Development of Pilot Plant Control for Milling Guayule Shrub d
U
K. W. TAYLOR AND R. L. CHUBB U.S . Natural Rubber Research Station, Salinas, Cali,f.
P
ILOT plant developments usually serve as a transitional step between the procedures developed at the laboratory bench and the ultimate commercial processes. In process research for the recovery of rubber from guayule, however, the pilot plant frequently supplants the laboratory because of the nature of the equipment and the character of the operations. It therefore became necessary to design the work so that the maximum usable information could be obtained with minimum cost and effort. It was immediately obvious that some method of recovery of rubber from guayule shrub needed to be developed which would provide criteria by which the effect of variations in treatments, machines, or methods could be judged. HISTORICAL
The U. S. Natural Rubber Research Station was established in 1947 to carry out investigations on a natural rubber which can be produced in the United States, and which would be available in the event supplies from the Far East were curtailed. The work reported here concerns guayule, but is applicable to other process development. It had been generally contended that guayule (Parthenium argentaturn, Gray) ( 7 ) shrub direct from harvesting could not be milled without loss of rubber unless the latex were first coagulated by some conditioning process or by the addition of acid or other chemicals during the milling (1, 6, 8, 1 2 ) . I n the work contemplated neither of these means was desirable because of the possible effect on the rubber hydrocarbon. It therefore became necessary to develop some form of control milling which would not only provide the maximum reproducibility, but would also produce criteria by which the results of various experiments could be judged even though some of the rubber were lost in the process.
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quently inadequate or improper sampling confused the interpretation of the experimental data. The vagaries of the weather during the period of field cure and the biologic nature of guayule were often overlooked. Consequently, unknown factors influenced the data and could not be taken into account in interpreting the information obtained. These inadequacies led to the conclusion that the objectives of current process research could best be met by the application of experimental designs and statistical treatments of the resultant data, a s have been used by the biological scientists with such success during the past three or more decades. Accordingly, a series of experiments was carried out to develop a control system of milling whereby the degree of reproducibility would be known and where the factors which caused uncontrolled variation could be segregated and their effects measured (2,4, 6). MATERIAL AND EQUIPMENT
Dryland guayule variety 593, grown in the Salinas Valley, Calif. was used throughout the course of the work here reported, The shrub was slightly over four years old, a customary age for commercial harvesting (11). Pilot plant equipment included a steam-jacketed vessel for parboiling, a screened trommel defoliator, an ensilage-type cutter, a rotary fly-knife cutter, corrugated crushing rolls, a small hammer mill, Jones binary sample divider, a rotary replicate divider, a battery of four 27-inch batch pebble mills, two hydraulic pressure decorking vessels called “pailas” (S),a single 27-inch batch pebble mill used as a scrub mill, a bottom-driven basket centrifuge, a through-circulation steam-heated drying oven, moisture tellers, thermometers, balances, and other minor pilot plant equipment. The pebble mills, pailas, and scrub mill are shown in Figure 1. In the field phases of the work, two mattocks and a pickup truck were all that was required. CONTROL MILLING PROCEDURE
As a result of this series of experiments, the following detailed NEED FOR CONTROL METHODS procedure for control milling has been established a~ standard. A block of plants in the field from which the material is to be Considerable work has been done in the past on what were used for the experiment is selected in which the plants appear to called cpntrol millings in the production of solid or ‘‘worm” be uniform as to plant size, vigor, and growth characteristics. This block is rogued to remove all plants that are off-type rubber from guayule. (Rubber from guayule is usually recovered when the experiment to be run as small pelletlike aggregates involves the testing of differconventionally referred to ent types of machines or difa b L‘worms.” This is classed ferent operative procedures. as recovery of rubber in solid When the experiment is of such nature that it takes field form as contrasted to the revariability into consideracovery of rubber as latex, tion the plants are selected by which has not proved comrandom number. I n either mercially feasible with case, the plants required are harvested and delivered to guayule.) Critical examinathe pilot plant as soon as postion of this work indicated sible after digging. almost consistent failure t o The plants are then parmaintain total rubber and boiled for 15 minutes a t a temperatureof 20Ooto205”F. solids balances throughout and then passed through the any given experiment, and trommel f o r d e f o l i a t i o n . there was rarely any deter(Parboiling is done primarily mination of the reproducito facilitate defoliation, but it is also advantageous in that bility of any method. Hardly i t removes adhering soil and ever were controls run in exinitiates coagulation of the actly the same way on raw Figure 1. Battery of Batch Pebble Mills, Flotation latex.) When defoliated, they Tanks, and Pailas Used in the Pilot Plant m a t e r i a l s t h a t had been are cut in a fly-knife cutter through a llrinch screen. handled exactly alike. FrePailas are t h e two small vessels to the right of switchboard panel
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Samples are taken fiioin the cut shrub for analytical determinations of rubber (IO),resin, and moisture. The remaining shrub is crushed twice a t a roll clearance of approximately O.OO2-inch, then hammer milled once through a 1/2-inch screen using straight hammers. After hammer milling, the material is divided into the required number of replicates. Each replicate is immediately sampled for an analytical moisture determination, and a rapid moisture determination obtained on a Dietert moisture teller. As soon as the results from the rapid moisture determination are available, the mill charges are weighed out on a dry weight basis according to the volume of the mill in which each replicate is to be processed. Each mill charge is labeled and placed in a covered can. All control charges are held overnight before milling.
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1and at a minimum temperature of 140 O F., after which the rubber is skimmed onto drying trays and spread to a uniform depth. The addition of preservative or antioxidant to the rubber at this point is governed by the nature of the experiment. The rubber is dried in a through-circulation dryer with gradually increasing temperatures ( 1 1 ) as follows: Initial temperature of 100" F. for 7l/2 minutes; 125' F. for 7l/2 minutes; 150' F. tor 71,l2 minutes; 175' F. for 7'/2 minutes; and 200' F. for 30 minutes. The trays of rubber are then weighed, replaced in the dryer at 200' F. for 5 minutes, then weighed again. This is repeated until a constant weight is reached. When dry, samples for chemical analysis are taken, and the total weight of the crude rubber is obtained. To determine rubber losses in process, the liquor from the p i imary flotation tank is passed through a perforated basket-type centrifuge. The flotation tank and the pipe lines are thoroughly washed out, The centrifuge is allowed to run for 5 minutes after the final wash water has passed through in order to reduce the bagasse to an approximately uniform moisture content. The wet weight of the bagasse is obtained and three 1-quart representative samples of bagasse are taken for the analytical determination of rubber, resin, and moisture. The effluent from the centrifugation is stored in a tank and the volume determined, after which it is agitated violently with an electric mixer and, while the agitation continues, three 1-quart samples are removed for analysis for rubber resin, and total solids. All flotation temperatures and ratios are held within the stated limits in the interest of uniformity in procedure, but are not necessarily optimum or applicable to factory operation. EXPERIMENTAL WORK
Figure 2.
Rotary
Sample
Divider
for
Homogeneous Lots of Shrub
Obtaining
The replicates are milled in 27-inch batch pebble mills. hlilling is accomplished a t a water-to-solids ratio of 4.5 to 1 and a pebble-to-solids ratio of 20 to 1. Mills are run a t 80% of their critical speeds and the initial mill temperature is 85" F. Milling is carried out for 60 minutes. Between 10 and 15 minutes after milling is started the mill is stopped, the lid is removed, and the shrub that becomes packed between mill and lid is carefully replaced in the mill. The lid is then replaced and milling continued. A record of the time for lid removal and replacement is kept so that the exact milling time of 60 minutes is maintained. At the end of the milling period the mill is filled approximately half full of water and the contents, except for the pebbles, discharged into the flotation tank. The mill is rinsed with copious amounts of water 5 times. On tho fourth rinse the mill lid is replaced and the mill is run for 5 revolutions before it is emptied. When rinsing is complete, hot or cold make-up water is added to the flotation tank until a water-to-solids ratio of 100 to 1 at a temperature of 100" F. is reached. When the correct flotation ratio and temperature are attained the floating rubber is skimmed off. When the initial skimming is completed, the flotation liquor is agitated with an electric mixer for 5 minutas, allowed to settle for 5 minutes, and then skimmed again. This agitation and settling process is repeated once, followed by a third skimming, The crude rubber and cork recovered by skimming are held in appropriate sample cam in clean warm water. If it is necessary to hold the rubber for any length of time before decorking is done, a hold-down cover is placed on the rubber to keep it submerged. Decorking is done in a paila preheated to 200' F. before the rubber is placed in it. The rubber, with about three times its volume of water, is introduced into the paila and brought to a rolling boil. Immediately after boiling, the rubber in the paila is placed under hydraulic pressure of 500 pounds per square inch for 90 minutes. This treatment collapses the cells of the cork granuules and renders them susceptible to waterlogging. After the decorking treatment, the rubber is discharged into a flotation tank a t an approximate water-to-solids ratio of 400 to 1 and a minimum temperature of 140" F. The rubber is skimmed from the flotation tank and held in warm water for scrub milling. The 27-inch scrub mill is preheated to a minimum temperature of 140" F. The rubber charge is then placed in the mill with a pebbles-to-solids ratio of approximately 20 to 1. Water is added and steam is introduced to the scrub mill until the charge is brought u p to a minimum temperature of 140' F. and a water-tosolids ratio of a t least 20 to 1. Scrub milling is done for 15 minutes. Then the mill is emptied and rinsed thoroughly. The rubber is floated a t an approximate water-to-solids ratio of 400 t o
In the evolution of the detailed procedure just described, it was necessary to run a number of experiments. Each successive experiment was the outgrowth of study of the data obtained from the preceding ones. Discussions of those of major importance follow: SHRUBS.4MPLING FOR CHEM~CAL ANALYSIS. It has been CUPtomary to riffle a mass of comminuted shrub through a Jones binary sample divider t o secure samples for chemical analysis of rubber hydrocarbon, resin, and moisture. I n some of the preliminary investigations there appeared to be some question whether representative samples could be obtained after the shrub was cut, crushed, and hammer milled. It was believed that crutching brought about some agglomeration of the rubber and resin S O that the chances of obtaining erratic samples were increased. To determine a t what point sampling would be most accurate, approximately 100 shrubs were harvested and prepared through cutting as described in the control procedure. The cut shrub was then divided into 16 replicates Eight of these replicates were sampled at once for chemical analysis, after which they were crushed twice, hammer milled once, and again sampled for chemical analysis. The other eight replicates were crushed six times, hammer milled once, and sampled for chemical ,analysk. Sampling was done by splitting each replicate through the Jones divider to a 1-quart sample. The results of this study are shown in Table I. For both rubber and resin the standard deviation increases with crushing. This inaccuracy of sampling was borne out in the
TABLEI. EFFECT OF VARIOCSTYPESOF COMMINUTIOX QN ACCURACY O F SAMPLING FOR CHEMICAL ANALYSIS O F GUAYELE SHREB Type of Comminution
Cut only
mushed twice, hammer milled once
Cut crushed' s:x times, hammer milled once
14.0 0.4
13 6 0.5
13.3 0.6
7.3
7.3 0.2 45.2 1.0
7.0 0.4
Cut,
Analytical Results Rubber, standard devietion, Resin, standard deviation,
Yo"
yoa
Moisture, standard deviation,
%"
0.1
45.2 0.6
a Values given are means of eight replicates.
42.9 0.5
A
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
analytical laboratory where small pellets of rubber were observed t o roll off the spatula when the final 2-gram sample of shrub Wac; weighed out. It is obvious that the most accurate results may be obtained by sampling after cutting and before crushing. In conjunction with the development of sampling techniques and the attainment of a set of procedures which would permit the maximum effective use of pilot plant facilities and personnel, it was necessary t o cut hand operations t o a minimum. Accordingly, a dividing mechanism was fabricated which ~ o u l dsplit a mass of shrub into eight approximately equal partB. The ‘divider consists of eight compartments or boxes with a common base so arranged that they can be rotated a t a constant speed by an electric motor and gear reducer. The outer end of each compartment is arranged to lift out for ready removal of the shrub after division. The cut shrub is fed into the eight compartments by utilizing an inclined screw conveyor 6 inches in diameter, fitted with a feed hopper, and electrically powered with a variable speed drive. After final adjustments in speed of rotation and rate of feed, it was found that more uniform results could be obtained than could be obtained with the Jones divider previously used. Figure 2 is a photograph of the finished rotary divider.
DRYWEIQHTCHANGES IN SHRUB.During some of the earlier experiments it was observed that the second set of four millings gave higher rubber hydrocarbon yields than did the f i s t four, although all eight were milled on the same day. To secure moisture determinations, make the necessary calculations, weigh, and run t h e first four replicates through the mills required about 4 hours. A t the end of this time the second set of mill charges was weighed aut. It was possible that the 4 hours’ difference between the two sets .of millings might result in more coagulation of rubber. There was, however, another possibility. It is known that parboiling does not kill all the cells and that there are losses in dry weight due t o respiration and microbiologic activity. It seemed that the dry weight losses might occur more rapidly than had originally been believed. If they did, then it appeared that the last four mill charges weighed out might represent more of the original dry weight wit4 no reduction in rubber; hence, there would be more wbber per mill charge t o recover. An effort was made to determine the facts in a separate experiment. In this experiment eight replicates were prepared as for milling. Moisture samples were taken and the dry weights established. T h e dry weights were again determined after 4, 20, 28, 44, 52, 116, 124, and 164 hours. The mean dry weight values for each period of observation are recorded in Figure 3. It is unique that u p t o 20 hours the prepared shrub appears t o gain weight rather than lose it. The individual dry weights showed a decided variation a t 4 and 20 hours. This variation increases with increasing time, but since the mill replicates in the various experiments were weighed out during a period of 4 to 20 hours after the shrub was prepared, it is reasonable to assume that in no caae were the dry weights of the mill charges the same as they would have been if weighed out immediately after preparation. The samples for chemical analysis for the various mill runs were taken as soon as shrub preparation was complete and the yields were computed on t h a t basis. If the dry weight changed between the time the sample was taken for chemical analysis and the time the mill charge was weighed out the yield figures would be erroneous. The reason for the gain in dry weight for the &st 20 hours is not apparent. It may be that oxidation of certain of the plant materials occurs rapidly enough t o overshadow the normal losses due to respiration and microbiotic activities. Whatever the reason for t h e variations, it is great enough t o necessitate weighing out the Teplicate mill charges as soon as possible after shrub preparation. INEQUALITIES IN MILL YIELDS. From analyses of the data obtained in preceding experiments certain factors, which previously b a d been considered of minor importance or were overlooked, now loomed up as contributors t o erroneous results. For instanoe, all four pebble mills are 27-inch batch mills, and it was assumed that their performance characteristics were com-
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TABLE11. Rr HBER HYDROCARBON RECOVERIES FROM MILLS CHARGED TYITII LQUAL SHRUB AND PEBBLE LOADSAND OPERATED AT THE SAMESPEEDSAND WATERRATIOS hIill Surnber 1 2 3 4 Q
Rubber Hydrocarbofi Recoverp, % 92.5 90.8 94.1 95.5
Values shown are meam of four replicates.
TABLE111. RUBBER HYDROCARBON R~COVERIES FROM THE FIRSTFOURAND LASTFOUR I N A SERIES OF EIQHTMILLINGS % ’ Recovery Mihings Mill No. 1 Mill No. 2 Mill No. 3 Mill No. 4 lstfour 89.6 85.7 89.0 91.0 2nd four 92.4 89.9 91.8 92.8 Difference 2.9 4.2 2.8 1.8 Mean difference 2.92 Standard error 0.492 Observed t value0 5.945 Student’s 1 teat for significance ( 9 ) . t value required for significance a t 1% level .= 6.841.
parable. Examination of the data indicated that mill performance was not exactly the same when mills were run a t the accepted optimum speed of 80% of the critical speed, and when shrub loading, water ratios, and other factors w e e identical. The data from the first and second experiments were combined and subjected to an analysis of variance (9). Average mill performance is shown in Table 11. The differences in these yields are statistically highly significant ( F > 0.01 level of probability). An investigation of the mills revealed slight differences in volume. Shrub and pebble loadings were changed to conform to exact mill volume in subsequent experiments. At this point, without going into the details of these experiments, it can be said that the changed loadings decreased but did not eliminate the differences in yield due to mill performance. It is probable that the remaining differencesbetween mills are due t o slight variations in pebble size, degree of smoothness in mill linings, or other factors which cannot be controlled readily. However, it is possible as a result of this investigation to take this factor into consideration when analyzing treatment differences in other experiments.
__ 24
48
72
--FA 96
120
144
-
166
nourn
Figure 3. Change in Dfy Weight of Guayule Shrub over a Period of 160 Hours In all the experiments where eight replicates were used there occurred the baffling situation of increased rubber hydrocarbon yields irom the last four replicates milled. Even when the procedure had been refined to the limits of practicability, this difference persisted as will be seen by scrutiny of Table 111. The increases in yield are real-that is, they are not due to chance although the phenomenon cannot be ascribed t o any known cause at present. As in the case of the variation among mills, the difference in yields between first and second millings can
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IN D,US 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
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a lesser degree, for thc rosins. Accuracy of accountability for total solids is influenced 111.t,hc Total Solids, Grams Rubber Hydrocarbon, Grams Resin, Grains __ ___ method of sampling, and the reFlotation Bagasse Flotation Bagasse Flotation Baeasse sults of sampling thc ontire liquor, plus liquor, pius liquor, i,ius Values not effluent, not effluent, not effluent, f l o t a t i o n l i q u o r a r ( >m o r e Obtained centrifuged centrifuged centrifuged centrifuged centrifuged centrifiiged erratic; hence centrifugation is Alean0 3541 3844 72.9 88.0 164.6 181.2 175 Standard deviation 8.6 365 8.8 12.1 13.4 judged more reliable. ... $303 Mean difference ... +I5.0 $16.5 The losses of rubber, resin, Standard error for paired samples . . . 149 ... 3.06 5.96 and total solids in the paila and 2.034 Observed t values ... 4.909 2.774 PC '. 0 . 0 5 P ='o.ol 2.447 3.707 P = 0.05 Required t value 2.447 scrub mill flotation tanks have Mean values for averages of three observations for each of eight ieplicates been nonexistent or so s m ~ l l b Student's t test for significance. that, they need not be considC Probability level. ered when interpreting the rcsults of an experiment. I t is necessary to takc the be segregated by proper experiment,al design and consequently hardncss of the process water into account when cornputing mxtedoes not preclude accurate interpretations of data from othtr exrials balances. Without deducting these solids, the tot,al accountperiments. ability runs about 470 higher than t,he values shown in Table IV. SAMPLING FOR MATERIALS BALANCE. One of thk first problems SAMPLING C R ~ DRUBBER. E Obtaining rrpresentativc samples encountered was that of taking sufficient samples a,long thc procof the crude rubber presented virtually no problem. When the ess line to permit total accountability of rubber, resin, and other trays of dried crude rubber are removed from the drier nlqirosiplant materials. It is apparent that much of the rubber and mately 50 grams are rcinoved for chemical analysis. Samples resin, together with a small amount of other plant parts can be taken show remarkable uniformity and accmracy, and it is noi. accounted for in the crude rubber. However, it is important to considered necessary to elaboratc here on this topic. know the total distribution of these components throughout a ACCOUKTABILITY O F R E S I K , RUBBER, A N D T O T A L S O L I D S milling system, and in the case of rubber hydrocarbon, this knowledge is imperative. It is considered that the objective of setting u p a control mothod Most of the plant constituents are in t,he primary flot,atioii of milling has been met as well as possible with the available liquor after the rubber has been skimmed off. I t had been beequipment. As a find check on thc accuracy of the method, 230 lieved that this liquor could be vigorously agitated and sampled plants were harvcsted and milled in the manner outlined in the while in agitation. The method showed rather high variabilitj-, procedure section. The rcsults of this work are summarized in however, and a better means was sought. Table V.
TABLEIQ. COMPARlSOS OF FLOTATION LOSSESAS FLOTATION LIQUORAND SEPARATE SAMPLISG
OBT.4INED FROM SAMPL1h.G \\-HOLE O F B.4GASSE .4NU 1':FFLUENT
Q
A basket-type centrifuge equipped with a perforated removable basket lined with 50-mesh stainless steel screen was installed in the process line. After primary flotation was completed, the flotation liquor was pumped into the centrifuge basket where most of the suspended solids could be retained. The effluent containing the water solubles and finely divided solids was pumped to a tank where it could be held for sampling. One hundred and twenty-five plants were harvested and processed as eight replicates. After the crude rubber was skimmcd from the primary flotation tank, the flotation liquor for each replicate was agitated violently with an electric mixer. While in agitation, three 1-quart samples were dipped out and analyzed for rubber, resin, and total solids. Then the flotation liquor was centrifuged as described and the tank and pipelines were thoroughly washed. The centrifuge was run for 5 minutes after the final wash water had passed through to reduce the bagasse to uniform moisture content. When centrifugation was complete the tared basket was removed and weighed, after which the bagasse cake was sampled by cutting through the cake and removing a complete section. Three 1-quart samples were removed in t,his manner and analyzed for rubber, resin, and moisture. The effluent from the centrifuge had its weight determincd volumetrically. It was then agitated with an electric mixer and three I-quart samples were removed for analysis of rubber, resin, and total snlids. When the analytical data were complete, a statistical analysis was made of the averages of the three values obtained for the flot,ation liquor from each replicate and the combined average values for the bagasse and effluent. This analysis is summarized in Table IV. Inspection of the values for the standard deviation indicates that for rubber and resin one method of sampling is as good as the other so far as reproducibility is concerned. For total solids, mmpling the bagasse and effluent after centrifugation gives more consistent results than does sampling the whole flotation liquor. However, the accuracy of accountability for rubber and resin is markedly influenced by the method of sampling. The statistically highly significant t (9) value of 4.909 for rubber hydrocarbon makes it obvious that the flotation liquor must be centrifuged for reliable estimates of the rubber lost. This is also true, though to
T A B L E I-.
RESIS,RUBBER, ASD T O T A L SOLIDS /iCCOUNT.LBILITYn
Rubber Resin Hydrocarbon 'Total Solids l l a t e r i a l Analyzed Distribution, % Distribution. Dhtrihrition, % ( h i d e rubber, stand52.7 90.1 18.1 ard deviation 1.50 2.30 12 13agase and effliient 3L6* 11 3 * 82 (j:,: liquor, standard 0.96 1 20 2 (10 de\-iation Total accountability. S8.3* 101.8* 100.7:3 standard deviation 1.84 1.30 2.70 a All d a t a shown are mean valiies for eight replicates sxcci't for tliobe marked witli asterisks (*) where only seven obnervations could ba used. The rubber a n d resin Yalues are expressed as a percentage of t h a t found in the shrub by chemical analysis, while total solids are expressed a s a percentage of the dry weight of the shrnb milled. Bagasse a n d effluent were analyzed scuaratelv. h u t the valiieb mere combined for accountabilitv purposes
Total solids balance can bo maintained with very reasoiiablc experimental error. Table 1' shows a high degree of reproducibility in rubber balances and also reflects good total accountability. There is a, strong negative correlation between the t w o components of the "total rubbcr." As an individual yirld of rubber hydrocarbon in t'he crude iiicreases, the corresponding amount of rubber hydrocarbon in waste products decreases. Accounting for all the resins has been consistently difficult ant1 Table i7 shows little improvenicnt in this regard. The error is fairly constant, howevcr, and the reproducibility is fair. S o specific reason is advanced for the low accountability of the resin, though it may bc speculated that a portion of thc rcsinoua material is hydrolyzed in the effluent liquor and cannot b(x accounted for analytically. Fortunately, accurate resin balances are not so vitally important, as rubber balances ; hence they n c ~ d not be considered further a t present. REPLICATES REQUIRED.In addition to the ilevelopmcnt of a controlled method of milling, it v a s the aim of t,his study to determine approximately the number of replicates required for future work. The data acquired show that when eight replicates are
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I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
April 1952
used a difference between means in rubber hydrocarbon recoveries of about 3y0 would be required for statistical significance a t the 5y0level. If only four replicates were used, and it is assumed that the standard deviations were the =me, then a difference of about 5Y0 between means would be required to establish significant differences at the 5% level. Inasmuch as most pilot plant work will o hydrocarbon probably be done in the range of 90 to 1 0 0 ~ rubber recovery, eight replications should be used to establish the effect of various treatments. CONCLUSIONS
These experiments are believed to demonstrate, though in a limited way, the applicability of biological experimental design and statistical analysis to process development research. Utilization of these principles enables the researcher to speak with more accurate knowledge of the meaning of his results, and a t the same time permits the elucidation of factors bearing on his work which might require years of trial and error experimentation to uncover. They emphasize the necessity for elimination of human bias wherever possible, and accentuate the fact that the nontechnical workers (who must be relied upon in much pilot plant research) must be very carefully trained in each operational step to achieve the maximum accuracy of results.
887
ACKNOWLEDGMENT
The authors are grateful to I. C. Feustel for his encourngement and leadership in this work, to the analytical section of this station-particularly to R. V. Crook and W. J. Gowaiis for running the chemical analyses, and to E. C. Taylor for statistical analyses. LITERATURE CITED
Allen, P. J., and Emerson, Ralph, IND.ENQ.CHEM.,41, 346-65 (1949). Bainbridge, J. R., Ibid., 43, 1300-6 (1951). Gumming, J. M., and Chubb, R. L., %hem. & Met. Eng., 53, 1256 (1946). Gore, W. L., IND.ENG.CHEM.,42, 320-3 (1950). Huhndorff, R. F., Ibid., 41, 1300-3 (1949). Jones, E. P., U. S. Patent 2,434,412 (Jan. 13, 1948). Lloyd, F. E., Carnegie Inst. Wash. Pub., No. 139 (1911). Nishimura, M. S., Hirosawa, F. N., and Emerson, Robe1t , IND. ENG.CHEM.,39, 1477 (1947). Snedeoor, George W.., “Statistical Methods,” 4th ed., Ames, Ia., Iowa State College Press, 1946. Spence, D., and Caldwell, M. L., IND.ENG.CHEM.,5, 371 (1933). Taylot, K. W., Econ. Botany, 5, 255-73 (~$951). Tint, H., and Murray, C. W., U. S. Patent 2,459,369 (Jan. 18, 1949). RECEIVED for review February 9, 1961.
ACCEPTED October 31, 1951.
Moisture in Oil-Treated Insulation FRANK M. CLARK General Electric Co., Schenectady, N. Y.
T
HE efforts of the engineer to obtain and maintain electrical
equipment in a moisture-free condition are severely handicapped by the chemical properties of the insulations which are present. Since the essential dielectric properties of the insulating materials are adversely affected by the presence of moisture, the solution of the problem may appear to lie in the exhaustive drying of the equipment during its manufacture. It has been demonstrated, however, that the excessive drying of the cellulose insulation results in the chemical degradation of the cellulose, leading to acid formation, gas evolution, increase in dielectric loss, and a decrease in the mechanical and dielectric strength of the insulation
(4,8). On the other hand, it may be suggested that the moisture problem may be alleviated by the selection of insulating materials having the least ability to attract and to hold moisture. Chemical and dielectric considerations, however, frequently prevent the engineer from applying the materials having the greatest water repellency. For example, the ability of mineral oil to dissolve moisture decreases with the increased severity of its refinement, and such a suggestion would lead to the application of highly refined mineral oil as the preferred impregnant for cellulose insulation. It has, however, become widely recognized that the chemical and dielectric instability of the highly refined mineral oil presents a major hazard in itself to the safe operation of oilfilled equipment in which such oil may be used ( 7 , 9 ) . Laboratory studies and practical experience unite in the conclusion that moisture in the insulation of electrical machines presents a dielectric hazard which is reflected in decreased dielectric strength, increased power factor, increased chemical deterioration of the cellulose and the mineral oil and, in general, a thoroughly undesirable chemical and electrical condition. Laboratory studies and practical experience also indicate, however, that the approach to the moisture problem must be made with full recognition of its complexity. The type and design of the equipment, its susceptibility to heat accumulation, its rated
.
voltage and the voltage stress on the insulation during operation, the temperature range of operation, and the possibility and degree of hazard presented by the oxidation and chemical deterioration of its insulation are engineering factors which must ultimately determine the moisture limitations necessary for the safe operation of the equipment and the type of test best suited for its evaluation and control. EXAMINATION OF T H E MOISTURE PROBLEM
Both dry mineral oil and dry cellulose will adsorb or dissolve moisture from the surrounding medium, whether this medium be gaseous or liquid. Given sufficient duration of exposure, a n equilibrium condition will be established between the oil or cellulose and the surrounding moisture-containing medium. T h e equilibrium conditions established in mineral oil which is in contact with air of varying degrees of humidity are illustrated in Figure 1 (6). Figure 2 illustrates typical data which have been reported for the moisture content of oil-free cellulose in contact with moist air. Because of its hygroscopicity, cellulose has been described as a n excellent dehydration agent. Immersed in mineral oil, cellulose will adsorb moisture from the oil and establish a moisture equilibrium with the oil (Figure 3). It has been suggested (16) that the presence of the oil medium merely slows up but does not change the ultimate moisture equilibrium of the cellulose-humid air system illustrated in Figure 2. This suggestion, if true, appears to be of little practical value because of the slowness with which the moisture equilibrium is established. The moisture content of vacuum-dried and oil-impregnated kraft paper immersed in mineral oil whose surface is exposed to air with a relative humidi6y of 70%, approaches its equilibrium conditions very slowly, and the moisture content of the oil-immersed paper after long periods of exposure is lower (approximately 1.1%moisture) than the moisture content (9 to 11%) which is established when
