IL44075A - Method for causing condensation in drops on heat-exchanger tubes - Google Patents

Description

A method for causing condensation in drops on heat-exchanger tubes COMMISSARIAT A L'EKERGIE ATOMIQUE & PRODUITS CHIMIQU S UGI'NE KUHIMANN The Inventors are: 1. Jean^Claude DERONZIER 2 »\ Louis FOULLETIER - 3. Jean HUYGHE 4* Jean-Marc NIEZBORALA C, 42238 This invention relates to a method for causing condensation in drops on heat-exchanger tubes as applicable in particular to installations for the distillation of sea water.

It is known that one of the major problems which arises in thermal power plants, chemical engineering complexes and. plants for the desalination of sea water. is the high cost price of heat exchangers, especially when they are constituted by tubes of noble metal (copper alloy, monel metal, titanium). It is therefore endeavored to achieve a substantial reduction of the necessary capital outlay by improving the quality of the heat transfer proces which accordingly . makes it possible to reduce the heat-transfer surface.

One of the means for achieving this improvement consists in preventing the formation of a condensation film on the external walls of heat-exchanger tubes and in replacing said film by condensation in drops.

In point of fact, if consideration is given to the conventional case of cylindrical-tube condensers, the overall heat-transfer coefficient U between the vapor to be condensed and the cooling fluid is calculated by means of the following relation : where h^ is the coefficient of" heat transfer inside the tube on the cooling-fluid side, h is the condensation heat-transfer coefficient outside the tube on the vapor side, r^ is the thermal resistance of the wall, r, is the thermal resistance of scale and dirt. fr- In the most common case in which the known practice consists in preventing excessive deposition, of scale or dirt by means of a suitable system for the pre- treatment of the cooling fluid and in which the wall is constituted by a material having sufficiently high heat conductivity (such as a copper alloy, for example), the relative values of the different terms of formula (1) are substantially as follows : 40% for the term 1/h. 1 20% for the sum r + r, p t 40% for the term 1/h c It is thus apparent that, if an improvement is achieved in the coefficient of condensation h_ by a factor of 2, for example, the effect thereby achieved will be to produce an increase in the overall heat-transfer coefficient by a factor of 1.25.

The improvement in the condensation coefficient is achieved by bringing about condensation in drops on the external or internal wall of the heat-exchanger tube. In order to produce said condensation in drops, the wall must be rendered hydrophobic, that is to say not wettable by water. The vapor thus condenses in the form of drops which form and grow at preferential points of the wall.

When the drops have reached a sufficient size, they run along the wall and are detached from this latter. The greater part of the tube wall is therefore freed from the liquid and thus directly in contact with the vapor, thereby increasing the heat-transfer coefficient.

The means which are at present known for making the external or internal walls of heat-exchanger tubes By way of example, the walls aforesaid can be provided with a hydrophobic coating which must either consist of a substance having fairly good conductivity or be deposited in extremely thin layers. This coating must also be of very high strength and have a practically infinite lifetime. At the present time, gold or chromium coatings produce good results but are extremely costly. ' It is also possible to make use of organic agents for promoting condensation in drops such as copper oleate, octadecylamine, mercaptan, the silicone oils, and so forth. These products are introduced into the vapor phase and come into contact with the heat-exchanger tube in a highly divided state in either the liquid phase or the vapor phase. Said products are mixed with the vapor and deposited on the external or internal wall of the tube in the form of a thin film or a monomolecular layer on which they are adsorbed. The organic products under consideration must be non-toxic and must not cause corrosion of the tubes.

The precise aim of the invention is to provide a method for initiating the formation of drops on the tubes of a heat exchanger by utilizing the action of a particularly advantageous agent for promoting condensation in drops.

The method according to the invention is characterized in that at least one chemical compound is incorporated in the feed vapor before this latter passes along the tube, said compound being constituted by a fluorinated derivative corresponding to the general formula ; ¾ *i - Cc¾)a- x in which n represents a whole number comprised between 2 causing the fluorinated product to adhere to the tube wall and is a member of the group consisting of -COOH, -SC^NCH C2H4OH, -SH, - H3, -COCOCH3, -S03H, -SC^NH (CH2) gOH , or -S-S-(CH ) -CnF_n+l and a is a whole number comprised between 2 and 20 or a = 0 and X is -C2H3C10P03H2.

The fluorinated derivatives hereinabove defined have been disclosed in French patent specification No. 2,163,808. When they are incorporated in the feed vapor, said compounds are fixed on the tube wall so as to form a coating which has. excellent hydrophobic and oleo-phobic properties. It is thus possible in accordance wit the method of the invention to cause condensation in drop not only of water vapor but also of vapors of organic liquids Among the fluorinated derivatives which are suitable for use in the method according to the invention can be mentioned the following products which are given without any limitation being implied : OF3 - The method in accordance with the invention can No. 2,175,332.

.» In accordance with an advantageous feature of the invention, there is employed as fluorinated derivative a polyfluorinated disulphide having the general formula: CnF2n+l - (CH2}a " S " S " ~ CnF2n+l in which cnF2n+l rePresents a straight or branched per-fluorinated chain, n is a whole number comprised between 2 and 20, and a is a whole number comprised between 2 and 20.

Said polyfluorinated disulphides have been disclosed in French patent specification No.

These polyfluorinated disulphides make it possible to obtain particularly advantageous results by means of the method according to the invention.

In accordance with the invention, the incorporation of one or a number of polyfluorinated products in the vapor phase is carried out.: - either by mixing with the liquid which is intended to form the feed vapor, in which case the fluorinated products are distilled under their own vapor tension ; - or by passing the vapor over a solution or emulsion of fluorinated products in water ; - or by mechanical injection of the fluorinated product or products into the feed vapor in the form of a mist, said product or products being dissolved in a solvent if necessary..

In accordance with an advantageous feature of the invention, the fluorinated product or products are employed in association with a standard agent for promoting condensation in drops. The introduction of these different com-pounds can be carried out either simultaneously or separately.

The incorporation of the fluorinated derivative in the feed vapor can be performed either continuously or non-contin ously , The operation is preferably non-continuous since the fluorinated products employed have a very long lifetime, thereby permitting addition at a very low rate and achieving an appreciable economy.

Depending on the use which is contemplated and in particular in the case of plants for the desalination of sea water, it is essential to ensure that the agent for promoting condensation in drops is not toxic. This is achieved by means of the method according to the invention by reason of the fact that the fluorinated derivatives employed have a sufficiently long perfluorinated chain in their molecules, thereby reducing or eliminating toxicity.

A few examples of practical execution of the method according to the invention are given hereinafter without any limitation being implied.

Example 1 The apparatus employed is illustrated in the accompanying figure and comprises a vapor generator 1, a condensation chamber 2 and a cooling system 3.

The vapor generator 1 which is constructed of stainless steel is a cylindrical enclosure 0.5 m in diameter and 1 m in length and containing approximately 100 liters of permuted water. The generator is heated by immersion-heating elements which are supplied with three-phase current and deliver a maximum power of 36 W. pyrex cylinder 1 m in length and 400 mm in diameter, the thermal insulation of which is ensured "by means of a layer of glass wool fitted with two viewing windows. The chamber 2 is connected to the generator 1 by means of a heat-insulated stainless steel duct 5» whose internal diameter (160 mm) is calculated so that the velocity of the vapor which penetrates into the chamber does not exceed 1 m per secondo The condensation chamber 2 is traversed by a measuring tube 6 on which the condensation is brought about. Said tube 6 has a length of 900 mm, an external diameter of 25 mm and a thickness of 1 mm. Heat insulation between said tube and the walls of the chamber 2 is provided by means of teflon rings 7.- A condensate collector 8 having a length which is appreciably greater than that of the tube 6 is placed beneath this' latter.

The water of the cooling loop 3 circulates through the tube 6 and said loop is: connected' directly to the municipal water-supply system. The water is circulated by a pump 9 which is capable of· delivering 15 m per hour under the operating conditions employed. Adjustment of the temperature of the cooling fluid is obtained by continuous introduction of water derived from the municipal water-supply system ; this adjustment is performed manually by producing action on the needle-valve 10 which controls the · removal of the hot water at the outlet of the condensation chamber 2.

The high flow rate of the water which circulates within each tube is chosen so as to ensure that the ' temperature rise resulting from condensation on the tube '6 is small com ared with the tem erature difference between >■?--the vapor and the cooling water.

In this example, the tube 6 is of copper.

A polyfluorinated disulphide compound having the formula (CgF-^CgH^S^ is' introduced through the duct 11 into the vapor which is supplied by the generator 1 . Approximately 38 mg of said compound must be injected in order to obtain 1 nr of condensate. A single injection of ^C8'F17G2¾^ ½ makes possible to obtain condensation in drops for a period exceeding 2500 hours, thus showing that the compound (CgF-^^H^S^ has a considerable lifetime.

Example 2 The same apparatus as that described in Example 1 is again employed but in this case the tube 6 is fabricated from an alloy of copper and nickel.

At the time of start-up of the apparatus, striped markings and spots of different colors appear on the surface of the tube 6 as a result of fine streams of condensates which are formed on the tube and oxidize this latter.

Under these conditions, the compound (¾F17C2H ^ alorie woul(i η0* readily be fixed on the tube 6 . There is therefore employed a mixture formed of the compound (CgF^C^S^ and the acid CgF-^C^ OOOH. This acid permits fixation of the disulphide derivative. A mixture formed of 1/3 acid and 2/ 3 disulphide is injected only once through the duct 11. The time of formation of condensate in drops is in excess of 2500 hours.

In the two examples given in the foregoing, the overall heat-transfer coefficient has been multiplied by three .

Example 3 Further tests have been carried out with other compounds and with a stainless steel condenser.

The promoting agent to be tested was introduced into the vapor phase. The type of condensation obtained, namely either in a mixed film or in drops, was observed.

The following table summarizes the tests : ^e^l? C¾^10 G00H s prod ces drops G8^17G2¾^3H '' m^xe^ condensation at the time of injection (stops as soon as the injec tion is discontinued) C6 15CHC1-CH20P05H2; idem C7F.15-COCH2-pOCH5 : idem (G8"^17G2¾^2 s ^ro s a^ ^e ime °f injection (stops as soon as the injection is discontinued) .

Example 4 The tests of Example 3 are repeated with a condenser of ordinary steel. In the case of (CgF-^C.pH^S^ good condensation in drops is obtained.

Example 5 The tests of Example 3 are repeated with an aluminum condenser. After a number of injections of (C8F17C2¾S » condensation in drops is obtained.

Example 6 The tests of Example 3 are repeated with a condenser of copper alloy.

The following table summarizes the tests : ^G8"F17°2¾S½ °° excellent condensation in drops. Very long lifetime C8F17(CH2)10COOH : condensation in drops for a few hours : idem

Claims (3)

1. 44075/2 What we claim is : 1. A method for causing condensation in drops on heat-exchanger tubes, wherein at least' one chemical compound is incorporated in the feed vapor before said vapor passes along the tube, said compound being constituted by a fluorinated derivative corresponding to the general formula CnF2n+l - (CH2)a -in which n represents a whole number comprised between 2 and 20, X represents a chemical function which is capable of causing the fluorinated product to adhere to the tube wall and is a member of the group consisting of -COOH, -S02NCH3C2H4OH, -SH, -NHj f -COCOCH3 , -S03H, -S02 H (CH2 ) gOH , or -S-S-(CH0) - C F_ ,,, and a is a whole number ί a n n+l. — comprised between 2 and 20 or a = 0 and X is -C2H3ClOP03H2.

2. A' method according to claim 1, wherein there is employed as fluorinated derivative a polyfluorinated di-sulphide having the general formula s °nF2n+l - ^CH2)a-S-S^CH2)a-CnF2n-:-l in which C I^.,-, represents a straight or branched per-fluorinated chain, n is a whole number comprised between 2 and 20, and a is a whole number comprised between 2 and 20 „

3. A method according to claim 1 or claim 2, wherein the fluorinated derivative is incorporated by mixing with the liquid which is intended to form the feed vapor. . A method according to claim 1 or claim 2, wherein the fluorinated derivative is incorporated by passing the feed vapor over an emulsion of said fluorinated derivative in water „ 5. A method according to claim 1 or claim 2, wherein the fluorinated derivative is incorporated by mechanical injection of said derivative into the feed vapor in the feed vapor in a continuous manner. 7. A method according to any one of claims 1 to 5» wherein the fluorinated derivative is incorporated in the feed vapor in a non-continuous manner. 8. A- method according to any one of claims 1 to wherein use is made of a fluorinated derivative consisting of a compound having the formula : ( CgF-^rjC^H^S ~) „ 9. A method according to any one of claims 1 to 7» wherein use is made of a mixture of fluorinated derivatives. 10. A method according to claim 9» wherein use is made of a fluorinated derivative consisting of a compound having the formula njunction with an acid having the for 11. An application of the method according to any one of claims 1 to 10 to plants for the desalination of sea water. 12. A method according to claim 1 for causing condensation in drops on heat-exchanger tubes, substantially as hereinbefore described with reference to the examples and accompanying drawing. B 4699.3 ppticanff