By A. E. BIRCH
A N DH. M. WEIR
A tla n tic R e fin in g Co.
P h ila d elp h ia , P a.
■ iJl F ig . 1— L o o k in g d o w n on tlie C e n trifix tu r b in e p la te
R
E C O G N I T I O N of the fact th at all black residuals a re not equally black has led to ex act d e te rm in a t i o n o f the contam ination of overhead stream s by asphalt in the continuous distillation o f cru d e in pipe stills. T h e findings have fo rm ed th e incentive fo r the installation o f an effective an d inexpensive cen trifu g a l device fo r th e rem oval of asphaltic fog fro m th e vapors, w hich has resu lted in the p ro d u ctio n of su p e rio r heavy lubricating stocks w ith consequent im p o rta n t econom ies in reducing such stocks to specification colors.M odern pipe stills o p erate so th a t th e liquid-vapor m ix tu re freq u en tly reaches a velocity o f 60-100 m iles p er h o u r at the outlet of the still. A t such velocities any sm all percentage of liquid is w hipped in to a m y riad of tin y d ro p lets w hich a re accelerated by th e to rre n t of vapors to speeds only slightly less th a n th a t o f th e vapors them selves.
I f the m ix tu re velocity is suddenly decreased by the insertion o f an expan sio n d ru m o r by in jectin g the v apors directly into a frac tio n a tin g colum n a considerable p ortion o f th e d roplets will settle as residue. I t has been show n, how ever, th a t an im p o rta n t sm all p ercen tag e o f residue in th e fo rm of a fine m ist will n o t settle out, b u t is carried fo rw a rd w ith th e decelerated v a p o r stream , finally to contam inate the heaviest liquid overhead stream tak en fro m th e colum n.
D esigners o f pipe still equipm ent have su rm ised fo r a long tim e th a t such phenom ena o ccurred in th e ir d istilla
tion equipm ent, b u t ap p aren tly have n o t ap p reciated the serious m ag n itu d e o f th e contam ination o f viscous oil stream s by residue. Som e baffle a rra n g e m e n t has o ften been in serted fo r en tra in m e n t rem oval an d th e m a tte r , dism issed as solved. H o w ev er, baffles of p ro v en effi
ciency in rem oving the e n train m en t w hich w ould o th e r
w ise occur fro m plate to p late in a high-velocity frac tio n a tin g colum n are q u ite-inadequate fo r rem oval the atom ized d roplets w hich are sw ep t fo rw a rd fro m th e pipe-still outlet. I t ap p ears th a t it is im practical to re m ove th is fog-like e n train m en t by any a rra n g e m en t of solid su rfaces in a stream of slow ly m oving vapors.
I f the concept of baffles in terp o sed in slow ly m oving stream s o f v ap o r is carried to its u ltim ate exp ressio n and
a layer o f steel wool used, excellent e n tra in m e n t rem oval is obtained fo r sh o rt periods. B u t an o p e ra tin g te m p e r
a tu re o f 750 deg. F ., o r above, cokes th e liquid enm eshed in the filam ents o f the wool, en tailin g sh u t dow ns and renew als o f th e m aterial.
A f te r a series of effo rts to obtain th e benefits o f th o r
ough rem oval o f en tra in m e n t w ith o u t th e d isadvantages ju s t m entioned, a suitably m odified C e n trifix se p a ra to r (C e n trifix C orp., C leveland) w as in serted in th e v ap o r line of an atm o sp h eric pipe still, ju s t b e fo re the fra c tio n atin g colum n. A ll of th e pipe-still charge passes th ro u g h the device, w hich sep arates liquid fro m v a p o r by c e n trif
ugal force g en erated by high-speed v apors. T h is it accom plishes as cleanly as does a la y e r of steel wool.
In F ig . 1 is a view looking vertically dow n into th e C entrifix w ith the top head rem oved. T h e re a re no m oving p a rts in the device. R a th e r, a ra p id ro ta tio n is im p arted to th e v ap o r-liq u id stre a m by a p a ir o f fan-like, alloy-steel elem ents w ith sta tio n a ry blades, th e u p p er elem ent o f w hich ap p ears in th e figure. T h e p h o to g rap h w as taken a fte r 1,119 h o u rs o f continuous o p eratio n (6 4 6 hours a t an average o f 746 deg. F . a n d 473 h o u rs a t 800 to 805 deg. F . ) , a n d y et th e only evidence th a t th e device h ad been in crude-oil d istillatio n service w as an e x trem ely thin film o f fluffy carbon.
F ig . 3 is a vertical section o f th e C entrifix. A is the tan g en tial v ap o r in let responsible fo r th e first sp in n in g m otion im p arted to th e v apor. A m a jo r p o rtio n o f th e liquid is th u s th ro w n to the sidew all and forced d o w n w ard to the base o f th e device. T h e w h irlin g v apor fogged w ith liquid d roplets th en passes dow nw ard th ro u g h th e sta tio n a ry tu rb in e plate, B , w hich restric ts the m otion o f the vapors to a sm all rad iu s, th u s in creas
ing the ro tatio n al velocity o f th e m ix tu re and decreasing th e distance th ro u g h w hich liquid p articles m u st m ove to escape fro m th e v ap o r stream . L iq u id fo rced to cylin
drical w all, C, by the ro ta tio n is fu rth e r accelerated d o w n w ard by th e v ap o rs an d passes o u t of th e m oving v apor stream th ro u g h the a n n u la r orifice, D . T h e final cleanup of fog is in su red by tu rb in e blade, E , a n d a n n u la r orifice, F .
V ertical sk irts, G and H , are sealed a g ain st v ap o r flow
366 C h e m ic a l & M e ta llu r g ic a l E n g in e e r in g — V o l.4 0 ,N o .7
by p artia l subm ergence in liquid in the bottom of the cylinder, w hich in tu rn is held a t com pensating levels by the liquid o u tlet seals, J, th ro u g h w hich th e unvaporized liquid passes to th e base o f th e colum n. T h e com pletely de-en train ed vapors pass d o w n w ard th ro u g h th e vapor outlet, K , w hich is closely coupled to th e colum n.
D ifferin g levels o f liquid a re held in th e an n u la r space form ed by skirts, G an d H , a condition th a t reflects the p re ssu re differen tial fro m in let to outlet of the de
vice, w hich is of th e o rd e r o f 0.5-1 lb. p e r sq.in. T h is increase of back p re ssu re on th e still has no com m ercial significance in atm o sp h eric distillation practice. T h e pow er cost o f the device is, th e re fo re , nil. T h e absence of an y m oving p a rts accounts fo r its low m aintenance cost, th e la tte r con sistin g only in th e in fre q u e n t replace
m ent o f the tu rb in e plates.
S avings b ro u g h t about by the C entrifix are larg e on account o f th e decreased w eight o f acid req u ired to finish the heaviest overhead lube oil, o r “ H iv is,” to g eth er w ith the g re a te r yield of oil obtainable by reaso n o f the sm aller am o u n t o f acid used. T h ese econom ies resu lt en tirely fro m th e com plete rem oval o f en train ed residue, an im provem ent so strik in g th a t the stocks have a green bloom fre q u e n tly m istak en on casual inspection fo r th at o f a finished oil. T h is applies to lube stocks fro m E a st T e x a s an d o th e r cru d es w hich have n o toriously d ark color resid u es as well as to th e lube stock fro m b e tter type crudes, such as R a n g e r and B urb an k .
B y w ay o f ex p lain in g th e accom plishm ents of the C entrifix, a little h isto ry is desirable. S everal y ears ago th is com pany ad o p ted color m e asu rin g equipm ent and a m ode o f ex p ressin g color values o f oils ch aracterized by d irect p ro p o rtio n a lity betw een th e readings an d th e actual
color concentration. T h is analytical tool has since been developed and im proved u n d e r the nam e of the O ptical D ensity, o r “ O .D .,” color scale. T h e relatio n betw een the O .D . and N .P .A . scales is N .P .A . = 1.081 (O .D . c o lo r)0-32. T h e im p o rtan t featu re of this scale is th a t it provides a m eans of accurately designating th e color of even the d arkest oil stock, by a n u m b er w hich is p ro portional to its color content. F o r exam ple, an oil of 5,000 O .D . color, w hen diluted w ith an equal volum e of w hite oil, will show 2,500 O .D . color, ju s t as a 10- color oil will show a 5-color re su lta n t if sim ilarly handled.
S hortly a fte r the scale w as established it w as applied to the study of distillates and residues fro m ex perim ental vacuum and atm ospheric-pressure pipe stills. T h e m ethods o f operation on the tw o stills and colum ns w ere such th a t w hen M id-C ontinent-type cru d es or topped crudes w ere being distilled to an 8 to 15 p er cent bottom s, the m axim um tem p eratu re attain ed in the vacuum col
um n w as 30 to 50 deg. F . low er th a n th e 800 deg. F . colum n-inlet tem p eratu re w ith the single-flash atrno- spheric-pressure equipm ent. T h e supposition th a t the tem p eratu re difference accounted fo r the m easurably d ark er O .D . colors o f distillate usually obtained fro m the atm ospheric-pressure unit, w as appealing since it carried the w eight of a conventional explanation. B u t it w as observed th at the O .D . colors of the bottom s fro m the several crudes w ere them selves quite d ifferen t a n d th a t the d arkest H ivis side stream invariably w as produced from crudes show ing very high color bottom s. Such results by them selves w ould h ard ly have excited com m ent had the special high-velocity design and operation of the vacuum colum n not been u n d e r scrutiny. A linear velocity o f 40 ft. per sec. o r h igher w as com m only being
4— V isco sity -co lo r re la tio n s R eag an a n d E ast T exas
of H iv is fro m c ru d es
F ig . 2— C e n trifix in s ta lle d on a p ip e s t i ll ; F ig . 3— C en trifix as- F ig.
liq u id seals in fo re g ro u n d se m b ly in cross-section
em ployed an d som e liquid en tra in m e n t w as anticipated.
H ence, as evidence accum ulated th a t bo th colum ns w ere p ro d u cin g an unnecessarily low q uality distillate, it becam e evident th a t an accu rate m eans o f m easu rin g v ery sm all am ounts o f e n train m en t m u st be developed to assist in th e design an d application o f e n train m en t rem oval devices. D istillations o f light h ydrocarbon-oil solutions of a non-volatile dyestuff w ere th e n c a rrie d out so th a t velocities w ere substantially th o se used in com m ercial equipm ent. W h e n th e re su lts w ere tra n sla te d
to fractio n atio n o f cru d e in the larg e atm o sp h eric-p res- su re colum n, it w as fo u n d th a t ab o u t 1 p e r cent by w eight of unvaporized liquid w ould be e n train ed a t a ch arg in g ra te of fro m 250,000 to 300,000 gal. p er day. T h e pipe still then in m ind had tw o parallel passes o f 4-in. tu b in g and th e crude w as flashed to ab o u t 90 p e r cent v a p o r in the 10-ft. diam eter colum n. F o r th e m o st p a rt, th is a m o u n t of en train m en t reflects the fog-like condition built up by th e high velocity in the pipe-still tubes, since the v ap o r velocity in the colum n influences th e e n tra in m ent to a relatively m in o r degree.
I n the ex p erim en ts w ith the dyestuff solutions it w as fu rth e r fo u n d by colorim etry th a t a 9-in. layer of steel wool packed h o rizontally above th e p o in t o f flash w ould reduce th e e n train m en t to a t least 1 /2 0 o f its original value. C alculations show ed th a t fu rth e r im provem ent in e n train m en t red u ctio n w as com m ercially u n im p o rtan t.
A period o f ex p erim en tatio n w ith steel wool, including o peration on larg e stills, show ed th e very real advantages w hich accrue to substantial!)" com plete rem oval o f e n train m en t. H o w ev er, th e coking difficulties w hich h ad been an ticip ated reduced the savings g re a tly because o f the “ dow n tim e ” on the u n it.
F ig . 5— A cid -lre atin g c u rv es f o r R e a g a n a n d E ast T exas H iv is
T y p ic a l C o lo r V a lu es, R e s id u e s a n d H iv is R e s id u e o r B o tto m s
P e r O .D . P e r
C e n t C o lo r, C e n t
O .D . on C e n trifix on
C r u d e C o lo r C r u d e O p e r a tin g C r u d e
R e a g a n ... 33,000 6 1,550 6
E a s t T e x a s . . . 81,500 10 1.650 6
L t . P a n h a n d le 66,400 12 4,000 5
B a r b e rs H i l l . . 29,000 17 1,500 8
H i v i s -O .D . C o lo r, C e n trifix B y p a s s e d *
5,800 11,900 13,400 3,600
P o u n d s o f A c id p e r B a r r e l
V is c o s ity a t 210 d e g . F .
140 140 125 145
♦ A p p ro x im a te ly I p e r c e n t e n t r a in m e n t o n b a s is o f t o t a l v a p o r ( c a lc u la te d ) .
T h is fact, to g eth er w ith the b ack g ro u n d o f experience w ith all types of usual baffle equipm ent, convinced us th a t it w as necessary to em ploy an en tirely new principle if success w ere to be attain ed . I t w as a t th is p o in t th a t th e efforts of the m a n u fa c tu re r w ere co n cen trated on a m odification o f th e C entrifix, w hich ultim ately m et all o u r dem ands o f efficiency a n d sim plicity.
W ith th is d ig ressio n w e pass to som e typical re su lts attain ed w ith th e aid o f th e device. F ig . 4 com pares th e O .D . colors of th e H iv is, o r hig h viscosity distillates, w ith an d w ith o u t th e use o f the C e n trifix device, in th e distillation o f R eag an an d E a s t T e x a s crudes. D istillate color in all cases is a fu n ctio n o f th e com position f o r a given viscosity so th a t fo r com parable resu lts, such as a re show n, th e q u ality o f th e lig h te r viscosity oil w hich v a porizes fro m the H iv is section of th e colum n m u st be held constant. C ertain facts in h e re n t in th e curves m e rit atten tio n . F irs t, a com parison of the p o in ts m ark ed by crosses, in d icatin g points obtained w ith the C en trifix on R eagan crude, show s th a t th e C en trifix rem oves e n tra in m en t as well as th e lay er o f steel w ool, th e cu rv e fo r w hich is the dashed line a t th e ex trem e left. Second, com parison of th e curves fo r th e tw o cru d es ap p aren tly w a rra n ts the conclusion th a t th e C en trifix is relatively m ore effective on E a s t T e x a s cru d e th a n it is on R eag an crude. A s a m a tte r o f fact, th e device rem oves su b sta n tially the sam e p ercen tag e e n tra in m e n t in bo th cases.
T h e difference in a p p a re n t color im p ro v em en t is due to th e h ig h er color d ifferen tial betw een residue an d H iv is in the case o f E a s t T e x a s th a n is th e case w ith R eag an crude.
A cid finishing re q u irem en ts o f b o th C en trifix d istil
lates an d o rd in a ry distillates co n tain in g en tra in e d b o t
tom s are show n in F ig . 5. F o r exam ple, th e C entrifix R eagan H iv is req u ired 38 lb. o f 66 deg. Be. acid p er 50-gal. b a rre l to p ro d u ce a 150 O .D . color p ro d u ct, w hereas th e o rd in a ry d istillate req u ire d 85 lb. o f acid to obtain this sam e resu lt. S av in g s fo r th e C e n trifix p ro d uct, w ith $10 acid, am o u n ted to $0.24 p e r b a rre l. T h is exam ple is by no m eans th e m ost strik in g th a t could be selected. M uch la rg e r acid savings a re possible w hen tre a tin g distillate fro m E a s t T e x a s crude.
D ecreased acid consum ption m u st alw ays be accom panied by increased yields. A lth o u g h th e e x te n t o f the increase will v a ry according to stock a n d tre a tin g p r o cedure, it is a p p a re n t th a t the to tal econom y w ill be a p preciably g re a te r th a n th e value o f th e acid saved. In som e cases th e indications are th a t acid tre a tm e n t m ay be en tirely elim inated an d the stock finished by clay tr e a t
m ent alone. I t is tru e th a t overall econom ies to be m ade by the use of the C e n trifix will depend upon th e circum stances of each application. B ut, in the m ain, it evidently re p resen ts an a u x ilia ry to d istillatio n a p p a ra tu s valuable out o f p ro p o rtio n to its cost, and one w hich m ay find u sefu l applications in distillation o p eratio n s o th e r th a n the case h ere discussed.
368 C h e m ic a l & M e ta llu r g ic a l E n g in e e r in g — V o l.4 0 ,N o .7