Refrigeration – Storage of solidified or liquified gas – Liquified gas transferred as liquid
Reexamination Certificate
1999-10-28
2001-05-29
Doerrler, William (Department: 3744)
Refrigeration
Storage of solidified or liquified gas
Liquified gas transferred as liquid
Reexamination Certificate
active
06237349
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a device for pressure build-up in a storage container for cryogenic liquids with a housing that has at least one intake opening for liquid and, inside, a heating device for evaporating liquid as well as a vapor line that is arranged in the upper third of the housing to remove vapor bubbles that are produced during evaporation.
BACKGROUND OF THE INVENTION
In the future, in view of the emissions burden that is caused by traffic, hydrogen will be used increasingly as a fuel for motor vehicles, aircraft and ships. The storage of hydrogen on board these vehicles is suitably carried out in liquid form, since based on the low density of gaseous hydrogen, the storage capacity would otherwise be only very limited. The hydrogen is therefore cooled to about 25 K and introduced into the storage tank that is in the vehicle at a pressure of 3 to 4 bar.
If a combustion engine is used as a drive assembly, the filling pressure of 3 to 4 bar is just enough for proper operation of the engine. When fuel cells are used to power vehicles, however, at this time a hydrogen supply under a pressure of 10 bar is necessary. Storage of hydrogen at a pressure of 10 bar and a corresponding equilibrium temperature of about 31 K is disadvantageous, however, since the storage capacity clearly drops because the density of the liquid hydrogen decreases with rising temperature.
In practice, therefore, the hydrogen that is stored at a pressure of 3 to 4 bar is first compressed to 10 bar, before it is fed to the fuel cell. The pressure increase can be achieved by, for example, introducing additional gaseous hydrogen into the storage tank or by evaporating a portion of the liquid hydrogen. Such a device for pressure build-up is known from, for example, DE 42 12 626 A1. In this connection, a gaslift is attached to the bottom of the storage tank, in which liquid hydrogen that enters into the gaslift with use of the pump heating is partially evaporated. The gas bubbles that are produced in this case entrain liquid hydrogen via a rising main upward into the gas chamber of the storage tank, where the latter is ultimately evaporated with use of an evaporator-heating system.
SUMMARY OF THE INVENTION
A feature of this invention is to provide a device of the type mentioned above, which offers a simple, quick and reliable option of pressure build-up in a storage container for cryogenic liquids.
This feature is achieved according to the invention in that the cross-section of a vapor line is large so that during operation of the device, more vapor than liquid, preferably basically only vapor, is drained off through the vapor line.
With use of the device according to the invention, only a small part of the cryogenic liquid that is in the storage container is evaporated in the container to increase pressure. For this purpose, a housing that has at least one intake opening for liquid is provided in the container. Inside the housing is a heating device, by means of which a portion of the liquid is evaporated. The vapor that is produced in this case is drained off via a vapor line into the header of the storage container. The cross-section of the vapor line is selected in this case so that the flow resistance of the gas bubbles is small so that the latter do not entrain any liquid or entrain only a little liquid. In contrast to the prior art gaslift or bubble column, no second heating in the header of the storage container is necessary for evaporation of liquid that is carried upward.
The cross-section area of the vapor line is advantageously larger than the sum of the cross-section areas of the vapor bubbles that simultaneously pass through the cross-section area of the steam line during operation of the device. The cross-section of the vapor line is consequently designed based on the output of the heating device.
The vapor line is preferably arranged at the highest point of the housing. In this way, essentially only steam is present at the introduction of the vapor line, so that it is almost completely impossible to carry the liquid through the vapor line.
Only a portion of the liquid used to increase pressure in the storage container is to be evaporated by the invention. Accordingly, the temperature of the residual liquid stored in the storage container is, as much as possible, not to be changed. With the evaporation of a portion of the liquid that is in the housing, heating of the remaining liquid in the housing unavoidably coincides. To avoid heating the liquid that is outside of the housing, the housing and/or vapor line is advantageously thermally insulated.
The insulation of the housing and/or the vapor line is carried out preferably in that the latter is designed with double walls, and the gap between the two walls has an intake opening for vapor and/or liquid.
It has proven advantageous to provide a device for retaining the liquid on the end of the vapor line that is not connected to the housing. This retention device collects liquid, which is carried along with the vapor despite the dimensioning of the vapor line according to the invention. In this way, a mixing of heated liquid, which was unintentionally transported by the vapor line into the header of the storage container, with the remaining liquid is avoided.
In extensive tests, it has been shown that with use of the device according to the invention for pressure build-up in a storage container for liquid hydrogen, the minimum expansion of the vapor line in the cross-section should be more than 5 mm, preferably more than 10 mm, especially preferably more than 20 mm. These conditions ensure that basically only gaseous hydrogen is carried through the vapor line.
In addition to the device for pressure build-up in a storage container, the invention also relates to a storage container for cryogenic liquids. Inside, preferably near its bottom, the storage container has a housing, which has at least one intake opening for liquid and, inside, a heating device for evaporating the liquid. Between the housing and a point above the maximum filling height of the storage container, there runs a vapor line for draining off vapor bubbles that are produced during evaporation. According to the invention, the cross section of the vapor line is large, so that more vapor than liquid is drained off through the vapor line.
After the pressure is increased, however, the liquid and the gaseous hydrogen in the storage tank are in a state of thermal imbalance, since the liquid has a temperature of about 25 K, while in the gas atmosphere, a temperature of 31 K exists. The thermodynamic system that is present in the tank is therefore induced by return condensation of hydrogen gas to create a balancing of the thermal ratios of gas and liquid.
Such a storage container is therefore advantageously improved in that the storage container is divided into cells by a considerable number of partitions, which consist of a low heat-conducting material, and the cells′ maximum expansion in a plane that is perpendicular to the partitions is less than 50 mm, preferably less than 10 mm, especially preferably less than 5 mm. Based on the use of partitions in the storage container and the dimensioning according to the invention of the cells that result from this, it is therefore ensured that a liquid that is stored in the storage container is kept in the temperature range that is already present or specifically created.
The invention is of value in particular in the storage of liquid hydrogen in a motor vehicle.
REFERENCES:
patent: 5937655 (1999-08-01), Weiler et al.
patent: 4212636A1 (1993-10-01), None
Posselt Heinz
Rudiger Horst
Doerrler William
Drake Malik N.
Linde Aktiengesellschaft
Millen White Zelano & Branigan P.C.
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