Hold-up studies, also known as flooding velocity studies, are a crucial aspect of designing and operating packed columns, trays, and other gas-liquid contactors in chemical processing and petroleum refining industries
Importance :
Usually, the low holdup is desired by reasonable holdup is necessary for efficient column operation. The weight of liquid held in the following must be considered when determining the support loads at the bottom of the packing as well as the column itself. The higher the holdup for any particular packing the greater will be the gas pressure drop and the longer the column drainage time when shutting down. Smaller size packing tends to have greater holdup than larger packing. Further, the prediction of liquid holdup is of interest to the designer in estimating the system's response to variations in operating conditions.
Hold-up:
Understanding Holdup in Packed Columns: A Key to Efficient Design
As a chemical engineer, you know how crucial it is to design and operate gas-liquid contactors efficiently. Holdup refers to the liquid retained in the packed column or contactor, which can be due to wetting of the packing material by the liquid or liquid pools caught in the crevices between packing materials. Accurate calculation of holdup is vital in designing and operating packed columns, as it directly affects mass transfer efficiency, pressure drop, and equipment sizing and design.
To ensure accurate holdup calculations, consider the following when creating equipment drawings and designs: choose packing materials with suitable surface area, void fraction, and wetting characteristics; ensure the column is properly sized and laid out to accommodate the expected holdup; and design efficient liquid distribution and collection systems to minimize holdup and ensure uniform liquid flow. By understanding and accurately calculating holdup, you can create more efficient, cost-effective, and safe packed column designs.
Understanding Flooding Velocity and Hold-up Studies
Every chemical engineer, process engineer, mechanical engineer, petroleum engineer, or corrosion engineer, should know how crucial it is to design and operate gas-liquid contactors efficiently and safely. Flooding velocity is the maximum superficial gas velocity at which a column or contactor can operate without experiencing flooding or liquid entrainment. When the gas flow rate is too high, it's like a traffic jam, causing the liquid to be carried upward and out of the column, leading to reduced efficiency, increased pressure drop, and potentially catastrophic consequences.
Hold-up studies involve measuring the hold-up (liquid fraction) in a column or contactor as a function of gas flow rate, liquid flow rate, and other operating conditions. By understanding how the liquid fraction changes with different operating conditions, you can optimize your column design and operation. When designing a column or contactor, consider factors like column diameter and height, packing material selection, liquid distributor and collector design, and gas inlet and outlet design to minimize flooding and optimize hold-up, ultimately creating a more efficient, cost-effective, and safe column design. Additionally, corrosion engineers can ensure that the materials selected for the column or contactor can withstand the corrosive effects of the process fluids, minimizing the risk of equipment failure and downtime.
There are three different types of liquid holdup :
- Total holdup (ht)
- Static holdup (hs)
- Operating holdup (ho)
All three are expressed as cubic meters of liquid per cubic meter of packing.
- The total holdup ht, is defined as the total liquid in the packing under operating conditions.
- The static holdup hs is defined as the liquid in the packing that does not drain from the packing when the liquid supply to the column is discontinued.
- The operating holdup ho, defined as the liquid that continuously moves through the packing and replacement regularly and rapidly by new liquid represents the liquid that will drain from the packing when the water flow is stopped.
The relation between the three holdups is given by:
- ht = hs +ho
Static holdup depends upon the balance between surface tension forces tending to hold liquid in the bed and gravity or other forces that tend to displace the liquid out of the bed.
An estimate of static holdup may be made from the following relationship of Shulman et al.
C1 µ1c2 σc3
hs = --------------
ρ10.37
Where µ1 = Liquid viscosity, Kg/m-s
σ = Surface tension, N/m
ρ1 = Liquid density, Kg/m3
Operating holdup contributes effectively to the mass transfer rate since it provides a residence time for phase contact and surface regeneration via agglomeration and dispersion. The static holdup is limited in its contribution to mass transfer rates. In laminar regions holdup, in general, has a negative effect on the efficiency of separation.
Experimental setup
- The experimental set up (fig) consists of a glass column 50 mm I.D. and 600 mm in length which is mounted vertically on a stand using support plates.
- The material of construction of all lines and pipe fittings is galvanized iron off. The valves and pressure tapings are of brass.
- At the bottom of the column, a water seal is provided to prevent the leakage of air.
- A rotameter is installed to measure the flow rate of water, which can measure the flow rate.
- The air inlet is connected to a balance designed to give a steady flow rate of air. An orifice meter is connected to a calibrated mercury U Tube Manometer to measure the flow rate of air.
- A lever arrangement is provided which consists of two vessels. One at the top of the column to drain and the other at the bottom of the column to collect operating holdup. Here both the vessels move simultaneously cutting off inlet water and facilitating the collection of operating holdup in the bottom vessel.
- The packing material is a Raschig ring of Outer dia 12 mm and Inner dia of 6 mm.
Measurement of the Operating liquid holdup
Ensuring the obtainability of reproducible data, the equipment is made ready for operation. The dry packing was thoroughly wetted by setting the water rate up to 120000 Kg/hrm2 for around ten minutes. Then the water rate was set to the desired value and the air rate was also set at approximately 100 Kg/hrm2. The water and air flow rates were kept constant for ten minutes before the operating holdup was measured. This ensured a steady flow rate. The time for collecting hold-up was fixed at five minutes as it ensured complete drainage. Simultaneously the manometer readings were recorded and the water rate was increased to the next higher value.
The different sets of packings of a series of runs were made in a similar fashion with and without air flows with increasing water rates. The water flow rate ranged from 30000 to 110000 Kg/hrm2. A lower than 30000 Kg/hrm2 water rate was not employed as it was felt that good liquid distribution could not be ensured at lower rates.
- Berl Saddle – 12mm
- Density of water at 28˚C = 996.26 Kg/m3 – ρw
- Volumetric flow rate of water ( R) = 2 lit/min
- Mass flow rate of water (L0) = 2 x 10-3 x 996.26 x 60 = 119.55 Kg/hr
Π (Dt)2 Area of packed column =
Π (67 x 10-3 )2
= --------------------
4
At = 3.52 x 10-3 m2
119.55 ho
L = -------------- --------
3.526 x 10-3 At
= 33905.27 Kg/hr-m2
The mass flow rate of air (G)
4.74 x 10-4 x 1.0593 x 3600
(G) = ------------------------------------------
3.526 x 10-3
= 516.61 Kg/hr-m2
Operating Hold up
Volume of water held up
ho = -----------------------------------
Volume of packed column
250 x 10-6
= ----------------------------------
3137.8 x 10-6 m3
= 0.0796 m3 / m3
Total holdup (ht) = ho + hg + hs + hst
Static holdup (hst), Solid holdp (hs) are assumed to be constant for a particular packing
hs = 0.390 m3 / m3hst = 0.239 m3 / m3
Therefore,
1 = 0.0796 + hg + 0.390 + 0.239hg = 0.2914 m3 / m3
Hold-up and Flooding Velocity Calculator:
Welcome to the Hold-up and Flooding Velocity Calculator!
This calculator is designed to help you estimate two important parameters in chemical engineering and process design:
- Hold-up: The fraction of the column or reactor volume occupied by the liquid phase.
- Flooding Velocity: The maximum gas velocity at which the column or reactor can operate without experiencing flooding or liquid entrainment.
How to use the calculator?
Simply enter the required input values, such as:
- Column diameter and height- Gas and liquid flow rates- Liquid and gas densities- Surface tension
Click the "Calculate" button, and the calculator will provide you with the estimated hold-up and flooding velocity values.
We hope you find this calculator helpful!
| Column Diameter (D): | m | |
| Column Height (H): | m | |
| Gas Flow Rate (Qg): | m³/h | |
| Liquid Flow Rate (Ql): | m³/h | |
| Liquid Density (ρl): | kg/m³ | |
| Gas Density (ρg): | kg/m³ | |
| Surface Tension (σ): | N/m |
Hold-UP and Flooding Velocity Results:
| Hold-up (ε): | - | |
| Flooding Velocity (Uf): | m/s |