Introduction
Certain gas components are transferred into the liquid phase as a result of a
liquid absorbing a gas. This process is known as gas absorption. In many
industrial applications, including pollution control, chemical processing, and
natural gas treatment, this phenomenon is essential. When a gas is absorbed,
some of the gas molecules dissolve into the liquid through the controlled
interaction of the gas with the liquid. Usually, the concentration gradient
between the gas and the liquid acts as the driving force behind absorption
(Cussler, 1997). The mass transfer during absorption can be described using
Fick's Law of diffusion.
dC
J=−D AB
dz
The nature of the gas and liquid, temperature, pressure, and flow rates are some
of the variables that affect the rate of absorption. Common equipment used for
gas absorption include packed columns, tray columns and membrane absorbers.
Each type offers different advantages depending on the specific application and
required efficiency. (Geankoplis, 2003)
Gas absorption is widely used in scrubbers to remove pollutants such as sulfur
dioxide (SO2) from flue gases. In this use, the pollutant is absorbed by a solvent,
lowering emissions into the atmosphere. Gas absorption is essential to the
synthesis of many chemicals. For instance, one of the most important processes
in the creation of fertilisers is the absorption of ammonia in water. (Perry, 2008)
Objective
1. Operate a gas absorption unit for carbon dioxide in water
2. Calculate the overall mass transfer coefficient of the carbon dioxide and
water system
Theory
Materials and Method
Required material:
SACED absorption system
Chemical elements; CO2. KOH (5%), NaOH (0,1N), HCL (0,1N), phenolphthalein
methyl orange.
Apparatus:
Tank: Contains water which is fed to the absorption column.
Packing material: Provides a large area of contact between the liquid and the solute-
containing gas entering the bottom of the absorber.
Pump: Transfers the absorbent to the top of the column.
Mixer: A specially made mixing tube, which comes after the flow meters but before
reaching the bottom of the tower, is used to combine the gases.
Tower: The column's raschig rings bed height is 115 cm, and its internal diameter is 8
cm.
Gas supply: Cylinders with regulators attached provide air and CO2. Flow control
valves are used to keep the flow rates at the desired levels.
Liquid Supply: Water is pumped to the top of the column from a tank as the liquid
supply. It can be directed to the drain or returned to the sump tank after passing
through the column.
Measurements: --
Temperature transmitter: The control panel provides a temperature reading.
Differential pressure gauge: Indicates the difference in pressure between the two
sections of the column.
Figure 1: Edibon Gas Absorption Column
Experimental Procedure
Started by setting the desired value for the water flow with the VR-3 valve or in the SACED
system (1 L/min) then checked if the air outlet valve VR-1 was completely closed, the airflow
was regulated by gradually opening this valve. We opened the CO2 line, fixed an outlet
pressure in the governor of 1 bar, and adjusted the desired flow through the VR-2 valve. The
system was allowed to reach a steady state (approximately 10 min). The following variables
were measured; the water flow rate (read directly in the flowmeter) and the air and CO 2 flow
which was read under normal conditions in the corresponding flowmeters which were
previously calibrated and set to the operation conditions, atmospheric pressure and
temperature. Filled the glass deposits with an aqueous dissolution of 5% KOH (with a
syringe) until the 100 ml mark. With the three-way valves VT-1, VT-2 and VT-3, the gas
sample-taking line was purged, and the sample was collected.
Smoothly added the sample with the syringe, until the liquid height in branch b coincides with
the obtained one in step 7. The decrease in the volume of the gas contained in the syringe
was the quantity of CO2 absorbed by the KOH solution. Through the sample-taking valve
VT-4, which was previously purged, a 50 ml sample of the liquid was collected. An identical
volume of NaOH 0.1N to this volume of CO2 aqueous solution was added. Repeated the
experiment for 2 different water flow rates, while keeping a constant flow of gaseous mixture.
Titrated the sample from steps (6 and 7) with 0.1N HCL using 2-3 drops of phenolphthalein
and methyl orange as indicators. Recorded the volume of HCL used at the neutralisation
point.
Results
Table 1: Data Collected from Column
Water flowrate V1-3 initial liquid level V1-3 final liquid level Gas Absorbed (initial -final)
(L/min) (ml) (ml) (ml)
1 105 64 36
2 100 73 27
Table 2: Column Specified Data
Data collected from practical Values and units
Water Flowrate 1 L/min
CO2 Flowrate 30 L/min
Air Flowrate 40 L/min
Height 115 cm
Diameter 8 cm
Concentration of NaOH 0,1 M
Concentration of HCl 0,1 M
Table 3: Titration
Trial 1 Values and Units
Initial volume (HCl) 50 ml
Final volume (HCl) 20 ml
Volume of NaOH 20 ml
Volume of CO2 20 ml
Trial 2 Values and Units
Initial volume (HCl) 50 ml
Final volume (HCl) 20,7 ml
Volume of NaOH 20 ml
Volume of CO2 20 ml
