WO2001096247A1

WO2001096247A1 - Supercritical water oxidation with reduced corrosion and enhanced oxidation rate

Info

Publication number: WO2001096247A1
Application number: PCT/US2001/018514
Authority: WO
Inventors: Ram Gupta; Poongunran Muthukumaran
Original assignee: Auburn University
Priority date: 2000-06-12
Filing date: 2001-06-08
Publication date: 2001-12-20
Also published as: AU2001268249A1

Abstract

A process and apparatus for the oxidation of hetero-atom (Cl, N, S, P, F, etc.) containing material in supercritical water that avoids corrosion and increases oxidation rate is disclosed. The process uses basic inorganic salts (e.g., sodium carbonate or calcium carbonate) that are insoluble in supercritical water. The process employs supercritical anti-solvent method to form fine particles of these basic salts in the reaction zone to provide high surface area for both catalysis of the oxidation and neutralization of the corrosive acidic species. An apparatus to accomplish this process is described which consists of two preheated inlet streams for the material and oxidant, and one non-preheated inlet nozzle for spraying the inorganic salt solution in the supercritical reactor. Most of the corrosive species collect on the particle surfaces. Upon cooling, the particles dissolve back in water, avoiding clogging in the downstream valves and pressure regulators.

Description

TITLE:

SUPERCRITICAL WATER OXIDATION WITH REDUCED CORROSION AND

ENHANCED OXIDATION RATE

CROSS-REFERENCE TO RELATED APPLICATION: This application claims benefit from the Provisional patent applications Serial No. 60/279,144 and entitled PROCESS AND APPARATUS FOR THE DESTRUCTION OF HETERO-ATOM (Cl, N, S, P, F, etc.) CONTAINING WASTES IN SUPERCRITICAL WATER WITH REDUCED CORROSION AND ENHANCED DESTRUCTION RATE, Serial No. 60 210,767 and entitled OXIDATION OF CHLORINATED AND NON- CHLORINATED WASTES IN SUPERCRITICAL WATER, WITH REDUCED CORROSION AND ENHANCED DESTRUCTION teachings of which are incorporated herein by reference.

FEDERALLY SPONSORED APPLICATION The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Grant No. UAB-EPSCOR-NSF-AU2 awarded by the National Science Foundation EPSCOR Grant, Grant No. CTS-9801067 awarded by the National Science Foundation and Grant No. 1R21RR13398-01 awarded by the National Institute of Health. BACKGROUND OF THE INVENTION Field of Invention

This invention relates to the oxidation of hetero-atom containing material in aqueous streams and more specifically, dispersing a basic inorganic salt in the reaction medium as fine particles for the oxidation rate enhancement and corrosion protection.

Background and Prior Art

Oxidation of the organic material to carbon dioxide and water is known for a long time and often used in heat generation, power generation and material disposal. Incineration is a common way of carrying out the material disposal. The downside is the production of toxic compounds including dioxins that ultimately find their way into the biosphere. Zimmermann (US 2665249) discloses a process known as wet air oxidation that involves reacting the material and the oxidant at high pressures and moderate temperatures for 0.5 to 1 hour. This process is effective in removing 70-95% of the initial organic matter. The reactors for this process are expensive and heat recovery from the combustion is also inefficient.

Supercritical water oxidation (SCWO) has been gaining importance as a feasible hazardous material disposal technique. In this technique, water at high temperatures (above 374 °C) and high pressures (above 221 bar) is used as a medium for spontaneous oxidation of the hazardous material. At these conditions, water is miscible with organic materials and oxygen, and has low viscosity and high mass transfer rates. Several studies have demonstrated that the oxidation efficiencies of 99.99% or higher can be easily achieved within less than a minute of the reaction time. Model (US 5746926) disclosed processing methods for the oxidation of organics in supercritical water. It has been estimated that U.S. industry produces about 600,000 tons of chlorinated material for disposal every year, and military has weapon chemicals to be disposed that have heterogeneous atoms including Cl, S, and N. Also there are numerous potential space and defense applications involving heterogeneous atoms that can utilize this technology. Yang and Eckert "Homogeneous catalysis in the Oxidation of p-Chlorophenol in Supercritical Water" Industrial and Engineering Chemistry, 27, 2009-2014 (1988) studied the oxidation kinetics of p-chlorophenol with a homogeneous catalyst; Jin et al. "Catalytic Supercritical Water Oxidation of 1,4-Dichlorobenzene", Chemical Engineering Science, 47 (9-11), 2659-2664 (1992) studied the catalytic oxidation of 1,4- dichlorobenzene; and Li et al. "2-Chlorophenol Oxidation in Supercritical Water: Global Kinetics and Reaction Products" AIChE Journal, 39 (1),178-187 (1993) studied 2- chlorophenol oxidation and provided a global kinetic model. Unfortunately, SCWO of chlorinated materials has a severe corrosion problem due to the formation of HC1 acid. In the course of this invention, it has been observed that stainless steel 316 surface can be easily corroded by SCWO of chlorinated materials as shown in Figures 1-2 (in Figure 1 passive protection layer is seen, whereas in Figure 2 this layer is broken in merely a 4 hour exposure to SCWO reactor for 2-chlorophenol). This led to the use of more- expensive alternate reactor materials such as titanium or Inconel. US 5527471 describes the use of iridium for SCWO. However, the problem of corrosion still remains. As early as in 1981, Dickinson disclosed (US 4292953) a process that utilizes the catalytic properties of alkali salts. In later efforts, NaOH was added to neutralize the acid but corrosion persisted due to the resulting sticky NaCl. Several inorganic salts were also tried due to their interesting solubility properties in water. These salts are highly soluble in ambient water, but insoluble in supercritical conditions due to the low dielectric constant of supercritical water (Figure 3).

Several researchers have studied the catalytic effect of the salts: Song et al. "Catalytic activity of alkali and iron salt mixtures for steam-char gasification," Fuel, 72(6), 797-803 (1993) have studied several alkali salts for their catalytic activity for steam-char gasification; Minowa et al. "Liquefaction of cellulose in hot compressed water using sodium carbonate," . Client. Eng. Japan, 30,186 (1997) have given a detailed description on the cellulose liquefaction using Na₂C0₃ catalyst in hot compressed water at different temperatures; Sealock et al. (US 5019135) disclose the use of alkali carbonates. These studies suggest that there is a good potential for alkali carbonates being used as catalysts in SCWO process. In 1981 and 1983, Dickinson (US 4292953, 4380960) disclosed a process for combustion of solid fuels containing sulfur utilizing supercritical water and alkalis like sodium carbonate and calcium carbonate for both catalysis and neutralization of corrosive gases like sulfur trioxide formed during the reaction. Much later, in 1998, Ross et al. (US 5746926, US 5837149) disclosed a method in which addition of Na₂C0₃ can enhance reaction rates and remove the corrosive gases significantly for SCWO of p- dichlorobenzene and hexachlorobenzene. The concentration of Na₂CO₃ used is so high that it tends to cake during the reaction and results in lower conversions after some time. The decrease in conversions can also be attributed to the used up surface of Na C0₃. These preliminary experiments were conducted in batch reactors and a fluidized bed reactor scheme was proposed. In a later disclosure (US 6010632), Ross et al. have modified the process in which Na₂C0₃ was placed in the plug flow reactor and heated prior to the addition of water and organic material. In this setup conversion is said to level off after it reaches 75%. A stirred tank reactor also did not seem to result in any optimum conversion. A corrosion free supercritical oxidation process will be a first step towards recovering the heat efficiently from supercritical water oxidation systems.

Summary of the Invention

The invention comprises of a process and apparatus for the oxidation of hetero- atom (Cl, N, S, P, F., etc.) containing material. A basic inorganic salt solution of predefined concentration at ambient temperature is sprayed into the reaction zone through a nozzle. The salt is readily soluble in ambient water and practically insoluble in near or supercritical water. As a result, the supercritical water inside the reactor acts as an anti- solvent and precipitates the salt as very fine particles. These particles provide very high surface area for organic molecules for adsorption and hence catalyze the oxidation. In addition to acting as a catalyst, the basic salt particles can adsorb the corrosive species and neutralize them.

The material is preheated and conveyed to this reaction zone and contacted with a predefined concentration of preheated oxidant. The apparatus is so designed that the basic inorganic salt solution is sprayed into the zone where the material and the oxidant come into contact. Rapid oxidation of the material takes place in this zone resulting in more than 99.9% conversion of the organic material.

When hetero-atom containing material is oxidized, corresponding acids are formed (e.g., HCl from Cl containing material). These acid gases at such high pressures and temperatures are very corrosive to reactor walls. The fine particles of the salt generated by supercritical anti-solvent technology provide enough surface area to adsorb and neutralize the corrosive acids. Neutralization reactions take place with corrosive species and micron size inorganic salt particles and as a result the sodium chloride formed is of very small size and it can be easily kept as a suspension in the reaction zone and transported out of the reactor.

In a preferred embodiment, the operating pressure and temperature are optimized for the best results in three factors.

1. To achieve the best oxidation (maximum conversion in oxidation).

2. To obtain fine inorganic salt particles for catalysis and corrosive species removal.

3. To maintain all the salts as small particles so that they can be transported out of the reaction zone easily.

Preferably, these optimum conditions include a temperature between 300° C and 650 °C and a pressure between 10 bar and 400 bar.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a Scanning Electron Microscope (SEM) picture of Stainless Steel 316 surface not exposed to SCWO. Passivation layer is seen here.

Figure 2 shows a SEM picture of Stainless Steel 316 surface exposed to conventional SCWO of 2-chlorophenol for 4 hours. Passivation layer is broken due to corrosion. Figure 3 shows sodium carbonate solubility in water versus dielectric constant function (ε-l)/( ε+l)

Figure 4 shows the schematic diagram of the apparatus for carrying out the process of the present invention.

Figure 5 shows the conversions of 2-chlorophenol with and without Na C0₃ in Supercritical water oxidation in a flow reactor.

Figure 6 shows conversions of phenol in supercritical water in a flow reactor.

Figure 7. SCWO of 2-Chlorophenol with potassium carbonate as the basic inorganic salt

Figure 8. SCWO of aniline with sodium carbonate as the basic inorganic salt. Comparison of conversions with and without sodium carbonate

Figure 9 shows the apparatus for the present invention that uses supercritical anti-solvent technology to obtain a high basic inorganic salt particle area for corrosion protection.

Figure 10 shows surface area of basic inorganic salt particles and ratio of particle surface area to reactor wall area as a functions of salt loading, based on 1 μm diameter particle size, for a cylindrical reactor (6 inch dia. and 5 feet length) at 400 °C and 300 bar.

DETAILED DESCRIPTION OF THE INVENTION

Definitions: "Hetero-atom containing material" means the organic or inorganic material containing atoms other than carbon and hydrogen. Cl, S, N, F or P are prominent examples of hetero-atoms. Hetero-atom containing material include aniline, nitrobenzene, nitroglycerin, tri-nitrotoluene, alkyl and aryl phosphates, mercaptons, chlorinated aromatic compounds like chlorophenol, chlorobenzene, chlorobenzodioxins, PCBs and other common industrial material. "basic inorganic salt" means an inorganic salt that is basic (pH > 7) in nature. They typically include alkali carbonates, alkali hydroxides, etc. Potassium carbonate, sodium carbonate and calcium carbonate are preferred.

"corrosion protection" means reducing the corrosion of the reactor wall in the supercritical water oxidation apparatus.

"enhanced oxidation rate" means increase in the oxidation rate of material in supercritical water by the addition of basic inorganic salt surface.

"backmixing" means thorough mixing of the reactants charged into the reaction zone.

"supercritical water" means water at above its critical temperature and pressure.

"antisolvent" means a fluid that has a poor dissolving power for a substance of interest.

"corrosive acidic species" means the acidic species formed as a result of the oxidation of heteroatomic material. These are highly corrosive at reaction temperature and pressures.

"thermodynamic nature of the basic inorganic salt" means the solubility and phase behavior of the basic inorganic salt in supercritical water medium.

"anti-solvent technology" means applying an antisolvent to a mixture containing a basic inorganic salt and precipitating the basic inorganic salt as fine particles. Description

This invention relates to method and apparatus for reacting a hetero-atom containing oxidizable material in a reactor at conditions above the critical point of water with an oxidant stream in the presence of basic inorganic salt particles that provide corrosion protection and oxidation rate enhancement. Many inorganic salts have decreasing solubility as the temperature is increased. Carbonates like sodium carbonate have a sharp decrease in solubility in the vicinity of the critical point of water. In supercritical water they are practically insoluble. Present invention involves dispersing the basic inorganic salts in the supercritical water reaction medium as fine particles and using their basic nature to neutralize the corrosive acids.

Supercritical anti-solvent technology is a method for forming micro or nanometer size particles of mainly organic materials like drug particles using supercritical carbon dioxide. In this method, drug material is dissolved in a conventional solvent like toluene and sprayed into a chamber of supercritical carbon dioxide through a nozzle of very small diameter in the range of 100 μm. Carbon dioxide acts as anti-solvent and removes the solvent from the solution and precipitates the drug as very small particles. It is a similar analogy to the present invention wherein water at ambient temperature acts as the solvent to basic inorganic salt and supercritical water acts as the anti-solvent and hence precipitates the basic inorganic salt as very small particles.

The small particles provide high surface area for

1. The organic molecules to get adsorbed and hence the salt acts as a catalyst.

2. The corrosive acidic species formed during the oxidation to get adsorbed and neutralized.

3. To keep the size of the salts to very small particles so that they can be transported easily out of the reaction zone.

In a preferred embodiment, the material is contacted with the oxidant at certain concentration in supercritical water. The oxidation takes place instantaneously and conversions of 99.9% or more are achieved in less than a minute. Since the basic inorganic salt forms "fresh surface" continuously, there is no drop in the conversion as the time proceeds.

When sodium carbonate is the basic inorganic salt used, the resultant sodium chloride, sodium sulfate, etc. are kept as small particles and transported out of the solution. The temperature is kept well above 400 C and as high as 650 C. The pressure is just above the critical pressure of water.

The key aspect in corrosion protection is the surface area of the Na₂C0₃ particles. For given loading of Na₂C0₃, a higher surface area (smaller particle size) will result in a higher corrosion protection. Assuming the average particle size to be 1 μm, the surface area of the particles for different loading of Na₂C0₃ in the feed solution is shown in Figure 9. For example, even 0.25 wt.% Na C0₃ in feed results in 97 m² of particle surface area. Though the surface area is shown to increase linearly with sodium carbonate loading, in the actual experiment the increase will be somewhat less than shown here because of particle agglomeration at the higher loadings.

The key factor in the corrosion protection is having a much larger particle surface area than the reactor wall area. For illustration, in Figure 10, the surface area ratios are shown for a cylindrical reactor with diameter of 6 inches and length of 5 feet for different Na₂C0₃ loading at 400 °C and 300 bar. A 0.25 wt.% loading of sodium carbonate can give particle surface area that is about 133 times the surface area of the reactor wall. This represents an extremely large corrosion protection in view of adsorption sites for the corrosive species. If the partitioning of the corrosive species in supercritical- fluid/reactor-wall is similar to that in supercritical-fluid/Na C0₃-particles, the amount of corrosive species depositing on the reactor wall can be reduced by a factor of 133. In practical applications Na₂C0₃ concentration is envisioned to be kept large enough to provide for both HCl neutralization and NaCl adsorption and small enough to avoid any plugging. Another issue is how Na₂C0₃ particles can be removed from the system. Usually, supercritical water oxidation reactor is followed by a heat exchanger. When the products are cooled, Na₂C0₃ particles will dissolve back.

Example 1 : Supercritical water oxidation was carried out in an isothermal, isobaric flow reactor above 400 C and pressures between 240-300 bar. Sodium carbonate was used as the basic inorganic salt. The material was 2-chlorophenol dissolved in water. Oxidant was 0.3 wt.% hydrogen peroxide. The residence times were varied from 20 seconds to 80 seconds and figure 5 shows the conversions achieved with and without sodium carbonate. In absence of sodium carbonate, conversion is only about 40% in 20 seconds and with a small amount of sodium carbonate the conversion increased to as high as 98% in 20 seconds. The reactor material was stainless steel 316 and several runs were carried out and no corrosion was noticed after these runs.

Example 2:

SCWO was carried out with phenol as the material and 0.3 wt.% hydrogen peroxide as the oxidant in an isothermal isobaric flow reactor. Figure 6 shows the conversions of phenol with respect to residence time inside the reactor. In the absence of sodium carbonate, the conversion was only 60% in 40 seconds and with the addition of even a small amount of sodium carbonate raised the conversion to more than 95% in 40 seconds.

Example 3:

Aqueous 2-chlorophenol was oxidized with 0.3 wt.% hydrogen peroxide in an isothermal isobaric flow reactor with potassium carbonate as the basic inorganic salt. Figure 7 shows the results. Even small amounts of potassium carbonate yielded very high conversions at low residence times. Conversions more than 95% were achieved in 20 seconds of residence time.

Example 4:

Aqueous aniline was taken as the material to be oxidized. Hydrogen peroxide (0.3 wt.%) was as the oxidant and SCWO was carried out in an isothermal isobaric flow reactor. Aniline is comparatively difficult to destroy than chlorophenol or phenol. Figure 8 shows the conversion results. Even in 50 seconds, the conversion is only 25% in absence of sodium carbonate. When sodium carbonate was added in very small quantities, the conversion increased to 60 o in 50 seconds.

Claims

CLAIMSWe claim:

1. A method for the oxidation of hetero-atom containing material by oxidation in near or supercritical water to environmentally benign material with corrosion protection and enhanced oxidation rate which comprises of:

(a) Preheating and pressurizing a hetero-atom containing material stream and the oxidant-water stream to the reactor temperature and pressure.

(b) Contacting the said preheated streams in a reaction zone where maximum backmixing is possible.

(c) Spraying an aqueous basic inorganic salt solution of desired concentration at ambient temperature through a nozzle into a reaction zone that consists of the material and oxidant in supercritical water, wherein the near or supercritical water acts as anti-solvent and precipitates the inorganic salt as fine particles with diameters in micron range.

(d) Keeping the operating pressures and temperatures in such a way to keep the inorganic particles suspended in the solution to provide enough surface area to adsorb and neutralize any corrosive acidic species formed during the oxidation, to allow hetero-atom containing material molecules to adsorb on the inorganic particles and effect a catalysis of oxidation, and to keep the resultant sodium chloride (or sulfide, nitrate, fluoride, phosphate, etc.) adhered to the particles as solids so as to transport them out of the reaction zone is possible with ease.

(e) Allowing sufficient residence time in the reactor to achieve more than 99% conversion of initial material to environmentally benign products such as carbon dioxide and water.

2. The method of claim 1 wherein the oxidant concentration is maintained sufficiently higher than the stoichiometric quantities required for the complete oxidation of the hetero-atom containing material.

3. The method of claim 1 wherein the temperature of the reaction zone is kept between 300 °C and 650 °C.

4. The method of claim 1 wherein the pressure of the reaction zone is kept between 10 bar and 400 bar and varied accordingly as the thermodynamic nature of the basic inorganic salt or the composition of the material.

5. The method of claim 1 wherein the feed concentration of the basic inorganic salt is just enough to make particles by anti-solvent technology and is just enough to remove the corrosive species.

6. The method of claim 1 wherein the oxidant is chosen from hydrogen peroxide, air, oxygen or a combination thereof.

7. The method of claim 1 wherein the oxidant is hydrogen peroxide.

8. The method of claim 1 wherein the basic inorganic salt is chosen from a list of potassium carbonate, calcium carbonate, sodium carbonate or a combination thereof.