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III V a c u u m P r o c e s s i n g T e c h n i q u e s - - III A~ractNo. - - and Reference8 Contd.

38 - - D I S T I L L A T I O N ~ 38

The Technique of Vacuum Still Operation United States. Dist i l la t ion a t reduced pressure pe rmi t s t he separa t ion of h igh boiling oils a t t e m p e r a t u r e s

suff ic ient ly low to avoid the dange r of t h e r m a l cracking. The overhead p roduc t s and side p roduc t s f rom the v a c u u m distillatio~a of c rude oil can be used for t he p roduc t ion of lubr ica t ing oils or as 2atalyt ic or thermal cracking cha rge -wi thou t fu r the r t r e a t m e n t . The b o t t o m produc t s are no rmal ly m a d e in to h e a v y fuel oil or asphal t . If a p ropane de -aspha l t ing un i t is used in con junc t ion wi th a v a c u u m still t he separa t ion of lubr ica t ing oil can be effected a t even lower t e m p e r a t u r e s t h a n in s t r a igh t v a c u u m disti l lation. The p re l iminary d a t a required for t he design of a v a c u u m dist i l lat ion column, such as pressure, t e m p e r a t u r e and s t e a m consumpt ion , can be der ived f rom f lash dist i l la t ion curves ob ta ined on e i ther ba t ch or con t inuous labora tory stills.. Opera- t ional t e m p e r a t u r e s are u sua l ly no t h igher t h a n 850°F. S t e a m requ i r emen t s are largely de t e rmined by economic considera t ions . For example , a dist i l lat ion tower r u n a t a ve ry low pressure needs a large cross-sect ional d i amete r to keep the vapou r veloci ty down. Also, t he p u m p i n g p l an t is larger and t h u s t he m a i n t e n a n c e costs go up. Sui table p u m p i n g m e a n s are : Mechanical pumps , baromet r ic condensers and s t e am jets . Usua l ly a combina t ion of s t e a m je t s and baromet r ic condensers is chosen. The ma jo r factors govern ing the design of v a c u u m co lumns are d iscussed briefly. In general, t h e tower m u s t be des igned to reduce t he residence t i m e of the processed ma te r i a l to a m i n i m u m . W h e n t he pressure is r is ing dur ing operat ion, it is of ten difficult to locate the cause of the trouble. One possible cause is low s t eam pressure. The je ts will no t work properly, if t he s t e a m pressure falls below t he m i n i m u m value for which t h e y are designed. A n o t h e r cause could be leaks. If necessary , leaks can be t raced by p u t t i n g pressure on t he v a c u u m tower for a shor t t ime. Finally, cracking can cause a pressure rise. D i ag rams are shown of three commerc ia l v a c u u m un i t s toge ther wi th an analys is of t he p roduc t s ob ta ined in t hem.

Sommaire: R e v u e g6n6rale des po in t s d e v a n t ~tre surveilMs lors de la cons t ruc t ion e t l 'opdrat ion d ' u n groupe de dist i l la t ion pour la sdpara t ion d 'hui les au po in t d '6bul l i t ion $1ev6, ~ t e m p 6 r a t u r e r6duite. Trois ensembles son t i l lustrds en forme de d i ag rammes .

Rectif ication at Reduced Pressures United States. Lit t le has been publ i shed so far on rect i f icat ion in packed towers a t reduced pressures. P rev ious

inves t iga to r s have found t h a t pe r fo rmance a t reduced pressure is be t te r t h a n a t a tmospher ic pressure, b u t the expe r imen t s referred to were carried ou t in labora tory-s ize equ ipmen t . As it could be expected t h a t certain fac tors such as p ressure drop and l iquid d i s t r ibu t ion have a considerably grea ter inf luence on e q u i p m e n t of pi lot p l an t size the a u t h o r s used compara t ive ly large co l umns in the i r exper iments . The s y s t e m employed was o r thod ich lo robenzene - -o r thod ie thy lbenzeue . T he vapour - l iqu id equi l ibr ium, a l t hough known f rom previous publ icat ions , was re -de te rmined in an O t h m e r still modif ied for v a c u u m work. The va lues ob ta ined are g iven in th ree g raphs showing re la t ive vola t i l i ty aga ins t compos i t ion for 10, 50 and 100mm. H g pressure respect ively. F r o m th i s d a t a a large cha r t was p repared for relat ive vola t i l i ty aga ins t pressure wi th compos i t ion as a pa ra - me t e r which served as a m e a n s of in te rp re t ing the resu l t s of the rect i f icat ion runs carried ou t in t h e course of th i s s tudy . Two co lumns were used, one of which was 6 ft. h igh and 3 inch. in d iameter . The pack ing consis ted of ½ inch carbon Rasch ig rings. T he packed sect ion was 63 inch. h igh and conta ined 10,200 pieces per ft. 8. The free vo lume was 73%. An o rd ina ry shower - type ref lux d i s t r ibu tor was ins ta l led a t t he top of the packed section. In i t ia l ly t he co l umn was r u n a t h igh t e m p e r a t u r e for abou t 30 m i n u t e s to ensure comple te we t t ing of the packing. T h e n t he h e a t i n p u t was reduced to the desired level, p ressure a d j u s t m e n t s comple ted and the co lumn allowed to r u n for abou t 3-4 hour s to ob ta in equi l ibr ium. Fo r ty -e igh t rect if icat ion runs were m a d e in the range of 16-100mm. Hg. The resul ts are expressed in t e rms of HOG (height of t r ans fe r un i t based on overal l d r iv ing force in gas phase) and KGa ( t ransfer coefficient based on overal l dr iv ing force in gas phase) and show t h a t pressure ha s a g rea t effect on HOG. In t he in te rp re ta t ion of t he a u t h o r s th is indicates t emper - a tu re effects of an order of m a g n i t u d e which would no t occur if t he gas f i lm was of control l ing impor tance . I t is concluded, therefore, t h a t the l iquid f i lm is t he control l ing factor. These f indings agree w i t h those of Sherwood and Hol loway publ i shed in Trans. A m . Inst . Chem. Engrs., 36, (1940), 39. The m i n i m u m tower he igh t required for rect i f icat ion opera t ions in t he m o s t favourab le pressure region was found to be 44ram. in the case of t he s y s t e m used in t he p re sen t exper iments .

Somrnaire : Recherches sur les pe r fo rmances de tours ~ dist i l la t ion p e n d a n t la rectif ication.

Improvements in the Simple Distillation of Fatty Acids by Continuous Methods United States. A v a c u u m p l an t is described for the con t inuous dist i l la t ion of f a t t y acids. The crude f a t t y acids

are fed to a d ryer and de-aera tor where t h e mate r ia l is hea t ed to abou t 200°F. a t a v a c u u m of 26-28 inch. Hg. S u b s e q u e n t l y t he processed ma te r i a l r eaches t he s t r ipp ing co lumn where the volati le c o m p o n e n t s are r emoved I n order to avoid po lymer i sa t ion to t a r s or c racking to hyd roca rbons the temperature in th i s process m u s t be low and t he res idence t ime of t he ma te r i a l in t he still a m i n i m u m . This is achieved in the present p l a n t by provid ing for a single pass on ly and f i t t ing bubble t r ay s of a design, where the heat is appl ied t h r o u g h the t r ay s t hemse lves by condens ing D o w t h e r m vapou r s wi th in t he t rays . An i l lus t ra t ion of t he t r ay s is shown. The pressure in t he co l umn is 5ram. Hg. T he m a x i m u m t e m p e r a t u r e to which t he ma te r i a l is exposed is 625°F. The residence t ime var ies f rom one m i n u t e for t he acid vapor ised f rom the top t r a y to abou t 30 m i n u t e s for t he res idue collecting a t the bo t tom. In t he conven t iona l direct-fired po t still residence t imes extend up to

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Article by Q. E. Benedict

Petrol. Ref. 3 x, Jan. 1952

103-106

44/III

Article by H. Bliss,

A. M. Eshaya .N.W. Fl~sch Chem. Engng.

Progr. 4 8 , Dec. 1952

627-632

45/III

January , 1953 Vacuum 98 Vol. I I I No. 1