Patent Application
20120210632
The present invention pertains to the field of supercritical water technologies. In particular, the present invention pertains to supercritical water gasification (SCWG) and supercritical water liquefaction (SCWL), in which fuel compounds or other desired products are made from a feedstock such as biomass. The present invention also pertains to the process of supercritical water oxidation (SCWO), in which waste materials are converted into innocuous mineral byproducts. The invention is particularly, but not exclusively, useful for synthesis reactions that can be carried out in a brine phase at temperatures above the critical point of water.
U.S. Pat. No. 4,113,446, issued Sep. 12, 1978, to Modell et al., discloses a process known as supercritical water gasification (SCWG). In this process, organic materials are contacted with water at supercritical conditions (T>374° C. and P>221 bar) and converted to gaseous materials comprised primarily of CH4, H2, CO2 and CO. For feed materials such as biomass, the solvating properties of supercritical water result in reduced formation of tar and char as compared to conventional, low pressure gasification. Hong and Spritzer (Proceedings of the 2002 U.S. DOE Hydrogen Program Review NREL/CP-610-32405, May 2002) describe a variant of the SCWG process in which a portion of the feedstock is oxidized within the gasifier in order to provide the heat needed for gasification. This process is known as supercritical water partial oxidation (SWPO).
U.S. Pat. No. 4,338,199, issued Jul. 6, 1982, to Modell, discloses a process known as supercritical water oxidation (SCWO). In this process an oxidant such as air or oxygen is added to the reaction mixture so that oxidation of organic feedstock occurs at conditions supercritical in temperature and pressure. SCWO has been shown to give complete oxidation of virtually any organic compound in a matter of seconds at 550-700° C. and 250 bar. A process related to SCWO, known as supercritical temperature water oxidation (STWO), can provide similar oxidation effectiveness for certain feedstocks but at lower pressure. This process has been described in U.S. Pat. No. 5,106,513, issued Apr. 21, 1992, to Hong, and utilizes temperatures in the range of 650° C. and pressures between 25 and 220 bar. As with SCWO, the overall goal of the process may be waste destruction, energy generation, or production of chemicals.
In addition to the above processes, supercritical water liquefaction (SCWL) may be used to produce liquid products at generally lower temperatures than SCWG, and supercritical water synthesis (SCWS) reactions may be used for various conversions of organic materials.
A feature of the preceding SCW processes is that due to the combination of temperatures and pressures used, e.g. 400-700° C. and 220-350 bar, the reactions are carried out in a supercritical steam phase with a density in the range of 60-500 kg/m3. This relatively low density medium (compared to liquid water) has the advantage of allowing complete miscibility of organics and gases with the steam phase, but also has disadvantages in terms of precipitation and plugging by salts, incompatibility with homogeneous catalysis (which precipitate out), and reduced heat transfer rates. In addition, reactions dependent on ionic-type mechanisms are retarded or prevented.
Both SCWO and SCWG have counterparts that operate in an aqueous liquid phase at subcritical temperatures. In the case of oxidation, the process known as wet oxidation has been used for the treatment of aqueous streams since the 1950s (see e.g. U.S. Pat. No. 2,665,249, issued Jan. 5, 1954, to Zimmermann). It involves the addition of an oxidizing agent, typically air or oxygen, to an aqueous waste stream at elevated temperatures and pressures, with the resultant “combustion” of oxidizable materials directly within the aqueous phase. The wet oxidation process is characterized by operating pressures of 30 to 250 bar and operating temperatures of 150 to 370° C. Typically, the amount of oxidant required to oxidize the waste exceeds the solubility limit of oxygen or air, so that both gaseous and liquid phases are present in the reactor. Because oxidation is carried out primarily in the liquid phase, some provision for mixing must be made to facilitate transfer of oxygen to the liquid phase. Bubble columns, baffles, packed beds and stirrers have been used to achieve this goal. Reaction is primarily carried out in the liquid phase since gas phase oxidation is quite slow. Thus, the reactor operating pressure is typically maintained at or above the saturated water vapor pressure, so that at least part of the water is present in liquid form. The largest single application of wet oxidation is for the conditioning of municipal sludge. The oxidation achieved in this process is only 5 to 15% complete, the primary objective being sterilization and disruption of the organic matrix to improve the dewatering properties of the sludge. Following wet oxidation, the sludge is used for soil improvement or landfill, or is incinerated. Other uses of wet oxidation are for the treatment of night soil, pulp and paper mill effluents, regeneration of activated carbon, and treatment of chemical plant effluents. In these applications higher temperatures are used but oxidation is still typically at most 90% complete. Wet oxidation is limited not only in the degree of oxidation achievable, but also by its inability to handle refractory compounds. Because of the low temperatures relative to those found in normal combustion, reaction times are on the order of an hour, rather than seconds. Even with these extended reaction times many refractory organics are poorly oxidized. One means for improving the low temperature oxidation has been the usage of homogeneous or heterogeneous catalysts in the liquid stream. The process is significantly complicated by this approach because of catalyst deactivation, attrition, and recovery.
Gasification in a subcritical aqueous phase, referred to as wet gasification, has been developed by Elliott et al. at the Pacific Northwest National Laboratory (Catalytic Hydrothermal Gasification of Lignin-Rich Biorefinery Residues and Algae, PNNL Final Report 18944, October 2009). Typical conditions for this process are 350° C. and 208 bar. Due to the low temperature, a catalyst must be used; even so, residence times of 15-20 minutes are required as compared to one to several minutes at supercritical temperatures.
In view of the preceding limitations of the existing art, it is an object of the present invention to provide a method and apparatus for using a reaction phase with liquid-like densities wherein the reaction phase retains the advantages of high reaction rates provided by supercritical water temperatures without the need for excessively high pressures. The combination of high density and high reaction rates imparts numerous benefits over the prior art, which entails low density or slow reaction rates.
In accordance with the present invention, a method and apparatus for processing a feed material includes a reactor vessel that is formed with a chamber. More particularly, the chamber is constructed to hold a brine phase in which feed material reacts to produce an effluence of gaseous or liquid byproducts. For the present invention, the brine phase is heated to a temperature that is greater than the supercritical water temperature of 374° C., and it is held under a relatively low pressure that is less than about 500 bar. Importantly, within the chamber of the reactor vessel, conditions are such that the brine phase has a density that is greater than about 500 kg/m3. Preferably, however, the density will be greater than about 700 kg/m3.
Structurally, the apparatus of the present invention includes a level controller to maintain the level of the brine phase in the upper part of the vessel. It may also include a downcomer pipe that is connected from the source of feed material into fluid communication with the chamber of the reactor vessel. With this connection, the downcomer pipe is used to introduce the feed material into the brine phase, where it will react to produce the effluence. In addition to the downcomer pipe, the apparatus has an exit pipe that is connected in fluid communication with the chamber for removing the gaseous effluence from the chamber.
In a preferred embodiment of the present invention, the brine phase and a gaseous effluence that is generated during the reaction of the feed material will coexist in the chamber. Consequently, a liquid/vapor interface is created between the gaseous effluence and the brine phase. In this case, a downcomer pipe extends from the source of feed material, through the gaseous effluence, and past the interface into the brine phase. Thus, the downcomer pipe is in direct fluid communication with only the brine phase. On the other hand, inside the chamber of the reactor vessel, the gas exit pipe is in fluid communication with only the gaseous effluence.
In addition to the structural components mentioned above, the apparatus of the present invention may also include several other components that are in fluid communication with the chamber of the reactor vessel. These additional components include an injection pipe that can be connected to the downcomer pipe for injecting super-heated water or brine into the feed material. As envisioned for the present invention, this injected fluid will have a temperature that is greater than 374° C., and will serve to rapidly preheat the feed material. Additionally, the apparatus can include an injection pipe that is connected in fluid communication with the chamber of the reactor vessel. In particular, the injection pipe is used to inject water or brine into the brine phase for control of the brine phase. Here again, the injected fluid preferably has a temperature that is greater than 374° C. Further, an oxidant pipe can be joined in fluid communication with the chamber of the reactor vessel for use in adding an oxidant to the brine phase. Still further, a drain pipe can be provided for removing residual solids and excess dissolved materials from the brine phase.
The brine phase inside the chamber of the reactor vessel is comprised of water mixed with at least one soluble inorganic compound. Preferably, the inorganic compound for the brine phase is selected from a group comprising salts, oxide compounds and hydroxide compounds. Also, a catalyst may be included in the brine phase in the reactor vessel. For one embodiment of the present invention the catalyst is an alkaline compound selected from a group comprising K2CO3 and NaOH. As an alternate embodiment, the catalyst may be based on a precious metal or a transition metal.
The brine phase inside the reactor vessel is maintained at a temperature greater than 374° C., and a pressure less than about 500 bar. Under these conditions, the brine phase will typically have a density that is greater than 700 kg/m3. The feed material is then introduced into the reactor vessel for reaction within the brine phase to produce the effluence. In order to accelerate and facilitate the reaction, water or brine at supercritical temperature can be injected into the feed material as it is being introduced into the reactor vessel. Further, the methodology of the present invention envisions removing residual solids and excess dissolved materials from the brine phase, and augmenting the brine phase with an oxidant. In some embodiments, the purpose of the apparatus and method is to collect or dispose of the product effluence that results from the reaction.
For simplicity in the following discussion, temperature and pressure will be referenced to the critical conditions for pure water, e.g. temperatures supercritical for water will be termed “supercritical temperature”. The key feature of the present invention is the use of soluble inorganic compounds, typically salts, but also oxide or hydroxide compounds, in mixtures with water to form dense brine reaction phases, i.e., with densities >500 kg/m3, at supercritical temperatures, i.e., >374° C., henceforth termed “supercritical brines”. The high temperatures allow high reaction rates to be attained in a dense medium. Depending on the desired reactions, particular potential advantages include the following:
A) Aqueous ionic reaction pathways can be carried out at supercritical temperatures;
B) Homogeneous catalysis at supercritical temperatures is enabled since catalytic salts can be solubilized along with the reactants;
C) Suspension of heterogeneous catalysts is facilitated by the dense reaction phase;
D) Use of precious metal catalysts may be avoided;
E) Salts can be selected to protect catalysts from poisoning, e.g., sulfur can be scavenged by the salt cations;
F) Soluble or insoluble additives may be introduced to the brine phase to protect catalysts from poisoning;
G) While supercritical pressures, i.e., >221 bar, will sometimes be desirable, dense reaction phases can also be maintained at supercritical temperatures, but subcritical pressures, i.e., <221 bar;
H) The presence of a dense phase yields high heat transfer rates, facilitating rapid heatup of feed materials and reducing the opportunity for tar and char formation;
I) The presence of high levels of soluble salts can help prevent deposition of low solubility salts; and
J) Combined with high reaction rates, the high density reaction phase allows small size reactors to be used.
These and other advantages will become apparent by means of the drawings and detailed description that follow.
The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which:
As conventionally practiced, supercritical water (SCW) processes involve a reaction phase with a density of <500 kg/m3, and usually <100 kg/m3. For example, typical supercritical water oxidation (SCWO) conditions are 650° C. and 235 bar, for which the density of the SCW medium is −70 kg/m3. Likewise, typical conditions for high-temperature supercritical water gasification (SCWG) are 650° C. and 235 bar, again with a density of the SCW medium of ˜70 kg/m3, while typical conditions for low-temperature SCWG are 400° C. and 345 bar, for which the density of the SCW medium is ˜470 kg/m3. Attaining liquid-like densities for conventional SCW processes requires substantially increased pressures, with corresponding increases in equipment and operating costs. For example, to attain a density of 900 kg/m3 at 400° C. requires a pressure of about 4,000 bar or 60,000 psi. The key feature of the present invention consists of carrying out reactions in a dense reaction phase containing water at temperatures beyond the critical temperature of water, i.e., 374° C., in which the dense phase is maintained by a high level of dissolved salts, oxides or hydroxides. Such reaction phases will hereinafter be referred to as “supercritical brines” and typically possess a density in the range of 1,000 kg/m3, which is the density of normal liquid water. Through use of supercritical brines, reactions can be accelerated by supercritical temperatures while excessive pressures, nominally >500 bar, are avoided. In addition to the general feature of accelerated reactions, the use of supercritical brines has a number of subsidiary advantages that are described in the following paragraphs.
By means of the present invention, aqueous ionic reaction pathways can be carried out at supercritical temperatures. Examples of such reactions include acid- and base-catalyzed reactions. Such reactions may be especially favorable in supercritical brines due to the enhanced ion product of water at dense supercritical conditions. For example, the ion product of water at 450° C. and 1000 kg/m3 is nearly 10−8 (mol/kg)2, almost 6 orders of magnitude higher than its ambient temperature liquid water value of 10−14 (mol/kg)2, and about 14 orders of magnitude higher than its value of 10−22 (mol/kg)2 in pure SCW at 450° C. and 250 bar (density ˜100 kg/m3). Thus, even though water is only a fractional component of the brine, relatively high concentrations of hydronium and hydroxyl ions will be present.
By means of the present invention, homogeneous catalysis at supercritical temperatures is enabled since catalytic salts can be solubilized along with the reactants. In some cases, the brine-forming solute may itself be the catalyst, as is the case for potassium carbonate. Alternatively, the homogeneous catalyst may be a soluble precious metal catalyst such as ruthenium trichloride. In addition, the dense reaction phase makes suspension of fine heterogeneous catalysts easier than with a typical low density SCW medium. The present invention is also compatible with fixed catalyst beds.
By means of the present invention, salts can be selected to protect precious metal catalysts from poisoning. For example, sulfur can be scavenged by the salt cations since sodium sulfide and potassium sulfide are more stable than ruthenium sulfide at the conditions of interest.
In the practice of SCWG it is well-known that formation of tar and char byproducts is minimized by rapid heatup to reaction temperatures. In the present invention the presence of a dense phase yields high heat transfer rates, facilitating rapid heatup of feed materials and reducing the opportunity for tar and char formation.
In the present invention, the presence of high levels of soluble salts can help prevent deposition of normally low solubility salts. This feature helps reduce operating problems due to solids accumulation and plugging.
In the present invention, the presence of high levels of soluble salts can allow brines at supercritical temperatures to persist at subcritical pressures. This feature can help reduce the capital and operating costs associated with a high-pressure process.
The preceding features of the invention are further illustrated with reference to the
Laboratory-scale batch tests were carried out on the apparatus shown in
Table 1 provides descriptions of a number of runs carried out on the apparatus of
Comparison of the results in Table 1 gives a preliminary indication of the effectiveness of the present invention. Runs 1-3 show the baseline results at several temperatures when no catalyst or salt is added to the apparatus. Runs 4-6, not a part of the present invention (since a brine phase is not involved), show how improved conversion is attained in the presence of supported ruthenium catalyst (2% Ru on extruded activated carbon pellets from BASF, referred to as Ru/C). The weight ratio of catalyst:feed of 8 includes the weight of the activated carbon support, and it should be noted that activated carbon itself possesses some catalytic activity (see e.g. Nakamura et al., Gasification of chicken manure using suspended activated carbon catalyst in supercritical water, 15th European Biomass Conference and Exhibition Proceedings, 2007). While the Ru/C catalyst is highly effective, it has drawbacks in that it utilizes an expensive precious metal and is subject to poisoning by certain commonly present elements such as sulfur. As previously noted, objects of the present invention include avoidance of the use of precious metal catalysts or, alternatively, protection of precious metal catalysts from poisoning.
Runs 7-8 with the neutral salt NaCl indicate little or no effect on the gasification attained by the baseline case without salt. Nevertheless, NaCl may still prove useful as a phase modifier or catalyst protector when mixed with other active salts or catalysts.
The alkaline compound NaOH is a well-known reactant with catalytic properties for organic decomposition and gasification. Runs 9-10 utilize a relatively large amount of NaOH with a 1:1 ratio of NaOH:wood. As expected, the tests show a substantial increase in gasification as compared to the baseline case. Use of NaOH as a catalyst/reactant in the present invention presents a significant difficulty, however, in that the carbon dioxide produced by the gasification reacts with the NaOH to produce insoluble solid Na2CO3; in fact Runs 9 and 10 include insufficient NaOH to obtain the full benefits of the present invention because all of the sodium is tied up as Na2CO3 part way through the gasification, with resultant disappearance of the brine phase. Nevertheless, brine is present for the initial portion of the gasification providing a partial indication of the advantages of the present invention. To obtain the full benefits of the present invention when using NaOH, the brine phase must be maintained by continual addition of fresh NaOH (a considerable expense for a full-scale plant), or mixed with one or more other salts to keep the Na2CO3 in solution, or both. Another noteworthy point is the absence of CO2 in the gas analysis results for Runs 9 and 10 in Table 2. As the reactor cools down below 270° C., sodium bicarbonate (NaHCO3) becomes stable and any excess CO2 is captured by the Na2CO3.
The alkaline compound K2CO3 is another material known to catalyze gasification reactions (see e.g. Sinag et al., Influence of the heating rate and the type of catalyst on the formation of key intermediates and on the generation of gases during hydropyrolysis of glucose in supercritical water in a batch reactor, Ind. Eng. Chem. Res. 2004, 43, 502-508). The same molar ratio of catalyst:feed was used as with NaOH, and Runs 11-15 also show a substantial increase in gasification as compared to the baseline case. K2CO3 is advantageous compared to NaOH because it acts as a true catalyst and is not consumed in the reactor. Furthermore, unlike Na2CO3, K2CO3 remains soluble in the supercritical brine as was previously shown in
A mixture of the dry salts Na2CO3 and K2CO3 has a eutectic melting point of 710° C. at about 57 mol % Na2CO3. Although data for the phase behavior of this salt mixture under high water pressure at supercritical temperatures is not available, by analogy with other known ternary water-salt-salt systems, the presence of water is expected to lower the minimum melting temperature and shift the corresponding composition slightly. Runs 16-17 were carried out with an equimolar mixture of Na2CO3 and K2CO3 to see if any beneficial effects were observed. The gasification appears to be a bit less than with K2CO3 alone, which may be due to selection of a non-optimal salt mixture. However, it is possible that sodium will be more active than potassium in some cases and that use of a mixture of Na2CO3 and K2CO3 could have certain advantages that are not apparent in Table 1.
Runs 18-20 were carried out with a mixture of K2CO3 and Ru/C to see if any synergistic effects were observed with both catalysts present. The mixture appears to give improved performance at lower temperatures, but about the same performance at higher temperatures. In actual practice, however, where contaminants such as sulfur may be present, the mixture of K2CO3 and Ru/C may provide superior performance at all conditions since poisoning of the Ru/C would be prevented by the K2CO3, as previously noted in conjunction with
It must be borne in mind that the laboratory batch apparatus is limited by its relatively slow heatup time and by the limited amount of brine present. It is anticipated that a continuous flow apparatus with more rapid heatup and excess brine as previously shown in
While the examples given are for use of wood biomass as a feed material, it is anticipated that the present invention will prove advantageous for all types of organic materials, including other types of biomass, municipal sludge and biosolids, coal, and waste plastic.
While the particular Method and Apparatus for High Temperature Brine Phase Reactions as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.
