Lab 5实验氨吸收 Experiment of Ammonia Absorption.doc

Lab 5 Experiment of Ammonia AbsorptionI. Objectives1. Understand the configuration of an absorption column2. Determine the overall volumetric gas phase mass transfer coefficient , KY3. Understand the characteristics of single-film resistance.II. Principles In gas absorption process, a solute component is absorbed by contacting with a liquid phase, in which the component is soluble. In this experiment, water is used to remove ammonia from air. The absorption of NH3 in water is gas phase controlled. The gas phase coefficient approximately equals to the overall mass-transfer coefficient. Based on this, the overall volumetric mass-transfer coefficient can be evaluated. When the mole fraction of solute in liquid is less than 10%, the equilibrium correlation obeys Henrys law. The overall mass-transfer coefficient is (1.26)where, h = height of packed section, m KY= overall volumetric gas phase mass transfer coefficient , kmol/m3hY = mole ratio of solute to solvent in gas phase; Y1, inlet value; and Y2, outlet valueYm = logarithm average driving force for gas phaseIII. ApparatusFigure 1.7 presents a diagram of the gas absorption unit to be employed in this experiment. This unit consists of 3 rotameters, 4 manometers, 2 thermometers, 2 buffer tanks, an effluent gas analyzer and an absorption tower. An air-ammonia mixture is introduced to the bottom of the column and pure water is sprayed from the top of the column. The air flowing upward contacts with water stream flowing downwards, which absorbs ammonia. Fig. 1.7 Gas absorption apparatus1packed tower; 2sprayer; 3packed section; 4grid; 5air blower; 6by-pass valve; 7buffer tank;8, 18, 26rotameters; 9,15thermometers; 10,17,28manometer(open to atmosphere);11ammonia cylinder;12, 13pressure gauge; 14ammonia buffer tank; 16ammonia control valve; 19pressure regulating valve;20sampling pipe for effluent gas; 21pressure accumulator; 22stopcock; 23absorption capsule;24wet-type gas flow meter; 25water control valve; 27drain pipe; 29differential pressure meterIV. Procedures The columns must be wetted for at least 10 minutes in order to ensure complete wetting of the packing. The system must be allowed to operate for another 10 minutes to reach an equilibrium state. Record all temperatures, flow rates, difference pressures.1. The effluent gas analyzer consists of an absorption capsule and a wet-type gas flow meter. Wash the absorption capsule with distilled water, drop 1 ml diluted sulfuric acid of a certain concentration with a suction pipette, and add distilled water to the tick mark. Joint the absorption capsule into the effluent gas analyzing pipeline after adding 12 drops of methyl red indicator.2. Open the water valve. Set the water flow rate to a value about 50 L/h on the rotameter. 3. Turn on the Jaeger blower. Set the air flow rate to a value about 20 m3/h on the rotameter by regulating the by-pass valve.4. Switch on the ammonia system under the supervising of the instructor. First, loose the spring of the reduction valve. Second, open the cut-off value. Third, regulate the reduction valve to the lower pressure of 0.05 to 0.08 MPa. Fourth, open the ammonia valve and set the ammonia flow rate to a value about 1 m3/h.5. Allow the system operating for 10 minutes to reach a steady state.6. Record all temperatures, difference pressures and flow rates.7. Analyze the effluent concentration. First, record the initial value of the wet-type gas flow meter. Second, open the analyzing valve. Third, record the end value when reaching the end point.8. Change either the gas flow rate or the liquid flow rate, and repeat the test.9. Close the ammonia, air and water systems in order.V. Data records Type of packing: Height of packed section: Column diameter: Packing size: Table 1.5 Data records for ammonia absorption experimentItemsParametersValueAir flow Inlet Gauge pressure of the rotameter /mmHgAir temperature /Reading of rotameter /m3h-1Ammonia flow Inlet gauge pressure of the rotameter /mmHgAmmonia temperature /Reading of rotameter / m3h-1purity of ammonia gasWater flowWater temperature /Reading of rotameter / Lh-1Concentration of effluent gas Mole concentration of H2SO4 /mmolml-1Volume of H2SO4 /mlVolume of effluent gas /LTower pressureGauge pressure at the top of tower /mmH2OPressure drop of packed section /mmH2OVI. Data processing1. Air flowIn order to obtain air flow rate, the volumetric flow rate under experimental conditions must be conversed to that under standard state ( 101.3kPa and 0):where V0,V2 = air flow rates under standard state and experimental conditions respectively, m3/h T = temperature of air, K; T0, under standard state, 273K; T1, under calibration state, 293K; T2,under experimental state p= pressure of air, Pa; p0, under standard state, 101 330 Pa; p1, under calibration state, 101 330 Pa; p2, under experimental state 2. Ammonia flow rate Ammonia flow rate can be calculated as follows:where V = ammonia flow rate, m3/h; , under standard state; , under experimental state =density, kg/m3; , density of air under standard state, 1.293 kg/m3; , density of ammonia under standard state, for ammonia purity of 98% 3. Molar mass velocity of airMolar mass velocity of air is expressed as followswhere D = column diameter, mG = molar mass velocity of air, kmol/m2.h 4. Mole ratio of ammonia to air in gas phaseThe mole ratios of ammonia to air in gas phase arewhere =reading for wet-type gas flow meter, L= volume of sulfuric acid ,mL= mole concentration of sulfuric acid, mmol/mL= average local atmosphere, PaY = mole ratio of ammonia to air in gas phase; Y1, at gas inlet; Y2, at gas outlet 5. Average forcing driver for gas phaseThe average forcing driver for gas phase, , is where = mole ratio of ammonia to air for gas phase in equilibrium with X; , with X1; , with X2 X = mole ratio of ammonia to water for liquid phase; X1, at liquid outlet; X2, at liquid inlet 1) The equilibrium value can be calculated as follows: where E = Henrys coefficient, Pa p = operation pressure, Pa m=phase equilibrium constant Table 1 shows Henrys coefficient varying with the temperature.Table 1.6 Henrys coefficients for ammonia-watert/ 0 10 20 25 30 40 E/Pa 29 690 50 868 78 835 95 960 12 660 196 380 2) X1 and X2 For pure water, X2=0. X1 can be calculated according to the overall material balance, i.e., where L = molar mass velocity of water, kmol/m2.h 6.