U.S. patent application number 09/816096 was filed with the patent office on 2001-11-08 for use of the lotus effect in process engineering.
This patent application is currently assigned to BASF Aktiengesellschaft. Invention is credited to Domschke, Thomas, Oost, Carsten.
Application Number | 20010037876 09/816096 |
Document ID | / |
Family ID | 7636993 |
Filed Date | 2001-11-08 |
United States Patent
Application |
20010037876 |
Kind Code |
A1 |
Oost, Carsten ; et
al. |
November 8, 2001 |
Use of the lotus effect in process engineering
Abstract
The invention provides an evaporator having a heatable heat
exchange surface, which has a self-cleaning microstructured surface
with elevations and depressions. The microstructured surface of the
heat exchange surface may be produced by powder coating. The
invention additionally provides a process for the evaporative
concentration of solutions in an evaporator having a self-cleaning
microstructured heat exchange surface, the solution for evaporative
concentration preferably comprising viscous or solid constituents
in solution, emulsion or suspension. Using the evaporators of the
invention, caking of the viscous or solid constituents on the heat
exchange surfaces during the evaporative concentration of the
solutions is prevented. As a result, such solutions may be
concentrated to a solvent content of <5% by weight.
Inventors: |
Oost, Carsten; (Bad
Duerkheim, DE) ; Domschke, Thomas; (Speyer,
DE) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Assignee: |
BASF Aktiengesellschaft
Ludwighshafen
DE
67056
|
Family ID: |
7636993 |
Appl. No.: |
09/816096 |
Filed: |
March 26, 2001 |
Current U.S.
Class: |
165/133 ;
165/95 |
Current CPC
Class: |
B08B 17/06 20130101;
B01D 1/30 20130101; B08B 17/02 20130101; F28F 19/00 20130101; F28F
2245/04 20130101; F28F 2245/02 20130101; B08B 17/065 20130101; F28F
13/187 20130101 |
Class at
Publication: |
165/133 ;
165/95 |
International
Class: |
F28G 001/00; F28F
013/18; F28F 019/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 30, 2000 |
DE |
100 15 855.2 |
Claims
We claim:
1. An apparatus or part thereof for chemical plant construction,
having a self-cleaning microstructured surface with elevations and
depressions.
2. An apparatus or part thereof as claimed in claim 1, selected
from the group consisting of internal apparatus, vessel and reactor
walls, discharge devices, fittings, circuit systems, evaporators,
filters, centrifuges, columns, dryers, internals, packings, and
mixing elements.
3. An apparatus as claimed in claim 2 in the form of an evaporator
having a heatable heat exchange surface, wherein the heat exchange
surface has a self-cleaning microstructured surface with elevations
and depressions.
4. An evaporator as claimed in claim 3 in the form of a forced
circulation, flash or coiled tube evaporator.
5. An evaporator as claimed in claim 3 in the form of an evaporator
tube in which an annular flow is able to develop.
6. An evaporator as claimed in claim 3, wherein the microstructured
surface of the heat exchange surface is produced by powder
coating.
7. A process for the evaporative concentration of solutions in an
evaporator having a self-cleaning microstructured heat exchange
surface as defined in claim 3.
8. A process as claimed in claim 7, wherein the solution for
evaporative concentration comprises viscous or solid constituents
in solution, emulsion or suspension.
9. A process as claimed in claim 7, wherein the solution for
evaporative concentration is an aqueous or aqueous-organic salt
solution.
10. A process as claimed in claim 8, wherein evaporative
concentration is conducted to a solvent content of <5% by
weight.
Description
[0001] The invention relates to evaporators having a
microstructured surface and to processes for the evaporative
concentration of solutions.
[0002] In process engineering, mixtures of substances including
viscous or solid constituents are frequently separated using an
evaporator. Examples are the concentration of polymer solutions,
the concentration of solutions laden with salts or resins, the
workup of column bottom products, contaminated substances or
sludges, and, in the food segment, the separation of solvents down
into the ppm range.
[0003] A problem which arises with these mixtures of substances is
that their highly viscous evaporation residues cake on the heat
exchange surfaces, thereby increasing the likelihood of clogging in
the evaporator. Moreover, the deposits and incrustations which form
drastically reduce the heat transfer coefficient.
[0004] In order to effect virtually complete evaporative
concentration of suspensions or solutions which are viscous and
tend to stick, it has hitherto been necessary to ensure, by
applying mechanical force, that the heat exchange surfaces are kept
free from attachments and incrustations. This is normally achieved
by installing rotating wipers or scrapers into the evaporators,
which keep the heat exchange surfaces clear. Examples of evaporator
designs of this kind are the evaporators with anchor stirrer and
evaporators with paddle stirrer shown on page 613 of Klaus Sattler,
Thermische Trennverfahren [Thermal Separation Processes], Wiley-VCH
Weinheim, Berlin, New York, 2nd Edition 1995. To date, where the
solution to be concentrated by evaporation was to attain a
pastelike or even solid state, the only suitable machines have been
screw machines. However, these are very expensive and their heat
transfer coefficient is poor.
[0005] It is an object of the present invention to provide
alternatives to the prior art evaporators for the evaporative
concentration of solutions comprising viscous or solid constituents
with a tendency to stick. It is a particular object of the present
invention to provide a heat exchanger which is inexpensive to
produce and in which the heat exchange surfaces are kept clear from
attachments and incrustations without applying mechanical
force.
[0006] We have found that this object is achieved by means of
apparatus and apparatus parts for chemical plant construction which
have a self-cleaning microstructured surface with elevations and
depressions.
[0007] Preferred apparatus and apparatus parts which may be
furnished with a self-cleaning microstructured surface are internal
apparatus, vessel and reactor walls, discharge devices, fittings,
circuit systems, evaporators, filters, centrifuges, columns,
dryers, internals, packings, and mixing elements.
[0008] Particularly preferred apparatus of the invention comprises
evaporators having a heatable heat exchange surface, wherein the
heat exchange surface has a self-cleaning microstructured surface
with elevations and depressions.
[0009] By apparatus and apparatus parts are meant:
[0010] Internal apparatus, vessel and reactor walls, discharge
devices, fittings, circuit systems, evaporators, pumps, filters,
compressors, centrifuges, columns, dryers, size reduction machines,
internals, packings, and mixing elements, preferably internal
apparatus, vessel and reactor walls, discharge devices, fittings,
circuit systems, evaporators, filters, centrifuges, columns,
dryers, internals, packings, and mixing elements, and, with very
particular preference, evaporators.
[0011] More specifically, said apparatus and apparatus parts may be
described as follows:
[0012] Vessels comprise, for example, reservoir or collecting
vessels such as troughs, silos, tanks, vats, drums or gasometers,
for example.
[0013] Apparatus and reactors comprise liquid, gas/liquid,
liquid/liquid, solid/liquid or gas/solid reactors, and gas
reactors, which are embodied, for example, in stirred, jet loop and
jet nozzle reactors, jet pumps, delay time cells, static mixers,
stirred columns, tube reactors, cylinder stirrers, bubble columns,
jet scrubbers and venturi scrubbers, fixed-bed reactors, reaction
columns, evaporators, rotary disk reactors, extraction columns,
kneading and mixing reactors and extruders, mills, belt reactors,
rotary tubes, or circulating fluidized beds.
[0014] Discharge devices comprise, for example, discharge ports,
discharge hoppers, discharge pipes, valves, discharge cocks or
ejector devices.
[0015] Fittings comprise, for example, stopcocks, valves, slide
valves, bursting disks, nonreturn valves, or disks.
[0016] Pumps comprise, for example, centrifugal, toothed wheel,
screw spindle, eccentric screw, rotary piston, reciprocating
piston, membrane, screw trough or jet liquid pumps, and also
reciprocating piston, reciprocating piston membrane, rotary piston,
rotary slide valve, liquid seal, lobe or pump-fluid vacuum
pumps.
[0017] Filter apparatus comprises, for example, fluid filters,
fixed-bed filters, gas filters, sieves or separators.
[0018] Compressors comprise, for example, reciprocating piston,
reciprocating piston membrane, rotary piston, rotary slide valve,
liquid seal, rotary, Roots, screw, jet or turbocompressors.
[0019] Centrifuges comprise, for example, screen-jacket or
solid-jacket centrifuges, preference being given to plate
centrifuges, full-jacket screw centrifuges (decanters), screen
screw centrifuges and reciprocal pusher centrifuges.
[0020] Columns comprise vessels with exchange trays, preference
being given to bubble, valve or sieve trays. The columns may also
have been packed with different packings, such as saddles, Raschig
rings, or beads.
[0021] Size reduction machines comprise, for example, crushers,
preference being given to hammer, impact, roll or jaw crushers, or
mills, preference being given to hammer, cage, pin, impact, tube,
drum, ball, vibratory and roll mills.
[0022] Internals in reactors and vessels comprise, for example,
thermocouple sheaths, flow disrupters, foam breakers, packings,
spacers, centering devices, flange connections, static mixers,
analytical instruments such as pH or IR probes, conductivity
meters, level measuring instruments, or foam probes.
[0023] The self-cleaning surfaces may be microstructured as
described in WO 96/04123. The surface described therein has
elevations and depressions, the distance between the elevations
being from 5 to 200 .mu.m, preferably from 10 to 100 .mu.m, the
height of the elevations being from 5 to 100 .mu.m, preferably from
10 to 50 .mu.m, and at least the elevations being of a hydrophobic
material. The water repellency of these surfaces is attributed to
the fact that the water drops lie only on the peaks of the
elevations and thus have only a small area of contact with the
surface. The water drop, occupying the smallest possible surface
area, forms a bead and rolls off from the surface at the slightest
vibration. The adhesion of solid particles to the surface is
similarly reduced. These particles have a more or less great
affinity for water, so that they are removed from the surface
together with the drops which roll off.
[0024] A self-cleaning microstructured surface in accordance with
one embodiment of the invention may be described as follows.
[0025] FIG. 1 shows the idealized representation of a section
through a self-cleaning microstructured surface in accordance with
one embodiment of the invention. The idealized microstructured heat
exchange surface (1) has hemispherical elevations (2) of radius R,
arranged with a spacing s, and depressions in between. The distance
s between the elevations (2) is such that a liquid (3) hanging down
between the elevations occupies a radius of curvature R*, and in
the depressions between the elevations (2) does not contact the
heat exchange surface (1). Preferably, s <4R. In the vapor space
(4), the vapor pressure Pv of the liquid (3) at the system
temperature is established; in the case of ideal mixtures, the sum
of the vapor pressures of the components. The downward-hanging
curve of the liquid is subject to the sum of this vapor pressure pv
plus the hydrostatic pressure P.sub.hy, i.e., in the case of a
horizontal heat exchange surface:
P.sub.v+P.sub.liqgh.
[0026] It is known that the vapor pressure over curved phase
boundaries is greater than over planar phase boundaries. The vapor
pressure over a curved surface is
p.sub.v(R*)=P.sub.v exp (2.PHI..sub.AB V.sub.liq/R* .THETA.T),
[0027] where
1 p.sub.v(R*) is the vapor pressure over the phase boundary with
the radius of curvature R*, P.sub.v is the vapor pressure over the
planar phase boundary, .sigma..sub.AB is the surface tension
between the liquid phase and the solid phase of the elevation (2),
V.sub.liq is the molar volume of the liquid phase, R* is the radius
of curvature of the downward-hanging liquid, .theta. is the ideal
gas constant, and T is the temperature.
[0028] The structure of the surface, then, is such that R* is so
small that at the anticipated film thicknesses of h the vapor
pressure p.sub.v(R*) always remains at least equal to the sum of
P.sub.v+P.sub.hy. In that case, the liquid (3) is unable to wet the
surface.
[0029] Rather than by a hydrostatic pressure P.sub.hy, the pressure
prevailing at the curve of the liquid may be increased by an
additional centrifugal pressure, brought about by a centrifugal
force. Such a centrifugal force acts, for example, in a coiled tube
evaporator. The above relationships apply accordingly.
[0030] The nonwettability of surfaces can therefore be attributed
to the vapor pressure increase in small drops. This effect may be
intensified by heating the surface. In evaporators, the heat
exchange surface is generally hotter than the liquid. In that case
the liquid repellency is supported by the additional increase of
the vapor pressure in the enclosed vapor bubble. If the system
reaches nucleate boiling, the surface is protected even more
effectively against wetting. The liquid curves become boil bubbles
of radius R*.
[0031] Similar laws apply to systems additionally comprising inert
gases. In that case, it is necessary to take account of the
desorption pressures of the soluble inert gases, as well as the
vapor pressures. With dissolved gases, too, the desorption pressure
is increased analogously by a curved phase boundary.
[0032] Real microstructured surfaces, depending on the nature of
their production, will generally have a geometry which deviates to
a greater or lesser extent from the idealized geometry indicated in
FIG. 1. In particular, the elevations (2) will not be exactly
hemispherical and their radius R and distance s will vary to a
greater or lesser extent. Moreover, the depressions lying between
the elevations (2) need not be planar. Preferably, however, the
elevations will have an essentially rounded form and will have on
average a radius R of from 5 to 100 .mu.m and a distance s of from
5 to 200 .mu.m.
[0033] The microstructured surface may be produced by powder
coating of adhesives and coating materials applied to the surface.
This can be done by, for example, blowing or powdering hydrophobic
pigments, Teflon powders, wax powders, polypropylene powders or
similar particulate substances of appropriate particle size onto
the surface wetted with the coating material or adhesive.
Preferably the powders have a narrow particle size distribution.
Microstructured surfaces may also be obtained by layer deposition
from solutions, electrolytic deposition, galvanic techniques,
etching techniques, or vapor deposition.
[0034] The particulate substance to be applied, and the polarity of
the microstructured surface, are chosen as a function of the
solvent to be evaporated. In the case of aqueous, aqueous-organic
or polar organic solvents, the microstructured surface will have
hydrophobic properties. For separation of nonpolar solvents,
however, it is also possible to equip the evaporator with a
hydrophilic microstructured surface.
[0035] The effect of the microstructure of the heat exchange
surface is that both solvents and solid particles or viscous
residues are almost completely unable to adhere to it. The surface
is unwettable. The result of this is that a subsequent flow of
liquid picks up, and removes, the solid or viscous constituents
occupying the surface.
[0036] In principle, all common types of evaporator may be equipped
with a microstructured surface. Particularly advantageous results,
however, are achieved with those types of evaporator in which there
is a continuous flow of liquid over the heat exchange surfaces.
Preference is given to forced circulation evaporators, flash
evaporators, and coiled tube evaporators. In a forced circulation
evaporator, the liquid intended for evaporative concentration is
conveyed over the surface by means of a pump. The coiled tube
evaporator has an evaporator tube which is curved in the form of a
coil. This type of evaporator is described in DE-C 2 719 968. By
virtue of a high gas flow rate (preferably >20 m/s), the liquid
is pressed against the tube wall, forcing the development of an
annular flow. The superimposed centrifugal force ensures the
development of a secondary flow, which improves the heat transfer.
In the case of a microstructured surface coiled tube evaporator in
accordance with a preferred embodiment of the invention,
surprisingly, the evaporative concentration of solutions containing
viscous residues is not accompanied by caking of the residues, and
at the same time a high heat transfer coefficient is realized.
These advantages are achieved similarly in all evaporator tubes
with a microstructured surface in which an annular flow is able to
develop. For example, the evaporator used may be a straight tube,
with an annular flow being forced by means of a high gas speed.
[0037] The present invention additionally provides a process for
the evaporative concentration of solutions in an evaporator having
a self-cleaning microstructured heat exchange surface. The solution
for evaporative concentration preferably comprises viscous or solid
constituents. These constituents may be present in suspension,
emulsion or solution in the solution for evaporative concentration.
Examples are aqueous or aqueous-organic solutions of inorganic
salts, such as aqueous butynediol solutions containing catalyst. If
a hydrophilic surface is employed, it is also possible to carry out
evaporative concentration of organic solutions, an example being
dehydrocholesterol acetate in xylene.
[0038] With the process of the invention it is possible to carry
out evaporative concentration of solutions down to a solvent
content of generally <10% by weight, preferably <5% by
weight, with particular preference <2% by weight.
[0039] The invention is illustrated by the following examples.
EXAMPLES
Experimental apparatus
[0040] The experiments were conducted in the experimental
installation depicted in FIG. 2. The solution containing viscous
residues was pumped from the reservoir vessel (1), which was
located on a balance, into the preheater (3) with the aid of the
gear pump (2). In the preheater (3), the solution was heated under
pressure and then released through the control valve (4) into the
evaporator (5) of the invention. The volume flows and the pressure
were set in such a way that an annular flow develops in the
evaporator. The evaporator comprised a coiled glass tube having an
internal diameter of 6 mm. 20 coils having a diameter of 70 mm and
a pitch of approximately 20.degree. were installed. The glass coils
were equipped with a microstructured surface produced by applying
UV-curable coating material (Laromer PO 84 F), coating it with
Teflon powder having an average particle size of 7 .mu.m (Dyneon TF
9205 PTFE), and then curing the system. The discharge from the
evaporator was passed into the heated phase separator (6), in which
liquid phase and vapor phase were separated. The residue (liquid
phase) was collected in the vessel (7). The vapor was condensed in
the condenser (8) and collected in the vessel (11). The apparatus
was operated under atmospheric pressure using the vacuum pump (10)
and the control valve (9).
Example 1
[0041] Evaporative concentration of an aqueous butynediol
solution
[0042] A 50% strength aqueous solution of butynediol containing
approximately 5% by weight Raney nickel (catalyst) was subjected to
evaporative concentration in the apparatus described above. For
this purpose, 13.8 g/h of the solution were preheated to
150.degree. C. at 10 bar and were released into the evaporator at
20 mbar. The salt-free aqueous butynediol solution was obtained as
the distillate. The residue was a viscous crystal slurry having a
residual water content of approximately 1% by weight. With the
evaporator having the microstructured surface, no incrustation of
the evaporator surface was observed.
[0043] In a comparative experiment with a conventional evaporator
corresponding to the geometries described above, the surface became
occupied. The only way to compensate this was by reducing the
degree of concentration, entailing a reduction in the product
yield.
Example 2
[0044] Evaporative concentration of a salt solution
[0045] A 10% strength aqueous NaCl solution was subjected to
evaporative concentration in the apparatus described above. For
this purpose, 20 g/h of the solution were preheated to 150.degree.
C. at 10 bar and released into the evaporator at 20 mbar. In this
way, the solution was concentrated to give a residue comprising a
viscous crystal slurry having a residual water content of
approximately 1% by weight. The crystals precipitating on the
evaporator surface were continually washed away by a subsequent
flow of water. No incrustation of the evaporator surface was
observed, even with a high degree of concentration. Salt-free water
was obtained as the distillate.
* * * * *