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Ind. Eng. Chem. Res. 2004, 43, 4580-4584
A High-Throughput Screening Technique for Conversion in Hot Compressed Water B. Potic, S. R. A. Kersten,* W. Prins, and W. P. M. van Swaaij Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
Conversion in hot compressed water (e.g., 600 °C and 300 bar) is considered to be a promising technique to treat very wet biomass or waste streams. In this paper, a new experimental method is described that can be used to screen the operating window in a safe, cheap, and quick manner (one measurement takes about 5 min). Small sealed quartz capillaries (i.d. ) 1 mm) filled with biomass or model compounds in water are heated rapidly in a fluidized bed to the desired reaction temperature. The reaction pressure can be controlled accurately by the initial amount of solution in the capillary. After a certain contact time, the capillaries are lifted out of the fluidized bed, rapidly quenched, and destroyed to collect the produced gases for GC analysis. Results of measurements for formic acid and glucose solutions have shown that the technique is reliable enough for screening purposes including trend detection. For conversions above 30%, three identical measurements are sufficient to produce reasonably accurate average values with a confidence level of 95%. Introduction Process intensification is a novel design approach in which reduction of equipment size leads to less energy consumption, improved safety, lower capital costs, and less pollution.1-3 In this paper, the intensification principle has been applied to a screening technique for the conversion of biomass in hot compressed water, in particular gasification of biomass/waste in supercritical water (SCWG). Very wet biomass (moisture content >70 wt %) cannot be converted economically by traditional techniques such as combustion and gasification due to the energy required for water evaporation (2.4 MJ/kg). In SCWG, water evaporation is avoided and intensive countercurrent heat exchange is practiced. Therefore, SCWG is considered as a promising technique to convert wet streams into a hydrogen-rich gas.4-6 Biomass is converted in the presence of water, e.g., via
C6H12O6 + 6H2O f 6CO2 + 12H2 The above stoichiometric equation is highly idealized; in practice, also CO, CH4, C2,3-components, liquids (including H2O), and polymers are formed next to CO2 and H2.7 The product distribution appears to be a strong function of the reactor temperature and the weight percentage of organic material in the feedstock.8 Antal and co-workers9 found that the wall material (Hastelloy, Inconel) of their bench-scale continuous flow reactor had a large effect on the obtained results. This finding points to an important role of catalysis in SCWG. Consequently, data obtained in small-scale metal reactors are obscured by undefined catalytic effects, and therefore difficult to interpret and extrapolate. The novel method uses quartz capillaries of 1 mm i.d., 2 mm o.d., and 150 mm length as batch reactors, which have no catalytic activity. Unfortunately, the number of experimental SCWG data published is small, while at the same time * To whom correspondence should be addressed. Tel.: +31 53 489 4430. Fax: +31 53 489 4738. E-mail: s.r.a.kersten@ utwente.nl.
the range of conditions explored is narrow. This is partly due to the severe operating conditions (300 bar and 600 °C), which make laboratory testing at bench scale (1100 g/h, 10 mL < Vreactor < 1 L) problematic, expensive, and time-consuming. The proposed technique will allow mapping of the complete operating area and trend detection of the process at moderate costs in just a short period of time. Because of the small diameter and the properties of quartz, these microreactors can withstand conditions up to 900 °C and 600 bar. Apart from SCWG of biomass, this technique can be used also to study other thermochemical conversion processes of wet feedstock, such as hydrothermal liquefaction, oxidation in supercritical water (SCWO), and possibly many other high-pressure processes. Hereafter, the novel technique will be explained and validated in detail. A subsequent paper (to be published) will report the results of extensive measurements meant to determine the complete operating window for supercritical gasification of some model compounds and wood. Experimental Section Methodology. The method uses quartz capillaries of 1 mm i.d., 2 mm o.d., and 150 mm length as batch reactors (see Figure 1). An experiment is started by charging a capillary with a known amount of aqueous solution (biomass or a model compound dissolved in water). Hereafter the capillary is sealed and rapidly heated in a high-temperature fluidized bed to the desired reaction temperature. In the heating trajectory, the pressure increases as a result of the evaporating water. By varying the amount of solution in the capillary, the final pressure is known and can be adjusted quite accurately, as the water vapor pressure predominantly determines the total pressure. If a more precise estimate of the pressure is required, thermodynamic calculations can be applied on the basis of the initial amount of water and the measured amount of product gases. After a certain contact time at a specific reaction temperature and pressure, the capillary is quenched to ambient conditions, which ensures that all reactions are
10.1021/ie030732a CCC: $27.50 © 2004 American Chemical Society Published on Web 04/13/2004
Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4581
Figure 1. Photograph of the capillaries together with a ruler.
stopped. Finally, the gas phase inside the capillary is analyzed with respect to the absolute amount and the composition. Advantages and Restrictions. This new experimental method has several advantages: (i) conducting an experiment is fast, cheap, and safe (the reactor volume is 0.12 mL), (ii) the quartz capillaries are strong enough to withstand extremely high pressures (600 bar) and high temperatures (900 °C), (iii) quartz has no or hardly any catalytic activity, (iv) quartz is resistant to corrosion, (v) catalysts can be easily added, and only very small amounts are required, (vi) the reactor content can be inspected visually (char, water-soluble organic compounds, and tars can be identified in the case of SCWG of biomass) and, (vii) the quartz reactors can be heated and cooled rapidly, leading to more precise results. A disadvantage of the technique is that analysis of the liquid phase is extremely difficult, due to very small sample volume, and has not been practiced up to now. Hence, when products are present in the liquid phase, complete mass balance closure cannot yet be obtained. Depending on the development/availability of liquid analysis methods with respect to the minimum required sample volume, this disadvantage can be overcome. Small (diameter) reactors are required to hold the large pressure built up and to ensure fast heat transfer. Fast heat transfer will guarantee accurate knowledge concerning the reaction temperature, which is necessary for correct data interpretation. It is common knowledge that in fluidized beds high heat transfer coefficients can be achieved, especially when the bed particles and the immersed objects are small.10-12 This has been checked by submerging a silver cylinder with the same dimensions as the used capillaries in a fluidized bed (dp ) 250 µm, u/umf ) 2). The external heat transfer coefficient derived from the heating curve appeared to be ca. 1000 W/(m2‚K), which is in good agreement with predictions from literature correlations.11,12 Inside the quartz wall of the capillaries, no important temperature gradients will be present because of the small dimensions (Dwall ) 0.5 × 10-3 m) and the relatively high thermal conductivity [λ ) 1.8 W/(m‚K)]. The heating time of the used capillaries in the FB was determined experimentally to be approximately 5 s at a reactor temperature of 600 °C. For this measurement, a thermocouple was welded into a water-filled capillary. The measured heating time (5 s) is in good agreement with the calculated one (4.5 s), which is based upon the measured external heat transfer coefficient, the known properties of the quartz wall, and minimum heat transfer conditions from the wall to the solution inside. The pressure-temperature and density trajectories of the feed material during heating to the reaction
conditions differ from those of continuous reactors. Continuous reactors are operated essentially isobaric, whereas the capillaries, because of the fixed volume and initial mass, are isochoric reactors. This could, in principle, be a cause for deviations in results between continuous reactors and the batch capillaries. However, from a few special SCWG tests in which the heating trajectory of the capillaries was varied, it appeared that the product composition is determined almost entirely by the final process pressure and temperature (see the Results and Discussion). Experimental Setup. As mentioned before, the capillaries were heated in a fluidized bed (sand particles of dp ) 250 µm, u/umf ) 2), which was placed in a temperature-controlled oven. For each measurement two capillaries were placed on a holder especially designed to allow quick and safe manual operation. The temperature inside the FB was measured with thermocouples. After a certain contact time at the desired reaction temperature and pressure, the capillaries were quenched in a water bath to ambient conditions, to ensure that no further reactions proceeded. The produced gases were analyzed by the following method: (1) a quenched capillary was put into a stainless steel chamber of 50 mL, (2) then this chamber was flushed with helium to remove traces of product gas from a previous run, (3) hereafter, the chamber was closed (with valves) and the capillary was crushed completely with a hammer mechanism, and (4) finally, a gas sample was taken from the chamber and led to a gas chromatograph. It is essential to crush the capillaries completely. Otherwise gas gaps, containing products, could be captured between liquid phases, and excluded from the gas analysis. Gaseous products were analyzed with a gas chromatograph using TCD cells (Varian Micro GC CP-2003). Two columns were used: a Molsieve 5A, operated at 90 °C and 155 mbar, and a Porapak Q, operated at 75 °C and 155 mbar. Helium was used as a carrier gas. The chromatograms were obtained and interpreted with Varian CP-Maıˆtre Elite software. Figure 2 is a schematic representation of the setup. Data Interpretation and Possible Errors. The degree of conversion of carbon in the biomass to permanent gases (carbon efficiency, Xc) is chosen here as the main process parameter for SCWG, because it indicates the distribution of carbon over the desired products (permanent gases) and the undesired product (liquids and polymers). In practice, the product composition can be modified catalytically, viz., by water-gas shift to produce CO2 and H2, or by methanation to produce CH4. Xc is defined by
Xc )
Nc,permanent gas (PV/RT)chamber ) (ω + ωCO2 + Nc,solution (msolutionf)nc/M CO ωCH4 + 2ωC2H4 + ...)
Like every other experimental technique, also the newly developed capillary reactor method contains certain error margins in the experimental parameters (see Table 1). As the main output of the method is the carbon efficiency, the error analysis will be focused on this parameter. The number of moles present in the sampling chamber is known accurately by precise measurement of T (within 1 °C), P (within 500 Pa), and V (within 0.1 mL). The reaction solution was prepared in batches of 50 g using high-precision balances
4582 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004
Figure 2. Schematic representation of the capillary reactor technique: 1, capillary; 2, nitrogen supply for the fluidized bed; 3, oven; 4, fluidized bed; 5, thermocouples; 6, capillary holder; 7, sampling chamber; 8, pressure indicator; 9, gas chromatograph; 10, temperature controller for the oven. Table 1. Possible Maximum Experimental Errors Estimated for an Experiment with a 17 wt % Glucose Solution in Water quantity (PV/RT)chamber (msolutionf)nc/M ∑ω TFB maximal possible error
value (x)
error (∆x)
3 × 10-3 9 × 10-5 varies
3 × 10-5 3.6 × 10-6 varies
units mol mol
∆x/x (%) 1 4 2 1 8
(accuracy (0.0001 g). For the mass of solution actually put into the capillary, the error is the highest (ca. 4%). This error is introduced mainly because the capillaries are sealed rapidly and carefully in a hydrogen flame, but nevertheless, some water will be evaporated. Furthermore, it has been found that small variations in the fluid bed temperature may cause differences in carbon efficiency ((1% every 3 °C for a 17 wt % glucose solution at Xc < 50%; see Figure 4). In the low conversion regime this error is significant, whereas near the maximum conversion, obviously, the effect of the exact process conditions disappears. Overall, the maximal possible experimental error in the carbon efficiency was estimated to be approximately 8% in a typical case (see Table 1), which is accurate enough for trend detection. Results and Discussion Visualization. As mentioned before, some products, such as tars and solid carbon (char), for SCWG, are clearly visible after the reaction has been carried out in the quartz capillaries. In practice, char formation in the heat exchanger (the feed-site exit temperature is typically 450 °C) will be one of the major bottlenecks of the SCWG process.13 Figure 3 shows capillaries with
Figure 3. Capillaries with a different level of char/tar formation after SCWG: (a, left) 20 wt % glucose in water at 600 °C; (b, right) 10 wt % glucose in water at 720 °C.
high and low levels of char/tar formation, indicating that possible problems can indeed be identified visually. Effects of the Heating Trajectory. To understand the effect of the heating trajectory of the capillaries on the carbon efficiency, special tests have been performed. The following results were compared: (a) capillaries with a short residence time in the FB (columns 1-3 of Table 2); (b) capillaries which were quenched after a short residence time (the same as in columns 1-3 of Table 2) and then reheated up to a 1 min total residence
Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4583 Table 2. Carbon Efficiency, Xc (%), Measured for a 10 wt % Glucose Solution in Watera 1s
2s
3s
60 s residence time 60 s residence time after the interruption without any interruption
0.08 0.08
0.69 0.76 0.69 0.71
0.1 0.15
0.69 0.72 0.73 0.73
a
The final conditions are 600 °C and 300 bar. Illustration of the effect of heating interruption after 1, 2, or 3 s. Table 3. Results of SCWG Measurement Series Illustrating the Reproducibility and Accuracy of the Experimental Technique (P > Pc) Nr required for 95% CI model compd formic acid glucose glucose glucose
concn T (wt %) (°C) Nr 1.2 0.8 18 17
700 700 550 460
24 23 24 18
〈Xc〉
σ
1.005 1.003 0.535 0.203
0.041 0.041 0.043 0.033
10% σ/〈Xc〉 5% (%) error error 4.1 4.1 8.1 16.2
3 3 10 41
1 1 3 10
time in the FB (column 4 of Table 2); (c) capillaries with a 60 s residence time without interruption (column 5 of Table 2). From Table 2 it can be concluded that the heating trajectory has no notable effect on the final carbon efficiency. Continuation of interrupted experiments results approximately in the same final carbon-to-gas conversion degrees as those of noninterrupted experiments. The differences between the values in the last two columns of Table 2 are all well below the experimental error. It should be realized that the results of Table 2 have been derived for isochoric conditions. Whether or not the corresponding conclusion is also valid for continuous tubular reactors should be proven in subsequent experimental work. Validation of the Technique. Mass balance closure was investigated by experiments with components which, according to the literature,14 should be converted completely to gas-phase components at temperatures above 700 °C and low feed concentrations. Additional measurements were carried out under conditions that should give a much lower carbon conversion. They were used to evaluate the reproducibility of the method for SCWG in case a significant amount of carbon remains in the liquid or solid phase. Formic acid and glucose were selected as model compounds. On basis of this analysis, the number of identical experiments could be defined whose average should represent a good estimate of the mean value. As mentioned before, the carbon efficiency was selected as an identifier for the process. Table 3 shows several series of measurements for which the average carbon efficiency and related experimental error were calculated. The first two series (100% carbon efficiency; see Table 3) show that all produced gases can be recovered with the novel method despite the very small volumes used. For all the series, the experimental error found is acceptable and not in disagreement with the estimated one, except for the low conversion range (
