Gas Absorption

Topics: Carbon dioxide, Water, Sodium hydroxide Pages: 6 (1447 words) Published: October 4, 2012



ChE 35 Chemical Engineering Laboratory II

Acabo, Dean Cris Aguirre, Ian Carlo Belarmino, Arniel Catan, Charles John

Engr. Marco Theodore E. Escaňo ChE 35 Instructor

October 2012

OBJECTIVE: To calculate rate of absorption of carbon dioxide into water from analysis of liquid solutions flowing down the absorption column. THEORETICAL BACKGROUND: Absorption refers to the transfer of a gaseous component from the gas phase to a liquid phase. The liquid surface area available for mass transfer and the time available for diffusion of the gaseous molecules into the liquid are important factors affecting performance. Absorption can be divided into two broad classifications: straight dissolution of absorbate (contaminant gas) into absorbent (liquid), and dissolution accompanied by irreversible chemical reaction. The gaseous contaminant being absorbed (absorbate) must be at least slightly soluble in the scrubbing liquid (absorbent). Mass transfer to the liquid continues until the liquid approaches saturation. At saturation, equilibrium is established between the two phases. The mass transfer rate of the contaminant into the liquid is equal to the mass transfer rate of the dissolved species back into the gas phase. Accordingly, the solubility of the contaminant in the liquid creates a limit to the amount of pollutant removal that can occur with a given quantity of liquid. This solubility limit can be overcome by providing reactants in the liquid phase that react with the dissolved gas contaminant, forming a dissolved compound that cannot exit the liquid. In this experiment, the performance of a packed gas-liquid absorption column is evaluated. A water steam entering the top of the column and exiting the bottom is used to absorb carbon dioxide (CO2) gas from air. The CO2 composition of the inlet gas stream is controlled by valves for CO2 and air. Flow rates of inlet gas and liquid streams can be measured using flowmeters.


Figure 1. The Gas Absorption Column

Figure 2. Obtaining a Gas Sample from the Top of the Column

MATERIALS AND APPARATUS: 1. Phenolphthalein indicator prepared from carbon dioxide free water 2. Standard 0.0277 M sodium hydroxide solution 3. Standard 0.01 M sodium bicarbonate solution 4. Water and CO2 PROCEDURE: 1. Fill the liquid reservoir tank at the base of the column to approximately three-quarters full with (preferably) deionized water. Note the volume added [VT liters]. 2. With gas flow control valves C2 and C3 closed, start the liquid pump and adjust the water flow through the column to approximately 6 liters/minute on flowmeter F1 by adjusting flow control valve C.

3. Start the compressor and adjust control valve C2 to give an air flow of approx. 10% of full scale on flowmeter F2. 4. Carefully open the pressure regulating valve on the carbon dioxide cylinder, and adjust valve C3 to give a value on the flowmeter F3 approx. one half of the air flow F2 Ensure the liquid seal at the base of the absorption column is maintained by, if necessary, adjustment of control valve C4. 5. After 15 minutes of steady operation, take samples at 10 minute intervals from S4 and Ss. Take 150ml samples at known times in each case. Analyze the samples according to the procedure detailed below.

Analysis of Carbon Dioxide Dissolved in Water

Note: Water used for absorption should be deionized as presence of dissolved salts could affect the analysis described below. If tap water is used, no metal ions should be present in greater quantities than 1.0 mg/liter and pH should be just alkaline: 7.1 to 7.8.

Chemical Solutions Needed 1. Phenolphthalein indicator A. R. grade. 2. Standard 0.0277M sodium hydroxide solution, prepared by diluting 3. 27.70ml 1M caustic soda standard solution to 1 liter with carbon dioxide free...
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