Abstract:
A heat recovery and upgrading method includes cycles of the subsequent steps of providing a working fluid including a liquid phase in a working fluid stream; transferring heat to the working fluid stream to partially evaporate working fluid in liquid phase to obtain a two-phase working fluid stream in liquid phase and gas phase; compressing the two-phase working fluid stream so as to increase a temperature and pressure of the working fluid and to evaporate working fluid in liquid phase; and transferring heat from the working fluid stream by element of condensation of working fluid. In the first step he working fluid is preferably in a predominantly single-phase working fluid stream in liquid phase when heat is transferred to the working fluid. In the third step working fluid in liquid phase is preferably evaporated so that a two-phase working fluid stream is maintained, especially a wet gas-phase working fluid.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a heat recovery and upgrading method comprising cycles of the subsequent steps of providing a fluid in a fluid stream; transferring heat to the fluid stream such as to evaporate the fluid; compressing the fluid; and transferring heat from the fluid. 
       BACKGROUND OF THE INVENTION 
       [0002]    Such method is known and is applied generally in industrial heat pump processes in which heat at a relatively low temperature is transferred to heat at a higher temperature. This is achieved by transferring heat at the relatively low temperature to a working fluid in liquid phase such that the working medium evaporates into the gas phase. Subsequently, the working fluid in gas phase is compressed, which causes the temperature and pressure of the fluid to rise, after which heat can be transferred by means of condensation from the working fluid to another medium for use of that medium at a relatively higher temperature. Limitations of the existing compression heat pump systems are the relative low condensation temperatures of about maximum 100° C. 
       SUMMARY OF THE INVENTION 
       [0003]    It is an objective of the invention to provide a heat recovery and upgrading method that allows providing heat at a high temperature, especially at a temperature above 80° C. or even 100° C. 
         [0004]    It is another or alternative objective of the invention to provide a heat recovery and upgrading method that allows providing heat at a temperature in excess of 150° C. or even 175° C. 
         [0005]    It is yet another or alternative objective of the invention to provide a heat recovery and upgrading method that allows providing heat at a higher temperature, from a medium having a lower temperature in the range of 60° C. to 120° C. 
         [0006]    It is yet another or alternative objective of the invention to provide a heat recovery and upgrading method that allows recovery and reuse of industrial waste heat streams in the order of 100° C. to a temperature that is in the order of 200° C. 
         [0007]    It is yet another or alternative objective of the invention to provide an efficient heat recovery and upgrading method in the high temperature range. 
         [0008]    It is yet another or alternative objective of the invention to provide a compressor for use in heat recovery and upgrading method that provides heat in an efficient way at a high temperature. 
         [0009]    At least one of the above objectives is achieved by a heat recovery and upgrading method comprising cycles of the subsequent steps of 
         [0010]    a.—providing a working fluid comprising a liquid phase in a working fluid stream; 
         [0011]    b.—transferring heat to the working fluid stream such as to partially evaporate working fluid in liquid phase to obtain a two-phase working fluid stream in liquid phase and gas phase; 
         [0012]    c.—compressing the two-phase working fluid stream so as to increase a temperature and pressure of the working fluid and to evaporate working fluid in liquid phase; and 
         [0013]    d.—transferring heat from the working fluid stream by means of condensation of working fluid. 
         [0014]    The method yields a temperature rise of the working medium upon compression, which causes working fluid in liquid phase to evaporate. Evaporation limits the temperature rises, but causes a pressure increase. The working fluid is compressed to yield a condensation regime of the working fluid at a desired temperature, for which a sufficiently high pressure is required. Compression of a gas-phase working fluid only would provide so-called superheating of the gas phase, which drastically lowers the efficiency of the process. The inventive method allows reaching a high temperature in a condensation regime of the gas-phase working fluid, so that heat at a high temperature can be recovered and upgraded to a high temperature and subsequently be transferred from the working fluid for reuse in another or same process. 
         [0015]    Preferably, step a comprises providing the working fluid in a predominantly single-phase working fluid stream in liquid phase for a very efficient transfer of heat to the working fluid stream. 
         [0016]    In further preferred embodiment step c comprises compressing working fluid to evaporate working fluid in liquid phase such that a two-phase working fluid stream is maintained, especially a wet gas-phase working fluid. Having all liquid-phase working fluid evaporated allows most efficient and accurate obtaining of the required condensation regime of temperature and pressure of the working fluid. In case some liquid-phase working fluid is still present after compression, it may evaporate after compression and adversely influence temperature and pressure of the working fluid. 
         [0017]    In an advantageous embodiment the working fluid comprises first and second components, a boiling temperature of the second component being lower than a boiling temperature of the first component at a same pressure. Advantageously, a boiling temperature of the working fluid is between boiling temperatures of the first and second components and dependent on the ratio in which the first and second components are present in the working fluid. Such binary working fluid allows setting of characteristics, such as a required boiling and condensation temperature, of the working fluid, and tuning of the working fluid to the specific heat recovery process in which it is employed. 
         [0018]    Preferably, the first and second components are selected such as to provide a non-separating mixture, which is efficiently achieved when the first and second components are alkali ionized components when mixed together. In an embodiment the first component is water and the second component is ammonia. 
         [0019]    In embodiments in step b heat is collected from a first medium and transferred to the working fluid stream and/or in step d heat is transferred to a second medium. 
         [0020]    In a preferred embodiment at least part of the liquid phase of the two-phase working fluid stream is provided as droplets in step c before and/or during compression of the working fluid stream and/or at least part of the liquid phase of the two-phase working fluid stream is separated from the two-phase working fluid stream and provided as droplets in step c before or during compression of the working fluid stream. The droplets provide a large droplet surface area to droplet volume ratio which yields an efficient heating and therefore evaporation of the droplets of liquid-phase working fluid. A larger volume of liquid-phase working volume will evaporate when presented in droplet form during compression of the working fluid. 
         [0021]    In an advantageous embodiment the droplets are provided at an inlet of and/or in a compression chamber of a compressor for compression of the working fluid. Introducing the droplets just at the inlet of and/or in the compression chamber guarantees that droplets are present during compression of the working fluid in the compression chamber, which otherwise might have merged into a larger volume of liquid-phase working fluid. 
         [0022]    In a further preferred embodiment the liquid phase of the two-phase working fluid stream is provided as a spray of tiny droplets, which provides an ever larger surface area to volume ratio of the droplets for an even further improved evaporation during compression. 
         [0023]    In an embodiment the method comprises subsequent to step c the step of expansion of the working fluid steam. This additional step is preferably carried out after heat transfer from the working fluid. Advantageously, power is recovered from expansion of the working fluid. In an embodiment, which can, for instance, be achieved when the working fluid is expanded in a positive displacement expander or turbine. 
         [0024]    In another aspect the invention provides for a compressor for use in step c of the above method, wherein the compressor is configured for compressing a two-phase working fluid so as to increase a temperature and pressure of the working fluid and to evaporate working fluid in liquid phase. 
         [0025]    In embodiments the compressor comprises a distribution arrangement configured for providing at least part of the liquid phase of the two-phase working fluid stream ( 12 ) as droplets in the compressor and the compressor may comprise a separation arrangement configured for separating at least part of the liquid phase of the two-phase working fluid stream ( 12 ) from the two-phase working fluid stream and a distribution arrangement configured for providing the separated liquid phase as droplets in the compressor. 
         [0026]    In a preferred embodiment the distribution arrangement is configured for providing droplets at an inlet of and/or in a compression chamber of the compressor. 
         [0027]    In a further preferred embodiment the distribution arrangement is configured to provide the liquid phase of the two-phase working fluid stream as a spray of tiny droplets. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    Further features and advantages of the invention will become apparent from the description of the invention by way of non-limiting and non-exclusive embodiments. These embodiments are not to be construed as limiting the scope of protection. Various other embodiments can be envisioned within the scope of the invention. Embodiments of the invention will be described with reference to the accompanying drawings, in which like or same reference symbols denote like, same or corresponding parts, and in which 
           [0029]      FIG. 1  shows a flow chart of an embodiment of the invention; 
           [0030]      FIG. 2  shows a flow chart of a modification of the embodiment of  FIG. 1 ; and 
           [0031]      FIG. 3  shows a flow chart another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0032]    An embodiment in which the heat recovery and upgrading method of the invention is implemented is shown in  FIG. 1 .  FIG. 1  shows a flow chart of a process cycle in which a working fluid is circulated in a main circuit  10 . The circuit  10  comprises a first heat exchanger  20 , a compressor  30 , a second heat exchanger  40 , an expander  50  and a third heat exchanger  60 . A pump  70  may be incorporated as well in the circuit  10  to provide working fluid stream within the circuit. In some processes a working fluid stream is induced by the process itself, so a pump  70  can in such occasions be dispensed with. 
         [0033]    A stream  21  of a first medium comprising hot gases, including vapor, at a temperature of about 120° C. and originating from a process is passed through the heat exchanger  20 . The stream  21  in the present embodiment a stream of hot gases and vapor coming from a frying oven, in which potato chips are produced. The gases and vapor are evacuated from the oven using one or more fans (not shown in the figures). The stream  21  of hot gases and vapor is fed into the first heat exchanger  20 , in which heat is transferred from the hot gases and vapors in stream  21  to working fluid of the working fluid stream in circuit  10 . The working fluid stream in circuit  10  may generally also be referred to as a working fluid stream  10 , which flows in a direction as indicated by the arrows in  FIG. 1 . The invention is not limited to heat transfer from a stream  21  of a first medium coming from a frying oven, but can be employed in a wide range of other applications as well. A first medium stream  22  that has released heat exits the first heat exchanger  20  and can be further used to release additional heat as will be described further below with respect to the embodiment of  FIG. 2 . 
         [0034]    The working fluid comprises first and second components, being water as the first component and ammonia as the second component in the embodiment described. The fraction of ammonia in the water ammonia working fluid can be in the range of 0.1% to about 50%. The first and second components of the working fluid are selected such as to provide a non-separating mixture of, preferably, alkali ionized first and second components when mixed together. A boiling temperature of the second component, being ammonia in the embodiment described, is lower than a boiling temperature of the first component, being water in the embodiment described, of the working fluid. A boiling temperature of the working fluid is in between boiling temperatures of the separate first and second components and dependent on the ratio in which the first and second components are present in the working fluid. 
         [0035]    The working fluid is provided in a predominantly liquid phase at a pressure of about 1 bar and a temperature of in the order of 30° C. to 70° C. in the working fluid stream  10  in circuit part  11  just before the first heat exchanger  20 . Actual temperatures and pressures disclosed may be dependent on the implementation of the process. Upon transfer of heat to the working fluid stream  10  working fluid in the liquid phase is partially evaporated. The process is embodied such that not all working fluid is evaporated into the gas phase. The amount of heat transferred in relation to the amount and flow rate of liquid phase working fluid provided in the first heat exchanger  20  should be such that some of the working fluid is still in liquid phase in circuit part  12  when having past the first heat exchanger  20 . A two-phase working fluid stream, comprising working fluid in liquid phase and gas phase, is therefore present in circuit part  12  after the first heat exchanger  20  at a pressure of about 1 bar and a temperature of about 97° C. 
         [0036]    It is noted that gas and vapor as used herein are identical in that both can be condensed from gas/vapor phase into liquid phase and the liquid phase can be evaporated into gas/vapor phase. The term vapor tends to be used for water vapor. 
         [0037]    The two-phase working fluid stream  12  is subsequently passed into compressor  30  to be compressed to a pressure with a predetermined condensation temperature of the gas-phase working fluid after compression. During compression the temperature of the working fluid will increase and at least part of the working fluid in liquid phase is evaporated into the gas phase. This is an important step to limit the temperature of the working fluid after compression. Preferably, only part of the liquid-phase working fluid evaporates at compression by compressor  30  to yield a wet gas-phase (two-phase) working fluid stream so as to avoid superheating of the working fluid. Having not all liquid-phase evaporate provides a working fluid stream in which gas phase and liquid phase are in equilibrium. After compression the temperature of the working fluid is about 185° C. and its pressure about 12 bar. 
         [0038]    At the compression stage part of the working fluid stream enters the compressor  30  in liquid phase. Evaporation of the liquid-phase working fluid upon compression will limit the temperature rise of the working fluid in the gas phase after compression to a desired and predetermined temperature or temperature range. The compression ratio of compressor  30  is set such as to achieve a desired and predetermined pressure or pressure range of the gas-phase working fluid in circuit part  13 . The amount of liquid-phase working fluid present before compression is such that pressure and temperature of the working fluid stream  13  after compression is at or within desired and predetermined levels or ranges. To achieve an efficient evaporation of the liquid-phase working fluid upon compression the liquid-phase working fluid is provided as droplets in the working fluid stream  12  just before and/or during compression by compressor  30 . An efficient evaporation of liquid-phase working fluid prevents superheating of gas-phase working fluid to a temperature that is not in equilibrium with the liquid-phase. The liquid-phase working fluid is preferably provided as a spray comprising very small droplets of liquid-phase working fluid to achieve a high droplet surface to droplet volume ratio so that a very efficient heat transfer to the droplet and therefore evaporation of a droplet is achieved. In the present embodiment the compression ratio of compressor is set to achieve a pressure of the gas-phase working fluid with a corresponding condensation temperature of about 180° C. in circuit part  13 . 
         [0039]    The compressed wet gas-phase working fluid subsequently enters a second heat exchanger  40 , in which the gas-phase working fluid is condensed to release its heat. Condensation is efficiently achieved when gas-phase working fluid is in equilibrium with the liquid-phase working fluid in the working fluid stream. The heat is released to a stream  41  of a second medium, being frying oil coming from the frying oven in the embodiment disclosed. The frying oil should have a temperature of about 180° C. in the frying oven, but is cooled to about 153° C. due to the frying process of potato chips. Stream  41  of frying oil from the frying oven has about this temperature of 153° C. and is heated to about 180° C. in frying oil stream  42  by heat exchanger  40  through heat release from the condensed working fluid. Frying oil stream  42  is passed to the frying oven (not shown in the figures) for reuse in the frying process. 
         [0040]    After heat release in the second heat exchanger  40  the compressed working fluid has a temperature of about 173° C. and is passed to an expander  50  to reduce the pressure of the working fluid from about 12 bar to about 1 bar. The expanding working fluid releases power to the expander  50 , which is used for power recovery. After expansion in expander  50  a two-phase working fluid continues as a working fluid stream having a liquid phase and a gas phase in circuit part  15 . The compressor  30  and the expander  50  are preferably of the positive displacement type, such as a Lysholm rotor or vane-type rotor. The expander may comprise a turbine. 
         [0041]    The power recovered by expander  50  is used to assist in driving compressor  30 . An electromotor (not shown) for driving compressor  30 , expander  50  and compressor  30  can be mounted in a common drive train (on a common axis). Alternatively, the expander can generate electrical power, for instance, when configured as an expander-generator. The electromotor drives the compressor assisted by (electrical) power from the expander  50 . Power released from the working fluid in expander  50  is thus recovered and reused in compressing working fluid by compressor  30 . 
         [0042]    A pressure sensor (not shown in the figures) is mounted in circuit part  13  to monitor a pressure of the compressed gas-phase working fluid, which is to be compressed to a predetermined pressure yielding a desired condensation temperature of the compressed gas-phase working fluid. The pressure measured by the pressure sensor is passed in a control loop (not shown in the figures) to the electromotor driving the compressor  30  to control a rotational speed of the electromotor and compressor  30  so as to set a compression ratio of the compressor  30  which yields the predetermined pressure of the compressed gas-phase working fluid in circuit part  13 . 
         [0043]    The expanded two-phase working fluid stream  15  is passed to a third heat exchanger  60 , in the embodiment shown, in which the working fluid is condensed to yield a substantially single-phase working fluid stream in circuit part  16 . In the third heat exchanger  60  heat is released from the two-phase working fluid stream  15  to another second medium, which is production water in the embodiment disclosed. A production water stream  61  enters heat exchanger  60  at a temperature of about 25° C., which is well below the boiling temperatures of both the first and second components, being water and ammonia, of the working fluid so as to allow condensation of the working fluid. A production water stream  62  having a temperature of about 60° C. leaves third heat exchanger  60 . Actual temperature of the production water stream  62  leaving heat exchanger  60  is governed by the design of the third heat exchanger and by flow conditions of working fluid stream and production water stream. The production water can be used for washing, cleaning and heating. The temperature of the working fluid after the heat exchanger is also in the order of about 60° C. 
         [0044]    The (substantially) single phase working fluid stream  16  is pumped by feed pump  70  towards circuit part  11 , where it is presented as a (substantially) single-phase working fluid stream  11  to the first heat exchanger  20 . Pump  70  hardly increases the pressure of the working fluid in the embodiment shown. At this point the cycle is repeated and continues as has been described. In the cycle heat is recovered and transferred from a first medium stream  21  resulting from a production process in first heat exchanger  20  to a liquid phase of a working fluid stream  11  so as to partly evaporate the liquid phase into the gas phase. The resulting two-phase working fluid stream  12  is upgraded by a considerable compression in compressor  30  to yield a working fluid stream  13  at a pressure having a high condensation temperature. Heat contained in the high-temperature working fluid stream  13  can be very efficiently employed in production processes, of which an example is given in the embodiments disclosed. 
         [0045]      FIG. 2  shows a modification of the embodiment shown in  FIG. 1 . Actually two modifications are implemented in the  FIG. 2  embodiment. In a first modification a bypass cycle  110  is provided. A bypass working fluid stream  111  from working fluid stream  16  is passed to a separator  120  to separate the gas-phase working fluid from the liquid-phase working fluid. Liquid-phase working fluid continues to circuit part  11  and a gas-phase working fluid stream  112  passes the separator  120  to an air-cooled condenser  130 , in which the working fluid releases heat to the atmosphere. A condensed liquid-phase working fluid stream  113  is merged again with working fluid stream  16  as shown in  FIG. 2 . The bypass cycle  110  may be required when not enough production water is available to provide condensation of working fluid in third heat exchanger  60 . The need for hot production water may be discontinuous, requiring an alternative to have the working fluid condense into a (substantially) single-phase working fluid stream  11 . 
         [0046]    In a second modification an auxiliary circuit  210  is connected to main circuit  10  through heat exchanger  220 . The first medium stream  22  of partly condensed frying gases and vapor from first heat exchanger  20  is led to auxiliary heat exchanger  220 , in which heat is further released to an auxiliary working fluid in auxiliary circuit  210 . The auxiliary working fluid is a refrigerant, which is pressurized in auxiliary circuit part  211 . Heat release in auxiliary heat exchanger  220  saturates the pressurized refrigerant. The pressurized refrigerant stream  212  is passed to an auxiliary expander  230  to reduce the pressure of the refrigerant stream and to release power to the auxiliary compressor  230 . A resulting two-phase refrigerant stream  213  is led to a separator  240 , separating the refrigerant stream into a liquid-phase refrigerant stream in auxiliary circuit part  214 . 1  and a gas-phase refrigerant stream  214 . 2 . The gas-phase refrigerant stream  214 . 2  is passed to air-cooled condenser  250  to condense the gas-phase refrigerant stream to a liquid-phase refrigerant stream  214 . 3 . 
         [0047]    Liquid-phase refrigerant stream  214  is pumped up by auxiliary mediate pump  270  to a required saturation pressure and to close the refrigerant loop towards auxiliary heat exchanger  220 . 
         [0048]    Power recovered by auxiliary expander  230  is also used to assist in driving compressor  30  in main circuit  10  by connecting auxiliary expander  230  to the drive train of compressor  30 . Power recovered by expanders  50  and  230  and used to assist in driving compressor  30  and heat recovery in heat exchangers  20 ,  40 ,  60  and  220  dramatically improves the energy efficiency of the whole process. 
         [0049]    First medium stream  21 , containing water vapor and predominantly air, is in two subsequent heat exchangers  20  and  220  condensed into a two-phase stream  23  that is passed to a separator  280  to yield an air stream  26  and a water stream  25 . Water stream  25  can be made available as production water after additional filtration (not shown in the figures), which further reduces a demand on resources. 
         [0050]      FIG. 3  shows another embodiment of which main circuit  10  is largely identical to the embodiment of  FIG. 1 . Main circuit  10  of the  FIG. 3  embodiment does not have an expander in the main circuit. An auxiliary circuit  310  is connected to main circuit  10  through heat exchanger  60 . Auxiliary circuit  310  comprises a working fluid that is a mixture of ammonia and water having a lower boiling and condensation temperature than the working fluid in main circuit  10 . In the embodiments of  FIG. 3  the working fluid of auxiliary circuit  310  comprises about 50% ammonia and 50% water. However, dependent on the application both components may be mixed in any ratio. 
         [0051]    In third heat exchanger  60  heat is transferred from the working fluid of main circuit  10  to the auxiliary working fluid of auxiliary circuit  310 . The auxiliary working fluid is at a pressure of about  71  bar at heat exchanger  60  and after the heat exchanger the temperature of the auxiliary working fluid is about 170° C. Subsequently, the auxiliary working fluid is passed to expander  320  to reduce pressure and temperature of the auxiliary working fluid to about 3.5 bar and 67° C., respectively, and to recover power from expansion of the auxiliary working fluid. After expansion the working fluid is passed to an air-cooled condenser to further reduce the temperature to about 30° C. Pump  340  then increases the pressure of the working fluid to about 71 bar at a slight temperature increase to about 31° C., after which the cycle of auxiliary circuit  310  is repeated again. In the  FIG. 3  embodiment power recovery in auxiliary circuit  310  is more efficient than power recovery in the  FIG. 1  embodiment. 
         [0052]    The working fluid in main circuit  10  after heat exchanger  60  in the  FIG. 3  embodiment has a temperature of about 34° C. and a pressure of about 12 bar. The pressure is further reduced by expansion valve  80  to about 1 bar to pass working fluid at a temperature and pressure of about 34° C. and 1 bar, respectively, to heat exchanger  20 , after which the cycle of the main circuit is repeated again.