Elastic scraper for high-capacity film device

Elastic scraper for high-capacity continuous molecular evaporation and reactors with a wiped-off film, the scraper being in the form of a plurality of wiping segments mounted upon a wiping device having a plurality of circumferentially spaced parallel supporting rods which rotate with respect to the surface of the apparatus from which the film is to be wiped. The wiping segments are held from above by an elastic spring which connected them to their respective supporting rods. The supporting rods may be floatingly mounted, as by connecting segments which are mutually joined whereby to provide the supporting rods and the wiping segments supported thereby with limited concumferential movement with respect to the surface to be wiped.

This invention relates to an elastic scraper for a continuous molecular 
evaporator and a reactor with wiped off film, such evaporator or reactor 
having a convex evaporating surface formed on the outer cylindrical 
surface or frusto-conical surface having a large diameter, e.g. over 200 
mm. 
In the devices known up to now having a wiped off film with a convex 
evaporating surface, the scraper is constructed either in the form of a 
continuous helix or in the form of helical segments. A scraper in the form 
of a continuous helix, according to Czechoslovak Pat. No. 90,917, and 
German Pat. No. 1,448,166, is technically unrealizable in large production 
devices. Production of a helix with a large diameter from a wire of 
approximately 10 mm in diameter encounters production problems; the helix 
undergoes deformation changes especially as a result of thermal expansion, 
and the helix is not able to accomodate itself to smaller deviations of 
the heating body from a circular section. Under these circumstances, 
wiping of the whole heating surface cannot be ensured. 
Further, a scraper in the form of a continuous helix closely ground in to 
the surface of the heating body does not permit the realization of a 
slowed down film when wiping off against the direction of natural film 
flow, the direction of rotation of a right-hand helix being to the right 
and the direction of rotation of a left-hand helix being to the right, or 
the scraper by its effect always accelerates the liquid film and helps it 
flow from a heating surface. A short dwell time of the film on the heating 
surface, a decreased intensity of film mixing, and poor heat and matter 
transfer result from such solution. 
Scrapers are disclosed in Czechoslovak patent applications Nos. 114,714 and 
181,054 in the form of a discontinuous metal helix, or in the form of 
helix segments placed in "Teflon" tubes, as a rule with the direction of 
rotation being opposite the direction of natural film flow, such scrapers 
ensuring the wiping off while keeping an optimum liquid dwell on the 
heating surface. The segments of the discontinuous helix with such scraper 
types are held on supporting rods which are placed in rotating rings; the 
segments together with the rods form a relatively rigid system rotating 
around the heating body. Relatively good concentricity of the heating body 
is required for this reason in order to ensure the satisfactory 
functioning of the scraper. With a large diameter of the heating body, 
e.g., around 1000 mm in high-capacity units, in which concentricity along 
the whole heating surface can hardly be guaranteed, zones with imperfect 
wiping off can occur on the heating surface as a result of the distortion 
(ovality) of the heating body. 
In addition, with higher liquid loading of the heating body periphery, a 
partial overflow of liquid onto the supporting rods takes place at the 
places where the wiping segments are connected to the supporting rods, 
such liquid having accumulated before the wiping off of the segments. A 
part of the fed liquid is thus wrenched out of the thermal treatment in 
the film on the heating surface; this manifests itself disadvantageously 
in the decreased capacity of the film evaporator, in lessened separation 
efficiency, and, in the case of a film reactor, in lower conversion and 
worse selectivity. 
The drawbacks of the prior art discussed above are overcome by the scraper 
according to the present invention, in which the wiping segments are held 
on the supporting rods by elastic couplings which include, for example, 
connecting springs. 
The scraper according to the invention has several advantages when compared 
with prior scrapers. The elastic clamping of wiping segments allows their 
accomodation to smaller deviations of the heating body from an accurate 
circular section, and ensures defined wiping over the whole heated 
surface. This effect is made even more pronounced by the elastic joining 
of single supporting rods. Liquid overflow over a segment onto its 
supporting rod is eliminated by holding the wiping segment by its upper 
half, the elastic coupling being directed upwardly. The provision of an 
elastic holding of a wiping segment allows a small distance, e.g. 30-40 mm 
to be maintained between the evaporating and condensing surface with a 
sufficient mechanical resistance; this is important in the case of 
molecular evaporators.

Turning first to FIG. 1, in such figure the righthand surface 1 of a 
heating means 1a is shown being wiped by a wiping segment 3. The outer 
portion 3a of wiping segment 3 is advantageously covered with a Teflon 
tube 3a which closely engages the heating surface 1. Segment 3 is held 
from above by an elastic coupling 4 which is fixedly secured at its upper 
end to a vertical supporting rod 2 and at its lower end to the inner body 
of the wiping segment 3. The body of the wiping 3, and the rod 2, may be 
made of metal, the elastic coupling 4 may be made of metal bands, and the 
opposite ends of the coupling 4 may be secured by welding to the rod 2 and 
the body of the wiping segment 3. 
The longitudinal axis of the wiping segment 3 is disposed at an angle of 
from 10.degree.-45.degree. with respect to the longitudinal axis of the 
supporting rod 2. This relationship is shown in FIG. 4, wherein another 
embodiment of the invention is illustrated. Also, as shown in FIG. 4, a 
number of wiping segments in the embodiment of FIG. 1 are mounted in 
spaced parallel relationship on a single supporting rod 2. The wiping 
segments on each two adjoining supporting rods are mutually conveniently 
shifted longitudinally of their respective rods. Also, as shown in FIG. 4, 
to ensure the effective wiping of the full surface 1. In FIG. 1 the 
elastic coupling 4 is advantageously made of bent steel strips. 
A second embodiment of wiper is shown in FIG. 2, wherein parts which are 
smaller to those shown in FIG. 1 are designated by the same reference 
characters. In the embodiment of FIG. 2, the elastic coupling 4 of FIG. 1 
is replaced by a cylinder and piston arrangement. A cylinder 7, which 
inclines downwardly from supporting rod 2 toward the heating surface 1, is 
rigidly connected at its upper end to rod 2. A piston 8 having a diameter 
marginally less than that of the inner diameter of the cylinder is loosely 
guided thereby and is held from rotation with respect to the cylinder by a 
radially direct pin 10 affixed to the piston, the pin being slidingly 
received within a longitudinal slot 9 in the upper wall of the piston. The 
wiping segment 3 is constantly resiliently urged into engagement with the 
surface 1 by a coil compression spring 11, which abuts the closed upper 
end of the cylinder 7 on the one end and the inner end of the piston 8 on 
the other. It will be seen that the cylinder 7, the piston 8, the guiding 
means therebetween, and the spring 11 constitute an elastic coupling, 
generaly designated 4'. This, like the elastic coupling 4 in FIG. 1, 
constantly resiliently presses the wiping segment 3 against the surface 1. 
In the embodiment of the device shown in FIGS. 3 and 4, each of the spaced 
parallel vertical supporting rods 2 has affixed thereto a connected 
segment 5 at each one of a plurality of locations (two shown) 
longitudinally of the rods 2. The connecting segments 5 in each set 
thereof are disposed in alignment perpendicular to the longitudinal axis 
of the rods 2 with the confronting ends of the connecting segments 5 being 
spaced from each other and connected by coiled compression springs 6. As 
shown in FIG. 3, the connecting segments 5 are disposed radially outwardly 
of the wiping segments 3, and are disposed coaxial of segments 3. 
It will thus be seen that in the embodiment of FIGS. 3 and 4 the supporting 
rods 2 are floatingly mounted and are capable of limited movement in a 
direction which is horizontal in FIG. 4 but is actually in a 
circumferential direction in FIG. 3. Such freedom of the supporting rods 
2, taken with the flexibility of mounting of the wiping segments 3 
afforded by the elastic coupling 4 of FIG. 1 or the elastic coupling 4' of 
FIG. 2 permits the wiping segments 3 faithfully to follow the surface 1 as 
the supporting rods 2 and the wiping segments carried thereby rotate with 
respect to such surface 1. It is to be understood that the means for 
driving the rods 2, the connecting segments 5, and the wiping segments 3 
about the axis of the heating member 1a is such as to permit the described 
freedom of movement of the rods 2 with respect to each other which is 
afforded by the resilient connection 6 between successive connecting 
segments 5. 
The invention also relates to a scraper with swinging wiping segments for 
short-path evaporator and a film reactor with wiped off film with convex 
evaporation surface formed at the outer evaporation cylinder surface. 
The scraper in short-path evaporator ensures at its rotary motion uniform 
distribution of distilled liquid at the whole evaporation surface and 
intensive mixing of the liquid. According to the Czechoslovak Patent 
Specification Nos. 114 714 and 181 054 a scraper for short-path evaporator 
is known having convex evaporation surface in the form of discontinuous 
metallic helix or in the form of helix segments placed in Teflon tubes, 
single wiping segments being held directly at supporting rods what forms 
relatively rigid system rotating around heating body. Regular function of 
scraper is however achieved only at relatively good roundness of the 
heating cylinder. In the case of large diameters of heating cylinder 
around 1000 mm in high-capacity units the required roundness along the 
whose evaporation surface can be hardly guaranteed. Besides it at higher 
liquid loading of heating cylinder periphery partial overflow of liquid 
onto supporting rod takes place at the place of holding of wiping segment 
to supporting rod. Wiping segment of the scrapers with segment holding at 
supporting rod from the above through an elastic coupling has, according 
to the Czechoslovak Author's Certificate No. PV 721-80, free motion in the 
plane perpendicular to the cylinder axis, free motion around the axis of 
supporting rod is however impossible. This fact can be the cause of 
non-uniform pressing of the wiping segment and of its abrasion. The 
exchange of worn out or damaged wiping segment is laborious. 
These drawbacks are eliminated with the scraper with swinging wiping 
segment for short-path evaporator and film reactor shown in FIG. 5. Such 
scraper is formed by supporting rods located around the perimeter of a 
heating cylinder with a system of wiping segments and elastic couplings 
according to the invention, the distinguishing feature of which is that 
every wiping segment of the scraper, held in a retainer such as a trough, 
is held by an elastic coupling upon a guiding tube which is movably 
supported upon the supporting rod. 
The scraper according to the invention has several advantages as related to 
the preceding types. Above all it keeps the advantageous properties which 
are connected with the holding of every wiping segment by an elastic 
coupling which enables its adaptation to smaller deviations of heating 
cylinder from accurate round section by what defined wiping at the whole 
evaporation surface is achieved. A new degree of freedom is achieved by 
holding wiping segment through an elastic coupling at guiding tube freely 
rotating upon supporting rod, whereby natural fitting of wiping segment to 
the heating surface is achieved without stress and non-uniform pressing of 
the segment and its abrasion is eliminated. Any necessary exchange of the 
wiping segment is simple. A system of condensation surfaces for 
micro-overflow capture according to the Czechoslovak Patent Specification 
No. 114 756 can be held at the scraper with swinging wiping segments 
according to the invention. 
Turning now to FIG. 5, a wiping segment 5 tightly held in a trough 6 is 
held through an elastic coupling 3 upon a guiding rube 4 freely rotatably 
upon a supporting rod 2. A whole series of guiding tubes 4 with elastic 
couplings made for example of springy metal, troughs 6 and wiping segments 
5, as in the embodiments of FIGS. 1-4, incl., is thus put on one 
supporting rod 2. The whole scraper for a short-path evaporator is formed 
by several supporting rods 2 uniformly angularly spaced about the 
perimeter of a heating cylinder. Wiping segment 5 is as a rule made of 
PTEP and it tightly conforms to the heating surface 1. As a rule it has a 
shape of an L-section rod with one edge cut so as to ensure an areal 
contact with the heating surface 1 on the one hand, further to provide the 
possibility of forming a reservoir of liquid in front of the wiping 
segment 5, and finally to provide its advantageous location in the 
profiled trough 6. The trough 6 is metallic as a rule, it has the shape of 
a segment of a helix and it increases the mechanical strength of the 
wiping segment 5. Wiping segment 5 has several cuts for liquid outflow at 
the side of its contact with the heating surface 1. The trough 6 together 
with wiping segment 5 is tipped through an angle 10.degree. to 45.degree. 
with respect to the axis of the supporting rod 2 and guiding tube 4. 
Although the invention is illustrated and described with reference to a 
plurality of embodiments thereof, it is to be expressly understood that it 
is in no way limited to the disclosure of such preferred embodiments but 
is capable of numerous modifications within the scope of the appended 
claims.