Gas Absorber Systems – Operation and Application

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What is a Gas Absorber?

Gas absorbers are used to eliminate pollutants from gas flows, such as methane from an exhaust discharge. A Gas absorption unit uses a column which often contains material for random or organized packing. Packed bed absorbers use fluxes of gas and liquid that flow counter current one to another. The contaminant gas is absorbed into the fluid stream, resulting in the exit gas being reduced as contaminant. The process of absorption is heavily dependent on the operating parameters which have to be studied to optimize the process. Let’s take an example of investigating the carbon dioxide absorption into water, and examine how the operating parameters affect system separation and efficiency.


Operation / Working

A Gas absorption unit uses a liquid solvent touch to absorb the material from a gas mixture. Energy is moved from the gas mixture to the solution, with equilibrium of the two phases. Instead, there is the isolation of the liquid gas phase. The overall material balance for the absorber is where the vapor and liquid flow rates respectively, thereby integrating the mole fraction in the vapor and liquid phase into the component material balance for the absorbed component.

The average coefficient of mass transfer is the rate at which the concentration of one material transfers from one substance to another. Here in this example, let’s consider KG as the total mass transfer coefficient, and PAG is the partial gas pressure that needs to be absorbed.

Mass transfer depends on the coefficients of mass transfer at each step, and the amount of interphase area available in the absorber. Henry's Law and Raoult's Law are applied to determine the concentration of the liquid phase partial pressures in equilibrium. 

A packed column Gas absorber will be used in the following experiment to absorb carbon dioxide from a flux of gas into water. From the bottom and top respectively the gas and water streams enter the column allowing counter flow. The composition of carbon dioxide at the inlet is controlled using the carbon dioxide and air valves. Then, the carbon dioxide concentration in the outlet is measured.

The device used in the above example is a column for the absorption of packed counterflow air. At a bed depth of 34 centimetres, the column is filled with 13 milliliter berl saddles. The valves at the column's entrance and exit allow gas to escape, while an infrared spectrometer is used to measure partial CO2 pressures during the gas process. 

Turn on the master switch to start the experiment then close the valve used to regulate the amount of water in the tank. Open the airflow valve full, and open the column pressure control valve. Set the airflow rate considering your desired level. Use at least 30 liters per minute, then raise the level as desired. Set column pressure to approximately 0.5 bar using a pressure control valve. Set the carbon dioxide flow rate to about 4 liters per minute, then set the water flow rate to about 4 liters per minute, too. Throughout the experiment change the water flow to maintain a steady water level within the tank using the in-line pressure gauges to sample and measure the carbon dioxide concentration at the base, middle and head of the column as needed.

Repeat the experiment by performing eight runs. Use two different gas flow rates, liquid flow rates, and carbon dioxide concentrations, thereby enabling the determination of the most important variables in the system. You must make sure that system is allowed to achieve its steady state whenever a flow rate is changed.

Repeat the experiment 8 times, using two distinct gas flow levels, carbon dioxide concentrations, and liquid flow rates to assess the system's most critical variables. Make sure that the system maintains a steady state if a flow rate is modified.

Let's have a look at the results of the experiment conducted. First, measure partial pressures and partial pressure balance for each phase, then use the partial pressures to determine the coefficients of mass transfer. The estimated values are represented here as triangles, while the expected values, shown as the solid line, derive from the operating and equilibrium lines measured. Confidence intervals were plotted with dotted lines for the model values and the mean coefficient of mass transfer.

There was no difference between the expected and actual values, suggesting that at the interface between the liquid and gas phases, the column is at a steady state of equilibrium. Now let's see what happens, when we compare the coefficients of the mass transfer under the same operating conditions. If the rate of gas was high or small, the model and experiment act the same, demonstrating that the rate of gas flow had little to no effect on the coefficient of mass transfer in the ranges tested.



Then, let's take a look at some industrial applications where the Gas absorber is used. Packaged bed absorbers are the most common piece of equipment used for controlling air pollution. Gas absorbers are also called scrubbers in such situations. Scrubbers are used to extract corrosive gases from toxic gasses and vents from oil refineries, chemical plants, and paper and pulp factories, such as nitric acid, sulfuric acid, and hydrochloric acid. The discharge process of the dissolved gas from the solvent is called stripping. Strippers are also used along with absorbers to extract the dissolved gas and recycle the liquid solvent. This is especially relevant when components of nitrogen and phosphorus are in the wastewater. This waste water used to be drained directly into oceans, but this resulted in excessive algae production, called eutrophication, which in turn was affecting the natural habitats severely.


Final thoughts

Ablaze Export Pvt. Ltd. focuses on the Gas absorption Unit analysis, in a simple way where only a single element in the gas solution is absorbed. All the other elements of the gas are considered as non-soluble in the liquid, and there isn’t any transfer of elements from the liquid phase to the gas phase as the liquid present inside becomes non-volatile. We design Gas absorbers that can be used for plenty of industrial purposes such as to extract corrosive gases from the toxic gases as well as vents from the oil refineries, and chemical plants. It is also used in the pulp & paper factories, to remove toxic gases such as hydrochloric acid, nitric acid, and sulfuric acid.

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