Method for vaporizing halide-free, silicon-containing compounds

Abstract: A vaporizer for halide-free, silicon-containing liquid reactants used in producing preforms is provided. The vaporizer includes a heated, vertically-oriented expansion chamber (20) and a vertical hollow shaft (42) which extends into the chamber and has a plurality of orifices (45) at its upper end (44). Preheated reactant is supplied to the vertical shaft (42) at an elevated pressure and is sprayed onto the chamber's heated wall (22) by the orifices (45). A portion of the liquid reactant vaporizes upon entering the internal volume (24) of the chamber (20) due to the pressure drop between the inside of the shaft and the inside of the chamber. The remainder of the liquid reactant vaporizes by being heated through contact with the chamber's wall (22). Higher molecular weight species present in the raw material or generated by the vaporization process are collected in the bottom portion of the chamber where they can be periodically removed.

Description: FIELD OF THE INVENTION This invention relates to providing reactant vapors for use in producing silicon dioxide containing preforms which can be used to produce optical or acoustic waveguide fibers either directly or through intermediate production of core cane. BACKGROUND OF THE INVENTION Historically, halide-containing raw materials, such as SiCl 4 or mixtures of SiCl 4 with various dopants, have been used in the manufacture of preforms by vapor phase deposition techniques, such as the MCVD (modified chemical vapor deposition), VAD (vapor axial deposition), and OVD (outside vapor deposition) techniques. In the MCVD technique, the halide-containing raw materials are vaporized and reacted with oxygen to form oxide particles which are deposited on the inside of a fused-silica tube. In the VAD and OVD procedures, vaporized, halide-containing raw materials are hydrolyzed in a burner to produce soot particles which are collected on a rotating starting rod (bait tube) in the case of VAD or a rotating mandrel in the case of OVD. In some OVD systems, the cladding portion of the preform is deposited on a previously-formed core preform, rather than on a mandrel. Various vaporizers have been developed which can be used in such processes, examples of which can be found in Aslami, U.S. Patent No. 4,212,663, Blankenship, U.S. Patent No. 4,314,837, Tsuchiya et al., U.S. Patent No. 4,938,789, and U.K. Patent Publication No. 1,559,978 (hot oil heating). In particular, French, U.S. Patent No. 4,529,427, discloses a vaporization system in which oxygen and at least one liquid reactant, e.g., SiCl 4 , are introduced into a heated chamber where the liquid reactant is flash vaporized. The resulting oxygen/reactant gas mixture is then used to produce the desired preform. Other flash vaporizers are disclosed in Japanese Patent Publication No. 58-125633 (see, in particular, Figure 4) and in Antos et al., U.S. Patent No. 5,078,092, where the material to be vaporized can be halide-free. Soubeyrand et al., U.S. Patent No. 5,090,985, discloses the use of a horizontal thin film evaporator for vaporizing various raw materials employed in the preparation of coated glass articles. The use of halide-containing raw materials generates substantial quantities of halide-containing by-products, e.g., hydrochloric acid. To avoid environmental pollution, these by-products must be collected, which increases the overall cost of the preform production process. Accordingly, halide-free materials and, in particular, halide-free, silicon-containing materials are desirable starting materials for the production of preforms. See Dobbins et al., U.S. Patent No. 5,043,002. As explained in the Dobbins et al. patent, particularly preferred halide-free, silicon-containing materials for use in producing preforms are polymethylsiloxanes, with the polymethylcyclosiloxanes being particularly preferred, and with octamethylcyclotetrasiloxane (OMCTS) being especially preferred. These same halide-free, silicon-containing raw materials are preferred for use with the present invention. Cain et al., U.S. Patent No. 5,356,451, discloses a vaporizer specifically developed for use with halide-free, silicon-containing raw materials, such as OMCTS. In accordance with this technology, a preheater in series with a flat plate vaporizer is used to change the state of the raw material from a liquid to a vapor. Both the preheater and the flat plate vaporizer use electrical resistance wires as their source of heat. In the preheater, the liquid is heated to below its boiling point. In the flat plate vaporizer, additional heat is added to (1) bring the fluid to its boiling temperature and (2) provide the necessary energy to overcome the material's latent heat of vaporization, whereupon the fluid becomes a vapor. Halide-free, silicon-containing materials are difficult to vaporize due to their sensitivity to cracking and polymerization when subjected to excessive temperatures. Polymerization results in the production of higher molecular weight species which form gums and gels. Additionally, higher molecular weight species may exist in the raw materials as originally manufactured. Such species do not easily vaporize and over time the polymerized materials tend to foul heat transfer surfaces and plug piping systems. Current data suggest that it may not be possible to produce vapor from materials such as OMCTS without incurring some polymerization. SUMMARY OF THE INVENTION In view of the foregoing considerations, it is an object of the invention to provide reactant vapors of halide-free, silicon-containing materials in a manner which (1) minimizes exposure of the material to high temperatures and (2) collects the gums and gels associated with such a material in a location which facilitates against fouling of heat transfer surfaces or plugging of delivery piping. The present invention provides a method for providing reactant vapors for making a silicon dioxide containing preform by decomposing said reactant vapors to form particles, comprising: providing a vertically-oriented expansion chamber comprising a wall surrounding an internal volume, said internal volume comprising a first zone located at the bottom of the chamber for collecting higher molecular weight species of the reactant and a second zone located above the first zone; heating the wall of the chamber; pre-heating a liquid reactant which is polymerizable to form higher molecular weight species; introducing said pre-heated liquid reactant into the second zone and projecting the reactant toward the wall of the chamber so that a first portion of the liquid reactant vaporizes as a result of a pressure drop upon being introduced into the internal volume of the chamber and a second portion vaporizes by being heated by the wall of the chamber; delivering vaporized reactant from said chamber to a vapor utilization site for the formation of said preform; and periodically removing higher molecular weight species of the reactant from the first zone. According to one embodiment of the invention (see Figure 2), the liquid reactant is heated to a sufficiently high temperature by the providing means, i.e. to a temperature above the boiling point of the liquid reactant at the operating pressure of the expansion chamber, such that a portion of the liquid reactant immediately vaporizes upon entering the internal volume (24) of the chamber due to a drop in pressure between the introducing means and the internal volume. The remainder of the liquid reactant is then vaporized by being heated by the chamber's heated wall (22). In a preferred embodiment (see Figure 3), the internal volume of the chamber (24) includes a third superheating zone (30), above the second zone (28), in which vaporized reactant is heated to a temperature above its boiling point at the operating pressure of the chamber. In further preferred embodiments, the introducing means is composed of a vertical hollow shaft (42) which has an upper end (44) which is (i) located in the second zone (28) and (ii) includes a series of spray orifices (45) which are directed towards the wall of the chamber (22). In some embodiments, the expansion chamber (20) includes a port for introducing a diluent gas, such as, oxygen. Methods disclosed herein have the following advantages over prior vaporizers: (1) the reduction of hot spots via the elimination of resistance heaters from the vaporization process; (2) the reduction in heat transfer surface fouling and delivery piping plugging via the addition of a collection sump in the vaporizer; and (3) the capability to greatly reduce the vaporization temperature by the addition of a diluent gas during the vaporization process. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of a vapor delivery system constructed in accordance with the present invention. Figure 2 is a schematic diagram of the pump, reactant preheater, diluent preheater, and expansion chamber components of the vapor delivery system of Figure 1. Figure 3 is a schematic diagram of the expansion chamber component of the vapor delivery system of Figure 1. The foregoing drawings, which are incorporated in and constitute part of the specification, illustrate the preferred embodiments of the invention, and together with the description, serve to explain the principles of the invention. It is to be understood, of course, that both the drawings and the description are explanatory only and are not restrictive of the invention. The drawings are not intended to indicate scale or relative proportions of the elements shown therein. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 illustrates an overall system for producing preforms in which the vaporizer of the present invention can be used. As shown therein, a reactant liquid, such as OMCTS, is stored in liquid feed tank 11. A metering pump 12 transfers the reactant liquid from tank 11 to preheater 14. Liquid flow meter 13 monitors the bulk liquid flow rate and sends a signal to the system's controller (not shown) which increases or decreases the rotation rate of the metering pump. As discussed in more detail below, the liquid delivered by pump 12 through liquid mass flow meter 13 is under sufficient pressure to prevent volatilization of the liquid in the preheater. Vaporizer 15 changes the liquid reactant to a vapor. Oxygen can be added to the vaporized reactant in the vaporizer or separately in mixer 16. When added in the vaporizer, the oxygen functions as a diluent and thus allows the vaporizer to operate at a lower temperature. Other diluents, such as an inert gas, can be used, if desired. The final step in the process is the delivery of the reactant/oxygen mixture to burners 10 which oxidize the reactant to form glass soot particles. In addition to the reactant/oxygen mixture, burners 10 are also supplied with an innershield gas, e.g., N 2 , an outershield gas, e.g., O 2 , and a mixture of CH 4 and oxygen to form a pilot flame. Figures 2 and 3 illustrate various components of the invention in more detail. In particular, these figures show pump 12, hot oil heat transfer system 32, reactant preheater 14, diluent preheater 46, vertical expansion chamber 20, and vertical hollow shaft 42 having spray orifices 45 at its upper end 44. Spray orifices 45 are sized to provide sufficient back pressure in preheater 14 so that the reactant does not vaporize (boil) therein. Such boiling results in flow oscillations in the system, which, in turn, can result in variations in the operation of burners 10 and thus in the optical fiber preform. The amount of back pressure needed depends upon the temperature to which the reactant is to be heated in preheater 14 and the vapor pressure of the reactant at that temperature. The size(s) of the orifice(s) needed to produce the desired back pressure depends upon the reactant flow rates which the system is to provide and, in particular, the lowest desired flow rate through the system since the back pressure produced by an orifice decreases as the flow rate decreases. In practice, preferred reactant flow rates for producing preforms using OMCTS as the halide-free, silicon-containing raw material can be in the range of from about 80 grams/minute to about 200 grams/minute or higher. A preferred operating temperature for preheater 14 is about 190°C for a system in which oxygen is added in mixer 16, rather than to vaporizer 15 (Figure 1). The boiling point of OMCTS at atmospheric pressure is about 175.5°C. To prevent OMCTS from boiling at 190°C requires a back pressure in preheater 14 of a gauge pressure of at least about 69 kPa (10 psig). Preferably, a somewhat higher minimum back pressure e.g. a back pressure of a gauge pressure of about 103.5 kPa (15 psig), is used. In this way, the liquid reactant is maintained as a subcooled (compressed) liquid in preheater 14, as is desired. Orifices having a diameter of about 0.25 mm, e.g. six orifices distributed around the circumference of vertical shaft 42, have been found to produce the desired back pressure of a gauge pressure of 103.5 kPa (15psig) for reactant flow rates as low as 80 grams/minute. Appropriate orifice sizes for other flow rates and back pressures can be readily determined by persons skilled in the art based on the disclosure herein. As show in figure 2, the basic process begins with the pumping of liquid reactant through preheater 14. Pump 12 delivers the liquid reactant at the desired flow rate called for by the system's controller. This pump, which can, for example, be a gear pump, must be sized to deliver the required head (pressure) at the desired flow rate. Preheater 14 is configured to add sufficient heat to the liquid reactant so that it will at least partially vaporize as its pressure drops across orifices 45 upon entering the internal volume 24 of expansion chamber 20. Preheater 14 is preferably a plate type heat exchanger sized to add all the heat required to bring the liquid reactant from room temperature to just below its boiling point at the back pressure established by orifices 45 at the system's minimum reactant flow rate. In particular, preheater 14 preferably heats the liquid reactant to a temperature in a range whose upper end is the temperature at which the reactant is a saturated liquid at the minimum back pressure established by orifices 45 (the saturation temperature) and whose lower end is the saturation temperature minus 50°C. The overriding consideration, however, is minimizing polymerization of the reactant and thus preheater 14 is preferably run at the minimum temperature for which the reactant will at least partially vaporize upon entering the chamber 20, e.g. at a temperature about 10°C higher than the boiling point of the reactant at the chamber's operating pressure. For OMCTS as the reactant and for an operating pressure in chamber 20 of a guage pressure of about 6.9 kPa (1 psig), 190°C is such a minimum temperature for preheater 14. The heat is preferably added to the liquid reactant by means of hot oil heat transfer system 32, which has an oil supply 34 and an oil return 36 which are connected to oil inlet port 56 and oil outlet port 58 of preheater 14 by conventional supply lines and fittings (not shown). A preferred oil for use in the hot oil heat transfer system is silicone oil, although other fluids can be used if desired. As discussed above, a preferred oil temperature is 190°C when OMCTS is the reactant liquid. It should be noted that as shown in Figure 2, a single hot oil heat transfer system is used for preheater 14, expansion chamber 20, and diluent preheater 46, so that all of these components operate at about the same temperature, e.g. 190°C. If desired, the components can be operated at different temperatures by using separate heating systems for each component. Use of a hot oil system is preferred for preheater 14 and expansion chamber 20. Other heating means, including electrical heating, can be used. Liquid reactant enters preheater 14 through liquid entry port 60 and leaves the preheater through liquid exit port 62. To minimize the residence time of the liquid reactant in the preheater and thus the opportunities for polymerization, a preheater having a single, straight through, reactant flow channel, surrounded by hot oil channels, is preferred, although other constructions can be used if desired. As indicated above, a plate type preheater is preferred, and a preferred spacing between the plates is approximately 1.2 mm. The use of fluid-to-fluid heat transfer provides uniform heating and ensures the absence of hot spots which can lead to polymerization of the reactant. The heated liquid reactant is delivered from preheater 14 to expansion chamber 20 by conduit 52. The expansion chamber includes internal volume 24 for free expansion of vapor and heated wall 22 for adding any additional heat required for complete vaporization and a slight amount of superheat. The chamber also acts as a collection area for gums and gel and provides for the addition of a diluent gas if desired. Heating of wall 22 is preferably performed by passing a heated fluid, e.g., hot oil from hot oil heat transfer system 32, over at least a portion of the outside surface of the wall. Preferably, the entire internal surface of wall 22 is heated to the same temperature, e.g., 190°C. A heating jacket surrounding the chamber and having an inlet 38 and an outlet 40 can be used for this purpose. In Figure 2, the inlet and outlet are connected to oil supply 34 and oil return 36 by conventional supply lines and fittings (not shown). As shown in Figures 2 and 3, chamber 20 has upper and lower conical portions and a cylindrical center portion. Distribution manifolds (not shown) can be used at the interfaces between the conical portions and the cylindrical portion to provide for a uniform distribution of hot oil around the circumference of wall 22. As shown in Figure 3, chamber 20 includes a lower gel collecting zone 26, a middle vaporization zone 28, and an upper superheating zone 30. Gel collection zone 26 includes port 80 for removing higher molecular weight species separated from the reactant vapor stream by the vertical vaporizer during, for example, periodic maintenance of the system. For OMCTS, a gel production rate of approximately 100 ppm has been observed for the vaporization process, that is, for every million grams of OMCTS vaporized by the system of Figure 1, approximately 100 grams of gel are collected in gel collection zone 26. The heated liquid reactant is introduced into chamber 20 by means of hollow vertical shaft 42 whose upper end constitutes an orifice head. A preferred inside diameter for shaft 42 is approximately 4 mm. The orifice head is located some distance into the expansion chamber and directs the liquid reactant against the inside surface of wall 22 in the region of vaporization zone 28. In particular, the orifice head needs to be located far enough into the chamber so that flow from the head does not pick up substantial amounts of gel from gel collection zone 26. For example, for a chamber having an overall length of about 75 cm composed of two conical portions, each 15 cm long, and a central cylindrical portion 45 cm long, an orifice location about 55 cm from the bottom of the chamber has been found to work successfully. The orifice head may contain multiple orifices to distribute the fluid along the wall of the chamber. Alternatively, a single orifice in combination with a set of slots and/or baffles to direct the fluid against the side wall of the chamber can be used. Other designs for spraying a liquid in a pattern like that illustrated at 54 in Figure 3 can be used if desired. Expansion chamber 20 should have a cross sectional area which is large enough to keep the velocity of the reactant relatively low so as to allow separation of any gel and to minimize gel carry over. In practice, for the 75 cm chamber described above, an inside diameter of about 8 cm has been found to produce the desired low flow velocity within the chamber 20, e.g. a flow velocity of less than about 0.5 meters/second, and preferably about 0.1 meters/second, for a reactant feed rate of about 200 grams/minute with oxygen being added in mixer 16. A portion of the heated liquid reactant immediately vaporizes upon entering vaporization zone 28 due to the drop in pressure between the inside of shaft 42 and the inside of chamber 20, e.g. the drop from a guage pressure of 103.5 kPa (15 psig) (or higher at high flow rates) inside shaft 42 to a guage pressure of 6.9 kPa (1 psig) inside chamber 20. (Note that, as discussed above, such immediate vaporization only takes place if preheater 14 has raised the temperature of the liquid reactant above the boiling point of the reactant at the expansion chamber's operating pressure.) The amount of reactant that immediately vaporizes will depend upon the amount of heating performed in preheater 14, i.e. it will depend on the temperature of the liquid reactant as it enters chamber 20 relative to the enthalpy of vaporization of the reactant. Although the temperature of the liquid reactant could be made sufficiently high to achieve substantially full immediate vaporization, such heating would defeat the purpose of avoiding polymerization of the reactant resulting from exposure to excessively high temperatures. For example, for OMCTS, full immediate vaporization would require heating to a temperature of about 250°C, at which temperature substantial polymerization would be much more likely than at 190°C. To avoid this problem, in accordance with the preferred embodiments of the invention, only a portion of the liquid reactant is immediately vaporized upon entry to chamber 20, e.g. about 15%, with the remainder being vaporized by being sprayed onto heated wall 22. The portion of liquid reactant that vaporizes upon entry into chamber 20 can be increased by introducing a diluent gas into the chamber so as to reduce the partial pressure of the liquid reactant in the chamber and thus lower its dew point. Alternatively, the diluent gas can be used to lower the temperature to which the liquid reactant must be heated in preheater 14 to achieve a desired level of immediate vaporization in chamber 20. For example, by introducing oxygen into chamber 20 at a flow rate of 0.3 slpm (standard liters per minute) of O 2 per gram/minute of OMCTS, vaporization of OMCTS through a combination of immediate vaporization and heating by wall 22 can be achieved with the temperature of the heating oil provided by hot oil heat transfer system 32 reduced from 190°C to about 150°C. As shown in Figure 2, the diluent gas is heated by preheater 46 which has a diluent entrance port 68 and a diluent exit port 70. Preheater 46 also has oil ports 64 and 66 which are connected to oil supply 34 and oil return 36 by conventional supply lines and fittings (not shown). Preheater 46 is preferably a plate type heater. Exit port 70 is connected to two-position valve 48. For the valve position shown in Figure 2, diluent gas is added to the flow of reactant vapor leaving chamber 20 through conduit 50, e.g., the diluent gas is added in mixer 16 of Figure 1. In the alternate position, the diluent gas is added to chamber 20 through port 82 through wall 22. Instead of a variable two-position valve, the system can be permanently configured to either supply or not supply diluent gas to chamber 20. Also, not all of the diluent gas needs to be added to chamber 20, but some can be added downstream in mixer 16. For example, valve 48 can provide flow to both port 82 and conduit 50. If the mole fraction of diluent gas introduced into chamber 20 is sufficiently high, a major portion of the liquid reactant can be immediately vaporized upon entry into the chamber without the need to raise the temperature of the liquid in preheater 14 to a point where polymerization becomes a serious problem. In such a case, the portion of the liquid reactant which is vaporized by being heated by the chamber's wall can be quite small. Adding diluent gas to chamber 20 can result in somewhat higher levels of carryover of higher molecular weight species of the reactant into the vapor stream leaving chamber 20 through conduit 50. Also, the addition of oxygen as the diluent gas may lead to higher levels of production of higher molecular weight species of the reactant. Once transformed into a vapor, the reactant leaves chamber 20 through conduit 50. In doing so, it passes through zone 30 where it is superheated to a temperature above its boiling point. A preferred amount of superheating in zone 30 can be on the order of about 5°C, e.g., for OMCTS and an operating pressure within chamber 20 of a gauge pressure of 6.9 kPa (1 psig), the reactant vapor can be heated to a temperature of about 185°C. The components of the vaporizer can be made of various standard materials known to those skilled in the art. For example, the reactant contacting portions of expansion chamber 20 and vertical shaft 42 are preferably made of stainless steel. Other materials can be used if desired.

Claims: A method for providing reactant vapors for making a silicon dioxide containing preform by decomposing said reactant vapors to form particles, comprising: providing a vertically-oriented expansion chamber comprising a wall surrounding an internal volume, said internal volume comprising a first zone located at the bottom of the chamber for collecting higher molecular weight species of the reactant and a second zone located above the first zone; heating the wall of the chamber; preheating a liquid reactant which is polymerizable to form higher molecular weight species; introducing said pre-heated liquid reactant into the second zone and projecting the reactant toward the wall of the chamber so that a first portion of the liquid reactant vaporizes as a result of a pressure drop upon being introduced into the internal volume of the chamber and a second portion vaporizes by being heated by the wall of the chamber; delivering vaporized reactant from said chamber to a vapor utilization site for the formation of said preform; and periodically removing higher molecular weight species of the reactant from the first zone. A method according to claim 1 wherein the internal volume of the chamber includes a third zone, above the second zone, and the vaporized reactant is heated to a temperature above its boiling point in said third zone. A method according claim 2 wherein the wall of the chamber is heated by flowing a heated fluid along at least a portion of the outer surface of the wall. A method according to any of claims 1 to 3 wherein the liquid reactant is sprayed into the second zone and toward the wall of the chamber. A method according to any of claims 1 to 3 wherein the liquid reactant is passed through a vertical hollow shaft and sprayed into the second zone and toward the wall of the chamber through a plurality of spray orifices located in the region of the upper end of the shaft. A method according to any of claims 1 to 5 wherein the reactant is provided at an elevated pressure above a predetermined minimum pressure and heated to a temperature in a range whose upper end is the temperature at which the reactant is a saturated liquid at said predetermined minimum pressure, this temperature being defined as the saturation temperature, and whose lower end is the saturation temperature minus 50°C. A method according to any of claims 1 to 6 which comprises the additional step of introducing a diluent gas into the internal volume. A method according to claim 7 wherein the diluent gas is oxygen, a polymethylsiloxane or octamethylcyclotetrasiloxane. A method for the production of a silicon dioxide containing preform which comprises providing reactant vapors by a method according to any of claims 1 to 8 and decomposing the reactant vapors to form silicon dioxide particles which are collected as a preform. A method for the production of an optical or acoustic waveguide fiber which comprises producing a silicon dioxide containing preform by a method according to claim 9 and producing optical or acoustic waveguide fiber therefrom either directly or with intermediate production of a core cane.

Inventors: WILLIAMS REED RICHARD, WILLIAMS, REED RICHARD
Applicants: CORNING INC, CORNING INCORPORATED
Application year: 1995
Main classification: B01D1
IPC codes: C03B8, C03B37, C03B19, C23C16, G02B6, B01D1, B01D3