ECH 4224L
Unit Operations Lab I
Packed Bed
Flow through a Packed Bed
Overview
Packed columns are typically used in industry for distillation, gas absorption, liquid-liquid
extractions, and catalytic reactions. Packing in a column improves a process by increasing the
contact area between phases or by containing catalyst for a reaction. The flow rate through a
packed column depends on several factors, such as the shape and size of the packing used. In this
experiment, you will investigate the effect of varying the packing height on the flow rate through
a packed column and compare the experimental results with predictions of the Ergun equation.
The experimental system contains two identical columns with diameter 1.5" packed with
spherical glass beads with diameter 3 mm, see Figure 1. The only difference between these
columns is the packing height. Water is supplied to the columns from a feed tank located above
them and the water flow rate is controlled by needle valves. The columns are drained into a
graduated collection tank.
Figure 1. Overview of the packed bed system.
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ECH 4224L
Unit Operations Lab I
Packed Bed
Objectives
•
Investigate effect of the packing height on the water flow rate.
•
Compare the experimental results with a theoretical prediction.
Theoretical Background
The pressure drop ΔP across a packed column is
∆𝑃 = 𝜌𝑓𝑝
𝐿𝑈𝑠2 (1 − 𝜀)
,
𝐷𝑝
𝜀3
(1)
where Us, ρ, and μ are, respectively, the superficial velocity, density, and viscosity of the fluid, fp
is the friction coefficient, Dp is the particle diameter, L is the height of the packing in the column,
and ε is the voidage (porosity) of the packing. The superficial velocity is defined as the volumetric
flow rate of the fluid divided by the cross-sectional area of the column.
The friction coefficient can be approximated by the Ergun equation,
𝑓𝑝 =
150
+ 1.75,
𝑅𝑒𝑝
(2)
𝜌𝑈𝑠 𝐷𝑝
(1 − 𝜀)𝜇
(3)
where
𝑅𝑒𝑝 =
is the Reynolds number. At a steady-state, the pressure drop in the column should be balanced by
the hydrostatic pressure above the packing,
∆𝑃 = 𝜌𝑔ℎ,
(4)
where h is the liquid level above the packing. Combining equations (1)-(4), one can obtain a
relationship between h and Us. This relationship should be verified experimentally.
The expressions (1)-(3) for the pressure drop in the column can be substantially simplified
in the case of small Reynolds numbers. In this case, the second term in Eq. (2) can be neglected
and Eqs. (1)-(3) reduce to Darcy’s law for flows in porous media:
∆𝑃 =
𝜇𝐿𝑈𝑠
.
𝜅
(5)
where
𝐷𝑝2 𝜀 3
𝜅=
,
150(1 − 𝜀)2
is permeability of the packed bed.
2
(6)
ECH 4224L
Unit Operations Lab I
Packed Bed
Experimental Procedure
1. Use the supply line to fill the feed tank with water.
2. Open one of the feed valves slowly to allow water to flow through one of the columns.
3. Adjust the valve until the level of water above packing reaches a steady-state.
4. After the water level above in the column has reached a steady-state, wait until the water level
in the collection tank reaches a tick mark. Then start a timer.
Note: Reading the markings near the bottom of the collection tank may be difficult. Therefore,
it is recommended to have 1-2 gallons of water in the tank before starting the timer to make
taking the measurements easier.
5. Continue filling the collection tank, logging times to fill each additional half gallon. It is
recommended to continue this process until amount of water in the collection tank changes by
at least 3 gallons from the moment you started the timer.
6. Close the feed valve.
7. Open the feed valve for the other column and repeat steps 3-6 for this column.
8. Drain water from the collection tank.
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