Performance of Sludge Based Activated Carbons in Catalytic Wet Air Oxidation of Phenol

INTERNATIONAL JOURNAL OF CHEMICAL

REACTOR ENGINEERING

Volume 8 2010 Article A135

Performance of Sludge Based ActivatedCarbons in Catalytic Wet Air Oxidation of

Phenol

R. R. N. Marques∗ J. Font† A. Fortuny‡

C. Bengoa∗∗ A. Fabregat†† F. Stuber‡‡

∗Universitat Rovira i Virgili, [email protected]†Universitat Rovira i Virgili, [email protected]‡Universitat Rovira i Virgili, [email protected]∗∗Universitat Rovira i Virgili, [email protected]††Universitat Rovira i Virgili, [email protected]‡‡Universitat Rovira i Virgili, [email protected]

ISSN 1542-6580Copyright c©2010 The Berkeley Electronic Press. All rights reserved.

Performance of Sludge Based Activated Carbons inCatalytic Wet Air Oxidation of Phenol∗

R. R. N. Marques, J. Font, A. Fortuny, C. Bengoa, A. Fabregat, and F. Stuber

Abstract

The catalytic potential of sludge based materials produced by steam or CO2

activation from non-treated dewatered raw sludge or dewatered mesophilic anaer-obically digested sludge were tested in the oxidation of phenol. Batch and con-tinuous experiments were conducted to assess activity and stability of these novelcarbons in terms of phenol conversion, carbon burn-off and metal leaching. Over-all, their catalytic activity can be considered satisfactory when compared to com-mercial carbons. However, further research on the preparation, activation andhardening of these sludge based materials is required to improve their thermal andmechanical stability and resistance to metal leaching during the catalytic wet airoxidation of phenol.

KEYWORDS: sludge based activated carbons, CWAO, phenol, metal leaching,burn-off

∗This work has been partly funded by the European Union through REMOVALS project (FP6-018525). Several authors are members of the CREPI group (Grup de Recerca Consolidat de laGeneralitat de Catalunya, SGR05-00792). Our special thanks are due to Prof. N. Graham, Dr. G.Fowler, Dr. K. Smith and S. Pullket from Imperial College for preparing the sludge based activecarbons.

1. INTRODUCTION Over the past decades, the number of people affected by water scarcity in Europe has increased by ca. 20% (EU, 2007). Accordingly, the domestic water consumption was growing due to higher standards of living (DEWA/GRID-Europe, 2004) requiring an intensification of the existing wastewater treatment sector. As a result, new sewage plants have been built that produce large amounts of primary and secondary sewage sludge. Their safe and economical disposal has therefore become the most pressing problem of actual wastewater treatment. Common disposal methods such as landfill, application to farmland and forestry and incineration are no longer feasible options due to the implementation of stricter European legislation rules for a safe sewage disposal. Thus, the development of new concepts that can sustainably integrate the disposal and revalorisation of sewage sludge is imperative.

In this context, the high costs of adsorbent and catalyst manufacture have prompted the preparation of low cost form of these materials from sewage sludge and other waste precursors (Ros et al., 2006a). Most of the related works have investigated the influence of activation conditions (Lillo-Rodenas et al., 2007, Bagreev et al., 2001a) on the production of high surface area carbons (high adsorption capacity) from different sludge precursors (Ros et al., 2006b). The performance of these carbons have been tested in the adsorption of organic pollutants and heavy metals from aqueous effluents (Dias et al., 2007, Rio et al., 2005, Chen et al., 2002, Martin et al., 2004) and H2S or NO2 from gaseous effluent streams (Ros et al., 2006a, Bagreev et al., 2002, Seredych et al., 2008, Pietrazak and Bandosz, 2008, Bagreev et al., 2001b).

In contrast, the application of sludge based activated carbons (SBAC) as catalyst support or direct catalyst has not been reported in the literature (Smith et al, 2009). Commercial parent and modified carbons have already shown a high catalytic activity in the liquid phase oxidation of organic pollutants (Stüber et al., 2005). Related studies suggest that the origin of raw material, preparation methods and activation conditions should determine the textural and chemical surface properties of carbons and thereby their performance as catalysts [Dias et al., 2007, Mohan and Pittman, 2006, Fortuny et al., 1999, Rodriguez-Reinoso, 1998, Pollard et al., 1992). Thus, Catalytic Wet Air Oxidation (CWAO) can be a potential application for activated carbons prepared from municipal sewage sludge precursors.

CWAO using activated carbon (AC) is a complex three phase reaction-adsorption system. Initial saturation of the active carbon occurs simultaneously with several oxidation reactions. The desired reaction is the oxidation or direct mineralization of parent pollutant to intermediates or water and CO2, respectively. Condensation reactions of parent phenolic compounds (called oxidative coupling)

1Marques et al.: CWAO of Phenol over Sludge Based Activated Carbons

Published by The Berkeley Electronic Press, 2010

to form long polymeric chains (Grant and King, 1999) that can irreversibly adsorb onto the AC surface and block the access to micro pores are also present during the CWAO process. In addition, AC itself, being an organic matter, can be burned off. The extent of burn off will depend on the temperature and oxygen partial pressure employed in CWAO and generally leads to a gradual decrease in the pollutant conversion (Stüber et al., 2005). The proper choice of operating conditions for a given activated carbon is then crucial for its successful application in CWAO.

The present work is part of the European research project REMOVALS and investigates the use of sludge based active carbons (SBACs) prepared and activated at Imperial College employing different raw sludge samples. Their catalytic activity was tested in the CWAO of phenol using a stirred batch reactor. Long term activity and stability in terms of conversion, carbon burn off and metal leaching of a particular selected carbon were also assessed through 72 h experiments in a laboratory Trickle Bed Reactor (TBR). 2. EXPERIMENTAL 2.1 Materials Analytical grade phenol (Aldrich) was used for the preparation of 5 g/L solutions. The synthetic air (Carburos Metálicos, Spain) fed to the reactor has a purity of 99.995%. SBACs were prepared at Imperial College from two precursors namely, non treated Dewatered RAW filter cake (DRAW) and Dewatered Mesophilic Anaerobically Digested sludge (DMAD). After drying and sterilization, each sample was subjected to steam or CO2 activation. The obtained carbons are labelled DM-SA (steam activated DMAD) and DM-CO2 (CO2 activated DMAD) and correspondingly DR-SA and DR-CO2. A commercial Merck carbon provided in the form of 2.5 mm pellets served as a reference catalyst. For the batch runs, all carbon samples were crushed down to a particle size less than 0.2 mm, in order to minimize internal and external mass transfer resistance limitations. Accordingly, experiments conducted in the TBR require a catalyst size, which ensures both acceptable pressure drop and external and internal mass transfer, so carbons were grinded and sieved to obtain the 0.3-0.7 mm fraction. 2.2 Characterization methods For characterization, the SBACs were crushed and sieved to a particle size below 150 μm. Their surface area was assessed by using a Coulter Omnisorp® 100 (Beckman-Coulter, UK). Prior to analysing, the SBACs were degassed at 150ºC for 7-8 hours until a pressure below 10-5 Torr was reached. N2 adsorption and

2 International Journal of Chemical Reactor Engineering Vol. 8 [2010], Article A135

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desorption isotherms for the determination of the BET surface area were obtained at liquid nitrogen temperature. The ash content of the carbons was measured using the ASTM standard D 2866-94 (ASTM, 2002). 2.3 Experimental procedures Batch tests were conducted in a 300 ml 316 Stainless Steel Autoclave (Autoclave Engineers, maximum T and P of 450ºC and 345 bar, respectively). A jacket-type furnace and a control device (Autoclave Engineers, MTCP Modular Controller Series) serve to monitor (thermocouple inserted in a well) and control (power supply of the furnace) the oven temperature. However, for a stable reaction temperature an internal coil that allows circulating cold water at a given flow rate was required. Vigorous stirring was ensured by a motor drive impelling a turbine-type agitator with straight blade turbines. The system pressure can be adjusted using a pressure reducer placed upstream of the reactor gas inlet and a valve fixed on the vessel top. Prior to initiating the reaction, phenol adsorption (and temperature) was allowed to develop during ca. 50 min. A liquid sample was then taken for roughly estimating the adsorption capacity of the SBACs. Subsequently, the reaction was started by increasing the total system pressure to 25 bar and the stirrer speed up to 1000 rpm. During the experiment, the pressure inside the reactor was monitored and liquid samples were regularly withdrawn after 0, 30, 60, 120 and 240 min through a valve assembly. For standard experiments, a known volume (100 mL) of 5g/L phenol solution and sludge based activated carbon (2 g) was loaded into the reactor vessel and operated for 4 h at a temperature of 160ºC and 4 bar of oxygen partial pressure (total pressure of ca. 25 bar). The remaining reaction solution was recovered, filtered and stored for eventual analysis of metal leaching.

The original experimental set-up for continuous experiments was designed by Fortuny et al. (1995) to study the CWAO of phenol using a laboratory Trickle Bed Reactor (TBR) and copper based catalysts. A 5 L stirred tank that contains the liquid feed solution is connected to a high precision metering pump (Eldex, Recipro HP Series model AA-100-S-2-CE) that can deliver up to 150 mL/h at a maximum operating pressure of 350 bar. After the pulse dampening section, the liquid flow rate is measured by means of a mass flow meter (National Instruments). The gas and the liquid streams are mixed and pre-heated before entering in downstream the reactor tube made of stainless steel (20 cm long and 1.1 cm I.D.) and normally loaded with 7-7.5 g of carbon. The catalyst is retained by a sintered metal disk at the bottom of the reactor. A thermocouple is inserted axially at the reactor top to measure the temperature at the bed half. The reactor unit with the preheating coil is placed in a temperature-controlled air convection oven (±1ºC). Downstream of the reactor exit, the line contains a 2 mL tube for



liquid sampling, a Gas-Liquid separator, two pressure reduction valves and a flow meter to measure and adjust the gas flow rate. Experiments with 5 g/L phenol solutions were conducted in the TBR over 72 h at a given liquid space time of 0.12 h (FL= 60 ml/h) and temperature and oxygen partial pressure ranging from 120 to 160 ºC and 2 to 8 bar, respectively. Figure 1 shows a scheme of the equipment used and Table 1 lists the conditions of both batch and continuous experiments.

Figure 1. Experimental set-up for continuous CWAO of phenol: (1) feed vessel, (2) high pressure liquid pump, (3) pulse dampener, (4) oven, (5) trickle bed reactor, (6) gas liquid separator, (7) sampling and (8) gas flow meter.



Table 1. Operating conditions for batch and TBR experiments.

Reactor TOS (h)

(h)

G (mL/s)

L (mL/s)

mAC

(g) T

(ºC) PO2

(bar)

Batch --- 4 --- 100 ml 2 160 4

TBR 72 0.12 2.4 60 7 120-160 2-8

2.4 Analytical methods After ending a continuous CWAO test, the spent carbon was collected, dried overnight and then weighted to calculate the eventual weight change (W) due to CWAO. Positive values indicate a gain probably due to dominating oxidative coupling reactions and irreversible adsorption, while negative values should reveal carbon consumption through enhanced burn-off. A small part of some samples (ca. 0.25 mg) was also subjected to thermo gravimetric analysis (TGA) using a Thermobalance Perkin-Elmer (model TGA7) and a heating rate of 10ºC/min from 100 to 900ºC under nitrogen flow.

Phenol concentration of liquid samples was determined by HPLC (model 1100 Agilent Technologies). The analysis was performed with a C18 reverse phase column (Hypersil ODS, Agilent Technologies) and a mobile phase of methanol and ultra-pure-water in proportion of 35:65 (v/v) at a flow rate of 1 ml/min. The wavelength in the diode array detector was set to 254 nm. TOC was quantified with a TOC analyzer following the protocol 5310B. The experimental raw data was compared in terms of phenol and TOC conversions.

Finally, to quantify the metal release of the carbons during CWAO all batch solutions and some liquid samples of TBR experiments were recovered and filtered after reaction. Detection and quantification of Fe and Cu were done on these samples by means of Atomic Absorption and standard solution of known metal concentration. 3. RESULTS AND DISCUSSION The prepared sludge based carbons and Merck carbon were first tested in batch operation. Subsequently, some selected carbons were also subjected to TBR runs over 72h of TOS to assess their long term activity and stability in continuous operation. The evaluation of carbon performance was based on phenol and TOC removal as well as metal leaching.



3.1 Characterization of SBACs For both fresh commercial Merck and prepared SBACs the BET surface area and ash content were determined (see Table 2). Table 2. Characterization of the commercial and sludge based carbons.

Sample DM-SA DM-CO2 DR-SA DR-CO2Merck

BET

(m2/g) 179 169 214 228 1140

Ash content (wt. %)

78.2 78.6 65.4 68.5 <4

From table 2 arises that the Merck carbon has a 5-6 times higher BET

surface area. This is because the DMAD and DRAW carbons have ash contents of ca. 60-80%. The ash fraction is essentially non porous, hence a big ash content is detrimental to surface area.

3.2 CWAO of phenol in batch reactor Following the experimental procedure described in section 2.3, batch CWAO runs were conducted over 4 h at 4 bar of oxygen partial pressure, 160ºC of temperature and 5 g/L of phenol concentration. Figures 2a and 2b compare phenol and TOC removal for both the commercial Merck carbon and the prepared SBACs.



a)

b) Figure 2. Influence of sludge precursor on removal of a) phenol and b) TOC: PO2=4 bar, T=160ºC, CPh,0=5 g/L, W=2g.



A first important result is that, although phenol and TOC conversions remain below those obtained with the commercial Merck carbon, all SBACs were active in the CWAO of phenol with final phenol and TOC removals in the range of 60 to 70% and 45 to 60%, respectively. It is worth to note that for all carbons a short induction period of 10-20 min is observed reflecting the radical nature of phenol oxidation over active carbon.

It also appears from figure 2 that the DRAW sludge yields more active carbons for phenol oxidation, whereas the physical steam or CO2 activation method does not seem to play an important role for its catalytic performance.

The best activity yields the commercial Merck carbon (95% and 85% of phenol and TOC removal after 4 h), which has also with difference the highest surface area (see table 2). This may suggest that a large BET surface area is one key variable for achieving a high catalytic activity in CWAO. In fact, the surface area accounting only for the carbon fraction the SBACs is actually in the same order as that of the Merck carbon attaining values of ca. 700 m2/g and 800 m2/g for DRAW and DMAD carbons, respectively. Thus, the observed activity of the SBACs with a 3-4 times lower organic content per gram of carbon is rather surprising.

Certainly, the high ash fraction containing active metals also plays an important role for the overall catalytic performance of the SBACs in the CWAO of phenol. Both DMAD and DRAW carbons were shown to have a high Fe content of 4 to 6%, which may be partially responsible for their demonstrated catalytic activity. The distribution of active metals (Fe, Cu, Zn, etc) and the ash free BET surface areas of SBACs, however, cannot explain the different activity observed for the DRAW and DMAD carbons. Probably, the textural location and oxidation state of the active metals also determines the intrinsic catalytic activity of the carbon.

Stability of SBACs towards leaching is quantified by measuring the concentrations of active metals released into the acidic medium during reaction and the results obtained are depicted in Figure 3. In case of the Merck carbon (not shown), no metal leaching was detected during the reaction, whereas the sludge based carbons, in particular DM-CO2, released a considerable amount of active metal (in particular Fe). In fact, the DRAW carbon in its actual form can not be used in Trickle Bed operation due to its very poor mechanical stability. TBR experiments were thus conducted with the mechanically resistant DM-SA carbon, although some experiments were also done with a hardened version of the steam activated DRAW carbon.



0

5

10

15

20

25

Fe Cu CaMetal released

Me

(ppm

)

DM -SA

DM -CO2

DR-SA

DR-CO2

Figure 3. Metal leaching measured after 4 h of reaction: PO2=4bar, T=160ºC,

CPh,0=5 g/L, W= 2g. 3.3 Continuous CWAO of phenol Most of the continuous CWAO runs of 72 h were conducted with the DM-SA carbon. A first test using two DM-SA carbons prepared from different sludge batches was considered to check for reproducibility of experiments. Then, several runs were undertaken at a given space time to obtain the catalytic activity of the carbon for different oxygen partial pressures and reactor temperatures. Finally, the performance of the DMAD carbon was compared with that of a hardened steam activated DRAW carbon. 3.3.1 Reproducibility test Figure 4 displays phenol conversions profiles measured at 160ºC and 2 bar of oxygen pressure using two DM-SA carbons prepared from sludge batches recollected at different days at the municipal WWTP.

Overall, the conversion profiles for the DMAD carbons are similar to those observed in case of commercial carbons. An initial apparent 100% conversion of phenol establishes, although the saturation of SBACs is reached very fast, reflecting their low surface area and hence low adsorption capacity (see Table 2). Then, both oxidation and adsorption become influent and phenol and



TOC conversion decreases in the absence of any deactivation or activation phenomena to steady state values after about 15-20 h of TOS. During this initial period, it was shown for a commercial carbon [20] that mainly micro pores (and certainly some active sites) may get blocked due to irreversibly adsorbed polymeric molecules formed by oxidative coupling reactions. Interestingly to point out is that Figure 4 evidences a certain activation of the DM-SA carbons during this initial step as phenol and TOC (not shown) conversions gradually increase over time to their pseudo steady state values. However, towards the end of the 72 h run, conversions starts to decrease again suggesting that simultaneous carbon consumption reactions, enhanced at 160ºC, become significant.

With respect to the reproducibility of the DM-SA carbons prepared from different sludge samples, it can be said that the two carbons show the same experimental evolution, although differences of up to 10-20% in conversions can be appreciated in Figure 4, in particular towards the end of the experiment. This is not very surprising because the composition of sewage sludge can vary significantly from one load to another and a reproducibility of 10-20% as observed should be considered as an acceptable variation. Even, commercial carbons, which are prepared following an optimised and reproducible protocol can show variation of up to 10% depending on the quality of the raw material used in the preparation steps.

0

20

40

60

80

100

0 12 24 36 48 60 72

time on stream (h)

X P

h (%

)

DM-SA (1s t lo ad )

DM-SA (2 nd lo ad )

Figure 4. Reproducibility test of DM-SA in CWAO of phenol in terms of pollutant removal: PO2 = 2 bar, T = 160ºC, = 0.12h, CPh,0 = 5 g/L.



3.3.2 Activity of DM-SA (Effect of T and PO2) Figure 5 illustrates phenol conversions measured as a function of temperature (120-160ºC) and different oxygen partial pressures (2-8 bar). At 120ºC and 4 bar of oxygen partial pressure, DM-SA does not yield any visible activity for phenol destruction and a further pressure increase to 8 bar results only in a small phenol removal of 11%. Temperature has a more visible effect on DM-SA activity than pressure. A significant improvement in phenol conversion up to 25% was observed in the case of 140 ºC and 4 bar of partial oxygen pressure, though DM-SA performed still worse with respect to the Merck carbon.

In order to establish comparable destruction efficiency for both commercial and SBAC carbons at 140ºC, DM-SA would require at least a oxygen partial pressure of 8 bar leading to a prohibitively high total system pressure of 45 bar. A temperature rise of only 20ºC to 160ºC at 2 bar of oxygen pressure seems thus a more feasible option to achieve the same effect (see Figure 5). In the latter case, final pH values measured at reactor outlet were also very close (2.5 and 2.6) suggesting a similar composition distribution of intermediates in the liquid phase, in particular of carboxylic acids, considered as responsible for the effluent acidity.

A further increase of oxygen pressure from 2 to 4 bar at 160ºC enhanced conversions after 40 h of TOS, but then conversion gradually dropped down over time from 60 to 40% after 72h (see Figure 5), most probably due to enhanced AC

Figure 5. Phenol conversion in TBR as a function of temperature and oxygen partial pressure using steam activated DMAD carbon: TOS= 72 h, = 0.12h, CPh,0= 5g/L.

0

20

40

60

100 120 140 160 180

T (ºC)

X P

h (%

)

Me rck 2 bar O2

2 bar O2

4 bar O2

8 bar O2

40 h

72 h



From these results, it can be concluded that the CWAO of a 5 g/L phenol solution over DM-SA proceeds in the kinetic regime and temperature is therefore the parameter of choice to tune the catalytic activity of DM-SA rather than oxygen pressure in agreement with its low reaction order of 0.5 typically reported in CWAO [8]. DM-SA has also enough mechanical stability and can provide sufficient activity at 160ºC, however its long term stability has to be assessed in detail under these operating conditions. 3.3.3 Stability of DM-SA carbon Stability of active carbon can be characterized by its mechanical strength, resistance towards metal leaching, burn-off and pore blockage due to carbonaceous deposits and all aspects are important in CWAO of phenolic compounds. Regarding burn-off and carbonaceous deposits, TGA and carbon weight measurements give valuable information. Table 3 lists the weight change due to CWAO and TGA. Figure 6 displays TGA profiles obtained for three DM-SA samples after being used over 72 h in CWAO at 4 bar of oxygen partial pressure and 120, 140 and 160 ºC, respectively. Table 3. Weight loss of SBACs during CWAO (W) and TGA (TWL).

*color of carbon sample changed from black to grey Table 3 and Figure 6 indicate that the carbon weight change during

CWAO and TGA seems to depend on both pressure and temperature. At low pressure and temperature DM-SA exhibited low but stable conversions

Experiment W (%) TWL (%)

DM-SA (fresh) - -4.0

DM-SA_120_4 17.6 -12.6

DM-SA_120_8 25.0 -12.3

DM-SA_140_2 12.4 -12.9

DM-SA_140_4 -8.9 -10.6

DM-SA_160_2* -18.3 -8.5

DM-SA_160_4* -33.4 -7.4

burn-off at higher pressures. The increased weight loss of the DM-SA carbon of 33% at 160 ºC and 4 bar compared to 18% at 160 ºC and 2 bar is in line with this observation.



accompanied by a pressure dependent weight gain (W), whereas the loss during TGA (TWL) was highest at these conditions. At higher pressures and temperatures these trends are inverted, i.e. W becomes now negative, but the carbons release fewer deposits during TGA. No apparent AC burn-off took place at 120ºC suggesting that oxidative coupling reactions and irreversibly adsorption of polymeric products prevailed over AC burn-off.

At 160ºC carbon burn-off becomes the dominant phenomena and goes in hand with a gradual decline of activity (see Figure 5), which seems mainly due to loss of catalyst and not to any deactivation of the carbon itself. However, at 160ºC and 4 bar of oxygen partial pressure, the weight loss is as high as 33%, whereas the organic content of the original DM-SA is only 22%. This means that also part of the ashes has been moved (by leaching or physical attrition) from the carbon during CWAO. The high carbon burn off is further confirmed by a colour change of the DM-SA carbon from dark black to white grey occurring only at these conditions.

85

90

95

100

100 200 300 400 500 600 700 800 900

T (ºC)

W (

%)

fres h DM-SA

DM-SA_160_4

DM-SA_140_4

DM-SA_120_4

Figure 6. Evolution of weight of original DM-SA and spent DM-SA during Thermo Gravimetric Analysis (TGA).

Observation of Figure 6 indicates significant changes of the carbon

surfaces exposed to continuous CWAO over 72h. The fresh DM-SA sample only starts to release surface groups at temperatures above 500ºC and more than half of the total weight loss during TGA occurs between 800 and 900ºC. All spent



carbons however show a significantly higher (2 to 3 times) weight decrease. The higher the reaction temperature of CWAO, the less is the total weight loss during TGA (see table 3). However, a higher carbon burn off is observed at the same time (see table 3). This suggests that oxidative coupling reaction and subsequent irreversible adsorption of condensation products are less influent at higher temperature compared to direct oxidation of the carbon surface. The DM-SA sample subjected to CWAO at 160ºC therefore shows the largest changes in the TGA profile compared to the fresh sample with a strong decrease of weight at lower temperatures (new groups) and the disappearance of the peak at 800ºC (elimination of original groups).

With respect to metal leaching it was illustrated in Figure 3 that the DM-SA carbon released metals (Fe, Cu, Ca and Mg) during batch oxidation of phenol and similar trends were also observed in the continuous experiments. After an initial period of 10 h, traces of Fe and Cu were found in the liquid effluent during 72 h of continuous CWAO of phenol at 160ºC and 4 bar of oxygen partial pressure. Ca and Mg were detected in higher concentration ranging from 10 to 30 ppm in the liquid phase from the start of the experiment. This strong leaching may indicate a certain disintegration of the carbon structure leading to the high weight loss of carbon observed at 160ºC and 4 bar of oxygen partial pressure.

Concluding on the stability of DM-SA, it can be said that at conditions of pressure and temperature that provide good activity for CWAO of phenol, burn-off and metal leaching are significant and further improvement of carbon preparation is required to achieve adequate long term stability. Nevertheless, it has to be pointed out that the overall catalytic activity and stability observed during CWAO of phenol over DM-SA at 140ºC can be considered as significant. 3.3.4 Comparison of DM-SA and hardened DR-SA carbons.

As pointed out in section 3.2, the DR-SA carbon performed a significantly better batch activity than the DM-SA, although the former showed very little mechanical resistance in pellet form. To improve the mechanical stability of the DRAW carbon and allow its safe utilisation in continuous TBR operation, a hardened version of DR-SA (DRSA-H) was thus prepared by mixing the milled DR-SA with 5% of a binder material (Poly vinyl acetate, PVA) prior to pelletizing. The resulting hardness of this new form of the DRAW carbon was comparable to that of the steam activated DMAD carbon. This new form was tested in a 72 h lasting TBR experiment at 160ºC and 4 bar of oxygen partial pressure in order to compare the results with those achieved with the DM-SA at the same operating conditions.

Phenol conversion and TOC removal for both carbons are illustrated in Figures 7 a) and b). As can be depicted there and in agreement with previous trends observed in batch phenol oxidation, the DR-SA-H carbon exhibited an



trends observed in batch phenol oxidation, the DR-SA-H carbon exhibited an almost two times higher activity than did the DM-SA carbon for phenol (ca. 80%) and TOC (ca. 60%) destruction during the first 24 to 36 h of continuous operation. After this initial period, a sudden decrease in catalyst activity (particularly for phenol conversion) occurred leading to final phenol and TOC conversions around 20% and 10%, respectively. Contrary, the initially less active DM-SA performed a phenol conversion of 40% over the last 10 h of operation and the deactivation was only 20% with respect to steady state activity.

0

20

40

60

80

100

0 12 24 36 48 60 72

X P

h (%

)

time on stream (h)

DR-SA-H

DM-SA

a)

0

20

40

60

80

100

0 12 24 36 48 60 72

X T

OC

(%

)

time on stream (h)

DR-SA-H

DM-SA

b)

Figure 7. Evolution of phenol (a) and TOC (b) conversion for DM-SA and DR-SA-H carbons in continuous CWAO: T= 160ºC, PO2= 4 bar, CPh,0= 5 g/L, t= 0.12 h, W= 7 g.



The behaviour of the DM-SA-H carbon was not expected, since all DRAW based carbons provided a better performance in batch oxidation. The deactivation is certainly related to the gradual carbon burn off and weight loss during CWAO that was found to be very similar for both SBACs carbons, i.e. -33.4% for DM-SA and -35.5% for DR-SA-H. However, this may not explain the sharp fall of conversion observed in the case of the DR-SA-H carbon. This induces the hypothesis that also disintegration of PVA binder material may have contributed to the deactivation of DR-SA-H carbon. PVA is an organic material and can undergo oxidation or dissolution during CWAO thereby causing the disintegration of the DR-SA-H carbon. Some experimental support provides the visual observation of the dried DM-SA-H carbon. While recovering the carbon particles from the reactor, they appeared to stick tightly together to form some big agglomerates. Overall, the use of the DRAW carbon seems to give excellent activity for continuous CWAO of phenol, but other inorganic binder materials must be tested in the carbon hardening in order to avoid undesired disintegration of the binder and related activity loss. 4. CONCLUSIONS Sludge Based Carbons prepared at Imperial College by steam or CO2 activation show promising activity in the CWAO of phenol. DRAW based carbons yield a better catalytic performance compared to DMAD independently of steam or CO2 activation. Due to their week mechanical stability, however they may be only recommended for use in slurry or fluidized bed reactors. The DM-SA carbons prepared from different batches and tested in Trickle Bed Reactor experiments confirm an acceptable reproducibility of the preparation method and conditions employed. In general, temperature rather than partial oxygen pressure should be used as the key parameter to tune activity of SBACs carbon in continuous operation, i.e. an increase of 10-20ºC allows to obtain conversion and mineralization degrees very similar to those obtained with the commercial Merck carbon. A hardened form of the DRAW carbon based on organic PVC binder material gave excellent initial conversions at 160ºC, but the binder was not stable enough at CWAO conditions and underwent disintegration probably caused by its oxidation. Moreover and as for commercials carbons, DM-SA and DR-SA-H suffer also notable carbon burn-off at temperatures above 150ºC. Regarding metal leaching, significant amounts of Fe were released during batch and continuous tests from DMAD carbons into the hot acidic reaction medium. It has to be concluded thus that the long term stability of SBACs is yet not sufficient and further research on activation and hardening conditions has to be conducted to improve the resistance of SBACs towards carbon burn off, metal leaching and binder disintegration.



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Performance of Sludge Based Activated Carbons in Catalytic Wet Air Oxidation of Phenol

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