Patent Application
20100071387
The present invention relates to a method and a device for cooling a liquid. Preferably, the corresponding liquid, which after cooling can also be a suspension of frozen liquid in liquid, is used as process liquid in further processes. Particular preference is given to the use for cooling water as process water for use in the production of freshly mixed concrete.
Various approaches are known from the prior art for cooling liquids. For example, it is known to cool liquids by means of compression refrigeration machines based on absorption refrigeration processes or by indirect cooling via a heat exchanger. The liquids are frequently water, alkali solutions, acids, solvents and/or liquid foods. All processes mentioned here are complex in terms of apparatus, which makes the methods described uneconomic in particular when a liquid must be cooled batchwise, since then high standing times coincide with high apparatus requirements.
Therefore, the object of the present invention is to specify a method for cooling liquids which is simple to carry out in terms of apparatus with high cooling efficacy. A corresponding device shall likewise be proposed.
These objects are achieved by the independent claims. The respective dependent claims are directed towards advantageous development.
The method according to the invention for cooling a liquid is based on introducing or injecting a coolant comprising a refrigeration-liquefied gas into the liquid.
Preferably, the coolant consists entirely of a refrigeration-liquefied gas such as, for example, liquid nitrogen.
Preferably, at least in part, in particular complete, vaporization of the refrigeration-liquefied gas occurs. By means of the vaporization enthalpy thereby applied and subsequent heating of the vaporized gas, cooling of the liquid proceeds, via which, in particular, an at least partial freezing of the liquid can be achieved, so that preferably a suspension of frozen liquid in liquid is formed.
According to a further preferred embodiment of the method according to the invention, the coolant and in particular the refrigeration-liquefied gas are introduced into the liquid in drop form.
It is particularly advantageous in this context to achieve a distribution as uniform as possible of the drops of the liquefied gas in the liquid to be cooled. Continuous freezing of the liquid can thereby be particularly effectively prevented. According to the invention, ice crystals or conglomerates of the frozen liquid form thereby which form a suspension with the still unfrozen liquid. This suspension can, particularly preferably, be used for cooling other substances, and in particular as process water or process liquid in other processes. Particularly advantageous in this case is a use of water correspondingly cooled by the method of the invention for producing concrete.
For instance it is firstly possible to produce here freshly mixed concrete of a defined temperature, and secondly the resultant concrete is advantageous when used for producing reinforced concrete, and in particular steel-reinforced concrete, since here the tendency to corrosion is decreased. This is due to the fact that when liquid nitrogen is used as coolant, the atmospheric oxygen dissolved in the water is stripped out. In particular, this is taken to mean displacement of the atmospheric oxygen dissolved in the water by nitrogen. As a result of the decreased proportion of atmospheric oxygen dissolved in the water, the tendency to corrosion on reinforcing the concrete is greatly decreased. In particular, the nitrogen proportion or the proportion of dissolved nitrogen in the water after cooling to about 0° C. is essentially 16 mg/m3 (milligrams per cubic metre).
According to a further advantageous embodiment of the method according to the invention, the drop diameter is in the range from 1 to 3 mm.
This special drop size causes an essentially uniform distribution of the liquid gas in the liquid, so that an essentially uniform freezing occurs without the formation of a continuously frozen block of frozen liquid.
According to a further preferred embodiment of the method according to the invention, the refrigeration-liquefied gas comprises nitrogen.
In particular, the coolant consists entirely of liquid nitrogen. Liquid nitrogen has a relatively high vaporization enthalpy and in addition is relatively inexpensive to produce or simple to obtain. This makes inexpensive cooling of liquids possible.
According to a further advantageous embodiment of the method according to the invention, the coolant is introduced into the liquid at a velocity of 20 m/s and above.
This relatively high velocity causes a uniform distribution of drops of the coolant, so that good mixing of coolant and liquid takes place. This leads to a good cooling result and leads to the formation of relatively small ice pieces, so that a well-mixed suspension of frozen liquid in liquid is obtained.
According to a further advantageous embodiment of the method according to the invention, the liquid is agitated or circulated.
By this means the mixing of the drops of the coolant with the liquid to be cooled can be further improved, so that an additionally improved cooling result can be achieved. In particular, the circulation can prevent relatively large pieces of frozen liquid from forming.
According to another advantageous embodiment of the method, the amount of coolant added is controlled as a function of a fraction of the liquid which is frozen.
The fraction of liquid which is frozen, hereinafter for simplicity called the ice fraction, determines the refrigeration capacity which can later be achieved with the cooled liquid, since when the ice is melted, in addition the melting enthalpy must be applied, which can advantageously also be used for cooling. Therefore, control as a function of the ice fraction is advantageous, since in this case, finally, the amount of heat and enthalpy available for cooling can be established. Ice fractions of up to 30%, preferably up to 35%, particularly preferably up to 40%, or even in particular up to 50%, are advantageously possible.
According to a further advantageous embodiment of the method, the amount of coolant to be added is controlled in such a manner that up to 30% of the liquid is frozen.
According to a further advantageous embodiment of the method according to the invention, the amount of coolant to be added is determined by means of a heat balance calculation.
In this case, in particular the temperature of the liquid, the input amount of coolant, the temperature of the coolant, the temperature of the vaporized cryogenically liquefied gas on leaving the cooling process and/or the like is taken into account. In particular, by means of the heat balance calculation, an ice fraction can be preset.
According to a further advantageous embodiment of the method according to the invention, the coolant is added by at least one nozzle.
Nozzles are suitable, particularly, for producing drops of the coolant, so that using a corresponding design of the at least one nozzle, preferably a plurality of nozzles, good mixing of the coolant with the liquid, and thus a good cooling result and a homogeneous suspension of frozen liquid in liquid can be achieved.
According to a further advantageous embodiment of the method according to the invention, when no coolant flows in, the gas comprised in the coolant is added in gas form through the nozzles.
In this case, this can be produced by external vaporization of the cryogenically liquefied gas, or added from other sources. It is advantageous in this case that an at least partial closure of the nozzles by icing can be prevented, since the nozzles can be kept open. The injection of gas is suitable for this, since as a result an intervening operation with the addition of coolant is possible, and there is no contamination of the liquid with a further gas. As a result it is possible to use in first time intervals the nozzles not for addition of liquefied gas, but instead to add the gas in gas form, while at other time points no gas is added in gas form, but rather in liquid form. This makes possible an operation of the method which is particularly favourable in terms of costs and energy, in which icing of the nozzles is prevented.
According to a further aspect of the present invention, a device for cooling a liquid is proposed, in particular for carrying out the method of the invention. The device comprises:
According to an advantageous embodiment, the nozzles have an outlet diameter of 1 to 3 mm. The nozzles are typically positioned in the lower two-thirds of the container, preferably in the lower half of the container and more preferably in the vicinity of the bottom of the container.
According to a further advantageous embodiment of the device according to the invention, the container comprises a gas collection space with gas removal line.
Preferably, this gas collection space is constructed above, or at, the top end, or at the top region of the container. This is advantageous, in particular, when the cryogenically liquefied gas, after vaporization, has a density which is less than the density of the liquid. This is the case, for example, when as coolant use is made of liquid nitrogen for cooling water.
According to a further advantageous embodiment, the device further comprises a transport unit for transporting and/or circulating the liquid.
This can ensure that, firstly, a mass flow rate of the cooled liquid can be ensured independently of a level of the liquid in the container, and secondly, by the circulation, complete freezing of the liquid can be prevented.
According to a further advantageous embodiment of the device according to the invention, means for comminuting frozen liquid are assigned to the transport unit which, for example, can comprise a pump.
Preferably, these means are combined with the transport unit or are constructed directly upstream of the transport unit. This advantageously leads to homogenization of the particle size distribution in the liquid, and in particular in the ice-water suspension, since it can thus be ensured that continuous freezing of the liquid is prevented, since relatively large ice fragments forming in the container are comminuted.
According to a further embodiment of the device according to the invention, this in addition comprises control means for controlling the feed of coolant. By this means, semiautomatic and/or fully automatic control of the addition of coolant may be implemented. Preferably, characteristics are transmitted to the control means which comprise, for example, the temperature in the container, the temperature of the coolant, the temperature of the exhaust gas, therefore of the vaporized coolant, the amount of liquid to be cooled and/or the amount of coolant to be added. A further characteristic can preferably be the level in the container, that is to say the amount of liquid present in the container.
According to an advantageous embodiment of the device, at least one temperature sensor is constructed in thermal contact with the container connected to the control means. Via this temperature sensor, preferably, the temperature of the liquid may be determined, either by measuring this directly and/or by determining the temperature of the liquid by the container temperature and if appropriate a computer-assisted model of the container.
Particularly advantageously, the method according to the invention and/or the device according to the invention can be used for cooling freshly mixed concrete. In particular, the method according to the invention and/or the device according to the invention can be used for cooling process water, the cooled process water being thereafter used in the production of fresh concrete, i.e. unset mixed concrete.
Hereinafter the invention will be described in more detail with reference to the accompanying drawing, without restricting it to the details and exemplary embodiments shown there. In the diagrams:
The water 2 to be cooled is fed to the container 1 via a feed line 8 continuously or discontinuously controlled by an automatic shutoff valve 9. The water inlet temperature is monitored via a second temperature sensor 10. Instead of an automatic shutoff valve 9, a manual shutoff valve or other flow control measures can also be formed.
The water 2 which is cooled to the desired preset temperature, for example 0° C., can be taken off via the outlet line 11 continuously via a transport means 12, in particular a transport pump. This can be controlled by a shutoff valve 13, preferably an automatic shutoff valve 13. In the case, in particular, of discontinuous pumping off, the water 2 is circulated via an overflow valve 14. In the process of circulation, in particular, in a preferred manner, the liquid 2 in the container 1 can also be incorporated.
For cooling the water 2, in accordance with the preset temperature to be set, liquid nitrogen, hereinafter called LIN, is injected in a temperature-controlled manner via the control valve 15 through the nozzles 3 which are installed at the bottom of the container 1. The temperature in the container 1, in particular of the liquid 2, is measured by at least one third temperature sensor 16, in the present exemplary embodiment, three third thermosensors, which are constructed at different heights of the container 1. The measurements of the third temperature sensors 16 are transmitted to a control means 17, via which the control valve 15 can also be controlled. In addition, the amount of injected LIN is also measured with a suitable measuring device 18. The measurements of this measuring device 18 are also transmitted to the control means 17 and analysed there. The control means 17 in addition takes into account the theoretical water temperature which is performed by means of a heat balance calculation by the control means 17 taking into account the amounts of water measured by the level sensor 4 and the temperature of the gaseous nitrogen which is determined via the first temperature sensor 7. The temperature thus calculated is continuously compared with the water temperature determined by the third temperature sensors 16 and, based thereon, if appropriate the amount of LIN to be injected is corrected. This is advantageous, in particular, in the vicinity of the ice point in order to prevent an excessive LIN amount from being injected into the water, which leads to uncontrolled ice formation.
When the preset temperature is reached, the LIN feed is closed by the control valve 15 and the valve 19 is automatically opened. As a result a small amount of gaseous nitrogen is permanently injected into the container 1. The liquid nitrogen is thereby forced out of the nozzles 3 and corresponding feed lines and thus prevents water 2 from penetrating into the nozzles 3 and the feed, which water, owing to the low temperature of the nozzles 3, would freeze immediately. When the cooling process is continued, the LIN control valve 15 is opened again and the GAN valve 19 closed. Liquid nitrogen thereby flows again into the container 1.
Owing to injection of the cryogenic liquid nitrogen into the water 2, ice crystals form on the outlet orifices of the nozzles 3 which build up to what is termed a mushroom. The liquid nitrogen exits through an opening in the centre of this mushroom. Via a small mechanical tension, this ice armour breaks off from the nozzle 3 and is regularly formed again.
Hereinafter, the production of process water having a preset temperature of 0° C. will be described. This water 2 is required for producing concrete and may contain additives. The production of quality concrete, for example for the construction of dams, bridges or what is termed white tank waterproof concrete is critical in particular in summer with high external temperatures without cooling the freshly mixed concrete. Excessively high concrete temperatures cause rapid evaporation of the water which can lead to contraction cracking. Especially in the case of high-volume structures, the heat of hydration released on setting must be taken into account, since the concrete expands owing to the temperature increase. If the concrete subsequently again cools down to ambient temperature, cracks form in the unreinforced concrete. Therefore, in the freshly mixed concrete, a temperature as low as possible is set.
In order to prevent crack formation in massive built structures such as dams or else watertight tanks, freshly mixed concrete temperatures between 8° C. and 23° C. are required. To maintain these temperatures, for example, the freshly mixed concrete in the transport vehicle is cooled down to 6° C. by indirect injection of liquid nitrogen into the rotating drum. In this method, however, it is necessary to note that excessive frost in the concrete can cause structural changes.
Cooling the aggregates such as, for example, gravel or the makeup water before mixing is therefore far less problematic. In addition, therefore, the essential method parameters for producing concrete in a stationary mixer and the results necessitated by cooling the makeup water are summarized in a table.
In the cooling of water by direct injection of LIN at a pressure of 5 bar, for a mixer volume of 4.5 m3 at a water rate of 540 kg per mixture and a mixing time of 3 minutes at a refrigeration capacity of 251 kW and a liquid nitrogen rate of 2580 kg/h for a rate of water to be cooled of 10.8 m3/h and a specific LIN consumption of 8.9 l of liquid nitrogen per ° C. and per m3 of concrete, a freshly mixed concrete temperature of 16.5° C. was achieved. In the comparative example, with a water temperature of 20.5° C., a freshly mixed concrete temperature of 20° C. was achieved. Therefore, the method according to the invention, using the device according to the invention for cooling freshly mixed concrete, enables significant cooling of the freshly mixed concrete with low requirements of apparatus, and in particular makes possible the production of high-value concrete in hot weather.
In addition, there is the possibility of utilizing, as circulation pump, a transport pump already present, which serves for transporting the water into a weighing device. After the charging of the weighing device, which customarily takes about 15 seconds, the transport pump is run in the circuit for the remaining time of the concrete mixing process and the water-ice suspension is pumped back into the container 1 via an overflow valve. A rotating blade device is likewise preferably connected upstream of this transport pump.
Hereinafter, the cooling effect on the freshly mixed concrete temperature will be described, when the process water required for concrete production, what is termed the makeup water, consists of a water-ice suspension having a 20% ice fraction.
In Comparative Example 2, without the cooling according to the invention, a freshly mixed concrete temperature of 20° C. was achieved at 20.5° C. water temperature. With cooling according to the invention, a water temperature of 0° C. and a water-ice suspension having an ice fraction of 20% was achieved. For a mixing volume of 4.5 m3 and a water rate of 432 kg per mixing, this was equivalent to an ice quantity of 108 kg. The mixing time was 3 minutes, the rate of water to be cooled per hour was 10.8 m3/h. This corresponded to a refrigeration capacity of 450 kW and an LIN consumption of 4640 kg/h. This is equivalent to a specific LIN consumption of 9.1 l of liquid nitrogen per ° C. and m3 of concrete. A freshly mixed concrete temperature of 13.5° C. was achieved thereby.
Hereinafter a description will be given of how, on the basis of a heat balance calculation, control of the rate of liquid nitrogen to be added can be carried out. Liquid nitrogen has a boiling point of 77.3 Kelvin and an enthalpy of vaporization of 198.6 kJ/kg at ambient pressure. In addition, liquid nitrogen can customarily be used as coolant, it is not vaporized indirectly in a heat exchanger, but is in direct contact with the liquid to be cooled. The liquid to be cooled is contaminated by the coolant, so that nitrogen is customarily well suited as coolant, since here an inert gas is present, which customarily does not react with the liquid to be cooled and frequently, in particular in the case of concrete also, for the most part outgases without problems.
With liquid nitrogen as coolant, it is possible to cool, in particular, without problems: process water which is used for producing concrete, sulphuric acid and/or alkali solutions. By direct input of LIN into the liquid 2 to be cooled, here, for example, a process water, heat is directly taken off from the water by the vaporizing nitrogen and the water 2 is thereby cooled. The resultant gaseous nitrogen first has boiling temperature and then ascends in the water as a host of gas bubbles. The host of gas bubbles warms and further cools the water. The temperature of the gaseous nitrogen equilibrates to that of the water bath and leaves it, in the case of sufficient height and therefore sufficient residence time, with the same temperature as that of the water bath. A cooling action as good as possible is achieved when the liquid nitrogen is introduced in drop form distributed as uniformly as possible over the cross section of the container 1. The drop diameter of the liquid nitrogen is determined by the exit diameter of the nozzles 3. Here, preferably, a diameter of the nozzles 3 of 1 to 3 mm and the use of a plurality to multiplicity of individual nozzles is proposed. These are preferably arranged at the bottom, or in the vicinity of the bottom, of the container 1.
Radial exit velocities are preferably in ranges from 20 m/s and more. The number of nozzles 3 depends on the required maximum refrigeration capacity. As a result of the vaporizing nitrogen drops, a gas bubble host is formed which ascends with high velocity in the liquid column. Because of this highly turbulent mixing process, the liquid temperature will rapidly equilibrate not only in the axial direction, but also in the radial direction. Experiments have shown that a preferred water height corresponds to 1.5 times the container diameter or more.
The cold energy available in the LIN is thereby optimally utilized. The preset temperature to be set is controlled not only via the temperature, but is determined via the heat balance by means of a process computer. The LIN rate required for cooling the water 2 is calculated according to the formula
m
LIN
=k*m
water
*ΔT/C1
Here, k is a correction factor which, especially, takes into account the wall cold losses of the container. k can be determined empirically. mwater is the water rate in kg/h, ΔT the temperature difference in water and C1(pT)=83.3 Kelvin. In this case a pressure of 5 bar in the liquid nitrogen is assumed. If after the ice temperature is reached, in addition liquid nitrogen is introduced into the cold water at 0° C., ice is abruptly formed in the form of snow slurry and ice pieces which are distributed in the manner of a suspension in the water owing to the intensive agitation.
Since the LIN rate required for a defined amount of ice can no longer be controlled via the temperature, since the temperature of the water 2 is unchanged at 0° C., in a particularly preferred manner the LIN rate required in the control means is calculated according to the formula
m
LIN
=k*[(mwater*ΔT/C1+Xice*mwater/C2)],
measured using a suitable measuring device 18 and added in accordance using a corresponding control valve 15, where is the fraction of ice in the water and C2(p, T)=1.05 at a pressure of 5 bar in the nitrogen tank.
It is possible thereby to set the ice fraction in the water at 0° C. (ice temperature) via the added rate of liquid nitrogen and thus in a simple manner to increase the cold energy available in the water or water-ice suspension.
The method according to the invention and the device according to the invention make possible in an advantageously simple manner the production of cooled liquids which, for example, can be used as process water in freshly mixed concrete production. In this case the enthalpy content of the coolant, preferably of liquid nitrogen, is utilized in a good manner by firstly using the enthalpy of vaporization and secondly utilizing the heating of the nitrogen after vaporization up to ice temperature for cooling the liquid.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2007 016 712.3 | Apr 2007 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2008/054049 | 4/3/2008 | WO | 00 | 11/30/2009 |
