BENCH SCALE EQUIPMENT AND TECHNIQUES Presented before the Division of Industrial and Engineering Chemistry a t t h e 111th Xleeting of the AMERICAN CHE.MICALSOCIETY, Atlantic City,
N. J.
High pressure reactions.. .. E. L. Clark, P. L. Golden, -4.M.Whitehouse, and H. H. Storch C E N T R i L EXPERIME3T STLTIOY. U. S. BUREiL' O F \lIYES, PITTSBURGH, Pi.
1 f$E design and operation of experimental equipment for the iuiestigation of h>drogenatioii of coal as a Source of &!rathetic liquid fuel is complicated b\ t h e rigorous requirenients of t h e process. The use of high pressure(1000 to 12,000 pounds per square inch gage) and high teniperatures (350" to 600" C.), coupled with t h e corrosi,eancl era-ive nature of the products, necessitates specialired anti carefully planned equipment. Three types of experimental high pressure units are i n w e : batch autoclaies or. in general, unit4 in H hich no flow of reactants or products takes place: semicontinuous units where part of either the rea(-tnnts or product3 are i r i c-ontinunus flow ;
T
HE iniportance of siiiall scale pilot plants iii our prcscrit icheiric of research and development work has been sui'ficiently stresscd to have become almost factual. -1wall designed H I I ~1 carefully planned bench scale layout can pt'rniit tremr,ndous -avings of time anti (sffort in investigating reactions anti processes 5 7 both ~ industrial sntl academic research. In the design of .-iiiali scale cquipnient for operation a t high temperatures and !Jrtwurc's the many mechanical problcni.2, the necessity foi, r r iebsing personnel saivty, and the care required in assembly antl uperation present aerious obstacle$. There are in the literatun, iiiany helpful references t o work done a t elevated p ,5agt. Obviously, untlt:r such condit i o i i , q the selcction of suitable matt&& of construction is importxiit. HI Cor the c:onstruction of equipment f ( ~ rt h c a hydrogt.nat ion of coal a t elevated temperatures and pwssut~c~i should'haw the following characteristics: ( a ) high creep strr~iigth lice to hydrogen, ( b ) high resistance t o abrasion, ( e ) good corrosion resistance i n the presence of appreciahli. quantities of ammonia, sulfur compounds, and \rater vapoi', ( d ) good niachininy qualities, and ( e ) good weld properties f l i t . joining parts rrwiily with presently available welding practic,t'i. .ilthough all parts of the equipmerit niay not be subjected t o elevated tcniperatures which imposc nianv of t hese requirement.es in uw are 2 to 3 inches in internal dlainrter and 12 t o 14 inches in length. For coal hydrogenation rcww-ch, I tit, required amounts of pon-tlrreti
Segmented retaining ring
Figure 2.
Detail of Pressure Seal
December 1947
Figure 3.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Segmented Ketainitig Ring
1557
the gaseous products are recoverrd by veiitirig th(* autoclavr through a suitable absorption train and thus reducing the pressure to atmospheric. The autoclave then opelied for removal and examination of the liquid and s d products. An obvious prccaution in handling esperimriital work of this type is a manipulation to permit good mass halances by keeping lo a minimum. A suitablc, balanw is usrd t o iveigh t i i t . it11 and contents at, several points i n tlitl charging and venting operations. The rrsults arr quit(. gratifl-ing; tiicy indicatr loshi’s of less than 0.5$ in many eas(’h. Thrsr indkate good sealing in both the autoclave closure and charging valvc., as n-r1I as .skillful and accurate manipulation. Closures for high pressur(’ \ $turd? bolts or studs and suitable flanges and gaskets. Such H closure for a converter 2 inches in internal c1ianietc.r is shown i r i Figure 1. This type of flianged closure is comriiorily used iri industrial work. The eight bolts are evenly tightened t o suitably deform tlic 3I(Jllt.l gasket. Torque w e n c h w are necessary t o assure evt.11 d(~foi,niatioria n d yocid seal. There are si,vc,ral disadvantagcxs in the use of this type of closure---increased prcmurv would tend to cause leakage, hrating or cooling must proceed slowly to avoid the effects of thermal shock, and the eight bolt9 needed for the closure are quite a nuisance in assembly and maintenance. This type of closure was extensively used in Grrnian experimrntal work, probably brcause of the ease of niachininy and t h r ppssibility of inschriing a side connection through the gasket will. 111 tliwe casw a gasket of so& t h r r r inchez in length and somewhat t h i r h r than the vessel nall was employed. with this type of closure are in usc at t h o Bureau of lIiiiev a i d a r t > giving troublt~-frwservice, thc ncctwitv for a e and thr amount of work required for a ,atisfactoryt hnri otlioal,catalyst, and oil vehicle are n-eighed into the autuclave; hydrogrn is added t o the required pressure, and then the tempera:ure raised to the point required for the reaction. T o provide niixing during thc reaction period, tlic autoclave is rotated n-ithin the furnace. Furnace temperatuws arr controlled by a temperaturr controller manually art. The internal temperature of the iutoclave is rccorded by a tlicrmocouplr and temperature re:order with mercury pools used for contacts to the couple, ivhich :oratre n-iih t h r autoclaw. .After th(, rcaction timil has elapsed,
Figure 5.
iutoclave Closure for 15,000 Pounds per Square Inch
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 39, No. 12
For increased pressures t iic segmented r' i'ttnnot be used for one of the retaining surfae
particularly if a Monel gasket, which i. pre vrablc. is spwified. Various types of threaded nuts have hccn used to supply thc retainirig surface t o effect the seal. .Is can be for( thesc. threaded nuts tend to freeze ~i-hcri\--tyjx thrtlada arc used. Buttrils;: threads help considerably and, together with hardened materials (300-325 Brinell) for this main or retaining n u i . solve thc problem t o a great oxtent. Figure 5 illustrates a closure designed for 15,000 pouiid. p r square inch and 500" C., using a Buttresstype thread for the main nut and a 11011c-l gasket. . I in P the previously discussed clo.wrC-. tt split ring and clamping ring are provided for the initial gasket deformation. These are raised by hex-head screws bearing on a thrust plat?. T h r main precaution in using thc threaded nu; as a retaining surface is to protert the thread,. froin any scratches or abrasions. The asscnihiy is simple and calls for only a minimum LlnlOl1nT of training and experience. T o eliminate entirely t h e us(% of thi,radrt! nuts, a so-called threadless closure is being eniployed b y the Bureau of Mines for all converters, scrubbers. and traps t o be used at higher p r v s u m . This replaces the threaded nut by a nut held in place by a gun-breech type of mechanism shown in Figures 6 and 7 . The nu: i. machined with three lugs fitting into three slotlocated in a projecting ridge i n tho vessel wall. Rotating th(. nut 60" effects the closurc-. .4 locking lug ic inserted in onc of the slots. a n , ] all that is needcd is to compress the hloiic,l gasket initially, which, in this case, is a(,voii~plished by a flat nut threaded on the hc,atl. A washer is provided between this flat nut and t hc threadless, main retaining nut to i d u w fi,iciion. This ?losure i:, amazingly 4nipIt. tfJ u.sc and offus few possibilities for I ' I T O I i n
Figure 6 . -4ssembly of Vessel Closiire
T h e general t?-pc of cloaurv pr(zt'c~rrcdib showIi i i i tl(.tail in Figure 2 . Although this closure is specifically dwigrit.d for 5000 pounds per square inch and 500" C., the method (Jf sealing can be adapted t o higher pressures. This closure, is sr.li'-sealed by the internal pressure. The pressure within the vessel, avting on the large surface of the head, is transmitted to the comparatively small surface of the gasket which, in this illustl%tion. is of copper. This gasket is confined by t,he head of the vessr~!, the wall of the vessel, and some retainingsurface. For this particular closure a segmented retaining ring is quite satisfactory. Figure 3 s h o w the construction of this ring. For the initial sealing of t l w closure before internal pressure is applied, some means for raising the head to obtain deformation of the gasket must bc j)r.ovided. A complete closure for a vessel 3 inches in internal ,[iameter is depicted iI1 Figure 4, and, in this case, the initial a t d is obtained by using a small split ring in the narrorv shank of 1)1(, head, with a clamping ring raising the head by pressure against thc split ring. The clamping ring itself is raised by three hex-head screws which, when tightened, bear against the centering ring. When internal pressure is applied, additional deformation This type of closure is originally of British design and is commented on in Tongue's (5) treatise. I t s virtues are ease of operation and suitability for use by rela1 ivrly inexperienced men. As mentioned previously, the copper sOaling ring must be replaced each time, and another fault is the irvcessity for occasional overhauling due t o deformation of thc wymented retaining ring. of the gasket takes place.
with these autoclavw it rra- t(ni!It! 111 expciidcd in removing the rcavTaii1 i from the autoclave and auitahly cleaning thr, autoelarc, n - t t l l ~ . The cleaning p r o c d u r e l i e c o m t ~important in coal hydroyt'iiatioi! I x ~ a u s eof tho largth ehangw in product caused hy rc.lati\-cl!iiiiriutc>aiiiounty of catalyst. Tlicb nwessity for rkan cqui~riiii~iit
TABLE I. PERMISSIBLE WORKISGPRESSGRES .4llonable Internal Working Pressure LjVarious Theories, Lb./Sq. I n . Service Max. blnx. Max. T e m p . , principal deforma- I f a x . strain Max F. stress tion work straln energy hear
\-ewel, Size, l'ressure, & Material o.d., L ' 2 - i n . wall, 2500 Ib. sq. i n . , t ~ p 3*7 e
lj-in
:3-i i n , type 34 I
in. 1000 sq. 1100
'2.in.standardpipe,
t::
'loo
1000
3:b$/sq,
i n . , 1200 l1O0 1300 1400 cj-in. o.d., l V * - i n , 1000 7500 1b./sq. type 347
2600
''O
""
2400
*"''
7070 .i300
5700 4430
5100 397.7
fi000
5060
4670
3980
7-180
6500
~480
6500
5700
3280 ,joSo 2090 1190 10800
4420 2860 1820 1040 i350
3280 5080 2090 1190 9900
4420 2860 1820 1040 8100
3870 2510 1600 912 6640
6630
6470
8100
6840
57X
9
5-in. o.d., 1-in. wall, 5000 Ib./sq. i n . , type 304
1000
December 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Figure 7 . Threacllesc ?Tiit
is increased further when various catalysts are beirig compared as to activity and distribution of product. I n addition, some catalytic effect, due to the stainless steel surface of the autoclave itPelf is perceptible. To avoid these difficulties a liner is now being used, consisting of a Pyrex vessel Jvhich fits quite snugly i n the autoclave. The top is flared, arid a glass well to cover the .stainless steel thermonell insertcid as shown in Figure 8. The vessel and glass thermo~vellare held together by spring clips, n-ith soft porous asbestos gaskets used to permit free f l o of ~ gas. The results have been quite gratifying, both as to more uniforni experinieritd data and a great saving of time in cleaning the metal surfaces of the autoclave and removing the reaction products. I n spite of these improvements,tho hatch operation leaves much t o he desircd in obtaining data. This is particularly true of work f i r elevated temperatures. Uecauhe of the large mass of thrb ~t.sseland the nerd to prevent undue atresses, hrating to reaction temperature is a fairly long process. The reverse, or coolingdown time, is of approxiniattTly equal length. For example, it takw slightly over a n hour to hring the autoclave and its con:eiir* T O 450" C., thix same aniount of tinie is required to cool to a itiic~temperature for venting the gahes and releasing the pres-ure. I s a result it is difficult to define reaction time except :L.< !I wlative or comparative valuel, In addition, the variation
Figure 8 .
Glass Liner for 5000-Pound .iutoclave
in kiydrogen partial pressure during the course of the reaction i i i somc instances makes interpretation of the results uncertain arid limits definitely the size of the coal sample that niay be used. The authors are thus limited to comparative data for the effects of most proress variables. The use of the hatch autoc,lave is, however, of great value in this iiivcstigation of rc,action mechanism and vatalyst activity. COYTINUOUS AND SE\IICONTINUOUS BENCH SCALE U h I T S
The continuous and semicontinuour units for high pressure and high temperature reactions utilize vessels simllar in dcqign to the batch autoclaves previously diseussd. Closurei ale usually provided at both ends, and any of the tvpc,s mentioned are used, depending on the temperature arid preisure. Liners, usually of carbon steel, are quite s u c c r d u l in decreasing maintenance requirements. Thwe lincrs are disccircled after rach
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
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Vol. 39, No. 12
personnel arc not permitted to enter any high prrs~ure-higlt temperaturr area, a leak in a conriection would n t'c'Ps9ltatt. _-.' shut-down of the equipment. These leaks are now practic~ally eliminated by thP use of a spring-loaded connection showrl ir, e of the hold-down nut is replarcti tiy B ich follo\vs any minor differcrii~i~ i t i t'xpansion caused by thermal changes. This type of connc:cti~~ii Iused at all rcactura and preheaters and is quite satisfactory. v a l v c ~suitable for use in high pressure-high tc:iiiiJ"rtirut~ice are commercially available but, in many cases, not q u i r t suitahlc~for bench scale equipment. This is particularly trutn.hcii tliroi tling valves for very small flows ai'e r e q u i r d , nhcn extrcnicly t,rosive service i. c~nc~outiterrd,or n.Iien standard a r r not desircd. Standard 1)ipe-thrcad or flanged i~o~iiic~rtioiis romnicrcial valves arr r i o t u~c'ilaliove pressures of 6000 pouritir per q u a r e inch in our cwal hydrogenation experimental work an(? have not been found satisfactory, in spite of many 81tiinipls. rl-br high teniperature ivork or fine, throttling service pressures. This unsatisfaci or? record ip due primarily t u tht. severe service accorded va1vc.s arid fittings as a rciult of freqiieii: tlimiantling and reassembly of espcii~inientalequipnicnt, u i i i thy wide range of f l o w d e m a n d d t u pvrniit the greatest pussibli! ~ i i c of equipment purchased. TI ci~nsiderationa:ut; responsible , t h c construc:ion by our ULVI, for tlie clr~sigiiand, in mail!' Iiersonnel of most of the v a l v c ~no\^ i i r use at the Bureau of lliiic-. This 1abor:itory requirtla a great niiiiiy shut-ofi, throt tling. and c-hecl; va1vc.s which are qiiirc siniilai, t o the ordinary coniinercial which are rnade valves anti differ only iii t hr c a i i t l c~)~iiiections, suitable for the sleeve antl c o u d t ul~ingunion prcviously ineiitioned arid illustrated iii Figurc !I: t l i c t pressure ratings, n-hil-h vary from 5000 to 15,000 p(JuntiS 1~c.i.q u x r i x inch: and perniissihle ti~nipcraturcs,rrhir~liare i n alniust all r;i-pc valvcs with fiiic111rc~ids oil thr solid - t i s n > anrl with typcs of plugs v:iryiiig u.itt
? I
nut
s!ee?'e
coned to/ 60°0ngle Figure 9 .
ibsrnilrl?
of Tubing Connrc*tioii.
ruii. Thc iiiaiii additionti1 featuws (ii' Ii i c > continuou~ti1111 svniicontinuous plant B whic~li rtquirc carc~i'ul design arc pipirig and instrument at ion. Two types ~ t pipiiig ' c t ~ i i i i i ~ i * t:LK> i ~ ~ i ni ~use iui. ( \ \ ( I .-izc,s of high pressure tubing. The tuliing ' arc' 5,g-inch o.d. b>inch i d . , bot11 fatihough this tubing i.> suitr squarr inch a1 500' (.'. purposes of unifornii~y. f cviiinc~c.tioiis u s t d . Thc sleeve-type connection for rhts '8-inc.1i o.d. tubing is quite simple and effieieiit. The tubing is coned to an angle of 58" anti fits into a 60" anglc seat i i i the fitting, valve, or vessel. A alec~ve is screwed oii t h e coned tubing and thc coned tuhing forced into the seat by using a hold-donn nut Ij,'8 inches in diametcr, The make-up of this connection requires a certain amount t i j experience, and usually bix t i 1 rzigl onth,* of training are ri.quired before nitsn are permitted I ( I riihlc high pressurts piping ul Tcw nnd crosses are made ~f without very c a i ~ ~ i supervi~ioii. stainless s i e d blocks suitably boi,cd, t h i w d r d , and coned. Thc, 6/16-inch o.d. tubing is connected by the use of a steel cinch and a hold-down nut which forces the cinch to compress, grip the tubing and seal against a rounded seat. Figure 10 s h o w this typo of connection for 7500 pounds per square inch. This type of connection has been adapted from the connection in use at t h r Fixed Sitrogcn Laboratories for many years. For these pressures the cinch is screwed on the tubing and silver-soldered in place. T h e hold-down nut forces the angle of the cinch against a rounded seat in the fitting. For lower pressures (below 5000 pounds) the compression of the cinch is used to hold thc tubing in place. A great deal of trouble is experienced in keeping connections tight when subject to thermal shock. This applies particularly t o the cone connection used for the b's-inch o.d. tubing. Since
bg
0.0. tubing
Tee
silver-soldered
\
Rounded ' s e a t for cinch Figure 10.
\ Weep
issenibly of Tubing Fittings
December 1947
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Thrust plate Spring-I oaded sleeve
the seat. On account of the small-borc inlet ( l , : ~inch), this valve is suited for only fairly clean fluids but finds a multitude of uses in our bench scale equipment even a t lo\!- pressures. for accuratcb regulation of small flow rates. T h e valve preferred for use with fluids containing solid material of a n erusive nature is sho\\-n in Figure 13 and is thc rcwlt of many changes and much esprrinientation. Ttir seat :mil ricedli. are provided ivith ins
