 pm x 328 [im. (a) The original image of trickle flow is first binary gated, so that only the liquid distribution within the image is seen (white) gas-filled pixels and pixels containing glass spheres show up as zero intensity (black), (b) The liquid distribu-...

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