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Ind. Eng. Chem. Res. 1999, 38, 2268-2274
Inhibition and Deactivation Effects in Catalytic Wet Oxidation of High-Strength Alcohol-Distillery Liquors Khaled Belkacemi,‡ Faı1c¸ al Larachi,*,† Safia Hamoudi,† Ginette Turcotte,‡ and Abdelhamid Sayari† Department of Chemical Engineering & CERPIC, and Department of Food Sciences Nutrition, Laval University, Sainte-Foy, PQ, Canada G1K 7P4
The removal efficiency of total organic carbon (TOC) from raw high-strength alcohol-distillery waste liquors was evaluated using three different treatments: thermolysis (T), noncatalytic wet oxidation (WO), and solid-catalyzed wet oxidation (CWO). The distillery liquors (TOC ) 22 500 mg/L, sugars ) 18 000 mg/L, and proteins ) 13 500 mg/L) were produced by alcoholic fermentation of enzymatic hydrolyzates from steam-exploded timothy grass. TOC-abatement studies were conducted batchwise in a stirred autoclave to evaluate the influence of the catalyst (7:3, MnO2/CeO2 mixed oxide), oxygen partial pressure (0.5-2.5 MPa), and temperature (453523 K) on T, WO, and CWO processes. Although CWO outperformed T and WO, TOC conversions did not exceed ∼60% at the highest temperature used. Experiments provided prima facie evidence for a gradual fouling of the catalyst and a developing inhibition in the liquors which impaired deep TOC removals. Occurrence of catalyst deactivation by carbonaceous deposits was proven experimentally through quantitative and qualitative experiments such as elemental analysis and X-ray photoelectron spectroscopy. Inhibition toward further degradation of the liquors was ascribed to the occurrence of highly stable antioxidant intermediates via the Maillard reactions between dissolved sugars and proteins. A lumping kinetic model involving both reaction inhibition by dissolved intermediates and catalyst deactivation by carbonaceous deposits was proposed to account for the distribution of carbon in the liquid, solid, and the vapor phases. Introduction Downstream processing of ethanol plants is known to generate large volumes of wastewater (or stillage) ranging from 6 to 15 m3/1 m3 of ethanol.1 This stillage is heavily loaded with oxygen-demanding organic compounds in the form of suspended solids and dissolved organic matter including sugars, proteins, carboxylic and dicarboxylic acids, phenols, lignins, furfurals, nitrogenous compounds, and so forth. Owing to the increasingly rigorous quality control of effluents, ethanol-manufacturing industries have to minimize waste in the first place through improved process design and eventually through the development of alternative processes. Meanwhile, stillage has to be freed from its organic content because of the following considerations: (i) direct discharge of such wastewaters into natural waters constituting a potential threat to the aquatic life; (ii) increasingly stringent regulations regarding mandatory discharge limits being enforced; (iii) the trend in the chemical manufacturing industries to use closed-loop operation by recycling and reusing waters. To reduce the organic content of stillage, several methods were evaluated. A list of such techniques along with their main drawbacks is given hereafter. (1) Concentration/incineration gives rise to harmful air emissions. In addition, concentration of the oxygendemanding compounds in the wastewaters must be adequately adjusted to fall within the concentration * Author for correspondence: Phone: (418) 656-3566. Fax: (418) 656-5993. E-mail:
[email protected]. † Department of Chemical Engineering & CERPIC. ‡ Department of Food Sciences and Nutrition.
operating window required for autothermal sustainability. Profitability of incineration is limited to wastewaters with chemical oxygen demand (COD) above 300 000 ppm.2 (2) Anaerobic digestion with biomethane recovery3-6 is a biological treatment whose main disadvantages are (i) large site area requirements, (ii) generation of large volumes of sludge with associated disposal problems, and (iii) inherent sensitivity of microorganisms to the type of substrates and concentration of effluents, often requiring dilution prior to the treatment. (3) Noncatalytic wet oxidation with steam generation followed by aerobic polishing7-9 is an aqueous-phase flameless combustion technique which is usually run under severe conditions such as elevated temperature, pressure, and residence time. (4) Noncatalytic supercritical water oxidation requires excessively high temperatures (up to 873 K) and pressures (>22.1 MPa).10 (5) Catalytic wet oxidation often uses either dissolved transition-metal cations or supported noble metals and various metal oxides:11 This method arose as a means to alleviate the wet oxidation severity by using efficient heterogeneous or homogeneous catalysts.11-16 A careful survey of the literature revealed that most investigations on wet oxidation treatments of alcoholdistillery liquors either were conducted in the absence of catalysts or used homogeneous catalysts.7,8 The purpose of the present study was therefore to evaluate the efficacy of solid-catalyzed wet oxidation for enhancing total organic carbon (TOC) removal from raw highstrength nondiluted alcohol-distillery liquors. Because of its effectiveness in treating various domestic and industrial contaminated wastewaters,17,18 a manganese
10.1021/ie980005t CCC: $18.00 © 1999 American Chemical Society Published on Web 04/21/1999
Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 2269 Table 1. Typical Composition of the Alcohol-Distillery Waste ASF Liquors sugars (g/L)
ash (g/L)
soluble lignin (g/L)
furfural (g/L)
proteins (g/L)
phenols (mg/L)
TKN (g/L)
TOC (ppm)
COD (ppm)
pH
18.0
0.4
5.2
0.3
13.5
NDa
2.1
22 500
50 000
4.5-5
a
Not determined.
and cerium composite oxide catalyst (7:3, MnO2/CeO2; atom ratio, 7Mn/3Ce) was chosen in the current work. Preliminary experiments showed that, during the catalytic wet oxidation process (CWO), organic carbon was also removed via two additional concurrent routes, namely, noncatalytic wet oxidation (WO) and thermolysis (T). Accordingly, the experimental methodology used in the current work as well as the proposed kinetic model were based on a stepwise approach. Experimental Section Materials. The alcohol-distillery waste liquors tested in this work were obtained from mature timothy grass (Phleum pratence L.) and were pretreated to separate cellulose from hemicellulose. Pretreatment by aqueous steam fractionation (ASF) was followed by enzymatic hydrolysis, ethanol fermentation, and distillation.19,20 Table 1 shows the typical composition of the ASF liquors. To assess the feasibility of solid-catalyzed wet oxidation of ASF liquors, a MnO2/CeO2 mixed oxide catalyst was used. It was prepared by coprecipitation of CeCl3 (Fisher Scientific Co.) and MnCl2 (Sigma Chemicals Co.) as described elsewhere.21 Procedure. Approximately 100 mL of nondiluted raw-filtered distillery ASF liquor was oxidized batchwise in a 300 mL stainless steel high-pressure Parr unbaffled autoclave (model 4842, Parr Instrument Inc., Moline, IL) equipped with an electrical heating jacket. The setup was designed to withstand pressures and temperatures up to 20 MPa and 620 K, respectively. The slurry (liquor + catalyst) was agitated with a magnetically driven 35 mm diameter six-bladed pitched turbine impeller. In a typical run, the liquor was loaded into the reactor, which was then sealed and preheated to the set temperature under liquor self-pressure while stirring at 750 rpm. This rotational speed was verified to be adequate to keep mass-transfer resistances marginal while inducing a sufficiently high oxygen sparging through the slurry.22 Prior to oxygen pressurization and catalyst introduction, a liquid sample was withdrawn, gravity-filtered, and analyzed for its initial TOC content. Oxygen was then introduced into the reaction vessel until it reached its preset partial pressure. Runs were performed at temperatures varying from 453 to 523 K, oxygen partial pressures from 0.5 to 2.5 MPa, and a catalyst concentration of 5 g/L. Thermolysis and noncatalytic runs were carried out in a similar fashion except that no catalyst was used in any of these experiments and no oxygen was added in purely thermal degradation tests. Analytical Methods. The distillery liquors were analyzed for their protein content by the micro-Lowry method (Protein Assay Kit, procedure #P 5656, Sigma Diagnostics, Sigma-Aldrich Canada, Oakville, Ontario). Their total Kjeldahl-nitrogen (TKN) and their COD were measured according to the APHA standard methods.23 Their sugar content was assessed by HPLC (model WAT-D85188, Waters Associates Inc., Milford, MA) and GC (model HP 5890 GC system, FID detector)
chromatographic methods. Raw and treated liquors were analyzed with respect to their total organic carbon content by means of a combustion/nondispersive infrared gas analysis technique (model Shimadzu 5050 TOC analyzer, Mandel Scientific Inc., Montre´al, PQ) using ultrapure air (Praxair Inc., Que´bec, PQ). Powdered and sieved catalysts were further characterized by nitrogen adsorption using an automated Coulter Omnisorp 100 series (Coulter Corp., Hialeah, FL) gas analyzer. The specific surface area for fresh MnO2/CeO2 as calculated using the BET model was 107 m2/g and the mean average diameter of particles was 63 µm. The carbon content of the carbonaceous deposits was quantified, after washing, filtering, and drying the spent catalyst, using a CHN elemental analyzer (model 1106 Carlo Erba, Technical Marketing Association Ltd., Pte-Claire, PQ). Metals leaching off from the catalyst to hot wet oxidation solutions were verified previously to be negligibly small for manganese and cerium after the concentration of dissolved metals was measured by plasma emission spectrometry.22,24 XPS spectra for fresh and thoroughly washed and airdried used catalysts were recorded using a V.G. Scientific Escalab Mark II system (East Greastead, UK) fitted on a Microlab system (vacuum generators) with a nonmonochromatized Mg KR X-ray source (hν ) 1253.6 eV). XPS measurements were made at room temperature and a background pressure below 7.5 × 10-8 Torr. The binding energies (B.E.) were calibrated by setting the Au 4f7/2 and Mn 2p3/2 peaks equal to 83.8 and 642.2 eV, respectively. For Au calibration, metallic gold was deposited on the catalysts using a sputter coater (Nanotech SEMPREP II, Cambridge, UK). To determine the surface atom composition, survey scans (0-1150 eV B.E.) recorded at a spectrometer resolution of 1 eV were used. Results and Discussion Thermolysis. The pyrolysis-hydrolysis tests were run in the temperature range 473-513 K under liquor autogenous pressure and without the addition of neither an oxidizing agent nor a catalyst. The time course of the TOC decline during thermal degradation of ASF liquor is displayed in Figure 1 for three different reaction temperatures, namely, 473, 493, and 513 K. The corresponding data shown as filled circles are referred to as T which stands for thermolysis tests. It is important to notice that the initial TOC values decreased as the reaction temperature increased as a result of precocious thermal degradation during the transient heat-up phase of the liquor. As seen, most of the TOC decline took place within the first 10 min of reaction. The amount of dissolved organic carbon decreased significantly. After 60 min of reaction at 473 or 513 K, it reached 72% and 68% of its original value, respectively. Such a drop-off in TOC was accompanied by a buildup of volatiles in the vessel headspace and the occurrence of insoluble charred “roasty-aroma” carbonaceous solid residues suspended in the liquor. The high content in soluble sugars and proteins in raw ASF
2270 Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999
Figure 1. TOC abatement for ASF liquor at different temperatures by thermolysis (T), wet oxidation (WO), and catalytic wet oxidation (CWO): (a) T ) 473 K; (b) T ) 493 K; (c) T ) 513 K. MnO2/CeO2 catalyst ) 5 kg/m3 and TO2 ) 1.5 MPa in T + WO and T + WO + CWO runs.
(Table 1) was propitious to the so-called Maillard reactions which are known to generate highly reducing (or antioxidative) soluble and insoluble materials known as Maillard reaction products (MRPs). This complex set of reactions involves a series of condensation, enolization, Amadori rearrangement, sugar dehydration and fragmentation, aldol condensation, polymerization, and so forth. At their ultimate stage, the Maillard reactions form roasted residues comprising insoluble reductones or melanoidin-like oligomers/polymers.25,26 Other insoluble residues may also be formed by reactions between soluble lignin, sugars, and proteins.27 Furthermore, as seen in Figure 1, thermolysis of ASF liquors under autogenous conditions leveled off after about 10 min of heating, the remaining soluble organic carbon (TOC) being refractory to further degradation. To improve the TOC abatement, treatment of the liquors in the presence of oxygen, referred to hereafter as noncatalytic oxidation or simply wet oxidation (WO), was explored.
Noncatalytic Oxidation. Noncatalytic wet oxidation experiments for raw ASF were conducted within the same range of temperature as that of the thermolysis tests in the presence of oxygen partial pressures ranging between 0.5 and 2.5 MPa. Figure 1 depicts the isothermal plots of the TOC decline versus the reaction time under the combined effect of thermal degradation and noncatalytic wet oxidation, referred to as (T + WO) in the figure. It is seen that the impact of oxygen addition, here 1.5 MPa, on the TOC abatement was quite strong. For instance, at 493 K, the TOC drop-off was 30% and 47% of its original level for T-runs and (T + WO)-runs, respectively (Figure 1b). The antioxidative nature of the MRPs and the production of stable soluble oxidation byproducts, such as carboxylic acids, resistant to further oxidation28 are likely to be at the origin of the inhibiting character developed by the liquor during reaction with or without oxygen. Hence, the reducing ability of the Maillard reactions generated compounds and the recalcitrance of oxidation intermediate products make the noncatalytic oxidation insufficient to deplete the liquors’ TOC content to acceptable levels within the temperature and oxygen pressure ranges of interest. The addition of heterogeneous catalysts was therefore explored as a third alternative to examine whether substantial improvements in TOC reduction could be achieved. Catalytic Oxidation. TOC abatement data for ASF liquor during oxidation over a MnO2/CeO2 composite oxide catalyst at PO2 ) 1.5 MPa in the temperature range 473-513 K are shown in Figure 1. The filled triangle symbols, referred to as (T + WO + CWO) in the legend, represent the combined outcome of three concurrent processes, namely, thermoylsis, noncatalytic wet oxidation, and catalytic wet oxidation. The initial drop in TOC content within the time interval of the first sampling event following injection of the catalyst into the reactor increased by up to 50% as compared to that of thermolysis alone. After this initial “flash” removal, the reaction proceeded at a lower rate to ultimately level off at TOC values significantly lower than those for thermolysis. Complementary tests were carried out using spent nonregenerated catalyst and treated liquor which has already undergone a first catalytic wet oxidation reaction. The objective of such experiments was to demonstrate whether the reaction breakdown was due to (i) catalyst deactivation, (ii) increased recalcitrance of treated liquors to further degradation, or (iii) combination of the above. Both the spent catalyst and the treated ASF liquor were collected by phase separation after a run using raw ASF for 2 h under (T + WO + CWO) conditions of Figure 1a. As shown in Figure 2, the TOC degradation profile of raw ASF liquor in the presence of the spent catalyst coincided, within measurement uncertainties, with that of the raw liquor oxidized noncatalytically. This indicates that the spent catalyst was already completely deactivated at the end of a single experiment using raw liquor. In addition, as shown in Figure 2, further oxidation of a treated ASF liquor under noncatalytic conditions or in the presence of a fresh catalyst led to low, but comparable, TOC conversions (
