Ind. Eng. Chem. Res. 1996, 35, 4471-4478
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Supercritical Water Oxidation of Solid Particulates Suresh A. Pisharody,† John W. Fisher,† and Martin A. Abraham*,‡,§ NASA Ames Research Center, Mail Stop 239-11, Moffett Field, California 94035, and Department of Chemical Engineering, The University of Tulsa, Tulsa, Oklahoma 74104
In the application of supercritical water oxidation (SCWO) to the treatment of aqueous solid wastes, particle size will be an important processing parameter. The particle size will impact feed preparation requirements such as slurry concentration, pumping requirements, and, in terms of the destruction of the solid particles, rate of reaction and reactor size. To address these issues, an experimental research program was undertaken to evaluate the effect of particle size on the reaction kinetics in SCWO of solid particulates (wheat straw and cellulose particles in this case). The experiments also included evaluation of the effects of temperature, pressure, and agitation rate. A two-step reaction mechanism was revealed, with a very rapid initial dissolution period followed by a longer particle reaction period. Empirically based mathematical relationships were developed that can be used for SCWO system design. Introduction In order for long-duration manned space missions to become possible, recycling of all or part of the produced waste is required. One life-support concept under development is the Controlled Ecological Life Support System (CELSS) involving the use of a small-scale ecological system that mimics ecological systems on earth. CELSS proposes the use of plants as a primary means of air purification and secondary means of water purification. A wide range of technologies exist for the treatment of aqueous or airborne waste streams. However, no base-line technology is currently available for the treatment of solid wastes. Thus, the development of solid wastes treatment technologies is especially important for space-based applications. Supercritical water oxidation (SCWO) and dry incineration are two technologies being considered for the clean and complete oxidation of carbonaceous wastes [Takahashi et al., 1987; Bubenheim et al., 1993]. In both cases, nearly complete conversion of the solid waste can be achieved by careful selection of the reaction conditions. For incineration, life support applications are limited by the formation of NOx and SO2, as well as the high-temperature and -energy requirements. SCWO achieves similar conversion at a lower temperature, essentially eliminating the formation of NOx and SO2. However, corrosion of the reactor [Mitton et al., 1994] and the formation of sticky salts [Armellini and Tester, 1990] can be a problem in a SCWO system. A major obstacle in the design of either system to process solid wastes is the design of a delivery system to handle solid particles. These solid particles include wastes such as inedible plant biomass and human feces. The delivery or pumping of a slurry of particles is difficult for any application. Pumping for a SCWO system is particularly difficult because of the high pressure (over 22 MPa) at which SCWO operates and because of the need to deliver slurry to the reactor at high concentrations. Particle size is a key factor controlling the maximum slurry concentration that may be pumped in a conventional unit. †
NASA Ames Research Center. The University of Tulsa. § Current address: Department of Chemical Engineering, 3055 Nitschke Hall, The University of Toledo, Toledo, OH 43606. ‡
S0888-5885(96)00269-2 CCC: $12.00
Particle size also affects the rate of reaction. For a reaction that takes place on the outside surface of a particle, larger particles generally react more slowly than smaller particles. A slow reaction will require a large reactor to achieve the necessary waste destruction efficiency; however, a large reactor cannot be accommodated in a space-based application. Although small particles may be favorable from the perspective of pump valves and reactor size, the requirement for very small particles creates a need for a grinding system of size, complexity, and energy consumption that would also be undesirable. The approach to this problem is to first determine a maximum particle size necessary for the SCWO reaction to proceed satsifactorily. For a number of reasons the reactor is the key component of the system. These reasons include the high pressure and temperature in the reactor, the relatively large diameter of the reactor, the possible formation of sticky solids as a part of the reaction, and other details of the chemical reaction. The maximum size of the reactor determines the maximum allowable particle size, and the particle size, in turn, determines the requirements for the grinding and pumping systems. Wheat straw was used in a series of experiments investigating the effects of particle size on SCWO. Previous studies at NASA Ames Research Center used polystyrene particles that were chosen because of their well-characterized size distribution [Fisher and Abraham, 1994]. Wheat straw, however, is a more representative example of space-based solid waste. Commercially available cellulose, which has a composition similar to that of wheat straw but a more uniform particle size, was used as a model compound to obtain more information about the reaction kinetics. A series of experiments was conducted to develop reaction rate expressions that can be used to predict the effects of particle size, temperature, and pressure. Experimental Section The experimental system consisted of a reactant injection system, a water and air feed system, a 2 L stirred autoclave, and a sampling system, as shown in Figure 1. The injection system consisted of a length of 0.635 cm (1/4 in.) internal diameter high-pressure tubing in © 1996 American Chemical Society
4472 Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996
Figure 1. Schematic diagram of an experimental system.
which the reactant was placed prior to the initiation of a run. The solid particles were ground to the desired mesh size (20-60 mesh) and were placed in the injection cell. The cell was then pressurized to an excess of 41.3 MPa (6000 psi) through the use of a high-pressure helium cylinder and a pressure generator. The pressure differential existing under the reaction conditions was used to force the dry particles into the reactor through the inlet ball valve, thus initiating the oxidation process. The reactor was a conventional 2 L stirred Autoclave Engineers’ batch reactor, constructed of Hastelloy C-276. The allowable working pressure of the reactor was 28.5 MPa at a working temperature of 540 °C. The reactor was equipped with a 4 kW electric heater, 1/4 in. thermowell, 1/4 in. inlet and outlet nozzles, and a 1/4 in. relief line fitted with an Inconel I-625 rupture disk set at 28.5 MPa at 540 °C. Pressure was measured using transducers on the inlet line, in the reactor, and in the sampling line. The autoclave was fitted with a six-blade magnetically driven agitator. The sampling system consisted of two sampling chambers which were evacuated prior to the sampling process. A sample was obtained by opening valve VH3 and allowing the pressure in the sampling chamber to increase to 0.34 MPa; samples were taken at regular time intervals. A sample of the collected fluid was then taken into an air-tight syringe and injected into the gas chromatograph (GC) for quantification. The reactor was initially evacuated to 30 mmHg using a vacuum pump. Then, 250-295 mL of water (depending upon the temperature of the reaction) was fed into the reactor through gravity feed. Air was injected to provide about 10 times stoichiometric excess. The reactor was then raised to the desired temperature.
Agitation was provided by a variable-speed magneticdrive paddle-bladed agitator. Approximately 0.275 g of dry particular matter was injected into the reactor by means of a helium pressure plug of 41.3 MPa (6000 psi). The injection cell was pressurized using a high-pressure helium cylinder with an outlet pressure of 41.3 MPa. The injection cell volume was roughly 4.2 cm3, and the amount of helium used to pressurize the cell was roughly 0.07 mol. This amount of material was deemed to be insignificant compared to the contents in the reactor. Wheat straw particles are nonporous; thus, helium diffusion into the particles is unlikely. The contents of the injection cell were injected into the reactor a few seconds after pressurization occurred. The injection procedure was tested by injecting the wheat straw particles into a container at room temperature and pressure, and no change was observed in the wheat straw fibers. The mass of reactant was kept low enough so that it would not raise the temperature of the reactant mixture due to the exothermicity of the reaction. Samples were collected at regular intervals and injected into the GC at room temperature. Expansion from the high-pressure reactor into the low-pressure sample bomb provided sufficient cooling to create a phase separation between the permanent gases and water condensate. The sampling sections were evacuated before each sampling. Condensed water was collected from the sample bomb, indicating phase separation had occurred in the sample bomb. The gas samples were completely dry, and no interference from water was observed during GC analysis. All reactions were performed in the single-phase region. The presence of a single fluid phase was
Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 4473
confirmed using the Peng-Robinson equation of state, as implemented using the Basic program VLMU [Sandler, 1989]. The program reports mixture molar volume and density from inputs of the mole percentages of O2, N2, helium (used to inject the wheat straw), and H2O and the temperature and pressure of the reaction. Binary interaction parameters of the various components are required; literature values were used [Sandler, 1989]. The calculations provided by the PengRobinson equation were checked with experimental measurements of the pressure, which always agreed with the calculation within the limits of experimental error. For experiments below the critical point of the mixture, the fluid phase could be characterized as a dense gas. After the desired reaction time, the reactor jacket temperature was lowered to room temperature, and the reactor was allowed to cool down. When the fluid was cooled to room temperature, the drain valve was opened, and the liquid effluent was collected. The injection system was then disconnected to recover uninjected solid particles. Gas analysis was accomplished using a Shimadzu Model 8A gas chromatograph equipped with a thermal conductivity detector (TCD) and a 5.3 m (20 ft) × 0.057 cm (1/8 in.) i.d. Heyesep D column. The oven was maintained isothermal at 65 °C, while the detector was maintained at 120 °C. This provided separation of air and carbon dioxide with retention times of approximately 3.5 and 8.2 min. The effluent liquid at the end of the reaction underwent several analytical tests. The total organic carbon was measured using a Dohrman TOC analyzer. Residual cations and anions were measured with a Dionex ion chromatograph. The metals concentration was determined using a Thermal Jarrell Ash atomic absorption spectrophotometer. The liquid at the end of the reaction was clear and showed no solids or evidence of char formation. Cellulose was commercially available (Aldrich) and used as received. Dried, mature wheat straw stalks grown in a hydroponic solution were used. Wheat straw samples were prepared by crushing the stalks in a hammer mill and sieving. The wheat straw was ground and sieved to mesh sizes from 40 to 60 mesh. The wheat straw particles were observed through a microscope (400× magnification). A large number of particles of each mesh size were observed and their dimensions averaged. The two particle sizes were approximately parallelepiped shapes, with 40 mesh having dimensions 0.6 × 0.275 × 0.065 mm and 60 mesh having dimensions 0.33 × 0.085 × 0.025 mm. The particles were treated mathematically as equivalent spheres with the same surface area to volume ratio and the same density as the actual particles [Aris, 1957], giving equivalent particle diameters of 148 and 55 µm for the 40 and 60 mesh particles, respectively. The carbon dioxide concentration was measured as a function of time. In selected cases, an attempt was also made to isolate carbon monoxide; only very low quantities were observed using the GC. This would be expected according to the work of Holgate and Tester [1994], that indicates any CO formed under these conditions would be rapidly converted to CO2 through the water-gas shift reaction. Thus, measurement of CO2 production allowed calculation of reactant conversion through stoichiometric considerations. Assuming complete conversion of cellulose to ultimate products, the stoichiometric equation
C6H10O5 + 6O2 f 6CO2 + 5H2O
(1)
indicates that production of 6 mol of carbon dioxide represents the disappearance of 162 g of cellulose. For wheat straw, elemental analysis of the wheat straw revealed a composition as indicated in Table 1. Thus, production of 3.36 mol of CO2 corresponds to the conversion of 100 g of wheat straw, again assuming complete conversion. If the reaction proceeds through the formation of organic intermediates, then these measures of conversion will represent only conversion of all aqueous organic matter to CO2, rather than the specific conversion of the reactant. Regardless, the stoichiometric amount of oxygen needed to convert all the organic carbon and hydrogen to carbon dioxide and water was calculated and 10 times stoichiometric excess of that amount was used. The carbon balance was obtained from the mass of reactant injected, final gas phase carbon dioxide composition, and the total organic carbon (TOC) of the effluent liquid stream obtained from the reactor. Results Kinetic experiments were conducted using both cellulose and wheat straw particles. The effects of particle size, reaction temperature, agitation speed, and reaction pressure on the conversion (measured in terms of CO2 production) were considered. The results from these experiments are presented first. Analysis of the liquid effluent was completed for only a few cases. These results follow the kinetics experiments. Effect of Kinetic Variables on Conversion. The effect of particle size on the rate of reaction at 382 °C and 22.8 MPa for cellulose and wheat straw particles is shown in Figure 2. The larger particles generally reacted more slowly than the smaller particles. This was clear in the case of the cellulose particles but not as apparent for the more heterogeneous wheat straw. However, data at other temperatures (not shown) indicates that the larger wheat straw particles did, in fact, react more slowly. In all cases, substantial conversion (25-60%) was observed even at the shortest sampling time of 10 s. For the well-characterized cellulose particles, the initial conversion was the same (within experimental error) for both particle sizes. This rapid conversion may be the result of a thermal decomposition of the dry solid on sudden exposure to the reaction temperature. This result was consistent with previous experiments with solid polystyrene beads, reported previously [Fisher and Abraham, 1994]. The subsequent reaction occurred more slowly and depended on the particle size. The reaction proceeded to completion in less than 1 h under all the conditions. The cellulose particles were converted at a slightly greater rate than were wheat straw particles of roughly equivalent size (50 µm for cellulose vs 55 µm for wheat straw). The initial instantaneous conversion was also slightly greater for the cellulose. The overall faster rate of reaction may have been partially due to the larger particle size of the wheat straw but can also be attributed to the greater heterogeneity of the wheat straw. The effluent at the end of the reaction was colorless with a slight odor. The carbon balance was calculated for each sample, based on an estimate of the mass of solid injected, the final CO2 concentration obtained in the gas, and the residual carbon content of the liquid. In all cases, the carbon balance was approximately 100% ((1%).
4474 Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 Table 1. Average Elemental Composition of Dried Wheat Straw element
composition (wt %)
carbon hydrogen oxygen nitrogen
40.37 5.61 44.69 1.72
element
composition (wt %)
sulfur inorganic species total
0.26 7.35 100
Figure 4. Effect of reaction temperature on the conversion (as measured by CO2 yield) of 40 mesh wheat straw particles.
Figure 2. Effect of solid particle size on the conversion of wheat straw and cellulose particles, T ) 382 °C.
Figure 5. Effect of agitation speed on the conversion (as measured by CO2 yield) of 40 mesh wheat straw particles at 382 °C.
Figure 3. Effect of reaction temperature on the conversion (as measured by CO2 yield) of 50 µm cellulose particles.
The effect of temperature on the conversion of 50 µm cellulose particles is indicated in Figure 3. An increase in temperature above the critical temperature from 382 to 400 °C led to a significant increase in conversion. A similar increase in temperature below the critical temperature from 343 to 360 °C showed roughly equivalent conversion. A substantial, essentially instantaneous, conversion of the cellulose was observed in all cases and displayed a temperature behavior that paralleled the behavior of the slower reaction. The effect of temperature on the rate of reaction of 40 mesh wheat straw particles is shown in Figure 4. As observed with the cellulose particles, the rate of reaction increased with increasing temperature above the critical temperature but was roughly independent of temperature below the critical temperature. At temperatures greater than 400 °C, the reaction was very rapid, giving complete conversion of the wheat straw in less than 3 min at 425 °C. The initial fast conversion was temperature-dependent over the entire range of temperatures considered, increasing with increasing temperature both above and below the critical temperature. A series of experiments were carried out using 40 mesh wheat straw particles at 382 °C to determine the
effect of agitation on the kinetics. Reactions were carried out at 1200, 600, and 0 rpm, maintaining a 10 times stoichiometric excess of oxygen and a total pressure of approximately 24.1 MPa. The effect of agitation on the kinetics of the 40 mesh wheat straw particles is as shown in Figure 5. Within the limits of experimental error, there was no effect of agitation on the rate of reaction. This implies that there was no external masstransfer limitation under the reaction conditions and that the surface reaction was the rate-controlling step. Two experiments were performed at 400 °C with 40 mesh wheat straw to determine the effect of reactor pressure on the rate of reaction. In these experiments, the reaction pressure was changed from 24 to 17 MPa, keeping all the other system variables the same. The results are reported in Figure 6. There was a slight decrease in the reaction rate when the reaction was performed at the lower (subcritical) pressure. This could indicate a density effect, as the density of the reaction mixture was approximately 76 kg/m3 at 17 MPa, approximately half the density of 135 kg/m3 at the higher pressure. This could also indicate a dependence on water concentration, as water concentration is directly related to fluid density. However, since this pressure dependence is so slight, the advantages of operating at low subcritical pressures (i.e., reduced energy requirements, decreased safety concerns, decreased corrosion concerns) may still apply in the current application. Analysis of the Liquid Effluent. The analysis of the liquid effluent sample obtained at the end of the reaction is shown in Table 2, for wheat straw particles. Most inorganic elements contained in the wheat straw formed soluble reaction products. The organic nitrogen
Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 4475 Table 2. Liquid Effluent Analysis of Wheat Straw Samples elementsa temp (°C)
mesh size
TOCb
343 360 382 400 343 360 382 400
60 60 60 60 40 40 40 40
2.5 1.7 2.3 1.2 4 3.2 2.5 2.8
a
oxidation products
corrosion products
K
Mg
NO3
NO2
PO4
SO4
NH4
Cr
Ni
Mo
66 43 74 50 53 40 26 52
1.1 1.1 1.3 1
