Surfactants with the ability to form extremely long, cylindrical micelles have, in recent years, attracted a great interest as drag-reducing additives to systems with circulating water, especially those destined for heat or cold distribution.

An important reason for this interest is that, although one desires to maintain a laminar flow in the conduits, one wishes at the same time to have turbulence in the heat exchangers to achieve therein a high heat transfer per unit area.

As may easily be understood, fibres or chain polymers are unable to provide this double function which, however, can be achieved with thread-like micelles, since the micelles, which are responsible for the drag reduction, can be destructed by mechanical devices either within the heat exchangers or immediately before them. Thus a turbulent flow will be created within said heat exchangers. In the tube after the exchanger the micelles will form again rather rapidly and the drag reduction will thus be restored.

The thread-like micelles are distinguished by operating in a fairly disorderly fashion at low Reynold's numbers (below 10 4), having no or only a very slight effect on the flow resistance. At higher Reynold's numbers (above 104), the micelles are paralleled and result in a drag reduction very close to that which is theoretically possible. At even higher Reynold's numbers (e. g. above 105) the shear forces in the liquid become so high that the micelles start to get torn and the drag-reducing effect rapidly decreases as the Reynold's number increases above this value.

The range of Reynold's numbers within which the sur- face-active agents have a significant drag-reducing effect is dependent on the concentration, the range increasing with the concentration.

By choosing the right concentration of surface-active agents and suitable flow rates in conduits and adequate devices before or in the heat exchangers, it is thus possible to establish a laminar flow in the conduits and turbulence in the heat exchangers. Thus, the dimensions of the conduits can be kept at a low level and the pump size, or the number of pump stations, and consequently the pump work, can alternatively be reduced while retaining the same tubular dimensions.

In WO 96/28527 a drag reducing agent is disclosed, which comprises a betaine surfactant in combination with a sulphonate or sulphate surfactant. This drag-reducing agent is effective within comparatively large temperature ranges.

However, the sulphate surfactant is rather sensitive to hard water, while the sulphonate surfactant is not regarded as easily biodegradable under anaerobic conditions.

It has now surprisingly been found that essential improvements are achieved by the use of a zwitterionic surfactant having the formula R is a group containing saturated or unsaturated aliphatic or acyl group with 10-24 carbon atoms, R6 and R7 are independently of each other an alkyl group of 1-4 carbon atoms or an hydroxyalkyl group of 2-4 carbon atoms, and R4 is an alkylene group of 1-4 carbon atoms, preferably CH2 or a group

where R5 is an alkyl group of 1-3 carbon atoms, in combina- tion with an anionic ether surfactant having the general structure Rl (OA) nB, R10 (AO) nCmH2mD or R3NH (AO) nCmH2 mD or a mixture thereof, where Ri is a hydrocarbon group of 10- 24 carbon atoms, R3 is an acyl group of 10-24 carbon atoms, A is an alkylene group having 2-4 carbon atoms, n is a number from 1 to 10, m is 1-4, B is a sulphate group OSO3M, D is a carboxylate group COOM, and M is a cationic, preferably mono- valent group, in a weight proportion between the zwitterionic surfactant and the anionic ether surfactant or ether surfactants of from 100: 1 to 1: 1, preferably from 50: 1 to 2: 1, as a drag-reducing agent in a flowing water-based liquid system. By"water-based"is meant that at least 50% by weight, preferably at least 90% by weight, of the water-based liquid system consists of water. The total amount of the zwitterionic surfactant and the anionic ether surfactants may vary within wide limits depending on the conditions but is generally 0.1-10 kg/m3 of the water-based system. The combinations of the zwitterionic and the anionic ether surfactants are especially suited for use in water-based systems flowing in long conduits, for distribution of heat or cold.

The group R in the zwitterionic surfactant is suitably an aliphatic group or a group R'NHC3H6, where R'designates an acyl group with 10-24 carbon atoms. Preferably the zwitterionic surfactant has the general formula where R is the aliphatic group or the group R'NHC3H6-where R'has the meaning mentioned above. In the anionic ether surfactant the hydrophobic group R1 can be aliphatic or aro- matic, straight or branched, saturated or unsaturated.

Furthermore, the groups A are preferably ethylene, n is

preferably a number from 1-5 and CmH2m is preferably methylene or the group -CH- R8 where R8 is an alkyl group of 1-3 carbon atoms. The group M is preferably sodium and potassium.

Both the zwitterionic surfactant and the anionic ether surfactants are readily biodegradable and tolerant towards hard water and electrolytes and said combination gives an excellent drag reducing effect within a wide temperature range. Thus, the drag-reducing additives may be used in a cooling media at temperatures below 20°C, when using surfactants, where the groups R and R'have 12-16 carbon atoms, and in a heat-transfer medium at a temperature in the range of 50-120°C, when using surfactants where the groups R, and R'contain 18,20 or 22 carbon atoms or more. The number of carbon atoms in the hydrophobic groups R, R', R1 and R3 will affect the useful temperature range for the mixture so that a high number will give products suitable for high temperatures and vice versa. The groups R and R1 can suitably be dodecyl, tetradecyl, hexadecyl, octadecyl, oleyl, eicosyl, docosyl, rape seed alkyl and tallow alkyl and the groups R'and R3 the corresponding acyl groups. Also aromatic groups, such as nonylphenol, may be used.

Furthermore, the zwitterionic and anionic surfactants are suitably chosen in such a manner that the crystallisation temperature for the combination is suitably below the lowest temperature for which the water-based system is intended.

Suitably the zwitterionic surfactant is combined with an anionic ether sulphate surfactant where n is 1-5 and OA oxyethylene, since the ether sulphate is easy to produce and gives in combination with the zwitterionic surfactant excellent drag-reducing effects.

The zwitterionic surfactant can be produced by reacting a compound of the formula RNR6R7, where R has the meaning mentioned above, with Na-chloroacetate at 70-80°C and a con-

stant pH-value of 9.5 in a medium of a lower alcohol and water. To obtain a good drag reducing effect it is essential that the amount of the amine reactant in the zwitterionic product used is low. If a low chloride content in the product is necessary the reaction can preferably be made in iso- propanol with the lowest water content possible, whereby the sodium chloride formed in the reaction will crystallise out of the product and may be removed by filtration or centri- fugation. Another route to a chloride-free product is to quaternize the amine reactant with ethylene oxide in the presence of an acid catalyst and then dehydrogenate the resulting product to the desired zwitterionic surfactant.

The anionic ether surfactants suitable for use in accordance with the invention are well-known products and so are also the production methods. Typical examples are aliphatic mono (oxyethylene) sulphates, alkyl di (oxyethylene) sulphates and alkyl tri (oxyethylene) sulphates derived from ethoxylated alcohols by sulphation with SO3 and the corresponding carboxylates obtained by reacting said ethoxylated alcohol and a halogenated carboxylate having the formula HalCmH2mCOOM, where Hal is chloride or bromide and M and m have the meanings mentioned above. The amido ether carboxylate may be produced according to well-known methods including the reaction of said halogenated carboxylate and the amidoalkoxylate R3NH (AO) nH, where R3, A and n have the meanings mentioned above.

The choice of the zwitterionic surfactant and the anionic ether surfactant will depend of the temperature of the water-based system. At low temperature the number of carbon atoms will normally be lower than at high temperature while the number of oxyalkylene will normally be higher at lower temperatures than at higher temperatures.

A convenient way to determine the right proportion between the zwitterionic surfactant and the anionic surfactant for a certain type of water is to make up a solution of e. g. 0.500 kg/m3 of the zwitterionic surfactant in the appropriate water in a 50 ml glass beaker with a

magnetic stirrer and keep the temperature in the middle of the intended temperature range for the system. This solution is then titrated with a solution of the anionic ether surfactant with a concentration of 10 kg/m3 in the appropriate water until the originally formed vortex has disappeared.

Apart from the zwitterionic and anionic surfactant, the water-based system may contain a number of conventional components such as corrosion inhibitors, anti-freeze and bactericides.

The present invention will now be further illustrated with the aid of the following examples.

Example 1 The drag reducing temperature interval was determined in the beaker test described above. In-the beaker test the surfactant mixture was stirred at a constant rotation speed of 700 r/min using a combined magnetic stirrer and heating plate. The absence of vortex or a vortex of max. 2 mm was equal to drag reducing conditions. In temperatures above 100°C a glass pressure reactor was used.

From stock solutions mixtures of betaine and anionic surfactant were prepared. The mixtures were diluted with water, with hardness according to the tables below, to 1000 ppm betaine and a total volume of 40 ml in a 50 ml beaker.

The amount of anionic surfactant is given as ppm in brackets.

The pH was adjusted to 9-10 with ammonia.

Table 1. N-behenyl betaine (1000 ppm) for heating systems DR interval (700 r/min) Anionic surfactant 0 °dH3 °dH8 °dH Sodium dodecyl sulphate 55-120 (30) 55-68 (30) 55-68 (30) Sodium dodecyl- (EO) 3- 55-123 (40) 55-108 (40) 50-78 (40) sulphate Dodecyl amide- (EO) 2-55-104 (40) 48-95 (40) 49-86 (40) carboxylate Nonylphenol- (EO) 3-53-97 (40) 49-94 (40) 49-74 (40) carboxylate

A drag reducing agent containing an anionic ether surfactant exhibit an essentially better drag reducing effect in water of 3 °dH and 8 °dH than the agent containing alkyl sulphate.

Example 2 The present example is performed according to the previously described screening test.

In order to determine the right amount of anionic surfactant the betaine solution was kept at 13°C in the test beaker with the magnetic stirrer running at 700 r. p. m and titrated with a water solution of the anionic surfactant until the vortex disappeared. The resulting concentration of anionic surfactant is given as ppm in brackets after the temperature range within which the composition has been found to give a drag reducing effect. The clear point (CP) of the solution is given in °C.

The concentration of the N-myristyl-betaine, the zwitterionic surfactant used in this example, was 1000 ppm in all tests.

Table 2. N-myristyl-betaine (1000 ppm) in mixture with anionic surfactant for cooling systems DR interval (700r/min) Anionic surfactant o °dH 3 °dH 8 °dH Dodecyl benzene No effect 0-29 (430) 3-25 (980) sulphonate (0-1130) CP 0°C CP 35°C CP 0°C Sodium dodecyl- (EO) 3-0-43 (288) 0-27 (550) 2-25 (510) sulphate CP 0°C CP 0°C CP 2°C Sodium dodecyl sulphate 6-43 (400) 0-39 (336) 4-43 (360) CP 6°C CP 14°C CP 20°C

These formulations are intended for comfort cooling circuits where the temperature range normally is between 4 and 15°C.

As can be seen, the dodecyl glycolether sulphate is working well in this temperature range whereas the dodecyl benzene sulphonate formulation gives no drag reduction in deionized water and sodium dodecyl sulphate does not work satisfactory in water of 8 °dH at low temperatures.

Furthermore the use of sodium dodecyl sulphate is hampered in practical applications by the precipitation both of the sodium and the calcium salts.