Upgrading of Tannery Wastes under Fast and Slow ... - ACS Publications

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Upgrading of Tannery Wastes under Fast and Slow Pyrolysis Conditions Antonio Marcilla,†,* Milagros León,‡ Á ngela Nuria García,† Elena Bañoń ,‡ and Pascual Martínez‡ †

Department of Chemical Engineering, University of Alicante, P.O. Box 99, E-03080 Alicante, Spain Footwear Technological Institute (INESCOP), Polígono Industrial Campo Alto, P.O. Box 253 E-03600 Elda, Spain

‡

ABSTRACT: The pyrolysis of a chromium-tanned waste (bovine split leather) under inert atmosphere has been carried out in a vertical lab scale reactor. The influence of process conditions, such as temperature, residence time, and heating rate on the pyrolytic product distribution has been studied, in order to optimize the liquid fraction obtained. Flash pyrolysis at three different temperatures (450, 500, and 550 °C) and slow pyrolysis up to 750 °C have been performed. Results indicate that, in the range of low temperatures, the product distribution is slightly dependent on temperature. In general, a reduction of the heating rate or residence time favors the presence of heavy gases (i.e., more than four carbon atoms hydrocarbons). The analysis of the pyrolytic liquid shows a wide spectrum of products (mainly nitrogenated and oxygenated compounds and phenols) which can be useful as a source of chemicals. A comparison between the results reached in the pyrolysis of tanned leather and a commercial collagen allows us to study the effect of the tanning process. According to the result obtained, the tanned leather produces more nitrogenated compounds and phenols and less ketones and linear hydrocarbons than pure collagen. A very good agreement between the evolution of pyrolytic gases produced in a slow pyrolysis and the weight loss of the solid measured by thermogravimetric analysis has been observed.

1. INTRODUCTION In Spain, more than 16000 tons per year (t/yr) of tannedleather wastes are produced. Around 12800 t/yr are generated in tannery industries (FECUC, 2010) and around 3800 t/yr are produced specifically by the footwear industries (FICE, 2009). Diverse literature about leather waste treatment and recycling can be found, for example: mechanical recycling where the recovery of shavings is mainly used to produce a base material reconstituted leather also known as ″leatherboard″,1 studies for the addition of leather wastes to the composition of ceramic pastes for the manufacture of building materials;2 protein recuperation and chromium reutilization like a novel three-step chromium-containing leather waste (CCLW) treatment process proposed for Mu et al.;3 residue treatment processes using alkali and proteolytic enzymes to obtain a hydrolyzed collagen protein and a chrome cake recyclable,4 use of pressure-driven membrane processes to reduce chromium content in wastewater from the tanning process and simultaneously improve the quality of the recycled chromium;5 or waste combustion for energetic use.6,7 Although the energetic efficiency in pyrolysis is lower than in combustion, the former can convert wastes into gas, liquid, and solid fractions with different applications. Additionally, the three fractions obtained can be used as fuels. Moreover, one of the advantages of this process is to obtain a liquid fuel with higher density than the original solid waste, which allows significant reduction of the transport costs per energy unit. Furthermore, gas and liquid fractions are also susceptible to generating useful chemicals. On the other hand, the char obtained used either alone or mixed with others can also be used as a cheap adsorbent.8,10 Thus, pyrolysis presents an interesting alternative to the conventional combustion process. © 2012 American Chemical Society

Pyrolysis of biomass is a widely studied process and many papers have been published about bio-oils generated in these processes. Instability and high content of oxygen are wellknown disadvantages of these bio-oils, though upgrading methods to improve their quality are being developed. As an example of the interest of this topic, in the last year many papers about pyrolytic bio-oils from biomass were published, studying different aspects of the subject (chemical characterization11 estimation of the production costs,12 catalytic upgrading,13 commercial developments,14 etc.) . However, specific information related to the pyrolysis of tannery wastes is scarce and only five references have been found: Muralidhara et al.15 evaluated the technical performance and cost effectiveness of a low temperature pyrolysis process which uses dry leather tanning wastes to provide energy and chrome tanning liquor for its reuse in tanneries. The kinetics of the global primary thermal decomposition of tanned leather was studied using TG-DTG under pyrolysis conditions as well as with different proportions of oxygen by Caballero et al.16 Pyrolytic products evolved from the thermal degradation of tannery wastes have been studied by Font et al.17 using a Pyroprobe 1000 connected to a secondary reactor and a small furnace. Yilmaz et al.8 investigated the production of useful materials from different kinds of leather waste (chromium and vegetable tanned shavings, and buffing dust), and Oliveira et al.9 used the wet blue leather waste after controlled pyrolysis to transform it into chromium-containing activated carbons (AC). Recently, using Pyroprobe equipment, our research Received: Revised: Accepted: Published: 3246

July 28, 2011 December 20, 2011 January 28, 2012 January 28, 2012 dx.doi.org/10.1021/ie201635w | Ind. Eng. Chem. Res. 2012, 51, 3246−3255

Industrial & Engineering Chemistry Research

Article

group has developed some works in analytical pyrolysis of tanned leather,18,19 In those papers, the effect of different organic and inorganic tanning agents was compared. Unlike most of the agricultural wastes, meat and bone meal is a nitrogenated biomass (animal origin), and its elemental analysis is closer to that of the leather. Thus, the pyrolytic products obtained from leather pyrolysis could be closer to those obtained from pyrolysis of meat and bone meal20−22 than from other wastes. Because the raw material used is a chromium-containing waste, some papers about pyrolysis/combustion of chromated copper arsenate (CCA) treated wood waste have been also revised.23−25 In these papers, the sample considered is metalimpregnated wood waste. Obviously, the sample is not subjected to a tanning process as in the case of leather, which involves a significant change in the polymeric structure of the original sample. However, they can give us information about the behavior of chromium under a pyrolytic process. Some conclusions of these papers are, for example, that (i) chromium is bounded even more strongly in the pyrolytic residue compared to the original CCA treated wood, (ii) chromium(III) oxide does not undergo any significant reactions during heating in inert or air atmosphere, (iii) at 1200 °C, under combustion conditions, only 0.05% of the total chromium was found in the gas phase, (iv) the metal compounds in the char may have migrated toward the ray cells during the pyrolysis process, so the metals appear as agglomerates rather than isolated elements, which can be useful to the development of a separation system to recover clean charcoal by separating the carbon and metal particles. Furthermore those authors conclude that, in the long term, the best available thermochemical conversion technology for the treatment of CCA-treated wood will be low-temperature pyrolysis or high temperature gasification. We consider that all these conclusions present the pyrolysis as a viable treatment technique for chromiumcontaining wastes. The aim of this paper is to study the pyrolysis of chromiumtanned wastes in order to convert them into a gas or liquid fuel or chemical source, optimizing the production of the liquid fraction, which can be called bio-oil since the raw material is a biopolymer. Unlike previous papers working on leather pyrolysis,8 an exhaustive analysis of the gas and liquid fractions has been performed in this work, identifying and quantifying the volatiles detected. Furthemore, different temperatures, heating rates, and residence time of volatiles in the reactor have been considered to evaluate the effect of these parameters on the volatiles generated. Moreover, a comparative analysis between the weight loss measured by TG and gas evolved in the slow pyrolysis performed in the reactor has been included.

Table 1. Ultimate and Immediate Analysis of the Materials Studied tanned leather Proximate Analysis, % (as Received) moisture 11.2 volatile mattera 76.7 ash 6.03 fixed carbonb 6.12 Ultimate Analysis, % (Dry Basis) carbon 41.5 hydrogen 6.85 nitrogen 12.9 sulfur 0.83 oxygenb 37.9 a

collagen 7.50 89.8 1.40 1.29 48.1 7.25 12.8 0.15 31.6

Calculated according to ASTM D3175-11. bBy difference.

atmosphere using a heating rate of 10 °C/min until 750 °C. The thermogravimetric analyses were carried out on a TGA/ SDTA 851 Mettler-Toledo, and the amount of sample degraded was around 8 mg. 2.3. Reactor. Samples of around 2 g were pyrolyzed in a stainless steel AISI 310 vertical reactor (see Figure 1), heated

Figure 1. Experimental system: (A) nitrogen; (B) rotameter; (C) oven; (D) reactor; (E) feed entrance; (F) top reactor heating system; (G) glass traps; (H) ice−salt bath; (I) manual valves; (J) thermocouple; (K) carrier gas preheater chamber.

by a cylindrical refractory oven allowing the collection of gas, liquid, and solid fractions for quantification. The body of the reactor was a cylinder 71 cm height and 5.8 cm internal diameter. Ceramic balls (0.6 cm diameter) were placed at the bottom of the reactor and a diffuser was located above them. At 46 cm from the bottom of the reactor, a lateral exit for the volatile compounds is located. The reactor temperature was controlled in these zones through a system of the corresponding three thermocouples located at the bottom, head, and the lateral exit of the reactor respectively. The gas fraction was collected in Tedlar bags and condensed volatiles were collected in glass traps with stainless steel Dixon rings. Solid residue generated remained inside the reactor body and it was taken out and weighted after each run. 2.4. Experimental Procedure. Pyrolysis experiments were carried out under nitrogen atmosphere in batch mode. Flash pyrolysis runs were performed at temperatures of 450, 500, and 550 °C, respectively. The slow pyrolysis run was carried out from room temperature up to 750 °C. All the experiments were repeated twice in order to check the reproducibility of the results. The error, calculated as (X1 − X2)/Xm (where Xi is the

2. MATERIALS AND METHODS 2.1. Materials. The experiments were carried out with a chromium-tanned waste (bovine split leather) supplied by Palomares Piel, S.L. and collagen powder from the Achilles tendon of a bovine species supplied by Sigma-Aldrich without any tanning treatment. Data about proximate and ultimate analysis of these materials are given in Table 1. The low heating value of the chromium-tanned leather was measured by using a calorimetric pump and the result was 5032 kcal kg−1. 2.2. Thermobalance. Chromium-tanned waste was subjected to thermal degradation in a thermobalance in a nitrogen 3247

dx.doi.org/10.1021/ie201635w | Ind. Eng. Chem. Res. 2012, 51, 3246−3255


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fraction distribution obtained in experiment i and Xm is the average value), is around 6%. The reactor exit is connected to a three-way valve. The first way is the vent to the atmosphere, the second way has two sets of traps in series. The first set consists of a first empty impinger followed by a second one with Dixon rings. The second set of traps, placed in series with the previous one, has two impinger in series filled with Dixon rings. The third way has a single set of traps filled with Dixon rings. All sets of traps are immersed in a bath of ice and salt. Hexane and acetone were checked as solvents to extract the stainless steel Dixon rings. Only acetone led to satisfactory results. The final exit of ways 2 and 3 are attached to the gas collection system consisting of a set of two manual valves connected to Tedlar bags. 2.4.1. Flash Pyrolysis. Flash pyrolysis experiments were carried out as follows. The reactor was programmed to the selected temperature, and the top reactor heating system was switched on. Reactor, glass traps, and gas sampling bag were connected on line, keeping the manual valve open on channel 1 (exit) and the rest closed. During the heating time, nitrogen flow purged the system to the exit. In those experiments, where an inert bed was used, about 500 g of silica sand (particle size in the range 70−210 μm) were placed in the reactor before heating. A 2 g portion of chromium-tanned leather was placed in the feed hopper. Prior to the experiment, the feed hopper was purged with nitrogen to guarantee inert atmosphere inside the reactor during the pyrolysis. When the reactor reached the selected temperature, the nitrogen flow was adjusted to be around 700 mL/min, and the experiment began by dropping the feed into the reactor and collecting the involved products. At this time, the valve on way 1 was closed, and the valve on way 2 was opened. After the first 2 min, the valve position was changed to lead the volatile flow into the second glass trap system (way 3) to avoid the gas flow sweeping out the volatiles already condensed in way 2. Gases were collected in the second gas sampling bag. One half hour later, the valves of ways 2 and 3 were closed and the valve of way 1 was opened sending the carrier gas to the atmosphere, when the experiment ended. 2.4.2. Slow Pyrolysis. The experimental procedure was similar to that described in the flash pyrolysis, but in this case, 4 g of dry sample was initially placed in the reactor and both, sample and reactor, were heated simultaneously from room temperature up to 750 °C at a heating rate of 10 °C/min, under inert atmosphere. Specific gas samples were collected at intervals of 50 °C, approximately, in Tedlar bags (12 bags were collected and analyzed along the run) and the condensed volatiles were trapped in the glass traps. In this case, the flow was changed from track 2 to track 3 when the reactor reached 500 °C. 2.5. Gas Analysis. Gases evolved from the reactor were identified and quantified by gas chromatography using standard gases. For the analysis of CO and CO2, a Shimadzu GC-14A gas chromatograph with TCD detector and a concentric column Alltech CTR1 were used. The program conditions were Tinyector, 28 °C; Tdetector, 110 °C; Toven, 110 °C; ttotal, 25 min; carrier gas flow (He), 40 mL/min. Gas components different from carbon oxides were identified by an HP-6890N gas chromatograph with mass spectrometry (HP-5973 MSD) and a capillary column GS-GASPRO

(nominal length, 30 and 0.320 mm internal diameter). The qualitative analysis of the chromatogram obtained was carried out using the mass spectra commercial libraries WYLEY275 and NIST02. For quantification of these gaseous compounds, an Agilent GC 6890N with FID detector and capillary column GASPRO was used. The program conditions were Tinyector, 150 °C; Tdetector, 210 °C; Tinitial (oven), 35 °C; Tfinal (oven), 180 °C; heating rate, 5 °C/min; ttotal, 51.5 min. For the quantification of the gas mixture, standards of aromatic, saturated and unsaturated hydrocarbons of known concentration supplied by Supelco were used. Those peaks that could not be identified with the standards employed were quantified by the response factor of the compound closest to them in the chromatogram. The estimation of the yield of NH3 was obtained using H2SO4 absorber. The analytical method of ammonia determination was the 4500-NH3·C Nesslerization direct method.26 NO and NO2 were measured with a continuous measure analyzer of NO−NO2−NOX by chemiluminescence, SIR S-5012 model. 2.6. Liquid Analysis. Moisture content of the liquid products was measured with a Karl Fisher Titrator using Hydranal composite 5 and Hydranal methanol rapid solutions for the analysis. The liquid fraction collected in the empty trap was diluted in acetone to be analyzed by gas chromatography with mass spectrometry (HP-5973 MSD gas chromatograph with a capillary column HP-5MS). The oven program was Tinitial = 37 °C; Tfinal = 320 °C; heating rate = 12 °C/min; and ttotal = 33.5 min. The liquid compounds were identified using the commercial mass spectra libraries WYLEY275 and NIST02. The quantification of the liquid was performed with commercial patterns of Fluka and Aldrich. A total of 27 standards were analyzed and their response factor was calculated. The standards were grouped according to their functional group as follows: • aromatics: toluene, benzene, naphtalene, 1,3-diisopropilbenzene, isobutylbenzene, 5-tert-butyl-m-xylene • phenols: phenol, 2-ethylphenol, guaiacol and syringol • oxygenated compounds: benzaldehyde, cyclopentanone, maltol, levoglucosane, 2-(5H)-furanone, furfural, acetic acid • nitrogenated compounds: indole, 2-methylindole, succinimide • sulfur compounds: bis(4-hydroxyphenyl) sulphide • alkanes: hexadecane, icosane and docosane • alkenes: decene, hexadecene and icosene An average response factor was calculated for each group. All the peaks in the chromatogram were integrated and quantified using the response factors obtained with these standards. When the compound was one of the specific standards, its response factor was used in the quantification. If the given compound did not correspond to any of the standards injected, the average response factor associated to its functional group was used. With these data, the total mass analyzed was estimated, as well as the mass fraction of each compound. To obtain the yield of each compound (gi/gsample), the mass fraction of the compound estimated was multiplied by the percentage of liquid fraction obtained on a dry basis. 2.7. Solid Residue Analysis. The low heating value of the char was measured using a calorimetric pump. X-ray photoelectron 3248



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spectroscopy (XPS, K-ALPHA, Thermo Scientific) was used to provide the chemical bonding state as well as the elemental composition of the char surface. All spectra were collected using Al-K radiation (1486.6 eV), monochromatized by a twin crystal monochromator, yielding a focused X-ray spot with a diameter of 400 m, at 3 mA × 12 kV. 2.8. Mass Balance. Experimental mass balance was calculated for every run. For flash pyrolysis experiments, this value reached very good results, being in the range 93−104%, while in the slow pyrolysis runs, the error was higher. Since in the slow pyrolysis 12 gas sampling bags have been analyzed versus 2 units in the flash pyrolysis, it was assumed that the main error in the mass balance was due to the gas volume measurement. Thus, the gas fraction yield was calculated by difference.

By increasing process temperature, the yield of char decreases, while gas percentage shows the opposite tendency. In the temperature range studied, the liquid fraction seems to show a maximum at 500 °C, although in general, only slight differences in the fraction distribution as a function of temperature are observed. The final temperature in the slow pyrolysis was higher than that in the flash ones, which leads to a higher gas percentage. Despite this, the increase of the gas yield is not too high, compared with other types of biomass, such as in the pyrolysis of wood feedstock of Pterocarpus indicus (the gas yield produced increases from around 40%−60% with a temperature increase of around 50 °C),28 sawdust of Radiata pine (45% −67% of gas yields when temperature increases from 562 to 661 °C),29 sugar cane waste (8.5% and 19% of gas yields at 425 and 575 °C, respectively),30 almond shells (gas yield produced increases from 16% to 32% when temperature increases from 400 to 600 °C).31 These values again seems to indicate that, in the pyrolysis of chromium-tanned leather, pyrolytic product distribution is not highly dependent on the process temperature. This tendency of no variability in the percentage of gas obtained as a function of pyrolysis temperature also occurs in other biomass samples, as in the case of safflower seed whose pyrolytic gas yield increases from 25 to 30% in a temperature range of 400 to 700 °C.32 When the sand bed was used, the nitrogen flow was not enough to fluidize it (the minimum sand fluidization velocities were 4.7 cm/s, calculated by Ergun equation, while the N2 velocity inside the reactor at 500 °C was 1.1 cm/s) and the free volume of the reactor was reduced about 11%. This fact reduced the residence time of the volatiles in the reactor. But residence time is not the only variable which is modified by adding the sand bed into the reactor, the temperature at which primary and secondary reactions take place also changes. Owing to the characteristics of the furnace, a temperature profile is observed inside the reactor where the maximum temperature is reached in the 10 centimeters located at the bottom of the reactor (central part of the oven). From this point, temperature decreases down to 400 °C, which is the temperature set up at the top of the reactor, out of the furnace. Therefore, the presence of a sand bed shifts the location where the primary as well as secondary reactions are taking place to the upper part of the reactor, that is, to lower temperatures. As a consequence, the gas yields in the experiments diminished, while an increase in the liquid fraction was noted. These results are in agreement with previous papers published where the influence of the residence time on the pyrolytic products obtained was studied. Onwudili et al.33 studied the effect of the residence time on low-density polyethylene and polystyrene pyrolysis observing that oil yield was reduced and gas yield increased by increasing residence time. Similar results were shown by other authors working with different raw materials.30−35 Previous works developed by our research group also show similar results.36−39 In those papers, a rigorous data treatment allowed us to develop a model, which included many aspects of the process: volume of primary gases and tars at the sand bed temperature, temperature profile in the empty part of the reactor, weight of sample discharged into the reactor, etc. This model allowed us to establish the ratio V/m as a parameter roughly proportional to the extent of the tar cracking (V is the free volume of the reactor without sand, and m is the mass of the sample discharged onto the bed) as well as

3. RESULTS AND DISCUSSION 3.1. Pyrolysis Yields. Table 2 shows the product distributions obtained in all the experiments carried out. The Table 2. Product Yields (wt %) of the Experiments chromium tanned leather

collagen

sample

flash 450 °C

flash 500 °C

flash 550 °C

slow 750 °C

flash 500 °C sand

gasa liquid char

20.5 41.0 38.5

25.1 44.5 30.4

26.6 42.3 31.0

41.8 29.6 28.6

9.8 55.4 34.8

a

flash 500 °C sand 12.1 53.5 34.0

Calculated from mass balance.

chromium leather samples were subjected to flash pyrolysis at 450, 500, 550 °C, and slow pyrolysis up to 750 °C. One flash pyrolysis at 500 °C including a sand bed in the reactor was also performed. In addition, a similar experiment to this run using collagen as raw material was carried out in order to compare similarities and differences between collagen and the tanned leather. The flash pyrolysis experiments were performed at low temperatures (in the range 450−550 °C) in order to find the optimum temperature to maximize the liquid fraction. The results indicate that, in the temperature range evaluated, the highest char yield is reached at 450 °C, while gas and liquid percentages show the lowest value. The average low heating value of the char was 4780 kcal kg−1, measured with a calorimetric pump. Obviously, chromium is the metal with the highest percentage in the char. No other heavy metals have been detected and only traces of Ca, Na, Si, Al, Fe, K, Mg, and Sr have been observed by X-ray microanalysis. Char XPS analysis was performed and, under operating conditions, Cr6+ was not detected. The spectrum obtained for the char was compared to the spectrum of the raw material (tanned-leather) where it is known that the chromium introduced is Cr3+. Both spectra were almost identical. This result was expected according to literature, since even in leather combustion processes the chromium found in the ashes was Cr3+.27 The char obtained presents different applications. Thus for example, besides being used as fuel, chromium presented in the raw material can be recovered from the char,15 or even activated carbon, which can be used as an adsorbent, can be obtained after activation of the char with CO2.8,9 Demineralization of the char can be carried out with HCl.8 3249



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Table 3. Pyrolytic Gas Composition. (g Compound/100 g Leather) chromium tanned leather

collagen

compound

flash 450 °C

flash 500 °C

flash 550 °C

slow 750 °C

flash 500 °C sand

flash 500 °C sand

CO2 CO methane ethane ethene propane propene isobutane n-butane trans-butene n-butene isobutene cis-2-butene isopentane n-pentane 1,3-butadiene n-pentene 2-butyne n-butyne n-hexane n-hexene bencene toluene cis-2-hexene heptane n-heptene acetaldehyde acetonitrile

8.5 2.1 0.30 0.19 0.14 0.16 0.20 0.039 0.033 0.082 0.074 0.15 0.019 0.008 0.010 0.063 0.014 0.0027 0.015 0.0054 0.010 0.0090 0.037 0.0012 0.0039 0.0056 0.081 0.42

8.9 3.2 0.41 0.23 0.20 0.25 0.24 0.049 0.040 0.015 0.091 0.35 0.022 0.0096 0.01 0.057 0.017 0.0028 0.016 0.0041 0.016 0.0059 0.16 0.00094 0.0032 0.0087 0.11 0.55

9.2 4.0 0.68 0.34 0.48 0.29 0.44 0.061 0.047 0.056 0.18 0.44 0.033 0.012 0.018 0.073 0.045 0.0049 0.033 0.016 0.0048 0.0064 0.29 0.0077 0.0045 0.00870 0.42 0.73

13 5.6 0.88 0.26 0.20 0.22 0.25 0.061 0.052 0.11 0.0910 0.22 0.025 0.012 0.019 0.075 0.0 0.0030 00.0 0.0088 0.015 0.0069 0.32 0.0019 0.0083 0.0091 0.76 0.6

6.2 1.2 0.34 0.19 0.16 0.17 0.22 0.041 0.037 0.015 0.082 0.33 0.021 0.0084 0.012 0.056 0.012 0.0070 0.054 0.029 0.015 0.015 0.23 0.0016 0.0061 0.010 0.12 0.51

4.4 0.9 0.22 0.18 0.062 0.15 0.10 0.046 0.037 0.0057 0.040 0.12 0.012 0.011 0.0061 0.032 0.0039 0.0013 0.011 0.0052 0.0014 0.0036 0.085 0.0037 0.0017 0.0 0.022 0.30

to obtain the residence time distribution of the volatile fractions in the reactor. In a comparison of the pyrolysis of the tanned leather and collagen it can be concluded that there are not significant differences among the amount of the three fractions obtained from both samples. 3.2. Gas Composition. Table 3 shows the yields of the gas components identified in the pyrolysis of leather (g compound i/100 g sample) for the experiments performed. As can be seen, there are not significant differences in the gas components obtained from flash and slow pyrolysis, although their yields depend on the heating rate and the process temperature. Carbon oxides are the major components found, followed by hydrocarbons (methane, ethane, ethylene, propane, propylene, isobutene) and some oxygenated components such as acetaldehyde and acetonitrile. Note the significant yield of acetonitrile obtained. Other nitrogenated, chlorinated, and sulfured gases, such as HCN, CH4S, COS, H2S, and ClCH3 are also detected. The ammonia and the NOx yields were measured in the experiment at 500 °C, reaching a value around 1.7% (g compound/100 g leather) for the ammonia and 5 and 0 ppm, NO and NO2 respectively, expressed as a mass parts per million of sample pyrolized. Owing to the inert reaction atmosphere as well as the low process temperature, a very low percentage of NOx compounds was expected. Obviously, ammonia and other nitrogenated compounds come mainly from the protein decomposition, except carbon oxides and ammonia of which the individual yield of each compound is lower than 1%. Table 4 shows the percentage of the identified gas compounds

Table 4. Pyrolytic Compound Gas Groups (g Compound/ 100 g Leather) compound carbon oxides alkanes alkenes alkines aromatics

flash flash flash slow 450 °C 500 °C 550 °C 750 °C

flash 500 °C sand

flash 500 °C sand (collagen)

11

12

13

18

7.5

5.4

0.75 0.76 0.018 0.046

0.97 0.79 0.016 0.16

1.5 1.8 0.038 0.30

1.5 1.0 0.023 0.32

0.83 0.92 0.061 0.25

0.66 0.39 0.012 0.089

classified according to their main functional group. As can be seen, in flash pyrolysis conditions, all the groups increase their yields with temperature. In the case of alkenes and alkynes a low or null increment is observed between 450 and 500 °C, while the most significant increase is produced between 500 and 550 °C. By comparing the results obtained from the flash pyrolysis at 500 °C with or without the sand bed, it can be observed that, in general, the sand bed (i.e., lower residence time) favors the presence of heavy gases (heavier than C4 hydrocarbons), facing the decline of the light elements together with carbon oxides. These results show the decrease in the residence time of volatiles, resulting in a reduced cracking of heavier products. Evolution of individual hydrocarbons versus V/m can be found in the literature,36−39 observing that compounds such as methane, ethane, or ethylene increase their yields by increasing free volume (higher residence time), while the yields of others such as propane or butene are reduced. 3250



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seems to follow different routes since its peak presents an asymmetrical form with two relative maxima: at 400−450 °C and at 700−750 °C. Aromatics and oxygenated compounds present a wider distribution as a function of temperature. It has to be pointed out that some of these components, such as acetaldehyde, are favored at high temperatures. Figure 3 shows the overlapping of the DTG curve and the evolution of gases (CO, CO2, and the addition of all the other

By comparing experiments performed under the same conditions (flash pyrolysis at 500 °C with sand bed) with leather and collagen, it is observed that, in the case of the pyrolysis of collagen, the yields obtained are always lower except for that of isobutane, isopentane, and cis-2-hexene. This decrease is more pronounced in the case of heavier compounds (e.g., n-hexane, n-heptene). Figure 2 shows the evolution, with the temperature, of the compounds detected in the slow pyrolysis. In general, it can be

Figure 3. DTG curve and the evolution of gases in the slow pyrolysis performed.

gases shown in Table 3) in the slow pyrolysis performed. Similarities between the evolution of pyrolytic gases and the solid weight loss are evident. It can be seen that the contribution of the pyrolytic gases to the solid weight loss at around 250−360 °C is mainly due to the CO2 formation together with a small contribution of CO. In the range 360− 540 °C, the evolution of other gases different from carbon oxides plays an important role in the weight loss of the sample. CO2 generation is also detected in this range. Above that temperature, the solid weight only suffers a slight variation, altough gases are detected in the reactor experiments (mainly CO and hydrocarbons). In this process, gases formed above 540 °C could be due to solid decomposition, but also to secondary reactions of heavy volatiles that remain in the hot zone of the reactor. In general, it could be said that the main application of the pyrolytic gas produced is to be used as fuel to provide the process with the energy needed. However, some of these compounds can be useful for other applications. Thus, for example, acetonitrile is a chemical used as a solvent in the purification of butadiene in refineries and battery applications as well as a solvent for the manufacture of pharmaceuticals and photographic film. Acetaldehyde is an important precursor of pyridine derivatives, and other chemicals. It is also used in combination with urea to obtain a useful resin. 3.3. Liquid Composition. The water content of the liquid products was around 53% for collagen pyrolytic liquids and 60% for those coming from pyrolysis of tanned leather. These percentages imply a water yield of around 28% and 27% g water/g sample, respectively. The liquid fraction after desiccation was analyzed by XPS to check the presence or absence of chromium in it. No chromium (neither Cr6+ nor Cr3+) was detected. The spectrum of liquid products found is complex. A typical chromatogram of the liquid fraction shows around 100 different peaks. It is mainly formed by ketones, aromatics, and heteroatoms containing compounds. As an example, Table 5 shows

Figure 2. Evolution of gases generated in slow pyrolysis with temperature.

said that this evolution presents a maximum around 400− 450 °C. This is easily observed with the light hydrocarbons (C2−C5). In the case of trans-butene, the maximum is shifted to low temperatures (350−400 °C). Formation of methane 3251



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Table 5. Pyrolytic Liquids Composition peak no.

retention time (min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61

4.33 4.43 4.56 4.73 5.07 5.15 5.25 5.31 5.38 5.93 6.08 6.14 6.19 6.35 6.45 6.61 6.72 6.95 7.02 7.08 7.28 7.33 7.52 7.55 7.59 7.78 7.84 7.97 8.12 8.17 8.23 8.39 8.49 8.53 8.77 8.87 8.97 9.11 9.56 9.70 9.74 9.86 9.93 10.31 10.53 10.70 10.74 10.92 10.99 11.53 11.62 11.78 11.85 11.93 12.05 12.77 12.88 12.95 13.08 13.24 13.45

library/ID 1H-pyrrole, 2-ethylacetonitrile, [(dimethylboryl)methylamino]1H-pyrrole, 3-ethyl1,2-cyclononadiene phenol piperidine, 3-methylpiperazine, 2,6-dimethylN,N′-dipropylidene-1,1,1diamine 2,2,5,5-tetramethyl-3-oxo-2,3,4,5-tetrahidropyrrole phenol, 2-methyl2-pyridinecarbonitrile cyclohexanone, 2-(dimethylamino)phenol, 3-methyl2.5-pyrrolidinedione,1-methyl 2-piridinamine,3-methyl phenol, 2,6-dimethyl2,2,6,6-tetramethyl-4-piperidinone phenol, 2-ethylbenzeneaceonitrile phenol, 2,4-dimethyl3-methyl-butanonitrile 2,2,6,6-tetramethyl-4-oxo-piperidine 2-piperidinone 1-dodecene naphthalene phenol, 2,4,6-trimethyl3,6-dimethyl-1H-indazole 4-methyl-benzenamine acetamide, N-methyl-N-[4-(1-pyrrolidinyl)-2-butynyl]1,3-cyclohexadiene, 1,2,4,6-tetramethylbenzenepropanenitrile cyclobutanone, 3,3-dimethylazocine, octahydrophenylenediamine, p2,3-dihydro-3-ethoxy-5(1 h)-indolizinone 1H-Indole 5-methyl-5-triazolo(1,5-A)pyrimidine naphthalene, 1-methylbiimidazole 3-azabicyclo[3.2.2]nonane s-triazolo[1,5-a]pyridine, 6-amino1-tetradecene 1H-Indole, 5-methyl1H-Isoindole-1,3(2H)-dione, 2-methylnaphthalene, 1,4-dimethylethyl-2-methyl-3-pentene-1 1H-isoindole-1,3(2H)-dione 1-pentadecene dodecane pyridine, 3-(1-methyl-2-pyrrolidinyl)pyrrolidine, 1-(6-methyl-1-cyclohexen-1-yl)1H-inden-1-one, 2,3-dihydro-4,7-dimethyl2-hydroxy-5-methylcyclohepta-2,4,6-trien-1-one 1-tridecene 1,2-trimethylenebicyclo[2.2.2]octane 1-indanone,3,3,5,6,7-pentamethyl1-acetoxybicyclo[3.3.1]nonan-3-one 1H-pyrrole-2-carboxylic acid, ethyl ester benzene, 1,2-dimethoxy-4-(2-propenyl)6-methyl-2,2′-bipyridine 1-oxide 7-methoxy-4-methylcarbonylcoumarin 3252

g compound/100 g leather

groupa

quality factor (%)

0.11 0.81 0.067 0.079 0.31 0.32 0.061 0.79 0.47 0.37 0.11 0.69 0.39 0.38 0.18 0.18 3.0 0.021 0.56 0.40 0.039 0.13 0.26 0.019 0.17 0.030 0.036 0.15 0.090 0.024 0.21 0.19 0.043 0.12 0.45 0.045 0.097 0.094 0.012 0.013 0.034 0.025 0.084 0.13 0.010 0.013 0.049 0.014 0.0083 0.014 0.018 0.096 0.38 0.027 0.018 0.051 0.077 0.051 0.094 1.4 0.45

N or N/O N or N/O N or N/O C and H phenols N or N/O N or N/O N or N/O N or N/O phenols N or N/O N or N/O N or N/O N or N/O N or N/O phenols N or N/O phenols N or N/O phenols N or N/O N or N/O N or N/O C and H aromatics phenols N or N/O N or N/O N or N/O C and H N or N/O O compounds N or N/O N or N/O N or N/O N or N/O N or N/O aromatics N or N/O N or N/O N or N/O C and H N or N/O N or N/O aromatics C and H N or N/O C and H C and H N or N/O N or N/O N or N/O O compounds C and H C and H N or N/O O compounds N or N/O O compounds N or N/O K

83 78 81 70 95 43 45 32 32 97 87 64 96 38 83 95 83 83 35 68 38 45 64 95 94 96 49 55 43 38 93 47 43 60 59 91 53 95 64 25 64 96 90 81 91 38 97 97 50 22 22 30 90 96 76 38 25 30 52 74 64



Article

Table 5. continued peak no.

retention time (min)

library/ID

g compound/100 g leather

groupa

quality factor (%)

62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84

13.68 13.79 14.15 14.27 14.36 14.39 14.75 14.92 15.09 15.20 15.26 15.40 15.53 15.88 15.92 16.18 16.22 16.39 16.52 16.78 17.01 17.14 18.27

1,4-diaza-2,5-dioxobicyclo[4.3.0]nonane 1-octadecene 2-dimethyl-3-methylidene-1-(2′,4′,6′-trimethylphenyl)azetidine 4,6,8-trihydroxy-1,2,3,4-tetrahydroisoquinolina phenol, 3,5-dimethoxythiazolo[5,4-f]quinoline, 7-methylpentadecanenitrile 8,9-dihydro-7H-cyclopent[a]acenaphthylene 1,4-diaza-2,5-dioxo-3-isobutylbicyclo[4.3.0]nonane 3,9-diazatricyclo[7.3.0.0(3,7)]dodecan-2,8-dione thiophene, 2-(phenylmethyl)2-methyl-naphtalene 5-oxo-7,7-dimethyl-5,6,7,8-tetrahydrocoumarin [1]benzothieno[2,3-d]pyridazine 1-butyl-2-ethyloctahydro-4,7-epoxy-1H-inden-5-ol hexadecane,1-(ethenyloxy) cyclotetradecane (CAS) heptadecanenitrile Tetradecanoic acid, 12-methyl-, methyl ester 2,4,6-trioxoheptadioic acid diethyl ester norleucine, 2-butyl-N,N-dimethyl-, methyl ester 2-cyclohexen-1,4-diol 9-octadecenamide,

0.29 0.051 0.026 0.053 0.023 0.064 0.16 0.027 0.009 0.53 0.050 0.044 0.092 0.11 0.034 0.053 0.059 0.099 0.028 0.14 0.040 0.18 0.093

N or N/O C and H N or N/O N or N/O phenols S compounds N or N/O aromatics N or N/O N or N/O S compounds aromatics O compounds S compounds O compounds O compounds C and H N or N/O O compounds O compounds O compounds O compounds N or N/O

93 95 52 38 50 68 93 90 80 97 42 55 53 59 40 53 91 99 53 18 55 35 93

a

N or N/O = compounds containing N or N and O; O compounds = compounds containing oxygen; C and H = compounds containing only C and H; phenols = phenol and its derivates; aromatics = aromatics; S compounds = compounds containing sulphur.

the list of the pyrolytic condensable volatiles identified in the flash pyrolysis run at 550 °C, as well as their yields obtained. The identification quality for each peak, according to the mass spectra libraries used, is also included. Around 33% of the peaks present a quality value higher than 90%. The compound with the highest yield is the 2,2,6,6-tetramethyl-1,4-piperidinone (triacetoneamine), followed by 6-methyl-2,2′-bipyridine 1-oxide, both of them result of the pyrolysis of proteins. In fact, the 10 major compounds obtained come from protein decomposition. Table 6 shows the percentage of identified compounds classified according to their composition. As can be seen, the

temperature of pyrolysis, a greater percentage of phenols is obtained, versus a slight decrease in the percentage of other oxygenated compounds (ketones, alcohols, aldehydes, and acids). The percentage of “N or N and O compounds” obtained under flash conditions shows a maximum at 500 °C. The yields of the other groups remain almost independent of the process temperature in the range studied. Slow pyrolysis up to 750 °C has led to yields of phenols similar to those obtained at 500− 550 °C and flash conditions, while percentages of aromatics, linear hydrocarbons, and sulfur-containing compounds have increased. The N and/or O compounds are reduced significantly under these conditions comparing with flash pyrolysis and lower temperatures. The comparison of both raw materials, tanned-leather and collagen, indicates that the first one produces higher yields of O compounds, linear hydrocarbons and phenols, while N or N/O compounds show percentages significantly lower. These results can be compared with those obtained from the pyrolysis of meat and bone meal.20−22 Although this biomass has suffered no tanned process and is different from that used in this work, it is also a nitrogenated biomass, with a similar origin, and the pyrolytic products obtained from leather pyrolysis could be closer to those obtained from pyrolysis of meat and bone meal than from other wastes. Thus, in the present work, the percentage of nitrogenated compounds in the liquid fraction is in the range 60−78%, being the major percentage, followed by oxygenated compounds (nonphenolic) in the range 12−22%, phenols in the range of 1.2−14%, and alkanes and alkenes between 0.5 and 3%. According to Ayllón et al.,20 the tar obtained in the pyrolysis of meat and bone meal in a fixed bed reactor is mainly composed of more than 60% nitrogenated aliphatic compound, 15% alkanes and alkenes, 10% oxygenated aliphatic compounds, and about 8% of phenols.

Table 6. Comparative of Pyrolytic Liquids (g Compound/ 100 g Leather) chromium tanned leather flash flash flash slow 450 °C 500 °C 550 °C 750 °C N or N/O compounds O compounds aromatics alkanes and alkenes phenols S compounds total

collagen flash 500 °C sand 10.2

flash 500 °C sand

12.9

15.1

12.4

7.1

21

2.7 0.2 0.1

2.1 0.2 0.1

1.7 0.3 0.3

1.4 0.85 0.35

3.7 0.11 0.2

1.8 0.10 0.02

0.2 0.1 16.24

1.5 0.23 19.32

1.7 0.23 16.76

1.6 0.36 11.72

2.3 0.560 17.07

0.43 0.37 23.64

group named as “N or N/O compounds ” is the major group. It includes those compounds containing nitrogen or nitrogen and oxygen atoms in their structure. The results of the analysis of liquids obtained from flash pyrolysis of tanned leather indicate that by increasing the 3253



Article

As can be seen, except for the group formed by alkanes and alkenes, the values presented are quite similar. Because of its composition, the liquid fraction would need a treatment before being used as a fuel. As Yilmaz el al.8 indicated, the oil obtained from leather waste can be used as fuel or chemical feedstock after retreatment, such as, steam cracking, hydrogenation, Fischer−Tropsch synthesis, etc. Besides the fuel use, pyrolytic liquid can be used as a source of different chemicals and applications.40 Thus for example, the abundance of NH2 groups in this liquid fraction, makes it especially attractive for the production of fertilizers. Furthermore carbonyl groups can also react with amonia, urea, or other sources of NH2 groups to convert the nitrogen to stable and biodegradable organic forms. Carbonyl and carboxylic groups can also react with alcohols to obtain acetals (R′-CH-(OR)2) and esters (R-COO-R′) which present important applications as solvents, components of resins and varnishes, and aroma and flavor chemicals. Other specific compounds can be useful in different industries, for example, pyrrole and its derivatives in the pharmaceutical industry or indols in the pharmaceutical, dye, or perfume industries.

used as fuel after treatment, and it can be used as an interesting source of different chemicals and applications. The solid residue obtained in the pyrolysis presents a heating value of around 4800 kcal kg−1. It can be demineralized and activated with CO2 to prepare activated carbon, or it can also be treated to recover the chromium content in the tanned leather.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: + 34 96 5903867. Fax: + 34 96 5903826. E-mail: [email protected].

■

ACKNOWLEDGMENTS Co-financial support for this investigation has been provided by IMPIVA, Generalitat Valenciana, and FEDER for research within the field of pyrolysis flash of leather waste.

■

REFERENCES

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4. CONCLUSIONS From the results of pyrolysis runs of chromium-tanned wastes shown in this paper, it can be concluded that, in the temperature range 450−550 °C, the pyrolytic product distribution is not highly dependent on the process temperature (around 25% gas fraction, 44% liquid fraction, and 30% solid fraction). In the gas fraction, carbon oxides are the main components, followed by hydrocarbons and a high number of oxygenated components. A significant proportion of nitrogenated and sulfured gases are also detected. Carbon oxides and ammonia present the higher yields. The gas from chromium leather could be suitable to be used as a combustion gas energy input to the process. It is worth emphasizing the yield of acetonitrile obtained. IA very good agreement has been detected between the evolution of pyrolytic gases produced in a slow pyrolysis and the weight loss of the solid measured by thermogravimetric analysis. This comparison indicates that the contribution of the pyrolytic gases to the solid weight loss at 250−360 °C is mainly due to CO2 formation, while at 360−540 °C the hydrocarbon formation plays an important role. Above 540 °C, gases evolved seem to be due to secondary reactions of heavy volatiles that remain in the hot zone of the reactor more than to the solid decomposition. A reduction of the residence time of volatiles in the reactor together with a shift of the location where primary and secondary reactions are taking place to the upper part of the reactor favors a gas distribution with heavier hydrocarbons (from C4), while carbon oxides and light hydrocarbons are reduced. The spectrum of liquid products obtained from the pyrolysis of tanned-leather is mainly formed by compounds that contain nitrogen atoms in their structure (7.1−15.1%) and oxygencontaining compounds (1.4−3.7%). By comparing the pyrolysis of chromium leather and collagen under the same conditions, the liquids obtained from collagen present a higher content of nitrogen-containing compounds (21 vs 10.2) and lower phenols (0.43 vs 2.3) and other oxygenated compounds (1.8 vs 3.7). The liquid fraction obtained from leather waste can be 3254



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