Catalytic Oxidation of Vapors Adsorbed on Activated Carbon Jerry N. Nwankwo and Amos Turk' Department of Chemistry, The City College of The City University of New York, New York, N.Y. 10031
Catalyst-impregnated activated carbons can serve as media for concentrating oxygenated organic vapors prior to oxidizing them in situ, without oxidizing the base carbon. The effectiveness of the system has been demonstrated for methylethyl ketone and for several acrylate monomer vapors. Metallic oxide and noble metal catalysts have been studied. The temperature interval between oxidation of the adsorbate and burnoff of the carbon, which is about 150°C, provides a basis for design of a cycling adsorption-oxidation system for purification of an air stream contaminated with organic vapors. Styrene removal, with regeneration of a platinized carbon substrate, was shown to maintain catalytic activity during repeated cycles.
Among the various practical methods available for removing organic vapors from air streams ( I ) , the most rapid and least selective are incineration by air oxidation and adsorption by activated carbon. The energy requirement for heating an air stream to the incineration temperature increases as the concentration, and hence the heat content, of its oxidizable contaminants decreases. On the other hand, the life of a carbon bed, and hence the cost of regeneration, increases as the contaminant load increases. These opposing trends lead to an equivalence point-a concentration a t which costs for the two methods are equal. Of course, there are various possible mitigating factors, such as the use of a catalyst to lower the incineration temperature, or the recovery of valuable adsorbed matter from a carbon bed. However, there are also various potential aggravations, such as the poisoning of catalysts or the deposition of polymer on the carbon. In many situations, the equivalence point is favorable for neither method. For example, the heat content of an air stream containing 100 ppm (vol/vol) of methylethyl ketone, MEK, is sufficient to raise its temperature only 10°C (based on AHcombustion for MEK of 582 kcal/mol), but a 1000 ft3/min (472 l./sec) flow of such a stream will saturate about 10 lb (4.54 kg) of activated carbon per hour. In practice, these circumstances lead to costs which, if not offset, are often prohibitive. MEK is worth recovering, but such benefit may not be applicable to vapors of mixed composition, or to reactive substances such as acrylates which can polymerize on the carbon surface. An earlier report (2) suggested that it may be feasible to combine the advantages of adsorption and incineration by using activated carbon as a substrate for both. The process would involve decontamination of an air stream by adsorption on a catalyst-impregnated carbon at ambient temperature, then heating the system to oxidize the adsorbate but not the carbon. To accomplish this partition effectively, the adsorbate must be oxidized a t a temperature comfortably below that at which the carbon ignites, and the system must be capable of maintaining its performance through repeated cycles. Methods
Calgon Corp. type BPL 6-16 mesh carbon (ignition temperature, 493OC; source, bituminous coke) was used in all of our preparations. Oxide catalysts were deposited on the 846
Environmental Science & Technology
carbon in the form of aqueous solutions of their nitrates or the ammonium salts of their oxyanions. Platinum was deposited from a solution of chkoroplatinic acid. The impregnation and activation procedures were previously described ( 3 ) .Palladium-impregnated coconut shell carbon containing 0.2% Pd by weight was obtained from Engelhard Industries. Adsorption runs were carried out by exposing the carbon in a modified ( 4 ) Du Pont No. 950 Thermogravimetric Analyzer (TGA). The TGA unit was plugged into a No. 900 Du Pont Modular Thermal Analysis System. This arrangement furnished a continuous plot of mass vs. time a t a fixed temperature, or mass vs. temperature at a fixed heating rate. Reaction temperatures were monitored by a differential scanning calorimeter (DSC) cell plugged into the thermal analyzer. The recorder plots the change in sample temperature ( A T )against furnace temperature, and any reaction is shown either as an exothermic ( 7 ) or an endothermic (4) peak. Heats of reaction were determined from the areas under the DSC peaks. Metals of known heats of fusion (indium, zinc, and tin) were used to calibrate the instrument at a heating rate of 10°C/min. The sensitivity of the DSC was set at a level that gave a measurable peak area within the chart during oxidation. The coefficients are used for calculating the heats of reaction, AH ( 5 ) .
A H = EATJS,
Ma = calibration coefficient, cal/"C-min = peak area, in.2 T,, = Y-axis sensitivity, "C/in. Ts7= X-axis sensitivity, %/in. M = sample mass, g a = heating rate, "C/min
where, E A
Methylethyl ketone (MEK) was selected for study as typical of an oxygenated solvent widely used in industry. Methylmethacrylate (MMA), ethylacrylate (EA), and nbutylacrylate were chosen as odorous, chemically reactive unsaturated oxygenates which, in their polymeric forms, could be difficult to remove from carbon. Styrene and toluene were used in the studies of multiple regeneration. E x p e r i m e n t a l Results
MEK. Table I displays the data for the oxidation of MEK on various carbon substrates. The activities of the catalysts are manifested by the heats of reaction shown in the last column. Among the noble metals, platinum was more effective than palladium. Figure 1 shows a typical DSC trace. Figure 2 is a TGA trace corresponding to the conditions of Figure 1. The rapid initial decline in weight that occurs from ambient temperature to 18OOC corresponds to desorption and air oxidation of MEK. The heat released in the oxidation pushes the temperature ahead of the program set for the furnace, and when the oxidation is substantially complete, there is no demand on the furnace, and the temperature drops ( 18O-13O0C, with no weight change). When the actual sample temperature meets the programmed temperature, the furnace becomes reenergized, and warming is resumed. What is significant about this curve is that
Table I. MEK Oxidation on Various Carbon Substrates Heating Rate = lO"C/min. Flow rate = 300 ml/min.
Atmosphere = air. AT-axis sensitivity Catalyst
None Cr203
v,o, v,o,
(2%) (1%)
c0304
cuo/co304 V2°S/K2S04
V,O,/KOH BaO/K,O C uO/C r 0, v2°5/cr203
Pd
Pt pd/v205
Oxidation temp. of MEK, 'Ca
Carbon support, burnoff, " C b
90-180 80-155 60-140 90-160 90-185 80-150 100-1 62 90-180 60-1 60 80-1 75 70-160 100-230 90-185 100-185
400 345 310 300 350 320 310 290 310 320 315 380 385 325
=
2"C/in. Heat of reaction, cal/g
*2 *9 * 36 *5
210 202 428 430 162 ?: 242 f 148 * 359 282 197 r 341 370 463 362 *
* * * * *
0 0 4 9 4 4 0 4 3 10
a Temperature range during which heat i s evolved b y oxidation
Figure 1. Typical DSC trace
TEMPERATURE , ' C
for oxidation of MEK absorbed on vana-
dized carbon (a) Oxidation of MEK. (b) Oxidation of chemisorbed or polymeric matter. (c) Oxidation of carbon. Heating rate, 10°C/min. A T-axis sensitivity, 2°C/in.
as evidenced b y the DSC peak. bTemperature at which carbon subport starts to oxidize, as evidenced b y the start o f the final D S C
peak.
some matter still remains on the carbon at the end of the first (large) exotherm (-13OoC), and is only gradually removed in a second broad exotherm between 180' and 26OOC (Figures 1 and 2). Table I1 gives desorption data for MEK with various catalysts; note that Vs05/KOH and Pt catalyze the complete removal of adsorbed matter at the end of the first exotherm. Figure 3(i), which refers to a V205/KOH carbon, shows a single exotherm a t 185OC, corresponding to the complete removal of adsorbate indicated by the TGA trace of Figure 3(ii). The second exotherm (Figure Ib) obtained with the unpromoted vandia catalyst would be due to chemisorbed MEK or to polymer. The KOH might inhibit acid-catalyzed polymerization of the MEK by neutralizing acidic groups on the carbon. Acrylates
V2O5 was a suitable catalyst for the selective oxidation of the adsorbed acrylates, but the other oxides tested, CuO, MnO2, CozO3, and Cr2O3, were not, as evidenced by lower heats of reaction obtained from DSC runs. Most suitable of all was platinized carbon, as shown for MMA by the DSC trace of Figure 4. GC analysis of the effluents during oxidation of MMA showed only C02 and desorbed monomer, but no intermediate oxidation products. The DSC trace obtained during the oxidation of a mixture of adsorbed
I WT
CALbON
ShTURIITED
OF
\
t l
loo,;,
I
-
,
I
600
500 ,OC
TEMPERITURE
I
I
I
400
300
,&ZOO
700
Figure 2. TGA trace corresponding to conditions of Figure 1 (a) Desorption and air oxidation of adsorbed MEK. (b) Oxidation of chemisorbed or polymeric matter. (c) Oxidation of carbon
Table II. Regeneration of Various Carbon Substrates Saturated with MEK
Initial temperature: ambient. Final temperature: 350°C. Heating rate: lO"C/min. Air f l o w rate: 300 rnl/min.
Catalysts
None Pt Pd
v,o, (1%) V,O,
(2%)
V,O,/KOH V,OS/K,SO, V,05/Cr2
3 '
Initial saturation level, % of carbon
% Residual w t of MEK, based on initial w t
30.0 30.4 30.8 26.8 29.0 29.5 29.3 29 .O
3.6 0.0 11.1 5.3 10.9 0.0 7.8 9.2
L
0
I
I
100
I
!
,
200
I
I
300
I
I
400
,
I
500
,
600
T E M P E R A T U R E , OC
Figure 3. DSC trace (i) MEK adsorbed on V205/KOH carbon. (ii) TGA of same carbon MEK. Heating rate 10°C/min. A T-axis sensitivity, 2'C/in. Weight scale, 4 mg/in.
+
Volume 9, Number 9, September 1975 847
100
I
0
,
I
,
100
,
200
-
,
,
do
,
,
400
I
500
-..
TEMPERATURE. OC
Figure 4. DSC trace (a) MMA adsorbed on platinized carbon. Heating rate, 10°C/min. AT-axis
200
300
400
500
600
TEMPERATURE ,OC
Figure 5. DSC trace (a) a mixture of adsorbed MMA, EA, and toluene on platinized carbon; (b) platinized carbon only
sensitivity, S°C/in; (b) platinized carbon only.
Table I l l . Styrene Oxidation on Platinized Carbon Concentration of styrene in N, stream, 8700 * 140 p p m . Temperature of adsorption, 25" C Cycle no.
Initial saturation level, % of carbon
Carbon w t recovered after ox idat ion at 310°C, %
36.8 34.0 33.5 32.5 34.6 34.2
100.0 100.0 101.0 100.5 103.0 103.5
MMA, EA, and toluene is shown in Figure 5. Both figures show a considerable temperature interval ( 150-2OO0C) between the oxidation of the adsorbate and that of the carbon.
0
100
200
300
400
500
600
TE?lPERASURE,eC
Figure 6. DSC traces (a) Initial and (b) final traces for styrene adsorbed on platinized carbon. Curve (b) was obtained after 6 saturation-oxidation cycles
Multiple Regeneration The procedure can be illustrated as follows to show the continuing recycle effect: Saturate carbon
check carbon capacity by TGA
Regenerate in air, warming at 15' C / m i n , until original carbon weight is reached (or
check catalyst activity by DSC
Data for regenerative studies on platinized carbon, using styrene as sorbate, are given in Table 111. The decrease in saturation capacity may be due to chemisorption of oxygen on the carbon or on the catalyst surface, leading to the formation of the metal oxide (6). Figure 6 shows the initial and final DSC curves. The respective heats of reaction are 830 f 4 and 834 f 8 cal/g, showing that the platinum catalyst is continuously active during the repeated cycles. 848
Environmental Science & Technology
0
Figure 7. DSC traces (a) Initial and (b) final DSC traces for toluene adsorbed on pailadized carbon. Curve (b) was obtained after 10 saturation-oxidation cycles
Table IV. Toluene Oxidation on Palladized Carbon Concentration of toluene in N, stream, 5300 ppm. Temperature of adsorption, 25°C Cycle no.
Initial saturation level, % of carbon
1 4 7 9 10
33.9 31.0 30.3 29.1 28.1
Carbon w t recovered after oxidation at 42OoC, %
100.0 101.5 100.0 100.0 100.0
Results obtained from multiple regeneration of a palladized carbon saturated with toluene are shown in Table IV and Figure 7 . It is evident that both the adsorptive capacity of the carbon and the heat of regeneration progressively deteriorate. Conclusion
The systems herein described offer a promising approach to conservation of energy while oxidizing dilute concentra-
tions of organic contaminants. The method should be most useful when the recovery of heat or solvent in conventional incineration or adsorption systems is infeasible. A further advantage over conventional adsorbers is that the capacity of the catalytic carbon medium is effectively multiplied by the number of cycles. Hence, the total mass of the carbon may be correspondingly less-a factor of particular significance for air purification in space vehicles. Literature Cited (1) Turk, A., Chem. Eng., 70-78 (Nov. 3,1969). ( 2 ) Turk, A., Ind. Eng. Chem., 47,966 (1955). (3) Nwankwo, J. N., Turk, A., Ann. N . Y . Acad. Sci., 237, 397 (1974). (4) . , Ruth. R. A.. Sauires. A. M.. Graff. R. A.. Enuiron. Sci. Technol., 6,'1009 (197i). ' (5) Levy, P. F., Am. Lab., 46-58 (1970). (6) Margolis, L. Ya., T r . Akad. N a u k . (S.S.S.R.)Otd. Khim. Nau., 225 (1959).
Received for review August 29, 1974. Accepted April 3, 1975. Paper presented at the 9th Middle Atlantic Regional Meeting of the American Chemical Society, April 25, 1974, Wilkes-Barre, Pa. Work supported, in part, by Celanese Corp., E. I. du Pont de Nemours h Co., Owens-Corning Fiberglas Corp., and the Pittsburgh Activated Carbon Division of Calgon Corp.
Removal of Phosphates and Metals from Sewage Sludges Donald S. Scott" and Harry Horlings' Department of Chemical Engineering, University of Waterloo, Waterloo, Ont., Canada ~~
Most of the metals and phosphorus in either unconditioned or conditioned final anaerobically digested sludges can be readily extracted by acid. The acid extract can be neutralized under controlled conditions of pH to yield a solid product low in organic material containing mostly iron and aluminum phosphates. By proper control of pH, it is possible to produce two solid products, one containing most of the iron and aluminum, and one containing most of the heavy metals. This technique of metal removal appears to be generally applicable to digested sludges. The crude metal-phosphate products can be treated by conventional means for recovery or recycle. From 80-90% of the iron, aluminum, zinc, and phosphate entering a treatment plant from all sources can be recovered in this way. The amounts of minor heavy metals removed from the sludge are more variable and depend on the acid used and the source of the sludge, although high removal efficiency is possible by ad-' justing extraction conditions. With the advent of chemical treatment for the removal of phosphates in wastewater treatment plants, the sludges produced from these plants will now contain the bulk of the phosphate in the original influent streams. In addition to its phosphate content, the sludge will also contain most of the metallic cations entering in the influent stream, as well as the metallic cations such as iron or aluminum that may have been added as treating chemicals to effect phosphate removal, or as coagulants. Whether the sludges are mixed sludges, activated sludges, or anaerobically digested Present address, Department of Chemical Engineering, University of Delft, Netherlands.
sludges, they will all contain the bulk of the metals and phosphorus that originally entered the treatment plant. The metals become concentrated to a considerable degree in the sludge, and some of them frequently reach appreciable levels (e.g., 5% by weight of dry solids or more). The case of anaerobically digested sludge is particularly interesting because of the reduction in sludge volume during digestion, and the correspondingly increased metal content. If it were possible to remove most of the heavy metal content from the sludge, this might be desirable for two reasons. There is some indication that one of the limiting factors in throughput rate in some types of sludge incinerators is due to the metal content of the sludge. Removal of most of the metals prior to incineration might result in significantly higher rates from existing incinerators. If sludge is to be disposed of by repeated application to agricultural land, a minimum metals content would be desirable to prevent a toxic accumulation from occurring over the years. There is not yet, apparently, any process which offers workable technology for removing metals and phosphates from underflow sludges or filter cake sludges. The principal objective of this study was to investigate means of carrying out a separation of the metals and phosphates from a typical sludge and, as a secondary consideration, to determine whether a ready means was available for economically purifying and recycling some of these materials. E x p e r i m e n t a l M a t e r i a l s and Methods
Because of its higher metal concentrations, an anaerobic sludge was selected for this study. Samples were obtained from a small plant (7 mgpd) which has been using ferric chloride addition for three years for phosphate removal (North Toronto plant of the Metropolitan Toronto Water Pollution Control Agency). Two types of samples were used Volume 9, Number 9, September 1975 849
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