Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1927.Includes bibliographical references (leaves 20-21).by David A. Shepard.M.S

Shepard, David A

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AFPCHIVESA STUDY OF SOI~E CHARACTERISTICSOFSUGAR DUST.A THESISSUBMITTED TO THE FACULTYOF THEMASSACHUSETTS INSTITUTE OF TECHNOLOGYIN PARTIAL FULFILLMENTOF THI REQUIR~EMENTS FOR TrE D-:GREE OFMASTER OF SCIENCE.SUBMITTED BYAPPROVED BY. .. ... .. • . . ..||In ack.no..1ledement for the :--nsrationand assistance of Professor Harold C. ,Weberza.nd tjae helfullness ac nd. o-operation of TheRevere Sugar Company, without which this workwould not have been possible.__________ __ ___ ______ -9I rDEXTITLE PAGESTJJECT . . . . . . . . . . . . 1OB CT . . . . . . . . . . . 2INTR-ODUCTIO . . . . . . . . . 3PPVIOUS WORK . . . . . . . . . 7SUT.'ARY OF PROCEDURFE . . . . . 9RESULTS . . . . . . . . . . 9APPENDIX . . . . . . . . . . .10~TMHOD OF PROCEDURE . .11SIKETCII OF APPARATUS . . . ... 13DISCUSSIOYN OF E XPrERIT =TATION 17DATA . . . . . . . . . .. .. 19BIBLIOGRAPfHY ....... 20m L-SUBJECT:Tie subject of this thesis is:"A Study of Some Characteristies of Sugar Dust."OBJECT:The object of this thesis is to study thecharacteristics of sugar dust with a view to examiningits settling properties and to determine whether ornot the dust behaves as a mist.INTRODUCTION:The refining of raw sugar in the UnitedStates is carried on with a very small margin ofprofit. Each operation in the refinery must progressin such an efficient manner that losses are reducedto a minimum. Because of the continuous, year-roundoperation in cane sugar refineries, waste apparentlyinsignificant over short periods of time may becomean important factor in the reduction of profitfor yearly operation.In the refinery of The Revere Sugar Com-pany, at Charlestown, Massachusetts, the refinedsugar crystals from the vacuum pans are partiallydried by centrifuging and are then sent through rotarygranulators for further drying and breaking up ofcrystal agglomerates. The granulators are arrangedin sets of two in series, the sets being in parallel.Each granulator is of cylindrical construction,steel shells, thirty-four feet long and seven feetin diameter, mounted on a shaft carrying a steam drumfor heating and drying. Each granulator is inclinedat an angle of about ten degrees with the horizontaland wet sugar, entering at the top of the granulator,i.e., the upper end of the slope, is picked up byfins on the inner surface of the shell and droppedfrom the fins as they pass the top of the orbit.0i '-Sturtevant fans draw a current of warm air throughthe granulators, counter-current to the flow of sugar,removing dust and assisting in the drying. Due tothe slope of the shells, the sugar passes slowly throughthe granulator, discharging at the low er end intobucket elevators which carry it to the dry sugarstorage bins.The air laden with dust passes through a dustcollector in which a large number of baffles deflectthe air current suddenly so that the large dust part-icles are thrown onto the baffle plates. Hammers,actuated by an exterior mechanism, strike thebaffle plates periodically and the sugar adheringthereon drops into hoppers in the bottom where screwconveyors carry it to storage before remelting.This apparatus serves well to remove the large dustparticles but has no effect on those particles thatare so extremely small that they are deflected withthe air because their mass is so small that theinertia is insufficient to throw them out of theair stream. Some entrapment is, of course, effectedby the roughness of the surfaces, by mechanicalentrainment by the larger particles, by settling fromminute eddy currents adjacent to the surfaces, and,perhaps, by some electrical effect due to the frictionof the dust laden air against the baffle plates.NL5.The amount of the smallest particles removed in thisway is, however, insignificant in comparison to theamount passing through tlhe dust collector unaffected.In series with and directly followingthe dust collector mentioned above, The Revere SugarCompany has recently installed a dust washer designedto remove the very small particles by passing thestream of dust-laden air through two screens of wateror dilute sugar solution. Long, narrow, perforatedtrays are attached close to each other on an endlesschain which makes two passes across the gas stream.Water or dilute sugar solution drips continuouslyfrom the perforations in the bottoms j1 ie traysand forms a screen of liquid through thici± thedust-laden air must pass.Before the above-rentioned washer was in-stalled, a series of tests was run by txhe staff ofThe Revere Sugar Company to determine the loss ofsugar aust from this dust recovery system. FromOctober 18 to November 5, 1926, inclusive, nine testsgave an average loss of 46.1 pounds per hour of sugarlost as dust from the recovery system when sugarproducec' by t:ie graiulators averaged 35,000 poundsper hour. This is a percentage loss of only 0.1317%,but it is an actual loss of aproximately U16,600per year, assuming t-_at there are three hundred workingM 0idays in a year and that the price of sugar is fivecents per pound, and neglecting the cost of p.ackingthe sugar lost.T:ie operation of the washer recently installedhas undoubtedlj improved these conditions, but theaplearance of the exit gases makes it quite obviousthat the loss is still very considerable and furtherimprovement would be highly desirable.At the outset of this investigation,it was thought that a scrutiny of thte settling mechan-ism of sugar dust would be of more value than any otherstudy. Acurdingly, a search of the literaturewas conducted and pertinent works were abstracted.Subsequently, it developed that a more advantageousresult would be tile determination of fwhether or notsugar dust in air behavea as a mist and, if so,whether or not it followed the action of sulfuricacid mists in sucl± apparati as coke boxes, etc.Due to the change in plan, the settling abstractshave not been included but the references are notedin tfle bibliography.INET 7.PREVIOUS WORK:-On the behaviour of sugar dust other thanin explosions, the literature is singularly silent.With regard to smokes and fogs, there is a better..selection, but the only z.rticle found which isparticularly relevant to the question at hand isone on mists by Harold C. Weber.1"A mist or fog is made up of small particlesof varying sizes. These particles may be eitlierliquid or solid. One concept is that each individ-ual particle is surrounded by an air film which eff-ectively insulates it from its neighbors. Accordingto this idea, then, anj metnod of analysis to beeffective must present some means of breaking throughthis air film. It is possible, howjever, to explainpractically all the peculiarities associated withmist analysis and absorption without assuming thieexistence of thiis air film. If we assume that themist particles are so small tl-at they ao not settleout readily by gravity and that they are so largethat their rate of diffusion is extremely smallcompared witli the diffusion rate for gases, it ispossible to see that extreme difficulty wouldbe experienced in dissolving them."1"QuAntitative Analysis of Mists and Fogs,"H. C. 'Weber, J. Ind. Eng. Chem., 16, 1215, 1921.MDusts, smokes, fogs and mists have oftenbden successfully removed by using some form ofa Cottrell electrical precipitator. This ayparatuswould not be acceptable in sugar dust operationsbecause of tPe danger of explosion. The greatestcare must be exercised in regard to this danger.SUMMARY OF PROCEDURE:By means of a rotary air pump, dust ladenair was drawn from the duct leaving the dust collector,through a sampling tube into an experimental coke-tower. Meters measured the amount of air out oftbe coke-tower. Sugar dust removed was determinedby correlation of analyses at the entrance and exitof the coke-tower.RESULTS:No results had been obtained at the conclu-sion of this investigation.10.APPENDIX11.IFour inches above the bottom of the tower, a screenwas set on supporting rods and a coke layer ofa desired thickness was placed on the screen. Inthis investigation, tro inches of three to foilr meshcoke were used. Connections for a manometer aboveand below the coke layer were built into the towerso that the pressure drop throughi the coke might beKMTHOD OF PROCEDURE:The sample pipe for the mist removal appar-atus consisted of a two and one-half foot lengthof one and one-quarter inch cast iron pipe. A by-passfrom the sampling line led to an alundum thimbleabsorption apparatus in series with a flow meter.The coke-tower itself was made up of a galvanizedsheet iron tube six inches in diameter and four feet,eight inches long. Connections with the one inchpipe on either side oi the coke box were madethrough special sheet iron reducing couplings.In the inlet coupling, a sheet iron cone was placedso that the entering gas was forced to pass throughan annular space between the cone and the coupling,thus preventing difficulties which might resultfrom channeling of the gas and insuring good distri-bution of the gas over the cross section of the tower.-L12.easily obtained. After the coke-tower, the gasline splits into three branches, each supplied witha valve. One line led through a wet meter for meas-uring the slower rates of flow. The second lineled through a mercury flow-meter for measuringhigh rates of gas flow. And the third line ledthirough an alundum thimble absorption apparatus inseries with a flow-meter for sa~ipling the gas leavingthe coke-tower. The three lines then converged intoa single pipe wcich was connected to the rotary airpump whence the gas was exhausted to the atmosphere.A sketch of this apparzatus appears on page 13.With the arrangement j..st described, itwas planned to investigate the effects of two varia-bles; (1.) Velocity of dust-laden gas through thecoke layer; and (2.) Thickness of the coke layer.An opening to the air near the blower wasprovided with a valve so that variable amounts ofair might be bled into the pump, thus varying theflow through the remainder of the apparatus andallowing the gas flow through the coke-tower tobe regulated throughout a range from zero to themaximum capacity of the pump. The capacity of thepump was such tnat a maximum velocity of forty feetNow IcIuruu·l- ··-- r*rrr-----·-"L····- rrrrrurrurri*u;rc·~iurrr*l··r -· -·----~·I- --· II-·-~---r----·-·-r.l -.·.~.·. . .rr--··---·~-r-·--·- ~ .I;~-~~.. .yU~L·--ri~L~· 'LL'L·-iW·WI U·IW-IY(YIY·~·ls·-YIUI-3··i·IIIW-·-IDI-L III^CII1-. -IX··*j··III~-U·I·· 1- ·I --·14.per minute might be obtained through the coke-tower.The thickness of the coke layer was deter-minea by measuring tihe distance from the top of thecylindrical part of the tower to the top of the cokelayer and subtracting this distance from the knowndistance to the screen on which the coke layerrested.The sacipling apparatus at the entrancerid the exit to the tower is shown in detail in asketch on the preceding page. The minute pores ofthe alundum thimble cause the gas to enter the waterused as the absorbing liquid in the formil of very smallbubbles, thus minimizing the distance necessary forthe sugar particles to diffuse trirough the air inorder to reach the interface of the absorbingmedi m. In this manner, complete and very rapidabsorption is obtainec. This apparatus has beenused successfully in tie analysis of hydrochloricand nitric acid mists. The alundum thimble isone-half inch in diameter and approximately twoinches long. Ordinary six-hundred cubic centimetersuction filter flasks were used to contain the ab-sorbing liquid. The flow-meter, indicating TLe rateof flow of gas through tiie absorption apparatus, waspl1cd a-fter the absorber so that any density correct-/1w/ -5 - An I -Cý /r16.ion for the sugar dust would be obviated. The volumeof the sugar dust was neglected in the determinationof the volume flowing.After an appropriate volume of the dustladen air had Ilowed through the sampling apparatus,the sample was to be analyzed by determining theamount of sugar absorbed in the distilled water witha Brix hydrometer, which would give values correctto one tenth of one percent of sugar. The hydro-meter determination was to be checked frequentlyby a determination with an immersion refractometer.WJith an analysis giving the sugar contentof the air entering and leaving the apparatus, theamount of sugar dust taken out of the air by thelayer of coke would hGve been calculatedThe t,.o flow-meters on the sampling appar-ati and the mercury flow-meter in parallel withthe wet meter were calibrated by connecting themin series with the wet meter and drawing air throughthem. This gave satisfactory calibrations fva thewater flow-meters but tihe mercury flow-meter wasfurther calibrated by connecting the water flow-meters and the wet meter in parallel and then connect-ing this arrangement in series with the mercuryflow-meter to give a calibration for it at highervelocities.m17.DISCUSSION OF EXPERIMENTATION:In spite of the care taken in the pre-paration for an investigation of this nature,uinforeseen difficulties usually arise. Theenumeration of these difficulties and a briefdiscussion with reference to them may be of ser-vice to future investigators in this field.The sampling and analysis of the streamflvring through the apparatus was unsatisfactory.During a run, vigorous bubbling through the alun-dum absorption thimbles in the sampling apparatiwas obvious but subsequent examination in animnersion refractometer gave ho indication ofthe presence of sugar., This was followed by thefamiliar alpha-naphthol test for sugar and gavea negative result. The latter test will showsugar present if there are as many as one hundredparts in a million by weight. The failure toobtain any trace of sugar during the absorptionsuggested possible channelling of the gas streamthrough the pipes and connections leading to thecoke box. The pipes and fittings were accordinglydismantled and, without exception, showed tracesof sugar adhering to the walls. Furthermore,the coke layer in the coke tower was covered with~18.sugar dust. These facts were regarde. as conclusivein indicating that channelling was not sufficientto prevent drawing some sugar dust through thesampling apparatus. The cause for this apparentanomaly was not determined.Although the sampling tube used to drawthe dust-laden air into the experimental apparatuswas as short as possible under the conditions,considerable sugar dust lodged on the inner surfaceof the tube and never reached the experimentalapparatus. This was not important in this investi-gation because it was not necessary to obtain arepresentative sapnle of the air in the duct.The difficulty would, however, have assum~ed moreimportance if a plant test had been run.M:iei-:i:iI IDATAFlovimeter Calibration:F1 one e e r Reading("Hp2o)Volumeflowing(cu.ft.)En t ran c eSam e r 1.22.26.2ExitSample r S0.41.42.44.47.40.8801.5602.1552.6100.2751.1102.2204.53504.9558.6404 0.220S 0.3125 0.4315 0.5221 0.2752 0.:5050.7405 0.9065 0.9918 1.080Time(min.)Rate(cu.ft ./in.)·_19.2CB IB3LI 0 GTAAP HYFaOR2 DUST IIVSTA bO~SITThe Industrial Treatment of Fumes and Dusty Gases,bry W. . Gibbs, D. SC.J. SOC. C=~E. I. 41, 189T-19~6(922)Has a good. theoretical treatment of theforces actin< on settling particlesfrom a mathematical point of view.A Jet Dust Counting Apparatus,by J. S. Owens, M.D.JOUR.ITD.TGI;EN 4, 522-534(19 2-23)A discussion of prevailin -• m:ethods forcounting and measuring dust -articlesand a descriction of the constructionand opera.tion of the author's a, paratusfor those determinations.A Method of Determining the Size of Particlesin omokes,by R. G. ray JT. B. SpeaIonan.PROC. ROY. SOC. of LOIN~D102A, 615 et se, (192~)A disoussion of the authors' method fordrtermrining the numboer, sixe and average~___~__~iFaR·Lc-21.mass of minute dust -art±cles in air.The method necessitates the use of amicro-balance and requires a highlydeveloped technique.(Many other articles under "I)ust " "Sm k "and"Powder" in the library at the Kassachusetts Instituteof Technology ,ere found irrelevant as far as sugar dustis concerned.)oh a c onVi ei ,i q i-

A study of some characteristics of sugar dust

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A study of some characteristics of sugar dust

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