Patent Application
20110017439
The invention relates to a heat exchanger for temperature-sensitive and/or polymerizable products.
The literature discloses many different embodiments of heat exchangers. Thus, for example, heat exchangers of the type consisting of plate heat exchangers or microheat exchangers are suitable for short residence times. However, the heat exchangers have the disadvantage that, owing to the narrow flow gap, they are suitable only for low-viscosity products. In the case of more highly viscous products, the pressure drop across these heat exchangers may be very high. In the case of products which tend to polymerise, such as monomers or polymer syrup which still contains monomers, there is the danger that the monomers will polymerise in the heat exchanger during operation or during stoppage. The removal of polymerised syrup from the plate heat exchangers and microheat exchangers is very complicated, if not impossible. For high viscosities, in particular for several Pa·s, and high pressures greater than 10 bar, no plate heat exchangers are available owing to the method of manufacture and the resultant forces.
U.S. Pat. No. 1,961,907 describes a tube-bundle heat exchanger having spirally grooved displacement bodies in the tubes. Owing to the helical flow, particularly effectively heat transfer is achieved. As a result of the flow of the medium to be thermostated within the displacement tube, however, an additional pressure drop and an additional residence time are produced, which may be harmful for the product. Moreover, the complex design also results in high costs, poor dismantleability and a difficult emptying procedure.
DE-G 87 12 815 (VIA Gesellschaft für Verfahrenstechnik) describes a tube-bundle heat exchanger for compressed-air dryers. In order to save material, the displacement body introduced into the tube consists in turn of a tube which is closed on the entry side. The displacement tube may have a fluted surface. However, the design, which was not developed for temperature-sensitive products, has a large product-filled volume since no trays with very small hold-up are used and the displacement rods are not closed at the bottom. In addition, the displacement tubes cannot be dismantled, which constitutes a major disadvantage in the case of temperature-sensitive polymers.
DE-G 89 03 349 (VIA Gesellschaft für Verfahrenstechnik) describes a tube-bundle heat exchanger, in particular for compressed-air dryers. In order to enable the heat-transfer medium to flow as uniformly as possible through the apparatus, a perforated plate which ensures uniform flow towards the tubes is arranged in the apparatus. In the case of this tube-bundle heat exchanger, however, heat transfer under mild conditions is not necessary so that there are no particular requirements regarding the cross section of the displacement rods and no displacement covers or flat trays with minimum hold-up are required. Moreover, the displacement rods cannot be dismantled.
Heat exchangers which show a small pressure drop even in the case of relatively highly viscous products are frequently, for example, of the tube-bundle type. In this embodiment, the product flows through a plurality of tubes arranged in parallel. However, a disadvantage here is that the tube-bundle heat exchangers usually have a small specific heat exchange area. The specific heat exchange area is defined here as the ratio of the heat exchange area to the volume in the tubes which the product fills. Owing to the small heat exchange area, as a rule large heat exchangers with consequently considerable hold-up in the tubes are required. The residence time is therefore very high in the tube-bundle heat exchangers.
In view of the prior art discussed, it was the object to develop a heat exchanger which enables the residence time for the product to be heated or to be cooled in the heat exchanger to be as short as possible. The heat exchanger should furthermore be designed in such a way that both low-viscosity and more highly viscous products can be heated or cooled.
An embodiment of the heat exchanger which
This object was achieved by a tube-bundle heat exchanger comprising specially designed displacement rods in the product-filled tubes. The displacement rods are designed in such a way that they occupy more than 40% of the volume present in the tubes, preferably occupy more than 50% of the volume present in the tubes and very particularly preferably occupy more than 60% of the volume present in the tubes. In order to keep the product-filled volume in the apparatus small, one or more displacement bodies are expediently arranged in the heat exchanger covers of the apparatus or at least one flat tray is used.
1. Design of the tube-bundle heat exchanger
The tube-bundle heat exchanger consists of a housing (4) and a tube bundle which is formed from one or more tubes which are arranged substantially parallel and through which the product to be thermostated flows. The tubes may be arranged flush, offset or on concentric circles of holes relative to one another. A minimum and substantially equal tube spacing is preferred, resulting in a small product-filled volume (6). An arrangement of the tubes on concentric circles is particularly preferred in order to obtain uniform flow towards the tubes and few dead zones in the bottom region.
The product flows through the tubes and is heated or cooled via the tube casing. The heating or cooling medium (5) flows through the external jacket of the tubes. The heating or cooling medium (5) can flow towards the tubes with cross-flow, countercurrently to or cocurrently with the product stream. The thermostating is preferably effected substantially with cross countercurrent flow since smaller driving temperature gradients between thermostating medium (5) and product space (6) are thus sufficient. To enable the emptying to be carried out in a simple manner, the product preferably flows through the heat exchanger from top to bottom. To enable the deaeration of the heating or cooling medium (5) to be effected in a simple manner, the thermostating medium (5) preferably flows through the heat exchanger from bottom to top.
At least one end of the tube bundle is enclosed by a tray through which the product enters or exits. This tray may be in the form of a heat exchanger cover (2) having a small wall thickness or in the form of a thick-walled but compact flat tray (17). The tray preferably has an apparatus flange so that it can be flange-connected to the main part of the heat exchanger or can be removed again. The tray may have a connecting piece which is preferably present on the axis and through which the product can enter or emerge. A plurality of connecting pieces in the vicinity of the axis through which product can emerge are also conceivable. The tray is preferably designed in such a way that it can be heated or cooled by a thermostating medium. However, electrical heating is also conceivable.
It is also conceivable for the heat exchanger to be connected directly to another apparatus so that a corresponding tray can be dispensed with on this side.
To compensate for expansion, a compensator can, if required, be used in the external jacket in order to compensate for the different thermal expansion between tube bundle and external jacket.
The pressure drop in the heat exchanger tubes is controllable for relatively highly viscous products by the choice of suitable tube diameters.
2. Design of the displacement rods
In order to reduce the volume of the product (6) in the heat exchanger tubes and to increase the heat transfer, displacement rods (7, 10, 12, 15) are introduced into the tubes. The displacement rods (7, 10, 12, 15) can project partly into the heat exchanger covers (2). The displacement rods (7, 10, 12, 15) are designed in such a way that they displace more than 40% of the volume in the heat exchanger tubes. Preferably, more than 60% of the empty volume of the tubes are displaced by the displacement rods (7, 10, 12, 15). Preferably, less than 95% of the volume are displaced, in order to maintain both a compact design of the heat exchanger and a small pressure drop. The outer contour of the displacement rods (7, 10, 12, 15) is designed so that the axis of the displacement rods (7, 10, 12, 15) is centred in the tubes in order to avoid dead zones and to achieve homogeneous flow over the cross section of the heat exchanger tube. The product stream flows in the gap (11) between displacement rod (7, 10, 12, 15) and inner wall of the heat exchanger tube.
In order to centre the displacement rods (7, 10, 12, 15) in the tube with a defined gap, the displacement rods (7, 10, 12, 15) can be designed, for example, as follows:
The displacement rods (7, 10, 12, 15) are preferably pushed into the tubes (9) so that they can, if required, be removed again for cleaning and testing purposes. The displacement rods (7, 10, 12, 15) may also consist of a plurality of individual rods connected in series. It is also conceivable to use hollow displacement rods which are filled with a medium which improves the heat transport. For example, they may contain water which vaporizes in the hot region and condenses in the cool region so that heat is transported in the axial direction. It is also conceivable additionally to transfer heat with the aid of a heat-transfer medium flowing through the displacement tubes. A further possibility consists in the use of electrically heated displacement rods, with the result that the specific heat transfer area is further increased and the residence time can be even further reduced. It is likewise conceivable to use combinations of the abovementioned displacement rods.
The displacement rods preferably produce a narrow cross section in the heated part of the tubes; in the entry region, an increase in the cross section can be provided for reducing the pressure drop in the region of the bottom of the tube.
The displacement rods (7, 10, 12, 15) reduce the hold-up of the product in the pipelines (6) and increase the specific heat-exchange area. The pressure drop across the tube-bundle heat exchanger with displacement rods (7, 10, 12, 15) is smaller than in the case of microheat exchangers and plate heat exchangers having the same thermal performance and number of tubes. In the case of microheat exchangers and plate heat exchangers, the pressure drop can be reduced to the level of the tube-bundle heat exchanger with displacement rods only by a substantial increase in the number of tubes in these heat exchanger types. The small tube diameter and the large number of tubes makes cleaning of these heat exchangers considerably more difficult.
The residence time in the tube-bundle heat exchanger with displacement rods is of course shorter than in tube-bundle heat exchangers without displacement rods of the same diameter. The residence time can be adjusted to the same level as in the case of tube-bundle heat exchangers with displacement rods only for empty tubes having a significantly smaller diameter, which, however, are then substantially longer.
3. Displacement bodies in the heat exchanger covers
In order to minimize the hold-up in the heat exchanger covers (2), displacement bodies (3) are installed in the covers (2). The covers (2) can likewise be heated or cooled. For centring, these may have, for example, metal sheets or pins on the outsides. In order to load the heat exchanger tubes uniformly with liquid, the side facing the heat exchanger tubes is preferably conical; cf.
Shorter residence time in the heat exchanger covers (2) and hence lower thermal loading of the products.
4. Flat tray
The zones for product entry (1) and product exit (8) can also be designed as a flat tray (17) having recesses (low volume head) (cf.
5. Operating parameters
Operating temperatures T=−20° C. to +400° C.;
Pressure in the product space of the tubes (6) and the covers (2) P=−0.95 barg to +100 barg.
The pressure in the space of the heat-transfer medium (5) may be between P=−0.95 barg and +50 barg. The temperature of the heat-transfer medium (5) may be T=−20° C. to +400° C.
The heat-transfer medium (5) may be fed in liquid or vapour form.
The heat exchanger described according to the invention is suitable for heating or for cooling products having a viscosity of η=0.1 mPa·s to 500 Pa·s. The residence time of the products in the heat exchanger may be 1 s to 300 s.
The heat exchanger makes it possible to establish a wide range of temperatures, pressures and viscosities.
Comparison between conventional heat exchangers of the prior art and heat exchangers having an annular gap
The following table summarizes results of mass and energy balances and of calculations of flow and heat transfer in tubes and annular gaps. The pressure drop calculations are based on the analytical solution of the pulse conservation equation for the tube with laminar flow (Hagen-Poiselle flow) or for the annular gap with a laminar flow. The transfer calculations are based on semiempirical Nusselt number relationships for hydrodynamically and thermally unformed laminar flow. Unless stated otherwise, a mass flow of 1000 kg/h, a residence time of 60 seconds in the tubes, a temperature increase of 100 K of the medium to be heated and a logarithmic temperature difference of 30 K between heat-transfer medium (5) and medium to be heated are assumed. The last two numerical values can be combined to give a quotient of 3.33. Moreover, a thermal conductivity of 0.15 W/mK, a density of 1000 kg/m3, a specific heat capacity of 2200 J/kgK and a constant dynamic viscosity of 1 Pa·s are used as material values, i.e. a Newtonian medium is assumed. Furthermore, it is assumed that the heat transmission resistance on the heat transfer side and the conduction resistance through the tube wall are negligible.
Example A shows that, in the case of conventional tube-bundle heat exchangers, very narrow, long tubes are required in order to achieve the specified conditions. However, these can be manufactured only with difficulty and are virtually impossible to clean.
Examples B and C show that shorter tubes are possible with shorter residence time (case B) or changed thermal conditions (case C). At the same time, however, the tube diameters no longer decrease and the number of tubes increases greatly so that this cannot be seen as an alternative to case A.
Examples D and E demonstrate that a larger tube diameter can be achieved with the aid of a longer residence time (case D) or higher wall temperatures (due to greater logarithmic temperature differences; case E). However, the advantage of the better cleanability owing to the larger diameter is overcompensated by the extremely increased tube length, which makes manufacturing very much more difficult, and deteriorations in the product quality owing to increased residence times and wall temperatures. Furthermore, the space requirement of such long apparatuses in buildings is problematic.
Examples F and G show that a smaller tube diameter, through shorter residence time (case F) or changed thermal conditions (case G) results in an extremely large number of tubes. Such a large number of delicate tubes cannot be manufactured in view of the high pressures and temperatures to which the tube-bundle apparatus would have to be exposed.
The tube length, which is not negligible, moreover still does not permit cleaning of the interior of the apparatus.
Example H shows that a reduced pressure drop due to an increased number of tubes and a shortened length does not result in smaller tube diameters. Owing to the large number of thin tubes having a length which is not negligible, here too the cleanability as well as the manufacturability is virtually impossible.
Comparison of a design according to the invention (Example I) with that of conventional tube-bundle heat exchangers (Examples A-H):
Example I shows by way of example a design for a heat exchanger according to the invention having displacement rods. Taking into account residence time, thermal conditions and pressure drop, this has a very large tube diameter, which ensures a good possibility for cleaning, in comparison with the examples of conventional heat exchangers (Examples A-H). In addition, the tube length is kept within limits compared with Examples A, D and E, with the result that good manufacturability and cleanability are permitted and little space is required. Moreover, compared with Examples A-C and F-H, the number of tubes is small so that simple and economical manufacture is possible.
The tube-bundle heat exchanger according to the invention can be particularly advantageously used in the synthesis of polymers since the short residence time in combination with effective heat transfer subjects the product to little thermal load and thus prevents undesired polymerisations.
Method of calculation
From the heat balance at the tube wall:
and with specification of two of the three geometrical parameters (external gap diameter da, internal gap diameter di tube length L), the third geometrical parameter can be calculated.
Here, L is the tube or annular gap length, τ is the residence time, da is the external diameter of the gap or tube diameter, di is the internal diameter of the annular gap (tube: di=0), ρ is the density, cp is the specific heat capacity, ΔTs is the temperature increase of the syrup, dn is the hydraulic diameter, λ is the thermal conductivity, ΔTlg is the logarithmic temperature difference between heating medium (5) and syrup.
The mean Nusselt number Num is calculated for a tube according to Baehr/Stefan (Heat and Mass Transfer, Springer-Verlag Berlin, 1994, pages 381-382) taking into account the hydrodynamic and thermal start-up using:
Here, Pr is the Prandtl number and X is a dimensionless length:
With K=d/da, the following is moreover approved for the mean Nusselt number in the externally heated annular gap:
Nu
m,RS=3.657+1.2·K1/2+(Num,tube−3.657)·(1+0.14·K1/3).
According to Martin (Wärmeübertrager [Heat exchangers], Georg Thieme Verlag Stuttgart, 1988, page 24), the following is true for the pressure drop in annular gaps or tubes (K=0):
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2008 011 341.7 | Feb 2008 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2008/066485 | 12/1/2008 | WO | 00 | 8/23/2010 |
