WordPress.com



Baual, Ann Christine

Evaristo, Carla Rae

Gregorio, Justin Edrik

Vinluan, Justin Timothy

Pressure Drop and Flooding in a Packed Column

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 Equation

The 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)

[pic] (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).

[pic] (2)

After several experiments with different packing material with different flow rates, Ergun was able to formulate the general form of the equation:

[pic] (3)

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

[pic] (4)

Leva-type Equation

Max 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:

[pic] (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:

[pic] (6)

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

[pic]

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 Equation

Robbins 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:

[pic]

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)

[pic]

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.

Methodology

Preliminary

The 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-Up

The 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.

Shutdown

The 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.

Treatment and Discussion of Results

Answers to Questions

These 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.

Conclusions

References

Geankoplis, C. J. (2003). Principles of Transport Processes and Separation Processes 4th Edition. New Jersery: Prentice Hall Professional Technical Reference.

Kolev, N. (2006). Packed Bed Columns: For absorption, desorption, rectification and direct heeat transfer. Oxford, UK: Elsevier B.V.

Mackowiak, J. (2010). Fluid Dynamics of Packed Columns: Principles of the Fluid Dynamic Design of Columns for Gas/Liquid and Liquid/Liquid Systems. Germany: Springer-Verlag Berlin Heidelberg.

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.

Encyclopedia of Chemical Engineering Equipment. (n.d.). Retrieved March 17, 2015, from

Packed Hydraulic. (2011, March 1). Retrieved March 17, 2015, from DESIGN GUIDELINE- PACKING HYDRAULIC Rev 1.0 web.pdf

Cussler, E. (1984). Diffusion, mass transfer in fluid systems. Cambridge. Cambridgeshire: Cambridge University Press.

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

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download