Topics in...
Chemical lnstrumentatidn Edited by S. Z. LEWIN, N e w York Universitv, N e w York 3, N. Y.
These m t i c h , most of which are to be contn'bu2ed by guest authors, are intended lo s m e the readers of this JOURNALby calling attentia lo new developments i n the theory, design, or mailability of chemical labo~atory instrumentation, o i by presenting useful insights and ezplanatias of topies lhai are of practical impo~lanceto those who use, or teach the w e of, modern i n s t r u m a a ' m and instrumental techniques.
XIX. Instrumentation for Condensing and Absorbing Vapors George A. Dalin, Yardney Electric Corp., The water-cooled glass condenser has been used in a variety of forms for many years, and the principal design considerations should by now be widely understood. Nevertheless, an examination of the condensers used in many laboratories will show that. the condenser selected is frequently ill-matched to the objectives. The mismatch may be due to poor design of the condenser, or to failure to understand what should he expected of the condenser.
Application of Water-Cooled Condensers The most frequent use of condensers is for downward condensation; vapor enters the upper end and is condensed to liquid which drains out of the lower end. Three requirements must be met: (1) the cooled surface area must be adequate to condense all of the vapor and t o cool the condensate sufficiently so that loss by evaporation is minimal, (2) the vapor passages must be large enough in cross section to accommodate the vapor flow without excessive pressure drop, and (3) no liquid should be trapped in such a way as to require washing out. This last condition is important in distillation where sharp cuts between fractions must be made. A second use of condensers is for returning reflux t o s. boiling vessel. Here the vapor enters the condenser a t the lower end and is condensed, but the condensate leaves by the same opening. The design conditions are: (1)the cooled surface area must be adequate for condensation only and (2) the passages must be large enaugh t o accommodate vapor and Liquid moving countercurrent to each other. The third use is for removing a. condensable vapor from a noncondensable (that is, noncondensable a t the temperature of the coolant) gas. Again, the surface area far cooling must be adequate,
N. Y.
but if the vapor concentration is Low, the area required will usually be small. This follows from the fact that the latent heat of the phase change is usually much greater than the heat transferred in coaling the gas. As above, the passages must be large enough to permit transit of the gas without excessive pressure drop. The passage-area requirement is greater if the condensate is returned to the vaporentrance passage. The principal difference between this type of condenser and the first two is the requirement that as much of the vapor as possible be brought to the coaled surface. This objective is met by using a gas passage which is long, of small diameter, and twisted.
in a nonvertical position in order t o avoid trapping condensate, as might happen with a bulb-type condenser.
Heat Transfer Considerations
Mass Transport Considerations
The common factor in the designs for the three uses just cited is the requirement for adequate cooling area. The rate of heat transfer csn be increased by increasing the rate of flow of the cooling water aver the heat-transfer surface, decreasing the thickness of the glass heat-transfer wall, and decreasing the thickness of the film of liquid condensate on the other heaetransfer surface. At the water flow rates commonly used, increase in flow rate gives only a small increase in heat transfer per unit of area. Also, condenser manufacturers are now aware of the advantage of thin glass heat-transfer walls; the glass used is held to the minimum consistent with strength requiremenh. However, operating the condenser in a horizontal, rather than vertioal, position causes condensate to drain off more rapidly; this decreases the average Iilm thickness and increases the heat-transfer rate. For a coil condenser with coolant in the coil, operating the condenser a t an angle such that each turn presents a lowest point from which condensate can drain, substantially increases the cooling capacity. Nevertheless, it is frequently preferable t o sacrifice the advantage of operation
Providing adequate cooling area is almost the only consideration in selecting a condenser for downward flaw. An upper opening adequate to accept the vapor flow is also necessary, but this is no problem, since it need only be no smaller than the vapor passage upstream. However, a problem can arise from failure to understand how the vapor reaches the cooling surface. I t is s, common misconceptiun that the vapor passage should, in effect, be baffled to ensure that no vapor travels in a straight line through the unit without encountering a cold surface. Actually, as vapor condenses, a sharp pressure drop a t the cooling surfsee ensues, causing vapor to travel from the middle of the vapor passage toward the cooling surface. As a result, the vapor concentration in the unit drops to that in equilibrium with the chilled condensate. Baffling, however, is necessary in the case where it is desired to remove vapor from a. permanent gas. The pressure drop due to condensation is relatively small, especially after most of the v%ppor has been removed. Moreover, the per-
George A. Dolin. 8. 5.. Howard (1 92L Ph. D.. Columbia (19341. Worked on 11 development of procesrer for blowing pia iicr at Plax Corporation; thir work result* in the development of the plwtic rqueer bottle. Developed precision reriltorr noble meiol 01ioy film. at Bolco Rereor< Presently Chief of Electro-Chemical Rereor, at Yardney Electric Corpomtion. Empho: is on high rpeciflc energy batteries such silver-zinc and silver cadmium. Consulto in laboratory and monufoctvring plant d sign.
(Continued m page A6)
Volume 42, Number 1 , January 1965
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Chemical Instrumentation
Fig. 1 Fig. 2
The Graham Condenser. The Uebig Condenser.
manent gas impedes flow of the vapor toward the cooling surface. Baffling, therefore, is desirable in this case. Also, the use of small passages brings every volume element of the gas close to the condensing surface. These objectives are well met by the jacketed coil condenser with the coolant outside the coil (as in the Graham condenser, Fig. 1). The case which most frequently intraduces serious problems is reflux condensation. The source of the difficulty is the counterflow of vapor and condensate through the same passages. At high vapor rates, due usually to vigorous hoiling, the condensate may be unable to flow downward against the "wind," and the condenser floods. At suHiciently high boil-up rates, liquid can be ejected from the top of the condenser-a highly dangerous condition. The difficulty cannot be solved by lengthening the condenser; this merely leaves more of the condenser unusable. Instead, adequate cross section must be provided for counterflow of vapor and liquid. This applies to the connection between the condenser and the flask, as well as to the condenser itself. Table 1.
Testing of Condensers A number of the more frequently purchased condensers were examined for conformance with the above principles. The surfsee areas per foot of cooled section were estimated and possible advantages or disadvantages were noted. These are summarized in Table 1. Some of the opinions given there may be considered subjective; the author prefers, for instance, that the top and bottom joints of condensers be ca-linear since such pieces of equipment can more easily be fitted into many arrangements. Some of the condensers were also tested under reflux, with the results shown in Table 2. The floodpoints were measured in terms of wattage input to the kettle. Where possible, the maximum wattage which the condenser would absorb was measured but, in general, flooding occurred before the entire surface of the condenser wru brought into action. The Liebig Condenser (Fig. 2) is prabably the most frequently used condenser. Its ends are co-linear and i t can be used either vertically or a t any angle to the horizontal. As generally made, the inner tube has an outer diameter of 12 mm. The condensing area per foot of jacket is
V Fig. 3 Fig. 4
The Allihn Condenser. The Kronbitter Condenser.
(Continued o n page A81
Chorocteristics of Common Commercial Condenser Types
..
Condenser design
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Area in square decimeters per foot of jacket
price per square Approximate decimeter (exclusive orice for one foot of of standard Joints condenser taper joints) in line?
Operable near harizantal?
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Liebig AUihn Hopkins Friedrichs
1.13 2.00 2.00 4.15
$10.00 11.00 14.00 14.50
$6.00 4.00 5.00 2.75
Yes Yes No No
Yes No Yes Yes
Return suiral center i u b e Cold finger coil and jacket Coil with bvpass join; Graham
4.97
15.25
2.50
Yes
Yes
15.04
30.00
1.75
No
Yee
4.97
23.00
2.25
Yes
No
4.97
12. 00
2.25
Yes
No
i;;-thj---
Chemical lnsfr~menfafion small and the condenser, under reflux, Hoods a t very low boil-up rates. The priee per unit of condensing area, is high. For mast applioations, this condenser should be considered obsolete. The bulbed or Allihn condenser (Fig. 3 ) is the second most frequently purchased condenser. I n the usual form the vapor p a e s through, rather than outside the bulbs. The condensing area per foot of jacket is about 200 em*. The priee per
unit of condensing area is about half that of the Liebie. The condenser is satisfactory for do&ward vapor flow provided the condenser is not inclined so much that condensate is trapped in the bulbs. The inside diameter of the constrictions between the bulbs is usually %bout 1 cm. A condenser 30 cm long u s u d y consists of five bulbs. However, when i t is used for reflux, it is generally impossible for the vapor to reach the fifth bulb because of flooding. The design, a t leest when more than one foot long, is therefore unsatisfactory far reflun operation. There are modifications in which the eonstrictions between the lower bulbs are larger, so that higher throughputs can be tolerated before flooding, but in view of the limitations, this design should also be considered obsolete. The Kranhitter design (Fig. 4) is a
Table 2. Condenser Liebig Allihn Allihn-Kronbitter Graham (coolant outside the coil) Friedricha Coil conderrser with by-pass joint Fig. 5 Fig. 6
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The Hopkinr Condenser. The Friedrichr Condenser.
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Applied wattage
296 387
modification of the Allihn which can be o~erittedin a nearly horizontal nosition x:ithout trapping candensate, Ghich is advantageous for downward condensation. It suffers from the other disadvantages of the Allihn in addition to having less condensing area. The Hopkins condenser (Fig. 5 ) resembles the Allihn except that the vapar travels over the outer surface of the bulhs. I t is generally made as a cold-finger type. It is much superior to the Ulihn with respect to condensing capacity u p to the floodpoint, and freedom from trapping of liquid when inclined. The ends are not co-linear. It is somewhat inferior to other designs with respect to henttransfer area.
(Continued on page A l O )
Testing of Condensers*
Flooding; vapar escaping 3 bnlbn in use: bezionine to flood a t constrictions between b u k Inoperable Beginning to flood a t constrictions between hulh.; Inoperable; 3 bulbs in use Beeinnine to flood in coil In&~erabie Some flooding in joint hoporxble; flaodmg s t joint; some vapor escaping 25 turns of coil in use 28 turns of coil in use Some vapar leaving top of oandenser No floodine
~.
~
T h a n k s are due to Mr. Willian Geyer, Sr., of Scientific Glms Apparatus Co., Inc., whn provided the equipment and laboratory space for the tests.
Chemical Instrumentation The Friedrichs condenser (Fig. 61, also a. cold-finger type, was one of the best tested insafar as capacity is concerned. In the largest size available, the length of the oooled section was only 22 cm. A helical indentation is formed in the cooled surface as B means of increasing the candensing area. The indentation is molded, which accounts for the relatively law price of this condenser. The clearance between the cold finger and the outer wall is too large for purposes of vapor strippings. On the other hand, i t is small enough so the condenser introduces an appreciable pressure drop a t throughputs imposed by a. 22-liter full-mantle heater operating a t rated capacity. I t is suggested that manufacturers should consider supplying longer units with larger clearance. The ends, as with all cnldfinger types, are not eo-linear. In one version of the spiral condenser (Fig. 7 ) with the water inside the spiral, the lower end of the tube is returned up through the axis of the spiral and is ringsealed out through the condenser wall near the top of the unit. This is advantageous when the condenser is used far reflux condensation of high bailing liquids because there is some possibility of breakage when very hot vapors meet a cooled ringseal. In test, this condenser flooded a t the lower joint. The West condensers (Fig. 8) are constructed with jackets of annular area much
Fig. 7 The Return Spiral Condenser. Fig. 8 The West Condenser.
smaller than the Liebig. The objective is to increase the linear velocity of the cooling water and thus the heat-transfer coefficient a t the glass-water interface. This certainly result,^ in some increase in the over-dl heat-transfer rate per unit of interfacial area. However, since the designs are based on the Liebig, the ratio of interfacial area to length is so low that the slight advanbage resulting from the improvement in the heat-transfer coeficient is swamped. The largest area of cooling surface per unit of length is obtained by inserting a cold finger and s. coil (Fig. 9) into the vapor space inside a water-cooled jacket. (Contimed on page A l d )
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Chemical Instrumentation -
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This type of condenser is also one of the cheapest in terms of turns of cooling surface per unit of cost.
Improvement of Condenser Design This list is by no means exhaustive, but is sufficiently representative to lay the basis fur making s, selection with respect, to condenser surface configuration. However, as was pointed out above, flooding in the lower part of a. condenser to be used far reflux can render the upper part inoperative. To study this problem, condensers were fitted at the lower end with 24/40 standard taper inner members and joined to a 12-liter flask equipped with a high-wattage heating mantle. This is, of course, a mismatch; a flask of this size should be fitted with a. much larger joint. However, the 24/40 is the most frequentlv used joint, and it is therefore pertinent to det,ermine what load it will carry. Mareover, i t may happen that a laboratory has standardized on 24/40 joints and has nothing else available. For the test, six liters of carbon tet.rachloride were put into the flask. Carbon tetrachloride has a low heat of vaporisatian (46.4 c d / g ) and a low boiling point, (76.8"C). The low heat of vaporisatkm means that the volume of liquid and vapor which must pass through the joints and the condenser for a given heat input will be large. The low boiling point means that the temperature difference zvailnble between the vapor and t,he cooling wat,er
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Fig. 9 The Cold Finger Coil and Jacket Design. Fig. 10 Conden,er Coil with B y p m Joint.
for driving the transfer of heat is small. The combination of t,hese two properties therefore comprises a severe test of the condenser. In carrying out the tests, wattage tu the heating mantle was gradually increased until flooding began. Table I1 shows the results. I t is evident that the Liebig and the Graham designs with coolant outside the coil are unsuitable for any but the lightest loads. The Allihn and the I'iranbitter designs are only slightly better. The Friedrichs and the Hopkins are sub(Catinued on page A 1 6 )
Chemical instrumentation stantially more effective, but they are limited by flooding in the 24/40 joint. The condensers with by-pass joints have by far the largest capacities. The by-pass joints (Fig. 10) provide separate passages for transit of vapor and liquid, thus avoiding the flooding problem. As s h o r n in the figure, the by-pass joint returns the liquid t o a point well below the neck eonstviction. I n this lower region, the vapor velocity is low, so there is little tendency for the vapor t o lift bhe liquid. As the results show, the by-pass joints can handle 2 to 3 times more condensate than the next best designs, the Friedrichs and the Hopkins. Vapor Removal
\-apors may be removed irom a gas stream by a. number of devices other than the water-cooled condenser. Perhaps the commonest of these is the cold-trap, Figure 11, which may he cooled by immersion in a Dewar flask containing one of the mixtures: ice water; dry ice-triehlorethylene; liquid nitrogen. Dry iceacetone, or liquid air are not recommended due t o fi1.e or explosion hazard.
Fie. 1 1 Fig. 12
Design of Conventional Vacuum Trap. Nerbitt Abmrption Bulb Design.
Where the vapor is t o be condensed to a liquid, the dimensions of the trap should be selected so that the liquid level dues not rise above the bottom of the inner tube; otherwise, an undesirable pressure drop may be introduced. The removable outer tube is desirable because it makes i t possible t o remove the condensate without removing the trap from the line. The joint should he held together with hooks and springs. I t is preierable to introduce the gas stream through the inner tuhe rather than the outer. I n this way, concurrent condensation takes place in the inner tuhe where the volume of condensate is greater because the gas stream is richer in condensable vapor; in the outer tuhe the flow of gas is countercurrent t o t h a t of the condensate, hut since the vapor has already been partially stripped, the danger of carry-over eauscd hv inadequate cooling or by flooding is reduced. (Continued on page A18) Circle No. 159 m Readen' service Card +
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Chemical Instrumentation I n general, the simple rold trap is used only where the volume of condensate and the gas flow rate are both small, so the possibility of inefficient stripping is small. For stripping large volumes of gas, one of the condensers described above can be used with a lnwer temperature r r s h n t than tap water. Thus, ice water, or trichlorethylene cooled by prior passage through a capper coil immersed in a dry ice-triehlorethylene mixture, can he pumped through a condenser. It is advisable to wrap the condenser with nsbestos tape or one of the siliaaeeouu pipe coverings to avoid transfer of heat from the surroundings. The insulation should be sealed with pitch, or with s. film such as Sarnn, to avoid condensation of water from the air. I n all cases, the gasshould he pasped downward through the condenser, thus avoiding countereunent Row of liquid and gas which might cause Hooding. I n general, large Rows can be accommodated hy passing the gas over the outside rather than i,he h i d e of a coolant roil. The coil should he used in conjunction with a ~:ooledjacket for maximum effectiveness. The condensate can he rolleeted in a eontainer underneath the condenser, or can be removed continuously by uverflow through a U-tube. Where the vapur is t , be ~ eondenfied to a solid, as in ireesing water vapor out of an air stream, the problem is rather different. Obviously, a condenser cannot be used. If the gas stream is introduced into a. cold trap through the inner tube, this will soon be plugged. The preferred technique where more than a trace of material is to he removed starts with a trap designed with a relatively narrow inner tube reaching within 3 or 4 cm of the bottom of a much wider outer tuhe. Suitable dimensions might be 12 mm o.d. for the inner tube, and 36 mm a d . for the outer tuhe. The gas is now introduced through the outer tuhe and the Dewar flask is positioned so that the coolant level is 3 or 4 cm above the bottom of the inner tube. Condensation will begin near the level of the coolant and will build up an the wdl of the outer tube. When the volume of the condensate threatens to plug the passage, the Dewar is raised, or more coolant is added to raise the liquid level. Condensation will now start higher up in the trap. The process can he continued until the trap is full. The quantity of vapor which can be removed in this wav is relativelv small. is available.
Vapor Absorbers Where the quantity oi vapor removed must be known precisely, as in quantitative determination of the hydrogen content of an organic compound, solid absorbers are used. The Nesbitt (Fig. 12), and the Turner (Fig. 13) absorption bulbs charged with s. sultable absorbent sre convenient, sturdy, and effective. When complete stripping oi a contsmi-
- ..-....-. .-- ..---. .. --....- --.A1 8
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(Continued on page A201
Chemical Instrumentation nant from a gas stream is the objective, liquid absorbents are more convenient. This case is encountered, for instance, in polarography where the nitrogen streams
the possibility of contamination. If the gas flow rate is great enough to cause flooding in the lower joint, then the nrrangement of Figure 16B can be substituted. I t will be noted that this is snother version of the by-pass joint shown in the condenser of Figure 10. The flaw of absorbent liquid should be adjusted so that, as a minimum, the saddles are completely wet and n steady drip from the bottom is visible. In addition, the exit gas should be examined for traces of the component to be absorbed. If still present, the liquid flow rate should be increased. Taking the height of the theoretical plate as roughly 2 inches for r/n-inch Berl saddles, a 48-inch column is equivalent to more than 20 "equilibrations" between gas and fresh absorbent. Incidentallv. this device can also be
Fig. 13 Turner Absorption Bulb Derign. Fig. 14 Go. Bubbling Apparatus with Gor Dispersing Tube.
must be completely freed of oxygen. An early method was to bubble gas from the end of an open tube through the absorbent liquid in a flask. Some mprovement in absorption efficiency results from the use of a disperser such as that shown in Figure 14. However, the Milligm gas washing bottle, Figure 15, is easily the most effective of the smell scale devices. In this absorber, the gas bubble travels in a long path between the conical outer vessel and a close-fitting inner molded spiral. Absorption of oxygen in a single bottle ahout
Fig. 15
Milligan Gor Washing Bottle
10 inches high is sufficiently complete so that a polarograph can detect no trace of oxygen in the stripped nitrogen stream.
High Flow Rates The absorbers described above are suitable far gas flow rates of the order of 1 to 2 cc/sec. Far higher rates, no standard equipment seems to be available, but effective equipment can easily be assembled. To start with, select a tube about 4 feet long with a diameter so that the linear velocity of the gas through the tube will be about 10 cm/sec, more or less. Figure 16A shows the general arrangement. It will be noted that the central member is simply a distilling column packed with Berl Saddles. Connections to the ends can he made by rubber stoppers, but ground joints decrease
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Fig. 16 Derign of Absorption equipment for efficient removal of a vapor component at high Row r-ter of the gar *Iream. A. General arrangement. 8. Alternative design of lower
.10,nt. .
Aerosols A problem which occasionally crops up in both condensation and absorption is the formation of aerosols. These can be very difiicult to trap, so it is best to avoid the conditions which can cause them. Two examples are here presented which illustrate causes and methods of prevention. The classical case is that of absorption of sulfur trioxide from air into water to form sulfuric acid. As a bubble ~f air is introduced into water, sulfur .rioxide diffuses toward the interface. However, water evepor~tesinto the bub,le substantially more rapidly, forming dfuric acid vapor. The vapor coalesces (Continued on page Ad4)
into minute droplets with a very low weight-tosurface ratio and a. negligible diffusion rate. As 8. result, most of the sulfur t r i o d e leaves the absorber as a sulfuric acid aerosol in the effluent air stream. This problem is solved by absorbing the sulfur triaxide in moderately cvneentrated sulfuric ac.id which hasalow watervapor pressure. Part of the concentrated sulfuric wid which is produced is removed as product. Water is added to the remainder to bring it to the original concentration and this is recirculated to the absorber. The second case is the removal of sulfur from a gas stream. If the stream is cooled strongly, a. remarkably stable aerosol results. The cause of the difficulty here is the great difference between the temper* ture of the gas stream and of what would normally be considered a suitable condenser temperature. Since sulfur boils a t 444T, the gas stream temperature will be above, say 350°C, if i t is to hold any quantity of sulfur vapor. Now, suppose the condenser wall to be a t 120PC, which is above the melting point of tbe sulfur. As s volume element of the gas moves toward the wall, the temperature of the element will begin to drop. With so large a temperature difference between the wall and the incoming gas stream, condensation will take place, in the volume element under consideration, a t some distance from the wall. As is well known from fluid flow studies, a stagnant film is present on s. surface in contact with a
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gas stream, even when the flow is turbulent. Molecules in the stream therefore reach the surface b y diffusion only, and not as part of the stream flow. Consequently, if the sulfur molecules, after condensing, can agglomerate into droplets before diffusing to the wall, a. stable aerosol will be formed. This will be the case when, as a result of a large temperature difference, condensstion takes place a t an appreciable distance from the wall. The last sentence points the way out of the difficulty. As %firststep, a tube about 18 inches long b y 36 mmi.d. is wound with resistor wire (Nichrome V from DriverHarrisor Tophet A from Wilbur B. Driver), packed with '/&nch Bed saddles and is mounted a t an angle of about 20" from horizontal. The tube is heated by the resistance wire controlled by a varizble transformer to it temperature which, as may be determined by experiment, is slightly below the sulfur condensation temperature. The temperature will he about 350'C. Under these oonditions, the gas will approach closely to the tube wall or to the slightly warmer saddles before condensation occurs. With the gas introduced a t the upper end of the tube, ahout 90y0 of the sulfur in the gas will condense out and run out the bottom of the tube. The remaining 10% forms an aerosol. As a second stage, the gai bearing the aerosol is run into a second tube, similarly constructed, in which the gas is reheated to vaporiae the aerosol. A third tube, heated t o a temperature slightly below t b s t of condensation now
removes 90% of the remainder of the sulfur. I n three condensation stages, with two reheatings, the sulfur content of the gas can be reduced t o about 0.1% which is adequate for most purposes. Of course, the condensed sulfur must be removed prior to reheating. This can be done by providing an overflow (heated) as in Figure 16. Incidentally, the five heated sections can d l constitute one tube. I t is only necessary to provide two overflows prior to each reheating section, and separate control far each heater. These last two cases can hardly be considered typical of the problems which may be encountered in attempting to condense part, or all of s gas stream. Nevertheless, these examples, together with the preceding discussion, should indicate the range of possibilities available in equipment which is either standsrdwith respect to catalogued items-or standard with easily obtained modifieations.
Acknowledgment
I have refrained from mentioning specific sources for the glass equipment because most of it is available from any scientific supply company which makes glassware to order. However, Scientific Glass Apparatus Co., Inc., of Bloomfield, New Jersey constructed all of the glassware, both standard and modified, on which the tests were run, thus warranting thanks in the form of special mention.
