INDUSTRIAL AND ENQINEERING CHEMISTRY
1112
ficultiea due to the fact tbat the aliphatic hydrocarbons have the m e b o i i range as the aromatic hydrocarbon, toluene, although some extrsotion methoda are suitable (f),and 88 mentioned above, aneotmpic distillation iteelf with an added entrainermaybeuaedforseparatinghydmaarbons.Thepderentisl nitration of the toluene to the mononitrate might be conducted using the comparatively inert aliphatic hydrocarbons as entrainer% for the water formed. The m o n o n i b toluene d t i n g could be readily separated by dietilling off the aliphatica. This step is little more expanaive than sew ration methods and is directed toward the 5 d end. The relatively innocuousmononitrateformedmay then be shipped or carried to a higher stage of nitration. Them nitration operations are described in patent applications (it,18). Dehydrating Oils
Castor oil has been dehydrated (f S) by the use of a cablyst such as sodium acid sulfate in the presence of -e, which sds as an entrainer for the water removed. The vapor of h y d m a r h n prewnta oxidation of the oil, and dehydration proceeds to u greater extent than is otherwise poseible. Other nondrying oils, such as soybean, have bean aimjJarly treated to give oils which have definite drying properties and poasible utility in the field of paints and varnishes. The dehydration prow%has been carried further to give plaatic materials of a rubbery nature, although the chemistry of the reactions themselves and of the 6nal products are not completelyknown at this time.
voL33,No.o
Literature Cited Bbtnsgar. 8. 8., .nd Ward, P. J.. IND. ENa. h. 31, ,196 (1988). Field, Edmond, U. 8. Patent 2,212,810(Aw.27,1940). %ob. E.E..IbX. l,fIB7,171 (April 9,19.36). Jaoob, J. J., patent application. Keyed, D. B.. Im. &a. h. 21.888 ,(19Zs). ma..u,1000 (1032). Kkr. M., “Fabrihtnn wn sbmlutem llllmhd m& Ver-dung da Zuaatrmittel N Motor-T~eibtoffen.” 2nd ad.. Hdle a. W e , W. Knapp. 1937. Kokatnur. V. R. (to Autoxym, ha.), Brit. Patent 409,862 (JUIYaa. 1837) and u. 8. Patent S P ~ ~ C S ~ ~ O W . Kokalauu. V.R., U.8.Patent 1,007.480 (April 24.1828).
w.. i,sia,w (JUIY~, ioai).
w., a.iii,97a (Maoh az. isas).
mid,pstent applioatioM. Kokatnur, V. R., and Jacobs, J. J., patazlt applloatio~. Main&0.E.,Chen. d Md.Bw., 28,778,841(1922). Other, D.F.,Ibid., 4&91 (June, 1941). O t h e r . D.F.. h. ha. h. 27.2E4 , (1086).
w.. 31,841 (1940).
Othmes, D.F., and Jwbs, J. r..lbid.. 32,164 (1940).
~ t b m p o ; ~ . ~ . . aJ.OO~S, n d J. J.. u. s.P.tent.PPlidothmar, D.F., mdisgaa, C. E., h. ENa. h. 35, .168
p?).
Rosmu. M&, and Qhsgow, R4m Natum2 Qadi- Mh., 10, 116 (1940). E-, MW, u.8. &tent 1.a94.232 (oct. is. 1921). WadS.J.,J.hSM.,87.1858(1806). waaa, J.. J . SOC.hrnd., u.iaaa (ioos). Wantworth, T. O., U. 8. Patent applioation. Went&, T. O., and Other, D. F.. Im. ENa. Cmu., 32, 1688 (1940). YOW, si&^, J . ci-. SW.. SI,707,7ao,763 (1803).
DESIGNING EFFICIENT ERUIPMENT qu+V. A
d
Buffalo Foundry & Machine Company. Buffalo, N. Y.
Problems of design of kettles for use in chemical plan@are reviewed from the standpoint of the designer and fabricator. Materials of construction, agitation, heat transfer, and details of design affecting operation are diuuaed.
of the most common pieces of zed for u multitude of purposes. It may be used for the mixing of e d y miscible liquids, for the diwlving of solids in liquids, and for the mixing or compounding of viscous liquids. Kettles are required to operate through large rangeeof temperature und pressure, and must be so designed that their contents may be heated or cooled in order that the prowas mction may procead properly. They must frequentlywithstand corrosion and are constructed of suitahle corrosion-resisting materkh. Kettles van‘ in size according to the requirements of the process for which they are to be used. Some kettles may hold only a few quarts, others thousands of gallons. Some kettles may be used for a variety of purposes. A reaction may be carried out, then it may be necessary to evaporate water with a thick viscous material resulting. Again chemicals may be charged as liquids which, as they react, produce 6mt a pasty m&88 which becomes almost solid and finally u granular material which must be discharged from the kettle. The kettle must operate under 80 wide a variety of conditions that it has been difEcult to analyze. Much has been published concerningheat transfer rates in heat exchangers, condensars, evaporators, etc., and on drying operations, distillation, and other unit operations. However, there is tittle in the litera-
ture to guide the designer of the humble and versatile kettle; yet frequently the initial operations upon which the yield and themfore economy of the whole process depends are c a r ried out in kettles. Materials of Construction
Until recently the only chemical-resistant material uvauahle at reasonable cost was cast iron. Veesels fabricated from steel plate were used for many purposes but were less reliable than cast iron since they gave trouhle at riveted or welded jointa. The cast iron vessels were seamless and uniform throughout. Today a large number of corrosion-&stant materials areuvailable. There is a large vuriety of stainless steels ranging from the simple 18 niOkel-8 chrome to the more complex alloys containing titanium and columbium. Each of the many alloys has definite properties which make it particularly suitable for certain applications. Since the price of the more complex alloys is much greater than tbat of the simpler ones, the purchaser of stainlw steel equipment should make sure tbat the apparatus is fabricated from the particular grade of stainless steel which is beat suited to the purpose. Many of the large chemical companies which purchase
s.Pbd=,
1941
1
.
6
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
1.0. k Or D. met Lt.1 Kettle
*
o h 4 d q@Bratue. Borne of thw mstQisls,prtieulsry nihelsnd #ai&xb9 w lrroaueed in the fom of clsd shed.in which a thib sheet of the Vauabb, dlw is h n l y bo& to a,hgvicn&e% d ate&, T h e w d d i b g o f t h e ~ t 4 d a l s i r l ~ vmsny e u metallurgical p m b b , such (L% the retention of the pro@of the original material in the weld, the heat treatment of the welded v e d to restore the properties of the metal, not to mention the commonplaca repuirememt of obtaining a sound weld. The preparation of the &e& for welding, the provisions which must be made to prevent distortion and buckling of the sheets under heat, and a host of other pmblema of welding teohniqoe are involved. The use of special met& snd SuOJrs is developing 80 rapidly and changeu come 80 freq U d l y thst oonstant Study is repuired to keep one 1'88801lably well informed on the late& ddopme.nta and methods. Suoh matarisla are all much more expensive than cast imn, stsel, or copper, but their use is justifiad by the higher quaJity of the C h r n n i d materinla pmduaed in venwla constructed from thean. The demand for better ohemid producta hss forcedtheengineertopmducabetterapparatus.
I
1113
Design Problems Another recent development has dected the design of chemical apparatus. Many states have adopted d e s for un6red preesure veesels similar to or the same as the A. 5.M. E.code, nearly all pressure vessels must be iaspected by representatives of insurance companies, and deaigne must be submitted to and approved by them before the vessels can be inspected and stsmped. The designer must therefore meet the following conditions: The apparatus must first suit the customer's requimmenta 88 to intemsl and jacket presmves and combinations of v ~ c u u m and pressure, &tation, openings, thermometer wells, etc., in addition to special arrangements for the use to which the kettle will be put. The designer must slso have a g o d working knowledge of the properties of the metals from which the vessel is to be constructed. For example, both staideen steel and aluminum are resietsnt to certain tm of c o d o n , but stainless steel is much stronger than aluminum. On the other hand, sin= it is a lighter metal, aluminum is lessexpensive than S&aideeu steel by the pound and much lees expeneive per square foot when in sheet form. If the V e d is to be uaed under preeaure conditions, the deaigner will prohbly recommend the use of stainless steel. If no preesure is involved, he may chooee aluminum. The choice of materials is frequently much more complicated and may involve practical considerations, such as the time requid for delimy of materials from the mill, the practicabiity of heat treating the completed vessel without distortion, and the limitations of the shop in which the fabrication is to be done. Afkr these questions have baen decided, tbe
I N D U S T R I A L A N D E N OI I N E E R I N G C H E M I S T R Y
1114
designer proceeds with the actual design of the apparatus. Reasure and temperature conditions will be the first consideration. The A. s.M. E. Code for Unhred Praasure Vessels contains the basic data and formulas to be used, and a complete familiarity with the provisions of the d e is neceaeary in order to arrive at an economical design. The kettle must be deaigned, first, to resist the i n t e d or bursting pressure. This determinea the shell tbicknw, the thickness of the upper and lower heads, the upper flange thickness, and the bolting between the kettle Ihnge and the
LJ upper head kuge. Next, the jacket pmsure is considered. Pressure in the jacket tends to c o l l a p the ixmer shell so that another determination of the inner shell and hottom head thickueases must be made. Frequently a kettle may have vacuum on the interior and pressure in the jacket. The jacket pressure must therefore be inmeased by 15 pounds per square inch. since both the vacuum and n m u r e will orobablv occur hitaneousIy. The code formulas treat the shell as a cvlinder when subjected to collapsing s t r e a m and take in6 account the unsupported length of the cylinder between sti5ening rings. It is easy to see that a large cylinder will not &t collapsing streas as well as one of smaller diameter, and that if the cylinder is reinforced by supporting rings, it is stmnger than one which is not so reinforeed. It is desirable to keep the diameter not only the of the kettle as and as possible. This &e& shell but the bottom head, since the latter ala0 is .under a collapsing stress. If the umr of the kettle will give the dedper a little leeway in determining the diameter of the kettle, he may obtain a moreeconomicalunit. FrequentJythedeaignariscalledn~ to furnish a kettle with a full jacket or one which corn the whole inner shell. This may not be abeolutely necessq from a process point of view and it inrreaaes the wst of the kettle unnecessslrily, since it increasee the uneupported length of the cylinder whicb forma the aids of the kettle. The flange by which the jacket is attsched to the sheU supports the inner shell; if it is not too far away from the lower head, it may be taken into account in the deeigu and a slightly thinner shell may be used. Consider now the lower head of the inner shell. Frequently it is made from a simple dished head where the 8pherid radius of the head is equal to the diameter of the d Such hesds are somewhat 9at snd will not resist coUaping pre& sures 80 well as a hemispherical head. It is mora emnomid from a metal point of view to use a hemiephericd head. From a cost standpoint this may not be 80 economical. The dished head can be purchaeed from many &with a small cost for forming all in one piece. The hemispherid head, however, is made up of segments like an orange peel, each
Vol. 33, No. 9
segment being part of the surface of a sphere. These eegmenta must be welded together to form the hemisphere. The cost of forming the segments for one hemispherical head may be more than is aaved by decreasing the thickness and weight of metal. If three or more heads are ordered at me time, the forming cost per head is 80 much reduced that the hemiapheric d head becomes the mast economical. Thin in explained when one considem that the cost of moat mechanical operations includs a charge for ‘‘Setup time’’-in this case, for placing in the p m the ~ diea which are required to produce a piece of the d& shape. Frequently the cost of setting up the machine will be greater than that of actually dobq the work. If the setup wst can be distributed over several heads, the cost per head w i l l be materislly reduced. If the war will mange his p r o w 80 that the kettlea will all he of one or two diameters, he will make B saving. D8erences in capacity may be made up by diflerenoesin depth. A.mt of charta bas been prepared to simplify the tgek of makmg comparativedesigns. Figure 1 is for a %inch kettle; similsr charta may be prepared for other diameters. The upper part of Figure 1 has to do with the heads of the kettle and jacket only. Entering the chart at an intemsl working prpssure of 200 pounds and passing horizontally to an intersection with the line repmentine; thioknasses for an inner heminpherid head and then proceeding vertically, it is found that the head is to be ‘/,&~ch thick; in the eame way a dished head would he l/,, inch thick. Either of these he& would withnhd an axtemal pressure (the pressure in the jacket) of 120 pounds per square inch. If, however, the pressure in the jacket was to be 160 pounds per square inch, thin pressure would determine the thickness of the head. Entering the chart at I W pounds per square inch and proceeding 48 be fore, it is found that the thicknew of a hemispherical inner head would have to be l/,, inch; for a dished head the thickness would be ’/,, inch. The thickness of the kettle side w d s
are determined from the lower part of the chart. Aaauming again an internal p w u r e of 200 pounds per square inch, entering the ohart from the left, and moving horisonbily to the line marked “internal pressure kettle” and from the interd o n vertically, it is found that the shell would he s/,,inch thick. If the jacket p m r e is to be 160 pounds, which CBUBBLI an external pressure in the inner shell, one procdn horizontally to the external presmre line and then down& it in found that the kettle side wall thickness would be 1% inch. At a lower jacket pressure the effect of the unaup p o d b g t h of the jacket becorns grester, and the cbnrt
INDUSTRIAL A N D ENQINEERINQ CHEMISTRY
1113
body at the top by meansof ssteel tin# welded to the body. At the hottom the jacket is welded to asteel hngu which ia $a0 welded to the body. h e r d di5-t conditicms of he444ingan coohg may eXi& The kettlemay be Eued with a eeol mahid aad a heating mediwn tumed into the jacket spaca. B e i i inarlated, the jacket quiohly resobea a temperature elmto that of tbe h*mediUm, while the Leth body tempersture rennsins close to that of the content& tlimllarly, a cooling mediummay be intmduaed into the jacket apsos which cools the jaoker quWy while the kettle body i i kept hot by ita contemts. 'lb womt condition exists when tho dabhasteel body mbot and the jaaket cool b t h e expaneion of the stsinlpee at4 aylindriosl body is muah greak.rthantbatofthecarbonebdjPCL& Thebottoreof the j&t is uoudly a dished hedsnd, being mbjeded only to a bunting pressure, b d t i * thin. It d l Sct Mpn expaneion jointand, by bemdin&adbtiheJftothe varyingtaw paatam oonditione. The welding, however, mnd be mre fully done no that the 8tracaes thua produced do not cpw CrsokE in the welds. If,however, the jacket is divided into two parta, the upper psrtbeingattsched to t h e w body at the topsad b o k of the otraigbt seetion while the -1 psrt con&b only ofa &nged dimbed heed c o m i n g the bottom of the kettle, a different condition gdsts. In ulill OPLLB the straight eylindrid porttons of the body and ja&t are faetaned rigidly togetku and tbem is no bottom head to take up the expaunion At+ slnnptim mu& be mnde the temperuturea whiob msy be reached by the two cyhdem, and the ntremm in them must be d&tad. These sbaaaa, c o m b 4 with thaee produced by the imponed psaasure%must notemeed the e b tic limit of the matarials; othefwk Permaneat d e f o d m d l be produced whioh may eduBB dimoulti€n. unu8uy,w h thio doutile-jacket conatmetion in ueed, an e x y e i o n joint i built into the jacket to pennit tbeae deformahom to taka pleae without ovemtmshg the platea or welds.
e
1116
INDUSTRIAL A N D PIIQINPPRINQ CHEMISTRY
the vapors m at high temperature, the temperaturegradient between the kettle and vapor is large and the rate of heat input is correspondingly hge. The temperature di5erenceu betareen the kettle body and j&t are increaeed with a correspondingly great i n c m in difierential atreaaes. These stresses should be care.fnUy worked out, and if they are ereeeSive, expaneion joints muat be provided. We have considered heretofom only the longitudinal EXpamion in the kettle and shell. Higher temperatures are encountered wben w o r k with Dowtherm and mercury, and when the kettle is made of stainless Bteel or a nickel alloy, another sort of difierentialstress must be coneidered. The COMeCtiOn s t the top Of the jacket batweNl the S a j&t and Btainlese Bteal body is ordinarily made by welding a deel ring to the body and then welding the jacket to the ring. Both Dowtherm and mercury are so difEcult to coniine, whether 88 liquid or vapor, that the h g d and bolted construction is avoided. As the cylindrical shell of the kettle is heated by the vapor, it expands at a rate approximately twice 88 great per degree temperature rise 88 that of the steel ring. This pduoea a compreusive streas in the body and tenuion in the ring; since the two are firmly attached by the welde, the deformation of the two must be equal. An equation msy be made from which the atmuom in the kettle body and in the ring may be dculated, sss~mp tiom heiig made for the width of kettle shell placed in compression. The streeses in both the kettle shell and ring under them conditionn m high and may approach the elaetio limit of both elements, particularly with beating vapor tempemtures of 800" F. The design of the weld between the kettle and ring must dso be c o n s i d d carefully. The designer may 6gum that the welds hetween the jacket and ring are only in shear and that the welds may be simple corner or Wet welda. Thie iS not the e888. An the jsaket expanda and puts a tenaion 8treaa into the ring, the welds must he strong enough to transmit these streeses. It is newmry, therefore, to make a J-ehaped cut on the top and bottom of the ring on the inside. m e n these grooves are 6lled with weld metal and the kettle expanda, the welda are in compression and are much 1-8 liable to failure m u m 3).
D.trilc of Conatruetion Wberever poasible it is deeirable to equip a kettle with a bottom outlet, since such an outlet permits the kettle to be quickly and thmugbly drained. For a numbar of yearn it was thought necmary to provide a stu5ing box around the outlet of the kettle where it pesaes through the jacket. This construction left a lengtb of pipe bethe bottom of the kettle and the valve. The pipe formed a pocket where solids could settle. Such solids would not enter the reaction and might plug the outlet. The stuffingboxes were 8 source of annoyance in operation. This deaign has been abandoned in favor of a deaign which may be used either for cast iron or fabricated steel kettles. A flange is oaat on the bottom of a cast iron kettle or welded onfo a fabricated kettle. The l b g e is diciently large so that its center can he faced and drilled for a standard valve Bsnge of the aise called for by the bottom opening. Around this flangea second ring is msobined, frequently slightly below tbe face, which has been machined for the valve. Thie face io also drilled for dud bolts. A corresponding faced and drilled Eange is welded to the imide of the %teel dished head forming the bottom of the jacket. When these two aangeS m made up together and properly gdeted, a tight joint in made around the bottom outlet of the kettle,and the valve may be bolted to the interior flange whioh is direotly on the
VoL 33, N a 9
bottom of the kettle.
This comtruction in of particular value where the kettle ia subjd.4 to severely oorrosive conditions and must be r e p l a c e d frequently, since the kettle jaoket Canbesalvsged resdilysnduaed on replaoement kettles. Asimilar construction may be uaed with stainless steel kettles, but in t h i s case a steel h n g e is welded t o the bottom outlet of the staidem steel kettle and the bottom head of t h e jacket is welded t o the periphery of the -e. A large numberof kettles have been made in tbi8 way with working steam pressure% up to 175 pounds, We do not know of any caws where leakage from the jacket haa Ween place from the jointa around the bottom outlet (Figure 4). With this type of bottom outlet it is easy to inatsll a flunheat valve whioh will completely 6ll the bottom outlet and be flush with the bottom of the kettle on the inside. Such valw m now baing made by neveral valve mannfmm. One manufaatnrer offan a removable and replaceable mt, machined to a conical surfam on the inside upon which the valve clisk eats. Another msnufscturerox%€# a plunger tspe ofvalve, in which the seat is made by means of packing rings around the plnnger. Both of t h w valvea have been used BUCWdUlIJ-.
Asitation
It is impoeaible to g e m r a h with regard to agitation. Agitators must be designed to meet the conditions under whi& the kettle is to be used. At one extreme we have a condition such as ecdsta in a nitrator where it is neoessary to &paw the in~~mingstresm of mixed d d instantenewaly throughout the kettle charge and at the 881118 time keap the whole oharge psssing rapidly over the oooled intmior of the shell and whatever additional cooling surfaces may have been PIP. vided, suah M cooling fingers OP coils. The d&er must atu~ keep in mind that, in general, a certain amount of corrosion is taking place on the rapidly moving agitator,and the deuign mnet be sufficient& rngged to prevent undue agitator r e p b menta. For this reaeon, nitrators m frequently quipped
. I N D U S T R I A L A N D E N Gl I N E E R I N Q C H E M I S T R Y
with propeller &tatom similar to the ordinary ahip propelk, notwithe fact that a t u r b t y p e impeuer is &nd y more e5cient ea m agitator. Sulfonatom are similar in many ways to the nitrator, in BO far ea agitation requirementsare concerned. Where the deeigner is required to mix Large quantities of m a t e d rapidly and &ciently, moh as in the diamlving of nitmootton cellulose acetate or natural and rutilicial ins, the conditions are quite merent. We now require an agitating means which will circulate Large quantities of liquid and dispersa through it qusptities of d i d material which must be kept separated and uniformly distributed until it is completely dissolved, and will Snauy produce a uniform solution. For mcb operations t u r b t y p e mixers would probably eave much time, power, and Boor spsoe bemuse of their rapid Sation. Varying requirementa in viscosity and plasticity have led to the development of sppeoialised types in order to give the e x d e n t results expected from t u r b t y p e mixera in this and other fields. Since the choice of type and Siae depends on many difterant factom, the designer should seek the advice of the manufa&um More deciding on the type or tank dimensions. The preoediog paragraph8 have bean devoted to descrip t i o of ~ agitator% for the mixing of a relatively fluid charge; frequently it is necessary to mix viscous materialn such as f d d e h y d e resins. One large manufacturerof resins U B ~ Ba kettle 7 feet 3 inches in diameter and 6 feet 6 inches deep. The kettle is cast iron with coils cantin the &. ordinars.steamat 40-Wpounds pmsure is uead ea the heating medium, but at times I50 pound8steam may be ueed. The castin coils are ueed in-
1117
tim so that w h e n d b a t d w a a r e m s d e , the steam is turned into only thw coils which am below the liquid level. If the lrettle is heated above the liquid level, the material forms a coating in the kettle walls above the liquid level. The kettle KBB provided origin& with an agitator which had &ble blade6 to scrape the kettle walls, but the agitator did not work so well ea waa expected. The steel scrapem wore down rapidly probbly b u m of conmion by acid in the
resin. After a period of operation in which the scrapers were used, it wea decided to provide heavier nonfladble scrapers and Bet them with a clearance wrsing from to '/, inch. The coating on the interior of tbrs kettle is limited to this clmance. The coating on the interior of older kettles which do not have the adjustable wsdea will be ea much ea '/a inch thick. Cooseauently there is B d a t e gain since the new kettle will ban& thfee to four times as msny batches before it must be cleaned. In the manufacture of a phenol-formatdehyde resin much of the hatch time is taken up in drJrine the resin after the reaction hss been coanpleted. The de in thickneza of the coating has a marked de.&on reducing the time requid for drying and fhereby increzaa the productivity of the kettle. Another manufacturer in operating a jacketed, cast imn kettle of 200 gdom working capacity. It 6as been found that with 19.5 pound8 of steam daSS' F.)in the jacket and-a %inch vacuum on the interior (at which pressure the dutiun boiled at 1M)" F.)he wea able to evaporate E M pounaS of water per how. When the kettle wea operated under atmaspheric conditions, he was able to evaporate about a00 pounds of watar per hour with the material boiling at a06* F. The &ace covered by the bat& wea 82.5 square feet. Prior to evaporation, the material had a apeoific gravity of 1.15, aftar 1.28. From these dab, using a steam&& d c i e n t of 2ooo and 88suming the thermal conductivity of iron at 833 B t. u. per hour per square foot per inch and metal thickneae of 1.5 inches, it wea detedned tbat the film d c i e n t in the interior of the kettle between the kettle wall and the boiling liquid waa 57 B. t. U. per hour when o m t h g under a 26-inch vacuum while it was 115 B. t. u. per square foot per hour when operating under atmospheric conditions. The foregoing showa that the p&blem hem is not so much one of thorough miXing ea it is of moving heated material from the waU of the veesel before it hss an opportunity to b come overheated and bakd onto the walls. The materials am so viscous that rapidity of agitation or mixing hae no effect; they have to be removed forcibly by the agitator. This requim that the agitator approach the kettle . . walls ea c l d y ea poeaihle without actually contacting them. The aame condition applies when a kettle is used ea a cryatallizar. An instsllation of this type wea made recant& in wEch it waa d e i d to cool a eaturatad solution of an organic material which, upon cooling, would form fine ayjtab. The kettles were provided with ding jackets containing spirsl bdea through which the cooling water w88 circdated. A bigh-qmed agitator could not be d d d becsusa when the aolution waa cooled, it had I the aoneistpmcv of a Slushv mow. Such an &iator wouid have broken up the Jb msstel8 to such an extent thatthey oouldnot have been separated from the mother Eqlm F I Q 8.~ Rumm Tvm OF AQITAMBME MOVINQh i8orm Lmm WmcH A m M Bm Mramo by a centrifugal. Also, it is probable that
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. 1118
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 33, No. 0
adaptation of old deaigns tonewuses. Ha14ngde cided upon the type of agitator to be used, the speed at which it is to be rotatad, and the horsepower mquirementa, the designer must choose the type of drive to be sup plied. For high-speed agitation (100 r. p. m. or higher) most deaignera would probably spcify a drive consisting of a geared motor supported on a ring Or other StNCturd Sup port resting on the kettle cover (Figure 9). This drive will frequently extend so far above the top of the kettle that there is hu5cjent haadroom in which to make the installation. If this is the cam, a vertical worm-gear drive may be substituted (Figure 10). Where slow speeds are required, from 10-15 to 2-3 r. p. m.drive, a bevel gear may be placed on the agitator shaft and driven by a pinion on a ahaft at right angles ta the agitator shaft. Thi pinion shaft may be driver FIGURE9. GRAREDMOTOR OB K ~ w r ~ m FIOW 10, VBRTICALw 0 r n - G ~D ~ m ~ COVERFOR H ~ Q F I - SAGWATION ~U by a gmed motor sup ported from the kettle cover. or hv anv other conveuientmeans. Thebevel~formtheisatsiephthereduca high-speed propeller would have formed a hole for itself in tion of speed between the motor and the agitator ahaft and it the slushy mass. In this csee a slow-speed agitator which is UeUsllJr more economical to UBB gesrs for this last step than came close to the wall of the vessel had to be used to remove to attempt to make the complete reduction in a gesr reducer. the eryatalllzed material from the walls and pmit warmer For intermedistespeeds, either of the two general methods material to come in contact with them. desmibed may be used, the choice depending on the wuiraIn addition to the more common typea of agitator m y special types have been built. In eame c88es where the ma- menta of the w r . If the kettle is to be i118t8lIed in a pharmaceutical plant, where appearance and quietnRss of operaterial is extremely viscous-for example, in the compounding tion are important, the geared motor or motor reducer would of grenae-a double-motion agitator may be wad. Another he uaed even if slightly uneconomical; if appearance and agitator is a modification of the homeshoe type in which a number of bara project upward from the homshoe and p s ~ e quietness are aecondary to firat cost, the bevel geam might be used e m on rslstively high-speed agitators. between downward-projecting arms which are fastened to a The kettle cover must be designed to meet the wuire stationary horizoutal har attached to the walls of the veasel menta of the operator. OPBnings for the introduction of (Figures 5 and 6). liquids or inert gases, for pressure gages,and the thermometer The mixing blade and stationary Mes may be made well are usually located near the front of the kettle where hollow so that steam, water, or brine may be circulated they are convenient for the operator. The manhole also is through them to provide heating or cooling during the miXing placed in front ea that it may be opened e d from ~ the op operation. It is claiied that a kettle fitted with this type of erating floor. Vmta or vapr pim and similar connections agitator will handle a reaction in which the material is a liquid are placed in the rear where they do not intarfere with opsaat the start, later becomes pasty, and Soauy b m h down into tian. If mght glsases are provided, one is pkced oonv&entb a powder. If the kettle is provided with a motor having two near the front, while another in placed at one side or toward or three speeds, the mixing speed may be accurately conthe rear. This me may be covemd with aahade containing trolled (Figure 7). a light which illwhaten the interior of the d. In some p r o w it is nec88881y to charge the kettle with When laying out the kettle cover, it is u a d y good prab large lumps of solid mate* which are to be melted. The tice to provide for an a d d i t i d nossle 01' two, though agitator must be sutsciently rugged to move this matmid no immediate UBB may be anticipated for thcrm. The addihoddv around the heated walls of the jacket until the rrmsa ia tional coet is mallwhen the kettle ia being built, but if they m e l d (~igure 8). bave to he added later, the cost Win be mu06 greater. Agitator design is very much a matter of expmience and the
