Objectives

The purpose of this experiment is to study the pressure drop and mass transfer characteristics required for design calculations of packed tower absorption of CO₂ from air into water. Two types of packing are used to compare performance.  Each must be analyzed separately during each part of the experiment.

Overall Operation (Revised 08/07)

A centrifugal pump takes water from the feed tank and delivers it to the system.  Pump operation is controlled by the feed pump switch.  Flow Rate is controlled by the water flow control valve located on the operator control panel.  Feed water flows continuously to the tank (bypass) or to the column dependant upon the position of the control valve.  This assures the pump operates at a constant pressure regardless of the column flow rate.

Turn the flow control to the right (CW) to feed water to the column.  Full left (CCW) is 100% bypass which gradually changes flow to the column as the valve is advanced to the right.  Observe the rotameter to establish the desired flow rate.

The water exiting the column flows into a surge tank.  Verify that the water recovery system is turned on anytime the feed pump is operating.  The “auto” setting of the switch will empty the surge tank as it fills and will shut off automatically.  The pump may be manually turned on by switching to “pump on”. Use caution if this mode is selected because the pump will run continuously until manually switched off.   Do not permit the pump to run dry as this will damage the seals within the pump.

Operational balance is maintained by adjustment of the various flow rates and control of the column water outlet valve.  This valve should be adjusted so the water level remains constant in the lower section of the column for the desired operational flow rates.  The suggested level is marked on the column.

The air to the bottom of the tower is provided by regulating building air to an acceptable system pressure. The air flow rate to the packed tower is controlled by the air flow control valve and rotameter.  Air flow is increased by turning the control to the left (CCW).  

Note that rotameter readings for air and carbon dioxide are based on gas volumes and read "actual" volume per time.  The flow figures for these gases must be corrected for composition, temperature and pressure to obtain values of "standard" gas volume. 

Pressure taps are located in the air stream at the top, middle and bottom of the tower.  Three differential pressure gauges are provided for measuring various operational pressures.  Measurement of unknown pressure values should initially be checked on the “high range” gauge to minimize slamming the more sensitive mid and low range gauges.   A meter zero momentary switch is provided for zeroing the meters prior to taking a reading.  Permit the pressure to stabilize after zeroing to insure the correct value is recorded.

Provisions for adding carbon dioxide to the column air consist of two high pressure supply cylinders, two electric pre-heaters, a parallel gas feed manifold, a regulator, a warming coil, the CO₂ control valve and, the rotameter.  Utilization of CO₂ requires both tank valves be open and the heater power applied a minimum of one minute prior to opening the CO₂ control valve.  These two heaters plus the warming coil assures the gas approximates ambient prior to entering the column air stream.

CO₂ flow is initiated by rotating the panel control valve to the left (CCW).  Once gas is flowing the manifold regulator pressure setting should be adjusted for a pressure of 40–50 lbs.  Adjustments to the CO₂ regulated pressure will affect the CO₂ flow rate and correction may be required to re-establish the intended value.

Proper operation of the system requires that a balance be established between the water flow rate, the air/CO₂ flow rates, and the column outlet drain rate.  Excessive flow rates and/or an inadequate outlet valve opening will lead to column overflow.  This is observed by water returning down the clear pipe coming from the top of the column. This should be avoided and when it does occur, flow rates and/or drain rate should be immediately changed to eliminate the condition.  Note that column overflow generally occurs at flow rate conditions above column “flooding”.  This is noted by increased and erratic pressures and slugs of gas traveling as bubbles up the column, rather than continuous gas flow with liquid trickling over the packing (see Perry’s for more information).

For mass transfer measurements, process water containing approximately 0.1 N soda ash flows counter current to the flow of the air-CO₂ mixture through the packed tower. Note that the CO₂ will react with the soda ash once the CO₂ dissolves in the water.

A Gas Chromatograph is used for analyzing CO₂ air composition from samples taken from the top, middle and bottom of the tower.  These samples are obtained from three water separator traps.  Individual trap lines route sample gases to a control panel sample valve which permits selection of the desired source.  The output of this valve feeds a small pump which assures adequate flow of the selected source to a sample port on the panel.  This pump is controlled by the sample pump switch on the control panel and should remain off when samples are not being collected.

It is important the sample pump be permitted to run a minimum of two minutes after selecting a sample source to insure the respective trap and lines are purged.  This insures the sample correctly represents the source conditions.  Empty all three water separator traps at the start of operations and again before beginning sample collections.  Drain these traps as required to minimize water accumulation.

Insure the sample valve is placed in the “closed” position when pressure readings are taken.  This eliminates pressure drops which may introduce error in the resulting pressure measurement.

Sample calibration may be made by calculating the inlet composition from the corrected air and CO₂ rotameter readings and verifying this figure with the GC.

Shut Down Procedure

Verify the following sequences have been completed when terminating system operation.

o  Turn the Water Flow control fully CCW to bypass.

      o  Switch off the Feed Pump.

      o  Switch off the Sample Pump.

      o  Switch off the CO₂  Heaters.

      o  Fully close (CW) the panel control valves for column air and CO₂ (force not necessary).

      o  Close both CO₂ cylinder valves.

o  Permit the column water to drain and the recycle pump to operate as may be required.  When draining is complete switch off the Water Recovery System.

o  Finally, verify all panel switches are off and no panel lights are lit.

The Henry's Law constant (atm/mole fraction) for CO₂ in water is given by the following equation:

where P_CO2 is the partial pressure (atm.) of CO₂ in the air and x_CO2 is the mole fraction of CO₂ in water.
At 0° C, H_CO2 = 728; at 10° C, H_CO2 = 1040; at 20° C, H_CO2 = 1420;
at 30° C, H_CO2 =1860; at 40° C, H_CO2 = 2330. Assume that this system obey's Henry's Law under the experimental conditions.