Gas: heating and illuminating – Processes – Fuel mixtures
Reexamination Certificate
1999-10-15
2001-10-09
Tran, Hien (Department: 1764)
Gas: heating and illuminating
Processes
Fuel mixtures
C048S189100, C048S189400, C137S098000, C137S099000, C137S109000, C137S110000, C137S111000, C138S038000, C165S109100, C165S166000, C422S215000, C422S224000, C422S225000, C422S228000, C366S144000, C366S337000, C366S340000
Reexamination Certificate
active
06299657
ABSTRACT:
To carry, out a chemical reaction in a continuous procedure, the reaction partners must be fed continuously to a chemical reactor and brought intimately into contact, i.e. mixed thoroughly, with the aid of a mixing element (mixer). A simple reactor is, for example, a tank with a stirrer as the mixing element. As a rule, several reactions, so-called main and side reactions, proceed in the reactor when the reactants come into contact. The aim of the process engineer here is to conduct the reactions and therefore also the mixing such that the highest possible yield of the desired product is achieved selectively.
The quality of the mixing and the influence of the mixing element on the yield of the desired product depends greatly here on the ratio of the rate of the chemical reaction, determined by the reaction kinetics, to the rate of mixing. If the chemical reactions are slow reactions, as a rule the chemical reaction is substantially slower than the mixing. The overall rate of reaction and the yield of desired product is then determined by the slowest step, that is to say the kinetics of the chemical reactions which proceed, and in addition by the global mixing properties (residence time distribution, macromixing) of the chemical reactor used. If the rates of the chemical reactions and the rate of mixing are of the same order of magnitude, complex interactions arise between the kinetics of the reactions and the local mixing properties. determined by the turbulence, in the reactor used and at the mixing element (micromixing). If the case occurs where the rates of the chemical reactions are substantially faster than the rate of mixing, the overall rates of the reactions which proceed and the yields are substantially determined by the mixing, i.e. by the local time-dependent speed and concentration field of the reactants, i.e. the turbulence structure in the reactor and at the mixing element [1].
According to the prior art. a number of mixing elements are employed for carrying out fast reactions in a continuous procedure. A distinction may be made here between dynamic mixers such as stirrers turbines or rotor-stator systems, static mixers, such as Kenics mixers, Schaschlik mixers or SMV mixers, and jet mixers, such as nozzle mixers or T mixers [2-4].
For rapid mixing of starting substances in rapid reactions with undesirable secondary or side reactions, nozzle mixers are preferably employed.
In jet or nozzle mixers, one of the two starting components is atomized into the other components at a high flow rate (cf. FIG.
1
). In this case, the kinetic energy of the stream (B) sprayed in is substantially dissipated behind the nozzle, i.e. is converted into heat by turbulent breakdown of the stream into eddies and further turbulent breakdown of the eddies into ever smaller eddies. The eddies contain the particular starting components, which are present side-by-side in the fluid balls (macromixing). A small degree of mixing by diffusion indeed occurs at the edges of these initially larger structures at the start of the turbulent breakdown of the eddies. However, complete mixing is achieved only when the breakdown of the eddies has progressed to the extent that, when eddy sizes of the order of magnitude of the concentration microdimension (Batchelor length) [5, 6] are reached, the diffusion is rapid enough for the starting components to be mixed completely with one another in the eddies. The mixing time required for complete mixing depends substantially on the specific energy dissipation rate, in addition to the substance data and the geometry of the apparatus.
The mixing processes in the mixers according to the prior art which are often used are in principle similar (in dynamic mixers and static mixers the eddies are also additionally divided mechanically, although as a rule with substantially lower specific energy dissipation rates). This means that in the mixers used according to the prior art, the time for breakdown of the eddies always elapses before complete mixing by diffusion. For very fast reactions, this means that either very high energy dissipation rates must be established, in order to avoid undesirable side and secondary reactions, or, in the case of reactions with even higher rates of reaction, the corresponding reactions are not carried out to the optimum, i.e. are carried out only with the formation of by-products or secondary products.
On the basis of this prior art, the object of the invention is to provide a process and a device with which mixing takes place rapidly and the formation of secondary products or by-products is suppressed or reduced. The achievement here must be that the educts are mixed homogeneously with one another so that, within the shortest time, local and time-related over-concentrations of the educts no longer occur. In the case of fluids which react chemically with one another, complete reaction of the fluids is to be achieved. If required, the heat of reaction should also be removed or supplied effectively and as rapidly as possible.
This object is achieved according to the invention by a process in which at least two educts A, B are divided in a microstructure mixer, by a system of slit-like microchannels (microslit channels) assigned to them, into spatially separate fluid lamellae, which then emerge with flow rates which are the same for the particular educt into a mixing/reaction space, each fluid lamella of an educt A being led into the mixing and reaction space in the immediate vicinity of a fluid lamella of another educt B, and the adjacent fluid lamellae mixing with one another by diffusion and/or turbulence. A microslit channel is understood here as meaning a rectangular microchannel having a depth d, its width b being >=10d (b/d>=10), preferably b>=20d (b/d>=20).
Laminar flow conditions for educts A, B are preferably maintained in the microslit channels. However, there is nothing against working with turbulent flows in the microslit channels, where appropriate.
An embodiment in which the fluid lamellae of educts A, B emerge into the mixing/reaction space in layers lying alternately one above the other or side by side has proved to be particularly suitable.
The geometry of the microstructure lamellae mixer is advantageously designed such that the thickness of the fluid lamellae d at the entry into the mixing/reaction space can be adjusted to a value between 10 &mgr;m and 1,000 &mgr;m, preferably between 10 &mgr;m and 100 &mgr;m. A thickness d which is of the order of magnitude of the concentration microdimension is preferably established, so that after exit from the microstructure mixer, micromixing of the components can take place rapidly by diffusion, without further eddy breakdown being necessary. The width b of the fluid lamellae or of the microslit channels via which the lamellae emerge from the microstructure lamellae mixer should be as wide as possible here, to keep the pressure loss in the mixer as low as possible by reducing the wall area per educt volume. The width b here can vary from values in the range of the order of 0.5 mm to high values in the range of several centimetres, and is substantially limited only by the mechanical stability of the structural component. A lowest possible thickness d of the fluid lamellae, and not the width b, is decisive here for the rate of mixing and therefore the mixing quality.
A further development of the process according to the invention comprises additionally feeding a fluid lamella of a temperature-controlled inert fluid, for example, for heating or cooling purposes, into the mixing/reaction space in the vicinity of a fluid lamella of an educt.
The process according to the invention is thus based on first dividing educt streams A, B convectively, by means of the microstructure lamellae mixer, into thin lamellae having a thickness d, which then mix with one another by diffusion and/or turbulence in the mixing/reaction space after their exit.
The task of the microstructure lamellae mixer here is to divide the educt stream
Bier Wilhelm
Herrmann Erhard
Koglin Bernd
Linder Gerd
Maul Christine
Bayer Aktiengesellschaft
Norris & McLaughlin & Marcus
Ridley Basia A
Tran Hien
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