GB2151007A - An absorption heat pump - Google Patents

Abstract

A beat pump using an azeotrope as working fluid and suitable for pumping low temperature heat comprises two condensers (40, 28), two evaporators (2, 14), a first absorber (5) paired with a first generator (18) and a second absorber (12) paired with a second generator (21). A vapour outlet of the first evaporator (2) communicates with a vapour inlet of the first absorber (5) and a vapour outlet of the second evaporator (14) communicates with a vapour inlet of the second absorber (12) and a vapour outlet of the first absorber (5) communicates with a vapour inlet of the second absorber (12). A conduit (26) having an expansion valve (27) interconnects a vapour outlet of the second generator (21) with the first generator (18) and a conduit (29) connects a vapour outlet of the second generator (21) with the second condenser (28). A conduit (30) having an expansion valve (31) interconnects the second condenser (28) with the second evaporator (14). Low temperature heat is supplied to the first evaporator (2) via (1) and high temperature heat is supplied to the second generator (21) via (33). There are means for delivering to the load heat of absorption from the second absorber (12), means for delivering to the load heat of condensation from the first condenser (40) and means for delivering to the first generator (18) latent heat of condensation from the second condenser (28). <IMAGE>

Description

SPECIFICATION An absorption heat pump This invention relates to absorption heat pumps using an azeotrope as its working fluid.

Since water has a modest vapour pressure at load temperatures below 1 00 C, is chemically stable, has a very high latent heat of evaporation and is not toxic or flammable, it is in some respects a most desirable working fluid for absorption heat pumps. The disadvantage of water however are its high freezing point and its very low vapour pressure at temperatures below 0 C.

Although it is possible to lower the freezing point of water by the addition of freezing point depressants, such as salts, unfortunately its vapour pressure is lowered even further. One alternative is to use an azeotrope which exhibits a large positive deviation from Raoult's law where the partial vapour pressures of the two components of a binary azeotrope are only marginally less than their full vapour pressures at the same temperature. An example of such an azeotrope is that of water and n -propanol. The problem however is to find an absorbent which will absorb both components in the correct proportions at the desired load temperature.

It is known that a temperature lift exceeding 60"C may be achieved by using water as the absorbate and sodium hydroxide as the absorbent. However sodium hydroxide does not absorb n- propanol. It is possible however to use certain fatty acid glycerides, for example cotton seed oil to absorb n- propanol from a solution thereof in water. Clearly however the fatty acid glyceride (which also contains fatty acids) and sodium hydroxide cannot be mixed as soaps would be formed making it useless as an absorbent.

A way has been found in which two absorbents such as sodium hydroxide and fatty acid glycerides may be used simultaneously and this is described and claimed in our copending application PM 8106.

In that invention heat is pumped from a low temperature to a higher temperature by the use of an absorption heat pump. The pump which is used comprises a condenser, an evaporator, two generators and two absorbers, each generator being independently connected to one of the absorbers by a conduit having a expansion valve and by a conduit having a pump and the condenser being connected to the evaporator via an expansion valve. In this pump the working fluid is an azeotrope of water and an organic compound having a positive deviation from Raoult's Law and one of the paired generator and absorber contains an absorbent for water and the other paired generator and absorber contains an absorbent for the organic compound.Low temperature heat is supplied to the evaporator and this causes the azeotrope to evaporate in the evaporator and the vaporised azeotrope passes to the absorber containing the absorbent which absorbs one of the components of the azeotrope and thereafter to the absorber containing the absorbent which absorbs the other component of the azeotrope. The heat of absorption is delivered to the load. In each paired absorber and generator absorbent solution is pumped from the absorber to the respective generator, high temperature heat is supplied to each generator whence absorbate is boiled off and this passes to the condenser. At the same time the stripped absorbent solution is returned from each generator to its absorber via the conduits containing an expansion valve.The absorbate (azeotrope) which is boiled off from the two generators condenses in the condenser delivering the latent heat of condensation to the load, usually via heat exchange. The condensate is passed from the condenser to the evaporator via the conduit having the expansion valve.

When water is paired with sodium hydroxide it is possible to pump heat from O"C to 100"C.

However it is not often if at all, that the temperature lift can be matched by the organic compound.

We have now discovered a process where heat can be pumped from O"C to about 70"C using as a working fluid an azeotrope of water and an organic compound having a positive deviation from Raoult's law. According to this invention heat is pumped from a low temperature to a higher temperature by the use of an absorption heat pump comprising a first condenser, a second condenser, a first evaporator, a second evaporator, a first absorber, a second absorber, a first generator and a second generator. This pump has as its working fluid an azeotrope of water and an organic compound having a positive deviation from Raoult's Law.In the process of the invention low temperature heat is supplied to the first evaporator, the evaporated azeotrope is allowed to pass to the first absorber where the organic compound is absorbed and water vapour is allowed to pass to the second absorber. The heat of absorption is delivered from the second absorber to the load and heat of absorption from the first absorber is transferred to the second evaporator. Evaporated water from the second evaporator is allowed to pass to the second absorber where it is absorbed. Absorbent solution is pumped from each absorber to the generator with which it is paired, heat is supplied to each generator and organic compound is evaporated from the first generator.The water which is evaporated from the second generator is in part (a) allowed to condense in the second condenser where its latent heat of condensation is used to evaporate the organic compound in the first generator and thereafter expand into the second evaporator and in part (b) allowed to expand into the first generator where it combines with the organic compound evaporated therefrom to reform an azeotrope which is allowed to condense in the first condenser. Stripped absorbent solution is allowed to pass after expansion from each generator to the absorber with which it is paired. The latent heat of condensation is delivered from the first condenser to the load and the condensate from the first condenser is allowed to expand from the first condenser to the first evaporator.

Various organic compounds can be used as the component of the azeotrope provided the azeotrope has a positive deviation from Raoult's Law, i.e. it displays a minimum boiling point relative to the surrounding mixture compositions. Although homogeneous binary azeotropes are preferred it is possible to use a heterogeneous binary azeotrope. Azeotropes of water and an organic compound having a relatively high minimum boiling point, e.g. above 70"C at 760 mm Hg and a relative high content of water, e.g. above 40 mole % are preferred.

The organic compound can be certain alcohols, e.g. those having 3 to 6 carbon atoms per molecule, ethers, carboxylic acids, ketones or hydrocarbons, for example. Suitable watercontaining azeotropes in approxinate increasing order of preference are: Heterogeneous azeotropes Mole % of Minimum b.p.

Second Component Second Component Temp "C (760 mm Hg) Ethyl acetate 76.00 70.4 Benzene 70.40 69.3 Toluene 44.40 84.1 Tert Amyl alcohol 35.00 87.0 n Butyl alcohol 25.00 92.3 iso Amyl alcohol 17.21 95.2 Homogeneous azeotropes Mole % of Minimum b.p.

Second Component Second Component Temp C (760 mm Hg) Ethanol 89.43 78.2 Isopropyl alcohol 68.54 80.4 Methyl ethyl ketone 67.00 73.5 Tert Butyl alcohol 64.59 79.9 Allyl alcohol 45.50 88.2 43.17 87.7 n Propyl alcohol 34.00 88.5 sec Butyl alcohol iso Butyl alcohol 32.86 89.9 A particularly convenient azeotrope is an azeotrope of water and n- propanol and this azeotrope has about 28 to 32 weight percent of water according to the temperature.

The absorbent for water is preferably an aqueous solution of an alkali metal hydroxide, e.g.

potassium hydroxide or especially sodium hydroxide. Alternatively, one may use the reaction mixture derived from reacting a Group Ia metal compound soluble in water, e.g. a halide such as calcium chloride with a Group la metal oxide or hydroxide, e.g. sodium hydroxide, to which water is added, as described in our U.K. patent application, GB 8321572.

The absorbent for the organic compound can vary and depends on the nature of the organic compound. Glycerides of fatty acids, e.g. cotton seed oil (glycerides of palmitic, oleic and linoleic acids), coconut oil (glycerides of lauric, capric, myristic, palmitic and oleic acids), olive oil (glycerides of oleic acid, palmitic acid and linoleic acid) and linseed oil (glycerides of linolenic acid, oleic acid, iinoleic acid and saturated fatty acids) for example, are suitable absorbents for alcohols, e.g. n-propanol. The simple, i.e. non-mixed, glycerides, e.g. olein, linolein, linolenin or palmitin would also be equally suitable.

In the process of the invention the source of low temperature heat may be for example atmospheric air at about 0 C whereas the high temperature heat source may be for example a boiler supplying heat at about 150"C.

In the case of an azeotrope of water and n- propanol using sodium hydroxide and fatty acid glyceride as absorbents, the water is the more strongly bound but it should be realised that it will not always be the water which is the more strongly bound component of the azeotrope. In this particular case it is desirable that the azeotrope leaving the first evaporator passes to the absorber containing absorbent for n- propanol, e.g. cotton seed oil and then to the absorber containing absorbent for water, e.g. aqueous sodium hydroxide. This is because it is desirable to remove the n- propanol before the azeotrope contacts the sodium hydroxide absorbent where if n- propanol were present in appreciable quantities it would combine with sodium hydroxide.

A heat pump suitable for carrying out the process of the invention comprises two condensers, two evaporators, and two paired absorbers and generators. Each paired absorber and generator is independently interconnected by a conduit having an expansion valve and by a conduit having a circulation pump. A vapour outlet-of the first evaporator communicates with a vapour inlet of the first absorber and likewise a vapour outlet of the second evaporator communicates with a vapour inlet of the second absorber. A vapour outlet of the first absorber communicates with a vapour inlet of the second absorber. There are means ensuring that vapour leaving the first evaporator passes through the first absorber before passing to the second absorber. There is a conduit having an expansion valve interconnecting a vapour outlet of the second generator with the first generator.There is a conduit connecting a vapour outlet of the second generator with the second condenser and a conduit having an expansion valve interconnecting the second condenser with the second evaporator. There is a conduit having an expansion valve interconnecting the first condenser with the first evaporator. There are means for supplying low temperature heat to the first evaporator, means for supplying high temperature heat to the second generator, means for delivering to the load heat of absorption from the second absorber (this absorber being paired with the second generator), means for delivering to the load heat condensation from the first condenser and means for delivering to the first generator latent heat of condensation from the second condenser.

The means for supplying heat to the first evaporator and to the generators are preferably heat exchangers. These are preferably located within the first evaporator and generators respectively and a fluid of appropriate temperature is caused to flow through these heat exchangers so that heat can be delivered to the condensed azeotrope in the first evaporator and to the absorbed components of the azeotrope in the generators. In order that the heat of condensation arising in the second condenser shall heat the first generator, it is preferred that the second condenser is in the form of a heat exchanger located within the first generator.

Similarly the means for delivering heat to the load from the second absorber and from the first condenser are preferably heat exchangers through which a heat transfer fluid such as a low vapour pressure liquid, for example a silicon liquid, e.g. Dow Syltherm 800 can flow. The heat exchangers are preferably located within the second absorber and within the first condenser respectively.

In order that the evaporated azeotrope leaving the first evaporator and the water evaporated from the second evaporator may be absorbed in both absorbers it is preferred that the two evaporators and the two absorbers be situated side-by-side in an enclosed container and for the absorbers to be the two inner units. Since there must be means ensuring that vapour leaving the first evaporator passes through the first absorber before passing to the second absorber, preferably each evaporator is separated from the nearer absorber by one baffle or preferably two offset baffles and the two absorbers are preferably separated by a pair of offset baffles.

Vaporised azeotrope and water leaving the first and second evaporators respectively then have to pass by way of at least one baffle before they enter the nearer absorber. Also it is preferred tha the organic compound leaving the first absorber has to pass by way of a pair of offset baffles before entering the second absorber.

It is also preferred thaqt the first condenser and the first generator be located side-by-side in an enclosed container and be separated from one another by one or two offset baffles. The presence of one or more baffles ensures that the organic compound pumped from the first absorber is delivered substantially only to the first generator and not to the adjacent first condenser.

It is also preferred that liquid azeotrope delivered to the first evaporator from the first condenser via an expansion valve, to the two absorbers from the paired generators via expansion valves, to the two generators from the paired absorbers via circulation pumps and to the second evaporator from the second condenser via an expansion valve be by way of sprinklers located above respectively the first evaporator, the two absorbers, the two generators and the second evaporator.

An apparatus in accordance with this invention is described with reference to the accompanying figure.

A heat exchanger 1 is located within a first evaporator 2. This evaporator 2 is located by the side of a pair of absorbers, first absorber 5 and second absorber 1 2 and offset baffles 3 and 4 separate the first evaporator 2 from the first absorber 5 which in turn is separated from the second absorber 12 by offset baffles 8 and 9.

There is a heat exchanger 6 located in the first absorber 5 and this is connected by conduits 7 and 11 to a heat exchanger 1 3 located in the second evaporator 14. Conduit 7 is provided with a circulation pump 10. The second evaporator 14 is separated from the second absorber by offset baffles 1 5 and 16. A heat exchanger 1 7 is located in the second absorber 12 and this heat exchanger is connected to the load L.

The first absorber 5 is connected to a first generator 1 8 by means of a conduit 1 9 having a circulation pump 20 and likewise the second absorber 1 2 is connected to a second generator 21 by means of a conduit 22 having a circulation pump 23. The working liquid being delivered from the first absorber 5 and from the second absorber 1 2 enters the first generator 1 8 and the second generator 21 respectively via sprinklers 24 and 25 respectively.

A conduit 26 having an expansion valve 27 interconnects the two generators 18 and 21 which are side by side. A heat exchanger 28 serving as the second condenser is housed in the generator 1 8 and is connected by a conduit 29 to the second generator 21 and is connected by conduit 30, expansion valve 31 and sprinkler 32 to the second evaporator 14. A heat exchanger 33 is located in the second generator 21.

A conduit 34 having an expansion valve 35 connects the first generator 1 8 to the first absorber 5 via a sprinkler 36 and a conduit 37 having an expansion valve 38 connects the second generator 21 to the second absorber 12 via a sprinkler 39.

A first condenser 40 situated by the side of the first generator 1 8 and separated therefrom by a baffle 41 houses a heat exchanger 42 connected to the load L. The first condenser 40 is connected to the first evaporator 2 by a conduit 43 having an expansion valve 44 via the sprinkler 45.

In this embodiment the working fluid which is used is an azeotrope of water and n- propanol and absorber 5 houses cotton seed oil as the absorbent for n- propanol and second absorber 1 2 houses aqueous sodium hydroxide as the absorbent for water.

The heat pump operates as follows: Cold air at about 0 C is circulated through the heat exchanger 1 located in the first evaporator 2. This cuases the azeotrope of water and n-propanol to evaporate and this passes via the baffles 3 and 4 to the first absorber 5. Here the n- propanol from the azeotrope is absorbed by the cotton seed oil housed in the first absorber 5. The heat of absorption in the first absorber 5 is delivered via the heat exchanger 6, conduits 7 and 11 and by means of the pump 10 to the heat exchanger 1 3 located in the second evaporator 1 4.

The azeotrope denuded of its n- propanol, i.e. water vapour passes via the baffles 8 and 9 to the second absorber 1 2. Here water is absorbed by the aqueous sodium hydroxide housed in the second absorber 1 2.

The heat delivered to the heat exchanger 1 3 causes the water to evaporate and this passes from the second evaporator 14 via baffles 1 6 and 1 5 to the second absorber 1 2. Here the water vapour is absorbed by the aqueous sodium hydroxide housed in the second absorber 1 2. The heat of absorption in the second absorber 1 2 is delivered to the load L via the heat exchanger 17.

Relatively weak working fluid (aqueous cotton seed oil and aqueous sodium hydroxide) is pumped separately from the first absorber 5 and the second absorber 1 2 respectively to the first generator 18 and the second generator 21 via the conduit 19, the pump 20 and the sprinkler 24 and via the conduit 22, the pump 23 and the sprinkler 25.

Relatively strong working fluid (cotton seed oil) is passed from the generator 1 8 through the conduit 34 and expanded via the expansion valve 35 and through the sprinkler 36 to the first absorber 5. Relatively strong working fluid (aqueous sodium hydroxide) is passed from the generator 21 through the conduit 37 and expanded via the expansion valve 38 and through the sprinkler 39 to the second absorber 1 2.

Relatively high temperature heat is supplied to the second generator 21 by the circulation of hot transfer fluid at about 1 50"C through the heat exchanger 33. This causes the working fluid to evaporate and water vapour passes from the second generator 21 via the conduit 26 and through the expansion valve 27, thereby experiencing a pressure reduction, to the first generator 18.

Also water vapour passes through conduit 29 to heat exchanger 28 which serves as the second condenser. The water condenses, the latent heat of condensation being supplied to the first generator 1 8. The condensed water then passes through the conduit 30 and expands through the expansion valve 31 and under reduced pressure enters the second evaporator 14 via the sprinkler 32.

Vaporised organic compound from the first generator 18 combines with the water vapour from the second generator 21 to reform the azeotrope and condenses in the first condenser 40 and the heat of condensation is delivered to the load L at a temperature of about 70"C via the heat exchanger 42.

Finally to complete the circuit, the condensed azeotrope in the condenser 40 passes through the conduit 43 and expands through the expansion valve 44 and under reduced pressure enters the first evaporator 2 via the sprinkler 45.

In this process therefore relatively low temperature heat at about 0 C is pumped to a load temperature of about 70"C.

Claims (14)

1. A heat pump comprising two condensers, two evaporators, a first absorber paired with a first generator and a second absorber paired with a second generator in which (1) each paired absorber and generator is independently interconnected by a conduit having an expansion valve and is also interconnected by a conduit having a circulation pump (2) a vapour outlet of the first evaporator communicates with a vapour inlet of the first absorber and a vapour outlet of the second evaporator communicates with a vapour inlet of the second absorber, (3) a vapour outlet of the first absorber communicates with a vapour inlet of the second absorber, (4) there are means to ensure that vapour leaving the first evaporator passes through the first absorber before it passes to the second absorber, (5) a conduit having an expansion valve interconnects a vapour outlet of the second generator with the first generator, (6) a conduit connects a vapour outlet of the second generator with the second condenser and a conduit having an expansion valve interconnects the second condenser with the second evaporator, (7) a conduit having an expansion valve interconnects the first condenser with the first evaporator, and in which there are (8) means for supplying low temperature heat to the first evaporator, (9) means for supplying high temperature heat to the second generator, (10) means for delivering to the load heat of absorption from the second absorber, (11) means for delivering to the load heat of condensation from the first condenser and (12) means for delivering to the first generator latent heat of condensation from the second condenser.

2. A heat pump according to claim 1 wherein the means for supplying heat to the first evaporator and to the generators are heat exchangers located within the first evaporator and the generators respectively.

3. A heat pump according to claim 1 wherein the second condenser is a heat exchanger located within the first generator so as to supply heat of condensation thereto.

4. A heat pump according to any one of the preceding claims wherein the means for delivering heat to the load from the second absorber and from the first condenser are heat exchangers located within the second absorber and the first condenser respectively.

5. A heat pump according to any one of the preceding claims wherein the two evaporators and the two absorbers are located side-by-side in an enclosed container with the two absorbers being the two inner units.

6. A heat pump according to claim 5 wherein the means for ensuring that vapour leaving the first evaporator passes through the first absorber before passing to the second absorber comprises one baffle or a pair of offset baffles between the first evaporator and the first absorber and a pair of offset baffles separating the absorbers.

7. A heat pump according to any one of the preceding claims wherein the first condenser and the first generator are located side-by-side in an enclosed container.

8. A heat pump according to any one of the preceding claims in which there are sprinklers located above the first evaporator, the two absorbers, the two generators and the second evaporator, said sprinklers terminating the conduits interconnecting respectively the first evaporator with the first condenser, the two absorbers with the two generators, the two generators with the two absorbers and the second evaporator with the second condenser.

9. A process of pumping heat from a low temperature to a higher temperature by the use of an absorption heat pump comprising a first condenser, a second condenser, a first evaporator, a second evaporator, a first absorber, a second absorber, a first generator and a second generator wherein the working fluid is an azeotrope of water and an organic compound having a positive deviation from Raoult's Law and wherein low temperature heat is supplied to the first evaporator, the evaporated azeotrope is allowed to pass to the first absorber where the organic compound is absorbed and water vapour is allowed to pass to the second absorber, heat of absorption is delivered from the second absorber to the load, heat of absorption from the first absorber is transferred to the second evaporator, evaporated water from the second evaporator is allowed to pass to the second absorber with where it is absorbed, absorbent solution is pumped from each absorber to the generator which it is paired. heat is supplied to each generator, organic compound is evaporated from the first generator, water which is evaporated from the second generator is in part (a) allowed to condense in the second condenser where its latent heat of condensation is used to evaporate the organic compound in the first generator and thereafter expand into the second evaporator and in part (b) allowed to expand into the first generator where it combines with the organic compound evaporated therefrom to reform an azeotrope which is allowed to condense in the first condenser, stripped absorbent solution is allowed to pass after expansion from each generator to the absorber with which it is paired, latent heat of condensation is delivered from the first condenser to the load and the condensate from the first condenser is allowed to expand from the first condenser to the first evaporator.

10. A process according to claim 9 wherein the organic compound component of the azeotrope is an alcohol having 3 to 6 carbon atoms per molecule.

11. A process according to claim 10 wherein the alcohol is n- propanol.

1 2. A process according to any one of claims 9 to 11 wherein the absorbent for water is an aqueous solution of an alkali metal hydroxide.

1 3. A process according to any one of claims 10 to 1 2 wherein the absorbent for the alcohol is a glyceride of a fatty acid or mixture of fatty acids.

14. A heat pump according to claim 1 substantially as hereinbefore described with reference to the drawings.