How To Design Crude Distillation Watkins - 1969

 

SPE I L REPORT

How to  esign  rude  istillation

Whether you are ma maki king ng prelim imin ina ary or det etai aile led d designs for atmospheric crud cr ude e dist distil illa lati tion on columns this process design gui uid de should aid you  

atkins Sug Sugar ar Land Texas

THE DESIGN an d ope operat ration ion of crude crude oil oil dist distil illa lati tion on units ar e still d on on e almost exclusively o n an emp empiri irical cal bas asis is.. This is be beca caus usee cr crud udee petr petrol oleu eum m an d its products products asi aside de from fro m ligh lightt en ends ds pr prod oduc ucts ts are are made made up of a large nu mb mber o f discrete hydrocarbons each co comp mpou ound nd bei eing ng pr pres esen entt

in rela relati tive vely ly sm smal alll am amou ount nt.. I t is sim simpler t hen hen to t l of boil iin ng r an ang es es when when describing t he he gross properties of a crude an d its fr fract actio ions ns.. B SIC SIC PRIN PRINCI CIPL PLES ES Firs Firstt let let us co con nsi sid der some of the the bas asic ic steps for se sepa para rattin g cru crude oil i nt nt o fractions wit wit h ho o ut ut r eg eg ar ar d for the effect o f side st stri ripp pper erss or st stri ripp ppin ing g stea steam m. Th e sep separa aratio tion n disdiscussed sed h er er e will involve a v aap p or or over erh hea ead d pr o od duct iv

sidestream sides tream products an d a resid residua uall liqu liquid id bo bott ttom om prod produc uct. t. A small amount of extra va vapo pori rizat zatio ion n cal calle led d ove verf rfla lash sh will b e u s e d to i n s u urr e th e de desi sire red d vapo vapori riza zati tion on occurs and and to p ro ro v vii de de some reflux for t he he bott om om section of the tower.  Present Addre .. : Cr Creo eole le Pe Petr trol oleu eum m Cor Corp. p.

Amu Amuay ay Vene Venezu zuel elaa

g Y D R O C A R B O N PROCESSING

Decembe Dec emberr 1969

 

HO W  

DESIGN CRUDE DISTILLATION

• • •

• •

 nv  n vestment

ost osts s must must  e

 he   rt rt of Crude Distillation T h e art of process cess design sign for crude crude dist distiil-

lation reli lation relies es heavi heavily ly upon upon empir empirica icall corr correl elaation tions. s. This This articl clee uses the the meth method odss pr prop opos osed ed 1 b y Packie an and d Edmi Edmist ster er 5 as a ba back ckgr grou ound nd for for maki making ng pr prel elim imiinary nary calculations. Late Laterr corr correl elat atio ions ns can be subs substi titu tute ted d int into o this this design proc proced edur ure. e.   an y desi design gner erss st stan anda dard rdiz izee their their procedure so that it can be pr ogramed for a computer. Then t h ey can give m o r e att ention to the establ establish ishmen mentt of an optimum design.  nany ev even entt the the method usua usuall llyy is si simi mi-la r to t h e one given here. • Basic Pr Prin inci cipl ples es . . . . . . . . . . . . . . . . .   • • • • •

Desi esign Pr Proc oced edur uree   ...........  Crude Oi l Evaluation Charge Co Ch Conditions Column Conditions Co Summary

Pa[Je 9 9

101

102 104 106

Usua Usuall lly y it is no t feasible to use a rebo il iler on a c r u de tower Thus th e cha charge rge usual usually ly is heated to its its maximu maximum m temp temper erat atur uree be befo fore re be bein ing g fed into into the the colu olumn. mn. All distil late products ar e vaporized as the hey y ente enterr the the tow ower er.. Thus th e t ot ot aall h ea ea t r eq eq ui uir ed ed by t he he process m us us t be contained in th e feed as it leaves th thee ch char arge ge furn furnac ace. e. Various ways ways of produ produci cing ng dist distil illa late te produc products ts fr from om th e feed v a p o r a re re shown in Fig. 1 Th e symbols used t o

weighed  g inst oper ting

lighter fra fracti ctions ons D 2   D s  D 4 an d DB Th e over overhead head fr from om the the seventh dr drum um is a va p o orr p rro od uc uc t VB Each of these separations occurs from cooling a nd so that

equilibriu equil ibrium condensati conde nsation on th the e onen equi eqents uili libr briu ium m iqui uid d co cont ntai ains ns sm mal all l co conc ncen entr trat atio ions ns of co comp mpon ts li ligh ghte terrliq th than an the de desi sire red d di dist stil illa late te.. Also th thee eq equi uili libr briu ium m vapor contains some of the   o m p o n e n t ~ wh whic ich h bel elo ong in th thee li liqu quid id so that the the ne next xt fr frac acti tion on to be con ond den enssed will ha have ve som some und ndes esiir ab able le he heav avy y ends ds.. Howe However ver it woul would d be po poss ssib ible le to us usee li liqu quid id re refl flux ux to was ash h ba back ck the he heav avy y comp compon onen ents ts from th thee va vapo porr an d to use str ipp in g vapors to strip light components fr from om the liqu quiid. Th e first appr approa oach ch to ge genu nuin inee fr frac acti tion onat atio ion n is shown in t h hee second t rraa iin n of Fig. 1 E ac ac h o f the ve vess ssel elss in this this tr train ain re repr pres esen ents ts on onee se sepa para rati tion on sect sectio ion. n. This type of unit is une unecon conomi omical cal si sinc ncee heat is re remo move ved d on only ly in the final co cond nden ense serr and is a v a i l a b l e at a te temp mper erat atur uree level is which too lo low w for for re reco cove very ry an d us usee el else sewh wher ere. e. Further Further m or or e the v aap po orr a nd nd l iiq q ui ui d traffic t h hrr ou ou gh gh o ou u t th e column  y p

U

increa easses ma mark rked edly ly fr from om th thee bo bottto tom m to th e to p stages. A plot of t h e vapor an d li liqu quid id tr traf affi ficc in each of the three de is desi sign gnat ated ed ty type pess of fraction fractionatio ation n sho w wn n in Fig . 2 Si Sizzing a to tow wer based on cond ndiiti tio ons at the to p tray re resu sult ltss in a m u c h l ar ge r towe r d i a m e t e r for Type U than re re q ui r e d for e i the r a T y p e A or a T ype R u ni t . T he s e l a t t e r two two types of un unit itss ac acco comp mpli lish sh heat re reco covc vclY lY by withdraw withdraw ing ing on onee of th thee in inte tern rnal al st stre ream amss of the tower tower cooling cooling it and ap o orr a nd nd liquid returning it t o t he column. Thus th e v ap traffic is dim dimini inishe shed d above above these these withdrawal withdrawal se sect ctio ions ns..

d cs cs ig ig n naa te te sowith me otfh ethety sepemof e thseparation: ods were cType h o s en U f o rdesig eas y association nates that th thee si side dedr draw aw po poin ints ts ar aree unrefluxed ev even en though though there is reflux at t h hee to p of the la last st co colu lumn mn.. Type A uses an exte external rnallyly-circ circulat ulated ed flui fluid d which which be beha have vess as an absorption oi oill for for heat remo remova vall pu purp rpos oses es.. Type Type R de desi sign gnat ates es th e use of an exte extern rnal al reflux which is an equil equilibri ibrium um liquid liquid with respect to t h he e t rra a y on wh ic ic h it re re-e -ent nter erss the tow tower er.. Flash h rums rums Befo   r i s o f Flas Before re disc discus ussi sing ng th e foregoing three me meth thod odss of separa separatio tion n co cons nsid ider er how fra fract ction ionss havin g th e de desi sire red d bo boil ilin ing g rang ranges es ca can n be made made us usin ing g a series of flash drum drumss. Cons Consid ider er the to p train of Fig. 1 in which th the e furnace eff efflue luent nt enters enters th e first drum wh wher eree the the re resi sidu dual al liquid W is sepa separa rate ted d from from the the va vapo por. r. T he first drum effluent v a ap po orr is cooled j us us t e no no u ug g h to condense th e over flash   o  so that i t will be s e p paa r a t e d as a l iqui d in the th e

second drum. T he ov over erfl flas ash h le leav aves es the syste stem as part of th e re resid sidual ual liquid liquid W T h e vapor f r o m t h e second drum is cooled oled agai again n to some prede predete term rmine ined d temper temperatu ature re and en ente ters rs a· third flash. Th er e the the heav heavies iestt disti distill llat atee product   is re moved as a ·liquid. Th e process is repeated to p l o d ~ e successively

Fig I Wa ys o f separating a crude i nt nto f rra a ct ct iio on s are com· pared with a series of fl flas ash h drum drums s

Bec

 

b\ll : J:969

HYDROG RBON PROCESSING

cost s to select the best type o crude distill tion   Th e mo more re comm common on way of at atta tain inin ing g inte terrme med d i heat a te recovery is shown by T y yp p e A. Hea Heavy vy li liqu quid id is w i thdraw n f rom t h e t ray above t h e d r a w tray cooled externally an d r et et ur ur n ned ed to t h hee column t h hrr eeee trays above t h e draw tray. This type is ca call lled ed a pump pumpar arou ound nd reflux remo mova vall of heat in th e si sid de co coo oler results in system. Th e re a smaller requ requireme irement nt for overhe overhead ad re refl flu ux. Th This is te tech chni niqu quee is generally employed at a low a n d at an intermediate p o i n t in t he he col col umn umn w hi hi ch ch makes h eeat at available at hvo yp

d i f f e r e n t temp temper erat atur uree levels for ex exch chaange aga against cr crud udee o i l charge. This met metho hod d ha hass the the ad adva vant ntag agee of stab abiilizing vapo vaporr an and d r e f l u x liq liquid uid flow thr throu ougho ghout ut th e sec sectio tion n where where it is used.

I t materially re redu duce cess vapor vapor an d liqui liquid d tr traff affic ic th thro roug ugho hout ut t h e enti entire re co colu lumn mn.. Th Thes esee two poi oin nts can be seen in Fig. 2 T h e disa disadv dvan anta tage ge of this his me meth thod od is that the th thre reee trays orr h e ea a t rem u s e fo remova ovall must must be co cons nsid ider ered ed as only one one tra ray y f o r fracti fraction onati ation on purp purpos oses es.. Th This is is because the pump mp a r o u n d liquid is for eign to t h hee zone i n which it is intro d u c e d . This liquid is good ood fo forr heat re remo mova vall pu purp rpos oses es but o f f e r s n o mass mass transfer transfer ad adva vant ntag ages. es.  yp A recti rectify fyin ing g sect sectio ion n em empl ploy oyin ing g su subc bcoo oole led d re refl flu ux is designated Type R. Th This is is a more more ef effficient meth method od for r ecover ing heat fr from om the the system bec ecau ausse it can be used   t every sect sectio ion n of th e column. I t has th thee ad addi diti tio ona nall ad

van.tage of bei n g t ru ru e reflux w h hii ch contributes to frac t i o n at i o n on all all trays. As shown in Fig. 2 a fa fair irly ly un unif ifor orm m v p O an d liquid liquid tra traffi fficc exists in al alll se sect ctiions of th thee tower t h u s m ak ak iin n g i t possible to design t he trays to be more

ef effi ficie cient nt.. This is the the type of se sepa para rati tion on whic which h will be use sed d as the basis for t he he design method to be disc discus usse sed d here. This This me meth thod od ha hass the the se seve vere re disa disadv dvan anta tage ge ho howe wev ver of of that the de re requ quir irin ing hi gher capita capisital l inv invest t. draw Note No te the desi upestmen ex exam ampl ple eggive gihigh ven n er later set toment. wi with thdr aw th e tota totall d osign wgn n liquid and and ro rout utee it to th e side stripper stripper follo followed wed by flow liq cr cru ude-t e-ta-p a-product h eat eat exch exchan ang ge. Th is is met met ho ho d incurs a la larg rgee strip strippi ping ng co cost st.. Th e alt alt er er nat natee meth ethod of s tr tr ip ippi ng ng only the the prod produc uctt while exch exchan angi ging ng ag agai ain nst both both prod produc uctt and refl eflux requ requir ires es two se setts of pu pump mpss and ex exch chan ange gers rs at ea each ch draw draw point oint.. Thus Thus an ev al u at i on of a T y p e R unit versus a T yp yp e A u ni ni t ca can n bec eco o me me very comp compllex an and d mus mus t consider inve invest stme men nt and oper operati ating ng costs fo forr the the plan plantt sit site underr stud unde study. y.  ESIG  ES IGN N

PRO E URE

In the sense that en engi gine neer erss un unde ders rsta tand nd dist distil illa lati tion on of

dis iscr cret etee comp compou ound ndss crude crude oil oil fra fracti ctiona onatio tion n is stil stilll a black black ar art. t. Pac ack kie s wor work1 at leas leastt prov provid ides es an elem elemen enta tary ry basis fo forr an anal alyz yzin ing g dist distil illa lati tion on prob proble lems ms.. Th e reader also is re refe ferr rred ed to NeJson son2 and Hengst Hengstebe ebeck ckS for for ba back ckgr grou ound nd in f0n11ation. Other references G- 7 ar e used to deriv rive the the prop prop er erty ty data us used ed in this this discu iscuss ssio ion. n. In the separa separatio tion n of chem chemical ically ly simila similarr materi materials als it is ge gene nera rall lly y true that the g rea reatt er er t he he difference i n boiling point ointss of indi indivi vidu dual al frac fracti tion onss the ea easi sier er it is to se sepa para rate te the fract fractio ions ns.. Th e sharpness of sepa separatio ration n us usua uall lly y is de fined as t he g a p o r o ve r l ap o f the boiling ranges of adjace adj acent nt frac fracti tion ons. s. For For a give given n se sepa para rati tion on a larg largee numb number er of trays will resu result lt in a low low refl reflux ux requ requir irem emen ent. t. Likewise

Fig. 3 These pressures a ld ld s tag tages es are recommended fo r maki making ng pre prelim limina inary ry calc calcula ulatI tIons ons.. V a p o r and liquid   ig liquid load loadin ings gs at th e t op o f a column using inte interst rstage age co coole olers rs be reduced by using

  n

95

H Y D R O C A R B O N PROCESSING

December 1969

 

HO W TO DESIGN CRUDE DISTIlLATION • • •

proper erti ties es of ea each ch fr frac acti tion on   roduct roduct Speci Specificat fications ions T he prop c an an be varied  s requ requir ired ed by sales dema demand nds, s, bu t only at

fe feas asib ible le amount amount of all distillates in th thee atm tmos osph pheeri ricc col ol-umn. T h hee n t h hee size of the va cuum column c a n be minimized. A side ben benef efiit is the maximization of the c ru rud e preheat preh eat prior prior to ente enterin ring g the fu furn rnac ace. e.

the the expense of th thee adja adjace cent nt fr fraaction ions. Genera nerallly the the prorodesigner must esti tim mate the the ma mate teri riaal b ala alan nc e o n the the basis of th thee spec pecific ificaati tio ons give given n for the the desi desire red d prod produc ucts ts.. Th e   sis for most p ro ro d du u cctt specifications for a crude co colu lumn mn are deriv erived ed from from the the me meth thod od prop ropos oseed by American So Socciety iety fo forr Tes Testin ting g Ma Mate teria rials ls  ASTM . Th This is me meth thod od re repo port rtss th thee tempe temperat rature uress at wh whic ich h cert certai ain n po porrtio tions of the the materiall are vapor materia vaporize ized. d. meth tho o d for setti tting p rro od du u ct ct A S TM TM E nd nd Points. On e me specifications to s t a t e t h e ma x i mu m allowable end poin points ts for th thee fra fracti ction ons. s. Th e heav heavie iest st dist distil illa late te pro produc duct, t, call called ed atmo atmosp sphe heri ricc gas gas oil, is ex excl clud uded ed fr from om thi this specification cat ion since it is wit withdr hdraw awn n fr from om th e colu column mn to prov provid idee adequate adeq uate fract fraction ionat ation ion betw between een the oth other er liquid liquid dist distil illa late tess an d the reduc reduced ed cru crude. de.   th thee crud crudee proc proces esssin ing g fa faci cili liti tiees co cont ntai ain n a vacu vacuum um colu mn mn , economics favor the the prod rodu ccti tio on o f the maxim imu um

For the example discussed here, the identity of th e crude oil fr frac acti tion onss sta startin rting g with th e li ligh ghte test st are: are: overh overhea ead d vapo vapor, r, li ligh ghtt naph naphtha, tha, heav heavy y nap naphth htha, a, light light di dist stil illa late te,, heavy heavy distillate, gas oil a nd nd re red duced crude. Isolation of these fractions should be c o m p u t e d on tw two o re repr pres esen enta tati tive ve crudes, one li ligh ghtt and and on onee heavy. Th e ma te r ia l balance will be based on alternately ma maxi xim miz izin ing g the the prod produc ucti tion on of ga gassoli lin ne (nap (naphth hthas) as),, light light dist distil illa late te and hea heavy vy dis distil tilla late te.. So Some me typica typicall ASTM boiling ra rang nges es fo forr thes hese al alte tern rnat ativ ives es are gi give ven n in Table   Key Tempe Temperat rature ures. s. Interme Intermedia diate te temper temperatu atures res usua usuall lly y are spe peci cifi fied ed re relat lative ive to th e prod product uct st stre ream ams. s. These These set set th e g aap p between th thee te temp mpee ra ra ttu u re re a t 5 ASTM for th e heav heavie ierr fr frac acti tion on and the the te temp mper erat atur uree a t 95 ASTM for the the li ligh ghte terr fr frac acti tion on.. Fo r pr prel elim imin inar ary y designs, th thee gaps gaps shown in Table 2 can be used whe n specifications are not given.

a hig he her reflux rate rate will req require a lower n umb umbee r o f trays.

cess

Fig. 4 ASTM  

l

'..;' :96'

VS. TBP 0  

to 0

. 100

to 100 ).

~ . ; . : . : ~ ~ :

 

 

Dece Decemb mber er 1969 1969

1

HYDROCARBON P R O

S ~ l N O

I

I

These spe peccif ific icat atio ions ns must must be tr tran ansf sfor orme med d into into AS ASTM TM point nt sp spec ecif ific icat atio ion n befo before re co conti ntinu nuin ing g with a design. e n d poi T h e method fo r making this ex extra trapo polat latio ion n will ill depe depend nd o n t h e wa y in which th e spec specif ific icat atio ions ns are writte written. n. One One

method uses uses proba probabil bility ity graph paper. On this type of graph th e plot of ASTM tem temper peratu ature re versus volum lume distilled fo r a well fra fra cti ctio o na na te te d product will app appea earr as a straight line or at least nearly stra straig ight ht.. Thus Thus it is possible to de deve velo lop p frac fracti tion on pr prop oper erti ties es from from a mini minimu mum m of information.

mater terial ial bala balanc ncee is estimated Mat eri al Balance. Th e ma

as a f u nc t i o n of th e desi desire red d ch char arac acte teri rist stic icss of the fr fraac tions an d th e amount of c ru ru de de that can be vaporized a t th e o u uttl et et conditions of t h hee furnace. I t is necessary to he n um um b be e r of a ctu estimate t he ctuaa l trays in t he he column and th e op opera eratin ting g pres ressures res of the the colu column mn in or de de r to set the conditions a t th e furn furnace ace ou outle tlet. t. A simplified version o f the the c ru ru d dee column is shown in Fig. 3 T h e number of tray rays that have have been been obse observ rved ed in ex exis isti ting ng inst instal alla lati tion onss are shown alon along g wi with th some recom mendations fo r t r a y numbers to use whe n making pre-

FIe

5 ASTM VS. T P

5 

TABLE l T y p l c a l Product Spa