LU84557A1 - HEAT EXCHANGER IN FLUORINATED THERMOPLASTICS - Google Patents

Description

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HEAT EXCHANGER IN N FLUORINATED THERMOPLASTICS

The present invention relates to improvements made to heat exchangers made of fluorinated thermoplastic materials.

Manufacturers have always sought to solve the problem of transferring calories between two fluids.

It is well known, to carry out this transfer, to use * exchange surfaces of metals such as copper, aluminum; 10 minium, steel or special alloys.

In the case where at least one of the fluids is very corrosive, the manufacturers have used increasingly sophisticated and increasingly expensive metals such as titanium, zirconium and tantalum.

Graphite is also widely used in this field. It has satisfactory chemical resistance but has the drawback of being more fragile and often porous.

When synthetic resins with high chemical resistance, such as polytetrafluoroethylene and tetrafluoroethylene-hexafluoropropene copolymers, were developed, consideration was also given to using them to construct heat exchangers. However, their poor mechanical properties and the fact that they are not or very difficult to weld have limited their use.

Indeed, the poor resistance to pressure from? tubes made of these polymers and their tendency to creep have forced manufacturers to use only tubes of small diameter and large wall thickness, which is eminently detrimental to heat transfer. Despite this, the pressures at which these devices can be used are still very limited.

In addition, since these polymers are practically not weldable, the manufacturers of exchangers have not been able to use the conventional and proven plate techniques; in which the tubes forming the surface | . are welded. They were forced to have; , χ use of very expensive construction methods, such as 9 2 which calls for the use of a screw connection for each tube, or techniques of compressing the tubes against each other to obtain a seal between them .

5 A whole new field of application has been opened up to the manufacturers of heat exchangers when fluorinated thermoplastic materials such as polyvinylidene fluoride (PVDF), ethylene-monochlorotrifluoroethylene copolymer (ECTFE), or 10 perfluoralkoxy polymers (PFA).

The latter have mechanical properties far superior to the previous ones, allowing their use in clearly enlarged temperature and pressure ranges. Being thermoplastic, these new polymers are also weldable.

Currently, it is well known to use these fluorinated thermoplastic polymers to build heat exchangers with bundles of straight parallel tubes, with pin bundles or with plies of spir-wound tubes.

It is also known to produce these exchange surfaces by means of fairly rigid smooth tubes or ringed tubes, which are much more flexible.

Realizing the numerous advantages presented by these devices, manufacturers are requesting more and more powerful exchangers, with an increasingly large exchange surface requiring an increasingly low production cost.

The applicants have developed a new heat exchanger 30 made at least partially of fluorinated thermoplastic material which meets these wishes.

The present invention consists of a heat exchanger, the exchange surface of which is produced by means of a fluorinated thermoplastic polymer and is characterized in that said exchanger is made up of a number of times two respective fixed concentric tubes - i ment to two superimposed tubular plates, the external tube at least being made of thermoplastic polymer * 3 fluorinated and being closed at its end opposite to the tubular plate, these tubes immersing in one of the fluids while the second fluid travels the free annular space between the outer tube and the inner tube.

5 Other details and additional characteristics of the invention will become apparent on reading the description which follows.

This will be done with reference to the accompanying drawings which illustrate by way of illustration without limitation, embodiments of the device according to the invention.

FIG. 1 shows the operating principle of the exchanger according to the invention and, as a variant,

Figure 2 illustrates a two pass apparatus.

Referring to FIG. 1, two concentric tubes (1 and 3) are fixed respectively to two corresponding superimposed tubular plates 5 and 7, the concentric tubes being able to be stored in a row or in staggered rows.

According to the invention, at least the outer tube is made of fluorinated thermoplastic material and is closed at its end opposite the tube plate. The concentric tubes are separated from each other by a distance which can vary from a few millimeters to a few centimeters depending on the acceptable pressure drop. These tubes 25 immerse in one of the two fluids while the second fluid traverses the free annular space between the outer tube 1 and the inner tube 3, as indicated by the arrows, the circulation being ensured between the plate on the one hand 5 and 7 and on the other hand the plate 7 and a cover 30 9.

The tubes can have any desired length, the most adequate being defined by the result of the thermal and mechanical calculation.

It is also, as a function of the results of these calculations, that the diameters of the pairs of tubes are determined, outer tube made of thermoplastic fluoropolymer fj and inner tube, to be used for construction.

By way of example, the following non-limiting dimensions • 4 may be cited: - outer tube: ^ ext: 15 mm d ^ nt: 13.4 mm; - inner tube: d. : 10 mm d. ,: 8 mm | ext int i I 5 - outer tube: dext: 12 mm d ^ nt: 10.4 mm inner tube: d,: 6 mm d. : 4 mm ext int - outer tube: d. z 8mm d.,: 6.4mm ext xnt d. : 4 mm d. ,: 3 mm ext int 10 By judicious partitioning of the space between the two tubular plates, apparatus can be constructed with a single pass or with several passes.

FIG. 2 illustrates an apparatus with two passes, it being understood that for the sake of clarity of the drawing, only a couple of concentric tubes have been shown per pass.

Identical references to those of FIG. 1 have been used for identical or similar constructive elements.

The heat exchanger which is the subject of the invention has a large number of advantages. Among these we can cite:

Unlike other types of exchangers built so far in synthetic materials, the exchanger according to the invention lends itself perfectly to the realization of large exchange surfaces by association of modular elements. For this purpose, the tubular plates from which tubes hang hang down side by side.

The length of the tubes is chosen according to the * '* Λ *.

thermal problem to be solved and the space available.

In the case of recovering calories from a hot aggressive liquid, such as sulfuric acid, the tubes can be placed vertically and hang in; the enclosure where the hot liquid is located, that this en- i | 35 enclosed either a parallelipipedic tank or round a canni i I n cal or any other volume. The tubes can then have a length of several meters (see example 1 below), i In the case of the recovery of calories from a hot corrosive gas, it is also preferable to let the tubes hang in a sheath conveying the gas or place them obliquely in a flue. The tubes are then shorter and may be only 5-20 or 30 cm long (see example 2 below).

In the case of the condensation of corrosive vapors, the tubes can be placed horizontally, for example at the top of a separation column (see example 3 below).

(10 Another advantage of the exchanger according to the invention is that it allows each of the tubes to expand and contract freely, depending on the temperature regime to which the device is subjected without the need to These compensation devices, which are necessary in the case of other devices, such as those with straight tube bundles, are complicated to build and increase their cost.

When treating dirty fluids laden with suspended solids or sludge, it is difficult to prevent deposition of these solids on the tubes. The exchanger according to the invention allows very rapid cleaning by giving immediate access to the external surface of the large diameter tubes by simple lifting of the tubular plates. In addition, the fluorinated thermoplastic polymers used resistant well to encrustation, it is possible to clean the tubes by simple washing.

Finally, the construction of the heat exchanger according to the invention is economical compared to the cost of the other 30 devices built so far. Indeed, in the very frequent case of heat exchange between a corrosive fluid and a non-corrosive fluid, the latter traversing the interior of the concentric tubes, only one of the two tubular plates, that to which the outer tubes are fixed. , must be, in whole or in part, made of fluorinated thermoplastic polymer. Likewise, the inner tube J, which is only in contact with the other fluid, and its tube plate, can be made of a material which is clearly less noble and therefore less expensive than a fluoropolymer. This may for example be, depending on the thermal and mechanical problem to be solved, polypropylene, polyvinyl chloride, copper, brass, carbon steel or stainless steel.

The fixing of the tubes to the tube plates can be carried out by any known technique such as welding, bonding, threading, etc. although welding is in principle preferred by the applicants. As fluorinated thermoplastics are expensive materials, it is advantageous from an economic point of view to limit their use for the production of parts which technically require it.

For example, to construct the tubular plate 15 to which the outer tubes of fluorinated thermoplastic polymer are fixed, it is possible to use a solid polymer plate of the thickness necessary for the mechanical strength and for carrying out the welds, but it will be preferred, if possible, use composite plates, including a minimum thickness of fluoropolymers, necessary for welding and chemical resistance, reinforced on its face in contact with the non-corrosive fluid by a metal such as steel or a resin thermosetting, for example a polyester or epoxy type resin.

The examples which follow are intended to illustrate the invention and do not limit its scope.

Example 1: Recovery of calories from H ^ SO ^ 98.5% at 140 ° C

A tank receives a flow of sulfuric acid at a concentration of 98.5% at a flow rate of 300 m / hour. This acid enters at the temperature of 140 ° C. The cover of this tank consists of a modular assembly of heat exchangers according to the invention constructed as follows: the outer tubes and the tube plate to which they are welded are made of ethylene-monochlorotrifluoroethylene copolymer (ECTFE). The inner tubes ^ are made of carbon steel as are their tube plate and the third plate forming a cover.

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Each of the 25 modules includes 1000 tubes 3.5 meters long, or 25,000 tubes in total. The ECTFE tubes have an outside diameter of 8 mm and an inside diameter of 6.4 mm. The carbon steel inner tubes have an outside diameter of 4 mm and an inside diameter of 3 mm.

Such an exchanger represents an exchange surface of 2,200 m ^.

Pressurized water at 100 ° C enters the exchanger modules against the flow of acid. It leaves the exchanger at 120 ° C. under a pressure of 4 bars 3 absolute at a flow rate of 420 m / h. It can be expanded into steam at 120 ° C. The heat exchanged is 4,880 KW.

Example 2: Recovery of calories from a fuel boiler smoke gas containing sulfur.

The sulfur contained in the fuel oxidizes to SO 2, and sulfuric acid is formed which corrodes conventional installations as soon as the dew point is reached.

To solve this problem, a heat exchanger 20 according to the invention is used, under the following conditions:

A smoke duct is traversed by 1.2 kg / sec of sulfuric fumes at 250 ° C. The top of the sheath is made up of exchanger modules according to the invention, constructed as follows: outer tubes and lower tubular plate in polyvinylidene fluoride (PVDF), outside diameter of the tabs-15 mm, inside diameter 13, 4 mm; inner tube and upper tube plate in carbon steel .u 30 carbon, outer diameter of the tubes 10 mm, inner diameter 8 mm. t, „

42 juxtaposed modules are provided, each comprising 2,100 tubes 50 cm long, or 100 m of exchange surface.

35 ‘The modules are traversed by 10 kg / sec. Of water entering at 70 ° C and heated to 74.5 ° C. The fumes are cooled to 95 ° C. and more than 90% of the acid jL is condensed. sulfuric content in the fumes, which brings a marked 9 i / 8 depolluting.

The entire installation includes a safety device interrupting or diverting the gas flow in the event of the water circulation stopping.

5 The power exchanged in the installation is 186 KW.

Example 3: Partial condensation of the vapors of a mixture of chlorinated and fluorinated solvents.

It is desired to partially condense, at 45 ° C., 600 10 kg / h of chlorinated and fluorinated solvent vapors rising in a separation tower.

This operation is carried out by placing at the top of the tower a heat exchange module according to the invention, constructed with a double pass as shown schematically in FIG. 2. The tubes placed horizontally are traversed by a methanolic brine at - 18 ° C.

2

The 5 m exchange surface is obtained by 220 tubes 50 cm long, per pass.

The outer tubes are made of polyvinylfluoride-20 lidene (PVDF), outer diameter 12 mm, inner diameter 10.4 mm. The inner tubes are made of AISI 316 stainless steel, outside diameter 6 mm, inside diameter 4 mm.

400 kg / h of vapor of a certain composition are condensed while 200 kg / h of vapor of another composition pass into another column.

Example 4: Heating and thermostatization at 35 ° C of a diethanolamine tank.

3

A tank contains 7.5 m of diethanolamine 30, the temperature of which must be maintained at 35 ° C; water heating fluid at 75 ° C is available.

The objective is achieved by an exchanger module according to the invention, with a single pass, of an exchange surface 2 of 13.5 m consisting of 360 tubes of 1 m long.

The outer tubes are made of ethylene-imonochlorotrifluoroethylene (ECTFE) copolymer, outer diameter 12 S mm, inner diameter 10.4 mm. The inner tubes are 1 in copper, outside diameter 6 mm, inside diameter 4itm.

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The power exchanged is 19.6 KW.

The module can be placed on the tank cover, with the tubes arranged vertically or laterally with the tubes arranged horizontally.

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Claims (5)

3. Heat exchanger according to claim 1: characterized in that the fluorinated thermoplastic polymer <, "is the ethylene-monochlorotrifluoroethylene copolymer. j 4. Heat exchanger according to any one of the preceding claims, characterized in that the concentric tubes (1,3) are both made of fluorinated thermoplastic N polymer. 5. Heat exchanger according to any one of • claims 1 to 3 characterized in that only the outer tube (1) of the concentric tubes (1,3) is .en re-fluorinated thermoplastic polymer, the inner tube (3 ) being made of another material such as polypropylene, polyvinyl chloride, copper, aluminum or steel. 6. Heat exchanger according to any one of the preceding claims, characterized in that the tubular plate (5) to which the outer tubes (1) of fluorinated thermoplastic material are fixed is a plate of composite material, the face of the side of tubes || being in fluorinated thermoplastic polymer, the other side! 35 being of another material, such as steel, ί. 7. Heat exchanger according to any one of i claims 1 to 5 characterized in that the tube plate (5) to which are fixed the outer tubes (1) .f N. i; i! 11 I in fluorinated thermoplastic polymer is a composite plate, the face of the side of the tubes being made of fluorinated thermoplastic material, the other face being made of a thermosetting resin. 8. Application of a heat exchanger according to claim 1 to the recovery of calories from highly concentrated and hot sulfuric acid * for the purpose of producing water vapor, characterized in that the pairs of concentric tubes (1,3) are made with an outer tube (1) of ethylene-monochlorotrifluoroethylene copolymer 10 and an inner tube (3) of steel. 9. Application of a heat exchanger according to claim 1 for the recovery of calories and the depo-lution of flue gases produced by fuels containing sulfur, characterized in that the pairs of concentric tubes (1,3) are made with an outer tube (1) of polyvinylidene fluoride and an inner tube (3) of steel. ^ "! ^ 3.