Wm. B. Wa r r e n, Coal Research Laboratory, Carnegie Institute of Technology, Pittsburgh, Pa.
I
N CONNECTION with studies of the influence of the rate of heating in the carbonization of coal, a control system has been developed to procure definite, predetermined, and reproducible rates of temperature rise in an electric furnace, and to provide a record of the actual temperature at all, times. The design of the apparatus is relatively simple, permitting it to'b e constructed from readily available parts, and the control mechanism has great flexibility so that it may be used for many purposes.
In the particular application of the control apparatus to be described, its performance has been satisfactory during 2 years of operation. The temperature of a furnace, drawing as high as 2250 watts at 110 volts, has been raised from room temperature to 1000° C. in 3 ,6 ,1 2 , and 24 hours, respectively, at constant rate throughout, and the temperature subse
quently maintained constant at 1000° C. for 1 hour. The recorded temperature never departed from the predetermined temperature at any instant by more than 10° C. Since the retort in the furnace has a fairly high heat capacity, the variation of the retort temperature itself was somewhat less.
A diagrammatic sketch of the apparatus used is shown in Figure 1. The furnace temperature is measured by a thermo
couple and recorded on the chart of a standard recording potentiometer. The controller is built onto the recorder as an integral part and consists essentially of a mandril, a control chart, two brushes, and a stylus. The mandril is identical with that which carries the chart under the re
cording pen and is supported so that both mandrils are rotated by the recorder-driving mechanism. The control chart is carried by this mandril and is constructed from a portion of the recorder chart by first drawing upon it the desired time-temperature curve and then cementing onto the paper strips of aluminum foil in such manner that the time-temperature curve lies in a slit 0.06 inch (0.16 cm.) wide between strips of foil. (Artists’ rubber cement has proved a very convenient adhesive, as it does not wrinkle the paper. Good electrical contact was obtained between two foil strips cemented together with it, in spite of the rub
ber.) To each of these strips is connected a second strip which is 1 inch (2.5 cm.) wide, laid along the adjacent edge of the paper parallel to the time axis. The brushes are supported above and make contact with the control chart through the marginal strips. The stylus is also supported above the control chart and is moved parallel to the tem
perature axis by the same mechanism which moves the re
cording pen, so that it will lie in the slit or on one or the other of the foil strips, depending upon the temperature and the time. Two relays, one connected to each brush and to a source of potential through the stylus, complete the set-up.
In use the control chart is placed upon its mandril at zero time reading, the brushes and stylus are lowered upon it, powrer is turned on to the furnace, and the recorder motor is started. The recorder pen and the stylus moving with it then take up a position which indicates the temperature.
If this is the proper temperature for this instant of the program, the stylus will lie on the slit between the foil strips and will not make contact with either; thus, neither relay is actuated and a medium value of current is provided by the shunt around resistance Ri and part of Rt- If the tem
perature continues to rise according to the desired program, the stylus will move so as always to lie on the slit as the chart is moved forward. Howrever, if at any given time the temperature is too low', the stylus will make contact with the left-hand or low-temperature foil, completing the circuit through the battery to relay 1, and thereby causing an increase in current, since the shunt around R3, Rtl and part of Ri is completed through closing C|. Likewise, if the tem
perature is too high, contact will be established between the stylus and the right-hand or high-temperature foil, com
pleting the circuit through the battery to relay 2, opening Ct while Ci remains open and thus providing a decrease in current, since both shunts are open and the only path for current is through R), R?, and part of R g Therefore, the system operates so as automatically and continuously to choose one of three graded values of current according to the instantaneous requirements of the program.
Fi g u r e 1. Dia g r a m o f Ap p a r a t u s
In practice the low value of current is chosen, through adjustment of Rt, such that the required rate cannot be maintained even at lowest temperature, while the high value is chosen, through adjustment of Ri, so that the rate of rise will be too rapid even at highest temperature. The medium value is then set half way between by adjustment of Ri. In the case of the author’s furnace of 6-ohm resistance, 5, 10, and 15 amperes wrere the values used for a program of 6 hours to 1000° C. This use of three different controlling conditions is particularly valuable where the range of tem
perature over which the controller operates is great, since thereby “hunting” is considerably reduced. In case of a narrower range of temperature it is possible that the edge of a single foil strip and one relay, hence two controlling conditions, would suffice. As a matter of fact, it is observed that during the early part of a program, the system alternates between the low and medium currents and toward the end between the medium and high currents, while during an intermediate range of temperature three values of current are used.
In the above paragraphs an apparatus has been described 285
Vol. 5, No. -1 for controlling the rate of heating of an electric furnace over
the temperature interval from room temperature to 1000° C.
However, the control system is not limited to this use but may be adapted to procure any desired temperature program or to control the rate of heating or cooling in metallurgical and chemical problems, wherever the method of adding or removing energy can be controlled by operation of electrical
relays. While the description of the apparatus and its use has been limited to programs involving temperature as a function of time, it appears evident that only slight modi
fications would be necessary to use the same system for controlling other variables as a function of time.
Re c e i v e d March 29, 1933.
A Microextractor
Le s l i e Ti t u s a n d V. W. Me l o c h e, Chemical Laboratory, University of Wisconsin, Madison, W is .
I
N CONJUNCTION with the biological and chemical study of the inland lakes of Wisconsin conducted by the Geological and Natural History Survey under the direction of E. A. Birge and C. Juday, it was found desirable to determine the ether extract of the lake water residues pre
pared by the evaporation of known volumes of water. Since the water of a large number of the lakes was soft, it was not convenient to prepare large residues, for this in turn would require the evaporation of large quantities of water. It was therefore necessary to select an extraction method which was applicable to small samples.
The macro-Soxhlet could not be used and early study was directed toward the use of a small-size Soxhlet extractor, as shown in Figure 1. The siphon C measures 4 cm. from the seal to its top and functions when it contains 8 cc. The extraction thimble was made by sealing a porous alundum disk into a section of glass tubing, X . A 15-mg. dried sample of lake residue was transferred to the thimble, 15 cc. of dry redistilled ether were placed in the flask, and the ether was refluxed over the sample for 24 hours. The flask Y was then placed over fused calcium chloride in a vacuum desiccator and the ether removed by evaporation. After the flask and extract had dried in this system for 12 hours, the flask was allowed to stand in the balance for 15 minutes and weighed.
The increase in weight of the flask represented the weight of the extract.
Although this system proved satisfactory, particularly for samples containing a reasonable amount of extractive material, certain difficulties appeared which made an ex
tension of the study imperative. The purpose of the present paper is to describe a new microapparatus and procedure which developed from a study of these difficulties.
Ne w Mi c r o a p p a r a t u s
The condenser and lower part of the extractor shown in Figure 2 are of Pyrex glass. The lower part, £, was blown from a standard Eck and Krebs 40-mm. joint, the outer member of the joint, R, sealed to the condenser, and the tube of the inner member A sealed to form a cup.
The extraction thimble T is made from a 16-mm. Pyrex tube. The inner part of the joint shown in the diagram is made by grinding the end of a short section of 15-mm. tube to fit inside the end of the longer 16-mm. tube. Hooks are sealed on the top of the thimble to support it on the aluminum ring E. An 18-mm. disk of filter paper is cut from larger sections of Whatman No. 50 or S. and S. No. 575. The disk is wet with water, laid over the ground end of the outer tube of the thimble, and forced into place by firmly pressing the inner ring into the ground end of the larger tube, thereby closing the bottom of the larger tube with paper. It is im
portant that the surfaces of the joint be very carefully
ground so that when the paper is in place finely divided par
ticles of sample cannot leak through the joint.
The weighing bottle D was blown from soft glass and has a total weight of 4 grams. When the bottle is in position in the extractor, the clearance between the bottle and the thimble is at least 1 mm.
The outer joint of the extractor is dry and must be ether- tight in order that extraction may proceed at reduced pres
sure without the loss of ether. In earlier models, a mercury seal was provided to prevent loss of ether at the joint, but this has since been eliminated. In order to prevent condensa
tion of ether at the joint, it is necessary to provide a moder
ate amount of heat. This is accomplished by passing one strand of No. 26 nichrome wrire around the top of the joint at P, fastening it by means of adhesive tape, and maintaining a temperature of about 50° C. by making an alternating current connection and adjusting the resistance in the line (contribution by Walter Militzer, limnology assistant).
The adhesive tape must cover the wire in order to keep the joint warm.
S u c t i o n . An ordinary water pump is used to reduce the pressure in the apparatus. The connection is made from the pump to an expansion flask, to the manometer, and finally to the extractor. A plug of cotton is placed in the outlet of the extractor so that dirt cannot be drawn into the extractor when the stopcock is opened at the end of the extraction pro
cedure. Connection may be so arranged that the one pump and manometer will serve several extractors. (The authors use a battery of six extractors.)
H e a t i n g . The extractor is heated by means of a 40-watt lamp placed in a small box which is covered with an asbestos board. The top of the box has a 3-inch (8-cm.) opening which makes it possible to place the extractor close enough to the lamp to provide the proper rate of distillation.
E t h e r . Ethyl ether was allowed to stand over fused calcium chloride for 2 weeks and finally distilled. Metallic sodium was added to the distillate and the dry ether was then dis
tilled as needed. The fraction distilling between 34° and 36° C. was used in the extractor. Thirty cubic centimeters of the freshly distilled ether gave no detectable residue upon evaporation.
Pr o c e d u r e
Before beginning a determination of ether extract, the entire apparatus is cleaned thoroughly with “cleaning solu
tion” and finally rinsed with water, alcohol, and ether in the order named. The apparatus, with the exception of the thimble T and weighing dish D, then needs no further clean
ing between determinations, since nothing but ether vapor comes in contact with it. The weighing dish D is cleaned in a similar manner, and dried in a vacuum desiccator over fused
July 15, 1933 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 287
Fi g u r e 1 . Mic r o- So x h l e t Ex t r a c t o r
calcium chloride at room temperature for 2 hours. It is transferred to the microbalance, where it is allowed to stand until it reaches equilibrium before it is weighed. The time required to bring the bottle to equilibrium must be deter
mined. It is usually about half an hour.
The g la ss p a rts of th e th im b le are clea n ed in a similar manner, and when the wet paper disk is pressed into position the thimble is again washed with alcohol and ether and allowed to dry. In the analysis of fat samples, the thimble was taken apart and the fat was weighed directly on the paper disk. However, in the case of the dry finely divided samples, the thimble was a sse m b le d and th e weighed sample introduced at the top. The introduc
tion of the sa m p le into the thimble is facilitated by the use of a small tro u g h -lik e piece of cut tube as the weigh
ing glass.
A fte r tr a n sfe r r in g the sample to the thimble, 5 cc.
of ether are ad d ed to th e p r e v io u s ly tared weighing bottle and the bottle placed in the bottom part of the extractor. The aluminum ring E is then placed on the top edge of A and the thimble is lowered into position until the hooks of the thimble are supported on the aluminum ring. A few drops of ether are now poured through the thimble to wash down any stray particles which may have lodged on the walls of the thimble.
The cup A is then joined to the upper part of the extractor.
The apparatus is evacuated to a pressure of about 20 cm.
of mercury and the stopcock is closed. The current for the heating lamp and the heating wire at the extractor joint is turned on and extraction begins. The extractor is adjusted at such a distance above the lamp that the rate of condensa
tion of the ether is not fast enough to cause the ether to^rise above the constriction in the thimble. (In the summer time, the tap water may be warm and to insure rapid condensation the water is allowed to run through a cooling coil before passing to the condenser.)
After 23 hours of extraction, the vacuum pump is started and the stopcock gradually opened. Evacuation is continued until all the ether has disappeared. Air is then carefully ad
mitted to the apparatus until atmospheric pressure is reached.
The lower part of the extractor is removed and the weighing bottle transferred to the vacuum desiccator where it is allowed to stand over calcium chloride for 2 hours. The bottle is then transferred to the microbalance, allowed to come to equilibrium, and weighed. The increase in weight of the bottle is the weight of the extract.
Two weighing bottles and two thimbles were prepared for each extractor, so that while one set is in use the other may be cleaned and weighed. One hour is ample time for remov
ing the old sample and adding the new one and it therefore is possible to make one determination each 24 hours for each extractor.
Re s u l t s o p Ex t r a c t io n
In order to test the operation of the extractor, samples were examined in the microextractor and the results obtained
compared to those of the macro-Soxhlet extractor, representative results are given in Table I.
Some
T a b l e I. R e s u l t s w it h Mac r o- a n d Mic r o e x t r a c t c Et h e r Ex t r a c t s
Sa m p l e Macro Micro
% %
Dry skim milk 0.0 8 0 .0 7
0.0 8 0.0 9
Casein 0.1 6 0 .1 6
0 .1 5 0 .1 9
Feed sample 439 5 .0 5 .4 3
5 .4 5
One to seven milligram samples of pure lard were weighed into the extraction thimbles and upon extraction recovery was obtained within ±0.002 mg. The following results
illus-□ 2
— A 24mm
Fi g u r e 2 . Ne w Mic r o e x t r a c t o r
trate the degree of agreement between duplicate samples of lake water residues.
T a b l e II. E x t r a c t i o n o f L a k e W a t e r R e s i d u e s Sa m p l e
Upper Gresham Clear Crooked Crystal
Dis c u s s io n
In using the direct extraction apparatus, the difficulties encountered with the micro-Soxhlet apparatus have been greatly minimized or entirely eliminated. The final evapora
tion of the solvent in the micro-Soxhlet required the removal of the 25-cc. weighing flask to the desiccator and sometimes the complete transfer of the ether extract to a .smaller dish
We i g h tor We i g h to f
Sa m p l e Ex t r a c t
Mg. Mo. %
15.671 0.164 1.0 5
12.785 0.1 3 8 1.0 8
14.903 0,219 J. .47
13.800 0.2 0 5 1.49
10.603 0.037 0 .3 5
13.605 0.050 0 .3 7
Vol. 5, No. 4 preliminary to evaporation and weighing. In the new ap
paratus, the weighing bottle is not moved until all the solvent has been evaporated. Possible mechanical loss is avoided.
The fact that the new weighing bottle weighs only 4 grams probably improves the chances of weighing with a precision of
=*=0.001 mg.
In the micro-Soxhlet apparatus, the extraction thimble was made by sealing an alundum disk into a section of glass tube X . It was difficult to select an alundum disk whose pores were sufficiently small to retain finely divided sample and at the same time allow the ether to flow through at a reasonable speed. The straight walls of the original thimbles also allowed occasional small sample losses due to creeping.
The new thimbles eliminated the first difficulty by the sub
stitution of the paper disk and the second difficulty by a constriction of the walls of the thimble T. There is little doubt that operation at reduced pressure also prevented creeping of the solvent to some extent. Upon condensation the ether flows into the thimble and directly into the weighing bottle. No ether collects in the bottom or at the joint of the extractor. If ether i s lost during the extraction, an addi
tional aliquot may be added at the top of the condenser with
out disturbing the sample.
While ether is the only solvent mentioned in this investiga
tion, it is possible to use other reagents effectively.
Re c e i v e d February 13, 1933. Presented before the Division of Physica and Inorganic Chemistry at the 84th Meeting of the American Chemical Society, Denver, Colo., August 22 to 26, 1932.