MASS TRANSFER CHARACTERISTICS OF A
ENTURI LIQUID-GAS CONTACTOR W. G. BAUER,' A. G. F R E D R I C K S O N , A N D H . M . T S U C H I Y A Chemical Engineering Department, Unioersity of Minnesota, .Minneapolis 14, M i n n .
Gas absorption b y water in a Venturi gas-liquid contactor was studied. The contactor produces a fine dispersion of gas bubbles in liquid by pumping the liquid through the Venturi and introducing the gas at the throat. The Venturi used was a Herschel meter type with '/s-inch i.d. inlet and exit tubes, a '/Ic-inch i.d. throat, and an included angle of 2 1 in the entrance cone and of 5' in the diffuser cone. A carbon dioxidewater system was chosen for study. The water flow rates ranged from 5.0 to 20.0 gallons per minute with introduction of 0 to 10,300 cc. per minute (0.364 cu. foot) of carbon dioxide saturated with water vapor, Radial and axial concentration profiles in the liquid yielding molar gas-liquid ratios of 0 to 1.23 X 1 O-4. phase were determined, as were axial pressure profiles in the Venturi. Effects of gas and liquid rates on absorption and over-all pressure drop are presented graphically. Water flow rate through the Venturi was found to be the most significant factor affecting absorption. Application to gas-liquid contacting in microbial propagators i s discussed.
i% AEROBIC
fermentations and stabilization of wastes effective
I transfer of oxygen from the gas phase to liquid phase is desired. so that the supply of dissolved gas will not limit the growth of the organism and decrease the yield of desirable products. I t is conventional practice in the fermentation and waste-treatment industry to sparge gas through the culture or waste in a n impeller-stirred fermentation vessel, or activated sludge plant. The gas sparger often consists of a ring or "spider" with numerous holes to form bubbles of a certain size. Certain features of this method of mass transfer might be improved with different types of absorption apparatus: reducing the power requirement for the compressors necessary to sparge the gas as well as the power requirement for impellers, where used ; increasing the absorption efficiency of the gas as it rises through the culture; and eliminating clogging of the sparger plate holes by growing cells. An experimental study of a different type of mass transfer device attempted to improve these undesirable features. The device is a simple meter-type Venturi through which liquid culture can be pumped. Introduction of gas a t the throat of the Venturi results in a mixture of finely dispersed bubbles in the liquid culture. Use of the Venturi contactor for a fermentation or in a wastetreatment process would have the following main advantages: Fouling of the gas introduction system is eliminated. Owing to the turbulence at the Venturi throat. the gas inlets cannot be clogged. Very fine dispersion of bubbles in the system increases the gas holdup and interfacial area, thus increasing mass transfer in all areas of the system. As a consequence, high absorption efficiency is attained, which substantially reduces the gas concentration in a single pass. Possible secondary advantages are: The circulation of the culture through a n external circuit obviates the necessity for impellers to keep the cells in suspension and to break up the gas bubbles. A compressor may not be necessary to introduce the gas, since the suction at the throat of the Venturi provides the driving force for introducing the gas. Additional mass transfer and dispersion should be realized in the pump. 1 Present address, James Ford Bell Research Center, General Mills, Inc., Minneapolis, Minn.
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I&EC PROCESS D E S I G N A N D D E V E L O P M E N T
Apparent disadvantages would be:
A low gas-liquid ratio is required for optimum performance, a t least with the apparatus studied. Possibly this difficulty could be overcome by employing multiple Venturis in series, but this would introduce higher pressure drops, create a need for compressors, and add to the over-all power requirement. An external circulation system might cause sterilization difficulties not associated with impeller systems. Some of these conjectures are substantiated by Jackson (5), who used various jet processes for the transfer of oxygen in a n air-water system. High absorption and transfer efficiencies were also observed. The transfer efficiency (pounds of oxygen per horsepower per minute) of the Venturi was shown to be superior to that of orifices or jets, making the Venturi contactor the most desirable of the jet devices. To establish the operating characteristics of the Venturi contactor a t various liquid and gas flow rates, ordinary tap water and pure carbon dioxide were used as the liquid-gas system. .4t each set of experimental conditions, samples of water were withdrawn a t several longitudinal and radial positions in the Venturi and analyzed for dissolved carbon dioxide, thus yielding information about the absorption process. Pressure profiles were also obtained at each set of operating conditions. The over-all absorption efficiencies and pressure drops were studied as functions of liquid and gas flow rates. The optimum operating conditions were obtained as a function of the gas-liquid ratio. Apparatus and Procedure
The investigation was conducted with a Herschel-type Venturi ( 7 ) bored out of a piece of 2 X 2 X 4 inch Plexiglas. A dimensioned drawing is shown in Figure 1. The transparent Plexiglas block permitted direct observation of the flow. A schematic diagram of the assembly is shown in Figure 2. Ordinary tap water continuously flowed into the 60-gallon holding tank. A 2-hp. LYorthington Monobloc centrifugal pump was used to pump \vater from the tank through the experimental system. The \vater flow rate was measured by a Fischer and Porter precision-bore Flowrator tube with a capacity of 0 to 30 gallons per minute of specific gravity 1.0 liquid. This Flowrator \vas calibrated over the experimental ran e of flow, and the actual flow rate was found to be within 0 . S k of the indicated flow rate. thus eliminating the necessity of a calibration curve. Upon leaving the Flowrator the \vater floived through the
To Manometer
t
To Manometer Pressure
Gas Inlet
Sampling
Figure 1 .
Schematic diagram
t
Manifold
Taps
Note: Samplipg Indicated by
Positions
of Venturi assembly showing location of sampling and pressure taps
Venturi, which was in a vertical position passed through a 2-foot section of i/8-inch i.d. Plexiglas pipe, then floMed into a drain through a long section of 1-inch vinyl tubing. A Taylor thermometer was located in the water stream immediately upstream from the Venturi. Carbon dioxide was obtained from Puritan cylinders of the pure gas. After passipg through a reducer valve on the cylinder, the carbon dioxide passed through a Flowrator into a humidifier to saturate the gas with water vapor and thus prevent the transfer of water vapor in the L-enturi. Full-View
Brooks rotameters were used to control the gas flow from 70 to 6500 cc. per minute. iZfter leaving the humidifier, the carbon dioxide stream was split into two equal parts with a glass Y and each line \vas attached to the Venturi block with copper nipples tapped into the Plexiglas block. Two holes, 0.0352-inch i.d. and 0.43-inch length, were drilled through the Plexiglas block on opposite sides to the center of the throat, to provide the jets for COZ introduction. Mercury manometers were located both upstream and down-
Water to Drain
TOP Water Inlet
Gas Humidifier
A
/
Venturi
Gas
Pressure Regulator
Overflow
J
I I I
Water Holding Tank
FIow ra t or
Downstream Manometer
I Figure 2.
'11 I'
iil Upstream Manometer
Gas Cylinder
i
Flow diagram of apparatus VOL. 2
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JULY 1963
179
stream from the rotameter. The carbon dioxide flow was controlled by needle valves on the cylinder regulator and the rotameter outlet. The upstream manometer was located between the two needle valves, yielding a reading which was used for a pressure correction on the carbon dioxide flow. The downstream manometer was located between the rotameter and the small jets drilled in the Venturi block. A glass centigrade thermometer was located in the gas stream immediately downstream from the rotameter. At every set of operating conditions, water samples Lvere withdrawn a t various longitudinal and radial positions for carbon dioxide determination. Six sampling taps were located on the Venturi: one upstream. and five downstream from the throat. 1 inch apart, starting 1 inch downstream from the throat. Two sampling taps were located on the section of Plexiglas pipe above the L'enturi, 16.7 and 28.8 inches downstream from the throat. Each sampling tap consisted of a '/g-inch hole drilled through the Venturi block with '/g-inch pipe threads tapped 3 / 8 inch into the block. T\vo pieces of rubber packing, 0.35 inch in diameter by ' t ! ~inch thick. were placed in the tap through which a 2-inch 22-gage (0.0160inch i.d., 0.0285-inch 0.d.) hypodermic needle was pierced. A packing nut was then screwed down, compressing the rubber and inducing a tight seal. The hypodermic needle could be moved to any radial position by loosening the packing nut. A 10-cc. Luer-lock syringe was attached to each hypodermic needle. The syringes were supported along the side of the Venturi by a piece of wood with properly spaced holes fitting the syringes. The syringe plungers were kept wet with bvater to maintain an air-tight seal. The hypodermic needles were removed from the moving liquid stream except when liquid samples were taken. Each hypodermic needle tip was beveled to 45O, and the bevel was faced upstream when a sample was withdrawn. This prevented gas bubbles accumulated in a slight wake downstream from the needle from entering the needle. The radial location of a needle was determined by measuring the distance from the tip of the needle to the right edge of the Venturi block with an accurate scale. Identical radial positions were used on all runs. Figure 1 shows the location of the sampling positions as well as the nomenclature used with the positions. Three radial positions were used a t taps 2 to 6, whereas only a center line sample was taken a t 1. 7 , and 8. Caution was exercised to avoid contaminating the samples with bubbles of CO,. By drawing the syringe plunger slowly and steadily. liquid was made to issue into the syringe, and generally no bubbles were observed. The rate of withdrawal was conducted to yield a linear velocity in the needle a t least five times slower than the linear velocity at that point in the Venturi. \Yith this criterion it is thought that the bubbles traveling a t about the same velocity as the water would not be draivn into the s1oLver-moving stream of the needle. Preliminary experiments using several different rates of withdrawal substantiated this theory. About 4 ml. of liquid was withdraivn for each sample. After the desired quantity of liquid was withdrawn into the syringe, the syringe was removed from the hypodermic needle and immediately transferred to the carbon dioxide electrode. which was used to determine the carbon dioxide content of the liquid. The carbon dioxide electrode was the Severinghaus ( 9 ) type manufactured by the National LVelding Equipment Co. Its readout instrument was the I. L. meter developed for this specific purpose by Instrumentation Laboratory. The temperature of the electrode was controlled with a water bath maintained a t 22.0' i 0.1' C. throughout the experiment. The electrode was calibrated with five different gases of known COS content having a p C O , in the range of the experimental values: 0.02035. 0.15. 1.38. 3.33, and 9.58%. The calibration gas was passed through a humidifier to saturate the gas with water vapor before it entered the cuvette of the electrode. The flow rate of gas was very small. only 2 to 3 bubbles per second. Thepco, could be determined from the follo\ving formula : $C02
(PBaro
- $CO,*)FCO?
where FcO,is the CO1 concentration fraction. A calibration curve was prepared by plotting p H us. log Pro,. A linear relationship is obtained throughout the range of 1.38 to 100% COz Below 1.38% the curve dropssharply, 180
l&EC
PROCESS DESIGN A N D DEVELOPMENT
as a 10-fold pco2 change is equivalent to approximately a 0.2 p H change. When a liquid sample was to be analyzed, the syringe was attached to a special female coupling on the electrode, and the liquid was sloivly forced through a catheter tube leading to the cuvette. Since the volume of the cuvette was only 0.2 ml., the cuvette could be flushed 20 times using only 4 ml. of liquid. Pressures in the Venturi \\.ere measured a t six locations by means of a manifold connected to a mercury manometer, shown schematically in Figure 1 . The pressure tap locations \\ere located exactly opposite the sampling tap locations at both upstream and dolvnstream positions on the Venturi. The taps consisted of 'js-inch holes drilled through the Plexiglas with *-inch pipe threads tapped 3 / g inch into the block. Needle valves were located at each tap and a '/.,-inch copper manifold \\as attached to the needle valves of the five downstream taps. The manifold was attached to one leg of the mercury manometer. Ivhile the upstream needle valve was
0.4 WATER R A T E :
A 0 I
0
I
X
10.0
O
5.0
81
00
10 15 20 DISTANCE D O W N S T R E A M , in
5
Figure 3.
20.0 gprn 15.0 18
GAS RATE:
(70°F, I a h . ) 1172 cc/rnin. 1173 1 8 1172 18 I I Y B '1 25
5,
30
Absorption profile at center line position for
1 179 f 19 cc. of CO2 per minute and various water rates
I
!
I-
2 e LL
I
I RADIAL
POSITION
I
r/R
e
L
n
a
w
8
0.8
v)
m
a
l
R A D I A L POSITION
, r/R
Figure 4. Radial absorption profilesfor 1 179 =t1 8 cc. of CO2 per minute and various water rates Inches a b o v e t h r o a t
0
2.0
A
3.0
Q 4.0
X 5.0
attached to the other manometer leg. Stopcocks and needle valves on the manifold enabled filling the manifold and pressure lines with water. By opening the upstream needle valve and one of the downstream needle valves, the differential pressure across the two locations could be determined. By closing all of the downm e a m needle valves and opening a valve exposing the manifold to the atmosphere, the static pressure at the upstream location could be determined. This static pressure reading was corrected for heights G f water above the manometer legs. Results and Discussion
The investigation was conducted primarily to obtain practical, operational information on the Venturi gas-liquid contactor rather than to obtain a wealth of experimental data on interfacial mass transport. However, many qualitative observations were made which are in agieement with theories on eddy diffusion and liquid film-controlled mass transfer, and these observations are discussed in detail. Since no detailed previous studies were discovered on the Venturi contactor, the initial investigation yielded basic quantitative information on its mass transfer characteristics. A simple gas-liquid system was used for rhe study with flow rates as the only variables. I n the light of this investigation, further studies are indicated to yield further information on the application of the Venturi contactor to a particular system. Calculation of Results. I n all runs the mass transfer data were reported as the absorption efficiency a t each sampling position. T h e absorption efficiency, or fraction of COz absorbed, was determined by the ratio of the actual partial pressure of C 0 2 exerted by the water as determined by the carbon dioxide electrode a t 22.0’ C. to the partial pressure of COz that the liquid would exert a t 22.0‘ C. if all the COz introduced were absorbed. This ratio is denoted asplpt. This was a valid procedure, since p i < p * a t all sets of gas-liquid conditions- that is, the water was capable of absorbing all of the COzwithout becoming saturated with respect to CO1. p t was calculated from the raw data by the dimensional equation:
p i = 153.495 L vL 1 / T‘ mP m . Hg This equation is based on Henry’s law, which is valid for the dilute solutions used. Q, is the gas Flowrator reading in standard liters per minute (air a t 70’ F., 1 atm.), L is the liquid Flowrator reading in gallons per minute, P is the Venturi pressure in millimeters of Hg, and T i s the ahsalute temperature in degrees Rankine. An analysis of the maximum probable error in $/fitshowed these values io be reliable within 5 to 10% a t all operating conditions except the runs a t very low gas rates. However, the error increased sharply at low gas flow rates, mainly because of increased difficulty in measuring the COz concentrations. Radial a n d Longitudinal Concentration Profiles. Typical longitudinal and radial concentration profiles (Figures 3 and 4) show the profiles for the indicated liquid rate at an approximately constant gas rate. The average gas rate for the runs presented was 1179 cc. per minute; the actual gas flow rates for the individual curves are indicated. The radial profiles were plotted as symmetrical across the Venturi tube. However, data points were determined on only one side of the center line as indicated on Figure 1. The radial profiles were found to be symmetrical within 0 to 6y0 by drawing samples a t symmetrical positions by temporary modification of the apparatus. T h e concentration profiles follow a general pattern at most gas and liquid conditions. No COBis present a t the center line below tap 2. (Actual analysis of inlet water a t tap 1 showed negligible COXcontent.) The inlet COr from the opposing jets in the throat is rapidly swept downstream, and the two COz streams do not “overlap” until a point beyond tap 2. This is clearly seen in the close-up photographs of Figures 5 to 8. After tap 2 is reached, the COSconcentration increases rapidly at the center line of the Venturi and at about the same rate for all gas-liquid conditions. However, the absorption rate decreases sharply downstream from the Venturi
4 Figure 5. Appearance of Venturi tube a t water rate of 5.0 gallons and CO? rate of 1 1 88 cc. per minute
b Figure 6. Appearance of Venturi tube a t water rate of 10.0 gallons and COZrate of 1 187 cc. uer minute
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4 Figure 7. Appearance of Venturi tube a t 'water rate of 15.0 gallons and COz rate of 1 196 CC. per minute
I b Figure 8. Appearance of Venturi tube at water rate of 20.0 gallons and COz rate of 5645 cc. per minute
as the longitudinal profiles become almost flat a t tap 8, 28.8 inches downstream from the throat. This indicates that most of the absorption occurs in the Venturi and that the mass transfer rate decreases in the straight section of tubing, because of the greater shear stresses and the turbulent conditions in the Venturi. Further substantiation of the rapid rate of transfer in the Venturi was derived from samples taken a t taps 2C and 2 0 . Samples were not available at these positions above gas rates of 250 cc. per minute because of the presence of large quantities of undissolved gas. When available, samples taken a t taps 2 8 and 2C indicated a very high pco,, which rapidly decreased farther downstream. This indicates that much of the absorption occurs in the throat immediately following the point of COZ introduction. As the liquid proceeds downstream, the dissolved COI becomes mixed throughout the bulk of the liauid. Awin the high shear stress present a t the throat " wall could account for this 1phenomenon. T1le axial profile for t h e ,,vater rate of 5.0 gallons per minute decr,zases immediately dovvnstream from the Venturi. This L. .~~-,.:~. .> L~~.~~~ ~~~. can bc S X ~ L ~ L I I Cuy U cxarrnning the radial profiles a t the water rate of 5.0 gallons per minute. The concentration drops sharply near the wall, and above tap 4 the center line values usually exhibit the maximum concentration of COS. However, as the liquid proceeds downstream, bulk mixing is promoted and a t tap 7,16.7 inches above the throat, the center line concentration decreases from the last Venturi tap. Farther downstream. as additional COS is absorbed, the center line concentration again in,TCaSCS. It is seen by examin;ition of the radial profiles that at water rates of 10.0, 15.0, an
