Pipehead:
I’m glad you brought out the subject of possibly using CO2 as a refrigerant because it allows us all to analyze its merits with other refrigerants on a level playing field. I started to envision using it as a universal refrigerant when I started out in the Compressed Gas industry, right out of college in 1960. When I saw it being used as the refrigerant in producing liquefied CO2 product at 250 psig and -8.5 oF at the Harrison, NJ Liquid Carbonic CO2 plant, I thought I had the universal solution. When I was later was sent abroad to operate and later install other CO2 and dry ice facilities I tried to incorporate it in the Dry Ice production cycle. That’s when I was forced to get into the phase equilibria of CO2 and its thermophysical properties. To my dismay, I found out the bitter truth – there was a tradeoff in applying CO2. The Harrison plant was successful in its application because it seized on a convenient property of CO2: it was readily and cheaply available and it fitted in the type of compressors employed.
Harrison plant used the conventional, 3-stage reciprocating compressors employed at that time to produce what was then called “high pressure liquid CO2” and distributed in high pressure steel cylinders. However, the industry saw the multiple advantages of radically changing the type of product and switched to “low pressure CO2”, which was saturated at 250 psig. This allowed the used of standard, ASME section VII vessels (called “bullets” in the LPG business) fabricated out of ASTM A-212 steel. This allowed the vessels to be designed for -20 oF. To produce the new level of product (called "LiquiFlow"), Liquid Carbonic designed the Harrison plant with the conventional 3-stage compression cycle – but with a major process change: the second stage, which conventionally took the gas from 65 to 250 psig was used as the last stage of compression in the production line. The third stage was converted to serve as a closed cycle, refrigeration stage while using CO2 in a closed cycle. The third stage took the vaporized CO2 from the main CO2 condenser at approximately 200 psig and recompressed it up to the critical pressure of 1,100 psig so that it could be readily condensed at 85 oF with cooling water. The high pressure liquid CO2 was stored in a liquid reservoir and served as the refrigerant in the CO2 condenser, completing the cycle.
When I tried to employ a CO2 refrigeration cycle in the production of Dry Ice I ran smack dab into the Triple Point and I couldn’t go any further. That ended that effort - real fast.
The above shows that indeed CO2 can be used as a refrigerant. However, the caveat is that it is limited by the type of compressor used and the compression suction pressure it has to withstand. A suction pressure of 200 psig on a compressor is a special case – and consequently will be reflected in pricing. Also, as I recall, the Coefficient of Performance (COP) for CO2 is terrible as compared with the conventional refrigerants – especially Ammonia (which I consider the best, practical refrigerant).
Unclesyd:
Nice to hear from you again. Yes, the preservation and transport of seafood has been a traditional application of direct, liquid CO2 refrigeration by expanding the refrigerant directly onto the material to be cooled. My old company, Liquid Carbonic pioneered this type of application many years ago – in the 1930’s, I believe. The method is nothing more than the same effect of using a CO2 fire extinguisher. The free, adiabatic expansion of liquid CO2 produces a 2-phase mixture of dry ice snow and cold CO2 vapor – both at approximately -109 oF. This effect was tried on both interstate trucking applications and rail cars. It was quickly found that direct application of dry ice on the consumable products produced a “burning” of the product and ruined it. The method worked, but not by direct contact; rather, the cold resulting vapors from the expansion and from sublimation were circulated about the consumable product and used to do the resultant cooling through a difficult gaseous film coefficient. The refrigeration and preservation of Ice Cream, however, was a different and more effective application. It is still the principal alternative method to preserving Ice Cream other than mechanical refrigeration.
I’m not familiar with any liquid CO2 or Dry Ice applications on fish or shrimp here in the Texas Gulf Coast. Although I was born, raised and educated in Galveston, Texas (the traditional home of Gulf of Mexico Shrimp and fishing industries) I never saw or heard of it being used there while I grew up in the 40’s and 50’s. However, when I was in Peru in 1963, I furnished a lot of Dry Ice and even some liquid CO2 for refrigerating fish in transport when the Peruvians had the largest fish industry in the world, based on their catches of anchoveta. Again, the system worked there, but it wasn’t cost competitive in most cases because of the cost of the CO2. Dry Ice is used today in such applications as those of hunters and fishermen who have to transport relatively small quantities of preserved meat product. I seriously doubt if one can allow the solid dry ice formed to come in contact with the fish or shrimp in order to cool it. The result, in my experience, would be “cold burns” which would make the product un-salable. I believe if it were to be still used today (which is likely in some situations) the liquid CO2 would have to be expanded in such a way that the solid dry ice were kept away from the product shrimp or fish. The cold, -109 oF vapors would be still used as the effective cooling medium alone. However, I could be wrong since I’ve been away from the CO2 industry for some time.
