V O L U M E 21, NO. 4, A P R I L 1 9 4 9
527
The results indicate that douhling the volume of wash water used after washing out all the excess ammonium oxalate does not . remove the excess oxalate in the precipit,ate.
To determine whether dilution of the calcium sulfate solution at the time of the calcium oxalate precipitation would reduce the sulfate ion error, the original calcium sulfate solution was divided into a number of 50-ml. portions. Ten were run by adding the reagents directly, using the urea hydrolysis techni ue. Then 350 ml. of water were added to give an eightfold Iilution, and a large number of determinations were made. The averages of the first and second groups were identical. T n ~ ointermediate dilutions were tried with the same results. I t is concluded that reasonable dilution of the calcium sulfate will not decrease the sulfate error. The urea hydrolysis technique for calcium oxalate formation in the presence of the sulfate ion has given a slightly smaller error than the standard technique on the first precipitation. On the second precipitation the oxalate error is still present with the standard technique, but thcoretical values can be obtained by the urea hydrolysis method. If different methods of crystal formation give different oxalate valucs from the same solution, the extra oxalate caused by the sulfate must be a part of the crystal. When the calcium ion is determined by the standard technique in the presence of a small amount of aluminum ion, t’here is always a precipitation of the oxalate ion equivalent to the sum of the calcium and aluminum ions. When the urea hydrolysis method was used, two groups of results were obtained. A possible explanation may be that the aluminum ion precipitates mmetimes with the oralatc ion but gcncrally with the hydroxyl ion.
In the presence of the tartrate ion, the calcium ion may be precipitated viith oxalate without any aluminum oxalate interference if 30 minutes are given for the aluminum tartrate formation. CONCLUSION s
The use of the hydrolysis of urea for raising the pH of a solution containing both the calcium and oxalate ions permits the formation of large, readily filtered crystals of calcium oxalate. These are less contaminated by magnesium or aluminum oxalate or excess oxalate due to the presence of the sulfate ion than the small crystals formed by the standard method, while the over-all time for a set of duplicate determinations for calcium is reducrd by the use of urea froin 90 minutes to 40 to 50 minutcs. ACKNOWLEDGMENT
The work of this paper is a part, of a project supported by a granbin-aid from the Sational Institute of Health of the United States Public Health Service for research upon rapid analytical procedures for n-atcr and sewage. LITERATURE CITED
(1) Am. Public Health Assoc., “Standard Methods for the Bnalysis of Water and Sewage,” 9th ed., New York, 1946. (2) Chan, F. L., dissertation, University of Michigan, 1932. (3) Willard and Furman, “Elementary Quantitative Analysis,” 3rd ed., p. 397, N e w York, D. V a n Nostrand Co., 1040. RECEIVED December 29, 1947.
All-Glass Laboratory Long Tube Evaporator WILLIAM H. BARTHOLOMEW, Werck & Co., Innc., Rahwuy, .V.J . 011 evaporating heat-sensitive iolutioiis in the laboratory, F the usual batchwise procedurc for Concentration in flasks often ii unsatisfactory. The retention of unstable materials a t -29 m m . 0 . D. TUBING 35/25
-ABOUT
20mm.
7
elevated temperatures for considerable periods of time can lead to undesirable decomposition. Long-tube or film type evaporators, which provide short contact time for the evaporating liquid a t moderate temperaturcs and high evaporation rates, have been used successfully in pilot plant and large scale operation to solve this problem (1, 3 ) . An all-glass corrosion-resistant film type evaporator has been designed for laboratory application.
ABOUT 3 8 m m .
APPARATUS
The assembled apparatus is shown in Figure 1. ..1 detailed sketch of the small glass separator i3 shown in Figure 2 .
E ~5lmm.O.D.TUBING
r 6 . 5 cm
LIT^
TUBING
2 m m . BORE STOPCOCKS
Figure 1.
Laboratory Long-Tube Evaporator
The evaporator consists of a &liter graduated feed funnel, A , which has a constant head air inlet tube. This is joined with flexible tubing to the bottom of the steam-jacketed heating tube, B. A small rotameter in the feed line allows rapid sett’ing of the feed rate, but this is not absolutely necessary for satisfactory operation. The vapor head of the heating tube has an opening for insertion of a thermometer. This head turns a t a sharp angle and connects with the tangential inlet of the separator, C. Prcvision is made for pressure measurement with a mercury manometer. The bottom outlet of the separator connects with the concentrat,e cooler, D, which joins the 2-liter concentrate receiver, E. The vapor outlet from the separator is connected through the right-angle adapter, F , to the vapor condenser, G , which is joined in turn t,o the 3-liter distillate receiver, H . A stopcock-controlled pressure-equalizing line connects the top of the right-angle adapter to the top of the concentrate receiver. The line connecting yith the vacuum source is attached a t the top of the distillate receiver. The ground-glass seiniball joints used to connect the various sections of the apparatus provide a measure of structural flexibility in the unit. Yo breakage has occurred during several months of operation.
528
ANALYTICAL CHEMISTRY
__
___
-~
Table I.
Performance Data for Laboratory Long-Tube E,apot.ator
Test S o . Feed t o evaporator Feed - - - - , litem ------
Concentrate, liters Distillate collected in receiver, liters Total distiilate. liters (feed-concentrate) Duration of test minutes Temp. a t vapor 'head, C. Pressure a t vaDor head. mm. H e Pressure a t dis't. receiver, mm. R g Feed rate, liters/hr. Vaporization rate, liters/hr. Vol. of feed per unit vol. of concentrate Point of initial boiling, inches from botto 111 of tube
___ 1 Water
2 Water
3 Water
4 Kater
3.00 1.11 1.88 1.89 38.0 51.5-53 97-101
3.00 1.39 1.59 1.61 31 ,.5 33 100
3.00 1.65 1.35 1.35 26.0 51-53 97-101
4:74
5:71 3.Oi
6:92 3.11 1.82 5
3.00 0.92 2.03 2.08 38.0 45-47 72-76 61-64 4.74 3.28 3 26 3
2.9:
2 , 3 .n
2.16 4
__
~~
J
LVater 3.00 1.17 1.60 1.83 32.3 46 74 62 5.67 3.40
1.56
4.0
7 T\-ater
6 Water
3.00 1.14 1.79 1.8G
34.5 46-47 72-76 64-66 5.22 3.24 2.63 3 5
3.00 1.36 1.37 1.64 27.8 47.5-48 77-80 67-69 6.48 3.54 2.20 4.5
~~_______
~
11 9 10 95% 95% PentEthanol Ethanol acetate 3.00 3.00 3.00 3.00 1.17 1.46 1.59 1.oo 1.72 1.28 1.54 0.96 1.83 1.41 1.61 2.00 14.5 9.75 9.0 27 63 45-46 45-46 44 68 75-76 170 176-183 53 63-64 165-173 160 1 8 . 5 20.0 6.67 12.4 12.2 8.68 3.58 8.28 2.56 1.89 2.16 3 5.5 6.5 4 .i 4.5
8 Water
Jacket steam temperature, inlet 101' C, exit 99' C . . Coolin& water triirperaturr, 220 t o 240 C
OPERATION
The system is evacuated and the water and steam services are turned on. The stopcock on the constant head air inlet tube and the bottom stopcock of A are opened. The flow rate is regulated by means of the stopcock a t the bottom of B. The desired feed rate can be determined either by use of a flowmeter in the inlet line or by estimation from observation of the initial boiling height in the evaporator tube. Ordinarily, flow rates are selected to give a concentration of 2 or 3 parts of feed to 1 part of concentrate. Main points of control during operation are the temperature and pressure a t the vapor head and the feed rate.
-I
SECTION
I--+ bjNO
DISTANCE UP BOILING TUBE
75mrn.
j
- INCHES
Figure 3. Temperature Gradient in LongTube Evaporator
29 m m . 0 . D.
DISCUSSION
I
4
0
L.-#--J
-75mm.-
TOP VIEW
Data for several tebts made in the evaporator are reported in Table I. Vaporization rates of 3 to 3.5 liters of water per hour a t pressures of 75 to 100 mm. of mercury are obtained in the evaporator. The concentration ratios of feed to concentrate are about 2 or 3 to 1. The tests with ethanol and pentacetate (Sharples Chemicals, Inc., 85% minimum ester) show the relatively rapid evaporation rate that can be obtained with solvents. In all tests free steam was used in the jacket for heating. With 20' to 25" C. condenser water and ordinary laboratory vacuum available, the equipment operates well over a range of 50- to 100-nim. pressure with aqueous solutions. Operation below this pressure range is not practical unless provision is made for brine cooling in the condensers and vapor lines of large diameter are used to connect the distillate receiver with a high capacity pumping system. A thermocouple traverse was made of the heating tube duri!ig test 5 to determine the temperature gradient through the length of the tube. The maximum temperature of 52' C. w ~ t sfound a t 10 to 12.5 cm. (4 to 5 inches) up the tube, as can be seen in Figure n
a.
Figure 2.
Detail of G l a s s Separator
A laboratory water aspirator has sufficient capacity to maintain a satisfactory vacuum on the system. An ice trap helps to reduce the load on the aspirator whenever the vapor volume exceeds the capacity of G . Although an aspirator holds the svstem a t substantially constant pressure after equilibrium conditions are reached, positive pressure control can be obtained by means of a vacuum regulator such as a Cartesian manostat (2). The procedure described and data obtained are for single-pass operation. However, installation of a recycle line from the concentrate receiver to the bottom of heating tube a t the low pressure side of the feed stopcock converts the evaporator for recycle operation.
A 5% sulfuric acid solution was evaporated a t about 3 liters p w hour. The distillate and concentrate were tested using barium chloride to precipitate the sulfate. .knalysis showed less than 0.1% entrainment through the separator. LITERATURE CITED
(1) Brooks and Badger, Trans. Am. Znst. Chem. Engrs., 33,
(1937).
392
(2) Gilmont, Roger, 1x11. ENG.CHEM.,ASAL. ED.,18, 633 (1946). (3) Porter, R. W., Chem. Eng., 53, N o , 10, 94 (1946).
RECEIVED April 1, 1948.
