The present invention relates to an improved heat pump for drying solid materials and liquid-solid mixtures.

Background The present invention relates to a heat pump for drying solid materials, in particular transformation and recovery of waste from fisheries, aquaculture and disposed foods as well as sludge, biodegradables and by-products. Solid and semi-solids materials can be pre-formed in suitable shapes for drying such as particulates, granulates or agglomerates to be properly dried. Additionally, the dryer processes Newtonian or non-Newtonian fluid-solid mixtures as they can be blended, cut, granulated or agglomerated prior to drying. In particular, this invention applies to industrial drying of distiller's byproducts and residues such as wet distillers wet spent grains.

There are many dryers around the world and special dryers as fully described on "Heat Pump Dryers - Theory, Design and Industrial Applications" with ISBN 13-978-1-4987-1133-3, written by Odilio Alves-Filho, 2016 CRC Press, NY, 2016. WO 2019/143254 discloses a modular system and a process of drying solids and liquid-solid mixtures, assigned to the same applicant as the present one. Whereas the dryer described there can be operated continuously, water is used as cooling medium in the condenser for extracting liquid from the vapor phase discharged from the dryer. However, the condenser requires a large volumetric rate of water to operate satisfactorily, which requires sufficient infrastructure to supply, and discharge water, which again results in loss of energy from the system in discharged water, and possibly cost per unit of water supplied.

Object

An object of the present invention is to provide an improved heat pump, arranged for use with dryers of solid material described above, with enhanced energy efficiency. The invention

The objects above are achieved by an improved heat pump according to the characterizing part of claim 1. Additional advantages and features appear from the dependent claims.

The present invention concerns an improved heat pump for use with dryers in general, using carbon dioxide as refrigerant. In the following, the heat pump according to the invention is described in connection with the dryer disclosed in WO 2019/143254 discussed above. However, it should be emphasized that the improved heat pump is not limited to that particular dryer, and can be integrated with any dryer in need of cooling and/or heating.

Drawings The invention is now explained in further details by means of drawings, where:

Fig. 1 is a schematic flow diagram illustrating an example of a prior art drying process,

Fig. 2A is a schematic flow diagram showing a first embodiment of the heat pump in accordance with the present invention integrated with the drying process of Fig. 1,

Fig. 2B is a diagram similar to Fig. 2A, but of a second embodiment of the heat pump in accordance with the invention,

Fig. 3A is a pressure-enthalpy diagram showing the course through the first embodiment of the heat pump of Fig. 2A,

Fig. 3B is a pressure-enthalpy diagram showing the course through the second embodiment of the heat pump of Fig. 2B Fig. 4 is a diagram showing specific cooling effect versus pressure,

Fig. 5 is a diagram showing specific heating effect versus pressure,

Fig. 6 is a diagram showing discharge temperature versus pressure,

Fig. 7 is a diagram showing cooling coefficient of performance versus pressure,

Fig. 8 is a diagram showing heating coefficient of performance versus pressure, Fig. 9 is a diagram showing combined coefficient of performance versus pressure,

Fig. 10 is a diagram showing actual work input versus pressure, and

Fig. 11 is a diagram showing compressor volumetric flowrate versus pressure. Detailed description

Example dryer

Initially it should be mentioned that instrumentation, such as temperature gauges, pressure gauges, humidity gauges, flow meters and controller have been omitted for the case of simplicity. However, the person skilled in the art would be in a position of including the instrumentation and controllers necessary to operate the process.

Now referring to Fig. 1, a simplified flow sheet a first embodiment of a dryer from the prior art disclosed in the above mentioned WO 2019/143254 is shown. A dryer chamber housing is indicated by reference numeral 100, having an inlet 101 for waste material with a hopper which feeds a screw conveyor 112 that is time- and flow-controlled to operate in semi-continuous or continuous mode. In a preferred embodiment, a feeder hopper 500 is arranged upstream of the drier chamber housing inlet 101, having an inlet 501, an outlet 502 in flow communication with the dryer housing inlet 101. The feeder hopper is kept in motion by an oscillator 503 to provide an initial de-clogging of the wet material to be dried. In Fig. 1, the feeder hopper 500 is illustrated as an elongate rectangular conduit, inclined in a direction downward in flow direction toward the outlet 502, and with an oscillator means arranged at the inlet end 501 to oscillate the feeder hopper 500 up and down urging the wet material toward the outlet 502 and de-bridging possibly agglomerated feed.

A rotary impeller 102 is arranged at the bottom of the drying chamber housing 100, which serves to disaggregate and circulate the waste material and, in combination with air flow, establishing a fluidized bed at the bottom of the drying chamber housing 100. A particle separator, here provided in the form of a main filter (not shown), arranged in the upper part of the drying, covering the cross-section of the dryer housing 100. A product outlet 104 is arranged to discharge the dried-hot product from the bottom of the drying chamber housing 100, e.g. a screw conveyor 113 transports the dried-hot product from the outlet of the drying chamber 100 to the inlet of the cooler-dryer 400. This screw conveyor 113 is controlled to semi-continuously or continuously discharge dried-hot material. A product cooler-end-dryer, arranged downstream of the product outlet 104, is indicated by reference numeral 400, having an inlet 401 connected with a drying chamber 100 and an outlet 402 for cooled-dried product discharge. A grid 403 is arranged inside the cooler-end-dryer chamber 400, supplied with medium-temperature air, coming from the CO2 heat pump gas-subcooler 802, to cool and end dry of the final product located inside the cooler- end-dryer chamber. Moreover, a fan 404 is arranged to draw medium-temperature air from the gas-subcooler 802, into grid 403 and through product in the cooler-end-dryer housing 400. Then, the fan 404 blows the warmer exiting air through the three-way valve and conduit 406, where the cooler-end-dryer exhaust air is mixed with the ambient air, and then the air flows through the inlet of CO2 heat pump gas-subcooler 802 to be pre-heated to the medium temperature and energy is recovered.

The product cooler-end-dryer 400 is advantageously kept in motion by an oscillator (not shown). The dryer housing 100 further comprises a gas outlet 105 connected with gas conduit 200. A box filter 107 (Fig. 1) is arranged in gas conduit 200, to remove and collect very fine particles down to 30 microns or smaller. The filter is described in more details below.

A condenser 300 is arranged downstream of the box filter 107, to condense vapour and remove liquid in air exhausted from the dryer chamber housing 100. Cooling water, at a temperature at least 5 °C lower than the dew point of the incoming condensing air stream, enters the condenser 300 at cooling water inlet 303, absorbs energy and increases its enthalpy and leaves the condenser 300 at cooling water outlet 304. Condensed liquid is drained at condense outlet 305 and from which the condensate energy can be recovered to preheat the material, screw conveyors or dis integrator. A bypass conduit 201 is arranged in the gas conduit 200, connecting the gas inlet 301 and gas outlet 302 of the condenser 300. A bypass valve means 202 is arranged in the bypass conduit 201, e.g. a flap or baffle valve, for partial control of flow, humidity and quality. The flap valve is opened in cases where the gas flow contains little humidity, as to control humidity and dry air mixture ratio and thus saving energy in the process.

Basis for the embodiments exemplified below

The present invention operates in wide range of high, medium and low pressures and variable gas- cooler outlet temperatures at the heat pump side and wide range of water removal and capacity at the drying side. To simplify explanation and principles of the present invention, the performance and specific enthalpy calculations below were made considering the following operation conditions: evaporating temperature ranges from 5 to 10 °C, gas-cooler outlet temperature between 35 and 50 °C, evaporating pressure from 40 to 45 bars, medium pressure between 55 and 58, bars and high pressure from 90 to 120 bars.

Integrated transcritical CO ₂ heat pump

With reference to Figs. 2A, 2B, 3A and 3B, the pressures are measured and controlled by the electronic expansion valves at state points 2 and 3 for high pressure, states 1 and 10 for low pressure, and states 5, 6 and 11 for medium pressure. The low pressure side is selected to attain the required evaporating temperature to satisfactorily condense the vapor and cool drying air to the operating temperature. The high pressure side is selected to provide a discharge temperature and gas-cooler outlet temperature sufficient to re-heat both the drying air and the cooler-end- dryer air to the set point temperatures.

In Figs. 2A, 2B, 3A and 3B, the reference numerals 1 through 12 are shaded to clarify the ducts in the heat pump in question, which corresponds to the states in the pressure versus enthalpy diagrams in Figs. 3A and 3B. The terms "state" and "conduit" are therefore used interchangeably.

First embodiment

Now with reference to Figs. 2A and 3A, the following describes a first embodiment of the integrated transcritical CO2 heat pump in accordance with the invention, integrated in the associated exemplary dryer described above and illustrated in Fig. 1 (prior art). Fig. 2A should be read in conjunction with Fig. 3A which is a pressure versus enthalpy diagram of CO 2.

The heat pump, indicated generally by reference number 800, comprises a closed-loop natural fluid conduit transporting refrigerant at varying pressures and temperatures. In further detail, the heat pump 800 comprises a compressor 801 having an upstream inlet 1 (at state 1 in Fig. 3A) and a downstream compressor outlet 2 with compressed CO2 (at state 2 in Fig. 3A) The levels of pressure and enthalpy at the compressor inlet and compressor outlet are indicated by reference numerals 1 and 2 in Fig. 3A, respectively. From the diagram we can see that the pressure has increased from about 45 bars to about 100 bars (example values only). The increase in enthalpy from the compressor inlet to the compressor outlet is in the example shown in Fig. 3A about 35 kJ/kg, which is the only workload in the heat pump 800.

Compressed refrigerant in conduit (state) 2 is then transported to air heater 206 in Fig. 1 (in the heat pump denoted as gas cooler 206) in a dryer circuit and cooled, in this example representing a heat loss from about 500 kJ/kg down to about 345 kJ/kg, representing a heat transfer of about 155 kJ/kg, heat which is transferred to the dryer recycle gas in conduit 207 (Fig. 1), fed to dryer 100. In this heat loss process the CO2 fluid changes from state 2 to state 3.

The cooled refrigerant in line 3 (or state 3) is then guided through the recovery gas-subcooler 802 above (in the heat pump denoted as external heat exchanger 802) to liberate additional heat to the drying air in conduit 406 to be supplied to the cooler-end-dryer 400 (Figs. 1 and 2A). The heat exchange in external heat exchanger 802 brings the specific enthalpy in the refrigerant further down from about 345 kJ/kg to about 315 kJ/kg, representing an additional extraction of energy in the CO2 heat pump of about 30 kJ/kg refrigerant. The loss of enthalpy at this stage in the circuit is illustrated in Fig. 3A from state 3 to state 4 at constant pressure.

The cooled gas at state 4 is then expanded in an expansion valve 803, e.g. an electronic expansion valve, from a pressure of about 100 bars to about 57 bars, reducing the refrigerant pressure to about 43 bars in state 5. At this stage, prior art heat pumps would proceed with boiling of refrigerant in an evaporator.

Moreover, a fan 404 is arranged to draw medium-temperature air from the gas-subcooler 802, into grid 403 and through product in the cooler-end-dryer housing 400. Then, the fan 404 blows the warmer exiting air through the three-way valve and conduit 406, where the cooler-end-dryer exhaust air is mixed with the ambient air, and then the air flows through the inlet of CO2 heat pump gas-subcooler 802 to be pre-heated to the medium temperature and energy is recovered.

According to the present invention, the expanded refrigerant comprising a mixture of gaseous and liquid refrigerant at state 5 is fed to a medium pressure gas-liquid separator 804, from which liquid refrigerant at equilibrium pressure at state 6 is fed to an internal heat exchanger 805, extracting additional heat in the refrigerant from about 315 kJ/kg to about 255 kJ/kg, indicated at state 7, representing an extraction of specific enthalpy of about 60 kJ/kg refrigerant.

The cooled liquid refrigerant in conduit 7 is throttled and cooled further to state 8 using a medium pressure expansion valve 806. At state 8 the fluid is a mixture of liquid and gas, in this example throttled from about 57 bars to about 45 bars, thus representing a pressure loss of about 12 bars at constant enthalpy.

The cooled refrigerant mixture at low pressure of 45 bars at state 8 is then supplied to the evaporator 300, which in the dryer circuit operates as a condenser. In the evaporator 300, the mixture is boiled reaching state 9. The boiling is achieved by taking heat from the externally flowing moist air in associated external process. During this process the moist air is cooled and the vapor is condensed and removed from the drying circuit 305 (Figs. 1 and 2A). The heat for boiling the CO2 in the evaporator is recycled in the gas-cooler 206 to reheat the drying air to feed the drying chamber 100 (Fig. 1). In this example, the specific enthalpy of the refrigerant is increased from about 220 kJ/kg at state 8 to about 435 kJ/kg in conduit or state 9, representing a heat extraction in the heat pump of about 215 kJ/kg. The gaseous refrigerant phase in medium pressure separator 804 at equilibrium pressure, indicated at state 11, is fed to a pressure control valve 807 to reduce gas pressure in conduit 12 (at state 12) to the pressure in conduit 10 described immediately above, whereupon the respective refrigerant flows from states 10 and 12, at similar pressure but different temperature, are combined at a junction 810 (Fig. 2A) as a feed at state 1 to compressor 801, thus completing the cycle.

Second embodiment

Now referring to Figs. 2B and 3B, a second, and preferred embodiment, of the integrated transcritical CO2 heat pump in accordance with the invention is illustrated. After boiling in the evaporator 300, the fluid exits at state 9, which can be a mixture, saturated or slightly superheated vapor. At any of these phases, the fluid flows into a gas-liquid-oil separator 808, also denoted as second separator 808, which assures that only saturated vapor is supplied to the internal heat exchanger 805 and, consequently, only superheated vapor enters the compressor 801 avoiding its damage by liquid droplets. Thus, the second separator 808 and its accompanying components replaces junction 810 in the first embodiment. Simultaneously, the saturated gaseous refrigerant phase at about 57 bars from medium pressure separator 804 at equilibrium pressure at state 11 is fed to a pressure control valve 807 to reduce gas pressure and to become a mixture at state 12. Then, the mixture enters the second separator 808, which advantageously allows the return of oil to the compressor. For this, from the bottom of separator 808, the mixture of liquid and oil in conduit 812 enters to the suction line and flows through the internal heat exchanger 805, reaches state 1 and enters the compressor 801.

The separator 808 supplies cooled saturated gaseous refrigerant in state 10 that undergoes heat exchange against liquid refrigerant in conduit 6, as it flows through the internal heat exchanger 805 described above. Then the refrigerant reaches state 1 as superheated vapor and re-enters the compressor 801 to repeat the cycle. In this process the enthalpy in conduit 10 is increased from about 424 kJ/kg to about 475 kJ/kg at state 1. The pressure in the separator 808 is kept to be the same as set point evaporating pressure by the CO2 safety control valve 809 to avoid fluctuating temperatures of the evaporator and the income drying air that is to be cooled and condensed. Technical effects

This invention is a green technology using carbon dioxide that is natural fluid with zero ozone depletion and no global warming. CO2 is non-toxic, non-corrosive, non-flammable, non-taxable and has no restriction on charge or re-charge. Also, CO2 has excellent thermal properties and high volumetric cooling capacity allowing compact components design, such as smaller diameter suction and discharge lines and smaller compressor size. CO2 has low critical temperature that allows heat pump operation at transcritical cycle that is desired to attain high gas-cooler pressure and high drying air temperature. The use of a closed circuit heat pump excludes the need for substantial infrastructure and cost of water supply. The invention operates in closed loop using only CO2, and therefore it avoids both discharge and loss of heated water as in conventional systems. The invention also obtains substantial energy savings because there is no need for heating water, only power supply to the compressor. A substantial part of the work input at the compression step is reused in the associated heat consuming in both drying and cooling-end- drying processes. In the exemplified drying process described in the example above, the drying time is reduced, the water removal or drying capacity is increased, and the energy consumption is reduced.

The technical effects are quantified in Figs. 4-11, which are diagrams showing values obtained from simulations at chosen operating conditions:

Figure 4 shows specific cooling effect versus pressure of a heat pump according to the invention and two prior art heat pumps. The invention exhibits a superior cooling effect, which is stable at varying pressure, in the example shown at about 220 kJ/kg, whereas the prior art cooling effects at 90 bars are substantially lower at a high pressure of 120 bars by about 18% and 38%, respectively, of the cooling effect provided by the invention, increasing to about 50% and 80%, respectively, of the cooling effect provided by the invention.

Figure 5 is a similar diagram, but where the specific heating effect versus pressure is compared between the invention and the two prior art heat pumps. Also here, the heat pump in accordance with the invention exhibits a substantial improvement compared to prior art.

Now moving on to Figure 6, the compressor discharge temperature is substantially higher than the prior art. Higher temperature is desired and advantageous for drying at higher water removal rates and capacities. Moreover, Figure 7 demonstrates that the invention exhibits a substantially higher cooling coefficient of performance as a function of pressure compared to the prior art, throughout the whole range of working pressure relevant to processes of this type.

Figure 8 provides an additional confirmation of the technical effect in that the heating coefficient of performance as a function of pressure is substantially higher than the prior art throughout the whole range of working pressure relevant to processes of this type.

Figure 9 show the combined coefficient of performance as a function of pressure shown in the Figures 7 and 8 above. The combined coefficient of performance represents two unique characteristics of the CO2 heat pump dryer in this invention. Firstly, it uses condensing energy for air heating the drying air and the cooling-end-drying air. Secondly, it uses the evaporating energy to cool the dry-air, to condense the vapor fraction and remove the liquid water from the drying loop. This indicates a unique aspect of the invention in which the CO2 heat pump dryer is capable to simultaneously use and recycle energy from both the refrigeration and heat pump cycles.

Figure 10 demonstrates a lower actual work input for the invention than the prior art heat pumps. The low actual work is accompanied by a significant increased cooling capacity due to the separator 804, which operates between the CO2 critical and evaporating pressures and feeds only liquid phase that is subcooled before throttling to evaporating pressure. In this way, the mixture released by the expansion valve approaches the saturation liquid line (see diagrams), and the cooling effect is maximized. Moreover, the prior art heat pumps may allow liquid droplets to enter and damage the compressor, whereas the invention includes superheat by using the internal heat exchange and separator, both assuring that only superheated vapor flows through the compressor protecting it. Also, both devices provide a higher discharge temperature desired for heating the drying air.

As is evident from Fig. 11, the heat pump can operate at a substantially higher efficiency and with a smaller compressor than the prior art. In the example shown, the volumetric flow rate at 90 bars is about 4.3 and 2.7 times larger for the prior art heat pumps, and at 120 bars the rates are about 1.4 and 1.2 times larger for the prior art systems.

Modifications

It should be emphasized that the associated drying process exemplified above is an example only. The heat pump in accordance with the invention can be used with any external process in need of heating and/or cooling. Abbreviations

COPall Combined Coefficient of Performance

COPH Heating Coefficient of Performance COP _L Cooling Coefficient of Performance Mass flowrate kg/s qH Specific heating effect kJ/kg qL Specific cooling effect kJ/kg tDIS Compressor discharge temperature °C mFR Compressor mass flowrate kg/s w Specific work input kJ/kg W Actual work input kJ vFR Volumetric flowrate m ³ /h