HK1217358B - Method for energy saving - Google Patents

Description

Method for saving energy

Technical Field

The present invention relates to a method for energy saving of an industrial process.

More specifically, the invention is intended for recovering energy by coupling a thermal industrial process with a cold industrial process.

Background

It is known that many industrial processes require heat. One example is the process of frying french fried potatoes in vegetable oil at 180 ℃.

It is also known that many industrial processes require cold. One example is the freezing of pre-fried french fries at a temperature of-33 ℃.

Traditionally, a large amount of energy is lost in industrial processes that require heat due to cooling and the dissipation of heat to the atmosphere. For example, in the frying of potatoes such as french fries or potato chips, the moisture present in the potato evaporates during frying and the resulting water and oil vapors cool in the air, causing the thermal energy therein to be dissipated to the atmosphere.

In order to make full or partial use of this thermal energy, it is known to exchange the heat in these vapours with another medium, so that the water and oil in the vapours condense. It is also known that when the other medium is water, hot water can be produced therefrom. If the other medium has a binary composition consisting of water and ammonia, a complete or partial phase change can take place, and then a higher pressure is reached by the compressor.

The compressed binary medium is then guided through a heat exchanger which acts as a heating means for the cooking oil still to be heated (i.e. cooled cooking oil from the fryer and fresh cooking oil compensating for the lost cooking oil), whereby a part of the heat from the compressed binary medium is dissipated to the cooled or fresh cooking oil, so that this binary medium is completely or partially condensed.

The fully or partially condensed binary medium is then expanded in an expander, thereby generating electrical energy. The fluid flow leaving the expander is a flow comprising two phases (liquid and vapor) which is conventionally fed back to a condenser where the vapor is condensed into liquid, whereby the energy recovery circuit is closed.

Also in industrial processes where refrigeration to the chilling temperature (about-30 ℃) is necessary, part of the energy that must be supplied to obtain refrigeration is recovered not by the condenser that generates electricity but by a pressure reducing valve that reduces the pressure in order to generate cold according to the joule-thomson effect. By using a condenser, the thermal energy generated by the compressor is dissipated to the atmosphere, where the heated and compressed coolant gas is cooled with it.

Refrigeration is obtained by compressing a suitable coolant gas, typically ammonia, after which the compressed and condensed coolant gas is expanded in a pressure reducing valve, whereby the temperature of the coolant gas drops sharply, which coolant gas is then further conducted to a phase separator, which separates the gas phase from a cold liquid phase (about-30 ℃), which can be used in various refrigeration devices, such as refrigeration lines, refrigerated storage areas and other cold storages.

The heated coolant gas produced after cooling can now be compressed again, in part, with the generated electrical power, in order to expand in the expander into compressed coolant gas, whereby the coolant gas circuit is closed.

Additional energy savings are possible by transferring heat from a first industrial process to which heat has been supplied to another industrial process that must produce cold. This is possible by converting the low value residual heat of a first industrial process into high value cold for a second industrial process that requires cold.

In the above example, the process for frying potatoes to prepare french fries is coupled to a process for freezing such french fries and placing them on the market as frozen products, resulting in additional energy savings.

In order to measure the efficiency of an industrial energy-saving process, it is often used the coefficient of performance of energy (COP), which represents the proportion of energy recovered relative to the energy that must be supplied for its recovery. Given the cost ratio of KWe to KWth, the recovery process is only economically cost effective when the COP is greater than two and one-half (2.5).

Many systems are known that recover heat from processes that require heat.

W02009/045196 and EP2514931 describe heat recovery from a heat source through a cascade rankine cycle with an organic energy carrier that is not compressed by a compressor.

W02013/035822 also describes heat recovery through a cascade rankine cycle, each of which uses pure material as an energy carrier and does not require a compressor.

CN202562132 describes the coupling of processes requiring heat (swimming pools) with processes requiring cold (rink) and uses compressors for gaseous energy carriers.

US4573321 recovers heat from a heat source through a coolant composed of a component having a high volatility and a component having a low volatility. This method does not use a compressor, but a counter-flow heat exchanger.

WO2011/081666 recovers heat with a rankine cycle using ammonia as an energy carrier and uses a compressor for compressing CO2 gas, whereby heat is exchanged between CO2 and ammonia in a heat exchanger.

Disclosure of Invention

The object of the present invention is to enable additional energy savings by providing a method for coupling a first industrial process requiring heat with a second industrial process requiring cold, whereby, a first circuit for energy recovery from a first industrial process transfers heat to a second circuit for cold production of a second industrial process requiring cold, wherein, in the first loop for energy recovery, the energy carrier is a binary fluid consisting of water and ammonia, which has two phases and is compressed by a compressor, such as a compressor with screw (Lysholm) rotors or with vanes, which is particularly suitable for compressing two-phase fluids, or a variant developed therefor, whereby all or part of the liquid phase is evaporated as a result of compression, so that overheating does not occur, and that less operating energy has to be supplied and that the total energy coefficient of performance or COP of the coupled process is increased relative to the total COP of the uncoupled process.

One advantage of using such a compressor suitable for two-phase fluids is that it consumes less energy to compress a two-phase fluid to a certain temperature and pressure than to compress a mere gaseous fluid to that temperature and pressure. In a two-phase fluid, all or part of the liquid phase evaporates due to compression, so that overheating does not occur and so that less working energy has to be supplied.

Preferably, the method whereby the loop for energy recovery of the first industrial process is coupled to the loop for cold production of the second industrial process, whereby the heat of the energy carrier in the first loop for energy recovery remaining after the energy carrier is expanded in an expander for power generation is additionally used for heating the energy carrier of the second industrial process by means of a heat exchanger between the first loop for energy recovery and the second loop for cold production, which additionally heats the energy carrier of the second industrial process before the energy carrier of the second loop for cold production is expanded in the expander of the second loop for power and cold production.

The advantage of this coupling of the two loops is that the total energy saving of the coupled loops is larger than the sum of the energy recovery of the loops when they are not coupled.

Preferably, in such a method for energy recovery, the energy carriers of the first and second circuits for energy saving are different from each other. For example, the energy carrier of the second circuit for energy saving may have a lower boiling point than the energy carrier of the first circuit for energy recovery, making it suitable for use in a refrigeration device.

Part of the heat remaining after expansion of the energy carrier in the first expander for power generation is recovered as electrical energy in the second expander by this coupling.

Preferably, in such a method for energy recovery, a part of the heat generated by the compressor in the energy carrier of the first circuit for energy recovery is used to heat a process fluid in liquid or gaseous form in the first industrial process, and this relies on a heat exchanger between the first circuit for energy recovery and a conduit for feeding the process fluid to a process vessel of the first industrial process, where it reaches a desired temperature for the production stage in the first industrial process.

An advantage of such a utilization of the recovered heat for use in the production phase of the first industrial process is that less energy needs to be supplied from the outside, which results in energy saving in the first industrial process.

The energy carrier of the first loop for energy saving, which is water and ammonia, is a two-phase fluid, i.e. consisting of a mixture of liquid and vapor or gas phases.

Such an energy carrier has the advantage that it can be brought into a liquid or gaseous state as desired by controlling the pressure and temperature.

In this method for energy recovery, the energy carrier for the cold-producing second circuit consists of ammonia, whereby all or part of the phase change between the gas phase and the liquid phase takes place and then a higher pressure is reached by the compressor.

At atmospheric pressure, ammonia has a boiling point of-33 ℃, so that low temperatures can be obtained due to the expansion of the energy carrier.

The advantage of ammonia as an energy carrier is that its low boiling point enables the energy carrier to be used in liquid form in industrial refrigeration processes, such as the freezing of food or other substances.

Preferably, the second circuit for cold production is equipped with an electric pump, by means of which the energy carrier of the second circuit for cold production is brought to a higher pressure before being expanded in the expander of the second circuit for cold production.

The advantage of this electric pump is that it brings the energy carrier to a higher pressure, so that more energy can be released by expansion in the expander, and it can be driven in part by recovered power originating from one or both expanders of the coupled industrial process.

Preferably, the second circuit for generating cold comprises a separator between the expander for expansion and the compressor for compressing the energy carrier for separating a liquid phase from a gaseous phase in the energy carrier, followed by one or more refrigeration devices for one or more generation stages in the second industrial process using the liquid phase for refrigeration.

The advantage of this separator is that the liquid phase of the energy carrier can be led to the industrial refrigeration unit thus cooled, while the gas phase can be led to a compressor to increase the pressure in the gas phase.

Preferably, after compression in the compressor to a pressure which is re-liquefied due to ambient cooling, the energy carrier of the second circuit for generating cold is further led to a heat exchanger, wherein the waste heat from the energy carrier can optionally be transferred to another process liquid used elsewhere in the coupled generation process, in this case softened water which is converted into steam.

An advantage of such a heat exchanger is that the waste heat can be directly utilized in the industrial process, so that less external energy needs to be supplied to reach the required temperature.

Preferably, the heat exchanger for the residual heat of the energy carrier is connected to the separator by means of a tap, wherein the saturated steam and the saturated demineralized water are separated from each other at a pressure of 400 kPa.

An advantage of such a separator is that steam can be generated for industrial use.

Preferably, the condensing part of the separator and the condensate from the consumed steam are fed back to the feed stream of the heat exchanger.

The water from the further separator is recovered together with the water vapour from the first production process (in this case water evaporated from the potatoes as a result of the frying process) and is available for industrial use after filtration, which reduces the need for drinking water for the first industrial process.

The energy carrier of the second circuit for generating cold is now led further in gaseous form to a condenser in which the gas is condensed into liquid, and further to a pump which drives the energy carrier further to a heat exchanger between the first circuit for energy recovery and the second circuit for generating cold, after which the energy carrier of the second circuit for generating cold is reused in a subsequent cycle.

An advantage of such a heat exchanger is that it enables heat to be transferred between a first circuit for energy recovery and a second circuit for cold production, so that two industrial processes are connected together.

Drawings

In order to better illustrate the characteristics of the invention, a preferred embodiment of a device for saving energy according to the invention is described hereinafter, by way of example without any limiting characteristics, with reference to the accompanying drawings, in which:

FIG. 1 schematically illustrates a flow diagram of two industrial processes coupled together according to the present invention;

fig. 2 to 5 show the heat flow as a function of temperature through the heat exchangers 5, 9, 13 and 33 of fig. 1;

figure 6 shows a pressure-enthalpy diagram for ammonia.

Detailed Description

Fig. 1 shows a flow diagram of a circuit 1 for heat recovery of a first industrial process, which circuit 1 is coupled to a second circuit 2 for cold production of a second industrial process. The first industrial process 3 supplies hot gas or vapour which flows through a pipe 4 to a heat exchanger 5, said heat exchanger 5 forming part of the first circuit 1 for heat recovery, and the energy carrier (mixture of water and ammonia) of this first circuit is heated therein and led through a pipe 6 to a compressor 7 adapted to compress a two-phase mixture, from where the compressed energy carrier is led via a pipe 8 to a second heat exchanger 9 for steam production and via a pipe 10 further to an expander 11 in which the energy carrier is expanded and via a pipe 12 further to a third heat exchanger 13 for transferring heat to the circuit 2 for cold production in the second industrial process and via a pipe 14 further to a pump 15 which drives the energy carrier of the first circuit via a pipe 16 to the first heat exchanger 5, so as to be heated again and to pass again through the first circuit 1 for energy recovery.

The pump 17 in the second circuit 2 for cold production drives the energy carrier of this second circuit for cold production, i.e. ammonia, through a conduit 18 to the heat exchanger 13, where the energy carrier absorbs heat from the first circuit 1 for energy recovery and is led through a conduit 19 to an expander in which the energy carrier expands and is further led through a conduit 21 to a separator 22 for separating a gas phase and a liquid phase in the energy carrier, from where the liquid phase of the energy carrier is led through a conduit 23 to an industrial refrigeration plant, in this case a freezing tunnel 24, a freezing storage area 25 and a fresh ice area 26 for order collection (galleddarea 26 for the collection of orders) and to further refrigeration devices 27, 28 which together form part of a second industrial production process in which cold is required.

The evaporated energy carrier from the refrigeration equipment is combined with the gas phase from the separator 22 via a conduit 29 and further led via a conduit 30 to a compressor 31, from where the compressed gas is led via a conduit 32 to a heat exchanger 33, where the waste heat can be dissipated to a softened water stream 34, which softened water stream 34 can flow via a conduit 35 to a steam generator 37 when a tap 36 is opened. The energy carrier of the second circuit for cold production is led from the heat exchanger 33 via a conduit 38 to a heat exchanger 39, where it is condensed by the air flow, after which the energy carrier is led further via a conduit 40 to the pump 17, from where it is led further via a conduit 18 and reused in the subsequent cycle of the second circuit 2 for cold production. Additional make-up of the energy carrier in the second circuit for cold production can be added to the liquid phase in the separator 22 via line 41. Through the pipe 42, the hot gas fed from the first production process 3 is used to heat the water in the generator 43 for hot water.

Fig. 2 to 5 show diagrammatically the relationship between the temperature in ° c of the energy carrier and the heat flow in KJ/s through the following heat exchangers, respectively: 5 (fig. 2), 9 (fig. 3), 13 (fig. 4) and 33 (fig. 5). The temperatures of the heated (outgoing) and cooled (incoming) streams in the heat exchanger are shown in each case.

Fig. 6 shows a Mollier diagram of the preferred energy carrier ammonia for the cold-producing second circuit, with enthalpy presented in kJ/kg along the horizontal axis and pressure in MPa along the vertical axis.

The curves represent all points of pressure and enthalpy for the liquid phase (below the curve) in equilibrium with the gas phase (above the curve).

The operation of the device 1 is very simple and as follows.

The first production process that requires heat can be an industrial frying apparatus for french-fry potatoes, for example, in which they are pre-fried, or it can be an apparatus for frying potato chips.

The first production process 3, which requires heat, is provided with a first circuit 1 for energy recovery, in which the energy present in the hot steam originating from the first production process 3 is partially recovered by transferring the heat of the hot gases in a heat exchanger 5 to an energy carrier, i.e. a mixture of water and ammonia present in this first circuit 1, which is then expanded in an expander 11, whereby electric energy can be generated which can be used again in the process.

Another part of the energy present in the hot steam is used for producing hot water by leading this part via a pipe 42 to a hot water generator 43.

Another part of the energy present in the hot gases is transferred from the energy carrier in the first circuit 1 for energy recovery to the energy carrier in the second circuit 2 for cold production, i.e. ammonia, via the heat exchanger 13, whereby the transferred heat is used to heat the energy carrier before the energy carrier of the second circuit 2 for cold production is expanded in the expander 20 for the production of electric energy which can be used again in the process.

The cooled energy carrier of the second circuit 2 is led to a separator 22 separating the liquid phase of the energy carrier from the gas phase, after which the liquid phase (-33 ℃) is used for a second industrial process requiring cold, and a refrigeration device is supplied with the liquid phase of the second energy carrier from said separator 22 via a conduit 23, so that applications such as a freezing tunnel 24, a freezing storage area 25, a collecting area 26 for frozen goods and other refrigeration devices 27, 28 can be cooled. The second industrial process requiring cold may be, for example, freezing and chilled storage of food products.

For maximum energy recovery of two coupled industrial processes, it is advantageous to have different energy carriers in the first circuit for energy recovery and in the second circuit for cold production. In the given example, the energy carrier in the first circuit is water with a portion of ammonia, while the energy carrier in the second circuit is ammonia.

After expansion in the expander 11, the first energy carrier is a two-phase flow that has been cooled, but more thermal energy can be dissipated from said first energy carrier to a second energy carrier (pure ammonia) with a much lower boiling point (-33 ℃), and this absorbs heat in the heat exchanger 13. This additional heat is used in the expander 20 of the second circuit for generating cold, in which the energy carrier of the second circuit is expanded.

The ammonia heated in the heat exchanger 13 for the cold-producing second circuit is expanded in an expander 20, whereby the energy carrier changes into two phases (liquid and gaseous), whereby these phases are separated from each other in a separator 22. The liquid phase (liquid ammonia) has a temperature of-33 ℃ and can be used in connected industrial refrigeration plants.

The pressure-enthalpy diagram of fig. 6 shows how much energy (work) can be recovered by lowering the pressure of ammonia in the liquid phase to a two-phase system, whereby this energy is extracted as electricity from the expander.

In the table below, the energy coefficient of performance, or COP, is calculated for two examples of processes from hot to cold.

Table 1 gives the energy account (energyaccount) for an apparatus for producing french fries coupled to a freezing apparatus. The energy recovery column gives the sum of all energy savings, while the energy supply column gives the sum of the energy that has to be supplied to enable energy recovery. In this case, the ratio of recovered energy to supplied energy, or COP, is 3.95 and higher than the COP of the total process in which the energy recovery is not coupled to the circuit for generating cold.

Table I: energy accounting for french fry potato production coupled to refrigeration device

Table II shows the energy accounting for the plant for french fries production not coupled to the second industrial process. The energy recovery column gives the sum of all energy savings, while the energy supply column gives the sum of the energy that has to be supplied to enable recovery. In this case, the ratio of recovered energy to supplied energy, or COP, is 4.59.

Table II: energy account for french fries production

It goes without saying that the invention can be used for coupling any industrial process in which one process requires heating and another process requires cooling.

The invention can also be applied in different temperature ranges and also with different energy carriers than those illustrated in the examples, as long as they can be two-phase for the first circuit for heat recovery.

The invention is in no way limited to the embodiments described as examples and shown in the drawings, but the device for saving energy according to the invention can be implemented in various forms and dimensions without departing from the scope of the invention as described in the following claims.

Claims (13)

1. A method for coupling a first industrial process requiring heat to a second industrial process requiring cold, wherein a first circuit (1) for energy recovery from the first industrial process transfers heat to a second circuit (2) for generating cold for the second industrial process requiring cold, characterized in that in the first circuit (1) for energy recovery a first energy carrier is two-phase and compressed by a first compressor (7) adapted to compress a two-phase fluid, the first compressor being a screw rotor or a bladed compressor, whereby all or part of the liquid phase evaporates as a result of the compression, so that overheating does not occur.

2. The method according to claim 1, wherein the first loop (1) for energy recovery of the first industrial process is coupled to the second loop (2) for cold production of the second industrial process, and wherein, the heat of the first energy carrier in the first circuit for energy recovery remaining after the first energy carrier is expanded in a first expander (11) for power generation is additionally used for heating a second energy carrier of the second industrial process by means of a third heat exchanger (13) between the first circuit for energy recovery (1) and the second circuit for cold production (2), the second energy carrier of the second industrial process is additionally heated before the second energy carrier of the second circuit (2) for generating cold is expanded in a second expander (20) to generate electricity and cold for the second circuit (2) for generating cold.

3. Method according to claim 1, characterized in that a first energy carrier of the first circuit (1) for energy recovery and a second energy carrier of the second circuit (2) for cold production are different from each other.

4. Method according to claim 1, characterized in that the second energy carrier of the second circuit (2) for cold production has a lower boiling point than the first energy carrier of the first circuit (1) for energy recovery.

5. Method according to claim 2, characterized in that a part of the heat generated by the first compressor (7) in the first energy carrier of the first circuit (1) for energy recovery is used to heat a process fluid in liquid or gaseous form in the first industrial process (3), and this relies on a first heat exchanger (9) between the first circuit (1) for energy recovery and a conduit for feeding the process fluid to a process vessel of the first industrial process (3), where it reaches a desired temperature for a production stage in the first industrial process.

6. Method according to claim 2, characterized in that the second energy carrier of the second circuit (2) for generating cold is ammonia.

7. Method according to claim 2, characterized in that the second circuit (2) for generating cold is equipped with an electric pump (17), by means of which the second energy carrier of the second circuit (2) for generating cold is brought to a higher pressure before being expanded in the second expander (20) of the second circuit (2) for generating cold.

8. The method according to claim 2, characterized in that the second circuit (2) for generating cold comprises a second separator (22) between the second expander (20) for expansion and a second compressor (31) for compressing a second energy carrier for separating a liquid phase from a gaseous phase in the second energy carrier, followed by one or more refrigeration devices (24, 25, 26, 27, 28) for one or more generation stages in the second industrial process.

9. Method according to claim 8, characterized in that the second energy carrier of the second circuit (2) for generating cold is led further to a second heat exchanger (33) after compression to a pressure of becoming liquid again in the second compressor (31), wherein the waste heat from the second energy carrier can be transferred to another process liquid used elsewhere in the coupled generation process.

10. Method according to claim 9, characterized in that the second heat exchanger (33) for the residual heat of the second energy carrier is connected to a first separator (37) by means of a tap (36), wherein saturated steam and saturated demineralized water are separated from each other at a pressure of 400 kPa.

11. Method according to claim 10, characterized in that the non-condensed part in the first separator (37) is used to heat hot water for plant use.

12. The method according to claim 11, characterized in that water from the generator (43) is recovered together with the fluid from the first industrial process (3) and after filtering is made available for industrial use.

13. Method according to claim 7, characterized in that the second energy carrier of the second circuit (2) for cold production is led in gaseous form from a condenser (39), where it becomes liquid, to the electric pump (17), which drives it further to a third heat exchanger (13) between the first circuit (1) for energy recovery and the second circuit (2) for cold production, after which the second energy carrier of the second circuit (2) for cold production is reused in a subsequent cycle.