COMMUNICATIONS
A Dilatometric Technique to Study Nucleation Phenomena in Urea-Ethanol Systems A dilatometric technique was devised to investigate the onset of nucleation in ureaethanol systems. A dilatometer was used to observe the volumetric behavior of solutions during constant rate coaling. The pre- and postcrystallization lines were found to be linear with their intersection occurring near the saturation temperature. The difference between this point and the noticed onset of nucleation was termed the supersaturation interval. This supersaturation interval was shown to be useful in relating effects of recrystallization (hysteresis) and the significance of impurities between sample lots. Effects of seeding and agitation were also investigated.
Knowledge of crystallization in industrial processes is still lacking and difficulty is encountered in determining the correct values of process variables for operating industrial crystallizers. Crystallization in pumps, pipes, and other undesirable locations still causes numerous problems. Kucleation, being the initial step of crystallization, must be understood if the entire process of crystallization is to be understood. Jackson (19651, Fielsen (1964), and Uhlmann and Chalmers (1965) reviewed the theoretical aspects of nucleation. Although considerable theoretical work on nucleation has been done, not much experimental work has been undertaken in liquid solutions. Some investigators noticed that such factors as agitation, mechanical shock, impurities, and seeding do affect nucleation and crystal growth. However, the main purpose of these investigations was not to determine which factors affected the onset of nucleation. This investigation’s objective was t o study the effect of seeding, degree of agitation, recrystallization, and other factors on the onset of nucleation by utilizing a dilatometric technique developed for that purpose. Experimental
A dilatometric technique. modified from one used by Vonnegut (1948), was used to measure the degree of supersaturation before nucleation occurred. The system consisted of a dilatometer, an insulated water bath, a variable area heat exchanger, and a coolant reservoir. The base of the dilatometer was formed from a 100ml. three-neck distilling flask. The upper part of the dilatometer was formed from 48 inches of either a No. 7-8 Pyrex capillary or a No. 8-10 Pyrex capillary. The capillary tube was inserted through a No. 4 rubber stopper which was fitted into the center neck of the distilling flask. A precision thermometer calibrated in tenths of a degree centigrade was inserted into one of the side necks of 136
Ind. Eng. Chern. Process Des. Develop., Vol. 10, No. 1, 1971
the flask by using a No. 2 rubber stopper. The other neck of the flask was stoppered tightly. Agitation in the dilatometer was provided by a 1-inch Teflon-coated magnetic stirring bar driven by a magnetic stirrer (E. H. Sargent and Co.). T o obtain a cooling curve for a particular solution, the dilatometer was first filled to the top of the capillary with the solution. The dilatometer was then submerged in the insulated water bath. The initial bath temperature was always a few degrees above the known saturation temperature of the solution. Cooling of the bath was provided by a copper tube heat exchanger through which a flow of ice water was maintained. The ice water was circulated through the heat exchanger and back into a coolant reservoir filled with ice. The heat exchanger was made so that its area could be increased (by lowering more exposed area into the bath) as the temperature in the water bath decreased. This procedure provided a constant cooling rate for each run. Figure 1 shows the piping arrangement for the dilatometric system. Rotameter A measured high flow rates while Rotameter B measured low flow rates. The flow rate of coolant was controlled by Valve 1, which allowed the pump to operate a t a constant flow rate. Exchanger B is the variable area heat exchanger used to obtain cooling curves while Exchanger A is the heat exchanger used for controlling temperature during a saturation run. The cooling curve was determined by cooling the water bath a t a constant rate and recording the decrease in height of the liquid in the capillary as a function of temperature. A typical cooling curve is shown in Figure 2. The apparatus used to seed the various solutions consisted of a water-soluble gelatin capsule suspended through the third neck of the dilatometer by a coneshaped, %-inch diameter stainless steel wire and thread. The seed was placed in the capsule, wetted down with
V= VALVE ROT. = ROTMETER EXC. = EXCHANGER
ROT.
A
I
Figure 1 . Coolant reservoir and piping
1
COOLANT RESERVOIR
1
system
INPUT EXC. A
b
INPUT EXC. B
OUTPUT LINE FOR EXCHANGERS
the test solution so that no air was trapped, and immersed in the dilatometer. At the desired temperature the seed was released by pulling the thread. This separated the capsule and released the crystals into the solution. The primary advantage of this seeding arrangement is that there is essentially no volume change upon introduction. Thus, the seed could be introduced during a run without disturbing the dilatometer reading. T o be sure of the saturation temperature of the solutions investigated, an apparatus was designed and built to saturate solutions a t a specified temperature. The saturation system consisted mainly of a constant temperature bath and two 1-liter round-bottomed flasks. The two flasks were connected by Pyrex tubing and a cotton filter plug. The solvent and excess solute were placed in one flask and both flasks were submerged in the water bath. The flask containing the solvent and solute was agitated with
SOLVENT = ETHYL ALCOHOL SAT". TEMP. = 3 0 . O O 0 C INTERSECTION = 3 0 . 9 6 ' C
0
21
I
I
23
25
I 27
I 29
I 31
TEMP. ('C)
Figure 2. Cooling curve for Run 4
I 33
use of a magnetic stirrer. The solution was agitated a t constant temperature for 8 to 1 2 hours to ensure saturation. The sa,turated solution was separated from the excess solute by forcing the liquid through the cotton filter plug by using nitrogen pressure. The urea used in this investigation was analytical grade. Absolute ethyl alcohol was used as the solvent. Numerous precautions were taken to keep contaminants out of the system. All solutions were kept in containers which were sealed with electrical tape. Dry, high-purity nitrogen was used to transfer solutions between containers when possible. The bottoms of all rubber stoppers were coated with epoxy resin to prevent sulfur contamination. Results and Discussion
As stated before, Figure 2 represents a typical cooling curve, An IBM 7094 Computer was used to fit the preand postcrystallization lines by the method of least squares and solve analytically for their intersection. The linearity of the pre- and postcrystallization curves is evident. The precrystallization line is the lower line since an increase in the ordinate corresponds to a decrease in volume. Since the precrystallization curves were linear, the onset of nucleation was observed with the dilatometer by noting the initial deviation from linearity. The difference in temperature between the initial deviation and the saturation temperature is a measure of the resistance to nucleation. This temperature difference was called the supersaturation interval and was the criteria for the conclusions reached in this experimental study. A series of eight replicated runs was made to obtain an estimate of the true experimental error (Table I ) . The mean square due to error was found to be fairly low which means that the supersaturation interval has a fairly high reproducibility. A series of 11 runs was made with a saturation temperature of 25.C. while a series of 13 runs was made with a saturation temperature of 30°C. The mean, 955 confidence limits for the mean, variance. and standard deviation were calculated for the intersection points a t each saturation temperature. They are listed in Table 11. The table shows that the 95% confidence limits for the mean encompass the true saturation temperature. Hence, it can be concluded that the pre- and postcrystallization lines intersect a t the saturation temperature. However, the variance and standard deviation are quite large. I n fact, these values are large enough that considerInd. Eng. Chem. Process Des. Develop., Vol. 10, No. 1, 1971
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Table I. Pairs of Repeated Experiments Supersaturation Interval, C.
Run No.“
4 5
6.0 6.2 1.2 1.4 1.0 1.2 0.6 1.4 2.4 2.8 3.8 3.6 5.0 5.4 4.4 4.2
n
1
8 9 10 11 12 13 14 23 24 31 32 33 34
“Even numbered runs are replicates of preceding odd numbered runs.
Table II. Results of line Intersections Saturated Temp. = 25°C.
Mean Variance Standard deviation 9 5 7 Confidence limits for mean
25.23 1.2870 1.1345 24.47 < 25.99
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