Baual, Ann ChristineEvaristo, Carla RaeGregorio, Justin EdrikVinluan, Justin Timothy

Pressure Drop and Flooding in a Packed Column

I. Introduction

Chemical processes usually occur as mixture of different components in the gas, liquid

or solid phase. Separation processes such as absorption, distillation, liquid-liquid extraction,

leaching, membrane processing, crystallization, adsorption and ion exchange can be used to

separate or remove one or more components from its original mixture by contacting this mixture

with another phase. For the solute/s to diffuse from one phase to the other, the two phases

which are somewhat miscible to each other are brought into more or less intimate contact. The

components of the original mixture reallocate between two phases during contact and these

phases are separated by simple physical methods. (Geankoplis, 2003)

Gas absorption is a unit operation in which soluble components of a gas mixture are

dissolved in a liquid. To make an intimate contact between the gas and liquid which usually

flows counter currently, packing elements are placed in the vertical, cylindrical columns or

towers. These packing elements also serve to provide development of interfacial surface

through which mass transfer takes place. (Perry, 2008)

A packed column are mainly use for counter-current gas-liquid flow for heat and mass

transfer. In a packed column, liquid, driven by gravity flows down through the random structured

packing and is characterized by low pressure drop and high operating range. (Mackowiak,

2010) Most commonly known packing materials are raschig rings, berl saddle, pall ring, intalox

metal (IMTP) and jaeger metal tri-pack. (Geankoplis, 2003) A packing has three important

geometrical characteristics: surface area, size and void fraction (free volume). The volume of

the free space of the packing is packing void fraction which is related to 1m3 of its volume. While

the area related to 1m3 of the volume the packing is its specific surface area. (Kolev, 2006)

One of the important hydrodynamic parameter of the packed bed is the packing pressure

drop which is equal the difference between the pressures at outlet and inlet of the packing.

Pressure drop is usually presented as a function of the gas superficial velocity (Kolev, 2006)

which starts to rise at a faster gas rate. Superficial gas velocity is the gas velocity through a pipe

assuming that there is no obstruction present in the system.

Gas flow rate is directly proportional to the liquid hold-up or accumulation which causes

flooding. The liquid can no longer flow down through the packing and is blown out with air at the

flooding point. The tower cannot operate above the flooding velocity. And the optimum

economic gas velocity is about one-half or more of the flooding velocity. (Geankoplis, 2003)

Ergun Type EquationThe Ergun equation is derived by Chemical Engineer Sabri Ergun in 1952. It is an

estimation of the pressure drop through a packed bed due to rate of fluid flow, fluid properties,

density of packing and physical properties of the packing material (Sandidge, Shin, Vega-

Fuentes, & Williams, 2005). Therefor Ergun modified Reynolds Number Rep that depends on

the superficial velocity νs, viscosity μ, density ρ, void fraction ε and particle diameter Dp. (Yuan

Jia & Hlavka, 2009)

(1)

Ergun also defined the friction factor fp as a function of pressure drop ΔP, the length of the

packed bed L, particle diameter, void fraction, superficial velocity and density given by equation

(2).

(2)

After several experiments with different packing material with different flow rates, Ergun was

able to formulate the general form of the equation:

(3)

To calculate for the pressure drop, the Ergun equation can be defined as:

(4)

Leva-type EquationMax Leva in 1959 created a semi-empirical equation for the prediction of minimum

fluidization velocity umf for gas fluidization as a function of particle diameter Dp, density ρ and

viscosity μ as shown below:

(5)

In order for the Leva equation to be used in liquid phase, it is modified using

experimental data for liquid phase and making an equation fitting the data (Mohammed, 2009).

The modified Leva equation can be written as follows:

(6)

The pressure drop prediction may be estimated by the correlation given by:

Where modified friction factor fm is a function of of particle diameter Dp, density ρ, shape

factor ϕ, exponent as a function of Re’ n, void fraction ε, pressure drop Δp, length of bed L and

fluid superficial mass velocity G.

Robbins EquationRobbins correlation for pressure drop in a random particle packed towers are based on a

dry packing factor unlike the most manufacturer’s published value which is based on packing

factor from wet data (Ludwig, 1997). Robbins generalized equation of pressure drop for random

tower packings is given by:

Where ΔP is the total pressure drop, Gf is gas loading factor, Lf is liquid loading factor, Fpd is a

dry packing factor, ρ is density and μ stands for viscosity.

There are several methods in using Robbins depending on different parameters and requires

careful attention to dimensions. However the use of the equation has been simplified through

the introduction of Fig. 1. (Perry, 2008)

Figure 1: The Robbins generalized pressure-drop correlation. (From L. Robbins Chem. Eng. Progr. May 1991. p.87)

This experiment aims to determine the void fractions of the packed beds, to determine the

effects of liquid holdups on the pressure drop of the packed column and to determine the

packing factor experimentally with the use of flooding velocity calculations.

II. Methodology

PreliminaryThe dimensions of the packed tower and packings were recorded. The sump tank

containing the water was cleaned and refilled to 75% of its capacity. On-off switch, knobs, flow

meter and drainage valves were closed/turned off while the valve if the return line and all

pressure taps were opened. All liquid in the tubes connected to the pressure taps were also

drained.

Start-UpThe equipment was turned on as well as the compressor and pump. The air flow rate

was set at maximum for 15 minutes to dry the remaining water in the tower.

Pressure Drop of dry and wet packings Starting from 20 L/min, the air flow rate was increased by 10 L/min until it reached the

maximum flow rate of 140 L/min. The water flow rate was set to zero. After each adjustment, the

pressure drop in the manometer, containing colored water, was measured and recorded.

Unlike the previous process, water flow rate was now increased periodically for all the given

air flow rates. The pressure drop of the manometer was measured and recorded. The air flow

rate increment was stopped whenever flooding occurs in the packed column.

ShutdownThe packed tower was drained of all liquid and the pumped was turned off. Similar to the

start-up process, the air flow rate was set at maximum for 15 mins to allow the packed tower to

dry. The compressor and on-off switch were turned off.

III. Treatment and Discussion of Results

IV. Answers to QuestionsThese are some of the characteristics that a packing should have for it to be employed in mass

transfer operation: (a) the shape of the packings should avoid stagnant pools of liquid, trap gas bubbles

and violent changes in the direction of the gas. (b) It should improve wetting and liquid distribution. (c) It

should provide more open area for vapor rise. (d) The ratio of tower diameter to random packing should

be greater than ten. (e) It should have highly corrosive service. (f) Due to the possibility of deformation,

plastic packing should be limited to an unsupported depth of 10-15 ft (3-4 m) while metal packing can

withstand 20-25 ft (6-7.6 m). (g) Packing factors. The capacity, efficiency and pressure drop

characteristics vary with packing size and type. The higher surface area, the more efficiency packing.

Packing factors are indicators of capacity. The lower packing factors, the higher capacity. Note that

ceramic packing will be the first choice for corrosive liquids, but ceramics are unsuitable for use with

strong alkalis. Plastic packing are attacked by some organic solvents and can only be used up to

moderate temperatures, so are unsuitable for distillation columns. Where the column operation is likely

to be unstable, metal rings should be specified, as ceramic packing is easily broken.

A packed bed column contains a support plate, a liquid distributor, and a mist eliminator. The liquid

stream flows down the column due to gravity while the gas flows upward, resulting in counter-flow.

Contaminants are transferred from the vapor to the liquid, due to equilibrium or kinetic mechanisms,

with the packing as the contactor.

Total liquid holdup could be determined by the difference between the dry and irrigated column

weight. Dynamic holdup could be found by collecting the liquid draining from the column after

interruption of input liquid while static holdup was then determined by the difference between total

and dynamic hold ups.

The liquid holdup is the fraction of liquid held up in packed column. The volume of liquid holdup

volume is often needed for calculating packed bed support beam loadings as well as for determining

how much liquid drains to the bottom of a tower when the vapor rate is stopped.

Channeling occurs when the gas or liquid flow is much greater at some points than at others. Such

channeling is undesirable, for it can substantially reduce interfacial area and hence mass transfer.

Loading is a requirement for good mass transfer. When loading begins, the flows may slightly decrease,

but the dramatic increase in the gas-liquid area means that the mass transfer is fast.

At low liquid velocities, the liquid’s physical properties will have little to no effect on the

pressure drop. This probably led to the considerable difference between the data acquired in the

experimental section compared to using the Leva-Robbins equation.

V. Conclusions

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Edition. New Jersery: Prentice Hall Professional Technical Reference.

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Perry, R. H. (2008). Perry's Chemical Engineers' Handbook 8th Edition. United States of America: The McGraw-Hill Companies, Inc. .

Leva, M. (1959). Fluidization. New York: McGraw-Hill.

Ludwig, E. (1997). Applied Process Design for Chemical and Petrochemical Plants, Volume 2. Houston: Gulf Publishing Company.

Mohammed, N. (2009). Estimated Equations for Water Flow Through Packed Bed of Multi-size Particles. Al-Qadisiya Journal For Engineering Sciences.

Sandidge, J., Shin, D., Vega-Fuentes, S., & Williams, L. (2005). Fluid Flow through Packed Beds: Experimental Data vs. Ergun’s Equation. 1-10.

Yuan Jia, Y. L., & Hlavka, D. (2009). Flow through packed beds.

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Schubert, C., Lindner, J., & Kelly, R. (1986). Experimental methods for measuring static liquid holdup in packed columns. AIChE Journal,32(11), 1920-1923.