ANCILLARY CRACKING OF PARAFFINIC NAPHTHA

IN coNJuσπoN WITHFCC UNIT OPERATIONS

FIELD OF THE INVENTION

This invention relates to increasing the production of lighter

hydrocarbons, such as ethylene, propylene and the butylenes, and gasoline in

conjunction with the operation of a fluidized catalytic cracking process.

BACKGROUND OF THE INVENTION

Propylene is second in importance only to ethylene as a petrochemical

raw material building block. Propylene has traditionally been obtained as a by¬

product from steam cracking to produce ethylene and from refinery fluidized

catalytic cracking processes to produce gasoline. The projected growth in

demand for propylene has started to exceed that of ethylene so that existing

processes cannot satisfy the future propylene demand. To meet the expected

market demand for propylene, a catalytic paraffmic naphtha cracking process

can be utilized.

The catalytic cracking of olefinic naphthas is well known and currently

practiced in all types of FCC units processing a variety of feedstocks. Recycled

cracked naphtha, olefkiic naphthas from the FCC, from visbreakers or cokers

are easily converted to propylene in the FCC reactor riser with the base

feedstock. The gasoline produced from recycling is high in octane and

aromatics. None of the prior art FCC processes can be used to crack light

straight run ("LSR") naphtha efficiently without significant modifications.

Fluidized catalytic cracking, or FCC, is a well-known and widely

practiced process for converting heavy hydrocarbons, gasoils and residues into

lighter hydrocarbon fractions. In general terms, the process for the cracking of

hydrocarbon feedstocks relies on contact with heated fluidized catalytic

particles in a reaction zone maintained at appropriate temperatures and

pressures. When the heavier feed contacts the hot catalyst and is cracked to

lighter products, carbonaceous deposits, commonly referred to as coke, form on

the catalyst and deactivate it. The deactivated, or spent, catalyst is separated

from the cracked products, stripped of removable hydrocarbons and passed to a

regeneration vessel where the coke is burned from the catalyst in the presence of

air to produce a substantially regenerated catalyst. The combustion products are

removed from the vessel as flue gas. The heated regenerated catalyst is then

recycled to the reaction zone in the FCC unit. A general description of the FCC

process is provided in USP 5,372,704, the complete disclosure of which is

incorporated herein by reference.

Various methods and apparatus have been proposed for increasing or

enhancing the output of particular product streams from the FCC unit. In some

cases, ancillary reactors and other treatment vessels have been provided to treat

a particular fraction or reaction product stream. In some instances, multiple

reactors are provided, each with a different feed, in order to derive a particularly

desired product stream.

For example, USP 4,090,949 describes a method for upgrading poor

quality olefinic gasoline using two reactor risers, one of which receives a typical

gas feedstream, while the second is used to crack a feedstream consisting

primarily of low quality olefinic gasoline. The reactor temperature is from 450°

F to 900° F and the catalyst-to-olefinic gasoline ratio is in the range of from 1 to

40, with a residence time in the range of from 1 to 30 seconds.

A process employing multiple cracking zones in either a fluidized bed or

in parallel riser reactors is disclosed in USP 3,856,659. In one aspect of this

integrated process, paraffmic naphtha feed from the crude unit is mixed with

recycled cracked naphtha (olefinic) and fed to one of the riser reactors. Typical

operating temperatures range from 900° F to 1,300° F with a catalyst/oil ratio of

from 3 to 20 and a residence time of from 1 to 10 seconds.

A process using separate multiple fluidized reactors with upward

direction in elongated transfer lines, or risers, is disclosed in USP 4,297,203.

Cracked naphtha feedstock from the first riser reactor is recovered and recycled

to the second riser along with another hydrocarbon feedstream. Reactor

temperatures are somewhat lower than in the prior example.

A method for upgrading or cracking virgin naphtha is disclosed in USP

4,830,728 using a single catalyst of the Y zeolite type, or a combination of

Y zeolite ZSM-5. In this process, ethylene is mixed with the virgin naphtha in

the riser reactor. This process mentions upgrading straight run naphtha, which

is mixed with recycled cracked material and injected into a separate upflow riser

reactor. It is apparently the objective of this process to upgrade naphtha in two

riser reaction zones with gasoil and/or resid catalytically cracked in the first

riser and ethylene and catalytically cracked naphtha recycle and/or other

naphtha(s) catalytically cracked in the second riser and in a dense fluidized

reactor.

A method is described in USP 5,372,704 that employs spent catalyst in a

re-cracking reactor for limited conversion of FCC naphtha or other thermally

produced olefinic naphtha to light products with an increase in the product

naphtha octane rating. The operating conditions for this process are relatively

mild with temperatures in the 800° F to 1,100° F range and a residence time of

from. 1 to 100 seconds.

A review of the disclosures of the patents discussed above, as well as

other prior art sources, has failed to identify a process in which a virgin

paraffinic naphtha feedstream is cracked in conjunction with an FCC unit to

produce primarily light olefins consisting of ethylene, propylene and butylenes,

and gasoline.

It is therefore an object of the present invention to provide a process in

which a virgin paraffinic naphtha feedstream withdrawn as a fraction from an

external source, such as a crude atmospheric distillation column, toppers, as a

by-product stream from a hydrotreater or hydrocracker units, or other high

paraffinic naphtha stream from an extraction process, is further cracked to

provide a light reaction product stream.

It is a further object of the invention to provide such a process that can be

run efficiently utilizing the same catalyst employed in the FCC unit.

Yet another object of the invention is to provide a novel process for

efficiently cracking a paraffinic naphtha feedstock to produce a lighter

hydrocarbon product stream consisting of ethylene, propylene, butylenes and

gasoline, which reaction product stream can either be recovered separately and

further fractionated to recover the individual components or combined with an

effluent stream from the FCC unit for further fractionation.

The term "paraffinic naphtha feed" shall be understood to include any

hydrocarbon charge stock boiling in the range of pentane (C ₅ ) hydrocarbons up

to about 450° F that contains 40% to 80% by weight of paraffinic components

with very little olefin components. The remaining components will be

naphthenes, aromatics and olefins in descending order of composition. This

paraffinic naphtha feed usually comes from crude or other atmospheric

fractionation columns, but can also be derived from other processes which

produce paraffinic-containing hydrocarbons. For example, hydrotreater

processes known in the refining and petrochemical art will produce paraffinic

hydrocarbons from olefinic and aromatic type feed streams that can be used in

the practice of the invention. The term "paraffinic naphtha" will also be

understood to include light straight run (LSR) naphtha, or virgin naphtha, such

as that obtained from a crude distillation unit, and will also include high

paraffinic naphtha feedstreams resulting from extraction processes, all of which

are known in the art.

SUMMARY OF THE INVENTION

The above objects and further advantages are achieved by the

improved process and apparatus of the invention in which a downward flow

fluidized catalyst reactor is added as an ancillary reactor to the existing FCC

process unit operation. The ancillary downflow reactor system, utilizes the same

hot regenerated catalyst as is used in the FCC unit. The regenerated catalyst and

a virgin paraffinic naphtha feedstream derived from a source that is independent

of the FCC unit are introduced and thoroughly mixed in an upper portion of the

downflow reactor that is above the reaction zone.

The mixture passes through the reaction zone with a residence time of 0.1

seconds to 5 seconds, and preferably in a range of 0.2 seconds to 2 seconds,

where the reaction zone operating temperature is from 900° F to 1,200° F. The

ratio of catalyst-to-naphtha, also referred to as the catalyst-to-oil ratio, in the

reaction zone is in the range of from 10 percent to 80 percent by weight, with a

preferred operating range of from 20 percent to 50 percent by weight. The

determination of the catalyst-to-oil ratio is an indication of operating severity

and its determination is well known to the art.

The efficient operation of this process is dependent upon the introduction

of a single feedstream consisting of a virgin paraffϊnic naphtha feed. The

relatively low residence times and higher catalyst-to-oil ratios of 20 to 50

percent by weight are specific to the paraffmic naphtha feedstream. The

introduction of other hydrocarbons into the feedstream of the secondary

downflow reactor will adversely affect the yields of the desired lighter

hydrocarbon reaction products.

The downfiow reactor provides several advantages including the relative

ease of separating the desired end products from other components.

The improved process of the invention can be utilized with prior art

FCC units, whether they employ riser cracking in an upward or downward flow

reaction scheme, or bed cracking to catalytically convert naphthas to the desired

lighter hydrocarbons.

Any existing FCC catalyst can be employed in the practice of the

improved process of the invention. Typical FCC catalysts with or without

catalyst additives are suitable for use in this process improvement.

BRIEF DESCRIPTION OF THE DRAWINGS

The apparatus and method of the invention will be described in further

detail below and with reference to the attached drawings in which:

FIG. 1 is a simplified schematic illustration of a typical FCC apparatus

and process of the prior art; and

FIG. 2 is a simplified schematic illustration of an embodiment of the

apparatus and process of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As indicated above, the method and apparatus of the present invention

can be employed with any number of FCC process units known to the prior art.

With reference to Fig. 1, a typical prior art FCC process is schematically

illustrated. The reactor vessel (10) receives the hydrocarbon, or oil, feedstock

(12) that is admitted into the lower end of reactor riser (14) where it is mixed

with fresh and/or regenerated catalyst that is transferred by a conduit (22). For

the purpose of this simplified schematic illustration and description, the

numerous valves, temperature sensors, electronic controllers and the like that

are customarily employed and well known to those of ordinary skill in the art

are not included.

In this continuous process, the mixture of catalyst and FCC reactor

feedstream proceed upward through the riser into a reaction zone in which the

temperature, pressure and residence time are controlled again within ranges that

S

are conventional and related to the operating characteristics of the one or more

catalysts used in the process, the configuration of the apparatus, the type and

characteristics of the feedstock and a variety of other parameters that are well

known to those of ordinary skill in the art and which form no part of the present

invention. The reaction product is withdrawn through conduit (16) for recovery

and/or further processing in the refinery.

The spent catalyst from the FCC unit is withdrawn via transfer line (18)

for delivery to the lower portion of regeneration vessel (20), most conveniently

located in relatively close proximity to FCC unit (10). The spent catalyst

entering through transfer line (18) is contacted by at least a stream of air

admitted through conduit (24) for controlled combustion of any accumulated

coke. The flue gases are removed from the regenerator via conduit (26), and the

temperature of the regenerated catalyst is raised to provide heat for the

endothermic cracking reaction in the reactor vessel (10).

The method of the present invention will now be described with reference

to Fig. 2. It will be understood that the reactor (10) and regeneration vessel (20)

include components common to those described in connection with Fig. 1 and

their description and functioning will not be repeated. The novel apparatus

component and method of operation depicted in Fig. 2 relates to the downflow

reactor (30) which hot receives regenerated catalyst via transfer line (28) that is

introduced into an upper portion of the vessel (30). Feedline (32) introduces a

parafflnic naphtha feedstream from a source other than the FCC unit for mixing

with the incoming regenerated catalyst from regenerator (20). The mixture of

naphtha and catalyst passes into a reaction zone (34) that is maintained at a

temperature that ranges from 900° F to 1,200° F. The ratio of the catalyst-to-

naphtha is in the range from 20 percent to 50 percent by weight The residence

time of the mixture in the reaction zone is from about 0.2 seconds to about 2

seconds.

Although a variety of catalysts can be utilized in the process, it will be

understood that the same catalyst used in the main FCC unit is also employed in

the catalytic cracking of the paraffinic naphtha feedstream in downflow reactor

(30). In the practice of the invention it is preferred that zeolite catalysts of the Y

type be used alone or in combination with ZSM-5 catalysts. As will be

understood by those of ordinary skill in this art, catalysts additives can also be

used with either of these systems. The choice of the catalyst(s) system does not

form a part of the present invention.

With continuing reference to Fig. 2, the light reaction product stream is

recovered via line (34). In accordance with the method of the invention, the

light hydrocarbon reaction product stream containing ethylene, propylene,

butylenes, gasoline, and any other by-products from the cracking reactions, is

withdrawn and can be either recovered separately in a segregated recovery

section (not shown) or combined with the reaction product stream from the FCC

unit for further fractionation and eventual recovery.

Stripping steam is admitted through line (36) to drive off any removable

hydrocarbons from the spent catalyst. These gases are discharged from the

downflow reactor (30) and introduced into the upper portion of the stripper

vessel (37) where these combined gases, or vapors, pass through cyclone

separators (39) and out of the stripper vessel via line (34) for product recovery

in accordance with methods known to the art.

The spent catalyst from the downflow reactor (30) is discharged through

transfer line (40) and admitted to the lower end of the diptube, or lift riser, (29)

which extends from the lower portion of the modified catalyst regenerator (20).

In this embodiment, air is introduced below the spent catalyst transfer line (40)

at the end of diptube or lift riser (29) via pressurized air line (25). A more

detailed description of the functioning of the secondary downflow reactor (30)

is provided below.

The configuration and selection of materials for the downflow reactor

(30), as well as the specific operating characteristics and parameters will be

dependent upon the specific qualities and flow rate of the paraffinic naphtha

feed introduced at feedline (32), which in turn will be dependent upon the

source of the feedstock. More detailed operating conditions are also set forth

below.

It is to be understood that the present invention broadly comprehends a

method of producing a product stream consisting primarily of the light olefins

ethylene, propylene and butylenes, and of gasoline in conjunction with the

processing of a petroleum feedstock in a fluidized catalytic cracking (FCC) unit

containing a catalyst of specified composition, the FCC and associated

downflow reactor catalyst feed being regenerated from spent catalyst, and the

method including the steps of:

a. providing a separate paraffinic naphtha feedstream and

directing it into an upper portion of a downflow reactor that

is proximate the FCC unit;

b. introducing regenerated catalyst of the same type used in the

FCC unit into the downflow reactor for mixing with the

paraffinic naphtha feedstream in a ratio of catalyst-to-

feedstream in the range from 10 percent to 80 percent by

weight;

c. passing the catalyst and paraffinic naphtha mixture through a

reaction zone in the downstream reactor that is maintained at

a temperature that ranges from 900° F to 1.200° F for a

residence time of from 0.1 seconds to 5 seconds to crack the

naphtha;

d. separating the reaction products stream containing light

olefins and gasoline from spent catalyst;

e. recovering the reaction product stream; and

f. passing the spent catalyst from the downflow reactor to a

separate regeneration vessel that also contains spent catalyst

from the FCC unit for regeneration and recycling to the FCC

unit and the downflow reactor.

With continuing reference to Fig. 2, the hot regenerated catalyst at

approximately 1250° F to 1500° F is transferred from the regenerator vessel

(20) of the FCC process by conventional means, e.g., through a downwardly

directed conduit or pipe (28), commonly referred to as a transfer line or

standpipe, to a withdrawal well or hopper (31) at the top of the downflow

reactor above the reaction zone (33) where the hot catalyst flow is allowed to

stabilize. in order to be uniformly directed into the mix zone or feed injection

portion of the reaction zone (33). A pressure stabilization line (38) connects the

top of the withdrawal well (31) to the existing regenerator (20).

The naphtha feedstock is injected into the mixing zone through feed

injection nozzles (32a) placed in the immediate vicinity of the point of

introduction of the regenerated catalyst into the downflow reactor (30). These

multiple injection nozzles (32a) result in the catalyst and oil mixing thoroughly

and uniformly. Once the paraffinic naphtha feedstock contacts the hot catalyst

the cracking reactions occur. The reaction vapor of hydrocarbon cracked

products and unreacted naphtha feed and catalyst mixture quickly flows through

the remainder of the downflow reactor and into a rapid separation section (35) at

the bottom portion of the reactor. The residence time of the mixture in the

reaction zone is controlled in accordance with apparatus and procedures known

to the art.

If necessary for temperature control, a quench injection (50) is provided

for the naphtha feed, recycle cracked naphtha or other light olefinic

hydrocarbon near the bottom of the reaction zone (33) immediately before the

separator. This quench injection quickly reduces or stops the cracking reactions

and can be utilized for controlling cracking severity and allows for added

process flexibility.

The reaction temperature, i.e., the outlet temperature of the downflow

reactor, is controlled by opening and closing a catalyst slide valve (not shown)

that controls the flow of regenerated catalyst from the withdrawal well (31) and

into the mix zone. The heat required for the endothermic cracking reaction is

supplied by the regenerated catalyst. By changing the flow rate of the hot

regenerated catalyst, the operating severity or cracking conditions can be

controlled to produce the desired yields of light olefinic hydrocarbons and

gasoline.

The rapid separator (35) along with the end portion of the downflow

reactor (30) is housed in the upper section of a large vessel referred to as the

catalyst stripper (37). The rapid separator directs the reaction vapor and catalyst

directly into the top part the stripper vessel (37).

The reactor vapor stream moves upward from the rapid separator outlet

into the stripper, combines with stripped hydrocarbon product vapors and

stripping gas from the catalyst stripping section of this vessel and passes

through conventional separating means such as cyclones (39), which further

separate any entrained catalyst particles from the vapors. The catalyst from the

separator that is captured in the cyclones is directed to the bottom of the stripper

vessel (37) through a cyclone dipleg for discharge into the bed of catalyst that

was recovered from the rapid separator in the stripping section.

After the combined vapor passes through the cyclones and out of the

stripper vessel, it is directed through a conduit or pipe commonly referred to as

a reactor vapor line (34) to a conventional product recovery section known in

the FCC art.

The catalyst from the rapid separator and cyclone diplegs flows to the

lower section of the stripper reactor vessel (37) that includes a catalyst stripping

section into which a suitable stripping gas, such as steam, is introduced through

streamline (36). The stripping section is provided with several baffles or

structured packing (not shown) over which the downwardly flowing catalyst

passes counter-currently to the flowing stripping gas. The upwardly flowing

stripping gas, which is typically steam, is used to "strip" or remove any

additional hydrocarbons that remain in the catalyst pores or between catalyst

particles.

The stripped catalyst is transported by the combustion air stream (25)

through a lift riser (29) that terminates in the existing regenerator (20) in a

typical FCC process to burn off any coke that is a by-product of the naphtha

cracking process. In the regenerator, the heat produced from the combustion of

the by-product coke produced in the first reaction zone (10 and 14) of a typical

FCC process from cracking heavy hydrocarbons and from the naphtha cracking

in zone (33) of the downflow reactor (30) is transferred to the catalyst.

The regenerator vessel (20) can be of any conventional previously known

design and can be used with the enhanced process and downflow reaction zone

of this invention. The placement of the regenerator-to-reactor conduit (28) or

regenerated catalyst transfer line for the regenerator will be such that it insures a

steady and continuous flow of a substantial quantity of regenerated catalyst that

is needed to meet the maximum design requirements of the downflow reactor.

The catalyst requirements for the process of the invention can be

determined in conjunction with any catalyst conventionally used in FCC

processes, e.g., zeolites, silica-alumina, carbon monoxide burning promoter

additives, bottoms cracking additives, light olefin-producing additives and any

other catalyst additives routinely used in the FCC process. The preferred

cracking zeolites in the FCC process are zeolites Y, REY, USY, and RE-USY.

For enhanced naphtha cracking potential, a preferred shaped selective catalyst

additive typically used in the FCC process to produce light olefins and increase

FCC gasoline octane is ZSM-5 zeolite crystal or other pentasil type catalyst

structure. This ZSM-5 additive is mixed with the cracking catalyst zeolites and

matrix structures in conventional FCC catalyst and is preferably used in the

method of the invention to maximize and optimize the paraffinic naphtha

cracking in the downflow reactor.

A particular advantage of this invention as an enhancement to an existing

FCC process for typical FCC heavy hydrocarbon feedstocks is the amount on

coke produced from these cracking reactions. In naphtha cracking, the overall

unit operational efficiency is adversely effected by the limited amount of coke

produced during the cracking reactions. The amount of coke produced is not

sufficient to produce enough heat during catalyst regeneration to allow for the

paraffinic naphtha cracking reactions to occur in the downflow reactor. By

comparison, the coke produced during the heavy oil or gasoil cracking in the

typical FCC process is more than adequate to provide the required heat to the

downflow reactor. In the method of the invention, this heat is transferred from

the regenerator to the downflow reactor by the regenerated catalyst by mixing

the spent catalysts from the two sources during the regeneration processing in

vessel (20).

A further advantage of the present invention as an enhancement to

existing FCC processes for co-processing paraffinic naphtha is that the products

can be recovered in the existing recovery section of the unit. The unconverted

paraffinic naphtha can be recycled with the olefinic naphtha in the FCC process

to produce additional light olefins from cracking the olefinic naphtha or for use

as a blending stock in finished gasoline. The process has the advantage of

providing for the separate recovery of the naphthas from each reactor for further

separate downstream processing, with the alternative of combining the two

streams for partial recycling to the FCC unit or for gasoline blending.

Examples

A bench scale pilot plant was used to determine the operating conditions

for obtaining desired product yields from cracking a typical paraffinic naphtha

feedstock. A pilot plant unit was used to represent the cracking conditions in

the downflow reactor.

In the following examples, two catalyst systems are utilized to

demonstrate the potential for cracking paraffmic naphtha to produce light olefin

yields. One catalyst was a typical low rare earth, low hydrogen transfer USY

zeolite catalyst that is commercially available. The second catalyst system was

the same commercially available USY zeolite cracking catalyst blended with a

shape selective ZSM-5 zeolite type cracking catalyst additive.

The following Table summarizes the effects of varying the cracking

severity by changing the reactor temperatures in the pilot unit for both catalyst

systems.

As used in the Table, the term "Selectivity" is defined as the ratio of the

amount of a particular Component to the Total Gas, e.g., Propylene/Total Gas.

It will be understood that the embodiments described above are

illustrative of the invention and that various modifications can be made by those

of ordinary skill in the art that will be within the scope of the invention, which

is to be determined by the claims that follow.