AU6480980A - Treatment of organic material in supercritical water - Google Patents

Description

TREATMENT OF ORGANIC MATERIAL IN SUPERCRITICAL WATER

Background of the Invention A number of alternate routes have been and are still being developed for converting organic feed materials such as coal and cellulosic materials including forest products, to liquid and gaseous fuels. Among the approaches are pyrolysis, gasification with steam and oxygen to form synthesis gas and liquefaction with hydrogen, carbon monoxide or hydrogen donor solvents. Each of these approaches have one or more drawbacks. In pyrolysis, there is a problem with the feed ending up as char in certain instances. In steam-oxygen gasification, high temperatures are necessary as for example 800 to 1000°C and the process thus requires considerable high-temperature heat. Liquefaction with hydrogen or carbon dioxide requires a separate processing step to supply these materials in relatively large quantities.

The prior art has also converted organic liquids to fuels by reaction with water, as for example, by reforming petroleum fractions catalytically to methane and carbon dioxide by a reaction with steam at 20 to 40 atmospheres. This avoids the use of a high temperature heat source and has met with some success where it is desirable to convert organic liquids to gaseous products.

In a still more recent development, it has been suggested that liquid or solid organic materials can be converted to high BTU gas with little or no formation of undesirable char or coke when the organic material is reacted with water at a temperature at or above the critical temperature of water and at or above the critical pressureof water to achieve the critical density of water as for example set forth in United States Patent 4,113,446.

In that patent, feed material was indicated to be converted to gaseous products in an amount of about 8 to 10% after one hour in a batch autoclave. In this prior gasification process, when a variety of reforming catalysts were used, 20 to 25% of the feed carbon could be gasified in relatively short times as for example 30 minutes. The formation of char can easily be avoided and useful high BTU gas is produced. However, as indicated in that patent, organic feeds were not completely transformed to gaseous products having high fuel value even after substantial periods of time at supercritical conditions. Attempts to separate substantial organic liquid products with methylene chloride resulted in recovery of only a small fraction, often no more than 6%, of liquid products, see Reforming of Glucose and Wood at The Critical Conditions of Wood, a paper presented at Intersociety Conference on Environmental Systems July 11-14, 1977, San Francisco, California, published by ASME, 1977. These results indicated that organic products could not be separated on a commercial scale.

Summary of the Invention It has now been found that when organic materials are dispersed in water and brought to supercritical conditions, the organic materials are rapidly broken down and restructured to form organic materials which have structures and properties which are different from those of the feed materials. Only some of the restructured products appear as gases such as CO, CO_2' H₂, CH₄, C₂ while the major portion of the products resulting are relatively volatile liquids. The gaseous as well as the volatile organic liquid products can be relatively easily separated from the water by reducing temperature and pressure below critical conditions of the reaction mixture and then carrying out conventional separation steps.

It is an object of this invention to use the reaction of organic materials with water in the region of the critical density of water to reform the organic materials and obtain useful volatile organic liquid materials.

It is a still further object of this invention to provide a method of reacting toxic organic materials with water in the region of the critical density of water to obtain non-toxic organic materials which reaction can be carried out when the toxic materials are in sub stantially pure form or appear as trace or other amounts in other materials such as river water, lake water or other mixtures to be purified.

According to the invention, an organic material which can be solid or liquid is treated by reacting in water to restructure the organic material and form resulting organic material. The feed organic material is admixed with water to form a reaction mixture for a time period while maintaining the water in the region of its critical density preferably by the use of a temperature at least as high as about the critical temperature of water and a pressure at least as high as about the critical pressure of water to form resulting more volatile organic material than the feed organic material which resulting material is selected from the group consisting essentially of non-toxic organic materials where the original feed material was toxic, and useful volatile organic liquids. The useful volatile organic liquids are recovered in amounts of at least 25 weight percent of the feed organic material.

Preferably, the organic material is a toxic material or a toxic material in at least trace amounts in a mixture as for example river water and the resulting product is a non- toxic material as for example cleansed water containing lower molecular weight organic materials which are non- toxic.

In another preferred process in accordance with this invention, the original organic material is a solid or heavy liquid material and the resulting products are lower molecular weight volatile organic liquids which can be used as fuels or for other purposes.

Preferably, the process is carried out by bringing the reacting temperature to at least 374°C rapidly and preferably substantially instantaneously to avoid the formation of char. When toxic materials are treated by the process of this invention, the resulting products may be safely discarded if no commercially useful products are formed.

The separation techniques for separating useful products from the reaction mixture include distillation, flashing, decanting and membrane separation. It is a feature of this invention that the liquid products formed are hydrophilic and can have volatilities such that they will distill off before water and thus be easily separated in distillation procedures. The term "volatile" as used in this application with reference to organic liquids means organic products which have a vapor pressure no less than 1/10 that of water at any temperature in the range of 25°C to 374°C.

Description of Preferred Embodiments In accordance with the invention, organic material is restructured in water to form different organic materials which include volatile organic liquids and in some cases, non- toxic organic materials which may or may not be volatile liquids.

The process can be carried out in batch or continuous operations. For example, the process can be carried out in an autoclave as described in U.S. Patent 4,113,446. Continuous methods can be used where a reaction slurry or liquid mixture of the reactants and water is treated under heat and pressure. A feed mixture of organic material to be treated is preferably brought to temperature of reaction as for example at least 374° C, quickly, as by adding it in a continuous stream to a flow of superheated water, heated for example to 600 to 900°C, to quickly and substantially instantaneously bring the organic material feed to temperature. The quick heating of the feed minimizes or substantially eliminates char formation.

The reaction conditions are such that the organic material feed is used in an amount up to about 25 weight percent of water and preferably from about 5 to 10 weight percent of water. Low concentrations may be used, as for example fractional percentages, in reforming toxic materials to non-toxic material as for example in the detoxification and purification of contaminated surface water or well water. Catalysts can be used during the reaction to promote reforming and hydrogenation of organic materials as well as to facilitate simple breakdown of organic chains. Representative suitable catalysts include nickel, molybdenum, cobalt, their oxides or sulfides, and noble metal catalysts such as platinum, palladium or the like or mixtures thereof either unsupported or supported on a base such as silica, alumina mixtures thereof and the like. The reaction often preferably is carried out at the critical density of water which means that the temperature must be at least the critical temperature and the pressure at least the critical pressure of water. Parameters at the near critical condition of water can also be used and should be considered the equivalent of exact critical condition. Thus, as referred to herein, the terms "substantially at its critical density", "about its critical temperature" and "about its critical pressure" refer to water in the near critical region. The near critical region or the term "in the region of the critical density of water" is encompassed by densities of from

3 0.2 to 0.7 grams per centimeter . In this near critical region or in the region of the critical density, pressures can be from 200 to

2500 atmospheres and temperatures can be from 374°C to at least 450°C.

A critical temperature range of 374°C to 450°C and a critical density range of .3 to .55 grams per centimeter are preferred for use.

While the reactions can be carried out in batch operations over long time periods of several hours, most efficient operation is achieved by rapid reaction over time periods of no more than 15 minutes and preferably from 1 to 10 minutes in flow- through, continuous reactors. Toxic material which can be treated by reaction with water under critical conditions include those on the EPA toxic chemical list of toxic organic substances as for example:

Aldrin Dieldrin DDT

2,4,5-T and esters

2,4-diaminotoluene

Lindane p-Aminobenzoic acid Anthranilic acid Alfatoxin

Heptachlor Malathion Nitros amines When toxic chemicals are treated under critical conditions in the process of this invention, restructuring of the toxic chemical occurs to form non-toxic materials. As used in this application, toxic materials are those recognized as hazardous by the U.S. Environmental Protection Agency as for example those set out in EPA publication EPA-560/11-79-001 entitled Test Data Development Standards: Chronic Health Effects Toxic Substances Control Act; Section 4. The reaction mixture after reaction can then be discarded safely. In some cases, the resulting materials can be removed as by biological oxidation, activated carbon adsorption and the like before discarding or reusing the remaining water. In other cases, the non-toxic products which result from reformation of the toxic organic starting materials can be used for commercial purposes. For example, volatile liquids formed can be used as fuels. It is found that since toxicity is highly structure-specific, simple altering of one or more chemical bonds in many organic toxic materials results in products which are non-toxic and which can be safely disposed of.

The non-toxic starting materials which are to be reformed by the process of this invention for use as fuels or for other commercial purposes, can be a wide variety of staring materials. Solid organics include coal or organic waste materials, cellulose, waxes, coal tars, shale, wood products including trees, leaves, bark and the like. Liquid organic materials including aryl or acyl hydrocarbons such as petroleum fractions up to and including asphalt fractions, aromatic hydrocarbons, sugars, black liquor from pulping of wood, green liquor used to pulp, organic acids, alcohols, aldehydes, ketones, amines, mixtures thereof and the like can be used.

The reaction is preferably effected by continuously intimately contacting the organic feed material with water. When employing solid organic material such as coal or organic waste material, it is preferred that the solid be in the form of small particles and the reaction be conducted so that the organic particles in water are fed to the reactor as a slurry. In order to promote intimate contact, the solid particles can be small and in the order of from submicron size to about 1 cm. Larger particles can be employed in some cases. In a specific example of this invention, a flow reactor was used with a one weight percent solution of glucose in water. At a flow rate of about 10 ml per minute, the feed solution was heated rapidly to supercritical conditions by pumping to 315 atmospheres and then passing the mixture through a feed preheater, which was constructed of 10 feet of 3/32 inch, inside diameter, stainless steel tubing immersed in a molten lead bath at 380°C. A residence time of less than 20 seconds was required to reach 374°C; within one minute, the feed was brought to essentially the temperature of the lead bath. Following preheat, the mixture was maintained at the supercritical conditions of 315 atmospheres and 380°C for a residence time of about 7 to 11 minutes in a reactor consisting of a ten foot section of 5/16 inch, inside diameter, stainless steel tubing, which was also maintained at constant temperature by immersion in the molten lead bath. The products were then passed through a water-cooled heat exchanger to bring the resulting mixture to room temperature and then to a throttling valve to bring the mixture to atmospheric pressure. Liquid and vapor phases were separated and the flow rates of each of these phases was measured. Samples of the vapor phase were analyzed by gas chromatography and the liquid phase was analyzed for total carbon. The results are shown in Tables 1 and 2.

At 380° C with these short residence times as indicated in the Tables, 7 to 10% of the feed organic material was gasified (Table 1, runs 1-7). This gasification is as much as had previously been observed for a one-hour residence time in batch autoclave experi ments with glucose. The observation that a significant quantity of gas can be obtained at a short residence time under supercritical conditions is the indication of the occurrence of rapid breakdown and restruc turing of the feed organic material.

Attempts were made to extract the organic liquid products so that they could be analyzed by a combination of gas chromatography, liquid chromatography and mass spectrome try. In prior work, a variety of solvents had been explored as extracting agents. Polar solvents could not be used because they are water-soluble. Methylene chloride was chosen because it is essentially immiscible with water and volatile enough so that the extracted products could be concentrated by evaporation of the solvent. In these prior tests, a 10 ml aliquot of aqueous product was extracted with 5 ml of methylene chloride. After concentrating the extract to 0.1 ml, analysis of the concentrate could account for no more than 0.5 to 6 weight percent of the carbon in the feed.

In an attempt to improve the extraction efficiency of liquid products, separate 20 ml aliquots of the aqueous product were acidified with 7 ml of 0.9 molar sulfuric acid and made basic with 7 ml 0.9 molar potassium hydroxide prior to extraction. Each aliquot was extracted with three 10 ml portions of methylene chloride. The extracts were then air-dried at ambient temperature and the residue weighed. The results, shown in columns 4 and 5 of Table 2, were converted to percent by weight of feed material. The acidic extractions resulted in a somewhat higher recovery of liquid products than the basic extractions, but in both cases, the recovery efficiency was extremely low (less than 10%).

To verify that the aqueous product actually contained significantly more organic liquid products than that which was extracted by methylene chloride, carbon analysis of feed and all products were made for run 3. Total carbon analyses were made for feed and liquid product and carbon content of the gaseous were calculated from the composition of the vapor, as determined by gas chromatography. The carbon content of the feed was found to be 0.42 weight percent, whereas the liquid product contained 0.36 weight percent and the vapor product 0.039 weight percent, for a total of 0.40 weight percent carbon in the products. Thus, it was established that the liquid products contained about 90% of the weight of the feed material, no more than 10% of which could be recovered by methylene chloride extraction. The reaction mixture after 7 to 11 minutes of reacting, was evaporated at ambient temperature. The fraction of organics remaining after evaporation are shown in Table 2 column 6. Only 10 to 26% of the original carbon feed is accounted for in the residue. A large fraction of organic liquid products are volatile; at least as volatile as water, if not more so. Thus, the last column in Table 2 is the fraction of the carbon feed that was lost during evaporation of the water, i.e., from 63 to 74% of the carbon feed can be recovered as volatile organic liquids. These materials are readily separable from the aqueous product by distillation or sequential flashing or in some cases by other conventional separation techniques at subcritical conditions. Moreover, these products are reformed products of the original organic feed and can be useful as fuels in their liquid or gaseous forms.

FIG. 1 illustrates a preferred flow diagram for a continuous process of reforming solid organic material to form liquid fuels and chemicals as shown in FIG. 1 at 10. The supercritical water reformer is noted at 21 and is preferably sized as a tubular reactor with a flow therethrough such that the material to be reformed spends only from seconds to minutes under supercritical conditions to rapidly dissolve and disperse the solid matter in the feed and rapidly break down high molecular weight components into gases and volatile liquids such as hydrogen, carbon monoxide, methane, carbon dioxide and other reformed lower molecular weight volatile liquid organic compounds. Portions of the products produced may be oxidized within the overall processing scheme and utilized for internal energy requirements as in heating water to supercritical conditions.

FIG. 1 shows a wood chip process where a feed of wood chips is fed to a slurry tank 20 and suspended in water. Feed pump 19 pumps the feed to a supercritical water reformer 21 which also receives super heated, supercritical water at high temperature so that the cold feed is instantly heated to supercritical conditions as previously described. A heat exchanger 22 is positioned in the line from the reformer 21 so as to heat combustion air going to the furnace 11 which in turn heats the water to supercritical conditions for use in the reformer 21. The flow from the reformer 21, is brought to a subcritical flashing unit 17 which removes gaseous products to a gas expander 16 and in turn uses these gases in the heating process of the furnace 11. The flashing unit 17 is maintained somewhat below the critical temperature of the aqueous solution, although the pressure may be above the critical pressure of water. Typically, the temperature may be 300 to 350°C in flashing unit 17. Organic liquids and water are taken through a liquid expander 18 where liquids are passed to a steam stripper 12 and condenser 13 arrangement which takes off organic liquids for use as fuel or chemicals. Off gases from the superheater 11 are passed to a boiler 14 which acts to aid in the stripping occurring in the stripper 12 while recycled water passes to water pump 15 for passage to the water heater with some water passing to the feed slurry tank.

The feed can be mixed to the proper proportion in the feed slurry tank bearing in mind the additional superheated water that will be added in the reformer. No predrying of the feed is necessary. The slurry can be pressurized and heated to supercritical conditions very rapidly to avoid char formation. Heating can be obtained by mixing the feed with the superheated, supercritical water at 600 to 900°C or by heating along the line leading from the slurry tank to the reformer. In all cases, short residence times of up to 15 minutes are preferably maintained in the supercritical water reactor.

FIG. 2 illustrates the processing of a slurry such as black liquor from pulping which contains relatively high concentrations of inorganic materials as well as organic materials. Black liquor from the holding tank 31 is pumped through pump 32 to the super- critical water reformer 33 which is maintained at near critical conditions and causes rapid breakdown of organics. The reformer 33 passes the reaction products to the heat exchanger 35 where heat is taken off to heat water passing to the supercritical water superheater 41 which water is in turn used in the reformer. Combustion air is used to heat the water in the superheater 41 along with combustible gases and volatile organics obtained from a flash drum number 2 at 40 with the oil phase from the flash drum passing to fuel storage. The feed from the reformer 33 after passing through heat exchanger 35 is reduced in pressure by a letdown valve 36 and then passed to flash drum number 1 at 38 whereupon an inorganic solution is removed through valve 42. Flash drum number 1 at 38 is maintained at temperatures and pressures below the critical conditions of water, where volatile organic material, gaseous products, and steam can be collected overhead. These conditions are typically in the range of 250 to 350°C and 30 to 150 atmospheres. The inorganic solution can be used to recover sodium and sulfur so as to obtain an acceptable green liquor which is subsequently to be mixed with wood feed. The temperature and pressure of the overhead steam from flash drum number 1 are reduced further and fed to flash drum number 2 at 40 through a heat exchanger 37. The conditions in flash drum number 2 at 40 are typically ambient temperature to

200°C and ambient pressure to 15 atmospheres. The aqueous phase con taining hydrophilic organics is taken from the flash drum number 2 passed through a booster 39 heat exchanger 37 and 35 and back to the superheater 41. It is repressurized reheated and superheated before recycled to the supercritical water heater 41.

While specific embodiments of the present invention have been shown and generally described above, many variations are possible. For example, the separation of the volatile liquid organics formed during the reactions with water at the critical density can be carried out under varying conditions. For example, steam stripping at atmospheric pressure can be used to remove the volatile organic liquids from the water in the reaction vessel after the reaction or in a subsequent vessel in processing. Distillation in a distillation tower can be carried out at pressures ranging from atmospheric to on the order of a 150 atmospheres to separate the liquid organic products from the water. The volatile organics can be collected in the vapor phase along with water to leave volatile organic residuals in the liquid phase after the reaction as for example at temperatures of from 250 to 350°C at 30 to 150 atmospheres. This can be followed by a distillation step in a distillation tower to separate the volatile organic liquids from the water. Flashing can be .used at temperature in the range of 300 to 350°C to take off gaseous products. Membrane separation such as reverse osmosis, can be used to separate volatile organic liquids. The volatile organic liquids being highly hydrophilic, require the separation step and are produced in sufficient quantity to make separation feasible. Preferably, the volatile organic liquids are produced in quantities at least as high as 25% but more preferably at least as high as 50% of the original organic feed.

Claims (5)

1. CLAIMS 1. A process for treating a toxic organic material by reacting said organic material in water to restructure said organic material, said process comprising admixing feed organic material with water to form a reaction mixture and reacting said organic material for a time period while maintaining said water in the region of its critical density to form resulting organic material which resulting organic material is non- toxic.
2. 2. A process in accordance with the process of claim 1 wherein said feed material is toxic and the resulting organic materials are non-toxic, and said critical density is obtained by the use of a temperature at least as high as about the critical temperature of water and a pressure at least as high as about the critical pressure of water.
3. 3. A process in accordance with the process of claim 2 wherein said feed material is selected from the group consisting essentially of EPA identified toxic organic materials.
4. 4. A process in accordance with the process of claim 3 wherein said toxic material in the reaction mixture is present in an amount of no more than 25 weight percent.
5. 5. A process in accordance with the process of claim 2 wherein said process is carried out without significant char formation by assuring that said reaction mixture is brought to reacting temperatures rapidly.