Industrial and Laboratory Pyrolyses

23 Pyrolysis Gasoline/Gas Oil Hydrotreating C. T. ADAMS and C. A. TREVINO

Downloaded by NORTH CAROLINA STATE UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0032.ch023

Shell Development Co., Westhollow Research Center, P.O. Box 1380, Houston, Tex. 77001

The use of heavier hydrocarbons as feeds to pyrolysis units for production of light olefins has increased significantly in recent years. Stocks such as atmospheric gas oils and even heavier fractions are key raw materials for several large-scale plants each with a capacity for producing over a billion pounds per year of ethylene. A highly significant feature of the use of the heavier feedstocks is the by-product yield structure. Table I based on data of Zdonik et al (1), presents yield data for cracking of naphtha, gas oil and vacuum distillate stocks. From these data, i t is apparent that the user of the heavier feedstocks must be prepared to deal with substantial quantities of C 5 plus liquid pyrolyzate. The liquid pyrolyzate is rich in diolefins, styrenes, and aromatics and its by-product value potential is substantial. For economic reasons, one may not choose to recover all of these by-products. Table II itemizes some of the potential uses and process requirements for components of the liquid pyrolyzate. Since the pyrolysis liquid is rich in diolefins, styrenes, and other reactive species, i t is highly unstable and tends to form polymers or gums when heated. Hydrogenation as a means of stabilizing the liquid is a key element in the downstream processing of the pyrolyzate and is of particular significance in optimization of by-product values. Several processes have been described for hydrotreatment of pyrolysis liquids over the past twenty-five years (2, 3, 4, 5, 6). Most of the processes outlined in the literature have concentrated on hydrorefining of pyrolysis gasoline derived primarily from pyrolysis of naphtha or lighter stocks. The trend to cracking of heavier feeds has provided the incentive for development of new techniques for handling the increased volume of pyrolysis gasoline and gas o i l . Further incentive has been provided by the fact that gasolines from gas oil pyrolysis have higher concentrations of olefins, diolefins and sulfur compounds than those derived from naphtha and lighter stock. (4)

412

In Industrial and Laboratory Pyrolyses; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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413

TABLE I TYPICAL ONCE-THROUGH YIELDS FROM PYROLYSIS OF HEAVY FEEDSTOCKS


(Range in l i q u i d by-product yields from typical l i q u i d feeds) Naphtha Yield Range, %wt C - 204°C Gasoline 5

Gas Oil

Vacuum D i s t i l l a t e

22-27

20-21

17-19

3-6

19-22

21-25

Pyrolysis

TABLE II POTENTIAL USES OF PYROLYSIS BY-PRODUCT LIQUIDS Component

Uses

Process Requirements

C Cut

petrochemicals, e . g . , isoprene, cyclopentadiene, cyclopentene

fractionation extraction selective hydrogénation

C - 213°C Gasoline

high octane motor fuel blending

hydrogenati on (selective)

CC-CQ

benzene, toluene, xylenes source

hydrogénation ( f u l l o l e f i n saturation) fractionation extraction

fuel o i l , fuel o i l blending, hydrocracker feed, carbon black production

fractionation hydrotreatment

5

5

Cut

Pyrolysis Oil 213°C plus


414

INDUSTRIAL AND LABORATORY PYROLYSES


During the past several years Shell has carried out development studies on the hydrotreatment of pyrolysis l i q u i d s from gas o i l cracking. Prime goals in the program were to minimize potent i a l fouling problems and to optimize by-product values. This a r t i c l e i s concerned with the hydrorefining of combined pyrolysis gasoline/gas o i l streams and describes a process scheme which provides a high degree of f l e x i b i l i t y in handling the l i q u i d pyrolyzate. Process Considerations Process r e l i a b i l i t y , p a r t i c u l a r l y as reflected in on-stream time, i s a major consideration in selection of any process for hydrotreatment of pyrolysis by-products. The on-stream factor i s of increased importance in the case of the increased volume and of the highly reactive nature of the l i q u i d pyrolyzate. The r e a c t i v i t y of the pyrolyzate from gas o i l cracking has two s i g n i f i c a n t aspects. The f i r s t part concerns the increased concentrations of reactive species ( d i o l e f i n s , e t c . ) , and the influence of these materials on the fouling tendency of the l i q u i d . The tendency of pyrolysis l i q u i d s to form deposits and foul heat exchange equipment and catalysts quickly at elevated temperatures i s well known and long has been a major concern as regards process r e l i a b i l i t y . Watkins (2J, for example, has reported that exposure of the l i q u i d to normal hydrodesulfurization conditions of 288-371°C could result in deposits amounting to 2%w of the feed charge. Such deposition rates would shut down a plant within 24 hours. Significant formation of polymeric deposits occur at much lower temperatures in the processing of pyrolysis l i q u i d s . Table III presents data on coking tendencies of.by-product liquids from gas o i l pyrolysis. The data show that the pyrolysis gasoline i s the most reactive of the components with s i g n i f i c a n t formation of deposits at a temperature of 177°C. The gas o i l , which contains fewer d i o l e f i n s and alkenyl aromatics than the gasoline, i s more stable and may be viewed as a diluent for the gasoline; i . e . , the blend of pyrolysis gasoline and gas o i l can be processed at a higher temperature than the gasoline alone. The pyrolysis gas o i l i s , however, a reactive l i q u i d and can form s i g n i f i c a n t deposits at r e l a t i v e l y mild conditions. Therefore, for many applications, the gas o i l requires hydrotreatment prior to further processing. The combined processing of the gasoline/ gas o i l thus appears to offer an advantage r e l a t i v e to the processing of the individual components. A second aspect of the r e a c t i v i t y of the gas o i l pyrolyzate i s the impact i t has on catalyst performance. The catalysts in many of the hydrorefining schemes developed for pyrolysis gasoline are p a r t i c u l a r l y sensitive to control of the d i s t i l l a t i o n end point and to the inclusion of substantial quantities of s u l f u r ,


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Gasoline IGas Oil Hydrotreating

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TABLE III COKING TENDENCIES OF BY-PRODUCT LIQUIDS FROM GAS OIL PYROLYSIS


Stream

Temperature of Incipient Formation of Deposits*

Pyrolysis Gasoline

177°C

Pyrolysis Gas Oil (204-371°C Cut)

260°C

Pyrolysis Gasoline/Gas Oil (yield proportions)

204°C

*Deposits measured on stainless steel inserts exposed to the pyrolysis liquids under typical flow conditions of a hydrorefining unit. nitrogen, and oxygen containing compounds that are found in gas o i l pyrolyzate. (4) These considerations led us to studies of the combined processing of pyrolysis gasoline and gas o i l . Catalyst performance, in p a r t i c u l a r , was recognized as a c r i t i c a l factor. When i t was determined that catalyst l i f e of available commercial hydrotreating catalysts was too short to give the desired plant onstream time, catalyst development became an integral part of the effort. Process Description Figure 1 outlines a multiple bed hydrotreating scheme which employs temperature staging for processing of a combined pyrolysis gasoline/gas o i l stream. For ease of description, the plant can be divided into two sections, hydrotreating and product separation. The basic elements in the hydrotreating sections are a t r i c k l e phase reactor train with two reaction stages, a third stage-vapor phase reactor (which processes only gasoline boiling range materials), quench and recycle o i l systems, and a hydrogen system to provide make-up and recycle hydrogen. The primary purpose of the f i r s t of the three stages (Stage 1) i s conversion of a l l d i o l e f i n s in the feed. The objective of the second stage i s to saturate mono-olefins in the gasoline and s u f f i c i e n t unsaturates in the gas o i l to make the o i l thermally stable. The third stage i s designed to desulfurize the gasoline portion, for example, to provide a sulfur-free concentrate for aromatics recovery.



QUENCH

GAS OIL RECYCLE

SECOND STAGE

Figure 1.

QUENCH

A

RECYCLE H

THIRD STAGE

2

AROMATICS CONCENTRATE

GAS OIL

HEAVY GASOLINE

\ J

LIGHT GASOLINE

Flow schematic of three-stage hydrotreater for full saturation of olefins

PYROLYSIS GASOLINE/GAS OIL

HYDROGEN

FIRST STAGE



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TREviNO

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Gasoline/ Gas Oil Hydrotreating

417

Figure 2 outlines a hydrotreater of similar design as presented in Figure 1 but with the basic difference that selectively hydrotreated gasoline or i t s components are recovered after Stage 1 processing. In both cases the pyrolysis gasoline/gas o i l feed enters at the top of the f i r s t reactor. A recycle stream of hydrotreated gas o i l i s injected with the feed. The recycle streams serve as a reactant diluent, a heat sink to aid in reactor temperature control and as a solvent for polymer removal. Multiple catalyst beds are employed in the reactors to aid in temperature control. Quench o i l i s injected between the beds for reaction heat control. The f i r s t stage i s operated at temperatures in the range of 107-177°C and at hydrogen partial pressures of the order of 48-68 atmospheres. After d i o l e f i n conversion in Stage 1, the products and hydrogen are heated to 232°C prior to entering Stage 2. In Stage 2, which also employs multiple beds, olefins are saturated and substantial sulfur (about 70%) i s removed from the gas o i l . Reaction quench between beds i s obtained by recycle of cooled hydrogenated gas o i l . The catalyst employed in the f i r s t two stages i s a proprietary nickel oxide-molybdenum oxide/alumina formulation developed for t h i s service. Separation of the gasoline (or other fractions) and gas o i l can be effected after either the f i r s t or second stage. Stage 3 processes either the f u l l range gasoline from Stage 2 or alternatively an aromatics cut. The feed i s heated and vaporized before entering Stage 3 at temperatures of the order of 316-343°C. Hydrogen pressure in the reactor i s approximately 44 atmospheres. A lower pressure could be used as far as the process i s concerned but the high pressure allows use of one compressor for a l l three stages. The t h i r d stage reactor has only one catalyst bed and quench i s not required. Aromatics hydrogénation also is minimal in this stage. The catalyst for the third stage i s a cobalt oxide-molybdenum oxide/alumina formulation. Performance Tests Product Properties. Properties of gasoline products from the f i r s t and second stage operations are compared with those of the feed in Table IV. The diene (measured by maleic anhydride value) and bromine number (a measure of mono-olefins) data show that essentially complete d i o l e f i n hydrogénation i s obtained with substantial mono-olefin retention in the f i r s t stage processing. The quality data indicate the selectively hydrotreated gasoline is a valuable high octane blending component. The data presented for processing the gasoline/gas o i l blend through both the f i r s t and second stages indicate that o l e f i n saturation i s complete. Gas-liquid chromatographic analyses of feed and product C - C hydrocarbons showed l i t t l e or no saturation of the 6

8



SECOND STAGE

THIRD STAGE

GAS OIL

HEAVY GASOLINE

AROMATICS CONCENTRATES

SELECTIVELY HYDROTREATED GASOLINE

Figure 2. Flow schematic of three-stage pyrolysis gasoline/gas oil hydrotreater for selective hydrogénation of gasoline and aromatics recovery

PYROLYSIS GASOLINE/GAS OIL

FIRST STAGE


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Gasolineι Gas Oil Hydrotreating

TABLE IV PROPERTIES OF HYDROTREATED GASOLINE FROM COMBINED PROCESSING OF PYROLYSIS GASOLINE/GAS OIL


Gasoline Feed

Gasoline Product 1st Stage 2nd Stage

Maleic Anhydride Value

199

0-2

0

Bromine Number

106

40-50

0

1

1

Existent Gum, mg/100 ml .01

Sulfur Content, %wt Research Octane No. (Unleaded)

.01 99

Industrial and Laboratory Pyrolyses

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