ISDUSTRIAL A,VD ESGIXEERING CHEMISTRY
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were milled on a very hot mill in order to preserve the original state as far as possible. The compound used was 100 parts of rubber, 5 parts of zinc oxide, 3 parts of sulfur, and 1 part of di-o-tolylguanidine, Vulcanization was carried out a t 140" C. Maximum tensile and stress a t 600 per cent elongation were taken from curves plotted to show changes of these properties with time of cure. Results are shown in Table T'. Table V-Rebound
of Unvulcanized Rubber a n d Physical Properties a f t e r Vulcanization
TYPEOF RUBBER
REBOUND M A X . ::N :E : AI 24' C. &
Smoked sheets Smoked sheets, gassy Smoked sheets, moldy Smoked sheets, clean, ribbed Smoked sheets, clean Smoked sheets, clean Smoked sheets, clean Smoked sheets, clean Pale crepe Pale crepe Para ham, washed No. 1 amber blanket No. 1 amber blanket No. 1 amber blanket No. 1 amber blanket No. 2 amber blanket No. 2 amber blanket No. 3 amber blanket Roll brown Roll brown Smoked sheets milled cold 40 min. Sheets 85, miAeral rubber 10, pine tar 5
MAX. TENSILE
Per cent 64 63 62 66 66 65 65 65 66 65 67 61 65 64 65 60 63 59 54 59 4s
Kg / s q . cm. 194 172 175 176 175 163 170 161 173 203 140 170 173 177 140 172 140 111 140 115
Kg./sy. cm. 296 268 260 311 302 290 290 295 294 288 336 252 292 279 303 246 238 276 131 224 130
61
161
238
188
These data are plotted in Figure 4. While the points do not fall ori a smooth curve, the tendency for decreased tensile and load-carrying capacity to accompany decreased rebound is shown. For rebound above 60 per cent the increase in tensile is very roughly 7 kg. per square centimeter for each per cent increase in rebound. The curves cannot be straight lines, since they do not incline toward the origin through which they must pass. It is true in general that in any given series of polymers the smaller molecules are more reactive chemically. Since w l canization involves a chemical combination of sulfur, the rubber having the largest proportion of the smallest molecules should combine with sulfur a t the greatest rate. While milling will greatly increase the plasticity of rubber, this increase should be a t the expense of the elastic network and of the largest aggregates more than of the aggregates of medium size. In spite of the fact that the number of the smallest aggregates may not be greatly increased by milling,
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the rate of combination of sulfur seems to be slightly increased. A sample OF rubber milled with 10 per cent of its weight of sulfur for 40 minutes on cold rolls had combined 1.86 per cent of sulfur after 2'/2 hours' cure a t 140' C. A similar mixture produced in a short time on hot rolls combined only 1.72 per cent of sulfur. This indicates that an increase of 8 per cent in the rate of combination has been caused by milling, although the physical state of the excessively milled rubber would indicate that no cure had taken place. Axelrod ( 1 ) showed a similar increase after the rubber was plasticized by heat. Conclusion
The data which have been presented indicate that elasticity is a property of rubber which persists through vulcanization and is not created to any great extent. The physical state of the rubber after any period of cure depends on the balance between suppression of plastic properties either directly or indirectly through the influence of combination of sulfur, and the creation of plastic properties through the influence of heat. It is probable that sulfur also combines a t a slower rate with the elastic portion of the rubber and finally results in a product of almost no mechanical strength. The transformation of energy by rubber depends upon the condition of the rubber as well as on the condition of the test. Heat liberated by elastic strain can be transformed almost quantitatively into mechanical work. Heat due to internalfrictional resistance which will be caused by both plastic and elastic flow is not reversible. Any reduction in time required for the transfer of energy reduces the amount lost through plastic flow. An increase in temperature increases the resistance of the elastic portion to strain, but also reduces the resistance to flow of the plastic portion. Sufficient increase in temperature causes a large energy loss due to a considerable but limited permanent flow of the rubber. This cannot be plastic flow of the elastic portion, since it is limited in extent, but can better be explained by some mechanical change such as a breaking of the anchorage between portions of the plastic material and elastic network. Literature Cited (1) (2) (3) (4)
Axelrod, Gummi-Ztg , 24, 352 (1909). Partenheimer, INO. ENG.CHEW, 20, 1245 (1928). Somerville, Rubber Chem Tech, 2, 1 (1929). Van Rossem and van der Mei~den,I b i d . , 1, 393 (1928).
An All-Glass Evaporator' D. F. Othmer EASTMAN KODAKCOMPANY, ROCHESTER. N. Y.
N T H E preparation of various fine chemicals, the processes often involve concentration by the evaporation of water, acid, or organic solvent from an acid solution. Usually the corrosion of equipment is secondary to impairment of product when ordinary types of evaporative equipment are used and the only recourse is to large glass or ceramic vessels heated externally. An experimental evaporator constructed of glass and stoneware is described by Badger and Cutting (1). It is too large for many small manufacturing operations, requires the molding of special stoneware shapes, and is built more for experi-
I
1 Received
April 12, 1929.
mental studies than for plant use. The one to be described is less complicated, less expensive t o build, and more simple t o operate. Furthermore, all rubber sleeves and gaskets have been eliminated and the material to be concentrated touches nothing but Pyrex glass. It has been used for some time as a general-purpose small evaporator for the concentration of various corrosive and non-corrosive solutions. Description of Apparatus
Standard Pyrex tubing is 91.4 cm. (36 inches) in length and the evaporator shell is constructed with tube sheet faces 76.2 cm. (30 inches) apart. This necessitates a large number of
I N D U S T R I A L A N D ENGINEERING C H E X I S T R Y
September, 1929
tubes for a given area of heating surface, but eliminates special tubing or welds in the steam space. The seven 2.5-em. 0 . d. Pyrex glass tubes with 2-mm. walls are held with asbestos pncking secured by a gland screwed into a packing box drilled and tapped in the thick tube sheet. The body is of 20.3-cm. (&inch) iron pipe, with flanges screwed on and bolted t o the tube sheet. (Tube sheets welded to the pipe would be a simpler construction.) The return bends are made and welded to the tubes assembled in place,* or if rubber is not objectionable the straight tubes and return bends may be connected with sleeves of rubber over tubing covering a butt joint. The thick liquor trap and receiver is constructed of two 5liter flasks welded together with insealed inlets and vapor and liquid outlets. The condenser, which is immersed in an
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returned through the second tube, which was only slightly lower, and so worked its way successively back and forth until it was discharged from the seventh tube, which was the last and lowest. The liquid was preheated in the first two tubes. Evaporation usually commenced in the end of the second or the beginning of the third, but "film evaporation" was most evident in the last tubes. The mixture of concentrating liquor and vapor passed through the return bends and tubes a t a very high velocity with the vapors traveling in the center of the tube and the liquid thrown against the wall in R spray and film. Usually a saturated solution a t the exit temperature was desired, and the control to obtain such a concentration was made very simple by the appearance of a slight fog of very tiny crystals in the last return bend if the concentration was carried too far. These crystals showed no tendency to scale; the velocity was far too great to allow of such formation. At their appearance the feed was slightly increased to such a point that the evaporator discharged a saturated but saltfree liquid. The 6-mm. cock between the two flasks of the thick liquor trap was normally open, and the lines to the ejector, the bottom outlet, and vent were closed. When the lower flask was filled with liquid, the middle cock was closed and the vent and discharge cocks open. These last two cocks were closed when the concentrate was discharged and the line t o the ejector opened t o evacuate this receiver before i t was again put in service. Advantages of This Evaporator
,#bZ
iron tank of water, consists of nine turns 55.9 em. (22 inches) in diameter of 2.5-cm. tubing terminatingin a bottom outlet. This condensW ing coil was originally used for another purpose and is several times as large as is needed for this evaporator. The condensate receiver is a standard 22-liter flask fitted with a two-holed rubber stopper. When comparatively large amounts of solution were to be evaporated, two of these flasks were connected in parallel with glass plug cocks suitably arranged so that one was being emptied while the other was filling, An alternate method which has proved advantageous for continuous operation is t o have the condensate receiver made of two smaller flasks blown in one piece and similar to the thick liquor rweiver. The steam e,iector is of brass, although iron, stainless steel. arid glass exh:tusters are also available.3 A vacuum of 55 to 60 cm. of mercury was usually maintained on the manometer connected to the vapor line (not shomi in the drawing).
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Operation
The system was first evacuated with the thin-liquor valve closed. The thin-liquor storage bottle was filled, steam adjusted t o the desired pressure, and feed cock regulated so that a thin stream came through the insealed sight feed. The common practice was to set the steam a t 80 to 90 pounds (5.6 to 6.3 kg. per sq. cm.) and introduce the feed a t about 50" F. (10' C.) Even this sudden temperature change-over 175" F. (97' C.) difference-never caused tube breakage, and thermal strains such as are encountered have no effect on any of the Pyrex apparatus used. Liquid passed through the first and uppermost tube, then 3 Glass work, except the condenser which was made by Corning Glass Co.. Corning, N. Y.,was initially done by Technical Glass Co., Rochester, N. Y. Subsequent welding of tubes due to changes in arrangement was satisfactorily done b y one with little experience in the technic of glass blowing. The operation of welding Pyrex tubing in place is readily mastered. Supplied by Schutte & Koerting Co , Philadelphia, Pa.
'
One of the principal advantages of this type of apparatus is that concentration a t a low temperature is accomplished owing to the vacuurn maintained on the system. This allows the evaporation of many delicate solutions which are injured by heating to the atmospheric boiling point. Another point in favor of vacuum evaporation is the large temperature drop which it allows between steam and liquid. The capacity of this small unit is enormous owing to this large temperature difference, and under favorable conditions, such as the feeding of a hot dilute thin liquor, evaporation u p to a pound (0.45 kg.) of water per minute has been accomplished. Organic solvents may be removed much faster, depending on the latent heat and boiling points. The limit is set by the maximum desirable speed of discharge from the last tube, which will be roughly shown by the vacuum obtainable if other conditions are constant. With 1pound (0.45 kg.) of water being evaporated per minute, the exit velocity through a 1.9-cm. (3/4-in~h) i. d. tube a t 50 em. vacuum is about 120 meters (400 feet) per second. Even such a small vapor separator as the top 5-liter flask is sufficiently large to separate vapor from thick liquor and only the most careful analysis will indicate entrainment. In the concentration of one very concentrated solution, the entrainment was calculated from analysis of thick liquor and condensate to be approximately 1 part of liquid in 25,000 parts of vapor by weight. The upper flask is evidently a very good entrainment separator, and its efficiency depends on the difference in the inertia of vapor and liquid a t the very high velocities encountered. The simplicity of operation of this equipment is indicated by the fact that it may be put in operation with a given solution and be discharging vapors and concentrate into the vapor separator in less than 3 minutes and may be shut down, emptied, washed with tap and distilled water, and ready for operation with another solution in less than 5 minutes. Literature Cited ( 1 ) Badger and Cutting, Trans. A m . Inst. Chem. Eng., 1926, 3 (advance COPY).
