EP0041005A1 - Method for mechanical energy production from heat using a mixture of fluids as the working fluid - Google Patents

Abstract

Mechanical power is generated by a process comprising (a) progressive vaporization of a mixture of fluids, (b) expansion of the resultant vapor, (c) condensation of the vapor and (d) recycling to step (a) of the liquid phase obtained in step (c). The heat exchanges are effected counter-currently, thus providing parallel evolutions of temperature. The condensation is effected in a temperature interval of from 7 DEG to 30 DEG C.

Description

The need to save energy and use new energy sources leads to the development of processes for the production of mechanical energy, which can be used directly or transformed into electrical energy, from relatively low level heat sources thermal, that is to say in a temperature range which can range, for example, from 50 to 400 ° C. Such sources of heat can be of various nature: industrial thermal discharges, heat transmitted by solar collectors, geothermal water, etc ... From such sources of heat, it is possible to produce mechanical energy by means of '' a Rankine cycle using a working fluid which is vaporized under pressure by taking heat from the heat source, expanded by producing mechanical energy, for example in a turbine, this mechanical energy can be used directly or transformed in electrical energy and condensed by means of a cooling fluid, water or air.

To improve the efficiency of the cycle and avoid operating at very low pressure, it is advantageous to replace the water, which is very generally used at higher temperature, with a fluid whose boiling point and critical temperature are many lower, such as butane or ammonia.

Such a fluid vaporizes and condenses at a substantially constant temperature.

However, the temperature of the external fluids with which the exchanges take place changes, as a general rule, during the exchange.

It has been discovered, and this is one of the objects of the present invention, that it is advantageous in this case to use a mixture of fluids which vaporizes and condenses progressively according to the temperature development of each of the external fluids with which exchanges take place.

The mixture is vaporized according to a temperature interval A by taking heat from an external fluid I which constitutes the heat source and the temperature of which changes according to a temperature interval A '. It is then relaxed by producing mechanical energy which can be used directly or transformed into electrical energy, then it is condensed according to a temperature interval B by yielding heat to an external fluid II which constitutes the cooling fluid and whose the temperature changes according to a temperature interval B '.

To take full advantage of the process according to the invention, it is however necessary to observe certain conditions.

For the yield obtained to be maximum, the temperature intervals A and B must be as close as possible to the temperature intervals A 'and B', which corresponds to the best conditions of thermal reversibility. The temperature interval A 'according to which the heat is supplied to the cycle being fixed, the composition of the mixture is chosen so as to obtain a vaporization interval A close to the temperature interval A'. In the case of a binary mixture, the temperature interval A generally evolves as shown in the diagram shown in Figure 1. For a given molar fraction x _I of the most volatile component 1 of a mixture formed of components 1 and II, the vaporization temperatures at the pressure in question are T _I and T _II , the vaporization of the mixture begins at the bubble temperature of the liquid TLB and ends at the dew temperature of the vapor T _VR . The vaporization interval is therefore equal to the difference between the temperatures T _LB and T _VR and can be adjusted by choosing the appropriate composition.

The condensation interval B is generally close to the vaporization interval A. In this case, it is advantageous to adjust the flow rate of the cooling fluid, water or air, used to carry out the condensation so that the interval of temperature B 'is close to the condensation interval B.

This also makes it possible, compared to operation with a pure body, to reduce the flow of water or cooling air and to reduce the energy consumption linked to cooling air ventilation or to pumping water. cooling. However, it is necessary to prevent the temperature interval B from becoming too large to avoid a drop in yield. For this reason, it im limits the temperature interval B to a value of 30 ° C. On the other hand, this interval must be at least 7 ° C so that the gain in yield brought about by the use of a mixture is significant. Consequently, in order to be placed in optimal yield conditions and to benefit from the advantages which the use of a mixture brings, it is important that the temperature range B is between 7 and 30 ° C. This condition will generally also be valid for the temperature interval A which is generally close to the temperature interval B, when the vaporization is carried out in a single step.

The implementation of the process can be described with reference to Example 1.

EXAMPLE 1

• Butane normal : 0,8
• Hexane normal : 0,2

The example is illustrated in Figure 2. Via the conduit 1 a flow of 5.67 m ³ / h of water arrives at a temperature of 85 ° C. Via line 4, 1254 kg / h of a mixture of the following composition are sent (in molar fractions):

• Normal butane: 0.8
• Normal hexane: 0.2

This mixture arrives at 20 ° C and begins to vaporize at 52 ° C by exchanging heat against the current with the water which arrives through line 1 in the exchanger E101. After exchange, the water leaves the exchanger E101 via the line 2 at a temperature of 60 ° C and the mixture leaves vaporized from the exchanger E101 through the line 3 at a temperature of 75 ° C and at a pressure of 4 , lbars.

The mixture is then expanded in the paddle motor M1 which drives the alternator AT1. An electrical power of 9 kW is collected at the terminals of the alternator. The mixture emerges from the M1 vane motor at a pressure of 1.6 bars. It is gradually condensed / in the exchanger E102 from where it is collected in the reserve tank B1. Cooling is ensured by water which enters through line 7 at 12 ° C and exits through line 8 at 32 ° C.

The liquid mixture is taken up, through line 6, by the pump P1 and recycled to the evaporator E101.

In this example, the use of a mixture of butane and hexane, makes it possible, during the vaporization and condensation stages, to follow the change in temperature of the external fluids, the mixture of fluids vaporizing according to an increasing change in temperature parallel to the decreasing change in temperature of the external fluid I and condensing according to a decreasing change in temperature parallel to the increasing change in temperature of the external fluid II. These changes in temperature necessitate operating the heat exchanges at the evaporator and at the condenser under conditions as close as possible to the counter-current. A pure counter-current heat exchange mode represents the preferred solution, but for implementation reasons, it is also possible to mount exchange surfaces in a generally counter-current arrangement, each of the exchange surfaces forming part of said arrangement operating under conditions different from the counter current, for example following a heat exchange with cross currents or with a circulation of one of the fluids taking place in U-shaped tubes.

The mixtures used can be mixtures of two, three (or more) constituents (separate chemical compounds). The constituents of the mixture can be hydrocarbons, the molecule of which comprises a number of carbon atoms of, for example, between 3 and 8, such as propane, normal butane and isobutane, normal pentane and i-sopentane, normal hexane and isohexane, normal heptane and isoheptane, normal octane and isooctane as well as aromatic hydrocarbons such as benzene and toluene and cyclic hydrocarbons such as cyclopentane and cyclohexane. When it is desired that the mixture is not flammable or is only hardly flammable, the mixture used can be a mixture of halogenated hydrocarbons of the "Freon" type such as chlorodifluoromethane (R-22), dichlorodifluoromethane (R- 12), chloropentafluoroethane (R-115), difluoroethane (R-152), trichlorofluoromethane (R-11), dichloro-tetrafluoroethane (R-114), dichlorohexafluoropropane (R-216), dichlorofluoromethane (R- 21), trichlorotrifluoroethane (R-113). One of the constituents of the mixture can be an azeotrope such as the R-502 azeotrope of R-22 and R-115 (48.8 / 52.2% by weight), the R-500 azeotrope of R-12 and of R-31 (78.0 / 22.0% by weight), the azeotropic R-506 of R-31 and R-114 (55.1 / 44.9% by weight).

Other types of mixtures are mixtures comprising water and at least one second component miscible with water such as mixtures formed of water and ammonia, mixtures formed of water and a amine such as methylamine or ethylamine, mixtures formed of water and an alcohol such as methanol, mixtures formed of water and a ketone such as acetone.

When the process operates according to the diagram shown in FIG. 2, the composition of the mixture is chosen so that the vaporization intervals A and of condensation B are as close as possible to the temperature intervals A 'and B' according to which evolve external fluids. To obtain a maximum gain in yield, it is preferable that the difference between the temperature intervals A and A ′ is less than 5 ° C.

It has also been discovered that in the case where a mixture is used as working fluid, other improvements can be envisaged when at one point of the circuit the mixture is divided into two fractions, which are remixed at another point of the circuit, one of said fractions covering all the different stages of the cycle and the other of these fractions covering only part of the stages of this cycle.

If the heat recovered by the evaporator is available in a relatively wide temperature range and the mixture is chosen to vaporize according to a neighboring temperature interval, operate according to the operating diagram shown in Figure 2 leads to operating with a wide temperature range B, which does not correspond to the conditions most favorable to yield. In this case, it is possible to operate according to the operating diagram represented in FIG. 3. The mixture is condensed in the exchanger E105 while being cooled by an external fluid which enters via the conduit 9 and leaves through the conduit 10. The condensed mixture emerges of the exchanger E105 via the conduit 11 and it is sent to the reserve tank B2. The pump P11 makes it possible to send a fraction of the liquid mixture via the conduit 12 into the exchanger E103 in which it vaporizes according to a temperature interval A ₁ by exchanging heat with an external fluid which enters through the conduit 13 and comes out through the conduit 14. The vaporized spring mixture of the exchanger E103 through the conduit 15 and it is sent to the engine stage M2. The pump P10 sends the remaining fraction of the liquid mixture through the pipe 16 into the exchanger E104, in which it vaporizes according to a temperature interval A ₂ by exchanging heat with the external fluid which arrives through the pipe 14 and res exits through line 17. The mixture leaves vaporized from the exchanger E104 and the steam thus obtained is mixed with the steam coming from the expansion through stage M2, then expanded at the same time as the steam coming from stage M2 in the engine stage M3 from which it emerges through the conduit 19.

Provided that the intermediate pressure level, that is to say the pressure at which the mixture vaporizes in the exchanger E104, is properly chosen, the temperature intervals A and A ₂ can be consecutive and it is thus possible to follow with the mixture an evolution of temperature parallel-to an evolution of temperature of the external fluid which provides heat to the cycle, corresponding to a temperature interval A 'approximately twice as wide as in the case of the operating diagram represented in the Figure 2.

It has also been discovered that in many cases it is particularly advantageous to operate according to the arrangement shown diagrammatically in FIG. 4. The condensed mixture is only partially vaporized in the exchanger E106 by taking heat from the external fluid which arrives via line 20 and leaves via line 21. On leaving the exchanger E106, the liquid and vapor fractions are separated in the separator flask S1. The steam fraction is expanded in the T3 turbine. The liquid phase is sent to the exchanger E107 in which it exchanges heat with the condensed mixture which is sent to the evaporator, then expanded through the expansion valve V1 and mixed with the expanded vapor phase leaving the turbine T3 . The mixture of the liquid vapor thus obtained is condensed by yielding heat to an external cooling fluid, collected in the reserve tank B3 and recycled by the pump P3 to the evaporator. The operating conditions of a device operating according to the arrangement shown diagrammatically in FIG. 4 are the subject of Example 2.

EXAMPLE 2

• NH₃ : 0,75
• H₂0 : 0,25

The example is illustrated in Figure 4. By pump P3, a flow rate of 3956 Kg / h is circulated of a mixture of water and ammonia of the following composition (in weight fractions):

• NH ₃ : 0.75
• H ₂ 0: 0.25

This mixture is sent via line 31 into the exchanger E107 from which it emerges via! E line 22 at a temperature of 55 ° C. It is then sent to the exchanger E106 in which it partially vaporizes by taking a thermal power of 1585 kW from a water flow which arrives via the conduit 20 at a temperature of 90 ° C and exits through the conduit 21 at a temperature of 65 ° C. The liquid-vapor mixture leaves the exchanger E106 through line 23 at a temperature of 85 ° C. and at a pressure of 20 bars. It is collected in the separator tank S1 in which the liquid phase and the vapor phase are separated. The liquid phase contains 52% ammonia by weight. It is evacuated via line 25 and sent to the exchanger E107.

The vapor phase is sent via line 24 to the turbine T3 in which it is expanded to a pressure of 8 bars. On the shaft of the turbine T3 is collected by means of the electric brake FE1 a power of 100 kW. The expanded steam is evacuated through the pipe 26. The liquid phase which leaves through the pipe 27 of the exchanger E107 is expanded through the expansion valve VI, from where it comes out through the pipe 28. It is then mixed with the vapor phase arriving through line 26 and the liquid-vapor mixture is sent through line 29 into the air cooler AR1, in which it is fully condensed B and from which it emerges through line 30 at a temperature of 28 ° C. The AR1 air condenser is made up of tubes provided with fins inside which the mixture circulates by condensing, these tubes being arranged in five layers placed transversely to the air circulation but mounted against the current, the mixture thus circulating generally against the flow of the cooling air. The condensed mixture is collected in the reserve tank B3 from where it is taken up by the feed pump P3.

The operating diagram, shown in Figure 4, makes it possible to adapt to variable operating conditions. In particular, by modifying the flow rate conveyed by the pump P3 through the conduit 31, it is possible to modify the pressure levels at the evaporator and at the condenser. In particular by increasing the flow rate of the pump P3, for fixed outlet temperatures to the evaporator and to the condenser, the pressure levels in the evaporator and in the condenser are reduced, which makes it possible to reduce the capacity of the system, c is the power delivered on the shaft.

In general, the operating mode shown diagrammatically in FIG. 4 considerably widens the possibilities offered by the use of the mixtures in the engine cycles.

It makes it possible to use mixtures of constituents whose boiling temperatures are very different, such as water and ammonia, in applications where the temperature intervals in the evaporator and the condenser are restricted, for example of on the order of 10 to 15 °, since in the evaporator only partial vaporization is carried out, which makes it possible to operate with a temperature range as reduced as desired.

On the other hand, as has already been indicated above, it is possible in such a system to adjust the pressure levels by varying the concentration of the solution which circulates. It is thus possible to place oneself in the optimal conditions making it possible to achieve a reduced volume flow rate and therefore a low volume expansion machine without bringing into play excessive pressures which would lead to excessively large investments.

Whatever the operating diagram, the operating conditions are generally chosen so that the pressure of the mixture in the evaporator is between 3 and 30 bars and so that the pressure of the mixture in the condenser is between 1 and 10 bars.L ^the temperature interval A is generally within the temperature range of 50 à.350 ° C and the temperature interval B is generally within the temperature range from 20 to 80 ° vs.

The operating diagrams given by way of example are not limiting and in particular all the improvements known to those skilled in the art in the case of Rankine cycles using a pure body as working fluid can be envisaged in the case mixtures. For example, when the engine in which the expansion of the vapor phase takes place comprises several stages, it is possible to preheat the liquid mixture sent to the evaporator by heat exchange with a vapor fraction withdrawn between two stages of the engine, the condensation of this vapor fraction to preheat the liquid mixture.

It is also possible to carry out different variants and combinations from the basic diagrams which have been described. For example, it is possible to carry out a vaporization in two or more stages at different pressure levels in order to widen the heat removal interval, the vaporization carried out during each of said vaporization stages being only partial and the liquid phase remaining at the end of said vaporization stages being recycled to the condensation stage according to the arrangement which was described in Example 2 in the case of a single vaporization stage.

On the other hand, different types of equipment known to those skilled in the art can be used in the method according to the invention.

The evaporator and the condenser can be, for example, tube and shell exchangers, double-tube exchangers or plate exchangers. When the heat exchange takes place with a fluid which is a gas, for example if air is used as coolant for the condenser, it is generally advantageous to provide the exchange surfaces with fins placed on the side of the gas to improve heat exchange with this gas.

The expansion of the vapor phase generated in the evaporator, which makes it possible to produce mechanical energy, can be carried out in all the machines known for this exchange: such a machine can be for example a turbine with one wheel or with several wheels, radial or axial, a screw machine of the same type as the screw compressors but operating in expansion, a vane motor or a reciprocating piston engine.

The mechanical power delivered can be very variable and range, for example, from a few kW to several megawatts.

In the claims which follow, it is indicated that the mixture of fluids must not form an azeotrope under the conditions of the vano- rization. This means that at least two constituents of this mixture do not form an azeotrope between them; however, each of the constituents can individually be an azeotrope.

Claims (12)

1. A method of producing mechanical energy characterized (a) in that at least a fraction of a fluid mixture (M) is gradually vaporized comprising at least two constituents which do not form an azeotrope under the vaporization conditions , by taking the heat of vaporization at least in part from an external fluid whose temperature changes according to a temperature interval A 'during the exchange, the temperature of the mixture evolving according to a temperature interval A, (b) in this that the vapor phase thus obtained is expanded by producing mechanical energy, (c) in that one gradually condenses, according to a temperature interval B, the vapor thus obtained by yielding heat to at least one external fluid II whose temperature varies according to a temperature interval B ', the width of the temperature interval B being at least 7 ° C and at most 30 ° C, and (d) in that the phase liquid from step (c) is recycled to step (a), the heat exchanges carried out with the external fluids 1 and 11 in stages (a) and (c) respectively being operated against the current, the mixture of fluids vaporizing in stage (a) according to an increasing temperature evolution parallel to the decreasing evolution of temperature of the external fluid 1 and condensing in step (c) according to a decreasing evolution of temperature parallel to the increasing evolution of temperature of the external fluid 11. 2. Method according to claim 1, in which the mixture of fluids is separated into two fractions at one point in the circuit and the resulting fractions are remixed at another point in the circuit, the first of these separate fractions passing through all of the steps ( a), (b) and (c) and the second of these fractions not traversing at least one of the stages traversed by the first of these fractions. 3. Method according to one of claims 1 and 2, wherein the difference between the temperature of the mixture traversing the interval A and the temperature of the fluid I traversing the interval A 'is at all times less than 5 ° C. 4. Method according to one of claims 1 to 3, wherein the mixture (M) is vaporized in at least two steps carried out at different pressure levels, a first fraction of the mixture being vaporized at the highest pressure level in taking heat in a first temperature interval, the vapor phase obtained being sent to the head of the driving machine in which the expansion takes place, said driving machine comprising a number of stages at least equal to the number of vaporization stages , the remaining fraction being vaporized in at least one step carried out at a pressure level lower than the pressure level of the first step, by withdrawing heat in a temperature interval situated at least partly below from the first temperature interval, the steam fraction (s) thus obtained being sent to the successive stages of the engine in which the expansion takes place at points corresponding to the pressure levels steam, the vapor mixture obtained after expansion being condensed and the liquid phase obtained after condensation being recycled to the vaporization stages. 5. Method according to one of claims 1 to 4, wherein the mixture (M) is partially vaporized in the evaporator by taking heat from an external fluid, the vapor phase and the liquid phase thus obtained being separated, the vapor phase being expanded by producing mechanical energy, the liquid phase being sent to an exchanger in which it exchanges heat with the condensed mixture (M) which is sent to the evaporator, the liquid phase then being expanded and mixed with the expanded vapor phase, the liquid-vapor mixture thus obtained being condensed by yielding heat to an external fluid, the condensed mixture (M) thus obtained being recycled to the evaporator. 6. Method according to one of claims 1 to 5, wherein the mixture is a mixture of hydrocarbons whose number of carbon atoms is from 3 to 8. 7. Method according to one of claims 1 to 5, wherein the mixture is a mixture of halogenated hydrocarbons. 8. Method according to one of claims 1 to 5, wherein the mixture is a mixture of water and at least one water-miscible constituent chosen from alcohols, ketones and amines. 9. Method according to one of claims 1 to 5, wherein the mixture is a mixture of water and ammonia. 10. Method according to one of claims 1 to 9, in which the temperature range A is in the temperature range from 50 to 350 ° C and in which the temperature range B is in the range temperature ranging from 20 to 80 ° C. 11. Method according to one of claims 1 to 10, wherein the pressure of the mixture in the evaporator is between 3 and 30 bar and the pressure of the mixture in the condenser is between 1 and 10 bar. 12. Method according to one of claims 1 to 11, wherein the mechanical energy produced by expansion of the vapor phase mixture is converted into electrical energy.

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