Gas − solid Co − current Flow Downer Units

The experimental data obtained from two independent downer reactor units was used to study the hydrodynamics of concurrent gas−solid flows over several operating

conditions. Both acrylic units were designed and built at the Chemical Reactor Engineering Center Laboratory of The University of Western Ontario. The main difference between them is that the first one has a one inch internal diameter, while the second unit has roughly the double (two inch internal diameter).

4.2.1 One inch Internal Diameter Downer Unit

A schematic diagram of the one inch (2.57 cm) internal diameter downer unit is presented in Figure 4-6. The solids in this unit are fed in a batch mode. This unit is operated at ambient conditions. Initially, the Fluid Catalytic Cracking (FCC) catalyst particles from a fluidized bed, are fed to the riser section through a feeding tube and a “Valve 1” located at the bottom of the riser section. A ball vibrator assists the ball valve, “Valve 1”, in supplying a constant amount of solids to the riser section. Regarding the catalyst particles, they are pneumatically transported upwards, using air fed through the combined action of a single orifice hole distributor and a ring gas distributor. The single orifice hole distributor is located at the bottom of the riser section and supplies the air to propel the catalyst particle upwards to the ring gas distributor.

The ring gas distributor is situated at a height of 34 cm above the single orifice hole distributor and contains twelve 0.5 mm holes distributed evenly around the ring distributor. These holes are also angled to 45 degrees from the vertical. This provides good and relatively uniform gas flow in the riser section.

At the top of the riser section (downer entry), the solid particles are disengaged from the air stream through the “Cyclone 2”. The air exiting the top of the cyclone passes through a bag filter where the remaining catalyst particles are collected. Moreover, the catalyst particles exiting the bottom of the “Cyclone 2” move downwards towards the downer

section. This occurs, as a result of the combined effect of gravity and some gas entrained with solid particles.

Figure 4-6: Schematic diagram of the showing the fiber optic port

The gas distributor is located 24 cm downstream from the downer top cyclone. This provides additional air to the downer section to complete the required gas flow rate. This air injection simulates the reactant injection and the intense mixing at the entrance region as a result of the combined effect of gravity and some gas entrained

: Schematic diagram of the one inch internal diameter downer unit showing the fiber optic port, adapted from Nova (2005)

The gas distributor is located 24 cm downstream from the downer top cyclone. This o the downer section to complete the required gas flow rate. This air injection simulates the reactant injection and the intense mixing at the entrance region as a result of the combined effect of gravity and some gas entrained

inch internal diameter downer unit

2005).

The gas distributor is located 24 cm downstream from the downer top cyclone. This o the downer section to complete the required gas flow rate. This air injection simulates the reactant injection and the intense mixing at the entrance region

of a downer unit. To secure a good particle−fluid mixing in the feeding section, the

downer top air distributor is configured with sixteen 0.5 mm holes distributed evenly around the circumference of the column and angled downwards 45 degree with respect to the vertical wall.

From this point onwards, the FCC catalyst particles continue their circulation down with the air stream, past the probes, until they reach another cyclone located at the bottom of the downer (Cyclone 3) where they are disengaged from the air stream and collected in a storage tank. This tank is placed on a scale for continuous weighing and monitoring of the catalyst flow rate. Moreover, the air coming from the cyclone 3 is directed to a filter to eliminate remaining solids and then to a rotameter to measure the gas flow in the downer section. More details regarding maintenance, adjustments and standard functioning of this experimental unit can be found in Islam (2010) and Nova (2005).

4.2.2 Two inch Internal Diameter Downer unit

A schematic diagram of the new downer unit is reported in Figure 4-7. Regarding pressure and temperature, this unit is also operated at ambient conditions. The main differences of this new downer unit when compared with the previous one are the following: 1) The internal diameter is almost two times larger, it has a two inch (5.08 cm) internal diameter instead of one inch. 2) The FCC catalyst particles follow a continuous loop 3) Two risers are used to secure a continuous solid loop circulation.

Initially, the FCC catalyst particles from a fluidized bed are fed to the first riser section through a feeding tube and a valve (Valve 1) located at the bottom of the riser section. The catalyst particles are pneumatically transported upwards, using air fed through the combined action of a single orifice hole distributor and a ring gas distributor. The single orifice hole distributor is located at the bottom of the riser section, and supplies the air to propel the catalyst particle upwards to the ring gas distributor.

At the top of the first riser section (downer entry), the solid particles are disengaged from the air stream by the Cyclone Separator 2. Moreover, the catalyst particles exiting the

bottom of the cyclone move downwards towards the downer section combined effect of gravity and

Figure 4-7: Schematic

The gas distributor is placed below the to the downer section, in order

simulates the reactant injection and the intense mixing at the entrance region of a downer unit. To secure a good particle

distributor is configured wit

of the cyclone move downwards towards the downer section, as a result of the combined effect of gravity and gas entrained with the solid particles.

: Schematic diagram of the two inch internal diameter downer unit.

The gas distributor is placed below the downer top cyclone (Cyclone 2).

, in order to achieve the required gas flow rate. This air injection simulates the reactant injection and the intense mixing at the entrance region of a downer unit. To secure a good particle−fluid mixing in the feeding section, the

istributor is configured with sixteen 0.5 mm holes distributed evenly around the as a result of the

inch internal diameter downer unit.

(Cyclone 2). This provides air the required gas flow rate. This air injection simulates the reactant injection and the intense mixing at the entrance region of a downer fluid mixing in the feeding section, the downer top air h sixteen 0.5 mm holes distributed evenly around the

circumference of the column and angled downwards 45 degree with respect to the vertical wall.

From this point onwards, the FCC catalyst particles continue circulating down with the air stream. They pass the probes, and continue onwards until they reach a two ways valve located at the bottom of the downer where: 1) they can continue and flow toward the second riser and then close the loop by going into the fluidized bed or, 2) the solids can be collected in a bag filter. The bag filter is then placed on a scale for weighing and monitoring the catalyst flow rate. Moreover, the air coming from the Cyclone 3, can be directed to a filter to retain the remaining solids and then, to a rotameter to measure the gas flow in the downer section.

A scaled representation of the two inch internal diameter downer unit is presented in Figure 4-8.

Figure 4-8: Scaled representation of the 2 inch internal diameter downer unit.