Sulfuric Acid Requirements for Industrial Alkylation Plants

16 Sulfuric

Acid

R e q u i r e m e n t s for Industrial

Alkylation

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 6, 2017 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0055.ch016

Plants

ORLANDO WEBB Rt. 4, F-12,Lee'sSummit, MO 64063

The most common u t i l i z a t i o n o f the l i g h t o l e f i n s produced from c a t a l y t i c c r a c k i n g and c o k i n g o p e r a t i o n s i s as f e e d s t o c k f o r a l k y l a t i o n u n i t s f o r r e a c t i o n w i t h i s o b u t a n e . The a l k y l a t e p r o d u c t has h i g h r e s e a r c h and motor o c t a n e numbers and i s used i n motor g a s o l i n e b l e n d i n g o r , to a l i m i t e d e x t e n t , r e r u n and used as t h e base s t o c k f o r m a n u f a c t u r i n g a v i a t i o n g a s o l i n e . Of the 125 commercial a l k y l a t i o n u n i t s o p e r a t i n g i n the U n i t e d S t a t e s t o d a y , 59 employ c o n c e n t r a t e d s u l f u r i c a c i d as the c a t a l y s t . T o t a l a l k y l a t e p r o d u c t i o n i s over 850,000 b a r r e l s p e r s t r e a m day, and of t h i s d o m e s t i c t o t a l the d a i l y c a p a c i t y o f the s u l f u r i c a c i d a l k y u n i t s i s i n excess o f 500,000 b a r r e l s ( 1 ) . To produce a s a t i s f a c t o r y p r o d u c t and a v o i d c o r r o s i o n and o t h e r problems the t i t r a t a b l e a c i d i t y o f the a c i d c a t a l y s t must be m a i n t a i n e d a t a r e l a t i v e l y h i g h l e v e l , n o r m a l l y above 88% ( e q u i v a l e n t wt. % H2SO4). I n most p l a n t s t h i s i s a c c o m p l i s h e d by c o n t i n u o u s a d d i t i o n o f f r e s h s u l f u r i c a c i d o f 98.0 t o 99.5% H2SO4 s t r e n g t h and c o n t i n u o u s d i s c a r d o f spent a c i d o f the m i n i mum a c c e p t a b l e s t r e n g t h . F i g u r e 1 i s a b l o c k d i a g r a m showing the g e n e r a l p r o c e s s arrangement. The d i s c a r d a c i d i s e i t h e r r e t u r n e d t o the a c i d s u p p l i e r , o r t o a much l e s s e r e x t e n t , u t i l i z e d e l s e where i n the r e f i n e r y . P u r c h a s e d a c i d c o s t s have i n c r e a s e d i n r e c e n t y e a r s from n o m i n a l l y $20/ton t o $30-40/ton i n many l o c a t i o n s . Typically, the c u r r e n t c o s t of make-up a c i d ranges from a low o f about $0.10 t o h i g h s of over $1.00 p e r b a r r e l o f a l k y l a t e . I t i s one o f t h e major o p e r a t i n g expenses f o r a s u l f u r i c a c i d a l k y l a t i o n u n i t . The purpose of t h i s paper i s t o d i s c u s s the f a c t o r s t h a t a f f e c t f r e s h a c i d r e q u i r e m e n t s and t o p o i n t o u t areas t h a t w a r r a n t close operator a t t e n t i o n . Plant Evaluation Considerations B e f o r e c o n s i d e r i n g s p e c i f i c a c i d r e a c t i o n s and t h e o r i e s , one s h o u l d pause b r i e f l y and t a k e n o t e o f the o p e r a t i n g p r o c e d u r e used on a t y p i c a l commercial u n i t and the e f f e c t t h i s can have on 271

Albright and Goldsby; Industrial and Laboratory Alkylations ACS Symposium Series; American Chemical Society: Washington, DC, 1977.


272

INDUSTRIAL

AND

L A B O R A T O R Y

A L K Y L A T I O N S

the apparent p l a n t p e r f o r m a n c e , i r r e s p e c t i v e o f f e e d i m p u r i t i e s , s i d e r e a c t i o n s , e t c . F i r s t , r e c o g n i z e t h a t the volume of the a c i d c a t a l y s t i n the r e a c t i o n s e c t i o n o f the p l a n t i s v e r y l a r g e compared t o b o t h the f r e s h a c i d make-up and the d i s c a r d a c i d flow rates. For example, t h e r e may be as much as 1,800 barrels o f a c i d i n v e n t o r y i n a 17,000 b a r r e l per day u n i t , and the f r e s h and d i s c a r d a c i d f l o w r a t e s may be o n l y r o u g h l y 30 b a r r e l s p e r h o u r . The c o n f i g u r a t i o n o f the l a r g e s e t t l e r s used f o r s e p a r a t i n g and r e c y c l i n g the a c i d u s u a l l y i s such t h a t even a s m a l l change i n the a c i d l e v e l , say one i n c h , may r e p r e s e n t 45 minutes of t o t a l f r e s h a c i d f l o w . Thus the m a i n t a i n i n g of a c o n s t a n t i n v e n t o r y i n the p l a n t i s a d i f f i c u l t o p e r a t i n g g o a l t o s a t i s f y . P l a n t performance e v a l u a t i o n s s h o u l d t a k e i n t o account the p o s s i b l e e f f e c t o f i n v e n t o r y changes. Second, even assuming t h a t an e s s e n t i a l l y c o n s t a n t volume o f a c i d c a t a l y s t i s m a i n t a i n e d , the o p e r a t o r must d e c i d e what the optimum d i s c a r d a c i d i t y s h o u l d be and attempt t o m a i n t a i n i t by making a d j u s t m e n t s i n f r e s h a c i d and d i s c a r d a c i d f l o w r a t e s . If the d i s c a r d a c i d i t y i s allowed to i n c r e a s e , excessive f r e s h a c i d i s used. I f i t i s a l l o w e d t o d e c r e a s e , a l k y l a t e q u a l i t y d e c l i n e s , u n d e s i r a b l e s i d e r e a c t i o n s i n c r e a s e , and a c i d usage increases-. A decrease i n average s y s t e m a c i d i t y o f 1% t y p i c a l l y reduces a l k y l a t e q u a l i t y by about 0.4 octane numbers. I n c e r t a i n s i t u a t i o n s , a c i d i t y may drop below the p o i n t where the a c i d i s no l o n g e r an e f f e c t i v e a l k y l a t i o n c a t a l y s t but c o n t i n u e s t o absorb olefins. C h a i n p o l y m e r i z a t i o n r e a c t i o n s can a l s o be i n i t i a t e d . T h i s i s accompanied by r a p i d t e m p e r a t u r e r i s e and a sharp f u r t h e r d e c l i n e i n t i t r a t a b l e a c i d i t y . In such s i t u a t i o n s , o p e r a t o r s commonly r e s o r t to v e r y l a r g e a c i d a d d i t i o n s i n an e f f o r t t o b r i n g the e n t i r e volume of a c i d up i n s t r e n g t h . R e c o g n i z i n g the enormous a c i d i n v e n t o r y i n the s y s t e m , i t i s o b v i o u s t h a t such u p s e t s , i n a d d i t i o n t o changes i n a c i d i t y at the b e g i n n i n g and end of an o p e r a t i n g p e r i o d , must a l s o be t a k e n i n t o account i n making a f a c t u a l p l a n t performance e v a l u a t i o n o f average a c i d usage. T h i r d , and now assuming e s s e n t i a l l y c o n s t a n t volume and a c i d i t y are m a i n t a i n e d , t h e r e i s the problem o f d e t e r m i n i n g an a c c u r a t e d i s c a r d a c i d a n a l y s i s . D i s c a r d a c i d samples a r e v i s c o u s and can c o n t a i n a p p r e c i a b l e e n t r a i n e d h y d r o c a r b o n . The h y d r o carbon phase must be c o m p l e t e l y removed by c e n t r i f u g i n g the sample i f the a n a l y s i s i s t o have any meaning o r r e p r o d u c i b i l i t y . Most a n a l y t i c a l l a b o r a t o r i e s r e p o r t the t i t r a t a b l e a c i d i t y o f the a c i d p o r t i o n o f the sample. T h i s i s c a l c u l a t e d from the amount of sodium h y d r o x i d e used i n n e u t r a l i z i n g t h e sample i n a s t a n d a r d t i t r a t i o n p r o c e d u r e . W h i l e the r e s u l t i s e x p r e s s e d as wt.% H2SO4 i t i s i m p o r t a n t t o r e c o g n i z e t h a t the d i s c a r d l i q u i d i s not a s i m p l e s u l f u r i c a c i d s o l u t i o n , but i s a c t u a l l y a v e r y complex m i x t u r e o f s u l f u r i c a c i d , weak o r g a n i c a c i d s , s u l f o n i c a c i d s , s u l f i d e s , a c i d s o l u b l e p o l y m e r i c o l e f i n compounds, c a r b o n , w a t e r , and p r o b a b l y o t h e r compounds depending on f e e d c o m p o s i t i o n and


16.

W E B B

Sulfuric Acid

Requirements

273


i m p u r i t i e s . The n a t u r e and r e l a t i v e amounts of t h e a c i d d i l u t i n g compounds can v a r y and have a pronounced e f f e c t on the make-up a c i d r e q u i r e m e n t and the c a t a l y t i c b e h a v i o r o f the a c i d i n the reactor. W i t h the a c i d i n v e n t o r y c o n t r o l , a c i d i t y c o n t r o l , and anal y t i c a l l i m i t s i n mind, and r e s p e c t i n g t h e i r p o t e n t i a l e f f e c t on r e p o r t e d a c i d make-up r e q u i r e m e n t s f o r any g i v e n o p e r a t i o n , p a r t i c u l a r l y o v e r o p e r a t i n g p e r i o d s o f l e s s t h a n t h i r t y d a y s , l e t us now t u r n t o some of t h e s p e c i f i c f a c t o r s a f f e c t i n g a c i d usage. Feed I m p u r i t y

Effects

The most s i g n i f i c a n t i m p u r i t i e s i n the f e e d streams t o a t y p i c a l a l k y l a t i o n u n i t are e t h y l e n e , d i o l e f i n s , s u l f u r compounds and w a t e r . C o r r o s i o n i n h i b i t o r s and o t h e r c h e m i c a l s used i n ups t r e a m p r o c e s s i n g can a l s o be p r e s e n t i n some c a s e s , and t h e s e can have h a r m f u l e f f e c t s . The amount o f each i m p u r i t y t h a t r e a c h e s the a l k y l a t i o n r e a c t o r v a r i e s c o n s i d e r a b l y from r e f i n e r y to r e f i n e r y . I f a c c u r a t e l y determined and p r o p e r l y accounted f o r , t h e s e i m p u r i t i e s can e x p l a i n an a p p r e c i a b l e p e r c e n t a g e o f the a c i d make-up r e p o r t e d by v a r i o u s o p e r a t i n g u n i t s . The impur i t y d a t a shown i n T a b l e I can be used t o e v a l u a t e the m e r i t of improved upstream p r o c e s s c o n t r o l and/or more e f f i c i e n t f e e d p r e t r e a t m e n t methods. Table I .

Feed I m p u r i t y E f f e c t s

Compound

S u l f u r i c A c i d D i l u t i o n Rates ( B a s i s 98.5% F r e s h , 90.0% D i s c a r d )

Ethylene Butadiene S u l f u r Compounds Water

3952 Lbs. per B a r r e l 2465 Lbs. per B a r r e l 15-60 Lbs. p e r Pound of 4116 L b s . per B a r r e l

Sulfur

Ethylene I f s u f f i c i e n t i s o b u t a n e i s a v a i l a b l e , most r e f i n e r s today are c h a r g i n g a f u l l C3-C4 cut t o a l k y l a t i o n . In t h e s e o p e r a t i o n s the c o n t r o l of the f e e d d e e t h a n i z e r can have a n o t i c e a b l e e f f e c t on the amount of f r e s h a c i d needed f o r the a l k y u n i t , and y e t the c o n c e n t r a t i o n o f e t h a n e - e t h y l e n e i n the f e e d may appear t o be i n s i g n i f i c a n t . E t h y l e n e r e a c t s w i t h s t r o n g s u l f u r i c a c i d t o form e t h y l s u l f u r i c a c i d , r a t h e r than combining w i t h i s o b u t a n e t o form an a l k y l a t e . The e t h y l s u l f u r i c a c i d becomes a p a r t o f the d i s c a r d a c i d m i x t u r e and t i t r a t e s as a weak a c i d , t h e r e b y d e c r e a s i n g the c a l c u l a t e d d i s c a r d a c i d i t y and r e q u i r i n g the a d d i t i o n o f more f r e s h a c i d f o r c o n t r o l . On a mole f o r mole b a s i s , and assuming an a c i d d i l u t i o n range of from 98.5 t o 90.0% ( w t . ) , the f r e s h a c i d make-up r a t e i s 3952 pounds of a c i d per b a r r e l of e t h y l e n e


INDUSTRIAL

274

A N D

L A B O R A T O R Y


i n the f e e d . T h i s c a l c u l a t e d f i g u r e i s c o n s i s t e n t w i t h r a t e s r a n g i n g from 2940 t o 4620 pounds p e r b a r r e l , w h i c h t h e w r i t e r has found i n common i n d u s t r i a l use. The a l k y l a t i o n a c i d p e n a l t y f o r e t h y l e n e i s so l a r g e i t makes c l o s e c o n t r o l o f t h e f e e d d e e t h a n i z e r a b s o l u t e l y mandatory f o r e c o n o m i c a l o v e r a l l o p e r a t i o n . The ethane and l i g h t e r c o n t e n t o f the C3-C4 c u t s h o u l d be m a i n t a i n e d n i l as measured by c o n t i n u o u s chromatography. Even a t r a c e amount o f C 2 s may r e p r e s e n t seve r a l b a r r e l s o f e t h y l e n e p e r day. I n a d d i t i o n , i f a s l u g o f e t h y l e n e i s a c c i d e n t a l l y charged t o the a l k y l a t i o n r e a c t o r i t can a l s o have t h e e f f e c t o f suddenly r e d u c i n g t h e s t r e n g t h o f t h e a c i d c a t a l y s t t o the p o i n t t h a t a l k y l a t i o n c e a s e s , o l e f i n a b s o r p t i o n r a p i d l y d i l u t e s t h e a c i d , and c h a i n p o l y m e r i z a t i o n o f o l e f i n s ( " a c i d runaways") can o c c u r .


f

Diolefins D i o l e f i n s a l s o occur i n r e l a t i v e l y s m a l l concentrations i n o l e f i n f e e d streams. T h e i r c o n c e n t r a t i o n i n c r e a s e s as c r a c k i n g s e v e r i t y i n c r e a s e s , such as i n f l u i d c o k i n g . As i n the case o f e t h y l e n e , t h e b u t a d i e n e s do n o t appear t o r e a c t w i t h i s o b u t a n e i n the presence o f s t r o n g s u l f u r i c a c i d . The d i e n e s a r e b e l i e v e d t o form r e a c t i o n p r o d u c t s , most o f w h i c h a r e a c i d s o l u b l e , and i f t h i s g e n e r a l p r e m i s e i s a c c e p t e d t h e f r e s h a c i d make-up r a t e f o r a d i l u t i o n range o f 98.5 t o 90.0% (wt.) can be c a l c u l a t e d t o be 2465 pounds o f a c i d p e r b a r r e l o f b u t a d i e n e . Industry p r a c t i c e i s t o use r a t e s i n t h e range o f 1890 t o 4200 pounds p e r b a r r e l . S e l e c t i v e hydrogénation has been s u c c e s s f u l l y employed comm e r c i a l l y t o convert the butadienes t o mono-olefins. Current s u l f u r i c a c i d p r i c e s give considerable j u s t i f i c a t i o n f o r adding these f a c i l i t i e s i n cases where s e v e r e c r a c k i n g c o n d i t i o n s a r e employed. S u l f u r Compounds B e f o r e b e i n g charged t o an a l k y l a t i o n u n i t , o l e f i n streams are n o r m a l l y t r e a t e d f o r removal o f s u l f u r compounds. The type o f t r e a t i n g used depends f o r t h e most p a r t on the amount and k i n d o f s u l f u r compounds p r e s e n t . Q u a n t i t i e s a r e n o r m a l l y r e p o r t e d i n parts per m i l l i o n u n i t s . According to industry sources, estimates o f t h e amount o f s u l f u r i c a c i d d i l u t e d l i e i n a range o f 15 t o 60 pounds p e r pound o f s u l f u r i n t h e f e e d . A v e r y commonly used f a c t o r i s 20 pounds p e r pound. I n t h e case o f m e t h y l m e r c a p t a n , i f one assumes a mole f o r mole r e a c t i o n o c c u r s and t h a t t h e r e a c t i o n p r o d u c t s a r e s o l u b l e i n t h e a c i d and do n o t t i t r a t e , then t h e c a l c u l a t e d amount o f a c i d d i l u t e d i s 53.7 pounds p e r pound o f mercapt a n s u l f u r ( d i l u t i o n range 98.5 t o 90.0%). This i s a very severe assumption however: commercial t e s t s i n d i c a t e about 45 pounds p e r pound and t h i s i s p r o b a b l y more r e a l i s t i c . Somewhat lower d i l u t i o n r a t e s a r e n o r m a l l y assumed f o r H2S and COS, t y p i c a l l y r a n g i n g between 15 and 18 pounds p e r pound o f s u l f u r p r e s e n t .


16.

W E B B

Sulfuric Acid

Requirements

275

In a d d i t i o n t o the d i l u t i o n e f f e c t , the p r e s e n c e o f the s u l f u r r e a c t i o n p r o d u c t s i n the a c i d c a t a l y s t promote p o l y m e r i z a t i o n and o t h e r u n d e s i r a b l e s i d e r e a c t i o n s . I t i s t h e r e f o r e o b v i o u s t h a t o p e r a t i n g companies s h o u l d check t r e a t e r performance f r e q u e n t l y and make w h a t e v e r changes are n e c e s s a r y t o a c c o m p l i s h a h i g h degree of d e s u l f u r i z a t i o n o f the a l k y l a t i o n f e e d s t o c k .


Water Water i n f e e d s t o c k s , p a r t i c u l a r l y t h a t i n excess o f s a t u r a t i o n , i s an i m p o r t a n t p o t e n t i a l a c i d d i l u e n t . There are s e v e r a l types o f s e p a r a t o r s and d r i e r s i n commercial use on a l k y l a t i o n u n i t s . The s i m p l e s t are c o a l e s c e r d e v i c e s packed w i t h e x c e l s i o r , o r o t h e r k i n d s of p a c k i n g m a t e r i a l . W i t h p r o p e r maintenance, these d e v i c e s can c o a l e s c e and remove s i g n i f i c a n t amounts o f f i n e l y d i v i d e d and e n t r a i n e d w a t e r d r o p l e t s i n the o l e f i n and i s o b u t a n e s t r e a m s . F o r b e s t r e s u l t s the f e e d streams are c h i l l e d by h e a t exchange w i t h the c o l d h y d r o c a r b o n e f f l u e n t b e f o r e they are charged t o the c o a l e s c e r s . I n one p l a n t t e s t , a 5 5 - g a l l o n drum o f w a t e r was drawn o f f such a d e v i c e every 12 h o u r s . The p l a n t was o p e r a t i n g a t an a l k y l a t e p r o d u c t i o n r a t e o f about 2,000 b a r r e l s per day. A problem w i t h these s e p a r a t o r s , however, i s t h a t they develop c h a n n e l i n g i n t i m e and t h e i r w a t e r removal e f f i c i e n c y d e c l i n e s r a p i d l y , u n f o r t u n a t e l y t h i s performance d e t e r i o r a t i o n and need f o r p a c k i n g replacement o f t e n goes u n d e t e c t e d , s i n c e t h e r e i s no s i g n i f i c a n t change i n p r e s s u r e d r o p , o r o t h e r d a t a t o a l e r t the o p e r a t o r . The r e s u l t i s t h a t f r e s h a c i d usage s t e a d i l y i n c r e a s e s i n many l o c a t i o n s . A more r e l i a b l e type o f s e p a r a t o r w h i c h i s sometimes used i s a guard chamber packed w i t h r o c k s a l t . These u n i t s can be b u i l t l a r g e enough so t h a t onstream time can be q u i t e l o n g . F i e l d d a t a i n d i c a t e e s s e n t i a l l y complete removal o f f r e e w a t e r and, i n some c a s e s , even a p o r t i o n o f the s o l u b l e w a t e r . The c o a l e s c e d w a t e r i s w i t h d r a w n as a b r i n e s o l u t i o n . Maintenance c o n s i s t s o f s i m p l y r e s t o r i n g s a l t l e v e l w h i l e the u n i t i s b e i n g by-passed f o r a s h o r t time. The i s o b u t a n e r e c y c l e from f r a c t i o n a t i o n can a l s o c o n t a i n a p p r e c i a b l e e n t r a i n e d f r e e w a t e r . I f the r e c y c l e c o n t a i n s no o l e f i n s (from o u t s i d e make-up s t r e a m s , f o r example) t h i s s t r e a m can be c h e m i c a l l y d r i e d by c o n t a c t i n g i t w i t h t h e d i s c a r d a c i d f r o m the u n i t . T h i s i s an e f f i c i e n t and a r e l a t i v e l y i n e x p e n s i v e p r o cedure . The most e f f i c i e n t d e v i c e s a v a i l a b l e f o r complete w a t e r removal employ m o l e c u l a r s i e v e s . These are w i d e l y used on h y d r o f l u o r i c a c i d u n i t s but have been d i f f i c u l t t o j u s t i f y f o r s u l f u r i c p l a n t s . They can a l s o be u t i l i z e d to remove any s u l f u r compounds r e m a i n i n g i n the f e e d a f t e r p r e t r e a t m e n t ( 2 ) . I f s u l f u r i c a c i d p r i c e s continue t o i n c r e a s e , f u r t h e r c o n s i d e r a t i o n of molecular s i e v e s seems w a r r a n t e d . The f r e s h a c i d make-up r a t e f o r w a t e r d i l u t i o n i s 4116 pounds



276

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o f a c i d p e r b a r r e l o f w a t e r i n t h e f e e d , a g a i n assuming an a c i d s p e n d i n g range o f 98.5 t o 90.0% ( w t . ) . F u r t h e r m o r e , i n t h e v i e w o f many t e c h n o l o g i s t s , i n c r e a s i n g t h e w a t e r content o f t h e a c i d i n the r e a c t o r has t h e a d d i t i o n a l u n d e s i r a b l e e f f e c t o f r e d u c i n g i t s e f f e c t i v e n e s s as an a l k y l a t i o n c a t a l y s t ( 3 ) . I f t r u e , t h i s c o u l d r e s u l t i n t h e f o r m a t i o n o f more a c i d s o l u b l e r e a c t i o n p r o d u c t s and t h e r e f o r e a f u r t h e r i n c r e a s e i n t h e a c i d make-up. Water a l s o i n c r e a s e s t h e c o r r o s i v e n a t u r e o f t h e a c i d i n t h e r e a c t i o n s e c t i o n . Metal l o s s e s i n the t u r b u l e n t high v e l o c i t y s e c t i o n s o f r e a c t o r s can be q u i t e r a p i d i n u n i t s c i r c u l a t i n g h i g h w a t e r content a c i d s . Reactor Operating

Conditions

The p r e v i o u s d i s c u s s i o n has been concerned w i t h i m p u r i t i e s i n f e e d streams and t h e i r e f f e c t s on a c i d make-up r e q u i r e m e n t . A l l o f t h e a c i d d i l u t i o n r a t e s a r e l a r g e and t h e r e s u l t i n g f r e s h a c i d c o s t s a r e adequate t o support e f f i c i e n t feed f r a c t i o n a t i o n and t r e a t i n g f a c i l i t i e s , f r e q u e n t maintenance and c l o s e o p e r a t o r o b s e r v a t i o n and c o n t r o l o f upstream o p e r a t i o n s . Assuming t h i s i s done, t h e major r e m a i n i n g concern i s t h e o p e r a t i o n o f t h e a l k y l a t i o n u n i t proper, p a r t i c u l a r l y the reactor s e c t i o n . In making a l l o p e r a t i n g and d e s i g n d e c i s i o n s , i t i s i m p o r t a n t to keep i n mind t h e d e f i n i t i o n o f t h e t r u e r e a c t i o n zone. Fundam e n t a l l y , t h i s i s t h e i n t e r f a c i a l a r e a between t h e i m m i s c i b l e h y d r o c a r b o n and a c i d c a t a l y s t l i q u i d phases i n the r e a c t o r . React a n t s and p r o d u c t s f l o w a c r o s s t h i s boundary. The o l e f i n s i n t h e feed stream react instantaneously w i t h the s u l f u r i c a c i d c a t a l y s t and combine w i t h t h e r e l a t i v e l y s m a l l amount o f i s o b u t a n e p r e s e n t i n s o l u t i o n i n t h e a c i d c a t a l y s t t o form a l k y l a t e . A l k y l a t e p a s s e s o u t through t h e i n t e r f a c i a l s u r f a c e r e a c t i o n boundary i n t o the h y d r o c a r b o n phase w h i l e i s o b u t a n e passes i n t o r e s a t u r a t e t h e c a t a l y s t . To s u p p r e s s u n d e s i r a b l e p o l y m e r i z a t i o n and o t h e r r e a c tions i t i s necessary t o : a. o p e r a t e t h e r e a c t o r and s e t t l e r i n such a manner as t o produce a f i n e l y d i v i d e d h y d r o c a r b o n - i n a c i d emulsion; b. m a i n t a i n v i g o r o u s and r e p e t i t i v e c o n t a c t between the h y d r o c a r b o n and a c i d c a t a l y s t phases and t h e r e b y a c c o m p l i s h continuous r e s a t u r a t i o n o f t h e a c i d as t h e o l e f i n s r e a c t and i s o b u t a n e i s consumed; c. keep t h e c o n c e n t r a t i o n o f o l e f i n u n i f o r m l y low i n the a c i d c a t a l y s t phase; d. m a i n t a i n a h i g h c o n c e n t r a t i o n o f i s o b u t a n e i n t h e h y d r o c a r b o n phase; and e. keep t h e t e m p e r a t u r e u n i f o r m l y low throughout t h e reactor.


16.

W E B B

Sulfuric Acid

Requirements

277


Emulsification Perhaps the most d i f f i c u l t o f the r e q u i r e m e n t s i s t h e e m u l s i f i c a t i o n and c o n t a c t i n g . F o r e f f i c i e n t o p e r a t i o n s the e m u l s i o n c h a r a c t e r i s t i c s must be u n i f o r m throughout the r e a c t o r . An e x c e s s of a c i d i s p r e f e r r e d , since t h i s r e s u l t s i n a hydrocarbon-in-acid e m u l s i o n . I n t h e s e m i x t u r e s t h e v i s c o s i t y and s u r f a c e t e n s i o n o f the c o n t i n u o u s a c i d phase are e f f e c t i v e i n m i n i m i z i n g t h e tendency of t h e d i s p e r s e d h y d r o c a r b o n d r o p l e t s t o c o a l e s c e and s e p a r a t e , a c t i n g under t h e i n f l u e n c e o f the v e r y l a r g e s p e c i f i c g r a v i t y d i f f e r e n t i a l between the l i g h t and heavy phases. The e f f l u e n t h y d r o c a r b o n - a c i d m i x t u r e i s s e p a r a t e d e i t h e r i n a s e t t l i n g zone i n t h e r e a c t o r o r i n a s e p a r a t e a c i d s e t t l e r v e s s e l . The o p e r a t i o n o f the a c i d s e t t l e r s e c t i o n can have a marked e f f e c t on t h e o v e r a l l performance o f t h e r e a c t o r . A v e r y s u c c e s s f u l o p e r a t i n g t e c h n i q u e i n v o l v e s o p e r a t i n g w i t h an a c i d e m u l s i o n i n the a c i d s e t t l e r and r e c y c l i n g the e m u l s i o n r a t h e r t h a n a s e p a r a t e d a c i d phase ( 4 ) . To a c c o m p l i s h t h i s t h e i n v e n t o r y o f a c i d i n the r e a c t o r s e t t l e r s e c t i o n i s l o w e r e d and t h e r e c y c l e r a t e i s i n c r e a s e d t o the p o i n t t h a t as much as 20 t o 30 p e r c e n t h y d r o c a r b o n i s e n t r a i n e d i n t h e a c i d as an e m u l s i o n r e c y c l e . Large improvements i n a c i d r e q u i r e m e n t s have been o b s e r v e d on b o t h butylène and mixed p r o p y l e n e - b u t y l e n e feeds when e m u l s i o n r e c y c l e has been employed. To m i n i m i z e a c i d make-up r e q u i r e m e n t s t h e i n t e r f a c i a l a r e a o f the c i r c u l a t i n g e m u l s i o n i n t h e r e a c t o r must a l s o be maximized. E m u l s i o n s e t t l i n g r a t e s a r e n o r m a l l y used*to measure t h e amount o f d i s p e r s e d h y d r o c a r b o n phase i n the e m u l s i o n . A sample i s t r a p p e d i n a t r a n s p a r e n t gauge g l a s s and t h e time i s r e c o r d e d as the hydrocarbons c o a l e s c e and s e p a r a t e i n a c o n t i n u o u s l a y e r on top o f the h e a v i e r a c i d e m u l s i o n l a y e r . E l e c t r i c a l c o n d u c t i v i t y measurements have a l s o been made c o n f i r m i n g the s e t t l i n g c u r v e d a t a . The e m u l s i o n s e t t l i n g c u r v e s p u b l i s h e d by C u p i t , Gwyn and J e r n i g a n (5) are t y p i c a l o f those o b s e r v e d on commercial r e a c t o r s . An example of a d e s i r a b l e homogenous a c i d - c o n t i n u o u s e m u l s i o n s e t t l i n g curve i s i l l u s t r a t e d i n F i g u r e 2. I n c r e a s i n g e m u l s i f i c a t i o n has t h e e f f e c t o f i n c r e a s i n g t h e a r e a a v a i l a b l e f o r mass t r a n s f e r o f i s o b u t a n e i n t o t h e a c i d c a t a l y s t phase. A l k y l a t e q u a l i t y improves and a c i d consuming s i d e r e a c t i o n s d e c r e a s e . Other i n v e s t i g a t o r s (5) have r e p o r t e d a l a r g e i n c r e a s e i n the r a t i o o f C8 compounds t o o t h e r 05+ compounds i n the p r o d u c t a l k y l a t e was o b s e r v e d when a homogeneous e m u l s i o n was produced. In t h e w r i t e r ' s e x p e r i e n c e , changing commercial r e a c t o r o p e r a t i o n t o produce t h e d e s i r e d t y p e o f e m u l s i o n h a s , i n some c a s e s , had the e f f e c t o f i m p r o v i n g a l k y l a t e q u a l i t y by 1.5 RON o r more, and r e d u c i n g a c i d make-up by 20 p e r c e n t w i t h o t h e r o p e r a t i n g v a r i a b l e s e s s e n t i a l l y unchanged.


INDUSTRIAL

A N D L A B O R A T O R Y

PROPANE.

NEUT.

EFFLUENT

REF'G.



REACTOR

TREATING

j SETTLER I

N-BUTAN^

RECYCLE ACID

ALKYLATE

DRYING 0LEF1NIC ISOBUTANE SULFURIC

DISCARQ

FEEDS

ACID RECYCLE

FEED

ISOBUTANE

ACID

Figure 1.

10

20

Simplifiedflowdiagram

30 SETTLING TIME,

Figure 2.

40

50

MINUTES

Preferred emuhion settling characteristics


*

16.

W E B B

Sulfuric Acid

Requirements

279


Contacting The o p e r a t i o n o f t h e m i x e r used i n t h e r e a c t o r can have a l a r g e e f f e c t on t h e degree o f e m u l s i f i c a t i o n . I t c o n t r o l s t h e r a t e o f c i r c u l a t i o n o f t h e e m u l s i o n w i t h i n t h e r e a c t o r , and t h e r e f o r e t h e average number o f passes t h e f e e d s t o c k s make through t h e m i x i n g i m p e l l e r d u r i n g t h e i r r e s i d e n c e t i m e i n t h e r e a c t o r . Most commercial r e a c t o r s have motor d r i v e n m i x e r s , and m i x e r speed cannot be v a r i e d . Some, however, a r e equipped w i t h t u r b i n e and gear d r i v e s , and some range o f speed v a r i a t i o n i s a v a i l a b l e t o t h e operator. I m p e l l e r speeds o f 620 RPM have been r e p o r t e d t o be v e r y e f f e c t i v e i n i m p r o v i n g a l k y l a t e q u a l i t y i n mixed f e e d o p e r a t i o n s ( 4 ) . Improved d i s p e r s i o n has been c r e d i t e d by K n o b l e and Hebert w i t h r e d u c i n g a c i d make-up from 1.85 t o 0.659 pounds p e r g a l l o n on t h i s commercial u n i t . Use o f e f f i c i e n t o l e f i n f e e d d i s t r i b u t o r s i s a l s o e f f e c t i v e i n r e d u c i n g a c i d consumption. The c o n d i t i o n s a t t h e p o i n t o f i n i t i a l o l e f i n contact w i t h the c i r c u l a t i n g c a t a l y s t are very i m p o r t a n t because o f t h e e x t r e m e l y r a p i d r a t e o f t h e i n i t i a l r e a c t i o n . P e r f o r a t e d s p a r g e r r i n g s a r e f r e q u e n t l y employed, and a c o n c e n t r i c f e e d n o z z l e i s a l s o a common f e a t u r e ( 6 ) . Space V e l o c i t y As a p p l i e d t o a l k y l a t i o n , space v e l o c i t y i s d e f i n e d as t h e g a l l o n s p e r h o u r o f o l e f i n i n t h e f e e d d i v i d e d by t h e g a l l o n s o f a c i d i n t h e r e a c t o r . T h i s t e r m i s s i m p l y a measure o f t h e concent r a t i o n o f o l e f i n i n t h e a c i d phase o f t h e r e a c t o r . As o l e f i n space v e l o c i t y i s i n c r e a s e d , t h e r e i s a g r e a t e r tendency f o r p o l y m e r i z a t i o n and o t h e r u n d e s i r a b l e r e a c t i o n s t o o c c u r and a c i d makeup r e q u i r e m e n t s i n c r e a s e . There i s a l s o an i n c r e a s e i n t h e h i g h e r b o i l i n g compounds i n t h e a l k y l a t e p r o d u c t . Space v e l o c i t y , i n one form o r a n o t h e r , e n t e r s i n t o almost a l l p r o c e s s performance c o r r e l a t i o n s and i s i m p o r t a n t t o n o t e i n any p l a n t e v a l u a t i o n s t u d i e s . F o r a g i v e n p l a n t i t i s sometimes used s i n g l y i n p r e d i c t i n g p l a n t behavior. F i g u r e 3 i s a t y p i c a l example o f t h e combined e f f e c t s of space v e l o c i t y and o t h e r v a r i a b l e s on a butylène o p e r a t i o n . Note c a r e f u l l y however t h a t space v e l o c i t y i s i n t i m a t e l y a s s o c i a t e d w i t h e m u l s i f i c a t i o n , c o n t a c t i n g , and m i x i n g horsepower. I t can be v e r y m i s l e a d i n g t o compare r e a c t o r s o r p l a n t s o f d i f f e r e n t d e s i g n s u s i n g space v e l o c i t y as a p r i m a r y c r i t e r i o n . Isobutane

Concentration

The d r i v i n g f o r c e t o complete t h e a l k y l a t i o n p r o c e s s i s t h e c o n c e n t r a t i o n o f i s o b u t a n e i n t h e h y d r o c a r b o n phase i n t h e r e a c t o r ( i . e . i n the hydrocarbon e f f l u e n t ) . This i s the source o f the i s o b u t a n e needed t o r e s a t u r a t e t h e a c i d c a t a l y s t as t h e a l k y l a t i o n r e a c t i o n p r o c e e d s . The η-paraffins i n t h e h y d r o c a r b o n phase a r e i n e r t and i f they a r e a l l o w e d t o b u i l d i n c o n c e n t r a t i o n they can


INDUSTRIAL

A N D

L A B O R A T O R Y



280



16.

W E B B

Sulfuric Acid

Requirements

281

c o n s t i t u t e a h a r m f u l d i l u e n t i n the r e a c t i o n zone. T h e i r e f f e c t i s n o t always r e c o g n i z e d , p a r t i c u l a r l y by those who, f o r the sake o f c o n v e n i e n c e , o v e r s i m p l i f y r e a c t o r b e h a v i o r by u s i n g the e x t e r n a l i s o b u t a n e t o o l e f i n r a t i o i n performance c o r r e l a t i o n . To c o n t r o l an o p e r a t i n g p l a n t f o r minimum a c i d make-up, the i s o b u t a n e c o n c e n t r a t i o n i n the r e a c t o r must be m a i n t a i n e d a t the maximum p o s s i b l e l e v e l . T h i s means o p e r a t i n g b o t h the depropani z e r and the d e i s o b u t a n i z e r under optimum c o n d i t i o n s . The opera t o r s h o u l d a d j u s t tower f e e d r a t e s and o p e r a t i n g c o n d i t i o n s , always u s i n g the i s o b u t a n e c o n c e n t r a t i o n i n e i t h e r the t o t a l e f f l u e n t h y d r o c a r b o n o r the n e t e f f l u e n t h y d r o c a r b o n f r o m the r e a c t o r as h i s p r i m a r y r e f e r e n c e . I s o b u t a n e - t o - o l e f i n r a t i o * d e i s o b u t a n i z e r overhead p u r i t y , d e p r o p a n i z e r r e c y c l e p u r i t y , and r e f r i g e r a n t r e c y c l e p u r i t y a r e s i g n i f i c a n t o n l y as they r e l a t e t o r e a c t i o n zone i s o b u t a n e c o n c e n t r a t i o n . The c o r r e l a t i o n s p u b l i s h e d by Putney (7) show the e f f e c t s o f i s o b u t a n e c o n c e n t r a t i o n on octane number, and o c t a n e number on a c i d consumption. These a r e f o r butylène f e e d s t o c k s . By comb i n i n g the d a t a i t i s p o s s i b l e t o c o n s t r u c t the curve shown i n F i g u r e 4 i l l u s t r a t i n g the i n f l u e n c e o f i s o b u t a n e c o n c e n t r a t i o n on a c i d make-up r e q u i r e m e n t , w i t h a l l o t h e r v a r i a b l e s c o n s t a n t . The p o t e n t i a l a c i d s a v i n g s p r o v i d e an o b v i o u s i n c e n t i v e f o r o p t i m i z i n g f r a c t i o n a t i o n o p e r a t i o n i n b o t h butylène and mixed f e e d o p e r a tions . Propane i s e l i m i n a t e d from the p l a n t by c h a r g i n g a p o r t i o n o f the s t r e a m c o n t a i n i n g the h i g h e s t p e r c e n t a g e o f propane, u s u a l l y the r e f r i g e r a n t s t r e a m , t o a d e p r o p a n i z e r tower. In some p l a n t s propane b u i l d - u p has been reduced and i s o b u t a n e c o n c e n t r a t i o n i n c r e a s e d by revamping and c h a r g i n g the d e p r o p a n i z e r tower at a h i g h e r f e e d r a t e , u t i l i z i n g the f u l l l i q u i d h a n d l i n g c a p a c i t y o f the tower e f f e c t i v e l y . I n the case o f η-butane the o p e r a t o r must f i n d the optimum mode o f o p e r a t i o n o f the d e i s o b u t a n i z e r , o r i s o s t r i p p e r column. For r e f l u x e d columns t h i s i n v o l v e s d e t e r m i n i n g the b e s t f e e d p o i n t l o c a t i o n , and the b e s t d i v i s i o n between r e f l u x and overhead r e c y c l e t o the r e a c t o r s e c t i o n . I n some cases i t has been found t h a t i s o b u t a n e c o n c e n t r a t i o n i n the r e a c t o r c o u l d be i n c r e a s e d by moving the f e e d p o i n t up, r e d u c i n g the r e f l u x and i n c r e a s i n g the r e c y c l e t o the maximum t h a t the r e a c t o r l o o p c o u l d h a n d l e . R e a c t i o n Temperature R e l a t i v e l y low temperatures a r e r e q u i r e d i n a l k y l a t i o n r e a c t o r s u s i n g s u l f u r i c a c i d c a t a l y s t . They a r e n e c e s s a r y i n o r d e r t o s l o w down p o l y m e r i z a t i o n r e a c t i o n s and reduce the f o r m a t i o n o f u n d e s i r a b l e a c i d s o l u b l e and h y d r o c a r b o n s o l u b l e b y - p r o d u c t s . T y p i c a l l y , most commercial u n i t s o p e r a t e w i t h r e a c t i o n tempera t u r e s i n the range o f 35°F. t o 65°F. D e s i g n temperatures are u s u a l l y s e t at 50°F. F o r minimum a c i d make-up the r e a c t o r s e c t i o n s h o u l d be o p e r a t e d as c o l d as p o s s i b l e . T h i s means o p e r a t i n g


282

INDUSTRIAL

A N D

L A B O R A T O R Y



the r e f r i g e r a t i o n compressor f o r peak performance and t a k i n g maximum advantage o f a v a i l a b l e c o o l i n g w a t e r . The c o s t o f the i n c r e m e n t a l u t i l i t i e s r e q u i r e d f o r l o w e r temperature o p e r a t i o n i s cons i d e r a b l y l e s s than the c o s t o f t h e i n c r e m e n t a l a c i d make-up. Some o p e r a t o r s have e x p r e s s e d concern o v e r the p o s s i b i l i t y o f a c i d f r e e z i n g and have r e s t r i c t e d o p e r a t i o n t o 45°F. Others howe v e r , have r e p o r t e d e x c e p t i o n a l l y low a c i d make-up r e q u i r e m e n t s at 32°F. t o 35°F. and have n o t r e p o r t e d any f r e e z i n g o r o t h e r o p e r a t i n g problems. V a r i a t i o n s i n the w a t e r c o n t e n t o f the a c i d c a t a l y s t may e x p l a i n why p l a n t e x p e r i e n c e v a r i e s . Fresh A c i d Composition Make-up a c i d f o r a l k y l a t i o n u n i t s t y p i c a l l y c o n t a i n s from 98.0 t o 99.5 wt. p e r c e n t H2SO4. R e c o g n i z i n g the d e s i r a b i l i t y o f m a i n t a i n i n g the l o w e s t p o s s i b l e w a t e r c o n t e n t ( 8 ) , some compan i e s s p e c i f y f r e s h a c i d i s t o be 99.0 t o 99.9 % H2SO4 but cont a i n i n g no f r e e SO3. Others s p e c i f y 98.0 t o 99.0 % based on the assumption t h a t t h i s s p e c i f i c a t i o n w i l l a s s u r e t h e r e i s , i n f a c t , no f r e e SO3 p r e s e n t . I n the w r i t e r ' s o p i n i o n , t h e f r e s h a c i d c o n c e n t r a t i o n s h o u l d be s p e c i f i e d as 99.0 t o 99.5 % H2SO4 and c o n t a i n i n g no f r e e SO3. F r e s h a c i d samples s h o u l d be s p o t checked r o u t i n e l y and c a r e f u l l y a n a l y z e d f o r t i t r a t a b l e a c i d i t y and SO3. Summary The o p e r a t o r o f a commercial s u l f u r i c a c i d a l k y l a t i o n u n i t can reduce t h e amount o f make-up s u l f u r i c a c i d he must charge i f he: 1. improves f e e d t r e a t m e n t f o r removal o f s u l f u r compounds and i n h i b i t o r s ; 2. m o n i t o r s f e e d s t o c k c o m p o s i t i o n and e l i m i n a t e s ethane and l i g h t e r compounds from the f e e d ; 3. o p e r a t e s f e e d d r i e r s f o r maximum w a t e r r e m o v a l ; 4. checks d i s c a r d a c i d i t y f r e q u e n t l y and c o n t r o l s i t at the optimum l e v e l d i c t a t e d by p l a n t e x p e r ience; 5. o p e r a t e s t h e a c i d s e t t l e r as a d e c a n t e r , maint a i n i n g an e m u l s i o n i n the a c i d s e t t l e r and r e c y c l i n g an e m u l s i o n t o t h e r e a c t o r ; 6. maintains a hydrocarbon-in-acid emulsion i n the r e a c t o r w i t h as h i g h an a c i d c o n t e n t as possible, consistent with s e t t l e r limitations; 7. i n c r e a s e s the speed of t h e r e a c t o r m i x e r t o as h i g h an RPM as i s p o s s i b l e , l i m i t e d by d r i v e r and s e t t l e r c a p a b i l i t y ; 8. o p e r a t e s t h e d e i s o b u t a n i z e r and d e p r o p a n i z e r as n e c e s s a r y t o a c c o m p l i s h a maximum c o n c e n t r a t i o n of i s o b u t a n e i n the h y d r o c a r b o n e f f l u e n t from the r e a c t o r s e c t i o n ;


16.

W E B B

9.

10.

Sulfuric Acid Requirements

283

l o w e r s t h e r e a c t i o n temperature t o t h e minimum consistent w i t h the a v a i l a b l e c o o l i n g water and r e f r i g e r a t i o n equipment l i m i t a t i o n s ; and uses f r e s h a c i d o f maximum s t r e n g t h and con t a i n i n g no f r e e SO3.


Abstract One of the largest operating costs of a s u l f u r i c acid alky l a t i o n unit for reacting l i g h t o l e f i n s with isobutane i s the cost of the fresh s u l f u r i c acid make-up needed to maintain the a c i d i t y of the catalyst at, or above, the required minimum l e v e l . Factors a f f e c t i n g fresh acid requirement are c l a s s i f i e d i n two categories: feed contaminants and their e f f e c t s ; and the reaction conditions that contribute to acid d i l u t i o n . Estimates of acid make-up are presented when the feed impurities are ethylene, butadiene, s u l fur compounds, and water. The importance of proper sampling, analysis, and control of the t i t r a t a b l e a c i d i t y of the acid cata l y s t and potential r e s u l t i n g operating problems are emphasized. Reaction conditions which a f f e c t acid make-up are: o l e f i n space v e l o c i t y i n the acid c a t a l y s t ; isobutane and n-paraffin concen tration i n the hydrocarbon phase; the amount of surface area at the hydrocarbon-acid catalyst i n t e r f a c e ; the degree of contacting between the two immiscible phases; and the reaction temperature. These conditions a f f e c t the formation of undesirable side reaction products; such products r e s u l t i n increased fresh acid require ments, lower aklylate product q u a l i t y , and more downstream oper ating problems. Methods of obtaining improved plant operation are considered. Literature Cited (1) (2) (3)

(4) (5) (6)

(7) (8)

"Gas Processing Refining and Worldwide Directory" (34th E d i t i o n ) , The Petroleum Publishing Company, Tulsa, (1976-77) C o l l i n s , J . J . , presented at NPRA annual meeting, San Antonio, Texas, (1962), Tech. 62-9 A l b r i g h t , Lyle F., Houle, Lawrence, Sumutka, Andrew Μ., and Eckert, Roger Ε. , Ind. Eng. Chem, Process Des. Develop., (1972), Vol I I , (No. 2). Knoble, W. S. and Hebert, F. E., Petroleum Refiner, (1959) Dec. Cupit, C. R., Gwyn, J . E., and Jernigan, E. C., Petro/Chem Engineer, (1961-1962), Dec. Jan. Kobe, Kenneth A. and McKetta, John J . , J r . , "Advances i n Petroleum Chemistry and Refining", Vol. 2, pages 315-355, Interscience Publishers, Inc., New York, (1959) Webb, Orlando, U. S. Patent 3,027,242 (1962) Goldsby, A. R., U. S. Patent 3,683,041 (1972)


Sulfuric Acid Requirements for Industrial Alkylation Plants

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