INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 19, No. 2
Solid Carbon Dioxide A New Commercial Refrigerant’ By D. H. Killeffer, Associate Editor
Advantage has been taken ofthe peculiarities of solid carbon dioxide by developing methods for its use as a commercial refrigerant in competition with water ice. OLID carbon dioxide, the laboratory curiosity shown to students of physics and chemistry for more than half a century, is difficult to consider new, but when this plaything of the laboratory and the lecture table goes so far as to threaten a revolution in methods of long-distance transportation of foodstuffs, even the most conservative must lend an ear. When the greatest single waste of industry finds a new use in the saving of food, the whole world must pay attention; for whether or not we believe with Malthus in the ultimate starvation of our race, the vital importance of economical production and handling of food touches us all. One is not inclined to attach any great significance to the blowing of liquid carbon dioxide into a canvas bag to form a very cold snow, with which liquid mercury may be frozen to a solid hammer. But when a similarly simple operation is applied economically in industry and tons of this snow are made and used every day, when by its use so perishable a commodity as ice cream can be economically transported from New York and Philadelphia to the warm climate of Cuba in such large quantities as to bring action from the Cuban Government in the form of an embargo on its import, and when carloads of frozen fish can be shipped by rail on a five days’ journey without re-icing and without thawing, the situation becomes essentially different. One must recognize that the elements of an industrial revolution are already present, but how extensive this revolution may be, what fields it may ultimately touch, and what economic structures it may seriously affect or overthrow, one can only guess after careful consideration of the facts upon which it is based. It is our purpose here to present a basis for such surmises by considering the remarkable properties of solid carbon dioxide, now being marketed as “Dry Ice,” as found in its practical use during the past two years.
S
Previous Efforts
Previous efforts to do what has now been done with solid carbon dioxide have failed, probably not for reasons inherent in the material itself, but rather from lack of appreciation of its peculiarities. I n India, in Germany, and in England attempts to produce carbon dioxide snow commercially were made with inefficient, unnecessarily complicated equipment, and, without efficient methods of application, a demand for the material as a refrigerant was so difficult to create that nothing of importance came of them. I n India a British army officer, interested in transporting this essential ingredient of his customary “whiskey and soda” from the seaboard to his station inland, hit upon converting it to a solid as a means of avoiding the impedimentum of the heavy steel cylinder required to retain the liquid. I n Germany and England the primary idea was refrigeration, but the conditions there encountered were not particularly favorable and the advantages of solid carbon dioxide were not great enough to warrant its additional cost as compared with other customary refrigerants. The methods of preparation employed here were almost too cumbersome to be practical] having been designed to save energy whose value was probably less than the cost of salvaging it, and in addition no new methods of 1
Received December 11, 1926.
use to take advantage of the valuable peculiarities of this material were developed, as has now been done. I n contrast to these failures the present situation in the United States approaches the ideal for the success of an efficient portable refrigerant and actual practice has already demonstrated the economies of solid carbon dioxide as compared with water ice. The growing necessity for transporting perishable foodstuffs over long distances requiring several days by rail has led to the development of extensive systems of refrigerator cars, based naturally upon water ice, and along with this development has come the further need for frequent re-icing stations for cars en route to assure the delivery of their cargoes in first-class condition. Two classes of perishable cargoes are recognized, one which must be kept frozen and the other which suffers damage if allowed to freeze. Both must be frequently sidetracked t o take on additional ice, a source of added cost largely obviated by the use of the new refrigerant. The fact that the latent heat of carbon dioxide from the solid to the gaseous state is nearly twice that of the water ice in melting gives the former a natural advantage by permitting a larger proportion of the cargo of a given car to be paying freight. The convenience of form, the slow rate of evaporation, and the release of a gas, which forms an insulating blanket around it, instead of a liquid, give carbon dioxide ice additional advantages over water ice which more than offset the difference in cost. Manufacturing Process
The steps in the manufacture of solid carbon dioxide in marketable form consist of (I) the preparation of pure gas, (2) its compression to a liquid, (3) the rapid evaporation of the liquid, taking advantage of the heat absorbed thereby, to convert part of it to a snow, and (4) the compression of the snow into more or less dense blocks for use. I n spite of many misconceptions and well-meant statements that carbon dioxide is prepared industrially from carbonates by treatment with acid, the commercial manufacture of this product is carried out almost entirely with coke as the raw material. The cheapness of many carbonates and the high price of coke would naturally lead one to suppose that the former would be the more economical raw material, but the fact that immense quantities of power are required to compress the gas to a liquid is overlooked in reaching such conclusions. It has been found that a properly designed plant utilizing all the heat produced by the burning of coke yields considerably less than enough power to separate and purify the carbon dioxide formed and compress it to a liquid. Hence it is only where the pure gas can be readily produced as a byproduct of some other operation that carbonates serve as a raw material. I n other words, the power required is too great to permit carbon dioxide to be easily transferred from the category of wastes to that of salable products. The utmost economy of energy must be practiced to make the process commercially successful. Liquid Carbon Dioxide
Commercially, carbon dioxide manufacture begins with the burning of coke under a steam boiler whose setting is designed
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February, 1927
INDUSTRIAL A N D ENGINEERING CHEMISTRY
to withstand high temperatures. No stack is used and the forced draft on the fire is so regulated that the flue gases contain a slight excess of oxygen in order to be sure that no carbon monoxide or hydrogen sulfide is present. The steam generated by this boiler operates the engines driving the compressors, blowers, circulating pumps, and other equipment required in the subsequent process. From the furnace the flue gases are conducted through an economizer to reduce their temperature and to recover as much of their heat content as possible. The exit gasesfrom theeconomiser atabout 150' C . cont.ain 17 to 18 per cent of carbon dioxide, and 1 or 2 per cent of oxygen, together with inert gases. The gas mixture at t.his point is scrubbed with cold water in two toners packed with trap rock to remove impurities and to cool the gas mixture further. The system up to this point is operated under a suacient. amount of suction to move the Rases at the reouisite speed
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which it is shipped, in addition to the actual manufacturing costs of the material. A steel cylinder weighing ahout 110 pounds contains 50 pounds of carbon dioxide, a tare of 220 per cent, and the freight chargeable to the material is consequently high in proportion. I n the form of pressed cakes of snow, this char@ for cylinders is completely obviated and t.he tare weight of the balsa wood boxes in which the solid is shipped is comparatively small. A balsa wood box holding 212 pounds of solid carbon dioxide weighs 60 pounds, a tare of a little more bhaii 28 per cent. 9 t ordinary outside temperatures such a container shows an exraporation loss of not more than 10 pounds per day for 5 days. Solidification o f Carbon Dioxide
The process of evaporating the liquid carbon dioxide and converting part of it to solid is quite simple but requires careSol provision aasinst absorption of external heat by evapora-
blower, Gliicli pumps the cooled, washed gases under pressure eylinder and canvas hag. The differences show up markedly i n the yields by the two methods. In the laboratory one gets into iiie absorbers which follow. The cool mixture OS carbon dioxide and iiitrogeii is pumped a conversion of 10 to 15 per cent of the liquid in the cylinder to absorbers consistiiig of two towers in series packed with into snow in the canvas bag, whereas in commercial practice coke, down diich a 10 per cent sodium carbonate solut,ion one-third or more of t,he liquid is obtained in the form of solid. trickles. The sodium carbonate in coiitact wibh the gases This result is attained by cooling the liquid to a temperature absorbs carbon dioxide, being thereby 70 per cent couverted well below its critical temperature and by increasing the to sodium bicarbonate. The nitrogen and residual oxygen pressure beyoiid i,he critical point. In the original installation the evaporators are sheet-steel are allowed to go to waste from the top of the second tower. The sodium bicarbonate solution is then pumped to the tanks 3 feet in diameter and 5 feet high, carefully Sagged on carbon dioxide generators, where it is heated by exhaust steam the outside with cork insulation. Within the evaporator froni the power equipment, supplerneiited by live steam, to is an inner shell ext,eiiding nearly to the top and providing an about 115' C . At this temperature carbon dioxide of a mnular space abuut an inch i idth between the two shells. sed by a filter cloth backed purity between 99.9 and 99.95 per cent is evolved and the The top of the inner shell is tom on one side is a manhole hot. regenerated sodium carbonate solution, still contttlining by a heavy screen. Piear the through both shells, and some 6carboiiate, is rethe nozzle for the admisturned to the absorbing sion of the liquid passes towers through a heat iut h r o u g h its center. In terchauger of the conceno p e r a t i o n , cooled liquid tric pipe type, where it carbon dioxide a t 1100 or gives up its heat t.o the more pounds pressure is cool sodium bicarbonate expanjed throiigh a 2-mm. solution on its way to t.he nozzle into the inner shell. generators. The rapid evaporation of The pure carbon dioxide, the liquid and the Jouleliberated at a pressure of Thompson effect of the a b o u t 1 0 p o u n d s per expanding gas cool it sudsquare inch, is cooled to d e n l y atid one-third or remove water and is then more of it is converted to conducted to the compresa snowlike solid. This is sors> where i t is liquefied filtered out by the filter a t a pressure of about 1100 cloth a t the top of the inner pounds per square inch. shell and the very cold gas The compressors used are released i n t h e process of the t h r e e - s t a g e t y p e passes down between the with suitable water cooltwo shells and out through ing between &ages. From Plant Manufneturing Solid Carbon Dioxide. Three Evaporaa pipe near tlie bottom, the compressors the liquid tor8 In the F'oregrround. with Molds and Finished Block t h u s p r o v i d i n g a cold ( A ) IIeilt ioterehanpern for cooling liquid COz carbon dioxide is run di(0) Hydraulic blacking press blanket of gas between the rectly ilito cylinders for actual evaporation and the storage or shipment, or o u t s i d e shell. The acproceeds in the process to the evaporators, where it is converted into snow. Up to cumulation of snow is controlled by weighings of the entire this point the process is practically automatic, involving hand evaporator. When 300 pounds of snow have accumulated, labor only in tending the fires and in handling the cylinders themanholeisopened and tlie chargejsacrii,pd outiuto suitable molds to be conipressed into convenient blocks. I n returning of finished product. The liquid carbon dioxide in cylinders has an average to the compressors t,he cold gas released froni the evaporators market value of 10 cent.6 per pound (as shown by Census at about 10 pounds pressure passes through a heat interchanger, Bureau surveys), hut this price is made up of a large item of where it cools the liquid about to he evaporated. This coolcost,, earriage, and depreciation on the heavy cylinders in ing of the liquid is essential to the success of the process. ~
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INDVSTRIB L AND ENGINEERING CHEMISTRY
Thus it is seen that the process employed in the plant difters only in detail, essential though that detail is, from the customary laboratory demonstration. The simple, but fnndamental, differences have been enough to make the difference between failure and commercial success. The steel molds containing proper quantities of snow are passed to hydraulic presses and blocks are formed under pressures between 500 and 800 pounds per square inch. These blocks have a specific gravity of approximately 1.1 as compared with a figure of 1.56 for the dense solid. Two forms of blocks are manufactured--cubes 10 inches on a side weighing 38 to 40 pounds each, and cylinders 3 inches in diameter and about 6 to 8 inches long. These standard blocks
may be cut on ordinary power wood saws into p renient size for use. In the preparation of the blocks from snow in the evaporators to finished sizes in storage, loss of solid by evaporation of less than 10 per cent Three such evaporators produce 7000 to 9000 p day and the blocking of the product has so far been done largely by hand. At so low a prodrr is not uneconomical, but details of carry out the operations involved demand forces production up, improvements both in the evaporators and in the blocking process will no doubt lead to decided economies. Even a t 3 production rate of 3.5 to 5 tons per day using hand labor, the product can be sold profitably a t one-half the price of liquid carbon dioxide nnd no doubt improved methods will make it possible to reduce this figure. At present the price of solid carbon dioxide is approximately ten times that of water ice, biit even a t that figure i t is able to compete successfully iii the market. Advantages in Use
The value of solid carbon dioxide as a refrigerant is based principally 011 two facts which make it commercially competitive wiihwater ice at 0.5 cent 3 pound. Probably its greatest advantage is in the fact that a dry gas is produced direct and
Vol. 19. No. 2
there is no liquid phase, carrying with it potential cooling capacity, to be drained away. The second great advantage is in the high latent heat of carbon dioxide in passing from the solid to the gaseous state-carbon dioxide absorbs a p proximately 152 calories per gram in passing from the pressed c&e to gas at 0" C., in contrast to water ice which absorbs only 80 calories per gram in melting a t 0' C . The evaporation of the solid carbon dioxide directly to a gas has a peculiar value, of which advantage is taken in refrigeration practice. The atmosphere of the refrigerated space is constantly replaced by fresh, pure, cold, dry carbon dioxide gas, which is quite harmless t,o products stored in it and is indeed considered to be an actual preservative of many foodstuffs. Each pound of snow evaporated yields about 8 cubic feet of gas, which is alioved to fill the refrigerated space and to overflow from vents situated as near its top as practicable. In this way any accumulation of odors in the stale air which nrould be confined within a11 ordinary refrigerator is avoided, and in addition the heat leaking through the walls is absorbed and vented along with the gas overflow. By venting the heat leakage in this manner, it is unnecessary for the refrigerant to absorb i t and thus the amount of evaporation needed to keep a cold space cold is only that required to keep up a flow of gas through the vents large enough to carry tbe heat leakage with it, the gas passing out a t the maximum temperature of the space. In other words, the incoming heat is carried out a t the highest possible temperature, which may be decidedly higher (80" C. or more) thaii the temperature of the evaporating solid itself. The case of water ice is quite different, for the liquid water leaving the refrigerator is a t or near the temperature of the melting ice itself and hence must remove heat a t the lowest feasible temperature differential, and in addition there is always the accumulation of undesirable odors in the necessarily confined air. In spite of its very low temperature-the internal temperature of a block of snow in an atmosphere of carbon dioxide is about -80' C.--the product is remarkably long-lived under most practicable conditions. The explanation of this phenomenon of slow evaporatioii appears to be the preFence within the cake itself of gas-filled voids making up about onefourth of its volume and the formation all around it of an insulating layer of cold gas. The beat conductivity oi the somewhat porous block is very low as compared with a solid block such as one of water ice, aid hence it is difficult for beat to penetrate it. The blanket of cold gas constantly snrrounding thc evaporating block prevents its contact with warm air, and possibly even it.s direct contact with a heatconducting solid upon which it may rest, a situation quite different from that of water ice, which on account of convection currents is in constant coritact with air much warmer than itself. The very high heat absorptiw capacity of the evaporation of solid carbon dioxide to gas makes it especially fortunate that these natural forces can be utilized to make the process a slow one. While the temperature of evaporating solid carbon dioxide in an atmosphere of its own gas is -80' C., if the block is placed in air its temperature can be reduced still further to -95" C. This effect of reducing the vapor pressure of the carbon dioxide in the atmosphere surrounding the solid can be used t.o advantage where rapid cooling to rery low temperatures is desired. The low temperature of evaporation of the carbon dioxide snow provides 3 valuable factor of safety as compared with water ice, where low temperatures must be maiiitained as in the transportation of frozen commodities. The necessity for using salt along with water ice to produce temperatures below the customary one of 10" C. (50" F.) not only involves additional labor but also reduces the over-all efficiency of the refrigerant. With carbon dioxide temperatures as low
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February, 1927
as -40" or -50" C. may be att'ained with reasonable efficiency and a t much higher efficiencies than freezing temperatures with ice and salt mixtures. Indeed, probably the most important field for carbon dioxide ice is in those temperatures below the melting point of ice so often required in commerce, as in the transportation of frozen meats, fish, ice cream, etc., for its efficiency is much higher than that of water ice below 0" C. I n all cases where freezing might be deleterious to the material cooled, it is essential that the carbon dioxide ice be carefully insulated so that only the gas evolved (having a specific heat of approximately 0.2) comes into contact with it and t'hat even then it be diluted considerably with the warmer gas in the upper parts of the refrigerated space. Refrigeration of Small-Lot S h i p m e n t s
The most spectacular of the uses to which solid carbon dioxide has been put, and one in which it is not at all competitive with any other refrigerant, is in the cooling of small parcels of ice cream in paper containers to be sent by express or mail or for considerable distances. The weight of refrigerant is small, its rate of evaporation slow, the product of its cooling action is a harmless gas and not a more or less disagreeable liquid, the weight of c.ontainer and necessary insulation is small, and it is capable of keeping tht: contents of the package cold for as much as 36 to 40 hours without difficulty. Its application to less than carload shipments of many commodities, obviating as it does the return of empty containers and at the same time the necessity for reicing en route, hasgroved very economical. Concrete Data
I n the transportation of ice cream between Xew York and Philadelphia, experience has shown that 200 pounds of solid carbon dioxide replaces 3000 pounds of water ice and 600 pounds of salt, an efficiency ratio of 15 to 1. I n the shipment of frozen fish from Xew York to Detroit, 17,000 pounds of water ice and 10 per cent of that weight of salt are ordinarily consumed per car, whereas the same quantity of fish may be shipped over the same route in a frozen condition with the use of only 1200 pounds of solid carbon dioxide, and furthermore, the carbon dioxide is all loaded a t the beginning of the trip and does not require frequent replenishment. The saving effected by the use of so convenient a refrigerant is evident from the fact that single charges of carbon dioxide have maintained perfect refrigeration in a car over a journey ordinarily requiring many re-icings. The standard practice in caring for refrigerator cars en route is represented by the following tabulation, the figures for water ice having been taken from the Car Builders' Cyclopedia: WATERICE Ice Original charge First day Second day Third day Fourth day
Pounds 12,000 1,995 1,950 1,750 __.
TOTAL
17,695
Salt
Unloaded
CARBONDIOXIDB ICE
Pounds 1200 200 195 175
Pounds
-
Unloaded
1770
1200
1200 ~
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Not only is there a saving in actual refrigerant used but ordinary practice with water ice requires that the car be taken off service approximately 20 per cent of the time of a journey for re-icing, and this is avoided by carbon dioxide icing with a corresponding saving in investment charges. The large amount of water ice required by a car adds to the weight of nonpaying freight and reduces the space available for pay freight. The very low temperature of solid carbon
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dioxide and the factor of safety which this introduces into refrigeration make it possible for a car to carry as much as 50 per cent more perishable freight than its present accustomed load. The evaporation of carbon dioxide t o a dry gas not only makes feasible its use in paper or cardboard containers for small-lot shipments without the necessity of the return of empty tubs or barrels, but for carload freight it also reduces the maintenance cost on refrigerator car equipment. The depreciation of refrigerator cars in service is very rapid, largely because of the moist cold supplied by water ice and the brine drippings from ice bunkers which must be cared for. There is also the possibility that drip pipes may become clogged and water collect around the cargo resulting in serious damage. There may, too, be some advantage in avoiding the drip of brine and ice water onto rails and ties, thus reducing corrosion and the cost of maintenance of way. Summary
Solid-carbon dioxide as a refrigerant possesses the following advantages to offset its higher cost as compared with water ice: (1) Insulating effect of gas as evaporated, permitting slower rate of heat absorption. ( 2 ) High heat-absorptive value per unit 01 weight. (3) Lower temperature as a factor of safety in shipping frozen cargoes. (4), Evaporation to a dry gas which reduces maintenance of cars and assists in venting heat leakage ( 5 ) Carbon dioxide itself has a slight preservative effect on foodstuffs. (6) On account of its dryness, refrigerated products may be safely kept in much lighter containers than are required with water ice.
The uses suggested for solid carbon dioxide to which ordinary portable refrigerants are inapplicable are : (1) Light-weight packages of butter, cheese, eggs, ice cream, confections, etc., tor shipment by mail or express. (2) Ship refrigeration to avoid the high cost of installation and the inefficient operation of refrigeration plants using the warm sea water in the region of the equator. (3) Shipment or storage of frozen products and others requiring extremely low temperatures. (4) Freezing quicksand in excavation work to avoid the cost of installing ice machines and brine coils now used. ( 5 ) Local anesthetic for surgery and dentistry. (ti) Miscellaneous low temperature work in various process industries, such as (a) separation of mixtures having low crystallizing points; ( b ) condensation of distillates having low boiling points; (c) low-temperature tests on electric cables; ( d ) routine tests on varnish.
Acknowledgment
The processes of manufacture and use of solid carbon dioxide described in this article are subjects of patents and applications controlled by the Dry Ice Corporation. To this corporation, and more particularly to J. W. Martin, Jr., and W. L. Hood of its staff, the author's thanks are extended for cooperation in preparing it. Our Felicitations-The Chemtker-Zeitung is well known among all circles of chemists as the principal periodical of the German chemical industry. With the close of 1926 it finished its fiftieth annual volume. We congratulate this publication upon rounding out a half-century of useful work. In commemoration of the event, Chemiker-Zeitung has issued a special December number in which the most famous chemists of Germany write of the past, present, and future of the science. Chemiker-Zeitung was founded in the closing days of 1576 by Prof. Dr. G. Krause, who still lives a t Cothen, having retired from active service. Beginning with 1906 Otto von Halem, the present publisher of the paper, assumed full responsibility and under his management it has been developed into a leading newspaper of chemical practice.
