Absorption of Chlorine in Ferrous Sulfate Solutions J. W. RIGGLE
AND
J. B. TEPE
E N Q l N E E R l N Q DEPARTMENT. E. I . DU PONT DE N E M O U R S
& COMPANY.
WILMINQION, DEL.
Experimental rate data suitable for design purposes were obtained on t h e absorption of chlorine from mixtures with nitrogen in ferrous sulfate solutions using a 6-inch diameter tower packed t o a depth of 66 inches with 0.5-inch Raschig rings. T h e effects on transfer unit height of gas rate, liquor rate, percentage conversion, and ferrous sulfate concentration were evaluated for certain restricted conditions by a method of successive approximation. A few values of transfer unit height for t h e absorptlon of chlorine in caustic solutions and in sodium carbonate solutions were determined using t h e same tower.
T
H E unit operation of gas absorption was considered in the early work of Whitman (14), who first advanced the present well-substantiated two-film theory of masa transfer between gas and liquid phases. For physical absorption alone, design procedures based on Whitman’s two-film theory have been developed by which sizes of plant absorbers can be calculated from fundamental equilibrium and rate data. Because of the complication of reaction resistance, however, a completely satisfactory design method has not been worked out for absorbers in which a liquid phase reaction takes place. The most reliable means for obtaining a good design of such absorbem is to determine over-all mass transfer rate data from the operation of semiworks towcrs, such as has been done, for example, for carbon dioxide-sodium hydroxide solutions by Tepe and Dodge (11) and for carbon dioxide-dicthanolamine solutions by Cryder and Maloney (6). Because data on other systems where chemical reactions occur are needed for adequate tower designs, the objective here is to present experimental rate data obtained in a semiworks tower on the absorption of chlorine from nitrogen In aqueous ferrous sulfate solutions. A few data obtained in the eame tower will be presented on the absorption of chlorine in sodium hydroxide solutions and in sodium carbonate solutions. The use of aqueous ferrous sulfate solutions for the absorption of chlorine possesses several advantages over the use of alkaline solutions, such as aqueous sodium hydroxide and aqueous sodium carbonate. From the commercial standpoint, i t is often desirable to bc ablc to absorb chlorine, a noxious gas, without using costly materials to abvorb carbon dioxide, which occurs in the mixture with chlorine and which can safely be discharged to the atmosphere. Ferrous sulfate solutions are selective absorbents for chlorine under these conditions because the reaction with chlorine that occurs is of thc oxidation-reduction type: Clz
+ 2FeS01 = 2FeClSOd
(1)
Carbon dioxide does not react with ferrous sulfate in this manner. The reactions of chlorine with alkaline absorbents, arc of the neutralization type:
+ 2SaOH = NaOCl + NaCl + HzO + NazCOa = NaOCl + NaCl + COZ
CIZ C12
(2) (3)
Carbon dioxide reacts with alkaline absorbents in thc same way:
+ 2NaOH = NazCOa + HzO COz + NazCOa + H10 = 2NaHCOa COz
(4)
(5)
Alkaline solutions, therefore, do not absorb chlorhic dcctively in the presence of carbon dioxide. Another advantage in the use of ferrous sulfate solutions for chlorine absorption is that chlorine is not liberated from FeSOdFeCISO, solutions in acid drainage systems. I t often happens
that a serious fume nuisance or serious stream pollution probleni occurs when solutions containing sodium hypochlorite, for example, are discharged into drains containing acid. Costwise, the use of the ferrous sulfate for chlorine absorption, rather than caustic or carbonate, frequently is advantageous In many localities ferrous sulfate, often in the form of copperas (FeS0+7HZO),can be obtained as a low-cost by-product material. In comparison with water, ferrous sulfate solutions offer the advantage of having a larger capacity for absorbing chlorine. Less volume of absorbent is required with attendant lower pumping requirements and possibly smaller tower dintmeter. EXPERIMENTAL
The experimenta apparatus was a tile- ipe absorption column 6 inches in diameter packed to the de t h of66 inches with 0.5-inch ceramic Raschig rings. The generarlayout of the equipment is shown in Figure 1. Liquor was distributed over the top of the packing through a 0.75-inch standard pipe cross drilled with twenty-four a/&nch holes. The packing was supported by a nickel plate drilled with 0.25-inch holes and having a free area of about 60%. The uniformity of liquor distribution and the condition of the packing were checked both before and during operation and found to be satisfactory. The experimental absorption system was designed to handle a maximum of 250 Ib./(hr.)(sq. ft.) of gas and 7500 lb./(hr.) (sq. ft.) of liquor. The solutions of ferrous sulfate were made up and stored in a wooden tank of 125-gallon capacity that was fitted with a propeller agitator. Liquor was pumped from the make-up tank to a constant-head tank from which i t flowed by gravity a t a regulated rate to the column. The gas stream was composed of nitrogen, supplied from a bank of five 200-cubic foot cylinders connected to a manifold, and chlorine, supplied from a single 150-pound cylinder. h’itrogen and chlorine rates were measured separately using orifice meters with stainless steel plates. These orifice meters were installed in %inch lines, had radius taps, and were constructed according to the standard specifications of the American Society of Mechanical Engineers (A.S.M.E.) (1). Flow rates were calculated using the formula presented in the A.S.M.E. ublication cited. A displacement meter that was installed in t1e inlet gas line for use in checking the orifice meter measurements was not used after initial tests. In the original design of the experimental equipment, dry plant air a t 5-pounds-per-square-inch gage pressure was used as the inert gas. After a few runs were made it was realized that oxygen in the air as well as chlorine might be reacting with ferrous sulfate, invalidating the chlorine absorption results. Also, .plant air p r e s sure varied widely, causing undesirable fluctuations in tower operation. Facilities t o permit the use of cylinder nitrogen as the inert gas, therefore, were installed and placed in operation. This measure resulted in steadier tower operation and eliminated any chance that the ferrous iron concentration was being depleted except by reaction with chlorine according to Equation 1. It has not been proved whether the presence of oxygen in the gaa to be treated for chlorine removal precludes the use of ferrous sulfate solutions as a chlorine absorbent. When the experimental work waa completed and the results of runs made using sir aa the inert gas were compafed with runs using nitrogen, the agree ment was fair1 good, indicating that oxygen absorption occura at a cornparativery slow rate.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
lune 1950
CONSTANT-HEAD
1037
the two analytical methods was satisfactor and the material baz ances could be closed; indicating that Orsat analyses of samples taken directly into the apparatus from the gas lines gave reliable results. T h e O r s a t method, being convenient and giving accurate results, was used exclusively during the tests.
TANK
EXIT GAS TO STAC
THER MOME TE R
CALCULATIONS CONSTANT- HEAD TANK
A satisfactory general method has not been developed for computing transfer unit heights for systems in which absorption is accompanied by chemical reaction in the liquid phase. The problem is simple in the special case when all of the diffusing gas reLIWOR OVERFLW TO EXIT LIWOR SAMRNG P O W acts with the liquid immediately and irreCYLINDCR versibly at the gasliquid interface. In this case there is no liquid phase resistance; the gas Figure 1. General Layout of Experimental Eauioment . . film resistance is controlling. The equilibrium A satisfactory operating procedure was develo ed during sevback pressure of the solute gas is zero, and transfer unit heights eral preliminary test runs. Before making ea& run, the d e can be calculated using as driving force the concentration of the sired quantity of ferrous sulfate was put into solution in the 1 2 5 solute gas in the main body of the gas phase. There is evidence gallon make-up tank b running the agitator for 10 to 20 minthat such system exist-for example, ammonia-sulfuric acid utes. The solution wasieated to 21 to 22" C. by spar g steam into the tank, and a liquid sample was taken for ana&. The (9), sulfur dioxide-aqueous sodium hydroxide (7),and chlorinesolution was umped into the constant-head tank and allowed aqueous sodium hydroxide (la). System of this type, however, to flow througi the tower untll a steady stream emerged from the are the exception rather than the rule. exit liquor line. The liquor flow rate was then adjusted to the deFor cases in which the zone of reaction is removed from the sired value. The nitrogen and chlorine cylinders were next opened to the gas manifold, and bot+ the desired chlorine coninterface, i t is difficult or impoasible to evaluate the driving tent of the inlet gas and the desired inlet gas flow rate were obforce on which to base the rigorous calculation of transfer unit tained b noting the individual orifice meter indications a?d adheights. The chlorine-ferrous sulfate and the chlorine-sodium justing t i e chlorine and nitrogen flows. After about 15 nunutes carbonate systems employed in the present work apparently are of operation, steady-state operation was attained. Readings of flow rates and temperatures then were taken every. 5 minutes of this type. The method of computing transfer unit heights throughout the run, which usually lasted from 30 mnutes to 1 used in this investigation, and also employed by numerous other hour. At the same time, three or more succeasiveOrsat anal ses investigators under similar circumstances ( 4 , 10, I I ) , is to of the inlet gas were made, the number depending on how welfthe compute over-all transfer unit heights based on a driving force individual analyses agreed. Liquid flow rate was measured every 10 minutes by catching the exit liquid from the tower in a equal to the difference between the concentration of the solute bucket over a measured length of time and weighing to detergas in the bulk of the gas phase and concentration representing mine the amount collected.. After the inlet gas composition and equilibrium with the bulk of the liquid. In the present case, this rate had become steady, exit gas analyses were made until three equilibrium value is equal to zero, This method is unsatisor more checks had been obtained. Before shutting down the inlet pas comDosition was checked and samples of the exit liquid factory for several reasons and admittedly is used simply because were Icoiiected. ti better method for reporting the results is not known. The procedures recommended b Pierce and Haenisch (8) were The most serious objection to the present method of comfollowed closely for making the foiowing liquor analyses: puting transfer unit heights is that the results can be used safely 1. Ferrous sulfate in inlet sulfate solutions for design purposes only under conditions that are the same as 2. Chloride in exit sulfate solutions the test conditions. This is because the true driving forces may 3. Chloride in exit sodium hydroxide solutions be different from the driving forces on which transfer unit height 4. Carbonate in inlet sodium carbonate solutions 5. Chlorine in exit sodium carbonate solutions calculations are based. Correct values of terminal concentrations can be calculated using reported values of HLOQas long as To ascertain the accuracy of Orsat gas analyses, an Orsat a p the same relationship exists between the true and the assumed paratus using potassium hydroxide as absorbent and an absorp tion train using 10% sodium hydroxide in sintered-glass absorp driving forces. If the present H6.w values are applied to an tion bottles were set up, and gas samples were taken simultaneoperation with appreciably different operating conditions, the ously by the two methods and analyzed during column operation. rclationship between the true and aasumed driving forces may be The Orsat apparatus was equipped with an expanded scale that allowed readin s to be taken to 0.1% of the total sample volume. different, and terminal concentrations obtained in operation may The largest teviation in successive analyses during a single run differ from the values calculated. waa about 0.3% chlorine out of 15 to 25% chlorine in the inlet Another objection is that the method of reporting the results gas and 5 to 10% chlorine in the rxit gas. Agreement between PACKING 1/2-IN RABCHIG RINGS PACKED DEPTH 6 6 IN COLUMN MAMETER 6 IN
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Table I .
Results of Statlstical Analysis of Each Correlation
Probability that Ht Is Not Influenced by Independent Vanable, %
Position of 95% Confidence Limits in Relation to Regression Curve, %
Vol. 42, No. 6
where y = concentration of solute gas in the gas stream, mole fraction representing equilibrium with the bulk y * = value of of the gquid, mole fraction (1
- y),
=
logarithmic mean of (1
- y) and
(1
- y*)
Equation 7 results when Equation 8 is integrated assuming that there is no equilibrium partial pressure of chlorine above US. % Conversion, FeSOi Simple mean would *20 the solutions used as absorbents. Equation 7 rigorously d e represent data * 25
