Refrigeration – Cryogenic treatment of gas or gas mixture – Solidification
Reexamination Certificate
2000-10-20
2002-04-23
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Solidification
Reexamination Certificate
active
06374633
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the field of dry ice manufacturing, and more particularly to a method and apparatus for producing pellets of dry ice.
2. Description of Related Art
Dry ice is the solid state of carbon dioxide (CO
2
). There are a vast array of applications for dry ice, including the processing and preservation of meats and other foods. Dry ice is the preferred means of cooling in such applications, since it imparts no color, odor, or taste, and has no lingering deleterious effect on the food. Dry ice also is desirable for the processing of food because its sublimes directly from the solid state to the gaseous phase, leaving no residue behind after yielding its cooling effect; therefore, no clean-up or removal of residual liquid is required. Furthermore, CO
2
is neither toxic, poisonous, reactive with other chemicals, nor flammable.
In its solid state, at standard temperature and pressure, carbon dioxide has a constant and stable temperature of −109.33° F. Carbon dioxide is normally transported in its liquid state, and stored in refrigerated vessels at a pressure of about 300 psia, and a corresponding temperature of about 0° F.
Once the liquid CO
2
reaches the manufacturing facility, dry ice is generally formed into one of the two final forms, blocks of dry ice or smaller pellets. Large blocks of dry ice typically are shipped long distances or stored for extended periods, as pellet size pieces sublimate faster.
The basic process for making dry ice blocks from liquid carbon dioxide has long been known. Sometimes, these blocks of dry ice from a block press are reduced to a smaller size that can more easily be handled and used in many types of applications. Other machines, for example the dry ice pelletizer, produces dry ice pellets. Dry ice pellets are easily packaged by the manufacturer and subdivided by the consumer into convenient portions for use. These dry ice pellets find a vast array of applications, including applications in the processing and preservation of meats and other foods because of the thermal, physical, and chemical properties of dry ice. In certain applications, the dry ice pellets come in intimate contact with the food being processed, such as in a meat packing house and in certain seafood processing plants. The dry ice pellets in these applications are delivered directly onto the food being processed to rapidly cool the food and to keep the food below a specified maximum temperature to prevent spoilage while processing and prior to refrigerated storage. Also, dry ice has long been the favored refrigerant for ice cream vendors and distributors.
Conventional dry ice pellet manufacturing processes incorporate several disadvantages and limitations. Prior art arrangements of the injection system and chamber typically dictate the use of only low CO
2
flow rates; thus, limiting pellet production. It would be beneficial to provide a pelletizing system that can handle increased flow rates in order to maximize pellet production.
One limitation of known pelletizers, for example, is the angle at which the liquid CO
2
is injected into the extrusion chamber. Conventional injection is generally perpendicular to the length (radial centerline) of the extrusion chamber. Such generally perpendicular injection is representatively shown in
FIG. 1
of U.S. Pat. No. 5,528,907. Alternatively, an injection path that is generally parallel to the length (radial centerline) of the extrusion chamber has been used. Such generally parallel injection is representatively shown in
FIG. 5
of U.S. Pat. No. 5,548,960. Both perpendicular and parallel injection suffer from limited flow stream interaction with the inner wall(s) of the chamber, improper snow piling and clogging problems.
For example, under the generally perpendicular injection conditions, approximately equal amounts (being one-half the total amount) of the injected CO
2
flow toward each end of a the chamber after the flow strikes the inside of the chamber. The CO
2
enters through the injection port, travels through the core of the chamber and collides into the inner wall of the other side of the chamber approximately normal to the inner wall. The flow then splits into two, opposite directional streams, each flowing toward an end of the chamber. It is problematic that CO
2
snow begins to pile up at the collision site, and the pile then grows in length toward either end of the chamber. As snow begins to pile up between the collision site and the vent port, any escaping gaseous CO
2
must first travel through this snow pile before it can be released from within the chamber through the vent port. This injection arrangement impedes maximum snow production because pressure builds up in the chamber prematurely as the volume of the chamber ever shrinks from both sides of the injection point due to piled snow, and because pressure does not have an unencumbered path to exit the chamber, but must pass through forming snow. This type of injection also can prematurely clog the exhaust vent(s) of the extrusion chamber with solid CO
2
, which clogging limits production. This orientation of injection also inefficiently cools the chamber at start-up, delaying the formation of the ice plug, as the injected CO
2
cools the chamber from the point of collision out toward the ends. Therefore, the die end of the chamber, the point at which the plug will form, is cooled last.
Another limitation of known pelletizers is the use of only a single injection port that also hampers attempts at increasing injection flow rates into the chamber. Additionally, the geometry of standard injection nozzles is inefficient. The current use of straight, or nontapered, pipe designs of nozzles frequently leads to blockages of the nozzle, completely stopping production. Not only can the non-tapered design clog, but another adverse effect of such a non-tapered pipe is the resultant random pressure variations inside the extrusion chamber. These variations can lead to frequent operator (manual) adjustment of the metering valve.
Further, there is a lack of automation with present pelletizers. An improvement over the conventional injection system and extrusion chamber would be the provision of automated control over the injection of liquid CO
2
into the chamber. Current designs have a manually adjustable metering valve that constantly must be adjusted to compensate for numerous operational variables including clogging of the injection port and changes in liquid pressure. Certain high volume dry ice production facilities have many machines producing tons of ice per day. Each one of these machines has at least one of these metering valves and each one of these valves must be adjusted several times per day. Labor cost to monitor and adjust these metering valves is very high. Replacing the manually operated metering valves with automated control process valves would significantly reduce the labor necessary to operate a pelletizer.
Other disadvantages of the conventional dry ice pellet manufacturing processes lie outside the injection system and extrusion chamber of pelletizers. For example, current pelletizing machines do not incorporate an automated start-up procedure. Yet, if injection is orientated for increased production (as the present invention provides), the production of a dry ice plug without manual, time consuming intervention becomes impractical. On machines with six inch bores and larger, the machine on its own may never build a plug. If fact, starting a machine in this manner is very wasteful and dangerous. An automated start-up system would allow the operator to begin the pelletizer run, and not intervene again.
The filter area of present pelletizers is yet another feature in the production of dry ice upon which improvements can be made. Conventional pelletizers have a ratio of filter screen area to chamber bore area that defeats efficient pellet production. As this ratio drops, so too does the production of dry ice. It would be beneficial to provide a pelletizer having a higher
Greer Brian
Tharp, Jr. James L.
Deveau Todd
Doerrler William
Drake Malik N.
Schneider Ryan A.
Tomco2 Equipment Co.
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