Sorption of Chloropicrin and Phosgene on Charcoal from a Flowing’ Gas Stream MALCOLM DOLE AND IRVING M. KLOTZ Northwestern University, Evanston, 111.
A comprehensive set of data for the adsorption and desorption rates of chloropicrin and phosgene on activated charcoal as a function of bed length, area of cross section, flow velocity, influent concentration, temperature, and relatively humidiZy is given. The critical bed length seems to be determined partly by the flow conditions and granule size and partly by a surface reaction rate-controlled process. The shape of the adsorption wave after a steady state has been reached is less influenced by the flow con-
ditions than by a change of concentration; this demonstrates the preponderating influence of a slow surface reaction. Phosgene curves are less sensitive to rate of flow than are chloropicrin curves. Desorption studies revealed the expected marked flattening of the desorption wave as it progressed through the adsorbent bed. From the proportion of recoverable phosgene and chloropicrin found experimentally, adsorption in absence of water vapor must be almost entirely physical in both cases.
T
“Dynamic isothermals”, which describe the amount of hydrogen sorbed in palladium as a function of pressure as the hydrogen determine more accurately than has been done before, is pumped out of the system at a known rate, have been deterparticularly in the region of low concentrations, the complete mined (16)as has the rise in pressure when a mass of palladium in history of the change in effluent concentration of a toxic gas with which hydrogen is sorbed is suddenly exposed to a vacuum (16). time, after i t has been carried by a flowing air stream through a The rate of desorption of bromine on charcoal was studied by bed of activated charcoal. The low concentration region was of means of the McBain-Bakr balance ( I S ) ,but the most extensive significant military interest and is of industrial and engineering rate of desorption studies were those of Wicke ( I “ ) , who measured interest in cases where as much gas or vapor as possible has to the concentration of carbon dioxide in the effluent air stream after be stripped from a stream of air. In addition it was hoped that the air had been swept over charcoal previously saturated with all factors influencing the rate of adsorption could be discovered carbon dioxide a t certain partial pressures of this gas. Similar and measured or calculated with sufficient accuracy to permit the studies of the desorption of chloropicrin and phosgene are deprediction of the “adsorption wave”-that is, the shape of the effluent concentration-time curve. Such calculations would be scribed in the present paper. useful in the design of gas mask canisters or industrial rgactors. EXPERIMENTAL The extent of removal by adsorption on charcoal of various All the charcoal used in this research was a zinc chloridegases from a flowing stream of air was measured by other workers activated, extruded wood charcoal, approximately 12-16 mesh, by (a) determining the increase in weight of the charcoal at difproduced by the National Carbon Company. It was dried in an ferent time intervals (8, 14, do), (b) by allowing the transmitted oven for 3 hours a t 150” C. and stored in a gas to accumulate over known time intervals desiccator before being poured into the adin chemical absorbing solutions which can be titrated (d), ( c ) by measuring the in160 sorption tube in the presence of air. Dry tensity of radioactivity of radium and air was then blown through the charcoal in the tube for about 30 minutes until the thorium emanations not adsorbed by the charcoal (S), and ( d ) by measuring the zero reading of the ultraviolet photometer thermal conductance of the effluent air I 20 reached a constant value. Methods used in saturating the dry air stream (5, 17). I n this paper the authors stream with chloropicrin vapor and mixing describe a method of studying adsorption this vapor-saturated air with dry air at rates involving the use of an ultraviolet 6 photometer which makes possible a rapid, d BO predetermined ratios, in measuring and quantitative, and continuous analysis of ; maintaining oonstant flow rates, were all standard and nzed not be described here. the effluent air stream for either chloropicrin or phosgene, with a hitherto unequaled The volume of the photometer absorption 40 cell and glass tubing leading to it was about sensitivity (10). 200 cc. so that, a t a flow rate of a liter a Theories for the shape of the adsorption minute, the gas in the photometer could be wave or equations dealing with systems of this kind have been given before in part, changed rapidly. It was possible to obtain either for charcoal adsorbents or for other 0 20 40 a system of pure dry air from a system filled with air containing chloropicrin a t the types, by Bohart and Adams (d), Wicke t , mlnuta Figure 1. Chloropicrin Conhighest concentration used and to bring the ( I 7 ) ,Furnas (1, 6 ) , and Mecklenburg (18). The rate of desorption as a function of centration as a Function of galvanometer deflection in the photometer Time back to zero in about 1 minute. time and other variables is also of interest Influent concentration, 7500 p.p.m.; The general procedure was to admit and significance but has not been studied flow rate, 62 liters per hour; bed dry air containing chloropicrin or phosgene nearly so much as has the rate of adsorption. depth, 7.0 bed diameter, 1.4
HE purpose of the research described in this paper was to
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I
I
ADSORPTION DATA
1
t , minuter
Figure 2. Effluent Concentrations of Chloropicrin (Solid Lines) and of Phosgene (Dotted Lines) as a Function of Time and Length of Charcoal Bed Cnfluent concentration of chloropicrin varied from 7500 to 8000 p.p.m.; flow rate, 62 liters per hour; tube diameter, 1.4 em. Influent concentration of phosgene, 1800 p.p.m.; other variables, same as for chloropicrin.
at known concentrations at’ zero time to the charcoal adsorption tube and photometer absorption cell system. Readings of the effluent concentrations were made with the photometer a t frequent enough intervals to make possible a detailed mapping of the effluent concentration-time curves. The sensitivity of the experimental system is shown in Figure 1 where the change of concentration in the neighborhood of the break is indicated. Thus, values of c/co a t low concentrations can be measured to *0.0002. Because of the steepness’of the effluent concentration-time curve and its close approach to a “square wave front” at these low concentrations, the determination of the break point a t 20 parts per million effluent concentration carries with it considerable significance. However, the chief uncertainty rested in the reproducibility of the packing density of the charcoal in the adsorbent tubes. Because of the irregular shape of the extruded granules, there was always some variation in the total amount of adsorbent used for any given length of bed. Details of the ultraviolet photometer are given in the paper by Klotz and Dole (10).
I
5
2
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e37 f3I
(1
I
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40
80 t , minutes
120
Vol. 38, No. 12
The most significant data obtained in this research were collected and tabulated in a report to the O.S.R.D. (4). Effluent concentration data are presented here in terms of c/c,, where cgis the influent concentration. INFLUENCE OF DEPTHOR LENGTH OF ADSORBENT B E D , Figure 2 illustrates the increasing time required for penetration of either chloropicrin or phosgene as the bed depth is increased. The phosgene curves are steeper a t the midpoint as compared to the chloropicrin curves, they are not symmetrical about the midpoint, and their shape becomes slightly flattened out as the bed depth increases. INFLUENCE OF FLOW RATS. The influence of this factor is illustrated for both chloropicrin and phosgene adsorption in Figure 3. The curves flatten out as the flow rate decreases. INFLUEXCE OF CROSS-SFCTIOXAL AREA. Increasing the crosssectional area should be equivalent to decreasing the flow rate and should, correspondingly, cause a flattening of the curves I t is evident from Figure 4 that a marked flattening does occur. INFLUENCE OF INITIAL CONCENTRATION.From Figure 5 it can be seen that the change of the influent concentration is more important in changing the shape of the c/c,-time curveB than is change of flow rate or bed depth. I N F L U E N C E O F T E M P E R A T U R E . The effect O f temperature OD the adsorption curves is small but can be detected (Figure 6). INFLUENCE OF WATERVAPOR. Presence of water vapor to the extent of 50% relative humidity in the influent air stream had no significant effect on the adsorption of chloropicrin, but, because of the well known catalytic hydrolysis, it did markedly alter the shape of the phosgene effluent concentration- time curves (Figure 7). 9 curious phenomenon of maxima and minima in the effluent concentration-time curve of Figure 8 was observed when air at 7% relative humidity (2190 parts per million of water vapor) was passed along with phosgene a t a concentration of 1800 parts per million over charcoal previously equilibrated with water vapor at 7% relative humidity. DESORPTION DATA
Figure 9 shows the general shape of the effluent concentrationtime curves for the desorption of phosgene and chloropicrin when pure dry air is passed over a bed of charcoal previously saturated with these gases. INFLUENCE OF BED DEPTH. Increase of bed depth cauBes a marked flattening of these desorption curves, as might be expected (Figure 9). The influence of other variables on the full desorption waves was not studied in this research because it wae
’8
160
Figure 3. Effluent Concentrations of Chloropicrin (Solid Lines) and of Phosgene (Dotted Lines) as a Function of Time and Flow Rate Chloropicrin influent concentration varied from 7700 to 8300 p.p.m.; bed depth, 5.3 em; bed diameter, 1.4 cm; phosgene influent concentrations, 1670 and 1760 p.p.m.; bed depth, 10.0 cm.; bed diameter, 1.4 em.
t , minutes
Figure 4. Effluent Concentration of Phosgene as a Function of Charcoal Bed Cross-Sectional Area Flow rate, 90 liters per hour; influent concentrations and bed length.
of the three curve- (from left to right), 1870, 1790, and 1560 p.p.m.,
and 5.0, 2.0, and 1.5 cm., respectively.
December, 1946
INDUSTRIAL AND ENGINEERING CHEMISTRY
more significant for military purposes to determine desorption from beds which had been brought just to the break 'point or from other similarly partly saturated beds. INFLUENCBI OF DELAY BETWEEN ADSORPTION AND DESORPTION.Figure LO illustrates the change in the form of the desorption wave produced by a 21-hour delay between adsorption and desorption. Apparently a migration Yr redistribution of the chloropicrin takes place in the bed on standing.
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40
80 160
120 200 t, minutea
.
INFLUENCE OF TEMPERATURE. Rise
in temperature definitely increases the rate of desorption (Figure 11). REVERSIBILITY AND REPRODUCI: BILITY OF DATA
160 240
320
280
Figure 5. Effluent Concentrations of Chloropicrin (Solid Lines) and Phosgene (Dotted Lines) as a Function of Time and Influe'nt Concentration, in Parte per Million Flow rete, 62 litere per hour; bed depth for phosgene, 5.0 om.; for chloropicrin, 5.3 om. time male refers to laat curve on right.
A repetition of a particular experiment usually gave duplicate results with good agreement; the primary error seemed to be in variations in packing density. I n regard to the reversibility of the adsorption and desorption process, integration of the C/COtime curves shows that, for a bed of 2.5 cm. in length, 45% of chloropicrin can be recovered after 190 minutes of desorption, and 72% after 54 hours. As chloropicrin was still being desorbed in detectable amounts when the experiment ended, it q n n o t be stated definitely whether 100% recovery would have. been possible. I n the case of dry phosgene 66% recovery had been achieved in 90 minutes. These calculated results indicate that by far the greater part of the chloropicrin and phosgene was physically adsorbed.
Temperature measurements by means of thermocouples in the special case of the adsorption of chloropicrin pn charcoal showed that (a) the temperature rose to a maximum slightly beforeAc/ A t was a maximum (probably when An/At was a maximum); 6)the total rise in temperature was 5-6" C., with the adsorptioo tube in a constant temperature bath; (c) the temperature fell to the ambient temperature as soon as the adsorption wave had passed; and (d) the temperature wave probably could not be used as an exact measure of the adsorption wave because of the conductance of heat along the bed by the air stream moving in advance of the wave. A rise in temperature of the eflluent would indicate the moment of break, but not to the accuracy with which the photometer could measure it. MATERIAL BALANCE
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which is the Mecklenburg equdion. At the moment of break, the distribution of gas in the adsorbent is illustrated by the solid line of Figure 12 (data obtained by weighing sections of the bed before and after adsorption), whereas the calculation of Mecklenburg is equivalent to the distribution shown by the lower dotted line. The part of the bed assumed to have np adsorbed gas in it is called the dead layer, but it must be emphasized that the dead layer, a convenient concept though it may be, is a mathematical fiction. If equilibrium were established instantly at all points throughout the bed, the adsdrption wave would have a square wave front-that is, at the moment of break the entire bed would be
1 . 0
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0.6
EQUATION
Mecklenburg ( I d ) gave a useful empirical equation derived in the following way: The amount of gas which has entered the adsorbing tube a t the moment of first detection of gas in the effluent stream is
0.4
d t B
where I B = break time
[f co is expressed in moles per liter, L in liters per minute, and t~ in minutes, then the above quantity is the number of moles of gas passed into the tube in t~ minutes. As all the gas is adsorbed (except for the negligible quantity transmitted up to the break concentration of 20 parts per million), we
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flow conditions than is the adsorption of phosgene. CRITICAL BED DEPTH
Critical bed depth is the lengt’hof the adsorbent just sufficient to prevent the transmission of gas a t a concentration greater than that taken for the break value, C B , at zero time. It is determined from a plot of the break time as a function of bed depth s; on extrapolation back to zero time, a finite value of the bed depth (the critical depth) is obtained. Such a plot is shown in Figure 13 where the break times obtained in the phosgene studies at three different flow rates are plotted as a function of bed depth. On extrapolation to zero time a finite value of the bed depth is obtained. This distance, which we shall call Q, is I mFH the h of hlecklenburg’s equation provided that the I I-, , , , ~, 160 240 320 400 plot is linear-that is, provided h is independent t . minuter of 2 . Figure 7. Phosgene Effluent Concentrations as a Function of From the equation given by Bohart and Adams Relati\e Humidity (2) for the effluent concentration as a function of Influent Concentration, 1800 p.p.m.; flow rate, 62 litera per hour; bed depth, time t , linear floiv rate v, rate of adsorption 5.0 cm. constant k , maximum adsorbed amount no, and influent concentration co, the following equation can be derived:
pi
I , ,
ln(ca/c - 1) = In(&@
v
- 1) - kcot
(2)
8 s eL’lo’’v is usually much greater than unity, Equation 2 may be simplified to
ln(c,/c - 1) = knLx/v
- kcot
(3)
For a definite break concentration C B , Equation 3 readily reduces to an equation similar to that of Mecklenburg:
t , minutes
Figure 8.
Phosgene Effluent Concentrations for Two Separate Experiments
Influent concentration, 1800 p.p.m.; relative humidity, 7%; depth, 5.0 cm.; flow rate, 6 2 liters per hour.
\Then L E is extrapolated to zero break time, Equation 2 reduces to ln(cr/c
Led
or
-
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-
1)
(5)
xo = (v/n
