Abstract:
In a high pressure and high temperature reaction system suitable for oxidative waste treatment, particularly a reaction system for supercritical water oxidation (SCWO), a method is disclosed for injecting a first fluid of a first temperature at a first flow rate into a second fluid of a second temperature at a second flow rate, mixing the first and the second fluids within a mixing length ( 115, 215 ), and wherein the first and second temperatures and the first and second flow rates are selected such that a temperature of the mixed fluids downstream of said mixing length ( 115, 215 ) is obtained, at which said first fluid being substantially non-corrosive.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention generally relates to an apparatus for mitigation of corrosion in a high pressure and high temperature reaction system, specifically in a system suitable for oxidative waste treatment under supercritical water conditions. The invention relates further to the reaction system itself and to a method in said reaction system. 
     DESCRIPTION OF RELATED ART AND BACKGROUND OF THE INVENTION 
     Several approaches for disposing of waste are available today, of which the major ones are landfilling and incineration. In recent years, another technique based on supercritical water oxidation (SCWO) has been commercialized, see, e.g. Supercritical   Water Oxidation Aims for Wastewater Cleanup , C. M. Caruana, Chem. Eng. Prog., April 1995. 
     Supercritical water oxidation is a novel and advanced process for, inter alia, effective destruction of toxic substances, particularly organic pollutions, in wastewater and sludge. The process converts, fast and effectively, organic materials containing substantially carbon and hydrogen to carbon dioxide and water, at a temperature above the critical point of water (374° C. and 22,13 MPa), while releasing energy. The process may be completely contained and the destruction efficiency is often higher than 99%. 
     Heavy metals present during the process are converted to their oxides whereas sulfur and phosphorous are converted to sulfate and phosphate, respectively. Halogens are converted to their corresponding acids, e.g., hydrochloric acid. Smaller amounts of nitrogen compounds, e.g. amines and ammonia, which exist in the waste material flow, are converted to molecular nitrogen, and not to NO x , which is an acidifying and fertilizing residual product and therefore undesirable in the effluent. 
     If, however, the waste material contains large amounts of ammonia and/or organic nitrogen compounds, substantial amounts of the nitrogen source may be found in the effluent as ammonia as a result of the destruction process. This phenomenon is undesirable as ammonia constitutes a fertilizing compound. Besides, discharge of ammonia without further purifying is very often imposed with restrictions. 
     It is known in the literature, e.g. through  Reactions of Nitrate Salts with Ammonia in Supercritical Water , P. C. Dell&#39;Orco et al., Ind. Eng. Chem., Vol. 36, No. 7, 1997, and references therein, that ammonia can be converted to molecular nitrogen during supercritical water oxidation conditions if nitric acid is used as a co-oxidant together with molecular oxygen, hydrogen peroxide or another suitable compound. The nitric acid has preferably to be supplied to the waste material flow firstly after that the organic contents have been destructed with oxygen as nitrate otherwise will compete with oxygen in the destruction of the organic contents. Furthermore, the nitric acid has to be dosed with high accuracy relative to the amount of ammonia (a stoichiometric amount is needed). If too little nitric acid is supplied, a remaining amount of ammonia will be left whereas too large amounts of nitric acid will result in an excess of nitrate in the effluent. 
     For purposes of strength and corrosion, nickel-based alloys, such as Hastelloy or Inconel, are employed for manufacturing of equipment for SCWO. Acids, and not at least nitric acid, are, however, in presence of oxygen strongly corrosive at high temperatures, though still subcritical ones, even if these corrosion resistant nickel alloys are used, see, e.g.  The Corrosion of Nickel - base Alloy  625  in Sub -  and Supercritical Aqueous Solutions of HNO   3    in the Presence of Oxygen , P. Kritzer et al., J. Mater. Sci. Lett., 1999, in print, and references therein. It was found in the temperature-resolved corrosion measurements reported that the corrosion due to nitric acid was most severe at temperatures between about 270° C. and 380° C., the same temperature range in which general corrosion is caused by the mixtures HCl/O 2  and H 2 SO 4 /O 2 , respectively. At supercritical temperatures the corrosion rates were low. 
     For this reason, particular solutions must be employed for the entry of nitric acid into the supercritical wastewater flow containing ammonia or ammonium salts to avoid or at least minimize the corrosion. 
     However, as regards corrosion, generally the most troublesome compound in the supercritical water oxidation process is the chlorine element, since it is very common in various chemical substances. If the chlorine is present as an ion at elevated temperatures, it will corrode the construction materials mentioned above. The chlorine may have been an ion originally, liberated during heat up or in the reactor. 
     U.S. Pat. No. 5,358,645 issued to Hong et al. disclose an apparatus and process for high temperature water oxidation, the apparatus (not in detail described) having a surface area, that may be exposed to corrosive material, composed of zirconia based ceramics. The ceramics may be employed as coatings or linings. 
     U.S. Pat. No. 5,461,648 issued to Nauflett et al. disclose a supercritical water oxidation reactor with a corrosion-resistant lining. The inner surface of the reactor vessel is coated with artificial ceramic or diamond. A cylindrical baffle for introducing the oxygenating agent extends axially within the interior of the vessel and has its exterior surface inside the vessel coated with said artificial ceramic or diamond. 
     U.S. Pat. No. 5,552,039 issued to McBrayer, Jr. et al. disclose a turbulent flow cold-wall reactor. It mentions, inter alia, that if the atmosphere in the reaction chamber is harsh and corrosive, the inside wall of the reaction chamber should preferably be made of or covered with a coating or a liner withstanding the harsh atmosphere. 
     None of these US patents, is, however, discussing corrosion problems in terms of temperature dependent corrosivity, or the particular corrosion caused by the corrosive compounds discussed above. 
     SUMMARY OF THE INVENTION 
     Embodiments of an apparatus and methods for mitigation of corrosion in a high pressure and high temperature reaction system that can be used for oxidative waste treatment under supercritical water conditions are described. 
     In an embodiment of a system and method for oxidative waste treatment, a first fluid may be transported through a first conduit at a first flow rate and at a first temperature. Construction of the first conduit may be such that the first conduit may have an end within the interior of the second conduit, and is in fluid communication with the second conduit. Fluid communication between the first conduit and second conduit may allow the first fluid to be injected into the second fluid. Transportation of the second fluid may occur in a second conduit at a second temperature and a second flow rate. The first fluid may be corrosive in a corrosive temperature range and the corrosive temperature range may exclude the second temperature. 
     The first and second fluids may be mixed in the second conduit at a mixing length downstream of the end of the first conduit. The second conduit may include a tube or liner having at least an inner surface area made of a corrosion resistant material and extending along the mixing length to inhibit corrosion of the second conduit. As used herein, “mixing length” is the distance necessary for a mixed fluid to reach a steady state temperature. 
     The first and second temperatures and the first and second flow rates may be selected such that the mixed fluids downstream of the mixing length are at a temperature that is substantially non-corrosive for the first fluid. 
     In an embodiment, a high pressure and high temperature reaction system suitable for oxidative waste treatment may include a first and a second conduit adapted to transport a first and a second fluid. The second conduit may be adapted to transport the second fluid at a second temperature and at a second flow rate. Transportation of the first fluid in the first conduit may occur at a first flow rate. The first fluid may be at a first temperature, which is corrosive in a corrosive temperature range, which excludes the second temperature. 
     The first conduit may have an end within the interior of the second conduit, which allows the first conduit to be in fluid communication with the second conduit. Fluid communication of the first and second conduits may be such that the first fluid and the second fluid can be mixed in the second conduit within a mixing length from the end of the first conduit. As a result of the mixing, the mixed fluids downstream of the mixing length may have a temperature substantially non-corrosive for the first fluid. 
     The high pressure and high temperature reaction system may have a tube or liner having at least an inner surface area made of a corrosion resistant material. The tube or liner may be part of the second conduit and may extend along the mixing length to inhibit corrosion of the second conduit. The second conduit may be made up of a conventional construction material (e.g., nickel based alloy) upstream and downstream of the tube or liner configured for high pressure and high temperature reaction systems suitable for supercritical water oxidation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description of embodiments of the present invention given hereinbelow and the accompanying  FIGS. 1–3  which are given by way of illustration only, and thus are not limitative of the invention. 
         FIG. 1  shows a simplified block diagram of a reaction system suitable for oxidative waste treatment under supercritical water conditions wherein the present invention may be employed. 
         FIG. 2  shows, in cross-section, a first embodiment of an apparatus according to the present invention. 
         FIG. 3  shows, in cross-section, a second embodiment of an apparatus according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the following description, for purposes of explanation and not limitation, specific details are set fourth, such as particular hardware, applications, techniques, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods, protocols, apparatus, and circuits are omitted so as not to obscure the description of the present invention with unnecessary details. 
     Considering  FIG. 1 , the operation of a high pressure and high temperature reaction system  10  such as a system suitable for oxidative waste treatment under supercritical water conditions, will briefly be overviewed so that in the subsequent detailed description of the present invention, the operation of the inventive apparatus may be better understood. 
     A conventional reaction system  10  comprises a primary tank  12 , a heat exchanger  14 , a heater  16  and a reaction chamber  18 . A primary wastewater stream  20  passes initially through the first compartment (not shown) of the heat exchanger  14 , then through the heater  16 , and enters the reaction chamber  18  under pressure, after it has been mixed with oxidant coming through feed line  22 . The organic matter contained in the primary waste stream  20  is oxidized, and in sequence, the hot effluence passes through the second compartment (not shown) of the heat exchanger  14 . As well known, heat exchangers usually have two compartments, physically isolated from each other, which, however, are heat-communicating. The second compartment transfers heat to the first compartment. 
     Constructing materials for the reactor and the tubing may comprise steel, nickel-based alloys, platinum, gold, titanium, zirconium, ceramics, ceramic composites and other corrosion resistant materials as the environment inside the reaction chamber and tubing may be hostile and corrosive. However, as many of the latter materials are highly expensive, an optimal compromise between cost, on one hand, and corrosion resistance, on the other hand, is to use nickel based alloys such as Hastelloy or Inconel, for the manufacturing of such equipment. 
     As already discussed in the prior art, there is a number of species that are very aggressive relative to these nickel based alloys within a finite temperature range, among them nitric acid, sulfuric acid and hydrochloric acid. All these three acids are strongly corrosive between about 270 and 380° C., but the corrosion rates for the latter two acids are lower by a factor of ten than the one found for nitric acid, see said Kritzer article. 
     It is clear from the description above of the operation of the system that the wastewater flow, as well as any additives, will be heated from initial low temperatures, which probably are close to ambient temperatures, up to supercritical temperatures (above 374° C.) for the oxidative treatment of the waste, whereafter the effluent is cooled either in a heat exchanger or by mixing it with quench water or a combination of both. 
     The present inventors have realized that if the initial temperatures and the temperature of the cooled effluent are kept preferably well below 270° C. and the temperature in the reaction chamber is kept preferably above 380° C., there is generally only two sections of a reaction system made of nickel-based alloy that may be attacked by corrosive agents such as those mentioned above contained in, or supplied to, the wastewater flow, namely a “heating” section and a “cooling” section, where the temperatures are within the temperature interval of said corrosion. 
     The present invention is thus concerned with such sections of the reaction system and how to design them in order to provide a reaction system of low cost and good corrosion resistance. The idea is to provide appropriate tubing (made of nickel based alloy or other, preferably relatively inexpensive, material that is not corrosive resistant) of the system with a corrosive resistant tube or liner. The number of tubes or liners, their positions and their lengths are chosen in order to protect the system from corroding. 
     Hereinbelow will follow a few implementation examples of the present invention. Note that the terms “corrosive” and “corrosive-resistant material” as used in the description below and in the appended claims should be understood as “corrosive” relative conventional construction material for high pressure and high temperature reaction systems suitable for supercritical water oxidation such as steel, nickel based alloys, nickel-chromium alloys and the like, at least within a given temperature interval, and “corrosive-resistant material” refers to unconventional expensive material which is corrosion-resistant relative a wide variety of harsh media such as acids, particularly the acids discussed above, halogens and the like, respectively. Examples of corrosion-resistant materials will be given below. 
     A first embodiment of the present invention, shown in  FIG. 2 , depicts an apparatus  101  for introducing nitric acid in a supercritical water flow containing ammonia or ammonium with the purpose of converting this to molecular nitrogen. 
     In a section of a reaction system tube  103 , which preferably is the conduit between the heater  16  and the reaction chamber  18 , or part of the reaction chamber itself, of  FIG. 1 , a separate tube or liner  105  of a corrosion resistant material is mounted, the outer surface of which is in fit with the inner surface of reaction system tube  103 . Alternatively, tube  105  constitutes part of the reaction system tube  103  itself (not shown). 
     A feeding pipe  107  of relatively small diameter, is mounted through an opening of tube  103  and extends substantially axially with tube  103  and liner  105 , and which ends in the interior of tube  103 . Preferably, feeding pipe  107  and tube  103  are concentrically arranged for transportation of fluids, the former nitric acid and the latter preheated wastewater feed, in the same directions, as indicated by arrows  109 – 113 . The temperature of the wastewater should preferably be above 380° C., and the temperature of the nitric acid should be low, preferably considerably lower than 270° C. Note that if the concentration of the corrosive agent is low, these temperature limits are not very crucial, i.e., the corrosion would be low at temperatures slightly lower than 380° C. and, particularly, at temperatures slightly higher than 270° C., e.g., 300° C. 
     By pumping nitric acid through the feeding pipe it will be preheated by the hot water flow and then get mixed with the supercritical water. The flow rates are such that the total flow (wastewater and nitric acid) becomes supercritical with a temperature of above 380° C. after having reached a steady temperature state a certain distance  115  from the end of the feeding pipe, said distance being referred to as the heat transfer or mixing length. Accordingly, to avoid any risk of corrosion of the inner walls of tube  103 , the length of the liner  105  should be of at least this length, and it should be localized to protect the inner walls of tube  103  within this length. For practical reasons, the liner  105  may have an offset  117  in the end facing the end of the feeding pipe, i.e. extend beyond (upstream of) said feeding pipe end to avoid any risk of corrosion in that region. 
     The material of the liner and preferably of the feeding pipe is chosen according to its corrosion resistance relative nitric acid at the occurring temperatures. Literature data shows that titanium, generally, is a suitable material, but also materials such as zirconium, platinum, tantalum, niobium and ceramics may be chosen. The entire liner, or an inner coating thereof, may be constructed of such material. 
     Even if a limited degree of corrosion may exist using these materials, the components are relatively cheap and easy to replace when so needed. 
     Preferably, there are means for positioning and/or holding the liner in place. In the embodiment showed, tube  103  is provided with an elbow at the downstream end of the mixing length to prevent liner  105  from moving further downstream. However, any suitable means for positioning and/or holding the liner, e.g. flanges at the inner walls of tube  103 , may be used. 
     In experimental work, an injection apparatus as the one shown in  FIG. 2 , was used, the liner and the feeding pipe being made of titanium. The ammonia destruction was performed by pumping 65% nitric acid into the reaction system during several hours without any detected corrosion. When the liner and the feeding pipe were demounted and inspected no corrosion of these components was discovered. In contrast thereto, in an experiment in which nitric acid was pumped into a supercritical water flow containing ammonia through a T-pipe of Inconel  625 , the pipe was destroyed through corrosion in just a few hours. 
     Consequently, by using an injection apparatus according to  FIG. 2 , nitric acid may safely be introduced without severe corrosion of the reaction system. 
     Furthermore, a substantial portion of the reaction between nitric acid and ammonia and/or ammonium will take part as early as in the section of the reaction system where the liner is localized, which further reduces the risk for severe corrosion. 
     Alternatively, feeding pipe  107  and tube  103  of  FIG. 2  may be arranged for transportation of a wastewater feed containing a corrosive agent such as a halogen, and water or a wastewater feed in lack of such a corrosive agent, respectively. The water or wastewater in tube  103  is preferably at a supercritical temperature, whereas the corrosive wastewater may be cooler. 
     Referring next to  FIG. 3 , which illustrates an apparatus  201  according to a second embodiment of the present invention, a separate tube or liner  205  of a corrosion resistant material is mounted in a section of a reaction system tube  203 , which is preferably at the effluent output or elsewhere in the exit path tubing. The outer surface of liner  205  is arranged to be in fit with the inner surface of the reaction system tube  203 . 
     A first input tube  207 , is mounted through an opening of tube  203  and extends substantially axially, preferably concentrically, with tube  203  and liner  205 , and which ends in the interior of tube  203 . A second input tube  208  is connected to tube  203  upstream from said end of input tube  207 . 
     Input tube  207  and input tube  208  are arranged for transporting effluent from reactor  18  containing corrosive compounds such as nitric acid, sulfur acid, or the like, and quench water, respectively, in the directions as indicated by arrows  209 – 213 . The effluent stream is supercritical or close to supercritical, and the temperature of the quench water is low, preferably at ambient temperature. 
     By pumping appropriate amounts of quench water through input tube  208 , the effluent input through tube  207  will be cooled effectively by the quench water and get mixed with it. The flow rates are such that the total flow (effluent and quench water) will have a temperature of below a certain temperature, e.g. 270° C., depending on concentration of corrosive compounds, after having reached a steady temperature state a certain distance  215  from the end of the input tube  207 , said distance being referred to as the mixing length. Accordingly, to avoid any risk of corrosion of the inner walls of tube  203 , the length of the liner  205  should be at least of this mixing length, and it should be localized to protect the inner walls of tube  203  within this length. For practical reasons, the liner  205  may have an offset  217  in the end facing the end of tube  207 , i.e. extend beyond (upstream of) said tube end, to avoid any risk of corrosion in that region. 
     The material of the liner and preferably of tube  207 , as well as suitable means for positioning and/or holding the liner in place may be chosen as in the first embodiment. 
     The first and the second embodiments of the present invention may be modified to include a heat exchange for assisting in increasing or decreasing the temperature in tubes  103  and  203 , respectively. Hereby, the lengths of liners  105  and  205 , respectively, may be shortened. 
     As a further example of an implementation of the present invention (not illustrated in the drawings), an effluent from the reactor containing chlorine ions is pre-cooled in a heat exchanger by part of the incoming waste stream, to a temperature well above 380° C., e.g., 400° C. The effluent is then cooled by an apparatus according to the present invention to a sufficient low temperature, e.g., 260° C., to minimize corrosion. After leaving the apparatus, the effluent water mixture is further cooled by the remaining of the waste stream. 
     It will be obvious that the invention may be varied in a plurality of ways. For instance, the geometry and function of the reaction system and the appearance of the tubing may deviate substantially from the description above. Such and other variations are not to be regarded as a departure from the scope of the invention. All such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the appended claims.