TECHNICAL FIELD OF THE INVENTION

This invention relates to the recovery of . energy from geothermal brines and other hot water sources, and is particularly concerned with a process and system of the aforementioned type, employing direct contact heat exchange between the hot brine or hot water feed, and a working fluid, and employing a plurality of separate stages in which the hot brine or hot water is intimately mixed with the working fluid, and the two fluids thereafter separated in each stage, whereby the heat from the hot brine or hot water is transferred in increments to the working fluid to raise the temperature thereof.

BACKGROUND OF THE PRIOR ART

Direct contact heat exchange (DCHE) has been proposed as a method for recovering thermal energy from scale depositing geothermal fluids, e.g. hot geothermal brine. Thus, U.S. Patent No. 3,988,895 to Sheinbaum discloses power generation from hot brines by passing a hot fluid containing salt such as geothermal brine, in direct contact heat exchange relation with a working fluid such as isobutane. In the copending U.S. application Serial

No. 589,068, filed June 23, 1975, by Samuel G. oinsky, there is described a process and system for recovery of energy from geothermal brine and other hot water sources, which comprises cooling the brine or hot water source in direct contact heat exchange relation with a working fluid in a direct contact heat transfer column, and having as an important feature the operation of the heat transfer column at or above the critical pressure of the working fluid.

In U.S. Patent No. 4,089,175 (application Serial No. 611,310, filed June 23, 1975, by Samuel G. Woinsky) , there -is described a similar process and system as in the above-mentioned application, and having as an important feature operation of the heat transfer column at a pressure below the critical pressure of the working fluid.

The direct contact heat exchangers of the prior art as exemplified above are generally in the form of the sieve tray type, wherein the liquid brine enters at the top of the column and

falls through a series of trays, with the working fluid travelling upward and mixing with the hot brine. The working fluid is then extracted at the top, e.g., as a vapor, where the temperature differences are such that the working fluid passes through its boiling zone. Another type of prior art direct contact heat exchanger employed is of the packed column type wherein mixing of the hot brine and working fluid and countercurrent■flow thereof through the column are achieved by passage of the fluids through a mesh or other type of packing.

Other related prior art patents include the Morris U.S. Patent 1,905,185 and the Fricke U.S. Patent 3,164,957, which disclose liquid-liquid direct contact heat exchange, wherein two liquids are permitted to mix for transfer of heat from one liquid to the other, followed by separation of the fluids due to differences in density.

BRIEF SUMMARY OF THE ^' NVENTION

It is accordingly an object of the present invention to provide a process and system for direct contact countercurrent heat exchange between a hot water containing fluid such as geothermal brine, and an immiscible working fluid, employing a plurality of self-contained stages, in each stage of which the working fluid is incrementally heated by the hot aqueous fluid. Another object of the invention is to provide a process and system of the above type, incorporating in each stage

mixing and settling of the fluids passing in countercurrent relation to each other. A still further object is the provision of a direct contact heat exchange process and system as described above, having certain advantages over the prior art direct contact heat exchange systems, including safety, reliability and overall economy of operation.

The above objects and advantages are achieved according to the invention by the feature of providing a plurality of separate stages, incor¬ porating in each stage a mixer-settler system for ^' the direct contact heat exchange of hot geothermal brine or other hot water containing source, with an immiscible working fluid such as a hydrocarbon working fluid. The process and system of the invention include in each stage intimately mixing hot water containing fluid with the working fluid, settling the mixture in a settling zone and separating the mixture into a hot water containing fluid or brine phase and a working fluid phase. The hot water containing fluid or hot geothermal brine is initially introduced ^' into one end stage such as the first stage of the above-noted plurality of separate stages, and the cold working fluid is introduced into the opposite end stage or last stage of the system. The hot water con¬ taining fluid or hot geothermal brine, and the working fluid flow in countercurrent relation through the respective stages and the working fluid is heated in increments by the hot water containing

fluid, to thereby raise the temperature of the working fluid.

The resulting hot working fluid discharged from the first stage is passed to an expander or heat engine where it is converted to useable energy or work, and the expanded working fluid is then recycled back into the system.

The mixer-settler system employed in each of the stages for direct contact heat exchange between the hot water containing fluid or geothermal brine and working fluid comprises a mixer in which thorough mixing of the two phases is achieved by means of an agitator, e.g. in the form of an impeller or by running the two phases through a line con- taining a turbulence generator, known as a static mixer. The mixed phases are then transferred to a settler, e.g. in the form of a settling tank, wherein the two phases are permitted to remain quiescent for a sufficient period of time to permit the heavy phase, e.g. the aqueous or. brine phase, to settle to the bottom, and the light working fluid phase to rise to the top. The apparatus can be designed so that the mixing and settling devices are separate and interconnected, or both the mixing and settling devices can be enclosed in a single vessel, if desired.

Any number of stages, each containing a mixer and settler, can be operated as a counter¬ current cascade by pumping one phase, that is the hot aqueous liquid or geothermal brine in one

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direction, and the other phase, the cold working fluid, in the opposite direction, as described in greater detail hereinafter.

The advantages of the process and system 5 of the invention include an economy saving in cost over the prior art direct contact heat exchangers noted above, such as the sieve tray or packed column, because in the present system each stage constitutes a separate unit and its size can be

10 tailored to correspond to the physical properties required for such stage. This is in contrast to the standard sieve tray and packed column arrange¬ ment wherein the size of the limiting stage determined the size of the overall or total apparatus,

15. Further, the direct contact heat transfer process and system of the invention provide safety and reliability from the standpoint that one or more separate stages of the system can be shut down by by-passing such stages, without forcing the shut

20 down of the entire system. Thus, the system, of the present invention incorporating a mixer and settler device in each stage is desirable for a plant which may require frequent shut downs, e.g. due to solids deposition problems.

25 BRIEF DESCRIPTION OF THE DRAWING

The above and other features and advantages of the invention will be more clearly understood by reference to the following detailed description of the invention, taken in connection with the acco - 30 panying drawing wherein:

FIG. 1 shows a schematic flow diagram of one preferred embodiment of the invention for direct contact heat exchange between hot geothermal brine and a cold working fluid; and FIG. 2 is a diagram illustrating work expansion of the hot working fluid, and recycling of the expanded working fluid to the direct contact heat exchanger.

DETAILED DESCRIPTION OF THE INVENTION Referring to Fig. 1 of the drawing, geothermal brine or hot water which may be degassed to partially remove non-condensible gases, is delivered from the well (not shown) through the line 10 to a pump 12.

The cold working fluid is introduced at 14 and is likewise pumped' ^" up to the above-noted high pressure corresponding to that of the hot brine, by pump 16. Such working fluids can be any suitable material which is immiscible with water, and having a density different from- the brine or hot water. Such -— working fluids preferably have a density less than the density of the hot brine or other hot water containing fluid.

Thus, the working fluid can be a hydrocarbon including aromatics, paraffins, naphthenes and olefins. Preferably, the working fluid is a paraffin or olefinic hydrocarbon containing from about 1 to about 8 carbon atoms, either straight or branched chain, such as methane, ethane, propane, n-butane, isobutane, n-pentane, isopentane, neopentane (2,2-dimethylpropane) , n-hexane, cyclohexane, n-heptane, n-octane, isooctane, and the analogous olefins such as n-butene, isobutene, and the ^" "" like. Most desirably, paraffinic or olefinic hydro¬ carbons containing from about 3 to about 6 carbon atoms

are employed. Aromatic hydrocarbons such as benzene, toluene, and xylene also can be used.

Mixtures of such hydrocarbons can also be utilized, such as, for example, a mixture of methane 5 and ethane, ethane and propane, or propane and n-butane, to optimize the critical -temperature and pressure conditions of the working fluid, and thereby obtain maximum energy recovery from the geothermal brine or hot water feed.

10 Working fluids other than hydrocarbons are also suitable provided they at least have the proper density relation to the hot aqueous fluid or brine as noted above, and are immiscible.

Preferred working fluids are n-butane and

15. isobutane.

Inlet temperature of the hot water or brine to one end of the system ranges from about 150°F. (65.6°C.) to about 600°F. (315.6°C), with a range of about 300°F. (148.9°C.) to about 500°F. (260°C), e.g.

20 about 400°F. (204.4°C), being customary for hot geothermal brine. Although the heat exchange system of the invention can be operated above, or below, or at the critical conditions of the working fluid, in one embodiment the system is operated above the

25 critical pressure of the working fluid, such pressure, in absolute terms, preferably ranging from about 450 psia. (31.638 kg/cm ->) to about 1,000 psia. (70.307 kg/cm2) , usually from about 500 to 700 psia. (35.154 to 49.215

2 kg/cm ) .

30 The incoming brine at 18 is introduced into the first stage A of a cascading countercurrent flow system according to the invention, comprised of five separate but interconnected stages including stages A,

B, C, D and E. The incoming brine at 18 in the first

stage A is combined with the working fluid at 20 coming from the second stage B and the combined mixture is fed to a mixer 22. The mixer 22 can be in the form of an impeller type mixer wherein the two phases, that is the hot brine and the warm working fluid, are thoroughly mixed.

The resulting mixture is then introduced at 24 into a settler in the form of a settling tank 26 wherein the two phase mixture is permitted to remain quiescent for a period of time sufficient to permit the heavy aqueous or brine phase to settle to the bottom of the settling tank and the light working fluid phase to rise and float on the aqueous brine phase in the upper portion of the settling tank. In the mixer 22 wherein the hot brine is thoroughly mixed with the warm working fluid from the second stage B, direct contact heat transfer takes place between the hot brine and the warm working fluid, to further heat the working fluid, while cooling the brine. Following settling in the settling tank 26, the hot working fluid light phase is withdrawn at 28. Referring to Fig. 2, the hot working fluid at 28 is introduced into an expander 30 for producing work which is taken off as shaft power at 32, and which can be employed for example to operate an electric generator (not shown) . The expanded working fluid at 33 is then introduced into a cooler at 34 for condensing the working fluid, and the condensed working fluid at 14 can then be recycled for introduction into the staged countercurrent direct contact heat transfer system shown in Fig. 1.

The cooled brine is withdrawn at 36 from the settling tank 26 of the first stage, and is pumped at 38 through line 40, where it is combined at 42 with working fluid in line 44 exiting the third stage C. The resulting mixture at 46 is introduced into the mixer 48 and the thoroughly mixed phases are introduced at 50 into the settling tank 52 of the second stage B, wherein the two phases are again allowed to separate as noted above. The further cooled heavy brine phase withdrawn at 54 is then pumped at 56 to the third stage C wherein it is first combined at 58 with the working fluid at 60 withdrawn from the fourth stage D. The combined mixture is then introduced into the mixer 62 of the third stage C and the resulting mixture is then allowed to settle in settling tank 64 of the third stage.

Again, further cooled brine is withdrawn at 66 from settling tank 64 and is pumped at 68 through line 70 where it is combined at 72 with working fluid at 74 exiting the fifth stage E. The resulting mixture is introduced into the mixer 76 for thorough mixing of the two phases, following which the mixture is passed into the settling tank 78 of the fourth stage for separation of the two phases.

The further cooled brine exiting settling tank 78, at 80 is pumped at 82 through line 84 and is combined at 86 with cold working fluid introduced into 86 as feed to the fifth stage through 88. The resulting mixture is then fed to the mixer 90 and the two thoroughly mixed phases are then introduced into the settling tank 92 of the fifth stage. The cold brine is then withdrawn at 94 from settling tank 92 and is sent to a sump or brine injection well (not shown) .

-li¬ lt is thus seen that the process and system of the invention comprises a series of successive like stages, each stage including a mixing device for thoroughly mixing the two phases of brine or hot water, and working fluid, and a settling tank, which can be a simple tank, and wherein the hot brine or other hot aqueous liquid is introduced into the stage at one end of the system and the working fluid is introduced into the stage at the other end of the system, and providing countercurrent flow of brine or other hot aqueous liquid, and working fluid through the successive stages, forming a cascading system, to heat the working fluid in increments in the successive stages. Thus, the hot working fluid exits the system from the same stage that the hot brine or other hot aqueous liquid enters the system.

The mixing section and settling section of each stage are preferably designed as separate components which permits a more optimum design of each section, resulting in reduced cost as compared to a continuous column having, for example, sieve trays. Thus, a mixer- settler countercurrent heat exchange system according to the invention has a lower capital cost as compared to the conventional sieve plate column for direct contact heat exchange.

Further, although the exact nature of the scale deposition problems are not known, such problems are much less severe in the mixer-settler system according to the invention, than in a sieve plate or packed column. The reason for this is that the deposi¬ tion of calcium carbonate type scales which occurs in the upper section of such column where the brine is first injected, usually occurs around the holes in the

sieve plate, thus altering the hole diameter and quickly altering the performance of the column. On the other hand, the mixer-settler system of the invention consists of a mixing section through which the fluid flows at a high rate and a large settling section, such as a simple vessel. The deposition of scale will occur in such vessel and thus permit it to be easily handled by mechanical removal means. Furthermore, the performance of the mixer-settler system of the invention will not degrade rapidly as in the case of the above-noted sieve plate and packed columns.

According to one modification of the invention system, instead of employing separate mixer and settling devices, both the mixing and settling sections can be - enclosed in a single vessel, such as a horizontal cylinder.

It will also be apparent that any number of stages each containing a mixer and settler, can be operated according to the invention as a countercurrent cascade system in the manner described above, by pumping one phase in one direction and the other in the opposite direction as illustrated in Fig. 1. Thus, at least two stages are required, the process and system of the invention utilizing five stages being a preferred embodiment.

The working fluid can. be any substance which is non-reactive and immiscible with the hot aqueous fluid and which has a density difference which enables countercurrent flow with the hot aqueous fluid. Although the working fluid can have a density greater than the brine or hot water containing fluid or liquid, the preferred working fluids have a density less than that -. of the hot brine or water containing fluid. The hot

3. __OMPT

water containing fluid can have salts such as sodium and magnesium chloride and/or other minerals dissolved therein.

In a specific example of operation of the countercurrent direct contact heat exchange process and system of the invention employing five stages as described above, hot brine at 325° to 400°F. (162.8 to 204.4°C.) is delivered from the well and is pumped up to a pressure, in absolute terms, of between about 600 and 700 psia. (42.184 to 49.215 kg/cm ) . The hot brine is introduced into the first stage of a five stage system as illustrated in Fig. 1. Cold isobutane at 130°F. (54.4°C.) is pumped up to the above-noted same high pressure as the hot brine and is introduced into the last stage of the system. The incoming brine at the first stage is combined with the isobutane coming from the second stage and is introduced into an impeller type mixer in the first stage. This mixture is then allowed to settle in a settling tank in the first stage, the light isobutane phase rising and the heavy brine phase settling. The hot isobutane at a temperature of about 310°F. (154.4°C.) is withdrawn from the first stage and passed through an expander for work expanding the isobutane and providing power. The brine from the first stage is combined with the isobutane from the third stage and the mixture introduced into a mixer in the second stage, and the resulting mixture then passed into the second stage settler where the phases are again allowed to separate. The brine from the second stage is then pumped to the third stage where it is combined with the isobutane of the fourth stage, and the process of heat exchange is repeated in the third, fourth and fifth stages as

described above, and illustrated in Fig. 1. The cold brine exiting the fifth stage is at a temperature of about 180°F. (82.2°C).

From the foregoing, it is seen that the invention provides a unique process and system for countercurrent direct contact heat exchange between hot brine or other hot aqueous liquid and an immiscible working fluid, incorporating a plurality of like stages, each containing like but separate mixer-settler devices, wherein the working fluid is heated in increments, the process and system being efficient, reliable and economical.

While we have described particular emobodiments of the invention for purposes of illustration, it will be understood that various changes and modifications within the spirit of the invention can be made, and the invention accordingly is not to be taken as limited except by the scope of the appended claims.