Single and Two-phase Flow in a Packed Bed Reactor

Overview

Source: Kerry M. Dooley and Michael G. Benton, Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA

The goal of this experiment is to determine the magnitude of maldistribution in typical packed bed reactors in both single phase and two-phase (gas-liquid) flow and evaluate the effects of this maldistribution on pressure drop. The concepts of residence time distribution and dispersion are introduced through the use of tracers, and these concepts are related to physical maldistribution.

Channeling in a single-phase flow can occur along walls or by preferential flow through a larger portion of the bed cross-section. Channeling in two-phase flow can result from even more complex causes, and simple two-phase flow theories seldom predict pressure drops in packed beds. A design goal is always to minimize the extent of channeling by finding the optimal bed and particle diameters for the design flow rates and by packing a bed in a way to minimize settling. It is always important to quantify how much maldistribution might occur and to over-design the unit to account for its occurrence.

The permeameter apparatus measures pressure drop, ΔP, and the concentration of tracer (dye) exiting horizontal packed beds of armored glass for either water, air, or two-phase flow (Figures 1 and 2). Water enters through a control valve and can be routed through manual valves to any of five beds (48" long, 3" I.D.) with different size glass bead dumped (random) packings. The pressure drop is measured using a pressure transmitter. The water flow is measured by a differential pressure (DP, orifice) transmitter and the air flow by a dry test meter (similar to a home gas meter). The dye sample is injected upstream by an automated sampling valve. The exit concentration of the dye from a bed is measured using a UV-Vis spectrometer. Residence time distributions are calculated from the tests and compared to the predictions of theories on dispersion in packed beds. Two-phase flow will be studied in bed 5, which contains the largest particles.

Figure 1: Process and instrumentation diagram of the apparatus.

Figure 2. 3-D rendering of the apparatus. Bed #1 is at the top, bed #5 at the bottom. The water control valve is on the left (red bonnet). The DP transmitter is at the top center (blue).

Procedure

1. Starting up the apparatus

The apparatus is primarily operated through the distributed control system interface. A Perm P&ID schematic appears and opening/closing automated valves is point and click.

To establish water flow to either bed #4 or #5, open the inlet and exit valves to the bed being tested and the water supply solenoid.
Use the flow controller to start water flowing through the bed, raising it gradually. Good starting points are 400 mL/m

Results

Obtain the RTDs (E-curves, using Equations 1-2) after subtracting an appropriate baseline (if necessary) from the spectrometer signals. An example of baseline correction for Bed #3 (not used here) is in Figure 3. Using Equations 1-3, calculate the average porosity, tracer mass, mean residence time, variance and variance divided by mean squared from the RTDs. Compare calculated tracer mass with injected mass - if they aren't within expected precision, exam

Application and Summary

In this experiment the real flow behavior of horizontal packed beds, both in single and two-phase flow, was contrasted to the simpler theoretical models for pressure drop and dispersion (flow spreading in the axial direction, deviating from plug flow). The utility of tracer tests in probing for maldistribution ("channeling") in such beds has been demonstrated, and it has even been shown that certain metrics calculated from the tracer tests can give some idea of the cause of the channeling. These calculations usin

References

Encyclopedia of Chemical Engineering Equipment." Distillation Columns. http://encyclopedia.che.engin.umich.edu/Pages/SeparationsChemical/DistillationColumns/DistillationColumns.html. Accessed 9/22/16.
Encyclopedia of Chemical Engineering Equipment." Absorbers. http://encyclopedia.che.engin.umich.edu/Pages/SeparationsChemical/Absorbers/Absorbers.html. Accessed 9/22/16.
Nevers, N., Fluid Mechanics for Chemical Engineers, 3^rd Ed., McGraw-Hill, 2004, Ch. 11. A derivation can be found in: M.M. Denn, "Process Fluid Mechanics", Prentice-Hall, 1980, Ch. 4.
Fogler, H.S., "Elements of Chemical Reaction Engineering", Prentice-Hall, 2006, Ch. 13.1-13.3 and 14.3-14.4 (dispersion models); Levenspiel, O., "Chemical Reaction Engineering", 3^rd Ed., John Wiley, 1999, Ch. 11 and 13 (dispersion models); Missen, R.W., Mims, C.A., and Saville, B.A., "Introduction to Chemical Reaction Engineering and Kinetics", John Wiley, 1999, Ch. 19 and 20.1.
Levy, S., "Two Phase Flow in Complex Systems", John Wiley, 1999, Ch. 3.

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