1. Wear safety glasses at all times. Know the safety shower and eye wash station locations.

2. Wear jeans or slacks, a long-sleeved shirt, and sturdy shoes that give good traction on possibly wet floors.

3. Guard against electrical hazards by making sure that all equipment is well grounded using three-wire plugs and other means.

4. Handle with great care any solvents or other potentially volatile, flammable, toxic, or otherwise dangerous chemicals. In this experiment air, water, and a solution of potassium chloride are used. Potassium chloride is toxic if ingested in large amounts.

5. Guard against falls, burns, cuts, and other physical hazards. Use heavy gloves to open or close hot steam or condensate valves.

6. THINK FIRST OF SAFETY IN ANY ACTION YOU TAKE. If not certain, ask the TA or faculty member before you act.

The packed bed absorber used in this experiment may be operated in four modes, of which you will use the first two:

(a) The pressure drop across the column is determined as a function of air and water flow rates.

(b) The response to impulse injection of an inert tracer is used to determine the water holdup on the packing.

(c) The column is operated at the steady state as an ammonia absorber, and the height of a theoretical plate is determined as a function of air and water flow rates.

(d) The column is operated under feedback control, and the transient response of the controlled system is determined as a function of the PID controller parameters.

Packed columns are commonly employed to carry out absorption and scrubbing operations. They provide a medium in which counter-current two-phase flow occurs, and in which a relatively large gas-fluid interface per unit volume of column volume exists. Because of the nature of the packing material (often a ceramic) a packed column can operate using strongly corrosive fluids. Packed columns are often more economical to build and operate than their plate or bubble-cap column counterparts, although pressure drops can be high, which requires larger gas blowers with high energy consumption.

The packed bed absorber in this experiment is to be used eventually to remove ammonia from a waste air stream. Before running ammonia, however, it is necessary to determine what water and air flow rates can be accommodated, to measure the pressure drop across the bed so that the air blower can be specified, and to test a noninvasive method for measuring the amount of water held up on the packing, so that incipient flooding can be detected. Your report to the design group should document your measurement methods and results, and provide the design group with data on which a good design can be based.

The packed column (see the schematic diagram below) consists of a 4" ID by 24" long section of borosilicate pipe sitting on a 4" by 1 1/2" borosilicate ell. Stainless steel top, bottom and side plates are sealed with Teflon or neoprene gaskets and provided with air, water, sample and pressure tap connections. About 24 inches of ceramic 3/8" Raschig ring packing is supported above the air entry point. (Note: The 36" packing height is better when pressure drop measurements are made, but the packing height may be reduced to 24" for the ammonia absorption runs, in order not to remove too much ammonia from the air stream.) The water effluent line is 1/2" stainless tubing with Swagelock fittings that permit the level of the exit line, and thus the level of the water in the column bottom, to be adjusted manually.

A syringe is used to inject potassium or sodium chloride solution at the top or bottom of the column. Typically 10 ml of 10 g/L KCl tracer is injected. The tracer concentration is measured by a conductivity cell located in the water effluent line, the cell being connected to a YSI conductivity meter. The meter can drive a strip chart recorder, but usually a computer-based data acquisition system is used. From the response to impulse tracer injection the volume of water between the injection point and measurement point may be calculated.

Regulators are mounted in water and air supply lines in order to reduce the effect of supply pressure variations on the flow rates. If the supply pressures are relatively constant these regulators may be removed.

A computer-driven valve may be used to control the water flow rate to the column. When not in use this valve should be bypassed by opening the bypass ball valve and using the valve on the rotameter to set the water flow rate.

Inspection

The apparatus should be examined carefully. Check that the effluent water line from the column is over the drain and that, if ammonia is to be used, the effluent air enters the laboratory chemical hood system. The ball valves on the tracer injection lines should be closed. The air pressure regulator should read 20 to 40 psig. Note that the air rotameter operates at essentially atmospheric pressure, regardless of the regulator setting, since the air valve is upstream of the rotameter.

With the air flow off, set the water flow rate to a low value using the water rotameter (read the center of the ball), and record the time needed to collect 250 ml of effluent water. Adjust the water exit line level if necessary to produce 5 to 10 cm of water in the bottom of the column. Then increase the water flow rate in about 6 increments over the range available (say 20, 40, 60, 80, 100, 120, and 140 mm), and measure the water flow rate (ml/min) at each setting. The water rotameter is now calibrated.

With the no water flow but with a water seal in the bottom of the column, set the air flow rate at eight or ten evenly spaced points (2.5, 3.0, .....5.5, 6.0, 6.5, 7.0) on the air rotameter, with a maximum rate typically 7.0 or 7.5 scfm. At each steady state the pressure drop across the dry packing should be measured using the water manometer. After the final high pressure drop has been recorded, the process should be reversed and data should be taken decreasing air flow rates over the same range. From these data a dry column pressure drop vs. air flow rate plot can be constructed.

Pressure Drop as a Function of Air and Water Rates

With the water flow at one of say 6 well-spaced rates, set the air flow rate at about 6 evenly spaced points on the air rotameter (e.g. 2, 3, 4, 5, 6, and 7 scfm), with a maximum rate typically 7 scfm. At each steady state the pressure drop across the packing should be measured using the water manometer. After the final high pressure drop has been recorded, the process should be reversed and data should be taken at decreasing air flow rates over the same range. From these data a pressure drop vs. air flow rate plot can be constructed. Note that the time needed to reach a new steady state is only a few seconds, except possibly when the air and water flow rates are high and water does not drain freely from the packing.

Please report your results in the following units: length = ft, volume = ft³, time = seconds, mass = pounds mass (lbm) and mass flux = lbm/(ft² s).

At a given steady state, the liquid holdup can be measured by an indicator dilution technique. Using a syringe, 10 ml of 10 g/L KCl is injected rapidly into either the top or bottom injection port. (Note that tracer concentration may be varied to give the best response, and that NaCl or NH₄Cl may also be used as tracers.) The conductivity of the water exiting from the column is measured using the YSI conductivity meter and recorded as a function of time using a strip chart recorder or computer-based data acquisition system.

Typically the computer used with the experiment is a 486 or Pentium IBM-compatible machine. We assume that subdirectory QB45 contains a recent version of Microsoft QUICKBASIC, and that program PKDTRC02.BAS (with working version TTT.BAS) is in QB45. After booting the computer, at the DOS prompt type CD \QB45 to connect to QB45, and then type QB TTT. This loads the program. Now hit ALT-R to run the program, and follow the instructions issued by the program. The program will collect baseline data, and then call for the injection of tracer. The program will calculate the mean residence time for each run, and will then calculate the volume of water between the injection and measurement point if the user supplies the volumetric water flow rate in ml/min.

From the data on tracer concentration as a function of time the mean residence time can be determined, as discussed in section H below. Since the water flow rate is known, the water volume between the injection and measurement points can be found. Then the holdup on the packing is found by difference. Note that these calculations are done by the program, and can also be done by loading the data written to a file by the program into a spreadsheet, e.g. EXCEL.

Several tracer injection runs should be made, most at higher air and water flow rates that produce significant hold up. Typically, at a high water flow rate, about four different air flow rates would be used, the last corresponding to incipient flooding, i.e. to a large hold up of water on the packing. During these runs the water exit line should be adjusted to give minimal water level in the column bottom, say 2 to 4 cm. But be sure that no air leaves the water effluent line, i.e. that the water seal at the bottom of the column is not lost. A lost seal can be recovered by turning down the air flow rate for a short time and allowing the water discharge line to fill.

You can verify, at least roughly, the holdup determined as above by shutting off the water flow until a steady state is reached, measuring the increase in water level in the column bottom, and collecting the water that drains from the column following water shut off.

1. The KCl tracer is typically 10 g/L, and 10 ml is injected. If the conductivity cell is linear, the results are independent of the tracer concentration.

2. Read the center of the ball on the water rotameter.

3. If the liquid seal at the bottom of the column is lost and air exits via the bottoms line, reduce the air flow rate briefly until a seal is reestablished. The exit end of the bottoms line may also have to be raised.

Show the calibration curve for the water rotameter.

Plot the pressure drop (converted from mm of water to inches of H₂0) as a function of air flow rate (SCFM) on a log-log plot, with water flow rate as a parameter. Include the dry-packing data. Note that, if pressure drop varies as the square of air flow rate, a straight line with a slope of 2 should be obtained. Departure (in the direction of higher pressure drop) from the curve defined by the low air flow points may indicate flooding. Leva (see reference below, page 28, Figure 10) provides data for a similar column. Compare your results also with the correlation in Perry's Handbook.

Present and discuss the results of the indicator dilution measurements. Provide a complete sample calculation for at least one run, including plots of tracer concentration vs. time for top and bottom injections. Use a run in which the liquid holdup was relatively large. Does the water holdup on the packing increase significantly as the flooding point is approached?

Identify areas of greatest experimental uncertainty, and the

discuss the degree to which they could alter your data and the conclusions derived from those data.

Discuss how such tracer techniques could be scaled up from the laboratory level to an industrial level.

1. Leva, Max, Tower Packings and Packed Tower Design, 2nd Edition, U.S. Stoneware Company, Akron Ohio (1953).

2. Perry, J. H., Editor, Chemical Engineers Handbook, 5th

Edition, McGraw Hill Book Company, pp 21-19 (1973).

3. McCabe and Smith, Unit Operations of Chemical Engineering

3rd Edition, pp 709-715 (1976).

Without deriving the theory, we consider an impulse injection of inert tracer to a flow system (well or poorly mixed) with volumetric flow rate Q and volume V. We can say, first, that all the tracer injected must leave eventually. Second, all the fluid in the system at time zero must eventually be washed out and replaced by new fluid. These statements can be shown to correspond to the equations

Q = D/INT (0-->inf):C(t)dt

(The notation INT(0 -->inf):C(t)dt connotes the integral over time from zero to infinity of C(t).)

and

V/Q = INT(0 -->inf): tC(t)dt / INT(0 -->inf): C(t)dt

where C(t) is the concentration of tracer in the effluent following the injection of amount D of tracer at the system inlet at time t = 0. Also, V/Q is the mean residence time of fluid in the system. Note that C may be replaced in the second equation by a multiple of C, say kC, without affecting the mean residence time V/Q. From V/Q, if Q is known, we can find V. For evenly spaced effluent concentration data we can use numerical integration, say Simpson's Rule, to find V/Q, as shown in the following example, where W denotes the Simpson's Rule weights (1 4 2 4 .... 2 4 1).

Then, using Simpson's Rule [(Delta t/3)(1 4 2 4 2 4 .... 2 4 1)], with Delta t = 2,

Int(0 --> inf): C(t)dt = (2/3) 38 = 25.333

and

V/Q = Int(0 --> inf): tC(t)dt / Int(0 --> inf): C(t)dt = [(2/3) 128]/25.333 = 3.368

If Q = 200 ml/min, the system volume V is 200*3.368 = 673.6 ml. Such calculations can be done easily using a spreadsheet if t and C(t) are given at evenly spaced times and W_k = 3 - (-1)^k except for the first and last points, i.e. for k = 1, 2, 3, .... Alternately trapezoidal rule numerical integration [(Delta t/2)( 1 2 2 2 ... 2 1)] can be used, with a small loss in accuracy.

In Perry's Handbook (Fifth Edition, Figure 18-39) we have a plot in which the abscissa X is

X = (L/G)(rho_g/rho_l)^1/2

and the ordinate Y is

Y = G²F_pPsi mu^0.2/rho_grho_lg

Here L is the liquid mass flux (lbm/ft² s) based on the area of the empty column, G is the gas mass flux (lbm/ft² s), Psi is the ratio of the density of water to the liquid density, F_p is a packing factor found in Perry, Table 18-5 (ft^-1), rho_g is the gas density (lbm/ft³), rho_l is the liquid mass density (lbm/ft³), mu is the liquid viscosity (centipoises), and g is the acceleration of gravity (32.2 ft/s²). Note that Y is not dimensionless.

As an example, consider an air flow rate of 6.0 ft³/min at STP, a water flow rate of 1200 g/s, a column ID of 4 inches. Then the column area is 0.0872 ft²,

G = ((6 29)/(359 60))(1/0.0872) = 0.0926 lbm/ft² s

L = 1200/(453.6 60 0.0872) = 0.506 lbm/ft² s

mu = 1.0 centipoise

F_p = 1000 (1/ft)

Psi= 1

rhol = 62.3 lbm/ft³

rhog = (29/359)(273.2/298.2) = 0.0740 lbm/ft³

and

X = (0.506/0.0926)(0.0740/62.3)^1/2 = 0.188

Y = (0.0926² 1000 1^0.2)/(0.074 62.3 32.2) = 0.0578

Based on Figure 18-39, the gas phase pressure drop per foot of packing is about 1.40 inches of water. If the packing height is 24 inches, the calculated pressure drop is 2.80 inches of H₂O, equivalent to 71.1 mm of water.