Recovery of Sulfuric Acid Alkylating Catalyst by Crystallization

18

Recovery of Sulfuric Acid Alkylating Catalyst by Crystallization

S. ROBERT STILES

Downloaded by PURDUE UNIV on June 29, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0055.ch018

Westinair Associates, P. O. Box 2091, Morristown, NJ 07960

A l k y l a t i o n of isobutane with o l e f i n i s accomplished by the carbonium ion mechanism when the s u l f u r i c acid catalyst p u r i t y i s maintained at a high l e v e l — generally above 90-92% t i t r a t a b l e a c i d i t y . At lower concentrations o l e f i n polymerization to the dimer i s accelerated and a l k y l a t i o n to tri-methyl-pentanes retarded. Alkylate product q u a l i t y i s effected. The free uncombined s u l f u r i c acid+ester+water composition of a t y p i c a l a l k y l a t i o n acid, present i n a commercial a l k y l a t i o n r e point data plotted i n the actor, is i l l u s t r a t e d by the check three component diagram of Figure 1. Water content ranges between 2-3% and ester diluents between 5-9%. Normally a continuous purge stream of reactor acid i s withdrawn, processed through an acid plant f o r r e j e c t i o n of hydrocarbon esters, and the regenerated acid a t 98.5-99.5% H SO purity returned to the reactor to control the c a t a l y s t a c i d i t y within t h i s range. A l k y l a t i o n c a t a l y s t a c t i v i t y can be maintained more economica l l y by concentrating these esters to 30-40% content l e v e l i n a small purge stream, p r i o r to being processed through the acid regeneration system. Savings i n acid regeneration and a l k y l a t e processing costs that r e s u l t are i l l u s t r a t e d by Figure 2. To accomplish these objectives recycle alky-acid is c h i l l e d , to freeze and extract 100% H SO crystals from the purge acid, according to procedures patented by S t i l e s , S k e l l y and F e l t e r (1-6). The s u l f u r i c acid i s returned to the a l k y l a t i o n reactor and the quantity of ester concentrate purge i s reduced to approximately 1/6th of its normal quantity. Operating data obtained from a commercial i n s t a l l a t i o n i s reported by 0 point composition data p l o t ted i n Figure 1. 2

4

2

4

Discussion

During a l k y l a t i o n a small quantity of reactants and impurit i e s remain trapped i n the acid phase as acid soluble esters. The term ester, as employed here, refers to any compound formed by the replacement of a t l e a s t one of the acid-hydrogen by a hydrocarbon 302

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

18.

STILES

Recovery by Crystallization

303

ς>

e

ο ο

1 ο


I §| i t • f i

&S ν. -Κ " S -ta

'§§ §1 1^

^ s ^ s Ο

•a

•S

I

Ο

3)

Ε


304

INDUSTRIAL

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

A L K Y L A T I O N S

r a d i c a l . Hydrocarbon r a d i c a l s may also contain s u l f u r , oxygen and/or nitrogen often found i n feed contaminants, such as amines, carbonyl s u l f i d e , carbamates, thio-carbamates and mercaptides. Being polar, esters are p r e f e r e n t i a l l y soluble i n the acid phase and accumulate to d i l u t e or contaminate the acid; and being a high molecular weight d i l u e n t , they have a temperature depressant e f f e c t on the freezing point of the acid l i q u i d phase. Generally these esters e x i s t only i n acids a t low water content, and r e a d i l y dissociate or hydrolyze when the water concent r a t i o n i s increased. Mono-esters with the HG-radical replacing only one hydrogen illustrated, by s C^Hg

+

HgSO^

-

(C^H )HS0^ 9


dissociate r e a d i l y a t r e l a t i v e l y low temperatures, whereas the dimer-ester i l l u s t r a t e d by the equations G^Hq

+

(C^H )HS0 9

(G4H ) S0^

4

9

2

requires higher temperatures and longer time f o r d i s s o c i a t i o n to occur. Trimer-esters, often c a l l e d t r i a l k y l esters, are formed from higher molecular weight r a d i c a l s such as:

These trimer-esters are extremely slow to dissociate and r e quire elevated temperatures a t b o i l i n g - a c i d conditions to complete the reactions. Tars are produced with release of s u l f u r dioxide. Differences i n hydrolysis c h a r a c t e r i s t i c s are used i n analyti c a l t e s t procedures to i d e n t i f y the e a s i l y dissociated esters by polymer and t i t r a t a b l e a c i d i t y , and the less reactive by t o t a l a c i d i t y procedures. Total hydrocarbon content i s determined by carbon analysis,and uncombined H^SO^ by the a n i l i n e sulfate procedure. Hydrolysis has been used i n some acid regeneration systems but the accumulation of excessive amounts of trimer-esters and tars have an adverse e f f e c t on alkylate q u a l i t y . Regeneration by combustion to t o t a l l y decompose the acid and esters to COp, SO^and H^O i s generally preferred. This decomposi t i o n i s performed i n an acid plant where the r e s u l t i n g SO^ i s then c a t a l y t i c a l l y converted to SOo n d regenerated to the 9 8 . 5 - 9 9 . 5 ^ HgSO^ that i s returned to the reactor system. a

A l k y l a t i o n and Ester Reaction Mechanism I t i s well known ( 7 » 8 , 9 ) that on start-up with f r e s h acid, the s u l f u r i c acid must be "'conditioned"" by adding the acid r a p i d l y to c i r c u l a t i n g isobutane containing less than 2% o l e f i n s to generate the carbonium-ion. Mixing acid with o l e f i n - f r e e isobutane w i l l not generate t h i s ion. However i f the acid i s added too slowly, to be overwhelmed by the o l e f i n , production of " r e d - o i l " r e s u l t s . I t i s also known that o l e f i n feed can be stopped a t any time without causing any problems, but i n t e r r u p t i o n of isobutane recycle


18.

Recovery by

STILES

Crystallization

305

flow — without immediate cut-off of o l e f i n feed flow — w i l l deteriorate the alkylate q u a l i t y which, i f carried on f o r any period of time, w i l l r e s u l t i n a disastrous "acid run-away . A reactor system can be kept on "stand-by" conditions f o r several days o r weeks, as long as isobutane r e c i r c u l a t i o n i s maintained. Alkylate i s slowly but continuously released from the a c i d phase into the isobutane l i q u i d during such operation and the catalyst " a c i d i t y " increases. A l k y l a t i o n , ester formation, polymerization, cracking, isomeri z a t i o n and other r e f i n e r y operations depend on the carbonium-ion. Whitmore (10) suggests that the carbonium-ion i s formed by the addi t i o n of hydrogen ion, from an a c i d , to an o l e f i n double bonds 11

H

Ç 3 H C-G=GH Downloaded by PURDUE UNIV on June 29, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0055.ch018

3

2

+

+ H

H

+

Χ

Ç 3 H C-£-GH

β

3

3

+ Χ

which then performs the basic r e f i n e r y reactions 1. Addition of negative ion s R-Î-G 2.

+

X"

—

R-Ç-C

Elimination of proton to produce olefins G-Î-G

G-G=G

+

H

+

3. Migration of proton (order of s t a b i l i t y : t e r t i a r y - secondary primary):

4.

C-Ç-Î-C - — - C-$-C-C C ~ C Migration of methyl group:

C-çL-î-C-C C-Ç-Ç-C-C e ce 5 . Addition of o l e f i n ; reverse reaction occurs by s c i s s i o n , two carbon atoms removed from charge: -

C-Î-C

6.

+

G=G-G

- — — "

G-Ç-G-Î-G C

Hydrogen t r a n s f e r reaction with t e r t i a r y hydrogens

C-Ç + G-G-G — G-Ç+ + G-G-G e û Any non-reversible reaction that terminates the e q u i l i b r i a by forming dimer o r trimer esters d i l u t e the acid and hinder the r e action of the carbonium i o n . Ideal reaction conditions of high isobutane concentration, and isobutane d i f f u s i o n into the a c i d phase a t a rate greater than the o l e f i n d i f f u s i o n rate, r e s u l t i n high q u a l i t y alkylate product. Ester formation, exclusive of those formed from feed contaminants,



306

INDUSTRIAL

A N D

L A B O R A T O R Y


l.3h

89

90

91

92

CLEAR RESEARCH

93 OCTANE

94

95

O F TOTAL

96

97

98

99

100

ALKYLATE

Figure 3. Ester production from olefin and isobutane reactants as a function of reaction conditions related to alkylate quality as measured by Research Octane Number



18.

STILES

Recovery by

Crystallization

307

i s n i l . As reaction conditions deteriorate and a l k y l a t e qualitydecreases, the rate of dimer and trimer ester formation increases. Ester production, r e l a t i v e to alkylate q u a l i t y , i s shown by Figure 3 & ^. Generally ester production from feed contaminants i s more d i f f i c u l t to p r e d i c t . Data obtained from commercial operations and plotted i n Figure ^, show the combined e f f e c t of ester production from these two sources. Y i e l d of trimer or t r i a l k y l esters o s c i l l a t e s as data of F i g ure 5 i n d i c a t e s . They are d i f f i c u l t to analyze and do not show i n the a c i d i t y t e s t s . However t h e i r e f f e c t on acid make-up require ments i s appreciable. Variations i n " f r e e " HpSCV content of acid are plotted as dotted lines i n the top of t h i s figure and show os c i l l a t i o n s that are t y p i c a l of most commercial operations. An ester concentrator that can extract these esters as they are gen erated to remove them from the reactor system, w i l l accomplish a valuable s e r v i c e . Smoother operation producing better q u a l i t y a l k ylate w i l l r e s u l t . Ester Concentrator - Process Description In t h i s process as shown by Figure 6, a portion of the A l k y l a t i o n Reactor acid recycle, that i s separated from the isobutaner i c h alky-reactor e f f l u e n t , i s c h i l l e d by d i r e c t contact with countercurrent flowing isobutane l i q u i d i n an agitated c h i l l e r v e s s a l or c r y s t a l l i z e r . Crystals of H^SO^ are formed. Referring to data points i n Figure 1, we have the alky-acid of composition "A" i n the area of the check (t/) point data, c h i l l e d to form c r y s t a l s of com p o s i t i o n "C" that are e s s e n t i a l l y 100^ H^SO^. As these c r y s t a l s form, the free H^SO^ content of the l i q u i d i s reduced and i t s ester plus water content increases along t i e - l i n e "CAB". This mixture of c r y s t a l s i n alky-acid l i q u i d flows downward thru colder upflowing isobutane l i q u i d and more c r y s t a l s are formed. The freezing point of the l i q u i d i s lowered as i t s ester plus water concentration i n creases, u n t i l at point "Β" the freeze-point temperature equals the temperature of the incoming c h i l l e d isobutane l i q u i d and no f u r t h e r c r y s t a l l i z a t i o n occurs. C r y s t a l l i z a t i o n i s performed i n ir-obutane l i q u i d to remove heat of fusion, but more important - to maintain a high isobutane concentration, so that-as the ester and water concentration i s i n creased, isobutane w i l l continue to react with the mono ester. The a l k y l a t e released from the acid phase, w i l l be absorbed by the isobutane l i q u i d and removed from the c r y s t a l l i z e r . Without t h i s removal of a l k y l a t e , c r y s t a l s i z e growth control i s d i f f i c u l t . A portion of the a l k y l a t i o n reactor auto-refrigerant recycle from the bottom of the depropanizer tower i s used f o r t h i s purpose. This stream i s free of o l e f i n s and moisture and i s c h i l l e d by ex change with evaporating propane before entering the bottom of the crystallizer. A portion of t h i s c h i l l e d isobutane l i q u i d flows upward to c h i l l the downflowing acid, remove the heat of fusion and convey the a l k y l a t e back to the a l k y l a t i o n reactor. The remainder conveys the a c i d c r y s t a l s plus ester concentrate mixture to the c e n t r i f u g a l


INDUSTRIAL

A N D

L A B O R A T O R Y



308



Figure 5.

AUGUST 2

Operating data showing effect of trimer esters on acid consumption and free H SO^ content

JULY


00


OLEUM

(0>

SOj

MIXER

-I

_|

cry/zs τ/911 /ΖΕ/?

REGENERATEA

-

r

.1 ,t ,T

r

Figure 6.

Γ

ι

* ESTER CONCENTRATE

R YY

1

Sulfuric acid type alkylation plant with acid purifier

ι )

r—;

/e&/e/a£/e/?T/os/


I

x

TRIMER

Ο

CO H-»


18.

Recovery by

STILES

Crystallization

311

f i l t e r . Here the acid crystals "C" are separated from ester concentrate "B" as the s l u r r y enters the apex of the spinning c o n i c a l screening centrifuge. Crystals larger than the screen openings are retained on the screen, while the ester concentrate and isobutane l i q u i d , containing smaller c r y s t a l s , pass thru the screen and flow into the decanter v e s s e l . Isobutane l i q u i d separate from the ester concentrate and i s returned to the a l k y l a t i o n reactor. Crystals "C" s l i d e across the surface of the c o n i c a l screen propelled by the angular force r e s u l t i n g from c e n t r i f u g a l force and the increasing diameter of the spinning c o n i c a l screen. The angle of the cone i s selected to allow c r y s t a l slippage while subjecting the c r y s t a l s to c e n t r i f u g a l force to remove adherent mother l i q u o r . Continuous flushing of isobutane i s maintained to a s s i s t i n t h i s l i q u i d - s o l i d separation. As the crystals reach the outer-diameter of the spinning cone, they are discharged into acid recycle flowing thru the c r y s t a l r e c e i v e r and are returned to the alky reactor. E s t e r concentrate "B" that decants from the isobutane, i s sent to acid regeneration f a c i l i t i e s f o r removal of hydrocarbons and water, The regenerated 9 8 . 5 - 9 9 . 5 ^ H^SO^ i s returned to the reactor. A portion of the ester concentrator i s continuously r e c i r c u l a t e d from the decanter to the c r y s t a l l i z e r i n l e t and mixed with incoming alky-acid, to provide "seed" crystals of the small crystals that had passed thru the screen. Freezing point temperature of ester concentrate i s lowered by depressant e f f e c t of both esters and water. Higher concentration of esters "B " can be obtained at a given c r y s t a l l i z a t i o n temperature, i f the water content i s reduced. Closed ft data points of Figure 1 show results of operations where oleum was mixed with alky a c i d , reacted with water: e

S0

3

+

H0 2

H S0 2

4

so that the r e s u l t i n g concentrate, obtained a f t e r subsequent c r y s t a l l i z a t i o n and separation of H^SO^ c r y s t a l s , had a higher ester content. Complete reaction must be accomplished before c h i l l i n g since oleum freezes at c r y s t a l l i z a t i o n temperatures, see Figure 7 . Water reaction with oleum "niaybe accomplished with incoming acid or with recycle acid-ester concentrate as shown i n Figure 6. Warmer c r y s t a l l i z a t i o n temperatures can be used to obtain a given ester concentrate l e v e l , i f the system i s operated on "block-flow" or intermittent procedure. In t h i s procedure ester concentrate c o l l e c t e d from alky acid i s accumulated i n the decanter f o r a time without discharging any to the acid p l a n t . Flow of alky-acid i s then stopped and the accumulated inventory run-off v i a the oleum contactor with HgSO^ c r y s t a l s returned to the reactor i n normal manner. For example ester concentrate of composition "B" obtained from alky acid "A" i s f o r t i f i e d to composition "A " and c r y s t a l s " C " séparated^to r e s u l t i n ester concentrate "Β'" that i s then sent to acid regeneration. A smaller quantity of concentrate r e s u l t s and processing costs are reduced. 1


312

A N D

L A B O R A T O R Y



INDUSTRIAL

Figure 7.

Freezing point temperature of sulfuric acid-water-oleum system


18.

STILES

Recovery by

Crystallization

313


CONCLUSION Today d u r a b l e l a m i n a t e d 200 mesh s c r e e n s a r e a v a i l a b l e t h a t can o p e r a t e i n c o n i c a l s c r e e n i n g c e n t r i f u g e s w i t h e x c e l l e n t s e r v i c e f a c t o r t o r e t a i n c r y s t a l s as s m a l l a s 75 microns · The c e n t r i f u g e a v a i l a b l e i n the e a r l i e r p l a n t r e q u i r e d a more rugged 40 mesh s i e v e t h a t c o u l d o n l y r e t a i n c r y s t a l s l a r g e r t h a n 400 m i c r o n s . G r e a t l y improved y i e l d o f c r y s t a l s p e r p a s s r e s u l t . This f a c t o r combined w i t h a c r y s t a l l i z e r d e s i g n t h a t c o n t r o l s a l k y l a t e r e l e a s e more e f f i c i e n t l y , g r e a t l y improves the performance and economics of the process. C o n s i d e r a b l e r e d u c t i o n i n a c i d r e q u i r e m e n t s and a l k y l a t i o n p r o d u c t i o n c o s t a r e o b t a i n e d by e s t e r c o n c e n t r a t i o n , a s shown by Figure 2. The o p e r a t i o n o f t h e e a r l i e r p l a n t demonstrated t h e p r o c e s s o p e r a b i l i t y o f t h e s y s t e m , b u t d i s c l o s e d m e c h a n i c a l and o p e r a t i o n a l l i m i t a t i o n s o f equipment a v a i l a b l e t h a t made the p r o cess uneconomical a t t h a t t i m e . Today e n e r g y c o s t s have changed and s u l f u r i c a c i d i s no l o n g e r worth $ 2 0 / t o n . This process, i n s t a l l e d i n an a l k y l a t i o n u n i t w i t h the equipment a v a i l a b l e t o d a y , can be a n i n t e g r a l p a r t o f t h e a l k y l a t i o n system t o s u b s t a n t i a l l y reduce a l k y l a t e p r o d u c t i o n c o s t s . LITERATURE

CITED

(1) (2) (3) (4) (5) (6) (7) (8)

F e l t e r , R.H., US Patent 2,593,128 (1952) S k e l l y , J.F. and S t i l e s , S.R., US Patent 2,716,592 (1955) S t i l e s , S.R., US Patent 2,831,043 (1958) S t i l e s , S.R., US Patent 2,862,791 (1958) S k e l l y , J.F. and S t i l e s , S.R. US Patent 2,863,724 (1958) S t i l e s , S.R., US Patent 2,903,339 (1959) S t i l e s , S.R., World Petroleum, Annual Refinery Review (1956) Durrett, L.R., Taylor, L.M., Wantland, C.F., Dvoretzky, I . , J.Am.Chem.Soc. 84 (1962) (9) Cupit, C.R., Gwyn, J.E., Jernigan, E.,PetroChem.Engr., 203 Dec (1961), 207 Jan. (1962) (10) Whitmore, F.,Chem.Eng.News, 26, 668 (1948): Ind.Engr.Chem. 26, 94 (1934)


Recovery of Sulfuric Acid Alkylating Catalyst by Crystallization

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