Octane Enhancement By Selective Reforming of Light Paraffins

.J.W. Ward (Editor). Catalysis 1987 1988 Elsevier Science Publishers B.\'.. Amsterdam - Printed in The Netherlands

OCTANE ENHANCEMENT BY SELECTIVE REFORMING OF LIGHT PARAFFINS

P. W. Tamm, D. H. Mohr, and C. R. Wilson Chevron Research Company, P.O. Box 1627, Richmond, California 94802-0627 ABSTRACT A new reforming process (AROMAXSM) is described for selectively converting C6-C7 paraffins to high octane aromatics. Since conventional processes are not very effective for these compounds and since lead additives are being phased out of gasoline, the AROMAX process fills an important industrial need. This process employs a catalyst comprising platinum on L-zeolite. Pure component studies are used to show that the high selectivity of the catalyst results from a different reaction network than conventional bifunctional reforming catalysts. Pilot-scale tests are used to show how unleaded gasoline or aromatic chemicals can be produced from refinery light naphthas with dramatically better yields than conventional reforming. INTRODUCTION Light naphtha has always been an important constituent of motor gasoline. It consists of a mixture of hydrocarbon molecules containing five to eight carbon atoms. An example of such a material is light straight-run naphtha (LSR). As typically fractionated, LSR contains mostly sixand seven-carbon atom species. It has a research octane rating which is usually in the range of 45-70 and needs considerable upgrading to meet the requirements of today's internal combustion engines. The modern catalytic reforming process, developed by Universal Oil Products in 1949 (ref. 1) and substantially improved by Chevron Research Company in 1968 (refs. 2 and 3) is very effective for improving the octane rating of heavy naphtha (Cg-CIO hydrocarbons), but it is much less effective for upgrading light naphtha. This is because the selectivity of the bifunctional catalyst used in the process is quite poor for producing high octane aromatic molecules from C6 and C7 hydrocarbons, particularly from the paraffinic species. This is clearly illustrated in Figure 1. As a result, light naphthas have not generally been reformed for gasoline production but have been upgraded instead by the addition of tetraethyl-lead antiknock compound. Their lead response is very good, increasing their octane rating by about ten numbers with the addition of 1.5 g of lead per gallon. However, soon this will no longer be possible due to the legislated phaseout of lead from gasoline.

336

Aromatization Selectivity. Mole "10 100 _ C6 . C . and Cs Cyclohexanes 90 Cyclopentanes - - __ - - -7 _ { 80 ~ 70 60

50 40 30 20 10

---

paraffinsE~

~~:: -.: :

........

-----C

---------_- _ _ _

---'C s -C

-----C 6 7

OL...-------'-------'-------'--------'

o

100

200

Pressure. psig

300

400

Fig. 1. Typical Effects of Pressure and Compound Type on Aromatization Selectivity for Conventional Catalysts The Environmental Protection Agency implemented a program for the orderly phasedown of tetraethyllead in gasoline in response to concerns about the health effects of lead emissions from automobiles. By January I, 1988, the maximum level of lead allowed in leaded gasoline will be 0.1 g/gal. Because of the small octane boost at this concentration and the higher octane specification for leaded fuel, many marketers are not even planning to offer leaded gasoline after lead phasedown is complete. With the elimination of lead, refiners must find another way to upgrade light naphthas if they are to be kept in the gasoline pool. Many refiners are turning to catalytic isomerization to upgrade light naphtha. Several commercial isomerization processes are available which will convert light naphtha to a mixture of singly and multiply branched isoparaffins. However, these processes have limitations. They are only applicable to Cs and C 6 hydrocarbons (C 7 compounds crack to an unaccceptable extent) and they only raise octane to the range of 80-90 RON. The greater the quantity of hexanes in the mixture the more difficult it is to achieve a high octane rating by isomerization. Therefore, as shown in Figure 2, an awkward region exists in the range of C 6 and C 7 hydrocarbons where neither conventional reforming nor isomerization is particularly effective. Chevron Research Company has developed a new reforming process which effectively upgrades these compounds to high octane aromatics. This process, referred to as AROMAXsM, utilizes a catalyst incorporating a molecular sieve material and operating by an entirely different mechanism than conventional reforming catalysts. This paper will use pure compound studies to illustrate the mechanism promoted by AROMAXTM catalyst and will use pilot plant data on refinery naphtha cuts to illustrate the advantages of AROMAX over conventional reforming.

337 Carbon Number

C7

c,

C9

Process Isomerization AROMAX'· Reforming Conventional Reforming Darkness of Bar Denotes Degree of Applicability

Fig. 2. Process Options for Improving Octane of Naphtha Feedstocks EXPERIMENTAL The data to be presented in this paper were obtained in two distinct types of experiments. Pure component studies were carried out in a microreactor unit. The catalyst charge was typically 0.7 g of crushed catalyst sieved to 24-80 mesh. The catalyst was positioned in a zone of constant wall temperature as measured by traverse of a thermocouple along the surrounding heater block. Quoted temperatures are the measured temperatures for the heater block. Since the reactions are endothermic, the actual catalyst temperatures will be somewhat lower. Feed was delivered by a high pressure syringe pump at a space velocity of 6.0 LHSV. The feed was introduced into a once-through flow of hydrogen set so as to give a hydrogen-to-hydrocarbon mole ratio of 6.0. On-line product analysis was obtained with a Hewlett-Packard 5880 gas chromatograph. The experiments were typically 100 hours in length with product analysis carried out hourly. Although the catalyst deactivated slightly during the course of the experiments, selectivities remained constant. Studies on refinery naphtha cuts were carried out in a fully integrated pilot plant capable of generating design-quality data. Operating temperatures and pressures, space velocities, and recycle gas ratios were all in the range commonly employed in semiregenerative reformers, The catalysts were tested in the form of 1/16-in. extrudates. Continuous on-line analyses were available for recycle gas composition, product gas composition, and total aromatic content of the Cs+ liquid product. Off-line chromatographic analysis was used to determine the detailed composition of the liquid product. Octane ratings of the liquid product were measured using standard ASTM procedures. DEFINITIONS For the purpose of evaluating the performance of aromatization catalysts it is useful to define conversion and selectivity in a somewhat different manner than the common conventions used for

338

these terms. First, we must introduce the concept of a "reactant pool." The reactant pool consists of all C6+ paraffins and alkylcyclopentanes. These compounds are all potential precursors to aromatic products. Cyclohexanes in the feed are considered as equivalent to feed aromatics because they aromatize so rapidly and selectively under the conditions employed in this study. "Pool conversion" is defined as the mole fraction of compounds in the reactant pool that react to form either aromatic products or cracked products (C5-)' Interconversion to other compounds in the reactant pool is not counted as conversion. The "pool aromatization selectivity" is defined as the moles of aromatics produced divided by the moles of the reactant pool converted. CATALYSTS AND REACfION MECHANISMS Conventional catalvsts The conventional catalysts used for catalytic reforming are referred to as bifunctional catalysts after the nomenclature established by Mills, Heinemann, Milliken, and Oblad in their classic paper of 1953 (ref. 4). These catalysts have a hydrogenation-dehydrogenation function provided by a noble metal and an acid isomerization function provided by an acidic support. Platinum has traditionally been used for the metal function and chlorided alumina for the acidic function. Virtually all of the reforming catalysts in use today also utilize a second metallic component to greatly improve the catalyst's resistance to deactivation by coke deposition. Rhenium is the predominant choice for this metal. The presence of the second metal does not change the mechanism by which the catalyst promotes the reforming reactions and, therefore, the newer catalysts have essentially the same selectivity as the older monometallic catalysts. The reaction network for bifunctional catalysis, proposed by Mills et al. and expanded by Hughes (ref. 5), is illustrated for hexanes in Figure 3. The aromatization of paraffins proceeds by a series of intermixed metal-catalyzed dehydrogenation steps and acid-catalyzed isomerization steps. A key feature of the mechanism is that for aromatization to occur the reaction must involve

c--~ 5

CS-_~.

C5--~~

~

::::Hexenes~6==6

-(?]-~

--v--0 c5 -

c5 -

Fig. 3. Reaction Network for Hexane Aromatization With Bifunctional Catalyst

339

formation of a cyclopentene structure. Hughes explained the poor selectivity of bifunctional catalysts for light paraffin aromatization in terms of the relative rates of acid-catalyzed cracking of the olefinic intermediate products versus acid-catalyzed ring closure or ring expansion of these species. His rationalization of the relative rates was based on an analysis of the stabilities of the various carbenium ion intermediates which would be involved. AROMAX catalyst AROMAX catalyst consists of platinum supported by L-type zeolite. Details of the preparation of this class of catalysts were described by Hughes et al. (ref. 6). This catalyst has no acid isomerization function; the entire catalytic function must be supplied by the noble metal. The exceptional selectivity of platinum/L-type zeolite catalyst for aromatizing light paraffins was first reported by J. R. Bernard in 1980 (ref. 7). The results of pure component studies help to elucidate the significantly different mechanism by which AROMAX catalyst operates. Aromatization of normal hexane (n-hexane), n-heptane, and n-octane was carried out over AROMAX catalyst at 920°F and pressures of 50, 100, and 150 psig. Figure 4 compares the aromatization selectivities which were obtained. All of the selectivities were above 90% and independent of carbon number. Referring back to Figure 1, this represents a range of improvement over the performance of the bifunctional catalysts from about 25% for n-octane at 50 psig to almost a factor of 4 for n-hexane at 150 psig. 920'F, 6 LHSV,6 H2/HC 100 95

;3'

0 1 0 0 psig 96

96

1.\1150 psig 96

90

~

85

tiQl a;

80

'0

75

:~

~50PSi9

CJ)

0 II.

70 65 60

nC 6

nC 7

nCB

Feed Compound

Fig. 4. Selectivity for Aromatization of Normal Paraffins With AROMAX ™

340

In a second series of experiments aromatization of the other hexane isomers; methylcyclopentane (MCP), 2-methylpentane (2-MP), and 3-methylpentane (3-MP); was measured under the same conditions. Figure 5 compares the aromatization selectivities of all the hexane isomers. The selectivity is highest for n-hexane, but note that the methylpentane selectivities are still much improved over those obtained with the bifunctional catalysts. (See Figure 1.) 920°F, 6 LHSV, 6 H2/HC

~50 100

psig

0100 psig

~150

psig

96

95 90

iu

85

oo

75

:~

Ql

Qj 1Il

0.

83

80

70 65 60

L..-........'-'-o
MCP

2MP

3MP

Feed Compound

Fig. 5. Selectivity for Aromatization of C6 Isomers With AROMAX ™ Table 1 shows the relative rates of aromatics production for the hexane isomers. The relative rates show the same trend as the selectivities-en-hexane is highest, MCP is slightly lower, and the methylpentanes are equal and lowest.

Table 1 Relative Rate of Aromatization of C6 Isomers With AROMAX ™ Feed Compound

Relative Rate of Aromatics Production* 1.0 (By Definition)

n-Hexane Methylcyclopentane

0.75

2-Methylpentane

0.6

3-Methylpentane

0.6

*920

0F,6

LHSV, 6 H:!HC, Relative Rate Defined

as (Pool Selectivity)·Log (t-Pool Conversion)

341

Finally, Figure 6 shows the effect of carbon chain length on aromatization selectivity. n-Hexane and 2-methylhexane, which each contain six-carbon chains, have selectivities near 95%. In contrast, 2-MP, with only a five-carbon chain, has a distinctly lower selectivity (83%) . 100 .... 951-

;f.

90 f-

s

851-

ti41 a;

80 I-

'0

751-

:~

-

94

.

-

, ------- -....

C-C-C-C-C-C

C-C-C-C-C-C I C r--97

C-C-C-C-C I ,....C 83

III 0

0-

70 I65160

2MP

nCa

2 MH

Feed Compound 100 psig, 920°F, 6 LHSV, 6 H 2/HC

Fig. 6. Effect of Chain Length on AROMAX TM Aromatization Selectivity

These results are all consistent with a reaction mechanism involving only metal-catalyzed ring closure, ring opening, and dehydrogenation. The proposed reaction network is illustrated for aromatization of hexanes in Figure 7. It is significantly different from the network for bifunctional catalysts shown in Figure 3. Benzene is formed by direct ring closure to cyclohexane followed by rapid dehydrogenation. Direct closure to a five-membered ring is also possible. When coupled with subsequent ring opening to n-hexane, it provides a reaction pathway for aromatization of the

~

~

c5 -

c5 -

Fig. 7. Reaction Network for Hexane Aromatization With AROMAX™ Catalyst

342

methylpentanes. All of the hexane isomers can also undergo metal-catalyzed hydrogenolysis to undesirable C5-cracked products. Since the rate of metal-catalyzed cracking is slow at normal reforming conditions and there are no sites to promote the faster acid-catalyzed cracking, a very high aromatization selectivity is achieved. The selectivities for aromatizing the normal paraffins are high and independent of carbon number (Figure 4) because they can each form a six-membered ring directly. The selectivities decrease with increasing pressure because the rates of the ring opening and hydrocracking reactions are accelerated by a higher hydrogen partial pressure. The selectivity for aromatizing MCP is lower than that of n-hexane (Figure 5) because it cannot form a six-membered ring directly. It must first undergo ring opening to n-hexane. Reversible ring opening to 2-MP or 3-MP is also possible, but a six-membered ring can only be formed from n-hexane, These extra reaction steps slow the rate of aromatization and allow a greater opportunity for cracking. The selectivity is more sensitive to pressure because two reaction steps are affected rather than one. The aromatization selectivities of the methylpentanes can be explained by similar arguments. To provide a more quantitative test of the proposed reaction network we have compared our data to a simplified kinetic model of the network. The reactions in Figure 7 were all assumed to be first-order reactions. It was assumed that the volume change due to the production of hydrogen could be neglected because of the introduction of 6 moles of auxiliary hydrogen per mole of hydrocarbon feed. With these assumptions, the reaction network can be described by a set of linear ordinary differential equations. The initial conditions are known from the feed composition and the equations can be numerically integrated for a given set of rate constants. An initial estimate of the rate constants was obtained by combining the results of low conversion experiments with available equilibrium data. Table 2 summarizes the results of the low conversion experiments and the conclusions which can be drawn from the data. Implicit in these

Table 2 Relative Rate Constant Estimates for AROMAX ™ Catalyst Obtained From Low Conversion Data Feed Temperature, OF Pool Conversion, % Mole Ratio in Product Benzene

nC6 870 6*

3 MP 870 28

2 MP 870 26

1.0

2 MP + 3 MP + MCP 0.6

2 MP nC6 + Benzene

0.6

3 MP nC6 + Benzene Conclusion

~=1.0 K_2

*10% of Normal Pt Loading

K...:!. =0.6 K2

K...:!. =0.6 K2

343

results is the assumption that the cracking rate constants are small. Shortly, it will be shown that this is a valid assumption. Table 3 summarizes the equilibrium constants for the reversible reactions of Figure 7 at 87QoF, 100 psig, and H:zIHC = 6. If one assigns the rate constant k t a value of 1.0 and combines the information of Tables 2 and 3, all of the other rate constants can be estimated on a relative basis. The rates of all of the hydrogenolysis reactions were assumed to be equal, and the value of this constant was adjusted so as to yield the observed pool selectivity.

Table 3 Equilibrium Relationships* Equilibrium Constants

Reaction nC G

~

Benzene

'S-=5 K_ 1

MCP ~ 3 MP

nC G

&=7

MCP

~=0.2

K_ 2

K_ 3

2 MP

MCP

~=0.1 K_ 4

'Conditions: 870°F, 100 psig, 6 H 2/HC

The results of the above exercise are shown in Figure 8. Note that the relative rates of forming five- and six-membered rings from n-hexane are approximately the same. The rates of opening MCP to n-hexane or the methylpentanes are faster. The rates of forming MCP from the methylpentanes are slower. Finally, the rates of the cracking reactions are small. - 0.02 C5 -

7

C5-~~~

0·0

~

0.2

lO.02

0

C 5-

Fig. 8. Approximate Relative Reaction Rates for Hexane Aromatization With AROMAX TM Catalyst*

344

The initial estimates of the rate constants were used in the model to predict the concentrations of all species versus reaction time for n-hexane, MCP, 2-MP, and 3-MP feeds. The results are shown in Figures 9-12. Also shown in each figure are the observed concentration for an actual high conversion experiment. The model predictions agree reasonably well with the actual data points. The order of the pool selectivities are correctly predicted (n-hexane > MCP > 2-MP "" 3-MP).

Table 4 summarizes the fit of the model predictions to the data and indicates the directional changes in rate constants necessary to improve the fit. Note that for every feed the same conclusion is reached--the fit would be improved by increasing k, and decreasing kj. This consistency in the results is encouraging. The estimates of the relative rate constants could be further improved by using a Levenberg-Marquardt parameter estimation algorithm, but this was not within the scope of the present paper. In summary, AROMAX catalyst is much more selective for converting light paraffins to aromatics than conventional reforming catalysts. A reaction network has been proposed that is consistent with the observed data. 1.0 0.9

I Experimental I Results

0.7 "l:I Ql Ql

lO. Ql

•I

(.) Pool Selectivity

0.8

0.20

i

I I

0.18 870°F 100 psig 6 H 2/HC

I

"0 :E 0.5 ...

I

Ql

l1.

0.14 "l:I

2MP(O)

I

0.6

0.16

u,

0.10

"0 :E

0.08

0.3

0.06

0.2

0.04

0.1

0.02

0.4

0.6

...

Ql

"0 0.4 :E

0.2

Ql

l1.

Ql

0.0 0.0

Ql Ql

0.12

0.8

1.0

Ql

"0 :E

0.00 1.2

Reaction Time (Arbitrary Units)

Fig. 9 Predictions of AROMAX ™ Model for n-Hexane Aromatization

0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.2

0.6

0.8

(0) 2 MP

I

I

I

I

1.0

1.2

MCP(v) 1.4

(0) 3MP(O)

870°F 100 psig 6 H 2/HC

Reaction Time (Arbitrary Units)

0.4

--

I

I

~I

I Experimental I

Res~

Selectivity

~::'-----="""o::::::::::-::M:::::::-Benzene

~Ol

Fig. 10. Predictions of AROMAX™ Model for Methylcyclopentane Aromatization

:ii:

'0

CIl

a.

:ii: Q;

'0

CIl

u,

CIl CIl

~

0.8

0.9

1.0

CIl CIl

0.1

0.2

0.3

0.4

0.5

0.6

0.71-

»<:

1.5

2.0

2.5

( 0) Benzene

3.0

3.5

IMCP lu\

I

\1

i

I

I::.

0.02

0.04

0.06

0.08

0.10

0 4.0

12 MP (0)

I ;n~)

0.12

0.14

I j3MP(o)

-; 0.16

I

(A) Pool Selectivity ~

Reaction Time (Arbitrary Units)

-,

I

-I

Experimental 870°F -, 0.20 Results 100 psig - - , 6 H2/ HCiO.18

Fig. 11. Predictions of AROMAX™ Model for 2-Methylpentane Aromatization

:ii:

'0

CIl

a.

:ii: Q;

'0

CIl

u,

~

t- \.

0.81-

0.9

1.01\

CIl CIl

~

:ii:

'0

CIl

a.

CIl

...

CIl

:ii:

'0

u,

co en

...

346 1.0

0.20 (.) Pool Selectivity

0.9 0.8

I

Experimental Results----J

0.7

I

"C III III

u.

0.5

III

0.4

I

0.10

0

0.08

I

"0

I

0.3

~ 0

:e... III

"0

:e

0.06

I

0.2

0.04

0.1 0.0 0.0

"C III III U.

III Q.

I

Q.

:e

0.14 0.12

III

"0

0.16

870'F 100 psig 6 H2/HC

I

0.6

:e... III

0.18

0.02

nC 6 (LI.) 0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.0 4.0

Reaction Time (Arbitrary Units)

Fig. 12. Predictions of AROMAX™ Model for 3-Methylpentane Aromatization

Table 4 Generate Next Estimate of Rate Constants by Comparing AROMAX™ Model With Data (Figures 9-12) Feed Compound:

n-Hexane

Methylcyclopentane

2-Methylpentane

3-Methylpentane

n-Hexane

Low

High

High

High

Methylcyclopentane

High

Low

Predicted Versus Measured Concentration of Products

2-Methylpentane

High

Ratio of 2 MP/3 MP

High

High

Low

Low

High

High

Necessary Change in Rate Constant Value

K1 K4

Increase

Increase

Increase

Increase

Decrease

Decrease

Decrease

Decrease

847

CATALYST STABILITY It has been known for many years that nonacidic noble metal catalysts such as platinum on carbon will catalyze the dehydrocyclization of paraffins to aromatic hydrocarbons (ref. 8), but heretofore these catalysts have not proved practical because of their extremely poor stability. One of the key features that the zeolite component contributes in AROMAX catalyst seems to be the ability to control coke fouling of the active catalytic component. Catalyst stability is not the focus of this paper; however, it is appropriate to briefly mention two important features of AROMAX stability previously reported by Law et al. (ref. 9). In contrast to the behavior of conventional, bifunctional catalysts, AROMAX catalyst is more stable when processing light hydrocarbons at low pressure than at high pressure. The catalyst is also much more sensitive to poisoning by sulfur compounds than either the monometallic or the bimetallic, bifunctional reforming catalysts. Chevron has developed a proprietary sulfur removal process that protects AROMAX catalyst from sulfur poisoning. With appropriate control of feed sulfur, AROMAX catalyst is sufficiently stable that all of the yield points subsequently reported in this paper for light refinery naphthas are potentially obtainable in a semiregenerative design reformer operating at conditions within the normal range for such reformers. OCfANE ENHANCEMENT OF LIGHT NAPHTHAS Feed properties Five different refinery naphthas were chosen to demonstrate the yield advantages for AROMAX over conventional catalysts. They included a mixed hexane feed, a raffinate from an aromatics extraction unit, two light straight-run naphthas, and a blend of a stright-run naphtha and a raffinate. The properties of these feeds are summarized in Table 5. The feeds span a boiling range from 1250F to 2800F and contain varying amounts of hydrocarbons in the Cs to Cg range. They are relatively paraffinic. The proportion of each feed which belongs to either the reactant pool (C6 + paraffins and cyclopentanes) or the product pool (aromatics and cyclohexanes) is indicated. Yield/octane relationships The relationship between reformate yield and reformate octane was obtained for each of the light naphthas, both with AROMAX catalyst and with a conventional bifunctional reforming catalyst. In each case, the feeds were reformed at a pressure of 150 psig. Operating temperatures fell within the range of 860 to 9600F. The comparative results are shown in Figure 13. In every case, the yield of reformate at a given octane is considerably greater with AROMAX catalyst than with the conventional catalyst. The yield difference broadens as the molecular weight of the feed is decreased and as the reformate octane is increased. Note that with the conventional catalysts one

86

on

68 70

70

72f

74

76

78

~l

82

••~_ixed Raffinate

•

75

85

90

Research Octane Number

80

95

~R.1

100

Open Symbols

AROMAX C''''y.1

\lSR_',

~

Bifunctional Catalyst: Closed Symbols

\

1

6~

~/Raffinate

Fig. 13. Reformate Yields With AROMAX ™ and Bifunctional Catalyst

U

+

II:

Qi

0

E

>= -;'"

-; -c a;

It)

o

...

e,

"iii

Cl

> 84 ::::!.

~

88l

90

e,

1200

1400

e'"

200

I

4001-

600

800

75

~

LS:Raffi~/

85

?LSR-1

90

95

and

Catalyst: Closed Symbols

p

Research Octane Number

80

~nctional

.......

AROMAX Catalyst: Open Symbols

Fig. 14. Hydrogen Yields With AROMAX™ Bifunctional Catalyst

J:

>.

-c

Cl

t::

>=

a; 1000

-c

It)

...o-;

Cl "iii

Ul

m 1600 u..

c

1800'

2000/-

2200 1

....cc C/O

349

seems to hit an "octane wall" for the lightest feeds, above which it is almost impossible to raise octane. All that happens upon raising operating temperature is that the feed is further degraded into light gases.

TableS Properties of Selected Light Naphthas

Description Average Mol. Wt Boiling Range, of (ASTM 0-86) Start 50% End Point Molecular Weight Distribution, LV % CS" Ca C7 Ca+ Species Distribution Paraffins Naphthenes Aromatics Ca+ Paraffins + Cyclopentanes, Mole % Aromatics + Cyclohexanes, Mole % RON

Mixed Hexanes 87 149 151 168

LSRI Raffinate Blend

Raffinate

91

92

152 175 258

126 176 265

---

LSR-2 LSR-1 - - - -101- 97

170 192 280

191 213 270

0 83.0 16.4 0.3

1.4 54.4 37.6 6.5

7.6 45.7 36.3 10.4

0.6 29.0 43.7 26.6

0 12.0 59.1 28.9

86.4 13.1 0.6 95.4 4.2 58.0

63.5 32.7 3.7 80.6 17.7 63.1

79.7 17.5 2.8 83.5 7.3 66.4

66.9 28.6 4.4 75.2 24.0 55.3

73.3 21.5 5.2 80.8 19.1 46.2

Gas yields Hydrogen is a valuable product from catalytic reforming. Figure 14 shows that in all cases AROMAX catalyst produces two to five times as much hydrogen as the bifunctional catalyst. The incremental hydrogen produced for a given increase in octane severity is clearly larger for AROMAX catalyst. This is another reflection of the selectivity difference between the two catalysts. Figures 15 and 16 summarize the light gas and butane yields in each case. Note that AROMAX catalystproduces much less of these low value products. Furthermore, the incremental light gas and butane produced for a given increase in octane severity is much smaller for AROMAX catalyst. Once again, this is a result of the selectivity difference between the two catalysts. Detailed comparisons A detailed comparison of the yields and product properties produced by each catalyst at the same product octane is given in Table 6 for the raffinate feed. This example is illustrative of the general characteristics of the differences seen for all the feeds studied. At equivalent octane the pool conversions were about the same for both catalysts, but the pool selectivities to aromatics

MD

70

oI

1DO f-

-

I

85

I

-

~"'1f.'-

Bifunctional Catalyst

.xr:

LSR-l

~

90

I

and

I

95 100

I

A':,0pen MAX e....y.I' Symbols

LSRIR.HI....

l -

Research Octane Number

I

~ ~~

I

Mixed Hexanes

" Bifunctional Catalyst" Closed Symbols .

Fig. 15. C1-C3 Yields With AROMAX™

> -

~o ,,~

Y

'"

_

1;1

o

~¥

. o,

~~

~

r

40D f-

500

~ I

70

oI

2

3

4

5 f-

7 6

I

75

•

~

•

00

.

Research Octane Number

I

~

I

~

Solo'Symbol.

I

I

I

100

..---<;;>-

LSR-1

R 2

%

LSRIR..,,,I.

•

eo",en'lo,,1 Catalyst;

Hexanes

7

M~

I ..c..» . :

L

Fig. 16. Butane Yields With AROMAX TM and Bifunctional Catalyst

"lD

~~-

;

c..

~ "w

-

>

~

#

10

11

0

~

351

were quite different. C5+ liquid yield, total aromatics, benzene, and hydrogen were all substantially greater with AROMAX catalyst. Note that the liquid product from AROMAX contains more aromatics. The product from the conventional catalyst shows higher concentrations of olefins and isoparaffins. Ethane, propane, and butanes were all substantially lower with AROMAX catalyst; but methane yields were about the same for both catalysts. Methane is produced primarily by metal-catalyzed hydrogenolysis, a process which is possible with both catalysts. The C 2-C4 gases are largely produced by acid-catalyzed cracking, a process which is absent in AROMAX catalyst. The above comparison was made at an octane which was near the maximum achievable from this feed with the conventional catalyst. The increased yield seen with AROMAX catalyst amounts to 15.5% more barrels of 89.7 RON product. However, it is important to note that AROMAX catalyst is also capable of making a higher octane product. For example, at the same liquid yield of 79 LV % AROMAX made a product which was 10.7 numbers higher than the product made by the conventional catalyst (Figure 13). It had an octane of 94.8 RON and still did not represent the maximum octane product which could be achieved with this catalyst. This is because the product still contained paraffins which AROMAX could aromatize. In contrast, at 89.7 RON the product from the conventional catalyst was already comprised largely of compounds which the catalyst would crack rather than aromatize.

Table 6 Yield Comparison With Raffinate Feed (150 psig, 89.7 RON) AROMAX'· Catalyst Liquids, LV % of Feed Cs+ C4 Aromatics Benzene Toluene Ca+ Aromatics Gases, SCFB

H2 C1 C2 C3 Cs+ Composition, LV % of C s+ n-Paraffins i-Paraffins Olefins Naphthenes Aromatics Reid Vapor Pressure, psig Pool Conversion, % Pool Selectivity, %

Bifunctional Catalyst

Difference

81.9 1.8

70.9 10.3

11.0 -8.5

18.2 17.4 2.7

8.0 13.8 6.2

10.2 3.6 -3.5

1530 180 30 25 15.6 33.1 1.3 3.1 46.8 5.0 60 85

740 140 165 135 18.3 36.7 3.3 2.1 39.6 5.7 61 51

790 40 -135 -110

352

The effects that the average molecular weight of the naphtha has on the comparative yields produced by AROMAX and the conventional catalyst are illustrated in Table 7. The three feeds

Table 7 Effect of Feed Molecular Weight on Yield Advantage of AROMAX ™ Over Bifunctional Catalyst* Mixed Hexanes

Feed

Raffinate

LSR-2

---

93.4

Product Research Octane Number

75.6

87.5

Feed Average Mol. Wt

87

92

101

12.8 885 -275 -9.5

9.2 715 -175 -7.3

5.3 820 -118 -6.0

8.4 8.9

8.3 7.9

11.0 4.0

82/44

84/53

88/58

Yield Difference C s+ , LV % of Feed H 2 , SCFB C,-C 3 , SCFB C 4 , LV % of Feed Total Aromatics, LV % of Feed Benzene, LV % of Feed Aromatization Selectivity, AROMAX/Bifunctional

*150 psig. Comparison at equal octane with octanes chosen such that C s+ yield of conventional catalyst is 74 LV % with each feed.

chosen for the comparison span the range of molecular weights studied. For each feed the two catalysts were operated so as to give the same product octane. Additionally, the comparison was made at the octane severities for which the C5+ liquid yields produced by the conventional catalyst were identical for all three feeds. Note that the aromatization selectivities differ by a factor of 2 on the lightest feed but only differ by 50% on the heaviest feed. Therefore, the difference in C 5+ liquid yield narrows as the feed becomes heavier. However, the difference in total aromatics (and hydrogen) remains relatively constant. As expected, AROMAX catalyst produces more benzene in all cases because of its excellent selectivity for converting hexanes. However, even when there are relatively few hexanes in the feed, the benzene production is substantial. This latter observation shows that AROMAX tends to dealkylate to a greater extent than the conventional catalyst. For those refiners who can accept additional benzene in their gasoline, AROMAX provides a much greater octane leverage than isomerization. Of course, some refiners extract benzene from reformate for sale as a chemical feedstock. AROMAX catalyst should prove particularly advantageous for them. An additional effect of feed composition is evident in Figure 13. Note that the LSR/raffinate blend gives a better liquid yield than the pure raffmate with both catalysts despite having almost the same molecular weight. This results from a higher concentration of aromatics and cyclohexanes in the blend. See Table 5. A similar difference is responsible for the comparative yields obtained with the two light straight-run feeds.

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CONCLUSIONS AROMAX catalyst represents a unique advance in the development of modem reforming catalysts. By employing a zeolite support it is able to take advantage of aromatization chemistry which is entirely different from and considerably more selective than that of the conventional catalysts. This chemistry allows one to upgrade light-paraffinic naphthas to high octane gasoline blend stocks with dramatically better yields and/or higher octane ratings than with conventional catalysts. Because AROMAX operates at process conditions which are essentially identical to those of conventional reforming, reformer designs and hardware need not be changed significantly to allow its use. In many cases, existing hardware should be adaptable to the new process without a large capital expense. ACKNOWLEDGMENTS The authors gratefully acknowledge the contribution of Mr. D. L. Holtermann in providing detailed gas chromatographic analyses of the reformate samples. REFERENCES 1.

2. 3. 4. 5. 6. 7. 8. 9.

V. Haensel, U.S. Patents 2,479,109 and 2,479,110 (1949); Oil and Gas J., 48 (47), (1950) 82. H. E. Kluksdahl, U.S. Patent 3,415,737 (1968). R. L. Jacobson; H. E. Kluksdahl; C. S. McCoy; and R. W. Davis, Proc. Am. Petrol. Inst., (1969),504. G. A. Mills; H. Heinemann; T. H. Millikan; and A. G. Oblad, Ind. Eng. Chem. 45 (1), (1953) 134. T. R. Hughes, paper presented at: Advances in Catalytic Chemistry III - A Symposium in Honor of Michel Boudart, Salt Lake City, Utah, May 20-24, 1985. T. R. Hughes; W. C. Buss; P. W. Tamm; and R. L. Jacobson, Proc. 7th Inter. Zeolite Conf., Tokyo, Japan, 1986, p 725. J. R. Bernard, Proc. Fifth Intern. Conf. Zeolites, Heyden, London, 1980, p 686. B. A. Kazansky and A. F. Plate, Chem Ber. 69B, (1936) 1862; Zh. Obshch. Khim. 9, (1939) 496. D. V. Law; P. W. Tamrn; and C. M. Detz, 1987 Spring Nat. Meet. AIChE. March 29-April 2, Houston, Texas.