Ammonia Gas Absorption

Transcription

1 Ammonia Gas Absorption by Oscar D. Crisalle Professor Chemical Engineering Department University of Florida Revision 12: September 24, 2013 Ammonia Absorption Rev 08-04/15/2013 Page 1

2 CONTENTS 1 Introduction 4 2 Experiment 1: Absorption of Ammonia (NH 3 ) 5 3 Operational Information 6 4 Thermodynamic Phase-Equilibrium 9 5 Henry's Law 11 6 Gas Densities 12 7 Colburn's NTU Equation Number of transfer units for gas-phase controlled transfer Height of a transfer unit for gas-phase controlled transfer Remarks on the NTU Equation (7) Overall Mass Transfer Coecient 15 9 Interpretation of the Absorption Factor Interpretation of NTU and HTU Characteristics of Flow in the Column Hold-up time (also called residence time) Number of hold-ups Scrubbing Eectiveness 19 Ammonia Absorption Rev 08-04/15/2013 Page 2

3 13 Theoretical Expectations Measurement of NH 3 Gas Compositions Rotameter: Water Flow Measurement Liquid solvent rotameter (RTM): water Measurements Rotameters: Gas Flow Measurements Gas feed-line rotameters (RTM): (NH 3 + N 2 ) Measurements Experimental Details Experimental Procedures Start-up and normal-operation procedures Shut-down procedure Anticipated Experimental Problems Objectives 33 Ammonia Absorption Rev 08-04/15/2013 Page 3

4 1 INTRODUCTION This experiment investigates the properties of gas absorption equipment where a gaseous solvent mixed with air or Nitrogen is absorbed by dissolution into a water stream There are two gas absorption experiments in the Unit Operations lab Experiment 1 Experiment 2 Absorption of ammonia in water Absorption of carbon dioxide in water The focus of this lab is the Experiment 1 which deals with the absorption of ammonia in water. It is MANDATORY to read the chapter entitled Gas Absorption in reference [3] before carrying out this experiment. Remark: Gas absorption is also referred to as gas scrubbing, or gas washing. Ammonia Absorption Rev 08-04/15/2013 Page 4

5 2 EXPERIMENT 1: ABSORPTION OF AMMONIA (NH 3 ) Counter-current absorption packed tower Raffinate (N 2 + NH 3 ) Solvent (W) V out, y out L in, x in Rotameter Nomenclature Solute: Ammonia (NH 3 ) Feed Carrier Gas: Nitrogen (N 2 ) Solvent: Water (W ) Thermodynamic Equilibrium Ammonia Sensor h N2 + NH3 Equilibrium Feed-solvent Phase (or raffinate phase) N2 + NH3 y W y = mx * W + NH3 x * Extract-solvent Phase (or extract phase) V in, y in Rotameter L out, x out Feed (N 2 + NH 3 ) Extract (W + NH 3 ) Assumption: Nitrogen is insoluble in W Ammonia Absorption Rev 08-04/15/2013 Page 5

6 3 OPERATIONAL INFORMATION Column available Height of the column : 800 mm Internal diameter: 100 mm Type of packing: Standard 6 mm Raschig rings Density The liquid water stream has a higher density than the N 2 + NH 3 gas stream. That is why the liquid stream is fed from the top. Insolubility We are making the assumption that N 2 is insoluble in W. This is only an approximation. Nonvolatility We are making the assumption that the solvent (W ) is nonvolatile at the temperature of the experimental conditions Ammonia Absorption Rev 08-04/15/2013 Page 6

7 OPERATIONAL INFORMATION Continuous and dispersed phases Two phases form inside the column: a CONTINUOUS phase and a DISPERSED phase. When a column is started up, it is FIRST lled with ONLY the gas. This denes the continuous phase. SECOND, the liquid stream is introduced, and it becomes the dispersed phase. Making the gas the continuous phase creates more interfacial area than when the liquid is the continuous phase (because the gas is constrained to reside in bubbles) Flooding by the water phase Occurs when the upward force exerted by the gas is sucient to prevent the liquid from owing downward The 100% ooding velocity of the gas stream can be determined for a given inlet liquid stream ow: Set the gas feed ow to a value that oods the column (water level is at the top of the packing surface) Ammonia Absorption Rev 08-04/15/2013 Page 7

8 OPERATIONAL INFORMATION Operation at 0% ooding Start gas ow Set 100% ooding conditions Progressively reduce the gas ow rate until a value where zero ooding (packing base level) occurs Space below packing base must be covered with water to prevent gas escape via the liquid exit pipe In the case of unpacked column For 0% ooding, the height of the column at which the inlet of the feed gas stream is located should be considered as the base level For 100% ooding, the height of the column at which the inlet of the water solvent stream is located should be considered as the top level Ammonia Absorption Rev 08-04/15/2013 Page 8

9 4 THERMODYNAMIC PHASE-EQUILIBRIUM Some NH 3 from the gas phase (Nitrogen + NH 3 ) absorbs into the liquid phase (Water + NH 3 ) establishing a phase equilibrium after a suciently long time x mole fraction of NH 3 in the liquid phase at equilibrium y mole fraction of NH 3 in the gas phase at equilibrium The equilibrium mole-fraction (x ) of absorbed NH 3 is known as the solubility of NH 3 in water Gas Phase N 2 + NH 3 y P Liquid Phase W + NH 3 x * T Ammonia Absorption Rev 08-04/15/2013 Page 9

10 THERMODYNAMIC PHASE-EQUILIBRIUM The solubility of NH 3 in water is high at room temperature and 1 atm of pressure The solubility increases with pressure and decreases with temperature It is possible to derive a relationship relating x to y, but we are mostly interested in cases of low values of y (use of dilute mixture of NH 3 and nitrogen) Focus: dilute gas-phase regime Ammonia Absorption Rev 08-04/15/2013 Page 10

11 5 HENRY'S LAW At constant T and at equilibrium, the amount of solute gas that dissolves into the liquid (x ) is proportional to the partial pressure (yp ) of the solute gas in the gas phase: i.e, or where m = H (T ) P where P is the operating pressure of the column and yp = H(T )x (1) y = mx (2) H (298.15) = atm Valid only for dilute solutions and when the solute (NH 3 ) does not react with the solvent (W ) Temperature dependence is given by the van't Ho equation and for NH 3 /water solution H (T ) = H ( T ref) exp Resource: [ C C = 3670 K ( 1 T 1 T ref )] (3) (4) Ammonia Absorption Rev 08-04/15/2013 Page 11

12 6 GAS DENSITIES Standard Temperature and Pressure NIST-STP: National Institute of Standards and Technology T = 20 C = 68 F = K P = 1 atm = bar = psi IUIPAC-STP: International Union of Pure and Applied Chemistry T = 0 C = 32 = K P = atm = 1 bar = psi Gas rotameter manufactures usually use dierent standards. Refer the instrument manual for details. Density Models Density of dry air (model using the specic air constant) where P is the feed gas pressure and ρ Air = P R Specific Air T R Specific Air = J/ (kg K) = m 3 atm/ (kg K) Density of ammonia gas ρ NH3 MW NH 3 MW Air ρ Air = ρ Air = 0.587ρ Air (6) (5) Ammonia Absorption Rev 08-04/15/2013 Page 12

13 7 COLBURN'S NTU EQUATION 7.1 Number of transfer units for gas-phase controlled transfer and N OG = A 1 A ln 1 1 A mx in y out mx in y ( in 1 mx ) (7) in y out mx in y in where A = L in mv in y out solute mole fraction in the ranate [dimensionless] y in solute mole fraction in the feed [dimensionless] x in solute mole fraction in the solvent [dimensionless] A absorption factor [dimensionless] V in [ ( in/a cross feed molar supercial velocity lbmole/ min ft 2 L in = L [ ( in/a cross solvent molar supercial velocity lbmole/ min ft 2 m equilibrium constant for dilute solution [dimensionless] (8) Ammonia Absorption Rev 08-04/15/2013 Page 13

14 COLBURN'S NTU EQUATION 7.2 Height of a transfer unit for gas-phase controlled transfer h is the height of the packed bed [ft] H OG = h N OG (9) 7.3 Remarks on the NTU Equation (7) Only valid for dilute feed streams It is assumed that the solute mole fraction in the solvent is zero, i.e., x in = 0 Number of transfer units is expressed in terms of concentration in the gas phase Solubility of ammonia in water is high As a result, the dominant resistance to diusion (mass transfer) resides within the gas Ammonia Absorption Rev 08-04/15/2013 Page 14

15 8 OVERALL MASS TRANSFER COEFFICIENT Overall mass transfer coecient on a gas-phase basis V in = 1 H OG A cross K y a = V in (10) H OG Interfacial area per unit volume of packing, a, is normally left lumped with the mass transfer coecient (Denition: A interfacial = aa cross ) Mass-transfer resistance: inverse of the mass-transfer coecient Resistance = 1 K y a Correlation (Solve using least-squares regression) where c 4 = ln c 1 (11) K y a = c 1 L c 2 in V c 3 in = ln (K ya) = c 4 + c 2 ln L in + c 3 ln V in (12) See tutorial on the Excel function LINEST (Least-squares regression using LINEST in Excel) posted in the course web site. You can also use MATLAB or OCTAVE Verication of correlation Carry out at least one additional experimental run (K y a) Error = correlation (K y a) run 100% (13) (K y a) run Ammonia Absorption Rev 08-04/15/2013 Page 15

16 9 INTERPRETATION OF THE ABSORPTION FACTOR The Absorption Factor A is dened as the ratio of the local slope of the operating curve to that of the equilibrium curve Slope of operating curve A = Slope of equlibrium curve = (L in /V in ) = L in m mv in For the transfer of NH 3 from the gas phase (V ) to the liquid phase (L), the driving force y y should be positive, which implies the operating line should be above the equilibrium line. This is possible when A > 1 mx in y out mx in y in Hence, the absorption of NH 3 from the gas phase into the liquid phase occurs only when the above condition on A is met. Observations When A < 1 mx in y out mx in y in (desorption or stripping) When A = 1 mx in y out mx in y in mass transfer occurs from the liquid phase into the gas phase there is no net mass transfer between the gas and liquid phases Ammonia Absorption Rev 08-04/15/2013 Page 16

17 10 INTERPRETATION OF NTU AND HTU Number of transfer units (NTU) Depend on the value of y out desired for a given y in Measure of the diculty of separation If a high-level of absorption (separation) is desired, then a larger number of NTUs is needed Height of a transfer units (HTU) Depend on the mass transfer coecient and the gas ow rate Measure of the separation eectiveness of the packing for the species being absorbed HTU is proportional to the resistance to mass transfer HTU is small (lower resistance) when There is a high rate of interface mass transfer There is a large amount of interfacial area (better contact) H OG = 1 V in (14) K y a A cross Ammonia Absorption Rev 08-04/15/2013 Page 17

18 11 CHARACTERISTICS OF FLOW IN THE COLUMN 11.1 Hold-up time (also called residence time) t hold up 11.2 Number of hold-ups t hold up = V P ack Q W ater (15) hold-up time (residence time) (min) Q W ater water (solvent) ow rate (GPM) V P ack packed volume (gal) N hold up = N hold up number of hold-ups (dimensionless) t SS time to steady-state (min) t SS t hold up (16) Ammonia Absorption Rev 08-04/15/2013 Page 18

19 12 SCRUBBING EFFECTIVENESS Denition of Scrubbing Eectiveness Formula Derivation: Dene ɛ := Overall rate of NH 3absorption into the liquid solvent Rate of NH 3 entering via the feed stream and use the mass-balance result V out = 1 y in 1 y out V in : ɛ = y inv in y out V out y in V in = = y in (1 y out ) y out (1 y in ) y in (1 y out ) (17) Y = y out y in (18) ( ) 1 yin y in V in y out V in 1 y out y in V in = y in y out y in (1 y out ) = 1 y out y in 1 y in y out y in = 1 Y 1 y in Y Ammonia Absorption Rev 08-04/15/2013 Page 19

20 Calculation 1. From experimental data Information needed: y in and y out Procedure: Calculate Calculate Y exp = y out y in (19) ɛ exp = 1 Y exp 1 y in Y exp (20) 2. From NTU predictions Information needed: y in, m, V in, L in, and N OG Procedure Calculate A and nd the value of Y by solving (graphically or numerically) from 1 N OG = A 1 + (A 1) A 1 ln Y (21) A Calculate 3. Prediction error Calculate the prediction error P E P E = ɛ pred = 1 Y 1 y in Y ɛ pred ɛ exp (22) ɛ exp 100% (23) Ammonia Absorption Rev 08-04/15/2013 Page 20

21 13 THEORETICAL EXPECTATIONS The mass transfer should increase for larger L in /V in ratios The mass transfer should more strongly aected by the gas-feed ow rate (V in ) than by the solvent ow rate (L in ) Ammonia Absorption Rev 08-04/15/2013 Page 21

22 14 MEASUREMENT OF NH 3 GAS COMPOSITIONS BACHARACH Ammonia gas monitor: Model AGMSZ Measures ammonia gas in the range of 25 to 10, 000 ppm Detector Type: Single pass, nondispersive infrared Sensitivity: 25 ppm Accuracy: ±10 ppm ± 10% of reading from ppm Response Time: 9 to 30 seconds, depending on tube length and gas concentration Operating Temperature: 32 to 122 F (0 to 50 C) Operating Humidity: 5 to 90% RH, noncondensing Ammonia Absorption Rev 08-04/15/2013 Page 22

23 15 ROTAMETER: WATER FLOW MEASUREMENT 15.1 Liquid solvent rotameter (RTM): water l Dwyer Rate-Master Flowmeter: Coarse Fine Model RMC l2 rotameters (coarse and ne adjust- ments) l Measurement m Coarse: m Fine: units Gallons per Minute (GPM) Gallons per Hour (GPH)... Ammonia Absorption Rev 08-04/15/2013 Page 23

24 ROTAMETER: WATER FLOW MEASUREMENT 15.2 Measurements Reading: Q RT M,solvent (graduation mark on the scale) For ne rotameter Q solvent (GP H) = Q RT M,solvent (24) For coarse rotameter Q solvent (GP H) = 60 min 1 hr Q RT M,solvent (25) Mass ow rate Q solvent (lb/hr) = ρ solvent Q solvent (GP H) (26) Ammonia Absorption Rev 08-04/15/2013 Page 24

25 16 ROTAMETERS: GAS FLOW MEASUREMENTS 16.1 Gas feed-line rotameters (RTM): Coarse Fine (N H3 + N2) l Dwyer Rate-Master Flowmeter: Model RMB l2 rotameters (coarse and ne adjust- ments) l Measurement units: Standard Cubic Feet per Hour (SCFH)... Ammonia Absorption Rev 08-04/15/2013 Page 25

26 ROTAMETERS: GAS FLOW MEASUREMENTS 16.2 Measurements Reference (from the instrument manual) T ref = 70 F = C = K P ref = 1 atm = bar = psi Reading: R RT M (oat position on the scale) P feed T ref (K) Q feed (SCF H) = R RT M P ref T feed (K) (27) Q feed (lb/hr) = f SCF H CF H ρ feed Q feed (SCF H) (28) where the conversion factor f SCF H CF H is f SCF H CF H = P ref T feed (K) P feed T ref (K) (29) Ammonia Absorption Rev 08-04/15/2013 Page 26

27 17 EXPERIMENTAL DETAILS Because NH 3 is highly soluble in water, one must operate at low solvent-to-feed rations (i.e., low L/V) to prevent complete mass transfer to the liquid (dominant resistance to mass transfer is in the gas phase) Measure the NH 3 composition in the feed and ranate stream using the sensor. Take repeated measurements to obtain statistical averages. Report concentration values at steady state (take great care of ensuring steady state is attained) Measure the volumetric mass ow rates of the the feed and solvent streams using the rotameters and convert the readings to mass and molar ow rates [lbmol/hr]. Then calculate the corresponding uxes need in Colburn's equation by dividing by the cross sectional area of the column. Determine the ooding velocity of the feed stream for each solvent ow rate considered. Ammonia Absorption Rev 08-04/15/2013 Page 27

28 EXPERIMENTAL DETAILS Run the column at various values of the absorption coecient. Note also that at steady state the NH 3 composition in the extract stream is estimated from the following expression (obtained from a mass balance) x out = y inv in y out V out L out (30) where V out is obtained from yet another mass-balance calculation as V out = 1 y in 1 y out V in (31) Assumptions The NH 3 /N 2 mixture behaves as an ideal-gas mixture The solvent stream contains no absorbed NH 3 on inlet to the column The extract stream contains no absorbed nitrogen Ammonia Absorption Rev 08-04/15/2013 Page 28

29 EXPERIMENTAL DETAILS Example of a data record Consider recording your data in a table similar to the one shown below Run T Q RT M,feed Q RT M,solvent y in y out P Ammonia Absorption Rev 08-04/15/2013 Page 29

30 18 EXPERIMENTAL PROCEDURES 18.1 Start-up and normal-operation procedures 1. Plug the power cable of the NH 3 sensor into the outlet. 2. Open the valve for the water outlet (extract) line. 3. Open fully the feed-gas cylinder (N 2 + NH 3 ). Set the regulator pressure to the desired setting. (Note: the pressure should not exceed 40 psi) 4. Adjust the rotameters to allow the desired ow of feed gas ow into the column. 5. Open the water inlet valve and adjust the rotameters to obtain the desired solvent (water) ow into the column. DO NOT allow water into the column when the feed ow rate is zero, as water might enter into the feed gas line until it reaches and damages the NH 3 sensor. 6. Switch on the dierential pressure gauge to measure the pressure drop across the column. 7. Open the appropriate sensor gas-valves to measure the concentration of NH 3 in either the feed stream or the ranate stream. 8. During operation always maintain the water level at the bottom of the column below the feed-gas inlet to prevent feed gas escaping the column through the extract-stream opening. Ammonia Absorption Rev 08-04/15/2013 Page 30

31 EXPERIMENTAL PROCEDURES 18.2 Shut-down procedure 1. Turn o the solvent ow into the column by closing the water valve completely. 2. Close completely the valve of the feed-gas cylinder (N 2 + NH 3 ). Important note: DO NOT turn o the feed gas before turning o the water. 3. Wait for the feed-gas and water ow into the column to go to zero on the rotameter scales; then turn o the rotameters. (Closing the inlet valves of water and feed gas before turning o the rotameters helps to release the pressure in the inlet lines in shut-down mode) 4. Switch o the pressure gauge. 5. Unplug the Ammonia sensor from the power outlet. Ammonia Absorption Rev 08-04/15/2013 Page 31

32 19 ANTICIPATED EXPERIMENTAL PROBLEMS Incorrect start-up sequence (creates the wrong dispersed phase) Not waiting suciently for steady-state conditions Experiments may not have been carried out at isothermal conditions The feed gas may escape through the extract outlet when a small amount of water level is not maintained at the extract outlet Ammonia Absorption Rev 08-04/15/2013 Page 32

33 20 OBJECTIVES NOTE: Address ONLY the objectives identied by the instructor (ignore the rest) Objective 1 Characterize the ooding condition of the column at each liquid ow rate by determining the ooding gas ow rate. Plot the ooding gas ow rate as a function of (a) liquid ow rate, (b) the liquid-to-gas molar ow ratios, and (c) the absorption factor A Objective 2 Determine the hold-up time and the number of hold-up times needed to achieve steadystate as a function of absorption factor A. Objective 3 Characterize the dependence of NTUs and HTUs on the absorption factor A: (a) Plot the NTU and HTU results as a function of A, (b) Plot the natural logarithm of the NTU and HTU results as a function of A. Ammonia Absorption Rev 08-04/15/2013 Page 33

34 OBJECTIVES Objective 4 Characterize the mass transfer coecient Find a correlation for the mass transfer coecient and verify the correlation using additional experimental test Establish the dependence of the mass transfer coecient on the absorption factor A: (a) Plot the mass transfer coecient as a function of A, (b) Plot the natural logarithm of mass transfer coecient as a function of A. Superimpose on these plots the correlation curve Objective 5 Plot the the scrubbing eectiveness as a function of A as a function of the NTUs. Objective 6 Plot the ranate and the extract compositions as a function of A Ammonia Absorption Rev 08-04/15/2013 Page 34

35 REFERENCES [1] Hodgman, C. D., Weast R. C., and Selby, S. M., editors, CRC Handbook of Chemistry and Physics, 42nd edition. CRC Press, Cleveland Ohio, (1961). [2] Geankoplis, C. J., Transport Processes and Unit Operations, Third Edition. Prentice-Hall Inc., Englewood Clis, NJ (1990). (Chapter 10) [3] McCabe, W. L., J. C. Smith, and P. Harriet, Unit Operations of Chemical Engineering, Fifth Edition. McGraw-Hill, Inc., New York, NY (1993). (Chapter 22) [4] Foust, A. S., L. A. Wenzel, C. V. Clump. L. Maus. and L. B. Anderson, Principles of Unit Operations. John Wiley & Sons, New York, page 552. [5] Onda, K., Takeuchi, H., and Okumoto, Y, Mass transfer coecients between gas and liquid phases in packed columns, Journal of Chemical Engineering of Japan, Vol 1, pp (1968). [6] Treybal, R. E., Mass Transfer Operations, 2nd. ed., McGraw-Hill, New York (1968). Ammonia Absorption Rev 08-04/15/2013 Page 35