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APPARATUS FOR THE LABORATORY STUDY OF GAS ABSORPTION IN PACKED TOWERS HOWARD 1. STRAUSS The Cooper Union, New York, New York
INSPITE of its tremendous industrial importance, very few satisfactory laboratory studies of ga.; absorption in packed towers have been developed for educational purposes (1, 2). Experiments dealing with mass transfer coefficients, H.T.U.'s, and equilibrium gas solubilities have also been neglected bv college chemical engineering laboratories, particul&ly at the undergraduate level. Perhaps the main reason for this is that the operation of conventional absorption units usually requires a considerable amount of skill and experience in order to obtain meaningful data. One of the biggest obstacles to overcome in this respect is setting up a procedure for obtaining the data required to calculate the material balances, ie., flow measurements and analyses of the inlet and outlet streams. Usually, correlation of the data is also complicated and tedious, particularly when dealing with a system in which two film resistances have to be taken into account. At the Cooper Union, we have developed an experimental gas absorption system for educational purposes which has overcome most of the objectionable features of conveutional laboratory units, and has many desirahle features, particularly for undergraduate instruction. First of these is the complete elimination of any analytical work, material balances being obtained from flow measurements of the inlet streams alone. Another outstanding feature of the system is the practically complete elimination of any gas flm resistance, thereby simplifying the correlation of the data. Among some of the minor, but nevertheless very desirable features of the unit, are that the experiment,^ are carried out on a pilot plant scale, and with a nontoxic, inexpensive system. Besides studying the characterist,ics of a gas absorption apparatus, the data obtained can also be used to calculate mass. transfer coefficients, H.T.U.'s, and gas solubility equilibria. Student interest is stimulated by the fact that the numerical values obtained are of rather good accuracy and reproducibility, enabling him to check his values against those reported in the literature (8-6).
ries a false bottom platform for supporting the pack-' ing, and a 1-in. liquid exit line. There is no gas exit line. A sight glass running the entire length of the column enables the actual liquid level in the tower to be determined at any particular time. The details of the Cooper Union tower are shown in Figure 1.
I
-%-in. Bolt Hole6
Fig"-
1.
Details of Towen
Absorption
The system studied is the absorption of carbon dioxide in water. As illustrated in Figure 2, the water supply consists of a a/pin. line with a flow-controlling globe valve and a suitable flow measuring device. In this connection, rotameters were chosen as the flow measuring instruments on both the water and gas lines. DESCRIPTION OF APPARATUS (In an earlier installation, ordinary orifice meters were The central unit. of the apparatus is a tower made of used, but until the student became familiar with the 6 ft. of standard 6-in. pipe with screwed flanges at- apparatus, these were blown too frequently, and retached to the top and bottom. A top cover plate car- sulted in a considerable amount of lost time and mories a a/pin. water inlet line and a perforated plate dis- tion.) The rotameters sufferno ill effectsif overloaded, tributor to insure uniform wetting of the packing. The and the direct correlation of the float movement with top cover plate also carries a '/a-in. vent line, a pressure changes in flow or pressure settings affords a very imtap (connected to a compound Bourdon type gage), pressive visual indication of how a change in one operand a '/%-in. gas inlet line. A bottom cover plate car- ating variable brings about a series of changes in the
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JOURNAL OF CHEMICAL EDUCATION
other process variables until a steady stat.e condition With the apparatus set up in this way, carbon diis re-established. oxide enters the tower, dissolves in the water flowing The carbon dioxide is supplied from a commercial down over the packing, and leaves only as dissolved cylinder (at about 900 psig). If the gas were throttled gas in the exit liquid. Thus the composition of the exit directly to the operating pressure, the attendant cooling liquid can be determined from the inlet flow measureeffects would seriously interfere with the operation of ments alone. I n practice, the height of packing, liquid the gas regulator through which the throttling is taking rate, and gas pressure are held constant a t any desired place. It was found that a two-stage pressure reduction value, and the system allowed to come t o a steady system eliminated this difficulty. I n such a system, the state condition, a t which time the carbon dioxide inlet gas is sent through a pressure regulator which reduces flow has also assumed a steady rate. With the ap$araits pressure t o about 250 psig. A small, heavy-walled tus under consideration, steady state conditions are cylinder receives the gas a t this intermediate pressure. usually established in 5-15 minutes after a change in A second regulator then reduces t,he pressure to the de- one of the operating variables. sired working level ( M O psig). The heavy-walled As can be seen, the entire gas space is filled with cylinder is made of Schedule 80 pipe, and acts as an in- carbon dioxide only, except for a small amount of water terheater between the two reduction stages. After the vapor which is also present. The gas film is therefore second regulator, the gas is sent, through a rotameter, also made up of almost pure carbon dioxide, ~vhichof to the top of the tower. As shown in the diagram, ther- course means that there is virtually no gas film remometers are strategically placed t o measure the water sistance. Another important virtue which accrues from inlet and outlet temperatures, the gas inlet tempera- the fact that the gas phase is pure carbon dioxide is that ture, and the tower temperature. Figure 3 is a photo- the gas composition (and therefore the equilibrium gas solubility) is constant throughout the length of the graph of the complete installation. tower, enabling the correlation of the mass transfer coOPERATION efficients and H.T.U.'s t o be made by direct integration In starting up, the tower is completely filled with of the material balance equation. water. The gas is then turned on, and the tower allowed to drain (the water being displaced by carbon di- CORRELATION OF THE DATA oxide) until the desired liquid level is obtained. The Considering a differential height of packed section, water is then turned on and the drain valve adjusted a material balance, using standard chemical engineering until the water level in the tower remains constant. nomenclature (7) yields: The packing below the water level is of course completely flooded and inoperative. Thus the effective height of packing can be adjusted at will, from zero to a maximum of 5 ft. (the actual height of packing in the tower). The end effects (spray on top, and water sur- If the inlet water contains no carbon dioxide, and if the face on bottom) can be evaluated from measurements outlet water contains x mols of carbon dioxide per mol with the liquid level maintained a t zero height of pack- of water (x being calculated from the inlet flow rates), ing. Besides enabling the student to determine end equation 2 can be directly integrated between the limits effects, the variable height of packing permits t,he cal- x = 0 and x = x (since x* is a constant): culation of the equilibrium solubility of carbon dioxide in water (as will be discussed subsequently). where cis the constant of integration and represents the end effects since it can be evaluated by running the tower with the packed section completely flooded (z = 0). I n general, c has been found t o be very small and can be neglected in undergraduate experiments, or when z> 0.5. Also, since no gas film resistance exists, kza = Kza. Thus equation 3 can be modified:
Or if the H.T.U. concept is t o be used for the correlation, equation 3 yields:
As can be seen from equation 4, KLa can be calculated from a single measurement of the outlet liquid concentration a t a given liquid rate and effective height of packing, using x* as given in the literature. However,
SEPTEMBER, 1950
recent investigations (6) have shown that. the literature equilibrium values (7, 8) differ somewhat from those actually observed for 1%-atertaken directly from ci1.y mains or other indust,rial sources. Consequently it may be advisable to extend the study by making' runs at t ~ oro more packing heights, keeping tho liquid rate constant. If xl and x2 are the exit liquid concentrations observed when the effective packed heights are zi and zz, respectively, equation 4 yields:
Although this equation is implicit in x*, it can be easily solved by trial and retrial, or by plotting ( x * / ~ * - x ~ ) ~ ' and ( x * / x * - ~ ~ )against ~' various values of x* on the same set of coordinates, and noting the value of x* where the txvo curves cross. The fact that equation 4 is implicit in x* makes it difficult to reverse the procedure to determine Kia. KLUcan, however, now be accurately determined from the calculated values of x*. REMARKS
As previously mentioned, the absorption of carbon dioxide by water has been extensively studied during recent years. The values of KLU,H,, and x * obtained with this apparatus, compare favorably with those reported in the literature, an important feature in impart,ing a sense of accomplishment t o the student. However, the apparatus, as described, serves a limited range of carbon dioxide pressures, namely from 0 t o 40 psig. Below 0 psig. it is difficult toremove water from the tower, and above 40 psig. it is difficult to get water into the tower, since only normal city pressure is available. These difficulties can of course be removed by the installation of a water discharge pump, and/or a water inlet pump. However, it is questionable if these modifications TT-odd extend the educational value of the apparatus. In operation, however, a very satisfactory range of liquid rates (from 100 to 2500 pounds per hour) can be obtained by using interchangeable steel and aluminum floats. It is important t o note that no temperature control is used. The installation of a water heater can extend the utility of the apparatus, but here again such an installation would be of questionable educational value, particularly a t the undergraduate level. Finally, the apparatus can be used to extend an innovation which has been used a t The Cooper Union to give the student practice and experience in correlating considerably more extensive and comprehensive data than can be taken during the two or three weeks ordinarily allotted for the usual laboratory study. Since the top and bottom cover plates are easily removed, the packing material can be changed after each squad has
Figure 3.
General View of App..at".
operated t,he tower. Thus, during the course of a semester (during which about 10 squads will have used the apparatus), a large variet-y of packing materials and sizes will have been used. All the data are then made available for the students' consideration, greatly improving their appreciation of the operation. LITERATURE CITED (1) MCCORMACK, H. "The Applications of Chemical Engineering," D. Van Nostrand, New York, 1940, p. 302. 0. T., AND I. LAYINE, "Unit Operations (2) ZIMMERMAN, Laboratory Equipment," 2nd ed., Section VII, University of North Dakota, Grand Forks, 1940. AND H. E. RUCKWAN, (3) SHERWOOD, T. K., F. C. DRAEMEL, Ind. Eng. Chem., 29, 282 (1937). (4) SIMMONS, C. W., AND H. B. OGBORNE, ibid., 26.529 (1934). ( 5 ) SHERWOOD, T. K., AND F.A. L. HOLLOWAY, Tram. Am. Inst. Chem. Engrs., 36, 39 (1940). H. A. BLUM,AND L. E. HUTCB(6) KOCH,H.A., L. F. STUTZMAN, INOS, C h m . Eng. P ~ Q 45,677 ., (1949). (7) PERRY, J., "Chemical Engineers' Handbook," 2nd ed., McGraw-Hill, New York, 1941, p. 1124. (8) National Researeh Council, "Intemtional Critical Tables," Vol. 111, McGraw-Hill, New York, 1928, p. 260.
