EP0913652B1 - Refrigerating and freezing method for water containing products - Google Patents

Abstract

The product is placed in a vacuum chamber in which the pressure is reduced by a vacuum pump until water vapor leaves the product. It is then cooled by direct evaporation cooling and absorbed in a zeolite bed before the vacuum pump. Evacuation of the vacuum chamber is matched to the absorption characteristics of the zeolite bed so that the product achieves the desired state e.g. temperature, water content etc. No water vapor flows through the zeolite to the vacuum pump.

Description

The invention relates to methods for cooling and / or freezing water-containing Products by direct evaporation under vacuum.

A method is known from DE 40 031 07, water by direct evaporation convert to ice. The one flowing off a water surface Water vapor is adsorbed in a zeolite bed. A vacuum pump sucks air and non-condensable gases out of the zeolite bed.

Zeolites are crystalline aluminosilicates, with a branched cavity structure, reversibly stores (adsorbs) the water molecules. The adsorption of Water vapor is a highly exothermic process. The stored water can by heating (regeneration) the zeolite crystals to above approx. 200 ° C again are evaporated from the crystal structure.

In practice, the above Devices that water vapor from the zeolite bed is not adsorbed or not adsorbed quickly enough and therefore up to Vacuum pump flows. However, vacuum pumps can only inject water vapor pump out very limited quantities. Limit excess water vapor the final vacuum of the pumps or soon lead to pump defects.

To protect the vacuum pump from harmful amounts of water vapor, you can increase the amount of zeolite. As a rule, however, this leads to that only parts of the amount of zeolite adsorb. In the subsequent regeneration process however, the entire filling must be heated. Because of the larger amounts of zeolite more heat must be introduced and in the following Cooling process more heat can be dissipated. The efficiency gets significantly worse.

Another way to protect the pumps is to fill the zeolite in its geometric extension the flow path of the water vapor adapt. The flow path of the steam within the The filling becomes longer and the pressure drop correspondingly higher. Around to reach the same evaporation pressure, the suction pressure of the pump be lowered. At the same time, however, it decreases with lower adsorption pressure the adsorption load. This also leads to an additional expenditure of desorption heat.

With long flow paths of water vapor through the zeolite bed so-called adsorption zones, in which the adsorption reaction takes place. The zeolite filling has already reached the saturation load in front of this zone, no water vapor flow can be measured behind the zone. The pressure is here the final pressure of the pump. Although the vacuum pump is continuous is in operation, its final pressure has no influence on the evaporation temperature in the product. The vacuum pump runs for a long time and with high operating costs.

Even with very long flow paths through the zeolite bed, the adsorption zone becomes reach the end of the filling after a certain time and the Water vapor can be sucked in by the vacuum pump.

Numerous types of zeolites with different adsorption characteristics are on the market today. Granules of the same zeolite type and identical Granulate diameter, which is produced by different granulation processes behave during water vapor adsorption under vacuum very different. An optimal geometric design of a zeolite bed it is therefore almost impossible for varying products.

The water vapor absorption capacity of a zeolite bed depends on it Dimensions from the driving steam pressure. This is, especially in terms of time Course, depending on the temperature, the amount and the structure of the to cooling product.

The object of the invention is to provide methods with which an optimal Loading the zeolite bed and at the same time protecting the vacuum pump from the damaging effects of high water vapor pressures is possible.

The task is solved by the characterizing features of the claim 1. In the subclaims are further process variants according to the invention demonstrated.

When cooling products by direct evaporation of water in the Vacuum can achieve very high cooling capacities. A freeze from Food can be done within a few minutes. The freezing process does not take place from the product surface as with conventional processes progressing slowly inwards, but simultaneously and homogeneously throughout the product. The final temperature of the product can be determined by the adjustable chamber pressure can be precisely regulated. The total pressure in the The vacuum chamber is characterized by the adsorption characteristics of the zeolite bed, the operating times and the achievable final pressure of the vacuum pump certainly.

The term product stands for all water-containing substances, indifferent whether it is organic or inorganic substances. The water content can vary widely. However, the minimum percentage must be as large be that the desired cooling temperature is achieved by direct evaporation can be. It is also advantageous to dry the surface only damp goods, such as B. plastic granules. It can make sense here be used to heat or pre-heat the product during the evaporation process to bring the drying to a higher temperature.

The common goal of all processes is, besides an optimal loading of the Zeolite bed and the protection of the vacuum pump, its running time on one Limit minimum. Especially with mobile and / or solar powered Devices is a low energy expenditure for the operation of the vacuum pump he wishes. Here it is of crucial advantage if the vacuum pump is only in operation until the water vapor flow from the product to the zeolite bed exceeds a given value. With appropriate devices flow monitors are used for this purpose, the pressure drop of the flowing water vapor.

Water vapor-sensitive sensors are also used, which are before or also are arranged after the pump. You can use the pump turn off as soon as they register water vapor in the flow. Sensors on Output of the pumps have the advantage that they are not suitable for vacuum have to. They can also be less sensitive because of the water vapor the pump is highly concentrated. A burst of water vapor the zeolite bed is always a signal that the zeolite filling is closed renew or regenerate.

Another very inexpensive way to protect the pump is the temperature rise of the zeolite bed during adsorption to be used as an output signal for switching off the pump. This regulation is particularly suitable if it is within the zeolite bed can form an adsorption zone. The temperature sensor is then to be placed at the end of the zeolite bed.

Instead of the temperature rise, there can also be an increase in the water concentration can be detected within the cavity structure at the end of the zeolite bed.

For liquid products with a high level or high initial temperatures There may be steam eruptions within the liquid as a result, liquid particles come with the outflowing Water vapor are carried away. In these cases, evacuation interrupted by the vacuum pump and after a short calming phase be evacuated further. The liquid particles can pass through known techniques are detected.

But it is also advantageous in the flow line between the product and Zeolite bed to install suitable droplet separators that carry away prevent non-gaseous particles.

It is particularly advantageous if the droplet separators are cooled. If the product temperature is above the liquefaction temperature of these cooled surfaces, the outflowing steam can condense on the cold surfaces and, if so desired, can drip back into the product to be cooled as condensate.
The evacuation of the system is controlled so that no water vapor flows to the zeolite bed during this phase. According to the invention, the further flow of the water vapor is prevented by an air cushion in the zeolite bed. The evacuation is controlled according to the invention so that the pressure in the vacuum chamber decreases continuously, but the expansion of the gas cushion in the zeolite bed is retained. A boundary layer a few centimeters wide forms between the flowing water vapor and the blocking gas cushion. On one side there is pure water vapor flowing at high speed and on the other side there is a relatively calm, water vapor-free gas cushion.
Only when the product temperature has almost reached the liquefaction temperature and consequently the cooling capacity decreases can water vapor flow into the zeolite bed and be adsorbed by controlled pumping out of the gas cushion. The evacuation according to the invention makes it possible to avoid the otherwise necessary flow valves and suction valves between the product and the zeolite filling.

Numerous products, especially food, tend to evacuate to foam up or inflate. In many cases this is a desirable one Process that the final product z. B. larger volume or tastier lets appear. Soft ice cream becomes conventional, for example, by introducing it compressed air foamed during the cooling process. According to the invention the foaming can now take place during the cooling or freezing process Vacuum only by controlling the evacuation process.

Dough is often treated with cold today. On the one hand, this is done around to stop or delay the fermentation process, on the other hand to keep the dough in the Store and transport refrigerated / frozen state for a long time. During the fermentation process, the dough is loosened by carbon dioxide the addition of leavening agents (yeast, baking powder, etc.) arises. With the invention Process, the fermentation process can be delayed and stopped or without the addition of blowing agents, because the Loosening (pore) in the direct evaporation by the expanding Water vapor takes place. Do the evaporation until completely freezing of the dough, the inflated structure of the dough remains the flooding of the vacuum chamber.

A number of other foods must be refrigerated / frozen at higher ones Temperatures are treated (cooking, baking, steaming, steaming, pasteurizing, Blanching, broth ect.). It is particularly advantageous if this Heat treatment already inside the still open vacuum chamber is possible. For the subsequent cooling process, only the Vacuum chamber are closed and the evacuation process started become. The vacuum chamber advantageously has the shape of, for example Cooking kettles, steamers or autoclaves with airtight closures Vacuum chamber are expandable. The suction line to the zeolite bed exists then, for example, from a flexible connection that is flanged to the lid becomes.

According to the z. B. also upgrade an oven to the vacuum chamber. Half-baked products can be baked in the oven by direct evaporation be frozen. The walls of the oven stay hot because of them no water can evaporate.

It is also particularly advantageous to use so-called vacuum packaging machines expand with a zeolite bed and pre-pack the food to cool or freeze. The products are sealed airtight here immediately after the direct evaporation according to the known Process in the same vacuum chamber.

Claims (13)

1. Method for cooling and freezing water-containing products by direct evaporation, in which method the water-containing product is introduced into a vacuum chamber and the chamber pressure is reduced by a vacuum pump until the water vapour escapes from the product and the product cools owing to the direct coldness of evaporation, the water vapour flowing from the product is adsorbed in a zeolite bed upstream from the vacuum pump, the evacuation of the vacuum chamber by the vacuum pump is matched to the adsorption characteristics of the zeolite bed such that the product adopts the desired temperature and/or the desired water content, no water vapour flows through the zeolite bed to the vacuum pump and the evacuation by the vacuum pump is ended as soon as the water charge in the zeolite bed in the bed region directly upstream from the vacuum pump exceeds a given value.
2. Method according to claim 1, characterised in that after the beginning of the direct evaporation, the evacuation is interrupted and is only resumed when the water vapour flow to the zeolite bed lessens.
3. Method according to any one of the preceding claims characterised, in that the evacuation is ended as soon as water vapour reaches the vacuum pump.
4. Method according to any one of the preceding claims characterised, in that the evacuation is ended as soon as the temperature of the zeolite bed in the region upstream from the vacuum pump increases owing to the adsorption heat being released.
5. Method according to any one of the preceding claims, characterised in that the evacuation is interrupted as soon as liquid components are separated from the product by the flow of water vapour and introduced into the zeolite bed.
6. Method according to any one of the preceding claims, characterised in that the evacuation of the product is only interrupted when the product is foamed, inflated or opened up.
7. Method according to claim 6, characterised in that the vacuum chamber is only flooded again when the foamed or opened product is frozen.
8. Method according to any one of the preceding claims, characterised in that the cooled or frozen product is enclosed still under vacuum in air-tight containers.
9. Method according to any one of the preceding claims, characterised in that before it can reach the zeolite bed, the water vapour flowing from the product is recondensed on a cold surface and can flow into the zeolite bed by further evacuation only when the evaporation temperature in the product has virtually reached the condensation temperature.
10. Method according to any one of the preceding claims, characterised in that the product is heated to higher temperatures before the cooling process.
11. Method according to any one of the preceding claims, characterised in that the product is introduced into the already pre-evacuated vacuum chamber.
12. Method according to any one of the preceding claims, characterised in that the water removed during the direct evaporation is added to the product prior to the cooling process.
13. Method according to any one of the preceding claims, characterised in that the product is stored cold for a relatively long period in the vacuum chamber after the direct evaporation.