1252
ANALYTICAL CHEMISTRY
The operation requires no further attention until the solvent has evaporated, as approximately the same level of liquid is maintained in the evaporator as in the receiver.
For other details concerning the operation of the circulating evaporator, the paper by Mitchell et al. should be consulted. DISCUSSION
When the liquid is heated in the steam-jacketed tube, it bursts into a froth, which not only pushes into the bowl but forces the liquid back in the opposite direction. A constriction (see diagram) prevents most of this back flow, so that the flow is almost entirely in one direction and a much less turbulent evaporation results. This smoother operation makes it possible to eliminate the centrifugal separator of the previous apparatus. Thus, after an iso-octane solution was evaporated a t the rate of 2 liters per hour, none of the nonvolatile solid could be detected in the distillate, although it was possible to detect as little as 0.01% of the solid in the original solution.
LIQUOR I N L E T
.WATER OUT
RESERVOIR
II ca 2
JACKET
LITER)
CONDENSER
CONDENSATE O U T
*
\
separator also makes it easier to recover the concentrate quantitatively and with less solvent. The apparatus has been run with a house vacuum that showed large pressure variations, with no adverse effects. It is therefore much less sensitive to pressure changes than the previous one. Thermometers, thermocouples, a manometer, or special heating systems may be used. Stopcocks with Teflon plugs would eliminate the need for lubricating stopcocks. The apparatus has been very useful for redistilling solvents. LIMITATIONS
To use the apparatus properly one must recognize certain limitations. The heated liquid froth, which spurts into the bowl, is forced to rotate rapidly within the small radius (4 cm.). The use of a bowl of larger radius would result in slower rotation of the liquid and therefore less efficient separation of liquid from vapor. The centrifugal separator of the previous apparatus may therefore be needed if a much larger bowl is employed. The final concentrate of heat-sensitive material should be evaporated with caution. Caution should also be exercised in evaporating solutions containing appreciable quantities of nonvolatile material. In operation, the circulating liquid in the evaporator loses its solvent, which is replaced by more liquid from the reservoir. The liquid in the evaporator becomes more and more concentrated. Should the volume of nonvolatile matter be greater than will remain below the steam jacket, the temperature of the liquid will approach that of the steam with Iittle further evaporation. Heat-sensitive materials will therefore be subject to attack. In such situations-for example, in using the apparatus as a solvent stripper-the adapter holding stopcock A should be replaced by a round flask, D (of sufficient capacity), fitted with a stopcock. Incoming air bubbles will agitate the liquid in the flask and thus maintain s fairly uniform solution in the evaporator.
V
CONSTRICTION (4 NU)
+WATER
IN
I 1
Compound Mercury Manometer. F. M. Tiller, Lamar State College of Technology, Beaumont, Tex. use of mercury manometers for measuring pressure has T generally been restricted to relatively low pressures. In the laboratory it is seldom convenient to exceed a differential of HE
0
5
'U
IO IS eo
CENTIMETERS
Figure 1.
Apparatus
During continuous operation of the previous apparatus frequent attention was required to maintain the proper solvent level; otherwise the leveloftendropped too low or the liquid overflowed into the receiver. I n the present apparatus a working level is maintained as long as there is liquid in the reservoir. Furthermore, the previous apparatus could not be fed continuously if it were necessary to introduce a small stream of air bubbles to prevent bumping, since the air bubbles and incoming liquid had to enter through the same inlet. In the present apparatus air bubbles enter through stopcock A , while liquid is introduced through stopcock C. The reservoir may be refilled without interrupting the distillation. With the present apparatus larger quantities of liquid can be handled with a smaller apparatus. With a smaller unit the liquor may be concentrated to a smaller volume and less liquid is needed to rinse the apparatus. When the unit is to be cleaned, the reservoir is first rinsed and stopcock B is then closed; the apparatus is then cleaned as described by Mitchell et al. The elimination of the centrifugal
25 pounds per square inch because of the height of the tubes. I n 1951 the author was faced with the necessity of measuring filtration pressures up to 75 pounds per square inch with a greater accuracy than was possible with available Bourdon gages. I n order to circumvent the difficulties a compound mercury manometer similar t o the one shown in Figure 1 was developed. By placing a number of manometers in series, it was possible to make measurements at as high pressures as were desired. HP
LP
11!4 Figure 1
V O L U M E 26, NO. 7, J U L Y 1 9 5 4
1253
By opening valve A t o the high pressure (HP) side and closing values B and C, the pressure is transmitted in series through the three manometers. If the high pressure is Po and 8 pressure differeutial of APl exists in the fust manometer, pressure P, rtt the entrance to the second manometer will he
PI
=
Po - API
(1)
pressure increases, C can be closed and B opened for the tw* manometer pressure range. This process can he continued for as many tubes as are available. I n Figure 2 an actual compound mercury manometer is shown, a8 used for measuring pressures in filtration research in the chemical engineering laboratories of Lamar State College of Technology. A number of modifications of the basic arrangement can he made. As more manometers are added, the accuracy of measurement decreases. An inclined or spiral manometer can be used for the last unit if increased accuracy is needed. By sealing off the low pressure side, the compound unit can he converted into a barometer or vacuum gage.
Improved Electrolytic Desalter. David H. Simmonds, Biochemistry Unit, Wool Textile Research Laboratory, Commonwealth Scientific and Industrial Research Organization, Melbourne, Australis. to the electrolytic desalter described by Astrup M Stage, and Olsen ( I ) have heen introduced which subODIPICATIONS
stantially reduce the temperature of operation and prevent the formation of air locks in the sulfuric acid circulating system. Viskine celloohane tubing is a more satisfactory semipermeable r re 2
81milwrly, ii AP, and AYa represent the pressure drops acro8s the second and third units, pressure P1 a t the entrance t o the third manometer and ~ r e s ~ u Pa r e a t the exit of the last manometer will he given by
P2 = P, - AP2
(2)
and
+
+
Pa = P, - APa = Po - (AP, APpp AP3)
(3)
or the aver-all pressure difference Po - P3 because
Po - Ps
+
= (0, APx
+ AP,)
(4)
If air is permitted t o remain above the mercury column, the values of the pressure drops will he different from tube t o tube because of the changes in absolute pressure. If a liquid ia placed above the mercury, the pressure drop will be the same for each manometer, provided a constant diameter tubing is used. For n manometers in series with equal pressure drops, AP, the over-all pressure drop Po - P. is given by Po - P,
=
nAP
(5)
Any number of manometers can he placed in series, thereby permitting the measurement of relatively high pressure differentials. The individual pressure drops, APi, may be read on each msr nometer, or if the AP's are all the same, i t is necessary to determine only one value. Actually the reading of height on one leg of the last monometer can he calibrated directly against the pressure differential, thereby eliminating the necessity for making more than one reading. If the desired pressure range corresponds to one or two manometers instead of d l three as illustrated in Figure 1, the valves can he manipulated t o short-circuit the first or second units. If valve A is closed and valve B is opened, the pressure difference will he recorded across the last two mauometers only. By opening valve C and closing A and B, low pressure readings can he made on the last manometer. Then a8 the
Figure 1. Apparatus
Several designs of electrolytio desalter have been suggested since Consden, Gordon, and Martin (S) devised the apparatus and described the theory underlying its operation. The most commonly used modifications are based on Deut's design (2, 4). Astrup, Stage, and Olsen (f ) improved the mercury circulating system and altered the design of the anode compartment to permit controlled circulation of sulfuric acid and measurement of the temperature of the effluent acid to avoid overheating. The apparatus described in this paper is based on the designs of Dent (2, 4 ) and Astrup, Stage, and Olsen (I) with modifications to permit more efficient cooling of the solution being desalted. The apparatus is constructed entirely of glass, aud, a8 shown in Figure 1, consists of four parts: the sample holder and cathode compartment, the mercury lift pump, the mercury reservoir and
