MTBE System Combined with ... - ACS Publications

Ind. Eng. Chem. Prod. Res.

Dev. 1981, 20, 474-481

temperature or one particular catalyst. Any model of the catalyst surface should accout for the temperature dependence of the surface composition, the mode of preparation, and the variations in active components.

~~

+A == HDS-3A HDS-SA

-

0.=-PA 0 HDS-2OA

Literature Cited

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/ ______r

/ ’D

315 0

232 5

371A

DEGREES C

F i g u r e 11. Bulk weight percent sulfur as a function of sulfiding temperature.

Increasing the sulliding temperature increases the degree of sulfiding at the catalyst surface (Figure 9) as might be expected. Combining the conclusion from the techniques employed in this study, it is apparent that the catalyst surface characteristics cannot be generalized from data at one

Bauwman, R.; Toneman, L. H. J. Catal. 1980, 61, 146. Brinen, J. S.;Armstrong, W. D. J. Catal. 1978, 54, 57. Cimino, A.; DeAngelis, 8 . A. J. Catal. 1975, 36, 11. Dale, J. M.; Huiett, J. D.; Fuller, E. L.; Richards, H. L.; Sherman, R. L. J. Catal. 1980, 61, 66. Davis, L. E.; McDonald, N. C.; Puimburg, P. W.; Rich, G. E.; Weber, R. E. “Handbook of Auger Electron Spectroscopy”; 2nd ed.; Physical Electronics, Inc.: Eden Prairie, Minn., 1976. Declerck-Grimee, R. I.; Canesson, P.; Friedman, R. M.; Fripiat, J. J. J. Phys. Chem. 1978a, 82, 885. Declerck-Grimee, R. I.; Canesson, P.; Friedman, R. M.; Friplat, J. J. J. Phys. Chem. 1978b, 82, 889. Delvaux, G.; Grange, P.; Delmon, B. J. Catal. 1979, 56, 99. Farragher, A. L.; Cossee, P. “Catalytic Chemistry of Molybdenum and Tungsten Sulfides and Related Ternary Compounds”; Preprint 98, Fifth International Congress on Catalysis Palm Beach, 1972. Grange, P. Catal. Rev. 1980, 21, 135. Harrison, D. P.; Hail. J. W.; Rase, H. F. Ind. fng. Chem. 1985, 57, 18. Jepsen, J. Scott M.S. Thesis, The University of Texas at Austin, Aug 1980. Lo Jacono, M.; Verbeek, J. L.; Schuit, G. C. A. J. Catal. 1973, 29, 463. Okamoto, Y.; Nakano, H.; Shimokawa, T.; Imanaka, T.; Teranishi, S. J . Catal. 1977, 50, 447. Patterson, T. A.; Carver, J. C.; Leyden, D. E.; Hercules, D. M. J. Phys. Chem. 1976, 80, 1700. Schuit, G. C. A.; Gates, 8. C. AIChE J. 1973, 19, 414. Seshadri, K. S.;Petrakis, L. J. Catal. 1973, 30, 195. Siegbahn, K.; Nordllng, C.; Fahlman, F.; Nordberg, R.; Hamrin, K.; Hedman, J.; Johansson, G.; Bergmark, T.; Karisson, E. S.;Lindgren, I.; Lindberg. 6. “ESCA. Atomic, Molecular, and Solid State Structure Studied by Means of Electron Spectroscopy”; Aimquist and Wlksells: Uppsela, 1967. Siegbahn, K. ”ESCA Applied to Free Molecules”; North-Holland Publishing Co.: Amsterdam, 1971; p 169. Wagner, C. D.; Rlggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E., Ed.; “Handbook of X-ray Photoelectron Spectroscopy”; Perkin-Elmer Corp., Physical Electronics Division, Eden Prairie, Minn., 1979. Received for review October 20, 1980 Revised M a n u s c r i p t Received M a r c h 17, 1981 Accepted May 15, 1981

Hydroisomerization/MTBE System Combined with Alkylation To Improve Octane Ronald M. Heck, Ronald G. McClung, Michael P. Wltt,” and Orlando Webb Engelhard Industries, Research & Development, Menlo Park, New Jersey 08817, Engelhard Industries, 429 Delancy Street, Newark, New Jersey 07 105, and Stratford/Graham Engineering Corporation, 4250 Madison A venue, Kansas City, Missouri 64 1 1 1

The product quality of alkylate produced from sulfuric or hydrofluoric acid cataiyzed alkylation plants can be improved by pretreating the olefin feed using an integrated system consisting of hydroisomerization‘ and MTBE process technologies. The pilot plant studies conducted to develop the hydroisomerization and MTBE technologies are described including figures and a discussion of the effect of key operating variables on each part of the system. A description and the results of a pilot plant study conducted to compare the quality of alkylates produced from normal butene isomers are presented. Proposed mechanisms for sulfuric and hydrofluoric acid catalyzed alkylation processes are discussed along with a comparison of the octane quality of the butene isomer alkylates produced by each process. Integration of the hydrobmeriition/MlBE system with akylation in an existing petroleum refinery is discussed along with the effect on gasoline blend stock quality and quantity.

Introduction As a result of the rapid escalation of petroleum feedstock prices and increased environmental and efficiency standards imposed by the government, petroleum refiners are faced with the task of operating their refineries to eco0196-4321/81/1220-0474$01.25/0

nomically produce higher quality motor fuels. Refiners will also be under pressure to produce higher quantities of motor fuels during periods of excess demand. To help refiners achieve the above goals, Engelhard has developed two process technologies which can be integrated as a @ 1981 American Chemical Society

Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 3, 1981 475 METHANOL

n

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1

HYDROCARBON FEEDSTOCK

N

GAS METER

QT’ Nz YTERINO PUMP

PUMP

Figure 1. Pilot plant unit flow diagram.

system to simultaneously improve gasoline quality and quantity. The first function in the integrated system is to upgrade the quality of the butene stream produced in the refinery. Engelhard’s hydroisomerization (simultaneous hydrogenation and isomerization) process technology utilizing HPN catalyst type IVB is used for this application to reduce the butadiene content in the butene stream while simultaneously isomerizing butene-1 to butene-2. Hydroisomerization results in a dual benefit for the refiner. Butadiene reduction accomplished by hydrogenation reduces the consumption of acid in the alkylation plant along with reducing the resultant formation of acid sludge. Alkylation plant operating costs are, therefore, reduced due to decreased acid use. The second benefit of hydroisomerization is the isomerization of butene-1 to butene-2. Under alkylation conditions of commercial interest, butene-2 alkylate octane is higher than butene-1 alkylate octane. The octane benefits associated with hydroisomerization depend on the alkylation unit configuration. This paper considers only the octane improvement benefit of hydroisomerization since the benefit of butadiene hydrogenation to reduce acid consumption has been evaluated in detail in a previous publication (Eleazar et al., 1979). The second part of Engelhard’s integrated system is the use of MTBE process technology. The product from the hydroisomerization section of the system is fed to the MTBE section of the system producing a high octane quality ether suitable for gasoline blending. The system is characterized by low utility requirements and is easily integrated in refinery operations. The economics indicate that Engelhard’s system is a profitable method which refiners can use to increase their production of high quality motor fuels. Hydroisomerization and MTBE Pilot Plant Studies Apparatus and Procedures. The experimental test rig used in this study is described in Figure 1. Detailed information on run procedures, sampling techniques, and GC analysis were previously published (Eleazar et al.,

1979). All the hydroisomerization studies were conducted in an upflow fixed bed tubular reactor using Engelhard HPN IVB precious metal catalyst. MTBE studies were conducted using both an upflow and downflow reactor configuration similar to Figure 1. The catalyst was commercially available ion-exchange resin. The isobutylene feedstock was similar to the feedstock used in the hydroisomerization study. ACS reagent grade methanol was used to react with the isobutylene over the ion exchange catalyst. Detailed results of this work are presented elsewhere (Eleazar et al., 1980). Results Hydroisomerization. The objectives of this study were threefold in that the experimental program was designed to determine operating conditions that could achieve complete butadiene hydrogenation, while simultaneously maximizing butene-1 isomerization to butene-2 with minimal loss of butenes to butane. Both the hydrogenation and isomerization reactions have been studied previously (Anderson et al., 1948; Bond et al., 1965; Haag et al., 1960); however, these studies involved vapor phase reactions as opposed to the liquid phase reactions of this work. Variables investigated in this study are operating temperature, operating pressure, space time (reciprocal space velocity), and hydrogen concentration. The actual responses from the gas chromatograph used to correlate the results were butadiene conversion, butene-1 remaining in the product, and total butenes in the product versus that in the feedstock. Figure 2 shows a typical response of reaction paths for hydroisomerization as a function of relative space time (relative space time is defined as the inverse of space velocity). Also depicted are the total butenes in the product, as well as the approach to isomerization equilibrium. Note that zero space time on the abscissa gives the feed conditions on the ordinate. Figure 3 depicts a typical reaction path for isomerization of butene-1 to the butene-2 isomers, cis and trans. Also shown in this figure is the calculated equilibrium composition of the three components and a line indicating the equilibrium ratio of cis- to trans-butene-2.

476

Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 3, 1981

,o

40t I Downloaded by UNIV OF NEBRASKA-LINCOLN on August 31, 2015 | http://pubs.acs.org Publication Date: September 1, 1981 | doi: 10.1021/i300003a010

2o

BUTADIENE

CONVERSION

PRESSURE

RELATIVE I TEMPERATURE, * C 40 RELATIVE H2 CONCENTRATION

0.6

ti RECIPROCAL RELATITIM

SPACE VELOCITY

Figure 2. Typical reaction profiles for hydroisomerization.

The relationship between butene-2 formation and the total butenes in the product after hydroisomerization is shown in Figure 4. The equilibrium butene-2 level is also indicated in this figure. MTBE. This study was aimed at determining the effect of process operating conditions (including recycle) in the formation of MTBE. Of particular interest was the incentive to minimize the unit operations involved in the

Discussion of Results Reaction Path for Hydroisomerization. A typical response for the hydrogenation and isomerization reaction for a butadiene/butene feedstock is shown in Figure 2. Note that as the reciprocal of the space velocity increases, both the degree of isomerization and hydrogenation increase. The butene-1 isomerization reaction begins to occur before the complete hydrogenation of butadiene (nominally 70 5% butadiene conversion). The final approach to isomerization equilibrium occurs after complete removal of the butadiene is accomplished. Also, the total butenes in the product appear to pass through a maximum, indicating that butene is an intermediate in butadiene hydrogenation. Isomerization of Butene-1. One projected reaction path (butanes normalized out) for the isomerization of

FEEDSTOCK COMDOS"

PROQSS CoN0ITK))IIs CATALYST H P N @ I V B TEMPERATURE, 4 0 . C RELATIVE $ C O N C E N T M "

downstream recovery of the MTBE product, yet having high MTBE selectivity from the isobutylene plus methanol reaction. These objectives distinguish this MTBE process from the previous literature. The actual variables considered in this investigation were operating temperature, space time (reciprocal weight hourly space velocity), methanol/isobutylene feed mole ratio, and reactor configuration (upflow and downflow). Gas chromatographic analysis was used to determine the MTBE selectivity, DIB dimer selectivity, isobutylene conversion, and methanol conversion. Figure 5 shows the response of MTBE selectivity for various feed methanol to isobutylene ratios. Also shown is the selectivity loss as defined by the isobutylene dimer formation.

EXPERIMENTAL RESULTS 0.6

RELATIVE PRESSURE M E D FROM 1.0 TO 3.0

(o.A,o) EQUKSRIUM RATIO OF

Cn TO TfWS BUTEH-2

srrmrl) A CIS

BVTPE-2

TRU(S-"-2

Figure 3. Typical reaction path for isomerization of butene-1 to butene-2 isomers (complete conversion of butadiene). Data normalized for three components.

Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 3, 1981 477 METHAM 97

-

I I

C A n s m nm@ IPI

96

-

sa

-

"*

I I

T a c L u T Y l , 40.C

PREHEPTER

BUT"/

m

c,

-

9.

Figure 6. Process flow diagram. TYPICAL

I

/

9

'

1

92

so

40

20

BUTENE

Do

80

- 2 / n - BUTENES,

7.

PROCESS

(BASE

FLOW

CASE1 ISOBUTANE

-

ACID

I

7

4

FLUID CATALYTIC CPACKING UNIT

ALKYLATION 1

1

1

ILKYLATE

I SPENT

ACID

Figure 4. Yield of butenes at complete removal of butadiene. Dl8

(DIMER)

MTBE SELECTIVITY

SELECTIVITY


(%I

IMPROVED

PROCESS

FLOW

I CASE

METHANOL

HYDROOEN FLUID CATALYTIC CRACKING UNIT

1-

I , CASE

21

IYJBUTAE

1-

HYDRO- ISOMERIZATION PLUS MTBE

7 ALKYLATION

ALKYLATE b

Figure 7. Typical and improved flow diagrams.

JP

\

"t

A

ti0

\ J

50 0 4

05

06 FEED

07

0 8

09

10

II

12

13

14

O I5

METHANOL/ ISOBUTYLENE MOLAR RATIO

Figure 5. MTBE (methyl-tert-butyl ether) and diisobutylene (dimer) selectivity vs. MEOH/IB molar ratio.

butene-1 to the isomers cis-butene and trans-butene-2 is shown in Figure 3 (at complete butadiene conversion) and compared with the calculated equilibrium values for the cis/trans ratio (see dashed line). It appears that during the early stages of isomerization that the trans-butene-2 is favored over the cis-butene-2. As the isomerization proceeds, an apparent equilibrium crossover is observed and then the cis-butene-2 is favored. Overall, the yield of trans-butene-2 is higher than the yield of cis-butene-2 for the reaction path depicted. The final composition approached the equilibrium ratio line from the cis side but did not reach the maximum trans level in this experimental program. The actual yield of butene-2 depends on the degree of isomerization and the starting concentration of butadiene in the feedstocks. As shown in Figure 4,the butene-2 formation can acutally pass through a maximum in the early stages of isomerization since the butadiene converted to butenes contributed to the butene-2 product concentration. As the degree of isomerization is pushed further to equilibrium, some butene-2 is apparently hydrogenated to butane and the totalbutenes in the product decline. For the feedstock used in the study, relative to the starting feed concentration of butenes, a 3 to 4% relative loss may occur a t complete butene-1 isomerization. MTBE Formation. Pilot plant evaluations were performed using a wide range of operating conditions. Of

significance is the fact that by operating within a given range of methanol to isobutylene feed ratios, capital investment for a commercial plant can be reduced. Figure 5 shows that operating below stoichometric feed methanol to isobutylene ratios, down to about 0.8, gives greater than 95% MTBE selectivity and negligible formation of DIB dimer. In addition, the methanol conversion is essentially 100% in this feed ratio range, resulting in a simplified distillation and recovery system. The MTBE selectivity is defined as the molar ratio of MTBE in the product to the isobutylene consumed in the feed.

Integrated MTBE/Hydroisomerization/Alkylation Process Description The process flow is depicted in Figure 6. The process is a combination of hydroisomerization and methyl-tertbutyl ether production. Hydrocarbon feed is pumped from storage and through the preheater to obtain the required inlet temperature. Hydrogen is fed to the reactor and the reactants are passed upflow through a fmed bed of precious metal catalyst. The highly active and selective precious metal catalyst allows the reaction to be conducted in the liquid phase. Hydrogenation is exothermic, resulting in a temperature increase across the reactor. The product from the reactor is cooled and then passes to a vapor disengaging drum. After vapor separation, methanol is combined with the liquid product (mixed C4stream) preheated to obtain the required inlet temperature and the stream is fed to a fixed bed reactor. In the reactor, MTBE is produced by reaction of isobutylene with methanol in the presence of an ion exchange resin-type catalyst. Various recovery schemes are then employed, depending on the integration of this system with the refinery. Currently in a petroleum refinery the C4 stream from the FCCU (fluid catalytic cracking unit) is processed, along with isobutane and an acid catalyst in the alkylation unit to produce alkylate (Figure 7). However, the hydroisomerization/MTBE system can be incorporated as indicated in Figure 7 to provide an improved process configuration.

478


Table I. Basis of Economics-Composition and Flow Raten,b

Table 11. Average Incremental Clear Research Octane Numbers for Alkylates Produced from Various Pure Butene Olefins in Various Acid Catalysts

Composition of C,’s component

wt %

is0 butane n-butane isobutylene butene-1 trans-butene-2 cis-butene-2 butadiene

38.2 12.1 14.7 12.7 12.0 9.9 0.4

total

Catalyst Type olefin feed butene-2 bLitene-l isobutene

100.0


%SO,

0.0 -0.5 -4.5c

Hutson et al. (1975).

Li

Table 111. Typical Butene Olefin Feeds Before MTBE Production and Hydroisomerization (Monoolefin Fractions Only)

improved case 1

-1.8

Roebuck et al. (1980). et al. (1970).

Process Flow Configuration current (base case)

HFb 0.0 -3.3

AlCl, a 0.0 -32.7 -4.8

case 2

(2,’s required, B/D 10000 1 0 0 0 0 1 4 4 7 9 18 18 excess isobutane required 1751 methanol required, lb /D -1 7 2 0 5 5 249095 alkylate produced, B/D 8677 5993 8677 1817 2630 MTBE produced, B/D _____ total gasoline blend stock, B/D 8 677 7 810 11 307 [(R + M)/2] of total blend 95.0 99.9 99.9 stock Alkylate octanes ( R + M)/2 for the base case, case 1 and case 2 (95.0, 96.4, and 96.4, respectively) are based strictly on the different C, isomer distribution to ai1 H,SO, alkylation unit. As discussed in this paper, no credits were taken for improved alkylate quality at the reduced case 1 alky unit load because this is a unit specific factor. A blending value of 111.6 (R + M)/2 was used for MTBE (Chase et al., 1980).

The basis for an economic evaluation is presented in Table I. The economics, however, will not be discussed since they were previously published (Heck et al., 1980). As indicated in Table I, two cases for the improved configuration are considered. Case 1 is based on charging loo00 B/D of C i s to the improved system. This case uses the same amount of FCCU C i s as the base case. Since the isobutylene contained in the Cis react to form MTBE, this case results in slack capacity in the alkylation plant (5993 B/D of alkylate) compared to the base case capacity of 8677 B/D of alkylate. Case 2 is based on obtaining enough C4’s (either by increasing the production of C4’s from the FCCU or by outside purchasing of a similar stream) to fill the alkylation unit to the base case capacity of 8677 B/D of alkylate. As indicated in Table I, the octane number of the combined products from both case 1 and case 2 are greater than the base case. Alkylate Octane Improvement Associated with MTBE Production and Butene Hydroisomerization Alkylate octane improvements associated with integrated MTBE production and hydroisomerization are the result of three factors. (1)MTBE production reduces the isobutene in the alkylation unit feed. For an existing alkylation unit, this will reduce the olefin space velocity in the reaction vessels thereby improving the alkylate quality. (2) MTBE production results in normal butene feed to an alkylation unit. It is well established that normal butene alkylates have higher octane than isobutene alkylates with H2SO4 catalyst and butene-2 alkylates have higher octane than isobutene alkylates with HF catalyst. (3) Hydroisomerization converts butene-1 to butene-2 in alkylation unit feeds. Under conditions of commercial interest butene-2 alkylates have higher octane than but-

olefin type

FCC monoole fins (also see Table I) wt 7%

butene-2 butene-1 isobutene

44.4% 25.8% 29.8% 100.0%

steam cracker monoolefins wt % 17.9% 33.1% 49.0% 100.0%

Table IV. Typical Alkylate Incremental Clear Research Octanes for Various Olefin Feeds with MTBE Production and Hydroisomerization Based on Table I1 and Table I11 Data FCC butenes

steam cracker butenes

HF H,SO, HF H,SO, catalyst catalyst catalyst catalyst no MTBE and no reference reference reference reference hydroisom. withMTBE pro- +0.2 +1.3 -0.1 +2.1 RON RON RON RON duction with hydroisom. +0.5 tO.1 +1.4 +0.2 process RON RON __-RON ___ RON ___ ~

total incremental +0.7 alkylate RON RON W/integrated processes

+1.4 RON

+1.3 RON

+2.3 RON

ene-1 alkylates in both H2SO4 and HF alkylation. The basis for this claim is discussed in this paper. Factor 1 (above) is significant for existing alkylation units but is unit specific. Further analysis in this paper will not include this effect. The alkylate octane improvements associated with integrated MTBE production and hydroisomerization processes are directly related to the incremental octane between alkylates produced from various pure butene feeds. Synergism between butene olefins in alkylation has not been demonstrated. The effect of olefin type on alkylate octane has been studied by several authors. Differences between butene isomer alkylates vary with conditions; however, the differences between butene isomer alkylates are primarily a function of catalyst type. Average incremental Research Octane Numbers are presented in Table 11.

Mechanisms proposed for each catalyst generally explain the different response to olefin feed type. The following sections will briefly discuss the incremental octanes claimed for HF and H2S04alkylation. There is presently little commercial interest in AlC&alkylation of butenes. AlC1, alkylation will not be further considered in this paper. MTBE production and hydrohomerization will normally be applied to butene streams that contain a mixture of butene olefins. The octane improvements associated with

Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 3, 1981 479

magnitude of the effect have been well established.

Table V reported HF alkylation data" trans-

butene-1 butene-2 total dimethylhexane, weight % trimethylpentane/ dimethylhexane (TMP/DMH) ratio calculated clear Research Octane Number


a

18.8

7.3

3.5

11.6

94.4

97.8

Hutson and Logan (1975).

this integrated technology are related to the olefin mix and catalyst type. Two examples of butene feeds and calculated alkylate octanes based on MTBE production and hydroisomerization are presented in Tables I11 and IV. Table IV was based on complete isobutene removal in the MTBE process and hydroisomerizationsuch that 80% of the normal butene mono-olefin product is butene-2. Improved alkylate quality due to MTBE production reducing the feed rate to the alkylation unit (previously discussed Factor 1)was not considered in preparing Table IV. Therefore, Table IV is applicable to new unita or to existing units where the decrease in alkylation unit feed with MTBE processing is balanced by an external source of normal butene olefin. Butene Isomer/Alkylate Quality Effects with Hydrofluoric Acid Catalyst Pilot studies by Hutson and Logan (1975) have demonstrated that butene-2 alkylate is superior to isobutene alkylate which in turn is superior to butene-1 alkylate with HF catalyst. This ranking of olefin feeds is well established and was used in Table I1 of this paper. Hutson and Logan (1975) published data concerning the H F alkylate composition and octane for each butene isomer. HF alkylation runs that formed the basis for their study were made at isobutene to olefin feed ratios between 11/1 and 13/1. Their study reported that cis- and trans-butene-2 produced similar alkylates; however, there was a significant difference between butene-2 alkylate and butene-1 alkylate. Butene-1 alkylates have significantly higher dimethylhexane contents than butene-2 alkylates with HF catalyst. (See Table V.) Hutson and Hays (1977) recently discussed the mechanism of HF alkylation and proposed that significant butene isomerization occurs in HF prior to the isobutane alkylation reaction. Further, they propose that dimethylhexanes are formed largely by olefin-olefin reactions involving butene-1. The above cited butene-2 vs. butene-1 alkylate composition data indicate that while butene-1 is a precursor to dimethylhexanes, butene-1 is participating in reactions before butene-l/butene-2 isomerization is completed. The results of radioactive tracer studies with pure butene olefins in HF have not been reported. As a result, it is not possible to determine if the excess dimethylhexanes produced during the HF alkylation of 1-butene are the result of olefin-olefin or olefin-isobutane reactions. Radioactive tracer studies with H2S04 (Hofmann and Schriescheim, 1962) indicate that dimethylhexanes produced during normal butene alkylation are composed of one molecule of olefin plus one molecule of isobutane (results are significantly different from isobutene alkylation in HzSO4). While there may be some question concerning the mechanism for excess dimethylhexane production with butene-1 olefin feed and HF catalyst, the existence and

Butene Isomer/Alkylate Quality Effects with Sulfuric Acid Catalyst I t has been long recognized (Cupit et al., 1962) that normal butene alkylates are similar and superior to isobutene alkylate with HzS04 catalyst. While there is not universal agreement concerning the mechanism of H2S04 alkylation, a proposed mechanism (Albright, 1977) based on radioactive tracer studies (Hofmann and Schriescheim, 1962) and other experimental results explain these phenomenon. Briefly, the mechanism for normal butene alkylation in sulfuric acid involves solution of the olefin in the acid phase followed by butene isomerization and reaction. Numerous reactions simultaneously occur with isomerization (Albright, 1977). These reactions involve intermediates present in the acid phase and the production of reaction intermediates and alkylate product. Normal butene isomerization reactions involve the production of significant quantities of isobutene (or more likely an isobutene-acid complex). This isobutene (or isobutene-acid complex) will form t-C4+ cations and other intermediates that participate in hydrogen transfer with isobutane in the acid phase. This isomerization to isobutene and hydrogen transfer is illustrated by the results of tracer studies (Hofmann and Schriescheim, 1962) that indicate that 23% of the normal butene carbon that enters a HzS04 reaction leaves the reaction zone as isobutane. Little normal butane formation has been cited with H2S04 catalyst (Albright, 1977). This mechanism predicts that a significant fraction of the reaction products from normal butene alkylation are produced from isobutene that is generated in the acid phase. A major difference between normal butene and isobutene alkylation is the fact that a major reactive component (isobutene or isobutene-acid complex) is slowly and uniformly introduced into the acid phase via isomerization from the normal butene feed while direct isobutene alkylation involves the introduction of large quantities of highly reactive isobutene into the acid phase at the start of the reaction or at the reactor mix point. On the basis of this mechanism, differences between butene-1 and butene-2 alkylates with HzS04 catalyst are attributable only to the small fraction of any butene-1 feed that is not isomerized to the thermodynamically favored olefins butene-2 and isobutene. H2S04alkylation mechanisms include a route to the low octane dimethylhexane (2-8's by direct reaction of butene-1 with t-C4+ cations. Mechanisms also include important non-butene-1 routes to dimethylhexane products. Conditions that favor the presence of butene-1 in HzS04 should result in increased dimethylhexane production; however, the production of dimethylhexanes by direct reaction of butene-1 is a second-order effect. It has been hypothesized (Albright, 1966) that an additional factor that affects the subtle difference between butene-2 and butene-1 alkylates is the fact that the rate of absorption of butene-1 into sulfuric acid is slower than the rate of absorption of butene-2 into sulfuric acid. Experimental results have demonstrated that butene-2 alkylates are in some cases superior (Cupit et al., 1962; Zimmerman et al., 1962; Albright et al., 1972) and in some cases inferior (Chase and Galvez, 1980) to butene-1 alkylates in HzSO4. Under relatively poor mixing conditions or high acidity (>97 wt % H2S04), butene-2 alkylates are inferior to butene-1 alkylates. Apparently under some circumstances, the positive aspects of slow butene-1 mass transfer to the acid phase (in terms of slow, uniform dispersion of olefin in the acid phase) outweigh the chemical

480


Table VI, Stratco Alkylation Pilot Plant Data Summary acidity level high

intermediate

low

5 90.0 99.8 94.4 97.1 343 91.3 40.0 14.5 3.9

4 85.8 99.2 93.6 96.4 368 87.8 36.0 13.2 5.6

6 89.9 99.6 94.4 97.0 34 2 91.2 40.3 13.8 4.1

3 87.2 98.4 93.4 95.9 370 87.6 32.7 11.0 6.8

Feed Butene-2 run 1 99.6 wt % H,SO, (average) Clear RON (engine test) 99.8 Clear MON (engine test) 94.8 97.3 (RON t MON)/2 ASTM end point (F) 301 wt % C, compounds 89.3 wt % 2,2,4-TMP on C,’s 42.2 wt %ratio C, TMP/DMH 15.9 4.1 wt % C , t compounds Feed Butene-1 run


wt % H,SO, (average)

Clear RON (engine test) Clear MON (engine test) (RON t MON)/2 ASTM end point ( F ) wt 9i C, compounds wt % 2,2,4-TMP on Cs’s wt %ratio C, TMP/DMH wt % C,+ compounds

2 99.7 99.6 94.5 97.1 303 89.9 42.8 13.7 4.8

Incremental Alkylate Product Octane a t Level. Butene-2 Less Butene-1 Engine 0.2 Clear RON Clear MON 0.3 (RON + MON)/2 0.2

Each Acidity Test Octane 0.2 0.8 0.0 0.2 0.1 0.5

inferiority of butene-1 to butene-2. In a laboratory study setup to investigate the effects of acid composition at relatively high intensity mixing (Albright et al., 1972), all butene-1 alkylate data points (13 data points) fall below corresponding butene-2 quality predictions. The difference between the butene-1 data points and the butene-2 prediction is between 0 and 2.3 clear research octane numbers with an average deviation of 1.1 clear RON. The experimental studies of Albright that involve comparison of butene-2 and butene-1 alkylate (Li et al., 1970; Albright et al., 1972) were executed on laboratory scale apparatus. Alkylation acid was not recycled to the reaction vessel. Reaction residence times were 3-5 min. The acid/hydrocarbon contacting vessels were designed for laboratory studies. Octanes reported for products were calculated based on the GC analysis of the alkylate products, clear component octanes, and the assumption that C,+ components were 80 RON. Most runs were made with 20:l or 1O:l isobutane to olefin feed ratios. All of the above conditions are significantly different from commercial HzS04-butene alkylation practice and there are reasonable questions concerning the direct application of this laboratory work to commercial alkylation units. Stratford/Graham Engineering recently executed pilot scale alkylation studies to compare butene-1 and butene-2 alkylates produced in an apparatus that simulated commercial Stratco units in terms of olefin space velocity, mixing geometry, feed isobutane to olefin ratio, temperature, ratio of acid settler inventory to contactor volume, and direct continuous recycle of acid from the acid/ hydrocarbon settler to the alkylation reactor. Each alkylation test run was made for approximately 16 h. Engine tests for Research and Motor Octane were made on alkylate samples taken throughout each run. Three experimental runs were made for butene-2 and three experimental runs were made for butene-1 at various acidity levels. Low acidity runs were made with spent alky acid. Intermediate acidity runs were made with spent alky mixed with fresh H2S04. Acid samples were analyzed for

Table VI1 ~

butene-1 C,+ alkylate HFa

butene-2 C , t alkylate

H,SO, HFa H,SO, run 6 (t-C,=2) run 5

weight % C,’s 85.0 91.2 91.4 91.3 weight % C , t 7.7 4.1 2.9 3.9 TMP/DMH ratio 3.5/1 13.8/1 11.6/1 14.5/1 i-C4/olefin feed ratio 11/1 5.7/1 lZ/l 5.7/1 ASTMendpoint(F) 357 342 328 343 Hutson and Logan (197 5).

titratable acidity during each run. Process variables held constant for each run include: operating temperature degrees, OF, 50; isobutane to olefin ratio, molar, 5.7/1; olefin space velocity, l/h, gal/h olefin/gal acid in reactor, 0.25. Test results are reported in Table VI. The data produced in the pilot study indicate the following results. (1) In pilot equipment simulating commercial Stratco equipment, butene-2 alkylates consistently showed higher engine test octane and higher trimethylpentane to dimethylhexane ratios than butene-1 alkylates run under similar conditions. As anticipated, the differences between butene-2 and butene-1 alkylates were small. All engine test Research and Motor Octanes show that butene-2 alkylates have been 0.0 and 0.8 octane number advantage over butene-1 alkylates. (2) Exceptionally high quality alkylate can be produced using HzS04 catalyst, normal butene olefin feeds (isobutene and butadiene free), reasonable reaction temperatures, and low isobutane to olefin feed ratios in a pilot apparatus set up to simulate a commercial Stratco alkylation unit. All pilot runs produced 98 to 100 Research Octane and 93 to 95 Motor Octane alkylate measured by engine tests. (3) The total C i s produced in these pilot H2SO4 studies were similar to pilot data reported for HF (Hutson and Logan, 1975; Hutson and Hayes, 1977). However, pilot study normal butene H2S04alkylates have a higher ratio of trimethylpentanes to dimethylhexanes than pilot study normal butene HF alkylates, even though the HF alkylates were produced at high isobutane to olefin feed ratios (Table VII). Table VI indicates that the average incremental octane between butene-2 and butene-1 alkylate with H2S04is 0.4 clear RON. Previous laboratory studies cited in this section (Li et al., 1970; Zimmerman et al., 1962; Albright et al., 1972) indicate 0.4 to 1.1 incremental clear RON between butene-2 alkylates and butene-l alkylates produced under intense mixing conditions and acidity levels of commercial interest. On the basis of these data and pilot data reported in this study, our judgment is that an incremental 0.5 RON between butene-2 and butene-1 alkylates is reasonable for planning the impact of hydroisomerization of alkylate quality with H$04 catalyst. This is the value presented in Table I1 and used in the preparation of Table IV. Conclusion Pilot study data indicate that MTBE can be produced from FCC butenes with little or no excess methanol feed and little isobutene dimer formation. MTBE production reduces the butene olefin available for alkylation such that the isobutane associated with the FCC butenes is close to the alkylation reaction requirement. Hydroisomerization of FCC butenes can result in reduced alkylation acid use as a result of butadiene removal. Hydroisomerization can also result in improved alkylate octane. Integrated hy-

Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 481-486

droisomerization/MTBE/alkylationcan result in a 4.9 (R

+ M)/2 octane advantage over straight alkylation of FCC

butenes.

Literature Cited


Albright, L. F. ACS Symp. Ser. 1077, No. 55, 128. Albrlght, L. F. Chem. Eng. July 4, 1088. 119. Albrlght, L. F.; Houle, H.; Sumutka, A. M.; Eckert, R. E. Ind. Eng. Chem. Process Des. Dev. 1972, 7 7 , 446. Anderson, J.; McAllister, S. H.: Derr. E. L.; Peterson, W. H. Ind. Em.Chem. 1048, 40, 2295. Bond, G. C.; Webb, G.; Wells, P. 8.; Winterbottom, J. M. J. Chem. Soc. 1085, 3218. Chase, J. D.; Gahrez, B. B. ”Processes for Gasoline Blending Ethers-TAME and MTBE-11,” Presented at Division of Petroleum Chemistry, 179th National Meeting of the American Chemical Society, Houston, Texas, March 1980. Cuplt, C. R.; Gwyn, J. E.; Jernigan, E. C. PeholChem. Eng. Dec 1961, 33; Jan 1082, 34. Ekazar, A. E.; Heck, R. M.; Wtt, M. P. “Hydrc-isomerization of C4 Hydrocarbons”, API 44th Midyear Meeting, May 1979, Proceedings-Reflnlng Department Vol. 58. 3-13.

401

Eleazar, A. E.; Heck, R. M.; McClung, R. G.; Olson, B. A. “Process Modlfications Can Improve MTBE Production”, Presented at the A.1.Ch.E. 88th National Meeting, June 1980. Haag, W.; Pines, H. J. Am. Chem Soc.1060, 82, 2488. Heck, R. M.; McClung, R. G.; Wkt, M. P.; Webb, 0. Hydrocarbon Process. 1980, 59(4) 185. Hofmann, J. E.; Schrbscheim, A. J. Am. Chem. Soc. 1082, 84, 957. Hutson, T.; Hayes, 0. E. ACS Symp. Ser. 1077, 55, 38. Hutson, T.; Logan, R. S., wrocarbon Process. 1075, 9, 107. Li, K. W.; Eckert, R. E.; Albrlght, L. F. Ind. fng. Chem. PIocess Des. Dev. 1070, 9, 441. Roebuck, A. K.; Evering, B. L. Ind. fng. Chem. Prod. Res. Dev. 1070, 9 , 76. Zimmerman, C. A.; Kelly, J. T.; Dean, J. C. Ind. Eng. Chem. prod. Res. Dev. 1082, 7 , 125.

Received for review February 4, 1980 Accepted April 6, 1981 Presented at the 179th National Meeting of the American Chemical Society, Houston, TX, Mar 23-28, 1980, Div. Petr. Chem.

Oxidation of Toluene by Cobalt(II1) Acetate in Acetic Acid Solution. Influence of Water Michael P. Crytko and Gunther K. Bub’ Lehrstuhl fur Technische Chemie, Ruhr-Universkat Bochum, 4630 Bochum, West Germany

The influence of water on the oxidation of toluene by cobalt(II1) acetate in acetic acid has been studied kinetically under aerobic and anaerobic conditions at 87 O C in the range 0.3 < [PhCHJ < 1.2 M, 0.05 < [Co], < 0.4 M, 0.03< [H,O] < 3 M, and 0 C Po, < 0.6 bar. Whereas no influence of water has been detected under anaerobic conditions, under aerobic conditions water enhances the oxidation at lower and inhibits at higher concentrations of water. A maximum rate of benzoic acid production was found at [H20] 1.3 M under the conditions investigated. Based on known findings, a reaction scheme describing the influence of water by waterlfree radical interaction is formulated of which the kinetic constants are determined experimentally.

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Introduction In the past the oxidation of toluene by cobalt(1II) acetate in acetic acid solution has been investigated by several authors (Morimoto and Ogata, 1967; Heiba et al., 1969; Sakota et al., 1969; Scott and Chester, 1972; Hendriks et al., 1978). The reaction sequences proposed by the authors have several common characteristics: the oxidation proceeds via a free radical sequence, the benzaldehyde formed as intermediate acts as cooxidant, and Co(II1) reacts with toluene yielding a radical cation by an electron transfer mechanism. The radical cation in a subsequent step loses a proton yielding a benzyl radical (Heiba et al., 1969). Though it was already recognized in earlier investigations that water which is formed during the oxidation has a marked effect on the absorption rate of oxygen and the reduction rate of cobalt(II1) acetate (see, e.g., Kamiya and Kashima, 1972),the role of water on benzoic acid formation has not been investigated to date: in most cases measurements were conducted with excess water to adjust a constant level during reaction (see, e.g., Hendriks et al., 1978) or its influence has been simply overlooked. I t is the intent of this study to give some first insights into the influence of water when oxidizing toluene by cobalt(II1) acetate in acetic acid under aerobic and anaerobic

* Chemische Werke Huls AG, Zentralbereich Forschung und Entwicklung FF 33, Lipper Weg, D 437 MARL, West Germany. 0196-4321/81/1220-0481$01.25/0

conditions to benzoic acid and to give a possible explanation of the effect of water on the oxidation. Experimental Section Materials. Toluene, acetic acid, chlorobenzene and cobalt(I1) acetate were used with AR quality as received. For preparation of cobalt(1II) acetate (Walker and Kopsch, 1932), air containing 5 % acetaldehyde was bubbled through a solution of 300 g of cobalt(I1) acetate in 3 L of acetic acid with 1% water at 80 “C. When 7040% conversion was reached the solution was evaporated a t 40 “C at a pressure of 2.7 X bar. The acetate obtained was dissolved in glacial acetic acid. The water content of all chemicals used was determined by Karl Fischer titration. Procedure. The oxidation was carried out at 87 “C in the reactor shown in Figure 1 (inner diameter 4 cm, length 25 cm). The whole reactor was made of glass, and the bearings consisted of Teflon. Since under the conditions used the reaction was so fast that oxygen depletion in the bulk of the liquid phase was observed, the six-blade stirrer together with four baffles as shown in Figure 1have been used, by which the oxygen concentration in the reaction mixture was maintained at 7590% of the saturation concentration. The liquid phase volume of 270 mL was stirred at a rate of 670 rpm. The oxygen-nitrogen mixture was blown in at the bottom of the reactor. The upper bearings were rinsed by a small N2stream. The exit gas stream of the reactor was cooled in such a way that with 0 1981 American Chemical Society

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