Refrigeration – Storage of solidified or liquified gas – Cryogen stored in both phases
Reexamination Certificate
1999-11-02
2001-07-17
McDermott, Corrine (Department: 3744)
Refrigeration
Storage of solidified or liquified gas
Cryogen stored in both phases
Reexamination Certificate
active
06260361
ABSTRACT:
STATEMENT OF FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
This invention relates to the apparatus and methods suitable for liquid carbon dioxide storage and process systems typically located at customer or user sites which supply very cold liquid or liquid and solid (slush) carbon dioxide to devices which then form dry ice snow (a form of solid carbon dioxide) and useful when creating refrigeration effects. Such systems, while they may have other beneficial uses, are especially useful as ground support/filling apparatus for trucks or rail cars utilizing carbon dioxide as an expendable refrigerant for cooling.
BACKGROUND-DESCRIPTION OF PRIOR ART
Solid carbon dioxide (dry ice) has long been used as an expendable refrigerant for many cooling applications because of its ease of application, its non-toxic nature, its very large refrigeration effect when subliming, its direct change to the gas phase and its desirable low range of refrigeration temperatures. Dry ice, at atmospheric pressure, sublimes at −110° F. and has a heat of sublimation of 244 btu/lb. Dry ice typically has been made at central points in the form of blocks and then transported to the customer or using sites, stored, then placed or mixed when and where cooling was desired. In some cases, the user was sufficiently large to have an on-site dry ice machine, usually making small extrusions, called nuggets, served by an on-site supply of liquid carbon dioxide.
One early use of carbon dioxide to cool rail cars or other transport was to place already formed dry ice blocks inside an insulated portion of the car or transport container, and optionally have a thermostat controlled fan to enhance circulation and control refrigeration provision. This practice has continued today, but more directed at smaller volume units, without fans.
Today liquid carbon dioxide is typically received and stored at customer sites in insulated storage vessels under about 300 psig pressure and at a temperature of about 0° F.; and then converted, when needed, to dry ice by the customer in a variety of machines, generally characterized within the carbon dioxide industry as “dispensing devices” or “dispensing equipment”. In many cooling applications, such as filling the dry ice bunker of a rail car or such, as shown in U.S. Pat. Nos. 4,704,876 (1987) to Hill, in 5,168,717 (1992) to Mowatt-Larssen and in 5,660,057 (1997) to the present inventor, the liquid carbon dioxide is piped to the rail car, then expanded inside the bunker to atmospheric pressure, where it partly turns to a solid, termed snow (a loose, non-compressed form of particulate dry ice), but with a substantial part of the liquid carbon dioxide flashing to vapor as it expands. This flash gas or vapor, at −110° F. can be used to cool the inside walls and the floor of the car as it exits the car, but its refrigeration is largely wasted. The amount of solid carbon dioxide (dry ice snow) needed to be provided in the car is determined by analysis of the intended trip, considering both en route time and ambient temperature anticipated; and the proper amount dry ice snow placed in the bunker, determined by measurement of liquid used (either by use of volumetric flow meters or timed injection into the bunker through orifices of known flow characteristics with liquid carbon dioxide), and with conversion of liquid carbon dioxide to snow calculations based upon the temperature of the liquid carbon dioxide. The dry ice snow deposited in the bunker provides the subsequent cooling needs of the rail car, subliming in the process. The use of liquid carbon dioxide at a temperature below the normal storage temperature of 0° F. is desirable in such applications because the use of such colder liquid carbon dioxide during the expansion process produces a larger percentage of solid carbon dioxide and a smaller percentage of vapor carbon dioxide, which is largely wasted; all resulting in reduced liquid carbon dioxide use and lower costs to the users. U.S. Pat. No. 3,660,985 (1972) to the present inventor represents an early method to achieve the convenience of liquid conveyance to the actual using device, but also provided improved dry ice conversion efficiency by reducing the temperature of the liquid carbon dioxide. U.S. Pat. No. 4,888,955 (1989) to the present inventor, et al, shows a different method of reducing the temperature of the liquid carbon dioxide before use. Reductions in carbon dioxide usage of up to about 20% are made possible by the use of very cold liquid carbon dioxide. In two early U.S. Pat., Nos. 3,810,365 (1974) to Hampton et al and 3,933,001 (1976) to Muska, a carbon dioxide slush (also termed a slurry or a multi-phase mixture) was created and then transported to a customer location for use. In U. S. Pat. No. 3,817,045 (1974) to Muska, a method of using slush of up to 85% solid is revealed in the manufacture of dry ice pellets (nuggets). In another early U.S. Pat., No. 3,984,993 (1976) also to Muska, a method a method of making high solid concentration carbon dioxide slush is revealed. However, the inherent problems of moving the slush mixture to many actual using devices (where the slush expands to atmospheric pressure) and the slush's use within the using device itself, were so severe and unsolved that these patents found no use. For some applications, such as shown in U.S. Pat. No. 4,695,302 (1987) to the present inventor-liquid carbon dioxide is converted to a triple point mixture and with the liquid and solid phase mixing so as to form a slush. This slush is then used to cool the liquid carbon dioxide used for snow making/bunker filling and for filling each car's individual small tank with liquid carbon dioxide. This results in the near 20% reduction stated above, with the reduction being in the amount of vapor formed. However, slush was not used in the '302 U.S. patent identified above to expand to snow in the bunker, only aiding in the production of cold liquid carbon dioxide.
While cooling carbon dioxide to low temperatures, or to the stage where slush is created may seem to be straightforward mechanical refrigeration problems and then moving the slush to a use point similar in nature to moving a water slush mixture; the highly unusual nature of carbon dioxide, and especially the problems in moving a slush mixture that instantly becomes a solid if allowed to depressurize even slightly below the triple point pressure, were such that no satisfactory solution was found. Some of the contributing problems unique to carbon dioxide usage include: 1) the fact that liquid carbon dioxide when depressurized to 75 psia (the triple point), it initially becomes a mixture of liquid and vapor; 2) as additional vapor is removed and the pressure drops, a layer of particulate solid carbon dioxide is created on the upper surface of the liquid; 3) the particulate solid carbon dioxide is heavier than the liquid, thus tends to sink to the bottom of the liquid; 4) the fact that slush carbon dioxide, when being moved, easily clogs lines at piping anomalies and at valves, etc.; and 5) subsequent pressure reduction to below the triple point due to flow induced pressure drop can cause carbon dioxide slush to turn entirely solid and block the conduit. Accordingly, most prior art inventions did not move the slush to a use point and then expand it directly to solid. Much the same type problems arose if an attempt was made to intermittently move or use liquid carbon dioxide whose condition was near the triple point.
A related problem is due to the nature of use of most expendable refrigerants, of which carbon dioxide is a member, whether used in liquid form or in solid form (dry ice). This problem is that expendable refrigerants are used precisely when the cooling is desired (or the need commences), thus the use rate can vary greatly. Low use rates can be followed by high use rates, varying quickly from no use to high use. U.S. Pat. Nos. 4,888,955 (1989) and 5,934,095 (1999) to the present inventor, et al, were directed at solving this prob
Drake Malik N.
McDermott Corrine
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