GB2057661A

GB2057661A - Apparatus for metering and controlling a feed of hydrogen fluoride vapour

Info

Publication number: GB2057661A
Application number: GB8022308A
Authority: GB
Original assignee: British Nuclear Fuels Ltd
Current assignee: British Nuclear Fuels Ltd
Priority date: 1979-07-23
Filing date: 1980-07-08
Publication date: 1981-04-01
Also published as: GB2057661B

Abstract

A boiler 1 for vaporising liquid hydrogen fluoride has a heat transfer surface 9 arranged to be covered increasingly by the liquid as the liquid level rises so that the heat supplied to the boiler can be adjusted to maintain a constant level of liquid in the boiler and thereby equate the feed rate of liquid hydrogen fluoride to that vaporised. Thus by metering and controlling the feed rate of the liquid the feed rate of the vapour can be metered and controlled. <IMAGE>

Description

SPECIFICATION Apparatus for metering and controlling a feed of hydrogen fluoride vapour This invention relates to apparatus for metering and controlling a feed of hydrogen fluoride vapour to a chemical plant.

According to the present invention apparatus for metering and controlling a feed of hydrogen fluoride vapour to a chemical plant comprises a boiler for vaporising substantially anhydrous liquid hydrogen fluoride having an inlet for the liquid hydrogen fluoride and an outlet for hydrogen fluoride vapour, a feed line including a control valve and a ftowmeterforthe liquid hydrogen fluoride, for connecting a source of substantially anhydrous liquid hydrogen fluoride with the boiler inlet, a heat transfer surface within the boiler arranged to be increasingly covered by the liquid hydrogen fluoride as the level of the liquid hydrogen fluoride rises in the boiler and means for supplying heat to the heat transfer surface.

The supply of heat to the heat transfer surface is so arranged that it is the heat transfer surface which controls the supply of heat to the liquid hydrogen fluoride in the boiler. With any specified feed rate an equilibrium can then be set up, the area of heat transfer surface covered by the liquid hydrogen fluoride determining the rate of boil-off of hydrogen fluoride vapour, and the rate of boiloff equalling the rate of feed of liquid hydrogen fluoride, so that the liquid level in the boiler is maintained at a substantially constant level. By means of the invention, therefore, a supply of hydrogen fluoride vapour to a reaction vessel may be metered by metering the liquid phase from which the vapour is derived.This is an advantage because the sensitivity of hydrogen fluoride vapour to changes in temperature and pressure affects the accuracy with which a flow of hydrogen fluoride can be metered in the vapour phase.

The heat supply and the heat transfer surface are both conveniently provided by a steam jacket for the boiler. The size of the steam jacket may be varied to vary the limits of the feed rate for a particular boiler. Means are preferably provided for maintaining the steam pressure in the jacket at a constant value.

To obtain a metered supply of hydrogen fluoride vapour by means of the invention it has been found in practice necessary to evaporate it from substantially anhydrous liquid hydrogen fluoride, that is to say, the liquid normally available commercially as "anhydrous hydrogen fluoride" and containing less than 2% by weight of water.

When used herein the term "substantially anhydrous liquid hydrogen fluoride" or "AHF" should be so understood.

The invention will now be described by way of example with reference to the accompanying drawings wherein, Fig. 1 is a diagram including apparatus in accordance with the invention.

Fig. 2 is a side view of the boiler shown within the dotted frame II of Fig. 1 and Fig. 3 is an end view of the boiler of Fig. 2 in the direction of the arrow Ill.

Fig. 1 of the drawings shows a boiler 1 having an inlet 2 leading to the bottom of the boiler and an outlet line 3. A feed line 2a including a control valve 4 and a flow recording controller 5 connects the inlet 2 with a header tank 8. AHF can be pumped from a storage tank 7 to the header tank 8 by a pump 6. The header tank 8 is provided with an overflow 8a whereby a constant head of the liquid AHF may be maintained in the header tank.

The boiler 1 is shaped to provide a heat transfer surface 9 which is surrounded by a steam jacket 11 for supplying heat to the surface 9. As is shown more clearly in Figs. 2 and 3 the heat transfer surface 9 forms the inner wall of the steam jacket 11. The steam jacket is made in three equal parts each extending around one third of the circumference of the heat transfer surface 9 and each is provided with a steam inlet 10, a condensate outlet 1 6 and a vent 20. The boiler is mounted on struts 22 and has a bottom drain 21.

Tapping points 12 and 1 5 for a level recorder 1 8 are provided on the boiler. This recorder is fitted with high and low level alarms. The high level alarm closes the valve 4. A pressure recorder 13 fitted with a high level alarm is connected at the top of the boiler. Also connected to the top of the boiler is an emergency vent line 1 7 leading to an emergency scrubber unit (not shown). This line carries a bursting disc 14 and a pressure relief valve 27 set to vent at a pressure below the safe working pressure of the boiler. A pressure indicator 28 is connected between the bursting disc and the pressure relief valve to enable the state of the bursting disc to be checked.

Referring now to the steam circuit, a low pressure steam feed circuit 24 controlled by a valve 26 is divided at junction 35. One part of the circuit includes a superheater 23 for hydrogen fluoride vapour in the boiler outlet line 3 and a steam trap 32 leading to a drain 37. The other part of the circuit includes a filter 36 to remove grit, a pressure controller 30, a pressure gauge 39 and a pressure relief valve 38, also a cut-off valve 33 for the inlets 10 of the steam jacket 11, which can be operated by the high level alarm of the pressure recorder 1 3. Condensate outlet lines 1 6 from the steam jacket 11 include a pressure regulation manometer 31 which leads to a drain 29.

In operation AHF is pumped from the storage tank 7 to the header tank 8. The flow recording controller 5 measures the flow through line 2a, compares it with the required flow and adjusts the control valve 4 accordingly. The inlet line 2 dips to the bottom of the boiler so that the cool liquid inlet will not prevent a steady vapourisation from the surface of the liquid.

Steam is passed to the steam jacket 11 from the circuit 24 and heat is transferred through the heat transfer surface 9 to vapourise the AHF in the boiler 1. As the AHF liquid level in the boiler rises, more and more heat is transferred to the liquid until a steady state is reached when liquid is vapourising from the surface (and vapour is passing out through the outlet line 3) at the same rate as liquid is being replaced through the inlet 2.

Condensate leaves the jacket 11 through the outlets 1 6. The three equal parts of the steam jacket 11, each with independent inlets 10, outlets 16 and vents 20 allow the heat capacity of the boiler to be varied since the closing of one set or, alternatively, two sets of the inlet and outlet valves 10 and 16 changes the area of heat transfer surface 9 in use and hence the ratio of heat transfer surface to AHF volume in the boiler.

The low pressure steam supply via valve 26 splits into two at junction 35. The first supply is used to super-heat the hydrogen fluoride leaving the outlet 3 from the boiler 1 before passing on to the chemical plant as reacting fluid (The hydrogen fluoride leaving the boiler at 3 is saturated i.e it is in equilibrium with liquid AHF). The condensate leaving the super heater is removed by the steam trap 32 to drain 37. The second supply to the steam jacket 11 is first filtered at 36 to remove grit then passed through the pressure controller 30.

Claims

1. Apparatus for metering and controlling a feed of hydrogen fluoride vapour to a chemical plant wherein the apparatus comprises a boiler for vaporising substantially anhydrous liquid hydrogen fluoride, having an inlet for the liquid hydrogen fluoride and an outlet for hydrogen fluoride vapour, a feed line including a control valve and a flowmeter for the liquid hydrogen fluoride, for connecting a source of substantially anhydrous liquid hydrogen fluoride with the boiler inlet, a heat transfer surface within the boiler arranged to be increasingly covered by the liquid hydrogen fluoride as the level of the liquid hydrogen fluoride rises in the boiler, and means for supplying heat to the heat transfer surface.

2. Apparatus as claimed in claim 1 wherein the heat transfer surface and the means for supplying heat to the heat transfer surface are provided by a steam jacket for the boiler.

3. Apparatus as claimed in claim 2 wherein the size of the steam jacket is variable.

4 Apparatus as claimed in claim 3 wherein the steam jacket is divided into three portions, each portion having its own inlet and outlet for steam.

5. Apparatus for metering and controlling a feed of hydrogen fluoride vapour to a chemical plant, substantially as hereinbefore described with reference to the accompanying drawings.