Title of Invention

" AN IMPROVED PROCESS AND APPARATUS FOR PRODUCING AND COOLING TITANIUM DIOXIDE"

Abstract

AN IMPROVED PROCESS AND APPARATUS FOR PRODUCING AND COOLING TITANIUM DIOXIDE PRODUCED FORM THE REACTION OF GASEOUS TITANIUM TETRCHLORIDE AND OZYGEN, IN WHICH THE TITANIUM DIOXIDE AND GASCOUS REACTOIN PRODUCTS ARE COOLED BY PASSING THESE MATERIALS THROUGTH A TUBULAR HEAT EXCHANGER ALONG WITH A SCOURINGT MEDIUM FOR REMOVING DEPOSITS FORM THE INSIDE SURFACES OF THR TUBULAR HEAT EXHANGER, AND THE PAFRTICULATE SCOURING MEDIUM, THE PARTICULATE TITANIUM DIOXIDE AND THE GASEOOUS REACTION PRODUCTS ARE ALL CAUSED TO FOLLOW A SPIRAL PATH THROUGTH THE USE OF SPIRALING VANES AND RECESSES ON AN INNER SURFACE OF THE EXCHANGER,

Full Text

"IMPROVED PROCESS AND APPARATUS FOR PRODUCING AND COOLING TITANIUM DIOXIDE" The present invention relates to processes and apparatus for producing and cooling titanium dioxide, and more particularly, to such processes and apparatus wherein the cooling of the titanium dioxide and gaseous reaction products produced is more efficiently carried out. In the conventional production of titanium dioxide utilizing the chloride process, heated gaseous titanium tetrachloridc and heated oxygen are combined in a tubular reactor at high flow rates. A high temperature oxidation reaction takes place in the reactor whereby paniculate solid titanium dioxide and gaseous reaction products arc produced. The titanium dioxide and gaseous reaction products are cooled by passing them through a tubular heat exchanger along with a scouring medium for removing deposits from the inside surfaces of the heat exchanger. The scouring medium is a paniculate solid such as sand, sintered or compressed titanium dioxide, rock salt or the like. In spite of the use of a scouring medium, the solid titanium dioxide and other deposits on the inside surfaces of the tubular heat exchanger have only been partially removed in the known processes, thereby leaving deposits which reduce the heat transfer efficiency of the heat exchanger. The present invention provides an improved process and apparatus for producing and cooling paniculate solid titanium dioxide in which gaseous titanium tetrachloride and oxygen are reacted, as before, at a high temperature to produce paniculate solid titanium dioxide and gaseous reaction products. The produced paniculate solid titanium dioxide and gaseous reaction products are then cooled by heat exchange with a cooling medium in a tubular heat exchanger. A scouring medium is injected into the heat exchanger for removing deposits of titanium dioxide and other materials from the inside surfaces of the heat exchanger. According to the present invention, however, in order to increase the removal of the deposits from the surfaces and thereby increase the heat transfer efficiency in the heat exchanger, the scouring medium is caused to follow a spiral path through the heal exchanger. After passing through the heat exchanger, the particulate solid titanium dioxide is separated from the gaseous reaction products according to known practice. The present invention is more clearly understood by reference to the accompanying drawings, in which: FIGURF. 1 is a side cross-sectional view of a tubular heat exchanger section which includes spiraling vanes and recesses in accordance with this invention; and FIGURE 2 is an end view taken along line 2-2 of FIG. 1. Titanium dioxide pigment is produced according to the chloride route by reacting heated gaseous titanium tetrachloride and heated oxygen in a tubular reactor at high temperatures. The titanium tetrachloride can include aluminum chloride in an amount sufficient to produce a rutile pigment containing between 0.3 percent to 3 percent by weight of aluminum oxide. Typically, the titanium tetrachloride is preheated to a temperature in the range of from 343 degrees Celsius (650°F) to 982 degrees Celsius (1800oF) depending upon the particular preheater apparatus utilized. The oxygen is typically preheated to a temperature in the range of from 954 degrees Celsius (1750°F) to 1871 degrees Celsius (3400°F). The oxidation reaction temperature at a pressure of 1 kg/sq. cm. (one atmosphere) is typically at least 1150 degrees Celsius, more typically being in the range of from 1260 degrees Celsius (2300°F) to 1371 degrees Celsius (2500°F). The reaction produces paniculate solid titanium dioxide and gaseous reaction products. The reaction products are conventionally immediately introduced into an elongated tubular heat exchanger, wherein the reaction products are cooled by heat exchange with a cooling medium such as cooling water. The elongated tubular heat exchanger is usually made up of a plurality of individual heat exchanger sections which are sealingly bolted together. The heat exchanger sections and overall length of the heat exchanger can vary widely depending on factors such as the titanium dioxide production rate, the desired discharge temperature and the diameter of the heat exchanger among other factors. Consequently, commercial producers of titanium dioxide that utilize the chloride process, that is, the process of oxidizing titanium tclrachloride, use heat exchangers of varying diameters and lengths to cool the reaction products. In known examples, the heat exchanger sections have an internal diameter of 18 centimeters (7 inches) and are from 2.1 meters (7 feet) to 4.9 meters (16 feet) long. The elongated tubular heat exchanger often also includes an adapter section which is from 0.3 meters (1 foot) to 1.2 meters (4 feet) long. While passing through the elongated tubular heat exchanger, the titanium dioxide and gascous reaction products are cooled to a temperature of 700 degrees Celsius (1300°F) or less. In order to prevent the build-up of deposits formed of titanium dioxide and other materials produced in the oxidation reaction, a scouring medium has been injected into the tubular heat exchanger along with the reaction products. Examples of scouring media which can be used include, but arc not limited to, sand, mixtures of titanium dioxide and water which are pelletized, dried and sintered, compressed titanium dioxide, rock salt, fused alumina, titanium dioxide and salt mixtures and the like. The salt mixed with titanium dioxide can be potassium chloride, sodium chloride and the like. The scouring medium impinges on the inside surfaces of the heat exchanger and removes deposits therefrom. While the scouring medium removes some of the deposits, it often does not remove all of the deposits and as a result, a layer of the deposits on the inside surfaces of the heat exchanger remains. The remaining layer of deposited material decreases the heat transfer rate from the reaction products being cooled through the walls of the heat exchanger and into the cooling medium. This in turn significantly decreases the efficiency of the heat exchanger and increases the overall costs of producing the titanium dioxide, by requiring the installation and maintenance of a longer heat exchanger and requiring a greater amount of the scouring medium. After the reaction products are cooled, the participate solid titanium dioxide is separated using conventionally known gas-solids separation apparatus from the gaseous reaction products and the scouring medium. The present invention is based on the discovery that the removal of the deposits from the inside surfaces of the heat exchanger can be improved by causing the scouring medium to follow a spiral path through the heat exchanger. While various techniques can be utilized for causing the scouring medium to follow a spiral path through the heat exchanger, a presently preferred technique is to provide one or more spiraling vanes on the inside surfaces of at least a portion of one or more of the individual heat exchanger sections. Preferably, for 18 cm (7 inch) to 28 cm (11 inch) internal diameter heat exchanger sections, two or more spiralling vanes having spiraling recesses therebetween are provided in 2,4 meter (8 foot) portions of two or more of the individual heat exchanger sections. Most preferably from, four to six spiraling vanes with four to six spiraling recesses therebetween are provided in the spiralcd portions of the sections. Referring now to the drawings, one of the individual 18 cm (7 inch) internal diameter by 4.8 meter (16 feet) long heat exchanger sections making up an elongated heat exchanger for cooling the reaction products is illustrated and generally designated by the numeral 10. The heat exchanger section 10 includes four spiraling vanes 12 with four recesses 14 therebetween extending over an 2.4 meter (8 foot) internal portion thereof. As shown in FIG. 1, the vanes 12 and recesses 14 rotate over the initial 2.4 meter (8 foot) internal surface length of the heat exchanger 10. The rate of rotation of the spiraling vanes and recesses is constant and is generally in the range of from 0.8 degrees per centimeter (2 degrees per inch) to 2.4 degrees per centimeter (6 degrees per inch), preferably being about 1.6 degrees per centimeter (4,5 degrees per inch). As shown in FIG. 2, the spiraling vanes 12 and recesses 14 have curved rectangular cross-sectional shapes. Generally, the heights, widths and rate of rotation of the spiraling vanes are such that for an individual heat exchanger section containing the vanes over its initial 2.4 meters (8 feet) of internal surface length, the maximum pressure drop at the maximum reaction products flow rate through the section is 14.1 g/sq. cm. (0.2 pounds per square inch). A further requirement is that "the scouring medium completely scours the inside surfaces of the heat exchanger section including the surfaces of the spiraling recesses. These criteria are met, for example, by a heat exchanger section having a length of 4.8 meters (16 feet), an internal surface diameter of 18 cm (7 inches) and having four curved rectangular vanes equally spaced over the initial 2.4 meters (8 feet) of internal surface therein when the vanes arc 1.3 cm (0.5 inch) high, 3,8 cm (1.5 inches) wide and have a rate of rotation of 1.7 degrees per centimeter (4.3 degrees per inch) and when a scouring medium having a specific gravity of 2 and a particle size of 0.7 mm (0.028 inch) is utilized with an inlet gaseous reaction product (low rate of 3 kg (6.6 pounds) per second at a temperature of 954 degrees Celsius (!750°F). As mentioned, all of the heat exchanger sections utilized to make up the elongated tubular heat exchanger can include spiraling vanes and recesses. Generally, however, the heat exchanger sections which include spiraling vanes and recesses in the elongated heat exchanger can be separated by several heat exchanger sections which do not include spiraling vanes and recesses. The number of heat exchanger sections which do not include vanes and recesses depends on whether those heat exchanger sections arc thoroughly cleaned by the scouring medium under the operating conditions involved. The vanes can be formed of a corrosion resistant alloy such as an alloy of nickel and chromium or they can be formed of a ceramic wear resistant material such as alumina, silicon carbide or the like. Also, the vanes can be hollow so that the cooling medium will keep them cooler, heat transfer will be increased and pigment deposits will be reduced. The improved process of this invention for producing and cooling participate solid titanium dioxide is comprised of the following steps. Heated gaseous titanium tetrachloridc and heated oxygen are reacted at a high temperature, that is, a temperature of at least 1200 degrees Celsius (2200°F), to produce particulate soiid titanium dioxide and gaseous reaction products. The titanium dioxide and gaseous reaction products are cooled by passing them through an elongated tubular heat exchanger along with a scouring medium for removing deposits from the inside surfaces of the heat exchanger. The scouring medium and the paniculate titanium dioxide and gaseous reaction products are caused to follow a spiral path as they flow through the elongated tubular heat exchanger. In accordance with the presently preferred process embodiment, the particulate titanium dioxidp and gaseous reaction products are caused to follow the spiral path by providing one or more spiraling vanes on the inside surfaces of all or spaced portions of the elongated tubular heat exchanger. A more specific process of the present invention for producing paniculate solid titanium dioxide comprises the steps of: (a) reacting gaseous titanium tetrachloride and oxygen at a temperature in the range of at least about 1200 degrees Celsius (2200°F) to produce paniculate solid titanium dioxide and gaseous reaction products; (b) cooling the produced paniculate solid titanium dioxide and gaseous reaction products with a cooling medium in a tubular heat exchanger to a temperature about 700 degrees Celsius (1300°F) or less; (c) injecting a scouring medium into the heat exchanger for removing deposits from the inside surfaces thereof; (d) causing the scouring medium to follow a spiral path through the heat exchanger ; and (c) separating the participate solid titanium dioxide from the scouring medium and the gaseous reaction products. In order to further illustrate the improved process and apparatus of the present invention, the following example is given. Example A series of tests were performed to increase the efficiency of an elongated tubular heat exchanger used for cooling the titanium dioxide and gaseous reaction products produced in the chloride process. The heat exchanger was instrumented to determine the effectiveness of heat transfer and consisted of a number of sections of water jacketed pipe. Cooling water flowed through the jacket and reaction products from the reactor consisting of a mixture of Cl2, TiO2 pigment, and 5 to 10 percent O2 flowed through the interior of the pipe. The heat exchanger sections were about 4.9 meters (16 feet) long and were connected together by flanges. An external water pipe called a jumper connected the water jacket of one section to the water jacket of the adjacent section. A thermocouple was placed in each jumper and total water flow through the heat exchanger sections was measured at the inlet to the sections. The amount of heat that was transferred from the reaction products stream to the water in each heat exchanger section was determined from the difference in temperature between the water inlet and outlet and the water flow rate. The gas temperature for the heat exchanger sections was calculated from a mass balance for the reactor, the amount of heat fed to the reactor with the reactant feed streams and the total heat lost from the reactor upstream of the sections. A heat transfer coefficient was calculated for each heat exchanger section from the temperature of the product stream and the amount of heat that.was transferred to the cooling water in that section. The calculated heat transfer coefficients were then compared to the heat transfer coefficients calculated from empirical heat transfer correlations available in the open literature for paniculate free gases. It was anticipated that the correlations for paniculate loaded gases would be different than for clean gases, but it seemed probable that there would be a relatively constant ratio between the coefficient measured for the heat exchanger sections and the coefficients calculated for clean gas. The results indicated that the deviation between the values calculated from empirical correlations and those determined experimentally were much greater for the sections near the exit of the elongated heat exchanger than for those at the inlet, it seemed likely that the difference could be due to deposits in the sections. Tests were then initiated to develop methods for improving heat transfer near the exit of the elongated heat exchanger. The tests were performed using the last 8 sections of the elongated heat exchanger. All of the sections were 18 centimeters (7-inches) in diameter and approximately 4.9 meters (16-feet) in length, except for the last section which was an adapter for attaching the elongated heat exchanger to the product collection section. The adapter section was 1,2 meters (4-fect) in length and slightly larger in diameter than the other sections. The results of all of the tests are given in the Table below. Test 1 A control test was performed using silica sand as the scouring medium. The product rate for the reactor was set at a level that could be maintained even if heat transfer rates were to change significantly. The ratios of the measured heat transfer coefficients to theoretical heat transfer coefficients were determined. The results indicate that the difference between the actual coefficients and the theoretical coefficients increases as the gases move down the elongated heat exchanger. Test 2 In the second test, a device was placed in the middle of a heat exchanger section to introduce N2 tangentially into the section to cause the scouring medium to follow a spiral path downstream of the nitrogen addition point. The reactor produced TiO2 pigment at a rate of about 59 to 68 kg (130 to 150 pounds) per minute. Approximately 5700 standard liters (200 standard cubic feet) of N2 was introduced into the section over a period of several minutes. The result was that the heat transfer improved measurably over the entire product cooler downstream of the point of injection. Without being limiting of the present invention, the increase in heat transfer is attributed principally to more efficient scouring rather than increased turbulence. Support for this conclusion was found first in that the increase in heat transfer was observed as far as 100 section diameters downstream from the point of N2 injection. Calculations and published data indicated that any increase in heat transfer due to turbulence decreases rapidly and disappears completely within about 20 pipe diameters downstream1.2. Additional support for the conclusion was found in that the increase in heat transfer was observed to continue for some time after the N2 flow had been stopped. Test 3 A scouring medium of TiO2 was prepared by agglomerating unfinished pigment, heat treating the material to produce a suitably hard material and then screening the material to provide a particle size distribution similar to that of the silica sand that had been used. The TiO2 scouring medium was fed at the front of the reactor. The results of this test were similar to the results of Test 1. Test 2 A heat exchanger section having spiraling vanes and recesses as shown in FIGS. 1 and 2 was installed in place of heat exchanger section No. 6, The portion of the heat exchanger section which included the spiraling vanes and recesses was the first 2.4 meters (8 feet) of the section. The scouring medium was the same as used in Test 3, and the product rate was approximately the same as in Tests 1 and 3. The results indicate that the average heat transfer coefficient for section No. 7 immediately downstream of section No. 6 was significantly higher than the average heat transfer coefficient for section No. 7 in Test 3. The average heat transfer coefficient for section No. 8 that was 9.8 meters (32 feet) or 55 pipe diameters from the end of the spiraling vanes and recesses was slightly higher than the average heat transfer coefficient for section No. 8 in Test 3. Test 5 The heat exchanger section including the spiraling vanes and recesses was installed in place of section No. 11 and a test similar to Test 4 was performed. The results indicate that a significant improvement was obtained even for section No. 13 that was 7.9 meters (26 feet) or more than 47 pipe diameters from the end of section No. 11. Additional Tests A test similar to Test 5 was performed using ceramic spiraling vanes (which may be advantageously used where excessive wear or chemical attack might be expected). The heat transfer results for sections No. 12 and No. 13 with the ceramic vanes were the same as for Test No. 5. The heat transfer within the section containing the vanes was dependent on the conductivity of the material used for the vanes and the design of the vanes. In another set of tests, the temperature of the gases exiting the bag filter was determined when the heat exchanger was operated without spiraling vanes. Vanes were then installed in place of section No. 11 and the production rate increased until the temperature of the gases exiting the bag filters had reached (hat same temperature. The results were that without the vanes, a production rate of 97 tons per day resulted in an exit temperature of 187 degrees Celsius (369°P), whereas with the vanes, a production rate of 119 tons per day (a 23 percent increase in productivity) could be achieved with an exit temperature of 184 degrees Celsius (363°F). "INCONEL™" vanes were operated for over 30 hours. No measurable wear was found on the vanes and raw pigment quality was excellent. No deposits were found on the vanes.The results of the tests confirm that the spiraling vanes and recesses increase the effectiveness of the scouring medium. References: 1. A. H. Algifri, R. K. Bhardwaj, Y.V. N. Rao; "Heat transfer in turbulent decaying swirl flow in a circular pipe," Int. J. Heat & Mass Transfer, Vol. 31(8.), pp. 1563-1568 (1988). 2.N. Hay, P. D. West; "Heat transfer in free swirling flow in a pipe," Tram ASME J. Heat Transfer, 97, pp. 411-416 (1975). WE CLAIM 1. A process for producing titanium dioxide wherein gaseous titanium tetrachloride and oxygen are reacted to produce particulate solid titanium dioxide and gaseous reaction products and the titanium dioxide and gaseous reaction products are cooled by passing them through a tubular heat exchanger along with a scouring medium such as herein described, for removing deposits from the inside surface of the tubular heat exchanger, characterized in that the scouring medium, the particulate titanium dioxide and gaseous reaction products follow a spiral path as these materials flow through said tubular heat exchanger. 2. The process as claimed in claim 1, wherein said scouring medium is selected from the group consisting of mixtures of titanium dioxide and water which are pelletized, dried and sintered, compressed titanium dioxide, rock salt, fused alumina and titanium dioxide and salt mixtures,; such as herein described. 3. The process as claimed in claim 2, wherein said scouring medium is a mixture of titanium dioxide and water which is pelletized, dried and sintered. 4. The process as claimed in any of claims 1-3, wherein the materials in the aggregate experience a rate of rotation associated with said spiraling movement of from 0.8 degrees per centimeter to about 2.4 degrees per centimeter. 5. The process as claimed in claim 4, wherein the gaseous titanium tetrachloride and oxygen are reacted at a temperature of at least 1150 degrees Celsius and the particulate solid titanium dioxide and gaseous reaction products are cooled to a temperature not exceeding 700° Celsius . 6. An elongated tubular heat exchanger for cooling particulate solid titanium dioxide and "gaseous reaction products produced by reacting gaseous titanium tetrachloride and oxygen in a reactor, characterized in that the exchanger comprises a plurality of connected-together heat exchanger sections, of which one or more but not all such sectionscomprisespiraling vanes on an inside surface thereof for causing the particulate titanium dioxide and gaseous reaction products to follow a spiral path as these materials flow past said spiraling vanes. 7. The exchanger as claimed in claim 6, wherein each of the sectionscomprising the spiraling vanes comprises from four to six spiraling vanes with four to six spiraling recesses between the vanes. 8. The exchanger as claimed in claim 6 or 7, wherein the spiraling vanes are spaced evenly around the inside surface of the heat exchanger sections bearing such vanes. 9. The exchanger as claimed in any of claims 6 through 8, wherein the spiraling vanes and recesses have curved rectangular cross-sectional shapes. 10. The exchanger as claimed in any of claims 6 through 9, wherein some of the spiraling vanes are hollow. 11. The exchanger as claimed in any of claims 6 through 10, wherein the spiraling vanes in each respective exchanger section bearing such vanes are characterized by a constant rate of rotation of from 0.8 to 2.4 degrees of rotation per centimeter of length of the relevant exchanger section. 12. The exchanger as claimed in any of claims 6 through 11, wherein the spiraling vanes are constructed from a corrosion resistant alloy, such as herein described, or a ceramic material, such as herein described. 13. The exchanger as claimed in any of claims 6 through 12, wherein the number, size and rate of rotation of the vanes in any given exchanger section bearing the vanes are all selected such that the maximum pressure drop experienced in any such section is 14.1 grams per square centimeter. An improved process and apparatus for producing and cooling titanium dioxide produced from the reaction of gaseous titanium tetrachloride and oxygen, in which the titanium dioxide and gaseous reaction products are cooled by passing these materials through a tubular heat exchanger along with a scouring medium for removing deposits from the inside surfaces of the tubular heat exchanger, and the particulate scouring medium, the particulate titanium dioxide and the gaseous reaction products are all caused to follow a spiral path through the use of spiraling vanes and recesses on an inner surface of the exchanger.

Documents:

258-kolnp-2003-granted-abstract.pdf

258-kolnp-2003-granted-assignment.pdf

258-kolnp-2003-granted-claims.pdf

258-kolnp-2003-granted-correspondence.pdf

258-kolnp-2003-granted-description (complete).pdf

258-kolnp-2003-granted-drawings.pdf

258-kolnp-2003-granted-examination report.pdf

258-kolnp-2003-granted-form 1.pdf

258-kolnp-2003-granted-form 13.pdf

258-kolnp-2003-granted-form 18.pdf

258-kolnp-2003-granted-form 3.pdf

258-kolnp-2003-granted-form 5.pdf

258-kolnp-2003-granted-gpa.pdf

258-kolnp-2003-granted-letter patent.pdf

258-kolnp-2003-granted-reply to examination report.pdf

258-kolnp-2003-granted-specification.pdf