SYMPOSIUM ON DRYING Drying Air and Commercial G a s e s
with Activated Alumina R. B. DERR Aluminum Research Laboratories, Aluminum Company of America, New Kensington, Pa.
The development of activated alumina and of improved equipment suited for its commercial use has resulted in practical gas drying and dehumidifying processes which are broadening the applications where removal of moisture can be accomplished economically. It is shown that the temperatures developed in the adsorbent by the heat of adsorption may exceed the boiling point of water, and that the elimination of this heat increases the capacity of the adsorbent. Also the relative quantity of gas dried under several conditions of flow rate and humidity are shown graphically. Complete drying and partial dehumidification are discussed and illustrated.
*T
HE relatively recent commercial production of solid
granular adsorbents has played an important part in the increased use of moisture-free air and industrial gases. Coincident with this production of adsorbents, decided progress has been made in the equipment necessary for successful moisture elimination. Thus the combination of adsorbents and equipment has resulted in improved drying processes which have superseded methods previously employed where some moisture elimination had been required. Moreover, these processes are making progress in applications where heretofore it has been possible to "get by" without removing moisture, and also in applications where the objectives are improved yields and quality of products, rather than the elimination of operating difficulties. Activated alumina is an important new adsorbent which is being employed successfully in these new drying processes. It is a granular, inert, porous solid capable of removing substantially 100 per cent of the moisture from air or gases passed through it, until it has taken up 12 to 14 per cent of its weight of water. This is a standard characteristic of the commercial product, and it is measured by passing air, a t the rate of 10 cubic feet per hour per pound, through a column of the adsorbent maintained a t 30" C. (86" F.) and then through phosphorus pentoxide to detect the first traces of moisture. After being used, the adsorbed water is removed by passing heated air through the column and, after being cooled, it is ready for repeated use. It is unnecessary to comment on other properties of activated alumina because they are generally well known or are readily available. I n discussing the drying of commercial gases, a distinction is made between the applications requiring substantially complete removal of moisture and those requiring only partial removal. Obviously, on the basis of equivalent pounds of gas dried, complete removal is the more difficult; as a result, the possible methods of accomplishment are greatly reduced. In this field a demand has existed for improved processes; therefore, it is natural that activated alumina should find initial success where extreme dryness is a requisite.
will be agreed upon by a number of investigators (1-5) is that a gas may be dried to contain not more than 0.002 to 0.005 mg. of moisture per liter. This is the moisture content of air in equilibrium with ice a t -70" and -64" C. (-94" and -83.2" F.), respectively. By carefully cooling a gas passed through activated alumina to the temperature of a solid carbon dioxide-ether mixture (at least -76" C., or - 104.8" F.), it was not possible to detect any condensation. This is believed to be a more reliable indication that the moisture content is less than 0.0008mg. per liter, than is shown by the methods employed in determining the above figures. It is unnecessary to debate here the final degree of dryness obtainable under optimum conditions. It is sufficient to state that it is not a difficult matter to design commercial equipment which will produce this degree of dryness and which will operate for practical periods before the partial saturation of the alumina permits higher concentrations of moisture to pass.
Complete Removal of Moisture To obtain dryness of a high order, it is necessary to provide a depth of adsorbent which will largely eliminate the effects of both the side wall and of channeling. For small volumes of adsorbent a height equal to three to four times the diameter is best suited for the complete removal of moisture, but where large volumes are required, beds four to ten feet deep are employed, and the height is selected on the basis of allow-
Degree of Dryness A question frequently asked is: how dry is gas which has been passed through activated alumina? An answer which 384
AND AIR CONDITIONING Fourth Chemical Engineering Symposium held under the auspices of the Division of Industrial and Engineering Chemistry of the American Chemical Society at the University of Pennsylvania, Philadelphia, Pa., December 27 and 28, 1937. Pages 384-397
plete drying. In this range a continuous supply of dry gas can readily be obtained with a unit consisting of two adsorbers, one of which is being reactivated while the other is operating on adsorption. I n Figure 2 the practical performance of a unit is shown when drying air containing about 5 grains of moisture per cubic foot and a t rates of about 15 and 20 cubic feet per hour per pound. This figure also compares the adsorption period and capacity when air of low moisture content is being dried. On the basis of air with equivalent moisture contents, the adsorption period is substantially proportional to the rate of flow, and only a slightly greater quantity of moisture is adsorbed a t 15 than a t 20 cubic feet per hour per pound. Also, a t equivalent flow rates only a slightly greater quantity of moisture is adsorbed a t high efficiency from the air containing 5.59 grains of moisture per cubic foot than from air containing 1.68 grains per cubic foot. Complementary of this, when the moisture concentration was reduced to about 33 per cent of its former value, about 2.8 times the quantity of dry air was produced. The temperatures existing in an activated alumina bed during reactivation are also of interest. I n Figure 3 temperatures are plotted against time when reactivating a bed of the same dimensions used to illustrate adsorption in Figure 1. The activated alumina had taken up moisture equal to 10.1 per cent of its dry weight. Air heated to between 450" and 475" F. was passed through the bed in the reverse direction to that employed during adsorption. The temperatures increase rapidly a t the first two levels (30 and 21 inches). The reason for this is that the moisture adsorbed in t h e dry air exit end of the bed is considerably less than the average. Where the alumina contains considerable moisture, rapid evaporation takes place and the exit air is thoroughly saturated. Thus the temperature does not rise above about 125' F. until the major portion of the bed is completely reactivated and substantially all of the moisture is removed. For all practical purposes, when the temperature of the exit air increases rapidly, the reactivation is complete.
able resistance to the flow of gas or ease of construction. Another important detail is that of reactivating with heated gas flowing in the reverse direction to that of the gas to be dried. This eliminates any possibility of incomplete removal of adsorbed moisture from the section of the bed nearest the exit of dry gas. The heat developed by adsorption has a marked influence on the capacity of an adsorbent unless means of dissipation are provided. I n the adsorption of 1 pound of moisture from any gas, heat approximately equivalent to the condensation of 1 pound of steam is liberated. If air containing 7 grains of moisture per cubic foot is being dried, then about 1000 B. t. u. will be converted from latent to sensible heat for every 1000 cubic feet of gas dried. This is sufficient to raise the temperature of air, at normal pressure, about 30" C. (54' F.). One might expect that the temperature of the dry exit gas would immediately show a corresponding rise, but this is not the case. The alumina stores the heat in the zone where adsorption a t high efficiency is taking place, and the temperature of the exit gas remains below the calculated figure throughout more than half the adsorption period. The temperature of the exit gas then rises above the mean temperature, but adsorption a t high efficiency continues until the exit gas approaches its maximum temperature. These relations are illustrated in Figure 1 where air a t 75" F. (dry bulb), containing 9 grains of moisture per cubic foot, was dried a t the rate of 5.2 cubic feet per hour per pound in an uncooled bed of activated alumina 32 inches deep. The maximum temperatures in the bed were above the boiling point of water, and a t the same time dry air was being delivered from the bed. Also, one hour after a trace of moisture was detectable in the exit, the moisture content was being reduced from 9 to 0.34 grain per cubic foot; i. e., the efficiency of removal was 96 per cent. Failure t o remove the heat of adsorption, or the drying of hot gases, reduces the capacity of activated alumina to adsorb moisture a t 100 per cent efficiency. The best results are obtained when the bed temperature is maintained between 32" and 100" F. One of the greatest improvements in the commercial development of adsorbent drying was the introduction of means of eliminating the heat of condensation. This is now being accomplished satisfactorily by cooling devices, which are used during adsorption and also to remove heat after reactivation. The duration of contact between the gas and activated alumina also has a bearing upon the efficiency of adsorption. Adsorption is a surfacc phenomena which takes place rapidly, but time is required to penetrate the inner pores. Thus higher flow rates tend to decrease the capacity a t high adsorptive efficiency. Flow rates between 10 and 20 cubic feet per hour per pound of activated alumina are suitable for com-
Applications An important application which requires dry gas is in the bright annealing of steels, particularly special carbon, silicon, and the various stainless steels. The blue discoloration which developed during annealing had been a constant cause of the rejection of large quantities of fabricated metal. Similar difficulties are encountered with some nonferrous metals. Calcium chloride and other desiccants had been used to remove moisture, but more positive means for maintaining uniformly low moisture concentrations were necessary before this trouble was eliminated. Activated alumina is now being used extensively for this purpose and performs equally well whether the annealing atmosphere is hydrogen, cracked ammonia, or natural gas. For the hydrogenation of vegetable oils and in certain reactions where gases such as ethylene, carbon monoxide, etc., are required, increased yields and fewer side reaction products are at times obtained when substantially moisture-free gas is employed. Many of these applications require the drying of gas a t elevated pressures, as is also the case where gases are being liquefied and moisture must be removed to prevent 385
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FIGURE 1 (TOP). TEMPERATURES EXISTINO AT VARIOUS CROSSSECTIONS OF AN UNCOOLED AcTIVATED ALUMINA BED(32 INCHES HIGH,1 FOOT IN DIAMETER) WHEN DRYINGAIR OF 75" F. DRY-BULB TEMPERATURE CONTAININQ 9 GRAINS OF MOISTURE PER CUBICFOOT, AND FLOW OF 5.2 CUBIC FEETPER HOUR PER POUNDOF ALUMINA
FIGURE 2 (Center). ADSORPTION OF MOISTURE 120 POUNDS OF ACTIVATED ALUMINA IN A BED 42 INCHES DEEP X 0.78 SQUARE FOOTCROSS SECTION(BEDTEMPERATURE, 90-100 ' F.) BY
FIGURE 3 (Bottom). TEMPERATURES EXISTING AT VARIOUS CROSSSECTIONS OF AN ACTIVATED ALUMINA BED WHENREACTIVATING WITH AIR HEATED TO 450-476 F.
the adsorption and condensation of the gas in the pores of the alumina and reduce its capacity or rate of moisture adsorption. An example is in the drying of carbon dioxide 3 at the final stage of compression for the pro4 duction of dry ice (about 1100 pounds per k square inch). Other adsorbents with lower * adsorptive intensity have been used with r" some satisfaction a t the final stage of comb pression. It was found, however, that actiEB vated alumina adsorbs and condenses carbon I dioxide when compressed approximately to F the critical pressure. Calculations readily show that, if the gas is dried after the second stage of compression, before mixing with the dry bIowback gas, the quantity of moisture to be removed is no greater than if the combined gases are dried after the final compression. Thus an activated alumina drier used after the second stage of compression is most economical, and the equipment is less expensive because of low-pressure design. Commercial units for the drying of gases under pressure are shown in Figures 4 and 5 . It is not possible to state the cost of producing dry gas until all of the required conditions are known. Naturally the cost per cubic foot is intimately related to the required frequency of reactivation. It has been pointed out that both the volume of gas dried and the adsorption period per unit flow are approximately inversely proportional to the initial moisture content. Thus minimum costs are obtained with equipment designed for and adjusted to the load requirements. It is apparent that heat for reactivation of the alumina is the major item of expense, and where a source of waste heat exists, the operating costs may be reduced to a minimum because all operations are susceptible of automatic control. If heated gases are employed for reactivation, only 20 to 25 per cent of the heat .is lost in the exit gas or by radiation. Thus the required heat can be calculated readily from the construction and performance of the unit. The several items which influence the cost of drying gases are now well understood, and ~ C C U rate estimates on equipment and operations can be made for specified conditions.
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freezing in the lines. Neither the pressures involved nor the combustibility or toxicity of the gas offers difficulties. The alumina is reactivated in place and, where necessary, the pressures need not be released and the gas remaining in the absorber a t the start of reactivation need not be lost. I n the drying of certain gases a t elevated pressures one precaution should be observed; namely, it is advisable not to approach the critical pressure too closely. This may permit
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Partial Removal of Moisture ;Dehumidification . A distinction has been made between applications which require complete drying and those where only partial removal of moisture is necessary. 'The reason is that different designs of equipment are required. Partial dehumidification is generally desired where relatively large quantities of gases are needed, and where any appreciable increase in the resistance to flow is quite objectionable. Thus the contacting of a gas at high velocities with a solid adsorbent necessitates the use of shallow beds. Generally, beds 2 to 3 inches deep and air velocities of 75 to 150 feet per minute are employed for dehumidification. Activated alumina weighs 50 pounds per cubic foot; therefore, the flow may be expressed as between about 300 and 1000 cubic feet per hour per pound. These conditions markedly reduce the period of contact of the gas with the adsorbent as compared with those employed for complete removal of moisture. As a result, the practical capacity of the adsorbent is reduced to between 3 and 5 per cent of the weight of the alumina per adsorption cycle. Equipment employed for partial dehumidification has consisted of pairs of shallow beds with damper control for alternately shifting the flow from heated air during reactivation to the air to be dehumidified during adsorption. Also circular rotating beds with a central distributor for both heated and cold gas, and with prorisions for FIGURE 4 (Above). EQUIPMENT FOR DRYIKG CARBON DIOXIDE GAS AT A PRESSURE OF 500 POUNDS PERSQUARE INCH FIGURE 5 (Left).
ACTI-
VATED ALUMINAEQUIPMENT FOR DRYING HYDROGEE AT A PRESSURE OF 2000 POUNDS PER SQUARE
FIGURE 6. ACTIVATED
-4LUMINA DEHUMIDIFIERS CAPABLE OF REMOVING ( A )45oPOUNDS AND ( B ) 600 POUNDS OF WATERPER HOUR FROM AIR AT ATMOSPHERIC
PRESSURE
separately discharging the moist reactivating gas to the outside atmosphere have found some application. More recently a single rotating valve mechanism has been developed to control the flow of large quantities of gases through stationary beds and permit the continuous discharge of gas of low humidity. The simplicity of operation and the compactness of the more modern units have special appeal. A particular unit of rectangular construction and capable of removing 450 pounds of water per hour from air has been located in a space 8 X 12 X 10 feet high (Figure 6 A ) . Another unit of circular construction and capable of removing 600 pounds of water per hour has been located in a space 16 feet in diameter and 16 feet high (Figure 6B). These dimensions include space for the motors, blowers, and automatic controls. The units may be located within or adjacent to the space to be dehumidified. They are neat in appearance and substantially noiseless in operation. Activated alumina dehumidifiers are especially well suited for maintaining lower than normal humidities in industrial plants where hygroscopic chemicals, candy, pharmaceuticals, leather, paper, glass, and costly equipment which may be injured by moisture, are being dried, packaged, fabricated, or stored. The commercial installations are too numerous to discuss specifically, but mention might be made of a few of the more novel ones which are effecting economies. I n one large plant where the rapid evaporation of a solvent takes place on the surface of a product, the cooling effect is sufficient to reduce the surface temperature about 15' F. below the dry-bulb temperature. When the dew point of the atmosphere increases to within this range, moisture con-
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denses on the surfaces and causes costly rejections. The lowering of the dew point to a t least 20" below the prevailing dry-bulb temperature, by means of an activated alumina dehumidifier, completely eliminated this difficulty. I n another plant 5 per cent relative humidity a t 76" F. is being maintained for the purpose of testing electrical apparatus under specific conditions. Another activated alumina installation is used in Conjunction with the drying of celluloid products where the dry air prevents blemishes which form on the product when the moisture concentration becomes too great. These several applications are merely indicative of the variety of operations which may be improved by controlled humidity.
obtain comfort a t the lowest possible cost. (c) Those who have been concerned with adsorbent drying have been engaged in the standardization of equipment to be used in existing and less competitive lines. It is believed that adsorbent dehumidification, in conjunction with refrigeration or other methods of cooling, has a place in comfort conditioning. This place will be fixed by the prevailing costs of power, gas, cooling water, and by any future progress which may be made in adsorption equipment specially adapted to this application. Adsorbents have been known for a long time but their commercial use is new. The success which has been attained so far is the basis for predicting a promising future for the greater use of complete and partially dried gases.
Literature Cited
Comfort Air Conditioning Much less progress has been made in the comfort air conditioning of houses and public buildings than in the industrial field. The major reasons are as follows: (a) I n addition to dehumidification, some sensible heat must be eliminated. (b) Each large installation must be separately engineered to
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(1) Bower, Bur. Standards J . Research, 12, 241 (1934). (2) Dover and Marden, J . Am. Chem. SOC., 39,1609 (1917). (3) Johnson, Ibid., 34, 911 (1912). (4) Munro and Johnson, IND.ENQ.CHEM.,17,88 (1925). (5) Yoe, Chem. News, 130,340 (1926). RXICEIVED February 7, 1938.
Drying Materials in Trays Evaporation of Surface Moisture
T
HE removal of moisture from a material by passage of heated air over the surface of the wet material represents the most common form of industrial drying today. The mechanism involved in this form of drying has been studied experimentally and theoretically by a number of investigators during the past ten years, but very little advance has been made in developing the results obtained to the point of practical application. This is probably due largely to the difficulty of developing theoretical formulas which will consistently cover the entire drying period of different materials. I n general, the drying of most solids may be divided into three distinct periods: a heating period, a constant rate of evaporation period, and a falling rate of evaporation period. The heating period is usually short in comparison with the total drying time and hence is of minor practical importance. The constant rate period varies in industrial drying processes from a negligible fraction to a major portion of the drying time. This is the period most susceptible to the application of theoretical considerations, and the influence of most of the variables affecting the mechanism of drying in this period has been fairly well established (9). The primary purpose of the present paper is to contribute new experimental data to this field and a t the same time to present a rational method for the practical application of these data. The falling rate period also may vary from a small proportion of the drying time to practically the entire drying period. Owing to the fact that the rate of drying in this period is continuously decreasing, the falling rate period may constitute the greater portion of the total drying time even when a large proportion of the total moisture is removed during the constant rate period. A considerable amount of experimental work has been performed in the study of the falling rate period, but the results have been so diverse that no great progress has been made in the development of a general theory.
C. B. SHEPHERD, C. HADLOCK, AND R. C. BREWER E. I. du Pont de Nemours & Company, Wilmington, Del.
The tray drying of surface moisture from nonhygroscopic materials and the effects of various drying conditions o n the constant drying rate have been studied. Two samples of Ottawa sand (20-30 and 50-70 mesh) were employed. The drying variables studied with the ranges covered were : air temperature, 115-300 O F.; relative humidity of air, 10-60 per cent; air velocity, 150-1375 feet per minute; material depth, 0.5-2 inches; and insulation of tray, none and 1-inch cork. The evaporation of water in trays under similar air conditions was also studied for comparison. I n the runs with sand, nearly all the water was removed during the constant rate period. The results obtained chiefly concern this period. The constant drying rate was found nearly identical for
