GAS ABSORPTION COLUMN UOP7

Transcription

1 INSTRUCTION MANUAL UOP7 GAS ABSORPTION COLUMN UOP7 ISSUE 19 DECEMBER 2009

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3 ARMFIELD LIMITED PRODUCT AND TEACHING MANUAL FOR UOP7 - GAS ABSORPTION COLUMN PAGE NO. PRODUCT MANUAL SAFETY IN THE USE OF EQUIPMENT SUPPLIED BY ARMFIELD 1 INTRODUCTION 7 RECEIPT OF EQUIPMENT 8 DESCRIPTION 9 INSTALLATION REQUIREMENTS 12 ASSEMBLY 14 CONNECTION TO SERVICES 16 COMMISSIONING 18 ROUTINE MAINTENANCE 20 TEACHING MANUAL INDEX TO TEACHING EXERCISES 21 NOMENCLATURE i

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5 SAFETY IN THE USE OF EQUIPMENT SUPPLIED BY ARMFIELD Before proceeding to install, commission or operate the equipment described in this instruction manual we wish to alert you to potential hazards so that they may be avoided. Although designed for safe operation, any laboratory equipment may involve processes or procedures which are potentially hazardous. The major potential hazards associated with this particular equipment are listed below. INJURY FROM ELECTRIC SHOCK Mains electrical supply to water pump and air compressor INJURY FROM INCORRECT HANDLING The equipment is heavy and must be handled properly when positioning. Bottles of compressed gas (not supplied by Armfield) are heavy and must be handled / stored correctly INJURY FROM MISUSE Glass components are delicate and can break if mistreated INJURY OR DAMAGE TO CLOTHING FROM CORROSIVE FLUIDS Dilute sodium hydroxide is used for gas analysis and is circulated through the column for one of the teaching exercises RISK OF INFECTION DUE TO LACK OF CLEANLINESS Water will stagnate if left in the sump tank or column. Water must be changed at regular intervals Accidents can be avoided provided that equipment is regularly maintained and staff and students are made aware of potential hazards. Armfield Ltd. suggests that a comprehensive list of Laboratory Safety Precautions and Rules be made available to all laboratory users. Local laws or regulations related to laboratory practice or laboratory safety must be incorporated in these safety rules. Please refer to the notes overleaf regarding the Control of Substances Hazardous to Health Regulations. 1

6 The COSHH Regulations The Control of Substances Hazardous to Health Regulations (1988) The COSHH regulations impose a duty on employers to protect employees and others from substances used at work which may be hazardous to health. The regulations require you to make an assessment of all operations which are liable to expose any person to hazardous solids, liquids, dusts, vapours, gases or micro-organisms. You are also required to introduce suitable procedures for handling these substances and keep appropriate records. Since the equipment supplied by Armfield Limited may involve the use of substances which can be hazardous (for example, cleaning fluids used for maintenance or chemicals used for particular demonstrations) it is essential that the laboratory supervisor or some other person in authority is responsible for implementing the COSHH regulations. Parts of the above regulations are to ensure that the relevant Health and Safety Data Sheets are available for all hazardous substances used in the laboratory. Any person using a hazardous substance must be informed of the following: Physical data about the substance Any hazard from fire or explosion Any hazard to health Appropriate First Aid treatment Any hazard from reaction with other substances How to clean/dispose of spillage Appropriate protective measures Appropriate storage and handling Although these regulations may not be applicable in your country, it is strongly recommended that a similar approach be adopted for the protection of the students operating the equipment. Local regulations must also be considered. Water-Borne Infections The equipment described in this instruction manual involves the use of water which under certain conditions can create a health hazard due to infection by harmful microorganisms. For example, the microscopic bacterium called Legionella pneumophila will feed on any scale, rust, algae or sludge in water and will breed rapidly if the temperature of water is between 20 and 45 C. Any water containing this bacterium which is sprayed or splashed creating air-borne droplets can produce a form of pneumonia called Legionnaires Disease which is potentially fatal. Legionella is not the only harmful micro-organism which can infect water, but it serves as a useful example of the need for cleanliness. Under the COSHH regulations, the following precautions must be observed:- 2

7 Any water contained within the product must not be allowed to stagnate, ie. the water must be changed regularly. Any rust, sludge, scale or algae on which micro-organisms can feed must be removed regularly, ie. the equipment must be cleaned regularly. Where practicable the water should be maintained at a temperature below 20 C or above 45 C. If this is not practicable then the water should be disinfected if it is safe and appropriate to do so. Note that other hazards may exist in the handling of biocides used to disinfect the water. A scheme should be prepared for preventing or controlling the risk incorporating all of the actions listed above. Further details on preventing infection are contained in the publication The Control of Legionellosis including Legionnaires Disease - Health and Safety Series booklet HS (G) 70. 3

8 USE OF A RESIDUAL CURRENT DEVICE FOR ELECTRICAL SAFETY The equipment described in this instruction manual operates from a mains electrical supply. The equipment is designed and manufactured by Armfield Ltd in accordance with appropriate regulations relating to the safe use of electricity. It is assumed that regulations applying to the safe operation of electrical equipment in a laboratory are observed by the end user. However, to minimise the possibility of injury to an operator in the event of a failure or an accident, Armfield Ltd recommends that a Residual Current Device (RCD, previously called an Earth Leakage Circuit Breaker or ELCB) is incorporated in the electrical supply to all mains operated equipment. If through misuse or accident equipment becomes electrically dangerous, an RCD will switch off the electrical supply and reduce the severity of any electric shock received by an operator to a level which, under normal circumstances, will not cause injury to that person. Armfield Ltd has supplied an RCD and protective cover (packed separately) with the equipment described in this instruction manual. If the electrical supply to the laboratory already incorporates an RCD then the device supplied by Armfield Ltd need not be fitted. If the electrical supply does not incorporate such protection then a competent electrician should install the RCD supplied by Armfield Ltd, either in the supply to the laboratory or in the supply to the individual item of equipment. The drawing overleaf (Armfield ref. BM20491) gives full installation instructions. Note: If any doubt exists as to whether the electrical supply incorporates a protection device then the RCD supplied by Armfield Ltd should be installed in the supply to the equipment. At least once each month, check that the RCD is operating correctly by pressing the TEST button. The RCD MUST trip when the button is pressed. Failure to trip means that the RCD is not operating correctly and a competent electrician must check it before the equipment is used. If the RCD trips in normal use and trips again when reset then a fault is likely to exist on the equipment and must checked by a competent electrician before the equipment is used. 4

9 Live (Brown) Neutral (Blue) Earth (Green/Yellow) Clip Live Neutral Earth To equipment Off position 1. Ensure equipm ent is d isconnected from electrical supply 2. Locate suitable position for RCD on or adjacent to equipment 3. Remove cover from m ounting base 4. Position base, mark off, d rill through 2 holes 5. Fix base at location using suitable screw s 6. Fix RCD to base 7. Remove 130mm of outer sheath of cable in line with RCD 8. Cut live and neu tral, connect to RCD. Do not cut earth. 9. Fit cover to base and RCD 10. Reconnect main pow er supply 11. Switch on RCD Remove 130mm outer sheath, cut Live (brown) & Neutral (blue) NOT EARTH (green/yellow) from BM20491 Fitting of Residual Current Circuit Device to Armfield Equipment 5

10 UOP7 GAS ABSORPTION COLUMN 6

11 INTRODUCTION The packed tower, in which two fluids flowing in opposite directions enable a chemical component to be transferred from one fluid phase to the other, occurs in almost all chemical plants. The process may be gas absorption, distillation, solvent extraction or chemical reaction. Knowledge of the characteristics of both fluid flow and of mass transfer in such towers is necessary for both plant operators and designers. The Armfield Gas Absorption Column has been designed to allow these studies to be made, and the instrumentation and layout enables students to follow both the hydrodynamic characteristics in the absence of mass transfer, and also, separately, to advise the performance of the mass transfer process involved in gas absorption. The size of the equipment has been chosen so that teaching exercises may be completed in a typical laboratory class period, while at the same time being capable of demonstrating full-scale plant behaviour. Considerable attention is directed towards matters of safety that are of crucial importance in the process industries. This instruction manual gives details of all the laboratory services required for operation of the column, together with commissioning procedures and instructions for the many teaching exercises possible with the equipment. 7

12 RECEIPT OF EQUIPMENT 1. SALES IN THE UNITED KINGDOM The apparatus should be carefully unpacked and the components checked against the Advice Note. A copy of the Advice Note is supplied with this instruction manual for reference. Any omissions or breakages should be notified to Armfield Ltd within three days of receipt. 2. SALES OVERSEAS The apparatus should be carefully unpacked and the components checked against the Advice Note. A copy of the Advice Note is supplied with this instruction manual for reference. Any omissions or breakages should be notified immediately to the Insurance Agent stated on the Insurance Certificate if the goods were insured by Armfield Ltd. Your own insurers should be notified immediately if insurance was arranged by yourselves. 8

13 DESCRIPTION All numerical references in brackets relate to the diagram on page 11. Details of the liquid overflow arrangement and air pipework are shown in the diagram on page 12. The packed absorption column is made of two 80 mm inside diameter clear acrylic sections joined end to end to give a total column length of 1.4 m and is installed vertically on a mild steel floor standing framework. The column is filled with 10 mm x 10 mm glass Rashig rings (11) which is representative of the type of packing used in gas absorption. The liquid for the process is normally water and is stored in a 50 litre rectangular sump tank (1). A valve (2) on the underside of the tank allows it to be drained for cleaning. A float valve on the side of the tank supplies fresh water as required. A centrifugal pump (15) is used to deliver the liquid to the top of the column (9) where it falls by gravity through the packing before returning to the sump tank via a 'U' trap that forms a liquid seal. A control valve C 4 downstream of the 'U' trap allows the head of water in the column to be varied. This valve is usually fully open when the column is in use but can be partially closed to restrict the flow of water returning back to the sump tank. Water can be drained from the 'U' trap after use by opening the drain cock (16) at the bottom. A variable area flowmeter F 1 gives a direct reading of the liquid flow rate entering the top of the column. The flow of liquid can be varied using the control valve C 1 above the liquid flowmeter. The gas to be absorbed is normally carbon dioxide and would be taken from a cylinder of compressed gas (not supplied by Armfield), located in a suitable storage rack adjacent to the column. The gas is connected to a low pressure regulator that is located at the rear of the vertical backboard. The gas passes through a calibrated variable area flow meter F 3 and is mixed with an air stream, also via a calibrated variable area flowmeter F 2 from a rotary compressor (14). The flow of air can be varied using the control valve C 2 above the air flowmeter. The ratio of gas to air in the mixture entering the column is therefore known and easily varied. Entering the base of the column, the gas mixture rises up through the packed bed of Raschig rings (11) and is counter-currently contacted with the liquid flowing down the column. Gas is prevented from escaping at the base of the column by the liquid in the 'U' trap. Tappings are provided at the base (13 with isolating valve), centre (12 with isolating valve) and top (8) of the column to allow indication of pressure drop in the two sections of column using a pair of 'U' tube manometers (3). These tappings also provide a means of extracting samples of gas from the column for analysis using the sampling syringe (7). The carbon dioxide content of the gas samples is determined using a Hempl apparatus (5) and readings are indicated on the scale (6). A sampling point (17) below the column allows analysis of the liquid outlet stream. A sampling pipette S 5 can be used to withdraw a sample of liquid from the sump tank for analysis. The flow meters, manometers and gas analysis equipment are all mounted on a vertical backboard at a convenient height for operation. The effluent gas leaves the top of the column via a cap with an overflow arrangement (19 & 20). This arrangement ensures that any liquid is safely returned to the sump tank if the column becomes flooded. When using a solution of caustic soda (sodium hydroxide) in the column it is suggested that a suitable container is located below the 9

14 overflow to intercept any liquid that spills from the top of the column. For safe operation the flooding characteristics of the column should be evaluated using clean tap water. Caustic soda should not be used for this series of tests. USE OF ALTERNATIVE GASES IN THE ABSORPTION COLUMN Other gases may be used as an alternative to carbon dioxide but Armfield Ltd. does not recommend this unless safe operating/disposal procedures are adopted. Where a gas is harmful to the operator an extraction system must be used to exhaust the effluent gas outside the laboratory building to a safe location. The equipment is not flameproof so flammable gases must not be used. If the gas outlet on the equipment is modified to allow connection to an extraction system then there must be no back pressure on the outlet connection (A restriction at the outlet may result in water entering and damaging the rotary air compressor). The Hemple apparatus is designed for use with carbon dioxide. If a different gas is used in the absorption column an alternative technique will be required for analysis of the particular gas / air mixture. 10

15 C C1 F1 F2 3 4 F3 13 S5 C

16 Diagram showing details of the overflow arrangement and air pipework

17 INSTALLATION REQUIREMENTS This apparatus is classified as Education and Training Equipment under the Electromagnetic Compatibility (Amendment) Regulations Use of the apparatus outside the classroom, laboratory or similar such place invalidates conformity with the protection requirements of the Electromagnetic Compatibility Directive (89/336/EEC) and could lead to prosecution. FACILITIES REQUIRED The equipment is designed for floor standing in a static location and requires a firm level floor. A waterproof floor and a floor drain are very desirable to simplify the drainage and re-filling operation between teaching exercises. Installation may be completed using a basic tool kit. The equipment requires connection to a single phase, fused electrical supply. A 4m length of supply cable is fitted to the equipment. The equipment requires a mains water supply of 10 L/min at 1 bar Overall dimensions of the equipment are as follows:- HEIGHT m WIDTH m DEPTH m 13

18 ASSEMBLY Refer to the diagrams on pages 11 and 12 when assembling the UOP7. The top half of the clear plastic column and its metal support bracket (10) will have been removed and packaged separately for shipping. Do not re-assemble the top half of the column until the bottom half has been filled with Raschig rings as described below. Attach the support bracket to the frame using two fixings supplied (60 mm long bolts with nuts and washers). Remove the two rigid air pipes from the clips at the bottom of the support bracket and slide the pipes upward to clear the clips. Clip the short length of rigid plastic air pipe (supplied loose) into the left-hand clip (looking from the rear) on the support bracket and connect the bottom of the pipe to the side connection at the base of the column by tightening the coupling (21). Clip the short pipe on the end of the flexible tube from the top of the flowmeter to the right hand clip on the support bracket. Slide the two rigid plastic air pipes down the support bracket and tightening the unions (18). Close the water outlet flow valve C 4 above the 'U' trap and ensure that the drain cock (16) at the bottom of the 'U' trap and all of the small sampling cocks (8, 12, 13 & 17) are closed. Pour water into the lower half of the column to about three-quarters of its height. Pour approximately half of the Raschig rings into the water filled column (to prevent damage to the rings) until the rings are 100 mm from the top of the column. Fit the 'O'-ring into its groove in the bottom flange of the upper column (Note: Apply a smear of grease to the O ring to ensure it remains in position during assembly). Place the plastic re-distributor on the top flange of the lower column with the serrations facing downwards then place the Raschig ring support (metal mesh that supports the Raschig rings in the top section of column) on top of the re-distributor. Now fit the upper half of the clear plastic column to the lower half. Ensure that the redistributor and Raschig ring support are located centrally in the recess. Bolt the two sections of the column to the lug on the metal support frame using three fixings supplied (70 mm long bolts with nuts and washers). Fit the 'O' ring into its groove on the top flange. Place the cap on top of the column with overflow tube (20) at the side of the frame then bolt the two flanges to the lug on the metal support frame using three fixings supplied (60 mm long bolts with nuts and washers). Pour water into to the upper column section through the air outlet (on the cap) to about three-quarters of its height then pour the rest of the Raschig rings into the column until the rings are 100 mm from the top flange. The water may now be drained out of the column by opening valve C 4 and allowing the water to enter the sump tank. Connect the flexible water supply tube (9) and small bore gas sample tube (8) to the appropriate inlets on the cap (8) at the top of the column. 14

19 Connect the lower half of the overflow arrangement (19) to the upper overflow pipe (20) by tightening the union. The lower section of the overflow will attach to the side of the main frame using the clips provided. When correctly assembled the end of the overflow should terminate just below the clip on the side of the frame. Locate the sampling pipette S 5 in the hole provided in the lid of the sump tank. Water leaving the column passes through a rigid U-tube section with a control valve; to the end of this pipe two different fittings can be attached. One fitting leads water to drain via a flexible pipe and the second returns water to the sump tank. Fit the drain pipe. The equipment is ready for commissioning. 15

20 CONNECTION TO SERVICES ELECTRICAL SUPPLY FOR VERSION UOP7-A: The equipment requires connection to a single phase, fused electrical supply. The standard electrical supply for this equipment is 220/240V, 50Hz. Check that the voltage and frequency of the electrical supply agree with the label attached to the supply cable on the equipment. Connection should be made to the supply cable as follows: - GREEN/YELLOW - EARTH BROWN - LIVE (HOT) BLUE - NEUTRAL Fuse Rating - 7 AMP ELECTRICAL SUPPLY FOR VERSION UOP7-B: The equipment requires connection to a single phase, fused electrical supply. The standard electrical supply for this equipment is 120V, 60Hz. Check that the voltage and frequency of the electrical supply agree with the label attached to the supply cable on the equipment. Connection should be made to the supply cable as follows: - GREEN/YELLOW - EARTH BROWN - LIVE (HOT) BLUE - NEUTRAL Fuse Rating - 13 AMP ELECTRICAL SUPPLY FOR VERSION UOP7-G: The equipment requires connection to a single phase, fused electrical supply. The standard electrical supply for this equipment is 220/240V, 60Hz. Check that the voltage and frequency of the electrical supply agree with the label attached to the supply cable on the equipment. Connection should be made to the supply cable as follows: - GREEN/YELLOW - EARTH BROWN - LIVE (HOT) BLUE - NEUTRAL Fuse Rating - 7 AMP SUPPLY OF COMPRESSED GAS A cylinder of carbon dioxide fitted with a suitable pressure regulator (cylinder and regulator not supplied by Armfield) should be connected to the inlet on the low pressure regulator at the rear of UOP7 using a length of suitable hose. The cylinder must be installed in a suitable rack for safe operation. Note: It is important that the cylinder is fitted with its own pressure regulator and that excessive pressure is not fed into the equipment. The small pre-set 16

21 regulator fitted to the apparatus is used as a final control of the gas pressure and will be damaged if full cylinder pressure is applied. If other gases are used as an alternative to carbon dioxide then appropriate extraction facilities must be provided for health and safety reasons. Refer to the notes in the Description section of this manual for further information. WATER The sump tank will require filling with clean cold tap water (maximum capacity 40 litres) for commissioning and all teaching exercises. Some teaching exercises require the use of a 0.2M solution of caustic soda (sodium hydroxide). This solution is created by adding 1M caustic soda to clean tap water in the sump tank. Details are given in the appropriate teaching exercise. A permanent supply of water (at least 10 L/min at 1 bar) should be attached to the float valve on the side of the sump tank. 17

22 COMMISSIONING All numerical references in brackets relate to the diagrams on pages 11 and 12. Clean tap water is used for safe commissioning of the equipment. 1. Fill the sump tank (1) with clean cold water by turning on the water supply attached to the float valve. 2. Connect the electrical supply cable to the appropriate mains supply. 3. Prime the water manometers (3) with clean water until the meniscus in both tubes is at mid height (250 mm on the scale). 4. Fill the Hempl gas analysis apparatus (5) with demineralised water up to the '0' mark on the scale (6). 5. Connect a CO 2 gas cylinder, fitted with a regulator, to the inlet regulator on the equipment with a flexible tube, and set the cylinder regulator to minimum pressure. 6. Open fully the gas flow control valve C 3 that is incorporated on the gas flowmeter F 3 (small, centre flowmeter) and open the main gas cylinder valve. 7. Increase the cylinder regulator output pressure to give maximum flow reading on the gas flowmeter then immediately close the main cylinder valve to avoid wasting gas (the remainder of the commissioning will be carried out using air and water). Close the gas flow control valve C 3 on F Close the air C 2 and water C 1 flow control valves. Close the gas sampling cocks (8, 12 and 13) on the absorption column. Check that the flow control valve C 4 in the discharge pipe from the 'U' trap into the sump tank is fully open. 9. Switch on the air compressor (14) using the appropriate mains switch (4). Check that the relief valve on the compressor operates - audible click with air escaping. Gradually open the air flow control valve C 2 and check that air is delivered to the base of the column. Open the air flow control valve fully and check that a maximum flowrate of 180 litres/min can be obtained on flowmeter F 2. Adjust C 2 to give a reading of 80 litres/min on F Switch on the water pump (15) using the appropriate mains switch (4). Gradually open the liquid flow control valve C 1 and check that water is delivered to the top of the column. Adjust C 1 to give a reading of 4 litres/min on F 1. Adjust valve C 4 if necessary to maintain a liquid seal in the 'U' trap (close the valve until air does not escape via the 'U' trap). 11. Open the sample cocks at the base (13), middle (12) and top (8) of the column. Check that the two water manometers (3) indicate the pressure drop across the column (the connecting valves above the manometers must be correctly set. Refer to the diagram on page G-1 for details). 12. Check that the Hempl gas analysis apparatus (5) operates correctly by following the instructions in exercise A. 13. Close the air flow control valve C 2 and switch off the air compressor. Open valve C 4 fully at the top of the 'U' trap. Open the liquid flow control valve C 1 fully and confirm that a maximum flowrate of 10 litres/min can be obtained on 18

23 flowmeter F 1. Switch off the water pump and close the liquid flow control valve C Drain the Hempl apparatus. 15. Drain the water seal by opening the small drain cock (16) at the bottom of the 'U' trap beneath the column. The equipment is now ready for use. 19

24 ROUTINE MAINTENANCE To preserve the life and efficient operation of the equipment it is important that the equipment is properly maintained. Regular servicing/maintenance of the equipment is the responsibility of the end user and must be performed by qualified personnel who understand the operation of the equipment. Little maintenance is required on this equipment apart from keeping it clean generally. If the equipment is likely to stand idle for any length of time then the column, the 'U' seal and the sump tank should be drained completely and the tank wiped dry. In the event of accidental misuse, water may get into the gas sampling tubing between the absorption column and the Hempl apparatus. To avoid incorrect results when performing gas analysis blow air through these flexible tubes to dry them if they have become wet. The air system is designed to prevent water from entering the rotary air compressor in normal operation. If water is accidentally allowed to enter the compressor via the inlet of the outlet connections then the compressor must be switched off immediately and dried by removing the coverplate to reveal the rotor and blades. Instructions are given in the Service Instructions supplied with the compressor. Failure to take immediate action can result in expensive repairs because of damage to the motor or corrosion of the compressor. 20

25 INDEX TO TEACHING EXERCISES EXERCISE A A-1 To measure the absorption of carbon dioxide into water flowing down the packed column, using the gas analysis equipment provided. EXERCISE B B-1 To calculate rate of absorption of carbon dioxide into water from analysis of liquid the solutions flowing down the packed column. EXERCISE C C-1 To calculate rate of absorption of carbon dioxide into caustic soda solutions from analysis of liquid solutions flowing down the packed column. EXERCISE D D-1 To show that the amount of carbon dioxide removed from the air stream equals the amount taken up by a liquid stream of caustic soda solution. EXERCISE E E-1 Determination of Overall Mass Transfer Coeffcient (K og ). EXERCISE F F-1 To determine the air pressure differential across the dry column as a function of the air flow rate. EXERCISE G G-1 To examine the air pressure differential across the column as a function of air flow rate for different water flow rates down the column. 21

26 NOMENCLATURE a = Effective interfacial area per unit packed column (cm -1 ) A = Cross sectional area of the tower (m 2 ) C c C d C N F G K og L M P R T V B V 1 V 2 X Y = Concentration of sodium hydroxide (g.moles/litre) = Concentration of dissolved 'free carbon dioxide' (g.moles/litre) = Concentration of sodium carbonate ions (g.moles/litre) = Flow (litres/second) = Gas flow rate (g.moles/second) = Gas-side mass transfer coefficient (g.moles/second.cm 2.atm) = Liquid flow (litre/second) = Molecular weight = Pressure (atm) = Mass transfer rate (g.moles/cm 2.second) = Standard acid titration volume (ml) = Volume of alkali solution added in liquid analysis (ml) = Volume of gas sample taken in Hempl apparatus (ml) = Corresponds to amount of gas absorbed in Hempl apparatus (ml) = Mole fraction of component in liquid phase = Mole fraction of component in gas phase Subscripts T i o N = Total = Inlet conditions to column = Outlet conditions to column = Rate of absorption (g.moles/second) i

27 UOP7 GAS ABSORPTION COLUMN - MASS TRANSFER EXERCISES EXERCISE A OBJECT OF EXERCISE: To measure the absorption of carbon dioxide into water flowing down the tower, using the gas analysis equipment provided. Gas outlet To gas analysis equipment V1 V 2 V 3 Closed Closed F 1 F 3 F 2 C 1 From pressure regulator and carbon dioxide cylinder C 3 C 2 R Compressor Tank Pump Obtaining a Gas Sample from the Top of the Column Gas outlet To gas analysis equipment V1 V 2 V 3 Open Closed F 1 F 3 F 2 C 1 From pressure regulator and carbon dioxide cylinder C 3 C 2 R Compressor Tank Pump Obtaining a Gas Sample from the Middle of the Column A-1

28 UOP7 GAS ABSORPTION COLUMN - MASS TRANSFER EXERCISES EQUIPMENT REQUIRED: 1. Carbon dioxide cylinder with integral pressure regulator, connected to regulator R on gas inlet on the apparatus. 2. Approximately 300ml of 1.0 molar caustic soda solution with safety gloves and goggles. Small funnel and tubing for filling analysis equipment. PROCEDURE: 1. First fill the two globes of the absorption analysis equipment on the left of the panel with 1.0 Molar caustic soda. Wear gloves and goggles while doing this. Adjust the level in the globes to the '0' mark on the sight tube, using drain valve C v into a flask to do this. (See step A in sketch overleaf).equipment SET-UP: 2. Turn on the mains water supply to the sump tank. Fit the drain pipe to the water return from the column so that water is not returned to the sump tank. 3. With gas flow control valves C 2 and C 3 closed, start the liquid pump and adjust the water flow through the column to approximately 4 litres/minute on flowmeter F 1 by adjusting control valve C Start the compressor and adjust control valve C 2 to give an airflow of approximately 25 litres/minute in flowmeter F Carefully open the pressure regulating valve on the carbon dioxide cylinder, and adjust valve C 3 to give a value on the flowmeter F 3 approximately one half of the air flow F 2. Ensure the liquid seal at the base of the absorption column is maintained by, if necessary, adjustment of control valve C After 5 minutes or so of steady operation, take samples of gas from the top and middle sample points. (Refer to the appropriate diagram for details of the valve positions.) Analyse these consecutively for carbon dioxide content in these gas samples as shown in the accompanying sketch and following notes. Diagrams relating to Steps A to F can be found on page A Flush the sample lines by repeated sucking from the line, using the gas piston and expelling the contents of the cylinder to atmosphere. Note that the volume of the cylinder is about 100 cc. Three full volumes of the gas piston is sufficient. (Steps B and C). 8. With the absorption globe isolated and the vent to atmosphere closed, fill the cylinder from the selected line by drawing the piston out slowly (Step B). Note volume taken into cylinder V 1, which should be approximately 20ml for this particular exercise (see WARNING note below). Wait at least two minutes to allow the gas to come to the temperature of the cylinder. 9. Isolate the cylinder from the column and the absorption globe and vent the cylinder to atmospheric pressure. Close after about 10 seconds (Step D). A-2

29 UOP7 GAS ABSORPTION COLUMN - MASS TRANSFER EXERCISES 10. Connect cylinder to absorption globe. The liquid level should not change. If it does change, briefly open to atmosphere again. 11. Wait until the level in the indicator tube is on zero showing that the pressure in the cylinder is atmospheric. 12. Slowly close the piston to empty the cylinder into the absorption globe. Slowly draw the piston out again (Steps E and F). Note the level in the indicator tube. Repeat steps E and F until no significant change in level occurs. Read the indicator tube marking = V. This represents the volume of the gas sampled. WARNING: If the concentration of CO 2 in the gas sampled is greater than 8%, it is possible to suck liquid into the cylinder. This will ruin your exercise and takes time to correct. Under these circumstances, do not pull the piston out to the end of its travel. Stop it at a particular mark, eg. V 1 = 20 on the coarse scale, and read the fine scale. 13. Once the final value has been read from the scale, slowly close the piston to empty the cylinder into the absorption globe. Reposition the valves as shown in step G to allow the globe to equilibrate with atmosphere. WARNING: It is important to follow the above steps closely in order to take representative gas samples and to achieve reproducible results. Deviation from the above can lead to erroneous results. A-3

30 UOP7 GAS ABSORPTION COLUMN - MASS TRANSFER EXERCISES A B C TO ATMOSPHERE 100 ml MAX SAMPLE VOL V1 FINE SCALE READS 8ml FROM SAMPLE POINTS ON COLUMN CONNECT TO MANOMETERS, SHUT DURING GAS ANALYSIS FROM SELECTED SAMPLE POINT 0 MARK CONNECT TEMPORARY TUBING AND FUNNEL (NOT SUPPLIED) REPEAT B AND C TO REMOVE UNREPRESENTATIVE GAS FROM THE SAMPLE LINES. THEN TAKE TRUE SAMPLE BY FURTHER REPEATING STEP B TO SELECTED VOLUME V 1 DO NOT LET LIQUID OVERSPILL V 2 VOLUME FRACTION OF ANALYSED GAS = V V 2 1 D E F REPEAT E AND F SLOWLY UNTIL V2 IS CONSTANT A. Fill with standard reagent to 0 on the scale. B. Taking gas sample C. Discharging to atmosphere D. Venting to atmosphere (balancing pressures) E. Charging the Hempl apparatus F. Withdrawing gas sample and taking reading V 2 G. Resetting scale prior to next sample G A-4

31 UOP7 GAS ABSORPTION COLUMN - MASS TRANSFER EXERCISES READINGS AND CALCULATIONS: A) CO 2 content of gas samples: From use of Hempl apparatus, volume fraction of CO 2 = V V 2 1 For ideal gases, volume fraction = mole fraction = Y. Check that the sample taken from the inlet to the absorption column should give the same value of CO 2 fraction as that indicated by the inlet flowmeters. ie. V V 2 1 = Y 1 F3 = F + F 2 3 F 1 (CO 2 ) litres/s READINGS AT INLET F 2 V 1 (air) litres/s ml V 2 ml CALCULATIONS F3 V 2 Y F2 + F = i 3 V 1 i From flowmeters From Hempl apparatus and sample point S 3 B) Calculation of amount of CO 2 absorbed in column from analysis of samples at inlet and outlet. From analysis with Hempl apparatus, volume fraction of CO 2 in gas stream at inlet V2 = = Yi V 1 i and at outlet, Y 0 = V2 V 1 0 If F a is litres/second of CO 2 absorbed between top and bottom, then:- [F 2 + F 3 ] Y i - [F2 + (F3 - F a )] Y o = F a CO 2 in CO 2 out CO 2 absorbed ( Yi Yo )( F2 + F3 ) ( Yi Yo ) = 1 Y ( 1 Y ) F = (total gas inlet flow) a o o A-5

32 UOP7 GAS ABSORPTION COLUMN - MASS TRANSFER EXERCISES Air F 2 CO 2 F 3 INLET CONDITIONS GAS SAMPLE TOTAL F 2 F 3 V2 Yi = V 1 i OUTLET GAS SAMPLE Y 0 V2 = V 1 0 ABSORBED CO 2 : F a litres/sec Note: Litres/second can be converted to g.moles/second as follows:- G a = av. column pressure mmhg av.column temp F a C absorbed CO 2 g.moles/second The assumption implicitly made here is that the volume flow is not affected by the pressure drop through the column as this drop should be small in comparison with atmospheric pressure. In Exercise D, a correct solution to this problem is made. A-6

33 UOP7 GAS ABSORPTION COLUMN - MASS TRANSFER EXERCISES EXERCISE B OBJECT OF EXERCISE: To calculate rate of absorption of carbon dioxide into water from analysis of liquid solutions flowing down absorption column. EQUIPMENT SET-UP: Gas outlet F 1 C 1 F 3 F 2 C 4 From pressure regulator and carbon dioxide cylinder R C 3 C 2 S 5 S4 Compressor Tank Pump PROCEDURE: 1. Turn on the mains water supply to the sump tank and allow the tank to fill. Ensure that the water outlet from the column is fitted with the drain pipe and not the pipe that returns water to the tank. 2. With gas flow control valves C 2 and C 3 closed, start the liquid pump and adjust the water flow through the column to approx. 6 litres/minute on flowmeter F 1 by adjusting flow control valve C Start the compressor and adjust control valve C 2 to give an air flow of approx. 10% of full scale on flowmeter F 2. B-1

34 UOP7 GAS ABSORPTION COLUMN - MASS TRANSFER EXERCISES 4. Carefully open the pressure regulating valve on the carbon dioxide cylinder, and adjust valve C 3 to give a value on the flowmeter F 3 approx. one half of the air flow F 2 Ensure the liquid seal at the base of the absorption column is maintained by, if necessary, adjustment of control valve C After 5 minutes of steady operation, take samples at 10 minute intervals from the mains water inlet and from S 4. Take 150ml samples at known times in each case. Analyse the samples according to the procedure detailed below. Analysis of Carbon Dioxide Dissolved in Water Note: Water used for absorption should be deionised as presence of dissolved salts affect the analysis described below. If tap water is used, no metal ions should be present in greater quantities than 1.0 mg/litre and ph should be just alkaline: 7.1 to 7.8. Chemical Solutions Needed a) Phenolphthalein indicator prepared from carbon dioxide-free distilled water ie. A. R. grade. b) Standard M sodium hydroxide solution, prepared by diluting 27.70ml 1M caustic soda standard solution to 1 litre with carbon dioxide free distilled water. Prepare daily and protect from carbon dioxide in the atmosphere by keeping in a stoppered Pyrex bottle. c) Standard 0.01M sodium bicarbonate solution, prepared by dissolving approximately 0.1 gram of anhydrous sodium bicarbonate in carbon dioxide free distilled water to 100ml. Procedure 1. Take a water sample from the mains water inlet, approximate volume of 150ml, or from liquid outflow point S Discharge the sample at the base of a 100ml graduated cylinder, flicking the cylinder to throw off excess liquid above the 100ml mark. 3. Add 5-10 drops of phenolphthalein indicator solution a) above; if the sample turns red immediately, no free CO 2 is present. If the sample remains colourless, titrate with standard alkali solution b) above. Stir gently with a glass rod until a definite pink colour persists for about 30 seconds. This colour change is the end point - note volume V B of alkali solution added. For best results, use a colour comparison standard, prepared by adding the identical volume of phenolphthalein solution a) to 100ml of sodium bicarbonate solution c) in a similar graduated cylinder. B-2

35 UOP7 GAS ABSORPTION COLUMN - MASS TRANSFER EXERCISES Calculation The amount of free CO 2 in the water sample is calculated from: g.mole/litre of free CO 2 = VB ml. of sample = C d Note: Solubility of CO 2 in water is a strong function of temperature. The accuracy of this titrimetric method is approximately ±10%. READINGS TO BE TAKEN: F 1 : TIME FROM START (minutes) litres/second FROM MAINS INLET (Correspond to conditions at top of tower) V B ml C d in tank [C di ] g mole/litre FROM LIQUID OUTLET SAMPLE POINT S 4 V B ml C d in tank [C d0 ] g mole/litre CALCULATIONS: A. CO 2 absorbed across the column at any particular time:- Inlet flow of dissolved CO 2 = F 1.C di g.mole/second Outlet flow of dissolved CO 2 = F 1.C di g.mole/second ie. absorption rate = F 1 [C di - C do ] g.mole/second B-3

36 UOP7 GAS ABSORPTION COLUMN - MASS TRANSFER EXERCISES EXERCISE C OBJECT OF EXERCISE: To calculate rate of absorption of carbon dioxide into caustic soda solutions from analysis of liquid solutions flowing down absorption column. EQUIPMENT SET-UP: Gas outlet F 1 F 3 F 2 C 1 C 4 From pressure regulator and carbon dioxide cylinder R C 3 C 2 S 5 S4 Compressor Tank Pump PROCEDURE: 1. Fill the sump tank at the base of the column to approximately three-quarters full with 0.2M caustic soda solution. The preparation of this requires great care and GLOVES AND GOGGLES MUST BE WORN. Use standard 1M caustic soda solution from normal laboratory supplies and add with care to the sump tank, eg. first add 30 litres of tap water to the sump tank, followed by successive additions of 1M caustic soda until a total volume of 7.5 litres of 1M caustic soda has been added, giving 37.5 litres of 0.2M. 2. With gas flow control valves C 2 and C 3 closed, start the liquid pump and adjust the flow of caustic soda through the column to approximately 3 litres/min on flowmeter F 1 by adjusting control valve C 1. C-1

37 UOP7 GAS ABSORPTION COLUMN - MASS TRANSFER EXERCISES 3. Start the compressor and adjust valve C 2 to give an air flow of approximately 30 litres/minute on flowmeter F 2. CAUTION: As the solution used in this exercise is caustic it is essential that the column is not allowed to flood. The air supply should be turned off immediately if any liquid is present in the clear tube above the top of the column. 4. Carefully open the pressure regulating valve on the carbon dioxide cylinder and adjust valve C 3 to give a flow of 3 litres/minute on flowmeter F 3. Ensure the liquid seal at the base of the absorption column is maintained by, if necessary, adjustment of control valve C After 15 minutes of steady operation, take samples at 20 minute intervals simultaneously from S 4 and S 5. Take 250ml samples at known times in each case. Analyse both samples according to the procedure detailed overleaf. Analysis of Carbon Dioxide Dissolved in Caustic Soda/Sodium Carbonate Solutions Theory: The absorption of carbon dioxide from mixtures with air into caustic solutions is characterised by the overall reaction (for most conditions) as: CO + 2 NaOH Na 2 CO 3 + H 2 O Under the conditions chosen for the absorption exercises, the amount of CO removed from the air stream can be estimated from the amounts of NaOH and NA 2 CO 3 in the liquid samples, as virtually no 'free' CO 2 will remain unreacted in the liquid. In using titration techniques of analysis, acid is first used to neutralise the caustic soda and at the same time converts all sodium carbonate to bicarbonate. Continuation of the titration with acid then neutralises all bicarbonate. The total concentration of carbonate can thus be determined, and hence the amount of CO 2 absorbed deduced. Solutions Needed: 1. Phenolphthalein indicator prepared from carbon dioxide-free distilled water. 2. Methyl orange indicator similarly prepared. 3. One litre of standardised 0.20M hydrochloric acid. 4. One litre of 5% by weight of barium chloride solution. C-2

38 UOP7 GAS ABSORPTION COLUMN - MASS TRANSFER EXERCISES Procedure: a) Take a 250ml sample of liquid from the absorption column liquid outlet or sump tank (as directed in the exercise) into a conical flask. Pipette two 50ml portions of this into two separate conical flasks. b) Flask 1 - Add a drop of phenolphthalein solution 1) to the contents and titrate until the pink colour just disappears with the standard hydrochloric acid 3) Note the volume of acid added - T 1, which is that needed to neutralise all hydroxide and convert carbonate to bicarbonate. Then add a drop of methyl orange 2) to the flask and continue to titrate with standard acid 3) until the end point is found. Note the total acid added to this second end point - T 2, which represents neutralisation of all bicarbonate as (T 2 - T 1 ). c) Flask 2 - Add about 10% more than the value of (T 2 - T 1 ) of the barium chloride solution 4), to the flask contents and shake well. This precipitates out all the original carbonate in the sample as barium carbonate. Now add two drops of phenolphthalein solution 1) and titrate against the standard acid solution 3) to the end point. Note the volume of acid added - T 2, which represents that needed to neutralise only the original caustic soda. (T 2 - T 3 ) represents the difference between total acid required for carbonate and hydroxide, and that required for hydroxide alone. Overall: NA 2 CO HCl 2NaCl + H 2 O + CO 2 It is advisable to repeat all of the above to check reproductibility. Calculations of Sample Composition a) Concentration of NaOH in original sample: T C c = 3 50 x 0.2M (gram-moles/litre) b) Concentration of NACO in original sample: C N = ( T T ) x 0.2M x 0.5 c) Amount of CO 2 removed from air mixture: During a time interval, or between the top and bottom of the column, C N will increase as CO 2 is absorbed in equi-molar proportions, while C c should decrease in twice-molar proportions. C-3

39 UOP7 GAS ABSORPTION COLUMN - MASS TRANSFER EXERCISES READINGS: Volume of solution in system litres (V T ) Flow of solution (F 1 ) : litres/second Time from start (mins) ANALYSIS OF LIQUID SAMPLES From sump tank (ie. column inlet) S 5 From liquid outlet S 4 T 1 ml T 2 ml T 3 ml C C C N T 1 T 2 T 3 C C C N Calculations: The amount of CO 2 absorbed across the column as measured from samples taken simultaneously from the sump tank feeding the column top and at the bottom outlet, is given by:- CO 2 absorbed = liquid flow rate x [(C N ) o - (C N ) i ] (gram-moles/sec) (litres/sec) (gram-moles/litres) = Liquid flow rate x 1 [(C 2 c ) i - (C c ) o ] Similarly, over a time period θ seconds after a first sample is taken from the sump S 5 :- CO 2 absorbed = Volume of liquid in whole system x [( ) ( C ) ] C N t= θ N t= O C-4

40 UOP7 GAS ABSORPTION COLUMN - MASS TRANSFER EXERCISES EXERCISE D OBJECT OF EXERCISE: To show that the amount of carbon dioxide removed from the air stream equals the amount taken up by a liquid stream of caustic soda solution. SUMMARY OF THEORY G o L i At steady state, the transfer of absorbing gas from the gas stream should equal that transferred to the liquid. S 1 Let: L 1 and L o be the volume flow of liquid entering and leaving the column respectively. Let: G i and G o be the total gas molal flow entering and leaving the column respectively. Let: Y i and Y o be the mole fractions of CO 2 entering and leaving the column in the gas stream. Amount of CO 2 removed from the gas stream:- G i - G o (gm.moles/sec)... (1) as no air dissolved in the solution. S 3 But from exercise B, the amount of CO 2 removed from the liquid stream equals the amount of carbonate ions produced:- S 4 L o.c No - L i.c Ni (gm.moles/sec).. (2) G i L o The object is to check that (1) equals (2). Note that, while the liquid flows in and out are the same in this exercise (L o = L i ), the gas flows are not equal because of the removal of CO 2, and also because of the pressure drop across the column. G o can be calculated from a molar balance on the air stream, none of which is absorbed: G o (1 - Y o ) = G i (1 - Y i ) G can be calculated from the fact that one gram mole occupies litres at 273K and 760mm Hg pressure: F2 + F column pressure drop G = And Y i and Y o are estimated by sampling as in Exercise A. D column temp K

41 UOP7 GAS ABSORPTION COLUMN - MASS TRANSFER EXERCISES PROCEDURE: The same procedure should be followed as for Exercise C, except that gas samples at the inlet and outlet, as described in Exercise A, are to be taken as well. As the liquid composition is slowly changing as progressive absorption of CO 2 takes place, the steady state can only be approximated by taking samples as close to each other in time as possible. Thus, after 5 minutes of circulating liquid and gases at the pre-set rates, take 1. a gas outlet sample, to give Y o, 2. a liquid outlet sample from the point S 4 as soon after 1) as possible, 3. a liquid sample from the sump tank at S 5 at the same time, 4. inlet gas sample, which should be constant and therefore can be taken last. READINGS TO BE TAKEN: QUANTITY UNITS SYMBOL COMMENTS Air flow rate litres/min From flowmeter air flow rate litres/sec F 2 (Divided by 60) CO 2 flow rate litres/min From flowmeter CO 2 flow rate litres/sec F 3 Caustic soda flow rate litres/min From flowmeter Caustic soda flow rate litres/sec L i = L o CO 2 outlet concentration vol. fraction Y o From Hempl analysis CO 2 inlet concentration vol. fraction Y i From Hempl analysis (should equal F 2 /F 3 +F 2 ) Liquid Samples: outlet Titration with HCl m T 1 (o) From Exercise C Procedure Titration with HCl ml T 2 (o) Titration with HCl ml T 3 (o) Liquid Samples: inlet = sump Titration with HCl ml T 1 (i) From Exercise C Procedure Titration with HCl ml T 2 (i) Titration with HCl ml T 3 (i) Barometric pressure mm H P Assumed equal to outlet pressure Column pressure drop mm H 2 O From manometer Pressure at base of column mm Hg P P Temperature of feed gas C θ i Temperature of feed gas K θ i SAMPLE CALCULATIONS: D-2

42 UOP7 GAS ABSORPTION COLUMN - MASS TRANSFER EXERCISES Readings: Comments: F 2 = 0.40 litres/second F 3 = litres/second Flowmeter readings divided by 60 L = litres/second Y o = Y i = = F and Y = = 0. F + F T 1 (o) = 33.2 ml T 2 (o) = 50.9 ml T 3 (o) = 35.0 ml T 1 (i) = 48.4 ml T 2 (i) = 47.2 ml T 3 (i) = 45.0 ml From Hempl apparatus readings (Use for Y i ) As in exercise C, 0.20M HCl used in titrations of 50ml sample. As in exercise C, 0.20M HCl used in titrations of 50ml sample. P = 759 mm Hg Lab. Barometer P = 100 mm H 2 O 100 = mm Hg 13.6 θi = 19 C 292 K Calculations: 1) Gas Flows: Gi (g.moles/second of gas mixture entering column) = = g.moles/se cond and G o = i ( 1 Yi ) ( 1 Y ) G = = g.moles/se cond Amount of CO 2 removed = g.moles/second.. (1) D-3