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Research Article - (2014) Volume 4, Issue 1

Catalytic Combustion of Tannery Sludge in a Rotating Reactor

Vincenzo Vaiano1*, Diana Sannino1, Daniela Caracciolo2, Biagio Naviglio2, Gianluigi Calvanese2 and Paolo Ciambelli1
1Department of Industrial Engineering, University of Salerno, Via Giovanni Paolo II, 132, 84084 Fisciano (SA), Italy, E-mail: [email protected]
2Research Institute of Leather Industry (SSIP), Via Poggioreale 39, 80143 Napoli, Italy, E-mail: [email protected]
*Corresponding Author: Vincenzo Vaiano, Department of Industrial Engineering, University of Salerno, Via Giovanni Paolo II, 132, 84084 Fisciano (SA), Italy, Tel: +39 089964027, Fax: +39 089964057 Email:

Abstract

This work is focused on the catalytic combustion of tannery sludge by using ferrites and CeO2 in a stainless steelfixed bed reactor in comparison with the same system set in rotation.

The presence of catalysts determined an increase of the reactivity with respect to the sludge alone. CeO2 catalystenhanced mainly the reaction rate associated to the combustion of volatiles while it is not active in the combustion ofthe char. On the contrary, both LaFeO3 and LnFeO3 perovskites have accelerated the latter step. In the case ofsludge alone, passing from the fixed to the rotating reactor, the mass of carbon burned increased from 18 wt% to 31wt% indicating that the related effect is the promotion of mass transfer rates among the different phases. Moreoverthe ratio CO/CO2 widely decreased in the rotating system demonstrating that the enhanced oxygen mass transferfrom gas to sludge particle surface determined an improvement of the combustion process since the selectivity toCO2 is enhanced.

Keywords: Catalytic combustion; Tannery sewage sludge; Rotating reactor.

Introduction

In the wastewater treatment system, the sewage passes through a series of treatment steps that use physical, biological, and chemical processes to remove pollutants from water. Many different treatment processes of industrial wastewater effluents are studied [1-13].

In any case, the final products are the purified water and sludge. Common disposal processes for sewage sludge from tannery wastewater are land-filling and thermal conversion (combustion, incineration, pyrolysis). The most used thermal processes are carried out without the presence of catalysts. For municipal sludge, several catalysts were tested, such as TiO2, Al2O3, Fe2O3, MnO2/CuO/CaO. Only in the case of MnO2/KOH, it has been found a clear demonstration that, with the addition of catalysts, the ignition of MSW is enhanced [14].

The influence of catalysts was also investigated in the thermal incineration of industrial sludge. In this case K2CO3, NaCl and Al2O3 were used. The catalytic performance of K2CO3 was better than NaCl and Al2O3 [15].

Very recently, we worked on the thermal treatment of tannery sewage sludge using CeO2 as catalyst and studying the influence of catalyst loading by thermogravimetric analysis coupled with mass spectrometry (TG-MS). The catalyst mixed with the dried sludge improved either the selectivity of the sludge combustion process, reducing the emission of cyclic and aromatic substances, either the combustion peak temperature, yielding a higher oxidation rate of the sludge organic fraction [16].

Moreover by adding the catalysts in the step of sludge formation, i.e. immediately after the addition of flocculants into the coagulation-flocculation step of tannery wastewater, the combustion peak of the sludge organic fraction occurred at lower temperature, about 300°C, while in the absence of catalyst the onset was at about 525°C). This points out to a significant improvement of the oxidation process due to the catalytic material [17]. The reaction occurs in a solid-solid-gas system and to this purpose, it is necessary to improve the contact among the solid phases. A way to obtain an improved physical contact among the sludge particles and catalysts during the combustion is to place in rotation the reactor. So, this work was focused on the catalytic combustion of tannery sludge by using ferrites and CeO2 in a stainless steel fixed bed reactor in comparison with the same reactor set in rotation.

Experimental

Materials and characterization methods

Tannery sewage sludge samples were supplied by an Italian tannery wastewater treatment plant. Sludge conditioning was accomplished by preliminary sample milling. In particular sludge was dried in a stove at 80°C for 8 hours and then pounded in an electric mortar to a size between 180-250 m.

Dried sludge was characterized by different techniques. Moisture content was evaluated by gravimetric analysis of samples before and after an isothermal treatment at 100°C ± 2°C up to constant weight. Total ashes percentage was obtained by weighing the inorganic residue after isothermal treatment at 550°C for 8 hours. Finally, the organic matter was calculated by difference with the moisture and inorganic matter content in the samples. The mineral content was analysed by ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) Liberty Series II Varian with axial torch after sludge acidic digestion. The organic fraction of sludge was analysed by GC-MS after solvent extraction (hexane). By cool-on-column injector the sample was introduced into GC column (Restek Rtx-5ms, 5% diphenyl and 95% dimethyl polysiloxane, 0.25 mm ID, 30 meter), then the separated compounds were analysed by a Quadrupole Mass Selective Detector (HP 5973). For the analysis of volatile compounds a Purge and Trap (P&T) device was used to introduce samples into the GC-MS, equipped with a GC column (Restek Rtx-624, 6% cyanopropylphenyl and 94% polysiloxane, 0.25 mm ID, 60 meter) via split/splitless inlet system.

CeO2, LaFeO3 and LnFeO3 (Ln=Ce, La, Nd, Pr, Sm) as catalysts were mixed with sludge by gently grinding in an agate mortar to obtain a contact between the two solid phases. The obtained samples are named Cex, Lay and Lnz, where where x, y and z represent the weight percentage of CeO2, LaFeO3 and LnFeO3 in the mixture catalyst-sludge, respectively. The catalyst loading was in the range 3-6 wt%. Physico-chemical characterization of sludge and catalysts was performed by different techniques. Specific surface areas were obtained by N2 adsorption measurement at -196°C with a Costech Sorptometer 1042 after pre-treatment at 120°C for 1 h in He flow (99.9990%). X-ray diffraction (XRD) was carried out using the X-ray microdiffractometer Rigaku D-max-RAPID.

TG-MS analyses were performed in air flow by a thermogravimetric analyzer (SDTQ600, TA Instruments) studying the evolved gas by a mass detector in the range m/z=29-209 (Pfeiffer Vacuum Benchtop Thermostar). Measurements were carried out on about 30 mg of sample in 100 (stp) cm3/min air flow (chromatographic grade) at 10°C/min heating rate, in the temperature range 20-1000°C. The parameters used for the evaluation of the catalytic activity from thermal analysis (TG-MS) were the first derivative maximum value of weight loss with respect to time (DTGmax), to describe the rate of conversion for the combustible matter, and the value of peak temperature (Tmax) associated to DTGmax. The emission of aromatic substances was monitored by analyzing the mass fragment m/z=78, characteristic of benzene.

Stainless steel reactor for constant temperature oxidation

A microreactor was employed in order to evaluate the reactivity of the mixture sludge-catalyst. The microreactor comprises a 500 mm length, 17 mm ID stainless steel tubular flow reactor, heated by an electrical furnace. Oxidation tests at constant set temperatures were carried out in air flow. ABB continuous analyzers determined exhaust gas concentrations: URAS 14 (for CO, CO2 and SO2 concentrations) and MAGNOS 106 (for oxygen). Signals from the analyzers were sent to a personal computer for data processing. Samples to be tested were previously diluted with quartz particles to avoid local raise of temperature during the tests. The diluted mixture was loaded to the reactor filling about 3 cm in length of its central zone. The test started raising the temperature to the desired value (T=350°C) in static air. Then the air stream was fed to the reactor starting the oxidation. The feeding air flow rate was 30 L/h (STP). Operating pressure was 1 atm. The initial mass of the sample was about 70 mg. In the case of tests with rotating reactor, the rotating rate was equal to 3 rpm.

Results And Discussion

Sludge and catalysts characterization

The analytical results obtained for furnished sludge are shown in Table 1.

advanced-chemical-engineering-Results-of-analytical

Table 1: Results of analytical tests on sludge.

The sample is mainly constituted by water (the portion loss at 102°C) and organic matter and the inorganic matter (residue at 550°C).

The abundance of chromium in the sludge depends of the initial composition of wastewater, as they are collected and equalized, i.e. from the prevalent tanning process in the relative industrial district. The presence of Al and Fe is mainly due to the use of Al2(SO4)3 as coagulant agent for wastewater, although it may derive, in small quantities, also from the tanning industry.

The values of specific surface area for sludge and catalysts are reported in Table 2.

advanced-chemical-engineering-Values-of-specific

Table 2: Values of specific surface area for catalysts.

The surface area of CeO2 is strongly higher. Consequently, the latter would seem able to make good contact with the sludge. XRD analysis carried out on CeO2 and LaFeO3 (Figure 1 and Figure 2) revealed the presence of only the signal due to the crystalline structure of the two substances. Since the catalyst LnFeO3 is a solid that includes several rare earth elements (La, Ce, La, Nd, Pr, Sm), the XRD peaks are not easy to attribute. From Figure 3 the diffraction patterns of the sample LnFeO3 are clearly related, to those of La, Ce, Nd, Pr, Sm ferrites, Fe2O3 and Fe3O4 [18]. Finally, the signals obtained for values of 2 equal to 32.1 and 55.5, are likely due to CeO2 present in the structure of the sample analyzed.

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Figure 1: XRD spectrum of CeO2 catalyst.

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Figure 2: XRD spectrum of LaFeO3 catalyst.

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Figure 3: XRD spectrum of LnFeO3 catalyst.

I*: integral of mass fragment m/z=78 profile as a function of run time; n.d: not detectable.

As general remarks on TG-MS results, the addition of catalytic material to dried sludge determined effects either with regard to the selectivity of the process, by reducing the emission of aromatic substances, either with respect to the operating conditions, resulting in lower values of Tmax. Table 3 summarizes the results obtained from the analysis carried out on all samples tested in terms of Tmax and the amount of aromatic compounds emitted, evaluated by the integral (I) of the signal associated to the mass fragment m/z=78 as a function of run time. Tmax decreased from 457°C for sludge alone to about 285°C for a CeO2 loading of 5 wt%. The amount of aromatic compounds emitted during the combustion decreased to zero for a CeO2 loading of 6 wt% and for all the mixtures containing LaFeO3 and LnFeO3 catalysts.

Figure

Table 3: Values of Tmax and aromatic compounds emission for sludge and catalysts.

Constant temperature oxidation tests

For all the samples, during the combustion tests in the reactor, there was the emission of CO2, CO and SO2. The behaviour of CO2 produced from the oxidation of sludge in the absence of catalyst is depicted in Figure 4.

advanced-chemical-engineering-oxidation-of-sludge

Figure 4: Behavior of CO2 produced during the oxidation of sludge in the absence of catalyst.

Two main stages in the behaviour of CO2 can be distinguished; the first, with high intensity, occurring in the first five minutes of reaction, is due to the combustion of volatiles, while the second, lower in intensity, is associated to the char combustion. From these data it is possible to observe that gas-phase combustion of volatiles is the dominant reaction in sludge combustion [19] while char combustion is the slowest reaction step [20]. Because the surface flux of water and volatiles, a probable prevention to the transfer of oxygen towards the exhaust char pellet, occurs [21]. A similar behaviour was obtained for all the samples.

Table 4 summarizes the results obtained as a function of type and catalyst loading in terms of percentage of carbon burned (%C), evaluated with respect to the total weight of sludge loaded in the reactor. The mass of carbon burned was calculated by the integration of the signal associated to the CO and CO2 produced as a function of run time.

advanced-chemical-engineering-carbon-burned

Table 4: Percentage of carbon burned evaluated with respect to the total sludge weight loaded in the fixed bed microreactor.

The amount of carbon burned increased from 18 to 25.6 wt% by increasing CeO2 loading. The highest value of %C was obtained in presence of LaFeO3 and LnFeO3 catalysts. In particular it increased up to 32 wt%, relevant more with respect to 18 wt% of the sludge alone.

These results clearly indicate that the presence of catalyst enhanced the combustion process allowing to burn a higher amount of organic substances contained in the sludge.

The results are also analysed considering the reactivity defined as the rate of change of the carbon conversion degree dX/dt (where X=(m0-m)/m0, m and m0 being the current and the total mass of carbon that can be burnt, respectively) as a function of X [22].

The behaviour of dX/dt for sludge, Ce3, La3 and Ln3 is shown in Figure 5a and 5b. In all cases, the presence of catalysts determined an increase of the reactivity with respect to the sludge. Ce3 sample presented the highest dX/dt up to X=0.24 in which the combustion of volatiles occurs.

advanced-chemical-engineering-Behaviour

Figure 5: a)Behaviour of dX/dt as function of X for sludge, Ln3, La3 and Ce3, b)Behaviour of dX/dt as function of X in the range 0-0.2 for sludge, Ln3, La3 and Ce3.

Figure 5b shows the profiles of dX/dt as a function of X in the range 0.2-1 for the same samples.

For X higher than 0.24, the reactivity of Ce3 sample is negligible, underlining that CeO2 catalyst enhanced mainly the reaction rate associated to the combustion of volatiles and that it is not active in the combustion of the char. On the contrary, both LaFeO3 and LnFeO3 accelerated also the latter reaction. In fact the two perovskites showed reactivity higher than sludge alone and allowed to burn all the organic fraction of the sludge.

In Figure 6a and 6b, the reactivity of sludge alone obtained with fixed bed in comparison with the rotating system, is reported.

advanced-chemical-engineering-rotating-reactor

Figure 6: a)Behaviour of dX/dt as function of X for sludge with fixed and rotating reactor, b)Behaviour of dX/dt as function of X in the range 0-0.2 for sludge with fixed and rotating reactor.

Passing from the fixed to the rotating reactor, both the reactivity associated to the combustion of volatiles (Figure 6a) and that one of char combustion (Figure 6b) was markedly enhanced. Moreover the mass of carbon burned, increased from 18 wt% to 31 wt% and the ratio CO/CO2 decreased from 0.33 to 0.17 indicating that the rotation system enhanced the oxygen mass transfer rate from gas to sludge particle surface. An improvement of the combustion process is so determined.

In the presence of CeO2 catalyst, neither the reactivity nor the ratio CO/CO2 show significant differences passing from fixed to rotating reactor. An important influence was instead obtained in presence of LaFeO3 catalyst with a content of 4 wt% (Figure 7a and Figure 7b). In this case, the reactivity of the sample was enhanced (Figure 7a) and in particular also the combustion rate of the char is higher when the rotating reactor was used, confirming that the mass transfer rate was increased with respect to the case in which the fixed bed was used.

advanced-chemical-engineering-fixed-and-rotating

Figure 7: a) Behaviour of dX/dt as function of X for La4 with fixed and rotating reactor, b) Behaviour of dX/dt as function of X in the range 0-0.2 for La4 with fixed and rotating reactor.

Conclusions

In the present study the combustion of tannery sewage sludge was evaluated in the absence and in the presence of cerium oxide or perovskites in a stainless steel fixed bed reactor. The oxidation profiles evidenced two main stages. The first occurring in the initial time of the reaction, due to the combustion of volatiles, while the second associated to the char combustion. The presence of catalysts determined an increase of the reactivity with respect to the sludge alone. CeO2 catalyst enhanced mainly the reaction rate associated to the combustion of volatiles while it is not active in the combustion of the char. On the contrary, both LaFeO3 and LnFeO3 perovskites also accelerated the latter step. Passing from the fixed to the rotating reactor, the mass of carbon burned, increased, while the ratio CO/CO2 decreased from 0.33 to 0.17 indicating that the rotation system enhanced the oxygen mass transfer rate from gas to sludge particle surface determining an improvement of the combustion process.

Catalytic thermal conversion presents many advantages, such as reduction of disposed solid mass and volume and energy recovery from the organic sludge fraction, determining lower disposal costs.

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Citation: Vaiano V, Sannino D, Caracciolo D, Naviglio B, Calvanese G and Ciambelli P (2014) Catalytic Combustion of Tannery Sludge in a Rotating Reactor. J Adv Chem Eng 4:103.

Copyright: © 2014 Vaiano, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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