Method of heat treating glass in a fluidized bed and apparatus therefore

A fluidized bed for the thermal treatment of glass articles is maintained in a quiescent uniformly expanded state of particulate fluidization by establishing a high pressure drop across a porous membrane through which fluidizing gas is supplied to the bed, of at least 60% of the pressure at which the fluidizing gas is supplied to a plenum chamber beneath the membrane.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to fluidised beds, and more particularly to a method 
of operating a fluidised bed and to fluidised bed apparatus. 
2. Description of the Prior Art 
In U.S. patent application Ser. No. 717,171, filed Aug. 24, 1976, now 
abandoned the disclosure of which is incorporated herein by reference, 
there is described a method of thermally treating glass in which the hot 
glass is immersed in a fluidised bed of particulate material. Fluidisation 
of the particulate material is effected within a tank by passing 
fluidising gas through a porous membrane forming the base of the tank. 
Prior to immersion of the glass the fluidised bed is in a quiescent 
uniformly expanded state of particulate fluidisation. 
The fluidised bed of particulate material in a quiescent uniformly expanded 
state of particulate fluidisation can be defined in terms of gas flow 
through the bed and the expanded height of the bed. the quiescent 
uniformly expanded state of particulate fluidisation has been found to 
exist between a lower limit of gas velocity at incipient fluidisation, 
that is the velocity at which the particles just become suspended in the 
uniformly distributed upwardly flowing gas, and an upper limit of gas 
velocity at which maximum expansion of the bed occurs and the top surface 
of the bed is tranquil and undisturbed by blubbing. A gas velocity higher 
than that which effects maximum expansion of the bed results in the 
development of extensive bubbling in the bed and at the onset of such 
bubbling there may be a partial reduction of the bed height. 
The invention of the above-mentioned Patent Application is particularly 
applicable for the thermal toughening of flat or bent glass sheets, such 
as those used singly as motor vehicle windscreens, sidelights or 
backlights, or as part of a laminated motor vehicle windscreen, or for use 
in construction of windscreen assemblies for aircraft and railway 
locomotives. 
In United Kingdom patent specification No. 774,305, thermal toughening of a 
glass sheet was proposed by a method in which a hot glass sheet is 
immersed in a freely bubbling bed of particulate material but such a 
process has not been brought into commercial use hitherto. 
The problem which we have found when attempting to operate such a freely 
bubbling fluidised bed for the thermal toughening of glass sheets is the 
high incidence of fracture of the glass sheets which occurs during their 
treatment in the fluidised bed. A freely bubbling bed has also been found 
to distort the shape of the glass sheets due to the irregular forces to 
which the glass sheets are subjected in a freely bubbling bed. 
By using a fluidised bed of particulate material which is in a quiescent 
uniformly expanded state of particulate fluidisation a successful 
commercial yield of whole glass sheets is achieved, there being very few 
fractures of the glass sheets while the toughening stresses are being 
developed in the glass sheets. It has also been found that the use of such 
a fluidised bed has very little effect on the shape of the glass sheets. 
For maintaining stable operation of a fluidised bed in the quiescent 
uniformly expanded state of particulate fluidisation there is a narrow 
range of fluidising gas velocities, between the lower limit of gas 
velocity at incipient fluidisation and the upper limit of gas velocity at 
the bed has a maximum expansion. For gas velocities above the upper limit 
there is general bubbling of the bed. Within the velocity limits for 
fluidisation in a quiescent uniformly expanded state of particulate 
fluidisation it has been found difficult to avoid the occurrence of 
localised bubbling in the bed, which in some cases can engender general 
bubbling in the bed. Another form of instability which arises is that of 
the formation of irregular currents of material in the bed. Both these 
effects are difficult to suppress once started. Such instabilities are 
particularly prone to arise in deep, e.g. 1 meter deep, fluidised beds 
such as are required for the processing of large glass sheets, for example 
of suitable size of motor vehicle windscreens. 
In a paper entitled "The Influence of the Gas Distributor on Bed Expansion, 
Bubble Size, and Bubble Frequency in Fluidised Beds" by D. Geldart, and J. 
R. Kelsey, I. Chem. E. Symposium Series No. 30 (1968) pages 114 to 125, 
there is a study of the types of instability occuring in bubbling 
fluidised beds of sand. The pressure drop across the distributor through 
which fluidising gas was supplied, and the geometry of that distributor 
was studied in terms of the stability of bubbling in a bed up to 1.5 
meters deep. The distributors employed included a perforated plate with 
underlying paper layers, a combination of a perforated plate with a porous 
plate, and a porous plate alone. The ratio of the pressure drop across the 
distributor to the pressure drop across the bed for minimum fluidisation 
was in the range 0.017 to 9.9 for a cylindrical bed and in the range 
0.0056 to 1.19 for a "two-dimensional" bed of rectangular cross section. 
For bubble-phase fluidisation observations were made with a range of 
values of that ratio from 0.08 to 23.0 for a cylindrical bed and from 
0.023 to 5.35 for a "two-dimensional" bed. The authors concluded that at 
low values of the ratio of pressure drop across the distributor to 
pressure drop across the bed, there was instability of bubbling, and that 
increasing the ratio from 0.1 to 10, measured at minimum fluidisation, had 
no observable effect on the behaviour of the bed. With the distributors 
and pressure ratios described only bubbling fluidisation could be achieved 
and such beds are unsuitable for the thermal processing of large glass 
sheets. 
In a book entitled "Fundamental Aspects of Fluidised Bed Coating" by 
Muharrem Elmas, Deletsche Hitgevers Maatschappii N.V., Delft, 1969, there 
is described the use of a shallow bed of polyethylene particles about 5 cm 
deep fluidised to a state of homogeneous fluidisation produced by varying 
the gas flow rate to a value in the range 1.0 to 1.4 producing minimum 
fluidisation. The pressure drop across the porous bronze plate used was at 
least equal to the weight of the bed. Hot objects of metal, glass ceramics 
or plastics were dipped into the bed in order to coat the objects. Such a 
bed has the disadvantage that the gas flow rate and pressure drop across 
the porous plate are inadequate to produce and maintain homogeneous 
fluidisation in a deep bed of material suitable for the thermal treatment 
of glass, in particular large sheets of glass for vehicle windscreens, 
sidelights or backlights. 
It has also been proposed in United Kingdom Patent Specification No. 
709,265 to employ rolled wire filter cloth as a gas-pervious support for a 
fluidised bed, with the pressure drop across the gas-pervious support of 
the same order as the pressure drop across the fluidised bed. 
The present invention is based on the discovery that stable operation of a 
fluidised bed in a quiescent uniformly expanded state of particulate 
fluidisation can be obtained by appropriate choice of membrane to create a 
high pressure drop across the membrane due to the flow of fluidising gas 
through the porous membrane through which the fluidising gas enters the 
bed. 
It is a main object of the invention to employ this discovery for improving 
stability of maintenance of a fluidised bed in a quiescent uniformly 
expanded state of particulate fluidisation for thermally treating glass 
articles. 
It is another object of the invention to promote uniform gaseous flow into 
the particulate material to be maintained as a deep homogeneous bed in 
said quiescent uniformly expanded state of particulate fluidisation. 
SUMMARY 
The invention provides a method of operating a fluidised bed of particulate 
material, for example for thermally treating a glass article by immersing 
the article in a fluidised bed of particulate material in a quiescent 
uniformly expanded state of particulate fluidisation. A high pressure drop 
is established in the fluidising gas flow across a porous membrane through 
which fluidising gas is supplied to the bed, of at least 60% of the 
pressure at which the fluidising gas is supplied beneath the membrane. 
The high pressure drop may be 85% of the pressure at which the fluidising 
gas is supplied beneath the membrane. 
Further according to the invention, the fluidised bed may be of particulate 
material of particle density at least 1.0 g/cm.sup.3 and may be of depth, 
at least 60 cm, sufficient for submersion of a hot glass sheet to be 
toughened. 
The bed is maintained throughout its whole depth in said quiescent state by 
uniform distribution of fluidising gas flow upwardly from the upper face 
of the porous membrane. 
The invention also comprehends fluidised bed apparatus including a membrane 
separating a plenum chamber to which fluidising gas is supplied, from a 
container for the fluidised bed, wherein the membrane comprises a 
perforated rigid member supporting a plurality of layers of material 
having a low permeability to gas flow such that the pressure drop in the 
fluidising gas flow across the porous membrane is at least 60% of the 
pressure at which fluidising gas is supplied to the plenum chamber. 
Preferably the layers of low permeability material are layers of paper. 
The membrane may include a protective covering laid on top of the layers of 
paper. The protective covering may be wire mesh. 
Because the pores in the paper are very fine and the paper has a low and 
uniform permeability a high pressure drop exists across the membrane. It 
is believed that this is a factor which contributes to stable operation of 
the fluidised bed in the quiescent uniformly expanded state of particulate 
fluidisation. 
Because of the high pressure drop across the membrane the layers of paper 
may be liable to bulge upwardly at the centre of the membrane. This could 
give rise to instability in operation of the fluidised bed. 
Such distortion may be prevented by the provision of stiffening means which 
engages the upper surface of the membrane and presents minimal hinderance 
to the flow of fluidising gas through the membrane. In a preferred 
arrangement the stiffening means comprises thin plate members which extend 
on edge across the upper surface of the membrane.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings the fluidised bed apparatus shown includes a 
rectangular tank 1 which is a container for a fluidised bed. A microporous 
membrane 2, which is described in greater detail with reference to FIG. 3, 
extends across the base of the tank 1. The edges of the membrane 2 are 
fixed between a flange 3 on the tank 1 and a flange 4 on a plenum chamber 
5 which forms the base of the tank 1. The flanges 3, 4 and the edges of 
the membrane 2 are bolted together by bolts 6. The membrane 2 thus 
separates the plenum chamber 5 from the container for the fluidised bed. A 
gas inlet duct 7 is connected to the plenum chamber 5 and fluidising air 
is supplied to the duct 7 at a regulated pressure. 
A preferred construction of the microporous membrane 2 is shown in FIG. 3 
and comprises a steel plate 8 having a regular distribution of holes 9. 
The margins of the plate 8 are drilled to provide passages for the bolts 
6. A gasket 10 is located between the lower face of the margins of the 
plate 8 and the flange 4 on the plenum chamber 5. 
A number of layers of strong microporous papers are laid on the plate 8. 
The membrane 2 includes a protective woven wire mesh 12 which is laid on 
top of the layers of paper 11. An upper gasket 13 is located between the 
margins of the wire mesh 12 and a spacer collar 14 located between the 
membrane 2 and the flange 3 on the tank 1. Stiffening means in the form of 
thin steel plates 15 engages the upper surface of the membrane and 
presents minimal obstruction to the flow of fluidising gas through the 
membrane. The plates 15 extend across the base of the tank 1 and are 
welded at their ends to the collar 14. The plates 15 extend on edge across 
the upper surface of the membrane, and are in edge contact with the upper 
surface of the membrane 2. In the preferred embodiment the plates 15 are 
5.0 cm in depth and 0.6 cm in thickness. 
In operation of the apparatus shown in FIGS. 1 and 2 particulate material 
16 in the tank 1 is fluidised by feeding fluidising air at a regulated 
pressure into the plenum chamber 5 through the inlet duct 7. The membrane 
2 is so constructed that fluidising air flows uniformly into the fluidised 
bed over the whole of the base of the bed to maintain the bed in a 
quiescent uniformly expanded state of particulate fluidisation. 
The particulate material 16 which constitutes the fluidised bed is an inert 
refractory material for example .gamma.-alumina of particle size in the 
range 20 .mu.m to 160 .mu.m, the mean particle size being 64 .mu.m. Other 
particulate materials which are suitable are disclosed in the 
above-mentioned patent application. 
The bed may be at least 60 cm deep. For example the size of the tank 1 
holding the fluidised bed was 38 cm .times. 215 cm .times. 100 cm deep. 
The membrane 2 was made up from 15 layers of paper 11 each layer of paper 
11 was 0.23 mm thickness and having an air permeability at normal 
temperature of 4.6 l/s/m.sup.2 at an applied pressure of 1.0 kN/m.sup.2. 
The fluidising air was fed into the plenum chamber 5 at a pressure of 24 
kN/m.sup.2. The resultant pressure drop through the member 2 was 14.4 
kN/m.sup.2 and the pressure drop through the depth of the fluidised bed 
was 9 kN/m.sup.2. The pressure drop across the membrane 2 was 60% of the 
feed air pressure in the plenum chamber 5. The fluidised bed surface was 
near the top of the tank. 
The high pressure drop which exists across the membrane 2 provides a 
uniform distribution of fluidising gas flowing upwardly from the upper 
face of the membrane 2 in the tank 1 so as to maintain the particulate 
material 16 in a quiescent uniformly expanded state of particulate 
fluidisation. By control of the plenum pressure in the plenum chamber 5, 
sensitive regulation of the velocity of upward gas flow through the 
particulate material is achieved. The particulate material is placed in 
the quiescent uniformly expanded state of particulate fluidisation through 
control of the plenum pressure such that the gas velocity through the bed 
is between that velocity corresponding to incipient fluidisation and that 
velocity corresponding to maximum expansion of the bed in which said 
quiescent state of fluidisation is maintained. 
The range of fluidising gas velocities in which such a state of 
fluidisation is obtained is narrow and any nonuniformity of fluidising gas 
flow rate into the particulate material might result in localised bubbling 
within the bed in regions where the fluidising gas velocity may exceed the 
upper limit of the range. Such localised bubbling can engender overall 
bubbling in the bed. 
Also any non-uniformity of gas flow might set up random flows of the 
particulate material within the bed. 
These forms of instability, observed hitherto, can give rise to 
difficulties when toughening glass sheets by immersion of the hot glass 
sheets in the fluidised bed. Fracture of the glass sheets can be caused or 
unacceptable changes in shape may result in the glass sheets. 
The construction of the membrane 2, as shown in FIG. 3, is such that 
although there may be localised variations in the porosity of the 
individual layers of paper 11 making up the membrane these variations are 
averaged out because the membrane comprises a plurality of layers of 
paper. The result of the high pressure drop which exists across the 
membrane and of the average uniform fine porosity of the membrane, is that 
there is a very uniform distribution of air velocity through the upper 
face of the membrane. This results in the particulate material being 
maintained in a stable quiescent uniformly expanded state of particulate 
fluidisation between the state of incipient fluidisation and the state of 
maximum expansion of the bed corresponding to the onset of bubbling in the 
bed. 
The high resistance to air flow of the membrane 2 minimises the effect of 
any transient changes which may occur in the pressure of the air fed into 
the plenum chamber 5. Such changes could otherwise result in changes in 
the velocity of fluidising air emerging through the membrane 2 into the 
fluidised bed, and could give rise to instabilities in operation of the 
bed. 
By way of comparison a fluidised bed of similar size and of the same 
particulate material has been operated using a low pressure drop membrane 
comprising a single sheet of porous plastics material. This membrane 
material had a much greater permeability to air flow than the 
corresponding permeability of the membrane used in the fluidised bed 
apparatus in accordance with the present invention. The air feed pressure 
required to establish fluidisation of the bed material in a quiescent 
uniformly expanded state of particulate fluidisation was 10 kN/m.sup.2 and 
the resultant pressure drop through the depth of the fluidised bed was 9 
kN/m.sup.2. Thus the pressure drop through the membrane was only 1 
kN/m.sup.2 which is only 10% of the air feed pressure. This bed was found 
to be very unstable in operation. Instabilities took the form of localised 
bubbling, and the setting up of random currents of material within the bed 
which could not be suppressed by reduction of the air feed pressure. When 
such instabilities occurred it was necessary to shut off the feed of 
fluidising air to the bed, thus allowing the bed to collapse and then to 
refluidise the bed. However it was found that the instabilities arose 
again shortly after fluidisation of the bed. 
It has been found, in operation of the method of the invention, that the 
higher the pressure drop which is established across the membrane 2 the 
better is the stability of fluidisation of the particulate material, up to 
a limit beyond which there is no improvement in stability. A membrane has 
been used comprising 20 layers of 0.05 mm thick paper having an air 
permeability of 0.25 l/s/m.sup.2 at normal pressure of 0.1 kN/m.sup.2. For 
fluidisation of the .gamma.-alumina material referred to above to a depth 
of 100 cms an air feed pressure of 52 kN/m.sup.2 was required. 
The resultant pressure drop through the depth of the fluidised bed was 
again 9 kN/m.sup.2, the pressure drop through the membrane being 43 
kN/m.sup.2. In this case the pressure drop through the membrane was 85% of 
the air feed pressure. The membrane can be constructed so that there is an 
even higher pressure drop than 85%, the only limitation on the percentage 
pressure drop to be used being that imposed by the resistance of the 
membrane to distortion by the pressure in the plenum chamber. 
It has also been found that as the percentage pressure drop across the 
membrane is increased the upper limit of gas velocity at which maximum 
expansion of the bed occurs before the onset of bubbling, also increases 
up to a limit. The invention provides an enhanced range of gas velocities 
within which the bed can be operated in the quiescent uniformly expanded 
state of particulate fluidisation. This also enhances the stability of 
operation of the bed. 
Some examples of operation are set out below. 
Two kinds of paper were used in constructing the membrane as follows: 
Paper A 
Thickness = 0.23 mm 
Air Permeability = 0.54 l/s/m.sup.2 at 0.1 kN/m.sup.2. 
Paper B 
Thickness = 0.05 mm 
Air Permeability = 0.25 l/s/m.sup.2 at 0.1 kN/m.sup.2. 
Working with .gamma.-alumina of particle density 2.2 g/cm.sup.3, particle 
size range 20 .mu.m to 160 .mu.m, and mean particle size 64 .mu.m, three 
experiments were conducted as follows: 
Table I 
______________________________________ 
Membrane Plenum Pressure drop 
Number of Pressure across Membrane 
Depth of bed 
Paper Layers KN/m.sup.2 
KN/m.sup.2 
% cm 
______________________________________ 
A 15 16.4 11.4 69.5 60 
B 10 35.1 26.8 76 100 
B 20 50.3 37.5 74 150 
______________________________________ 
Similar experiments with a porous powdered aluminosilicate material, each 
particle containing 13% by weight alumina and 86% silica, with particle 
size range up to 150 .mu.m, mean particle size 60 .mu.m and particle 
density 1.22 g/cm.sup.3, gave the following results 
Table II 
______________________________________ 
Membrane Plenum Pressure drop 
Number of Pressure across Membrane 
Depth of bed 
Paper Layers kN/m.sup.2 
kN/m.sup.2 
% cm 
______________________________________ 
A 15 8.65 6.0 69.5 60 
B 10 18.4 14.0 76 100 
B 20 29.1 22.3 74 150 
______________________________________ 
Further experiments were carried out with non-porous .alpha.-alumina of 
mean particle size 29 .mu.m and particle density 3.97 g/cm.sup.3. The 
results were as follows 
Table III 
______________________________________ 
Membrane Plenum Pressure drop 
Number of Pressure across Membrane 
Depth of bed 
Paper Layers kN/m.sup.2 
kN/m.sup.2 
% cm 
______________________________________ 
A 20 20.9 12.9 61.5 60 
B 10 38.5 25.2 65 100 
B 20 56.0 35.6 63 150 
______________________________________ 
It was found that the percentage pressure drop across the membrane is 
related to the toughening stresses induced in a glass sheet quenched in 
the fluidised bed. The higher the pressure drop, up to a permissible 
limit, the nearer does the state of the bed become to a state of maximum 
expansion at which the quiescent state of fluidisation is maintained. At 
maximum expansion the bed is of low viscosity so that the hot glass sheets 
can enter the bed easily with minimum effect on the bent or flat shape of 
the sheet. The nearer the state of the bed approaches to maximum 
expansion, the higher the central tensile stress in the glass as 
illustrated by the following Table IV which gives the result of 
experiments using the same .gamma.-alumina material as was used when 
carrying out the experiments of Table I. Sheets of glass 3 mm thick were 
heated to 660.degree. C. and lowered into the bed which was 60 cm deep and 
was at ambient temperature or just above. 
Table IV 
______________________________________ 
Pressure drop 
across Paper B Bed 
Membrane Number of Expansion Central Tensile Stress 
% Layers % MN/m.sup.2 
______________________________________ 
69 5 15 41 
82 10 18 44 
88 20 49 
______________________________________ 
In general the pressure drop of at least 60% across the membrane 2 makes 
possible the thermal treatment of glass articles, in particular the 
toughening of glass sheets for vehicle windscreens, in a fluidised bed at 
least 60 cm deep, for example of depth in the range 60 cm to 150 cm, of 
particulate material having a particle density of at least 1.0 g/cm.sup.3, 
for example in the range 1.0 g/cm.sup.3 to 4.0 g/cm.sup.3, which bed is in 
a quiescent uniformly expanded state of particulate fluidisation.