Dehydration of Organic Liquids with Activated Alumina R. B. DERR AND C. B. WILLMORE Aluminum Company of America, New Kensington, Penna.
A
LTHOUGH the solubility of water in many organic compounds is very low, it is generally recognized that water is soluble and is present in amounts which have considerable commercial significance. Moreover it is frequently desirable to eliminate the last traces of moisture from those organic compounds which are readily miscible with water and from which water has been substantially removed by distillation or rectification processes. Commercially the production of absolute alcohol is one of the few instances in which broader uses of a product have resulted from the removal of water. Many similar applications are now being investigated, and it is being found that certain catalytic reactions proceed more rapidly or completely in the absence of moisture. Likewise some solvents have greater solution capacity. Thus there appears to be a real need for an economical means of completely dehydrating organic materials whether they mix readily with, or are nonmiscible with water. Since it was believed that activated alumina had real possibilities of accomplishing this on a commercial scale, the dehydration of a number of typical solvents was investigated and the results are here described.
Dehydration Procedure In conducting the dehydration tests the solvents were introduced at the top of a column of freshly reactivated alumina at a uniform rate. When the liquid covered the adsorbent, it was then withdrawn from the bottom at the same rate it was introduced at the top. The moisture content was determined on fractions passed through the adsorbent; in some cases when moisture was detected in the effluent, the fractions were passed through an additional adsorber in the same order in which thev were withdrawn from the preceding one: From the data obtained the useful moisture adsorptive capacity of the alumina was determined as well as the practical volume of anhydrous solvent producible per unit weight of alumina. In general, critical solution temperature methods were employed for the determination of moisture. These are reasonably satisfactory for the determination of moisture in certain compounds and where suited are sensitive t o about *0.01 per cent. For the determination of very small percentages of water the method of Smith and Bryant' was employed. This method is based upon the liberation of acetic acid by reaction of the water in the sample with acetyl chloride in the presence of pyridine and the subsequent titration of the acetic acid with standard sodium hydroxide solution. The sensitivity of this method was found to be greater than that claimed by Smith and Bryant, especially when very small quantities of water are present 200400 and the size of sample can be increased t o 200 cc. or more. The method requires 1
J . Am. Chem. SOC.,57, 841 (1935).
careful technique and the use of dehydrated reagents. Nevertheless it is ca able of detecting as little as 0.0002 per cent moisture in a sogent such as gasoline.
Drying of Ethyl and Butyl Acetates Figure 1 shows graphically the results of two experiments on the dehydration of ethyl acetate. In one experiment the ethyl acetate contained 5 per cent water and in the other only 1 per cent water. In both experiments activated alumina took up 2.5 per cent of its weight of water before a detectable quantity of moisture passed in the effluent. When 5 per cent water was present, only one fifth as much anhydrous solvent was produced as with material containing 1 per cent water. It is of further interest that the activated alumina approached saturation after taking up about 7 per cent water in the first instance and after taking up only about 3.5 per cent from the solvent containing 1per cent water. This variation of total capacity with concentration is analogous to the influence of relative humidity on the total capacity of adsorbents for removing moisture from gases. It is characteristic of the dehydration of solvents which dissolve considerable amounts or are completely miscible with water. Moreover it shows the value of connecting a freshly reactivated adsorber in series after the first adsorber ceases to produce dry solvent. Obviously this practice is economical only when the water content is high and a much greater quantity of water can be taken up by the alumina after it ceases to produce anhydrous solvent.
800
cc. OF
800 n n y L ACETATE/
loo0
lxlo
1400
Boo
IeLwXKx,
LB. OF ACTIVATED ALUMNA
FIQERE1. DEHYDRATIOX OF ETHYL ACETATE CONTAINING 1 AND 5 PERCENTWATER BY PERCOLATION THROUGH 8-14 MESHACTIVATED ALUMINA 866
INDUSTRIAL AND ENGINEERING CHEMISTRY
JULY, 1939
O F BUTYL ACETATEB Y PERCOLATION FIGURE 2. DEHYDRATION ACTIVATEDALUMINA
Figure 2 illustrates the drying of butyl acetate which is not miscible with water but which will hold in solution 0.9 per cent water when saturated at about 250 C. The curves are similar to that of ethyl acetate, but another characteristic difference in the drying of solvents is shown by the fact that about 3.7 per cent moisture is adsorbed by the alumina before a detectable quantity of moisture passes through the adsorber. This compares with 2.5 per cent adsorbed a t high efficiency from ethyl acetate. Thus it is apparent that the capacity of the aiumina for adsorbing water from different solvents varies. Undoubtedly these differences relate to surface forces and the ease with which the solvent may be displaced by water. It .will also be observed that the efficiency of moisture removal decreases rapidly after moisture appears in the effluent and that the alumina is approaching saturation a t a point only slightly above its capacity a t high efficiency. In this case no practical advantage can be gained by connecting a second adsorber in series.
THROUGH
8-14
867
completely dried material equal to that originally present, the distillation characteristics are as shown in curve 3. It is to be observed that t h e b oi l i n g r a n g e s u b s t a n t i a 11y duplicated that of the original pyridine. Most of the water was removed from this pyridine when 30 to 40 per cent of the total volume had distilled and the boiling point had increased to about 135" C. At this point the distillation curves on all products substantially coincide. This is indicative that activated alumina had no catalytic action on the pyridine homologs. Similar curves were obtained with pure pyridine which had an i n i t i a l b o i l i n g r a n g e of 100" t o 117" C . and contained 5 per cent water.
MESH
Drying Gasoline For the dehydration of gasoline a c o ~ ~ e r c igrade al containing no tetraethyllead was employed. The solubility of water in this gasoline was first determined a t several temperatures and was found to be follows: Temp.,
7
25 40
C.
Sol. Water, % 0.00097 0.0043 0,0109
Drying of Pyridine Some experiments on the dehydration of pyridine are of interest because this compound forms hydrates rather than merely dissolving IO 20 3 0 4 0 50 60 70 8 0 9 0 103 0 water. The material used was of commercial CUMULATIVE QUANTITY OLnlLLED - % quality and was composed mostly of the higher FIGURB 3. A. S. T. M. DISTILLATION TESTS ON COMMERCIAL PYRIDINE CONhomologs. It was found to contain 3.35 per TAININff VARIOUS QUANTITIES O F WATER cent water. Activated alumina was capable of reducing the moisture to about 0.03 uer cent and 02 taking up about 6.8 per cent a t saturation. Figure 4 illustrates the dehydration of the gasoline when containing 0.0048 per cent water which is equivalent to saturaIt should be mentioned that water chemically combined in the form of hydrates is removed by adsorption with much tion a t 26" C. This saturated gasoline was passed through a bed of activated alumina a t the very rapid rate of 80 gallons more difficulty than dissolved water. The influence of the water content on the distillation range per hour per pound. No trace of moisture was detected in of pyridine is of some interest to those not familiar with the the effluent gasoline until the alumina had taken up moisture behavior of this material and also in connection with the acequal to 7.4 per cent of its weight and about 180 gallons had been dehydrated per pound of alumina. The activated alucuracy of the data. Figure 3 shows several A. S. T. M. dismina continued to adsorb moisture until it had taken up a tillation curves. It is to be noted that the original commertotal of 9.6 per cent and had produced an additional 112 galcial pyridine containing 3.35 per cent water had an initial lons of partially dehydrated gasoline. When this partially boiling point of 103' C. The initial boiling point of the portion dried to 1.5 per cent water was increased to 112" C., and dehydrated gasoline was passed through a freshly reactivated when dried to 0.2 per cent water the initial condensate apadsorber followed by a further quantity of saturated gasoline, a total of 260 gallons of anhydrous gasoline was produced peared a t 130" C. When water was again added to the more
IXDUSTRIAL AND ENGINEERING CHEMISTRY
860
(test B). The alumina had again taken up an average of 7.9 per cent of its weight of water before passing any moisture. In confirmation of the above experiments the adsorbed moisture was determined in the alumina which, according to calculations from the moisture content of the gasoline, had taken up 9.6 per cent of its weight of water. Actually water equal to 9.9 per cent of the alumina was recovered. In the second adsorber which had been saturated only slightly above its capacity to produce dry gasoline, the alumina bed was divided into three portions to obtain a measure of the distribution of adsorbed water. I t was found that the inlet portion had taken up 10.9 per cent, the middle section 8.8 per cent, and the effluent end 6.6 per cent moisture. Again the total
GALLONS
OF
WSOLINE/LB.
OF ACTIVATED ALUMIN*
OF GASOLINE SATURATED AT 26" C. FIGURE4. DRYING ACTIVATED ALUMIKA
WITH
Saturation, 0.0048 per cent water b y volume; flow, 80 gallons per hour per pound
water accounted for by direct determination of adsorbed water was in very close agreement with that calculated to have been introduced from the analysis and quantity of gasoline passed through the bed. The interesting item in these tests is the rapid rate a t which the moisture is removed. At the flow rate of 80 gallons per hour per pound of alumina the total period of contact of gasoline with alumina was less than 3 seconds. The fact that the alumina a t the effluent end of the bed was about 75 per cent saturated when moisture began to pass, indicates that complete dehydration occurs in a narrow zone. From the period required for moisture to pass with the effluent, it may be calculated that the effective dehydrating zone passed through the bed a t the rate of 0.048 inch per minute. It is not possible to calculate the minimum period of contact for complete dehydration from these figures, but they show that adsorption is extremely rapid.
A Commercial Solvent Drying Unit Although the above results are based upon experimental work, the practicability has been confirmed by several years of commercial experience. A clearer understanding of this may be obtained from a description of a commercial installation in which benzene and toluene are being dried. The unit consists of two adsorbers connected in series but operated as a single unit. The adsorbers are about 40 inches in diameter and 9 feet high, and each contains 2500 pounds of 4-8 mesh activated alumina. The alumina is supported on a grating covered with a fine metallic screen located several inches from the bottom of the adsorber. Steam coils providing a radiating surface of 1 square foot for each 5 to 8 pounds of activated
VOL. 31, NO. 7
alumina are distributed uniformly throughout the bed. Each adsorber contains ports at the top and bottom for introducing and withdrawing the solvent, and they are insulated to retain heat during reactivation. The flow of benzene is upward through the adsorbers. The upward flow has the practical advantage of providing a sump for entrapping water, in excess of solubility, which may accidentally be carried into the system. Benzene containing about 0.04 per cent water, or toluene containing 0.03 per cent water, is pumped through the adsorbers at the rate of 15,000 gallons per hour. The moisture content of the effluent is an undetectable quantity throughout a considerable portion of the adsorption cycle. Adsorption, however, is continued until the maximum moisture in the benzene reaches 0.025 per cent. This quantity of moisture is permitted to pass so that the average product meets the final specifications of less than 0.01 per cent moisture. The capacity of the unit per reactivation cycle is approximately 100,000 gallons of benzene or 200,000 gallons of toluene. For reactivation the liquid benzene retained in the adsorbers is pumped back to the storage tanks; 175-pound steam is then introduced through the embedded pipe coils, and a vacuum is applied to the bottom of the bed through a surface condenser. The condensed water and solvent are separated, and the recovered saturated solvent is returned to the initial storage. The bed is then cooled by passing water through the coils, after which it is in condition for further dehydration. An alternative method of reactivation is that of introducing an inert gas such as methane and then circulating this gas, by means of a pump, through a heater, the alumina bed, and a condenser. I n this manner moisture and solvent are trapped out in the condenser and the inert gas is reheated until the alumina reaches a temperature of about 175" C. Heating is then discontinued and the gas is circulated through the condenser and bed until the alumina is cooled. The operation of an activated alumina dehydration unit requires little control. If the feed is of a uniform water content the period of adsorption can be standardized and the labor is merely that of transferring from adsorption to reactivation at intervals. The interval and quantity of solvent dried per cycle are dependent upon the moisture content of particular solvent. I n the case of gasoline, a unit of the size described is capable of dehydrating about 1,000,000 gallons of gasoline per cycle. At the high flow rate employed, the million gallons could be dehydrated in about 2.5 hours. The cost of dehydrating benzene is a fraction of a cent per gallon and the cost of dehydrating gasoline would be only one twentieth that of benzene. The scope of this experimental work has been limited to a relatively small number of the organic liquids which are sold commercially. However, the dehydrations which have been obtained seem to justify the conclusion that activated alumina is capable of removing all of the moisture from liquid organic compounds which do not chemically combine with water and do not react with alumina. This leaves a broad field for further research having the objective of improving catalytic processes of manufacturing many of these organic materials and of improving the quality of products in which they are used. Such investigations have the assurance of a most practical and economical means of obtaining dehydrated raw materials.
Acknowledgment The authors wish to acknowledge the assistance of the following members of the Aluminum Research Laboratories staff: A. H. Riesmeyer on the drying of ethyl and butyl acetate; R. B. Unangst on the drying of pyridine, and F. D. Chew on the drying of gasoline.
