The invention relates to evaporators having a microstructured surface and to processes for the evaporative concentration of solutions.
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.
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.
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.
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.
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.
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.
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.
By apparatus and apparatus parts are meant:
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.
More specifically, said apparatus and apparatus parts may be described as follows:
Vessels comprise, for example, reservoir or collecting vessels such as troughs, silos, tanks, vats, drums or gasometers, for example.
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.
Discharge devices comprise, for example, discharge ports, discharge hoppers, discharge pipes, valves, discharge cocks or ejector devices.
Fittings comprise, for example, stopcocks, valves, slide valves, bursting disks, nonreturn valves, or disks.
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.
Filter apparatus comprises, for example, fluid filters, fixed-bed filters, gas filters, sieves or separators.
Compressors comprise, for example, reciprocating piston, reciprocating piston membrane, rotary piston, rotary slide valve, liquid seal, rotary, Roots, screw, jet or turbocompressors.
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.
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.
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.
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.
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 μm, preferably from 10 to 100 μm, the height of the elevations being from 5 to 100 μm, preferably from 10 to 50 μm, 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.
A self-cleaning microstructured surface in accordance with one embodiment of the invention may be described as follows.
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 Phy, i.e., in the case of a horizontal heat exchange surface:
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
pv(R*)=Pv exp (2ΦAB Vliq/R* ΘT),
|pv(R*) ||is the vapor pressure over the phase boundary with the radius of |
| ||curvature R*, |
|Pv ||is the vapor pressure over the planar phase boundary, |
|σAB ||is the surface tension between the liquid phase and the solid |
| ||phase of the elevation (2), |
|Vliq ||is the molar volume of the liquid phase, |
|R* ||is the radius of curvature of the downward-hanging liquid, |
|θ ||is the ideal gas constant, and |
|T ||is the temperature. |
The structure of the surface, then, is such that R* is so small that at the anticipated film thicknesses of h the vapor pressure pv(R*) always remains at least equal to the sum of Pv+Phy. In that case, the liquid (3) is unable to wet the surface.
Rather than by a hydrostatic pressure Phy, 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.
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*.
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.
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 μm and a distance s of from 5 to 200 μm.
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.
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.
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.
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.
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.
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.
The invention is illustrated by the following examples.