Methods of containing fluids deleterious to the container

Fluid permeable wall members through which an auxiliary fluid is passed against pressure exerted by a fluid to be contained, which wall member is composed of inorganic fibres, may be used to contain hot fluids or fluids otherwise having a deleterious effect on the wall members. Vacuum formed shapes composed of refractory inorganic fibres may be used to contain fluids having temperatures of over 2500.degree. C. such as gases which have been heated by electrical discharge means for use in the vapor phase manufacture of oxides of titanium, iron, aluminium, silicon or zirconium and offer low capital costs combined with resistance to thermal shock.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the containment of fluids. 
2. Brief Description of the Prior Art 
Problems may arise in connection with the containment of fluids which have 
a deleterious effect on the materials of construction of the containing 
vessel by reason of, for example, the reaction of the fluid, or of a 
constituent therein, with the material of construction of the containing 
vessel, the deposition of a solid from the fluid onto the wall of the 
containing vessel, or changes which may occur in the material of the 
containing vessel by reason of the fluid having either a sufficiently low 
temperature or a sufficiently high temperature to cause breakdown of the 
walls of the containing vessel by, for example, melting. 
One method of overcoming the above problem is to reduce the contact between 
a first such fluid and the walls of the containing vessel by use of a 
permeable wall member through which an auxiliary fluid is passed into said 
vessel. 
The material of construction of the permeable wall member may be permeable 
amorphous carbon, or graphite, or may be a metal or a refractory ceramic 
foam with continuous interconnecting pores. Such materials may be 
expensive and difficult to fabricate, and, particularly in the case of a 
refractory ceramic foamed structure, may be very sensitive to either 
thermal or physical shock. Such difficulties have limited the application 
of this technique of fluid containment. 
SUMMARY OF THE INVENTION 
The invention provides a method and apparatus for containing a fluid by 
means of a permeable wall member wherein the fluid to be contained would 
have a deleterious effect on the wall member on contact therewith but 
wherein the degree of the said contact is reduced by passing an auxiliary 
fluid through the wall member against the pressure exerted on the wall 
member by the fluid to be contained and wherein the permeable wall member 
is constructed of inorganic fibres.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
The auxiliary fluid must not cause any undesired effect on the fluid to be 
contained or on the wall member by reason, for example, of chemical 
reaction with the constituents of the fluid to be contained or of physical 
change in the wall member due to temperature effects. The auxiliary fluid 
may react with the first fluid to prevent solids deposition, or may act as 
an inert barrier to reaction between the first fluid with material of 
construction of the wall member, or may, by reason either of having a 
higher temperature or a lower temperature than that of the first fluid, 
maintain the wall member at a temperature at which it is physically 
stable. The auxiliary gas may be similar in composition to the gas to be 
contained. 
While the invention is applicable to any of the problem situations 
envisaged above and while in the broadest aspect of the invention the 
fluid may be either a liquid or a gas, the invention is particularly 
applicable to the containment of hot gases; including hot vapours; for 
example the containment of gases having a temperaure at least equal to the 
minimum sintering, melting or thermal decomposition temperature of the 
wall member, the auxiliary gas having a temperature below the said minimum 
sintering or melting temperature. Hereafter the invention will be 
particularly described with reference to the containment of gases, but 
without thereby intending any limitation thereto. 
The words "sintering", "melting" and "thermal decomposition" are used as 
alternatives since different inorganic fibres may deteriorate in different 
ways on heating to above the temperature at which they are stable. To 
eliminate confusion the lowest temperature at which the porosity and 
strength of a fibre is affected to any practical extent due to any one of 
the above properties is indicated hereafter by the general term "minimum 
deterioration temperature". 
The effect of the auxiliary gas is thought to be two-fold. Where a hot gas 
is to be contained the auxiliary gas exerts a cooling effect on the 
material of the wall. If a hot gas is flowing through a vessel or duct 
comprising the permeable wall member it may be possible to stabilise the 
boundary layer of the hot gas adjacent to the permeable wall by means of 
the auxiliary gas. The Reynolds number of the boundary layer may be 
reduced by the introduction of the auxiliary gas and it may be possible to 
introduce sufficient gas through the porous wall member to convert a 
turbulent boundary layer into a laminar boundary layer, the effect being 
to reduce the rate of convective heat transfer from the gas to the wall. 
The combination of these two effects may result in the establishment of a 
wall temperature which is substantially below that of the hot fluid and 
also below the minimum deterioration temperature of the fibres, of which 
the permeable wall is constructed, occurs. 
Structures composed of inorganic fibres have been used as heat insulators 
in a wide variety of applications. The present invention differs from 
previous practice in utilising such structures in applications where an 
auxiliary fluid is passed through the structure against the pressure of a 
fluid to be contained. In such applications readily formed durable 
structures composed of inorganic fibres may replace the costly and fragile 
types of permeable materials referred to above. 
There may be utilised in the practice of this invention any of a wide range 
of materials made of inorganic fibres. Examples of such materials are 
mineral wool, asbestos felt, glass fibre, silicon nitride or carbide 
fibres or carbon fibres. Preferably there is utilised in the practice of 
this invention refractory ceramic fibres made, for example, of alumina, 
silica, zirconia or silica/alumina. Such refractory ceramic fibres are 
recommended by their manufacturers for continuous use as heat insulators 
in the range of about 1000.degree. C. to 1500.degree. C. or, for short 
duration use only in the range of from 1500.degree. C. up to, in the case 
of some fibres, above 2000.degree. C. Above such recommended temperatures 
the fibres may sinter or melt. Refractory ceramic fibres may be 
manufactured by passing a molten ceramic oxide through an orifice and 
subjecting the resulting stream to a high pressure jet of steam or gas so 
as to attenuate the stream to fine fibres. Organic binders are commonly 
sprayed in to assist in the eventual formation of the resulting material 
into a felt or into a more dense compressed structure, made, for example, 
by vacuum moulding techniques. Such organic binders burn out at a 
temperature of about 300.degree. C. There may be slight (i.e. up to 6% 
linear) irreversible shrinkage at a temperature of somewhat above 
1000.degree. C. At temperatures between this and the minimum deterioration 
temperature of the fibres structures made from the fibres are 
dimensionally and chemically stable. Structures formed from such fibres by 
vacuum forming techniques may have a density in the range of from 0.16 to 
0.80 g/cu cm and considerable mechanical strength in relation to their 
weight. 
The following Tables list suitable commercially available refractory 
ceramic fibres: 
______________________________________ 
Ref. Trade Mark Manufacturer 
______________________________________ 
1. Triton Kaowool 
Moryanite Fibres 
2. Saffil Fibres I.C.I. Mond Division 
3. Refrasil Chemical and Insulating 
4. Procal Foseco F.S. Ltd. 
5. Fiberfax Carborundum Ltd. 
6. McKechnie McKechnie Refractory 
Fibres Ltd. 
______________________________________ 
Recommended Oper- 
Composition 
Ref. M.P. .degree.C. 
ating Temp. .degree.C. 
Wt. % 
______________________________________ 
1. 1760 1260 (al.sub.2 O.sub.3 43-47 
(SiO.sub.2 50-54 
2. &gt;2500 1400-1600 SrO.sub.2 
&gt;2000 1000-1400 Al.sub.2 O.sub.3 
3. 1700 1000 SiO.sub.2 
4. 1260 SiO.sub.2 75 Al.sub.2 O.sub.3 22 
5. &gt;1750 1260 Al.sub.2 O.sub.3 51 SiO.sub.2 47 
6. 1780-14 1925 
1260-1400 (Al.sub.2 O.sub.3 50-61 
(SiO.sub.2 49-38) 
______________________________________ 
In conducting processes based on an endothermic, or on an insufficiently 
exothermic gas phase reaction it may be necessary to supply heat to the 
reaction by heating one or more of the reactants, for example air or 
oxygen in the case of an oxidation reaction, or by conducting the reaction 
in the presence of a heated inert gas, for example nitrogen or argon. An 
example of such a gas phase reaction is manufacture of an oxide of an 
element selected from the group consisting of a titanium, iron, aluminum, 
silicon or zirconium by reacting a chloride of the element with an 
oxidising gas in the vapor phase to produce a gaseous suspension of solid 
particles of the oxide. In conducting such a process it is necessary to 
supply sufficient heat to raise the temperature of the mixed reactants to 
at least 700.degree. C. were they to be mixed without reaction occurring. 
In such processes there are difficulties associated with pre-heating the 
metal chloride and it has therefore been proposed to preheat either a 
diluent gas or the oxidising gas to a temperature above 2000.degree. C. by 
electrical discharge means. In our copending U.K. Patent Application No. 
26413/75, which corresponds to our pending U.S. patent application Ser. 
No. 579,635, now U.S. Pat. No. 4,056,704 there are described an apparatus 
and process suitable for heating the very large quantities of gases 
required for use in the very large scale production units which have been 
developed in the chemical industry, for example for use in processes for 
the production of titanium dioxide by the oxidation of titanium 
tetrachloride, on a scale which produces a quantity of product in the 
range of from 20,000 to 50,000 tons per annum from a single production 
unit. The said copending application discloses an apparatus comprising a 
heating zone, means for the supply of a gas to he heated thereto and means 
for the removal of the heated gas therefrom, and upstream and downstream 
electrode means adapted for the establishment of a distributed electrical 
discharge in the heating zone, the means arranged for the supply of gas to 
the heating zone comprising a plurality of adjacently positioned conduits 
arranged to constrain the gas supplied to the heating zone at its point of 
entry thereto into a plurality of substantially parallel streams and the 
upstream electrode means comprising a plurality of electrodes positioned 
respectively in the said plurality of gas streams. 
Advantageously the present invention may be used in containing a gas stream 
heated, by the technique described in the copending application referred 
to above, by other electrical discharge techniques or by combustion 
techniques, the gas stream being in the heating zone or in gas transport 
ducts downstream thereof. Preferably the zone in which the heating occurs 
and any transport ducts downstream of the heating zone are bounded by 
permeable side walls formed of a compressed mass of refractory ceramic 
fibres and an auxiliary gas having a temperature below the melting point 
of the refractory ceramic fibres is passed through the permeable wall 
member into the heating zone. The invention is particularly suitable for 
use where electrical discharge heating means are used in view of the high 
electrical resistivity of many inorganic fibres and particulary of the 
refractory ceramic fibres referred to above. 
Preferably the permeable wall member is composed of one or more 
vaccum-formed shapes, which can be cemented together, or jointed by means 
of a spigot joint construction. 
The use of a permeable wall duct to contain gases having a temperature 
above the melting point of the refractory fibre is readily achieved in the 
case of straight ducting. The cross-sectional shape of the duct preferably 
should be circular but an elipitical section is acceptable. A square or 
rectangular section is less satisfactory because the complex circulatory 
flow paths in the hot gas stream in the vicinity of each of the four 
corners of the cross section may interfere with the desired action of the 
cool gas in stabilising the boundary layer. Much greater care is required 
in the design of permeable wall ducts involving a change in direction. 
Small radii of curvature at a change in direction should be avoided if the 
protective action of the cool gas is to be effective. 
The permeability of the porous wall member is not particularly critical 
always provided that it is uniform throughout. Conveniently, where the 
auxiliary fluid is a gas the permeability is within the range 10.sup.-5 to 
10.sup.-9 cm.sup.2 and preferably within the range 10.sup.-7 to 10.sup.-8 
cm.sup.2 calculated from the equation 
##EQU1## 
(Micromeritics. J. M. Dallavalle 2nd Ed. page 265 published by Pitman). 
The wall thickness for any value of permeability can be so selected to 
promote a uniform distribution of the flow of the auxiliary gas through 
the entire surface area of the wall member and is suitably in the range of 
5 mm to 25 mm. This is an important feature in the practice of the 
invention since if the flow of auxiliary gas through the wall member is 
uneven there will be a tendency for hot spots to be formed, possibly 
resulting in localised deterioration of the refractory ceramic fibres 
forming the wall member. Suitably the flow of the auxiliary gas is in the 
range from 1 to 4 cm.sup.3 /sq. cm internal surface of wall member/second. 
Conveniently the wall member is surrounded externally by a jacket to enable 
the establishment of a positive pressure in the auxiliary gas relative to 
the pressure within the vessel. The jacket may be of mild steel if the 
temperature of the auxiliary gas is sufficiently low. Preferably a filter 
is installed in the conduit supplying the auxiliary gas to the jacket. 
Such a filter may take the form of an easily replaceable portion of the 
permeable material of which the wall member is composed. The presence of a 
filter tends to reduce the tendency for the pores of the wall member to 
block with, for example, dust particles. In order to achieve a flow of 
auxiliary gas through the porous wall member the pressure gradient in the 
permeable wall member need not be very great. Therefore, even when the 
pressure of gas to be heated inside the vessel is relatively high the net 
stress to which the permeable wall itself is subjected is low. It is 
possible to operate this invention with low auxiliary gas temperatures 
even when the temperature of the gas being heated inside the vessel is 
high. In this event it is not necessary to take special precautions to 
insulate the outside of the jacket. The temperature of the auxiliary gas 
may be for example at from 10.degree. C. to 100.degree. C. 
We have disclosed above that refractory ceramic fibres commercially 
available may tend to shrink slightly irreversibly at a temperature above 
about 1000.degree. C. Preferably therefore the wall member is preheated to 
said temperature to allow this irreversible shrinkage to take place before 
being fitted into such support means as may be necessary. It is a 
particular advantage of the use of wall members made from refractory 
ceramic fibres that no further expansion or contraction occurs with 
temperature changes, and the wall members are therefore not subject to 
thermal shock problems. The low density of the permeable ceramic makes 
possible the use of lighter supporting members. Suitably shaped permeable 
wall members made by vacuum forming techniques are readily and cheaply 
obtainable and the use of such members represents a considerable plant 
cost saving. 
When practising the invention there are low heat losses from the containing 
vessel even in the absence of external lagging with resulting high thermal 
efficiency in a process, for example, the vapour phase oxidation of 
titanium tetrachloride, with which the present invention may be used. 
A particular embodiment of the invention above described will now be 
illustrated by means of the following Example, and with reference to the 
accompanying drawing. 
EXAMPLE 
FIG. 1 is a part longitudinal section, not to scale, through apparatus 
according to the invention. 
The apparatus comprises an electrical discharge heater with ancilliary 
equipment and ducting 10 according to the invention surrounding the 
heating zone 11. 
The electrical discharge heater comprises an electrode 1 cooled internally 
and having coolant fluid inlet and outlet 2, the electrode being mounted 
by insulating gasket 3 in the wall of container 4 equipped with gas inlet 
5. The electrode extends out of the container through aperture 6 and is 
surrounded, so as to leave an annular space, by cooling jacket 7 of 
stainless steel. The end of the electrode 1 is flat and recessed within 
the cooling jacket so as to form in combination with the jacket a 
construction in the form of a bluff body flame holder. The cooling jacket 
7 is equipped with inlet and outlet 9 for coolant fluid. A heating zone 11 
is defined by ducting 10, which is a hollow cylindrical vacuum-formed 
sleeve of refractory ceramic fibres available under the Trade Mark Triton 
Kaowool, by the top 8 of the electrode 1 and the electrode 12. The 
electrode 12 is an annular hollow structure provided with inlets and 
outlets 13 for coolant fluid and an outlet 14 for heated gas. The ducting 
10 is jacketed by jacket 15 and the space 16 is packed loosely with 
ceramic fibre wool available under the Trade Mark Triton Kaowool and is 
provided with inlet 17 for coolant fluid and is supported by impermeable 
heat resistant electrically non-conductive flanges 18. 
The electrodes 1 and 12 are made of a silver/aluminum alloy, the jacket 7 
is made of stainless steel and the jacketing 15 is made of mild steel. In 
use the coolant fluid fed to jackets 7 and electrode 12 was water and 
nitrogen at 280.degree. K. was fed to space 16 so as to give a pressure of 
25.5 kW/m.sup.2 gauge at the outer surfaces of ducting 10. This 
represented a feed rate of 30 1 min.sup.-1 into space 16. 
After start-up of the electrical discharge heater in known manner the 
apparatus was run at the steady state of a feed of 80 1 min.sup.-1 of 
nitrogen through inlet 5, to give a nitrogen pressure of 24.8 kN/m.sup.2 
gauge in the interior of the heating zone 11, and a potential gradient of 
450 volts was maintained between the anode 8 and the cathode 12 to give a 
current of 28 amperes passing through the gas flowing between cathode and 
anode. Allowing for heat losses to the coolant fluid, 9.45 k VA of 
electrical power was introduced into the 80 1 min.sup.-1 of nitrogen 
flowing through inlet 5 and thence through the heating zone 11. The 
temperature within the heating zone portion of the heating zone nearest to 
the electrode 1 was 3027.degree. C. and that of the gas leaving the 
heating zone through opening 14 was 2727.degree. C. The apparatus was 
operated for 6 hours during which time the ducting 10 showed no tendency 
to deteriorate.