阅读说明：本技术 光纤用多孔玻璃基材的制造方法及制造装置 (Method and apparatus for producing porous glass substrate for optical fiber ) 是由 野田直人 井上大 饭沼均 水上博公 于 2019-08-02 设计创作，主要内容包括：根据本发明,抑制了供给到气化器的有机硅氧烷液体原料的流量变动,并且使二氧化硅微粒的沉积密度均匀。在根据本发明的制造多孔玻璃基材的方法中,内压为P_1的原料罐中储存的有机硅氧烷液体原料被质量流量控制器控制为预定的流量,经过内压为P_2的配管并被供给到气化器。液体原料在气化器中气化,并作为气体原料供给到燃烧器。气体原料被燃烧器燃烧以沉积由此生成的二氧化硅微粒并形成多孔玻璃基材。本发明的特征在于在上述制造方法中P_1≤P_2。(According to the present invention, the fluctuation of the flow rate of the organosiloxane liquid raw material supplied to the vaporizer is suppressed, and the deposition density of the silica fine particles is made uniform. In the method for producing a porous glass substrate according to the present invention, the internal pressure is P 1 The organosiloxane liquid raw material stored in the raw material tank is controlled to a predetermined flow rate by a mass flow controller, and the internal pressure is P 2 And is supplied to the vaporizer. The liquid feedstock is gasified in a gasifier and supplied to the burner as a gaseous feedstock. The gaseous raw material is combusted by the burner to deposit the silica microparticles thus generated and form a porous glass substrate. The present invention is characterized in that P is used in the above-mentioned production method 1 ≤P 2 。)

1. A method for manufacturing a porous glass substrate for an optical fiber, comprising:

controlled to a predetermined flow rate by a mass flow controller, with an internal pressure of P₂Will be stored at an internal pressure of P₁The liquid feedstock of organosiloxane in the feedstock tank of (a) is fed to a vaporizer;

gasifying the liquid feedstock in the gasifier and supplying the gasified feedstock as a gaseous feedstock to a burner; and

forming a porous glass substrate by depositing silica microparticles formed by combusting the gaseous raw material in the burner,

wherein, P is satisfied₁≤P₂。

2. The method for producing a porous glass substrate for an optical fiber according to claim 1, comprising a step of feeding the liquid material in the material tank to the mass flow controller by using a liquid feeding pump, wherein the pressure of the liquid material supplied to the mass flow controller is P₃When, satisfy P₁≤P₂＜P₃。

3. The manufacturing method according to claim 2, wherein in the step of conveying the liquid material, the liquid-sending pump once raises the pressure of the liquid material to P₄And reducing the pressure to P via the pressure loss section₃Supplying the liquid raw material to the mass flow controller so as to satisfy P₁≤P₂＜P₃＜P₄。

4. The manufacturing method according to claim 3, wherein P is satisfied₃≤0.6P₄。

5. The manufacturing method according to claim 3 or 4, wherein the pressure is increased to P by the liquid-sending pump₄Is returned to the raw material tank, and the remaining part is supplied to the raw material tankThe mass flow controller is described.

6. The production method according to any one of claims 1 to 5, wherein P is satisfied₁≤0.1MPa。

7. The manufacturing method according to claim 6, wherein P is satisfied₁≤0.05MPa。

8. The manufacturing method according to any one of claims 1 to 7, wherein a pipe through which the liquid raw material flows is heated and kept at a temperature maintained above a freezing point of the liquid raw material.

9. The manufacturing method according to any one of claims 1 to 8, wherein the liquid raw material is Octamethylcyclotetrasiloxane (OMCTS).

10. An apparatus for manufacturing a porous glass substrate for an optical fiber, comprising:

a raw material tank that stores a liquid raw material that is a liquid organosiloxane and fills the remaining space with an inactive gas;

a liquid feed pump that feeds the liquid raw material from the raw material tank;

a circulation pipe for returning a part or all of the liquid material sent by the liquid sending pump to the material tank;

a first supply pipe branching from the circulation pipe;

a pressure loss unit provided downstream of the first supply pipe;

a second supply pipe provided downstream of the pressure loss section;

a mass flow controller provided downstream of the pressure loss unit via the second supply pipe to control the flow rate of the liquid material to a predetermined flow rate value;

a third supply pipe provided downstream of the mass flow controller;

a vaporizer provided downstream of the mass flow controller via the third supply pipe to vaporize the liquid raw material; and

a burner that burns the raw material gas gasified by the gasifier to deposit silica microparticles,

wherein when the internal pressure of the raw material tank is P₁The internal pressure of the circulation piping is P₄And the internal pressure of the second supply pipe is P₃And the internal pressure of the third supply pipe is P₂When, satisfy P₁≤P₂<P₃<P₄。

11. The manufacturing apparatus according to claim 10, further comprising a heating/keeping-warm unit for heating and keeping the circulation pipe, the first supply pipe, the second supply pipe, and the third supply pipe at a temperature higher than a freezing point of the liquid raw material.

Technical Field

The present invention relates to a method and an apparatus for producing a porous glass substrate for optical fibers using an organosiloxane raw material.

Background

The glass substrate for optical fibers is produced by sintering a porous glass substrate. The porous glass substrate to be sintered is fabricated by depositing silica microparticles on a starting material, and is fabricated by the VAD or OVD method.

Silica fine particles deposited on a starting material are formed by burning an organosiloxane raw material in a burner (for example, see patent documents 1 to 4).

One method for forming silica fine particles is a method using a liquid organosiloxane raw material (hereinafter referred to as a liquid raw material) such as Octamethylcyclotetrasiloxane (OMCTS) (see, for example, patent document 1). In the method of patent document 1, silica fine particles are formed through the following steps: the method includes the steps of supplying a liquid feedstock to a gasifier, gasifying the liquid feedstock in the gasifier to form a feedstock gas, and combusting the gasified liquid feedstock in a combustor. In the method of patent document 1, the liquid raw material supplied to the vaporizer is controlled by a mass flow controller.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2013-177297

Patent document 2: japanese laid-open patent publication No. 2015-505291

Patent document 3: japanese patent laid-open publication No. 2017-036172

Patent document 4: japanese patent laid-open publication No. 2017-197402

Disclosure of Invention

Problems to be solved by the invention

However, in the method of patent document 1, for example, when a gas is dissolved in a liquid raw material and bubbles of the dissolved gas are mixed, the flow rate of the liquid raw material may fluctuate. As the flow rate of the supplied liquid raw material varies, the vaporization amount of the liquid raw material in the vaporizer also varies accordingly, and the amount of the raw material gas supplied to the combustor varies. When the amount of the raw material gas supplied to the burner fluctuates, the combustion reaction in the burner becomes unstable, and the formation amount of the silica fine particles also fluctuates. As a result, the density of the silica microparticles becomes uneven, which leads to a reduction in manufacturing efficiency and defects such as poor dehydration during sintering of the resulting porous glass substrate for optical fibers into transparent glass and the generation of bubbles in the glass.

The present invention has been made in view of the above problems, and an object of the present invention is to suppress variation in the flow rate of an organosiloxane liquid raw material supplied to a vaporizer and to make uniform the deposition density of silica fine particles.

Means for solving the problems

In order to solve the above-described problems, in the method for producing a porous glass substrate according to the present invention, the internal pressure is controlled to P by a mass flow controller at a predetermined flow rate₁The liquid organosiloxane raw material stored in the raw material tank, and the internal pressure of the liquid organosiloxane raw material passing through is P₂Is transported to a vaporizer, a liquid raw material is vaporized in the vaporizer, and then supplied as a gas raw material to a burner, and silica fine particles formed by burning the gas raw material in the burner are deposited to form a porous glass substrate. The present invention is characterized in that the above-mentioned production method satisfies P₁≤P₂。

Preferably, the above manufacturing method includes a step of conveying the liquid material in the material tank to the mass flow controller using a liquid-conveying pump. When the pressure of the liquid material supplied to the mass flow controller is P₃When P is satisfied, it is preferable₁≤P₂＜P₃. Further, in the feeding step, it is more preferable that the liquid-sending pump once raises the pressure of the liquid raw material to P₄And reducing the pressure to P via the pressure loss section₃While supplying the liquid raw material to the mass flow controller so as to satisfy P₁≤P₂<P₃<P₄. In this case, it is preferable to satisfy P₃≤0.6P₄。

It is also preferable that the pressure is increased to P by the liquid-sending pump₄A part or all of the liquid raw material of (a) is returned to the raw material tank, and the remaining part is supplied to the mass flow controller.

In the present invention, P₁Preferably ≦ 0.1MPa, and P₁More preferably 0.05MPa or less.

In the present invention, it is preferable that the pipe through which the liquid material flows is heated and maintained at a temperature higher than the freezing point of the liquid material. In the present invention, the liquid starting material may be Octamethylcyclotetrasiloxane (OMCTS).

An apparatus for manufacturing a porous glass substrate for an optical fiber includes: a raw material tank that stores a liquid raw material and fills the remaining space with an inactive gas, the liquid raw material being organosiloxane in a liquid state; a liquid feed pump that feeds the liquid raw material from the raw material tank; a circulation pipe for returning a part or all of the liquid material conveyed by the liquid conveying pump to the material tank; a first supply pipe branching from the circulation pipe; a pressure loss unit provided downstream of the first supply pipe; a second supply pipe provided downstream of the pressure loss section; a mass flow controller provided downstream of the pressure loss section via a second supply pipe to control the flow rate of the liquid raw material to a predetermined flow rate value; a third supply pipe provided downstream of the mass flow controller; a vaporizer provided downstream of the mass flow controller via a third supply pipe to vaporize the liquid raw material; and a burner that burns the raw material gas gasified by the gasifier to deposit the silica microparticles. When the internal pressure of the raw material tank is P₁The internal pressure of the circulation piping is P₄The second supplyThe internal pressure of the supply pipe is P₃And the internal pressure of the third supply pipe is P₂When, satisfy P₁≤P₂<P₃<P₄。

In the present invention, it is preferable to provide a heating/heat-retaining unit for heating and holding the circulation pipe, the first supply pipe, the second supply pipe, and the third supply pipe at a temperature higher than the freezing point of the liquid material.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, it is possible to stably supply an organosiloxane raw material represented by Octamethylcyclotetrasiloxane (OMCTS) while suppressing variation in the flow rate of the liquid raw material supplied to the vaporizer.

Drawings

Fig. 1 shows the structure of the apparatus for producing a porous glass substrate for an optical fiber according to the present embodiment.

Fig. 2 shows the configuration of a porous glass substrate manufacturing apparatus for an optical fiber used in the examples.

FIG. 3 shows the raw material supply flow around the vaporizer of the apparatus for producing a porous glass substrate for optical fiber in the example.

Fig. 4 shows a raw material supply flow of the apparatus for producing a porous glass substrate for an optical fiber in the comparative example.

Detailed Description

Hereinafter, the present invention will be described in more detail based on embodiments. In the following description, the same parts are denoted by the same reference numerals, and the description of the parts already described is omitted accordingly.

FIG. 1 shows a raw material supply flow of a porous glass substrate manufacturing apparatus for an optical fiber. The apparatus for manufacturing a porous glass substrate for an optical fiber shown in fig. 1 includes: a raw material tank 1; a liquid feeding pump 3; an accumulator 15; pressure loss sections 5, 11, 17; a mass flow controller 7; a gasifier 9; the burner 12 and the intermediate tank 18 are connected by a pipe.

A liquid organosiloxane raw material (hereinafter, simply referred to as a "liquid raw material") 101 is supplied from a raw material injection pipe 2 and stored in a raw material tank 1. At this time, the internal pressure of the raw material tank is set to P₁. The liquid material 101 stored in the material tank 1 is pressurized by the liquid feed pump 3 and is fed to the pressure loss unit 5 through the pipe 4. Here, let P₄The internal pressure in the pipe 4 for supplying the liquid material 101 to the pressure loss section 5.

The pressure loss unit 5 depressurizes the supplied liquid raw material and supplies the liquid raw material to the mass flow controller 7 via the pipe 6. Here, let P₃The internal pressure of the pipe 6 for supplying the liquid material 101 to the mass flow controller 7.

The mass flow controller 7 controls the supplied liquid material 101 to a predetermined flow rate, and supplies the liquid material 101 to the vaporizer 9 through the pipe 8. Here, let P₂The internal pressure of the pipe 8.

The vaporizer 9 vaporizes the supplied liquid raw material 101 to produce a raw material gas 102. The vaporized raw material gas 102 is depressurized while passing through the pipe 10 and the pressure loss section 11, and is supplied to the combustor 12. The burner 12 burns the raw material gas 102 to produce silica microparticles 103, and the silica microparticles 103 are deposited on a starting material (not shown) by a combustion reaction. As described above, a porous glass substrate for an optical fiber can be manufactured.

In the production method using the apparatus for producing a porous glass substrate for an optical fiber as described above, if bubbles are mixed into the liquid raw material 101 flowing through the pipe 8, the actual flow rate of the liquid raw material 101 supplied to the vaporizer 9 fluctuates. As a result, the amount of the liquid raw material 101 vaporized in the vaporizer 9 (i.e., the amount of the generated raw material gas 102) fluctuates and becomes unstable, and the flow rate of the raw material gas 102 flowing through the pipe 10 also becomes unstable. If the flow rate of the raw material gas 102 flowing through the pipe 10 varies, the generation speed of the silica fine particles 103 in the burner 12 becomes unstable, and therefore the density of the silica fine particles 103 deposited on the starting material varies and becomes uneven. As a result, the density and shape of the produced porous glass substrate become uneven.

Therefore, it is preferable to take out and convey the liquid raw material 101 from the bottom of the raw material tank 1 sufficiently apart from the liquid surface of the liquid raw material 101 stored in the raw material tank 1 to prevent bubbles from mixing with the liquid raw material 101 conveyed from the raw material tank 1 as much as possible.

However, it is inevitable that a part of the gas 104 that comes into contact with the liquid surface of the liquid raw material 101 stored in the raw material tank 1 will be dissolved in the liquid raw material 101. This dissolved gas 104 may foam in the piping on the way to the vaporizer 9 and cause an unstable amount of the raw material gas generated in the vaporizer 9.

Therefore, in the apparatus for producing a porous glass substrate for an optical fiber according to the present invention, the liquid material 101 taken out from the material tank 1 is pressurized by the liquid-sending pump 3. Here, the internal pressure P of the pipe 8 which is located downstream of the mass flow controller 7 and supplies the liquid raw material 101 to the vaporizer 9₂Should not be less than the internal pressure P of the raw material tank 1₁(i.e., P)₁≤P₂). Specifically, if the internal pressure P of the raw material tank 1 is high₁When the pressure is set to be higher than the atmospheric pressure, the inert gas adjusted by the pressure regulator is supplied from the inert gas supply pipe 13 to adjust P₁. The discharge pressure of the liquid-sending pump 3 is adjusted according to the pressure loss in the flow path from the liquid-sending pump 3 to the piping 8, and a pressure loss portion 11 such as a needle valve is provided on the way of the flow path from the piping 8 to the combustor 12 to adjust the pressure. By these methods, P₂Adjusted to be sufficiently higher than the atmospheric pressure at the outlet of the combustor 12 and not less than the internal pressure P of the raw material tank 1₁. In this way, the gas 104 dissolved in the liquid material 101 can be effectively prevented from foaming in the pipe 8.

In addition, if the liquid containing bubbles flows through the mass flow controller 7, accurate flow measurement becomes difficult, and the flow rate adjusting operation of the mass flow controller 7 may become unstable. Thus, in addition to setting P₁≤P₂In addition, the internal pressure P of the pipe 6 upstream of the mass flow controller 7₃Should not be less than the internal pressure P of the raw material tank 1₁(i.e., P)₁≤P₃). Specifically, the internal pressure P of the raw material tank 1 is adjusted by supplying an inert gas adjusted by a pressure regulator from an inert gas supply pipe 13₁. Then, the discharge pressure of the liquid-sending pump 3 is adjusted in accordance with the pressure loss in the flow path from the liquid-sending pump 3 to the mass flow control 7, whereby the internal pressure P of the pipe 6 is adjusted₃Adjusted so that the internal pressure P₃Not less than the internal pressure P of the raw material tank 1₁. Further, a pressure loss portion 11 such as a needle valve is provided on the way of the flow path from the mass flow controller 7 to the combustor 12 to adjust the pressure. By these methods, P₂Adjusted to be sufficiently higher than the atmospheric pressure at the outlet of the combustor 12 and not less than the internal pressure P of the raw material tank 1₁. In this way, the bubbles can be hindered from entering the liquid raw material 101 passing through the mass flow controller 7.

Here, it is preferable to set the internal pressure P upstream (on the side of the pipe 6) of the mass flow controller 7₃Higher than the internal pressure P of the downstream (pipe 8 side)₂(i.e., P)₁≤P₂＜P₃). In this way, the flow rate adjusting operation of the mass flow controller 7 is further stabilized. In particular, since the flow rate adjusting operation of the mass flow controller 7 is stable, P is preferably set to₃Set to the ratio P₂The pressure is higher than 0.05 MPa. To realize P₃And P₂In this relation, the power of the liquid-sending pump 3 can be adjusted so that the discharge pressure of the liquid-sending pump 3 becomes sufficiently high in accordance with the pressure loss in the flow path from the liquid-sending pump 3 to the mass flow controller 7.

In the manufacturing method using the apparatus for manufacturing a porous glass substrate for an optical fiber as described above, the flow rate of the liquid material 101 may vary depending on the operation of the liquid-feeding pump 103. That is, if the upstream pressure of the mass flow controller 7 fluctuates in a short period of time due to the fluctuation of the discharge pressure of the pump caused by the internal motion of the pump when the liquid sending pump 3 boosts the liquid raw material 101, the flow rate regulation by the mass flow rate regulation controller 7 may not be able to keep up with the pressure fluctuation. Therefore, the pressure loss section 5 (e.g., a pressure reducing valve, an orifice, etc.) should be provided between the pipe 4 through which the liquid material 101 is discharged from the liquid sending pump 3 and the pipe 6 through which the liquid material 101 is supplied to the mass flow controller 7 so that the internal pressure P of the pipe 4 is set₄Higher than the internal pressure P of the piping 6₃(i.e., P)₃＜P₄). Thus, the internal pressure P of the pipe 6₃The flow rate control operation by the mass flow controller 7 can be stabilized without being affected by variations in the discharge pressure of the liquid feed pump 3. In particularPreferably, the pressure loss section 5 is set so that the pressure P is₃Is P₄About 0.6 times or less, since P can be effectively suppressed₃A variation of (c). In view of the above, it is preferable that P is₁≤P₂＜P₃＜P₄。

The end of the raw material injection pipe 2 that injects the liquid raw material 101 into the raw material tank 1 is mounted such that the end of the raw material injection pipe 2 is below the liquid surface of the liquid raw material 101 stored in the raw material tank 1. Thus, the gas 104 present in the space above the liquid surface of the liquid raw material 101 can be prevented from being involved, and bubbles of the gas 104 can be prevented from being mixed into the liquid raw material 101.

When a highly flammable liquid raw material such as Octamethylcyclotetrasiloxane (OMCTS) is used, the gas 104 in the space above the liquid level of the liquid raw material 101 in the raw material tank 1 may be an inert gas such as nitrogen, argon, helium, or the like. Thus, an unexpected oxidation reaction in the raw material tank 1 can be prevented. In order to supply the inert gas to the upper space of the raw material tank 1, an inert gas supply pipe 13 may be provided as shown in fig. 1.

Internal pressure P of raw material tank 1₁A positive pressure higher than atmospheric pressure may be maintained. Thus, even if the raw material tank 1 has an unexpected pinhole or the like, the inflow of the external air with oxygen into the raw material tank 1 can be prevented.

On the other hand, it is preferable to reduce the internal pressure P of the raw material tank 1₁And P₁To prevent the gas 104 from being excessively dissolved into the liquid raw material 101 stored in the raw material tank 1. In particular, it is preferable to adjust the internal pressure P of the raw material tank 1₁Is kept below 0.1MPa, and even more preferably is kept below 0.05 MPa. It is preferable to add P₁Is kept within + -0.01 MPa, and it is even more preferable to keep P within₁The pressure fluctuation of (A) is kept within. + -. 0.005 MPa.

When the liquid raw material 101 is gasified in the gasifier 9, the gas 104 dissolved in the liquid raw material 101 is also released. The gas 104 in the upper space of the raw material tank 1 should have a constant gas species (in the case of gas mixture, each gas species)Class and mixing ratio thereof) and the internal pressure P of the raw material tank 1₁Should be small. Thus, the amount of the gas 104 dissolved in the liquid raw material 101 is stable. Since the amount of the gas 104 dissolved in the liquid raw material 101 is stable, the partial pressure of the gas 104 released in the vaporizer 9 is also stable. As a result, the flow rate of the raw material gas 102 supplied to the combustor 12 can be stabilized.

For adjusting the internal pressure P in the raw material tank 1₁The pressure of the inert gas supplied from the inert gas supply pipe 13 may be adjusted by a pressure reducing valve (not shown) to maintain a constant pressure. A safety valve 14 and a back pressure valve (not shown) may also be provided to maintain the internal pressure P by depressurizing the raw material tank 1₁Thereby generating an internal pressure P₁The internal pressure P being exceeded unexpectedly₁To below a predetermined pressure.

As the liquid-feeding pump 3, a diaphragm pump can be used as a metering pump. Alternatively, a plunger pump or a gear pump may be used. If the pump 3 is coupled to the pressure P in the pipe 4₄When the pulsation of (3) is large, the pressure P in the pipe 6 becomes large₃And may be pulsed accordingly. To inhibit P₃Preferably P, preferably P₄Is limited to within 0.1MPa, even more preferably to within 0.05 MPa.

To inhibit P₃The pulsation of (2) may be a pump without pulsation, or an accumulator 15 may be installed between the discharge side of the pump 3 and the pipe 4, and an orifice or other pressure loss portion may be installed. The accumulator 15 is a buffer device that suppresses pulsation of liquid by repeatedly expanding and contracting a diaphragm (rubber film) when the pulsating liquid passes through.

As shown in fig. 1, a pipe 16 may be branched from the middle of the pipe 4, and a pressure loss part 17 such as an orifice, a safety valve, a back pressure valve, a needle valve, or the like may be installed in the pipe 16 to discharge a part of the liquid raw material 101 fed from the liquid feeding pump 3 to the pipe 4. Thus, the internal pressure P of the raw material tank 1₁Can be kept small.

Further, the discharged liquid raw material 101 may be held in the intermediate container 18 and, for example, left to stand to remove air bubbles unintentionally mixed, and then the liquid raw material 101 may be returned to the raw material tank 1 for reuse.

Fig. 2 shows a modification of the apparatus for manufacturing a porous glass substrate for optical fiber shown in fig. 1. The apparatus for producing a porous glass substrate for an optical fiber shown in fig. 2 has a structure in which the liquid material 101 discharged through the pressure loss section 17 is returned to the material tank 1 through the pipe 19. The liquid raw material 101 sent from the raw material tank 1 circulates through the liquid sending pump 3, the pipe 4, the pipe 16, the pressure loss section 17, and the pipe 19, and returns to the raw material tank 1. Thus, the discharged liquid raw material 101 can be easily reused.

As in the case of the raw material injection pipe 2, the end of the pipe 19 for returning the discharged liquid raw material 101 to the raw material tank 1 is preferably attached below the liquid surface of the liquid raw material 101 in the raw material tank 1. With this structure, the liquid material 101 returned from the pipe 19 to the material tank 1 can prevent bubbles from entering the liquid material 101 due to entrainment of the gas 104 present in the space above the liquid surface.

Examples

The present invention will be described in detail below by way of examples. In the examples, a porous glass substrate manufacturing apparatus for an optical fiber having the configuration shown in fig. 2 was used. As liquid starting material, liquid Octamethylcyclotetrasiloxane (OMCTS) was used.

First, the liquid raw material 101 is supplied to the raw material tank 1 through the raw material injection pipe 2 and stored. Then, the liquid material 101 stored in the material tank 1 is pumped into the pipe 4 by the liquid feeding pump 3. The pipe 16 branches from the middle of the pipe 4, and the portion of the liquid material 101 that passes through the pipe 16 and the orifice of the pressure loss unit 17 is returned to the material tank 1 through the pipe 19.

On the other hand, the remaining part of the liquid raw material 101 pumped into the pipe 4 is depressurized by a pressure reducing valve as a pressure loss section 5, pumped into the pipe 6, and supplied to a vaporizer 9 through a pipe 8 by controlling the flow rate by a mass flow controller 7. The raw material tank 1 and the flow path from the raw material tank 1 to the vaporizer 9 are heated and maintained at a temperature of 30 ℃ to 40 ℃ as needed. Preferably, the temperature range to be maintained is above the freezing point of the liquid feedstock 101 and below the flash point. When OMCTS is used as the liquid feedstock 101, it is preferred to heat and maintain the temperature between 17 ℃ and 52 ℃ because the freezing point of OMCTS is 17 ℃ and the flash point is 52 ℃.

The space above the liquid surface of the liquid raw material 101 in the raw material tank 1 is filled with nitrogen supplied from the inert gas supply pipe 13. Internal pressure P of raw material tank 1₁Maintained at a gauge pressure of 0.045MPa on average, and during the manufacture period P₁(P) of₁Maximum value of-P₁Minimum of (d) is kept within ± 0.005 MPa.

The discharge rate of the liquid-sending pump 3 is set to 100 cc/min, and the accumulator 15 is installed directly below the liquid-sending pump 3. Internal pressure P of the pipe 4₄Is maintained at a gage pressure of 0.5. + -. 0.005 MPa. As shown in examples 1 to 10 in table 1 described later, the internal pressure P of the pipe 6₃Is maintained in a gage pressure range of 0.19MPa to 0.40 MPa. Then, as shown in examples 1 to 10, the liquid raw material 101 was supplied toward the vaporizer 9 at a flow rate in the range of 15 g/min to 70 g/min by the mass flow controller 7.

When the apparatus is started, the liquid material 101 circulates through the liquid sending pump 3, the pipe 4, the pipe 16, and the pressure loss section 17 (orifice) by operating the liquid sending pump 3 with the valve 20 attached upstream of the pressure loss section 5 (pressure reducing valve) closed, and returns from the pipe 19 to the material tank 1. By circulating the liquid raw material 101 in this manner, residual gas in the piping can be pushed out, and the piping can be filled with the liquid raw material without bubbles.

FIG. 3 shows a feed stream of feedstock around a gasifier in an embodiment. The flow rate of the liquid raw material 101 pumped into the pipe 6 is adjusted to a predetermined value (g/min) by the mass flow controller 7, and the liquid raw material 101 pumped into the pipe 6 is supplied to the vaporizer 9 through the pipe 8.

The temperature of the gasifier 9 was set to 200 ℃. From the viewpoint of efficiently vaporizing the raw material OMCTS and preventing the polymerization reaction, it is preferable to set the temperature of the vaporizer 9 to 150 ℃ to 250 ℃.

Nitrogen gas heated to 200 ℃ in a heat exchanger at a constant flow rate (0 ℃, 1 atm standard equivalent, L/min) was supplied as a carrier gas 105 from a pipe 21 connected to the vaporizer 9. In this way, the liquid raw material 101 and the carrier gas 105 are mixed in the vaporizer 9 to promote vaporization of the liquid raw material 101.

As the carrier gas 105, an inert gas such as argon or helium, oxygen, or a mixed gas of oxygen and an inert gas may be used in addition to nitrogen. The flow of carrier gas 105 is controlled by a mass flow controller (not shown). The carrier gas 105 is supplied by heating with a heat exchanger (not shown).

A raw material gas 102, which is a mixture of gas OMCTS obtained by vaporizing a liquid raw material 101 and nitrogen as a carrier gas 105, is supplied to the combustor 12 through a pipe 10 and a needle valve as a pressure loss section 11. The pipe 10, the pressure loss part 11 (needle valve), and the pipe 22 were heated to 190 ℃ to prevent the raw material gas 102 from condensing.

The raw material gas 102 passed through the pressure loss section 11 (needle valve) is further mixed with a constant flow rate of oxygen 106 heated to 200 ℃ through the pipe 23. The feed gas 102 (mixed gas of gas OMCTS and carrier gas) mixed with additional oxygen 106 is then fed to the burner 12.

From the viewpoint of preventing recondensation of the liquid raw material 101, the oxygen gas 106 mixed here may be supplied in the following state: is previously heated to a temperature higher than the liquefaction temperature expected from the partial pressure of the raw material gas 102 in the mixed gas by using a heat exchanger (not shown) or the like. By mixing oxygen with the raw material gas 102 in advance before supplying to the combustor 12, the combustion reaction of the raw material gas 102 in the combustor 12 can be promoted.

In addition to the gas mixture gas, if necessary, a combustible gas for combustion, oxygen for combustion, and a seal gas are supplied to the combustor 12. Hydrogen, methane, ethane and propane can be used as combustible gases for combustion. As the sealing gas, an inert gas such as nitrogen, argon, helium, or the like, or oxygen or a mixed gas of oxygen and an inert gas is preferably used.

In the flame of the burner 12, the raw material gas 102, a combustible gas for combustion, an oxygen gas for combustion, and the like are mixed and combusted, thereby forming silica fine particles 103. The formed silica microparticles 103 are deposited on a starting material to form a porous glass substrate for an optical fiber.

Further, the porous glass substrate was heated at 1500 ℃ in a helium-containing atmosphere to manufacture a transparent glass substrate for an optical fiber.

In the above method for producing a glass base material for an optical fiber, the internal pressure P of the raw material tank 1 is measured by a pressure gauge₁The internal pressure P of the pipe 4₄The internal pressure P of the pipe 6₃And the internal pressure P of the pipe 8₂As gauge pressure. (P)₃-P₂) Corresponding to the pressure difference across the mass flow controller 7. The flow rate of the mixed gas of the raw material gas 102 and the carrier gas 105 passing through the pipe 10 is measured by a mass flow meter. The measurement was performed for 10 minutes and the rate of change of mass flow meter reading (═ (max-min)/average x 100%) was measured. The mass flow meter used is of a thermal type and is formed by mixing a raw material gas 102 and a carrier gas 105 in the amount of N₂The heat capacities were scaled and summed to make the measurement. No actual flow scaling with scaling factors and other factors is performed.

Comparative example 1 is an example of producing a porous glass substrate under different conditions using the same apparatus as in example. In comparative example 1, a porous glass substrate was produced with the pressure loss part 11 (needle valve) released, and the internal pressure P of the pipe 8 was adjusted₂The pressure was set to 0.020MPa gauge. In comparative example 1, the internal pressure P of the raw material tank 1₁Is 0.045MPa, therefore P₁And P₂The relation between is P₁＞P₂. As a result, the rate of change in the flow rate exceeded 5%.

Comparative example 2 is an example in which a porous glass substrate was produced using an apparatus shown in fig. 4 without the liquid-feeding pump 3. In the apparatus having such a structure, the liquid raw material 101 is supplied with the supply pressure of nitrogen supplied from the inert gas supply pipe 13 set to 0.3 MPa. As a result, the rate of change in the flow rate exceeded 10%.

Table 1 shows the conditions and flow rate variations of examples 1 to 10 and comparative examples 1 and 2.

[ Table 1]

As shown in examples 1 to 10 in table 1, it can be seen that by setting P₁≤P₂The flow rate of the raw material gas supplied to the burner can be suppressed from varying. In addition, it can be seen from the comparison between each of examples and comparative example 2 that P is preferable₁≤P₂＜P₃。

As described above, according to the present invention, it is possible to suppress variation in the flow rate of the liquid raw material supplied to the vaporizer and to stabilize the supply of the raw material gas vaporized and supplied to the combustor.

Description of the reference numerals

1 stock tank

2 raw material injection piping

3 liquid feeding pump

4. 6, 8, 16, 19 liquid raw material piping

5. 11, 17 pressure loss part

7 mass flow controller

9 gasifier

10. 22 gas raw material piping

12 burner

13 inert gas supply pipe

14 safety valve

15 energy accumulator

18 intermediate container

20 valve

21 carrier gas supply pipe

23 oxygen supply pipe

101 liquid raw material

102 raw material gas

103 silica fine particles

104 gas in the raw material tank 1

105 carrier gas

106 oxygen gas