ARTICLE pubs.acs.org/IECR
Catalyzed Partial Oxidative Gasification of Phenol in Supercritical Water DongHai Xu, ShuZhong Wang,* Yang Guo, XingYing Tang, YanMeng Gong, and HongHe Ma School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an Shaanxi province 710049, China ABSTRACT: Phenol, a substance difficult to be gasified, is often an intermediate product from biomass gasification in supercritical water. In this study, 1 wt % phenol is gasified in supercritical water with different oxidation coefficients (0 e n e 0.8) at 430-550 °C, 25 MPa in a continuous tubular-flow reaction plant with or without 0.1 wt % Na2CO3. The results show that TOC (total organic carbon) removal rate and COD (chemical oxygen demand) removal rate of reactor effluent will rise with the increase of oxidation coefficient and reaction temperature. When n rises from 0.3 to 0.8 at 500 °C, hydrogen yield and hydrogen molar fraction will first increase and then decrease. They reach their respective maximum values of 0.245 mol/mol and 46.2% at n = 0.6 with the residence time of 2.96 min. There is an appropriate oxidation coefficient for obtaining the highest hydrogen yield for phenol supercritical water partial oxidative gasification (SWPO). Na2CO3 can serve as a catalyst or an additive for phenol SWPO. When 0.1 wt % Na2CO3 is added, hydrogen yield is near 0.477 mol/mol at n = 0.6, 500 °C with the residence time of 2.96 min. Furthermore, the potential conversion pathways for phenol SWPO with Na2CO3 catalyst are also proposed.
1. INTRODUCTION As a promising technology for hydrogen production, renewable biomass gasification in supercritical water has attracted great attention in recent years. Supercritical water (T > 374.15 °C, P > 22.12 MPa) is not only a benign solvent but also a reactant, from which about one-half of the hydrogen in gaseous products comes.1-4 As compared to other biomass treatment technologies such as pyrolysis, air gasification, and steam reforming, biomass gasification in supercritical water can treat wet biomass with high moisture content more than 90% without a drying procedure and obtain a higher gasification efficiency at a lower temperature.5 Many researchers have investigated gasification processes of model biomass,3,6-11 real biomass,11-14 and organic wastes4,12,13,15,16 in supercritical water. Phenol is not only an intermediate product4,17-20 but also a key compound2,18,12,21,22 representing reaction pathway as well as a kind of undesired tar2 formed by pyrolysis of organic molecular products.23 Moreover, phenol is a model compound of lignin6 and phenolic wastewater.24 Although phenol can be gasified in supercritical water,7,15 its gasification needs a high temperature (above 600 °C) and proper catalyst.6,8,17 Therefore, it is necessary to further investigate phenol gasification in supercritical water to improve biomass gasification efficiency and reveal its reaction mechanism. Phenol can be almost completely oxidized into water and carbon dioxide at a temperature as low as 400 °C if there is profuse oxidant and proper catalyst,25-29 or no catalyst30,31 in supercritical water. DiLeo at al.6 have reported that phenol is mainly gasified into H2, CO2, and CH4 under certain conditions. Thus, it is possible to produce a hydrogen-enriched gas by gasifying phenol in supercritical water with a small amount of oxidant at a low temperature. The potential advantages of adding some oxidant for biomass gasification in supercritical water are as follows: it helps to decompose organic matter, provide heat energy for gasification process, lead to less tar formation, improve hydrogen yield,32 and reduce operating cost.33 In addition, alkali compound has r 2011 American Chemical Society
been proved to be an effective catalyst,2,16,19,21,33 which contributes to improve biomass gasification in supercritical water. Thus, to achieve high biomass gasification efficiency and hydrogen yield in supercritical water at a lower temperature, 1 wt % phenol gasification experiments have been conducted with a small amount of oxidant (a simulated air) in supercritical water at 25 MPa and temperatures ranging from 400 to 550 °C with or without 0.1 wt % Na2CO3.
2. EXPERIMENTS 2.1. Apparatus and Experimental Procedures. In this work, 10 g of phenol and 0.1 g of Na2CO3 are dissolved into 1 L of deionized water to form 1 wt % phenol solution with 0.1 wt % Na2CO3. Phenol gasification is performed in a continuous tubular-flow reaction plant in supercritical water, as shown in Figure 1. This plant, equipped with an oxidant transportation system, is markedly different from our previous one in the literature.9 The new oxidant preheater (1 m length and 12.3 mm i.d.) can withstand higher temperature and higher pressure, up to 600 °C and 36 MPa, respectively. A tubular-flow reactor (1.7 m length and 12.3 mm i.d.) is adopted, which is made of Hastelloy C276. By mixing a certain proportion of O2 and N2, a type of “simulated air” is used as oxidant. O2 and N2 are added into a supercharger in turn after the outlet pressures of their pressure reducing valves are set. The volume ratio of O2 to N2 is determined by the supercharger pressure only in the presence of O2, by the total pressure after N2 is mixed, and finally by gas chromatography. Oxidant is compressed to reach its reaction pressure by high pressure metering pump 9, which pushes the Received: July 30, 2010 Accepted: February 16, 2011 Revised: January 26, 2011 Published: March 15, 2011 4301
dx.doi.org/10.1021/ie1016255 | Ind. Eng. Chem. Res. 2011, 50, 4301–4307
Industrial & Engineering Chemistry Research
ARTICLE
Figure 2. Different oxidation coefficients dependences on hydrogen yield, TOC removal rate, and COD removal rate for phenol SWPO.
Suppose that phenol is completely oxidized into CO2 and H2O in supercritical water, then eq 2 will be obtained: Figure 1. Schematic diagram of tubular-flow reaction plant for phenol gasification in supercritical water: (1) feed tank, (2) homogenizer, (3) high pressure metering pump, (4) check valve, (5) safety valve, (6) pressure gauge, (7) pin valve, (8) water tank, (9) high pressure metering pump, (10) oxygen cylinder, (11) nitrogen cylinder, (12) nitrogen pressure reducing valve, (13) oxygen pressure reducing valve, (14) supercharger, (15) thermocouple, (16) oxidant preheater, (17) feedstock preheater, (18) heating wire, (19) safeguarding structure, (20) mixer, (21) bursting disk device, (22) tubular reactor, (23) cooler, (24) back pressure valve, (25) gas-liquid separator, (26) volumetric flow meter, (27) gas bag, (28) effluent tank.
piston of the supercharger. The oxidant preheating temperature is the measured average value of the two thermocouples on the outside wall of the oxidant preheater. The oxidant is preheated to 200 °C with a heating rate of 2 K/min. Its flow rate is calibrated through the volumetric flow meter before the subsequent gasification experiments. The experiment is conducted twice. The volume flow, pressure, and temperature are controlled within (0.02 L/h, (0.1 MPa, and (2 °C, respectively. In our experiments, 1 wt % phenol is gasified at a constant pressure of 25 MPa. Residence time is calculated by the following eq 1.9 t ¼ ðV =Qo Þ ðVo =Vr Þ
ð1Þ
where V is reactor volume, and Q0 and V0 are the volumetric flow rate and specific volume of feedstock under room temperature and atmospheric pressure conditions, respectively. Vr is the specific volume of feedstock under reaction conditions. Because mass concentrations of phenol and Na2CO3 in feedstock are very low, feedstock properties are similar to those of water. Thus, Vr is calculated by a common calculation software of water physics properties when reaction temperature and pressure are provided. In this work, V = 0.2 L, Q0 = 0.00733 L/min, and V0 ≈ 0.001 m3/kg. Furthermore, it can be considered that the simulated air can completely dissolve into supercritical water, so the oxidant volume is neglected when we calculate the residence time under supercritical water condition. As a result, the residence times in Figures 4-7 at 430, 450, 480, 500, 520, and 550 °C are 2.74, 2.54, 1.92, 1.77, 1.68, and 1.58 min, respectively.
C6 H6 O þ 7O2 ¼ 6CO2 þ 3H2 O
ð2Þ
Oxidation coefficient (n) is defined as the ratio of O2 added to that needed for complete oxidation of phenol. Thus, when n = 0, the treatment technology is supercritical water gasification (SCWG); when 0 < n < 1, the treatment technology is supercritical water partial oxidative gasification (SWPO); when n g 1, the treatment technology is supercritical water oxidation (SCWO). In this work, the ratio of N2 to O2 is about 3, and oxidation coefficients correspondingly change from 0.5 to 0.8 when oxidant volumetric flow rates vary in the range of 0.5-0.8 mol/h. Phenol gasification experiments are carried out with n ranging from 0 to 0.8 to determine the hydrogen production potential of phenol in supercritical water with or without 0.1 wt % Na2CO3. The identification and quantification of gaseous products are conducted in the same way as our previous experiment.9 COD concentration of liquid after reaction is measured by a multiparameter water analyzer (model NOVA60). TOC concentration is monitored by a TOC analyzer (model ET1020A). Liquid products are identified and quantified by high-performance liquid chromatography (model LC-3000) with an ultraviolet detector (model UV3000) and two high pressure pumps (model P3000). Its operating conditions are as follows: C18 liquid phase chromatography volume adopted, 1 mL/min for the total flow rate of the two high pressure pumps, 220 nm for the wavelength of the ultraviolet detector, and 60 min for the volume flow ratio of methanol to water to change from 9 to 0.7. Errors in measurement such as gas yield, concentration of phenol, COD concentration, and TOC concentration are lower than 5%. Sample is collected and measured twice when experimental conditions are stable enough. The average values are utilized as the final data for plotting.
3. RESULTS AND DISCUSSION Figure 2 shows influences of oxidation coefficients on hydrogen yield, TOC removal rate, and COD removal rate for phenol SWPO with the residence time of 2.96 min at 500 °C. With n increasing from 0.3 to 0.8, hydrogen yield first increases rapidly and then decreases evidently. It has a peak value of 0.245 mol/ mol at n = 0.6. TOC removal rate and COD removal rate change 4302
dx.doi.org/10.1021/ie1016255 |Ind. Eng. Chem. Res. 2011, 50, 4301–4307
Industrial & Engineering Chemistry Research
Figure 3. Influences of different oxidation coefficients on gas yield and gas molar fraction for phenol SWPO.
ARTICLE
Figure 5. Influences of oxidant and Na2CO3 on COD removal rate for phenol gasification in supercritical water.
Figure 4. Influences of oxidant and Na2CO3 on hydrogen yield for phenol gasification in supercritical water.
similarly. They increase evidently with n rising from 0.5 to 0.8 and reach their respective maximum values of 86.7% and 88.9% at n = 0.8. Besides the lower and higher oxidation coefficients, there is an appropriate oxidation coefficient, about 0.6, at which the highest hydrogen yield is obtained. As demonstrated in Figure 3, when oxidation coefficient increases, hydrogen yield and hydrogen molar fraction change similarly, both of which increase first and then decrease. They reach their respective maximum values of 0.245 mol/mol and 46.2% at n = 0.6; CO2 yield and molar fraction evidently rise and approach 0.3 mol/mol and 50% respectively at n = 0.8; CH4 yield is very low and slowly reduces to 0.004 mol/mol at n = 0.8; its molar fraction is less than 3%. CO, a gaseous product for phenol SCWG,6 is also found in phenol SCWG in this experiment, and even in phenol SWPO at n = 0.5 and 500 °C. No CO is detected when n is higher than 0.6. The higher amount of oxidant may promote CO and CH4 to be oxidized into CO2. Dileo et al.6 have calculated the hydrogen equilibrium yield of phenol SCWG by the RGIBBS block in ASPEN Plus. The results indicate that H2, CO2, and CH4 are the main products. Furthermore, Wei et al.4 report that oxidant will decrease the hydrogen yield of o-cresol wastewater SCWG. In this research, gaseous products of phenol SWPO also include H2, CO2, and CH4, among which H2 and CO2 are the main components. However, hydrogen yield first increases and then decreases following the increase of oxidation coefficient, as shown in Figures 2 and 3. The
Figure 6. Influences of oxidant and Na2CO3 on phenol conversion efficiency for phenol gasification in supercritical water.
reason may be that the oxidation reaction may promote C-C bonds of phenol to break into small molecular compounds,34-37 and parts of them will further decompose into H2, CO2, CH4, CO, and H2O. However, when the oxidant amount is inappropriate, oxidant may convert CO into CO2 to inhibit watergas shift reaction, or even react with H2, which will result in low H2 yield. Figures 4-6 show the influences of different reaction conditions on hydrogen yield, COD removal rate, and phenol conversion efficiency for phenol gasification in supercritical water. It can been seen that without oxidant and 0.1 wt % Na2CO3, hydrogen yield, COD removal rate, and phenol conversion efficiency are comparably low and rise evidently only at 500 °C or above, and reach about 0.18 mol/mol, 75.5%, and 80.4%, respectively, at 550 °C. Hydrogen yield, COD removal rate, and phenol conversion efficiency follow the increase of reaction temperature to increase under four reaction conditions introduced in Figures 4-6. Oxidant (n = 0.5) or Na2CO3 can improve hydrogen yield, COD removal rate, and phenol conversion efficiency, and they reach maximum values 0.64 mol/mol, 90.7%, and 96.7%, respectively, at 550 °C, n = 0.5 with 0.1 wt % Na2CO3. As shown in Figures 4-6, low hydrogen yield, COD removal rate, and phenol conversion efficiency for phenol SCWG imply 4303
dx.doi.org/10.1021/ie1016255 |Ind. Eng. Chem. Res. 2011, 50, 4301–4307
Industrial & Engineering Chemistry Research that phenol is difficult to be gasified to produce hydrogen at the experimental temperatures. The conclusion agrees with those of the previous studies. Goodwin et al.38 find that the activation energy and the first-order rate constant of phenol SCWG are 264 ( 20 kJ/L and 0.66 ( 0.03 s-1, respectively, in the absence of oxidant at 750 °C, 25 MPa. Sy nag et al.17 suggest that phenol can be gasified at 600 °C. Xu et al.15 report that phenol only has a conversion efficiency of 80% in supercritical water at 600 °C. Catalyst has an important influence on phenol gasification in supercritical water.17 With a proper catalyst, phenol can be gasified.8 If the catalyst is Ni, a complete gasification can be achieved within 10 min.6 Reactor wall made of Hastelloy C276 (a Ni-base alloy) is also testified to have catalysis on glucose steam reforming in supercritical water.39 Thus, it can be supposed that the reactor wall in our experiments might have catalysis on phenol SCWG and SWPO. However, the catalysis can be neglected according to the subsequent proving experiments with Hastelloy C276 material in a batch reactor. Alkali compound can improve the water-gas shift reaction to promote hydrogen yield and inhibit formation of tars for biomass SCWG.9 In Figures 4-6, hydrogen yield, COD removal rate, and phenol conversion efficiency for phenol SCWG are improved by 0.1 wt % Na2CO3. They reach up to 0.38 mol/mol, 84.5%, and 87.4%, respectively, at 550 °C, 25 MPa in the absence of oxidant. Therefore, Na2CO3 has a catalytic effect for phenol SCWG. Similar to o-cresol SCWG,4 the effect of Na2CO3 for phenol SCWG may also be attributed to the improvement of the watergas shift reaction. Phenol SCWO has been studied widely. It is proved that the amount of oxidant must be larger than the stoichiometric coefficient of oxidizing phenol into CO2 and H2O in a short residence time and at a low reaction temperature. SWPO is utilized to partially oxidize organic matters to obtain hydrogen in an oxygendeficient environment by the unique properties of supercritical water.39 It is also found that hydrocarbon SWPO at low temperatures is favorable to improve hydrogen selectivity.33,40-42 As indicated in Figures 4-6, it has been proved that hydrogen yield, COD removal rate, and phenol conversion efficiency are improved by promoting decomposition of phenol40-43 when some oxidant (n = 0.5) is added. Therefore, as compared to SCWG, SWPO is feasible to convert phenol into more hydrogen at a low reaction temperature. Phenol only has one site (hydroxyl functional group) for initial reaction with Na2CO3. Muthukumaran et al.44 report that Na2CO3 highly speeds up the phenol SCWO process in the presence of oxidant, and phenol conversion efficiency can reach about 98% with 45 mg/kg Na2CO3 in 30 s. Thus, Na2CO3 may promote phenol conversion with a small amount of oxidant (0 < n < 1) in supercritical water at a lower temperature. As demonstrated in Figures 4-6, hydrogen yield, COD removal rate, and phenol conversion efficiency in supercritical water are evidently improved by oxidant (n = 0.5) and 0.1 wt % Na2CO3. They approach 0.64 mol/mol, 90.7%, and 96.7%, respectively, at 550 °C, 25 MPa. Therefore, Na2CO3 can be employed as a catalyst or an additive for phenol SWPO. The catalytic effect may be achieved through the following two pathways: first, Na2CO3 promotes phenol conversion under 0 < n < 1 condition, which further improves the water-gas shift reaction; second, oxidant may enable phenol to decompose partially or be oxidized into CO directly, and then Na2CO3 speeds up the water-gas shift reaction. Figure 7 depicts the influences of oxidant and Na2CO3 on gas yield, TOC removal rate, and COD removal rate at different
ARTICLE
Figure 7. Oxidant and Na2CO3 dependences on gas yield, TOC, and COD removal rates for phenol SWPO at different reaction temperatures.
Figure 8. Gas yields and hydrogen molar fraction with residence time increasing for phenol SWPO.
reaction temperatures for phenol SWPO. TOC removal rate and COD removal rate increase with reaction temperature rises, which reach 87.9% and 89.5%, respectively, at 520 °C. H2 yield and CO2 yield increase slowly, while CH4 yield will decrease. They reach up to 0.44, 0.94, and 0.026 mol/mol, respectively, at 520 °C. As displayed in Figures 4-7, increasing reaction temperature is helpful to improve hydrogen yield, COD removal rate, phenol conversion efficiency, and TOC removal rate probably because the total reaction process for phenol SWPO is endothermic. Thus, increasing reaction temperature leads to higher hydrogen equilibrium yield and even releases hydrogen atoms from supercritical water.10 It can be known from Figures 2-6 that the maximum hydrogen yield is obtained at n = 0.6, and 0.1 wt % Na2CO3 has a catalytic effect for phenol SWPO. Figure 8 further indicates different gas yields and hydrogen molar fraction with residence time increasing for phenol SWPO at n = 0.6, 500 °C with 0.1 wt % Na2CO3. It can be seen that H2 yield and CO2 yield rise slowly, while CH4 yield and CO yield, both of which are very low, decrease gradually. Hydrogen yield can reach up to approximately 0.477 mol/mol for 1 wt % phenol SWPO at n = 0.6, 500 °C with residence time of 2.96 min and 0.1 wt % Na2CO3. 4304
dx.doi.org/10.1021/ie1016255 |Ind. Eng. Chem. Res. 2011, 50, 4301–4307
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. Product Distributions for Phenol SWPO at 25 MPa Experimental Conditions reaction temperature (°C)
500
520
500
residence time (min)
2.96
1.68
2.96
oxidation coefficient
0.8
0.5
0.6
Na2CO3 catalyst (wt %)
0
0.1
0.1
Gaseous Product Yield (mol/mol) H2
0.21
0.441
CO
0
0.01
0
CH4 CO2
0.004 0.3
0.026 0.94
0.032 0.894
C2
0.29
0.49
0.54
0.477
C3-C4 Liquid Product Yield (mol/mol) benzene yield
0.05
0.03
0.02
dibenzofuran yield
0.01
0.02
0.02
other liquid products Phenol SWPO Results carbon in gaseous products (%)
14.7
32.4
33.4
carbon in gaseous and liquid products (%)
27.3
44.1
43.1
TOC removal rate (%)
86.7
87.9
89.4
phenol conversion efficiency (%)
94.6
95.8
96.7
COD removal rate (%)
88.9
89.5
90.1
The maximum hydrogen yield (0.477 mol/mol) is much higher than the corresponding maximum value of 0.245 mol/mol in Figure 3. Hydrogen molar fraction increases smoothly and reaches up to the maximum value of 34.0% at 2.96 min, which is much lower than 46.2% obtained without Na2CO3 in Figure 3. This suggests the increase rate of CO2 yield is more rapid than that of H2 yield when 0.1 wt % Na2CO3 is added. Like biomass gasification in supercritical water,14,45 increasing residence time is helpful for hydrogen production for phenol SWPO. A possible explanation for gases yield changes shown in Figures 7 and 8 is that, when reaction temperature or residence time increases, the reverse process of methanation reaction and the water-gas shift reaction will be reinforced,12,14,46,47 as shown in eqs 3 and eq 4. CO þ 3H2 T CH4 þ H2 O
ð3Þ
CO þ H2 O T CO2 þ H2
ð4Þ
Although previous investigations have proved that organic matter (especially glucose) SCWG can obtain high hydrogen yield, it is also found that hydrogen yields are very low for glycerol SCWG,13 lignin SCWG,42 phenol SCWG,6,10 and so on. Phenol as an undesired intermediate product for some biomass SCWG even is considered as the best obstacle of complete gasification,2,13 even under oxidant condition.24 The very low hydrogen yields (always
