CHANGING FUTURE FOR PYROLYSIS GASOLINE

In addition to producing many of the basic building blocks for the polymer industry, naphtha steam-cracking yields significant amounts of olefin- and aromatics-rich gasoline (pygas). Table 1 shows a typi-cal pyrolysis (pygas) yield and composition.

Table 1. Typical naphtha cracker pygas (C5-200°C) yield and composition

Total yield, wt % of cracker feed

22

Component wt %

Paraffins + naphthe-nes

11.8

Olefins 5.5

Diolefins 18.1

Benzene 28

Toluene 13.9

Xylenes 7.2

Styrene 3

C9+ Aromatics 12.5

Total Aromatics 64.6

Pygas has an excellent research octane number due to its high aromatics and olefins contents. Until recently, pygas was often used as a gasoline pool blending stock after undergoing selective hydrogenation to eliminate gum-forming compounds such as diolefins, styrene compounds and indenes. The trend to more stringent environmental regulations concerning gaso-line has led to a reduction in the amount of pygas that can be added to the gasoline pool.

The European specifications planned for 2005 will

limit this practice even further, for the following rea-sons:

• high benzene content compared to the 1% volume maximum in the gasoline pool

• sulfur content is generally higher than the pro-posed 50 ppm limitation

Pygas is rich in light olefins having low motor octane numbers (MON).

AXENS - THE WORLD'S LEADING HYDROGENATION TECHNOLOGY AND CATALYST SUPPLIER

There are several reasons for Axens' position, among which are that it offers a variety of industrially tested, dependable pygas upgrading routes that enable the refiner to:

• upgrade pygas while avoiding the production of negative value products, i.e., those with a lower value than that of the steam-cracker feed

• offer a wide variety of processing options

• offer the most profitable schemes for various site-specific constraints.

Several examples of attractive processing routes and new catalysts are presented for the following applica-tions: upgrading C5 diolefins; complete C5 olefin conversion; benzene production; C6 to C9 aromatics production; increased performance and service life of existing applications.

PROCESSING ROUTES FOR UPGRADING C5 DIOLEFINS

Steam-cracker C5 cuts contain unsaturated hydrocar-bons that can be converted to high value-added derivatives. The compositions in Table 2 show that two components, isoprene and cyclopentadiene, are

UPGRADING C5+ STREAMS

CREATING VALUE WITH PYROLYSIS GASOLINE

present in reasonably high concentrations and have high value-added potential.

Table 2 Boiling points, octane numbers, and weight per cent values for compounds found in steam-

cracked C5 cuts.

Compound Boiling point,

°CRON MON Compositon,

wt %

3-methyl-1-butene 20.2 97.5 - 0.5

Isopentane 27.9 92.3 90.3 8.5

1-pentene 30.0 90.9 77.1 2.7

2-methyl-1-butene 31.2 102.5 81.9 6.0

Isoprene 34.1 99 81 15.4

n-pentane 36.1 61.7 62.6 12.3

cis- trans 2-pentene 36.4/37.0 98 80.0 2.0

2-methyl-2-butene 38.6 97.3 84.7 4.0

Cyclopentadiene 41.0 103 86 20.0

1,3-pentadiene 42.1 - - 14.0

Cyclopentene 44.3 93 70 13.4

Cyclopentane 49.3 101 85 1.2

Isoprene and cyclopentadiene are not easily separated by distillation due to the small difference in boiling points; however, cyclopentadiene is dimerized easily to higher-boiling dicyclopentadiene (DCPD). DCPD can be separated from the other C5 components by simple distillation.

Global consumption of DCPD is forecast to grow by around 8% per year until 2005 after previously grow-ing at a rate of 10% per year. There are several DCPD derivatives, including hydrocarbon resins and unsatu-rated polyester resins.

Most of these are marketed as two grades of purity, 92% and 83%. Axens' selective cyclopentadiene dimerization process for raw C5 cuts is shown in Fig-ure 1.

The raw pyrolysis gasoline (RPG) is first depentan-ized to produce a C5 cut free of heavier components that would adversely affect DCPD product purity. The C6+ cut is sent to a hydrogenation plant.

RPG

DCPDCPDDimerization

Hydrogen-ation Plant

C5s to Isoprene Extraction

C5 Saturates toSteam Cracker

Olefins andSulfur-Free Aromatics

C6+

CrudeC5s

Figure 1 - Upgrading C5 Diolefins

After the CPD dimerization, the remaining isoprene-enriched C5 cut can be sent either to an extraction plant for isoprene recovery or to a hydrogenation plant.

The DCPD process has the flexibility to cope with varying product targets; for CPD conversions of 85 to 90%, DCPD purity is 83%. In the same unit, a DCPD of 91-92% is obtained by limiting the CPD conversion to 50-60%. A unit put on stream in 1991 produces both 83% and 92% purity DCPD depending on the application.

Upgrading the isoprene-enriched C5 cut

After the CPD separation step, the remaining C5 cut is concentrated in isoprene and pentadiene (I-P) enriched C5 cut and can be sent to an extractive distillation unit similar to that used for 1-3 butadiene extraction. There, an isoprene-pentadiene stream is removed from the C5 olefins and paraffins. Isoprene and pentadiene can then be separated by superfractionation.

When it is not necessary to upgrade the I-P cut, the most economical route is to send it to a hydrogenation plant for complete C5 olefin saturation. The saturated hydrocarbons obtained can then be recycled as feed to the cracker furnaces.

PROCESSING ROUTE FOR COMPLETE C5 OLEFIN CONVERSION

Previously, the C5 cut was separated from the aro-matic rich C6+ cut after hydrogenation and sent to the gasoline pool. This practice is now limited ow-ing to the poor motor octane quality of this olefinic stream, see Table 2. The high vapor pressure of this cut, as indicated by the boiling points of the compo-nents, is undesirable as well. Therefore, a lower value is attributed to the olefinic C5 cut compared to that of the steam-cracker naphtha feed. Conse-quently, the C5 stream is recycled to the steam-cracker furnaces.

Although recycling this C5 stream to the cracker is a convenient way to dispose of the cut, it comes at the expense of a lower overall ethylene yield. Ethylene yields from naphtha components generally decrease in the following order starting with the highest yield: n-paraffins>iso-paraffins>naphthenes >ole-fins>diolefins>aromatics. There is therefore an incentive to saturate the C5 olefins, because it re-places naphtha feed without reducing ethylene yield. In the past, the most common method was to send the C5 cut, mixed with the aromatics stream, to a second-stage hydrogenation unit for olefins satura-tion and hydrodesulfurization. This processing scheme (Figure 2) results in complete C5 olefin satu-ration.

Cyclopentane Production

Under certain circumstances, it may be advanta-geous to capitalize on the high cyclopentane content of the treated C5 pyrolysis cut to produce cyclopen-tane. This product can represent one-third of the total weight of the C5 cut. Cyclopentane has a high market value as a chlorofluorocarbon (CFC) blow-ing agent replacement in the manufacture of rigid polyurethane foams.

Cyclopentane can be separated by distillation from the lower boiling hydrocarbons, i. e., isopentane and pentane. If necessary, benzene traces present in the

cyclopentane can be removed by simple hydrogena-tion to cyclohexane.

RPG1st StageHydro.

C5s to Steam

Cracking Furnaces

C10+ (Optional)

C5-C9

2nd StageHydro.

C6-C9Aromatics

F. G.H2SFuel Gas

Figure 2 - Conventional RPG hydrogenation scheme with complete saturation of C5 olefins

An economically improved scheme that provides a C5 cut containing about 75% saturated hydrocarbons is now available, Figure 3. This cut is well suited for recycle to the cracker furnaces.

RPG1st StageHydro.

C5s to SteamCracking Furnaces

C10+(Optional)

C6 -C9

2nd StageHydro.

C6 -C9Aromatics

F.G.H2SF.G.

Figure 3 - Improved RPG hydrogenation scheme for

the production of 75% saturated C5 cuts

The improved first-stage hydrogenation (GHU-1) process is very similar to that employed convention-ally. The major difference is that the catalyst, LD 365, features much higher diolefin and styrene compounds’ hydrogenation rates as well as high activity for light olefins hydrogenation.

The performance obtained for a typical C5 cut is reported in Table 3:

Table 3 - C5 product composition from an improved GHU-1t

Compound wt %

Isopentane 14

n-Butenes 22

Pentenes 5

Pentane 26

Cyclopentene 0

Cyclopentane 34

100

Total saturated Hydrocarbons 74 wt %

HYDROGENATION ROUTE FOR COMBINED BUTADIENE, BUTENES AND C5 OLEFINS CUT

In geographical areas where there is no market for butadiene or butenes, the debutanizer can be elimi-nated from the steam-cracker separation train and the combined C4 and C5 stream can be sent to a full hydrogenation unit. The product from this unit con-tains at least 85% saturated hydrocarbons and can be recycled to the cracking furnaces to produce more ethylene and propylene with less naphtha feed. sev-eral units based on this approach are in commercial operation.

PROCESSING ROUTES TO BENZENE

Raw pyrolysis gasoline contains over 60 wt% of C6 to C10 aromatics, over half of which is benzene. A typical C6 cut contains 75% benzene and - after removal of diolefins, olefins and sulfur - is an excel-lent feed for extractive distillation processes.

The C7+ gasoline cut has an excellent octane rating; typical RONs and MONs are 102 and 88, respec-

tively. Therefore, this cut could be advantageously sent to the gasoline pool after hydrodesulfurization. When demand for benzene is high, the C7+ cut can be sent to a hydrodealkylation process to convert the alkylbenzenes to benzene.

Improved schemes and technologies are available for two major options:

• High purity benzene/C7+ cut for the gasoline pool, Figure 4

• Maximum benzene production, Figure 5. Option 1

In this scheme, the C6+ product from the GHU-1 is re-run to remove the heavy aromatics. The re-run step can be avoided by good control of the gasoline end point in the primary distillation section of the steam-cracker.

The second stage hydrogenation step (GHU-2) pro-duces a sulfur and olefin-free cut, rich in benzene. This cut can be sent directly to a benzene extractive distillation unit. The C6 cut from the GHU-2 typi-cally contains less than 0.5 ppm thiophene, less than 0.2 ppm total sulfur and an acid wash color (AWC) of less than one. The extracted benzene requires no additional processing, such as clay treatment. The C7+ cut contains less than 5 ppm sulfur, and is an excellent blending stock for the ultra-low sulfur gasoline pool.

RPG1st StageHydro.

C5s to S. C. Furnaces

C10+(Optional)

C6 -C9

2nd StageHydro.

C6 -C9Aromatics

F.G.H2SF.G.

C6s toBenzeneExtraction

C7 -C9Gasoline

Pool

Figure 4 - High purity benzene / C7+ to gasoline

pool

C6 -C9

Benzene Product

RPG1st StageHydro.

C5s to S. C. Furnaces

C10+(Optional)

2nd

StageHydro.

F.G.H2SF.G.

C6+

HDA Aro-fining

Heavy Aromatics

Recycle

Figure 5 - Maximum benzene production

Option 2

The block flow diagram in Figure 5 shows how maximum high purity benzene production can be achieved. The C6+ cut is sent to a hydrodealkylation (HDA) unit. This non-catalytic process shows dis-tinct advantages over other HDA processes. The technology produces high molar yields of high-purity benzene product.

Unlike catalytic HDA processes, the Axens HDA process operates without removal of the H2S pro-duced in the GHU-2. Therefore, the HDS reactor is integrated completely in the HDA unit; conse-quently the second stage unit is reduced to a simple reactor system integrated in the HDA unit's heating train and using the same H2 circuit.

The benzene product from the HDA unit contains traces of diolefins, olefins and, occasionally, thio-phene that are reformed in the HDA reactor by the addition of H2S and unsaturated hydrocarbon under high H2S partial pressures.

Axens' unique hydrofining technology enables com-plete hydrogenation of trace diolefins and olefins in the HDA product as well as conversion of thiophene to thiophane. The hydrorefining reactor is easily integrated in the HDA unit's cooling train. The reac-tor product is cooled further and sent to the separation train stabilizer and a benzene tower that

produces high purity benzene having an acid wash color near 0 and thiophene content less than 0.5 ppm. The thiophane produced in the hydrofining reactor is removed in the C7

+ cut, –its boiling point is close to that of toluene– and recycled to the sec-ond stage hydrogenation unit where it is destroyed. This simple and efficient technology avoids the need for clay treatment and its associated cost and spent clay disposal problems.

THE ROUTE TO HIGH PURITY C6 TO C10 AROMATICS

The objective of the scheme, shown in Figure 6, is to maximize production of high purity C6 to C9 aro-matics. The C6+ cut is sent directly to the GHU-2. The second stage is designed for complete olefin hydrogenation and desulfurization of the aromatic cut.

RPG1st StageHydro.

C5s to SC Furnaces

C6 - C9

2nd StageHydro.

High PurityC6 - C9

Aromatics

F.G.H2SF.G.

Figure 6 - Processing route for high purity C6 to C10

aromatics production

Typical performance obtained is given in Table 5. The C6-C9 aromatic rich cut is suitable for produc-tion of high purity benzene, toluene, xylene and C9 aromatics.

Table 5. C6 – C9 cut characteristics after second stage hydrogenation

Diene value 0

Bromine index, mg/100 g 100

Sulfur, ppm < 1

Thiophene, ppm < 0.2

Acid Wash Color (AWC) 1

C6 to C9 aromatics recovery, % 99.5

Benzene recovery, % 99.7

C6 cut Bromine index 20

C6 cut AWC 1-

INCREASED PERFORMANCE AND SERVICE LIFE IN EXISTING UNITS

Over 30 years have been committed to the continu-ous improvement of Axens' processes and catalysts as well as to meet the evolving needs of the petro-chemical market. Our first pygas hydrogenation unit (GHU) was put on stream in early 1967. Some cur-rent market trends are given below:

• revamping to increase steam-cracking capacity

• increased hydrogenation unit capacity with minimum investment

• feed diversification. This continuing trend re-sults in new contaminants being introduced in steam-cracker effluents, which must be taken into account.

First stage hydrogenation

The heart of the GHU is the reaction section. Be-sides making mechanical and physical modifications such as the installation of improved pumps, distilla-tion towers, distribution trays and quench boxes, the replacement of existing catalyst by more active, stable or poison-resistant catalyst can offer signifi-

cant economic benefits. To this end, a full line of hydrogenation catalysts has been commercialized recently. The previous generation palladium LD 265 and nickel LD 241 catalysts, which have been the industry's benchmark products, have undergone major improvements and led to new generation LD 365 and LD 341 products. Their major charac-teristics and performance are compared in Table 6.

Table 6. Relative catalyst performance for first stage pygas hydrogenation

Trade name

Metal

Rela-tive

Activ-ity per volume

Rela-tive

LHSV

Cycle length, years *

LD 241 Ni 1 1 0.5

LD 265 Pd 2 1.5-2 0.8-1.5

LD 341 Ni 2 1.5-2 0.8-1.5

LD 365 Pd 3 2.5-3 1-2

* Values obtained on full range gasoline i.e. C5-200 °C.

These new generation catalysts show remarkable performance improvements over catalysts that have been the industry's standards. Table 7 shows how the new generation catalysts provide revamping and grassroots solutions for all petrochemical applica-tions.

Table 7. Choosing the most economically attractive catalysts for various applications

Case Previous Catalyst New Catalyst

Unit revamp Up to 150% capacity New feed with con-taminants

LD 241 LD 265

LD 341 LD 341

Grassroots unit Uncontaminated feedContaminated feed

LD 365/341

LD 341

If the pygas contains temporary poisons such as As, P, or Hg or permanent poisons such as Si or Pb, the nickel catalyst, LD 341, is the product of choice. Because of its high metal content compared to pal-ladium catalysts, LD 341 has a cumulative tolerance to poisons that is higher by a factor of five to ten. Our accumulative experience in pygas hydrogena-tion enables us to propose the best alternatives for all site-specific cases.

Second stage hydrogenation

Good operation of the GHU-2 for complete olefin saturation and hydrodesulfurization depends on the performance of the operation of the first stage. For several years, dual catalyst systems that ensure long cycle lengths have been commercialized. The first bed contains a formulation that is dedicated to olefin hydrogenation. This product significantly limits polymer formation. Polymer formation, which is the main source of plugging and pressure drop prob-lems, is avoided when employing this catalyst. The second bed contains a selective HDS catalyst, which avoids aromatics hydrogenation. The high stability and high poison tolerance of the dual catalyst sys-tem are features that enable the elimination of a spare reactor and thus reduced investment costs.

ECONOMICS

Based on a 1 million metric ton per year naphtha steam cracker producing a 620 000 ton per year (~15 000 BPSD) first-stage pyrolysis gasoline stream:

€/Mt

ISBL Investment 16.4

Utilities and catalysts 7.3

For a second-stage hydrogenation unit treating the product from the first-stage unit (445 000 tpa.):

€/Mt

ISBL Investment 18.6

Utilities and catalysts 2

INDUSTRIAL EXPERIENCE

The first Axens pyrolysis gasoline hydrogenation unit started up in 1967. Since then, over 90 first-stage and over 60 second-stage units have been li-censed.

CONCLUSION

Interest in pygas upgrading is strong due to chang-ing gasoline pool requirements. With more than 30 years experience in pygas process development and application, and over 60% of the market share, Ax-ens has acquired a vast expertise that has led to a complete portfolio of pygas hydrogenation tech-nologies that cover many different applications. A full line of high performance catalysts that meet most site-specific profitability requirements accom-panies these technologies.

Oct

04-C

5HD

T