An evaporator comprises a plurality of substantially parallel plates mounted for rotation about a common axis and in which provision is made for a condensible vapor to flow across a first face of each of said plates and for a liquid, at least a portion of which is to be evaporated, to flow across the second face of each of said plate, which plates are arranged to be rotatable at such a mean acceleration, measured in a radial direction with respect to said axis of rotation, greater than the acceleration due to gravity, said first face of each of said plates has a surface designed to discourage formation of a continuous liquid film thereon. The evaporator may be used to concentrate solutions, e.g. of depleted brine or aqueous caustic soda.

The present invention is an improved evaporator, more specifically a 
centrifugal apparatus for this purpose. 
Within manufacturing industry it is frequently desired to convert a liquid 
into vapor form. For example a liquid feedstock may be required to enter a 
reaction in the vapor phase; or liquid components of a mixture may be 
separated or purified by distillation; or a solution may be concentrated 
by removing a proportion of the solvent by evaporation therefrom. For 
these diverse purposes, a range of equipment has been developed over many 
years. Most of such equipment is of a stationary type but more recently, 
for some applications, rotary evaporators of different types have been 
proposed or used. 
Thus an article by K C D Hickman et al in "Advances in Chemistry Series" 
No. 27, pages 128-46 (1960) described the so-called "Hickman Still", which 
was designed and operated to study its possible use for the distillation 
of brackish and saline waters. The inferior yield achieved by that 
apparatus as compared with that theoretically achievable was attributed by 
the authors to a number of possible factors. 
Another rotary device is the so-called CentriTherm ultra-short-time 
evaporator, which has been offered by the firm Alfa-Laval for the 
treatment of heat-sensitive liquids. In that evaporator, the heat transfer 
is from steam across the thickness of conical discs in a nested stack. 
Our object in devising the present invention has been to produce a rotary 
evaporator suitable for a wide range of applications and having a high 
rate of heat transfer. 
According to the present invention, there is provided an evaporator which 
comprises a plurality of substantially parallel plates mounted for 
rotation about a common axis and in which provision is made for a 
condensible vapour to flow across a first face of each of said plates and 
for a liquid, at least a portion of which is to be evaporated, to flow 
across the second face of each of said plates, which plates are arranged 
to be rotatable at such a rotary speed as to subject any liquid thereon to 
a mean acceleration, measured in a radial direction with respect to said 
axis of rotation, greater than the acceleration due to gravity, said first 
face of each of said plates has a surface designed to discourage the 
formation of a continuous liquid film thereon. 
Mean acceleration is defined by the equation: 
##EQU1## 
where N is the rotational speed of the plates about the said axis in 
revolutions per minute, and r.sub.1 is the distance in meters from the 
axis of rotation to the radially outer portion of the plates. 
The evaporator of our invention may be used in general in any situation 
where it is desired to convert a liquid into its vapour. Thus it may be 
used, for example, as a rotary still for purifying a liquid product, for 
example saline or brackish water. The evaporator is of particular value 
for concentrating a solution by removing a portion of the solvent. Such 
solution may be a liquid dissolved in a liquid solvent, provided that the 
solvent is more volatile than the solute. More usually, however, the 
solution will be of a solid solute. Thus in one particular application, 
the evaporator of the present invention may be used for concentrating 
brine, for example so-called "depleted" brine from electrolytic cells used 
in the production of chlorine; in another, it may be employed to 
concentrate aqueous caustic soda solutions from membrane cells. 
The condensible vapour which flows across the first face of each of the 
plates of the evaporator and by means of which heat is introduced to the 
system may with, advantage be steam. Alternatively it may be the vapor 
form of one of the commercially available or appropriate compounds or 
mixtures of compounds specifically offered as heat transfer fluids. Thus 
it may be one of the refrigerants of the chlorofluorohydrocarbon range, 
especially the C.sub.1 to C.sub.2 compounds from that range. These include 
Refrigerant 22, which is chlorodifluoromethane, Refrigerant 12 
(dichlorodifluoromethane), Refrigerant 115 (chloropentafluoroethane) and 
Refrigerant 114 (dichlorotetrafluoroethane) and those refrigerants which 
are azeotropic mixtures of such compounds. 
As already indicated, the first face of the plates which are a feature of 
the evaporator according to the present invention, that is the face over 
which the condensible vapor flows, has a surface designed to discourage 
the formation of a continuous liquid film thereon. Preferably the first 
face of the plates is treated such that (a) condensation of the 
condensible vapour thereon occurs in a dropwise fashion and (b) its 
wettability is reduced such that formation of any continuous, stable 
liquid film is discouraged. Such treatments include provision of a coating 
of inter alia a suitable silicone or polytetrafluoroethylene on the 
surface. 
Alternatively, in order to disrupt any liquid film which otherwise would be 
formed, the surface of said first face of the plates may have protrusions 
from said surface, indentations in said surface or the plate may be 
corrugated. These features designed to disrupt the film, where they are 
present, are preferably disposed generally transversely to the radial flow 
of any liquid across the plate surface. More preferably, they are disposed 
in one or more circles which are concentric with the axis of rotation of 
the plates or in a continuous spiral configuration about said axis. Thus 
in one form, where the surface features are one or more channels in the 
plate surface, it is preferred that they are continuous channels disposed 
concentrically about said axis of rotation or a continuous spiral channel 
about that axis as centre. 
When these features, for example corrugations, protrusions or indentations, 
are disposed in concentric circles or a spiral configuration, the circles 
or successive laps of the spiral are preferably spaced at a density, 
measured in a radial direction, of between 50 and 1,000 per meter, 
preferably more than about 100 per meter. Thus the pitch of the pattern of 
these surface features, that is the distance between repeated features of 
the pattern, is preferably between 1 mm and 20 mms, more preferably less 
than about 10 mms. When the surface features are channels, the depth of 
each channel is preferably between 0.05 and 5 mms, especially between 0.2 
and 5 mms and more especially between 0.5 and 2.5 mms. Very shallow 
channels, for example of the order of 0.05 to 0.25 mm, may if desired be 
formed by etching the plate surface. 
We do not exclude the possibility that in addition to modifying the profile 
of the plate surface, the plate surface coming into contact with the 
condensible vapor may be treated to reduce its wettability. 
The second face of the plates, that is the face over which flows the liquid 
of which at least a proportion is to be evaporated, may advantageously be 
treated so as to assist (i.e. enhance) the retention of a continuous film 
of liquid thereon. Such treatment, which may be chemical, e.g. etching, or 
physical, e.g. sand-blasting, will in general be aimed at giving the 
surface an overall fine roughness. 
The thickness of the plates employed in the device according to the present 
invention is generally between 0.5 and 5 mms, depending upon the material 
of construction, the specific evaporation to be carried out and the form 
of surface features chosen. While the thickness of the plate may vary--and 
obviously will vary with some forms of surface features--in general when 
referring to plate thickness we refer to the plate thickness as it would 
be without those features. The plate thickness is preferably between 0.25 
and 1.5 mms, especially between 0.5 and 1.0 mm. 
The outer diameter of the plates used in the evaporator of the present 
invention is typically in the range 10 cm to 5 meters and is preferably 
between about 50 cm and 100 cm and where the plates are in the form of an 
annulus the inner diameter thereof is typically in the range 5 cm to 1 
meter. 
The plurality of plates in the evaporator of the present invention are 
mounted substantially parallel to each other along the common axis about 
which they are able to rotate and are closely adjacent to one another to 
form narrow passages. Preferably the mean axial depth of the passages 
between adjacent plates is less than about 50 mms and more preferably is 
between 0.25 mm and 5 mm. Where the axial depth of a passage varies along 
the radial length thereof, for example both of the two opposing surfaces 
which define the passage have peaks and troughs, the troughs of the first 
of the said surface being aligned with the troughs of the second of the 
said surfaces and the peaks of the first surface being aligned with the 
peaks of the second surface, the narrowest gap is often about 2 mm and the 
largest gap is often about 8 mm. 
It is indeed often preferred that the surfaces of adjacent plates, where 
those surfaces are both contoured, are so aligned that protrusions on one 
surface are aligned with protrusions on the other. In this way, a fresh 
spray of the liquid formed by condensation of the condensible vapor can be 
continuously formed since as the liquid flows through the passageway 
between two plates, the opposed surfaces of which have a multiplicity of 
contours, it is ejected from a protruding contour on one plate and 
deposited on a contour on the opposed surface of the adjacent plate, from 
a protruding contour of which it is rapidly ejected. This ejection 
alternates between the protruding contours of the two opposed surfaces as 
the liquid flows along the passageway therebetween and in this way the 
transfer of heat via the plates to the liquid to be vaporised is enhanced. 
In a useful modification of this last-described arrangement, linear 
protrusions on one surface are arranged to cross similar protrusions on 
the other surface at a shallow angle and the protrusions on these adjacent 
surfaces are in contact with each other at the points of crossing. In this 
way, an element of additional support is imparted to the structure as a 
whole and as a result it is possible to use thinner plates, thereby 
further improving the transfer of heat across each plate. 
In general, when a plate bearing liquid upon its surface is rotated, the 
centrifugal effect tends to move that liquid in a direction generally away 
from the axis of rotation. Thus the liquid to be evaporated in the 
presently-described evaporator is conveniently fed to the plates at a 
point adjacent to their axis of rotation, for example to the centre of the 
plates when the latter are annular. The vapor generated may be withdrawn 
at a point adjacent to the radially inner edge of the plates while any 
liquid remaining unvaporised, for example a more concentrated solution 
than that fed to the evaporator, may be withdrawn from a point or points 
adjacent to the outer edge of the plates. 
The condensible vapor, on the other hand, is conveniently fed to the outer 
edge of the plates so that, as it passes under pressure in a direction 
generally towards the axis of rotation, it flows counter-current to the 
liquid formed as it condenses, which latter may be collected at the outer 
edge of the plates. 
The material of which the plates are constructed should have good thermal 
conductivity and should be such that each plate can withstand the stress 
generated during operation of the evaporator. Preferably also the material 
is substantially resistant to attack by or reaction with the materials 
with which it comes into contact in use. With these considerations in mind 
suitable materials include steel, aluminium, copper, nickel and titanium. 
The plates, in operation, are rotated at speed as to subject any liquid 
thereon to a mean acceleration, measured in a radial direction with 
respect to the axis of rotation, greater than the acceleration due to 
gravity, `g`. The particular value selected depends upon such 
considerations as the size of the plates, the heat flow therethrough and 
the desired capacity of the evaporator in terms both of total liquid 
throughput and of quantity of liquid to be evaporated. In general, the 
acceleration may lie within the range from 5 to 1000 g, especially from 50 
to 750 g and more preferably from 100 to 600 g.

Referring firstly to FIGS. 1 to 6 of the drawings, a rotor 1, mounted on a 
shaft 2 by means of which it is rotated in a housing 3, is formed from a 
plurality of annular plates 4 mounted on a cylindrical member 5. The 
member 5 is provided with ports 6 and the annular plates 4 are formed with 
a plurality of orifices 7, e.g. each plate 4 has six orifices disposed 
uniformly adjacent the outer perimeter thereof. Alternate spaces between 
the annular plates 4 are sealed at their outer perimeter by rims 8 to form 
sealed spaces 9, which are in fluid flow connection with the ports 6, and 
open spaces 10. 
Circular washers 11 and C-washers 12 are mounted in orifices 7 to form a 
set of, for example six, manifolds which join the spaces 9. A liquid feed 
pipe 13, provided at its lower end with orifices 14, projects into the 
center of the rotor and is, in part, surrounded by a vapor discharge pipe 
15. The housing 3 is provided with ports 16 and 17 for the entry and exit 
respectively of a condensible vapor. Gas-tight seals 18 allow rotation of 
the rotor 1 and shaft 2 within the housing 3. A stationary scoop 19, 
leading to a liquid discharge pipe 20 is disposed between the top two 
plates 4. 
In operation, the rotor is rotated, a liquid to be vaporised, for example 
an aqueous solution of caustic soda, is fed via feed-pipe 13 and orifices 
14 to impinge on the inner surface of the member 5 and moves outwards 
through the ports 6 into the spaces 9 where it flows under the centrifugal 
effect arising from rotation of the rotor, across one surface of each of 
the plates 4. A condensible vapour is fed into the housing through port 16 
and enters the spaces 10. The vapor loses a portion of its heat, condenses 
to form a film which moves rapidly outwards under centrifugal force 
through the open spaces 10 as a thin film on one surface of each of the 
plates 4, and is then removed via the port 17. The heat lost by the 
condensible vapor vaporises a portion of the liquid in spaces 9 and the 
vapor so produced is discharged from the apparatus via the ports 6 and 
pipe 15. The concentrated solution in the spaces 9 is discharged via 
orifices 7 and is forced, by the pressure generated by the dynamic head of 
the liquid, through the stationary scoop 19 and discharge tube 20. 
The plate 21 illustrated in FIG. 7 is one example of a plate suitable for 
use in the above described evaporator. The surface 22 of plate 21 is the 
surface upon which the condensible vapor condenses and in the illustrated 
embodiment it is flat but has a very thin film of polytetrafluoroethylene 
thereon. Liquid condensed on surface 22 rapidly gathers into drops, which 
are quickly flung from the surface by centrifugal force. The opposite 
surface 23 of plate 21 has a close-packed succession of channels, which 
may be in the form of concentric circles or successive turns of a spiral 
i.e. surface discontinuities disposed transversally to radial flow of 
liquid across the surface 23. Each of the channels has a more gently 
sloped side 23a nearer to the axis of rotation and a more steeply sloped 
side 23b nearer to the outer edge of the plate 22. The whole of surface 23 
is roughened on a fine scale by sand-blasting. On rotation of plate 21, 
the liquid to be vaporised flows in an outward direction over surface 23 
(that is, left to right as illustrated). A continuous film of liquid is 
maintained but within that film, a good mixing is achieved at the point 
where liquid on slope 23a encounters the foot of slope 23b--as indicated 
by the curved arrows in FIG. 7. The peak of each slope 23b, that is where 
liquid climbing that slope encounters the start of slope 23a, is rounded 
to assist retention of the film on the surface. 
The plate 24 illustrated in FIG. 8 has been shaped by stamping and is 
corrugated. The profile of the corrugations (represented by alternating 
series of protrusions 26a and indentations 26b) is such that the surface 
26 over which flows the liquid to be evaporated is similar to the surface 
23 of FIG. 7, that is it is shaped to maintain a continuous liquid film, 
while encouraging (i.e. enhancing) mixing within the film as indicated. 
The surface 26 has been sand-blasted. The other surface 25 of plate 24 has 
the complementary profile to surface 26 and has been coated with 
polytetrafluoroethylene. This combination of profile and coating ensures 
that any film of liquid on surface 25 is quickly disrupted and that the 
liquid is rapidly thrown from the plate.