Ind. Eng. Chem. Res. 2002, 41, 6503-6509
6503
KINETICS, CATALYSIS, AND REACTION ENGINEERING Formation of Organic Acids during the Hydrolysis and Oxidation of Several Wastes in Sub- and Supercritical Water Lourdes Calvo* and David Vallejo† Supercritical Water Oxidation Projects, Environmental and Water Resource Engineering Program, University of Texas at Austin, Austin, Texas 78758
The objective of this work was to evaluate the transformation characteristics of four organic substances in supercritical water. The purpose was to demonstrate the yield and stability of the acetic acid produced under hydrolytic and oxidative conditions. Other organic acids, such as formic, glycolic, and lactic acids, were monitored. Cellulose and coconut oil solutions, as well as brewery and dairy effluents, were used as feedstocks. Batch tests were performed at fixed conditions of 400 °C, 27.6 MPa, and 5 min of reaction time. Hydrogen peroxide was the oxidant. Under hydrolytic conditions, 70% of the initial carbon remained as the liquid product. On the contrary, in the presence of excess oxygen, there was a 95% conversion to the gaseous product. Typically, less than 15% of the initial total organic carbon was converted to the acids. The use of catalysts (i.e., TiO2) and additives (i.e., H2SO4) did not enhanced the organic acid yield. However, catalysts addition facilitated feedstock breakdown at lower oxygen levels. To evaluate the effect of alkali addition and the use of lower temperatures, continuous flow tests were conducted using glucose as the substrate. Under alkaline conditions, organic acid production increased. For example, at 250 °C and 27.6 MPa with the addition of NaOH (55.6 wt % glucose) and providing 25% stoichiometric oxygen, about 77% glucose was converted to acetic acid (17%), glycolic acid (22%), and formic acid (38%). These preliminary results indicate that valuable compounds could be obtained during the degradation of organic wastes in sub- and supercritical water instead of complete oxidation to CO2 and water. Introduction Water is a unique solvent. Its chemical and physical properties vary considerably at high temperatures and pressures when compared to water at ambient conditions. Above its critical conditions (374 °C and 22.1 MPa), supercritical water (SCW) shows low-viscosity and high-diffusivity values, similar to those of ambient organic solvents, e.g., acetone. The low polarity of SCW results in greatly decreased solubility of inorganic salts and increased solubility of organic compounds. Also, SCW exhibits a relatively high heat capacity, and this results in efficient heat transfer. As ambient water transforms into a supercritical fluid, about two-thirds of the hydrogen bonds are destroyed, causing a drastic drop in the dielectric constant. These properties vary with density, which is a strong function of temperature and pressure in the critical region; thus, the reaction environment can be manipulated by varying the operating conditions. An additional advantage of using SCW as the solvent is that the material to be processed may already be in aqueous solutions and the product can be * To whom correspondence should be addressed. Current address: Departamento de Ingenieria Quı´mica, Universidad Complutense de Madrid, Avenida Complutense s/n, 28040 Madrid, Spain. Phone: +34 91 394 4185. E-mail: lcalvo@ quim.ucm.es. † Current address: Montgomery Watson Harza Global, Dallas, TX.
easily separated and recovered from the reactor effluent. In some cases, water needs not to be removed from the final product. Supercritical water oxidation (SCWO) uses the unique properties of SCW to transform organic waste streams into environmentally innocuous products, and if carried to completion, the residuals are mainly carbon dioxide, nitrogen, water, and inorganic salts or ash. Under such conditions, water serves as a solvent for organic compounds and oxygen, creating an optimum medium for oxidation reactions. Destruction efficiencies greater than 99.99% have been achieved using short reactor times of 1 min or less.1-3 The SCWO reactor facility can operate as an enclosed facility. Additionally, the SCWO concept is flexible and capable of handling a wide range of organic concentrations.3 Because the SCWO process is exothermic, a waste stream with an organic concentration of 10% or more and an appropriate heat exchanger will need little or no external heat beyond that required to initiate the reactions.4 In addition to the potential complete transformation of organic substances to carbon dioxide, other aspects of the hydrothermal reactions may be of practical interest, e.g., the use of SCW as a medium to carry out synthesis. Historically, much of the effort was directed to gasification and hydrolysis of the biomass.5-8 Most of these studies used cellulose and glucose as the model compounds because of the high content of cellulosebased polymeric materials in the biomass.
10.1021/ie020441m CCC: $22.00 © 2002 American Chemical Society Published on Web 11/15/2002
6504
Ind. Eng. Chem. Res., Vol. 41, No. 25, 2002
Table 1. Characteristics of the Feedstocks for Batch Tests type of feedstock cellulose coconut oil brewery wastewater dairy wastewater
composition
initial TOC (mg/L)
initial COD (mg/L)
(C6H10O5)n C45H86O6
3200 6200
8400 23 500
36000 4700
137 800 13 000
Sasaki et al.9 showed that cellulose hydrolysis products in SCW were essentially glucose or oligomers of this monomer. On the other hand, the products of glucose hydrolysis depended on the operating conditions. Thus, Kabyemela et al.10 found that this compound underwent the following: (a) isomerization, producing fructose; (b) dehydration, producing 1,6-anhydroglucose; (c) C-C bond cleavage, producing erythrose and glyceraldehyde. The conditions were temperatures of 300400 °C and pressures of 25-40 MPa for very short residence times between 0.02 and 2 s. At higher temperatures (425-600 °C) and longer reaction times (510 s), shorter and more refractory compounds were detected, namely, formic, acetic, lactic, and propenoic acids, as well as furan derivatives.11 The stability of those species decreased sharply at higher temperatures where transformation to gas products was high. Holgate et al.11 studied the oxidation of glucose under SCW conditions. As with hydrolysis, the conversion of glucose was almost complete. Liquid-phase products were greatly reduced compared to hydrolysis conditions. Again, acetic acid and furan derivatives were identified as intermediates. At temperatures above 550 °C, no liquid-phase products were detected. Other authors reported the formation of acetic acid as one of the main intermediates remaining during SCWO and wet air oxidation of organic wastes.12-14 The highest yields ranged from 8 to 10 wt %. These studies demonstrated the refractory characteristics of this acid, but none of them focused on enhancing acetic acid production. The purpose of this work was to demonstrate the yield and stability of the acetic acid produced during the conversion of selected organic wastes under hydrolytic and oxidative conditions. Other organic acids, such as formic, glycolic, and lactic acids, were also monitored. Cellulose and glucose were chosen as feedstocks because they were shown to produce acetic acid in SCW conditions and were present in several wastes such as pulp and paper mill wastes and municipal sludges. Additional feedstocks such as brewery waste, oils, and dairy waste were also used in the survey. The identification of the feedstocks and process conditions was discussed previously.15 First, the oxygen requirements and catalyst effectiveness were investigated. For hightemperature oxidation processes carried out in the presence of water, the most common catalysts were vanadium-based systems (single, binary, or tertiary), such as V2O5, V2O5-Cr2O3, V2O5-Sb2O5, and Co-VAl. However, previous experience in SCWO showed that V2O5-based catalysts may be unstable in the SCW medium.16 In the case of oxidative hydration in the gaseous phase, Mo and Co oxides were used.16 Therefore, a survey of several catalysts (e.g., TiO2 and Mn/ CeO2) and additives (e.g., H2SO4) was planned for this investigation. On the other hand, it is well-known that organic acids can be produced by heating cellulosic materials with an alkali solution at atmospheric pressure or in closed
notes TOC and COD calculated assuming 95% hydrolysis to glucose average molecular formula estimated from the free fatty acid composition sample contained yeast cells simulated using a solution of skim milk in water [1/10 (v/v)]
systems. Little conversion of the cellulosic materials to organic acids occurred at 100 °C or lower. Above 300 °C, a substantial part of the acids produced (except acetic acid) was destroyed.17 At temperatures ranging from 240 to 280 °C and autogenous pressure, the products of totally converted cellulosic materials rendered a complex mixture of mono- and dicarboxylic acids.18,19 Formic, acetic, glycolic, and lactic acids were important alkaline degradation products, with lactic (15-20 wt %) and formic (10-14 wt %) acids being the major reaction products. Acetic acid production was typically less than 4 wt %. Depending on the raw material, the maximum yield of these acids varied from 40 to 50 wt %. The final objective of this work was to evaluate the effect of alkaline environments on organic acids formation. Materials and Methods Both batch and continuous-flow SCW reactor systems were used. A batch reactor was utilized to evaluate the influence of oxygen concentration and catalysts on organic transformation involving SCW. To evaluate the alkaline transformation of glucose, a continuous-flow apparatus was used. Liquid samples were analyzed to determine the organic carbon content and acids formation. Materials. Four materials were selected as surrogates of wastewater and feedstocks. Powdered cellulose and glucose (analytical grade; Sigma) served as biomass model compounds. Coconut oil (Copra oil; Sigma) was used to simulant oily vegetable wastes. Brewery wastewater and a solution of milk [10% (v/v)] completed the selected feedstocks. In oxidation tests, a 35% solution of hydrogen peroxide was used, assuming that a molecule of hydrogen peroxide provides half a molecule of oxygen. For cellulose, oxygen requirements were based on the glucose molecular formula. For coconut oil and the remaining feedstocks, the oxygen requirements were based on the free fatty acid composition and the chemical oxygen demand (COD), respectively. Sulfuric acid (0.02 wt %) was used in the acid-catalyzed tests. TiO2 (Chemical Process Products Norton) and Mn/CeO2 (Carus Chemical Co.) were the catalysts. The initial total organic carbon (TOC) and characteristics of the feedstocks are summarized in Table 1. Batch Reactor System. Figure 1 depicts details of the batch reactor assembly. The major components included a reactor vessel assembly, a fluidized sand bath, an ice bath, a wrist-action shaker, a temperature controller, and temperature/pressure readout devices. The reactor consisted of 316 stainless steel coiled tubing of 76.2-cm length, 1.27-cm outside diameter, and 0.21cm wall thickness. The volume of the reactor was 34 cm3. A high-pressure valve connection permitted sampling of gaseous products. A thermocouple was inserted into the reaction chamber. The reaction pressure, as defined by the initial volume of the solution and
Ind. Eng. Chem. Res., Vol. 41, No. 25, 2002 6505
Figure 2. Schematic of the continuous-flow reactor system.
Figure 1. Schematic of the batch reactor system.
temperature, was recorded. The pressure varied within (5% from the preestablished value, which was set at 27.6 MPa. An amount of 0.1 ( 0.0001 g of the solid feedstocks (cellulose and coconut oil) was loaded individually in the batch reactor. Then, a mixture of deionized (DDI) water and hydrogen peroxide, when applicable, was added. The volume of liquid was calculated based on the density of water at the target temperature and pressure. Usually, 10-12 mL was required. In the case of the brewery and dairy wastes, this volume was added directly. When catalysts were used, 0.1 g was placed into the reactor, and DDI water was substituted for a 0.02% solution of sulfuric acid. Atmospheric air trapped in the reactor was less than 5% of the oxygen demand. After the loading procedure was completed, the reactor was immersed in the heated sand bath and shaken for 5 min. Following the prescribed reaction time, the reactor was mechanically transferred from the sand bath to the ice-water bath. The time required to cool the mixture to room temperature was about 1 min. Gases were collected with a syringe. The liquid samples were displaced from the reactor with pressurized air, transferred into 20 cm3 glass vials, and analyzed immediately or refrigerated for subsequent analyses. The continuous-flow apparatus is shown in Figure 2. The feed solution was delivered to the preheater by an injection pump. Both the preheater and reactor consisted of 316 stainless steel coiled tubing. The tubing diameters were 0.318-cm outside diameter and 0.140cm inside diameter. The lengths of the preheater and reactor tubes respectively were 280 and 400 cm. The temperature of the sand bath was 2 °C higher than the target reaction temperature. A thermocouple was placed at each reactor inlet and outlet. The reaction temperature was the average of the upstream and downstream fluid temperatures. Typically, the difference between these two temperatures was less that 3 °C. The pressure was controlled manually by adjusting a backpressure regulator and recorded by the operator. The pressure
was within (0.5 MPa. The effluent was quenched to ambient temperature through the use of a countercurrent heat exchanger. Prior to releasing the effluent to the environment, the effluent was passed through a gas-liquid separator. To initiate an experiment, DDI water was pressurized to 27.6 MPa. Then, the pump was switched to a feed solution, consisting of a dilute glucose solution (9 g/L). In alkaline tests, different quantities of NaOH (1-10 g/L) were added. In the oxidation tests, hydrogen peroxide was used. During hydrolytic experiments, the dissolved oxygen was eliminated from the feed solution by purging with helium. Effluent samples were collected at 5-min intervals. During sampling, the pressure drop was less than 0.7 MPa. The yield was reported as the average of three or more samples. Analytical Procedures. Effluent and influent TOC analyses were performed on all samples. A TOC analyzer (Shimadzu TOC-5050) with an automatic sampler (ASI-5000) was used. Test procedures were performed in accordance with Standard Methods 5310 C. The detection limit for these measurements was 1 mg/L. For brewery and dairy wastewaters, influent COD measurements were used to estimate oxygen requirements. The COD analyses were made using Standard Methods 5220 D and HACH (model 45600). A Bausch & Lomb spectrophotometer (Spectronic 88) was used for COD colorimetric analyses. The detection limit for COD measurements was 2 mg/L. A Dionex System 14 ion chromatograph (AS-3 separatory column, AG-1 guard column, and micromembrane suppresser AMMS-11), equipped with a conductivity detector, was used to identify and quantify the anionic species derived from the dissociation of acetic, formic, lactic, and glycolic acids in effluent samples. A 2.5 mM sodium borate solution was used as the eluant. The flow rate was 1.1 cm3/min. Peak identification was accomplished by comparison of sample peak retention times with those of standard solutions. The detection limit was about 10 mg/L. The analytical procedure required careful attention to the superposition of the acetic and lactic acid peaks. A gas chromatographic analysis was used to verify results.
6506
Ind. Eng. Chem. Res., Vol. 41, No. 25, 2002
Figure 3. Effect of oxygen addition on the transformation of the feedstocks. Experimental conditions: 400 °C, 27.6 MPa, reaction time 5 min.
Results and Discussion Batch tests were conducted to investigate the transformation of the feedstocks under hydrolysis and oxidation conditions. These experiments were performed at temperatures and pressures of 400 °C and 27.6 MPa, respectively. These levels are slightly above the critical point of water. The reaction time was 5 min. For the cellulose and coconut oil wastes, the effect of catalysts was also evaluated. Continuous-flow tests were carried out to evaluate the impact of alkaline environments on organic acid formation. The carbon conversion to watersoluble products and the formation of selected shortchain carboxylic acids were monitored. Organic destruction was quantified in terms of TOC. The yield was expressed as a percentage of the initial TOC converted into the acid carbon. The proportion of the organic acid in the effluent was expressed as a fraction of the effluent TOC. The results are discussed in detail below. Effect of the Oxygen Content. The amount of oxygen added and the carbon conversion to watersoluble products for each feedstock are presented in Figure 3. In hydrolysis tests, carbon conversion to watersoluble products was relatively high, about 70% of the initial TOC; however, the addition of oxygen enhanced conversion of the carbonaceous feedstocks to gaseous products. In batch reactors, because of mixture and heat-transfer inefficiency, it is usually necessary to use higher temperature and/or higher oxygen supply than those in continuous-flow systems to obtain a similar conversion. Thus, to achieve higher conversion to gas products, it was necessary to provide big excess oxygen, as based on the estimated stoichiometric requirements. Convertibility of the feedstocks to CO2 was dependent on the influent composition. Cellulose, because of its polymeric structure, required more than estimated oxygen levels. Organic acid residuals were relatively low during the hydrolytic and oxidative tests. Generally, a maximum of about 5-15% of the initial TOC was converted to those acids. Only acetic and formic acids were detected during the oxidative tests, while little lactic and glycolic acids appeared during hydrolytic tests. In all cases, the acetic acid yield depended upon the organic carbon
Figure 4. Impact of carbon conversion to water-soluble products on the acetic acid yield. Experimental conditions: 400 °C, 27.6 MPa, reaction time 5 min.
conversion to water-soluble products. Figure 4 depicts a somewhat uniform relationship between the acetic acid yield and liquid product. For example, in the case of cellulose, a maximum acid yield was 10.5% while the carbon conversion to water-soluble products was 15%. For the other feedstocks, this maximum occurred ranging from 15 to 30% carbon conversion. After the maximum, the acetic acid yield dropped because of its own degradation. The same pattern was found during the degradation of industrial excess activated sludge from two independent sources.14,15 Moreover, Vallejo15 obtained different levels of carbon conversion not only by increasing the amount of oxygen supplied but also by varying the reaction temperature and time, and there was always a clear correlation between acetic acid formation and carbon conversion. The fraction of the total effluent TOC in the form of acetic acid increased as the organic substrate was transformed to gaseous products. For low conversions, the organic carbon as acetic acid in the effluent decreased because of the presence of several other intermediate compounds. Acetic acid was relatively refractory, and its fraction increased in the effluent (Figure 5), as the rest of the reaction intermediates were oxidized to CO2. These findings agree with the kinetic model developed by Li et al.20 In this case, acetic acid was considered to be a rate-limiting intermediate. Therefore, to maximize the acetic acid yield, the optimum carbon conversion of the cellulose was 11.7%. However, because the proportion of acetic acid in the effluent continued to increase as cellulose was transformed to gas products, this implies a tradeoff. There were various color changes as different feedstocks were processed. For cellulose, low-conversion samples were accompanied with a dark brown color and visible char. As the oxygen content and conversion to gaseous products increased, the color and char disappeared. Oily emulsions appeared during the treatment of coconut oil. Such evidence of incomplete conversion occurred during hydrolytic and low oxygen content tests. At high conversions, the effluents were transparent and contained no suspended solids. For brewery wastewater, the initial reaction produced a dark brown color and suspended solid particles. As conversion increased, the liquid became transparent. Colored effluents as well as suspended solid products observed in low-conversion
Ind. Eng. Chem. Res., Vol. 41, No. 25, 2002 6507
Figure 5. Fraction of effluent TOC as acetic acid during the transformation of the feedstocks. Experimental conditions: 400 °C, 27.6 MPa, reaction time 5 min.
Figure 6. Effect of catalyst addition on the carbon conversion to water-soluble products, using cellulose as the substrate. Experimental conditions: 400 °C, 27.6 MPa, reaction time 5 min.
samples probably corresponded to high molecular weight polymerized compounds that at high oxidant conditions were less produced or rapidly destroyed. In general, liquid samples were acid due to organic acid presence. Effect of Catalysts. Catalysts have been used to (a) enhance the conversion of complex compounds, (b) reduce the reaction time, (c) lower the reaction temperature, and (d) optimize the reaction pathways.16 Using cellulose as the substrate, the impact of TiO2 was evaluated on carbon conversion and organic acid formation. Results are shown in Figure 6. Less oxygen was required to achieve higher conversions to gas products in oxidation tests. For example, only 15% of the initial TOC remained as water-soluble products with 100% oxygen. To achieve a similar result, more than 1000% stoichiometric oxygen was required without catalyst. Similar results were found while testing coconut oil. Catalysts, TiO2 and Mn/CeO2, improved oil digestion and increased the conversion with less oxygen requirements. To evaluate acid-catalyzed effects, sulfuric acid (0.02%) was used during the transformation of cellulose. The
findings were similar to those obtained using solid catalysts. Higher conversion to gas products was found with lower oxygen addition. The use of catalysts exhibited a negligible impact on the production of organic acids as reported in Table 2. The acetic acid yield was similar to that of previous tests. On the contrary, sulfuric acid increased the formation of formic acid. The presence of char was also more evident. It seemed that the acid addition changed the degradation pathways, favoring pyrolytic routes which conducted to higher formic acid and char formation. Effect of Alkali Addition. To enhance the production of organic acids, tests were carried out under alkaline conditions. Glucose was used as a raw material. A continuous-flow reactor was utilized. To provide background information, glucose transformation was first studied under conditions similar to those of batch experiments, that is, 27.6 MPa and 400 °C. Increasing the oxygen content from 0% (hydrolysis) to 100% stoichiometric requirements was assayed. The flow rate was 3 cm3/min, providing 23 s of preheating time and 43 s
Table 2. Carbon Conversion to Liquid Products and Organic Acid Formation from Cellulose in SCWa conditions amount of oxygen (%) 0 100 200 400 1000 2000 3000 4000 5000 0 20 50 100 250 0 50 100 200 a
catalyst none
TiO2 (0.1 g)
H2SO4 (0.02%)
carbon acid yield fraction of effluent TOC as acid conversion to (wt %, carbon basis) (wt %, carbon basis) water-soluble acetic formic lactic glycolic total acetic formic lactic glycolic total products (%) 4.1 5.7 8.0 7.2 9.5 10.5 9.8 4.5 1.6 4.9 5.9 10.6 8.6 7.2 ND 1.4 2.9 5.8
0.6 2.7 5.5 6.3 5.1 3.4 2.6 0.4 0.2 2.4 2.0 0.6 1.6 2.3 7.3 10.1 16.6 19.4
2.5 ND ND ND ND ND ND ND ND 0.3 ND ND ND ND 1.2 1.3 ND ND
2.9 0.6 0.2 0.2 ND ND ND ND ND 0.1 ND ND ND ND 1.0 0.5 0.4 ND
10.1 2.3 9.0 4.2 13.7 8.6 13.7 8.6 14.6 19.0 13.9 36.0 12.4 37.5 4.9 38.6 1.9 47.2 7.7 3.3 7.9 5.0 11.3 11.0 10.2 12.3 9.5 11.3 9.5 4.5 13.3 ND 19.9 2.2 25.2 8.6
0.2 1.3 3.9 4.9 6.6 7.5 11.5 2.4 4.7 1.1 1.1 0.8 2.9 5.8 2.9 6.8 16.2 37.7
1.4 ND ND ND ND ND ND ND ND 0.2 ND ND ND ND 0.7 1.3 ND ND
3.8 1.1 0.3 0.3 ND ND ND ND ND 0.2 ND ND ND ND 1.3 1.0 1.1 ND
7.7 6.5 12.8 13.8 25.6 43.4 49.0 41.0 51.9 4.7 6.1 11.8 15.2 17.1 9.5 9.1 19.4 46.3
69.8 54.4 37.2 33.9 20.1 11.7 5.8 4.6 1.4 59.8 47.2 21.5 14.8 10.2 64.3 38.9 26.8 13.4
ND: not detected. Experimental conditions: pressure, 27.6 MPa; temperature, 400 °C; reaction time, 5 min.
color and shape dark brown, char dark brown, char brown, char brown, char light brown, char light brown, char white, few char transparent transparent brown, turbid brown, turbid gray, turbid transparent transparent light brown, char light brown, char dark yellow, char light yellow, few char
6508
Ind. Eng. Chem. Res., Vol. 41, No. 25, 2002
Table 3. Carbon Conversion to Liquid Products and Organic Acid Formation from Glucose in SCWa amount of oxygen (%)
acetic
0 25 50 75 100
0.9 2.0 6.1 1.5 1.6
acid yield (wt %, carbon basis) formic lactic glycolic 1.4 0.5 0.2 ND ND
1.2 ND ND ND ND
2.4 3.1 0.4 ND ND
total
acetic
6.0 5.5 6.3 1.5 1.6
1.0 4.0 25.8 65.4 92.1
fraction of effluent TOC as acid (wt %, carbon basis) formic lactic glycolic 1.5 1.0 0.7 0.3 0.4
1.3 ND ND ND ND
2.6 6.3 1.7 0.4 0.5
total
carbon conversion to water-soluble products (%)
6.4 11.3 28.2 66.1 93.0
92.8 48.8 23.7 2.3 1.0
a ND: not detected. Experimental conditions: continuous flow; glucose concentration, 9 g/L; temperature, 400 °C; pressure, 26.6 MPa; reaction time, 43 s.
Table 4. Carbon Conversion to Liquid Products and Organic Acid Formation during the Alkaline Transformation of Glucose in Sub- and Supercritical Watera conditions alkaline concn (g/L) 1 5 10 5 5 5 5 5 a
temp (°C)
amount of oxygen (%)
acetic
400 400 400 250 350 250 350 400
0 0 0 0 0 25 25 25
27.1 32.3 36.1 ND ND 16.6 18.6 16.8
acid yield (wt %, carbon basis) formic lactic glycolic 2.3 2.1 2.8 5.6 5.1 38.3 21.3 5.2
ND ND ND 18.0 23.9 ND ND ND
2.0 0.6 ND 2.1 3.1 22.4 18.7 12.8
total 31.4 35.1 38.9 25.7 32.2 77.3 58.6 34.8
fraction of effluent TOC as acid (wt %, carbon basis) acetic formic lactic glycolic total 51.5 58.0 60.7 0 ND 19.0 27.2 27.8
2.8 2.5 3.1 3.5 3.9 28.5 20.4 5.6
ND ND ND 33.8 42.4 ND ND ND
3.0 0.8 ND 1.6 2.9 20.1 21.6 16.7
57.4 61.3 63.8 39.0 49.3 67.5 69.2 50.1
carbon conversion to water-soluble products (%) 52.6 55.8 59.4 103.6 85.0 87.7 68.3 60.6
ND: not detected. Experimental conditions: glucose concentration, 9 g/L; pressure, 27.6 MPa; flow rate, 6 cm3/min.
of reaction time. Glucose transformation was complete with or without oxygen. The carbon conversion to watersoluble products, organic acid formation, and their concentration in the effluent are shown in Table 3. Average uncertainties were 9 and 4%, in organic acid yield and carbon conversion to water-soluble products, respectively. During hydrolysis, about 93% of the initial TOC remained as transformation products in the liquid phase. When using 100% oxygen demand, 99% initial carbon was converted to gaseous form. Intermediate oxygen contents were explored to track the formation of organic acids. Less than 10% of the TOC in the liquid effluent corresponded to those acids. Formic, glycolic, and lactic acid contents decreased in oxidation tests. Similar to batch tests, acetic acid depended upon the degree of conversion; thus, the yield reached a maximum of 6.1% at 23.7% carbon conversion to water-soluble products. Also, the acetic acid concentration increased in the liquid effluent as the conversion to gas products increased. To study the effect of alkali addition, the pressure was maintained constant at 27.6 MPa and the flow rate was fixed at 6 cm3/min. This provided reaction times of 56, 47, and 13 s at 250, 350, and 400 °C, respectively. The effects of different alkaline concentrations, decreasing temperatures, and oxygen additions were explored. The results are presented in Table 4. The most significant findings are summarized as follows. Sodium hydroxide, in concentrations ranging from 1 to 10 g/L, was added to the glucose solution. The temperature was set at 400 °C. The liquid effluent contained little formic and glycolic acids. The yield of acetic acid ranged from 27% using 1 g/L NaOH to 36% at 10 g/L NaOH concentration. Acetic acid represented 60% of the organic carbon in the effluent. The decrease in temperature up to subcritical conditions was studied by fixing the alkaline concentration at 5 g/L NaOH. The objective was to avoid lactic, glycolic, and formic acid degradation occurring at high temperatures. The results showed that, at subcritical
temperatures (250 and 350 °C), the lactic acid yield was about 20%, together with small quantities of formic and glycolic acids. No acetic acid was found. Previous studies indicated that acetic acid could be a product of the lactic acid degradation, being more refractory at higher temperatures.15 Hydrogen peroxide was added to the feed solution in order to provide 25% stoichiometric oxygen. In hot pressurized water systems, even small quantities of oxygen can promote molecule fragmentation and create an oxidizing atmosphere. At 250 °C, about 77% of the initial glucose resulted in the production of acetic (17%), glycolic (22%), and formic (38%) acids. Increasing the temperature up to 400 °C decreased the yield of acids. Both oxygen and temperature promoted rapid degradation to gaseous products. All effluent samples were characterized by color, pH, and char or solids formation. Generally, the color of the liquid samples varied from dark to light brown, except the oxygenated samples, which ranged from light yellow to colorless. The effluent was acid when oxygen was added. No char, solids, or turbidity was visible. Conclusions High levels (>95%) of TOC conversion of carbonaceous wastes to gaseous products were reached using excess oxygen at moderate temperatures in SWC. The oxygen requirements were decreased through the use of a solid catalyst or sulfuric acid, as based on cellulose. The process could be more attractive economically if, instead of complete oxidation to CO2 and water, other valuable compounds could be obtained. The production of organic acids was investigated because acetic and formic acids were found as intermediate products during the degradation of several wastes and proven to be quite refractory in SCW. Nevertheless, the formation of the shortchain carboxylic acids was typically less than 15% of the initial TOC. The use of solid or acid catalysts did not improve this result. The aggressive conditions
Ind. Eng. Chem. Res., Vol. 41, No. 25, 2002 6509
dominating at supercritical conditions made it very difficult to control the chemistry to direct the feedstocks transformation. Conversely, preliminary results in the alkaline degradation of glucose using subcritical water showed that the reaction could be more selective and efficient for the production of organic acids. Acknowledgment The authors acknowledge Prof. Dr. Earnest F. Gloyna for his suggestions while writing this paper and his permission to use the SCWO facilities at the Center for Energy Studies of the University of Texas at Austin. The Environmental Health Engineering Program of the University of Texas at Austin provided graduate research assistantship support. Also, the Ministry of Science and Education of the Government of Spain awarded postdoctoral funds. Literature Cited (1) Modell, M. Supercritical Water Oxidation. In Standard Handbook of Hazardous Waste Treatment and Disposal; Freeman, H. M., Ed.; McGraw Hill: New York, 1989; Section 8.11. (2) Krietemayer, S.; Wagner, T. Supercritical Water Oxidation. EPA/504/S-92/006, 1992. (3) Tester, J. W.; Holgate, H. R.; Armellini, F. J.; Webley, P. A.; Killilea, W. R.; Hog, G. T.; Barner, H. E. Supercritical Water Oxidation Technology: Process Development and Fundamental Research. In Emerging Technologies in Hazardous Waste Management III; Tedder, D. W., Pohland, F. G., Eds.; ACS Symposium Series 518; American Chemical Society: Washington, DC, 1993; p 35. (4) Cocero, M. J.; Sanz, T.; Alonso, E.; Fdz-Polanco, F. Supercritical Water Oxidation Process for Wastewater Treatment. Energetically Self-Sufficient Process. J. Supercrit. Fluids 2002, 24 (1), 37. (5) Kruse, A.; Meier, D.; Rimbrecht, P.; Schacht, M. Gasification of Pyrocatechol in Supercritical Water in the Presence of Potassium Hydroxide. Ind. Eng. Chem. Res. 2000, 39 (10), 4842. (6) Xu, X.; Antal, M. J., Jr. Gasification of Sewage Sludge and Other Biomass for Hydrogen Production in Supercritical Water. Environ. Prog. 1998, 17 (4), 215. (7) Schmieder, H.; Abeln, J.; Boukis, N.; Dinjus, E.; Kruse, A.; Kluth, M.; Petrich, G.; Sadri, E.; Schacht, M. Hydrothermal
Gasification of Biomass and Organic Wastes. J. Supercrit. Fluids 2000, 17, 145. (8) Yoshida, T.; Matsamura, Y. Gasification of Cellulose, Xylan, and Lignin Mixtures in Supercritical Water. Ind. Eng. Chem. Res. 2001, 40 (23), 5469. (9) Sasaki, M.; Kabyemela, B.; Malaluan, R.; Hirose, S.; Takeda, N.; Tadafumi, A.; Arai, K. Cellulose Hydrolysis in Subcritical and Supercritical Water. J. Supercrit. Fluids 1998, 13, 261. (10) Kabyemela, B. M.; Adschiri, T.; Malaluan, R.; Arai, K. Glucose and Fructose Decomposition in Subcritical and Supercritical Water: Detailed Reaction Pathway, Mechanisms and Kinetics. Ind. Eng. Chem. Res. 1999, 38, 2888. (11) Holgate, H. R.; Meyer, J. C.; Tester, J. W. Glucose Hydrolysis and Oxidation in Supercritical Water. AIChE J. 1995, 41 (3), 637. (12) Foussard, J. N. Study on Aqueous Phase Oxidation at Elevated Temperatures and Pressures. Ph.D. Dissertation, National Institute of Applied Science, Toulouse, France, 1983. (13) Shanableh, A.; Gloyna, E. F. Supercritical Water Oxidations Wastewaters and Sludges. Water Sci. Technol. 1991, 23 (1-3), 389. (14) Tonghamachart, C. Supercritical Water Oxidation of Anaerobically Digested Municipal Sludges. Ph.D. Dissertation, University of Texas at Austin, Austin, TX, 1990. (15) Vallejo, D. Synthesis of Acetic Acid from Nontraditional Feedstocks in Subcritical and Supercritical Water. Master Thesis, University of Texas at Austin, Austin, TX, 1996. (16) Ding, Y. D.; Frisch, M. A.; Li, L.; Gloyna, E. F. Catalytic Oxidation in Supercritical Water. Ind. Eng. Chem. Res. 1996, 35, 3257. (17) Chesley, K. G.; Ark, C.; Montgomery, C. W.; Rouge, B.; Sandborn, L. T. Production of Organic Acids and Salts Thereof from Cellulosic Materials. U.S. Patent 2,750,414, 1956. (18) Niemela¨, K. The Formation of Hydroxy Monocarboxylic Acids and Dicarboxylic Acids by Alkaline Thermochemical Degradation of Cellulose. J. Chem. Technol. Biotechnol. 1990, 48, 17. (19) Krochta, J. M.; Tillin, S. J.; Hudson, J. S. Degradation of Polysaccharides in Alkaline Solution to Organic Acids: Product Characterization and Identification. J. Appl. Polym. Sci. 1987, 33, 1413. (20) Li, L.; Chen, P.; Gloyna, E. F. Generalized Kinetic Model for Wet Oxidation of Organic Compounds. AIChE J. 1991, 37 (11), 1687.
Received for review June 18, 2002 Revised manuscript received October 3, 2002 Accepted October 8, 2002 IE020441M
