Fluent material handling – with receiver or receiver coacting mea – Processes – Filling dispensers
Reexamination Certificate
2001-07-18
2003-07-15
Maust, Timothy L. (Department: 3751)
Fluent material handling, with receiver or receiver coacting mea
Processes
Filling dispensers
C141S009000, C141S082000, C141S104000, C141S105000, C137S005000, C048S189100
Reexamination Certificate
active
06591872
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a plant for dynamically manufacturing and for packaging medical gas mixtures, particularly N
2
O/O
2
gas mixtures that can be used in the medical field, especially in analgesia, preferably N
2
O/O
2
mixtures containing approximately 50 vol % nitrous oxide (N
2
O) and 50 vol % oxygen.
BACKGROUND OF THE INVENTION
From the industrial standpoint, there are at the present time various methods and processes for manufacturing and packaging gas mixtures.
However, in these the amounts of gas to be mixed are fed into a gas mixer and are monitored by measuring the pressure and the temperature of the gases. The metering is therefore based on two measuring instruments which add their measurement errors, thus possibly leading to quite random results.
Moreover, the choice of measurement points in the plant does not allow the desired physical quantities to be determined, or does so only incompletely, and therefore does not allow the mixture to be produced effectively or reliably.
Thus, the temperature is usually measured at the gas-filling injection rail by a temperature probe which does not reflect, or does so only very inaccurately, the effective gas temperature in the packaging containers.
Sometimes this measurement is performed directly on the surface of the container (bottle) by an infrared thermal probe; it will be understood that this measurement is not a precise reflection of that of the gas in the container.
Moreover, a pressure sensor is used to constantly measure the pressure in the filling injection rails, the gases while flow through the pipes.
Consequently, there is therefore an inevitable difference between the final pressure in the containers, at the end of filling, and the pressure measured during filling, which depends on the pressure drops, the flow rate and the temperature of the gases.
Not knowing the pressure-drop coefficients and the temperature of the gases precisely therefore requires the pressure to be checked in static mode, at the end of the injection cycle, that is to say a posteriori.
Thus, if the amounts of gas mixed are off-specification, it is then necessary to top up the amounts of gas mixed by adding the amount of gas lacking, something which is not practical or not always easily achievable.
However, conversely, any excess gas completely falsifies the precision of the desired gas mixture and either results in the gas mixture thus obtained being scrapped or requires a readjustment in order to try to re-establish the equilibrium. This is not always possible.
Furthermore, a process for packaging gas mixtures based on carbon dioxide (CO
2
) is also known, this process being called a dynamic packaging process, in which the CO
2
is packaged above a supercritical state at a pressure of 270 bar and at a temperature between about 70° C. and 120° C., both the pressure and the temperature of the gas being determined by the conditions under which the packaging process is carried out.
The pressures of the various gas sources must be balanced at 270 bar since the pressure is defined in such a way that containers can be filled at a pressure of 200 bar even in summer when they are hot, since they are usually stored outdoors. This means that the filling source downstream of the mixing chamber must therefore be able to reach 240 bar (for a container at a temperature ranging up to 60° C.).
In addition, the pressure drops across such a dynamic mixer often reaches 20 bar and consequently the pressures of the gas sources must reach a minimum of 260 bar.
Hitherto, CO
2
is the only liquefied gas that has already been packaged dynamically.
During a filling cycle, the pressures of the gas sources are reduced downstream of the mixing chamber down to the pressure of the containers and the pressure downstream of the chamber varies, during the cycle, from a few mbar to the final filling pressure of the gas mixture.
In the case of CO
2
at a pressure of 270 bar, the temperature is 70° C. and this is chosen so that the expansion is not accompanied by a change of state of the CO
2
, passing into the solid state (carbon dioxide snow) especially when the pressure is less than 5 bar.
This is because any formation of carbon dioxide snow runs the risk of obstructing the taps of the bottles, thus resulting in disparities in the contents of the gas mixtures produced in the bottles for the same filling injection rail.
Consequently, only gas mixtures containing a CO
2
content generally not exceeding 30% can be produced, since otherwise the temperature reached downstream of the expansion chamber would be below the demixing temperature.
Gas mixtures whose content of a given component is greater than 30% (by volume) are usually produced by more conventional manufacturing methods, for example by gravimetry with a check of the masses injected into the bottles by weighing or by a temperature-corrected pressure measurement. However, these methods have the drawbacks of making it almost mandatory to roll the bottles after mixing in order to homogenize the contents thereof and to carry out an analytical check on the containers in order to ensure that they conform to the intended specifications. Such procedures are therefore not very practical and are expensive in terms of time and of productivity.
Furthermore, during dynamic mixing there is also the problem of demixing of the gas mixture downstream of the mixing chamber, that is to say inopportune demixing or separation of the various components of the mixture downstream of the site where the said mixing takes place.
Demixing of a gas mixture is characterized by the separation of the said mixture into two separate phases, namely a gas phase and a liquid phase.
Demixing occurs as soon as the temperature of the mixture drops below a temperature threshold. The higher the gas content of the mixture the higher the demixing temperature.
For a binary gas mixture formed from 50% O
2
and 50% N
2
O, this demixing threshold is about −5.5° C., as explained in the document “
Equilibria for mixtures of oxygen with nitrous oxide and carbon dioxide and their relevance to the storage of N
2
O/O
2
cylinders for use in analgesia
”, March 1970.
Now, gas packaging using a dynamic mixer is always accompanied by an expansion downstream of the mixing chamber and therefore in general a reduction in the temperature of the gases, even to below the demixing temperature in the case of an analgesic mixture.
The flow of the gases through the filling injection rails into the bottles is therefore a two-phase flow, the liquid phase and the gas phase moving at different flow velocities.
Consequently, the bottles are no longer filled homogeneously and relatively large differences are observed in the final contents of the mixtures produced in each of the bottles filled from the same injection rail during the same manufacturing cycle or process.
These disparities may be explained by preferential flows of certain constituents of the gas mixture with respect to others in the pipes of the injection rails for filling the containers, namely gravitational flows or flows in the form of droplets in the case of liquefied gases.
Thus, under high filling-rate conditions or in the case of type B5 small-volume (5 liter) containers, the resulting contents of mixtures in a few containers of the same manufacturing batch may be outside the production tolerances imposed by the Pharmacopoeia, namely a maximum deviation of 1% in the case of a 50 vol %/50 vol % O
2
/N
2
O mixture. Consequently, it is essential to carry out an analytical check on each container. This is tiresome and not very practical from the industrial standpoint.
SUMMARY OF THE INVENTION
Thus, it is an object of the present invention to be able to produce gas mixtures, in particular gas mixtures intended for the medical field, and then to package them rapidly, reliably and effectively, that is to say without encountering the problems that arise with the conventional packaging processes.
Put another way, the problem that arises is to be able to produce and package, dynamically, gases for
Air Liquide Sante ( International)
Maust Timothy L.
Young & Thompson
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